1
Introduction
Rare earth elements (REEs) are valuable metals that are
relatively scarce in the earth’s crust or have low distribution owing to the high cost of mining and refining. REEs comprise the 15 lanthanoids (Lns), from lanthanum (La) to lutetium (Lu), scandium (Sc), and yttrium (Y). These REEs are used in various 2021 © The Japan Society for Analytical Chemistry
† To whom correspondence should be addressed. E-mail: [email protected]
1 Introduction 119
2 Ionic Liquids as Extraction Solvents 121 2·1 Hydrophobic ionic liquids
2·2 Extraction mechanisms in ionic liquid–water extraction systems
3 Ionic Liquid Extraction of Rare Earth Elements 122 3·1 Extraction systems using anionic ligands
3·2 Extraction systems using neutral ligands
3·3 Synergistic extraction systems 3·4 Extraction systems without extractants 3·5 Extraction systems using task-specific
ionic liquids
4 Conclusions 128
5 Acknowledgements 128
6 References 128
Recent Progress in Ionic Liquid Extraction for the Separation of
Rare Earth Elements
Hiroyuki O
KAMURA*
†and Naoki H
IRAYAMA**
* Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319–1195, Japan
** Department of Chemistry, Faculty of Science, Toho University, 2-2-1 Miyama, Funabashi 274–8510, Japan
This review summarizes recent progress in solvent extraction of rare earth elements (REEs) using an ionic liquid (IL) as the extraction solvent. These IL extraction systems are advantageous owing to the affinity of ILs for both charged and neutral hydrophobic species, in contrast to conventional organic solvent extraction systems. Herein, REE extraction studies using ILs are detailed and classified based on the type of extraction system, namely extraction using anionic ligands, extraction using neutral ligands, synergistic extraction, extraction without extractants, and a specific system using task-specific ionic liquids (TSILs). In IL extraction systems, the extracted complexes are often different from those in organic solvent systems, and the REE extraction and separation efficiencies are often significantly enhanced. Synergistic IL extraction is an effective approach to improving the extractability and separability of REEs. The development of novel TSILs suitable for IL extraction systems is also effective for REE separation.
Keywords Ionic liquids, task-specific ionic liquids, solvent extraction, liquid–liquid extraction, separation, rare earth
elements, extractants, anionic ligands, neutral ligands, synergistic effect
(Received August 31, 2020; Accepted October 13, 2020; Advance Publication Released Online by J-STAGE October 23, 2020)
Hiroyuki OKAMURA is a Researcher at Advanced Science Research Center, Japan Atomic Energy Agency (JAEA). He received his Ph.D. degree in 2013 from Kanazawa University. He was a Research Fellowship at JAEA (2008 – 2013) and a JSPS Research Fellow (2011 – 2013). He has been working at JAEA since 2013. His current research interest is separation chemistry based on liquid–liquid distribution using ionic liquids.
Naoki HIRAYAMA is a Professor of Faculty of Science, Toho University. He received his Doctor of Science degree in 1993 from Kyoto University. After that, he worked at Kanazawa University as Research Associate (1993 – 2002) and Associate Professor (2002 – 2010). Since 2010, he has been working at Toho University. His research interest is separation chemistry using liquid–liquid biphasic system. Recently, he has been focusing on fundamental research on nature of ionic liquids as extraction phase solvents.
Anal. Sci., 2021, 37, 119–130 DOI:10.2116/analsci.20SAR11
high-tech products, such as permanent magnets, catalysts, glasses, and fluorescent materials, owing to their unique physicochemical properties.1–3 In many countries, securing a sustainable supply of REEs is considered politically important because REEs are crucial in state-of-the-art products used in daily life.4 Although the development of alternative materials to REEs has been extensively investigated, the use of REEs in industry remains indispensable. The global demand for REEs continues to rise, resulting in a large gap between demand and supply.5 Although natural REE resources are limited,6 large amounts are present in spent industrial products, known as urban mines. Therefore, to solve these supply problems, the recovery of REEs from secondary resources, such as spent industrial products, has become important in recent years.7 Today, traditional hydrometallurgical methods are still widely applied to recover and separate REEs.
Solvent extraction, or liquid–liquid extraction, is among the most promising techniques for recovering and separating metal ions,8–10 including REEs.11,12 This technique has been broadly employed in many fields, including hydrometallurgy, analytical chemistry, and nuclear fuel reprocessing. However, solvent extraction methods generally require water-immiscible solvents, such as hydrophobic organic solvents. Because most organic solvents are volatile and flammable, their use involves potential risks to human health and safety. Furthermore, organic solvents negatively affect the environment. Accordingly, the use of organic solvents has tended to be avoided in recent decades, with many researchers exploring and developing novel liquid media as alternatives to organic solvents.
Ionic liquids (ILs) are among the potential candidates for novel liquid media, and have attracted much attention in many fields, including analytical chemistry13–16 and materials science.17–20 In general, ILs are defined as “salts with melting temperatures below 100°C”21 or “liquids composed entirely of ions that are fluid around or below 100°C”.22 ILs have unique properties, such as extremely low vapor pressure, incombustibility, high thermal stability, and a wide electrochemical window, in contrast to conventional molecular organic solvents.23 As a further benefit, the solvent properties of ILs can be tuned by varying the constituent ions because their physicochemical properties depend on the combination of cation and anion.24 Therefore, the polarity, hydrophobicity, and solvent miscibility can be adjusted depending on the intended use. ILs are solvents capable of overcoming the problems of organic solvents described above. Therefore, the possibility of using ILs as alternatives to organic solvents has been extensively studied in various fields of chemistry. Typical ILs are composed of an asymmetric and bulky organic cation, such as imidazolium, ammonium, phosphonium, and pyrrolidinium, and a halide-containing inorganic or organic anion. The chemical structures of representative cations and anions of ILs used in this review article are shown in Fig. 1.
The first IL exhibiting air and water stability is generally recognized to be 1-ethyl-3-methylimidazolium tetrafluoroborate ([C2mim][BF4]), reported by Wilkes and Zaworotko in 1992.25 In 1995, Chauvin et al. reported the “first hydrophobic IL” 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]-[PF6]).26 Furthermore, a series of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)imides ([CnCmim][Tf2N]) with higher hydrophobicity was reported by Bonhôte et al. in 1996.27 Currently, various types of IL are commercially available, but can be easily synthesized by alkylation of nitrogen- and phosphorus-containing compounds using alkyl bromides or alkyl chlorides.28,29 These halide ILs can be further ion-exchanged with appropriate anions to produce other ILs. In
particular, hexafluorophosphate (PF6–) and bis(perfluoroalkane-sulfonyl)imide (such as bis(trifluoromethanebis(perfluoroalkane-sulfonyl)imide (Tf2N–)) anions result in ILs that are water-immiscible. These hydrophobic ILs can be applied as novel extraction solvents. Some hydrophobic ILs are also immiscible with nonpolar organic solvents, enabling the construction of IL–water–organic-solvent triphasic extraction systems.30 Furthermore, the unique features of ILs are promising as functional extraction media for solvent extraction.
