- Email: [email protected]
Degradation of chlorpyrifos in humid tropical soils
Journal of Environmental Management 125 (2013) 28e32
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Degradation of chlorpyrifos in humid tropical soils Lian-Kuet Chai a, *, Mee-Hua Wong a, Hans Christian Bruun Hansen b a b
Agriculture Research Centre, Semongok, Department of Agriculture, Sarawak, Borneo Height Road, 93250 Kuching, Sarawak, Malaysia Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Copenhagen, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2012 Received in revised form 27 March 2013 Accepted 6 April 2013 Available online 28 April 2013
The insecticide chlorpyrifos is extensively used in the humid tropics for insect control on crops and soils. Chlorpyrifos degradation and mineralization was studied under laboratory conditions to characterize the critical factors controlling the degradation and mineralization in three humid tropical soils from Malaysia. The degradation was fastest in moist soils (t1/2 53.3e77.0 days), compared to dry (t1/2 49.5e120 days) and wet soils (t1/2 63.0e124 days). Degradation increased markedly with temperature with activation energies of 29.0e76.5 kJ mol1. Abiotic degradation which is important for chlorpyrifos degradation in sub-soils containing less soil microbial populations resulted in t½ of 173e257 days. Higher chlorpyrifos dosages (5-fold) which are often applied in the tropics due to severe insects infestations caused degradation and mineralization rates to decrease by 2-fold. The mineralization rates were more sensitive to the chlorpyrifos application rates reflecting that degradation of metabolites is rate limiting and the toxic effects of some of the metabolites produced. Despite that chlorpyrifos is frequently used and often in larger amounts on tropical soils compared with temperate soils, higher temperature, moderate moisture and high activity of soil microorganisms will stimulate degradation and mineralization. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Pesticide Mineralization Abiotic degradation First order kinetic Soil contamination
1. Introduction In Malaysia, chlorpyrifos is used intensively for pests control in soils and crops. The conducive climate and continuous cultivation of crops throughout the year have resulted in proliferation of pests and diseases. To minimize economic losses, chlorpyrifos is usually applied at higher rates and larger volumes to crops and soils to eradicate and prevent the proliferation of pests. This practice may increase the risk of pesticide contamination of soil and water resulting in toxicity to non-target organisms. In addition, if the pesticides persist for a prolonged time in soil, they may be taken up by new crops. Chlorpyrifos which sorbs strongly to soil organic matter is known to migrate into the subsoils. This is attributed to the macropore transport of chlorpyrifos sorbed to particulate matter (Chai et al., 2009a). For the soils studied, cracks, root channels, earth worm burrows, and other macropores which could facilitate leaching of chlorpyrifos are found abundantly in top and subsoils. Data on chlorpyrifos degradation is critical for predicting residue levels likely to remain in the soil, enabling assessment and
* Corresponding author. Tel.: þ60 82 611171; fax: þ60 82 611178. E-mail addresses: [email protected], [email protected], chailk@ sarawaknet.gov.my (L.-K. Chai). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.04.005
modeling of leaching risks to the aquatic environment as well as determining the residue’s toxicity effects. Chlorpyrifos (O, Oediethyl O-3, 5, 6-trichloro-2-pyridyl phosphorothioate) is a broad spectrum insecticide used to control pests in the soil or on foliage. Chlorpyrifos has a low water solubility of 2.0 103 g L1 (25 C) and an intermediate organic carbonewater partitioning coefficient of log Koc 3.78 and hence sorbs strongly to humic particles (Tomlin, 1994). Chlorpyrifos is rapidly hydrolyzed to its primary metabolite, 3, 5, 6-trichloropyridinol (TCP), which is moderately mobile (log Koc: 1.43e2.59) and persistent in the soil (Jin and Webster, 1998; Racke, 1993). For temperate soils, chlorpyrifos is moderately persistent and the half-lives of chlorpyrifos depend on the type of soil and the environmental conditions (Chu et al., 2008; Fang et al., 2009; Racke, 1993). In laboratory studies, the reported half-lives for chlorpyrifos in temperate soils range between 3 and 40 d while in more clayey soils the half-lives are longer and in the range of 120e450 d (Baskaran et al., 2003; Chu et al., 2008; Fang et al., 2009; Racke et al., 1994; Sardar and Kole, 2005). In field studies, shorter half-lives of less than 10 days have been reported for chlorpyrifos degradation in tropical cultivated soils (Ciglasch et al., 2006; Laabs et al., 2002; Chai et al., 2009b). Longer half-lives of 13e20 days have been reported for chlorpyrifos applied at higher concentrations for treatment in soils without
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
vegetation (Chai et al., 2009a). To our knowledge, there is little information available on the factors affecting the chlorpyrifos degradation for tropical soils under laboratory conditions. Variations in soil temperatures, moisture contents, application rates, soil properties, and soil microorganisms are likely to affect the chlorpyrifos degradation, but the significance of the different factors have not been quantified. In our earlier field study, chlorpyrifos was reported to dissipate rapidly in cultivated humid tropical soils (Chai et al., 2009a, b). This was attributed to photodegradation, leaching through macropores and chemical/enzymatic degradation. In this study, experiments were conducted under controlled laboratory conditions to study the effects of moisture, temperature, application rates and microbial activity on the degradation of chlorpyrifos in three representative humid tropical mineral topsoils from Malaysia. The objective was to identify factors responsible for the chlorpyrifos degradation and mineralization and suggest possible remedial actions to reduce their environmental risks. 2. Materials and methods 2.1. Chemicals Chlorpyrifos (purity 99.0%) standards were obtained from Ehrenstorfer, Germany. Analytical and residue grades of sodium sulfate, ethyl acetate, sodium hydroxide and acetone were purchased from J.T. Baker, USA. Sodium hydroxide was obtained from Merck, Germany. Radio-labeled chlorpyrifos (activity 9.25 MBq or 1.0 mCi mL) with carbon labeled at 1-ethyl, purchased from Izotop, Hungary, was used for the mineralization experiments. 2.2. Apparatus and instrumentation An orbital shaker (Lab-line Instruments Inc., USA) was used for shaking soil suspensions during extraction. A Rotavapor RE 111 rotary evaporator (Switzerland) coupled to a Buchi 461 water bath (Switzerland) and a refrigerated cooler (Polyscience, USA) was used to concentrate extracts. Soil samples were incubated in an incubator (Memmert, Germany; 1.0 C). An Agilent (U.S.A.) Model 6890 gas chromatograph (GC) equipped with a flame photometric detector (FPD) was used for the determination of chlorpyrifos. A non-polar fused-silica capillary column, HP5, 15 m 0.53 mm 1.5 mm purchased from J & W Scientific U.S.A. was used with nitrogen as carrier gas at a flow of 4.0 mL min1. The column temperature was first maintained at 120 C for 1.0 min, then programmed at 30 C min1e150 C. This was later followed by another temperature ramp of 5 C min1e270 C and held at 270 C for 10 min. Injector and detector temperatures were maintained at 260 C and 250 C, respectively. Air and hydrogen gas flow were set at 80 mL min1 and 67 mL min1, respectively. 2.3. Soils Three soils previously used for vegetable cultivation were used in this study. They were classified as clayey red yellow podzolic (Typic Paleudult located at Semongok; N 01230 05.900 , E 110190 44.70 ), alluvial (Typic Udorthent located at Tarat; N 01120 01.900 , E 110 31015.30 ) and Red Yellow Podzolic soil (Typic Kandiudult located at Balai Ringin; N 01020 48.900 , E 110 480 21.70 ) (Soil Survey Staff, 2010). Top soils (0e10 cm) collected from the field were air-dried and sieved (2 mm) to remove stones, plant and root residues and used for the degradation experiments. Prior to use, soil moisture was adjusted to the experimental conditions and left for a week to reactivate biological activity. Air-dried soil was used for soil physiochemical characterization (Page et al., 1982); results are shown in Table 1.
