Mechanistic study on the removal of Cs from contaminated soil by rapid ion exchange in subcritical water

Abstract

ABSTRACT As a result of the Fukushima nuclear power plant accident in March 2011, massive amounts of soil were contaminated with radioactive Cs. To remediate the contaminated soil, we are developing a desorption technique for removing Cs from soil using subcritical water. In the contaminated soil, the radioactive Cs is stably trapped in the interlayers of the clay minerals, making the use of traditional desorption approaches ineffective. However, we found that rapid ion exchange of Cs with multivalent cations in subcritical water is an effective desorption technique. To understand the mechanism and structural aspects of this process, in situ observations of the soil layer in subcritical water were carried out by high-energy X-ray diffraction together with pair distribution function analysis, and extended X-ray absorption fine structure analysis at SPring-8. Simulated contaminated soil (vermiculite) loaded with non-radioactive Cs+ was packed into a column equipped with observation ports and MgCl2 solution was passed through the column at a fixed flow rate under various conditions. Both the desorption experiments and structural studies indicated that conformational changes of the layer containing Cs+ under subcritical water conditions promoted intercalation of the hydrated Mg and trapped Cs+, which facilitated a rapid ion-exchange reaction of Mg2+ and Cs+.