Environ. Sci. Techno/. 1995, 29, 2706-2708
TABLE 1
Urhium d Thorium from Nitric
0
ats
Reagents
Y U E H E L I N , + N E I L G . S M A R T , + , *A N D C H I E N M. WAI*>' Department of Chemistry, University of Idaho, Moscow, Idaho 83843, and Company Research Laboratory, British Nuclear Fuels plc, Springfields Site, Salwick, Preston, PR4 OXJ U.K.
Extraction techniques for the recovery of uranium and transuranic elements from acid waste solutions are important in nuclear waste management. Current technology for the removal of uranium and plutonium from acidic nuclear wastes and from dissolved spent fuel relies on solvent extraction with tributyl phosphate (TBP) diluted in a hydrocarbon solvent such as kerosene (I). Recent studies in supercritical fluid extraction (SFE) indicate that lanthanides and actinides in solid and liquid materials can be effectively extracted by C02 containing a mixture of TBP and a fluorinated P-diketone at 60 "C and 150 a m (2,3). Supercritical C02 modified with 30% TBP v/v (-9% mol fraction) at 60 "C and 350 atm has also been shown to extract trivalent lanthanide ions in acid solutions yielding extraction efficiencies varying from 61 to 92% (4). One advantage of using supercritical COa for metal extraction is that the production of organic solvent waste can be minimized. Carbon dioxide on the other hand is environmentally benign, economical, and easily recyclable. This paper examines the feasibility of extracting uranyl and thorium ions from nitric acid solutions with supercritical COz containing the different organophosphorus reagents shown below: RO
\
RO-P=O
/
TBP
(R = n - C d H g )
TBPO TOPO TPPO
(R = n- C4H9)
RO
R\
R-P=O
R/
( R = n-C~H17) (R = C6H5)
TBP is a viscous liquid at room temperature (bp 289 "C). The phosphine oxides TBPO (tributylphosphine oxide), TOPO (tri-n-octylphosphine oxide), and TPPO (triphenylphosphine oxide) are solids with melting points of 64-69, 50-54, and 156- 158 "C, respectively. These organophosphorus reagents are known to form coordinativelysolvated salts with lanthanides and actinides through the P-0 group (5, 6). The solubilities of the phosphine oxides in supercritical COZ are not known. The phase behavior of TBPC02 was reported by Page et al. for mixtures containing upto 15.5 mol % TBP (7). In this study, an organophosphorus reagent is dissolved in supercritical COz by passing +
University of Idaho. British Nuclear Fuels plc.
2706
compounds
SFE condition
TBPO TOPO
200 atm, 60 "C 150 atm, 40 "C 200 atm, 45 "C 200 atrn, 60 "C
TPPO
solubility (mole fraction) 5.1 x
IO-*
2.9
10-3 10-3 10-4
3.5 4.6
the fluid through a reagent vessel placed upstream of the sample vessel in the extractor.
Experimental Section
Introduction
7
Solubility of TBPO, TOPO, and TPPO in Supercriiical Carbon Dioxide
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 10.1995
All nitric acid solutions used in this study contain 50 pg/ mL eachuranyl and thorium ions. All other cited chemicals used were of analytical reagent grade. The SFE experiments were performed with a lab-built SFE apparatus described in a previous paper (2).A restrictor of stainlesssteel capillary (ISCO) was used to maintain the extraction pressure. The flow rate of the supercritical COZwas approximately 1.4 mL min-I at 60 "C and 200 atm. The main body of the liquid extraction vessel was made from a stainless steel column (0.75 cm i.d. and 14 cm in length, cell volume 6.2 mL,Alltech),and the infittingswere obtained from Swagelok (Seattle,WA). The vessel was modified for used with liquid samples using the same procedure reported by Hendrick et al. (8).TBP placed in a liquid vessel connected upstream of the extraction vessel was dissolved in supercritical Con by bubblingthrough. Using this approach, supersaturation of the extractant in the fluid phase wouid not take place. The solid phosphine oxides (TBPO, TOPO and TPPO) were placed in a 3.5-mL stainless steel high-pressure cell. Supercritical COZflowed first through the cylinder containing a phosphine oxide and then bubbled through the liquid extraction cell containing 4.0 mL of an acid solution of uranium and thorium. The complexation and the extraction processes were allowed to occur under a static condition for 15 min by pressurization with supercritical COZdissolved complexant without opening the exit valve. After that, the exit valve of the extraction cell was opened, and the sample was extracted and flushed under dynamic condition for 15 min with supercritical COaflowingthrough the cell. When the extraction was completed, the sample was removed from the extraction cell and analyzed by neutron activation analysis (NAA). The extraction efficiency was determined by the amount of uranium and thorium ions remaining in the sample solution after extraction divided by the amount of uranium and thorium ions present prior to the extraction. The procedure of solubility measurement and the details of NAA have been reported previously (9).
