Alternatives to HDEHP and DTPA for Simplified ... - ACS Publications

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Alternatives to HDEHP and DTPA for Simplified TALSPEAK Separations Jenifer C. Braley,† Travis S. Grimes, and Kenneth L. Nash* Chemistry Department, Washington State University, Pullman, Washington, United States ABSTRACT: The TALSPEAK process is an established option for lanthanide/minor actinide separations using solvent extraction. In this process, selective extraction of lanthanides is achieved by contacting a water-soluble aminopolycarboxylate complexant in a concentrated carboxylic acid buffer with a liquid cation exchanging extractant in an immiscible organic diluent. Although TALSPEAK process development has been successful on several levels, studies of the detailed fundamental chemistry have revealed undesirable complex interactions between aqueous and organic solute species. These complications threaten to impair process modeling and could impact engineered operations. In the present work, results are reported describing equilibrium partitioning and phase transfer kinetics trends for trivalent lanthanide ions and americium into bis-2-ethyl(hexyl) phosphoric acid (HDEHP) or structural analog 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) organic phases from aqueous lactate solutions containing diethylenetriamine-N,N,N0 ,N00 ,N00 -pentaacetic acid (DTPA), triethylenetetramine-N,N,N0 ,N00 ,N000 , N000 -hexaacetic acid (TTHA), or N-(2-hydroxyethyl)ethylenediamine-N,N0 ,N0 -triacetic acid (HEDTA). The undesirable partitioning of Naþ, lactic acid, and water into the organic phase is greatly reduced when HEH[EHP] replaces HDEHP as the extractant. TTHA appears to offer little advantage over DTPA in conventional TALSPEAK, but both DTPA and TTHA are too strong for use in combination with HEH[EHP]. The combination of HEDTA with HEH[EHP] achieves good balance and exhibits a nearly flat pH dependence between 2.5 and 4.5, in contrast with conventional TALSPEAK. The latter combination demonstrates more predictable performance than is seen in conventional TALSPEAK, while providing acceptable americium/lanthanide separation factors. The HEDTA/HEH[EHP] combination offers the additional advantage of more rapid phase transfer kinetics for the heavier lanthanides without the need for high concentrations of a lactate buffer.

’ INTRODUCTION To enable transmutation of transplutonium actinides, an efficient separation of trivalent actinides (An3þ) from fission product lanthanides (Ln3þ) is needed.1 Because of the similar hard-acid2 nature and comparable charge density of An3þ and Ln3þ, this separation is unusually challenging. The most successful approaches to group separations developed to date have involved exploiting the slightly greater covalency of An3þ bonding interactions, which was first reported by Diamond et al.3 This difference in bonding characteristics is believed to arise from the greater radial extension of the 5f orbitals (relative to the 4f orbitals in the lanthanides).47 It has been noted that the interaction of nitrogen and sulfur donor ligands with An3þ is stronger than with Ln3þ, thus potentially enabling a successful separation.5 This fundamental difference has inspired a considerable amount of research addressing the development of softdonor molecules or ions for separations applications.4,712 There are two complementary approaches to the separation of americium (Am3þ) from Ln3þ based on the manipulation of this chemistry, using soft donor extractant molecules (actinides are selectively extracted into the organic phase) or soft donor complexants (actinides are retained in the aqueous solution while lanthanides are extracted). [In this report, the behavior of Am3þ is taken as representative of that of trivalent transplutonium actinides. Additional adjustments might be necessary to accommodate heavier actinides. However, on the basis of the half-life of its primary long-lived isotopes (241Am, t1/2 = 433 years, 243Am, t1/2 = 7370 years), it can be argued that Am represents the most important driver for considering such r 2011 American Chemical Society

separations.] In the European advanced fuel cycle research program, this important separation has been approached primarily through the design of soft-donor extractant molecules, of which the bistriazinyl-pyridine (and bipyridine) derivatives have seen the most extensive development.8,12 Extraction systems based on dialkyldithiophosphinic acids have also been demonstrated.10,11 Unfortunately, it has been noted that the relatively polarizable molecular orbitals encountered in sulfurand pyridine-based separations reagents increase the susceptibility of these ligands to radiolytic damage.8,12,13 One of the most extensively tested approaches of employing soft donor holdback reagents (aqueous complexants) is the TALSPEAK (Trivalent ActinideLanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) solvent extraction process and related systems. The origins of TALSPEAK are traced to Oak Ridge National Laboratory in the 1960s.14 There is some evidence that TALSPEAK may be less severely impacted by radiolysis effects than the soft donor extractant systems.1518 The essential features of the TALSPEAK process have been reviewed recently.18 In TALSPEAK, actinide selectivity over lanthanides arises from the polyaminopolycarboxylate holdback reagent which is Special Issue: Alternative Energy Systems: Nuclear Energy Received: February 9, 2011 Accepted: May 23, 2011 Revised: May 17, 2011 Published: May 23, 2011 629

dx.doi.org/10.1021/ie200285r | Ind. Eng. Chem. Res. 2012, 51, 629–638

Industrial & Engineering Chemistry Research

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Figure 1. Structures of the components used in these studies. (A) Bis-2-ethyl(hexyl) phosphoric acid (HDEHP). (B) 2-Ethyl(hexyl)phosphoric acid mono-2-ethyl(hexyl) ester (HEH[EHP]). (C) Triethylenetetramine-N,N,N0 ,N00 ,N000 ,N000 -hexaacetic acid (TTHA). (D) Diethylenetriamine- N,N,N0 , N00 ,N00 -pentaacetic acid (DTPA). (E) Lactic acid (HL). (F) N-(2-Hydroxyethyl) ethylenediamine-N,N0 ,N0 -triacetic acid.

balanced against an extractant that does not discriminate between the groups. The most thoroughly studied TALSPEAK separations are based on bis-(2-ethyl(hexyl)) phosphoric acid (HDEHP, organic extractant), diethylenetriamine-N,N,N0 N00 , N00 -pentaacetic acid (DTPA, holdback reagent), and lactic acid (carboxylic acid buffer), whose structures are shown in Figure 1. This combination of reagents will be referred to in this report as “conventional TALSPEAK”. Recent reports addressing fundamental aspects of TALSPEAK chemistry have found metal distribution data are not always reliably modeled using the expected mono- and biphasic thermodynamic equilibria.19 Complex interactions between solute species and increased partitioning of water into the extractant phase have been noted as particular obstacles to producing an accurate thermodynamic model.1820 Ongoing studies address aggregation phenomena, metal ion coordination chemistry in the extracted complex and in the aqueous phase, the identification of possible ternary complex species, the determination of system-relevant thermodynamic parameters, and the origins of the slow phase transfer kinetics of the heavier lanthanides. The concentrated lactic acid buffer is a factor in each of these issues, in both a positive and negative sense. In the following report, the impact of changing the extractant from HDEHP to the less acidic but structurally analogous phosphonic acid (2-ethyl(hexyl)phosphonic acid mono-2-ethyl(hexyl) ester, HEH[EHP], Figure 1) on the basic features of lanthanide and Am3þ extraction in a TALSPEAK extraction system is examined. In addition, the potential impact of changing in parallel the polyaminopolycarboxylate holdback reagent has been investigated.

