On the mechanism of the dissolution of gypsum (calcium sulfate dihydrate)

The dissolution of minerals is central to many fields of research interest, including hydrometallurgy, materials science, and geochemistry. The successful development of a comprehensive understanding of the mechanism of dissolution will have an impact on these fields. The challenge for the dissolution of gypsum is that rate of dissolution is proportional to the molar concentration of the dissolved salt, yet the solubility product is proportional to the square of the molar concentration of dissolved salt. Deriving an expression that is consistent with both the kinetics and thermodynamics has vexed researchers for decades. Furthermore, the zeta potential of gypsum shows no clear dependence on pH. In this paper, we show that the experimental data for the kinetics of dissolution, the thermodynamics describing the solubility product, and the zeta potential describing the surface charge are reconciled by accounting for the surface charge using the surface vacancy model of dissolution, and in doing so provide insight into the elementary steps involved in the dissolution of gypsum.