I. INTRODUCTION
Lanthanides (Ln) recycling from industrial residues has become economically interesting with the widely application of Ln compounds [1-3]. In addition, the Ln and actinides (An) should be co-extracted from nuclear waste for the subsequent An/Ln separation [4, 5]. For these reasons, various organic compounds have been investigated to obtain an appropriate extractant for Ln. Musikas et al. [6, 7] extracted Ln with some diamides dissolved in alkanes and found that 3-oxapentanediamides showed high extractability for some Ln. Drew et al. [8] studied the complexation of various aza-aromatic based on the Ln series (La-sm). However, an effective extraction in conventional organic diluent needs to be accomplished under high acidic conditions, as the predominant mode of cation transfer from aqueous phase into organic phase has been proved as neutral complex mechanism. For example, Narita et al. [9] noted that chloroform solution containing N,N’-dimethyl-N,N’-diphenyl-3-oxapentanediamide showed the distribution ratio of Lu about 100 from 3 M HNO3. Therefore, the subsequent acid degradation of extractants under high acidity will severely restrict their industrial application.
Ionic liquids (ILs) have been identified as ideal alternative solvents in liquid-liquid extraction due to their unique properties such as negligible vapor pressure, good thermal, chemical stability and good dissolving ability, etc. [10-13]. Also, it has been noted that Ln cations can be extracted from aqueous phase into the ILs phase through cation exchange mechanism without adding acids [14-16]. Dietz et al. [14] reported that 1-amyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imides ([C5mim][PF6]) solution containing dicyclohexano-18-crown-6 showed a high distribution coefficient of strontium at low acidity as the cation exchange between [C5mim]+ and Sr2+. Nakashima et al. [15] reported that 1-butyl-3-methyl-imidazolium hexafluorophosphate [C4mim][PF6] solution containing 3 mM octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide could extract 90% of Ce3+ from deionized water, since the [PF6]- anion plays a key role in the electrically neutralization of the Ln3+·3CMPO complex. Therefore, the application of ILs would construct the low acidity extraction system to enhance the stability of extraction system.
In present study, an extraction system consisted of 2,6-bis(5,6-dihexyl-1,2,4-triazin-3-yl) pyridine (isobutyl-BTP) as extractant (Fig. 1(a)) and 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2]) as diluents (Fig. 1(b)) was designed. The extraction behavior of several Ln ions (La, Eu, Dy, Lu) from low acidity HNO3 solution was studied. The isobutyl-BTP/cyclohexane system was investigated for comparison purpose.
II. EXPERIMENTAL
A. Materials
The isobutyl-BTP (>95%) was synthesized according to Ref. [17]. [Cnmim][NTf2] (purity > 99%) was purchased from Lanzhou Greenchem ILs, LICP, CAS, China (Lanzhou, China). No impurities were detected using 1H NMR spectrometry. The other organic and inorganic compounds were commercial products of reagent grade and used without further purification. Stock solutions of La3+, Eu3+, Dy3+ and Lu3+ were prepared by dissolving their nitrate into HNO3 solution.
B. Method
1. Solvent extraction
The extraction phase (2.0 mL) containing 25 mM isobutyl-BTP was prepared by dissolving isobutyl-BTP in [Cnmim][NTf2], and the aqueous solution (2.0 mL) containing La3+, Eu3+, Dy3+ and Lu3+ (2 mM for each species) was prepared in HNO3 medium in concentration of 0.01, 0.1, 0.5, 1, 2 and 3 M. The extraction experiments were conducted by contacting the extraction phase with aqueous solution for designated time in a constant temperature incubator shaker which maintained a thermo-stated water bath at 25 ℃ with a rotating speed of 120 rpm, followed by centrifuging for 3 min to ensure that two phases were completely separated. After phase separation, the aqueous solution was diluted with deionized water, and Ln concentration in the diluted aqueous solution was measured by Prodigy high dispersion inductively coupled plasma atomic emission spectrometer (ICPS-7510, SHIMADZU, JPN). The extraction experiment against Ln using isobutyl-BTP/cyclohexane extraction system was carried out in the same condition.
The distribution ratios (D) and extraction efficiencies (E) were calculated with Eqs. (1) and (2), respectively, where M means the cation species in aqueous phase, the subscript org/aq is short for organic phase/aqueous phase, while the subscript "i/f" designates the initial/final concentration of metal ions in the diluted solution.