The results are correlated with predictions based on thermodynamic data from the literature.

’ EXPERIMENTAL SECTION Materials. The HDEHP used in this study was 97% pure material from Sigma-Aldrich. The copper purification method was used to purify the sample to >99% as verified by potentiometric titrations.21 Eichrom Technologies provided HEH[EHP] (Ionquest 801, Albright and Wilson) at 90% purity. The third phase formation procedure was used to purify the material to >98% as verified by NMR spectroscopy.22 The n-dodecane was obtained from Alfa-Aesar (99% pure) and used as received. Sodium nitrate (Ricca Chemical Company) was dissolved in deionized water, filtered through a 1.0 μm membrane, recrystallized from H2O, and redissolved in H2O to prepare a 5.28 mol NaNO3/kg stock solution, as determined using Eichrom cation exchange resins (Hþ form) and potentiometric titrations. Lanthanide oxides, 99.999%, purchased from Arris International, were used to prepare lanthanide nitrate stock solutions (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y). The metal content, acid content, and nitrate content of lanthanide solutions were determined using an HP Agilent 4500þ ICP-MS and cation-exchange resin coupled with potentiometric titrations. Nitric acid solutions were prepared from Trace Metal grade HNO3 (Fisher Scientific) using deionized water obtained from a Milli-Q2 water purification system. Sodium lactate (a mixture of D/L isomers) was obtained as an aqueous solution from J.T. Baker, and solutions were standardized using cation-exchange 630

dx.doi.org/10.1021/ie200285r |Ind. Eng. Chem. Res. 2012, 51, 629–638


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chromatography (Dowex 50, Hþ form) with analysis by potentiometric titration. Stock solutions of polyaminopolycarboxylic acid were prepared by dissolving the solid acid in deionized water and adding 50% w/w NaOH (Sigma Aldrich) to raise the pH to 3.6. Analytical grade potassium hydrogen phthalate (KHP) was used to standardize the base solutions for potentiometric titrations. Potentiometric titrations were performed using a semimicro Ag/AgCl electrode and standardized acid or base. Radioactive 152/154 Eu and 24Na were produced by neutron activation of 99.999% Eu2O3 (Arris International) and recrystallized NaNO3, respectively, using a Teaching, Research, Isotopes General Atomics (TRIGA) reactor with a neutron flux of 5 1012 n/cm2 3 s at the Nuclear Radiation Center at WSU. The 241Am tracer in 0.1 M HNO3 was obtained from Pacific Northwest National Laboratory. Methods. All aqueous solutions were prepared by weight. Experiments were conducted at a 1:1 aqueous/organic phase ratio with variations including pH, lactate, and time. Lactate solutions were prepared fresh using sodium lactate rather than lactic acid to minimize possible complications from the lactate esters that form spontaneously in acidic lactic acid solutions.23 Radiotracer experiments using 152/154Eu and 241Am were analyzed on a NaI(Tl) solid scintillation counter and a Packard Cobra-II auto gamma, for gross gamma counting. Radiotracer experiments using 14C (lactic acid) were analyzed using a Beckman LS6500 liquid scintillation counter for β detection with 5 mL of EcoScint scintillation fluid. Water distribution to the organic phase was examined by Karl Fisher titration using a Mettler Toledo Model 4200 KF titrator. The instrument was standardized immediately before analysis using 1000 ppm water standards. The calibrations were consistent within (25% error. The distribution ratios for extraction using radiotracer techniques were calculated by measuring the amount of radioactivity in both the aqueous and organic phases. The counting efficiency is identical for both phases. The distribution ratio is defined as the ratio of specific activity of metal in the organic phase to that of the aqueous phase: DM ¼

½Mf , org ½Mf , aq

Figure 2. trans-Lanthanide distribution patterns for alternative and conventional configurations of the TALSPEAK process. The following conditions were common to all systems. (a) Aqueous phase: 1 M NO3 , pH = 3.6, 1 M total lactate, 20 mM DTPA or TTHA. Organic phase: 0.1 M HDEHP in n-dodecane. (b) Aqueous phase: 1 M NO3 , pH = 3.6, 1 or 0.1 M total lactate, 20 mM HEDTA. Organic phase: 0.1 M HEH[EHP] in n-dodecane. Error bars indicate (1σ uncertainty. Lines associated with data are added to highlight the trans-lanthanide pattern.

examining Eu distribution as a function of pH were performed using both radiotracer and inactive metal methodologies with good agreement observed (within (10%). The following conditions are valid for all experimental results reported: (i) experiments were run in triplicate; (ii) all elements are present simultaneously in trans-lanthanide experiments; (iii) concentrations of HDEHP or HEH[EHP] were maintained at 0.1 M; (iv) nitrate (NO3) was present at 1 M; (v) all experiments were performed at ambient temperature (21 ( 1)°C; (vi) contact times for distribution studies were 30 min, except where otherwise noted. For 30 min contacts, samples were vortex mixed. Longer contacts utilized a carousel mixer. Previous studies establish that biphasic equilibria for conventional TALSPEAK are attained within these constraints.19 The effect of total lactate concentration (pH 3.6) on trans-lanthanide uptake kinetics was examined for all combinations of DTPA and TTHA with HDEHP and HEH[EHP]. A pH profile for lanthanide partitioning between a DTPA/lactate aqueous phase and an HDEHP or HEH[EHP] organic phase was also completed. trans-Lanthanide partitioning kinetics and pH profile studies for HEDTA-containing systems were completed only with an HEH[EHP] organic phase. The partitioning of water, lactate, and sodium into HDEHP or HEH[EHP] organic phases as a function of total lactate concentration was studied from both DTPA and HEDTA aqueous media.

