Superhydrophobic modification of TiO2 nanocomposite PVDF membranes for applications in membrane distillation

Hyperbranched polymeric membranes for industrial water purification

This work aims at the preparation and characterization of dual-layer (DL) nano-fibrous mat (NFM) of hydrophobic and mechanical stable polyacrylonitrile (PAN) nano-fibers (NFs), as a supporter, and polyamide 6 (PA)/chitosan (Ch) NFs as a top hydrophilic coating layer. PAN and PA fibers, as residual wastes from textile processes, were collected and dissolved in their proper solvents. PAN was electro-spuned under certain conditions of electro-spinning (voltage, flow rate, and distance between spinneret and collector) to obtain PAN-NFM. Different ratios of PA/Ch composite were prepared and then electro-spun above the PAN-NFM that was previously prepared to obtain hydrophobic/hydrophilic functional dual-layer nano-fibrous membrane (DLNFM). The efficiency of the prepared DLNFM for capturing dye residues and heavy metals from wastewater was investigated. The viscosities of the prepared composite solutions were measured. The prepared dual-layer nano-fiber membranes (DLNFMs) were chemically and physically characterized by Fourier transform infrared spectroscopy, scanning electron microscope, X-ray diffraction, and thermogravimetric analyzer. The potential of the prepared mats for the adsorption of some heavy metal ions, i.e., Cu+2, Cr+3, and Pb+2 cations in addition to dyes from wastewater was evaluated. The effect of using different concentrations of PA/Ch composite as well as the thickness of the obtained DLNFM on the filtration efficiency was studied. The results of this study show the success of functional DLNFM in dye and heavy metal removal. The maximum removal efficiency of acid dyes was reached to 73.4 % and of reactive dye was approximately 61 % for PAN/PA-1.25%Ch DLNFM after 3 days at room temperature. The removal efficiency percent of heavy metal ions reached to 54 % by DLNFM. Additionally, the results showed that 0.08 mm is the ideal thickness for maximum absorption capacity. This value is correlated with the membrane's highest Ch percentage, which is (PAN/PA-1.25%Ch). Furthermore, the results demonstrate that the presence of the Ch polymer strengthened the produced bi-layered membrane to achieve the highest thermal stability when compared to the other nano-fibrous membranes (NFMs), with the breakdown temperature of the Ch functionalized dual-layer membranes (DLMs) reaching approximately 617 °C and a maximum weight loss of 60 %.

Performance investigation of poly(vinylidene fluoride-cohexafluoropropylene) membranes containing SiO2 nanoparticles in a newly designed single vacuum membrane distillation system

The current study focuses on the development of a superhydrophobic poly(vinylidene fluoride-cohexafluoropropylene) nanocomposite membrane suitable for vacuum membrane distillation by incorporating SiO2 nanoparticles. At loading hydrophobic nano-SiO2 particle concentration (0.50-1.50 wt.%), the developed nanocomposite membranes are optimized in terms of vacuum membrane distillation performance. The influence of temperature, vacuum pressure, and feed water flow is studied for desalinating high-salinity brine. The results show that the developed vacuum distillation membrane is capable of 95% salt rejection during the treatment of a highly saline feed (65,000 ppm) at fixed flow rates of 120 L/h saline feed and different operating conditions consisting of feed inlet temperatures ranging from 40°C to 70°C and distillate inlet temperatures of 7-15°C. The vacuum membrane distillation process achieves 0.38-1.66% water recovery with increasing concentration factor, meaning that recovery is increased, and shows a specific electrical energy consumption of 5.16-23.90 kWh/m3 for product water. Overall, the newly designed membrane demonstrates suitability for a vacuum membrane distillation system. PRACTITIONER POINTS: Desalinate high-salinity brine (TDS > 35,000 ppm) using a vacuum membrane distillation system. A hydrophobic PVDF-HFP/SiO2 nanocomposite membrane development for vacuum membrane distillation. A newly designed single vacuum membrane distillation system for RO brine treatment.

