Liquid–Liquid Extraction of Actinides, Lanthanides, and Fission Products by Use of Ionic Liquids: from Discovery to Understanding

Anal Bioanal Chem (2011) 400:1555–1566 DOI 10.1007/s00216-010-4478-x

REVIEW

Liquid–liquid extraction of actinides, lanthanides, and fission products by use of ionic liquids: from discovery to understanding

Isabelle Billard & Ali Ouadi & Clotilde Gaillard

Received: 13 October 2010 /Revised: 19 November 2010 /Accepted: 25 November 2010 /Published online: 4 January 2011 # Springer-Verlag 2010

Abstract Liquid–liquid extraction of actinides and lantha- policies favor direct disposal in deep geological reposito- nides by use of ionic liquids is reviewed, considering, first, ries. Whatever the choice made nowadays, the future of phenomenological aspects, then looking more deeply at the nuclear energy, stimulated by the world demand for energy, various mechanisms. Future trends in this developing field are will require new waste-management policies, guided by presented. new regulations and economic obligations, centered on future nuclear reactors, most probably emerging from the Keywords Actinides . Lanthanides . Liquid–liquid generation IV program. In this context, ionic liquids, extraction . Ionic liquids already used in various industrial fields [1], may prove useful. Thus, this paper reviews the emerging field of Abbreviations liquid–liquid extraction of actinides (An), lanthanides (Ln), CMPO Carbamoyl-methyl phosphine oxide and fission products (mainly Sr and Cs but in a few cases TBP Tributyl phosphate we will refer to works devoted to other metallic species) by TODGA N,N,N′,N′-Tetra(n-octyl)diglycolamide use of ionic liquids (ILs). Owing to limited space, the TTA Thenoyltrifluoroacetone reader is referred to some recent excellent reviews on ILs or on An and Ln in ILs [2–5]. We simply define ionic liquids as salts with melting points below 100°C. Although ILs have a large variety of Introduction structures, extraction studies predominantly concern the imidazolium family, for which we adopt the CnmimX n Reprocessing is a key industrial feature of nuclear waste nomenclature, where X is the counteranion and indicates management in some countries, whereas other national the length of the second alkyl chain on the imidazolium cation (Fig. 1). We make a clear distinction between two types of liquid–liquid extraction systems, depending on Published in the special issue Radioanalytics - Dedicated to Marie whether the IL phase is used as a pure liquid or contains Skłodowska-Curie with Guest Editors Boguslaw Buszewski and one (or more) solutes dissolved in it, the extracting Philippe Garrigues. properties of which are examined. In this sense, the fact : I. Billard (*) A. Ouadi that the solute in the organic phase is, or is not, an ionic IPHC-DRS/CHNU, liquid by itself does not make any difference for us. Finally, 23 rue du Loess, although many studies have dealt with PF -based ILs in the 67037 Strasbourg Cedex 2, France 6 e-mail: [email protected] past, it has recently become clear that these ILs are liable to degradation in contact with an aqueous phase, especially C. Gaillard when high nitric acid concentrations are used [6–8]. Results IPN Lyon, in such solvents are therefore to be considered with caution, 4 rue E. Fermi, 69622 Villeurbanne Cedex, France although we nevertheless cite some of these works, owing e-mail: [email protected] to their fundamental interest. 1556 I. Billard et al.

+ - extracting phases. Although there is experimental evi- CH3 N N R X dence for an ion-exchange mechanism in some cases [13,

Fig. 1 Chemical structure of the CnmimX ionic liquid family 15], the possibility of better solvation/solubilization of ions in the IL phase than in the aqueous phase cannot be disregarded. Extraction into a pure IL phase Although the experimental background is rather limited, this topic is of dramatic importance to the field: ILs are very Surprisingly enough, there is an increasing number of unusual compared with traditional solvents, for which papers presenting systems in which there is no need of any extraction always involves a diluent, an extracting agent, extracting agent dissolved in the IL phase to obtain and, in some cases, a synergist. It is worth noting that a (possibly very high) extraction efficiencies. Although this very common IL family, namely CnmimX, enables good phenomenon is rather scarce in classical organic phases, we extraction of various cations, for example Hg2+,Ce4+, and suspect more and more liquid–liquid extraction systems Th4+. It is probable that more and more surprisingly based on pure IL phases will be discovered in the future, simple pure IL systems will be discovered in the future. either with rather simple ILs or with more complex ones. Understanding the deep physicochemical reasons for such At the moment, Hg(II) seems to be the most common a phenomenon in ILs would provide clues to solvation cation for which pure IL phases have been identified as and complexation in these media that are obviously of extracting media, some of these systems displaying decisive importance for better design of future extracting distribution ratio, D, values above 103 or extraction systems, either with or without extracting complexing efficiency, E≈100%. Very simple ILs, for example moieties.

