Ionic Liquid Based Separations of Trivalent Lanthanide and Actinide

Article pubs.acs.org/IECR

Ionic Liquid Based Separations of Trivalent Lanthanide and Actinide Ions. Ilya A. Shkrob,*,† Timothy W. Marin,†,‡ and Mark P. Jensen*,† †

Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ‡ Chemistry Department, Benedictine University, 5700 College Road, Lisle, Illinois 60532, United States S Supporting Information *

ABSTRACT: Group separations of lanthanides from minor actinides is required in the currently considered scenarios for closing of the nuclear fuel cycle. TALSPEAK is a well-known and historically first process suggested for such separations. The process is based on competitive complexation of trivalent f-group ions by an aminopolycarboxylate (such as the base of diethylenetriamineN,N,N′,N″,N″-pentaacetic acid, DTPA) in an aqueous buffer and a dialkylphosphate (such as the base of bis(2ethylhexyl)phosphoric acid, HDEHP) in an organic phase. Unfortunately, this method exhibits excessive sensitivity to pH and composition of the aqueous feed. In this study, we ″reinvent″ TALSPEAK, retaining the competitive ion binding but changing considerably the chemical implementation of the underlying general principles. The DTPA moiety is integrated into a functionalized ionic liquid (IL) that is immiscible with an organic phase containing dialkylphosphate ligands. Choline and betainium bistriflimides double as IL diluents and synthetic reagents. The integration of the aminopolycarboxylate moiety into these ILs is achieved in situ through the reactions of the cyclical dianhydride of DTPA with IL functional groups, either through the formation of a mixed dianhydride (for the betainium cation) or a diester (for the choline cation). The deprotonated DTPA− betainium conjugate forms 1:1 complexes with trivalent f-element cations whereas these metal ions form 1:2 complexes with the DTPA−choline conjugates. Large separation factors for Eu/Am partitioning between the two phases are observed, approaching 120−150 for DTPA−betainium and 250−270 for DTPA−choline. In the latter system, as in the traditional aqueous TALSPEAK, there is a characteristic ″parabolic″ dependence of the phase distribution ratios as a function of ionic radius that allows separations of the largest lanthanide ions. Group separations of all lanthanides from americium has been demonstrated, and a separation process that is based on this chemistry is suggested.

1. INTRODUCTION

Scheme 1. Structural Formulas of Pentetic Acid (H5DTPA), Its Cyclical Dianhydride (cDTPAA), and Bis(2ethylhexyl)phosphoric Acid (HDEHP)

Efficient group separations of minor actinides (An) such as americium and curium from lanthanide (Ln) fission products is perhaps the most challenging part of the nuclear fuel cycle.1 Such separations would enable transmutation of the long-lived actinides in fast neutron reactors, eliminating many of the present concerns regarding long-term nuclear waste storage. In this study, we demonstrate how functionalized (″taskspecific″)2 ionic liquids (ILs) can be used to reinvent one of the oldest approaches to this challenge, TALSPEAK. To save space, many of the supporting tables, figures, and details of synthetic and analytical protocols have been placed in the Supporting Information (SI). When referenced in the text, these materials have the designator ″S″, as in Figure 1S. 1.1. TALSPEAK. The TALSPEAK (Trivalent Actinide− Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) process was developed in the late 1960s at Oak Ridge National Laboratory.3 This process is based on competitive extraction of trivalent ions (M3+) from diethylenetriamine-N,N,N′,N″,N″-pentaacetic (“pentetic”) acid (DTPA) in a buffered aqueous solution containing lactic acid (LA). Bis(2-ethylhexyl)phosphoric acid (HDEHP) diluted in ndodecane or 1,4-diisopropylbenzene (DIPB) serves as the extracting phase. See Scheme 1 for structural formulas of these compounds. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 3641

October 30, 2013 January 28, 2014 February 11, 2014 February 11, 2014 dx.doi.org/10.1021/ie4036719 | Ind. Eng. Chem. Res. 2014, 53, 3641−3653

Industrial & Engineering Chemistry Research

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Nilsson and Nash1 summarized the current state of knowledge about this process, and we refer the reader to their overview. Simplistically, DTPA5− (or another aminopolycarboxylate)4 serves as an actinide holdback reagent, HDEHP (or another phosphorus-containing chelating agent)4,5 is a lanthanide extractant, and lactic acid is a buffer. However, each component has multiple functions, some of which have been recognized only recently.1,4−12 The typical extraction conditions are listed in Table 1S of the SI. At pH ∼ 3.5, the M3+ ions form M(DTPA)2‑ and M(HDTPA)− aqua complexes.1,10 Ideally,11,12 the M3+ ions are extracted into the organic phase as M(DEHP...HDEHP)3 complexes as sketched in Scheme 2.

