Controllable integration of nano zero-valent iron into MOFs with different structures for the purification of hexavalent chromium-contaminated water: Combined insights of scavenging performance and potential mechanism investigations

This work combined the stability of the porous structure of metal-organic frameworks with the strong reducibility of nano zero-valent iron, for the controllable integration of NZVI into MOFs to utilize the advantages of each component with enhancing the rapid decontamination and scavenging of Cr(VI) from wastewater. Hence, four kinds of MOFs/NZVI composites namely ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, were prepared for Cr(VI) capture. The results indicated that the stable structure of ZIF67, MOF74, MIL101(Fe), CuBTC, was beneficial for the dispersion of NZVI that could help more close contact between MOFs/NZVI reactive sites and Cr(VI), subsequently, MOFs/NZVI was proved to be better scavengers for Cr(VI) scavenging than NZVI alone. The Cr(VI) capture achieved the maximum adsorption capacity at pH ~ 4.0, which might be due to the participation of more H+ in the reaction and better corrosion of NZVI at lower pH. Mechanism investigation demonstrated synergy of adsorption, reduction and surface precipitation resulted in enhanced Cr(VI) scavenging, and Fe(0), dissolved and surface-bound Fe(II) were the primary reducing species. The findings of this investigation indicated that the as-prepared composites of ZIF67/NZVI, MOF74/NZVI, MIL101(Fe)/NZVI, CuBTC/NZVI, with high oxidation resistance and excellent reactivity, could provide reference for the decontamination and purification of actual Cr(VI)-containing wastewater.

Effects of Fe(II) and humic acid on U(VI) mobilization onto oxidized carbon nanofibers derived from the pyrolysis of bacterial cellulose

The effects of Fe(II) and humic acid on U(VI) immobilization onto oxidized carbon nanofibers (Ox-CNFs, pyrolysis of bacterial cellulose) were investigated by batch, spectroscopic and modeling techniques, with results suggesting that, Ox-CNFs exhibited fast adsorption rate (adsorption equilibrium within 3 h), high adsorption performance (maximum adsorption capacity of 208.4 mg/g), good recyclability (no notable change after five regenerations) in the presence of Fe(II) towards U(VI) from aqueous solutions (e.g., 40 % reduction and 10 % adsorption at pH 8.0), which was attributed to the various oxygen-containing functional groups, excellent chemical stability, large specific surface area and high redox effect. U(VI) adsorption increased with increasing pH from 2.0 to 5.0, then high-level plateau and remarkable decrease were observed at 5.0-6.0 and at pH > 6.0, respectively. According to FT-IR and XPS analysis, a negative correlation between U(VI) reduction and organic in the presence of Fe(II) implied that U(VI) reduction was driven by Fe(II) while inhibited by humic acid. The interaction mechanism of U(VI) on Ox-CNFs was demonstrated to be adsorption and ion exchange at low pH and reduction at high pH according to XPS and surface complexation modeling. These findings filled the knowledge gaps pertaining to the effect of Fe(II) on the transformation and fate of U(VI) in the actual environment. This carbon material with distinctive performance and unique topology offers a potential platform for actual application in environmental remediation.

Significance of MOF adsorbents in uranium remediation from water

Uranium is considered as one of the most perilous radioactive contaminants in the aqueous environment. It has shown detrimental effects on both flora and fauna and because of its toxicities on human beings, therefore its exclusion from the aqueous environment is very essential. The utilization of metal-organic frameworks (MOFs) as an adsorbent for the removal of uranium from the aqueous environment could be a good approach. MOFs possess unique properties like high surface area, high porosity, adjustable pore size, etc. This makes them promising adsorbents for the removal of uranium from contaminated water. In this paper, sources of uranium in the water environment, human health disorders, and application of the different types of MOFs as well as the mechanisms of uranium removal have been discussed meticulously.

