# Recovery of Rare Earth Elements (REEs) Using Ionic Solvents

Guilhem Arrachart, Julien Couturier, Sandrine Dourdain, Clément Levard, S Pellet-Rostaing

► To cite this version:

Guilhem Arrachart, Julien Couturier, Sandrine Dourdain, Clément Levard, S Pellet-Rostaing. Recovery of Rare Earth Elements (REEs) Using Ionic Solvents. Processes, 2021, 9 (7), pp.1202. 10.3390/pr9071202. [https://doi.org/10.3390/pr9071202](https://doi.org/10.3390/pr9071202)

HAL Id: hal-03395395

https://hal.science/hal-03395395

Submitted on 22 Oct 2021

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Review

# Recovery of Rare Earth Elements (REEs) Using Ionic Solvents

Guilhem Arrachart<sup>1,* ID</sup>, Julien Couturier<sup>1,2</sup>, Sandrine Dourdain<sup>1</sup>, Clément Levard<sup>2</sup>
and Stéphane Pellet-Rostaing<sup>1 ID</sup>

**Abstract:** Rare earth elements (REEs) are becoming more and more significant as they play crucial roles in many advanced technologies. Therefore, the development of optimized processes for their recovery, whether from primary resources or from secondary sources, has become necessary, including recovery from mine tailings, recycling of end-of-life products and urban and industrial waste. Ionic solvents, including ionic liquids (ILs) and deep-eutectic solvents (DESs), have attracted much attention since they represent an alternative to conventional processes for metal recovery. These systems are used as reactive agents in leaching and extraction processes. The most significant studies reported in the last decade regarding the recovery of REEs are presented in this review.

**Keywords:** rare earth elements; ionic liquid; deep-eutectic solvent; leaching; extraction; separation

<figure><img src="image_2.png" /></figure>

check for
updates

**Citation:** Arrachart, G.; Couturier, J.; Dourdain, S.; Levard, C.; Pellet-Rostaing, S. Recovery of Rare Earth Elements (REEs) Using Ionic Solvents. *Processes* **2021**, *9*, 1202.
https://doi.org/10.3390/pr9071202

Academic Editor: Marco Trifuoggi

Received: 3 June 2021

Accepted: 5 July 2021

Published: 12 July 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

<figure><img src="image_3.png" /></figure>

**Copyright:** © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## 1. Use and Sourcing of Rare Earth Elements

Rare earth elements (REEs) as defined by the International Union of Pure and Applied Chemistry (IUPAC) include metals characterized by similar properties, namely scandium (Sc), yttrium (Y) and all the lanthanides. The latter correspond to the chemical elements listed in the periodic table of Mendeleev that have an atomic number ranging from 57 for lanthanum (La) to 71 for lutetium (Lu) [1]. REEs are often subdivided into “light rare earths elements” (LREEs) and “heavy rare earths elements” (HREEs) according to their atomic numbers. Yttrium is oftentimes associated with HREEs due to chemical similarities, including ionic radii. In some cases, the elements from samarium to terbium are considered as the “middle rare earth elements” (MREEs) [2].

With the increasing uses of these elements, global demand is growing and their access is becoming crucial, leading to diversified sources of supply (Figure 1).

The properties of REEs are linked to their electronic configuration and in particular to the specificity of their electronic 4f sublayer, which allows numerous optical transitions and gives them particular magnetic and catalytic properties [3]. REEs are mainly trivalent; however, different oxidation states can occur naturally for cerium and europium.

Due to these remarkable properties, REEs are used in many high-tech applications [4,5], such as lasers [6,7], permanent magnets [8,9] energy storage [10,11] and so on. REEs are therefore part of the so-called “technological” metals, the supply of which has become strategic, as it is threatened by the growth in world demand for these particular metals [12,13]. REEs are also considered as “critical” elements by a number of institutions, such as the EU, which has identified these elements as being of high economic importance with a high risk associated with their supply. The latter includes geopolitical considerations, such as the quasi-monopoly over the production of these elements maintained by China.

REEs are commonly mined as co-products or as by-products of other materials. Primary sources of REEs include hard rock deposits of bastnaesite as well as alluvial deposits of monazite and xenotime [14]. In addition to these ores, which contain the highest concentrations of REEs, other mineral resources can be considered, such as phosphate or apatites,

<footnote n="1"><sup>1</sup> ICSM, Univ Montpellier, CEA, CNRS, ENSCM, 30207 Marcoule, France; couturier@cerege.fr (J.C.); sandrine.dourdain@cea.fr (S.D.); stephane.pellet-rostaing@cea.fr (S.P.-R.)</footnote>

<footnote n="2"><sup>2</sup> Aix-Marseille Univ, CNRS, IRD, INRAE, CEREGE, 13545 Aix-en-Provence, France; levard@cerege.fr</footnote>

<footnote>* Correspondence: guilhem.arrachart@umontpellier.fr; Tel.: +33-4-66791568</footnote>
which are admittedly poorer in REEs, but whose exploitation and processing in the future
may be cost-effective [15,16].

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Use and sourcing of rare earth elements.</figcaption></figure>

Due to the critical need for these materials, alternative sources need to be investigated, particularly in the context of the circular economy [17,18]. Valorisation of secondary or unconventional sources becomes more and more relevant, both from economic and environmental perspectives [19].

REE recovery from industrial landfilled stocks containing lower REE concentrations requires larger volumes. Among industrial wastes, bauxite residue (red mud) generated during the production of alumina from bauxite (the Bayer process) represents a promising source for REEs [20,21]. It is mainly composed of the metallic oxides aluminium, iron and titanium, as well as up to 1000 ppm of REEs [22,23].

Like bauxite residue, phosphogypsum, which is a waste generated by the fertilizer industry, has been identified as a significant source of REEs due to presence of these elements and the large volumes generated worldwide [24,25].

Other important secondary sources that are challenging for REE recovery are acid mine drainages (AMDs) [26] and wastewater sludges [27]. AMD and mine water from operating and closed mining sites represent a potential secondary source for many elements of economic interest, including REEs [28]. There is also significant potential for the recovery of these elements from coal and coal combustion by-products (coal fly ash) and the feasibility and implementation of industrial processes are under study [29,30].

As REEs are also present in a large amount of technological equipment, recovery from recycling of specific wastes, and in particular from waste electrical and electronic equipment (WEEE), also called urban mines, represents an unconventional and alternative source with high potential [18,31,32]. In this context, much research has recently been performed on end-of-life products containing high concentrations of REEs, such as magnets [33,34] or fluorescent lamps [35,36].

The processes currently used to recover rare earths from these natural or industrial ores (conventional and unconventional sources) consist in subjecting the ores, which have been previously crushed, to chemical treatments (with acidic or basic reagents) in order to obtain mineral concentrates. The chemical attack or leaching process is conventionally carried out
by means of one or more acid reagents, including nitric acid, sulphuric acid, phosphoric acid, hydrochloric acid or a mixture of them (aqua regia). The so-called "pregnant leach solutions" are then subjected to an extraction/separation step using solvent extraction, ion exchange and/or precipitation [37].

Among such separation techniques, solvent extraction or liquid-liquid extraction is the most commonly used process for the recovery of REEs from the different sources (primary and secondary mining, waste streams, end-of-life materials and so on). The solvent extraction process consists in bringing the aqueous phase constituted by the pregnant leach solution into contact with an organic phase comprising one or more extractants, in order to achieve an efficient and selective extraction of REEs. Although widely applied on an industrial scale, liquid-liquid extraction suffers from certain limitations, such as the use of large amounts of toxic solvents and/or flammable solvents, which are a potential threat to human health and the environment [38]. For example, aliphatic diluents, such as isooctane, are classified as highly flammable liquids and vapours with safety concerns (they may be fatal if they are swallowed and enter the airways, very toxic to aquatic life, etc.), while *n*-dodecane is classified as a combustible liquid and can be used as an alternative; however, it also presents health risks. Another major issue associated with the use of organic solvents in hydrometallurgy is their loss by evaporation. Besides the economic aspect, this leads to ecological concerns. Elimination or replacement of hazardous solvents in terms of toxicity and flammability, which has a clear impact on worker safety and environmental issues, is therefore one of the main challenges for metallurgical processes.

## 2. Description and Properties of Ionic Solvents

Due to their quasi non-flammability [39] and non-volatility [40,41], ionic solvents, including ionic liquids (ILs) and deep-eutectic solvents (DESs), are increasingly being used as alternative diluents or extractants to conventional volatile organic compounds in extraction processes [42,43]. Recently, the term "ionometallurgy" was introduced by Abbott et al. for metallurgical processes using ionic solvents [44]. One of the interesting features of using ionic solvents is that an electrochemical process for the dissolution and the purification of metallic species can be implemented thanks to their large electrochemical window [45].

Ionic solvents can be considered as salts with melting points lower than 100 °C [46,47] and modular physical and chemical properties. Two categories can be considered: the "conventional" ILs composed of a discrete cation and anion, and DES systems formed from an eutectic mixture of Lewis or Brønsted acids and bases, which can contain a diversity of ionic species (anionic and/or cationic) [48–50].

ILs exhibit a wide variety of chemical structures, but share some common characteristics, such as bulky organic cations (ammonium, phosphonium, imidazolium, pyridinium, pyrrolidinium, piperidinium, triazolium) combined with smaller inorganic (chloride, hexafluorophosphate, tetrafluoroborate, etc.) or organic (acetate, triflate, bis(trifluoromethyl) sulfonyl imide, etc.) anions (Figure 2).

Appropriate combinations of the cation and anion allow adjustment of the physicochemical properties of the ILs (viscosity, density, hydrophilicity and solubility). The multiple possibilities make it possible to consider them as "designer solvents" that are attractive potential alternatives for organic molecular solvents [51].

In conventional approaches applied for metal extraction, ILs are usually hydrophobic to avoid their miscibility in the aqueous phases. Their hydrophobicity is governed by the length of the alkyl chain carried by the cation, as well as by the shape, symmetry and the nature of the anion associated. Typically the applied organic cations present long carbon chains, and the anions are usually hydrophobic anions such as hexafluorophosphate $[PF_6^-]$ or bis(trifluoromethyl)sulfonyl imide $[NTf_2^-]$. IL density and viscosity are essential parameters to take into account in the implementation of an extraction process, as they affect the transport properties, such as diffusion [52]. Longer alkyl chain lengths result in lower density and higher viscosity while, for different anions, these parameters may
evolve independently from their hydrophobicity [53]. Viscosity behaviours have also been assigned to the molecular structure of ionic liquids and to the presence of hydrogen bonding and van der Waals interactions [54]. In addition, for an identical cation, ILs based on a weak or non-coordinating anion have a lower viscosity than those composed of a complexing anion.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Structures of common cations and anions used in ILs.</figcaption></figure>

Eutectic mixtures are usually considered a new class of ILs and they gained attention because some of them can be prepared from natural sources, which tends to make them more biodegradable, biocompatible and sustainable [55,56]. In this case, they are defined as natural DESs (NADESs) [57,58]. DESs usually result from a mixture of compounds that interact through hydrogen bonding, which leads to charge delocalization resulting in a depression of the melting point of the mixture [49,59–62].

A four-group classification has been proposed by Abbott et al. based on their constituents [48,49].

Types I to III DESs can be defined by the general formula $Cat^{+}X^{-}Y^{-}$, where:

- $Cat^{+}$ is usually a quaternary ammonium or phosphonium salt;

- X is the anionic moiety (generally a halide anion);

- Y is a metal chloride for type I, a metal chloride hydrate for type II and a hydrogen bond donor for type III.

Type IV DESs involve a metal chloride hydrate and hydrogen bond donor.

Most common DESs imply a hydrogen bond donor (HBD) and an ionic hydrogen bond acceptor (HBA), leading to type III DESs as defined by Abbott et al. [48]. DESs are mainly hydrophilic mixtures as they are based on at least one ionic species and strong hydrogen bonding interactions. Most popular DESs are based on quaternary ammonium salts and choline chloride [63] for their HBA part, and on urea, carboxylic acids, alcohols and glycols for their HBD. The most common structures of the HBA and HBD are illustrated in Figure 3. As for the ILs, judicious choices for each component allow the control of parameters such as density, viscosity, acidity or basicity, hydrophobicity, polarity, volatility and the extractability properties of the DES.
<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Structures of some hydrogen bond acceptors (halide salts) and hydrogen bond donors (amines, alcohols, carboxylic acids) used in the formation of deep-eutectic solvents.</figcaption></figure>

Among these properties, the acidic character of DESs, also called acidic deep-eutectic solvents (ADESs), has been highlighted for applications in dissolution, extraction and metal electrodeposition [60]. The acidity of ADESs can be controlled with the acidity of the HBD and these DESs are often divided into two classes: Brønsted and Lewis DESs.

Also, since 2015, with the concept of hydrophobic deep-eutectic solvents (HDESs) proposed by van Osch et al. [64], interest in the synthesis and application of HDESs is strongly increasing [65–67]. It is assumed that the hydrophobic character depends on the hydrophobicity of the components forming the DES. HDESs are mainly type III, involving bulky quaternary ammonium or phosphonium halide ILs as HBAs and hydrophobic carboxylic acids as HBDs. This trend for hydrophobic eutectic mixtures was recently investigated for mixtures solely composed of non-ionic species [68,69]. A fifth class of DES has therefore been proposed to complete the Abbott classification so that it includes such fully organic hydrophobic eutectic solvents involving strong interaction between a hydrogen bond donor and a hydrogen bond acceptor [70]. These type V non-ionic DESs represent a relatively new class of eutectics which have strong potential for the recovery of REEs through solvent extraction.

### 3. Use of ILs and DESs for the Recovery of REEs

#### 3.1. *Leaching Processes*

Conventional leaching processes are based on the use of strong mineral acids (sulphuric, nitric or hydrochloric) as well as alkali media (such as ammonia or sodium hydroxide), despite their poor selectivity and adverse effects for the environment and health. In this regard, ionometallurgy appears as an interesting alternative that deserves to be further explored. In particular, DESs and ILs have been shown to be good solvents for metal oxides [71–75]. Few examples of DES and IL systems have been proposed for the dissolution of materials that incorporate REEs (Table 1).
The solubility of metal oxides, including rare earth oxides (REOs), has been studied using ILs such as betaine bis(trifluoromethylsulfonyl)imide ([Hbet] [NTf₂]). [Hbet] [NTf₂] is a weak Brønsted acidic ionic liquid due to the carboxyl group attached to its cation. The formation of a carboxylate complex between the carboxylic acid group of the ionic liquid and the metal oxides allows the dissolution of metal oxides, which is favoured by the presence of water in the IL phase [76].

Such an [Hbet] [NTf₂] ionic liquid has been applied for the leaching of bauxite residue in order to perform a selective dissolution of REEs against iron [77]. In aqueous solution, the betainium cation exhibits the necessary acidity (higher acidity (pKa = 1.83) than acetic acid (pKa = 4.75)) for the metals' dissolution, as well as complexing properties for the metals' complexation. It has been found that the temperature is a key parameter: the efficiency is much more pronounced when dissolution is carried out at temperatures above 100 °C.

Another application of [Hbet] [NTf₂] highlighted is its role as part of the selective leaching for the recovery of REEs from iron in NdFeB permanent magnets [78]. The useful effect of the roasting of NdFeB permanent magnets, which makes it possible to avoid the formation of the insoluble ternary NdFeO₃ phase, was supported by leaching with an aqueous solution containing the ionic liquid [Hbet] [NTf₂] [79]. The same research group showed that the use of protic ionic liquids, such as pyridine hydrochloride (PyHCl), at high temperature (165 °C) allows the leaching of NdFeB magnets and then the recovery of the REEs by solvent extraction [80].

DES also have excellent dissolution properties due to their capacity to accept/donate protons or electrons to form hydrogen bonds [49,59]. The leaching behaviour of bastnaesite-type deposits, which are the main source of rare earths, was investigated using choline-chloride-based deep-eutectic solvents [81]. For this purpose, single-salt and multicomponent mineral mixtures of REEs (Y, La, Ce, Nd and Sm) in their carbonate form were studied. Dissolution experiments have shown that the DESs based on [choline chloride (ChCl):malonic acid (MA)] and [choline chloride (ChCl):urea + malonic acid mixture (UMA)] display higher performances in dissolving REEs. Bastnaesite ore was therefore subjected to a dissolution step using [ChCl:UMA], highlighting a potential DES system for selective dissolution of heavy REEs in comparison to light REEs and competitive elements resulting from the gangue minerals, such as Ca, Mg and Fe.

Dissolution of REOs has been investigated using a series of Brønsted acidic DESs based on polyols and organic acids [82]. Among the DESs studied, the [ethylene glycol (EG):maleic acid (Maleic) (4:1)] system was highlighted as the optimal solvent due to its strong acidity and low viscosity. Notable differences in the solubilities of REOs have been observed, with considerable solubilization ability identified for the LREEs (except for CeO₂) and MREEs in comparison to the HREEs, which allows the consideration of their separation. The solubility and separation of REE oxides and salts were also recently demonstrated by Söldner et al. [75,83] for various DESs.

NdFeB magnet leachates have also been studied with a deep-eutectic solvent. It has been shown that the complete dissolution of NdFeB magnets can be performed with [choline chloride (ChCl):lactic acid (LAC) (molar ratio 1:2)], while the dissolution is not observed in the case of [ChCl:urea] or [ChCl:ethylene glycol] [84].

It was assumed that the protons present in the lactic acid could react with the metal oxides to dissolve them, and that the coordinating abilities of lactic acid and of choline may have favoured this dissolution. After the complete dissolution, liquid-liquid extraction was applied to separate the REEs from Fe, Co and B and to separate Dy from Nd in a second step.

In a similar approach, a recovery process for simulated NdFeB permanent magnets was proposed using [guanidine hydrochloride (GUC):lactic acid (LAC)]. Such DESs exhibit a high selectivity for the dissolution of Nd in comparison to Fe. Starting from a mixture of Nd₂O₃ and Fe₂O₃ with a mole ratio of 1:7, it was found that 95% of Nd can be dissolved while less than 1% could be for Fe [85]. The particular interest of this application has also
been demonstrated through a patent which is based on the use of the choline chloride/lactic acid system, thus showing the maturity of such a process for the recovery of REEs from NdFeB magnets [86].

