Extraction of Tetra-Oxo Anions into a Hydrophobic, Ionic Liquid-Based

May 5, 2010 - E-mail: [email protected] (D.C.S.), [email protected] (I.A.S.). Cite this:Ind. .... The Journal of Physical Chemistry B 2011 115 (37), 109...
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Ind. Eng. Chem. Res. 2010, 49, 5863–5868

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Extraction of Tetra-Oxo Anions into a Hydrophobic, Ionic Liquid-Based Solvent without Concomitant Ion Exchange Dominique C. Stepinski,* George F. Vandegrift, and Ilya A. Shkrob* Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 S. Cass AVe., Argonne, Illinois 60439

James F. Wishart and Kijana Kerr Chemistry Department, BrookhaVen National Laboratory Upton, New York 11973

Mark L. Dietz, Diab T. D. Qadah, and Sarah L. Garvey Department of Chemistry and Biochemistry, UniVersity of Wisconsin Milwaukee, 3210 N. Cramer Str., Milwaukee, Wisconsin 53211

Hydrophobic ionic liquids (IL) have the potential to simplify certain separations by serving as both an extraction solvent and an electrolyte for subsequent electrochemical reductions. While IL-based solvents are known to be efficient media for metal ion extraction, separations employing these solvents are frequently complicated by the loss of constituent IL ions to the aqueous phase, resulting in deteriorating performance. In this study, we have examined the extraction of pertechnetate and related tetra-oxo anions from aqueous solutions into IL-based solvents incorporating tetraalkylphosphonium bis[(trifluoromethyl)sulfonyl]imide and a crown ether. In contrast to various previously studied IL-based cation extraction systems, facile anion extraction without significant transfer of the IL ions to the aqueous phase has been observed. In addition, the solvents exhibit high distribution ratios (100-500 for pertechnetate), significant electrical conductivity (>100 µS/cm), and a wide (∼4 V) electrochemical window. The results suggest that these solvents may provide the basis for improved approaches to the extraction and recovery of a variety of anions. 1. Introduction Hydrophobic ionic liquids (IL) are gaining increasing attention as replacements for the conventional (i.e., molecular) organic solvents employed in traditional solvent extraction processes for the separation of ions.1-17 Studies of the extraction of cations into various ILs have shown that these solvents can provide distribution ratios much greater than those observed in conventional solvent extraction processes employing organic molecular liquids.6,14 Subsequent work, however, has demonstrated that ILs suffer from certain drawbacks as extraction solvents, in particular, the loss of the constituent cations into the aqueous phase, thus limiting their utility.1-4 It has been shown, for example, that the partitioning of a metal cation (e.g., Sr2+) into 1-alkyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imides (Cnmim NTf2, Scheme 1) containing a Sr2+-selective ionophore such as the crown ether (CE) dicyclohexano-18crown-6 (DCH18C6), proceeds Via the ion-exchange reaction depicted in eq 11-3,5 + + 2+ Sr · CE2+ aq + 2Cnmimorg T 2Cnmimaq + Sr · CEorg

(1)

where the subscript “org” indicates a species present in the IL phase. The release of Cnmim+ to the aqueous phase results in a gradual dissolution of the IL, which has negative implications for the practical use of these ILs. While distribution ratios, DSr, of 104 have been reported for 1-ethyl-3-methylimidazolium ILs,6 dialkylimidazolium ILs are poorly suited for practical applications unless the alkyl side arm of the Cnmim+ cation is * To whom correspondence should be addressed. Tel.: 630-252-3087 (D.C.S.), 630-252-9516 (I.A.S.). E-mail: [email protected] (D.C.S.), [email protected] (I.A.S.).

sufficiently long (n > 8) to prevent a significant loss of the IL cation in reaction 1. Such an increase in hydrophobicity, however, causes the extraction efficiency for Sr2+ to decline until it becomes comparable to that observed for ordinary molecular liquids such as 1-octanol, thus nullifying one of the key advantages of using ILs.1-3 Additionally, ion exchange is considered undesirable in many extraction applications due to the necessity of stripping the organic phase with concentrated acids or bases. For ILs, this is a particularly vexing problem, as, depending on the ionic composition, biphasic aqueous-IL systems incorporating these liquids are sometimes unstable at extremes of pH. Electrochemical deposition, chemical redox reactions, and precipitation thus become more attractive ways of stripping the extracted ions from such solvents.18-21 As new hydrophobic ionic liquids that are more resistant to cation exchange are being developed (e.g., by incorporation of strongly coordinating anions, such as hexafluoroacetylacetonate),22 a question of both fundamental and practical interest is whether the same potential limitation of ILs (which is obviously related to their being composed of ions)17 adversely impacts the extraction of aqueous anions (A-). That is, in analogy to reaction 1, does an anion exchange reaction of the type shown in eq 2 occur + + Aaq + C Xorg T Xaq + C Aorg