Modification of the distribution of humic acid complexations by introducing microbubbles to membrane distillation process for effective membrane fouling alleviation

Membrane fouling caused by inorganic ions and natural organic matters (NOMs) has been a severe issue in membrane distillation. Microbubble aeration (MB) is a promising technology to control membrane fouling. In this study, MB aeration was introduced to alleviate humic acid (HA) composited fouling during the treatment of simulative reverse osmosis concentrate (ROC) by vacuum membrane distillation (VMD). The objective of this work was to explore the HA fouling inhibiting effect by MB aeration and discuss its mechanism from the interfacial point of view. The results showed that VMD was effective for treating ROC, followed by a severe membrane fouling aggravated with the addition of 100 mg/L HA in feed solution, resulting in 45.7% decline of membrane flux. Analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and zeta potential distribution of charged particles proved the coexistence of HA and inorganic cations (especially Ca2+), resulting in more serious membrane fouling. The introduction of MB aeration exhibited excellent alleviating effect on HA-inorganic salt fouling, with the normalized flux increased from 19.7% to 37.0%. The interfacial properties of MBs played an important role, which altered the zeta potential distributions of charged particles in HA solution, indicating that MBs adhere the HA complexations. Furthermore, this mitigating effect was limited at high inorganic cations concentration. Overall, MBs could change the potential characteristics of HA complexes, which also be used for other similar membrane fouling alleviation.

Performance of modified hollow fiber membrane silver nanoparticles-zeolites Na-Y/PVDF composite used in membrane bioreactor for industrial wastewater treatment

Membrane bioreactor (MBR) deteriorates due to fouling on the membrane pores, which can reduce the membrane performance. To reduce membrane fouling, the addition of inorganic filler can enhance the antifouling properties. This study investigates two different membrane preparation by thermally induced phase separation (TIPS) and dip coating methods to modify hollow fiber membrane with Silver Nanoparticles (AgNPs)-Zeolites used in MBR for industrial wastewater treatment. Performance was evaluated by analyzing the flux of water and wastewater, rejection, water content, and antifouling properties. Characterization result represented the synthesized silver nanoparticles had similar diffraction peak with commercial AgNPs, then the micrograph of AgNPs and zeolites addition membrane showed that the inorganic material had an octahedral shape representing zeolite crystal and irregular shape representing AgNPs. The addition of zeolites and AgNPs resulted in satisfying performance, increased flux, rejection, and antifouling properties.

Harvesting Blue Energy Based on Salinity and Temperature Gradient: Challenges, Solutions, and Opportunities

Greenhouse gas emissions associated with power generation from fossil fuel combustion account for 25% of global emissions and, thus, contribute greatly to climate change. Renewable energy sources, like wind and solar, have reached a mature stage, with costs aligning with those of fossil fuel-derived power but suffer from the challenge of intermittency due to the variability of wind and sunlight. This study aims to explore the viability of salinity gradient power, or "blue energy", as a clean, renewable source of uninterrupted, base-load power generation. Harnessing the salinity gradient energy from river estuaries worldwide could meet a substantial portion of the global electricity demand (approximately 7%). Pressure retarded osmosis (PRO) and reverse electrodialysis (RED) are more prominent technologies for blue energy harvesting, whereas thermo-osmotic energy conversion (TOEC) is emerging with new promise. This review scrutinizes the obstacles encountered in developing osmotic power generation using membrane-based methods and presents potential solutions to overcome challenges in practical applications. While certain strategies have shown promise in addressing some of these obstacles, further research is still required to enhance the energy efficiency and feasibility of membrane-based processes, enabling their large-scale implementation in osmotic energy harvesting.

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.