ð1Þ

The distribution ratios ([M]f,org/[M]f,aq) for nonradioactive metal ions were calculated by analyzing the aqueous phase before ([M]i,aq) and after ([M]f,aq) contact with the organic phase. Assuming no losses to sorption or precipitation, the difference between [M]i,aq and [M]f,aq defines the concentration of metal in the organic phase. The distribution ratio can then be defined as DM ¼

½Mi, aq ½Mf , aq ½Mf , aq

ð2Þ

Single isotopes were used in the radiotracer experiments, while the ICP-MS experiments were conducted using mixtures of all lanthanides plus yttrium (Pm was excluded due to lack of availability). Acknowledging the potential for isobaric interferences, ICP-MS analysis utilized masses that were not susceptible to such issues. The total concentration of all metal in the lanthanide experiments was maintained at 1 mM to minimize the possible effects of extractant loading. To further confirm the comparability of radiotracer and ICP-MS results, experiments

’ RESULTS trans-Lanthanide Partitioning. Lanthanide distribution ratios as a function of atomic number for equilibration of HDEHP with DTPA or TTHA in 1 M buffered lactate/NaNO3 solution are shown in Figure 2a. A similar investigation was completed for the HEH[EHP]/HEDTA system (Figure 2b). The concentration of each lanthanide was maintained at 20 ppm for a total 631



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Figure 4. Distribution of selected rare earth ions as a function of pH for the HEH[EHP]/DTPA TALSPEAK system using a 48 h contact time. The following conditions were common to all systems. Aqueous phase: 1 M lactic acid, 1 M NO3 , 20 mM DTPA. Organic phase: 0.1 M HEH[EHP] in n-dodecane. All error bars indicate (1σ uncertainty. Metal-DTPA constants included in calculations were reported for 0.1 M ionic strength. Lines indicate the predicted extraction patterns based on thermodynamic data. La3þ results have been plotted on a separate axis for clarity.

Figure 3. Equilibrium distribution ratios of Eu3þ (9) and Am3þ (0) as a function of pH for several versions of the TALSPEAK process after 30 min of mixing. The following conditions were common to all systems. Aqueous phase: 1 M total lactic acid, 1 M NO3 , 20 mM DTPA or TTHA. Organic phase: 0.1 M HDEHP or HEH[EHP] in n-dodecane. Error bars indicate (3σ uncertainty. Thermodynamic equilibrium constants used in distribution calculations for DTPA and TTHA were at 0.1 and 0.5 M ionic strength, respectively.2426 Solid lines indicate prediction of thermodynamic data for Eu3þ. Dashed lines indicate calculated trends for Am3þ in conventional TALSPEAK. Trends for Am3þ extraction in the HEH[EHP] system are not calculated, as Kex for Am3þ and HEH[EHP] have not been reported.

metal concentration in solution of approximately 1 mM. All lactate concentrations represent the total molar concentration of lactate/lactic acid. The Influence of pH. The results of the 241Am/152/154Eu radiotracer studies and parallel calculations of distribution ratios based on thermodynamic data from the literature are shown in Figure 3 for HDEHP or HEH[EHP] equilibration with DTPA or TTHA. Metal ion extraction by HEH[EHP] is seen to increase with increasing pH. In contrast, metal extraction by HDEHP decreases with increasing pH (above pH = 3.6). As was described in an earlier report,19 thermodynamic data from the existing literature predict a constant to slightly increasing extraction for both systems as the pH increases. As the disagreement between prediction and experiment in the HDEHP system is seen for both TTHA and DTPA results, it is reasonable to conclude that the ultimate explanation for the discrepancy in pH dependence lies in reactions occurring predominantly in the HDEHP extractant phase (after equilibration with concentrated lactate buffer). To confirm that the agreement between prediction and experimental results in the pH dependence of HEH[EHP]-based systems is not an artifact of the radiotracer experiments with 152/154 Eu3þ and 241Am3þ, a similar examination of the influence of pH was done for several other lanthanides using ICP-MS (La3þ, Dy3þ, Y3þ, and Lu3þ, Figure 4). A 48 h contact was allowed to ensure thermodynamic equilibrium. It is seen that for lanthanides across the series the pH dependence in this system is nearly flat. The influence of pH on Am3þ and Eu3þ extraction was also investigated for the combination of HEH[EHP] with the weaker holdback reagent HEDTA (Figure 5). This combination is also seen to produce a nearly flat pH dependence and attains the

Figure 5. Distribution of Eu3þ (9) and Am3þ (0) as a function of the pH for the HEH[EHP]/HEDTA modification of the TALSPEAK process after a 30 min contact. The following conditions werecommon to all systems. Aqueous phase: 1 M total lactic acid, 1 M NO3 , 20 mM HEDTA. Organic phase: 0.1 M HEH[EHP] in n-dodecane. Error bars indicate (3σ uncertainty. Thermodynamic equilibrium constants used in distribution calculations for HEDTA were reported at 0.1 ionic strength.24,25 Dashed lines indicate predicted extraction patterns based on thermodynamic data for Eu3þ.

expected higher distribution ratios. The Am3þ/Eu3þ separation factor is nearly constant between pH 2.5 and 4.2. Predicted metal distribution patterns were calculated for europium in this system using stability constants from the NIST database and extraction constants derived from the previous literature.24,25 Europium partitioning was overestimated using these equilibrium constants 632



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Figure 6. Water and lactate partitioning as a function of total lactate concentration. The following conditions were common to all systems. Aqueous phase: 1 M total lactic acid, 1 M NO3 , 20 mM DTPA, pH(final) = 3.5. Organic phase: 0.2 M HDEHP or 0.1 M HEH[EHP] in n-dodecane. Samples were contacted for 15 min. Error bars indicate (1σ uncertainty. HDEHP: red square, H2O; red circle, HL. HEH[EHP]: blue sqaure, H2O; blue circle, HL. ( 3 3 3 ) Calculated concentration of lactic acid in the organic phase based on Kd into n-dodecane.20 (---) Calculated concentration of water in the organic phase based on Kd into kerosene.27

Figure 7. Sodium partitioning as a function of pH. Aqueous phase: 1 M NO3 , 1.0 M total lactate 20 mM DTPA. Organic phase: 0.1 M HDEHP (9) or HEH[EHP] (red b) in n-dodecane. Samples were contacted for 15 min. Error bars indicate (1σ uncertainty.

