Engineering a superwettable polyolefin membrane for highly efficient oil/water separation with excellent self-cleaning and photo-catalysis degradation property

Sources, colloidal characteristics, and separation technologies for highly hazardous waste nanoemulsions

Nanoemulsions play a crucial role in various industries. However, their application often results in hazardous waste, posing significant risks to human health and the environment. Effective management and separation of waste nanoemulsions requires special attention and effort. This review provides a comprehensive understanding of waste nanoemulsions, covering their sources, characteristics, and suitable treatment technologies, intending to mitigate their environmental impact. This study examines the evolution of nanoemulsions from beneficial products to hazardous wastes, provides an overview of the production processes, fate, and hazards of waste nanoemulsions, and highlights the critical characteristics that affect their stability. The latest advancements in separating waste nanoemulsions for recovering oil and reusable water resources are also presented, providing a comprehensive comparison and evaluation of the current treatment techniques. This review addresses the significant challenges in nanoemulsion treatment, provides insights into future research directions, and offers valuable implications for the development of more effective strategies to mitigate the hazards associated with waste nanoemulsions.

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.

Super-wetting and self-cleaning polyvinyl alcohol/sodium alginate nanofiber membrane decorated with MIL-88A(Fe) for efficient oil/water emulsion separation and dye degradation

Membrane separation is considered an effective approach to water purification. Nevertheless, membrane fouling dramatically decreases the separation efficiency and lifetime of membranes, thus limiting its further development and application. Herein, a multifunctional self-cleaning MIL-88A(Fe) decorated polyvinyl alcohol/sodium alginate (MIL-88A(Fe)@PVA-SA) nanofiber membrane was prepared by electrospinning and in-situ growth methods for the separation of oil/water emulsions and photo-Fenton degradation of dyes. The membrane possesses superhydrophilicity with a water contact angle (WCA) of 0° and superoleophobicity with underwater oil contact angle (UCA) of 161.7°, and exhibits superior separation efficiency (>99.5 %) and permeation flux (1140-2455 L/m2/h) for different oil/water emulsions. Moreover, the membrane exhibited an outstanding photo-Fenton performance under visible light, with degradation efficiencies (~99.9 %) towards methylene blue (MB) and reactive red 24 (RR24) within 90 min. Importantly, the membrane can be easily regenerated by simple rinsing and photo-Fenton self-cleaning treatment. In this study, MIL-88A(Fe)@PVA-SA nanofiber membrane has a promising application in dye removal and oil/water separation, providing a new idea to develop novel membrane materials.

Facile Fabrication of a Robust Superhydrophilic/Underwater Superoleophobic Material for Oil-Fouling Expulsion

The reduction of oil fouling in pipes and tanks is essential for the oil storage and transportation industry. In this study, a superhydrophilic/underwater superoleophobic surface (SUSS) with high wearability, weatherability, and durability was developed using a facile two-step synthesis method and used to expel fouled oil from the surface using water without a surfactant. Some typical oils, including kerosene and white oil, can be spontaneously expelled by static water; however, rapeseed oil requires motive water for expulsion because of its high affinity for the SUSS. Different occurrences can be estimated based on a correlated parameter, φ(Pe), which is calculated using an introduced dimensionless number, P e = σ L V u μ . A positive value of φ indicates the occurrence of fouled-oil expulsion by water replacement, whereas a negative value indicates no occurrence of this phenomenon. This study provides a facile strategy for the rapid cleansing of oil-fouled pipes and tanks without using a detergent, thereby lowering costs and environmental risks.

