Applications of metal-organic framework based membranes in water purification: A review

Organic and Metal-Organic Polymer-Based Catalysts-Enfant Terrible Companions or Good Assistants?

This overview provides insights into organic and metal-organic polymer (OMOP) catalysts aimed at processes carried out in the liquid phase. Various types of polymers are discussed, including vinyl (various functional poly(styrene-co-divinylbenzene) and perfluorinated functionalized hydrocarbons, e.g., Nafion), condensation (polyesters, -amides, -anilines, -imides), and additional (polyurethanes, and polyureas, polybenzimidazoles, polyporphyrins), prepared from organometal monomers. Covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and their composites represent a significant class of OMOP catalysts. Following this, the preparation, characterization, and application of dispersed metal catalysts are discussed. Key catalytic processes such as alkylation-used in large-scale applications like the production of alkyl-tert-butyl ether and bisphenol A-as well as reduction, oxidation, and other reactions, are highlighted. The versatile properties of COFs and MOFs, including well-defined nanometer-scale pores, large surface areas, and excellent chemisorption capabilities, make them highly promising for chemical, electrochemical, and photocatalytic applications. Particular emphasis is placed on their potential for CO2 treatment. However, a notable drawback of COF- and MOF-based catalysts is their relatively low stability in both alkaline and acidic environments, as well as their high cost. A special part is devoted to deactivation and the disposal of the used/deactivated catalysts, emphasizing the importance of separating heavy metals from catalysts. The conclusion provides guidance on selecting and developing OMOP-based catalysts.

A review on the potential of cellulose nanomaterials for the development of thin film composite polyamide membranes for water treatment

Membrane-based separation technologies have drawn significant interest because of their compactness, low energy consumption, and ability to be easily integrated with existing processes. There has been significant interest in the utilization of natural materials derived from sustainable and renewable resources for membrane fabrication. Cellulose is one of the promising polymers which has been extensively studied in membrane fabrication and modification due to its abundant availability, non-toxicity and biodegradability. While there have been several reviews in recent years separately on TFC membranes and cellulose-based materials for different applications, reviews exclusively focusing on cellulosic nanomaterials-based TFC membranes are still lacking. This review provides an overview of the types of cellulose nanomaterials exploited for the development and modification of TFC membranes, particularly those used for desalination and wastewater treatment. We have presented a brief description of cellulose-based nanomaterials followed by a detailed discussion of different studies addressing each cellulose nanomaterial separately. In addition, we have summarized the performance of different studies in the literature, paying particular attention to the enhancement achieved by the incorporation of cellulose nanomaterial in the membrane.

Innovative strategy for the effective utilization of coal waste slag in the Fenton-like process for the degradation of trichloroethylene

In response to environmental concerns at the global level, there is considerable momentum in the exploration of materials derived from waste that are both sustainable and eco-friendly. In this study, CS-Fe (carbon, silica, and iron) composite was synthesized from coal gasification slag (CGS) and innovatively applied as a catalyst to activate PS (persulfate) for the degradation of trichloroethylene (TCE) in water. Scanning electron microscope (SEM), fourier transmission infrared spectroscopy (FTIR), energy dispersive x-ray spectroscopy (EDS), brunauer, emmet, and teller (BET) technique, and x-ray diffractometer (XRD) spectra were employed to investigate the surface morphology and physicochemical composition of the CS-Fe composite. CS-Fe catalyst showed a dual nature by adsorption and degradation of TCE simultaneously, displaying 86.1% TCE removal in 3 h. The synthesized CS-Fe had better adsorption (62.1%) than base material CGS (36.4%) due to a larger BET surface area (770.8 m2 g-1), while 24.0% TCE degradation was recorded upon the activation of PS by CS-Fe. FTIR spectra confirmed the adsorption and degradation of TCE by investigating the used and fresh samples of CS-Fe catalyst. Scavengers and Electron paramagnetic resonance (EPR) analysis confirmed the availability of surface radicals and free radicals facilitated the degradation process. The acidic nature of the solution favored the degradation while the presence of bicarbonate ion (HCO3-) hindered this process. In conclusion, these results for real groundwater, surfactant-added solution, and degradation of other TCE-like pollutants propose that the CS-Fe composite offers an economically viable and favorable catalyst in the remediation of organic contaminants within aqueous solutions. Further investigation into the catalytic potential of coal gasification slag-based carbon materials and their application in Fenton reactions is warranted to effectively address a range of environmental challenges.