Results and Discussion According to Page et al., about 11% mole fraction of TBP is miscible with C02 at 60 "C and 120 atm (7). Under our experimental conditions, supercritical CO2 in contact with TBP is therefore expected to contain this quantity of TBP. The solubilities of TBPO, TOPO, and TPPO in supercritical COz are shown in Table 1. The solubilities of TOPO and
0013-936W95/0929-2706$09.00/0
0 1995 American Chemical Society
Tiam2
Percent Extraction of Uranyl and Thorium Ions from Nitric Acid Solutions with Supercritical COZ Containing Organophosphorus Reagents at 60 and 200 atnf sample matrix 6 M "03
+ 3 M LiNO3
U(VI)
Th(lV)
TBP TBPO TBP l T A c TOPO TPPO TTA TBP TBPO TOPO TPPO TTA TBP TBP TTAC TBPO TOPO TPPO TPPO TTAd TTA TBP TBP TTAC TBPO TOPO TPPO TTA
98 99 97 99 99 1 91 99 99 98 2 52 70 99 98 97 99 4 12 87 97 89 92 24
93 98 90 100 88 2 80 98 98 68 2 26 85 99 98 30 90 12 20 96 91 93 20 65
+
+
+
0.1 M " 0 3
% extractionb
extraction reagent
+
Extraction conditions: 15 min static followed by 15 min dynamic at 60 "C and 200 atm. Sample volume: 4 mL containing 50yglmL each U and Th. TBP, tributyl phosphate; TBPO, tri-n-butylphosphine oxide; TOPO, tri-n-octylphosphine oxide; TPPO, triphenylphosphine oxide: lTA, thenoyltrifluoroacetone. Average data from a minimum of two runs. SFC02saturatedwithTBPflowedthrough a ligandcellcontaining 0.5 g of TTA. Ligand cell contained 0.5 g of TPPO + 0.5 g of lTA. a
TPPO in supercritical Con are much lower when compared with TBPO. For the conventional solvent extraction of uranyl ions from nitric acid solutions by organophosphorus reagents, it was demonstrated that the reaction proceeds by the following reaction:
where L is an organophosphorus reagent such as TBP (1). The reaction mechanism in supercritical fluid extraction is probably similar to that in solvent extraction. According to reaction 1, the equilibrium tends to shift to the right with a higher concentration of nitric acid or nitrate salt. This is consistent with the results shown in Table 2, which indicate that the extraction efficiencies for uranyl and thorium ions decrease with decreasing concentration of nitric acid when TBP is used as an extracting agent. In 6 M HN03, the extraction efficiencies for the uranyl and thorium ions by TBP are 91% and 80%,respectively, under our experimental conditions at 60 "C and 200 atm. When the acid concentration is reduced to 1 M, the extraction efficiencies for the uranyl and thorium ions are lowered to 52% and 26%, respectively. At 0.1 M "03, the percent extraction of uranylions is further reduced to 12%. Addition of LiN03 increases the extraction efficiency of uranyl and thorium ions from 91% and 80% to 98% and 93%,
0 0
1
2
3
4
5
7
6
8
0
10
Nitrate Concentration (M)
n 4 + l%tmctiw
from Nitric Add
P '01
I
0 0
1
2
5
4
I
7
8
8
0
10
Nitrate Concentration(M) FIGURE 1. Comparison of the percent extraction of uranyl (A) and thorium (B) ions using supercritical COZ saturated with TBP with that observed for conventionalsolvent extraction. Solvent extraction data obtained for 19% v/vTBP in kerosene: (U)SFE data; ( 0 )solvent extraction data.
