Selective separation and immobilization process of 137Cs from high-level liquid waste based on silicon-based heteropoly salt and natural minerals

A macro porous silica-based ammonium phosphomolybdate (AMP/SiO2) adsorbent was synthesized and tested for separation and recovery of 137Cs from real radioactive liquid waste. The excellent adsorption ability and high selectivity of AMP/SiO2 for 137Cs were confirmed. AMP/SiO2 had relatively large decontamination efficiency of 137Cs exceeding 99% in radioactive wastes, and the decontamination factors of 137Cs were estimated to be 688 and 270 after adsorption in radioactive waste at 1 and 3 mol/L HNO3, respectively. In column operation, the separation and recovery of Cs(I) and coexisting ions could be realized using different eluents, and the recovery rate of Cs(I) was more than 90%. The Cs 3d and Mo 3d X-ray photoelectron spectroscopic spectra of AMP/SiO2 before and after Cs(I) adsorption, suggesting that the ion exchange process of cesium with crystal lattice cations was the governing mechanism, and cesium ions could spontaneously react with approximately two ammonium ions in each AMP lattice by density functional theory simulation. Further, the crystal binding energy of each revised model showed an upward trend with the progress of the adsorption. After sintering at 1200 °C, CsAlSiO4, which could stably solidify eluted cesium, was formed in the allophane-based solidified product. The crystalline properties and volume reduction effect of the sintered products were enhanced with the increase in the allophane content. Ba(II) hardly entered the lattice of CsAlSiO4 and occupied the Cs(I) site, resulting in minor changes in the crystal structure with substitution of Ba(II) for Cs(I).