Journal of Nuclear Fuel Cycle and Waste Technology

Understanding how saline water intrusion alters the performance of bentonite buffer is critical for the reliable safety assessment of a high-level radioactive waste repository. This study investigated sorption thermodynamics of Cs(I) and Sr(II) onto Na-exchanged Bentonil-WRK montmorillonite. Batch sorption experiments were conducted under ambient conditions (T = 25℃, pH 4–10, S/L = 5 g·L⁻¹, I = 0.01 mol·L⁻¹ NaCl), followed by inverse modeling of the resulting data. The sorption of Cs(I) was dominated by cation exchange (XNa + Cs⁺ ⇌ XCs + Na⁺, log K = 1.48 ± 0.02) with a minor contribution from edge surface complexation at silanol site (≡SiOH + Cs⁺ ⇌ ≡SiOCs + H⁺, log K = −5.36 ± 0.70). In contrast, Sr(II) uptake involved a more complex mechanism comprising cation exchange (2XNa + Sr²⁺ ⇌ X2Sr + 2Na⁺, log K = 0.47 ± 0.06), interlayer ion-pairing with chloride (XNa + Sr²⁺ + Cl− ⇌ XSrCl + Na⁺, log K = 3.05 ± 0.33), and edge complexation (≡SiOH + Sr²⁺ ⇌ ≡SiOSr⁺ + H⁺, log K = −6.04 ± 0.94). The derived thermodynamic parameters complement the radionuclide sorption database for Ca-type bentonite, reducing uncertainty by enabling process-based evaluation of clay buffer performance under long-term geochemical evolution scenarios.

Keywords

Sorption, Montmorillonite, Cesium, Strontium, Ion exchange, Surface complexation