respectively, at 6 M HN03. The standard deviation based on triplicate runs of TBP in 6 M HN03 3 M LiN03is about 2%. The strong dependence of nitric acid concentration on the extraction of uranyl ions by TBP in supercritical COzis similar to that observed in the solvent extraction of uranyl ions using TBP and dodecane as the organic extractant. The SFE efficiency for uranyl by TBP-modified COZ correlates strongly with that obtained from solvent extraction with TBP in dodecane at different HN03 concentrations (Figure 1). The solvent extraction D values were obtained from the literature for 19%vlv TBP in dodecane (10).The distribution coefficients in the literature were converted to % extraction, as shown elsewhere (11). Figure 1 also shows the correlation between SFE efficiency of Th4+by TBPmodified COZ and by solvent extraction with TBP in dodecane (12). The strong correlations observed between SFE and solvent extraction for uranyl andTh4+ions suggest that the solvation behavior of supercritical COZis similar to that of dodecane for the TBP system. Also, the TBP shows similar chemical properties in supercritical COz to those observed in noncritical systems. It should be noted that, without TBP, uranyl and thorium ions in 6 M nitric acid solutions cannot be extracted ( ~ 2 %by ) supercritical COz. When a stronger Lewis base, TBPO, TOPO, or TPPO is used as the extraction reagent in supercritical COZ, the extraction efficiencies of uranyl and thorium ions are generally higher than those of TBP. The SFE efficiencies of uranyl by TBPO in supercritical COZare 99%, 99%, and 97% in 6, 1, and 0.1M "03, respectively, at 60 "C and 200
+
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atm. Equally high extraction efficiencies for thorium ions (98% 99%, and 91% at 6, 1,and 0.1 M HN03, respectively) were observed under the same SFE conditions. High SFE efficiencieswere also observed for uranyl and thorium ions in 0.1-6 M HN03 when TOPO was used as the extractant. The stronger complexingability of TBPO and TOPO relative to TBP is probably due to the direct bonding between the alkyl and phosphoryl groups, which results in an increase in the electron cloud density of the phosphoryl oxygen compared to the electron-withdrawing property of the alkoxy1oxygeninTBP. Using TBPO or TOPO in supercritical COz, effective extraction of uranyl and thorium ions can be achieved even in dilute "03 solutions, thus yielding the possibility of reducing acidic waste volumes in nuclear waste treatment. TPPO in supercritical C o n also shows high extraction efficiencies for uranyl ions from 0.1 to 6 M HN03 media. The percent extractionsare 98%,97% and 92%,respectively, in 6,1,and 0.1 M HN03at 60 "C and 200 atm. These results are interesting because TPPO is a high melting solid with a much lower solubititythan TBPO or TOPO in supercritical C o r . Solid extractants are easier to handle in SFE experiments than liquid extractants in a one-pump system. The extraction efficiency of TPPO for thorium is lower and decreases rapidywith decreasing nitric acid concentration (from 68% at 6 M to 30% at 1 M HN03). The addition of another chelating agent such as a solid fluorinated P-diketone like TTA (thenoyltrifluoroacetone)can greatly enhance the extraction efficiency of thorium. Using a mixture of TPPO and TTA, about 90%of the thorium ions in 1M "03 can be extracted by supercritical COz at 60 "C and 200 a m . This synergisticeffect is probably due to adduct formation of TTA and TPPO similar to that observed previously in the SFE of thorium in aqueous solutions with a mixture of TBP and a fluorinated /3-diketone ( 2 , 3 ) .When a mixture of TBP and TTA was used for the extraction of uranyl and thorium ions in dilute "03 solutions, a positive synergistic effect was also observed. For example, in 0.1 M "03, the extraction efficiencies of uranyl and thorium ions increased from 12%and 20%, respectively,for TBP alone to 87%and
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+
96% respectively, for TBP 'ITA in COz at 60 "C and 200 atm. The results presented in this paper may form the basis of a novel extraction process for the treatment of acidified nuclear wastes, while minimizing the production of secondary wastes.
Acknowledgments This work was supported by British Nuclear Fuels plc and by NSF-Idaho EPSCoR Program under NSF Cooperative Agreement OSR-9350539. Neutron irradiations were performed at the Washington State University Nuclear Radiation Center under a Reactor Sharing Program supported by DOE.
literature Cited (1) Benedict, M. In Nuclear Chemical Engineering, Benedict, M., Pigford, T. H., Levi, H. W., Eds.; McGraw-Hill Book Co. Inc.: New York, 1981. (2)Lin, Y.;Wai, C. M.; Jean, F. M.; Brauer, R. D. Environ. Sci. Technol. 1994, 28, 1190. (3) Lin, Y.; Wai, C. M. Anal. Chem. 1994, 66, 1971. (4) Laintz, K. E.; Tachikawa, E. Anal. Chem. 1994, 66, 2190. (5) Cheng, K. L.; Ueno, K.; Imanura, T. Handbook of Organic AnalydcalReagents; CRC Press: Boca Raton, FL, 1982; pp 431443. (6) Mason G. W.; Lewey, S.; Peppard, D. F. J. Inorg. Nucl. Chem. 1964, 26, 2271. (7) Page, S. H.; Sumpter, S. R.; Goats, S. R.; Lee, M. L. 1. Supercrit. Fluids 1993, 6, 95. (8) Hedrick, J. L.; Mulcahey, L. J.; Taylor, L. T. In Supercriticalfluid Technology-Theoretical and Applied Approaches to Analytical Chemistry;Bright, F. V., McNally, M. E., Eds.; ACS Symposium Series 488; American Chemical Society: Washington, DC, 1991; pp 206-220. (9) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M.Anal. Chem. 1993, 65, 2549. (10) Hesford, E.; McKay, H. A.; Scargill, D.J. Inorg. Nucl. Chem. 1957, 4 , 321. (11) Wai, C. M. In Preconcentration Techniquesfor Trace Elements; CRC Press: Boca Raton, FL, 1991; Chapter 4, pp 101-132. (12) Sato, T. 1.Inorg. Nucl. Chem. 1959, 9, 188.
Received for review April 12, 1995. Revised manuscript received June 27, 1995. Accepted July 9, 1995.
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