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Chapter 7
Supercritical Fluid Extraction of Mixed Wastes: PAH, PCB, Uranium and Lanthanum in Solid Matrices Joanna S. Wang,1 WenYen Chang,2 HuaKwang Yak,3 and KongHwa Chiu*,4 1Department
of Chemistry, University of Idaho, Moscow, ID 83844, USA of Natural Sciences, National Science Council, Taipei, Taiwan 107, ROC 3Department of Chemistry, ChungYuen Christian University, ChungLi, Taiwan 330, ROC 4Department of Applied Science, National DongHwa University, Hualien, Taiwan 907, ROC *
[email protected] 2Department
A sequential two-step supercritical fluid extraction technique has been developed for treating mixed wastes containing organic compounds (polychlorinated biphenyls and polyaromatic hydrocarbons) and metals (uranium and lanthanum). In the first step, the organic contaminants are extracted with neat supercritical fluid carbon dioxide at 250 atm and 150 °C. The extraction efficiencies for the organic compounds are around 93-100%. In the second step, a TBP (tri-n-butylphosphate)-nitric acid ligand is added to the system to extract uranium and lanthanum from the solid at 75 °C and a supercritical fluid density of 0.77 g/mL. The extraction efficiency for uranium is over 90% and lanthanum around 80%. This sequential supercritical fluid extraction technique appears promising for large-scale treatment of mixed wastes.
Introduction Radioactive wastes arise in many forms from a wide range of activities, particularly from the operation of nuclear power plants and processes associated © 2010 American Chemical Society Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
with nuclear research and isotope production. Uses of radioisotopes in medical diagnostic research, pharmaceutical and biotechnology developments also generate radioactive wastes (1–4). Mixtures of trans-uranium elements (TRU) or low level wastes (LLW) with toxic and hazardous substances are defined as mixed wastes by the Resource Conservation and Recovery Act (RCRA). Spent nuclear fuel, high-level radioactive waste and trans-uranic radioactive waste are three categories of nuclear wastes. Hazardous wastes include metallic compounds, organic solvents, and toxic organic materials such as polychlorinated biphenyls (PCBs) and poly aromatic hydrocarbon (PAHs). PCBs are particularly harmful to human health and environments. The Department of Health and Human Services (DHHS) (5) has concluded that PCBs can be anticipated to be carcinogens. Many existing mixed wastes containing such toxic compounds and radioisotopes have to be treated to acceptable levels for safe discharge into the environment. Solvent extraction is extensively used in the nuclear waste treatment and nuclear fuel reprocessing (6, 7). In the PUREX (Plutonium Uranium Extraction) process, acid dissolution followed by solvent extraction, typically kerosene containing an organophosphorous reagent, is used to recover uranium and plutonium separately from irradiated fuel. Organic chelation has been widely used for selective separation of metals from aqueous solutions in solvent extraction (6, 8). Similar technologies have also been proposed for treating soils containing radioactive wastes. Acid dissolution usually leads to many hazardous by-products and environmental problems. The use of liquid–liquid solvent extraction can generate large quantity of organic wastes. To remove toxic organic compounds from solid wastes, solvent reflux methods are often used. The Soxhlet process, for example, would generate organic liquid wastes. Therefore, there is a great need to have an improved method for treatment of mixed wastes using green solvents which would minimize secondary waste generation. Supercritical fluid carbon dioxide (Sc-CO2), as a green extraction medium, offers an attractive alternative over the conventional methods such as the PUREX and Soxhlet processes. The SFE technique has several advantages over the conventional solvent extraction methods (9). Extraction efficiency in SFE is enhanced due to the rapid mass transfer in the supercritical fluid phase. Efficient and rapid separation of extracted substances from CO2 can be achieved by simply venting CO2 into the atmosphere. SFE for extraction of organic compounds such as PCBs and PAHs is well-established in the literature (10–13). However, it has seldom been applied to the mixed waste treatment. This paper describes a sequential SFE method to efficiently separate organic and radioactive materials in mixed wastes into different fractions. PCBs (IUPAC #77, IUPAC #209) and PAH congeners (chrysene) are used as model organic compounds. We select uranium and lanthanum as representative metals in this mixed waste study. The matrix effect is studied by spiking the toxic organic compounds, uranium and lanthanum in different solid matrices including filter paper, sea sand, and soil. Extraction conditions including temperature, pressure, CO2 density, and time are optimized, and two different ligands (TTA-TBP and TBP/HNO3) are used for metal extraction.
