Behavior of the Chiral Herbicide Imazamox in Soils: pH-Dependent, Enantioselective Degradation, Formation and Degradation of Several Chiral Metabolites.

Many pesticides show a pronounced biphasic degradation in soil, typically with a faster initial phase, followed by a slower decline. For chiral compounds, a biphasic decline of the total concentration may result from enantioselective degradation. In this study with the chiral herbicide imazamox, biphasic degradation was observed in most of the 18 soils investigated. In neutral soils, degradation was, in fact, enantioselective with faster degradation of (+)-imazamox. In slightly acidic soils, differences between enantiomers were not pronounced, and in strongly acidic soils, degradation was again enantioselective, but with reversed preference. Additional experiments with pure enantiomers indicated no interconversion. Enantioselective degradation thus contributed to the biphasic decline of the total concentration in certain soils. However, this was not the only factor since degradation of the individual enantiomers was biphasic in itself. In addition to the observed correlation between enantioselectivity and pH, degradation was generally faster in neutral than in acidic soils with half-lives ranging from only 2 to >120 days. Half-lives were also determined for two known metabolites and a further chiral metabolite, the structure of which was characterized by high resolution tandem mass spectrometry. As for the parent compound, half-lives of the metabolites varied considerably in the different soils.