New mechanism for the dissolution of sparingly soluble minerals

Abstract

A requirement for the determination of the solubility of minerals is to ensure that equilibrium has been reached. Recent constant composition dissolution studies of sparingly soluble calcium phosphates have revealed an interesting and unusual behavior in that the rates decreased, eventually resulting in effective reaction suppression, even though the solutions remained undersaturated. Traditional theories of dissolution assume a volume diffusion-controlled mechanism with reaction continuing until true equilibrium has been reached. The new results for sparingly soluble salts point to the importance not only of particle size on the dissolution rate but also the participation of critical phenomena. Although the crystal size decreases during dissolution, when the reaction is controlled by poly-pitting (the formation and growth of pits), the edge free energy increases at the very first stage of reaction owing to the creation of pits and dissolution steps. The constant composition experimental results demonstrate the development of surface roughness as the dissolution steps are formed, implying an increase of the total edge length during the reaction. This is an exactly analogous mechanism to that of crystal growth, in which the formation of embryos of critical size plays a key role in the overall mechanism. In contrast to crystal growth, dissolution is a process of size reduction, and, when the particle size is sufficiently reduced, critical phenomena become important so that the influence of size must be taken into consideration. It is interesting to recognize that these critical phenomena are readily apparent for sparingly soluble minerals for which the critical conditions are attained much more readily. The results point to the importance of understanding the detailed mechanism of dissolution when attempts are made to measure, experimentally, the solubilities of sparingly soluble minerals.