Biomimetic hydrophobic membrane: A review of anti-wetting properties as a potential factor in membrane development for membrane distillation (MD)

Treatment of wastewater from oil palm industry in Malaysia using polyvinylidene fluoride-bentonite hollow fiber membranes via membrane distillation system

Membrane distillation (MD) is gaining increasing recognition within membrane-based processes for palm oil mill effluent (POME) treatment. This study aims to alter the physicochemical characteristics of polyvinylidene fluoride (PVDF) membranes through the incorporation of bentonite (B) at varying weight concentrations (ranging from 0.25 wt% to 1.0 wt%). Characterization was conducted to evaluate changes in morphology, thermal stability, surface characteristics and wetting properties of the resulting membranes. The resulting membranes were also tested using direct contact membrane distillation (DCMD) with POME as the feed solution, aiming to generate high-purity water. Results indicated that the PVDF-0.3B and PVDF-0.5B membranes achieved the highest water vapor flux. The finger-like structure and macrovoids present in these membranes aid in minimizing mass resistance during vapor transport and enhancing permeate flux. All membranes demonstrated exceptional performance in removing contaminants, eliminating total dissolved solids (TDS) and achieving over 99% rejection of chemical oxygen demand (COD), nitrate nitrogen (NN), color, and turbidity from the feed solution. The permeate water analysis showed that the PVDF-0.3B membrane had superior removal efficiency and met the standards set by the local Department of Environment (DOE). The PVDF-0.3B membrane was chosen as the preferred option because of its consistent flux and high removal efficiency. This study demonstrated that incorporating bentonite into PVDF membranes significantly enhanced their properties and performance for POME treatment.

An Opportunity for Synergizing Desalination by Membrane Distillation Assisted Reverse-Electrodialysis for Water/Energy Recovery

Industry, agriculture, and a growing population all have a major impact on the scarcity of clean-water. Desalinating or purifying contaminated water for human use is crucial. The combination of thermal membrane systems can outperform conventional desalination with the help of synergistic management of the water-energy nexus. High energy requirement for desalination is a key challenge for desalination cost and its commercial feasibility. The solution to these problems requires the intermarriage of multidisciplinary approaches such as electrochemistry, chemical, environmental, polymer, and materials science and engineering. The most feasible method for producing high-quality freshwater with a reduced carbon footprint is demanding incorporation of industrial low-grade heat with membrane distillation (MD). More precisely, by using a reverse electrodialysis (RED) setup that is integrated with MD, salinity gradient energy (SGE) may be extracted from highly salinized MD retentate. Integrating MD-RED can significantly increase energy productivity without raising costs. This review provides a comprehensive summary of the prospects, unresolved issues, and developments in this cutting-edge field. In addition, we summarize the distinct physicochemical characteristics of the membranes employed in MD and RED, together with the approaches for integrating them to facilitate effective water recovery and energy conversion from salt gradients and freshwater.

Advancements in Inorganic Membrane Filtration Coupled with Advanced Oxidation Processes for Wastewater Treatment

Membrane filtration is an effective water recycling and purification technology to remove various pollutants in water. Inorganic membrane filtration (IMF) technology has received widespread attention because of its unique high temperature and corrosion resistance. Commonly used inorganic membranes include ceramic membranes and carbon-based membranes. As novel catalytic inorganic membrane processes, IMF coupled with advanced oxidation processes (AOPs), can realize the separation and in situ degradation of pollutants, thus mitigating membrane contamination. In this paper, the types and performance of IMF are discussed. The influencing factors of inorganic membranes in practical wastewater treatment are summarized. The applications, advantages, and disadvantages of the coupled process of IMF and AOPs are summarized and outlined. Finally, the challenges and prospects of IMF and IMF coupled with AOPs are presented, respectively. This contributes to the design and development of coupled systems of membrane filtration with inorganic materials and IMF coupled with AOPs for practical wastewater treatment.

