The kinetics of the reaction CO 2 + H 2 O H + + HCO 3 - as one of the rate limiting steps for the dissolution of calcite in the system H 2 O---CO 2 ---CaCO 3

Details The kinetics of the reaction CO 2 + H 2 O H + + HCO 3 - as one of the rate limiting steps for the dissolution of calcite in the system H 2 O---CO 2 ---CaCO 3 The kinetics of the reaction CO 2 + H 2 O H + + HCO 3 - as one of the rate limiting steps for the dissolution of calcite in the system H 2 O---CO 2 ---CaCO 3

Sep 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996gecoa..60.3375d&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 60, Issue 18, pp.3375-3381

Mathematics

Logic

13

Scientific paper

Dissolution of CaCO 3 in the system H 2 O---CO 2 ---CaCO 3 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 H 2 O + CO 2 = H + + HCO 3 - . A theoretical model of Buhmann and Dreybrodt (1985a, b) predicts that the dissolution rates depend critically on the ratio V / A 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 0.01 cm < V / A < 0.1 cm. We have performed measurements of dissolution rates in a porous medium of sized CaCO 3 particles for V / A in the range of 2·10 -4 cm and 0.01 cm in a system closed with respect to CO 2 using solutions pre-equilibrated with an initial partial pressure of CO 2 of 1·10 -2 and 5·10 -2 atm. The results are in satisfactory agreement with the theoretical predictions and show that especially for V / A < 10 -3 cm dissolution is controlled entirely by conversion of CO 2 into H + and HCO 3 - , whereas in the range from 10 -3 cm up to 10 -1 cm both CO 2 -conversion and molecular diffusion are the rate controlling processes. This is corroborated by performing dissolution experiments using 0.6 molar solutions of carbonic anhydrase, an enzyme enhancing the CO 2 -conversion rates by several orders of magnitude. In these experiments CO 2 conversion is no longer rate limiting and consequently the dissolution rates of CaCO 3 increase significantly. We have also performed batch experiments at various initial pressures of CO 2 by stirring 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 CO 2 -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 c eq . 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.

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