Parameters affecting azoxystrobin and imidacloprid degradation in biobed substrates in the North Indian tropical environment

Abstract

This study reports degradation of azoxystrobin (AZOXY) and imidacloprid (IMIDA) in the rice straw (RS)/corn cob (CC) and peat (P)/compost (C)-based biomixtures. The effect of biomixture preconditioning (10?days incubation prior to pesticide application), pesticide concentration and moisture content was evaluated. Results suggested that conditioning of biomixture greatly affected IMIDA degradation where half-life (t_1/2) was reduced by 5–9 times. This was attributed to higher microbial biomass carbon content and dehydrogenase activity in the conditioned biomixtures. Pesticide application in the conditioned biomixture did not show any negative impact on soil microbial parameters. Both pesticides degraded at faster rate in the rice straw-based biomixtures than in the corn cob-based biomixtures. Degradation slowed down with increase in initial concentration of pesticides in biomixture and 1.6–3.0 (AZOXY) and 2.4–3.6 (IMIDA) times increase in t_1/2 values was observed. The moisture content of biomixture showed positive effect on degradation which increased when moisture content was increased from 60 to 80% water holding capacity. The effect was significant for IMIDA degradation in the corn cob-based biomixtures and AZOXY degradation in the peat biomixtures. The rice straw-based biomixtures were better in degrading AZOXY and IMIDA and can be used in biopurification systems.     

Earthworms to assess the innocuousness of spent biomixtures employed for glyphosate degradation

In this study, the innocuousness of different biomixtures employed for glyphosate degradation was tested through Eisenia fetida earthworms. Eight biomixtures were prepared with local materials: alfalfa straw (AS), wheat stubble (WS), river waste (RW) and two different soils (A and B). Each biomixture was divided into two equal portions: one without glyphosate application (control substrate) and the other was sprayed with a commercial glyphosate formulation of 1,000 mg glyphosate a.i. kg^?1 biomixture (applied substrate). The bioassay started when all sprayed biomixtures reached high percentages of glyphosate degradation (spent biomixtures). Three parameters were studied: survival, adults and juveniles biomass and reproduction. The results allowed the identification of three biomixtures (AWS, BWS and BWSRW) for good maintenance and development of E. fetida. In addition, at the end of the bioassay two of the viable biomixtures (AWS and BWS) showed the highest performance of juvenile earthworms compared to a reference soil. The Principal Component Analysis (PCA) indicated that the biomixtures containing high silt and clay percentages and minor density renders higher values of earthworm growth and reproduction. Therefore, these innocuous biomixtures can be used as organic amendments or recycled materials for new treatments on biobeds.     

Chlorpyrifos degradation in a biomixture of biobed at different maturity stages

The biomixture is a principal element controlling the degradation efficacy of the biobed. The maturity of the biomixture used in the biobed affects its overall performance of the biobed, but this is not well studied yet. The aim of this research was to evaluate the effect of using a typical composition of Swedish biomixture at different maturity stages on the degradation of chlorpyrifos. Tests were made using biomixture at three maturity stages: 0 d (BC0), 15 d (BC15) and 30 d (BC30); chlorpyrifos was added to the biobeds at final concentration of 200, 320 and 480 mg kg⁻¹. Chlorpyrifos degradation in the biomixture was monitored over time. Formation of TCP (3,5,6-trichloro-2-pyrinidol) was also quantified, and hydrolytic and phenoloxidase activities measured. The biomixture efficiently degraded chlorpyrifos (degradation efficiency >50%) in all the evaluated maturity stages. However, chlorpyrifos degradation decreased with increasing concentrations of the pesticide. TCP formation occurred in all biomixtures, but a major accumulation was observed in BC30. Significant differences were found in both phenoloxidase and hydrolytic activities in the three maturity stages of biomixture evaluated. Also, these two biological activities were affected by the increase in pesticide concentration. In conclusion, our results demonstrated that chlorpyrifos can be degraded efficiently in all the evaluated maturity stages.     