minimum D(Ln3+) reached for midsize Ln3+ ions (SI Figure 1S). Consequently, D(La3+)/D(Eu3+) ∼ 10, as the equilibrium constant for La3+−DTPA binding is 103 times smaller for La3+ than Eu3+ (SI Figure 2S), whereas the equilibrium constant for HDEHP extraction into the organic phase is only 102 times smaller for this ion. In the following, we will refer to this characteristic pattern as the ″La bend″. This property allows TALSPEAK to satisfy condition 1 for all trivalent f-ions. Equally important is the “soft-donor ligand” selectivity of DTPA5− for the trivalent actinide cations that increase η to 80− 120.13−17 The most technically challenging aspect of TALSPEAK is the requirement for very tight control over pH, especially for larger ions.1 Other recognized problems are poor DTPA solubility at lower pH, an insufficiently understood extraction mechanism, the need for multiple separation stages, pronounced kinetic effects, reliance on relatively expensive and hard-to-recover reagents, and considerable radiation damage effects.18 While alternative approaches to Ln/An partitioning using sulfur- or nitrogen-containing ″soft″ donors exist (see, e.g., ref 19), the TALSPEAK process remains a top contender. 1.2. ″Reinventing″ TALSPEAK. We reasoned that TALSPEAK’s unwanted extreme pH sensitivity could be reduced or eliminated entirely through substitution of two of the carboxylate groups in DTPA5−‑ since then there would be an equivalent exchange of three protons between the M3+-binding complexes in the two phases, as shown in Scheme 2. The pH sensitivity can be further reduced by replacement of water with a less protic solvent. Because DTPA exhibits very low or no solubility in organic solvents other than water, DTPA derivatization in Scheme 2 is needed in order to introduce the aminopolycarboxylate moiety into such solvents, so this chemical modification achieves two goals at one stroke. The simplest way to accomplish such a modification is a reaction between the commercially available cyclical dianhydride of DTPA (cDTPAA) and a functionalized diluent. We conjectured that in order to minimize energetic costs for deprotonation of the tentative DTPA conjugate shown in Scheme 2, this diluent needs to exhibit self-buffering properties (in analogy to the lactate/lactic acid buffer in the traditional aqueous TALSPEAK), and this concern led us to choose a fragment of the ligand molecule in its zwitterion form, as illustrated in SI Scheme 1S, because such a fragment would have maximum structural similarity and acid−base properties to the ligand molecule itself. These considerations led us to investigate a recently discovered class of functionalized ionic liquid solvents (sections 1.3 and 1.4). 1.3. Room-Temperature Ionic Liquids. Room-temperature ILs are organic salts with low melting points. Because such liquids have low volatility, excellent solvation properties, and other benefits,20,21 development of IL-based diluents for separations is an active area of research.22−46 The simplest strategy for adapting a known process to take advantage of the properties of ILs (the so-called “conservative approach”) is by replacing an organic diluent with a hydrophobic IL. For TALSPEAK, this strategy has been pursued by Sun et al.25 Poor solubility of HDEHP in 1-alkyl3-methylimidazolium ILs (Cnmim+) with short aliphatic arms required the use of cations with long aliphatic arms (n = 4−10) that tend to be excessively viscous.22 The most pronounced “parabolic” dependencies of D(Ln3+) on r−1 were observed for C4mim N(SO2C2F5)2 and N-methyl-N-butylpyrrolidinium

Scheme 2. The Concept for TALSPEAK Inspired pH Insensitive Competitive Binding

The process conditions are adjusted so the phase distribution ratios D(M 3+ ) for trivalent ions (the ratios of total concentrations of M3+ in the organic and aqueous phases) obey condition 1: D(Ln 3 +) > 1 > D(An 3 +)

(1)

with more than a 20-fold difference between the minima and the maxima of the D’s for the entire Ln3+ and An3+ series, as illustrated in SI Figure 1S. Because the Eu3+/Am3+ separation is among the most problematic, and Am3+ and Cm3+ are the prevalent trivalent minor actinides in spent fuel, the separation factor η = D(Eu 3 +)/D(Am 3 +)

(2)

is a useful metric for characterizing the overall Ln/An separations.1,6 D(M3+) can be changed without changing η by varying the pH or the DTPA or HDEHP concentrations.1,3 In the absence of DTPA, log D(M3+) linearly increases with r−1 (SI Figure 2S), where r is the crystallographic radius of M3+ ions for a coordination number of 8 (which systematically decreases across the period due to the 4f- or 5f-contractions). Had not DTPA5‑ been selectively binding trivalent ions in the aqueous phase, this steep dependence of D(Ln3+) on r−1 would cause D(La3+) and D(Ce3+) to be comparable to D(Am3+), as seen from SI Figure 2S. However, for these lightest lanthanides, the corresponding DTPA binding constant strongly decreases with r−1 (before reaching a plateau, as shown in SI Figure 2S), so in the presence of DTPA5‑, the log D(Ln3+) vs r−1 dependence has a characteristic “parabolic” shape with a 3642

dx.doi.org/10.1021/ie4036719 | Ind. Eng. Chem. Res. 2014, 53, 3641−3653


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bistriflimide. The extraction of Ln3+ into the IL originated through a combination of neutral and ion exchange mechanisms. Rout et al.34 used radiotracers to estimate D(M3+) for 1:1 (v/v) extraction from 5 mM DTPA (pH 3.0) using 50 mM HDEHP in C8mim NTf2, obtaining η ≈ 35. Separation factors η ≈ 150 were reported for analogous systems involving bis(2-ethylhexyl)diglycolamic acid instead of HDEHP, but such high selectivity was observed only at impracticably low concentrations of DTPA (95%, suggesting ″click″ chemistry pathways to

Figure 1. Luminescence lifetime τ of Eu3+ complexes in solutions of Hbet NTf2 (squares) and Hcar NTf2 in Chol NTf2 (vs the mole fraction of carboxylated cations). The empty symbols are for solutions containing 10× molar excess of cDTPAA; the filled symbols are for the IL solvent itself. The lines are guides for the eye.

