On the Acid−Base Chemistry of Permanently Charged Minerals

Abstract

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.

A Model for Metal Adsorption on Montmorillonite

“…Finally, and probably most important, is the ability of the clay model to reproduce the variations in pH zsc with ionic strength and the increase in H ϩ adsorption with decreasing ionic strength that are seen in the potentiometric data (33). The Gouy-Chapman model (or its variations) always predicts that all the acid-base titration curves should intercept at a fixed pH zsc , independent of I and that, at a given pH, the absolute value of the surface charge should always decrease with decreasing I.…”

Kinetic Study of Denatonium Sorption to Smectite Clay Minerals

“…Consequently absent of any significant changes in surface charge density, the sorption capacity should be independent of pH. However, for permanently charged clays, the surface potential should decrease with pH (Kraepiel et al, 1998). Data presented in Table 3 for Syn-1 suggest a sorption capacity that is independent of pH.…”

Metals distribution in the organic and inorganic fractions of soil: a case study on soils from Sicily

“…In spite of their considerable cation exchange capacity, the humic substances are a minor part of soil and sediment bulk. Most of soils or sediments are constituted by electrically charged inorganic components, such as carbonates, silicates (chiefly clays) and secondary metastable iron manganese sesquihydroxides, which also contribute, to different extent, in controlling the metal absorption-release surface process (Kraepiel et al, 1998;Sposito, 1984). These mineral particles have very large specific area per unit volume which significantly influence the partitioning of trace metals between soil solutions and the surface of minerals.…”