THE SOLUBILITY OF CEO₂ IN NANO₃ MEDIUM

Large quantities of plutonium, actinides, and fission products are in present in high level waste tanks at locations such as Hanford, Washington and Savannah River, South Carolina. These wastes are characterized by high pH values, elevated temperatures, and the presence of various organic ligands. Knowledge of the chemistry of plutonium and the actinides in these tank systems is a prerequisite to removal and treatment of this waste. The hydrolysis and solubility of actinides (IV) in the High Level Nuclear Waste tank systems should be understood. To better understand hydrolysis of tetravalent actinides, we use Ce (IV) as a chemical analogue because the ionic radius of Ce (IV) is close to the ionic radius of Pu (IV). Various experiments were conducted at 25 °C, a pH range from 2.5 to 13, and ionic strength of 0.1 and 1.0 molal in NaNO₃ medium. After seventy (70) days, the experiments with low pH values appear to have reached equilibrium. The solubility of CeO₂ in 1.0 molal NaNO₃ at pH=2.47 is 36 mg/L (± 2.16) Ce whereas in 0.1 molal NaNO₃ at pH=2.68 it is 24 mg/L (± 1.92) Ce. The solubility of CeO₂ in 1.0 molal NaNO₃ at pH=3.83 is 6.8 mg/L (± 0.476) Ce whereas in 0.1 molal NaNO₃ at pH=3.91 is 4.3 mg/L (± 0.344) Ce. The solubility of CeO₂ in NaNO₃ medium is shown to be strongly dependent on acidity, and moderately dependent on ionic strength. The slope of a plot of log m_Ce vs. pH suggests that Ce(OH)₃⁺ and Ce(OH)₄⁰ are the predominant species under these conditions. Cerium oxide residues in the experiment were examined by XRD and they exhibited the same XRD pattern as that of the original cerium oxide powder. Experiments at higher pH values are still in progress.