lifetime is close to the known lifetime of the Eu[(DTPA)(H2O)]2− complex in aqueous solutions.51 A complex is observed when cDTPAA is added to the IL phase after the completion of reaction 3, suggesting that DTPA can readily replace bet and Hbet+ from the coordination sphere of the metal ion. When an 18:1 mol/mol excess of betaine was added to this cDTPAA solution, the lifetime of the luminescent complex further increased to 865 μs, suggesting water replacement; nearly the same lifetime of 895 μs was observed when Eu3+ was introduced into dry Hbet NTf2 in a small amount of DMSO (SI Figure 10S). For dry Hcar NTf2, the lifetimes of Eu3+ complexes with and without cDTPAA were 580 and 1093 μs, respectively. The hydroxyl group of carnitinium is likely to be involved in the complex formation, and the same relates to choline, as the lifetime in Chol NTf2 is very long, ca. 1.62 ms (without cDTPAA addition). As the fraction of Chol NTf2 increases in the solution, so does the luminescence lifetime (Figure 1). 3645



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We first undertook a kinetics study to estimate the period of time required for equilibration (SI Figure 11S). As seen from this plot, it takes 10 min of vigorous agitation to achieve the equilibrium separation factor η of 125. We used this protocol for other measurements. Figure 2 shows the dependence of D(Am3+) and D(Eu3+) vs [cDTPAA] for [HDEHP] = 0.1 m (in cumene). These are

These observations indicate that in neat Hbet NTf2 and Hcar NTf2 containing cDTPAA, Ln3+ ions are bound by DTPA− (Hbet+)2 and DTPA−(Hcar+)2 mixed anhydride conjugates formed via reaction 6 and that the nature of the luminescent complex changes in the presence of Chol+ cations, which promote reaction 7. As DMSO-d6 is added to Hbet NT2 solutions, there is a continuous increase in the luminescence lifetime, as shown in SI Figure 10S. This plot suggests that addition of 0.1−0.5 m DMSO-d6 has little effect on Eu3+ coordination in the IL. This provided a convenient way of introducing Ln3+ into the organic solutions. Before we proceed to consider the results of trivalent ion extraction experiments, we need to address the miscibility of the IL and organic solvents used in these separations. 3.4. Miscibility of Ionic Liquid Solutions with Organic Solvents. Miscibility of Hbet NTf2 solutions with organic solvents was determined using 1H NMR. The results are summarized in SI Table 12S. Neat HDEHP has low solubility in Hbet NTf2 (∼3 mol % after 1:1 mol/mol equilibration), and this uptake further decreases as HDEHP is diluted by a hydrocarbon solvent. Phase distribution coefficients of 20−40 were observed for these solvents and HDEHP (see SI Table 12S). Toluene had excessive solubility in the ionic liquids, whereas DIPB formed stable emulsions. Both n-decane and cumene (and solutions of HDEHP in these solvents) readily mixed with the ionic liquids and rapidly separated (as there is nearly a 2-fold difference in the densities). The concentration of the ILs extracted into the organic phase was negligible, and the concentration of HDEHP partitioning into the IL was in the millimolar range; see SI Table 12S. Even less HDEHP is extracted into Hcar NTf2 and Chol NTf2, and the mixtures of Chol NTf2 with the other two ILs, with corresponding D’s over 100 (SI Tables 13S and 14S). As shown in section 3.6, mixtures of Chol NTf2 and Hbet NTf2 present particular interest, and we inquired whether betaine can be used as a much cheaper substitute for Hbet NTf2 in such solvents. When 28 mol % betaine was introduced into Hbet NTf2 containing 25 mm cDTPAA, HDEHP was present only in trace amounts in the IL phase (0.24 mol % by 1H NMR). When this experiment was carried out for Chol NTf2 containing 22 mol % betaine (SI Table 15S), HDEHP was quantitatively stripped into the IL phase from 0.1 m HDEHP/ cumene solution; for 1 m HDEHP/cumene solution, the phase distribution ratio changed from 740 (no betaine) to ∼3. These observations indicate the occurrence of equilibrium reaction 8 (bet)IL + (HDEHP)org ↔ (Hbet+ DEHP−)IL

Figure 2. cDTPAA concentration dependencies for D(Am3+) (triangles) and D(Eu3+) (circles) for 1:1 (w/w) extraction (Hbet NTf2, 0.1 m HDEHP in cumene). Two different series of measurements using different batches of ionic liquid are superimposed to illustrate reproducibility of these measurements. The D’s linearly increase with [cDTPAA]. The distribution coefficients for Eu3+ and Am3+ in the IL solution without cDTPAA are indicated with the arrows in the plot (corresponding to 82.8 and 2.0, respectively). The separation factor η does not change significantly as the function of [cDTPAA].

