Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 38, No. 12, p. 1097–1102 (December 2001)
Dissolution Behavior of Uranium Oxides with Supercritical CO2 Using HNO3 -TBP Complex as a Reactant Osamu TOMIOKA1 , Yoshihiro MEGURO2 , Youichi ENOKIDA3, ∗ , Ichiro YAMAMOTO1 and Zenko YOSHIDA2 2
1 Department of Nuclear Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai-mura, Naka-gun, Ibaraki 319-1195 3 Research Center for Nuclear Materials Recycle, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603
(Received July 10, 2001 and accepted in revised form September 24, 2001) Dissolution behavior of U3 O8 and UO2 using supercritical CO2 medium containing HNO3 -TBP complex as a reactant was studied. The dissolution rate of the oxides increased with increasing the HNO3 /TBP ratio of the HNO3 -TBP complex and the concentration of the HNO3 -TBP complex in the supercritical CO2 phase. A remarkable increase of the dissolution rate was observed in the dissolution of U3 O8 when the HNO3 /TBP ratio of the reactant was higher than ca. 1, which indicates that the 2:1 complex, (HNO3 )2 TBP, plays a role in facilitating the dissolution of the oxides. Half-dissolution time (t1/2 ) as an indication of the dissolution kinetic was determined from the relationship between the amount of uranium dissolved and the dissolution time (dissolution curve). A logarithmic value of a reciprocal of the t1/2 was proportional to the logarithmic concentration of HNO3 , CHNO3 , in the supercritical CO2 . The slopes of the ln(1/t1/2 ) vs. ln CHNO3 plots for U3 O8 and UO2 were different from each other, indicating that the reaction mechanisms or the rate-determining steps for the dissolution of U3 O8 and UO2 are different. A principle of the dissolution of uranium oxides with the supercritical CO2 medium is applicable to a method for the removal of uranium from solid matrices. KEYWORDS: supercritical carbon dioxide, dissolution, uranium oxides, HNO3 -TBP complex
I. Introduction Recently, much attention has been paid to a supercritical CO2 fluid as a medium for the separation of metals.1–12) Supercritical fluid extraction (SFE) using the supercritical CO2 exhibits several advantages as follows over a traditional solvent extraction procedure using an organic solvent. The extraction efficiency and the extraction rate are expected to be improved due to rapid mass transfer in the supercritical fluid phase. The rapid and complete removal of the extraction medium is attained by gasification of CO2 at ambient temperature and pressure after the extraction process. The extraction efficiency or selectivity can be optimized by changing the properties of the CO2 medium optionally by tuning the pressure.1, 6, 13) The use of SFE process in the field of nuclear technology exhibits a particular significance for minimizing the amount of the radioactive solvent waste.12, 14) The authors have developed the SFE of U(VI) and Pu(IV) from nitric acid solutions using the supercritical CO2 containing tri-n-butylphosphate (TBP).1, 6, 13) An alternative and more attractive method based on the SFE is “direct dissolution (extraction)” of metals from the solid samples. In this method, acid for the pretreatment of the sample is not necessary to be employed. The authors have studied the dissolution behavior of lanthanide oxides, zirconium oxide, strontium oxide, etc.2, 7, 15) with the supercritical CO2 containing the HNO3 -TBP complex as a reactant, and a method has been developed for the removal of the uranium oxides from the solid samples named supercritical fluid leaching (SFL) method, which was reported preliminarily in ∗
Corresponding author, Tel. +81-52-789-3786, Fax. +81-52-7893785, E-mail:
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Ref. 12). In the present work, the dissolution behavior of U3 O8 and UO2 with a flow of the supercritical CO2 containing HNO3 TBP complex was investigated. A relationship between the amount of uranium dissolved and the dissolution time, which is called as “dissolution curve” hereafter, was examined and the parameters determining the rate were evaluated. A feasibility of the direct dissolution method for the efficient and selective separation of uranium oxides from various samples was discussed.
