Breathable and asymmetrically superwettable Janus membrane with robust oil-fouling resistance for durable membrane distillation

Advances in membrane distillation for wastewater treatment: Innovations, challenges, and sustainable opportunities

Water pollution and the growing demand for zero liquid discharge solutions have driven the development of advanced wastewater treatment technologies. Membrane distillation (MD) is a promising thermal-based process capable of treating high-salinity brines and wastewater. This review provides an in-depth analysis of MD configurations, operating principles, and membrane characteristics while addressing key challenges such as fouling and pore wetting which hinder large-scale implementation. To overcome these limitations, various membrane fabrication and modification strategies, including physical and chemical approaches, have been explored. The integration of MD with other processes (hybrid MD) for wastewater treatment is also examined. A comprehensive discussion on the mechanisms of organic, inorganic, and biological fouling and their impact on MD performance is presented. Additionally, recent advancements in antifouling strategies, including surface modifications, novel materials, and operational optimizations, are reviewed. Furthermore, the review critically analyzes membrane wetting, its governing mechanisms, and mitigation techniques. By summarizing the current challenges and future prospects, this work provides valuable insights into improving MD performance for practical applications. The findings serve as a foundation for further research and technological advancements in the field of wastewater treatment using MD.

Interfacial fluid manipulation with bioinspired strategies: special wettability and asymmetric structures

The inspirations from nature always enlighten us to develop advanced science and technology. To survive in complicated and harsh environments, plants and animals have evolved remarkable capabilities to control fluid transfer via sophisticated designs such as wettability contrast, oriented micro-/nano-structures, and geometry gradients. Based on the bioinspired structures, the on-surface fluid manipulation exhibits spontaneous, continuous, smart, and integrated performances, which can promote the applications in the fields of heat transfer, microfluidics, heterogeneous catalysis, water harvesting, etc. Although fluid manipulating interfaces (FMIs) have provided plenty of ideas to optimize the current systems, a comprehensive review of history, classification, fabrication, and integration focusing on their interfacial chemistry and asymmetric structure is highly required. In this review, we systematically introduce development and highlight the state-of-the-art progress of bioinspired FMIs. Firstly, the biological prototype and development timeline are presented, and the underlying mechanism of on-surface fluid control on versatile structures is analyzed. Secondly, the definition and classification of FMIs as well as the strategy for controlling fluid/interface interaction are discussed. Thirdly, emergent applications of FMIs in practical scenarios including fog/vapor collection, fluid diodes, interfacial catalysis, etc. are presented. Furthermore, the challenges and prospects of interfacial liquid manipulation are concluded. We envision that this review should provide guidance for the incorporation of FMIs into suitable situations, which enlightens interdisciplinary research and practical applications in the fields of interface chemistry, materials design, bionic science, fluid dynamics, etc.

Progress in the Preparation and Application of Breathable Membranes

Breathable membranes with micropores enable the transfer of gas molecules while blocking liquids and solids, and have a wide range of applications in medical, industrial, environmental, and energy fields. Breathability is highly influenced by the nature of a material, pore size, and pore structure. Preparation methods and the incorporation of functional materials are responsible for the variety of physical properties and applications of breathable membranes. In this review, the preparation methods of breathable membranes, including blown film extrusion, cast film extrusion, phase separation, and electrospinning, are discussed. According to the antibacterial, hydrophobic, thermal insulation, conductive, and adsorption properties, the application of breathable membranes in the fields of electronics, medicine, textiles, packaging, energy, and the environment are summarized. Perspectives on the development trends and challenges of breathable membranes are discussed.

