The absorption of metolachlor, 2-chloro-6'-ethyl-N-(2-methoxy-1 -methy1ethyl)acet-o-toluidide was studied on three soils of various physical and chemical properties. Adsorption isotherms conformed to the Freundlich equation. The k, values increased with increasing organic carbon content of the soils. However, variations in k,, values by a factor of four were observed within three natural soils. For a given soil, the k,, values remained essentially unchanged after lowering the carbon content by hydrogen peroxide oxidation. These results suggest that the contribution of organic matter to total metolachlor adsorption process is influenced by association mechanisms between organic and inorganic soil constituents. Studies with mixtures of clay and organic matter indicate that the interaction between the two soil components reduces the surfaces available for metolachlor adsorption.

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

The adsorption and desorption of imazapyr, (()-2- [4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-pyridinecarboxylic acid, by six soils were studied. Adsorption isotherms conformed to the Freundlich equation. A multiple regression analysis indicated that K f(ads) values were better correlated if cation exchange capacity and pH (r ) 0.984) were simultaneously taken into account. The results show that the herbicide adsorption is strongly affected by the pH and the charge of the adsorbing component. The organic matter is effective in imazapyr retention, but only at pH <5. Enhanced adsorption was observed in amorphous iron oxide, most likely because of a ligand exchange process.