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 25C under dark conditions with the 14C-labeled chemicals. The traps isomers of permethrin, cypermethrin and deltamethrin degraded more rapidly than the corresponding cis isomers, and the aS epimers of cypermethrin, fenvalerate and deltamethrin degraded faster than the corresponding chR epimers. The degradation rate of cypermethrin or deltamethrin isomers decreased in order of (trans. aS) > (trans. aR) > (cis, aS) > (cis, c R). The cis Jtrans or aR/aS 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 traps or aS isomers, whereas oxidation products retaining the ester linkage such as diphenyl-ether bondcleavage products and ring-hydroxylated products were of more importance in the soils treated with more stable cis or ceR isomers. A larger amount of bound 14C was formed with cis or caR 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.
Degradation of cypermethrin (I) and fenvalerate (II) isomers by isolated soil bacteria was examined.Two out of 103 bacterial strains showed little metabolic activity, but the remaining strains all degraded I and II isomers with high substrate specificity.The (1R, trans, c S), (1R, cis, c S) and (1 S, traps, ciS) isomers of I were degraded faster than the other five isomers of I, and the (2R, ctS) isomer of II than the other three isomers of II. Ester hydrolysis was the main route of degradation.Cell-free extracts were prepared from four bacterial strains and fractionated by gel filtration or ion-exchange column chromatography. The enzyme assay showed the presence of several enzyme fractions capable of preferentially degrading the cS isomers, whereas two enzyme fractions degraded the traps isomers faster than the cis isomers.It appears that soil microorganisms have several esterase enzymes that can hydrolyze I and II isomers with high stereospecificity.
The degradation of fenitrothion (I) and 3-methyl-4-nitrophenol (II), one of the major degradation products in soil, was studied, using 14C preparations labeled at the phenyl ring. In two kinds of aerobic upland soils, I and II degraded with an initial half-life of less than 7 days. A main degradation route of I was a cleavage of the P-O-aryl linkage, leading to the formation of II which underwent further metabolism to 14C02. In soils treated initially with II, a trace amount of 3-methyl-4-nitrocatechol was detected. When soils containing the bound 14C residues arising from I were mixed with fresh soil, 17.5 to 22.4° of the bound 14C was mineralized to 14C02 over 22-week period. In two kinds of submerged soils, I degraded mainly via reduction of the nitro group to the amino group with an initial half-life of less than 7 days. The resultant product was bound to the soils or metabolized further to the formylamino and acetylamino derivatives. In addition, a relatively large amount of 14C02 was evolved from the soils. When the conditions of the submerged soils were altered to those of aerobic upland by draining the flooded water, the bound 14C residues were partially mineralized to 14C02. Furthermore, the bound 14C residues in upland and submerged soils were hardly taken up by bean and rice plants, respectively.
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