Article pubs.acs.org/IECR
Study of the Interaction between HDEHP and CMPO and Its Effect on the Extraction of Selected Lanthanides Peter Tkac,†,* George F. Vandegrift,† Gregg J. Lumetta,‡ and Artem V. Gelis† †
Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois, United States Radiochemical Sciences and Engineering Group, Pacific Northwest National Laboratory, Richland, Washington, United States
‡
ABSTRACT: Separation of the trivalent actinides from the trivalent lanthanides relevant to used nuclear fuel reprocessing remains still a challenging task. One of the options currently being investigated is the combination of two extraction processes, TRUEX and TALSPEAK. However, the two extractants used in the individual processes, when combined, result in a system with complex extraction behavior. To better understand the interactions in this combined extraction system, FT-IR spectroscopy was used in combination with small-scale solvent extraction tests. The data indicate that in the presence of CMPO, HDEHP dimer cleaves and interacts with CMPO through hydrogen bonding between POH and phosphoryl group of CMPO. The formation of a new HDEHP-CMPO adduct (log β = 3.4) at [HDEHP] > [CMPO] significantly lowers the concentration of free CMPO available for complexation with metal. The distribution ratios of Eu(III) decrease significantly with increasing nitric acid concentration and point to the acidic properties of this mixed extractant. The data presented suggest that several species containing HDEHP-CMPO adduct, HDEHP, and nitrate are formed upon extraction of Eu(III) by the mixture of HDEHP and CMPO.
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INTRODUCTION There is a need for developing a method to separate the trivalent actinides (An3+, such as Am3+ and Cm3+) from the trivalent lanthanides (Ln3+) for transmutation of long-lived actinides through reprocessing of used nuclear fuel. Several extraction processes have been developed in the past for this separation. However, due to the difficulty of the separation, a series of solvent extraction steps was required. The Fuel Cycle Research and Development (FCR&D) Program under US Department of Energy’s Office of Nuclear Energy, Science and Technology established the Minor Actinide Separation Sigma Team to investigate and develop cost-effective methods for separation of An3+ from Ln3+. One of the options investigated under this effort is the combining of the neutral extractant CMPO [octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide] used in the transuranic extraction (TRUEX) process and the acidic extractant, HDEHP [bis-(2-ethylhexyl)phosphoric acid] (Scheme 1), used in the trivalent actinide lanthanide separation with phosphorus-reagent extraction from aqueous complexes (TALSPEAK) process, to form a single process for separating An from Ln. The TALSPEAK process, developed in 1964 by Weaver and Kappelmann,1 uses a solvent composed of HDEHP in n-dodecane (DDC) to separate trivalent lanthanides from trivalent actinides in aqueous solutions. For a comprehensive review of the TALSPEAK process, the reader is referred to the work of Nilsson and Nash.2 In this process, the important factors that allow separation are a slight preference of HDEHP for the smaller lanthanides over the larger actinides, and a small difference in Lewis acidity between An3+ and Ln3+. Because lanthanides act as harder Lewis acids compared to the trivalent actinides, a soft Lewis base such as DTPA (diethylene triamine pentaacetic acid), introduced as an aqueous complexant, will preferentially bond An3+ and prevent extraction by HDEHP. The extraction © 2012 American Chemical Society
Scheme 1. (a) HDEHP Dimer: Bis-(2-ethylhexyl)phosphoric Acid (b) CMPO: Octyl(phenyl)-N,Ndiisobutylcarbamoylmethylphosphine Oxide
of trivalent lanthanides by HDEHP was proposed by Peppard et al.,3 and a general equation describing the mechanism where metal (M 3+) is coordinated to three HDEHP dimers (HDEHP)2 can be represented by eq 1: M3 + + 3(HL)2 ⇆ M· (HL 2)3 + 3H+
(C8H17O)2PO2−,
(1) 3+
where L represents the anion and M , the trivalent metal. The TALSPEAK process originally proposed as Received: Revised: Accepted: Published: 10433
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a part of the UREX+ suite was successfully tested4 at Argonne National Laboratory; however, the complex chemistry and extraction and stripping mechanism in this extraction system is still not fully understood. The TRUEX process was invented by Horwitz et al.5 and uses CMPO (Scheme 1) as an extractant and tri-n-butyl phosphate (TBP) as a phase modifier diluted in n-dodecane. The TRUEX process was designed to selectively extract trivalent actinides and lanthanides from the transition metals in nitric acid media. The general equation describing the extraction of trivalent metal by CMPO was proposed by Horwitz et al.6 and can be represented by eq 2: M3 + + 3NO3− + 3CMPO ⇆ M(NO3)3 ·3CMPO
Article
EXPERIMENTAL SECTION
