Development of adsorption and solidification process for decontamination of Cs-contaminated radioactive water in Fukushima through silica-based AMP hybrid adsorbent

Developing highly efficient cesium (Cs) adsorbent and a stable solidification method for Cs is vital for the advancement of the decontamination system for Fukushima Daiichi Nuclear Power Plant. A novel porous silica loaded with ammonium molybdophosphate (AMP/SiO2) was prepared through impregnation/precipitation/freeze-drying method. The composite with 32.4 wt% AMP loading amount had superior large 254.2 Å pore width and 72.2 m2/g surface area. The 137Cs removal efficiency in actual seawater was 97.1% within 30 min, indicating that AMP/SiO2 effectively treated contaminant water with low-level concentration of Cs even in the presence of highly concentrated competing ions. The AMP/SiO2 adsorption of 137Cs in seawater conformed well with Redlich-Peterson model. The ion exchange ratio of Cs+ and NH4+ in AMP was estimated to be 71% from the equivalent of Cs2NH4MP. Pyrogenetic AMP/SiO2-adsorbed Cs (Cs-AMP/SiO2) was decomposed above 400 °C sintering temperature, and Cs immobilization (%) decreased from 100% to 40% after sintering at 1200 °C; porous silica support matrix had no immobility for Cs. In contrast, adding natural mordenite (NM) can depress the volatilization of Cs, resulting to the steady immobilization ratio of Cs at nearly 100% for the sintered products. The crystal phase immobilizing Cs was identified as Cs4Al4Si20O48 through X-ray diffractometry. The Cs leachability from the sintered products was less than 0.1% even at 90 °C in distilled water and 0.1 M sodium chloride.