Supercritical Fluid Extraction of Radionuclides - American Chemical

Chapter 9

Supercritical Fluid Extraction of Radionuclides: A Green Technology for Nuclear Waste Management

Downloaded by UNIV OF ARIZONA on August 10, 2012 | http://pubs.acs.org Publication Date: September 21, 2006 | doi: 10.1021/bk-2006-0943.ch009

Chien M. Wai Department of Chemistry, University of Idaho, Moscow, ID 83844

Supercritical fluid carbon dioxide is capable of dissolving uranium dioxide directly with a CO -soluble n-tributylphosphate-nitric acid complex. The extracted uranyl compound UO (NO ) (TBP) has a solubility of about 0.4 mol/L in supercritical CO at 40 °C and 200 atm. Cesium and strontium can also be extracted by supercritical CO using a crown ether and a fluorinated counteranion. Using dicyclohexano-18-crown-6 as the ligand and pentadecafluoron-octanoic acid as the counteranion, Sr can be selectively extracted over Ca and M g by supercritical C O at 60 °C and 100 atm. Supercritical CO which can penetrate into small pores of solid materials and extract radionuclides with minimal liquid waste generation appears to provide an attractive green technology for nuclear waste management. 2

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© 2006 American Chemical Society In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Introduction Utilizing supercritical fluids as solvents for extraction, separation, synthesis, and cleaning has been a very active research area in the past two decades (1-3). This technology development is mainly due to the changing environmental regulations and increasing costs for disposal of conventional liquid solvents. Carbon dioxide is widely used in supercritical fluid extraction (SFE) applications because of its moderate critical constants (T =31.1 °C, P =72.8 atm, φ = 0.471 g/mL), inertness, low cost, and availability in pure form. However, because C 0 is a linear triatomic molecule (with no dipole moment), it is actually a poor solvent for dissolving polar compounds and ionic species. A method of dissolving metal ions in supercritical C 0 was developed in the early 1990s using an in situ chelation technique. In this method, a C0 -soluble chelating agent is used to convert metal ions into soluble metal chelates in the supercritical fluid phase. Quantitative measurements of metal chelate solubilities in supercritical C 0 were first made by Laintz et al. in 1991 using a high-pressure view cell and UV/Vis spectroscopy (4). In this pioneering study, the authors noted that fluorine substitution in the chelating agent could greatly enhance (by 2 to 3 orders of magnitude) the solubility of metal chelates in supercritical C 0 . A demonstration of copper ion extraction from solid and liquid materials using supercritical C 0 containing a fluorinated chelating agent bis(trifluoroethyl)dithiocarbamate was reported in 1992 (5). Since then, a variety of chelating agents including dithiocarbamates, β-diketones, organophosphorus reagents, and macrocyclic ligands have been tested for metal extraction in supercritical fluid C 0 (6). Highly C0 -soluble metal complexes involving organophosphorus reagents have been found and reported in the literature (7). Recently, direct dissolution of metal oxides such as uranium dioxide and lanthanide oxides in supercritical C 0 using a tri-nbutylphosphate-nitric acid complex as the extractant has also been demonstrated (8, 9). These studies have greatly expanded potential uses of the supercritical fluid extraction technology for metal related applications. The in situ chelation-SFE technique appears attractive for nuclear waste management because it can greatly reduce the secondary waste generation compared with the conventional processes using organic solvents and aqueous solutions. Other advantages of using the SFE technology for nuclear waste management include fast extraction rate, capability of penetration into small pores of solid materials, and rapid separation of solutes by depressurization. The tunable solvation strength of supercritical fluid C 0 also allows potential separation of metal complexes based on their difference in solubility and partition between the fluid phase and the matrix. This article summarizes recent developments regarding SFE of radionuclides, particularly with respect to uranium extraction to illustrate the potential of the SFE technology for removing long-lived radioisotopes from contaminated wastes. c

