Surface Decontamination of Radioactive Metal Wastes Using Acid-in

Dec 14, 2007 - Supercritical fluid surface decontamination (SFSD) of radioactive metal wastes was successfully demonstrated using acid-in-CO2 emulsion...
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Ind. Eng. Chem. Res. 2008, 47, 278-283

APPLIED CHEMISTRY Surface Decontamination of Radioactive Metal Wastes Using Acid-in-Supercritical CO2 Emulsions Moonsung Koh, Jaeryong Yoo, Minsu Ju, Bokyoung Joo, Kwangheon Park,* Hakwon Kim, and Hongdoo Kim Green Nuclear Research Laboratory, EIRC, Kyung Hee UniVersity, Yongin-shi, Kyungki-do, 449-701, South Korea

Bruno Fournel CEA Valrhoˆ , DEN/DTCD/SPDE/Laboratorie des Fluides Supercritiques et Membranes, BP 111, 26702 Pierrelatte Cedex, France

Supercritical fluid surface decontamination (SFSD) of radioactive metal wastes was successfully demonstrated using acid-in-CO2 emulsions. NP-series(ethoxylated nonyl phenol series) formed stable acid-in-CO2 emulsions, contrary to F-AOT(sodium bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate). An ultrasonic horn was utilized to enhance mechanical and chemical reactions in Sc-CO2 emulsions. A Cu-coated specimen for a mock-up test was prepared and examined with acid-in-CO2 microemulsions. Over 90% of the Cu coating was easily dissolved away from the specimen. For actual radioactive contaminants, 6 M HNO3 in Sc-CO2 microemulsions enhanced by ultrasonic force was more than 95% decontaminated. Sc-CO2 macroemulsions were also applied with 0.1 M HNO3 or 5% oxalic acid. The decontamination efficiencies were about 90% with HNO3 and over 95% (up to background level) with oxalic acid. After decontamination, the used surfactants were recovered up to about 73% through the recovery process, and they can be reused as well as CO2. So the volume of radioactive wastes can be positively reduced. This SFSD technique may be an effective candidate for surface decontamination of radioactive metals in the near future. 1. Introduction Nuclear power plants have been used as a major source of energy all over the world. In Korea, nuclear energy has grown to account for over 40% of the total electric power production, in its 20 operating plants, after initially being introduced in 1978. As the operating age of plants and facilities increases, the amounts of radioactive contaminants also are accumulating accordingly, which not only degrades the system’s materials but also decreases efficiency.1 Major subjects containing radioactivity include contaminated components, tools, equipment, containers, and facilities and nuclear wastes such as uranium scrap, lagoons, and even clothing. The contamination in these subjects usually exists in the form of metallic salts and oxides. According to the origin of creation, the contaminants can be divided into three typesscorrosion products (Co, Mn, and Zr), fission products (Sr, Cs, Ce, Zr, and Ru), and transuranium nuclides (U, Pu, Np, Am, etc.). Decontamination techniques can assist in minimizing waste volume by concentrating the radioactivity. However, the generation of secondary radioactive waste from decontamination processes is inevitable. Some current methods present many problems such as secondary wastes and the use of many toxic organic solvents. Thus, it is desired to improve decontamination techniques to resolve these issues.2,3 * To whom correspondence should be addressed. Tel.: +82-31201-2563. Fax: +82-31-202-2410. E-mail: [email protected].

To decontaminate radioactive wastes, we applied supercritical CO2 (Sc-CO2), which is gaining attention as an alternative solvent. The advantages of the process using CO2 are as follows: (1) it is nontoxic, (2) it is stable against radioactivity, (3) it produces no secondary wastes, and (4) it can recover and recycle CO2 and surfactants simply. CO2, because of its nonpolarity, has limited solubility with polar materials such as metallic ions. Generally, metal extraction using chelating ligands in Sc-CO2 through a few fluorinated ligands tended only to enhance the extraction efficiency. However, a number of groups have been studying other approaches to metal extraction because of the high cost and potential toxicity of fluorinated ligands. Microemulsions are especially advantageous for the extraction of metals because the amount of water they required is proportional only to the amount of metal to be extracted, not to the amount of waste to be cleaned. To enhance the solubility of polar materials in CO2, Johnston et al. and Eastoe et al. developed a new method using a CO2-water microemulsion, while various studies of metal extraction were being done.4,5 Water-in-CO2 microemulsions can solubilize most nonvolatile polar and nonpolar substances because their aqueous and organic phases can be dispersed in CO2 given sufficient stabilization of the interface.6 Yates and Campell and co-workers7,8 demonstrated the extraction of metal ions from a variety of solid substrates using microemulsions in Sc-CO2. More than 99% of the copper was extracted from filter paper, and 81% of the used surfactant was recovered and generated. Liu et al.9 also used microemulsions with a nonionic

