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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 1min. Furthermore, nearly 97.3±2.9% of UO22+ was stripped from [HOOCmim][NTf2] phase by 1M 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 complexes1,2,3,4. They have potential as solvents for separation of metal ions from ores5,6,7. To date, most RTILs have only been used as diluents during liquid-liquid extraction8,9,10,11,12,13,14,15,16. Various types of functionalised task-specific ionic liquids (TSILs) have been designed to improve the properties of ionic liquids17,18,19,20. 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-H2O mixture can be converted to one homogeneous phase by raising temperature, and the two-phase equilibrium can be re-established by reducing temperature21,22,23. 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 cycle10,19, and there have been many publications on the extraction of U(VI) species6,24,25,26,27. 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 studies10,24. 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 H2O17,32. This system showed that effective extraction of trivalent rare-earth, indium, gallium, neodymium ions18, and uranyl species33 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) imide34. Recently, Dupont et al. used a functionalised ionic liquid for the selective dissolution and revalorization of Y2O3:Eu3+ from lamp phosphor waste35. Nockemann et al.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 UO22+ 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 1-carboxymethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([HOOCmim][NTf2], Fig. 1) both as diluent and extractant has been designed. Fast homogeneous extraction and traditional liquid-liquid extraction for the removal of UO22+ were separately studied in this work, and the selectivity of [HOOCmim][NTf2] and the influence of metal ions on the extraction of UO22+ were also carefully assessed. Furthermore, a theoretical study was conducted on the selectivity of [HOOCmim][NTf2].
The [HOOCmim][NTf2]-H2O system forms a homogeneous phase when the temperature is increased to 75°C (Fig. 2), and two-phase equilibrium can be re-established by reducing the temperature. Accordingly, the phase-transition behaviour of the [HOOCmim][NTf2]-H2O mixture was used to remove UO22+ from the aqueous phase. The two-phase [HOOCmim][NTf2]-H2O mixture was kept at a constant temperature of 75°C for 10min and then homogenised by a vibrating mixer for 1min. Extraction efficiency (EU) of 86.8±4.8% for UO22+ was obtained at 60°C, and 83.2±4.0% efficiency was achieved after cooling to 30°C. Treatment of cooling to 30°C was chosen in the following extraction process. The phase behaviour of the [HOOCmim][NTf2]-H2O mixture is of great importance for the design of extraction experiments, so the percentage rate ([R]) of [HOOCmim]+ from organic phase to aqueous phase was studied first. The [R] of [HOOCmim]+ from organic phase to aqueous phase are calculated based on the solubility ([S]) of [HOOCmim]+ in water and the mass of aqueous phase after equilibrium. The [R] is calculated as follows: [R]=maq×([S]/(1+[S]))/mTSILs×100%, where maq is the mass of aqueous phase after equilibrium. mTSILs is the initial mass of [HOOCmim][NTf2].
As shown in Fig. S1, the [R] of [HOOCmim]+ decreased with the increase of phase ratio VRTILs/VH2O, where VTSILs and VH2O represent the initial volumes of [HOOCmim][NTf2] and water, respectively. When VRTILs/VH2O=1, the solubility of [HOOCmim]+ in water was about 4.7±0.1% (Figs S2 and S3). The [R] of [HOOCmim]+ was calculated at near 7.5±0.1% and changed slightly as the equilibration time increased (Fig. S3), indicating that the [HOOCmim][NTf2]-H2O system can maintain a low [R] at VRTILs/VH2O=1. For the purpose of comparative analysis, the solubility of [HOOCmim]+ in 1.0 HNO3 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 HNO3, possibly due to cation exchange between [HOOCmim]+ and H+9,36.
