Pulsed corona-induced degradation of organic materials inwaterCitation for published version (APA):Hoeben, W. F. L. M. (2000). Pulsed corona-induced degradation of organic materials in water. Eindhoven:Technische Universiteit Eindhoven. https://doi.org/10.6100/IR535691

DOI:10.6100/IR535691

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Pulsed corona-induced degradation of organic materials in water

PROEFSCHRIFT

ter verkrijging van de graad van doctoraan de Technische Universiteit Eindhoven,

op gezag van de Rector Magnificus, prof.dr. M. Rem,voor een commissie aangewezen door het College

voor Promoties in het openbaar te verdedigen op donderdag 15 juni 2000 om 16.00 uur

door

Wilhelmus Frederik Laurens Maria Hoeben

geboren te Geldrop

Dit proefschrift is goedgekeurd door de promotoren:

prof.dr. W.R. Rutgersenprof.dr.ir. C.A.M.G. Cramers

Copromotor:dr.ir. E.M. van Veldhuizen

CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN

Hoeben, Wilhelmus Frederik Laurens Maria

Pulsed corona-induced degradation of organic materials in water / byWilhelmus Frederik Laurens Maria HoebenEindhoven: Technische Universiteit Eindhoven, 2000. -Proefschrift.-ISBN 90-386-1549-3NUGI 812Trefw: gepulste corona / elektrische ontladingen / AOP / fenol / afbraak /oxidatie / conversie / efficiëntie / vloeistofchromatografieSubject headings: pulsed corona / electrical discharges / AOP / phenol / degradation /oxidation / conversion / efficiency / liquid chromatography

This project has been financially supported by “Technologie voor DuurzameOntwikkeling (TDO)”, Technische Universiteit Eindhoven.

Ontwerp omslag: B. Mobach / TUEDrukwerk: Universiteitsdrukkerij Technische Universiteit Eindhoven

“A company has control of its production only if it also knows the make-up of its waste water”

[Ullmann, Encyclopedia of Industrial Chemistry 1994, 5th edn. Vol. B6, Weinheim: Verlag Chemie, ISBN 3-527-20136-X, 474]

Aan mijn ouders, broer en zus

Contents

1. Introduction .......................................................................................... 11.1 Advanced oxidation processes ..................................................................... 21.2 Electrical discharges ................................................................................... 61.3 Model compounds...................................................................................... 71.4 Thesis scope ............................................................................................. 9

2. Theory ..................................................................................................112.1 Corona discharges.....................................................................................112.2 Oxidizers..................................................................................................132.3 Degradation of organic compounds ..............................................................152.4 Oxidation of model compounds ...................................................................20

2.4.1 Phenol ..........................................................................................202.4.2 Atrazine ........................................................................................232.4.3 Malachite green..............................................................................232.4.4 Dimethyl sulfide .............................................................................24

2.5 Diagnostics ..............................................................................................242.5.1 Chemical diagnostics ......................................................................252.5.2 Electrical diagnostics.......................................................................312.5.3 Optical diagnostics .........................................................................32

3. Experimental setup.............................................................................373.1 Reagents and reactors ...............................................................................373.2 Chemical diagnostics .................................................................................403.3 Electrical diagnostics .................................................................................453.4 Optical diagnostics ....................................................................................46

4. Results .................................................................................................494.1 Pulsed corona discharges ...........................................................................49

4.1.1 Hydroxyl radicals............................................................................494.1.2 Ozone...........................................................................................554.1.3 Corona pulse energy .......................................................................594.1.4 Corona treatment of deionized water.................................................63

4.2 Oxidation of phenol ...................................................................................654.2.1 Chromatography.............................................................................654.2.2 Mass spectrometry .........................................................................874.2.3 Spectroscopy.................................................................................924.2.4 Electrical conductometry ...............................................................1024.2.5 Microtox ecotoxicity .....................................................................1054.2.6 Total organic carbon .....................................................................108

4.3 Oxidation of other model compounds .........................................................1094.3.1 Atrazine ......................................................................................1094.3.2 Malachite green............................................................................1104.3.3 Dimethyl sulfide ...........................................................................112

5. Discussion ............................................................................................1135.1 Pulsed corona discharges .........................................................................1135.2 Corona-induced phenol oxidation ...............................................................1165.3 Phenol oxidation pathways .......................................................................1225.4 Analysis techniques.................................................................................1375.5 AOP comparison .....................................................................................141

6. Conclusions..........................................................................................1456.1 Pulsed corona discharges .........................................................................1456.2 Oxidation of model compounds .................................................................1466.3 Analytical techniques...............................................................................1476.4 Outlook .................................................................................................148

7. References ...........................................................................................149

Summary ...................................................................................................159

Samenvatting............................................................................................161

Dankwoord / Acknowledgements .........................................................163

Curriculum Vitae ......................................................................................164

1. Introduction

Since a long time, natural processes have not been able anymore, to rectify theenvironmental load caused by the ever-increasing world population. Our water reservesare a main issue of interest, because pollution from both the atmosphere and soil willeventually enter the aqueous phase by deposition and percolation respectively.

Sources of pollution are both nature and mankind. Examples of natural pollution arevolcanic activity, forest fires and decomposition of vegetation. Pollution by mankind iscaused by e.g. nutrition, transportation, accommodation, synthesis and energyexploitation. Although probably not always acknowledged, chemical activity isindispensable to sustain life; also it is needed to comply with a high standard of living.Examples are medicaments, cleaning and disinfecting products, cosmetics, stabilizers,artificial fertilizers, pesticides, fuel, batteries, polymers (thermoplastics, thermosettingresins, elastomers, fibers), paint and dyes.

Both synthesis and application of these product classes inevitably yield pollution. Inaddition to biological waste like carbohydrates, proteins, urea, fats, food & vegetationresidues and carbon dioxide, we also encounter priority compounds. These materialsexhibit carcinogenic, mutagenic and/or teratogenic properties, which implies that a no-effect-level in fact does not apply for these compounds. In addition, priority compoundscan be highly persistent.Some organic priority compounds are for instance [1]: halogenated dioxins/benzofurans/xanthenes from the incineration of halogenated phenols, polychlorinatedbiphenyls (PCB’s) used as dielectric media, fire retardants; polycyclic aromatichydrocarbons (benzo[a]pyrene, dibenzo[a:h]anthracene) in soot and coal tar/pitch fromthe incomplete combustion of hydrocarbons and from coal gasification; simple aromatichydrocarbons (benzene, nitrobenzene, p-dichlorobenzene, o-phenylenediamine) used asprecursors in organic chemical synthesis; chlorinated aliphatics (chloroform,tetrachloromethane, trichloroethylene) applied as solvent and/or stain remover;pesticides (DDT, kepone, lindane) for crop protection and pest control; ammunition(TNT, picric acid, nitroanilines); monomers (acrylonitrile, vinylchloride, urethane) fromthe synthesis, processing and incomplete combustion of polymers, dyes (benzidine-based) for the colorization of e.g. textile, leather and polymers.Inorganic priority compounds are for instance heavy metals & salts (Cd, Ni, Cr),asbestos, arsenic/compounds, beryllium/compounds and radioactive materials.

Although we left the ages of unscrupulous operation long ago, we inherit innumeroushighly polluted waste sites of former gasworks, ammunition and pesticide plants, oil/gasdrill and refinery locations, mining sites, fuel stations, dry-cleaning facilities, wastedump and incineration sites. Conventional microbiological degradation desperately needsthe assistance of new technologies, like for instance advanced oxidation processes, todegrade hazardous persistent materials by chemical oxidation.

2 Chapter 1.

1.1. Advanced oxidation processes

Advanced Oxidation Processes (AOP’s) aim at the in-situ production of strong oxidizers.The oxidizing power is reflected by the standard reduction potential E0. Table 1.1 showssome oxidizers in decreasing power order and E0 values, expressed for reduction half-cell reactions [2,3]. The potential is defined relative to the standard hydrogen electrodepotential [4]. The Gibbs free energy change ∆G of the redox-reaction is calculated fromthe resulting electromotive force of both half-cell reactions corrected for activitydependence (E), the number of electrons involved (n) and the Faraday constant(F=96485 C/mol), see Equation 1.1.

Table 1.1 Standard reduction potential values for some oxidizers at T=298.15 K,for acidic conditions pH=0 applies.

Reduction half-cell reaction E0 (V)XeF+ e- → Xe + F- 3.42OF2 (g) + 4H+ + 4e- → O2 (g) + 4HF 3.29OH + H+ + e- → H2O 2.56O (g) + 2H+ + 2e- → H2O 2.43O3 + 2H+ + 2e- → O2 + H2O 2.08H2O2 + 2H+ + 2e- → 2H2O 1.76HClO2 + 2H+ + 2e- → HClO+H2O 1.67HO2 + H+ + e- → H2O2 1.44Cl2 + 2e- → 2 Cl- 1.40

(1.1)

The strongest oxidizers known are xenonfluoride (XeF) and possibly H4RnO6, but theseoxidizers are not commercially attractive for water treatment because of both extremereactivity and remaining toxicity in reduced form. Also, halogen-based oxidizers are notacceptable as oxidizer, because they halogenate organic materials to e.g.trihalomethanes [5] which are very harmful compounds; in addition their reaction leadsto salt formation. It is obvious, that metal-based oxidizers like permanganate (MnO4

-)and dichromate (Cr2O7

2-) also are not desirable. Of interest are thus oxygen-basedhalogen/metal-free oxidizers like the hydroxyl radical (OH), atomic oxygen (O), ozone(O3) and hydrogen peroxide (H2O2).

Next, a concise description is presented for major AOP’s with regard to the generationof oxygen-based halogen-free oxidizers, particularly hydroxyl radicals. A comparison ofAOP’s is discussed in section 5.5.

EFnG ⋅⋅−=∆

Introduction 3

Ozone-UV oxidation

In the ozone-UV technology [6,7], hydroxyl radicals are produced from ozone, waterand UV photons; high-pressure mercury or xenon lamps deliver the photons, seeEquation 1.2.

O3 + H2O + hν → 2OH + O2 λ≤310 nm (1.2)

Ozone is produced on location by an ozonizer, which converts atmospheric or pureoxygen into ozone by corona discharges [8,9]. These electrical discharges are producedin a barrier discharge electrode setup, where the electrodes are separated by a dielectricmaterial e.g. glass or ceramic at a thickness of about 0.5-3 mm. The applied voltage is8-30 kV and the frequency range is 60-2000 Hz. The energy efficiency is about 60g/kWh for air or 120 g/kWh for oxygen [10]. The theoretical maximum efficiency iscalculated from the standard formation enthalpy change ∆Hf

0=144.8 kJ/mol for thereaction 3O2→2O3 and is about G=1193 g O3/kWh. Commercial ozone generators arebased on different electrode configurations, e.g. fluid-cooled shell & tube typegenerators for generation of large ozone amounts and air-cooled plate type generatorsfor small amounts. Cooling is very important, to prevent decomposition of ozone.

Hydrogen peroxide-UV and Fenton oxidation

Hydrogen peroxide is decomposed by UV photons into hydroxyl radicals [11], seeEq.1.3a. Also, the reaction of hydrogen peroxide with iron (II) ions produces hydroxylradicals; this reaction is known as the Fenton reaction (Eq.1.3b) [12]. In addition, Fe(III)ions contribute to hydroxyl radical formation by Eq.1.3c/d (Fenton like reaction) andindirectly by regeneration of Fe(II).

H2O2 + hν → 2OH 250 nm<λ<300 nm (1.3a)Fe2+ + H2O2 → OH + OH- + Fe3+ (1.3b)Fe3+ + OH- Fe(OH)2+ (1.3c)Fe(OH)2+ + hν → OH + Fe2+ λ=350 nm (1.3d)

The advantage of photo-Fenton/Fenton like reactions over hydrogen peroxide-UV ismainly explained by the efficient use of light quanta, because the absorption of Fe(III)chelates (hydroxo, carboxyl) extends to the near UV-visible region and their molarabsorption coefficient is relatively high compared to the molar absorption coefficient ofhydrogen peroxide. Synthesis of hydrogen peroxide is mainly performed according tothe following processes [13,14]:*Anthraquinone (AO) process: Reduction of a 2-alkyl-9,10-anthraquinone to thecorresponding hydroquinone by hydrogen, followed by the oxidation of thehydroquinone by oxygen to hydrogen peroxide and the anthraquinone.*2-Propanol process: Oxidation of 2-propanol by oxygen produces 2-propanol-2-hydroperoxide, which decomposes into hydrogen peroxide and acetone.*Electrochemical processes: Anodic oxidative coupling of sulfate ions to persulfate ions,followed by hydrolysis of the persulfate via the peroxomonosulfate to hydrogenperoxide and bisulfate ions.The theoretical maximum efficiency, calculated from the standard formation enthalpychange ∆Hf

0=98.3 kJ/mol for the reaction H2O (l) +½O2 (g) → H2O2 (l) is aboutG=1246 g H2O2/kWh.

4 Chapter 1.

Photocatalytic oxidation

Photocatalytic oxidation produces hydroxyl and hydroperoxyl radicals at an irradiatedsemiconductor surface in contact with water [15,16]. Excitation of electrons in thesemiconductor surface layer by UV photons will promote electrons from the valenceband to the conductivity band. In this way electron-deficient holes (h+) are created inthe valence band and free electrons (e-) will be available in the conductivity band.Equations 1.4a-f are the main reactions, that take place at the irradiated semiconductorsurface. Water is absorbed onto the surface, resulting in the formation of H+ and OH-

ions, see Eq.1.4a/b. Hydroxyl radicals are produced by oxidation of water (Eq.1.4c) oroxidation of hydroxyl ions (Eq.1.4d), while hydroperoxyl radicals are obtained from thesuperoxide anion (O2

-), see Eq.1.4e/f.

2H2O + 4h+ → 4H+ + O2 (1.4a)2H2O + 2e- → 2OH- + H2 (1.4b)H2O + h+ → OH + H+ (1.4c)OH- + h+ → OH (1.4d)O2 + e- → O2

- (1.4e)O2

- + H+ → HO2 (1.4f)

Some applied semiconductors are titanium oxide (TiO2), zinc oxide (ZnO) and cadmiumsulfide (CdS). The most well-known is the anatase crystal structure of TiO2. Its band-gap energy is 3.2 eV; the irradiation wavelength λ<385 nm. TiO2 has favourablephotochemical stability and photocatalytic activity.

Wet oxidation

In wet oxidation, water with dissolved oxygen is used to oxidize the target compound[17,18]. The process can be performed at e.g. subcritical (4 MPa<p<20 MPa,513K<T<593K) or supercritical conditions (p>22.1 MPa, T>647K). These conditionsenable optimal solubility of oxygen and organic compounds in water. Metal ions can beadded to catalyze the oxidation. Equations 1.5a-h are the main reactions. Hydroxylradicals are produced from the dissociation and oxidation of water (Eq.1.5a/b).Hydroperoxyl radicals are formed from the oxidation of water (Eq.1.5b) and the targetcompound RH (Eq.1.5f). Hydroxyl radicals are also produced from hydrogen peroxide(Eq.1.5d) and from the reaction of atomic oxygen with the target compound (Eq.1.5h).Hydrogen peroxide is produced by recombination of hydroperoxyl radicals (Eq.1.5c) orby reaction of hydroperoxyl radicals with the target compound (Eq.1.5g). Atomicoxygen is produced from the dissociation of oxygen (Eq.1.5e). Although thehydroperoxyl radical is less reactive than the hydroxyl radical, it plays an important rolebecause of its relative abundance.

H2O → OH + H (1.5a)H2O + O2 → OH + HO2 (1.5b)2HO2 → H2O2 + O2 (1.5c)H2O2 → 2OH (1.5d)O2 → 2O (1.5e)RH+O2 → R + HO2 (1.5f)RH + HO2 → R + H2O2 (1.5g)RH + O → R + OH (1.5h)

Introduction 5

Radiolysis

Irradiation of water by high-energy photons or electrons dissociates water moleculesinto hydroxyl radicals and hydrogen atoms or ionizes water molecules, see Eq.1.6a/b[19,20]. Ionized water molecules react with water to produce hydroxyl radicals, seeEq.1.6c. By saturation of the water with nitrous oxide (N2O), solvated electrons(Eq.1.6d) are converted into hydroxyl radicals (Eq.1.6e). Also the target compound isdissociated or ionized. Halogenated target compounds RXn react rapidly with solvatedelectrons, see Eq.1.6f.

H2O → OH + H (1.6a)H2O → H2O+ + e- (1.6b)H2O+ + H2O → H3O+ + OH (1.6c)e- + H2O → eaq

- (1.6d)N2O + eaq

- + H2O → N2 + OH + OH- (1.6e)RXn + eaq

- → RXn-1 + X- (1.6f)

High-energy photons are obtained from a radioactive source (60Co-γ) and electrons areproduced by an electron beam accelerator or a Van de Graaff generator.

Ultrasonic irradiation

The introduction of ultrasonic energy into a liquid causes electrohydraulic cavitation[21,22]. The applied frequency range is from 15 kHz up to 1 MHz. The generation ofultrasound energy can be performed by electromechanical (piezoelectric or magneto-strictive) or liquid-driven (liquid whistle = low intensity) transducers.The cavitation process involves the oscillation of the radii of pre-existing gas cavities bythe periodically changing pressure field of the ultrasonic waves. The rapid implosion ofthe eventually instable gas bubbles causes adiabatic heating of the bubble vapourphase. In this way, localized and transient high temperatures and pressures are reached,e.g. p>300 bar and T>3300 K in aqueous solution. These vigorous conditions invokedissociation and pyrolysis of the liquid phase molecules and present target compounds.Water will be dissociated into hydroxyl radicals and hydrogen atoms, see Eq.1.7a.Organic compounds are dissociated into radicals (Eq.1.7b/c) and functional groups likecarboxyl and nitro groups are removed, see Eq.1.7d/e.

H2O → OH + H (1.7a)AB → A + B (1.7b)RXn → RXn-1 + X (1.7c)RCOOH → RH + CO2 (1.7d)RNO2 → RO + NO (1.7e)

6 Chapter 1.

1.2. Electrical discharges

The discharge of electric energy into a dielectric medium may cause dissociation,ionization and excitation of the dielectric molecules or atoms [23]. Depending on theenergy input, the produced plasma is non-thermal or thermal. In thermal plasmas theionization level is high, about 10-2. Examples of thermal electrical discharges arelightning and arc discharges. Typical numbers of electron density (ne) and electronenergy (Te) for lightning discharges are about ne=1⋅1017-5⋅1017 cm-3 and Te=2.2 eV(corresponding to 25000 K). Corona and glow discharges are non-thermal plasmas.Their ionization level is very low, about 10-6. The electron density of a corona plasma isabout ne=1013 cm-3. The chemical reactivity of corona discharges is based on the fact,that the electric field strength at the discharge streamer heads is extremely high viz.about 200 kV/cm, corresponding to 1000 Td. This implies an average electron energyof about Te=10 eV, which reaches beyond the dissociation energy of water (5.16 eV),oxygen (5.17 eV) and nitrogen (9.80) [24]. Within the energy distribution of electronsin the streamer head, even higher energetic electrons exist that cause ionization [25] ofoxygen (12.07 eV), water (12.62 eV) and nitrogen (15.58 eV).A very particular advantage of pulsed corona discharges is the fact, that a highlyreactive streamer discharge medium is created, while the bulk gas is at ambienttemperature and pressure [26,27]. Therefore, pulsed corona promises higher efficiencythan other advanced oxidation processes.Corona discharges in water produce hydroxyl radicals and hydrogen atoms from thedissociation and ionization of water molecules, see Eq. 1.8a-c. In a humid gas phase,corona discharges additionally create radicals, ions and metastables from thedissociation and ionization of the gas phase molecules or atoms. In humid air, thefollowing main oxidizer species are produced: hydroxyl radicals, ozone, atomic oxygen,singlet oxygen and hydroperoxyl radicals, see Eq. 1.8a-n. Also, small amounts ofnitrogen oxides like NOx and N2O are formed according to Eq. 1.8o-r.

H2O + e- → OH + H + e- dissociation (1.8a)H2O + e- → H2O+ + 2e- ionization (1.8b)H2O+ + H2O → H3O+ + OH dissociation (1.8c)N2 + e- → N2

* + e- excitation (1.8d)O2 + e- → O2

* + e- excitation (1.8e)N2 + e- → 2N + e- dissociation (1.8f)O2 + e- → 2O + e- dissociation (1.8g)N2 + e- → N2

+ + 2e- ionization (1.8h)O2 + e- → O2

+ + 2e- ionization (1.8i)O2 + e- → O2

- attachment (1.8j)O2 + e- → O- + O dissociative attachment (1.8k)O2 + O → O3 association (1.8l)H + O2 → HO2 association (1.8m)H + O3 → HO3 association (1.8n)N + O → NO association (1.8o)NO + O → NO2 association (1.8p)N2

+ + O2- → 2NO recombination (1.8q)

N2 + O → N2O association (1.8r)

Next to these reactions, many others exist [28].

Introduction 7

Figure 1.1 shows a typical CCD image of pulsed positive corona discharges in air over awater surface. The bright areas are a superposition of about 100 streamer dischargechannels. The streamer channels are thin; their diameter is 1 mm or less. The streamerdischarges start from a 30 pins anode and then propagate towards the gas-liquidinterface. The discharges do not enter the aqueous phase due to the high relativepermittivity of water. The cathode plate is situated directly outside and underneath theglass reactor vessel.

Figure 1.1 CCD image of pulsed positive corona discharges in air over a watersurface. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm. The CCD camera settings are: diaphragm f/5.6, exposure time1 s. The image has been taken by A.H.F.M. Baede.

The first applications of electrical discharges in water date from Russian experiments inthe seventies on pre-breakdown phenomena in liquid dielectrics by Klimkin [29] andAlkhimov [30] and also for the destruction of bacteria in water. In the early eightieswater ozonation was introduced for waste water treatment, while from about 1987 thepulsed corona discharge technology was introduced for the degradation of waterpollutants by Clements and Sato [31].Till now, pulsed corona discharge technology has been in an experimental stage, mainlyfor the removal of nitrogen oxides and sulfur dioxide from flue gas and the destructionof hydrocarbons and odor components in waste gas [27].

1.3. Model compounds

In this thesis, the oxidizing power of pulsed corona discharges is investigated withregard to organic compounds. The choice has been made to study some well-knowncompound classes viz. the bulk chemical phenol, the herbicide atrazine, the dyemalachite green and the odor component dimethyl sulfide, see Figure 1.2. Phenol,atrazine and malachite green have been degraded in aqueous solution, dimethyl sulfidehas been degraded in the gas phase. A summary of particular compound properties andapplications is discussed now.

8 Chapter 1.

phenol atrazine malachite green dimethyl sulfide

N

N

N

Cl

NHNHS

OH

X-

N+

N

Figure 1.2 Model compounds applied for corona-induced oxidation.

Phenol (hydroxybenzene) [32,33] is a moderately toxic crystalline solid, readilyabsorbed by the skin. Investigations and carcinogenic or co-carcinogenic properties giveambiguous indications. The human lethal dose value is about 140 mg/kg. Phenol is amajor precursor in synthesis of e.g. polymers (phenol-formaldehyde, polycarbonate andepoxy resins) including light stabilizers, pharmaceuticals (acetylsalicylic acid, vitamin E,antioxidants), microbicides/fungicides (chlorinated phenols, hydroxybiphenyls), dyes(azo, nitro, triarylmethane, from aniline raw material), photochemicals (diazocoupling,developers) surfactants (alkylphenols), fragances (phenolic ethers); therefore phenoloccurs in waste flows released from the synthesis of these compounds.Also during oil refinery and cokes production phenol is set free. Lignins, thebiopolymeric construction materials of plant cell walls and wood tissue, consist of e.g.hydroxybenzene-functional monomeric units [34]. Lignins occur in waste flows frompaper mills. The proper water solubility of phenol makes it ideally suitable for corona-induced aqueous phase oxidation.

Atrazine (6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine) is a harmfuland persistent herbicide [35,36]. It is widely used for selective weed control (broadleafand grassy weeds) in corn, sugar cane and asparagus but is also used for nonselectiveweed control on noncropped land. Atrazine is mobile in sand and loam and low tointermediately mobile in clay loam; irrigation induced leaching and dilution has led toatrazine residues persisting for 3 years in soil of irrigation ditches at depths certainly upto 90 cm. Atrazine is a possible human carcinogen [1]. The maximum permissibleconcentration of atrazine in drinking water is 0.1 ppb.

Malachite green [37,38] is a triphenylmethane cationic dye. These dye types favour ahigh color strength and brilliance, but their light fastness is generally poor. Its greencolor is due to the absorption of the wavelengths λ=427.5 nm (FWHM=40 nm, log(ε)=4.30) and λ=621 nm (FWHM=60 nm log (ε)=5.02). Malachite green is applied forcoloring paper, leather, inks, waxes and polyacrylonitrile fibers. Malachite green exhibitsacute oral toxicity and is a suspected human carcinogen due to its photosensitizingproperties. It causes acute lethal toxicity in fish, inhibits algae growth and also inhibitsactivated sludge.

Introduction 9

Dimethyl sulfide [39,40] is a highly volatile liquid spreading a very unpleasant odor. Theodor detection limit is about 0.75 mg/m3. The boiling point is 37°C and partial pressureis p293K=55.5 kPa. On industrial scale, dimethyl sulfide is emitted from e.g. Kraftpulping paper mills, fish processing and from incineration of cattle cadavers. Dimethylsulfide is used as marker for odourless gases. Organic sulfides also occur in abundancefrom natural sources. Inhalation of dimethyl sulfide may cause narcosis and paralysis ofthe nerve system controlling the respiration and circulation. The maximum allowableconcentration is suggested to equal the odor detection limit, about 0.75 mg/m3 equallyto 0.29 ppm.

1.4. Thesis scope

The scope of this thesis is an investigation of the applicability and technical feasibilityof pulsed positive corona discharges for the degradation of organic materials at lowconcentration in aqueous solution. Although research on aqueous phase remediation byAOP’s is a topic of growing interest since about 20 years, corona research for thesepurposes is still rather exotic. For this reason a multidisciplinary project has beenperformed in which the following knowledge sources have participated: the physics ofelectrical discharges, physical-chemical analysis and organic chemistry. In this way anadequate fundamental comparison can be made between corona discharge technologyand other more known AOP’s; a financial consideration of corona with regard to otherAOP’s and conventional waste water treatment has not been part of this project. Thestudy is based on the model compounds phenol, atrazine, malachite green and dimethylsulfide; the corona-induced degradation of phenol has been studied in detail. Key partsare the conversion efficiency and identity of the oxidation products.

In chapter 2, a concise description is presented with regard to corona discharges,oxidizer properties, degradation of organic compounds and applied chemical/electrical/optical diagnostics. Chapter 3 describes the applied reagents, reactors, diagnostics andconfigurational settings.Chapter 4 represents the experimental results of corona-induced oxidizer production andmodel compound degradation. The corona-induced production of the oxidizers hydroxylradicals and ozone is discussed in section 4.1. In-situ electron spin resonance and ex-situ molecular probe fluorescence spectrometry have been applied for the detection ofhydroxyl radicals, while the formation of ozone has been quantified using in-situabsorption spectrometry.Section 4.2 describes the corona-induced degradation of phenol. The conversionefficiency has been determined from the conversion and required energy input. Theconversion has been determined by liquid chromatography, namely ion-exclusionchromatography and reversed-phase high performance liquid chromatography. Theenergy input has been determined from corona pulse voltage and current waveformmeasurements. In addition to ex-situ conversion determination, oxidation progress hasalso been monitored by application of in-situ laser-induced fluorescence spectroscopy.The obtained phenol oxidation product mixtures have been analyzed by severalanalytical techniques. Primary product identification has been performed by IonSprayand electron-impact mass spectrometry. The environmental impact has been studied byMicrotox ecotoxicity tests and total organic carbon measurements.

10 Chapter 1.

Electrical conductometry has been applied to monitor phenol oxidation progress by theformation of carboxylic acids. The analysis of gaseous phenol oxidation products hasbeen studied by Fourier transform infrared spectroscopy and an aldehyde screeningtest. Oxidation pathway models have been constructed in order to account for thecomposition of the phenol oxidation product mixture.In section 4.3 the corona-induced degradation of atrazine, malachite green and dimethylsulfide is described. Atrazine conversion has been determined by reversed-phase HPLC,malachite green degradation by several reactor geometries has been measured bydecolorization using absorption spectrometry. The oxidation of dimethyl sulfide hasbeen measured by gas chromatography.The influence of different corona parameters and reactor configurations has beendetermined from phenol conversion and malachite green decolorization measurements;this has resulted in a preferred corona configuration.In chapter 5, a discussion is presented about pulsed corona discharges, corona-inducedphenol degradation, phenol oxidation pathways, analysis techniques and a fundamentalcomparison of different AOP’s. The conclusions are summarized in chapter 6.

2. Theory

This chapter starts with a fundamental description of corona discharges in air. Oxidizerproperties and general degradation pathways of organic compounds are discussed,followed by specific model compound oxidation products reported from literature.Finally the applied chemical, electrical and optical diagnostics are concisely summarized.

2.1. Corona discharges

The requirements for the formation of corona discharges in air at atmosphericconditions are a sharply non-uniform electrical field and a starting condition [23]. Thesharply non-uniform E-field is achieved by applying a high voltage to e.g. a point-to-plane electrode configuration in air. Corona triggering is enabled, if an ion-electron pairis produced within the inception region of the high voltage electrode, where theelectron can gain enough energy to ionize molecules of the dielectric. The ion-electronpair is usually produced by cosmic rays or natural radioactivity, which cause ionizationof air at a rate of about 109 m-3s-1. The polarity of a corona can be either positive ornegative. The positive corona consists of cathode-directed streamer discharge channels,while the negative polarity corona is anode-directed.

Positive polarity corona

Once the ion-electron pair is formed within the inception area, the electron isaccelerated in the electric field, see Figure 2.1. On its way to the anode the electroncollides with other molecules from the dielectric; if the applied field is high enough,primary avalanche electrons are produced. The molecules excited in this primaryavalanche cause photoionization by emitting high energy photons and secondaryavalanche electrons are produced.The primary avalanche electrons sink in the anode and leave behind a primary avalancheof positive ions having low mobility. The secondary avalanche of electrons runs into theprimary avalanche positive ions and a quasineutral plasma channel is produced. Theremaining secondary avalanche positive ions form a positive space charge at the headof this plasma channel. If the electical field induced by the space charge reaches avalue of the order of the external field, a streamer can be produced. Then, the numberof positive ions in the head should reach a value of at least 108, according to Meek[41]. By recurrence of the described processes, the streamer channel grows from theanode tip into the direction of the cathode (cathode directed streamer), see Figure 2.2.At the streamer head, an intense electrical field strength of about E=200 kV/cm isreached that accounts for chemical reactivity viz. radical formation. The streamer growsfrom the anode at a speed of about 108 cm/s, but stops where the E-field drops belowthe critical value.Corona discharges have been applied in pulsed form, to prevent transport of ions, thatis Ohmic dissipation.

12 Chapter 2.

A

C

+-

A

C

-+

eI-

iI+

A

C

eI-

iI+ -

+

-+

eII-

iII+

1 2 3 4 5

A

C

+ -+ -+

iI+/eII-

iII+

A

C

+ -+ -+ Ni≥108

Figure 2.1 The formation of a cathode-directed streamer in air; A = anode tip, C =cathode plate. (1) The starting condition: an ion-electron pair within theinception area. (2) The production of a primary avalanche of electrons eI

-

leaving behind a primary avalanche of ions iI+; the electron avalanchecauses photo-ionization. (3) The primary avalanche electrons sink into theanode, while secondary avalanche electrons eII

- produced by photo-ionization, run into the primary avalanche ions. (4) Recombination ofsecondary avalanche electrons eII

- and primary avalanche ions iI+ producesa quasineutral plasma channel; at the end of this channel a positive spacecharge is formed by the secondary avalanche ions iII+. (5) If the number ofpositive ions in the channel head is at least 108, then a streamer can beproduced.

Figure 2.2 The E-field of a cathode-directed streamer in air. At the streamer head thefield strength is about 200 kV/cm. The breakdown field strength of air atambient conditions is Ec≈30 kV/cm.

Negative polarity corona

Positive ions produced by the primary avalanche of electrons decrease the field strengthat the negatively charged point-shaped electrode. Therefore, the electrons lose energyon their way to the anode and get attached to electronegative oxygen. These negativeions drift slowly towards the anode, while the primary avalanche positive ions sink intothe cathode. Then the field at the cathode recovers and the processes restart at theformation of a new ion-electron pair within the inception area. For the case of non-pulsed DC operation, this recurrent process is known as the Trichel pulse regime.Opposite to positive polarity corona, the electrons can only gain kinetic energy withinthe inception area and therefore the formation of radicals is limited to this region.

Theory 13

2.2. Oxidizers

Some characteristic properties are described for the following oxidizers produced byadvanced oxidation processes: the hydroxyl radical, the ozone radical ion, ozone,atomic oxygen, hydrogen peroxide and the hydroperoxyl radical.

Hydroxyl radical

The hydroxyl radical (OH) is one of the strongest oxidizers among the oxygen-basedoxidizers [42]; its standard reduction potential is E0=2.56 V in acidic environment, seealso Table 1.1. The hydroxyl radical is extremely reactive: its life in water is about 2 nsand the radius of diffusion is about 20 Å [43]. In its reaction with inorganic ions,electrons are transferred from the ion to the hydroxyl radical, via an intermediate adductconsisting of the ion, the hydroxyl radical and -depending on the coordinating propertiesof the ion- a solvent molecule. With regard to organic molecules, the hydroxyl radicalreacts electrophilic and adds to unsaturated bonds of e.g. alkenes and aromatic rings.The hydroxyl radical also abstracts hydrogen atoms from organic molecules. In astrongly alkaline environment, the hydroxyl radical exists in its conjugated form: theoxygen radical ion O-, according to Equation 2.1. The acid dissociation constant of thehydroxyl radical is about pKa=11.9.

OH + OH- O- + H2O (2.1)

The oxygen radical ion is a nucleophilic particle that preferentially abstracts hydrogenatoms from organic molecules. It reacts much more slowly than the hydroxyl radical.

Ozone radical ion

Although the hydroxyl radical is generally assumed to be the strongest oxygen-basedhalogen-free oxidizer, the ozone radical ion (O3

-) is reported to be an even morepowerful oxidizer in acidic solution [2]. It has a standard reduction potential E0=3.3 V.The reduction half-cell reaction is given by Equation 2.2a. The O3

- ion is produced fromthe reaction of the oxygen radical ion and oxygen, according to [42], see Equation2.2b. In aqueous solution, the O3

- ion will oxidize water by which a hydroxyl radical, ahydroxide ion and oxygen are produced, see Equation 2.2c.

O3- + 2H+ + e- → O2 (g) + H2O E0=3.3V at T=298.15 K and pH=0 (2.2a)

O- + O2 → O3- (2.2b)

O3- + H2O → OH + OH- + O2 (2.2c)

14 Chapter 2.

Ozone

Ozone (O3) [8,9] is a strong oxidizer, as is indicated by E0 = 2.08 V. It oxidizes waterto hydrogen peroxide. Therefore the bulk solubility of ozone in water is rather low viz.about 0.1 mM at T= 293 K [44]. By irradiation with photons of wavelength λ≤310 nm,ozone is decomposed into a singlet oxygen atom and a singlet oxygen molecule(Eq.2.3a) [6]. In humid air, the singlet oxygen atom reacts with water to hydroxylradicals (Eq.2.3b); in the aqueous phase initially hydrogen peroxide can be produceddue to recombination of hydroxyl radicals that cannot escape from the solvent cage,see Eq.2.3.c. The singlet oxygen molecule is also very reactive, its life in water is about4.4 µs [43].

O3 + hν → O (1D) + O2 (1∆g) λ≤310 nm (2.3a)O (1D) + H2O(g) → 2OH (2.3b)O (1D) + H2O(l) → H2O2 (2.3c)

In acidic environment at normal temperatures, ozone reacts selectively with organiccompounds as an electrophilic molecule [45]. The electrophilic behaviour of ozone isexplained by the positively charged oxygen atom in the possible resonance structures,which are mainly represented by (A) and to a small extent by (B), see Figure 2.3.

O

O+

O

O

O+

O

O

O

O+

O+

O

O(A) (B)

Figure 2.3 Ozone resonance structures; the positively charged oxygen is electrophile.

Ozone is destroyed by hydroxyl radicals, according to the Equations 2.4ab. The netreaction is the conversion of ozone into oxygen.

O3 + OH → O2 + HO2 (2.4a)HO2 + O3 → OH + 2O2 (2.4b)

Ozone mass transfer from the gas phase into water is diffusion controlled; the Henrycoefficient KH, expressing the equilibrium partitioning of a compound between the gasand liquid phase, is large viz. KH≈3.76⋅103 at 20°C [46] which implies a negligibleresistance to mass transfer in the gas film compared to the liquid film. The masstransfer rate is influenced by e.g. the gas phase ozone concentration, temperature,pressure, gas dispersion, solution ionic strength, solution acidity and presence ofreactive compounds in the liquid phase.

Atomic oxygen

Atomic oxygen (O) is produced by dissociation of molecular oxygen, which requires anenergy of about 498.4 kJ/mol [24] corresponding to 5.2 eV. In acidic environment theoxygen atom is a stronger oxidizer than ozone, E0 =2.43 V. Its stability is howeververy limited. In the gas phase, atomic oxygen directly reacts with molecular oxygen toozone, where the activation energy of this reaction is only Ea=16.7 kJ/mol [47].Atomic oxygen oxidizes water to hydrogen peroxide.

Theory 15

Hydrogen peroxide

Hydrogen peroxide (H2O2) [13,14], the dimerization product of hydroxyl radicals, is lessreactive than the hydroxyl radical. Its standard reduction potential is E0=1.76 V inacidic environment. By photolysis, hydrogen peroxide decomposes into hydroxylradicals; the HO-OH bond strength is only 213 ±4 kJ/mol [24], which corresponds to2.2 eV. Concentrated hydrogen peroxide (>90%) is extremely instable; thedecomposition into water and oxygen is strongly exothermic viz. 98.3 kJ/mol. It is dueto the ability of hydrogen peroxide to simultaneously oxidize and reduce itself.Hydrogen peroxide is a weak acid: its acid dissociation constant is about pKa=11.75 atT=293 K; however in 50% aqueous solution pKa~9 [47], see Equation 2.5.

H2O2 + H2O H3O+ + HO2- (2.5)

Hydroperoxyl radical

The hydroperoxyl radical (HO2) is a much less strong oxidizer than the hydroxyl radical,ozone or hydrogen peroxide; its standard reduction potential is E0=1.44 V in acidicenvironment and thus just excels chlorine as oxidizer, see Table 1.1. The HO2 radical isproduced in oxygen enriched water from hydrogen atoms that are formed bydissociation of water molecules, see Equation 2.6ab. In alkaline environment, thehydroperoxyl radical exists as the superoxide radical ion O2

-, see Equation 2.6c. Theacid dissociation constant of the hydroperoxyl radical is about pKa=4.4. Hydroperoxylradicals often react with each other to hydrogen peroxide and oxygen, see Equation2.6d [48].

H2O → H + OH (2.6a)H + O2 → HO2 (2.6b)HO2 + OH- O2

- + H2O (2.6c)2HO2 → H2O2 + O2 (2.6d)

2.3. Degradation of organic compounds

Chemical oxidation is an important method to degrade organic compounds. Theobjective of degradation is mineralization i.e. conversion of the target compound tocarbon dioxide, water and -depending on the nature of the compound- inorganic ionslike e.g. chloride, nitrate, phosphate and sulfate; the inherent toxicity of possiblyobtained fluoride and bromate ions cannot be overcome by the oxidation process. Inpractice, complete mineralization is normally not requested, except for extremelydangerous materials. In many cases it is both justified and efficient to partially degradethe target compound in order to enable further degradation by microbiologicaltreatment. For that case, the chemical oxidation step is needed to destroy persistentmolecular structures, to remove high ecotoxicity and enhance water solubility.Examples of degradation pathways of organic compounds are discussed now. Thedegradation of unsaturated bonds by ozone, hydroxyl radicals and oxygen is discussed.In addition to chemical oxidation, reduction and pyrolysis are briefly mentioned.

16 Chapter 2.

Chemical oxidation

Ozone can react both directly and indirectly. The indirect way takes place under neutralor alkaline conditions via hydroxyl radicals. The direct way in acidic environment is theelectrophilic addition of ozone to unsaturated bonds of alkenes and aromaticcompounds. This addition reaction initially produces a molozonide, which rearrangesimmediately to an ozonide, see Figure 2.4. The ozonide decomposes by ring- cleavageand a zwitterion and an aldehyde or a ketone are produced. In water, the zwitterionhydrolyzes to a hydroxyalkyl hydroperoxide. Depending on the substituent groups, thehydroxyalkyl hydroperoxide decomposes into an aldehyde or a ketone by elimination ofhydrogen peroxide or a rearrangement to carboxylic acids occurs [49].

R1R4

R3 R2

OO

O+

OO

O

R1R4

R3R2

+O OC+

R12

R43

OR2

1

R34

COH

OOHR1

2

R43

OH2

alkene molozonide ozonide zwitterion aldehyde/ketone

O OC+

R12

R43

+

zwitterion hydroxyalkyl hydroperoxide

OO

R1R4

OR3

R2

RCOOH, RCHO, R2CO

Figure 2.4 The reaction of ozone with an unsaturated bond of an alkene or anaromatic compound yields bond cleavage. Products are carboxylic acids,aldehydes or ketones. R is a substituent group.

Hydroxyl radicals attack regions of high electron density and therefore add tounsaturated bonds of aromatic compounds and alkenes. Attack of a hydroxyl radical onan aromatic compound produces hydroxycyclohexadienyl radicals; attack of oxygen onthese radicals yields endoperoxyalkyl and endoperoxyl radicals; the endoperoxyl radicalsyield endoperoxides [19,50], see Figure 2.5. The very instable endoperoxidesdecompose by ring-cleavage to unsaturated aliphatic hydrocarbons with polyfunctionalgroups like carboxyl, aldehyde, carbonyl or alkanol groups. Also carbon monoxide maybe eliminated.