The first report on the distribution of substances between hydrophobic ILs and water was reported for organic compounds by Huddleston et al. in 1998.31 At that time, ILs were only used as environmentally friendly solvent alternatives to harmful volatile organic solvents. In contrast, Dai et al. reported in
Fig. 1 Chemical structures of representative cations and anions of
ILs.
Fig. 2 Number of publications concerning [“ionic liquid*”] (solid
circles) and [“ionic liquid*” extraction] (open circles) retrieved using the Web of Science.
1999 that the extraction efficiency of Sr2+ using extractant dicyclohexano-18-crown-6 (DC18C6) was significantly enhanced when using ILs as the extraction media compared with organic solvents.32 Since these reports, the IL extraction of metal ions has been extensively investigated. The number of publications on IL extraction annually is shown in Fig. 2, as obtained by searching the Web of Science using [“ionic liquid*” extraction] and [“ionic liquid*”] as keywords (note that the vertical axis of the graph is a logarithmic scale). Since 1999, the number of publications on ILs has increased rapidly. Publications on IL extraction show a similar trend, with this prosperity in IL extraction research originating from the aforementioned first report on hydrophobic ILs. Many review articles on the IL extraction of metal ions,33–44 especially REEs,45–48 have been published to date. The chemical structures of representative extractants employed in IL extraction studies on REEs are shown in Fig. 3. These extractants can be mainly divided into two categories, namely, anionic (Brønsted acid-type) ligands and neutral ligands.
In this review article, recent progress, particularly that since 2012, in solvent extraction systems using ILs as extraction media for REE separation with various extractants is described. First, the characteristics of hydrophobic ILs as extraction solvents and their extraction mechanisms in IL extraction systems are explained. Various types of IL extraction systems for REEs are then specifically described.
2
Ionic Liquids as Extraction Solvents
2·1 Hydrophobic ionic liquidsHydrophobic ILs 1-alkyl-3-methylimidazolium hexafluoro-phosphate ([Cnmim][PF6]) and 1-alkyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide ([Cnmim][Tf2N]) are most widely used in IL extraction studies. However, according to the above definition of ILs, many hydrophobic ILs were reported before [C4mim][PF6], although they were not known as ILs at the time. For example, a typical classical hydrophobic IL is trioctylmethylammonium chloride (TOMAC), known as a liquid ion exchanger. The major difference in [Cnmim][PF6] and [Cnmim][Tf2N] compared with such classical hydrophobic ILs is the low viscosity that is acceptable for a solvent. Classical ILs, such as TOMAC, are too viscous (2088 mPa s),49 and are therefore diluted with organic solvents for use as extractants in solvent extraction.
Although [Cnmim][PF6] and [Cnmim][Tf2N] are hydrophobic, the solubility of water in these ILs is considerably higher than that in hydrophobic organic solvents. The polarity of hydrophobic ILs [Cnmim][PF6] and [Cnmim][Tf2N], evaluated using relative permittivity, is as high as those of hydrophobic polar organic solvents and long-chain alcohols such as 1-octanol.50 In recent years, Tf2N–-type ILs have mainly been used because PF6–-type ILs have the problem of producing hydrogen fluoride through hydrolysis of PF6–. When using hydrophobic ILs composed of a bulky cation and hydrophilic anion, such as Cl–, Br–, NO3–, and SCN–, in solvent extraction systems, anions contained in the aqueous phase can exchange with the IL constituent anions. Dupont et al. investigated the effect of the salt anions on anion exchange in biphasic IL–water solvent extraction systems.51 Anion exchange by aqueous phase salts increases in efficiency in the order SO42– < Cl– < Br– < NO3– < I– < ClO4– < SCN– < Tf2N–, which is consistent with decreasing charge density (Hofmeister series). ILs consisting of Tf2N– anions undergo anion exchange with difficulty, whereas chloride ILs readily undergo anion exchange.
2·2 Extraction mechanisms in ionic liquid–water extraction systems
2·2·1 Neutral complex extraction mode
Hydrophobic ILs have an affinity for hydrophobic neutral species and can extract neutral complexes in IL extraction systems. An n-valent metal cation (Mn+) forms a neutral complex with anionic ligands. For the extraction of Mn+ with a monobasic Brønsted acid chelating extractant (HA), the extraction equilibrium and extraction constant (Kex) can be expressed as follows:
Mn+ + nHAIL MAn,IL + nH+, (1)
Kex = [MAn]IL[H[Mn+][HA]+n]n
IL, (2)
where species with the subscript IL and without subscript are those in the IL phase and aqueous phase, respectively. Neutral complex extraction mode in the IL systems is essentially the same as that in conventional organic solvent systems. In this extraction mode, the hydrophobic IL acts as a simple nonaqueous solvent.
2·2·2 Cation-exchange extraction mode
In addition to their affinity for hydrophobic neutral species, hydrophobic ILs have an affinity for hydrophobic charged species because ILs can also act as liquid ion exchangers. Therefore, in the IL extraction systems, charged species can be extracted into the IL phase, and the extraction of cationic complexes must be considered. In general, cationic complexes of Mn+ are formed with neutral extractants (L), and the extraction equilibrium and Kex can be expressed as follows:
Mn+ + mLIL + nIL-C+IL MLmn+IL + nIL-C+, (3) Kex = [M[MLmn+][L]n+]IL[IL-CmIL[IL-C++]]nn
IL, (4)
where IL-C+ denotes the IL constituent cation. Cation-exchange extraction is not observed in conventional organic solvent systems and is specific to IL systems. Regarding cation-exchange extraction systems, Hamamoto et al. investigated the partition of 1-ethylpyridinium monocation and paraquat (1,1′-dimethyl-4,4′-bipyridinium) dication in IL–water biphasic systems.52 The cation extraction behavior from aqueous solution to hydrophobic ILs was quantitatively explained using a model in which cation extraction proceeds through both ion exchange with the IL cations and ion-pair transfer with the IL anions, while retaining a constant value of the solubility product (Ksp) for the IL in the aqueous phase.
