The degradation of chlorpyrifos and diazinon in aqueous solution by ultrasonic irradiation: Effect of parameters and degradation pathway

Chemosphere 82 (2011) 1109–1115

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The degradation of chlorpyrifos and diazinon in aqueous solution by ultrasonic irradiation: Effect of parameters and degradation pathway Yuanyuan Zhang a, Yaxi Hou a, Fang Chen a,b,⇑, Zhiyong Xiao c, Jianing Zhang a, Xiaosong Hu a,⇑ a College of Food Science & Nutritional Engineering, Key Laboratory of Fruits and Vegetables Processing, Ministry of Agriculture, Engineering Research Centre for Fruits and Vegetables Processing, Ministry of Education, China Agricultural University, Beijing 100083, China b National Soybean Engineering Research Center, Harbin 150030, China c Beijing Agro-Environmental Monitoring Station, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 28 June 2010 Received in revised form 28 November 2010 Accepted 29 November 2010 Available online 21 December 2010 Keywords: Chlorpyrifos Diazinon Ultrasonic irradiation Degradation products Toxicity

a b s t r a c t In this paper, elimination of two types of organophosphorus pesticides (OPPs), chlorpyrifos and diazinon spiked in aqueous solution by ultrasonic irradiation was investigated. Results showed that chlorpyrifos and diazinon could be effectively and rapidly degraded by ultrasonic irradiation, and the degradation of both pesticides was strongly influenced by ultrasonic power, temperature and pH value. Furthermore, two and seven products for the degradation of chlorpyrifos and diazinon formed during ultrasonic irradiation have been identified by gas chromatography-mass spectrometry, respectively. The hydrolysis, oxidation, hydroxylation, dehydration and decarboxylation were deduced to contribute to the degradation reaction and the degradation pathway for each pesticide under ultrasonic irradiation was proposed. Finally, the toxicity evaluation indicated that the toxicity decreased for diazinon solution after ultrasonic irradiation, but it increased for chlorpyrifos solution. The detoxification of OPPs by ultrasonic irradiation was discriminative. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction The widespread occurrence of pesticides in the environment water such as surface water, groundwater and drinking water (Mukherjee and Gopal, 2002; Hildebrandt et al., 2008; Gao et al., 2009) is a recognized problem owing to their widely applied in agriculture and hygiene throughout the world. Thereinto, organophosphorus pesticides (OPPs) are one group of the most frequently encountered pesticides. Recent studies have demonstrated that OPPs have immunosuppressive effects (Neishabouri et al., 2004), cytotoxicity (Jacobsen et al., 2004), mutagenicity (Okamura et al., 2005) and endocrine-disrupting effects (Kitamura et al., 2003). Furthermore, it should be noted that the combined effects of multipesticides may show higher adverse effects on human health (Dolara et al., 1992). Therefore, greater attention should be paid to removing OPPs, especially in the production of drinking water. Some techniques for removal of OPPs in water have been developed. Firstly, physical methods including nanofiltration (Košutic´ et al., 2005) and activated carbon adsorption (Foo and Hameed, 2010) are very common for eliminating pesticides from water. Then, chemical methods performed by ozonation (Ku et al., 1998), aqueous chlorine (Zhang and Pehkonen, 1999) and Fenton ⇑ Corresponding authors. Tel./fax: +86 10 62737645x18 (F. Chen), tel.: +86 10 62737434x12 (X. Hu). E-mail addresses: [email protected] (F. Chen), [email protected] (X. Hu). 0045-6535/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2010.11.081

treatment (Wang and Lemley, 2002) have been intensively investigated. More recent studies have focused on some new methods such as photocatalysis, biological methods (Li et al., 2010) and irradiation techniques through X-ray (Trebse and Arcon, 2003) and gamma-ray (Basfar et al., 2007) and so on. Ultrasonic irradiation is an attractive technique as the degradation of contaminants may occur under ultrasonic irradiation without the addition of other chemicals. The most important mechanism responsible for the degradation of contaminants is considered as acoustic cavitation, which leads to the pyrolysis reactions (of water) and the formation of free-radical species (Serpone and Colarusso, 1994). Generally, the degradation of dissolved organic compounds in aqueous media involves pyrolysis inside the bubble and/or at the bubble-bulk interface region, and free radical-mediated reactions at the bubble-bulk interface region and/or in the bulk liquid (Adewuyi, 2001). Hitherto, ultrasonic irradiation has been found to be effective for the removal of some OPPs in aqueous solution (Schramm and Hua, 2001; Liu et al., 2008). However, these previous researches mainly paid attention to the effect of ultrasonic irradiation on the removal of parent compounds, and very few studies addressed the intermediate products and the evaluation of toxicity after ultrasonic irradiation. Chlorpyrifos (O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate) and diazinon (O,O-diethyl O-[6-methyl-2-(1-methylethyl)-4-pyrimidinyl] phosphorothioate) are two permittable and important OPPs in China. They have been detected in the aquatic