II. Experimental 1. Chemicals Uranium oxides; The UO2 powder was obtained by grinding mechanically the UO2 nuclear fuel pellet using a “vibrating sample mill” (Heiko Co., TI-100). The UO2 powder was fractionated using a sieve of 500 mesh and the grain size of the UO2 powder was found to be 1–5 µm by microscopic observation. The O/U ratio of the UO2 powder was determined to be 2.005±0.003 by spectrophotometry after the dissolution of the sample with strong phosphoric acid.16) The U3 O8 was prepared by heating the UO2 pellet at 753 K in air for 2 h. The U3 O8 was gently ground by the mill and fractionated using the sieve. The grain size of the U3 O8 powder was found to be 2–20 µm. HNO3 -TBP complex; Hundred cm3 of an anhydrous trin-butylphosphate (TBP, Koso Chemical Co.) was contacted with 100 cm3 of 60%HNO3 (Wako Pure Chemicals Co.), i.e. 13.1 mol·dm−3 (M) HNO3 , by shaking vigorously in a conventional extraction tube for 30 min to prepare a stock solution of the HNO3 -TBP complex. The HNO3 -TBP stock solution was a mixture of the HNO3 -TBP-H2 O complexes of
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different compositions. The concentration of HNO3 in the stock solution was determined to be 3.9 M by a titration with NaOH solution, after HNO3 in the stock solution was backextracted into the aqueous solution. The total content of TBP in the stock solution was found to be 2.8 M from the result of the determination of the phosphorus concentration by ICPAES (Shimadzu, ICPS-8000E). From these results the molar ratio of HNO3 /TBP was determined to be 1.38, which implies that the stock solution consists of approximately 60 molar fraction % of the (1:1) complex, HNO3 TBP, and 40% of the (2:1) complex, (HNO3 )2 TBP.17–19) The water was contained in the HNO3 -TBP stock solution and the water content in the stock solution was determined to be 1.3 M by KarlFischer titration method. The HNO3 -TBP stock solution was diluted with either water-saturated TBP or a mixture of anhydrous TBP+water-saturated TBP of a known ratio in order to prepare the HNO3 -TBP solution of given concentrations of 0 0 ), TBP (C TBP ), and water (C H0 2 O ) for HNO3 (denoted as C HNO 3 the dissolution experiments. All chemicals used were of reagent grade. 2. SFE Apparatus A schematic diagram of the apparatus is shown in Fig. 1. A reaction vessel used was a column-type, mini-guard column of GL Sciences, of 4 mmφ I.D. and 10 mm in length. An employment of the column-type reaction vessel enables a measurement of a reproducible dissolution curve by minimizing a dead space inside the reaction vessel. The other parts of the apparatus were essentially identical to those employed in the previous work,7) and were mainly consisted of a thermostated water bath, a syringe pump (ISCO Co. Ltd., model 260D), a plunger-type pump, a restrictor of stainless steel tubing of 0.1 mmφ I.D. and 2–4 m in length, and a collection vessel. 3. Procedure The oxide powders sample of U3 O8 (200±1 mg) or UO2 (520±1 mg) was taken in the reaction vessel. Neat CO2 from CO2 cylinder was filled in the SFE system and then CO2 was pressurized up to a given pressure, P, in the range of 12– 23 MPa using the syringe pump. Then CO2 was allowed to
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where F = f comp /{ f CO2 (dCO2 [283, P]/dCO2 [T, P]) + f comp }. (4) Here, dCO2 [T, P] is the density of CO2 at P and T .20) The HNO3 -TBP complex in the supercritical CO2 reacted with U3 O8 or UO2 in the reaction vessel and U(VI)-NO− 3TBP complex was formed and dissolved in the supercritical CO2 . The effluent from the reaction vessel was depressurized down to an atmospheric pressure by passing through the restrictor, and then bubbled into n-dodecane in the collection vessel. Because CO2 gasified in the collection vessel and such solutes contained in the CO2 medium as U(VI)-NO− 3TBP complex and HNO3 -TBP complex were recovered in the collection vessel. Uranium(VI) was quantitatively stripped as U(VI)-carbonate from the U(VI)-NO− 3 -TBP complex using (NH4 )2 CO3 aqueous solution. The concentration of uranium in the stripped solution was determined by ICP-AES. A dissolution efficiency (U dissolved %), defined as a ratio of the cumulative amount of uranium collected in the collection vessel to that initially loaded, was determined from the result of the amount of uranium collected.
