Role of chlorine for reactions between clay mineral and alkaline chloride

Shimoyama, Iwao   ; Baba, Yuji  

Soil decontamination is generally difficult because radioactive Cs can be strongly fixed in clay minerals, whereas efficient decontamination is possible when clay minerals are converted to different minerals which cannot include Cs in their crystal structures by reacting with some alkaline salts at high temperatures. So far, it is not clear why reaction temperature is largely decreased if some chloride is chosen as a reactant. In this work, we analyzed chemical bonding states of Cl using X-ray absorption spectroscopy (XAFS) for the samples after heat treatments of weathered biotite and NaCl-CaCl mixed salt and interpreted XAFS spectra using molecular orbital (MO) calculations to study the role of chlorine on phase transformation of clay minerals. We found that there existed not only monovalent-negative Cl sites but also high-valent positive Cl sites. We also estimated structural stabilities of some silica model clusters with chlorine using semiempirical MO calculations and clarified that formation of Cl-O bonds destabilizes silica network to decrease reaction temperature. These results suggest that Cl has some catalytic effect on phase transformation of clay minerals.