10.1021/ie070865m CCC: $40.75 © 2008 American Chemical Society Published on Web 12/14/2007

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Figure 1. Apparatus for phase behavior measurement in Sc-CO2: (1) CO2 cylinder, (2) syringe pump, (3) volume variable container, (4) oven, (5) collection trap, (6) volume controller, (7) visible camera, and (8) monitor. Table 1. Compositions of Radioactive Metal Wastes radionuclides Mn-54 Co-58 Co-60 NB-95 Sb-125 Zr-95

half-life

concentration (Bq/g)

312.7 days 70.8 days 5.271 years 35.06 days 2.77 years 64.02 days

1622.7 249.6 12361.2 904.2 3012.7 459.5

surfactant (X-100) to extract copper ions. Lately, Shimizu et al.10 decontaminated the ferrite fixed on the iron pipes using a reactive microemulsion into Sc-CO2. With use of the microemulsion containing the organic acid, the efficiencies of the specimens were 56 and 92% for NP-2 + citric acid + Sc-CO2 and PFOA + H2O + Sc-CO2, respectively, at 353 K. Waterin-CO2 microemulsions were presented as a new strategy for a promising method of metal ion extraction. Also, microemulsions are efficient because only a small amount of water needs to be used in proportion to the amount of metal to be extracted, enabling the extraction of grams of waste with ∼µL of water. This study presents the surface decontamination of metal oxides from radioactive wastes using Sc-CO2 micro-/macroemulsions. To accomplish this, fundamental experiments for surfactants, solutions, and agitations were performed to find the optimal conditions. A Cu-coated specimen as a mock-up test was examined in acid-CO2 microemulsions, and then radioactive wastes were demonstrated in micro-/macroemulsions. Also, a recovery test on the used surfactants was performed after decontamination. 2. Experimental Section 2.1. Chemicals and Materials. Carbon dioxide of 99.98% purity was purchased from Air Tech., Korea. In our experiments, we used two kinds of surfactants: one is the nonyl phenol series (NP-4, 10), which contains commercially available nonionic surfactants supplied from Nicca Korea and Hannong Chemicals, and the other is a fluorinated-AOT, which is sodium bis(2,2,3,3,4,4,5,5-octafluoro-1-pentyl)-2-sulfosuccinate, synthesized by modification of the procedures given by Erkey11 and Eastoe and co-workers.12,13 All other chemicals used were analytical grade reagents. A Cu-coated specimen (nut) plated by electroplating was prepared for the mock-up test. The standard conditions and materials for electroplating were 2.5 mA/dm,2 time of 7 min, and nut surface area of 4.4 ( 0.5 cm2, and the materials were placed into a Cu electroplating solution. Radioactive metal wastes (bolts, nuts, connectors, and parts), having radioactive isotopes (Table 1), were obtained from Unit 2 of the Kori Nuclear Power Plant during an overhaul period.

Figure 2. Experimental apparatus using a magnetic bar: (1) CO2 cylinder, (2) syringe pump (260D), (3) view cell (10 mL), (4) oven, (5) stirrer, and (6) collection vial.

Figure 3. Experimental apparatus using ultrasonic horn: (1) CO2 cylinder, (2) syringe pump, (3) high-pressure cell (87 mL), (4) ultrasonic horn, (5) ultrasonic power supply, (6) oven, and (7) collection vial.