Plasma-Induced Superhydrophobicity as a Green Technology for Enhanced Air Gap Membrane Distillation

Superhydrophobicity has only recently become a requirement in membrane fabrication and modification. Superhydrophobic membranes have shown improved flux performance and scaling resistance in long-term membrane distillation (MD) operations compared to simply hydrophobic membranes. Here, we introduce plasma micro- and nanotexturing followed by plasma deposition as a novel, dry, and green method for superhydrophobic membrane fabrication. Using plasma micro- and nanotexturing, commercial membranes, both hydrophobic and hydrophilic, are transformed to superhydrophobic featuring water static contact angles (WSCA) greater than 150° and contact angle hysteresis lower than 10°. To this direction, hydrophobic polytetrafluoroethylene (PTFE) and hydrophilic cellulose acetate (CA) membranes are transformed to superhydrophobic. The superhydrophobic PTFE membranes showed enhanced water flux in standard air gap membrane distillation and more stable performance compared to the commercial ones for at least 48 h continuous operation, with salt rejection >99.99%. Additionally, their performance and high salt rejection remained stable, when low surface tension solutions containing sodium dodecyl sulfate (SDS) and NaCl (down to 35 mN/m) were used, showcasing their antiwetting properties. The improved performance is attributed to superhydrophobicity and increased pore size after plasma micro- and nanotexturing. More importantly, CA membranes, which are initially unsuitable for MD due to their hydrophilic nature (WSCA ≈ 40°), showed excellent performance with stable flux and salt rejection >99.2% again for at least 48 h, demonstrating the effectiveness of the proposed method for wetting control in membranes regardless of their initial wetting properties.

Anti-Wetting Performance of an Electrospun PVDF/PVP Membrane Modified by Solvothermal Treatment in Membrane Distillation

Membrane distillation (MD) is attractive for water reclamation due to the fact of its unique characteristics. However, membrane wetting becomes an obstacle to its further application. In this paper, a novel hydrophobic polyvinylidene fluoride/poly(vinyl pyrrolidone) (PVDF/PVP) membrane was fabricated by electrospinning and solvothermal treatment. The electrospun membranes prepared by electrospinning showed a multilevel interconnected nanofibrous structure. Then, a solvothermal treatment introduced the micro/nanostructure to the membrane with high roughness (Ra = 598 nm), thereby the water contact angle of the membrane increased to 158.3 ± 2.2°. Owing to the superior hydrophobicity, the membrane presented high resistance to wetting in both NaCl and SDS solutions. Compared to the pristine PVDF membrane, which showed wetting with a flux decline (120 min for 0.05 mM surfactant solution treatment), the prepared membrane showed outstanding stability over 600 min, even in 0.2 mM surfactant solutions. These results confirm a simple method for anti-wetting hydrophobic membrane preparation, which presented universal significance to direct contact membrane distillation (DCMD) for industrial application.

Theoretical Investigation of Wenzel and Cassie Wetting States on Porous Films and Fiber Meshes

In this study, the wetting phenomenon on a thin, porous film covering a substrate was investigated on the basis of a classical equilibrium theory. The equilibrium contact angles of the Wenzel, hydrophobic Cassie, and hydrophilic Cassie states and the transition points between them were derived as functions of parameters, such as the porosity and specific surface area of the porous film. These expressions were applied to describe wetting on fiber mats/meshes. The equilibrium contact angles and transition points of the three wetting states on the fiber mesh covering the substrate are expressed as functions of parameters, such as fiber radius, roughness factor, and volume porosity of the fiber mesh. A fiber mesh can attain superhydrophobicity when it is composed of thin hydrophobic fibers with surface roughness and has an appropriate volume porosity.

Formation and Antibacterial Activity of AlOOH/Ag Composite Coating on Macroporous α-Al2O3 Ceramics

In this study, the modification of macroporous α-Al2O3 ceramics with AlOOH nanostructures impregnated with silver particles is carried out using bicomponent Al/Ag nanoparticles obtained by the simultaneous electrical explosion of Al and Ag wires. Nanoparticle suspension impregnation of porous ceramics followed by oxidation with water is shown to lead to the formation of a continuous AlOOH nanosheet coating on the ceramic surface, with silver releasing on the surface of nanosheets in the form of individual particles sized 5–30 nm. Modified with AlOOH/Ag nanostructures, macroporous α-Al2O3 pellets with a diameter of 11 mm and a thickness of 5 mm show 100% efficiency for water purification from bacteria with a concentration of 105 CFU/mL for 7.5 min at a flow rate of 6.7 mL/min.

