Chromium Speciation in Zirconium‐Based Metal–Organic Frameworks for Environmental Remediation

Metal-Organic Frameworks for Dual Photo-Oxidation and Capture of Arsenic from Water

Despite rapid technological progress, heavy metal water pollution, and particularly arsenic contamination, remains a significant global challenge. In addition, the stabilization of trivalent arsenic as neutral arsenite (AsIII) species hinders its removal by conventional sorbents. While adsorption of anionic arsenate (AsV) species is in principle more feasible, there are only few adsorbents capable of adsorbing both forms of arsenic. In this work, we explore the potential of two well-known families of Metal-Organic Frameworks (MOFs), UiO-66 and MIL-125, to simultaneously adsorb and photo-oxidize arsenic species from water. Our results demonstrate that the formation of AsV ions upon light irradiation promotes the subsequent adsorption of AsIII species. Thus, we propose the combined utilization of photocatalysis and adsorption with Metal-Organic Framework photocatalysts for water remediation purposes.

Defect-enabling zirconium-based metal-organic frameworks for energy and environmental remediation applications

This comprehensive review explores the diverse applications of defective zirconium-based metal-organic frameworks (Zr-MOFs) in energy and environmental remediation. Zr-MOFs have gained significant attention due to their unique properties, and deliberate introduction of defects further enhances their functionality. The review encompasses several areas where defective Zr-MOFs exhibit promise, including environmental remediation, detoxification of chemical warfare agents, photocatalytic energy conversions, and electrochemical applications. Defects play a pivotal role by creating open sites within the framework, facilitating effective adsorption and remediation of pollutants. They also contribute to the catalytic activity of Zr-MOFs, enabling efficient energy conversion processes such as hydrogen production and CO2 reduction. The review underscores the importance of defect manipulation, including control over their distribution and type, to optimize the performance of Zr-MOFs. Through tailored defect engineering and precise selection of functional groups, researchers can enhance the selectivity and efficiency of Zr-MOFs for specific applications. Additionally, pore size manipulation influences the adsorption capacity and transport properties of Zr-MOFs, further expanding their potential in environmental remediation and energy conversion. Defective Zr-MOFs exhibit remarkable stability and synthetic versatility, making them suitable for diverse environmental conditions and allowing for the introduction of missing linkers, cluster defects, or post-synthetic modifications to precisely tailor their properties. Overall, this review highlights the promising prospects of defective Zr-MOFs in addressing energy and environmental challenges, positioning them as versatile tools for sustainable solutions and paving the way for advancements in various sectors toward a cleaner and more sustainable future.

Metal-Organic Framework/Polyvinyl Alcohol Composite Films for Multiple Applications Prepared by Different Methods

The incorporation of different functional fillers has been widely used to improve the properties of polymeric materials. The polyhydroxy structure of PVA with excellent film-forming ability can be easily combined with organic/inorganic multifunctional compounds, and such an interesting combining phenomenon can create a variety of functional materials in the field of materials science. The composite membrane material obtained by combining MOF material with high porosity, specific surface area, and adjustable structure with PVA, a non-toxic and low-cost polymer material with good solubility and biodegradability, can combine the processability of PVA with the excellent performance of porous filler MOFs, solving the problem that the poor machinability of MOFs and the difficulty of recycling limit the practical application of powdered MOFs and improving the physicochemical properties of PVA, maximizing the advantages of the material to develop a wider range of applications. Firstly, we systematically summarize the preparation of MOF/PVA composite membrane materials using solution casting, electrostatic spinning, and other different methods for such excellent properties, in addition to discussing in detail the various applications of MOF/PVA composite membranes in water treatment, sensing, air purification, separation, antibacterials, and so on. Finally, we conclude with a discussion of the difficulties that need to be overcome during the film formation process to affect the performance of the composite film and offer encouraging solutions.

Saiz P, Reizabal A, Wuttke S, Celaya-Azcoaga L, Valverde A, Fernández de Luis R. A State-of-the-Art of Metal-Organic Frameworks for Chromium Photoreduction vs. Photocatalytic Water Remediation

Hexavalent chromium (Cr(VI)) is a highly mobile cancerogenic and teratogenic heavy metal ion. Among the varied technologies applied today to address chromium water pollution, photocatalysis offers a rapid reduction of Cr(VI) to the less toxic Cr(III). In contrast to classic photocatalysts, Metal-Organic frameworks (MOFs) are porous semiconductors that can couple the Cr(VI) to Cr(III) photoreduction to the chromium species immobilization. In this minireview, we wish to discuss and analyze the state-of-the-art of MOFs for Cr(VI) detoxification and contextualizing it to the most recent advances and strategies of MOFs for photocatalysis purposes. The minireview has been structured in three sections: (i) a detailed discussion of the specific experimental techniques employed to characterize MOF photocatalysts, (ii) a description and identification of the key characteristics of MOFs for Cr(VI) photoreduction, and (iii) an outlook and perspective section in order to identify future trends.

