Solubility of salts in water: Key issue for crystal growth and dissolution processes

Abstract

The formation of sparingly soluble salts from aqueous solutions and their dissolution has attracted broad research interest. Of particular importance is the formation and transformation of minerals exhibiting polymorphism or encountered in more than one crystalline phase as, for example, in the case of calcium phosphates, formed in biological mineralization and in industrial-scale deposits. Understanding of these processes depends primarily on the equilibrium between the mineral phases considered and the aqueous medium in contact. Precipitation takes place in supersaturated solutions with rates depending on the solution supersaturation. The experimental investigation may reveal mechanistic details if done at sustained supersaturation. The kinetics of crystal growth depends either on surface diffusion or on bulk diffusion, which in turn is controlled by the medium fluid dynamics. In the case of magnesium ammonium phosphate (struvite), the presence of water-soluble organic compounds is responsible for the retardation both of the time needed for the onset of precipitation and for the kinetics of growth of the supercritical nuclei. Dissolution processes are controlled by the same mechanisms. In the case of calcitic marble, the dissolution in alkaline solutions is controlled by surface diffusion. Compounds active at the marble/water interface may in this case be used as protective agents.