Underwater superoleophobic-underoil superhydrophobic Janus ceramic membrane with its switchable separation in oil/water emulsions

Nanosheet BiOBr Modified Rock Wool Composites for High Efficient Oil/Water Separation and Simultaneous Dye Degradation by Activating Peroxymonosulfate

The development of superlyophobic materials in liquid systems, enabling synchronous oil/water separation and dye removal from water, is highly desirable. In this study, we employed a novel superwetting array-like BiOBr nanosheets anchored on waste rock wool (RW) fibers through a simple neutralization alcoholysis method. The resulting BiOBr/RW fibers exhibited superoleophilic and superhydrophilic properties in air but demonstrated underwater superoleophobic and underoil superhydrophobic characteristics. Utilizing its dual superlyophobicity, the fiber layer demonstrated high separation efficiencies and flux velocity for oil/water mixtures by prewetting under a gravity-driven mechanism. Additionally, the novel BiOBr/RW fibers also exhibited excellent dual superlyophobicity and effective separation for immiscible oil/oil systems. Furthermore, the BiOBr/RW fibers could serve as a filter to continuously separate oil/water mixtures with high flux velocity and removal rates (>93.9%) for water-soluble dye rhodamine B (RhB) simultaneously by directly activating peroxymonosulfate (PMS) in cyclic experiments. More importantly, the mechanism of simultaneous oil/water separation and RhB degradation was proposed based on the reactive oxygen species (ROS) quenching experiments and electron paramagnetic resonance (EPR) analysis. Considering the simple modified process and the waste RW as raw material, this work may open up innovative, economical, and environmentally friendly avenues for the effective treatment of wastewater contaminated with oil and water-soluble pollutants.

Janus Membranes with Asymmetric Superwettability for High-Performance and Long-Term On-Demand Oil/Water Emulsion Separation

Current single-function superwettable materials are typically designed for either oil removal or water removal and are constrained by oil density, limiting their widespread applications. Janus membranes with opposite wettability on their two surfaces have recently emerged and present attractive opportunities for on-demand oil/water emulsion separation. Here, a combination strategy is introduced to prepare a Janus membrane with asymmetric superwettability for switchable oil/water emulsion separation. A mussel-inspired asymmetric interface introduction cooperating with the sequence-confined surface modification not only brings about an asymmetric superwettability Janus interface but also guarantees an outstanding stable interface and remarkable chemical stability surfaces. Specifically, the superhydrophilic surface with underwater superoleophobicity can separate surfactant-stabilized oil-in-water emulsions. Conversely, other surface displays opposite superhydrophobicity and superoleophilicity to treat surfactant-stabilized water-in-oil emulsions. Significantly, this superwettable Janus membrane presents superior long-term on-demand oil/water emulsion separation without obvious flux decline and high recovery ability because of its superwettability and superior stability. Furthermore, the asymmetric superwettability enhances the interfacial floatability at air-water interfaces, enabling the design of advanced interfacial materials. The as-prepared superwettable Janus membrane has established a cooperated separation system, overcoming the monotony of conventional superwettable membranes and expanding the application of these specialized membranes to oily wastewater treatment.

Special Superwetting Materials from Bioinspired to Intelligent Surface for On-Demand Oil/Water Separation: A Comprehensive Review

Since superwetting surfaces have emerged, on-demand oil/water separation materials serve as a new direction for meeting practical needs. This new separation mode uses a single porous material to allow oil-removing and water-removing to be achieved alternately. In this review, the fundamentals of wettability are systematically summarized in oil/water separation. Most importantly, the two states, bioinspired surface and intelligent surface, are summarized for on-demand oil/water separation. Specifically, bioinspired surfaces include micro/nanostructures, bioinspired chemistry, Janus-featured surfaces, and dual-superlyophobic surfaces that these superwetting materials can possess asymmetric wettability in one structure system or opposite underliquid wettability by prewetting. Furthermore, an intelligent surface can be adopted by various triggers such as pH, thermal and photo stimuli, etc., to control wettability for switchable oil/water separation reversibly, expressing a thought beyond nature to realize innovative oil/water separation by external stimuli. Remarkably, this review also discusses the advantages of all the materials mentioned above, expanding the separation scope from the on-demand oil/water mixtures to the multiphase immiscible liquid-liquid mixtures. Finally, the prospects of on-demand oil/water separation materials are also concluded.

