Anal. Chem. 1994,66, 2190-2193
Extraction of Lanthanides from Acidic Solution Using Tributyl Phosphate Modified Supercritical Carbon Dioxide K. E. Lalntz' and E. Tachlkawa Japan Atomic Energy Research Institute, Tokakmura, Nakagun, Ibarakkken, 3 19- 11 Japan
The feasibility of using supercritical carbon dioxide as a substitute extractionsolvent in nuclear reprocessingwas tested by the extraction of lanthanide ions from acidic solution. Lanthanides were extracted from 6 M HNOr3 M UNO3 solutio^ using tributyl phosphate- (TBP-) modified C02. Synergisticeffects were also investigated using a combination of thenoyltrifluoroacetone (TTA) and TBP-modified C02 as the extractant. It was found that near-quantitativeextraction , Gd,' and DyMwas achievedwhile the extraction of S 3 + E@, efficienciesfor h 3 + , C P , Yb,' and LuM were much lower. The light lanthanide# extracted as Ln(NOs)r3TBP and the heavy lanthanides extracted as Ln(N03)~.2TBpwhen TBPmodified CO, was used as the extractant, while Ln(TTA)r 3TBP and La(ITA)r2TBP adducts were extracted when I T A was added to TBP-modified C02. Trivalent lanthanides and actinides are byproducts of nuclear processes and must be separated from dissolved fuel solutions during reprocessing operations. One drawback to such processing systems is the generation of large amounts of organicwaste in the form of spent solvents. It may be possible to minimize the generation of liquid wastes by substituting a less expensive, easily recyclable, and generally nontoxic supercritical fluid such as C02 in nuclear waste and fuel processing schemes. In addition, extractions using supercritical C02 are generally more efficient in terms of speed as compared to conventional solvent extraction due to the more favorable mass transport properties of a supercritical fluid. Since the direct extraction of metal ions using supercritical C02 is inefficient due to weak van der Waals interactions and due to the need for cationic charge neutralization, it has been suggested that a chelating agent be introduced into the supercritical fluid extraction stream in order to extract metals.2 While research involving the extraction of metal ions using supercritical C02 containing a dissolved ligand is in its infancy, the extraction of transition metals from aqueous and solid materials293 and f-block elements from solid matrices' using a dissolved ligand in the C02 phase has been accomplished. The extraction of metal ions using this in situ chelation and extraction method has been shown to be more efficient with the use of alcohol-modified C02; however, it may be possible to use a modifier which is itself a complexing agent. One such modifier that has been used in the supercritical fluid Current addresa: L a Alamoa National Laboratory, Chemical Science and Technology Division, CST-1, MS E537, Lcs Alamcs, N M 87545. (1) Brennecke, J. F.; Eckert, C. A. AIChE J . 1989, 35, 1409. (2) Laintz, K. E.; Wai; C. M.; Yonker, C. R.; Smith, R. D. AMI. Chem. 1992, 64, 2875. (3) Wai, C. M.; Lin. Y.; Braucr, R.; Wang, S.;Bcckert, W. F. Talanta 1993,40, 1325. (4) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. AM!. Chem. 1993,155,2549.
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extraction of polar organic analytes is tributyl phosphate (TBP).S Since one method of removing rare earth elements from various process streams involves solvent extraction from an acidic solution with TBP diluted with an organic solvent? it is reasonable to assume that lanthanides could be extracted using TBP-modified C02. In this case, supercritical C02 would function as the diluent in a manner analogous to normal TBP extraction processes. It is also known that the addition of another ligand to a TBP-organicsolvent system can increase the extraction of metal ions. In synergistic solvent extractions of rare earth elements using TBP, thenoyltrifluoroacetone (TTA) is often added as a second extractantS6In order to test the feasibility of waste minimization through the use of supercritical C02 in nuclear processing, the extraction of lanthanides using TBP-modified C02 and TBP-modified CO, containing TTA from an acidic aqueous matrix was studied.