A solar-driven degradation-evaporation strategy for membrane self-cleaning in the efficient separation of viscous crude oil/water emulsions

Membrane separation techniques are promising methods for effectively treating hazardous emulsified oily wastewater, but membrane fouling remains a serious challenge because the high viscosity and complex composition of crude oil make it easy to adhere to membranes and difficult to be removed by conventional physical or chemical cleaning means. Herein, a two-stage solar-driven (photo-Fenton degradation/evaporation) strategy was proposed to realize the self-cleaning of membranes fouled by viscous crude oil (>60,000 mPa s), wherein the photo-Fenton process helped to degrade the heavy components into light components, and all light components removed during the solar-driven evaporation process. A 1D/2D heterostructure membrane with photo-Fenton activity and anti-crude-oil-fouling performance was prepared via a facile self-assembly vacuum-assist method. The addition of rod-like g-C₃N₄ (RCN) increased the interlayer distance of α-FeOOH/porous g-C₃N₄ (FPCN) nanosheets, resulting in a high permeation flux. The FPCN-RCN membrane exhibited both high permeation flux of 779 ± 19 L m^-2h^-1bar^-1 and a separation efficiency of 99.4% for highly viscous crude oil-in-water emulsion. Importantly, the viscous crude oil fouled on the membrane was completely removed by the photo-Fenton degradation/solar-driven evaporation strategy, and the flux recovery rate of the membrane was ∼100%. Therefore, the FPCN-RCN membrane combined with the novel self-cleaning strategy exhibits great potential for practical emulsified oily wastewater treatment.

Facile Enhancement of Electrochemical Performance of Solid-State Supercapacitor via Atmospheric Plasma Treatment on PVA-Based Gel-Polymer Electrolyte

A facile oxygen (O₂) atmospheric plasma treatment is applied to a polyvinyl alcohol (PVA) matrix to enhance its wettability and hydrophilicity. The optimal plasma treatment conditions are determined by varying the applied plasma power and plasma treatment time. A PVA matrix treated with a plasma power of 120 W for 5 s shows the most hydrophilicity owing to successful formation of carbonyl (-CO, >C=O) functional groups without any structural degradation. The plasma-treated PVA matrix is used as the gel-polymer electrolyte of a solid-state supercapacitor (SSC) by immersing solid matrix into various liquid electrolytes, such as sodium sulfate (Na₂SO₄), sulfuric acid (H₂SO₄), and potassium hydroxide (KOH). Compared with the pristine PVA-based device, PVA-120W5/Na₂SO₄-, PVA-120W5/H₂SO₄-, and PVA-120W5/KOH-based SSCs show 2.03, 2.05, and 2.14 times higher specific capacitances, respectively. The plasma-treated PVA matrix shows increased specific capacitance owing to the increased wettability, which in turn increases the ion transportation and reduces the electrical resistance. This study successfully demonstrates that the electrochemical performance of a SSC can be readily enhanced through plasma treatment for a short time (≤5 s).

Carbon electrochemical membrane functionalized with flower cluster-like FeOOH catalyst for organic pollutants decontamination

Decorating active catalysts on the reactive electrochemical membrane (REM) is an effective way to further improve its decontamination performance. In this work, a novel carbon electrochemical membrane (FCM-30) was prepared through coating FeOOH nano catalyst on a low-cost coal-based carbon membrane (CM) through facile and green electrochemical deposition. Structural characterizations demonstrated that the FeOOH catalyst was successfully coated on CM, and it grew into a flower cluster-like morphology with abundant active sites when the deposition time was 30 min. The nano FeOOH flower clusters can obviously boost the hydrophilicity and electrochemical performance of FCM-30, which enhance its permeability and bisphenol A (BPA) removal efficiency during the electrochemical treatment. Effects of applied voltages, flow rates, electrolyte concentrations and water matrixes on BPA removal efficiency were investigated systematically. Under the operation condition of 2.0 V applied voltage and 2.0 mL·min^-1 flow rate, FCM-30 can achieve the high removal efficiency of 93.24% and 82.71% for BPA and chemical oxygen demand (COD) (71.01% and 54.89% for CM), respectively, with only a low energy consumption (EC) of 0.41 kWh·kgCOD^-1, which can be ascribed to the enhancement on OH yield and direct oxidation ability by the FeOOH catalyst. Moreover, this treatment system also exhibits good reusability and can be adopted on different water background as well as different pollutants.

