Local polarity modulation in triangular pores of covalent organic frameworks for iodine capture from water

Radioactive iodine from nuclear fission processes can enter water systems in various ways, adversely affecting human health and the environment. Herein, we construct a series of covalent organic frameworks (COFs) with triangular pores and different local polarities for efficient iodine (I2 + I- ⇌ I3-) capture from aqueous solution. Triangular pores containing weakly polar units show a strong affinity for nonpolar iodine via hydrophobic interactions, whilst the periodically ordered frameworks and intrinsic interlayers based on aromatic moieties and nitrogen-containing groups capture I2 and I3- through van der Waals forces and charge transfer. COF-1 with weak local polarity delivers a high iodine (131I) adsorption capacity, fast adsorption kinetics, and excellent selectivity in tap water, groundwater, and seawater. Moreover, COF-1 exhibits an exceptional trace iodine removal efficiency (from ~200 ppb to below ~65.9 ppb in tap water and ~58.3 ppb in groundwater), making it a promising adsorbent for efficiently treating aqueous iodine waste streams. Porous materials are actively researched for iodine adsorption, but the pore characteristics of frameworks required for efficient iodine capture in water are not well understood. Here, the authors construct a series of covalent organic frameworks with triangular pores and different local polarities for efficient iodine capture from aqueous solution.