Beyond permanent magnets, other end-of-life products, such as fluorescent lamps, have been studied using chloride-based DESs and [Hbet] [NTf₂] ionic liquid. In this case, a [choline chloride:levulinic acid (LevA) (1:3)] system was found to be appropriate for the solubility of the phase containing the REEs, such as red phosphor Y₂O₃:Eu³⁺ (YOX) against halophosphate phosphor (Sr,Ca)₁₀(PO₄)(Cl,F)₂:Sb³⁺, Mn²⁺ (HALO), which does not contain any REEs [87]. Extraction of REEs from the loaded lixiviant DES phase was also examined and showed that metal recovery was possible via solvent extraction, thus allowing the recycling of the DES. The [Hbet] [NTf₂] efficiency for the oxide dissolution was valued for lamp phosphor waste with the selective dissolution of YOX without dissolving the other phosphors, such as the green phosphor LaPO₄:Ce³⁺, Tb³⁺ (LAP), the blue phosphor BaMgAl₁₀O₁₇:Eu²⁺ (BAM) or halophosphate phosphor (HALO) [88]. Also, as an alternative to conventional sulphuric acid leach, the recovery of REEs from waste phosphor can be performed after their leaching in 1-methylimidazolium hydrogen sulphate [C₁Him] [HSO₄] and by oxalic acid precipitation [89].

Coal and coal combustion by-products have been studied as potential sources of REEs. The dissolution of coal in ILs has been recently reviewed [90] and the possibility of recovering REEs has been investigated using an IL and a DES, respectively 1-butyl-3-methylimidazolium chloride [C₁C₄im] [Cl] and a mixture of choline chloride:urea (molar ratio 1:2) [91].

Due to the low melting points of these ionic solvents, they can be used as undiluted or low-dilution leaching agents, which is very different from conventional mineral acid solutions. It implies new mechanisms that are, for now, still not well understood or mastered. Irrespective of the hydrophobic or hydrophilic character of the ionic solvent, water content seems to be a critical point for all the systems, influencing the viscosity, the dissolution rate, the selectivity and even the triggering or the inhibiting of the mechanisms, depending on the situation. Moreover, these low water content ionic systems change the nature of the pregnant leach solution, which therefore strongly affects the following steps of the metal recovery compared to conventional processes [80,84,87].

<table><caption>**Table 1.** Overview of ionic solvent for REE leaching.</caption><thead><tr><td>Ionic Solvent</td><td>Extracted/Feed</td><td>Matrix</td><td>Author</td></tr></thead><tbody><tr><td>ChCl:U + MA (1:1:0.5)</td><td>HREEs leaching against Ca, Mg and Fe</td><td>Carbonate salts</td><td>Entezari-Zarandi et al. [81]</td></tr><tr><td>EG:Maleic (4:1)</td><td>LREEs leaching against HREEs</td><td>REO</td><td>Chen et al. [82]</td></tr><tr><td>ChCl:U</td><td>LREEs leaching</td><td>REO</td><td>Söldner et al. [83]</td></tr><tr><td>[Hbet] [NTf<sub>2</sub>]</td><td>REE leaching</td><td>REO</td><td>Nockemann et al. [76]</td></tr><tr><td>[Hbet] [NTf<sub>2</sub>]</td><td>REE leaching against Fe</td><td>Bauxite residue</td><td>Davris et al. [77]</td></tr><tr><td>[Hbet] [NTf<sub>2</sub>]</td><td>REE leaching against Fe</td><td>NdFeB magnet</td><td>Dupont et al. [78]</td></tr><tr><td>[Hbet] [NTf<sub>2</sub>]</td><td>Nd leaching against Fe</td><td>NdFeB magnet</td><td>Orefice et al. [79]</td></tr><tr><td>[Py] [Cl]</td><td>Complete dissolution</td><td>NdFeB magnet</td><td>Orefice et al. [80]</td></tr><tr><td>ChCl:LAC (1:2)</td><td>Complete dissolution</td><td>NdFeB magnet</td><td>Riaño et al. [84]</td></tr><tr><td>GUC:LAC (1:2)</td><td>Nd leaching against Fe</td><td>NdFeB magnet</td><td>Liu et al. [85]</td></tr><tr><td>ChCl:LevA (1:3)</td><td>YOX phosphor (Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>)</td><td>Fluorescent lamps</td><td>Pateli et al. [87]</td></tr><tr><td>[Hbet] [NTf<sub>2</sub>]</td><td>YOX phosphor (Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>)</td><td>Fluorescent lamps</td><td>Dupont et al. [88]</td></tr><tr><td>[C<sub>1</sub>Him] [HSO<sub>4</sub>]</td><td>YOX phosphor (Y<sub>2</sub>O<sub>3</sub>:Eu<sup>3+</sup>)</td><td>Fluorescent lamps</td><td>Schaeffer et al. [89]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [Cl]; ChCl:U (1:2)</td><td>Complete dissolution</td><td>Coal by-products</td><td>Rozelle et al. [91]</td></tr></tbody></table>
## 3.2. *Solvent Extraction Processes*

As mentioned before, solvent extraction or liquid-liquid extraction is the most commonly used process for the recovery of rare earth elements after the leaching step.

Since the pioneering work of Dai et al. [92] highlighting the potential of ILs for solvent extraction in the context of metal recovery, many studies have been developed showing their potential in terms of efficiency in comparison to classical organic solvents [93].

As already mentioned, solvent extraction exploits the different distribution of a solute between two immiscible liquids. In most cases, the pregnant leach solutions, which contain the REEs to be extracted, are in an aqueous phase that makes contact with the hydrophobic ILs, leading to the formation of biphasic systems [94].

Some hydrophobic ILs that are immiscible with nonpolar organic solvents can also be engaged in triphasic extraction systems and are named water/organic solvents/ILs [95]. Moreover, water-miscible ILs can also form two-phase systems with aqueous solutions in the presence of kosmotropic ions, which contribute to the stability and structure of water-water interactions, such as $K_3PO_4$, $K_2CO_3$ and $Na_2S_2O_3$ in the aqueous solution. The hydrophilic ILs can thus be separated from the aqueous phase and be further re-used [96].

Ionic liquid-based aqueous biphasic systems (IL-ABSs) are also encountered as systems for the extraction and separation of different metal species but much more marginally compared to triblock copolymer-based ABSs [97].

In conventional solvent extraction, as the hydrated metallic cations tend to stay in the aqueous solution, extractants are usually required in the organic phase to form complexes with the metal and increase its hydrophobicity. This therefore favours the transfer of the metallic ions from the aqueous phase to the hydrophobic IL phase.

It has been highlighted that, in some cases, the solvation of extractant-metal complexes is more thermodynamically favoured in ILs than in conventional organic solvents. [92] This property is a major advantage of using ILs for metal ion extractions since in many cases they exhibit significantly higher extraction performance and improved metal selectivity than conventional solvents [98]. Consequently, IL-based extraction systems have been prepared using several extractants or synergistic systems diluted in ILs, or by using task-specific ionic liquids (TSILs), which refer to ILs that incorporate the extracting moieties into the cation or the anion of the ionic liquid.

In conventional diluents, solvent extraction mechanisms are based on the molecular and supramolecular interaction properties of the extractant molecule aggregates due to their amphiphilic nature. This aptitude has also been demonstrated for the IL-based organic phase [99].

Usually, the extraction mechanisms in an IL differ considerably from those in traditional organic diluents [100,101]. ILs are capable of solubilizing neutral or charged complexes, while only neutral species (or neutral ion pairs) are extracted in conventional solvents. These extraction modes and mechanisms are based on the principle of electroneutrality. Therefore, if charged species are extracted in the IL phase, they have to be counterbalanced in the aqueous phase. Depending on the aqueous leach solutions, the nature of the extractant and of the IL involved in the extraction systems, various mechanisms can therefore be encountered, such as ion exchange, when the cations or anions of ILs might be transferred into the aqueous phase during extraction; neutral extraction; or neutral coextraction [102,103]. Three main kinds of molecular mechanisms have been identified when ionic liquids are used as extraction diluents. Taking as an example an extractant in an imidazolium-type ionic liquid, these mechanisms are illustrated within the Figure 4 and can be described as follows:

- Neutral extraction mechanism: A neutral complex is formed and extracted in the IL phase. This mechanism is similar to those encountered in conventional solvents; however, the detailed mechanisms are often different. In this extraction mode, the IL acts as a polar non-aqueous solvent [104];
- Cation exchange: This extraction mode is specific to IL systems. The IL acts as a liquid ion exchanger with cationic complexes extracted from the aqueous phase into the IL phase. Consequently, an IL cation has to be transferred to the aqueous phase while, in exchange, a metallic cation is transferred to the IL phase to respect electroneutrality [105];

- Anion exchange: This extraction mode is also specific to IL systems. An anionic extractant forms over-neutralized anionic complexes that can be transferred to the IL phase, in exchange for the transfer of an IL anion to the aqueous phase [106]. Anion exchange is less common than cation exchange.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Schematic representation of mechanisms encountered in IL-based extraction systems.</figcaption></figure>

In comparison with the extraction behaviours of conventional organic systems, the mechanism in IL-based extraction systems exhibits high complexity. In some cases, the IL-based systems involve polyvalent metals and cationic or anionic extractants or polyvalent ILs can be encountered, which leads to mixed extraction mechanisms.

Ion-exchange mechanisms are the main drawback of the IL-based extraction systems, since a part of the IL is lost in the aqueous phase and can be considered as sacrificial. The main factor influencing this exchange mechanism is the solubility in the aqueous phase of the IL component. It has been shown that hydrophobic cations can moderate these exchanges, whereas the phenomenon is increased with hydrophilic anions [107,108]. The increase of the hydrophobicity, such as the alkyl chain length of the IL cation, can lead to a switchover of the mechanism from ion exchange to neutral complex extraction [109]. Aqueous phase acidity has also been shown to play a role in the ion-exchange mechanisms of some systems [110].

### 3.3. IL-Based Extraction Systems for REE Recovery

Many systems, including cation exchangers (Table 2) and neutral extractants (Table 3), as well as synergistic systems (Table 4), have been studied for the recovery of REEs and have shown their potential in common solvents. This is why the feasibility of using these various systems has been implemented within the framework of IL-based extraction for the recovery of REEs [111,112]. Applications of mixtures of ILs and molecular extractants in metal solvent extraction technology have been recently reviewed by Wang et al. [113].

#### 3.3.1. Ionic Exchangers

The cationic exchangers may exhibit a function with an acidic character (possessing a labile proton), such as carboxylic (diglycolamic acid), phosphoric (HDEHP: bis (2-ethylhexyl)phosphoric acid), phosphonic (PC-88A: 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester) and phosphinic (Cyanex-272: bis(2,4,4-trimethylpentyl) acids), or they may
function as chelating extractants, such as beta-diketone (HTTA: 2-thenoyltrifluoro acetone, HBA: benzoylacetone). In these cases, metal ions are complexed by the conjugate bases.

Among these extractants, numerous studies have been dedicated to the use of the beta-diketone HTTA in imidazolium-based IL ([C₁Cₙim] [NTf₂]) [114–118]. Since the work of Jensen et al. with HTTA in ILs showing the presence of anionic lanthanide complexes in [C₁C₄im] [NTf₂] [106], comprehensive investigations of REE extraction have suggested the unusual extraction mechanisms of such ionic exchangers. Anionic lanthanide complexes are made possible by the exchange of the ionic liquid anions into the aqueous phase for the lanthanide complex at high HTTA concentrations, while at lower concentrations, Ln³⁺ was extracted as a neutral Ln(TTA)₃ complex.

Eu³⁺ chelation with HTTA into [C₁Cₙim] [NTf₂] has been investigated with time-resolved laser-induced fluorescence spectroscopy (TRLFS) and highlighted that a mixture of neutral Eu(TTA)₃ and anionic Eu(TTA)₄⁻ complexes can be encountered [114]. The stoichiometry of extracted complexes, such as the Ln(TTA)₄⁻ anionic complex, was confirmed by emission profiles and spectrophotometric titration respectively for Eu³⁺/HTTA and Nd³⁺/HTTA complexes [115]. The role of the IL anion with regard to Ln chelates and the extraction mechanism has been evidenced for the bis(trifluoromethanesulfonyl)imide anion (NTf₂) [116], as well as nonafluoro-1-butanesulfonate (CF₃(CF₂)₃SO₃⁻) [117]. The efficiency of HTTA has thus been demonstrated for various REEs [118]; other beta-diketones, such as benzoylacetone (HBA), also show the interest of this type of extractant. This aptitude for REE extraction has also been studied within the implementation of synergistic systems in combination with neutral extractants.

The feasibility for the selective recovery of REEs using [C₁C₈im] [NTf₂] with DODGAA as extractant has been demonstrated with sulphuric and nitric acid solutions [119,120].

The extraction performance of DODGAA in [C₁C₄im] [NTf₂] for REEs (Y, La, Ce, Eu and Tb) with an acidic leach solution of phosphor powders coming from waste fluorescent lamps was examined and demonstrated selective extraction toward metal impurities such as Fe, Al and Zn for both leaching nitric and sulphuric acid solutions [121].

Recently, solvent impregnated resins (SIRs) based on DODGAA and [C₁C₄im] [NTf₂] have been developed for extraction chromatography of lanthanides [122], as well as for the recovery of REEs from leach solutions of thin film phosphors [123]. Another diglycolamic acid (HDEHDGA) has been studied by Rout et al., confirming the potential of this type of extractant in ionic liquid media such as tetraoctylammonium dodecyl sulphate, [N₈₈₈] [DS] [124] or methyltrioctylammonium bis (trifluoromethanesulfonyl)imide ([N₁₈₈₈] [NTf₂]) [125].

Despite Cyanex-272 and HDEHP being widely used as metal extractants, there are only a few studies reporting their use in the context of IL-based extraction systems for REE recovery. Cocalia et al. [104] have studied these extractants and described identical extraction behaviour and coordination of f-element cations using 1-decyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₁C₁₀im] [NTf₂]) and n-dodecane. Other studies have shown their potential in various ionic liquid diluents, such as [C₁Cₙim] [NTf₂] and [C₁Cₙim] [BETI] (n = 4–10) [126], or compared to diglycolamic acid extractant [124,125]. In some cases the conjugate bases of acidic and chelating extractants have been used to replace the IL anion in order to develop TSIL (see below).

Stripping from the extraction phase is an important aspect to take into consideration. REE extraction efficiency often depends on acid feed concentration, which usually allows the stripping of some REEs by playing with the pH. The stripping can be attributed partly to a lack of dissociation of the chelating extractant or protonation of the acidic extractant. Indeed, for acidic extractants, the extraction ability usually decreases for high acidic concentrations in the feed solution. REEs can therefore be stripped from the ionic phase by strongly acidic solutions. The stripping percentage can be gradually increased by increasing the acidity of the stripping phase.
The efficiency of the stripping is also related to the affinity of the extracting phase for the metal. Indeed, with greater affinities, the back-extraction will be more difficult. In some cases, complex-forming agents, such as ethylenediaminetetraacetic acid (EDTA) and di-ethylenetriaminepentaacetic acid (DTPA), need to be added to the aqueous stripping phase.

With regard to the reuse of the ionic phase, it must generally undergo an additional clean-up step (a scrubbing step with a base, for example) in order to deprotonate the acidic extractant engaged in the ionic based-extraction system.

<table><caption>**Table 2.** Examples of acidic and chelating extractants in ILs for the recovery of REEs.</caption><thead><tr><th>Extractants</th><th>Ionic Solvent</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td rowspan="5">[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 2–10<br>[C<sub>1</sub>C<sub>4</sub>im] [NfO]<br>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 2, 4</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Eu(III) chelate</td><td>Okamura et al. [114]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Nd(III) chelate</td><td>Gujar et al. [115]</td></tr><tr><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 2–10</td><td>Eu(III) chelate</td><td>Okamura et al. [116]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NfO]</td><td>Nd; Eu (NaClO<sub>4</sub><sup>-</sup>)</td><td>Jensen et al. [117]</td></tr><tr><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 2, 4</td><td>Ce (NO<sub>3</sub><sup>-</sup>)</td><td>Hidayah et al. [118]</td></tr><tr><td rowspan="3">[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 8, 12<br>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 8, 12<br>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 8, 12</td><td>REEs (SO<sub>4</sub><sup>2-</sup>)</td><td>Yang et al. [119]</td></tr><tr><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 8, 12</td><td>Y, Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Kubota et al. [120]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Y, La, Ce, Eu, Tb (SO<sub>4</sub><sup>2-</sup>; NO<sub>3</sub><sup>-</sup>)</td><td>Yang et al. [121]</td></tr><tr><td rowspan="2">[N<sub>8888</sub>] [DS]<br>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>[N<sub>8888</sub>] [DS]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [124]</td></tr><tr><td>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [125]</td></tr><tr><td rowspan="3">[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4–10<br>[N<sub>8888</sub>] [DS]<br>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4–10</td><td>Ln (NO<sub>3</sub><sup>-</sup>)</td><td>Sun et al. [126]</td></tr><tr><td>[N<sub>8888</sub>] [DS]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [124]</td></tr><tr><td>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [125]</td></tr><tr><td rowspan="2">[N<sub>8888</sub>] [DS]<br>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>[N<sub>8888</sub>] [DS]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [124]</td></tr><tr><td>[N<sub>1888</sub>] [NTf<sub>2</sub>]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [125]</td></tr></tbody></table>

### 3.3.2. Neutral Extractants

A typical neutral extractant has donor groups (O, S, P, N) without labile hydrogen. The extractant (a hydrophobic Lewis base) acts through its donor power, giving rise to interactions of the acceptor-donor type with the neutral metal species of the aqueous phase. The extracted species is solvated in an electrically neutral molecular form.

The most widely used solvating compounds are the neutral organophosphorus derivatives, including phosphines, phosphine oxides, phosphinates, phosphonates and phosphates. The extraction of neutral metal complexes by solvation is performed through their O-donor character [127]. This ability for the extraction and separation of REEs has also been examined in ILs.

Tri-*n*-octylphosphine oxide (TOPO) was engaged with [C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] (n = 4, 8) for the extraction of REEs from a nitrate medium, showing the importance of the cation of the IL in the extraction mechanism. REEs were extracted through the exchange of the metal ion and the imidazolium cations of the IL. In addition, for the IL-based TOPO extraction systems, an inversion of selectivity for the REEs was observed in comparison to an organic solvent such as *n*-dodecane, which can be explained by the difference in the coordination sphere [128].

Cyanex 923, which is a mixture of four trialkylphosphine oxides, exhibits extraction properties similar to TOPO. Cyanex 923 has been used in ionic liquid phases for the extraction of REEs from nitric acid medium using ILs containing a bis(trifluoromethylsulfonyl) imide anion ([C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>], [N<sub>1444</sub>] [NTf<sub>2</sub>] and [P<sub>66614</sub>] [NTf<sub>2</sub>]) [129], while nitrate
ILs, such as trioctylmethylammonium nitrate ([N<sub>1888</sub>] [NO<sub>3</sub>]) and trihexyl(tetradecyl) phosphonium ([P<sub>66614</sub>] [NO<sub>3</sub>]), were employed for concentrated chloride aqueous solutions [130].

