Robust dual-layered omniphobic electrospun membrane with anti-wetting and anti-scaling functionalised for membrane distillation application

High Pressure Resistance in Omniphobic Distillation Membranes with Re-entrant Nanostructures

We developed pressure-resistant omniphobic membranes that enable stable distillation of low-surface-tension liquids at applied pressures exceeding 15 bar. Membranes were synthesized by grafting re-entrant nanostructures onto porous alumina membranes, followed by hydrophobic modification. The membranes exhibited a high liquid entry pressure of 36.2 bar with water and withstood an applied pressure up to 15.5 bar with a low-surface-tension 15 wt % ethanol-water mixture. Simulations revealed that the enhanced wetting resistance is due to the presence of re-entrant structures, which facilitated a 220% increase in wetting pressure for the low-surface-tension liquid compared to a control membrane with cylindrical pores. We further demonstrated stable pressure-driven distillation of low-surface-tension liquids, achieving higher than 97% salt rejection. This work is the first demonstration of distillation membranes operating with low-surface-tension liquids under high applied pressures and provides critical validation of wettability theory under extreme pressures.

A review on recent progress and techniques used for fabricating superhydrophobic coatings derived from biobased materials

Superhydrophobic coatings with remarkable water repellence have emerged as an increasingly prominent field of research with the growth of the material engineering and coating industries. Superhydrophobic coatings address the requirements of several application areas with characteristics including corrosion resistance, drag reduction, anti-icing, anti-fogging, and self-cleaning properties. Furthermore, the range of applications for superhydrophobic coatings has been substantially broadened by the inclusion of key performance features such as flame retardancy, thermal insulation, resistance to water penetration, UV resistance, transparency, anti-reflection, and many more. Numerous research endeavours have been focused on biomimetic superhydrophobic materials because of their distinct surface wettability. To develop superhydrophobic coatings with a long lifespan, scientists have refined the processes of material preparation and selection. To accomplish water repellency, superhydrophobic coatings are usually fabricated using harmful fluorinated chemicals or synthetic polymers. Utilising materials derived from biomass offers a sustainable alternative that uses renewable resources in order to eliminate the consumption of these hazardous substances. This paper provides an insight of several researches reported on the construction of superhydrophobic coatings using biomass materials such as lignin, cellulose, chitosan and starch along with the techniques used for the constructing superhydrophobic coatings. This study is a useful resource that offers guidance on the selection of various biobased polymers for superhydrophobic coatings tailored to specific applications. The further part of the paper put a light on different application of superhydrophobic coatings employed in various disciplines and the future perspectives of the superhydrophobic coatings.

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.

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.

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.

Research Progress of Water Treatment Technology Based on Nanofiber Membranes

In the field of water purification, membrane separation technology plays a significant role. Electrospinning has emerged as a primary method to produce nanofiber membranes due to its straightforward, low cost, functional diversity, and process controllability. It is possible to flexibly control the structural characteristics of electrospun nanofiber membranes as well as carry out various membrane material combinations to make full use of their various properties, including high porosity, high selectivity, and microporous permeability to obtain high-performance water treatment membranes. These water separation membranes can satisfy the fast and efficient purification requirements in different water purification applications due to their high filtration efficiency. The current research on water treatment membranes is still focused on creating high-permeability membranes with outstanding selectivity, remarkable antifouling performance, superior physical and chemical performance, and long-term stability. This paper reviewed the preparation methods and properties of electrospun nanofiber membranes for water treatment in various fields, including microfiltration, ultrafiltration, nanofiltration, reverse osmosis, forward osmosis, and other special applications. Lastly, various antifouling technologies and research progress of water treatment membranes were discussed, and the future development direction of electrospun nanofiber membranes for water treatment was also presented.

Coaxial Electrospun Nanofibrous Membranes for Enhanced Water Recovery by Direct Contact Membrane Distillation

Membrane distillation (MD) is an emerging technology for water recovery from hypersaline wastewater. Membrane scaling and wetting are the drawbacks that prevent the widespread implementation of the MD process. In this study, coaxially electrospun polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) nanofibrous membranes were fabricated with re-entrant architecture and enhanced hydrophobicity/omniphobicity. The multiscale roughness was constructed by incorporating Al2O3 nanoparticles and 1H, 1H, 2H, 2H Perfluorodecyltriethoxysilane in the sheath solution. High resolution transmission electron microscopy (HR-TEM) could confirm the formation of the core-sheath nanofibrous membranes, which exhibited a water contact angle of ~142.5° and enhanced surface roughness. The membrane displayed a stable vapor flux of 12 L.m−2.h−1 (LMH) for a 7.0 wt.% NaCl feed solution and no loss in permeate quality or quantity. Long-term water recovery from 10.5 wt.% NaCl feed solution was determined to be 8−10 LMH with >99.9% NaCl rejection for up to 5 cycles of operation (60 h). The membranes exhibited excellent resistance to wetting even above the critical micelle concentration (CMC) for surfactants in the order sodium dodecyl sulphate (SDS) (16 mM) > cetyltrimethylammonium bromide (CTAB) (1.5 mM) > Tween 80 (0.10 mM). The presence of salts further deteriorated membrane performance for SDS (12 mM) and Tween-80 (0.05 mM). These coaxial electrospun nanofibrous membranes are robust and can be explored for long-term applications.

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.

Janus Photothermal Membrane as an Energy Generator and a Mass-Transfer Accelerator for High-Efficiency Solar-Driven Membrane Distillation

Membrane distillation (MD) is an emerging membrane-based evaporation technology with great promise for the desalination and separation industries. However, its widespread application still depends on substantial development to increase the distillation flux, reduce the energy consumption, and extend the lifespan of the membrane. Herein, we report for the first time the integration of multiple functions, that is, energy-saving, flux-enhancing, and anti-fouling properties, into a single membrane. Such a membrane was fabricated by coating the top surface of a poly(vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) nanofibrous mat with photothermal and hydrophobic graphitic carbon spheres and subsequently coating the bottom surface with a hydrophilic polydopamine layer, yielding a novel Janus photothermal membrane (JPTM). Owing to the high photothermal efficiency and accelerated mass transport across the membrane, the JPTM demonstrated an excellent desalination performance when assembled into a solar-driven MD system, with a distillation flux of 1.29 kg m^-2 h^-1, which is 10 times higher than that of the conventional un-modified PVDF-HFP membrane, requiring only 1 kW m^-2 solar illumination as the energy input.