Architecture-dependent humidity-induced transport limitations in porous sorbents for methyl iodide capture: A molecular dynamics study

The capture of volatile radioiodine species, particularly methyl iodide (CH ₃I), remains a critical challenge in nuclear safety and off-gas treatment systems. Although porous sorbents such as activated carbons, zeolites, and covalent organic frameworks have demonstrated promising adsorption capacities under dry conditions, their performance often deteriorates in the presence of humidity. In this work, molecular dynamics simulations are employed to investigate the influence of water on the transport and adsorption of CH ₃I in three representative porous materials with distinct pore architectures: triethylenediamine-impregnated activated carbon TEDA-AC, the all-silica zeolite EMM-17, and the boron-linked covalent organic framework COF-103. Simulations were performed under dry and humid conditions to elucidate the molecular origins of humidity-induced transport limitations. Analysis of molecular trajectories, density distributions, radial distribution functions, and mean-squared displacements reveals that water modifies CH ₃I uptake through mechanisms that depend strongly on pore topology and confinement. In activated carbon, water preferentially occupies transport-critical regions, thereby reducing the accessibility of the pore network. In the zeolite, strong confinement amplifies competitive exclusion effects, leading to reduced occupancy of intersection volumes and altered transport. In contrast, the larger pore volume of COF-103 preserves transport pathways but promotes boundary-templated water clustering and central volumetric bypass of CH ₃I. These results demonstrate that the impact of humidity cannot be captured by a single universal adsorption penalty; rather, it arises from the interplay among pore architecture, molecular transport, and competitive adsorption. The present study provides molecular-level insight into the mechanisms governing CH ₃I mobility in high water-loading conditions representative of humid operation and highlights the importance of accounting for transport phenomena alongside equilibrium adsorption when designing porous sorbents for radioiodine abatement.