Advances in the research of carbon-, silicon-, and polymer-based superhydrophobic nanomaterials: Synthesis and potential application

With the rapid development of science and technology, superhydrophobic nanomaterials have become one of the hot topics from various subjects. Due to their distinct properties, such as superhydrophobicity, anti-icing and corrosion resistance, superhydrophobic nanomaterials are widely used in industry, agriculture, defense, medicine and other fields. Hence, the development of superhydrophobic materials with superior performance, economical, practical features, and environment-friendly properties are extremely important for industrial development and environmental protection. Aimed to provide a scientific and theoretical basis for the subsequent study on the preparation of composite superhydrophobic nanomaterials, this paper reviewed the latest progress in the research of superhydrophobic surface wettability and the theory of superhydrophobicity, summarized and analyzed the latest development of carbon-based, silicon-based and polymer-based superhydrophobic nanomaterials in terms of their synthesis, modification, properties and structure sizes (diameters), discussed the problems and unique application prospects of carbon-based, silicon-based and polymer-based superhydrophobic nanomaterials.

Treatment to surfactant containing wastewater with membrane distillation membrane with novel sandwich structure

Surfactant containing wastewater widely exists in textile industry, which hardly to be treated by membrane technology due to its high in salinity and wetting potential. In this study, PVDF membrane was modified by constructing a PDMS-SiO₂-PDMS "sandwich" structure on top of its surface via coating to achieve resistance to surfactant induced wetting. The "sandwich" layer was optimized based on the membrane performance during membrane distillation. Compared to the pristine PVDF membrane with contact angle of 92°, the water contact angle of the membrane with a "sandwich" layer of 0.44 μm increased to 153°. For the feed contained 0.5 wt% NaCl and 0.25 wt% surfactant, there was no membrane wetting occurred during the experiment period using the membrane with a "sandwich" structure, in comparison to the pristine PVDF membrane being wetted from beginning. For a challenge experiment to the developed membrane lasting for 100 h using a surfactant containing feed, there is no wetting sign observed and the stable flux is 20 kg·m^-2·h^-1.

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.

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.

Biotechnological advancements towards water, food and medical healthcare: A review

The global health status is highly affected by the growing pace of urbanization, new lifestyles, climate changes, and resource exploitation. Modern technologies pave a promising way to deal with severe concerns toward sustainable development. Herein, we provided a comprehensive review of some popular biotechnological advancements regarding the progress achieved in water, food, and medicine, as the most substantial fields related to public health. The emergence of novel organic/inorganic materials has brought about significant improvement in conventional water treatment techniques, anti-fouling approaches, anti-microbial agents, food processing, biosensors, drug delivery systems, and implants. Particularly, a growing interest has been devoted to nanomaterials and their application for developing novel structures or improving the characteristics of standard components. Also, bioinspired materials have been widely used to improve the performance, efficiency, accuracy, stability, safety, and cost-effectiveness of traditional systems. On the other side, the fabrication of innovative devices for precisely monitoring and managing various ecosystem and human health issues is of great importance. Above all, exceptional advancements in designing ion-selective electrodes (ISEs), microelectromechanical systems (MEMs), and implantable medical devices have altered the future landscape of environmental and biomedical research. This review paper aimed to shed light on the wide-ranging materials and devices that have been developed for health applications and mainly focused on the impact of nanotechnology in this field.

Multi-Layered Branched Surface Fluorination on PVDF Membrane for Anti-Scaling Membrane Distillation

Membrane distillation (MD) has emerged as a promising technology for hypersaline wastewater treatment. However, membrane scaling is still a critical issue for common hydrophobic MD membranes. Herein, we report a multi-layered surface modification strategy on the commercial polyvinylidene fluoride (PVDF) membrane via plasma treatment and surface fluorination cycles. The repeated plasma treatment process generates more reaction sites for the fluorination reaction, leading to higher fluorination density and more branched structures. MD tests with CaSO₄ as the scaling agent show that the modification strategy mentioned above improves the membrane scaling resistance. Notably, the PVDF membrane treated with three cycles of plasma and fluorination treatments exhibits the best anti-scaling performance while maintaining almost the same membrane flux as the unmodified PVDF membrane. This study suggests that a highly branched surface molecular structure with low surface energy benefits the MD process in both membrane flux and scaling resistance. Besides, our research demonstrates a universal and facile approach for membrane treatment to improve membrane scaling resistance.

