Diverse sorption capacities and contribution of multiple sorptive sites on illitic clays to assess the immobilization of dissolved cesium in subsurface environments

The multiple sorptive sites on illitic clays (e.g., frayed edge [FES], type II [TS], and planar sites [PS]) are related to variable ¹³⁷Cs immobilization in subsurface environments. This study investigated the diverse Cs⁺ sorption using 10 illitic clays under various competing K⁺ (distilled water−10⁻¹ mol L⁻¹) and Cs⁺ concentrations (10⁻⁷−10⁻³ mol L⁻¹). In addition, the multisite cation exchange model was simulated the best-fit sorption models compared to experimental datasets, and subsequently, optimized the sorption capacities of multiple sorptive sites on the illitic clays. The best-fit sorption model exhibited that diverse Cs⁺ sorption of 10 illitic clays was closely linked to the individual sorption capacities at the FES (1.76 × 10⁻⁵ to 1.12 × 10⁻⁴ eq kg⁻¹), TS (1.59 × 10⁻³ to 9.76 × 10⁻³ eq kg⁻¹), and PS (2.14 × 10⁻² to 1.51 × 10⁻¹ eq kg⁻¹). The FES predominantly sorbed Cs⁺ at low aqueous-phase concentrations, whereas the TS and PS contributed to Cs⁺ sorption at relatively high concentrations. These sorption capabilities were correlated to illite contents and crystallinity of 10 illitic clays so that such parameters could be significant factors to predict the Cs⁺ sorption for illitic clays. Finally, the 1-D transport simulations showed significantly diverse Cs⁺ retardation (R_d ≈ 20–100) at low dissolved Cs⁺, implying that the various FES capacities of illitic clays could play an important role on Cs⁺ distribution in actual radioactive contamination sites.