Antifouling, high-flux oil/water separation carbon nanotube membranes by polymer-mediated surface charging and hydrophilization

High-Flux Steady-State Demulsification of Oil-In-Water Emulsions by Superhydrophilic-Oleophobic Copper Foams with Ultra-Small Pores Under Pressure

3D superwetting materials struggle to maintain high-flux steady-state demulsification for oil-in-water emulsions because the accumulated oil within the material is difficult to discharge rapidly. The water flow shear force can swiftly remove the oil from the anti-fouling surface. In this study, by introducing nanofibers and carbon nanotubes and chemical modification, a superhydrophilic-oleophobic copper foam with pores of several micrometers is prepared, which can achieve a continuous demulsification process with steady-state flux over 57000 L m-2 h-1 for oil-in-water emulsions and rapid hydraulic-driven oil release under an additional pressure of 5 kPa. Thanks to the ultra-small pores of the copper foam, the steady-state demulsification efficiency can be still maintained at over 97.5%. During the demulsification process, the accumulation of oil and surfactants within the copper foam can be maintained at low levels, achieving dynamic equilibrium. With the aid of second-stage superhydrophilic copper mesh, the demulsified oil-water mixtures can be rapidly separated. This high-flux, steady-state, and efficient demulsification process shows great potential for industrial applications.

Enhancing membrane performance for oily wastewater treatment: comparison of PVDF composite membranes prepared by coating, blending, and grafting methods using TiO2, BiVO4, CNT, and PVP

This comparative study investigates the modification of polyvinylidene fluoride (PVDF) membranes with different nanoparticles (TiO2 or TiO2-based composites containing BiVO4 and/or CNT), using three distinct methods (blending, coating, and grafting) and polyvinylpyrrolidone (PVP). The objective was to enhance the photocatalytic and filtration performance for the separation of oil-in-water emulsions. Regarding the UV activity, the PVDF-TiO2/CNT/PVP-coated membrane presented the best performance. Overall, the addition of 2 wt.% CNT to the TiO2 notably enhanced the photocatalytic activity of the membranes for both UV and visible irradiations. Meanwhile, the presence of 2 wt.% BiVO4 was beneficial only for photocatalysis under visible light irradiation. Regarding the filtration of the oil-in-water emulsions, 2 wt.% CNT or BiVO4 addition resulted in the highest fluxes in the series of the PVDF-TiO2-grafted membranes. The presence of pore former PVP led to relatively high fluxes and photocatalytic activities for all series. Regarding the modification methods, coated membranes showed the highest photocatalytic efficiency and lowest fluxes. Grafted membranes showed relatively high photocatalytic efficiencies and the best filtration performances.

Formation of Microporous Poly Acrylonitrile-Co-Methyl Acrylate Membrane via Thermally Induced Phase Separation for Immiscible Oil/Water Separation

An interconnected sponge structure and porous surface poly (acrylonitrile-co-methyl acrylate) (P(AN-MA)) microfiltration membranes (MF) were fabricated via thermally induced phase separation (TIPS) by using caprolactam (CPL), and acetamide (AC) as the mixed diluent. When the ternary system was composed of 15 wt.% P(AN-MA), 90 wt.% CPL, and 10 wt.% AC and formed in a 25 °C air bath, the membrane exhibited the highest water flux of 8107 L/m2·h. The P(AN-MA) membrane contained hydrophobic groups (-COOCH3) and hydrophilic groups (-CN), leading it to exhibit oleophobic properties underwater and hydrophobic properties in oil. The membrane demonstrates efficient separation of immiscible oil/water mixtures. The pure water flux of the petroleum ether/water mixture measured 870 L/m2·h, and the pure oil flux of the petroleum tetrachloride/water mixture measured 1230 L/m2·h under the influence of gravity. Additionally, the recovery efficiency of diluents through recrystallization was 85.3%, significantly reducing potential pollution and production costs.