A cost-effective and high efficient Janus membrane for the treatment of oily brine using membrane distillation

Membrane distillation technology could utilize low-grade heat to desalinate brine, but the membrane material often suffers from disadvantages of low permeation flux and weak robustness to contaminants. To address these issues, the commercial polytetrafluoroethylene (PTFE) membrane was modified by cost-effective chemicals of tannic acid and (3-Aminopropyl)-triethoxysilane (APTES) to construct hydrophilic/underwater superoleophobic nano-rough structures on the surface to enhance its flux and oil-fouling resistance in direct contact membrane distillation. The results show that a high underwater oil contact angle of 180° is observed to the membrane surface due to the rough nanostructures functionalized by abundant hydroxyl groups. Despite the additional mass transfer resistance provided by the rough nanostructures, the flux was increased noticeably. This is mainly attributed to the strong interactions between the abundant hydroxyl groups of hydrophilic layer surface and water molecules, leading to a part of free water staying at intermediate transition state (IW). The mass transfer resistance of the hydrophilic layer itself is reduced as a consequence of decreased evaporation enthalpy of water, thereby increasing the flux. Moreover, while the flux of the pristine membrane is reduced by 84.18%, the flux of Janus membrane remains the same when treating mineral oil brine emulsions with oil concentration up to 1500 ppm in comparison with the result for 35 g l-1brine solution, indicating that the Janus membrane is safe from the oil contamination. Our work provides a fine guidance for membrane distillation to treat high oily brine.

Molecular insights into the adsorption and penetration of oil droplets on hydrophobic membrane in membrane distillation

Membrane fouling induced by oily substances significantly constrains membrane distillation performance in treating hypersaline oily wastewater. Overcoming this challenge necessitates a heightened fundamental understanding of the oil fouling phenomenon. Herein, the adsorption and penetration mechanism of oil droplets on hydrophobic membranes in membrane distillation process was investigated at the molecular level. Our results demonstrated that the adsorption and penetration of oil droplets were divided into four stages, including the free stage, contact stage, spreading stage, and equilibrium stage. Due to the extensive non-polar surface distribution of the polytetrafluoroethylene (PTFE) membrane (comprising 95.41 %), the interaction between oil molecules and PTFE was primarily governed by van der Waals interaction. Continuous oil droplet membrane fouling model revealed that the new oil droplet molecules preferred to penetrate into membrane pores where oil droplets already existed. The penetration of resin (a component of medium-quality oil droplets) onto PTFE membrane pores required the "pre-paving" of light crude oil. Finally, the ΔE quantitative structure-activity relationships (QSAR) models were developed to evaluate the penetration mechanism of pollutant molecules on the PTFE membrane. This research provides new insights for improving sustainable membrane distillation technologies in treating saline oily wastewater.

Engineering Multinanochannel Polymer-Intercalated Graphene Oxide Membrane for Strict Volatile Sieving in Membrane Distillation

Graphene oxide (GO) membranes enabled by subnanosized diffusion channels are promising to separate small species in membrane distillation (MD). However, the challenge of effectively excluding small volatiles in MD persists due to the severe swelling and subsequent increase in GO interlamination spacing upon direct contact with the hot feed. To address this issue, we implemented a design in which a polymer is confined between the GO interlaminations, creating predominantly 2D nanochannels centered around 0.57 nm with an average membrane pore size of 0.30 nm. Compared to the virginal GO membrane, the polymer-intercalated GO membrane exhibits superior antiswelling performance, particularly at a high feed temperature of 60 °C. Remarkably, the modified membrane exhibited a high flux of approximately 52 L m-2 h-1 and rejection rates of about 100% for small ions and 98% for volatile phenol, with a temperature difference of 40 °C. Molecular dynamics simulations suggest that the sieving mechanisms for ions and volatiles are facilitated by the narrowed nanochannels within the polymer network situated between the 2D nanochannels of GO interlaminations. Concurrently, the unrestricted permeation of water molecules through the multinanochannel GO membrane encourages high-flux desalination of complex hypersaline wastewater.