III. Results and Discussion
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flow through the system at a flow rate, f CO2 , and the HNO3 TBP complex was supplied to the CO2 flow at a constant flow rate, f comp , using the plunger-type pump (at dissolution time t=0). Here, f CO2 was the flow rate at the outlet of the syringe pump (283 K). It was not easy to keep both the pressure and the flow rate of CO2 constant at the same time in the flow SFE process, since the hydraulic resistance of the flow system was different, though slightly, between different experiments and varied with the passage of time during the dissolution experiment. The pressure P was kept constant throughout the dissolution experiment in the present work. The mixture medium was heated up to a dissolution temperature, T , ranging between 303 and 333 K by passing through the pre-heating coil, and then introduced into the reaction vessel. During the experiment, the temperature inside the reaction vessel was kept constant. The concentrations of the HNO3 , TBP and water in the supercritical CO2 in the reaction vessel at pressure P and temperature T were calculated using the following equations:
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Solids of UO2 and U3 O8 (U3 O8 is a 1:2 mixture of UO2 and UO3 ) react with the HNO3 -TBP complex to form UO2 (NO3 )2 (TBP)2 complex soluble in the supercritical CO2 phase. In this connection, tetravalent uranium such as U4+ once dissolved in the supercritical CO2 containing the HNO3 TBP complex is liable to be oxidized to U(VI) under the condition studied. The overall dissolution reactions of UO2 and U3 O8 in the supercritical CO2 are expressed by Eqs. (5) and (6):
Fig. 1 Apparatus for the dissolution of uranium oxide powder with supercritical CO2 containing the HNO3 -TBP complex JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
Dissolution Behavior of Uranium Oxides with Supercritical CO2
UO2 + (8/3)HNO3 + 2TBP → UO2 (NO3 )2 (TBP)2 + (2/3)NO + (4/3)H2 O, (5) UO3 + 2HNO3 + 2TBP → UO2 (NO3 )2 (TBP)2 + H2 O. (6) The species UO2 (NO3 )2 (TBP)2 was confirmed as the extracted species in the supercritical CO2 extraction system13) identically to the species in the traditional solvent extraction system of U(VI) from nitric acid solution into organic solvent containing TBP. A lot of studies on the dissolution kinetics and mechanism of UO2 and U3 O8 in nitric acid solution have been accumulated.21–25) The mechanism and the rate-determining step of these dissolution reactions, however, are still of controversy. 1. Dissolution Curve for the Dissolution of U3 O8 with the HNO3 -TBP Complex of Various Compositions The U3 O8 powders sample was dissolved in a stream of a mixture of supercritical CO2 , f CO2 =1.4–2.0 cm3 /min, and the HNO3 -TBP complex, f comp =0.5 cm3 /min, at 323 K and 12.0 MPa. In these experiments, the HNO3 -TBP complexes of various molar ratios of HNO3 /TBP in the range of 0.60 to 1.38 were employed. The dissolution curves as shown in Fig. 2 were obtained by plotting the dissolution efficiency (U dissolved %) against the dissolution time (t). It was found that the dissolution rate was strongly dependent on the HNO3 /TBP ratio of the complex and the dissolution rate increased with an increase of the ratio. When U3 O8 was dissolved with supercritical CO2 containing the HNO3 -TBP complex of the HNO3 /TBP ratio larger than ca. 1, quantitative dissolution was attained within 40 mins. The U dissolved % was in the range of 95–97% even after the
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Fig. 2 Dissolution curves for U3 O8 with supercritical CO2 containing the HNO3 -TBP complex of various compositions CHNO3 , CTBP , CH2 O (M): (1) 0.32, 0.53, 0.43, (2) 0.37, 0.45, 0.30, (3) 0.43, 0.45, 0.27, (4) 0.48, 0.40, 0.20, (5) 0.56, 0.43, 0.20, (6) 0.72, 0.53, 0.24 f CO2 (cm3 /min): (1) 1.6±0.2, (2) 1.9±0.5, (3) 1.8±0.3, (4) 2.0±0.2, (5) 1.8±0.3, (6) 1.4±0.1 f comp =0.5 cm3 /min, T =323 K, P=12.0 MPa VOL. 38, NO. 12, DECEMBER 2001