Recent Advances in Metal-Organic Framework (MOF)-Based Composites for Organic Effluent Remediation

Environmental pollution caused by organic effluents emitted by industry has become a worldwide issue and poses a serious threat to the public and the ecosystem. Metal-organic frameworks (MOFs), comprising metal-containing clusters and organic bridging ligands, are porous and crystalline materials, possessing fascinating shape and size-dependent properties such as high surface area, abundant active sites, well-defined crystal morphologies, and huge potential for surface functionalization. To date, numerous well designated MOFs have emerged as critical functional materials to solve the growing challenges associated with water environmental issues. Here we present the recent progress of MOF-based materials and their applications in the treatment of organic effluents. Firstly, several traditional and emerging synthesis strategies for MOF composites are introduced. Then, the structural and functional regulations of MOF composites are presented and analyzed. Finally, typical applications of MOF-based materials in treating organic effluents, including chemical, pharmaceutical, textile, and agricultural wastewaters are summarized. Overall, this review is anticipated to tailor design and regulation of MOF-based functional materials for boosting the performance of organic effluent remediation.

Recent advances in nanocellulose-based two-dimensional nanostructured membranes for sustainable water purification: A review

Nanocellulose (NC), a one-dimensional nanomaterial, is considered a sustainable material for water and wastewater purification because of its promising hydrophilic surface and mechanical characteristics. In this regard, nanostructured membranes comprising NC and two-dimensional (2D) nanomaterials emerged as advanced membranes for efficient and sustainable water purification. This article critically reviews the recent progress on NC-2D nanostructured membranes for water and wastewater treatment. The review highlights the main techniques employed to fabricate NC-2D nanostructured membranes. The physicochemical properties, including hydrophilicity, percent porosity, surface roughness, structure, and mechanical and thermal stability, are summarized. The key performance indicators such as permeability, rejection, long operation stability, antifouling, and interaction mechanisms are thoroughly discussed to evaluate the role of NC and 2D nanomaterials. Finally, summary points and future development work are highlighted to overcome the challenges for potential practical applications. This review contributes to the design and development of advanced membranes to solve growing water pollution concerns in a sustainable manner.

From environmental issue to purification aid: Novel positively charged functionalized algal biochar as robust modifier of composite nanofiltration membranes

Robust membrane modifiers were achieved for the first time by functionalizing the algal biochar of unique porous structure. The biochar was prepared through the pyrolysis of Cladophora glomerata, the most widespread freshwater macroalga, functionalized by diethylenetriamine and dendrimer poly(amidoamine), and employed to fabricate positively charged composite nanofiltration membranes. The presence of hydrophilic functionalizers of positive charge on the membrane was verified through Fourier transform infrared and energy dispersive X-ray analyses and atomic force microscopy and zeta potential measurements were performed to determine surface roughness and confirm positive charge of the modified membranes. Dispersion of modifiers on the surface and morphology of the were also revealed through field-emission scanning electron microscopy images. It has shown that, compared to the pristine membrane, pure water fluxes were increased by 214% and 185%, and water contact angles were reduced from 66.1° to 39.5° and 43.3° in those modified by biochar functionalized with dendrimer poly(amidoamine) and diethylenetriamine, respectively. More than 90% dye rejections and salt and heavy metals removals were recorded for the membranes possessed 0.6 wt% of modifiers. Finally, a comparative study conducted between the novel modifier introduced in this study and those reported in the literature, indicated that C. glomerata biochar decorated with amine functional groups could be considered as a robust and practical alternative to the common modifiers used to manipulate nanocomposite membranes characteristics.

Acidic Properties of Known and New COOH-Functionalized M(IV) Metal-Organic Frameworks

Herein, we report two new COOH-functionalized metal-organic frameworks (MOFs) of composition [M6 O4 (OH)6 (PMA)2 (H2 PMA)]×H2 O, M=Zr, Hf), denoted CAU-61, synthesized by using pyromellitic acid (H4 PMA), a tetracarboxylic acid, as the linker and acetic acid as the solvent. The structure was determined from powder X-ray diffraction data and one-dimensional inorganic building units are connected through tetracarboxylate as well as dicarboxylate linker molecules, resulting in highly stable microporous framework structures with limiting and maximum pore diameter of ∼3.6 and ∼5.0 Å, respectively, lined with -COOH groups. Thermal stabilities of up to 400 °C in air, chemical stability in water at pH 1 to 12 and water uptake of 17 mol/mol prompted us to study the proton exchange of the μ2 -OH, μ3 -OH of the IBU and -COOH groups of the linker by titration with LiOH. Comparison of the pKa values with three UiO-66 derivatives confirms distinct pKa value ranges and trends for the different acidic protons. Furthermore, the preparation of Zr-CAU-61 membranes and first results on permeation of dyes and ions in aqueous solutions are presented.