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environment due to their high usage frequency in agriculture (Xue et al., 2005; Wang et al., 2007). Therefore, the degradation of chlorpyrifos and diazinon in aqueous solution under ultrasonic irradiation was explored in the present study. The effect of parameters including electric power, temperature and pH of solution on the degradation of chlorpyrifos and diazinon was investigated. In addition, degradation products were identified by gas chromatography–mass spectrometry (GC–MS) and the change in toxicity of the samples after ultrasonic irradiation were evaluated. 2. Materials and methods 2.1. Reagents and materials Chlorpyrifos (99.0%), 3,5,6-trichloro-2-pyridinol (TCP, 99.4%), chlorpyrifos oxon (O,O-diethyl O-(3,5,6-trichloro-2- pyridinyl) phosphate, 94.5%), diazinon (99.0%) and 2-isopropyl-6-methylpyrimidine-4-ol (IMP, 99.5%) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). Diazoxon (O,O-diethyl O-[6-methyl-2(1-methylethyl)-4-pyrimidinyl] phosphate) was obtained from Chem Service (West Chester, PA) at 98.8% purity. Anhydrous sodium sulphate was pesticide-grade and purchased from CNW Technologies (Düsseldorf, Germany). Disodium hydrogen phosphate, potassium dihydrogen phosphate, hydrochloric acid and sodium hydroxide were analytical grade and obtained from Sinopharm Chemical Reagent Co. (Beijing, China). Pesticide-grade dichloromethane and acetone were purchased from Fisher (Fair Lawn, New Jersey). Nylon syringe filter (13 mm  0.22 lm) was obtained from Shanghai Anpel Scientific Instrument Co. (Shanghai, China). HPLC-grade and 0.20 lm filtered water was used to prepare solutions. The stock solutions of chlorpyrifos (2380 mg L–1) and diazinon (10600 mg L–1) were prepared in acetone and stored in glassstoppered flasks at 18 °C. The solutions with pH 5, 6, 7, 8 prepared by disodium hydrogen phosphate and potassium dihydrogen phosphate (25.0 mM) were spiked with the stock solutions of both pesticides to reach the final concentrations of 2–3 mg L–1, respectively. 2.2. Ultrasonic irradiation Ultrasonic irradiation was carried out with a high-intensity ultrasonic probe system (Beijing Hongxianglong Biotechnology Co., Beijing, China). It consists of an ultrasonic generator, an ultrasonic transducer and a titanium horn microtip. The ultrasonic generator operated at a fixed frequency of 20 kHz and a variable electric power up to 1.2 kW. Ultrasonic irradiation time could be adjusted to a desired value in the range of 1–120 min. In all case, 500 mL aqueous solution containing pesticides was placed in a water-jacketed glass vessel and subjected to ultrasonic irradiation. During experiments, the temperature of solution could be set between 4 and 50 °C through connecting the water-jacketed vessel to a thermostatic circulator HX-1050 (Beijing Detianyou Technology Co., Beijing, China). Samples were periodically drawn from the vessel and all the samples were analyzed in 1 d. Each treatment was conducted in triplicate. 2.3. Sample preparation The method for pesticides analysis is a modification of the standard method established by Ministry of Agriculture of China (2003). An aliquot of aqueous solutions (10 mL) were extracted three times using dichloromethane (10 mL). The combined dichloromethane phase was passed through 5.0 g of anhydrous sodium sulphate, and concentrated in a rotary evaporator to approximate

