ADVANCED CHEMICAL OXIDATION AND TOXICITY REDUCTION OF 2,4,6-TRICHLOROPHENOL BY USING FENTON'S REAGENT- PART IV:

REACTION MECHANISM AND KINETICS

Somnath Basu1•2, and Irvine W. Wei 2

/{ .1!5MV! Ct-L L Deer Island Treatment Plant, Massachusetts Water JJ-eatm<m.t Authority,

Winthrop, MA 02152

2· Department of Civil and Environmental Engineering, Northeastem University, Boston, MA 02115

Prepared for Presentation at

A!ChE 1996 Annual Meeting Chicago, Illinois

November 10 - 15, 1996

Session on

Advanced Oxidation Processes

Introduction

Fenton's Reagent is an aqueous solution of hydrogen peroxide and ferrous ions. It is an effective oxidant of various organic compounds dissolved in water. This property makes it an excellent candidate as a treatment agent for industrial and hazardous wastes. The ferrous ions act as a homogeneous catalyst, while the hydrogen peroxide serves the role of the oxidant.

This paper repotts the results of ongoing research on the reactions of Fenton's Reagent on 2,4,6 Trichlorophenol (TCP) in aqueous phase. TCP is a highly toxic chemical. It is a priority pollutant used as a biocide, and is a designated RCRA Hazardous Waste. It is a common industrial chemical, primarily used as a wood preservative agent. It can also be traced in pulp bleach wastes.

Background Chemistry

Haber and Weiss (1934) described the interaction between hydrogen peroxide and ferrous ions in aqueous phase as follows:

Fe 2+ + HO - OH --> Fe 3+ + (OH) · + OH' (eqn. l )

The free hydroxyl radical (OH') is an unstable, highly active species. In the absence of other reactants, it further reacts with ferrous ions to produce more ferric ions and hydroxyl ions:

Fe'• + OH' --> Fe 3+ + OH · (eqn.2)

or, with hydrogen peroxide, according to the following reaction:

OH' + H202 -> H 20 +HO ,· (eqnJ)

and, HO,' + H202 --> 0 2 + H20 + OH ' (eqn.4)

In its summarized version, the Haber and Weiss model can be represented by a set of chain reactions consisting of equations 1 ,2,3, and 4.

Merz and Waters (194 7) showed that in presence of various organic substances, the hydroxyl radicals abstract hydrogen atoms from the organic compounds and produce organic free radicals (R') by the following mechanism:

OH' + Organic Compound--> R ' + H20 R' + HO - OH --> R -OH + OH '

(eqn.5) (eqn.6)

Simultaneous generation of hydroxyl and organic radicals initiates chain reactions leading to the oxidation of large amounts of organic substances, such as primary and secondary alcohols, ethers, etc.

1

Merz and Waters demonstrated that certain compounds, e.g. tertiary alcohols, esters, carboxylic acids, etc., oxidize via non-chain reactions, as expressed by equations 3, 5, and 7.

2R' � Stable Dimer Product, or Disproportionation Reaction Products. (Eqn. 7)

They also identified a third category of compounds that are not attacked by the active hydroxyl radicals. A few examples of such compounds are: acetic, malonic, maleic, etc., among acids; acetone, MEK, etc., among ketones; and urea, acetamide, etc., among amides. Kolthoff and Medalia (1949) te1med the primary alcohol-type compounds as promoters, and the acetic acid­type compounds as suppressors. However, they reported that while no organic substrate (including acetic acid-type compounds) acts as a suppressor in the presence of oxygen, chloride ions suppress oxidation reactions by Fenton's Reagent, both in the presence and absence of dissolved oxygen.

The model of Merz and Waters has been significantly improved by others, most notably by Walling and Kato (1971). They proposed that the active organic radical R'reduces fen-ic ions to fen-ous ions, as follows:

R' +Fe 3+ � R+ +Fe 2+ (Eqn.8)

In this case, reaction 8 assumes the role of the chain propagating step.

They also proposed a more complex model for organic molecules containing hydrogen atoms at different locations. According to this model, the active hydroxyl radicals can attack the hydrogen atoms of the same H-R molecules at different locations, giving rise to a unique kind of organic radical by attacking upon the hydrogen atom at a particular location. Under this condition, reaction 5 is substituted by a series of simultaneously occUlTing reactions, as follows:

H-R+OH' � R,' + H,O

H-R + OH' � R; + H,O

H-R + OH' � R3' + H20 (Eqn.9).

The three organic radicals are different from each other. Subsequently, each radical was assumed to pursue a different reaction path from each other. R1' was assumed to follow through the chain propagation path represented by equation 8. R2' was assumed to tmminate the chain according to reaction 7, while R3' was assumed to oxidize ferrous ions, leading to the re­formation of the original substrate, according to the following sequence of reactions:

[H+] R3'+Fe2+� FeJ++R,-� R 3H (eqn.IO).

2

[W] R:+Fe2+� Fe3++R,-� R H (eqn 10) J J 3 • •

This can be well understood by considering the example of hydroxylation of benzene molecules, which have hydrogen atoms at multiple locations of the molecular structure. As shown in equation 11, the reaction proceeds through the formation of an intermediate hydroxycyclohexadienyl radical, as a result of addition of a hydroxyl radical with benzene molecules. This intermediate reacts further to generate the products, following three parallel pathways. The relative amounts of the products formed are determined by the reaction conditions.

DJMERJZATION Q-----0

OH* + 0- A F•"•C""- �OH

� >< OXIDATION lVJ H OH\__ � ,,,. 0

H+ H20 + �- - Fe�·+ � (eqn.l l)

Application of Fenton's Reagent to industrial wastewaters that are recalcitrant or toxic to biological treatment subject the organic molecules to oxidative degradation, leading to the formation of total or partial oxidation products. Even if the pollutants are partially oxidized, the products, e.g. alcohols, acids, etc., are generally less toxic and more biodegradable, compared to the original organic substrates. This renders the wastewaters containing such pollutants acceptable to municipal sewer systems (Bowers, et al. , 1987).

