The Influence of Internal Charge on the Sorption Properties of Clays

Abstract

The surface complexation model (SCM) has been used successfully to describe the adsorption properties of oxides and other solids, such as carbonates and sulphides. This model describes adsorption as a superposition of the electrostatic interactions at the solid-water interface with chemical reactions of solutes with reactive groups at the surface of the solid. The activities of the species at the interface depend on the electrostatic potential of the surface, and are calculated from a so-called 'coulombic term' whose exact form depends on the electrostatic description of the interface (e.g. double layer or triple layer). The surface complexation model has been used to model two kinds of experimental data: acid-base titration curves and metal or ligand sorption edges. It predicts that as the ionic strength, I, decreases, the absolute surface charge density also decreases at a given pH. At low ionic strength, the surface charge is less efficiently shielded and accumulation of charge on the surface is energetically more difficult. In the absence of any strongly binding ion, the SCM also predicts that the titration curves at various I intercept at the same pH, the zero proton condition (ZPC), where the concentrations of adsorbed H + and OHat the surface are equal. Sorption edges of a cation M n+ usually show a strong increase of adsorption with pH, as the competition of H + with the cation for the surface sites decreases. The slope of the [M ~+] adsorbed v s pH curve reflects the net number of H + ions displaced by the adsorption of M n+ which depends on the stoichiometry of the reaction and the variation of the surface potential with pH. In spite of the success of the SCM in describing sorption properties of oxides, its application to clays has not been straigthforward. In particular, some titration data on clays such as kaolinite and montmorillonite show an increase in the ZPC as ionic strength decreases; concomitantly, at low pH, the charge density is higher at low ionic strength, contrary to what is usually observed on oxides. Furthermore, fitting metal sorption on clays with the SCM has proven difficult (Bradbury and Baeyens, 1997). Here, we propose that the different sorption properties of clays compared to oxides may be explained by the porous character of clays, whose interlayers are permeable to water and electrolyte ions, together with their fixed internal charge resulting from isomorphic substitution. The electrostatics of a system consisting of a homogeneous porous solid bearing a fixed internal charge and immersed in a 1:1 electrolyte solution can be described by the Poisson-Boltzmann equation, leading to a generalization of the Gouy-Chapman theory. We thus obtain expressions for the internal and surface potential as a fimction of ionic strength, internal charge density and surface charge density.