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Chemical Engineering and Processing 56 (2012) 10– 18

Contents lists available at SciVerse ScienceDirect

Chemical Engineering and Processing:Process Intensification

j ourna l h o me pa ge: www.elsev ier .com/ locate /cep

arbamazepine removal from water by dielectric barrier discharge: Comparisonf ex situ and in situ discharge on water

anan Liua,∗, Shufang Meia, Djakaou Iya-Soub, Simeon Cavadiasb, Stéphanie Ognierb

School of Environmental Science and Engineering, Dong Hua University, C-201620, Shanghai, ChinaUPMC Univ Paris 06, EA 3492, Laboratoire de Génie des Procédés et Traitements de Surface, F-75005, Paris, France

r t i c l e i n f o

rticle history:eceived 16 February 2011eceived in revised form 5 March 2012ccepted 7 March 2012vailable online 23 March 2012

a b s t r a c t

Dielectric barrier discharges (DBD) were used for the degradation of carbamazepine (CBZ) in aqueoussolution. The electric discharge was generated either ex situ or in situ directly on the water surface. Tomaintain the same ozone concentration of 40 ppm in both instances, the power injected was 0.7 W inthe ex situ discharge and 12 W in the in situ discharge. The results showed 100% CBZ removal after 3 minof treatment with the ex situ discharge, while the in situ discharge only removed 81% of the CBZ after

eywords:ielectric barrier dischargearbamazepinedvanced oxidation processeszonationlasma processes

60 min. According to measurements of UV absorbance at 285 nm and 254 nm, and of total organic carbon,the ex situ discharge system also proved to be more effective than the in situ system. The measurement ofnitrogen oxides in both gaseous and liquid phases indicated that high energy in situ discharges produceda large amount of NOx. These species contributed to decreased pH and significantly slowed the CBZoxidation rate, due to their competition with ozone. Production of NOx should be avoided when usingthe DBD technique for wastewater treatment.

© 2012 Elsevier B.V. All rights reserved.

. Introduction

With the development of analytical techniques and environ-ental concerns, more attention has been paid to micropollutants

n the environment [1,2], including pharmaceutically active com-ounds (PACs), which have been detected in water bodies aroundhe world [3]. Although no evidence of adverse human healthffects from PACs in water has been found, some PACs haveeen proven to be potential endocrine-disrupting substances [4].oor removal of PACs in municipal wastewater treatment plantsWWTPs) allows their release in the environment [5]. Carba-

azepine (CBZ) (Fig. 1) has been proven to be one of the mostifficult pharmaceuticals to biodegrade in WWTPs [6,7]. CBZ is anntiepileptic drug, and 1014 ton of the drug are consumed aroundhe world yearly [8]. It can be found in most water bodies in rela-ively high concentrations (a range of 30–1100 ng/L) compared tother PACs [9]). Thus, new techniques are needed for the removalf this kind of pollutant to insure the safety of WWTP discharges.Inhis challenging context, the development of new advanced oxida-

ion processes (AOPs) is the subject of intense scientific activityt national and international levels. Among the different tech-iques, the “non-thermal plasma techniques” are very promising

∗ Corresponding author. Tel.: +86 21 67792537; fax: +86 21 67792522.E-mail address: liuyanan@gmail.com (Y. Liu).

255-2701/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.cep.2012.03.003

because the resulting processes are simple, effective and easy forfurther technological transfer and do not require the use of otherchemical agents [10,11]. These techniques involve generating non-thermal plasma in direct contact with the water to be treated.Although this is not a trivial task, the technique unequivocallypossesses several advantages compared to the conventional ozonetreatments, which are considered to be today’s “gold-standard” forwater treatment. In fact, in all the ozone-based methods, ozone isfirst produced by a non-thermal plasma discharge and then injectedinto the liquid stream to ensure the treatment. However, the ozoneyield is low because in the plasma environment, energy is dissi-pated among different dispersive mechanisms, such as radiation,and other highly energetic chemical reactions [12]. On the con-trary, if the plasma discharge is generated in direct contact withthe water to be treated, all the involved energies (UV radiation,shock waves, radical species, ions, free electrons, and others) willconverge to oxidize the pollutants. Therefore, the energetic effi-ciency could be increased when compared to the standard ozonemethods.

In non-thermal plasma reactors with direct electrical dischargeon the surface of the water, the role of ozone still remains unclear[13]. The question remains: is ozone the main species responsible

for the initiation of chain reactions leading to the degradation oforganic pollutants? In fact, ozone, the most popular agent for AOP,is generated ex situ from electrical discharges in air or oxygen, andplays an important role in non-thermal plasma reactors [14,15].

