Tuning of gaseous and aqueous iodine sequestration in triazine-linked covalent organic frameworks via ligand...

Nuclear energy is a clean and low-carbon source of energy, but operating nuclear reactors can pose safety issues due to the release of radioactive species. Radioactive iodine in the form of molecular I₂ and organic iodide, such as methyl iodide (CH₃I), is potentially harmful to both people and the environment due to its long-term effects and water solubility. Hence, the sequestration of iodine and organic iodide present in the Off-gas produced during spent nuclear fuel reprocessing and nuclear disaster is critical for the environment. In this context, herein we demonstrated three triazine-based covalent organic frameworks (COFs), IISERP-COF1, IISERP-COF1(Me)₂, and IISERP-COF6, developed by engineering the linker, for efficient sequestration of iodine and CH₃I in both gaseous and aqueous forms. IISERP-COF1(Me)₂ displayed the highest iodine uptake of 5.45 g/g and 6.24 g/g at 70 and 100° C in the gas phase. While in the gas phase CH₃I sequestration, IISERP-COF1 displayed the uptake of 1.32 g/g at 75 °C, surpassing that of IISERP-COF1(Me)₂ and IISERP-COF6. In the aqueous phase iodine sequestration, IISERP-COF1(Me)₂ showed the maximum uptake of 2.04 g/g at ambient conditions. These nitrogen-rich COFs displayed facile recyclability and good humidity tolerance. Different spectroscopic investigations revealed strong charge transfer interaction between imine N, triazine N, the π-electron density of the frameworks and iodine or iodide species, which resulted in the formation of polyiodide species. Molecular simulation analysis further helps elucidate the I₂ interactions with the different binding sites in the frameworks and the corresponding binding energies, which offer a comprehensive understanding of the adsorption mechanism.