Zwitterionic Tröger's Base Microfiltration Membrane Prepared via Vapor-Induced Phase Separation with Improved Demulsification and Antifouling Performance

The fouling of separation membranes has consistently been a primary factor contributing to the decline in membrane performance. Enhancing the surface hydrophilicity of the membrane proves to be an effective strategy in mitigating membrane fouling in water treatment processes. Zwitterionic polymers (containing an equimolar number of homogeneously distributed anionic and cationic groups on the polymer chains) have been used extensively as one of the best antifouling materials for surface modification. The conventional application of zwitterionic compounds as surface modifiers is intricate and inefficient, adding complexity and length to the membrane preparation process, particularly on an industrial scale. To overcome these limitations, zwitterionic polymer, directly used as a main material, is an effective method. In this work, a novel zwitterionic polymer (TB)-zwitterionic Tröger's base (ZTB)-was synthesized by quaternizing Tröger's base (TB) with 1,3-propane sultone. The obtained ZTB is blended with TB to fabricate microfiltration (MF) membranes via the vapor-induced phase separation (VIPS) process, offering a strategic solution for separating emulsified oily wastewater. Atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle, and zeta potential measurements were employed to characterize the surface of ZTB/TB blended membranes, assessing surface morphology, charge, and hydrophilic/hydrophobic properties. The impact of varying ZTB levels on membrane surface morphology, hydrophilicity, water flux, and rejection were investigated. The results showed that an increase in ZTB content improved hydrophilicity and surface roughness, consequently enhancing water permeability. Due to the attraction of water vapor, the enrichment of zwitterionic segments was enriched, and a stable hydration layer was formed on the membrane surface. The hydration layer formed by zwitterions endowed the membrane with good antifouling properties. The proposed mechanism elucidates the membrane's proficiency in demulsification and the reduction in irreversible fouling through the synergistic regulation of surface charge and hydrophilicity, facilitated by electrostatic repulsion and the formation of a hydration layer. The ZTB/TB blended membranes demonstrated superior efficiency in oil-water separation, achieving a maximum flux of 1897.63 LMH bar-1 and an oil rejection rate as high as 99% in the oil-water emulsion separation process. This study reveals the migration behavior of the zwitterionic polymer in the membrane during the VIPS process. It enhances our comprehension of the antifouling mechanism of zwitterionic membranes and provides guidance for designing novel materials for antifouling membranes.

Carbon nanomaterials for designing next-generation membranes and their emerging applications

Carbon nanomaterials have enormous applications in various fields, such as adsorption, membrane separation, catalysis, electronics, capacitors, batteries, and medical sciences. Owing to their exceptional properties, such as large specific surface area, carrier mobility, flexibility, electrical conductivity, and optical pellucidity, the family of carbon nanomaterials is considered as one of the most studied group of materials to date. They are abundantly used in membrane science for multiple applications, such as the separation of organics, enantiomeric separation, gas separation, biomolecule separation, heavy metal separation, and wastewater treatment. This study provides an overview of the significant studies on carbon nanomaterial-based membranes and their emerging applications in our membrane research journey. The types of carbon nanomaterials, their utilization in membrane-based separations, and the mechanism involved are summarized in this study. Techniques for the fabrication of different nanocomposite membranes are also highlighted. Lastly, we have provided an overview of the existing issues and future scopes of carbon nanomaterial-based membranes for technological perspectives.

C-shaped porous polypropylene fibers for rapid oil absorption and effective on-line oil spillage monitoring

Development of efficient absorbent materials for detection and treatment of offshore oil spillages remained a challenge. In this work, C-shaped polypropylene oil-absorbent fibers with sub-micron internal pores were prepared by combining spun-bonding technique and thermally induced phase separation (TIPS). The effect of drawing speed on the phase separation and the porous morphology of the shaped fiber non-woven fabric (NWF) was investigated. C-shaped NWF with porous morphology had large water contact angle, higher porosity, larger specific surface area, and increased oil absorption speed and capacity. An online oil spillage detection system was developed using porous C-shaped NWF and an oxygen sensing probe, showing shorter response time and higher signal-to-noise (STN) ratio. The response time for detecting the spillage of soybean oil and diluted crude oil (0.5 mL/0.8 L) in water were only 24 s and 10 s, respectively. The reliable oil detection low detection limit (RLDL) of the oxygen sensing probe was reduced 173 times (from 36.5 g/L to 0.21 g/L) when combined with C-shaped porous fiber NWF.