29
Table 1 Chemical and physical properties of the three soils investigated.
pHa % moistureb % carbonc % clayd % siltd % sandd CEC7e % base saturationf AlCBDg FeCBDg Clay mineralsh
Semongok
Tarat
Balai Ringin
4.8 33 2.2 23 30 47 11.8 40 56 197 kaolinite, vermicullite
5.6 32 1.8 14 15 71 16.2 70 63 118 kaolinite, vermicullite, illite
5.6 22 1.4 6 16 78 5.0 88 12 29 kaolinite, vermiculite
a
pH determined in 0.01 M CaCl2 in a 1:2.5 soil:water suspension. Natural moisture content. Amount of water per mass of dry soil (gravimetric water content of field moist soil). c Mass percentage of carbon determined by dry combustion. d Mass percentage of particle size distribution determined by sieving and sedimentation (clay < 2, mm, 2 mm < silt < 20 mm, 20 mm < fine sand < 200 mm, 200 mm < coarse sand < 2000 mm). e CEC7 : Cation exchange capacity determined by the ammonium acetate method (pH 7) (cmol(þ)/kg). f Sum of exchangeable base cations (Ca2þ, Mg2þ, Naþ, Kþ)/CEC7*100. g Extractable aluminum and iron determined by the dithionite-citratebicarbonate method (mmol/kg). h Clay minerals determined by powder X-ray diffraction. b
2.4. Degradation studies Degradation kinetics of chlorpyrifos was quantified using laboratory incubation experiments. Soils were prepared according to the procedures by Racke et al. (1994). A 400 g of moist soil was weighed into a 1 L amber glass flask with PTFE-lined caps (Wheaton, USA). Twenty mL of 100 mg L1 (for initial soil concentration of 5 mg g1) or 500 mg L1 (for initial soil concentration of 25 mg g1) chlorpyrifos solutions (in acetone) were added to one quarter of the soils, left for half an hour to allow the acetone to evaporate and then mixed with the rest of the soils. The flasks were then shaken on a horizontal shaker at 200 rpm for half an hour to ensure chlorpyrifos was mixed homogenously with the soil. The flasks were stored in a fixed-temperature incubator (1 C) according to experimental conditions. For experiments with sterilized soil, incubation flasks were sealed and only aerated during sampling in a sterilized chamber to prevent microbial contamination. Sterilization was performed by autoclaving the soil for one hour at 121 C for three consecutive days following the procedure described earlier (Chai et al., 2010). Investigations were carried out on the effects of soil moisture (dry, moist and wet), sterilization, temperature (15, 25, 35 C) and chlorpyrifos application rates (5, 25 mg g1; fresh weight) on chlorpyrifos degradation. The soil moisture contents refer to air-dry soil, soils at field moisture contents, and wet soils with gravimetric water contents of 61e68% (Table 2). Soils were incubated in darkness and 10 g of soils were retrieved for analysis from each flask on day 0, 5, 15, 25, 40, 70, 100, 130 and 160. The weight of the incubation flasks was recorded to permit periodic addition of water so that constant moisture contents of soils could be maintained. For sterilized soils, all equipment used was autoclaved and samplings were conducted in a sterilized chamber (Gleeman, UK). All experiments were carried out in triplicates.
2.5. Mineralization studies A 10 g of moist soil was weighed into a 100 mL serum flask. A 495.9 mL of non radio-labeled chlorpyrifos solution (5000 mg L1 in
30
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
Table 2 First-order rate constants (k), half-lives (t1/2) with 95% confidence limits (cl) and coefficients of determination (r2) for first-order fitting of chlorpyrifos degradation in soil at 5 mg g1 initial concentration (unless specified) for different experimental conditions. Soil Sterilization Balai Ringin Tarat Semongok Moisture Balai Ringin
Variable
r2
k (day1)
Sterilized Non-sterilized Sterilized Non-sterilized Sterilized Non-sterilized
0.88 0.98 0.83 0.98 0.86 0.96
0.0040 0.010 0.0038 0.013 0.0027 0.009
0.78 0.98 0.95 0.92 0.98 0.96 0.84 0.96 0.92
0.0082 0.010 0.011 0.014 0.013 0.011 0.0058 0.009 0.0056
0.87 0.98 0.85 0.93 0.98 0.96 0.97 0.96 0.96 0.99 0.98 0.96 0.98 0.98 0.96
Dry Moist Wet Tarat Dry Moist Wet Semongok Dry Moist Wet Temperature C Balai Ringin 15 25 35 Tarat 15 25 35 Semongok 15 25 35 Concentration mg g1 Balai Ringin 25 5 Tarat 25 5 Semongok 25 5
t1/2 (day)
t1/2 95% cl (day)
0.0006a 0.001b 0.0006a 0.002c 0.0005d 0.001e
173 69.3 182 53.3 256 77.0
128e276 56.3e81.5 182e451 38.5e87.7 131e301 59.7e99.0
0.002a 0.001b 0.001b 0.002c 0.002d 0.001b 0.001e 0.001f 0.007e
84.5 69.3 63 49.5 53.3 63 120 77.0 124
54.1e193 56.3e81.5 47.8e84.1 36.5e78.8 38.5e87.7 49.5e82.5 83.5e211 59.7e99.0 84.9e78
0.0036 0.010 0.030 0.0083 0.013 0.019 0.0031 0.009 0.0185
0.001a 0.001b 0.008c 0.001d 0.002e 0.003f 0.0002g 0.001d 0.002h
193 69.3 23.1 83.5 53.3 36.5 224 77.0 37.5
142e308 56.3e81.5 14.4e63 62.4e124 38.5e87.7 28.9e54.6 192e267 59.7e99.0 28.9e53.3
0.0058 0.010 0.0091 0.013 0.0082 0.009
0.0003a 0.001b 0.0005c 0.002d 0.0005e 0.001c
120 69.3 76.2 53.3 84.5 77.0
108e133 56.3e81.5 74.7e97.6 38.5e87.7 69.3e86.6 59.7e99.0
Moist soils were used unless specified; experiments were conducted at 25 C unless specified; different letters in superscript indicate the rate constants are significantly different within the specific experiment (p < 0.05).
acetone) and 214.8 mL radio-labeled chlorpyrifos solution (4.13 mg L1 in acetonitrile) were added to each flask to give an initial chlorpyrifos concentration of 5 mg g1 and an initial 14C activity of 5.0 104 dpm. The chlorpyrifos solutions were spread well onto the soil and a small test tube with 1 mL of sodium hydroxide (1 M) was placed in the serum flask to trap 14CO2 evolved by mineralization as a result of microbial activity. The flasks were closed tightly and incubated at 25 C in darkness. The sodium hydroxide solutions were removed and replaced with fresh solution on day 1, 3, 5, 7, 10, 14, 21, 30, 45 and 60. Five mL of scintillation cocktail (Optisafe, Wallace, Finland) was added to the sodium hydroxide solution before radioactivity was measured on a scintillation counter (Wallace, Finland) for 20 min. The background counts of the sodium hydroxide and scintillation cocktail were measured and subtracted from the sample results. The experiments were carried out in triplicates at two initial concentrations of chlorpyrifos (5 and 25 mg g1). Samples containing quartz were used as control.
2.6. Pesticide extraction and analysis To determine the total contents of chlorpyrifos in incubated soils, 10 g of soil sample was weighed into a 500 mL glass shaking flask where 10 mL of water and 100 mL of ethyl acetate were added. The mixture was shaken for 1 h on an orbital shaker at 300 rpm.