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In Nuclear Waste Management; Wang, P., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Supercritical CO2 Extraction of Lanthanides and Actinides Lanthanide and actinide ions in solid and liquid materials can be extracted using a chelating agent such as a β-diketone dissolved in supercritical C 0 (6, 10, 11). Fluorine containing β-diketones are more effective than the nonfluorinated acetylacetone for SFE of the f-block elements. In several reported SFE studies for lanthanide and actinides, thenoyltrifluoroacetone (TTA) was used as a chelating agent. One reason for using TTA is that it is a solid at room temperature (m.p. 42 °C) and is easy to handle experimentally. Other commercially available fluorinated β-diketones, often in liquid form at room temperature, have also been used for SFE of lanthanides and actinides. A strong synergistic effect was observed for the extraction of lanthanides from solid samples when a mixture of tri-n-butylphosphate (TBP) and a fluorinated β-diketone was used in supercritical C 0 (10, 11). TBP is highly soluble in supercritical C 0 with a solubility close to 10 mole percent under normal SFE conditions. Actually at a given temperature, TBP becomes miscible with supercritical C 0 above a certain pressure according to a recent phase diagram study of the TBP/C0 system (12). The synergistic extraction effect is probably due to an adduct formation with TBP replacing a coordinated water molecule in the lanthanide-TTA complex, thus increasing the solubility of the adduct complex. Uranyl and thorium ions in solids and in aqueous solutions can also be extracted by supercritical C 0 containingfluorinatedβ-diketones. For example, spiked U 0 and Th in sand can be extracted by supercritical C 0 containing TTA with efficiencies around 70-75% at 60 °C and 150 atm with 10 min of static and 20 min of dynamic extraction (//). Using a mixture of TTA and TBP, the extraction efficiencies of U 0 and Th are increased to > 93%. The feasibility of extracting uranyl ionsfromnatural samples was tested using mine wastes collected from an abandoned uranium mine. The uranium concentrations in two mine waters tested were 9 ^ g / m L and 18μg/mL, respectively. The mine waters were extracted with a 1:1 mixture of TTA and TBP in neat C 0 at 60 °C and 150 atm for a static time of 10 min followed by 20 min of dynamic extraction. Under these experimental conditions, the percent extraction of uraniumfromthese samples was 81% ± 4% and 78% ± 5%, respectively. The mine waters were also added to a soil sample and dried at room temperature for SFE tests. The percent extraction of uranium using TTA/TBP was in the range of 77-82% at 60 °C and 150 atm. Recently, Fox and Mincher showed that plutonium spiked in a soil can also be extracted by TTA/TBP with efficiencies around 70-80% (13). In highly acidic solutions (1-6 M HNO3), organophosphorus reagents such as TBP and TBPO (tri-butylphosphine oxide) dissolved in supercritical C 0 can extract uranyl ions (U0 ) and thorium ions (Th ) effectively (14). Uranyl nitrate alone does not show an appreciable solubility in supercritical C 0 . But, when it is coordinated with TBP, the uranyl nitrate TBP complex


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164 (υθ )(Ν0 ) ·2ΤΒΡ becomes highly soluble in supercritical C 0 (7). The extraction efficiencies for U 0 and Th using TBP saturated supercritical C 0 are comparable to those observed in solvent extraction with kerosene containing 19%v/vTBP (14). The solubilities of (U0 )(N0 ) -2TBP in supercritical C 0 in the temperature range 40-60 °C and pressure range 100-200 atm were measured using a spectroscopic method (7). The solubility of this important uranyl complex in supercritical C 0 is about 0.4 mol/L at 40 °C and 200 atm. This uranium concentration is similar to that used in the conventional purex (plutonium uranium extraction) process. In comparison with U0 (N0 ) -2TBP, the solubility of U0 (TTA) TBP in supercritical C 0 is about an order of magnitude lower (75). This information indicates that uranium and transuranics in solid materials can be extracted directly by supercritical C 0 containing a mixture of TBP and a fluorinated β-diketone or TBP and nitric acid. 2

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Direct Dissolution of Uranium Dioxide in Supercritical C 0

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Direct dissolution of solid U 0 in supercritical C 0 is difficult because uranium at +4 oxidation state does not form stable complexes with commonly known ligands. An oxidation step is needed to convert uranium from +4 to+6 oxidation state to make it extractable in supercritical C 0 . Recent reports show that TBP forms a complex with nitric acid which is soluble in supercritical C 0 and is capable of dissolving lanthanide oxides and uranium oxides directly (8, 9). The complex is prepared by mixing TBP with a concentrated nitric acid solution. Nitric acid dissolves in the TBP phase forming a complex of the general formula TBP(HN0 ) (H 0) that is separated from the remaining aqueous phase. The TBP-nitric acid complex is soluble in supercritical C 0 , and it is capable of dissolving solid uranium dioxide directly. The U 0 dissolution process probably involves oxidation of the tetravalent uranium in U 0 to the hexavalent uranyl followed by formation of U0 (N0 ) -2TBP, which is known to have a high solubility in SF-C0 . Dissolution of U 0 by a TBP-nitric acid complex depends on the stoichiometry of the complex and the density of the supercritical fluid phase. Figure 1 shows the amount of U 0 dissolves in the supercritical fluid stream containing the TBP(HN0 )o. (H 0)o.7 complex at a flow rate of 0.4 mL/min. The supercritical C 0 solution was produced by bulbing liquid C 0 through a cell containing the TBP(HN0 )o. (H 0)o.7 complex at room temperature (around 23 °C). The dissolution of U 0 using the TBP(HNO ) (H O) . complex in supercritical C 0 is more rapid because of a higher nitric acid concentration provided by the complex. The alkali metals, the alkaline earth metals, and a number of transition metals cannot be extracted by the TBP-nitric acid complex in supercritical C 0 . Sonification can enhance the dissolution rate of tightly 2

