Complexation of uranyl ion by tetrahexylmalonamides: an equilibrium modeling and infrared spectroscopic study

www.elsevier.nl/locate/ica Inorganica Chimica Acta 293 (1999) 195 – 205

Complexation of uranyl ion by tetrahexylmalonamides: an equilibrium modeling and infrared spectroscopic study Gregg J. Lumetta a,*, Bruce K. McNamara a, Brian M. Rapko a, James E. Hutchison b a

Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA b Department of Chemistry, Uni6ersity of Oregon, Eugene, OR 97403, USA Received 24 March 1999; accepted 21 May 1999

Abstract We investigated the extraction of uranyl nitrate from aqueous sodium nitrate with a series of tetrahexylmalonamides. The tetrahexylmalonamides considered were N,N,N%,N%-tetrahexylmalonamide (THMA), N,N,N%,N%-tetrahexyl-2-methylmalonamide (MeTHMA), and N,N,N%,N%-tetrahexyl-2,2-dimethylmalonamide (DiMeTHMA). This series allowed for a systematic determination of the effects of alkyl substitution of the methylene carbon. Equilibrium modeling of the extraction data indicates that at 1 M NaNO3, two extracted species are formed: UO2(NO3)2L2 and UO2(NO3)2L3. The relative abundance of these two species depends on the nature of the tetrahexylmalonamide ligand. The UO2(NO3)2L2 species is dominant in the DiMeTHMA system, with very little formation of the UO2(NO3)2L3 species. In contrast, the UO2(NO3)2L3 species is more predominant in the MeTHMA case. The case of THMA lies in between. The greater propensity of MeTHMA versus THMA to bind in a 3:1 fashion to uranyl ion might reflect the greater basicity of the carbonyl oxygens in MeTHMA. The fact that DiMeTHMA binds primarily in 2:1 fashion suggests that steric constraints are more important in that ligand. As the nitrate concentration is increased, the ligand-to-metal ratios tend to decrease, i.e. the UO2(NO3)2L2 species tends to predominate, while the UO2(NO3)2L3 species becomes less important. In the case of THMA and MeTHMA, equilibrium modeling suggests the existence of a UO2(NO3)2L species at higher nitrate concentrations. FTIR spectral studies confirm that at least two uranyl – THMA complexes formed, one of which has been identified as UO2(NO3)2(THMA) by thermogravimetric analysis (TGA). The identity of the second species has not been definitively determined, but is most likely UO2(NO3)2(THMA)2. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Extraction equilibria; Uranium complexes; Amide complexes

1. Introduction A number of research groups have been investigating the extraction of f-block ions by amide ligands. These extractants are of potential use in separating problematic radionuclides from high-level radioactive wastes. Monoamides are known to extract tetravalent and hexavalent actinides [1– 6], whereas diamides extract trivalent actinides and lanthanides along with the tetravalent and hexavalent actinides [7 – 13]. One of the perplexing features of these extractants is the fact that slope analysis methods generally indicate that the amide-to-metal

* Corresponding author. Tel.: + 1-509-376 6911; fax: + 1-509-372 3861. E-mail address: [email protected] (G.J. Lumetta)

stoichiometry in liquid–liquid extraction systems is greater than that indicated by isolated amide–metal complexes, metal-loading studies, or spectroscopic measurements. Furthermore, the amide-to-metal stoichiometries indicated by slope analysis are often nonintegral. This behavior has been variously explained as (i) nonideality in the organic phase, (ii) aggregation of the extractant in the organic phase, and (iii) outer-sphere complexes forming with the amide ligand located in the second coordination sphere of the metal ion. Spectroscopic (IR, UV–Vis, and NMR) and vapor-pressure osmometry measurements have failed in providing hard evidence for second-sphere coordination of amides [5,7]. Recent small angle X-ray scattering studies reported by Erlinger et al. have indicated that N,N%dimethyl-N,N%-dibutyl-2-tetradecylmalonamide (DMD-

0020-1693/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 2 3 8 - 8

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volumetric flask and filled to the mark with deionized water. The solution was determined to contain 0.0013 M 233U (by alpha spectroscopy) and 0.34 M 238U (by laser fluorimetry). The final stock solution was 0.34 M in HNO3. All FTIR spectra were obtained using a Nicolet 750 FTIR spectrometer. Scheme 1.

