Supercritical CO2 Extraction of Uranium(VI) from HNO3 Solution

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SEPARATIONS Supercritical CO2 Extraction of Uranium(VI) from HNO3 Solution Using N,N,N′,N′-Tetrabutyl-3-oxapentanediamide Moonsung Koh, Jaeryong Yoo, Yong Park, Daeil Bae, Kwangheon Park,* Hakwon Kim, and Hongdoo Kim Green Nuclear Research Laboratory, EIRC, Kyung Hee UniVersity, Yongin, Kyungkido 449-701, Korea

Distribution ratio (DU) and extraction efficiency (EU) of uranium(VI) were investigated using N,N,N′,N′tetrabutyl-3-oxapentanediamide (TBOD) in supercritical carbon dioxide (Sc-CO2). Five diamide derivatives were synthesized; TBOD was found to be the most suitable extractant through solubility data. The composition ratio of uranium(VI) extraction was described as a relation of UO2(NO3)2TBOD, through the measurement of DU as the concentration ranges of HNO3 and TBOD. Comparing this with the extractability of TBP, the DU of TBOD was superior to that of TBP in Sc-CO2 by 2 orders of magnitude. The results showed that TBOD could be substituted for tri-n-butyl phosphate (TBP) in Sc-CO2 extraction of uranium(VI). Uranium, fission products (Cs, Cd, Mo, Ba), and corrosion products (Ni, Fe, Cr, Co) were extracted with TBOD in Sc-CO2. Over 90% each of actinides (U, Th) and lanthanides (La, Ce, Gd) were extracted. But, fission products and corrosion products were extracted at a low efficiency of less than 20%, which showed the possibility of selective separation in these metals. 1. Introduction Solvent extraction is a major process in the nuclear industry, involving metal purification, separation, recovery, and extraction. The extraction process of uranium, in particular, marks one of the most important fields from mining the uranium to reprocessing the spent fuel.1-3 The importance of uranium has also been growing gradually due to the increasing applications of atomic energy and the rising price of uranium due to the high cost of oil. However, solvent extraction produces prodigious volumes of liquid waste from n-dodecane which is used as a solvent. Carbon dioxide (CO2) may be a promising substitute due to the ease with which it can be recycled, and the fact that it leaves negligible amounts of secondary wastes.4,5 CO2 also has several attractive features over conventional solvents, such as the following: it has moderate critical conditions (TC ) 31.1 °C; PC ) 72.8 bar), it is environmentally and economically benign, it allows for rapid mass transport and separation, and possesses chemical and radiochemical stability.6 Taking advantage of these traits, many studies on the extraction of uranium ions or oxides in Sc-CO2 have been reported since the 1990s by Wai’s group7-11 and by Meguro and Enokida12-14 using tri-n-butyl phosphate (TBP) or β-diketones. Many effective uranium solvent extractants were investigated after diethyl ether was reported in early 1805. With the rapid progress of nuclear technology in the 1940s, the first extractant used for separation of uranium was diethyl ether with carbonbonded oxygen donor atoms. These were replaced by more effective extractants such as TBP due to their weak efficiency and inflammable property. From that time until the early 1980s, a variety of solvating extractants were exploited, such as nitrogen-, sulfur-, selenium-, and phosphorus-bonded oxygen * To whom correspondence should be addressed. Tel.: +82-31-2012563. Fax: +82-31-202-2410. E-mail: [email protected].

donor atoms. Most of these are capable of extracting uranyl saltssnot only effectively but also with a good selectivity. Of these, TBP is probably the most important uranium extractant because of the good efficiency and inexpensive commercial availability of the chemicals.15 TBP used as an extractant, however, presents several drawbacks, as follows: (i) it produces a large amount of secondary waste in the form of P2O5 or H2PO4 during thermal treatment (incineration) of the spent solvent, and (ii) it is degraded by hydrolysis and radiolysis. The degradation of TBP results in byproducts, mainly monobutyl phosphate (H2MBP) and dibutyl phosphate (HDBP).16 Thus, there is incentive to explore alternative extractants in order to overcome the difficulties associated with the use of TBP. For the past 20 years, there has been a considerable number of studies attempting to do just that.17-21 Among the various research activities, it has been observed that diamide derivatives and the new organophosphorus derivatives for the extraction of actinides from high acid solutions have both proven to be effective at solvent extraction.16,18 By comparison with organophosphorus extractants, however, the diamide derivatives have some remarkable advantages: (i) they have a high irradiation stability and strong affinity to metallic ions such as actinides in a strong acidic solution, (ii) they are ligands that can be completely incinerated without secondary solid waste, because they consist of C, H, O, and N elements (this feature presents an important advantage in the nuclear industry which needs to minimize waste due to storage costs and environmental concerns), and, also, (iii) they have a high dipole moment and tautomerize at a high concentration of mineral acid.22,23 Therefore, diamide derivatives can be promising substitutes for TBP in uranium extraction. Nevertheless, there have been no studies done on uranium extraction in Sc-CO2. In this study, we synthesized the diamide derivatives as a new CO2 soluble chelating ligand and measured the ligand’s solubility in Sc-CO2. The distribution ratios of uranium(VI), DU,