Reagents. Before its use, CMPO was purified by the method reported by Horwitz et al.12 with slight modification.13 HDEHP was purified according to Partridge and Jensen.14 Several stock solutions of 0.3 and 0.5 mol L−1 HDEHP, 0.2 mol L−1 CMPO, 0.5 mol L−1 CMPO + 0.3 mol L−1 HDEHP, and 0.5 mol L−1 HDEHP + 0.2 mol L−1 CMPO in n-dodecane were prepared in volumetric flasks. Solutions with various ratios of CMPO and HDEHP were then prepared by combining the stock solutions using pipettes. La(NO3)3·6H2O from Aldrich was used to prepare a stock solution of La3+. Eu-152 was obtained from Eckert and Ziegler in 0.5 mol L−1 HCl. A small aliquot of Eu-152 was diluted with nitric acid to form a stock solution in ∼0.1 mol L−1 HNO3. All other chemical reagents used in this work were of analytical-reagent-grade purity and were used without further purification. All aqueous solutions were prepared with deionized water with a resistivity ≥18 MΩ•cm. Solvent Extraction. Extraction experiments were performed with various mixtures of CMPO and HDEHP diluted with n-dodecane, unless otherwise noted. Prior to extraction, the organic phase was pre-equilibrated with an aqueous phase containing HNO3 and NaNO3 having the same concentration as the final aqueous phase for which the distribution ratios were measured. However, it should be noted that for extractions with solutions containing high metal concentrations (∼0.05 mol L−1), when HDEHP is coordinated with the metal, a significant amount of H+ is displaced by the metal, leading to an increase in the acidity of the aqueous phase. The initial concentration of La(III) in the aqueous phase was ∼0.05 mol L−1. Extraction experiments with 0.05 mol L−1 Eu(III) were performed using Eu(NO3)3 with Eu-152 as a radioactive tracer, or at trace Eu(III) concentration using only Eu-152. All extraction experiments were performed by vigorous mixing (vortex mixer) for 4 min at the 1:1 aqueous to organic volumetric ratio at ambient temperature conditions (22 ± 2 °C). The typical volumes of the organic and aqueous phases for the extraction were in the range of 0.2−1 mL. If Eu-152 was used, 10 μL of the stock solution in 0.1 mol L−1 HNO3 was added to the aqueous phase prior to extraction. After agitation, phases were separated by centrifugation, and aliquots from the aqueous phase were taken for determination of concentration of La using ICP-MS. The data obtained by ICP-MS are reported with 10% uncertainty. The concentration of La extracted into organic phase was calculated as the difference between the aqueous concentration of La before and after extraction. The distribution ratio of La was calculated as the ratio of the total equilibrium analytical concentrations of metal in the organic to that in the aqueous phase. The activity of Eu152 in both phases was analyzed using liquid scintillation counting of 100−200 μL aliquots of the aqueous and organic phases after addition of 15 mL of Ultima Gold liquid scintillation cocktail. Determination of Organic Nitrate Concentration. A 0.5 mL aliquot of the organic phase, after equilibration with an aqueous phase containing 0.05 mol L−1 La(III), was vigorously mixed with 2 mL of 1.0 mol L−1 sodium citrate and 0.05 mol L−1 DTPA (pH = 5.9) for 3 min using a vortex mixer. Then, the phases were separated by centrifugation, and the organic phase was washed with 8 mL of deionized water and then combined with the citrate fraction. It was experimentally determined that the combination of 1.0 mol L−1 sodium citrate
(2)
In this work, we investigated the interaction between HDEHP, CMPO, and Ln(III) species that are being extracted by this mixture. Originally, it was expected that by combining these two extractants the extraction chemistry of CMPO would dominate under conditions of higher nitric acid concentrations (≥0.5 mol L−1 HNO3), resulting in coextraction of the transplutonium and lanthanide elements into the organic phase. On the other hand, the extraction properties of HDEHP would prevail at higher pH and could therefore be utilized to strip the solvent with DTPA in the presence of lactate or citrate.4,7−9 The concept, using 0.1 mol L−1 CMPO + 1 mol L−1 HDEHP in n-dodecane, has been recently intensively tested7,9 confirming that the distribution ratios for Am(III) and Eu(III) from 1 mol L−1 HNO3 are comparable to those obtained by the TRUEX process (0.2 mol L−1 CMPO in 1.4 mol L−1 TBP/ndodecane) and are much higher than those achieved with 1 mol L−1 HDEHP. The separation factors for Ln/An during the actinide stripping step obtained using 0.05 mol L−1 DTPA at pH = 3−4 were higher when 1.5 mol L−1 citrate was used (SFEu/Am = 18) compared with the lactate system (SFEu/Am = 15). The separation factor (SFEu/Am) is defined as the distribution ratio of Eu divided by that for Am; a high SFEu/Am in a stripping operation means that Am can be stripped from the solvent far more readily than Eu. Samarium represents the worst case in terms of the Ln(III)/An(III) separation factor with SFSm/Am = 12 for citrate and 10 for lactate.10 However, addition of CMPO to HDEHP increases the extractability of Am, compared to the extraction from 1 mol L−1 HDEHP and complicates the separation from Ln during stripping. Without the addition of CMPO, the SFEu/Am would be ∼100; as the concentration of CMPO increases, the SFEu/Am decreases significantly. Moreover, Dhami et al.11 reported that increasing the aqueous nitric acid concentration in the extraction section, for a solvent containing 0.3 mol L−1 HDEHP and 0.2 mol L−1 CMPO, led to a decrease of the distribution ratios of metals. They explained this result as a lowering of the extraction capabilities of CMPO due to its interaction with HDEHP resulting in the decrease of the active concentrations of the reagents in the solvent phase. Therefore, there are many indications that the extraction system containing both CMPO and HDEHP does not behave ideally, and the extractants do not act independently but instead interact with each other which affects the performance of the separation process. An effort was made to study the interactions between HDEHP and CMPO and to explore potential impacts of these interactions on the extraction properties of the mixture. Formation of metal species in the organic phase for various HDEHP, CMPO, and H+/NO3− concentrations is discussed. 10434
