Sustainable nuclear energy development is hampered by challenges in managing radioactive waste, particularly the sequestration of radioiodine isotopes (e.g., ¹²⁹I and ¹³¹I). These isotopes pose significant environmental and safety risks due to their long half-life, high mobility, and rapid bioaccumulation. Water discharged from nuclear reactors also contains radioactive iodine, leading to widespread contamination. Efficient and selective sequestration of iodine from both vapor and aqueous phases is therefore a crucial research priority, with applications ranging from nuclear waste management to water purification and medical uses. While porous materials have shown promise in iodine adsorption, developing an adsorbent with superior properties remains challenging due to the lack of effective design strategies. This study aims to address this challenge by designing and synthesizing a highly efficient iodine adsorbent that combines several key features identified in the literature: high surface area, optimal pore size distribution, strong binding sites (via charge transfer complexes, π-electron-rich conjugated frameworks, and specific heteroatoms like nitrogen in imine, amine, and other functionalities), and the inclusion of cationic functionalities with exchangeable anions. The use of a crystalline aerogel structure is also incorporated to enhance adsorption kinetics. This approach moves beyond single-material adsorbents to investigate the potential of a hybrid composite material.

Existing literature highlights various porous materials for iodine capture, focusing on improving kinetics, capacity, high-temperature capture, and selectivity. However, few materials fully satisfy these criteria. Previous studies indicate that high iodine adsorption capacity is related to textual features like surface area, pore size, and pore volume, and strong interactions between electron-deficient iodine and electron-rich porous materials (charge transfer complexes). The incorporation of nitrogen-containing moieties (imine, triazine, pyridine, amine) and cationic functionalities with free anions (enhancing electrostatic interactions with polyiodide species) are shown to improve iodine capture efficiency. The crystalline aerogel structure has also been demonstrated to improve I₂ capture with rapid kinetics. This research incorporates these findings, focusing on overcoming limitations encountered with previously reported materials.

This study synthesized a crystalline hybrid ionic aerogel material (IPcomp-7) through covalent grafting of an amino-functionalized Zr(IV)-based metal-organic polyhedron (MOP) with a dual-pore imine-functionalized covalent organic framework (COF). The Zr(IV)-MOP, featuring Cp₃Zr₃O(OH)₃ SBUs, free Cl⁻ ions, and amine groups, acted as the guest component, while the 2D COF served as the host matrix. The synthesis involved a stepwise process: first, synthesizing the individual amino-functionalized Zr(IV)-MOP and the COF aerogel; then, covalently linking them via terephthaldehyde; and finally, converting the hybrid wet-gel into an aerogel using supercritical CO₂ drying. The resulting material was characterized using various techniques, including PXRD, FT-IR, XPS, TGA, ¹³C CP-MAS NMR, FESEM, TEM, confocal laser microscopy, X-ray computed tomography (CT), N₂ gas sorption, UV-Vis spectroscopy, steady-state photoluminescence, and Raman spectroscopy. Iodine sequestration studies were conducted under static and dynamic conditions in both vapor and aqueous phases, assessing adsorption capacity, kinetics, selectivity, retention, recovery, and recyclability at various temperatures. The adsorption mechanism was investigated by FESEM-EDX, 3D X-ray CT imaging, FT-IR, XPS, Raman spectroscopy, EPR, and solid-state ¹³C CP-MAS NMR. Density Functional Theory (DFT) calculations were performed to support the experimental findings and determine binding energies.

IPcomp-7 demonstrated exceptional iodine sequestration capabilities. In static vapor-phase experiments at 75 °C, it achieved a maximum adsorption capacity of 9.98 g·g⁻¹, significantly exceeding the capacities of the individual MOP (2.31 g·g⁻¹) and COF (6.11 g·g⁻¹) components. This capacity remained high (4.27 g·g⁻¹ at 25 °C and 2.89 g·g⁻¹ at 150 °C) even in dry and humid conditions. The material displayed excellent recyclability, maintaining high sorption capacity (>7.61 g·g⁻¹) after five consecutive capture-release cycles. In dynamic vapor-phase tests, IPcomp-7 achieved an adsorption capacity of 3.76 g·g⁻¹ at 75 °C. In static aqueous-phase experiments, IPcomp-7 exhibited a maximum adsorption capacity of 4.74 g·g⁻¹ for I₂ and 5.16 g·g⁻¹ for I₃⁻. The material demonstrated superior selectivity for I₃⁻, even in the presence of other competing anions (NO₃⁻, Cl⁻, SO₄²⁻, Br⁻) at high concentrations. It maintained high removal efficiency (>85%) in diverse water systems, including seawater. Dynamic aqueous-phase column tests showed >99% triiodide removal efficiency. Characterization studies revealed that the high performance is attributable to the combined effect of hierarchical macro-micro porosity, a large surface area, and multifunctional binding sites involving interactions with heteroatoms (nitrogen in imine and amine moieties), the Zr(IV)-SBU, and free Cl⁻ ions. DFT calculations confirmed the strong interactions between iodine/polyiodides and the composite's functional groups.

The findings demonstrate that the synergistic combination of a cationic MOP and a hierarchical porous COF aerogel in IPcomp-7 results in superior iodine sequestration compared to individual components. The hierarchical pore structure facilitates rapid mass transfer and provides ample access to the multifunctional binding sites. The strong interactions identified through various characterization techniques, along with DFT calculations, underscore the importance of the designed molecular interactions. The exceptional performance in both vapor and aqueous phases, as well as in diverse water matrices, highlights the potential of IPcomp-7 for practical applications in radioiodine remediation. The high recyclability and retention efficiency demonstrate the material's suitability for long-term use. This work contributes significantly to the rational design of advanced hybrid materials for efficient iodine sequestration.

This study successfully synthesized an ultralight, crystalline hybrid aerogel (IPcomp-7) exhibiting unprecedented iodine sequestration capabilities. Its superior performance stems from the cooperative effects of its hierarchical porosity, high surface area, and multifunctional binding sites. The material shows great promise for real-world applications in nuclear waste management and water purification. Future research could explore scaling up the synthesis, investigating its performance in more complex environmental matrices, and optimizing the material for specific applications.

While IPcomp-7 shows remarkable performance, further studies are needed to evaluate its long-term stability under various environmental conditions and potential degradation mechanisms. The scalability of the synthesis and the cost-effectiveness of the process should also be investigated for practical implementation. A comprehensive lifecycle assessment would further refine its overall sustainability.