DISSOLUTION KINETICS OF CALCIUM CARBONATE MINERALS IN H2O-CO2 SOLUTIONS IN TURBULENT FLOW: THE ROLE OF THE DIFFUSION BOUNDARY LAYER AND THE SLOW REACTION H2O + CO2 ↔ H+ + HCO-3

Abstract

Dissolution and precipitation of calcium carbonate minerals in aqueous solutions with turbulent flow are controlled by a diffusion boundary layer (DBL) adjacent to the surface of the mineral, across which mass transfer is effected by molecular diffusion. A rotating disk technique was used to investigate the effect of the DBL on the dissolution rates of CaCO3. This technique allows an exact adjustment of the thickness of the DBL by controlling the rotation speed of a circular sample of CaCO3. Measurements of the dissolution rates in H2O-CO2-Ca2+ -solutions in equilibrium with various partial pressures of CO2 from 1 . 10-3 up to 1 atm showed a dependence of the rates R on the rotation frequency ω, given by R # ωn. The exponent n varies from 0.25 at low PCO2 to about 0.01 at a PCO2 of 1 atm. This reveals that the rates are not controlled by mass transport only, which would require n = 0.5. The experimental data can be explained employing a theoretical model, which also takes into account the slow reaction CO2 + H2O ↔ H+ + HCO-3 and the chemical reactions at the surface (Dreybrodt and Buhmann, 1991). Interpretation of the experimental data in view of this model reveals that conversion of CO2 plays an important role in the control of the rates. At high PCO2 and large DBL thickness (ε > 0.001 cm), conversion of CO2 occurs mainly in the DBL and, therefore, becomes rate limiting. This is corroborated by the observation that upon addition of the enzyme carbonic anhydrase, which catalyzes CO2-conversion, the dissolution rates are enhanced by 1 order of magnitude. From our experimental observations we conclude that the theoretical model above enables one to predict dissolution rates with satisfactory precision. Since the precipitation rates from supersaturated solutions are determined by the same mechanisms as dissolution, we infer that this model is also valid to predict precipitation rates. The predicted rates for both dissolution and precipitation can be approximated by a linear rate law R = α . (ceq - c, where ceq) is the equilibrium concentration with respect to calcite and α a rate constant, dependent on temperature, PCO2, DBL thickness (ε), and the thickness of the water sheet flowing on the mineral. Values of α are listed that can be used for a variety of geologically relevant conditions.