Acquisition of these results led to the implementation of a process in continuous mode for the separation of Dy and Nd from chloride media using Cyanex 923 diluted in [P<sub>66614</sub>] [SCN] [131].

Tri-*n*-butyl phosphate (TBP) is widely used for metal recovery. It has been employed in [N<sub>1888</sub>] [NO<sub>3</sub>] for the recovery of REEs from an aqueous nitrate phase from spent Nd-Fe-B magnets [132].

This possibility has also been investigated for Pr(III), Nd(III) and Dy(III) in aqueous phase containing a bis(trifluoromethyl-sulfonyl)amide anion with the aim of undertaking electrodeposition after their extraction by TBP in triethyl-pentyl-phosphonium [P<sub>2225</sub>] [NTf<sub>2</sub>]. A combination ion-pair and cation-exchange mechanism was observed and the direct electrodeposition of the extracted [Nd(TBP)<sub>3</sub>]<sup>3+</sup> in the TBP IL-based system was demonstrated [133].

The efficiency and selectivity of REE(III) extraction from chloride media has also been demonstrated using the neutral extractant octyl(phenyl)-N,N-diisobutyl-carbamoylmethylphosphine oxide (CMPO) in [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>] [134,135]. A CMPO IL-based system ([C<sub>1</sub>C<sub>4</sub>im] [PF<sub>6</sub>]) has been previously used for the recovery of REEs from nitrate aqueous solution with an enhancement in the efficiency in comparison to conventional organic solvents [136].

Moreover, the usual neutral organophosphorus derivatives, phosphoryl-containing po-
dands, have been also investigated for REE recovery from a nitric acid, such as triphosphine
trioxide (TPO) [137] in *N*-butyl, *N*-octyl-*N*-ethylpiperidinium bis(trifluoromethylsulfonyl)
imide [C<sub>2</sub>C<sub>4</sub>Pip] [NTf<sub>2</sub>] or [C<sub>2</sub>C<sub>8</sub>Pip] [NTf<sub>2</sub>], or diethyl 2- [(diphenylphosphoryl)methoxy]-
5-ethylphenylphosphonate [138] in [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>].

Diglycolamides (DGAs), with their tridentate O-donating ligands, have the ability to form stable complexes with lanthanides and actinides. Therefore, diglycolamide-based systems in IL have been investigated for the recovery of such metals [139]. It was found that N,N,N',N'-tetraoctyl diglycolamide (TODGA) allows the enhancement of the extraction performance for REEs when it is diluted in [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>] in comparison to an isooctane system. Significant differences were observed in the extraction mechanisms, since a cation-exchange mechanism is involved for the TODGA-ILs systems while it is ion pair extraction in isooctane. Also, a difference in the selectivity was noticed, with a preference for MREEs in the ILs systems and HREEs in the isooctane system [140]. A kinetics study of metal transfer (Eu<sup>3+</sup>) from a nitric solution to a TODGA- [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>] phase showed that it depends on the nitric acid and TODGA concentrations [141].

The possibility of transferring REEs from a Cl⁻ aqueous phase was investigated using the systems TODGA- [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>] [135] and TODGA- [N<sub>1114</sub>] [NTf<sub>2</sub>], or TODGA- [C<sub>1</sub>C<sub>4</sub>Pyro] [NTf<sub>2</sub>], by studying the solvation of REEs with electronic absorption spectroscopy [142]. Murakami et al. [143] studied the possibility of using a TODGA- [P<sub>2225</sub>] [NTf<sub>2</sub>] system to recover REEs from an aqueous phase containing a bis(trifluoromethyl-sulfonyl)amide anion with a similar approach as in their study with TBP- [P<sub>2225</sub>] [NTf<sub>2</sub>] [133].

Other studies have focused on REE complexation by diglycolamides in [C<sub>1</sub>C<sub>8</sub>im] [NTf<sub>2</sub>], showing that DGA ligands are bonded with the metal ion in a tridentate fashion via 2 carbonyl O atoms and one etheric O atom, as observed in organic solvent [144]. Also, a high loading of metals is possible without third phase formation, contrary to what can be observed with molecular diluents [145]. Similarly, other DGAs have been used, such as N,N'-dimethyl-N,N'-dioctyl-3-oxadiglycolamide (DMDODGA) in [C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>] [146]. The use of DGAs has been studied by taking advantage of their preorganization on macro-cyclic platforms, such as calix [4]arene. As demonstrated in other studies, DGA-based extraction systems result in different behaviours in ILs with regard to the efficiency and selectivity for REEs [147].
There are on-going studies investigating the use of malonamide as part of an IL-based extracting system for the recovery of REEs, with a focus on the molecular and supramolecular mechanisms involved in such systems [148,149].

As already discussed, stripping is an important step to consider for the recovery of REEs and the reusability of the ionic phase. As neutral extractants are able to extract acids, stripping can be performed by using a high acidic stripping phase. Promoting the extraction of acid in the ionic phase allows the release of metals extracted from this phase.

The mechanism of the stripping reaction is attributed to exchange of REE ions with protons in the ionic phase. However, the presence of acid in the ionic phase can have a negative impact; for systems with an NTf₂⁻ anion, this leads to the formation of triflimide (HNTf₂), which is partially lost in the aqueous stripping phase due to its high solubility in water.

<table><caption>**Table 3.** Examples of neutral extractants in ILs for the recovery of REEs.</caption><thead><tr><th>Extractants</th><th>Ionic Solvent</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td>TOPO</td><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 8</td><td>Y, Eu, Dy, Nd (NO<sub>3</sub><sup>-</sup>)</td><td>Yang et al. [128]</td></tr><tr><td>Cyanex 923</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>], [N<sub>1444</sub>] [NTf<sub>2</sub>],<br>[N<sub>1888</sub>] [NTf<sub>2</sub>], [P<sub>66614</sub>] [NTf<sub>2</sub>]<br>[N<sub>1888</sub>] [NO<sub>3</sub>], [P<sub>66614</sub>] [NO<sub>3</sub>]<br>[P<sub>66614</sub>] [SCN], [P<sub>66614</sub>] [NO<sub>3</sub>]</td><td>Nd (NO<sub>3</sub><sup>-</sup>)<br>La, Ce, Pr (Cl<sup>-</sup>)<br>Nd, Dy (Cl<sup>-</sup>)</td><td>Rout et al. [129]<br>Regadío et al. [130]<br>Riaño et al. [131]</td></tr><tr><td>n=8; n'=6; n=6; n'=8/n=n'=6/n=n'=8</td><td>[N<sub>1888</sub>] [NO<sub>3</sub>]<br>[P<sub>2225</sub>] [NTf<sub>2</sub>]</td><td>Pr, Nd, Dy (NO<sub>3</sub><sup>-</sup>)<br>Pr, Nd, Dy (NTf<sub>2</sub>)</td><td>Kikuchi et al. [132]<br>Matsumiya et al. [133]</td></tr><tr><td>CMPO</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>6</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>REEs (Cl<sup>-</sup>)<br>Ce, Eu, Lu (Cl<sup>-</sup>)<br>REEs (NO<sub>3</sub><sup>-</sup>)</td><td>Turanov et al. [134]<br>Atanassova et al. [135]<br>Nakashima et al. [136]</td></tr><tr><td>TPO</td><td>[C<sub>2</sub>C<sub>n</sub>Pip] [NTf<sub>2</sub>] n = 4, 8</td><td>La, Nd, Eu, Dy, Yb (NO<sub>3</sub><sup>-</sup>)</td><td>Turgis et al. [137]</td></tr><tr><td>TODGA</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>6</sub>im] [NTf<sub>2</sub>] n = 2, 4, 6<br>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]<br>[N<sub>1114</sub>] [NTf<sub>2</sub>], [C<sub>1</sub>C<sub>4</sub>Pyro] [NTf<sub>2</sub>]<br>[P<sub>2225</sub>] [NTf<sub>2</sub>]</td><td>REEs (NO<sub>3</sub><sup>-</sup>)<br>Ce, Eu, Lu (Cl<sup>-</sup>)<br>La, Eu, Lu (NO<sub>3</sub><sup>-</sup>)<br>Eu (NO<sub>3</sub><sup>-</sup>)<br>Sm, Eu, Yb (Cl<sup>-</sup>)<br>Pr, Nd, Dy (NTf<sub>2</sub>)</td><td>Atanassova et al. [135]<br>Shimojo et al. [140]<br>Sypula et al. [141]<br>Pan et al. [142]<br>Murakami et al. [143]</td></tr><tr><td>TRDGA</td><td>[C<sub>1</sub>C<sub>8</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>8</sub>im] [NTf<sub>2</sub>]<br>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Nd, Eu (NO<sub>3</sub><sup>-</sup>)<br>Eu (NO<sub>3</sub><sup>-</sup>)<br>La, Nd, Sm, Gd, Yb (NO<sub>3</sub><sup>-</sup>)</td><td>Ansari et al. [144]<br>Rama et al. [145]<br>Chen et al. [146]</td></tr><tr><td></td><td>[C<sub>2</sub>C<sub>8</sub>Pip] [NTf<sub>2</sub>]</td><td>La, Eu, Yb (NO<sub>3</sub><sup>-</sup>)</td><td>Whebie et al. [147]</td></tr></tbody></table>
### 3.3.3. Combination of Extractant and Synergistic Systems

As mentioned above, many studies have demonstrated remarkable favourable changes, in terms of extraction mechanisms, efficiency and selectivity, for a variety of IL-based systems [150]. These extractants can also be used in combination to achieve, under certain conditions, synergistic systems in which the interactions of the two extractants produce a better effect than the sum of their individual effects. In some cases, the term “synergism” is used when the extractants are engaged in a simple combination to improve their efficiency or selectivity in cooperative ways. Synergistic mixtures of extractants, for which improvements of extraction efficiency have been demonstrated in organic solvents, have been studied in IL-based systems.

Taking into account the efficiency of beta-diketone (HTTA) and benzoylacetonone (HBA), these extractants have been combined or engaged in synergistic systems with different neutral extractants, such as TOPO or CMPO.

Accordingly, we can notably cite the systems HTTA-TOPO [151–154], HBA-TOPO [154,155], HTTA-CMPO [156] and HBA-CMPO [157], which have all been diluted in various [C₁Cₙim] [NTf₂].

A combination of neutral extractants has also been investigated and demonstrated that the addition of TBP to a TODGA- [C₁C₈im] [NTf₂] system makes it possible to increase the efficiency for REE extraction [145], while a TODGA-CMPO mixture in [C₁Cₙim] [NTf₂] phases highlighted antagonism behaviour in the transfer of REEs (Ce, Eu and Lu) from Cl⁻ aqueous solutions [135]. Recently, the selective separation of Gd(III) from Sm(III) in a nitric acid solution was described in the presence of a mixture of organophosphorus extractants (TBP-Cyanex 272 or TBP-HDEHP) in the [C₁C₆im] [PF₆] [158].

<table><caption>**Table 4.** Examples of combinations of extractants in ILs for the recovery of REEs.</caption><thead><tr><th>Extractants</th><th>Ionic Solvent</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td rowspan="4">HTTA-TOPO</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Ln (HTTA)</td><td>Okamura et al. [151]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Sc, Y, La, Nd, Eu, Dy (NO<sub>3</sub><sup>-</sup>)</td><td>Zhao et al. [152]</td></tr><tr><td>[P<sub>2225</sub>] [NTf<sub>2</sub>]</td><td>Pr, Nd, Dy (NTf<sub>2</sub>)</td><td>Matsumiya et al. [153]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>La, Eu, Lu (Cl<sup>-</sup>)</td><td>Okamura et al. [154]</td></tr><tr><td rowspan="2">HBA-TOPO</td><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>Eu (Cl<sup>-</sup>)</td><td>Okamura et al. [154]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>im] [NTf<sub>2</sub>]</td><td>La, Nd, Eu, Dy, Lu (Cl<sup>-</sup>)</td><td>Hatakeyama et al. [155]</td></tr><tr><td>HTTA-CMPO</td><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4–10</td><td>Eu (Cl<sup>-</sup>)</td><td>Atanassova et al. [156]</td></tr><tr><td rowspan="2">HBA-CMPO</td><td>[C<sub>1</sub>C<sub>n</sub>im] [NTf<sub>2</sub>] n = 4, 10</td><td>Eu (Cl<sup>-</sup>)</td><td>Atanassova et al. [157]</td></tr><tr><td>[C<sub>1</sub>C<sub>4</sub>pyrro] [NTf<sub>2</sub>]</td><td>Eu (Cl<sup>-</sup>)</td><td>Atanassova et al. [157]</td></tr><tr><td>TBP-TODGA</td><td>[C<sub>1</sub>C<sub>8</sub>im] [NTf<sub>2</sub>]</td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Rama et al. [145]</td></tr><tr><td>CMPO-TODGA</td><td>[C<sub>1</sub>C<sub>6</sub>im] [NTf<sub>2</sub>]</td><td>Ce, Eu, Lu (Cl<sup>-</sup>)</td><td>Atanassova et al. [135]</td></tr><tr><td>TBP-Cyanex 272</td><td>[C<sub>1</sub>C<sub>6</sub>im] [PF<sub>6</sub>]</td><td>Gd, Sm (NO<sub>3</sub><sup>-</sup>)</td><td>Asadollahzadeh et al. [158]</td></tr><tr><td>TBP-HDEHP</td><td>[C<sub>1</sub>C<sub>6</sub>im] [PF<sub>6</sub>]</td><td>Gd, Sm (NO<sub>3</sub><sup>-</sup>)</td><td>Asadollahzadeh et al. [158]</td></tr></tbody></table>

### 3.3.4. Task-Specific Ionic Liquids (TSILs)

As they refer to ILs that incorporate a chelating site, TSILs combine the properties of ionic liquids with those of conventional extracting compounds [159]. TSILs can therefore act both as the organic phase as well as the extracting agent, avoiding the difficulties that can be encountered through extractant/solvent miscibility. Extraction experiments have thus been performed using undiluted TSILs or diluted in an IL. The incorporation of functional groups, which act as complexation sites for the REEs, can be undertaken on the anionic or cationic parts of the IL.

* TSILs with functionalized anion (Table 5)

TSILs with functionalized anions usually exhibit an anionic chelation site, which has strong coordinating properties for the REEs. REEs can be coordinated by anions with
complexation abilities, such as thiocyanate [SCN⁻] or nitrate [NO₃⁻], as demonstrated by various studies from Binnemans' research group. These studies are mainly focused on recycling processes, such as the recovery of REEs from magnets [160,161], batteries [162] and fluorescent lamp waste [163]. Also, the conventional IL anion can be replaced by an anionic extractant.

HDEHP is certainly one of the most useful and efficient acidic extractants. However despite the ability of ILs to solubilize a wide range of solutes, it appears that HDEHP is poorly soluble in these media [164].

To overcome such a drawback, the extractant has been engaged as an anion in various TSIL systems. The extraction behaviour of neodymium(III) from a nitrate medium was studied using 1-hexyl-3-methylimidazolium bis(2-ethylhexyl)phosphate ([C₁C₆im] [DEHP]), 1-hexyl-1-methylpyrrolidinium bis(2-ethylhexyl)phosphate ([C₁C₆pyr] [DEHP]) and tetrabutylammonium bis(2-ethylhexyl)phosphate ([N₄₄₄] [DEHP]), diluted in their corresponding bis(trifluoromethylsulfonyl)imide IL [165]. Sun et al. studied various DEHP-type ionic extractants with ammonium (tetrabutylammonium [N₄₄₄], tetraoctylammonium [N₈₈₈]) or phosphonium (trihexyl(tetradecyl)phosphonium [P₆₆₆₁]) cations diluted in [C₁Cₙim] [NTf₂] or [C₁Cₙim] [BET]. Higher REE extraction performances were obtained in ILs with better solubilities and stabilities for REE complexes in comparison to organic solvents [166–168]. It has been highlighted that the driving force of DEHP-based TSIL extraction systems is mainly ion interaction rather than ion exchange. Also, DEHP-based TSILs have been used in a synergistic extraction using Cyanex 923 as molecular extractant, which made it possible to increase the efficiency of the separation of middle to heavy REEs (Eu³⁺, Dy³⁺, Er³⁺) from light REEs (La³⁺) [169].

Recently, more and more studies have described the use of Cyphos® IL 104 as an extracting phase. This phosphonium-based ionic liquid [P₆₆₆₁] uses Cyanex 272 as part of the anion of the IL.

Solvent extractions of Y from chloride and nitrate solutions were investigated with this TSIL and compared to ammonium-based ionic liquids [N₁₈₈] with Cyanex 272 as an anion [170]. Other studies have shown the efficiency of such systems for various REEs from various feed solutions [171,172]. Also, it has been shown that Cyphos® IL 104 can be used for the separation of rare earths, with an ionic liquid [C₁C₂im] [Cl] phase allowing an ionic liquid-ionic liquid extraction system, for the recycling of valuable metals from nickel metal hydride (NiMH) batteries [173].

Other phosphorous acidic extractants have been considered as anionic species for the development of TSILs, such as 2-ethylhexyl phosphoric acid mono(2-ethylhexyl) ester, which is named P507 or PC-88A. As an example, the extraction of mid-heavy REEs from H₂SO₄ media has been highlighted using [N₁₈₈] [P507] [174]. In some cases, the TSIL is used as an extractant that can be diluted in organic solvent. This was proposed by Dutta et al. [175] for the liquid-liquid extraction of gadolinium from a nitric acid medium to an organic phase consisting of [N₁₈₈] [P507] extractant dissolved in *n*-dodecane.

As shown in different studies, beta-diketones are also widely used for the recovery of REEs. They have been studied as anions for the implementation of TSIL-based extraction systems [176,177]. This extractant was used for the application of a homogeneous liquid-liquid extraction process involving a hexafluoroacetylacetone [hfac] as anion in an IL-based system involving [Ch] [hfac] and [Ch] [NTf₂]. Extraction of Nd(III) was performed by taking advantage of [Ch] [NTf₂] thermomorphic behaviour in water and of the formation of an anionic tetrakis(hexafluoroacetylacetonato)Nd(III) complex, with a choline cation as counter ion. It was accompanied by an ion exchange of three choline ions from the IL phase to the aqueous phase [177].

Other coordinating anions have also been considered, such as diglycolamate or carboxylate anions. A TSIL based on trioctylmethylammonium dioctyl diglycolamate [N₁₈₈] [DGA] diluted in [N₁₈₈] [NO₃] was proposed for the recovery of Nd(III), and it showed good extraction properties from nitrate media in comparison to chloride media [178]. Undiluted tetraoctylphosphonium oleate [P₈₈₈] [oleate] was used as a bifunctional extractant
for the recovery of metals from an aqueous chloride feed solution [179]. The separation of transition metals from REEs was considered by adjusting the chloride concentration and pH.

