Molecular Engineering of Pyridinium-Functionalized Metal-Organic Cages for Iodine Capture in Multiple Phase Media

Abstract

Radioactive iodine isotopes generated during nuclear fission present significant challenges due to their high radioactivity, mobility, and long-term ecological hazards. Their efficient capture and safe disposal are therefore critical for nuclear waste management. The adsorption method is considered the most promising approach owing to its operational simplicity and low energy consumption. Herein, we propose a multi-strategy synergistic molecular engineering design that enhances aqueous stability through a hydrophobic sulfonylcalix[4]arene capping ligand, strengthens electrostatic adsorption for iodine via a positively charged pyridinium-functionalized endo cavity, and fine-tunes the cavity structure through conformational modification of the bridging ligands. Based on this strategy, a series of cobalt-based metal-organic cages (MOCs) with precisely tailored endo cavity configurations were successfully synthesized. Systematic investigations of their iodine adsorption behaviors in gaseous, organic, and aqueous phases revealed maximum adsorption capacity of 650.5 mg g^-1 for gaseous I₂ and 1345.2 mg g^-1 for aqueous I₃^-. Mechanistic studies indicate that gaseous adsorption primarily relies on the synergistic effect of Co²⁺ sites and pyridinium sites, whereas the highly efficient aqueous adsorption is mainly driven by strong electrostatic interactions between the pyridinium-functionalized cationic endo cavity and I₃^- anions. Recirculating flow system adsorption experiments further demonstrated that this pyridinium-functionalized MOC achieves rapid and efficient capture of low-concentration I₃^- with good resistance to ionic interference and robust cycling stability. This work provides a clear molecular model for understanding the structure-performance relationship in iodine adsorption and offers promising candidate materials for the aqueous remediation of radioactive iodine.