Physical surface-complexation models for sorption at the mineral–water interface

Abstract

NONE of the traditional models of surface complexation of ions at oxide–water interfaces, such as the constant-capacitance, double-diffuse-layer and triple-layer models^1–5, provides an explicit, quantitative treatment of ion solvation. Here I show that this process can be included quantitatively in surface-complexation theory by describing it using the Born theory of ion solvation^6,7. In this way, the standard Gibbs free energy of sorption can be decomposed into three terms: the standard coulombic term, a Born solvation contribution and a term intrinsic to the ion alone. Consideration of the Born solvation term shows that the equilibrium constant for sorption depends linearly on the inverse of the dielectric constant of the solid. By this means, all three contributions to the free energy can be estimated empirically or calculated theoretically. Inclusion of this physical description of ion solvation should facilitate the application of the theory of ion sorption to complex natural oxide and silicate minerals.