Mechanism of surface reactions and dissolution of fluorite surface in an aqueous electrolyte solution

Abstract

Environmental contextSolubility and dissolution rates of mineral surfaces depend on both the surface properties of the mineral and the composition of the aqueous solution. We investigated the link between the interfacial reactions and dissolution of a fluorite crystal. The study provides a detailed microscopic picture of the dissolution phenomena at the fluorite surface, and the results have wider application to general mineral dissolution processes taking place in the environment. AbstractDissolutions of the fluorite (111) crystallographic plane and fluorite (CaF2) colloidal particles were studied as a function of pH. The process was examined by measuring the concentration of released fluoride and calcium ions by ion-selective electrodes. Additionally, electrokinetic and inner surface potentials were measured by means of electrophoresis and a fluorite single crystal electrode respectively. The rate of fluorite dissolution was analysed assuming a reaction mechanism with a series of elementary steps, which included the reaction of surface groups with H+ ions, the formation of F− vacancies, the dissociation of surface groups and the release of calcium and fluoride ions into the interfacial region as well as the diffusion of ions from the interfacial region. The proposed reaction mechanism indicates that H+ ions play a necessary role in allowing the dissolution to take place, a concept not possible to confirm by looking at the overall equation of fluorite dissolution. The order of the total reaction with respect to H+ ions was found to be 0.37, which is in good accordance with the value derived from the reaction mechanism (1/3). The experimentally determined rate coefficient of fluorite dissolution was found to be kdis=9×10−6mol2/3dmm−2s−1.