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Rapid Removal of Carbofuran from Aqueous Solution by Pulsed Corona Discharge Treatment: Kinetic Study, Oxidative, Reductive Degradation Pathway and Toxicity Assay Raj Kamal Singh, Ligy Philip, and Ramanujam Sarathi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01191 • Publication Date (Web): 05 Jun 2016 Downloaded from http://pubs.acs.org on June 8, 2016

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Industrial & Engineering Chemistry Research

254x190mm (300 x 300 DPI)

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Rapid Removal of Carbofuran from Aqueous Solution by Pulsed Corona Discharge Treatment: Kinetic Study, Oxidative, Reductive Degradation Pathway and Toxicity Assay Raj Kamal Singha, Ligy Philipa,*, Sarathi Ramanujamb, * Corresponding Author a b

Department of Civil Engineering, Indian Institute of Technology Madras, India - 600036,

Department of Electrical Engineering, Indian Institute of Technology Madras, India -6000 36, E-mail: [email protected]; # +91-44-22574274; Fax No: +91-44-22574252

Abstract This work demonstrates the application of plasma generated by pulsed corona discharge for the rapid degradation of carbofuran from aqueous solution. Carbofuran with a concentration of 0.5 mg/L to 30 mg/L was effectively removed within 4 min to 10 min of treatment time, at 101.5 W input power. Degradation yield (2.95 g/kWh) was much higher for 10 mg/L carbofuran concentration compared to degradation yield (0.37 g/kWh) for 0.5 mg/L carbofuran. The effect of pulsed voltage, pulsed frequency, initial pesticide concentration, pH and effect of radical scavengers (HCO3-, CO32- and humic acid) was also investigated in detail. Appearance of some key intermediates confirmed the possibility of formation of some demethylated products due to direct electron impaction during degradation process. Seven intermediates were identified, and possible oxidation and reduction mechanism of carbofuran degradation pathway was proposed. Microalgae ecotoxicity study confirmed the complete detoxification of carbofuran after 14 min corona discharge.

Keywords: Carbofuran degradation, pulsed corona, streamer, mineralization, degradation yield, ecotoxicity

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1. Introduction Presence of emerging contaminants (ECs) such as pharmaceutically active compounds (PACs), pesticides, personal care products etc. in water bodies are known to cause irreversible harmful effects to living beings and are recognised as a worldwide threat

1–3

. Direct application of

pesticides in agricultural fields, rain water run-off from the fields, and discharges of wastewater from the industries are recognised as the major sources of pesticide contamination in surface water and groundwater 4. United States geological survey (USGS) revealed that more than 80% of water and fish samples from all streams sampled in the U.S. contain at least one pesticide 5. These EC’s are toxic and tend to remain persistent in water bodies even at a lower concentration (µg/L to ng/L). Further, the studies conducted on aquatic organisms revealed the nature of toxicity with respect to its growth rate, bioaccumulation, reproduction, geno-toxicity along with the

morphological

and

physiological

abnormalities

6

.

Carbofuran

(2,3-dihydro-2,2-

dimethylbenzofuran-7-yl methyl carbamate), a carbomate group pesticide, used worldwide and also in India poses a severe threat to the aquatic organisms and has also been reported to have carcinogenic properties

7,8

. Carbofuran is highly soluble in water (700 mg/L)

9

and

concentrations as high as 6.8µg/L has been reported in river waters 10. The commonly employed tertiary water treatment systems such as sand filtration, biodegradation, membrane bioreactor, microfiltration, and activated carbon adsorption are not sufficient enough to produce treated water with concentrations of EC’s under acceptable limit 11,12

. Considering the above problems, the application of advanced oxidation processes (AOPs) to

mitigate adverse effect of pesticides has gained momentum in municipal water and wastewater treatment sectors. AOP is an efficient and environmental friendly method with the capability of breaking down toxic pesticides into less harmful by-products or completely mineralize it The degradation of carbofuran and other pesticides by ozone, UV processes

15,16

, photo Fenton

17

, solar photo-catalysis

18,19

12,13

.