EXPERIMENTAL SECTION Extraction Samples. An aqueous solution consisting of 3 X 10-4 M each of the rare earth elements used in this study (La, Ce, Sm, Eu, Gd, Dy, Yb, Lu) was prepared by dissolving theappropriate amount of the rareearth oxidesand Ce(CO&, all of which were obtained from Shin-Etsu Chemical Co. (Tokyo, Japan), in 1 M HN03. A second solution containing 3 X 10-4 M each of the same lanthanides was prepared by dissolving the appropriate amount of rare earth oxides and Ce(C0,)s in 6 M HN03. Lithium nitrate (reagent grade, Kanto Chemical Co., Tokyo, Japan) was also added to this solution to a concentration of 3 M. An 8-mL aliquot of the appropriate lanthanide stock solution was placed into a 10mL stainless steel aqueous sample extraction vessel (Jasco, Tokyo, Japan) for each extraction experiment. SupercriticalFluid Extractislrs. Off-line supercritical fluid extractions of the lanthanide solutions were performed using an Isco Model 260D syringe pump and controller (Isco, Lincoln, NE). The pump was first filled with an appropriate volume of TBP (extrapure reagent grade, Kanto Chemical Co.) followed by the addition of C02 (four nines grade, ShinTokyo Teisan, Tokyo, Japan) to create lo%, 2096, or 30% (v/v) TBP-modifiedCO2. To prevent the formation of a twophase system within the pump, the pump pressure was ramped up and down several times to ensure mixing. While TBP has high solubilities in many nonpolar solvents, the binary-phase behavior of TBP-modified C02 solvent systems is largely Levy, J. M.; Doh, L.; Ravcy, R. M.; Storozynsky, E.; Holowczak, K. A. J . High Resolut. Chromatogr. 1993, 16, 368. Schulz, W. W., Navratil, J. D., &Is. Science and Technology of Tributyl Phosphate. Volume I: Synthesis, Properties, Reactions. und Analysis;CRC Press, Inc.: Boca Raton, FL,, 1984; p 5.
0003-2700/94/03662190$04.50/0 Chemlcal Society
(8 1994 American
Syringe
Restrictor
Collection Vaael
Sample Extraction Cell
Fluid Heating Coil
Chelating Agent
Extraction Cdl
Flguro 1. Schematic dlagram of the experimental system used for the SFE of lanthanides from an aqueous sample.
uninvestigated. It stands to reason that a 10%volume of TBP in C 0 2should be a single-phase solvent system; however, the phase behavior of 20% and 30% TBP-modified C02 is unknown. For the purpose of this experiment, it was assumed that the TBP-modified COz extraction fluids existed as singlephase systems. The pump was refilled with fresh TBP and C02 prior to each extraction. The resulting TBP-modified C02 was delivered to the aqueoussample extraction vessel after passing through a 1-m coil of l/la-in.-o.d. stainless steel tubing contained within the extraction oven in line prior to the extraction vessel to prewarm the fluid to the desired extraction temperature as in the method of Burford et ala7Each sample was extracted under static conditions (extraction vessel pressurized with solvent having no flow through the cell) for 15 min with the TBP-modified C02, followed by a 30-min dynamic (solvent flow through the cell) extraction. These extraction times were arbitrarily chosen. To study the synergisticeffect of an additional ligand within the TBP-modified COz, a 1-mL ligand extraction vessel made from an HPLC precolumn (GL Sciences, Tokyo, Japan) containing 50 mg of solid TTA was placed in the extraction oven in line between the fluid heating coil and the aqueous extraction vessel, similar to the method used previously for the extraction of Cu*+ using bis(trifluoroethy1) dithiocarbamate.2 This amount of TTA corresponds to slightly more than a 3-fold molar excess of the amount required to form 3:l IanthanideTTA complexes with all of the lanthanides present in the aqueous sample. The TTA sample was extracted under static conditions for 15 min. A valve placed between the TTA extraction cell and thelanthanide solution cell was then opened, and the lanthanide solution was subjected to a 15-min static extraction with TBP-modified C02 containing the dissolved TTA. The overall system was then subsequently extracted for 30 min under dynamic conditions. While the solubility of TTA in C02 and in TBP-modified C02 has yet to be investigated, no residual TTA was found in the ligand extraction vessel after the extraction process, which suggested sufficient solubilities for applicability in this study. (7) Burford, M. D.; Hawthorne, S. B.; Miller, D. J.; Braggins, T. J. Chromorogr. 1992, 609, 321.