Integration of quantum dots with Zn2GeO4 nanoellipsoids to expand the dynamic detection range of uranyl ions in fluorescent test strips

Fluorescent colorimetric test strips normally have a narrow dynamic detection-range due to the limited responsive range from single responsive materials, which cannot meet the wide detection requirement in practical applications. Herein, we developed an approach to detect uranyl ions (UO₂²⁺) with a broad detection range using the synthesized ZnS:Mn quantum dots (QDs) modified Zn₂GeO₄ nanoellipsoids (Zn₂GeO₄ @ZnS:Mn NEs), containing two responsive materials with the opposite signal responses at different UO₂²⁺ concentrations. Specifically, a red to chocolate color change was observed at low analyte concentrations (0.01-100 μM) resulting from the photoinduced electron transfer effect from ZnS:Mn QDs to UO₂²⁺. A sequentially olive drab to green color change has been observed when further increasing the UO₂²⁺ concentration (100-1000 μM) as a result of the antenna effect between Zn₂GeO₄ nanoellipsoids and UO₂²⁺. In addition, a low-cost and portable fluorescent test strip has been further fabricated through embedding Zn₂GeO₄ @ZnS:Mn NEs on a microporous structure membrane, demonstrating a facile yet effective colorimetric response to UO₂²⁺ in lab water, lake water, and seawater with a wide dynamic range. Therefore, it is potentially attractive for real-time and on-site detection of UO₂²⁺ in sudden-onset situations.

Fe3C-porous carbon derived from Fe2O3 loaded MOF-74(Zn) for the removal of high concentration BPA: The integrations of adsorptive/catalytic synergies and radical/non-radical mechanisms

In this study, novel Fe₃C-porous carbon composites (Fe₃C-C) were prepared via the pyrolysis of Fe₂O₃ loaded MOF-74(Zn), which could integrate both strong adsorption properties and excellent peroxymonosulfate (PMS) activating performance for the removal of bisphenol A (BPA) in water. Results indicated that the composite obtained at 1000 °C (Fe₃C-C1000) exhibited optimal catalytic capability. Specifically, 0.1 mM BPA could be completely removed by 0.1 g/L Fe₃C-C1000 within 10 min after the adsorption enrichment. Afterwards, the mechanism of Fe₃C-C/PMS system was unveiled based on quenching tests, electron spin resonance analysis, electrochemical analysis, PMS consumption detection and solvent exchange (H₂O to D₂O) test. The BPA degradation pathways were also analyzed through identifying its decomposition intermediates. Results showed that the Fe₃C and porous carbon constituents could activate PMS via radical and non-radical mechanisms respectively, and BPA was readily degraded through both pathways. Additionally, it was found that the Fe₃C-C1000/PMS system could maintain conspicuous catalytic performance in a variety of complicated water matrices with wide pH application range and long-time use stability. This study suggests a new insight for the design and development of novel catalyst which can be used for the removal of refractory organic contaminants with high concentrations in water media.

Facile preparation and adsorption performance of low-cost MOF@cotton fibre composite for uranium removal

A novel composite MOF@cotton fibre (HCF) was prepared and characterized by FTIR, SEM, XPS and TGA. The effect of various parameters on the adsorption efficiency, such as the solution pH, contact time, initial U(VI) concentration and temperature, was studied. The maximal sorption capacity (Qm) is 241.28 mg g-1 at pH 3.0 for U(VI) according to the Langmuir isotherm adsorption model, and the kinetic and thermodynamic data reveal a relatively fast entropy-driven process (ΔH0 = 13.47 kJ mol-1 and ΔS0 = 75.47 J K-1 mol-1). The removal efficiency of U(VI) by HCF is comparable with that of pure cotton fibre and as-prepared MOF (noted as HST). However, the HST composite with cotton fibre significantly improved the treatment process of U(VI) from aqueous solutions in view of higher removal efficiency, lower cost and faster solid-liquid separation. Recycling experiments showed that HCF can be used up to five times with less than 10% efficiency loss.

Zeolite-Like Boron Imidazolate Frameworks (BIFs): Synthesis and Application

This review is devoted to discussion of the latest advances in design and applications of boron imidazolate frameworks (BIFs) that are a particular sub-family of zeolite-like metal–organic frameworks family. A special emphasis is made on nanostructured hybrid materials based on BIF matrices and their modern applications, especially in environment remediation and energy conversion.