1099 60 mins’ dissolution using the HNO3 -TBP complex with the HNO3 /TBP ratio of 1.38 (plot 6 in Fig. 2). Slightly lower U dissolved % than 100% was due to an experimental error, and the quantitative dissolution was confirmed from the experimental result that the amount of uranium remained in the reaction vessel after the dissolution experiment was determined to be less than 0.1 mg. As mentioned in the figure caption, the C H2 O in the supercritical CO2 also varied together with the HNO3 /TBP ratio in each experiment. In this connection, the dissolution curve of U3 O8 was measured using the HNO3 -TBP complex containing the water of various C H0 2 O . In these experiments, the HNO3 /TBP ratio of the HNO3 -TBP complex was kept constant at 0.60, and the C H2 O was changed in the range of 0.14 to 0.43 M. It was found that the dissolution rate increased with an increase of C H2 O . For example, the U dissolved % of U3 O8 was 50% with 110 mins’ dissolution and less than 2% with 120 mins’, respectively, when the HNO3 -TBP complex of C H2 O =0.43 and 0.14 M were used. Water molecules may facilitate a dissociation of the HNO3 -TBP complex or HNO3 and a generation of H+ or NO− 3 in the nonpolar supercritical CO2 phase, which leads to an enhancement of the dissolution of the oxides. The results in Fig. 2 imply both effects of the HNO3 /TBP ratio and the water content on the dissolution rate. The experimental fact indicating the increase of the dissolution rate with an increase of the HNO3 /TBP ratio despite a decrease of the water content suggests that the effect of the HNO3 /TBP ratio to enhance the dissolution of U3 O8 is dominant under the experimental condition for Fig. 2. 2. Effect of the Temperature on the Dissolution Rate of U3 O8 The U3 O8 powders sample was dissolved with a mixture of supercritical CO2 of f CO2 =1.1–2.5, the HNO3 -TBP complex of f comp =0.5, and water of C H2 O =0.17–0.24 M, at various temperatures in the range of 303 to 333 K and 12.0 MPa. The HNO3 /TBP ratio was fixed at HNO3 /TBP=1.38. The dissolution curves are shown in Fig. 3. A remarkable temperature effect was observed for the dissolution at 303 and 313 K using the supercritical CO2 . The dissolution rate at 303 K was smaller than that at temperature higher than 313 K. The dissolution rate was hardly influenced by the temperature in the range of 313 to 333 K. The maximum values of U dissolved % of plots 2 to 4 were in the range of 90–96%, which were lower than 100%. This was attributable to the experimental error as described in the previous section. 3. Effect of the Pressure on the Dissolution Rate of U3 O8 Dependence of the dissolution rate on the pressure was examined in the dissolution of U3 O8 at 323 K. The f CO2 was 1.7 cm3 /min and f comp =0.5. Experiments at different pressures without changing the f CO2 were performed by changing the pressure with the use of the restrictors of different lengths. The pressure of 12.0, 20.3 and 22.6 MPa were attained using the restrictors of 3, 3.5 and 4 m long, respectively. The results are shown in Fig. 4. The pressure dependence of the solubility of a solute in the supercritical CO2 has been widely studied empirically and
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Fig. 3 Effect of temperature on the dissolution of U3 O8 T (K): (1) 303, (2) 313, (3) 323, (4) 333, P=12.0 MPa CHNO3 , CTBP , CH2 O (M): (1) 0.61, 0.45, 0.20, (2) 0.52, 0.38, 0.17, (3) 0.72, 0.53, 0.24, (4) 0.66, 0.48, 0.22, HNO3 /TBP ratio=1.38 f CO2 (cm3 /min): (1) 2.3±0.3, (2) 2.5±0.1, (3) 1.4±0.1, (4) 1.1±0.2 f comp =0.5 cm3 /min
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Fig. 5 Effect of the concentration of the HNO3 -TBP complex in supercritical CO2 on the dissolution of U3 O8 CHNO3 , CTBP , CH2 O (M): (1) 0.43, 0.45, 0.27, (2) 0.21, 0.22, 0.27 HNO3 /TBP ratio=0.96 f CO2 (cm3 /min): (1) 1.8±0.3, (2) 2.0±0.1, f comp (cm3 /min): (1) 0.5, (2) 0.25, T =323 K, P=12.0 MPa
C H2 O with an increase of the pressure. A density of the CO2 molecules increases and the reactivity of the HNO3 -TBP complex in the supercritical CO2 decreases with an increase of the pressure, which is a possible explanation of the negative pressure dependence of the dissolution rate observed in the present system.
