Robust superhydrophilic and underwater superoleophobic membrane optimized by Cu doping modified metal-organic frameworks for oil-water separation and water purification

Illuminating for purity: Photocatalytic and photothermal membranes for sustainable oil-water separation

The integration of photocatalytic and photothermal materials with oil-water separation membranes marks a significant advancement in sustainable separation technologies. These hybrid membranes exhibit exceptional functionalities, including resistance to oil fouling, self-cleaning, antibacterial properties, and reduced oil viscosity. Based on their reaction mechanisms, current photocatalytic and photothermal membranes are categorized into four types, i.e., photocatalytic membranes, photo-Fenton membranes, PMS-assisted photocatalytic membranes, and photothermal membranes. Under light irradiation, photocatalytically functionalized membranes generate reactive oxygen species (ROS) that degrade organic pollutants and inactivate bacteria on the membrane surfaces, enabling in-situ cleaning and regeneration. In addition to the above benefits, photothermal membranes achieve reduction of oil viscosity for higher membrane permeation and removal of light oil from membrane surfaces through light-induced heating. This review first explores the mechanisms underlying light-driven advanced oxidation processes (AOPs) and photothermal effects, followed by an in-depth discussion on the fabrication methods of these membranes. Additionally, the applications of photocatalytic and photothermal membranes in oil-water separation are examined, with an emphasis on how the photocatalytic and photothermal materials contribute to membrane functionality. Finally, this review presents the challenges currently faced by photocatalytic and photothermal membranes and outlines future research directions.

Novel polyarylether nitrile/layered bimetallic oxide/2-Methylimidazole composite membrane for efficient synergistic adsorption and degradation of organic pollutants under visible light

The difficulty of recycling and the finite photocatalytic performance of primitive nano-photocatalysts restrict their application in wastewater purification. In this study, a multifunctional membrane with efficient synergistic adsorption and degradation performance was constructed. The nano-photocatalyst layered bimetallic oxide (LDO) was combined with the matrix membrane polyarylether nitrile (PEN) by delayed phase transition technology. The introduced 2-Methylimidazole (2-MeIm) provided a virtual electron transfer pathway between PEN and LDO and enhanced the photocatalytic performance. The results suggested that PEN/LDO/2-MeIm has outstanding removal performance to organic dyes methylene blue (MB). After three consecutive cycles, the reacted membrane can be readily recovered from the system. The MB removal rate remained high at 89.38%, suggesting that the functional membrane is eligible for recycling and reuse. Finally, based on liquid chromatography-mass spectrometry (LC-MS) analysis and density functional theory (DFT) calculations, the mechanism and pathway of MB photodegradation by the PEN/LDO/2-MeIm system were proposed. Therefore, constructing PEN/LDO/2-MeIm membranes in this study may offer a novel perspective on creating eco-friendly and functional PEN-based membranes for practical use in wastewater purification.

Preparation of multifunctional PET membrane and its application in high-efficiency filtration and separation in complex environment

Nowadays, effluent treatment is a severe challenge mainly because of its complex composition, which includes oil, heavy metal ions, and dyes. Developing new intelligent membranes is one of the strategies to tackle these significant challenges in wastewater treatment. In this study, we fabricated asymmetric polyethylene glycol terephthalate (PET) membranes by grafting cross-linked poly (itaconic anhydride) (CL-PITA) nanoparticles onto the irradiated face. These nanoparticles were then functionalized with polyethyleneimine (PEI) and protonated with HCl to introduce numerous active electropositive amine groups. The fundamental purpose was to increase surface roughness, introduce numerous hydrophilic groups, and modify it to create a multi-functional PET membrane to separate complex environments. The promising results demonstrated that the protonated PET-g-ITA/DVB(10)-cat membrane exhibited excellent separation efficiencies (SE) for water/light oil, water/heavy oil and oil-in-water (O/W) emulsion. Compared to PET-g-ITA/DVB(0)-cat, it showed superior performance in SE for O/W emulsion and flux decay for water/light oil after 10 cycles. More interestingly, owing to numerous positively charged active amino groups and negativley charged carboxylate groups, the intelligent membrane exhibited a high removal rate of ca. 90 % for anionic dye (congo red) and heavy metals (Cu2+ and Co2+), showing great potential in complex water treatment environments.

