Ind. Eng. Chem. Res. 2008, 47, 2803-2807
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Extraction of Trivalent Lanthanides with Oxa-Diamides in Supercritical Fluid Carbon Dioxide Guoxin Tian,*,† Weisheng Liao,‡ Chien M. Wai,*,‡ and Linfeng Rao† Chemical Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Department of Chemistry, UniVersity of Idaho, Moscow, Idaho 83843
This paper describes the results of the extraction of trivalent lanthanides Ln(III) by supercritical fluid CO2 (SF-CO2) containing N,N,N′,N′-tetrabutyl-3-oxapentanediamide (TBODA), one of a group of neutral oxadiamide ligands that have the potential to replace traditional organophosphorus extractants and make the SF-CO2 extraction process more environmentally benign. According to this study, TBODA dissolves in neat SF-CO2 easily, but the solubility of Ln(III)/TBODA complexes in neat SF-CO2 is low and little Ln(III) can be extracted in the SF-CO2 phase. Addition of small amounts of acetone as a modifier in SF-CO2 can significantly improve the extraction of Ln(III) from either aqueous nitric acid solutions or oxide solids. The low solubility of Ln(III)/TBODA complexes in neat SF-CO2 and the improved solubility of Ln(III)/TBODA complexes in acetone-modified SF-CO2 are discussed in terms of the structure of Ln(III)/TBODA complexes and its effect on solute-solvent interactions. 1. Introduction In recent years, extraction of lanthanides and actinides by supercritical fluid carbon dioxide (SF-CO2) has become a subject of study of significantly high interest,1,2 because advanced nuclear energy systems call for the development of separation processes that are innovative, more efficient, and environmentally sustainable. Extraction of lanthanides and actinides by SFCO2 has several advantages over the traditional separation processes in the nuclear industry using hazardous organic solvents, including nontoxicity, recyclability, and ease of solvent removal. Besides, the SF-CO2 medium exhibits gaslike mass transfer properties that could facilitate the kinetics of mass transfer, yet has liquidlike solvation capabilities that make its properties “tunable”. All these advantages could make the SFCO2 process a revolutionary separation process in advanced nuclear energy systems. There have been a number of studies in the past decade investigating the extraction of lanthanides and actinides by SFCO2, but the majority of them involve the use of organophosphorus ligands (e.g., tri-n-butylphosphate (TBP) and carbamoylmethylene phosphineoxide (CMPO)) or fluorinated organic ligands (e.g., thenoyltrifluoroacetylacetone (TTA)) in SFCO2.2-6 These ligands are known to be hazardous to the environment and troublesome in spent nuclear fuel reprocessing. To develop cleaner and more efficient separation processes for advanced nuclear energy systems, it is necessary to explore the use of environmentally sustainable ligands in SF-CO2. For this purpose, we have started the investigations of the extraction behavior of lanthanides and actinides with N,N,N′,N′-tetraisobutyl-3-oxapentanediamide (TiBODA) and N,N,N′,N′-tetrabutyl3-oxapentanediamide (TBODA) in SF-CO2. Oxa-diamide ligands satisfy the “CHON” principle (i.e., consisting of only C, H, O, and N atoms) so that they are completely incinerable and do not generate “secondary” solid or liquid wastes. Replacing hazardous TBP/TTA ligands with oxa-diamides in SF-CO2 could make the SF-CO2 extraction process more environmentally * To whom correspondence should be addressed. † Lawrence Berkeley National Laboratory. ‡ Department of Chemistry.
sustainable. However, though oxa-diamide ligands have previously been shown to extract actinides and some fission products well in conventional solvent extraction,7-13 their use in SF-CO2 has not been systematically studied. In a recent study, Koh et al. obtained very large distribution coefficients for U(VI) and lanthanides in SF-CO2 containing N,N,N′,N′-tetrabutyl-3-oxapentanediamide (TBODA) by observing the decrease in the metal concentrations in a nitric acid solution in contact with SF-CO2.14 The results are encouraging, but they need to be corroborated because the amount of the dissolved metal complexes in SF-CO2 was not measured directly but was obtained by the difference of the metal ion concentration in the aqueous phase before and after the extraction. There was no direct evidence that TBODA complexes of U(VI) and lanthanide ions were actually present in the SF-CO2 phase in their extraction experiments. In this work, we have studied the extraction behavior of lanthanide ions from nitric acid solution and lanthanide oxide solids using TBODA in SF-CO2. The lanthanide species in both aqueous and SF-CO2 phases were examined by absorption spectroscopy. Mass balance in the extraction process was checked by quantifying the amounts of lanthanides in various fractions including the trapping solution, aqueous phase, SFCO2 phase, and a “third phase” that was found stuck to the inner wall of the reaction vessel in the absence of a modifier. The use of a modifier (acetone) in SF-CO2 to improve the extraction and the possibility of using TBODA in SF-CO2 for decontamination and solid dissolution were also evaluated. 2. Experimental Section 2.1. Chemicals. TBODA was synthesized by following the procedure in a previous work.9 Powder Nd2O3 and Eu2O3 (Aldrich, 99.9%) were used as purchased. Nd(III) and Eu(III) nitrate solutions were prepared by dissolving weighed oxides with concentrated HNO3, fuming the solutions dry, and dissolving in 1 M HNO3. The TBODA-HNO3 complex with TBODA/HNO3 molar ratio about 1:1 was prepared by mixing weighed TBODA and concentrated HNO3 (68-70%). In this case, TBODA is used as a Lewis base to carry nitric acid into the SF-CO2 phase. A similar tri-n-butylphosphate (TBP)-nitric
10.1021/ie071422l CCC: $40.75 © 2008 American Chemical Society Published on Web 03/13/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 8, 2008
Figure 1. Setup for the dynamic extraction/dissolution experiments.
