title: Insights into the mechanism of extraction of uranium (VI) from nitric acid solution into an ionic liquid by using tri-n-butyl phosphate.
abstract: 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/[C4 mim][Tf2 N]) 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 C4 mim(+) . At high acid concentrations, the extraction proceeds through a cationic exchange between [UO2 (NO3 )(HNO3 )(TBP)2 ](+) and one C4 mim(+) . As a consequence of this mechanism, the variation of DU as a function of TBP concentration depends on the C4 mim(+) 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.
authors.affiliation: Institut de Physique Nucléaire de Lyon, Université de Lyon, CNRS-IN2P3, 69622 Villeurbanne cedex (France). c.gaillard@ipnl.in2p3.fr.
authors.forename: Dr Clotilde
authors.initials: DC
authors.lastname: Gaillard
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).
authors.forename: Maria
authors.initials: M
authors.lastname: Boltoeva
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).|Univ. Grenoble Alpes, LEPMI, F-38000 Grenoble (France).|CNRS, LEPMI, F-38000 Grenoble (France) (present address).
authors.forename: Isabelle
authors.initials: I
authors.lastname: Billard
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).
authors.forename: Sylvia
authors.initials: S
authors.lastname: Georg
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).
authors.forename: Valérie
authors.initials: V
authors.lastname: Mazan
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).
authors.forename: Ali
authors.initials: A
authors.lastname: Ouadi
authors.affiliation: Université de Strasbourg, IPHC, 23 rue du Lœss, 67037 Strasbourg (France).|CNRS, UMR 7178, 67037 Strasbourg (France).|Institute of Organic Chemistry, NASU, Kyiv (Ukraine).
authors.forename: Dariia
authors.initials: D
authors.lastname: Ternova
authors.affiliation: Helmholtz-Zentrum Dresden-Rossendorf, Institute of Resource Ecology, P.O. Box 510119, 01314 Dresden (Germany).
authors.forename: Christoph
authors.initials: C
authors.lastname: Hennig
keywords: ionic liquids; solvent extraction; tri-n-butyl phosphate; uranium