Dynamic immobilization of simulated radionuclide ~(133)Cs in soil by thermal treatment/vitrification with nanometallic Ca/CaO composites

摘要

Although direct radiation induced health impacts were considered benign, soil contamination with ~(137)Cs, due to its long-term radiological impact (30 years half-life) and its high biological availability is of a major concern in Japan in the aftermath of the Fukushima nuclear power plant disaster. Therefore ~(137)Cs reduction and immobilization in contaminated soil are recognized as important problems to be solved using suitable and effective technologies. One such thermal treatment/vitrification with nanometallic Ca/CaO amendments is a promising treatment for the ultimate immobilization of simulated radionuclide ~(133)Cs in soil, showing low leachability and zero evaporation. Immobilization efficiencies were 88%, 95% and 96% when the ~(133)Cs soil was treated at 1200 ℃ with activated carbon, fly ash and nanometallic Ca/CaO additives. In addition, the combination of nanometallic Ca/CaO and fly ash (1:1) enhanced the immobilization efficiency to 99%, while no evaporation of ~(133)Cs was observed. At lower temperatures (800 ℃) the leachable fraction of Cs was only 6% (94% immobilization). Through the SEM-EDS analysis, decrease in the amount of Cs mass percent detectable on soil particle surface was observed after soil vitrified with nCa/CaO + FA. The ~(133)Cs soil was subjected to vitrified with nCa/CaO + FA peaks related to Ca, crystalline phases (CaCO_3/Ca(OH)_2), wollastonite, pollucite and hematite appeared in addition to quartz, kaolinite and bentonite, which probably indicates that the main fraction of enclosed/bound materials includes Ca-associated complexes. Thus, the thermal treatment with the addition of nanometallic Ca/CaO and fly ash may be considered potentially applicable for the remediation of radioactive Cs contaminated soil at zero evaporation, relatively at low temperature.