Thermodynamic model for aqueous Cd2+−CO32− ionic interactions in high-ionic-strength carbonate solutions, and the solubility product of crystalline CdCO3

The aqueous solubility of CdCO3(c) was studied in 0.01M NaClO4, in solutions equilibrated with N2-CO2(g) mixtures that contained 0.0003, 0.001, or 0.138 atmospheres of CO2(g). The pH ranged from about 4.5 to 9, and the studies were conducted from both the oversaturation and the undersaturation directions, with the equilibration periods ranging from 6 to 57 d. To determine the carbonato complexes of Cd(II), the solubility of CdCO3(c) was also studied in 0.0016 to 1.00M Na2CO3 solutions at fixed hydroxide ion concentration, and in solutions with fixed aqueous C concentrations (0.1 and 0.01M C) over a range of OH− from 1×10−5 to 1.0M. The equilibrium Cd concentrations were reached in less than about 6 d. Pitzer's ion-interaction model was used to interpret these solubility data, as well as CdCO3(c) solubility data reported in the literature, which extended to 5.0M K2CO3 with an ionic strength of approximately 18.6 m. Our thermodynamic model required the introduction of two aqueous Cd2+-CO 3 2− complexes, CdCO3(aq) and Cd(CO3) 2 2− , as well as ion-interaction parameters for Cd(CO3) 2 2− with the bulk cations Na+ and K+. This model gave excellent agreement with all available experimental data extending to 5.0M K2CO3. The logarithms of the thermodynamic equilibrium constants for the reactions: $$\begin{gathered} CdCO_3 \left( c \right) \rightleftarrows Cd^{2 + } + CO_3^{2 - } \hfill \\ Cd^{2 + } + CO_3^{2 - } \rightleftarrows CdCO_3 \left( {aq} \right) \hfill \\ Cd^{2 + } + 2CO_3^{2 - } \rightleftarrows Cd\left( {CO_3 } \right)_2^{2 - } \hfill \\ \end{gathered} $$ were found to be −12.24±0.1, 4.71±0.1, and 6.49±0.1, respectively. The β0 ion-interaction parameters for Cd(CO3) 2 2− −Na+ and Cd(CO3) 2 2− −K+ were found to be −0.14 and −0.06, respectively.