Preparation and Modification of PVDF Membrane and Study on Its Anti-Fouling and Anti-Wetting Properties

Membrane distillation (MD) has unique advantages in the treatment of high-salt wastewater because it can make full use of low-grade heat sources. The high salinity mine water in western mining areas of China is rich in Ca2+, Mg2+, SO42− and HCO3−. In the MD process, the inorganic substances in the feed will cause membrane fouling. At the same time, low surface tension organic substances which could be introduced in the mining process will cause irreversible membrane wetting. To improve the anti-fouling and anti-wetting properties of the membrane, the PVDF omniphobic membrane in this paper was prepared by electrospinning. The water contact angle (WCA) can reach 153°. Direct contact membrane distillation (DCMD) was then used for treating high-salinity mine water. The results show that, compared with the unmodified membranes, the flux reduction rate of the omniphobic membrane was reduced by 34% in 20 h, showing good anti-fouling property. More importantly, the omniphobic membrane cannot be wetted easily by the feed containing 0.3 mmol/L SDS. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was used to analyze the free energy of the interface interaction between the membrane and pollutants, aiming to show that the omniphobic membrane was more difficult to pollute. The result was consistent with the flux variation in the DCMD process, providing an effective basis for explaining the mechanism of membrane fouling and membrane wetting.

Performance study of fouling resistant novel ultrafiltration membranes based on the blends of poly (ether ether sulfone)/poly (vinyl pyrrolidone)/nano-titania for the separation of humic acid, dyes and biological macromolecular proteins from aqueous solutions

This study explains the use of a ultrafiltration membrane made of polyvinyl pyrrolidone (PVP) and poly(ether ether sulfone) (PEES)/Nano-titania (n-TiO₂) for the separation of organic compounds. The results of the tests for porosity, water content, surface chemistry, membrane morphology, and contact angle demonstrated that the developed membranes have more hydrophilicity than PEES membranes due to the redundant hydrophilic nature of PVP and n-TiO₂. The membrane pure water flux, which contains 5 wt% PVP and 1.5 wt% n-TiO_2, was 312.76 Lm^-2h^-1, about three-fold higher than that of pristine membrane (95.71 Lm^-2h^-1). Employing bovine serum albumin as a model foulant, the fouling resistance of the PEES/PVP/n-TiO₂ membrane was examined. According to the analysis of flux recovery ratio and irreversible resistance, modified membranes were less likely to foul, and the PEES/n-TiO₂ membrane with 5% PVP addition was recommended as optimal. The fabricated membranes effectively removed more than 95% of various organic compounds such as humic acid, safranin O, egg albumin, pepsin, and trypsin from aqueous solution. Permeability of safranin O and humic acid of PEES/PVP/n-TiO₂ membranes was about 118 Lm^-2h^-1 and 138 Lm^-2h^-1, respectively.

Non-Solvent Influence of Hydrophobic Polymeric Layer Deposition on PVDF Hollow Fiber Membrane for CO2 Gas Absorption

The implementation of hydrophobicity on membranes is becoming crucial in current membrane technological development, especially in membrane gas absorption (MGA). In order to prevent membrane wetting, a polypropylene (PP) dense layer coating was deposited on a commercial poly(vinylidene fluoride) (PVDF) hollow fiber membrane as a method of enhancing surface hydrophobicity. The weight concentration of PP pellets was varied from 10 mg mL-1 to 40 mg mL-1 and dissolved in xylene. A two-step dip coating was implemented where the PVDF membrane was immersed in a non-solvent followed by a polymer coating solution. The effects of the modified membrane with the non-solvent methyl ethyl ketone (MEK) and without the non-solvent was investigated over all weight concentrations of the coating solution. The SEM investigation found that the modified membrane surface transfiguration formed microspherulites that intensified as PP concentration increased with and without MEK. To understand the coating formation further, the solvent-non-solvent compatibility with the polymer was also discussed in this study. The membrane characterizations on the porosity, the contact angle, and the FTIR spectra were also conducted in determining the polymer coating properties. Hydrophobic membrane was achieved up to 119.85° contact angle and peak porosity of 87.62% using MEK as the non-solvent 40 mg mL-1 PP concentration. The objective of the current manuscript was to test the hydrophobicity and wetting degree of the coating layer. Hence, physical absorption via the membrane contactor using CO2 as the feed gas was carried out. The maximum CO2 flux of 3.33 × 10-4 mol m-2 s-1 was achieved by 25 mg modified membrane at a fixed absorbent flow rate of 100 mL min-1 while 40 mg modified membrane showed better overall flux stability.