Thermal and magnetic dual-responsive switchable device with superhydrophilicity/underwater superoleophobicity and excellent targeted oil–water separation performance

In view of the increasingly serious problem of oil–water separation, it is a convenient and practical method to introduce a hydrogel coating on the surface of materials to make super-wetting materials. Nowadays, researchers of super-wetting materials pay more attention to the research and development of responsive materials. Here, a thermal and magnetic dual-responsive superhydrophilicity/underwater superoleophobicity switchable device (Fe3O4@PNIPAM-Cu) was simply fabricated using the Fe3O4 nanoparticles, poly-N-isopropylacrylamide (PNIPAM) hydrogel as the functional coating and copper foam as the skeleton through a one-step solution immersion method. The separation efficiency of the benzene-water mixture of this dual-responsive device can reach up to 99.98%. Even after 10 separation cycles, it maintained an efficiency of more than 99.90%. At temperatures above ~34°C, the device can stop oil–water separation. The experiments presented here demonstrate this dual-responsive device possesses excellent superhydrophilicity/underwater superoleophobicity, thermal-responsive property and magnetic navigation function.

A superwetting stainless steel mesh with Janus surface charges for efficient emulsion separation

Design of charged materials for demulsification of ionic surfactant-stabilized oil-in-water emulsions is emerging in recent years. Herein, a superwetting stainless steel mesh with Janus surface charges (Janus SSM) was prepared by respectively brush-coating polyethyleneimine/aminated carbon nanotubes (PEI/CNTs-NH₂) coating and polyacrylic acid (PAA) coating on its two sides. Two demulsification mechanisms, i.e., electrostatic attraction-repulsion and electrostatic repulsion-attraction based on the synergism of two oppositely charged sides were proposed. Combined with the superwettability and optimized pore size, the Janus SSM can successfully be used to demulsify, coalesce and separate emulsions. In detail, the Janus SSM exhibited separation efficiencies of up to 99.29%, 97.12% for SDS- and DTAC-stabilized oil-in-water emulsions respectively under the electrostatic attraction-repulsion mechanism, and up to 97.10%, 98.57% under the electrostatic repulsion-attraction mechanism. The results indicated that the electrostatic attraction-repulsion mechanism proposed in this study is conductive to achieving higher efficiency in emulsion separation. Furthermore, excellent durability extend the operation life of Janus SSM. This Janus SSM, which combines opposite charges on its two sides, may advance the development of charged materials for emulsion separation.

Novel dopamine-modified cellulose acetate ultrafiltration membranes with improved separation and antifouling performances

Cellulose acetate (CA) is widely used in the preparation of ultrafiltration membranes due to its many excellent characteristics, especially chemical activity and biodegradability. To improve the inherent hydrophobic and antifouling properties of CA membrane, in this work, CA was successfully modified with dopamine (CA-2,3-DA) through selective oxidation and Schiff base reactions, which was confirmed by FTIR and ¹H NMR measurements. Then, CA-2,3-DA membrane with high water permeability and excellent antifouling property was prepared by the phase inversion method. Compared with the original CA membrane, the CA-2,3-DA membrane maintained a higher rejection ratio for BSA (92.5%) with a greatly increased pure water flux (167.3 L m^-2 h^-1), which could overcome the trade-off between permeability and selectivity of the traditional CA membrane to a certain extent. According to static protein adsorption and three-cycle dynamic ultrafiltration experiments, the CA-2,3-DA membrane showed good antifouling performance and superior long-term performance stability, as supported by the experimental results, including flux recovery ratio, flux decline ratio, and filtration resistance. It is expected that this approach can greatly expand the high-value utilization of modified natural organic polysaccharides in separation engineering.

