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SEPARATIONS Feasibility of Ionic Liquids as Alternative Separation Media for Industrial Solvent Extraction Processes Kazunori Nakashima, Fukiko Kubota, Tatsuo Maruyama, and Masahiro Goto* Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan
Extraction of rare earth metals into ionic liquids (ILs) from aqueous solutions was investigated using octyl(phenyl)-N,N-diisobutylcarbamoylmethyl phosphine oxide (CMPO) as an extractant. Use of ILs greatly enhanced the extraction efficiency and selectivity of CMPO for metal ions compared to when n-dodecane was used as the extracting solvent. The extraction mechanism has been studied by slope analysis and extraction tests, and these confirmed that the metal extraction proceeds via a cation-exchange mechanism. Furthermore, stripping of metals from ILs into an aqueous phase by complexing agents and recycling of the extracting ILs phase was successfully accomplished. Introduction In recent years, the concept of “green chemistry” has become well-known among scientists in many fields. In particular, exploration of novel solvents that can replace volatile organic compounds (VOCs) used in synthesis, catalysis, and separation processes has been avidly pursued. Ionic liquids (ILs),1-4 as well as supercritical fluids and fluorous compounds, attract much attention as alternatives to conventional organic solvents. ILs are usually composed of heterocyclic organic cations and various anions and have unique properties such as nonvolatility, nonflammability, and a wide temperature range for liquid phase. The most attractive property of ILs with respect to separation techniques is that ionic liquids can be designed to be water-immiscible salts by extending the alkyl chain length of the cation and/or incorporating hydrophobic anions.5,6 These hydrophobic ILs are therefore available for separation techniques that use a biphasic system, such as liquid-liquid extraction. Since the pioneering work by Rogers7 and co-workers, several attempts have been made to utilize ILs as alternatives to traditional organic diluents in solvent extraction of organic molecules7-10 and metal ions.11-21 Dai et al.11 examined the extraction of alkaline and alkaline earth metals into ILs using crown ethers as an extractant and achieved high extraction efficiency compared to that of ordinary organic diluents. Rogers et al. reported pH-switching partitioning of an indicator dye.8 They also developed novel ILs incorporating metal coordination groups, and these ‘task-specific’ ILs showed great extraction efficiency when used on their own as an extracting phase or when doped into nonspecific 1-alkyl-3-methylimidazolium-based ILs.13 To date, applications of ILs in separation techniques have extended * To whom correspondence should be addressed. Tel/Fax: +81-(0)92-642-3575. E-mail: mgototcm@ mbox.nc.kyushu-u.ac.jp.
Figure 1. Molecular structures and abbreviations of ILs and extractant CMPO.
to gas chromatography,22 capillary electrophoresis,23 and supported liquid membrane (SLM) systems.24,25 With the increasing dependence on nuclear energy, recovery of trivalent actinides from acidic solutions generated by reprocessing spent nuclear fuels is an inevitable problem. The neutral bidentate reagent, octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) (Figure 1) is the most promising extractant for the recovery of trivalent actinides in the TRUEX process.26-28 However, the present extraction system using CMPO employs toxic or flammable diluents such as diethylbenzene, tetrachloromethane, or normal paraffinic hydrocarbons. The intention of our research is to demonstrate the feasibility of ILs as alternatives to traditional organic diluents employed in industrial liquid-liquid extraction processes. In the present study, we investigated the extraction behavior of trivalent rare earth metals with CMPO using ILs as an extracting phase. Metal extraction was performed by varying some extraction parameters such as extractant concentration in the ILs phase and acid concentration in the aqueous feed solution. The extraction behavior in the ILs system is compared to that in dodecane, which has been widely used with CMPO in the TRUEX process. On the basis of the experimental results, the extraction mechanism in the ILs system is discussed. Furthermore, the recovery of metal ions extracted into the ILs phase and the reusability of ILs are also demonstrated.