Potter and Roth (1993) extensively investigated the kinetics of oxidation of several mono- and di-chlorinated phenolic isomers by Fenton's Reagent. For the three monochlorophenols, the original substrates disappeared more than 90% in about an hour.

The Present Research

This paper reports the findings from a portion of an ongoing research project on this subject. Results from the earlier phases of this research have already been reported as Parts I (Chen, Basu, and Wei, 1995), II (Basu, Sreenivasan, and Wei, 1996), and III (Basu, Sreenivasan, and Wei, 1996), of this project. Pmis I and II rep01ied the effects of various reaction parameters on the rates and extents of reactions of TCP with Fenton's Reagent. Part III presented a comparative study of the advanced oxidation of TCP by Fenton's Reagent, and by UV-Peroxide treatment. For all of the reactions, the effects were monitored in terms of degradation of TCP, release of chlorine atoms from the organic structure, and reductions in COD, TOC, and toxicity (EC 50) values.

A summary of the imp01iant observations from the above three pmis are presented below:

- The reaction involves continuous destruction of TCP and intermediate products, and

.3

release of organically bound chlorine atoms to chloride ions, as well as oxidation of the organic structure, resulting in a simultaneous drop of pH. The effects on the chlorine atoms and the system pH are shown in Figures 1 A and B.

- TCP disappeared almost completely within an hour, with simultaneous appearance and disappearance of unidentified intermediate organics.

- Treatment by Fenton's Reagent causes significant reductions in original COD, TOC, and toxicity (as measured by EC 50) of the substrate.

- Rate of reaction is a function of the molar ratio of hydrogen peroxide (oxidant) to TCP (substrate), molar ratio of ferrous ions (catalyst) to hydrogen peroxide (oxidant), reaction temperature, and pH.

- The optimum oxidant to substrate ratio and catalyst to oxidant ratio for oxidation of TCP with Fenton's Reagent are 5.5:1, and 0.1:1, respectively. For oxidation of TCP, the optimum range of pH is 2 to 3.5. However, it is not necessary to attempt any pH adjustment, as the reaction drives itself to the optimum pH range.

- The rate of reaction increases significantly with an increase of temperature. In the case of a real industrial discharge, the decision should be made, based upon the effluent temperature, whether preheating of the effluent will be necessary or not.

- Ferric ions can also act as a catalyst for oxidation of TCP by hydrogen peroxide, but are not practical because of the requirement of large amounts of both oxidant and the catalyst.

- Utilization of hydrogen peroxide as the oxidant can be enhanced by addition in smaller increments rather than a single addition at the start of the reaction. This is an important consideration, for practical application, which can reduce the application of hydrogen peroxide while delivering the same level of treatment, leading to more economic operation.

- Dissolved Oxygen (DO) participates in the reaction and enhances oxidation. However, it is not necessary to pre-aerate since the normal DO level in an industrial wastewater is sufficient to drive the reaction forward. The reaction creates an oxygen demand which helps additional oxygen from air to diffuse into the reaction system.

- Advanced oxidation by Fenton's Reagent, under the optimum conditions of reaction, effects about a three-fold reduction in the toxicity of wastewater containing TCP, making it more acceptable to biotreatment. A major component of the toxicity reduction can be attributed to dechlorination.

- UV-Peroxide is a more aggressive treatment process than Fenton's Reagent for

4

removal of TCP from wastewater, but the toxicity reduction by Fenton's Reagent closely matches that by UV -Peroxide treatment. Therefore, as a pretreatment process, Fenton's Reagent is more attractive from an economic standpoint.

The present paper (Part IV) reports the postulated mechanism for the reaction of TCP with Fenton's Reagent. An empirical kinetic equation for the disappearance of TCP with time is also presented here.

Experimental Section

1. Reagents

a) TCP stock solution ( l mM) in deionized water was prepared in the laboratory from 98% pure 2,4,6-Trichlorophenol in solid form, manufactured by Aldrich Chemical Inc.

b) 3% stock solution of Hydrogen Peroxide was prepared by diluting a 3 0% reagent grade solution manufactured by Fisher Chemical.

c) The Ferrous ions, which act as catalyst in the Fenton's Reactions, were supplied in the form of Ferrous Sulfate crystals (FeS04,7H20), manufactured by J.T.Baker.

d) Ceric Sulfate solution (0.1 N in Sulfuric Acid), used as a titrant for the determination of Hydrogen Peroxide concentrations, was purchased from VWR.

e) Sodium Sulfite crystals, to quench the withdrawn samples for future analyses and to stop further oxidation in the sample vials, were purchased from J. T. Baker.

f) Deionized (DI) Water was used to fulfill all the needs of water to carry out the experiments, e.g. for preparation of solutions, cleaning of glassware, etc. DI Water was prepared in the laboratory from distilled water with the help of a Millipore Milli­Q water purification system.

2. Experiments

This phase of the research consisted of tln·ee experimental runs under various conditions, as clearly outlined in Table I. The experiments were conducted in one-liter stirred batch reactors. Each reaction was carried out with a 500 ml batch of an aqueous solution of 1 mM 2,4,6-TCP on a magnetic stirring platform.

The steps to statt a reaction consisted of measuring out 500 mL of TCP solution (1 mM) in a I L glass beaker, and adding pre-calculated amounts of the 3% hydrogen peroxide solution and ferrous sulfate crystals to the TCP solution. The starting amounts of oxidant and catalyst represented an oxidant to substrate molar ratio of 5.5:1 and a catalyst to oxidant molar ratio of 0.1: 1.

