Design and fabrication of phosphazene-based porous organic materials for iodine adsorption†

Abstract

Iodine plays a critical role in nuclear industries, medicine, and environmental monitoring, often occurring in trace amounts in wastewater and contaminated environments. Despite the exploration of various adsorbents for iodine capture, many of them still exhibit limitations such as low adsorption capacities, poor performance, and limited reusability. Therefore, novel materials with enhanced iodine extraction capabilities are needed. Polyphosphazene polymer-based adsorbents offer substantial promise due to their unique chemical structures and rich functional groups. In this study, via nucleophilic substitution of hexachlorocyclotriphosphazene (HCCP) with various amines, five polyphosphazene-based polymers—PDD–HCCP, BDP–HCCP, BDD–HCCP, TAPA–HCCP, and TAPDA–HCCP—were synthesized. These polymers exhibited excellent iodine adsorption capacities, with TAPDA–HCCP achieving the highest theoretical capacities in both iodine vapor and aqueous phases (I₂: 7.83 g g⁻¹, CH₃I: 1.26 g g⁻¹, iodine water: 3.69 g g⁻¹, iodine cyclohexane: 1.15 g g⁻¹). In aqueous iodine adsorption experiments, the kinetics followed a pseudo-second-order model, indicating chemical adsorption as the dominant mechanism. Specifically, equilibrium was reached within 240 min, with PDD–HCCP and TAPDA–HCCP achieving iodine removal efficiencies greater than 90%. The adsorption isotherms fitted the Langmuir model, suggesting monolayer adsorption. FT-IR and XPS analyses confirmed that the –NH, PN–P, and sp³ N groups play a crucial role in forming charge-transfer complexes with iodine. These results highlight the potential of polyphosphazene-based adsorbents for efficient iodine capture in environmental applications.