A comprehensive review on state-of-the-art antifouling super(wetting and anti-wetting) membranes for oily wastewater treatment

One of the most dangerous types of pollution to the environment is oily wastewater, which is produced from a number of industrial sources and can cause damage to the environment, people, and creatures. To overcome this issue, membrane technology as an advanced method has been considered for treating oily wastewater due to its stability, high removal efficiency, and simplicity in scaling up. Membrane fouling, or the accumulation of oil droplets at or within the membrane pores, compromises the efficiency of membrane separation and water flux. In the last decade, the fabrication of membranes with specific wettability to reduce fouling has received much consideration. The purpose of this article is to offer a literature overview of all fabricated anti-fouling super(wetting and anti-wetting) membranes for applicable membrane processes for the separation of immiscible and emulsified oil/water mixtures. In this review, we first explain membrane fouling and discuss methods for preventing it. Afterwards, in all membrane separation processes, including pressure-driven, gravity-driven, and thermal-driven, membranes based on the form and density of oil are categorized as oil-removing or water-removing with special wettability, and then their wettability modification with different materials is particularly discussed. Finally, the prospect of anti-fouling membrane fabrication in the future is presented.

Construction of Binary RGO/TiO2 Fibrous Membranes with Enhanced Mechanical Properties for E. coli Inactivation

For environmental remediation, it is significant to design membranes with good mechanical properties and excellent photocatalytic activity. In this work, RGO/TiO2 membranes with heterogeneous structures and good photocatalytic efficiency were synthesized using the method of electrospinning combined with a thermal treatment process. In the binary nanocomposites, RGO was tightly adhered to TiO2 fibers and by simply adjusting the loading of RGO, the strength and modulus of the fibrous membranes were improved. Notably, the RGO-permeated TiO2 fibers exhibited 1.41 MPa in tensile strength and 140.02 MPa in Young's modulus, which were 705% and 343% of the original TiO2 fibers, respectively. Benefiting from the enhanced light response and the homogeneous and compact heterogeneous structure, the synthesized RGO/TiO2 membranes displayed good antibacterial performance with a photocatalytic inactivation rate of 6 log against E. coli within 60 min. This study offers a highly efficient alternative to inactivate E. coli for the synthesis of TiO2-based membranes.

Designing of anisotropic gradient surfaces for directional liquid transport: Fundamentals, construction, and applications

Many biological surfaces are capable of transporting liquids in a directional manner without energy consumption. Inspired by nature, constructing asymmetric gradient surfaces to achieve desired droplet transport, such as a liquid diode, brings an incredibly valuable and promising area of research with a wide range of applications. Enabled by advances in nanotechnology and manufacturing techniques, biomimetics has emerged as a promising avenue for engineering various types of anisotropic material system. Over the past few decades, this approach has yielded significant progress in both fundamental understanding and practical applications. Theoretical studies revealed that the heterogeneous composition and topography mainly govern the wetting mechanisms and dynamics behavior of droplets, including the interdisciplinary aspects of materials, chemistry, and physics. In this review, we provide a concise overview of various biological surfaces that exhibit anisotropic droplet transport. We discussed the theoretical foundations and mechanisms of droplet motion on designed surfaces and reviewed recent research advances in droplet directional transport on designed plane surfaces and Janus membranes. Such liquid-diode materials yield diverse promising applications, involving droplet collection, liquid separation and delivery, functional textiles, and biomedical applications. We also discuss the recent challenges and ongoing approaches to enhance the functionality and application performance of anisotropic materials.

Electrospun Nanofiber Materials for Photothermal Interfacial Evaporation

Photothermal interfacial evaporation with low cost and environmental friendliness has attracted much attention. However, there are still many problems with this technology, such as heat loss and salt accumulation. Due to their different structures and adjustable chemical composition, electrospun nanofiber materials generally exhibit some unique properties that provide new approaches to address the aforementioned issues. In this review, the rational design principles for improving the total efficiency of solar evaporation are described for thermal/water management systems and salt-resistance strategies. And we review the state-of-the-art advancements in photothermal evaporation based on nanofiber materials and discuss their derivative applications in desalination, water purification, and power generation. Finally, we highlight key challenges and opportunities in both fundamental research and practical applications to inform further developments in the field of interfacial evaporation.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