5

The experimental set-up consisted of two probes, one for pH, and the other for chloride ions, dipped into the reacting liquid for on-line, real time measurement of these variables.

Experiment Nos. 1 and 3 were conducted under open-to-air condition, in a constant room temperature at 25°C. The constant temperature inside the laboratory was adjusted with the help of a temperature controller. The natural tendency of pH is to drop continuously as the reactions proceed. No attempt was made to adjust the pH; it was allowed to float freely.

The goal of Experiment No. I was to investigate the identity of the reaction products, in order to define a reaction pathway. Experiment No. 3 was conducted to study the reaction kinetics of the disappearance of TCP.

The purpose of the Experiment No. 2 was to investigate the effect of dissolved oxygen on the distribution of the reaction products. It was determined in the earlier phases of this research that dissolved oxygen participates in the oxidation of TCP by Fenton's Reagent. Therefore, this experiment was conducted under conditions identical to those in Experiment Nos. 1 and 3 , except for dissolved oxygen, which was eliminated from the reaction system by continuously sparging nitrogen gas through a diffuser.

TABLE 1

Expt Oxidation Starting Starting pH Temp Cata- Mode Chem-ical No. Condition Molar Molar Con- (OC) lyst of H202 analyses

Ratio of Ratio of clition State Addi-H202:TCP Catalyst: tion

H,02

1. Open-to- 5.5 :1 0.10:1 Floating 25 Fe2+ Single pH, ct·, au Batch HPLC/MS

2. Nitrogen 5.5 :1 0.10:1 Floating 25 Fe2+ Single pH, ct·,

Sparged Batch HPLC/MS

3 . Open-to- 5.5 :1 0.10:1 Floating 25 Fe2+ Single pH, c1·, air Batch HPLC

3. Analytical Methods

The chemical analyses performed for this phase of the research included the real time on-line monitoring of pH and chloride ions. Off-line analyses included determination of residual hydrogen peroxide concentration by titration, and determination of TCP and oxidation products by high performance liquid chromatography (HPLC) and mass spectrometry (MS). A summary of the analytical methods is furnished below.

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a) Chloride Ion Concentration and pH measurement

These were continuously monitored with the help of a pH/ISE analyzer (Orion Model 720A) with a pH probe (Orion pH combination electrode) and a chloride electrode (Orion electrode Model 9617BN). The probes were dipped directly into the reaction system.

b) Residual Hydrogen Peroxide Concentration Measurement

Hydrogen peroxide solution standardization, and also determination of residual hydrogen peroxide in the reaction solution, were accomplished by titration with standard (O. lON) eerie sulfate solution. This technique has been especially recommended when organic matter is present in the matrix.

One drop of ferroin (Ferrous 1,10 Phenanthrolin) was used as the indicator for every titration. The end point is characterized by a change of the dark red color of eerie ions to a light pink color.

c) TCP and Reaction Product Analyses

The samples preserved from the experiments were analyzed later for identification and quantification of the various chemical species involved in the reactions, by HPLC. On withdrawal from the reactor, the samples were preserved in sealed glass vials, which contained sodium sulfite crystals to quench further reaction with any residual oxidant. The vials were stored in a refrigerator at 4°C during the time between their withdrawal and the actual analysis. The aqueous samples were directly injected into the column without any extraction by organic solvent.

The analytical conditions were as follows:

Column: Waters NovaPak (5 Jlm, 150 mm x 3.9 mm)

Column Temperature: Ambient

Mobile Phase Flow: 1.5 mL/min.

Injection Volume:

Mobile Phase: 40/60 ACN/K2HP04 Buffer (0.1 M, pH 5.0)

Detection: UV (A. = 295 nm )

7

The following analytical equipment was used:

Injector: Hewlett-Packard (HP) M-1050

Detector: HP M-1050

Pump: HP M-1050

Integrator: HP M-3396

HPLC separation, followed by UV detection as mentioned above, is a good technique for qualitative and quantitative analyses of known compounds, e.g. TCP in this case. This technique was used in Experiment No. 3 to study the kinetics of the disappearance of TCP. The reaction was observed to be very fast in the earlier phases of this research. Therefore, Experiment No. 3 was pre-designed in order to capture the maximum amount of information at the beginning of the reaction. Accordingly, samples were collected at very short intervals of time at the beginning. However, in order to define a reaction pathway, it is necessary to identify the various chemical species, formed at different stages of the reactions, as intermediate and final products. To fulfill this objective, samples generated under selected reaction conditions in Experiment Nos. 1 and 2 were analyzed by HPLC/MS.

HPLC/MS is a very powerful analytical technique which utilizes the separation capability of HPLC of separating detected matter from a solution into bands of organic compounds, and the capability of Mass Spectrometry (MS) to detect the composition of the eluted bands. The two analytical instruments are physically interfaced, such that the eluted peaks from the HPLC column directly enter the rapid-scan mass spectrometer for a real time identification of the peaks. The identification is facilitated by a computer interfaced with the mass spectrometer.

Mass Spectrometry operates on the principle of splitting up organic compounds into component ions and resolving the ions on the basis of mass to charge ratio. A given compound follows a pattern in producing a set of ionized species. Therefore, Mass Spectrometric detection relies on the study of the fragmented ions to identify the nature of the parent compound. A computer significantly aids in the calculations and data processing, and in interpretation of the results. In many cases, the computer is also equipped with a database of the fragmentation patterns of various possible organic compounds, in the form of standard software. Scanning through the database, it quickly matches the results of an unknown compound with those in the software, and identifies a few possible compounds based upon best match. One of those compounds can be selected on the basis of the experience and judgement of the analyst.