The extraction pressure was maintained with a ca. 30 cm 50 pm i.d. fused silica tubing restrictor (GL Sciences). This restrictor allowed for extraction flow rates of about 2 mL/ min (measured at the pump) extraction fluid during dynamic extraction. Extracts were collected by inserting the restrictor through a silicon stopper into a 5050 (v/v) solution of ethanol and water contained in a 100-mL Erlenmeyer flask. Because the resulting C02 gas in the collectionvessel contained noxious TBP vapors, the collection vessel was vented via I/a-in.-o.d. polyethylene tubing into a fume hood after passing through a solvent trap. It should be noted that supercritical fluid extractions of samples with high concentrations of water are known to plug fused silica restrictors: and the restrictors are often heated to prevent this from occurring. In this study, the collection vessel was placed inside of the extraction oven. In this manner, both the restrictor and collection solvent were heated, and restrictor plugging did not occur. A schematic diagram of the overall experimental apparatus used for this extraction experiment is shown in Figure 1. After extraction, the system was allowed to slowly depressurize for about 1 h. Complete depressurization was necessary to avoid the generation of a noxious C02 aerosol containing TBP and 6 M HN03 when the system was opened for sampling and cleaning. ICP-AESAnalysis. A 5-mL aliquot of the stock extraction solution was diluted to 25 mL with 1 M HNO3 in a volumetric flask and analyzed by ICP-AES to determine the lanthanide concentration prior to extraction. Following extraction, 5 mL of the aqueous solution in the sample vessel was removed and diluted to 25 mL with 1 M H N 0 3 in a volumetric flask. This solution was then also analyzed by ICP-AES to determine the amount of lanthanides remaining after extraction. Since the goal of this study was to determine the feasibilityof extracting lanthanides from an aqueous matrix and to determine the extraction efficiency of such a process, the overall collection efficiency was not studied, and the extract solution in the collection vessel was not analyzed. The extraction efficiency was then determined by the amount of lanthanides remaining in the samplesolution after extractionmultiplied by lo0divided by the amount of lanthanides present prior to extraction. The instrumentation used for the analysis of the lanthanides in solution before and after extraction was a KOL Model UOP-2 X
AmWilcaiChemisby, Voi. 66, No. 13, Ju& 1, 1994
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T a b 1. Analyak ParamtwS Used for ICP-AES “ n l n a t k n
ol
lanthanide La Ce
Sm Eu Gd
DY Yb
Lu
emission wavelength (nm)
observn ht above load coil (mm)
333.749 413.765 359.260 38 1.967 342.247 353.170 328.937 261.542
11 11 11 11 11 11 12 12
entrance ‘lit (pm) vertical horizontal 300 500 500 200 300 300 300 300
50 100 100 2s
so so so
50
Derivative ICP-AES from Kyoto-Koken, Inc. (Kyoto, Japan) equipped with an echelle monochromator. Single-channel current signals from the PMiube (R-1463,Hammatsu,Tokyo, Japan) were processed by a current amplifier, digitized and collected by an ADC, and processed using an NEC PC9801DA personal computer. The argon plasma was operated at 1.3 kW with a plasma gas flow rate of 10 L/min, a nebulizer gas flow rate of 0.4 L/min, and an auxiliary gas flow rate of 0.1 L/min. The sample uptake was at 1.5 mL/min, and the vertical and horizontal exit slits were fixed at 500 and 100 pm, respectively. The remaining analysis parameters of the lanthanides investigated are summarized in Table 1.
RESULTS AND DISCUSSION Most industrial-scale applications and the majority of literature citations that involve the TBP extraction of lanthanides areextractions from nitratesolutions. For this reason, lanthanide extraction solutions were prepared using HNO3. In initial experiments, it was found that the various lanthanides used for this study could not be extracted from a 1 M HNOs solution using any of the described experimental procedures. However, the distribution coefficients (generally, the concentration of metal ion in the organic phase divided by its concentration in the aqueous phase) for lanthanides are known to increase with increasing acid concentration and are also known to increase with the addition of salting out agents. The maximum distribution coefficients tend to occur for extractions from solutionscontaining 6-8 M HNO3, with the most effective salting out agents being 3 M LiNO3 and 1 M Al(N03)3.8 Therefore, the extraction of lanthanides from a 6 M HNO3-3 M LiNOs solution was studied. The results of the extraction of the eight lanthanides from a 6 M HNOs-3 M LiNO3 solution using TBP-modified C02 are summarized in Table 2. In addition, the results of the synergistic extraction using TTA in addition to TBP-modified C02 are also shown in Table 2. As shown in the table, the lanthanideions could not be extracted using neat supercritical C02 without a chelating agent. On the other hand, using C0,modified with 10%TBPat 350atmand6OoC,extractions in the range of 20%for the heavier lanthanides were observed. The extraction efficiency increased with increasing TBP concentrations to near-quantitative extraction with C02 modified with 30% TBP and containing TTA. However, at a 30% TBP modifier concentration, the overall extraction enhancement due to the addition of I T A was not that large. The largest extraction enhancement due to the addition of (8) Schulz, W. W.; Navratil, J. D. J. Chromatogr. 1992, 609, 174.
