Fast selective homogeneous extraction of UO22+ with carboxyl-functionalised task-specific ionic liquids

The carboxyl-functionalised task-specific ionic liquid of 1-carboxymethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([HOOCmim][NTf2]) was used as solvent and extractant for UO22+ extraction from aqueous solution. A homogeneous phase of [HOOCmim][NTf2]-H2O system could be achieved at 75 °C, and 86.8 ± 4.8% of UO22+ was separated from the aqueous solution after vibrating for only 1 min. Furthermore, nearly 97.3 ± 2.9% of UO22+ was stripped from [HOOCmim][NTf2] phase by 1 M HNO3 solution. K+, Na+, Mg2+, Dy3+, La3+, and Eu3+ have little influence on the homogeneous extraction of UO22+, and the extraction efficiency of UO22+ still remained at ca. 80%. Experimental and theoretical study on the selectivity of [HOOCmim][NTf2]-H2O system were performed for the first time. Density functional theory calculation indicates that the solvent effect plays a significant role on the selectivity of [HOOCmim][NTf2]-H2O.

Room-temperature ionic liquids (RTILs) are liquid salts at or around room temperature. In recent years, RTILs have received increasing attention because of their unique physicochemical properties, such as negligible vapour pressure and strong ability to solubilise metal complexes [1][2][3][4] . They have potential as solvents for separation of metal ions from ores [5][6][7] . To date, most RTILs have only been used as diluents during liquid-liquid extraction [8][9][10][11][12][13][14][15][16] . Various types of functionalised task-specific ionic liquids (TSILs) have been designed to improve the properties of ionic liquids [17][18][19][20] . The presence of functional groups in either the cation or anion of these ionic liquids allows them to be used both as solvent and extractant in solvent extraction systems without additional extractant. The solubility of TSILs in water can be adjusted by incorporating functional groups into the ionic liquids, enabling creation of temperature-sensitive TSILs. A two-phase TSILs-H 2 O mixture can be converted to one homogeneous phase by raising temperature, and the two-phase equilibrium can be re-established by reducing temperature [21][22][23] . The long equilibration time for extraction can be greatly reduced by formation of a homogeneous phase.
There is an urgent need for rapid extraction of U(VI) species for separation of uranium from ores in the nuclear fuel cycle 10,19 , and there have been many publications on the extraction of U(VI) species 6,[24][25][26][27] . The fast, selective separation of U(VI) species is of great interest for applications in the nuclear fuel cycle, and has been the subject of several theoretical and experimental studies 10,24 . Most studies have proposed methods requiring an extractant in the RTILs phase for selective extraction of U(VI) [28][29][30][31] . Unfortunately, traditional extraction processes commonly need a long equilibration time, which limits their practical application. In addition, RTILs are only used as diluents in these processes, while the required additional extractant. Hoogerstraete et al. designed a homogenous extraction system by using binary mixtures of betainium bis(trifluoromethylsulfonyl)imide ionic liquid and H 2 O 17,32 . This system showed that effective extraction of trivalent rare-earth, indium, gallium, neodymium ions 18 , and uranyl species 33 can be achieved by homogeneous extraction without additional any extractant. Homogeneous liquid-liquid extraction of neodymium(III) has also been achieved using choline hexafluoroacetylacetonate in the ionic liquid choline bis(trifluoromethylsulfonyl) imide 34 19 found that U(VI) oxide could be dissolved in three different ionic liquids functionalised with a carboxyl group, and three carboxyl groups coordinated bidentately to the uranyl species in the crystal structure of U(VI)-TSILs complexes. Sasaki et al. 33 reported that the extractability of UO 2 2+ at near 62% was achieved by using betainium bis(trifluoromethylsulfonyl)imide ionic liquids. Fast selective homogeneous extraction of U(VI) species from lanthanides by TSILs without addition of any extractant will be of great significance in the nuclear fuel cycle.
Herein, a new fast homogeneous extraction system using For the purpose of comparative analysis, the solubility of [HOOCmim] + in 1.0 HNO 3 was determined to be approximately 6.3 ± 0.1%, which is higher than that in water. A general chemical cation exchange model, involving a combination of the H + and cationic species from an acidified aqueous phase toward an ionic liquid phase, was proposed by Billard et al. 11 . Accordingly, the solubility of [HOOCmim] + increased with the addition of HNO 3 , possibly due to cation exchange between [HOOCmim] + and H + 9,36 .
The traditional liquid-liquid extraction kinetics of [HOOCmim][NTf 2 ] for the removal of UO 2 2+ has not previously been reported and was investigated at a constant temperature of 30 °C for comparison. Both E U and distribution ratios (D U ) increased rapidly and reached a plateau, with values exceeding 82.8 ± 3.2% and 3.4 ± 0.1, respectively, after 60 min (Fig. S4). This result indicates that the extraction equilibrium at 30 °C could be achieved in 60 min. Compared to traditional liquid-liquid extraction (Table 1), equilibration time for extraction is dramatically shortened through homogeneous extraction.
The mechanism of extraction using the [HOOCmim][NTf 2 ]-H 2 O system is of great importance for its practical application, so it was studied by varying the H + concentration of aqueous solution. As shown in Fig. 3, the partitioning of UO 2 2+ into the organic phase decreased rapidly as the H + concentration was increased by addition of HNO 3 . Interestingly, the partitioning of UO 2 2+ into the organic phase increased after decreasing the H + concentration by addition of NaOH solution. Nockemann et al. 19  . Therefore, it can be proposed that deprotonation of the carboxyl groups is necessary for coordination of UO 2