linear dependences with slopes near −1 indicating the formation of 1:1 M3+:ligand complexes in the IL phase. Similar plots were observed for TALSPEAK; e.g., see Figure 5 in ref 1. As in these previous studies, the separation factor η is approximately constant as a function of ligand concentration and varies little from the value observed in the absence of the functionalized cDTPAA ligand, suggesting that most of the Eu3+/Am3+ separation observed in this system originates from size selectivity in the M(DEHP...HDEHP)3 complexes. Also similarly to TALSPEAK, the phase distribution ratios of M3+ ions strongly depend on HDEHP concentration. A cubic dependence1 on [HDEHP] can be expected for the exchange reaction shown in Scheme 2. Figure 3 exhibits a log D(M3+) vs log [HDEHP] plot that reveals a steep concentration dependence for the ions. For Am, the slope is 2.87 ± 0.1, suggesting the formation of the M(DEHP...HDEHP)3 complex in the organic phase. For Eu3+, a smaller slope of 2.19 ± 0.09 was observed, which implies extraction of a different complex into the organic phase, either a complex featuring bidentate chelation of the metal ion by DEHP anions, M(DEHP)2(DEHP...HDEHP)12,60,61, or a complex containing HDEHP dimers and one IL anion, M(DEHP...HDEHP)2(NTf2). The optimum HDEHP concentration (in terms of condition 1) is 50−200 mm, which is comparable to the TALSPEAK working range.62 The luminescence lifetime for the extracted Eu3+ complexes in HDEHP/cumene was 1.67 ms, which is also consistent with the known parameters for E u-

(8)

Due to the occurrence of this equilibrium, the betaine moiety can only be introduced as betainium cation, in order to avoid the protonation of the zwitterion by HDEHP via reaction 8. For the same reason, the carboxylate moiety (for coupling reaction 6) can be only part of a cation or an anion of a strong acid (e.g., trifluoroacetate) that cannot be readily protonated. 3.5. Partitioning of Trivalent Ions between Hbet NTf2cDTPAA and HDEHP/Cumene. While Hbet NTf2 is prohibitively viscous, it was the simplest system to investigate, so it served as a useful reference system. In a typical experiment, Hbet NTf2 containing 25−30 mm cDTPAA was put in contact with 0.1 m HDEHP in cumene (1:1 (w/w)). Both phases were transparent, and no third phase formation was observed. 3646



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behavior suggests that little ion size selection occurs in the IL phase: in that regard DTPA−(Hbet+)2 is an indiscriminate holdback agent. This lack of selectivity can occur due to different ligand design as compared to DTPA5−, but there is also another possibility. In Hbet NTf2, the constituent Hbet+ cations themselves bind metal ions, so even if the binding constant for DTPA−(Hbet+)2 decreases for larger lanthanide ions, this decrease may not translate into the increased D’s, as the Hbet+ cations compete for these ions with HDEHP. This complication does not exist in aqueous solutions, as lactic acid is too feeble a complexant to compete with HDEHP even for the largest lanthanide ions. This reasoning suggested that diluting Hbet+ cations may increase La3+ extraction. 3.6. Hbet NTf2−Chol NTf2 Mixtures. Following this hunch, we examined mixtures containing Hbet NTf2 (or Hcar NTf2) and Chol NTf2. For carnitinium mixtures, the r−1 dependence also suggested no size selectivity (SI Figure 12S). Fortunately, entirely different results were obtained for Hbet NTf2−Chol NTf2 mixtures. There is a clear increase in relative D values for larger ions that scales with decreasing mole fraction of betainium, as seen in SI Figure 13S, where log D(Ln3+)/D(Eu3+) is plotted for several mole fractions of Hbet NTf2. The same can be seen in Figure 5, where we compare log

Figure 3. HDEHP concentration dependences for D(Am 3+) (triangles) and D(Eu3+) (circles) and separation factor η (filled squares, to the right) for 1:1 (w/w) extraction (28 mm cDTPAA in Hbet NTf2, 0.1 m HDEHP in cumene). The filled symbols correspond to 10 min vortexing, and open circles correspond to 3 min vortexing.

(DEHP...HDEHP)3 complexes. Due to the difference of the slopes for Eu3+ and Am3+, the separation factor η systematically decreases with an increasing HDEHP concentration. The most efficient separation occurs when D(Eu3+) ∼ D(Am3+)−1 (Figure 3), with [HDEHP] = 0.1 m and η ∼ 100. While this system exhibits good Eu3+/Am3+ specificity, its overall performance across the lanthanide period is unsatisfactory, as indicated by Figure 4. With or without cDTPAA

Figure 5. Ion radius dependence for D(Ln3+)/D(Eu3+) ratios for (a) neat Hbet NTf2 and (b) 1:2 mol/mol mixture of Hbet NTf2 and Chol NTf2 (in both panels the arrows indicate the position of europium data). These solutions contained either zero (circles) or 30 mm cDTPAA (squares). The lines are guides for the eye. The experimental conditions and methods are indicated in the plot. Figure 4. Phase distribution coefficients for Ln3+ and Am3+ ions determined from eq 4 under the extraction conditions indicated in the plot using the three methods given in section 2.3. D(Am3+) was determined from a separate radiotracer measurement.

D(Ln3+)/D(Eu3+) with and without cDTPAA, in neat Hbet NTf2 and in a 1:2 mol/mol mixture of Hbet NTf2 and Chol NTf2 (panels a and b, respectively). For neat Hbet NTf2 (Figure 5a), the plots obtained with and without cDTPAA are almost identical, whereas for the mixture (Figure 5b) there is a clear upward trend for ions larger than Sm3+, which is observed only in the presence of cDTPAA. The ratio D(La3+)/ D(Eu3+), which quantifies the ″La bend″, systematically increases as the mole fraction of Hbet NTf2 decreases, being the largest at 5−15 mol %. For more systematic studies, we settled on 20 mol % mixtures since the solubility of cDTPAA

present in this IL, the corresponding log D(Ln3+) vs r−1 dependencies ascend linearly (cf. SI Figure 2S), showing no ″La bend″ (section 1.1) so that D(La3+) ≪ D(Eu3+). Because this ascent is less steep than in the aqueous system, D(La3+)/ D(Am3+) is 3.1 (assuming η = 120 determined from radiotracer experiments): i.e., even in this system there is separation of all lanthanides from Am3+ and Cm3+; however, this group separation is too inefficient to present practical interest. This 3647



