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

Springtail-inspired omniphobic slippery membrane with nano-concave re-entrant structures for membrane distillation

Omniphobic membranes, due to their exceptional properties, have drawn significant attention for overcoming the bottleneck in membrane distillation (MD) technology. This study demonstrates an innovative method for fabricating an omniphobic membrane that is simple and facile compared to other methods such as wet/dry etching and photolithography. The surface morphology of springtails was imitated using electrospraying technique to coat a polyvinylidene fluoride substrate with concave-shaped polystyrene beads that were successfully developed by controlling the electrical traction (voltage) and air resistance (humidity). Then, the lipid coating of springtail surfaces was mimicked by dip-coating the membrane in a low-toxicity short-chain perfluoropolyether lubricant. The concave structure's tiny air pockets increased membrane hydrophobicity significantly, indicated by the fact that the first round of water bouncing took only 16.3 ms. Finally, in MD treatment of seawater containing 1.0 mM sodium dodecyl sulfate, the optimized omniphobic membrane maintained a stable 99.9% salt rejection rate.

Maximizing the wetting resistance of fluorine-free omniphobic membranes for hypersaline wastewater desalination

Membrane distillation (MD) equipped with omniphobic (non-wetting) membranes has found a niche in water reclamation from hypersaline industrial wastewater. Here, we examined the efficacy of non-fluorinated materials as surface coating agents for omniphobic MD membrane fabrication, and identified necessary mechanisms to attain a maximized wetting resistance using fluorine-free materials. We first prepared MD membranes with different surface chemistries using a series of linear alkylsilanes and polydimethylsiloxane (PDMS) as representative fluorine-free, low surface energy materials. Membranes modified with a longer chain alkylsilane exhibited a lower surface energy and demonstrated a greater wetting resistance in direct contact MD experiments using feedwaters of various surface tensions. Despite the nearly identical surface energy measured for the longest alkylsilane and PDMS, PDMS-modified membrane exhibited an extended antiwetting performance as compared to the membrane treated with the longest alkylsilane. To elucidate the source of the distinctive wetting resistance, we examined the nucleation and condensation kinetics on the surfaces with the different surface chemistries via environmental scanning electron microscopy. Our analysis suggests that the membranes treated with long chain alkylsilanes contain surface defects (i.e., hydrophilic regions) whereas the high mobility of the PDMS effectively minimizes the defect exposure, slowing down the condensation and subsequent surface wetting.

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.

Advancements in Nanoenabled Membrane Distillation for a Sustainable Water-Energy-Environment Nexus

The emergence of nano innovations in membrane distillation (MD) has garnered increasing scientific interest. This enables the exploration of state-of-the-art nano-enabled MD membranes with desirable properties, which significantly improve the efficiency and reliability of the MD process and open up opportunities for achieving a sustainable water-energy-environment (WEE) nexus. This comprehensive review provides broad coverage and in-depth analysis of recent innovations in nano-enabled MD membranes, focusing on their role in achieving desirable properties, such as strong liquid-repellence, high resistance to scaling, fouling, and wetting, as well as efficient self-heating and self-cleaning functionalities. The recent developments in nano-enhanced photothermal-catalytic applications for water-energy co-generation within a single MD system are also discussed. Furthermore, the bottlenecks are identified that impede the scale-up of nanoenhanced MD membranes and a future roadmap is proposed for their sustainable commercialiation. This holistic overview is expected to inspire future research and development efforts to fully harness the potential of nano-enabled MD membranes to achieve sustainable integration of water, energy, and the environment.

A comprehensive review on state-of-the-art antifouling super(wetting and anti-wetting) membranes for oily wastewater treatment

One of the most dangerous types of pollution to the environment is oily wastewater, which is produced from a number of industrial sources and can cause damage to the environment, people, and creatures. To overcome this issue, membrane technology as an advanced method has been considered for treating oily wastewater due to its stability, high removal efficiency, and simplicity in scaling up. Membrane fouling, or the accumulation of oil droplets at or within the membrane pores, compromises the efficiency of membrane separation and water flux. In the last decade, the fabrication of membranes with specific wettability to reduce fouling has received much consideration. The purpose of this article is to offer a literature overview of all fabricated anti-fouling super(wetting and anti-wetting) membranes for applicable membrane processes for the separation of immiscible and emulsified oil/water mixtures. In this review, we first explain membrane fouling and discuss methods for preventing it. Afterwards, in all membrane separation processes, including pressure-driven, gravity-driven, and thermal-driven, membranes based on the form and density of oil are categorized as oil-removing or water-removing with special wettability, and then their wettability modification with different materials is particularly discussed. Finally, the prospect of anti-fouling membrane fabrication in the future is presented.