The dissipation of three fungicides in a biobed organic substrate and their impact on the structure and activity of the microbial community

Biopurification systems (BPS) have been introduced to minimise the risk for point source contamination of natural water resources by pesticides. Their depuration efficiency relies mostly on the high biodegradation of their packing substrate (biomixture). Despite that, little is known regarding the interactions between biomixture microflora and pesticides, especially fungicides which are expected to have a higher impact on the microbial community. This study reports the dissipation of the fungicides azoxystrobin (AZX), fludioxonil (FL) and penconazole (PC), commonly used in vineyards, in a biomixture composed of pruning residues and straw used in vineyard BPS. The impact of fungicides on the microbial community was also studied via microbial biomass carbon, basal respiration and phospholipid fatty acid analysis. AZX dissipated faster (t _1/2?=?30.1 days) than PC (t _1/2?=?99.0 days) and FL (t _1/2?=?115.5 days). Fungicides differently affected the microbial community. PC showed the highest adverse effect on both the size and the activity of the biomixture microflora. A significant change in the structure of the microbial community was noted for PC and FL, and it was attributed to a rapid inhibition of the fungal fraction while bacteria showed a delayed response which was attributed to indirect effects by the late proliferation of fungi. All effects observed were transitory and a full recovery of microbial indices was observed 60 days post-application. Overall, no clear link between pesticide persistence and microbial responses was observed stressing the complex nature of interactions between pesticides in microflora in BPS.     

Reduction of the movement and persistence of pesticides in soil through common agronomic practices

Laboratory and field studies were conducted in order to determine the leaching potential of eight pesticides commonly used during pepper cultivation by use of disturbed soil columns and field lysimeters, respectively. Two soils with different organic matter content (soils A and B) were used. Additionally, soil B was amended with compost (sheep manure). The tested compounds were cypermethrin, chlorpyrifos-methyl, bifenthrin, chlorpyrifos, cyfluthrin, endosulfan, malathion and tolclofos-methyl. In soil B (lower organic matter content), only endosulfan sulphate, malathion and tolclofos-methyl were found in leachates. For the soil A (higher organic matter content) and amended soil B, pesticide residues were not found in the leachates. In addition, this paper reports on the use of common agronomic practices (solarization and biosolarization) to enhance degradation of these pesticides from polluted soil A. The results showed that both solarization and biosolarization enhanced the degradation rates of endosulfan, bifenthrin and tolclofos-methyl compared with the control. Most of the studied pesticides showed similar behavior under solarization and biosolarization conditions. However, chlorpyrifos was degraded to a greater extent in the solarization than in biosolarization treatment. The results obtained point to the interest in the use of organic amendment in reducing the pollution of groundwater by pesticide drainage and in the use of solarization and biosolarization in reducing the persistence of pesticides in soil.     

Soil temperature effect in calculating attenuation and retardation factors

The effect of annual variation of daily average soil temperature, at different depths, in calculating pesticides ranking indexes retardation factor and attenuation factor is presented. The retardation factor and attenuation factor are two site-specific pesticide numbers, frequently used as screening indicator indexes for pesticide groundwater contamination potential. Generally, in the calculation of these two factors are not included the soil temperature effect on the parameters involved in its calculation. It is well known that the soil temperature affects the pesticide degradation rate, water-air partition coefficient and water-soil partition coefficient. These three parameters are components of the retardation factor and attenuation factor and contribute to determine the pesticide behavior in the environment. The Arrhenius equation, van't Hoff equation and Clausius-Clapeyron equation are used in this work for estimating the soil temperature effect on the pesticide degradation rate, water-air partition coefficient and soil-water partition coefficient, respectively. These dependence relationships, between results of calculating attenuation and retardation factors and the soil temperature at different depths, can aid to understand the potential pesticide groundwater contamination on different weather conditions. Numerical results will be presented with pesticides atrazine and lindane in a soil profile with 20 degrees C constant temperature, minimum and maximum surface temperatures varying and spreading in the soil profile between -5 and 30 degrees C and between 15 and 45 degrees C.     

Sorption kinetics and its effects on retention and leaching.

