Solubility and surface adsorption characteristics of metal oxides

This chapter provides an overview of the recent developments in the understanding of the interaction of metal oxide and hydroxide minerals with hydrothermal solutions. With a few exceptions, metals exposed to aqueous solutions generally form an oxide surface film, which may or may not passivate the surface toward further oxidation. This phenomenon is driven by the relative Gibbs energies of the solid phases and the redox state of the system. Metal oxides formed in the presence of water are often hydrated regardless of whether the hydrous phase is thermodynamically stable relative to the pure oxide or a less hydrated phase. The ideal method of defining a solubility product is to demonstrate that the reaction is "reversible," i.e., to show that the solution achieves the same IAP = K by approaching the equilibrium condition from under and super-saturation. The solubilities of metal oxides cannot be described without a detailed understanding of the hydrolysis and complexation of the aqueous reactants and products at elevated temperatures. At this time, there are no systems for which both dissolution/precipitation rate data and detailed information on the pH-dependent charging and adsorptive characteristics of the solid phase are known at elevated temperatures. However, the HECC and other emerging experimental techniques make such studies feasible and important targets for future research in hydrothermal oxide-water interactions.