(2)

thus resulting in the loss of the IL anion X- to the aqueous phase and a gradual deterioration of the separations performance? Preliminary studies by Jensen et al.4 of the extraction of lanthanides by 2-thenoyltrifluoroacetone into an ionic liquid (CnmimNTf2) indicate that such anion exchange can indeed occur, but its extent and its impact on extraction performace, if

10.1021/ie1000345  2010 American Chemical Society Published on Web 05/05/2010

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hydrophobic IL solvent comprising trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide, P666,14 NTf2 (Scheme 1). The solvent containing the extracted pertechnetate can subsequently be used for reductive Tc stripping.31

Scheme 1. Structure of the IL Ions and Extracting Agents

2. Experimental Section Caution! 99Tc is a β-emitter (Emax ) 294 keV, t1/2 ) 2 × 10 years). All operations were carried out in radiochemical laboratories equipped for handling this isotope. 2.1. Materials. All reagents were obtained from Aldrich and used as received. 99mTc was obtained by eluting a Technelite (Lantheus Medical Imaging Inc.) generator with 0.1 M HCl, bringing the solution to dryness, and redissolving the residue in an appropriate aqueous solution. 99Tc (as NH4TcO4) was obtained from Argonne stocks. Rhenium was obtained as NH4ReO4 and NaReO4 from Sigma-Aldrich. Optima Gold liquid scintillation cocktail was obtained from Perkin-Elmer Corporation. 2.2. Methods. The distribution ratio, DTc, was calculated as the ratio of the activity of 99mTc or 99Tc in the IL solution of the extractant to that of an aqueous NH4OH and/or NaOH solution at 22 ( 1 °C. Typically, a solution of the extractant in IL was pre-equilibrated twice with an equal volume of the same aqueous solution used in the extraction experiment. A 500 µL aliquot of the pre-equilibrated IL phase was vortexed with an equal volume of the aqueous phase containing 99mTc and/or 99Tc where appropriate. A total of 10 min of vortexing was adequate to achieve equilibrium. The samples were centrifuged to facilitate phase separation, and aliquots of each phase were assayed by counting the 99mTc 140 keV γ-ray using a 1480 Wizard 3″ NaI detector γ-counter (Perkin-Elmer) or 99Tc using a Model 3100 liquid scintillation counter (Perkin-Elmer). Satisfactory mass balance was observed for all experiments. The experimental determinations of the concentration of ReO4(which was used as a model for TcO4- due to its comparable ion size and properties) were carried out by reversed-phase highperformance liquid chromatography using either an aqueous solution of 0.5 M NaCl (for determination of ReO4- in 1 M base solutions) or methanol (for both aqueous and IL phases) as a mobile phase. The absorption band of perrhenate at 220 nm was used for in-flow detection. Diisopropylbenzene or carbon tetrachloride was used as the internal chromatographic standard. No reduction of ReO4- by the solvent during the extraction was observed. As a complementary method, inductively coupled plasma-optical emission spectroscopy using an Optima 3300 DV ICP-OES spectrometer (Perkin-Elmer) was employed. Concentrations were established by cross-checking emission intensities versus standard solutions (Ultra Scientific) using two emission wavelengths (197.25 and 227.53 nm). Emission signals were averaged for 1 min following sample injection, giving detection limits typically less than 3 ppb. The distribution ratios were determined in the same fashion as discussed above. 19F nuclear magnetic resonance (NMR) spectroscopy measurements of [NTf2-] in the aqueous phase were carried out using 10 mM NH4PF6 as the internal calibration standard. The lowest detection level for this anion was ca. 1.0 µM. 5

any, remain unclear. Of course, aqueous anions can be extracted into molecular solvents (e.g., 1-octanol) containing ionophores (such as crown ethers) with rings designed to accommodate ammonium and alkali cations (M+), either by formation of a neutral coordination complex or, when the anion and/or cation are sufficiently large and of low charge, by ion-pairing, as depicted in the following equation:1-3,6,23 + M+ aq + Aaq + CEorg T M · CE Aorg