Green and Sustainable Membranes: A review

Membranes are ubiquitous tools for modern water treatment technology that critically eliminate hazardous materials such as organic, inorganic, heavy metals, and biomedical pollutants. Nowadays, nano-membranes are of particular interest for myriad applications such as water treatment, desalination, ion exchange, ion concentration control, and several kinds of biomedical applications. However, this state-of-the-art technology suffers from some drawbacks, e.g., toxicity and fouling of contaminants, which makes the synthesis of green and sustainable membranes indeed safety-threatening. Typically, sustainability, non-toxicity, performance optimization, and commercialization are concerns centered on manufacturing green synthesized membranes. Thus, critical issues related to toxicity, biosafety, and mechanistic aspects of green-synthesized nano-membranes have to be systematically and comprehensively reviewed and discussed. Herein we evaluate various aspects of green nano-membranes in terms of their synthesis, characterization, recycling, and commercialization aspects. Nanomaterials intended for nano-membrane development are classified in view of their chemistry/synthesis, advantages, and limitations. Indeed, attaining prominent adsorption capacity and selectivity in green-synthesized nano-membranes requires multi-objective optimization of a number of materials and manufacturing parameters. In addition, the efficacy and removal performance of green nano-membranes are analyzed theoretically and experimentally to provide researchers and manufacturers with a comprehensive image of green nano-membrane efficiency under real environmental conditions.

ZIF-67 blended PVDF membrane for improved Congo red removal and antifouling properties: A correlation establishment between morphological features and ultra-filtration parameters

Dye effluents from various sectors have constantly imperilled the environment and ecosystem. Nano-composite membrane technology incorporating metal-organic frameworks (MOFs) has shown tremendous potential for toxic pollutant remediation. This study details the impact of ZIF-67 MOF nanoparticles on the structural properties of polyvinylidene fluoride (PVDF) ultrafiltration membrane during the non-solvent induced phase separation (NIPS) process. In order to outline the properties that determine the performance parameters in a MOF-modified mixed matrix membrane, the corresponding changes in mean pore size (MPS), surface porosity, solvent viscosity, and hydrophilicity have been discussed with appropriate surface characterization analysis. The suitability of ZIF-67 as filler nanoparticles were established based on polymer compatibility, dispersibility, and water stability studies. The ZIF-67 incorporated PVDF mixed matrix membranes (MMM) showed 99.5% CR dye removal with 2.6 times DI water permeability than the neat. The flux recovery ratio (FRR) improved by 1.9 times and the membranes were found suitable for up to 5 filtration cycles. Based on the overall results, a correlation analysis between the MMM surface properties and membrane performance parameters were established to determine the key performance parameters. It was observed that in comparison to MPS, surface porosity was more correlated to J_d/J_w (r = 0.96) and FRR (r = 0.95).

Recent advances in Metal Organic Framework (MOF)-based hierarchical composites for water treatment by adsorptional photocatalysis: A review

Architecting a desirable and highly efficient nanocomposite for applications like adsorption, catalysis, etc. has always been a challenge. Metal Organic Framework (MOF)-based hierarchical composite has perceived popularity as an advanced adsorbent and catalyst. Hierarchically structured MOF material can be modulated to allow the surface interaction (external or internal) of MOF with the molecules of interest. They are well endowed with tunable functionality, high porosity, and increased surface area epitomizing mass transfer and mechanical stability of the fabricated nanostructure. Additionally, the anticipated optimization of nanocomposite can only be acquired by a thorough understanding of the synthesis techniques. This review starts with a brief introduction to MOF and the requirement for advanced nanocomposites after the setback faced by conventional MOF structures. Further, we discussed the background of MOF-based hierarchical composites followed by synthetic techniques including chemical and thermal treatment. It is important to rationally validate the successful nanocomposite fabrication by characterization techniques, an overview of challenges, and future perspectives associated with MOF-based hierarchically structured nanocomposite.