2·2·3 Anion-exchange extraction mode
As described above, hydrophobic charged species can be extracted into the IL phase. Therefore, the extraction of anionic complexes also occurs in IL extraction systems. Anionic complexes of Mn+ can be formed by over-neutralizing with anionic ligands, including HA. In this case, the extraction equilibrium and Kex can be expressed as follows:
Mn+ + iHAIL + (i – n)IL-A–IL
MAi(i–n)–IL + iH+ + (i – n)IL-A– (i > n), (5) Kex = [MAi(i–n)–]IL[H+]i[IL-A–]i–n
[Mn+][HA]iIL[IL-A–]i IL–n
, (6)
where IL-A– denotes the constituent anion of the IL. Anion-exchange extraction mode is also specific to IL systems. Katsuta
et al. reported that a similar extraction model held for anion-exchange extraction systems, with anion extraction proceeding through both ion exchange with the IL anions and ion-pair transfer with the IL cations, while retaining a constant value of Ksp for the IL in the aqueous phase.53,54
3
Ionic Liquid Extraction of Rare Earth Elements
3·1 Extraction systems using anionic ligandsThe chelate extraction system is a well-known solvent extraction system that uses anionic ligands. In chelate extraction, a metal ion is complexed with multidentate anionic (Brønsted acid-type) chelating extractants, and the resulting complex is extracted into the extraction solvent. The chelate extraction system has the advantage of extraction selectivity for metal ions being controllable by adjusting the aqueous phase pH, and the back extraction and recovery of extracted metals being easy to achieve using acidic solution. We describe extraction systems applying chelate extraction to ILs as “ionic liquid chelate extraction systems”.55–57 IL extraction studies on REEs using anionic ligands are summarized in Table 1.
Jensen et al. performed pioneering research on the chelate extraction of Nd3+ and Eu3+ with anionic ligand 2-thenoyl-trifluoroacetone (Htta) using IL [C4mim][Tf2N].58 Slope analysis, luminescence spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy showed the formation of anionic Nd(tta)4– and Eu(tta)4– complexes with no coordinated water molecules in the IL, and extraction through anion exchange at high Htta concentrations (approx. > 0.1 mol dm–3). However, at lower Htta concentrations, Ln3+ was extracted as a neutral Ln(tta)3 complex in neutral complex extraction mode.59 Okamura et al. quantitatively investigated the IL chelate extraction of Eu3+ with Htta into [Cnmim][Tf2N] using regular solution theory and time-resolved laser-induced fluorescence spectroscopy (TRLFS).60,61 Slope analysis of log D vs. log[tta–]aq plots in each [Cnmim][Tf2N] (n = 4, 6, and 8) indicated that Eu3+ was extracted with Htta as a mixture of neutral Eu(tta)3 and anionic Eu(tta)4– complexes, as shown in the following equations:
Eu3+ + 3HttaIL Eu(tta)3,IL + 3H+, (7) Eu3+ + 4HttaIL + Tf2N–IL Eu(tta)4–IL + 4H+ + Tf2N–. (8) The extraction constants of both these complexes were determined by equilibrium analysis, with the Eu(tta)3 complex almost completely dehydrated by solvation with Tf2N–, which has a coordination ability62 in the ILs. In contrast, the IL chelate extraction of Nd3+ and Dy3+ into triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide ([P2225][Tf2N]) with benzoyl-trifluoroacetone (Hbfa), classified as a β-diketone similar to Htta, showed that neutral Ln(bfa)3 complexes were extracted and Tf2N– anions did not participate in the extraction mechanism.63 In the IL chelate extraction system with Htta, the stability constants of four [Nd(tta)i](3–i)+ complexes, where i = 1 – 4, in water-saturated [C4mim][Tf2N] were successfully determined by absorption spectra analysis.64
Interestingly, Jensen et al. reported that, for IL chelate extraction with Htta into 1-butyl-3-methylimidazolium nonafluoro-1-butanesulfonate ([C4mim][NfO]), Nd3+ and Eu3+ were extracted as fully hydrated aqua cations Ln(H2O)93+, hydrated cationic Ln(tta)2(H2O)2+, and dehydrated Ln(tta)3 depending on the Htta concentration and aqueous phase pH.65 This finding was attributed to the presence of a large amount of
water in the [C4mim][NfO] phase (approx. 10 mol dm–3 H2O) and the higher hydrophobicity of NfO– compared with that of Tf2N–. The hydration state of the first coordination sphere of free Ln3+ ions in ILs, unlike that of Ln complexes, has been well studied.66–68 The hydration properties of Eu(Tf2N)3 in IL 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)-imide ([C4mpyr][Tf2N]) were examined by Brandner et al., showing that water added to the solution bound to Eu3+ ions quantitatively.66 Ansari et al. reported that the complexation of Ln(Tf2N)3 with nitrate in water-saturated [C4mim][Tf2N] proceeded via replacement of the water molecules by bidentate nitrate anions from the inner solvation spheres of Ln3+ ions.67,68 Furthermore, the stability constants of four [Nd(NO3)i](3–i)+ complexes, where i = 1 – 4, in water-saturated [C4mim][Tf2N] were several orders of magnitude higher than those in aqueous solution,69 but much lower than those observed in dry [C4mim]-[Tf2N].70 The dehydration behavior of the neutral complex Eu(tta)3 in the IL suggests that the coordination environment of Ln(III) complexes in ILs is quite different to that of Ln3+ ions. However, further study of the interactions between Ln(III) complexes and other potential ligands, such as water, halide ions, and IL components in ILs, is required.
4-Acyl-5-isoxazolone-type extractants are a class of β-diketone analogs with potential applications in strong acidic media.71,72 Atanassova et al. demonstrated that the application of [C4mim][Tf2N] greatly enhanced the extraction performance of 4-benzoyl-3-phenyl-5-isoxazolone (HPBI) toward Ln3+ compared with that in the chloroform system.73 The composition of the extracted species in the IL was established to be anionic
Ln(PBI)4–. The extraction of Ln3+ by an 8-quinolinol derivative (5-octyloxymethyl-8-quinolinol; HO8Q), which is a typical chelating extractant, has also been investigated in [C8mim]-[Tf2N].74
Goto et al. investigated the applicability of N,N-dioctyl-diglycolamic acid (DODGAA) as extractant in the IL extraction system for REEs.75–79 In the organic solvent extraction system, DODGAA exhibits high extractability and selectivity for trivalent REE ions (REE3+).80–84 In IL extraction systems, DODGAA selectively extracts REEs3+ as neutral complexes,76–78 with extraction through supported liquid membranes achieved.75,77 Furthermore, DODGAA in the IL can be applied to efficient recovery of REEs from acidic leach solution of phosphor powders from waste fluorescent lamps.79 Mutual separation of Eu3+ and Am3+,85 and extraction of Eu3+ into fluorine-free IL tetraoctylammonium dodecylsulfate ([N8888][DS]),86 were investigated using bis(2-ethylhexyl)diglycolamic acid (DEHDGA), which has a similar structure to DODGAA.
