Solubility and Complexation of the Bismuthate Ion in Nitric Acid Systems

The dissolution rate and solubility of NaBiO₃ have been investigated in nitric acid systems ranging from 4 to 6 M HNO₃ and were found to be 58-76 μg/cm²·d and 490-830 mM, respectively. The presence of 50 mM U(VI) drastically increased the solubility to 540-1200 mM, while rates of dissolution were relatively unchanged. The solubility of NaBiO₃ increased with an increase in U(VI) concentrations at 4 M HNO₃, with log-log analysis indicating a one-to-one complex between Bi and U and infrared spectroscopic evidence monitoring uranyl stretching, suggesting complex formation. Absorbance spectra were obtained experimentally and computationally with an absorbance band in the range of 450-600 nm that has been attributed to Bi(V). The ingrowth and decay of Bi(V) in solution was also studied as a function of mass of solid NaBiO₃ present, acidity, and temperature. The activation energies of dissolution and decomposition were calculated to be 39 ± 4 and 61 ± 6 kJ/mol, respectively. These results indicate that dissolution of NaBiO₃ into the respective Na⁺ and BiO₃^-occurs prior to undergoing reduction, a process which conventionally has been believed to occur in the reverse order.