1 mL and evaporated to dryness under a stream of nitrogen in a water bath at 30 °C. The residue was re-dissolved and made up to 2 mL with acetone. The final test solutions were filtered through 0.22 lm syringe filter for GC analysis. For the GC–MS analysis of degradation products of both pesticides, the samples were acidified with hydrochloric acid to pH 6 2 (for chlorpyrifos) and to pH = 2.5 (for diazinon) prior to extraction. Then the following steps were performed same as to the above-mentioned method for GC analysis. The residue was re-dissolved and made up to 200 lL with acetone. The final test solutions were filtered through 0.22 lm syringe filter for GC–MS analysis. 2.4. GC analysis Quantitative analysis of chlorpyrifos and diazinon was detected with GC 9790 (Fuli Analytical Instrument Co., Zhejiang, China) equipped with flame photometric detector. A HP-5 fused silica capillary column (30 m  0.53 mm, 1.5 lm, Hewlett Packard, Avondale, USA) was employed for GC separation. Nitrogen carrier gas was set at constant pressure mode with pressure of 0.05 MPa. The GC oven temperature program was as follows: initial temperature of 120 °C held 1 min, then from 120 to 240 °C at 20 °C min–1, and held 7 min at 240 °C. The injector and detector temperatures were set at 250 and 260 °C, respectively. Sample solution (1 lL) was injected in splitless mode. The recoveries of this method ranged from 87% to 107% for chlorpyrifos and from 93% to 106% for diazinon when the concentration of pesticides spiked in the aqueous solution was between 0.1 and 5.0 mg L–1. The relative standard deviations were ranged within 2–10% for chlorpyrifos and 2–7% for diazinon, respectively. 2.5. GC–MS analysis The qualification analysis of degradation product of both pesticides after ultrasonic irradiation was performed by Shimadzu GC– MS-QP2010 Plus (Shimadzu, Kyoto, Japan) configured with a programmed temperature vaporization injector (Shimadzu, Kyoto, Japan). The sample solution (10 lL) was injected in splitless mode with an AS 2000 autosampler (Shimadzu, Kyoto, Japan). The Rxi5 ms fused silica capillary column (30 m  0.25 mm  0.25 lm) coated with 5% diphenyl-methylpolysiloxane stationary phase (Restek International, Bellefonte, USA) was used for GC separation. Helium was used as the carrier gas with the flow rate of 1.75 mL min–1. For the products of chlorpyrifos detection, the GC temperature program was set as follows: initial temperature 82 °C, held 5 min; rate of 8 °C min–1 up to 280 °C, held 1 min, then rate of 25 °C min–1 up to 300 °C, held 2 min. For the products of diazinon detection, the temperature program was 5 min at 82 °C and then 8 °C min–1 to 280 °C (14.5 min). The mass spectrometer was operated in electron impact ionization mode at 70 eV and the temperatures of transfer line, ion source and quadrupole were set at 200, 250 and 250 °C, respectively. Mass spectra in full-scan mode were collected at the rate of 2400 scan min–1 over the mass range (m/z) of 75–400. To avoid the filament damage, the mass spectrometer was set to be off at the elution time of chlorpyrifos (21.1–21.3 min) and diazinon (19.5–19.8 min), respectively. Mass spectral search was carried out in the National Institute for Standard Technology mass spectral library (search program version 1.5, Gaithersburg, Maryland, USA). 2.6. The analysis of toxicity and total organic carbon (TOC) The aqueous solutions (pH = 5) were spiked with individual chlorpyrifos (2.4 mg L–1) and diazinon (65 mg L–1) for toxicity measurement. The respective spiking concentrations of both pesticides

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were selected to obtain the similar inhibition for the photobacteria. These solutions were treated at 600 W for 30 and 60 min at 25 °C, respectively. The unspiked aqueous solution was treated at the same conditions and used as blank control. The photobacterium bioassay was used for the evaluation of toxicity of samples, according to the decrease in light emission from the luminescent bacterium as a result of exposure under pollutants (Zhang et al., 2008). All samples were adjusted to pH 6.8 ± 0.1 with 2.0 M NaOH, and then 17% NaCl was added to reach optimum salinity (0.85%) for rearing the luminescent bacterium Vibrio qinghaiensis sp. Nov-Q67 (Q67) (Beijing Hamamatsu Photon Tec., Beijing, China). The freeze-dried Q67 was activated by 0.85% NaCl solution. Then, the solution (50 lL) of Q67 solution was added to 2 mL of sample in a cuvette and mixed thoroughly. After holding 15 min, the relative luminescence intensity (RLI) of samples was measured by Water toxicity analyzer BHP 9511 (Beijing Hamamatsu Photon Tec., Beijing, China). The toxicity of sample was indicated by inhibition ratio (IR, %) for the bacteria Q67 and was described as follows:

