Conductive-diamond electrochemical oxidation of chlorpyrifos in wastewater and identification of its main degradation products by LC–TOFMS

Chemosphere 89 (2012) 1169–1176

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Conductive-diamond electrochemical oxidation of chlorpyrifos in wastewater and identification of its main degradation products by LC–TOFMS José Robles-Molina a, María J. Martín de Vidales b, Juan F. García-Reyes a, Pablo Cañizares b, Cristina Sáez b, Manuel A. Rodrigo b, Antonio Molina-Díaz a,⇑ a b

Analytical Chemistry Research Group, Department of Physical and Analytical Chemistry, University of Jaén, Campus Las Lagunillas, 23071 Jaén, Spain Department of Chemical Engineering, University of Castilla–La Mancha, Enrique Costa Building, Avda. Camilo José Cela 12, 13071 Ciudad Real, Spain

h i g h l i g h t s " The electrochemical degradation of the insecticide chlorpyrifos was investigated in wastewater. " Identification of degradation products was performed by liquid chromatography–time of flight-mass spectrometry (LC–TOFMS). " Six degradation products (with Mw 154, 170, 197, 305 321 and 333) were identified.

a r t i c l e

i n f o

Article history: Received 26 March 2012 Received in revised form 2 August 2012 Accepted 3 August 2012 Available online 1 September 2012 Keywords: Chlorpyrifos Priority contaminant Water Electrochemical degradation Mass spectrometry Transformation products

a b s t r a c t The electrochemical transformation of the organophosphorous insecticide chlorpyrifos (CPF) was investigated in wastewater. The oxidation of CPF was carried out in a single-compartment electrochemical flow cell working under batch operation mode, using diamond-based material as anode and stainless steel as cathode. In order to evaluate its persistence and degradation pathway, two different concentration levels (1.0 mg L1 and 0.1 mg L1) were studied. Liquid chromatography/mass spectrometry was used for evaluation of the initial and electrolyzed solutions. The identification of CPF transformation products was performed by liquid chromatography–time of flight-mass spectrometry (LC–TOFMS). Results showed that CPF is completely removed at the end of treatment time. Analysis by LC–TOFMS allowed the identification of six degradation products (with Mw 154, 170, 197, 305 321 and 333). Three of the identified intermediates (Mw 170, 305 and 321) were completely removed at the end of electrolysis time. Interestingly, the formation of diethyl 3,5,6-trichloropyridin-2yl phosphate (chlorpyrifos oxon) and 3,5,6-trichloropyridin-2-ol was also found in previous reported degradation pathways using different degradation technologies. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Chlorpyrifos (CPF) is an organophosphorous (OPPs) insecticide included as priority pollutant in the European Water framework Directive 2000/60/CE (WFD) (EU, 2000). CPF is one of the most widely used pest control products in the world, due to its effective and cost-competitive broad spectrum of activity when compared with alternative products. These features have promoted the use of CPF as a replacement for persistent organochlorinated compounds (Testai et al., 2010). However, it is well known that OPPs have strong inhibitory activity effects to cholinesterase (Pope et al., 2005), reproductive toxicity (Piña-Guzmán et al., 2005) cyto-

⇑ Corresponding author. Tel.: +34 953 212147; fax: +34 953 212940. E-mail address: [email protected] (A. Molina-Díaz). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.08.004

toxicity (Giordano et al., 2007), immunotoxicity (Girón-Pérez et al., 2007) and genotoxicity (Çakir and Sarikaya, 2005). Due to its extensive use, CPF may contaminate surface and ground water (Sultatos, 1994), and it also is often present in wastewater (Teijon et al., 2010). The negative impact caused by pesticide contamination in the environment has fostered the development and use of new treatment technologies for pesticide degradation and/or removal. Several techniques have been proposed for the removal of OPPs in waters. Amongst them, physical methods such as nanofiltration (Košutic´ et al., 2005) and activated carbon adsorption (Foo and Hameed, 2010), chemical methods like ozonation (Chelme-Ayala,

