Degradation of Pyrethroid Optical Isomers in Soils

Abstract

Pyrethroid isomers (four isomers of permethrin and fenvalerate, eight isomers of cypermethrin, and deltamethrin and its seven isomers) and a racemic mixture of fenpropathrin were examined for degradation rate and route in two Japanese upland soils under the same experimental conditions. Degradation of each pyrethroid isomer was studied at 25°C under dark conditions with the 14C-labeled chemicals. The trans isomers of permethrin, cypermethrin and deltamethrin degraded more rapidly than the corresponding cis isomers, and the αS epimers of cypermethrin, fenvalerate and deltamethrin degraded faster than the corresponding xR epimers. The degradation rate of cypermethrin or deltamethrin isomers decreased in order of (trans, xS)> (trans, xR)>(cis, xS)>(cis, xR). The cis/trans or xR/xS isomerization hardly occurred in the soils. Based on the minimum and maximum half-lives of individual isomers of each pyrethroid, permethrin was judged to have decomposed most rapidly, followed in decreasing order by cypermethrin, deltamethrin and fenvalerate, with only a slight difference between cypermethrin and deltamethrin. Larger amounts of 14C02 and ester cleavage products were formed in the soils treated with less persistent trans or αS isomers, whereas oxidation products retaining the ester linkage such as diphenyl-ether bond-cleavage products and ring-hydroxylated products were of more importance in the soils treated with more stable cis or αR isomers. A larger amount of bound 14C was formed with cis or xR isomers. Degradation of four isomers of a dichlorovinyl analogue of chrysanthemic acid, the acidic half of permethrin and cypermethrin molecules, was also examined in two soils. © 1992, Pesticide Science Society of Japan. All rights reserved.