PRECIPITATION KINETICS OF CALCITE IN THE SYSTEM CACO3-H2O-CO2 - THE CONVERSION TO CO2 BY THE SLOW PROCESS H--]CO2+H2O AS A RATE-LIMITING STEP(+HCO3)

Precipitation rates of CaCO3 from supersaturated solutions in the H2O - CO2 - CaCO3 system are controlled by three rate-determining processe s: the kinetics of precipitation at the mineral surface, mass transpor t of the reaction species involved to and from the mineral surface, an d the slow kinetics of the overall reaction HCO3- + H+ --> CO2 + H2O. A theoretical model by Buhmann and Dreybrodt (1985a,b) taking these pr ocesses into account predicts that, due to the slow kinetics of this r eaction, precipitation rates to the surface of CaCO3 minerals depend c ritically on the ratio V/A of the volume V of the solution to the surf ace area A of the mineral in contact with it, for both laminar and tur bulent flow. We have performed measurements of precipitation rates in a porous medium of sized particles of marble, limestone, and synthetic calcite, with V/A ratios ranging from 3.10(-4) to 1.2-10(-2) cm at 10 degrees C. Calcite was precipitated from supersaturated solutions wit h [Ca2+] approximate to 4 mmol/L and an initial P-CO2 of 5.10(-3) or 1 .10(-3) atm, respectively, using experimental conditions which prevent ed exchange of CO2 with the atmosphere, i.e., closed system. The resul ts are in qualitative agreement with the theoretical predictions. Agre ement with the observed data, however, is obtained by modifying the ra te law of Plummer et al. (1978) to take into account surface-controlle d inhibition effects. Experiments with supersaturated solutions contai ning carbonic anhydrase, an enzyme which enhances the conversion of HC O3- into CO2, yield rates increased by a factor of up to 15. This prov ides for the first time unambiguous experimental evidence that this re action is rate limiting. We have also measured precipitation rates in batch experiments, stirring sized mineral particles in a solution with V/A ranging from 0.03 to 0.75 cm. These experiments also give clear e vidence on the importance of the conversion of HCO3- into CO2 as rate limiting step. Taken together our experiments provide evidence that th e theoretical model of Buhmann and Dreybrodt (1985a,b) can be used to predict reliable rates from the composition of Ca++-HCO3- solutions wi th low ionic strength in many geologically relevant situations. Copyri ght (C) 1997 Elsevier Science Ltd.