BTDMA) forms aggregates containing approximately five DMDBTDMA molecules per aggregate in dodecane [14]. Similarly, Nigond et al. used NMR investigations to determine the aggregation number for DMDBTDMA to be approximately six in benzene and approximately four in a tetrahydrogenated propylene (a mixture of branched dodecanes) [15]. However, no attempt has been reported in correlating the observed aggregation behavior to the solvent extraction behavior of DMDBTDMA. Condamines et al. have proposed a statistical mechanics model to account for organic phase nonideality; this model gave satisfactory results for the extraction of U(VI) by certain monoamides and diamides in aliphatic hydrocarbon diluents [16]. We are investigating the extraction behavior of a series of tetrahexylmalonamides with f-block metal ions. In this work, we have used equilibrium modeling and FTIR spectroscopy to gain insight into the extraction chemistry. We report here the extraction of uranyl ion with N,N,N%,N%-tetrahexylmalonamide (THMA), 2methyl-N,N,N%,N%-tetrahexylmalonamide (MeTHMA), and 2,2-dimethyl-N,N,N%,N%-tetrahexylmalonamide (DiMeTHMA). Scheme 1 presents the structures of these compounds. The stoichiometries of the extracted species have been deduced by equilibrium modeling using the SX Solver computer program, which we have recently described [17]. IR spectroscopic measurements support the findings of the equilibrium modeling.

2. Experimental

2.1. General materials and methods Tetraalkylmalonamides were synthesized using methods analogous to those reported in the literature as we have described elsewhere [18]. The purity of THMA, MeTHMA, and DiMeTHMA were determined by 1H and 13C NMR and gas chromatography – mass spectrometry (GC–MS) to be \99%. A stock solution of 233UO2(NO3)2 was made by dissolving a small quantity of isotopically pure 233U3O8 in 10 ml of nitric acid. A 0.1 ml aliquot of this 233U stock and 1.65 g of 238UO2(NO3)2·6H2O (Alfa Inorganics, Beverly, MA) as carrier was mixed in a 10 ml

2.2. Preparation of UO2(NO3)2(THMA) UO2(NO3)2(THMA) was prepared using a procedure similar to the literature method [19]. Uranyl nitrate hexahydrate (0.05 g, 0.1 mmol) was dissolved in 1 ml of a 5 M NaNO3 solution with 0.01 M HNO3. A yellow precipitate formed when this aqueous solution was mixed with an equal volume of 0.52 M THMA in t-butylbenzene. The precipitate was dissolved in CH3CN, and a crystalline film remained as the solvent slowly evaporated. This material was analyzed by thermogravimetric analysis (TGA) using a Seiko 320 series thermogravimetric calorimeter. The TGA was operated at a scan rate of 1.0°C min − 1 in the temperature range 23–1000°C; air was used as the carrier gas. A 14.72 mg aliquot of this material was analyzed using TGA, yielding 5.10 mg of residue. The theoretical value was 4.96 mg as U3O8.

2.3. Distribution measurements Distribution coefficients were determined by measuring the relative 233U activity in the organic and aqueous phases. Following equilibration, the mixtures were centrifuged; then aliquots of each phase were counted by liquid scintillation alpha spectroscopy. The distribution coefficient was calculated as the 233U activity in the organic phase divided by that in the aqueous phase.

2.3.1. Kinetics Kinetic experiments were performed, in duplicate, to establish the time required to reach equilibrium under the vortex mixing conditions used. The results for each diamide indicated that at 23°C, equilibrium was achieved in less than 5 min. Accordingly, for all subsequent extraction experiments, vortex mixing was used for 5 min. 2.3.2. Extraction experiments The diamide solutions were first pre-equilibrated with sodium nitrate solution by contacting 0.4 ml of aqueous NaNO3 (1.0–5.0 M) with 0.6 ml of t-butylbenzene solutions of the diamide (0.1–0.5 M). Phase separation was facilitated by centrifugation; then 0.5 ml of the pre-equilibrated organic phase was contacted for 5 min with 0.4 ml of the appropriate aqueous phase. The aqueous phase consisted of the desired concentration of NaNO3, 0.33 mM 238UO2(NO3)2, and a 0.0012 mM

G.J. Lumetta et al. / Inorganica Chimica Acta 293 (1999) 195–205 233

UO2(NO3)2 tracer. The aqueous phase also contained 1.54 mM HNO3 to prevent hydrolysis of the metal ion. Four contacts were performed for each condition investigated. For multiple runs, deviations from the mean values were between 4 and 25%.

