Degradation of organophosphorus pesticides in soils with special reference to unaerobic soil conditions.
Organophosphorus pesticides are generally transformed by the reactions including oxidation, reduction, hydrolysis, hydroxylation, dehydrochlorination, dealkylation, methylation, isomerization, and conjugate formation. Although the degradation process of pesticides in soils is complicated, main factors may be soil constituents, soil microflora, and chemical structures of pesticides. Chemical structures are especially important for soil metabolism of organophosphorus pesticides, because the priority of the reactions mentioned above is decided. Although organophosphorus pesticides are generally hydrolyzable, the order of hydrolysis varies with chemical structures. It might be said that the slower the hydrolysis rate of the molecule, the more the possibility to be attacked by reactions other than hydrolysis. In such cases, oxidation and reduction are primarily important for the degradation of organophosphorus pesticides. Flooded soils in paddy fields give a favourable environment for the reduction of organophosphorus pesticides having labile substituents such as nitro groups. The threshold of reduction in-flooded soil is expressed as redox potential. Eh, the Eh of paddy soil fluctuates to a great extent, depending on seasons and soil types, especially organic matter content. The result of laboratory experiments with fenthion, disulfoton, Kitazin P (0,0-diisopropyl S-benzyl phosphorothiolate), edifenphos (0-ethyl S,S-diphenyl phosphorodithiolate) and amiprophos (0-ethyl 0-(2-nitro-p-tolyl) N-isopropyl phosphoramidothionate) suggested the participation of several factors mentioned above in the degradation of organophosphorus pesticides.