Collagen Fiber-Based Advanced Separation Materials: Recent Developments and Future Perspectives

Separation plays a critical role in a broad range of industrial applications. Developing advanced separation materials is of great significance for the future development of separation technology. Collagen fibers (CFs), the typical structural proteins, exhibit unique structural hierarchy, amphiphilic wettability, and versatile chemical reactivity. These distinctive properties provide infinite possibilities for the rational design of advanced separation materials. During the past 2 decades, many progressive achievements in the development of CFs-derived advanced separation materials have been witnessed already. Herein, the CFs-based separation materials are focused on and the recent progresses in this topic are reviewed. CFs widely existing in animal skins display unique hierarchically fibrous structure, amphiphilicity-enabled surface wetting behaviors, multi-functionality guaranteed covalent/non-covalent reaction versatility. These outstanding merits of CFs bring great opportunities for realizing rational design of a variety of advanced separation materials that were capable of achieving high-performance separations to diverse specific targets, including oily pollutants, natural products, metal ions, anionic contaminants and proteins, etc. Besides, the important issues for the further development of CFs-based advanced separation materials are also discussed.

Polyphenylene Sulfide-Based Membranes: Recent Progress and Future Perspectives

As a special engineering plastic, polyphenylene sulfide (PPS) can also be used to prepare membranes for membrane separation processes, adsorption, and catalytic and battery separators because of its unique properties, such as corrosion resistance, and chemical and thermal stability. Nowadays, many researchers have developed various types of PPS membranes, such as the PPS flat membrane, PPS microfiber membrane and PPS hollow fiber membrane, and have even achieved special functional modifications. In this review, the synthesis and modification of PPS resin, the formation of PPS membrane and the research progress of functional modification methods are systematically introduced, and the future perspective of PPS membrane is discussed.

The High Flux of Superhydrophilic-Superhydrophobic Janus Membrane of cPVA-PVDF/PMMA/GO by Layer-by-Layer Electrospinning for High Efficiency Oil-Water Separation

A simple and novel strategy of superhydrophilic-superhydrophobic Janus membrane was provided here to deal with the increasingly serious oil-water separation problem, which has a very bad impact on environmental pollution and resource recycling. The Janus membrane of cPVA-PVDF/PMMA/GO with opposite hydrophilic and hydrophobic properties was prepared by layer-by-layer electrospinning. The structure of the Janus membrane is as follows: firstly, the mixed solution of polyvinylidene fluoride (PVDF), polymethylmethacrylate (PMMA) and graphene oxide (GO) was electrospun to form a hydrophobic layer, then polyvinyl alcohol (PVA) nanofiber was coated onto the hydrophobic membrane by layer-by-layer electrospinning to form a composite membrane, and finally, the composite membrane was crosslinked to obtain a Janus membrane. The addition of GO can significantly improve the hydrophobicity, mechanical strength and stability of the Janus membrane. In addition, the prepared Janus membrane still maintained good oil-water separation performance and its separation efficiency almost did not decrease after many oil-water separation experiments. The flux in the process of oil-water separation can reach 1909.9 L m-2 h-1, and the separation efficiency can reach 99.9%. This not only proves the separation effect of the nanocomposite membrane, but also shows its high stability and recyclability. The asymmetric Janus membrane shows good oil-water selectivity, which gives Janus membrane broad application prospects in many fields.

Underoil Directional Self-Transportation of Water Droplets on a TiO2-Coated Conical Spine

Directional self-transportation of tiny droplets is significant in many fields. However, almost all existing studies focus on the phenomenon in air, and to realize similar performance in complex environments, such as oil, is still extremely rare. Here, we report a TiO₂-coated conical spine (TCS) and demonstrate underoil directional self-transportation of water droplets on its surface. It is found that high surface hydrophilicity resulting from UV irradiation is necessary to achieve the self-transportation of water in oil. The critical water contact angle in oil is about 57°, and the maximal transport velocity can reach 1.4 mm/s. Mechanism analysis reveals that the excellent self-transportation property is ascribed to the combined effect between the Laplace force (F_L) caused by the conical gradient structure and the hysteresis reduction resulting from the high hydrophilicity. Moreover, based on the special underoil self-transportation performance, a droplet-based microreaction and demulsification of water-in-oil emulsions were demonstrated using the TCS. This work reports the self-transportation of water in oil, which could provide some fresh ideas for designing new superwetting self-transportation materials.

Nanoarchitectonics for Electrospun Membranes with Asymmetric Wettability

Membranes with asymmetric wettability have attracted significant interest by virtue of their unique transport characteristics and functionalities arising from different wetting behaviors of each membrane surface. The cross-sectional wettability distinction enables a membrane to realize directional liquid transport or multifunction integration, resulting in rapid advance in applications, such as moisture management, fog collection, oil-water separation, and membrane distillation. Compared with traditional homogeneous membranes, these membranes possess enhanced transport performance and higher separation efficiency owing to the synergistic or individual effects of asymmetric wettability. This Review covers the recent progress in fabrication, transport mechanisms, and applications of electrospun membranes with asymmetric wettability and provides a perspective on future development in this important area.