There are various ways to produce ions of an analyte molecule. Two of the

8

traditional, and most commonly used, methods employ rapid addition of energy to the molecule, e.g. bombardment of the molecules with a fast moving jet of electrons, and the chemical ionization method. For this research, a relatively new method, called 'electro spray' (ES) method has been utilized. With this method, when the entering analyte sample in the ionization chamber is subjected to an intense electric field, typically a few kilovolts of potential difference, the analyte becomes dispersed into charged droplets. Simultaneously, the solvent, containing the analyte molecules, is evaporated from the surfaces of the charged droplets, leaving behind macro-ions of the solute analyte molecules.

One major advantage of ESMS is the "soft" ionization of parent molecules. Some of the traditional teclmiques, e.g. fast atom bombardment by a beam of electrons, are very aggressive, and sometimes the fragmentation becomes so extensive that identification of the parent molecule becomes a problem. The fragmentation pattern in ESMS is so benign that protein molecules can retain even their tertiary structures up to solvent evaporation. Thus, ESMS has emerged as a very powerful analytical tool for large biomolecules, or polymers.

ESMS also produces multiply charged ions for a given compound, resulting in a series of adjacent peaks, each representing one charged state. The spectra can be de­convoluted by the interfacing computer to arrive at the molecular weights of the parent compounds. The ability to determine the molecular weights is so precise that it can very effectively identify individual components from a solution of various oligomers of the same compounds. It is because of these advantages that ESMS is also finding applications in areas outside of bioanalytical chemistry.

The details of the ESMS equipment and analytical conditions used for this research are listed as follows:

Equipment Name: Single Quadrupole LC/MS Mass Spectrometer

Manufacturer: Perkin-Elmer Sciex Instruments

Model: 014256 Rev. 1

Software: LC2 Tune 1.1

For Experiment No. I, three samples were tested by HPLC/MS. The samples were withdrawn at 10, 30, and 60 minutes after the start of the reaction. A duplicate 30 minute sample from the same reaction was also analyzed by GC/MS for confirmation of the reaction products. The details of the GC/MS conditions are listed below.

Equipment Name: Single Quadrupole, Electron Impact Mode GC/MS Mass Spectrometer

9

Manufacturer:

Model:

Hewlett-Packard

HP 5890 Series II Gas Chromatograph HP 5971 Mass Spectrometer

As mentioned before, samples from Experiment No. 2 were detected by UV detector, and not mass spectroscopy, because the goal was to analyze TCP only. For Experiment No. 3, only one sample was withdrawn from the reaction system after 60 minutes of the start of the reaction, and was analyzed by HPLC/MS teclmique using the same analytical conditions as were used in the case of Experiment No. 1.

Results and Discussions

1. Definition of Reaction Pathway

The first step is the identification of reaction products. Figures 2 A, B, and C are the mass spectra of the various components in samples 1, 2, and 3, of this reaction. Figure 2 D is the mass spectrum of TCP in the original solution.

As mentioned before, the ESMS uses a very soft ionization teclmique to ionize the analyte molecules. Therefore, the extent of fragmentation is not severe, and it does not create a very extensive fragmentation pattern for any given compound as is sometimes used in conventional MS teclmique to identify the compound. Instead, it produces a very dominant peak for the molecular ion. When coupled with HPLC it produces the mass spectra of all the compounds,

· in the order that they elute from the chromatographic column. Thus, the group of peaks under each assigned molecular weight represents one compound that possesses the assigned molecular weight for its negative molecular ion. Since a negative molecular ion can be created by simply displacing a few hydrogen atoms from the molecular structure, it is relatively simple to predict the actual molecular structure of the compounds from the mass spectral information.

From the molecular weights of the different peaks, it appears that the oxidation of TCP by Fenton's Reagent proceeds via the formation of corresponding hydroxycyclohexadienyl radicals by attachment of hydroxyl radicals to the benzene ring. This results in progressive replacement of the aromatic chlorine atoms. Thus, all three types of compounds -- single (MW= 179), double (MW=l60.5), and triple-hydroxyl radical substituted (MW=l42) TCP -­

could be detected in the mass spectra. The corresponding weights of the molecular ions, as they appear in the spectra, are 179, 157, and 141. The weight of the molecular ion for the parent compound is shown to be in the range of 194 to 201.

The possible mono-substituted (by hydroxyl group) compounds that can form from 2,4,6-Trichlorophenol, are 2,6 Dichloro 1,4 Benzenediol (also known as 2,6 Dichlorohydroquinone) and 3,5 (or 4,6) Dichloro 1,2 Benzenediol (also known as 3,5 Dichloro Catechol). It is supposed that both were formed, although it is impossible to distinguish between the two in

10

the mass spectra because of their similar structure and identical molecular weights. However, that the peak at MW= 179 is at least due to the dichloro catechol isomer can be concluded with a fair degree of confidence, since chlorocatechol molecular ions are very stable (Knuutinen and Korhonen, 1983). Even by conventional mass spectroscopy, the dominant peak for all chlorocatechols are those of their molecular ions, with very minor fragment peaks, as evident from Figure 3 A. Similar fragmentation behavior (to chlorocatechols) has been reported by Svec, et a!. (1976) for the mass spectra of chlorohydroquinones. The Svec team presented the mass spectrum of 2,5 Dichlorohydroquinone as a representative of all dichlorohydroquinones, as shown in Figure 3 B. Here also, it can be observed that the molecular ion is the dominant fragment.