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decreases in proportion to the decrease in the Hbet NTf2 fraction, limiting the range of cDTPAA concentrations. As pointed out in section 3.3 and SI section 4S, cDTPAA reacts with Chol+ cations forming a diester; this reaction occurred in all of the Hbet NTf2−Chol NTf2 mixtures, so the identity of the metal binding ligand was the same in all of these solutions. Furthermore, it is clear that these DTPA−(Chol+)2 conjugates are size-selective, as otherwise there would be no La bend. The fact that this La bend becomes more pronounced at a lower mole fraction of Hbet NTf2 suggests that Hbet+ does compete with DTPA−(Chol+)2 for the largest lanthanide ions (as we hypothesized in section 3.5), and this competition can be suppressed through dilution of the Hbet+ cations. To probe this rationale, we obtained the dependencies of log D(M3+) (without cDTPAA) vs the mole fraction of Hbet NTf2 (SI Figure 14S). The D’s obtained in neat Chol NTf2 were, in fact, even lower than in neat Hbet NTf2. Therefore, Chol NTf2 was not an “inert” diluent, which is also suggested by Figure 1. However, as the fraction of Hbet NTf2 increased to 10−20 mol %, the D’s for all lanthanides increased by a large factor. It is this increase that made La bend possible, greatly favoring the extraction of Ln3+ ions over their retention by the IL. By itself this effect, observed for all Ln3+ ions, would not favor the extraction of large lanthanides had not the DTPA−(Chol+)2 been size-selective. An even greater surprise was brought out by radiotracer experiments. The HDEHP concentration dependencies of log D(M3+) for Eu3+ and Am3+ (Figure 6) revealed anticipated

Figure 7. Like Figure 2 for 20 mol % Hbet NTf2 in Chol NTf2.

Figure 8. Like Figure 4 for 20 mol % Hbet NTf2 in Chol NTf2.

system outperforms the traditional aqueous TALSPEAK by a factor of 2. A particular concern for practical applications is the overall reproducibility of the process (cycling stability). Addressing this issue, we studied how D(Eu3+) changed when the IL solution was reused. To this end, after each separation, the remaining M3+ ions were stripped by 1 m HDEHP in cumene (1:1 (w/ w)) and the IL phase was scrubbed with cumene (1:1 (w/w)) to remove traces of HDEHP. The results of this trial are shown in SI Figure 15S. Following the first cycle, D(Eu3+) decreased by a factor of 2, but after this initial decrease, the phase partitioning stabilized, and we did not observe further changes after four more cycles. While more studies are required to explain this initial decrease, the overall process appears to be sufficiently robust and predictable with regard to recycling of the IL solvent. 3.7. 1:2 Binding of Trivalent Ions to DTPA−(Chol+)2 Conjugates. The results of section 3.6 suggest the occurrence of 1:2 complexation involving DTPA−(Chol+)2 conjugates. As this mode of metal ion complexation by DTPA derivatives is unprecedented, we sought independent verification of its occurrence in IL solutions. To this end, we varied the mole ratio, ρ, of [cDTPAA] and [M3+] keeping [M3+] constant. For Nd3+ ion, the absorption band at 560−600 nm was analyzed, as it is known that this band is particularly sensitive to the ligand

Figure 6. Like Figure 3 for 20 mol % Hbet NTf2 in Chol NTf2 (1:1 (w/w) separations). The experimental conditions are indicated in the plot.

slopes of ≈3 (section 3.5); however, the cDTPAA concentration dependencies yielded slopes of ≈2 (Figure 7), suggesting the formation of 1:2 complexes in the IL phase (instead of 1:1 complexes as envisioned in Scheme 2). That the nature of the IL complex has changed is also suggested by a remarkable increase in the separation factor η that was typically over 200 and in some regimes reached 250−270. Figure 8 presents the most important result of this study: the log D(M3+) vs r−1 dependence for lanthanide ions from La3+ to Gd3+ with a data point for Am3+ (obtained using the separation factor of 250 estimated in our radiotracer experiments under the same experimental conditions). For all lanthanides other than La, D(M3+)/D(Am3+) > 20, whereas for La3+ this ratio is ∼10. Thus, this IL-based system achieves group separations for Ln3+ and An3+ (section 1.1). For Eu3+/Am3+ separations, this 3648



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environment.63−65 Likewise, for Eu, the 620 nm fluorescence emission kinetics were sampled and analyzed as explained in SI section 5S. To introduce these ions into the IL, aqueous solutions of the nitrates were reduced to dryness and dissolved in 20 mol % Hbet NTf2 in Chol NTf2; this stock solution was diluted either by this IL or a solution of cDTPAA in this IL. SI Figure 16S(a) exhibits the progression of absorption spectra observed from solutions of 10.5 mm Nd3+ in the IL as a function of ρ. It is seen that the spectral changes continue well after 1:1 stoichiometry is attained. The same can be discerned from SI Figure 16S(b), where we plotted optical density at the specified wavelengths vs ρ. While 1:1 binding clearly occurs at lower ρ, resulting in an initial linear decrease of the optical density, the absorption keeps changing as ρ increases. In Figure 9, normalized absorption spectra at 585 nm are plotted in order

Figure 10. (a) Dependences of lifetimes for luminescent complexes of Eu3+ in 20 mol % Hbet NTf2 in Chol NTf2 as a function of ρ = [cDTPAA]/[Eu3+]. Traces i and ii correspond to solutions containing no betaine (i) and 12:1 mol/mol Eu3+ excess of betaine (ii), respectively. These “effective” lifetimes correspond to a singleexponential fit of the multiexponential kinetics for t > 100 μs. (b) Speciation in IL solution as determined in our model: (i) L3−, (ii) H3L, (iii) solvated M3+, (iv) ML, and (v) ML...H3L. See the inset in the plot for disambiguation.

