Enantiomeric differences in permethrin degradation pathways in soil and sediment

Chirality occurs widely in synthetic pyrethroids. Studies have shown significant differences in both aquatic toxicity and degradation rates between enantiomers from the same diastereomer of selected pyrethroids. To better understand chiral selectivity in biodegradation of pyrethroids, ¹⁴C-labeled permethrin was used to characterize enantiomeric differences in the formation of transformation intermediates in two soils and a sediment. Individual enantiomers of permethrin were spiked into soil and sediment samples, and transformation products were identified with known standards. Enantioselectivity was observed in most treatments when the dissipation of the parent enantiomers, the amount of intermediates and bound residues formed, and mineralization rates were compared between the enantiomers. The results show that all enantiomers of permethrin hydrolyzed rapidly and that the hydrolysis products were quickly further transformed. The direct hydrolysis products, cyclopropanic acid (Cl₂CA), 3-phenoxybenzyl alcohol (PBalc), and 3-phenoxybenzoic acid (PBacid), were recovered at small percentages, ranging from 1 to 14% for Cl₂CA and from 0.2 to 6% for PBalc and PBacid. The R-enantiomer of both cis- and trans-permethrin was mineralized more quickly than the S-enantiomer after hydrolysis. The degradation products from cis-permethrin were more persistent than those from trans-permethrin. As some transformation intermediates of permethrin may have greater acute and chronic toxicity than the parent compound, enantioselectivity in the formation of degradation intermediates may lead to different overall toxicities and merit further investigation. This study suggests that for chiral compounds, enantioselectivity may be reflected not only in the dissipation of the parent enantiomers but also in the kinetics of formation of intermediate transformation products.