The extract was left to settle and then filtered through a filter paper (Whatman, grade 41) containing sodium sulfate into a round bottomed receiving flask. The shaking flask and its contents were rinsed twice with extraction solvents and the washes were decanted and filtered into the same receiving flask. The filtrate was evaporated using a rotary evaporator until it was almost dry and later made up to 10 mL with acetone. Two mL of the final extract was injected into GC-FPD. Good recoveries of 76e102% with standard deviation (SD) of less than 8.9% were obtained for chlorpyrifos in the three soils (Chai et al., 2008). The limit of quantification was 0.01 mg g1. 2.7. Data analysis The chlorpyrifos degradation data was modeled using a simple first-order model: Ct ¼ Coeekt where Ct is the chlorpyrifos concentration at time t, Co is the initial concentration of chlorpyrifos, and k is the rate constant. Curve fitting was performed using a non-linear least squares regression analysis of chlorpyrifos concentration against time using TableCurve (Systat Software Limited, USA). The activation energy was calculated based on the Arrhenius equation; k ¼ AeeEa/RT, where k is the rate constant, A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature in Kelvin. 3. Results 3.1. Soil characteristics The soils used in our study were all slightly acidic with gravimetric moisture and carbon contents ranging from 22 to 33% and 1.4e2.2%, respectively. The Balai Ringin soil had the lowest contents of clay (5.9%), CEC (5.0 mmol (þ) kg1), CBD extractable aluminum and iron (12.0 and 29.1 mmol kg1, respectively), but had higher content of sand (76.3%) and base saturation (88%) compared to Tarat and Semongok soils. The clay fraction in the soils was dominated by kaolinite with traces of vermiculite; the less developed Tarat soil also contained illite. 3.2. Effect of sterilization on chlorpyrifos degradation (abiotic degradation) Degradation of chlorpyrifos in sterilized soil was much slower than in non-sterilized soil (Table 2). On day 70, 40.5e46.2% of the chlorpyrifos remained in non-sterilized soils in the three soils investigated compared to 67.5e72.1% in sterilized soils. Half-lives in the sterilized soils ranged between 173 and 256 days reflecting 2e 3-fold slower degradation compared to non-sterilized soils. The half-lives of chlorpyrifos in sterilized soils were in the order of Semongok (256 days) > Tarat (182 days) > Balai Ringin (173 days). Significant differences in rate constants and half-lives among the sterilized and non-sterilized soils were observed demonstrating soil microbial activity significantly affected the degradation of chlorpyrifos. 3.3. Effect of moisture on chlorpyrifos degradation Degradation of chlorpyrifos in soils under different moisture contents followed first-order kinetics (r2 > 0.78). The rates of chlorpyrifos degradation for different moisture levels were soil specific, but in general degradation rates decreased in the order of Tarat > Balai Ringin > Semongok soils. Longer half-lives were obtained for wet soils (63.0e124 days) compared to air dry (49.5e120 days) and moist soils (53.3e77.0 days). The rate constants and halflives for dry, moist and wet soils were significantly different among
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
the three soils except for the Balai Ringin moist and wet soils and Semongok dry and wet soil indicating that degradation depends on combination of soil type and soil moisture regime.
Table 3 Mineralization rate constants (k), half-lives (t1/2) with 95% confidence limits (cl), and coefficients of determination (r2) for chlorpyrifos in field moist soils at two chlorpyrifos application rates at 25 C.
3.4. Effect of temperature on chlorpyrifos degradation Degradation of chlorpyrifos was faster at 35 C compared to at 15 C (Table 2). Half-lives of 83e224 days, 53e77 days and 23e38 days were obtained at 15 C, 25 C and 35 C, respectively, with significant differences observed between 15 C, 25 C, and 35 C for all the three soils. The activation energy of degradation was in the order of Tarat (76.5 0.6 kJ mol1) > Semongok (64.8 0.2 kJ mol1) > Balai Ringin (29.0 0.4 kJ mol1) soils.
31
Balai Ringin Tarat Semongok
Application rate (mg g1)
r2
k (day1)
5 25 5 25 5 25
0.99 0.99 0.99 0.99 0.99 0.99
0.0062 0.0023 0.0034 0.0033 0.0036 0.0019
0.00014a 0.00004b 0.00010a 0.00007a 0.00009a 0.00005b
t1/2 (day)
t1/2 95% cl (day)
112 301 204 210 193 365
107e118 289e301 193e217 198e217 182e204 351e395
Different letters in superscript indicate the rate constants are significantly different (p < 0.05).
3.5. Effect of application rate on chlorpyrifos degradation The higher chlorpyrifos application rate of 25 mg g1 resulted in decrease of the rates of chlorpyrifos degradation compared with the application rate of 5 mg g1 (Table 2). Thus a 1e2-fold increase in the half-lives of chlorpyrifos from 53 e 77 days to 76e120 days was observed. Significant differences in half-lives among the application rate were observed for all the three soils. 3.6. Mineralization of chlorpyrifos Mineralization of chlorpyrifos was measured as carbon dioxide evolved. Mineralization of chlorpyrifos at the application rate of 5 mg g1 was gradual in the three soils (Fig. 1). Mineralization followed first-order kinetics in the three soils with fastest mineralization in the Balai Ringin soil. The Tarat and Semongok soils showed similar rates of mineralization. On day 60, almost similar amounts of chlorpyrifos had mineralized in the Tarat and Semongok soils (18%) compared to higher mineralization in the Balai Ringin soil (30%). At the higher chlorpyrifos application rate of 25 mg g1 the highest mineralization rate was observed for the Tarat soil. For the Semongok and Tarat soils, the mineralization rates were 2e3 times lower at the higher application rate compared with the low application rate (Table 3). The amount mineralized was almost the same for the Tarat soil on day 60 for both application rates. At the low spiking level of 5 mg g1, the mineralization half-lives ranged between 112 and 204 days with the shortest half-life for the Balai Ringin soil while at the higher spiking level of 25 mg g1, halflives increased up to 2.7 times (Table 3). The soils fortified with 5 mg g1 of chlorpyrifos were extracted after incubation for 40 days with 1 M NaOH, 30 mL water, followed by 30 mL methanol:water (60:40) and 10 mL 5 M sodium hydroxide to determine the amount of 14C present in the different pools. The highest amount of 14C was found in the 1 M NaOH extract (18e 30.6%) and water þ methanol (19.4e33.9%), followed by 5 M NaOH (14.1e20.2%), and water (1.7e3.1%) (Fig. 2). The total amount of 14C
Fig. 2. Percentage of 14C present in different soil extracts at end of the mineralization experiment (chlorpyrifos loading of 5 mg g1). Vertical bars represent standard deviations (n ¼ 3).
recovered in various pools was 68.7, 68.8 and 68.5% for the Semongok, Tarat and Balai Ringin soils, respectively.
4. Discussion 4.1. Effect of soil properties, moisture content and temperature In our investigation, degradation rate decreased in the order Tarat > Balai Ringin > Semongok soils; in most cases the differences were significant. The differences in degradation rates between soils were attributed to soil pH and clay contents. The soil with the lowest pH e the Semongok soil e had the slowest degradation in agreement with previous investigations on temperate soils (Chu et al., 2008; Fang et al., 2009; Racke, 1993; Sardar and Kole, 2005). The Tarat soil had higher clay contents than in the Balai Ringin soil (Table 1) and also had highest counts of
Fig. 1. Mineralization kinetics of chlorpyrifos at spiking levels of 5 mg g1 and 25 mg g1 in Balai Ringin (:), Tarat (-), and Semongok (A) soils. Full lines are due to first-order fitting. Vertical bars represent standard deviations (n ¼ 3).
