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

Abstract: [Background]: Stable 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. [Purpose]: This study aims to investigate the reversibility and mechanism of bentonite colloid aggregation induced by typical cations Ca2+ and Na+. [Methods]: This study combined synchrotron radiation small-angle X-ray scattering and dynamic light scattering techniques to analyze the aggregation structure of single-layer bentonite colloids influenced by Ca2+ and Na+ at the nanoscale to microscale, and compared the structural changes before and after dialysis. [Results]: The results demonstrated that Ca2+ promoted 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 exhibited partial disaggregation, while the cyclic lamellar structures maintained their structural integrity. In contrast, Na+ promoted the formation of loosely packed edge-to-face aggregates that underwent 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+. [Conclusions]: Considering the characteristics of the groundwater at the Beishan site, it can be inferred that bentonite colloids near the disposal site are likely to remain in an aggregated and sedimented state for long time, which to some extent inhibits the migration of radionuclides adsorbed onto the bentonite colloids.