(3)

As will be shown below, by tuning its consituent ions, it is possible to suppress ion exchange in an IL, thereby replicating the extraction behavior typical of molecular liquids. The main advantage of using ionic liquids for such (neutral) extractions, it should be noted, is the expected increase in the efficiency of the extraction, arising from strong solvation of the ion pair by the IL. Specifically, we demonstrate rapid (99%) sequestration of the pertechnetate anion (TcO4-) from alkaline aqueous phases into an IL solvent without a concurrent loss of IL ions into the aqueous phase. Although interesting from a fundamental perspective, these results are also of considerable practical significance given that this tetra-oxo anion is an important component of spent nuclear fuel reprocessing streams. Indeed, 99Tc, a long-lived β-emitting radionuclide with high mobility in the environment, is one of the main fission products (having a yield of 6.3%) of uranium. Eliminating the release of 99Tc into the environment24-27 represents an important step toward improving the safety of spent nuclear fuel processing and restoring public confidence in the sustainable nuclear power option.24 The current baseline flowsheet for the processing of spent nuclear fuel in the U.S. involves coextraction of uranyl and technetium as one of the stages.27 The U separation process (UREX) involves the extraction of uranyl nitrate from 1-3 M nitric acid with tri(n-butyl) phosphate (TBP) in n-dodecane. 99 Tc is coextracted by TBP as UO2(TcO4)(NO3) · 2TBP, UO2(TcO4)2 · 2TBP, and HTcO4 · 3TBP complexes, hence, its presence in the U stream designated for calcining to solid UOx and further recycling. In the current implementation of the UREX process, 99Tc is present in the U/Tc stream as ∼0.6 mM pertechnetate (TcO4-) in the 0.34 M UO2(NO3)2 solution generated by stripping the organic phase using 0.01 M HNO3. TcO4- is removed from this aqueous stream using a column of the weakly basic poly(vinylpyridine) resin (Reillex HPQ) and subsequently eluted using 1 M NH4OH.28-30 The resulting solution is only ∼10 times more concentrated in TcO4- than the initial feed. Although further processing steps have not been decided upon, one scheme currently under consideration involves precipitating TcO4- using tetrabutylammonium hydroxide, followed by calcination of NBu4TcO4 and steam reforming of TcO2 to Tc(0).29,30 The present study examines an alternative to this approach in which TcO4- is extracted from the Reillex HPQ column effluent (1 M NH4OH) and concentrated in a

3. Results and Discussion 3.1. Solvent Screening. In an effort to identify an ionic liquid exhibiting satisfactory physicochemical properties and yielding acceptable extraction efficiencies, a range of acid- and basecompatible ionic liquids were examined (Scheme 1), among them alkyl-substituted ammonium, pyrrolidinium, and phos-

