Neptunium(V) hydrolysis and carbonate complexation: Experimental and predicted neptunyl solubility in concentrated NaCl using the Pitzer approach

The solid-liquid equilibrium of Np (V) has been studied in NaCl at 25°C and 0.01 atm CO 2. The equilibrium solids were characterized using X-ray powder diffraction, and the Np (V) solution species were characterized using NIR absorption spectroscopy. The solid phases NaNPO 2CO 3 · nH 2O at [CO 3] < 10 −3 M and Na 3NPO 2(CO 3) 2 · nH 2O at [CO 3] > 10 −3 M were found as solubility-limiting solid equilibrium phases. Pure Np (V) carbonato complexes were formed in solution; hydroxo, bicarbonato, or mixed hydroxocarbonato Np (V) complexes were determined not to be significant. The comparison of Np (V) solubility data in NaCl and NaClO 4 solutions indicated a stabilization of Np (V) in solution due to the interaction with chloride ions. Absorption spectra of NpO 2 + and NpO 2CO 3 in 5 M NaCl were shifted slightly towards higher wavelengths, also suggesting an interaction with chloride ions. The Pitzer approach was applied to parameterize experimental data and to predict Np (V) solubility in brine solutions. The activity of water changes with electrolyte concentration; thus incorporation of the reported molecules of hydration water in NaNPO 2CO 3 · nH 2O ( n = 3.5) to determine the chemical potential of this solid improved the agreement between predicted solubility and experimental data. NaNPO 2CO 3 · nH 2O and NaAmO 2CO 3 · nH 2O showed the same solubility in 3 and 5 M NaCl.