Y. Liu et al. / Chemical Engineering an

Fig. 1. CBZ chemical structure.

Hembbtatc

tubular DBD reactor already described in the literature [13]. Theelectrical discharge takes place in the gaseous gap between the

owever, few studies compare conventional ozonation and directlectrical discharge on the water surface. In this study, the treat-ent of an aqueous solution polluted by CBZ has been performed

y conventional ozonation (“ex situ” discharge) and direct dielectricarrier discharge (DBD) on the water surface (“in situ” discharge)o (i) compare the efficiencies of the two processes for CBZ removalnd (ii) analyze the possible mechanism of CBZ oxidation under

he two kinds of discharge systems. The conditions were chosen toonfirm that ozone production was identical in the two systems.

Fig. 2. Experimental set-up of in situ

d Processing 56 (2012) 10– 18 11

2. Experimental methods

2.1. Materials and procedures

CBZ and bis(trimethylsilyl) trifluoroacetamide (BSTFA) werepurchased from Sigma–Aldrich (US); tert-butyl alcohol, acetoni-trile, sodium nitrite, indigo trisulfonate acid, phosphoric acid, andsodium dihydrogenphosphate were provided by VWR (France). Allthe chemicals were of analytical grade.

Solutions of 20 mg/L CBZ (8.47 × 10−5 M) were prepared by dis-solution in tap water (TW) using magnetic mixing for 24 h. Thecharacteristics of the tap water are as follows: Cl2 0–0.2 mg/L,iron 0–0.6 mg/L, nitrates 10–40 mg/L, calcium 90 mg/L, bicarbon-ates 228–233 mg/L, chlorides 12–22 mg/L, potassium 2.0–2.1 mg/L,sodium 8.9–9.1 mg/L and sulfates 14–25 mg/L.

Fig. 2 shows the experimental set-up used to perform thetreatments by ex situ and in situ discharges. In the treatment bythe ex situ discharge (Fig. 2(a)), 1 L/min of air is fed into a glass

high-voltage electrode, a metallic disc centered in the glass tube,and the glass tube acting as the dielectric. The air containing active

and ex situ discharge on water.

1 ing and Processing 56 (2012) 10– 18

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2 Y. Liu et al. / Chemical Engineer

pecies, mainly ozone, is then introduced in a glass bottle contain-ng 100 mL of the CBZ solution. At the bottom of the bottle, a piecef sintered glass allows the distribution of the gas as tiny bub-les, ensuring good contact between the gas and the liquid to bereated.

To study the in situ discharge, the falling film gas–liquid dielec-ric barrier discharge reactor described previously by Ognier et al.13] was used. The solution to be treated was contained in theubbler and continuously pumped in the gas–liquid DBD reactorith a liquid flow rate of 6 mL/min; a treatment by direct dis-

harge on the water surface was performed. The gas–liquid DBDeactor was also fed by an airflow rate of 1 L/min. This airflowate was fed through the bubbler before being discharged in thetmosphere.

The power (P) injected in the discharge was obtained by thentegration of two periods of applied voltage and discharge current.he energy density (ED) is defined by (P × t)/V, where P is the power,

is the time of treatment and V is the volume of the solution in theubbler.

To compare the two processes, the ozone concentration mea-ured at the gas outlet was fixed at the same value of 40 ppmor the two systems. To obtain an ozone concentration of 40 ppm,he power injected was 0.7 W in the ex situ discharge and2 W in the in situ discharge. The energy density corresponds to.5 × 104 J/Lsolution in the ex situ discharge and 4.3 × 105 J/Lsolution inhe in situ discharge.

.2. Analysis

The CBZ solution was continuously pumped out for the onlineeasurement of the UV absorbance. The CBZ concentration wasonitored online by a UV spectrometer at a wavelength of 285 nm

Shimadzu SPD-6A) connected to a computer. Aromatic compoundsere qualitatively measured at a wavelength of 254 nm. Liquid

amples were collected for analysis by liquid chromatography withV detection, liquid chromatography–tandem mass spectrometry

LC–MS, Agilent 1100 series; PE SCEX API 3000), GC–MS (FocusC–DSII, ThermoScientific), ion chromatography (Metrohm 850)nd pH meter (METTLER TOLEDO). Sodium thiosulfate was imme-iately added in the collected samples to stop the reaction withesidual ozone. O3 and NOx (NO and NO2) were measured in theas outlet using GASTEC colorimetric tubes (Japan).