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2010), aqueous chlorine (Zhang and Pehkonen, 1999) and Fenton (Wang and Lemley, 2002), and biological treatments (Cycon´ et al., 2009; Jiyeon, 2009; Ahmad, 2010) have been proposed. Recently, studies have been aimed on some new methods such as photocatalysis (Devipriya and Yesodharan, 2005; Wei et al., 2009), electrochemical (Balaji et al., 2009), and irradiation techniques including X-ray (Trebse and Arcon, 2003) and gamma-ray (Basfar et al., 2007). Considering the particular importance of CPF, many recent studies have paid special attention on its individual degradation (Liu et al., 2001; Anwar et al., 2009; Samet et al., 2010; Sulaiman, 2010; Tang et al., 2011; Zhang et al., 2011) in different types of samples. Liu et al. studied the natural degradation of CPF in environmental waters, being hydrolysis the main degradation pathway. Interestingly, in this study, the half life of CPF in aqueous systems ranged from few hours to up to 210 d. On the other hand, Samet et al. showed the effects on electrochemical degradation of CPF of different parameters such as current density, temperature and CPF initial concentration (at least in the initial concentration range studied), although the formation of transformation products was not studied in detail. Amongst electrochemical treatment techniques, conductivediamond electrochemical oxidation (CDEO) is an emerging technology, characterized by its great efficiency and robustness, with many potential applications in the removal of pollutants from water and wastewaters (Cañizares et al., 2007; Rodrigo et al., 2010b). Presently, it can compete satisfactorily with most advanced oxidation technologies and it usually overcomes Fenton and ozonation at alkaline pH in the destruction of organic pollutants (Rodrigo et al., 2010a). The most frequently degradation products (DPs) found for CPF degradation upon hydroxyl radical-based oxidative processes are 3,5,6-trichloro-2-pyridinol and diethylthiophosphate. However, the formation of others DPs might be also expected. Interestingly, there is a lack of detailed mechanistic studies of the CPF pathway degradation in water samples (Samet et al., 2010; Sulaiman, 2010; Zhang et al., 2011). The elucidation of intermediates formed during degradation processes is relevant since DPs formed during treatment may be more toxic than parent compounds or they can also have insecticidal properties, as well as high persistence in the environment. Bearing this in mind, the goal of this work was to evaluate the degradation of CPF during treatment of wastewater with conductive-diamond electrochemical oxidation (CDEO), and determine the DPs formed, in order to establish the degradation pathway followed by this compound during treatment. Different experiments at selected concentration levels of CPF were carried out, being the process monitored by liquid chromatography high resolution mass spectrometry. In this sense, a liquid chromatography time-of-flight mass spectrometry (LC–TOFMS) system has been used, given its usefulness for the identification of intermediates during degradation experiments, because of its high-resolution, accurate mass measurements capabilities and high-sensitive full-scan spectrum acquisition (García-Reyes et al., 2007; Gómez et al., 2010; González et al., 2011).