2.2. Phase Behavior Measurement. A variable volume cell (4.2-22.4 mL, Hanwoul Eng.) to continuously measure several points of the microemulsions was used as shown in Figure 1. The desired amounts of water or nitric acid solution and surfactant were loaded into the cell, which then was sealed. The cell was heated in an oven and pressurized by a syringe pump. The contents were stirred with a magnetic bar placed inside. The surfactant was partially dissolved and started to form the microemulsions with water into CO2. After agitating for about 20 min, microemulsions were formed clearly after a cloud state was observed for a moment as the pressure was gradually increased. When the microemulsions were observed at a specific pressure, we slowly decreased the pressure until the cloud point appeared at a fixed temperature. That is, while the transition state reached the critical point of the mixture, the formation pressure of microemulsions could be determined by direct observation through sapphire windows placed on both sides. So the microemulsions were measured continuously while their volume increased from the minimum volume (4.2 mL). The apparatus was placed into the oven and its temperature was controlled to (0.5 °C. Pressure in the cell was controlled by a syringe pump (260D, ISCO, USA) to (1 bar.14 2.3. Decontamination Experiments. Experimental apparatuses using different agitating methods are shown in Figures 2 and 3. One is a magnetic stirrer and the other is an ultrasonic generator. The ultrasonic high-pressure cell, with an ultrasonic horn directly connected inside, was developed to enhance the stability of microemulsions and the efficiency of decontamination. The horn’s frequency was 20 kHz. The amount of energy dissipated by the horn inside the cell was not measured; however, we guess approximately 10-20 W, which is an energy

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Figure 4. Recovery process of surfactants in Sc-CO2 microemulsions: (1) Microemulsions having contaminants, (2) separation of water (contaminants) from microemulsions, and (3) recovery of surfactants and collection of water (contaminants).

efficiency of 5-10% of the sonar’s total energy consumption (200 W) of the sonar.15,16 2.4. Separation and Recovery of Surfactants. Figure 4 shows the process of recovering the surfactants after decontamination. Microemulsions were prepared by a molar ratio (W ) 20) between the surfactant (NP-4, 445.6 mg) and water (0.4 mL) into the cell (125 mL) at 40 °C and 200 bar. After 1 h, microemulsions were transferred to a separator by depressurization, and thus water droplets in unstable microemulsions started to agglomerate to each other and were finally separated from the surfactant. Then with the pressure and temperature being maintained, the surfactant still dissolved in CO2 was separated from the water droplets and was collected through dynamic flushing of CO2. Finally, the collected surfactants were analyzed by HPLC. 2.5. Analysis. The microstructure of the Cu-coated surface treated by the CO2-acid microemulsion was observed using a scanning electron microscope (SEM, Steroscan 440, Leica, Cambridge, England). The optical microscopic images were taken using a camera (Eclipse ME600, Nikon, Japan). With use of these results, the dissolution effect was observed. The efficiency of radioactive wastes was analyzed by measuring their gamma-spectrum using a Ge-detector (HPGE P-TYPE [EGPC 30-1, 85-R], Eurisys Measures, France). So decontamination efficiency was calculated from the radioactivity of specimens measured for 24 h using a detector before and after decontamination by using eq 1,

Decon. Efficiency (%) )

(Abs - Aas) × 100 Abs

(1)

where Abs and Aas are the radioactivity of Co-60 γ rays in the specimens before and after decontamination, respectively. 3. Results and Discussion 3.1. Microemulsions Formation with Acidic Solutions. To confirm the stability of microemulsions in strongly acidic conditions, microemulsions using acidic solutions instead of pure water were observed using two kinds of surfactants. In general, fluorinated surfactants combined with fluorine (F) are quite soluble in CO2 because the weak dispersion forces of these tails are well-matched to those of CO2.17 Therefore, F-AOT has good

Figure 5. Stability of microemulsions with F-AOT as acidity increases.

solubility in Sc-CO2 as shown in Figure 5. Also, microemulsions were stable with water. However, acid-in-CO2 microemulsions were not stable when the concentration of surfactant exceeded 1.0 × 10-2 M in the 3 M of HNO3 because the water core was completely destroyed by the proton ratio of the surfactant. That is, anionic surfactant was shifted by nitric acid from an ionic state to a nonionic state in reaction with nitric acid, and then it was decomposed due to mutual action with water, and microemulsions became unstable (eq 2). Thus, formation pressure elevated rapidly at high acid concentration. This result shows that an ionic surfactant was unsuitable because it has several problems such as high price, complicated synthesis, and unstable microemulsions under highly acidic conditions.

R-SO3-Na+ + HNO3 T R-SO3H + NaNO3

(2)

On the contrary, NP-4 formed microemulsions continuously in reaction with increasing concentrations of nitric acid, as shown in Figure 6. Although the polyether group of the surfactant was decomposed partly due to its reaction with nitric acid and, accordingly, its formation pressure was higher than that in the reaction with water, it was confirmed to remain stable. Consequently, we decided to use NP-series for this purpose. Inorganic acid can easily corrode the metal surface. In particular, because a corroded surface is unacceptable when

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Figure 6. Stability of microemulsions with NP-4 as acidity increases.