Effect of Electrofiltration on the Dewatering Kinetics of Arthrospira platensis and Biocompound Recovery

Arthrospira platensis (A. platensis) is a microalga with a wide range of commercial uses. One of the main concerns that needs to be addressed in microalgae biorefineries is the costs associated with the harvesting and concentration steps. Filtration has been shown to be an effective technique for concentrating microalgae and recent studies have attempted to enhance membrane filtration by applying an external electric field to the filtration cell. This study consisted of assessing the use of electrically assisted filtration (electrofiltration) at 60 A/m2 and 1 bar for the dewatering of A. platensis, as well as the effect of pretreating the microalgae with ultrasounds (US) on the filtration process. Untreated A. platensis exhibited better filtration kinetics than US-treated A. platensis, and electrofiltration was found to increase the cake dryness. More protein and pigments were present in the US-treated microalgae solution compared to the untreated microalgae, which led to the presence of higher concentrations of protein and pigments in the filtrate streams after pressure filtration at 1 bar without the application of an external electric field. Electrofiltration was found to consume less energy compared to traditional drying techniques used for A. platensis. However, electrofiltration degrades the biocompounds present in the filtrate and cake due to pH changes and other electrophoresis phenomena, which shows the need to optimize the process in future work.

Crosslinked biomimetic coating modified stainless-steel-mesh enables completely self-cleaning separation of crude oil/water mixtures

The development of high-flux, durable and completely self-cleaning membranes is highly desired for separation of massive oil/water mixtures. Herein, differently crosslinked poly(2-methacryloyloxylethyl phosphorylcholine) (PMPC) brush grafted stainless steel mesh (SSM) membranes (SSM/PMPCs) were fabricated by integrating of mussel inspired universal adhesion and crosslinking chemistry with surface-initiated activators regenerated by electron transfer atom transfer radical polymerization (SI-ARGET-ATRP). The durability and self-cleaning performance of the prepared SSM membranes were evaluated by separating sticky crude oil/water mixtures in a continuous recycling dead-end filtration device. The water filtration flux driven by gravity reached 60,000 L⋅m^-2⋅h^-1 with a separation efficiency of over 99.98%. Furthermore, zero-flux-decline was observed during a 5 h continuous filtration when assisted by mechanical stirring. More significantly, such a completely self-cleaning separation of the well crosslinked SSM/PMPC2 membrane under optimized flux and stirring conditions had been operated cumulatively for 190 h in 30 days without any additional cleaning. These significant advances are more promising for practical applications in crude oil-contaminated water treatments and massive oil/water mixture separation.

Membranes for Oil/Water Separation: A Review

Recent advancements in separation and membrane technologies have shown a great potential in removing oil from wastewaters effectively. In addition, the capabilities have improved to fabricate membranes with tunable properties in terms of their wettability, permeability, antifouling, and mechanical properties that govern the treatment of oily wastewaters. Herein, authors have critically reviewed the literature on membrane technology for oil/water separation with a specific focus on: 1) membrane properties and characterization, 2) development of various materials (e.g., organic, inorganic, and hybrid membranes, and innovative materials), 3) membranes design (e.g., mixed matrix nanocomposite and multilayers), and 4) membrane fabrication techniques and surface modification techniques. The current challenges and future research directions in materials and fabrication techniques for membrane technology applications in oil/water separation are also highlighted. Thus, this review provides helpful guidance toward finding more effective, practical, and scalable solutions to tackle environmental pollution by oils.