The traditional liquid-liquid extraction kinetics of [HOOCmim][NTf2] for the removal of UO22+ has not previously been reported and was investigated at a constant temperature of 30°C for comparison. Both EU and distribution ratios (DU) increased rapidly and reached a plateau, with values exceeding 82.8±3.2% and 3.4±0.1, respectively, after 60min (Fig. S4). This result indicates that the extraction equilibrium at 30°C could be achieved in 60min. 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][NTf2]-H2O 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 UO22+ into the organic phase decreased rapidly as the H+ concentration was increased by addition of HNO3. Interestingly, the partitioning of UO22+ into the organic phase increased after decreasing the H+ concentration by addition of NaOH solution. Nockemann et al.19 found that three carboxyl groups coordinated bidentately to the uranyl species in the crystal structure of [UO2([OOCmim])3]2+ complexes formed between [HOOCmim][NTf2] and UO22+. Therefore, it can be proposed that deprotonation of the carboxyl groups is necessary for coordination of UO22+. The deprotonation of [HOOCmim]+ can be inhibited by HNO3, but promoted by addition of NaOH. As a result, EU decreases with addition of HNO3, 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 literatures11,36,41,42. Therefore, the decrease of EU 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 UO22+ from the ionic liquid phase was performed by using nitric acid solution. The organic phase, containing UO22+, was mixed with different concentrations of nitric acid solution. As illustrated in Fig. S5, the stripping of UO22+ from carboxyl-functionalised task-specific ionic liquids was easily achieved using HNO3 solution, and nearly 97.3±2.9% of the UO22+ was stripped from the organic phase by 1M HNO3. This approach provides a valuable method to strip the extracted UO22+ and recycle carboxyl-functionalised task-specific ionic liquids.
The influence of metal ions on the extraction of UO22+ was also assessed. As shown in Fig. 4, K+, Na+, Mg2+, Dy3+, La3+, and Eu3+ had little influence on the separation of UO22+ from the aqueous phase, and the EU remained at ca. 80%. These results suggest the potential for separation of UO22+ in the presence of K+, Na+, Mg2+, Dy3+, La3+, and Eu3+. Furthermore, Eu3+ has been widely used as a representative of the trivalent lanthanides43. Accordingly, the extraction of Eu3+ was also studied under the same conditions to explore the selectivity of [HOOCmim][NTf2]-H2O. The results demonstrated that [HOOCmim][NTf2]-H2O had lower selectivity for Eu3+ (EEu=42.1±2.0%; DEu=0.48±0.02%) than UO22+ (EU=83.2±4.0%; DU=3.4±0.2%), indicating the possibility for fast separation of UO22+ from aqueous solution containing trivalent lanthanides.
The selectivity of [OOCmim] for UO22+ and Eu3+ was further investigated using DFT calculations. Figure 5 shows the optimised structures of [OOCmim], [UO2([OOCmim])3]2+, and Eu([OOCmim])4]3+. Table 2 lists the changes in enthalpies (Hg), entropies (Sg), and binding energies (Gg) 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 TΔSg. The [OOCmim] showed high selectivity for Eu3+ (ΔGg=−3005.2kJ/mol) over UO22+ (ΔGg=−1610.9kJ/mol) in the gas phase. In addition, considering solvent effects in [HOOCmim][NTf2] solution, the solvation structure was optimised in [HOOCmim][NTf2] 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. As shown in Table S2, the binding energies (ΔGsol) were much lower than the corresponding gas-phase binding energies. Interestingly, the difference in the Gibbs free energy for the complexation reactions in [HOOCmim][NTf2] proves that [OOCmim] has higher extractability for UO22+ (ΔGsol=−443.5kJ/mol) compared to Eu3+ (ΔGsol=−80.3kJ/mol), which is remarkably consistent with the experimental results. Furthermore, the conformation of [UO2([OOCmim])3]2+ optimised in [HOOCmim][NTf2] agreed well with the reported crystal structure of [UO2([OOCmim])3]2+ (Fig. S6) in the literature19, which indicates that the solvation effect plays a significant role in the extraction of UO22+. 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 Table S3, for the formation of [UO2([OOCmim])3]2+ and Eu([OOCmim])4]3+, the changes of the Gibbs free energy in water were −152.2 and −47.5kJ/mol, respectively, which are less negative compared to that of in [HOOCmim][NTf2]. The difference of the Gibbs free energy in different solvents suggests that these complexes are preferred in [HOOCmim][NTf2].
In conclusion, a new fast homogeneous system with [HOOCmim][NTf2] both as solvent and extractant is designed for the removal of UO22+ from aqueous solution. The homogeneous phase of [HOOCmim][NTf2]-H2O system can be achieved at temperature higher than 75°C, and 86.8% of UO22+ was separated from the aqueous solution after vibrating for only 1min. Compared to traditional liquid-liquid extraction, homogeneous extraction provides an extremely short equilibration time. Furthermore, nearly 97.3±2.9% of UO22+ can be stripped from organic phase by 1M HNO3. K+, Na+, Mg2+, Dy3+, La3+, and Eu3+ have slight influence on the separation of UO22+ from aqueous phase, and the EU still remained at ca. 80%. [HOOCmim][NTf2]-H2O shows a high selectivity for UO22+ rather than Eu3+, indicating the possibility for fast separation between UO22+ and Eu3+. According to the results of DFT calculation, the solvent effect plays a significant role in the selectivity of [OOCmim]. The difference in the Gibbs free energy for the complexing reactions in [HOOCmim][NTf2] proves that [OOCmim] shows higher extractability for UO22+ (ΔGsol=−443.5kJ/mol) than Eu3+ (ΔGsol=−80.3kJ/mol). Therefore, the fast homogeneous extraction system of [HOOCmim][NTf2]-H2O presents an opportunity for removal of UO22+ in aqueous solution containing rare earth metal ions.