Graphene-Coated PVDF Membranes: Effects of Multi-Scale Rough Structure on Membrane Distillation Performance

Graphene-coated membranes for membrane distillation have been fabricated by using a wet-filtration approach. Graphene nanoplatelets have been deposited onto PVDF membrane surfaces. Morphology and physicochemical properties have been explored to evaluate the changes in the surface topography and related effects on the membrane performance in water desalination. The membranes have been tested in membrane distillation plants by using mixtures of sodium chloride and humic acid. The multi-scale rough structure of the surface has been envisaged to amplify the wetting and fouling resistance of the graphene-coated membranes so that a better flux and full salt rejection have been achieved in comparison with pristine PVDF. Total salt rejection and an increase of 77% in flux have been observed for coated membrane with optimized graphene content when worked with NaCl 0.6 M (DCMD, ΔT ≈ 24 °C) over a test period of 6 h. The experimental findings suggest these novel graphene-coated membranes as promising materials to develop functional membranes for high-performing water desalination.

One-step laser etching of a bionic hierarchical structure on a silicone rubber surface with thermal and acid/alkali resistance and tunable wettability

Superhydrophobic silicone rubbers with robust physical and chemical stability have promising application potential in the field of flexible electronics. A one-step laser etching strategy is proposed for successfully fabricating superhydrophobic silicone rubbers with bioinspired hierarchical micro/nanostructures. Regular and dense micro/nano spheres gradually accumulate on the modified silicone rubber surface with the increase of laser etching cycles. Owing to the bioinspired hierarchical micro/nano spheres, a 5 μL water droplet on the modified silicone rubber surface exhibits a contact angle of 158 ± 3° and a sliding angle of 5 ± 1°. Moreover, the modified silicone rubber can maintain a stable superhydrophobic state in acid/alkali (pH = 2, 4, 6, 8, and 10) and thermal environments (50-90 °C). Importantly, the contact angle and sliding angle are adjustable depending on the number of laser etching cycles, which is beneficial for the different application requirements. The proposed method for the fast fabrication of superhydrophobic silicone rubbers with tunable wettability can provide an excellent candidate for the protection of flexible electronics in rainy and acid/alkali environments.

Recent Progress, Challenges, and Opportunities of Membrane Distillation for Heavy Metals Removal

Water is essential for the presence of life on this earth. However, water contamination due to the presence of heavy/toxic metals is one of the serious environmental issues for living beings. Several methods have been devoted to separating or removing those heavy metals from wastewater. Among them, membrane distillation (MD) has become one of the most attractive approaches due to its higher rejection rate than processes driven by pressure, lower energy consumption than traditional distillation processes. MD has gained significant attention for removing heavy metals than other techniques like ion exchange and adsorption in the last two decades. This review provides insight knowledge to the reader and focuses on how heavy metals impact humans and the environment, sources of heavy metals, current and especially removal methods using the MD method. Moreover, recent studies, challenges, and opportunities on MD membrane modules and heavy metal removal systems are discussed. More importantly, in this review, we have identified the gaps and opportunities that are required for enhancing the MD approach and its practical suitability for heavy metal removals. MD module and system showed high performance, proving their possible applications to remove heavy metal ions in water/wastewater treatment.