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Fig. 4 Effect of pressure on the dissolution of U3 O8 P (MPa): (1) 12.0, (2) 20.3, (3) 22.6, T =323 K CHNO3 , CTBP , CH2 O (M): (1) 0.45, 0.47, 0.29, (2) 0.57, 0.60, 0.37, (3) 0.59, 0.62, 0.38, HNO3 /TBP ratio=0.96 f CO2 =1.7 cm3 /min, f comp =0.5 cm3 /min
theoretically,26–28) and it has been generally accepted that the solubility increases with an increase of the pressure. According to this general understanding, it was predicted that the higher pressure resulted the more effective dissolution of the oxides into the supercritical CO2 . In fact, the solubility of such complexes as UO2 (NO3 )2 (TBP)2 was confirmed experimentally to increase with an increase of the pressure.28) The results in Fig. 4 do not coincide with the above prediction on the basis of the pressure effect on the solubility. The dissolution rate of U3 O8 decreases with an increase of the pressure, regardless of the increase of C HNO3 , C TBP and
4. Effect of the Ratio f comp / f CO2 on the Dissolution Rate The dissolution curves as shown in Fig. 5 were obtained for the dissolution of U3 O8 at f CO2 =1.8 and f comp =0.50 (plot 1) or f CO2 =2.0 and f comp =0.25 (plot 2) using the supercritical CO2 with C H2 O =0.27. The dissolution rate is lower with supercritical CO2 containing the lower content of HNO3 -TBP complex. It should be noted that the maximum U dissolved % was 80% and the oxide powders undissolved still remained in the reaction vessel after the experiment for plot 2 in Fig. 5. 5. Dissolution of UO2 Dissolution experiments for UO2 were performed identically to those for U3 O8 described in the previous sections. The results for the dissolution of UO2 with the HNO3 TBP complex of various HNO3 /TBP ratios are shown in Fig. 6. The dissolution rate increased with an increase of the HNO3 /TBP ratio of the complex. Employing the HNO3 TBP complex of the HNO3 /TBP ratio larger than ca. 1, almost complete dissolution of UO2 was attained within 40 mins. 6. Comparison between the Dissolution Rates of U3 O8 and UO2 For more quantitative treatment of the results of the dissolution rate, a half-dissolution time (t1/2 ), when 50% of the sample was dissolved, was defined and determined from the results for U3 O8 (Fig. 2) and UO2 (Fig. 6). A reciprocal of t1/2 of the dissolution rate is plotted against the HNO3 /TBP ratio in Fig. 7(a). JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
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Fig. 6 Dissolution curves for UO2 with supercritical CO2 containing the HNO3 -TBP complex of various compositions CHNO3 , CTBP , CH2 O (M): (1) 0.40, 0.67, 0.53, (2) 0.56, 0.58, 0.36, (3) 0.63, 0.46, 0.21, (4) 0.77, 0.51, 0.28 f CO2 (cm3 /min): (1) 1.2±0.1, (2) 1.3±0.1, (3) 1.6±0.2, (4) 1.5±0.1 f comp =0.5 cm3 /min, T =323 K, P=12.0 MPa
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The increase of the dissolution rate of U3 O8 is remarkable when the HNO3 /TBP ratio becomes higher than ca. 1. It should be noted that the HNO3 -TBP complex of the HNO3 /TBP ratio more than ca. 1 contains the higher content of (HNO3 )2 TBP. The (HNO3 )2 TBP complex is considered to be particularly strong reactant enhancing such reaction steps involved in the dissolution of uranium oxide as a protonation of the oxide, an oxidation of U(IV) and a formation of UO2 (NO3 )2 (TBP)2 complex. An effect of the HNO3 /TBP ratio on the dissolution rate of U3 O8 is more remarkable than that of UO2 . Different aspect of the dependence of the dissolution rate on the HNO3 /TBP ratio leads to an idea that the mechanism or the rate-determining step are different between the dissolution of UO2 and U3 O8 . Figure 7(b) involves plots between ln(1/t1/2 ) and ln C HNO3 . Both ln(1/t1/2 ) vs. ln C HNO3 plots for the dissolution of U3 O8 and UO2 show clear linearity with the slopes of 3.8 (correlation coefficient R=0.978) and 2.0 (R=0.975), respectively. These results suggest that the numbers of HNO3 involved in the rate-determining step of the dissolution of these oxides are different. As far as the results in Fig. 7(a) concern, the dissolution rates of U3 O8 and UO2 are of similar extent, when the HNO3 /TBP ratio is less than ca. 1. Further studies for the characterization of the oxide solids such as an evaluation of the specific surface area or the crystallographic property are necessary for a comparison between the dissolution rates of U3 O8 and UO2 . Referring to the results of the dissolution behavior of Nd2 O3 in the previous work,2, 15) uranium oxides and Nd2 O3 are expected to be dissolved separately with the supercritical CO2 containing different concentration of the HNO3 -TBP reactant. Nd2 O3 is dissolved in the supercritical CO2 containing the HNO3 -TBP complex, e.g., with the HNO3 /TBP ratio
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ln C HNO3 (b) Fig. 7 Correlation between the dissolution rate and (a) the HNO3 /TBP ratio of the complex or (b) the concentration of HNO3 in the supercritical CO2 phase, for the dissolution of U3 O8 and UO2 with supercritical CO2 containing the HNO3 -TBP complex Plot 1: for U3 O8 , Plot 2: for UO2
of 0.60, in which U3 O8 and UO2 are not dissolved. After the dissolution of Nd2 O3 , U3 O8 and UO2 can be dissolved with the supercritical CO2 containing the HNO3 -TBP complex of the higher HNO3 /TBP ratio. These results imply a potential of a new method for the separation of uranium and lanthanides from the mixture of their oxides using the supercritical CO2 medium.
