Degradation of chlorobenzene in aqueous solution by pulsed power plasma: Mechanism and effect of operational parameters

Journal Pre-proof Degradation of chlorobenzene in aqueous solution by pulsed power plasma: Mechanism and effect of operational parameters Jerin Jose, Ligy Philip

PII:

S2213-3437(19)30599-8

DOI:

https://doi.org/10.1016/j.jece.2019.103476

Reference:

JECE 103476

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

13 July 2019

Revised Date:

19 September 2019

Accepted Date:

11 October 2019

Please cite this article as: Jose J, Philip L, Degradation of chlorobenzene in aqueous solution by pulsed power plasma: Mechanism and effect of operational parameters, Journal of Environmental Chemical Engineering (2019), doi: https://doi.org/10.1016/j.jece.2019.103476

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Degradation of chlorobenzene in aqueous solution by pulsed power plasma: Mechanism and effect of operational parameters Jerin Jose, Ligy Philip* * Corresponding Author Department of Civil Engineering, Indian Institute of Technology Madras, 600036, India

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E-mail: [email protected]; # +91-44-22574274; Fax No: +91-44-22574252

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Abstract

The degradation of chlorobenzene in aqueous solution by pulsed power plasma was

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studied. 100% degradation of 200 mg/L chlorobenzene happened in both liquid and gas phase after 12 min and dechlorination efficiency of 85.3% was obtained after 20 min. The energy efficiency corresponding to 50 % chlorobenzene degradation was 3.14 g/kWh. The plasma

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degradation efficiency of chlorobenzene was inversely proportional to the initial concentration, conductivity and alkalinity of the solution. The study of effect of pH and scavengers on chlorobenzene removal revealed the major role of •OH radicals in the degradation. The major intermediate compounds were benzene derivatives including various chlorinated and nitrogenous organics and their concentration decreased significantly after 20 min of plasma treatment.

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Organic acids such as acetate, formate and oxalate were also produced, which contributed to a part of remaining TOC (total organic carbon) in the solution. The degradation pathway of chlorobenzene involving various oxidative and reductive species is proposed. The disc diffusion test confirmed the complete detoxification of chlorobenzene solution after treatment. Keywords:

Chlorobenzene, plasma, degradation, •OH radical, intermediates, degradation

mechanism

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1. Introduction Chlorinated volatile organic compounds (VOCs) are widely used in various industries like pharmaceutical, textile, chemical manufacturing, petroleum refining, agrochemicals and plastic manufacturing [1]. They are used as solvents and as raw material in the production of other chemicals. Once released to the environment, many of these compounds do not readily degrade and pose a threat to humans and the ecosystem; most of them are toxic and some are even carcinogenic [2]. Some of these chlorinated VOCs are partially degraded by physical,

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chemical or biological processes. But the degradation intermediates formed tend to accumulate and sometimes they are multiple times toxic than the parent compound [3].Chlorobenzene, one

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of the most common chlorinated aromatic VOCs, is used primarily as a solvent, a degreasing agent, and a chemical intermediate. It is used as an intermediate in the synthesis of various

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pesticides and dyes, as a dielectric material and heat exchanger [4]. The extensive use of chlorobenzene in various industrial applications has led to their widespread release into the

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environment and subsequent contamination of soil and water resources. It has raised serious concerns because it is chemically stable, toxic, persistent and bio-accumulative nature. Chronic exposure of humans to chlorobenzene results in adverse effect on central nervous system (CNS);

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it includes numbness, cyanosis, muscle spasms etc. Exposure via inhalation can cause headache, irritation of eyes and the mucosa of the upper respiratory tract in humans. The CNS, liver, and

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kidneys of animals were affected due to chronic exposure to chlorobenzene. The Reference Dose (RfD) for chlorobenzene is 0.02 mg/kg of body weight per day in the liver in dogs [5]. Chlorobenzene has been identified as a priority pollutant by the US Environmental Protection Agency (EPA) with a maximum allowed concentration of 100 μg/L [6]. Different technologies such as biological treatment [7], pyrolysis [8], and various

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advanced oxidation processes (AOP) such as electrochemical treatment [9], Fenton’s process [10], ultrasonic degradation [11], photocatalysis [12], microwave irradiation [13] etc. have been evaluated for degradation of chlorobenzene. Among various biological treatment methods for chlorinated aromatic compounds, biotrickling filters are the main waste air treatment technology [14-16]. Even though biodegradation is a cost-effective technology for treatment of contaminants, chlorobenzene is a xenobiotic substrate, and there are only a few specific efficient strains available for chlorobenzene removal. Moreover, biological processes are sensitive to 2

various environmental factors [7]. Dilmeghani and Zahir [17] employed UV alone and a combination of UV with H2O2 and O3 for decomposition of chlorobenzene and demonstrated that the treatment time was too longer when UV alone was adopted. Drijvers et al. [18] conducted ultrasonic degradation of aqueous mixture of TCE (trichloroethylene) and chlorobenzene and found out that higher initial concentrations decreased the sonolysis rate and the presence of chlorobenzene lowered the degradation rate of TCE. La–N–co-doped TiO2 nanotubes were not effective for treatment of gaseous chlorobenzene at higher initial concentrations [19]. The major disadvantages of commonly employed AOPs for pollutant degradation include high operating

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cost, generation of sludge (Fenton’s process), incomplete mineralization, and longer treatment time [20]. In many times a combination of different AOPs are needed to achieve the desired level

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of treatment [21]. AOPs are expensive because of high energy consumption (microwave, ultrasound), continuous supply of costly chemicals (O3, H2O2, iron salts), acidic pH dependency

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and slow kinetics [22, 23]. So a major consideration for research on AOPs for recalcitrant

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compounds is a proper balance between energy requirement and removal efficiency. Plasma technology based on high voltage electrical discharges is an emerging technology for the fast and efficient degradation of recalcitrant compounds present in water and wastewater

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[24]. Various studies based on non-thermal plasma technology have been carried out for the degradation of chlorinated organic compounds in different types of reactors [25-27]. Plasma process is an advanced oxidation process (AOP) and it produces free radicals like •OH, O•, H•,

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molecules like H2O2, O3, aqueous electrons (e-aq) and physical effects including production of UV light, shockwave etc [28, 29]. In case of pulsed power plasma, the use of pulsed voltage having fast rise time and narrow width will produce strong electric field and it will result in increased

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energy efficiency [30].