32
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
bacteria and fungi in the untreated soil (results not shown) resulting in highest degradation among the three soils studied. The soil moisture level affected the degradation of chlorpyrifos. The degradation half-lives for chlorpyrifos for most soils were in the order of moist soils < air-dry soils < wet soils. The slower degradation in wet soils was attributed to inactivation of the soil microbial activities under anaerobic condition. Slower degradation in air-dry soils was also due to inactivation of soil microbial activities under low moisture conditions. In earlier studies on temperate soils, the rate of chlorpyrifos degradation for low spiking levels of 0.25 mg g1 in three soils (sandy, sandy loam and clayey) with pH of 6e9, organic matters 0.9e4.0% and clay contents of 20e33% was air-dry < field capacity < submerged (Awasthi and Prakash, 1997). However, it was reported that the effect of moisture (9e22%) was not pronounced for chlorpyrifos (10 mg g1) in clayey temperate soils with pH 8, 1.7% organic matter, and 42% clay (Racke et al., 1994). It is likely that in the latter investigation alkaline hydrolysis overruled the effect of moisture. The effect of temperature obtained in this study was similar to those reported for chlorpyrifos in temperate soils (Racke et al., 1994). The activation energy of degradation was in the range of 29.0e76.5 kJ mol1 which was about 3e4 times higher than the activation energy reported earlier for acephate (16.8e28.3 kJ mol1) in similar tropical soils (Chai et al., 2010). This implies that shading has great effect on the degradation of chlorpyrifos in soils. 4.2. Mode of degradation Enzymatic degradation contributed substantially to the degradation of chlorpyrifos as the half-lives were 2e3-fold higher for the sterilized soils than the non-sterilized soils (Table 2). However, degradation still occurred in the sterilized soils resulting from abiotic degradation processes which are attributed to hydrolysis. The ratios between degradation rates in non-sterilized and sterilized soils were 2.4e4.8 demonstrating that enzymatic degradation accounted for about 60e80% of the total degradation. A 5-fold increase in the chlorpyrifos application rate had only minor effect on chlorpyrifos degradation in agreement with results previously observed for clayey temperate soils and for organophosphorus pesticides (Chai et al., 2010; Racke, 1993; Vijay et al., 2006). Although the higher doses may have toxic effects on soil microorganisms, the relatively high abiotic degradation and the sorption of chlorpyrifos to clays seemed to make the chlorpyrifos degradation less sensitive to toxic dosing effects. Mineralization rates were more sensitive to chlorpyrifos application rates than degradation rates except for the Tarat soil. This indicates that degradation of metabolites but not degradation of the parent compound is rate limiting and toxic effects of some of the metabolites produced cannot be excluded. Chlorpyrifos mineralization levels were similar to the levels of organophosphorus pesticide mineralization typically seen for temperate soils (Singh et al., 2003) but higher than what have been previously reported for acephate mineralization in tropical soils (Chai et al., 2010; Marchetti and Luchini, 2004). Most of the non-mineralized chlorpyrifos was found in NaOH extracts, indicating that chlorpyrifos residues have been incorporated into humic substances and microbial tissue in agreement with other studies for temperate soils (Jensen et al., 2004). 5. Conclusions Soil moisture, temperature, and soil microbial activity are critical variables controlling chlorpyrifos degradation in humid tropical soils. The degradation and mineralization of chlorpyrifos in the three soils at various conditions can be described by first-order
kinetics. Degradation of chlorpyrifos is slowest in the absence of soil microbial activity and at low soil temperatures. The results from this study implied that the degradation of chlorpyrifos is slower for water logged soils, acidic soils, soils with high clay, soils at low temperatures and at high application rates. Under these conditions, leaching, accumulation and possible toxic effects of metabolites of chlorpyrifos may occur. Thus, increasing the soil pH by liming and using low application rate will accelerate chlorpyrifos degradation. The use of chlorpyrifos on soils with dense vegetation resulting in shade and hence lower surface soil temperatures and less photolysis is expected to retard degradation. Considerably longer half-lives found in this study compared to field study demonstrates that in addition to degradation, leaching, photodegradation and volatilization of chlorpyrifos are important dissipation process in the field. Acknowledgment The authors wish to thank University of Copenhagen (KU), Denmark and State Government of Sarawak for funding this project. The authors also wish to acknowledge assistance from laboratory technicians of Department of Agriculture Sarawak and Faculty of Life Sciences of KU. References Awasthi, M.D., Prakash, N.B., 1997. Persistence of chlorpyrifos in soils under, different moisture regimes. Pestic. Sci. 50, 1e4. Baskaran, S., Kookana, R.S., Naidu, R., 2003. Contrasting behaviour of chlorpyrifos and its primary metabolites, TCP (3,5,6-trichloro-2-pyridinol) with depth in, soil profiles. Aust. J. Soil Res. 41, 749e760. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2008. Determination of chlorpyrifos, and acephate in tropical soils and application in dissipation studies. Int. J. Environ. Anal. Chem. 88, 549e560. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2009a. Degradation and migration of, the phosphorothioates acephate and chlorpyrifos in tropical field soils. J. Environ. Qual. 38, 1160e1169. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2009b. Dissipation of acephate, chlorpyrifos, cypermethrin and their metabolites in a humid-tropical vegetable, production system. Pest Manag. Sci. 65, 189e196. Chai, L.K., Wong, M.H., Mohd-Tahir, N., Hansen, H.C.B., 2010. Degradation and, mineralization kinetics of acephate in humid tropic soils of Malaysia. Chemosphere 79, 434e440. Chu, X.Q., Fang, H., Pan, X.D., Wang, X., Shan, M., Feng, B., Yu, Y.L., 2008. Degradation of chlorpyrifos alone and in combination with chlorothalonil and, their effects on soil microbial populations. J. Environ. Sci. 20, 464e469. Ciglasch, H., Busche, J., Amelung, W., Totrakool, S., Kaupenjohann, M., 2006. Insecticide dissipation after repeated field application to a Northern Thailand Ultisol. J. Agric. Food Chem. 54, 8551e8559. Fang, H., Yu, X.Q., Wang, X.G., Yang, X., Yu, J.Q., 2009. Degradation of, chlorpyrifos in laboratory soil and its impact on soil microbial functional diversity. J. Environ. Sci. 21, 380e386. Jensen, P.A., Hansen, H.C.B., Rasmussen, J., Jacobsen, O.S., 2004. Sorption-, controlled degradation kinetics of MCPA in soil. Environ. Sci. Technol. 38, 6662e6668. Jin, H., Webster, G.R.B., 1998. Dissipation of chlorpyrifos, oxon and 3,5,6-, trichloro2-pyridinol in litter and elm forest soil. Int. J. Environ. Anal. Chem. 69, 307e316. Laabs, V., Amelung, W., Pinto, A., Zech, W., 2002. Fate of pesticides in tropical soils, of Brazil under field conditions. J. Environ. Qual. 31, 256e268. Marchetti, M., Luchini, L.C., 2004. Sorption/desorption and mineralization of the, insecticide acephate in the soil. Revista de Ecotoxicologia e Meio Ambiente 14,1e6. Page, A.L., Miller, R.H., Roberts, T.R., Kearney, P.C., Keeney, D.R., 1982. Method, of Soil Analysis. Soil Society of America, Madison, Wisconsin, USA. Racke, K.D., 1993. Environmental fate of chlorpyrifos. Rev. Environ. Contam. Toxicol. 131, 1e151. Racke, K.D., Fontaine, D.D., Yoder, R.N., Jack, R.M., 1994. Chlorpyrifos, degradation in soil at termiticidal application rate. Pestic. Sci. 42, 43e51. Sardar, S., Kole, R.K., 2005. Metabolism of chlorpyrifos in relation to its effect on the availability of some plant nutrients in soil. Chemosphere 61, 1273e1280. Singh, B.K., Walker, A., Morgen, J.A.W., Wright, D.J., 2003. Effects of soil pH on, the biodegradation of chlorpyrifos and isolation of a chlorpyrifos-degrading bacterium. App. Environ. Microbiol. 69, 5198e5206. Soil Survey Staff, 2010. Keys to Soil Taxonomy, 11th ed. Pocahontas Press Inc., Blacksburg, Virginia, U.S.A. Tomlin, C., 1994. The Pesticide Manual. The British Crop Protection Council, Surrey, UK and The Royal Society of Chemistry, Cambridge, United Kingdom. Vijay, A.K.B., Gundi, I., Reddy, B.R., 2006. Degradation of monocrotophos in soils. Chemosphere 62, 396e403.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Degradation of chlorpyrifos in humid tropical soils Lian-Kuet Chai a, *, Mee-Hua Wong a, Hans Christian Bruun Hansen b a b