10.1021/ie050992n CCC: $33.50 © 2006 American Chemical Society Published on Web 06/23/2006

Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5309 Table 1. Standard Solutions Containing Metal Ions metal

concn/ppm

company

U(VI) Th(IV) Ce(III) La(III) Cr(III) Ba(II) Mo(VI) Cs(I) Cd(II) Fe(III) Ni(II) Co(II) Gd(III)

973 1019 10061 1006 1003 1000 1000 1000 1006 1003 1003 1004 1006

Aldrich Aldrich Aldrich Aldrich Aldrich Fluka Fluka Cica-Reagent Cica-Reagent Cica-Reagent Cica-Reagent Cica-Reagent Cica-Reagent

were determined between the aqueous phase (HNO3 solution) and the organic phase (Sc-CO2 or n-dodecane) for optimization of extraction conditions using N,N,N′,N′-tetrabutyl-3-oxapentanediamide (TBOD), and for a comparison of extractability between TBP and TBOD. Finally, we discuss the potential of metal ions extraction using TBOD in Sc-CO2. 2. Experimental Section 2.1. Reagents and Materials. Carbon dioxide of 99.98% purity was supplied by Air Tech. Various ionic metals (AAS or ICP analysis grade) that were used for the experiments are shown in Table 1. TBP of 98% purity was obtained from the Aldrich Chemical Co. Nitric acid and organic solvents (HPLC grade) were obtained from Ducksan Pure Chemical Co. Ltd. Other chemicals were purchased from Aldrich and were used as received. 2.2. Synthesis of Diamide Derivatives. According to the known method, by reaction of the appropriate diacid chloride with amines in the presence of triethylamine in dichloromethane, the corresponding N,N,N′,N′-tetraalkyldiamides were prepared in a laboratory scale.17,24,25 Typical experimental procedure for the synthesis of TBOD has been described as follows: The reaction mixture of dibutylamine (3.3 g, 2.2 equiv) and triethylamine (3.0 g, 2.5 equiv) in dichloromethane (0.1 M) was stirred and cooled. Diglycolyl chloride (2.0 g, 1 equiv) was added dropwise at 0 °C and was stirred at room temperature for 4 h. The reaction mixture was poured into water and was then washed with aqueous 1 M HCl to remove the remaining dibutylamine and triethylamine. It was extracted with dichloromethane, dried over anhydrous MgSO4, and concentrated in vacuo to afford TBOD in an 80% yield. 1H NMR (300 MHz, CDCl3) δ 0.90-0.96 (multiplet, 12H), 1.32 (sextet, J ) 7.2 Hz, 8H), 1.52 (quintet, J ) 7.2 Hz, 8H), 3.20 (triplet, J ) 7.5 Hz, 4H), 3.32 (triplet, J ) 7.5 Hz, 4H), 4.31 (singlet, 4H); IR (film) 2907, 1651 cm-1. N,N,N′,N′-Tetraethyl-3-oxapentanediamide (TEOD) was synthesized by the reaction of diglycolyl chloride and diethylamine in a 76% yield. 1H NMR (300 MHz, CDCl3) δ 1.11-1.20 (multiplet, 12H), 3.31 (quartet, J ) 7.2 Hz, 4H), 3.39 (quartet, J ) 7.2 Hz, 4H), 4.32 (singlet, 4H); IR (film) 2910, 1651 cm-1. N,N,N′,N′-Tetrahexyl-3-oxapentanediamide (THOD) was synthesized by the reaction of diglycolyl chloride and dihexylamine in a 92% yield. 1H NMR (300 MHz, CDCl3) δ 0.88 (broad singlet, 12H), 1.28 (broad singlet, 24H), 1.52 (broad singlet, 8H), 3.16 (triplet, J ) 7.6 Hz, 4H), 3.28 (triplet, J ) 7.6 Hz, 4H), 4.32 (singlet, 4H); IR(film) 2900, 1650 cm-1. N,N,N′,N′-Tetraoctyl-3-oxapentanediamide (TOOD) was synthesized by the reaction of diglycolyl chloride and dioctylamine in a 76% yield. 1H NMR (300 MHz, CDCl3) δ 0.85-0.89 (multiplet, 12H), 1.26 (broad singlet, 40H), 1.52 (broad singlet,

