Solubility of PuO₂(am,hyd) and the Formation of Pu(IV) Carbonate Complexes in Carbonate Solutions Containing 0.1–5.0 mol·dm⁻³ NaNO₃

Abstract

The solubility of plutonium hydrous oxide (PuO₂(am, hyd)) was investigated in the presence of carbonate in neutral to alkaline, dilute to concentrated sodium nitrate (NaNO₃) solutions. The observed dependence of the Pu(IV) solubility in low to medium concentration NaNO₃ agreed with previously reported data and was well reproduced by the thermodynamic model curves, containing the species Pu(CO₃)₄⁴⁻, Pu(CO₃)₅⁶⁻, and Pu(OH)₂CO₃)₂²⁻. Significant deviations between experimental and modeled Pu(IV) solubility were, however, observed for 2 mol.dm⁻³ NaNO₃ solutions. A more comprehensive chemical model including the complex Pu(OH)₂(CO₃)₄⁴⁻ was established, that reproduces the experimental results reasonably well. The conditional complex formation constants for Pu(OH)₂(CO₃)₂⁴⁻ were extrapolated to I = 0 using the specific ion interaction theory (SIT) and Pitzer approaches, and the corresponding ion interaction parameters were evaluated.