Negative Pressure Membrane Distillation for Excellent Gypsum Scaling Resistance and Flux Enhancement

Membrane distillation (MD) has potential to become a competitive technology for managing hypersaline brine but not until the critical challenge of mineral scaling is addressed. The state-of-the-art approach for mitigating mineral scaling in MD involves the use of superhydrophobic membranes that are difficult to fabricate and are commercially unavailable. This study explores a novel operational strategy, namely, negative pressure direct contact membrane distillation (NP-DCMD) that can minimize mineral scaling with commercially available hydrophobic membranes and at the same time enhance the water vapor flux substantially. By applying a negative gauge pressure on the feed stream, NP-DCMD achieved prolonged resistance to CaSO₄ scaling and a dramatic vapor flux enhancement up to 62%. The exceptional scaling resistance is attributable to the formation of a concave liquid-gas under a negative pressure that changes the position of the water-air interface to hinder interfacial nucleation and crystal growth. The substantial flux enhancement is caused by the reduced molecular diffusion resistance within the pores and the enhanced heat transfer kinetics across the boundary layer in NP-DCMD. Achieving substantial performance improvement in both the scaling resistance and vapor flux with commercial membranes, NP-DCMD is a significant innovation with vast potential for practical adoption due to its simplicity and effectiveness.

Superhydrophobic modification of electrospun nanofibrous Si@PVDF membranes for desalination application in vacuum membrane distillation

Superhydrophobic nanofibers have received prominent attention owing to their exceptional properties and researchers are focused on developing high-performing MD membranes. Herein, we fabricate superhydrophobic electrospun nanofibrous membranes using polyvinylidene fluoride (PVDF) solutions with silica nanoparticles (0 wt% to 6 wt%) to create multiscale (or hierarchical) surface roughness. For superhydrophobicity, the composite membranes (Si@PVDF) were subjected to a two-step modification that included acid pre-treatment and silanization with fluoroalkylsilane (FAS) compound of low surface energy. The acid pre-treatment enhances the hydroxyl group of SiO₂ nanoparticles and create active sites in abundance for silanization. The modified membranes (FAS-Si@PVDF-A) having 6 wt% SiO₂ showed excellent wetting resistance with water contact angle (WCA) up to 154.6 ± 2.2°, smaller average pore size of 0.27 ± 0.3 μm, and high liquid entry pressure (LEP) of 143 ± 4 kPa. It was observed, increasing silica content decreased the fiber diameter and average pore size and increased WCA and LEP of modified membranes. The modified superhydrophobic membranes gave stable permeate flux, exhibited strong wetting resistance and excellent salt rejection in vacuum membrane distillation (VMD) test. The optimal FAS-Si@PVDF-A membrane (6 wt% SiO₂) of thickness 98 ± 5 μm produced a stable permeate flux of more than 11.5 kg m^-2 h^-1 and salt rejection as high as 99.9% after 22 h of continuous operation using NaCl solution (3.5 wt%) as feed. Therefore, this modification provided superhydrophobic membranes possessing robust anti-wetting properties with significant permeability and has encouraging application in membrane distillation for desalination.

Improving superamphiphobicity by mimicking tree-branch topography

when a droplet impacts on a superhydrophobic structured surface below a certain impact velocity, the droplet can bounce off completely from the surface. However, above such velocity a fraction of the droplet will pin on the surface. Surfaces capable of repelling water droplets are ubiquitous in nature or have been artificially fabricated. However, as the surface tension of the liquid is reduced, the capability of the surface to remain non-wetting gets hindered. Despite progress in previous research, the understanding and development of superamphiphobic surface to impacting low surface tension droplets remains elusive. It is proposed that multi-layer re-entrant like roughness can further enhance the anti-wetting properties also for low surface tension fluids. In this work, we produce patterned conical micro-structures with lateral nano-sized roughness. Furthermore, the droplet impact experiments are conducted on various surfaces with variable surface tensions (27 mN/m - 72 mN/m) by using droplets with different Weber numbers (2-170). We show that conical microstructures with lateral roughness mimicking tree-branches provides a surface topology capable of absorbing the force exerted by the droplet during the impact which prevents the droplet from pinning on the surface at higher impact velocity even for low surface tension droplets. Our study has significance for understanding the liquid interaction mechanism with the surface during the impact process and for the associated surface design considerations.