.2.1. LC–MS conditionWater (A) and acetonitrile (B) were used as mobile phases. The

ethod program was as follows: A was linearly decreased from0% to 50% in 15 min, held for 10 min, and then increased to 90% in

5 min. The total run time was 40 min, and the flow rate was.7 mL/min. The column temperature was 25 ◦C. The HP1000utosampler properties were the following: 100-�L-syringe size,5-�L injection volume and 200 �L/min sample speed. The HPLCystem was coupled to a hybrid quadrupole orthogonal accelera-ion time-of-flight mass spectrometer (PE SCEX API 3000), whichas used with an electrospray ionization interface operating in theositive and negative ion mode. The MS spectra were acquired overn m/z range of 100–500.

.2.2. GC–MS condition

[UV285 ab

The analyses were carried out using a Focus GC–DSII, Thermo-cientific gas chromatograph equipped with a quadrupole masspectrometer. An aliquot containing 1 �L of the derivatized sam-les was injected in the splitless mode (250 ◦C) into a CPSil 5 CB

Fig. 3. 285- and 254-nm UV ABS of CBZ degradation under in situ and ex situ dis-charge on water.

MS 30 m length × 0.25 mm ID × 0.25 �m phase thickness column.The GC temperature program was as follows: 50 ◦C for 1 min, rampat 10 ◦C/min to 300 ◦C, hold at 300 ◦C for 10 min. The mass spectrawere obtained in the electron ionization mode (70 eV). Additionalanalyses were conducted in full scan mode (m/z 35–800 with0.35 s/scan) to acquire the mass spectra for each derivative. Thederivatization procedure was based on Kevin’s methods [16], withsome minor modifications: 200 mL of sample was evaporated withhelium and air; then 1 mL pyridine and 1 mL BSTFA were added,and from the mixed solution, 250 �L of the sample was removedand heated at 70 ◦C for 2 h.

3. Results

3.1. 285-nm UV absorbance, 254-nm UV absorbance and CBZdegradation

The decrease in the 285-nm UV absorbance can be directlyrelated to the degradation of CBZ. As the online 285-nm UVabsorbance data (Fig. 3) show, in the ex situ discharge, CBZ (line3) was degraded very rapidly in the water matrix. On the contrary,in the in situ discharge, the CBZ (line 4) degradation was very slow,even when the ozone production was the same as in the ex situ dis-charge. These results were confirmed by the HPLC measurements.As the HPLC results clearly show (Fig. 4), CBZ disappeared in 3 minwith the low-energy ex situ discharge (Fig. 4(a)); however, withthe high-energy in situ discharge, there was still CBZ after 60 min(Fig. 4(b)).

After 60 min, the 285-nm UV removal with the low-energy exsitu discharge was much higher than that for the high-energy in situdischarge. The 285-nm UV removal is defined by the followingequation:

ance]before treatment − [UV285 absorbance]after treatment

[UV285 absorbance]before treatment(1)

The 254-nm UV absorbance has been widely used to char-acterize the presence of aromatic organics in water [17–19]. AsFig. 3 shows, the 254-nm UV absorbance (line 1) for the in situdischarge increased slowly from 0.5 to 0.6 during the 60-min dis-charge, while in the ex situ discharge, the 254-nm UV absorbance(line 2) increased rapidly during the first 6 min up to more than0.8 and then decreased steadily in the following oxidation pro-cedure to reach 0.3 after 60 min. The increase in the 254-nmUV absorbance observed for both experimental conditions can beexplained by the formation of mono-aromatic compounds follow-ing the degradation of the CBZ molecule. The following decrease

in the 254-nm UV absorbance observed after 6 min in the ex situdischarge was due to the further oxidation of the primary aromaticbyproducts. In the in situ discharge, only a slight increase in the254-nm UV absorbance was observed due to the slow degradation

Y. Liu et al. / Chemical Engineering and Processing 56 (2012) 10– 18 13

rum o

ohlrr

TC

Fig. 4. HPLC spect

f CBZ. The TOC removal ratio (Table 1) after the 60-min treatment

ad the same trend as the 285-nm UV and the 254-nm UV; the

ow-energy ex situ discharge was more efficient (48.2% TOC removalatio) than the high-energy in situ discharge (19.4% TOC removalatio).

able 1omparison between ex situ and in situ discharge.

ED (J/L) O3 (ppm) UV285 degradationratio(after 60 min)

UV254 degratio(after

Ex situ discharge 2.5E+04 40 77.7 46.3

In situ discharge 4.3E+05 40 40.7 -18.9

f CBZ degradation.

