Self-roughened omniphobic coatings on nanofibrous membrane for membrane distillation

Constructing spherical-beads-on-string structure of electrospun membrane to achieve high vapor flux in membrane distillation

Hydrophobic membranes with a reentrant-like structure have shown high hydrophobicity and high anti-wetting properties in membrane distillation (MD). Here, PVDF spherical-beads-on-string (SBS) fibers were electrospun on nonwoven fabric and used in the MD process. Such a reentrant-like structure was featured with fine fibers, a low ratio of bead length to bead diameter, and high bead frequency. It was revealed that the SBS-structured membranes exhibited an exceptional capability for vapor flux, due to the formation of a network of more interconnected macropores than that of fibers and fusiform-beads-on-string structures, ensuring unimpeded vapor diffusion. In the desalination of formulated seawater (3.5 wt.% NaCl solution), a vapor flux of 61 ± 3 kg m-2 h-1 with a salt rejection of >99.98 % was achieved at a feed temperature of 60 °C. Furthermore, this SBS structured membrane showed satisfactory seawater desalination performance with a stable flux of 40 kg m-2 h-1 over a 27 h MD process. These findings suggest a viable approach for fabricating SBS-structured membranes that significantly enhance vapor flux in MD for desalination applications. Besides, the hydrophobic membranes with SBS structure can be prepared by single-step electrospinning, and it is facile to scale-up manufacture. This strategy holds promise for advancing the development of high-performance MD membranes tailored for efficient seawater desalination processes.

Enhanced Anti-Wetting Methods of Hydrophobic Membrane for Membrane Distillation

Increasing issues of hydrophobic membrane wetting occur in the membrane distillation (MD) process, stimulating the research on enhanced anti-wetting methods for membrane materials. In recent years, surface structural construction (i.e., constructing reentrant-like structures), surface chemical modification (i.e., coating organofluorides), and their combination have significantly improved the anti-wetting properties of the hydrophobic membranes. Besides, these methods change the MD performance (i.e., increased/decreased vapor flux and increased salt rejection). This review first introduces the characterization parameters of wettability and the fundamental principles of membrane surface wetting. Then it summarizes the enhanced anti-wetting methods, the related principles, and most importantly, the anti-wetting properties of the resultant membranes. Next, the MD performance of hydrophobic membranes prepared by different enhanced anti-wetting methods is discussed in desalinating different feeds. Finally, facile and reproducible strategies are aspired for the robust MD membrane in the future.

Recent advance in biomass membranes: Fabrication, functional regulation, and antimicrobial applications

Both inorganic and polymeric membranes have been widely applied for antimicrobial applications. However, these membranes exhibit low biocompatibility, weak biodegradability, and potential toxicity to human being and environment. Biomass materials serve as excellent candidates for fabricating functional membranes to address these problems due to their unique physical, chemical, and biological properties. Here we present recent progress in the fabrication, functional regulation, and antimicrobial applications of various biomass-based membranes. We first introduce the types of biomass membranes and their fabrication methods, including the phase inversion, vacuum filtration, electrospinning, layer-by-layer self-assembly, and coating. Then, the strategies on functional regulation of biomass membranes by adding 0D, 1D, and 2D nanomaterials are presented and analyzed. In addition, antibacterial, antifungal, and antiviral applications of biomass-based functional membranes are summarized. Finally, potential development aspects of biomass membranes are discussed and prospected. This comprehensive review is valuable for guiding the design, synthesis, structural/functional tailoring, and sustainable utilization of biomass membranes.

Tuning PVDF Membrane Porosity and Wettability Resistance via Varying Substrate Morphology for the Desalination of Highly Saline Water

Here, we report the fabrication of a series of highly efficient polyvinylidene fluoride (PVDF) membranes via substrate morphology variations. A wide range of sandpaper grit sizes (150-1200) were utilized as casting substrates. The effect of the penetration of abrasive particles present on the sandpapers on the casted polymer solution was tuned, and the impact of these particles on porosity, surface wettability, liquid entry pressure and morphology were investigated. The membrane distillation performance of the developed membrane on sandpapers was evaluated for the desalination of highly saline water (70,000 ppm). Interestingly, the utilization of cheap and widely available sandpapers as a substrate for casting can not only help in tuning the MD performance, but also in producing highly efficient membranes with stable salt rejection (up to 100%) and a 210% increase in the permeate flux over 24 h. The findings in this study will help in delineating the role of substrate nature in controlling the produced membrane characteristics and performance.

