Effect Of Environmental Conditions On The Chiral Degradation Of Pyrethroid Insecticides

Pyrethroid insecticides are widely used in modern agricultural production because of their high efficiency,low toxicity characteristics.Most pyrethroid insecticides are chiral compounds,which contain one to three chiral centers and exist as two to eight enantiomers.Different isomers often show different insecticidal activity and degradation behavior in the environment.Moreover,the environmental conditions,such as soil organic matter content,pH,temperature and humidity may have extentive influence on the degradation rate and mechanism.In this paper,three typical pyrethroids,i.e.,permethrin,cypermethrin,and cyfluthrin,were selected as pesticide models for the enantioselectivle degradation study in soils.The research results will provide a theoretical guidance for the metabolism of pyrethroid insecticides in the environment.The future research directions in this field were discussed in order to provide a theoretical basis for the remediation of pesticide contamination.The main contents are as follows:(1)Enantioselective degradation of permethrin in soilsPermethrin has four stereoisomers and the degradation of permethrin in two soils documented a significant isomer selectivity.The half-lives of permethrin were 7.4d in Shijiazhuang yellow soil and 12.6d in Wuhan red soil.Further determination of ER(Enantiomer Ratio)showed that the enantioselectitity in Wuhan red soil is higher than that in Shijiazhuang yellow soil,and the degradation of trans-permethrin is faster than the cis-isomer in the two soils.(2)Enantioselective degradation of cypermethrin in soilsCypermethrin has eight stereoisomers and the degradation of cypermethrin also showed a significant isomer selectivity.The half-lives of cypermethrin were 8.39d in Shijiazhuang yellow soil and 21.5d in Wuhan red soil.Similar to permethrin,the trans-isomers showed a faster degradation rate and greater enantioselectivity.The results also showed that the cis-1 isomer exhibited a weakest enantioselectivity and a slowest degradation rate in both the two soils.(3)Enantioselective degradation of cyfluthrin in soilsCyfluthrin also has eight stereoisomers and has a very similar structure to cypermethrin.The half-lives of cyfluthrin were 7.9d in Shijiazhuang yellow soil and 17.7d in Wuhan red soil.Also,the degradation rate of trans-isomers in both two soils was faster than the cis-isomers and the degradation rate of trans-4 was the fastest among the four enantiomers.It was found that the degradation rate of cyfluthrin is faster than cypermethrin,suggesting the introduction of a fluorine atom in cyfluthrin could improve the specificity of microbial degradation in soils.(4)Enantioselective degradation of cyfluthrin by changing the soil pH valuesWe adjusted the pH value of Wuhan red soil to 7.97(soil 2a)and 5.03(soil 2b)and studied the effect of pH on the degradation of cyfluthrin.The half-lives of cyfluthrin were 10.7d in soil 2a and 27.1d in soil 2b,suggesting the degradation is faster in alkaline condition.During the degradation process,the isomer selectivity became more obvious.The degradation rate of the trans-cyfluthrin was faster than the cis-isomer in two soils,and the degradation rate of trans-4 is the fastest.Although the pH of soil 2a is greater than soil 1,the degradation rate in the soil 1 is still faster than soil 2a,which showed that pH was the only one factor to effect degradation of pesticides.(5)Effects of fertilization on degradation of cyfluthrin in soilsThe enantioselective degradation of cyfluthrin was studied after applying certain fertilizers to the soils.By contrast with the degradation of cyfluthrin without fertilizer,it was found that half-lives of cyfluthrin were 9.7d and 12.5d,but the degradation rate of trans-3 was the fastest.The fertilization mainly influenced the degradation rate of cyfluthrin and had little effect on the enantioselectivity.