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TTA occurred at a TBP modifier concentrationof 201,with an average increase in the distribution coefficient (percent lanthanide extracted divided by the percent remaining) by a factor of about 2. The overall synergistic effect due to the addition of TTA to the TBP-CO2 solvent stream for all three TBP concentrations studied was an average increase in the lanthanide distribution coefficient of about 1.7. These values tend to be much lower than those reported for conventional solvent extraction?-ll which tend to increase by orders of magnitude. However, the solubility of 1anthanideTTA complexes in supercritical C02 is believed to be small, based on the supercritical fluid chromatographic performance of some transition metal-TTA complexes.12 A low solubility in supercritical CO2 would most likely result in a lower distribution coefficient when compared to liquid solvent extraction. For this reason, to more fully understand the extraction processes, solubility studies of lanthanide-TTA complexes in supercritical C02 are warranted. In any event, theextraction of lanthanides using TTA with 30% (v/v) TBPmodified C02 was nearly quantitative. Also worth noting from the data presented in Table 2 is that the extraction process using TBP-modified C02 would seem to be more selective for the lanthanides that fall in the middle of the group over the heavy and light lanthanides. These results were expected since the distribution coefficients for the extraction of lanthanides from dilute H N 0 3 solutions in the presence of salting out agents are known to increase from La to Ho and then decrease to Lu.13 In addition, this type of extraction behavior has also been observed for the extraction of lanthanides from about 3-6 M HNO3 solutions into CC14 using TBPI4 and in synergistic extractions using TTA and TBP into CCb from 1 M C104- solutions? This extraction phenomenon is based on the “half-filled shell e f f e ~ t ” ~ and ~ J 5 parallels the order of decreasing basicity.” This characteristic extraction profile is often employed in the separation of lanthanides from each other. It was found that the best overall extraction results for the described experimental procedures were observed using a pressure of 350 atm and a temperature of 60 OC for all extractions. At pressures less than 350 atm, the extraction efficiency was markedly less. For example, using C02 modified with 10% TBP at 300 atm, all lanthanides showed extractions of less than 1% as compared to extractions in the range of 20% for the heavier lanthanides using the same conditions at 350 atm. While the extraction temperature of 60 OC was arbitrarily chosen, a large difference in efficiency was not seen using a temperature of 50 OC. Higher temperatures were avoided in an attempt to lessen water loss from the extraction vessel. However, it should be noted that an extraction temperature of 60 OC is likely below the critical temperatures of the various COrTBP mixtures used in this experiment. For this reason it is p s i b l e that a two-phase ( 9 ) Farbu, L.; Alstad, J.; Augutson, J. H. J. Inorg. Nucl. Chem. 1974,36,2091.
29, 1457. (11) Sckioe, T.; Dytsscn, D. J. Inorg. N w l . Chcm. 1%7,29, 1481.
(IO) Sekine, T.; Dytssen, D. 1. Inorg. Nucl. Chem. 1%7,
(12) Lain% K. E.; Meguro, Y.;Iso,S.;Tachikawa,E. J. High Resolut. Chromatogr. 1993, 16, 372. (13) Schulz, W.W.,Navratil, J. D.,Ed,. Science and Technology of Tributyl Phosphate, Volume I I . Part 3: Technical and Industrial Uses;CRC Rcss, Inc.: Boca Raton, FL, 1984; p 13. (14) Peppard, D. F.; Driscoll, W. J.; Sironen, R. J.; McCarty, S.1. Inorg. Nucl. Chem. 1957.4, 326. (IS) Bommer, H.Z . Anorg. Chem. 1939, 241, 273.
T a w 2. P e r d Extractkn of Lanthalck Ions hwn 6 M H atm and 60 O C
W M UNot Solution Udng sUp.rcrltkal COz modllkd wlth TBP at 350
% extraction
C02 modifier neat C02 10% TBP 20% TBP 30% TBP lO%TBP+TTA 20% TBP TTA 30% TBP TTA
+ +
Table 3.
plot
La3+