Sorption of pesticides to substrates used in biopurification systems is important as it controls the system's efficiency. Ideally, pesticide sorption should occur fast so that leaching of the pesticide in the biopurification system is minimized. Although modeling of pesticide transport commonly assumes equilibrium, this may not always be true in practice. Sorption kinetics have to be taken into account. This study investigated the batch sorption kinetics of linuron, isoproturon, metalaxyl, isoxaben and lenacil on substrates commonly used in a biopurification system, i.e. cow manure, straw, willow chopping, sandy loam soil, coconut chips, garden waste compost and peat mix. The first-order sorption kinetics model was fitted to the observed pesticide concentrations versus time resulting in an estimated kinetic rate constant alpha. Sorption appeared to be fast for the pesticides linuron and isoxaben, pesticides which were classified as immobile, while less mobile pesticides displayed an overall slower sorption. However, the substrate does not seem to be the main parameter influencing the sorption kinetics. Coconut chips, which is a substrate with a high organic matter content showed slow sorption for most of the pesticides. The effect of different estimated alpha values on the breakthrough of pesticides through a biopurification system was evaluated using the HYDRUS 1D model. Significant differences in leaching behavior were observed as a result of the obtained differences in sorption kinetics.     

Kinetics and isotherm modeling of azoxystrobin and imidacloprid retention in biomixtures

The paper reports the kinetics and adsorption isotherm modeling for imidacloprid (IMIDA) and azoxystrobin (AZOXY) in rice straw (RS)/corn cob (CC) and peat (P)/compost (C) based biomixtures. The pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich and intraparticle diffusion models were used to describe the kinetics. The adsorption data were subjected to the Langmuir and the Freundlich isotherms. Results (r²_Adj values) suggested that the modified Elovich model was the best suited to explain the kinetics of IMIDA sorption while different models explained AZOXY sorption kinetics in different biomixtures (PFO in RS?+?C and RS?+?P; PSO in CC?+?P and Elovich in CC?+?C). Biomixtures varied in their capacity to adsorb both pesticides and the adsorption coefficient (K_d) values were 116.8–369.24 (AZOXY) and 24.2–293.4 (IMIDA). The Freundlich isotherm better explained the sorption of both pesticides. Comparison analysis of linear and nonlinear method for estimating the Freundlich adsorption constants was made. In general, r²_Adj values were higher for the nonlinear fit (AZOXY?=?0.938–0.982; IMIDA?=?0.91–0.970) than the linear fit (AZOXY?=?0.886–0.993; IMIDA?=?0.870–0.974) suggesting that the nonlinear Freundlich equation better explained the sorption. The rice straw-based biomixtures performed better in adsorbing both the pesticides and can be used in bio-purification systems.     

Leaching and degradation of 21 pesticides in a full-scale model biobed

Filling and cleaning of pesticide sprayers presents a potential risk of pollution of soil and water. Three different solutions for handling sprayers have been suggested: Filling and cleaning in the field, filling and cleaning on hard surfaces with collection of the waste water, and filling and cleaning on a biobed, which is an excavation lined with clay and filled with a mixture of chopped straw, sphagnum and soil with turf on top, and with increased sorption capacity and microbial activity for degradation of the pesticides. In the present study the degradation and leaching of 21 pesticides (5 g of each) was followed in an established full-scale model biobed. Percolate was collected and analysed for pesticide residues, and the biobed material was sampled at three different depths and analysed by liquid chromatography double mass spectrometry (LC-MSMS). During the total study period of 563 days, no traces of 10 out of 21 applied pesticides were detected in the percolate (detection limits between 0.02 and 0.9 μg l⁻¹) and three pesticides were only detected once and at concentrations below 2 μg l⁻¹. During the first 198 days before second application, 14% of the applied herbicide bentazone was detected in the leachate with maximum and mean concentrations of 445 and 172 μg l⁻¹, respectively. About 2% of the initial mecoprop and fluazifop dose was detected in the percolate, with mean concentrations of 23 μg l⁻¹, while MCPA and dimethoate had mean concentrations of 3.5 and 4.7 μg l⁻¹, respectively. Leachate concentrations for the remaining pesticides were generally below the detection limit (0.02–0.9 μg l⁻¹, below 1% of applied). Sorption studies of five pesticides showed that compounds with a low K_d value appeared in the leachate. After 169 days, all pesticides in the biobed profile were degraded to a level below 50% of the calculated initial dose. Pesticides with K_oc values above 100 were primarily found in the uppermost 10 cm and degraded slowest due to the low bioavailability. The 11 most degradable pesticides were all degraded such that less than 3% remained in the biobed after 169 days.

Following second pesticide application of the biobed, leachate was sampled 215 and 365 days after the treatment. This showed the same pesticides to be leached out and at concentrations comparable to those of the first treatment. The same pesticides as after the first treatment were retained in the biobed.     