10.1021/ie049050t CCC: $30.25 © 2005 American Chemical Society Published on Web 05/07/2005
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Experimental Section Chemicals. 1-Methylimidazole and hexafluorophosphoric acid were obtained from Aldrich Chemical Co., and lithium bis[(trifluoromethyl)sulfonyl]amide was purchased from Fluka Chemical Co. Octyl(phenyl)-N,Ndiisobutylcarbamoylmethyl phosphine oxide (CMPO) (Figure 1) was obtained from Strem Chemicals (Newburyport, MA). All other chemicals were reagent grade and used without further purification. The deionized water in the experiments was purified with a Millipore Milli-Q system. Synthesis of Ionic Liquids. Ionic liquids used in the present study were synthesized according to the procedure previously reported.5,7 The molecular structures and abbreviations of ionic liquids are shown in Figure 1. 1-Butyl-3-methylimidazolium chloride, [Bmim][Cl], was prepared by the reaction of equimolar amounts of 1-methylimidazole and 1-chlorobutane by heating at 60 °C for 48 h. The product was washed with ethyl acetate (100 mL × 3), the remaining ethyl acetate was removed by heating at 60 °C under vacuum, and the product was freeze-dried for 24 h. A slightly yellow crystalline ([Bmim][Cl]) was obtained. For the synthesis of 1-butyl-3-methyl-imidazolium hexafluorophosphate (abbreviated as [Bmim][PF6]), [Bmim][Cl] was dissolved in 500 mL of deionized water in a plastic flask and then hexafluorophosphoric acid, HPF6 (60% aqueous solution, 1.1:1 molar ratio), was added slowly to the solution in an ice bath. The reaction mixture was stirred for 2 h. The upper acidic aqueous phase was decanted, and the lower ionic liquid phase was washed with water repeatedly until the upper aqueous phase became neutral. The final product, [Bmim][PF6], was dried by vacuum evaporation for 3 h and freeze-dried for 24 h. For the synthesis of 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]amide (abbreviated as [Bmim][Tf2N]), the same procedure as that described above for [Bmim][PF6] was used, except that lithium bis[(trifluoromethyl)sulfonyl]amide (LiTf2N) was used instead of HPF6. Liquid/Liquid Extraction. Competitive extraction of rare earth metals (mainly Ce3+, Eu3+, and Y3+) from an aqueous solution into ILs was performed by the batch method. The extracting phase was prepared by dissolving the extractant CMPO in each IL with the assistance of ultrasonic agitation. For comparison with ILs, an ordinary organic solvent, n-dodecane (preferred solvent for CMPO), containing CMPO was prepared in the same manner. The composition of the aqueous phase differed between the ILs system and the dodecane system, for a reason that will be described later. An aqueous solution containing 0.1 mM of metal ions was prepared for the ILs system by dissolving their nitrate salt hexahydrates in deionized water, while for the dodecane system, 1 M HNO3 solution was used. Equal volumes (3 mL) of the aqueous and IL solutions were mixed in a stoppered test tube and vigorously shaken by a vortex mixer (2000 rpm) for 15 min, so the extraction reached equilibrium. The samples were then left standing for 30 min for complete phase separation. After phase separation, the concentrations of rare earth ions in the aqueous phase were measured by ICP-AES to evaluate the degree of extraction (E [-] ) [M3+]org,eq/[M3+]aq,ini) and the distribution ratio (D [-] ) [M3+]orq,eq/[M3+]aq,eq). Stripping Test. For efficient stripping of metal ions, acetohydroxamic acid (AHA), diethylenetriamine pentaacetic acid (DTPA), ethylenediamine tetraacetic acid (EDTA), and citric acid were used as stripping reagents
Figure 2. Effect of nitric acid concentration on metal extraction with CMPO (10 mM) in [Bmim][PF6] (filled symbols) and ndodecane (open symbols).