Fluoropolymer Membranes for Membrane Distillation and Membrane Crystallization

Fluoropolymer membranes are applied in membrane operations such as membrane distillation and membrane crystallization where hydrophobic porous membranes act as a physical barrier separating two phases. Due to their hydrophobic nature, only gaseous molecules are allowed to pass through the membrane and are collected on the permeate side, while the aqueous solution cannot penetrate. However, these two processes suffer problems such as membrane wetting, fouling or scaling. Membrane wetting is a common and undesired phenomenon, which is caused by the loss of hydrophobicity of the porous membrane employed. This greatly affects the mass transfer efficiency and separation efficiency. Simultaneously, membrane fouling occurs, along with membrane wetting and scaling, which greatly reduces the lifespan of the membranes. Therefore, strategies to improve the hydrophobicity of membranes have been widely investigated by researchers. In this direction, hydrophobic fluoropolymer membrane materials are employed more and more for membrane distillation and membrane crystallization thanks to their high chemical and thermal resistance. This paper summarizes different preparation methods of these fluoropolymer membrane, such as non-solvent-induced phase separation (NIPS), thermally-induced phase separation (TIPS), vapor-induced phase separation (VIPS), etc. Hydrophobic modification methods, including surface coating, surface grafting and blending, etc., are also introduced. Moreover, the research advances on the application of less toxic solvents for preparing these membranes are herein reviewed. This review aims to provide guidance to researchers for their future membrane development in membrane distillation and membrane crystallization, using fluoropolymer materials.

Structural design of the electrospun nanofibrous membrane for membrane distillation application: a review

Although membrane distillation (MD) is a promising technology for water desalination and industrial wastewater treatment, the MD process is not widely applied in the global water industry due to the lack of a suitable membrane for the MD process. The design and appropriate manufacture are the most important factors for MD membrane optimization. The well-designed porous structure, superhydrophobic surface, and pore-wetting prevention of the membrane are vital properties of the MD membrane. Nowadays, electrospinning that is capable of manufacturing membranes with superhydrophobic or omni phobic properties is considered a promising technology. Electrospun nanofibrous membranes (ENMs) possess the characteristics of cylindrical morphology, re-entrant structure, and easy-shaping for a specific purpose, benefiting the membrane design and modification. Based on that, this review investigates the current state and future progress of the superhydrophobic, multi-layer, and omniphobic ENMs manufactured with various structural designs for seawater desalination and wastewater purification. We expect that this paper will provide some recommendations and guidance for further fabrication research and the configuration design of ENMs in the MD process for seawater desalination and wastewater purification.

Facile fabrication of self-roughened surfaces for superhydrophobic coatings via polarity-induced phase separation strategy

Rough structures have gained increasing attention since they are essential for surfaces with special wettability, which can be used for various applications. It is still a challenge to find a low-cost and simple way to fabricate rough surfaces despite extensive efforts. Herein, we report a facile strategy to fabricate self-roughened surfaces based on polarity-induced phase separation. The strategy relies on the migration of flexible chains of the nonpolar polysiloxane to airside, driven by surface tension and polarity difference with the polar crosslinker, which forms a self-roughened surface with numerous protrusions. It is worth noting that this strategy does not require strict control of procedures, since it is insensitive to environmental changes unlike other phase separation methods, as shown by the results of systematic studies on several key parameters. Modified fabrics and coatings exhibit excellent superhydrophobicity with a water contact angle higher than 160°. Moreover, due to the strong hydrogen bonds formed by the polar urea groups of the crosslinker with substrates, the abrasion resistance of the coating is significantly enhanced. It is believed that the proposed novel and facile strategy will be a promising candidate for industrial manufacturing.

Long-Chain PFASs-Free Omniphobic Membranes for Sustained Membrane Distillation

Omniphobic membranes possessing high wetting resistance have been created for the treatment of challenging hypersaline feedwaters with low surface tension through membrane distillation (MD). However, virtually all such membranes are fabricated with long-chain per- and polyfluoroalkyl substances (PFASs, ≥8 fluorinated carbons). The environmental risks and high bioaccumulation potential of long-chain PFASs have raised increasing concerns. Developing highly wetting-resistant MD membranes while avoiding the use of long-chain PFASs is essential to improve the viability of MD for resilient and sustainable water purification. We demonstrate that MD membranes with exceptional wetting resistance can be designed through the combination of hierarchically structured membranes consisting of re-entrant texture at different length scales and (ultra)short-chain fluorocarbons, which have lower acute toxicity and bioaccumulation potentials than long-chain PFASs. Our hierarchically structured membrane with three-tier micro/nanostructure fabricated with short-chain fluorocarbon possesses superior wetting resistance, which is comparable to or higher than the long-chain PFASs-based omniphobic membranes reported in the literature. Furthermore, the hierarchically structured membranes fabricated with ultrashort-chain fluorocarbons display improved wetting resistance against feedwaters with low surface tension. Our findings indicate that long-chain PFASs are not required when designing wetting-resistant membranes and that the balance between sustainability and wetting resistance should be tailored to the wetting potential of the feedwater.