IV. Conclusion The dissolution behaviors of U3 O8 and UO2 in the supercritical CO2 with the HNO3 -TBP reactant were investigated under various conditions of the HNO3 /TBP ratio of the reactant and the concentration of the reactant in the supercritical CO2 as well as pressure and temperature. The dissolution rate increases with increasing the HNO3 /TBP ratio and the con-
1102 centration of the reactant in the supercritical CO2 . The dissolution rate does not depend largely on the temperature in the range of 313 to 333 K, and decreases with an increase of the pressure in the range of 12 to 23 MPa. The HNO3 -TBP complex has multi-functions in promoting such elementary reaction steps involved in the dissolution reaction as a protonation of the oxide, an oxidation of U(IV) to U(VI) and solubilization of U(VI) through a formation of UO2 (NO3 )2 (TBP)2 . From the results of the dependence of the dissolution rate on the HNO3 /TBP ratio and the total concentration of HNO3 in the supercritical CO2 , it is concluded that the mechanisms or the rate-determining steps of the dissolution of U3 O8 and UO2 are different from each other. A new method on the basis of the efficient and selective dissolution of uranium oxides with the supercritical CO2 containing the HNO3 -TBP complex is promising for the separation and the removal of the uranium oxides from the solid samples such as various uranium-containing solid wastes generated from the nuclear plants. References 1) Y. Meguro, S. Iso, Z. Yoshida, “Correlation between extraction equilibrium of uranium (VI) and density of CO2 medium in a HNO3 /spercritical CO2 -tributylphosphate system,” Anal. Chem., 70[7], 1262 (1998). 2) O. Tomioka, Y. Enokida, I. Yamamoto, “Solvent extraction of lanthanides from their oxides with TBP in supercritical carbon dioxide,” J. Nucl. Sci. Technol., 35[7], 515 (1998). 3) M. D. Burford, M. Z. Ozel, A. A. Clifford, K. D. Bartle, Y. Lin, C. M. Wai, N. G. Smart, “Extraction and recovery of metals using a supercritical fluid with chelating agents,” Analyst, 124, 609 (1999). 4) C. M. Wai, Y. M. Kulyako, B. F. Myasoedov, “Supercritical carbon dioxide extraction of cesium from aqueous solutions in the presence of macrocyclic and fluorinated compounds,” Mendeleev Commun., 180 (1999). 5) M. A. Khorassani, L. T. Taylor, “SFE of mercury(II) ion via in situ chelation and pre-formed mercury complexes from different matrices,” Anal. Chim. Acta, 379, 1 (1999). 6) S. Iso, S. Uno, Y. Meguro, T. Sasaki, Z. Yoshida, “Pressure dependence of extraction behavior of plutonium(IV) and uranium(VI) from nitric acid solution to supercritical carbon dioxide containing tributylphosphate,” Prog. Nucl. Energy, 37[1–4], 423 (2000). 7) O. Tomioka, Y. Enokida, I. Yamamoto, T. Takahashi, “Cleaning of materials contaminated with metal oxides through supercritical fluid extraction with CO2 containing TBP,” Prog. Nucl. Energy, 37[1–4], 417 (2000). 8) M. Shamsipur, A. R. Ghiasvand, Y. Yamini, “Extraction of uranium from solid matrices using modified supercritical fluid CO2 ,” J. Supercrit. Fluids, 20, 163 (2001). 9) Z. J. Cui, J. C. Liu, L. C. Gao, Y. Q. Wei, “Extraction of copper ions by supercritical carbon dioxide,” J. Environ. Sci., 12[4], 444 (2000). 10) C. Erkey, “Supercritical carbon dioxide extraction of metals from aqueous solutions: A review,” J. Supercrit. Fluids, 17, 259 (2000).
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