Coumarin-modified microporous-mesoporous Zn-MOF-74 showing ultra-high uptake capacity and photo-switched storage/release of U(VI) ions

Efficiency and mechanism of adsorption of low concentration uranium in water by extracellular polymeric substances

Extracellular polymeric substances (EPS) of uranium adsorbent was first extracted from the aerobic activated sludge of municipal wastewater treatment plant as raw material. The structure and surface morphology of EPS was characterized by FTIR, SEM-EDX, 3D-EEM, and XPS. The 3D-EEM spectra of EPS revealed that there are Tryptophan-like protein and Humus which can adsorb uranium in the EPS. The results of XPS indicated that the EPS surface contained active functional groups (COOH,CONH₂,-H₂PO₄,OH,NH₂ and so on) which all react with uranium, and the C, N, O elements play an important role in the reaction. The static batch test was used to study the adsorption behavior of uranium on the EPS, and the effects of pH, dosage of EPS and initial concentration of the solution on the removal of uranium by EPS were investigated. The adsorption isotherm, thermodynamics and kinetic models were used to match the mechanism of the interaction between EPS and uranium. Batch adsorption experiments revealed that the pH value had a great influence on the adsorption effect of EPS, and the optimal solution pH for uranium adsorption was around 6.0 with the removal efficiency of uranium was about 93% in the condition of neutral. Freundlich (R² ≈ 0.997) and Langmuir (R² ≈ 0.9931) models can get a good fitting effect, indicating that the adsorption of uranium by EPS had both monolayer adsorption and multilayer adsorption. EPS and uranium were combined disorderly and ion exchange mechanism could be involved. In this study, the active groups on the surface of EPS were also involved in the chemisorption process of uranium adsorption. The maximum adsorption capacity of EPS by Langmuir fitting was 333.3 mg/g. We conclude EPS is a potential adsorbent for radionuclide treatment.

Metal–organic framework-based materials for the recovery of uranium from aqueous solutions

This review focuses on the recent progress in MOFs and MOF-based materials as superior adsorbents for the efficient removal of uranium from aqueous solutions.

Porous coordination/covalent hybridized polymers synthesized from pyridine-zinc coordination compound and their CO2 capture ability, fluorescence and selective response properties

Currently, porous polymers are mainly synthesized by linking coordination or organic covalent bonds. In this study, we propose the synthesis of a porous coordination/covalent hybridized polymer from di(4-vinylpyridine)-dichloro-zinc (ZnVP2) by "coordination-polymerization" method. The resulting porous polymer demonstrated CO2 capture ability and multi-responsive properties.

Metal-organic framework-based materials: superior adsorbents for the capture of toxic and radioactive metal ions

Highly efficient removal of metal ion pollutants, such as toxic and nuclear waste-related metal ions, remains a serious task from the biological and environmental standpoint because of their harmful effects on human health and the environment. Recently, highly porous metal-organic frameworks (MOFs), with excellent chemical stability and abundant functional groups, have represented a new addition to the area of capturing various types of hazardous metal ion pollutants. This review focuses on recent progress in reported MOFs and MOF-based composites as superior adsorbents for the efficient removal of toxic and nuclear waste-related metal ions. Aspects related to the interaction mechanisms between metal ions and MOF-based materials are systematically summarized, including macroscopic batch experiments, microscopic spectroscopy analysis, and theoretical calculations. The adsorption properties of various MOF-based materials are assessed and compared with those of other widely used adsorbents. Finally, we propose our personal insights into future research opportunities and challenges in the hope of stimulating more researchers to engage in this new field of MOF-based materials for environmental pollution management.

Functionalization of Metal-Organic Frameworks for Photoactive Materials

Metal-organic frameworks (MOFs) are intriguing platforms with multiple functionalities. Additional functionalization of MOFs generates novel materials for various applications. Here, three main topics are examined regarding the functionalization of MOFs for use as photoactive materials. The first is chemical approaches for postsynthetic modification of the metal clusters and organic linkers in MOFs; that is, sites on pore surfaces and chemical trapping of photoactive moieties within the pores, which create materials with chemical functionalities for water splitting and CO₂ reduction by light. The second topic focuses on the functionalization of MOFs for photochemical response and the versatile applications of such materials. State-of-the-art research on functionalizing MOFs through photochemical reactions on the pore surface and within the pores as guests is also summarized. The third topic introduces the functionalization of MOFs for photofunctional materials, including photoluminescent tuning and integration, photoluminescent LED devices and barcodes, and photophysical applications for chemical sensing. Finally, conclusions and perspectives on the fields are given.