Figure 9. Evolution of the spectral envelope for 560−600 nm absorption band of Nd3+ (10.5 mm) in 20 mol % Hbet NTf2 in Chol NTf2 as a function of ρ = [cDTPAA]/[Nd3+] These spectra were normalized at 585 nm (indicated by the filled arrow). The open arrows indicate spectral regions where evolution continues for ρ > 1.

binds to the metal ion modifying the absorption and emission properties. This suggests the direct involvement of the second ligand in ion coordination as opposed to weak interactions between the 1:1 complex and free conjugates in the solution. To put these observations on a more quantitative footing, we assumed that metal binding by these DTPA derivatives involves the exchange of exactly three protons, as suggested by Figure 6. Assuming that the identity of the metal binding ligand is a deprotonated DTPA−(Chol+)2 conjugate (L3−), the 1:1 complex can be represented as ML (with the stability constant of K1), and the 1:2 complex that is involved in the separations is ML...H3L (with the stability constant of K2). In the latter, the structural formula does not mean to specify the mode in which the three protons are distributed between the two ligands. In betaine solutions the ligand is deprotonated, and the formation of ML...H3L is suppressed. Without betaine, there are protic equilibria that ensure a fraction of the ligand exists in the protonated form, so ML can react with H3L. As little is known about such protic equilibria in ILs, we made a further assumption that this IL is self-buffering, so that the proticity does not change as cDTPAA is added and, therefore, the ratio Ka = [H3L]/[L3−‑] remains constant. Using this assumption, we were able to quantitatively fit both our Eu3+ and Nd3+ data (see SI section 5S for details). For the fluorescence kinetics data, we assumed that each complex exhibits single-exponential decay and optimized equilibria constants, lifetimes, and relative quantum yields for 620 nm

to facilitate the comparison of spectral envelopes. There are at least three ranges (indicated by arrows) where spectral evolution continues in the high-ρ regime. When a 5-fold molar excess of betaine was added to this solution, a different behavior was observed. Very little spectral evolution was seen once ρ > 1 was reached (SI Figure 17S(a)) and the individual curves for a fixed wavelength (SI Figure 17S(b)) were consistent with 1:1 binding with a high binding constant. Comparison of the absorption spectra with and without betaine (SI Figure 18S) suggests that two different complexes are present in IL solutions containing no cDTPAA (ρ = 0), whereas the spectra become almost identical at ρ = 3, suggesting structural similarity once the conjugate complexes the ion. Even more striking were the results for Eu3+ luminescence kinetics. The latter are, in fact, multiexponential, but even one exponent provides a reasonably good fit for delay times > 100 μs. In Figure 10a, we plotted this effective lifetime vs ρ for 4 mm Eu3+ solutions containing no betaine (trace i) and 5-fold excess of betaine (trace ii). In the presence of betaine, there are changes in the lifetime that are consistent with the occurrence of 1:1 binding, whereas in the solution without betaine, the lifetime continues to increase after attaining 1:1 stoichiometry. Both of these experiments indicate that while 1:1 binding prevails in solutions containing an excess of betaine, in solutions without betaine more than one DTPA conjugate 3649



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luminescence. Without betaine in solution, we estimated K1 ≈ 1700 M−1, K2 ≈ 65 M−1, and Ka ≈ 11, which yields the speciation plot shown in Figure 10b. Addition of betaine clearly changes the nature of “free” metal ion complex in IL, suggesting that betaine zwitterions are involved in the complexation (see section 3.2). Thus, it is not surprising that the binding constant becomes smaller (we estimated K1 ≈ 140 M−1) in the presence of betaine. For Nd3+ absorption, we estimated K1 ≈ 3310 M−1 and K2 ≈ 9 M−1, although these estimates are less reliable than for Eu3+ emission, due to the small changes in the absorbance spectra of ML and ML...H3L species (Figure 9). Despite the uncertainty, these analyses do suggest that competitive 1:1 and 1:2 metal ion binding is sufficient to rationalize the observed trends for both lanthanide ions. These results (i) confirm the occurrence of 1:2 complexes in the presence of DTPA−(Chol+)2 conjugates observed in the extraction experiments, (ii) demonstrate that binding of the second ligand occurs through the metal ion as opposed to molecular interactions of the complex with another ligand, (iii) indicate that the 1:2 complex is protonated, and (iv) suggest that 1:1 binding is promoted while 1:2 binding is suppressed when this ligand is present in deprotonated form. Given the large molar excess of the ligand over the metal ions in the extraction experiments presented in section 3.6 and the fact that most of DTPA−(Chol+)2 is protonated in this IL, it is not surprising that only the 1:2 complex participated in the extraction equilibria. The discussion of the structural aspects of complexation and the possible soft-donor effects66−69 in such complexes are continued in SI section 6S.