lipophilic extractant (HDEHP or HEH[EHP]) that binds the lanthanides and the hydrophilic aminopolycarboxylic acid complexant (DTPA, TTHA, HEDTA) that complexes and retains the trivalent actinides in the aqueous phase. The extractant molecules behave as hard base donors and exhibit little selectivity between Ln3þ and An3þ cations of similar size. However, both HDEHP and HEH[EHP] are highly sensitive to the size of the cation, demonstrating increasing extraction strength as the atomic number of the ions increases.26 The nearly 20% decrease in cation radius that occurs between La3þ and Lu3þ results in a 105 increase in Kex for both HDEHP and HEH[EHP].31 Actinide extraction constants are nearly identical for like-sized ions (Nd3þ ≈ Am3þ). The basic phase transfer equilibrium is

(dashed line in Figure 5). The deviation between experimental and calculated values is seen to increase as the pH rises. Lactate, Sodium, and Water Partitioning. The partitioning of lactate and water between a concentrated lactate buffer containing DTPA and HDEHP/n-dodecane or HEH[EHP]/ndodecane is shown in Figure 6. It is seen that the partitioning of both lactate and water into HDEHP/n-dodecane increases as [lactate]tot rises, in agreement with an earlier study of conventional TALSPEAK in 1,4-diisopropyl benzene.20 In contrast, the water content of the HEH[EHP] system does not increase with increasing [lactate]tot. Lactic acid partitioning does increase with increasing [lactate]tot, but at a level nearly identical with the partitioning of HLac into n-dodecane.20 The enhanced partitioning of HLac into HDEHP is clearly established in these results, as is the absence of a similar effect in the more basic HEH[EHP] extractant. The sodium ion distribution into 0.1 M HDEHP/n-dodecane or 0.1 M HEH[EHP]/n-dodecane organic phases was seen to be independent of [lactate]tot with average log D values of 2.0 ((0.2) (HEH[EHP]) and 1.8 ((0.2) (HDEHP) over the [lactate]tot range of 0.5 to 2.0 M at pH = 3.5. The distribution of Naþ into the extractant phase increases with increasing pH in the HDEHP system (Figure 7). In the HEH[EHP]/n-dodecane system, pH has no effect on the partitioning of Naþ in the pH 2.54.5 range. In fact, Naþ partitioning is (within the accuracy of the measurements) identical to that of the pure diluent (as represented by kerosene).2830

þ M3þ aq þ 3ðHAÞ2org / MðAHAÞ3org þ 3Haq

ð3Þ

with the phase transfer equilibrium constant: Kex ¼

½MðAHAÞ3 org ½Hþ 3aq ½M3þ aq ½ðHAÞ2 3org

ð4Þ

where (HA)2 is the normal hydrogen bonded dimer that typically dominates the speciation of lipophilic carboxylic and organophosphorus acids in organic solvents.3235 The partitioning of the metal ion is thus dependent on both pH and the concentration of the extractant. Balanced against this in the TALSPEAK aqueous phase is the dominating formation of aminopolycarboxylate complexes. The aqueous phase metal complexation equilibria under most conditions are straightforward, involving the formation of 1:1 (ML) complexes, which can under some conditions be protonated (MHL).24 This process is also pH dependent by virtue of the protonation equilibria of the free ligand Ln. As has been discussed in detail in prior publications on TALSPEAK,18,19 the predominant metal complexation equilibrium for DTPA in the pH 3.53.6 range (the baseline for normal TALSPEAK

’ DISCUSSION Basic Operation and Modeling of the TALSPEAK Process. As noted above, the fundamental chemistry of the TALSPEAK process can be characterized as a “tug-of-war” between the 633



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between La3þ and Eu3þ that is seen for the DTPA complexes. If the reported stability constant for Am3þ with TTHA (log KAm = 26.624) is correct, the pattern indicates that TTHA should be an efficient competitor for HDEHP under the right circumstances and could in principle provide an improved lanthanide/americium separation relative to DTPA. However, the comparative increase in strength of the light lanthanide complexes with TTHA predicts a poorer separation of Am3þ from La3þ, Ce3þ, and Pr3þ than is seen with DTPA, as is observed in Figure 2a. Since the potentially nona- or decadentate TTHA could completely saturate the coordination sphere of a lanthanide with carboxylate and amine donor groups, examining the kinetic behavior of TTHA could also advance the understanding of kinetics issues that have been observed for the heavier lanthanides in conventional TALSPEAK.18,3638 The separation efficiency of two phosphonic acid extractants, 2-ethyl(hexyl) (phenyl)phosphonic acid (HEH[jP]) and decyl(decyl)phosphonic acid HD[DP], was studied in the initial Oak Ridge TALSPEAK report.14 Separation factors for HEH[jP] were comparable to those seen for HDEHP, though the phosphonic acid gave considerably higher distribution ratios than HDEHP. Results for HD[DP] indicated slightly reduced separation factors and much lower distribution ratios than those of HEH[jP]. These two phosphonic acid demonstrations are the only examples from the prior literature in which a phosphonic extractant has been used to perform TALSPEAK-type chemistry. The application of HEH[EHP] for TALSPEAK separations is of interest for several fundamental and practical reasons. First, HEH[EHP] is structurally related to HDEHP; therefore, differences in metal partitioning can be explained primarily by the difference in the basicity of the extractants rather than alteration of the structure or solubility of organic phase metal complexes. The 2-ethyl(hexyl) groups of HEH[EHP] are attached via one POC ester linkage and one PC bond. The PC bond accounts for the greater basicity of HEH[EHP] but perhaps more importantly should exhibit improved resistance to radiolytic and hydrolytic attack of HEH[EHP] relative to HDEHP. It is expected that the phosphonic acid should be more robust in application to actual dissolved nuclear fuel solutions. trans-Lanthanide Partitioning. trans-Lanthanide trends for polyaminopolycarboxylate ligands different from DTPA in TALSPEAK-like solvent extraction systems have not been reported previously. In Figure 2, the experimental lanthanide distribution patterns that arise when TTHA is paired with HDEHP or HEDTA with HEH[EHP] have been shown in comparison with the equivalent pattern for conventional TALSPEAK. On the basis of Weaver’s reported ineffectiveness of HEDTA as a holdback reagent in competition with HDEHP, all of the HEDTA experiments in this study were conducted with the less acidic extractant HEH[EHP], as a better balance of aqueous/organic complex strength was expected. Even though Weaver had observed TTHA to be an inadequate competitor for HDEHP, the thermodynamic literature24 suggests that TTHA should be capable of retaining the actinides in the aqueous phase; thus TTHA was used with HDEHP. It is important to attain a balance between the extractant and holdback reagents in TALSPEAK. If the holdback reagent is too strong relative to the extractant, no metal ions will be extracted, and there will be no group separation. Likewise, if the extractant is too strong, all metal ions will be extracted, and there will be no productive separation. The results in Figure 2a indicate that group separation is possible for the TTHA-based conventional TALSPEAK. However, the