In addition, the potential of ionic liquids exhibiting a carboxylic acid group on their anion has been shown in various studies. Sec-Octylphenoxy acetic acid (CA-12) and sec-nonylphenoxy acetic acid (CA-100) have been applied in combination with trioctyl-methylammonium as the bifunctional ionic liquid extractants [N<sub>1888</sub>] [CA-12] and [N<sub>1888</sub>] [CA-100] for the selective extraction of La(III) from REEs into a chloride medium [180].

The same system has been involved for the extraction of REEs from a nitrate medium [181]. In a similar approach, the feasibility of a selective extraction of Y(III) from a chloride medium was demonstrated using a [N<sub>1888</sub>] [CA-12] system [182], with an improvement in the efficiency and phase separation when tri-n-butyl phosphate (TBP) was used as a phase modifier [183]. Another example that can be cited is the use of benzoate anion in phosphonium-based ionic liquid in the context of Nd(III) extraction from an aqueous solution of NdCl<sub>3</sub> with and without hydrochloric acid (HCl) [184].

Synergistic extraction strategies have been proposed using TSILs with functionalized anions, such as mixtures of [N<sub>1888</sub>] [CA-12] and Cyphos® IL 104 for HREE separation from chloride solution [185].

<table><caption>**Table 5.** Examples of TSILs with functionalized anions for the recovery of REEs.</caption><thead><tr><th>TSIL Anion</th><th>TSIL Cation</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td rowspan="5">DEHP</td><td rowspan="5">[C<sub>1</sub>C<sub>6</sub>im]<sup>+</sup>, [C<sub>1</sub>C<sub>6</sub>pyrro]<sup>+</sup> [N<sub>4444</sub>]<sup>+</sup><br/>[N<sub>4444</sub>]<sup>+</sup>, [N<sub>1888</sub>]<sup>+</sup>, [N<sub>66614</sub>]<sup>+</sup><br/>[N<sub>1888</sub>]<sup>+</sup><br/>[N<sub>2222</sub>]<sup>+</sup>, [N<sub>4444</sub>]<sup>+</sup>, [N<sub>6666</sub>]<sup>+</sup>,<br/>[N<sub>8888</sub>]<sup>+</sup>, [N<sub>1888</sub>]<sup>+</sup><br/>[N<sub>2222</sub>]</td><td>Nd (NO<sub>3</sub><sup>-</sup>)<br/>REEs (DTPA-glycolic acid)</td><td>Rout et al. [165]<br/>Sun et al. [166]</td></tr><tr><td>REEs (DTPA-glycolic acid)</td><td>Sun et al. [167]</td></tr><tr><td>REEs (DTPA-glycolic acid)</td><td>Sun et al. [168]</td></tr><tr><td>Nd, Eu, Dy, Er (Cl<sup>-</sup>)</td><td>Sun et al. [169]</td></tr><tr><td>Cyphos® IL 104</td><td></td><td>Y (Cl<sup>-</sup>,NO<sub>3</sub><sup>-</sup>)</td><td>Devi et al. [170]</td></tr><tr><td rowspan="3">Cyanex 272</td><td rowspan="3">[P<sub>66614</sub>]<sup>+</sup></td><td>La, Nd, Gd, Lu (Cl<sup>-</sup>)</td><td>Kumari et al. [171]</td></tr><tr><td>Ce, La (NO<sub>3</sub><sup>-</sup>)</td><td>Makowka et al. [172]</td></tr><tr><td>Y, La, Nd, Eu, Dy, Ho, Yb ([C<sub>1</sub>C<sub>2</sub>im] [Cl])</td><td>Rout et al. [173]</td></tr><tr><td rowspan="2">P507</td><td>[N<sub>1888</sub>]<sup>+</sup></td><td>Y, La, Gd, Tb, Tm, Yb, Lu (SO<sub>4</sub><sup>2-</sup>)</td><td>Shen et al. [174]</td></tr><tr><td>[N<sub>1888</sub>]<sup>+</sup></td><td>Gd (NO<sub>3</sub><sup>-</sup>)</td><td>Dutta et al. [175]</td></tr><tr><td rowspan="2">F<sub>5</sub>C<sub>2</sub>CF<sub>3</sub> TTA</td><td rowspan="2">[C<sub>1</sub>C<sub>n</sub>im]<sup>+</sup> n = 1-17,<br/>[C<sub>1</sub>C<sub>1</sub>C<sub>4</sub>im]<sup>+</sup>, [C<sub>1</sub>C<sub>4</sub>pyrro]<sup>+</sup><br/>[Ch]<sup>+</sup></td><td>Nd (NTf<sub>2</sub><sup>-</sup>)</td><td>Mehdi et al. [176]</td></tr><tr><td>Nd (NO<sub>3</sub><sup>-</sup>)</td><td>Onghena et al. [177]</td></tr><tr><td>DGA</td><td>[N<sub>1888</sub>]<sup>+</sup></td><td>Nd (NO<sub>3</sub><sup>-</sup>)</td><td>Rout et al. [178]</td></tr><tr><td>CA-12</td><td>[P<sub>8888</sub>]<sup>+</sup></td><td>La, Nd, Sm, Dy, Er, Yb (Cl<sup>-</sup>)</td><td>Parmentier et al. [179]</td></tr><tr><td rowspan="4">BA</td><td rowspan="4">[N<sub>1888</sub>]<sup>+</sup><br/>[N<sub>1888</sub>]<sup>+</sup><br/>[N<sub>1888</sub>]<sup>+</sup><br/>[N<sub>1888</sub>]<sup>+</sup></td><td>REEs (Cl<sup>-</sup>)</td><td>Wang et al. [180]</td></tr><tr><td>Y, La, Eu, Ho (NO<sub>3</sub><sup>-</sup>)</td><td>Yang et al. [181]</td></tr><tr><td>REEs (Cl<sup>-</sup>)</td><td>Wang et al. [182]</td></tr><tr><td>Y (Cl<sup>-</sup>)</td><td>Chen et al. [183]</td></tr><tr><td></td><td>[P<sub>66614</sub>]<sup>+</sup></td><td>Nd (Cl<sup>-</sup>)</td><td>Panigrahi et al. [184]</td></tr></tbody></table>
* TSILs with a functionalized cation (Table 6)

A great variety of functional groups have been incorporated onto the cation of ILs, mainly hard base oxygen derivatives, since REEs are considered hard acids according to Pearson's HSAB concept.

TSILs bearing a carboxylic group have been proposed, such as hydrophobic [PF₆], which requires saponification to be considered for the extraction of yttrium [186]. Chen et al. described a pH-controlled selective separation of Nd(III) from Fe(III) by using carboxyl-functionalized imidazolium ionic liquids [C₁(CH₂)ₙCOOHim] [NTf₂] (n = 3, 5, 7) as the solvent and complexing agent [187]. Taking advantage of carboxylic acids for the extraction of REEs, a carboxyl-functionalized phosphonium IL [P₄₄C₁COOH]Cl showed good extraction properties for scandium in an aqueous biphasic system (ABS) [188]. Betaine incorporates a carboxylic acid.

This functional cation, which was originally developed for the dissolution of metal oxides [76], has been also used for the extraction and recovery of REEs. Many examples of IL-based systems using [Hbet] [NTf₂] can be cited [189–191]. Homogeneous liquid–liquid extraction was implemented on the basis of the significant solubility in water and the thermomorphic behaviour of [Hbet] [NTf₂]. The [Hbet] [NTf₂]/water mixture exhibits an upper critical solution temperature (UCST) of 55 °C [190]. The extraction efficiency of the zwitterionic betaine has been highlighted for various REEs and applications [78,88,192,193]. Other Brønsted acids can be also encountered; Dupont et al. have reported the use of ILs incorporating sulfonic acid (-SO₃H) [194], alkylsulfuric acid (-OSO₃H) [195] and N-alkylated sulfamic acid (NH–SO₃) [196].

Neutral extractants have also been immobilized on ILs cations. Most TSILs containing neutral extractants have been studied for the partitioning of actinides/lanthanides [197]. As mentioned above, CMPO was used as the extractant in ILs for the extraction of REEs; moreover, ILs with grafted CMPO moieties have been synthesized and used for the extraction of Ln [198–200].

Phosphoryl-based TSILs, involving the introduction of coordinating P=O groups on the IL cations, were found to help Nd(III) ion extraction [201].

Solvent extraction experiments have indicated the possibility of extracting Eu(III) with diglycolamide-functionalized task-specific ionic liquids, as shown in a study of the Eu³⁺–DGA-TSIL complex in [C₁C₄im] [NTf₂] [202]. In contrast, Yun et al. indicated that no Ln can be extracted by using a diglycolamide-grafted imidazolium IL in [C₁C₆im] [NTf₂]. It has been suggested that TSILs suppress the extraction of lanthanides because of the formation of water-soluble complexes [203].

<table><caption>**Table 6.** Examples of TSILs with a functionalized cation for the recovery of REEs.</caption><thead><tr><th>TSIL Cation</th><th>TSIL Anion</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td>R-N-[N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>COOH]<br>R = C<sub>4</sub>H<sub>9</sub>, C<sub>6</sub>H<sub>13</sub>, C<sub>8</sub>H<sub>17</sub><br>[1-alkyl-3-(1-carboxylpropyl)im]</td><td>[PF<sub>6</sub>]<sup>-</sup></td><td>Y (NO<sub>3</sub><sup>-</sup>)</td><td>Wang et al. [186]</td></tr><tr><td>-N-[N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>COOH]<br>n=3, 5, 7<br>[C<sub>1</sub>(CH<sub>2</sub>)<sub>n</sub>COOHim]</td><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Sm, Nd</td><td>Chen et al. [187]</td></tr><tr><td>F-C<sub>4</sub>H<sub>9</sub>COOH<br>[P<sub>44</sub>C<sub>1</sub>COOH]</td><td>[Cl]<sup>-</sup></td><td>Sc (Cl<sup>-</sup>)</td><td>Depuydt et al. [188]</td></tr></tbody></table>
<table><caption>**Table 6. Cont.**</caption><thead><tr><th>TSIL Cation</th><th>TSIL Anion</th><th>Extracted (Feed)</th><th>Author</th></tr></thead><tbody><tr><td rowspan="5">[Hbet]<br/>N<sub>2</sub>O<sub>2</sub>COOH</td><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Nd (Cl<sup>-</sup>)</td><td>Fagnant et al. [189]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Nd (NTf<sub>2</sub><sup>-</sup>)</td><td>Vander Hoogerstraete et al. [190]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Nd (NTf<sub>2</sub><sup>-</sup>)</td><td>Vander Hoogerstraete et al. [191]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Sc (Cl<sup>-</sup>, NO<sub>3</sub><sup>-</sup>)</td><td>Onghena et al. [192]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Sc (SO<sub>4</sub><sup>2-</sup>)</td><td>Onghena et al. [193]</td></tr><tr><td rowspan="2">sulfonic acid<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>S<sub>2</sub>O<sub>2</sub>OH</td><td rowspan="2">[NTf<sub>2</sub>]<sup>-</sup></td><td rowspan="2">Dy, Nd, Sc (Cl<sup>-</sup>)</td><td rowspan="2">Dupont et al. [194]</td></tr><tr><td></td></tr><tr><td rowspan="2">alkylsulfuric acid<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>S<sub>2</sub>O<sub>2</sub>OH</td><td rowspan="2">[NTf<sub>2</sub>]<sup>-</sup></td><td rowspan="2">Y, Dy, Nd, La (Cl<sup>-</sup>, SO<sub>4</sub><sup>2-</sup>)</td><td rowspan="2">Dupont et al. [195]</td></tr><tr><td></td></tr><tr><td>N-alkylated sulfamic acid<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>S<sub>2</sub>O<sub>2</sub>H</td><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>La, Nd, Dy (Cl<sup>-</sup>, NO<sub>3</sub><sup>-</sup>)</td><td>Dupont et al. [196]</td></tr><tr><td rowspan="3">CMPO<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>O<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>Ph<sub>2</sub>Ph</td><td>[PF<sub>6</sub>]<sup>-</sup></td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Odinets et al. [198]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Eu (NO<sub>3</sub><sup>-</sup>)</td><td>Mohapatra et al. [199]</td></tr><tr><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>La, Eu, Tb, Ho, Er, Lu (NO<sub>3</sub><sup>-</sup>)</td><td>Turanov et al. [200]</td></tr><tr><td>PFILs<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>O<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>Ph<sub>2</sub>Ph</td><td>[PF<sub>6</sub>]<sup>-</sup>, [NTf<sub>2</sub>]<sup>-</sup></td><td>Nd (NO<sub>3</sub><sup>-</sup>)</td><td>Wang et al. [201]</td></tr><tr><td>DGA<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>O<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>Oct<sub>2</sub>Oct</td><td>[NTf<sub>2</sub>]<sup>-</sup></td><td>Y, La, Ce, Pr, Nd, Sm (NO<sub>3</sub><sup>-</sup>)</td><td>Sengupta et al. [202]</td></tr><tr><td>DGA<br/>R<sub>2</sub>N<sub>2</sub>O<sub>2</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>N<sub>2</sub>C<sub>6</sub>H<sub>5</sub>O<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>Oct<sub>2</sub>Oct</td><td>[NTf<sub>2</sub>]<sup>-</sup>, [BETI]<sup>-</sup></td><td>La, Eu, Lu (NO<sub>3</sub><sup>-</sup>)</td><td>Yun et al. [203]</td></tr></tbody></table>

## 4. Outlook—Open Questions and Needed Research

Ionic solvents offer an interesting alternative to the various conventionally used organic solvents and extractants and acid or alkali leaching solutions. Indeed, such systems can be used independently as solvents for the solvent leaching and extracting phases in the recovery of metallic species; these methods are part of the so-called “ionometallurgy” process.

However, in the case of REE recovery, few studies report both leaching and extraction steps using ionic solvents [80,84]. Indeed, changes in pregnant leach solution using DESs or ILs affect extraction and separation possibilities and can lead to emerging non-aqueous processes, introduced as “solvometallurgy” by Binnemans and Jones [204]. The following sections provide an overview of the potential of iono-/solvo-metallurgy for REE recovery, including the identification of potential gaps that need to be further explored.

### 4.1. Future of Leaching

Using ionic solvents during the leaching step often shows lower dissolution properties but higher selectivities toward targeted metals compared to the use of strong mineral acid solutions that dissolve the entire matrix. Such solvents could particularly be appropriated for low-grade REE resources, like mine tailings or industrial wastes, since complete leaching requires large amounts of chemicals and energy in these cases. Moreover, partial decomposition of the matrix by ionic solvents should theoretically reduce the difficulty and/or the number of steps during liquid-liquid extraction and then reduce the impact of recovery. In the best-case scenarios, we can imagine that the extraction step could be suppressed by efficient selective leaching. However, REE speciation in secondary sources is often overlooked. A better consideration of chemical speciation of REEs coupled with a
good characterization of the hosting matrices would help in identifying the proper IL or DES system for leaching selectivity.

In addition, a concern to take into account in the context of the use of ionic solvents as leaching agents is that some of them can remain in the leaching residues, which implies the need to clean the solid residue.

## 4.2. *Future of Liquid-Liquid Extraction*

The number of studies involving IL-based extracting systems for the recovery of REEs is constantly increasing. Many studies have focused on imidazolium-based cations with hydrophobic anions, including PF₆⁻ and NTf₂⁻, but more recent studies involve other types of cations, such as ammonium or phosphonium. Khazalpour et al. published a recent review focused on phosphonium-based ionic liquid PILs for various applications, including ion extraction and, therefore, REE recovery [205].

Recent studies rely on the use of ionic liquid-based aqueous biphasic systems (IL-ABS) to avoid having to resort to hydrophobic ILs, which are mainly based on fluorinated anions that are expensive and present a significant environmental impact. Currently, the use of this type of IL-ABS system is not well-reported for the recovery of REEs [206], except for a few examples [207,208].

In the majority of the studies for the recovery of REEs, DESs are involved in the leaching process but not as extracting phases. Hydrophobic DESs (HDESs) demonstrated promising results as extracting phases for the extraction and separation of metallic elements [209–213]. However, REE recovery has not been studied apart from a recent study showing the potential of HDES for recovery of waste SmCo magnets [214]. Also, non-ionic hydrophobic eutectics should be considered, as recent studies have proposed a hydrophobic eutectic solvent-based on TOPO as an extractant for liquid-liquid separation of uranyl nitrate from aqueous acid [215] or platinum group and transition metals in HCl media [216].

In addition, emphasis should be placed on intra-rare earth separation. A recent review devoted to this issue showed that high selectivities can be obtained when separating LREEs from HREEs; however, the separation between adjacent REEs requires more attention [217].

## 4.3. *Electrochemical Behaviour*

Beyond their efficiency for the extraction of REEs, ionic solvents can also be used for the electrodeaching [218] or electrodeposition of metals from non-aqueous electrolytes [44,219]. Electrodeposition of REEs cannot be performed in aqueous solutions because the electropositivity of these metals is greater than that of hydrogen, but ILs can provide suitable conditions for their electrodeposition [220]. Therefore, the electrochemical behaviour of various IL systems and REEs have been investigated [221–224]. Zhang et al. [225] recently described the application of non-haloaluminate ILs as solvents for low-temperature electrodeposition of REEs.

Although electrodeaching has been demonstrated for different metals using ionic solvents, there are only a few studies focused on REEs. Further research on this topic should therefore be further considered.

## 4.4. *Mechanism and Predictive Models*

Another open question concerns the mechanisms involved in ionic solvents, which are not fully described in most of the studies on this topic. Comprehension and prediction of solvent extraction in ionic solvents would help in identifying a priori which ionic solvent can be used to transpose the extractant systems known in conventional solvents to ionic solvents. Transposition from conventional diluents to ionic solvents would therefore become more straightforward and less expensive for industrial actors.

Recent studies have been undertaken focused on supramolecular mechanisms and the formation of aggregates [149]. In addition, the contributions of modelling and theoretical chemistry are important to take into account in order to better understand these mechanisms and acquire predictable models.
To render the application of ILs in solvent extraction predictable and reliable, it is therefore of crucial importance to understand their impact in the solvent extraction mechanisms. Specifically, it is also of prime industrial importance to use non-sacrificial ILs, for which it is necessary to be able to predict why and with which ILs ion exchange may occur or not.

### 4.5. Industrial Applications and Prospects

Although the performances and expectations are significant, the feasibility of these experiments when transferred from the laboratory scale to the pilot or even industrial levels is not well addressed in the various studies. One of the often reported drawbacks for ILs is their cost. Their recyclability and non-volatility make it possible to reduce the cost of use. The recovery of the extracted REEs and the recycling of the ionic phase are important in industrial separation processes. However, this point is not always investigated in detail in most studies. It is important to further investigate the stripping of REEs from the loaded ionic phase, as well as, in some cases, the scrubbing of the ionic phase before reusing it in new extracting steps.

