Continuous-Flow Iodine Removal from Aqueous Wastes Using Structured MOF Composites

The effective immobilization and recovery of radioactive iodine under continuous-flow conditions remain critical challenges in nuclear waste management. Although metal-organic frameworks (MOFs) exhibit outstanding iodine capture performance, their application in practical flow-through systems is limited by the inherent challenges associated with powder handling, shaping, and the structural changes induced during fabrication into usable forms. In this work, the structural evolution of UiO-66-NH2 (U66-NH2) based structured adsorbents fabricated via extrusion, phase inversion, and surface coating was systematically elucidated, and revealed how distinct structural characteristics govern I2 adsorption performance. The extruded sample (U66-Clay) exhibited a densely packed particle structure, whereas the phase inversion-derived samples (U66-PVDF and U66-PES) formed interconnected macroporous networks with MOF particles partially embedded within a continuous polymer phase. In contrast, the surface-coated sample (U66-PVDF-C) maximized the exposure of MOF particles while maintaining structural stability, although its MOF loading was relatively low. Batch adsorption experiments revealed that iodine uptake was primarily governed by the loading and accessibility of UiO-66-NH2, with U66-Clay exhibiting an adsorption capacity (46.3 mg/g) comparable to that of pristine UiO-66-NH2 powder. Under continuous-flow conditions, U66-Clay exhibited the fastest adsorption kinetics and the highest Thomas rate constant (3.75 × 10-3 L/mg·min), owing to its greater exposure of active sites and more efficient mass transfer, significantly outperforming the polymer-based structured adsorbents (U66-PVDF, U66-PES). Spectroscopic analysis further revealed that iodine capture proceeded through a synergistic mechanism involving charge-transfer chemisorption at the electron-rich amino and aromatic sites, together with physical adsorption within the porous framework. Furthermore, all structured adsorbents exhibited rapid regeneration (∼30 min), minimal iodine leaching (∼6%), and excellent cyclic stability over repeated cycles. Overall, this work elucidates the structural evolution of MOFs during shaping process and establishes the relationship between structuring strategies and adsorption performance, providing general design principles for developing high-performance structured adsorbents for the continuous capture of iodine and other environmental pollutants.