Janus membranes at the water-energy nexus: A critical review

Membrane technology has emerged as a highly efficient strategy for alleviating water and energy scarcity globally. As the key component, the membrane plays a fatal role in different membrane systems; however, traditional membranes still suffer from shortcomings including low permeability, low selectivity, and high fouling tendency. Janus membranes are promising to overcome those shortcomings and appealing for applications in the realm of water-energy nexus, due to their special transport behaviors and separation properties as a result of their unique asymmetric wetting or surface charge properties. Recently, numerous research studies have been conducted on the design, fabrication, and application of Janus membranes. In this review, we aim to provide a state-of-the-art summary and a critical discussion on the research advances of Janus membranes at the water-energy nexus. The innovative design strategies of different types of Janus membranes are summarized and elucidated in detail. The fundamental working principles of various Janus membranes and their applications in oil/water separation, membrane distillation, solar evaporation, electrodialysis, nanofiltration, and forward osmosis are discussed systematically. The mechanisms of directional transport properties, switchable permeability, and superior separation properties of Janus membranes in those different applications are elucidated. Lastly, future research directions and challenges are highlighted in improving Janus membrane performance for various membrane systems.

Interlayered Interface of a Thin Film Composite Janus Membrane for Sieving Volatile Substances in Membrane Distillation

Hypersaline wastewater treatment using membrane distillation (MD) has gained significant attention due to its ability to completely reject nonvolatile substances. However, a critical limitation of current MD membranes is their inability to intercept volatile substances owing to their large membrane pores. Additionally, the strong interaction between volatile substances and MD membranes underwater tends to cause membrane wetting. To overcome these challenges, we developed a dual-layer thin film composite (TFC) Janus membrane through electrospinning and sequential interfacial polymerization of a polyamide (PA) layer and cross-linking a polyvinyl alcohol/polyacrylic acid (PP) layer. The resulting Janus membrane exhibited high flux (>27 L m^-2 h^-1), salt rejection of ∼100%, phenol rejection of ∼90%, and excellent resistance to wetting and fouling. The interlayered interface between the PA and PP layer allowed the sieve of volatile substances by limiting their dissolution-diffusion, with the increasing hydrogen bond network formation preventing their transport. In contrast, small water molecules with powerful dynamics were permeable through the TFC membrane. Both experimental and molecular dynamics simulation results elucidated the sieving mechanism. Our findings demonstrate that this type of TFC Janus membrane can serve as a novel strategy to design next-generation MD membranes against volatile and non-volatile contaminants, which can have significant implications in the treatment of complex hypersaline wastewater.

Preparation and Characterization of a Janus Membrane with an "Integrated" Structure and Adjustable Hydrophilic Layer Thickness

Oil-water emulsions are types of wastewater that are difficult to treat. A polyvinylidene fluoride hydrophobic matrix membrane was modified using a hydrophilic polymer, poly(vinylpyrrolidone-vinyltriethoxysilane), to form a representative Janus membrane with asymmetric wettability. The performance parameters of the modified membrane, such as the morphological structure, the chemical composition, the wettability, the hydrophilic layer thickness, and the porosity, were characterized. The results showed that the hydrolysis, migration, and thermal crosslinking of the hydrophilic polymer in the hydrophobic matrix membrane contributed to an effective hydrophilic layer on the surface. Thus, a Janus membrane with unchanged membrane porosity, a hydrophilic layer with controllable thickness, and hydrophilic/hydrophobic layer "structural integration" was successfully prepared. The Janus membrane was used for the switchable separation of oil-water emulsions. The separation flux of the oil-in-water emulsions on the hydrophilic surface was 22.88 L·m^-2·h^-1 with a separation efficiency of up to 93.35%. The hydrophobic surface exhibited a separation flux of 17.45 L·m^-2·h^-1 with a separation efficiency of 91.47% for the water-in-oil emulsions. Compared to the lower flux and separation efficiency of purely hydrophobic and hydrophilic membranes, the Janus membrane exhibited better separation and purification effects for both oil-water emulsions.