Investigation of a modified metal-organic framework UiO-66 with nanoscale zero-valent iron for removal of uranium (VI) from aqueous solution

A novel composite material (nZVI/UiO-66) of nanoscale zero-valent iron (nZVI) with a functionalized metal-organic framework was synthesized by this study via a coprecipitation method, which was used for the efficient removal of U(VI) in the aqueous solution. The nZVI/UiO-66 had an excellent removal capacity of 404.86 mg g^-1 with an initial U(VI) concentration of 80 mg L^-1, 313 K and pH = 6. The transmission electron microscopy (TEM) revealed that nZVI particles were inhomogeneously distributed on the surface of UiO-66. The analysis by the X-ray diffraction (XRD) has further illustrated that the introduction of nZVI did not change the structure of UiO-66. The adsorption process closely followed the pseudo-second-order kinetic and the Freundlich isotherm model. The removal process of U(VI) by nZVI/UiO-66 was spontaneous and endothermic. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses have illustrated that the mechanism was mainly related to adsorption of U(VI) from UiO-66 and reduction of U(VI) by nZVI. The Zr-O bonds were shown to play a vital role in the uranium removal. nZVI/UiO-66 could be recycled. The uptake rate could be maintained at around 80% after 5 cycles of use. Therefore, these results manifested that the nZVI/UiO-66 is a promising sorbent for the efficient and selective removal of U(VI) in radioactive wastewaters.

Construction of hybrid DNAs@CP for the rapid synchronous sensing of multiplex microRNAs based on experimental studies and molecular simulation

A water stable one-dimensional (1D) ladder-shaped coordination polymer (CP) has been synthesized and exhibits a strong affinity to two fluorescein-tagged single-stranded probe DNAs (P-DNAs), giving a sensing platform of P-DNAs@1. Such a hybrid sensing platform is capable of simultaneous detection of breast cancer related microRNA-221 (miRNA-221) and miRNA-222 in a specific and synchronous manner, without observable cross-reactions, as supported by experimental evidences. The interaction mode and the electronic energy between CP 1 with nucleic acid were confirmed by molecular simulation and the universal force field (UFF).

In-situ synthesis of metal nanoparticles@metal-organic frameworks: Highly effective catalytic performance and synergistic antimicrobial activity

M-NP@Zn-BIF (M-NP = Ag or Cu nanoparticle; Zn-BIF is a zinc-based boron imidazolate framework, Zn₂(BH(2-mim)₃)₂(obb); 2-mim = 2-methylimidazole; obb = 4,4'-oxybis(benzoate)) composites were successfully in-situ synthesized by utilizing the reducing ability of the BH bond contained in the Zn-BIF at room temperature without any additional chemical reduction reagents. These composites (225 μg/mL) exhibited excellent catalytic activity to convert 4-nitrophenol to 4-aminophenol in 2.5 min and 6 min with a conversion rate of 99.9 %, respectively. In addition, Ag@Zn-BIF (50 μg/mL) showed highly synergistic antibacterial activity against both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) with a bactericidal rate of approximately 99.9 %. An antibacterial mechanism was proposed for the generation of intracellular reactive oxygen species (ROS) levels. Superoxide radicals (O^2-) and hydroxyl radicals (OH) formed during the antibacterial process were shown to accelerate the death of bacteria. They also exhibited highly photocatalytic activity for Rhodamine B (RhB). When the concentration of the composites is 1000 μg/mL, the photocatalytic efficiency of Ag@Zn-BIF and Cu@Zn-BIF increased by 31.62 and 18.13 times compared with Zn-BIF, respectively. All in all, this study developed a simple and versatile integrated platform for the removal of nitrophenols, organic dyes, and the effective inactivation of bacteria in water.

