The coupling action mechanism of NaOH/NaNO 3 on the hydrothermal synthesis of fly ash-based zeolites and the Sr-Na exchange capacity

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 Sr 2 * 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 Sr 2 * 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 Sr 2 *. As Sr 2 * 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.