| Pesticides have been widely used in agriculture,but pesticides will affect the environment and human health.In recent years,the proportion of chiral pesticides has increased.Due to the significant differences in the biological activity,toxicity,degradation and metabolism of the enantiomers,the enantioselectivities of chiral pesticides in the environment attract lots of attention.The present study was conducted to evaluate the degradation and toxicity of fenoxaprop-ethyl(FE)and its metabolites fenoxaprop(FA),ethyl-2-(4-hydroxyphenoxy)propanoate(EHPP),2-(4-hydroxyphenoxy)propanoic acid(HPPA)and 6-chloro-2,3-dihydrobenzoxazol-2-one(CDHB)in aquatic environment and aquatic organisms on an enantiomeric level.In order to study the stereoselective degradation of FE and its metabolites,the analytical methods of FE and its metabolites were established.Four pairs of chiral compounds were separated on two chiral columns.Pesticides in water environments are not only distributed in the water,but also accumulate in sediments.The environmental fate of the herbicide FE and metabolites in water,sediment and water-sediment microcosm was studied.FE,FA,EHPP,HPPA were chiral and the environmental behavior was investigated on an enantiomeric level.In water,sediment and water-sediment microcosms,FE degraded very fast with half-lives less than 1 day and it was found the herbicidally inactive S-enantiomer degraded faster.FA was the main primary degradation product which was quickly formed and the further degradation was relatively slow with half-lives of 6.4-12.4 days,and the S-enantiomer degraded faster too.EHPP,HPPA and CDHB could be found and S-EHPP and S-HPPA were degraded preferentially.The effects of microorganism and water content were investigated and it was found that the enantioselectivity was attributed to microorganisms.In sediment,the main degradation pathway of FE was hydrolysis and the degradation rate of FE increased with water content.Pesticides in the water environment will be harmful to aquatic organisms.The bioaccumulation and elimination of FE and its metabolites in zebrafish were studied.FA,HPPA and CDHB were bioaccumulated in zebrafish,while FE and EHPP were not.R-FA and R-HPPA were preferentially bioaccumulated,and S-FA and S-HPPA were preferentially degraded.The EF values fluctuated between 0.65 and 0.85,and EF values of FA were higher then that of HPPA.In addition to aquatic organisms,amphibians will also be exposed to pesticides in the water environment.The bioaccumulation and elimination of FE and its metabolites in frogs and tadpoles were studied.After both oral administration and aqueous solution exposure,FE was not found in frogs,while FA was formed and accumulated in liver,kidney,brain,eggs,skin,thigh muscle and blood with preferential accumulation of R-FA.A dominant tissue distribution of FA and CDHB was observed in the liver and kidney.The presence of FA in frog eggs suggested maternal transfer in females and potential impacts to offsprings.The elimination of FA in frog tissues was also enantioselective with a preferential metabolism of R-FA(kidney)or S-FA(liver,eggs,skin,muscle and whole blood).FE and FA were hardly detectable in tadpoles after aqueous solution exposure,while CDHB was accumulated and eliminated as first-order kinetics with half-life of 37.1 hours.Pesticides that do not enter the water environment and remain in the soil will be harmful to soil organisms.The degradation of FE and its metabolites in earthworm was studied.FE was not found in earthworms,while FA was in relatively high levels.FA was accumulated almost exclusively with R-enantiomer in earthworms and the bio-concentration factors of R-FA and S-FA were 1.39 and 0.17 respectively with the enantiomer fraction(EF)values about 0.99.The degradation of FA in earthworms followed first-order kinetics with half-life of 1.82 day.The other metabolites could not be detected in earthworms.Pesticides in the environment or organisms will bring toxic effects.The acute toxicity and oxidative stress of FE and its metabolites to zebrafish was studied.The toxicity of all metabolites was higher than FE.The order of acute toxicity is CDHB>FA>EHPP>HPPA>FE.The toxicity of S-enantiomers of FE and its metabolites is greater than that of R-enantiomers.The activity of CAT and SOD and the content of MDA in gill and liver were significantly enhanced by FE and metabolites,and S-enantiomer showed greater effects.The acute toxicity and genotoxicity of FE and its metabolites to tadpoles was studied.Mortality of tadpoles and micronucleus rate in peripheral blood erythrocytes of tadpoles were used to evaluate the enantioselective acute toxicity and genotoxicity of FE,FA and CDHB.Only CDHB induced significant acute toxicity to tadpole with 96 h LC50 value of 30.4?g/mL,and rac-FA,S-FA and CDHB showed genotoxicity.The acute toxicity of FE and its metabolites to earthworm were performed.The earthworm was more sensitive to all metabolites than FE.Furthermore,earthworm was more sensitive to the R-form of the chiral metabolites than the S-form and rac-form.The results suggested metabolites and enantioselectivity should be taken into consideration to better predict the exposure concentration and apply ecological indicators in toxicological studies.Wastewater from sewage treatment plants will not only enter the water environment,but also as agricultural irrigation water affected soil.The effects of wastewater irrigation and sewage sludge on the dissipation behavior of FE and its metabolites in soil were studied on an enantiomeric level during a 148-day exposure experiment.FE decreased with half-life below 6 hours without enantioselectivity in soil under tap water irrigation.FA was formed with great preference of R-enantiomer.The degradation of FE and FA was restrained by both wastewater and treated wastewater irrigation,but the enantioselectivities are less affected.Surprisingly,there is enantioselectivity with preference degradation of S-FE in sewage sludge.More FA was generated in sewage sludge than in soil,and there was difference of enantioselectivity in sewage sludge and soil.A sterilization experiment indicated that the enantioselectivity of FE to FA were determined by the microorganisms in soil and sewage sludge.Farming practices like wastewater irrigation and sewage sludge application might not only influence the fate of pesticide,but also the enantioselectivity of chiral pesticide enantiomers and thus the risks of pesticide residues posed to the environment.Since FE and its metabolites remain in the water environment for a long time and produce toxic effects,it is necessary to repair the pollution of FE and its metabolites in the water environment.The adsorption of four kinds of biochar on FE and its metabolites was studied.The adsorption on FE and its metabolites was balanced in 2 days.The order of percentage of adsorption was bamboo charcoal biochar>straw biochar>hair biochar>rice husk biochar.The desorption of bamboo charcoal biochar on FE and its metabolites was balanced in 2 days.Bamboo charcoal biochar is not only nontoxicity to zebrafish,but also reduces the toxicity of FE and its metabolites to zebrafish.Bamboo charcoal biochar may be used as an adsorbent for the remove of FE and its metabolites in the water environment. |
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