Reinforced superhydrophobic membrane coated with aerogel-assisted polymeric microspheres for membrane distillation

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.

Desalination technologies, membrane distillation, and electrospinning, an overview

Water is a critical component for humans to survive, especially in arid lands or areas where fresh water is scarce. Hence, desalination is an excellent way to effectuate the increasing water demand. Membrane distillation (MD) technology entails a membrane-based non-isothermal prominent process used in various applications, for instance, water treatment and desalination. It is operable at low temperature and pressure, from which the heat demand for the process can be sustainably sourced from renewable solar energy and waste heat. In MD, the water vapors are gone through the membrane's pores and condense at permeate side, rejecting dissolved salts and non-volatile substances. However, the efficacy of water and biofouling are the main challenges for MD due to the lack of appropriate and versatile membrane. Numerous researchers have explored different membrane composites to overcome the above-said issue, and attempt to develop efficient, elegant, and biofouling-resistant novel membranes for MD. This review article addresses the 21st-century water crises, desalination technologies, principles of MD, the different properties of membrane composites alongside compositions and modules of membranes. The desired membrane characteristics, MD configurations, role of electrospinning in MD, characteristics and modifications of membranes used for MD are also highlighted in this review.

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.

A review on fabrication, characterization of membrane and the influence of various parameters on contaminant separation process

In most developing countries, the availability of drinking water is a major problem. This creates the need for treatment of wastewater, reusability of water, etc. The membrane technology has its place in the market for treating such water. This review compares polymeric membrane fabrication techniques, characteristics, and factors responsible for effective membrane separation for different materials. Although extensive knowledge is available on membrane fabrication, fabricating a membrane is still more challenging, which is more prone to antifouling properties. The competency in different fabrication methods like phase inversion, interfacial polymerization, stretching, track etching and electrospinning are elucidated in the current study. Further, the challenges and adaptability of different application fabrication methods are studied. Important surface parameters like surface wettability, roughness, surface tension, pore size, surface charge, surface functional group and pure water flux are analyzed for different polymeric membranes. In addition, the properties responsible for fouling the membrane are also covered in detail. Flow direction and velocity are the main factors that characterize a membrane's antifouling nature. Antifouling separation can still be achieved by characterizing feed properties such as pH, temperature, diffusivity, ion concentration, and surface content. Understanding fouling properties is a key to progress in membrane technology to develop an effective membrane separation.

Design and Study of a Photo-Switchable Polymeric System in the Presence of ZnS Nanoparticles under the Influence of UV Light Irradiation

Recent progress in the field of photosensitive materials has prompted a need to develop efficient methods to synthesize materials with basic intermolecular architectural designs and novel properties. Accordingly, in this work we design and study a photoactive polymer as a photo-switchable polymeric system in the presence and absence of ZnS nanoparticles (average size < 10 nm) at 5 wt.%. The influence of UV light irradiation on its properties were also studied. The photoactive block copolymer was obtained from styrene (S) and methyl methacrylate (MMA) as monomers and 1-(2-hydroxyethyl)-3,3-dimethylindoline-6-nitrobenzopyran (SP) was grafted to the block copolymer backbone as a photochromic agent. Furthermore, the incorporation of ZnS (NPs) as photo-optical switch component into the system enhances the purple colored photo-emission, with the open form of the spiropyran derivative (merocyanine, MC). The ZnS stabilize the isomeric equilibrium in the MC interconversion of the photochromic agent. The photo-switchable properties of the PS-b-PMMA-SP in the presence of ZnS (NPs) were examined using UV-VIS spectroscopy, Photoluminescence (PL) spectroscopy, optical fluorescence and scanning electronic microscopy (SEM-EDX.). The observed changes in the absorbance, fluorescence and morphology of the system were associated to the reversible interconversion of the two states of the photochromic agent which regulates the radiative deactivation of the luminescent ZnS NPs component. After UV irradiation the photoactive polymer becomes purple in color. Therefore, these basic studies can lead to the development of innovative functional and nanostructured materials with photosensitive character as photosensitive molecular switches.