Membrane Distillation: Recent Configurations, Membrane Surface Engineering, and Applications

Membrane distillation (MD) is a developing membrane separation technology for water treatment that involves a vapor transport driven by the vapor pressure gradient across the hydrophobic membrane. MD has gained wide attention in the last decade for various separation applications, including the separation of salts, toxic heavy metals, oil, and organic compounds from aqueous solutions. Compared with other conventional separation technologies such as reverse osmosis, nanofiltration, or thermal distillation, MD is very attractive due to mild operating conditions such as low temperature and atmospheric pressure, and 100% theoretical salt rejection. In this review, membrane distillation’s principles, recent MD configurations with their advantages and limitations, membrane materials, fabrication of membranes, and their surface engineering for enhanced hydrophobicity are reviewed. Moreover, different types of membrane fouling and their control methods are discussed. The various applications of standalone MD and hybrid MD configurations reported in the literature are detailed. Furthermore, studies on the MD-based pilot plants installed around the world are covered. The review also highlights challenges in MD performance and future directions.

Influence of Preparation Temperature on the Properties and Performance of Composite PVDF-TiO2 Membranes

Composite PVDF-TiO₂ membranes are studied extensively in literature as effective anti-fouling membranes with photocatalytic properties. Yet, a full understanding of how preparation parameters affect the final membrane structure, properties and performance has not been realized. In this study, PVDF-TiO₂ membranes (20 wt% TiO₂/PVDF) were fabricated via the non-solvent-induced phase separation (NIPS) method with an emphasis on the preparation temperature. Then, a systematic approach was employed to study the evolution of the membrane formation process and membrane properties when the preparation temperature changed, as well as to establish a link between them. Typical asymmetric membranes with a high porosity were obtained, along with a vast improvement in the permeate flux compared to the neat PVDF membranes, but a reduction in mechanical strength was also observed. Interestingly, upon the increase in preparation temperature, a significant transition in membrane morphology was observed, notably the gradual diminution of the finger-like macrovoids. Other membrane properties such as permeability, porosity, thermal and mechanical properties, and compression behavior were also influenced accordingly. Together, the establishment of the ternary phase diagrams, the study of solvent-nonsolvent exchange rate, and the direct microscopic observation of membrane formation during phase separation, helped explain such evolution in membrane properties.

Metal oxide and carbon nanomaterial based membranes for reverse osmosis and membrane distillation: A comparative review

Commercial membranes typically suffer from fouling and wetting during membrane distillation (MD). In contrast, reverse osmosis (RO) can be subject to the fouling issue if applied for highly saline feed solutions containing foulants (e.g., organics, oils, and surfactants). Among the diverse treatment options, the nanomaterial-based membranes have recently gained great interest due to their advantageous properties (e.g., enhanced flux and roughness, better pore size distribution, and higher conductivity). This review focuses on recent advances in the mechanical properties, anti-fouling capabilities, salt rejection, and economic viability of metal oxide (SiO₂, TiO₂, and ZnO) and carbon nanomaterial (graphene oxide/carbon nanotube)-based membranes. Current challenges in applying nanomaterial-based membranes are also discussed. The study further describes the preparation methods, mechanisms, commercial applications, and economical feasibility of metal oxide- and carbon nanomaterial-based membrane technologies.

Exfoliated Bi2Te3-enabled membranes for new concept water desalination: Freshwater production meets new routes

Water scarcity forces the science to find the most environmentally friendly propulsion technology for supplying plentiful freshwater at low energy costs. Membrane Distillation well meets criteria of eco-friendly management of natural resources, but it is not yet competitive on scale. Herein, we use a dichalchogenide compound (Bi₂Te₃) as a conceivable source to accelerate the redesign of advanced membranes technologies such as thermally driven membrane distillation. A procedure based on assisted dispersant liquid phase exfoliation is used to fill PVDF membranes. Key insights are gained in the crucial role of this topological material confined in hydrophobic membranes dedicated to recovery of freshwater from synthetic seawater. Intensified water flux together with reduced energy consumption is obtained into one pot, thereby gathering ultrafast production and thermal efficiency in a single device. Bi₂Te₃-enabled membranes show ability to reduce the resistance to mass transfer while high resistance to heat loss is opposite. Permeate flux is kept stable and salt rejection is higher than 99.99% during 23 h-MD test. Our results confirm the effectiveness of chalcogenides as frontier materials for new-concept water desalination through breakthrough thermally-driven membrane distillation, which is regarded as a new low-energy and sustainable solution to address the growing demand for access to freshwater.

