Ultralight crystalline hybrid composite material for highly efficient sequestration of radioiodine

The paper addresses the urgent need to efficiently capture radioactive iodine (notably 129I and 131I) released during nuclear fuel reprocessing and from reactor water discharges due to its radiological persistence, mobility, and bioaccumulation. Prior materials show strengths in capacity, kinetics, temperature performance, or selectivity, but integrating all desirable features into a single adsorbent remains challenging. The study aims to design a single hybrid composite combining high surface area, hierarchical porosity, electron-rich functional sites, cationic frameworks with exchangeable anions, and Zr-based secondary building units to achieve rapid, high-capacity, and selective sequestration of iodine from both vapor and aqueous phases, with strong retention and recyclability.

Iodine uptake in porous materials depends on textural properties (surface area, pore size/volume) and strong specific interactions, often via charge-transfer between electron-rich frameworks and electron-deficient iodine. Incorporation of π-conjugation and N-containing moieties (imine, triazine, pyridine, amine) enhances uptake. Cationic frameworks with free counteranions strengthen interactions with in situ formed polyiodides in vapor and enable anion exchange for I3− in water. Crystalline aerogels often outperform powders due to rapid mass transport. Zr-SBU with hydroxyls exhibits selective interactions with iodine over off-gas interferents (e.g., NO2). However, MOPs can aggregate and block sites; embedding MOPs in porous matrices (MOFs, silica, polymers) has been explored, but covalent strapping of MOPs into COFs to form crystalline hybrid aerogels has remained limited. The work leverages these insights to integrate multiple beneficial features within a single COF–MOP hybrid.

Synthesis: An amino-functionalized, cationic Zr(IV) metal–organic polyhedra (NH2-Zr(IV)-MOP; {[Cp3Zr3O(OH)3]4(NH2-BDC)}·Cl4) and a dual-pore, imine-linked 2D COF aerogel (from ETTA and terephthalaldehyde, TPD) were prepared. A stepwise covalent hybridization strategy grafted the NH2-Zr(IV)-MOP to TPD (forming TPD@NH2-Zr(IV)-MOP), followed by co-gelation with COF precursors (ETTA and TPD) in DMSO with acetic acid to form a hybrid wet-gel. Supercritical CO2 drying yielded a crystalline, ultralight hybrid composite aerogel (IPcomp-7). Multiple loadings of MOP were screened; ~15 mg MOP per batch gave optimal iodine capture. Characterization: PXRD confirmed crystallinity and retention of COF’s dual-pore structure. FT-IR showed imine formation (C=N at ~1622 cm−1) and MOP features (Zr–O ~1380 cm−1; NH2 stretches). XPS survey detected Zr and Cl (from MOP), and Zr 3d shifts indicated host–guest interaction. TGA showed elevated thermal behavior characteristic of both components. Solid-state 13C CP-MAS NMR confirmed imine linkages and the presence of both MOP and COF fingerprints (new signal at ~171 ppm). FESEM/TEM/HRTEM revealed sponge-like, interconnected fibrous morphology with hierarchical macropores (~1–20 μm; CT-derived void fraction ~80.2±2%). HAADF-TEM/EDX mapping showed homogeneous Zr/Cl distribution with Zr/Cl ~3. N2 sorption (77 K) indicated micro/mesoporosity (type I/IV isotherm), BET area 1463 m2 g−1, and NLDFT pore distribution confirming micro- and mesopores; combined with CT these demonstrated hierarchical macro–micro porosity. Mechanical flexibility was verified by compression tests. Iodine adsorption tests: Static vapor-phase iodine uptake was measured at 75 °C (and at 25 °C, 150 °C; dry/humid). Dynamic vapor capture at 75 °C was assessed under flow. Retention and recyclability were evaluated via thermal/solvent desorption (hexane) and multiple capture–release cycles. Aqueous-phase iodine capture was studied for I2 in water and in n-hexane, and for I3− formed from I2/KI in water; kinetics (pseudo-second order), isotherms (Langmuir), distribution coefficients (Kd), and selectivity in the presence of competing anions (NO3−, Cl−, SO42−, Br−, ClO4−) at equimolar and 100-fold excess were determined. Performance was also evaluated in complex water matrices (seawater, lake, river, wastewater, drinking water). A column breakthrough setup assessed dynamic aqueous I2 and I3− capture with regeneration using NaCl and hexane. Mechanistic studies: Post-adsorption FT-IR showed shifts/intensity changes consistent with charge-transfer interactions (C=N, C=C, C–H, C–N; Zr–O). XPS identified iodine states (I 3d peaks for molecular iodine and polyiodides), N 1s shifts and N–I features, Zr 3d and Cl 2p shifts indicating involvement of Zr-SBUs and Cl− in binding. Raman spectra identified I3−, I5−, [I2Cl]−, and [2I2Cl]− species. EPR indicated radical formation upon iodine loading (reduced after desorption). Solid-state UV–vis showed broad CT absorption; 13C CP-MAS NMR peaks broadened after loading. PXRD of I2@IPcomp-7 lacked iodine crystallization peaks, indicating amorphous iodine within pores; TGA confirmed high iodine loading; conductivity increased to ~7.83×10−5 S cm−1 upon iodine uptake. Confocal microscopy and 3D CT imaging demonstrated widespread, accessible macroporosity aiding rapid mass transport. Numerical simulations (in-silico mass transport) quantified average flow velocities and permeability. DFT calculations (modeling COF TPE backbone and NH2-MOP fragments) yielded strong binding energies for interactions with iodine/polyiodides: imine functionality −34.8 eV, Zr–OH sites −26.8 eV, Cl− −25.2 eV, and free NH2 −23.3 eV, supporting multiple cooperative binding sites.