Figure 2. Absorption spectra of Nd(III) in TBODA/SF-CO2 extraction. Spectrum I, Nd(III) in 1 M aqueous HNO3 solution before extraction; spectrum II, aqueous phase after extraction; spectrum III, SF-CO2 phase after extraction; spectrum IV, acetone wash of the oily material on the inner wall of the reaction cell; spectrum V, Nd(III)/TBODA complex in acetone. The absorbances of spectra are not normalized to correct for the difference in optical path in different measurements.
acid complex for dissolution of lanthanide oxides and uranium oxides in SF-CO2 was described in previous reports.15-17 Nitric acid is needed to dissolve lanthanide oxides and to provide nitrate counteranions for SF-CO2 extraction of the Ln(TBODA)3 complex. 2.2. Static Extraction of Nd(III) from Nitric Acid Solutions and from Solid Nd2O3. The experimental setup for static extraction of Nd(III) from nitric acid solution and from Nd2O3(s) is the same as described in previous publications.4,15 For the experiments with nitric acid solutions, an aqueous solution of Nd(III) in 1 M HNO3 and the TBODA ligand were placed in the reaction cell first. The cell was sealed and heated to a preset temperature while pumping CO2 into the cell to an expected pressure. The extraction was conducted with 0.18 g of TBODA and 5 mL of 0.01 M Nd(NO3)3 in 1 M HNO3 in a 10 mL reaction cell, with optical fiber connected to a spectrophotometer. The temperature and pressure of the extraction system vary from 40 to 70 °C and from 150 to 200 atm. The window on the view cell allowed visual observation of the phases during the experiments. For the experiments where the absorption spectra of Nd(III) needed to be collected on-line, a cell with optical fibers connected with a CCD-array UV-vis spectrometer (Model 440, Spectral Instruments, Inc., Tucson) was used to replace the view cell. The optical path was adjustable, varying from 2 to 5 mm according to specific experimental conditions. For the experiments with Nd2O3(s), a secondary container (a beaker) was placed in the reaction cell to contain Nd2O3 solid while pre-prepared TBODA-HNO3
complex was placed in the reaction cell but outside the beaker. This setup helped to minimize the contact of Nd2O3(s) with liquid TBODA-HNO3 complex before the latter completely dissolved in CO2. 2.3. Dynamic Dissolution/Extraction of Eu(III) from Solid Eu2O3. To evaluate the efficiency of decontamination of lanthanides and actinides from paper/fabric tissues using TBODA in SF-CO2, dynamic dissolution/extraction experiments were conducted with a slightly modified experimental setup (Figure 1). Two cells are connected in tandem. The first cell contains pre-prepared TBODA-HNO3 complex (with or without acetone as the modifier) and is connected with the CO2 pump. The second cell is cylindrical in shape (6.9 mL) and a small piece of filter paper or fabric tissue loaded with sorbed Eu(NO3)3 or Eu2O3 solids was placed at the entrance end of the cell. During the experiment, SF-CO2 saturated with TBODAHNO3 complex from the first cell contacts the solid Eu(III) compound in the second cell. After dissolution/extraction, the paper or fabric tissue was taken out of the cell and the europium remaining on the tissue was examined by taking photographs under a UV lamp. Both the static and dynamic experiments were conducted with neat SF-CO2 and SF-CO2 + acetone as the extraction media. 3. Results and Discussion 3.1. Static Extraction of Nd(III) from Nitric Acid Solutions with TBODA in Neat SF-CO2. 3.1.1. Solubility of TBODA. By visual observation of the homogeneity of the extraction system through the viewing window while gradually increasing the amount of TBODA, the solubility of TBODA was found to be about 1 g in 17 mL of SF-CO2, i.e., ∼0.15 mol‚dm-3, at 50 °C and 150 atm. This value is comparable to the solubility of TBODA reported by Koh et al.14 3.1.2. Distribution of Nd(III) in the Extraction. The steady state of extraction was usually achieved within 20 min. Before extraction, the absorption spectrum of Nd(III) in the aqueous solution was collected to verify the existence and the concentration of Nd(III). This spectrum is shown in Figure 2 as spectrum I. The resolution of the transition band of Nd(III) at 575 nm is not high due to instrumental limitations, but the position and intensity are in fairly good agreement with those for free Nd3+ in acidic solutions reported in the literature.18 After the steady state of extraction was achieved, absorption spectra of both the aqueous and SF-CO2 phases were collected on-line. As expected, no Nd(III) was found in the aqueous phase (spectrum II). However, it was surprising that Nd(III) was also absent from the SF-CO2 phase (spectrum III in Figure 2) across the range of temperature and pressure in these experiments. The absence
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Figure 3. (top) Change of absorption spectra of Nd(III) in aqueous phase as a function of amount of TBODA in the reaction cell (50 °C, 200 atm). The ligand concentration varies from 0 to 3.6 g in the extraction cell. (bottom) Plot of absorbance at 575 and 797 nm vs molar ratio of TBODA/Nd(III).
Table 1. Distribution of Nd(III) or Eu(III) in the TBODA/SF-CO2 Extraction System experiment static extraction of Nd(III) from HNO3 static extraction of Nd(III) from Nd2O3(s) dynamic extraction of Nd(III) from Nd2O3(s) dynamic extraction of Eu(III) from Eu2O3(s)
Nd(III) after extraction (%) neat SF-CO2 with acetone neat SF-CO2 with acetone with acetone neat SF-CO2 with acetone
T/P (oC/atm)
aq phase
SF-CO2 phase
trapping soln
“third” phase
50/200 50/200 50/150 50/150 50/200 50/150 50/150
