Stability Analysis of GMZ Bentonite Colloids: Aggregation Mechanism Transition and the Edge Effect in Strongly Alkaline Conditions

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.

Stability of Eu(III)-silicate colloids: Effect of Eu content, pH, electrolyte and fulvic acid

Dissolved silicic acid in the environment has strong affinity for actinides (An), but An(III)-silicate colloids have been scarcely investigated. In this study, Eu(III)-silicate colloids, an analogue to An(III)-silicate, were prepared and the aggregation kinetics of the colloids was investigated as a function of Eu content (Si/Eu molar ratio), pH, background electrolyte (NaCl, NaNO₃, NaClO₄, KCl and CsCl) and fulvic acid (FA). Results indicated that the colloids with higher Si/Eu molar ratio exhibited higher stability under the same conditions. The stability of the colloids increased with increasing aqueous pH (7.1-9.4) and decreasing ionic strength, and the inhibition effect of monovalent electrolytes on the colloid stability followed the order of Na⁺ < K⁺ < Cs⁺ and Cl^- < NO₃^- < ClO₄^-. In addition, the presence of FA significantly increased the stability of the colloids. The dependence of the stability on the chemical conditions in all cases could be illustrated by DLVO theory. Disaggregation kinetics showed that the aggregation process of the colloids was not fully reversible, because a time-dependent size memory effect led to a bigger mean size of disaggregated colloids as compared to the initial ones. The present work provides detailed insight in the formation and stability of An(III)-silicate colloids under the alkaline conditions relevant to geological disposal of radioactive waste, which is critical for understanding the behavior of this type of colloids in the environment.

Bentonite colloids immobilization and release in quartz column and its influence on selenite migration

Immobilization and release of colloids are important for colloids-facilitated migrations, and in the safety assessment of geological disposal for high-level radioactive waste, the association between the immobilization and release process of the bentonite colloids with selenite migration has not been well revealed. In this work, the migration of bentonite colloids under different conditions is evaluated, and the effects of colloids immobilization and release on selenite migration are studied. In addition, the cases of in-migration (colloids are immobilized in the quartz sand, and then selenite migrates through the quartz sand with immobilized colloids) and co-migration (colloids bearing selenite are immobilized in the quartz sand) are investigated. The results show that in the systems containing 3.0 mM Mg²⁺, the mobility of the colloids is highly hindered and the colloids are immobilized in the quartz sand mainly by straining effect. The immobilization of bentonite colloids affects selenite migration differently according to the immobilization process (in-migration or co-migration). A more significant retardation effect is observed in the co-migration process than in-migration due to the additional inner-sphere complexed selenite in the co-migration. The immobilized colloids can be more easily released by alkaline DI-water (pH 11.0) than acidic one (pH 6.0) as a result of the more negative surface charges of the immobilized bentonite colloids. The average size of the released colloids is larger than the initial colloids at the same pH. Selenite is found to be released ahead of colloids in either in- or co-migration process, and part of selenite is discovered migrating with released colloids in co-migration process. Since colloids immobilization and release would influence radionuclides migration, further research about colloids immobilization and release with broad range of pH and ionic strength in the host rock and its influence on the migration of other radionuclides are needed.