The effect of added dissolved calcium on calcite dissolution kinetics in aqueous solutions at 25°C

Abstract

The kinetics of calcite dissolution in solutions containing dissolved Ca 2+ has been investigated at 25°C, using a rotating disc apparatus. In acid solutions no effect of Ca 2+ in solution is observed. The rate is dependent on the transport of H + to the surface. In neutral to alkaline solutions the dissolution reaction is controlled by mixed kinetics and the conventional empirical representation of the dissolution rate results through the interaction of chemical and transport gradients in the diffusion boundary layer. The chemical reaction rate is a function of the gradient between the equilibrium and the surface concentrations of calcium carbonate, whereas the transport reaction can be described in terms of a series of gradients between the surface and the bulk of dissolved calcium and carbonate species. The presence of dissolved Ca 2+ decreases the rate of the transport reaction, making the dissolution process more transport-controlled. The chemical rate constant is independent of the Ca 2+ concentration in solution. The chemical rate constant for Carrara marble dissolution is determined to ∼2·10 −2 cm s −1 and the diffusion coefficient for the transport reaction to ∼7.6·10 −6 cm 2 s −1. In natural systems with high Ca 2+ concentrations and in absence of inhibitors of the surface chemical reaction, the dissolution of calcite may approach a transport-controlled reaction, especially in environments with restricted flow.