Importance of halogen for soil decontamination using cesium-free mineralization; A Role of chlorine on phase transformation of weathered biotite

Shimoyama, Iwao   ; Baba, Yuji*  

Management of radioactive contaminated soil is an unsolved problem after the nuclear incident in Fukushima, but soil decontamination is generally difficult due to the existence of micaseous clay minerals in which radiocesium is strongly fixed. Cesium-free mineralization (CFM) can remove Cs from the soil through the conversion process from clay minerals to other minerals and the choice of reactant is critically important for this method. Because CaCl has higher decontamination efficiency than CaCO and Ca(OH) and facilitates phase transformation of clay minerals around 700C, it is deduced that Cl has a catalytic effect on phase transformation, however, the detail is not clear. In this work, Cs-sorbed weathered biotite (WB) was used as a model soil and chemical bonding states of Cl was studied using X-ray absorption spectroscopy and molecular orbital calculations. We clarified that Cl interacted with WB at 400C and Cl and higher valent Cl sites were formed by bonding with O in WB besides Cl sites in the early stage of the reaction. Because formation of Cl-O bonds can destabilize the silica network, Cl can facililate decomposition and phase transformation of WB at moderate temperatures.