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and 0.05 mol L−1 DTPA is sufficient for quantitative stripping of La from the organic phase. It has to be noted that in some cases, especially at high HDEHP concentrations, a white gelatinous precipitate was formed after stripping with citrate. By letting the sample sit overnight, the gel disappeared and two clear phases were formed. The formation of the white gel could be caused by saponification of HDEHP and was not observed when higher concentrations of CMPO were present. The aqueous phase remaining after stripping was analyzed for nitrate by ion chromatography (Dionex ICS-2000) with an IonPac AS18 anion separation column, an IonPac AG18 as a guard column, and using ASRS-300 as an electrolyte suppressor. Potassium hydroxide (RFIC EGC III KOH) was used as eluent. It was experimentally determined that the presence of citrate and DTPA did not interfere with the determination of nitrate concentration. The data for the nitrate concentration reported are within 5% uncertainty and represent the total nitrate concentration in the organic phase that was extracted with the metal and that which extracted as HNO3. However, since the initial aqueous concentration of HNO3 was 0.01 mol L−1 (for experiments with 0.05 mol L−1 La(III)), the organic concentration of HNO3 is expected to be very low. Determination of Organic HNO 3 Concentration. Typically a 1 mL aliquot of the organic phase was titrated using Methrom 836 Titrando using a mixture of ∼70% acetone in water. Prior to determination of the organic HNO3 concentration, mixtures of HDEHP and CMPO were titrated to determine the concentration of H+ from HDEHP. The organic HNO3 concentration was calculated as the difference between the total acidity and the HDEHP molarity in the organic phase. All titrations were performed in duplicate. Infrared Spectroscopy. A Nicolet 6700 Fourier transform infrared spectrometer equipped with a Smart iTR (attenuated total reflectance) sampling accessory with a diamond crystal was used to collect the spectra. A few microliters of the organic phase were deposited on the diamond disk for every measurement. Thirty-two scans with a resolution of 4 cm−1 in the range of 650−4000 cm−1 were taken for each sample. A scan of the empty diamond iTR accessory was used to collect background spectrum before every sample. The interaction of water with HDEHP and CMPO in the organic phase could not be identified by the single reflectance method. Therefore, a transmission technique using a KBr liquid cell with 0.015 mm spacer was used for some samples. Analysis of the Infrared Spectra. The interaction between HDEHP and CMPO in the organic phase was evaluated by fitting the infrared spectra with the nonlinear leastsquares program HypSpec.15 HypSpec allows calculation of the conditional interaction constant and the molar absorption coefficient spectrum of each absorbing species using experimental absorbances, analytical concentrations of reagents, and the proposed chemical model as inputs. The spectra were analyzed in the region of 800−1700 cm−1 (1868 points per spectrum), where the most significant changes were observed. To properly analyze the infrared spectra, three absorbing species were initially considered to be present in the organic phase without the presence of metal: dodecane, CMPO, and HDEHP. A fourth absorbing species formed by the interaction between HDEHP and CMPO was identified by HypSpec.15
by mixtures of CMPO and HDEHP at higher aqueous nitric acid concentrations yields lower distribution ratios than that reported for the TRUEX system (CMPO in TBP). It was suggested that this behavior could be due to the interaction between CMPO and HDEHP, as the alkylphosphoric acids are known to have a tendency to form mixed moieties resulting in the decrease of the active concentrations of the extractants.11,16 In this work, to better understand this interaction, FT-IR spectroscopy in the combination with solvent extraction was used. FT-IR spectra of the system containing 0.3 mol L−1 HDEHP and various CMPO concentrations in n-dodecane are shown in Figure 1, and characteristic peaks in the infrared spectra and their assignments for bonds involved in [HDEHP]/[CMPO]/ metal systems are listed in Table 1 (M = La(III), Eu(III)).
Figure 1. FT-IR spectra of HDEHP and CMPO mixtures at 0.3 mol L−1 HDEHP and various CMPO concentrations. Also the spectrum of 0.05 mol L−1 CMPO in DDC is shown at the very bottom.
Table 1. Characteristic Frequencies of HDEHP, CMPO, and Their Mixtures in DDC in the Absence and Presence of La frequencies, cm−1 group CO CO···M CO···M PO PO···M OH PO COP a
HDEHP
1230 1216−1218 1695 909−1040 1032−1034
CMPO
HDEHP + CMPO
1642−1644, 1634a 1589a 1604a sh 1208, 1180 1147−1153
1636−1640 1593−1600 ∼1575 sh 1132, 1144, 1265
1028−1032
Tetrachloroethylene (TCE), M = La(III), Eu(III); sh = shoulder.
