Photodegradation of Fenitrothion in Water and on Soil Surface, and Its Hydrolysis in Water

Abstract

On exposure to sunlight, fenitrothion was photodecomposed with the half-lives of 0.6-1.0 days in distilled water, 1.5, 1.0, and 0.9 days in buffer solutions at pH 3, 7 and 9 respectively, and 0.9-1.1 days in natural river and sea water. The quantum yield was determined to be 8.0 × 10-4 (at 313 nm in distilled water), and the half-life of disappearance at 40 degree north latitude was calculated to be 1.4 days in fall. Fenitrothion applied on two kinds of soil thin layer plates promptly disappeared through photodegradation and volatilization, with the half-life of approximately 1 day. The insecticide underwent photo-induced oxidation of P=S to P=0, oxidation of the aryl methyl group to the carboxyl group, reduction of the nitro group to the amino group, coupling of the carboxy group with the amino group leading to the formation of the dimeric compound, cleavage of the P-O-aryl or P-O-methyl linkage, isomerization, formation of the benzoisoxazole derivative with subsequent formation of the seven-membered ring (azepine derivative) through Beckmann rearrangement, and photomineralization of the aromatic ring to carbon dioxide in water, and oxidation of P=S to P=0 and cleavage of the P-O-aryl linkage on soil. The formation of the major photoproducts in water was pH dependent. The carboxy derivative oxidized at the aryl methyl group of fenitrothion was predominant in distilled water at pH 5.9 and in buffer solutions at pH 3 and 7, while the dimeric compound, composing of the carboxy derivative of fenitrothion and the corresponding amino analog, was more predominantly formed in buffer solutions at pH 7 and 9, and natural river (pH 7.4) and sea (pH 7.8) water. On prolonged exposure to sunlight, these photoproducts were degraded further to 14CO2 and the unextracted residues, which were considered to be the polymerized compounds like humic acid and other highly polar products. Fenitrothion underwent hydrolysis through neutral (pH independent) and basecatalyzed processes below pH 7 and above pH 10 respectively, while both reactions occurred between pH 7 and pH 10. The half-lives of disappearance by hydrolysis within the ranges of pH 5–9, normally found in natural water, were about 200–630 days at 15°C, 17–61 days at 30°C, and 4–8 days at 45°C. The product cleaved at the P-O-aryl linkage was predominant above pH 10, while the product cleaved at the P-O-methyl linkage was predominant below pH 8. The rates and pathways of degradation of fenitrothion by photolysis and hydrolysis in natural river and sea water were similar to those in buffer solutions at the same pH. © 1985, Pesticide Science Society of Japan. All rights reserved.