Nanofiltration Mixed Matrix Membranes from Cellulose Modified with Zn-Based Metal–Organic Frameworks for the Enhanced Water Treatment from Heavy Metal Ions

Nowadays, nanofiltration is actively used for water softening and disinfection, pre-treatment, nitrate, and color removal, in particular, for heavy metal ions removal from wastewater. In this regard, new, effective materials are required. In the present work, novel sustainable porous membranes from cellulose acetate (CA) and supported membranes consisting of CA porous substrate with a thin dense selective layer from carboxymethyl cellulose (CMC) modified with first-time synthesized Zn-based metal–organic frameworks (Zn(SEB), Zn(BDC)Si, Zn(BIM)) were developed to increase the efficiency of nanofiltration for the removal of heavy metal ions. Zn-based MOFs were characterized by sorption measurements, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The obtained membranes were studied by the spectroscopic (FTIR), standard porosimetry and microscopic (SEM and AFM) methods, and contact angle measurement. The CA porous support was compared with other, prepared in the present work, porous substrates from poly(m-phenylene isophthalamide) and polyacrylonitrile. Membrane performance was tested in the nanofiltration of the model and real mixtures containing heavy metal ions. The improvement of the transport properties of the developed membranes was achieved through Zn-based MOF modification due to their porous structure, hydrophilic properties, and different particle shapes.

One-Step Synthesis of Al-Doped UiO-66 Nanoparticle for Enhanced Removal of Organic Dyes from Wastewater

In this study, a series of Al-doped metal-organic frameworks (Al_xZr_(1-x)-UiO-66) were synthesized through a one-step solvothermal method. Various characterization techniques, including X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and N₂ sorption measurement, suggested that the Al doping was uniform and barely influenced the crystallinity, chemical stability, and thermal stability of the materials. Two cationic dyes, safranine T (ST) and methylene blue (MB), were selected for investigating the adsorption performances of Al-doped UiO-66 materials. Al_0.3Zr_0.7-UiO-66 exhibited 9.63 and 5.54 times higher adsorption capacities than UiO-66, 498 mg/g and 251 mg/g for ST and MB, respectively. The improved adsorption performance can be attributed to π-π interaction, hydrogen bond, and the coordination between the dye and Al-doped MOF. The pseudo-second-order and Langmuir models explained the adsorption process well, which indicated that the dye adsorption on Al_0.3Zr_0.7-UiO-66 mostly occurred through chemisorption on homogeneous surfaces. A thermodynamic study indicated the adsorption process was spontaneous and endothermic. The adsorption capacity did not decrease significantly after four cycles.

Room temperature synthesis of monolithic MIL-100(Fe) in aqueous solution for energy-efficient removal and recovery of aromatic volatile organic compounds

The removal and recovery of volatile organic compounds (VOCs) are widely used in many industrials. Unfortunately, most conventional porous materials not only have low VOCs uptake, but also need to be regenerated at relatively high temperature. Metal-organic frameworks (MOFs) have great potential for the removal and recovery of VOCs as their record-breaking gas adsorption capacity, easy regeneration, tunable pore structure and functional groups. Whereas, powdered MOFs are hardly implemented in industrial fields owing to their low bulk density and high pressure drop. Exploring a green method to prepare granular MOFs for the removal and recovery of VOCs is still a challenge. Herein, we report the room temperature green synthesis of a stable Fe-based MOF monolith by using water as the solvent without applying high pressure and chemical binders. The static and dynamic experiments show that the optimized centimeter-scale monolith has high porosity and mechanical strength, and exhibits much better adsorption performance for representative aromatic VOCs (benzene, toluene and p-xylene), than commercial activated carbon and activated carbon fiber under the same conditions. Remarkably, as-synthesized monolith can be rapidly regenerated at lower temperature. These results clearly demonstrate the advantages of MOF monoliths in removing and recovering VOCs, and also provide new insight into the effects of drying temperature, washing and centrifugation procedures on MOF shaping.

Porphyrinic metal–organic frameworks as molybdenum adsorbents for the 99Mo/99mTc generator

Two porphyrinic metal–organic frameworks (PCN-222 and PCN-224) were prepared and their potential as molybdenum adsorbents for the 99Mo/99mTc generator was explored.

An Overview of the Modification Strategies in Developing Antifouling Nanofiltration Membranes

Freshwater deficiency has become a significant issue affecting many nations' social and economic development because of the fast-growing demand for water resources. Nanofiltration (NF) is one of the promising technologies for water reclamation application, particularly in desalination, water, and wastewater treatment fields. Nevertheless, membrane fouling remains a significant concern since it can reduce the NF membrane performance and increase operating expenses. Consequently, numerous studies have focused on improving the NF membrane's resistance to fouling. This review highlights the recent progress in NF modification strategies using three types of antifouling modifiers, i.e., nanoparticles, polymers, and composite polymer/nanoparticles. The correlation between antifouling performance and membrane properties such as hydrophilicity, surface chemistry, surface charge, and morphology are discussed. The challenges and perspectives regarding antifouling modifiers and modification strategies conclude this review.

