Photodegradation of the Pyrethroid Insecticide Cypermethrin in Water and on Soil Surface

Abstract

Photodegradation of (1R, cis, αRS)- and (1R, trans, αRS)-isomers of cypermethrin [(RS)-α-cyano-3-phenoxybenzyl (1RS)-cis, trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate] in water and on soil surface was studied, using 14C preparations labeled separately at the cyclopropyl C-l, cyano or benzyl ring. The cis-isomer was photodecomposed 1.4-1.7 times faster in sunlight than the trans-isomer in water. The half-life of the cis-isomer was 2.3-2.6 days in distilled water and 1 ppm humic acid aqueous solution, and 0.6-0.7 day in natural river and sea water, and <0.5 day in 2% aqueous acetone. A triplet photosensitizer acetone together with the naturally occurring substances in river and sea water enhanced the photodegradation of both isomers. On three kinds of soil surface, both isomers were rapidly photodegraded with the initial half-life of 0.6-1.9 days. The photoreactions involved were: 1R/IS and cis/trans isomerization of the cyclopropane ring, cleavage of the ester or diphenyl ether linkage, oxidation of the CHO group to the COOH group, hydration of the CN group to the CONH2 group, hydrolysis of the CONH2 group to the COOH group, oxidative cleavage of the halogenated side chain, dehalogenation, intramolecular cyclization to form γ- or δ-lactone, and photomineralization of the cyclopropyl C-l, cyano and benzyl ring to 14CO2. © 1985, Pesticide Science Society of Japan. All rights reserved.