Halogen-Bonding Interaction-Mediated Efficient Iodine Capture of Highly Nitrogen-Functionalized Hyper-Crosslinked Polymers

Abstract

Radioactive iodine species, ¹²⁹I and ¹³¹I, are volatile radioactive nuclides generated from nuclear fission processes. The exposure of these isotopes has caused severe effects on the environment as a result of the long half-life of ¹²⁹I and high radiation energy of ¹³¹I. Therefore, ideal adsorbents capable of effectively adsorbing iodine from gas and solution phases have received particular attention. In this study, we applied the concept of supramolecular noncovalent interactions to design the functional polymeric adsorbents for efficient iodine removal. A series of nitrogen-functionalized hyper-crosslinked polymers (HCPs) containing hydrazine (P-Hz), azide (P-Az), and amine (P-Am) were synthesized from the reactive tosylated HCP (P-OTs) through facile organic transformations. After being characterized by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), UV–vis, scanning electron microscopy (SEM), and Brunauer–Emmett–Teller (BET) surface area analysis, iodine adsorption in the gas phase and solutions was investigated, and the results revealed that the interplay between the electron-donating ability of nitrogen functional groups of HCPs and the molecular iodine (I₂) resulted in enhanced iodine adsorption compared to the nitrogen-free HCPs. Density functional theory (DFT) computational studies and UV–visible spectroscopic titrations revealed the formation of the N···I–I halogen bonding, where the electron-donating nature of nitrogen in hydrazine, azide, and amine, as well as the solvent medium, significantly governed the strength of interactions. Importantly, P-Am exhibited a high iodine adsorption capacity of 2.83 g·g^–1 in the gas phase and 506.8 mg·g^–1 in the hexane phase, albeit with low porosity, suggesting the importance of specific functional groups in the adsorption capacity. X-ray fluorescence (XRF) and Raman spectroscopic analysis of P-Am after iodine adsorption suggested that iodine species are stabilized on the polymer matrix in the form of polyiodides such as I₃^– and I₅^–.