Grafting (S)-2-Phenylpropionic Acid on Coordinatively Unsaturated Metal Centers of MIL-101(Al) Metal-Organic Frameworks for Improved Enantioseparation

Chiral metal-organic frameworks (cMOFs) are emerging chiral stationary phases for enantioseparation owing to their porosity and designability. However, a great number of cMOF materials show poor separation performance for chiral drugs in high-performance liquid chromatography (HPLC). The possible reasons might be the irregular shapes of MOFs and the low grafting degree of chiral ligands. Herein, MIL-101-Ppa@SiO₂ was synthesized by a simple coordination post-synthetic modification method using (S)-(+)-2-Phenylpropionic acid and applied as the chiral stationary phase to separate chiral compounds by HPLC. NH₂-MIL-101-Ppa@SiO₂ prepared via covalent post-synthetic modification was used for comparison. The results showed that the chiral ligand density of MIL-101-Ppa@SiO₂ was higher than that of NH₂-MIL-101-Ppa@SiO₂, and the MIL-101-Ppa@SiO₂ column exhibited better chiral separation performance and structural stability. The binding affinities between MIL-101-Ppa@SiO₂ and chiral compounds were simulated to prove the mechanism of the molecular interactions during HPLC. These results revealed that cMOFs prepared by coordination post-synthetic modification could increase the grafting degree and enhance the separation performance. This method can provide ideas for the synthesis of cMOFs.

Preparation and Adsorption Performance Study of Graphene Quantum Dots@ZIF-8 Composites for Highly Efficient Removal of Volatile Organic Compounds

Based on the large specific surface area and excellent adsorption potential of graphene quantum dots (GQDs) and zeolitic imidazolate framework-8 (ZIF-8) materials, a GQDs@ZIF-8 composite was constructed to achieve optimal matching of the microstructure and to acquire efficient adsorption of volatile organic compounds (VOCs). GQDs and ZIF-8 were synthesized and then compounded by the solution co-deposition method to obtain GQDs@ZIF-8 composites. GQDs were uniformly decorated on the surface of the ZIF-8 metal-organic framework (MOF), effectively restraining the agglomeration, improving the thermal stability of ZIF-8 and forming abundant active sites. Thus, the VOC removal percentage and adsorption capacity of the GQDs@ZIF-8 composites were significantly improved. Toluene and ethyl acetate were chosen as simulated VOC pollutants to test the adsorption performance of the composites. The results showed that, after the addition of GQDs, the adsorption property of GQDs@ZIF-8 composites for toluene and ethyl acetate was obviously improved, with maximum adsorption capacities of 552.31 mg/g and 1408.59 mg/g, respectively, and maximum removal percentages of 80.25% and 93.78%, respectively, revealing extremely high adsorption performance. Compared with raw ZIF-8, the maximum adsorption capacities of the composites for toluene and ethyl acetate were increased by 53.82 mg/g and 104.56 mg/g, respectively. The kinetics and isotherm study revealed that the adsorption processes were in accordance with the pseudo-first-order kinetic model and the Freundlich isotherm model. The thermodynamic results indicated that the adsorption process of the GQDs@ZIF-8 composites was a spontaneous, endothermic and entropy increase process. This study provides a new way to explore MOF-based adsorption materials with high adsorption capacity which have broad application prospects in VOC removal fields.

Novel silver-modified carboxymethyl chitosan antibacterial membranes using environment-friendly polymers

The rapid reproduction of foodborne bacteria in food packaging threatens the health of consumers, the massive use and waste of packaging also causes serious environmental pollution. In this study, novel biodegradable antibacterial membranes based on silver-modified carboxymethyl chitosan (Ag-CMCS) were prepared. Polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT) were used as the base membrane materials. Characterization of the prepared membranes was performed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), water contact angle, and so on. Especially, the silver on the surface of Ag-CMCS was proved to be metallic silver. For the first cycle of zone of inhibition test, the diameter of inhibition zone could reach up to 17 mm while the mass of silver released was negligible. The prepared antibacterial membranes could kill almost 100% of bacteria under certain conditions and inhibition zone still existed after more than 7 cycles of tests, indicating the prepared antibacterial membranes were effective. This study could provide new ideas for preparing efficient and environment-friendly antibacterial food packaging membranes.