Studies looking into both liquid and gaseous phase chlorobenzene degradation as well as

into the oxidative and reductive degradation pathways of chlorobenzene are scanty. Some studies based on non-thermal plasma and its combination with various metal catalysts, glow discharge plasma and a combination of plasma catalysis with biofiltration etc. have been reported for chlorobenzene degradation [4, 31, 32]. Plasma degradation studies of chlorobenzene in aqueous phase are scarce [31]. Liu and Jiang [31] employed plasma produced by contact glow discharge 3

electrolysis for degradation of 50 mg/L of chlorobenzene and it took 120 min for less than 95% removal and 240 min for complete removal. The low input voltage of 500 V used for plasma generation might be the reason for the prolonged reaction time to achieve desired degradation in their study. Fazekas et al. [33] studied the decomposition of chlorobenzene in radiofrequency thermal plasma both in neutral and oxidative conditions. Thermal plasma required very high input power and the temperature was extremely high (≥ 9000 oK). Moreover, various polycyclic aromatic hydrocarbons (PAH) and chlorinated PAH molecules were produced as byproducts.

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The plasma degradation studies already carried out for chlorobenzene degradation have limitations like incomplete removal, longer treatment time, requirement of catalysts for improved degradation, formation of byproducts etc. Hence, it is important to develop appropriate plasma

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technology for chlorobenzene degradation, which overcomes these limitations.

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Most of the previous plasma studies on chlorobenzene degradation were focused only on gaseous chlorobenzene degradation [4, 34]. In this study, degradation of high concentration of

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chlorobenzene in aqueous solution by pulsed power plasma was evaluated along with monitoring of gas phase degradation. Using the needle-plate electrode configured reactor, electrical discharge happened in air medium above the solution and the reactive species produced were

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able to react with chlorobenzene present in gas phase, gas-liquid interface and bulk liquid phase. High concentration of 200 mg/L of chlorobenzene was taken to study the suitability of plasma

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technology for the treatment of pharmaceutical wastewater containing high concentration of organic solvents like chlorobenzene. Gupta et al. [35] reported that the concentration of chlorinated solvents in untreated drug waste is in the range 600 – 700 mg/L. Batch studies were carried out to optimize different process parameters like input voltage, pH, initial concentration and conductivity, which affected the degradation efficiency. The study of effect of active species

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scavengers and solution pH on chlorobenzene degradation revealed details about the degradation mechanism. TOC analysis was carried out to evaluate the mineralization potential. The various intermediates produced during chlorobenzene degradation were identified and using those, the possible degradation pathway of chlorobenzene which involves oxidative as well as reductive species is proposed. 2. Materials and Methods 2.1. Materials 4

Analytical grade chlorobenzene, isopropanol, NaNO3, K2HPO4, KH2PO4, KMnO4, H3PO4, DMSO (dimethyl sulfoxide), DNPH (2, 4-Dinitrophenylhydrazine), NaOH and HCl procured from Rankem, catalase enzyme (Sigma-aldrich), Muellere-Hinton agar and sterile cotton swab (Himedia) were used in present study. Stock solutions were prepared using Millipore water. K2HPO4 was used to prepare solutions with different initial conductivities. 2.2. Reactor setup The pulsed power plasma for chlorobenzene degradation was produced using electrodes

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in needle – plate configuration as shown in Fig. 1. High voltage was supplied to the seven tungsten needles each having a length of 2 cm. A circular aluminium plate, placed at bottom

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(diameter = 5.2 cm) was used as the ground electrode. The solution to be treated was enclosed in the reactor using glass cylinder and the depth of the solution was 2.36 cm. Chlorobenzene

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degradation was evaluated for different gap distances of 2 mm, 4 mm and 7 mm and highest degradation rate constant was obtained for 4 mm gap and hence 4 mm was selected as the

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optimum gap for the studies. The rate constant of chlorobenzene (initial concentration = 200 mg/L) degradation at 17 kV for gap between the needles and the solution surface of 2 mm, 4mm

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and 7 mm was 0.177 min-1, 0.249 min-1 and 0.221 min-1 respectively. Ice water jacket was provided around the reactor in order to control the increase in temperature during corona discharge. Liquid samples were collected using a syringe and gas samples were collected into 4

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mL vacuum vial using a double-ended needle. The details about electrical circuit used for generation of high voltage pulses are provided in our previous study [36]. The pulse signals were generated by charging of 10000 pF capacitor which was discharged by using a rotating spark gap

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(RSG) switch. The speed of rotation of the RSG decided the frequency of pulse signals.

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(a) Fig. 1. Experimental setup for plasma treatment 2.3. Experimental procedure The chlorobenzene degradation efficiency in liquid and gas phase was determined. The effect of various parameters (input voltage, pH, conductivity, and initial concentration of chlorobenzene) affecting plasma degradation efficiency was studied by conducting batch studies.

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The details of all these experiments are given in Table S1 (Supplementary data). The diameter of the reactor was 5.2 cm and the height of the glass cylinder portion was 6.3 cm. The solution was

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filled in the reactor up to a height of 2.36 cm and the multiple needle electrode assembly was provided above the solution. The volume of chlorobenzene solution taken was 50 mL. Samples

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withdrawn at regular time intervals were analyzed using GC-FID (gas chromatograph with flame ionization detector).

concentration of chlorobenzene at time ‘t’.

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where Co is the concentration of chlorobenzene solution at time t = 0 min and Ct is the

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Scavenger studies using isopropanol and nitrate were conducted in order to elucidate the mechanism of degradation of chlorobenzene.

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In addition to calculating the degradation efficiency, the energy efficiency was also determined in order to estimate the operating cost and to evaluate the feasibility of scale up. Energy efficiency is the quantity of pollutant degraded per unit energy delivery and it is expressed in terms of gram per kWh. The input energy is obtained by integrating current and voltage waveforms over one pulse cycle and multiplying with the pulse frequency and the

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treatment time.

The effect of various operating parameters on degradation efficiency was evaluated by

calculating the rate constant in each cases. The slope of line plotted between ln( treatment time gave the first order rate constant. 2.4. Analytical procedure

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Liquid and gas chlorobenzene samples were analyzed using GC-FID (Perkin Elmer Clarus 500) connected with Elite-624 column (30m x 0.53mm, 3.00µm). The carrier gas used was nitrogen (2 mL/min). The chlorobenzene in aqueous samples was extracted using n-pentane (1:1 ratio) and injected using auto sampler. Using a gas tight syringe, 500 µL of the gas sample was injected in manual mode. The GC column was maintained at a temperature of 150 oC for 7 min. Temperatures of injector and detector were held at 200 and 250 oC, respectively and a split ratio of 3 was given.

