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

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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].
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