Supercritical fluid extraction and separation of uranium from other actinides.

The feasibility of separating U from nitric acid solutions of mixed actinides using tri-n-butylphosphate (TBP)-modified supercritical fluid carbon dioxide (sc-CO2) was investigated. The actinides U, Np, Pu, and Am were extracted into sc-CO2 modified with TBP from a range of nitric acid concentrations, in the absence of, or in the presence of, a number of traditional reducing and/or complexing agents to demonstrate the separation of these metals from U under sc-CO2 conditions. The separation of U from Pu using sc-CO2 was successful at nitric acid concentrations of less than 3M in the presence of acetohydroxamic acid (AHA) or oxalic acid (OA) to mitigate Pu extraction, and the separation of U from Np was successful at nitric acid concentrations of less than 1M in the presence of AHA, OA, or sodium nitrite to mitigate Np extraction. Americium was not well extracted under any condition studied.

Uranium dioxide in ionic liquid with a tri-n-butylphosphate-HNO3 complex--dissolution and coordination environment.

Uranium dioxide can be dissolved directly in an imidazolium-based ionic liquid (IL) at room temperature with a tri-n-butylphosphate(TBP)-HNO(3) complex. The dissolution process follows pseudo first-order kinetics initially. Raman spectroscopic studies show the dissolved uranyl ions are coordinated with TBP in the IL phase with a molar ratio of (UO(2))(2+) : TBP = 1 : 2. The dissolved uranyl species can be effectively transferred to a supercritical fluid carbon dioxide (sc-CO(2)) phase. No aqueous phase is formed in either the IL dissolution or the supercritical fluid extraction process. Absorption spectra of the extracted uranyl species in the sc-CO(2) phase suggests the presence of a UO(2)(TBP)(2)(NO(3))(2) and HNO(3) adduct probably of the form UO(2)(TBP)(2)(NO(3))(2)·HNO(3). The adduct dissociates in a water-dodecane trap solution during pressure reduction resulting in UO(2)(TBP)(2)(NO(3))(2) collected in the dodecane phase.

Characterization of uranyl(VI) nitrate complexes in a room temperature ionic liquid using attenuated total reflection-Fourier transform infrared spectrometry.

Room temperature ionic liquids form potentially important solvents in novel nuclear waste reprocessing methods, and the solvation, speciation, and complexation behaviors of actinides and lanthanides in room temperature ionic liquids is of current interest. In this study, the coordination environment of uranyl(VI) in solutions of the room temperature ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide containing either tetrabutylammonium nitrate or nitric acid was characterized using attenuated total reflection-Fourier transform infrared spectrometry. Both UO(2)(NO(3))(2) and UO(2)(NO(3))(3)(-) species were detected in solutions containing tetrabutylammonium nitrate. ν(as)(UO(2)) for these two species were found to lie at 951 and 944 cm(-1), respectively, while ν(as)(UO(2)) arising from uranyl(VI) coordinated by bis(trifluoromethylsulfonyl)imide anions in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was found to lie at 968 cm(-1). In solutions containing nitric acid, only UO(2)(NO(3))(2) was detected, due to the high water content. The UO(2)(NO(3))(+) species was not detected under the conditions used in this study. From the results shown here, we conclude that infrared spectroscopy forms a valuable addition to the suite of tools currently used to study the chemical behavior of uranyl(VI) in room temperature ionic liquids.