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For the identification of intermediates, GC-MS (Agilent 7820A) connected with purge and trap facility was employed. The carrier gas used was ultra-high pure helium and HP5-MS

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column (30 m * 0.25 mm, 0.25 µm) was employed. The injector temperature was 200 oC. The oven initial temperature was set as 32 oC for 5 min, a ramp of 10 oC/min was given and the

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temperature was increased up to 250 oC and held for 5 min. 50:1 split ratio was set. The 20 mL filtered aqueous samples were subjected to purge and trap analysis, and the purge gas used was

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nitrogen.

Various ions produced (chloride, nitrate, acetate and formate) were analyzed by ion

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chromatography (IC) (Thermo Scientific Integrion HPIC) using AS18 column (4*250 mm). The eluent used for IC analysis was 35 mM NaOH (at a flow rate of 1 mL/min). TOC analyzer connected with Non-Dispersive Infrared Detector (Shimadzu) was used for TOC (total organic

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carbon) analysis. The concentration of oxalate was estimated using titrimetric method using 0.0095 M KMnO4 as the titrant. The quantification of

•OH radicals was carried out by derivatization technique as

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explained by Sahni and Locke [37]. 2.5. Toxicity study

Agar disk-diffusion [38] test was carried out for evaluating the toxicity of chlorobenzene

solution before and after the treatment. E. coli was used as model organism for toxicity studies due to its high relevance in indication of faecal contamination and thus sanitary standards.. Since the chlorobenzene solution after treatment contained H2O2 produced during electrical discharge, catalase enzyme from bovine liver (2,000-5,000 units/mg protein) (Make: Sigma Aldrich, India) 7

was added to the solution to remove the effect of H2O2. The pH of the treated solution was adjusted to 7 using NaOH. The H2O2 removal and pH adjustment were carried out in order to avoid their interference on toxicity study. E. coli was grown on Muellere-Hinton agar (MHA) media. E.coli inoculum of approximately 108 cfu/mL was applied to the surface of MHA plates using a sterile cotton swab. Pulsed power treatment (23 kV, 25 Hz) was carried out for 200 mg/L of chlorobenzene solution and liquid samples were taken. Sterile discs impregnated with samples taken at 0, 7, 14, 21 and

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30 min were fixed on the MHA where E.coli was already plated. Then the plates were incubated at 37 oC for 12 hours. After incubation, the diameter of clear zone around the discs was

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measured.

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3. Results and Discussion

3.1. Degradation of chlorobenzene in aqueous solution and gas phase

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Chlorobenzene solution of 200 mg/L concentration was prepared and plasma treatment (23 kV, 25 Hz) was carried out for 12 min in air medium. Since chlorobenzene is a VOC, it

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could have partitioned between the liquid and gas phase. Hence, the analysis of chlorobenzene concentration was carried out in liquid and gas phases. Due to gas phase electrical discharge, chlorobenzene degradation could have happened in gas phase, gas-liquid interface and bulk

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solution depending on the diffusion of reactive species. Fig. 2 depicts the degradation profile of chlorobenzene in liquid and gas phase. The initial concentration measured in liquid phase was 172.4 mg/L and that in gas phase was very small, i.e. 0.747 µg/g. The reason for small initial concentration in gas phase may be due to its low vapour pressure of 9 mm Hg. 100 % chlorobenzene degradation was obtained in liquid and gas phase after 12 min of treatment. The

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measured voltage and current profiles are given in Jose et al. [36].The energy efficiency for complete degradation of chlorobenzene in liquid phase was 0.543 g/kWh. As the treatment time increased, the gaseous phase chlorobenzene concentration showed a continuous decrease. Since the reactor was completely sealed, there was no volatilization loss during treatment. Since microsecond pulses were used for plasma generation, there was an initial increase in temperature in the solution (Fig. S1 in Supplementary data) despite ice jacket was provided. Thagard et al. [39] demonstrated that PPT (pulsed power technique) was able to treat hydrophobic compounds 8

effectively. Such compounds were in the interface in higher concentrations where it could directly react with the reactive species. It can be inferred that PPT is suitable for the treatment of VOCs like chlorobenzene. The amount of •OH, O2•‾ and H2O2 generated in 10 min of electrical discharge (23 kV, 25 Hz) were determined to be 0.260 mM, 0.046 mM and 0.353 mM respectively [36]. Energy efficiency corresponding to 50% degradation is chosen for comparing different plasma processes, because interference from various intermediate compounds may be significant

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at more than 50% degradation [25]. The energy efficiency corresponding to 50 % chlorobenzene degradation in the present study was 3.14 g/kWh. Zhu et al. [4] reported an energy yield of 0.38

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g/kWh for plasma alone case for 32 % chlorobenzene degradation and 0.87 g/kWh for plasma catalysis (CeO2/HZSM-5) for 72 % degradation. The energy efficiency obtained by Duan et al.

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[40] during chlorobenzene degradation by dielectric barrier discharge plasma along with MnO2-γ Al2O3 catalyst varied from 7.9 (40% decomposition) to 10.7 g/kWh (76% decomposition))

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according to catalyst position. Song et al. [34] studied the degradation of 300 ppm of chlorobenzene using dielctric barrier discharge plasma and also with catalysts. They obtained

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chlorobenzene removal efficiency of 78.3%, 81.4% and 97.1% in case of plasma alone, plasma combined with CeMn/TiO2 and CoMn/TiO2 respectively at an input power of 36 W. Liu and Jiang [31] carried out glow discharge plasma degradation of chlorobenzene solution (initial

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concentration of 50 mg/L) and reported 50 % degradation after 40 min and complete degradation after 240 min. They have used ferrous and ferric ions as catalysts for better removal efficiency. In the present study, 100 % chlorobenzene degradation was obtained in 12 min without the use of catalysts. Karuppiah et al. [41] reported that non-thermal plasma is suitable for removal of mixture of VOCs (toluene, benzene and chlorobenzene) than individual VOCs because of the

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formation of reactive intermediates like aldehydes, peroxides, etc.

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Fig. 2. Change in chlorobenzene concentration during plasma treatment (23 kV, 25 Hz, initial concentration = 200 mg/L)

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3.2.1. Effect of discharge voltage / input power

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3.2. Effect of operational parameters on chlorobenzene degradation

The degradation efficiency of chemical compounds by plasma processes depends on the

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energy input which in turn depends on input voltage and frequency. Hence, the degradation study of 200 mg/L chlorobenzene solution was conducted at three pulse peak voltages such as 17, 20 and 23 kV (frequency: 25 Hz). The input power corresponding to 17, 20 and 23 kV was 69, 81

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and 92 W respectively. The chlorobenzene decomposition efficiency was directly proportional to the applied voltage or the input power (Fig. 3a). At 17 kV, only 51 % of chlorobenzene was degraded after 4 min, whereas 91% degradation was achieved at 23 kV with the same treatment time.