Agriculture Research Centre, Semongok, Department of Agriculture, Sarawak, Borneo Height Road, 93250 Kuching, Sarawak, Malaysia Department of Basic Sciences and Environment, Faculty of Life Sciences, University of Copenhagen, Thorvaldsensvej 40, DK-1871, Frederiksberg C, Copenhagen, Denmark
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2012 Received in revised form 27 March 2013 Accepted 6 April 2013 Available online 28 April 2013
The insecticide chlorpyrifos is extensively used in the humid tropics for insect control on crops and soils. Chlorpyrifos degradation and mineralization was studied under laboratory conditions to characterize the critical factors controlling the degradation and mineralization in three humid tropical soils from Malaysia. The degradation was fastest in moist soils (t1/2 53.3e77.0 days), compared to dry (t1/2 49.5e120 days) and wet soils (t1/2 63.0e124 days). Degradation increased markedly with temperature with activation energies of 29.0e76.5 kJ mol1. Abiotic degradation which is important for chlorpyrifos degradation in sub-soils containing less soil microbial populations resulted in t½ of 173e257 days. Higher chlorpyrifos dosages (5-fold) which are often applied in the tropics due to severe insects infestations caused degradation and mineralization rates to decrease by 2-fold. The mineralization rates were more sensitive to the chlorpyrifos application rates reflecting that degradation of metabolites is rate limiting and the toxic effects of some of the metabolites produced. Despite that chlorpyrifos is frequently used and often in larger amounts on tropical soils compared with temperate soils, higher temperature, moderate moisture and high activity of soil microorganisms will stimulate degradation and mineralization. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Pesticide Mineralization Abiotic degradation First order kinetic Soil contamination
1. Introduction In Malaysia, chlorpyrifos is used intensively for pests control in soils and crops. The conducive climate and continuous cultivation of crops throughout the year have resulted in proliferation of pests and diseases. To minimize economic losses, chlorpyrifos is usually applied at higher rates and larger volumes to crops and soils to eradicate and prevent the proliferation of pests. This practice may increase the risk of pesticide contamination of soil and water resulting in toxicity to non-target organisms. In addition, if the pesticides persist for a prolonged time in soil, they may be taken up by new crops. Chlorpyrifos which sorbs strongly to soil organic matter is known to migrate into the subsoils. This is attributed to the macropore transport of chlorpyrifos sorbed to particulate matter (Chai et al., 2009a). For the soils studied, cracks, root channels, earth worm burrows, and other macropores which could facilitate leaching of chlorpyrifos are found abundantly in top and subsoils. Data on chlorpyrifos degradation is critical for predicting residue levels likely to remain in the soil, enabling assessment and
* Corresponding author. Tel.: þ60 82 611171; fax: þ60 82 611178. E-mail addresses: [email protected], [email protected], chailk@ sarawaknet.gov.my (L.-K. Chai). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.04.005
modeling of leaching risks to the aquatic environment as well as determining the residue’s toxicity effects. Chlorpyrifos (O, Oediethyl O-3, 5, 6-trichloro-2-pyridyl phosphorothioate) is a broad spectrum insecticide used to control pests in the soil or on foliage. Chlorpyrifos has a low water solubility of 2.0 103 g L1 (25 C) and an intermediate organic carbonewater partitioning coefficient of log Koc 3.78 and hence sorbs strongly to humic particles (Tomlin, 1994). Chlorpyrifos is rapidly hydrolyzed to its primary metabolite, 3, 5, 6-trichloropyridinol (TCP), which is moderately mobile (log Koc: 1.43e2.59) and persistent in the soil (Jin and Webster, 1998; Racke, 1993). For temperate soils, chlorpyrifos is moderately persistent and the half-lives of chlorpyrifos depend on the type of soil and the environmental conditions (Chu et al., 2008; Fang et al., 2009; Racke, 1993). In laboratory studies, the reported half-lives for chlorpyrifos in temperate soils range between 3 and 40 d while in more clayey soils the half-lives are longer and in the range of 120e450 d (Baskaran et al., 2003; Chu et al., 2008; Fang et al., 2009; Racke et al., 1994; Sardar and Kole, 2005). In field studies, shorter half-lives of less than 10 days have been reported for chlorpyrifos degradation in tropical cultivated soils (Ciglasch et al., 2006; Laabs et al., 2002; Chai et al., 2009b). Longer half-lives of 13e20 days have been reported for chlorpyrifos applied at higher concentrations for treatment in soils without
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
vegetation (Chai et al., 2009a). To our knowledge, there is little information available on the factors affecting the chlorpyrifos degradation for tropical soils under laboratory conditions. Variations in soil temperatures, moisture contents, application rates, soil properties, and soil microorganisms are likely to affect the chlorpyrifos degradation, but the significance of the different factors have not been quantified. In our earlier field study, chlorpyrifos was reported to dissipate rapidly in cultivated humid tropical soils (Chai et al., 2009a, b). This was attributed to photodegradation, leaching through macropores and chemical/enzymatic degradation. In this study, experiments were conducted under controlled laboratory conditions to study the effects of moisture, temperature, application rates and microbial activity on the degradation of chlorpyrifos in three representative humid tropical mineral topsoils from Malaysia. The objective was to identify factors responsible for the chlorpyrifos degradation and mineralization and suggest possible remedial actions to reduce their environmental risks. 2. Materials and methods 2.1. Chemicals Chlorpyrifos (purity 99.0%) standards were obtained from Ehrenstorfer, Germany. Analytical and residue grades of sodium sulfate, ethyl acetate, sodium hydroxide and acetone were purchased from J.T. Baker, USA. Sodium hydroxide was obtained from Merck, Germany. Radio-labeled chlorpyrifos (activity 9.25 MBq or 1.0 mCi mL) with carbon labeled at 1-ethyl, purchased from Izotop, Hungary, was used for the mineralization experiments. 2.2. Apparatus and instrumentation An orbital shaker (Lab-line Instruments Inc., USA) was used for shaking soil suspensions during extraction. A Rotavapor RE 111 rotary evaporator (Switzerland) coupled to a Buchi 461 water bath (Switzerland) and a refrigerated cooler (Polyscience, USA) was used to concentrate extracts. Soil samples were incubated in an incubator (Memmert, Germany; 1.0 C). An Agilent (U.S.A.) Model 6890 gas chromatograph (GC) equipped with a flame photometric detector (FPD) was used for the determination of chlorpyrifos. A non-polar fused-silica capillary column, HP5, 15 m 0.53 mm 1.5 mm purchased from J & W Scientific U.S.A. was used with nitrogen as carrier gas at a flow of 4.0 mL min1. The column temperature was first maintained at 120 C for 1.0 min, then programmed at 30 C min1e150 C. This was later followed by another temperature ramp of 5 C min1e270 C and held at 270 C for 10 min. Injector and detector temperatures were maintained at 260 C and 250 C, respectively. Air and hydrogen gas flow were set at 80 mL min1 and 67 mL min1, respectively. 2.3. Soils Three soils previously used for vegetable cultivation were used in this study. They were classified as clayey red yellow podzolic (Typic Paleudult located at Semongok; N 01230 05.900 , E 110190 44.70 ), alluvial (Typic Udorthent located at Tarat; N 01120 01.900 , E 110 31015.30 ) and Red Yellow Podzolic soil (Typic Kandiudult located at Balai Ringin; N 01020 48.900 , E 110 480 21.70 ) (Soil Survey Staff, 2010). Top soils (0e10 cm) collected from the field were air-dried and sieved (2 mm) to remove stones, plant and root residues and used for the degradation experiments. Prior to use, soil moisture was adjusted to the experimental conditions and left for a week to reactivate biological activity. Air-dried soil was used for soil physiochemical characterization (Page et al., 1982); results are shown in Table 1.