at specific concentrations of NaNO3 in the aqueous phase. If multiple species of the type UO2(NO3)2Lp are formed in the organic phase, it can be shown that the distribution coefficient (D) is given by the following expression: % Ki [UO22 + ]a[NO3− ]2[L]poi

3. Results D=

3.1. Sol6ent extraction studies The U(VI) extraction data were modeled to the following general equation using the SX Solver program. UO22 + (aq)+2NO3 − (aq) +pL(org) UUO2(NO3)2Lp(org)

(1)

The conditional equilibrium constant for this reaction is defined as K=

[UO2(NO3)2Lp]o [UO22 + ]a[NO3− ]2a[L]po

197

(2)

where quantities in brackets refer to the molar concentration of the individual species. The subscripts ‘a’ and ‘o’ refer to species in the aqueous and organic phases, respectively. Strictly speaking, the equilibrium constant should be based on the activities of each component rather than their concentrations. However, if constant ionic strength is maintained, the activity coefficients can be assumed constant. Under these conditions, the conditional equilibrium constant as expressed in Eq. (2) becomes valid. For this reason, data sets were collected

i

[UO22 + ]a

(3)

where Ki is the conditional extraction constant for species i. In this expression, [UO22 + ]a represents the total analytical concentration of U in the aqueous phase, including not only free UO22 + , but also any UO22 + /NO3 − complexes. In modeling the U(VI) extraction data, conditional extraction constants were calculated for each specific NaNO3 concentration considered. A number of different possible extracted complexes were considered, but the modeling indicated that the dominant extracted species were UO2(NO3)2L2 and UO2(NO3)2L3 as discussed below, where L was either THMA, MeTHMA, or DiMeTHMA. In some cases, the modeling indicated the presence of UO2(NO3)2L; this was especially true at high nitrate concentration. Fig. 1 compares the measured uranium distribution coefficients with the calculated curves for the models that best fit the data. Table 1 lists the calculated extraction constants along with their standard deviations for these models. The standard deviations were determined using the Microsoft Excel® macro developed by Billo [20]. The extraction constants depend on the nitrate concentration, which

Fig. 1. Extraction of U(VI) by THMA, MeTHMA, and DiMeTHMA: comparison of measured and calculated values. The calculated curves were obtained using the extraction constants listed in Table 1.

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Table 1 Extraction constants for the extraction of U(VI) with tetrahexylmalonamides a [NaNO3] (M) THMA 1 2 3 4 5 MeTHMA 1 2 3 4 5 DiMeTHMA 1 2 3 4 5

K1,1

c c c

1.1 9 0.5 0.5 90.9 c c c c

2.39 0.3

K1,2

3.99 2.1 7.590.9 11.390.8 12.091.4 15.89 2.4 3.492.2 2.490.9 6.691.1 11.692.5 4.590.7

K1,3

r

b

24.29 5.3 11.39 2.1 2.69 2.0

0.986 0.997 0.999 0.994 0.988

48.595.5 27.99 2.2 15.79 2.8 4.59 7.5

0.996 0.998 0.998 0.994 0.996

c c

c

c

0.07990.008

0.0073

c

0.169 0.01 0.2690.02 0.3690.03 0.6099 0.004

9 0.0020 0.0299 0.016 1.000 0.0859 0.034 0.999 0.0979 0.056 0.999 c 1.000

c c c

0.999

a K1,1, K1,2, and K1,3 are the extraction constants for the UO2(NO3)2L, UO2(NO3)2L2, and UO2(NO3)2L3 species, respectively. b r= correlation coefficient. c SX Solver program indicates that this species is insignificant — not included in the fit.

mainly reflects changes in the solution activities with changing ionic strength. Nitrate complexation of U(VI) in the aqueous phase is probably also partially responsible for the variation of the extraction constants with the nitrate concentration. As the SX Solver program does not explicitly correct for nitrate complexation in the aqueous phase, the aqueous-phase nitrate complexation constants are incorporated into the extraction constants.