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When the voltage increased, the discharge energy of pulse also increased and it was calculated as 2.76, 3.25 and 3.68 joule per pulse for voltages of 17, 20 and 23 kV, respectively. The amount of reactive species produced increased with increase in input energy, and as a result chlorobenzene degraded faster at higher input voltage [42]. Liu and Jiang [43] reported that rise in applied voltage increased the number of H2O+ ions in the glow discharge and its kinetic energy, which contributed to the increase in concentration of radicals.

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Energy efficiency is the amount of parent compound (in this case chlorobenzene) degraded (in grams) at a particular time divided by the amount of input energy. Higher energy efficiency obtained at 23 kV was because of greater concentration of reactive species produced at 23 kV was required to degrade higher concentration of chlorobenzene present in the solution. The maximum energy efficiency (3.14 g/kWh) corresponding to 50 % chlorobenzene degradation was obtained in case of 23 kV. With increase in percentage degradation, the energy efficiency decreased (Fig.3b). This is because energy efficiency is calculated only by considering the amount of chlorobenzene degraded and it does not consider the degradation of intermediate

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compounds. Initially more chlorobenzene molecules would be present and all the •OH radicals produced would have reacted with it and higher energy efficiency was obtained. With increase

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in percentage degradation, the amount of remaining chlorobenzene molecules decreased but the amount of •OH radicals produced would be the same due to constant input energy. Hence at

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higher percentage degradations, the amount of chlorobenzene degraded by unit input energy decreased. As a result, the energy efficiency also decreased. In case of 17 kV where there was

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small increase initially thereafter it also started decreasing (Fig. 3b). In case of 17 kV, the amount of reactive species produced initially was less to react with 200 mg/L of chlorobenzene

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present and hence the energy efficiency decreased. With increase in time, due to the production

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of more reactive species, energy efficiency increased.

3.2.2. Effect of initial concentration of chlorobenzene Since the concentration of chlorobenzene in industrial wastewater can vary over a wide range, its effect on the degradation was studied. Chlorobenzene solutions of initial concentrations

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40, 200 and 400 mg/L were subjected to corona discharge (17 kV, 25 Hz) for 12 min. The degradation efficiency of chlorobenzene decreased with increase in initial concentration (Fig. 3c). After 2 min of treatment, the removal efficiency was 58% in case of 40 mg/L chlorobenzene whereas it was only 21% for 400 mg/L solution. Similar observation of decrease in chlorobenzene removal with increase in concentration was reported by Zhu et al. [4] in plasma treatment. For a fixed input energy, a particular amount of reactive oxygen species (ROS) would be produced and initially this might not be sufficient to treat higher concentrations of chlorobenzene. When chlorobenzene concentration increased above the stoichiometric 11

requirement of reactive species produced, the removal efficiency decreased. Fig. 3d shows the change in energy efficiency with percentage degradation of chlorobenzene. The energy efficiency decreased with increase in percentage degradation and highest energy efficiency was obtained in case of 400 mg/L chlorobenzene solution. Even though the percentage degradation was a little lower in case of 400 mg/L among different initial concentrations studied, the degradation yield (energy efficiency) was high in case of 400 mg/L. This is because of the higher quantity of chlorobenzene degraded in case of 400 mg/L for a particular % degradation. For 50

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% degradation, the energy efficiency was 2.25, 1.45 and 0.55 g/kWh in case of 400, 200 and 40 mg/L chlorobenzene, respectively. When the % degradation is converted into concentration in terms of ‘gram’ and divide with the energy delivery, the energy efficiency obtained was high in

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case of 400 mg/L solution. In case of 40 mg/L solution, even though faster degradation happened, since the total amount of chlorobenzene degraded was lesser, lower energy efficiency

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was obtained.

(b)

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(a)

(c)

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Fig. 3. Removal efficiency (a) and energy efficiency (b) of chlorobenzene (25 Hz, initial concentration = 200 mg/L) at various input power or pulse voltages. Removal efficiency (c) and energy efficiency (d) for different chlorobenzene concentrations (17 kV, 25 Hz) 3.2.3. Effect of pH and alkalinity of solution 3.2.3.1 Effect of initial pH of solution The pH of solution is an important parameter that affects the plasma degradation in

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various ways. Eventhough the effects of pH are complex, an attempt was made to explain the effect of initial pH on chlorobenzene degradation, based on the concentration of reactive species

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produced, the structural variations of the organic compound and the properties of microbubbles formed during electrical discharge. Since pH of the solution became acidic during initial 4 min of

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treatment, the effect of pH evaluated in this study is the effect of initial pH of the solution. The pH of the pharmaceutical wastewater varies over a wide range (3.9 – 9.2) and chlorobenzene is

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one of the priority pollutants present in such wastewater [44]. Hence the change in degradation efficiency of 200 mg/L chlorobenzene solution was analyzed fordifferent initial pH values of 3, 7, 9 and 12 at an input voltage of 12 kV. The pH of solution affects the degradation efficiency

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mainly in two ways - (1) by affecting the concentration of various reactive species formed by their acid-base equilibrium and (2) by affecting the structure of the organic compound depending on its pKa value [45]. The structure of an organic compound changes by protonation and

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deprotonation according to the changes in pH of the solution. [46, 47]. Higher degradation efficiency was obtained in acidic pH followed by neutral pH and alkaline pH. For example, the removal efficiency of chlorobenzene reached 81 % after 6 min when the initial pH was 3, whereas it was only 58 % at pH of 12. In acidic pH, more •OH radicals were generated and H2O2 decomposition was hindered [48, 49]. When pH is in acidic range, higher amount of H• are

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produced (Eq.3) which are subsequently converted to •OH and H2O2 as shown by Eqs. 2-5 [28]. Thagard et al. [50] reported that higher concentration of H2O2 was produced in acidic pH compared to alkaline pH during electrical discharges at gas-liquid interface .

e-aq

+ H+  H•

k = 2.6 * 1010 M-1 s-1

(1) H• + H2O2  H2O + •OH

k = 5 * 107 M-1 s-1

(2) 13

H• + H2O  H2 + •OH

k = 1 * 1010 M-1 s-1

(3)

H• + O2  HO2•

k = 1.2 * 1010 M-1 s-1

(4)

H• + HO2•  H2O2

k = 1 * 1010 M-1 s-1

(5)

In alkaline pH, the chlorobenzene removal efficiency reduced due to the scavenging of •OH by hydroxide ions and also due to the formation of reducing agents like aqueous electrons (e-aq) [36]. The behavior of chlorobenzene degradation at different pHs points to the fact that

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major reactive species responsible for chlorobenzene degradation by plasma are •OH radical and H2O2. In acidic pH, the microbubbles formed during corona discharge became positively charged

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by adsorbed H+ ions and hence they might not form bigger bubbles by coalescence. As a result, high mass transfer of reactive species might occur from bubbles to the chlorobenzene containing

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solution resulting in its faster degradation [51]. The maximum energy efficiency was obtained in acidic pH and it decreased with increase in percentage degradation (Fig. 4b).