3·2 Extraction systems using neutral ligands
If some water molecules coordinating to the metal ion are replaced with neutral ligands to increase hydrophobicity, the metal ion can be extracted into the IL phase by cation exchange. In fact, as described above, the first report of IL extraction of metal ions32 was that of Sr2+ from nitric acid solution using DC18C6. The extraction of alkali metal ions and alkaline earth metal ions by neutral ligands, such as crown ethers (CEs), has long been studied. IL extraction studies on REEs using neutral ligands are summarized in Table 2.
Table 1 Summary of IL extraction studies on REEs using anionic ligands
Year Metal ions Extractants ILs Author Ref.
2003 Nd3+, Eu3+ Htta [C4mim][Tf2N] Jensen et al. 58
2005 Nd3+, Eu3+ Htta [C4mim][Tf2N] Jensen et al. 59
2008 La3+, Eu3+, Lu3+ Htta [C4mim][Tf2N] Hirayama et al. 107
2010 Y3+, Eu3+, Zn2+ DODGAA [Cnmim][Tf2N] (n = 4, 8, 12) Kubota et al. 75
2010 Ln3+ Htta [C4mim][Tf2N] Okamura et al. 108
2011 Y3+, Eu3+, Zn2+ DODGAA [Cnmim][Tf2N] (n = 2, 4, 8, 12) Kubota et al. 76
2011 Nd3+, Dy3+, Fe3+ DODGAA [C8mim][Tf2N] Baba et al. 77
2011 Eu3+, Am3+ DEHDGA [C8mim][Tf2N] Rout et al. 85
2012 Eu3+ Htta [C4mim][Tf2N] Okamura et al. 60
2012 Nd3+, Eu3+ Htta [C4mim][NfO] Jensen et al. 65
2012 Y3+, Ln3+ DODGAA [Cnmim][Tf2N] (n = 4, 8, 12) Yang et al. 78
2013 Nd3+, Eu3+, Dy3+ HO8Q [C8mim][Tf2N] Yang et al. 74
2013 Y3+, Ln3+, Fe3+, Zn2+, etc. DODGAA [C4mim][Tf2N] Yang et al. 79
2014 Eu3+ DEHDGA [N8888][DS] Rout et al. 86
2014 La3+, Eu3+, Lu3+ Htta, Hnta, Hba [C4mim][Tf2N] Okamura et al. 111
2014 La3+, Eu3+ HL1 [C4mim][Tf2N] Atanassova et al. 115
2015 La3+, Eu3+, Lu3+ HL2 [C4mim][Tf2N] Atanassova et al. 116
2016 Nd3+, Dy3+ Hbfa [P2225][Tf2N] Matsumiya et al. 63
2016 Eu3+ HPMMBP, HPMFBP, Htta [C4mim][Tf2N] Atanassova et al. 117
2016 Ce3+, Eu3+, Lu3+ HPMMBP, HPMFBP [Cnmim][Tf2N] (n = 4, 6, 8, 10) Petrova 118
2017 Eu3+ Htta [Cnmim][Tf2N] (n = 2, 4, 6, 8, 10) Okamura et al. 61
2017 Eu3+ Hba [C4mim][Tf2N], [C4mpyr][Tf2N] Atanassova and Kurteva 120
2018 La3+, Eu3+, Lu3+ HPBI [C4mim][Tf2N] Atanassova et al. 73
2018 La3+, Nd3+, Eu3+, Dy3+, Lu3+ Hba, Hacac, Hdbm [C4mim][Tf2N] Hatakeyama et al. 113
2018 La3+, Eu3+, Lu3+, etc. HPMMBP [C4mim][Tf2N] Atanassova et al. 121
2019 Nd3+ Eu3+, Am3+ Htta [C4mim][Tf2N] Gujar et al. 64
2019 La3+, Eu3+, Lu3+ Hipt [C4mim][Tf2N] Higuchi et al. 114
2019 Eu3+ Htta [Cnmim][Tf2N] (n = 4, 6, 8, 10) Atanassova and Kurteva 122
2020 Ln3+ Htta [Cnmim][Tf2N] (n = 4, 10) Okamura et al. 112
HL1: 3-methyl-1-phenyl-4-(4-trifluoromethylbenzoyl)-pyrazol-5-one; HL2: 3-methyl-1-phenyl-4-(4-phenylbenzoyl)-pyrazol-5-one; HPMMBP: 3-methyl-4-(4-methylbenzoyl)-1-phenyl-pyrazol-5-one; HPMFBP: 4-(4-fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one; Hacac: acetylacetone; Hdbm: dibenzoylmethane.
Trioctylphosphine oxide (TOPO) and tributyl phosphate (TBP) are typical neutral ligands that have been applied to the IL extraction of REEs. Yang et al. studied the extraction of REE3+ from a nitrate-containing solution into [Cnmim][Tf2N] using TOPO.87 In [C4mim][Tf2N], tricationic complex REE(TOPO)63+ was extracted by cation exchange, whereas in [C8mim][Tf2N], which has more hydrophobic cations, monocationic complex REE(TOPO)4(NO3)2+ was extracted. Kikuchi et al. reported the extraction of REE3+, such as Pr3+, Nd3+, and Dy3+, from spent Nd–Fe–B magnets using TBP in trioctylmethylammonium nitrate ([N1888][NO3]).88 The extractability of REE3+ was confirmed to be enhanced by adding NaNO3 to the aqueous phase, suggesting the extraction of REE nitrates coordinated by TBP into the IL phase. The same group investigated the extraction of Pr3+, Nd3+, and Dy3+ using TBP in [P2225][Tf2N] for REE recovery from spent Nd–Fe–B magnets by direct electrodeposition.89,90 The REE3+ extraction mechanism was found to be based on ion-pair formation with Tf2N– combined with cation exchange. REE extraction using TBP has also been reported using ILs [N1888][NO3],91,92 trihexyl(tetradecyl)-phosphonium nitrate ([P66614][NO3]),91 trioctylmethylammonium thiocyanate ([N1888][SCN]),92 and trihexyl(tetradecyl)-phosphonium thiocyanate ([P66614][SCN]).92 Extraction using these NO3– and SCN–-type ILs will be described in detail later.
Binnemans et al. reported the extraction of REEs using
commercially available neutral extractant Cyanex 923 (a mixture of four trialkylphosphine oxides) in various types of IL.91–93 A study on the effect of IL cations on Nd3+ extraction using Cyanex 923 in five different ILs containing the Tf2N– anion showed that ILs with small hydrophilic cations extracted Nd3+ efficiently from aqueous nitrate solution via cation exchange, whereas ILs with hydrophobic cations extracted Nd3+ much less efficiently owing to the low solubility of the IL cation in the aqueous phase suppressing ion exchange.93 Furthermore, IL extraction studies on REEs using N,N,N′,N′-tetraoctyl diglycol-amide (TODGA),94–99 DC18C6,100 imidazoles,101 N,N,N′,N′ -tetra-(2-ethylhexyl)malonamide (TEHMA),102 and 1,10-phenan throline-2,9-dicarboxamide103 have been reported by various researchers.