Insight into the effect of rice-straw ash on enhancing the anaerobic digestion performance of high salinity organic wastewater

Anaerobic digestion is an economic method for treating high salinity organic wastewater (HSOW), but performance enhancement is needed because of the inhibitory effect of high salinity. In this study, rice-straw ash (RSA) was applied to alleviate the inhibitory effect during HSOW anaerobic digestion. The results showed that, when the NaCl content increased from 0% to 3.0%, the methane production decreased by 87.35%, and the TOC removal rate decreased to 34.12%. As a K+ and alkalinity source, RSA addition enhanced the anaerobic digestion performance, and the optimal dosage was 0.88 g/L. Under this dosage, the methane production increased by 221.60%, and TOC removal rate reached 66.42% at 3.0% salinity. The addition of RSA increased the proportion of living cells in the high salinity environment, and enhanced the activity of key enzymes and electron transfer efficiency in the anaerobic digestion process. The addition of RSA with a dosage of 0.88 g/L promoted the accumulation of acetoclastic methanogen Methanothrix. The abundance of substrate transporters, ion transporters and electron transfer related functional genes were enriched, which might be key for promoting HSOW anaerobic digestion performance. The results also showed that RSA addition played an important role in maintaining the stability of the anaerobic digestion system, and it could be a potential strategy for enhancing the anaerobic digestion performance under high salinity conditions.

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.

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.

Spraying Fluorinated Silicon Oxide Nanoparticles on CuONPs@CF-PVDF Membrane: A Simple Method to Achieve Superhydrophobic Surfaces and High Flux in Direct Contact Membrane Distillation

Desalinization of seawater can be achieved by membrane distillation techniques (MD). In MD, the membranes should be resistant to fouling, robust for extended operating time, and preferably provide a superhydrophobic surface. In this work, we report the preparation and characterization of a robust and superhydrophobic polyvinylidene fluoride membrane containing fluoroalkyl-capped CuONPs (CuONPs@CF) in the inner and fluorinated capped silicon oxide nanoparticles (SiO₂NPs@CF) on its surface. SiO₂NPs@CF with a mean diameter of 225 ± 20 nm were prepared by the sol method using 1H,1H,2H,2H-perfluorodecyltriethoxysilane as a capping agent. Surface modification of the membrane was carried out by spraying SiO₂NPs@CF (5% wt.) dispersed in a mixture of dimethyl formamide (DMF) and ethanol (EtOH) at different DMF/EtOH % v/v ratios (0, 5, 10, 20, and 50). While ethanol dispersed the nanoparticles in the spraying solution, DMF dissolved the PVDF on the surface and retained the sprayed nanoparticles. According to SEM micrographs and water contact angle measurements, the best results were achieved by depositing the nanoparticles at 10% v/v of DMF/EtOH. Under these conditions, a SiO₂NPs covered surface was observed with a water contact angle of 168.5°. The water contact angle was retained after the sonication of the membrane, indicating that the modification was successfully achieved. The membrane with SiO₂NPs@CF showed a flux of 14.3 kg(m²·h)^-1, 3.4 times higher than the unmodified version. The method presented herein avoids the complicated modification procedure offered by chemical step modification and, due to its simplicity, could be scalable to a commercial membrane.

