Study on stable solidification of silica-based ammonium molybdophosphate adsorbing cesium: Micromechanics and density functional theory modeling

Silica-based ammonium molybdophosphate (AMP/SiO ₂) adsorbent can effectively remove ¹³⁷Cs from simulated wastewater discharged during the Fukushima NPP-1 accident. A pressing/sintering method for the final disposal of the spent Cs adsorbent was described herein. Immobilization of Cs in the stable ceramic solid form of Cs ₄Al ₄Si ₂₀O ₄₈ was accomplished using natural mordenite. Different sintering temperatures and duration times exerted different effects on the final microstructures and the mechanical and loading abilities of the solidified products. At the sintering condition of 1 h duration time at 1,200 °C, the Cs immobilization ratio was almost 100%, and a relatively high hardness and elastic modulus of 3.50 and 35.38 GPa, respectively, were achieved. The formation of crystals as temperature increased corresponded to the micromechanical properties of the sintered product. Density functional theory calculations revealed that Cs ₄Al ₄Si ₂₀O ₄₈ is orthogonal with cubic unit cells, and Cs–O is more likely to form an ionic bond in the crystal. The dissolution of ceramic matrix was determined to be a temperature-dependent solidification mechanism that involves a corrosion medium. These findings provide a basic technological support for the stabilization of radioactive wastes.