Amyloid fibril-UiO-66-NH2 aerogels for environmental remediation

A sustainable hybrid aerogel based on β-lactoglobulin amyloid fibril/UiO-66-NH₂ is developed for environmental remediation. The hybrid aerogel's CO₂ capture and water purification performances were investigated. The hybrid aerogel can achieve CO₂ capture and possesses excellent adsorption capacities for several heavy metals, dyes, and organic solvents.

Reactive Extrusion Printing for Simultaneous Crystallization-Deposition of Metal-Organic Framework Films

Reactive extrusion printing (REP) is demonstrated as an approach to simultaneously crystallize and deposit films of the metal-organic framework (MOF) Cu3 btc2 (btc=1,3,5-benzenetricarboxylate), also known as HKUST-1. The technique co-delivers inks of the copper(II) acetate and H3 btc starting materials directly on-surface and on-location for rapid nucleation into films at room temperature. The films were analyzed using PXRD, profilometry, SEM and thermal analysis techniques and confirmed high-quality Cu3 btc2 films are produced in low-dispersity interconnected nanoparticulate form. The porosity was examined using gas adsorption which showed REP gives Cu3 btc2 films with open interconnected pore structures, demonstrating the method bestows features that traditional synthesis does not. REP is a technique that opens the field to time-efficient large-scale fabrication of MOF interfaces and should find use in a wide variety of coating application settings.

Enhancement of wastewater treatment performance using 3D printed structures: A major focus on material composition, performance, challenges, and sustainable assessment

In order to enhance the performance and sustainability of wastewater treatment technologies, researchers are showing keen interest in the development of novel materials which can overcome the drawbacks associated with conventional materials. In this context, 3D printing gained significant attention due to its capability of fabricating complex geometrics using different material compositions. The present review focuses on recent advancements of 3D printing applications in various physicochemical and biological wastewater treatment techniques. In physicochemical treatment methods, substantial research has been aimed at fabricating feed spacers and other membrane parts, photocatalytic feed spacers, catalysts, scaffolds, monoliths, and capsules. Several advantages, such as membrane fouling mitigation, enhanced degradation efficiency, and recovery and reusability potential, have been associated with the aforementioned 3D printed materials. While in biofilm-based biological treatment methods, the use of 3D printed bio-carriers has led to enhanced mass transfer efficiency and microbial activities. Moreover, the application of these bio-carriers has shown better removal efficiency of chemical oxygen demand (∼90%), total nitrogen (∼73%), ammonia nitrogen (95%), and total phosphorous (∼100%). Although the removal efficiencies were comparable with conventional carriers, 3D printed carriers led to ∼40% reduction in hydraulic retention time, which could significantly save capital and operational expenditures. This review also emphasizes the challenges and sustainability aspects of 3D printing technology and outlines future recommendations which could be vital for further research in this field.

Nano-biotechnology, an applicable approach for sustainable future

Nanotechnology is one of the most emerging fields of research within recent decades and is based upon the exploitation of nano-sized materials (e.g., nanoparticles, nanotubes, nanomembranes, nanowires, nanofibers and so on) in various operational fields. Nanomaterials have multiple advantages, including high stability, target selectivity, and plasticity. Diverse biotic (e.g., Capsid of viruses and algae) and abiotic (e.g., Carbon, silver, gold and etc.) materials can be utilized in the synthesis process of nanomaterials. "Nanobiotechnology" is the combination of nanotechnology and biotechnology disciplines. Nano-based approaches are developed to improve the traditional biotechnological methods and overcome their limitations, such as the side effects caused by conventional therapies. Several studies have reported that nanobiotechnology has remarkably enhanced the efficiency of various techniques, including drug delivery, water and soil remediation, and enzymatic processes. In this review, techniques that benefit the most from nano-biotechnological approaches, are categorized into four major fields: medical, industrial, agricultural, and environmental.

