A Continuous and Mechanistic Representation of Calcite Reaction-Controlled Kinetics in Dilute Solutions at 25 degrees C and 1 Atm Total Pressure

Calcite dissolution rates were measured using a free-drift technique at 25 degrees C, 1 atm total pressure, and various Pco, in deionized water. The data were corrected for gas phase disequilibrium and fitted to a kinetic expression derived by coupling the mechanistic models of Plummer etal. (1987a) and Chou etal. (1989) to the surface complexation model of Van Cappellen eral. (1993). Corrected dissolution and precipitation rate measurements from previous investigations were combined to our data set and fitted to the same expression. The following reactions provide an adequate description of the calcite dissolution and precipitation mechanism in dilute solutions: > CO3- + 2H(+) -> <-(k1)(k5) -> > Ca+ + H2CO3 (1a) > Ca+ + H2CO3 -> <- > CO3H0 + CaHCO3+ (1b) > CO3H0 + CaHCO3+ -> <-> Ca+ + H2CO3 + CaCO3 (1c) CaCO3(Solid) < CaCO3, for which the overall reaction rate is given by Rate = k(1) > CO3- (a(H)+)(2) + (k(2)-k(5)) > Ca+ aH(2)co; + k(4) - (k(6) - k(3)) where > i are the densities of surface complexes (mol/m(2)), a(i) are the activities of dissolved species and, k(i) are the rate constants corresponding to the above reactions. This rate equation satisfies the principle of microscopic reversibility and applies to both dissolution and precipitation reactions over a wide range of Pco(2), pH and saturation states. The rate constants obtained from fitting the data set to Equation (3) are compatible with values reported by Plummer et al. and Chou et al., as well as yielding a very good estimate of the thermodynamic solubility constant of calcite, K-sp(0).