Rational Construction of Porous Metal-Organic Frameworks for Uranium(VI) Extraction: The Strong Periodic Tendency with a Metal Node

Although metal-organic frameworks (MOFs) have been reported as important porous materials for the potential utility in metal ion separation, coordinating the functionality, structure, and component of MOFs remains a great challenge. Herein, a series of anionic rare earth MOFs (RE-MOFs) were synthesized via a solvothermal template reaction and for the first time explored for uranium(VI) capture from an acidic medium. The unusually high extraction capacity of UO₂²⁺ (e.g., 538 mg U per g of Y-MOF) was achieved through ion-exchange with the concomitant release of Me₂NH₂⁺, during which the uranium(VI) extraction in the series of isostructural RE-MOFs was found to be highly sensitive to the ionic radii of the metal nodes. That is, the uranium(VI) adsorption capacities continuously increased as the ionic radii decreased. In-depth mechanism insight was obtained from molecular dynamics simulations, suggesting that both the accessible pore volume of the MOFs and hydrogen-bonding interactions contribute to the strong periodic tendency of uranium(VI) extraction.

Facile synthesis of amorphous MoSx–Fe anchored on Zr-MOFs towards efficient and stable electrocatalytic hydrogen evolution

MoS_x–Fe nanoparticles dispersively, firmly anchored on a UiO-66-(OH)₂ support via phenol–Fe(iii) coordination exhibit superior electrocatalytic hydrogen evolution performance in an acidic electrolyte.

Radioactive Uranium Preconcentration via Self-Propelled Autonomous Microrobots Based on Metal-Organic Frameworks

Self-propelled micromachines have recently attracted attention for environmental remediation, yet their use for radioactive waste management has not been addressed. Engineered micromotors that are able to combine highly adsorptive capabilities together with fast autonomous motion in liquid media are promising tools for the removal of nuclear waste, which is one of the most difficult types to manage. Herein, we fabricate self-propelled micromotors based on metal-organic frameworks (MOFs) via template-based interfacial synthesis and show their potential for efficient removal of radioactive uranium. A crucial challenge of the MOF-based motors is their stability in the presence of fuel (hydrogen peroxide) and acidic media. We have ensured their structural stability by Fe doping of zeolitic imidazolate framework-8 (ZIF-8). The implementation of magnetic ferroferric oxide nanoparticles (Fe₃O₄ NPs) and catalytic platinum nanoparticles (Pt NPs) results in the magnetically responsive and bubble-propelled micromotors. In the presence of 5 wt % H₂O₂, these micromotors are propelled at a high speed of ca. 860 ± 230 μm·s^-1 (i.e., >60 body lengths per second), which is significantly faster than that of other microrod-based motors in the literature. These micromotors demonstrate a highly efficient removal of uranium (96%) from aqueous solution within 1 h, with the subsequent recovery under magnetic control, as well as stable recycling ability and high selectivity. Such self-propelled magnetically recoverable micromotors could find a role in the management and remediation of radioactive waste.

In situ Preparation of Chitosan/ZIF-8 Composite Beads for Highly Efficient Removal of U(VI)

With the rapid growth of nuclear power generation and fuel processing, the treatment of nuclear industry wastewater has become a major problem, and if not handled properly, it will pose a potential threat to the ecological environment and human health. Herein, a chitosan (CS)/ZIF-8 composite monolithic beads with ZIF-8 loading up to 60 wt% for U(VI) removal was prepared, which can be easily removed after use. It possesses a very high adsorption capacity of 629 mg•g⁻¹ at pH = 3 for U(VI) and a well recyclability is demonstrated for at least four adsorption/desorption cycles. X-ray photoelectron spectroscopy (XPS) was carried out to study the adsorption mechanism between uranium and adsorbent, and the chelation of U(VI) ions with imidazole, hydroxyl, and amino groups was revealed. This work shows that CS/ZIF-8 composite can be used as an effective adsorbent for uranium extraction from aqueous solution, and has a potential application value in wastewater treatment.