which form a water-soluble complex with metal ions. In the forward extraction, equal volumes of [Bmim][PF6] containing 10 mM CMPO and the 0.1 mM metal solution were mixed and shaken vigorously for 15 min. A 5 mL portion of the ionic liquid phase was taken and contacted with 5 mL of stripping solution containing a complexing agent, followed by vigorous shaking to reach equilibrium. The concentration of metal ions in the aqueous solution was measured to determine the degree of stripping (Stripping ratio [-] ) Caq,eq/Corg,ini). Results and Discussion Metal Extraction with CMPO into IL or nDodecane. To assess the partitioning behavior of rare earth metals with CMPO in the IL ([Bmim][PF6]) system and the organic solvent (n-dodecane) system, extraction equilibria of metal ions (Ce3+, Eu3+, Y3+) were measured as a function of the aqueous HNO3 concentration (Figure 2). Since CMPO is a neutral extractant, anionic species are required to extract the cationic metal ions into a nonpolar organic solvent. Therefore, in a conventional organic diluent, CMPO shows high extraction ability under a high nitric acid concentration, although a decrease in the extraction efficiency will be observed under much more acidic conditions as a result of protonation of the PdO group in the CMPO molecule.29 In the IL extraction system, however, metal cations could be sufficiently extracted into the IL phase at a very low HNO3 concentration. Further investigation revealed that metal ions can even be extracted from deionized water, where no anionic species are present. On the basis of these results, deionized water was used as the aqueous solvent in the IL system for the examination of the extraction mechanism, while 1 M HNO3 was employed in the dodecane system to attain efficient extraction under suitable acidic conditions, unless noted otherwise. Figure 3 shows the extraction behavior of Ce3+, Eu3+, and Y3+ with increasing CMPO concentration in [Bmim][PF6] and in n-dodecane. The extraction ability of CMPO in [Bmim][PF6] is exceedingly high compared to that in n-dodecane. For example, quantitative extraction of Ce3+ ions into the extracting phase needs at least 100 mM of CMPO in the dodecane system, whereas only 3 mM of CMPO in [Bmim][PF6] was sufficient for achieving an equivalent degree of extraction. As shown in Figure 3B, plots of log D vs log[CMPO] produced
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Figure 4. Selectivity for lanthanides in [Bmim][PF6] with 5 mM of CMPO (filled symbols) and n-dodecane with 50 mM of CMPO (open symbols).
Figure 3. (A) Extraction profile as a function of CMPO concentration and (B) plots of log D vs log[CMPO] in [Bmim][PF6] (filled symbols) and n-dodecane (open symbols).
Figure 5. Effect of anionic component in ILs on the extraction efficiency of Ce3+ using [Bmim][PF6] (filled symbols) and [Bmim][Tf2N] (open symbols) as the extracting phase.
straight lines with a slope of 3 for all metals. This result indicates that CMPO forms a 3:1 complex with each metal cation (Ln3+‚(CMPO)3). One of the reasons for the high extractability in the IL might be attributable to the polarity or the dielectric constant of the ionic liquid. In general, an ion-pair complex, which is formed predominantly by Coulomb interaction, is apt to partition to the organic solvent with high dielectric constant, such as chloroform or nitrobenzene. Therefore, we presumed that the dielectric constant of the ILs is high enough to achieve the high extractability as compared with that of dodecane. However, there is still uncertainty with respect to the high extractability. Thus, further investigations are needed to elucidate why the efficiency is enhanced in the ILs system. We examined the selectivity of CMPO for lanthanide ions by comparing the extraction efficiency of each metal ion. The relationship between the ionic radius and the extraction efficiency of lanthanides is depicted in Figure 4. The ILs solution containing 5 mM of CMPO (filled symbols) showed a high extraction efficiency with excellent selectivity, while the dodecane solution with the same concentration of CMPO could not extract any metals at all (data not shown). Although the extraction efficiency in dodecane was improved by increasing the CMPO concentration to 50 mM (open symbols), the selectivity was still not comparable to that in the IL. While some researchers12,14,15 have examined the effect of the alkyl chain length in ILs on the extraction efficiency, little attention has been paid to the effect of variations in the anionic component of ILs. To our