CnmimPF6 (n=4,6,8)[9] are highly efficient but one may also find pyridinium or piperidinium-based ILs, either − − with BF4 or Tf2N [10]. It is very interesting to note that Phenomenological results for IL systems with extracting ILs specifically designed for Hg(II) extraction and with agents glycol [11] or thiourea [12] units inserted in their structure do not act better than very simple ILs, with D values not However, chronologically speaking, efficient IL extracting above 300. Clearly, “functionalization” is not mandatory systems comprising an extracting agent were reported first and for extraction. some papers presented spectacular extraction efficiencies that Extraction from pure IL phases is not restricted to Hg(II) urge the community to perform further studies. “Unprece- as it is achieved for La(III) using imidazolium-based ILs, dented large D values” were obtained for Sr2+ extraction by a with D ranging from 0 to 30 [13], for Ce(IV) [7] with D crown ether (CE) dissolved in various imidazolium-based − − 4 from 0 to 90, and for Pu(IV) [6]withD from 0.01 to 7.8 ILs (with either PF6 or Tf2N ), rising up to D=10 ,whilein using C8mimPF6 in both cases, all variations being studied toluene or chloroform, D only reaches values of the order of −1 as a function of [HNO3] in the aqueous phase. Pu(IV) is 10 [17]. The better efficiency of various CE dissolved in 2+ also efficiently extracted into C8mimTf2N, with D up to ILs was latter confirmed for Sr extraction in comparison −1 37.5 for [HNO3]=5 mol L [14]. Efficient extraction is with octanol [20, 21] and dichloroethane [22], but in this last obtained for Am3+ into a pure IL phase with a hydroxyl- paper, it was noted that selectivity towards competitive ions amine structure [15]. This example is one of the rare cases such as K+,Cs+,orNa+ was lower, an usual compromise in for which extraction occurs at basic pH. A distribution ratio molecular solvents. All these systems involved ILs from the 2+ of approximately 2 is obtained for UO2 with an IL imidazolium family and, to the best of our knowledge, no bearing a phosphoryl unit [16] and extraction is also paper has been published so far comparing the efficiency of observed for Sr(II) [17] in imidazolium-based ILs, the a given extracting compound dissolved in classical solvents − − anionic part being either PF6 or Tf2N . In this latter case, and in ILs other than imidazolium. the D values are approximately unity, which however Furthermore, it would be necessary to precisely define means that approximately half of the cation of interest is what a “better” system is, as compared with traditional extracted into the IL phase without the help of any solvents. Should the IL system display extraction efficien- extracting agent. Apart from fission products and An/Ln, cies above that of the reference system for the whole range one may also find recent publications evidencing efficient of investigated chemical conditions or is one optimized set extraction by use of simple pure IL phases (for example, of chemical conditions with higher D values enough to + − − − Cnmim , with either Tf2N ,BF4 or PF6 ) for Zn(II), Cd consider the IL system to be better than another? In this (II), Fe(III) [18] or Ni(II), Mn(II) or Cu(II) [19]. respect, one should note that TBP–CnmimTf2N(n=4,5,8, 2+ The question of the extraction mechanism has not 10) extraction systems for UO2 do not display extraction been studied in great detail so far for these pure efficiencies above that of the industrial TBP–dodecane system Liquid–liquid extraction of actinides, lanthanides, and fission 1557

100 for the same value of TBP concentration (30% v/v) and the same range of acidic conditions of the aqueous phase [23– 90 Eu in IL phase 80 25]. Nevertheless, ILs are considered better than traditional Lu in IL phase solvents for quite a large variety of cation–extracting 70 Eu in isooctane 2+ compound combinations. One may find examples for Sr – 60 Lu in isooctane 3+ 2+ 4+ 4+ 3+ CE, as discussed above, (Am ,UO2 ,Pu ,Th ,Ce , 50 3+ 3+ Eu ,Y )–CMPO [26–28]incomparisonwithdodecane E (%) 40 and (La3+,Eu3+,Lu3+)–TODGA [29]incomparisonwith isooctane. In some cases, the advantage is expressed as a 30 lower concentration of the extracting moiety to be dissolved 20 in the IL as compared with the usual solvent in order to 10 obtain an identical D value, with a concentration benefit in 0 the range of a factor of 4 to 30 [27, 28]oreven500[29]. 0,001 0,01 0,1 1 10 100 Apart from a better efficiency as solely measured via the [TODGA] (mM) D values, other aspects of IL systems deserve consideration Fig. 3 Schematic diagram of variation of the extraction coefficient for and could have potential advantages. extraction of Eu3+ and Lu3+ by an IL phase or by isooctane, as a First, pH dependence can be reversed and selectivity function of the ligand concentration from one cation to another can be reshuffled as compared with traditional solvents. This is better illustrated in the case with TBP and CE into various CnmimTf2N media [31]. of TODGA dissolved in either C2-mimTf2N or isooctane Although synergism between CMPO and TBP was not for extraction of Eu3+,La3+, and Lu3+ [29]. Variation of the demonstrated for the extraction studies of various actinides extraction efficiency as a function of [HNO3] or [TODGA] into C4mimPF6 phases, there is evidence of enhancement for the IL and molecular solvent phases is presented by addition of TBP to the CMPO–IL solution [26]. schematically in Figs. 2 and 3, respectively. In the IL Third, and probably most importantly, adjustment of the system, extraction can be performed under much less acidic D values has been proved to be possible by changes in the conditions (Fig. 2) and the selectivity of TODGA for the IL cations and anions. This is a fascinating perspective for a lanthanide series is in favor of the heavier Ln in isooctane chemist and a major reason for the justified growing and the lighter ones in CnmimTf2N[29]. Similar results can interest in ILs. Such effects have been studied for CE only be found for transition metals in Ref. [30]. but this is probably not the only example. It has been Second, synergism between different extracting moieties shown that the alkyl chain length has an effect on the is also observed in ILs, as in the case of Sr2+ extraction extraction and selectivity of K+,Cs+,Rb+, and Na+ by CE in a imidazolium-PF6 series [32] and D values greatly depend on both the alkyl chain length and the IL anion 100 − − 2+ (Tf2N vs PF6 ) in the case of Sr extraction [17]. Other 90 publications studied Sr/Cs selectivity in great detail; it was