⋅OH

XO2

O2

OH

X

OO

OO⋅OH

X

aromatic hydroxy endoperoxy endoperoxyl endoperoxidescompound cyclohexadienyl alkyl radical radical radical

OH

X

OO

polyfunctional aliphatichydrocarbons

OH

X

OO

-OH,=O⋅

Figure 2.5 The attack of a hydroxyl radical and oxygen on an aromatic compoundproduces endoperoxides, which decompose to unsaturated aliphatichydrocarbons with polyfunctional groups.

Theory 17

Attack of the hydroxyl radical and oxygen on an alkene produces hydroxyalkylperoxylradicals. These radicals dimerize to a tetraoxide intermediate. The tetraoxide maydecompose in many ways. However, an important pathway is a fragmentation reactionthat yields α-hydroxyalkyl radicals, aldehydes/ketones and oxygen. The α-hydroxyalkylradical scavenges oxygen and produces an α-hydroxyalkylperoxyl radical that yields analdehyde or a ketone by elimination of hydroperoxyl radicals, see Figure 2.6 [50,51].

R1R4

R3 R2

OH R12R4

3

R21R3

4

OH

⋅O2

R12R4

3

R21R3

4

OH

OO⋅alkene hydroxyalkyl hydroxyalkyl α-hydroxyalkyl aldehyde radical peroxyl radical radical or ketone

+ HO2⋅R12R4

3OH

OO⋅ O

R12R4

3

R12R4

3OH

⋅O

R21R3

4+ +O2

2x

R12R4

3OH

⋅ + O2

2 2

α-hydroxyalkyl α-hydroxyalkyl aldehyde hydroperoxyl radical peroxyl radical or ketone radical

tetraoxide

Figure 2.6 The attack of a hydroxyl radical and oxygen on an alkene produces ahydroxyalkylperoxyl radical; dimerization of hydroxyalkylperoxyl radicalsyields a tetraoxide intermediate. The tetraoxide decomposes into α-hydroxyalkyl radicals, aldehydes/ketones and oxygen. The attack ofoxygen on an α-hydroxyalkyl radical yields an aldehyde or a ketone byelimination of a hydroperoxyl radical.

Hydroxyl radicals also abstract hydrogen atoms from a saturated hydrocarbon chain, bywhich radical sites are created on the hydrocarbon chain where oxygen can attack. Thisresults in the formation of unsaturated bonds and hydroperoxyl radicals, see Figure 2.7.The produced unsaturated hydrocarbon will be cleaved by ozone attack, see Figure 2.4.

H

H H

OO

H

H+ HO2

•

saturated radical peroxy radical unsaturated hydroperoxylhydrocarbon hydrocarbon hydrocarbon hydrocarbon radical

O2

-H2O

OH H

•

Figure 2.7 Hydrogen abstraction from a saturated hydrocarbon chain by a hydroxylradical, followed by oxygen attack produces an unsaturated hydrocarbonand a hydroperoxyl radical.

During oxidation, covalently bonded halogens, nitrogen, phosphorous and sulfur -ifpresent- are removed from the target molecule and converted to inorganic ions likehalides, nitrates, phosphates and sulphates. Figure 2.8 shows the oxidation ofdichloromethane by hydroxyl radicals and oxygen, eventually yielding carbon monoxide,carbon dioxide and hydrogen chloride; the intermediate phosgene is highly toxic, but itrapidly hydrolyzes to carbon dioxide and hydrogen chloride [50,51].

18 Chapter 2.

C HCl

HCl

+ OH

C Cl

HCl

C H

ClH

O2

O2

-H2O

-ClOH

Cl

O

ClCO2 + 2H+ +2Cl-

CO + H+ Cl-+ +dichloro-methane

phosgene

H2O

OH

CCl

HCl

OO•

CH

ClH

OO•

OH -

Figure 2.8 The oxidation of dichloromethane by hydroxyl radicals and oxygeneventually yields carbon monoxide, carbon dioxide and hydrogen chloride;phosgene is a highly toxic intermediate, which rapidly hydrolyzes tocarbon dioxide and hydrogen chloride.

Reduction

Reduction of unsaturated hydrocarbons by hydrogenation does not invoke bond/ring-cleavage but only saturation takes place [52]. Nevertheless aromaticity is destroyed inthis way. Figure 2.9 shows the hydrogenation of benzene to cyclohexane. In contrastto benzene, cyclohexane is less harmful [1].

hyd hyd

benzene hydroxycyclo cyclohexadienes cyclohexene cyclohexane hexadienyl radicals

HH

⋅H H

Figure 2.9 Reduction of benzene to cyclohexane by hydrogenation (hyd).

Reduction of azo dyes invokes cleavage of the azo bond (-N=N-), which impliesfragmentation of the azo dye molecule into two amino (RNH2) compounds, see Figure2.10. This reaction explains reductive fading (decolorization) of the dye, induced byketyl or carboxy radicals [53]. However, this degradation reaction produces e.g. highlyharmful aniline.

Acid Red 1

+

aniline

reduction NH2

OH

SO3NaSO3Na

NH

O

NH2

NOH

N

SO3NaSO3Na

NH

O

4H+ + 4e-

Figure 2.10 The reduction of the azo dye Acid Red 1; this reaction is an example ofreductive fading (dye decolorization).

Theory 19

Pyrolysis

Pyrolysis is thermal decomposition of an organic compound in the absence of oxygen.By pyrolysis, molecules are dissociated into radicals and elimination of functional groupsmay take place like e.g. decarboxylation of carboxylic acids, dehydration of alkanolsand esters, dehalogenation, loss of nitro and sulfone groups, nitrogen, carbonmonoxide, see Figure 2.11. Thermal cracking of alkanes yields lower molecular weightalkenes, but pyrolysis of simple aromatics leads to polymerization viz. polycyclicaromatic hydrocarbons [54].

CCl4 •CCl3 + •Cl1.

2.

3.

4.

+ NO

NO2

OH OH

O

OH

O

O

OHOH

O+ CO2

+

Figure 2.11 Pyrolysis of organic compounds: 1. Dissociation of tetrachloromethane; 2.Nitric monoxide release from p-nitrophenol; 3. Decarboxylation of malonicacid yields acetic acid; 4. Cracking of n-butane to ethane and ethylene.

Knowledge of the target compound conversion level and/or conversion efficiency is notsufficient to qualify an advanced oxidation process. It is also very important to identifythe intermediate and final oxidation products. During the oxidation of organiccompounds intermediates may be produced, which exhibit higher toxicity than thetarget compound. Examples are dibenzofurans and dioxins, produced from supercriticalwater oxidation of phenol, as will be discussed in section 2.4.1. The oxidation ofhalogenated hydrocarbons yields highly harmful halogenated aldehydes/carboxylic acids[51,55]. Therefore oxidation progress must have proceeded, until these harmfulintermediate products have been converted.

20 Chapter 2.

2.4. Oxidation of model compounds

A survey from literature is presented about the general oxidation pathways of theapplied model compounds phenol, atrazine, malachite green and dimethyl sulfide.

2.4.1. Phenol

The oxidation of phenol produces a wide oxidation product range, consisting ofpolyhydroxybenzenes/quinones, ring-cleavage products and polymerization products.Literature data originate from radiolysis [50,56,20], oxidation by hydrogen peroxide[11,57,58], ozone [7] and other chemical oxidizers [34,52], photocatalytic oxidation[15], photolysis [59] and oxidation by supercritical water [18,60,17].

Polyhydroxybenzenes and quinones

Among the polyhydroxybenzenes [61] are the dihydroxybenzenes (DHB’s): catechol(1,2-DHB), resorcinol (1,3-DHB), hydroquinone (1,4-DHB) and the trihydroxybenzenes(THB’s): pyrogallol (1,2,3-THB), hydroxyhydroquinone (1,2,4-THB) and phloroglucinol(1,3,5-THB), see Figure 2.12. These compounds are produced by attack of the hydroxylradical on the benzene ring. With increasing amount of hydroxyl groups attached to thebenzene ring, the stability of the hydroxybenzene towards oxidation strongly decreases[52]. Therefore higher hydroxylated benzenes are not likely to be found during vigorousoxidizing conditions.Quinones are produced by oxidation of polyhydroxybenzenes. The following quinonesare reported: 1,4-benzoquinone, 1,2-benzoquinone and hydroxybenzoquinone. 1,4-benzoquinone is produced by oxidation of hydroquinone. 1,2-benzoquinone is a veryunstable oxidation product of catechol. Hydroxybenzoquinone is produced byhydroxylation of 1,4-benzoquinone or partial oxidation of hydroxyhydroquinone; it isreported to undergo polycondensation in aqueous solutions. 1,3-benzoquinone does notexist, because the structure would be nonplanar and highly strained [62].

catechol resorcinol hydroquinone pyrogallol hydroxy- phloroglucinol hydroquinone O

O

OO

O

O

OH

1,4-benzo- 1,2-benzo- hydroxybenzo- quinone quinone quinone

OHOH

OH

OH

OH

OH

OH

OH

OHOH

OH

OH

OH

OHOH

Figure 2.12 Polyhydroxybenzenes and quinones produced by the oxidation of phenol.

Theory 21

Ring-cleavage products

The oxidation of polyhydroxybenzenes and quinones produces ring-cleavage products.Observed products are unsaturated and saturated C1-C6 hydrocarbons withpolyfunctional groups like carboxyl-, aldehyde-, ketone- or alkanol- groups, see Figure2.13. Alkanol-functional groups are oxidized to aldehyde groups, while aldehydes areoxidized to carboxylic acids. The following classes can be mentioned:Saturated monocarboxylic acids: formic, acetic, propionic and glyoxylic acid. Saturateddicarboxylic acids: oxalic, malonic, ketomalonic, D,L-malic, succinic, glutaric and adipicacid. Unsaturated monocarboxylic acids: acrylic acid. Unsaturated dicarboxylic acids:maleic, fumaric and cis,cis-muconic acid. Saturated aldehydes: formaldehyde,acetaldehyde and glyoxal. Unsaturated hydrocarbons: acetylene and butadiene.Acetylene is reported to be produced under supercritical conditions by addition ofoxygen to catechol [17]. Butadiene is reported as a decomposition product ofhydroxyhydroquinone [16].

OH

O

H OH

O

OH

O

OH

O

O

H

OH

OOH

OOH

O

O

OHOH

O

OHO

OHO

OHO

OH

OH

O

O

OH

OH

OOH

O

OH

O

OH OOH

OH

O

O

OH

O

O

H H

O

H

O

OH

H

formic acid acetic acid propionic acid glyoxylic acid

oxalic acid malonic acid succinic acid glutaric acid adipic acid

acrylic acid maleic acid fumaric acid cis,cis-muconic acid

formaldehyde acetaldehyde glyoxyal

OH

O

O

OH

O

ketomalonic acid D,L-malic acid

OH

O

OOH

Figure 2.13 Aldehydes and carboxylic acids produced by the oxidation of phenol.

22 Chapter 2.

Polymerization products

The radical-induced oxidation of phenol also invokes molecular coupling, see Figure2.14. Reported dimerization products are 4,4’/2,4’/2,2’-dihydroxybiphenyl and 4/2-hydroxydiphenylether. These products are formed by dimerization of phenoxy radicals.Purpurogallin is produced from the dipolar dimerization of the ortho-quinone ofpyrogallol. Supercritical water oxidation of phenol yields the following multiringcondensation products: dibenzofuran, dibenzofuranol, dibenzo-p-dioxin, 9H-xanthene-9-one, 2,3-dihydro-1H-indene-1-one. Polymerization products are the so-called “synthetichumic acids” consisting of hydroquinone and (hydroxy)benzoquinone monomeric units;these amorphous products exhibit a dark-brown color. The toxicity of the couplingproducts is higher to much higher than the toxicity of phenol. Especially thebenzofurans and dioxins are highly unwanted, but these compounds are avoided ordestroyed by supercritical conditions over T=600°C.

OHOH

OH

OHO

OH

OOH

OH

OH

O O

O

OOH

O

O

OHOH

OH

O OH

O

4,4'-DHBP 2,4'-DHBP 2,2'-DHBP 4-HDE 2-HDE

O

On

OH

OH

O

OO

O

n

OH

OH

dihydroxybiphenyl hydroxydiphenylether

dibenzofuran dibenzofuranol dibenzo-p-dioxin 9H-xanthene-9-one 2,3-dihydro-1H- Indene-1-one

purpurogallin

synthetic humic acids

Figure 2.14 Polymerization products formed during the oxidation of phenol.

Theory 23

2.4.2. Atrazine

The oxidation of atrazine involves deaminoalkylation, dechlorination and hydroxylationof the s-triazine ring. Ring-cleavage has not been reported. Literature data have beenobtained from photocatalytic oxidation [36] and photo-Fenton oxidation [63,64]. Someimportant oxidation products are shown by Figure 2.15.

By oxidation of the aminoalkyl groups the following products are formed: 4-acetamido-2-chloro-6-(isopropylamino/ethylamino)-s-triazine. Partial dealkylation products aredeethylatrazine and deisopropylatrazine. Complete oxidation of the alkyl groups yieldsdiaminoatrazine. The dechlorination preferentially occurs after considerable degradation.Due to deaminoalkylation and dechlorination nitrate and chloride ions are produced.Also ethane has been detected. The final oxidation product is cyanuric acid (2,4,6-trihydroxy-1,3,5-triazine).

atrazine

N

N

N

Cl

NH2NH2

N

N

N

OH

OHOH

diaminoatrazine

deethylatrazine

deisopropylatrazine

4-acetamido-2-chloro-6-(isopropylamino)-s-triazineN

N

N

Cl

NHNH

N

N

N

NH NH

O

Cl

N

N

N

NH NH

O

Cl

N

N

N

NH2 NH

Cl

4-acetamido-2-chloro- 6-(ethylamino)-s-triazine

N

N

N

Cl

NH NH2

cyanuric acid

Figure 2.15 Atrazine and some major oxidation products.

2.4.3. Malachite green

The oxidation of malachite green is described in literature by the lowering of lightfastness [53]. Two degradation mechanisms may apply viz. dealkylation (methyl groupsattached to nitrogen) and molecular fragmentation, starting from the carbinol base.Reported products are N,N-di- and N-monomethyl-4-aminobenzophenone and N,N-dimethyl-4-aminophenol, see Figure 2.16.

24 Chapter 2.

O

N

O

N

OHN

NNOH

malachite green MG carbinol base

N,N-di- and N-monomethyl- N,N-dimethyl- 4-aminobenzophenone 4-aminophenol

[ox]

N+

N

OH

H+

Figure 2.16 Malachite green and some of its oxidation products.

2.4.4. Dimethyl sulfide

Literature data on the oxidation of dimethyl sulfide have been obtained from[39,40,65]. Dimethyl sulfide is initially oxidized to dimethyl sulfoxide, which can befurther oxidized to dimethyl sulfone, methanesulfonic acid and finally sulfuric acid, seeFigure 2.17.

S SO

dimethyl dimethyl dimethyl methane- sulfuric acid sulfide sulfoxide sulfone sulfonic acid

S OHO

OHO

SO

OS OHO

O[ox] [ox] [ox] [ox]

Figure 2.17 Dimethyl sulfide and some of its oxidation products.

2.5. Diagnostics

An overview is presented of major chemical, electrical and optical diagnostics, whichhave been applied to study the formation of oxidizers and the oxidation of modelcompounds by pulsed corona discharges. Applied chemical diagnostics are liquidchromatography, mass spectrometry, aldehyde screening, electron spin resonance,Microtox ecotoxicity, total organic carbon content and acidity. Electrical diagnostics arecorona pulse voltage & current measurements and conductometry. Applied opticaldiagnostics are UV absorbance spectrometry and fluorescence & infrared spectroscopy.

Theory 25

2.5.1. Chemical diagnostics

Liquid chromatography

Separation of the liquid-phase oxidation product mixture into its components isnecessary for determination of the conversion of the target compound and identificationof the oxidation products. In this way every product can be detected separately withmaximum sensitivity, because mutual influence is not possible. In this thesis the liquid-phase oxidation product mixture has been separated by reversed-phase highperformance liquid chromatography (rp-HPLC) and ion-exclusion chromatography (ICE)[66,67,68].In liquid chromatography, a sample in a carrier flow i.e. eluent or mobile phase isintroduced into the separation column containing a stationary phase. The samplecomponents will partition between the stationary phase and the mobile phase due tocomponent-specific physical interaction mechanisms. In this way component-specificretention thus separation is established. Detection of the eluting components iscommonly performed by a UV absorbance detector but the detection by e.g. massspectrometry, fluorescence, electrical conductivity or refractive index are also optional.

Rp-HPLC has been initially applied for exploration of the complex oxidation productmixture. It is the most applied and versatile liquid chromatography technique, suitablefor separation of a wide group of organic compound classes including phenols,polycyclic aromatic hydrocarbons, alkanols and alkanes. The retention mechanism isbased on non-specific hydrophobic interaction (dispersive forces) but also on dipole-dipole and proton donor/acceptor interaction.In rp-HPLC the non-polar stationary phase is commonly an alkyl-bonded silica packinge.g. a C8- or C18-alkane grafted on silica; the polar mobile phase is a mixture of waterand an organic modifier. The retention behaviour of the components, thus theseparation, can be adjusted by changing the eluent composition i.e. eluent strength.The eluent strength is the power of the eluent to displace components interacting withthe stationary phase. The used eluents often consist of mixtures of water andacetonitrile or methanol, which do not absorb the UV light of the absorbance detectorat analytically-important wavelengths (cutoff wavelength: acetonitrile: 190 nm,methanol: 210 nm, water: 191 nm [69]). The eluent dosage is performed at constantcomposition (isocratic conditions) or by means of a gradient (increasing eluentstrength). The gradient dosage is applied to force the elution of compounds thatstrongly interact with the stationary phase. Unfortunately rp-HPLC is not suitable forthe separation of carboxylic acids, which are important oxidation products.

In order to resolve the carboxylic acids, ion-exclusion chromatography (ICE) has beenapplied. The retention mechanism is mainly based on the exclusion of anions from acationic-exchange resin. The anions cannot penetrate the resin, because they encounterthe Donnan potential, which guarantees the electrical neutrality within the resin. Theapplied ICE column has a polystyrene-divinylbenzene partially crosslinked resin withsulfonate (RSO3

-H+)-functional groups. An aqueous solution of a strong acid, heretrifluoroacetic acid, is used as eluent. Strong carboxylic acids exist in ionic form (H+A-)and will be excluded from the stationary phase. On the contrary, weak carboxylic acidsin molecular form will be able to diffuse into the resin pores.

26 Chapter 2.

The carboxylic acids are thus separated by their acidic strength, which is reflected bythe acid dissociation constant pKa. Other ICE retention mechanisms are hydrophobicinteraction and molecular size, both originating from the partially cross-linked resin. Theseparation can be influenced by the eluent acidity, because the eluent aciditydetermines the dissociation behaviour of the carboxylic acids according to theHenderson-Hasselbalch relationship, see Equation 2.7. α Equals the dissociationfraction.

−+−

−

+++

=α

α−α−= AOHOHHA

AHAA

andpHpKa 32][][][

1log (2.7)

Although the eluent strength will also influence the retention behaviour of ion-exclusionchromatography, the addition of organic modifiers to the acidic aqueous mobile phasehas not been applied for the used column. The partially cross-linked polymeric resin isexpected to swell due to the absorption of organic modifier, which may result incracking of the resin.

Next to UV absorbance detection, the carboxylic acids have been detected using aconductivity detector. However, the acidic eluent necessary for ICE separation causes ahigh background conductivity, which makes the detection of separated carboxylic acidsimpossible. Therefore the eluent conductivity has to be decreased by removal of thehighly conductive hydronium ions, which is accomplished by a suppressor. The appliedsuppressor is a micromembrane suppressor, which consists of cation-exchangemembranes and is supplied with an aqueous ammonia solution. According to theDonnan equilibrium, these membranes exclude the trifluoroacetate anions of thetrifluoroacetic acid eluent but allow hydrogen ions to pass by exchange with ammoniumions, while electrical neutrality is maintained. The highly conductive hydronium ion isthus replaced by the less conductive ammonium ion.

Liquid chromatography / mass spectrometry

In order to identify the oxidation products after separation by the liquid chromatograph,a mass spectrometer is connected to the LC system by means of a special interface.This LC-MS coupling has to introduce a huge eluent flow (typically about F=1 ml/min)containing tiny amounts of separated components, into the high vacuum of thequadrupole mass spectrometer. In this work an IonSpray interface has been utilized[70,71].The IonSpray interface is a pneumatical and electrostatical nebulizer. A splitted fractionof the eluent carrying the separated components from the LC system is introduced intoa hollow needle, which is energized at high voltage. Together with the eluent anebulizer gas flow is introduced; if necessary in combination with an organic solvent-based make-up liquid to improve the sprayability of high water content eluents. A mistof highly charged droplets is produced in the direction of the mass spectrometer. Due toevaporation the droplets decrease in size and the electrical field at the surface of thedroplet increases. When a critical field has been reached, ions are emitted from thesurface of the droplets. These ionization conditions are very mild: no fragmentationtakes place and thus molecular ions are produced. These ions are transferred to theorifice, where they can enter the quadrupole mass-spectrometer. Eluent molecules areprevented from entering the mass spectrometer by a gas curtain interface.

Theory 27

IonSpray is particularly suitable for the analysis of thermolabile and ionic components.For the identification of carboxylic acids and hydroxybenzenes, the IonSpray interfacehas been operated in the negative ion mode: the applied needle voltage is negative withrespect to the grounded wall. Also ammonia has been added to the aqueous oxidationproduct mixture, to facilitate the production of anions (e.g. phenolate, carboxylates).

Next to the IonSpray interface, the Atmospheric Pressure Chemical Ionization (APCI)interface has been tested for applicability. Here, the splitted flow from the LC system isnebulized and evaporated by heating. The evaporated flow is directed to a coronadischarge needle, where chemical ionization of sample and solvent takes place. APCI isparticularly suitable for the processing of high eluent flow rates containing highconcentration of electrolytes. However, the sample components have to bethermostable.

Electron-impact mass spectrometry

In addition to IonSpray-MS, electron-impact mass spectrometry (EI-MS) has beenperformed for the identification of phenol oxidation products. After evaporation in a pre-vacuum compartment, the mixture components are introduced into the main vacuumcompartment by differential pumping. Here the components are ionized by an electronbeam. Produced are molecular fragments but also molecular ions.

Solid phase extraction

Solid phase extraction (SPE) [66] is a sample preparation technique implying a traceenrichment procedure and solvent transfer step, in order to obtain samples withdetectable amounts of components in a suitable solvent.The SPE column contains a sorbent bed material that is similar to the stationary phasesused high performance liquid chromatography. The procedure involves four steps. Firstthe SPE sorbent bed is conditioned, where an organic solvent is used to increase theinteraction surface area and displace contaminants; excess solvent is then removed byrinsing with a liquid similar to the sample solvent. The second step is the sorption of themixture components onto the sorbent bed. The third step is the removal of undesiredsample matrix with a weak solvent. In the last step, the components are desorbed intoa small volume by means of a solvent with sufficient strength. Adsorption anddesorption are performed by vacuum suction.

Aldehyde screening test

The production of volatile aldehydes during the oxidation of phenol has been verified bya gas sampling aldehyde screening test according to NIOSH [72]. This test is suitable toidentify C1-C7 aliphatic saturated aldehydes (e.g. formaldehyde, acetaldehyde) but alsounsaturated aldehydes like acrolein and crotonaldehyde. The test involves the collectionof gas phase components in a sampling tube, filled with the derivatization agent 2-(hydroxymethyl)-piperidine (HMP) on a carrier phase. HMP specifically reacts withaldehydes to an oxazolidine compound, see Figure 2.18.

28 Chapter 2.

NH

OH

+ OR

H

+ OH2

HMP aldehyde oxazolidine

NO

R

Figure 2.18 The chemical derivatization of an aldehyde with HMP.

The oxazolidine can be identified by gas chromatography-mass spectrometry. Themolecular ion of a specific aldehyde equals the molecular weight of the originalaldehyde plus 97. For example formaldehyde (HCHO) is recognizable by a base peak atm/z=97 and other characteristic ions have m/z values 126 and 127 (molecular ionC7H13NO).

Electron spin resonance

Electron spin resonance (ESR) has been applied in-situ, to identify oxidizer radicalsproduced by the corona discharges and radical intermediate phenol oxidation products.Of interest are inorganic oxidizer radicals e.g. the hydroxyl radical (HO•) / oxygen anion(O-•) and hydroperoxyl radical (HO2•) / superoxide anion (O2

-•); organic radicalintermediates that are produced by oxidation of phenol are e.g. thedihydroxycyclohexadienyl[peroxyl] radicals C6H5(OH)2[OO]•. Next to the mentionedradicals, hydrogen atoms may be detectable.

A spin trap has been applied to trap the short living radicals and form a stablemeasurable adduct. Literature references originate from in-situ glow dischargeelectrolysis [73], in-situ radiolysis [74] and ex-situ sonolysis [22] of aqueous spin trapsolutions. 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap has been used. Theformation of the DMPO-OH adduct from DMPO and the hydroxyl radical is shown byFigure 2.19. The DMPO-OH adduct exhibits a half-life of about 2 hours [75]. TheDMPO-OOH adduct, however, is reported to be highly unstable [76].

NO

H +

DMPO DMPO-OH

NO

OHH

OH

Figure 2.19 The trapping of a hydroxyl radical by the spin trap DMPO.

The ESR spectrum of the DMPO-OH adduct consists of a 1:2:2:1 quartet, due to theequivalence of the 14N and 1H hyperfine coupling constants viz. aN=aH=1.49 mT.DMPO-H has a nine-line spectrum due to coupling with one 14N nucleus (aN=1.66 mT)and 2 identical protons (aH=2.25 mT). However, no hyperfine coupling constants areknown for the adduct of DMPO and the organic DHCHD(P) radicals.

Theory 29

Microtox ecotoxicity test

In order to investigate the detoxification progress during the degradation of phenol,Microtox ecotoxicity tests have been performed [77]. In the Microtox test, theecotoxicological effect of a chemical substance or a mixture is determined from thebioluminescence behaviour of the marine bacterium Vibrio fischeri before and afterexposure to the medium. The bioluminescence output decreases with increasing toxicityof the chemical compound. The ecotoxicity is expressed as an effect concentration (EC)at a particular effect level, in this thesis as a 20 % bioluminescence decrease (E20). LowEC values imply high ecotoxicity. The EC value is determined by interpolation of thedose-effect relationship, see Equation 2.8. The effect is expressed by the gamma value(Γt), that is related to the inhibitory effect (Ht) according to Equation 2.9. The inhibitoryeffect is calculated from the bioluminescence intensity after the exposure (ITt) and acorrected background intensity (Ict), according to Equation 2.10.

)log()log()log( abc tt +Γ⋅= (2.8)

t

tt

HH

−=Γ

100(2.9)

100⋅−=ct

Ttctt I

IIH (2.10)

Evidence for acute ecotoxicity is obtained, by measuring the effect level at differentexposure times. Table 2.1 shows EC50

5min values for phenol and some of its oxidationproducts [78]. Although 50% effect levels are more commonly reported in literature,the measurement of 20% effect levels implies higher sensitivity of the Microtox test. Itshould be noted, that a comparison of effect concentrations determined at differentlevels i.e. 20% or 50% and different exposure times like 5 min, 15 min or 30 minutes isnot possible. The deviation of the reported EC50 values is probably caused by the originof the applied bacteria cultures viz. the supplier and supplied state (freeze-dried, liquid-dried or fresh).

Table 2.1 EC505min effect concentrations of phenol and some of its oxidation

products. The phenol EC505min value is an average value determined from a

set of 22 literature references [78].

Compound EC505min (mg/l)

Phenol 28.6 ±7.9Catechol 32.0Resorcinol 310; 375Hydroquinone 0.042; 0.0791,4-Benzoquinone 0.0085; 0.020; 0.08; 1.4Formaldehyde 3.0; 8.7; 9.0; 10; 10.1; 904.17Glyoxal 754 ±55.1Glyoxylic acid 11.2 ±0.16Oxalic acid 11.3 ±0.22Formic acid 7.91 ±0.22Acetic acid 9.24 ±0.38

30 Chapter 2.

Total organic carbon

A different way to measure oxidation progress of an organic compound is thedetermination of the carbon content of the oxidation product mixture. The total carboncontent TC is defined as the sum of the total organic carbon TOC (hydrocarbons) andtotal inorganic carbon TIC (carbonate, bicarbonate, carbon dioxide) [79].

Due to oxidation, the carbon skeleton of an organic compound is gradually chopped intoshorter carbon chain molecules containing oxygen-based functional groups viz. aliphaticaldehydes and carboxylic acids. The last member in the oxidation sequence is formicacid, which upon oxidation yields the unstable carbonic acid. The TOC level of theoxidation product mixture decreases by release of carbon dioxide (mineralization) andvolatile or gaseous intermediates.The measurement of total organic carbon does not reveal information about thechemical or toxicological properties of the sample. However, if the total carbon contentequals the total inorganic carbon content, the organic carbon has been completelymineralized and the remaining toxicity is only due to inorganic ions originating fromcovalently bonded elements.

Acidity

The acidity of the oxidation product mixture is an indication for oxidation progress,because the oxidation of an organic compound yields carboxylic acids. Unfortunately, itis impossible to deconvolute the overall acidity into concentrations of all carboxylicacids present in the oxidation product mixture. The produced carboxylic acids aregenerally weak acids, compared to mineral acids like hydrochloric acid and nitric acid.The strongest acid, produced by oxidation of phenol is oxalic acid. The acidic strengthis expressed by the acid equilibrium constant Ka, according to Equation 2.11. The aciddissociation constant pKa=-10log(Ka) of phenol and some of its acidic oxidation productsis shown by Table 2.2 [80].

−+−+

++⋅= AOHOHHAHA

AOHKa 32

3

][][][

2.11

Table 2.2 Acid dissociation constants of phenol and some of its acidic oxidationproducts; dibasic acids (H2A) show a two-step dissociation.

Compound pKa,I pKa,II

Oxalic acid 1.23 4.19Maleic acid 1.83 6.07Glyoxylic acid 3.18 -Formic acid 3.75 -Phenol 9.89 -

Theory 31

2.5.2. Electrical diagnostics

Corona pulse energy & conversion efficiency

The energy of a single corona discharge pulse (Ep) is determined from the pulse voltageV(t) and corona current Icor(t). The corona discharge current Icor(t) is calculated from thedifference of total current I(t) and capacitive current Icap(t), see Equation 2.12. Thecapacitive current is determined by measuring the capacitance of the reactor geometryCg at voltages below the corona onset, where I(t)=Icap(t), see Equation 2.13. Thecorona pulse energy Ep is then calculated by integration of pulse voltage times coronacurrent over the pulse time, see Equation 2.14 [81].

)()()( tItItI capcor −= (2.12)

onsetgcap VVdt

tdVCtI <∧⋅= )(

)( (2.13)

∫ ⋅=pulse

corp dttItVE )()( (2.14)

The conversion efficiency is defined according to the G yield value. G expresses thenumber of target compound molecules converted with regard to the required energyinput, illustrated by Equation 2.15. X is the conversion of the target compound, C0 isthe initial target compound concentration (mol/l), Vol is the solution volume (l), Ep is thepulse energy (J), f is the pulse repetition rate (Hz) and t is the oxidation time (s). Inliterature G units are expressed as mol/J, “molecules per 100 eV” or g/kWh. Table 2.3shows the interconversion of the efficiency units.

GX C Vol

E f tp=

⋅ ⋅⋅ ⋅0 (2.15)

Table 2.3 The interconversion of G yield efficiency units.

mol/J (100eV)-1 g/kWh

mol/J= 1 eNA1001

FW6106.31

⋅

(100eV)-1= 100NAe 1 FWeNA

6106.3100

⋅

g/kWh = 3.6⋅106FW eNFW

A100106.3 6⋅

1

NA = Avogadro’s constant: 6.022⋅1023 mol-1

e = 1.6022 ⋅10-19 J/eVFW = target compound molecular weight (g/mol)

32 Chapter 2.

Electrical conductometry

Electrical conductometry has been applied to measure the conductivity of the oxidationproduct mixture in order to monitor oxidation progress. The oxidation of an organiccompound yields carboxylic acids, which are partially dissociated in aqueous solutiondepending on their acid strength and therefore contribute to the electrical conductivity.The conductance G (unit Siemens) is the reciprocal value of the electrical resistance Raccording to Ohm’s law, see Equation 2.16. ρ Equals the resistivity, A and l are theconductor’s area and length.

Kl

A

A

lRG

11

)(

11 ⋅=⋅=⋅

== σρρ

(2.16)

The conductivity σ equals the product of the conductance G and the cell constant K.With regard to the phenol oxidation product mixture, conductivity is nearly exclusivelydue to protons. The carboxylic acid anions and other anions like chloride and nitratecontribute to a limited extent to conductivity, as is illustrated by the molar conductivityat infinite dilution λ0=(σ/c)c→0, see Table 2.4 [82].

Table 2.4 Molar conductivity at infinite dilution for some ions observed duringoxidation of an organic compound.

Ion Formula λ0 (m2Smol-1)⋅104

Proton H+ 349.7Chloride Cl- 76.3Oxalate (½)C2O4

2- *) 74.1Nitrate NO3

- 71.4Formate HCOO- 54.6Acetate CH3COO- 40.9H-Oxalate HC2O4

- 40.2*) Double charge

2.5.3. Optical diagnostics

UV absorbance spectrometry

Quantitative ozone measurements have been performed by UV absorbancespectrometry. By absorption of energy, a molecule is transferred from its ground stateto an excited state. The relaxation of the molecule to the ground state may involveradiationless transfer, fluorescence or phosphorescence, depending on the electronicstructure of the molecule. According to Lambert-Beer’s law, the intensity of theabsorbed radiation (I) is related to the number density of the absorbing molecule (N), theabsorption cross section (σ(λ)) and the optical path length (x) see Equation 2.17. Thisrelation is valid for monochromatic light and diluted solutions viz. concentration ≤10-2

mol/l [83]. The cross section for ozone at λ=260 nm is σ260=1.14⋅10-21 m2 [9,84].

)exp(0 xNII σ−= (2.17)

Theory 33

Laser-induced fluorescence spectroscopy

Laser-induced fluorescence (LIF) spectroscopy has been applied for in-situ conversionmeasurements of phenol in aqueous solution.

The fluorescent properties of a molecule are determined by both electronic andstructural requirements [85]. Electronic requirements are that the molecule absorbsenergy frequencies lower than the strength of the weakest bond, the molecule’s firstexcited singlet state S1 has a life-time of about 10-8 s and the first excited singlet state(S1) and lowest triplet state (T1) are well separated. In all media, except for low-pressure gasses, organic molecules emit fluorescence from the lowest vibrational levelof the first excited singlet state to the singlet ground state (S0), see Equation 2.18.

S1 → S0 + hνfl (2.18)

Structural requirements are that the molecule contains a planar rigid conjugated systemof double bonds, preferentially in a cyclic structure and especially in linear polycyclicmolecules where the π electrons can readily circulate inside the molecule; groupssubstituted to the conjugated system shall be electron donating like e.g. hydroxyl and(dimethyl)amino groups and polysubstitution shall not influence the π electron mobility.

The application of LIF spectroscopy for monitoring the phenol conversion duringoxidation in aqueous solution is not straightforward, because of following reasons. Thefluorescence radiation from phenol excited states is quenched by other phenolmolecules, oxidation products and water. The excitation laser beam is absorbed byphenol and the oxidation products in aqueous solution. Some oxidation products alsoshow fluorescence by laser excitation. The different processes that occur afterexcitation of a phenol molecule by the laser beam are described by Figure 2.20. Fourenergy levels are described viz. the ground state (0), the initial phenol excited state (2),a lower excited phenol state (1) and the electronic state of a quenching molecule Q (3).

2

0

A10n1

n2/τkQnQn2

B02 n0 ρ A20n2

kQnQn1

1

Phenol Q

3

Figure 2.20 A Jablonski diagram for different energy states of phenol and aquenching compound Q.

34 Chapter 2.

ni equals the population density of state i. A20 and A10 are the Einstein coefficients forspontaneous emission from state 2 to 0 and state 1 to 0 respectively. kQ is thequenching coefficient. τ is the characteristic time constant for decay from state 2 tostate 1. B02 is the Einstein coefficient for induced absorption for the transition fromstate 0 to 2. ρ equals the laser photon density, which can be expressed according toLambert-Beer’s law by Equation 2.19. ρ0 equals the laser intensity before entering theaqueous solution, σ02 is the absorption cross section of phenol in aqueous solution andn0 is the population density of the phenol ground state 0.

)exp( 0020 Lnσρρ −= (2.19)

The population density of excited state 1 grows by decay of excited state 2 to theexcited state 1 according to n2/τ. The population density of excited state 1 decreasesby decay to the groundstate 0 by A10n1 and by quenching due to molecule Q accordingto an amount kQnQn1, see Equation 2.20.The population density of excited state 2 grows by laser excitation by an amountB02n0ρ, The population density of the excited state 2 decreases by decay to the excitedstate 1 by n2/τ, by decay to the ground state 0 by A20n2 and by quenching due tomolecule Q according to an amount kQnQn2, see Equation 2.21.

111021 nnknA

ndtdn

QQ−−=τ

(2.20)

22202

0022 nnknA

nnB

dtdn

QQ−−−=τ

ρ (2.21)

The assumption of steady-state conditions is valid for collision-dominated conditions,which is justified for the liquid phase. Then, the derivates of population density withrespect to time are equal to zero and the fluorescence intensity can be expressed byEquation 2.22.

( )( )( )ττ

σρηη2010

0020002110LIF 1

expAnkAnk

LnnBnAI

QQQQ +++−

== (2.22)

η is a proportionality constant. From Equation 2.22 it can be derived that thefluorescence intensity versus the population density n0 reaches a maximum value atn0=(σ0L)-1. At higher phenol concentrations absorption and quenching overrule thefluorescence.

Phenol complies with the requirements for fluorescence. When excited at a wavelengthof about λex=270 nm in aqueous solution, phenol emits a maximum fluorescence near300 nm<λfl<310 nm [85-90]. The excitation wavelength of the dihydroxybenzenes isin same range i.e. 265 nm<λex<285 nm and these compounds show a somewhatweaker fluorescence at the wavelengths 315 nm<λfl<340 nm.

Theory 35

The trihydroxybenzenes are weakly to non-fluorescent. Phloroglucinol shows theweakest fluorescence, possibly because it also reacts in a tautomeric keto-form [61]which is non-aromatic, see Figure 2.21.

O

OO

OH

OHOH

phloroglucinol 1,3,5-cyclohexanetrione

Figure 2.21 Keto-enol tautomerism of phloroglucinol (1,3,5-trihydroxybenzene).

1,4-Benzoquinone is not a hydroxybenzene but forms a redox couple withhydroquinone; the compound is non-aromatic viz. a cyclic diene and also shows weakfluorescent properties. Saturated hydrocarbons do not comply with the requirements forfluorescence and therefore major literature references on fluorescence involve solelyaromatic compounds.

Fluorescent molecular probe

A fluorescent molecular probe has been applied to identify the hydroxyl radical incorona-exposed aqueous solution. This molecule is a non-fluorescent compound, thatspecifically reacts with hydroxyl radicals in a very sensitive way to produce a stronglyfluorescent molecule. In this thesis the molecular probe coumarin-3-carboxylic acid,abbreviated as CCA, has been utilized [91,92]. CCA reacts with the hydroxyl radical to7-hydroxycoumarin-3-carboxylic acid, abbreviated as 7-OHCCA, see Figure 2.22.

O O

O

OH

O O

O

OH

OH

CCA 7-OHCCA

Figure 2.22 The non-fluorescent CCA and the strongly fluorescent product of OH andCCA: 7-OHCCA.

The excitation wavelength of 7-OHCCA is λex=396 nm. The fluorescence maximum isnear λfl=450 nm. Using CCA, time resolved measurement of the hydroxyl radicalproduction rate kinetics is possible.However, a problem is the very limited solubility of CCA in water. The choice for abetter solvent or solvent addition are no option: the hydroxyl radical will react with thesolvent molecule and quantitative measurements of the hydroxyl radical concentrationare not accurate anymore.

36 Chapter 2.

Infrared spectroscopy

For identification of gaseous phenol oxidation products, infrared spectroscopy has beenapplied [93-95]. The gross selection rule for IR spectroscopic activity is that amolecule’s dipole moment shall change during the normal mode of vibration. Thechange of the dipole moment establishes an electrical field that can interact with theelectrical vector of the radiation. Normal modes of vibration are mutually-independentsynchronous motions of atom groups. Some typical molecular vibrations are stretchvibrations (symmetric and anti-symmetric) and deformations (scissoring, rocking,wagging, twisting). Linear molecules consisting of N atoms vibrate in 3N-5 differentways while non-linear molecules show 3N-6 different vibrations.The wavelength range of interest to organic compounds is 4000-600 cm-1

corresponding to 2.5-17 µm. This region can be divided into two parts viz. theabsorption range due to characteristic functional groups (4000-1200 cm-1) and thefinger-print area, where absorptions provide information about the overall constitutionof the molecule (1200-600 cm-1).

With regard to the oxidation of phenol, the following compound classes are of majorinterest: carbon oxides, aldehydes and carboxylic acids. Indirectly involved are ozoneand nitrogen oxides produced by corona in air.

Carbon dioxide shows two stretch vibrations and two perpendicular bending motions.The stretch vibrations are located at 2349 cm-1 (anti-symmetric) and 1333 cm-1

(symmetric). The bending modes occur at 667 cm-1.

Carbon monoxide shows one stretch vibration at 2143 cm-1.

Aldehydes, ketones and carboxylic acids are recognizable by carbonyl stretch vibrationsin the range 1760-1690 cm-1.

Carboxylic acids show a broad absorption at 3000-2500 cm-1 due to O-H stretchvibrations. At 1300-1080 cm-1 C-O stretch vibrations occur.

