Discriminating adsorption, hydration-state transitions, and ion exchange during cesium retention on montmorillonite: Implications for mechanistic understanding and sorbent design

Abstract

Understanding the mechanisms governing radionuclide retention by clay minerals is essential for the long-term safety of geological repositories. This study proposes an operational, multi-criteria assessment framework to interpret the relative contributions of surface adsorption, hydration-state–mediated interlayer uptake (1W→2W transitions), and ion exchange during cesium retention in Wyoming montmorillonite (SWy-2). Rather than relying on any single technique, the framework integrates complementary signatures from batch experiments, X-ray diffraction (used strictly as hydration-state indicators), PHREEQC geochemical modeling, and multilayer statistical-physics fitting. Batch experiments were conducted using both non-radioactive CsCl and 137 Cs across concentrations from 10 -6 to 10 -2 M, pH 4–9, and temperatures of 278–338 K. At trace concentrations (<10 -6 M), cesium retention is consistent with rapid surface adsorption, characterized by short half-lives and strong pH sensitivity. At intermediate concentration (≈10 -4 M), a basal-spacing evolution from 12.5 to 15.2 Å reflects hydration-state transitions accompanying partial Na + /Cs + exchange, supported by slower uptake kinetics and thermodynamic trends. At high concentration (>10 -2 M), ion-exchange signatures predominate, as evidenced by Cs + /Na + selectivity (Kex = 2.8 ± 0.3), Na + release stoichiometry, and persistent 2W domains. Dry-state XRD patterns are interpreted exclusively as hydration-state constraints rather than mechanistic proof, and mechanistic assignments arise only from convergence among kinetic, thermodynamic, structural, and exchange-based evidence. PHREEQC simulations confirm that Cs remains >99.8% as hydrated Cs + across all pH values and that precipitation is thermodynamically unfavorable (SI = –6.22 to –10.21). Statistical-physics modeling reproduces the equilibrium isotherm with R 2 > 0.95 and yields physically consistent layer-site distributions after unit correction. Overall, this integrated approach provides a structured framework for synthesizing multi-signature datasets to interpret cesium retention mechanisms on montmorillonite, while explicitly recognizing the limitations of individual techniques (particularly dry-state XRD) and avoiding over-interpretation of single experimental indicators. • Integrated experimental, XRD, geochemical, and statistical physics approaches to discriminate radionuclide retention mechanisms in montmorillonite • Clear concentration-dependent transition from surface adsorption to intercalation and ion exchange • Quantitative d 001 basal spacing evolution used as a structural fingerprint of intercalation • Kinetic and thermodynamic signatures establish definitive mechanistic criteria • Implications for predictive modeling of radionuclide behavior in geological disposal systems