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.

The Capture of Cadmium from Solution during the Replacement of Calcite by Apatite

“…We predicted the formation of a Cd-bearing phosphate phase due to the similar ionic sizes of Ca 2+ and Cd 2+ , which provides a good exchange capacity as seen with the (Ca,Cd)CO 3 solid solution, which can theoretically form a perfect solid solution. 53,54 Raman analysis of the sample surface could not provide information about the composition of the precipitate layer, as it is too thin compared to the penetration depth of the Raman laser used. However, after reaction observations by SEM and EDX showed the presence of Cdcontaining crystals, probably HAP and/or TCP, on the surface of the samples reacted in a 0.5 M NH 4 H 2 PO 4 and 5 mM CdCl 2 solution.…”

Carbonate Coprecipitation for Cd and Zn Treatment and Evaluation of Heavy Metal Stability Under Acidic Conditions

“…Cadmium ions have a similar ionic radius to Ca ions (94 vs 100 pm, respectively), whereas Zn ions are significantly smaller (74 pm), making these elements compatible with calcite. As a result, otavite can form a continuous solid solution with calcite for all possible mole fractions, whereas smithsonite forms a solid solution with calcite with immiscibility above 0.2 mole fraction . Factors such as the ratio of the solubility products, the distribution coefficient, , the degree of supersaturation, temperature, nucleation rates, charge similarity, and metal complexes influence the extent of metal incorporation.…”