Stable solidification of silica-based ammonium molybdophosphate by allophane: Application to treatment of radioactive cesium in secondary solid wastes generated from fukushima

•Pressing/sintering method was used to solidify secondary solid wastes.•The kegging structure of AMP had no effect for formation of stable ceramic solid form.•Crystal phase of Cs4Al4Si20O48 was formed and decreased by 300 °C compared with zeolite.•The ceramic solid form exhibited excellent leaching resistance properties.

Silica-based ammonium molybdophosphate (AMP/SiO2) is an absorbent material that can effectively remove Cs from radioactive-contaminated wastewater (RCW) generated by Fukushima nuclide accident. Pressing/sintering method was used for final disposal of secondary waste (spent absorbent) to achieve the volume reduction of AMP-Cs/SiO2 (AMP/SiO2 saturation adsorption of Cs) and stable solidification of Cs by adding natural allophane. The structure of AMP-Cs completely collapsed at approximately 700 °C, and most Mo and P species in AMP sublimed. The optimal sintering temperature was estimated as 900 °C. The stable crystalline phase of Cs4Al4Si20O48 was recrystallized by the reaction of Cs2O, Al2O3, and SiO2, and the immobilization ratio of Cs was approximately 100%. The leachability of Cs from the sintered product in distilled water was approximately 0.41%. The high immobilization and low leachability of Cs were attributed to the excellent solidification properties of the sintered products of AMP-Cs/SiO2-allophane.

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