Most of the significant changes in the spectra occurred in the range of 800−1800 cm−1 and are discussed in more detail. The spectra of HDEHP and CMPO mixtures in the region of 1500−1700 cm−1 are represented mainly by the strong carbonyl band of CMPO at ∼1640 cm−1 and a weak but broad OH bending band (HDEHP) with a maximum at 1695 cm−1. The phosphoryl group of HDEHP in the absence of CMPO is observed at 1230 cm−1, and that of CMPO in the absence of HDEHP at 1180 and 1208 cm−1. The COP vibration gives rise to a strong band at 1033 cm−1. Interaction of the HDEHP with the CMPO causes significant changes in the IR spectrum (Figure 1) mainly in the 900−1300 cm−1 region. The changes at ∼1000 cm−1 are most likely due to the changes in P(O)OH group vibration involving a PO
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RESULTS AND DISCUSSION Interaction between HDEHP and CMPO. It has been previously reported11 that the extraction of metal−nitrate salts 10435
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stretch17 at 909−1040 cm−1, which is overlapped by the intense band of the COP group. At a constant 0.3 mol L−1 HDEHP and increasing CMPO concentration, the intensity of the peak at 1034 cm−1 decreases and a new peak is formed at ∼1000 cm−1. Also, changes in the OH-bonded (HDEHP) at 2333 cm−1 were noticeable (using KBr windows) as the peak sharpens and shifts to the lower frequency when the CMPO concentration is increased (not shown). This could be explained by the change to the hydrogen bond in (HDEHP)2, where the dimer breaks forming two HDEHP monomers, and each HDEHP monomer (POH) is bonded to the phosphoryl group of CMPO. The intensity of the phosphoryl peak of HDEHP at 1230 cm−1 decreases as the concentration of CMPO is increased and shifts to higher frequency (∼1265 cm−1) due to the formation of HDEHP monomer leaving the PO group free. On the other hand, the phosphoryl group of CMPO is shifted to lower frequencies, indicating an interaction with HDEHP (Table 1). Also, a small shift in the carbonyl group peak (ΔνCO = −4 cm−1) was observed in the presence of HDEHP and could indicate a weak interaction between HDEHP monomer and the carbonyl group of CMPO. However, these changes could also be due to a solvent effect, since the presence of HDEHP increases the polarity of the solvent. Infrared spectra obtained from 21 different mixtures of CMPO and HDEHP in the concentration range of 0.05−0.5 mol L−1 were processed using HypSpec15 software. To properly analyze the spectral data, a dimerization constant for HDEHP in n-dodecane of log β = 4.4318 was applied. Since the changes in the infrared spectra obtained for different HDEHP and CMPO mixtures indicate that in the presence of CMPO the (HDEHP)2 dimer breaks, the following equilibria in mixed organic phase were considered: HDEHP + HDEHP ⇆ HDEHP − HDEHP HDEHP + CMPO ⇆ HDEHP − CMPO
Figure 2. Experimental (symbols) and calculated (lines) spectra for mixtures of 0.3 mol L−1 HDEHP and 0.05−0.5 mol L−1 CMPO obtained by fitting the experimental spectra using HypSpec software.15
± 50. Furthermore, the formation of 1:1 adducts between HDEHP and CMPO has been previously reported by Lumetta et al.7 They did not observe the formation of an adduct between HDEHP dimer and CMPO. They reported7 an interaction constant log β = 3.07 ± 0.05 for a 1:1 HDEHP:CMPO adduct in n-dodecane determined from NMR spectra for a system containing 0.1 mol L−1 CMPO and various HDEHP concentrations that agrees reasonably well with the value determined by fitting the infrared spectra in this work. Both constants indicate that when HDEHP is in excess over CMPO, a significant fraction of the CMPO is associated with HDEHP, and only a small portion of CMPO is available to form a complex with metal ions. This implies that the extraction of metal ions under conditions where HDEHP is in large excess over the CMPO concentration is governed primarily by the formation of a complex between the HDEHP−CMPO adduct and the metal. This observation will be discussed in more detail. The extinction spectrum of the HDEHP−CMPO adduct, resolved using HypSpec15 software, and the experimental spectra of (HDEHP)2, and CMPO, all in dodecane, are shown in Figure 3 (since the FT-IR spectra were obtained using iTR with an unspecified, constant path length, molar absorption coefficients were not determined). Major differences in spectra can be seen in the region of ∼950−1300 cm−1, with peak
β(dimer) (3) β(adduct) (4)
