Aerobic metabolism and adsorption of pyrethroid insecticide imiprothrin in soil

Abstract

Imiprothrin, a unique type of pyrethroid having an imidazolidinyl ring in its alcohol moiety, was rapidly degraded in two aerobic U.S. soils. The half-lives of biologically active trans isomer were estimated by assuming the first-order kinetics to be 1.6–2.5 days, shorter than those of the cis isomer (3.3–12.5 days). The primary metabolic pathway was ester cleavage followed by instantaneous elimination of a hydroxylmethyl group from the alcohol moiety to form PGH (1-propargylimidazolidine-2,4-dione). The opening of the imidazolidinyl ring with subsequent release of the carbamoyl group resulted in the formation of PG (N-propargylglycine). The soil adsorption coefficients (Koc) of the trans isomer, the main component of imiprothrin (80%), to the two soils were determined to be 376 and 428 (ml/g o.c.) by the batch equilibrium method and these lower values as compared with other pyrethroids were likely to stem from the hydrophilic character of the alcohol moiety. Based on the metabolic half-lives and Koc values, the groundwater concentration of imiprothrin was calculated to be 0.039 μg/L by using the screening simulation model SCI-GROW known to give a conservative groundwater concentration. This significant lower concentration clearly indicates that imiprothrin is most unlikely to contaminate the groundwater, mainly due to its rapid degradation in soil.