SOLUBILITY OF NP(V) IN K-CL-CO3 AND NA-K-CL-CO3 SOLUTIONS TO HIGH-CONCENTRATIONS - MEASUREMENTS AND THERMODYNAMIC MODEL PREDICTIONS

The solubility of Np(V) in K-Cl-CO3 and Na-K-Cl-CO3 solutions was meas ured to confirm a published thermodynamic model for Np(V) solubility a nd solubility-controlling solid phases. Undersaturation experiments st arting with NpO2OH(am), KNpO2CO3(s), or Na3NpO2(CO3)(2)(s) were conduc ted in KCl + K2CO3 and NaCl + KCl + Na2CO3 solutions to high electroly te concentrations. Steady-state dissolved Np(V) concentrations were ob tained within 180 days, at which time the final solid phases were char acterized by X-ray diffraction techniques. The thermodynamic model and data base, based on the Fitter ion interaction formalism for activity coefficients, has been parameterized in the NaCl, NaClO4, NaCl-CO2(g) , NaClO4-CO2(g), Na2CO3, and K2CO3 systems but has not been parameteri zed using data from K-CI-CO, and Na-K-Cl-CO3 solutions. Predictions fr om the thermodynamic model match measured Np(V) solubilities in K-Cl-C O3 solutions to within 0.3 log units, confirming recent parameter valu es. Np(V) solubilities in Na-K-Cl-CO3 solutions agree within 0.5 log u nits when KNpO2CO3(s) was the equilibrium solid phase. A deviation of up to 0.8 log units resulted when Na3NpO2(CO3)(2)(s) was the equilibri um solid phase. Additionally, the data base provides satisfactory pred ictions of oversaturation solubility experiments in three complex synt hetic brines. This work demonstrates that thermodynamic models can pro vide reliable predictions of Np(V) solubility in concentrated groundwa ters such as may exist in the near- and far-field environments of nucl ear waste repositories located in salt beds.