The advent of thermoplasmonic membrane distillation

Freshwater scarcity is a vital societal challenge related to climate change, population pressure, and agricultural and industrial demands. Therefore, sustainable desalination/purification of salty/contaminated water for human uses is particularly relevant. Membrane distillation is an emerging hybrid thermal-membrane technology with the potential to overcome the drawbacks of conventional desalination by a synergic exploitation of the water-energy nexus. Although membrane distillation is considered a green technology, efficient heat management remains a critical concern affecting the cost of the process and hindering its viability at large scale. A multidisciplinary approach that involves materials chemistry, physical chemistry, chemical engineering, and materials and polymer science is required to solve this problem. The combination of solar energy with membrane distillation is considered a potentially feasible low-cost approach for providing high-quality freshwater with a low carbon footprint. In particular, recent discoveries about efficient light-to-heat conversion in nanomaterials have opened unprecedented perspectives for the implementation of sunlight-based renewable energy in membrane distillation. The integration of nanofillers enabling photothermal effects into membranes has been demonstrated to be able to significantly enhance the energy efficiency without impacting on economic costs. Here, we provide a comprehensive overview on the state of the art, the opportunities, open challenges and pitfalls of the emerging field of solar-driven membrane distillation. We also assess the peculiar physicochemical properties and synthesis scalability of photothermal materials, as well as the strategies for their integration into polymeric nanocomposite membranes enabling efficient light-to-heat conversion and freshwater.

Effect of TiO2 Nanoparticles on Capillary-Driven Flow in Water Nanofilters Based on Chitosan Cellulose and Polyvinylidene Fluoride Nanocomposites: A Theoretical Study

In this study, a novel concept of nanofiltration process of drinking water based on capillary-driven nanofiltration is demonstrated using a bio-based nanocomposites’ nanofilter as free power: a green and sustainable solution. Based on Lifshitz and Young–Laplace theories, we show that the chitosan (CS), cellulose acetate (CLA), and Polyvinylidene fluoride (PVDF) polymer matrixes demonstrate hydrophobic behavior, which leads to the draining of water from nanopores when negative capillary pressure is applied and consequently prevents the capillary-driven nanofiltration process. By incorporating 10%, 20%, and 30% volume fraction of titanium dioxide (TiO₂) nanoparticles (NPs) to the polymers’ matrixes, we demonstrate a wetting conversion from hydrophobic to hydrophilic behavior of these polymer nanocomposites. Subsequently, the threshold volume fraction of the TiO₂ NPs for the conversion from draining (hydrophobic) to filling (hydrophilic) by capillary pressure were found to be equal to 5.1%, 10.9%, and 13.9%, respectively, for CS/TiO₂, CLA/TiO₂, and PVDF/TiO₂ nanocomposites. Then, we demonstrated the negligible effect of the gravity force on capillary rise as well as the capillary-driven flow for nanoscale pore size. For nanofilters with the same effective nanopore radius, porosity, pore shape factor, and tortuosity, results from the modified Lucas–Washburn model show that the capillary rise as well as the capillary-driven water volume increase with increased volume fraction of the TiO₂ NPs for all nanocomposite nanofilter. Interestingly, the capillary-driven water volume was in range (5.26–6.39) L/h·m² with 30% volume fraction of TiO₂ NPs, which support our idea for capillary-driven nanofiltration as zero energy consumption nano-filtration process. Correspondingly, the biodegradable CS/TiO₂ and CLA/TiO₂ nanocomposites nanofilter demonstrate capillary-driven water volume higher, ~1.5 and ~1.2 times, respectively, more than the synthetic PVDF/TiO₂ nanocomposite.

A self-healing PVDF-ZnO/MXene membrane with universal fouling resistance for real seawater desalination

Seawater desalination is regarded as a possible way to overcome current shortages of fresh water, and membrane-based air humidification-dehumidification desalination (MHDD) represents a promising technique owing to its high-quality freshwater and cost-effectiveness; however, its development is restricted by membrane fouling. While a superhydrophobic membrane provides resistance to hydrophilic fouling, it remains susceptible to hydrophobic fouling. Here, a polyvinylidene fluoride-ZnO/MXene (PVDF-ZM) membrane, with a reversible conversion between superhydrophobicity and hydrophilicity was fabricated to achieve universal fouling resistance. It earned a competitive permeate flux (3.93 kg·m^-2·h^-1) and an excellent salt rejection (>99.5%). The membrane exhibited a strong anti-hydrophilic fouling ability, benefiting from its superhydrophobicity and rough surface. The adsorbed hydrophobic contaminants could desorb from the membrane surface under UV irradiation when transforming the surface wettability into hydrophilicity, exhibiting an anti-hydrophobic fouling ability. Subsequently, the membrane surface returned to the hydrophobic state under dark conditions. The membrane recovered 90% of the original permeation flux, while maintaining a salt rejection of >99.5%, thus realizing membrane self-healing. The PVDF-ZM membrane holds promise for sustainable desalination applications.

