Investigation of Cesium Sorption Mechanisms into Clay Minerals

摘要

To investigate detailed sorption behaviors of cesium (Cs) into clay minerals, thereby contributing to resolution of soil contamination issues due to Fukushima Daiichi Nuclear Power Station accident, we applied synchrotron-based X-ray analysis along with molecular orbital calculation (MO) using density functional theory and SEM/TEM-EDS combined with imaging plate (IP) methods. Conventional extended X-ray absorption fine structure (EXAFS) results suggest that Cs is preferentially incorporated into interlayers of vermiculite clay, which has water-swollen interlayers, but, by contrast, is dominantly sorbed onto the surface of clays such as illite in which most of the interlayers are closed off. The processes of incorporation of Cs into the swollen interlayers, followed by collapse of the interlayers, were first observed with time resolved- dispersive X-ray fine structure (TR-DXAFS). Small angle X-ray diffraction (SAXS) results show that successional sorption of Cs into clay minerals follows if one Cs ion can be sorbed into the clay minerals. Molecular orbital calculation based on density functional theory (DFT) methods suggests, furthermore, that the Cs incorporated into clay minerals can stabilize in the interlayers of clay minerals by forming covalent bonds with the clay minerals after the incorporation process. From analysis of litter soil collected from a forest in Iitate, Fukushima Prefecture, using SEM/TEM-EDS and IP sensitive to radioactivity, weathered biotite clay was confirmed to be important as a super-selective absorbent for Cs.