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Figure 1. Dissolution ofU0 and U0 in supercritical C0 using TBP(HN0 )OJ(H 0)O.7 as an extractant at 60 °C and 150 atm, flow rate -0.4 mL/min (Reproduced with permissionfromreference 9. Copyright 2001 Royal Society of Chemistry.) 2

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packed U 0 powders significantly, probably by increasing the transport of U0 (N0 )2(TBP) from the solid surface to the supercritical fluid phase (16). Dissolution of U 0 by a TBP-nitric acid complex (e.g. TBP(HNO ) .7(H O) .7 or ΤΒΡίΗΝΟ^.βζΗ^ο.β) in supercritical C 0 is more efficient than the dissolution of U 0 . This can be attributed to the fact that the uranium in U 0 is already in the +6 oxidation state, thus no oxidation is required in the formation of the uranyl complex U0 (N0 ) (TBP) . In the conventional purex process, aqueous nitric acid (3-6 M) is first used to dissolve and oxidize U 0 in the spent fuel to uranyl ions (U0 ) . The acid solution is then extracted with TBP in dodecane to remove the uranium as U0 (N0 ) -2TBP into the organic phase. The SF-C0 dissolution of U 0 with a TBP-nitric acid complex has advantages over the purex process since it combines dissolution and extraction into one step with minimum waste generation. This green SF process that requires no aqueous solution and organic solvent may have potential applications for processing spent nuclear fuel, decontamination of U 0 contaminated wastes, and even for processing of rare earth ores. 2

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Characterization of TBP-Nitric Acid Complex The chemical nature of the TBP-HNO3 complex in a SF-C0 solution is not well known. It is known that water can dissolve in TBP during mixing. The solubility of water in TBP at room temperature is about 64 grams per liter, which is close to a 1:1 mole ratio of a TBP:H 0 complex. The complex is most likely formed through hydrogen bonding of H 0 with the P=0 group of TBP. When concentrated nitric acid is mixed with TBP by shaking, nitric acid also dissolves in the TBP phase forming a TBP(HN0 ) (H 0) complex that is separated from the remaining aqueous phase. The χ and y values in the complex can be determined by acid-base titration and by Karl Fischer titration, respectively. According to a recent report, the χ and y values in the complex depend on the relative amount of the acid and TBP used in the preparation. For example if 5 mL of TBP is mixed with 5 mL of concentrated nitric acid (15.5 M), the TBP phase has a composition of TBP(HN0 ) (H 0)o. . If 5 mL of TBP is mixed with 1 mL of the concentrated nitric acid, the TBP phase has a composition of ΤΒΡ(ΗΝ0 )ο. (Η 0)ο. . Proton NMR studies indicated that the protons of H N 0 and H 0 in the complex showed only a single peak, suggesting a rapid exchange of the protons like in a nitric acid solution (77). This single proton resonance peak shifts upfield with increasing x/y ratio in the complex. The proton resonance peak of ΤΒΡΉ 0 complex appears at 3.85 ppm. In a TBP(HN0 ) (H 0)y complex, the proton resonance peak shifts upfield and reaches around 11.2 ppm when the x/y ratio is 3. When the TBP(HN0 )o.7(H 0)o.7 complex is added to a low dielectric constant solvent such as chloroform (ε = 4.18 at 20 °C), fine droplets of nitric acid are formed as indicated by the NMR spectrum shown in Figure 2. The small peak at 6.49 ppm in Figure 2 is the nitric acid droplets formed in the system. The formation of these fine droplets of acid in chloroform can be attributed to an anti-solvent effect because H N 0 and H 0 are sparsely soluble in chloroform. The same anti-solvent effect is expected to occur in supercritical C 0 , which has a very small dielectric constant. When the TBP(HNO ) . (H O) .7 complex was added to supercritical C 0 , the solution becomes cloudy, suggesting formation of small acid droplets in the fluid phase. These small droplets of nitric acid formed in the supercritical C 0 phase can oxidize U 0 to ( U 0 ) that is followed by the formation of U0 (N0 ) (TBP) which becomes soluble in supercritical C 0 . Assuming the TBP-HN0 complex has a 1:1 stoichiometry, the dissolution of U 0 by this complex may be expressed by the following equation: 2