2. Experimental

City, CA, USA). Chlorpyrifos was obtained from Sigma Aldrich (Steinheim, Germany). 2.2. Analytical procedures To measure pH an InoLab WTW pH-meter was used. Relative abundances of CPF and its intermediates generated during the electrolysis were measured by a high-performance liquid chromatography (HPLC) system (consisting of vacuum degasser, autosampler and a binary pump) (Agilent series 1200, Agilent Technologies, Santa Clara, CA) equipped with a reversed-phase XDB-C18 analytical column 1.8 lm, 50 mm  4.6 mm (Agilent Technologies, Santa Clara, CA). Mobile phases A and B were respectively water with 0.1% formic acid and acetonitrile. The chromatographic method held the initial mobile phase composition (10% B) for 3 min, followed by a linear gradient to 100% B up to 25 min that was then held for 3 min. The flow rate used was 0.5 mL min1. The HPLC system was connected to a timeof-flight mass spectrometer Agilent 6220 accurate mass TOF (Agilent Technologies, Santa Clara, CA) equipped with an electrospray interface operating in either positive and negative ionization modes, using the following operation parameters: capillary voltage, 4000 V(+)/3000 V(); nebulizer pressure, 40 psig; drying gas flow rate, 9.0 L min1; gas temperature, 325 °C; skimmer voltage, 65 V; octapole 1 rf, 250 V; fragmentor voltages: 160, 190 and 230 V. LC–MS accurate mass spectra were recorded across the m/z range of 50–1000 in positive ion mode and 50– 1100 in negative ion mode. The instrument performed the internal mass calibration automatically, using a dual-nebulizer electrospray source with an automated calibrant delivery system, which introduces the flow from the outlet of the chromatograph together with a low flow (approximately 45 lL min1) of a calibrating solution which contains the internal reference masses TFANH4 (ammonium trifluoroacetate, C2O2F3NH4, at m/z 112.985587 in negative ion mode), purine (C5H4N4, at m/z 121.050873, in positive ion mode) and HP-0921 (Hexakis(1H,1H,3H-tetrafluoropropoxy)phosphazine, C18H18O6N3P3F24, at m/z 922.009798 in positive ion mode and 1033.988109 in negative mode). The full scan data were recorded with Agilent Mass Hunter Data Acquisition software (version B.04.00) and processed with Agilent Mass Hunter Qualitative Analysis software (version B.04.00). Samples were directly introduced with no previous treatment. 2.3. Electrochemical cell The oxidation of CPF was carried out in a single-compartment electrochemical flow cell working under a batch operation mode (Cañizares et al., 2005). Diamond-based material was used as anode and stainless steel (AISI 304) as cathode. Both electrodes were circular (100 mm diameter) with a geometric area of 78 cm2 each and an electrode gap of 9 mm. Boron-doped diamond (BDD) films were provided by Adamant Technologies (Neuchatel, Switzerland) and synthesized by the hot filament chemical vapor deposition technique (HF CVD) on single-crystal p-type Sih1 0 0i wafers (Siltronix). Thickness of the diamond layer was 2.71 lm and the ratio between diamond and graphite carbon was sp3/sp2 = 226.

2.1. Chemicals 2.4. Experimental procedures HPLC-grade acetonitrile, methanol, n-hexane, ethyl acetate and formic acid, all were purchased from Sigma–Aldrich (Madrid, Spain). Dichlorometane was obtained from Merck (Darmstadt, Germany). HPLC-grade water was obtained by purifying demineralizated water in a Milli-Q Gradient A10 (Millipore, Bedford, MA, USA). Cartridges used were 500 mg Bond elut C18 (Varian, Harbor

Bench-scale electrolyses of 5 L of wastewater were carried out under galvanostatic conditions. Sodium sulphate (5000 mg L1 Na2SO4) was used as supporting electrolyte. The cell voltage did not vary during electrolysis, indicating that conductive-diamond layers did not under go appreciable deterioration or pas-

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sivation phenomena. Prior to use in galvanostatic electrolysis assays, the electrode was polarized during 10 min in a 1 M Na2SO4 solution at 15 mA cm2 to remove any kind of impurity from its surface. The wastewater was stored in a glass tank and circulated through the electrolytic cell by means of a centrifugal pump (flow rate 2.5 L min1). A heat exchanger coupled with a controlled thermostatic bath (Digiterm 100, JP Selecta, Barcelona, Spain) was used to maintain the temperature at the desired set point (25 °C). 2.5. Degradation experiments Lab-scale electrolyses of 5000 mL of synthetic wastewater were carried out under galvanostatic conditions for 720 min each one. The concentration of CPF was ranged from 0.1 to 1.0 mg L1, and 5000 mg L1 Na2SO4 was used as supporting electrolyte. Thirteen aliquots of 25 mL of the treated sample were collected at different times during electrolysis experiments. The current densities employed were of 15 and 30 mA cm2. The cell voltage did not changed throughout the course of the electrolysis experiment, indicating that conductive-diamond layers did not undergo appreciable deterioration or passivation phenomena. Prior to use in galvanostatic electrolysis assays, the electrode was polarized during 10 min in a 0.035 M Na2SO4 solution at 15 mA cm2 to remove any kind of impurity from its surface. 2.6. Calculation of current efficiency The faradaic efficiencies were calculated to all the cases following the equation:

Chlorpyrifos Ethy] / Relative area

100

(a)

80

60

40

20

0 0

1

2

3

Ef ð%Þ ¼

F  R½Dn  z  100 I  Dt

ð1Þ

where F is the Faraday constant (96487 C mol1), Dn is the moles of Chlorpyrifos removed at time Dt, z is the number of electrons exchanged (39) and I is the current applied. 3. Results and discussion 3.1. Degradation of chlorpyrifos Fig. 1 shows the changes in the degradation of Chlorpyrifos (CPF) during the electrochemical oxidation with CDEO of synthetic solutions containing different initial concentrations of this compound (0.1 and 1.0 mg L1) and a supporting electrolyte (5000 mg L1 of sodium sulphate) to increase the ionic conductivity of the reaction media up to the values required for an efficient electrolyses. Moreover, in this figures the influence of the current density applied is also shown. Results are shown in terms of relative area instead of TOC or COD due to the low reproducibility and low accuracy of these measurements at low concentration of organic load. Moreover, COD measurements can also suffer from interferences at high salinity used in this work. It can be observed that the compound is removed to the detection limit of the LC–TOFMS technique, independently of the initial concentration of the contaminant studied and of the current density applied. This fact demonstrates the high efficiency of the electrochemical oxidation with conductive-diamond anodes. In the concentration range studied, the current charge requirement does not increase with the increase in the initial concentration of pollutant. On contrary, the efficiency of the degradation process decreases with the increase in the current density, since large current charges are required to obtained the same removal, mainly in the electrolysis of 1.0 mg L1 of CPF. This clearly indicates a contribution of mass-transfer limitations to the process kinetics (Samet et al., 2010). Fig. 2 shows the effect of the organic content and current density on the current efficiency. It can be observed that both parameters strongly influence the efficiency of the process. The efficiency increases with the organic load and decreases with the current density. This corresponds to the typical behavior of discontinuous experiments in which the process effectiveness decreases as a result of the decrease in the concentration of pollutants ready to be oxidized. The theoretical current charge passed required to attain

4

Q / Ah L-1 2.5

(b)

2

80

Efficiency / %

Chlorpyrifos Ethyl / Relative area

100

60

40

1.5

1

0.5

20

0

0 0

1

2

3

4

Q / Ah L-1 Fig. 1. Chlorpyrifos relative area profile vs. current charge during electrolysis process at 15 mA cm2 (j) and 30 mA cm2 (h) at different initial concentrations: (a) 1 mg L1, (b) 0.1 mg L1.

0

1

2

3

4

Q / Ah L-1 Fig. 2. Effect of the organic content and current density on the current efficiency. (j) 1 mg L1, 15 mA cm2; (h) 1 mg L1, 30 mA cm2; (N) 0.1 mg L1, 15 mA cm2, (4) 0.1 mg L1, 30 mA cm2.

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mental ones. This may indicate that it is completely-pure masstransfer controlled process and that the role of inorganic oxidants (which extend the oxidation process from the nearness of the electrode surface to the complete reaction volume) is not significant.

the complete removal of CPF has been estimated assuming the direct mineralization of CPF to carbon dioxide. The theoretical values (2.981  103 and 2.981  104 Ah L1 for electrolysis of 1 and 0.1 mg L1 respectively) are significantly lower than the experi-

Table 1 Mass spectral features of chlorpyrifos and in-source CID fragmentation at three different voltages. Ion

Fragmentation (fragmentor voltage) 160 V

ESI+

190 V

Elemental composition

Error (ppm)

220 V

m/z

Abundance

Relative abundance

Abundance

Relative abundance

Abundance

Relative abundance

Formula (MFG)

Diff. (ppm)

349.9333 231.9022 293.8707 197.9274 153.0133 124.9702 96.9503

49569 1921 0 1473 0 0 1778

100.0 3.9 0.0 3.0 0.0 0.0 3.6

26997 8503 3529 12745 3598 3266 11382

100.0 31.5 13.1 47.2 13.3 12.1 42.2

3380 4314 5926 33701 2285 4011 27830

10.0 12.8 17.6 100.0 6.8 11.9 82.6

C9H12Cl3NO3PS C7H8Cl3NO3PS C5H4Cl3NO3PS C5H3Cl3NO C4H10O2PS C5HO2PS –

0.66 0.01 0.24 0.93 0.56 0.66 –

(XIC) of m/z 195.9132

x10 4 1.4

15.24

(c)