3.2. Characterization of the active species produced by in situ and

ex situ discharges

The experimental results presented in Section 3.1 indicate thatthe performance of the ex situ discharge in terms of pollutant

radation60 min)

CBZ removal rate(after 3 min) (%)

CBZ removal rate(after 60 min) (%)

TOC removal ratio(after 60 min) (%)

100 100 48.267.2 90.7 19.4

14 Y. Liu et al. / Chemical Engineering and Processing 56 (2012) 10– 18

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ig. 5. Nitrite and nitrate ion concentrations during discharge in the in situ dischargeystem.

egradation are much better than the ones obtained with the in situischarge, even if energy density injected in the in situ discharge isigher. To understand these results, the species generated by theischarge in the ex situ and in situ modes were characterized.

.2.1. NOx measurements in the outlet gasThe concentrations of NOx (NO + NO2) were measured in ex situ

nd in situ configurations using the energy densities used previ-usly during the treatment of the CBZ solutions. The concentrationf NOx was negligible (below 5 ppm) in the ex situ discharge, butt was 50 ppm in the in situ discharge. Higher energy was requiredn the in situ discharge system not only to reach the same ozoneoncentration but also to initiate the discharge. This unavoidableigher energy caused a higher NOx production, as Kogelschatz et al.20] supposed: the concentration of NOx increases when the energyensity in J/Lgas increases.

Nitrate (NO3−) and nitrite (NO2

−) were measured in the CBZolution during the treatment by the in situ discharge. The presencef nitrate and nitrite ions can be explained by the dissolution of NO2n the water. Different reactions have been proposed [21]:

NO2 + H2O → HNO2 + H+ + NO3− (R1)

HNO2 → H+ + NO3− + H2O + 2NO(g) (R2)

NO2 ↔ H+ + NO2− (R3)

Nitrous acid HNO2 is highly unstable and will convert into NO3−

nd NO by reaction (R2) or dissociate into NO2− by reaction (R3)

epending on the pH of the solution.In the presence of ozone dissolved in the water, nitrite ions can

lso be oxidized according to the following reaction:

O2− + O3 → NO3

− + O2 (R4)

As Fig. 5 shows, the concentration of nitrate ions in the solu-ion increases steadily during the treatment from 2 to 12 mM.he concentration of nitrite and nitrous oxide increases rapidlyt the beginning of the reaction to reach approximately 0.8 mMfter 10 min; it then remains almost stable from 10 to 30 min. After0 min, the concentration dropped rapidly, reaching zero at 60 min.he stabilization after 10 min, followed by the decrease after 30 minf NO2

− and HNO2, may be explained by the decrease in the pH ofhe solution. As shown in Fig. 6, during the treatment of the CBZolution, the pH decreased in the in situ discharge DBD reactor from.9 to 2.8 (Fig. 6) due to the generation of H+ in the water causedy the dissolution of NO2. Nitrous acid (HNO2) is a weak acid char-cterized by a pKa of 3.35. As a result, when the pH decreases, lessnd less nitrous acid is dissociated. The consequence is that NO2

−

s no longer produced in the solution because HNO2 is completelyonverted into nitrate ions and NO by reaction (R2).

After 60 min of the discharge treatment, the molar concentra-ion of NO3

− was 12.269 mM (after substraction of the 0.362 mM

Fig. 6. pH change during discharge.

of NO3− initially present in the tap water), while the molar con-

centration of H+ was 4.78 mM (pH was 2.32). As the formula ofHNO3 supposes, the same molar concentrations of the hydroniumand nitrate ions are produced when the nitric acid dissociates.Because bicarbonate ions (HCO3

−) are present in tap water inSouthwest Paris with a concentration of approximately 4.07 mM[16], 4.07 mM of H+ reacted with HCO3

− during the treatment toproduce H2O and CO2. However, there was still 3.42 mM H+ thatcould not be balanced. Moreover, if we suppose that the concentra-tions of Cl−, SO4

2−, Ca2+, K+, Na+ and Fe2+ do not change during thetreatment, the balance between the positive and negative charges(electroneutrality of the solution) cannot be verified. The result ofthis calculation gives 13.6 mM for the negative charges but only9.6 mM for the positive charges. This means that new cations arepresent in the solution. To explain these results, the followinghypothesis is proposed: an electrochemical reaction generatinghydroxide ions (OH−) and metallic cations (M2+) could take placeon the surface of the metallic electrode covered by the water [22].As an example, the following redox system is proposed:

H2O + 0.5O2 + M → 2OH− + M2+ (R5)

where M is a metal.Concerning the treatment by the ex situ discharge, the con-

centration of [NO3− + NO2

−] after 60 min of treatment was only1.02 mM, and the pH was stable at 8 during the operation. Thisexplanation makes sense, as the amount of NO2 absorbed in thesolution was too low to modify the pH due to the buffer effect ofthe bicarbonate ions. In this experiment, the molar concentrationof CBZ was 8.47 × 10−2 mM, and in this low-energy discharge con-dition, there was a very small amount of NOx produced [20]. Evenif all CBZ molecules were oxidized to acidic molecules, which mayhave a maximum of 12 acidic groups, they were 3 times lower thanbicarbonate molecules. Therefore, in the ex situ discharge system,there was no great pH change.

3.3. Identification of CBZ oxidation byproducts

For both discharge systems, GC–MS and LC–MS analyses wereperformed to detect the intermediate oxidation byproducts. In viewof the chromatograms, it was easier to detect the intermediate oxi-dation byproducts in the ex situ discharge compared to the in situdischarge. The identified compounds are summarized in Table 2.

Six oxidation products could be identified in the case of theex situ discharge and only two in the case of the in situ dis-charge. In the case of the ex situ discharge, the byproductsdetected contained either quinazoline-based functional groups(after cyclization) and/or benzaldehyde- or benzoic acid-based

functional groups. Byproducts of no. 1, no. 2, no. 4, no. 6 and no.7 have been already detected and identified in studies concern-ing the oxidation of carbamazepine by ozone alone [23] or by Fe(VII) and Mn (VI) [24]. Byproduct no. 4 was detected by McDowell

Y. Liu et al. / Chemical Engineering and Processing 56 (2012) 10– 18 15

Table 2CBZ oxidation byproducts.

No. Elemental formula Structure Molecular weight GC–MS LC–MS Ex situ discharge In situ discharge

1 C14H11NO2

N

H

OH OH

225√ √

2 C15H10N2O2

N

NO

O

250√ √ √

3 C15H12N2O2N

O NH2

O

252√ √ √

4 C15H10N2O3

NH

NO

O

O

266√ √ √

5 C15H11ClN2O4

NH

NO

OH

O

OH

Cl

319√

6 C15H12N2O4N

O NH2

O

OH

O

284√ √

7 C15H12N2O5N

O NH2

O

OH

O

OH

300√ √ √

16 Y. Liu et al. / Chemical Engineering and Processing 56 (2012) 10– 18

from t

eB

wcob11gowdp

tipwib

4

otrdlidmo

Fig. 7. Mass spectra of the fragments

t al. [23]. It may be formed by the oxidation of byproduct no. 2.yproducts no. 1, no. 6 and no. 7 were detected by Hu et al. [24].

In the case of the in situ discharge, byproducts no. 5 and no. 7ere identified. Byproduct no. 7, already identified in the ex situ dis-

harge, is often detected in ozonation treatments. The identificationf byproduct no. 5 based on its mass spectrum indicated that thisyproduct might have a chlorine group. In fact, the base peak was56, and the key fragment peaks were 43, 57, 71, 97, 121, 135 and41. The two fragments at m/z 156 and 141 may contain a chlorineroup (Fig. 7). Byproduct no. 5 could be formed after cyclizationf byproduct no. 7 and attack by a chlorine radical Cl•. The tapater contains free chlorine, and it could attack the aromatic ringuring the in situ discharge under high-energy conditions with theresence of ferric chloride acting as catalyst.

During the experiment, the solution slowly became yellow inhe ex situ discharge system and immediately changed to yellown the in situ discharge system. This coloration could indicate theresence of aromatic compounds containing nitro–NO2 functions,hich makes sense considering the presence of nitrite and nitrate

n the solution. However, no such byproducts could be identifiedy GC–MS or LC–MS.

. Discussion

By comparing the in situ and ex situ DBD on water with the samezone production, it was found that the low-energy ex situ sys-em behaved much better than the high-energy in situ system withegard to CBZ, aromatic carbon and TOC degradation. The main CBZegradation pathway in the ex situ discharge system was due to the

ong life-time oxidant ozone, which can react directly with CBZ and

nitiate chain reactions [20] to oxidize CBZ and intermediate oxi-ation products efficiently. The main role of O3 in the oxidationechanisms in the ex situ discharge was confirmed by the nature

f the intermediate oxidation byproducts, which could be detected

he in situ discharge treatment of CBZ.

and identified. These products were identical to those generallyobserved in studies concerning the conventional ozonation of CBZsolutions.