Figure 7. Stability of microemulsions with organic acids (citric acid, oxalic acid).

reuse of the target materials is required, organic acids are more commonly used. So microemulsions using oxalic acid and citric acid were measured and plotted in Figure 7. For organic acid, a concentration of 5 wt % was used, and the other conditions were the same as those for inorganic acids. Organic acid also formed stable microemulsions with NP-4. However, the microemulsions became unstable and thereby showed little higher formation pressure because the pH of oxalic acid is generally lower than that of citric acid. As acidity became very high when the two acids were mixed, microemulsions became slightly unstable. 3.2. Effect of Ultrasonic Force on Micro-/Macroemulsions Formation. In surface decontamination using micro-/macroemulsions, microemulsions with contaminants can be more unstable after decontamination than before decontamination. Therefore, feasible and effective techniques for stable microemulsions are in great demand. So ultrasonic force was used to enhance the stability of microemulsions. The effect was compared with a magnetic stirrer, which took about 20 min to make microemulsions; the stirrer should be operated at a speed of over 400 rpm. In contrast, microemulsions enhanced by ultrasonic force were formed in about 10 min (after 30 s of continuous ultrasound) clearly and were more stable than that formed by the stirrer. Compared to the stirrer system, ultrasonic force was twice as effective because of the acoustic streaming effect of ultrasonic force on stability.18,19 The formation pressure of microemulsions as increasing W values was compared to find out the detailed agitation effect of

Figure 8. Agitation effect on the microemulsions formation by stirrer and ultrasonar (40 °C).

the stirrer and ultrasonar, as shown in Figure 8. We found out interesting results through the comparison of formation pressure. Unexpectedly, microemulsions formed by stirring were formed at lower pressures (about 20 bar) than those formed by ultrasound at low W values (W ) 5, 10). At W ) 15, both stirrer and ultrasonar showed similar effects. Microemulsions by ultrasonar were formed at lower pressures (20-60 bar) than those by stirring for W > 15. It was demonstrated that the stability of microemulsions formed by ultrasonar was revealed by a streaming effect at high W values. Also, ultrasonic force showed the same effect in the macroemulsions as well as microemulsions. Macroemulsions by ultrasonar were formed more quickly (∼30 s) and more stably than those by the stirrer. The reason was because ultrasonic force broke the surface tension between CO2 and water, breaking down the solution more vigorously than did stirring. 3.3. Surface Dissolution from Cu-Plated Specimen. As a mock-up test, a Cu-plated specimen (in this case, a machine nut) was examined with microemulsions containing HNO3 (24.5µL, 1 M or 6 M,). After 1 h, most of the Cu plating was dissolved clearly into the microemulsions. We observed the surface via optical microscopy and SEM to ascertain the effect upon the metal surface as the concentration of nitric acid changed. 1 M HNO3 removed over 90% of the Cu plating without damaging the metal’s surface. However, 6 M HNO3 not only removed the Cu plating but also damaged some parts of the surface due to its high concentration. This result reminded us that we need to control the concentration of HNO3. 3.4. Decontamination of Radioactive Metal Wastes. Based on the result of the test of surface dissolution of the Cu-plated specimen, we decontaminated actual radioactive metal wastes such as bolts, nuts, and other parts. Various radioactive peaks (Co-60, Sb-125, Mn-54, and Zr-95) from the subjects were detected through a Ge detector. Among the nuclides, we calculated the decontamination efficiency from the activity of Co-60. Table 2 shows the results of decontamination efficiencies using micro-/macroemulsions. With use of microemulsions, the specimens were decontaminated with a very small amount (24.5 µL) of nitric acid (1 M, 6 M) for 1 h, respectively. While the efficiency of 1 M HNO3 was about 78%, 6 M HNO3 was decontaminated up to almost background level (over 95%). However, the surface of the parts was partially damaged by the high concentration of nitric acid (6 M), which brought us to the conclusion that the concentration of nitric acid should be regulated carefully pursuant to the features of the specimen. Next, macroemulsions with diluted acidic solutions were used

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Figure 9. Photos of the surfactants recovery in water-in-CO2 microemulsions after decontamination. Table 2. Decontamination Efficiencies of Radioactive Metal Wastes (Nut, Bolt, and Parts) by Acid-in-CO2 Micro-/Macroemulsionsa efficiency (%) decontamination methods

acid types

agitation

nuclides

first

Sc-CO2 microemulsions

HNO3 (1 M) HNO3 (6 M)

ultrasonic force ultrasonic force

Co-60 (1.17, 1.32 MeV) Co-60 (1.17, 1.32 MeV)