Under the conditions investigated in this study, the initial concentration of monomeric HDEHP was less than 1% of the total HDEHP concentration present (based on a dimerization constant for HDEHP of log β = 4.43) and was not included as an absorbing species for the analysis of the spectra. Fitting the experimental data with HypSpec15 suggests the formation of a 1:1 HDEHP:CMPO adduct between HDEHP monomer and CMPO with a formation constant determined as log β = 3.4 ± 0.02. The spectra were fit by optimization of the spectra accounting for a new HDEHP−CMPO adduct. All other absorbing species (CMPO, (HDEHP)2, and DDC) were inserted as species with known spectra. Experimental and calculated data obtained by HypSpec15 software for 0.3 mol L−1 HDEHP and various CMPO concentrations are shown in Figure 2. A very good fit was obtained using the equilibria described in eqs 3 and 4 and by optimization of the absorption spectra of HDEHP−CMPO adduct. The possible formation of adducts between HDEHP and other extractants is well-known. El-Reefy et al.19 discuss how the formation of adducts between HDEHP and CYANEX-921 (commercial trioctyl phosphine oxide) can affect the distribution ratios of uranium. The formation of adducts between HDEHP and trioctyl phosphine oxide (TOPO) was also reported by Brown et al.20 They suggest that (HDEHP)2−TOPO and HDEHP−TOPO adducts are formed with formation constants: K1 = 30 ± 5 and K2 = 100
Figure 3. Extinction spectra of (HDEHP)2 and CMPO in DDC compared with the spectrum of the HDEHP−CMPO adduct resolved using HypSpec software:15 (solid line) HDEHP−CMPO adduct; (dashed line) HDEHP dimer; (dotted line) CMPO. Initial concentrations of HDEHP and CMPO were in the range of 0.05− 0.5 mol L−1. 10436
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maxima for the adduct at ∼997, 1031, 1104, 1132 and 1144, and 1265 cm−1. The peak at 1265 cm−1 can be assigned to the PO group of HDEHP which is shifted to a higher frequency due to breaking of the hydrogen bond between the two HDEHP molecules. The new peak at 997 cm−1 probably belongs to the PO stretch of the P(O)OH group17 that corresponds to hydrogen bonding between HDEHP and CMPO, as discussed earlier. Peaks at 1132 and 1144 cm−1 could be assigned to the PO group of CMPO that are shifted from 1180 and 1208 cm−1 due to the formation of a new hydrogen bond between HDEHP monomer and CMPO. The proposed interaction between POH group of the HDEHP monomer and the PO group of CMPO through the hydrogen bonding is illustrated in Figure 4. The interaction
Table 2. Organic Concentrations of HNO3 in Various Mixtures of HDEHP and CMPO after Pre-equilibration with HNO3 sample
total HDEHP
total CMPO
aq. HNO3, mol L−1
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0.3026 0.3026 0.5 0.3017 0.3017 0.3017 0.3017 0.3017 0.292 0.3052 0.3052 0.3052 0.3052 0.3052 0.3052 0.394 0.491 0.490
0 0 0 0.05 0.05 0.05 0.05 0.05 0.15 0.2 0.2 0.2 0.2 0.2 0.2 0.15 0.15 0.1
1 4.0 1.0 0.2 0.5 1.0 2.0 4.0 1.0 0.05 0.2 0.5 1.0 2.0 4.0 1.0 1.0 1.0
a
org. HNO3, mol L−1 NDa 1.54 × 5.00 × ND ND 1.38 × 3.33 × 7.29 × 5.09 × ND ND 2.36 × 6.23 × 1.36 × 2.38 × 5.40 × 4.96 × 2.96 ×
10−2 10−3
10−2 10−2 10−2 10−2
10−2 10−2 10−1 10−1 10−2 10−2 10−2
Not determined.
Figure 4. Illustration of the interaction between CMPO and HDEHP monomer through hydrogen bonding between phosphoryl group of CMPO and P−O−H group of HDEHP monomer.
between HDEHP and CMPO, and formation of the HDEHP− CMPO adduct, significantly lowers the concentration of CMPO available for the complexation with metal ions and considerably affects the ability of CMPO itself to participate in the extraction of metal. Extraction of Nitric Acid. A very important factor in the understanding of the metal partitioning into the organic phase is to know the speciation of the extractant. The extraction of nitric acid by CMPO and HDEHP has been investigated previously21−23 and is well understood, but because of the interaction between HDEHP and CMPO, the extraction of HNO3 by their mixtures must also be investigated. The organic HNO3 concentration for various combinations of HDEHP and CMPO was determined by potentiometric titration as described above. The organic HNO3 concentrations for various initial HDEHP, CMPO, and HNO3 concentrations are listed in Table 2. Figure 5 shows the FT-IR spectra of the 0.3 mol L−1 HDEHP + 0.2 mol L−1 CMPO system for samples 10−15 from the Table 2. It was confirmed that the extent of extraction of HNO3 by HDEHP is very low, and the data obtained agree with the previously reported data.21 On the other hand, CMPO can extract significant quantities of nitric acid, and extraction constants for the CMPO·HNO3 and CMPO·2HNO3 adducts, KH1 = 1.9 and KH2 = 0.009, respectively, were determined previously by Chaiko et al.23 The data in Table 2 show that the organic acid concentration increases significantly with increasing initial CMPO concentration. Since in the presence of an excess of HDEHP, the CMPO is mostly present as HDEHP− CMPO adduct, it is important to investigate what portion of
Figure 5. FT-IR spectra of 0.3 mol L−1 HDEHP + 0.2 mol L−1 CMPO in n-dodecane after pre-equlibration with 0.05, 0.2, 0.5, 1, 2, and 4 mol L−1 nitric acid. The dashed line represents the spectrum of 0.3 mol L−1 HDEHP + 0.2 mol L−1 CMPO in n-dodecane in the absence of HNO3.
