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Photochemical properties and degradation by-products of triasulphuron and thifensulphuron-methyl

The effect of light on two sulphonylurea herbicides, triasulphuron and thifensulphuron methyl, was studied under both UV and solar simulator irradiation (Suntest). Energies of first singlet and triplet state transitions were calculated from fluorescence and phosphorescence spectra. Experiments were performed in the presence of either a singlet or a triplet quencher showing that photodegradation of both herbicides begins from a triplet state, T1. The photolysis process of both herbicides occurred through first-order kinetics. The investigation stressed the relevance of the light exposition on the degradation rate of both herbicides. Half-lives of photolysis reactions (Suntest) in the organic solvent used in the experiments (22 and 54 h for triasulphuron and thifensulphuron methyl, respectively) are comparable with the hydrolysis rate in aqueous environment. With UV irradiation, the degradation time of both herbicides can be greatly reduced to several minutes, thus suggesting that this technique can be adopted as an efficient method of detoxification. The main photoproducts, identified by LC-ESI-MS, were: (4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea and 4-methoxy-6-methyl-1,3,5-triazin-2-amine, common to triasulphuron and thifensulphuron-methyl; 2-(2-chloroethoxy) benzenesulphonamide and (2-chloroethoxy) benzene, arising from triasulphuron degradation; 4-sulphamoyl-thiophene-3-carboxilic acid methyl ester and thiophene-3-carboxilic acid methyl ester, occurring from thifensulphuron-methyl transformation. The presence of minor by-products was also ascertained.