80 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Experimental Chemicals Uranium and lanthanum standard solutions were obtained from EM Science (Gibbstown, NJ, USA) and used as received. Sea sand (washed, S25–500) was purchased from Fisher Scientific Inc. (NJ, USA). Chloroform and hexane were purchased from EM Science (Gibbstown, NJ, USA), and from J.T. Baker, respectively. SFE grade carbon dioxide (purity of 99.99%) was supplied by Oxarc, Spokane, WA. USA. Tri-n-butylphosphate (TBP) (≥ 97%) was obtained from Aldrich Chemical Company, Inc. (Milwaukee, WI) and used as received. HNO3 (Sigma-Aldrich) was used directly. IUPAC #77 (3, 3′, 4, 4′-tetrachlorobiphenyl), IUPAC #209 (decachlorinated PCB), and a PAH compound chrysene (about 300 µg/mL) were obtained from Ultra Scientific (North Kingston). Internal standard fluoranthene was supplied by Sigma Chemicals Co. (USA). De-ionized water (Millipore Milli-Q system, Bedford, MA) was used for the preparation of all aqueous solutions. Instruments and Facilities Supercritical fluid extractions were performed by an ISCO syringe pump (model 260D, Lincoln, NE, USA) connected to a series of stainless steel vessels placed in an oven (Varian 3700, USA). The flow of Sc-CO2 was controlled manually by a high pressure outlet valve. A schematic diagram of the experimental apparatus is shown in Figure 1. A fused silica capillary (inner diameter: 100 µm, length 25 cm) from Polymicro Technologies, Phoenix, USA, was used as a restrictor in the extraction of PCB and PAH processes. For the extraction of U and La, a PEEK tube, as a restrictor, was used. The outlet end of the restrictor was placed in a glass tube containing a trap solvent (hexane or chloroform) to collect the extracted analytes. A high-resolution gas chromatograph combined with a flame ionization detector (FID) (Hewlett Packard, HP 5890A) was used for analyzing the organic compounds. Sample Preparation Three spiked solid matrices were studied: filter paper (Whatman No.1), washed sea sand and soil. A standard mixture of PCBs and PAHs containing two PCB congeners, IUPAC #77 (3, 3′, 4, 4′-tetrachlorobiphenyl) and IUPAC #209 (decachlorinated biphenyl), and one PAH compound: chrysene (about 300 µg/mL/congener) was used as the source of the organic compounds. Samples were prepared by spiking with 100 µL (or 100 µg, at 1000 µg/mL concentration) of each of the standard mixtures of PCBs, PAHs, UO2 and lanthanum. The solvent was evaporated at room temperature for 2 h. After that the sample was loaded into the extraction vessel and placed in the oven. Two ligands, i.e., TBP(HNO3)0.7(H2O)0.7 and TTA-TBP, were used in the extraction of metals. The extractant, TBP(HNO3)0.7(H2O)0.7, was prepared as described previously (14). A TTA-TBP complex was prepared by adding 0.111 g 81 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Figure 1. Schematic diagram of the experimental system for the mixed waste treatment in Sc-CO2: (1) CO2 cylinder, (2) syringe pump, (3) ligand cell, (4) extraction cell, (5) collection vial, (6) heater for the PEEK restrictor, (7) heating oven. of solid TTA to 1 mL TBP. The ligand (3 mL of TBP(HNO3)0.7(H2O)0.7 or 1 mL TTA-TBP) was placed in a separate vessel in the oven as shown in Figure 1.
Supercritical Fluid CO2 Extraction In the first step, extraction of PCBs and PAHs was performed at 250 atm, 150 °C with 20 min static time followed by 20 min dynamic time with a flow rate of 1.5 mL/min. During dynamic extraction, the analytes were trapped in hexane (20 mL). The PCB/PAH extract was concentrated by evaporating the solvent to about 2 mL using a slow nitrogen flow. Internal standard fluoranthene (100 µL) was added to the sample before the analysis. The PCB/PAH extract was analyzed by a high-resolution gas chromatograph/flame ionization detector. Separation of the compounds was carried out on a DB-5 column (J&W Scientific, Folsom, CA, USA, 30 m, 0.32 mm, 0.25 µm). Sample (1 µL) was injected at 280 °C. Helium gas(purity 99.9%, Oxarc, WA, USA) was used as a carrier gas. The oven temperature was held at 70 °C for 1 min, then increased at a rate of 10 °C/min to 300 °C, where it was held for 5 min. Concentrations of the analytes were calculated by comparing the peak area ratio between extracted compounds to the internal standard. In the second step, CO2 was delivered to the sample vessel through the ligand vessel, carrying the dissolved TBP(HNO3)0.7(H2O)0.7 into the extraction cell and converting uranium and lanthanide into CO2-soluble metal complexes. Static extraction time was fixed at 10 min and the dynamic time was optimized from 5 min up to 80 min with flow rate of 1 mL/min. Extracted metals were collected in 20 mL of chloroform. Back extraction of the extracted metals was performed by adding 3 mL of HNO3 (8 N) to the trap solution and shaking for 3 min. The aqueous phase was separated and the organic phase was washed twice with 3 mL of deionized water. The amount of the extracted metal ions was later analyzed by an ICP-AES (Perkin Elmer ICP optima 3000 XL, USA). The wavelengths of 385.9 nm and 408.7 nm were used for U and La analyses, respectively. 82 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Results and Discussion Two steps are used for treatment of mixed wastes in this study. The first step is for separation of organic compounds, PCBs and PAHs, which is followed by the second step, i.e., extraction of metals.