RLIt IRt ¼ ð1  Þ  100% RLIC where RLIC and RLIt were RLI values of the blank and the sample after irradiation time t, respectively. TOC concentrations were determined using a TOC analyzer (TOC–VCPH, Shimadzu, Kyoto, Japan) programmed by TOC-Control V software, and hydrogenophthalate potassium was adopted as the reference standard. 2.7. Statistical analysis ANOVA of the experiment data was performed using SAS software (version 8.0, Statistical Analysis System, Cary, North Carolina, USA) and significant differences were established at a p = 0.05 level using Duncan’s test. 3. Results and discussion 3.1. Effect of electric power The electric power is one of the most crucial parameters for the application of ultrasonic irradiation. Thus, the effect of electric power on the degradation of chlorpyrifos and diazinon was investigated and the results are shown in Fig. 1. It can be seen that the increase of power had a favorable effect for the degradation of both pesticides. When the sample was treated for 60 min, the concentration of chlorpyrifos at 300 W was 1.2 and 1.9 times higher than

a

1.8

10

20

30

b

40

50

3.3. The effect of pH The changes of degradation percentage of chlorpyrifos and diazinon with pH value are shown in Fig. 3. It was observed that both

a

o

15 C

2.0

0.6

2.0

To make a clear description of the effect of temperature on the degradation of both pesticides, experiments were carried out at temperature of 15, 25 and 35 °C and the results are shown in Fig. 2. It was found that the degradation of both pesticides was strongly temperature-dependent. Increasing temperature from 15 to 25 °C, the degradation of chlorpyrifos and diazinon was considerably improved. However, the degradation percentage for both pesticides declined as the temperature further increased to 35 °C. This result indicated the complicated effect of temperature on degradation of pesticides under ultrasonic irradiation. In some studies there was the similar phenomenon that the increase in temperature within a certain range favored the degradation reaction, whereas the further increase in temperature above a certain value leaded to a decrease in the reaction rate (Sehgal and Wang, 1981; Adewuyi, 2001). On one hand, the increased temperature could result in the reduction of surface tension lowering the threshold intensity required to produce cavitation (Goel et al., 2004), and thus promote the occurrence of cavitation. On the other hand, more vapor of the target compound and water has an opportunity to diffuse into the cavitation bubble when temperature is increased over a range. This improves the resistance of the bubble to the inward motion during the collapse and weakens the cavitational collapse (Adewuyi, 2001; Goel et al., 2004), thus resulting in the decrease in degradation efficiency.

2.4

300 W 600 W 900 W

1.2

2.4 0

3.2. The effect of temperature

60

1.6

Concentration (mg L-1)

Concentration (mg L-1)

2.4

that at 600 and 900 W, respectively. The similar result was shown for the degradation of diazinon. It is obvious that the degradation percentages of chlorpyrifos and diazinon were only 35% and 11% when the samples were treated at 300 W for 10 min, fortunately, the degradation efficiency of pesticides could be improved by prolonging irradiation time (Fig. 1). This is consistent with the degradation of dichlorvos in water under ultrasonic irradiation (Schramm and Hua, 2001). The positive effect of electric power could be explained that ultrasonic energy transmitted into the solution increases and the cavitation activity augments with the increases of applied power. Consequently, the number of collapsing bubbles increase and cause a high concentration of free-radicals into the bubble-bulk interface region and aqueous solution (Dükkanci and Gündüz, 2006), thus improving the degradation of pesticides.

o

25 C

1.6

o

35 C

1.2 0.8 2.4 0

10

20

b30

40

50

60

20

30

40

50

60

2.0 1.6

1.2 1.2

0.8 0

10

20

30

40

50

60

Irradiation time (min) Fig. 1. Effect of electric power on the degradation of chlorpyrifos (a) and diazinon (b) at 25 °C and pH 5.0.

0

10

Irradiation time (min) Fig. 2. Effect of temperature on the degradation of chlorpyrifos (a) and diazinon (b) at 600 W and pH 5.0.

