Mechanism of Cs removal from Fukushima weathered biotite by heat treatment with a NaCl-CaCl mixed salt

Honda, Mitsunori   ; Okamoto, Yoshihiro  ; Shimoyama, Iwao   ; Shiwaku, Hideaki   ; Suzuki, Shinichi; Yaita, Tsuyoshi

An in situ extended X-ray absorption fine structure (in situ EXAFS) spectroscopic analysis at high temperature was conducted to investigate the mechanism of Cs removal from weathered biotite (WB) from Fukushima, induced by heating with a mixed salt of NaCl and CaCl. This indicated that most Cs remained in WB during heating at 200-700C. In addition, the in situ EXAFS spectra gradually changed on heating with the mixed salt and a completely different spectrum was observed for the sample after cooling from 700C to room temperature. Ex situ EXAFS measurements and X-ray fluorescence analyses were also conducted on samples after heat treatment and removal of the mixed salt to clarify the temperature dependence of the Cs removal ratio. Based on the results of radial structure function analysis obtained from in situ EXAFS, we concluded that almost all Cs was removed from WB by heating at 700C with the mixed salt, and that Cs formed CsCl bonds after cooling to room temperature from 700C. In contrast, although more than half of the Cs present was removed from WB by heat treatment at 500C, most Cs was surrounded by silica tetrahedrons, maintained by Cs-O bonds. On the basis of these results, different Cs removal processes are suggested for the high-temperature (600-700C) and low-temperature (400-500C) regions.