Reversibility study of bentonite colloids aggregation induced by calcium and sodium ions: insights from scattering techniques

BackgroundStable and mobile bentonite colloids can adsorb various radionuclides and facilitate their migration. Therefore, a thorough understanding of the stability and aggregation behavior of bentonite colloids in groundwater environments is of great significance for accurately assessing the long-term safety performance of high-level radioactive waste disposal repositories.PurposeThis study aims to investigate the reversibility and mechanism of bentonite colloid aggregation induced by typical cations Ca2+ and Na+.MethodsFirstly, single-layer bentonite colloidal suspensions were prepared from sodium bentonite by high-speed centrifugation followed by dialysis against ultrapure water to remove impurity ions. Then, the colloids were exposed to Ca2+ (1~6 mmol·L-1) and Na+ (10~50 mmol·L-1) electrolyte solutions at representative concentrations found in Beishan groundwater. Subsequently, the aggregation structures were characterized by synchrotron radiation small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS). The phase structure of the sample was measured using an X-ray diffractometer (XRD), and a transmission electron microscope (TEM) was employed to observe the morphology. Finally, the aggregated colloids were re-dialyzed against ultrapure water, and the structural changes before and after dialysis were compared to evaluate the reversibility of aggregation.ResultsThe results show that Ca2+ promotes the assembly of face-to-face aligned lamellar structures with a characteristic interlayer spacing of 1.86 nm, mediated by a distinctive ionic bridging mechanism. Following dialysis, only microscale secondary aggregates exhibit partial disaggregation whilst the cyclic lamellar structures maintain their structural integrity. In contrast, Na+ promotes the formation of loosely packed edge-to-face aggregates that undergo complete dissociation into individual monolayers upon dialysis. In mixed Ca2+-Na+ electrolyte systems, the colloidal aggregation behavior of bentonite was found to be dominantly controlled by Ca2+.ConclusionsResults of this study demonstrate that bentonite colloids are likely to remain in an irreversibly aggregated state with a characteristic face-to-face stacking periodicity (d = 1.86 nm) for a long time with consideration of the characteristics of the Beishan groundwater (Ca2+~6 mmol·L-1, Na+~30 mmol·L-1), which to some extent inhibits the migration of radionuclides adsorbed onto the bentonite colloids.