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.

Water-energy nexus: desalination technologies and renewable energy sources

Rapid population growth and industrialization have contributed to a dramatic decline in the supply of freshwater. As a result, desalination is an important choice to solve the global problem of water scarcity. Nevertheless, the hyper-saline by-product, the high capital costs, and the high energy demands currently met by fossil fuels are key obstacles to the widespread adoption of desalination systems. Furthermore, desalination plants powered by fossil fuels have negative environmental impacts due to greenhouse gases (GHGs) emissions. In contrast to fossil fuels, renewable energy is abundant and clean and is therefore a promising alternative for powering desalination plants. This is why the water-energy nexus is a crucial step towards a sustainable future. Therefore, the integration of renewable energy sources (RES) into desalination is very important. The main objective of this review to analyze and evaluate desalination technologies (thermal-based and membrane-based) and RES (solar, wind, hydropower, geothermal, and biomass) that could be combined as an integrated process. Social-economic factors, environmental concerns, current challenges, and future research areas for both desalination and RES are discussed.

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.

Flexible and Superhydrophobic Composites with Dual Polymer Nanofiber and Carbon Nanofiber Network for High-Performance Chemical Vapor Sensing and Oil/Water Separation

Polymer nanofiber composites with superhydrophobicity are promising for the chemical vapor sensing or oil/water separation, but it remains challenging to develop superhydrophobic, anticorrosive, and durable nanofiber composites that can achieve both the organic solvent vapor detection and oil (organic solvent)/water separation with high separation flux and excellent recyclability. Here, a flexible, stretchable, and superhydrophobic/superoleophilic nanofiber composite membrane with excellent photothermal conversion performance is fabricated by decorating carbon nanofibers (CNFs) with a hollow structure onto the polyurethane nanofibers and subsequent polydimethylsiloxane (PDMS) modification. The combination of CNFs and PDMS greatly improves the membrane's tensile strength and Young's modulus without sacrificing its stretchability. The dual polymer nanofiber and CNF network are beneficial to the chemical vapor or liquid diffusion into the membrane and thus can be used for high-performance chemical vapor sensing and oil/water separation. The nanofiber composite is responsive to different organic vapors with a low detection limit and good selectivity. Also, the material can achieve fast oil/water separation with the oil (dichloromethane) permeate flux as high as 6577.3 L m^-2 h^-1. In addition, the separation flux and efficiency remain stable during the 30 separated oil/water separation tests, exhibiting excellent recyclability.

Macro-corrugated and nano-patterned hierarchically structured superomniphobic membrane for treatment of low surface tension oily wastewater by membrane distillation

A hierarchically assembled superomniphobic membrane with three levels of reentrant structure was designed and fabricated to enable effective treatment of low surface tension, hypersaline oily wastewaters using direct contact membrane distillation (DCMD). The overall structure is a combination of macro corrugations obtained by surface imprinting, with the micro spherulites morphology achieved through the applied phase inversion method and nano patterns obtained by fluorinated Silica nanoparticles (SiNPs) coating. This resulted in a superomniphobic membrane surface with remarkable anti-wetting properties repelling both high surface tension water and low surface tension oils. Measurements of contact angle (CA) with DI water, an anionic surfactant, oil, and ethanol demonstrated a robust wetting resistance against low surface tension liquids showing both superhydrophobicity and superoleophobicity. CA values of 160.8 ± 2.3° and 154.3 ± 1.9° for water and oil were obtained, respectively. Calculations revealed a high liquid-vapor interface for the fabricated membrane with more than 89% of the water droplet contact area being with air pockets entrapped between adjacent SiNPs and only 11% come into contact with the solid membrane surface. Moreover, the high liquid-vapor interface imparts the membrane with high liquid repellency, self-cleaning and slippery effects, characterized by a minimum droplet-membrane interaction and complete water droplet bouncing on the surface within only 18 ms. When tested in DCMD with synthetic hypersaline oily wastewaters, the fabricated superomniphobic membrane demonstrated stable, non-wetting MD operation over 24 h, even at high concentrations of low surface tension 1.0 mM Sodium dodecyl sulfate and 400 ppm oil, potentially offering a sustainable option for treatment of low surface tension oily industrial wastewater.

Omniphobic Nanofibrous Membrane with Pine-Needle-Like Hierarchical Nanostructures: Toward Enhanced Performance for Membrane Distillation

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO₂) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO₂ nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.

