Species- and Site-Dependent Adsorption of Iodine-Bearing Molecules on Porous Organic Polymer Fragments and Amorphous Graphene

Volatile and mobile iodine species, which can occur in chemically distinct molecular forms, remain a persistent challenge in nuclear-fuel-cycle and reprocessing environments. Here, dispersion-corrected density functional theory compared the adsorption of I2, CH3I, HOI, and HIO3 on finite fragments of NPOP-1 (a piperazine-bridged nitrogen-rich porous network prepared by nucleophilic substitution polymerization of piperazine with 2,4,6-tris(bromomethyl)mesitylene, retaining pendant bromomethyl groups), POP-VDX (a porous 1-vinylimidazole-divinylbenzene copolymer; modeled member POP-VD2, porogen/monomer molar ratio = 2), and POP-VDF (its fluorinated ternary copolymer incorporating 2,2,3,4,4,4-hexafluorobutyl acrylate; modeled member POP-VDF2, crosslinking degree 20%), and amorphous graphene. Of 170 independent site/orientation families, 168 satisfied the ionic convergence criterion, and all 16 material–adsorbate combinations gave exothermic minima whose binding strengths and motifs varied markedly with speciation and local environment. CH3I was consistently the weakest adsorbate (−0.175 to −0.338 eV), undergoing little distortion and thus predominantly dispersive physisorption. HIO3 bound most strongly on the imidazole-based polymers (POP-VDX −1.362 eV; POP-VDF −1.260 eV with full proton transfer), POP-VDF offered the most favorable I2 minimum (−0.791 eV), and amorphous graphene was most favorable for HOI and CH3I. Broad site/orientation energy ranges imply that single starting configurations may miss favorable motifs. Grand canonical Monte Carlo simulations on the periodic crosslinked models over 298.15–473.15 K resolved temperature-dependent type-I uptake for the four preferred host–guest pairs, with simulated uptakes of 418–668 mg/g at 298.15 K and 100 kPa. POP-VDF and amorphous graphene achieve selective immobilization of different iodine species through complementary mechanisms—charge-transfer complexation and proton-transfer chemisorption at the imidazole N sites of POP-VDF, and dispersion-, polarization-, and confinement-driven capture at the defect sites of amorphous graphene—providing guidance for designing species-specific iodine-capture adsorbents.