Dissolution and Solubility of the Calcite–Otavite Solid Solutions [(Ca1−xCdx)CO3] at 25 °C

Abstract

A complete series of the calcite–otavite solid solutions [(Ca1−xCdx)CO3] were prepared, and their dissolution processes lasting nine months were experimentally investigated. For the dissolution in the N2-degassed water, the Ca concentrations of the aqueous phases increased up to the steady states after 5040 h of dissolution, and the Cd concentrations of the aqueous phases increased up to the highest values and then decreased gradually to the steady states of 0.017–6.476 μmol/L after 5040 h of dissolution. For the dissolution in the CO2-saturated water, the Ca and Cd concentrations of the aqueous phases increased up to the peak values and then decreased gradually to the steady states of 0.94–0.46 mmol/L and 0.046–9.643 μmol/L after 5040 h of dissolution, respectively. For the dissolution in the N2-degassed water at 25 °C, the mean solubility products (log Ksp) and the Gibbs free energies of formation (ΔGfθ) were estimated to be −8.45–−8.42 and −1129.65–−1129.48 kJ/mol for calcite [CaCO3] and −11.62–−11.79 and −671.81–−672.78 kJ/mol for otavite [CdCO3], respectively. Generally, the log Ksp values decreased non-linearly, and the ΔGfθ values increased linearly with the increasing Cd/(Ca+Cd) mole ratio (XCd) of the (Ca1−xCdx)CO3 solid solutions. In the Lippmann diagrams constructed for the sub-regular (Ca1−xCdx)CO3 solid solutions with the estimated Guggenheim coefficients a0 = −0.84 and a1 = −3.80 for the dissolution in the N2-degassed water or a0 = −1.12 and a1 = −3.83 for the dissolution in the CO2-saturated water, the (Ca1−xCdx)CO3 solid solutions dissolved incongruently, moved progressively up to the quasi-equilibrium curves for otavite and then along the quasi-equilibrium curve from right to left, approached the solutus curve and finally reached the minimum stoichiometric saturation curve for calcite. The considerably Cd-poor aqueous phases were finally in equilibrium with the CdCO3-rich solid phases.