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Degradation percentage (%)

60 Chlorpyrifos Diazinon 50

40

30

20

10

0

5

6

7

8

pH Fig. 3. Effect of initial pH of the aqueous solutions on the degradation percentage of chlorpyrifos (C0 = 2.5 mg L–1) (a) and diazinon (C0 = 2.0 mg L–1) (b) after treatment at 600 W for 40 min at 25 °C.

pesticides were significantly influenced by the pH value of the solution, and the highest degradation efficiency was achieved at pH 7. Increasing the pH value from 5 to 7, the degradation percentages obviously increased from 41% to 55% and from 23% to 38% for chlorpyrifos and diazinon, respectively. However, the degradation percentages decreased for both pesticides, as the pH value increased from 7 to 8. For chlorpyrifos, the rate of hydrolysis is constant in acidic to neutral waters, but increases in alkaline waters. However, the breakdown rate of diazinon at acidic waters is quicker than that in a neutral solution (Howard, 1991). Under the ultrasonic irradiation, the influence of pH range of solution on the degradation of the both pesticides was not consistent with their basic characteristics. This probably resulted from the occurrence of complex degradation pathway under the ultrasonic irradiation. 3.4. Degradation products and pathways of chlorpyrifos and diazinon The knowledge on the degradation products of pesticides is important to better understand the degradation pathways and evaluate the practical applications of ultrasonic irradiation for cleaning up contaminated water. Hence, the aqueous solutions spiked with individual chlorpyrifos (6 mg L–1) and diazinon (21 mg L–1) were treated with ultrasonic irradiation at 600 W for 60 min at 25 °C for products analysis by GC–MS.

3.4.1. Chlorpyrifos The GC–MS identification of chlorpyrifos products suggested the formation of two products (chlorpyrifos oxon and TCP). Their retention times and the fragment ions of the mass spectra were summarized in Table 1. Based on the identification of degradation products, the degradation pathway of chlorpyrifos was summarized in Fig. 4a. When chlorpyrifos was exposed to the ultrasonic irradiation, the P@S bond of chlorpyrifos was inclined to be oxidized to P@O bond by the free radical arose from pyrolysis reactions of water during ultrasonic irradiation (Hua et al., 1995). In addition, TCP produced due to the hydrolysis of chlorpyrifos, which also has been reported during the degradation of contaminants under ultrasonic irradiation (Hua et al., 1995; Schramm and Hua, 2001). Probably, the hydrolysis of chlorpyrifos oxon was another way to form TCP, since Duirk et al. (2008) have demonstrated that it occurred in chlorpyrifos degradation by aqueous chlorine. 3.4.2. Diazinon Seven degradation products (D1–D7) have been identified for diazinon under ultrasonic irradiation and their fragment ions along with their retention times are summarized in Table 1. It can be concluded that all the degradation products belonged to the thiophosphate except IMP (D1) and diazoxon (D2). This was in accordance with some products identified after the exposure of diazinon to UV with TiO2 (Kouloumbos et al., 2003). Based on the identification of degradation products, the degradation pathway of diazinon under ultrasonic irradiation was postulated in Fig. 4b. The oxidation of the P@S bond to P@O bond through free-radical attack was facile and aroused the formation of diazoxon. According to the research of Hua et al. (1995), hydrolysis reactions could also be enhanced, particularly for compounds that partition to the bubble interface. Therefore, IMP was produced by the hydrolysis of diazinon or diazoxon through the scission of the PAO bond (pyrimidine group). Hydroxylation reaction is essential and has been reported in the sonolysis of contaminants (Vassilakis et al., 2004; Torres et al., 2008). For diazinon, hydroxylation at the tertiary and primary carbon atoms of the isopropyl group occurred and resulted in the formation of hydroxydiazinon and 2-hydroxydiazinon, respectively. The former could undergo dehydration reaction and transform into isopropenyl derivative of diazinon. And then the oxidation plus decarboxylation reaction on isopropenyl derivative of diazinon occurred resulting in formation of hydroxyethyl derivative of diazinon, which had been found in the metabolism of diazinon in animals and plants (Miyazaki

Table 1 Degradation products of chlorpyrifos and diazinon identified by GC–MS. Degradation products of chlorpyrifos

Rt (min)

Characteristic ions (m/z)

C1 C2

3,5,6-trichloro-2-pyridinol (TCP) O,O-diethyl O-(3,5,6-trichloro-2-pyridinyl) phosphate (chlorpyrifos-oxon)

14.12 21.01

Degradation products of diazinon D1 D2 D3 (Isopropenyl derivative of diazinon) D4

Rt (min)