A duplicate 30 min. sample from Experiment No. 1 was also analyzed by GC/MS technique, after following the EPA Method 625 for sample preparation. The GC/MS system has the capability of scanning the fragmentation patterns of various organic compounds in the library of its computer, and matching the results of the unknown compounds with those of the known compounds in the library. On the basis of that scan, the computer predicted 2,5 Dichiara 1,4 Benzenediol (2,5 Dichlorohydroquinone) as one of the possible compounds in the 30 min. sample from the reaction product of Experiment No. I. Because all dichlorohydroquinones have very similar spectra, it is suspected that the actual compound was the 2,6 isomer. On the basis of this evidence, it is safe to assume that both 3,5 Dichiara Catechol (DCC) and 2,6 Dichiara Hydroquinone (DCH) are formed as reaction intermediates in the aqueous phase oxidation of TCP by Fenton's Reagent.

Sorokin et a!. (1996) conducted an extensive investigation on the types of products formed when TCP is oxidized in the aqueous phase by hydrogen peroxide using Iron Sulfothiocyanine as the catalyst. They identified 2,6 Dichiara 1,4 Benzoquinone (DCQ) as the main intermediate product. Because of the highly oxidizing environment, it can be safely assumed that DCH will exist in equilibrium with DCQ, with the equilibrium heavily weighted towards DCQ. Incidentally, these two compounds are very close to each other in terms of their molecular weights. Sorokin, et a!. detected maleic acid, chloromaleic acid, fumaric acid, and chlorofumaric acid among other compounds in the final product. They also suggested a reaction pathway. Unfortunately, the suggested weights of the molecular ions obtained from the HPLC/MS analyses of samples from the present research do not correspond to the molecular weights of compounds predicted according to the mechanism suggested by Sorokin, et a!. (1996). The discrepancy may be due to the state of iron in the catalyst structure. In the case of Fenton's Reagent, iron alternates between ferrous and ferric states, whereas the catalyst used by Sorokin and co-workers contains iron embedded in a complex organic structure.

Two di-substituted (by hydroxyl groups) isomers- 2 Chiaro 1,4,6 Benzenetriol and 4 Chloro 1,2,6 Benzenetriol, and only one tri-substituted (by hydroxyl groups) - 1,2,4,6 Tetro! are possible to form from 2,4,6-Trichlorophenol. Substitution of aromatic ring chlorine atoms by hydroxyl groups, in the case of treatment of aqueous chi oro benzene by Fenton's Reagent, was also reported by Johnson, Stein, and Weiss (1951 ). Based on this information, and also on

11

the evidence of formation of dichloro-substituted TCP compounds, it is accepted that progressive substitution of chlorine atoms by hydroxyl groups is the first step of the reaction mechanism, with the possible existence of DCQ in equilibrium. It appears that the substitution reaction progresses in series, from mono-, to di-, and finally to tri-substituted phenol. It is observed that, even after one hour of reaction, some small amount of mono­substituted compound remains in the system. Perhaps, this is the only chlorinated organic compound that remains in the product after treatment, because the reaction essentially completes in an hour. After total dechlorination, the next step involves aromatic bond cleavage to produce two predominant compounds, the weights of the molecular ions of which are 118.8, and 81, respectively. The species with a mass of 96.5 is a fragment from the parent compound, 2,4,6-Trichlorophenol. This can be verified from Figure 2 D, which is the mass spectrum of TCP by HPLC/MS. The corresponding GC/MS analysis, as presented in Figure 2 E, also shows a dominant peak at a mass of 97. Interestingly, the difference in the extent of fragmentation caused by the two mass analyzers is obvious. The analyzer linked with the HPLC created only one fragment (96.5) other than the molecular ion (197.5), whereas the analyzer with the GC created a very extensive set of fragments. Thus, the peak at the mass number of 96.5 does· not represent any pure compound. However, since this peak is observed from the different product spectra also, even long after all TCP disappeared (as shown in Figures 2 A, B and C), it is expected that this ion is a fragment common to all chlorinated phenols.

The compound with MW 120 is produced at a slower rate, reaches a peak, and eventually gets further oxidized and removed from the system within one hour. Because of the sharp rate of decline of the pH, this compound is expected to be an acid. To identify this compound, all

. compounds possessing the molecular formulae of C3H405, C4H804, and C, H12 � were investigated from the literature (Chapman and Hall, 1996). Because this compound appears in the system at a slower rate, and then disappears, it is also expected to be a highly oxidized product of aromatic bond cleavage. Based on the search, it appears that the most probable compound is Hydroxy Propanedioic Acid.

The other major compound (MW = 82) starts appearing after 10 minutesform the start of the reaction, and continues to grow even at 60 minutes. From a search of all possible compounds possessing molecular formulae of C4H,02 and C5H60, it is concluded that this compound is another product of aromatic ring breakdown, and it can be identified as either 1,3 Butadiene, or 2 Butynedial, or a combination of both. On the basis of these findings, a reaction pathway for oxidation of 2,4,6-Trichlorophenol by Fenton's Reagent is proposed (equation 12), containing the structures of all of the major compounds. It may be noticed here that the first intermediate compound proposed after the aromatic bond cleavage is 2,4 Hydroxy Muconic Acid. This breaks further to form the two compounds of MW 120 and 82, which are proposed to be Hydroxy Propanedioic Acid and 1,3 Butadiene (or, 2 Butynedial), respectively. This is in agreement with the mechanism for oxidation of phenol to 2,4 hydroxy muconic acid by Fenton's Reagent as proposed by Eisenhauer (1964). The main difference between Eisenhauer's findings and the present hypothesis is in the presence of two hydroxyl groups at the 2 and 4 positions of the muconic acid molecule. The best possible explanation for this is

12

Ci�CI

Y +oH·

Cl

OH CI�CI

y.oH

Cl

OH CI:QCI [H']

+ "'I -• Cl H

+ Clt¢rcl

+ HCI OH

DCH

�------------------��

J j � OH

CI:QrOH

"'I • Cl OH

OH CI�CI

+ yaH

OH

[H•]

OH c1*0\

HCI oH*

OH

H'-.