Modelling pesticide volatilization after soil application using the mechanistic model Volt'Air

Volatilization of pesticides participates in atmospheric contamination and affects environmental ecosystems including human welfare. Modelling at relevant time and spatial scales is needed to better understand the complex processes involved in pesticide volatilization. Volt'Air-Pesticides has been developed following a two-step procedure to study pesticide volatilization at the field scale and at a quarter time step. Firstly, Volt'Air-NH₃ was adapted by extending the initial transfer of solutes to pesticides and by adding specific calculations for physico-chemical equilibriums as well as for the degradation of pesticides in soil. Secondly, the model was evaluated in terms of 3 pesticides applied on bare soil (atrazine, alachlor, and trifluralin) which display a wide range of volatilization rates. A sensitivity analysis confirmed the relevance of tuning to K_h. Then, using Volt'Air-Pesticides, environmental conditions and emission fluxes of the pesticides were compared to fluxes measured under 2 environmental conditions. The model fairly well described water temporal dynamics, soil surface temperature, and energy budget. Overall, Volt'Air-Pesticides estimates of the order of magnitude of the volatilization flux of all three compounds were in good agreement with the field measurements. The model also satisfactorily simulated the decrease in the volatilization rate of the three pesticides during night-time as well as the decrease in the soil surface residue of trifluralin before and after incorporation. However, the timing of the maximum flux rate during the day was not correctly described, thought to be linked to an increased adsorption under dry soil conditions. Thanks to Volt'Air's capacity to deal with pedo-climatic conditions, several existing parameterizations describing adsorption as a function of soil water content could be tested. However, this point requires further investigation. Practically speaking, Volt'Air-Pesticides can be a useful tool to make decision about agricultural practices such as incorporation or for the estimation of overall pesticide volatilization rates, and it holds promise for time specific dynamics.     

Hydroponic uptake of atrazine and lambda-cyhalothrin in Juncus effusus and Ludwigia peploides

Phytoremediation encompasses an array of plant-associated processes known to mitigate contaminants from soil, sediment, and water. Modification of pesticides associated with agricultural runoff includes processes directly associated with aquatic macrophytes in addition to changes in soil geochemistry and associated rhizospheric degradation. Remediation attributes of two vegetative species common to agricultural drainages in the Mississippi Delta, USA, were assessed using atrazine and lambda-cyhalothrin. Concentrations used in 8-d hydroponic exposures were calculated using recommended field applications and a 5% runoff model from a 0.65-cm rainfall event on a 2.02-ha field. While greater atrazine uptake was measured in Juncus effusus, greater lambda-cyhalothrin uptake occurred in Ludwigia peploides. Maximum pesticide uptake was reached within 48h for each exposure and subsequent translocation of pesticides to upper plant biomass occurred in macrophytes exposed to atrazine. Sequestration of 98.2% of lambda-cyhalothrin in roots of L. peploides was measured after 8d. Translocation of lambda-cyhalothrin in J. effusus resulted in 25.4% of pesticide uptake partitioned to upper plant biomass. These individual macrophyte remediation studies measured species- and pesticide-specific uptake rates, indicating that seasonality of pesticide applications and macrophyte emergence might interact strongly to enhance mitigation capabilities in edge-of-field conveyance structures.     

City refuse compost and sodium dodecyl sulphate as modifiers of diazinon leaching in soil

The effect of a city refuse compost (CRC) and of an anionic surfactant (sodium dodecyl sulphate (SDS) on the leaching of diazinon (0,0-diethyl 0-2-isopropyl-6-methylpyrimidin-4-yl-phosphorothioate) in the soil was studied using packed soil columns. Breakthrough curves showed the existence of various regimes of pesticide adsorption related to the pesticide and organic material nature and the soil properties. Leaching rate and mass transfer of diazinon decrease following the addition of CRC to the soil and increase after the addition of SDS. The degree of increase or decrease was found to depend strongly on the amendment dose added, especially in the case of SDS. The results afford basic data on which to base the possible use of the organic amendments studied in physicochemical methods designed to prevent the pollution of water by hydrophobic pesticides (immobilization) or to restore soils contaminated by these compounds (leaching).     

