Synthesis of N-A-S-H Gels via Sol–Gel and Hydrothermal Methods: Composition, Microstructure, and Dynamic Adsorption Characteristics for Sr2+

Fly ash-based geopolymers are widely used as solidification matrices for intermediate- and low-level radioactive waste liquids. Their performance relies heavily on the adsorption capacity of the primary component, N-A-S-H gels, which directly governs the long-term stability of the solidified waste form. This study prepared N-A-S-H gels using both the sol–gel synthesis and the hydrothermal synthesis method. The effects of the Si/Al ratio, alkalinity, and NaNO3 on the structure of N-A-S-H gels were analyzed using EDS, FT-IR, and XRD. Additionally, the dynamic adsorption capacity of N-A-S-H gels for Sr2+ under varying solution temperatures and alkaline conditions was evaluated and fitted using the Thomas and Yoon–Nelson models. For sol–gel-synthesized gels, a lower initial Si/Al ratio led to a higher theoretical maximum Sr2+ adsorption capacity of 121.71 mg/g at room temperature and neutral pH. For hydrothermally synthesized gels from raw fly ash, the high initial alkalinity reduced the Si/Al ratio but adversely decreased Sr2+ adsorption capacity. Raising the temperature to 60 °C improved Sr2+ uptake for all gel samples, with a maximum increase of 28.75 mg/g. Under alkaline conditions, Sr2+ uptake for all gels decreased significantly, with a minimum theoretical reduction of 31.55 mg/g. The findings of this study provide a theoretical foundation for understanding the radionuclide migration mechanisms of fly ash-based geopolymer solidification forms in complex geological environments.