Low-fouling biomimetic membranes fabricated by direct replication of self-cleaning natural leaf

The membrane fouling has always been a big issue for developing membrane applications. Surface morphology and roughness affect remarkably on the membrane tendency to fouling. In this study, a biomimetic technique, as a simple, cost-effective and time-saving method was employed to replicate Tropaeolum majus (nasturtium) leaf surface on the surface of a commercial thin-film composite (TFC) reverse osmosis (RO) membrane using polyethersulfone (PES) moulds. Morphology of surface and hydrophilicity of membranes were investigated by scanning electron microscope (SEM), atomic force microscope (AFM) and water contact angle measurements. AFM and SEM photos of membrane surface declared that replication of nasturtium leaf improved the surface characteristics of membranes. The average roughness of membranes heated at 130°C and 150°C was 81.1 and 152.4 nm, respectively. The similar measurement was lower for the virgin membrane. Also, the roughened membranes showed higher hydrophilicity than the virgin membrane. In addition, the performance of the membrane was assessed by evaluating pure water flux (PWF) and flux recovery (FR) after the filtration of whey solution as a severe foulant for membranes. The findings exhibited that the replicated membranes had higher PWF and FR values, indicating the lower fouling tendency of modified membranes.

Fabrication of a novel one-step coating hyper-hydrophobic fluorine-free TiO2 decorated hollow composite membrane for use in longer-term VMD with enhanced flux, rejection, anti-wetting and anti-fouling performances

Despite recent efforts, there are still significant challenges in preparing hyper-hydrophobic membranes using environmental-friendly materials and simple methods. In this work, using phase separation theory, we prepared a fluorine-free hyper-hydrophobic porous hollow composite membrane using one-step ultrasound dip-coating. Then, fluorine-free modified titanium dioxide, polydimethylsilane and polypropylene was used to construct the porous membrane with a water contact angle of 161°. The distribution of surface elements, morphology, wetting and the scale of titanium on the membranes was characterized using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), the water contact angle and acid-alkali stability, wetting resistance, and so on. The membrane was evaluated for desalination in the presence of organic-pollutants. Under longer-term vacuum membrane distillation, compared with the general polypropylene membrane, the flux of the hyper-hydrophobic membrane increased to 12.17 kg (m² h)^-1, and the rejection rate reached 99.99%. These results indicated that the free-fluorine hyper-hydrophobic membrane could be used for seawater desalination. Finally, our results indicate that the hyper-hydrophobic modified membrane has good potential for use in industrial desalination.

Advances in seawater membrane distillation (SWMD) towards stand-alone zero liquid discharge (ZLD) desalination

Seawater membrane distillation (SWMD) is a promising separation technology due to its ability to operate as a stand-alone desalination unit operation. This paper reviews approaches to improve laboratory-to-pilot-scale MD performance, which comprise operational strategies, module design, and specifically tailored membranes. A detailed comparison of SWMD and sea water reverse osmosis is presented to further analyze the critical shortcomings of SWMD. The unique features of SWMD, namely the ability to operate with extremely high salt rejection and at extreme feed concentration, highlight the SWMD potential to be operated under zero liquid discharge (ZLD) conditions, which results in the production of high-purity water and simultaneous salt recovery, as well as the elimination of the brine disposal cost. However, technical challenges, such as thermal energy requirements, inefficient heat transfer and integration, low water recovery factors, and lack of studies on real-case valuable-salt recovery, are impeding the commercialization of ZLD SWMD. This review highlights the possibility of applying selected strategies to push forward ZLD SWMD commercialization. Suggestions are projected to include intermittent removal of valuable salts, in-depth study on the robustness of novel membranes, module and configuration, utilization of a low-cost heat exchanger, and capital cost reduction in a renewable-energy-integrated SWMD plant.