1.2 1 0.8

13.12

(b)

0.6 0.4

(a)

0.2

10.52

0 x10 2

9

164.8363

10

11

12

13

14

15

3

16

17 Cl

DIAGNOSTIC ION

O

2.5

18

19

20

21

Counts vs. Acquisition Time (min)

303.9121 Cl

N

a

P

2

HO

O

195.9132

1.5

O

C4 Cl

1

220.9448

350.7633

249.9642

0.5 0 160

170

180

x10 3 3.5

190 200

210

220

230 240

250

260

270 280

290

300

310

320 330

Cl

195.9129 S

3 2.5

HO

2

350

360 370

380

b

319.8882

Cl

N

340

Counts vs. Mass-to-Charge (m/z)

P

C6 O

O

Cl

1.5 1 0.5

251.0971

160.8423

0 x10 4 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5

160

170

180

190 200

210

195.9132

220

230 240

250

260

270 280

290

300

310

320 330

340

350

360 370

380

Counts vs. Mass-to-Charge (m/z)

221.1188

c

Cl Cl N

C3

HO Cl

160

170

180 190

200

210

220

230

240 250

260

270

280

290

300

310

320

330

340

350

360

370

380

Counts vs. Mass-to-Charge (m/z) Fig. 3. Example of using diagnostic ions for the identification of transformation products (DPs) of chlorpyrifos (CPFs). The extracted ion chromatogram (XIC) of m/z 195.9132, yielded 3 peaks that corresponded to CPF DPs C4, C6 and C3 respectively. (a) ESI–TOF–MS spectrum of C4 (R.T. 10.523 min.); (b) ESI–TOF–MS spectrum of C6 (R.T. 13.123 min.); (c) ESI–TOF–MS spectrum of C3 (R.T. 15.243 min.).

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Ionization mode

RT (min)

m/z Experimental

Proposed formula

m/z Theoretical

C1

ESI+

18.40

333.9553

C9H11Cl3NO4P

333.957

C2

ESI

1.34

153.028

C4H11O4P

C3

ESI

15.24

195.9132

C4

ESI

10.53

C5

ESI

C6

ESI

Diff. (ppm)

DBEa

1.63

4

153.0322

3.47

0

C5H2Cl3NO

195.9124

1.36

4

303.9091

C7H7Cl3NO4P

303.9134

1.76

4

2.04

169.0096

C4H11O3PS

169.0088

2.33

0

13.11

319.8882

C7H7Cl3NO3PS

319.8872

1.72

4

Proposed structure

b

Cl Cl S HO

N

P O O

Cl

a

DBE: double-bound and ring equivalency. b IUPAC names of DPs: C1 diethyl 3,5,6-trichloropyridin-2yl phosphate; C2 diethyl hydrogen phosphate; C3 3,5,6-trichloropyridin-2-ol; C4 ethyl 3,5,6-trichloropyridin-2-yl hydrogen phosphate; C5 O,O-diethyl O-hydrogen phosphorothioate; C6 O-ethyl O-3,5,6-trichloropyridin-2-yl O-hydrogen phosphorothioate.