knowledge, there are no more than two hydrophobic anionic species, hexafluorophosphate (PF6-) and bis[(trifluoromethyl)sulfonyl]imides (Tf2N-), which form a biphasic liquid system with water, in combination with [Bmim]+ cations. Then, we examined the influence of anionic components in the ILs on the Ce3+ extraction using [Bmim][PF6] or [Bmim][Tf2N] as the extracting phase. The results are shown in Figure 5. The extractability in the [Bmim][Tf2N] system is higher than that in the [Bmim][PF6] system. The difference observed here might be attributed to the metal coordination environment in the ILs which could be affected by the hydrogen bond basicity and/or dipolarity of the extracting IL phase.5,30 Employment of ILs for the extracting phase greatly improved not only the extraction efficiency but also the selectivity in the extraction of rare earth ions with CMPO. In industrial separations, the high extraction ability must be a great advantage in the cost efficiency, owing to the reduction in the amount of the expensive extractant CMPO required. However, many issues with respect to what the dominant factor in these improvements and differences is remain to be clarified. Extraction Mechanism. The marvelous extraction efficiency and selectivity for metal ions with CMPO in ILs was demonstrated. The question arising next is how the metal cations were extracted into the IL. As described above, extraction of cationic metal ions (M3+) with CMPO into a nonionic organic phase requires anion coextraction to neutralize the charge of the cationic M3+‚ (CMPO)3 complex. This means that effective extraction must be achieved when a large amount of anion (e.g.
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Figure 7. Structures and abbreviations of complexing agents used in this study.
Figure 6. Influence of [Bmim]+ cation in the aqueous phase on the extraction efficiency in [Bmim][PF6] with 5 mM CMPO (filled symbols) and n-dodecane with 50 mM CMPO (open symbols).
NO3-) is present in the aqueous feed phase, that is, under highly acidic conditions. However, the extraction mechanism in the IL system appears to differ considerably from that in traditional organic solvent. Because the extraction efficiency was hardly affected by HNO3 concentration in the IL system, except for a little decrease in extraction ability at higher acid concentrations (Figure 2), the extraction of metal ions does not seem to be influenced by the anionic species in the feed phase. Some research groups14,15 have also reported that the metal ion partitioning is independent of a counteranion in an aqueous phase. The most convincing extraction mechanism for the ILs-based system is metal cation transfer to the ILs phase via a cation-exchange mechanism; a metal cation coordinated with CMPO (M3+‚(CMPO)3) is exchanged for a cationic component of the IL, [Bmim]+. Thus, the expression for the metal ion partitioning to the IL can be represented as follows:
M3+aq + 3CMPOorg + 3Bmim+org a M3+‚(CMPO)3org + 3Bmim+aq (1) When this extraction mechanism holds, an increase in the [Bmim]+ concentration in the aqueous phase would cause a decrease in the metal ion partitioning, according to the equilibrium expression. Therefore, we examined the effect of the [Bmim]+ concentration in the aqueous phase on the extraction in the IL-based system and in the dodecane system by adding [Bmim][Cl] to the aqueous phase as a source of [Bmim]+ (Figure 6). As shown in Figure 6, the extraction efficiency of the IL system did indeed decrease with increasing [Bmim]+ concentration (A), whereas the dodecane system was not affected by [Bmim]+ at all (B). This result indicates that
Figure 8. Stripping of metal ions from [Bmim][PF6] phase using various strippants.