14

, ultrasound, Fenton

, and electrochemical process

20,21

has

been studied by various reseachers. There are certain advantages and disadvantages associated with all the AOPs in terms of efficiency and energy consumption 22. Among the AOP’s, plasma process that generates hydroxyl radical and other reactive species (OH˙, H2O2, O3, HO2., NO˙, NO2˙, O2˙- etc) in water has the potential of completely removing the pollutants and thus resulting in a cleaner technology 23. Pulsed corona discharges, a method used for creating plasma 2 ACS Paragon Plus Environment


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in the air

24

, finds its application in water disinfection

very recently for the removal of EC’s from water

25–28

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, organic dye degradation

32–34

29 - 31

and

. Although plasma treatment can

completely remove the pollutants, its potential to completely mineralize the pollutants still remain debatable with limited literature avialable on this aspect. For complete understanding of pollutant degradation, it is essential to find the degradation pathyway and the governing mechanism. Previous studies and findings have

mostly reported the oxidative pathway of

pollutant degradation where the hydroxyl radicals play a major role

16,18,19

. However, in plasma

treatment processes, free electrons are generated along with the hydroxyl radicals

23,24

. These

electrons may have a potential role in the degradation of ECs in the form of hydrated electrons following a reduction pathway. Many a time, the intermediates formed during the degradation processess are more toxic than the parent compound

35,36

. Hence, it is essntial to carryout

ecotoxicity assays on microorganisms to confirm the mineralisation of the compound. With the ecotoxicity assays, the toxic nature of the parent compound and the intermediates obatined during the treatment procceses could be evaluated, thus indicating the safety of the treated water to be used for various beneficiary uses. Also, to the best of author’s knowledge, insights on ecotoxicity of the intermediate generated during the degradation of carbofuran by plasma treatment processes are very scanty. The present study focused on the treatment of carbofuran by the application of pulsed corona discharge. The objectives of this study include (i) Effect of environmental parameters like carbonates, bicarbonates, natural organic matter, pH etc. commonly found in water and process parameters such as voltage, frequency and time on pesticide removal by plasma corona discharge (ii) confirmation of mineralization of carbofuran by performing the TOC analysis and mass spectroscopy analysis, (iii) to understand the possible role of e- in carbofuran degradation (reduction pathway), which can widen the understanding about the degradation pathway of an organic molecule during plasma treatment process, and, (iv) evaluation of ecotoxicity of carbofuran and its intermediate degradation products using Chlorella vulgaris as the model micro-alga.

2. Materials and Methods 2.1. Reactor set up


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Pulsed corona discharges for water treatment study was generated through multiple-needle plane electrode configuration, as shown in Figure 1, and the detailed description of the experimental set-up has been provided in our previous studies

25,26

. The top tungsten needle electrodes have

radius of curvature of 50 µm and are connected to high voltage. The local electric field at the tip of the needle electrodes was high and could cause continuous streamer propagation in the liquid solution. A rotating spark gap (RSG) was used for the generation of square pulses. The applied voltage and injected current to the test cell were measured using voltage probe (EP-50K, PEEC, Japan) and current probe (Pearson Electronics, USA, model no - 101), respectively. Voltage and current profiles produced by high voltage discharge were recorded by a digital storage oscilloscope (Tektronics, TBS 1102B, 100 MHz). The frequency and the duration of square pulses were controlled by varying the speed of RSG. To maintain a constant temperature during the degradation process, iced water jacket was provided around the reactor. The inner diameter of the reactor was 6 cm, effective height was 7 cm and the spacing between needle electrodes was 1.5 cm. The cylindrical part of the reactor was made up of glass and aluminium was used as the ground electrode. The experiments were carried out with a liquid volume of 50 mL throughout the study.