3.2. Degradation products: identification and oxidation With the aim of evaluating the CPF degradation and to study the formation of intermediates, aliquots collected at different times were analyzed by LC–TOFMS in order to obtain accurate mass measurements of tentative CPF intermediates, which assists to assign the elemental composition and the chemical structure to each intermediate. The identification of DPs of CPF were carried out using two strategies, diagnostic ion search (García-Reyes et al., 2007) and automated data mining using molecular feature extraction (MFE) software (Gómez et al., 2010). Diagnostic ions search consists on the use of possible diagnostic ions of CPF (Table 1). An in-source fragmentation experiment was performed to find possible diagnostic ions. In this experiment, three fragmentor voltages were evaluated in both positive and negative ionization modes: 160, 190 and 220 V. The diagnostic ions are those which are common to a family of species. Therefore, several DPs may contain the nuclei of the original molecule, so that part of its fragmentation is common. The fragmentation pathways of the parent species can be used to predict possible transformation products, since the bonds that are easily cleaved in the instrument (in-source fragmentation) are those that might be broken in reactions or in ambient conditions. For instance, Fig. 3 shows the extracted ion chromatogram of ion m/z 195.9132 (negative ion mode), a fragment ion typical in CPF, which was found to be common to some of the DPs identified (C3, C4 and C6, see Table 2). A thorough evaluation of the acquired LC-MS chromatograms obtained in both positive and negative ionization mode at different treatment times combined with the use of the mentioned search

tools (diagnostic ion search and molecular feature extraction) enabled the identification of six DPs of chlorpyrifos. Their proposed structures and the information provided by LC–TOFMS analysis are summarized in Table 2. All the proposed structures and elemental compositions were confirmed by accurate mass measurements of ions with relative mass error below 2 ppm in most cases. Isotope pattern matching was also used for unambiguous confirmation of the proposed species. From the identified compounds, degradation compound C1 (diethyl 3,5,6-trichloropyridin-2yl phosphate (chlorpyrifos oxon (CPF-O)), was detected at a retention time of 18.4 min. This compound was formed by oxidation of P@S bond of CPF. The proposed structure for C1 confirmed with a mass error of 1.6 ppm, is shown in Table 2. It is consistent with the calculated elemental composition (C9H11Cl3NO4P) and a DBE value (double bond and ring equivalent) of 4. Its sodium adduct could be also found with an m/z value of 355.9383. Intermediate C2 was detected at retention time of 1.34 min, which evidences the high polarity of the compound. The calculated elemental composition (C4H11O4P, DBE 0) and the appareance of a characteristic ion fragment of 125.0023 m/z are consistent with the proposed structure (diethyl hydrogen phosphate) formed by hydrolysis of CPF-O. C3 (see Fig. 4) was detected at 15.2 min, with a calculated formula of C5H2Cl3NO and a DBE value of 4. This compound corresponds to a secondary hydrolysis of CPF-O, is a typical degradation product in oxidation processes as described elsewhere (Gomathi-Devi et al., 2009; Zhang et al., 2011). Degradation product C4 was identified at 10.53 min. Its elemental composition (C7H7Cl3NO4P) and DBE value (4) are consistent with a loss of ethyl group from CPF-O (C1), its structure is

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(a)

3 x10

4 x10 4 3.5 3 2.5 2

23.37 Cl

3 2.5

Cl

2

N

S

O P

1.5 1

1.5 1 0.5 0

O

O

Cl

0.5 0 4

6

8

10

12

14

16

18

349.9336

20

22

24

26

40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400

Counts vs. Acquisition Time (min)

(b)

4 x10 8 7 6 5 4 3 2 1 0

15.24

Cl Cl N

HO Cl 1

3

5

7

9

11

13

15

Counts vs. Mass-to-Charge (m/z) 221.1188

4 x10 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 17

19

21

23

195.9132

116.9072 40

25

60

80

100

120

140

160

4 x10

2.04

(c)

3

6 x10 2.4

S

180

200

220

240

260

280

300

320

Counts vs. Mass-to-Charge (m/z)

Counts vs. Acquisition Time (min) 96.9618

2

2.5 HO

2 1.5

P

1.6

O

1.2

O

1

0.8

0.5

0.4

0

68.9963

112.9857

169.0098

0 1

3

5

7

9

11 13 15 17 19

60

21 23 25 27

80

100 120 140 160 180 200 220 240 260 280 300 320 340

Counts vs. Mass-to-Charge (m/z)

Counts vs. Acquisition Time (min)

Chromatographic area

Chromatographic area

Fig. 4. (a) Extracted ion chromatogram and accurate mass spectrum of CPF; (b) extracted ion chromatogram, accurate mass spectrum and information provided by LC–TOFMS software for C3 transformation product; (c) extracted ion chromatogram, accurate mass spectrum and information provided by LC–TOFMS software for C5 transformation product.

t/min

t/min

Fig. 5. Intermediates chromatographic area vs. oxidation time during electrolysis process at 15 mA cm2 and an initial concentration of CPF of 1 mg L1.