Figure 1. Apparatus for measuring the phase behavior: (1) liquid CO2 cylinder, (2) syringe pump, (3) volume variable cell, (4) oven, (5) collection trap, and (6) volume controller.

8H), 3.16 (triplet, J ) 7.6 Hz, 4H), 3.28 (triplet, J ) 7.6 Hz, 4H), 4.32 (singlet, 4H); IR (film) 2900, 1651 cm-1. N,N,N′,N′-Tetrabutylglutardiamide (TBGD) was synthesized by the reaction of glutaryl chloride with dibutylamine in a 96% yield. 1H NMR (300 MHz, CDCl3) δ 0.89-0.96 (multiplet, 12H), 1.23-1.37 (multiplet, 8H), 1.43-1.56 (multiplet, 8H), 1.98 (triplet, J ) 6.9 Hz, 2H), 2.39 (triplet, J ) 6.9 Hz, 4H), 3.15 (triplet, J ) 7.6 Hz, 4H), 3.30 (triplet, J ) 7.6 Hz, 4H); IR (film) 2907, 1637 cm-1. A new synthetic method has been developed for efficient large-scale synthesis of TBOD. This method consists of the following two steps in a one-pot reaction from diglycolic acid as a starting material: 100 g of diglycolic acid was slowly added at 0 °C to a solution of 1.0 L of dibutylamine in 1.3 L of dichloromethane. After the complete dissolution, 350 mL of thionyl chloride was introduced for 1 h, and the resulting reaction mixture was stirred for an additional 2 h at room temperature. It was quenched with water (700 mL) and was then stirred for 30 min. The organic layer was separated, and the aqueous layer was extracted with dichloromethane (2 or 3 times). The collected organic layers were mixed with 1 L of distilled water at 0 °C, and the pH of reaction solution was adjusted to 9-10 by the addition of NaOH. The organic layer was separated, washed with water, and neutralized with 1 M HCl to remove any remaining water. The organic solvent was evaporated in vacuo, and 1.2 L of petroleum ether, 700 mL of water, and 690 mL of HNO3 were added and stirred for 30 min to separate three layers. The second phase was separated, poured into 1.5 L of water, and neutralized with NaOH. The resulting mixture was extracted with dichloromethane, washed with water, dried over anhydrous MgSO4, and concentrated in vacuo to obtain TBOD in a 73% yield. 2.3. Phase Behavior (Solubility) Measurement. A variablevolume cell (from V0 (4.2 mL) to Vmax (22.4 mL), Hanwoul Engineering) was installed to continuously measure solubility points of the extractant in a high-pressure media (Figure 1). Using a micropipet, the desired amounts of ligand were placed into the cell, which was then sealed. The cell was then heated in a constant-temperature oven at a defined temperature, and CO2 was introduced by a syringe pump (260D, ISCO) from a liquid CO2 cylinder. When a single phase was observed at a specific pressure, as the pressure was gradually increased with stirring the magnetic bar, we slowly decreased the pressure until two phases appeared at a fixed temperature. That is, while the transition state reached the critical point of the mixture, the solubility point could be determined by direct observation through sapphire windows placed on both sides. After the pressure was determined at a given volume, the procedure was

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Figure 3. Solubilities of diamide derivatives in Sc-CO2. Temperature, 40 °C. Figure 2. Apparatus for measuring the distribution ratio: (1) liquid CO2 cylinder, (2) syringe pump, (3) extraction cell, (4) high-pressure circulation pump, (5) two-position, six-way valve, (6) sample loop (200 µL), (7) sample vial, (8) syringe, (9) back-pressure regulator, (10) oven, (11) magnetic stirrer, and (12) collection vial.