Figure 1 – Experimental setup



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2.2. Chemicals Analytical grade carbofuran (98% purity) procured from Sigma Aldrich, India, was used for the study. Sodium bicarbonate (Rankem, India) was used to understand the effect of alkalinity on MB degradation. Humic acid (Himedia, India) was used as natural organic matter. NaOH (Rankem, India) and HCl (Merck, India) was used to adjust the pH of pesticide solution.

2.3. Batch study To characterize the effect of different operating parameters such as voltage, frequency, time and environmental parameters such as alkalinity, pH and natural organic matter (NOM), batch studies were carried out by varying the level of each parameter. The variation of the parameters for experimentation is shown in Table 1.

Table 1 - Three levels for each of the operating and environmental parameters Parameters

Lower level

Medium level

Higher level

Voltage (kV)

15

20

25

Frequency (Hz)

20

25

30

pH

4

7

9

Alkalinity (mg/L as

0

200

400

0

5

10

CaCO3) Humic acid (mg/L)

The experiments were carried out using an initial carbofuran concentration of 1 mg/L. Since total reaction volume (50 mL) was less, the solution was manually mixed during each sampling (after 2 min of discharge) to maintain uniform concentration of carbofuran in the reactor. The pesticide degradation efficiency was calculated using Eq. 1

= 1 − × 100


(1)

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Where, C0 = initial concentration of carbofuran solution and Ct = treated concentration of carbofuran at time t. The efficiency of pesticide degradation was better illustrated by the degradation yield (Y), defined as the amount of pesticide degradation per unit of energy consumed in streamer discharge (Eq. 2).

(/ℎ) =

(2)

where C0 is the initial concentration of the pesticide in g/L, η is the degradation efficiency of pesticide in %, V is the solution volume in L, Wp is the input power in kW and t is the treatment time in h. The input power can be calculated using Eq. 3 and Eq. 4. &'

/ = !&( "(#)$(#)%# &'

) = !&( "(#)$(#)%# × *

(3) (4)

By integrating the voltage (U) and current profile (I) over one pulse cycle, deposited energy per cycle (Ec/pulse) can be calculated (Eq. 3). Input power (Wp) can be calculated using Eq. 4 and multiplying with treatment time provides total energy consumption for the treatment period.

2.4. Analytical methods Carbofuran concentration was analysed using HPLC – UV/Vis (Dionex, Ultimate 3000) using C18 (220 mm × 4.6 mm) reverse phase column at 220 nm wavelength. Acetonitrile and water ratio of 70:30 and a constant flow rate of 1 mL/min were used for the analysis. 20 µL of carbofuran solution was used for the analysis. Degradation products of carbofuran were analysed using Thermo elite ion trap mass spectrometer (Thermo Scientific, USA) in the mass range of m/z = 50 to m/z = 500. The orbitrap mass spectrometer was operated in a positive heating electro-spray ionization mode. Electro-spray voltage of +1.5 kV, ion source heater temperature of 360ºC and capillary temperature of 350ºC was maintained during the analysis. The flow rates for sheath, auxiliary and sweep gases were 35, 10 and 0 arbitrary unit, respectively. The S-lens RF level was set at 60 %. The samples were injected with absolute methanol (Sigma Aldrich, India). Organic carbon contents (TOC) of carbofuran and its degradation products were analysed using total organic carbon analyser (Shimadzu, Japan) equipped with NDIR analyser.

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Microalgae, the first level of the trophic chain in water bodies, are commonly used to assess the toxicity of hazardous substances in the water 37. Commonly found micro-algae Chlorella vulgaris was used for the assessment of toxicity of carbofuran along with their intermediates formed during the treatment process. Chlorella vulgaris culture was obtained from Department of Biotechnology, Indian Institute of Technology, Madras, India. The culture was inoculated in sterile BG 11 broth (Himedia, India) and allowed to grow in an orbital shaker (150 rpm) for 20 days. The algal cells were incubated under white fluorescent light (12 h light: 12h dark) at 25°C. For toxicity assay, 106 order of initial microalgae cell count and 10 mg/L initial carbofuran concentration were used. Toxicity of contaminants was confirmed by percentage loss in the algal cell viability. Cells were incubated (72 h) with the treated and untreated samples, which were obtained after different time of plasma treatment. After interaction period, intact cells were counted on a haemocytometer under an optical microscope (Nikon Eclipse LV100, Japan). Appropriate control experiments were performed in distilled water containing micronutrients, to confirm the toxicity contributed by ECs alone. Prior to cells incubation, pH of the treated solutions were adjusted to 7 and catalase (Bovine lever) (Sigma, India) was added to eliminate the effect of H2O2, which gets produced from corona discharge. Percentage loss of cell viability was also calculated with respect to control. Trypan blue stain was used to distinguish between live and dead cells prior to microscopy.