Hydrophilization of Hydrophobic Mesoporous High-Density Polyethylene Membranes via Ozonation

This work addresses hydrophilization of hydrophobic mesoporous membranes based on high-density polyethylene (HDPE) via ozonation. Mesoporous HDPE membranes were prepared by intercrystallite environmental crazing. Porosity was 50%, and pore dimensions were below 10 nm. Contact angle of mesoporous membranes increases from 96° (pristine HDPE) to 120° due to the formation of nano/microscale surface relief and enhanced surface roughness. The membranes are impermeable to water (water entry threshold is 250 bar). The prepared membranes were exposed to ozonation and showed a high ozone uptake. After ozonation, the membranes were studied by different physicochemical methods, including DSC, AFM, FTIR spectroscopy, etc. Due to ozonation, wettability of the membranes was improved: their contact angle decreased from 120° down to 60°, and they became permeable to water. AFM micrographs revealed a marked smoothening of the surface relief, and the FTIR spectra indicated the development of new functionalities due to ozonolysis. Both factors contribute to hydrophilization and water permeability of the ozonated HDPE membranes. Hence, ozonation was proved to be a facile and efficient instrument for surface modification of hydrophobic mesoporous HDPE membranes and can also provide their efficient sterilization for biomedical purposes and water treatment.

A review on super-wettable porous membranes and materials based on bio-polymeric chitosan for oil-water separation

Appropriate surface wettability of membranes and materials are of an extreme importance for targeting separation of mixtures/emulsions such as oil from water or conversely water from oil. The development of super-wettable membranes and materials surfaces have shown remarkable potential for recovering water from oil-water emulsion while offering maximum resistance to fouling. The availability of clean and potable water has been regarded as an important global challenge for coming human generations. Oil and gas industry is continuously producing immense quantities of waste stream regarded as produced water which contains oil dispersed in water along with other several components. Treating such immense quantities of oily wastewater is of utmost need for recovering precious water for possible reuse or safe disposal. Various technologies have been developed for targeting the separation of oil-water emulsions or mixtures to harness useful potable water and oil as products. Membrane-based separations or use of porous materials such as mesh have been explored in literature for separation of oil-water mixtures/emulsions. Given the unique features of special hydrophilicity, ease of tunability, control of molecular weight, abundant availability, and potential for commercial scale up, chitosan has been extensively used for modifying membranes/meshes or preparing composites with other materials for oil-water separations. This review has described in detail the synthesis, methods of modification and application of chitosan-based super-wettable membranes/meshes and porous materials for oil-water separation. The special wettability features including super-hydrophobicity/superoleophilicity, super-oleophobicity/super-hydrophilicity and super-hydrophilicity/underwater super-oleophobicity of various chitosan-based membranes and materials have been discussed in detail in the review. The strategies for enhancing or developing special wettability for target specific applications have also been discussed. Finally, the challenges, their respective importance have been identified along with a discussion on possible solutions to these challenges.

Tannic acid-based functional coating: surface engineering of membranes for oil-in-water emulsion separation

Recent progress in the tannic acid-based functional coating for surface engineering of membranes toward oil-in-water emulsion separation is summarized.

Absolute and Fast Removal of Viruses and Bacteria from Water by Spraying-Assembled Carbon-Nanotube Membranes

Membrane separation is able to efficiently remove pathogens like bacteria and viruses from water based on size exclusion. However, absolute and fast removal of pathogens requires highly permeable but selective membranes. Herein, we report the preparation of such advanced membranes using carbon nanotubes (CNTs) as one-dimensional building blocks. We first disperse CNTs with the help of an amphiphilic block copolymer, poly(2-dimethylaminoethyl methacrylate)-block-polystyrene (PDMAEMA-b-PS, abbreviated as BCP). The PS blocks adsorb on the surface of CNTs via the π-π interaction, while the PDMAEMA blocks are solvated, thus forming homogeneous and stable CNT dispersions. We then spray the CNT dispersions on porous substrates, producing composite membranes with assembled CNT layers as the selective layers. We demonstrate that the optimized membrane shows 100% rejection to phage viruses and bacteria (Escherichia coli) while giving a water permeance up to ∼3300 L m^-2 h^-1 bar^-1. The performance of the resultant BCP/CNT membrane outperforms that of state-of-the-art membranes and commercial membranes. The BCP/CNT membrane can be used for multiple runs and regenerated by water rinsing. Membrane modules assembled from large-area membrane sheets sustain the capability of absolute and fast removal of viruses and bacteria.