repeated at an increased volume until values could no longer be obtained. The apparatus was placed in an oven, and the temperature was controlled to (0.5 °C. Pressure in the cell was controlled by a syringe pump to (1 bar. 2.4. Distribution Ratio Measurement. The apparatus for consecutively measuring the distribution ratio in a high-pressure state was set up, as in Figure 2. Experimental procedures were as follows: 15 mL of HNO3 solution containing uranium ions (1.5 mg, 4.2 × 10-4 M) was loaded in a 30 mL extraction cell. After confirming the operation of a high-pressure circulation pump (PU-2080 plus, JASCO), the desired amounts of TBOD were added to the HNO3 solution. With control of the cell until a defined temperature was reached using a constant-temperature oven, CO2 was then introduced by a syringe pump to obtain the desired pressure. The mixture was stirred with a magnetic bar until the sampling time was reached, when the stirrer was stopped for several minutes to stabilize the aqueous phase. The circulation pump was run at a rate of 5 mL/min. Then, a 200 µL sample solution containing the uranium ions from the aqueous solution was collected into a sampling vial through the sample loop (200 µL) using the two-position, six-way valve (Rheodyne). A 1 mL aliquot of 1 M nitric acid was flushed by the syringe to clean the uranium(VI) remaining in the sample loop. The procedures were repeated at the defined conditions (time, pressure, and temperature). The amounts of uranium ions collected in the sample solution were analyzed by ICP-AES (LTIM2C, JOBIN YVON/HORIBA) at KAERI (Korea Atomic Energy Research Institute).

quite soluble in CO2 (mole concentration from 1 × 10-2 to 1.7 × 10-1 in Sc-CO2). According to the previous paper for solvent extraction, DU by diglycolamide (DGA) substituted with ether oxygen is very much higher than that obtained for extraction by glutalamide (GLA) with ether carbon.24 Narita et al. reported on the basis of the extended X-ray absorption fine structure (EXAFS) and X-ray diffraction (XRD) studies that the ether oxygen as well as two carbonyl oxygens in the DGA molecule coordinated to metal ions; that is, DGA shows tridentate coordination.26 Also, solubility of TBOD with ether oxygen was more soluble than TBGD with ether carbon. Therefore, it seemed that ether oxygen in the main frame of diamide has an influence on the enhancement of solubility and extractability. Solubility pressure increases as the length of the alkyl chain attached to the amidic N atoms increases. In particular, the pressure increases sharply over the butyl (C4 alkyl) chain. The difference in solubility between diamides can be explained by two factors: First, the number of hydrocarbons. According to Dandge’s study27 and Bamberger’s study,28 the solubility in CO2 decreases rapidly with an increase in the carbon number or the molecular weight. It was proven that the solubility decreased with an increasing alkyl chain using fatty acids and triglycerides (n ) 12, lauric acid; n ) 14, myristic acid; n ) 16, palmitic acid). The other factor is the difference in vaporization energy. In 1990, King’s group estimated the solubility parameter (δ2) corresponding to the molecular structures. The solubility parameter for the subject was calculated as the square root of the ratios of the vaporization energy (Σi(∆Ev)i), and the molar volume of the overall structure (Σi(∆V)i), as shown in eq 1. As the alkyl chain increases, the vaporization energy increases so that solubility tends to decline due to a decrease of δ2.29 Also, the diamide substituted with ether oxygen, TEOD, was more soluble than TBGD. From these results, TEOD was the most soluble extractant among the diamide derivatives in Sc-CO2.

3. Results and Discussion 3.1. Solubility of Diamide Derivatives in Sc-CO2. Supercritical CO2 generally has a limited solubility for polar materials such as extractant and/or metal-extractant complexes due to its nonpolar property. Therefore, it is very important to confirm the stability of the extractant and measure the range of solubility in Sc-CO2. The solubilities of diamide derivatives (TEOD, TBOD, THOD, TOOD, and TBGD) synthesized by modified procedures at 40 °C were determined, and the results were represented in Figure 3. As expected, all of the derivatives turned out to be

δ2 )

∑i(∆EV)i ∑i(∆V)i

x

(1)

Because metal ions should be extracted in an aqueous solution, the solubilities of extractants in water should be compared. In accordance with the literature,24 the solubility (mM) of the diamide derivatives in water were >60 for TEOD, 2.3 for TBOD, 0.11 for THOD, and 0.042 for TOOD. These values support the fact that the diamide with a shorter alkyl chain attached to amidic N atoms is more hydrophilic. Therefore,

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Figure 4. Solubility of the TBOD + HNO3 complex at various temperatures (20, 40, and 60 °C).