3. Results and Discussion 3.1. Current - voltage (I – V) characteristics Figure 2 shows the typical voltage and current pulses measured during the tests. It is observed that irrespective of applied voltage magnitude, the rise time and the duration of the voltage wave shape is the same. The rise time of the pulse is calculated as per international standards [IEC Standards, 60060]. In the present study, the duration of the applied voltage is measured at the time required for the pulse to rise from 10% of its peak voltage to reduce to the 10 % of the peak voltage. The applied voltage magnitude and the duration of the applied voltage have high impact on efficiency of the system. The duration of the applied voltage gets altered due to conductivity 7 ACS Paragon Plus Environment

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of the liquid. The applied voltage magnitude and duration of the pulse have high influence on amount of energy deposited to the test setup. In addition, in the present study, due to conductivity of the liquid, the applied voltage wave shape is more of double exponential pulse than a square pulse. In the present study, the rise time of the double exponential pulses were measured as 0.6 µs, which is generally considered as an optimum choice over the other waveforms

38,39

. The shape of the pulse voltage and pulse current waveforms remained

unchanged with increasing voltage, while amplitudes of the waves increased and pulse width decreased with increasing applied voltage. The distortion in the wave shapes may be due to injection of current due to corona discharge and streamer propagation in the test cell. The effect of applied voltage on the energy discharge was evaluated using Eq. 3 and 4. With the increasing applied voltage, the energy deposited per pulse also increased. The calculated values are 2.8, 3.4, 3.7 and 4.05 J for 17, 20, 23 and 25 kV, respectively. The estimated total energy discharges are 25.2, 30.6, 33.6, and 36.5 kJ at 17, 20, 23 and 25 kV, respectively for 6 min duration with pulse frequency of 25 Hz.

(a)

(b)

Figure 2 – Time dependent pulse voltage and pulse current at different input power (a) pulse voltage (b) pulse current ; Pulse forming capacity = 10000 pF and pulse frequency = 25 Hz

3.2. Effect of voltage on pesticide degradation With the pulse power technique, the supply voltage has high significance effect on pesticide degradation. Carbofuran degradation efficiency and degradation yield (Y) at different applied voltage with respect to time is shown in Figure 3. The experiments were carried out with double exponential voltage wave shape with a frequency of 25 Hz and an initial carbofuran concentration of 1 mg/L. The rate of carbofuran degradation was high in first few minutes, and then rate of degradation decreased with increase in time. It was also observed that the increase in 8 ACS Paragon Plus Environment


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voltage significantly increased the rate of carbofuran degradation. The time required for the complete degradation of carbofuran was 6 minutes at 25 kV, whereas it increased to 10 minutes when the voltage was reduced to 15 kV. Increase in voltage magnitude increased the generation of reactive species such as hydroxyl radical, hydrogen peroxide and ozone concentration in aqueous solution 25. These reactive oxygen species have very high oxidation potential, which are capable of oxidizing the pollutants in very short time. UV generated in reactor may also assist the hydroxyl radical formation by radiolysis of water 29. UV incident energy flux from streamer discharge was quantified. The flux also increased with the increase in voltage

25

. Though

pollutant degradation efficiency increased with increase in pulsed voltage magnitude and it was highest at 25 kV, degradation yield was highest at a relatively lower supply voltage of 15 kV. The lower degradation yields at 20 kV and 25 kV may be attributed to the low initial concentration of carbofuran and the low mass transfer rate in the simple reactor.

