Efficient iodine adsorption from seawater by simple macrocycle and mechanistic insights

Abstract

The challenge of iodine capture in seawater is accentuated by the existence of competing species and material stability in the seawater environment. Here, we present a 3,3′-bipyridinium-based cyclophane with an inherently rigid cavity for the effective adsorption of iodine from artificial and natural seawater. This macrocycle demonstrates superior iodine adsorption capacity and reusability compared to 4,4′-bipyridinium-based cyclophane, with notably enhanced efficiencies in seawater relative to pure water. Remarkably, static adsorption in natural seawater achieved a record-high iodine uptake of 10.4 g g⁻¹, while dynamic experiments revealed iodine removal efficiency exceeding 99%. Substantial density functional theory (DFT) calculations and controlled experiments revealed that halogen ions (X⁻) and ionic concentrations critically influence adsorption performance through the formation of different [I_2nX]⁻ complexes, which modulate adsorption energies. X-ray crystallography of iodineloaded cyclophanes indicated that efficient iodine capture arises from multiple noncovalent interactions and the macrocycle’s loosely packed structure. This study highlights a promising strategy for designing efficient and practical iodine adsorbents for real-world applications.