Desalination brine disposal methods and treatment technologies - A review

Brine, also known as concentrate, is the by-product of the desalination process that has an adverse impact on the environment due to its high salinity. Hence, viable and cost-effective brine management systems are needed to reduce environmental pollution. Currently, various disposal methods have been practiced, including surface water discharge, sewer discharge, deep-well injection, evaporation ponds and land application. However, these brine disposal methods are unsustainable and restricted by high capital costs and non-universal application. Nowadays, brine treatment is considered one of the most promising alternatives to brine disposal, since treatment results in the reduction of environmental pollution, minimization of waste volume and production of freshwater with high recovery. This review article evaluates current practices in brine management, including disposal methods and treatment technologies. Based upon the side-by-side comparison of technologies, a brine treatment technology framework is introduced to outline the Zero Liquid Discharge (ZLD) approach through high freshwater recovery and wastewater volume minimization. Furthermore, an overview of brine characteristics and its sources, as well as its negative impact on the environment is discussed. Finally, the paper highlights future research areas for brine treatment technologies aiming to enhance the effectiveness and viability of desalination.

Recent advances in membrane development for treating surfactant- and oil-containing feed streams via membrane distillation

Membrane distillation (MD) has been touted as a promising technology for niche applications such as desalination of surfactant- and oil-containing feed streams. Hitherto, the deployment of conventional hydrophobic MD membranes for such applications is limited and unsatisfactory. This is because the presence of surfactants and oils in aqueous feed streams reduces the surface-tension of these media significantly and the attachment of these contaminants onto hydrophobic membrane surfaces often leads to membrane fouling and pore wetting, which compromises on the quantity and quality of water recovered. Endowing MD membranes with surfaces of special wettabilities has been proposed as a strategy to combat membrane fouling and pore wetting. This involves the design of local kinetic energy barriers such as multilevel re-entrant surface structures, surfaces with ultralow surface-energies, and interfacial hydration layers to impede transition to the fully-wetted Wenzel state. This review critiques the state-of-the-art fabrication and surface-modification methods as well as practices used in the development of omniphobic and Janus MD membranes with specific emphasis on the advances, challenges, and future improvements for application in challenging surfactant- and oil-containing feed streams.

Design of omniphobic interfaces for membrane distillation - A review

Membrane distillation (MD) has a great potential in treating high salinity industrial wastewater due to its unique characteristics. Nevertheless, the implementation of MD for industrial wastewater reclamation must be conducted with precaution because low-surface-tension contaminates in feed solutions may easily wet the membranes. In recent years, omniphobic membranes that exhibit strong repellence towards liquids with a wide range of surface tensions have been proposed as a promising solution to deal with the wetting problem. In this paper, we aim to provide a comprehensive review of omniphobic interfaces and illustrate their fundamental working principles, innovative design approaches and novel applications on membrane distillation. The review may provide insights in designing stable solid-liquid-vapor interfaces and serve as a guidance for the development of robust anti-wetting membranes for industrial wastewater reclamation via membrane distillation.

A review of membrane development in membrane distillation for emulsified industrial or shale gas wastewater treatments with feed containing hybrid impurities

Investigations on membrane materials for membrane distillation (MD) and its applications have been ongoing since the 1990s. However, a lack of materials that produce robustly stable and up-to-the-mark membranes for MD for different industrial applications remains an ongoing problem. This paper provides an overview of materials developed for MD applications. Although key aspects of published articles reviewed in this paper pertain to MD membranes synthesized for desalination, future MD can also be applied to organic wastewater containing surfactants with inorganic compounds, either with the help of hybrid treatment processes or with customized membrane materials. Many industrial discharges produce effluents at a very high temperature, which is an available driving force for MD. However, there remains a lack of cost-effective membrane materials. Amphiphobic and omniphobic membranes have recently been developed for treating emulsified and shale gas produced water, but the problem of organic fouling and pore wetting remains a major challenge, especially when NaCl and other inorganic impurities are present, which further deteriorate separation performance. Therefore, further advancements in materials are required for the treatment of emulsified industrial wastewater containing surfactants, salts, and for oil or shale gas wastewater for its commercialized reuse. Integrated MD systems, however, may represent a major change in shale gas wastewater and emulsified wastewater that are difficult to treat.

Trade-off in membrane distillation with monolithic omniphobic membranes

Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance. However, a design framework for MD membrane remains incomplete, due to the complexity of omniphobic membrane fabrication and the lack of fundamental relationship between wetting resistance and water vapor permeability. Here we present a particle-free approach that enables rapid fabrication of monolithic omniphobic membranes for MD desalination. Our monolithic omniphobic membranes display excellent wetting resistance and water purification performance in MD desalination of hypersaline feedwater containing surfactants. We identify that a trade-off exists between wetting resistance and water vapor permeability of our monolithic MD membranes. Utilizing membranes with tunable wetting resistance and permeability, we elucidate the underlying mechanism of such trade-off. We envision that our fabrication method as well as the mechanistic insight into the wetting resistance-vapor permeability trade-off will pave the way for smart design of MD membranes in diverse water purification applications.