The limitations of these processes should be taken into account, such as the loss of ionic liquid through ionic exchange in some of the systems examined. This phenomenon can be slightly mitigated by changing the pH [110] but, in practice, ion exchange is often attenuated by increasing the hydrophobic character of the ILs: by acting on the anion nature or on the length of the cation chains. This latter possibility generally leads to an increase in viscosity. The viscosities of the ionic liquids used in the extraction processes are indeed relatively high compared to the viscosities of water and traditional organic solvents, which may have an impact on the mixing, phase transfer and separation. They can also be reduced by using suitable IL mixtures [226]. The industrial applications of some ILs [227] may also be limited due to other characteristics, such as corrosiveness, hygroscopicity and, in some cases, toxicity.

The industrial applications of ionic liquids are steadily increasing in many sectors [228]: a pre-commercial permanent magnet recycling plant for REE recovery was opened in 2018 by Seren Technologies Limited, based on patents from Nockemann's group at Queen's University Belfast [229,230], and a pilot-scale plant for rare earth ore separation is currently being built.

The rise of DESs is also moving in this direction since they involve lower viscosities and cheaper precursors than the ILs, making technical considerations for the implementation of industrial process less restrictive [231]. It appears that the application of DESs for the recovery of REEs could address some of the existing challenges of ionic liquids.

### 4.6. Environmental Impact

The low pressure of ionic liquid vapours is often considered a major advantage of ionic liquids from an environmental point of view, and these liquids are considered as environmentally friendly alternatives to volatile organic solvents. However, the toxicity of these ionic systems must be considered [232,233]. It has been shown that the overall toxicity of ILs depends on both their cation—and particularly the cation alkyl chain length—and anion parts [234,235]. It seems important to take into account these factors when implementing IL-based extracting systems and choosing the cation and anion. Once again, this tends to encourage the use of DESs and, more particularly, NADESs. Indeed, DESs are usually known to be less toxic and more biodegradable than ILs [236]. However, statements about DESs' low toxicity are based on the low toxicities of their individual components; however, DESs are not necessarily as low in toxicity as their constituents. The toxicities of their mixtures must be taken into account [237].

Given the importance of REEs in various advanced technologies and the potential of ionic solvent recovery processes highlighted in various studies, investigation of such systems, and particularly DES systems, should continue to grow.
**Funding:** The authors acknowledge the financial support from the CNRS through the MITI interdisciplinarity programs (PRIME 2020: ExtraMet project), the Agence Nationale de la Recherche through the RECALL project (ANR-20-CE04-0007), the SILEXE project (ANR-13-CDII-0010) and the Labex Project CheMISyst (ANR-10-LABX-05-01).

**Conflicts of Interest:** The authors declare no conflict of interest.

## References

1. Connelly, N.G.; Damhus, T.; Hartshorn, R.M.; Hutton, A.T. Nomenclature of Inorganic Chemistry—IUPAC Recommendations 2005. *R. Soc. Chem.* **2005**, *27*, 25–26. [CrossRef]

2. Rollinson, H.R. *Using Geochemical Data: Evaluation, Presentation, Interpretation*; Longman Scientific & Technica: Harlow, Essex, UK, 1993; ISBN 9780582067011.

3. Balaram, V. Rare Earth Elements: A Review of Applications, Occurrence, Exploration, Analysis, Recycling, and Environmental Impact. *Geosci. Front.* **2019**, *10*, 1285–1303. [CrossRef]

4. Chakhmouradian, A.R.; Wall, F. Rare Earth Elements: Minerals, Mines, Magnets (and More). *Elements* **2012**, *8*, 333–340. [CrossRef]

5. Watari, T.; Nansai, K.; Nakajima, K. Review of Critical Metal Dynamics to 2050 for 48 Elements. *Resour. Conserv. Recycl.* **2020**, *155*, 104669. [CrossRef]

6. Jha, A.; Richards, B.; Jose, G.; Teddy-Fernandez, T.; Joshi, P.; Jiang, X.; Lousteau, J. Rare-Earth Ion Doped TeO2 and GeO2 Glasses as Laser Materials. *Prog. Mater. Sci.* **2012**, *57*, 1426–1491. [CrossRef]

7. Anashkina, E.A. Laser Sources Based on Rare-Earth Ion Doped Tellurite Glass Fibers and Microspheres. *Fibers* **2020**, *8*, 30. [CrossRef]

8. Coey, J.M.D. Perspective and Prospects for Rare Earth Permanent Magnets. *Engineering* **2020**, *6*, 119–131. [CrossRef]

9. Trench, A.; Sykes, J.P. Rare Earth Permanent Magnets and Their Place in the Future Economy. *Engineering* **2020**, *6*, 115–118. [CrossRef]

10. Lucas, J.; Lucas, P.; Le Mercier, T.; Rollat, A.; Davenport, W. Chapter 10—Rare Earths in Rechargeable Batteries. In *Rare Earths*; Lucas, J., Lucas, P., Le Mercier, T., Rollat, A., Davenport, W.B.T.-R.E., Eds.; Elsevier: Amsterdam, The Netherlands, 2015; pp. 167–180. ISBN 978-0-444-62735-3.

11. Zhao, H.; Xia, J.; Yin, D.; Luo, M.; Yan, C.; Du, Y. Rare Earth Incorporated Electrode Materials for Advanced Energy Storage. *Coord. Chem. Rev.* **2019**, *390*, 32–49. [CrossRef]

12. Alonso, E.; Sherman, A.M.; Wallington, T.J.; Everson, M.P.; Field, F.R.; Roth, R.; Kirchain, R.E. Evaluating Rare Earth Element Availability: A Case with Revolutionary Demand from Clean Technologies. *Environ. Sci. Technol.* **2012**, *46*, 3406–3414. [CrossRef]

13. Binnemans, K.; Jones, P.T. Rare Earths and the Balance Problem. *J. Sustain. Metall.* **2015**, *1*, 29–38. [CrossRef]

14. Kanazawa, Y.; Kamitani, M. Rare Earth Minerals and Resources in the World. *J. Alloys Compd.* **2006**, *408–412*, 1339–1343. [CrossRef]

15. Dutta, T.; Kim, K.-H.; Uchimiya, M.; Kwon, E.E.; Jeon, B.-H.; Deep, A.; Yun, S.-T. Global Demand for Rare Earth Resources and Strategies for Green Mining. *Environ. Res.* **2016**, *150*, 182–190. [CrossRef]

16. Khan, A.M.; Bakar, N.K.A.; Bakar, A.F.A.; Ashraf, M.A. Chemical Speciation and Bioavailability of Rare Earth Elements (REEs) in the Ecosystem: A Review. *Environ. Sci. Pollut. Res.* **2017**, *24*, 22764–22789. [CrossRef]

17. Hennebel, T.; Boon, N.; Maes, S.; Lenz, M. Biotechnologies for Critical Raw Material Recovery from Primary and Secondary Sources: R&D Priorities and Future Perspectives. *New Biotechnol.* **2015**, *32*, 121–127. [CrossRef]

18. Binnemans, K.; Jones, P.T.; Blanpain, B.; Van Gerven, T.; Yang, Y.; Walton, A.; Buchert, M. Recycling of Rare Earths: A Critical Review. *J. Clean. Prod.* **2013**, *51*, 1–22. [CrossRef]

19. Gaustad, G.; Williams, E.; Leader, A. Rare Earth Metals from Secondary Sources: Review of Potential Supply from Waste and Byproducts. *Resour. Conserv. Recycl.* **2021**, *167*, 105213. [CrossRef]

20. Rivera, R.M.; Ounoughene, G.; Malfliet, A.; Vind, J.; Panias, D.; Vassiliadou, V.; Binnemans, K.; Van Gerven, T. A Study of the Occurrence of Selected Rare-Earth Elements in Neutralized–Leached Bauxite Residue and Comparison with Untreated Bauxite Residue. *J. Sustain. Metall.* **2019**, *5*, 57–68. [CrossRef]

21. Akcil, A.; Akhmadiyeva, N.; Abdulvaliyev, R.; Abhilash; Meshram, P. Overview On Extraction and Separation of Rare Earth Elements from Red Mud: Focus on Scandium. *Miner. Process. Extr. Metall. Rev.* **2018**, *39*, 145–151. [CrossRef]

22. Binnemans, K.; Jones, P.T.; Blanpain, B.; Van Gerven, T.; Pontikes, Y. Towards Zero-Waste Valorisation of Rare-Earth-Containing Industrial Process Residues: A Critical Review. *J. Clean. Prod.* **2015**, *99*, 17–38. [CrossRef]

23. Evans, K. The History, Challenges, and New Developments in the Management and Use of Bauxite Residue. *J. Sustain. Metall.* **2016**, *2*, 316–331. [CrossRef]

24. Cánovas, C.R.; Pérez-López, R.; Macías, F.; Chapron, S.; Nieto, J.M.; Pellet-Rostaing, S. Exploration of Fertilizer Industry Wastes as Potential Source of Critical Raw Materials. *J. Clean. Prod.* **2017**, *143*, 497–505. [CrossRef]

25. Cánovas, C.R.; Chapron, S.; Arrachart, G.; Pellet-Rostaing, S. Leaching of Rare Earth Elements (REEs) and Impurities from Phosphogypsum: A Preliminary Insight for Further Recovery of Critical Raw Materials. *J. Clean. Prod.* **2019**. [CrossRef]

26. León, R.; Macías, F.; Cánovas, C.R.; Pérez-López, R.; Ayora, C.; Nieto, J.M.; Olias, M. Mine Waters as a Secondary Source of Rare Earth Elements Worldwide: The Case of the Iberian Pyrite Belt. *J. Geochem. Explor.* **2021**, *224*, 106742. [CrossRef]

27. Kawasaki, A.; Kimura, R.; Arai, S. Rare Earth Elements and Other Trace Elements in Wastewater Treatment Sludges. *Soil Sci. Plant Nutr.* **1998**, *44*, 433–441. [CrossRef]
28. Royer-Lavallée, A.; Neculita, C.M.; Coudert, L. Removal and Potential Recovery of Rare Earth Elements from Mine Water. *J. Ind. Eng. Chem.* **2020**, *89*, 47–57. [CrossRef]

29. Zhang, W.; Rezaee, M.; Bhagavatula, A.; Li, Y.; Groppo, J.; Honaker, R. A Review of the Occurrence and Promising Recovery Methods of Rare Earth Elements from Coal and Coal By-Products. *Int. J. Coal Prep. Util.* **2015**, *35*, 295–330. [CrossRef]

30. Rybak, A.; Rybak, A. Characteristics of Some Selected Methods of Rare Earth Elements Recovery from Coal Fly Ashes. *Metals* **2021**, *11*, 142. [CrossRef]

31. Tunsu, C.; Petranikova, M.; Gergorić, M.; Ekberg, C.; Retegan, T. Reclaiming Rare Earth Elements from End-of-Life Products: A Review of the Perspectives for Urban Mining Using Hydrometallurgical Unit Operations. *Hydrometallurgy* **2015**, *156*, 239–258. [CrossRef]

32. Sethurajan, M.; van Hullebusch, E.D.; Fontana, D.; Akcil, A.; Deveci, H.; Batinic, B.; Leal, J.P.; Gasche, T.A.; Ali Kucuker, M.; Kuchta, K.; et al. Recent Advances on Hydrometallurgical Recovery of Critical and Precious Elements from End of Life Electronic Wastes—A Review. *Crit. Rev. Environ. Sci. Technol.* **2019**, *49*, 212–275. [CrossRef]

33. Rademaker, J.H.; Kleijn, R.; Yang, Y. Recycling as a Strategy against Rare Earth Element Criticality: A Systemic Evaluation of the Potential Yield of NdFeB Magnet Recycling. *Environ. Sci. Technol.* **2013**, *47*, 10129–10136. [CrossRef] [PubMed]

34. Bandara, H.M.D.; Darcy, J.W.; Apelian, D.; Emmert, M.H. Value Analysis of Neodymium Content in Shredder Feed: Toward Enabling the Feasibility of Rare Earth Magnet Recycling. *Environ. Sci. Technol.* **2014**, *48*, 6553–6560. [CrossRef] [PubMed]

35. Binnemans, K.; Jones, P.T. Perspectives for the Recovery of Rare Earths from End-of-Life Fluorescent Lamps. *J. Rare Earths* **2014**, *32*, 195–200. [CrossRef]

36. Tan, Q.; Li, J.; Zeng, X. Rare Earth Elements Recovery from Waste Fluorescent Lamps: A Review. *Crit. Rev. Environ. Sci. Technol.* **2015**, *45*, 749–776. [CrossRef]

37. Gupta, C.K.; Krishnamurthy, N. Extractive Metallurgy of Rare Earths. *Int. Mater. Rev.* **1992**, *37*, 197–248. [CrossRef]

38. Joshi, D.; Adhikari, N. An Overview on Common Organic Solvents and Their Toxicity. *J. Pharm. Res. Int.* **2019**, *28*, 1–18. [CrossRef]

39. Smiglak, M.; Reichert, W.M.; Holbrey, J.D.; Wilkes, J.S.; Sun, L.; Thrasher, J.S.; Kirichenko, K.; Singh, S.; Katritzky, A.R.; Rogers, R.D. Combustible Ionic Liquids by Design: Is Laboratory Safety Another Ionic Liquid Myth? *Chem. Commun.* **2006**, *24*, 2554–2556. [CrossRef]

40. Earle, M.J.; Esperança, J.M.S.S.; Gilea, M.A.; Lopes, J.N.C.; Rebelo, L.P.N.; Magee, J.W.; Seddon, K.R.; Widegren, J.A. The Distillation and Volatility of Ionic Liquids. *Nature* **2006**, *439*, 831–834. [CrossRef]

41. Esperança, J.M.S.S.; Canongia Lopes, J.N.; Tariq, M.; Santos, L.M.N.B.F.; Magee, J.W.; Rebelo, L.P.N. Volatility of Aprotic Ionic Liquids—A Review. *J. Chem. Eng. Data* **2010**, *55*, 3–12. [CrossRef]

42. Domínguez de María, P. Chapter 6—Ionic Liquids, Switchable Solvents, and Eutectic Mixtures. In *The Application of Green Solvents in Separation Processes*; Pena-Pereira, F., Tobiszewski, M., Eds.; Elsevier: Amsterdam, The Netherlands, 2017; pp. 139–154. ISBN 978-0-12-805297-6.

43. Singh, S.K.; Savoy, A.W. Ionic Liquids: Synthesis and Applications: An Overview. *J. Mol. Liq.* **2020**, *297*, 112038. [CrossRef]

44. Abbott, A.P.; Frisch, G.; Gurman, S.J.; Hillman, A.R.; Hartley, J.; Holyoak, F.; Ryder, K.S. Ionometallurgy: Designer Redox Properties for Metal Processing. *Chem. Commun.* **2011**, *47*, 10031–10033. [CrossRef] [PubMed]

45. Leclerc, N.; Legeai, S.; Balva, M.; Hazotte, C.; Comel, J.; Lapicque, F.; Billy, E.; Meux, E. Recovery of Metals from Secondary Raw Materials by Coupled Electroleaching and Electrodeposition in Aqueous or Ionic Liquid Media. *Metals* **2018**, *8*, 556. [CrossRef]

46. Hallett, J.P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. *2. Chem. Rev.* **2011**, *111*, 3508–3576. [CrossRef] [PubMed]

47. Plechkova, N.V.; Seddon, K.R. Applications of Ionic Liquids in the Chemical Industry. *Chem. Soc. Rev.* **2008**, *37*, 123–150. [CrossRef]

48. Abbott, A.P.; Barron, J.C.; Ryder, K.S.; Wilson, D. Eutectic-Based Ionic Liquids with Metal-Containing Anions and Cations. *Chem. Eur. J.* **2007**, *13*, 6495–6501. [CrossRef]

49. Smith, E.L.; Abbott, A.P.; Ryder, K.S. Deep Eutectic Solvents (DESs) and Their Applications. *Chem. Rev.* **2014**, *114*, 11060–11082. [CrossRef]

50. Martins, M.A.R.; Pinho, S.P.; Coutinho, J.A.P. Insights into the Nature of Eutectic and Deep Eutectic Mixtures. *J. Solut. Chem.* **2019**, *48*, 962–982. [CrossRef]

51. Plechkova, N.V.; Seddon, K.R. Ionic Liquids: "Designer" Solvents for Green Chemistry. In *Methods and Reagents for Green Chemistry: An Introduction*; John Wiley & Sons: Hoboken, NJ, USA, 2007; pp. 103–130. ISBN 9780471754008.