Figure 3 - Percentage degradation and degradation yield of carbofuran with pulse voltage at 25 Hz frequency

3.3. Effect of pulse frequency Effect of pulse frequency on pesticide degradation was further investigated at 20 kV supply voltage, and the results are presented in Figure 4. With increase in number of pulses applied to the test cell indicates higher degradation efficiency. The significant effect of frequency could be observed up to 6 minutes of treatment time. The increase in frequency also increased the total energy delivered for plasma generation as shown in the Eq. 4. Increase in energy increased the rate of ionisation, which in turn increased the rate of pesticide degradation. In addition, increasing the pulse frequency led to a short delay time of discharge initiation, which could enhance the reactive species concentration in the reactor cell 9 ACS Paragon Plus Environment

29

. It was observed that the

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degradation yield at 30 Hz (0.45 g/kWh) was higher than the degradation yield (0.40 g/kWh) obtained at 20 Hz frequency. Thus, pulsed frequency also affects the plasma process. From the results presented in Figs. 2 and 4, it can be concluded that a combination of lower pulse voltage (15 kV) and higher pulse frequency (30 Hz) resulted in a higher degradation yield for the plasma process. Therefore, degradation yield has to be considered and optimized before implementation of this technique.

Figure 4 - Percentage degradation and degradation yield of carbofuran with pulse frequency at 20 kV pulsed voltage

3.4. Effect of initial pesticide concentration and degradation kinetics of carbofuran In order to study the effect of initial pesticide concentration on degradation efficiency, the pesticide concentration was varied from 0.5 mg/L to 30 mg/L. A voltage of 25 kV and pulse frequency of 25 Hz was employed for the study and the result is presented in Figure 5. It was observed that degradation time did not linearly increase with increase in the concentration of pesticide (Figure 5). The degradation yield was also observed to be higher with increase in pesticide concentration. During complete degradation of carbofuran, degradation yield (2.95 g/kWh) for initial pesticide concentration of 10 mg/L was much higher compared to degradation yield (0.37 g/kWh) for 0.5 mg/L concentration. Zeng et al, recently reported a higher degradation yield (4.5 g/kWh) in a cylindrical wetted-wall corona discharge reactor for the complete degradation of ibuprofen

40

. The novel reactor design with better mass transfer efficiency is

responsible for higher degradation yield. In cylindrical wetted-wall reactor, a higher mass transfer of reactive species could take place from air to aqueous phase, which significantly enhanced the degradation yield. The simple reactor configuration used in the present study may be the reason for lower yield.



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Figure 5 - Percentage degradation and degradation yield of carbofuran for different initial concentrations at 25 kV pulse voltage and 25 Hz frequency Table 2 – The zero, first and second order rate constants and corresponding R2 for carbofuran degradation at different initial concentrations Initial First order concentration rate (mg/L) constant (min-1) 0.5 2.68

R2

t1/2 (min)

1.00

1.0

1

1.71

0.93

1.2

2

0.82

0.97

1.3

5

0.57

0.97

1.6

10

0.61

0.92

2.5

20

0.23

0.91

3.5

30

0.32

0.95

5.6

Reactive species generated in the system get quenched by pollutant present in aqueous solution or they undergo a series of reaction to form different reactive species 11 ACS Paragon Plus Environment

41

. When high

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concentrations of pollutants are present in the system, the probability of the reactive species reacting with pollutant is high. When the pollutant concentration is low, more number of the reactive species could get converted to other stable species and lesser species could be available for pollutant degradation. Therefore, higher carbofuran degradation was achieved at higher initial concentration. To understand the kinetics of carbofuran degradation, the experimental data was fitted to pseudo first order kinetics and the result is given in supplementary material (Fig S1). First order rate constant was determined from the slope of the graph plotted between –ln (C0/C) and time. The R2 values and half life degradation time values were also determined for each initial carbofuran concentrations. The rate constants, half life degradation time values and their corresponding R2 values are presented in Table 2. For lower initial concentration of carbofuran, higher rate constant was observed (Table 2). The degradation was faster for the lower carbofuran concentration. However, half life degradation time for 30 mg/L carbofuran concentration was much higher than the 0.5 mg/L carbofuran concentration, which means higher efficiency and yield in case of the higher initial concentration. Similar reaction kinetics has been reported in many literatures for the plasma treatment of organic compounds 30,31,34,40.