operations) is M3þ þ H3 L2 / ML2 þ 3Hþ

ð5Þ

with the corresponding equilibrium constant expression K1 ¼

½ML2 ½Hþ 3 ½M3þ aq ½H3 L2

ð6Þ

Different complexation equilibria will be important in the aqueous phase for other aminopolycarboxylate complexants, and under some circumstances, it may not be permissible to ignore the competing complexation by the lactate ion. This basic approach can be used to characterize and predict any variant of TALSPEAK. The aqueous (eq 5) and organic phase (eq 3) reactions can now be combined to develop the expression that defines the competition between DTPA and HDEHP in conventional TALSPEAK at a baseline pH of 3.6: 2 ML2 aq þ 3ðHAÞ2org / MðAHAÞ3org þ H3 Laq

ð7Þ

which can be readily recognized as a reaction that should have no pH dependence while H3L2 is the dominant free ligand species of DTPA. In principal, the lactate buffer should help to control small deviations in pH. Species calculations reported previously establish that H3L2 is the dominant protonated DTPA species between pH 3 and 5. This model predicts that conventional TALSPEAK should be relatively immune to pH variation between pH 3 and 5. It can be seen in Figure 3 that while the HEH[EHP] variant of TALSPEAK exhibits minimal pH dependence, there is a pronounced deviation in the HDEHP system above pH 3.5. Ongoing work seeks to establish an explanation for this deviation in conventional TALSPEAK. Previous studies have noted that this equilibrium model does indeed predict the observed translanthanide extraction pattern for conventional TALSPEAK (at pH 3.6) shown in Figure 3a; hence under standard TALSPEAK conditions, the thermodynamic model has some predictive value.18 Previous Investigations of Phosphonic Acids, TTHA, and HEDTA for TALSPEAK. The original Oak Ridge report provided a brief examination of TTHA and HEDTA as holdback reagents for the TALSPEAK process.15 In that report, it was indicated that TTHA and HEDTA were too weak to compete with HDEHP for Am3þ. The original report did observe that insertion of HEDTA in place of DTPA as the holdback reagent in conventional TALSPEAK provided cerium/americium separation factors of about 20. Each of these original TTHA and HEDTA studies was conducted in sodium glycolate media. The poor competition of HEDTA and TTHA with HDEHP discouraged further investigation. To predict their separation performance in TALSPEAK applications, the trans-lanthanide patterns of lanthanide and Am3þ stability constants for the log β101 metal complexes with DTPA, TTHA, and HEDTA were examined.24 The pattern for DTPA complexes includes a steady increase in stability from La3þ through Eu3þ with little further variation from Eu3þ through Lu3þ; a similar pattern is seen for HEDTA, though the equilibrium constants are lower than those for DTPA and the change from La3þ to Eu3þ less significant. TTHA complexes for the lanthanides increase only slightly from La3þ through Lu3þ; thus they do not demonstrate the relative increase in stability 634



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efficiency of the separation of Am3þ from the lightest lanthanides is lower than is seen in the conventional HDEHP-DTPA system, as was predicted from the trans-lanthanide pattern of TTHA stability constants. As the light lanthanides represent the majority of lanthanide waste in used nuclear fuel, this result could indicate that TTHA is a less suitable holdback reagent than DTPA in conventional TALSPEAK. Of course, the studies of TTHA-based TALSPEAK conducted herein have not been exhaustive and should not be considered definitive. It is also conceivable that mixtures of aminopolycarboxylate molecules might provide greater efficiency, though at the cost of increased complexity. The HEDTA/HEH[EHP] system (Figure 2b) shows lanthanide extraction trends comparable in some respects to conventional TALSPEAK. As is the case in TTHA-based conventional TALSPEAK, HEDTA appears to compete more successfully with HEH[EHP] for the light lanthanides than is seen in conventional TALSPEAK, resulting in lower distribution ratios and reduced separation from Am3þ relative to conventional TALSPEAK. It is seen in this system that the trans-lanthanide extraction pattern is flatter than is seen for conventional TALSPEAK. Depending on the concentration of lactate, La3þ/Am3þ (the least separated pair) separation factors range from 10 to 40 in the example shown in Figure 2b. For Eu3þ and Gd3þ (fission product lanthanides with high neutron capture cross-sections), the separation factor is higher with HEH[EHP]/HEDTA than is seen in conventional TALSPEAK (Figure 2a), ranging between 50 and 100 depending on the concentration of lactate used. In marked contrast to conventional TALSPEAK, heavy lanthanide extraction is rapid in this system even at low total lactate concentrations. In the prior TALSPEAK literature, it has been noted that total lactate concentration must be at least 0.5 M to maintain fast extraction of the heavier lanthanides.3638 The required high lactate concentration is believed to contribute significantly to the complexity of conventional TALSPEAK. It can be inferred from the results in Figure 2b that the group separation factors are slightly higher at 0.1 M lactate than at 1.0 M in the HEDTA/HEH[EHP] version of TALSPEAK. Metal Partitioning in TALSPEAK as a Function of pH. As has been noted repeatedly above, previous work has shown that speciation calculations based on thermodynamic data (aqueous and biphasic) from the literature do not describe the observed pattern of pH dependence in conventional TALSPEAK separations.19 The thermodynamic modeling calculations used to project this discrepancy assume the predominance of lanthanide extraction as the Ln(AHA)3 complex throughout the pH range and independent of lactate concentration.3235 Weaver and Kappelmann have indicated that an alternate metal ion extraction pathway exists at high lactate and high metal concentrations in the system, in effect, the extraction of lanthanide complexes as mixed lactate/HDEHP species (e.g., Ln(Lac)(AHA)2).39 Kosyakov and Yerin have postulated similar species in radiotracer studies of curium and samarium extraction in conventional TALSPEAK.35 In both cases, the species proposed is based on the results of slope analysis. Such a species could account for reduced extraction at the high end of the TALSPEAK pH range. An extraction process involving lactate would likely only be important if considerable partitioning of lactate into the organic phase was occurring. The results in Figures 35 show the pH profiles for Eu and Am for several variations of the TALSPEAK process examined in this investigation. Following from the analysis of complexation equilibria in eqs 37 above, it is appropriate to consider the

observed pH trends in terms of the metal ion distribution ratios. Assuming that the equilibria defined above are applicable in all systems, the theoretical distribution ratios shown in Figures 35 have been calculated on the basis of the following model expression: D¼

