Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri-n-butyl phosphate
Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri-n-butyl phosphate
Gaillard, C.; Boltoeva, M.; Billard, I.; Georg, S.; Mazan, V.; Ouadi, A.; Ternova, D.; Hennig, C.
We present new results on the liquid–liquid extraction of uranium (VI) from a nitric acid aqueous phase into a tri-n-butyl phosphate/1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (TBP/[C4mim][Tf2N]) phase. The individual solubilities of the ionic-liquid ions in the upper part of the biphasic system are measured over the whole acidic range and as a function of the TBP concentration. New insights into the extraction mechanism are obtained through the in situ characterization of the extracted uranyl complexes by coupling UV/Vis and extended X-ray absorption fine structure (EXAFS) spectroscopy. We propose a chemical model to explain uranium (VI) extraction that describes the data through a fit of the uranyl distribution ratio DU. In this model, at low acid concentrations uranium (VI) is extracted as the cationic complex [UO2(TBP)2]2+, by an exchange with one proton and one C4mim+. At high acid concentrations, the extraction proceeds through a cationic exchange between [UO2(NO3)(HNO3)(TBP)2]+ and one C4mim+. As a consequence of this mechanism, the variation of DU as a function of TBP concentration depends on the C4mim+ concentration in the aqueous phase. This explains why noninteger values are often derived by analysis of DU versus [TBP] plots to determine the number of TBP molecules involved in the extraction of uranyl in an ionic-liquid phase.
Keywords: ionic liquid; uranium; exafs
Involved research facilities
- Rossendorf Beamline at ESRF DOI: 10.1107/S1600577520014265
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Permalink: https://www.hzdr.de/publications/Publ-22409