80 shown to be sensitive first to the IL anion and second on the alkyl length [33, 34], with intricate effects. 70 Finally, for Na+ extraction, an isomer effect of some CE 60 on D variations was evident in C10mimTf2N, which was very different from that observed in 1-octanol [35]. 50

E (%) in IL solvant As a conclusion to this phenomenological presentation 40 in isooctane of extraction in IL systems, one should keep in mind the huge variety of possible systems compared to those already 30 investigated, which is both a fascinating stimulus and a 20 discouraging observation. All the extraction features discussed above can be considered either as advantages or 10 disadvantages, depending on priorities or needs. One may 0 be looking at extraction from very acidic conditions or 0,001 0,01 0,1 1 10 might be interested in extraction from almost pure aqueous [HNO3] (M) phases. Similarly, the change in the selectivity order is not an intrinsic advantage but may be helpful in some circum- Fig. 2 Schematic diagram of variation of the extraction coefficient for extraction of the Ln3+ ion by an IL phase or by isooctane as a function stances. As a whole, IL-based extracting systems present a of the nitric acid concentration large variety of behavior, some of which are very different 1558 I. Billard et al. from that in conventional molecular solvents, and one has not too high [24]. These salt-in/salt effects have been to decide whether or not these are an important issue for confirmed for other aqueous–IL systems [37]. one’s own specific problem. These intricate effects deserve three comments. First, in traditional extraction systems, it is assumed that the dehydration of metallic cations, when passing from the In search for mechanisms: preliminary studies aqueous to the water phase, is the main factor limiting to focus on “natural” extraction (i.e. without extracting agent) and this experimental fact is one rationale for the need for The variety of behavior highlighted in the section above extracting agents. Considering the high water solubility was a great impulse to a deeper investigation of IL-based in ILs, dehydration is no more a handicap to cation extraction systems and as time passed, mechanism under- extraction and this might be one of the clues to the standing became almost compulsory. As the number of surprising phenomenon of cationic extraction into pure papers on these topics increases, IL systems are obviously IL phases detailed above. Second, it would be desirable appearing more complex, if not unpredictable, thus less to better understand the state of water in the IL phase, appealing at first sight. It thus tends to be a commonplace and its possible interactions with the extracting agent, if nowadays to conclude that the “greenness” aspects of ILs any. Should the water be bound to the extracting moiety, might not be as convincing as one used to think a few years this would largely reduce the concentration of “free” ago. By contrast to this rather disappointing balance, we extractant able to bind metallic cations, thus affecting the would like to reaffirm how fascinating ILs are, assuming stoichiometry of the extracted species. Third, such large objectives are clearly defined. amounts of water dissolving in the IL phase modify the On another hand, the “unusual” behavior of ILs (as density and the viscosity of the IL phase, which might be compared with traditional solvents; as time is passing, ILs of industrial concern, and also lead to volume variations may come more and more traditional, thus “usual”)is of both the aqueous and IL phases once equilibrium is sometimes so unexpected that these aspects are simply not reached. checked, which may have disastrous consequences on correct understanding of extraction mechanism. In this Acid solubility in the IL phase section, we pinpoint important issues of this kind that have to be studied first to correctly decipher the IL extraction Similarly, very few data have yet been published. This mechanism. is partly because of the lack of analytical procedures in ILs so that, most of the time, only the variation of Water solubility in the IL phase the aqueous concentrations before and after phase