Modified Electrospun Membranes Using Different Nanomaterials for Membrane Distillation

Obtaining fresh drinking water is a challenge directly related to the change in agricultural, industrial, and societal demands and pressure. Therefore, the sustainable treatment of saline water to get clean water is a major requirement for human survival. In this review, we have detailed the use of electrospun nanofiber-based membranes (ENMs) for water reclamation improvements with respect to physical and chemical modifications. Although membrane distillation (MD) has been considered a low-cost water reclamation process, especially with the availability of low-grade waste heat sources, significant improvements are still required in terms of preparing efficient membranes with enhanced water flux, anti-fouling, and anti-scaling characteristics. In particular, different types of nanomaterials have been explored as guest molecules for electrospinning with different polymers. Nanomaterials such as metallic organic frameworks (MOFs), zeolites, dioxides, carbon nanotubes (CNTs), etc., have opened unprecedented perspectives for the implementation of the MD process. The integration of nanofillers gives appropriate characteristics to the MD membranes by changing their chemical and physical properties, which significantly enhances energy efficiency without impacting the economic costs. Here, we provide a comprehensive overview of the state-of-the-art status, the opportunities, open challenges, and pitfalls of the emerging field of modified ENMs using different nanomaterials for desalination applications.

Efficacy of Electrospun Nanofiber Membranes on Fouling Mitigation: A Review

Despite the advantages of high contaminant removal, operational flexibility, and technical advancements offered, the undesirable fouling property of membranes limits their durability, thus posing restrictions on their usage. An enormous struggle is underway to conquer this major challenge. Most of the earlier reviews include the basic concepts of fouling and antifouling, with respect to particular separation processes such as ultrafiltration, nanofiltration, reverse osmosis and membrane bioreactors, graphene-based membranes, zwitterionic membranes, and so on. As per our knowledge, the importance of nanofiber membranes in challenging the fouling process has not been included in any record to date. Nanofibers with the ability to be embedded in any medium with a high surface to volume ratio play a key role in mitigating the fouling of membranes, and it is important for these studies to be critically analyzed and reported. Our Review hence intends to focus on nanofiber membranes developed with enhanced antifouling and biofouling properties with a brief introduction on fabrication processes and surface and chemical modifications. A summary on surface modifications of preformed nanofibers is given along with different nanofiller combinations used and blend fabrication with efficacy in wastewater treatment and antifouling abilities. In addition, future prospects and advancements are discussed.

The High Flux of Superhydrophilic-Superhydrophobic Janus Membrane of cPVA-PVDF/PMMA/GO by Layer-by-Layer Electrospinning for High Efficiency Oil-Water Separation

A simple and novel strategy of superhydrophilic-superhydrophobic Janus membrane was provided here to deal with the increasingly serious oil-water separation problem, which has a very bad impact on environmental pollution and resource recycling. The Janus membrane of cPVA-PVDF/PMMA/GO with opposite hydrophilic and hydrophobic properties was prepared by layer-by-layer electrospinning. The structure of the Janus membrane is as follows: firstly, the mixed solution of polyvinylidene fluoride (PVDF), polymethylmethacrylate (PMMA) and graphene oxide (GO) was electrospun to form a hydrophobic layer, then polyvinyl alcohol (PVA) nanofiber was coated onto the hydrophobic membrane by layer-by-layer electrospinning to form a composite membrane, and finally, the composite membrane was crosslinked to obtain a Janus membrane. The addition of GO can significantly improve the hydrophobicity, mechanical strength and stability of the Janus membrane. In addition, the prepared Janus membrane still maintained good oil-water separation performance and its separation efficiency almost did not decrease after many oil-water separation experiments. The flux in the process of oil-water separation can reach 1909.9 L m-2 h-1, and the separation efficiency can reach 99.9%. This not only proves the separation effect of the nanocomposite membrane, but also shows its high stability and recyclability. The asymmetric Janus membrane shows good oil-water selectivity, which gives Janus membrane broad application prospects in many fields.

