RESUMO
Environmentally contaminated aqueous samples are examined for evidence of stereoselective degradation of metolachlor. The unique chemical structure of metolachlor, a chloroacetamide herbicide, consists of four stereoisomers due to axial and/or C-chirality. The degradation of metolachlor is monitored over time in agricultural runoff water that is applied to a subsurface flow constructed wetland. Metolachlor stereoisomers are isolated from aqueous samples by achiral reversed-phase solid-phase extraction and analyzed by normal-phase high-performance liquid chromatography using a chiral stationary phase. The analyses of 64 post-application samples, collected over a period of four weeks, are reported. The samples are filtered (0.45 microm) prior to analysis and thereby represent metolachlor in solution and/or associated with dissolved organic carbon. Sixteen samples demonstrate total racemic metolachlor concentrations greater than 10 ppb. Of these 16 samples, one sample is determined statistically to demonstrate enantioselective degradation. Significant contributions made by this study include the evaluation of stereoselectivity based on mathematically derived fractions, rather than ratios, and statistical evaluation of precision establishing the variability resulting from chromatographic processes versus metabolic processes. The research demonstrates that distribution of metolachlor between the solid phase composed of chemical and/or biological particulates and the aqueous phase is not primarily stereoselective, and that stereoselectively enriched metolachlor does not dominate in the aqueous phase.