Membrane Distillation of Saline Water Contaminated with Oil and Surfactants

Application of the membrane distillation (MD) process for the treatment of high-salinity solutions contaminated with oil and surfactants represents an interesting area of research. Therefore, the aim of this study is to investigate the effect of low-concentration surfactants in oil-contaminated high-salinity solutions on the MD process efficiency. For this purpose, hydrophobic capillary polypropylene (PP) membranes were tested during the long-term MD studies. Baltic Sea water and concentrated NaCl solutions were used as a feed. The feed water was contaminated with oil collected from bilge water and sodium dodecyl sulphate (SDS). It has been demonstrated that PP membranes were non-wetted during the separation of pure NaCl solutions over 960 h of the module exploitation. The presence of oil (100–150 mg/L) in concentrated NaCl solutions caused the adsorption of oil on the membranes surface and a decrease in the permeate flux of 30%. In turn, the presence of SDS (1.5–2.5 mg/L) in the oil-contaminated high-salinity solutions slightly accelerated the phenomenon of membrane wetting. The partial pores’ wetting accelerated the internal scaling and affected degradation of the membrane’s structure. Undoubtedly, the results obtained in the present study may have important implications for understanding the effect of low-concentration SDS on MD process efficiency.

Carnauba Wax/Halloysite Nanotube with Improved Anti-Wetting and Permeability of Hydrophobic PVDF Membrane via DCMD

The hydrophobic membranes have been widely explored to meet the membrane characteristics for the membrane distillation (MD) process. Inorganic metal oxide nanoparticles have been used to improve the membrane hydrophobicity, but limited studies have used nano clay particles. This study introduces halloysite nanotube (HNT) as an alternative material to synthesis a hydrophobic poly(vinylidene fluoride) (PVDF)-HNT membrane. The PVDF membranes were fabricated using functionalized HNTs (e.g., carnauba wax and 1H,1H,2H,2H-perfluorooctyl-trichlorosilane (FOTS)). The results were determined by Fourier transform infrared-attenuated total reflection, scanning electron microscope, goniometer and porometer to determine the desired hydrophobic membrane for direct contact membrane distillation (DCMD). The addition of FOTS-HNT (f_s-HNT) and carnauba wax-HNT (f_w-HNT) in the PVDF membrane enhanced the water contact angle (CA) to 127° and 137°, respectively. The presence of f_w-HNT in the PVDF membrane exhibited higher liquid entry pressure (LEP) (2.64 bar) compared to f_s-HNT in the membrane matrix (1.44 bar). The PVDF/f_w-HNT membrane (Pf_w-HNT) obtained the highest flux of 7.24 L/m²h with 99.9% salt removal. A stable permeability in the Pf_w-HNT membrane was obtained throughout 16 h of DCMD. The incorporation of f_w-HNT in the PVDF membrane had improved the anti-wetting properties and the membrane performance with the anti-fouling effect.

Nanoparticles functionalized ceramic membranes: fabrication, surface modification, and performance

Membrane technologies are used intensively for desalination and wastewater treatment. Water filtration using ceramic membranes exhibited high performance compared with polymeric membranes due to various properties such as high resistance to fouling, permeability, rejection rate, and chemical stability. Recently, the performance of nanocomposite ceramic membranes was improved due to the development in nanotechnology. This article focusses on the development of porous ceramic membranes and nanomaterial functionalized ceramic membranes for water filtration applications. At the beginning, various fabrication methods of ceramic membranes were described, and the effect of surface modification techniques on the membrane intrinsic properties was reviewed. Then, the performance of nanoparticles functionalized ceramic membranes was evaluated in terms of physicochemical properties, rejection rate, and water permeability. This work can help new entrants and established researchers to become familiar with the current challenges and developments of nanoparticle-incorporated ceramic membranes for water filtration applications.