Isomer- and enantioselective degradation and chiral stability of fenpropathrin and fenvalerate in soils

Fenpropathrin and fenvalerate are two widely used pyrethroid insecticides. Due to one and two asymmetrical centers, fenpropathrin and fenvalerate consist of two and four stereoisomers, respectively. In the present study, the degradation of fenpropathrin and fenvalerate in two soils, an alkaline soil and an acidic soil, was investigated using enantioselective high-performance liquid chromatography (HPLC) and the following results were obtained. (i) Degradation of fenpropathrin and fenvalerate in alkaline soil was slightly enantioselective with S-fenpropathrin and αS,2R-fenvalerate being faster degraded, respectively. (ii) Fenpropathrin and fenvalerate were found configurationally unstable in alkaline soil and significant racemization at the chiral α-C position took place along with the degradation process. The racemization is chemically induced since it occurred under both nonsterilized and sterilized conditions. (iii) No racemization was observed in acidic soil. The soil pH dependent effect of the racemization was further validated by incubating S-fenpropathrin and αS,2R-fenvalerate in methanol–buffer solutions with different pH values. (iv) Fenvaleric acid was identified as the main metabolite of fenvalerate, formed by cleavage of the ester bond, during fenvalerate degradation in alkaline soil. Formation of fenvaleric acid in alkaline soil was slightly enantioselective, resulting in an enrichment of S-fenvaleric acid in the soil.