Characteristic ions (m/z) 14.56 19.19

D5 D6 D7

2-isopropyl-6-methyl-pyrimidine-4-ol (IMP) O,O-diethyl O-[6-methyl-2-(1-methylethyl)- 4-pyrimidinyl] phosphate (Diazoxon) O,O-diethyl O-[2-isopropenyl-6-methylpyri- midin-4-yl] thiophosphate 20.79 O,O-diethyl O-[2-(1-hydroxy-1-methylethyl)-6- methylpyrimidin-4-yl] thiophosphate (Hydroxydiazinon) O,O-diethyl O-[2-(1-hydroxyethyl)-6- methylpyrimidin-4-yl] thiphosphate (Hydroxyethyl derivative of diazinon) O,O-diethyl O-[2-acetyl-6-methylpyrimidin- 4-yl] thiophosphate (Diazinon methyl ketone) O,O-diethyl O-[2-(2-hydroxy-1-methylethyl)-6- methylpyrimidin-4-yl] thiophosphate (2-Hydroxydiazinon)

302, 246, 197, 177, 150 21.05

197, 169, 134, 107, 98 333, 298, 270, 242, 197, 169, 109, 81

152, 137, 109, 84 288, 273, 260, 152, 151, 137

21.23

320, 305, 292, 276, 260, 246, 195, 178, 153 306, 289, 262, 181, 164, 137

21.93

304, 276, 248, 199, 180, 153

22.81

320, 290, 195, 167, 151, 122

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a Cl

N

OH

Cl

S Cl chlorpyrifos

O P

O

n io

Cl

Cl

O

at id ox

is lys ro d hy

N

Cl

hydrolysis

Cl

N

Cl

O P O O Cl

O

chlorpyrifos oxon

TCP HO

b

O

oxidation

oxidation

N N O O decarboxylation O O P P O O S S isopropenyl derivative of diazinon (3) hydroxyethyl derivative of diazinon (5) N

N

N

N

O O P O S diazinon methyl ketone (6)

dehydration

OH HO N

N

hydroxylation

O O P O S

N

hydroxylation

O O P O S

diazinon

N

N

O O P S

O

2-hydroxydiazinon (7)

i ox n tio da

is lys ro d hy

hydroxydiazinon (4)

N

hydrolysis

N

N

N OH

IMP(1)

N

O O P O O diazoxon (2)

Fig. 4. Schemes for the degradation pathways of chlorpyrifos (a) and diazinon (b) in aqueous solution under ultrasonic irradiation.

et al., 1970). Lastly, hydroxyethyl derivative of diazinon was oxidized to diazinon methyl ketone. Obviously, there are two consistent characteristics in the degradation pathway of chlorpyrifos and diazinon under ultrasonic irradiation. On the one hand, both chlorpyrifos (2.7 mPa, 25 °C) and diazinon (12 mPa, 25 °C) have low vapor pressure (Tomlin, 2006), thus they were mainly present in the bubble-bulk interface region and bulk region, where free-radical reactions were predominant. Therefore, the degradation of both pesticides was mainly promoted by the formation of free-radicals. On the other hand, the most of degradation products of both pesticides are heterocyclic compounds. This might be related to the good stability of heterocyclic compounds. So it should be kept in mind that complete mineralization by ultrasonic irradiation was a slow process. Similar result was also observed in the sonochemical degradation of parathion by Kotronarou et al. (1992). 3.5. Mineralization study The most common way of estimating the mineralization is to monitor the reduction in TOC. Thus, the solutions of chlorpyrifos (1.4 mg L–1) and diazinon (2.4 mg L–1) were sonicated at 900 W for 60 min. Results showed that no significant TOC difference was observed between the untreated samples and the samples treated by sonication for 60 min (p = 0.05), although the degradation percentages for chlorpyrifos and diazinon reached 85% and 57%, respectively. This indicated the incomplete mineralization occurring during sonication. The identified products in this study also verified the presence of organics as intermediates. The similar