3·3 Synergistic extraction systems
The synergistic effect occurs when two types of extractant used together achieve extractability greater than the sum of the extractabilities of each extractant. The synergistic effect has proven to be quite effective for increasing not only the extractability of each Ln, but also separability among Lns.104,105 Stepinski et al. investigated the synergistic extraction of Sr2+ into several [Cnmim][Tf2N] and [C5mim][PF6] ILs, indicating that the extraction efficiency of Sr2+ was enhanced by adding TBP to the ILs containing DC18C6.106 Synergistic IL extraction studies on REEs are summarized in Table 3.
Table 2 Summary of IL extraction studies on REEs using neutral ligands
Year Metal ions Extractants ILs Author Ref.
2012 La3+, Ba2+ TODGA [Cnmim][Tf2N] (n = 2, 3, 4, 6, 8, 10),
[Cnmim][BETI] (n = 2, 3, 4, 6, 8, 10)
Bell et al. 94
2013 Y3+, Nd3+, Eu3+, Dy3+ TOPO [Cnmim][Tf2N] (n = 2, 4, 8) Yang et al. 87
2013 Eu3+ TODGA [C4mim][Tf2N] Sypula et al. 95
2014 Pr3+, Nd3+, Dy3+ TBP [N1888][NO3] Kikuchi et al. 88
2014 Pr3+, Nd3+, Dy3+ TBP [P222n][Tf2N] (n = 5, 8, 12),
[Cnmim][Tf2N] (n = 2, 6)
Matsumiya et al. 89
2014 Y3+, Ln3+, etc. 1-mim, 2-mim, imidazole,
benzimidazole
[Cnmim][Tf2N] (n = 2, 4, 8) Shen et al. 101
2014 Eu3+ TEHMA [P66614][NO3] Rout and Binnemans 102
2014 La3+, Eu3+ S1, S2 [C4mim][Tf2N] Atanassova et al. 115
2015 Y3+, Ln3+ Cyanex 923 [Cnmim][Tf2N] (n = 4, 10),
[N1nnn]-[Tf2N] (n = 4, 8), [P66614][N1nnn]-[Tf2N]
Rout and Binnemans 93
2015 La3+ SIV [C4mim][Tf2N] Atanassova et al. 116
2016 Pr3+, Nd3+, Dy3+, B(OH)3, Fe3+ TBP [P2225][Tf2N] Matsumiya et al. 90
2016 Eu3+ TODGA, TEHDGA,
DEHDODGA, TBP, DHOA
[Cnmim][Tf2N] (n = 4, 6, 8) Rama et al. 96
2016 Pr3+, Nd3+, Dy3+ TODGA [P2225][Tf2N] Murakami et al. 97
2016 Ln3+, Am3+
1,10-Phenanthroline-2,9-dicarboxamide
[Cnmim][Tf2N] (n = 4, 6, 8) Dehaudt et al. 103
2016 Ce3+, Eu3+, Lu3+ TODGA, CMPO, SIV [Cnmim][Tf2N] (n = 4, 6, 8, 10) Atanassova 119
2017 Eu3+ CMPO, SIV [C4mpyr][Tf2N] Atanassova and Kurteva 120
2018 La3+, Ce3+, Pr3+ TBP, Cyanex 923 [N1888][NO3], [P66614][NO3] Regadío et al. 91
2018 Ln3+, Am3+, Cm3+, etc. TODGA [N1888][NO3] Zsabka et al. 98
2018 Y3+, Ln3+ TODGA [Cnmim][Tf2N] (n = 2, 4, 6, 8, 10),
[Cnmim][BETI] (n = 2, 4, 6, 8, 10)
Dehaudt et al. 99
2018 Sm3+, Eu2+, Eu3+, Zn2+ DC18C6 [N1888][NO3] de Voorde et al. 100
2018 La3+, Eu3+, Lu3+, etc. NH-urea-containing
molecule 1, 2
[C4mim][Tf2N] Atanassova et al. 121
2019 Eu3+ CMPO [Cnmim][Tf2N] (n = 4, 10) Atanassova and Kurteva 122
2020 Nd3+, Dy3+ TBP, Cyanex 923 [N1888][NO3], [P66614][NO3],
[N1888][SCN], [P66614][SCN]
Riaño et al. 92
1-mim: 1-methylimidazole; 2-mim: 2-methylimidazole; S1: 5,11,17,23-tetra-tert-butyl-25,26,27-tris(dimethylphosphinoylpropoxy)-28-hydroxy-calix[4]arene; S2: 5,11,17,23-tetra-tert-butyl-25,27-bis(dimethylphosphinoylpropoxy)-26,28-di5,11,17,23-tetra-tert-butyl-25,26,27-tris(dimethylphosphinoylpropoxy)-28-hydroxy-calix[4]arene; SIV:
5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene; TEHDGA: N,N,N′,N′
-tetra(2-ethylhexyl)-diglycolamide; DEHDODGA: N,N,N′,N′-di(2-ethylhexyl)dioctyldiglycolamide; DHOA: N,N-dihexyloctanamide; [P222n][Tf2N]:
Okamura et al. reported the synergistic IL extraction of Ln3+ by a combination of Htta and several macrocyclic neutral ligands, such as CEs, in [C4mim][Tf2N].107,108 The synergistic effect was only observed for lighter Ln3+ ions using 18-crown-6 (18C6) or DC18C6 as macrocyclic neutral ligands, leading to the selective separation of lighter Ln3+ ions from their heavier counterparts. In these extraction systems, the extraction of lighter Ln3+ ions was indicated to proceed through ion exchange of cationic ternary complexes, such as Ln(tta)2(CE)+ and Ln(tta)(CE)2+, into the IL. Interestingly, Okamura et al. observed an “intramolecular cooperative effect” in the extraction of Sr2+ by a novel macrocyclic receptor (H2βDA18C6) composed of diaza-18-crown-6 and two β-diketone fragments in ILs.109,110 An intramolecular cooperative effect denotes the phenomenon that combining two different types of extractants leads to an increase in the extraction performance due to their cooperative interactions within the molecule, which is discriminated from general synergistic effect.
