Abstract:
Dissolution rates of single calcite crystals, limestones, and compressed calcite powders were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed in aqueous HCl solutions over the bulk solution pH range -1 to 3, and at temperatures of 25°, 50°, and 80°C. Corresponding rates of the three different sample types are identical within experimental uncertainty. Interpretation of these data using equations reported by Gregory and Riddiford [Gregory, D.P., Riddiford, A.C., 1956. Transport to the surface of a rotating disc. J. Chem. Soc. London 3, 3756-3764] yields apparent rate constants and H+ diffusion coefficients. The logarithms of overall calcite dissolution rates (r) obtained at constant disk rotation speed are inversely proportional to the bulk solution pH, consistent with r=k2'aH+,b, where k2' stands for an apparent rate constant and aH+,b designates the hydrogen ion activity in the bulk solution. This variation of dissolution rates with pH is consistent with corresponding rates reported in the literature and the calcite dissolution mechanism reported by Wollast [Wollast, R., 1990. Rate and mechanism of dissolution of carbonates in the system CaCO3-MgCO3. In: Stumm, W. (Ed.), Aquatic Chemical Kinetics. Wiley, pp. 431-445]. Apparent rate constants for a disk rotation speed of 340 rpm increase from 0.07+/-0.02 to 0.25+/-0.02 mol m-2 s-1 in response to increasing temperature from 25° to 80°C. H+ diffusion coefficients increase from (2.9 to 9.2)x10-9 m2 s-1 over this temperature range with an apparent activation energy of 19 kJ mol-1.