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Shimoyama, Iwao; Baba, Yuji*
no journal, ,
Cs-free mineralization enable soil decontamination at lower temperature than conventional heat treatments because Cs is removed through conversion of clay minerals to different minerals, but the efficiency depends on additives. CaCl
derives high decontamination ratio accompanying phase transformation of clay minerals at 700
C, however, the effect of Cl is unclear. In this work, Cs-sorbed weathered biotite (Cs-WB) was used as a model soil, and the interaction between Cl and Cs-WB before phase transformation was investigated using NEXAFS spectroscopy. Cs-WB was reacted with NaCl-CaCl and excessive salt was removed by water rinsing. NEXAFS spectrum of the sample prepared at 400C showed three kind of Cl sites which are attributed to -1, +1, and high valencies based on the DV-X
molecular orbital calculations. Because +1 and high valence Cl sites with Cl-O bonding destabilize silica framework, this interaction can facilitate decomposition of clay minerals at moderate temperatures. The comparison of other chloride reagents suggests that the effect of Cl depends on the valence of counter cation in the reagent.
; Effect of chlorine on phase transformation of a clay mineralShimoyama, Iwao
no journal, ,
Radiocesium-contaminated soil is a severe unsolved problem in environmental remediation of Fukushima. Cesium-free mineralization (CFM) is a soil decontamination method by heating, and the characteristics of CFM is that Cs is removed by converting clay minerals which fix Cs in soil to other minerals through chemical reactions. CFM has succeeded Cs removal from weathered biotite (WB) at much lower temperature (700C) with CaCl reactant than conventional heat treatments. By contrast, phase transformation of WB did not occur at the temperature with Ca(OH) and CaCO
reactants, which suggests some effect of Cl for phase transformation, however, the detail is unclear. The interaction between Cl and WB was studied using X-ray absorption spectroscopy. The Cl spectrum of the sample prepared at 400C with CaCl showed multiple components which are interpreted as negative monovalent, positive monovalent, and positive high-valent Cl sites based on molecular orbital (MO) calculations. Comparison of structural stability of some silica model clusters by MO calculations indicates that formation of O-Cl bonds destabilizes the system more efficiently than formation of Si-Cl bonds. These results suggest that O-Cl bonds destabilize clay minerals and facilitates phase transformation of clay minerals at lower temperatures.