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phonium salts of NTf2, dicyanamide (DCA ), and bis(2,4,4trimethyl pentyl)phosphinate (TMP-), with and without cosolvents (such as TBP) and ionophores (such as 18-crown-6 and 15-crown-5). Some of these results are given in Table 1S of the Supporting Information. The N-methyl-N-butylpyrrolidinium salt of NTf2 (of interest due to its low viscosity) proved to be the least effective IL, both as the extracting agent and solvent, whereas the quaternary phosphonium ILs consistently showed good performance, with distribution ratios typically in the 10-50 range. In the absence of an ionophore, the addition of ammonium salts to the aqueous phase decreased the extraction efficiency into all neat ILs investigated, suggesting partitioning Via an ion-exchange process. The addition of 0.1 M crown ethers and quaternary ammonium salts to P666,14TMP resulted in only small changes in the distribution ratios, which also suggests anion exchange (i.e., eq 2) as the prevalent mode of extraction for TcO4-, even in the presence of the ionophore. The addition of TBP (a typical cosolvent for the extraction of TcO4- into molecular liquids at low pH)25 had little effect on the distribution ratios observed for the ionic liquid solvents (both at high and low pH) but resulted in the formation of emulsions at the interface. The DCA- based ILs were ruled out because these solvents were found to extract TcO4- Via anion exchange (i.e., eq 2), thereby resulting in a loss of the IL anion to the aqueous phase. (This was demonstrated by the formation of silver dicyanamide precipitate upon the addition of silver nitrate to the aqueous phase.) P666,14NTf2 yielded the highest uptake of TcO4- both in the presence and absence of DCH18C6 and was thus selected for subsequent studies. This IL, it should be noted, is especially attractive because of its commercial availability and chemical and electrochemical stability. One concern regarding the use of ILs as extraction solvents is their very high viscosity. For P666,14NTf2, for example, the viscosity is 4 P at 20 °C. Fortunately, this viscosity decreases markedly upon the addition of certain cosolvents. For example, the addition of only 10% n-hexane reduces the viscosity of P666,14NTf2 to 1.5 P, thereby improving both its ion-transport properties (and thus electrical conductivity) and phase separation behavior. In this study, carbon tetrachloride, 1,2-dichloroethane, m-xylene, diisopropylbenzene, phenylcyclohexane, and saturated hydrocarbons (e.g., dodecane) and TBP were evaluated as cosolvents, with 0.1-0.5 M of the IL added to the organic solvent. These mixed solvent systems were then subjected to a preliminary screening for their metal ion extraction efficiency using as a substitute for pertechnetate and perrhenate the brightly colored MnO4- anion, which allowed for visual evaluation of ion transfer. As expected, not all molecular cosolvents provided equally efficient extraction. In fact, saturated hydrocarbons yielded very inefficient separations. Unexpectedly, the addition of TBP as a cosolvent resulted in the formation of TBP aggregates in the aqueous phase that extracted the ionic liquid from the organic phase. Overall, only chlorinated and aromatic cosolvents were found to exhibit acceptable behavior. Of the nonchlorinated solvents, diisopropylbenzene, phenylcyclohexane, and m-xylene were selected for more detailed examination because of their comparatively high flashpoint. 3.2. Mechanistic Aspects. The results displayed in Tables 1 and 2 represent distribution ratios obtained for ReO4- (used as surrogate for TcO4-) from an aqueous solution of NH4ReO4 (Table 1) and from a 7:1 v/v mixture of 1 M NaOH and 1 M NH4OH (Table 2). (A mixture of NH4OH and NaOH was used to avoid emulsion formation with the ILs, which is observed when NH4OH solution alone is used as the aqueous phase.) The DRe (and by analogy the DTc) values obtained for this solvent

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Table 1. Distribution Ratios (DRe) for ReO4 for Extraction from Aqueous Solutions of NH4ReO4 Using 0.1 M DCH18C6 and 0.4 M P666,14NTf2 in Specified Solvent co-solvent

DRe, 4.2 mM ReO4-

DRe, 22 mM ReO4-

carbon tetrachloride m-xylene diisopropylbenzene phenylbenzene

6.7 3.9 2.3 2.8

6.3 4.0 2.3 2.7

Table 2. Distribution of ReO4- from a 7:1 v/v Mixture of 1 M NaOH and 1 M NH4OH Using 0.1 M DCH18C6 and 0.4 M P666,14NTf2 in Specified Solvent co-solvent

DRe, 3.8 mM ReO4-

DRe, 18.5 mM ReO4-

carbon tetrachloride diisopropylbenzene phenylbenzene

53 24 30

15 8.2 9.7

system are sufficient to permit the design of a robust extraction process for removal of 5-20 mM of the pertechnetate from the Reillex HPQ column effluent. The capacity of the IL solvent at saturation is 70-90 mM ReO4-, corresponding to the extraction of a 1:1:1 ReO4-/M+/CE complex. As seen from the data given below, the extraction efficiency for TcO4- is greater than that for ReO4-; at infinite dilution of pertechnetate, distribution ratios in the 350-500 range are observed. In an effort to identify the mode of TcO4- partitioning into the P666,14NTf2-based solvent systems, the effect of various system parameters on the extraction of technetium was examined. Figure 1 shows the effect of increasing the concentration of the crown ether, DCH18C6, in a P666,14NTf2-m-xylene mixture on the extraction of TcO4- from basic aqueous solution (see also Tables 2S and 3S in the Supporting Information). As can be seen, DTc exhibits a first-power dependency upon crown ether concentration, a dependency consistent with either the extraction of an ion pair, eq 4: + Na+ aq + TcO4(aq) + CEorg T Na · CE TcO4(org)

(4)

or an anion-exchange process, eq 5: Na · CE+OHorg + TcO4(aq) T Na · CE+TcO4(org) + OHaq

(5)

Analogous processes for the extraction of TcO4- from NaNO3 solution into 1-octanol containing DCH18C6 have been described previously.32 Figure 2 depicts the effect of NaOH concentration on the extraction of TcO4- into the P666,14NTf2-m-

Figure 1. Extraction of 99mTc from a 7:1 v/v mixture of 1 M NaOH and 1 M NH4OH as a function of DCH18C6 concentration in 0.4 M P666,14NTf2 in m-xylene (b) or 1,2-dichloroethane.