29
Table 1 Chemical and physical properties of the three soils investigated.
pHa % moistureb % carbonc % clayd % siltd % sandd CEC7e % base saturationf AlCBDg FeCBDg Clay mineralsh
Semongok
Tarat
Balai Ringin
4.8 33 2.2 23 30 47 11.8 40 56 197 kaolinite, vermicullite
5.6 32 1.8 14 15 71 16.2 70 63 118 kaolinite, vermicullite, illite
5.6 22 1.4 6 16 78 5.0 88 12 29 kaolinite, vermiculite
a
pH determined in 0.01 M CaCl2 in a 1:2.5 soil:water suspension. Natural moisture content. Amount of water per mass of dry soil (gravimetric water content of field moist soil). c Mass percentage of carbon determined by dry combustion. d Mass percentage of particle size distribution determined by sieving and sedimentation (clay < 2, mm, 2 mm < silt < 20 mm, 20 mm < fine sand < 200 mm, 200 mm < coarse sand < 2000 mm). e CEC7 : Cation exchange capacity determined by the ammonium acetate method (pH 7) (cmol(þ)/kg). f Sum of exchangeable base cations (Ca2þ, Mg2þ, Naþ, Kþ)/CEC7*100. g Extractable aluminum and iron determined by the dithionite-citratebicarbonate method (mmol/kg). h Clay minerals determined by powder X-ray diffraction. b
2.4. Degradation studies Degradation kinetics of chlorpyrifos was quantified using laboratory incubation experiments. Soils were prepared according to the procedures by Racke et al. (1994). A 400 g of moist soil was weighed into a 1 L amber glass flask with PTFE-lined caps (Wheaton, USA). Twenty mL of 100 mg L1 (for initial soil concentration of 5 mg g1) or 500 mg L1 (for initial soil concentration of 25 mg g1) chlorpyrifos solutions (in acetone) were added to one quarter of the soils, left for half an hour to allow the acetone to evaporate and then mixed with the rest of the soils. The flasks were then shaken on a horizontal shaker at 200 rpm for half an hour to ensure chlorpyrifos was mixed homogenously with the soil. The flasks were stored in a fixed-temperature incubator (1 C) according to experimental conditions. For experiments with sterilized soil, incubation flasks were sealed and only aerated during sampling in a sterilized chamber to prevent microbial contamination. Sterilization was performed by autoclaving the soil for one hour at 121 C for three consecutive days following the procedure described earlier (Chai et al., 2010). Investigations were carried out on the effects of soil moisture (dry, moist and wet), sterilization, temperature (15, 25, 35 C) and chlorpyrifos application rates (5, 25 mg g1; fresh weight) on chlorpyrifos degradation. The soil moisture contents refer to air-dry soil, soils at field moisture contents, and wet soils with gravimetric water contents of 61e68% (Table 2). Soils were incubated in darkness and 10 g of soils were retrieved for analysis from each flask on day 0, 5, 15, 25, 40, 70, 100, 130 and 160. The weight of the incubation flasks was recorded to permit periodic addition of water so that constant moisture contents of soils could be maintained. For sterilized soils, all equipment used was autoclaved and samplings were conducted in a sterilized chamber (Gleeman, UK). All experiments were carried out in triplicates.
2.5. Mineralization studies A 10 g of moist soil was weighed into a 100 mL serum flask. A 495.9 mL of non radio-labeled chlorpyrifos solution (5000 mg L1 in
30
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
Table 2 First-order rate constants (k), half-lives (t1/2) with 95% confidence limits (cl) and coefficients of determination (r2) for first-order fitting of chlorpyrifos degradation in soil at 5 mg g1 initial concentration (unless specified) for different experimental conditions. Soil Sterilization Balai Ringin Tarat Semongok Moisture Balai Ringin
Variable
r2
k (day1)
Sterilized Non-sterilized Sterilized Non-sterilized Sterilized Non-sterilized
0.88 0.98 0.83 0.98 0.86 0.96
0.0040 0.010 0.0038 0.013 0.0027 0.009
0.78 0.98 0.95 0.92 0.98 0.96 0.84 0.96 0.92
0.0082 0.010 0.011 0.014 0.013 0.011 0.0058 0.009 0.0056
0.87 0.98 0.85 0.93 0.98 0.96 0.97 0.96 0.96 0.99 0.98 0.96 0.98 0.98 0.96
Dry Moist Wet Tarat Dry Moist Wet Semongok Dry Moist Wet Temperature C Balai Ringin 15 25 35 Tarat 15 25 35 Semongok 15 25 35 Concentration mg g1 Balai Ringin 25 5 Tarat 25 5 Semongok 25 5
t1/2 (day)
t1/2 95% cl (day)
0.0006a 0.001b 0.0006a 0.002c 0.0005d 0.001e
173 69.3 182 53.3 256 77.0
128e276 56.3e81.5 182e451 38.5e87.7 131e301 59.7e99.0
0.002a 0.001b 0.001b 0.002c 0.002d 0.001b 0.001e 0.001f 0.007e
84.5 69.3 63 49.5 53.3 63 120 77.0 124
54.1e193 56.3e81.5 47.8e84.1 36.5e78.8 38.5e87.7 49.5e82.5 83.5e211 59.7e99.0 84.9e78
0.0036 0.010 0.030 0.0083 0.013 0.019 0.0031 0.009 0.0185
0.001a 0.001b 0.008c 0.001d 0.002e 0.003f 0.0002g 0.001d 0.002h
193 69.3 23.1 83.5 53.3 36.5 224 77.0 37.5
142e308 56.3e81.5 14.4e63 62.4e124 38.5e87.7 28.9e54.6 192e267 59.7e99.0 28.9e53.3
0.0058 0.010 0.0091 0.013 0.0082 0.009
0.0003a 0.001b 0.0005c 0.002d 0.0005e 0.001c
120 69.3 76.2 53.3 84.5 77.0
108e133 56.3e81.5 74.7e97.6 38.5e87.7 69.3e86.6 59.7e99.0
Moist soils were used unless specified; experiments were conducted at 25 C unless specified; different letters in superscript indicate the rate constants are significantly different within the specific experiment (p < 0.05).
acetone) and 214.8 mL radio-labeled chlorpyrifos solution (4.13 mg L1 in acetonitrile) were added to each flask to give an initial chlorpyrifos concentration of 5 mg g1 and an initial 14C activity of 5.0 104 dpm. The chlorpyrifos solutions were spread well onto the soil and a small test tube with 1 mL of sodium hydroxide (1 M) was placed in the serum flask to trap 14CO2 evolved by mineralization as a result of microbial activity. The flasks were closed tightly and incubated at 25 C in darkness. The sodium hydroxide solutions were removed and replaced with fresh solution on day 1, 3, 5, 7, 10, 14, 21, 30, 45 and 60. Five mL of scintillation cocktail (Optisafe, Wallace, Finland) was added to the sodium hydroxide solution before radioactivity was measured on a scintillation counter (Wallace, Finland) for 20 min. The background counts of the sodium hydroxide and scintillation cocktail were measured and subtracted from the sample results. The experiments were carried out in triplicates at two initial concentrations of chlorpyrifos (5 and 25 mg g1). Samples containing quartz were used as control.
2.6. Pesticide extraction and analysis To determine the total contents of chlorpyrifos in incubated soils, 10 g of soil sample was weighed into a 500 mL glass shaking flask where 10 mL of water and 100 mL of ethyl acetate were added. The mixture was shaken for 1 h on an orbital shaker at 300 rpm.