Mitigation of membrane biofouling via immobilizing Ag-MOFs on composite membrane surface for extractive membrane bioreactor

The extractive membrane bioreactor (EMBR) combines an extractive membrane process and bioreactor to treat highly saline recalcitrant organic wastewater, in which the organic contaminations diffuse through a semi-permeable polydimethysiloxane (PDMS) composite membrane from the feed wastewater to the receiving biomedium. During the long-term EMBR operation, membrane biofouling is an inevitable phenomenon, which is one of the main obstacles impeding its wide applications. The excessive biofilm deposited on membrane surface could significantly reduce the organic mass transfer coefficient of composite membranes by more than 40%. Therefore, in this work, the silver (Ag)-metal organic frameworks (MOFs) were synthesized and immobilized on the PDMS surface of nanofibrous composite membranes to mitigate the membrane biofouling. The robustness of Ag-MOFs coating on membrane surface was well demonstrated by ultrasonic treatment. In addition, the silver nanoparticles (AgNPs) were coated on the PDMS surface of composite membranes for comparison. In contrast with the unmodified composite membrane #M0, the AgNPs-coated (#M1) and Ag-MOFs modified (#M2) composite membranes possessed less hydrophobic and negatively charged surfaces due to the coating layers. Although the modified membranes exhibited lower phenol mass transfer coefficients (k0's) in the aqueous-aqueous extractive membrane process due to these additional modification layers, both #M1 and #M2 displayed better long-term performance in the 12-days continuous EMBR operations due to their excellent anti-biofouling properties. Moreover, #M2 exhibited the most stable EMBR performance among the composite membranes developed in this work and other reported membranes with a finally stabilized k0 of 33.0 × 10-7 m/s (89% of initial k0). The least amounts of proteins, polysaccharides and total suspended solids (TSS) on the surface of tested #M2 also demonstrated its outstanding biofouling resistance. This excellent anti-biofouling performance should be attributed to the stable, controlled and long-lasting Ag+release from Ag-MOFs, as well as its less hydrophobic and negative charged surface properties, which made #M2 undergo the k0's increasing and gradual stabilization stages in the long-term EMBR operations.

Preparation and application of porous materials from coal gasification slag for wastewater treatment: A review

In recent years, coal gasification has been gradually promoted as clean technology, and coal gasification slag (CGS) emissions have increased accordingly. CGS, including coarse slag and fine slag, is rich in SiO₂ and Al₂O₃ and has pozzolanic activity, and thus CGS can be regarded as a cheap source of aluminosilicate. Also, CGS, especially the fine slag, usually contains higher contents of residual carbon which has a large specific surface area and low volatility and hence can be considered as a favorable precursor of activated carbon. Benefiting from these characteristics, CGS can be used to prepare high value-added porous materials, such as zeolite, mesoporous silica, carbon-silicon composite, and porous ceramics, and the obtained structures accommodate both sufficient adsorption capacity and low cost. Here, we review the research advances in characteristics of CGS and preparation methods of CGS-based porous materials, as well as their adsorption performance of heavy metal ions, organic dyes, ammonia nitrogen, and other water pollutants. The current studies indicate that CGS-derived adsorbents are effective and economical alternatives for removing aqueous pollutants. In addition, further research prospects on CGS-based porous materials are proposed.

Novel application of ion exchange membranes for preparing effective silver and copper based antibacterial membranes

Ion exchange membranes (IEMs) are widely used in water treatment applications such as electrodialysis. However, the exploration of IEMs as effective antibacterial food contact materials (e.g., food packaging membranes) against pathogenic bacteria to ensure food safety has not been reported. Here, we report a simple but effective method to prepare high performance antibacterial membranes via ion exchange coupled with in-situ reduction. The general membrane properties are characterized using SEM, EDS, FTIR, XPS, XRD, DSC, TGA, water uptake, etc. The distribution of silver and copper in the membranes are generally in line with the distribution of sulfur, indicating that the antibacterial ions are introduced into the membranes via ion exchange and are bonded with the sulfonate groups in the membranes. The antibacterial performance is investigated using zone of inhibition tests and continuous bacteria growth inhibition tests. All of the prepared membranes show obvious antibacterial activities compared to the bare cation exchange membranes. The diameters of inhibition zone against Staphylococcus aureus (S. aureus) are all larger than those of Escherichia coli (E. coli), indicating that the prepared membranes are more efficient in inhibiting S. aureus compared to E. coli. Furthermore, the silver-based membrane shows more sustainable antibacterial activities compared to the copper-based membrane. Especially, the results clearly reveal that the silver-based membrane is capable of killing bacteria instead of just inhibiting the growth of bacteria. We have shown for the first time that membranes derived from IEMs have the potential as food contact materials to inhibit the growth of pathogenic bacteria so as to eliminate the risk of bacterial infections and meanwhile delay food spoilage due to bacteria growth.