Alkanols show O-H stretch vibrations at 3640-3610 cm-1 and vibrations of hydrogenbonds at 3600-3200 cm-1. C-O stretch vibrations occur at 1300-1080 cm-1.

Nitrogen dioxide has two stretch vibrations viz. at 1618 cm-1 (anti-symmetric) and at1318 cm-1 (symmetric) and a bending mode at 750 cm-1.

Nitrous oxide has two stretch vibrations viz. an XY stretch at 2224 cm-1 and YZ stretchat 1285 cm-1 and two bending modes at 589 cm-1.

Ozone has two stretch vibrations viz. at 1103 cm-1 (symmetric) and 1042 cm-1 (anti-symmetric) and a bending mode at 701 cm-1.

Finally water vapour, produced by application of pulsed corona discharges over theaqueous solution, causes absorptions due to stretch vibrations at 3657 cm-1

(symmetric) and 3756 cm-1 (anti-symmetric) and a bending mode at 1595 cm-1.

3. Experimental setup

A description of the applied reagents, experimental configurations and chemical,electrical and optical analysis techniques is presented.

3.1. Reagents & reactors

Table 3.1 shows a list of applied model compounds and oxidation products,chromatographic eluents and other reagents.

Table 3.1 Applied reagents: name, Chemical Abstract Service registry number,molecular weight, density of liquids, purity and origin.

Compound CAS no. FW (g/mol) ρ (g/cm3) Grade(w%)1) ManufacturerAcetic acid 64-19-7 60.05 1.05 100 MerckAcetonitrile 75-05-8 41.05 0.782 sg BiosolveAcrylic acid 79-10-7 72.06 1.051 99 AldrichAmmonia 7664-41-7 17.03 0.91 25 MerckArgon 7440-37-1 39.95 - 99.999 MesserAtrazine 1912-24-9 215.69 - 99.9 Riedel-de Haën1,4-Benzoquinone 106-51-4 108.10 - 98 AldrichCatechol 120-80-9 110.11 - 99+ AldrichCCA 531-81-7 190.15 - 99 AldrichDimethyl sulfide 75-18-3 62.13 0.846 - PolyScienceDMPO 3317-61-1 113.16 1.015 97 AldrichFormic acid 64-18-6 46.03 1.22 98-100 MerckGlyoxal 107-22-2 58.04 1.265 40 AldrichGlyoxylic acid 298-12-4 74.04 1.342 50 AldrichHelium 7440-59-7 4.00 - 99.995 HoekloosHydrogen peroxide 7722-84-1 34.01 1.11 30 MerckHydroquinone 123-31-9 110.11 - 99+ AldrichHydroxyhydroquinone 533-73-3 126.11 - 99 AldrichMaleic acid 110-16-7 116.07 - 99 AldrichMalonic acid 141-82-2 104.06 - 99 AldrichMethanol 67-56-1 32.04 0.79 sg BiosolveNitrogen 7727-37-9 28.01 - 99.9 HoekloosOxalic acid 144-62-7 90.04 - 99+ AldrichPhenol 108-95-2 94.11 - 99+ AldrichPhloroglucinol 6099-90-7 126.11 - >99 MerckPropionic acid 79-09-4 74.08 0.992 >99 FlukaPyrogallol 87-66-1 126.11 - 99 AldrichResorcinol 108-46-3 110.11 - 99+ AldrichSuccinic acid 110-15-6 118.09 - 99+ AldrichToluene 108-88-3 92.14 0.865 99.7 MerckTrifluoroacetic acid 76-05-1 114.02 1.490 >98 Merck

1) sg=HPLC supra gradient

38 Chapter 3.

Malachite green has been obtained as a gift from the faculty of Chemistry/departmentof Instrumental Analysis/TUE. No accurate information about the malachite green anionis known, therefore data have not been further specified. It may regard e.g. malachitegreen carbinol hydrochloride CAS no. [123333-61-9] FW=382.94 g/mol or malachitegreen oxalate CAS no. [2437-29-8] FW=927.03 g/mol.

General remarks

All corona experiments have been performed at ambient conditions. The compoundsolutions have been prepared from deionized water (Millipore, resistivity 10 MΩcm),except for a few exploratory experiments for which tap water has been applied (asindicated). The reactor contents is continuously homogenized by a magnetic stirring bar.The sample volume is 1 ml or less. Due to the different analytical approaches, severalreactor types have been applied. Table 3.2 shows the used reactor types anddimensions.

Table 3.2 Applied reactor+anode types, listed by reactor configuration numberR. no.; VL=liquid phase volume, Vtot=total reactor contents, ∅=internaldiameter, L=length, h=height.

Reactor configurationR.no.

ExperimentType Dimensions

(mm)Anode1) VL (ml) Vtot (ml)

DMPO ESR Cyl+cir2) ∅=45, h=305 30p Fe 500 6401a1b DMPO oxidation Beaker ∅=85, h=122 30p Fe 249 500

Cube 70x80x100 30p Fe 242 560CCA probeVessel ∅≤95, h~200 30p Fe 500 1000

2a

2b CCA oxidation Vessel ∅≤95, h~200 30p Fe 500 10003a3b

OzonePhenol /EI-MS

Cube 57x62x100 30p Fe 100 353

4 Calorimetry Cube3) 75.5x80x99 1p W variable 5985a5b5c

Conducto-waterConducto-HBMicrotox test

Beaker ∅=85, h=122 1pFe/W/Pt1p W30p Fe

250 500

6a6b6c

PhenolAtrazinePhenol/LC-MS

Vessel ∅≤95, h~200 31p Fe31p Fe30p Fe

500 1000

7 Aldehydescreening

Cylinder ∅=45, h=305 30p Fe 500 570

Vessel ∅=90, h~100 30p Fe 300 6008a8b

LIFCube 57x62x100 30p Fe 100 353

9 FTIR Vessel ∅=70, h=121 30p Fe 250 50010 TOC Beaker ∅=100, h=142 30p Fe 498 100011 Malachite green Bar 30x80x100 various 125 24012 Dimethyl sulfide Cylinder ∅=37, L=310 wire gas phase 330

Experimental setup 39

1) Anode

•30,31p FeThe generally applied anode consists of an aluminum plate: diameter 30 mm, thickness1.5 mm. 30 or 31 steel pins are separated at 5 mm mutual distance; the global pindimensions are: length=14-15 mm, thickness=0.6 mm, 60 µm tip.

•variousDifferent electrode configurations have been tested: single pin/multipin/wire anode inthe liquid phase or gas phase at several distances from the liquid/gas interface, thecathode is situated inside/outside the reactor. Details are given in the appropriatesection.

•wireThe applied reactor is a cylindrical gas phase reactor, equipped with a 128 mmeffective length wire anode situated in the centre. A cathode wire mesh surrounds theglass tube on the outside.

•1p Fe/W/PtThe steel tip equals the generally used anode tips. The tungsten tip has been obtainedfrom a welding rod by grinding and polishing: the material is an alloy of tungsten and asmall amount of thorium; global dimensions are: length=15 mm, thickness=1 mm, 30µm tip. The platinum tip has been obtained from platinum wire; global dimensions:length=15 mm, thickness=0.5 mm, 70-100 µm tip.

In all configurations except for reactor configuration 11 and some setups of reactorconfiguration 4, the cathode is situated directly outside and underneath the reactorglass vessel bottom and therefore is dielectrically separated from the anode.

2) In-situ ESR

The reactor volume is about 570 ml; teflon tubing (L~5 m, ∅=4 mm) and viton pumptubing (L~20 cm, ∅~5 mm) have been applied, the circuit volume is about 70 ml. Dueto the pulsating pump flow, the anode-to-liquid distance d is somewhat fluctuating.

3) Calorimetry

The reactor heat capacity Creactor has been determined from the construction materials(glass and quartz) and the contents (water) [96].Quartz: (2.3x80x99+2.3x75.5x99)mm3 ≡ 34.6 cm3; cp

quartz=0.17 Jg-1K-1; Cquartz=13.0J/K. Glass: (3.0x80x99+ 3.0x75.5x99 +3.0x85.6x75.6)mm3 ≡ 65.6 cm3; cp

glass=0.48Jg-1K-1; Cglass=78.1 J/K. Water: cp

water=4.184Jg-1K-1; CH2O=4.18⋅volume; thereforeCreactor=(91.1+4.18⋅volume) J/K. The heat loss due to imperfect insulation is small,about 0.2 Kelvin per 15 minutes.

40 Chapter 3.

3.2. Chemical diagnostics

Liquid chromatography

The applied column types, liquid chromatography systems and conditions aresummarized by Table 3.3.

Table 3.3 Applied LC setups, listed by the LC configuration number; Col=columnno., Sys=system no., F=eluent flow, T=column temperature,con=conductivity, ACN=acetonitrile, TFA=trifluoroacetic acid.

SettingsLCno.

Experiment1) Col SysEluent (v/v%) F

(ml/min)Detector λ (nm) T (°C)

1 DMPO 1.0 mM 2b 3 H2O/ACN=90/10 1.0 230 202 CCA 1.0 mM 1 3 TFA 1 mM 0.8 210,270 503 Water 1 4 TFA 1 mM 0.8 200,210,220,

255,270,con35

4 Phenol 0.05 mM 3 1b H2O/ACN=80/20 1.0 254, 2804) 202a 2 H2O/ACN grad3) 1.0 210 205a

5bPhenol 1.0 mM

12) 1a TFA 1 mM 0.8 210 356 Phenol 1.0 mM air/Ar 1 4 TFA 1 mM 0.8 200,210,220,

255,270,con35

7 TOC 1.0 mM phenol 1 3 TFA 1 mM 0.8 270 358 CC 1.0 mM HB 1 3 TFA 1 mM 0.8 225, 270, 278 359 MT phenol ≤0.4 mM 2b 3 H2O/ACN=70/30 1.0 270 35

10 Atrazine 0.12 mM 3 1b H2O/ACN=55/45 1.0 216 20

1) TOC=total organic carbon, CC=conductometry & conversion, HB=hydroxybenzenes,MT=Microtox

2) Including OA-HY guard column L=20 mm, ∅=3 mm3) Grad= gradient: 4 min 0→5%, 3 min 5→20 min, 8 min 20→50%, 1 min 50→75% ACN4) 0-5 min: 254 nm, 5-10 min: 280 nm

Columns:

1 Merck Polyspher OA-HY (ICE)Dimensions: column L=300 mm, ∅=6.5 mm;Stationary phase: Polystyrene-divinylbenzene with -SO3

-H+ functional groups,dp=8 µm, crosslink ratio 8%; Column no. 150095

2 Zorbax Rx-C18 (rp-HPLC)Dimensions L=150 mm, ∅=4.6 mm, dp=5µm,Stationary phase: octadecyl grafted silicaa Column no. PN883967.902 - DU5023b Column no. PN883967.902 - DU4041

3 Zorbax SB-C18 (rp-HPLC)Dimensions L= 150 mm, ∅= 4.6 mm, dp=5 µmStationary phase: octadecyl-grafted silica; Column no. -

Experimental setup 41

LC-systems:

The injection volume is 20 µl for all analyses.

1a. Philips PU4100 Liquid Chromatograph and Philips PU4110 UV/VIS detector1b. System 1 including autosampler: Marathon ser.no. 0106, Spark Holland

2. Merck Hitachi L-6200A Liquid Chromatograph and Merck Hitachi L-4250UV/VIS detector

3. Hewlett-Packard HP1100 series Liquid Chromatograph, HP-G1315A diode arraydetector, G1311A quaternary pump, G1322A solvent degasser, G1313Aautosampler, G1316A column compartment.

4. System 3 in series with a micromembrane suppressor (Dionex AMMS-ICE II) andconductivity detector (Dionex CDM-2). The conductivity detector has been set tooutput range=300 µS and 1 Volt full scale; software: 106 units per Volt. Thesuppressor is supplied with a 5 mM aqueous ammonia solution at a flow rateF=2.0 ml/min.

Mass spectrometry

Configuration 1: Perkin Elmer Sciex API 300

The oxidation product sample is introduced into the IonSpray compartment by means ofa liquid chromatograph (Shimadzu LC-10AT); the acetonitrile flow is F=1.0 ml/min; theflow split ratio is 1:10 because the maximum allowable IonSpray flow is 0.2 ml/min.The settings are: NC=-3.5 kV, step=0.3 amu, dwell time=3.0 ms, pause=2.0 ms.The scanned mass-range=40-300 amu. 0.1 v/v% ammonia has been added to theoxidation product mixture to promote the ionizability.

Configuration 2: Balzers PPM 421

By means of a PEEK capillary (L=3 m, ∅=0.17 mm), the oxidized solution isintroduced into the prevacuum compartment where p<10-2 mbar; this compartment isseparated from the mass spectrometer by a 100 µm orifice. The pressure at thedetector is p<3⋅10-5 mbar. Ionization is performed by an electron beam of 100 eV. Theions are detected in a count mode. The scanned mass-range is 1-500 amu.

Solid phase extraction

The following SPE column types have been used: 500 mg C18 and 500 mg C6H5 (Baker).50 ml of the oxidized phenol solution has been extracted. Desorption has beenperformed using 0.5 ml methanol.

42 Chapter 3.

Gas chromatography

Configuration 1: Aldehyde screening

The toluene extract from the sorbent tubes has been analyzed by gas chromatography-mass spectrometry (GC-MS), using following configurations:

Instrument: Shimadzu QP5000Column: CP-Sil5; L=25 m, dc=220 µmDetector: QuadrupoleTemperature: T0=50°C, dT/dt=10°C/min, T1=275°C, F=2 ml/min

Instrument: Hewlett-Packard 6890+Leco Pegasus IIAColumn: CP-Sil5; L=8 m, dc=50 µmDetector: Time of flightTemperature: T0=60°C, dT/dt=20°C/min, T1=275°C, F=4 ml/min

Configuration 2: Conversion of dimethyl sulfide

Instrument: Hewlett-Packard 5890Column: HP Ultra-1; WCOT MeSil.; L=20m, dc=320 µm, df=0.52 µm,

He 100 barDetector: Sulfur Chemiluminescence, T=800°C, pcell=234 mBarTemperature: Isotherm T=40°CInjection: PTV: cold introduction, splitless mode, injection volume=100 µl

T0=40°C, dT/dt=12.5°C/s, T1=320 °C, flow 250 ml/min

10 ppm DMS standards have been prepared in methanol, because DMS is insoluble inwater. The retention time of DMS for the specified conditions is tR= 0.492 ± 0.009minutes.

Aldehyde screening test

The presence of volatile aldehydes during the oxidation of phenol has been investigatedusing aldehyde-specific gas sampling tubes (Supelco ORBO-23, no. 2-0257-U). Thesolid sorbent tubes contain 10% 2-(hydroxymethyl)piperidine (HMP) on Supelpak 20N(20/40) carrier divided over two sections: the main section contains 120 mg and thebackup section contains 60 mg. The required gas sampling volume is 5 liter. By meansof a flow controller (Brooks 5850) and controller unit (Gossen 5875) the reactor hasbeen purged with an argon 5.0 flow at a rate F=200 ml/min, see reactor configuration7. Figure 3.1 shows the applied setup. After sampling, HMP and possible oxazolidinesare extracted from the sorbent tube contents using 1 ml toluene by ultrasonic agitationduring 60 minutes. The extract is analyzed by GC-MS.

Experimental setup 43

STFC

R

Ar

Figure 3.1 Setup for sampling of volatile aldehydes from an oxidized phenol solution;ST=sampling tube; Ar=argon 5.0 flow, FC= flow controller, R=coronareactor.

Electron spin resonance

The identification of oxidizer radicals and phenol oxidation intermediate radicals hasbeen studied by in-situ Electron Spin Resonance (ESR). The applied reactor isconstructed from a glass tube (QVF) and two teflon covers with o-rings, see reactorconfiguration 1a. By means of chemically-inert tubing and a peristaltic pump, acontinuous flow system has been created between the corona reactor and an ESRquartz flat cell (flow area: 1x10 mm), see Figure 3.2. The flow rate is about 8 cm3/s(N~2 s-1). The applied spin trap is 5,5-dimethyl-1-pyrroline N-oxide. Measurementshave been performed using a Bruker ESP300 ESR spectrometer. The settings aremicrowave frequency: 9.75 GHz; modulation frequency: 100 kHz, modulationamplitude: 5 G. The centre field has been set to 3355 G/width 100 G and to 3450G/width 500 G. The temperature is T=295 K.

R

ESR

P

Figure 3.2 In-situ ESR setup for the detection of radicals; R=reactor, P=peristalticpump.

44 Chapter 3.

Microtox test

For determination of the reference bioluminescence intensity, two Vibrio fischerisolutions have been prepared using freeze-dried bacteria: a commercial Microtox sea-water diluent and the Millipore water used to prepare the standard phenol solutions withNaCl added (20g/l). Before the Microtox test is performed, the degree of acidity ofevery solution has been adjusted to 5.0<pH<5.5 using a diluted NaOH solution. Alsothe oxygen content of the solutions has been verified.

A series of 250 ml phenol solutions at concentrations 0.02 mM, 0.05 mM, 0.1 mM,0.2 mM and 0.4 mM have been exposed to pulsed corona discharges during 30minutes, see reactor configuration 5c. To investigate the possible ecotoxicity increase,caused by corona treatment of Millipore water i.e. the formation of hydrogen peroxideand nitric acid, the 0 mM sample has been included in the test. The 0.4 mMconcentration is the upper limit for observation of a 20% effect. All samples have beenstored away under argon to bridge the time before analysis.

The bioluminescence intensity of untreated and oxidized phenol solutions has beendetermined in duplicate, with regard to the reference bioluminescence intensity. TheEC20 effect value has been reported at the Vibrio fischeri exposure times tVF=5 min,t=15 min and t=30 minutes.

Total Organic Carbon

The carbon content of phenol solutions has been determined versus the oxidation time(reactor configuration 10) using a Dohrmann TC190 analyzer. Prior to analysis thesamples have been diluted by a factor 4 with deionized water. The total organic carbon(TOC) content is determined from the difference of total carbon content (TC) and totalinorganic carbon content (TIC). The TC content is calculated from the amount of carbondioxide that is released from catalytic combustion of the sample. The carbon dioxideconcentration is measured by infrared spectroscopy. The TIC content is determinedfrom the carbon dioxide release that occurs after acidification of the sample withphosphoric acid.

Acidity

A pH measuring device (Metrohm 691) has been used to measure the solution acidity ofuntreated and oxidized phenol solutions. Before the measurements, the instrument iscalibrated using calibration buffer solutions (Merck).

Experimental setup 45

3.3. Electrical diagnostics

Electrical circuit

The electrical circuit for generation of pulsed corona discharges is shown by Figure 3.3.By means of a high voltage power supply of negative polarity (Wallis), a capacitor(Thomson CSF-LCC) is charged through a 10 MΩ load resistor (Metallux). By triggeringthe spark gap, the energy stored in the capacitor is discharged into a glass reactorvessel, by means of a capacitive electrode configuration. In this way, conductivecurrents are prevented. Positive corona is produced at the anode, situated at a distanced over the aqueous solution. The anode is single-pin or multipin consisting of 30 or 31steel pins at 5 mm mutual distance. The cathode plate is located outside andunderneath the reactor vessel. A 50 MΩ tail resistor (Philips) is installed parallel to thereactor, to rezero the voltage after every discharge pulse. The spark gap is triggered byfrequency-adjustable 9 kV pulses. The reactor contents is stirred by a 1 cm magneticstirring rod. The circuit is settled in welded-alumina EMC compartments.

The pulse voltage is measured at the anode by means of a high voltage probe(Tektronix P6015A: 1000x, 3.0 pF, 100 MΩ; compensation box 015-049). The currentis determined at the cathode by means of a coil probe (Pearson 2877: 1 V/A). Dataacquisition is performed by a 400 MHz 2 Gs/s digital oscilloscope (Tektronix TDS380).Data signals are obtained in eightfold averaging mode and 20 MHz filtering enabled. Theprobe cables are shielded with copper non-woven jackets to avoid interception of highfrequency noise, emitted by the spark gap. A coil (L) is inserted in series with thecapacitor to damp parasitic high frequency oscillations.For reasons of discrete sampling times, the assumption has been made that the pulseenergy is constant in between two measurements: Ep(ti-1<t≤ti) = Ep(ti). With regard tothe observed small changes in the pulse energy during oxidation runs of several hours,this assumption is justified.

R = 10 MΩ

R =

50

MΩ

C = 1 nF

L

V-probe

I-probe

anode

cathode

triggeredsparkgap

-

+

highvoltagesource

reactor

Figure 3.3 The high voltage circuit to generate pulsed corona discharges.

46 Chapter 3.

Probes calibration

For calibration of the voltage probe, a 20 V 100 kHz square waveform from a functiongenerator (Textronics CFG250) is presented to the probe. The voltage probe calibrationfactor is determined from a set of successive acquisitions of function generator voltageand probe-indicated voltage. The observed calibration factor is about 909x.

The current probe is calibrated by application of the 20 V 100 kHz square waveformacross a 1 kΩ resistor. The current probe calibration factor is determined from a set ofsuccessive acquisitions of voltage across the resistor and current indicated by the probein series with the resistor. The observed calibration factor is about 1.0 V/A. The currentprobe cable is terminated with a 50 Ω impedance to avoid signal reflection; then 1 voltcorresponds to 2 A.

Due to a difference in probes cable length, a time lag exists between the voltage andcurrent signal. Assuming that electric charge travels through the probe cable with thespeed of light, the time lag is about 3.3 ns/m. The time lag correction is very importantfor accurate pulse energy determination. The actual time lag has been determined fromthe voltage and current signals observed during the current probe calibration. Theobserved time lag is about 8 ns.

Electrical conductometry

The electrical conductivity of corona-exposed deionized water and aqueoushydroxybenzene solutions has been measured using a conductivity meter (Cole Parmer01481-92) equipped with a gold-plated electrodes dip-cell (CP 01481-93). The cellconstant of the used dip cell equals K=10 cm-1. This conductivity meter is temperaturecompensated: dG/dT=2%/°C. Before every experiment, the device is calibrated using a445 µS/cm NaCl standard solution (CP 01489-93) at 25°C.

3.4. Optical diagnostics

UV absorption spectrometry

The ozone production by pulsed corona discharges in air has been measured by UVabsorption spectrometry. The applied setup is shown by Figure 3.4, see also reactorconfiguration 3a. A high-pressure mercury lamp (Philips 3110E) has been used assource. By means of quartz optics, the light beam is transmitted through a quartz wallreactor (57x62x100 mm, optical path=57.6 mm) and then imaged on the slit of a0.5 meter monochromator (Jarrell Ash 82025), equipped with a 1200 mm-1 grating andphotomultiplier (Hamamatsu R636). The absorption at 260 nm is measured. Afteramplification (Textronics AM502) of the photomultiplier signal, data acquisition isperformed by and A/D converter (TSC500).

Experimental setup 47

R

Hg

+ +

M-JAPM

AM A/D

Figure 3.4 UV absorbance setup for quantitative ozone measurements. Hg=high-pressure Hg lamp, +=positive quartz lens, R=reactor, M-JA=monochromator, PM=photomultiplier, AM=amplifier, A/D=analog-to-digital converter.

Laser-induced fluorescence

A Nd:YAG solid-state pulsed laser (Continuum 9030) has been applied as excitationsource for LIF measurements on aqueous phenol solutions. The applied excitationwavelength λ=266 nm is produced from a twofold frequency doubling of thefundamental wavelength λ=1064 nm. The laser pulse width is about 6 ns and thelinewidth is 1.0 cm-1. The beam repetition frequency is set to 10 Hz. The measuredlaser pulse energy is about 0.8-1.1 mJ. The beam waist at the detection volume isabout 3 mm. Figure 3.5 shows a schematic drawing of the setup.By means of a Pellin-Broca prism, the excitation wavelength is separated from the otherharmonics (355 nm and 532 nm). The applied reactors are equipped with quartzobservation windows, see reactor configuration 8. The fluorescence intensity ismeasured at a 90° angle relative to the laser beam. By means of a quartz lens andquartz fibre the fluorescence radiation is introduced into a monochromator (Jobin-YvonH25) equipped with a 150 mm-1 grating. Detection is performed by an ICCD camera(Andor ICCD-452/DH534-18) operated in the gated mode: the ICCD is externallytriggered by the Nd:YAG laser. The exposure time is 5 seconds, which yields thefluorescence intensity due to 50 laser pulses. The laser triggers the ICCD camera via apulse delay generator (Stanford DG535). A low-pressure mercury lamp (Cathodeon93109) has been used for wavelength calibration.

Nd:YAG

M-JY ICCD DG

D

PB

PC

ex.tr.QF

R

+

Figure 3.5 LIF setup for measuring phenol degradation by pulsed corona.Nd:YAG=laser, PB=Pellin-Broca prism, R=reactor, +=positive quartzlens, QF=quartz fibre, M-JY=monochromator, ICCD=detector,DG=delay generator; PC=computer, D=laser dump.

48 Chapter 3.

Fluorescent molecular probe

The detection of hydroxyl radicals has been performed by means of the molecular probecoumarin-3-carboxylic acid (CCA). 1 mM aqueous solutions have been prepared. Beforeoxidation, a sample is taken from the corona reactor, see reactor configuration 2a. Thisnon-fluorescent CCA sample is used for background determination. The fluorescenceintensity is measured by a Perkin Elmer LS50B fluorescence spectrometer. Theexcitation wavelength is λex=396 nm. The applied quartz containers (Hellma QS111)have an optical path L=10 mm and the volume is 3500 µl. The scanspeed is 100nm/min. The excitation and emission slitwidth are 5 nm.For the hydroxyl radical concentration determination, 100 ml standards have beenprepared consisting of CCA and unstabilized hydrogen peroxide. In order to have equalinitial CCA concentrations, all standards have been prepared from the same 1.0 mMCCA stock solution by a 10-fold dilution. The standard solutions contain 0.1 mM CCAand increasing amounts of hydrogen peroxide i.e. 4.7⋅10-5 M, 6.6⋅10-5 M, 7.9⋅10-5 M,2.0⋅10-4 M, 3.6⋅10-4 M and 9.7⋅10-4 M.

Infrared spectroscopy

The gas phase over oxidized phenol solutions has been analyzed by Fourier-transforminfrared spectroscopy (FTIR, Bruker IFS 66). A DTGS detector has been applied. Thescanned wavelength range is 3000-1500 cm-1. The resolution is 4 cm-1.The volume of the applied reactor is 500 ml and the optical path is 120 mm, seereactor configuration 9; the reactor is equipped with sapphire windows. Thespectrometer is purged with nitrogen at a rate F=500 l/h to exclude carbon dioxide andwater vapour. After corona-exposure, the sealed reactor is immediately transferred fromthe high voltage setup to the spectrometer. Spectra have been recorded 15 minutesafter installation of the reactor into the spectrometer, in order to recover a carbondioxide-free measurement compartment. Background spectra have been recorded fromthe empty reactor, the reactor filled with the phenol solution at t=0 and the emptyspectrometer compartment in between the measurements.

4. Results

In this chapter the results will be presented concerning the production of oxidizers bypulsed corona discharges in humid air. A detailed analysis of the oxidation of the modelcompound phenol is given regarding conversion efficiency, oxidation products andanalysis techniques. In addition, the oxidation of the model compounds atrazine,malachite green and dimethyl sulfide is described.

4.1. Pulsed corona discharges

This section describes the results of experiments on the effects of pulsed coronadischarges in air over deionized water. The production of hydroxyl radicals and ozone isdescribed, reactor geometrical capacitances and pulse energies are reported and theanalysis of corona-exposed deionized water is discussed.

4.1.1. Hydroxyl radicals

In order to detect hydroxyl radicals in corona-treated water two approaches have beenapplied. The first approach is trapping of the hydroxyl radical by the spin trap DMPO,followed by in-situ ESR detection of the produced DMPO-OH adduct. The secondapproach is the reaction of the hydroxyl radical with the fluorescent molecular probecoumarin-3-carboxylic acid (CCA), followed by ex-situ fluorescence spectrometry onproduced 7-hydroxy CCA (7OHCCA).

The spin trap approach

A 500 ml 1.0 mM aqueous DMPO solution has been exposed to pulsed coronadischarges in a continuous flow system, see reactor configuration 1a. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. During the oxidation for70 minutes, several ESR scans have been made in order to detect the DMPO-OHadduct. No ESR-absorption signals have been observed. Hereafter, an extra 2.5 mmolDMPO amount has been added to the continuous flow system in order to be sure, thatenough fresh spin trap is available for the trapping of radicals. Also this extra additionhas not resulted in any absorption signals after 50 minutes of continuation.A second experiment has been performed, in which a solution containing 1.0 mMphenol and 1.0 mM DMPO spin trap has been oxidized by pulsed corona discharges.Now the spin trap is intended to trap radical intermediates produced by the attack ofhydroxyl radicals on phenol i.e. dihydroxycyclohexadienyl(peroxyl) radicals. Theseorganic radicals are more stable in aqueous solution than hydroxyl radicals. Again, noESR absorption signals have been detected within 75 minutes of corona-exposure time.The possibility may exist that DMPO is destroyed by the pulsed corona discharges. Thishas been verified by application of liquid chromatography to untreated and oxidizedDMPO solutions. A reversed-phase HPLC column and UV absorbance detector havebeen used according to LC configuration 1. Figure 4.1 shows the conversion of DMPOas a function of time.

50 Chapter 4.

0

10

20

30

40

0 15 30 45 60 75 90Time (min)

Con

vers

ion

(%)

Figure 4.1 The conversion of a 250 ml 1 mM DMPO solution by pulsed coronadischarges. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm.

The observed conversion course will mainly imply the degradation of DMPO byoxidizers, but the production of the DMPO-OH adduct will also be included. Figure 4.2shows a possible degradation pathway of DMPO by ozone.

O3OOO

NO

H OHOOH

HNO2

OHO

NO2

OHNO2

OH2-

H2O2-DMPO molozonide ozonide zwitterion hydroperoxy alkanol

OH2OO

ON

O

H

NO

H OO

C+

HNO O

Figure 4.2 Possible degradation pathway of the spin trap DMPO by ozone.

Degradation of DMPO by ozone causes ring-cleavage of the 5-membered ring and 4-nitro-4-methyl valeric acid or -valeraldehyde may be produced. Also singlet oxygen inknown to degrade DMPO [97]. Identification of oxidation products has not beenperformed.

Although conversion increases significantly with the exposure time, within short timesthe conversion is still low (X10min<5%) and enough spin trap is available to traphydroxyl radicals. In addition, the half-life of the DMPO-OH adduct is reported to beabout 2 hours [75].

Results 51

The fluorescent molecular probe approach

A 242 ml 1.0 mM CCA solution has been exposed to pulsed corona discharges for 40minutes, see reactor configuration 2a. The corona parameters are V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm. The fluorescence intensity of the non-treated and oxidizedsolutions as a function of time is illustrated by Figure 4.3.

0123456789

10

400 450 500 550 600Wavelength (nm)

Inte

nsity

(arb

.u.)

40 min

30 min

20 min

10 min

0 min

Figure 4.3 Fluorescence spectra of a 1 mM CCA solution after different corona-exposure times (0-40 min). The excitation wavelength λex=396 nm. Thecorona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

The fluorescence from non-treated CCA is marked by t=0 min and is very weakcompared to the fluorescence from the reaction product of CCA and the hydroxylradical: 7-OHCCA. The fluorescence intensity of the oxidized solutions with regard tothe non-treated CCA solution (background) is illustrated by Figure 4.4.

0123456789

10

400 450 500 550 600Wavelength (nm)

Inte

nsity

(arb

.u.) 40 min

30 min

20 min

10 min

Figure 4.4 Fluorescence spectra of a 1 mM CCA solution after different corona-exposure times (10-40 min). Subtracted background is the untreated CCAsolution. The excitation wavelength λex=396 nm. The corona parametersare V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

52 Chapter 4.

The increase of the fluorescence intensity with the exposure time is almost linear. Thisimplies that the production rate of the hydroxyl radicals is constant. This experimenthas been repeated applying a longer exposure time, see Figure 4.5.

0

5

10

15

20

25

400 450 500 550 600Wavelength (nm)

Inte

nsity

(arb

.u.)

90 min

60 min45 min

30 min

15 min

Figure 4.5 Fluorescence spectra of a 1 mM CCA solution after different corona-exposure times (15-90 min). Subtracted background is the untreated CCAsolution. The excitation wavelength λex=396 nm. The corona parametersare V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Here, the increase of the fluorescence intensity is still reasonably proportional to theexposure time, although it may be expected, that CCA will be degraded after longerexposure times. The stability of CCA towards pulsed corona discharges has beendetermined by ion-exclusion chromatography using the LC configuration no. 2b. A500 ml 1.0 mM CCA solution has been oxidized by pulsed corona discharges for 3hours. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. Theconversion of CCA versus the oxidation time is shown by Figure 4.6.

0

5

10

15

20

25

0 30 60 90 120 150 180

Time (min)

Con

vers

ion

(%)

210 nm

270 nm

Figure 4.6 The conversion of a 500 ml 1 mM CCA solution by pulsed coronadischarges. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm.

Results 53

CCA appears to be fairly stable towards pulsed corona discharges. After 3 hours only20% conversion occurs. Possible degradation of CCA may involve multifoldhydroxylation and ring-cleavage of the benzene ring, hydroxylation of or ozone attackon the double bond present in the pyran ring followed by ring-cleavage, see Figure 4.7.Identification of oxidation products has not been applied.

O O

O

OH

CCA

O O

O

OH

OH

c) d)

b)a)O O

O

OHHO O

O O

O

OH

O

O

O O

O

OH

O OO

Figure 4.7 Possible degradation of CCA: a) hydroxylation of the benzene ring, b) ring-cleavage of the benzene ring by ozone, c) hydroxylation of the pyran ringdouble bond, d) ring-cleavage of the pyran ring by ozone.

Attempts have been made to quantify the production of hydroxyl radicals. Thefluorescence intensity of 7-OHCCA has been calibrated versus the amount of hydroxylradicals produced per solution volume. Fluorescence standard solutions have beenprepared from CCA and hydrogen peroxide, where hydrogen peroxide has been used assource for hydroxyl radicals. With regard to the quantitative approach, unstabilizedhydrogen peroxide has been utilized, to avoid scavenging of hydroxyl radicals by thestabilizer normally present in commercial hydrogen peroxide solutions. Differentamounts of hydrogen peroxide i.e. known amounts of hydroxyl radicals have beenadded to 100 ml 0.1 mM CCA solutions. A pure 0.1 mM CCA solution has beenexposed to corona discharges. The corona parameters are V=25 kV, C=1 nF, f=100Hz, d=1.0 cm. The fluorescence intensity of both the oxidized CCA solution and thestandards is shown by Figure 4.8.

0

1

2

3

4

5

400 450 500 550 600

Wavelength (nm)

Inte

nsity

(arb

.u.)

30 min20 min10 min 0 min9.7E-4 M OH3.6E-4 M OH2.0E-4 M OH7.9E-5 M OH6.6E-5 M OH4.7E-5 M OH

Figure 4.8 Fluorescence spectra of an 0.1 mM CCA solution after different corona-exposure times (0-40 min) The corona parameters are V=25 kV, f=100Hz, C=1 nF, d=1.0 cm. Also shown are the fluorescence spectra of theCCA/H2O2 fluorescence standards, indicated by a virtual hydroxyl radicalconcentration. The excitation wavelength λex=396 nm.

54 Chapter 4.

Unfortunately, the fluorescence standards appear to show no correlation between thefluorescence intensity and the amounts of hydroxyl radicals added (as hydrogenperoxide). Also, the fluorescence intensity of the untreated 0.1 mM CCA solution i.e.background is even higher than the intensity of several fluorescence standards.

These contradictory results are explained by the fact that the preparation of thefluorescence standards is critical. The addition of hydrogen peroxide to the CCAsolution should be in stoichiometric proportions, here equal molarities. However, thesolubility of CCA in water is poor: higher concentrations than 1 mM are not feasible.This implies, that hydrogen peroxide should be added in very diluted form to the CCAsolution. Very dilute hydrogen peroxide solutions are not stable, because hydrogenperoxide will oxidize the water, according to the redox reactions shown by Equations4.1a-c. The addition of hydrogen peroxide to the 0.1 mM CCA solutions is thusunverifiable.

H2O2 + 2e- → 2 OH- reduction of hydrogen peroxide (4.1a)H2O2 + 2H+ + 2e- → 2 H2O reduction of hydrogen peroxide (4.1b)2H2O → O2 + 4H+ + 4e- oxidation of water (4.1c)

Summary

The formation of hydroxyl radicals during exposure of water by pulsed coronadischarges has been demonstrated by fluorescence spectroscopy using the OH-specificfluorescent molecular probe CCA. The production of hydroxyl radicals appears to berather constant during 90 minutes of corona exposure. It has not been possible toattribute the observed fluorescence intensity to certain amounts of hydroxyl radicals. In-situ Electron Spin Resonance using the spin trap DMPO has not been able to identifythe hydroxyl radical under the same conditions.

Results 55

4.1.2. Ozone

The production of ozone by pulsed corona discharges in air has been determined forseveral reactor conditions using UV absorbance spectrometry, see reactor configuration3a. First, ozone measurements have been performed for corona discharges in anambient air-filled reactor at different load voltages viz. -25 kV, -20 kV, 15 kV, 20 kV,25 kV and 30 kV. The ozone concentration versus time is shown by Figure 4.9. It hasbeen observed, that the maximum ozone concentration increases with increasingabsolute load voltage. At a given voltage, the negative polarity corona produces lessozone than the positive polarity corona. After about 16 minutes the corona has beenswitched off: a decrease in ozone concentration follows as a result of the backwardreaction of ozone into oxygen by wall recombination.

0.0E+00

2.0E+22

4.0E+22

6.0E+22

0 5 10 15 20

Time (min)

[Ozo

ne]

(m-3)

15 kV

20 kV

-20 kV

-25 kV

25 kV

30 kV

Figure 4.9 Ozone concentration versus time in a reactor filled with ambient air,energized at different load voltages. The corona parameters areV (indicated), C=1 nF, f=100 Hz, d=1.0 cm. After about 16 min thecorona discharges have been stopped.

Hereafter, ozone concentrations have been measured for the case of a reactor filledwith ambient air and 100 ml deionized water. The load voltages have been 20 kV,25 kV and 30 kV, see Figure 4.10. The ozone concentration over the deionized waterincreases with the load voltage just as for the case of the ambient air-filled reactor, butthe maximum ozone concentration over water is lower than the ozone concentration inair. Also it has been observed, that the initial ozone production rate i.e. initial curveslope is higher for the water-filled reactor compared to the ambient air-filled reactor.

The difference in maximum ozone concentration can be explained by the fact thatozone is destroyed by hydroxyl and hydroperoxyl radicals produced in humid air overwater, according to Equations 2.4ab. Also ozone reacts with water, see Equations2.3a-c. The water-filled reactor has a smaller gas phase volume (253 ml) than theambient air-filled reactor (353 ml), therefore initially the ozone production rate is higher.

56 Chapter 4.

0.0E+00

2.0E+22

4.0E+22

6.0E+22

0 5 10 15

Time (min)

[Ozo

ne] (m

-3)

30 kV water

30 kV air

25 kV air

25 kV water

20 kV air

20 kV water

Figure 4.10 Ozone concentration versus time in a reactor filled with ambient air and100 ml deionized water or solely ambient air, energized at different loadvoltages. The corona parameters are V (indicated), C=1 nF, f=100 Hz,d=1.0 cm.

Next, the influence of phenol in water on the gas phase ozone concentration has beenmeasured. 100 ml 1 mM phenol solutions have been oxidized by corona in ambient airat voltages 25 kV and 30 kV. The ozone concentration versus time for deionized waterand phenol solutions is shown by Figure 4.11. The presence of phenol in the deionizedwater additionally decreases the ozone concentration, so it is clear that phenol inaqueous solution consumes ozone produced over the aqueous solution. The reactionproducts of phenol oxidation will be discussed in sections 4.2 and 5.3.

0.0E+00

2.0E+22

4.0E+22

6.0E+22

0 5 10 15Time (min)

[Ozo

ne]

(m

-3)

30 kV water

30 kV 1 mM phenol

25 kV 1 mM phenol

25 kV water

Figure 4.11 Ozone concentration versus time in a reactor filled with ambient air and100 ml deionized water or 100 ml 1 mM phenol solution, energized atdifferent load voltages. The corona parameters are V (indicated), C=1 nF,f=100 Hz, d=1.0 cm.

Results 57

Application of pulsed corona discharges in pure oxygen may produce higher ozonelevels. This has been verified by purging the reactor with oxygen at the flow ratesF=100 ml/min and F=200 ml/min. The load voltages are 20 kV, 25 kV and 30 kV, seeFigure 4.12. With increasing load voltage and constant oxygen flow rate the ozoneconcentration increases. Increasing the oxygen flow rate appears to have a minorpositive effect on the ozone production at 20 kV, but has a strongly negative effect onthe ozone production at the load voltages 25 kV and 30 kV.

This is explained by the fact, that at higher flow rates the produced ozone is removedfrom the reactor by the oxygen purge. This effect can also be illustrated by comparisonof the ozone production at V=30 kV & F=200 ml/min and the ozone production atV=25 kV in an oxygen saturated reactor (F=0 ml/min). The positive effect of thehigher voltage is overruled by the negative effect of a high flow rate.

0.0E+00

2.0E+22

4.0E+22

6.0E+22

8.0E+22

1.0E+23

0 2 4 6 8 10Time (min)

[Ozo

ne]

(m-3)

20 kV 100 ml/min20 kV 200 ml/min

25 kV 0 ml/min

25 kV 100 ml/min

25 kV 200 ml/min

30 kV 200 ml/min

30 kV 100 ml/min

Figure 4.12 Ozone concentration versus time in a reactor purged with oxygen atdifferent flow rates, energized at different load voltages. The coronaparameters are V (indicated), C=1 nF, f=100 Hz, d=1.0 cm.