equilibration is investigated. HNO3 dissolves easily in Although ILs used for liquid–liquid extraction are not C4-mimTf2N[38] and large amounts of acid are readily miscible with water, most are hygroscopic. The solubility transferred to the IL phase during the phase equilibration of water in ILs is quite high, a situation rather different stage of extraction experiments. For example, for an initial −1 from that for dodecane for example. Furthermore, the HNO3 concentration of 7.4 mol L , the remaining solubility of pure water in a pure IL phase is not an HNO3 concentration in water after equilibration with pure −1 indication of the situation under the chemical conditions C4mimTf2N is only 5.8 mol L [24]. However, in contrast of extraction, when the aqueous phase contains large with the dodecane case, TBP has been shown not to quantities of acids and/or salts and when the IL phase is, participate into this transfer, which thus should be most of the time, diluted with an organic extracting regarded as a physical phenomenon instead of an agent. However, experimental results are very scarce on extraction process. Nonetheless, the quantity of H+ and − this point and data are available for the imidazolium NO3 ions present in the IL phase is far from negligible family only. The solubility of pure water in pure ILs and this aspect has to be kept in mind when considering increases slightly as a function of temperature [36]. metallic cation extraction processes. Whether H+ and − Various salts (NaCl, KCl, CaCl2, etc.) slightly reduce the NO3 remain dissociated or not when entering the IL solubility of water in pure C4-mimTf2N, from ca. phase is an unresolved question that deserves consider- 14,000 ppm (~1.1 mol L−1) (pure water) to 13,000 ppm ation. Finally, to our knowledge, no data are available on −1 (~1.0 mol L )at1.5m(molalunity)ofaddedsalt.HNO3 the possible effect of extracting agents other than TBP on has a very different effect, because water solubility HNO3 or other acid or salt solubilities in various ILs. −1 increases to ca. 32,000 ppm (~2.5 mol L )at[HNO3]= Published data concern solely C4mimTf2Nsofurther 7.5 mol L−1 and addition of TBP to the IL phase also greatly studies should also focus on other IL for the sake of increases the water solubility, if the HNO3 concentration is comparison. Liquid–liquid extraction of actinides, lanthanides, and fission 1559

IL solubility in the aqueous phase extraction, which remains very high (100% for Eu3+ for −1 −3 −1 [CMPO]=10 mmol L ) from [HNO3]=10 to 1 mol L , Measuring IL solubility in a pure aqueous phase is easily while further investigation revealed that metal ions could achieved by UV–visible spectroscopy, because most the IL even be extracted from deionized water. Unfortunately, no cations absorb strongly in the near UV. However, solubility indication of the exact pH of the aqueous samples can be values are scattered, ranging from 15 mmol L−1 [29, 39]to found in this paper but we assume it should be approxi- −1 47 mmol L for C4mimTf2N in pure water [24]. Other mately 5, owing to the carbonate species that easily −1 values include 47 mmol L for C2mimTf2Nand dissolve in “pure” water. This slightly acidic pH prevents −1 5 mmol L for C6mimTf2N[29]. hydrolysis and precipitation of the lanthanides. Slope Large amounts of HNO3 render such a determination analysis has been used to derive the CMPO stoichiometry − difficult because NO3 also strongly absorbs in the UV of the extracted species. According to the authors, the slope range so that, once again, data obtained under conditions is equal to 3 for all the cations investigated, but a closer close to those of extraction experiments are very limited. To look to the log–log plot would rather indicate slopes − overcome this problem, determination of the Tf2N con- between 2 and 2.7, depending of the cation of interest. centration in water can be performed by ATR spectroscopy. The suggested extraction equilibrium is:

Surprisingly enough, C4mimTf2N solubility in water, as 3þ þ − Ln þ 3CMPO þ 3C4mim measured via the Tf2N aqueous concentration, depends neither on the HNO3 nor on the TBP concentrations and , LnðÞ CMPO 3þ þ C mimþ ð Þ −2 −1 3 3 4 1 remains equal to ca. 4×10 mol L in the range [HNO3]= − 1 3+ + 0 to 7 mol L and [TBP]=0% to 30% [24]. Because IL To ascertain the proposed scheme of Ln vs C4mim + contamination of the aqueous phase by ion exchange exchange, the authors added C4mim to the aqueous phase extraction is considered to be a problem for industrial (by dissolution of C4mimCl) and showed that the extraction applications of ILs, it might be important to extend such efficiency is thus reduced. This is a clear evidence of the + solubility studies, possibly determining which chemical role of C4mim in the extraction scheme, although the conditions reduce IL solubility in water, thus enabling stoichiometry was not ascertained. purification of aqueous phases [40]. Another study has dealt with CMPO dissolved in

C8mimPF6 for extraction of Pu(IV) and the authors have Characterization in the IL phase also concluded it is a cation-exchange mechanism [6]. In a first step, for a fixed CMPO concentration, they showed