87 ( 5 95 ( 5

Sc-CO2 macroemulsions

HNO3 (3 M) HNO3 (0.1 M) oxalic (5%)

stirrer ultrasonic force ultrasonic force

Co-60 (1.17, 1.32 MeV) Co-60 (1.17, 1.32 MeV) Co-60 (1.17, 1.32 MeV)

63 ( 5 89 ( 5 95 ( 5

a

second

third

78 ( 5

85 ( 5

Microemulsions (NP-4, W ) 20), macroemulsions (NP-10, acidic solution: 10 v/o), 200 bar, 40 °C, decontamination for 1 h.

instead of microemulsions. The decontamination was done by using the two agitation methods and inserting 10 v/o of nitric acid or organic acid. Macroemulsions using 3 M HNO3 and a stirrer decontaminated 75% of the specimen after repeating the process three times. The reason was that macroemulsions formed unstably due to the weak mechanical force of the stirring. On the other hand, macroemulsions using 0.1 M nitric acid and ultrasonic force decontaminated about 90% of the specimen after only one cycle because macroemulsions had formed very stably by taking advantage of the strong agitating and cleaning power and improved mechanical decontamination efficiency. Using oxalic acid achieved nearly 100%, up to background level, decontamination without damaging the metal’s surface because oxalic acid is also used for cleaning metallic surfaces, by virtue of the strength of the acid: its reductive capacity and its retention of metallic ions (chelates) in solution. 3.5. Surfactant Recovery after Decontamination. To minimize secondary wastes and reuse the surfactants, the used surfactants were separated and recovered from the contaminated microemulsions. Photographs of the surfactant recovery process are shown in Figure 9. Photo (a) shows that microemulsions are formed stably after about 20 min. By depressurization through controlling the volume, microemulsions are separated, as shown in photo (b). At this moment, the surfactant is still soluble in CO2. When the surfactants dissolved in CO2 are

collected, a part of nitric acid is precipitated at the bottom, as shown in photo (c). Partial microemulsions are formed at a low concentration of surfactant. Surfactants were recovered at a rate of 73% through the depressurized process. Finally, reutilizing the decontaminated metals below the clearance level, and reusing the recovered surfactants and CO2, remarkably reduces the radioactive waste volume. 4. Conclusions The SFSD technique was proven to be a suitable candidate to decontaminate radioactive metal wastes. Furthermore, the SFSD was shown to be an environmentally friendly process by reducing secondary wastes by recovering the surfactants and CO2. Compared to anionic (F-AOT) and nonionic (NP-series) surfactants, NP-series was more stable than F-AOT in acidCO2 microemulsions. Because the ionic state of F-AOT was changed by nitric acid, the core of microemulsions was destroyed and unstable. An ultrasonic horn was used as an agitating tool to enhance chemical and mechanical effects in microemulsions. In a mock-up test, over 90% of the Cu plating was removed from the Cu-coated specimen using acid-in-CO2 microemulsions. Radioactive metal wastes were subjected to decontamination by acid-in-CO2 micro-/macroemulsions. The efficiency of microemulsions was proportional to the concentra-