Finally the ozone production has been monitored versus the integrated corona pulseenergy for the reactor filled with ambient air and 100 ml deionized water. The loadvoltages are 15 kV, 20 kV, 25 kV and 30 kV, see Figure 4.13. From the initial slope ofthe ozone concentration versus corona energy curves, the following ozone productionefficiency has been calculated: 5.45⋅1020 ±0.05 m-3J-1. This value equals about40 g/kWh for the applied reactor and is quite high, because this efficiency has beenobtained in humid air. Nowadays commercial ozone generators reach efficiencies of50-60 g/kWh in dry air [98].

58 Chapter 4.

0.0E+00

2.0E+22

4.0E+22

6.0E+22

0 50 100 150 200

E (J)

[Ozo

ne]

(m-3)

15 kV

20 kV

25 kV

30 kV

Figure 4.13 Ozone concentration versus the integrated corona pulse energy in areactor filled with ambient air and 100 ml deionized water, energized atdifferent load voltages. The corona parameters are V (indicated), C=1 nF,f=100 Hz, d=1.0 cm.

Summary

The production of ozone by pulsed corona discharges in air and oxygen has beendemonstrated. The ozone concentration increases with the corona load voltage.Negative corona produces less ozone than positive corona at equal absolute loadvoltage. Ozone produced by corona over water is destroyed by hydroxyl andhydroperoxyl radicals, which are also formed by the corona in humid air. Ozoneproduced in the gas phase over an aqueous phenol solution is consumed by phenol.Pulsed corona discharges in oxygen produce higher ozone concentrations thandischarges in air, but high oxygen purge flow rates remove ozone from the reactor. Theachieved ozone production efficiency for humid air is about 40 g/kWh.

Results 59

4.1.3. Corona pulse energy

In order to calculate the pulse energy of corona discharges applied in the gas phase,first the capacitance of the applied setup has been determined. The severalconfigurations mainly differ by liquid/gas phase volume and anode type. Figure 4.14shows a selection of best fits of I(t) and Cg⋅dV(t)/dt for estimation of the geometrycapacitances. Table 4.1 shows the measured average geometry capacitances for allapplied setup configurations.

Table 4.1 Average geometry capacitances (Cg) and 95% confidence intervals.

ReactorReactorno.

Anode(pins) liquid vol (ml) liquid and gas type

Cg

(pF)6ab 31 500 tap water, air 4.3 ±0.16c 30 500 deionized water, air 2.5 ±0.16c 30 500 phenol 1 mM, air 2.6 ±0.13b 30 100 phenol 1 mM, air 2.3 ±0.23b 30 100 phenol 1 mM, Ar purge 2.4 ±0.05ab 1 250 deionized water, air 1.4 ±0.1

All configurations have the anode situated d=1.0 cm over the aqueous solution, whilethe cathode plate is situated outside and directly underneath the glass reactor vessel.Although the determined capacitances are rather small, the capacitive current for theseconfigurations is not negligible, because the slope of the voltage is very steep, that isdV(t)/dt≈1011-1012 V/s. The observed oscillations are due to parasitic impedances fromthe circuit in combination with fast voltage rise times created by the triggered sparkgap.

The presence of phenol in the water does not influence the capacitance, see reactorconfiguration 6c; the relative permittivity of phenol is about εR,phenol≈12.4 while waterhas a relative permittivity εR,H2O≈80.1. In addition, the applied molar fraction of phenol towater is negligible viz. 10-3 M:55.4 M=1.82⋅10-5. With regard to the oxidation ofaqueous phenol solutions, neither formic acid (εR≈51.1) nor hydrogen peroxide (εR≈74.6)will contribute to the permittivity of water, because of their low concentration. Datahave been obtained from [99]. From the equal capacitances of reactor configuration 3bwith air or argon, it can be derived that the capacitance is not affected by vigorouslypurging of the reactor. Although the relative permittivities of argon and air are nearlyequal, the argon purge at a flow rate F=100 ml/min. results in a gas-dispersed phenolsolution that may have a different capacitance than the immobile phenol solution in theair-filled reactor. Finally, the capacitances cannot be related to the indicated liquidphase volumes, because the reactors have different dimensions.From the measured capacitances, the capacitive current has been calculated that is partof the total current recorded during the corona experiments. The pulse energy has beenestimated from the pulse voltage and corona current. Table 4.2 shows pulse energies,averaged over the different observation times, during the applied pulsed corona-inducedoxidation experiments discussed in chapter 4. Characteristic voltage and currentwaveforms are presented together with the efficiency values in the appropriatesections.

60 Chapter 4.

-0.2

0.0

0.2

0.4

0.6

0.8

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=4.2 pFVload=9 kV

Config. 6ab

-0.05

0.00

0.05

0.10

0.15

0.20

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=1.6 pFVload=7 kV

Config. 5ab

-0.1

0.0

0.1

0.2

0.3

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=2.5 pFVload=7 kV

Config. 6cwater

-0.1

0.0

0.1

0.2

0.3

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=2.5 pFVload=7 kV

Config. 6c1 mM phenol

-0.10

0.00

0.10

0.20

0.30

0.40

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=2.4 pFVload=7 kV

Config. 3bAr purge

-0.10

0.00

0.10

0.20

0.30

0.40

-200 0 200 400

Time (ns)

Cur

rent

(A

)

I(t)Cg⋅dV(t)/dt

Cg=2.3 pFVload=7 kV

Config. 3bair

Figure 4.14 A selection of best fits of I(t) and Cg⋅dV(t)/dt; the result is Cg. Indicatedare reactor configuration numbers.

Table 4.2 Time-averaged pulse energies and 95% confidence intervals for theperformed experiments.

reactorno.

Experimentcorona in gas phase over aqueous solution

Corona parametersV,C,f

Ep

(mJ)3a Ozone measurements in a reactor, filled with

air and 100 ml deionized water15 kV, 1 nF, 100 Hz20 kV, 1 nF, 100 Hz25 kV, 1 nF, 100 Hz30 kV, 1 nF, 100 Hz

0.6 ±0.21.4 ±0.43.5 ±0.35.9 ±1.1

6a Phenol oxidation, 500 ml 0.05 mM; air 30 kV, 100 pF, 50 Hz 13.4 ±1.46b Atrazine oxidation, 500 ml 0.12 mM; air 30 kV, 100 pF, 50 Hz 10.7 ±0.56c Phenol oxidation, 500 ml 1 mM; air 25 kV, 1 nF, 100 Hz 10.0 ±0.13b Phenol oxidation, 100 ml 1 mM; air 25 kV, 1 nF, 100 Hz 5.8 ±0.23b Phenol oxidation, 100 ml 1 mM; argon flow 25 kV, 1 nF, 100 Hz 10.6 ±0.35a Deionized water oxidation, 250 ml; air 25 kV, 1 nF, 100 Hz 4.3 ±0.25b Hydroxybenzenes oxidation, 250 ml 1 mM; air 25 kV, 1 nF, 100 Hz 5.6 ±0.3

Results 61

Although this thesis deals with the application of pulsed corona discharges in the gasphase over aqueous solutions of the target compound, also some pulse energycalculations are shown related to the application of corona in the liquid phase. Thereason for this approach is to compare pulse energies of both applications. Muchenvironmental corona research is performed as liquid phase corona [100-102].

Both a deionized water and tap water liquid phase have been regarded. Reactorconfiguration 4 has been used. The anode is made from a W/Th alloy (welding rod) andhas a 30 µm tip. Both cathode plate and anode tip are situated in the liquid phase at amutual distance ∆=2.0 cm. It has not been possible to calculate the pulse energyaccording to the procedure described by section 2.5.2, because the capacitive currentis high as a result of the high capacitance of the water-filled reactor with immersedelectrodes. Therefore the pulse energy has been estimated by two different methodsviz. a calorimetric determination and a calculation based on the average direct currentdelivered by the high voltage power supply.

At a voltage V=19 kV, a pulse repetition rate f=100 Hz and electrode distance ∆=2.0cm the pulse energy has been calculated for the reactor, filled with different amounts ofdeionized water i.e. Vol=200 ml, 250 ml, 300 ml, 400 ml. Table 4.3 shows the energyper pulse calculated by the two methods.

Table 4.3 Pulse energy measured in deionized water at different volumes. Thecorona parameters are V=19 kV, C=1 nF, f=100 Hz, ∆=2.0 cm.

Pulse energy (mJ)Volume(ml) Calorimetry Power supply200 61 ±9 90 ±8250 56 ±8 88 ±7300 55 ±8 85 ±7400 58 ±9 87 ±7

With regard to both methods the following remarks can be made. A possible error madeby the calorimetric method is due to imperfect thermal insulation. However, theobserved temperature decrease rate after stopping the experiment is only about 0.2 Kper 15 minutes, while the duration of the experiment is comparable, viz. 10-20minutes. The error involved in the pulse energy obtained from the power source averagedirect current is due to the assumption, that power dissipation only takes place in theexternal circuit by the 10 MΩ load resistor.

Calorimetric- and average direct current-based measurements on 250 ml tap waterunder equal corona conditions have yielded pulse energies of 130 ±30 mJ and 260 ±30mJ respectively. The pulse energy in tap water is much higher than the pulse energy indeionized water, because of the higher electrical conductivity of tap water. The applieddeionized water exhibits a conductivity of about 30-100 µS/cm, while the used tapwater has a conductivity of about 2580 µS/cm. The reported pulse energies fordeionized water and tap water have been determined at a voltage V=19 kV and are yetmuch higher than the pulse energies in air, determined at V=25-30 kV.

62 Chapter 4.

The formation of corona discharges in water requires evaporation of water at the anodetip, thus a high pulse energy. Figure 4.15 shows a CCD image of corona discharges atan anode tip immersed in deionized water. Stroboscopic illumination has been appliedusing a He/Ne laser chopped at 10 Hz. The discharges appear as bright irregular-shapedchannels. Vapour bubbles appear as rows of dots. The size of the vapour bubbles isapproximately several tenths of a millimeter.

Figure 4.15 CCD image of corona discharges at an anode tip (top) immersed indeionized water. Stroboscopic illumination has been applied. Thedischarges appear as bright irregular-shaped channels. The vapour bubblesappear as rows of dots. The corona settings are V=20 kV, f=0.1 Hz,d=2.6 cm. The exposure time is 1 s. The image has been taken byA.H.F.M. Baede.

The reason for the application of corona in the liquid phase is to produce the oxidizersi.e. hydroxyl radicals directly at the location where they are needed to avoid loss due torecombination. However, in section 4.1.1 it has been shown that by application ofpulsed corona discharges over water, yet the action of hydroxyl radicals in aqueoussolution can be demonstrated, by means of the hydroxyl radical specific molecularprobe CCA. The hydroxyl radicals are produced directly by dissociation of watermolecules and indirectly from ozone, water and UV photons.

Summary

Two different methods have been applied to estimate the pulse energy for application ofcorona in water viz. a calorimetric determination and a method based on the averagedirect current from the power supply. By comparison of the pulse energies of coronadischarges applied over and in water, it has been shown that the application of coronaover water is much more favourable. This is caused by the fact that the production ofcorona in water requires evaporation of water at the anode tip. The pulse energymeasured for corona in tap water is higher than the pulse energy for corona in deionizedwater, due to a higher electrical conductivity of tap water compared to deionized water.

Results 63

4.1.4. Corona treatment of deionized water

The application of pulsed corona discharges in an air gas phase over deionized waterchanges the water in several ways. Corona discharges in air produce ozone and smallamounts of nitrogen oxides. In water, the ozone is converted into hydrogen peroxidewhile the nitrogen oxides are converted into nitric acid. The corona discharges strike thewater surface and dissociate water molecules into hydroxyl radicals and hydrogenatoms. Hydroxyl radicals recombine to hydrogen peroxide. Hydrogen atoms react withdissolved oxygen to hydroperoxyl radicals, which recombine to yield hydrogen peroxideand oxygen. Recombination of hydrogen atoms produces hydrogen. Also, the metal ofthe anode tip may be sputtered due to the high electric field strength, so elementarymetal or metal oxides may be found in the water. The chemical change of corona-exposed deionized water has been investigated by application of electricalconductometry, spectrochemical ICP analysis and ion-exclusion chromatography.250 ml deionized water samples have been exposed to pulsed corona discharges, seereactor configuration 5a. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm. Three different anode tips have been tested: steel, tungsten and platinum.The following relative temperature increase (∆T/T0) values have been measured after 60minutes of corona exposure: Fe: 0.14, W: 0.15, Pt: 0.08. Figure 4.16 shows theelectrical conductivity of the deionized water as a function of the corona-exposure time.

0

100

200

300

400

500

0 20 40 60Time (minutes)

Con

duct

ivity

(µS/c

m)

Fe

W

Pt

Figure 4.16 The electrical conductivity of 250 ml deionized water samples versus theoxidation time. Used anode tip materials are steel, tungsten and platinum.The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Measured pulse energies are: EpFe=4.5 ±0.3 mJ, Ep

W=4.1 ±0.2 mJ, EpPt=4.2 ±0.3 mJ.

The conductivity of the deionized water samples increases with the corona-exposuretime. The increase is highest when corona is produced using the steel anode tip, whilethe platinum tip brings about the lowest conductivity increase. The conductivitydifferences using different anode tip materials may imply the effect of anode materialthus the sputtering of anode material into the water. However, the anode tip geometriesare far from identical, because this is very difficult to achieve. Therefore the amounts ofsputtered material may be different. Spectrochemical ICP analysis has been applied tothe deionized water samples oxidized by the steel and tungsten tips, but no metals havebeen identified. Therefore it is likely, that the conductivity increase arises from nitricacid. The mutually different anode tips may have produced different amounts ofnitrogen oxides versus the oxidation time.

64 Chapter 4.

The 30 minutes corona-exposed deionized water samples have been analyzed by ion-exclusion chromatography using a diode array UV absorbance detector and conductivitydetector in series, see LC configuration 3. Figure 4.17 shows the chromatograms ofoxidized and untreated deionized water obtained by using steel, tungsten and platinumanode tips.

The UV absorbance chromatogram shows three similar strong absorptions at abouttR=3.06 minutes. The UV absorbance intensity decreases according to the order200 nm>210 nm>220 nm>>255 nm>270 nm. This first peak represents excludedanions and is only present in corona-exposed aqueous solutions. Nitrate ions are likelyto account for this peak. Nitrite is less probable, because it is oxidized to nitrate by thepulsed corona discharges.The conductivity chromatograms reveal a very strong negative peak at about tR=3.5min and a weak conductivity signal at tR=8.7 minutes, for all solutions. The strongnegative peak represents the elution of water from the sample. This peak overrules apossible conductivity signal due to the components eluting at tR=3.06 min, consideringthe time difference of about 0.24 minutes between the signals from the UV absorbanceand conductivity detector. The components eluting at tR=8.7 min are retained by theICE column, thus their identity equals either an organic molecule or a cation. However,the fresh deionized water is, except for traces, free from any organic or inorganiccompounds. Metal ions are likely to be retarded by the ICE mechanism but have notbeen identified by spectrochemical ICP analysis.

Summary

By exposure of deionized water to pulsed corona discharges, the electrical conductivitysignificantly increases. This is likely due to nitrate ions, originating from nitrogen oxides,which are produced by corona in air. Corona-induced anode metal sputtering istheoretically possible, but metals have not been identified.

Figure 4.17 UV absorbance (λ=210 nm, left) and conductivity (right) chromatogramsof 250 ml 30 minutes exposed deionized water samples. Used anode tipmaterials are steel, tungsten and platinum. The parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

0

20

40

60

80

100

0 5 10 15 20

Retention time (min)

UV

abs

orba

nce

(mA

U)

Fe: 3.06 min W: 3.05 minPt: 3.06 min

4.20E+05

4.25E+05

4.30E+05

4.35E+05

4.40E+05

0 5 10 15 20Retention time (min)

Con

duct

ivity

(arb

.u.)

FeWPt

Fe: 8.72 minW: 8.74 minPt: 8.75 min

Results 65

4.2. Oxidation of phenol

This section gives a detailed analysis of the oxidation of the model compound phenolregarding conversion, energy efficiency, oxidation products and analysis techniques.

4.2.1. Chromatography

Reversed-phase HPLC

The initial measurements of phenol conversion have been performed using a standardreversed-phase HPLC column according to LC configuration 4. This column is suitablefor the separation of phenol from its oxidation product components e.g.polyhydroxybenzenes and carboxylic acids. Phenol conversion is calculated from the UVabsorbance detector peak area as a function of time. 500 ml 5 mg/l (0.05 mM) phenolsolutions have been prepared using tap water. The pulsed corona discharges take placein air over the phenol solution, see reactor configuration 6a. The influence of theparameters voltage (V), pulse repetition rate (f), anode-tip-to-water distance (d) andsolution acidity (pH) on the phenol conversion are shown by Figure 4.18. During theseexperiments, only one parameter is varied at a time, while the others are kept constantat the standard values V=30 kV, f=50 Hz, d=1.0 cm, pH=n.a. (not adjusted), t=30min (except V series: t=45 min). The plots show individual trends. The absolute valuescannot be compared mutually, because singular data acquisition has been applied.

Figure 4.18 The conversion of phenol as a function of the indicated parameters.500 ml 5 mg/l (0.05 mM) phenol solutions have been oxidized.

0

20

40

60

80

100

2 4 6 8 10pH (-)

Con

vers

ion

(%)

t=30 minV=30 kVf=50 Hzd=1.0 cm

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5Distance (cm)

Con

vers

ion

(%)

t=30 minV=30 kVf=50 HzpH=n.a.

0

20

40

60

80

100

15 20 25 30 35 40Voltage (kV)

Con

vers

ion

(%)

t=45 minf=50 Hzd=1.0 cmpH=n.a.

0

20

40

60

80

100

0 50 100 150 200 250Frequency (Hz)

Con

vers

ion

(%)

t=30 minV=30 kVd=1.0 cmpH=n.a.

66 Chapter 4.

The conversion of phenol appears to increase non-linear with the applied corona loadvoltage. This is likely due to the radical formation processes by the corona plasma. Byincreasing the voltage the radical production indeed grows, but the probability ofrecombination also increases. Therefore the radical production at higher voltages islikely to be less efficient.

The conversion increases with the pulse repetition rate for the range 0 Hz<f<100 Hz.The unexpected decrease of the conversion at 200 Hz might be related to coronainstability at higher repetition frequencies as a result of the application of a pressurizedtriggered spark gap.

Conversion is considerably higher in an alkaline solution than in an acidic solution. Inalkaline solution ozone reacts by hydroxyl radicals which are far more reactive thanozone. In addition, at high pH the existence of the phenolate anion (C6H5O-) is extrafavourable with regard to the electrophilic nature of hydroxyl radicals [7].

Adjusting the anode-tip-to-water distance at about d=1.0 cm yields the highestconversion. Application of pulsed corona discharges at the gas-liquid interface limits theproduction of oxidizers e.g. ozone, therefore a certain gas phase volume i.e. distance isfavourable. This effect has also been observed by experiments on the influence ofelectrode configurations on the decolorization of malachite green dye, which will bedescribed in section 4.3.2. The application of pulsed corona discharges in an aqueoussolution is energetically unfavourable, because this causes evaporation of water at theanode tip [103].

In addition to this experiment, the influence of the solution volume on the conversion ofphenol has been determined in threefold, according to the parameters V=30 kV,C=100 pF, f=50 Hz, d=1.0 cm, t=30 min. The conversion of the 500 ml 5 mg/lsolution is 60.7% ±7.0% while the conversion of the 250 ml 10 mg/l solution is73.5% ±4.0%. The oxidizers are more efficiently consumed for the case of the 250 mlsolution, compared to the 500 ml solution. Application of pulsed corona discharges tothin liquid films will be favourable to conversion.

In the subsequent experiments, pulsed corona discharges have been applied in the gasphase over the aqueous solution of the target compound [104], considering an anode-tip-to-water distance d=1.0 cm. Although alkaline conditions favour the conversion ofphenol, no such pH adjustments have been made, because the applicability of thepulsed corona technology should be qualified in an intrinsic way.The oxidation of a 5 mg/l (0.05 mM) phenol solution in tap water has been performed inthreefold. The corona parameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm. Theconversion and 95% confidence interval are plotted as a function of time, see Figure4.19.

After an oxidation time of 1 hour, a conversion X=92% ±7.9% has been reached. Therelationship between ln(C/C0) and the oxidation time t is linear, implying first orderreaction kinetics with a rate constant k1≈4.1⋅10-2 min-1. Characteristic pulse voltage,current and power are shown by Figure 4.20. The efficiency of phenol conversion,expressed by the G yield value is shown by Table 4.4.

Results 67

0

20

40

60

80

100

0 20 40 60 80Time (min)

Con

vers

ion

(%)

Figure 4.19 The conversion of a 500 ml 5 mg/l (0.05 mM) phenol solution (tap water-based). The corona parameters are V=30 kV, C=100 pF, f=50 Hz,d=1.0 cm.

T

Tbcl

F

-1.0E+04

0.0E+00

1.0E+04

2.0E+04

3.0E+04

-100 0 100 200 300 400

Time (ns)

Vol

tage

(V

)

-10

0

10

20

30

Cur

rent

(A

)

V(t)

I(t), Icor(t)

-2.0E+05

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

-100 0 100 200 300 400

Time (ns)

Pow

er (

W)

igure 4.20 Typical pulse voltage, current and power waveforms for corona in air,recorded after 1 hour of oxidation. The corona parameters are V=30 kV,C=100 pF, f=50 Hz, d=1.0 cm.

able 4.4 Conversion (X), pulse energy (Ep) and efficiency (G) during oxidation of a500 ml 5 mg/l (0.05 mM) phenol solution (tap water-based); averagevalues and 95% confidence intervals are presented. The coronaparameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm.

t (min) X (%) Ep (mJ) G (mol/J)⋅108 G (100eV)-1 G (g/kWh)15 24.5 ±18.3 13.7 ±4.2 1.08 ±1.14 0.10 ±0.11 3.7 ±3.930 60.7 ±7.0 13.1 ±5.1 1.39 ±0.54 0.13 ±0.05 4.7 ±1.845 85.8 ±35.6 14.3 ±25.4 1.20 ±2.63 0.12 ±0.25 4.1 ±8.960 92.3 ±7.9 12.4 ±7.4 1.10 ±2.08 0.11 ±0.20 2.5 ±5.5

he waveforms show rather much parasitic high frequency oscillations. These haveeen largely suppressed in the subsequent experiments. The 95% confidence levels ofonversion and efficiency are rather large. The reason for the poor reproducibility isikely due to corona instability during these introductory experiments.

68 Chapter 4.

The first quantitative measurements have been applied to phenol, hydroquinone, 1.4-benzoquinone and resorcinol. 500 ml 25 mg/l (0.27 mM) phenol solutions (tap water-based) have been exposed to pulsed corona discharges in air for 5 hours. The coronaparameters are V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm. The concentrationdetermination has been performed in threefold, every 30 minutes.

The first analysis showed the production of hydroquinone and resorcinol. The duplicateand triplicate measurements only showed hydroquinone and the conversion rate ofphenol and hydroquinone was somewhat lower than for the case of the firstmeasurement, see Figure 4.21. It has not been possible to distinguish hydroquinonefrom 1.4-benzoquinone, because these compounds show co-elution using the reversed-phase HPLC column according to LC configuration 4. Hydroquinone is initially producedand may be oxidized to 1.4-benzoquinone. The carboxylic acids formic acid and aceticacid have been qualitatively identified by Capillary Zone Electrophoresis.

0.0E+00

1.0E-04

2.0E-04

3.0E-04

0 60 120 180 240 300

Time (min)

Con

cent

ration

(m

ol/l) phenol

hydroquinone

resorcinol

Figure 4.21 The oxidation of a 500 ml 25 mg/l (0.27 mM) phenol solution (tap water-based). The corona parameters are V=30 kV, C=100 pF, f=50 Hz,d=1.0 cm.

Results 69

Ion-exclusion chromatography

The applicability of a reversed-phase HPLC column for separation of the phenoloxidation product mixture has appeared to be limited. Although polyhydroxybenzenescan be properly separated, the carboxylic acid-functional ring-cleavage products cannotbe retained. In order to separate both of these mutually different product groups, anion-exclusion column has been applied.

The conversion of phenol has been measured simultaneously using two different LCconfigurations with a reversed-phase HPLC column (5a) and an ion-exclusion column(5b). 500 ml 1.0 mM (94 mg/l) phenol solutions have been oxidized by pulsed coronadischarges in air for 3 hours, see reactor configuration 6c. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion determination has beenperformed in threefold, every 15 minutes during 3 hours. The samples have beendirectly analyzed after sampling. At startup and at the end of the experiment thesolution acidity, conductivity and temperature have been recorded. Figure 4.22 showsthe observed conversion as a function of the oxidation time measured by both columns.Characteristic pulse voltage and current waveforms are shown by Figure 4.23. Theefficiency, conversion and pulse energy are listed by Table 4.5.There appear to be no significant differences between the conversion measurementsusing a reversed-phase HPLC column or an ion-exclusion column. Therefore the morepowerful ICE column has been used for subsequent separations. The conversion timerelationship seems to obey first order kinetics with a rate constant k1≈2.9⋅10-3 min-1.After three hours of oxidation, the conversion is about 39%. The efficiency is in therange of 1.9⋅10-8-2.7⋅10-8 mol/J and slowly decreases as a function of the oxidationtime, because during oxidation progress less phenol molecules are available.

0

10

20

30

40

50

0 60 120 180

Time (min)

Con

vers

ion

(%)

rp-HPLC

ICE

Figure 4.22 Phenol conversion measured simultaneously by rp-HPLC and ICEchromatography. Oxidation of a 500 ml 1.0 mM (94 mg/l) phenolsolution. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm.

70 Chapter 4.

Table 4.5 Phenol conversion (X), pulse energy (Ep) and efficiency (G) during theoxidation of a 500 ml 1.0 mM (94 mg/l) phenol solution; average valuesand 95% confidence intervals are presented. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion has beendetermined by ICE chromatography.

t (min) X (%) Ep (mJ) G (mol/J)⋅108 G (100eV)-1 G (g/kWh)0 0.0 ±0.0 - - - -

15 4.6 ±1.1 - - - -30 9.5 ±4.4 9.9 ±0.5 2.67 ±1.10 0.26 ±0.11 9.06 ±3.7445 13.3 ±0.5 - - - -60 17.5 ±3.3 10.2 ±0.1 2.41 ±0.44 0.23 ±0.04 8.15 ±1.5075 20.7 ±2.3 - - - -90 24.1 ±2.8 10.1 ±0.8 2.21 ±0.20 0.21 ±0.02 7.50 ±0.67

120 30.0 ±3.2 10.0 ±0.3 2.10 ±0.22 0.20 ±0.02 7.10 ±0.75150 34.3 ±1.9 10.0 ±0.7 1.92 ±0.07 0.19 ±0.01 6.50 ±0.24180 39.3 ±1.9 9.9 ±0.3 1.85 ±0.04 0.18 ±0.01 6.27 ±0.13

The change in solution acidity, conductivity and temperature are respectively∆pH=-3.1 ±0.1, ∆σ=+1773 ±106 µS/cm and ∆T/T0=+0.1 ±0.1. The production ofcarboxylic acids is evidently shown by both the drastic pH decrease and conductivityincrease. The energy dissipation, illustrated by the small solution temperature increaseis very favourable, compared to the necessarily forced cooling that is applied for pulsedcorona discharges in water.

An approach has been made to identify a number of important oxidation products ofphenol, by comparison of the retention times of unknown components in the ion-exclusion chromatogram by the retention times of pure possible candidate oxidationproducts. The following components have been verified: the polyhydroxybenzenes:catechol, resorcinol, hydroquinone, pyrogallol and hydroxyhydroquinone; the quinone:1,4-benzoquinone; the carboxylic acids: succinic acid, maleic acid, malonic acid,propionic acid, oxalic acid, acrylic acid, acetic acid and formic acid. Figure 4.24 showsa representative ICE chromatogram, recorded after 3 hours of oxidation time.

Figure 4.23 Typical pulse voltage, current and power waveforms for corona in air,recorded after 1 hour of oxidation. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

0.0E+00

1.0E+05

2.0E+05

3.0E+05

-100 0 100 200 300 400Time (ns)

Pow

er (

W)

0.0E+00

1.0E+04

2.0E+04

3.0E+04

-100 0 100 200 300 400Time (ns)

Vol

tage

(V

)

0

10

20

30

Cur

rent

(A

)

V(t)

I(t)Icor(t)

Results 71

0 10 20 30Retention time (min)

Abs

orba

nce

(arb

.u.)

9.3

38.5

07.1

46.8

85.9

45.4

74.8

14.1

93.4

33.2

4

10.2

39.7

2

14.9

915.6

7

20.4

821.2

7

31.8

6

Table 4.6 shooxidation prod3 hours of oxi

Table 4.6 Rsap

Oxidation prpeak no

123456789

1011121314151617

Figure 4.24

A representative ion-exclusion chromatogram of a 500 ml 1.0 mM(94 mg/l) phenol solution, oxidized for three hours. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The UVabsorbance detector is set at 210 nm.

ws a survey of retention times. The retention times observed from theuct mixture are average values, recorded in threefold after 2 h, 2.5 h anddation time. Also shown are pure candidate oxidation products.

etention times observed in the chromatograms of oxidized phenololutions and retention times of pure candidate oxidation products;verage values and 95% confidence intervals are presented. Unidentifiedeaks are labelled with a question mark.

oduct mixture Pure compoundtR (min) tR (min) name3.24 ±0.00 nitrate, hydrogen peroxide3.45 ±0.01 3.61 (as peak 1), oxalic acid4.20 ±0.00 ?4.81 ±0.00 4.75 maleic acid5.47 ±0.01 ?5.94 ±0.00 5.99 malonic acid6.88 ±0.01 ?7.15 ±0.01 7.21 succinic acid8.49 ±0.01 8.04, 8.69 formic acid, acetic acid9.34 ±0.01 ?9.72 ±0.01 ?

10.23 ±0.01 10.25 propionic acid14.94 ±0.03 14.36 pyrogallol15.72 ±0.03 16.22 hydroxyhydroquinone20.53 ±0.04 20.49 hydroquinone / 1,4-benzoquinone21.31 ±0.04 21.26 catechol / resorcinol31.88 ±0.03 31.86 phenol

72 Chapter 4.

The first peak in the ion-exclusion chromatogram represents a component, which leavesthe column without retention. The peak may also represent different co-elutingunretarded components. It may be assumed that this peak represents negative ions,because the ion-exclusion column excludes these ions, see section 4.1.4. Anasymmetric second peak directly follows the first peak.The candidate anion is nitrate (NO3

-) that is formed in aqueous solution by nitrogenoxides (NOx), produced by corona discharges in air. It may also be possible, that thispeak is caused by hydrogen peroxide or dissolved ozone. An experiment has beenperformed, in which an oxidized phenol solution has been purged by helium. The peakarea of the first peak remains unchanged by the helium purge; thus ozone cannotexplain this peak, because it has been removed by the purge. However, by continuationof the corona discharges in helium, the peak area of the first peak increases. Hydrogenperoxide might thus also explain the existence of this peak, because it is produced fromhydroxyl radicals, which are formed by the dissociation of water molecules. The ion-exclusion chromatogram of a diluted hydrogen peroxide solution reveals the sametypical set of two adjacent peaks, as observed in the chromatogram of oxidized phenolsolutions.The asymmetric second peak may also represent oxalic acid, the strongest carboxylicacid present in the phenol oxidation product mixture: its first dissociation constant ispKa,I=1.23 [80].

It has been observed that the elution regions of the carboxylic acids and the differentpolyhydroxybenzenes are distinct. With regard to the used setup and conditions, thecarboxylic acids elute between 3-10 minutes, the trihydroxybenzenes between 14-17minutes, the dihydroxybenzenes between 20-22 minutes and monohydroxybenzenephenol at about 32 minutes.

However, the identity of every single peak cannot be completely guaranteed withoutthe definite proof of mass spectrometry. As yet, the production and conversion of theoxidation products have been reported by plotting the total peak area of the mentionedproduct classes as a function of the oxidation time, see Figures 4.25 and 4.26.

Figure 4.25 The production of dihydroxybenzenes (left) and trihydroxybenzenes (right)as a function of time during the oxidation of a 500 ml 1.0 mM (94 mg/l)phenol solution. The corona parameters are V=25 kV, C=1 nF, f=100Hz, d=1.0 cm.

0.0E+00

1.0E+06

2.0E+06

3.0E+06

0 30 60 90 120 150 180

Time (min)

UV

abs

orba

nce

(cou

nts·

s)

0.0E+00

5.0E+05

1.0E+06

1.5E+06

0 30 60 90 120 150 180

Time (min)

UV

abs

orba

nce

(cou

nts·

s)

Results 73

The dihydroxybenzenes catechol, resorcinol and hydroquinone seem to reach amaximum concentration after about 75 minutes. They may be transformed intotrihydroxybenzenes, quinones or may undergo ring-cleavage by oxygen or ozone attack.The trihydroxybenzenes pyrogallol/hydroxyhydroquinone will definitely undergo ring-cleavage, because they are stronger reducing agents than the dihydroxybenzenes. Thetrihydroxybenzene phloroglucinol (1,3,5-THB) is an exception, because it also reacts ina tautomeric keto-form, see section 2.5.3 Figure 2.21. By ring-cleavage of thehydroxybenzenes, a large variety of carboxylic acids is produced.

From corona experiments in helium, also phenol conversion has been reported. Acomparison of conversion values by corona in helium and air is reported in threefoldafter 30 minutes and 60 minutes of oxidation time, see Table 4.7.

Table 4.7 Phenol conversion (X) by corona in helium and air. Oxidation of a 500 ml1.0 mM (94 mg/l) phenol solution; presented are average values and 95%confidence intervals. The parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm.

Time (min) X (%) helium X (%) air0 0 0

30 9.6 ±6.4 9.0 ±2.160 13.4 ±3.4 17.5 ±2.5

The conversion by corona discharges in helium cannot be caused by ozone, becauseoxygen has been removed from the reactor by the helium purge. Therefore it is likely,that hydroxyl radicals are produced by helium ions/metastables bombardment of watermolecules.

Figure 4.26 The production of carboxylic acids as a function of the time during theoxidation of a 500 ml 1.0 mM (94 mg/l) phenol solution. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

0.0E+00

5.0E+06

1.0E+07

1.5E+07

0 30 60 90 120 150 180

Time (min)

UV

abs

orba

nce

(cou

nts·

s)

74 Chapter 4.

Quantitative ion-exclusion chromatography

Six major phenol oxidation products have been selected for quantitative analysis i.e.hydroquinone, hydroxyhydroquinone, formic acid, oxalic acid, glyoxylic acid andglyoxal. The analysis has been performed using the ion-exclusion column and a seriesconnection of a UV absorbance and conductivity detector, according to LCconfiguration 6. The UV absorbance detector has been applied to detect thehydroxybenzenes, while the conductivity detector has been utilized for detection of thecarboxylic acids.Pulsed corona discharges have been applied in both an air and argon gas phase over100 ml 1.0⋅10-3 M phenol solutions, see reactor configuration 3b. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. By application of the coronain both air and argon, important aspects of the degradation mechanism can be resolved.The question is, whether either the hydroxyl radical or ozone accounts for degradation,or both oxidizer species contribute together to the chemical conversion of phenol inaqueous solution. The concentration of phenol and the selected oxidation products hasbeen measured as a function of the oxidation time [105].

The calibration lines of the standard solutions are shown by Figures 4.27 and 4.28. Ithas been observed that the calibration lines of phenol, hydroquinone and formic acidhave high correlation coefficients (r2). The standard series of oxalic acid and glyoxylicacid show moderate correlation. The correlation of the hydroxyhydroquinone standardsis poor and the glyoxal data show no correlation.

Figure 4.27 UV absorbance detector calibration lines of phenol, hydroquinone,hydroxyhydroquinone and glyoxal. Indicated are UV absorbance detectorwavelengths.

0

1000

2000

3000

4000

5000

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[Hydroxyhydroquinone] (mol/l)

Abs

orba

nce

(mA

Us) 200 nm

270 nm

y = 1E+07x + 58.69R2 = 0.9996

y = 2E+06x + 15.101R2 = 0.9994

0

4000

8000

12000

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[phenol] (mol/l)

Abs

orba

nce

(mA

Us) 200 nm

270 nm

y = 2E+07x + 244.39R2 = 0.9978

y = 1E+06x + 11.995R2 = 0.9984

0

5000

10000

15000

20000

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[Hydroquinone] (mol/l)

Abs

orba

nce

(mA

Us) 200 nm

270 nm

0

5

10

15

20

25

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[Glyoxal] (mol/l)

Abs

orba

nce

(mA

Us)

200 nm210 nm

Results 75

The correlation quality of the standard is dependent on the chemical stability of thestandard compound. Hydroxyhydroquinone is susceptible to oxidation, even by oxygenpresent in water. Glyoxal hydrolyzes and consequently exists in several oligomerizedforms in aqueous solution [106]. Due to the large number of standards and samples incombination with long oxidation and analysis times, ageing of the samples isunavoidable. Nevertheless, ageing has been minimized by storage of both the standardsand samples before analysis in dark vials under nitrogen at 0°C.

Characteristic UV absorbance and conductivity chromatograms, obtained from phenolsolutions oxidized by corona in air and argon during 2 hours, are shown by Figures 4.29and 4.30. It has been observed, that the oxidized solutions have different colors: thesolution oxidized by corona in air is pale yellow, while the solution oxidized by corona inargon is pale beige.

The observed retention times of the standards are shown by Table 4.8. Both thehydroxybenzenes and glyoxal do not exist in ionic form in aqueous solution, thereforethese compounds are not detectable by the conductivity detector. The time lagbetween the signals of conductivity and UV absorbance detector is caused by theintermediate suppressor.

Figure 4.28 Conductivity detector calibration lines of formic acid, oxalic acid andglyoxylic acid.

y = 8E+08x - 91872R2 = 0.8966

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[Oxalic acid] (mol/l)C

ondu

ctiv

ity

(arb

.u.)

y = 5E+08x - 13546R2 = 0.9625

0.0E+00

2.0E+05

4.0E+05

6.0E+05

0.0E+00 4.0E-04 8.0E-04 1.2E-03

Glyoxylic acid (mol/l)

Con

duct

ivity

(arb

.u.)

y = 6E+08x - 3180.4R2 = 0.9983

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

0.0E+00 4.0E-04 8.0E-04 1.2E-03

[Formic acid] (mol/l)

Con

duct

ivity

(arb

.u.)

76 Chapter 4.

Figure 4.30 Conductivity chromatograms of 100 ml 1.0 mM phenol solutions, oxidizedby corona in air and argon during 2 hours. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Table 4.8 Absorbance (UV) and conductivity (CON) detector average retention timesof the standard compounds. Also shown are 95% confidence intervals.

UV tR (min) CON tR (min) Standard compound 3.18 ±0.09 3.44 ±0.06 Oxalic acid 4.84 ±0.01 5.07 ±0.02 Glyoxylic acid 7.26 ±0.01 7.49 ±0.01 Formic acid10.03 ±0.01 - Glyoxal17.25 ±0.01 - Hydroxyhydroquinone20.83 ±0.03 - Hydroquinone32.30 ±0.07 - Phenol

Figure 4.29 UV absorbance chromatograms of 100 ml 1.0 mM phenol solutions,oxidized by corona in air (left) and argon (right) during 2 hours. Thecorona parameters are: V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.Indicated are UV absorption wavelengths.

Results 77

The conversion of phenol in aqueous solution by pulsed corona discharges in both an airand argon atmosphere is shown by Figure 4.31. The conversion by corona in argonappears to be higher than the conversion in air. After 2 hours, the corona in air hasconverted 59% of the initial present phenol amount, while the corona in argon hasconverted 88%. The conversion seems to obey first order kinetics: the rate contantsare k1

air ≈8.2⋅10-3 min-1 and k1

argon ≈1.8⋅10-2 min-1.

0

20

40

60

80

100

0 30 60 90 120

Time (min)

Con

vers

ion

(%)

200 nm air270 nm air200 nm Ar270 nm Ar

Figure 4.31 Phenol conversion during oxidation of 100 ml 1.0 mM phenol solutions bycorona in air and argon. The corona parameters are V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm. Indicated are UV absorbance wavelengths.

The DHB hydroquinone elutes just before its isomers resorcinol (1,3-DHB) and catechol(1,2-DHB), which cannot be separated by the ion-exclusion column, see Figure 4.29.An estimation of the order of magnitude of the total DHB concentration has beenperformed by relating the total DHB peak area to the calibration line of hydroquinone,assuming similar extinction coefficients for the three DHB isomers. Figures 4.32 and4.33 show the hydroquinone concentration and the total DHB concentration versus theoxidation time, respectively. During the oxidation the maximum hydroquinoneconcentration is 8.4⋅10-6 M in air and 3.1⋅10-5 M in argon, after about 45 minutes.

0.0E+00

1.0E-05

2.0E-05

3.0E-05

4.0E-05

5.0E-05

0 30 60 90 120

Time (min)

[Hyd

roqu

inon

e] (m

ol/l) 270 nm air

270 nm argon

Figure 4.32 The production of hydroquinone during the oxidation of 100 ml 1.0 mMphenol solutions by corona in air and argon. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

78 Chapter 4.

The total DHB concentration reaches a maximum value of 1.3⋅10-4 M in air and3.4 ⋅10-4 M in argon after about 60 minutes. It is remarkable, that the DHB amountsproduced by corona in argon are considerably higher than the DHB amounts producedby corona in air.

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

0 30 60 90 120

Time (min)

[DH

B]

(mol

/l)

270 nm air

270 nm argon

Figure 4.33 An estimation of the production of total dihydroxybenzenes during theoxidation of 100 ml 1.0 mM phenol solutions by corona in air and argon.The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

The THB hydroxyhydroquinone has not been identified in the phenol oxidation productmixture. A possible explanation for the absence of this compound is the limited stabilityof this strong reducing agent. Nevertheless, it has been observed by ICE analysisimmediately after corona treatment in air, see Figure 4.24. With regard to the ring-cleavage products of phenol, the following observations have been made. Theproduction of formic acid by corona in air is much higher than the production by coronain argon. After 2 hours of corona discharges, a concentration of 2.3⋅10-4 M is reachedby corona in air and a concentration of 8.9⋅10-5 M is reached by corona in argon. Forthe observed time span, the production rate increases almost linearly with time, seeFigure 4.34.