52. Micheau, C.; Arrachart, G.; Turgis, R.; Lejeune, M.; Draye, M.; Michel, S.; Legeai, S.; Pellet-Rostaing, S. Ionic Liquids as Extraction Media in a Two-Step Eco-Friendly Process for Selective Tantalum Recovery. *ACS Sustain. Chem. Eng.* **2020**, *8*, 1954–1963. [CrossRef]

53. Jacquemin, J.; Husson, P.; Padua, A.A.H.; Majer, V. Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. *Green Chem.* **2006**, *8*, 172–180. [CrossRef]

54. Bonhôte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Grätzel, M. Hydrophobic, Highly Conductive Ambient-Temperature Molten Salts. *Inorg. Chem.* **1996**, *35*, 1168–1178. [CrossRef]

55. Paiva, A.; Matias, A.A.; Duarte, A.R.C. How Do We Drive Deep Eutectic Systems towards an Industrial Reality? *Curr. Opin. Green Sustain. Chem.* **2018**, *11*, 81–85. [CrossRef]
56. Florindo, C.; Lima, F.; Ribeiro, B.D.; Marrucho, I.M. Deep Eutectic Solvents: Overcoming 21st Century Challenges. *Curr. Opin. Green Sustain. Chem.* **2019**, *18*, 31–36. [CrossRef]

57. Liu, Y.; Friesen, J.B.; McAlpine, J.B.; Lankin, D.C.; Chen, S.-N.; Pauli, G.F. Natural Deep Eutectic Solvents: Properties, Applications, and Perspectives. *J. Nat. Prod.* **2018**, *81*, 679–690. [CrossRef]

58. Paiva, A.; Craveiro, R.; Aroso, I.; Martins, M.; Reis, R.L.; Duarte, A.R.C. Natural Deep Eutectic Solvents—Solvents for the 21st Century. *ACS Sustain. Chem. Eng.* **2014**, *2*, 1063–1071. [CrossRef]

59. Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jérôme, F. Deep Eutectic Solvents: Syntheses, Properties and Applications. *Chem. Soc. Rev.* **2012**, *41*, 7108–7146. [CrossRef] [PubMed]

60. Qin, H.; Hu, X.; Wang, J.; Cheng, H.; Chen, L.; Qi, Z. Overview of Acidic Deep Eutectic Solvents on Synthesis, Properties and Applications. *Green Energy Environ.* **2020**, *5*, 8–21. [CrossRef]

61. Abbott, A.P.; Capper, G.; Davies, D.L.; Rasheed, R.K.; Tambyrajah, V. Novel Solvent Properties of Choline Chloride/Urea Mixtures. *Chem. Commun.* **2003**, *1*, 70–71. [CrossRef] [PubMed]

62. Hansen, B.B.; Spittle, S.; Chen, B.; Poe, D.; Zhang, Y.; Klein, J.M.; Horton, A.; Adhikari, L.; Zelovich, T.; Doherty, B.W.; et al. Deep Eutectic Solvents: A Review of Fundamentals and Applications. *Chem. Rev.* **2021**, *121*, 1232–1285. [CrossRef]

63. Abbott, A.P.; Boothby, D.; Capper, G.; Davies, D.L.; Rasheed, R.K. Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids. *J. Am. Chem. Soc.* **2004**, *126*, 9142–9147. [CrossRef]

64. van Osch, D.J.G.P.; Zubeir, L.F.; van den Bruinhorst, A.; Rocha, M.A.A.; Kroon, M.C. Hydrophobic Deep Eutectic Solvents as Water-Immiscible Extractants. *Green Chem.* **2015**, *17*, 4518–4521. [CrossRef]

65. Dwamena, A.K. Recent Advances in Hydrophobic Deep Eutectic Solvents for Extraction. *Separations* **2019**, *6*, 9. [CrossRef]

66. Makos, P.; Slupek, E.; Gebicki, J. Hydrophobic Deep Eutectic Solvents in Microextraction Techniques—A Review. *Microchem. J.* **2020**, *152*. [CrossRef]

67. Florindo, C.; Branco, L.C.; Marrucho, I.M. Quest for Green-Solvent Design: From Hydrophilic to Hydrophobic (Deep) Eutectic Solvents. *Chem. Sus. Chem.* **2019**, *12*, 1549–1559. [CrossRef] [PubMed]

68. Ribeiro, B.D.; Florindo, C.; Iff, L.C.; Coelho, M.A.Z.; Marrucho, I.M. Menthol-Based Eutectic Mixtures: Hydrophobic Low Viscosity Solvents. *ACS Sustain. Chem. Eng.* **2015**, *3*, 2469–2477. [CrossRef]

69. Martins, M.A.R.; Crespo, E.A.; Pontes, P.V.A.; Silva, L.P.; Bülow, M.; Maximo, G.J.; Batista, E.A.C.; Held, C.; Pinho, S.P.; Coutinho, J.A.P. Tunable Hydrophobic Eutectic Solvents Based on Terpenes and Monocarboxylic Acids. *ACS Sustain. Chem. Eng.* **2018**, *6*, 8836–8846. [CrossRef]

70. Abranches, D.O.; Martins, M.A.R.; Silva, L.P.; Schaeffer, N.; Pinho, S.P.; Coutinho, J.A.P. Phenolic Hydrogen Bond Donors in the Formation of Non-Ionic Deep Eutectic Solvents: The Quest for Type V DES. *Chem. Commun.* **2019**, *55*, 10253–10256. [CrossRef] [PubMed]

71. Abbott, A.P.; Capper, G.; Davies, D.L.; McKenzie, K.J.; Obi, S.U. Solubility of Metal Oxides in Deep Eutectic Solvents Based on Choline Chloride. *J. Chem. Eng. Data* **2006**, *51*, 1280–1282. [CrossRef]

72. Jiang, J.; Bai, X.; Zhao, X.; Chen, W.; Yu, T.; Li, Y.; Mu, T. Poly-Quasi-Eutectic Solvents (PQESs): Versatile Solvents for Dissolving Metal Oxides. *Green Chem.* **2019**, *21*, 5571–5578. [CrossRef]

73. Pateli, I.M.; Thompson, D.; Alabdullah, S.S.M.; Abbott, A.P.; Jenkin, G.R.T.; Hartley, J.M. The Effect of PH and Hydrogen Bond Donor on the Dissolution of Metal Oxides in Deep Eutectic Solvents. *Green Chem.* **2020**, *22*, 5476–5486. [CrossRef]

74. Kim, B.-K.; Lee, E.J.; Kang, Y.; Lee, J.-J. Application of Ionic Liquids for Metal Dissolution and Extraction. *J. Ind. Eng. Chem.* **2018**, *61*, 388–397. [CrossRef]

75. Söldner, A.; Zach, J.; König, B. Deep Eutectic Solvents as Extraction Media for Metal Salts and Oxides Exemplarily Shown for Phosphates from Incinerated Sewage Sludge Ash. *Green Chem.* **2019**, *21*, 321–328. [CrossRef]

76. Nockemann, P.; Thijs, B.; Pittois, S.; Thoen, J.; Glorieux, C.; Van Hecke, K.; Van Meervelt, L.; Kirchner, B.; Binnemans, K. Task-Specific Ionic Liquid for Solubilizing Metal Oxides. *J. Phys. Chem. B.* **2006**, *110*, 20978–20992. [CrossRef] [PubMed]

77. Davris, P.; Balomenos, E.; Panias, D.; Paspaliaris, I. Selective Leaching of Rare Earth Elements from Bauxite Residue (Red Mud), Using a Functionalized Hydrophobic Ionic Liquid. *Hydrometallurgy* **2016**, *164*, 125–135. [CrossRef]

78. Dupont, D.; Binnemans, K. Recycling of Rare Earths from NdFeB Magnets Using a Combined Leaching/Extraction System Based on the Acidity and Thermomorphism of the Ionic Liquid [Hbet][Tf2N]. *Green Chem.* **2015**, *17*, 2150–2163. [CrossRef]

79. Orefice, M.; Van den Bulck, A.; Blanpain, B.; Binnemans, K. Selective Roasting of Nd-Fe-B Permanent Magnets as a Pretreatment Step for Intensified Leaching with an Ionic Liquid. *J. Sustain. Metall.* **2020**, *6*, 91–102. [CrossRef]

80. Orefice, M.; Binnemans, K. Solvometallurgical Process for the Recovery of Rare-Earth Elements from Nd-Fe-B Magnets. *Sep. Purif. Technol.* **2021**, *258*, 117800. [CrossRef]

81. Entezari-Zarandi, A.; Larachi, F. Selective Dissolution of Rare-Earth Element Carbonates in Deep Eutectic Solvents. *J. Rare Earths* **2019**, *37*, 528–533. [CrossRef]

82. Chen, W.; Jiang, J.; Lan, X.; Zhao, X.; Mou, H.; Mu, T. A Strategy for the Dissolution and Separation of Rare Earth Oxides by Novel Bronsted Acidic Deep Eutectic Solvents. *Green Chem.* **2019**, *21*, 4748–4756. [CrossRef]

83. Söldner, A.; König, B. Optical Analysis and Separation of Trivalent Lanthanides in Deep Eutectic Solvents. *J. Rare Earths* **2020**, *38*, 784–792. [CrossRef]
84. Riaño, S.; Petranikova, M.; Onghena, B.; Vander Hoogerstraete, T.; Banerjee, D.; Foreman, M.R.S.; Ekberg, C.; Binnemans, K. Separation of Rare Earths and Other Valuable Metals from Deep-Eutectic Solvents: A New Alternative for the Recycling of Used NdFeB Magnets. *RSC Adv.* **2017**, *7*, 32100–32113. [CrossRef]

85. Liu, C.; Yan, Q.; Zhang, X.; Lei, L.; Xiao, C. Efficient Recovery of End-of-Life NdFeB Permanent Magnets by Selective Leaching with Deep Eutectic Solvents. *Environ. Sci. Technol.* **2020**, *54*, 10370–10379. [CrossRef]

86. Siriwardana, A.; Sánchez Cupido, L.; Hidalgo Betanzos, J.; Nieto Maestre, F.J. Extraction of Rare Earth Elements with Deep Eutectic Solvents. *European Patent EP 17382134*, 19 September 2018.

87. Pateli, I.M.; Abbott, A.P.; Binnemans, K.; Rodriguez Rodriguez, N. Recovery of Yttrium and Europium from Spent Fluorescent Lamps Using Pure Levulinic Acid and the Deep Eutectic Solvent Levulinic Acid-Choline Chloride. *RSC Adv.* **2020**, *10*, 28879–28890. [CrossRef]

88. Dupont, D.; Binnemans, K. Rare-Earth Recycling Using a Functionalized Ionic Liquid for the Selective Dissolution and Revaluation of Y2O3:Eu3+ from Lamp Phosphor Waste. *Green Chem.* **2015**, *17*, 856–868. [CrossRef]

89. Schaeffer, N.; Feng, X.; Grimes, S.; Cheeseman, C. Recovery of an Yttrium Europium Oxide Phosphor from Waste Fluorescent Tubes Using a Brønsted Acidic Ionic Liquid, 1-Methylimidazolium Hydrogen Sulfate. *J. Chem. Technol. Biotechnol.* **2017**, *92*, 2731–2738. [CrossRef]

90. Zhao, H.; Franklin, M.S. Ionic Liquids for Coal Dissolution, Extraction and Liquefaction. *J. Chem. Technol. Biotechnol.* **2020**, *95*, 2301–2310. [CrossRef]

91. Rozelle, P.L.; Khadilkar, A.B.; Pulati, N.; Soundarrajan, N.; Klima, M.S.; Mosser, M.M.; Miller, C.E.; Pisupati, S.V. A Study on Removal of Rare Earth Elements from U.S. Coal Byproducts by Ion Exchange. *Metall. Mater. Trans. E* **2016**, *3*, 6–17. [CrossRef]

92. Dai, S.; Ju, Y.H.; Barnes, C.E. Solvent Extraction of Strontium Nitrate by a Crown Ether Using Room-Temperature Ionic Liquids. *J. Chem. Soc. Dalt. Trans.* **1999**, 1201–1202. [CrossRef]

93. Hirayama, N. Chelate Extraction of Metals into Ionic Liquids. *Solvent Extr. Res. Dev. Jpn.* **2011**, *18*, 1–14. [CrossRef]

94. Zhao, H.; Xia, S.Q.; Ma, P.S. Use of Ionic Liquids as 'green' Solvents for Extractions. *J. Chem. Technol. Biotechnol.* **2005**, *80*, 1089–1096. [CrossRef]

95. Toita, M.; Morita, K.; Hirayama, N. Mutual Separation of Fe(II) and Fe(III) Using Cyclohexane/Water/Ionic-Liquid Triphasic Extraction System with 2,2'-Bipyridine and Tri-n-Octylphosphine Oxide. *Anal. Sci.* **2020**, *36*, 1387–1391. [CrossRef]

96. Gutowski, K.E.; Broker, G.A.; Willauer, H.D.; Huddleston, J.G.; Swatloski, R.P.; Holbrey, J.D.; Rogers, R.D. Controlling the Aqueous Miscibility of Ionic Liquids: Aqueous Biphasic Systems of Water-Miscible Ionic Liquids and Water-Structuring Salts for Recycle, Metathesis, and Separations. *J. Am. Chem. Soc.* **2003**, *125*, 6632–6633. [CrossRef]

97. Karmakar, R.; Sen, K. Aqueous Biphasic Extraction of Metal Ions: An Alternative Technology for Metal Regeneration. *J. Mol. Liq.* **2019**, *273*, 231–247. [CrossRef]

98. Makanyire, T.; Sanchez-Segado, S.; Jha, A. Separation and Recovery of Critical Metal Ions Using Ionic Liquids. *Adv. Manuf.* **2016**, *4*, 33–46. [CrossRef]

99. Sukhbaatar, T.; Dourdain, S.; Turgis, R.; Rey, J.; Arrachart, G.; Pellet-Rostaing, S. Ionic Liquids as Diluents in Solvent Extraction: First Evidence of Supramolecular Aggregation of a Couple of Extractant Molecules. *Chem. Commun.* **2015**, *51*, 15960–15963. [CrossRef] [PubMed]

100. Billard, I.; Ouadi, A.; Gaillard, C. Is a Universal Model to Describe Liquid-Liquid Extraction of Cations by Use of Ionic Liquids in Reach? *Dalt. Trans.* **2013**, *42*, 6203–6212. [CrossRef]

101. Dietz, M.L. Ionic Liquids as Extraction Solvents: Where Do We Stand? *Sep. Sci. Technol.* **2006**, *41*, 2047–2063. [CrossRef]

102. Cocalia, V.A.; Holbrey, J.D.; Gutowski, K.E.; Bridges, N.J.; Rogers, R.D. Separations of Metal Ions Using Ionic Liquids: The Challenges of Multiple Mechanisms. *Tsinghua Sci. Technol.* **2006**, *11*, 188–193. [CrossRef]

103. Janssen, C.H.C.; Macías-Ruvalcaba, N.A.; Aguilar-Martínez, M.; Kobrak, M.N. Metal Extraction to Ionic Liquids: The Relationship between Structure, Mechanism and Application. *Int. Rev. Phys. Chem.* **2015**, *34*, 591–622. [CrossRef]

104. Cocalia, V.A.; Jensen, M.P.; Holbrey, J.D.; Spear, S.K.; Stepinski, D.C.; Rogers, R.D. Identical Extraction Behavior and Coordination of Trivalent or Hexavalent F-Element Cations Using Ionic Liquid and Molecular Solvents. *Dalt. Trans.* **2005**, *11*, 1966–1971. [CrossRef]

105. Sun, X.; Ji, Y.; Guo, L.; Chen, J.; Li, D. A Novel Ammonium Ionic Liquid Based Extraction Strategy for Separating Scandium from Yttrium and Lanthanides. *Sep. Purif. Technol.* **2011**, *81*, 25–30. [CrossRef]

106. Jensen, M.P.; Neuefeind, J.; Beitz, J.V.; Skanthakumar, S.; Soderholm, L. Mechanisms of Metal Ion Transfer into Room-Temperature Ionic Liquids: The Role of Anion Exchange. *J. Am. Chem. Soc.* **2003**, *125*, 15466–15473. [CrossRef]

107. Garvey, S.L.; Dietz, M.L. Ionic Liquid Anion Effects in the Extraction of Metal Ions by Macrocyclic Polyethers. *Sep. Purif. Technol.* **2014**, *123*, 145–152. [CrossRef]

108. Dupont, D.; Depuydt, D.; Binnemans, K. Overview of the Effect of Salts on Biphasic Ionic Liquid/Water Solvent Extraction Systems: Anion Exchange, Mutual Solubility, and Thermomorphic Properties. *J. Phys. Chem. B* **2015**, *119*, 6747–6757. [CrossRef]

109. Dietz, M.L.; Dzielawa, J.A.; Laszak, I.; Young, B.A.; Jensen, M.P. Influence of Solvent Structural Variations on the Mechanism of Facilitated Ion Transfer into Room-Temperature Ionic Liquids. *Green Chem.* **2003**, *5*, 682–685. [CrossRef]

110. Gaillard, D.C.; Boltoeva, M.; Billard, I.; Georg, S.; Mazan, V.; Ouadi, A.; Ternova, D.; Hennig, C. Insights into the Mechanism of Extraction of Uranium (VI) from Nitric Acid Solution into an Ionic Liquid by Using Tri-n-Butyl Phosphate. *Chem. Phys. Chem.* **2015**, *16*, 2653–2662. [CrossRef] [PubMed]
111. Okamura, H.; Hirayama, N. Recent Progress in Ionic Liquid Extraction for the Separation of Rare Earth Elements. *Anal. Sci.* **2021**, *37*, 119–130. [CrossRef]

112. Wang, K.; Adidharma, H.; Radosz, M.; Wan, P.; Xu, X.; Russell, C.K.; Tian, H.; Fan, M.; Yu, J. Recovery of Rare Earth Elements with Ionic Liquids. *Green Chem.* **2017**, *19*, 4469–4493. [CrossRef]

113. Wang, L.Y.; Guo, Q.J.; Lee, M.S. Recent Advances in Metal Extraction Improvement: Mixture Systems Consisting of Ionic Liquid and Molecular Extractant. *Sep. Purif. Technol.* **2019**, *210*, 292–303. [CrossRef]

114. Okamura, H.; Sakae, H.; Kidani, K.; Hirayama, N.; Aoyagi, N.; Saito, T.; Shimojo, K.; Naganawa, H.; Imura, H. Laser-Induced Fluorescence and Infrared Spectroscopic Studies on the Specific Solvation of Tris(1-(2-Thienyl)-4,4,4-Trifluoro-1,3-Butanedionato)Europium(III) in an Ionic Liquid. *Polyhedron* **2012**, *31*, 748–753. [CrossRef]

115. Gujar, R.B.; Verma, P.K.; Ansari, S.A.; Mohapatra, P.K. Complexation of 2-Thenoyltrifluoroacetone (HTTA) with Trivalent f-Cations in an Ionic Liquid: Solvent Extraction and Spectroscopy Studies. *New. J. Chem.* **2019**, *43*, 13675–13680. [CrossRef]

116. Okamura, H.; Aoyagi, N.; Shimojo, K.; Naganawa, H.; Imura, H. Role of Tf2N—Anions in the Ionic Liquid–Water Distribution of Europium(III) Chelates. *RSC Adv.* **2017**, *7*, 7610–7618. [CrossRef]

117. Jensen, M.P.; Borkowski, M.; Laszak, I.; Beitz, J.V.; Rickert, P.G.; Dietz, M.L. Anion Effects in the Extraction of Lanthanide 2-Thenoyltrifluoroacetone Complexes into an Ionic Liquid. *Sep. Sci. Technol.* **2012**, *47*, 233–243. [CrossRef]

118. Hidayah, N.N.; Nurihan, M.F.S.; Abidin, S.Z. Liquid-Liquid Extraction of Cerium Using Synergist Extractant. *J. Mech. Eng. Sci.* **2018**, *12*, 3302–3312. [CrossRef]

119. Yang, F.; Baba, Y.; Kubota, F.; Kamiya, N.; Goto, M. Extraction and Separation of Rare Earth Metal Ions with DODGAA in Ionic Liquids. *Solvent Extr. Res. Dev.* **Jpn.** **2012**, *19*, 69–76. [CrossRef]

120. Kubota, F.; Shimobori, Y.; Baba, Y.; Koyanagi, Y.; Shimojo, K.; Kamiya, N.; Goto, M. Application of Ionic Liquids to Extraction Separation of Rare Earth Metals with an Effective Diglycol Amic Acid Extractant. *J. Chem. Eng. Jpn.* **2011**, *44*, 307–312. [CrossRef]