extracted HNO3 is associated with the adduct. For the system containing 0.3 mol L−1 HDEHP and 0.2 mol L−1 CMPO, no significant changes in the spectra were observed until the organic HNO3 concentration reached ∼0.06 mol L−1 (1 mol L−1 initial aqueous HNO3, see Table 2, Figure 5). New peaks at ∼945 and ∼1290 cm−1 are characteristic of the presence of molecular HNO3.24,25 The increase in the absorbance at ∼1030 cm−1 is apparently due to the formation of a new peak at ∼945 cm−1. As the organic HNO3 concentration increases the peak at 1265 cm−1 (PO, HDEHP in the HDEHP−CMPO adduct) continuously decreases, and a new peak is formed at ∼1210 cm−1, which indicates a direct interaction between the nitric acid and the phosphoryl group of the HDEHP−CMPO adduct. An increase in the absorption at ∼1143 cm−1 could be due to the interaction between free CMPO (not in the adduct with HDEHP) and HNO3. The infrared spectra show clearly that 10437
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nitric acid is interacting with both free CMPO and the HDEHP−CMPO adduct. To determine the available concentrations of free HDEHP, CMPO, and HDEHP−CMPO for the complexation with metal, several equilibria were considered upon extraction of nitric acid: CMPO + H+ + NO3− ⇆ CMPO· HNO3 CMPO + 2H+ + 2NO3− ⇆ CMPO· 2HNO3
KH1
(5)
KH2 (6)
(H(DEHP)2 ) + H+ + NO3− ⇆ (H(DEHP)2 ) ·HNO3 KH3
(7) +
HDEHP − CMPO + H + ·HNO3
NO3−
Figure 6. Correlation between calculated and experimental organic HNO3 concentrations for various mixtures of HDEHP and CMPO from Table 2.
⇆ (HDEHP−CMPO)
KH4
(8)
HDEHP − CMPO + 2H+ + 2NO3− ⇆ (HDEHP−CMPO)· 2HNO3
KH5
(9)
The constants23 KH1 = 1.9 and KH2 = 0.009 were used for the equilibria between HNO3 and CMPO. The constants KH3−KH5 were resolved using the Microsoft Excel Solver plug-in. The solver was set to find a solution so that the differences between the experimental and calculated organic HNO3 concentrations were minimal. The total organic HNO3 concentration can be written as follows: [HNO3]org = KH1[CMPO]f {H+}{NO3−} + KH2[CMPO]f Figure 7. Speciation diagram for the system containing 0.3 mol L−1 HDEHP and 0.2 mol L−1 CMPO after extraction of HNO3 as a dependence on activity of HNO3: (○) free (HDEHP)2, (□) free CMPO, (△) free HDEHP−CMPO, (■) CMPO·HNO3, (orange ▲) HDEHP−CMPO·HNO3, (+) (HDEHP)2·HNO3, (◇) CMPO·2HNO3, (green ▲) HDEHP−CMPO·2HNO3. The relative species distribution is based on initial concentrations of HDEHP dimer, CMPO, and HDEHP−CMPO adduct by applying HDEHP dimerization constant (log β = 4.43) and formation constant for HDEHP−CMPO adduct (log β = 3.4). The plot shows the distribution of HDEHP dimer, CMPO, and HDEHP−CMPO adduct between species associated with HNO3 and free species (not associated with HNO3).
{H+}2 {NO3−}2 + KH3[(H(DEHP)2 )]f {H+} {NO3−} + KH4[HDEHP−CMPO]f {H+} {NO3−} + KH5[HDEHP−CMPO]f {H+}2 {NO3−}2
(10)
where [CMPO]f, [(H(DEHP)2)]f, and [HDEHP−CMPO]f are free concentrations and {H+} and {NO3−} are activities. The concentration of free CMPO, HDEHP, and HDEHP−CMPO was obtained by a simultaneous solution of eq 10 and the appropriate mass balance equation for every species. From the analysis of the extraction data in Table 2, the values of KH3, KH4, and KH5 were found to be 5.29 × 10−3, 0.83, and 1.06 × 10−2, respectively. A comparison between the experimental and calculated organic HNO3 concentrations is presented in Figure 6. The distribution diagram for the species in the organic phase containing 0.3 mol L−1 HDEHP and 0.2 mol L−1 CMPO after extraction of nitric acid is shown in Figure 7. The distribution of organic species was calculated using the formation constant for HDEHP−CMPO adduct of log β = 3.4 and the constants KH1−KH5 for the extraction of nitric acid. Metal−HDEHP−CMPO System. After pre-equilibration of HDEHP and CMPO mixtures with aqueous solution containing HNO3 and NaNO3, the presence of water in the organic phase was apparent from the appearance of the bonded OH band near ∼3450 cm−1 (using KBr windows). Water was associated mainly with CMPO, causing broadening and a small down-frequency shift of ΔνCO and ΔνPO = −2 cm−1. By the extraction of metal, the water content decreases significantly indicating that water molecules are replaced by the metal.
