Efficient removal and stable immobilization of Cs+ by silica-based ammonium phosphomolybdate and geopolymer

¹³⁷Cs, as the main heat-generating nuclide in radioactive wastewater, has a long half-life, strong radioactivity and toxicity. Thus, Cs⁺ adsorption–immobilization from radioactive wastewater is essential for human health and environmental safety. In this study, the pore structure of hollow mesoporous silica spheres (HMSS) was firstly adjusted by the use different amount of pore-expanding agents, and then ammonium phosphomolybdate (AMP) was loaded onto HMSS to prepare a novel adsorbent (AMP-HMSS) for cesium. Taking advantage of the mesoporous and hollow structure and AMP loading, AMP-HMSS showed outstanding adsorption ability for Cs⁺ with maximum adsorption capacity of 179.21 mg/g and high adsorption selectivity. AMP-HMSS showed much better adsorption performance than HMSS in the strong acidic solution (up to 1 mol/L HCl). The governing removal mechanism of AMP-HMSS for Cs⁺ was exchange between Cs⁺ and NH₄⁺ along with participation of hydroxyl group. Moreover, spent adsorbents (AMP-HMSS/4-Cs) was immobilized in fly ash-based geopolymer. The geopolymer displayed good anti-leaching performance even in the salty environment, which indicated that Cs⁺ could be stably immobilized in the geopolymer. After calcination of 900 ℃ for 8 h, new phases zeolite and nepheline was generated in the geopolymer and subsequently enhanced its compressive strength.