Differences between ILs and traditional solvents may arise that Pu extraction increases as a function of either HNO3 − + from a difference in the stoichiometry of the extracted and NaNO3 (thus proving the effect of NO3 instead of H ). species. Although slope analysis may be sufficient to Second, for a fixed HNO3 value, the authors also showed determine such stoichiometries, as stressed above, the exact that Pu(IV) extraction increases as a function of CMPO amounts of (nitrate) ions in the aqueous phase and of the concentration. Graphical analysis leads to slope values of “ ” free ligand in the IL phase are largely unknown, which ~0.94 (~0.87) and ~2.2 for the HNO3 (NaNO3) and CMPO hampers use of this simplistic graphical method. Two dependences, respectively, so the proposed mechanism is: publications have highlighted the great benefit of using EXAFS [41, 42]orUV–visible spectroscopy [41]to þ Pu4 þ NO þ 2CMPO þ 3C mimþ determine the stoichiometries of extracted species of Sr(II) 3 8 and U(VI) in ILs. We would like also to emphasize the need 3þ þ , PuðÞ NO3 ðÞCMPO þ 3C8mim ð2Þ for analytical techniques in ILs, which are still in their 2 infancy owing to the ionic nature of IL solvents. In this paper, there is no further proof of the role of + C8mim by addition of this cation in the aqueous phase, as was done in the study discussed before, but the evidence Archetypal examples of extraction mechanism studies for a cation-exchange mechanism is, nevertheless, very convincing. Cation exchange A slightly different exchange mechanism occurs for La (III), Eu(III), and Lu(III) extraction by TODGA dissolved

Some of the most striking evidence for a cation-exchange in C2mimTf2N[29]. As in the previous studies, slope mechanism is to be found in the paper by Nakashima et al. analysis has been used to derive the TODGA stoichiometry, [28] dealing with the extraction of Ce(III), Eu(III), and Y which is equal to 3. Interestingly enough, the authors note

(III) by CMPO dissolved in C4-mimPF6. The concentration there is some evidence of HNO3 dissolution in the IL phase of HNO3 in the aqueous phase has no effect on the but they did not study this aspect further. As a matter of fact, 1560 I. Billard et al. the integer value derived for the TODGA stoichiometry a occurring. This, once again, does not prove that anion posteriori demonstrates that TODGA is not involved in HNO3 exchange is scarce in IL systems, owing to the still very dissolution. The extraction ratio decreases as a function of limited number of systems studied. The first system of + [H ] on use of either HNO3 or H2SO4. This shows that no concern is Eu(III) extracted by HTTA dissolved in anion is involved in the extraction mechanism. The C4mimTf2N[43]. Slope analysis indicates a +4 dependence + + extraction efficiency also decreases as C2mim is added to on HTTA concentration and a −4 dependence on H the aqueous phase. However, the exchange mechanism concentration. This shows that extraction is associated with proposed by the authors is: the release in water of four H+ per extracted Eu unit, in agreement with the deprotonation of four HTTA moieties to 3þ þ Ln þ 3TODGA þ 3C2mim form a negatively charged complex in the IL phase. Anion exchange is also evident from the increase of the , LnðÞ TODGA 3þ þ C mimþ ð Þ − 3 3 2 3 concentration of Tf2N in water as the lanthanide is extracted. The slope of +1 is consistent with the stoichi- This equation duly accounts for the C mim+ dependence 2 ometry derived from the HTTA dependence. The authors of the extraction efficiency but does not reproduce the took advantage of optical spectroscopy and EXAFS effect of H+ obtained with HNO and H SO . Therefore, we 3 2 4 measurements to further assess the stoichiometry of the propose that the extraction is driven by a cation-exchange extracted species, which thus led them to propose the process involving both H+ and C mim+, following the 2 following extraction equilibrium: general scheme below: Ln3þ þ HTTA þ C mimþTf N 3þ þ þ 4 4 2 Ln þ 3TODGA þ nH þ ðÞ3 n C2mim þ , C4mimLnðÞ TTA þ 4H þ Tf2N ð5Þ , LnðÞ TODGA 3þ þ nH þ þ ðÞ n C mimþ ð Þ 4 3 3 2 4 Although perfectly in agreement with the experimental Actually, dissolution of the acid in the IL phase (see the data, this equation deserves a comment. The lanthanide section “Acid solubility in the IL phase”) provides large species in the IL phase is written as a neutral entity, quantities of H+ likely to be backward extracted to although the authors have clear evidence of an anion- 3+ + counterbalance the Ln extraction. exchange mechanism, and the role of C4mim is limited to Two other very interesting examples of cation exchange artificial neutralization of the Ln complex. Strictly speak- can be found in the literature: ing, Eq. (5) is formally identical with Eq. (6): + 1. Cs extracted by calixarenes in CnmimTf N[22]; and 2 3þ þ 2+ Ln þ 4HTTA þ Tf2N , LnðÞ TTA þ 4H þ Tf2N 2. Sr extracted by crown ethers in CnmimTf2N[20]. 4 ð6Þ In the Cs+ extraction, the authors have observed that + + although Cnmim is predominantly exchanged with Cs , However, the picture in Eq. (6) is very different, in which leads to undesirable contamination of the aqueous nature, from that in Eq. (5). In the former, the question of phase by the costly imidazolium cation, Na+,whendissolved the solvation sphere of the Ln complex is not tackled, in the IL phase, reduces imidazolium transfer into water (of the whereas Eq. (5) gives the image of an ion pair dissolved in − − order of 25% for 0.12 mol L 1 Na+ and 2.5 mmol L 1 Cs+) a ionic medium. The question of the onion shell structure of because it is also exchanged with Cs+. This offers the solvated ions in ILs is a wide subject which is beyond the possibility of using sacrificial ions to reduce the loss of ILs scope of this review but the reader can find some hints in to aqueous phases. This point would deserve further studies. Ref. [44]. Another attempt to describe ion exchange with Two remarks are worth making as a conclusion of this neutral species can be found in Ref. [7]. section. The various cation exchanges discussed in this A second convincing example of anion exchange is Pu section all involve neutral extractants, which, however, (IV) extraction, in the form of nitrate anionic species, into does not prove that this mechanism is always true and pure simple IL phases such as C4mimTf2N and C8mimTf2N solely for neutral extractants. The charge of the extracted [14]. Interestingly, it seems that the nitrato complex of Pu is − species is either 1+, 2+, or 3+, which is evidence of the different in the two ILs, being [Pu(NO3)5] in C4mimTf2N 2− versatility of ILs to accept positively charged species. and [Pu(NO3)6] in C8mimTf2N.