Confined Channels Induced Coalescence Demulsification and Slippery Interfaces Constructed Fouling Resist-Release for Long-Lasting Oil/Water Separation

Superwetting membranes based on steric exclusion and affinity difference have drawn substantial interest for oil/water separation. However, the state-of-the-art membranes fail to literally sort out fouling and permeability decline and so limit their viability for long-term separation. Inspired by Dayu's philosophy of "draining rather than blocking water", herein, we achieve a long-lasting and efficient separation for viscous emulsions by designing poly(hydroxyethyl methylacrylate) (PHEMA)- and polydimethylsiloxane (PDMS)-compensated poly(vinylidene fluoride) membranes based on coalescence demulsification via chemical coordination phase separation. The symmetric and torturous microporous structure facilitated oil spatial confining and coalescence demulsification, while the synergistic compensation of PHEMA and PDMS coordinated the fouling resist and release properties, which was confirmed by multichannel confocal laser scanning microscopy. The developed membrane shows an unprecedented permeability half-life (τ) for viscous emulsions (e.g., decamethylcyclopentasiloxane, soybean oil paraffin, n-hexadecane, and isooctane) under cross-flow operation, far more beyond common superwetting membranes under applied bench-scale dead-end filtration. Our technique for designing "nonfouling" membranes opens up opportunities for advancing next-generation membranes for oil/water separation.

Constructing Scalable Superhydrophobic Membranes for Ultrafast Water-Oil Separation

Superhydrophobic membranes are desirable for separation of water-in-oil emulsions, membrane distillation, and membrane condensation. However, the lack of large-scale manufacture methods of superhydrophobic membranes hampers their widespread applications. Here, a facile method of coaxial electrospinning is provided to manufacture superhydrophobic membranes for the ultrafast separation of water-in-oil emulsions. Under the high-voltage electric field, the polydimethylsiloxane (PDMS)-coated polyvinylidene fluoride (PVDF) nanofibers and PDMS microspheres with PVDF nanobulges were integrated together during the electrospinning process. Moreover, asymmetric composite membranes with selective layers are designed to reduce the resistance of the mass transfer. Consequently, the as-prepared asymmetric composite membrane exhibits an ultrafast permeance and excellent separation efficiency of about 99.6%, outperforming most of the state-of-the-art membranes reported previously. Most importantly, the membrane could be as large as 770 cm², could be manufactured continuously, and could be easily enlarged further via tailoring the roller receptor, showing strong promise in the separation of water-in-oil emulsions.

Mechanically Robust Janus Poly(lactic acid) Hybrid Fibrous Membranes toward Highly Efficient Switchable Separation of Surfactant-Stabilized Oil/Water Emulsions

An ideal oil/water separation membrane should possess the characteristics of high flux and separation efficiency, recyclability, as well as good mechanical stability. Herein, a facile method is applied to fabricate a Janus polylactic acid (PLA) fibrous membrane for efficiently separating surfactant-stabilized oil/water mixtures. The Janus PLA fibrous membrane architecture was prepared by electrospinning a PLA/carbon nanotubes (CNTs) fibrous membrane and the subsequent electrospinning of a PLA/SiO₂ nanofluids (nfs) membrane onto one side of the PLA/CNTs fibrous membrane. Due to the strong electrostatic interaction between SiO₂ nfs and CNTs, synchronous enhancement and plasticization of PLA fibrous membranes were achieved, which was far superior to that reported in the literature. The introduction of CNTs had caused an upshift of the hydrophobicity of the PLA/CNTs fibrous membrane (water contact angle (WCA) > 140°). In contrast, SiO₂ nfs bearing long-chain organic anions and cations located onto the surface of the fibers during electrospinning to achieve superhydrophilicity (WCA ≈ 0°). Benefiting from completely opposite wettability on both sides of the Janus membrane, the obtained asymmetric Janus membranes exhibited a high flux (1142-1485 L m^-2 L^-1) and excellent oil/water separation efficiency (>99%), which were superior to those reported for other Janus membranes. Furthermore, the Janus membranes showed desirable flux recovery without any treatment (>80% for water-in-oil emulsions and >90% for oil-in-water emulsions, respectively, after 11 cycles), showcasing promising applications for water treatment in the future.