Superhydrophilic polyethersulfone (PES) membranes with high scale inhibition properties obtained through bionic mineralization and RTIPS

Reverse thermally induced separation (RTIPS) was used to obtain a separation membrane with a better internal structure for a higher water flux and a surface that could easily form a hydration layer. In comparison to the traditional modification method, this work focused on the aspect that the internal structure obtained by changing the membrane-making method provided easier adhesion conditions for the dopamine/TiO₂ hybrid nanoparticles (DA/TiO₂ HNPs) obtained by biomimetic mineralization. It provided a basis for exploring the variation in adhesion with the water bath temperature and the amount of titanium added through the study of turbidity point, SEM images, water contact angle, thermogravimetric test, EDX, AFM, XPS, FTIR and other test results. The SEM images proved that the membrane obtained through the RTIPS method had a porous surface and spongy internal structure, furthermore, additional polymers were adsorbed. Use of EDX demonstrated that biomimetic mineralization prevented the production of agglomerated titanium dioxide. XPS and FTIR spectra confirmed the introduction and immobilization of HNP aggregation. Moreover, a decrease in the surface roughness and water contact angle further suggested an improvement in the hydrophilicity of the modified membrane. The introduction of HNP at a higher water bath temperature helped increase the water flux up to ten times, moreover, the oil-water separation efficiency could still reach over 99.50%. Lastly, a cycle test of the modified membrane under the optimal conditions helped confirm that the membrane forming conditions at this time could provide a better environment for the formation of the hydrophilic layer, which was conducive to the recycling of the separation membrane. In summary, more fixed more hydrophilic particles could be obtained through the RTIPS method based on biomimetic mineralization to prevent the accumulation of titanium dioxide, thus helping improve permeability and anti-fouling of the membrane.

Fabrication of photo-Fenton self-cleaning PVDF composite membrane for highly efficient oil-in-water emulsion separation

The anti-fouling performance of membranes is an important performance in the separation of oil/water.

Hydrophilic and underwater superoleophobic porous graphitic carbon nitride (g-C3N4) membranes with photo-Fenton self-cleaning ability for efficient oil/water separation

Due to the great fouling resistance property, (super)hydrophilic/underwater superoleophobic membranes are prevalent candidates for oil-polluted wastewater treatment. Even so, membrane fouling inevitably occurs during long-term operation. Therefore, it is of great significance to construct anti-fouling membranes with robust flux recovery. Herein, a polyvinyl pyrrolidone (PVP) coated porous potassium-doped g-C₃N₄ (PKCN) membrane was fabricated for the first time by vacuum filtration. The as-prepared membrane displays enhanced hydrophilicity and underwater superoleophobicity. The permeability of the membrane increased significantly after sonication treatment, which is attributed to the increased pore volume and small nanosheets size that shorten the transport pathway of water molecules. Importantly, owing to the high photo-Fenton activity, the PKCN membrane exhibits fast (within 15 min) and excellent flux recovery (96.5%) after the photo-Fenton cleaning process. Furthermore, after 10 repeated usages, the PKCN membrane still keeps stable permeability and excellent purification efficiency. This work opens a door for developing self-cleaning membranes with the superior anti-fouling ability for effective oil/water separation.

Efficient Fenton-Like Catalysis Boosting the Antifouling Performance of the Heterostructured Membranes Fabricated via Vapor-Induced Phase Separation and In Situ Mineralization

A photocatalytic membrane with significant degradation and antifouling performance has become an important part in wastewater treatment. However, the low catalyst loading on the polymer membrane limits its performance improvement. Herein, we fabricated poly(vinylidene fluoride) (PVDF) and poly(acrylic acid) (PAA) blend membranes with a rough surface via a vapor-induced phase separation (VIPS) process. Then Fe³⁺ was cross-linked with the carboxyl groups on the membrane surface and further in situ mineralized into β-FeOOH nanorods. The resultant membranes exhibit not only hydrophilicity and underwater superoleophobicity but also favorable separation efficiency and high water flux in oil-in-water emulsions separation. Under visible light irradiation, the membrane can degrade methylene blue (MB) to 95.2% in 180 min. More importantly, the membrane has a significant photocatalytic self-cleaning ability for crude oil with a flux recovery ratio (FRR) as high as 94.1%. This work brings a new strategy to fabricate the rough and porous surface for high loading of the hydrophilic photo-Fenton catalyst, improving the oil/water emulsion separation and antifouling performance of the membranes.

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.