The extract was left to settle and then filtered through a filter paper (Whatman, grade 41) containing sodium sulfate into a round bottomed receiving flask. The shaking flask and its contents were rinsed twice with extraction solvents and the washes were decanted and filtered into the same receiving flask. The filtrate was evaporated using a rotary evaporator until it was almost dry and later made up to 10 mL with acetone. Two mL of the final extract was injected into GC-FPD. Good recoveries of 76e102% with standard deviation (SD) of less than 8.9% were obtained for chlorpyrifos in the three soils (Chai et al., 2008). The limit of quantification was 0.01 mg g1. 2.7. Data analysis The chlorpyrifos degradation data was modeled using a simple first-order model: Ct ¼ Coeekt where Ct is the chlorpyrifos concentration at time t, Co is the initial concentration of chlorpyrifos, and k is the rate constant. Curve fitting was performed using a non-linear least squares regression analysis of chlorpyrifos concentration against time using TableCurve (Systat Software Limited, USA). The activation energy was calculated based on the Arrhenius equation; k ¼ AeeEa/RT, where k is the rate constant, A is the frequency factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature in Kelvin. 3. Results 3.1. Soil characteristics The soils used in our study were all slightly acidic with gravimetric moisture and carbon contents ranging from 22 to 33% and 1.4e2.2%, respectively. The Balai Ringin soil had the lowest contents of clay (5.9%), CEC (5.0 mmol (þ) kg1), CBD extractable aluminum and iron (12.0 and 29.1 mmol kg1, respectively), but had higher content of sand (76.3%) and base saturation (88%) compared to Tarat and Semongok soils. The clay fraction in the soils was dominated by kaolinite with traces of vermiculite; the less developed Tarat soil also contained illite. 3.2. Effect of sterilization on chlorpyrifos degradation (abiotic degradation) Degradation of chlorpyrifos in sterilized soil was much slower than in non-sterilized soil (Table 2). On day 70, 40.5e46.2% of the chlorpyrifos remained in non-sterilized soils in the three soils investigated compared to 67.5e72.1% in sterilized soils. Half-lives in the sterilized soils ranged between 173 and 256 days reflecting 2e 3-fold slower degradation compared to non-sterilized soils. The half-lives of chlorpyrifos in sterilized soils were in the order of Semongok (256 days) > Tarat (182 days) > Balai Ringin (173 days). Significant differences in rate constants and half-lives among the sterilized and non-sterilized soils were observed demonstrating soil microbial activity significantly affected the degradation of chlorpyrifos. 3.3. Effect of moisture on chlorpyrifos degradation Degradation of chlorpyrifos in soils under different moisture contents followed first-order kinetics (r2 > 0.78). The rates of chlorpyrifos degradation for different moisture levels were soil specific, but in general degradation rates decreased in the order of Tarat > Balai Ringin > Semongok soils. Longer half-lives were obtained for wet soils (63.0e124 days) compared to air dry (49.5e120 days) and moist soils (53.3e77.0 days). The rate constants and halflives for dry, moist and wet soils were significantly different among
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
the three soils except for the Balai Ringin moist and wet soils and Semongok dry and wet soil indicating that degradation depends on combination of soil type and soil moisture regime.
Table 3 Mineralization rate constants (k), half-lives (t1/2) with 95% confidence limits (cl), and coefficients of determination (r2) for chlorpyrifos in field moist soils at two chlorpyrifos application rates at 25 C.
3.4. Effect of temperature on chlorpyrifos degradation Degradation of chlorpyrifos was faster at 35 C compared to at 15 C (Table 2). Half-lives of 83e224 days, 53e77 days and 23e38 days were obtained at 15 C, 25 C and 35 C, respectively, with significant differences observed between 15 C, 25 C, and 35 C for all the three soils. The activation energy of degradation was in the order of Tarat (76.5 0.6 kJ mol1) > Semongok (64.8 0.2 kJ mol1) > Balai Ringin (29.0 0.4 kJ mol1) soils.
31
Balai Ringin Tarat Semongok
Application rate (mg g1)
r2
k (day1)
5 25 5 25 5 25
0.99 0.99 0.99 0.99 0.99 0.99
0.0062 0.0023 0.0034 0.0033 0.0036 0.0019
0.00014a 0.00004b 0.00010a 0.00007a 0.00009a 0.00005b
t1/2 (day)
t1/2 95% cl (day)
112 301 204 210 193 365
107e118 289e301 193e217 198e217 182e204 351e395
Different letters in superscript indicate the rate constants are significantly different (p < 0.05).
3.5. Effect of application rate on chlorpyrifos degradation The higher chlorpyrifos application rate of 25 mg g1 resulted in decrease of the rates of chlorpyrifos degradation compared with the application rate of 5 mg g1 (Table 2). Thus a 1e2-fold increase in the half-lives of chlorpyrifos from 53 e 77 days to 76e120 days was observed. Significant differences in half-lives among the application rate were observed for all the three soils. 3.6. Mineralization of chlorpyrifos Mineralization of chlorpyrifos was measured as carbon dioxide evolved. Mineralization of chlorpyrifos at the application rate of 5 mg g1 was gradual in the three soils (Fig. 1). Mineralization followed first-order kinetics in the three soils with fastest mineralization in the Balai Ringin soil. The Tarat and Semongok soils showed similar rates of mineralization. On day 60, almost similar amounts of chlorpyrifos had mineralized in the Tarat and Semongok soils (18%) compared to higher mineralization in the Balai Ringin soil (30%). At the higher chlorpyrifos application rate of 25 mg g1 the highest mineralization rate was observed for the Tarat soil. For the Semongok and Tarat soils, the mineralization rates were 2e3 times lower at the higher application rate compared with the low application rate (Table 3). The amount mineralized was almost the same for the Tarat soil on day 60 for both application rates. At the low spiking level of 5 mg g1, the mineralization half-lives ranged between 112 and 204 days with the shortest half-life for the Balai Ringin soil while at the higher spiking level of 25 mg g1, halflives increased up to 2.7 times (Table 3). The soils fortified with 5 mg g1 of chlorpyrifos were extracted after incubation for 40 days with 1 M NaOH, 30 mL water, followed by 30 mL methanol:water (60:40) and 10 mL 5 M sodium hydroxide to determine the amount of 14C present in the different pools. The highest amount of 14C was found in the 1 M NaOH extract (18e 30.6%) and water þ methanol (19.4e33.9%), followed by 5 M NaOH (14.1e20.2%), and water (1.7e3.1%) (Fig. 2). The total amount of 14C
Fig. 2. Percentage of 14C present in different soil extracts at end of the mineralization experiment (chlorpyrifos loading of 5 mg g1). Vertical bars represent standard deviations (n ¼ 3).
recovered in various pools was 68.7, 68.8 and 68.5% for the Semongok, Tarat and Balai Ringin soils, respectively.
4. Discussion 4.1. Effect of soil properties, moisture content and temperature In our investigation, degradation rate decreased in the order Tarat > Balai Ringin > Semongok soils; in most cases the differences were significant. The differences in degradation rates between soils were attributed to soil pH and clay contents. The soil with the lowest pH e the Semongok soil e had the slowest degradation in agreement with previous investigations on temperate soils (Chu et al., 2008; Fang et al., 2009; Racke, 1993; Sardar and Kole, 2005). The Tarat soil had higher clay contents than in the Balai Ringin soil (Table 1) and also had highest counts of
Fig. 1. Mineralization kinetics of chlorpyrifos at spiking levels of 5 mg g1 and 25 mg g1 in Balai Ringin (:), Tarat (-), and Semongok (A) soils. Full lines are due to first-order fitting. Vertical bars represent standard deviations (n ¼ 3).