3.5 Effect of radical scavengers and pH on pesticide degradation The effect of radical scavengers on the degradation of carbofuran was investigated. Carbofuran degradation efficiency (% degradation) and degradation yield (Y) as a function of time are shown in Figure 6(a) – (c). Major water constituents like alkalinity (HCO3- and CO32- ions) and natural organic matter (NOM) like humic acid affect the oxidation processes by quenching the hydroxyl radicals 42. To evaluate the effect of alkalinity and humic acid on pesticide degradation by pulsed corona technology, experiments were performed, and the results are shown in Figure 6a and 6b. 98.7 % of degradation efficiency was achieved in 10 min of treatment time for an alkalinity of 400 mg/L, whereas in the presence of 10 mg/L humic acid, 99.4 % degradation efficiency was achieved. It was also observed that the carbofuran degradation and yield decreases with increase in alkalinity and humic acid concentrations (Figure 6a and Fig 6b). Since the carbofuran molecules, water constituents, and the decomposed intermediate products compete with each other to quench the reactive species, the presence of such constituents will affect the carbofuran degradation Though, the concentration of humic acid was much lower than the concentration of HCO3- and CO32- ions, the degradation time and yield was obtained 12 ACS Paragon Plus Environment


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almost same. This may be attributed to the complex structure of humic acid molecule, where a high concentration of intermediates might generate. Those intermediates could quench most of the OH˙ generated during discharge, whereas HCO3- and CO32- ions could be mineralized fast by consuming lesser concentration of OH˙ produced during discharge (eq. 7 and eq. 8). CO32- + OH˙

CO3˙- + OH-

(7)

HCO3- + OH˙

CO3˙- + H2O

(8)

Common pollutants such as xeno – biotic organic matter like humic acid and alkalinity scavenge the ROS. Thus carbofuran degradation efficiency decreased with increasing water constituents. It is reported that pH conditions of aqueous solution also affected the oxidation processes

42

.

Therefore, the effect of pH was also evaluated by varying the solution pH between 5 and 9. Pesticide degradation efficiency was observed maximum in acidic pH (Figure 6c), and it is in good agreement with reported studies

43–45

. The influence of pH on the degradation of any

organic compound can have the following two plausible reasons (i) pH affecting the availability of reactive species (OH˙ radicals, H2O2, O3 and O2˙- etc.) and/ or (ii) the organic compound undergoing structural variations because of the pH. The pKa value of carbofuran molecule was found to be 11.95 ± 0.46

46

. Due to very high pKa value, the effect of pH on carbofuran

degradation, in terms of structural variability could be neglected. Therefore, the effect of pH on carbofuran degradation can be attributed to higher amount of reactive species formation in acidic condition. Effect of initial pH on the oxidation process was not very significant, however, for 2 and 4 min, the better degradation efficiency was achieved in acidic pH. It was found that the pH decreases drastically during the initial 4 min of treatment and then it became a constant. Similar result was obtained in our previous study also

26

. The pH after 4 min of plasma treatment was

almost same for the all three solutions with different initial pH’s (Figure 6 d). Therefore, the effect of pH can be neglected after 4 min of plasma treatment. Secondly, more than 99 % carbofuran was degraded after 4 min of treatment in all the three cases. Therefore, the effect of initial pH was not obvious after 4 min of discharge.


