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.

Extraction of uranium(VI) from nitric acid solutions using N,N-dihexyloctanamide in ionic liquids: Solvent extraction and spectroscopic studies

“…On the basis of the UV-vis spectrophotometric data, and taking advantage of the previous studies, [17] we suggest that at low acidities the extraction the behavior is explained in terms of the double cation-exchange mechanism as shown in Eq. (2): [9,10]…”

“…[18] In addition to the influence of slight changes in the alkyl chain length on the imidazolium cation (from 4 to 5) and in the TBP concentration (from 1.1 M to 1.2 M) from one experiment to the other, we think that this difference in the D values is mainly due to the change in the IL anion, from PF 6 for the work of Giridhar [16] to NTf 2 for the other two works. [17,27] In a separate study, Giridhar et al reported a D U value of ca. 6 under identical conditions (i.e., with 1.1 M TBP in [C 4 mim][NTf 2 ] at 4 M HNO 3 ), [28] which is close to the values of Billard and Dietz in the same IL (see Table 3).…”

“…and concentration of the acid used, [38] and also strongly depend on the nature of the ligand. [17,39] It was therefore necessary to estimate the C n mim + solubilities under the chemical conditions of this study. Our data show ( Fig.…”

“…The extraction efficiency of organic ligands has been shown to be enhanced manifold when room-temperature ionic liquids (RTILs), [7][8][9][10][11] which are solvents with negligible vapor pressure, nonflammable properties, and tendency to remain in the liquid phase over a wide range of temperature, [12][13][14][15] have been used as the diluents in place of the conventional diluents such as n-dodecane. [16,17] The extraction of actinide ions such as uranium(VI) has been investigated using TBP in RTILs, and an unusual cation-exchange mechanism has been proposed for the transfer of the actinide ion into the ionic liquid phase with a simultaneous partitioning of the cationic part of the IL, namely C n mim + , into the aqueous phase. [18] There is no report, however, on the extraction behavior of any metal ion, particularly the actinides, with DHOA in RTILs.…”

Highly Efficient Extraction of Trivalent f‐Cations Using Several N‐Pivot Tripodal Diglycolamide Ligands in an Ionic Liquid: The Role of Ligand Structure on Metal Ion Complexation

“…As discussed below, the extraction of the metal ions was independent of nitrate ion concentration suggesting that the lowering in the D values with increasing HNO 3 concentration could only be explained by an increasing tendency of the extraction of H + ion towards the ionic liquid phase with increasing HNO 3 concentration (as HL + species) thereby making the metal ion extraction as a competing reaction and hence, becoming less important at higher nitric acid concentrations. Such competing cation‐exchange mechanism where the H + ion gets extracted into ionic liquid medium is well known but not relevant in molecular diluents. The extraction equilibrium defining a cation‐exchange mechanism is given as: while that involving a solvation or ion‐pair mechanism is given as: where the subscripts “aq” and “IL” represent species those in the aqueous and in the RTIL phases, respectively.…”