½MðAHAÞ3 ½M3þ þ ½ML2þ þ ½ML2 þ þ ½ML3 þ ½MR 2 þ ½MHR

ð8Þ where L is lactate, R is DTPA, and AHA is the singly ionized conventional hydrogen bonded dimer of HDEHP (H(DEHP)2). A detailed discussion of this equation and the equilibrium constants necessary for application have been reported previously.18,19 Equilibrium constants applied in the calculations were taken from the NIST database24 and, for the extraction equilibrium, from Stary26 or Kubota et al.25 For the HEDTA calculations, the MHR species was omitted from modeling calculations, as no equilibrium constant describing such a species is reported in the NIST database. Several hypotheses have been offered in the literature to account for the mismatch observed between the model and experimental results in conventional TALSPEAK. One possible option suggested was the undocumented presence of aqueous phase competition reactions, for example, Ln(Lac)4 species or mixed Ln-DTPA-Lac complexes.19 Jensen and co-workers have reported the results of spectrophotometric studies that appear to confirm the absence of either ternary Ln-DTPA-Lac or Ln(Lac)4 species in TALSPEAK aqueous phases.40 Alternatively, the discrepancy could arise from complications in the organic phase of TALSPEAK. The presence of mixed Ln(Lac)n(AHA)3n extracted species (as suggested by Kosyakov and Yerin35 and Weaver and Kappelmann39), dramatic changes in the activity of the organic phase brought on by increased partitioning of lactic acid, or the increase of Naþ concentration in the organic phase at higher pH (forming the complex Na(DEHP)(HDEHP)32830) have all been suggested. The contrast between HDEHP and HEH[EHP] observations in Figure 3 (relative to theoretical calculations) imply that uncharted interactions of HDEHP with other ions or molecules in the organic phase may be largely responsible for the mismatch between predicted performance and observed behavior. When HEH[EHP] is substituted for HDEHP in conventional TALSPEAK (Figure 4), the high pH deviation from model predictions disappears. The distribution ratios of La3þ, Eu3þ, Dy3þ, Lu3þ, and Y3þ calculated on the basis of the thermodynamic data for the DTPA/HEH[EHP] system (as a function of pH) show the best agreement with the experimental observations for the lightest lanthanides. The agreement between experimental and calculated values is poorer for the middle to heavier lanthanides. In contrast to model calculations on conventional TALSPEAK, the model tends to underestimate metal extraction, though it predicts the correct pH profile of the lanthanides studied. It is noteworthy that the distribution ratios for the lanthanides in this system are at least 102 lower than in conventional TALSPEAK, implying that DTPA is too strong for this coupling. When HEH[EHP] is instead matched with the pentadentate HEDTA holdback reagent, Eu3þ distribution ratios greater than 1 are achievable without apparently compromising the integrity of the organic phase (Figure 5). An upward trend is indicated in both experimental data and predicted performance. The pH 635



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dependence of the experimental extraction results is in general agreement with calculations of the model, but the difference between the experimental and model-prediction values increases with increasing pH. It is premature to speculate on the origins of this difference, though an obvious suggestion is the possible presence of ternary complexes (e.g., Ln(Lac)(HEDTA)). The literature notes several examples of ternary Ln(X)(HEDTA)/2 complexes, where X is a carboxylic acid or an aminopolycarboxylate, but the HEDTA/lactate example is not specifically observed.41,42 As HEDTA is only pentadentate, the formation of ternary complexes will be more probable in the lanthanide HEDTA complexes than would be expected for the octadentate DTPA. Further examination of this possibility is warranted. Organic Phase Composition. The isostructural nature of the HDEHP and HEH[EHP] extractant molecules tends to implicate the difference in basicity of the extractants in the significant pH dependence deviation that is seen for HDEHP but not HEH[EHP]. It can be noted from the prior literature that HEH[EHP] has a lower tendency toward dimerization and higher pKa (log KD = 3.37; pKa = 4.51) than HDEHP (log KD = 3.68; pKa = 3.47).43 The lanthanide extraction trends are similar for the two extractants but displaced toward higher pH for HEH[EHP]. Previous work has shown that there is a notable tendency of lactate, sodium, and water to partition into the HDEHP/1,4 diisopropylbenzene organic phase when lactate is present.20 One possible explanation for the more predictable behavior of the HEH[EHP] system relative to HDEHP is that the higher basicity and lowered dimerization tendency might lead to reduced partitioning of lactic acid, sodium ions, and water into the organic phase for HEH[EHP]. The partitioning of lactate and water (as a function of the total lactate concentration) is seen in Figure 6 to be higher into HDEHP/n-dodecane than into HEH[EHP]/n-dodecane. It was noted above that water and lactate partitioning into 0.1 M HEH[EHP] in n-dodecane is (within error) comparable to or less than the Kd value for water or lactic acid into kerosene.27 This is significantly less than the water and lactate partitioning into HDEHP/1,4-diisopropylbenzene that was reported previously.20 As lactic acid partitioning is 3 times lower in the HEH[EHP] system and the water content of the HEH[EHP] is constant and about 100 times lower, it is tempting to postulate that these differences may account for the relative accuracy of thermodynamic modeling in the two systems. Sodium ion partitioning into HDEHP is seen in Figure 7 to increase with rising pH, while the corresponding plot for HEH[EHP] shows no correlation between sodium extraction and solution pH. Literature reports indicate that log Kex for Naþ extraction by HDEHP is 3.56 ( 0.08 from 1 M NaClO4 and for HEH[EHP] is 5.46 ( 0.09 from 0.5 M NaNO3, both as in eq 9.2830 Literature reports indicate that the dominant phase transfer reaction for Naþ in these systems is þ Naþ aq þ 2ðHAÞ2 org / NaA 3 3ðHAÞorg þ Haq

ð9Þ

Using eq 9 to calculate the organic phase concentration of sodium for extraction from simple salt media (under the conditions of these experiments, 1 M NO3, 0.1 M HDEHP, 20 mM DTPA), the predicted concentration of the extracted [NaA(HA)3] species ranges from 0.46 to 39.4 mM as the pH is increased from 2.7 to 4.4. The experimentally measured distribution ratio of Naþ is 84 mM at a pH of 4.4. As this value

is twice that predicted, another process must contribute to Naþ extraction when lactate is present. Since there appears to be no correlation between Naþ extraction and [lactate]tot, it is likely that whatever process contributes, it does not directly involve lactate. The marked contrast in pH dependence for metal ion extraction between the HDEHP and HEH[EHP] systems is noteworthy in this regard. Neutron, X-ray scattering, and complementary distribution/spectroscopic experiments have been completed to address possible aggregation phenomena in these systems; those results are being analyzed and will be the subject of future publications.