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When the initial pH is adjusted to 12 using NaOH, the conductivity of the solution became 4.57 mS/cm because of sodium and hydroxide ions. At higher conductivity, the

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streamers propagating to the surface of the solution became thicker and brighter. Higher conductivity causes larger discharge of current, intense UV light and increased temperature but results in reduced rates of generation of active species [29]. Intense streamers were produced at

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high pH because of high conductivity and and hence the chlorobenzene degradation efficiency was decreased at higher pH. When pH was 3, conductivity was 422 µS/cm and more •OH radicals were produced in acidic condition which resulted in increased degradation efficiency. Even though pH affected the degradation efficiency of chlorobenzene initially, more than

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90% removal was obtained after 14 min in all cases. This indicates that it is possible to adopt PPT for treatment of chlorobenzene containing wastewater with varying pH. 3.2.3.2 Effect of initial alkalinity of solution The effect of alkalinity on chlorobenzene degradation was assessed for three different alkalinity values - 0, 300 and 1000 mg/L as CaCO3. From Fig. 4c it can be inferred that the removal efficiency reduced with increase in alkalinity. As alkalinity increased, higher concentrations of HCO3- and CO32- scavenged the •OH radicals produced (Eqs. 6 and 7) [52, 53] 14

and only less concentration of oxidizing species was present to degrade chlorobenzene molecules. The oxidation potential of the secondary radical •CO3- is very low, Eo (•CO3-/ CO32-) =1.59 V [54]. CO32- + •OH  •CO3- + OH-

k = 4.2 * 108

(6)

HCO3- + OH-  •CO3- + H2O

k = 8.5*106 - 4.9*107

(7)

Since •OH radical was the main species (as seen in section 3.2.3.1 and 3.3) responsible

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for chlorobenzene degradation, its removal by above reactions resulted in decreased degradation efficiency. Singh et al. [55] reported that 99.8% and 98.0% reduction in inorganic carbon was

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obtained during plasma treatment in case of solutions with 150 and 300 mg/L alkalinity, respectively. In addition to that, since alkalinity acts as a buffer against decrease in pH, this also

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might be another reason for reduced chlorobenzene degradation rate because highest degradation efficiency was obtained at acidic pH of 3. According to Jiang et al. [46] ions such as CO32-, HCO3-, PO43- etc. can react with •OH radical and form some non-active products and stop the

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activity of •OH.

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The energy efficiency of chlorobenzene showed a decreasing trend with increase in percentage degradation, irrespective of alkalinity of the reaction media (Fig. 4d). The energy efficiency was inversely proportional to the alkalinity of the solution. This might be due to the

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wastage of oxidative species by reacting with carbonate ions. Wang and Chen [56] demonstrated that energy efficiency of phenol degradation reduced drastically with increase in Na2CO3 concentrations in both plasma and plasma-photo catalysis processes. 3.2.4. Effect of initial conductivity of solution

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The conductivity of the solution is significant in the sense that it influences the nature of

the discharge and the generation of chemically active species. Since the wastewater conductivity can be very high (0.5 – 1.0 mS/cm), it is relevant to evaluate its effect on the pollutant degradation [57]. In the present study, 200 mg/L chlorobenzene solutions with three different initial conductivities such as 2, 1000 and 10000 µS/cm were subjected to pulse corona discharge. The ionization of air by electrical discharge and subsequent dissolution of ionized species in water can alter the pH and conductivity of the solution [58]. Since the conductivity of the 15

solution increased during treatment by corona discharge, the effect of conductivity examined in this study is the effect of initial conductivity of the solution. The conductivity of chlorobenzene solution was adjusted using 0.3 M K2HPO4. It is clear from Fig. 4e that highest degradation efficiency was obtained when initial conductivity was minimal, i.e. 2 µS/cm. When conductivity increased, the degradation efficiency reduced and there was not much difference in case of 1000 and 10000 µS/cm solutions. When the conductivity of the solution was increased, the streamers propagating to the surface of the

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solution became thicker and brighter. Higher conductivity causes larger discharge current, intensive UV light and increased temperature but results in reduced rates of generation of active

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species [29]. Another reason could be scavenging of active species by ions present in solution. Ions such as CO32-, HCO3-, PO43-, Cl- and NO2- are well known •OH scavengers [53, 59]. These

-p

ions can scavenge the •OH converting it to some unreactive products. The degradation efficiency of 1000 and 10000 µS/cm chlorobenzene solutions are closer which indicates that very high

re

conductivity such as 10000 µS/cm did not scavenge all the •OH radicals produced. In case of gas discharge reactor, the effect of solution conductivity on reactive species concentration and thereafter on pollutant degradation depends on the volatility, hydrophobicity and initial

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concentration of the pollutant compound. Since chlorobenzene is volatile and hydrophobic, and its initial concentration was 200 mg/L, it would have diffused into the plasma channel above

ur na

liquid surface and rapidly reacted with the •OH radicals. Because chemical reactions in plasma channel are assumed to be independent of the solution conductivity, the higher conductivity of the solution did not have significant influence on chlorobenzene degradation [60]. Hence PPT is

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suitable for treatment of chlorobenzene containing wastewater having high conductivity.

16

(b)

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of

(a)

(d)

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lP

re

-p

(c)

(e)

(f)

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Fig. 4. Effect of solution pH on removal efficiency (a) and energy efficiency (b) of chlorobenzene (12 kV, 25 Hz, initial concentration = 200 mg/L). Effect of alkalinity on removal efficiency (e) and energy efficiency (f) of chlorobenzene (17 kV, 25 Hz, initial concentration = 20 mg/L). Effect of conductivity of solution on removal efficiency (e) and energy efficiency (f) of chlorobenzene (17 kV, 25 Hz, initial concentration = 200 mg/L). In order to evaluate the chlorobenzene removal rate at various operational conditions, the

degradation values were fitted to Eq. 8: = kt

17

(8)

where

= concentration of chlorobenzene (mg/L) at time = 0 min,

= concentration of

treated solution at time ‘t’ min, and k = rate constant (min-1). The plot of ln(

/

) vs time

showed good fitting to first order rate equation and hence the plasma degradation of chlorobenzene showed first order kinetics. The rate constants obtained at different levels of input peak voltage, conductivity, pH and initial concentrations are shown in Table S2. Highest rate constant of 0.624 min-1 was obtained when input voltage was 23 kV.