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

0 30 60 90 120

Time (min)

[For

mic

aci

d] (

mol

/l) CON airCON argon

Figure 4.34 The production of formic acid during the oxidation of 100 ml 1.0 mMphenol solutions by corona in air and argon. The corona parameters are:V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Results 79

The estimation of the oxalic acid concentration involves an error margin, because oxalicacid appears to co-elute with a component leaving the column unretarded, as can beobserved from Figures 4.29 and 4.30. This component is likely to be the nitrate ion,according to the discussion in section 4.1.4. Oxalic acid elutes rapidly, because it is thestrongest organic acid observed in the phenol oxidation product mixture. Oxalic acid hasonly been detected during phenol oxidation by corona in air: after two hours ofoxidation a concentration of about 3.9⋅10-4 M has been reached. Figure 4.35 shows theproduction of oxalic acid versus the oxidation time for corona discharges in air.

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

0 30 60 90 120

Time (min)

[Oxa

lic a

cid]

(m

ol/l) CON air

Figure 4.35 The production of oxalic acid during the oxidation of a 100 ml 1.0 mMphenol solution by corona in air. Oxalic acid has not been detected duringoxidation by corona in argon. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

The dialdehyde glyoxal has not been found in traceable amounts. Glyoxylic acid, anoxidation product of glyoxal, appears in small amounts in both the phenol solutionoxidized by corona in air viz. 4⋅10-5 M to 5⋅10-5 M and the phenol solution oxidized bycorona in argon viz. 3⋅10-5 M, see Figure 4.36.

0.0E+00

2.0E-05

4.0E-05

6.0E-05

0 30 60 90 120

Time (min)

[Gly

oxyl

ic a

cid]

(m

ol/l)

CON airCON argon

Figure 4.36 The production of glyoxylic acid during the oxidation of 100 ml 1.0 mMphenol solutions by corona in air and argon. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

80 Chapter 4.

A very strong absorption signal has been found in the UV absorbance chromatogramsof the phenol solution oxidized by corona in air, at the retention time tR=13.73 ±0.01min and only pronounced at the wavelengths λ=255 nm and λ=270 nm. There is acorresponding conductivity signal at the retention time tR=13.97 ±0.03 min. Thesignal appears to be almost absent in the UV absorbance and conductionchromatograms of the phenol solution oxidized by corona in argon. Figure 4.37 showsthe observed peak areas of both detectors versus the oxidation time.

0

400

800

1200

0 30 60 90 120Time (min)

Abs

orba

nce

(mA

U)

0.0E+00

2.0E+04

4.0E+04

6.0E+04

Con

duct

ivity

(arb

. u)255 nm

270 nmCON

Figure 4.37 The production of an unknown compound during the oxidation of a100 ml 1.0 mM phenol solution by corona in air. The corona parametersare V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

The identity of this component might be revealed in the following way. The componentappears at the end of the carboxylic acid retention time range (3<tR<15 min) of theUV absorption chromatogram. It exists in partially dissociated form in aqueous solution,because it is detectable by the conductivity detector. Its strong UV absorption atwavelengths λ=255 nm and λ=270 nm indicates the presence of unsaturated carbon-carbon bonds. The compound is formed under ring-cleavage conditions.It is known from literature that the oxidation of phenol, benzene or catechol yields thering-cleavage product cis,cis-muconic acid (cis,cis-1,3-butadiene-1,4-dicarboxylic acid)[11,107,108]. According to the observations, the unknown compound possibly iscis,cis-muconic acid. Cis,cis-muconic acid has not been introduced in the standardseries, because of its presumed but ambiguous limited stability [7].

Typical pulse voltage, current and power waveforms for corona in air and argon areshown by Figure 4.38. The corona current in argon is higher than the current in air. Thisis explained by the electronegative character of oxygen, that tends to inhibit the coronadischarges in air compared to the discharges in argon.

Results 81

From the pulse energy and conversion, the efficiency of phenol oxidation by corona inair and argon has been calculated, see Table 4.9 and Figure 4.39.

Table 4.9 Efficiency (G), pulse energy (Ep) and phenol conversion at λ=270 nm (X270)during oxidation of 100 ml 1.0 mM phenol solutions by corona in air and bycorona in argon. The corona parameters are V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm.

corona in air corona in argonTime(min)

Ep

(mJ)X270

(%)G ⋅108

(mol/J)G

p100eVG

(g/kWh)Ep

(mJ)X270

(%)G ⋅108

(mol/J)G

p100eVG

(g/kWh)15 6.3 14.4 2.60 0.25 8.8 10.2 18.8 2.09 0.20 7.130 6.2 26.1 2.35 0.23 8.0 10.3 43.1 2.35 0.23 8.045 5.8 35.0 2.23 0.21 7.5 10.8 50.2 1.72 0.17 5.860 5.7 42.1 2.01 0.19 6.8 10.4 65.5 1.72 0.17 5.875 5.8 48.0 1.79 0.17 6.1 10.7 75.5 1.53 0.15 5.290 5.4 52.6 1.74 0.17 5.9 10.8 82.2 1.36 0.13 4.6

105 5.7 56.2 1.48 0.14 5.0 10.9 85.8 1.20 0.12 4.1120 5.6 59.2 1.38 0.13 4.7 11.0 88.5 1.05 0.10 3.6

Figure 4.38 Typical pulse voltage, current and power waveforms for corona in air (left)and argon (right), recorded after 1 hour of oxidation. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

0.0E+00

1.0E+04

2.0E+04

3.0E+04

-100 0 100 200 300 400

Time (ns)

Vol

tage

(V

)

0

10

20

30

Cur

rent

(A

)V(t)

I(t)Icor(t)

corona in air

0.0E+00

2.0E+05

4.0E+05

6.0E+05

-100 0 100 200 300 400

Time (ns)

Pow

er (W

)

corona in air

0.0E+00

1.0E+04

2.0E+04

3.0E+04

-100 0 100 200 300 400

Time (ns)

Vol

tage

(V

)

0

10

20

30

Cur

rent

(A

)V(t)

I(t)Icor(t)

corona in argon

0.0E+00

2.0E+05

4.0E+05

6.0E+05

-100 0 100 200 300 400

Time (ns)

Pow

er (W

)

corona in argon

82 Chapter 4.

Only minor differences in efficiency appear to exist for phenol conversion by corona inair and argon. With increasing conversion, less phenol molecules are available foroxidation and a competition with intermediate oxidation products exists, thus theefficiency decreases.

0.0E+00

1.0E-08

2.0E-08

3.0E-08

0 20 40 60 80 100Conversion X270 (%)

G (m

ol/J

)

airargon

Figure 4.39 Efficiency of phenol conversion during oxidation of 100 ml 1.0 mM phenolsolutions by corona in air and argon. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. The conversion has beenmeasured by a UV absorbance detector set at λ= 270 nm.

Nevertheless, the difference between the phenol oxidation pathways for corona in airand argon is distinct. This is explained by simplified degradation pathways of phenol byhydroxyl radicals, oxygen, ozone and argon ion bombardment. A detailed phenoloxidation mechanism is presented in section 5.3.The attack of the hydroxyl radical on phenol will initially produce para- and ortho-dihydroxycyclohexadienyl (DHCHD) radicals, see Figure 4.40. The meta-DHCHD radicalis not considered here, because it is likely to be less stable than the others.

OHOH

phenol para-DHCHD ortho-DHCHD radical radical

OH H

OH

•

OHH

OH

•

Figure 4.40 The formation of dihydroxycyclohexadienyl (DHCHD) radicals from theattack of the hydroxyl radical on phenol.

In an air atmosphere, oxygen will attack these radicals to produce dihydroxy-cyclohexadienylperoxyl (DHCHDP) radicals, see Figure 4.41. The ortho-DHCHDP radicalmay produce catechol by elimination of the hydroperoxyl radical (HO2). The para-DHCHDP radical probably yields hydroquinone by dimerization to a tetraoxide followedby decomposition; hydroperoxyl elimination is not likely here, because of the larger -Hto •OO- distance of the para-DHCHDP radical compared to the ortho-DHCHDP radical.By action of oxygen, the DHCHDP radicals are eventually converted to endoperoxides.These very instable intermediates decompose by ring-cleavage.

Results 83

Direct ozone attack on phenol will also invoke ring-cleavage by addition, see Figure4.42. In this way, a complex mixture of aliphatic unsaturated and saturated C1-C6

hydrocarbons will be produced, having polyfunctional groups like carboxyl, aldehyde,ketone or hydroxyl groups. Glyoxal, glyoxylic acid and oxalic acid may result frommultifold attack of ozone on phenol. Formic acid may be produced by carbon monoxideloss from glyoxylic acid or by decarboxylation of oxalic acid.

O2

O2

O2 O2

- HO2

endoperoxides

C1-C6 ring cleavage products

COOH, CHO, CO, OH

DHCHD

catechol

DHCHDP

OH

OH

OHOH

hydroquinone

T

OHH

OH

•

OH H

OH

•

HOH

OH OO•

HOH

OH OO•

OH

OO

OH

OO•

Figure 4.41 A simplified mechanism of the oxidation of DHCHD and DHCHDP radicalsto endoperoxides, followed by aromatic ring-cleavage.

O

OHO

O

-H2OOH

OO O

phenol

molozonides ozonides

zwitterions hydroperoxy alkanols

unsaturated polyfunctional

aliphatic hydrocarbons

O3H2O

OH

OH

OO

O

OH

OO

O

O3

CO

HOC

+O

H

HO

O

O

OHH

HOHO

O

OOHOHHH

Figure 4.42 A simplified mechanism of phenol oxidation by ozone.

84 Chapter 4.

On the contrary, when pulsed corona discharges take place in an argon atmosphereover an oxygen-free phenol solution, the argon ions/metastables created by the coronadischarges will dissociate water molecules to produce hydroxyl radicals (and hydrogenatoms) but no ozone can be formed. Hydroxylation of phenol will be the maindegradation pathway and hydroxybenzenes will be found in much higher amounts thanfor the case of corona oxidation in air, see Figure 4.43.

OH H

OH

OH OH

OH H

OH

OH H

O

OHH

OHOH

OHH

OH OH

OH

HO

OH

OH

OHOH

-H2O

-H2O

isomerizationDHCHD

hydroquinone

catechol

•

•

Figure 4.43 A simplified mechanism of phenol oxidation by hydroxyl radicals underoxygen-free conditions.

The fact, that still some ring-cleavage products are found during corona oxidation inargon, can be explained by ring fragmentation by the argon ions/metastablesbombardment. Ring-cleavage also takes place by small amounts of oxygen, that cannotbe removed by argon purging. The differences in amounts of polyhydroxybenzenesbring about the color difference between the oxidized solutions.

As can be derived from both the chromatograms and a carbon mass balance, a certainnumber of unknown products remains, whose identity is difficult to resolve. They arelikely to be carboxylic acids, because the conductivity detector is able to observe thesepartially ionic compounds. Candidates are C1-C6 mono- and dicarboxylic acids withalkanol-, aldehyde- or ketone-functional groups.

Finally, by comparison of the conductivity chromatograms of oxidized phenol solutionsand deionized water exposed to corona in air (section 4.1.4), the following observationhas been made: the conductivity signal at tR=8.73 min from deionized water exposedto corona in air is present in the chromatogram of the phenol solution oxidized bycorona in argon, but is absent in the chromatogram of the air-oxidized phenol solution.This might be explained by anode metal sputtering differences between corona in argonand air. Also it should be considered, that the regarding peak may be due to anoxidation product with equal retention time.

Results 85

Gas chromatography

By oxidation of phenol in aqueous solution, the following oxidation products may beintroduced in the gas phase over the oxidized solution: carbon oxides, unsaturatedhydrocarbons and volatile aldehydes. A preliminary direct gas chromatography analysisof a helium flow reactor purge did not reveal any information. Therefore a specificprocedure has been chosen according to NIOSH for screening the presence ofaldehydes. The identification of other candidate oxidation products has been performedby infrared spectroscopy and is discussed in section 4.2.3.The aldehyde screening test involves a chemical derivatization reaction of aldehydes.Gas sampling tubes containing the derivatization agent HMP are exposed to an argon5.0 purge, originating from the reactor.

A 500 ml 1.0 mM phenol solution has been oxidized for 3 hours, see reactorconfiguration 7. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.Before oxidation, the reactor and phenol solution are purged with argon 5.0 and thepurge gas is directed through a gas sampling tube; this is the background sample. Dueto this purge the corona discharges take place in argon 5.0. After the oxidation thereactor is purged again and the collected purge gas is directed through a different freshtube; this is the corona sample. The background and corona sample tube contents areextracted with toluene and the extract is analyzed by gas chromatography-massspectrometry, see GC configuration 1. The obtained chromatograms are shown byFigure 4.44.

0.0E+00

1.0E+06

2.0E+06

3.0E+06

4.0E+06

130 150 170 190 210 230

Retention time (s)

FID

sig

nal (

arb.

u. )

backgroundsample

coronasample

Figure 4.44 GC chromatograms of toluene-extracted sampling tube contents. Thetubes have been exposed to an argon 5.0 purge originating from thereactor, before oxidation (background sample) and after oxidation (coronasample). The reactor contains 500 ml 1.0 mM phenol solution. Thecorona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

86 Chapter 4.

The chromatograms reveal non-derivatized HMP present in both samples, elutingbetween about 170 and 210 seconds. HMP-aldehyde derivatization products shouldoccur just before and after the elution of non-derivatized HMP. Although many smallpeaks are visible, there appear to be no differences between the corona sample and thebackground sample. All peaks below 145 seconds definitely originate from theextraction solvent, viz. toluene together with xylene impurities.

It is concluded that no volatile aldehydes have been detected during oxidation of aphenol solution by corona in argon, within the lower detection limit of this procedure.This limit is not specified, because this test is an overall screening technique. A specificacetaldehyde test (method 2538) using the same derivatization technique but differentparameters has a lower detection limit of 0.74 ppm ≡ 1.3 mg/m3.

Summary

The conversion of phenol in aqueous solution by pulsed corona discharges increases byincreasing the corona load voltage, corona pulse repetition rate and solution alkalinity.The location of the discharges is best at some distance from the liquid-gas interface.Ion-exclusion chromatography has proven to be considerably more powerful forseparation of the complex phenol oxidation product mixture than reversed-phase HPLC.Identified phenol oxidation products are di- and trihydroxybenzenes, mono- anddicarboxylic acids. The phenol oxidation pathways strongly depend on the compositionof the gas phase, where the corona discharges are produced. Corona in argon or heliumalso invokes oxidation by the formation of hydroxyl radicals due to ions/metastablesbombardment of water. The maximum obtained phenol conversion efficiency is aboutG=2.7⋅10-8 mol/J ≡ 0.26 (100eV)-1 ≡ 9.1 g/kWh at X=9.5% conversion by corona inair. According to an aldehyde screening test, no volatile aldehydes have been detectedduring oxidation of a phenol solution by corona in argon.

Results 87

4.2.2. Mass spectrometry

The first attempts to identify phenol oxidation products have been performed by liquidchromatography coupled mass spectrometry (LC-MS) analyses on samples obtained bySolid Phase Extraction (SPE), see MS configuration 1. Phenol degradation by pulsedcorona discharges involves oxidation by highly reactive, thus non-specific, hydroxylradicals. This means, that a wide variety of oxidation products is formed. However, theoxidation of low content phenol solutions, according to the scope of this thesis, impliesthat the produced amounts of oxidation products are very low. The oxidation of highcontent phenol solutions may produce a different oxidation product mixture as a resultof polymerization. Also these solutions require a long treatment time to achievereasonable conversion. Pre-concentration of the oxidation product mixture of lowcontent phenol solutions by SPE is attractive. Unfortunately, LC-MS analyses of SPE-processed oxidation product samples have not revealed the identity of any of thecomponents. Also it has been observed, that certain oxidation products will be lost tothe SPE matrix. This has been concluded from chromatograms taken from an oxidizedphenol solution before and after SPE processing. Therefore, SPE has not been applied tosubsequent analyses of phenol oxidation product mixtures despite its advantages.Different LC-MS approaches have been made, but all of them have been performed off-line i.e. without ICE column. The acidic eluent, required for separation according to theion-exclusion mechanism, causes a high background noise that complicates the analysisof the weak signals of the oxidation product components. Also the spraying of theaqueous eluent has appeared to be problematic, due to the high surface tension ofwater. Only sparingly results have been achieved by application of negative IonSpray ofa phenol oxidation product sample in a 100% acetonitrile flow. Prior to the MS analysis0.1 v/v % concentrated ammonia has been added to the sample, to promote the ionformation by converting carboxylic acids and phenols into their anionic form. Thesample has been prepared by oxidation of a 500 ml 1.0 mM (94 mg/l) phenol solutionfor 3 hours, see reactor configuration 6c. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm. With regard to the interpretation of the mass-spectrumshown by Figure 4.45, it should be remarked that the indicated masses are equal toMass-1, because [M-H]- ions are produced.

0.0E+00

2.0E+05

4.0E+05

6.0E+05

0 50 100 150 200 250 300m/z (amu)

Inte

nsity

(arb

.u.)

69

62

113

11593

141 17589

9799

Figure 4.45 IonSpray mass spectrum of a 500 ml 1.0 mM phenol solution, oxidizedduring 3 hours. The corona parameters are V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm.

88 Chapter 4.

An extensive list of probable phenol oxidation products has been mainly derived fromliterature references (section 2.4.1), see Table 4.10. This list is used to identify theobserved masses (FW). m/z=62 might be a (CO2)(H2O) cluster. m/z=69 has not beenidentified. If the peak at m/z=89 is no noise peak, it may be due to singly deprotonatedoxalic acid or deprotonated lactic acid (90). m/z=93 is due to phenol (FW=94 g/mol,76.6%C, 6.4%H, 17.0%O). m/z=97 and 99 have not been identified. m/z=113 islikely to be trifluoroacetic acid (114), that has been left behind in the IonSpraycompartment from previous attempts. m/z=115 may originate from singlydeprotonated maleic/fumaric acid or deprotonated dioxobutyric acid (116). m/z=141 isprobably a singly deprotonated muconic acid enantiomer (142). The peak at m/z=175may contain tetrahydroxybenzoquinone (172). Higher masses are likely to bepolymerized benzoquinones/hydroquinones [34], although polymerization is not likely tobe important during the oxidation of 1 mM phenol solutions.

Other attempts have included the application of APCI mass spectrometry in combinationwith the TFA eluent and addition of acetonitrile to favour the sprayability. Also thesample preparatory options i.e. fractional collection and freeze drying have beenapplied. No spectra could be obtained from these experiments. There are severalreasons for the fact, that identification by LC-MS has been problematic. Although therange of oxidation products is broad, the individual component concentration is low.Negative IonSpray, especially suitable for ionizing phenols and carboxylic acids is lesssensitive than positive ion spray. The available interfaces IonSpray and APCI appear tobe unsuitable for analyzing masses below 200 amu.

Table 4.10a A list of possible phenol oxidation products, sorted by increasingmolecular weight FW (g/mol). FW<FWphenol. Also shown are abbreviatedcompound structure and w/w-% carbon, hydrogen and oxygen.

Compound name Compound structure %C %H %O g/molAcetylene CH≡CH 92.3 7.7 0 26Carbon monoxide CO 42.9 0.0 57.1 28Ethylene CH2=CH2 85.7 14.3 0 28Formaldehyde HCHO 40.0 6.7 53.3 30Carbon dioxide CO2 27.3 0.0 72.7 44Acetaldehyde CH3-CHO 54.5 9.1 36.4 44Formic acid HCOOH 26.1 4.3 69.6 46Butadiene CH2=CH-CH=CH2 88.9 11.1 0 54Glyoxal HC(O)-CHO 41.4 3.4 55.2 58Acetic acid CH3-COOH 40.0 6.7 53.3 60Acrylic acid CH2=C(H)COOH 50.0 5.6 44.4 72Malonaldehyde HC(O)-CH2-CHO 50.0 5.6 44.4 72Glyoxylic acid HC(O)-COOH 32.4 2.7 64.9 74Propionic acid C2H5-COOH 48.6 8.1 43.2 74Maleic aldehyde HC(O)-CH=CH-CHO 57.1 4.8 38.1 84Pyruvic acid CH3-C(O)-COOH 40.9 4.5 54.5 88Butyric acid C3H7-COOH 54.5 9.1 36.4 88Oxalic acid HOOC-COOH 26.7 2.2 71.1 90Lactic acid CH3-CH(OH)-COOH 40.0 6.7 53.3 90

Results 89

Table 4.10b A list of possible phenol oxidation products, sorted by increasingmolecular weight FW (g/mol). FW> FWphenol. Also shown are abbreviatedcompound structure and w/w-% carbon, hydrogen and oxygen.

Compound name Compound structure %C %H %O g/molOxobutyric acid HC(O)-(CH2)2-COOH 47.1 5.9 47.1 102Valeric acid CH3-(CH2)3-COOH 58.8 9.8 31.4 102Malonic acid HOOC-CH2-COOH 34.6 3.8 61.5 104o,p-Benzoquinone O=C6H4=O 66.7 3.7 29.6 108Muconaldehyde HC(O)-CH=CH-CH=CH-CHO 65.5 5.5 29.1 110Dihydroxybenzenes C6H4(OH)2 65.5 5.5 29.1 110Cyclohexadienediol HO(H)-C6H4-(H)OH 64.3 7.1 28.6 112Cyclohexanedione C6H8(O)2 64.3 7.1 28.6 112Dioxosuccinic aldehyde HC(O)-C(O)-C(O)-CHO 42.1 1.8 56.1 114Maleic / fumaric acid HOOC-CH=CH-COOH 41.4 3.4 55.2 116Dioxobutyric acid HC(O)-C(O)-CH2-COOH 41.4 3.4 55.2 116Caproic acid CH3-(CH2)4-COOH 62.1 10.3 27.6 116Ketomalonic acid HOOC-C(O)-COOH 30.5 1.7 67.8 118Succinic acid HOOC-(CH2)2-COOH 40.7 5.1 54.2 118Hydroxybenzoquinone O=C6H3(OH)=O 58.1 3.2 38.7 124Trihydroxybenzenes C6H3(OH)3 57.1 4.8 38.1 126Oxalacetic acid HOOC-CH2-C(O)-COOH 36.4 3.0 60.6 132Glutaric acid HOOC-(CH2)3-COOH 45.5 6.1 48.5 132Malic acid HOOC-CH2-CH(OH)-COOH 35.8 4.5 59.7 134Dihydroxybenzoquinones C6H2(OH)2(O)2 51.4 2.9 45.7 140Muconic acid HOOC-CH=CH-CH=CH-COOH 50.7 4.2 45.1 142Dioxosuccinic acid HOOC-C(O)-C(O)-COOH 32.9 1.4 65.8 146Adipic acid HOOC-(CH2)4-COOH 49.3 6.8 43.8 146Tartaric acid HOOC-CH(OH)-CH(OH)-COOH 32.0 4.0 64.0 150Trihydroxybenzoquinone C6H(O)2(OH)3 46.2 2.6 51.3 156Dibenzofuran C6H4(O)C6H4 85.7 4.8 9.5 168Tetrahydroxybenzoquinone C6(O)2(OH)4 41.9 2.3 55.8 172Dibenzo-p-dioxin C6H4 (O,O)C6H4 78.3 4.3 17.4 184Dihydroxybiphenyl HOC6H4-C6H4OH 77.4 5.4 17.2 186Dimer BQ-BQ C6H3(O)2-C6H3(O)2 67.3 2.8 29.9 214Dimer HQ-BQ C6H3(O)2-C6H3(OH)2 66.7 3.7 29.6 216Purpurogallin C6H(OH)3C5H3(OH)(O) 60.0 3.6 36.4 220

With regard to hydroxylated phenols and C1-C6 ring-cleavage products, the phenoloxidation product mixture exhibits a mass range of about 30 amu (formaldehyde) till172 amu (tetrahydroxybenzoquinone).

Next to off-line LC-MS, mass analyses have been performed by electron-impact massspectrometry (EI-MS), see MS configuration 2. In order to have sufficient detectorsignal from the oxidation products, a 100 ml 5.0⋅10-1 M (concentrated) phenol solutionhas been oxidized for 3 hours, see reactor configuration 3b. The corona parameters areV=30 kV, C=1 nF, f=300 Hz, d=1.0 cm. By oxidation the initial colorless phenolsolution turns deep brown. Mass spectra have been recorded for the mass range 1-500amu, see Figure 4.46.

90 Chapter 4.

Figure 4.46 EI-mass spectra of a phenol oxidation product mixture, obtained byoxidation of a 100 ml 0.5 M phenol solution by corona in air. The coronaparameters are: V=30 kV, C=1 nF, f=300 Hz, d=1.0 cm.

The most evident masses found are m/z=94 (phenol) and m/z=95 (protonated phenol).Protonation is caused by the ionization of water molecules by the 100 eV electronbeam. m/z=110 may originate from the hydroxybenzene isomers, but muconicaldehyde is also possible. m/z=105 might be protonated malonic acid. m/z=106cannot be identified. The observed masses below 94 amu may contain phenol oxidationproducts, but are mainly due to EI-fragmentation of phenol.

Results 91

The mass analyses from 110-500 amu sometimes reveal traces of higher molecularweight species, but these results are not reproducible when the detector integrationtime is increased. This is remarkable, because the deep brown color of the oxidizedsolution clearly implies the presence of polymerized benzoquinones/hydroquinones i.e.synthetic humic acids [34]. It is possible that the observed suspended polymericparticles cannot enter the mass spectrometer by means of the applied capillary tubing.

Summary

Several IonSpray/APCI-MS and EI-MS measurements have not confirmed the presenceof both theoretically possible and literature-reported oxidation products, in spite of thefact that significant phenol conversion has been demonstrated. This has limited thequantitative measurements, because the identification of compounds from complex ICEchromatograms by comparison of retention times is problematic. As main reason for thenegative results can be mentioned that, although the product range is broad, theconcentration of individual oxidation products is low.

92 Chapter 4.

4.2.3. Spectroscopy

Laser-Induced Fluorescence spectroscopy

Next to chromatography, the conversion of phenol has also been measured by Laser-Induced Fluorescence (LIF) spectroscopy [109,110]. Regarding the fact, that LIFliterature only regards gas phase studies, the application of LIF for liquid phase analysisis considered to be a new approach. LIF spectroscopy enables in-situ and time-resolvedmeasurements, which is very favourable to batch-wise sampling and analysis timeinherent to liquid chromatography. In addition to phenol conversion measurements, thein-situ monitoring of total hydroxybenzenes fluorescence is an approach to oxidation ordetoxification progress.A typical LIF spectrum of phenol in aqueous solution at 1.0⋅10-5 M concentration isshown by Figure 4.47. The excitation is performed by the fourth harmonic wavelengthof a Nd:YAG laser i.e. λ=266 nm. No fine structure of rotational lines can be observed,because a low resolution grating (150 mm-1) has been applied to scan the broadwavelength range.

Figure 4.47 A typical LIF spectrum of an 1.0⋅10-5 M aqueous phenol solution, excitedat λ=266 nm.

The fluorescence of phenol excited states in aqueous solution will be quenched bywater, phenol and oxidation product molecules. The measured dependence of the LIFpeak intensity at 298 nm on the phenol concentration is shown by Figure 4.48. Theintensity appears to be linear for the range 1.0⋅10-6 M to 1.0⋅10-5 M.The extent of quenching by oxidation products has been investigated as follows. Thefluorescence peak intensity of phenol-resorcinol and phenol-pyrogallol mixtures hasbeen measured at constant total concentration of 1.0⋅10-5 M while the partialconcentrations have been mutually varied, see Figure 4.48. Resorcinol and pyrogallolare the polyhydroxybenzenes that are both most stable and most fluorescent. Byincreasing the concentration of resorcinol or pyrogallol thus decreasing the phenolconcentration, the LIF peak intensity at 298 nm decreases linearly. Collisionalquenching of phenol molecules by the oxidation products resorcinol and pyrogallol thusappears not to influence linearity at these concentration levels. Also, the testedconcentration range of oxidation products is far higher than actually observed duringphenol oxidation.

Results 93

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

phenol phenol-resorcinol phenol-pyrogallol

LIF

sign

al (ar

b.u.

)

Phenol concentration (10-5 mol/l)

Figure 4.48 The LIF peak intensity of phenol versus the concentration. Also shown isthe LIF peak intensity of several phenol-resorcinol and phenol-pyrogallolaqueous mixtures at a total concentration of 1.0⋅10-5 M.

A 100 ml 1.0⋅10-5 M phenol solution has been oxidized for 20 minutes, see reactorconfiguration 8b. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm. The LIF spectrum of the phenol solution after 4 minutes of oxidation isshown by Figure 4.49. Also indicated is the LIF spectrum due to the remaining amountof phenol after 4 minutes of oxidation. This decomposed spectrum has been derived byfitting the LIF spectrum of the unoxidized phenol solution at t=0 minutes to the LIFspectrum of the oxidized solution, over the wavelength range 250-280 nm.

Figure 4.49 The LIF spectrum of a 100 ml 1.0⋅10-5 M phenol solution, oxidized for 4minutes. Also shown is the decomposed spectrum of non-oxidized phenol.The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

With regard to Figure 4.48, it is justified to assume that the decomposed phenolfluorescence intensity is equal to the phenol concentration. Figure 4.50 shows thephenol concentration versus the oxidation time.

94 Chapter 4.

The phenol concentration seems to decrease according to two different decay timeconstants. For the first 10 minutes the decay time constant is about 25 minutes, from10 to 17 minutes the decay time constant is about 9.8 minutes.

0 5 10 15 20

0.1

1

Con

cent

ration

(10-5

mol

/l)

Time (min)Figure 4.50 The phenol concentration versus time during oxidation of a 100 ml

1.0⋅10-5 M phenol solution. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

Next, the oxidation of 300 ml 1.0⋅10-4 M phenol solutions has been studied undersimilar conditions, see reactor configuration 8a; the oxidation time is 3 hours. Theeffect of the laser on phenol degradation has been determined by measuring the LIFpeak intensity at λ=298 nm of a 300 ml 1.0⋅10-4 M phenol solution versus the laserexposure time at a laser beam pulse rate of 10 Hz, see Figure 4.51.

0 50 100 150 2000.1

1

LIF

sign

al (

arb.

u.)

Time (min)

Figure 4.51 Decrease of the LIF peak intensity of a 300 ml 1.0⋅10-4 M phenol solutiondue to degradation by exposure to a Nd:YAG laser beam at wavelengthλ=266 nm and 10 Hz pulse rate.

The degradation effect is substantial for long exposure times. The 266 nm photonshave an energy of 4.7 eV, which is very close to the bond strength of a hydrogen atomattached to a benzene ring (465 ±3 kJ/mol [24]). Therefore, for all experiments theexposure time has been kept very short viz. 5 seconds and the laser pulse energy hasbeen minimized to 0.8-1.1 mJ.

Results 95

The influence of quenching effects with regard to the 1.0⋅10-4 M phenol solution hasbeen tested for the range 10-6-10-4 M, see Figure 4.52. The LIF intensity is linear to thephenol concentration for the range 1⋅10-6-4⋅10-5 M. At higher concentrations deviationfrom linearity occurs due to absorption of the excitation photons; the phenol data pointsfit well to Equation 2.22 where kQ=0, as is shown by the dotted line. Both resorcinoland pyrogallol quench the fluorescence of phenol excited states significantly; theirdegree of quenching is mutually comparable.

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

phenol phenol-resorcinol phenol-pyrogallol

LIF

sign

al (

arb.

u.)

Phenol concentration (10-4 mol/l)

Figure 4.52 The LIF peak intensity of phenol versus the concentration. The dotted lineshows the absorption effect of the laser intensity. Also shown is the LIFintensity of several phenol-resorcinol and phenol-pyrogallol aqueousmixtures at a total concentration of 1.0⋅10-4 M.

The LIF peak intensity of the 300 ml 1.0⋅10-4 M phenol solution is plotted versus theoxidation time by Figure 4.53. The solution fluorescence has been decomposed into thefluorescence of phenol and its oxidation products for the time range 0-150 minutes.With regard to this plot, no corrections have been performed for either laser absorptionby the solution or quenching due to oxidation products.

0 50 100 1500.01

0.1

1 Phenol Oxidation products

LIF

sign

al (

arb.

u.)

Time (min)

Figure 4.53 The decomposed LIF peak intensity due to phenol and oxidation products,during oxidation of a 300 ml 1.0⋅10-4 M phenol solution. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

96 Chapter 4.

It is observed, that the oxidation products reach a maximum concentration after about20 minutes of oxidation. After this time these intermediate products are furtheroxidized. The intermediate products comply -next to phenol- with the requirements forfluorescence and are thus likely to be polyhydroxybenzenes.

Application of the absorbance correction to the decomposed phenol LIF intensity versustime course according to Equation 2.22, yields the absolute phenol concentration, seeFigure 4.54. The error bars mark the deviation in the concentration due to quenchingeffects. Characteristic decay time constants τ apply for three intervals, viz. τ0-50min=19minutes, τ50-120min=25 minutes and τ120-150min=100 minutes. These different decay ratesoriginate from the production and consumption of intermediate polyhydroxybenzenes;after disappearance of phenol non-fluorescent saturated carboxylic acids remain thateventually mineralize.

0 20 40 60 80 100 120 140 1600.0

0.2

0.4

0.6

0.8

1.0

Phen

ol c

once

ntra

tion

(10

-4 m

ol/l)

Time (min)

Figure 4.54 The absolute phenol concentration versus time during oxidation of a 300ml 1.0⋅10-4 M phenol solution. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

Figure 4.55 shows the LIF spectra of 1.0⋅10-4 M solutions of the dihydroxybenzenesand trihydroxybenzenes. Also included is the quinone 1,4-benzoquinone. The LIFintensity decreases by the following order: phenol > hydroquinone > resorcinol >catechol > hydroxyhydroquinone > pyrogallol > 1,4-benzoquinone, phloroglucinol.

The trihydroxybenzenes are much weaker fluorescent than the dihydroxybenzenes andtheir maximum fluorescence intensity is at about 350 nm. Especially hydroquinone andhydroxyhydroquinone seem to account for the observed increased LIF intensity athigher wavelengths.

Results 97

Figure 4.56 shows LIF spectra due to the phenol oxidation products, recorded after40 minutes and 100 minutes of exposure time for corona in air and argon. Thesespectra have been derived by subtraction of the decomposed phenol spectrum from theobserved solution LIF spectrum.

The LIF spectcompared to tby corona in athat the degamounts of dwavelengths the trihydroxy

Figure 4.56 L1cp

250 300 350 400 450 5000

1Ar t=40 min

Ar t=100 min

LIF

sign

al (

arb.

u.)

Wavelength (nm)

250 300 350 400 450 5000

1

air t=40 min

air t=100 min

LIF

sign

al (

arb.

u.)

Wavelength (nm)

Figure 4.55 LIF spectra of 1.0⋅10-4 M dihydroxybenzene (left) and trihydroxybenzene(right) solutions.

250 300 350 400 450 5000

400

800

1200Hydroxyhydroquinone

Pyrogallol

Phloroglucinol

LIF

sign

al (

arb.

u.)

Wavelength (nm)

250 300 350 400 450 5000

10000

20000

30000

40000

Hydroquinone

Resorcinol

Catechol

1,4-Benzoquinone

LIF

sign

al (

arb.

u.)

Wavelength (nm)

IF spectra due to phenol degradation products, recorded after 40 min and00 min, during degradation of 1.0⋅10-4 M phenol solutions by pulsedorona discharges in an air (left) and argon (right) atmosphere. The coronaarameters are V=25 kV, V=1 nF, f=100 Hz, d=1.0 cm.

ra of oxidized phenol solutions reveal fluorescence at higher wavelengthshe LIF spectrum of phenol. This is especially the case for phenol degradedrgon. From the ICE measurements (section 4.2.1) it has been concluded,radation of phenol by corona in argon produces considerably higherihydroxybenzenes. According to Figure 4.55 the fluorescence at higheris likely due to the dihydroxybenzene hydroquinone. The contribution ofbenzenes to fluorescence is negligible.

98 Chapter 4.

300 ml 1.0⋅10-4 M polyhydroxybenzene solutions and a 1,4-benzoquinone solution havebeen oxidized by pulsed corona discharges in air. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm. Figure 4.57 shows the LIF peak intensity of thesolution versus the oxidation time. Peak wavelengths are according to Figure 4.55.Although no spectrum decomposition has been performed here, the LIF intensity decaycan be related to the disappearance of the hydroxybenzenes by oxidation. The oxidationstability of the DHB’s decreases by the order resorcinol > 1,4-benzoquinone,hydroquinone > catechol while the stability of the THB’s decreases according to theorder phloroglucinol > pyrogallol > hydroxyhydroquinone. The mutual stability withinthe DHB and THB classes may be explained by the resonance structures and by ringstrain due to sterical hindrance of hydroxyl groups. It is remarkable that the THBstability is rather similar to the DHB stability, because THB’s are much strongerreducing agents than DHB’s [61]. This contradiction may arise from the fact that theoxidation of the hydroxybenzenes in aqueous solution by ozone is mass transfer limited.

Figure 4.57 The solution LIF peak intensity versus time during the oxidation of 300 ml1.0⋅10-4 M dihydroxybenzene (left) and trihydroxybenzene (right)solutions. The corona parameters are V=25 kV, C=1 nF, f=100 Hz,d=1.0 cm.

Summary

Although LIF spectroscopy is generally applied for gas phase studies, it has beendemonstrated that it is also applicable to in-situ conversion measurements of phenol inaqueous solution at concentrations up to 1.0⋅10-4 M. For that case, the phenol LIF peakintensity has been obtained from the solution LIF peak intensity by spectrumdecomposition, as a correction with regard to quenching effects. The LIF spectrum dueto the phenol oxidation products indicates the presence of polyhydroxybenzenes, whichis consistent with the results from ICE measurements. The polyhydroxybenzenes areless resistant to oxidation than phenol, but the decrease of the LIF intensity of DHB’sand THB’s has appeared to be comparable, although these compounds differ in reducingstrength. This may be due to mass transfer limitation of the oxidation reaction byozone.

0 30 60 90 120

10-2

10-1

100

resorcinol catechol hydroquinone 1,4-benzoquinone

LIF

sign

al (

arb.

u.)

Time (min)

0 30 60 90 120

10-2

10-1

100

pyrogallol hydroxyhydroquinone phloroglucinol

LIF

sign

al (ar

b.u.

)

Time (min)

Results 99

Infrared spectroscopy

In addition to the aldehyde screening test, the gas phase over oxidized phenol solutionshas also been analyzed by Fourier-transform infrared spectroscopy (FTIR). 250 ml1.0 mM phenol solutions have been oxidized by pulsed corona discharges in both an airand argon gas phase, see reactor configuration 9. Figure 4.58 shows the FTIR spectrarecorded after three hours of oxidation.

0

0.02

0.04

0.06

1500200025003000Wavenumber (cm-1)

Abs

orba

nce

(arb

.u.)

corona in aircorona in argon

Figure 4.58 FTIR spectra of the gas phase over 250 ml 1.0 mM phenol solutions,oxidized by pulsed corona discharges in air or argon during 3 hours. Thecorona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

The strong absorption at 2349 cm-1 is due to carbon dioxide (anti-symmetric stretchvibration). Carbon monoxide (2143 cm-1) has not been detected. The very weakabsorption at 2224 cm-1, observed in the spectrum from corona in air, may originatefrom traces of nitrous oxide (N2O). Within the scanned wavenumber range, no othercompounds have been identified. Table 4.11 shows carbon dioxide concentrations.

Table 4.11 The production of carbon dioxide during oxidation of 250 ml 1.0 mMphenol solutions by pulsed corona discharges in air or argon. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

[CO2] (ppm v/v)Time(min) air argon

0 330 030 - 16060 590 230

120 - 330180 1100 410

The mineralization of phenol by corona in air is faster than the mineralization of phenolby corona in argon.

100 Chapter 4.

For the case of air the oxidizers ozone, hydroxyl radicals and oxygen can producecarbon oxides from phenol. In argon, oxidizers can only be produced from thedissociation of water by argon ions and from trace amounts of oxygen dissolved in thewater.

Although carbon monoxide has not been detected, the production of this compound ispossible from the decomposition of endoperoxides [19] produced by phenol oxidation,see Figure 4.59. However carbon monoxide is oxidized to carbon dioxide by ozone orby hydroxyl radicals and oxygen, see Equations 4.2a-d.

OH H

OO

OOH

H

O

OH

OOHHH + CO

phenol endoperoxide

OHOH

O2

Figure 4.59 The production of carbon monoxide from the decomposition of anendoperoxide produced during the oxidation of phenol by hydroxyl radicalsand oxygen.

CO + O3 → CO2 + O2 (4.2a)CO + HO• → HCO2• (4.2b)HCO2• + O2 → CO2 + HO2• (4.2c)HCO2• + HO• → CO2 + H2O (4.2d)

The production of carbon dioxide has also been measured during the oxidation of250 ml 0.1 mM phenol solutions, under the same conditions. The observed carbondioxide concentrations are shown by Table 4.12 and Figure 4.60.

Table 4.12 The production of carbon dioxide during oxidation of 250 ml 0.1 mMphenol solutions by pulsed corona discharges in air or argon. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm

[CO2] (ppm v/v)Time(min) air argon

0 330 1830 467 13260 591 17390 687 240

120 787 287180 922 350

It is clear, that the carbon dioxide production depends on the initial phenolconcentration. The concentration increase versus the oxidation time is non-linear.

Results 101

0

200

400

600

800

1000

0 30 60 90 120 150 180

Time (min)

[CO

2]

(ppm

v/v

)

corona in argon

corona in air

Figure 4.60 The production of carbon dioxide during oxidation of 250 ml 0.1 mMphenol solutions by pulsed corona discharges in air or argon. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

The theoretical amount of carbon dioxide that can be produced from phenol iscalculated according to Equation 4.3.