was shown by spectroscopy (section 3.7), and their involvement in the extraction equilibria (eq 10) was attested through the changed log D(M3+) vs [cDTPAA] slopes observed in Figure 7, by the doubling of η and by the emergent La bend that indicates size selectivity for large ions that the 1:1 complexes lacked (section 3.6). While further study is needed to characterize the admittedly complex chemistry of this system, the results of section 3.6 are sufficient for suggesting a process based on this chemistry (Figure 11). In this process, trivalent ions are extracted from

4. DISCUSSION To recapitulate the results of section 3, the cyclical dianhydride of DTPA quantitatively reacts with functionalized ILs as shown in Scheme 4. In Chol NTf2−Hbet NTf2 mixtures, Hbet+ catalyzes esterification with the formation of the DTPA−(Chol+)2 conjugate; without Chol+ cations present in the reaction mixture, the DTPA−(Hbet+)2 mixed anhydride is formed instead. The latter species forms 1:1 complexes with trivalent ions in the IL phase, showing large Eu3+/Am3+ separation factors (>100) but insufficient size selectivity for larger Ln3+ ions, whereas DTPA−(Chol+)2 also yields a 1:2 complex in which one of the DTPA−(Chol+)2 ligands likely remains protonated. Such 1:2 complexes exhibit improved size selectivity and increased Eu3+/Am3+ separation efficiency, which makes it possible to achieve group separations of lanthanides from minor actinides with D(Ln3+)/D(Am3+) > 10. Unlike aqueous DTPA (section 1), such separations should not be complicated by excessive pH sensitivity, as no extra protons are lost or gained when a trivalent metal ion partitions between the two phases:

Figure 11. Conceptual representation of IL-based process for group separations of lanthanides from minor actinides.

the aqueous feed to 0.1 m HDEHP solution in an organic solvent (the “loading” stage). This M3+ ion loaded solvent is placed in contact with the functionalized IL containing DTPA conjugates that selectively separate An3+ ions into the IL phase, while Ln3+ ions remain in the organic phase. As in reverse TALSPEAK, these Ln3+ ions are subsequently stripped from the organic phase by 1 M nitric acid. To strip An3+ ions from the IL solvent, the IL is put in contact with 1 m HDEHP, so all of the trivalent ions are extracted into the organic phase; nitric acid is then used to strip extracted An3+ ions into nitric acid solution. The end results of this cycle are two aqueous waste streams containing An3+ and Ln3+ ions. To complete this cycle, organic diluent is used to scrub the IL of traces of HDEHP, and the process is repeated. The large difference in the densities of the phases results in short settling times. We named this process LABILE for Lanthanide Actinide separations Based on Ionic Liquid Exchange.

[ML]IL + 3(HDEHP...HDEHP)org ⇌ [H3L]IL + [M(DEHP...HDEHP)3 ]org

5. CONCLUDING REMARKS Recently, it has been suggested that ionic liquids might have been given too much acclaim regarding their tentative ability to achieve dramatic improvements in nuclear cycle separations.22 While this criticism is mostly deserved, with few exceptions the uses reviewed amounted to replacement of the organic diluent in the existing processes; rare success has been achieved down this path. However, ionic liquids can and should be used in more imaginative ways that take better advantage of their remarkable properties. Such is the use of functionalized ILs in

(9)

[ML...H3L]IL + 3(HDEHP...HDEHP)org ⇌ 2[H3L]IL + [M(DEHP...HDEHP)3 ]org

(10)

To our knowledge, 1:2 complexes of trivalent metal ions involving DTPA derivatives have not been reported. Our results point to the occurrence of such complexation very strongly (sections 3.6 and 3.7). The formation of such 1:2 complexes 3650



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this study that was itself inspired by the latest advances in understanding of such ILs.37,39−44,70 In our system, ILs still serve as a diluent, but the separations occur not between the IL and aqueous phases, as in the “conservative” approach pursued by others,23−25,34 but instead between immiscible IL and organic phases. Furthermore, the IL serves not only as a diluent, but also as a reagent and a catalyst, achieving the desired modification of the precursor molecule. The aminopolycarboxylate moiety becomes conjugated with the constituent cations, acquiring new chemical properties, on which the extraction efficacy and selectivity of the process critically depend. In this reaction, a functionalized IL becomes a task-specific one. Typically, such ILs are obtained through painstaking synthetic effort; in our case, the reaction occurs in situ, effortlessly. We used this chemistry to address one of the most challenging problems in the entire field of nuclear separations chemistry, viz., group separation of trivalent lanthanide from actinide ions. Having departed from the familiar TALSPEAK process, we arrived at a method that retains only certain features of this process, “reinventing” it from scratch. The competitive extraction between aminopolycarboxylic chelating agents in one phase and phosphate-based extracting agents in another phase (that gives TALSPEAK its name) is retained, but the chemical realization of this competition is quite different. The ligand is integrated into the IL. The resulting species retains three carboxyl groups, so the pH-dependent proton exchange that complicates aqueous TALSPEAK is eliminated (Scheme 2). Furthermore, as the conjugate forms 1:2 complexes (as opposed to 1:1 complex formation with DTPA5‑), the system exhibits more pronounced Eu3+/Am3+ selectivity than aqueous Ln−DTPA complexes. Separation factors 250−270 for Eu3+/Am3+ partitioning have been demonstrated. This system also displays the characteristic La bend that is the hallmark of the traditional aqueous TALSPEAK. Group separation of lanthanides from trivalent actinides with D(Ln3+)/D(An3+) > 10 was demonstrated. While more complete characterization of this system is required for process development, the system exhibits favorable cycling properties that allow one to consider a process based on this chemistry (Figure 11). Even taken separately, these results would be remarkable in any extraction system. Taken together, they suggest that ionic liquids have the potential to enable robust nuclear separations that have not been realized previously.