’ CONCLUSIONS The conventional TALSPEAK process for separation of lanthanides from trivalent actinides balances the strong liquid cation exchanging extractant molecule HDEHP against the water-soluble aminopolycarboxylate complexant DTPA in a concentrated lactic acid buffer. The high lactate concentration is necessary to allow acceptable phase transfer kinetics. To address the inherent complexity of this system (demonstrated in earlier and ongoing fundamental studies), the impact of altering the relative strength of the aminopolycarboxylate holdback reagent and of the extractant has been investigated. These studies have revealed that substituting the higher denticity TTHA for DTPA in conventional TALSPEAK (based on HDEHP) results in slightly improved Am/Ln separation factors for heavy lanthanides but lowered group separation efficiency for the more important light lanthanides. Substitution of the phosphonic acid extractant HEH[EHP] for HDEHP results in lower distribution ratios overall in the conventional TALSPEAK pH buffer range, probably indicating that DTPA is too strong to be matched against HEH[EHP]. Substitution of HEH[EHP] for HDEHP nearly eliminates the substantial pH dependence of conventional TALSPEAK extraction in favor of an almost flat pH profile. The pH dependence of the HEH[EHP]-DTPA system is predicted more reliably by thermodynamic data than has been seen for the HDEHP-DTPA system. The final adjustment of substituting the weaker holdback reagent HEDTA for DTPA in the HEH[EHP] system results in further improvements in predictability of the process performance based on available thermodynamic data, a flat dependence of lanthanide extraction on equilibrium pH, sufficiently improved phase transfer kinetics to contemplate a significant reduction in the concentration of lactate buffer required, dramatically reduced partitioning of both water and lactic acid into the HEH[EHP] extractant phase (relative to HDEHP), and a nearly flat translanthanide extraction trend that is approximately consistent with predictions made using thermodynamic data from the existing literature. The combined results of these investigations indicate that a considerable potential for improvement of TALSPEAK-type separations is achievable by better matching of extractant and holdback reagent complex strength while reducing the acidity of the organophosphorus extractant. The application of a holdback reagent with lower denticity appears also to offer improvement in phase transfer kinetics. These results suggest that the greater extraction strength of HDEHP (resulting from its higher acidity) may actually be detrimental to the performance potential of TALSPEAK separations. Further investigation of the HEH[EHP]/HEDTA system is recommended. 636



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’ AUTHOR INFORMATION

(14) Weaver, B.; Kappelmann, F. A. TALSPEAK, A New Method of Separating Americium and Curium from the Lanthanides by Extraction from an Aqueous Solution of an Aminopolyacetic Acid Complex with a Monoacetic Organophosphate or Phosphonate; ORNL-3559, Oak Ridge National Laboratory: Oak Ridge, TN, 1964. (15) Tachimori, S.; Nakamura, H. Radiation effects on the separation of lanthanides and transplutonides by the TALSPEAK-type extraction. J. Rad. Nucl. Chem. 1979, 52, 343–354. (16) Martin, L. R.; Mincher, B. J.; Mezyk, S. P.; Elias, G.; Tillotson, R. D. Effects of Aqueous Phase Radiolysis on Lactic Acid Under TALSPEAK Conditions. ACS Symposium Series; American Chemical Society: Washington, DC, 2010; Vol 1046, Chapter 20, pp 243253. (17) Martin, L. R.; Mezyk, S. P.; Mincher, B. J. Determination of Arrhenius and Thermodynamic Parameters for the Aqueous Reaction of the Hydroxyl Radical with Lactic Acid. J. Phys. Chem. A 2009, 113, 141–145. (18) Nilsson, M.; Nash, K. L. Review article: A review of the development and operational characteristics of the TALSPEAK process. Solvent Extr. Ion Exch. 2007, 25, 665–710. (19) 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. (20) Grimes, T. S.; Nilsson, M.; Nash, K. L. Lactic Acid Partitioning in TALSPEAK Extraction Systems. Sep. Sci. Technol. 2010, 45, 1725–1732. (21) Partridge, J. A.; Jensen, R. C. Purification of Bis(2-ethylhexyl) Hydrogen phosphate by precipitation of copper(II) bis(2-ethylhexyl) phosphate. J. Inorg. Nucl. Chem. 1969, 31, 2547–2589. (22) Hu, Z.; Pan, Y.; Ma, W.; Fu, X. Purification of organophosphorus acid extractants. Solvent Extr. Ion Exch. 1995, 13, 965–976. (23) Carothers, W. H.; Van Natta, F. J. Studies on polymerization and ring formation. III. Glycol esters of carbonic acid. J. Am. Chem. Soc. 1930, 52, 314–326. (24) Martell, A. E.; Smith, R. M. NIST Critically Selected Stability Constants of Metal Complexes Database, version 8.0; National Institute of Standards and Technology: Gaithersburg, MD, 2004. (25) Kubota, F.; Goto, M.; Nakashio, F. Extraction of Rare Earth Metals with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl Ester in the Presence of Diethylenetriaminepentaacetic Acid in Aqueous Phase. Solvent Extr. Ion Exch. 1993, 11, 437–453. (26) Stary, J. Separation of the transplutonium elements. Talanta 1966, 13 (3), 421–437. (27) Groschuff, E. The solubility of water in benzole, petroleum, paraffin. Z. Elecktrochem. 1911, 17, 348–354. (28) McDowell, W. J.; Coleman, C. P. Sodium and strontium extraction by di(2-ethylhexyl) phosphate: mechanism and equilibria. J. Inorg. Nucl. Chem. 1965, 27, 1117–1139. (29) Gao, Z.; Sun, S.; Kong, W.; Wang, B.; Shen, J. Study on the Solvent Extraction of Sodium with HDEHP; Shandong University: Shandong, P.R. China, 1983; Vol. 4, p 80. (30) Komasawa, I.; Otake, T.; Higaki, Y. Equilibrium studies of the extraction of divalent metals from nitrate media with Di-(2-ethylhexyl) Phosphoric Acid. J. Inorg. Nucl. Chem. 1981, 43, 3351–3356. (31) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Crystallogr. 1976, A32, 751–767. (32) Peppard, D. F.; Mason, G. W.; Maier, J. L.; Driscoll, W. J. Fractional extraction of the lanthanides as their di-alkyl orthophosphates. J. Inorg. Nucl. Chem. 1957, 4, 334–343. (33) Svantesson, I.; Persson, G.; Hagstr€om, I.; Liljenzin, J. O. Distribution ration and empirical equations for the extraction of elements in PUREX high level waste solution II: HDEHP. J. Inorg. Nucl. Chem. 1980, 42, 1037–1043. (34) Kosyakov, V. N.; Yerin, E. A. Separation of transplutonium and rare-earth elements by extraction with HDEHP from DTPA solutions. J. Radioanal. Chem. 1978, 43, 37–51. (35) Kosyakov, V. N.; Yerin, E. A. On the mechanism of trivalent actinide extraction in the system HDEHP—lactic acid with DTPA. J. Radioanal. Chem. 1980, 56, 93–104.

