Anion adsorption on oxide surfaces: inclusion of the water dipole in modeling the electrostatics of ligand exchange.

Adsorption of aqueous anions, such as sulfate, arsenite, and oxalate, to oxide surfaces is important in the retardation of toxic species in the environment, but predicting the surface speciation as a function of environmental parameters is a major challenge. Recent laboratory spectroscopic studies defining surface speciation must be integrated with surface complexation models. However, the latter have neglected the electrostatic work of desorption of water dipoles in treating anion adsorption by ligand exchange. Taking this effect into account permits close quantitative description of anion adsorption and the prediction of anion surface speciation as a function of pH, ionic strength, and surface coverage in agreement with spectroscopic results.