Extraction Efficiency for Organic Compounds Extraction of the natural soil showed that there was no detectable level of PCBs/PAHs in the solid matrix. Extraction efficiencies of PCBs and PAHs standards directly from the vessel (without the soil) were about 96-100%. This indicated that the SFE trapping technique was workable and no loses of PCBs/PAHs occurred during the sample treatment and analysis. PCB and PAH compounds were effectively extracted (93-100%) from all three solid matrices under the conditions of 250 atm pressure and 150 °C. Milder conditions were studied as well, but the extraction efficiency of chrysene decreased as the temperature or pressure descended to 100 °C or 200 atm. Since 150 °C and 250 atm showed the best results for extraction of the organic analytes in soil, this condition was adopted for mixed waste applications.
Extraction Efficiency for Metals The behavior of La and U in the complex formation with TBP/HNO3 can be described by the following equations:
where SF stands for supercritical fluid. Extraction of metals from solid matrix with different ligands showed that a higher extraction efficiency can be achieved with the TBP/HNO3 complex than the TTA-TBP complex (Figure 2). This is probably due to the higher solubility of TBP/HNO3 complex in the supercritical fluid (15). The extraction of metals from sea sand and filter paper was easier than from natural soil. This is probably because natural soil has more active sites to bind the metal ions than filter paper or sea sand matrices. Extraction efficiency of uranium was near 80% from natural soil even with the ligand TTA-TBP. Only 55-70% of the spiked lanthanum was extracted under the conditions of 65 °C and 250 atm using TBP(HNO3)0.7(H2O)0.7. The extraction efficiency for lanthanum was lower than that of uranium in all cases (Figure 2). The trap solvent (hexane) was analyzed after the first extraction. Results confirmed that lanthanum was not dissolved in neat CO2 during the process of extraction of organic compounds. The low SFE efficiency for lanthanum was most likely due to its strong interactions with the solid matrices. 83 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Figure 2. SFE of uranium and lanthanum from solid matrices at 65 °C and 250 atm. The left part is for uranium extraction and the right part is for lanthanum extraction. The extraction efficiencies are calculated for the first and second step extractions. Mixed Waste Extraction In Table I, the optimal conditions for Sc-CO2 extraction of the mixed waste are presented. Effects of Density and Temperature In SFE, one of the most promising advantages is that the density of CO2, can be continuously tuned over a wide range by adjusting the pressure and temperature. The solubility of an analyte in the Sc-CO2 phase depends on the density of the Sc-CO2. The following equation explains the correlation between the solubility of solute and the CO2 density (16).
where SSF is the solubility of a solute in Sc-CO2 with a specific density (D). Constant p is related to the solvation strength of the solute and q is a temperature-dependent constant that involves thermal properties including solvation heat and volatility of the solute. Therefore, to improve the extraction efficiency of metals from solid matrices, the major Sc-CO2 extraction parameters, i.e. temperature, pressure, and fluid density, should be studied. When the Sc-CO2 density was below 0.6 g/mL, the extraction efficiency of uranium was over 60%, whereas less than 20% of lanthanum was extracted from the sea sand (Figure 3). When the fluid density exceeded 0.6 g/mL, the extraction of lanthanum was significantly enhanced, reaching 67 % at 0.77 g/mL. 84 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Table I. Sequential SFE of spiked mixed waste (PCBs and PAHs + U and La) as % recovery from different solid matrices First Extraction % with Neat CO2a
Second Extraction % with TBP-HNO3 in CO2b
Matrices
PCB #77
PCB #209
Chrysene
U
La
Filter Paper
100
100
99
90
85
Sea Sand
98
97
92
90
85
Natural soil
95
96
93
85
77
a 150 °C, 250 atm, 20 min static and 20 dynamic extraction time. min static and 80 dynamic extraction time.