Okamura et al. also reported that combining several
β-diketones, including Htta, 2-naphthoyltrifluoroacetone (Hnta), and benzoylacetone (Hba), with TOPO in [C4mim][Tf2N] resulted in a significant enhancement in both extraction efficiency for heavier Ln3+ ions and separability among Ln3+ ions through the selective synergism.111 In the Htta–TOPO– [C4mim][Tf2N] system, the extracted species were determined to be neutral complexes Ln(tta)3(TOPO)2 and cationic charged complexes Ln(tta)2(TOPO)2+, Ln(tta)2(TOPO)3+, and Ln(tta)(TOPO)32+ by three-dimensional extraction equilibrium analysis.112 The selective synergism for heavier Ln3+ ions was attributed to the formation of hydrophobic charged adducts, such as Ln(tta)2(TOPO)3+ and Ln(tta)(TOPO)32+, in the IL, where the extraction equilibria can be expressed as follows:
Ln3+ + 2HttaIL + 3TOPOIL + C4mim+IL
Ln(tta)2(TOPO)3+IL + 2H+ + C4mim+, (9)
Ln3+ + HttaIL + 3TOPOIL + 2C4mim+IL
Ln(tta)(TOPO)32+IL + H+ + 2C4mim+. (10) In contrast, in the Hba–TOPO–[C4mim][Tf2N] system, Ln3+ ions were extracted as cationic ternary complexes, such as Ln(ba)2(TOPO)2+ and Ln(ba)(TOPO)42+, except for Lu3+, for which only Ln(ba)(TOPO)42+ was extracted, as expressed in the following equations:113
Ln3+ + 2HbaIL + 2TOPOIL + C4mim+IL
Ln(ba)2(TOPO)2+IL + 2H+ + C4mim+, (11)
Ln3+ + HbaIL + 4TOPOIL + 2C4mim+IL
Ln(ba)(TOPO)42+IL + H+ + 2C4mim+. (12) Extracted species Lu(ba)(TOPO)42+ is the most stable complex in the IL, and this synergistic extraction system is superior for the separation among heavier Ln3+ ions. In a similar Ln3+ extraction system, the synergistic effect of 4-isopropyltropolone (Hipt) and TOPO in [C4mim][Tf2N] was observed by the formation of two types of cationic complex, Ln(ipt)2(TOPO)+ and Ln(ipt)(TOPO)32+.114 Furthermore, the Ln3+ extraction efficiency has been enhanced by adding TOPO to the HO8Q– [C8mim][Tf2N] extraction system.74
The synergistic effect in IL systems has also been studied by Atanassova et al. using various β-diketones, octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO), TODGA,
Table 3 Summary of synergistic IL extraction studies on REEs
Year Metal ions Extractants ILs Author Ref.
2008 La3+, Eu3+, Lu3+ Htta–18C6 [C4mim][Tf2N] Hirayama et al. 107
2010 Ln3+ Htta–18C6, Htta–DC18C6, Htta–DB18C6, Htta–15C5 [C4mim][Tf2N] Okamura et al. 108
2013 Nd3+, Eu3+, Dy3+ HO8Q–TBP, HO8Q–TOPO, HO8Q–LIX 63 [C8mim][Tf2N] Yang et al. 74
2014 La3+, Eu3+, Lu3+ Htta–TOPO, Hnta–TOPO, Hba–TOPO [C4mim][Tf2N] Okamura et al. 111
2014 La3+, Eu3+ HL1–S1, HL1–S2 [C4mim][Tf2N] Atanassova et al. 115
2015 La3+ HL2–SIV [C4mim][Tf2N] Atanassova et al. 116
2016 Eu3+ TODGA–TBP, TODGA–DHOA [C8mim][Tf2N] Rama et al. 96
2016 Eu3+ HPMMBP–Htta, HPMFBP–Htta [C4mim][Tf2N] Atanassova et al. 117
2016 Ce3+, Eu3+, Lu3+ HPMMBP–CMPO, HPMFBP–CMPO, HPMMBP–SIV,
HPMFBP–SIV
[Cnmim][Tf2N] (n = 4, 6, 8, 10)
Petrova 118
2016 Ce3+, Eu3+, Lu3+ TODGA–CMPO, SIV–TODGA, SIV–CMPO [Cnmim][Tf2N] (n = 4, 6,
8, 10)
Atanassova 119
2017 Eu3+ Hba–CMPO, Hba–SIV [Cnmim][Tf2N] (n = 4, 10),
[C4mpyr][Tf2N]
Atanassova and Kurteva 120 2018 La3+, Nd3+, Eu3+,
Dy3+, Lu3+
Hba–TOPO, Hacac–TOPO, Hdbm–TOPO [C4mim][Tf2N] Hatakeyama et al. 113
2018 La3+, Eu3+, Lu3+,
etc.
HPMMBP–NH-urea-containing molecule 1, HPMMBP–NH-urea-containing molecule 2
[C4mim][Tf2N] Atanassova et al. 121
2019 La3+, Eu3+, Lu3+ Hipt–TOPO, Hipt–TONO, Hipt–phen, Hipt–dpp [C4mim][Tf2N] Higuchi et al. 114
2019 Eu3+ Htta–CMPO [Cnmim][Tf2N] (n = 4, 6,
8, 10)
Atanassova and Kurteva 122
2020 Ln3+ Htta–TOPO [Cnmim][Tf2N] (n = 4, 10) Okamura et al. 112
DB18C6: dibenzo-18-crown-6; 15C5: 15-crown-5; LIX 63: 5,8-diethyl-7-hydroxydodecan-6-one oxime; HL1: 3-methyl-1-phenyl-4-(4-trifluoromethylbenzoyl)-pyrazol-5-one; S1: 5,11,17,23-tetra-tert-butyl-25,26,27-tris(dimethylphosphinoylpropoxy)-28-hydroxy-calix[4]-arene; S2: 5,11,17,23-tetra-tert-butyl-25,27-bis(dimethylphosphinoylpropoxy)-26,28-dihydroxy-calix[4]5,11,17,23-tetra-tert-butyl-25,26,27-tris(dimethylphosphinoylpropoxy)-28-hydroxy-calix[4]-arene; HL2: 3-methyl-1-phenyl-4-(4-phenylbenzoyl)-pyrazol-5-one; SIV: 5,11,17,23-tetra-tert-butyl-25,26,27,28-tetrakis-(dimethylphosphinoylmethoxy)calix[4]arene, DHOA: N,N-dihexyloctanamide; HPMMBP: 3-methyl-4-(4-methylbenzoyl)-1-phenyl-pyrazol-5-one; HPMFBP: 4-(4-fluorobenzoyl)-3-methyl-1-phenyl-pyrazol-5-one; Hacac: acetylacetone; Hdbm: dibenzoylmethane; TONO: trioctylamine oxide; phen: 1,10-phenanthroline; dpp: 4,7-diphenyl-1,10-phenanthroline.
calixarenes, and NH-urea-containing molecules as extractants.115–122 In a recent paper, the synergistic effect of Htta and CMPO was reported in the extraction of Eu3+ into [Cnmim][Tf2N].122 The synergistic coefficients in this system were compared with those in our reported extraction systems.107,111
As discussed later, ILs with nitrate anions can act as complex-forming agents as well as extraction solvents. Zhu et al. reported that, in REE3+ extraction, the mixture of [N1888][NO3] and di(2-ethylhexyl) 2-ethylhexylphosphonate (DEHEHP) caused a synergistic effect.123 Extraction mechanism analysis showed that Pr3+ could be extracted as neutral complexation species Pr(NO3)3·xDEHEHP and ion-type species [N1888]y·Pr(NO3)3+y. This mixed IL extraction system has an obviously synergistic effect toward lighter REEs (La–Eu), but an anti-synergistic effect toward heavier REEs (Gd–Lu, Y), indicating that this synergistic extraction system is helpful for the separation of lighter REEs from heavier REEs.
