Stable nitrogen-containing covalent organic framework as porous adsorbent for effective iodine capture from water

Efficient Removal of Iodine from Water by a Calix[4]pyrrole-Based Nanofilm

The efficient removal of radioactive iodine from an aqueous solution is largely dependent on the adsorbent materials employed. In this work, we report a calix[4]pyrrole-based nanofilm and its application for the rapid removal of iodine from water. The nanofilm was synthesized through a confined dynamic condensation of tetra hydrazide calix[4]pyrrole with 1,3,5-tri-(4-formylphenyl) aldehyde at the air/dimethyl sulfoxide (DMSO) interface. The thickness of the obtained nanofilm is ∼35 nm, enabling fast mass transfer and a high ratio of accessible binding sites for iodine. The pseudo-second-order rate constant of the nanofilm for iodine is ∼0.061 g g-1 min-1, 3 orders of magnitude higher than most reported adsorbent materials. Flow-through nanofiltration tests demonstrated that the nanofilm has an adsorption capacity of 1.48 g g-1, a high removal efficiency, and good reusability. The mechanism study revealed that the moieties of Schiff base, pyrrole, and aromatic rings play a key role for binding iodine. We believe this work provides not only a new strategy for the efficient removal of radioactive iodine from water but also new ideas for designing efficient iodine adsorbents.

[M8L4]8+-Type Squares Self-Assembled by Dipalladium Corners and Bridging Aromatic Dipyrazole Ligands for Iodine Capture

Square-like metallamacrocyclic palladium(II) complexes [M₈L₄]⁸⁺ (1-7) were synthesized by reacting aromatic dipyrazole ligands (H₂L¹-H₂L³ with pyromellitic arylimide-, 1,4,5,8-naphthalenetetracarboxylic arylimide-, and anthracene-based aromatic groups, respectively) with dipalladium corners ([(bpy)₂Pd₂(NO₃)₂](NO₃)₂, [(dmbpy)₂Pd₂(NO₃)₂](NO₃)₂, or [(phen)₂Pd₂(NO₃)₂](NO₃)₂, where bpy = 2,2'-bipyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine, and phen = 1,10-phenanthroline) in aqueous solutions via metal-directed self-assembly. Metallamacrocycles 1-7 were fully characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry, and the square structure of 7·8NO₃^- was further confirmed via single crystal X-ray diffraction. These square-like metallamacrocycles exhibit effective performance for iodine adsorption.

Polymorphic Covalent Organic Frameworks: Molecularly Defined Pore Structures and Iodine Adsorption Property

Radioactive iodine-capturing materials are urgently needed for the emerging challenges in nuclear waste disposal. The various pore structures of covalent organic frameworks (COFs) render them promising candidates for efficient iodine adsorption. However, the detailed structure-property relationship of COFs in iodine adsorption remains elusive. Herein, two polymorphic COFs with significantly different crystalline structures are obtained based on the same building blocks with varied molecular ratios. The two COFs both have high crystallinity, high specific surface area, and excellent chemical and thermal stability. Compared with the [C₄+C₄] topology (PyT-2) with an AA stacking form, the [C₄+C₂] topology (PyT-1) with an AB stacking form has more twisted pore channels and complex ink-bottle pores. At ambient conditions, PyT-1 and PyT-2 both exhibit good adsorption properties for iodine capture either in a gaseous or liquid medium. Remarkably, PyT-1 presents an excellent maximum adsorption capacity (0.635 g g^-1), and the adsorption limit of PyT-2 is 0.445 g g^-1 in an n-hexane solution with an iodine concentration of 400 mg L^-1, which is highly comparable to the state-of-the-art iodine absorption performance. This study provides a guide for the future molecular design strategy toward novel iodine adsorbents.

Enabling Solution Processable COFs through Suppression of Precipitation during Solvothermal Synthesis

Covalent organic frameworks (COFs) are crystalline, nanoporous materials of interest for various applications, but current COF synthetic routes lead to insoluble aggregates which precludes processing for practical implementation. Here, we report a COF synthesis method that produces a stable, homogeneous suspension of crystalline COF nanoparticles that enables the preparation of COF monoliths, membranes, and films using conventional solution-processing techniques. Our approach involves the use of a polar solvent, diacid catalyst, and slow reagent mixing procedure at elevated temperatures which altogether enable access to crystalline COF nanoparticle suspension that does not aggregate or precipitate when kept at elevated temperatures. On cooling, the suspension undergoes a thermoreversible gelation transition to produce crystalline and highly porous COF materials. We further show that the modified synthesis approach is compatible with various COF chemistries, including both large- and small-pore imine COFs, hydrazone-linked COFs, and COFs with rhombic and hexagonal topologies, and in each case, we demonstrate that the final product has excellent crystallinity and porosity. The final materials contain both micro- and macropores, and the total porosity can be tuned through variation of sample annealing. Dynamic light scattering measurements reveal the presence of COF nanoparticles that grow with time at room temperature, transitioning from a homogeneous suspension to a gel. Finally, we prepare imine COF membranes and measure their rejection of polyethylene glycol (PEG) polymers and oligomers, and these measurements exhibit size-dependent rejection and adsorption of PEG solutes. This work demonstrates a versatile processing strategy to create crystalline and porous COF materials using solution-processing techniques and will greatly advance the development of COFs for various applications.