J. Robles-Molina et al. / Chemosphere 89 (2012) 1169–1176

confirmed by the characteristic ion fragment of 195.9132 m/z. Compound C5 (see Fig. 4) was O,O-diethyl O-hydrogen phosphorothioate with C4H11O3PS as proposed formula and DBE of 0, resulted of CPF hydrolysis together with C3. C6 was detected at 13.11 min, formed by ethyl group loss from CPF (O-ethyl O-3,5,6-trichloropyridin-2-yl O-hydrogen phosphorothioate), with an elemental composition of C7H7Cl3NO3PS, confirmed by the characteristic ion fragment of 195.9132 m/z. Fig. 5 shows the profile peak areas of the DPs generated during the CDEO experiment at a initial concentration of CPF of 1 mg L1 carried out at 15 mA cm2 as measured by LC–TOFMS (similar results were found at 30 mA cm2). As can be observed, some intermediates (C5 and C6) are removed efficiently in the first stages of the electrolyses. C4 intermediate shows a slower complete degradation, while, the degradation of intermediates C1, C2 and C3 is not complete and they remain at the end of the electrolysis time. This might indicate that the oxidation of these compounds competes with the oxidation of the rest of organics simultaneously present in the reaction media and that higher current charges would be required to remove them from the solution. It should be stressed that the presence of some of these DPs at the beginning of the experiment indicate they are rapidly formed from CPF by natural hydrolysis, which is coherent with studies from Liu et al. (2001) already commented in this paper.

4. Conclusions Electrochemical oxidation with boron doped diamond can be successfully used to treat wastewaters polluted with chlorpyrifos, being able to degrade completely this species. The efficiency of the degradation process decreases with the increase in the current density, so indicating a contribution of mass-transfer limitations to the process kinetics. The use of LC–TOFMS allowed the accurate identification of six DPs, three of them were completely eliminated during the CDEO process. Specially attention must be paid on 3,5,6-trichloropyridin-2-ol (C3) which is not completely eliminated and although it does not induce cholinesterase inhibition, has a potential exposure in food and residential settings. Acknowledgments The authors acknowledge funding support from Regional Government of Andalusia (Spain) ‘‘Junta de Andalucía’’ (Research Group FQM-323) and from the national Spanish Ministry of Education and Science (Project CSD2006-00044 CONSOLIDER INGENIO 2010 ‘‘TRAGUA Project’’). References Ahmad, T.S., 2010. Removal of pesticides from water using anaerobic–aerobic biological treatment. Chin. J. Chem. Eng. 18, 672–680. Anwar, S., Liaquat, F., Khan, O.M., Khalid, Z.M., Iqbal, S., 2009. Biodegradation of chlorpyrifos and its hydrolysis product 3,5,6-trichloro-2-pyridinol by Bacillus pumilus strain C2A1. J. Hazard. Mater. 168, 400–405. Balaji, S., Chunga, S.J., Ryuc, J.Y., Moon, I.S., 2009. Destruction of commercial pesticides by cerium redox couple mediated electrochemical oxidation process in continuous feed mode. J. Hazard. Mater. 172, 1470–1475. Basfar, A.A., Mohamed, K.A., Al-Abduly, A.J., Al-Kuraiji, T.S., Al-Shahrani, A.A., 2007. Degradation of diazinon contaminated waters by ionizing radiation. Radiat. Phys. Chem. 76, 1474–1479. Çakir, S., Sarikaya, R., 2005. Genotoxicity testing of some organophosphate insecticides in the drosophila wing spot test. Food Chem. Toxicol. 43, 443–450. Cañizares, P., Lobato, J., Paz, R., Rodrigo, M.A., Sáez, C., 2005. Electrochemical oxidation of phenolic wastes with boron-doped diamon anodes. Water Res. 39, 2687–2703.

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