3·4 Extraction systems without extractants
ILs consisting of anions with complexation abilities, such as NO3– and SCN– ions, can be used as sources of complex-forming anions to extract metal cations.92 The advantage of this approach is that ILs can extract REEs without using additional extractants. This is due to NO3– and SCN– anions having a stronger affinity for the IL phase.51 In this IL system, REE3+ is coordinated by nitrate or thiocyanate ions in the IL phase.
Banda et al. reported the extraction separation of Y3+ and Eu3+ using two undiluted ILs, [N1888][SCN] and [P66614][SCN].124 In this system, anions from the aqueous phase were involved in the extraction, allowing the formation of anionic complexes in the IL phase. Therefore, salting-out agents played a significant role in the extraction. In fact, the extractability of Y3+ and Eu3+ increased with increasing CaCl2 concentration in the aqueous phase for both ILs. This increase in extractability was attributed to a combination of the adjusted water activity levels resulting from the addition of a salting-out agent, resulting in weaker hydration of the REE ions, and chloride anions being involved in the extraction equilibrium and needing to be coextracted to enable metal extraction. The extraction mechanism by ILs [N1888][SCN] and [P66614][SCN] can be expressed as follows:
REE3+ + 3Cl– + x[IL-C][SCN]IL
[IL-C+]x–3[REE(SCN)x3–x]IL + 3[IL-C]ClIL, (13) where [IL-C] is [N1888] or [P66614]. Furthermore, the extraction of Y3+ and Eu3+ into [P66614][SCN] was also increased by increasing the thiocyanate concentration in the aqueous phase at different CaCl2 concentrations (2 – 6 mol dm–3). The increased extractability was due to thiocyanate ions being involved in the extraction process. Because SCN– has higher hydrophobicity than Cl–, in addition to a higher affinity for coordination with REE ions, SCN– is more likely to be coextracted to the IL phase compared with Cl–. The expected extraction mechanism can be expressed as follows:
REE3+ + 3SCN– + (x – 3)[IL-C][SCN]IL
[IL-C+]x–3[REE(SCN)x3–x]IL. (14) Interestingly, Depuydt et al. reported that short-chain symmetrical dialkylimidazolium IL 1,3-dihexylimidazolium nitrate [C6C6im]-[NO3] can preferentially extract REEs over first row transition metals.125 This IL extraction system can be applied to the separation of Sm3+/Co2+ and La3+/Ni2+ pairs, which is relevant for the recycling of Sm–Co magnets and Ni–MH batteries. A series of ILs with similar structures, [N1888][NO3],91,92,98,100,126
[P66614][NO3],91,92,102,127,128 [N1888][SCN],92 [P66614][SCN],92 and 3,5-diethyl-1-(2-ethylhexyl)-4-methyl-1,2,3-triazolium iodide or thiocyanate ([T1][I], [T1][SCN])129 was used to study the extraction of REEs even without extractants.
3·5 Extraction systems using task-specific ionic liquids 3·5·1 Incorporating a neutral ligand into an ionic liquid
cation
Task-specific ionic liquids (TSILs) are ILs functionalized with specific groups to impart specialized properties.130,131 However, salts prepared by introducing functional groups into ILs are now also referred to as TSILs, even if they do not exhibit IL properties (such as solid state). For solvent extraction of metal ions, TSILs have functions as novel extractants with high IL-philicity and/or as extraction solvents. Figure 4 shows the chemical structures of representative TSILs employed in REE extraction classified by type. In early IL extraction research, many TSILs in which a neutral ligand was introduced onto an IL cation were reported.132 However, electrostatic repulsion between the positively charged TSIL cation and the positive charge of the complexed site derived from metal cations often leads to lowering of the complexation ability for metal cations and, therefore, a decrease in the extractability of metal cations compared with the original extractants.133 This type of TSIL requires sufficient IL affinity and a strong complexation ability to counteract electrostatic repulsion.
Yun et al. reported novel TSILs comprising a diglycolamide moiety grafted onto the alkyl chain of an imidazolium cation (DGAILs).134 Unfortunately, no Ln extraction was observed from aqueous solution into [C6mim][Tf2N] containing these TSILs, implying that TSILs suppressed Ln extraction through the formation of water-soluble complexes. Wu et al. synthesized a novel TSIL with an N,N,N′,N′ -tetrakis(2-pyridylmethyl)-1,3-diaminopropane-2-amido structure ([IL-TPTNA][Tf2N]), and investigated its Eu3+ extraction performance from nitric acid solution by dissolving in [C6mim][Tf2N].135 The extractability of Eu3+ was shown to increase with increasing aqueous phase pH, and this TSIL selectively extracted heavier Ln ions over lighter Ln ions.
New preorganized 1,2-diamide-functionalized bidentate-ligand-embedded hydrophobic ILs (DAILs) were reported by Boyd et al., indicating that DAILs dissolved in trihexyl(tetradecyl)-phosphonium bis(trifluoromethanesulfonyl)imide ([P66614][Tf2N]) can efficiently extract Lns from the aqueous phase.136 The selective metal binding ability of DAILs was examined by NMR, FTIR, and structure optimization calculations, and their selective extraction behavior was demonstrated.
3·5·2 Incorporating an anionic ligand into an ionic liquid cation
When an anionic (Brønsted acid-type) ligand is introduced onto an IL cation, the electrostatic repulsion problem described above is reduced because charge neutralization of metal cations occurs during complex formation. However, few reports of this type of TSIL exist.137 Recently, Brønsted-acid ILs have been well studied.138 The acid group is located on the IL cation such that zwitterions, which can be expected to coordinate to metal ions, are formed upon deprotonation.