A Crosslinked Ionic Organic Framework for Efficient Iodine and Iodide Remediation in Water

Iodine is widely used as an antimicrobial reagent for water disinfection in the wilderness and outer space, but residual iodine and iodide need to be removed for health reasons. Currently, it is challenging to remove low concentrations of iodine and iodide in water (≈5 ppm). Furthermore, the remediation of iodine and iodide across a broad temperature range (up to 90 °C) has not previously been investigated. In this work, we report a nitrate dimer-directed synthesis of a single-crystalline ionic hydrogen-bonded crosslinked organic framework (H_C OF-7). H_C OF-7 removes iodine and iodide species in water efficiently through halogen bonding and anion exchange, reducing the total iodine concentration to 0.22 ppm at room temperature. Packed H_C OF-7 columns were employed for iodine/iodide breakthrough experiments between 23 and 90 °C, and large breakthrough volumes were recorded (≥18.3 L g^-1 ). The high iodine/iodide removal benchmarks recorded under practical conditions make H_C OF-7 a promising adsorbent for water treatment.

Multi-Functionalization Integration into the Electrospun Nanofibers Exhibiting Effective Iodine Capture from Water

Efficient capture of radioiodine from aqueous solutions is of importance for sustainable development of nuclear energy and protection of the environment. However, current adsorbents under exploration suffer from limited adsorption capacity and powder form that are unfavorable for practical applications. Herein, we applied a "multi-functionalization integration" idea to construct novel electrospun fiber adsorbents (N-MOF-PAN fibers) containing cationic quaternary ammonium groups, uncharged amine groups, and porous MOF material (UiO-66-NH₂), which in synergy adsorb iodine effectively from both saturated I₂ aqueous solution and I₃^- aqueous solution. Iodine species (94.6%) could be removed from saturated I₂ solution within 180 min, and 98.7% of iodine species were captured from I₃^- solution within 240 min. Additionally, the N-MOF-PAN fibers exhibited high iodine uptake capacities of 3.56 g g^-1 from a concentrated KI/I₂ aqueous solution and 3.61 g g^-1 from the Langmuir isotherm model, surpassing many reported iodine adsorbents in the aqueous medium. Characterization and mechanism analysis indicated that multiple active sites simultaneously attribute to the high binding affinity toward iodine species through physical adsorption and chemical adsorption. Furthermore, benefiting from their macroscopic architecture, N-MOF-PAN fibers were used as the adsorption column for dynamic iodine capture with a bed volume of 1490 mL, which is much higher than commercially activated carbons. Overall, this work provides guidance for the development of novel fiber adsorbents for related applications.

Adsorption of iodine in metal-organic framework materials

Nuclear power will continue to provide energy for the foreseeable future, but it can pose significant challenges in terms of the disposal of waste and potential release of untreated radioactive substances. Iodine is a volatile product from uranium fission and is particularly problematic due to its solubility. Different isotopes of iodine present different issues for people and the environment. 129I has an extremely long half-life of 1.57 × 107 years and poses a long-term environmental risk due to bioaccumulation. In contrast, 131I has a shorter half-life of 8.02 days and poses a significant risk to human health. There is, therefore, an urgent need to develop secure, efficient and economic stores to capture and sequester ionic and neutral iodine residues. Metal-organic framework (MOF) materials are a new generation of solid sorbents that have wide potential applicability for gas adsorption and substrate binding, and recently there is emerging research on their use for the selective adsorptive removal of iodine. Herein, we review the state-of-the-art performance of MOFs for iodine adsorption and their host-guest chemistry. Various aspects are discussed, including establishing structure-property relationships between the functionality of the MOF host and iodine binding. The techniques and methodologies used for the characterisation of iodine adsorption and of iodine-loaded MOFs are also discussed together with strategies for designing new MOFs that show improved performance for iodine adsorption.

Construction of manganese-based metal organic frameworks derived from aromatic dicarboxylic acids and application for the adsorption of iodine

In this work, we selected terephthalic acid or 2-amino-terephthalic acid as ligand, transition metal manganese salt as metal source under the solvothermal conditions to successfully construct two kinds of manganese-based metal-organic frameworks (Mn-MOFs): Mn3(BDC)3(H2O)2 (1) and Mn3(NH2-BDC)3(DMF)4 (2) (H2BDC = terephthalic acid; NH2-BDC = 2-amino terephthalic acid; DMF = N, N-dimethyl formamide). It was characterized by elemental analysis, IR spectrum, thermogravimetric analysis (TG), X-ray powder diffraction (PXRD) and UV-vis absorption spectrum. It was found that the packing structures of compounds 1 and 2 were constructed by the trinuclear Mn3O16 building block and exhibited different spatial structure: compound 1 was a three-dimensional structure, and 2 was a two-dimensional network structure. The iodine adsorption in cyclohexane solution properties of compounds 1 and 2 were investigated. Research results showed that the uncoordinated amino group in the structure of framework compounds has a great influence on the iodine adsorption capacity and compound 2 had good adsorption property and reusability.

Recent Advances in Covalent Organic Frameworks for Heavy Metal Removal Applications

Covalent organic frameworks comprise a unique class of functional materials that has recently emerged as a versatile tool for energy-related, photocatalytic, environmental, and electrochromic device applications. A plethora of structures can be designed and implemented through a careful selection of ligands and functional units. On the other hand, porous materials for heavy metal absorption are constantly on the forefront of materials science due to the significant health issues that arise from the release of the latter to aquatic environments. In this critical review, we provide insights on the correlation between the structure of functional covalent organic frameworks and their heavy metal absorption. The elements we selected were Pb, Hg, Cr, Cd, and As metal ions, as well as radioactive elements, and we focused on their removal with functional networks. Finally, we outline their advantages and disadvantages compared to other competitive systems such as zeolites and metal organic frameworks (MOFs), we analyze the potential drawbacks for industrial scale applications, and we provide our outlook on the future of this emerging field.