phenomena have been found by Vajnhandl and Le Marechal (2007) and Madhavan et al. (2010). These results implied that the complete mineralization may require longer sonication time or higher sonication power. 3.6. The toxicity evaluation The degradation of both pesticides occurred rapidly under ultrasonic irradiation, however, the problem is that some more toxic compounds were produced, such as chlorpyrifos oxon and diazoxon (Sparling and Fellers, 2007). Hence, it is necessary to compare the toxicity of the aqueous solution after treatment to that before treatment, in order to completely evaluate ultrasonic irradiation as the detoxification technique. After irradiation for 30 and 60 min at 600 W, the degradation percentage of chlorpyrifos raised by 65% and 72%, but the IR of solutions increased by 19% and 33% compared with the untreated sample, respectively. Compared with the sample treated for 30 min, the degradation percentage of chlorpyrifos treated for 60 min grew by 1.7 times, but the toxicity of the sample increased by 1.1 times. This may contribute to the formation of degradation products. Although TCP was much less toxic than chlorpyrifos, chlorpyrifos oxon have been demonstrated to be 100 times more toxic than its parent compound (Sparling and Fellers, 2007). Furthermore, the total ion chromatogram (TIC) showed that the peak area of chlorpyrifos oxon after irradiation for 60 min was 1.3 times higher than that after the irradiation for 30 min (data not shown). Therefore, it could be deduced that the chlorpyrifos oxon might contribute to the increase of toxicity of sample after ultrasonic irradiation. Similarly, the increase in

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toxicity of chlorpyrifos solution after photodegradation was found by Kralj et al. (2007) for the formation of chlorpyrifos oxon. For the solutions spiked with diazinon, the degradation percentage of diazinon increased by 22% and 47% after irradiation for 30 and 60 min at 600 W, and the IR reduced by 1% and 14% compared with the untreated samples, respectively. Moreover, the degradation percentage of diazinon increased by 2.2 times and the toxicity of the samples decreased by 9.6 times when the treatment time increased from 30 to 60 min. Although it is known that diazoxon is 10 times more toxic than diazinon (Sparling and Fellers, 2007), and the peak area of diazoxon after the irradiation for 60 min was 2.3 times higher than that after the irradiation for 30 min (TIC data not shown). Fortunately, the toxicity of the solution spiked with diazinon declined significantly after ultrasonic irradiation. This indicated that the amount of diazoxon formed during ultrasonic irradiation could not result in the increase of the toxicity of sample, since the oxidation might be not the predominant reaction in the degradation of diazinon. However, the quantification of degradation products will be needed for the explanation of toxicity changes in future. 4. Conclusions This study demonstrated that chlorpyrifos and diazinon could be effectively degraded by ultrasonic irradiation. The different parameters including electric power, temperature and pH value had significant effect on the degradation of both pesticides. Based on GC–MS analysis, two and seven degradation products for chlorpyrifos and diazinon were detected, respectively. The oxidation of P@O and hydrolysis were proposed for the degradation pathway of chlorpyrifos, and the hydrolysis, oxidation, hydroxylation, dehydration, and decarboxylation were deduced in the degradation pathway of diazinon. However, the detoxification for chlorpyrifos and diazinon by ultrasonic irradiation was discriminative depending on the toxicity and the degradation products. The longer irradiation times or combination with other detoxification techniques is probably applied to thoroughly dissipate the pesticides. Acknowledgements The work was supported by the Graduate Scientific Research and Innovation Foundation of China Agricultural University Funds (kycx09100), Chinese Universities Scientific Fund (2009-1-77) and Beijing Science and Technology Project (101105046610001). References Adewuyi, Y.G., 2001. Sonochemistry: environmental science and engineering applications. Ind. Eng. Chem. Res. 40, 4681–4715. 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. Dükkanci, M., Gündüz, G., 2006. Ultrasonic degradation of oxalic acid in aqueous solutions. Ultrason. Sonochem. 13, 517–522. Dolara, P., Salvadori, M., Capobianco, T., Torricelli, F., 1992. Sister-chromatid exchanges in human lymphocytes induced by dimethoate, omethoate, deltamethrin, benomyl and their mixture. Mutat. Res. Mutat. Res. Lett. 283, 113–118. Duirk, S.E., Tarr, J.C., Collette, T.W., 2008. Chlorpyrifos transformation by aqueous chlorine in the presence of bromide and natural organic matter. J. Agric. Food Chem. 56, 1328–1335. Foo, K.Y., Hameed, B.H., 2010. Detoxification of pesticide waste via activated carbon adsorption process. J. Hazard. Mater. 175, 1–11. Gao, J., Liu, L., Liu, X., Zhou, H., Lu, J., Huang, S., Wang, Z., 2009. The occurrence and spatial distribution of organophosphorous pesticides in Chinese surface water. Bull. Environ. Contam. Toxicol. 82, 223–229. Goel, M., Hongqiang, H., Mujumdar, A.S., Ray, M.B., 2004. Sonochemical decomposition of volatile and non-volatile organic compounds – a comparative study. Water Res. 38, 4247–4261.

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