Cs+ adsorption mechanisms on todorokite: an MD simulation study bridging atomistic and macroscopic scales

초록

Understanding the long-term fate of radioactive cesium (137Cs) following nuclear accidents requires mechanistic knowledge of its adsorption on soil minerals. Although todorokite is a major tectomanganate, the mechanisms governing Cs+ adsorption at the trace levels typical of environmental contamination remain poorly understood. This study examined the concentration-dependent adsorption mechanisms of Cs+ on todorokite through molecular dynamics (MD) simulations. Selectivity-coefficient calculations based on potential of mean force (PMF) profiles showed that sodium ions (Na+) were energetically favored on external (100) and (010) todorokite surfaces, whereas this selectivity was reversed dramatically in favor of Cs+ within tunnel nanopores. Na+ encountered no significant kinetic barriers upon tunnel entry, but the interior was thermodynamically unfavorable; in contrast, Cs+ faced high entry barriers but gained strong interior binding, resulting in slow desorption and high retention. Thus, the exceptional Cs+ selectivity of todorokite arises from the contrasting energetic landscapes. Isotherm modeling of published experimental data, constrained by MD-derived equilibrium adsorption constants, suggested a concentration-dependent shift in dominant uptake sites, with intra-tunnel sites governing high-affinity adsorption at low Cs+ concentrations, whereas external surfaces (where both inner- and outer-sphere complexes form) become dominant at higher Cs+ concentrations.

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