The Extraction of Metal Contaminants Using Supercritical Carbon Dioxide

The Extraction of Metal Contaminants Using Supercritical Carbon Dioxide

Sesik Kang (Catholic University of Pusan, Busan, Korea), Minsu Ju (Dongnam Institute of Radiological & Medical Sciences, Busan, Korea) and Junghoon Kim (Catholic University of Pusan, Busan,Korea)
DOI: 10.4018/IJCCE.2018010104
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Conventional decontamination methods utilize water-based systems, which generate high amounts of secondary wastes. Herein, the authors describe an environmentally benign decontamination method using liquid and supercritical CO2. The use of CO2 as a solvent affords effective waste reduction by its ability to be recycled, thereby leaving behind only the contaminants upon its evaporation. In this study, a CO2 solution process was assessed using t-butyl salen (t-salen), dicyclohexano-18Crown6 (DC18C6), 8-hydroxyquinoline (8-HQN), perfluoro-1-octanesulfonic acid tetra-ethylammonium salt (NEt4PFOSA), and pentadecafluorooctanoic acid ammonium salt (NEt4PFOA) to extract spiked radioactive contaminants (Nb, Zr, Co, Sr) from an inert sample matrix, namely with filter paper. With the static extraction method, Sr was extracted with a maximum extraction rate of 97%, and Nb was extracted with a maximum extraction rate of 75%. Additionally, the authors were also able to extract Co and Zr with maximum extraction rates of 73% and 64%, respectively.
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Disposal and handling of industrial wastes imposes high costs and efforts on the society. For nuclear power industries in particular, it is quite expensive and difficult to process the waste products, as they are often radioactive. Because nuclear power plants use aqueous solutions for the decontamination of items such as clothing, hats, shoes, tools, and other parts, there exists a problem of producing secondary radioactive waste, such as contaminated radioactive water. Therefore, it is necessary to develop a new technology through which the decontamination solvent can be recycled, thereby reducing costs. Recently, we have seen vast improvements in metal extraction technology using supercritical CO2. As most radioactive wastes contain metal compounds, it is possible to decontaminate radioactive waste with supercritical CO2. If we extract metals using CO2 as a solvent, we can then easily separate CO2 from the extracted metals through vaporization, and the separated CO2 can subsequently be reused in the extraction process. Typically, metal extraction using CO2 is known to be a nontoxic, nonflammable, and environmentally benign process. Furthermore, this method of employing supercritical CO2 is energy efficient, is applicable to low temperature processes, and can be employed to processes containing red-short materials. However, CO2 (which is a non-polar compound) has difficulty dissolving metals, which are polar. Therefore, extracting agents are required to extract metals when CO2 is used as a solvent. Table 1 displays a summary of radioactive nuclides that typically exist in contaminated clothing at nuclear power plants.

All radioactive nuclides mentioned in the table, except I, are metal nuclides. Of them, Nb and Zr exhibit high radioactivity values in contaminated clothing. Therefore, in this investigation, we selected Zr, Nb, Co (the strongest -emitter of all corrosion products), and Sr (a representative nuclide of a fission product) as the nuclides to be subjected to decontamination.

Table 1.
Radiation of various nuclides found in clothing contaminated at a nuclear power plant
NuclidesRadioactivity (Bq/g)Half Life
Nb-953.1678E+0235 d
Zr-952.4521E+0264 d
Zr-975.0162E-0117 h
Co-601.4243E+005 y
Cr-514.0247E+0128 d
Zn-652.7022E+00244 d
I-1315.9759E-018 d
Cs-1375.7893E-0130 y
Ba-1403.7442E+0013 d

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