Figure 6. Plots of DU vs HNO3 (M) in dodecane (left) and Sc-CO2 (right). Experimental conditions: aqueous phase, 0.5-6 M HNO3 containing 4.2 × 10-4 M U(VI); organic phase, (A) dodecane (15 mL), (B) Sc-CO2 (15 mL, 200 bar, 40 °C); extractant, 0.1 mL of TBOD; reaction time, 60 min.

Figure 5. Solubility of the TBOD + HNO3 + U(VI) complex at various temperatures (20, 40, and 60 °C).

Figure 7. Plots of DU vs TBOD (mL) in dodecane (left) and Sc-CO2 (right). Experimental conditions: aqueous phase, 6 M HNO3 containing 4.2 × 10-4 M U(VI); organic phase, (A) dodecane (15 mL), (B) Sc-CO2 (15 mL, 200 bar, 40 °C); extractant, 0.01-0.1 mL of TBOD; reaction time, 60 min.

the TEOD was unsuitable for extracting uranium ions from an aqueous solution. TBOD has a very similar solubility to TEOD in Sc-CO2, while it has very low solubility in water. Consequently, TBOD was determined to be the most suitable extractant among the synthesized derivatives. Although TBOD requires a higher pressure for full dissolution as compared to TBP, TBOD can be applied as a promising extractant because it has the advantages of superior irradiation resistance and better stability within a strong acid solution. 3.2. Solubility of TBOD’s Complexes in Sc-CO2. Nitric acid is an important solvent for oxidization of the metals. Solubility of nitric acid in Sc-CO2, however, is very low because of its polar property. If HNO3 is complexed with an organic ligand, it may become quite soluble in Sc-CO2. So, the TBOD + HNO3 complex and the TBOD + HNO3 + U(VI) complex were prepared by the same method reported for the preparation of UO2(NO3)2(TBP)2 given by Meguro,30 and their solubilities are shown in Figure 4 and Figure 5. As shown in Figure 4, although the TBOD + HNO3 complex has lower solubility than TBOD, it has an appropriate stability in Sc-CO2. This finding implies that we can suggest HNO3 in Sc-CO2 in order to dissolve metals including uranium oxides. Figure 5 illustrates the solubility of the TBOD + HNO3 + U(VI) complex in the temperature range of 20-60 °C and in the pressure range of 70-350 bar. The solubility of the TBOD + HNO3 + U(VI) complex was very analogous to TBOD + HNO3, which showed that uranium oxides could be extracted

directly using TBP + HNO3 under moderate conditions. Its slope introduces the operating conditions for extraction. 3.2. Distribution Ratio of Uranium(VI) (DU) in Dodecane and in Sc-CO2. For a metal type (M), the distribution ratio (D) can be written as follows:

DM )

concn of all types containing [M]total org M in organic phase ) concn of all types containing [M]total aq M in aqueous phase

(2)

where [M]total indicates the sum of concentrations of all metal types in both the aqueous phase and the organic phase. The distribution ratios of (VI), DU, were plotted as a function of TBOD concentration and HNO3 concentration. Dodecane and Sc-CO2 were employed in order to make a comparative study of distribution ratios. The results between DU and HNO3 (M) or TBOD (mL) are given in Figure 6 and Figure 7. The slopes were about 2.0 and 1.0, respectively. Therefore, the overall reaction can be expressed as follows:

UO22+ + 2HNO3 + TBOD(org) f UO2(NO3)2TBOD(org) (3) The close correlation of acid concentration on the extraction equilibrium of U(VI) is shown in Figure 6. The slope of DU in Sc-CO2 is entirely analogous to that in dodecane. Extraction

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Figure 8. Reaction scheme of the chemical equilibrium of uranium ions between the aqueous and organic phases.

