Kinetics of Chemical Dissolution of Oxides: Comparison of Theory and Experiment
AbstraktThe mechanism of chemical dissolution of oxides in aqueous solutions of acids has been the subject of significant interest during the last three decades. The reactivity and dissolution behaviour of oxides is crucial for a wide variety of processes including, e.g, hydrometallurgy, corrosion of oxide materials, geochemical processes, catalysis, environmental processes or drug design. In the theoretical study of the mechanism of the process, electrochemical, colloid-science and surface-science approach have been used. Current electrochemical theories assume that dissolution rates are controlled by solid-liquid interaction in the interphase region, predicting their inverse exponential dependence on solution pH. The colloid-science approach explains the dissolution of oxides by ionic adsorption at the oxide/solution interface. The surface-science approach is based on the categorisation of oxides into groups (ionic, covalent, semiconducting), characterized by bulk and surface defects and structural changes in both regions. General principles that govern the kinetics of the dissolution of oxides, are not sufficiently known. A major difficulty is that no single oxide was extensively studied and the range of the conditions for the individual oxides studied varies. Very often, the rate-controlling step cannot be determined at all. There are also the regimes that could not be explained by existing kinetic theories. This study treats theoretical aspects of the dissolution from the oxide type viewpoint, brings a review of the existing kinetic theories, classifies the published experimental data and shows discrepancies between theory and experiment, which seem to be worth detailed analysis.