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U0 (s) + 8/3 TBP-HN0 -> U0 (N0 ) (TBP) + 3/2 NO + 4/3 H 0 + 2/3 TBP 2

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For complexes with other TBP:HN0 ratios, similar equations can be written. 3



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F/gare 2. Proton NMR spectra ofTBP(HNO ) . ?(H2O) .7. Top: 300 MHz NMR spectrum of the complex using an insert; bottom: 300 MHz NMR spectrum of the complex in CDCl . 3

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Supercritical C0 Extraction of Cesium and Strontium 2

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Crown ethers are selective extractants for Cs and Sr in conventional solvent extraction processes. For example, it is known that 18-membered crown ethers with cavity diameters in the range of 2.6 to 2.8 Â are the most suitable hosts for Sr * (2.2 Â cationic diameter). For Cs* with a 3.34 À cationic diameter, the 21-crown-7 host with a cavity diameter in the range of 3.4-4.3 À is suitable for selective complexation of the cation. However, these crown-metal complexes are not soluble in supercritical C0 . One method of making the crown-metal complexes soluble in supercritical C 0 is to use a fluorinated counteranion to extract the complexes in the supercritical fluid phase as ionpairs. Τ he fluorinated c ounteranions can η eutralize t he charge ο f t he c rownmetal complexes and make the resulting ion-pair soluble in supercritical C0 . Fluorinated counteranions such as pentadecafluroro-n-octanoic acid and perfluoro-l-octanesulfonic acid (ammonium or potassium salt) are effective for selective extraction of Cs and Sr * using crown ether 1 igands in supercritical C0 . The extraction efficiency depends on the amount of crown ether and counteranion used in the extraction. Table I shows some data related to the SFE of Sr * from water using dicyclohexano-18-crown-6 (OC18C6) as the ligand and pentadecafluoro-n-octanoic acid (PFOA-H) as the counteranion in supercritical C 0 at 60 °C and 100 atm. The extraction is selective for Sr * over Ca * and Mg *. Selective extraction of Sr * in acidic solutions can also be achieved using DC18C6 and a potassium salt of PFOA with a high efficiency (18). 2

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Table I. Extraction of Sr *, Ca *, and Mg * from Water by Supercritical C0 Containing DC18C6 and Pentadecafluoro-n-octanoic acid at 60 °C and 100 atm % Extraction Mole Ratio S^ Co * PFOA-H : DC18C6 : 1 0 10 0 0 1 1 0 10 1±1 1±1 4±1 1 5 10 36±2 1±1 1±1 1 10 10 52±2 2±1 1±1 1 10 50 7±2 2±1 98±2 2

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Note: The aqueous solution contained a mixture of Si^Ca *, and Mg * with a concentration of 5.6 χ ΙΟ" M each; pH of water under equilibrium with SF C 0 = 2.9; 20-min static followed by 20-min dynamic flushing at a flow rate of 2mL/min. PFOA-H = CF (CF ) COOH. (Datafromreference 18.) 5

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169 Dicyclohexano-21-crown-7 (DC21C7) with perfluoro-l-octanesulfonic acid as a counteranion can be used to extract Cs in supercritical C 0 . However, potassium ion (K ) can also be extracted with Cs in this case (19). A more selective macrocyclic ligand is needed in order to achieve selectivity and efficiency for Cs extraction in supercritical C 0 . +