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3.3. Effect of scavengers on chlorobenzene removal Wastewater often contains various scavenger compounds which can quench the reactive

ro

species produced in plasma. Hence, the effect of such compounds on chlorobenzene removal was evaluated. In the present study, 1 M isopropanol was used as the •OH radical scavenger [61, 62] and 0.1 M nitrate was employed for scavenging the aqueous electrons [63-65]. The reactions of e-aq are shown in Eqs. 9 and 10 respectively.

-p

IPA with •OH and that of nitrate with

(CH3)2CHOH + •OH  (CH3)2•COH + H2O

re

k = 1.9 * 1010 M-1s-1

NO3− + e-aq  (NO3)•2−

(10)

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k = 1 * 1010 M-1s-1

(9)

20 mg/L chlorobenzene was used for scavenger studies (12 kV, 25 Hz). Fig. 5a shows that the presence of nitrate accelerated the degradation rate. This was due to the scavenging of

ur na

e-aq by nitrate ions and thereby increasing the net concentration of •OH radicals which caused faster removal of chlorobenzene. The amount of •OH generated after 8 min of electrical discharge (12 kV, 25 Hz) was 0.099 mM and 0.136 mM in absence and presence of 0.1 M nitrate respectively. Even though nitrate was generated during electrical discharge in air medium, it was not sufficient to scavenge the e-aq generated. The amount of nitrate produced after 4 min of

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discharge was 68.8 mg/L and after 10 min it increased to 239.8 mg/L (12 kV and 25 Hz). Since the nitrate produced was not sufficient to scavenge e-aq generated, 6200 mg/L (0.1 M) of nitrate was added to the solution to scavenge the e-aq. The lowest degradation rate was obtained in IPA containing solution. IPA competed with chlorobenzene for the •OH radicals and as a result, chlorobenzene degradation rate was decreased [62]. From these results, it can be inferred that •OH radical is the major species involved in chlorobenzene degradation. Liu and Jiang [43] reported similar inhibition effect of n-butanol on phenol degradation by glow discharge plasma;

18

n-butanol competed with phenol for the •OH radicals and, subsequently decreased the phenol degradation rate. Fig. 5c shows the degradation behavior of IPA. 69.2 % degradation of 1 M IPA was obtained after 14 min of plasma treatment (12 kV, 25 Hz). 3.3.1 Degradation of chlorobenzene in real matrices In addition to the mechanistic investigation about the effect of scavengers on chlorobenzene removal, the degradation studies were carried out using real matrices such as surface water and secondary effluent. Water sample collected from Cauvery river, Karnataka,

of

India (surface water) and secondary effluent collected from outlet of sequential batch reactor of sewage treatment plant of Indian Institute of Technology, Madras were used as real matrices for

ro

chlorobenzene degradation studies. 20 mg/L chlorobenzene solution was prepared in distilled water, surface water and secondary effluent and plasma degradation was carried out at 12 kV

-p

input voltage for 12 min. Fig.5d shows the degradation behavior of chlorobenzene in real matrices in comparison to that in distilled water. The degradation efficiency decreased

re

significantly in case of secondary effluent. The conductivity and COD of secondary effluent were 2.29 mS/cm and 40.9 mg/L respectively and since the ions and remaining organic matter in

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secondary effluent scavenged the •OH radicals, the chlorobenzene degradation efficiency was decreased significantly. In case of surface water, there was only a slight decrease in degradation

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efficiency

(a)

(b)

19

(d)

of

(c)

Fig. 5. Effect of scavengers on removal efficiency (a) and energy efficiency (b) of chlorobenzene

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(c) Change in percentage degradation of IPA (d) Change in percent degradation of chlorobenzene in surface water and secondary effluent (12 kV, 25 Hz, initial concentration = 20

re

-p

mg/L)

3.4. Dechlorination, TOC removal and organic acid formation during plasma treatment of

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chlorobenzene

For any treatment process, it is important to assess the mineralization potential or the degradation of intermediate compounds along with the target compound removal [66, 67]. In

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order to assess mineralization potential of plasma process, dechlorination efficiency and TOC removal was determined. Fig. S2(a) and S2(b) in Supplementary data show the generation of chloride ion and percentage TOC removal with respect to time during PPT of 200 mg/L chlorobenzene solution (92 W or 23 kV). After 16 min of treatment, 46.1 mg/L of chloride was produced and after that its concentration was almost constant. The initial chlorobenzene

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concentration in liquid phase was 172.4 mg/L and that in gas phase was very small, i.e. 0.747 µg/g. So for 172.4 mg/L, if complete dechlorination happened, 54.3 mg/L of chloride would be generated. The chloride concentration measured after 20 min of plasma treatment was 46.3 mg/L, corresponding to a dechlorination efficiency of 85.3%. The remaining chloride/chlorine might have been present in some chlorinated intermediate compounds produced during chlorobenzene degradation. The chlorinated intermediates such as benzene 1-chloro 4-nitroso, benzene 1-chloro 3-nitro, 1, 2-dichlorobenzene, 2-chlorophenol etc. were identified in this study 20

and their concentration was found to decrease with time. The various non-chlorinated intermediates detected such as nitrobenzene, nitrophenol, benzene, benzoquinone, toluene and organic acids show the dechlorination potential of plasma treatment of chlorobenzene (section 3.5). Liu and Jiang [31] studied the degradation of 100 mg/L chlorobenzene by glow discharge plasma and obtained almost complete dechlorination (30 mg/L chloride) after 200 min. Compared to their studies, faster dechlorination happened in 20 min in the present study. The longer time required for complete dechlorination in the study by Liu and Jiang [31] might be

of

due to the low input power used for plasma generation. The TOC removal obtained in case of 200 mg/L chlorobenzene solution after 12 min was

ro

37.6 % and it became 49.1 % after 30 min. This shows that some intermediate compounds produced during chlorobenzene degradation were resistant to plasma treatment and remained in

-p

solution. The intermediates identified included many chlorinated and nitro group containing aromatic and aliphatic compounds (Table S3). One such intermediate compound which

re

contributed to a part of final TOC was short chain carboxylic acid. Carboxylic acids such as oxalate, acetate and formate were detected during chlorobenzene degradation and formation kinetics of acetate and formate is shown in Fig. S2(c). The concentration of acetate increased

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slowly and reached 5.80 mg/L after 18 min of treatment and formate concentration was 7.19 mg/L after same time. The TOC contributed by 7.19 mg/L formate was 1.92 mg/L and that by

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5.80 mg/L of acetate was 2.36 mg/L and together they contributed 4.38 mg/L of final TOC. According to previous studies, short chain carboxylic acids like formic, acetic and oxalic acids, are more difficult to degrade than phenol [68, 69]. Ceriani et al. [70] reported that during nonthermal plasma degradation of phenol, the concentration of acetic acid remained stable after reaching a particular concentration and amount of oxalic and formic acids increased

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continuously. Zhu et al. [4] reported a gradual decrease in TOC with increase in specific input energy and it was because of the incomplete oxidation of chlorobenzene at low specific input energy. In Fenton’s degradation of chlorobenzene, Sedlak and Andren [10] reported that the initial TOC of 110 mg/L became 58 mg/L after 7 hours of treatment and there was no significant change in TOC after 5 hours. They put forth that the stabilization of TOC after 5 hours is due to the inability to regenerate Fe2+ used in Fenton’s process.

