Vaterite growth and dissolution in aqueous solution II. Kinetics of dissolution

The kinetics of dissolution of vaterite (CaCO3) crystals in aqueous potassium chloride solutions were studied at temperatures between 15 and 45°C and ionic strengths between 50 and 200 mol/L. The vaterite preparate used consisted of spherulites of vaterite crystals with initial radii of 2.7 to 4.6 μm. The progress of the dissolution was followed by recording pH as a function of time. From pH the concentration of the dissolved matter was calculated, and from this we found the particle size, r, and the dissolution rate, - dr/dt, where r = average particle radius and t = time. Straight lines were obtained by plotting -r(dr/dt) as a function of undersaturation, cs - c, which may be explained by diffusional rate control. Both the values of the apparent diffusion coefficients determined and their dependence on temperature (activation energy = 24.38 kJ/mol) support the assumption that the rate-determining process for the dissolution is the diffusion of the dydrated calcium and carbonate ions away from the crystal surface into the bulk solution.