Abstract:
Precipitation rates of CaCO3 from supersaturated solutions in the H2O - CO2 - CaCO3 system are controlled by three rate-determining processes: the kinetics of precipitation at the mineral surface, mass transport of the reaction species involved to and from the mineral surface, and the slow kinetics of the overall reaction HCO-3 + H+ -> CO2 + H2O. A theoretical model by Buhmann and Dreybrodt (1985a,b) taking these processes into account predicts that, due to the slow kinetics of this reaction, precipitation rates to the surface of CaCO3 minerals depend critically on the ratio VA of the volume V of the solution to the surface area A of the mineral in contact with it, for both laminar and turbulent flow. We have performed measurements of precipitation rates in a porous medium of sized particles of marble, limestone, and synthetic calcite, with VA ratios ranging from 3 . 10-4 to 1.2 . 10-2 cm at 10°C. Calcite was precipitated from supersaturated solutions with [Ca2+] # 4 mmol/L and an initial PCO2 of 5 . 10-3 or 1 . 10-3 atm, respectively, using experimental conditions which prevented exchange of CO2 with the atmosphere, i.e., closed system. The results are in qualitative agreement with the theoretical predictions. Agreement with the observed data, however, is obtained by modifying the rate law of Plummer et al. (1978) to take into account surface-controlled inhibition effects. Experiments with supersaturated solutions containing carbonic anhydrase, an enzyme which enhances the conversion of HCO-3 into CO2, yield rates increased by a factor of up to 15. This provides for the first time unambiguous experimental evidence that this reaction is rate limiting. We have also measured precipitation rates in batch experiments, stirring sized mineral particles in a solution with VA ranging from 0.03 to 0.75 cm. These experiments also give clear evidence on the importance of the conversion of HCO-3 into CO2 as rate limiting step. Taken together our experiments provide evidence that the theoretical model of Buhmann and Dreybrodt (1985a,b) can be used to predict reliable rates from the composition of Ca++-HCO-3 solutions with low ionic strength in many geologically relevant situations.