Evaluation of soil temperature effect on herbicide leaching potential into groundwater in the Brazilian Cerrado

The effect of annual variations in the daily average soil temperatures, at different depths, on the calculation of pesticide leaching potential indices is presented. This index can be applied to assess the risk of groundwater contamination by a pesticide. It considers the effects of water table depth, daily recharge net rate, pesticide sorption coefficient, and degradation rate of the pesticide in the soil. The leaching potential index is frequently used as a screening indicator in pesticide groundwater contamination studies, and the temperature effect involved in its calculation is usually not considered. It is well known that soil temperature affects pesticide degradation rates, air-water partition coefficient, and water-soil partition coefficient. These three parameters are components of the attenuation and retardation factors, as well as the leaching potential index, and contribute to determine pesticide behavior in the environment. The Arrhenius, van't Hoff, and Clausius-Clapeyron equations were used in this work to estimate the soil temperature effect on pesticide degradation rate, air-water partition coefficient, and water-soil partition coefficient, respectively. The relationship between leaching potential index and soil temperature at different depths is presented and aids in the understanding of how potential pesticide groundwater contamination varies on different climatic conditions. Numerical results will be presented for 31 herbicides known to be used in corn and soybean crops grown on the municipality of S?o Gabriel do Oeste, Mato Grosso do Sul State, Brazil.     

Review of advances in the thin layer chromatography of pesticides: 2012–2014

Publications reporting techniques and applications of thin layer chromatography (planar chromatography) for the separation, detection, qualitative, and quantitative determination, and preparative isolation of pesticides and their metabolites are reviewed for the period from November 1, 2012 to November 1, 2014. Analyses are described for a variety of sample types and pesticide classes. In addition to references on residue analysis, studies such as pesticide structure – retention relationships, identification and characterization of natural and synthesized pesticides, metabolism, bioactivity, degradation, soil mobility, and lipophilicity are covered.     

The adsorption and degradation of chlorpyriphos-methyl, pendimethalin and metalaxyl in solid urban waste compost

To evaluate the feasibility of using compost to prepare substrates for the disposal of pesticide residues, adsorption and degradation studies were carried out on three widely used agricultural pesticides: chlorpyriphos-methyl, pendimethalin and metalaxyl. Obtained from solid urban waste, this compost has been shown to be able to adsorb high levels of chlorpyriphos-methyl and pendimethalin (85%, 100%) whereas metalaxyl was only adsorbed at a level of 37%. However, adding smectite to the compost increased the adsorption of metalaxyl by 117%. Chlorpyriphos-methyl and pendimethalin degraded quickly with half-lives of 1.7 and 14.5 days, respectively, whereas metalaxyl proved more persistent (a half-life of 84 days). Adding ammonium nitrate to the compost accelerated metalaxyl degradation to a half-life of 15 days.     

A pesticide emission model (PEM) Part II: model evaluation

In the first part of the paper, the development of a numerical pesticide emission model (PEM) is described for predicting the volatilization of pesticides applied to agricultural soils and crops through soil incorporation, surface spraying, or in the furrow at the time of planting. In this paper the results of three steps toward the evaluation of PEM are reported. The evaluation involves: (i) verifying the numerical algorithms and computer code through comparison of PEM simulations with an available analytical solution of the advection/diffusion equation for semi-volatile solutes in soil; (ii) comparing hourly heat, moisture and emission fluxes of trifluralin and triallate modeled by PEM with fluxes measured using the relaxed eddy-accumulation technique; and (iii) comparison of the PEM predictions of persistence half-life for 29 pesticides with the ranges of persistence found in the literature. The overall conclusion from this limited evaluation study is that PEM is a useful model for estimating the volatilization rates of pesticides from agricultural soils and crops. The lack of reliable estimates of chemical and photochemical degradation rates of pesticide on foliage, however, introduces large uncertainties in the estimates from any model of the volatilization of pesticide that impacts the canopy.     