C6H5OH + 7O2 → 6CO2 + 3H2O (4.3)

1 mol phenol can produce a maximum amount of 6 moles carbon dioxide. The molargas volume of carbon dioxide is about Vm=24.3 liter at p=p0 and T=293.15 K [111].The 250 ml 1.0 mM phenol solution contains 2.5⋅10-4 mol phenol that corresponds to36.5 ml carbon dioxide, while the 0.1 mM phenol solution can produce only one tenthof this amount. The gas phase volume is also equal to 250 ml. The percentage ofcarbon converted by oxidation of both 0.1 mM and 1 mM phenol solutions during 180minutes in air and argon is shown by Table 4.13.

Table 4.13 Percentage of carbon converted into carbon dioxide during oxidation of250 ml 0.1 mM and 1.0 mM phenol solutions by corona in air or argon.The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

t=180 min 0.1 mM 1.0 mMair 922 / 14600 ≡ 6.3 % 1100 / 146000 ≡ 0.8 %

argon 350 / 14600 ≡ 2.4 % 410 / 146000 ≡ 0.3 %

However, if the solubility of carbon dioxide in water [44] is taken into account viz.7.07⋅10-4 mol CO2/mol H2O ≡ 3.92⋅10-2 mol/l ≡ 1.72 g/l ≡ 952.0 ml/l at T=293.15 Kand p=p0, the liquid phase may also contain large amounts of dissolved carbon dioxide.Then the percentage of carbon dioxide converted is much higher. The 1.0 mM phenolsolution is clearly less mineralized than the 0.1 mM solution. The converted carbon ratioair-to-argon is about 2.7 for both phenol concentrations.

Summary

Only CO2 has been detected in the gas phase over oxidized phenol solutions. Theamounts produced by corona in argon are distinctly lower than the amounts producedby corona in air. The observed CO2 levels possibly imply a low level of mineralization.

102 Chapter 4.

4.2.4. Electrical conductometry

Electrical conductometry has been applied in order to monitor oxidation progress ofphenol and some of its intermediate oxidation products, by the increase of solutionconductivity due to the production of carboxylic acids.

The electrical conductivity of the hydroxybenzene solutions has been determined as afunction of the oxidation time. Simultaneously, the hydroxybenzene conversion hasbeen determined by ion-exclusion chromatography, see LC configuration 8.Experiments have been performed using 250 ml 1 mM aqueous hydroxybenzenesolutions, see reactor configuration 5b. Tested compounds are phenol (PHE), catechol(CAT), hydroquinone (HQ), pyrogallol (PG) and hydroxyhydroquinone (HHQ). The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm and the maximum oxidationtime is 40 minutes. A single tungsten anode tip has been used. The following relativetemperature increase (∆T/T0) values have been measured after 40 minutes of coronaexposure: PHE: 0.10, CAT: 0.08, HYD: 0.09, PYR: 0.10, HHQ: 0.11.

As reported in section 4.1.4, the solution conductivity also increases by the formationof nitrate ions, due to the application of corona discharges in air. Therefore theconductivity measurements on oxidized hydroxybenzene solutions have been reportedwith regard to the following background: the conductivity of deionized water as afunction of the oxidation time, under the same conditions the hydroxybenzenes havebeen oxidized. The conductivity measurements as a function of the oxidation time areshown by Figure 4.61.

0

100

200

300

400

500

0 10 20 30 40

Time (min)

Con

duct

ivity

(µS/c

m)

PHECATHYDPYRHHQ

Figure 4.61 The conductivity of 250 ml 1.0 mM hydroxybenzene solutions versus theoxidation time. Tested hydroxybenzenes are phenol (PHE), catechol(CAT), hydroquinone (HYD), pyrogallol (PYR) and hydroxyhydroquinone(HHQ). The conductivity is reported with regard to the conductivity ofdeionized water oxidized under the same conditions. The coronaparameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Results 103

It has been observed that the conductivity increases with the oxidation time for alltested compounds. Also, the conductivity at a fixed oxidation time increases accordingto the order phenol, catechol/hydroquinone, pyrogallol, hydroxyhydroquinone. This orderresembles the increase in reducing properties of the regarded hydroxybenzenes.Hydroxyhydroquinone is the strongest reducing agent of the tested hydroxybenzenesand the oxidation of this compound i.e. production of carboxylic acids will thus befastest. The untreated hydroxyhydroquinone solution already shows conductivity ofabout 35 µS/cm, which is caused by oxidation of this compound by oxygen dissolved inthe water. The conversion of the hydroxybenzenes is shown by Table 4.14.

Table 4.14 Conversion X of 250 ml 1.0 mM hydroxybenzene solutions. A tungstensingle-pin anode has been used. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

Time (min) XPHE (%) XCAT (%) XHYD (%)0 0.0 0.0 0.0

10 1.7 3.0 3.120 3.3 6.1 6.230 5.0 9.4 9.240 49.3 12.2 12.4

Unfortunately the conversion determination of hydroxyhydroquinone and pyrogallolfailed. The phenol conversion at 40 minutes is unlikely. The pulse energy has beenrather constant during all oxidation experiments, so the oxidation conditions arecomparable. Average pulse energies with 95% confidence interval are as follows: forphenol oxidation 5.1 ±0.3 mJ, for catechol oxidation 5.5 ±0.0 mJ and forhydroquinone oxidation 6.1 ±0.1 mJ. Figure 4.62 shows the hydroxybenzeneconversion versus the solution conductivity observed during oxidation.

0

5

10

15

0 100 200 300Conductivity (µS/cm)

Con

vers

ion

(%)

PHE

CAT

HYD

Figure 4.62 The hydroxybenzene conversion versus the solution conductivity.PHE=phenol, CAT=catechol and HYD=hydroquinone. The conductivityis reported with regard to the conductivity of deionized water oxidizedunder the same conditions. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

104 Chapter 4.

There appears to be a relationship between conversion and conductivity. Themonohydroxybenzene phenol shows a different course than the dihydroxybenzeneisomers catechol or hydroquinone with mutually comparable course. Theorizing,because of the missing conversion data, the slope of the trihydroxybenzene conversionversus conductivity relationship will be much smaller than the slope of thedihydroxybenzenes conversion course.The relationship is applicable for a component-specific oxidation progress range wherethe conversion increases with conductivity, knowing the initial conductivity. Due tomineralization, the carboxylic acids will eventually disappear thus the conductivity willdecrease from that time. Also it should be noted, that after 100 % conversion of thetarget compound, still carboxylic acids may be present that contribute to conductivity.

Finally the conversion efficiency values are reported for phenol, catechol andhydroquinone by the G yield, see Table 4.15.

Table 4.15 The conversion efficiency of phenol, catechol and hydroquinone; averagevalues and 95% confidence intervals are presented.

Compound G mol/J ⋅108 G (100 eV)-1 G (g/kWh)Phenol 1.4 ±0.07 0.13 ±0.01 4.6 ±0.3Catechol 2.3 ±0.07 0.22 ±0.01 9.1 ±0.3Hydroquinone 2.2 ±0.06 0.21 ±0.01 8.6 ±0.2

From these data it is confirmed, that the dihydroxybenzenes catechol and hydroquinoneare stronger reducing agents than phenol. The observed efficiency values are generallyless favourable than the efficiency values observed in section 4.2.1; this may beexplained by application of a single pin anode.

Summary

Electrical conductometry has been tested as an alternative conversion measurement.There appears to be a relationship between solution conductivity and target compoundconversion for a certain oxidation progress range. This relationship is based on theformation of carboxylic acids providing conductivity by deprotonation in aqueoussolution.

Results 105

4.2.5. Microtox ecotoxicity

The degree of detoxification during oxidation of phenol by pulsed corona discharges hasbeen investigated by Microtox ecotoxicity tests. The ecotoxicity is expressed as aneffect concentration EC20, defining the concentration at which 20% inhibitory effecttakes place. The EC20 value is determined from a concentration series by interpolationof the dose-effect relationship, see Equation 2.8. In order to quantify the ecotoxicity ofphenol during oxidation, an EC20 value has been determined before and after oxidationof a series of different phenol concentrations. 250 ml solutions at 0.02 mM, 0.05 mM,0.1 mM, 0.2 mM and 0.4 mM initial phenol concentration [PHE]0 have been oxidized for30 minutes, see reactor configuration 5c. The corona parameters are V=25 kV, C=1nF, f=100 Hz, d=1.0 cm. Table 4.16 shows the phenol conversion X and absoluteconverted phenol amount mX, see LC configuration 9.

Table 4.16 Phenol conversion (X) and absolute converted amount (mX) after 30minutes of oxidation of 250 ml solutions. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

[PHE]0 (mM) X (%) mX (mg)0.02 64.6 1.40.05 72.8 3.50.10 53.0 5.00.20 20.2 3.80.40 25.3 9.6

The untreated phenol solutions show a dose-effect relationship according to Equation2.8. The EC20 values, dose-effect parameters b,a and correlation coefficient r2 areshown by Table 4.17. As the EC20 values do not change with the Vibrio fischeriexposure time tVF, the ecotoxicity effect is acute.

Table 4.17 EC20 values of phenol in deionized water versus the Vibrio fischeriexposure time. Also shown are the dose effect parameters b,a andcorrelation coefficient r2.

tVF (min) EC20 (mM) b (-) a (-) r2 (-) 5 0.06 0.70 -0.79 0.999915 0.07 0.90 -0.62 0.982530 0.07 0.91 -0.62 0.9784

On the contrary, no EC20 value can be determined for the oxidized phenol solutions,because there appears to be no dose-effect relationship. All samples except for the0.1 mM sample show very low bioluminescence intensity, implying high ecotoxicity ofthese oxidized phenol solutions. Table 4.18 shows the observed effects, indicated bythe gamma value, for the different phenol solutions and Vibrio fischeri exposure times.It should be noted that the effect increases with the exposure time tVF. This means thatthe ecotoxicity effect of the oxidized samples is not acute, which may be explained bymass transfer-limited diffusion of the oxidation products within the Vibrio fischeribacterium.

106 Chapter 4.

Table 4.18 Ecotoxicity effects of oxidized phenol solutions, indicated by the gammavalue, for different Vibrio fischeri exposure times; average values (AVG)and standard deviations (σn-1) are presented. 250 ml solutions have beenoxidized for 30 minutes. The corona parameters are V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm.

Γ 5 min (-) Γ 15 min (-) Γ 30 min (-)[PHE]0

(mM) AVG σn-1 AVG σn-1 AVG σn-1

0.02 24.9 13.1 31.3 16.7 49.3 29.90.05 85.6 59.6 105.2 57.0 151.4 91.20.10 1.1 0.0 1.4 0.0 2.1 0.10.20 6.2 0.1 15.9 0.1 51.0 1.80.40 30.9 8.0 52.5 20.3 94.5 50.3

An explanation for the absence of the dose-effect relationship after oxidation is thefact, that all oxidized samples contain a complex range of oxidation products. Especiallyhydroquinone and 1,4-benzoquinone are highly toxic towards Vibrio fischeri, see Table2.1. By oxidation progress, these compounds disappear but the oxidation productmixture increases in acidity which is also unfavourable to these bacteria.As part of the Microtox test, the solution acidity and oxygen content have beenmeasured for both untreated and oxidized samples, see Table 4.19. If the sampleacidity exceeds the range 6<pH<8.5 and/or the sample has a low oxygen content(<0.5 mg/l), these effects are inhibitory and bias the effect of the target compound.It appears, that oxidation increases the acidity of both the Millipore water and thephenol solutions. For the case of Millipore water, the production of nitrogen oxides bycorona discharges in air results in the formation of nitric acid. The acidity increase byoxidation of phenol is caused by the formation of carboxylic acids.

Table 4.19 Solution acidity and oxygen content (expressed as percentage of thesaturation concentration) for untreated and oxidized samples. 250 mlsolutions have been oxidized for 30 minutes. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Untreated Oxidized[PHE]0

(mM) pH (-) [O2] (%) pH (-) [O2] (%)0 5.7 65 4.2 710.02 5.9 63 4.0 600.05 5.9 62 4.0 610.10 6.2 61 3.9 480.20 6.2 51 3.8 650.40 6.2 60 3.8 51

The small differences in oxygen content between the oxidized and untreated phenolsolutions, if significant, might be caused by different amounts of polyhydroxybenzenesproduced during the oxidation. These compounds are strong reducing agents and thushave a high affinity for oxygen. The oxygen content of oxidized Millipore water issomewhat higher than the oxygen content of untreated Millipore water. This might bedue to the presence of hydrogen peroxide.

Results 107

The influence of the solution acidity on the Microtox test has been verified bydetermination of the gamma values of non-pH adjusted oxidized phenol solutions at anexposure time tVF =5 minutes. The acidity of these solutions is about pH=4, see Table4.19. The gamma values of oxidized phenol solutions that have not been pH-adjusted,are shown by Table 4.20.

Table 4.20 Gamma values of non-pH adjusted oxidized phenol solutions, after 5minutes exposure time; average values (AVG) and standard deviations(σn-1) are presented. 250 ml solutions have been oxidized for 30 minutes.The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

Γ 5 min[PHE]0 (mM)AVG σn-1

0.02 13.7 6.10.05 1.4 0.10.10 5.9 1.20.20 395.6 127.50.40 193.1 25.5

The gamma values are highest for the 0.2 mM and 0.4 mM samples, which are moreacidic than the other samples. Also, these gamma values are very different from thegamma values of pH-adjusted oxidized solutions, illustrated by Table 4.18. The pHeffect is thus very relevant.

Finally, it has been observed, that the oxidation of Millipore water also inducesecotoxicity. The bioluminescence intensity at tVF=5 minutes of fresh Millipore water isIt5=100.8 / σn-1=0.9. The bioluminescence intensity of pH-adjusted oxidized Milliporewater is It5=100.1 / σn-1=2.5. However, if oxidized Millipore water is not pH-adjusted,the bioluminescence intensity is It5 = 67.8 / σn-1=5.2.This effect is also caused by acidity. The corona discharges produce nitrogen oxides inair, which dissolve in the water and form nitric acid. By adjusting the acidity of thesolution the ecotoxicity vanishes, so the effect is not caused by the nitrate ion.Although hydrogen peroxide is produced by the corona discharges and hydrogenperoxide exhibits ecotoxicity (EC50

15min=16 mg/l) [78], this effect has not beenobserved: the bioluminescence intensities of untreated Millipore water and pH-adjustedoxidized Millipore water are equal. Hydrogen peroxide at low concentrationsdecomposes rapidly into oxygen and water.

Summary

After oxidation during a fixed time (30 min) of a series of different concentration phenolsolutions, no dose-effect relationship appears to exist anymore. Although mutuallydifferent conversions have been measured, the Microtox ecotoxicity effect is high forboth low and high initial phenol concentration. This effect is especially caused byhydroquinone and 1,4-benzoquinone, that are highly toxic to Vibrio fischeri bacteria.Although these compounds disappear by oxidation progress, carboxylic acids areproduced that cause pH-toxicity.

108 Chapter 4.

4.2.6. Total organic carbon

During the oxidation of a 498 ml 1.0 mM phenol solution, see reactor configuration 10,the total organic carbon level (TOC) has been measured every 15 minutes during2 hours. The corona parameters are V=25 kV, C=1 nF, f=100 Hz, d=1.0 cm. Inaddition, the conversion has been measured by ICE chromatography, according to LCconfiguration 7. Table 4.21 shows the TOC and conversion (X) values versus theoxidation time. X2 is a repeated conversion measurement, 21 hours after the firstmeasurement X1.

Table 4.21 Total organic carbon (TOC) level and conversion (X) during the oxidationof a 498 ml 1.0 mM phenol solution. The time span between the twoconversion measurements is 21 hours. The corona parameters areV=25 kV, C=1 nF, f=100 Hz, d=1.0 cm.

t(min) TOC (mg/l) X1 (%) X2 (%)0 52.0 0.0 0.0

15 58.4 3.3 3.230 62.8 7.0 7.145 57.2 10.8 11.060 59.2 13.9 14.475 53.6 17.0 17.290 67.6 19.9 20.3

105 62.4 21.3 22.3120 56.8 25.2 25.7

It should be remarked, that the theoretical TOC level of an 1.0 mM phenol solutionequals 72.1 mg/l. The measured value at t=0 however is somewhat lower. This maybe due to an incidental sample preparation error, because the samples had to be dilutedbefore analysis.

The TOC level appears to be rather constant during oxidation, while the conversionincreases. This means, that the oxidation products are likely to remain in the liquidphase for the observed conversion range. This is in accordance with the results fromthe infrared analyses of the gas phase over oxidized phenol solutions (section 4.2.3)and the aldehyde screening test (section 4.2.1).From the conversion measurements (X1 and X2) it is clear, that after stopping of theoxidation the conversion of phenol does not significantly proceed within 21 hours.Possible conversion progress after stopping the corona discharges might be due toremaining amounts of ozone and hydrogen peroxide. In between these two conversionmeasurements the TOC measurements have been performed.

Summary

At a conversion up to 25%, the TOC level of an oxidized 1 mM phenol solution appearsto be rather constant. This implies that the oxidation products are likely to remain in theliquid phase for the observed conversion range.

Results 109

4.3. Oxidation of other model compounds

In this section a global survey is given of the oxidation of atrazine herbicide, malachitegreen dye and dimethyl sulfide odor component. The conversion of atrazine in aqueoussolution has been measured by reversed-phase HPLC, while the oxidation progress ofmalachite green in aqueous solution has been determined by in-situ absorptionspectrometry. Dimethyl sulfide has been oxidized in the gas phase and the conversionhas been measured by gas chromatography.

4.3.1. Atrazine

A 500 ml 25 mg/l (0.12 mM) atrazine solution (tap water-based) has been oxidized for5 hours, see reactor configuration 6b. The corona parameters are V=30 kV, C=100pF, f=50 Hz, d=1.0 cm. Every 15 minutes (0-3 hours) or 30 minutes (3-5 hours) theconversion has been determined in threefold using a rp-HPLC column according to LCconfiguration 10. Figure 4.63 shows the conversion versus the oxidation time. Theconversion efficiency versus time is shown by Table 4.22.

0

20

40

60

80

100

0 60 120 180 240 300

Time (min)

Con

vers

ion

(%)

Figure 4.63 The conversion of a 500 ml 25 mg/l (0.12 mM) atrazine solution (tapwater-based). The corona parameters are V=30 kV, C=100 pF,f=50 Hz, d=1.0 cm.

Table 4.22 Conversion (X), pulse energy (Ep) and efficiency (G) during oxidation of a500 ml 25 mg/l (0.12 mM) atrazine solution (tap water-based); averagevalues and 95% confidence levels are presented. The corona parametersare V=30 kV, C=100 pF, f=50 Hz, d=1.0 cm.

t (min) X (%) Ep (mJ) G (mol/J)⋅1010 G (100eV)-1⋅103 G (g/kWh)60 12.7 ±5.5 10.8 ±2.1 7.67 ±5.14 7.40 ±4.96 0.60 ±0.40

120 27.2 ±15.3 10.7 ±0.5 8.22 ±5.17 7.93 ±4.99 0.64 ±0.40180 38.5 ±21.0 10.4 ±1.2 7.89 ±3.29 7.62 ±3.18 0.61 ±0.26240 47.8 ±28.8 11.2 ±1.9 6.83 ±1.93 6.59 ±1.86 0.53 ±0.15300 56.5 ±32.5 10.2 ±0.6 7.21 ±4.86 6.96 ±4.69 0.56 ±0.38

110 Chapter 4.

As a function of the oxidation time, the error bars grow to considerable size. Thedeviation increase cannot be due to sample inhomogenity because the reactor contentis continuously mixed by a magnetic stirring bar. The time dependent effect may be arelated to a UV absorbance detector stability problem.

The conversion time relationship seems to obey first order kinetics with a rate constantk1≈2.7⋅10-3 min-1. After 5 hours of oxidation the atrazine conversion is about 57%. Theefficiency values clearly show that atrazine is a very stable compound. At least sevenoxidation products have been found in the chromatogram, but the identification of theseproducts has not been possible. Pelizzetti [36] has concluded from detailed GC-MSexperiments, that photocatalytic oxidation of aqueous atrazine will not destroy thetriazine ring and that the final oxidation product is cyanuric acid. Cyanuric acid (2,4,6-trihydroxy-1,3,5-triazine) is a questionable carcinogenic compound [112].

Summary

Atrazine is a rather stable herbicide. The conversion efficiency is about one order ofmagnitude lower than the conversion efficiency of phenol. It has not been possible, toidentify the variety of oxidation products.

4.3.2. Malachite green

The influence of different electrode configurations on the decolorization of malachitegreen dye has been determined by absorption spectrometry. From a LED light source, aphotodiode detector, an amplifier and a recorder a simple setup has been constructedthat may be a reasonable alternative to a precious and laboratorybound liquidchromatograph or absorption spectrometer.

125 ml 1.94 mg/l malachite green solutions have been exposed to pulsed coronadischarges during 1.5 hours, see reactor configuration 11. The load voltage has beenset to V=20-30 kV, depending on the configuration. The time span t24 has beendetermined, after which 24% decrease in absorption at wavelength λ=590 nm hasbeen measured. This percentage is based on the configuration which showed slowestdecolorization.The configuration variables are as follows: the anode has been located at a distanced=-4 mm, +1 mm, +5 mm, +10 mm relative to the solution surface; the used anodetypes are single pin, 4 pins, 8 pins and a wire anode; in one case an air flow has beensupplied at a single pin anode tip (configuration no. VII); the cathode location is in thewater on the bottom of the reactor or directly outside/underneath the reactor. Table4.23 shows the power input (P), decolorization time span (t24) and efficiency (G).

It can be observed that capacitive configurations, where the cathode plate is situatedoutside/underneath the glass reactor vessel, are generally more favourable to theefficiency than configurations where the cathode is inside the aqueous phase. This iscaused by the fact that for the case of a capacitive configuration, conductive current isnot possible.

Results 111

Table 4.23 Power input (P), decolorization time span (t24) and efficiency (G) for theoxidation of malachite green in aqueous solution. The corona parametersare V (configuration dependent), C=5 pF, f=150 Hz, d (indicated).

Anode P t24 GConf.nr.

Cathodeposition type d (mm) (mW) (min) (mg/kWh)

I outside 4 pins +1 395 12.1 412II outside 8 pins +1 366 15.9 337III outside wire +1 493 32.2 123IV outside 1 pin +1 383 57.9 89V outside 1 pin -4 432 94.0 48VI inside 1 pin +1 1789 26.4 67VII inside 1 pin+air -4 6944 35.0 16VIII inside 1 pin +10 3963 61.9 13IX inside 1 pin +5 6301 66.7 7

Immersion of the anode in the aqueous solution is less favourable than situating theanode in the gas phase over the solution. The production of corona discharges at ananode tip immersed in an aqueous phase, demands extra energy for evaporation of theliquid, which is less efficient than the formation of the corona discharge in the gasphase. A multipin anode is more efficient than a single pin anode. The multipin anodeseems to produce a more efficiently-dimensioned reactive volume (plasma) for theproduction of oxidizers than a single pin, while the energy input differences arecomparable. The anode wire efficiency is in between the performances of multipin andsingle pin; although it spans a favourable space, it lacks the pin shape necessary for thecreation of high electric field strengths. Although the addition of air to the discharge islikely to favour the production of ozone by provision of oxygen, configuration VII hasappeared to show low performance. In this way, large amounts of oxygen-based ionsmay have been produced, that have caused high conduction currents within this non-capacitive configuration.It should be noted, that the decolorization of malachite green does not imply thatdetoxification takes place. Decolorization is caused by the destruction of chromophoricgroups; at that time, mineralization of the large dye molecule is certainly not obvious.The observed decolorization efficiency is about one to two orders of magnitude lowerthan the conversion efficiency of phenol.The degradation of malachite green is complex, because the number of probable attacksites for oxidizers like the hydroxyl radical, ozone and oxygen is very large. Themethyne central carbon is prone to attack by the hydroxyl radical or ozone yieldingbond cleavage, thus removal of the main chromophoric part i.e. C=C6H4=N+(CH3)2

from the dye molecule. Also, the aromatic rings can be hydroxylated or cleaved. Inaddition to the oxidation products mentioned in section 2.4.3, the following productsmay be formed: nitro/hydroxybenzophenones, nitrophenols, benzoic acid derivatives,aliphatic aldehydes/carboxylic acids and nitrate ions.

Summary

A multipin anode over the aqueous phase, combined with a cathode directly outside andunderneath the reactor is the most favourable configuration for the decolorization ofmalachite green.

112 Chapter 4.

4.3.3. Dimethyl sulfide

This experiment differs from the others, because the target compound dimethyl sulfide(DMS) has been oxidized in the gas phase using a concentric electrode tubular reactoraccording to reactor configuration 12. A gas chromatograph equipped with sulfurchemiluminescence detector has been applied for analysis, see GC configuration 2.10 ppm and 100 ppm standard solutions of DMS in methanol have been prepared. Thecorona parameters are V=30 kV, C=100 pF, f=50 Hz. Before every coronaexperiment, the reactor contains about 2.5 ppm DMS in ambient air. Three oxidationtimes have been investigated. Every experiment has been performed in threefold. Table4.24 shows the results.

Table 4.24 Conversion (X) of dimethyl sulfide in air versus the oxidation time. Alsoshown are initial and final average concentrations [DMS] and 95%confidence intervals. The corona parameters are V=30 kV, C=100 pF,f=50 Hz.

t (s) [DMS]0 (ppm) [DMS]t (ppm) X (%)15 2.33 ±0.60 1.28 ±0.25 4530 2.59 ±1.05 0.77 ±0.25 7060 2.54 ±0.22 0.14 ±0.03 94

Next to the oxidation products mentioned in section 2.4.4, oxidative demethylationlikely yields formaldehyde and formic acid. The aim of DMS removal from industrialwaste gas flows is odor destruction. Although destruction of dimethyl sulfide has beendemonstrated, thorough oxidation progress is necessary, because the oxidationproducts are still harmful. Dimethyl sulfoxide is an odourless, strongly hygroscopicliquid that is readily absorbed by the skin; it causes especially irritation of the mucousmembranes; dimethyl sulfoxide is reported to exhibit mutagenic and teratogenicproperties [113]. Formaldehyde is strongly irritating and has mutagenic, teratogenic andprobably carcinogenic properties [1]. Formaldehyde is oxidized to formic acid and formicacid will eventually disappear by mineralization, but sulfuric acid has to be removed bya gas scrubber.

Summary

2.5 ppm Dimethyl sulfide in 330 ml ambient air has been converted for about 94 %after 60 seconds using gas phase corona discharges at V=30 kV and f=50 Hz. Thedestruction of this organosulfur compound initially yields harmful intermediates;although qualitative product analysis has not been performed, likely final oxidationproducts are sulfuric acid, carbon dioxide and water.

5. Discussion

A survey of the results is presented in combination with points of special interest andnew approaches. The discussed topics are general pulsed corona discharge application,corona-induced phenol oxidation, the oxidation mechanism of phenol, a presentation ofthe applied analysis techniques and a comparison of some advanced oxidationprocesses.

5.1. Pulsed corona discharges

Oxidation by corona in air

For the case of corona in air, possibly nitrate ions have been identified in the oxidationproduct mixture, see section 4.1.4. The amounts of nitrogen oxides produced by coronain air are small viz. a few ppm’s [114]. Nitrate seems to exhibit no ecotoxicity,according to the Microtox tests described in section 4.2.5 on corona exposed deionizedwater; the observed toxicity appearing from corona-exposure of deionized water is dueto H3O+ from nitric acid. The question arises, whether nitrogen might be introduced intothe oxidation products by means of nitro groups, resulting in a very significant increaseof toxicity. This should be accomplished by the attack of radical species like nitrogenoxide radicals, produced by oxidation of nitrite or nitrate ions. However, the electronaffinities of NO2 and NO3 are very high viz. 2.27 eV and 3.94 eV respectively [115],therefore the production of these radicals is highly unlikely. In addition, nitrite ions areoxidized to nitrate ions by oxidizers like the hydroxyl radical and ozone. NO has a lowelectron affinity (0.03 eV), but will also be oxidized to nitrate and exhibits poorsolubility in water. Compared to corona in oxygen, the application of corona in air, thushas no extra negative impact on the toxicity of the oxidation products. The productionof nitrate can be avoided by application of corona in an oxygen atmosphere, of coursefor the case of nitrogen-free target compounds.

Liquid/gas phase corona oxidation products

By application of corona in aqueous solution or in the gas phase (air, oxygen) over thesolution it is reasonable to argue, whether the way of application may give rise tooxidation product mixtures with different chemical composition. For the case ofaqueous phase corona, the discharge channels in the solution will locally createtransient extreme temperatures and pressures; in addition to oxidation by hydroxylradicals, the target compound may undergo pyrolytical decomposition, comparable tothe supercritical conditions favoured by ultrasonic irradiation or wet oxidation.For the case of corona in air or oxygen, in addition to hydroxyl radicals and ions, thecorona discharges produce ozone that diffuses into the liquid phase and oxidizes thetarget compound. Due to their high reactivity, hydroxyl radicals produced in the gasphase cannot reach the liquid, but may produce hydrogen peroxide over the solution.

114 Chapter 5.

Hydroxylation in aqueous solution is explained by the reaction of ozone, water and UVphotons or hydrogen peroxide and UV photons. At the gas-liquid interface, thedischarge streamers produce limited amounts of reactive species by ions/metastablesbombardment; pyrolytical decomposition of the target compound will not be relevant,due to the small contact area between the discharge channels and the target compoundsolution.

Additives

The conversion efficiency of pulsed corona discharges can likely be improved by theapplication of additives. Fe(II,III) salts invoke the decomposition of produced hydrogenperoxide into hydroxyl radicals according to Fenton chemistry, see Equations 1.3b-d.Extra hydrogen peroxide can be added, which is decomposed into hydroxyl radicals byUV photons produced by the corona. Application of corona in an oxygen atmosphereincreases the production of ozone, atomic oxygen and oxygen ions. In alkaline solution,ozone reacts by hydroxyl radical chemistry and weakly acidic compounds (e.g. phenols)then exist in anionic form that is preferentially attacked by the electrophilic oxidizerspecies. Nitrous oxide addition, as applied in radiolysis, may also yield extra hydroxylradicals for the case of corona treatment, according to Equation 1.6e. By turbulentmixing using a compressed gas, the gas-liquid interface area is increased, which is likelyto be favourable to conversion.

Corona-induced synthesis

It may be suggested that pulsed corona discharges can also be utilized for the synthesisof organic compounds. Application of pulsed corona discharges in an argon atmosphereproduces polyhydroxybenzenes in considerably higher amounts than corona in air, ashas been described in section 4.2.1. Although unwanted ring-cleavage takes place, thismay be suppressed by removal of remaining oxygen from the reactor by degassing oradditional purging with inert gas and minimizing the corona energy input. Ring-cleavagealso inevitably occurs during hydrogen peroxide-based commercial production ofpolyhydroxybenzenes from benzene or phenol. The advantages of pulsed coronadischarges over ex-situ hydrogen peroxide addition are a fine-tunable hydroxyl radicaldosage and the in-situ production of hydrogen peroxide which is very safe andconvenient.

By application of pulsed corona discharges to non-aqueous media under oxygen-freeconditions, the medium is dissociated into radical species, while the absence ofoxidizers prevents mineralization. For instance the exposure of a monomer to pulsedcorona discharges under oxygen-free conditions is likely to invoke polymerization; alsocoupling reactions may be performed between different reactants. The advantages ofcorona-induced synthesis to conventional synthesis may be simplified synthesispathways and reduced waste.

Of course, radical formation can also be achieved by pulse radiolysis or electron-beamtreatment. However these technologies are expensive, complex and demand shieldingof radiation, while corona application is straightforward and comparatively inexpensive.

Discussion 115

The synthesis of several organic compounds has been applied already for several yearsby electrochemical reactions viz. by electrolysis. Pathways include cathodic coupling(adiponitrile from acrylonitrile), cathodic hydrogenation (aniline from nitrobenzene) andanodic functionalization by hydroxylation (hydroquinone from benzene), halogenation(prefluorinated dialkyl ethers from dialkylethers) or oxygenation (dimethyl sulfoxide fromdimethyl sulfide) [116].

Corona discharges versus electrolysis

A major application of electrical energy for chemical purposes is electrolysis. Thedifferences between electrolysis and corona are very distinct:Electrolysis deals with low voltage (a few volts) and high current densities: typicalvalues for inorganic electrolysis are j=1-10 kA/m2 and for organic electrosynthesisj=0.1-1 kA/m2 [116]. Pulsed corona features high voltage and low time-averagedcurrents: typical values are a voltage of 10-100 kV and a time-averaged current of afew milliamperes in gaseous dielectrics. For the case of electrolysis both oxidation andreduction take place at the anode and cathode respectively; for the case of coronaeither oxidizing or reducing species can be produced, depending on the constitution ofthe dielectric medium where the discharges take place.Electrolysis is applied in molten salts, aqueous solution with electrolytes or for somecases in pure organic liquid phases; corona is applied either in the gas phase or inaqueous solution.The location where reactions occur is the electrode surface for the case of electrolysis;the corona discharge channel tips and -to a less extent- the discharge channels form avariable volume where reactive species are produced.For electrolysis, ion migration in solution and electron transfer at the electrodes takeplace; the application of pulsed corona discharges in the gas phase minimizes ionmigration, electron transfer occurs from the streamer head by the discharge channeltowards the electrode.Electrolysis is operated at DC voltage, while corona can be applied as either fastpositive polarity pulses (gas and water remediation) or at AC voltage (ozonizers).

An unconventional form of electrolysis is contact glow discharge electrolysis (CGDE)[117]. This phenomenon represents DC glow discharges across a gaseous envelopebetween anode or cathode and the surrounding aqueous, non-aqueous or molten phase.CDGE is initiated from normal electrolysis by increasing the voltage to a level, where adistinct current drop and growth of a gaseous sheath over an electrode are observed.The applied voltage is much higher than the voltage of normal electrolysis viz. somehundreds of volts, while the current is several milliamperes. By application of CGDE toaqueous solutions of e.g. ammonia [118] or ethanol [119], products are formed, whichare explained by the dissociation and ionization of liquid phase molecules by electronand ion bombardment.As far as known, CGDE has not been qualified as AOP, probably because of therequirement of a high solution conductivity and an unfavourable conversion efficiencydue to solvent evaporation by resistive heating.

116 Chapter 5.

5.2. Corona-induced phenol oxidation

Ozone to phenol ratio

An estimation has been made of the ratio R of ozone molecules consumed by phenol tothe amount of converted phenol molecules. Data have been obtained from ozonemeasurements over 100 ml deionized water and over a 100 ml 1.0 mM phenol solutionand a phenol oxidation experiment using a 100 ml 1.0 mM phenol solution. Allexperiments have been performed under equal conditions: V=25 kV, C=1 nF,f=100 Hz, d=1.0 cm.

The amount of ozone molecules consumed by phenol is calculated from the differenceof the ozone production efficiency for the case of 100 ml deionized water (GO2

water) andthe ozone production efficiency for the case of a 100 ml 1.0 mM phenol solution(GO2

1mM phenol) . The phenol conversion efficiency (Gphenol1mM) is known from Table 4.9. The

ratio calculation is shown by Equation 5.1. Regarding this estimation, the assumption ismade that phenol is exclusively converted by ozone.

mMphenol

phenolmMO

waterO

G

GGR 1

1

33−

= (5.1)

The ozone production rate is determined from the initial slope (S) of the ozone densityversus time plots given by Figure 4.11. The determined slopes are aboutSwater=1.54⋅1020 m-3s-1 and S1mM phenol=1.22⋅1020 m-3s-1. The ozone production efficiencyis calculated from the initial slope S (m-3/min), the pulse energy (Ep

water=3.5 mJ,Ep

1mM phenol=5.6 mJ), the pulse repetition rate (f=100 Hz) and the gas phase volume(Volg=253 cm3).

The ozone production efficiency for the case of 100 ml deionized water is aboutGO3

water=1.12⋅1017 J-1≡1.85⋅10-7 mol/J≡1.79 (100eV)-1, while the efficiency regardingthe 100 ml 1.0 mM phenol solution is about GO3

1mM phenol=5.50⋅1016 J-1≡9.13⋅10-8 mol/J≡0.88 (100 eV)-1. The amount of ozone molecules consumed by phenol equals ∆GO3=5.67⋅1016 J-1≡9.42⋅10-8 mol/J≡0.91 (100 eV)-1. The initial phenol conversion efficiencyis about Gphenol

1mM=1.44⋅1016 J-1≡2.39⋅10-8 mol/J≡0.23 (100 eV)-1. From these values ithas been calculated that about 4 ozone molecules are used to convert 1 phenolmolecule: R=4. A theoretical stoichiometric ratio is impossible to determine. The reasonfor this is the complex phenol oxidation mechanism, as will be discussed in section 5.3.

Hydroxyl radical production efficiency

An estimation of the order of magnitude of the hydroxyl radical production efficiencycan be made as follows. The initial slope of the phenol concentration versus timecourse indicates the rate at which phenol is converted at the start of corona oxidation.At that time, produced oxidizers are exclusively consumed by phenol and byrecombination but not yet by oxidation products.

Discussion 117

For the case of oxidation in argon, it may be assumed that phenol is mainly convertedby hydroxylation. The exact stoichiometric ratio is unknown but ranges theoreticallyfrom OH:phenol=1-5, although ratios higher than 1 are unlikely.Then, including radical loss processes and polyfold OH attack on phenol, a lower limitestimation of the hydroxyl radical production rate equals 1/5th of the phenol decreaserate in argon at t=0. This production rate and the measured energy input, which isassumed to be completely used for radical production, yield an approximation of thehydroxyl radical production efficiency.

The first order exponential fit of the phenol conversion versus time has been determinedfrom Figure 4.31, while the initial phenol concentration C(0)=1.0 mM, see Equation5.2. The pulse energy at time t=0 is about 10 mJ, see Table 4.9. Therefore at a pulserepetition rate f=100 Hz, 60 Joules are consumed every minute. The reactor volume is100 ml.

1351

0

1211

mindm mol108.1)0()(

min108.1)exp()0()(

−−−

=

−−

⋅−≈⋅−=

⋅≈⋅−=

Ckdt

tdC

ktkCtC

t

(5.2)

Then, the lower limit of the hydroxyl radical production efficiency GOH is estimated to be1/5x1.8⋅10-5 mol dm-3 min-1x0.1 dm3x1 min/60 J ≈ 6⋅10-9 mol OH/J ≡ 0.06 OH /100 eV.For the case of OH/phenol=1 stoichiometry, the efficiency is about 3⋅10-8 mol/J ≡0.29 OH/100 eV and is equal to about 345 eV/OH. By comparison of this value to thephenol conversion efficiency values listed by Tables 4.4, 4.5 and 4.9 and assuming thatphenol exclusively reacts with hydroxyl radicals, the OH-to-phenol ratio is globally 1-2.

Phenol conversion efficiency

A comparison of the observed efficiency values for the conversion of phenol is givenbelow. The relevant data are given by Tables 4.4, 4.5 and 4.9. It has appeared, thatthe efficiency of oxidation of a 500 ml 0.05 mM phenol solution at V=30 kV andf=50 Hz is considerably less favourable than the efficiency of oxidation of a 500 ml1 mM phenol solution at V=25 kV and f=100 Hz. The removal of phenol from dilutedaqueous solutions is probably less efficient, because here the highly reactive oxidizershave less probability to encounter a phenol molecule before they disappear byrecombination or decomposition. Ozone may decompose into oxygen by wallrecombination, by homogeneous catalysis by light and by nitrogen oxides producedfrom corona in air. In humid air, hydroxyl radicals react with nitrogen and oxygen toproduce nitrogen oxides. Hydroxyl radicals oxidize water to hydrogen peroxide.

Within this context it is also important to state, that the removal of target compoundsis likely to be more efficient, when pulsed corona discharges are applied to thin films ofthe aqueous target solution. This has been demonstrated by the conversionmeasurements on two oxidized phenol solutions having equal initial amounts of phenolbut a different solution volume and phenol concentration, see section 4.2.1. Theinfluence of the gas-liquid interface area on the mass transfer of oxidizers maycontribute to the differences between the efficiency values of Table 4.5 and 4.9.

118 Chapter 5.

Phenol conversion efficiency in corona literature

The phenol efficiency values, determined from this thesis, have been compared to someliterature references on phenol oxidation by liquid phase corona and ozonation. Theefficiency has been determined at 50% phenol conversion (G50). Table 5.1 shows theliterature references, an overview of the applied conditions and the phenol conversionefficiency [104].

Table 5.1 A comparison of phenol conversion efficiency G50 values, obtained fromliquid phase corona literature and from this thesis. Symbol description:C0=initial phenol concentration Vol=phenol solution volume, Ep=pulseenergy, f=pulse repetition rate, t50=oxidation time to reach 50% phenolconversion.

corona in solution corona in air over solution ozone

References [100]1) [101] [102] [120] Thesis [45]C0 (mM) 0.03 0.53 0.53 0.02 0.05 1.0 1.0 0.5Vol (cm3) 550 1000 2502) 500 500 100 500 4000Ep (mJ) 1750 800 8802) 30 13 6 10 -f (Hz) 60 50 48 50 50 100 100 -t50 (min) 180 260 7 180 26 83 237 8G50 (mol/J) 7.3·10-12 4.2·10-10 3.7·10-9 3.1·10-10 1.2⋅10-8 1.7⋅10-8 1.8·10-8 5.2·10-8

G50 (100eV)-1 7.0·10-5 4.1·10-3 3.6·10-2 3.0·10-3 1.2⋅10-1 1.6⋅10-1 1.7·10-1 5.0·10-1

1) The energy consumption inside the power supply is included in the pulse energy.2) Additional information [121]. In this reference the yield is given as G = 1.0·10-2 (100 eV)-1.

The phenol conversion efficiency by ozonation has been calculated using an ozoneconsumption of 61.5 mg/min x 8 minutes and assuming an ozone production efficiencyof 100 g/kWh.