Design of chitosan-based drug-loaded laminated materials with superhydrophilic/superhydrophobic properties for simultaneous effective hemostasis and antiadhesion

Hemostatic materials play a crucial role in trauma medicine. However, existing materials have poor hemostatic efficacy and a tendency to adhere to the wound surface, limiting their clinical effectiveness. Herein, a drug-loaded, superhydrophilic/superhydrophobic laminated material (DSLM), consisting of a superhydrophobic inner layer with a micropore array, a superhydrophilic chitosan-based sponge layer loaded with hemostatic/antimicrobial drugs, and a superhydrophobic outer layer, was developed. Furthermore, the DSLM allows unidirectional flow of blood and exudates from the wound bed through the superhydrophobic inner layer while facilitating efficient drug delivery. In addition, it possesses excellent biocompatibility and antiadhesion properties, as confirmed by in vivo and in vitro experiments. Compared with traditional hemostatic materials, the DSLM remarkably increased the survival time by over threefold in the acute femoral transaction wound bleeding model, and simultaneously prevented secondary wound damage by reducing peeling force to one-eighth incomparison to pristine gauze. The DSLM holds promise as a versatile clinical biomaterial for prehospital acute trauma treatment, with its simple structure facilitating manufacturing and expanding applications in biomedicine.

Ultra-high flux mesh membranes coated with tannic acid-ZIF-8@MXene composites for efficient oil-water separation

Oil/water separation has become a global concern due to the increasing discharge of multi-component harmful oily wastewater. Super wetting membranes have been shown to be an effective material for oil/water separation. Ultra-high flux stainless-steel meshes (SSM) with superhydrophilicity and underwater superoleophobicity were fabricated by tannic acid (TA) modified ZIF-8 nanoparticles (TZIF-8) and two-dimensional MXene materials for oil/water separation. The TZIF-8 increased the interlayer space of MXene, enhancing the flux permeation (69,093 L m-2h-1) and rejection of the composite membrane (TZIF-8@MXene/SSM). The TZIF-8@MXene/SSM membrane showed an underwater oil contact angle of 154.2°. The membrane maintained underwater superoleophobic after stability and durability tests, including various pH solutions, organic solvents, reusability, etc. In addition, the oil/water separation efficiency of TZIF-8@MXene/SSM membranes was higher than 99% after treatment in harsh conditions and recycling. The outstanding anti-fouling, stability, durability, and recyclability properties of TZIF-8@MXene/SSM membrane highlight the remarkable potential of membranes for complex oil/water separation process.

Superhydrophilic Photothermal-Responsive CuO@MXene Nanofibrous Membrane with Inherent Biofouling Resistance for Treating Complex Oily Wastewater

MXene, a recently emerged 2D material, has garnered substantial attention for a myriad of applications. Despite the growing interest, there remains a noticeable gap in exploring MXene-based membranes for the simultaneous achievement of photomodulated oil/water separation, bacterial resistance, and the removal of pollutants in the treatment of oily wastewater. In this work, we have successfully synthesized a novel multifunctional CuO@MXene-PAN nanofibrous membrane (NFM) featuring unique nanograin-like structures. Benefitting from these unique structures, the resultant membrane shows excellent superwetting properties, significantly enhancing its performance in oil/water separation. In addition, the membrane's photothermal property boosts its permeance by 40% under visible light illumination within 30 min. Furthermore, the resultant membrane shows decent dye removal efficiency in an aqueous solution, e.g., Rhodamine B (RhB), promoting efficient degradation with high reusability under visible light. Most remarkably, the resultant membrane exhibits superior anti-biofouling capability and consistently resists the adhesion of microorganisms such as cyanobacteria over a 14 day period. Thus, the combined effect of superior superwetting properties, photothermal responsivity, photocatalytic activity, and the antibacterial effect in CuO@MXene-PAN NFM contributes to the efficient treatment of intricate oily wastewater. This synergistic combination of superior properties in the membrane could be an appealing strategy for the broad development of multifunctional materials to prevent fouling during actual separation performance.

