A model for trace metal sorption processes at the calcite surface: Adsorption of Cd 2+ and subsequent solid solution formation

Abstract

The rate of Cd 2+ sorption by calcite was determined as a function of pH and Mg 2+ in aqueous solutions saturated with respect to calcite but undersaturated with respect to CdCO 3. The sorption is characterized by two reaction steps, with the first reaching completion within 24 hours. The second step proceeded at a slow and nearly constant rate for at least 7 days. The rate of calcite recrystallization was also studied, using a Ca 2+ isotopic exchange technique. Both the recrystallization rate of calcite and the rate of slow Cd 2+ sorption decrease with increasing pH or with increasing Mg 2+. The recrystallization rate could be predicted from the number of moles of Ca present in the hydrated surface layer. A model is presented which is consistent with the rates of Cd 2+ sorption and Ca 2+ isotopic exchange. In the model, the first step in Cd 2+ sorption involves a fast adsorption reaction that is followed by diffusion of Cd 2+ into a surface layer of hydrated CaCO 3 that overlies crystalline calcite. Desorption of Cd 2+ from the hydrated layer is slow. The second step is solid solution formation in new crystalline material, which grows from the disordered mixture of Cd and Ca carbonate in the hydrated surface layer. Calculated distribution coefficients for solid solutions formed at the surface are slightly greater than the ratio of equilibrium constants for dissolution of calcite and CdCO 3, which is the value that would be expected for an ideal solid solution in equilibrium with the aqueous solution.