Layered double hydroxide-modified membranes for water treatment: Recent advances and prospects

Layered double hydroxides (LDHs) represent an exciting class of two-dimensional inorganic materials with unique physicochemical properties. They have been widely employed in water treatment due to their high surface areas, excellent ion exchange capacities, and highly tunable structures. They have also been employed in the fabrication and development of membranes for water treatment. 2D nanostructures as well as tailorable "structure forming units", surface functionalization with desired moieties, and interlayer galleries with adjustable heights and internal compositions make them attractive materials for membrane separations. This paper critically overviews the recent advancements in the synthesis and applications of LDH based membranes in water purification. The synthesis techniques and the effect of LDH incorporation into different membrane compositions have been described. LDH-based membranes showed excellent antifouling capability and improved water flux due to enhanced hydrophilicity. Such membranes have been successfully used for the treatment of inorganics, organics from environmental water samples. This review will be useful for understanding the current state of the LDH-based membranes for water purification and defining future research dimensions. In the end, we highlight some challenges and future prospects for the efficient application of LDH-based membranes in water decontamination.

Fabrication of a novel heteroepitaxial structure from an MOF-on-MOF architecture as a photocatalyst for highly efficient Cr(vi) reduction

Ce-on-Zr-MOF-808, a novel MOF-on-MOF hybrid used for efficient chromium reduction under visible-light irradiation.

2D Material Nanofiltration Membranes: From Fundamental Understandings to Rational Design

Since the discovery of 2D materials, 2D material nanofiltration (NF) membranes have attracted great attention and are being developed with a tremendously fast pace, due to their energy efficiency and cost effectiveness for water purification. The most attractive aspect for 2D material NF membranes is that, anomalous water and ion permeation phenomena have been constantly observed because of the presence of the severely confined nanocapillaries (<2 nm) in the membrane, leading to its great potential in achieving superior overall performance, e.g., high water flux, high rejection rates of ions, and high resistance to swelling. Hence, fundamental understandings of such water and ion transport behaviors are of great significance for the continuous development of 2D material NF membranes. In this work, the microscopic understandings developed up to date on 2D material NF membranes regarding the abnormal transport phenomena are reviewed, including ultrafast water and ion permeation rates with the magnitude several orders higher than that predicted by conventional diffusion behavior, ion dehydration, ionic Coulomb blockade, ion-ion correlations, etc. The state-of-the-art structural designs for 2D material NF membranes are also reviewed. Discussion and future perspectives are provided highlighting the rational design of 2D material membrane structures in the future.

Biopolymer-Based Membrane Adsorber for Removing Contaminants from Aqueous Solution: Progress and Prospects

The demand for energy-efficient water treatment as well as the limitation in adsorption of existing membranes has motivated the pursuit of membranes that can break the selectivity-permeability trade-off and provide high selective adsorption for chemicals of interest. The membrane adsorbers have received a lot of attention for removing contaminants from aqueous solution due to combine both advantages of adsorption and membrane separation. Membrane adsorbers constructed by biopolymer with many functional groups are widely used in water purification, because the biopolymers are easily available from biomass materials in nature, degradable, and low-cost. This paper summarizes the characteristics and important development direction of these types of biomass-based membrane adsorption materials to adsorb organic/inorganic contaminants of water and analyzes the preparation methods of natural biomacromolecule cellulose, chitosan, sodium alginate, and protein to construct the membrane adsorption materials, as well as the application of pollutant removal from aqueous solutions. According to the current problems and shortcomings in the research of biopolymer-based membrane adsorbers, it is proposed to improve the understanding of the adsorption mechanism of biopolymer-based membrane adsorbers and accelerate the development of practical applications as the focus of future research.

A comprehensive review on the synthesis and applications of ion exchange membranes

Ion exchange membranes (IEMs) are undergoing prosperous development in recent years. More than 30,000 papers which are indexed by Science Citation Index Expanded (SCIE) have been published on IEMs during the past twenty years (2001-2020). Especially, more than 3000 papers are published in the year of 2020, revealing researchers' great interest in this area. This paper firstly reviews the different types (e.g., cation exchange membrane, anion exchange membrane, proton exchange membrane, bipolar membrane) and electrochemical properties (e.g., permselectivity, electrical resistance/ionic conductivity) of IEMs and the corresponding working principles, followed by membrane synthesis methods, including the common solution casting method. Especially, as a promising future direction, green synthesis is critically discussed. IEMs are extensively applied in various applications, which can be generalized into two big categories, where the water-based category mainly includes electrodialysis, diffusion dialysis and membrane capacitive deionization, while the energy-based category mainly includes reverse electrodialysis, fuel cells, redox flow battery and electrolysis for hydrogen production. These applications are comprehensively discussed in this paper. This review may open new possibilities for the future development of IEMs.