The Extraction of Metal Contaminants Using Supercritical Carbon Dioxide

Introduction

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.