VATERITE GROWTH AND DISSOLUTION IN AQUEOUS-SOLUTION .2. KINETICS OF DISSOLUTION

The kinetics of dissolution of vaterite (CaCO3) crystals in aqueous potassium chloride solutions were studied at temperatures between 15 and 45 degrees 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 mu m. 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, c(S)-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 hydrated calcium and carbonate ions away from the crystal surface into the bulk solution.