Mechanochemical synthesis of heteroatom-rich amide-linked covalent organic frameworks for efficient iodine sequestration.

Abstract

The efficient capture of radioactive iodine is of great importance for environmental protection and the safe management of nuclear waste due to its high volatility, environmental persistence, and radiotoxicity. Herein, we report the solvent-assisted mechanochemical synthesis of three heteroatom-rich amide-linked covalent organic frameworks (COFs), namely METP-COF, MEOB-COF, and MEPY-COF, as sustainable adsorbents for iodine sequestration. The incorporation of sulfur-, oxygen-, and nitrogen-containing functional groups provides abundant electron-rich active sites, enhancing iodine adsorption. The successful formation of amide-linked frameworks was confirmed by 13C solid-state NMR, which exhibited characteristic amide carbonyl resonance at 165 ppm, while XPS analysis of METP-COF revealed the presence of C, N, O, and S, with characteristic S 2p3/2 and S 2p1/2 peaks at 161 and 163 eV, respectively. The iodine adsorption performance was evaluated using the static vapour method at 75 °C, where METP-COF exhibited the highest uptake of 2.65 g g-1, followed by MEOB-COF (2.54 g g-1) and MEPY-COF (2.04 g g-1). Raman and XPS analyses confirmed the formation of polyiodide species within the framework, indicating strong charge-transfer interactions between iodide and the electron-rich COFs. Density functional theory calculations further revealed stronger iodine binding at the heteroatom-rich site (-112.80 KJ mol-1) than at the carbonyl site (-99.80 KJ mol-1), accompanied by a reduction in the highest occupied molecular orbital-lowest unoccupied molecular orbital band gap from 3.08 to 3.01 eV, which indicates favorable electronic interactions. These findings demonstrate that heteroatom-rich amide-linked COFs synthesized through a green mechanochemical strategy are promising adsorbents for efficient iodine capture and retention.

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