One-Step Fabrication of a Micro/Nanosphere-Coordinated Dual Stimulus-Responsive Nanofibrous Membrane for Intelligent Antifouling and Ultrahigh Permeability of Viscous Water-in-Oil Emulsions

Membrane fouling is a major challenge for long-term oil/water separation. The incomplete degradation of organic pollutants or membrane damage exists in the common methods of membrane regeneration. Herein, a dual-responsive nanofibrous membrane with high water-in-oil emulsion separation efficiency and smart cleaning properties is reported, which shows complete restoration of its original separation performance. The pH-responsive and upper critical solution temperature (UCST)-type thermoresponsive nanofibrous membrane with a micro/nanosphere structure was developed via a one-step-blending electrospinning strategy. The membrane displays high hydrophobicity/oleophilicity at pH 7 and 25 °C and hydrophilicity/oleophobicity at pH 3 and 55 °C. As a result, it exhibits an ultrahigh permeability of 60528.76 L m^-2 h^-1 bar^-1 and a separation efficiency of 99.5% for water-in-D5 emulsions at room temperature (25 °C). Moreover, the contaminated membranes could be easily reclaimed by being rinsed with warm acidic water (pH 3 and 55 °C). The membrane maintained high separation performance after being used for multiple cycles, indicating its scalable application for purifying emulsified oil. This study provides a facial method of constructing membranes with multiscale hierarchical structures and a new idea for the design of recyclable oil/water separation membranes.

Superwetting Bi2MoO6/Cu3(PO4)2 Nanosheet-Coated Copper Mesh with Superior Anti-Oil-Fouling and Photo-Fenton-like Catalytic Properties for Effective Oil-in-Water Emulsion Separation

Superwetting materials with excellent anti-oil-fouling performance for the treatment of oily wastewater are urgently demanded in practice. In this work, aiming at effectively separating diverse oil-in-water emulsions, a multifunctional Bi₂MoO₆/Cu₃(PO₄)₂ nanosheet-coated copper mesh was successfully fabricated by the combination of chemical oxidation and ultrasonic irradiation deposition methods. The resultant copper mesh exhibited superior superhydrophilicity/underwater superoleophobicity and, more importantly, preferable anti-oil-fouling property benefitting from the stable and firm hydration layer. A series of oil/water separation experiments for the highly emulsified surfactant-free and surfactant-stabilized oil-in-water emulsions were conducted, with the respective permeation fluxes of up to 3000 and 700 L·m^-2·h^-1 and the corresponding separation efficiencies of 99.5 and 98.6% solely driven by gravity. Meanwhile, considering the photo-Fenton-like catalytic activity of Bi₂MoO₆, the as-fabricated copper mesh exhibited excellent degradation ability toward organic pollutants under visible light irradiation. More importantly, stability tests were performed to evaluate the ability to cope with the harsh environments for practical applications. With the outstanding performances of high separation efficiency, desirable photo-Fenton-like catalytic capacity, and strong stability, the Bi₂MoO₆/Cu₃(PO₄)₂ nanosheet-coated copper mesh holds promising potential for purifying emulsified wastewater.

Synchronous oil/water separation and wastewater treatment on a copper-oxide-coated mesh

Despite remarkable progress in oil/water separation and wastewater treatment, the ability to carry out the two processes in a synchronous manner has remained difficult. Here, synchronous oil/water separation and wastewater treatment were proposed on mesh surfaces coated with copper-oxide particles, which possess superwetting and catalytic properties. The superwetting performance generates additional pressure to achieve the permselectivity of the designed mesh, on which the oil phase is selectively repelled while the water phase passes though easily. Moreover, the catalytic performance of the copper oxide forms reactive oxygen species to purify the water during oil/water separation process. We show that the oil/water separation and catalytic degradation efficiencies for organic pollutants can reach more than 99% by adjusting the content of copper oxide on the mesh surfaces. Such a unique design for integrating multifunctionality on single mesh surfaces strongly underpins the synchronization of oil/water separation and wastewater treatment, which will provide a new insight for separating pure water from industrial oil/water mixtures.

An integrated multifunctional copper-oxide-coated mesh was designed via facile immersing and burning methods, which manifests synchronous oil/water separation and wastewater treatment.