Adsorption of Re(Ⅶ) and Se(Ⅳ) on Fault Gouge of Beishan Granite

In the research on geological disposal of high-level radioactive waste, granite is one of the candidate host rocks for deep geological repositories in China. However, the fractures and faults existing in granite bodies may become favorable channels for the migration of nuclides with groundwater, which adversely affects the retention performance of nuclides. To evaluate the retention performance of natural barriers, this study took the fault fillings(fault gouge) in the Beishan area of Gansu Province, China, as the research object and used batch experiments to investigate the adsorption behavior of Re(Ⅶ)(as a chemical analog of Tc(Ⅶ)) and Se(Ⅳ). The results show that the adsorption behavior of Re(Ⅶ) and Se(Ⅳ) by fault gouge is significantly affected by solution pH. At a solution pH of 7.5, the equilibrium adsorption capacity of Re(Ⅶ) and Se(Ⅳ) are 0.320 mg/g and 1.07 mg/g, respectively. The equilibrium adsorption capacity decreases with the increase of solid-liquid ratio and initial concentration. The adsorption process conforms to the Freundlich isothermal adsorption model, indicating that it allows multilayer adsorption. Thermodynamic studies show that increasing temperature favors the adsorption process, which is a spontaneous endothermic process. In addition, the presence of organic acids(formic acid, oxalic acid, citric acid, and humic acid) enhances the adsorption of Re(Ⅶ) and Se(Ⅳ) by fault gouge, with a more significant enhancement effect on Se(Ⅳ). The addition of zero-valent iron powder significantly enhances the equilibrium adsorption capacity of Se(Ⅳ), likely attributable to reductive immobilization.