Nanoarchitectonics for Electrospun Membranes with Asymmetric Wettability

Membranes with asymmetric wettability have attracted significant interest by virtue of their unique transport characteristics and functionalities arising from different wetting behaviors of each membrane surface. The cross-sectional wettability distinction enables a membrane to realize directional liquid transport or multifunction integration, resulting in rapid advance in applications, such as moisture management, fog collection, oil-water separation, and membrane distillation. Compared with traditional homogeneous membranes, these membranes possess enhanced transport performance and higher separation efficiency owing to the synergistic or individual effects of asymmetric wettability. This Review covers the recent progress in fabrication, transport mechanisms, and applications of electrospun membranes with asymmetric wettability and provides a perspective on future development in this important area.

Membrane Distillation for Wastewater Treatment: A Mini Review

Water serves as an indispensable part of human life and production. On account of the overexploitation of traditional water sources, the demand for wastewater recycling is expanding rapidly. As a promising water treatment process, membrane distillation (MD) has been utilized in various wastewater treatments, such as desalination brine, textile wastewater, radioactive wastewater, and oily wastewater. This review summarized the investigation work applying MD in wastewater treatment, and the performance was comprehensively introduced. Moreover, the obstructions of industrialization, such as membrane fouling, membrane wetting, and high energy consumption, were discussed with the practical investigation. To cope with these problems, various strategies have been adopted to enhance MD performance, including coupling membrane processes and developing membranes with specific surface characteristics. In addition, the significance of nutrient recovery and waste heat utilization was indicated.

Membrane Distillation: Pre-Treatment Effects on Fouling Dynamics

In the research reported in this paper, membrane distillation was employed to recover water from a concentrated saline petrochemical effluent. According to the results, the use of membrane distillation is technically feasible when pre-treatments are employed to mitigate fouling. A mathematical model was used to evaluate the fouling mechanism, showing that the deposition of particulate and precipitated material occurred in all tests; however, the fouling dynamic depends on the pre-treatment employed (filtration, or filtration associated with a pH adjustment). The deposit layer formed by particles is not cohesive, allowing its entrainment to the bulk flow. The precipitate fouling showed a minimal tendency to entrainment. Also, precipitate fouling served as a coupling agent among adjacent particles, increasing the fouling layer cohesion.

Anisotropic Janus materials: from micro-/nanostructures to applications

Janus materials have led to great achievements in recent years owing to their unique asymmetric structures and properties. In this review, recent advances of Janus materials including Janus particles and Janus membranes are summarized, and then the microstructures and applications of Janus materials are emphasized. The asymmetric wettability of Janus materials is related to their microstructures; hence, the microstructures of Janus materials were analyzed, compared and summarized. Also presented are current and potential applications in sensing, drug delivery, oil-water separation and so on. Finally, a perspective on the research prospects and development of Janus materials in more fields is given.

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.

Development of Janus Cellulose Acetate Fiber (CA) Membranes for Highly Efficient Oil-Water Separation

A new type of Janus cellulose acetate (CA) fiber membrane was used to separate oil–water emulsions, which was prepared with plasma gas phase grafting by polymerizing octamethylcyclotetrasiloxane (D4) onto a CA fiber membrane prepared by centrifugal spinning. The Janus–CA fiber membrane was described in terms of chemical structure using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) analysis, energy dispersive X-ray spectroscopy (EDX) analysis and morphology by field emission scanning electron microscopy (FESEM). In this contribution, we examine the influence of spinning solution concentration, spinning speed and nozzle aperture on the centrifugal spinning process and the fiber morphology. Superhydrophobic/hydrophilic Janus–CA fiber membrane was used to separate water and 1,2-dibromoethane mixture and Toluene-in-water emulsion. Unidirectional water transfer Janus–CA fiber membrane was used to separate n-hexane and water mixture. The separation for the first-time interception rate was about 98.81%, 98.76% and 98.73%, respectively. Experimental results revealed that the Janus cellulose acetate (CA) fiber membrane gave a permeate flux of about 43.32, 331.72 and 275.27 L/(m²·h), respectively. The novel Janus–CA fiber membrane can potentially be used for sustainable W/O emulsion separation. We believe that this is a facile strategy for construction of filtration materials for practical oil–water separation.