Removal of pesticides from rinsate by adsorption using agricultural residuals as medium

Abstract

The feasibility of using agricultural residual with highly organic matter content ‐ spent mushroom compost (SMC) as adsorption medium for removal of pesticides including carbaryl, carbofuran, and aldicarb with a concentration range of 0–30 mg/L from rinsate was investigated. Bottle‐point method was used to determine adsorption isotherms for single‐pesticide and multi‐pesticides. It was found that SMC might potentially be used for on‐farm treatment of pesticide rinsate. The adsorption capacity of SMC was increased compared to most soils, possibly related to the increased organic matter content. The rapid equilibrium of pesticide solution and SMC was consistent with a physical type of adsorption mechanism. The adsorption of carbamate pesticides on SMC was found to exhibit nonlinear “favorable”; adsorption behavior that could be characterized well by the Freundlich isotherm model. In addition, the multi‐pesticide adsorption tests displayed the characteristics of competitive behavior. The competitive ability of these three pesticides in multi‐components adsorption was in the order carbaryl > carbofuran > aldicarb, which was consistent with the adsorbability of pesticides.     

Biomassbed: a biological system to reduce pesticide point contamination at farm level

A potential method for cleaning water from point-source pollution by organic compounds is using biological reactors. In this study, four reactors were tested for their ability to retain and degrade pesticides. The pesticides tested were the insecticide chlorpyrifos, the fungicide metalaxyl and the herbicide imazamox. The reactors were filled with differing mixtures of vine-branch, citrus peel, urban waste and public green compost. The reactor volume was 188 l. Forced circulation of the contaminated solution was programmed to decontaminate the solution. Both retention and degradation of the compounds by the reactors was studied. Chlorpyrifos was the best retained, due to its physico-chemical characteristics, while only one substrate effectively retained metalaxyl and imazamox (citrus peel+urban waste compost). Degradation of the pesticides in the reactors was faster than published values for degradation in soil. The half-life of all pesticides in the reactors was less than 14 days, compared to literature values of 60-70 days in soil. The combined retention and fast degradation make the biofilter a feasible technique to reduce spill-related and point environmental contamination by pesticides. The technique is most effective against persistent pesticides, while for mobile pesticides, the efficiency can be improved with several passages of the contaminated solution through biofilters.     

Influence of the application of sewage sludge on the degradation of pesticides in the soil

A study was made of the influence of the application of sewage sludge on the degradation of pesticides in the soil. Two kinds of sludge were used, with different characteristics, one from an urban treatment plant and one from a food processing plant. Three organophosphorus insecticides, fenitrothion, diazinon and dimethoate, were studied. The relative importance was determined of the chemical and biological degradation processes, which involved experiments on soil and sterile soil samples. A comparative study was also made of the degradation of pesticide residues and the evolution of the microbial population. The application of sludge seems to have a complex effect on the degradation of pesticides, determined by the bioavailability and biodegradability of their active ingredient. The biodegradation of pesticide residues brings about alterations in the microorganism population of the soil.     

Obsolete pesticides and application of colonizing plant species for remediation of contaminated soil in Kazakhstan

In Kazakhstan, there is a problem of finding ways to clean local sites contaminated with pesticides. In particular, such sites are the deserted and destroyed storehouses where these pesticides were stored; existing storehouses do not fulfill sanitary standards. Phytoremediation is one potential method for reducing risk from these pesticides. Genetic heterogeneity of populations of wild and weedy species growing on pesticide-contaminated soil provides a source of plant species tolerant to these conditions. These plant species may be useful for phytoremediation applications. In 2008–2009 and 2011, we surveyed substances stored in 80 former pesticide storehouses in Kazakhstan (Almaty oblast) to demonstrate an inventory process needed to understand the obsolete pesticide problem throughout the country, and observed a total of 354.7 t of obsolete pesticides. At the sites, we have found organochlorine pesticides residues in soil including metabolites of dichlorodiphenyltrichloroethane and isomers of hexachlorocyclohexane. Twenty-four of the storehouse sites showed pesticides concentrations in soil higher than maximum allowable concentration which is equal to 100 μg kg^?1 in Kazakhstan. Seventeen pesticide-tolerant wild plant species were selected from colonizing plants that grew into/near the former storehouse’s pesticides. The results have shown that colonizing plant annual and biannual species growing on soils polluted by pesticides possess ability to accumulate organochlorine pesticide residues and reduce pesticide concentrations in soil. Organochlorine pesticides taken up by the plants are distributed unevenly in different plant tissues. The main organ of organochlorine pesticide accumulation is the root system. The accumulation rate of organochlorine pesticides was found to be a specific characteristic of plant species and dependent on the degree of soil contamination. This information can be used for technology development of phytoremediation of pesticide-contaminated soils.