Zhao Zheng

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.

The operation of nuclear power plants generates a large amount of low- and intermediate-level radioactive waste liquid. Zeolite-rich geopolymers, which are synthesized under hydrothermal conditions from industrial waste fly ash, can effectively immobilize radioactive nuclides. In this study, the synthesis law of zeolite-rich geopolymers and the adsorption/desorption performances of radioactive nuclide Cs⁺ were researched using XRD, SEM and ICP. The results show that the increase in curing temperatures and NaOH concentrations leads to the transformation of Y-type zeolite to chabazite and cancrinite at low NaNO₃ concentrations. However, at high NaNO₃ concentrations, NaOH above 2 M has no obvious effect on the phase transformation of the main zeolite of chabazite and cancrinite. In the adsorption and desorption experiment of Cs⁺ on the chabazite/garronite-rich geopolymer, it was found that the adsorption of Cs⁺ in the low initial concentration range is more suitable for the Freundlich equation, while the Langmuir equation fits in the adsorption process at the high initial concentration range. Moreover, the desorption kinetics of Cs⁺ are in good agreement with the pseudo-second-order rate equation. Thus, the adsorption of Cs⁺ on chabazite/garronite-rich geopolymers is controlled by both physical and chemical reactions, while desorption is a chemical process.

Due to its high retention capacity and well durability, fly ash-based geopolymers rich of zeolites are considered to be potential solidified materials for low- and intermediate-level radioactive waste stream. This study mainly focused on the phase transformation, pore structure, and leaching characteristic of Sr²⁺ and Cs⁺ in solidified fly ash/cement matrices under different sodium hydroxide contents or sodium nitrate concentrations after thermal curing of 28d. The results obtained from the experiments indicated that the geopolymers prepared from pure fly ash had the potential to transform into ordered structures at high temperatures of 90°C instead of room temperature, such as Na-P1 zeolite and chabazite. With the increase in sodium hydroxide contents, more sodium nitrate participated in the reaction to form a zeolite phase in solidified fly ash/cement matrices, while the coexistence phenomenon of chabazite and Na-P1 zeolite appeared under the condition of high sodium hydroxide contents of more than 5. Just a proper number of zeolites can effectively inhibit the leaching of radionuclides Sr²⁺, as too many zeolites would lead to the increase in porosity, which was harmful to the retention of radionuclides. Solidified fly ash/cement matrices at a sodium hydroxide content of 5 or at a sodium nitrate concentration of 300g/L obtained the lowest cumulative leaching fraction of Sr²⁺. Moreover, sodium hydroxide increased the cumulative leaching fraction of Cs⁺ which reversely decreased significantly with the increase in sodium nitrate concentrations.

The utilization of cement waste forms with high content of fly ash is a potential method in large-volume immobilization of low- and intermedium-level radioactive waste at near-surface. However, the migration behavior of radionuclides in fly ash/cement waste forms under geological environment is still unclear. This study researched the transformation of products, pore structure as well as the leachability of Ca²⁺, Na⁺, and simulated radioactive nuclides (Sr²⁺ and Cs⁺) in fly ash/cement waste forms under simultaneous effect through X-ray diffractometer (XRD), mercury intrusion porosimetry (MIP), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Furthermore, the relationship between the decalcification and the leaching of Sr²⁺ and Cs⁺ was established by the fitting curves of apparent diffusion coefficient. The results indicated that the products of fly ash/cement waste forms were kept stable under simultaneous effect and the pore structure has excellent resistance to temperature. The increasing salt concentrations of leachant and temperature promoted the leaching of radionuclides while the protection layer formed at 6080°C inhibited this phenomenon. The relationship of apparent diffusion coefficient between Sr²⁺ and Ca²⁺ was quadratic nonlinear, while the relationship between Cs⁺ and Ca²⁺ showed a linear relationship. Compared to cement waste forms, fly ash/cement waste forms were more stable. In addition, the long-term retention ability of fly ash/cement waste forms to Cs⁺ was more outstanding than Sr²⁺.