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.

Nanobubble-assisted scaling inhibition in membrane distillation for the treatment of high-salinity brine

In this study, we report the use of nanobubbles (NBs) as a simple and facile approach to effectively delay scaling in membrane distillation (MD) during the treatment of highly saline feed (100 g L-1). Unlike conventional gas bubbling in MD for improving the hydrodynamic flow conditions in the feed channel, here we generated air NBs with an average size of 128.81 nm in the feed stream and examined their impact on membrane scaling inhibition during MD operation. Due to their small size, neutral buoyancy, and negative surface charge, NBs remain in suspension for a longer time (14 days), providing homogenous mixing throughout the entire feed water. The MD performance results revealed that severe membrane scaling happened during the DCMD treatment of high salinity brine in the absence of nanobubbles, which dramatically reduced the distillate flux to zero after 13 h. A one-time addition of air NBs in the saline feed significantly reduced salt precipitation and crystal deposition on the PVDF membrane surface, delayed the occurrence of flux decline, prevented membrane wetting, thereby prolonging the effective MD operating time. With similar feed concentration and operating conditions, only 63% flux decline after 98 h operation was recorded in nanobubble-assisted MD. Two key explanations were suggested for the delayed membrane scaling upon addition of air NBs in the MD feed: (1) NB-induced turbulent flow in the feed channel that increases the surface shear forces at the membrane surface, alleviating both temperature and concentration polarization effect, (2) electrostatic attractions of the counterions to the negatively charged NBs, which reduces the availability of these ions in the bulk feed for scale formation.

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.

Surfaces with Adjustable Features-Effective and Durable Materials for Water Desalination

Materials based on PVDF with desirable and controllable features were successfully developed. The chemistry and roughness were adjusted to produce membranes with improved transport and separation properties. Membranes were activated using the novel piranha approach to generate OH-rich surfaces, and finally furnished with epoxy and long-alkyl moieties via stable covalent attachment. The comprehensive materials characterization provided a broad spectrum of data, including morphology, textural, thermal properties, and wettability features. The defined materials were tested in the air-gap membrane distillation process for desalination, and improvement compared with pristine PVDF was observed. An outstanding behavior was found for the PVDF sample equipped with long-alkyl chains. The generated membrane showed an enhancement in the transport of 58-62% compared to pristine. A relatively high contact angle of 148° was achieved with a 560 nm roughness, producing a highly hydrophobic material. On the other hand, it was possible to tone the hydrophobicity and significantly reduce adhesion work. All materials were highly stable during the long-lasting separation process and were characterized by excellent effectiveness in water desalination.

Janus Membrane with a Dense Hydrophilic Surface Layer for Robust Fouling and Wetting Resistance in Membrane Distillation: New Insights into Wetting Resistance

Although membrane distillation (MD) has been identified as a promising technology to treat hypersaline wastewaters, its practical applications face two prominent challenges: membrane wetting and fouling. Herein, we report a facile and scalable approach for fabricating a Janus MD membrane comprising a dense polyvinyl alcohol (PVA) surface layer and a hydrophobic polyvinylidene fluoride (PVDF) membrane substrate. By testing the Janus membrane in direct contact MD experiments using feeds containing a sodium dodecyl sulfate (SDS) surfactant or/and mineral oil, we demonstrated that the dense Janus membrane can simultaneously resist wetting and fouling. This method represents the simplest approach to date for fabricating MD membranes with simultaneous wetting and fouling resistance. Importantly, we also unveil the mechanism of wetting resistance by measuring the breakthrough pressure and surfactant permeation (through the PVA layer) and found that wetting resistance imparted by a dense hydrophilic layer is attributable to capillary pressure. This new insight will potentially change the paradigm of fabricating wetting-resistant membranes and enable robust applications of MD and other membrane contactor processes facing challenges of pore wetting or/and membrane fouling.

