An investigation of Tc adsorption by Beishan granite in Beishan groundwater

Investigating the adsorption and migration processes of Tc on Beishan granite is a key issue for the safety assessment of Beishan high-level radioactive waste repository. Nevertheless, there is a lack of in-depth research on Tc adsorption on Beishan granite with authentic Beishan groundwater and a comprehensive characterization of the granite. In this work, we systematically investigated the adsorption of Tc on Beishan granite in real groundwater through batch experiments combined with multi-technique characterization. The results show that Beishan granite is mainly composed of quartz, plagioclase and mica, with the SiO₂ and Al₂O₃ as the main chemical composition. Adsorption reaches equilibrium within 3 days and is better described by the pseudo first order kinetic model. Tc exhibits weak affinity for the granite, with an equilibrium distribution coefficient (K_d) of approximately 1.4 mL/g and a linear isotherm over the initial concentration range of 10-100 Bq/mL. The adsorption capacity decrease with increasing pH, NaCl concentration, and solid-liquid ratio, whereas colloid concentration exerts negligible influence. The adsorption process is endothermic and spontaneous. Characterization after adsorption indicates that Tc(VII) is retained on the granite surface primarily via physical adsorption, rather than through reduction or chemisorption. Under oxidizing, alkaline groundwater conditions, granite provides limited retardation of Tc. These results fill a data gap on Tc behavior in actual Beishan groundwater and supply key parameters for the long-term safety assessment of the Beishan HLW repository. The findings also underscore that reducing conditions or engineered barriers are essential for effective Tc retention.