PVDF-Modified TiO2 Nanowires Membrane with Underliquid Dual Superlyophobic Property for Switchable Separation of Oil-Water Emulsions

Separation membranes with underliquid dual superlyophobicity have recently caused widespread concern due to their switchable separation of oil-water mixtures and emulsions. However, the fabrication of the reported underliquid dual superlyophobic membranes is difficult, and the design of the underliquid dual superlyophobic surface of these membranes is challenging because of their complex surface composition. Theoretically, underliquid dual superlyophobicity is an underliquid Cassie state attainable by the synergy of the underliquid dual lyophobic surface and the construction of a high-roughness surface. Herein, we fabricated an underliquid dual superlyophobic membrane by combining underliquid dual lyophobic polyvinylidene fluoride (PVDF) and TiO₂ nanowires. PVDF-modified TiO₂ nanowire membranes with underliquid dual superlyophobicity were prepared via a simple adsorption and filtration approach. PVDF was coated onto TiO₂ nanowires to form a PVDF layer with a thickness of 6 nm. The PVDF modification provided flexibility to the fragile TiO₂ nanowires membrane and changed its wettability from underwater superoleophobicity/underoil superhydrophilicity to underliquid dual superlyophobicity. The PVDF-modified TiO₂ nanowires membrane efficiently separated both oil-in-water and water-in-oil emulsions. The binary cooperative effect between the TiO₂ nanowires and the coated PVDF layer was responsible for the underliquid dual superlyophobicity.

Non-Isothermal Treatment of Oily Waters Using Ceramic Membrane: A Numerical Investigation

Currently, the oil industry deals with the challenge of produced-water proper disposal, and the membrane-separation technology appears as an important tool on the treatment of these waters. In this sense, this work developed a mathematical model for simulating the oil/water separation by a ceramic membrane. The aim was to investigate the thermal aspects of the separation process via computational fluid dynamic, using the Ansys CFX® 15 software (15, Ansys, Inc., Canonsburg, PA, USA). Oil concentration, pressure, and velocity distributions, as well as permeation velocity, are presented and analyzed. It was verified that the mathematical model was capable of accurately representing the studied phenomena and that temperature strongly influences the flow behavior.

Bioinspired Superwettable Covalent Organic Framework Nanofibrous Composite Membrane with a Spindle-Knotted Structure for Highly Efficient Oil/Water Emulsion Separation

Covalent organic frameworks (COFs) have attracted broad interest in a number of fields including gas access, catalysis, and ionic adsorption. However, owing to the low stability in water, the application of COFs in the field of oil/water separation is extensively impeded. In this paper, we synthesized COF-DhaTab/polyacrylonitrile (PAN) nanofibrous composite membranes with a bioinspired spindle-knotted structure via a facile blending electrospinning method. The COF-DhaTab/PAN composite membrane shows prewetting-induced superoleophobicity under water and superhydrophobicity under oil. It possesses outstanding rejection ratio (>99.9%), excellent antifouling performance, and ultrahigh oil/water mixture flux up to 4229.29 L/m²h even though driven only by gravity. Specifically, an extraordinary oil contact angle under water (152.3°) and a satisfied water contact angle under oil (153.7°) were offered by the composite membrane. These are mainly attributed to the spindle-knotted structures induced by COFs. To the best of our knowledge, the application of COF/PAN composite membrane in the field of oil/water separation has never been reported. It is an innovative approach for oily wastewater treatment and oil purification.

Design of a Janus F-TiO2@PPS Porous Membrane with Asymmetric Wettability for Switchable Oil/Water Separation

Extremely asymmetric wettability, high wetting selectivity, instantaneous superwetting behaviors (low transmembrane resistance), and superior resistance to chemicals and solvents are needed for Janus membranes for switchable oil/water separation. However, it is still challenging to obtain Janus membranes with such properties. In this study, a surface with metastable hydrophobicity is constructed on one side of the chemically stable superhydrophilic TiO₂@PPS membrane by adjusting the hydrophobization depth and the morphology of the hydrophobic layer via a water-oil interfacial grafting. The prepared Janus membrane exhibits a high water contact angle difference of ∼150° between its two surfaces with maintaining the superior penetrability of the original membrane, which makes it capable to separate both oil-in-water and water-in-oil emulsions with high fluxes and accuracy. The separation efficiency is higher than 98% for the separation of the two kinds of emulsions. The fluxes of the surfactant-free toluene-in-water and water-in-toluene emulsions are up to 6.4 × 10² and 9.5 × 10² L m^-2 h^-1, respectively. Furthermore, the Janus membrane exhibits desirable antifouling performance and reusability during usage.