It has been observed that the efficiency of phenol conversion by application of coronain air over the solution is one to two orders of magnitude higher than the efficiency ofliquid phase corona. In references [100-102] it is mentioned, that the liquid phase isexternally cooled during application of the corona discharges. The temperature increaseof the aqueous solution due to exposure to gas phase corona, has been measured to beunsignificant, see section 4.1.4, 4.2.1 and 4.2.4.

It is expected, that further optimization of the corona reactor will lead to a comparableor even higher efficiency than that of ozonation. First trial runs by corona in oxygenhave already yielded higher phenol conversion efficiency values than ozonation [104].This is explained by the broad range of reactive species that are produced by coronadischarges in air viz. hydroxyl radicals, ozone, atomic oxygen, UV photons, ions andmetastables.

Discussion 119

LIF spectra of oxidized phenol solutions

Oxidized phenol solutions, especially those exposed to corona in an argon atmosphere,show an increased fluorescence intensity at wavelengths in the range 400-500 nm, seeFigure 4.56. This fluorescence is not due to phenol, see Figure 4.47. This fluorescenceis partially explained by the presence of polyhydroxybenzenes. The strongestfluorescent polyhydroxybenzene is hydroquinone, which shows a fluorescencemaximum at about 335 nm but does not show fluorescence at wavelengths higher than450 nm. The trihydroxybenzene hydroxyhydroquinone shows a similar fluorescencecourse but at a much lower intensity.Other molecules that can explain for the fluorescence have to be in accordance with therequirements for fluorescence, see section 2.5.3. Muconic acid may likely be nocandidate fluorescent product, although it has a conjugated system of π electrons (it isa diene) and the carbon chain is cyclic and flat, because the carbon chain is not linked-up. Regarding the fluorescence range a remaining possibility is, that synthetic humicacids account for this fluorescence behaviour. These polymeric molecules viz. containhydroquinone monomeric units, see section 2.4.1. On the contrary the occurrence ofthese compounds in oxidized 1 mM phenol solutions is likely to be limited.

Microtox toxicity units

Although it has not been possible to determine the toxicity of the phenol oxidationproduct mixture by an effect concentration EC20 value, yet a quantitative estimation ofecotoxicity can be made by means of a toxicity units (TU) calculation [122]. The TUx

value describes the amount of dilution that is needed for a sample to reach a definedeffect percentage x. High TU values imply high toxicity. The TUx value is calculated bysummation of the quotients (tux,i) of component concentration and component ECx valuefor each product mixture component i, see Equation 5.3. The general assumption ismade that the toxicity of the individual components is additional.

∑∑ =

=

componentsix

components ixx tu

ECtC

TU ,

)((5.3)

A TU50 calculation has been performed with regard to the quantitative ICE analysis of1.0 mM phenol solutions exposed to pulsed corona discharges in air and argon,described in section 4.2.1. However, this calculation is a rather rough estimation,because some oxidation products have not been identified. Also the actual catechol andresorcinol concentrations are not known, because these components could not beseparated; the same is true for hydroquinone and 1,4-benzoquinone. For a worst caseapproach it has been assumed that only hydroquinone and catechol represent thedihydroxybenzenes; resorcinol viz. has the lowest ecotoxicitity among thedihydroxybenzenes. 1,4-benzoquinone and hydroquinone have comparable ecotoxicity.Muconic acid is likely to be present but neither Microtox ecotoxicity nor concentrationsare known. Finally, the addition of individual toxicity units is only justified, if thecomponents show comparable toxicity effects. Although this assumption isquestionable, the essence of ecotoxicity change during oxidation is demonstrated.

120 Chapter 5.

A global ecotoxicity estimation has been performed on the phenol oxidation productmixture, obtained after 1 hour and 2 hours of exposure to corona in air and argon, usingthe EC50 values from Table 2.1. The following compounds are concerned: phenol,hydroquinone, catechol, formic acid, oxalic acid and glyoxylic acid. From the differenthydroquinone effect values reported in literature [78], an average value EC50

5min ≈ 0.061mg/l has been used for the calculations. Table 5.2 shows the individual componenttoxicity contributions (tu50) and the total mixture ecotoxicity (TU50).

Table 5.2 An estimation of the toxicity unit value TU50 of 100 ml 1.0 mM phenolsolutions, after exposure to pulsed corona discharges in air or argon fort=60 min and t=120 min. The corona parameters are V=25 kV,C=1 nF, f=100 Hz, d=1.0 cm.

t=60 min t=120 minCorona in airC (mg/l) tu50 (-) C (mg/l) tu50 (-)

Phenol 58.0 2.0 40.8 1.4Hydroquinone 0.4 6.6 0 0Catechol 13.6 0.4 8.6 0.3Formic acid 5.9 0.7 10.8 1.4Oxalic acid 31.2 2.8 34.9 3.1Glyoxylic acid 3.1 0.3 4.0 0.4

TU50= 12.8 TU50= 6.6

t=60 min t=120 minCorona in ArC (mg/l) tu50 (-) C (mg/l) tu50 (-)

Phenol 34.0 1.2 11.4 0.4Hydroquinone 2.1 34.4 1.5 24.6Catechol 35.2 1.1 29.1 0.9Formic acid 1.7 0.2 4.0 0.5Oxalic acid 0 0 0 0Glyoxylic acid 0 0 2.2 0.2

TU50= 36.9 TU50= 26.6

The total ecotoxicity of the phenol oxidation product mixture seems to be mainlycaused by hydroquinone or benzoquinone, due to their very low EC value. The oxidationof phenol by corona in argon produces much higher amounts of dihydroxybenzenes thanthe oxidation of phenol by corona in air, which results in high ecotoxicity as is reflectedby the high TU values. Regarding the fact, that at t=0 minutes TU50=3.5, the totalecotoxicity initially increases considerably but decreases again during oxidationprogress.

Finally, the ecotoxicity is discussed for some hypothetical situations, where a 1.0 mMphenol solution is completely converted into one of the described oxidation products,according to stoichiometry, see Table 5.3. By this approach, a comparison of phenoloxidation products ecotoxicity is illustrated.

Discussion 121

Table 5.3 Ecotoxicity of hypothetical situations, where a 1 mM phenol solution iscompletely converted into one of the described oxidation products,according to stoichiometry.

Oxidation productfrom 1 mM phenol

Stoichiometry(mM)

TU50

(-)Hydroquinone 1 1805.1Catechol 1 3.4Resorcinol 1 0.3Formic acid 6 34.9Oxalic acid 3 23.9Glyoxylic acid 3 19.8

The stoichiometric conversion of a 1 mM phenol solution (TU50=3.3) into a 1 mMhydroquinone solution thus creates the most dramatic ecotoxicity, while the conversionof a 1 mM phenol solution into a 1 mM resorcinol solution should result in a decrease ofecotoxicity according to literature EC50 values [78]. The ecotoxicity increase byconversion of a 1 mM phenol solution into a solution of the mentioned carboxylic acidsat stoichiometric concentration, is maximum 2 % of the ecotoxicity increase due to theformation of a 1 mM hydroquinone solution.

Final oxidation products

Phenol oxidation has only been performed during a limited time range in order to ensurestable oxidation conditions, which is important for accurate efficiency measurements.By continued oxidation of the investigated product mixtures by corona in air, theobserved polyhydroxybenzenes disappear by ring-cleavage, which is favourable toecotoxicity. Also the possibly present quinones will be converted. The produced ring-cleavage products, viz. polyfunctional aliphatic hydrocarbons, gradually degrade andfinally all organic carbon has been converted into carbon dioxide. If all carbon dioxidehas left the water, ecotoxicity has disappeared. Theoretically, remaining minorecotoxicity may appear from carbonic acid i.e. pH toxicity. However, carbonic acid is aweak acid: pKa,I=6.46 and pKa,II=10.22; only about 1 % of the dissolved carbondioxide appears to react with water [123].Continued degradation by corona in argon will also eventually result in mineralization,due to hydroxylation and argon ions & metastables bombardment. By absence ofoxygen, oxygenation now mainly proceeds by hydroxyl radicals, originating from thedissociation of water. Without detailed information about the composition of the phenoloxidation product mixtures, it is not justified to state that oxidation of phenol by coronain argon results in oxidation products with a different oxygen content than the productsobtained from oxidation of phenol by corona in air. On the contrary it is clear, that themineralization rate due to corona in argon is different from the mineralization rate due tocorona in air.

Previous to complete mineralization, a large variety of organic oxidation products exists,which will be discussed in section 5.3.

122 Chapter 5.

5.3. Phenol oxidation pathways

A complete description of the oxidation mechanism of phenol, containing all possibleintermediate products is probably unattainable, because of the huge amount of likelyreactions involved in radical-induced oxidation. Simplification of the oxidationmechanism is only possible, when the most relevant intermediates have been identifiedin the oxidation product mixture. Although a considerable number of oxidation productshas been found by ICE chromatography, the revelation of the identity by massspectrometry has appeared to be very complicated.Therefore, from a theoretical point of view the oxidation of phenol by hydroxylradicals/oxygen and ozone is described in this section. The deduction has beenperformed by analogy with the studies of Pan and Von Sonntag on the oxidation ofbenzene & alkenes [19,50] and fundamental ozone chemistry according to Bailey [49].

The attack of the hydroxyl radical and oxygen on phenol

The action of the hydroxyl radical is always reported in combination with oxygen. Thehydroxyl radical concentration is very low due to its high reactivity, while oxygen ispresent in high excess amounts: at T=293K the solubility of oxygen in water is about1.4 mM [44].Attack of the hydroxyl radical on the benzene ring of phenol, produces 1,2- and 1,4-dihydroxycyclohexadienyl (DHCHD) radicals, see Figure 5.1. The formation of the 1,3-DHCHD radical is not discussed in literature, probably because this radical is less stable.Oxygen adds to these radicals to form dihydroxycyclohexadienylperoxyl (DHCHDP)radicals. For easy reference, the intermediates and products are coded with a numberaccording to the attack position of the hydroxyl radical and oxygen.

OH

OH

O2

O2

1,2-DHCHD DHCHDP-12 radical radical

1,4-DHCHD DHCHDP-14 radical radical

5

4

4

3

21

OHOH H

OH

•

HOH

OH OO•

HOH

OH OO•

OHH

OH

•

Figure 5.1 The production of 1,2- and 1,4-dihydroxycyclohexadienyl (DHCHD)radicals from the attack of the hydroxyl radical on phenol. Also shown isthe production of dihydroxycyclohexadienylperoxyl (DHCHDP) radicalsfrom the attack of oxygen on the DHCHD radicals.

Discussion 123

The DHCHDP radical may react in different ways, see Figure 5.2. Catechol can beproduced from the DHCHDP-12 radical by elimination of a hydroperoxyl radical. For thecase of the DHCHDP-14 radical, hydroperoxyl elimination is less probable due to thelarger -H to •OO- distance compared to the DHCHDP-12 radical; hydroquinone is likelyto be formed from the decomposition of a tetraoxide (T) that is produced from thedimerization of two DHCHDP-14 radicals.Also possible is the formation of α,α’-endoperoxyalkyl (EPA) radicals. This reaction isreversible, because of the very weak endoperoxyl bond. The EPA radicals may scavengeoxygen again and then endoperoxyalkylperoxyl (EPAP) radicals are produced accordingto an irreversible reaction.

O2

OH

OH

catechol

OH H

OH

OO

H•

EPA-14 radical EPAP-14 radical

HO2•+

OH

OO

HOH

H•

O2

EPA-12 radical EPAP-12 radical

DHCHDP-14

DHCHD-12

HOH

OH OO•

HOH

OH OO•

OH H

OH

OO

H

OOH

•

OH

OO

HOH

OO

HH

•

T

hydroquinone

OH

OH

2x

Figure 5.2 The production of dihydroxybenzenes or α,α’-endoperoxyalkyl (EPA)radicals from dihydroxycyclohexadienyl (DHCHD) radicals. Also illustratedis the formation of endoperoxyalkylperoxyl (EPAP) radicals from the attackof oxygen on EPA radicals.

The produced dihydroxybenzenes hydroquinone and catechol will also undergo oxidationand yield trihydroxybenzenes and ring-cleavage products. These pathways are notdiscussed, because they are comparable to the described mechanisms.

124 Chapter 5.

The dimerization of EPAP radicals produces a tetroxide (T), that decomposes into twoendoperoxides (EP) and oxygen, see Figure 5.3. The unstable endoperoxides undergoring-cleavage; regarding EP-14b/12b, ring-cleavage occurs together with the eliminationof carbon monoxide. In this way, aliphatic monounsaturated C5 & C6 hydrocarbons (P-14/12) with carboxyl-, aldehyde-, ketone- or alkanol-functional groups are produced.

OH H

OH

OO

OH

H

OH H

OH

OO

O

OH

OO

HOH

OH

H

OH

OO

HOH

O

O2

O2

OH H

OH

OH OH

OH

OH H

OH

OH

O

H

OHOH

OHO

OHO

OHO

H H-CO

-H2O

-CO

-H2OOH

HOH

OHOH

OHHOH

OH

OH

O

O

OHHOH

O

H

OHHOH

OH

HO

OH

OH H

OH

OOHH

O

H

T

T

EPAP-14

EPAP-12

2

2

EP-14a

EP-14b

EP-12a

EP-12b

P-14dP-14c

P-14e

P-14f P-14g

P-12c P-12d

P-12e

P-12f

OH H

OH

OO

H

OOH

•

OH

OO

HOH

OO

HH

•

Figure 5.3 The dimerization of endoperoxyalkylperoxyl (EPAP) radicals produces atetroxide (T) that decomposes into endoperoxides and oxygen. Theendoperoxides undergo ring-cleavage and aliphatic monounsaturatedC5 & C6 hydrocarbons (P-14/12) with polyfunctional groups are produced.

Discussion 125

The monounsaturated products P-14/12 can also be attacked by a hydroxyl radical andoxygen. Also ozone attack is very likely but is not discussed here. Every product P canproduce two hydroxyperoxyl C5 & C6 radicals HP-14/12, because the hydroxyl radicaland oxygen can attack on both sides of the alkene bond, see Figures 5.4 and 5.5.

OH H

OH

OH

O

OHO

OHO

H H

OH H

OH

OOHH

O

H

O2O

H

H

O

HOH•

OH

OH

OH+ +2 2

•

O

H

HOHH

OHOH

O

H

OO2+ +2 2

+OH O2

+OH O2

O

HOH

H

HOHH

OO

OH

OH

H•

O2+ +2 2

O

OH

H

OOH

HOH H

O

H OHH

•

2 2 O2+ +

+OH O2

O

H

OH

H

H

O

O

OH

OH

H•2 2+ + O2

O

OH

H

O •

O

H

OHH

H

OH

2 2+ + O2

T

2x

T

2x

T

2x

T

2x

T

2x

T

2x

P-14d

P-14e

P-14g

αHA-14d1

αHA-14d2

αHA-14e1

αHA-14e2

αHA-14g1

αHA-14g2

monounsaturated saturated saturated polyfunctional hydroxyperoxyl polyfunctional α-hydroxyalkylC5,C6 hydrocarbons C5,C6 radicals C2,C3,C4 hydrocarbons C2,C3,C4 radicals

HP-14d1

HP-14d2

HP-14e1

HP-14e2

HP-14g1

HP-14g2

P-14d1-I

P-14d2-I

P-14e1-I

P-14e2-I

P-14g1-I

P-14-g2-I

OH H

OH

OOHH

O

HHOO

HOH

•

OH H

OH

OH

OOOHOHH

•

OH H

OH

OH

OOH

HOO

H•

OHO

OHO

H HH

OOH

OH

•

OHO

OHO

H HHOHHOO•

OH H

OH

OOHH

O

H

HOO

HOH

•

Figure 5.4 The attack of the hydroxyl radical and oxygen on the monounsaturatedring-cleavage products (P-14) produces hydroxyperoxyl C5 & C6 radicals(HP-14). Dimerization of HP-14 produces a tetraoxide intermediate (T)that decomposes into polyfunctional saturated hydrocarbons (P-14-I), α-hydroxyalkyl radicals (αHA-14) and oxygen.

126 Chapter 5.

The HP-14/12 radicals dimerize to a tetraoxide, that decomposes into saturatedaliphatic polyfunctional C2-C4 hydrocarbons (P-14/12-I), saturated aliphatic α-hydroxyalkyl-functional C2-C4 radicals (αHA-14/12) and oxygen.

HOH

OHOH

O

O

OHHOH

OH

OHHOH

OH

HO

OH

+OH O2

H

O

O

H

•

OH

O

HOH

OH H

O2+ +2 2

OHOH

H O

H

O•OOH

H

H2 2+ + O2

+OH O2

OO

H

HHOH

HOH

OH H

OH

O

•

2 2+ + O2

O

OHHOH

O H •O

HHOH

H

OH2 2 + O2+

+OH O2

O

H

H

O O

OHHOH

OH H•

2 2+ + O2

O

OHHOH

O H

•O

H

H

OH2 2+ + O2

T

2x

T

2x

T

2x

T

2x

T

2x

T

2x

αHA-12d1

αHA-12d2

αHA-12e1

αHA-12e2

αHA-12f1

αHA-12f2

P-12d

P-12e

P-12f

monounsaturated saturated saturated polyfunctional hydroxyperoxyl polyfunctional α-hydroxyalkylC5,C6 hydrocarbons C5,C6 radicals C2,C3,C4 hydrocarbons C2,C3,C4 radicals

HP-12d1

HP-12d2

HP-12e1

HP-12e2

HP-12f1

HP-12f2

P-12d1-I

P-12d2-I

P-12e1-I

P-12e2-I

P-12f1-I

P-12f2-I

HOH

OHOH

O

HOH

OO H•

HOH

OHOH

O

HOO

HOH•

OHHOH

OH

HO

OH

HOHHOO•

OHHOH

OH

HO

OH

HOOHOH

•

O

OHHOH

OH

HOH

HOO•

O

OHHOH

OH

HOO

HOH•

Figure 5.5 The attack of the hydroxyl radical and oxygen on the monounsaturatedring-cleavage products (P-12) produces hydroxyperoxyl C5 & C6 radicals(HP-12). Dimerization of HP-12 produces a tetraoxide intermediate (T)that decomposes into polyfunctional saturated hydrocarbons (P-12-I), α-hydroxyalkyl radicals (αHA-12) and oxygen.

Discussion 127

Finally, by scavenging oxygen, the α-hydroxyalkyl radicals (αHA-14/12) are convertedinto α-hydroxyalkylperoxyl radicals (αHAP-14/12), see Figure 5.6.

•

OH

OH

OH

•

O

H

HOHH

OH

O

OH

OH

H•

OH

HOH H

O

H OHH

•

•

O

H

OHH

H

OH

αHA-14d1

αHA-14d2

αHA-14e1/14g1

αHA-14e2

αHA-14g2/12e2

•

OH

O

HOH

OH H

•OOH

H

H

HOH

OH H

OH

O

•

αHA-12d1

αHA-12d2/12f2

αHA-12e1/12f1

OH

OH

O+ •HO2

O2

O2+ •HO2

O

H

HOHH

O

O2O

OHO

H+ •HO2

O2

O

HOH H

O

H OHH

+ •HO2

O2

O

H

OHH

H

O+ •HO2

OH

O

HOH

O H

O2 + •HO2

O2

OO

H

H

+ •HO2

O2 HOH

O H

OH

O + •HO2

α-hydroxyalkyl α-hydroxyalkylperoxyl saturated polyfunctional radicals radicals C2,C3,C4 hydrocarbons

P-14d1-II

P-14d2-II

P-14e1/14g1-II

P-14e2-II

P-14g2/12e2-II

P-12d1-II

P-12d2/12f2-II

P-12e1/12f1-II

αHAP-14d1

αHAP-14d2

αHAP-14e1/14g1

αHAP-14e2

αHAP-14g2/12e2

αHAP-12d1

αHAP-12d2/12f2

αHAP-12e1/12f1

OH

OH

OHOO•

O

H

HOHH

OHOO•

O

OH

OH

HOO•

OH

HOH H

O

H OHH

OO•

O

H

OHH

H

OHOO•

OH

O

HOH

OH HOO•

OOH

H

H

OO•

HOH

OH H

OH

O

OO•

Figure 5.6 The attack of oxygen on α-hydroxyalkyl radicals produces α-hydroxyalkyl-peroxyl radicals that decompose into saturated polyfunctional C2-C4

hydrocarbons by elimination of a hydroperoxyl radical.

128 Chapter 5.

The α-hydroxyalkylperoxyl radicals (αHAP-14/12) decompose into saturated aliphaticC2-C4 hydrocarbons with carboxyl-, aldehyde-, ketone- or alkanol-functional groups byelimination of hydroperoxyl radicals. A summary of all presented saturatedpolyfunctional oxidation products of hydroxyl radical & oxygen - induced phenoloxidation is given by Figure 5.7. The occurrence frequency of the compounds isindicated by the compound code used in the oxidation pathways.

O

H

H

O

HOHP-14d1-I, P-14d2-II

O

OHHOH

O H

P-12e2-I, P-12f2-I, P-12e1/12f1-II

OH

O

H

O

P-14d2-I, P-14d1-II

H

O

O

HP-12d1-I, P-12f1-I, P-12d2/12f2-II

O

OH

H

O P-14e2-I, P-14g2-I, P-14e1/14g1-II

O

H

OH

H

H

O

P-14g1-I, P-12e1-I, P-14g2/12e2-II 3-hydroxy-1,2-propanedione(88 g/mol)

hydroxymalonaldehyde(88 g/mol)

ketomalonaldehyde(86 g/mol)

glyoxal(58 g/mol)

glyoxylic acid(74 g/mol)

2-hydroxy-3-oxopropionic acid(104 g/mol)

OHOH

H O

H

OP-12d2-I, P-12d1-II 2-hydroxy-1,3,4-butanetrione

(116 g/mol)

O

HOH

H

HOHH

OP-14e1-I, P-14e2-II 2,4-dihydroxy-1,3-butanedione

(118 g/mol)

Figure 5.7 Saturated polyfunctional C2, C3 & C4 hydrocarbons produced from theoxidation of phenol by hydroxyl radicals and oxygen.

Discussion 129

The attack of ozone on phenol

Next, the electrophilic addition of ozone on phenol is described. Although the phenolateanion (C6H5O-) reacts considerably faster with ozone than phenol [7], these reactionshave not been described here, because this anion only exists in an alkaline environment.The two most likely ozone attack positions of the phenol molecule are the 2,3- and 3,4-position. The 1,2- position is not considered here, because the hydroxyl group is likelyto cause sterical hindrance. For easy reference, now all intermediates and products arecoded with a number according to the attack position of ozone.The attack of ozone on phenol produces the molozonides M-23 and M-34, see Figure5.8. These molozonides rearrange immediately to the ozonides O-23 and O-34. Theunstable ozonides decompose by ring-cleavage to zwitterion-functional products. Thezwitterion group can be formed on both sides of the former attack position, resulting infour compounds viz. Z-23a/b and Z-34a/b. In water, the zwitterion groups hydrolyze tohydroxyalkylhydroperoxyde-functional ring-cleavage products (H-23a/b and H34a/b).

O3

O

OHO

O

OC

+

OHO

O

H

H

OH

OO O

C+

OH

OO

OHHO3

C+

OH

O

O

OHH

C+

OH

OO

H

HO

OH

O

OOHOH

H

H

OH

OH

O

OOH

H

H

OH

O

OOHOHH

H

OH

OHHOO

HHO

H-23a

H-23b

H-34a

H-34b

M-23

M-34

Z-23a

Z-23b

Z-34a

Z-34b

OH

OO

O

OH

OO

O

O-23

O-34

5

4

4

3

21

OH

Figure 5.8 The attack of ozone on the 2,3- or 3,4-position of phenol initiallyproduces molozonides (M-23/34), which rearrange immediately toozonides (O-23/34). The unstable ozonides decompose to zwitterion-functional ring-cleavage products (Z-23/34). Hydrolysis of the zwitterion-functional products yields hydroxyalkylhydroperoxide-functional ring-cleavage products (H-23/34).

130 Chapter 5.

The first generation hydroxyalkylhydroperoxides (H-23ab and H-34a/b) decomposes intomonounsaturated C6 ring-cleavage products (P-23c/d/e and P-34c/d/e) by release ofwater or hydrogen peroxide, see Figure 5.9. Every set of hydroperoxides produces threedifferent products P, because P-23d and P-34d can be produced from both of thehydroperoxides. The markers c/d/e identify the six possible products by the position ofa carboxyl or aldehyde endgroup.

OH

O

OOHOHHH

OH

OH

O

OOH

HH

OH

O

OOHOH

HH

OH

OHHOO

OH

H

H-23a

H-23b

H-34a

H-34b

H2O2

H2O

H2O

H2O2

H2O

H2O

O

O

OOHH

O

OOHO

H

O

OOH

H

O

O

O

OHH

O

O

OH

H

O

OHO

OH

P-23c

P-23d

P-23e

P-34c

P-34d

P-34e

+

+

+

+

+

+

Figure 5.9 The decomposition of the first generation hydroxyalkylhydroperoxides(H23 and H34) into monounsaturated C6 ring-cleavage products (P23 andP34) with carboxylic acid- or aldehyde-functional endgroups by release ofwater or hydrogen peroxide respectively.

Discussion 131

The monounsaturated C6 ring-cleavage products P23c/d/e and P34c/d/e can also beattacked by ozone. Attack by a hydroxyl radical and oxygen is also possible, but is notdiscussed here. Then, a second generation molozonides M-45c/d/e, M-56c/d/e andozonides O-45c/d/e, O-45c/d/e is produced, see Figures 5.10 and 5.11. Decompositionof the second generation of ozonides splits the former phenol molecule into two parts.A second generation zwitterions (Z-45f/g/h/i and Z-56f/g/h/i) is produced, of which theZ-45g/56g and Z-45h/56h zwitterions can be formed in two ways. Together with thezwitterions a first generation of saturated polyfunctional C2 & C4 hydrocarbons isproduced (P-45c1/c2/d1/d2/e1/e2-I and P-56c1/c2/d1/d2/e1/e2-I).

O

O

OOHH

O

O

O

OHH

O

O

OH

H

O

OOOO

OOHH

O

OOOO

O

OHH

O

OOOO

OH

H

O

O

O

OOH

OOH

O

O

O

O

OO

H

H

O

O

O

O

OO

OHH

O3

O3

O3

C+

O

O O

OOH

H

C+ O

O O

H

H

OO

H

H

O

O

OH

OH

C+

O

O O

O

H

C+ O

O O

H

H

OO

H

H

O

O

O HH

+

+

+

+

C+

O

O O

OH

H

C+

O

O O

OHH

O

O

OH

H

OO

OH

H+

+

M-45c

M-45d

M-45e

O-45c

O-45d

O-45e

P-23c

P-23d

P-23e

Z-45f

Z-45g

Z-45h

Z-45g

Z-45h

Z-45i

P-45c1-I

P-45c2-I

P-45d1-I

P-45d2-I

P-45e1-I

P-45e2-I

Figure 5.10 Ozone attack on the 4,5-position of the monounsaturated C6 ring-cleavageproducts (P-23) yielding a second generation molozonides (M-45) andozonides (O-45). The ring-cleavage of the 4,5-ozonides produces a secondgeneration zwitterions (Z-45) and a first generation saturated C2 & C4

hydrocarbons (P-45-I)

132 Chapter 5.

O

OOHO

H

O

OO

H H

O

OHO

OH

O

O

OOH

O

OO

H

O

O

OO

O

OHH

O

OHO

O

O

OO

H

O

O

OH

OO

O

OH

O

O

O

O

O

O

HH

O

O

O

O

OHO

OH

O3

O3

O

C+

O

OHO

O

H

C+

OO O

H

H

OO

H

H

O

O

OHOH

O

C+

O

OO

HH

C+

OO O

H

H

OO

H

H

O

O

OH

H

C+

O

OO

O

HH

C+

OO

OH

O

H

OO

OH

H

O

O

O HH

+

+

+

+

+

+

M-56c

M-56d

M-56e

O-56c

O-56d

O-56e

P-34c

P-34d

P-34e

Z-56f

Z-56g

Z-56h

Z-56g

Z-56h

Z-56i

O3 P-56c1-I

P-56c2-I

P-56d1-I

P-56d2-I

P-56e1-I

P-56e2-I

Figure 5.11 Ozone attack on the 5,6-position of the monounsaturated C6 ring-cleavageproducts (P-34), yielding a second generation molozonides (M-56) andozonides (O-56). The ring-cleavage of the 5,6-ozonides produces a secondgeneration zwitterions (Z-56) and a first generation saturated C2 & C4

hydrocarbons (P-56-I)

The second generation zwitterions (Z-45 and Z-56) hydrolyzes to a second generationhydroxyalkylhydroperoxides (H-45f/h, H-56f/h, H-4556g/i), which decompose into asecond generation saturated polyfunctional C2 & C4 hydrocarbons (P-45f1/f2/h1/h2-II,P-56f1/f2/h1/h2-II, P-4556g1/g2/i1/i2-II) by elimination of water or hydrogen peroxide,see Figure 5.12.

Discussion 133

O

O

OH

OH

OO

OH

H

C+

O

O O

OOH

H

C+ O

O O

H

H

OOOH

OH OOHH

O

OOH

OH

H

H

OO

H

H

O

O

OH

OHO

C+

O

O O

OH

H

O

O

OH OOHH

H

O

O

O OHH

O

O

OH

H

C+

O

OO

OHH

O

OOH

OHOH

H

OO

OH

H

O

OH

O

OH

Z-45f

Z-45g=Z-56g

Z-45h

Z-45i=Z-56i

H2O2

H2O

H2O

H2O2

H2O

+

+

+

+

+

H2O2+

H2O

H2O2

+

+

H-45f

H-45g=H56g

H-45h

H-45i=H-56i

O

C+

O

OHOO

HO

OH

O

HOO

OHH

O

O

OH

OHO

O

O

OHOHZ-56f H-56f

O

C+

O

OOH

HO

O

HOO

OHH

H O

O

OH

H

O

O

OH

O H

Z-56h H-56h

H2O2

H2O

H2O2

H2O

+

+

+

+

P-45f1-II

P-45f2-II

P-56f1-II

P-56f2-II

P-45g/56g1-II

P-45g/56g2-II

P-45h1-II

P-45h2-II

P-56h1-II

P-56h2-II

P-45i/56i1-II

P-45i/56i2-II

Figure 5.12 Hydrolysis of the second generation zwitterions (Z-45 and Z-56). Alsoillustrated is the decomposition of the second generationhydroxyalkylhydroperoxides (H-45 and H-56) into a second generationsaturated C2 & C4 ring-cleavage products (P-45/56-II) with carboxylic acid-or aldehyde-functional endgroups, by release of water or hydrogenperoxide respectively.

134 Chapter 5.

A summary of all presented saturated polyfunctional oxidation products of ozone-induced phenol oxidation is given by Figure 5.13. The occurrence frequency of thecompounds is indicated by the compound code used in the oxidation pathways.

OO

H

H

P-45c1-I, P-45d1-I, P-56c1-I,P-56d1-I,P-45g/56g2_II

OO

OH

H

P-45e1-I, P-56e1-I, P-45g/56g1-II,P-45i/56i2-II

O

O

O HH

P-45d2-I, P45-e2-I, P-56d2-I,P-56e2-I, P-45h2-II, P-56h2-II

O

O

OH

O H

P-45c2-I, P-45f2-II, P-56h1-II

O

O

OHOH P-56c2-I, P-56f2-II, P-45h1-II

O

OH

O

OH

P-45i/56i1-II

O

O

OH

OHO

P-45f1-II, P56f1-II

glyoxal(58 g/mol)

glyoxylic acid(74 g/mol)

oxalic acid(90 g/mol)

1,2,4-butanetrione(100 g/mol)

2,4-dioxobutyric acid(116 g/mol)

3,4-dioxobutyric acid(116 g/mol)

oxo-succinic acid(132 g/mol)

Figure 5.13 Saturated polyfunctional C2 & C4 hydrocarbons produced from theoxidation of phenol by ozone.

Discussion 135

The described reaction chemistry of the oxidation of phenol by hydroxyl radicals,oxygen and ozone accounts for the formation of a broad range of polyfunctionaloxidation products, but is still incomplete. Many of the described products are notstable and will further degrade due to the rigorous oxidizing conditions. With regard tothe possible oxidation products, a following summary can be made:

The oxidation of phenol by hydroxyl radicals and oxygen initially yieldsdihydroxybenzenes and monounsaturated aliphatic C5 and C6 hydrocarbons. Thedihydroxybenzenes are either oxidized to trihydroxybenzenes or undergo ring-cleavagelike phenol. Repeated attacks of the hydroxyl radical and oxygen result in the formationof saturated aliphatic C2, C3 and C4 hydrocarbons with carboxyl-, aldehyde-, ketone-and/or alkanol-functional groups.The oxidation of phenol by ozone initially yields di- and monounsaturated aliphatic C6

hydrocarbons. Attack of ozone on these ring-cleavage products yields saturatedaliphatic C2 and C4 hydrocarbons with carboxyl-, aldehyde- and/or ketone-functionalgroups.

Glyoxal, glyoxylic acid and oxalic acid are well-known stable oxidation products ofphenol, see section 2.4.1. In addition these products form an oxidation decay series tocarbon dioxide. Ketomalonaldehyde and hydroxymalonaldehyde are precursors toketomalonic acid, which is described in literature, see section 2.4.1. The products withaldehyde-functional endgroups will be oxidized to carboxylic acids. Carboxyaldehydesare mentioned in literature [124] as products from the cleavage of aromatic compoundsby ozone. With regard to the produced oxocarboxylic acids (α: RC(O)COOH, β:RC(O)CH2COOH, γ: RC(O)-(CH2)2COOH) the α-form is relatively stable, the β-formdecomposes by decarboxylation to corresponding ketones and the γ-form is again stableand does not decarboxylate [125]. β-hydroxy aldehydes (R-C(OH)-CH2-CHO) generallydimerize or polymerize [126].

Oxalic acid, glyoxylic acid and polyhydroxybenzenes have been identified in this study,see section 4.2.1. Although the other observed products viz. formic, acetic, propionic,malonic, succinic and maleic acid cannot be directly traced in the presented oxidationpathways, these compounds are eventually explained from continued degradation of theproducts described in the models or from combined OH/O3 oxidation reactions.

The phenol degradation pathways have been described for either oxidation by thehydroxyl radical or ozone, in order to present a clear model. For the case of practicalconditions for corona oxidation in humid air, all described reactions occur mixed up andalso other reactive species will be present like e.g. singlet oxygen, metastables, ionsand UV photons.

Modeling of the reaction kinetics of phenol oxidation by pulsed corona discharges isonly meaningful, if the relevant reaction pathways are known from experimentalobservations. This has been the main problem, because both separation andidentification of the theoretically possible intermediates/products is analyticallyconsidered to be very difficult. Detailed clarification of the phenol oxidation mechanismby e.g. using labelled compounds, blocking of reactive molecular sites and chemicalderivatization reaches beyond the scope of this thesis.

136 Chapter 5.

Toxicological aspects of phenol degradation

The discharge of oxidized phenol solutions into surface waters may lead to oxygendepletion, due to the presence of polyhydroxybenzenes, which are strong reducingagents. Hydroquinone and 1,4-benzoquinone show very high ecotoxicity according tothe Microtox test. Although several carboxylic acids produced during mineralization ofphenol occur in nature, a concentrated discharge flow causes ecotoxicity due to acidity.Although direct human intoxification by phenol oxidation products is not likely, indirectexposure via the food chain may eventually occur. Therefore a short discussion abouthuman toxicity is presented [1,61,127-129]:

All hydroxybenzenes cause irritation or etching of skin and mucous membranes and areskin poisons. After resorption respirational paralysis takes place. 1,4-benzoquinonecauses severe eye injury. Global fatal doses are: phenol 5-10 g, resorcinol 12 g,hydroquinone 5-12 g, pyrogallol 2-3 g. Hydroxybenzenes exhibit mutagenic properties,this means that these compounds cause chromosomic and genetic abberations;mutagenic activity is reported according to the order hydroquinone >hydroxyhydroquinone > pyrogallol.

For the dimerization products 2,2’- and 4,4’-dihydroxybiphenyl human mutation dataare reported. The multiring condensation product dibenzo-p-dioxin is a questionablecarcinogenic compound.

Although endoperoxides cannot be isolated from the oxidation product mixture becauseof their instability thus reactivity, these compounds probably are the most harmfulintermediates. Endoperoxides may also release singlet oxygen which is highly reactivetowards physiological materials [43].

Unsaturated carboxylic acids like muconic, maleic, fumaric and acrylic acid are harmfuldue to their reactivity of the alkene bond. By metabolysis, the alkene bond may beconverted into a carcinogenic epoxide, as has been explicitly described for benzene andpolyaromatic hydrocarbons. The epoxide causes DNA damage, because it reacts withnucleic acids.

Aldehydes exhibit mutagenic properties, cause strong irritation of the mucousmembranes and skin and act on the central nervous system; examples areformaldehyde, acetaldehyde, acrolein and glyoxal. Unsaturated aldehydes (enals) arereported to be particularly reactive with some biological molecules [124].

Formic and acetic acid are entitled as etching protoplasma poisons due to their acidity,but their salts show low toxicity. Skin poisoning is possible for the case of formic acid,because of fat-dissolving properties. Acetic acid exhibits a lethal dose value of about20-50 g. In nature, formic acid occurs in stinging-nettle and ants, while acetic acid is animportant ingredient of vinegar.Oxalic acid shows a very strong etching effect (low pKa), combines calcium and causeskidney damage or cardial paralysis; the lethal dose value is about 5-15 g. Oxalic acidsalts are toxic. In nature it occurs in small amounts in rhubarb and spinach.Glyoxylic acid is a metabolite in mammalian biochemical pathways. It also occurs in inplants e.g. in unripe food e.g gooseberries. It is oxidized in the human body to oxalicacid and thus causes comparable effects. The lethal dose LD50=2500 mg/kg (oral,rat).Glyoxylic acid has possibly mutagenic properties.

Discussion 137

5.4. Analysis techniques

Liquid chromatography

The identification of corona-induced liquid phase oxidation products of organiccompounds is best performed by ion-exclusion chromatography (ICE). The oxidationmechanism induced by pulsed corona discharges results in a diverse product range,consisting of several aliphatic carboxylic acids, which can be properly separated by ion-exclusion chromatography. The polyhydroxybenzene intermediate oxidation products donot interfere with the carboxylic acids.The application of a conductivity detector in series with a UV absorbance detectoroffers specific sensitivity for the carboxylic acids. For the case of a target compoundwith fluorescent properties like e.g. polycyclic aromatic hydrocarbons, benzene, phenoland laser dyes, a fluorescence detector offers conversion measurements with muchhigher sensitivity, although the non-fluorescent degradation products require yetidentification by a UV absorbance or conductivity detector.

The required analysis time of phenol-containing oxidation products is long viz. about30 minutes, due to strong interaction of phenol with the partially-crosslinkedpolystyrene-divinylbenzene stationary phase resin of the ICE column. Depending on thecolumn type, it may be possible to introduce organic modifier into the acidic aqueouseluent by means of a gradient elution program. In this way the retention of phenoland/or polyhydroxybenzenes may be forced after elution of the complex carboxylic acidrange, in order to shorten the analysis time. However, this option has not beeninvestigated, because identical column performance cannot be guaranteed anymoreafter operation with organic modifier-based acidic eluents.

Mass spectrometry

The product range complexity rapidly increases with the complexity of the targetcompound molecular structure. The fact, that the number of possible oxidation productsfrom a very simple molecular structure like a benzene ring is already extensive,inevitably demands the application of mass spectrometry for identifcation afterseparation. Mass spectrometry has been applied both directly by electron-impact MSand via IonSpray and APCI interfaces with the liquid chromatograph (off-line MS).Direct mass analysis of oxidized high initial concentration phenol solutions has notrevealed significant information, although the appearance of the oxidized solutionimplies thorough chemical changes. Concerning off-line MS, the signals of separatedcomponents are overruled by the high background signal due to the acidic ICE eluent.The available interfaces are not suitable for handling the separated low molecularweight components.

138 Chapter 5.

Gas chromatography

Although GC-MS technology is more straightforward than LC-MS technology, the GCanalysis of high water content samples is problematic. Therefore the oxidation productshave to be transferred from the aqueous phase to an organic phase, by solid-phaseextraction or freeze-drying. Several attempts have failed, because certain compoundswithin the complex mixture have been lost during this sample preparation step. Forapplication of GC-MS, also chemical derivatization of the oxidation product compoundsmay be necessary, for the production of thermally stable compounds and for improvedseparation and detectability.

Aldehyde screening test

For gas phase analysis, both gas chromatography-mass spectrometry (GC-MS) andFourier transform Infrared spectroscopy (FTIR) have been applied. Chemicalderivatization has been applied to trap possible aldehydes present in the gas phase. Gassampling can be performed in-field; the storage life of tubes after sampling depends onthe analyte and the sampling/derivatization technique: sampled aldehyde screeningtubes are stable for at least one week at 25°C. Nevertheless, here the performed testshave been directly processed. Direct gas sampling may be disadvantageous, due to thepresence of water vapour that is produced by the corona discharges over the aqueoussolution.

Hydroxyl radical identification

The fact that in-situ ESR has not revealed hydroxyl radicals can be explained by the lowsensitivity of ESR compared to fluorescence spectrometry. Also it is reported by Sun[130], that the spin trapping efficiency η of DMPO for hydroxyl radicals is only η=33±3 % for OH generated by photolysis of hydrogen peroxide and η=28 ±3 % for OHgenerated by photocatalytic oxidation of water. The trapping of hydroxyl radicals byDMPO is thus likely to be far from effective, although DMPO is a well-known andwidely applied spin trap.

Fourier transform infrared spectroscopy

Gas phase analysis by FTIR can be performed in-situ and is highly sensitive. Simplemolecules like oxides of carbon, nitrogen and sulfur are completely identified.Unfortunately, full revelation of the identity of a complex hydrocarbon, especially in amixture, is nearly impossible. On the contrary, the presence of functional groups likee.g. hydroxyl (carboxylic acids, alkanols), carbonyl (aldehydes, carboxylic acids, esters,ketones), nitrogen-based groups (nitriles, amines, amides, nitro compounds), sulfur-based groups (thiols) and the presence of aromaticity (mono, polycyclic) or aliphaticunsaturated hydrocarbons (alkenes, alkynes) can be monitored accurately. It should benoted, that in-situ FTIR measurements may encounter disturbance by water vapour.