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(a)

(b)

(c)

(d)

Figure 6 – Percentage degradation and degradation yield of carbofuran with (a) humic acid concentrations (b) alkalinity (c) different pH; at impulse voltage of 25 kV and 25 Hz frequency (d) Variation of pH during discharge in carbofuran solutions of different initial pH

3.6 Oxidation and reduction pathway of carbofuran and its fate

In AOPs, hydroxyl radical mediated oxidative degradation pathway of carbofuran degradation has been reported in some studies

18,19,48

. However, knowledge about the role of e- in reductive

degradation is meagre. During plasma treatment, hydrated electrons also get produced along with other reactive species

23,24

. As a result of the reaction between the hydrated electrons and

carbofuran molecules, the reductive degradation products can be expected along with oxidation products due to attack of OH˙ radicals. Based on the analyses of intermediates generated during the degradation process, some key steps in the degradation are postulated and discussed below. (I) Scheme 1: oxidative pathway and (II) Scheme 2: reductive pathway. All the molecules associated with carbofuran appeared with sodium (Na+) adduct in mass spectra (Figure S2, supplementary material). Therefore, for the determination of parent compound and different intermediates, m/z of 23 was subtracted from the m/z obtained in each case. After the treatment of carbofuran, some of the unstable and stable intermediates were detected in the mass



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spectra. Intermediates mass (m/z) and their chemical structures are represented in Table S1 (Supplementary information). Scheme 1. Oxidative pathway The characteristics peak of carbofuran (m/z = 221) completely vanished in 4 min of streamer discharge (Figure S2(c), supplementary material), and this reflects that the carbofuran was broken down into some intermediates or got mineralized partially. Some of the peaks identified as m/z less than 221, along with some other higher peaks (m/z > 221) suggested that the carbofuran degradation reaction can take place by two pathways: (a) breaking the bonds of carbofuran molecule and (b) hydroxylation of carbofuran molecule. Possible oxidative pathway for degradation of carbofuran molecule has been shown by Alvarez et al

19

during solar

photocatalytic degradation, where they have detected 2,2-dimethyl-3-oxo-2,3-dihydro-1benzofuran-7-ol, as a final intermediate with an m/z = 178 and they could not detect the lower molecular weight intermediates. Three lower molecular weight stable intermediates (m/z = 110, 142, 176) were analysed in this study, and their structure is shown in Table 2. In our study, we observed catechol (m/z = 110) as a small ringed compound and with the continuous OH˙ attack, ring opening of catechol occurred leading to the formation of aliphatic acids (m/z = 142, 176), lower aliphatic acids and finally leading to the complete mineralization resulting in CO2 and H2O. The complete proposed oxidative pathway is shown with black colour (Figure 7). Scheme 2: Reductive pathway Some of the intense peaks with m/z = 136 and m/z = 209 observed in the mass spectra, support the reductive degradation pathway of carbofuran due to the impact of the hydrated electron where demethylation was found to be the major reductive degradation mechanism for carbofuran molecules. The carbon-centered radicals were formed after hydrated electron attack, which was further stabilized by abstraction of hydrogen atoms from water molecules. Although, the first intermediate (m/z = 193) could not be detected, but the peak with m/z =209 was detected, which could be the transformed product of m/z = 193. Therefore, the m/z = 193 could also be considered as one of the intermediate of reductive degradation pathway. Similar reductive degradation pathway for diclofenac degradation by pulsed radiolysis is recently reported by Yu et al 49. The proposed pathway of reductive degradation is shown in Figure 7 and indicated with blue colour.