/COOH

/c, ----Ho COOH

Hydroxy Propanedioic Acid (MW = 120)

# c c-OH

I + c

�C-OH

1.3 Butadiyne (MW= 82)

c1*o + c1*c1

Cl 0

DCQ

OH OH CI¢0H �OH OH::,..I � 0

+ OH OH

Hy�OH OH* [0,]

OH 2,4 Hydroxy Muconic Acid

/ CHO c Ill c

\ CHO 2 Butynedial (MW = 82)

13

Methane Triol (MW= 64)

DCQ

oo\

l¢?";Q�', HCI -

OH

! H20 +

H�

OH

- C02 + H20

(eqn. 12)

that, in the present situation, the parent compound is 1,2,4,6 tetrol, which is an intermediate product, as opposed to phenol.

All of the above HPLC/MS analyses were performed in real time as the reaction was progressing. This was pre-planned in order to avoid any possible loss of sample integrity due to further reaction, within the sample vial, while in storage. Therefore, the mass spectra in Figures 2 A, B, and C are those of the actual chemical species formed at the respective times of sampling. The same samples were quenched and preserved in a refrigerator and analyzed again the next day. The mass spectra of those preserved Samples Nos. 1, 2, and 3 are shown in Figure Nos. 4 A, B, and C. Quite interestingly it is observed that, although they represent the analyses of the same sample, the spectra 2 A and 4 A are not identical to each other. The same discrepancies are observed between the spectra 2 B and 4 B, and between the spectra 2 C and 4 C. This suggests that, even after neutralizing the residual hydrogen peroxide in the sample, the reaction still continues in the sample. This may be simple rearrangement of molecules, or it may be further oxidation with the help of oxygen in the headspace air.

In each pair of spectra, the major differences are in the peak sizes. Moreover, one extra peak at MW = 63 .8 is observed in Figure Nos. 4 A, B, and C. From the large size peaks for Benzenetetrol, it is clear that the substitution reaction of chlorine by hydroxyl group proceeds all the way until all the chlorine atoms are totally replaced by hydroxyl groups. The reduced sizes of the 2,4 fragment peak (MW = 98) suggest further dechlorination of aromatic chlorine compound(s), and the appearance of a peak with 63.8 as the weight of the molecular ion suggests oxidative degradation of the compounds with MW of 120, or 82, or both. The molecular weight of this compound is assumed to be 64. The only possible molecular formula for this molecular weight is CH403• From an extensive search of the possible compounds, this compound has been identified as Methane Trio! (CH(OH)3).

Experiment No. 2 was conducted with continuous nitrogen sparging. The purpose of this reaction was to generate a sample for subsequent product identification by HPLC/MS, for investigation of any possible differences in the products due to the depleted DO condition. The sample was collected in a vial after sixty ( 60) minutes from the start of the reaction, quenched with sodium sulfite, and preserved in the refrigerator. The analysis was performed after several days. Interestingly, the product peaks in this case appear at the same locations as those in Figures 4 A, B, and C. This indicates that, even with the DO depleted condition, the major intermediate product species are the same as those formed under open-to-air condition. The major difference is in the size of the peak with MW= 63.8. This is probably due to the progress of oxidation during the time gap between the experiment and the analysis, which resulted in the conversion of higher molecular weight intermediate compounds to the smallest breakdown product (methanetriol). This confirms that the reaction pathway and product distribution are the same regardless of the level of DO, as long as some oxygen is available during the reaction from the atmosphere.

The intermediate products of the low molecular weights, e.g. 82 and 64 have been anticipated based upon possible match from the literature. There is no other evidence of formation of

14

these compounds. Moreover, the compounds 1 ,3 butadiene, or 2 butynedial are very unstasble under the conditions of Fenton's reactions. Therefore, it cannot be stated with certainty that this particular species is a compound, and not a fragment ion of a larger compound.

2. The Kinetic Model for TCP Removal

It was observed, during the earlier phase of this research, that TCP is oxidized very quickly by Fenton's Reagent. It was felt that the sampling time interval of ten minutes (10), used during the earlier phase, was not adequate to follow the kinetics of degradation of TCP at the beginning of the reactions. Moreover, samples used to be collected, quenched, preserved, and analyzed much later by HPLC for TCP. As discussed in the preceding section, the reaction continues in the sample vial, affecting the integrity of the samples. Experiment No. 3 was carried out to overcome these problems. The samples were collected at intervals of every two minutes for the first ten (1 0) minutes, after which the sampling intervals were increased as the reaction slowed down. The samples were also analyzed in real time by HPLC as they were withdrawn.

By a linear regression analysis, the kinetics of disappearance of TCP have been determined to be an overall second order reaction. For the purpose of this kinetic modeling only the concentrations of hydrogen peroxide and TCP were considered. DO was not considered for the kinetic modeling. The rate equation, represented by the straight line in Figure 5, is first order with respect to each of the reactants, hydrogen peroxide and TCP, as follows:

- d Ca --zrc- = k .ca. cb (eqn. 12)

Where, Ca = Cone. Of TCP

Cb = Cone. Of HP2

and, k = Rate Coefficient

The regression analysis yielded 0.13 Liter/(mM).(Min.) as the value of k at 25°C. The value of the regression coefficient (R2) was obtained to be 99.25%.

This kinetic equation has been obtained empirically, without considering the details of the reaction mechanism. The main usefulness of this equation is in the scale-up, particularly in the reactor sizing for the design of an actual treatment system. The lack of the effect of DO concentration in the equation should not be a problem in the design calculations as long as the wastewater contains normal level of DO.