Figure 9. Plots of DU vs reaction time (min) in supercritical CO2. Experimental conditions: aqueous phase, 6.0, 9.0 M HNO3 containing 4.2 × 10-4 M U(VI); organic phase, supercritical CO2 (15 mL, 200 bar, 40 °C); extractant, 0.1 mL of TBOD; sampling time, 0, 3, 6, 10, 20, 40, and 60 min.

constants of U(VI) obtained by Sc-CO2 are 1 or 2 orders of magnitude smaller than those obtained by dodecane over the entire range. In the plot of DU for the concentration of TBOD, the slope displays a slight difference, which is slightly higher in value in dodecane than in Sc-CO2 (Figure 7). DU as a function of HNO3 concentration ranges from 1.3 (at 1 M) to 26.8 (at 6 M) in dodecane and from 0.06 (at 0.5 M) to 11.9 (at 6 M) in Sc-CO2. The difference of DU between dodecane and Sc-CO2 is similar to the plot as a function of TBOD. The process of the extraction equilibrium of uranium ions can be described in detail in four steps, as shown in Figure 8. The first step is the complexation of U(VI) by NO3 in the aqueous phase to form the uncharged UO2(NO3)2 complex. TBOD is slightly dissolved in the aqueous solution (second step), and then UO2(NO3)2TBOD is formed through the reaction between UO2(NO3)2 and TBOD. The final step is extraction of the complex from the aqueous phase to the organic phase. The kinetic behavior of the reaction equilibrium as a function of time was investigated as shown in Figure 9. At the low concentration of HNO3, the kinetic effect was not indicated, due to the low distribution ratio. Conversely, it was steeply increased within a few minutes at the high concentration. The chemical equilibrium of uranium(VI) between the two phases was reached completely in about 40 min. Thus, this implies that the Sc-CO2 can be more effective than dodecane in achieving a higher efficiency, which is attributable to a difference in the kinetic factor of a component in the media, due to rapid mass transport in the Sc-CO2 phases. 3.3. Comparison of Distribution Ratios between TBOD and TBP in Sc-CO2. The extractabilities of TBOD and TBP

Figure 10. Plots of log(DU) vs chelating ligand (mol, TBOD, TBP) and HNO3 (M) in Sc-CO2. Experimental conditions: aqueous phase, 0.1-9.0 M HNO3 containing 4.2 × 10-4 M U(VI); organic phase, Sc-CO2 (15 mL, 200 bar, 40 °C); extractant, 0.01-0.1 mL of TBOD or TBP; reaction time, 60 min.

expressed by the distribution ratio were compared in Sc-CO2 and are shown in Figure 10. It can be seen that the DU values of TBOD and TBP increased regularly with chelating ligand molarity. Although the solubility of TBOD in Sc-CO2 was lower than that of TBP, the DU values of TBOD over the entire range of acidity are significantly higher in difference by over 2 orders of magnitude, as compared to TBP. The effect of the high DU of TBOD may be explained because of its affinity to metal ions, which appears to be better than the affinity of TBP in Sc-CO2. The DU ranges from 1.3 (at 2.7 × 10-5 mol) to 11.9 (at 2.7 × 10-4 mol) for TBOD and from 0.01 (at 3.7 × 10-5 mol) to 0.07 (at 3.7 × 10-4 mol) for TBP. 3.4. Extraction of Actinides, Lanthanides, Fission Products, and Corrosion Products from Nitric Acid Solution in Sc-CO2. The results of the TBOD extracted actinides (U, Th), lanthanides (La, Ce, Gd), fission products (Cs, Cd, Mo, Ba), and corrosion products (Fe, Ni, Cr, Co) in Sc-CO2 are summarized in Tables 2 and 3. As shown in Table 2, over 83% of uranium ions were extracted, and only approximately 20% of fission and corrosion products were extracted. The results showed that uranium ions could be separated selectively among these metals. Also, it was confirmed that TBOD was distinguished as an extractant for the actinides and lanthanides (Table 3). Over 90% of all elements were extracted, although the efficiency of uranium ions was partially reduced due to the higher affinity of other metals with 0.1 mL of TBOD. The stripping process for recovery of extracted metals is also of importance in commercial operations. The actinides and

Ind. Eng. Chem. Res., Vol. 45, No. 15, 2006 5313 Table 2. Extraction of Actinides and Fission Products in TBOD/ Sc-CO2-6 M HNO3 (Pressure, 200 Bar; Temperature, 40 °C; Metal, 3 × 10-4 M; HNO3, 4 M) extraction efficiency (%) series actinides fission products

corrosion products

nuclide

TBOD/(0.1 mL)

TBOD/(0.3 mL)

U(VI) Cs(I) Cd(II) Mo(VI) Ba(II) Fe(III) Ni(II) Cr(III) Co(II)

83