121. Yang, F.; Kubota, F.; Baba, Y.; Kamiya, N.; Goto, M. Selective Extraction and Recovery of Rare Earth Metals from Phosphor Powders in Waste Fluorescent Lamps Using an Ionic Liquid System. *J. Hazard. Mater.* **2013**, *254–255*, 79–88. [CrossRef]

122. Friend, M.T.; Parker, T.G.; Mastren, T.; Mocko, V.; Brugh, M.; Birnbaum, E.R.; Fassbender, M.E. Extraction Chromatography of 225Ac and Lanthanides on N,N-Dioctyldiglycolamic Acid /1-Butyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide Solvent Impregnated Resin. *J. Chromatogr. A* **2020**, *1624*, 461219. [CrossRef]

123. Schaeffer, N.; Grimes, S.M.; Cheeseman, C.R. Use of Extraction Chromatography in the Recycling of Critical Metals from Thin Film Leach Solutions. *Inorg. Chim. Acta* **2017**, *457*, 53–58. [CrossRef]

124. Rout, A.; Souza, E.R.; Binnemans, K. Solvent Extraction of Europium(III) to a Fluorine-Free Ionic Liquid Phase with a Diglycolamic Acid Extractant. *RSC Adv.* **2014**, *4*, 11899–11906. [CrossRef]

125. Rout, A.; Ramanathan, N. Liquid-Liquid Extraction of Europium(III) in an Alkyl Ammonium Based Ionic Liquid Containing Diglycolamic Acid. *J. Mol. Liq.* **2020**, *319*, 114016. [CrossRef]

126. Sun, X.; Bell, J.R.; Luo, H.; Dai, S. Extraction Separation of Rare-Earth Ions via Competitive Ligand Complexations between Aqueous and Ionic-Liquid Phases. *Dalt. Trans.* **2011**, *40*, 8019–8023. [CrossRef] [PubMed]

127. Batchu, N.K.; Li, Z.; Verbelen, B.; Binnemans, K. Structural Effects of Neutral Organophosphorus Extractants on Solvent Extraction of Rare-Earth Elements from Aqueous and Non-Aqueous Nitrate Solutions. *Sep. Purif. Technol.* **2021**, *255*, 117711. [CrossRef]

128. Yang, F.; Kubota, F.; Kamiya, N.; Goto, M. A Comparative Study of Ionic Liquids and a Conventional Organic Solvent on the Extraction of Rare-Earth Ions with TOPO. *Solvent Extr. Res. Dev.* **Jpn.** **2013**, *20*, 225–232. [CrossRef]

129. Rout, A.; Binnemans, K. Influence of the Ionic Liquid Cation on the Solvent Extraction of Trivalent Rare-Earth Ions by Mixtures of Cyanex 923 and Ionic Liquids. *Dalt. Trans.* **2015**, *44*, 1379–1387. [CrossRef] [PubMed]

130. Regadío, M.; Vander Hoogerstraete, T.; Banerjee, D.; Binnemans, K. Split-Anion Solvent Extraction of Light Rare Earths from Concentrated Chloride Aqueous Solutions to Nitrate Organic Ionic Liquids. *RSC Adv.* **2018**, *8*, 34754–34763. [CrossRef]

131. Riaño, S.; Sobekova Foltova, S.; Binnemans, K. Separation of Neodymium and Dysprosium by Solvent Extraction Using Ionic Liquids Combined with Neutral Extractants: Batch and Mixer-Settler Experiments. *RSC Adv.* **2020**, *10*, 307–316. [CrossRef]

132. Kikuchi, Y.; Matsumiya, M.; Kawakami, S. Extraction of Rare Earth Ions from Nd-Fe-B Magnet Wastes with TBP in Tricaprylmethylammonium Nitrate. *Solvent Extr. Res. Dev.* **Jpn.** **2014**, *21*, 137–145. [CrossRef]

133. Matsumiya, M.; Kikuchi, Y.; Yamada, T.; Kawakami, S. Extraction of Rare Earth Ions by Tri-n-Butylphosphate/Phosphonium Ionic Liquids and the Feasibility of Recovery by Direct Electrodeposition. *Sep. Purif. Technol.* **2014**, *130*, 91–101. [CrossRef]

134. Turanov, A.N.; Karandashev, V.K.; Yarkevich, A.N. Extraction of Rare-Earth Elements from Hydrochloric Acid by Carbamoyl Methyl Phosphine Oxides in the Presence of Ionic Liquids. *Russ. J. Inorg. Chem.* **2018**, *63*, 406–413. [CrossRef]

135. Atanassova, M. Investigation of Synergism and Selectivity Using Mixture of Two Neutral Extractants in IL Media for Lanthanoids Extraction. *Sep. Purif. Technol.* **2016**, *169*, 253–261. [CrossRef]

136. Nakashima, K.; Kubota, F.; Maruyama, T.; Goto, M. Ionic Liquids as a Novel Solvent for Lanthanide Extraction. *Anal. Sci.* **2003**, *19*, 1097–1098. [CrossRef]

137. Turgis, R.; Arrachart, G.; Dubois, V.; Dourdain, S.; Virieux, D.; Michel, S.; Legeai, S.; Lejeune, M.; Draye, M.; Pellet-Rostaing, S. Performances and Mechanistic Investigations of a Triphosphine Trioxide/Ionic Liquid System for Rare Earth Extraction. *Dalt. Trans.* **2016**, *45*, 1259–1268. [CrossRef] [PubMed]
138. Turanov, A.N.; Karandashev, V.K.; Baulin, V.E.; Baulin, D.V.; Khvostikov, V.A. Extraction of Rare-Earth Elements(III) from Nitric Acid Solutions with Diethyl 2-[(Diphenylphosphoryl)Methoxy]-5-Ethylphenylphosphonate. *Russ. J. Inorg. Chem.* **2019**, *64*, 1297–1303. [CrossRef]

139. Mohapatra, P.K. Diglycolamide-Based Solvent Systems in Room Temperature Ionic Liquids for Actinide Ion Extraction: A Review. *Chem. Prod. Process. Model.* **2015**, *10*, 135–145. [CrossRef]

140. Shimojo, K.; Kurahashi, K.; Naganawa, H. Extraction Behavior of Lanthanides Using a Diglycolamide Derivative TODGA in Ionic Liquids. *Dalt. Trans.* **2008**, *5083–5088*. [CrossRef]

141. Sypula, M.; Ouadi, A.; Gaillard, C.; Billard, I. Kinetics of Metal Extraction in Ionic Liquids: Eu3+/HNO3//TODGA/[C1C4im][Tf2N] as a Case Study. *RSC Adv.* **2013**, *3*, 10736–10744. [CrossRef]

142. Pan, Y.; Hussey, C.L. Electrochemical and Spectroscopic Investigation of Ln3 + (Ln = Sm, Eu, and Yb) Solvation in Bis(Trifluoromethylsulfonyl)Imide-Based Ionic Liquids and Coordination by N,N,N',N'-Tetraoctyl-3-Oxa-Pentane Diamide (TODGA) and Chloride. *Inorg. Chem.* **2013**, *52*, 3241–3252. [CrossRef]

143. Murakami, S.; Matsumiya, M.; Yamada, T.; Tsunashima, K. Extraction of Pr(III), Nd(III), and Dy(III) from HTFSA Aqueous Solution by TODGA/Phosphonium-Based Ionic Liquids. *Solvent Extr. Ion Exch.* **2016**, *34*, 172–187. [CrossRef]

144. Ansari, S.A.; Gujar, R.B.; Mohapatra, P.K. Complexation of Tetraalkyl Diglycolamides with Trivalent F-Cations in a Room Temperature Ionic Liquid: Extraction and Spectroscopic Investigations. *Dalt. Trans.* **2017**, *46*, 7584–7593. [CrossRef]

145. Rama, R.; Rout, A.; Venkatesan, K.A.; Antony, M.P.; Vasudeva Rao, P.R. Loading Behavior of Eu(III) at High Aqueous Concentrations in Diglycolamide/Ionic Liquid Systems. *J. Radioanal. Nucl. Chem.* **2016**, *308*, 835–842. [CrossRef]

146. Chen, Q.; Lu, C.; Hu, Y.; Liu, Y.; Zhou, Y.; Jiao, C.; Zhang, M.; Hou, H.; Gao, Y.; Tian, G. Extraction Behavior of Several Lanthanides from Nitric Acid with DMDODGA in [C4mim][NTf2] Ionic Liquid. *J. Radioanal. Nucl. Chem.* **2021**, *327*, 565–573. [CrossRef]

147. Wehbie, M.; Arrachart, G.; Ghannam, L.; Karamé, I.; Pellet-Rostaing, S. An Ionic Liquid-Based Extraction System Using Diglycolamide Functionalized Macrocyclic Platforms for the Extraction and Recovery of Lanthanides. *Dalt. Trans.* **2017**, *46*, 16505–16515. [CrossRef] [PubMed]

148. Usma, C.L.; Dourdain, S.; Arrachart, G.; Pellet-Rostaing, S. Liquid-Liquid Extraction of Rare Earths Elements by Use of Ionic Liquids. In Proceedings of the Hydroprocess, Santiago, Chile, 19–21 June 2019.

149. Usma, C.L.; Dourdain, S.; Arrachart, G.; Pellet-Rostaing, S. Liquid-Liquid Extraction of Rare Earths Elements from Nitrate Media in DMDOHEMA/Ionic Liquid System: Performance and Mechanism Studies. Unpublished work.

150. Atanassova, M.; Kurteva, V.; Lubenov, L.; Billard, I. Comparing Extraction, Synergism and Separation of Lanthanoids Using Acidic and Neutral Compounds in Chloroform and One Ionic Liquid: Is the Latter Always "Better"? *RSC Adv.* **2014**, *4*, 38820–38829. [CrossRef]

151. Okamura, H.; Mizuno, M.; Hirayama, N.; Shimojo, K.; Naganawa, H.; Imura, H. Synergistic Enhancement of the Extraction and Separation Efficiencies of Lanthanoid(III) Ions by the Formation of Charged Adducts in an Ionic Liquid. *Ind. Eng. Chem. Res.* **2020**, *59*, 329–340. [CrossRef]

152. Zhao, Z.; Baba, Y.; Kubota, F.; Kamiya, N.; Goto, M. Synergistic Extraction of Rare-Earth Metals and Separation of Scandium Using 2-Thenoyltrifluoroacetone and Tri-n-Octylphosphine Oxide in an Ionic Liquid System. *J. Chem. Eng. Jpn.* **2014**, *47*, 656–662. [CrossRef]

153. Matsumiya, M.; Nomizu, D.; Tsuchida, Y.; Sasaki, Y. Separation of Rare Earth Elements by Synergistic Solvent Extraction with Phosphonium-Based Ionic Liquids Using a β-Diketone Extractant and a Neutral Ligand. *Solvent Extr. Ion Exch.* **2021**, 1–21. [CrossRef]

154. Okamura, H.; Takagi, H.; Isomura, T.; Morita, K.; Nagatani, H.; Imura, H. Highly Selective Synergism for the Extraction of Lanthanoid(III) Ions with Beta-Diketones and Trioctylphosphine Oxide in an Ionic Liquid. *Anal. Sci.* **2014**, *30*, 323–325. [CrossRef]

155. Hatakeyama, M.; Nishiyama, Y.; Nagatani, H.; Okamura, H.; Imura, H. Synergistic Extraction Equilibrium of Lanthanide(III) Ions with Benzoylacetone and a Neutral Ligand in an Ionic Liquid. *Solvent Extr. Res. Dev. Jpn.* **2018**, *25*, 79–89. [CrossRef]

156. Atanassova, M.; Kurteva, V. Synergism in the Solvent Extraction of Europium(III) with Thenoyltrifluoroacetone and CMPO in Methylimidazolium Ionic Liquids. *J. Solut. Chem.* **2019**, *48*, 15–30. [CrossRef]

157. Atanassova, M.; Kurteva, V. Peculiar Synergistic Extraction Behavior of Eu(III) in Ionic Liquids: Benzoylacetone and CMPO Fusion. *Sep. Purif. Technol.* **2017**, *183*, 226–236. [CrossRef]

158. Asadollahzadeh, M.; Torkaman, R.; Torab-Mostaedi, M.; Hemmati, A.; Ghaemi, A. High Performance Separation of Gadolinium from Samarium with the Imidazolium Ionic Liquid through Selective Complexation of Organophosphorus Extractants. *Environ. Technol. Innov.* **2020**, *19*, 100979. [CrossRef]

159. Chiappe, C.; Pomelli, C.S. Point-Functionalization of Ionic Liquids: An Overview of Synthesis and Applications. *Eur. J. Org. Chem.* **2014**, *2014*, 6120–6139. [CrossRef]

160. Riaño, S.; Binnemans, K. Extraction and Separation of Neodymium and Dysprosium from Used NdFeB Magnets: An Application of Ionic Liquids in Solvent Extraction towards the Recycling of Magnets. *Green Chem.* **2015**, *17*, 2931–2942. [CrossRef]

161. Raiguel, S.; Depuydt, D.; Vander Hoogerstraete, T.; Thomas, J.; Dehaen, W.; Binnemans, K. Selective Alkaline Stripping of Metal Ions after Solvent Extraction by Base-Stable 1,2,3-Triazolium Ionic Liquids. *Dalt. Trans.* **2017**, *46*, 5269–5278. [CrossRef]

162. Depuydt, D.; den Bossche, A.; Dehaen, W.; Binnemans, K. Metal Extraction with a Short-Chain Imidazolium Nitrate Ionic Liquid. *Chem. Commun.* **2017**, *53*, 5271–5274. [CrossRef]
163. Banda, R.; Forte, F.; Onghena, B.; Binnemans, K. Yttrium and Europium Separation by Solvent Extraction with Undiluted Thiocyanate Ionic Liquids. *RSC Adv.* **2019**, *9*, 4876-4883. [CrossRef]

164. Smith, C.D.; Foersterling, F.H.; Dietz, M.L. Solvent Structural Effects on the Solubility of Bis(2-Ethylhexyl)Phosphoric Acid (HDEHP) in Room-Temperature Ionic Liquids. *Sep. Sci. Technol.* **2021**, *56*, 800–810. [CrossRef]

165. Rout, A.; Kotlarska, J.; Dehaen, W.; Binnemans, K. Liquid-Liquid Extraction of Neodymium(III) by Dialkylphosphate Ionic Liquids from Acidic Medium: The Importance of the Ionic Liquid Cation. *Phys. Chem. Chem. Phys.* **2013**, *15*, 16533–16541. [CrossRef] [PubMed]

166. Sun, X.; Luo, H.; Dai, S. Solvent Extraction of Rare-Earth Ions Based on Functionalized Ionic Liquids. *Talanta* **2012**, *90*, 132–137. [CrossRef] [PubMed]

167. Sun, X.; Luo, H.; Dai, S. Mechanistic Investigation of Solvent Extraction Based on Anion-Functionalized Ionic Liquids for Selective Separation of Rare-Earth Ions. *Dalt. Trans.* **2013**, *42*, 8270–8275. [CrossRef] [PubMed]

168. Sun, X.; Do-Thanh, C.-L.; Luo, H.; Dai, S. The Optimization of an Ionic Liquid-Based TALSPEAK-like Process for Rare Earth Ions Separation. *Chem. Eng. J.* **2014**, *239*, 392–398. [CrossRef]

169. Sun, X.; Waters, K.E. Synergistic Effect between Bifunctional Ionic Liquids and a Molecular Extractant for Lanthanide Separation. *ACS Sustain. Chem. Eng.* **2014**, *2*, 2758–2764. [CrossRef]

170. Devi, N.; Sukla, L.B. Studies on Liquid-Liquid Extraction of Yttrium and Separation from Other Rare Earth Elements Using Bifunctional Ionic Liquids. *Miner. Process. Extr. Metall. Rev.* **2019**, *40*, 46–55. [CrossRef]

171. Kumari, A.; Sinha, M.K.; Sahu, S.K.; Pandey, B.D. Solvent Extraction and Separation of Trivalent Lanthanides Using Cyphos IL 104, a Novel Phosphonium Ionic Liquid as Extractant. *Solvent Extr. Ion Exch.* **2016**, *34*, 469–484. [CrossRef]

172. Makowka, A.; Pospiech, B. Studies on Extraction and Permeation of Lanthanum(III) and Cerium(III) Using Cyphos IL 104 as Extractant and Ion Carrier. *Sep. Sci. Technol.* **2020**, *55*, 2193–2203. [CrossRef]

173. Rout, A.; Wellens, S.; Binnemans, K. Separation of Rare Earths and Nickel by Solvent Extraction with Two Mutually Immiscible Ionic Liquids. *RSC Adv.* **2014**, *4*, 5753–5758. [CrossRef]

174. Shen, L.; Chen, J.; Chen, L.; Liu, C.; Zhang, D.; Zhang, Y.; Su, W.; Deng, Y. Extraction of Mid-Heavy Rare Earth Metal Ions from Sulphuric Acid Media by Ionic Liquid [A336][P507]. *Hydrometallurgy* **2016**, *161*, 152–159. [CrossRef]

175. Dutta, B.; Ruhela, R.; Yadav, M.; Singh, A.K.; Sahu, K.K.; Padmanabhan, N.P.H.; Chakravartty, J.K. Liquid-Liquid Extraction Studies of Gadolinium with N-Methyl-N,N,N-Triocyt Ammonium-Bis-(2-Ethylhexyl) Phosphonate—Task Specific Ionic Liquid. *Sep. Purif. Technol.* **2017**, *175*, 158–163. [CrossRef]

176. Mehdi, H.; Binnemans, K.; Van Hecke, K.; Van Meervelt, L.; Nockemann, P. Hydrophobic Ionic Liquids with Strongly Coordinating Anions. *Chem. Commun.* **2010**, *46*, 234–236. [CrossRef]

177. Onghena, B.; Jacobs, J.; Van Meervelt, L.; Binnemans, K. Homogeneous Liquid-Liquid Extraction of Neodymium(Ii) by Choline Hexafluoroacetylacetonate in the Ionic Liquid Choline Bis(Trifluoromethylsulfonyl)Imide. *Dalt. Trans.* **2014**, *43*, 11566–11578. [CrossRef]

178. Rout, A.; Binnemans, K. Solvent Extraction of Neodymium(III) by Functionalized Ionic Liquid Trioctylmethylammonium Dioctyl Diglycolamate in Fluorine-Free Ionic Liquid Diluent. *Ind. Eng. Chem. Res.* **2014**, *53*, 6500–6508. [CrossRef]

179. Parmentier, D.; Vander Hoogerstraete, T.; Metz, S.J.; Binnemans, K.; Kroon, M.C. Selective Extraction of Metals from Chloride Solutions with the Tetraoctylphosphonium Oleate Ionic Liquid. *Ind. Eng. Chem. Res.* **2015**, *54*, 5149–5158. [CrossRef]