In Situ Three-Dimensional Welded Nanofibrous Membranes for Robust Membrane Distillation of Concentrated Seawater

Membrane distillation (MD) is a promising technology for treating the concentrated seawater discharged from the desalination process. Interconnected porous membranes, fabricated by additive manufacturing, have received significant attention for MD technology because of their excellent permeability. However, their poor hydrophobic durability induced by the deformation of pores constrains their water desalination performance. Herein, an in situ three-dimensional (3D) welding approach involving emulsion electrospinning is reported for fabricating robust nanofibrous membranes. The reported method is simple and effective for welding nanofibers at their intersections, and the reinforced membrane pores are uniform in the 3D space. The results show that the in situ 3D welded nanofibrous membrane, with a stability of 170 h and water recovery of 76.9%, exhibits better desalination performance than the nonwelded (superhydrophobic) nanofibrous membrane and the postwelded (superhydrophobic) nanofibrous membrane. Furthermore, the stability mechanism of the in situ 3D welded nanofibrous membrane and the two different wetting mechanisms of the nonwelded and postwelded nanofibrous membranes were investigated in the current work. More significantly, the in situ 3D welded nanofibrous membrane can further concentrate the actual concentrated seawater (121°E, 37°N) to crystallization, demonstrating its potential applications for the desalination of challenging concentrated seawater.

PVDF Composite Membranes with Hydrophobically-Capped CuONPs for Direct-Contact Membrane Distillation

Water scarcity is an imminent problem that humanity is beginning to attempt to solve. Among the several technologies that have been developed to mitigate water scarcity, membrane distillation is of particular note. In the present work, CuO nanoparticles capped with 1-octanethiol (CuONPs@CH) or 1H,1H,2H,2H-perfluorodecanethiol (CuONPs@CF) are prepared. The nanoparticles are characterized by FT-IR and TGA methods. Two weight losses are observed in both cases, with the decomposition of the organic fragments beginning at 158 °C and 230 °C for CuONPs@CF and CuONPs@CH, respectively. Flat sheet PVDF composite membranes containing nanoparticles are prepared by the casting solution method using nanoparticle concentrations that ranged between 2-20% with a non-woven polyester fabric as support. The obtained membranes showed a thickness of 240 ± 40 μm. According to water contact angle (87° for CuONPs@CH and 95° for CuONPs@CF, both at 10% w.t) and roughness (12 pixel for CuONPs@CH and 14 pixels for CuONPs@CF, both at 10% w.t) determinations, the hydrophobicity of membranes changed due to a decrease in surface energy, while, for naked CuONPs, the roughness factor represents the main role. Membranes prepared with capped nanoparticles showed similar porosity (60-64%). SEM micrographs show asymmetric porous membranes with a 200-nm surface pore diameter. The largest finger-like pores in the membranes prepared with CuONPs, CuONPs@CH and CuONPs@CF had values of 63 ± 10 μm, 32 ± 8 μm, and 45 ± 10 μm, respectively. These membranes were submitted to a direct contact membrane distillation module and flux values of 1.8, 2.7, and 3.9 kg(m2·h)-1 at ΔT = 30 °C were obtained for the CuONPs, CuONPs@CH, and CuONPs@CF, respectively. The membranes showed 100% salt rejection during the testing time (240 min).

Effective separation of water-DMSO through solvent resistant membrane distillation (SR-MD)

The treatment of organic waste or wastewater with high organic solvent content has been challenging in industries as it cannot be done effectively using conventional wastewater treatment technologies such as biodegradation and advanced oxidation process. Solvent resistant membrane distillation (SR-MD) was proposed as an energy-efficient alternative to treat these waste streams but its application is hampered by the lack of solvent-resistant membranes, and there is a research gap in studying the feeds with water-solvent mixtures. In this work, ceramic tubular membranes with different pore sizes and structures were molecularly grafted with 1H,1H,2H,2H-perfluorodecyltriethoxysilane to obtain hydrophobic ceramic membranes for SR-MD. The modified membranes exhibited excellent hydrophobicity and solvent resistant properties, and they were tested for SR-MD performance with a wide range of dimethyl sulfoxide (DMSO) feed concentrations, from 3.5 to 85 wt%. The membranes exhibited a high DMSO rejection of >98% and the separation factor of >170, with permeation flux >4.4 kg m^-2 h^-1 when the DMSO concentration in feed was below 65 wt%. The separation performance was found strongly dependent on the evaporation step and the vapour-liquid equilibrium near the interface. The DMSO rejection was also comparable to pervaporation while the permeation flux was much higher at the feed concentration of 50 wt%. This study establishes the strategy of using SR-MD as a promising membrane process in treating complex industrial wastes with high organic solvent content.