Anion exchange Extraction of neutral species

Anion exchange is also possible but to our knowledge, very So far, only one IL system has been demonstrated to behave few publications report systems in which anion exchange is exactly as a traditional molecular solvent does [35, 45], that Liquid–liquid extraction of actinides, lanthanides, and fission 1561 is, the mechanism at work is extraction of a neutral species. 1000 This is for U(VI) extraction by use of phosphinic acid, for which the variation of the distribution ratio in C10mimTf2N closely resembles that in dodecane in shape and quantita- 100 tive values, and the UV and EXAFS spectra of the extracted species are similar in both solvents. All together, this is a C5mimTf2N very convincing proof of an identical extraction mechanism 10 in both systems. Another paper claims an identical C10mimTf2N partitioning mechanism in IL and a conventional organic 1-octanol solvent in the case of Na+ extraction by some specific Sr 1 isomers of the crown ether DCH18C6, in either 1-octanol D or C10mimTf2N[35]. However, this assertion is based solely on comparison of the distribution coefficient variations and is thus not as convincing as the previous case. 0,1 In these two papers, the authors congratulate themselves on finding, at last, IL systems avoiding ion exchange, owing to the disadvantage of aqueous phase contamination 0,01 that is inherent to ion-exchange mechanisms. It is true that dissolution of costly ILs in the aqueous phase may be regarded as a drawback of such liquid media but this is a 0,001 rather partial way to consider the whole problem. It might 0,001 0,01 0,1 1 10 be more economically advantageous to waste part of the IL [HNO3] (M) component in the aqueous phase, while dramatically Fig. 4 Schematic diagram of the variation of the Sr extraction ratio as increasing the efficiency of the whole process, thus a function of [HNO3] for different extracting solvents reducing costs and overall pollution. Conversely, one could wonder what is the true advantage of IL systems behaving as traditional solvents. In such a perspective, ILs simply do in the case of a cation-exchange mechanism. In contrast remain costly viscous solvents with no advantages in terms with the C5mimTf2N case, DSr also increases as a function of efficiency. of HNO3 when using C10mimTf2N. However, the other Whatever is sought in terms of extraction with ILs, the experimental results of this study are not in favor of an very restricted list of studies discussed above does not identical extraction mechanism in C10mimTf2N and 1- enable any reasonable conclusion to be drawn about the octanol. The slope of the DSr vs HNO3 variation is clearly conditions that may be required to achieve (or avoid) lower for C10mimTf2N than for 1-octanol and Sr extraction extraction of neutral species by ILs. However, it is worth in C10-mimTf2N reaches a plateau above [HNO3]= −1 noting that C10mimTf2N is the IL under investigation in 1 mol L , which is not observed in 1-octanol, up to −1 these two studies. Obviously, the long alkyl chain on the [HNO3]=6 mol L at least. The authors ascribe this imidazolium ring limits solubility of the IL cation in leveling off to the “decreased of free DCH18C6 arising aqueous phases, thus somehow hampering, but not exclud- from nitric acid extraction by the crown ether” but this ad-hoc + ing, a cation-exchange mechanism in which C10mim hypothesis must be regarded with great caution. First, it would would be involved. As discussed in the section “ Anion be important to understand why no leveling off is observed in + + exchange”, dual cation exchange (with H and Cnmim )is 1-octanol, in which DCH18C6 is also susceptible to nitric acid also possible and might be favored by long alkyl chains on extraction [46] and, second, recalling that TBP does not the imidazolium ring (Eq. 4). extract HNO3 in C4mimTf2N (see the section “ Acid As first evidence that C10mimTf2N does not always lead solubility in the IL phase”), the assumed HNO3 extraction to the extraction of (neutral) species similar to that extracted by DCH18C6 in C10mimTf2N remains to be proved in molecular solvents, we would like to point the very experimentally . More importantly, the EXAFS spectra of interesting paper reporting extraction of Sr(II) by the extracted species in 1-octanol and C10mimTf2N differ − DCH18C6 in C5mimTf2N, C10mimTf2N, and 1-octanol substantially, Sr being coordinated to ~2.1 NO3 entities in [21], as schematically illustrated in Fig. 4. In 1-octanol, the 1-octanol and to 0.3±0.5 nitrate ions in C10mimTf2N, thus Sr distribution ratio increases as a function of [HNO3], supporting (but not proving) the extraction of a cationic consistent with the extraction of the neutral Sr entity containing only one nitrate, in order to qualitatively

(NO3)2DCH18C6 species, as is also characterized by comply with the DSr vs HNO3 variation. In such a case, the EXAFS, whereas in C5mimTf2N, DSr decreases, as it would cation-exchange mechanism would possibly involve one 1562 I. Billard et al.