32
L.-K. Chai et al. / Journal of Environmental Management 125 (2013) 28e32
bacteria and fungi in the untreated soil (results not shown) resulting in highest degradation among the three soils studied. The soil moisture level affected the degradation of chlorpyrifos. The degradation half-lives for chlorpyrifos for most soils were in the order of moist soils < air-dry soils < wet soils. The slower degradation in wet soils was attributed to inactivation of the soil microbial activities under anaerobic condition. Slower degradation in air-dry soils was also due to inactivation of soil microbial activities under low moisture conditions. In earlier studies on temperate soils, the rate of chlorpyrifos degradation for low spiking levels of 0.25 mg g1 in three soils (sandy, sandy loam and clayey) with pH of 6e9, organic matters 0.9e4.0% and clay contents of 20e33% was air-dry < field capacity < submerged (Awasthi and Prakash, 1997). However, it was reported that the effect of moisture (9e22%) was not pronounced for chlorpyrifos (10 mg g1) in clayey temperate soils with pH 8, 1.7% organic matter, and 42% clay (Racke et al., 1994). It is likely that in the latter investigation alkaline hydrolysis overruled the effect of moisture. The effect of temperature obtained in this study was similar to those reported for chlorpyrifos in temperate soils (Racke et al., 1994). The activation energy of degradation was in the range of 29.0e76.5 kJ mol1 which was about 3e4 times higher than the activation energy reported earlier for acephate (16.8e28.3 kJ mol1) in similar tropical soils (Chai et al., 2010). This implies that shading has great effect on the degradation of chlorpyrifos in soils. 4.2. Mode of degradation Enzymatic degradation contributed substantially to the degradation of chlorpyrifos as the half-lives were 2e3-fold higher for the sterilized soils than the non-sterilized soils (Table 2). However, degradation still occurred in the sterilized soils resulting from abiotic degradation processes which are attributed to hydrolysis. The ratios between degradation rates in non-sterilized and sterilized soils were 2.4e4.8 demonstrating that enzymatic degradation accounted for about 60e80% of the total degradation. A 5-fold increase in the chlorpyrifos application rate had only minor effect on chlorpyrifos degradation in agreement with results previously observed for clayey temperate soils and for organophosphorus pesticides (Chai et al., 2010; Racke, 1993; Vijay et al., 2006). Although the higher doses may have toxic effects on soil microorganisms, the relatively high abiotic degradation and the sorption of chlorpyrifos to clays seemed to make the chlorpyrifos degradation less sensitive to toxic dosing effects. Mineralization rates were more sensitive to chlorpyrifos application rates than degradation rates except for the Tarat soil. This indicates that degradation of metabolites but not degradation of the parent compound is rate limiting and toxic effects of some of the metabolites produced cannot be excluded. Chlorpyrifos mineralization levels were similar to the levels of organophosphorus pesticide mineralization typically seen for temperate soils (Singh et al., 2003) but higher than what have been previously reported for acephate mineralization in tropical soils (Chai et al., 2010; Marchetti and Luchini, 2004). Most of the non-mineralized chlorpyrifos was found in NaOH extracts, indicating that chlorpyrifos residues have been incorporated into humic substances and microbial tissue in agreement with other studies for temperate soils (Jensen et al., 2004). 5. Conclusions Soil moisture, temperature, and soil microbial activity are critical variables controlling chlorpyrifos degradation in humid tropical soils. The degradation and mineralization of chlorpyrifos in the three soils at various conditions can be described by first-order
kinetics. Degradation of chlorpyrifos is slowest in the absence of soil microbial activity and at low soil temperatures. The results from this study implied that the degradation of chlorpyrifos is slower for water logged soils, acidic soils, soils with high clay, soils at low temperatures and at high application rates. Under these conditions, leaching, accumulation and possible toxic effects of metabolites of chlorpyrifos may occur. Thus, increasing the soil pH by liming and using low application rate will accelerate chlorpyrifos degradation. The use of chlorpyrifos on soils with dense vegetation resulting in shade and hence lower surface soil temperatures and less photolysis is expected to retard degradation. Considerably longer half-lives found in this study compared to field study demonstrates that in addition to degradation, leaching, photodegradation and volatilization of chlorpyrifos are important dissipation process in the field. Acknowledgment The authors wish to thank University of Copenhagen (KU), Denmark and State Government of Sarawak for funding this project. The authors also wish to acknowledge assistance from laboratory technicians of Department of Agriculture Sarawak and Faculty of Life Sciences of KU. References Awasthi, M.D., Prakash, N.B., 1997. Persistence of chlorpyrifos in soils under, different moisture regimes. Pestic. Sci. 50, 1e4. Baskaran, S., Kookana, R.S., Naidu, R., 2003. Contrasting behaviour of chlorpyrifos and its primary metabolites, TCP (3,5,6-trichloro-2-pyridinol) with depth in, soil profiles. Aust. J. Soil Res. 41, 749e760. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2008. Determination of chlorpyrifos, and acephate in tropical soils and application in dissipation studies. Int. J. Environ. Anal. Chem. 88, 549e560. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2009a. Degradation and migration of, the phosphorothioates acephate and chlorpyrifos in tropical field soils. J. Environ. Qual. 38, 1160e1169. Chai, L.K., Mohd-Tahir, N., Hansen, H.C.B., 2009b. Dissipation of acephate, chlorpyrifos, cypermethrin and their metabolites in a humid-tropical vegetable, production system. Pest Manag. Sci. 65, 189e196. Chai, L.K., Wong, M.H., Mohd-Tahir, N., Hansen, H.C.B., 2010. Degradation and, mineralization kinetics of acephate in humid tropic soils of Malaysia. Chemosphere 79, 434e440. Chu, X.Q., Fang, H., Pan, X.D., Wang, X., Shan, M., Feng, B., Yu, Y.L., 2008. Degradation of chlorpyrifos alone and in combination with chlorothalonil and, their effects on soil microbial populations. J. Environ. Sci. 20, 464e469. Ciglasch, H., Busche, J., Amelung, W., Totrakool, S., Kaupenjohann, M., 2006. Insecticide dissipation after repeated field application to a Northern Thailand Ultisol. J. Agric. Food Chem. 54, 8551e8559. Fang, H., Yu, X.Q., Wang, X.G., Yang, X., Yu, J.Q., 2009. Degradation of, chlorpyrifos in laboratory soil and its impact on soil microbial functional diversity. J. Environ. Sci. 21, 380e386. Jensen, P.A., Hansen, H.C.B., Rasmussen, J., Jacobsen, O.S., 2004. Sorption-, controlled degradation kinetics of MCPA in soil. Environ. Sci. Technol. 38, 6662e6668. Jin, H., Webster, G.R.B., 1998. Dissipation of chlorpyrifos, oxon and 3,5,6-, trichloro2-pyridinol in litter and elm forest soil. Int. J. Environ. Anal. Chem. 69, 307e316. Laabs, V., Amelung, W., Pinto, A., Zech, W., 2002. Fate of pesticides in tropical soils, of Brazil under field conditions. J. Environ. Qual. 31, 256e268. Marchetti, M., Luchini, L.C., 2004. Sorption/desorption and mineralization of the, insecticide acephate in the soil. Revista de Ecotoxicologia e Meio Ambiente 14,1e6. Page, A.L., Miller, R.H., Roberts, T.R., Kearney, P.C., Keeney, D.R., 1982. Method, of Soil Analysis. Soil Society of America, Madison, Wisconsin, USA. Racke, K.D., 1993. Environmental fate of chlorpyrifos. Rev. Environ. Contam. Toxicol. 131, 1e151. Racke, K.D., Fontaine, D.D., Yoder, R.N., Jack, R.M., 1994. Chlorpyrifos, degradation in soil at termiticidal application rate. Pestic. Sci. 42, 43e51. Sardar, S., Kole, R.K., 2005. Metabolism of chlorpyrifos in relation to its effect on the availability of some plant nutrients in soil. Chemosphere 61, 1273e1280. Singh, B.K., Walker, A., Morgen, J.A.W., Wright, D.J., 2003. Effects of soil pH on, the biodegradation of chlorpyrifos and isolation of a chlorpyrifos-degrading bacterium. App. Environ. Microbiol. 69, 5198e5206. Soil Survey Staff, 2010. Keys to Soil Taxonomy, 11th ed. Pocahontas Press Inc., Blacksburg, Virginia, U.S.A. Tomlin, C., 1994. The Pesticide Manual. The British Crop Protection Council, Surrey, UK and The Royal Society of Chemistry, Cambridge, United Kingdom. Vijay, A.K.B., Gundi, I., Reddy, B.R., 2006. Degradation of monocrotophos in soils. Chemosphere 62, 396e403.