Modification of PAN electrospun nanofiber membranes with g-C3N4 nanotubes/carbon dots to enhance MBR performance

This study is dedicated to the enhancement of electrospun polyacrylonitrile (PAN) nanofiber membranes for their application in membrane bioreactor (MBR) processes. The improvement is achieved through the incorporation of graphitic carbon nitride nanotubes/carbon dots (g-C3N4 NT/CDs) and subsequent heat post-treatments at varying temperatures. Notably, the hot-pressing methodology effectively mitigates surface roughness and significantly reduces issues related to peeling during nanofiber experimentation. Our results demonstrate that the introduction of 0.5 wt% of g-C3N4 NT/CDs leads to a substantial enhancement in water flux. In particular, nanocomposite membranes subjected to hot-pressing at 90 °C for 10 min exhibited an impressive flux recovery ratio (FRR) of 70%. Furthermore, the heat-treated nanocomposite membranes exhibited remarkable antifouling properties and significantly reduced fouling rates when compared to their heat-treated bare counterparts. This study underscores the noteworthy potential of g-C3N4 NT/CDs-modified PAN nanofiber membranes to substantially elevate MBR performance, firmly positioning them as highly promising candidates for critical applications in the domains of water and wastewater treatment. However, it is imperative to underscore that the existing written material necessitates a comprehensive overhaul to align with the provided structural framework.

Enhanced management and antifouling performance of a novel NiFe-LDH@MnO2/PVDF hybrid membrane for efficient oily wastewater treatment

Layered double hydroxides (LDHs) have gained significant recognition for their facile synthesis and super-hydrophilic two-dimensional (2D) structure to fabricate antifouling membranes for oily wastewater separation. However, conventional PVDF membranes, due to their hydrophobic nature and inert matrix, often exhibit insufficient permeance and compatibility. In this study, a novel NiFe-LDH@MnO2/PVDF membrane was synthesized using ultrasonic, redox, and microwave-hydrothermal processes. This innovative approach cultivated grass-like NiFe-LDH@MnO2 nanoparticles within an inert PVDF matrix, promoting the growth of highly hydrophilic composites. The presence of NiFe-LDH@MnO2 resulted in pronounced enhancements in surface morphology, interfacial wettability, and oil rejection for the fabricated membrane. The optimal NiFe-LDH@MnO2/PVDF-2 membrane exhibited an extremely high pure water flux (1364 L m-2•h-1), and increased oil rejection (from 81.2% to 93.5%) without sacrificing water permeation compared to the original PVDF membrane. Additionally, the NiFe-LDH@MnO2/PVDF membrane demonstrated remarkable antifouling properties, evident by an exceptional fouling resistance ratio of 96.8% following slight water rinsing. Mechanistic insights into the enhanced antifouling performance were elucidated through a comparative "semi-immersion" investigation. The facile synthesis method, coupled with the improved membrane performance, highlights the potential application prospects of this hybrid membrane in emulsified oily wastewater treatment and environmental remediation.

Robust PVA/GO@MOF membrane with fast photothermal self-cleaning property for oily wastewater purification

The poor mechanical durability and weak fouling resistance of oil/water separation membranes severely restrict their applications in industry. Herein, a robust PVA/GO@MOF membrane with fast photothermal self-cleaning capability was developed through facile chemical crosslinking and suction-filtration strategies. Attributed to the powerful underwater superoleophobicity, the PVA/GO@MOF membrane exhibited extraordinary anti-oil adhesion even for high-viscosity crude oil and continuous crude oil emulsion purification capability with stable flux (1020 L m-2 h-1 bar-1) and exceptional efficiency (> 99.3%) even after 60 min. Most importantly, in comparison to reported photocatalytic self-cleaning oil/water separation membranes, the PVA/GO@MOF membrane can degrade organic contaminants more rapidly with a higher degradation rate (99.9%) in 50 min due to the superior photothermal conversion capacity. The synergistic photothermal and photocatalytic effects significantly enhanced photodegradation efficiency, which created opportunities for in-depth treatment of complex oily wastewater. Besides, the obtained membrane displayed excellent chemical and mechanical durability with underwater oil contact angle (UWOCA) above 150° even in harsh environments, such as corrosive solutions, UV irradiation, ultrasound treatment, abrasion experiment and bending test. Therefore, the developed PVA/GO@MOF membrane with robust durability and fast photocatalytic self-cleaning property is highly expected to purify oily wastewater and degrade organic pollutants.