+ C10mim entity per extracted Sr moiety. The reader can find acid concentration depends on the IL of interest. This in Ref. [47] another example in which C10mimTf2N does not system, which is the IL analog of one of the most behave as 1-octanol. characteristic extraction systems of the nuclear fuel cycle, It seems, therefore, that the exact nature of the extraction namely the Purex process (in which kerosene–dodecane is mechanism is not simply governed by the choice of the IL used instead of IL) has been examined in great detail by but is the result of intricate interactions between the IL two research groups [23, 24]. Both groups agree that the U solvent, the extracting moiety (if any), and the element to shape of the distribution ratio is because of a change in the be extracted. For example, note the extraction of various extraction mechanism, from cation exchange at low metal ions by CnmimPF6 (n=4, 6, 8) with HTTA, where acidities to either extraction of the neutral species 2+ 2+ 2+ Ni ,Cu , and Pb are extracted as the neutral species M [UO2(NO3)2.2TBP] [23] or to anion exchange, implying 2+ 2+ 2+ 2+ − − (TTA)2(H2O)x and Mn ,Co ,Zn ,andCd are [UO2(NO3)3(TBP)n] and Tf2N [24]. − extracted as M(TTA)3 [48]. Again, this can be regarded as of huge potential or of discouraging complexity. Stripping

Mixed extraction mechanisms The number of studies devoted, at least in part, to stripping and recycling is very limited. D varying from values below The studies discussed in the above three sections always unity (approx. 0.01 or lower) to values well above unity as present a single mechanism in the whole range of ligand a function of chemical conditions (typically, the acidity of and acidic conditions investigated. This however does not the aqueous phase) is actually one of the main character- indicate that extraction is not susceptible to mechanistic istics needed for a system to be well-suited to both changes depending on the chemical conditions. A striking extraction and back extraction. In this respect, the IL 2+ example of this is seen for UO2 extraction by TBP in system presented in Fig. 2 is a good candidate, whereas the various CnmimTf2N(n=4,5,8,10)[23, 24]. In any of IL systems in Fig. 4 are not so interesting. Of all the these ILs, the distribution coefficient first decreases as a systems depicted in Fig. 5,onlyC10mimTf2N is likely to be function of [HNO3], then increases (the variations are suitable for extraction and stripping, but this aspect has not displayed schematically in Fig. 5) and the turnover nitric been studied in detail. Favorable results for stripping and recycling of the IL phase were found for Ag+ extracted into 2+ 100 C8mimPF6 [49] but are far from convincing for Cu and C4mimPF6 [50]. Whatever the system, the stripping mechanism has not been sought and apart from the study with Ag+, for which there is evidence that the extraction mechanism is simply reversed during stripping [49], there is C -mimTf N 10 4 2 no indication that cationic or anionic species extracted in the IL phase are actually back-extracted with the same stoichiometry. Actually, in the case of C mimTf N for C -mimTf N 10 2 5 2 uranyl extraction, as illustrated in Fig. 5, it is clear that favorable extraction should be performed from an highly −1

U acidic aqueous phase ([HNO ]>1 mol L )butback 1 3 D extraction with two different mechanisms can be envisaged, depending whether the aqueous reception phase is acidified C8-mimTf2N −1 with HNO3 above or below ca. 0.02 mol L . Furthermore, should the extraction mechanism be anion exchange, − implying Tf2N [24],thenapossiblewayofback 0,1 2+ extracting UO2 from the IL phase could be by use of aqueous phases acidified with HTf2N, as suggested for other systems [43]. At the moment, none of these aspects C10-mimTf2N have been examined in detail.