Lanthanum Extraction. For the system containing only HDEHP in DDC, the spectra confirm a coordination of metal (La(III)) with the phosphoryl group of HDEHP. Characteristic frequencies for HDEHP/CMPO/La system are listed in Table 1. The PO absorption band associated with bonding to the metal ion shifts from 1230 cm−1 to lower frequencies at 1216− 1218 cm−1, and correspondingly, the intensity of the peak for free PO decreases (Table 1). As expected, after the extraction of metal by CMPO in n-dodecane, the formation of a white interfacial crud was observed, which is in accordance with the data reported elsewhere for CMPO solvents where TBP is not used as a phase modifier.26 Although a small fraction of La−CMPO complex was found to be dissolved in the DDC fraction, to properly identify the absorption features of the complex between CMPO and La, and to avoid the formation of interfacial crud, tetrachloroethylene (TCE) was also used as a diluent in some experiments where the HDEHP concentration 10438
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Figure 8. FT-IR spectra of 0.3 mol L−1 HDEHP and 0.15 mol L−1 CMPO in n-dodecane before (solid line) and after (dashed line) extraction of La (0.054 mol L−1) from 0.01 mol L−1 HNO3 and 1.0 mol L−1 NaNO3. Spectra on the right show the decrease in the absorption of the carbonyl peak at 1636 cm−1 after the extraction of metal by normalizing the spectra to CH3 and CH2 vibrations at 1378 and 1466 cm−1.
was low. The FT-IR spectrum of the La−CMPO system in the absence of HDEHP exhibits absorption bands with maxima at ∼1634 and 1589 cm−1 with a shoulder at 1604 cm−1 and absorption at 1147−1153 cm−1, which are assigned to carbonyl and phosphoryl stretching modes, respectively. Carbonyl and phosphoryl stretching frequencies decrease by 40−50 and 30 cm−1 after extraction of metal, respectively, indicating that the La is bonded to CMPO in a bidentate mode to both the phosphoryl and carbonyl group. However, the involvement of the carbonyl group in coordination to metal can be different at different organic metal concentrations, as previously discussed by Horwitz et al. for the extraction of Nd(III) by CMPO,6 and supported by FT-IR studies.27 In general, the features of the FT-IR spectra obtained for M(III)−CMPO and M(III)− HDEHP systems are consistent with previous literature data.27,28 FT-IR spectra of the mixtures of CMPO and HDEHP after extraction of La from a solution of 0.05 mol L−1 La, 0.01 mol L−1 HNO3, and 1 mol L−1 NaNO3 indicate that both HDEHP and CMPO are involved in the complex with the metal ion. Comparison of the spectra for the 0.3 mol L−1 HDEHP + 0.15 mol L−1 CMPO mixture before and after extraction of La from 0.01 mol L−1 HNO3 and 1 mol L−1 NaNO3 (Figure 8) reveals formation of absorption features at 1104, 1133, and 1149 cm−1 with high intensities and new peaks at 1220, 1293, ∼1480, and 1597 cm−1 and a broad absorption in the whole region of 800− 1800 cm−1. Furthermore, after extraction of La, the intensity of the peak at 1032 cm−1 with a shoulder at 996 cm−1 further decreases. This could indicate that upon extraction of metal into the organic phase, the hydrogen bond in the HDEHP− CMPO adduct breaks and hydrogen is displaced by the metal (Scheme 2). This suggests that the adduct is an acidic extractant. The peaks at 1133 and 1149 and 1220 cm−1 are assigned to the interaction of La with the PO group of CMPO and HDEHP, respectively (Table 1). The assignment for the peak at 1104 cm−1 is unclear, as it is visible also in the spectrum of HDEHP−CMPO adduct (Figure 3) in the absence of La and its intensity increases significantly upon extraction of La. The formation of a new peak at 1597 cm−1 suggests that La is also bonded to the CO group of CMPO. However, it is important to point out that the intensity of the peak at 1636 cm−1 assigned to the free CO group of CMPO does not change considerably after the extraction of La (organic La concentration 0.052 mol L−1), even though a new peak for C OLa is formed at 1597 cm−1 (Figure 8a). This is most likely
Scheme 2. Illustration of Metal Complexation by HDEHP− CMPO Adducta
a
R1 = octyl, R2 = isobutyl, R3 = ethylhexyl.
due to the presence of an underlying broad absorbance that, as can be seen from Figure 8, also affects the absorbance of the peaks at 1378 and 1466 cm−1, which belong to CH3 and CH2 vibrations. If the spectrum is normalized to CH3 and CH2 vibrations at 1378 and 1466 cm−1, the spectrum shows a decrease of absorbance at ∼1640 cm−1 upon extraction of La (Figure 8b). The infrared spectrum of the La−CMPO precipitate (in DDC) in the region of 1400−1700 cm−1 is dominated by the intense peak at 1587 cm−1 suggesting a strong involvement of the CO group in binding with La. Similar behavior was observed for extraction in TCE diluent. From the data listed in Table 1, it is apparent that the bond between the carbonyl oxygen and La is stronger for the pure CMPO system with a peak maxima at ∼1590 cm−1 compared to the feature at ∼1600 cm−1 when HDEHP was present (due to a stronger bond between oxygen and metal, the CO bond weakens and appears at lower frequency). Although not very apparent from the FT-IR data obtained by iTR, the spectra for the system at various concentrations of HDEHP and CMPO, obtained with KBr windows, indicate that there is a shift in the COLa frequency at ∼1600 cm−1 for different [HDEHP]/ [CMPO] ratios (Table 3). The difference between the position of the CO bond for the La−CMPO complex and the system containing both CMPO and HDEHP can be explained as follows. Upon addition of HDEHP to CMPO, the HDEHP−CMPO adduct is formed, which lowers the concentration of CMPO and limits the interaction between metal and free CMPO considerably. The weaker interaction between the CO group and La(III) in the presence of HDEHP is due to the formation of a complex between La and the HDEHP−CMPO adduct, and a partial replacement of nitrate molecules in the organic phase by HDEHP, which is a stronger base than the nitrate. Therefore, 10439
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Table 3. Frequencies of the Carbonyl Group Coordinated with La for the System Containing Various Concentrations of CMPO and HDEHP in n-Dodecane Obtained Using KBr Windows
a
concentration, mol L−1 CMPO; HDEHP
[HDEHP]/ [CMPO]
COLa cm−1
ratio of absorbance ∼1600/∼1640 cm−1
0.05; 0.5 0.1; 0.5 0.2; 0.5 0.3; 0.5 0.3; 0.3 0.4; 0.3a 0.5; 0.3a 0.2; 0.1a 0.2; 0.0a
10 5 2.5 1.67 1 0.75 0.6 0.5
1598.7 1598.7 1596.8 1596.8 1594.9 1592.9 1591.0 1589.1 1587.1
1.02 0.75 0.52 0.43 0.4 0.48 0.69 1.46
Table 4. Composition of the Organic Phase after Extraction of 0.054 mol L−1 La(III) from 0.01 mol L−1 HNO3 and 1.0 mol L−1 NaNO3 by Various Mixtures of HDEHP and CMPO in n-Dodecane organic concentration, mol L−1 sample CMPO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Tetrachloroethylene used as a diluent.
