On the Acid-Base Chemistry of Permanently Charged Minerals

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 acidbase 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 acidbase 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 acidbase calculations exhibit the same qualitative behavior as observed experimentally for clays. Models 1 and 2 are then shown to describe parsimoniously published acidbase titration data for kaolinite and montmorillonite, respectively.