Noninvasive Real-Time Monitoring of Wetting Progression in Membrane Distillation Using Impedance Spectroscopy

Membrane distillation (MD) is a promising technology for the treatment of high salinity wastewater using a hydrophobic membrane; however, the occurrence of wetting due to surfactants in polluted or low surface tension liquid impedes MD application. Common monitoring approaches, such as conductivity and flux measurement, cannot explain the wetting phenomenon that occurs during the wetting process in detail. Recently, impedance spectroscopy has been proposed for early wetting detection, as it depends on the change of water/air composition in the membrane pores. An earlier and larger variation was observed with precise signal detection. In this study, we proposed an analytical approach to estimate the wetting front, which is the average feed intrusion distance, by the impedance value recorded in real-time operation. With this proposed approach, the wetting mechanism in the presence of a surfactant and the effect of pore size on a commercial polyvinylidene fluoride membrane could be quantified, which cannot be explained in detail using conductivity and flux measurements.

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 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.

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.

Facile fabrication of a Janus mesh for water fluid unidirectional transportation

A Janus membrane/mesh is a type of functional membrane/mesh composed of opposing wetting properties formed into a single layer in order to achieve novel properties. Janus membranes/meshes have attracted increasing attention from materials scientists due to their promising applications in the fields of microfluid transportation, water-oil separation and cleaning energy applications. Herein, we report a simple method to fabricate a Janus mesh by combining opposite wettability functions into one copper mesh substrate. The superhydrophilicity is achieved by chemical etching and the superhydrophobicity is fabricated by hydrophobic SiO₂ nanoparticle spraying. Due to its special composition and structure, the prepared mesh demonstrates distinct wetting properties on its two sides. Meanwhile, aqueous fluids can pass through the mesh from the hydrophobic side to the hydrophilic side spontaneously, whilst being blocked by the mesh when coming from the other direction. This unique property can realize unidirectional transportation of water fluids. The mechanism of the unique property based on Janus wettability is proposed and the stability of the prepared Janus mesh was also tested. The prepared Janus mesh can be used in the fields of microtidal energy, the chemical industry and in astronautics, demonstrating promising practical prospects.

Enhancing the mechanical and thermal properties of waterborne polyurethane composites with thermoset epoxy resin microspheres

A facile way to prepare WPU with good mechanical and thermal properties by adding epoxy microspheres.

Omniphobic re-entrant PVDF membrane with ZnO nanoparticles composite for desalination of low surface tension oily seawater

In this study, an omniphobic membrane was fabricated by electrospraying fluorinated zinc oxide (ZnO) nanoparticles (NPs) mixed with polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) on the surface of an organosilane functionalized polyvinylidene difluoride (PVDF) membrane. Our results revealed that the functionalized ZnO NPs membrane exhibited a rough hierarchical re-entrant morphology with low surface energy which allowed it to achieve high omniphobic characteristics. It was observed that the addition of 30% ZnO (w/w of PVDF-HFP) was found to be optimal and imparted a high repulsive characteristic. The optimized PVDF/ZnO(30)/FAS/PVDF-HFP referred as cPFP-30Z membrane exhibited a high contact angle values of 159.0 ± 3.1°, 129.6 ± 2.2°, 130.4 ± 4.1° and 126.1 ± 1.2° for water, sodium dodecyl sulfate (SDS) saline solution (0.3 mM SDS in 3.5% NaCl), ethanol, and vegetable oil, respectively. The low surface energy and high surface roughness (Ra) of optimised membrane was assessed as 0.78 ± 0.14 mN m^-1 and 1.37 μm, respectively. Additionally, in contrast with the commercial PVDF membrane, the cPFP-30Z membrane exhibited superior anti-wetting/anti-fouling characteristics and high salt rejection performance (>99%) when operated with a saline oil solution (0.015 v/v) and SDS (0.4 mM) feed solutions.

Robust Superhydrophobic Membrane for Membrane Distillation with Excellent Scaling Resistance

We report in this study a scalable and controllable approach for fabricating robust and high-performance superhydrophobic membranes for membrane distillation (MD). This novel approach combines electro-co-spinning/spraying (ES²) with chemical vapor welding and enables the formation of robust superhydrophobic (r-SH) membranes that are mechanically strong, highly porous, and robustly superhydrophobic. Compared with superhydrophobic membranes obtained using surface deposition of fluorinated nanoparticles, the r-SH membranes have more robust wetting properties and higher vapor permeability in MD. MD scaling experiments with sodium chloride and gypsum show that the r-SH membrane is highly effective in mitigating mineral scaling. Finally, we also discuss the mechanism of scaling resistance enabled by superhydrophobic membranes with a highlight on the roles of the surface-bound air layer in reducing the crystal-membrane contact area, nucleation propensity, and ion-membrane contact time.