Fabrics with Novel Air-Oil Amphibious, Spontaneous One-Way Water-Transport Capability for Oil/Water Separation

Porous media with directional water-transport capability have great applications in oil-water separation, moisture-harvesting, microfluidics, and moisture-management textiles. However, the previous directional water-transport materials chiefly work in the air. The materials with directional water-transport capability in the oil phase have been less reported. Here, we fabricated a novel Janus fabric with amphibious directional water-transport capability that can work both in the air and oil phases. It was prepared using dip coating and spraying to develop an oleophobic-hydrophobic to oleophobic-hydrophilic gradient across the thickness of the fabric substrate. The fabric allowed water droplets to rapidly transport from the hydrophobic to the hydrophilic side when the fabric was either in the air environment or fully immersed in oil. However, it hindered water transport in the opposite direction. More importantly, the fabric can overcome gravity to capture water from oil. Such an air-oil amphibious water-transport fabric showed excellent water collecting capability. In oil, it does not require any prewetting or extra pressure to perform directional water transport, which is vital for water-oil separation and microfluidics. Such amphibious directional water-transport function may be useful for the development of smart membranes and directional liquid delivery.

Janus membranes for membrane distillation: Recent advances and challenges

Membrane distillation (MD) is a promising hybrid thermal-membrane separation technology that can efficiently produce freshwater from seawater or contaminated wastewater. However, the relatively low flux and the presence of fouling or wetting agents in feed solution negate the applicability of MD for long term operation. In recent years, 'two-faced' membranes or Janus membranes have shown promising potential to decrease wetting and fouling problem of common MD system as well as enhance the flux performance. In this review, a comprehensive study was performed to investigate the various fabrication, modification, and novel design processes to prepare Janus membranes and discuss their performance in desalination and wastewater treatment utilizing MD. The promising potential, challenges and future prospects relating to the design and use of Janus membranes for MD are also tackled in this review.

Recent advances in femtosecond laser-structured Janus membranes with asymmetric surface wettability

Janus wettability membranes have received much attention because of their asymmetric surface wettability. On the basis of this distinctiveness from traditional symmetrical membranes, relevant scholars have been inspired to pursue many innovations utilizing such membranes. Femtosecond laser microfabrication shows many advantages, such as precision, short time, and environmental friendliness, over traditional fabrication methods. Now this has been applied in structuring Janus membranes by researchers. This review covers recent advances in femtosecond laser-structured Janus membranes with asymmetric surface wettability. The background in femtosecond laser-structured Janus membranes is first discussed, focusing on the Janus wettability membrane and femtosecond laser microfabrication. Then the applications of Janus membranes are introduced, which are divided into unidirectional fluid transport, oil-water separation, fog harvesting, and seawater desalination. Finally, based on femtosecond laser-structured Janus membranes, some existing problems are pointed out and future perspectives proposed.

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Transport Analysis of Anti-Wetting Composite Fibrous Membranes for Membrane Distillation

Composite electrospun fibrous membranes are widely studied for the application of membrane distillation. It is an effective approach to enhance the membrane distillation performance in terms of anti-wetting surface and permeate flux by fabricating composite fibrous membranes (CFMs) with a thin skin layer on a thick supporting layer. In this work, various membranes prepared with different pore sizes and porosities by polyacrylonitrile and polyvinylpyrrolidone were prepared. The membrane characteristics and membrane distillation performance were tested. The mass transfer across the membranes was analyzed experimentally and theoretically in detail. It is shown that the skin layer significantly increases liquid entry pressure of the CFM by 5 times. All the membranes have a similar permeate flux. The permeate flux of membranes is stable at 19.2 ± 1.2 kg/m2/h, and the salt rejection ratios remain above 99.98% at 78 ± 1 °C for 11 h. The pore size and porosity of membranes have an insignificant effect on the temperature distribution of membrane. The porosity and pore size of the skin layer have an insignificant effect on the mass transfer process of the CFM. The mass transfer process of the CFM is governed by the supporting layer.