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AUTHOR INFORMATION

Corresponding Authors

*Shkrob, I. A. Tel.: (630) 252-9516. E-mail: [email protected]. *Jensen, M. P. Tel. (630) 252-3670. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank S. M. Brombosz, M. Antonio, R. Wilson, L. Soderholm, R. Ellis, M. L. Dietz, S. Dai, H. Luo, and J. F. Wishart for helpful discussions, J. V. Muntean and Y. Tsai for technical assistance with NMR and ICP-MS analyses, respectively, and R. Chiarizia for his many valuable suggestions and critical reading of the manuscript. The work at Argonne was supported by the U.S. Department of Energy (DOE) Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under Contract No. DE-AC02-06CH11357. Programmatic support via a DOE SISGR grant “An Integrated Basic Research Program for Advanced Nuclear Energy Separations Systems Based on Ionic Liquids” is gratefully acknowledged.

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REFERENCES

(1) Nilsson, M.; Nash, K. L. A Review of the Development and Operational Characteristics of the TALSPEAK Process. Solvent Extr. Ion Exch. 2007, 25, 665−701. (2) Davis, J. H. Task-Specific Ionic Liquids. Chem. Lett. 2004, 33, 1072−1077. (3) Weaver, B.; Kappelmann, F. A. Preferential Extraction of Lanthanides over Trivalent Actinides by Monoacidic Organophosphates from Carboxylic Acids and from Mixtures of Carboxylic and Aminopolyacetic Acids. J. Inorg. Nucl. Chem. 1968, 30, 263−272. (4) Braley, J. C.; Grimes, T. S.; Nash, K. L. Alternatives to HDEHP and DTPA for Simplified TALSPEAK Separations. Ind. Eng. Chem. Res. 2012, 51, 629−638. (5) Grimes, T. S.; Tian, G.; Rao, L.; Nash, K. L. Optical Spectroscopy Study of Organic-Phase Lanthanide Complexes in the TALSPEAK Separations Process. Inorg. Chem. 2012, 51, 6299−6307. (6) Nilsson, M.; Nash, K. L. Trans-Lanthanide Extraction Studies in the TALSPEAK System: Investigating the Effect of Acidity and Temperature. Solvent Extr. Ion Exch. 2009, 27, 354−377. (7) Grimes, T. S.; Nilsson, M. A.; Nash, K. L. Lactic Acid Partitioning in TALSPEAK Extraction Systems. Sep. Sci. Technol. 2010, 45, 1725− 1732. (8) Leggett, C. J.; Liu, G.; Jensen, M. P. Do Aqueous Ternary Complexes Influence the TALSPEAK Process? Solvent Extr. Ion Exch. 2010, 28, 313−334. (9) Tian, G.; Martin, L. R.; Rao, L. Complexation of Lactate with Neodymium(III) and Europium(III) at Variable Temperatures: Studies by Potentiometry, Microcalorimetry, Optical Absorption, and Luminescence Spectroscopy. Inorg. Chem. 2010, 49, 10598−10605. (10) Tian, G.; Rao, L. Effect of Temperature on the Protonation of the TALSPEAK Ligands: Lactic and Diethylenetrinitropentaacetic Acids. Sep. Sci. Technol. 2010, 45, 12−13. (11) Grimes, T. S.; Jensen, M. P.; Debeer-Schmidt, L.; Littrell, K.; Nash, K. L. Small-Angle Neutron Scattering Study of Organic-Phase Aggregation in the TALSPEAK Process. J. Phys Chem. B 2012, 116, 13722−13730. (12) Marie, C.; Hiscox, B.; Nash, K. L. Characterization of HDEHPLanthanide Complexes Formed in a Non-Polar Organic Phase Using 31 P NMR and ESI-MS. Dalton Trans. 2012, 2012, 1054−1064. (13) Nash, K. L. A Review of the Basic Chemistry and Recent Developments in Trivalent f-Elements Separations. Solvent Extr. Ion Exch. 1993, 11, 729−768. (14) Choppin, G. R. Comparative Solution Chemistry of the 4f and 5f Elements. J. Alloys Compd. 1995, 223, 174−179.

ASSOCIATED CONTENT

S Supporting Information *

Sections 1S−6S giving abbreviations, synthetic methods and data, TRLF measurements, and reactions, Tables 1S−15S listing typical conditions for the TALSPEAK process, NMR resonances, bond energies, speciations, and miscibilities, Schemes 1S−4S showing various additional structures and reactions, and Figures 1S−19S showing equilibrium distributions, exchange equilibrium constants, ion radius dependences, ESI MS1+ spectra, optimized structures, luminescence lifetimes, equilibrium kinetics, D(Ln3+) as a function of mole fraction, the cycling test described in section 3.6, and absorption spectra and ρ dependence. This material is available free of charge via the Internet at http://pubs.acs.org. 3651



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Ionic Liquid Based Separations of Trivalent Lanthanide and Actinide

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