Investigating the role of Na-bentonite colloids in facilitating Sr transport in granite minerals through column experiments and modeling

Colloids play a crucial role in influencing the mobility of radionuclides in high-level radioactive waste re-positories. However, the co-transport behavior of radionuclides and colloids in geological media remains insufficiently understood. This study investigated the transport of Strontium (Sr) in four types of granite minerals (quartz, biotite, K-feldspar, and plagioclase) in the presence and absence of Na-bentonite colloids (Na-BC) using column experiments. Employing a stepwise modeling strategy, this study first determined the basic flow and transport parameters through breakthrough curve analysis of the conservative tracer (Br). Then, the experi-mental data of Sr in the colloid-free Sr transport experiments and Na-BC in the Sr-colloid co-transport experi-ments were quantitatively explained using a two-site sorption model and a two kinetic sites model, respectively. Finally, the co-transport behavior of Sr and Na-BC was fitted using the colloid-facilitated solute transport model. The stepwise modeling allowed quantification of the kinetics of Sr sorption onto mobile and immobile Na-BC and highlighted the role of straining in Na-BC retention. In the absence of Na-BC, Sr transport experienced the greatest retardation in biotite, followed by plagioclase, K-feldspar, and quartz, respectively, which positively correlated with the specific surface area of the minerals. Moreover, Na-BC enhanced Sr transport with the same retardation order in all tested minerals, with recovery rate increments of 68.61%, 19.21%, 6.67%, and 4.33%, respectively. The transport of Sr in K-feldspar was found to be less affected by Na-BC compared to the other tested minerals, likely due to the strong cation exchange capacity of K+ among the cation components of min-erals, making hydrated K+ more likely to exchange with Sr2+ on K-feldspar surfaces. These findings hold sig-nificant implications for assessing the risks associated with the transport of radionuclides in deep geological repositories.