THE KINETICS OF THE REACTION CO2-]H++HCO3- AS ONE OF THE RATE-LIMITING STEPS FOR THE DISSOLUTION OF CALCITE IN THE SYSTEM H2O-CO2-CACO3(H2O)

Dissolution of CaCO3 in the system H2O-CO2-CaCO3 is controlled by thre e rate-determining processes: The kinetics of dissolution at the miner al surface, mass transport by diffusion, and the slow kinetics of the reaction H2O + CO2 = H+ + HCO3-. A theoretical model of Buhmann and Dr eybrodt (1985a,b) predicts that the dissolution rates depend criticall y on the ratio V/A of the volume V of the solution and the surface are a A of the reacting mineral. Experimental data verifying these predict ions for stagnant solutions have been already obtained in the range 0. 01 cm < V/A < 0.1 cm. We have performed measurements of dissolution ra tes in a porous medium of sized CaCO3 particles for V/A in the range o f 2 . 10(-4) cm and 0.01 cm in a system closed with respect to CO2 usi ng solutions pre-equilibrated with an initial partial pressure of CO2 of 1 . 10(-2) and 5 . 10(-2) atm. The results are in satisfactory agre ement with the theoretical predictions and show that especially for V/ A < 10(-3) cm 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 mu molar solutions of carbonic anhydrase, an enzyme enhancin g the CO2-conversion rates by several orders of magnitude. In these ex periments CO2 conversion is no longer rate limiting and consequently t he dissolution rates of CaCO3 increase significantly. We have also per formed batch experiments at various initial pressures of CO2 by stirri ng sized calcite particles in a solution with V/A = 0.6 cm and V/A = 0 .038 cm. 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 a re valid for concentrations c less than or equal to 0.9 c(eq). Summari sing we find good agreement between experimental and theoretically pre dicted 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.