Dissolution and Solubility of the Calcite-Otavite Solid Solutions [(Ca1-xCdx)CO3] at 25 degrees C

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 N-2-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 mu mol/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 mu mol/L after 5040 h of dissolution, respectively. For the dissolution in the N-2-degassed water at 25 degrees C, the mean solubility products (log K-sp) and the Gibbs free energies of formation (Delta G(f)(theta)) 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 K-sp values decreased non-linearly, and the Delta G(f)(theta) values increased linearly with the increasing Cd/(Ca+Cd) mole ratio (X-Cd) 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 a(0) = -0.84 and a(1) = -3.80 for the dissolution in the N-2-degassed water or a(0) = -1.12 and a(1) = -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.