3.1.1. Extraction of U(VI) with THMA Fig. 2 shows the uranium distribution coefficients as a function of THMA concentration at several different NaNO3 concentrations. Also shown are selected calculated curves considering different extraction models. Calculated curves are presented for the uranyl extraction at 1, 3, and 5 M NaNO3. For 1 and 3 M NaNO3, calculated curves for three different extraction models are presented: (i) one extracted species — UO2(NO3)2(THMA)2 (2:1 species), (ii) one extracted species — UO2(NO3)2(THMA)3 (3:1 species), and (iii) two extracted species — UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 (2:1 + 3:1 species). For 5 M NaNO3, calculated curves for four different extraction models are presented: (i) one extracted species — UO2(NO3)2(THMA) (1:1 species), (ii) one extracted species — UO2(NO3)2(THMA)2, (iii) two extracted

Fig. 2. Extraction of U(VI) by THMA. Calculated curves are given for 1, 3, and 5 M NaNO3.

species — UO2(NO3)2(THMA) plus UO2(NO3)2(THMA)2 (1:1+ 2:1 species), and (iv) two extracted species — UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 (2:1+ 3:1 species) (this curve is almost indistinguishable from the 2:1 curve). At 1 M NaNO3, the single-species model with UO2(NO3)2(THMA)2 gives a reasonable fit to the experimental data, but at the lower THMA values examined, the calculated values tend to be somewhat higher than the experimental values. Similarly, the single-species model with UO2(NO3)2(THMA)3 deviates from the experimental values at lower THMA concentration, but in the opposite direction, i.e. the calculated values are lower than the measured values. The best overall fit to the U(VI)/THMA extraction data at 1 M NaNO3 is obtained when two extracted species are considered — UO2(NO3)2(THMA)2 and UO2(NO3)2(THMA)3. The extraction model with UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 also yields the best fit for the U(VI)/THMA extraction data at 2 M NaNO3. However, in this case, the calculated line for the two-species model is closer to the calculated line for the UO2(NO3)2(THMA)2 model, while the calculated line for the UO2(NO3)2(THMA)3 model shows a marked deviation. (For the sake of clarity, these curves are not presented in Fig. 2.) This suggests that the UO2(NO3)2(THMA)2 species is more prominent at 2 M

G.J. Lumetta et al. / Inorganica Chimica Acta 293 (1999) 195–205

NaNO3 than at 1 M NaNO3. This trend becomes even more pronounced at 3 M NaNO3, where the calculated lines for the single-species UO2(NO3)2(THMA)2 model is nearly the same as that for the two-species UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 model (see Fig. 2). At 4 M NaNO3, the SX Solver program assigns a value of zero to the extraction constant for the UO2(NO3)2(THMA)3 species when the UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 model is considered, indicating that this species is insignificant. However, at the lower THMA concentrations, the calculated distribution coefficients are lower than the measured values. A slightly better fit is obtained by considering the presence of UO2(NO3)2(THMA) in the model along with UO2(NO3)2(THMA)2. This trend continues at 5 M NaNO3 (see Fig. 2) where the deviation for the UO2(NO3)2(THMA)2 plus UO2(NO3)2(THMA)3 model is even greater. The UO2(NO3)2(THMA) plus UO2(NO3)2(THMA)2 model yields the best fit at 5 M NaNO3. The extraction constants (Table 1) also reflect the trend of less propensity to form the UO2(NO3)2(THMA)3 species, i.e. K1,3 decreases with increasing NaNO3 concentration. At the same time, K1,2 increases with increasing NaNO3 concentration, indicating the dominance of the UO2(NO3)2(THMA)2 species at high nitrate concentration. In some cases, the standard deviations for the calculated extraction constants are relatively large. This is especially true for the UO2(NO3)2(THMA) species at 4 and 5 M NaNO3, indicating that the evidence for the existence of this species in the solvent extraction system is weak. (But as discussed below, the existence of UO2(NO3)2(THMA) has been confirmed by TGA and FTIR.)

3.1.2. Extraction of U(VI) with MeTHMA The U(VI) extraction behavior with MeTHMA is similar to that with THMA, but there are some notable differences. First, as with THMA, there is a decreased propensity to form the UO2(NO3)2(MeTHMA)3 species as the NaNO3 concentration increases. This is reflected in the fact that K1,3 decreases with increasing NaNO3 concentration (Table 1). But MeTHMA has a greater propensity than THMA to form the UO2(NO3)2L3 species, as indicated by the higher K1,3 values. A general trend is also seen for increasing K1,2 with increasing NaNO3 concentration up to 4 M NaNO3. K1,2 decreases in going to 5 M NaNO3, where the UO2(NO3)2(MeTHMA) species becomes significant. Second, at the lower nitrate concentrations, the DU values for MeTHMA are similar to those for THMA, but as the nitrate concentration is increased, the DU values for MeTHMA become slightly less than those for THMA. This might reflect the greater propensity of MeTHMA to form the MeTHMA/U complex of higher

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ligand-to-uranyl stoichiometry, i.e. the complexation of the additional MeTHMA decreases the effective MeTHMA concentration in the system. This is perhaps due to the increased basicity of the carbonyl oxygen upon methylation. Alternatively, the lower DU values for MeTHMA might be due to increased steric constraints introduced by the methyl group.