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.

Oily Wastewater Treatment: Overview of Conventional and Modern Methods, Challenges, and Future Opportunities

Industrial developments in the oil and gas, petrochemical, pharmaceutical and food sector have contributed to the large production of oily wastewater worldwide. Oily wastewater pollution affects drinking water and groundwater resources, endangers aquatic life and human health, causes atmospheric pollution, and affects crop production. Several traditional and conventional methods were widely reported, and the advantages and limitations were discussed. However, with the technology innovation, new trends of coupling between techniques, use of new materials, optimization of the cleaning process, and multiphysical approach present new paths for improvement. Despite these trends of improvement and the encouraging laboratory results of modern and green methods, many challenges remain to be raised, particularly the commercialization and the global aspect of these solutions and the reliability to reduce the system’s maintenance and operational cost. In this review, the well-known oily wastewater cleaning methods and approaches are being highlighted, and the obstacles faced in the practical use of these technologies are discussed. A critical review on the technologies and future direction as the road to commercialization is also presented to persevere water resources for the benefit of mankind and all living things.

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.

The use of carbon nanomaterials in membrane distillation membranes: a review

Membrane distillation (MD) is a thermal-based separation technique with the potential to treat a wide range of water types for various applications and industries. Certain challenges remain however, which prevent it from becoming commercially widespread including moderate permeate flux, decline in separation performance over time due to pore wetting and high thermal energy requirements. Nevertheless, its attractive characteristics such as high rejection (ca. 100%) of nonvolatile species, its ability to treat highly saline solutions under low operating pressures (typically atmospheric) as well as its ability to operate at low temperatures, enabling waste-heat integration, continue to drive research interests globally. Of particular interest is the class of carbon-based nanomaterials which includes graphene and carbon nanotubes, whose wide range of properties have been exploited in an attempt to overcome the technical challenges that MD faces. These low dimensional materials exhibit properties such as high specific surface area, high strength, tuneable hydrophobicity, enhanced vapour transport, high thermal and electrical conductivity and others. Their use in MD has resulted in improved membrane performance characteristics like increased permeability and reduced fouling propensity. They have also enabled novel membrane capabilities such as in-situ fouling detection and localised heat generation. In this review we provide a brief introduction to MD and describe key membrane characteristics and fabrication methods. We then give an account of the various uses of carbon nanomaterials for MD applications, focussing on polymeric membrane systems. Future research directions based on the findings are also suggested.

Fluorinated silica-modified anti-oil-fouling omniphobic F-SiO2@PES robust membrane for multiple foulants feed in membrane distillation

Direct-contact membrane distillation (DCMD) can be eminent solution for oily wastewater treatment if the membrane provided is slippery and tolerant to low surface tension complex solutions. This study describes preparation of an anti-oil-fouling omniphobic polyethersulfone membrane using fluorinated silica nanoparticles (F-SiO₂@PES) combined with perfluorodecyl triethoxysilane and polydimethylsiloxane for application against oil-In-water (o/w) emulsions. Feed solutions consist of different concentrations of oil (hexadecane), different charge surfactants (anionic sodium dodecyl benzenesulfonate, non-ionic Tween 20, and cationic hexadecyltrimethylammonium bromide, and salt (NaCl). The hierarchical re-entrant micro structured surface of the omniphobic F-SiO₂@PES membrane and functional groups are confirmed by atomic force microscopy, scanning electron microscopy, and Fourier-transform infrared spectroscopy. The anti-oil-fouling and anti-wetting performance of omniphobic F-SiO₂@PES membranes are investigated using contact-angle, sliding angles, DCMD tests with multiple foulants of surfactants. Omniphobic F-SiO₂@PES membrane exhibited effective anti-oil-fouling and anti-wetting performance against emulsions as no severe fouling and a conductivity rises were evident regardless of surfactant charge and the concentration of components. Flux reduction and rejection rates for the omniphobic F-SiO₂@PES membranes are in a range of 5-15% (only) and >99%, respectively, for various combinations of feed solution components.

