Multiple activated complex dissolution of metal (hydr) oxides: A thermodynamic approach applied to quartz

Abstract

The rate of proton-promoted dissolution of metal (hydr)oxides can be related to the charge and speciation of surface groups at the solid/solution interface. In the literature this dissolution is thought to be rate limited by the decomposition of only one charged activated complex. A new model for the description of the dissolution kinetics of metal (hydr)oxides has been developed in which all potential activated complexes are in principle able to decompose. The new model with multiple activated complexes (MAC) uses the classical activated complex theory. Variable charge theory and lattice statistics are combined in our MAC model with a thorough analysis of the thermodynamics of surface reactions presented in this paper. The MAC model predicts the pH and salt dependency of the dissolution rates on the basis of the thermodynamics-derived quantities (Gibbs free energies, enthalpies, and entropies) for proton (de/ad)sorption reactions without any recourse to the dissolution data if the surface geometry and surface composition of the metal (hydr)oxides are known. The MAC model is applied to quartz. The relevant surface characteristics of quartz are relatively simple. The kinetics of dissolution of silica and quartz are evaluated quantitatively. On the basis of surface geometry five activated complexes are distinguished, differing in the charge of the surrounding ligands. For well-cleaned quartz the MAC theory can describe the pH and salt dependency of the kinetics of proton-promoted dissolution adjusting only one parameter which determines the absolute level of the rate of dissolution. It follows from the model that all activated complexes may be of importance in determining rate. The relative importance of the various activated complexes is strongly pH dependent. The model predicts that the experimentally determined activation energy is a function of the solution composition. For each decomposition reaction the activation enthalpy has been determined.