MOF-Based Photocatalytic Membrane for Water Purification: A Review

Photocatalytic membranes can effectively integrate membrane separation and photocatalytic degradation processes to provide an eco-friendly solution for efficient water purification. It is of great significance to develop highly efficient photocatalytic membranes driven by visible light to ensure the long-term stability of membrane separation systems and the maximum utilization of solar energy. Metal-organic framework (MOF) is an emerging photocatalyst with a well-defined structure and tunable chemical properties, showing a broad application prospect in the construction of high-performance photocatalytic membranes. Herein, this work provides a comprehensive review of recent advancements in MOF-based photocatalytic membranes. Initially, this work outlines the main tailoring strategies that facilitate the enhancement of the photocatalytic activity of MOF-based photocatalysts. Next, this work introduces commonly used methods for fabricating MOF-based photocatalytic membranes. Subsequently, this work discusses the application and mechanisms of MOF-based photocatalytic membranes toward organic pollutant degradation, metal ion removal, and membrane fouling mitigation. Finally, challenges in developing MOF-based photocatalytic membranes and their practical applications are presented, while also pointing out future research directions toward overcoming these existing limitations.

Covalent organic framework functionalized smart membranes with under-liquid dual superlyophobicity for efficient separation of oil/water emulsions

The smart membrane with under-liquid dual superlyophobicity, which can achieve on-demand separation of oil/water emulsions only by simple liquid pre-wetting, is of essential value for the treatment of complicated real oil/water systems. Here, we first fabricated a stable suspension of imine-linked covalent organic framework nanospheres (TPB-DMTP-COF), and subsequently fabricated COF functionalized smart membranes with under-liquid dual superlyophobicity by immersing polyacrylonitrile-based (PAN-based) membranes into TPB-DMTP-COF nanosphere suspension. Accordingly, effective switchable separation of both oil-in-water and water-in-oil emulsions by TPB-DMTP-COF/PAN membranes can be achieved by employing pre-wetting processes (both the oil contact angle under water and the water contact angle under oil are over 150°). Specifically, the separation flux and the separation efficiency are higher than 1200 L/m2‧h and 98.0 %, and 2100 L/m2‧h and 97.4 % for the surfactant-stabilized oil-in-water and water-in-oil emulsions, respectively. Furthermore, the ultralow adhesions in liquid contributed to the outstanding reusability and antifouling resistance of the prepared TPB-DMTP-COF/PAN membranes. This work provides a feasible approach for fabricating a smart membrane with under-liquid dual superlyophobicity for oily wastewater treatment.

Biomimetic materials in oil/water separation: Focusing on switchable wettabilities and applications

Clean water resources are crucial for human society, as the leakage and discharge of oily wastewater not only harm the economy but also disrupt our living environment. Therefore, there is an urgent need for efficient oil-water separation technology. Surfaces with switchable superwetting behavior have garnered significant attention due to their importance in both fundamental research and practical applications. This review introduces the fundamental principles of wettability in the oil-water separation process, the basic theory of switchable wettability, and the mechanisms involved in oil-water separation. Subsequently, the review discusses the research progress, challenges, and issues associated with three conventional types of special wettability materials: superhydrophobic/superoleophilic materials, superhydrophilic/superoleophobic materials, and superhydrophilic/underwater superoleophobic materials. Most importantly, it provides a detailed exploration of recent advancements in switchable wettability smart materials, which combine elements of traditional special wettability materials. These include stimulus-responsive smart materials, pre-wetting-induced materials, and Janus materials. The discussion covers key response factors, detailed examples of representative works, design concepts, and fabrication strategies. Finally, the review offers a comprehensive summary of switchable superwetting smart materials, encompassing their advantages and disadvantages, persistent challenges, and future prospects.

Rapid Construction of Caffeic Acid/p-Phenylenediamine Antifouling Hydrophilic Coating on a PVDF Membrane for Emulsion Separation

The current methods of constructing modification strategies for hydrophilic membranes are time-consuming, complex in operation, and poor in universality, which limit their application on membranes. In this work, inspired by the adhesion properties and versatility of caffeic acid (CA) and p-phenylenediamine (PPDA), a simple, rapid, and universal method was designed for the separation of oil-in-water emulsion by preparing a stable hydrophilic coating separation membrane. The preparation time of the membrane was shortened to 40 min. The developed PVDF-PCA/PPDA membrane showed superhydrophilic and underwater superoleophobic properties. When applied to petroleum ether-in-water emulsion, isooctane-in-water emulsion, and dodecane-in-water emulsion separation, the oil rejection was more than 99.0%. In the circulating separation of 10 g/L soybean oil-in-water emulsion, the oil rejection was more than 99.3%, and the highest flux was 1036 L·m-2·h-1. The prepared PVDF-PCA/PPDA membrane performed well in the separation test of oily wastewater. The proposed strategy is simple and rapid; it may become a universal method for preparing membranes with super strong antifouling properties against viscous oil and accelerate the research progress of membrane separation of oil-in-water emulsions.