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Ind. Eng. Chem. Res., Vol. 49, No. 12, 2010 Table 3. 19F NMR Measurements of NTf2- Concentration in the Aqueous Phase after Contacting the Solution of 0.4 M P666,14NTf2 and 0.1 M DCH18C6 in Diisopropylbenzene or Carbon Tetrachloride with Specified Aqueous Phase aqueous phase Water 20 mM NaReO4 20 mM NH4ReO4 1 M NaOH 20 mM NaReO4, 1 M NaOH 1 M NaOH 20 mM NaReO4b

[NTf2-] in aqueous phase (M) diisopropylbenzene CCl4 2.9 2.4 2.1 a 2.4 a

× 10-5 × 10-4 × 10-4 × 10-4

3.6 × 10-5 9.1 × 10-5 3.65 × 10-4 a a a

No detectable NTf2-. b In a 7:1 v/v solution of 1 M NaOH and 1 M NH4OH. a

99m

Figure 2. Extraction of Tc from NaOH solutions using 0.1 M DCH18C6 in 0.4 M P666,14NTf2 in m-xylene.

Figure 3. Extraction of tracer levels of 99mTc from a 7:1 (v/v) mixture of 1 M NaOH and 1 M NH4OH by 0.1 M DCH18C6 and 0.4 M P666,14NTf2 in m-xylene (b) or 1,2-dichloroethane (9) and extraction of 5.2 mM 99TcO4from 1 M NaOH by 0.1 M DCH18C6 and 0.4 M P666,14NTf2 in m-xylene (O).

xylene mixture in the presence of a fixed concentration of DCH18C6. As can be seen from this plot, extraction becomes more efficient with increasing aqueous NaOH concentration, thus ruling out TcO4-/OH- exchange (eq 5) as the dominant mode of partitioning. The extraction of technetium, as reflected in DTc, also increases as the concentration of the IL increases (see also Table 3S in the Supporting Information). At first glance, this would seem to suggest that TcO4- partitioning into the IL involves an anion exchange process, eq 6 + TcO4(aq) + P+ 666,14NTf2(org) T NTf2(aq) + P666,14TcO4(org)

(6)

and, thus, dissolution of the IL in the aqueous phase. To investigate this possibility, 19F NMR measurements were carried out to determine the amount of NTf2- transferring into the aqueous phase from 0.1 M DCH18C6 and 0.4 M P666,14NTf2 in CCl4 or diisopropylbenzene upon the extraction of 20 mM ReO4- (Table 3). A high concentration (20 mM) of perrhenate was added to maximize the interphase transfer of the NTf2anion, thus facilitating its determination. The loss of NTf2- to the aqueous phase was found to be negligible in all of the solvent systems tested. Thus, the observed increase in the technetium distribution ratio is likely due to the improvement in the