21

The concentration of nitrate generated after 10 min of electrical discharge at 23 kV (frequency: 25 Hz) was 799 mg/L. This was because of the electrical discharge in air medium resulting in dissociation of O2 and N2 which led to the production of nitrate in the solution [71]. The production of nitrate can be hindered by using either argon or helium instead of air in the gas phase where discharge happens. Pothanamkandathil et al. [72] reported that during corona discharge (16 kV, 25 Hz) the concentration of nitrate produced reduced significantly when oxygen and argon (8.62 mg/L and 9.31 mg/L respectively) were present instead of air (134

3.5. Intermediates analysis during chlorobenzene degradation

of

mg/L).

ro

The low TOC removal indicates formation of some intermediates which did not get mineralized during plasma treatment. During chlorobenzene degradation, the intermediates

-p

produced were identified using ion chromatography (IC) and GC-MS and it included benzene and its derivatives, chlorinated aliphatic and aromatic compounds, nitrogenous organics,

re

aldehydes, organic acids etc. The formation of nitrogenous organic compounds was expected because high voltage discharge in air medium would dissociate nitrogen molecules and forms

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nitrogen oxide radicals and its subsequent attachment to aromatic rings. The details about formation of organic acids are explained in section 3.4. Zhu et al. [4] reported that the major chlorobenzene degradation products by nonthermal plasma catalysis were benzene derivatives,

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nitrogenous organics, COx and O3. The intermediates identified are given in Table S3 (Supplementary data). Liu and Jiang [31] demonstrated that the chloride ions, formic acid, acetic acid, oxalic acid, three isomeric chlorophenols and phenol were the intermediates produced during chlorobenzene degradation by glow discharge plasma and the concentration of chlorophenols increased to the maximum at 40 minutes and decreased slowly with longer

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discharge time. Chlorobenzoquinone, dichlorobiphenyl and chlorophenol, were detected as intermediates during oxidation of chlorobenzene with Fenton’s reagent [10]. Dilmeghani and Zahir [17] detected compounds such as phenol, chlorobiphenyl, biphenyl, benzaldehyde, chlorophenol, dichlorobiphenyl isomers during various UV based treatment of cholorobenzene. Apart from these intermediates reported in literature, many other intermediates as shown in Table S3 are identified in this study.

22

The change in concentration of various intermediates identified using GC-MS is shown in Fig. 6 and Fig.S3. The intermediates were formed at trace levels compared to the chlorobenzene transformed. Among the intermediates, the concentration of five compounds – 1-chloro 3nitrobenzene, nitrobenzene, toluene, naphthalene and 1, 2-dichlorobenzene was determined and their change in concentration is plotted in Fig.6. In the case of other compounds, the change in area of the chromatogram peak is plotted in Fig.S3 which corresponds to the change in concentration of those compounds. It can be seen that the concentration of all the intermediates

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identified using GC-MS initially increased, reached a maximum value (at 4 min or between 4 and 8 min) and thereafter it decreased and almost completely degraded after 20 min. Among the intermediates whose concentrations were determined, the major one was 1-chloro 3-

ro

nitrobenzene. The concentration of 1-chloro 3-nitro benzene reached 1.308 mg/L at 8 min and thereafter its concentration decreased. The change in concentration of acetate and formate is

ur na

lP

re

decreased slightly after 18 min compared to 12 min.

-p

depicted in Fig. S2(c). Acetate concentration increased with time, and formate concentration

(a)

(b)

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Fig. 6. Change in concentration of chlorobenzene degradation intermediates identified using GC-MS

3.6. Possible degradation mechanism / Decomposition pathway The study of effect of different scavengers on chlorobenzene removal has showed that it got degraded faster by •OH radicals and also undergone reductive degradation by e-aq, though at a lower rate. Hence, it is likely that the degradation products were formed through oxidation and

23

reduction during plasma treatment of chlorobenzene. The degradation pathway is proposed using the identified intermediates (Table S3). The major degradation routes are formation of nitrophenol (aqueous electron reaction), benzene (H• reaction), toluene, chlorophenol (•OH reaction) and nitrogen containing chlorinated compounds (•OH reaction). These aromatic intermediates were converted to benzoquinone or benzoic acid and further ring opening reactions happened and further oxidation resulted in formation of CO2 and water.

of

There are mainly five reaction routes shown in the proposed pathway (Fig. 7). •OH radicals are the primary species responsible for chlorobenzene degradation in the plasma

ro

treatment. Two different reaction mechanisms are possible for the reaction between •OH and chlorobenzene – (1) addition of •OH radical to the aromatic ring and formation of chloro

-p

hydroxyl cyclo hexadienyl (ClHCD) radical, and (2) the abstraction of hydrogen atom by •OH radical (Routes 4 and 3 respectively) [73]. The organic radicals formed through these two

re

mechanisms might undergo several possible reactions as shown in the pathway. The •C6H4Cl radical formed through route 3 could have converted to 1,2-dichlorobenzene, 1-chloro 4-

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nitrosobenzene and 1-chloro 3-nitrobenzene by reaction with •Cl, •NO and •NO2 radicals, respectively and all these compounds might have undergone further oxidation with •OH radicals to form hydroquinone and subsequent degradation. In an ozonation degradation study of 1-chloro

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4-nitrobenzene, Shen et al. [74] proposed degradation through hydroquinone and benzoquinone and subsequent ring opening to form various carboxylic acids, ketones and aldehydes. In the presence of oxygen or other strong oxidants, ClHCD radical could have been converted to chlorophenol. In Fenton’s degradation study of chlorobenzene, Sedlak and Andren [10] reported the formation of chlorophenol and chlorobenzoquinone from ClHCD radical