Nockemann et al. developed novel TSIL betainium bis(trifluoromethanesulfonyl)imide ([Hbet][Tf2N]) for the dissolution of metal oxides.139 The main functional group of [Hbet][Tf2N] is a carboxy group, and carboxylic acid-type extractants have been used to extract and separate REEs.140 Furthermore, Ln ions are known to form complexes with betaine.139 This TSIL [Hbet][Tf2N] can dissolve various metal oxides owing to the coordinating ability of betaine.141,142
Hoogerstraete et al. applied [Hbet][Tf2N] to the homogeneous liquid–liquid extraction of REEs based on the fact that binary mixtures of [Hbet][Tf2N] and water show an upper critical solution temperature (UCST) of 55.5°C.143 Although only low extraction efficiencies for Lns were obtained with pure [Hbet][Tf2N] (distribution ratio, <1), using zwitterionic betaine as an extractant increased the distribution ratio of Lns to >10. In this homogeneous liquid–liquid extraction system with betaine–[Hbet][Tf2N], several fundamental extraction parameters, such as the kinetics and loading, were studied for Nd3+, and a plausible extraction mechanism was proposed.144 The extraction and recovery of Sc3+ from aqueous solutions, including sulfation-roasted leachates of bauxite residue, with [Hbet][Tf2N] were also studied.145,146 Furthermore, taking advantage of the unique properties of [Hbet][Tf2N], the [Hbet][Tf2N]–H2O system was used to recover REEs from waste fluorescent lamps147 and Nd–Fe–B magnets.148 Other Brønsted-acid ILs, such as sulfonic Brønsted-acid ILs,149 alkylsulfuric acid ILs,150 and N-alkylated sulfamic acid ILs,151 have been reported by Dupont et al. and applied to the solvent extraction of REEs.
3·5·3 Replacing an ionic liquid anion with an anionic ligand
Novel TSILs for metal extraction can also be obtained by replacing an IL anion with a deprotonated Brønsted acid-type ligand. This type of TSIL has recently emerged and been extensively studied. However, when an IL is used as a diluent for this type of TSIL, it is essentially the same as when a salt of a TSIL anion or Brønsted conjugate acid is dissolved in IL. Therefore, it is advisable to use TSIL itself as the extraction
solvent.
Sun et al. synthesized three TSILs, tetrabutylammonium di(2-ethylhexyl)phosphate ([N4444][DEHP]), trioctylmethylammonium di(2-ethylhexyl)phosphate ([N1888][DEHP]), and trihexyl-(tetradecyl)phosphonium di(2-ethylhexyl)phosphate ([P66614]-[DEHP]).152 The distribution ratios for REEs with di(2-ethylhexyl)phosphoric acid (HDEHP), [N4444][DEHP], [N1888][DEHP], and [P66614][DEHP] in [C6mim][Tf2N] were found to be much higher than those in diisopropylbenzene (DIPB). The solubilities of the DEHP-based TSILs in [C6mim]-[Tf2N] were much better than that of HDEHP in [C6mim][C6mim]-[Tf2N]. Furthermore, Sun et al. investigated the extraction and separation of REEs with [N1888][DEHP] in [Cnmim][Tf2N] (n = 4, 6, 8, and 10) and 1-alkyl-3-methylimidizolium bis(perfluoroethane-sulfonyl)imide ([Cnmim][BETI], n = 4, 6, 8, and 10).153 In stark contrast to conventional molecular extractants, very similar extraction behavior was observed even as the carbon chain length on the IL cation was increased from butyl (C4) to decyl (C10). Extraction systems for REEs using DEHP-based TSILs dissolved in ILs have also been reported by Sun et al.154 and Rout et al.155 Interestingly, Abney et al. investigated the molecular structure in DEHP-based TSIL extraction systems using small-angle neutron scattering (SANS) and X-ray absorption fine structure (XAFS) spectroscopy.156
As an example of using undiluted TSILs, the extraction of a series of metal ions, including REEs, from an aqueous chloride solution with nonfluorinated fatty acid-based IL tetra octyl-phosphonium oleate ([P8888][oleate]) was reported by Parmentier
Fig. 4 Chemical structures of representative TSILs employed in REE extraction classified by type:
(a) Incorporating a neutral ligand into an IL cation; (b) incorporating an anionic ligand into an IL cation; (c) replacing an IL anion with an anionic ligand.
et al.157 Water-saturated [P8888][oleate] had very low viscosity owing to the large unsymmetrical oleate anion and greater water uptake of the IL, which was comparable to the viscosity of water-saturated [C4mim][Tf2N] and [C4mim][PF6].158 All REEs were extracted with [P8888][oleate] via the oleate anion although the extractable pH region was relatively high (pH > 5). Depuydt et al. reported ILs in which the docusate (dioctylsulfosuccinate or DOSS) anion is incorporated into several phosphonium ILs, including tetrabutylphosphonium and phosphonium cations functionalized with ester, carboxylic acid, or ethylene glycol groups.159 The IL with tributyl{2-[2-(2-methoxyethoxy)ethoxy]-ethyl}phosphonium as the cation, [P444E3][DOSS], exhibited a lower critical solution temperature (LCST) phase behavior upon mixing with water, yielding a homogeneous phase at temperatures below 19°C and resulting in its use for homogeneous liquid–liquid extraction. In contrast, all other ILs were hydrophobic and immiscible with water. Generally, trivalent metal ions, including REEs, were extracted better than divalent transition metals in both homogeneous liquid–liquid extraction and conventional solvent extraction systems.
Representing this type of TSIL, trioctylmethylammonium dioctyldiglycolamate ([N1888][DGA]),126 choline hexafluoro-acetylacetonate ([Chol][hfa]),160 protic ILs based on organic superbases, either 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadione (Hfod) or 1,1,1,5,5,5-hexafluoroacetylacetone (Hhfa),161 tetraethylammonium bis(2,4,4-trimethylpentyl)phosphinite ([N2222]-[BTMPP]), and tetraethylammonium bis(2,4,4-trimethylpentyl)-dithiophosphinite ([N2222][BTMPDTP])154 have also been reported for REE extraction.
4
Conclusions
In this review article, we have presented an overview of recent research progress on IL extraction systems for the separation of REEs focusing on extractant category. The development of IL extraction systems using novel extractants exhibiting higher extractability and separability and the development of novel extraction systems using TSILs are extensively investigated. The latter is a new approach in IL extraction systems, with synergistic IL extraction systems found to be quite effective for REE separation. ILs in the extraction systems are often reproducible as the extraction solvents, which is advantageous for practical applications. Although many researchers have studied IL extraction systems, the potential of ILs as extraction solvents for REE separation remains not fully clarified. Further fundamental consideration of IL extraction systems is needed for their practical use in industrial separation and purification processes. We anticipate that this article will contribute to the further development of IL extraction research.
5 Acknowledgements
This work was partially supported by JSPS KAKENHI Grant Numbers JP19K15605 and JP18K05183. We thank Simon Partridge, PhD, from Edanz Group (https://en-author-services. edanzgroup.com/ac) for editing a draft of this manuscript. 6 References
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