Thermodynamic model for the solubility of Cr(OH)3(am) in concentrated NaOH and NaOH-NaNO3 solutions

The main objective of this study was to develop a thermodynamic model for predicting Cr(III) behavior in concentrated NaOH and in mixed NaOH–NaNO3 solutions for application to developing effective caustic leaching strategies for high-level nuclear waste sludges. To meet this objective, the solubility of Cr(OH)3(am) was measured in 0.003 to 10.5 m NaOH, 3.0 m NaOH with NaNO3 varying from 0.1 to 7.5 m, and 4.6 m NaNO3 with NaOH varying from 0.1 to 3.5 m at room temperature (22 ± 2°C). A combination of techniques, X-ray absorption spectroscopy (XAS) and absorptive stripping voltammetry analyses, were used to determine the oxidation state and nature of aqueous Cr. A thermodynamic model, based on the Pitzer equations, was developed from the solubility measurements to account for dramatic increases in aqueous Cr with increases in NaOH concentration. The model includes only two aqueous Cr species, Cr(OH)4− and Cr2O2(OH)4− (although the possible presence of a small percentage of higher oligomers at >5.0 m NaOH cannot be discounted) and their ion–interaction parameters with Na+. The logarithms of the equilibrium constants for the reactions involving Cr(OH)4− [Cr(OH)3(am) + OH− ⇌ Cr(OH)4−] and Cr2O2(OH)42− [2Cr(OH)3(am) + 2OH− ⇌ Cr2O2(OH)42− + 2H2O] were determined to be −4.36 ± 0.24 and −5.24 ± 0.24, respectively. This model was further tested and provided close agreement between the observed Cr concentrations in equilibrium with Cr(OH)3(am) in mixed NaOH–NaNO3 solutions and with high-level tank sludges leached with and primarily containing NaOH as the major electrolyte.