Effects of Dissolved Carboxylates and Carbonates on the Adsorption Properties of Thiuram Disulfate Pesticides

The adsorption of thiram and disulfiram onto α-Al₂O₃ and montmorillonite clay has been studied in the presence of small carboxylate anions, bicarbonate, formate, and oxalate. At natural concentrations, HCO₃^- enhances dramatically the adsorption of both pesticides on α-Al₂O₃ and clay. An analogous significant enhancement of pesticide adsorption is also observed in the presence of formate and oxalate. Density functional theory calculations demonstrate that in solution a stable molecular complex between one molecule of thiram and one molecule of HCO₃^- is formed with interaction energy −35.6 kcal/mol. In addition, two H₂O molecules further stabilize it by an interaction energy of −3.6 kcal/mol. This clustering [thiram−HCO₃^-−2H₂O] leads to a change of the electronic structure and the ultraviolet−visible spectrum of thiram that is observed experimentally. Surface complexation modeling shows that the molecular cluster [thiram−HCO₃^-−2H₂O], which bears a total net charge of −1, is responsible for the observed enhanced adsorption on the charged surface of alumina and clay at pH below their points of zero surface charge. The results reveal a novel pervasive role of carboxylate anions and particularly HCO₃^- on the adsorption of dithiocarbamate pesticides in natural waters.