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tion of HNO3, which was ∼87% by 1 M and ∼95% by 6 M HNO3, but the surface was damaged partially due to the strong acidity of the 6 M HNO3. Next, acid-in-CO2 macroemulsions were examined with inorganic and organic acids. In a comparison of the result of efficiency to that of the agitating method, the experiment by stirring decontaminated about 85% after three times, but the ultrasonar decontaminated more than 89% after just one cycle. Also, organic acid-in-CO2 macroemulsions achieved over 95% efficiency up to below background level. To reduce the waste volume and reuse the surfactants, the surfactants used in surface decontamination were separated and collected. So 73% of the total surfactants was recovered and could be reused, which means that secondary waste can be minimized considerably by using a minute amount of solution and reusing the CO2 and surfactants. Acknowledgment This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF2006-DOO211) and by MOCIE through the EIRC program. Literature Cited (1) RadioactiVe Metal Waste Recycling Technology DeVelopment; KAERI/AR-474/97; Korea Atomic Energy Research Institute: Daejeon, Korea, 1997. (2) Bjerler, J.; McCooey, S.; Oertel, K.; Walthery, R.; Costes, J. R.; Hirabayashi, T. Decontamination Techniques Used in Decommissioning ActiVities; A Report by the NEA Task Group on Decontamination; OECD/ NEA: Paris, 1999. (3) Bayliss, C. R.; Langley, K. F. Nuclear Decommissioning, Waste Management, & EnVironmental Site Remediation; Elsevier, ButterworthHeinemann Ltd: Amsterdam, Boston, 2003. (4) Johnston, K. P.; Yates, M. Z.; Li, G.; Shim, J. J.; Maniar, S.; Lim, K. T.; Webber, S. Ambidextrous Surfactants for Water-Dispersible Polymer Powders from Dispersion Polymerization in Supercritical CO2. Macromolecules 1999, 32, 1018. (5) Eastoe, J.; Cazelles, B. M. H.; Steytler, D. C.; Holmes, J. D.; Pitt, A. R.; Wear, T. J.; Heenan, R. K. Water-in-CO2 Microemulsions Studied by Small-Angle Neutron Scattering. Langmuir 1997, 13, 6980. (6) Sandro, R. P.; Psathas, P. A.; Klein, E.; Johnston, K. P. Concentrated CO2-in-Water Emulsions with Nonionic Polymeric Surfactants. J. Colloid Interface Sci. 2001, 239, 241.

(7) Yates, M. Z.; Apodaca, D. L.; Campbell, M. L.; Birnbaum, E. R.; McCleskey, T. M.; Metal Extractions using Water in Carbon Dioxide Microemulsions. Chem. Commun. 2001, 25. (8) Campbell, M. L.; Apodaca, D. L.; Yates, M. Z.; McCleskey, T. M.; Birnbaum, E. R. Metal Extraction from Heterogeneous Surfaces Using Carbon Dioxide Microemulsions. Langmuir 2001, 17, 5458. (9) Liu, J.; Wang, W.; Li, G. A New Strategy for Supercritical Fluid Extraction of Copper Ions. Talanta 2001, 53, 1149. (10) Shimizu, R.; Sawada, K.; Enokida, Y.; Yamamoto, I. Decontamination of Radioactive Contaminants from Iron Pipes using Reactive Microemulsion of Organic Acid in Supercritical Carbon Dioxide. J. Nucl. Sci. Technol. 2006, 43, 694. (11) Erkey, C. Supercritical Carbon Dioxide Extraction of Metals from Aqueous Solutions: A Review. J. Supercrit. Fluids 2000, 17, 259. (12) Downer, A.; Eastoe, J.; Pitt, A. R.; Simister, E. A.; Penfold, J. Effects of Hydrophobic Chain Structure on Adsorption of Fluorocarbon Surfactants with Either CF3- or H-CF2- Terminal Groups. Langmuir 1999, 15, 7591. (13) Nave, S.; Eastoe, J.; Penfold, J. What Is So Special about AerosolOT? 1. Aqueous Systems. Langmuir 2000, 16, 8733. (14) Koh, M. S.; Yoo, J. R.; Park, Y.; Bae, D. I.; Park, K. H.; Kim, H. W.; Kim, H. D. Supercritical CO2 Extraction of Uranium(VI) from HNO3 Solution using N,N,N′,N′-Tetrabutyl-3-Oxapentanediamide. Ind. Eng. Chem. Res. 2006, 45, 5308. (15) Park, K. H.; Koh, M. S.; Yoon, C. H.; Kim, H. W.; Kim, H. D. Solubilization Study by QCM in Liquid and Supercritical CO2 under Ultrasonar. In Supercritical Carbon Dioxide: Separations and Processes; American Chemical Society: Washington, DC, 2003; Chapter 14, p 201. (16) Park, K. H.; Koh, M. S.; Yoon, C. H.; Kim, H. W.; Kim, H. D. The Behavior of Quartz Crystal Microbalance in High Pressure CO2. J. Supercrit. Fluids 2004, 29, 203. (17) Liu, Z.; Erkey, C.; Water in Carbon Dioxide Microemulsions with Fluorinated Analogues of AOT. Langmuir 2001, 17, 274. (18) Abramov, O. V. Ultrasound in Liquid and Solid Metals. CRC Press: Boca Raton, FL, 1994. (19) Awad, S. Ultrasonic Cavitations and Precision Cleaning. Precis. Clean. 1996, (Nov.), 12.

ReceiVed for reView June 23, 2007 ReVised manuscript receiVed September 20, 2007 Accepted September 22, 2007 IE070865M