Photochemistry and photophysics of MOFs: steps towards MOF-based sensing enhancements

In combination with porosity and tunability, light harvesting, energy transfer, and photocatalysis, are facets crucial for engineering of MOF-based sensors.

Defect engineering in metal–organic frameworks: a new strategy to develop applicable actinide sorbents

A greatly enhanced U(vi) loading in MOFs was achieved by tuning the missing-linker defects of highly porous and stable UiO-66.

Ship-in-a-bottle CMPO in MIL-101(Cr) for selective uranium recovery from aqueous streams through adsorption

Mesoporous MIL-101(Cr) is used as host for a ship-in-a-bottle type adsorbent for selective U(VI) recovery from aqueous environments. The acid-resistant cage-type MOF is built in-situ around N,N-Diisobutyl-2-(octylphenylphosphoryl)acetamide (CMPO), a sterically demanding ligand with high U(VI) affinity. This one-step procedure yields an adsorbent which is an ideal compromise between homogeneous and heterogeneous systems, where the ligand can act freely within the pores of MIL-101, without leaching, while the adsorbent is easy separable and reusable. The adsorbent was characterized by XRD, FTIR spectroscopy, nitrogen adsorption, XRF, ADF-STEM and EDX, to confirm and quantify the successful encapsulation of the CMPO in MIL-101, and the preservation of the host. Adsorption experiments with a central focus on U(VI) recovery were performed. Very high selectivity for U(VI) was observed, while competitive metal adsorption (rare earths, transition metals...) was almost negligible. The adsorption capacity was calculated at 5.32mg U/g (pH 3) and 27.99mg U/g (pH 4), by fitting equilibrium data to the Langmuir model. Adsorption kinetics correlated to the pseudo-second-order model, where more than 95% of maximum uptake is achieved within 375min. The adsorbed U(VI) is easily recovered by desorption in 0.1M HNO₃. Three adsorption/desorption cycles were performed.

Extending the Use of Highly Porous and Functionalized MOFs to Th(IV) Capture

Thorium separation has recently become a hot topic because of the potential application of thorium as a future nuclear fuel, while metal-organic framework (MOF) materials have received much attention in the separation field due to their unique properties. Herein, a highly porous and stable MOF, UiO-66, and its carboxyl derivatives (UiO-66-COOH and UiO-66-(COOH)₂) were synthesized and explored for the first time for Th(IV) capture from a weak acidic solution. Although the introduction of carboxyl groups into UiO-66 leads to an obvious decrease in the surface area and pore volume, the adsorbability toward Th(IV) is greatly enhanced. At pH = 3.0, the saturated sorption capacity for Th(IV) into UiO-66-(COOH)₂ reached 350 mg/g, representing one of the largest values for Th(IV) capture by solid extraction. Moreover, the functionalized MOFs show fast sorption kinetics and desirable selectivity toward Th(IV) over a range of competing metal ions. A possible mechanism for the selective recognition of Th(IV) by these MOFs was explored on the basis of extended X-ray absorption fine structure and Fourier transform infrared analysis. It is concluded that UiO-66-COOH and UiO-66-(COOH)₂ sorb Th(IV) through the coordination of carboxyl anions in the pores of the MOFs, whereas in the case of UiO-66, both the precipitation and the exchange with the organic solvent contribute to the Th(IV) uptake. This study contributes to the assessment of the feasibility of MOFs applied in actinides separation and better understanding of actinides sorption behavior in this kind of hybrid porous solid materials.

Metal–organic frameworks for radionuclide sequestration from aqueous solution: a brief overview and outlook

¹³⁷Cs, ⁹⁰Sr, ²³⁸U, ⁷⁹Se, and ⁹⁹Tc sequestrations from aqueous solution by metal–organic framework materials are summarized in this Frontier article.

Photoswitching adsorption selectivity in a diarylethene–azobenzene MOF

The first photoactive MOF composed of both azobenzene and diarylethene units is reported and used to modulate adsorption selectivity via light.

Conversion of supramolecular organic framework to uranyl-organic coordination complex: a new “matrix-free” strategy for highly efficient capture of uranium

Conversion of hydrogen-bonded supramolecular organic frameworks (HSOF) to a uranyl-organic coordination complex (UOCC) by uranyl-induced disassembly and reassembly: innovative “matrix-free” strategy for highly efficient uranium capture.

