THE KINETICS OF THE REACTION CO2 + H2O → H+ + HCO-3 AS ONE OF THE RATE LIMITING STEPS FOR THE DISSOLUTION OF CALCITE IN THE SYSTEM H2O-CO2-CACO3

Abstract

Dissolution of CaCO3 in the system H2O-CO2-CaCO3 is controlled by three rate-determining processes: The kinetics of dissolution at the mineral surface, mass transport by diffusion, and the slow kinetics of the reaction H2O + CO2 = H+ + HCO3−. A theoretical model of Buhmann and Dreybrodt (1985a, b) predicts that the dissolution rates depend critically on the ratio of the volume V of the solution and the surface area A of the reacting mineral. Experimental data verifying these predictions for stagnant solutions have been already obtained in the range . We have performed measurements of dissolution rates in a porous medium of sized CaCO3 particles for in the range of 2·10−4 cm and 0.01 cm in a system closed with respect to CO2 using solutions pre-equilibrated with an initial partial pressure of CO2 of 1·10−2 and 5·10−2 atm. The results are in satisfactory agreement with the theoretical predictions and show that especially for dissolution is controlled entirely by conversion of CO2 into H+ and HCO3−, whereas in the range from 10−3 cm up to 10−1 cm both CO2-conversion and molecular diffusion are the rate controlling processes. This is corroborated by performing dissolution experiments using 0.6 μmolar solutions of carbonic anhydrase, an enzyme enhancing the CO2-conversion rates by several orders of magnitude. In these experiments CO2 conversion is no longer rate limiting and consequently the dissolution rates of CaCO3 increase significantly. We have also performed batch experiments at various initial pressures of CO2 by stirring sized calcite particles in a solution with and . These data also clearly show the influence of CO2-conversion on the dissolution rates. In all experiments inhibition of dissolution occurs close to equilibrium. Therefore, the theoretical predictions are valid for concentrations c ≤ 0.9 ceq. Summarising we find good agreement between experimental and theoretically predicted dissolution rates. Therefore, the theoretical model can be used with confidence to find reliable dissolution rates from the chemical composition of a solution for a wide field of geological applications.