Bioinspired Hybrid Micro/Nanostructure Composited Membrane with Intensified Mass Transfer and Antifouling for High Saline Water Membrane Distillation

Membrane distillation (MD) holds great promise for high-saline solution treatment, but it is typically impeded by the trade-off between the high mass transfer and antifouling properties of the membrane. Herein, a new MD utilized membrane with bioinspired micro/nanostructure (lotus leaf and fish gill) was constructed on commercial PP membrane, which can simultaneously enhance the permeation flux and antifouling in the hypersaline MD operation. On the basis of the classic nucleation theory and hydrodynamics simulation, the nanoscale structure can intensify the interfacial nanoscale turbulent flow and hinder the crystal deposition, which works like the fish gill. In addition, the optimized nanoscale feature size renders the membrane with the heterogeneous nucleation barrier very similar to the homogeneous system, which works like the lotus leaf and hinders the induced nucleation effectively. The microscale structure as the supporting platform of nanostructure can additionally enlarge the effective evaporative surface with superior hydrophobicity and then promote the permeation transfer through the membrane. The hybrid micro/nanostructures render the fabricated membrane with excellent high-permeation flux and significantly prolonged fouling induction time, which sheds light on a new approach for the development of ideal MD utilized membrane.

Sacrifice Template Strategy to the Fabrication of a Self-Cleaning Nanofibrous Membrane for Efficient Crude Oil-in-Water Emulsion Separation with High Flux

The superhydrophilic/underwater superoleophobic membrane materials have attracted considerable attention in oil/water separation. However, most materials are extremely susceptible to pollution during oil-water separation, which drastically restricts their widespread applications. Herein, a momordica-charantia-like nanofibrous membrane (MCNM) with underwater superoleophobic performance was fabricated through a sacrifice template strategy by the electrospinning solution of zeolitic imidazolate framework-8 (ZIF-8) and polyacrylonitrile particles. The opened voids and wrinkles left after removing the template of nanocrystals ZIF-8 not only increased the porosity and roughness of the as-prepared fibrous membrane but also tremendously improved the underwater superoleophobicity. Therefore, the as-prepared MCNM showed excellent self-cleaning performance toward crude oil under water, avoiding the decrease of the separation efficiency and flux caused by membrane fouling during oil-water separation. Meanwhile, the separation efficiency of various surfactant-stabilized oil-in-water emulsions was higher than 99.6% with a flux up to 1580 ± 30 L m^-2 h^-1 solely driven by gravity. Moreover, no obvious wrinkles and cracks were observed on the resulted nanofibrous membrane after the sand impact and bent testing. More importantly, the as-prepared MCNM still maintained exceptional underwater superoleophobicity in harsh environment (3.5 wt % NaCl, 4 M HCl, 50 °C hot water) even after ultrasound for 1 h. The robust mechanical and chemical stability makes the antifouling MCNM exhibit tremendous potential for practical applications in dealing with oily wastewater in the future.

Bioinspired superwetting fibrous skin with hierarchical roughness for efficient oily water separation

Developing superwetting membranes with interconnected pore and multi-scale roughness for efficient oily water separation is significant but challenging owing to the limitations of low water flux and membrane fouling. Herein, we report a scalable method to develop superwetting membranes with superhydrophilicity and underwater superoleophobicity for oily water separation. This novel approach, composed of electrospinning/electrospraying of polyacrylonitrile (PAN), was to fabricate rough sphere membrane substrate, followed by in-situ polymerization of dopamine/polyethyleneimine (DA/PEI) to positively charge the fiber skin and then subsequent immersed into the negatively charged Ludox solution to construct rough membrane surface via electrostatic attraction. Benefiting from the rough sphere surface of the fibrous skin layer, the resultant membrane displayed micro/nanostructured surfaces with intriguing in-air superhydrophilicity of 0° and underwater superoleophobicity of 166° as well as robust oil-proof pressure of 83.55 kPa. As a proof-of-concept, the resultant membrane achieved high water flux and oil rejection efficiency as well as fantastic durability and antifouling performance toward the separation of highly emulsified oily water. The integration of electrospinning/electrospraying with bioinspired method is also expected to fabricate superwetting sphere surface membrane with interconnected pores for other selective separation applications.