Reusable composite membranes for highly efficient chromium removal from real water matrixes

Natural or industrial hexavalent chromium water pollution continues to be a worldwide unresolved threat. Today, there is intense research on new active and cost-effective sorbents for Cr(VI), but most still exhibit a critical limitation: their powdered nature makes their recovery from water cost and energy consuming. In this work, Al(OH)₃, MIL-88-B(Fe), and UiO-66-NH₂ Cr(VI) sorbents were immobilized into a poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymeric substrate to develop an easily reactivable and reusable water filtering technology. The immobilization of the sorbents into the PVDF-HFP porous matrix modified the macro and meso-porous structure of the polymeric matrix, tuning in parallel its wettability. Although a partial blocking of the Cr(VI) adsorptive capacity was observed for of Al(OH)₃ and MIL-88-B(Fe) when immobilized into composite membranes, PVDF-HFP/UiO-66-NH₂ filter (i) exceeded the full capacity of the non-immobilized sorbent to trap Cr(VI), (ii) could be reactivated and reusable, and (iii) it was fully functional when applied in real water effluents.

Recent Progress in the Development of MOF-Based Photocatalysts for the Photoreduction of Cr(VI)

There has been a direct correlation between the rate of industrial development and the spread of pollution on Earth, particularly in the last century. The organic and inorganic pollutants generated from industrial activities have created serious risks to human life and the environment. The concept of sustainability has emerged to tackle the environmental issues in developing chemical-based industries. However, pollutants have continued to be discharged to water resources, and finding appropriate techniques for the removal and remedy of wastewater is in high demand. Chromium is one of the high-risk heavy metals in industrial wastewaters that should be removed via physical adsorption and/or transformed into less hazardous chemicals. Photocatalysis as a sustainable process has received considerable attention as it utilizes sunlight irradiation to remedy Cr^(VI) via a cost-effective process. Numerous photocatalytic systems have been developed up to now, but metal-organic frameworks (MOFs) have gained growing attention because of their unique versatilities and facile structural modulations. A variety of MOF-based photocatalysts have been widely employed for the photoreduction of Cr^(VI). Here, we review the recent progress in the design of MOF photocatalysts and summarize their performance in photoreduction reactions.

Bimetallic organic framework Cu/UiO-66 mediated "fluorescence turn-on" method for ultrasensitive and rapid detection of carcinoembryonic antigen (CEA)

Carcinoembryonic antigen (CEA) is a key serum tumor marker which is overexpressed in all types of adenocarcinomas. Therefore, establish the ultrasensitive, accurate and rapid method for CEA detection is essential for reducing the mortality of cancer. Here, a bimetallic organic framework Cu/UiO-66 was synthesized through the simple two-step hydrothermal method and used to construct a "fluorescence turn-on" analytical method for CEA detection. Cu/UiO-66 can adsorb CEA aptamers modified with FAM (CEA/FAM-Apt) and take place photoinduced electron transfer (PET) between Cu/UiO-66 and FAM, resulting in the fluorescence of the FAM is quenched. When CEA is present, CEA and CEA/FAM-Apt are tightly combined, making CEA/FAM-Apt far away from the Cu/UiO-66 surface. As a result, the fluorescence intensity of the system was significantly restored. Under optimal conditions, the proposed "fluorescence turn-on" method can detect CEA as low as 0.01 ng mL^-1 in a range of 0.01-0.3 ng mL^-1. Besides, this analytical method owns good selectivity, reproducibility and serum applicability, which provides a new platform for the direct detection of clinical diagnosis-related markers.