3.1.3. Extraction of U(VI) with DiMeTHMA Interestingly, adding a second methyl group to the methylene carbon in the tetrahexylmalonamide structure results in a significant change in extraction behavior. This is immediately evident in the approximately one order-of-magnitude decrease in the uranium distribution coefficients. The results presented in Table 1 clearly indicated that UO2(NO3)2(DiMeTHMA)2 dominates this system. Thus, it appears that substituting a second methyl group to the methylene carbon results in weaker binding of the malonamide to the uranyl ion, leading to a much lower propensity for forming UO2(NO3)2(DiMeTHMA)3. However, equilibrium modeling did not indicate a significant contribution of the 1:1 U/DiMeTHMA complex in the extraction of U(VI). 3.2. FTIR spectral studies A series of FTIR experiments was undertaken to try to better understand the complexation of uranyl ion by THMA. The top half of Fig. 3 shows the carbonyl region of the FTIR spectrum as THMA/U is varied from 1.0 to 3.0 at a constant U concentration of 0.2 mM in CH3CN. The spectrum of free THMA in CH3CN is also shown in the top half of Fig. 3. These spectra suggest that at least two species formed, indicated by the appearance of bands at 1623 and 1603 cm − 1. The former is predominate at THMA/U = 1.0, while the latter is more apparent at THMA/U = 2.0 and 3.0. As discussed below the band at 1579 cm − 1 is associated with the band at 1623 cm − 1. When a 5 M NaNO3 solution containing 0.027 M UO2(NO3)2 and 0.01 mM HNO3 was contacted with 0.3 M THMA in t-butylbenzene, a third phase was formed, which could be isolated. This third-phase material was washed with water, dried at ambient temperature, and then rinsed with pentane and dried again. The FTIR spectrum of this third-phase material dissolved in CH3CN is shown in the bottom part of Fig. 3. Also shown in the bottom half of Fig. 3 are (i) the spectrum obtained when the spectrum of THMA is subtracted from the spectrum of the THMA/U = 3.0 solution and (ii) the spectrum obtained when the spectrum of THMA and the spectrum of the third-phase species are subtracted from the spectrum of the THMA/U = 3.0 solution. In this manner, we were able to isolate the spectra of two different species — Species 1 (the third-

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Fig. 3. FTIR spectra of THMA/UO2(NO3)2 mixtures in CH3CN — carbonyl region.

phase material) and Species 2. Species 1 displays two distinct carbonyl bands at 1623 and 1579 cm − 1. Species 2 also appears to have two carbonyl bands, but they are not well resolved. These appear as a distinct band at 1602 and a shoulder at 1585 cm − 1. The ‘negative’ absorbances in the lower half of Fig. 3 are due to subtraction of the spectrum of free THMA. Because the concentration of free THMA in the system was not known, the THMA spectrum was subtracted until its absorbances were negative. This ensured that the positive absorbances observed were due to uranyl/THMA complexes and not the free THMA. Fig. 4 shows the FTIR spectral region from 1100 to 1550 cm − 1, which contains the n1 and n4 nitrate stretching bands [21]. The spectrum of UO2(NO3)2 in CH3CN displays two broad bands at 1532 (n1) and 1270 (n4) cm − 1. The 262 cm − 1 difference between these two

bands is typical for a bidentate nitrate ligand. In the case of species 1, the n1 is similarly broad, but is shifted to 1525 cm − 1. The n4 band in Species 1 is split into two bands at 1286 and 1270 cm − 1. The nitrate region in Species 2 is significantly different. In this case, a broad band is observed centered at approximately 1365 cm − 1. This indicates a significant change in the nitrate coordination. We postulate that this change in the nitrate bands indicates a shift from bidentate to monodentate coordination or perhaps even displacement of nitrate ion from the primary uranyl coordination sphere. For comparison, the bottom portion of Fig. 4 shows the spectrum obtained when the spectrum of tetrabutylammonium chloride (TBAC) is subtracted from that of tetrabutylammonium nitrate (TBAN); this yields the spectrum of nitrate ion in CH3CN. In the nitrate spectrum, distinct bands appear at 1382 and 1341 cm − 1.

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Fig. 4. FTIR spectra of THMA/UO2(NO3)2 mixtures in CH3CN — nitrate region.