the decrease of the extraction efficiency in the IL system (A) was not due to competition between the metal cation and the [Bmim]+ cation, but due to the mobile equilibrium. These results indirectly support the extraction equilibrium described in eq 1. The cation exchange mechanism we propose for the ILs system is consistent with the arguments previously reported.15,31 Stripping of Metal Ions and Reuse of the IL. The use of ionic liquids as sustainable green solvents have been discussed, because of their negligible vapor pressure, which leads to the elimination of some problems such as vapor loss and atmospheric pollution. To use the ILs for a liquid-liquid extraction process, recovery of extracted metal ions, and recycling of the extracting phase is of great importance. Nevertheless, only a few studies16,18,20 have so far been made on the stripping of metal ions in an ILs system. Our preliminary tests suggested that metal ions could not be recovered from the IL phase into the aqueous phase using only deionized water or acidic solutions. This negligible stripping can be explained by the extraction profiles for the IL system shown in Figure 2. Consequently, we used a metal recovery strategy that used stripping solutions containing a complexing agent which forms a watersoluble complex with metal cations. Moreover, the results shown in Figure 6A demonstrate that the decrease of extraction efficiency at a high [Bmim]+ concentration can be available for stripping. That is, addition of excess [Bmim]+ will cause a decrease in extraction efficiency, resulting in effective back extraction. The chemical structures of the water-soluble complexing agents used in this study and the stripping results are shown in Figures 7 and 8, respectively. The nitric acid solution 1 which contained no complexing agents could strip almost no metal ions. In
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contrast, metals could be stripped sufficiently with solutions 2, 3,32 and 4, which contain acetohydroxamic acid (AHA), diethylenetriamine pentaacetic acid (DTPA), or ethylenediamine tetraacetic acid (EDTA), respectively. The optimum strippant 533 containing citric acid could quantitatively recover the metal cations from the IL phase. As expected, it was also possible to strip metal ions using solution 6, which contains a large amount of [Bmim]+. Reuse of the IL has been examined for the recycling of the extracting phase. After the extraction of metal ions with 5 mM of CMPO in [Bmim][PF6], the metal ions in the ILs were completely removed from the ILs using the strippant 5. The IL phase was taken out and then used again for the next extraction operation with a fresh metal solution. These procedures were repeated four times. During these repeated extraction and stripping processes, no appreciable decline in the extraction efficiency and no extraction hindrance by the complexing agent were observed. These results demonstrate that ILs can be reused as the extracting phase in an aqueous/IL extraction system. Conclusion In the present work, we examined the feasibility of ionic liquids as novel media for the extraction of rare earth elements. CMPO dissolved in an ionic liquid, [Bmim][PF6], showed extremely high extraction ability and selectivity of metal ions as compared to in an ordinary diluent, n-dodecane. Partitioning of the metal cations into the IL with CMPO appears to proceed via a cation-exchange mechanism, which is quite different from that of the conventional solvent extraction system. We also observed that the employment of a watersoluble complexing agent was valid for the stripping of metal cations and that the addition of [Bmim]+ cation into the aqueous phase also promotes the stripping. Moreover, the reusability of ILs in liquid/liquid extraction was demonstrated. The present study suggests that ionic liquids are a promising medium for actinide and fission product separation. Acknowledgment This research was partly supported by a Grant-inAid for Science Research (No. 15560656) from the Ministry of Education, Science, Sports and Culture of Japan. Literature Cited (1) Seddon, K. R. J. Chem. Technol. Biotechnol. 1997, 68, 351. (2) Welton, T. Chem. Rev. 1999, 99, 2071. (3) Wasserscheid, P.; Keim, W. Angew Chem. Int. Ed. 2000, 39, 3772. (4) Blanchard, L. A.; Hancu, D.; Beckman, E. J.; Brennecke, J. F. Nature 1999, 399, 28.
