Formation of trialkyl phosphorothioate esters from organophosphorus insecticides after exposure to either ultraviolet light or sunlight

Abstract

Five different phosphorothioate insecticides, methyl parathion, fenthion, methidathion, azinphos-methyl, and malathion, and a phosphoramidothioate, acephate, were exposed as thin films to irradiation by sunlight and ultraviolet light. With the exception of acephate, measurable amounts of different trimethyl phosphorothioate and dithioate esters, some of them having delayed toxic activity, were formed from these insecticides as a result of irradiation. The extent of photoalteration to these low molecular weight trialkyl phosphorothioates had a strong link with the UV extinction coefficient, as well as absorption maxima of the various insecticides. On the basis of known oral toxicities of the trimethyl phosphorothioate and dithioate esters and the recommended field use rates of the test insecticides, the levels of the photoalteration products formed are not expected to pose toxic hazards to farm workers.Previous reports from this laboratory and the Medical Research Council Toxicology Unit (Carshalton, England) have described the unusual delayed toxic properties of 0,0,S-trimethyl phosphorothioate (1) (Mallipudi et al., 1979;Umetsu et al., 1981) and 0,S,S-trimethyl phosphorodithioate (2) (Aldridge et al., 1979). These compounds, which have been shown to exist as minor contaminants in malathion and other organophosphorus insecticides, caused delayed death in rats following a single oral dose as low as 15 mg/kg. Moreover, 1, 2, and other related organophosphorus impurities also were found to potentiate the acute mammalian toxicity of carboxylic acid ester containing organophosphorus insecticides such as malathion and phenthoate (Pellegrini and Santi, 1972; Umetsu et al., 1977). These impurities are introduced as side products formed during the various synthetic steps leading to the technical material.

Photodegradation of the organophosphorus pesticides chlorpyrifos, fenamiphos and vamidothion in water

“…To our knowledge no other studies on aquatic photodegradation of vamidothion have been undertaken. The formation of sulfoxide and sulfone TPs is a common feature in the photolysis of organophosphorus pesticides that can lead to such TP (11). However, as we have noticed, no sulfone TP has been obtained in our experiments.…”

Comparative Photodegradation Study of Fenthion and Disulfoton under Irradiation of Different Light Sources in Liquid- and Solid-Phases

“…The individual photolysis reactions of fenthion and disulfoton have been studied, [20][21][22][23] although little is known of their comparative decomposition under the conditions of light source, exposure time of light and reaction phase. Most of the photolysis studies have used natural sunlight, so that it is unclear what wavelength range of light actually contributes to the photolytic reaction.…”

Reviews of Environmental Contamination and Toxicology

“…Glass plates (Chukwudebe et al 1989) and silica gel (Hirayama et al 1998) were utilized to examine the comparative photoreactivity of various pesticides.…”