Thus, the ‘free’ nitrate bands are in a position similar to those seen for Species 2. Finally, Fig. 5 displays the FTIR spectral region from 700 to 1100 cm − 1. The portion of the spectrum further shows the existence of two distinct uranyl complexes as evidenced by the n(OUO) at 936 cm − 1 for Species 1 and 927 cm − 1 for Species 2. Species 1 displays a n2 nitrate band at 1028 cm − 1, which is similar to that at 1027 cm − 1 in UO2(NO3)2.

4. Discussion Direct measurement of the FTIR spectra of the uranyl complexes in the organic phases from the solvent extraction experiments was difficult because the uranyl concentrations achieved in the organic phases were not high enough to obtain clear spectra. However, FTIR spectroscopy has confirmed that two species formed in the solvent extraction system. As mentioned above, Species 1 was obtained as a third phase when a 5 M NaNO3 solution containing 0.027 M UO2(NO3)2 and 0.01 mM HNO3 was contacted with 0.3 M THMA in t-butylbenzene. Similarly, when a 3 M NaNO3 solu-

tion containing 0.027 M UO2(NO3)2 and 0.01 mM HNO3 was contacted with 0.3 M THMA in t-butylbenzene, an isolatable third phase was formed. The FTIR spectrum of this material indicated the presence of both Species 1 and 2, confirming the existence of at least two uranyl complexes in the solvent extraction system. Nigond et al. observed similar FTIR spectral features in the DMDBTDMA/uranyl nitrate system [9]. The FTIR spectrum reported for an equimolar mixture of DMDBTDMA and UO2(NO3)2 in t-butylbenzene was very similar to that reported here for Species 1, i.e. two carbonyl bands were present at 1623 and 1590 cm − 1, with the former band being about twice as intense as the latter band. The nitrate n1 was at 1535 cm − 1, and the n4 band was split into two bands at 1278 and 1261 cm − 1. This pattern is the same as that observed for Species 1 in this work. At a DMDBTDMA/U ratio of 2.2, the FTIR spectrum was quite different than that obtained with equimolar quantities of DMDBTDMA and UO2(NO3)2. Indeed, this spectrum was remarkably similar to that which we obtained for a 2:1 mixture of THMA and UO2(NO3)2 in acetonitrile. Nigond et al. used a deconvolution routine to isolate the individual bands in this spectrum. In addition to bands associated

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Fig. 5. FTIR spectra of THMA/UO2(NO3)2 mixtures in CH3CN — UO2 region.

with uncomplexed DMDBTDMA, bands at 1621, 1588, and 1604 cm − 1 were reported. The bands at 1621 and 1588 cm − 1 were attributed to the species observed at 1:1 DMDBTDMA/U. The band at 1604 cm − 1 was attributed to DMDBTDMA bound in an outer sphere manner. However, based on our work with THMA, we believe it is more likely that this represents an entirely different uranyl complex. That is, this is likely analogous to Species 2 reported here, which appears to contain two malonamide ligands bound to the uranyl ion. As mentioned previously, examples of isolated uranyl diamide complexes typically indicate stoichiometries of the type UO2(NO3)2L. One notable exception is UO2(NO3)2(THMA)2, which has been reported by Ruikar and Nagar [19]. These authors argued that, based on the IR spectrum, the 2:1 THMA/U complex should be formulated as [UO2(NO3)2(THMA)]THMA, with one THMA being bound in an outer sphere manner. However, there are inconsistencies in that paper. First, the carbonyl stretch in the IR spectrum of the THMA/U complex is reported to be at 1535 cm − 1. This is at significantly lower energy for the other amide

and diamide complexes reported in the same paper and those observed in our studies discussed above. We believe that the band at 1535 cm − 1 is actually the n1 band for the coordinated nitrate ligand. Secondly, thermochemical data were presented by Ruikar and Nagar [19] for a number of isolated monoamide and diamide uranyl compounds, but the THMA/U complex was not reported. This is unfortunate because the thermal analysis should easily show that the outer sphere THMA is more easily removed than the bound THMA. We attempted to reproduce the synthesis of the 2:1 THMA/U complex using the method described by Ruikar and Nagar [19], except that t-butylbenzene was used as diluent instead of benzene. A solid third phase formed when the uranyl nitrate solution was contacted with the THMA solution. The FTIR spectrum indicated this material to be the same as Species 1 discussed above. Thermogravimetric analysis of this material indicated the THMA/U ratio to be one; thus this material (Species 1) is formulated to be UO2(NO3)2(THMA). It is instructive to compare the FTIR spectrum for this material to those reported for other UO2(NO3)2(diamide) complexes reported by Ruikar and Nagar.

