A consistent thermodynamic model is developed for metal sorption on expanding 2:1 layer clays such as montmorillonite. The particle of clay, including lamellae and interlayers, is represented as a porous solid bearing a permanent negative charge (resulting from isomorphic substitution) with an infinite plane interface (i.e., edges) with the solution. Cation exchange occurs inside the clay particle as the result of the negative potential of the clay. Surface complexation reactions take place at the interface whose surface charge and potential are pH dependent. The potential in the bulk of the clay and near the interface, as well as the surface potentialsurface charge density relation, are calculated taking into account the effect of the permanent negative charge. The results are discussed and compared with the classic Gouy-Chapman theory. A subroutine (Clayeql) with the new potential-charge relationships is implemented in the thermodynamic equilibrium program Mineql ؉3.0 and is used to fit an extensive published experimental data set on adsorption of transition metals on montmorillonite. The model is shown not only to fit satisfactorily all the data, but also to explain specific features of adsorption on clays compared to oxides. In particular, the increase in the surface concentration of protons with decreasing ionic strength is successfully reproduced and the weaker dependence of metal sorption on pH compared to oxides is correctly fitted.

The acid−base properties of oxides are well described by the surface complexation model, which superposes a thermodynamic description of acid−base reactions at the oxide surface with a double-layer model of the electrostatics at the solid−solution interface. So far, however, this model has not been extended to include the effects of permanent charges such as result, for example, from isomorphic substitution in clays. Contrary to oxides, solids with permanent charge often exhibit an increasing degree of protonation with decreasing ionic strength at low pH. They also show an increase in their zero proton condition (ZPC) with decreasing ionic strength. Here we examine the influence of the pH-independent charge of a solid on its acid−base properties. We consider two simple cases:  model 1 in which all the acid−base groups and pH-independent charges are distributed at the surface of a nonpenetrable solid, at the interface with the solution; Model 2 in which the solid is porous (i.e., penetrable by water and electrolyte ions), and the pH-independent charges are distributed inside the bulk of the solid, while the acid−base groups are on the surface of the solid. For model 1, the Gouy−Chapman theory yields the surface potential as a function of surface charge and ionic strength; for model 2, the solution to the Poisson−Boltzmann equation applied both inside and outside the solid yields expressions for the internal and surface potentials as a function of internal charge, surface charge, and ionic strength. When these equations are used with reasonable physical and chemical parameters for models 1 and 2, the resulting acid−base calculations exhibit the same qualitative behavior as observed experimentally for clays. Models 1 and 2 are then shown to describe parsimoniously published acid−base titration data for kaolinite and montmorillonite, respectively.

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