as the metal is coordinated to both CMPO and HDEHP, the bond between the metal and the CO of CMPO is weaker compared to the pure metal ion−CMPO system. As the ratio of [HDEHP]/[CMPO] further decreases, the concentration of free CMPO becomes significant and the peak at ∼1590 cm−1 start to dominate due to the coextraction of the La(NO3)3·3CMPO complex. Absorptions at 1293 and ∼1480 cm−1 (Figure 8) could be assigned to ν1 symmetric and ν4 asymmetric stretching vibrations, respectively, of nitrate bonded to metal (C2ν symmetry).29 A criterion often used to distinguish between monodentate and bidentate mode of bonding is the separation between the ν1 and ν4 frequencies of the coordinated nitrate group. This separation is usually greater (>180 cm−1) for bidentate nitrates relative to that of monodentate nitrate ( [CMPO] significantly lowers the concentration of free CMPO available for the complexation of metal ions and essentially eliminates the ability of CMPO to participate in the extraction of trivalent f-block ions as M(NO3)3·3CMPO. Distribution ratios of M3+ decrease significantly with increasing nitric acid concentration and point to an essential role played by HDEHP in the partitioning of metals to the organic phase for mixed CMPO/HDEHP system. At low H+/NO3− concentrations, M(H(DEHP)2)3 and Eu(NO3)(DEHP)2(CMPO)2 species predominate, while at high H+/NO3− concentrations the main species present at [HDEHP] > [CMPO] is M(NO3)2·(DEHP)·(CMPO).
basis of the modeling data, the extraction data for trace Eu concentration could be explained as follows. Extraction of Eu from 1 mol L−1 HNO3 for the various [HDEHP]free/ [CMPO]free ratios shown in Figure 12 could be satisfactorily explained by the extraction of only two species Eu(NO3)(DEHP)2(CMPO)2 and Eu(NO3)2·(DEHP)·(CMPO) and (eqs 13 and 14), which explain a slope of ∼1.6 for the HDEHP−CMPO adduct. At >1 mol L−1 HNO3 or ∼1 mol L−1 HNO 3 and higher nitrate concentrations, the Eu(NO3)2·(DEHP)·(CMPO) species are the most predominant. At lower acidities and higher [HDEHP]:[CMPO] ratio, Eu(H(DEHP)2)3 and Eu(NO3)·(DEHP)2·(CMPO)2 are the main species present in the organic phase. The Eu(NO3)3·3CMPO species forms only when [HDEHP] ≪ [CMPO] and at moderate or higher HNO3 concentrations. For HDEHP in excess, most of the CMPO is bound to HDEHP in the HDEHP−CMPO adduct and associated with HNO3, which minimizes its availability for the complexation with metal. The proposed M3+ complexes formed after extraction from nitrate media by mixtures of HDEHP and CMPO, based on FT-IR spectra, distribution ratios, and extraction modeling are presented in Figure 15. The formation of ternary species including Ln(III)−HDEHP−CMPO:Ln(CMPO)(DEHP) (H(DEHP)2)2, with bidentate coordination of CMPO (COM, POM) to the metal, was recently proposed for the extraction of Nd(III) and Eu(III) from 1.5 mol L−1 lactic acid (pH = 1) and absence of nitrate by mixture of HDEHP and CMPO.39
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357. This work was supported by the U.S. Department of Energy, Office of Nuclear Energy, Fuel Cycle Research and Development Project under Contract DE-AC02-06CH11357. The authors wish to express their sincere gratitude to Alena Paulenova and Martin Precek (Oregon State University) for their kind assistance with determination of nitrate concentrations in the organic phase using ion chromatography, Kurt Frey (ANL) for his assistance with the determination of activity coefficients, Renato Chiarizia (ANL) and Candido Pereira (ANL) for their reviews.
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