Bentonite colloids mediated Eu(III) migration in homogeneous and heterogeneous media of Beishan granite and fracture-filling materials

Accurate prediction of radionuclide migration directed by colloids in a repository environment is critical for the long-term safety assessment of radioactive waste disposal. Yet, there remains a paucity of research focusing on the migration and release processes of radionuclides combined with colloids associated with the repository, e.g., granite, and especially fracture-filling materials (FFMs). Further, the impact of heterogeneity on radionuclide migration in these media remains unclear. Thus, laboratory-scale column experiments were conducted to explore the migration behaviors of Eu(III) and bentonite colloids (BCs) in both homogeneously and heterogeneously configured columns. It was observed that the migration of BCs can be influenced by the configuration of media and ionic strength of solution. FFMs exhibited a higher retardation capacity for BCs than granite, and the retention ability of the heterogenous media (Granite-FFMs) was intermediate between that of granite and FFMs. The retardation of BCs increased with increasing ionic strength. Despite the "irreversibility" of Eu(III) adsorption on these media with insensitivity on ionic strength, the presence of BCs can grab the immobilized Eu(III) and carry more amount of adsorbed Eu(III) into the mobile phase at higher ionic strength. Even with the rinse of BCs flow, FFMs still revealed a better retardation ability for Eu(III) than granite. These findings are essential for predicting the geological fate and the release risk of radionuclides in the Beishan repository environment.