A general mechanism of interaction of carbonates with non-polar S-containing pesticides

Based on a combination of Density Functional Theory (DFT) and Surface Complexation Modeling (SCM), a general mechanism is suggested for (a) the interaction of non-polar S-containing pesticides, Methyl/Ethyl-Parathion, Fenthion, Methidathion, and Thiram with carbonates in solution and (b) their adsorption on chargeable oxides. Surface Complexation Models show that in the presence of carbonates, the species [pesticide–HCO3−] is formed in solution. At pH 5–7, the convolution of the pH-dependence of the [pesticide–HCO3−] concentration with the pH-dependent concentration of the {≡ XOH2+} sites of a-Al2O3 determines the pH profile of the adsorption. The Density Functional Theory calculation for Methyl-Parathion and Thiram reveals that a significant fraction of negative charge is accumulated on the S-atoms of the pesticides while a net positive charge is localized on the –CH3 groups. Thus the combination of –CH3 and S is ultimately responsible for the charge imbalance on these pesticides.