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Table 2 IR spectroscopic data for UO2(NO3)2(diamide) compounds a Nitrate bands Compound

n(CO)

n1

n4

n2

n6

n3

n(UO2)

UO2(NO3)2(THMA) UO2(NO3)2(TBMA) UO2(NO3)2(TIBMA)

1623, 1579 1575 1580

1525 not reported not reported

1286, 1270 1280 1285

1028 1030 1030

834 815 820

745 750 750

936 940 935

a Data for the tetrabutylmalonamide (TBMA) and tetraisobutylmalonamide (TIBMA) complexes were taken from Ref. [19]. All frequencies are given as wavenumbers (cm−1).

Table 2 presents such a comparison. Clearly, there are similarities between the spectrum we obtained for UO2(NO3)2(THMA) and those reported for UO2(NO3)2(TBMA) and UO2(NO3)2(TIBMA) (TIBMA= N,N,N%,N%-tetraisobutylmalonamide), but there appears to be gaps in the data reported for the latter compounds. The carbonyl stretch reported at 1575 cm − 1 for the TBMA complex and at 1580 cm − 1 for the TIBMA complex agrees with that which we observed at 1579 cm − 1 for the THMA complex. However, the higher energy carbonyl stretch (1623 cm − 1) we observed for the THMA complex (see Fig. 4) was not reported by Ruikar and Nagar. Likewise, these authors did not report values for the nitrate n1 stretch, which we clearly observed at 1525 cm − 1 for the THMA compound (Fig. 4). Single bands were reported for the nitrate n4 stretches in the TBMA and TIBMA compounds, whereas we observed splitting of this band for the THMA compound. Perhaps this splitting was simply not resolved in the previous work. All other bands reported for the TBMA and TIBMA compounds agree well with those for the THMA compound. The equilibrium modeling indicates that UO2(NO3)2L3 was formed. The existence of this species is more speculative. A third type of complex has not been identified by FTIR, and there is no precedence in the literature for such a species forming. Given our understanding of the coordination chemistry of the UO22 + ion, it is unlikely that three tetrahexylmalonamide ligands would bind in bidentate fashion. However, it is possible that one or more of the tetrahexylmalonamide ligands is bound in a monodentate fashion. The notion that bifunctional extractant molecules can bind in a monodentate fashion is certainly not new. NMR spectroscopic studies have indicated that dihexylN,N-diethyl-carbamoylmethylphosphonate (DHDECMP) binds to promethium(III) predominately in a monodentate fashion through the phosphoryl oxygen [22], and IR spectroscopy has indicated the same for lanthanum(III)/nitrate/DHDECMP complexes in dilute solution [23]. Similarly, IR spectroscopic studies have indicated that both DHDECMP and octyl(phenyl) - N,N - diisobutylcarbamoylmethyl - phosphine

oxide (CMPO) bind to neodymium(III) and samarium(III) in a monodentate fashion through the phosphoryl oxygen, except at high metal loading [24]. Monodentate binding of diisopropyl-N,N-diethylcarbamoylmethylphosphonate to erbium(III) has been established by single-crystal X-ray diffraction [25]. Further evidence for monodentate binding of CMPO was provided by small-angle neutron-scattering studies of CMPO/praseodymium complexes [26,27]. These studies indicated that at high Pr loading, the CMPO coordinates in a bridging fashion, with the phosphoryl oxygen bound to one Pr ion and the carbonyl oxygen binding to a second Pr. We have recently reported the crystal structures of the lanthanide complexes Ln2(NO3)6(TMSA)3 (TMSA= tetrahexylsuccinamide), which demonstrate that similar bridging coordination can occur for diamide ligands [28]. Indeed, Kannan and Ferguson have recently demonstrated monodentate binding of a malonamide ligand in the compound [{UO2(C6H5COCHCOC6H5)2}2(C6H5NHO)2CH2] in which the malonamide bridges between two uranyl ions in the solid state [29]. Likewise, Charpin et al. have reported the structure of a polymeric (UO2(NO3)2TBGA)x (TBGA=tetrabutylglutaramide) material containing bridging TBGA ligands [30]. Regardless of whether the tetrahexylmalonamides are bound bidentate or monodentate, the formation of a UO2(NO3)2L3 species would certainly require that at least one nitrate ion be displaced from the primary coordination sphere of the uranyl ion. This would explain the observed nitrate-dependence on the extracted species formed, i.e. higher nitrate concentrations would disfavor displacement of nitrate from the primary coordination sphere and, thus, fewer diamide ligands would bind to the uranyl ion. This is supported by the equilibrium modeling results.