2. UV-visible analysis of cation in the IL
The content of [C2mim]+ in aqueous phase after extraction was quantified using UV-Visible spectrophotometry (SHIMADZU UV-3600 PC spectrophotometer). Calibrations were performed using the standards with a series of initial concentration of the [C2mim]Cl dissolved in water. The characteristic band was selected at 210 nm due to its relative high signal intensity.
3. Back extraction experiment of isobutyl-BTP/[C2mim][NTf2] against Dy3+
Extraction experiments have been described above. After extraction of Dy3+ at 0.01 M HNO3, it was centrifuged to separate out aqueous phase and IL phase and then took the IL phase (1mL) into back extraction agent (1 mL) H2O, 1 M HNO3, 3 M HNO3, 0.01 M DTPA (Diethylene triamine pentacetate acid) and 0.5 M Thiourea to oscillate under the same condition for 48 h. The concentration of Dy3+ was analyzed by ICPS-7510 (SHIMADZU, Japan).
III. RESULTS AND DISCUSSION
Firstly, we investigated the extraction of Ln using ILs alone in the absence of isobutyl-BTP,to find that the ILs alone can hardly extract Ln. This is similar to the work reported by Shimojo et al. [18]. But in a solubility experiment, we found that [Cnmim][NTf2] (n=2, 6, 8) was of excellent solubility against isobutyl-BTP.
A. Extraction of Dy3+ in different extraction systems
The distribution ratio of Dy3+ ion (DDy) as function of HNO3 concentration for the isobutyl-BTP/[Cnmim][NTf2] systems and the isobutyl-BTP/cyclohexane system are shown in Fig. 2.
In the cyclohexane system, the DDy increased with HNO3 concentration, indicating that the anion NO3- enhances extractability due to neutral complex structure. The extraction mechanism can be expressed as Eq. (3) [19]:
In the isobutyl-BTP/[Cnmim][NTf2] systems, DDy decreased rapidly with increasing HNO3 concentration from 0.01 M to 1 M, where it began to increase slightly. The curves differ from each other because of the mechanism transition depending on acidity variation. H+ is a competitive species in cation exchange, hence the higher DDy with the lower concentration of H+; but when the H+ concentration is great enough to restrain the cation exchange, the neutral extraction becomes the dominant extraction mechanism. As a consequence, the DDy increases slightly with the acidity, just like in the cyclohexane system.
Based on the above discussion, the HNO3 concentration of 0.01 M was used in the following experiments using isobutyl-BTP/[Cnmim][NTf2] system.
B. Extraction kinetics of isobutyl-BTP/[Cnmim][NTf2] extraction system
The extraction kinetics reflects basic properties of the extraction system, so the isobutyl-BTP/[Cnmim][NTf2] extraction experiments in different contacting time were carried out. In the isobutyl-BTP/[C2mim][NTf2] system, significant advantage in extraction kinetics was observed (Fig. 3). Its extraction kinetics of Dy3+ reached equilibrium in 5 min, about 1000 times faster than that in the isobutyl-BTP/[Cnmim][NTf2] (n=6, 8) system, which spent dozens of hours to reach equilibrium. This indicates that both hydrophobicity and viscosity of imidazolium are impeditive to the cation exchange [21]. Longer alkyl chain of IL would improve the hydrophobicity and viscosity. Therefore, [C2mim][NTf2] was chosen as the diluent for the following Ln extraction.
C. Determination of thermodynamic parameters
Temperature effect on the extraction of Dy from 0.01 M HNO3 using 20 mM isobutyl-BTP/[C2mim][NTf2] system was studied. As shown in Fig. 4, DDy increased with temperature, indicating an endothermic process of the extraction. The change in enthalpy (ΔH) during the extraction can be calculated by the Van’t Hoff equation
A plot of log D vs. 1/T gives a straight line with a slope of -ΔH/2.303R, while the change in Gibb’s free energy (ΔG) can be calculated by
The change in entropy (ΔS) at a particular temperature can be calculated by
From Eqs. (4), (5) and (6), the values of ΔH, ΔG and ΔS are 28.91, 19.86 and 30.37 kJ/mol, respectively. Therefore, the extraction is spontaneous endothermic. Temperature increase is good for extraction.
