NEPTUNIUM(V) HYDROLYSIS AND CARBONATE COMPLEXATION - EXPERIMENTAL AND PREDICTED NEPTUNYL SOLUBILITY IN CONCENTRATED NACL USING THE PITZER APPROACH

摘要

The solid-liquid equilibrium of Np(V) has been studied in NaCl at 25 degrees C and 0.01 atm CO2. 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 NaNpO2CO3 . nH(2)O at [CO32-] < 10(-3) M and Na3NpO2(CO3)(2) . nH(2)O at [CO32-] > 10(-3) M were found as solubility-limiting solid equilibrium phases. Pure Np(V) carbonate 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 NaClO4 solutions indicated a stabilization of Np(V) in solution due to the interaction with chloride ions. Absorption spectra of NpO2+ and NpO2CO3- in 5 M NaCl were shifted slightly towards higher wavelengths, also suggesting an interaction with chloride ions. The Fitter 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 NaNpO2CO3 . nH(2)O (n = 3.5) to determine the chemical potential of this solid improved the agreement between predicted solubility and experimental data. NaNpO2CO3 . nH(2)O and NaAmO2CO3 . nH(2)O showed the same solubility in 3 and 5 M NaCl. [References: 32]