15

Conclusions

1. Treatment of TCP by Fenton's Reagent is a multistep oxidation process consisting of dechlorination, aromatic bond cleavage, followed by progressive oxidation reactions, and ultimately leading to the formation of carbon dioxide and water molecules.

2. At the optimum oxidant to substrate molar ratio (H202:TCP) of 5.5:1, and the catalyst to oxidant molar ratio(Fe2+:H202) of 0.1: I, the dechlorination is almost complete within about an (I) hour of the start of the reaction. The extents of progress of the subsequent steps depend upon the amount of oxidant used, and the length of the reaction time.

3 . I t has been shown that the oxidation reaction continues even after two hours, but that the rate becomes very slow. Also, it has been shown in the earlier phase of this research that higher amounts of oxidant make the reactions faster and more complete, but the utility of hydrogen peroxide goes down when applied at rates above that determined by the optimum oxidant to substrate ratio. Therefore, although the use of longer reaction time and larger amounts of hydrogen peroxide would carry the reaction further towards completion, such measures would make the process very expensive. Longer retention time would require a larger size reactor, involving higher capital cost, while higher dosages of hydrogen peroxide would involve higher operating cost.

4. Earlier phases of this research have shown that treatment of a wastewater containing about I mM TCP by Fenton's Reagent significantly removes toxicity, and renders the wastewater biotreatable, under optimum conditions. Therefore, from an economic point of view, it is adequate to use the optimum dosages of chemicals for a pretreatment lasting two hours, and to discharge the treated stream to the municipal sewer system for subsequent biotreatment.

16

5

4

3 ::c a.

2

0

0

Fig. 1A pH PROFILE AS A FUNCTION OF OXIDANT TO SUBSTRATE RATIOS Catalyst to Oxidant Ration of 0.1:1

\ k��-- _.._-...:;

�

I 50

--tl .....

I 100

1ime in Minutes

I 150 200

.. 2.75:1.0 +5.5:1.0 :.i.8.25:1 911.0:1.0

Fig. 1 B RELEASE OF CHLORIDES IONS N3 A FUNCTIONOF OXIDANT TO SUBSTRATE MOLAR RATIOS catalyst to Oxidant Ratio of 0.1:1

100 '0 Q) l@ 80 r-

:/· Q) Q) 0: 60 Q) :g

v 0 40

..c () 20

*-0 I

0 50

•

• •

I 100

Time In Minutes

I 150

17

.....

•

.2.75:1.0 +5.5:1.0 911.0:1.0

200

4.0e6 96.5

3.8e6 3.6e6 3.4e6 3.2e6 3.0e6 2.8e6 2.6e6

(f) 2.4e6 0. () » 2.2e6 � (f) 2.0e6 c ' 118.8 Q) \ - 1.8e6 c i

1.6e6 1.4e6 1.2e6 1.0e6 8.0e5 6.0e5 176.8 4.0e5 194.8 81.0 157.0 2.0e5 A,. A, 1�9.0 1 I' � � � . '

90 120 150 180 210 m/z, amu

Figure 2 A. Mass Spectrum of Sample after Reaction Time= 10 Mins

18

96.5 4.2e6 4.0e6 3.8e6 119.0

3.6e6 3.4e6

3.2e6

3.0e6 2.8e6 2.6e6

(/) 2.4e6 0. () � 2.2e6 (/) 2.0e6 c Q) 80.8 -

1.8e6 c

1.6e6 1.4e6 1.2e6 1.0e6 8.0e5 6.0e5 : .

4.0e5 } 179.0 kA 201.0 2.0e5 141.0 163.0 J ��� � � �II "--'n 90 120 150 180 210

m/z, aniu

Figure 2 B. Mass Spectmm of Sample after:R,eaction Time= 30 Mins

19

en a.. 0 � en c Q) -c

5.4e6 5.1e6

4.8e6

4.5e6 4.2e6 3.9e6

3.6e6 3.396 3.0e6 2.7e6 2.4e6 2.1e6 1.8e6 1.5e6 1.2e6 9.0e5 6.0e5 3.0e5

... ' 63.8 . " .

96.5

81.0

'

i" .� )f 90

141.0 200.8

178.8 I 118.8 .

A�l .�l • j .,j 163.0

120 150 180 m/z, amu

Figure 2 C. Mass Spectrum of Sample after Reaction Time = 60 Mins

20

210

100

50

30

i'l. 6e5 0

.ff 4e5 � .gl - 2e5

80.0 �

80

96.8

I I 1\ 119.0

100 120

196.6

140.8 158.8 178.8 I� ' 140 160 180 200

m/z, amu

. Figure 2 D. HPLC/MS Mass Spectrum of2,4,6 TCP in 1 mM Aqueous Solution

.: 2 4 6·Trichlorophenol

Scan 150 (16.140 min) of cab231901021.d

� \0 1.2 0 "' 1.0 0 - 0.8 X

_.-., 0.6 62 ·� 0.4 48 /

97

� 0.2 "' 1[ c§., o.o 1.1. ,\, I JU .i\. .....

40 ·eo 80

" 132

/

109 1 / l!i·

100 120 140

m/z

/ 196

160 / h.

160 180

203 �

200

Figure 2 E GC!MS Mass Spectrum of2,4,6 TCP in 1 mM Aqueous Solution

114

142 132 149 150

. 178 [M]

Fig. 3 A Electron Impact Mass Spectrum of 3,5 Dichloro 1,2 Benzenediol

21

OH ,d._(OH

CI).�CI

100 M+

75

50 179\ 53 c �

25 d l j (M-2ot I

.11 ,\ .t .J ik b � � l ' ' ' '

40 60 80 100 120 140 160 180 200 m/e

Fig. 3 B Electron Impact Mass Spectrum of2,5 Dichloro 1,2 Benzenediol

(/) 0.. 0

� (/) c (]) +' c

(/) 0.. 0

-� (/) c (]) +' c

(/) 0.. 0

� (/) c (]) +' c

Plot of Spectrum from 3.86 min (2 scans) from 3-7-96 01, subtracted

1t .o 1.2e6 81.8.