Reversible photo/thermoswitchable dual-color fluorescence through single-crystal-to-single-crystal transformation

A rare example of the insertion of a 1D polymer in a MOF using a light stimulus and subsequent characterization using single crystal X-ray diffraction is reported. An uncommon reversible photo/thermoswitchable dual-color green-to-blue fluorescence is observed.

UiO-66 and its Br-modified derivates for elemental mercury removal

Phenyl bromine-appended metal-organic frameworks (Br-MOFs) were synthesized and applied in elemental mercury (Hg⁰) removal from simulated flue gas, considering the stability of bromine on the materials at the same time. The techniques of PXRD, nitrogen adsorption, TGA and XPS were used to characterize the materials. Phenyl bromide on the MOFs was the main active site for Hg⁰ capture. The optimal Br-MOF showed high Hg⁰ removal efficiency of more than 99% for 48h at 200°C, whereas the efficiency of un-functionalized MOF and conventional bromine impregnated active carbon dropped to 59.8% and 91.2% within 5h, respectively. The crystalline integrity of the Br-MOF was maintained after Hg⁰ adsorption. Br-MOF exhibited enhanced Hg⁰ removal efficiency when SO₂ was introduced to the flue gas. However, exposure Br-MOF to flue gas with steam resulted in low Hg⁰ removal efficiency. Bromine leaching experiments proved that Br-MOFs have high bromine stability over the Hg⁰ adsorption process, avoiding the possible bromine pollution caused by the conventional bromine impregnated adsorbents. All of these results demonstrated the phenyl bromine-appended MOFs to be potential Hg⁰ adsorbent regarding its high Hg⁰ capture efficiency and low environmental risk.

A Combined Experimental and Theoretical Study on the Extraction of Uranium by Amino-Derived Metal-Organic Frameworks through Post-Synthetic Strategy

A novel carboxyl-functionalized metal-organic framework for highly efficient uranium sorption was prepared through a generic postsynthetic strategy, and this MOF's saturation sorption capacity is found to be as high as 314 mg·g^-1. The preliminary application illustrated that the grafted free-standing carboxyl groups have notably enhanced the sorption of uranyl ions on MIL-101. In addition, we have performed molecular dynamics simulation combined with density functional theory calculations to investigate the molecular insights of uranyl ions binding on MOFs. The high selectivity and easy separation of the as-prepared material have shown tremendous potential for practical applications in the nuclear industry or radioactive water treatment, and the functionalized MOF can be extended readily upon the versatility of click chemistry. This work provides a facile and purposeful approach for developing MOFs toward a highly efficient and selective extraction of uranium(VI) in aqueous solution, and it further facilitates the structure-based design of nanomaterials for radionuclide-containing-medium pretreatment.

Light-triggered Supramolecular Isomerism in a Self-catenated Zn(II)-organic Framework: Dynamic Photo-switching CO2 Uptake and Detection of Nitroaromatics

A self-catenated Zn(II)-organic framework formulated as [Zn₂(3,3′-bpeab)(oba)₂]·DMF (1) exhibiting a six-connected 4⁴·6¹⁰·8 topology has been successfully synthesized through the mixed-ligand of kinked 3,3′-bis[2-(4-pyridyl)ethenyl]azobenzene (3,3′-bpeab) and 4,4′-oxybis-benzoic acid (H₂oba) under solvothermal condition. UV light triggers isomerization of complex 1 in a single-crystal-to-single-crystal (SCSC) manner, giving rise to a conformational supramolecular isomer 1_UV through the pedal motion of photoresponsive double bonds. Dynamic photo-switching in the obtained light-responsive supramolecular isomers leads to instantly reversible CO₂ uptake. Furthermore, the ligand originated fluorescence emission of water-resistant complex 1 is selectively sensitive to 4-nitrotoluene (4-NT) owing to a higher quenching efficiency of the perilous explosive over other structurally similar nitroaromatics, prefiguring the potentials of 1 as a fluorescence sensor towards 4-NT in aquatic media.

MOF catalysis of FeII-to-FeIII reaction for an ultrafast and one-step generation of the Fe2O3@MOF composite and uranium(vi) reduction by iron(ii) under ambient conditions

Surface is for the first time attested to be one of outstanding merits of metal–organic frameworks (MOFs).