Phototransformation of three herbicides: Chlorbufam, isoproturon, and chlorotoluron. Influence of irradiation on toxicity

Abstract

The direct phototransformations in aqueous solution of chlorbufam (I), chlorotoluron (II), and isoproturon (III) are compared. (Chemical Equation Presented) With I and II photohydrolysis (substitution of chlorine atom with formation of the corresponding hydroxylated derivative) is initially almost quantitative. It does not depend on irradiation wavelength in the range 250-300 nm. In contrast with III the main reaction is a free radical rearrangement, the reaction being more efficient at 254 nm than at wavelengths longer than 270 nm. Compounds II and III were also irradiated on sand in the absence of water. Several photoproducts were identified, they result from elimination or oxidation of methyl groups of urea moiety. The toxicity of photoproducts was determined using the standard Microtox® test (toxicity on the bacterium Vibrio fischeri). Photohydrolysis of I and II does not change much the toxicity of solutions whereas the oxidation, or at a lower scale the elimination of methyl group on the urea moiety, dramatically increases the toxicity.