Functional PVDF ultrafiltration membrane for Tetrabromobisphenol-A (TBBPA) removal with high water recovery

Tetrabromobisphenol-A (TBBPA) is one of the most important brominated flame retardants (BFRs), accounting for 60% of the total commercial BFR market. Increasing amounts of TBBPA and byproducts are released to the aquatic environment due to their extensive utilization in various sectors. However, research on the treatment of TBBPA contaminated wastewater using membrane filtration is still lacked. Herein, a PVDF10-PAA-ZrO₂ membrane was successfully developed and applied for the treatment of high-concentration TBBPA wastewater with super-high water recovery. The membrane was obtained through surface functionalization with nano-ZrO₂ from commercial PVDF ultrafiltration (UF) membrane. Compared to the commercial PVDF membrane, the developed membrane exhibited 4 times of permeate flux which was up to 200 L/m² min with comparable TBBPA rejection rate. Furthermore, the mechanisms of membrane development and TBBPA rejection were explored through synchrotron-based ATR-FTIR and X-ray analyses. It was revealed that ZrO₂ NPs were immobilized into membrane surface through binding with PAA layer, where the O of the carboxyl group combined with the Zr⁴⁺ on the ZrO₂ NP surface to form C-O-Zr bond through monodentate and bridging-bidentate modes. The sieving function of membrane could be the main mechanism of TBBPA removal. This research demonstrated a practical route and solid insight toward the development of highly efficient membrane for TBBPA removal. The proposed PVDF10-PAA-ZrO₂ membrane can also be promising for other industrial separation and purification applications.

Recent Developments in Nanomaterials-Modified Membranes for Improved Membrane Distillation Performance

Membrane distillation (MD) is a thermally induced membrane separation process that utilizes vapor pressure variance to permeate the more volatile constituent, typically water as vapor, across a hydrophobic membrane and rejects the less volatile components of the feed. Permeate flux decline, membrane fouling, and wetting are some serious challenges faced in MD operations. Thus, in recent years, various studies have been carried out on the modification of these MD membranes by incorporating nanomaterials to overcome these challenges and significantly improve the performance of these membranes. This review provides a comprehensive evaluation of the incorporation of new generation nanomaterials such as quantum dots, metalloids and metal oxide-based nanoparticles, metal organic frameworks (MOFs), and carbon-based nanomaterials in the MD membrane. The desired characteristics of the membrane for MD operations, such as a higher liquid entry pressure (LEPw), permeability, porosity, hydrophobicity, chemical stability, thermal conductivity, and mechanical strength, have been thoroughly discussed. Additionally, methodologies adopted for the incorporation of nanomaterials in these membranes, including surface grafting, plasma polymerization, interfacial polymerization, dip coating, and the efficacy of these modified membranes in various MD operations along with their applications are addressed. Further, the current challenges in modifying MD membranes using nanomaterials along with prominent future aspects have been systematically elaborated.

Retardation of wetting for membrane distillation by adjusting major components of seawater

Wetting by fouling is phenomenon specific to membrane distillation (MD) and are regarded as challenges to the seawater membrane distillation (SWMD) process. To understand fouling and wetting, the influence of Mg and Sr crystals, which can potentially cause scaling, as well as Ca crystals deposited on the membrane surface were investigated. Mg(OH)₂ and CaSO₄ had significant impact on fouling and wetting. Even if CaCO₃ and SrSO₄ had no effects on fouling and wetting as single salts, CaCO₃ and CaSO₄ were dominant in synthetic seawater without Mg(OH)₂. However, the occurrence of Mg(OH)₂ scales became a cause for concern if Ca ion was removed from seawater for the prevention of fouling and wetting. Therefore, Mg as well as Ca should be removed for proper fouling and wetting control. NaOH/Na₂CO₃ softening was used for the removal of Ca and Mg ions. In addition, based on the inhibition effects of Mg ions on Ca scales, a new pretreatment method involving the injection of MgCl₂ to increase the Mg /Ca ratio was examined.

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.