• Static vapor-phase iodine capacity at 75 °C: 9.98 g·g−1 (equilibrium within 24 h; 7.87 g·g−1 at 6 h; 9.18 g·g−1 at 12 h), exceeding pristine MOP (2.31 g·g−1) and COF aerogel (6.11 g·g−1).
• Temperature/humidity effects (dry): 4.27 g·g−1 at 25 °C; 2.89 g·g−1 at 150 °C; humid conditions caused only slight reductions.
• Kinetics (vapor): pseudo-second-order, k ≈ 0.0567 g·g−1·h−1, indicating rapid uptake.
• Dynamic vapor capture at 75 °C: 3.76 g·g−1; kinetic constant ≈ 0.0627 g·g−1·h−1; after five dynamic cycles capacity remained >2.48 g·g−1.
• Retention: >94% of loaded iodine retained over 7 days; higher than pristine components.
• Reusability: ~97% desorption in first cycle; >96% iodine released into hexane within 2 h; capacity >7.61 g·g−1 after five static cycles.
• Aqueous I2/I3− capture: rapid removal of I2 from water and n-hexane; for I3− (from I2/KI) pseudo-second-order kinetics with k ≈ 1.9437 g·mg−1·min−1; Langmuir maximum capacity 5.16 g·g−1; distribution coefficients Kd ~10^5 mL·g−1.
• Selectivity: >97.9% I3− removal in presence of equimolar and ~100-fold excess competing anions (NO3−, Cl−, SO42−, Br−, ClO4−); Kd values ~10^5–10^6 mL·g−1.
• Complex water matrices: >85% I3− removal within 1 min across seawater, lake, river, wastewater, and drinking water; in seawater capacity 5.09 g·g−1 and Kd ~10^5 mL·g−1.
• Column breakthrough: >96% removal of I2 from water across concentrations; for I3−, >99% removal over >200 mL in first cycle; regenerable with aqueous NaCl and hexane over multiple cycles.
• Mechanistic evidence: multiple cooperative interactions (heteroatom sites, imine/amine, π systems, Zr(IV)-SBU, and exchangeable Cl−) drive strong charge-transfer and electrostatic binding; Raman identifies I3−, I5−, and iodine–chloride adducts; conductivity increase and EPR signals support polyiodide formation and CT complexes.
• Structure–function: hierarchical macro–micro porosity (void ~80%) and ultralight aerogel scaffold enable fast mass transport and accessibility of active sites, yielding ultrafast kinetics and high capacities.

The hybrid COF–MOP aerogel (IPcomp-7) integrates hierarchical macro–micro porosity with multiple complementary binding motifs—electron-rich imine/amine/phenyl groups, cationic framework with exchangeable Cl−, and Zr(IV)-SBUs bearing hydroxyls—resulting in synergistic physisorption and chemisorption of iodine and polyiodides. The macroporous, low-density aerogel architecture accelerates diffusion and access to active sites, while the covalent strapping of MOPs within the COF prevents MOP aggregation and preserves active functionalities. Spectroscopic (FT-IR, XPS, Raman, EPR, UV–vis), structural (CT, PXRD), and computational (DFT) analyses corroborate multi-site charge-transfer and electrostatic interactions, including formation of I3−/I5− and iodine–chloride adducts, and strong binding at Zr–OH sites. This cooperative mechanism explains the observed high capacities, rapid kinetics, excellent selectivity against competing anions, strong retention, and regenerability in both vapor and aqueous systems, underscoring the material’s relevance for practical radioiodine remediation.

A crystalline hybrid ionic aerogel (IPcomp-7) was developed by covalently grafting amino-functionalized Zr(IV)-MOPs into a hierarchical imine-COF aerogel, yielding an ultralight, high–surface area, mechanically robust composite. The material exhibits ultrafast and selective capture of iodine/polyiodides from vapor and water with high capacities (9.98 g·g−1 vapor; up to 5.16 g·g−1 for I3− in water), robust retention (>94% over 7 days), and efficient recyclability and recovery, including dynamic column performance and regeneration. Mechanistic studies and DFT support cooperative interactions among Zr–OH SBUs, amine/imine sites, π systems, and exchangeable Cl−, enabled by hierarchical porosity that facilitates rapid mass transport. The insights provide a foundation for rational design of multifunctional hybrid composites for radioiodine sequestration and related separations requiring porous macroscopic scaffolds.