Corresponding Author

*E-mail: [email protected]. Present Addresses †

Pacific Northwest National Laboratory, Richland, Washington, United States

’ ACKNOWLEDGMENT Work supported at Washington State University by the U.S. Department of Energy, Office of Nuclear Energy Science and Technology, Nuclear Energy Research InitiativeConsortium (NERI-C), contract number DE-FC07-02ID14896, and the Fuel Cycle Research and Development (FCRD) Program, Minor Actinide Separations Sigma Team ’ REFERENCES (1) Nash, K. L.; Madic, C.; Mathur, J. N.; Lacquement, J. Actinide Separation Science and Technology. In The Chemistry of Actinides and Transactinide elements; Morss, L. R., Katz, J. J., Edelstein, N., Fuger, J., Eds.; Springer: Dordrecht, The Netherlands. 2006; Vol 4, Chapter 24, pp 26222798. (2) Aspinall, H. Chemistry of the f-Block Elements, 5th ed; Gordon & Breach: Australia, 2001. (3) Diamond, R. M.; Street, K. J.; Seaborg, G. T. An Ion Exchange Indication of Possible 5f Hybridized Bonding in the Actinides. J. Am. Chem. Soc. 1954, 76, 1461–1469. (4) Jensen, M. P.; Bond, A. H. Influence of aggregation on the extraction of trivalent lanthanide and actinide cations by purified Cyanex 272, Cyanex 301, and Cyanex 302. Radiochim. Acta 2002, 90, 205–209. (5) Jensen, M. P.; Bond, A. H. Comparison of covalency in the complexes of trivalent actinide and lanthanide cations. J. Am. Chem. Soc. 2002, 124, 9870–9877. (6) Kozimor, S. A.; Yang, P.; Batista, E. R.; Boland, K. S.; Burns, C. J.; Clark, D. L.; Conradson, S. D.; Martin Wilkerson, M. P.; Wolfsberg, L. E. Trends in covalency for d- and f-Element metallocene dichlorides identified using chlorine K-edge x-ray absorption spectroscopy and time-dependent density functional theory. J. Am. Chem. Soc. 2009, 131, 12125–12136. (7) Choppin, G. R.; Nash, K. L. Actinide separation science. Radiochim. Acta 1995, 70/71, 225–236. (8) Ekberg, C.; Fermvik, A.; Retegan, T.; Skarnemark, G.; Foreman, M. R. S.; Hudson, M. J.; Englund, S.; Nilsson, M. An overview and historical look back at the solvent extraction using nitrogen donor ligands to extract and separate An(III) from Ln(III). Radiochim. Acta 2008, 96, 225–233. (9) Mathur, J. N.; Murali, M. S.; Nash, K. L. Actinide Partitioning A Review. Solvent Extr. Ion Exch. 2001, 19, 357–390. (10) Modolo, G.; Kluxen, P.; Geist, A. Demonstration of the LUCA process for the separation of americium(III) from curium(III), californium(III), and lanthanides(III) in acidic solution using a synergistic mixture of bis(chlorophenyl)dithiophosphinic acid and tris(2ethylhexyl)phosphate. Radiochim. Acta 2010, 98, 193–201. (11) Peterman, D. R.; Martin, L. R.; Klaehn, J. R.; Harrup, M. K.; Greenhalgh, M. R.; Luther, T. A. Selective separation of minor actinides and lanthanides using aromatic dithiophosphinic and phosphinic acid derivatives. J. Rad. Nucl. Chem. 2009, 282, 527–531. (12) Hill, C.; Guillaneux, D.; Berthon, L.; Madic, C. Sanex-BTP process development studies. J. Nucl. Sci. Technol. 2002, No. Suppl. 3, 453–461. (13) Mincher, B. J.; Modolo, G.; Mezyk, S. P. Review article: The effects of radiation chemistry on solvent extraction: 1. Conditions in acidic solution and a review of TBP radiolysis. Solvent Extr. Ion Exch. 2009, 27, 1–25. 637



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(36) Kolarik, Z.; Koch, G.; Kuhn, W. Acidic organophosphorus extractants. XVIII. Rate of lanthanide (III) extraction by bis(2ethylhexyl) phosphoric acid from complexing media. J. Inorg. Nucl. Chem. 1974, 36, 905–909. (37) Danesi, P. R.; Cianetti, C.; Horwitz, E. P. Distribution equilibriums of europium(III) in the system: bis(2-ethylhexyl)phosphoric acid, organic diluent-sodium chloride, lactic acid, polyaminocarboxylic acid, water. Sep. Sci. Technol. 1982, 17, 507–519. (38) Nilsson, M.; Heydon, A.; Nash, K. L. Studies of the lactic acid concentration on extraction equilibria and kinetics in TALSPEAK chemistry. Manuscript in preparation. (39) 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. (40) Leggett, C. J.; Liu, G.; Jensen, M. P. Do Aqueous Ternary Complexes Influence the TALSPEAK Process? Solvent Extr Ion Exch. 2010, 28, 313–334. (41) Thompson, L. C.; Loraas, J. A. Complexes of the Rare Earths. III. Mixed Complexes with N-Hydroxyethylethylenediaminetriacetic Acid. Inorg. Chem. 1963, 2, 89–93. (42) Choppin, G. R.; Thakur, P.; Mathur, J. N. Complexation thermodynamics and structural aspects of actinideaminopolycarboxylates. Coord. Chem. Rev. 2006, 78, 936–947. (43) Miralles, N.; Sastre, A.; Martinez, M.; Aguilar, M. The aggregation of organophosphorus acid compounds in toluene. Anal. Sci. 1992, 8, 773–777.

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