[HOOCmim][NTf2] (with a purity >99%) were purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). UO2(NO3)2·6H2O was obtained from Beijer Chemapol Co. NaNO3, KNO3, Dy(NO3)3·6H2O, Eu(NO3)3·6H2O, and La(NO3)3·6H2O (Beijing chemical corp., >99%) were used to assess the influence of metal ions on the extraction of UO22+. These compounds were used without further purification. All other solvents were analytical-grade reagent and used as received.
Aqueous phase containing 2mM UO22+ was prepared by dissolving UO2(NO3)2·6H2O with deionized water in plastic container. 0.40mL organic phase of [HOOCmim][NTf2] and 0.40mL aqueous phase containing 2mM UO22+ were added into a tube. Then the tube was heated at 75°C for 10min, followed by vibrating for 1min in a vibrating mixer. Hereafter, samples were kept in 60°C and 30°C thermostat for cooling. After that, samples were centrifuged for 5min to ensure the complete separation of two phases. Then the aqueous solution was diluted ca. 40 times by deionized water, and the concentration of UO22+ 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+, Mg2+, Dy3+, La3+, and Eu3+ (2mM) on the extraction of UO22+ was assessed. The extraction of Eu3+ was also studied under the same condition for exploring the selectivity of [HOOCmim][NTf2]. The EU and DU are calculated as follows:
where ni and nf designate the initial and final amount of metal ions in the aqueous solution, respectively. Corg and Caq represent the concentration of metal ions in the organic phase and the aqueous phase after extraction, respectively. All above experiments were carried out in plastic container, and all obtained values were in duplicate with uncertainty within 5%.
0.40mL organic phase of [HOOCmim][NTf2] was mixed with 0.40mL aqueous phase containing 2mM UO22+. The extraction experiments were oscillated with a rotating speed of 120rpm in air bath at 30°C. Afterwards, the samples were centrifuged for 5min to ensure the complete separation of two phases. The EU and DU were calculated by using the same method as that of fast homogeneous extraction.
After extraction, the organic phase containing UO22+ 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:
where no and nw designate the initial amount of UO22+ in the organic phase and the final amount of UO22+ in the aqueous phase, respectively.
The solubility of [HOOCmim]+ in water (Fig. S2) during extraction were analysed by 1H NMR recorded on a Bruker AV-400 instrument.
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 theory44. Calculations were carried out with the Gaussian 09 program package using DFT at the B3LYP level45,46. 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 groups47,48,49,50,51. The adopted large-core RECPs include 52 electrons50,51 and 60 electrons47,48,52 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 (Hg), entropies (Sg), and Gibbs free energies (Gg) were calculated at the B3LYP/6-311G(d,p)/RECP level in the gas phase (298.15K). For obtaining the enthalpies (Hsol), entropies (Ssol), and Gibbs free energies (Gsol) of these species in solvents ([HOOCmim][NTf2] and water) at 298.15K, 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 SMD53, which was known to predict energies of solvation well54. The static dielectric constant at 66.4 determined by PCM-1A dielectric constant detector and refractive index at 1.4454 determined by Abbe refractometer were adopted for [HOOCmim][NTf2].
How to cite this article: Ao, Y. et al. Fast selective homogeneous extraction of UO22+ with carboxyl-functionalised task-specific ionic liquids. Sci. Rep. 7, 44100; doi: 10.1038/srep44100 (2017).
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The National Natural Science Foundation of China (NNSFC, Project No. 91126014, 11375019 and 11475112) and Innovation Foundation of Institute of Nuclear Physics and Chemistry (Grant No. 2015CX04) are acknowledged for supporting this research.
The authors declare no competing financial interests.
Author Contributions Yinyong Ao planned and performed experiments, performed data analysis and wrote the paper. Jian Chen, Min Xu, Jing Peng and Wei Huang contributed to analyzing experiments and writing paper. Jiuqiang Li and Maolin Zhai contributed to planning experiments and writing the paper.