Surface ionization and complexation at the oxide/water interface. 3. Adsorption of anions

Abstract The site-binding model for the electrical double layer of hydrous oxides reported in a previous paper is applied to adsorption of anions from dilute solution. Generally, more than one stoichiometric surface reaction is needed to describe the adsorption behavior of divalent weak acid anions. If mass law equations for surface reactions are corrected for effects of the electrostatic field at the interface, the calculated adsorption density depends upon the type of surface species formed. It is shown that calculations which consider formation of surface complexes by protonated anionic forms, e.g., HCrO 4 − , HSeO 4 − , HSO 4 − , are more consistent with experimental adsorption data than complexation by bidentate surface sites. Modeling results predict that adsorbed anions are more easily protonated than those in bulk water, and a qualitative explanation for this phenomenon is presented. The model applies over a wide range of solute concentrations and accounts for effects of changes in composition of the supporting electrolyte. In addition calculated results for a shift in pH PZC upon specific adsorption of sulfate are in reasonable agreement with other experimental studies.

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