b
75 °C, 295 atm, 10
Figure 3. Effect of Sc-CO2 density on the extraction of U and La from sea sand at a constant temperature of 65 °C. At a constant CO2 density (0.77 g/mL), elevation of temperature resulted in a significant improvement on the extraction of both uranium and lanthanum from sea sand (Figure 4). Uranium can be efficiently extracted at a low temperature (55 °C), whereas a high extraction efficiency for lanthanum can only be obtained at 75 °C. We did not perform experiments higher than 75 °C due to the apparatus limitation. However, with extended extraction time (up to 80 min dynamic time), approximately 89% of the spiked lanthanum could be extracted at temperature of 75 °C and CO2 density of 0.77 g/mL. The result indicates that in extraction of lanthanum from solid matrices, more harsh extraction conditions and a longer extraction time are required in order to reduce the interactions between lanthanum ions and matrix. Formation of lanthanum complex with TBP(HNO3)0.7(H2O)0.7 is not as effective as that of uranium. As temperature increases, a better extraction efficiency is obtained which suggests that lanthanum probably can react more effectively with the ligand at higher temperatures. 85 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
Figure 4. Effect of temperature on the extraction of La from sea sand at a constant CO2 density of 0.77 g/mL.
Conclusions A sequential two-step Sc-CO2 extraction process can be used to treat mixed-wastes which contain both radioactive materials and hazardous organic compounds. First, organic compounds such as PAHs and PCBs are separated from solid matrices by neat Sc-CO2 extraction. In the second step, radioactive elements such as uranium and lanthanides are extracted using a proper complexing agent. Generally speaking, the Sc-CO2 extraction efficiency is lower in the “naturally” occurring matrices such as soil. This could be caused by strong interactions between the matrix and the contaminants. The density of the Sc-CO2 phase can be adjusted by temperature and pressure to achieve optimum extraction efficiencies. This sequential extraction method represents an environmentally friendly green technology for treating mixed wastes. The Sc-CO2 technology appears suitable for large-scale treatment of mixed wastes containing lanthanides, actinides, and toxic organic compounds.
References 1. 2. 3. 4. 5. 6.
Kemp, R. V.; Bennet, D. G.; White, M. J. Environ. Int. 2006, 32, 1021–1032. Andrade, C.; Martínez, I.; Castellote, M.; Zuloaga, P. J. Nucl. Mater. 2006, 358, 82–95. Plećaš, I.; Dimovic, S. Prog. Nucl. Energy 2006, 48, 629–633. Melville, G.; Liu, S. F.; Allen, B. J. Appl. Radiat. Isot. 2006, 64, 979–988. U.S. Department of Health & Human Services. http://www.hhs.gov/. Minezewski, J.; Chwastowska, J.; Dybezynski, R. In Separation and Preconcentration Methods in Inorganic Trace Analysis; Ellis Harwood/Halsted Press: Chichester, England, 1982; pp 97−282. 86 Wai and Mincher; Nuclear Energy and the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 2010.
7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
Alegret, S.; Masson, M. R. In Development in Solvent Extraction; Ellis Harwood Limited: Chichester, England, 1988; Chapter 11, pp 177−188. Wai, C. M. In Preconcentration Techniques for Trace Elements; Alfassi Z. B., Wai, C. M., Eds.; CRC Press: Boca Raton, FL, 1991; Chapter 4, pp 101−32. Lin, Y.; Smart, N. G.; Wai, C. M. Trends Anal. Chem. 1995, 14, 123–133. Berg, B. E.; Lund, H. S.; Kringstad, A.; Kvemheim, A. L. Chemosphere 1999, 38, 587–599. Miller, D.; Hawthorne, S. B.; Clifford, A. A. J. Supercrit. Fluids 1997, 10, 57–63. Morselli, L.; Sabbioni, M.; Zappoli, S.; Quattroni, G. Fresenius Environ. Bull. 1995, 4, 463–468. Berset, J. D.; Holzer, R. J. Chromatogr., A. 1999, 852, 545–558. Enokida, Y.; ElFatah, S. A.; Wai, C. M. Ind. Eng. Chem. Res. 2002, 41, 2282–2286. Samsonov, M. D.; Wai, C. M.; Lee, S. C.; Kulyako, Y.; Smart, N. G. Chem. Commun. 2001, 1868–1869. Rao, A.; Kumar, P.; Ramakumar, K. L. Radiochim. Acta 2008, 96, 787–798.
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