solvation properties of the nonpolar organic solvent upon addition of the IL. Consistent with this idea is the observation that neat m-xylene yields a low DTc value (approaching 0.1 in the absence of IL) and that this value is greatly increased by IL addition (Figure 3). In contrast, in 1,2-dichloroethane, which is already polar, an increasing concentration of IL does not have a significant effect on the DTc values observed in the extraction of trace TcO4-. Taken together with our other observations, these results strongly suggest that the predominant mode of TcO4- transfer in these systems is the extraction of an ion pair (as per eq 4), an unexpected result given the well-established propensity of ILs toward ion exchange.33 For a 0.4 M P666,14NTf2 solution in m-xylene containing 0.1 M DCH18C6, the infinite dilution distribution ratio for pertechenate, DTc, in a 7:1 v/v mixture of 1 M NaOH and 1 M NH4OH reached 474 and decreased to 140 for a 5.2 mM solution of the pertechnetate. Therefore, using this solution to strip 99Tc from the ion-exchange column employed for separation of 99Tc from a U/Tc waste stream will generate an effluent from which extraction and significant preconcentration of 99Tc using the ILbased solvent is readily achievable. The pertechnetate extracted and concentrated in this manner can be subsequently stripped from the IL using chemical reduction to insoluble hydrous TcO2, chemistry which is considered elsewhere.31 3.3. Electrochemical Properties. To assess the electrochemical properties of the IL-m-xylene mixed solvent, the conductivity of a 0.4 M P666,14NTf2 solution in m-xylene was determined before and after its equilibration with either water or 50 mM NH4ReO4 (1:1 v/v). The resulting conductivity was 124, 129, and 129 µS/cm, respectively. The addition of 0.1 M DCH18C6 changed the conductivity to 124, 138, and 193 µS/ cm, respectively. The increased conductivity in the presence of the crown ether can be understood from the smaller size of the perrhenate anion and the crown-NH4+ complex relative to the bulky ions comprising the ionic liquid. Regardless of the m-xylene concentration, when the concentration of the ionic liquid present was varied between 0.1 and 1 M, the electrochemical window was ∼4 V, a window sufficiently wide for electrochemical reduction of many anions of practical interest,20,21 although not for tetra-oxo anions that need oxygen-accepting (or proton donating) reagents for electrochemical reduction. In aqueous solutions, hydrogen atoms generated by cathodic reduction of hydronium ions in the aqueous phase can mediate the reduction of such anions, but in acidified ILs, these hydrogen atoms promptly react with the solvent, lowering the electrochemical efficiency of tetra-oxo anion reduction (see the cyclic voltammograms given in Figures 1S and 2S in the Supporting Information).

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4. Conclusion -

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We have demonstrated that TcO4 and ReO4 extraction into certain IL-based solvent systems proceeds Via partitioning of a sodium (or ammonium) crown-ether pertechnetate ion pair. When the solvent is suitably formulated, extraction is not accompanied by a loss of the IL anion (NTf2-) into the aqueous phase, an unexpected result in the view of previous reports of the difficulty in extracting neutral complexes of various cations into ILs and of the loss of IL cations to the aqueous phase in various IL-based extraction systems.1-5,33 These results have important ramifications for emerging applications of ILs in the extraction and separations of various radionuclides, as well as for other (e.g., biochemical) applications. In this work, ion-pair extraction (and thus suppression of anion exchange) was achieved by judicious choice of the cosolvent and the use of a highly hydrophobic IL anion. The extracted species is stabilized by strong interactions with the constituent ions in the IL solvent, resulting in high distribution ratios for the extracted ions. These solvents thus combine the favorable solvation properties of the IL component with a propensity of molecular solvents toward extraction of neutral species. The resulting solvent is a strong electrolyte exhibiting good conductivity (>100 µS/cm) and a wide electrochemical window (∼4 V), thereby opening up the possibility of subsequent electrochemical treatment of the extracted ions. Acknowledgment The work at Argonne was supported by the U.S. Department of Energy, Office of Nuclear Energy, under contract DEAC0206CH11357 and AFCI NE-DOE grant No. AN0915020606. The work performed at Brookhaven National Laboratories was carried out under contract DE-AC02-98CH10886 with the U.S. Department of Energy. Programmatic guidance provided by T. Todd and J. Vienna is gratefully acknowledged. We thank H. Luo and S. Dai of ORNL for electrochemical measurements. I.A.S. thanks M. Jensen, R. Chiarizia, L. Soderholm, M. Goldberg, T. Marin, J. Muntean, W. Ebert, J. Fortner, J. Cunnane, and A. Guelis for technical expertise and review. Nomenclature CE ) crown ether Cnmim ) 1-alkyl(n)-3-methylimidazolium cation D ) distribution ratio DCA ) dicyanamide DCH18C6 ) dicyclohexano-18-crown-6 IL ) ionic liquid NTf2 ) bis[(trifluoromethyl)sulfonyl]imide P14 ) N-methyl-N-butylpyrrolidinium P666,14 ) trihexyl(tetradecyl)phosphonium TMP ) bis(2,4,4-trimethyl pentyl)phosphinate TBP ) tributyl phosphate UREX ) URanium EXtraction

Supporting Information Available: A PDF file containing Tables 1S-3S and Figures 1S and 2S. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Dietz, M. L.; Stepinski, D. C. A Ternary Mechanism for the Facilitated Transfer of Metal Ions into Room-Temperature Ionic Liquids (RTILs): Implications for the “Greenness” of RTILs as Extraction Solvents. Green Chem. 2005, 7, 747.

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ReceiVed for reView January 7, 2010 ReVised manuscript receiVed February 22, 2010 Accepted April 25, 2010 IE1000345