Discussion 139

Liquid chromatography versus LIF-spectroscopy

Reversed-phase HPLC features a very powerful separation of the oxidation productmixture components combined with identification by a wide range of detectors. On thecontrary, samples have to be analyzed off-line and the analysis times are always longerthan that of in-situ spectroscopic measurements. Sample ageing is relevant for the caseof e.g. the phenol oxidation product mixture, because after stopping the coronadischarge experiment i.e. the oxidation time, residual generated oxidizers continue thedegradation in the time lag before analysis. Especially the trihydroxybenzenes aresusceptible to further degradation after sampling. In addition, all hydroxybenzenes arelight-sensitive.

LIF spectroscopy features a high sensitivity, time- and space-resolved analysis offluorescent priority compounds like e.g. polyaromatic hydrocarbons, benzene andaniline. Although LIF spectroscopy is actually applied for the study of gas phasereactions, its application for the study of fluorescent molecules in aqueous solution ispossible, but requires some points of attention. Only low concentration solutions can bestudied, because laser absorption by a high concentration solution results in conversion-dependent excitation. Despite fluorescence quenching, the conversion determinationcan be justified by decomposition of the solution fluorescence spectrum into thespectrum of the target compound and oxidation products. Analyte photolysis due to theexcitation photons can be minimized, by application of short exposure times and lowlaser power.

By electronic excitation, the acid dissociation constant pKa of phenol is reported tochange considerably that is, phenol becomes a much stronger acid [131]. In the groundstate S0 pKa=10, while for the first excited singlet state S1 pKa=3.6-4.0. For thelowest triplet state T1 pKa=8.5. By laser excitation, excited phenol molecules may beproduced that will dissociate into phenolate anions according to the Equations 5.4ab.

C6H5OH* + H2O H3O+ + C6H5O-* (5.4a)

][]][[

*56

*563

OHHCOHCOH

Ka

−+

= (5.4b)

The phenolate anions are preferentially attacked by electrophilic oxidizer species. Inliquid chromatography, the detection of separated components by UV absorbance orfluorescence indeed also creates excited phenol molecules, but this is an ex-situmeasurement.

If phenol conversion measurements by in-situ LIF spectroscopy are biassed by thementioned effect, the excitation volume should be sufficiently high and also oxidizersshould be present in that volume. It is likely, that oxidation mainly takes place at thegas-liquid interface; the laser beam enters the reactor vessel some centimeters belowthe water surface. The excitation volume is estimated from a 3 mm laser beam waistand 10 cm optical path length to be about 0.7 ml which is only 0.2 v/v% or 0.7 v/v %of the total reactor volume (300 or 100 ml). Therefore this effect can be ignored.

140 Chapter 5.

Conductometry

Conductometry measurements in general may be applied as very cheap and simple in-situ tests to globally monitor oxidation progress of C/H/O-based target compounds in-field, by the increase of the carboxylic acid concentration. For the case of halogen/nonmetal-based organic target compounds (viz. chlorobenzene and aniline), conductivityincrease during oxidation progress is also due to the formation of inorganic ions(chloride and nitrate respectively) and then specific pH conductometry is to be preferredfor monitoring the disappearance of total carbon. For that case, a certain conductivitythus remains after complete mineralization of the target compound.

Microtox ecotoxicity

The toxicity change due to degradation of phenol by pulsed corona discharges has beenmonitored by a bacteria test viz. the Microtox ecotoxicity test. Actually, thedetermination of environmental toxicity should also take place for other trophic levelslike for fish, crustaceans, algae and activated sludge, however this approach reachesbeyond the scope of this thesis. The toxicity effects revealed by the Microtox testcannot be related to human toxicity, because these are based on bacteria viz. Vibriofischeri. This can be illustrated by the EC50

5min effect values [78] and human lethal dosevalues [127] of some compounds with very high human toxicity: potassium cyanide:EC50

5min= 4.77 mg/l CN- while 0.15-0.25 g is an average lethal dose for adults;aflatoxin B1: EC50

5min=21.97 mg/l while 5 µg/kg is an upper exposure limit; arsenic (III)oxide: EC50

5 min=73.73 mg/l while 0.12-0.3 g is a lethal dose for adults.

General remarks

It can be summarized that all applied analysis techniques have generally yielded veryconsistent results. Hydroxyl radicals account for the formation of polyhydroxybenzenesfrom phenol and have been demonstrated by fluorescence spectrometry using themolecular probe CCA. Ozone is consumed by phenol in aqueous solution according toUV absorbance spectrometry and accounts for the formation of carboxylic acidstogether with hydroxyl radicals and oxygen. Polyhydroxybenzenes have been identifiedby rp-HPLC, ICE, LIF and the Microtox test. Carboxylic acids have been demonstratedby ICE and conductometry. No other gaseous phenol oxidation products than carbondioxide have been identified according to FTIR, the aldehyde test and the TOCmeasurements. The less consistent results, viz. radical identification by ESR and theidentification of oxidation products by MS, are explained by sensitivity problems and/ornon-optimal conditions.Of particular interest for global in-field monitoring of oxidation progress are in-situ UVabsorbance or fluorescence spectrometry and conductometry. UV absorbance isgenerally applicable to organic compounds, while fluorescence spectroscopy has a veryhigh dynamic range but is only applicable to fluorescent molecules; on the contraryseveral aromatic compounds exhibit fluorescent properties [85-87,131]. Unfortunately,spectroscopic analysis of turbid waste water is not possible; TOC measurements thenare more favourable but these measurements are not in-situ and are more complicated.Conductometry is cheap, simple and suitable for monitoring oxidation progress of anyorganic compound, but the results are rather emperical.

Discussion 141

5.5. AOP comparison

Fundamental merits and problems are described for the advanced oxidation processes[132,12] discussed in section 1.1 together with corona discharge technology. Afinancial comparison of these technologies has not been part of this study. Generalproblems concerning oxidation are the conversion of nitrogen from air into nitrogenoxides and nitrous oxide. Nitrogen oxides contribute to environmental acidification;nitrogen dioxide is suspected to exhibit human reproductive toxicity [133]. Nitrousoxide is a greenhouse gas. Oxidation of halide-containing waste water may result inhalogenation of organic target compounds [124]; also problematic is the oxidation ofbromide to carcinogenic bromate (BrO3

-).

Ozone/UV

Pro: Ozone is a powerful oxidizer that can be produced from a simple ozonizer setupand air. Also, hydrogen peroxide is produced from the oxidation of water by ozone.Contra: The reactions of ozone in aqueous solution are mass transfer limited. Molecularozone reacts much more slowly than hydroxyl radicals. Unreacted/undecomposed ozoneleaving the reactor has to be detoxified e.g. by chemical reduction. The UV-lamp powerefficiency and lamp life are limited; the penetration depth of UV radiation in turbidaqueous solutions is low.

Hydrogen peroxide/UV

Pro: Hydrogen peroxide is a pure source of hydroxyl radicals. Activation can be appliedby UV photons and/or iron (II,III) salts. The quantum yield for generation of hydroxylradicals from photolysis of hydrogen peroxide is about 1.0.Contra: The transport, storage and handling of hydrogen peroxide require special safetyprecautions. Hydrogen peroxide shows only weak absorption in the range 200-300 nmand also absorption at wavelengths higher than 300 nm is not significant; by addition ofiron salts the hydroxyl radical production efficiency is greatly enhanced, but a high ironsalt concentration is required according to stoichiometry. The application of other saltsthan iron hydroxo/carboxyl chelates causes unnecessary release of anions like e.g.sulfate, chloride or nitrate. Lamp power efficiency and lamp life are limited. Turbidwaste water is problematic.

Photocatalytic oxidation

Pro: A simple setup is required.Contra: The quantum yield for the generation of hydroxyl radicals on the surface of themost generally applied photocatalyst titanium dioxide is low. Indicated numbers are only4-8% for TiO2 slurries or even lower for immobilized photocatalyst particles. Also thequantum yield is dependent on the light intensity. Mainly for these reasons, the scale-upprocedure from laboratory setup to industrial application is problematic. Mass transferlimitation occurs for immobilized photocatalysts. After reaction, suspendedphotocatalyst particles have to be separated from the oxidation product mixture. Again,lamp power efficiency and lamp life are limited. Turbid waste water is problematic.

142 Chapter 5.

Wet oxidation

Pro: Supercritical water conditions favour high solubility of organic compounds plusoxygen and extreme chemical reactivity.Contra: The setup requires autoclave-proof housing including e.g. high-pressure pumps,(pre)heaters, compressors thus only has large scale applicability. Oxygen addition isnecessary. After oxidation, vapour/liquid/solids separation and cooling of the productmixture are necessary. Due to depressurization, only batch flow operation is possible.The vigorous conditions may enable polymerization and multiring condensation oforganic compounds.

Electron beam treatment

Pro: The generation efficiency of hydroxyl radicals by high energy electrons is about2.7 OH per 100 eV. The E-beam technology is especially suitable for the degradation ofhalogenated hydrocarbons, because these compounds react rapidly with solvatedelectrons. Waste water containing solid matter up to 5 % is accepted without pre-treatment. Continuous flow operation is possible.Contra: The setup involves complex equipment and the technology only has large scaleapplicability. The shielding of β- radiation is necessary. The appearance of electrons fromthe setup through the titanium foil high vacuum separator causes energy dissipation.Idle E-beam setups require power due to vacuum maintenance and cathode heating. Theapplication of nitrous oxide for improved hydroxyl radical production from aqueouselectrons requires additional facilities.

Ultrasonic irradiation

Pro: With a simple setup, vigorous conditions can be created in aqueous solution.Continuous flow operation is possible.Contra: A main issue is the fact that the scale-up procedure is complex. The shieldingof ultrasound is important, because the intense first subharmonic of the applied drivingfrequency and white noise cause hearing impairment; also, a potential hazard is theformation of aerosols from the harmful solution by surface wave activity [134]. Thetransducer device undergoes erosion by intense cavitation.

Pulsed corona

Pro: The pulsed corona technology features a simple setup and produces a broad rangeof oxidizers. Oxidizers are produced by highly efficient processes. Target compoundscan be oxidized in both the liquid phase and gas phase. Continuous flow operation ispossible.Contra: Shielding of electromagnetic radiation is necessary to avoid interference withadjacent electronic devices. The life of pulsed-corona circuits is limited, because long-term generation of steep pulses will eventually strain the electrical components. Anodecorrosion may occur.

Discussion 143

Electric arc

Pro: The thermal plasma offers the highest destruction power. Conversion of gas, liquidand solid phase target compounds is possible.Contra: The energy consumption is very high. After decomposition, cooling andstripping of the gaseous reaction products is necessary. The required facilities requirelarge scale operation. The process is batch-wise. The electrodes life is limited.

General remarks

It is sensible to state, that one particular ideal AOP does not exist. The applicabilitydepends on e.g. the nature of the target compound(s), the pollution magnitude andconcentration, geographical location of the pollution and AOP performance stability.Extremely dangerous materials like 2,3,7,8-tetrachlorodibenzo-p-dioxin [1] have to bedestructed regardless of the costs. With regard to high quantities, continuous-flowoperation is preferable to batch-wise processing. Treatment of low concentrationintermediate toxic waste flows should be performed with maximum efficiency. The AOPsetup complexity and pollution magnitude determine whether it is affordable to build orposition the setup at/near the location where the pollution is situated. The AOPperformance stability is dependent on e.g. the process-technological complexity but alsoon the input flow composition stability; therefore every AOP should be continuouslymonitored by chemical-analytical measurement techniques on the processed waste flowto guard performance stability.

With regard to the obtained results described in this thesis and mentioned coronaliterature references, pulsed corona technology can be considered as a true AOP. Itsstrongest feature is the efficient conversion of low concentration target compoundsfrom both the liquid or gas phase, using simple technology. It is worth-wile to studyscale-up possibilities. Then also a cost analysis can be derived. At this experimentalstage of investigation, it is not possible to determine the process costs for a specificdetoxification: unknown parameters are viz. the size of power supply, power charges,reactor design, target compound(s), destruction level, flow rate, pollution magnitude,maintenance costs, setup depreciation and analytical monitoring costs.

144 Chapter 5.

6. Conclusions

6.1. Pulsed corona discharges

It has been demonstrated that the application of pulsed corona discharges in the gasphase over aqueous target compound solutions is able to degrade the target compoundby oxidation. This application of corona in air has a more powerful effect than an ozonegenerator, because apart from ozone a wide variety of highly reactive species areproduced like hydroxyl radicals, singlet oxygen, metastables, ions and UV photons.Hydroxyl radicals are produced in the gas phase by ionization and dissociation of watermolecules at the corona discharge streamer tips. Also, hydroxyl radicals are producedindirectly, in the aqueous phase from ozone and UV photons, where ozone has beenproduced by the dissociation of oxygen molecules.The production of hydroxyl radicals in water has been investigated by ex-situfluorescence spectrometry using a molecular probe and in-situ electron spin resonance.The molecular probe coumarin-3-carboxylic acid (CCA) has revealed a linearfluorescence increase with the corona exposure time, implying a constant hydroxylradical production rate for the observed time span. Quantitative hydroxyl radicalmeasurements using CCA-hydrogen peroxide standards have failed, due to instability ofdilute hydrogen peroxide solutions and poor CCA solubility in water. In-situ electronspin resonance using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has notrevealed hydroxyl radicals, which has been attributed to an insufficient sensitivitycompared to fluorescence spectrometry.The production rate of ozone has been determined by UV absorbance spectrometry.The ozone concentration increases with the load voltage, while negative corona alwaysyields less ozone than positive corona at the same absolute value of the load voltage.The ozone concentration over aqueous phenol solutions has appeared to be lower thanthe ozone concentration over deionized water, which has been explained by thereaction of ozone with phenol. In the applied setups, ozone concentrations up to about5.5⋅1022 m-3 in air and about 8.0⋅1022 m-3 in oxygen have been measured. The obtainedozone production efficiency is about 40 gO3/kWh which is quite favourable, because ithas been achieved in humid air.The action of radicals, metastables and ions has been illustrated by the application ofcorona in argon and helium for the degradation of phenol in aqueous solution. Althoughunder inert gas conditions no ozone can be produced, considerable conversion has beenmeasured. This conversion is caused by the production of hydroxyl radicals from thedissociation of water by ions and metastables bombardment.From analyses of corona-exposed deionized water by electrical conductometry, ion-exclusion chromatography and spectrochemical ICP analysis it has been derived, thatnitric acid is formed from nitrogen oxides, produced by the corona discharges in air.From experiments on the effect of different electrode configurations on the conversionof phenol and decolorization of malachite green, it has been concluded, that afavourable electrode configuration consists of a capacitive configuration with a multipinanode. The dielectrical separation of cathode and anode avoids conductive currents inthe water, which dissipate energy at the expense of contributing to radical production.The multipin anode produces a more efficiently-dimensioned reactive volume for theproduction of oxidizers than a single pin.

146 Chapter 6.

With regard to the generally applied corona conditions, these are a voltage V=25 kV, apulse repetition rate f=100 Hz, an anode-tip-to-water distance d=1.0 cm and asolution volume Vol≤500 ml, the measured pulse energy range is about 5 -15 mJ.The application of corona in the gas phase over aqueous target compound solutions hasappeared to be more efficient than application of corona in the aqueous phase. This hasbeen demonstrated by both a comparison of phenol efficiency values from literaturewith experimentally obtained values and pulse energy measurements in aqueoussolution. Calorimetry-based pulse energy measurements in deionized water have yieldedvalues of about 55-61 ±9 mJ at a voltage V=19 kV and pulse repetition rate f=100Hz. The application of corona discharges in the liquid phase indeed produces radicals inthe direct vicinity of the target compound. However, for creation of the discharges theliquid phase at the anode tip has to be evaporated, which is less efficient than theproduction of an oxidative environment in the gas phase over the aqueous targetcompound solution. The order of magnitude difference in pulse energy between liquidphase and gas phase corona is in accordance with the observed efficiency differences,see Table 5.1.

6.2. Oxidation of model compounds

The oxidation of phenol in aqueous solution has been studied in detail. The phenoloxidation product mixture has a complex composition. Observed oxidation products aredihydroxybenzenes, trihydroxybenzenes, carboxylic acids and carbon dioxide. Thefollowing polyhydroxybenzenes have been identified: catechol, resorcinol,hydroquinone, pyrogallol and hydroxyhydroquinone. The possible existence of quinonoidcompounds could not be confirmed. Several aliphatic carboxylic acids have beenobserved: formic, acetic, propionic, oxalic, malonic, maleic, succinic and glyoxylic acid;possibly also a muconic acid enantiomer. Except for the carboxyaldehyde glyoxylic acid,no aldehydes have been detected.Pathways have been constructed for the oxidation of phenol by the hydroxyl radical andoxygen and for phenol oxidation by ozone. The described mechanisms account for theproduction of polyhydroxybenzenes and a broad range of aliphatic carboxylic acids withpolyfunctional groups.

The measured conversion efficiency for a 500 ml 1.0 mM phenol solution by corona inair at a conversion X=24% is G=2.2⋅10-8 J/mol ≡ 0.21 (100eV)-1 ≡ 7.5 g/kWh.After 2 hours of oxidation of a 100 ml 1.0 mM phenol solution by corona in air andargon using the generally applied corona parameters, a quantitative product analysis hasyielded the following approximate concentrations (air/argon values in mmol/l): phenol:0.43/0.12 mM; hydroquinone: 0/0.01 mM; total dihydroxybenzenes: 0.08/0.28 mM;formic acid: 0.23/0.09 mM; oxalic acid: 0.39/0 mM; glyoxylic acid: 0.05/0.03 mM;glyoxal: 0/0 mM.The oxidation mechanism of phenol by corona in argon appears to be very differentfrom the oxidation of phenol by corona in air. Degradation of phenol by corona in argonmainly yields polyhydroxybenzenes due to hydroxyl radicals produced from thedissociation of water by ion/metastables bombardment; in argon, ring-cleavage is ofminor importance because of the absence of oxygen and ozone.

Conclusions 147

Application of a Microtox ecotoxicity test on a series of 1 mM phenol solutions exposedto corona in air for different times, has yielded a substantial toxicity increase. Thiseffect is mainly due to the presence of polyhydroxybenzene- and possibly also quinone-intermediate products; especially hydroquinone and 1.4-benzoquinone show very highecotoxicity. The formation of carboxylic acids also accounts for the toxicity increase,but to a far less extent. The toxicity increase is only temporal, because thepolyhydroxybenzenes, quinones and eventually also the carboxylic acids will bemineralized. The temporal toxicity increase is not a specific problem of coronatechnology, but is inherent to oxidation.Carbon dioxide has been detected in the gas phase over oxidized phenol solutions, butcarbon monoxide and volatile aldehydes have not been identified. After 3 hours ofoxidation using the standard corona conditions, about 0.8% carbon from a 1.0 mMphenol solution and about 6.3% carbon from a 0.1 mM phenol solution have beenconverted into CO2. The amount of CO2 produced by phenol oxidation by corona in airis about 2.7 times higher than the amount produced by corona in argon. The totalorganic carbon content appears to be rather constant during oxidation, which impliesthat for the observed oxidation time span the oxidation products mainly remain in theliquid phase, except for the small amounts of CO2 produced.

The degradation of the atrazine herbicide, malachite green dye and dimethyl sulfide odorcomponent by pulsed corona discharges has resulted as follows. The conversionefficiency of a 500 ml 0.12 mM atrazine solution oxidized by corona in air at aconversion level X=13 % is about 7.7⋅10-10 mol/J ≡ 7.4⋅10-3 (100eV)-1 ≡ 0.6 g/kWh.This low efficiency is inherent to persistent triazine herbicides. In order to achieve a24% decolorization of malachite green at λ=590 nm, the degradation efficiency usingseveral electrode configurations has been determined. An immersed electrodeconfiguration with a single pin anode has yielded the lowest efficiency viz. about7 mg/kWh; a capacitive electrode configuration with a 4-pin anode has yielded thehighest efficiency: about 412 mg/kWh. Dimethyl sulfide has been degraded in the gasphase. The conversion of air containing 2.5 ppm dimethyl sulfide has been measured tobe 45% after 15 seconds, 70% after 30 seconds and 94% after 1 minute. Coronatreatment appears to be very effective for stench abatement.

6.3. Analytical techniques

Ion-exclusion chromatography (ICE) has appeared to be much more powerful thanreversed-phase HPLC for separation of the phenol oxidation product mixture. AlthoughICE has been designed for the separation of complex mixtures of carboxylic acids, alsothe polyhydroxybenzenes are well-separated mutually and do not interfere with theacids. On the contrary, the separation of individual polyhydroxybenzene isomers is bestperformed by reversed-phase HPLC.Identification of the complex phenol oxidation product mixture by mass spectrometry isnecessary before quantitative analyses can be performed. However, both off-line LC-MSand electron-impact MS have yielded very limited information about the composition ofthe oxidation product mixture. The applied IonSpray and APCI LC-MS interfaces appearto be not applicable to the relatively low molecular weight phenol oxidation products.Also, the individual product concentrations are low.

148 Chapter 6.

The applied solid phase extraction technique has appeared to be not successful.Therefore, based on literature, major oxidation products have been identified bycomparison of retention times.For global in-field monitoring of oxidation progress of an organic compound, acidityincrease by electrical conductometry is a very unpretentious option. In addition, simpledevices based on detection by UV absorbance or fluorescence (when applicable) can beapplied to monitor the conversion of organic materials. However for the case of turbidwaste water, spectroscopic analysis techniques fail; then, TOC measurements are to bepreferred.Due to sensitivity differences, in-situ ESR has not been able to resolve hydroxylradicals, while fluorescence spectrometry using the CCA molecular probe has distinctlydemonstrated hydroxyl radical production in aqueous solution.

6.4. Outlook

The very favourable conversion efficiency justifies the continuation of pulsed corona-induced water treatment; topics of interest may be described as follows. Optimizationof the reactor/electrode configuration can be achieved by increasing the contact areabetween the target compound solution and the corona discharges. Also an investigationof corona pulse duration and pulse shape may result in an even higher oxidizerproduction efficiency. Continuous flow application, scale-up possibilities and processstability should be studied. With regard to commercially applied organicelectrosynthesis, it is interesting to investigate the possibilities of corona-inducedsynthesis. As electric discharge source for waste water treatment, the application of aflat dielectric barrier lamp may be promising; these xenon-excimer-based lamps produce172 nm radiation at a very high efficiency of about 60%. High resolution LIFspectroscopy should be studied, in order to investigate the possibility to resolve theoxidation products by their rotational spectra. The identification of unresolved oxidationproducts might be improved by application of chemical derivatization prior to liquid- orgaschromatography / mass spectrometry.

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Summary

Since recently, some advanced oxidation processes (AOP’s) have been applied for thedegradation of harmful materials, as alternative to microbiological waste watertreatment or as add-on technology. The AOP focuses on the degradation of persistenttoxic materials by radical-induced oxidation; for this purpose, halogen- and metal-freeoxygen-based oxidizers are utilized, especially hydroxyl radicals. By advanced oxidationa persistent toxic compound is converted into microbiologically degradable products, oris even mineralized to carbon dioxide, water and -depending on the nature of the targetcompound- inorganic ions like e.g. nitrate, sulfate and phosphate. Hydrogenperoxide/UV, ozone/UV and wet oxidation AOP’s have already been applied on modestscale; the status of electrical discharge, electron-beam and photocatalytic AOP’s is stillexperimental.

This thesis describes the degradation of organic materials in aqueous solution by pulsedcorona discharges. These electrical discharges have been applied in the gas phase overthe target compound solution. As a result of the extremely high electric field strength(about 200 kV/cm) at the head of the discharge channels, locally present molecules ofthe dielectric are dissociated, excited or ionized. In humid air the following reactivespecies are produced: hydroxyl radicals, oxygen atoms, ozone, nitrogen metastables,ions and UV photons. The pulsed corona technology has been studied in detail using themodel compound phenol (hydroxybenzene), which is an important precursor in organicchemical synthesis. Also the degradation of some other model compounds has beenstudied viz. atrazine (herbicide), malachite green (dye) and dimethyl sulfide (odorcomponent). A favourable electrode configuration has been derived from theexperiments.In addition to the oxidation of model compounds, also the production of oxidizers hasbeen investigated. The production of hydroxyl radicals in aqueous solution has beenstudied by fluorescence spectrometry in combination with the fluorescent molecularprobe coumarin-3-carboxylic acid (CCA); also in-situ electron spin resonance has beenapplied using the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). UV absorptionspectrometry has been applied for the quantitative analysis of ozone.

The conversion level, conversion efficiency and oxidation mechanisms of the modelcompound phenol have been investigated. Conversion and oxidation pathways havebeen determined by means of the liquid chromatographic techniques reversed-phaseHPLC and ion-exclusion chromatography; in addition, IonSpray and electron-impactmass spectrometry have been applied. Also, phenol oxidation has been studied in-situusing laser-induced fluorescence spectroscopy. The gas phase over the oxidizedaqueous phenol solution has been analyzed by infrared spectroscopy and an aldehydescreening test. From an environmental point of view, Microtox ecotoxicity tests andtotal organic carbon measurements have been performed on fresh and oxidized phenolsolutions. Reaction mechanisms have been derived for phenol oxidation by hydroxylradicals, oxygen and ozone.

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The conversion of atrazine has been performed by reversed-phase HPLC. Dimethylsulfide has been oxidized in the gas phase and the conversion has been determined bygas chromatography. The decolorization of malachite green due to oxidation by coronahas been determined for several electrode configurations using absorption spectrometry.The most efficient electrode configuration consists of a multipin anode situated in thegas phase over the aqueous solution of the target compound, while the cathode isdielectrically separated from the anode. In this way an effectively dimensioned plasmais produced and energy dissipation due to vapour formation is avoided.

During oxidation of aqueous solutions of the fluorescent molecular probe CCA, thefluorescence intensity appears to increase linearly with the oxidation time, whichimplies a constant hydroxyl radical production rate in water. Calibration of thefluorescence intensity as a function of the amount of hydroxyl radicals added usinghydrogen peroxide and CCA has appeared to be impossible. Identification of hydroxylradicals by in-situ ESR has been unsuccessful, although the spin trap DMPO has beenutilized to produce a stable adduct.The ozone concentration in air over water increases with the applied load voltage, whilepositive corona produces more ozone than negative corona. The measured ozoneproduction efficiency is about 40 g/kWh which is high, because the ozone has beenproduced in humid air. During oxidation the ozone concentration over aqueous phenolsolutions is significantly lower than the ozone concentration over deionized water,which is explained by the reaction of ozone with phenol.The oxidation of phenol in aqueous solution by pulsed corona discharges in air yields acomplex mixture of oxidation products. Polyhydroxybenzenes are produced, which yieldaliphatic aldehydes and carboxylic acids by ring-cleavage. The derived oxidationpathways explain the formation of polyhydroxybenzenes and aliphatic polyfunctionalhydrocarbons. The intermediate oxidation products are more harmful than phenol,therefore thorough oxidation progress is required; the temporal toxicity increase is nospecific problem occurring from corona technology, but is inherent to oxidation. Exceptfor small amounts of carbon dioxide, no other gaseous phenol oxidation products havebeen identified. This is in accordance with an observed nearly constant total organiccarbon level of the aqueous solution during oxidation. There appear to be largedifferences between oxidation products obtained by corona in air and corona in argon.By corona in argon mainly hydroxylation of phenol takes place, while in the presence ofair also ring-cleavage takes place. The measured efficiency at 24% phenol conversionby corona in air is about 22 nanomol/J, which corresponds to 7.5 g/kWh.Atrazine and malachite green are very stable compounds, which appears from theconversion efficiency: the efficiency of atrazine is 0.6 g/kWh at 27% conversion, whilethe efficiency of malachite green is 0.4 g/kWh at 24% absorption decrease at λabs=590nm with regard to the most favourable electrode configuration. On the contrary,dimethyl sulfide is readily converted.

Samenvatting

Ten behoeve van de afbraak van schadelijke chemische verbindingen worden sindsenige tijd enkele geavanceerde oxidatieprocessen (AOP’s) toegepast als vervanger voor-of in combinatie met- conventionele microbiologische afvalwaterbehandeling. Een AOPbeoogt de degradatie van persistente toxische verbindingen via radicalaire oxidatie;hierbij wordt gebruik gemaakt van halogeen- en metaalvrije, op zuurstof gebaseerdeoxidatoren, met name hydroxylradicalen. Door geavanceerde oxidatie wordt eenpersistente toxische verbinding gedegradeerd tot microbiologisch afbreekbareverbindingen, of eventueel gemineraliseerd tot koolstofdioxide, water en -afhankelijkvan de aard van de verbinding- anorganische ionen zoals bijvoorbeeld nitraat, sulfaat enfosfaat. Van de AOP’s worden waterstofperoxide/UV, ozon/UV technologie ennattelucht oxidatie reeds op beperkte schaal toegepast; elektrische ontladingen,elektronenbundel technologie en fotokatalytische oxidatie bevinden zich nog in deexperimentele fase.

In dit proefschrift wordt de afbraak beschreven van organische verbindingen in waterigeoplossing met behulp van gepulste corona-ontladingen. Deze elektrische ontladingen zijntoegepast in de gasfase boven the oplossing van de doelcomponent. Ten gevolge vande extreem hoge elektrische veldsterkte (ongeveer 200 kV/cm) aan de kop van decorona-ontladingskanalen, worden aldaar aanwezige moleculen van het diëlectricumgedissocieerd, geëxciteerd of geïoniseerd. In vochtige lucht worden aldus de volgendereactieve deeltjes geproduceerd: hydroxylradicalen, zuurstofatomen, ozon,stikstofmetastabielen, ionen en UV fotonen. De gepulste corona technologie isgedetailleerd getoetst op de modelstof fenol (hydroxybenzeen), een belangrijkeprecursor in de organisch-chemische synthese. Tevens is de degradatie van enkeleandere modelcomponenten bestudeerd, te weten atrazine (herbicide), malachiet groen(kleurstof) en dimethyl sulfide (geurcomponent). Uit de experimenten is een gunstigeelektrodenconfiguratie naar voren gekomen.Naast de oxidatie van doelcomponenten is ook de produktie van oxidatoren onderzocht.De produktie van hydroxylradicalen in waterige oplossing is bestudeerd metfluorescentie spectrometrie in combinatie met de fluorescerende moleculaire probecoumarine-3-carboxylzuur (CCA); tevens is in-situ elektron spin resonantie toegepastmet behulp van de spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). UV absorptie-spectrometrie is toegepast voor de kwantitatieve analyse van ozon.

Van de modelstof fenol zijn omzettingsgraad en efficiëntie alsmede deoxidatiemechanismen onderzocht. De conversie en oxidatieroutes zijn bepaald metbehulp van de vloeistofchromatografische technieken reversed-phase HPLC en ion-exclusie chromatografie; ook zijn IonSpray en electron-impact massaspectrometrietoegepast. Tevens is in-situ de oxidatie van fenol bestudeerd met laser-geïnduceerdefluorescentiespectroscopie. De gasfase boven de geoxideerde fenoloplossing isgeanalyseerd met infrarood spectroscopie en een aldehyde test. Vanuit milieutechnischoogpunt zijn Microtox ecotoxiciteitstests en totaal organisch koolstof analysesuitgevoerd op onbehandelde en geoxideerde fenoloplossingen. Reactiemechanismen zijnopgesteld voor de oxidatie van fenol door hydroxylradicalen, zuurstof en ozon.

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De conversie van atrazine is bepaald met reversed-phase HPLC. Dimethylsulfide isgeoxideerd in de gasfase en de conversie is gemeten met gaschromatografie. Deontkleuring van malachietgroen ten gevolge van oxidatie door corona ontladingen isgemeten voor diverse elektrodenconfiguraties door middel van absorptiespectrometrie.De meest efficiënte elektrodenconfiguratie bestaat uit een meerpunts anode in degasfase boven de waterige oplossing van de doelcomponent, waarbij de kathodediëlektrisch gescheiden is van de anode. Aldus wordt een effectief gedimensioneerdplasma gevormd en wordt energieverlies door dampvorming voorkomen.

Bij oxidatie van waterige oplossingen van de fluorescerende moleculaire probe CCA,blijkt de fluorescentie intensiteit lineair toe te nemen met de oxidatietijd, waaruit eenconstante produktiesnelheid van hydroxylradicalen in water is gebleken. Calibratie vande fluorescentie-intensiteit als functie van de hoeveelheid toegevoegde hydroxyl-radicalen met behulp van waterstofperoxide en CCA is niet mogelijk gebleken. Het isniet gelukt, om hydroxylradicalen aan te tonen met behulp van in-situ ESR, ondanks dateen spin trap is toegepast voor de vorming van een stabiel adduct.De ozonconcentratie in lucht boven water neemt toe met de aangelegde spanning,terwijl positieve corona meer ozon produceert dan negatieve corona. De gemetenproduktie efficiëntie van ozon bedraagt ongeveer 40 g/kWh en is hoog, temeer daar deozon in vochtige lucht geproduceerd is. Tijdens oxidatie blijkt de ozonconcentratieboven fenoloplossingen significant lager dan de ozonconcentratie boven gedeïoniseerdwater, hetgeen verklaard wordt door de reactie van ozon met fenol.De oxidatie van fenol in waterige oplossing door middel van gepulste coronaontladingen in lucht levert een complex mengsel van oxidatieprodukten. Geproduceerdworden polyhydroxybenzenen, waaruit door ringopening alifatische aldehyden encarboxylzuren ontstaan. De opgestelde modellen verklaren de produktie vanpolyhydroxybenzenen en alifatische polyfunctionele koolwaterstoffen. De intermediaireoxidatieprodukten blijken schadelijker dan fenol, waardoor grondige oxidatie dus vereistis. De aanvankelijke toxiciteitstoename is geen specifiek probleem van coronatechnologie, doch is inherent aan oxidatie. Behalve kleine hoeveelheden koolstofdioxide,zijn geen andere gasvormige oxidatieprodukten van fenol aangetroffen. Dit stemtovereen met een waargenomen nagenoeg constante waarde van het totaal organischkoolstofgehalte van de vloeistoffase tijdens oxidatie. Er blijken grote verschillen tebestaan tussen de oxidatieprodukten van fenol, die ontstaan zijn door behandeling metcorona in lucht en corona in argon. Voor het geval van corona in argon treedthoofdzakelijk hydroxylering van fenol op, terwijl in aanwezigheid van zuurstof ookringopening optreedt. De gemeten conversie efficiëntie bij 24% fenol conversie doorcorona in lucht bedraagt ongeveer 22 nanomol/J, wat overeenkomt met 7.5 g/kWh.Atrazine en malachietgroen zijn zeer stabiele verbindingen, hetgeen blijkt uit deconversie efficiëntie: deze bedraagt voor atrazine 0.6 g/kWh bij 27% conversie en voormalachietgroen 0.4 g/kWh bij 24% absorptievermindering bij λabs=590 nm voor demeest gunstige elektrodenconfiguratie. Dimethylsulfide wordt daarentegen snelomgezet.

Dankwoord / Acknowledgements

Dit multidisciplinaire proefschrift is tot stand gekomen mede dankzij de inzet van eengroot aantal personen.

Allereerst dank ik eerste promotor Wijnand Rutgers en co-promotor Eddie vanVeldhuizen bijzonder voor toekenning van de promotieplaats, intensieve begeleiding,enthousiasme en een zeer prettige samenwerking. Tevens dank ik Gerrit Kroesen, Fritsde Hoog en Bram Veefkind voor hun belangrijke bijdrage aan mijn promotieonderzoek.

Tweede promotor Carel Cramers en begeleider Henk Claessens (beiden faculteitScheikundige Technologie / capaciteitsgroep Instrumentele Analyse) dank ik voor hunondersteuning op chemisch-analytisch gebied en het beschikbaar stellen vanlaboratoriumfaciliteiten.

René Janssen (faculteit Scheikundige Technologie / capaciteitsgroep Macromoleculaireen Organische chemie) dank ik voor de diverse discussies en voor de leiding bij de ESRexperimenten.

Voor chemisch-analytische assistentie ben ik verder dank verschuldigd aan Marion vanStraten, Eric Vonk, Jan Jiskra, Ber Vermeer, Marc van Lieshout, Joost van Dongen,Gius Rongen, Henri Snijders en Hans Damen. Ook dank aan Denise Tjallema.

Het was geweldig vertoeven bij de capaciteitsgroep Elementaire Processen inGasontladingen; bedankt voor de leuke tijd en samenwerking: Loek Baede (speciaal voorde technische realisatie van corona en voor fotografie), Lambert Bisschops, LeonBakker, Rina Boom, Jean-Charles Cigal, Jurgen van Eck, Marjan van de Elshout, HansFreriks, Marc van de Grift, Charlotte Groothuis, Gerjan Hagelaar, Daiyu Hayashi (specialthanks for his important contribution to LIF spectroscopy), Marcel Hemerik, CaroleMaurice, Gabriela Paeva, Koen Robben, Eva Stoffels, Winfred Stoffels en GeertSwinkels. En niet te vergeten mijn oud-collega’s van Elektrotechniek (EG): VadimBanine, Hub Bonné, Frank Commissaris, Ad Holten, Ad van Iersel, Herman Koolmees,Roel Moors en Mariet van Rixtel.

Dank voor jullie bijdrage: afstudeerders Roy Janssen, Hens Renierkens, Ralph van Eijk;stagiairs Geert Dooms, Coen van de Vin, Chris Peters, Marco van Steen, Martijn Siffels.

Verder dank ik: Marius Bogers en medewerkers van de faculteitswerkplaats N voor devervaardiging van onderdelen ten behoeve van experimentele opstellingen. Marco vande Sande (N/ETP) voor uitvoering van de spectrochemische ICP metingen. GerardHarmsen (Directoraat-Generaal RWS-RIZA, Lelystad) voor advies over ecotoxicologischetests. KEMA KPG/CET Arnhem voor uitvoering van de TOC metingen.

Thanks to Pavel Šunka (Institute of Plasma Physics of the Czech Academy of Sciences,Prague CR) and Bruce Locke (Florida State University, Tallahassee USA) for receivedhospitality and pleasant discussions.

Tenslotte dank ik speciaal mijn ouders, broer en zus voor hun grote belangstelling enondersteuning.

Curriculum Vitae:

Daar op de basisschool al behoefte bestond aan een eigen chemisch laboratorium, is deauteur (geboortedatum 02-07-1966) na afronding van het VWO Peelland College teDeurne in 1985 Scheikundige Technologie gaan studeren aan de toenmalige THEindhoven. Na zijn afstuderen in 1990 als polymeertechnoloog is hij werkzaam geweestbij NKF Kabel in Delft als chemisch technoloog en ook als plaatsvervangend chef vanhet materialen laboratorium energie aldaar. In november 1995 is hij gestart met ditpromotieonderzoek bij de faculteit Elektrotechniek / vakgroep Elektrische Energie-systemen, in samenwerking met de faculteit Scheikundige Technologie / capaciteits-groep Instrumentele Analyse, waarna het onderzoek is voortgezet en afgerond bij defaculteit Technische Natuurkunde / capaciteitsgroep Elementaire Processen inGasontladingen (EPG). Het promotieonderzoek heeft geleid tot dit proefschrift.

A youthful passion to own a private chemical laboratory convinced the author (date ofbirth 02-07-1966) to apply for the chemical engineering curriculum at EindhovenUniversity of Technology, after finishing his pre-university education at Peelland CollegeDeurne in 1985. After his MSc degree in polymer engineering in 1990, he joined NKFKabel in Delft as chemical engineer and also as deputy supervisor of the materialslaboratory. In November 1995 he started this PhD project at the faculty of ElectricalEngineering / Electrical Energy Systems group in cooperation with the faculty ofChemical Engineering / Instrumental Analysis group; he continued and finished theproject at the faculty of Applied Physics / department of Elementary Processes in GasDischarges (EPG). The PhD project has resulted in this dissertation.

Stellingen

behorende bij het proefschrift

“Pulsed corona-induced degradation of organic materials in water”

door W.F.L.M. Hoeben

1. De gepulste corona-ontladingen technologie omvat meer dan alleen de produktievan ozon uit zuurstof. [dit proefschrift]

2. Ten behoeve van waterreiniging is het toepassen van gepulste corona-ontladingenin de gasfase efficiënter dan toepassing in de vloeistoffase. [dit proefschrift]

3. Bij toepassen van gepulste corona-ontladingen in de gasfase boven een waterigeoplossing van de doelverbinding, is de chemische samenstelling van de gasfasevan grote invloed op het degradatiemechanisme van de doelverbinding.[dit proefschrift]

4. Een ion-exclusie kolom is vanwege het carboxylzuur-specifieke retentiemechanisme zeer geschikt voor de scheiding van oxidatieprodukten mengselsvan organische verbindingen. [dit proefschrift]

5. Bij degradatie van een organische verbinding door oxidatie kunnen intermediaireoxidatieprodukten ontstaan, die schadelijker zijn dan de doelverbinding.[dit proefschrift]

6. Simulatie beperkt milieuvervuiling.

7. Kunstmatige mineralisatie is energetisch gezien inefficiënt.

8. Het potentieel van geavanceerde oxidatie technologie is hoofdzakelijk gelegen inhet onschadelijk maken van organische probleemstoffen.

9. Milieubewustheid komt nog steeds niet altijd gelegen.

10. Het is niet mogelijk, om milieukundig onderzoek te verrichten zonder enigchemisch afval te produceren.

11. World Wide Web adressen zijn vanwege het vluchtige karakter geen bruikbareliteratuurreferenties.

12. Het weggooien van een proefschrift is een vorm van energiedissipatie.

13. Hoge schoorstenen verlangen veel wind.

14. Allesreinigers reinigen niet alles.

15. Een goed tandarts is zeker zo belangrijk als een goed gebit.