The surface chemistry of oxides, hydroxides, and oxide minerals

Abstract

The initial and reversible steps of the interaction of (hydr)oxide surfaces with electrolytes, that is, adsorption-desorption equilibria with H<sup>+</sup> (OH<sup>-</sup>), metal ions, and ligands (anions), are discussed in terms of the coordination chemistry of the oxide-water interface. This interface is characterized by different types of surface hydroxyl groups, and metallic centers (Lewis acids). Competitive complex formation equilibria (metal ion versus H<sup>+</sup>, or anion versus OH<sup>-</sup>) explain the strong dependence of metal ion and anion binding, respectively, on pH. Some experimentally determined representative intrinsic surface equililbrium constants reflecting the acid-base characteristics of surface hydroxyl groups and the stability of metal complexes, of anion complexes, and of ternary surface complexes, respectively, are tabulated. Such constants can also be estimated from equilibrium constants for corresponding reactions in solution. Additional support for the existence of inner-sphere surface complexes comes from molecular methods (spectroscopy) and investigations on the kinetics of adsorption of ligands and of metal ions.<br />(Hydr)oxides are abundant components of the earth's crust. The rates of processes occurring at these surfaces, such as precipitation (heterogeneous nucleation) and the dissolution of mineral phases - of importance in the weathering of rocks, the formation of soils and sediments, the corrosion of metals, and their inhibition - are critically dependent on the coordinative interactions taking place on these surfaces.