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The hydrolysis of the sulfonylurea herbicide triasulfuron [(2-(2-chloroethoxy)-N-[[4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide] was studied in aqueous buffers of pH values 2, 3, 4, 5, 6, 7, and 9. The reaction was of first-order and pH-dependent. Triasulfuron was more persistent in neutral or weakly basic than in acidic solution. Five metabolites have been isolated and identified. At all pH values studied, the primary pathway of degradation was the cleavage of the sulfonylurea bridge. However, minor degradation pathways have also been observed like O-demethylation and opening of the triazine ring. The product distribution was pH-dependent. Keywords: Sulfonylureas; herbicides; triasulfuron; hydrolysis; degradation

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Adsorption and catalytic decomposition of 4-nitrobenzenesulphonylmethylcarbamate (herbicide Nisulam) on Upton, Wyoming, bentonite saturated with different cations was studied using thin-layer chromatography and infrared spectroscopy. Nisulam is adsorbed at room temperature by coordination through the NO~ group to the exchange cation regardless of the cation's nature. On moderate heating (75~176 this molecule decomposes to 4-nitrobenzenesulphonamide whereas a similar compound (herbicide Asulam) containing the NHz functional group instead of NOz is adsorbed by protonation at room temperature and decomposes into different products. For cations having a high polarizing power, a coordination bond between the Asulam molecule's C=O group and the exchange cation is established, and the molecule decomposes to sulphanilic and carbamic acid. In contrast, for cations having a low polarizing power there is no coordination, and the molecule decomposes mainly into sulphanilamide. Nisulam's coordination to the exchange cation through the NO2 group instead of C=O is ascribed to inductive and conjugation effects, typical of the nitro group.

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The degradation of chiorsulfuron wa.s studied in laboratory experiments in three oxisois from south and south-east Brazil. Three soil profiles were sampled by horizon, and classified according to USDA soil taxonomy and the Brazilian system. Degradation assays were made to evaluate the influence of temperature, humidity and liming on chlorsutfuron decotnposition. Further experiments were set up to study enhanced biodegradation. Abiotic degradation was also .studied in sterile soils, to evaluate, by comparison with non-sterile soils, the role of microorganisms in degradation. The degradation always followed first-order kinetics and was generally faster in samples from A than B horizons. An increase in temperature (from 25 to40°C) increased chlorsulfuron degradation. Further, an increase in moisture content increased chlorsuifuron degradation in samples from the A horizons of all soils, whereas for two out of three soils, degradation in samples from the B horizon was greater at lower water content. The biotic contribution to degradation was significant only for the soil with higher fertility. Soil liming significantly increased chlorsulfuron half-life in all samples. Significant enhancement of degradation (decrease in half-iife on reapplication) was observed oniy in sosi from A horizons, where a higher microbial activity was likely.