(5) Bonhoˆte, P.; Dias, A.-P.; Papageorgiou, N.; Kalyanasundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168. (6) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.; Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156. (7) Huddleston, J. G.; Willauer, H. D.; Swatloski, R. P.; Visser, A. E.; Rogers, R. D. Chem. Commun. 1998, 1765. (8) Visser, A. E.; Swatloski, R. P.; Rogers, R. D. Green Chem. 2000, 2, 1. (9) Carda-Broch, S.; Berthod, A.; Armstrong, D. W. Anal. Bioanal. Chem. 2003, 375, 191. (10) Smirnova, S. V.; Torocheshnikova, I. I.; Formanovsky, A. A.; Pletnev, I. V. Anal. Bioanal. Chem. 2004, 378, 1369. (11) Dai, S.; Ju, Y. H.; Barnes, C. E. J. Chem. Soc., Dalton Trans. 1999, 1201. (12) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Griffin, S. T.; Rogers, R. D. Ind. Eng. Chem. Res. 2000, 39, 3596. (13) Visser, A. E.; Swatloski, R. P.; Reichert, W. M.; Mayton, R.; Sheff, S.; Wierzbicki, A.; Davis, J. H.; Rogers, R. D. Chem. Commun. 2001, 135. (14) Chun, S.; Dzyuba, S. V.; Bartsch, R. A. Anal. Chem. 2001, 73, 3737. (15) Dietz, M. L.; Dzielawa, J. A. Chem. Commun. 2001, 2124. (16) Nakashima, K.; Kubota, F.; Maruyama, T.; Goto, M. Anal. Sci. 2003, 19, 1097. (17) Visser, A. E.; Rogers, R. D. J. Solid State Chem. 2003, 171, 109. (18) Wei, G.-T.; Yang, Z.; Chen, C.-J. Anal. Chim. Acta 2003, 488, 183. (19) Shimojo, K.; Goto, M. Chem. Lett. 2004, 33, 320. (20) Luo, H.; Dai, S.; Bonnesen, P. V. Anal. Chem. 2004, 76, 2773. (21) Hirayama, N.; Deguchi, M.; Kawasumi, H.; Honjo, T. Talanta 2005, 65, 255 (22) Armstrong, D. W.; He, L.; Liu, Y.-S. Anal. Chem. 1999, 71, 3873. (23) Yanes, E. G.; Gratz, S. R.; Baldwin, M. J.; Robinson, S. E.; Stalcup, A. M. Anal. Chem. 2001, 73, 3838. (24) Branco, L. C.; Crespo, J. G.; Afonso, C. A. M. Angew. Chem., Int. Ed. 2002, 41, 2771. (25) Miyako, E.; Maruyama, T.; Kamiya, N.; Goto, M. Chem. Commun. 2003, 2926. (26) Horwitz, E. P.; Kalina, D. G.; Diamond, H.; Vandegrift, G. F.; Schulz, W. W. Solvent Extr. Ion Exch. 1985, 3, 75. (27) Horwitz, E. P.; Diamond, H.; Martin, K. A. Solvent Extr. Ion Exch. 1987, 5, 447. (28) Schulz, W. W.; Horwitz, E. P. Sep. Sci. Technol. 1988, 23, 1191. (29) Horwitz, E. P.; Kalina, D. G.; Kaplan, L.; Mason G. W.; Diamond, H. Sep. Sci. Technol. 1982, 17, 1261. (30) Anderson, J. L.; Ding, J.; Welton, T.; Armstrong, D. W. J. Am. Chem. Soc. 2002, 124, 14247. (31) Jensen, M. P.; Dzielawa, J. A.; Rickert, P.; Dietz, M. L. J. Am. Chem. Soc. 2002, 124, 10664. (32) Law, J. D.; Herbst, R. S.; Todd, T. A.; Romanovskiy, V. N.; Esimantovskiy, V. M.; Smirnov, I. V., Babain, V. A.; Zaitsev, B. N. INEEL/EXT-99-00954, LMITCO, Idaho Falls, ID, 1999. (33) Chitnis, R. R.; Wattal, P. K.; Ramanujam, A.; Dhami, P. S.; Gopalakrishnan, V.; Bauri, A. K.; Banerji, A. J. Radioanal. Nucl. Chem. 1999, 240, 721.
Received for review September 29, 2004 Revised manuscript received March 3, 2005 Accepted April 15, 2005 IE049050T