A metal-organic-framework approach to engineer hollow bimetal oxide microspheres towards enhanced electrochemical performances of lithium storage

Nanosized electrode materials with a hollow structure, larger specific surface area, and lower energy density as well as more void space are widely adopted for high-performance lithium-ion batteries. In this work, we obtained bimetal-organic frameworks of Fe/Mn-MOF-74 with a hollow microsphere morphology via a facile one-step microwave method and further used it to fabricate hollow Fe-Mn-O/C microspheres. Endowed with the metal-organic-framework-derived carbon-coated nanoparticle-assembled hollow structure with hierarchical porous characteristics and synergistic effects between two different metal species, the Fe-Mn-O/C electrode exhibits outstanding electrochemical performances as the anode of lithium-ion batteries. It achieves improved cycling performance (1294 mA h g-1 after 200 cycles at 0.1 A g-1) and good rate capability (722, 604, and 521 mA h g-1 at 0.2, 0.5 and 1 A g-1). The smart design of a hollow morphology with uniform two metal species can promote the synthesis of multimetal oxides and their carbon composites, as well as their further potential application for energy-storage.

Water-stable metal-organic frameworks for aqueous removal of heavy metals and radionuclides: A review

Heavy metals and radionuclides in water are a global environmental issue, which has been receiving considerable attention worldwide. Water-stable MOFs are green and recyclable materials to eliminate the environmental impacts caused by the hazardous heavy metal ions and radionuclides in water. This paper presents a systematical review on the current status of water-stable MOFs that capture and convert a wide range of heavy metal ions (e.g., As(III)/As(V), Pb(II), Hg(II), Cd(II), and Cr(III)/Cr(VI)) and radionuclides (e.g., U(VI), Se(IV)/Se(VI) and Cs(I)) in aqueous solution. Water-stable MOFs and MOF-based composites exhibit the superior adsorption capability for these metal species in water. Significantly, MOFs show high selectivity in capturing target metal ions even in the presence of multiple water constituents. Mechanisms involved in capturing metal ions are described. MOFs also have excellent catalytic performance (photocatalysis and catalytic reduction by formic acid) for Cr(VI) conversion to Cr(III). Future research is suggested to provide insightful guidance to enhance the performance of the MOFs in capturing target pollutants in aquatic environment.

Beyond Crystal Engineering: Significant Enhancement of C2H2/CO2 Separation by Constructing Composite Material

Different from the established crystal engineering method for enhancing gas-separation performance, we demonstrate herein a distinct approach. In contrast to the pristine MOF (metal-organic framework) material, the C₂H₂/CO₂ separation ability for the resultant Ag NPs (nanoparticle)@Fe₂O₃@MOF composite material, estimated from breakthrough calculations, is greatly enhanced by 2 times, and further magnified up to 3 times under visible light irradiation.

Applying MOF+ technique for in situ preparation of a hybrid material for hydrogenation reaction

We demonstrate herein the preparation of a hybrid material for catalytic purpose by our recently developed MOF⁺ method. Impressively, the results show that the resultant catalyst displays superior catalytic performances for hydrogenation on olefin and selective semihydrogenation on phenylacetylene.

Iron Containing Metal-Organic Frameworks: Structure, Synthesis, and Applications in Environmental Remediation

Metal-organic frameworks (MOFs) with Fe content are gradually developing into an independent branch in environmental remediation, requiring economical, effective, low-toxicity strategies to the complete procedure. In this review, recent advancements in the structure, synthesis, and environmental application focusing on the mechanism are presented. The unique structure of novel design proposed specific characteristics of different iron-containing MOFs with potential innovation. Synthesis of typical MILs, NH₂-MILs and MILs based materials reveal the basis and defect of the current method, indicating the optimal means for the actual requirements. The adsorption of various contamination with multiple interaction as well as the catalytic degradation over radicals or electron-hole pairs are reviewed. This review implied considerable prospects of iron-containing MOFs in the field of environment and a more comprehensive cognition into the challenges and potential improvement.

Fe3O4@ZIF-8: a magnetic nanocomposite for highly efficient UO22+ adsorption and selective UO22+/Ln3+ separation

Fe₃O₄@ZIF-8 nanoparticles with outstanding UO₂²⁺ adsorption capacity and selectivity for lanthanide ions could be simply separated by magnet from a solution containing lanthanide ions.