Measurement and thermodynamic modeling of Np(V) solubility in aqueous K2CO3 solutions to high concentrations

The solubility of Np(V) solids was studied in 0.01 to 6.38 mol-kg-1 K2CO3 solutions. Two series of experiments were conducted from undersaturation using NpO2OH(s) or KNpO2CO3(S) as the starting solid. The Np(V) solid phases observed at steady state are KNpO2CO3(s) and K3NpO2(CO3)2(s). The activity coefficient formalism of Pitzer was used to obtain a thermodynamic interpretation of these data, including the solubility constants: KNpO2CO3(s) K+ + NpO2+ + CO2-3 log Ks = -13.6±0.1(2σ) K3NpO2(CO3)2(s) 3K+ + NpO+2 + 2CO2-3 log Ks = -15.9±0.1(2σ) To model these data, we propose values for several binary and ternary ion interaction parameters among the carbonate ion, potassium ion, and the neptunyl triscarbonato ion. With these new parameters, the thermodynamic data base for Np(V) solubility in concentrated aqueous solutions has been extended to the NaK-Cl-ClO4-CO3-H-OH-H2O system.