0,01 Task-specific ILs 0,001 0,01 0,1 1 10 [HNO3]aq,init (M) Although the term task-specific IL (TSIL) is quite popular 2+ in the literature, its exact definition is rather vague. Fig. 5 Extraction ratio of UO2 as a function of [HNO3] for different IL from the imidazolium family CnmimTf2N Generally, a TSIL is an IL (thus, referring to the definition Liquid–liquid extraction of actinides, lanthanides, and fission 1563 adopted in this review, a salt which melts below 100 °C) β-diketone [62] or azolate [63] patterns. for example. but with functionality which confers additional properties to the none of these has been tested so far for detailed extraction compound, for example catalytic activity or extraction studies, although promising results have been obtained with properties. The new compound thus combines the solvent the β-diketones [62]. As a matter of fact, ILs with properties of the IL with the extracting properties of the extracting functionality on the cation efficiently extract pendant arm. Various examples of this idea can be found in cations, which would seem counter-intuitive at first glance. the literature, which provides ILs grafted with hydroxyl- As far as molecular dynamic simulations can prove, cations amine [15], phosphoryl [14, 16], CMPO [51, 52], crown- in ILs adopt an onion-shell solvation structure, the first ethers [53], calixarenes [54, 55], etc. A general overview of solvation sphere being entirely composed of the anionic the field can be found elsewhere [56, 57]. entity of the IL [44, 64–66]. The nature of the solvation– Following this definition, TSILs would be a sub-class of complexation structure of the extracted species in the case ILs but the chemical variety of TSILs is as large as that of of those fancy ILs is a question yet to be answered and for ILs so this distinction seems rather unjustified. On the other which the effect of dissolved water should not be neglected. hand, ILs, by definition, can be regarded as task-specific We hope that information on this point could be obtained entities—strictly speaking, they are specifically designed by study of ILs grafted on to the anion instead. for given viscosity or solubility properties, or electrochem- ical window, so that ILs should be regarded as a sub-class of TSILs. As a consequence, although TSIL is a convenient Conclusion notion, it lacks real significance. The terms “dual-functionalized”, “bifunctional”, and “multi task specific” recently As a conclusion, we will pinpoint some future trends of An used in the literature [58–60] are similarly useless. and Ln extraction by use of ionic liquids but we will not Despite the reservations expressed above, the idea of enter into the details of some (although very interesting) grafting an extraction pattern on to the IL skeleton is other aspects of IL extraction systems, namely liquid–solid obviously a intelligent one and has led to a brilliant renewal extraction [51, 67] and extraction with supercritical CO2 of the concept of liquid–liquid extraction. This is first seen [68–70]. in the possibility of reducing the solubility of costly One subject of increasing interest in the literature is the extractants in the aqueous phase, as emphasized in some use of ILs as solutes in more traditional solvents, a quite publications [9, 11]. This strategy is similar to traditional surprising idea at first sight. From the above discussions, IL attachment of pendant alkyl chains to reduce the solubility extraction systems seem to favor ion exchange, either of extracting moieties in the aqueous phase [61]. However, cationic or anionic. Thus, the primary function of ILs is the it adds more to the idea because, in some cases, the reservoir they provide for ion exchange, and their role as compound obtained is a liquid which is fluid enough to be solvents is of second order. Considering costs, it might be used as the extracting phase, without the use of a diluent better to reduce the quantity of IL in the process, to limit it [15, 16]. In this case, the extraction system is thus a pure IL to the exact need for extraction, and no more. In this phase; the reader is referred to the section “ Extraction into respect, dissolution of ILs in traditional solvents and, by a pure IL phase” for a deeper discussion of the mechanism. adjusting their concentration, getting the better of the two Of course, one could argue that the cost of such compounds worlds could be envisaged. At the moment, few papers greatly exceeds their benefits in terms of extraction have explored this area but we think such studies will be efficiency but this debate about economics has to be developed in the near future, owing to the promising results examined for each specific case. Note also that grafting obtained so far. Tetraalkylammonium salts have been extraction patterns on to the IL skeleton is not always a dissolved in cyclohexane and chloroform and shown to panacea to all extraction problems, as stressed by Luo et al. extract Eu(III) from nitric aqueous phases [71]. The authors for the crown-ether pattern: “These TSILs gave lower suggest Eu is extracted through ion pairing, which would extraction efficiency than the IL extraction systems using deserve some additional experiments to be fully proved. the crown ether alone as extractants” [53]. Various lanthanides were efficiently extracted by use of Apart from their potential in applied extraction studies, several phosphine oxides dissolved in dichloroethane to those fancy ILs are of fundamental interest as they raise which small amounts of ILs have been added [72]. In −3 −1 many questions yet to be solved. At the moment, extraction particular, the addition of 10 mol L C4mimTf2N was patterns have been grafted only on to the cation component shown to dramatically increase the europium distribution of ILs, most probably because its basic structure was in ratio (over three orders of magnitude). Again, the authors accord with what organic synthesis usually works with. suggest that a neutral species is extracted. Another study − − Extracting functionalities grafted on to the anion have just concerns the extraction of ReO4 (as a surrogate of TcO4 ) recently appeared in the literature, with the introduction of into CCl4 or diisopropylbenzene in which a crown ether 1564 I. Billard et al. and tetraalkylphosphonium ILs are dissolved. Results using phosphonate based task specific ionic liquid. Radiochim suggest the formation of an ion pair but the role of the IL Acta 98:459 15. Ouadi A, Gadenne B, Hesemann P, Moreau EJJ, Billard I, in the extraction mechanism is unclear and the possibility of Gaillard C, Mekki S, Moutiers G (2006) Task-specific ionic an ion-exchange mechanism is not ruled out [73]. liquids bearing 2-hydroxybenzylamine units: synthesis and americium By dissolving small amounts of ILs in molecular extraction studies. 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