2+
. The deprotonation of [HOOCmim] + can be inhibited by HNO 3 , but promoted by addition of NaOH. As a result, E U decreases with addition of HNO 3 , but increases with addition of NaOH. Billard  et al. 11 proposed a cation exchange model between H + and cationic species during extraction. Inhibition of the cation exchange mechanism by hydrogen ions has been proved in the literatures 11,36,41,42 . Therefore, the decrease of E U into the organic phase is caused by both protonation of the carboxyl groups and inhibition of the cation exchange mechanism by hydrogen ions. Based on the study of the extraction mechanism, the stripping of UO 2 2+ from the ionic liquid phase was performed by using nitric acid solution. The organic phase, containing UO 2 2+ , was mixed with different concentrations of nitric acid solution. As illustrated in Fig. S5, the stripping of UO 2 2+ from carboxyl-functionalised task-specific ionic liquids was easily achieved using HNO 3 solution, and nearly 97.3 ± 2.9% of the UO 2 2+ was stripped from the organic phase by 1 M HNO 3 . This approach provides a valuable method to strip the extracted UO 2 2+ and recycle carboxyl-functionalised task-specific ionic liquids. The influence of metal ions on the extraction of UO 2 2+ was also assessed. As shown in Fig. 4, K + , Na + , Mg 2+ , Dy 3+ , La 3+ , and Eu 3+ had little influence on the separation of UO 2 2+ from the aqueous phase, and the E U remained at ca. 80%. These results suggest the potential for separation of UO 2 2+ in the presence of K + , Na + , Mg 2+ , Dy 3+ , La 3+ , and Eu 3+ . Furthermore, Eu 3+ has been widely used as a representative of the trivalent lanthanides 43   The selectivity of [OOCmim] for UO 2 2+ and Eu 3+ was further investigated using DFT calculations. Figure Table 2 lists the changes in enthalpies (H g ), entropies (S g ), and binding energies (G g ) for the metal-ligand complexation reactions  in gas phase. As presented in Table 2, the gas-phase reaction enthalpies were relatively large, negative gas-phase binding energies that were significantly more negative than TΔ S g .  (Fig. S6) in the literature 19 , which indicates that the solvation effect plays a significant role in the extraction of UO 2 2+ . Consequently, the conformations of these species were affected by the solvation effect, leading to the clear changes of the Gibbs free energy for the complexation reactions and the selectivity of [OOCmim]. As shown in . These compounds were used without further purification. All other solvents were analytical-grade reagent and used as received.
Fast homogeneous extraction. Aqueous phase containing 2 mM UO 2 2+ was prepared by dissolving UO 2 (NO 3 ) 2 ·6H 2 O with deionized water in plastic container. 0.40 mL organic phase of [HOOCmim][NTf 2 ] and 0.40 mL aqueous phase containing 2 mM UO 2 2+ were added into a tube. Then the tube was heated at 75 °C for 10 min, followed by vibrating for 1 min in a vibrating mixer. Hereafter, samples were kept in 60 °C and 30 °C thermostat for cooling. After that, samples were centrifuged for 5 min to ensure the complete separation of two phases. Then the aqueous solution was diluted ca. 40 times by deionized water, and the concentration of UO 2 2+ in the diluted aqueous solution was measured by Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICP-AES) (Teledyne Leeman Labs, USA) at room temperature. Moreover, the influence of K + , Na + , Mg 2+ , Dy 3+ , La 3+ , and Eu 3+ (2 mM) on the extraction of UO 2 2+ was assessed. The extraction of Eu 3+ was also studied under the same condition for exploring the selectivity of [HOOCmim][NTf 2 ]. The E U and D U are calculated as follows: . The extraction experiments were oscillated with a rotating speed of 120 rpm in air bath at 30 °C. Afterwards, the samples were centrifuged for 5 min to ensure the complete separation of two phases. The E U and D U were calculated by using the same method as that of fast homogeneous extraction.
Stripping experiment. After extraction, the organic phase containing UO 2 2+ was mixed with deionized water and different concentration of nitric acid solutions. The two phases were conducted in a vibrating mixer in order to make two phases completely contacted. The stripping efficiencies (Es) are calculated as follows: Theoretical calculations. Electron correlation effects are included by employing density functional theory (DFT) methods, which have shown that the main features of actinide complexes can be accurately reproduced at this level of theory 44 . Calculations were carried out with the Gaussian 09 program package using DFT at the B3LYP level 45,46 . For the U and Eu atoms, relativistic effects were considered with the quasirelativistic effective core potentials (RECPs) and the associated valence basis sets developed by the Stuttgart and Dresden groups [47][48][49][50][51] . The adopted large-core RECPs include 52 electrons 50,51 and 60 electrons 47,48,52 in the core for Eu(III) and U(VI) were used for geometry optimizations, respectively. The 6-311G(d,p) basis set was used for all carbon, hydrogen, oxygen, and nitrogen atoms. Geometry optimizations and electronic calculations for all of the species were carried out firstly in the gasphase at the B3LYP/6-311G(d,p)/RECP level. The enthalpies (H g ), entropies (S g ), and Gibbs free energies (G g ) were calculated at the B3LYP/6-311G(d,p)/RECP level in the gas phase (298.15 K). For obtaining the enthalpies (H sol ), entropies (S sol ), and Gibbs free energies (G sol ) of these species in solvents ([HOOCmim][NTf 2 ] and water) at 298.15 K, these structures were optimised in solvents and calculated by frequency analysis at the B3LYP/6-311G(d,p)/RECP level of theory based on the universal continuum solvation model of SMD 53 , which was known to predict energies of solvation well 54