I

� 8.0e5 � 96.8

! Jt fl ,! 4.0e5 127.0 Jt 116.8 162.8 ,...

8-� 100 120 1�0 160 m/z, amu

Plot of Spectrum from 2.77 min (2 scans) from 3-7-96 01, subtracted

81.0 14r .0 1.2e6 � •! 8.0e5 !] 96.8 il

I. jl_ � il I 4.0e5 ·.� 'I 116.8 127.0 Jl ... rl. ••

80 100 120 140 160 m/z, amu-

Plot of Spectrum from 1.73 min (3 scans) from 3-7-96 01, subtracted

1.6e5

1.2e6 ! 3.0e5 � 4.0e5 !j! ,... �

80.8 ' • J .� 11i IL

ao

141.0

f, 96.8 • l

� l 1 II 113.0 126.8 153.0

I • 0 0 I 100 120 140 160

m/z, amu

Fig. 4 A, B, and C (from Top to Bottom)

184.0 -· 180

185.0 0 0 0

180

185.0 ,

180

Mass Spectra of Samples after Reaction Times of 10, 30, and

60 mins. from Experiment No. I (Analyzed after 1 Day)

22

200

200.6 •

200

• 200

[Fig. 5 Reaction Kinetic Data Analysis for Disappearance of TCPJ

0.5 r-----------------,

� c i

0.4

. 0 3 ....1 •

� E � 0.2 0 "0 0.1

0.5

II

1 1.5 2 2.5 Ca . Cb (mM/L)A2

3 3.5

23

References

1. Basu, S., Sreenivasan, V., and Wei, I. W. (1996) Advanced Chemical Oxidation and Toxicity Reduction of 2,4,6-Trichlorophenol by Using Fenton's Reagent- Part II. Presented at the 1996 AIChE Spring National Meeting, New Orleans, February, 1996.

2. Basu, S., Sreenivasan, V., and Wei, I. W. (1996) Advanced Chemical Oxidation and Toxicity Reduction of 2,4,6-Trichlorophenol by Using Fenton's Reagent - Part III. Presented at the 51 st. Purdue Industrial Waste Conference, West Lafayette, IN, May, 1996.- '-

3. Bowers, A. R., Gaddipati, P., Eckenfelder, W. W., Jr., and Monsen, R. M. (1989) Treatment of Toxic or Refractory Wastewaters with Hydrogen Peroxide. Wat. Sci. Tech., 21, 477-486.

4. Chapman & Hall Electronic Publishing Division (1996) Directory of Organic Compounds, London, U.K.

5. Chen, K. W. (Stone), Basu, S., and Wei, I. W. (1995) Advanced Chemical Oxidation and Toxicity Reduction of 2,4,6-Trichlorophenol by Using Fenton's Reagent- Part I. Presented at the 1995 AIChE Summer National Meeting, Boston, August, 1995.

6. Haber, F., and Weiss, J. (1934) The Catalytic Decomposition of Hydrogen Peroxide by Iron Salts. Proc. Royal Soc. (Land.), A 147, 332 - 351.

7. Johnson, G. R. A., Stein, G., and Weiss, J. (1951) Some Free Radical Reactions of Chlorobenzene. The Action of Hydrogen Peroxide - Ferrous Salt Reagent and X-Rays on Aqueous Solutions of Phenols. Proc. Roy. Soc., 3275 - 3278.

8. Kolthoff, I. M., and Medalia, A. I. (1949) The Reaction between Ferrous Iron and Peroxides. Reaction with Hydrogen Peroxide in Absence of Oxygen. J. Am. Chern. Soc., 71, 3777 - 3792.

9. Knuutinen, J., and Korhonen, I. 0. 0. (1983) Mass Spectra of Chlorinated Aromatics Formed in Pulp Bleaching I - Chlorinated Catechols. Organic Mass Spectroscopy, 18(10), 438.

10.Merz, J. H., and Waters, W. A. (1947) The Mechanism of Oxidation of Alcohols with Fenton's Reagent. Faraday Soc. Disc. (Land.), 2, 179.

11.Potter, F. J., and Roth, J. A. (1993) Oxidation of Chlorinated Phenols Using Fenton's Reagent. Haz. Was. & Haz. Mat. 10(2), 151 - 170.

12.Sorokin, A., Suzzoni-Dezard, S. D., Poullain, D., Noel, J. P., and Meunier, B. (1996) C02 as the Ultimate Degradation Product in the H202 Oxidation of 2,4,6-Trichlorophenol Catalyzed by Oiron Tetrasulfophthalocyanine. J. Am. Chern. Soc. 118, 7410.

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13.Svec, P., Adamek, P. , and Zbirovsky, M. (1976) Spectnilni Projevy Chlorovanych Hydrochinonu (Spectral Properties of Chlorinated Hydroquinones). Sborn ik Vysok e Skoly Chemicko-Technologick e v Praze. C, Organick a Chemie a Technologie, C23, 27 - 36.

14.Walling, C., and Kato, S. (1971) The Oxidation of Alcohols by Fenton's Reagent. The Effect of Copper Ion. J. Am. Chern. Soc., 93 (17), 4275- 4281.

Acknowledgement

The authors sincerely acknowledge Professor Phillip LeQuesne, Associaciate Director, Barnett Institute of Chemical Analysis and Material Science, Northeastern University, Boston, for the helpful discussions and comments on the results from time to time.

25