Synthesis of high mechanical strength and excellent radiation resistance NaX zeolite microspheres via geopolymer in situ conversion and their adsorption study of ¹³⁷Cs and ⁹⁰Sr

For the remediation of radionuclides (e.g., ¹³⁷Cs and ⁹⁰Sr) in nuclear wastewater, conventional materials struggle to simultaneously exhibit excellent adsorption performance, high mechanical strength and good irradiation resistance. Herein, a kind of high-strength NaX zeolite microspheres with 75-150 μm particle size (GXU-GNaXs, compressive strength: 19.21 MPa, Vickers hardness: 216.30) were prepared for the first time by the in-situ conversion of geopolymer technology under normal pressure at 105 ºC for 12 h. The results showed that GXU-GNaXs achieved removal efficiency exceeding 90 % for Cs⁺ and Sr²⁺ within 20 min at dosage of 0.8 g·L^-1, following pseudo-second-order and Langmuir model with maximum saturated adsorption capacities of 138.30 and 153.60 mg·g^-1 at 45 and 30 min, respectively. Meanwhile, GXU-NaXs maintained >98 % structural stability and adsorption capacity after 500 kGy γ-irradiation, which had the potential for excellent stability in the nuclear environment. GXU-NaXs also exhibited good dynamic adsorption effect at flow rate of 6 mL·min^-1, and the removal efficiency after three cycles remained 97.89 and 56.39 % for Cs⁺ and Sr²⁺, respectively, demonstrating GXU-NaXs had good feasibility for industrial applications. GXU-NaXs showed good removal capacity and selectivity in complex seawater matrices for Cs⁺ and Sr²⁺. Density functional theory calculations revealed adsorption energies of -2.43 (Cs⁺) and -3.52 eV (Sr²⁺), while SEM/EDS, TEM, XPS and FT-IR methods confirmed the adsorption mechanism was ion exchange and chemisorption. This study pioneers the advanced synthesis technology in nuclear adsorbents, offering a promising direction for radioactive wastewater remediation.