Size-dependent sorption behavior and isotherms of Cs(I), Sr(II), and La(III) onto bulk solid and colloidal bentonite buffer materials

Bentonite buffer materials play a critical role in controlling radionuclide migration and retardation in deep geological disposal systems for high-level radioactive waste (HLW). Under saturated groundwater conditions, physicochemical erosion of bentonite produces a broad particle size distribution, including colloidal fractions that may act as mobile carriers for radionuclides. This study investigates the sorption behavior of Cs(I), Sr(II), and La(III) on size-fractionated bulk solid and colloidal bentonite under weakly alkaline conditions. Bentonite fractions with controlled particle sizes were prepared by a centrifugation method and characterized using various methods. The sorption of Cs(I) was described by the Langmuir isotherm within the tested concentration range, supporting monolayer-type sorption dominated by interlayer cation exchange. In contrast, Sr(II) sorption followed the Freundlich isotherm, reflecting contributions from ion exchange, surface complexation, and electrostatic interactions associated with the electric double layer (EDL). La(III) sorption appeared to depend on particle size within the investigated bentonite fractions, potentially indicating an increased role of edge-related sites, while the combined effects of ion exchange, surface complexation, and electrostatic accumulation in the diffuse layer led to Freundlich-type sorption behavior. The experimental findings provide sorption parameters and mechanistic insight that can be used to improve models of radionuclide migration and retardation that explicitly account for both bulk solid and colloidal bentonite in deep geological disposal environments.