Recent Developments in Two-Dimensional Materials-Based Membranes for Oil-Water Separation

The industrialization witnessed in the last century has resulted in an unprecedented increase in water pollution. In particular, the water pollution induced by oil contaminants from oil spill accidents, as well as discharges from pharmaceutical, oil/gas, and metal processing industries, have raised concerns due to their potential to pose irreversible threats to the ecosystems. Therefore, the effective treating of these large volumes of oily wastewater is an inevitable challenge to address. Separating oil-water mixtures by membranes has been an attractive technology due to the high oil removal efficiency and low energy consumption. However, conventional oil-water separation membranes may not meet the complex requirements for the sustainable treatment of wastewater due to their relatively shorter life cycle, lower chemical and thermal stability, and permeability/selectivity trade-off. Recent advancements in two-dimensional (2D) materials have provided opportunities to address these challenges. In this article, we provide a brief review of the most recent advancements in oil-water separation membranes modified with 2D materials, with a focus on MXenes, graphenes, metal-organic frameworks, and covalent organic frameworks. The review briefly covers the backgrounds, concepts, fabrication methods, and the most recent representative studies. Finally, the review concludes by describing the challenges and future research directions.

Robust self-cleaning membrane with superhydrophilicity and underwater superoleophobicity for oil-in-water separation

The discharge of oily wastewater has increased dramatically and will bring serious environmental problems. In this work, a self-cleaning and anti-fouling g-C₃N₄/TiO₂/PVDF composite membrane was fabricated via the layer-by-layer approach. The surface of as-prepared composite membrane displayed a superhydrophilic and underwater superoleophobic behavior under irradiation with visible light. Also, upon irradiation with visible light, the fabricated g-C₃N₄/TiO₂/PVDF composite membrane displayed enhanced permeation flux and improved oil removal efficiency as a result of the generation of hydroxyl free radicals during the photocatalytic filtration process. Significantly, irradiation with visible light remarkably improved reusability of the composite membrane by initiating photocatalytic decomposition of deposited oil foulants, which enabled removal of over 99.75% of oils, thus reaching a nearly 100% flux recovery ratio. Furthermore, the g-C₃N₄/TiO₂/PVDF composite membrane exhibited great anti-fouling behavior in photocatalysis-assisted filtration. The mechanistic study revealed that underwater superhydrophobicity and the generation of free hydroxyl radicals jointly contributed to membrane anti-fouling. The greatest advantages of this g-C₃N₄/TiO₂/PVDF composite membrane are that not only does it degrades the oil pollutants, but it also makes the membrane less vulnerable to fouling.

C,N co-doped TiO2 hollow nanofibers coated stainless steel meshes for oil/water separation and visible light-driven degradation of pollutants

Complex pollutants are discharging and accumulating in rivers and oceans, requiring a coupled strategy to resolve pollutants efficiently. A novel method is proposed to treat multiple pollutants with C,N co-doped TiO₂ hollow nanofibers coated stainless steel meshes which can realize efficient oil/water separation and visible light-drove dyes photodegradation. The poly(divinylbenzene-co-vinylbenzene chloride), P(DVB-co-VBC), nanofibers are generated by precipitate cationic polymerization on the mesh framework, following with quaternization by triethylamine for N doping. Then, TiO₂ is coated on the polymeric nanofibers via in-situ sol-gel process of tetrabutyl titanate. The functional mesh coated with C,N co-doped TiO₂ hollow nanofibers is obtained after calcination under nitrogen atmosphere. The resultant mesh demonstrates superhydrophilic/underwater superoleophobic property which is promising in oil/water separation. More importantly, the C,N co-doped TiO₂ hollow nanofibers endow the mesh with high photodegradation ability to dyes under visible light. This work draws an affordable but high-performance multifunctional mesh for potential applications in wastewater treatment.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.