D. Extraction mechanism of Ln using isobutyl-BTP/[C2mim][NTf2] system
In order to check the proposed cation exchange mechanism, we carried out extraction experiments in deionized water by mixing the aqueous solutions of EuCl3 with [C2mim][NTf2]. Since DyCl3 has a poor water solubility, and ultraviolet absorption of NO3- has strong interference on [C2mim]+ cation, herein we chose EuCl3. As shown in Fig. 5, calibration curves of [C2mim]+ was used to measure the concentration of [C2mim]+ in the aqueous phase transferred from [C2mim][NTf2]. We measured the Eu3+ concentration in [C2mim][NTf2] transferred from the aqueous phase, and [C2mim]+ in the aqueous phase transferred from [C2mim][NTf2]. The results were plotted against each other in Fig. 6, being linear with a slope of 2.84. Due to experiment error, we considered the slope of 3. This indicates that a cation exchange mechanism is occurring with three cations from the IL involved. Therefore, the extraction equilibrium equation at low HNO3 concentration can be suggested as
In the equation, the stoichiometric ratio of Dy3+ to BTP is based on previous investigation [20].
E. Extraction of Ln in isobutyl-BTP/[C2mim][NTf2] system
To assess the extraction performance of isobutyl-BTP/[C2mim][NTf2] extraction system for Ln, ion species of La3+, Eu3+, Dy3+ and Lu3+ were employed as light, middle, middle, and heavy Ln, respectively. As shown in Table 1, the order of extraction efficiency for Ln was Eu3+ > Dy3+ > Lu3+ > La3+, indicating that isobutyl-BTP/[C2mim][NTf2] system is inclined to extract Eu3+ and Dy3+. Therefore, we suppose that the ionic size of dysprosium and europium are particularly suitable for coordination.
| Ln | Ionic radius (Å) | Extraction efficiency (%) |
|---|---|---|
| La | 1.061 | 80.79 |
| Eu | 0.950 | 99.78 |
| Dy | 0.908 | 99.08 |
| Lu | 0.848 | 94.92 |
F. Back extraction
The stripping of Dy3+ from the [C2mim][NTf2] was carried out for further knowledge of recycling. Several complexing agents were chosen, including 0.01 M DTPA, 0.5 M Thiourea and high concentration of HNO3. Pure water was chosen as comparison. As shown in Table 2, the Dy3+ was stripped completely at 3 M HNO3, and other back extraction agent did not function. Considering the extraction behavior referred above, we think that Dy and isobutyl-BTP complexation ([Dy(BTP)3]3+) in the IL phase is instability at high acidity, hence the strip of Dy ion at 3 M HNO3. Maybe, this cannot be called back extraction. Further efforts will be made to find good back extraction agent of Dy without resulting in instability of [Dy(BTP)3]3+.
| Stripping solution | Stripping efficiency (%) |
|---|---|
| H2O | 0.43 |
| 1 M HNO3 | 0.69 |
| 3 M HNO3 | 100 |
| 0.01 M DTPA | 0.35 |
| 0.5 M Thiourea | 0.39 |
IV. CONCLUSION
A novel extraction system based on isobutyl-BTP/[Cnmim][NTf2] was developed for Ln extraction. The system provided high extraction performance for Ln compared with cyclohexane. Of all the isobutyl-BTP/[Cnmim][NTf2] systems (n=2, 6, 8), isobutyl-BTP/[C2mim][NTf2] system provides the best extraction performance and the fastest extraction kinetics within 5 min towards Dy3+. It was found that the isobutyl-BTP/[C2mim][NTf2] system is inclined to extract at low acidity conditions, and can be stripped at high HNO3. The extraction mechanism based on the cation exchange was reasonably proposed for the novel extraction system. In addition, Dy3+ was found to be favor extracted in current extraction system. The extraction is spontaneous endothermic and increasing the temperature is good for extraction. Therefore, the isobutyl-BTP/[C2mim][NTf2] extraction system is effective to be used at low acidity condition for extraction. These unique properties of the IL based extraction system may help us to design new separation process for hydrometallurgy industry or advanced nuclear fuel cycle application in the future.
C2-symmetric zirconium bis(amidate) complexes with enhanced reactivity in aminoalkene hydroamination
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