Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by using Tri-n-butyl phosphate

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/[C(4)mim][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 D-U. 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 C(4)mim(+). At high acid concentrations, the extraction proceeds through a cationic exchange between [UO2(NO3)(HNO3)(TBP)(2)](+) and one C(4)mim(+). As a consequence of this mechanism, the variation of D-U as a function of TBP concentration depends on the C(4)mim(+) concentration in the aqueous phase. This explains why noninteger values are often derived by analysis of D-U versus [TBP] plots to determine the number of TBP molecules involved in the extraction of uranyl in an ionic-liquid phase.