In the in situ system, high concentrations of gaseous NOx wereproduced due to the use of high energy. In fact, when a water filmcovered the ground electrode, it was necessary to apply a highervoltage to initiate the discharge. Due to this higher voltage, thedischarge was highly energetic, and not only ozone but also NOx

species were produced. The formation of nitrogen oxides in an air-DBD discharge, also called a “poisoning effect”, is a well-knownphenomenon [20]. The nitrogen oxides are NO, NO2, NO3 (formedby the reaction between NO2 and O3), and N2O5 (formed by therecombination of NO3 and NO2). Some of these species may pro-mote the oxidation of CBZ. For example, the NO3 radical can reactdirectly with many organic species [25] and NO2

− and NO3− are

likely to be photolyzed by the UV radiation emitted by the dis-charge and to produce radicals such as HO•, •NO or •NO2 [26]. Buthow do we explain the low removal efficiency observed when bothnitrogen oxides and ozone are produced by the discharge? First ofall, nitrogen oxides show lower reactivity and more selectivity thanhydroxyl radicals. However, the main reason is that nitrogen oxidesprevent or slow down the chain reactions initiated by ozone due tothe following reactions:

- Direct reaction with ozone [27]

NO2− + O3 → NO3

− + O2 K = 5.83(±0.04) × 105 M−1 s−1 (R6)

- Reaction with hydoxyl radicals [28]

NO2− + •OH → NO3

• + OH− K = (6.6–10) × 109 M−1 s−1 (R7)

In the in situ discharge, the comparison of the reaction rates ofozone with CBZ, on one hand, and ozone with NO2

−, on the other,indicates that the fast reaction between ozone and CBZ does not

Y. Liu et al. / Chemical Engineering and Processing 56 (2012) 10– 18 17

adatio

ow

0mtOtrc6

a

Fig. 8. CBZ degr

ccur. The ratio R is defined as follows (the rate constant of CBZith ozone is from Ref. [29]).

After 2 min, the concentrations of NO2− and CBZ were 0.6 and

.0847 mM, respectively. At this time, the value of R is approxi-ately 14, meaning that the reaction rate of NO2

− with O3 is 13imes higher than that of CBZ with O3. Thus, in the in situ system,3 is mainly used to oxidize nitrates to nitrites, and there is lit-

le ozone available to oxidize CBZ and initiate chain reactions. As aesult, because nitrogen oxides are not effective as oxidant species

ompared to O3 and •OH, there is still CBZ in the in situ system after0 min.

The analysis of the oxidation byproducts by GC–MS supports thessumption that the major oxidant species and then the oxidation

n mechanisms.

pathways are different in the two systems; in fact, the oxidationproducts were different in the ex situ and in situ discharge reactors.Finally, the following CBZ oxidation pathways can be proposed:

In the ex situ discharge, CBZ is oxidized by O3 and/or radicalsproduced by ozone decomposition, while in the in situ discharge,the oxidation of CBZ is performed by nitrogen species, such as theNO2

• radical (Fig. 8).

5. Conclusions

By comparing the ex situ DBD and in situ DBD on water withthe same ozone production, it was found that even though muchmore energy was injected in the in situ discharge, the ex situ

1 ing an

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[28] J. Mack, James R. Bolton, Photochemistry of nitrite and nitrate in aqueous solu-

8 Y. Liu et al. / Chemical Engineer

ystem behaved much better with regard to CBZ, aromatic carbonnd TOC degradation. This was explained by the high concentra-ions of gaseous NOx produced when high energy was injected toenerate the discharge directly on the surface of the water. In thisase, the main oxidants were nitrogen oxides, which are less effec-ive compared to ozone and can prevent ozone and other radicalsrom reacting with CBZ. Due to this high amount of NOx, the pH inhe in situ discharge system decreased significantly, whereas theH in the ex situ discharge did not change. The main conclusion ofhis work is that it is crucial to avoid the production of NOx in thereatment of wastewater by the NTP technique.

cknowledgments

This study is realized with the financial support of the Keyasic Research of Shanghai Science and Technology CommitteeNo. 11JC1400100), the Natural Science Foundation of Shanghai10ZR1401100), the National Natural Science Foundation of ChinaNo. 51108070, 51150110581, 51178093) and the Fundamentalesearch funds for Central Universities (12D11306).

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