Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH--CO2- HCO3-- CO32--H2O System: A Thermodynamic Model

Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3-13), at two different fixed partial pressures of CO2(gas) (0.003 or 0.03 atm.), and as functions of K2CO3 concentrations (0.01 to 5.8 mol kg-1) in the presence of 0.01 mol dm-3 KOH and KHCO3 concentrations (0.001 to 0.826 mol kg-1) at room temperature (22 ± 2°C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH) (CO3)22-) and Cr(OH)4CO33-] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO2(gas) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log10 K0 values of reactions involving these species [{Cr(OH)3(am) + 2CO2(gas) = Cr(OH)(CO3)22- + 2H+}and {Cr(OH)3(am) + OH- + CO32- = Cr(OH)4(CO3)3-}] were found to be –(19.07 ± 0.41), -(4.19 ± 0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.