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Potential Applications The direct dissolution of UO in supercritical CO using a TBP-nitric acid complex described in this section has many potential applications. For example, this technique can be used to remove uranium from U O contaminated wastes, such as those generated by nuclear fuel fabrication plants. Ash samples spiked with U O can be quantitatively extracted by supercritical CO containing the TBP(HNO ) (H 0) complex at 60 °C and 200 atm using a combination of static and dynamic extraction (20). Contaminated uranium dioxide in real ash samples from a fuel fabrication plant could also be removed from the ash with efficiencies up to 85%. The possibility of using supercritical fluid solutions for reprocessing spent nuclear fuels was suggested by Smart et al. several years ago (21). Recent development in direct dissolution of U 0 using a TBP-nitric acid complex makes this suggestion more acceptable. The possibility of dissolving UO directly from spent nuclear fuels for their reprocessing in supercritical C O is currently being tested by a team in Japan. The Japanese demonstration project (2002-2005) involves Mitsubishi Heavy Industries, Japan Nuclear Cycle Corp., and Nagoya University (16). The project is aimed at extracting uranium and plutonium from the mixed oxide fuel as well as the irradiated nuclear fuel using the TBP(HNO ) (H O ) complex as an extractant in supercritical CO . This project will provide valuable information regarding safe handling of nuclear spent fuels in a high-pressure supercritical fluid extraction system. This information should also be useful for considering other applications of the supercritical fluid technology for treatment of a variety of nuclear waste related problems. 2

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Acknowledgments This work was supported by the U.S. Department of Energy Environmental Management Science Program (grant number DE-FG07-98ER14913).

References 1. 2.

McHugh, Μ. Α.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworth-Heinemann: Oxford, U.K., 1994. Darr, J.; Poliakoff, M . Chem. Rev. 1999, 99, 495-541.


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Phelps, C. L.; Smart, N . G.; Wai, C. M . Chem. Edu. 1996, 12, 1163-1168. Laintz, K. E.; Wai, C. M . ; Yonker, C. R.; Smith, R. D. J. Supercrit Fluids 1991, 4, 194-198. 5 Laintz, K. E.; Wai, C. M.; Yonker, C. R.; Smith, R. D. Anal. Chem. 1992, 64,2875-2878. 6. Wai, C. M . ; Wang, S. J. Chromatography A 1997, 785, 369-383. 7. Carrott, M . J.; Waller, Β. E.; Smart, N . G.; Wai, C. M . Chem. Commun. 1998, 373-374. 8. Tomioka, O.; Meguro, Y.; Iso, S.; Youshida, Z.; Enokida, Y.; Yamamoto, I. J. Nucl. Sci. Technol. 2001, 38, 1097. 9. Samsonov, M . D.; Wai, C. M.; Lee, S. C ; Kulyako, Y.; Smart, N. G. Chem. Commun. 2001, 1868-1869. 10. Lin, Y.; Wai, C. M . Anal. Chem. 1994, 66, 1971-1975. 11. Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1994, 28, 1190-1193. 12. Joung, S. N.; Kim, S. J.; Yoo, K. P. J. Chem. Eng. Data 1999, 44, 10341036. 13. Fox, R. V.; Mincher, B. J. In Supercritical Carbon Dioxide Separations and Processes, Gopalan, S., Wai, C. M., Jacobs, H. K., Eds.; ACS Symposium Series 860; American Chemical Society: Washington, DC, 2003; chapter 4, pp 36-49. 14. Lin, Y.; Smart, N . G.; Wai, C. M . Environ. Sci. Technol. 1995, 29, 27062708. 15. Wai, C. M . ; Waller, B. Ind. Eng. Chem. Res. 2000, 39, 3837-3841. 16. Enokida, Y.; Sami, A. E.; Wai, C. M . Ind. Eng. Chem. Res. 2002, 41, 22822286. 17. Enokida, Y.; Tomioka, O.; Lee, S. C ; Rustenholtz, Α.; Wai, C. M . Ind. Eng. Chem. Res. 2003, 42, 5037-5041. 18. Wai, C. M.; Kulyako, Y.; Yak, Η. K.; Chen, X., Lee, S. J. Chem. Commun. 1999,2533-2535. 19. Wai, C. M.; Kulyako, Y. M.; Myasoedov, M . Commun. 1999, 180, 181. 20. Meguro, Y.; Iso, S.; Yoshida, Z.; Ougiyanagi, J.; Uehara, Α.; Enokida, Y.; Yamamoto, I.; Tomioka, O.; Yamamoto, Α.; Wada, R.; Yamaguchi, K. Decontamination of Uranium Waste Based on Supercritical Carbon Dioxide Fluid Leaching Method, Proceedings of Super Green Conference 2002, Kyung Hee University, Korea, Nov 3-6,2002, pp 179-183. 21. Smart, N . G.; Wai, C. M . ; Phelps, C. Chem. Britain 1998,34, 34-36.


Supercritical Fluid Extraction of Radionuclides - American Chemical

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