Chitin/Metal-Organic Framework Composites as Wide-Range Adsorbent

Composites based on chitin (CH) biopolymer and metal-organic framework (MOF) microporous nanoparticles have been developed as broad-scope pollutant absorbent. Detailed characterization of the CH/MOF composites revealed that the MOF nanoparticles interacted through electrostatic forces with the CH matrix, inducing compartmentalization of the CH macropores that led to an overall surface area increase in the composites. This created a micro-, meso-, and macroporous structure that efficiently retained pollutants with a broad spectrum of different chemical natures, charges, and sizes. The unique prospect of this approach is the combination of the chemical diversity of MOFs with the simple processability and biocompatibility of CH that opens application fields beyond water remediation.

Efficient Removal of Chromium(VI) Anionic Species and Dye Anions from Water Using MOF-808 Materials Synthesized with the Assistance of Formic Acid

This study presents a simple approach to prepare MOF-808, an ultra-stable Zr-MOF constructed from 6-connected zirconium clusters and 1,3,5-benzene tricarboxylic acid, with tailored particle sizes. Varying the amount of formic acid as a modulator in the range of 200-500 equivalents results in MOF-808 materials with a crystal size from 40 nm to approximately 1000 nm. Apart from the high specific surface area, a combination of a fraction of mesopore and plenty of acidic centers on the Zr-clusters induces a better interaction with the ionic pollutants such as K2Cr2O7 and anionic dyes. MOF-808 shows uptakes of up to 141.2, 642.0, and 731.0 mg/g for K2Cr2O7, sunset yellow, and quinoline yellow, respectively, in aqueous solutions at ambient conditions. The uptakes for the ionic dyes are significantly higher than those of other MOFs reported from the literature. Moreover, the adsorption capacity of MOF-808 remains stable after four cycles. Our results demonstrate that MOF-808 is a promising ideal platform for removing oxometallates and anionic dyes from water.

Processing Strategies to Obtain Highly Porous Silk Fibroin Structures with Tailored Microstructure and Molecular Characteristics and Their Applicability in Water Remediation

The present work reports on the control of silk fibroin (SF) porous structures performance through various processing methods. The study includes the analysis of two dissolving techniques (CaCl₂/H₂O/EtOH ternary and LiBr/H₂O binary solutions), three regeneration methods (gelation, lyophilization and gas foaming) and one post-processing (EtOH). In all the cases, followed steps lead to SF structures with porosity values above 94% and large surface areas. Also, results about samples microstructure, secondary organization, crystallinity and water behavior, reveal a direct correlation between processing and SF properties. Thanks to the achieved progress, the SF varying porous structures were evaluated for metalloids (As⁵⁺ and As³⁺) and heavy metals (Cr⁶⁺ and Cr³⁺) adsorption, observing a direct relationship between samples processing and ionic species adsorption ability. Thus, it is shown that the control of the properties of SF based porous structures through processing, represents a suitable and ecofriendly approach for the development of bio-based materials for environmental applications.

Modulation of the Bifunctional CrVI to CrIII Photoreduction and Adsorption Capacity in ZrIV and TiIV Benchmark Metal-Organic Frameworks

The presence of hexavalent chromium water pollution is a growing global concern. Among the currently applied technologies to remove CrVI, its adsorption and photocatalytic reduction to CrIII less mobile and toxic forms are the most appealing because of their simplicity, reusability, and low energy consumption. However, little attention has been paid to bifunctional catalysts, that is, materials that can reduce CrVI to CrIII and retain both hexavalent and trivalent chromium species at the same time. In this work, the dual CrVI adsorption–reduction capacity of two iconic photoactive water-stable zirconium and titanium-based metal–organic frameworks (MOFs) has been investigated: UiO-66-NH2 and MIL-125. The bifunctionality of photoactive MOFs depends on different parameters, such as the particle size in MIL-125 or organic linker functionalization/defective positions in UiO-66 type sorbents. For instance, the presence of organic linker defects in UiO-66 has shown to be detrimental for the chromium photoreduction but beneficial for the retention of the CrIII phototransformed species. Both compounds are able to retain from 90 to 98% of the initial chromium present at acidic solutions as well as immobilize the reduced CrIII species, demonstrating the suitability of the materials for CrVI environmental remediation. In addition, it has been demonstrated that adsorption can be carried out also in a continuous flux mode through a diluted photoactive MOF/sand chromatographic column. The obtained results open the perspective to assess the bifunctional sorption and photoreduction ability of a plethora of MOF materials that have been applied for chromium capture and photoreduction purposes. In parallel, this work opens the perspective to develop specific chemical encoding strategies within MOFs to transfer this bifunctionality to other related water remediation applications.