Recent Progress in Superhydrophilic Carbon-Based Composite Membranes for Oil/Water Emulsion Separation

The purification of stabilized oil/water emulsions is essential to meet the ever increasing demand for monitoring water in the environment, which has been addressed with superwetting carbon-based separation membranes. These include superhydrophilic carbon-based membranes whose progress in recent years and perspectives are reviewed in this paper. The membrane construction strategy is organized into four parts, vacuum-assisted self-assembly, sol-gel process, electrospinning, and vacuum-assisted filtration. In each section, the design strategies and their responding disadvantages have been comprehensively discussed. The challenges and prospects concerning the superhydrophilic carbon-based separation membranes for oily wastewater purification are also summarized to arouse researchers to carry out more studies.

ZnO Nanoneedle-Modified PEEK Fiber Felt for Improving Anti-fouling Performance of Oil/Water Separation

Membrane separation has been considered to be the most effective decontamination method for oily waste water. The most significant point of membrane separation is the resistance against membrane fouling. Fabricating hierarchical architectures on the membrane surface is an available approach to improving its anti-fouling property. In this study, ZnO nanoneedles were successfully anchored onto surface-sulfonated poly(ether-ether-ketone) (PEEK) felt via UV/ozone cleaning and hydrothermal synthesis. The modified felt (PEEK-f-Z) showed much better anti-fouling properties and far higher rejection height (33 cm) than the unmodified felt (17 cm) with a separation efficiency up to 99.99%. The enhanced separation properties could be attributed to the stronger water locking capability of the hierarchical architectures on the surface. Furthermore, benefiting from the great chemical stability of PEEK substrates and ZnO nanoneedles, the as-prepared membrane exhibited admirable solvent resistance, mechanical strength, and thermal stability. As a result, PEEK-f-Z could even separate immiscible organic liquids with different polarities and collect hot water from the oil/water mixture, promising to be used under severe conditions.

Engineering and Characterization of Antibacterial Coaxial Nanofiber Membranes for Oil/Water Separation

In the present study, a coaxial nanofiber membrane was developed using the electrospinning technique. The developed membranes were fabricated from hydrophilic cellulose acetate (CA) polymer and hydrophobic polysulfone (PSf) polymer as a core and shell in an alternative way with addition of 0.1 wt.% of ZnO nanoparticles (NPs). The membranes were treated with a 2M NaOH solution to enhance hydrophilicity and thus increase water separation flux. Chemical and physical characterizations were performed, such as Fourier transform infrared (FTIR) spectroscopy, and surface wettability was measured by means of water contact angle (WCA), mechanical properties, surface morphology via field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and microscopy energy dispersive (EDS) mapping and point analysis. The results show higher mechanical properties for the coaxial nanofiber membranes which reached a tensile strength of 7.58 MPa, a Young's modulus of 0.2 MPa, and 23.4 M J.m-3 of toughness. However, treated mebranes show lower mechanical properties (tensile strength of 0.25 MPa, Young's modulus of 0.01 MPa, and 0.4 M J.m-3 of toughness). In addition, the core and shell nanofiber membranes showed a uniform distribution of coaxial nanofibers. Membranes with ZnO NPs showed a porous structure and elimination of nanofibers after treatment due to the formation of nanosheets. Interestingly, membranes changed from hydrophobic to hydrophilic (the WCA changed from 90 ± 8° to 14 ± 2°). Besides that, composite nanofiber membranes with ZnO NPs showed antibacterial activity against Escherichia coli. Furthermore, the water flux for the modified membranes was improved by 1.6 times compared to the untreated membranes.

