Surface-Catalyzed Transformations of Aqueous Endosulfan

We report the effect of suspended solids on the oxidation and hydrolysis of the insecticide endosulfan (alpha and beta isomers) and its degradation products: endosulfan diol, endosulfan sulfate, endosulfan ether, and endosulfan lactone in 0.001 M NaHCO3 buffer (pH 8.15). Suspensions of sea sand, TiO2, alpha-Fe2O3, alpha-FeOOH, Laponite, and SiO2 all catalyzed the hydrolysis of endosulfan to the less toxic endosulfan diol. Suspended creek sediment (Bread and Butter Creek SC, 4% OC) inhibited endosulfan hydrolysis. Heterogeneous and homogeneous rate constants of endosulfan hydrolysis were measured and indicate that beta-endosulfan hydrolyzes faster than alpha-endosulfan. This observation was explained by a more stable transition state for beta-endosulfan that was confirmed with ab initio molecular orbital calculations (STO-6G) on the anionic intermediates of endosulfan hydrolysis. Rates of endosulfan hydrolysis over the different surfaces corresponded to their tritium-exchange site-density and suggest a mechanism involving surface coordination prior to nucleophilic attack. The oxidation of alpha-endosulfan and beta-endosulfan to the persistent pollutant endosulfan sulfate was not observed in this study.