180. Wang, W.; Yang, H.; Cui, H.; Zhang, D.; Liu, Y.; Chen, J. Application of Bifunctional Ionic Liquid Extractants [A336][CA-12] and [A336][CA-100] to the Lanthanum Extraction and Separation from Rare Earths in the Chloride Medium. *Ind. Eng. Chem. Res.* **2011**, *50*, 7534–7541. [CrossRef]

181. Yang, H.-L.; Wang, W.; Cui, H.-M.; Chen, J. Extraction Mechanism of Rare Earths with Bifunctional Ionic Liquids (Bif-ILs) [A336][CA-12]/[A336][CA-100] in Nitrate Medium. *Chin. J. Anal. Chem.* **2011**, *39*, 1561–1566. [CrossRef]

182. Wang, Y.; Huang, C.; Li, F.; Dong, Y.; Zhao, Z.; Sun, X. The Development of Sustainable Yttrium Separation Process from Rare Earth Enrichments Using Bifunctional Ionic Liquid. *Sep. Purif. Technol.* **2016**, *162*, 106–113. [CrossRef]

183. Chen, J.; Huang, C.; Wang, Y.; Huang, B.; Sun, X. Extraction Behavior of Bifunctional Ionic Liquid [N1888][SOPAA] and TBP for Rare Earth Elements. *J. Rare Earths* **2016**, *34*, 1252–1259. [CrossRef]

184. Panigrahi, M.; Grabda, M.; Kozak, D.; Dorai, A.; Shibata, E.; Kawamura, J.; Nakamura, T. Liquid-Liquid Extraction of Neodymium Ions from Aqueous Solutions of NdCl3 by Phosphonium-Based Ionic Liquids. *Sep. Purif. Technol.* **2016**, *171*, 263–269. [CrossRef]

185. Ma, L.; Zhao, Z.; Dong, Y.; Sun, X. A Synergistic Extraction Strategy by [N1888][SOPAA] and Cyphos IL 104 for Heavy Rare Earth Elements Separation. *Sep. Purif. Technol.* **2017**, *174*, 474–481. [CrossRef]

186. Wang, W.; Liu, Y.; Xu, A.; Yang, H.; Cui, H.; Chen, J. Solvent Extraction of Yttrium by Task-Specific Ionic Liquids Bearing Carboxylic Group. *Chin. J. Chem. Eng.* **2012**, *20*, 40–46. [CrossRef]

187. Chen, Y.; Wang, H.; Pei, Y.; Ren, J.; Wang, J. PH-Controlled Selective Separation of Neodymium (III) and Samarium (III) from Transition Metals with Carboxyl-Functionalized Ionic Liquids. *ACS Sustain. Chem. Eng.* **2015**, *3*, 3167–3174. [CrossRef]

188. Depuydt, D.; Dehaen, W.; Binnemans, K. Solvent Extraction of Scandium(III) by an Aqueous Biphasic System with a Nonfluorinated Functionalized Ionic Liquid. *Ind. Eng. Chem. Res.* **2015**, *54*, 8988–8996. [CrossRef]

189. Fagnant, D.P.; Goff, G.S.; Scott, B.L.; Runde, W.; Brennecke, J.F. Switchable Phase Behavior of [HBet][Tf2N]–H2O upon Neodymium Loading: Implications for Lanthanide Separations. *Inorg. Chem.* **2013**, *52*, 549–551. [CrossRef]
190. Vander Hoogerstraete, T.; Onghena, B.; Binnemans, K. Homogeneous Liquid-Liquid Extraction of Metal Ions with a Functionalized Ionic Liquid. *J. Phys. Chem. Lett.* **2013**, *4*, 1659–1663. [CrossRef] [PubMed]

191. Vander Hoogerstraete, T.; Onghena, B.; Binnemans, K. Homogeneous Liquid-Liquid Extraction of Rare Earths with the Betaine-Betainium Bis(Trifluoromethylsulfonyl)Imide Ionic Liquid System. *Int. J. Mol. Sci.* **2013**, *14*, 21353–21377. [CrossRef] [PubMed]

192. Onghena, B.; Binnemans, K. Recovery of Scandium(III) from Aqueous Solutions by Solvent Extraction with the Functionalized Ionic Liquid Betainium Bis(Trifluoromethylsulfonyl)Imide. *Ind. Eng. Chem. Res.* **2015**, *54*, 1887–1898. [CrossRef]

193. Onghena, B.; Borra, C.R.; Van Gerven, T.; Binnemans, K. Recovery of Scandium from Sulfation-Roasted Leachates of Bauxite Residue by Solvent Extraction with the Ionic Liquid Betainium Bis(Trifluoromethylsulfonyl)Imide. *Sep. Purif. Technol.* **2017**, *176*, 208–219. [CrossRef]

194. Dupont, D.; Raiguel, S.; Binnemans, K. Sulfonic Acid Functionalized Ionic Liquids for Dissolution of Metal Oxides and Solvent Extraction of Metal Ions. *Chem. Commun.* **2015**, *51*, 9006–9009. [CrossRef]

195. Dupont, D.; Renders, E.; Binnemans, K. Alkylsulfuric Acid Ionic Liquids: A Promising Class of Strongly Acidic Room-Temperature Ionic Liquids. *Chem. Commun.* **2016**, *52*, 4640–4643. [CrossRef]

196. Dupont, D.; Renders, E.; Raiguel, S.; Binnemans, K. New Metal Extractants and Super-Acidic Ionic Liquids Derived from Sulfamic Acid. *Chem. Commun.* **2016**, *52*, 7032–7035. [CrossRef]

197. Iqbal, M.; Waheed, K.; Rahat, S.B.; Mehmood, T.; Lee, M.S. An Overview of Molecular Extractants in Room Temperature Ionic Liquids and Task Specific Ionic Liquids for the Partitioning of Actinides/Lanthanides. *J. Radioanal. Nucl. Chem.* **2020**, *325*, 1–31. [CrossRef]

198. Odinets, I.L.; Sharova, E.V.; Artyushin, O.I.; Lyssenko, K.A.; Nelyubina, Y.V.; Myasoedova, G.V.; Molochnikova, N.P.; Zakharchenro, E.A. Novel Class of Functionalized Ionic Liquids with Grafted CMPO-Moieties for Actinides and Rare-Earth Elements Recovery. *Dalt. Trans.* **2010**, *39*, 4170–4178. [CrossRef]

199. Mohapatra, P.K.; Kandwal, P.; Iqbal, M.; Huskens, J.; Murali, M.S.; Verboom, W. A Novel CMPO-Functionalized Task Specific Ionic Liquid: Synthesis, Extraction and Spectroscopic Investigations of Actinide and Lanthanide Complexes. *Dalt. Trans.* **2013**, *42*, 4343–4347. [CrossRef]

200. Turanov, A.N.; Karandashev, V.K.; Artyushin, O.I.; Sharova, E. V Extraction of U(VI), Th(IV), and Lanthanides(III) from Nitric Acid Solutions with CMPO-Functionalized Ionic Liquid in Molecular Diluents. *Solvent Extr. Ion Exch.* **2015**, *33*, 540–553. [CrossRef]

201. Wang, J.; Zhao, J.; Feng, D.; Kang, X.; Sun, Y.; Zhao, L.; Liang, H. Enhancing Extraction Ability by Rational Design of Phosphoryl Functionalized Ionic Liquids and Mechanistic Investigation on Neodymium (III) Extraction. *J. Rare Earths* **2016**, *34*, 83–90. [CrossRef]

202. Sengupta, A.; Mohapatra, P.K.; Kadam, R.M.; Manna, D.; Ghanty, T.K.; Iqbal, M.; Huskens, J.; Verboom, W. Diglycolamide-Functionalized Task Specific Ionic Liquids for Nuclear Waste Remediation: Extraction, Luminescence, Theoretical and EPR Investigations. *RSC Adv.* **2014**, *4*, 46613–46623. [CrossRef]

203. Yun, W.; Youwen, Z.; Fuyou, F.; Huimin, L.; Peizhuo, H.; Yinglin, S. Synthesis of Task-Specific Ionic Liquids with Grafted Diglycolamide Moiety. Complexation and Stripping of Lanthanides. *J. Radioanal. Nucl. Chem.* **2014**, *299*, 1213–1218. [CrossRef]

204. Binnemans, K.; Jones, P.T. Solvometallurgy: An Emerging Branch of Extractive Metallurgy. *J. Sustain. Metall.* **2017**, *3*, 570–600. [CrossRef]

205. Khazalpour, S.; Yarie, M.; Kianpour, E.; Amani, A.; Asadabadi, S.; Seyf, J.Y.; Rezaeivala, M.; Azizian, S.; Zolfigol, M.A. Applications of Phosphonium-Based Ionic Liquids in Chemical Processes. *J. Iran. Chem. Soc.* **2020**, *17*, 1775–1917. [CrossRef]

206. Schaeffer, N.; Passos, H.; Billard, I.; Papaiconomou, N.; Coutinho, J.A.P. Recovery of Metals from Waste Electrical and Electronic Equipment (WEEE) Using Unconventional Solvents Based on Ionic Liquids. *Crit. Rev. Environ. Sci. Technol.* **2018**, *48*, 859–922. [CrossRef]

207. Chen, Y.; Wang, H.; Pei, Y.; Wang, J. A Green Separation Strategy for Neodymium (III) from Cobalt (II) and Nickel (II) Using an Ionic Liquid-Based Aqueous Two-Phase System. *Talanta* **2018**, *182*, 450–455. [CrossRef]

208. Vargas, S.J.R.; Quintao, J.C.; Ferreira, G.M.D.; Da Silva, L.H.M.; Hespanhol, M.C. Lanthanum and Cerium Separation Using an Aqueous Two-Phase System with Ionic Liquid. *J. Chem. Eng. Data* **2019**, *64*, 4239–4246. [CrossRef]

209. van Osch, D.J.G.P.; Parmentier, D.; Dietz, C.H.J.T.; van den Bruinhorst, A.; Tuinier, R.; Kroon, M.C. Removal of Alkali and Transition Metal Ions from Water with Hydrophobic Deep Eutectic Solvents. *Chem. Commun.* **2016**, *52*, 11987–11990. [CrossRef]

210. Tang, N.; Liu, L.; Yin, C.; Zhu, G.; Huang, Q.; Dong, J.; Yang, X.; Wang, S. Environmentally Benign Hydrophobic Deep Eutectic Solvents for Palladium(II) Extraction from Hydrochloric Acid Solution. *J. Taiwan Inst. Chem. Eng.* **2021**. [CrossRef]

211. Geng, Y.; Xiang, Z.; Lv, C.; Wang, N.; Wang, Y.; Yang, Y. Recovery of Gold from Hydrochloric Medium by Deep Eutectic Solvents Based on Quaternary Ammonium Salts. *Hydrometallurgy* **2019**, *188*, 264–271. [CrossRef]

212. Shi, Y.; Xiong, D.; Zhao, Y.; Li, T.; Zhang, K.; Fan, J. Highly Efficient Extraction/Separation of Cr (VI) by a New Family of Hydrophobic Deep Eutectic Solvents. *Chemosphere* **2020**, *241*, 125082. [CrossRef] [PubMed]

213. Tang, W.; An, Y.; Row, K.H. Emerging Applications of (Micro) Extraction Phase from Hydrophilic to Hydrophobic Deep Eutectic Solvents: Opportunities and Trends. *TrAC Trends Anal. Chem.* **2021**, *136*, 116187. [CrossRef]

214. Ni, S.; Su, J.; Zhang, H.; Zeng, Z.; Zhi, H.; Sun, X. A Cleaner Strategy for Comprehensive Recovery of Waste SmCo Magnets Based on Deep Eutectic Solvents. *Chem. Eng. J.* **2021**, *412*, 128602. [CrossRef]
215. Gilmore, M.; McCourt, É.N.; Connolly, F.; Nockemann, P.; Swadźba-Kwaśny, M.; Holbrey, J.D. Hydrophobic Deep Eutectic Solvents Incorporating Trioctylphosphine Oxide: Advanced Liquid Extractants. *ACS Sustain. Chem. Eng.* **2018**, *6*, 17323–17332.
[CrossRef]

216. Schaeffer, N.; Conceição, J.H.F.; Martins, M.A.R.; Neves, M.C.; Pérez-Sánchez, G.; Gomes, J.R.B.; Papaiconomou, N.; Coutinho, J.A.P. Non-Ionic Hydrophobic Eutectics—Versatile Solvents for Tailored Metal Separation and Valorisation. *Green Chem.* **2020**, *22*, 2810–2820.
[CrossRef]

217. Quijada-Maldonado, E.; Romero, J. Solvent Extraction of Rare-Earth Elements with Ionic Liquids: Toward a Selective and Sustainable Extraction of These Valuable Elements. *Curr. Opin. Green Sustain. Chem.* **2021**, *27*, 100428.
[CrossRef]

218. Pateli, I.M.; Abbott, A.P.; Jenkin, G.R.T.; Hartley, J.M. Electrochemical Oxidation as Alternative for Dissolution of Metal Oxides in Deep Eutectic Solvents. *Green Chem.* **2020**, *22*, 8360–8368.
[CrossRef]

219. Abbott, A.; Endres, F.; Macfarlane, D. (Eds.) *Electrodeposition from Ionic Liquids*; Wiley: Weinheim, Germany, 2008; ISBN 9783527682706.

220. Bourbos, E.; Giannopoulou, I.; Karantonis, A.; Paspaliaris, I.; Panias, D. Electrodeposition of rare earth metals from ionic liquids. In *Rare Earths Industry*; De Lima, I., Filho, W., Eds.; Elsevier: Boston, MA, USA, 2016; pp. 199–207.

221. Legeai, S.; Diliberto, S.; Stein, N.; Boulanger, C.; Estager, J.; Papaiconomou, N.; Draye, M. Room-Temperature Ionic Liquid for Lanthanum Electrodeposition. *Electrochem. Commun.* **2008**, *10*, 1661–1664.
[CrossRef]

222. Kurachi, A.; Matsumiya, M.; Tsunashima, K.; Kodama, S. Electrochemical Behavior and Electrodeposition of Dysprosium in Ionic Liquids Based on Phosphonium Cations. *J. Appl. Electrochem.* **2012**, *42*, 961–968.
[CrossRef]

223. Chou, L.-H.; Hussey, C.L. An Electrochemical and Spectroscopic Study of Nd(III) and Pr(III) Coordination in the 1-Butyl-1-Methylpyrrolidinium Bis(Trifluoromethylsulfonyl)Imide Ionic Liquid Containing Chloride Ion. *Inorg. Chem.* **2014**, *53*, 5750–5758.
[CrossRef] [PubMed]

224. Bengio, D.; Dumas, T.; Arpigny, S.; Husar, R.; Mendes, E.; Solari, P.L.; Schlegel, M.L.; Schlegel, D.; Pellet-Rostaing, S.; Moisy, P. Electrochemical and Spectroscopic Study of Eu(III) and Eu(II) Coordination in the 1-Ethyl-3-Methylimidazolium Bis(Trifluoromethylsulfonyl)Imide Ionic Liquid. *Chem. A Eur. J.* **2020**, *26*, 14385–14396.
[CrossRef] [PubMed]

225. Zhang, Q.; Hua, Y.; Xu, C.; Li, Y.; Li, J.; Dong, P. Non-Haloaluminate Ionic Liquids for Low-Temperature Electrodeposition of Rare-Earth Metals—A Review. *J. Rare Earths* **2015**, *33*, 1017–1025.
[CrossRef]

226. Niedermeyer, H.; Hallett, J.P.; Villar-Garcia, I.J.; Hunt, P.A.; Welton, T. Mixtures of Ionic Liquids. *Chem. Soc. Rev.* **2012**, *41*, 7780–7802.
[CrossRef] [PubMed]

227. Gutowski, K.E. Industrial Uses and Applications of Ionic Liquids. *Phys. Sci. Rev.* **2018**, *3*, [CrossRef]

228. Greer, A.J.; Jacquemin, J.; Hardacre, C. Industrial Applications of Ionic Liquids. *Molecules* **2020**, *25*, 5207. [CrossRef] [PubMed]

229. Nockemann, P.; Brolly, D.; Bradley, E.; McCourt, E. Enhanced Separation of Rare Earth Metals 2019. International Patent WO2019239150A1, 19 December 2019.

230. Nockemann, P.; Brolly, D.; Bradley, E.; McCourt, E. Countercurrent Rare Earth Separation Process 2019. International Patent WO2019239151A1, 19 December 2019.

231. Nomngongo, P.N.; Biata, N.R.; Sihlahla, M.; Mpupa, A.; Mketo, N. Recent Advances in the Application of Greener Solvents for Extraction, Recovery and Dissolution of Precious Metals and Rare Earth Elements from Different Matrices. In *Nanotechnology-Based Industrial Applications of Ionic Liquids*; Inamuddin, M., Asiri, A.M., Eds.; Springer International Publishing: Cham, Switzerland, 2020; pp. 299–309. ISBN 978-3-030-44995-7.

232. Flieger, J.; Flieger, M. Ionic Liquids Toxicity-Benefits and Threats. *Int. J. Mol. Sci.* **2020**, *21*, 6267. [CrossRef]

233. Pham, T.P.T.; Cho, C.-W.; Yun, Y.-S. Environmental Fate and Toxicity of Ionic Liquids: A Review. *Water Res.* **2010**, *44*, 352–372.
[CrossRef] [PubMed]

234. Frade, R.F.; Afonso, C.A. Impact of Ionic Liquids in Environment and Humans: An Overview. *Hum. Exp. Toxicol.* **2010**, *29*, 1038–1054. [CrossRef] [PubMed]

235. Viboud, S.; Papaiconomou, N.; Cortesi, A.; Chatel, G.; Draye, M.; Fontvieille, D. Correlating the Structure and Composition of Ionic Liquids with Their Toxicity on Vibrio Fischeri: A Systematic Study. *J. Hazard. Mater.* **2012**, *215–216*, 40–48. [CrossRef] [PubMed]

236. Plotka-Wasylka, J.; de la Guardia, M.; Andruch, V.; Vilková, M. Deep Eutectic Solvents vs Ionic Liquids: Similarities and Differences. *Microchem. J.* **2020**, *159*, 105539. [CrossRef]

237. Macário, I.P.E.; Jesus, F.; Pereira, J.L.; Ventura, S.P.M.; Gonçalves, A.M.M.; Coutinho, J.A.P.; Gonçalves, F.J.M. Unraveling the Ecotoxicity of Deep Eutectic Solvents Using the Mixture Toxicity Theory. *Chemosphere* **2018**, *212*, 890–897. [CrossRef] [PubMed]