The coupling action mechanism of NaOH/NaNO3 on the hydrothermal synthesis of fly ash-based zeolites and the Sr-Na exchange capacity

Abstract

The treatment of low- and intermediate-level radioactive waste from nuclear power plants using fly ash-based zeolite involves issues related to product evolution and cation exchange. This study aims to elucidate the evolution of hydration products and the interaction mechanism between Sr²⁺ and Na⁺ in fly ash-based zeolite under the regulation of NaOH/NaNO₃. The fly ash-based zeolites were characterized using XRD, IR, and SEM. The adsorption capacity and chemical binding energy before and after Sr²⁺ adsorption were analyzed using ICP and XPS, respectively. The results demonstrated that the main products in fly ash-based zeolite influenced only by NaOH undergo a transformation from Faujasite to Na-P1 zeolite, and finally to Sodalite. However, when NaOH/NaNO₃ were added simultaneously, the main product first transformed from Faujasite to Na-P1 zeolite, and then into Sodalite or Cancrinite as the Na/Al ratio increased. Furthermore, the adsorption process of fly ash-based zeolite influenced solely by NaOH followed the Langmuir isotherm model, while the adsorption process affected by both NaOH /NaNO₃ followed the Freundlich isotherm model. Among these, Na-P1 zeolite showed the most favorable adsorption performance for Sr²⁺. As Sr²⁺ adsorption increases, the fly ash-based zeolites exhibit the appearance of Sr-O bonds. These findings offer a new perspective for enhancing the nuclide immobilization capacity of fly ash-based geopolymers through the adjustment of salt and alkali concentrations.