5. Summary and conclusions In this paper, we have examined the extraction of uranyl ion with tetrahexylmalonamides by equilibrium modeling and by FTIR spectroscopy. The equilibrium

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modeling results can be summarized as follows. At 1 M NaNO3, two extracted species are indicated by the equilibrium modeling: UO2(NO3)2L2 and UO2(NO3)2L3. The relative abundance of these two species depends on the tetrahexylmalonamide ligand. The UO2(NO3)2L2 species is dominant in the DiMeTHMA system, with very little formation of the UO2(NO3)2L3 species. In contrast, the UO2(NO3)2L3 species is more predominant in the MeTHMA case. The case of THMA lies in between. The greater propensity of MeTHMA versus THMA to bind in 3:1 fashion to uranyl ion might reflect the greater basicity of the carbonyl oxygens in MeTHMA. The fact that DiMeTHMA (which is even more basic than MeTHMA) only binds almost exclusively in 2:1 fashion suggests that steric constraints are more important for that ligand. As the nitrate concentration is increased, the ligandto-metal ratios in the extracted species tend to decrease. That is, the UO2(NO3)2L2 species tends to predominate, while the UO2(NO3)2L3 species becomes less important. In the case of THMA and MeTHMA, equilibrium modeling suggests the existence of a UO2(NO3)2L species at higher nitrate concentrations. This trend is consistent with third phases isolated from the solvent extraction systems when sufficient UO2(NO3)2 is present in the system. That is, the third phase isolated in the THMA extraction of uranyl nitrate from 3 M NaNO3 consisted of a mixture of Species 1 [UO2(NO3)2(THMA)] and Species 2 [UO2(NO3)2(THMA)2], whereas, the third phase isolated in the THMA extraction of uranyl from 5 M NaNO3 consisted only of Species 1. The FTIR spectral study corroborates that at least two uranyl/diamide complexes formed in this system. The complex UO2(NO3)2(THMA) has been isolated as a third phase in the solvent extraction system, and the THMA/U stoichiometry was unequivocally determined to be 1:1 by thermogravimetric analysis. FTIR spectroscopy indicates that UO2(NO3)2(THMA) is the predominant species formed when equimolar quantities of THMA and UO2(NO3)2 are mixed together in CH3CN. At 2:1 and 3:1 THMA/U ratios in CH3CN, the FTIR spectra clearly indicate that a second species formed. This species was also observed in the third phase isolated from a solvent extraction experiment at 3 M NaNO3. This second species has not been exactly formulated, but the nitrate coordination is significantly different than that for UO2(NO3)2(THMA) and UO2(NO3)2. Indeed, the FTIR spectral data suggest that nitrate might be displaced from the primary uranyl coordination sphere. It is significant to note that this second species has been identified in the solvent extraction system by FTIR spectroscopy. Equilibrium modeling indicates that the 2:1 complex dominates at the conditions used to obtain the extract containing this species. Thus, this second species is tentatively assigned

as UO2(NO3)2(THMA)2. Because the FTIR spectrum indicates the possible displacement of the nitrate ions, this species would perhaps be more correctly formulated as [UO2(NO3)2 − x (THMA)2](NO3)x (where x= 1 or 2). Alternatively, the nitrate ions might shift from bidentate to monodentate coordination to accommodate the second bound THMA ligand. Repeated attempts were made to obtain spectra of solutions under the conditions of the liquid–liquid extraction experiments. The species in solution in the organic phase could not be definitively identified because of the high tetraalkylmalonamide concentrations used in these experiments. However, by small increases in the ratio [UO2(NO3)2]/[THMA] the extraction system is forced into a ternary phase region, allowing assessment of the aqueous/organic insoluble components. Whether the species trapped in this phase are captured en-route to the final extracted product or are indeed the predominant extracted species under the extraction conditions is not known. Nevertheless, their spectra correlate well to single-phase experiments carried out in CH3CN in which UO2(NO3)2 and THMA are mixed in stoichiometric quantities. Acknowledgements Pacific Northwest National Laboratory is operated for the US Department of Energy by Battelle under Contract DE-AC06-76RLO 1830. This work was funded by the US Department of Energy through the Environmental Management Science Program. The authors thank S.A. Bryan and W.C. Cosby for reviewing the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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