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As shown in Figure 7, in the first step, carbofuran degradation could follow four routes. Firstly, the C – O bond in the carbamate group might get attacked by ˙OH and form compound A and carbamic acid. Carbamic acid, could then transform to methylamine and carbon dioxide 44. Secondly, due to electrophilic addition of OH on electron cloud of benzene ring, product B forms as a result of hydroxylation. Thirdly, the abstraction of H atom and replacement of OH atom occurs at the C3 group in the furan ring resulting in product C 19. In fourth route, demethylation of carbofuran resulted in compound D following a reductive degradation. Further hydroxylation of compound D led to compound F, which further transformed into compound G by removing carbamic acid by hydroxylation with OH. On the other hand, demethylation of compound A lead to the formation of simple compound E. Ring opening of E and G, lead to the formation of compound H and I respectively, which further transformed to aliphatic compound K and L due to successive hydrolysis, decarboxylation, and ring opening. The cause of ring opening by successive hydrolysis and decarboxylation is also reported during sonochemical treatment of methylparaben 50. Although carbofuran degraded in 4 min, while some smaller molecules (m/z = 110, 176) were persistent in the reactor till 10 min of streamer discharge (Figure S2(c), S2(d), S2(e), S2(f), supplementary material). These smaller molecules were formed by the continuous attack of hydroxyl radicals. After 14 min of discharge, most of the molecules were completely mineralized to CO2, H2O and other inorganic ions. This proved the breakdown of carbofuran to smaller innocuous fragments in the reactor.



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Figure 7 – Proposed pathway for degradation of carbofuran by pulsed corona discharge

3.7 Toxicity assay The eco-toxicity of the plasma treated effluent was evaluated by algal cell viability to find out the extent of carbofuran detoxification occurred during the treatment process. The viability (%) and toxicity (%) of algal cells at different treatment time are shown in Figure 8. A slight decrease in cell viability (or a slight increase in toxicity) was observed in first 2 min of treatment time. Subsequently, a sudden increase in cell viability (or a decrease in toxicity) was observed after 2 min of treatment time, which indicate that the intermediates produced during treatment process, are more toxic than the parent compound. However, the toxicity decreased gradually and reached a minimum toxicity of 3.04%, 4.43% and 7.30% for incubation time of 24 h, 48 h and 72 h, respectively, after 10 min of treatment time. Similar trend of toxicity by carbofuran and its degradation products was observed during the photo-Fenton process 6. In many AOPs studies, it has been reported that the intermediates are more toxic than the parent compounds


35,36

. In the

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present study, though carbofuran/intermediaes toxicity increased in the first 2 min of treatment, the toxicity was significantly decreased after further treatment.

Figure 8 – The graph shows the effects of carbofuran and its intermediates on percentage viability of Chlorella vulgaris and their toxicity, after plasma treatment of different durations

To confirm the residual organic carbon remaining in the system after treatment, which might be responsible for the toxicity, TOC was analysed and results are shown in Figure 9. Almost complete TOC removal was achieved at the end of 14 min treatment, which demonstrates that the plasma treatment could be a promising technique for the carbofuran mineralization and detoxification.

Figure 9 –The percentage reduction of total organic carbon (TOC) as a function of treatment time

4. Conclusion A multiple needle pulsed corona discharge above the water surface was used to remove carbofuran from aqueous solution. More than 99% of carbofuran degradation was achieved after 10 min of plasma treatment with a degradation yield of 1.5 g/kWh. Degradation was enhanced 18 ACS Paragon Plus Environment


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by increasing voltage and frequency while degradation yield decreased with increasing voltage. For 90 % carbofuran degradation, the yield increased from 0.74 to 8.1 g/kWh, when initial carbofuran concentration was varied from 0.5 to 30 mg/L. Carbofuran degradation followed pseudo first order kinetics and rate constant ranged between 0.32 min-1 (for 30 mg/L) and 2.68 min-1 (for 0.5 mg/L). The effects of pH and radicals scavengers were also investigated. The effect of pH was insignificant, while degradation efficiency and yield was largely affected by alkalinity (HCO3- and CO32- ions) and humic acid. From the mass spectra, seven degradation intermediates of carbofuran were identified from which the oxidation and the reductive degradation pathways of carbofuran were elucidated. From this study, it can be concluded that the system could rapidly degrade recalcitrant pollutants like pesticides and can provide good energy efficiency for high pollutant load. The significant decrease in acute toxicity confirmed that the current system is a promising technique for the treatment of carbofuran contaminated water.

Acknowledgement The authors express their deep gratitude to Department of Science and Technology (DST), Government of India (GoI) for the financially supporting this study.

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