Prewetting Polypropylene-Wood Pulp Fiber Composite Nonwoven Fabric for Oil-Water Separation

The removal of oil from the water surface is vital to protect the environment and living organisms against the threats posed by industrial oily wastewater and offshore oil spills. High cost, low efficiency, and environmental pollution limit the widespread use of the commercial methods of oil-water separation. In this study, the prewetting polypropylene-wood pulp fiber composite nonwoven fabric (PWNF) has been used in the gravity-driven separation of the oil-water mixture. The prewetting PWNF displayed superior underwater oleophobic properties, and the underwater kerosene contact angle was 137.65° ± 4.27°. The oil-water interfacial tension in the microchannels among the PWNF fibers prevented the oil from passing through the microchannels; however, it allowed the passage of water. The PWNF membrane maintained an excellent oil-water separation performance after repeated separation and long-term soaking cycles. The separation membrane maintained 75% and 50% of the initial separation performance after 40 repeat cycles and water immersion for more than 20 days, respectively. The separation rate of the PWNF membrane was also investigated as a function of salt solution concentration, temperature, and pH. Meanwhile, the influences of prewetting time, prewetting temperature, and different dyeing condition of the mixture on the separation rate were clarified. The destruction of the oil-water contact interface was suggested as the main failure mode of the developed PWNF separation membrane. The maximum kerosene height that the PWNF separation membrane could sustain was 800 mm. The obtained results confirmed that the PWNF separation membrane exhibited the high potential of widespread use in various environments for achieving efficient and stable separations, especially for the oily wastewater treatment.

Fabrication of pH-Sensitive Superhydrophilic/Underwater Superoleophobic Poly(vinylidene fluoride)-graft-(SiO2 Nanoparticles and PAMAM Dendrimers) Membranes for Oil-Water Separation

The efficient treatment of oil-water emulsions under acidic condition remains a widespread concern. Poly(amidoamine) (PAMAM) dendrimer with hyperbranched structures and a large amount of primary and tertiary amino groups has exhibited advantages to solve this issue. Here, a novel poly(vinylidene fluoride)-graft-(SiO₂ nanoparticles and PAMAM dendrimers) (PVDF-g-SiO₂ NPs/PAMAM) membrane was fabricated using a surface-grafting strategy. SiO₂ NPs were immobilized on PVDF-g-poly(acrylic acid) (PAA) membranes for improving the surface roughness, and PAMAM dendrimers were further immobilized on the membrane surface by interfacial polymerization (IP) for improving the surface energy. The obtained membrane demonstrated a water contact angle and a stable underwater-oil contact angle of 0° and >150°, respectively. These characteristics endowed the membrane with excellent water permeability [>3100 L/(m²·h) at 0.9 bar] and separation efficiency (>99%) during oil-water separation. Furthermore, the PAMAM chain will extend from a collapsed state into a fully extension state because of the protonation of amine groups under acidic condition, thus achieving a low underwater oil-adhesion property, fouling resistance, desirable stability, and recyclability (over 12 cycles) during usage. This work shows a promising prospect for the treatment of corrosive emulsions under acidic condition.

A hierarchical structured steel mesh decorated with metal organic framework/graphene oxide for high-efficient oil/water separation

A hierarchical structured steel mesh decorated with metal organic framework (UiO-66-NH₂) nanoparticles/graphene oxide (GO) nanosheets was successfully prepared via a simple self-assemble method. Because water molecules tend to build hydrogen bonds with the amine, carboxyl and hydroxyl functional groups of UiO-66-NH₂/GO hierarchical structure, the hierarchical structure can easily capture water and tightly lock the water to build a stable water layer on the steel mesh surface and block oil in contact with the steel mesh. Therefore, the obtained hierarchical structured steel mesh exhibits super-hydrophilicity, underwater super-oleophobicity, excellent oil resistance and outstanding oil/water separation performance with a superior high permeating flux (54,500 L m^-2  h^-1) and rejection (>99.9%) under gravity force, indicating the mesh possesses great potential for treating oily wastewater.