Calcite dissolution kinetics in view of Gibbs free energy, dislocation density, and pCO2

The dissolution of calcite in a full range of saturation conditions is investigated to explore the kinetic effect of Gibbs free energy, dislocation density, and the presence of atmospheric pCO(2). Experiments are carried out in a mixed-flow reactor at room temperature (25 degrees C) in both closed and open (to air) settings, and calcite samples are prepared by fragmentation and milling to generate different defect densities. Experimental observations show a highly nonlinear dependence of the dissolution rates on the Gibbs free energy; however, the kinetics does not seem to be affected by the samples' dislocation density, nor the presence of atmospheric pCO(2) at any saturation condition. Fitting the conventional transition state model (TST) to the observed rate - free energy relationship indicates that, though the TST rate equation is sufficient to describe the dissolution kinetics near and far from equilibrium, it clearly overestimates the dissolution rate when the system sits in between. These results suggest that: (i) the classic TST model may not be sufficient to depict the relation between dissolution rate and Gibbs free energy once solution saturation falls from its extrema. (ii) The steps associated with increased crystal defects may be overwhelmed by those regenerated at corners and edges of calcite particles through layer-by-layer dissolution along the cleavage directions. (iii) The presence of CO2 in ambient environments bears little importance to calcite dissolution possibly due to the slow response of aqueous HCO3- to pCO(2) change at low CO2 partial pressure conditions. (C) 2012 Elsevier B.V. All rights reserved.