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In route 1, •C6H5 radical was formed by e-aq mediated reaction and its subsequent reaction

with •NO2 radical would have resulted in formation of nitrobenzene. Hydroxylation of nitrobenzene by •OH radical could have formed nitrophenol. Its subsequent oxidation and ring opening might have formed 1-hexanol 2-ethyl and butanal. The H• radical has one electron on 1s orbital and it tends to leave and as a result reaction of two H• radicals with chlorobenzene resulted in formation of benzene (route 2) [12]. Quinone would be formed by reaction of reactive species such as O2•- with benzene. In this study, benzoquinone was identified which could have 24

formed through oxidation of hydroquinone. Benzoquinone was reported as one intermediate during chlorobenzene and chlorophenol degradation in various studies [10, 75]. 1, 2 propane diamine could be formed by reaction between nitrobenzene and benzoquinone. Toluene would have formed through the reaction of CH3 group released from any of the ring opened products with benzene (route 5) and it might have degraded through benzaldehyde, benzoic acid and finally to various organic acids [76]. The various mechanisms of ring opening of aromatic compounds could be continuous

of

attack of •OH radicals on aromatic intermediates, direct ozone attack on the aromatic compounds by cycloaddition and successive hydrolysis and decarboxylation which produces aliphatic acids

Jo

ur na

lP

re

-p

ro

[45, 75]. In this study various aliphatic acids such as oxalate, acetate and formate were detected.

25

of ro -p re lP ur na Jo Fig. 7. Proposed degradation pathway of chlorobenzene

26

3.7. Toxicity assay The toxicity of the chlorobenzene solution after pulsed power treatment was evaluated by conducting disc diffusion test using E.coli. The solution from the disc diffused into the agar and inhibited the growth of E.coli bacteria and the diameter of the zone corresponded to the toxicity of the solution [38, 77]. The toxicity of the solution at regular time intervals is shown in Fig. S4. The zone of inhibition at 0 min indicates the extent of toxicity of untreated chlorobenzene solution and its diameter was 10 mm. But there was no clear zone observed around the discs for

of

samples taken at 7, 14, 21 and 30 min. It shows that toxicity of the solution was removed due to plasma treatment and the intermediates produced or the final degradation byproducts were not

ro

toxic to E.coli. Thus pulsed power plasma treatment of chlorobenzene is efficient and nontoxic solution is produced by PPT.

-p

3.8 Energy consumption and electricity cost

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The estimation of operating cost of a treatment technology is a very important aspect regarding its implementation. The energy efficiency of chlorobenzene degradation (92 W or 23 kV) was 3.14 and 0.527 g/kWh for 50 % and 100 % degradation respectively. Various previous

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studies have demonstrated the economic aspects of different wastewater treatment technologies [78-80]. Muga and Mihelcic [79] reported that the cost of mechanical treatment (activated sludge process) was about 4–5.5 times greater than lagoon system and 4–6.5 times greater than a land

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treatment processes. The energy consumption (kWh/m3) in this study was calculated by dividing the total input energy by the volume of the solution treated (50 mL) [81]. The total input energy was determined by multiplying the single pulse energy with pulse frequency and the treatment duration. Fig. S5 in supplementary data shows the variation in energy consumption with respect to percentage degradation for different input powers of 69, 81 and 92 W. When the input power

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was 92 W, the energy consumption for 100 % degradation of chlorobenzene was 245 kWh/m 3, which is corresponding to 21.7 $/m3. The electricity tariff for industries was considered as Rs 6.35/unit (https://www.tangedco.gov.in /linkpdf /ONE_PAGE_STATEMENT.pdf). Table 1 shows the comparison of treatment cost of various technologies employed for degradation of different pollutants in wastewater.

27

Table 1

Pollutant compound/Wastwater

Treatment cost ($/m3)

Reference

Aerobic / Anaerobic treatment

Soluble and suspended organics

0.02 – 0.2

[82]

Ultrasonication + Photocatalysis

Phenol

1955

[83]

Ultrasonication + UV

Phenol

793

Ultrasonication

TCE

24

Electrochemical treatment

Phenol–formaldehyde resin manufacture

8.1a

Electrochemical treatment

Oil refinery

9.4a

[84]

Electrochemical treatment

Bulk drug manufacture

[84]

Ultrafiltration

Metal working oily waste

Plasma

ro

[83]

-p

[83]

lP

12.5a

[84]

2.8

[85]

Phenol derivatives

24.9 – 40.5a

[86]

Chlorobenzene

21.7a

Present study

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Plasma

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Treatment technology

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Economic comparison of various technologies employed for degradation of different pollutants in wastewater

a – electricity cost

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4. Conclusion

The present study demonstrated the suitability of pulsed corona discharge generated by

needle-plate configured reactor for the treatment of chlorobenzene in both liquid and gas phase. Complete degradation of chlorobenzene happened in 12 min with an energy efficiency of 0.528 g/kWh. The reactive species generated in plasma channel degraded chlorobenzene molecules in gas phase, gas-liquid interface and in bulk liquid phase. The percentage degradation and energy efficiency increased with increase in input voltage. The chlorobenzene degradation was found to 28

decrease with increase in initial concentration, initial pH, initial conductivity and alkalinity of the solution. The scavenging of aqueous electrons by nitrate resulted in increased concentration of •OH radicals which resulted in enhanced chlorobenzene degradation. The major reactive species involved in chlorobenzene degradation was found out to be •OH radicals. Chlorobenzene degradation studies in real matrices revealed that, the removal was affected significantly in case of secondary effluent and slightly in surface water. After 30 min of plasma treatment, 49.1 % TOC removal was obtained and the remaining TOC was due to the production of recalcitrant

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intermediate compounds like oxalate, acetate, formate etc. 85.3% dechlorination efficiency was achieved after 20 min. 17 degradation intermediates were identified which included chlorinated and nitrogenous compounds and their concentration decreased to very small values (1 chloro 3

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nitro benzene: 67.1 µg/L, toluene: 0.452 µg/L, 1,2 dichloro benzene: 0.398 µg/L) after 20 min of treatment. Using the intermediate compounds, the possible degradation pathway of

-p

chlorobenzene, which included reactions of oxidative and reductive species, is proposed. Disc

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diffusion test using E.coli revealed the nontoxic nature of treated solution.

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Acknowledgements

The authors express their deep gratitude to Dr. Sarathi Ramanujam, Professor, Department of Electrical Engineering, Indian Institute of Technology Madras, 600036, India for providing the

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high voltage power supply and the pulse signal producing facility for the experimental work. The authors acknowledge the financial support received from Department of Science and Technology (DST), Government of India, Grant No. DST/TM/WTI/WIC/2K17/82(G) for this study.

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