Characterization of PVDF/Graphene Nanocomposite Membranes for Water Desalination with Enhanced Antifungal Activity

Seawater desalination is a worldwide concern for the sustainable production of drinking water. In this regard, membrane distillation (MD) has shown the potential for effective brine treatment. However, the lack of appropriate MD membranes limits its industrial expansion since they experience fouling and wetting issues. Therefore, hydrophobic membranes are promising candidates to successfully deal with such phenomena that are typical for commercially available membranes. Here, several graphene/polyvinylidene (PVDF_G) membranes with different graphene loading (0–10 wt%) were prepared through a phase inversion method. After full characterization of the resulting membranes, the surface revealed that the well-dispersed graphene in the polymer matrix (0.33 and 0.5 wt% graphene loading) led to excellent water repellence together with a rough structure, and a large effective surface area. Importantly, antifungal activity tests of films indicated an increase in the inhibition percentage for PVDF_G membranes against the Curvularia sp. fungal strain. However, the antifungal surface properties were found to be the synergistic result of graphene toxicity and surface topography.

Polyvinylidene fluoride nanocomposite super hydrophilic membrane integrated with Polyaniline-Graphene oxide nano fillers for treatment of textile effluents

Water pollution from the fashion industries containing dyes has become a major source of water pollution. These anthropogenic contaminated waters directly enter irrigation and drinking water systems, causing irreversible environmental damage to human health. Nanomembrane technology has attracted extensive attention to remove these toxic chemicals but new approaches are still required for improving removal efficiency and control the channel size. The work deals with the fabrication of a novel hybrid polyvinylidene fluoride (PVDF)-polyaniline (PANI) membrane with graphene oxide (GO). Incorporation of PANI-GO as a nanofiller has significantly improved antifouling properties and a solvent content of the fabricated membrane. Besides, pure water flux also increases from 112 to 454 L m^-2 h^-1 indicating the hydrophilic nature of the nanocomposite membrane. Among various compositions, the nanocomposites membrane with 0.1 %w/v GO demonstrated a maximum of 98 % dye rejection at 0.1 MPa operating pressure. After multiple testing of the membrane, the flux recovery ratio reached about 94 % and dyes rejection improved with the addition of PANI-GO. The removal efficiency of the composite membrane for Allura red is 98 % and for methyl orange is 95 %. Based on the above results the PVDF/PANI/GO membranes are recommended for practical use in wastewater treatment, particularly for anionic dyes removal from textile effluents.

Membrane Contactors for Maximizing Biomethane Recovery in Anaerobic Wastewater Treatments: Recent Efforts and Future Prospect

Increasing demand for water and energy has emphasized the significance of energy-efficient anaerobic wastewater treatment; however, anaerobic effluents still containing a large portion of the total CH4 production are discharged to the environment without being utilized as a valuable energy source. Recently, gas–liquid membrane contactors have been considered as a promising technology to recover such dissolved methane from the effluent due to their attractive characteristics such as high specific mass transfer area, no flooding at high flow rates, and low energy requirement. Nevertheless, the development and further application of membrane contactors were still not fulfilled due to their inherent issues such as membrane wetting and fouling, which lower the CH4 recovery efficiency and thus net energy production. In this perspective, the topics in membrane contactors for dissolved CH4 recovery are discussed in the following order: (1) operational principle, (2) potential as waste-to-energy conversion system, and (3) technical challenges and recent efforts to address them. Then, future efforts that should be devoted to advancing gas–liquid membrane contactors are suggested as concluding remarks.