Robust bio-inspired superhydrophilic and underwater superoleophobic membranes for simultaneously fast water and oil recovery

Robust nanoparticles growth in the interior of porous sponges for efficient dye adsorption and emulsion separation

Emulsified oils and dye contaminants already pose a huge threat to global ecosystems and human health. It is a significant research topic to develop efficient, rapid, versatile methods for emulsion separation and dye adsorption. The membrane material modified with common methods only modified the outer surface of the membrane, while the interior is hardly fully decorated. In this investigation, a solvent exchange method was used to in situ grow nanoparticles in the interior of a porous sponge. These nanoparticles were obtained with polyethyleneimine, gallic acid, and tannic acid via Michael addition and Schiff base reaction. The prepared nanoparticle-coated sponges provided efficient separation of dyes, emulsions, and complex contaminants. The separation efficiency of the dye reached 99.49%, and the separation efficiency of the emulsion was as high as 99.87% with a flux of 11140.3 L m-2 h-1. Furthermore, the maximum adsorption capacity reached 486.8 mg g-1 for cationic dyes and 182.1 mg g-1 for anionic dyes. More importantly, the nanoparticles were highly robust on the surface of the porous sponge, and the modified sponge could have long-term applications in hazardous environments. Overall, it is envisioned that the nanoparticles-modified porous sponge exhibited considerable potential for emulsion and dye wastewater treatment.

Biomimetic materials in oil/water separation: Focusing on switchable wettabilities and applications

Clean water resources are crucial for human society, as the leakage and discharge of oily wastewater not only harm the economy but also disrupt our living environment. Therefore, there is an urgent need for efficient oil-water separation technology. Surfaces with switchable superwetting behavior have garnered significant attention due to their importance in both fundamental research and practical applications. This review introduces the fundamental principles of wettability in the oil-water separation process, the basic theory of switchable wettability, and the mechanisms involved in oil-water separation. Subsequently, the review discusses the research progress, challenges, and issues associated with three conventional types of special wettability materials: superhydrophobic/superoleophilic materials, superhydrophilic/superoleophobic materials, and superhydrophilic/underwater superoleophobic materials. Most importantly, it provides a detailed exploration of recent advancements in switchable wettability smart materials, which combine elements of traditional special wettability materials. These include stimulus-responsive smart materials, pre-wetting-induced materials, and Janus materials. The discussion covers key response factors, detailed examples of representative works, design concepts, and fabrication strategies. Finally, the review offers a comprehensive summary of switchable superwetting smart materials, encompassing their advantages and disadvantages, persistent challenges, and future prospects.

Rapid Construction of Caffeic Acid/p-Phenylenediamine Antifouling Hydrophilic Coating on a PVDF Membrane for Emulsion Separation

The current methods of constructing modification strategies for hydrophilic membranes are time-consuming, complex in operation, and poor in universality, which limit their application on membranes. In this work, inspired by the adhesion properties and versatility of caffeic acid (CA) and p-phenylenediamine (PPDA), a simple, rapid, and universal method was designed for the separation of oil-in-water emulsion by preparing a stable hydrophilic coating separation membrane. The preparation time of the membrane was shortened to 40 min. The developed PVDF-PCA/PPDA membrane showed superhydrophilic and underwater superoleophobic properties. When applied to petroleum ether-in-water emulsion, isooctane-in-water emulsion, and dodecane-in-water emulsion separation, the oil rejection was more than 99.0%. In the circulating separation of 10 g/L soybean oil-in-water emulsion, the oil rejection was more than 99.3%, and the highest flux was 1036 L·m-2·h-1. The prepared PVDF-PCA/PPDA membrane performed well in the separation test of oily wastewater. The proposed strategy is simple and rapid; it may become a universal method for preparing membranes with super strong antifouling properties against viscous oil and accelerate the research progress of membrane separation of oil-in-water emulsions.

Robust Janus Superwetting Textile with Large Pore Sizes for Oil-in-Water Emulsion Separation

Developing advanced oil-water separation technology is significant for environmental conservation. According to the synergetic effects of the size-sieving mechanism, superwetting materials with small pore sizes have been designed to realize high-efficiency separation for oil-water emulsions. However, the separation flux limited by the pore size and the weakness of the superwetting material impede its practical application severely. Herein, we construct a robust Janus superwetting textile with large pore sizes for oil-in-water emulsion separation. The pristine textile is coated by the as-prepared CuO nanoparticles as the bottom layer with superhydrophilicity and then grafted by 1-octadecanethiol as the top layer with superhydrophobicity to construct the Janus textile. When used as a filter, the superhydrophobic layer acts as the nucleation site to coalesce the small oil droplets facilely. Then, the coalesced oil fills the pores of the superhydrophobic layer and selectively permeates it but is blocked by the superhydrophilic layer with large pore sizes. Utilizing the unique separation mechanism, the Janus textile realizes efficient and rapid separation. Even after multicycle separation, hot liquid immersion for 24 h, tribological test for 60 min, and sandpaper abrasion for 500 cycles, the Janus textile still retains the superwettability and excellent separation performance, manifesting outstanding stability to resist severe damage. This separation strategy provides a novel guideline for high-efficiency and high-flux emulsion separation and practical application.

Interfacial design for detection of a few molecules

Major advances in molecular detection are being driven by goals associated with the development of methods that are amenable to miniaturization and automation, and that have high sensitivity and low interference. The new detection methods are confronted by many interfacial issues, which when properly addressed can lead to improved performance. One interfacial property, special wettability, can facilitate precise delivery and local enrichment of molecules to sensing elements. This review summarizes applications of unique features of special wettability in molecular detection including (1) chemical and electrochemical reactions in anchored microdroplets on superwetting surfaces, (2) enrichment of analytes and active materials at low contact areas between droplets and superwetting surfaces, (3) complete opposite affinities of superwetting surfaces toward nonpolar/polar solutes and oil/water phases, and (4) directional droplet transportation on asymmetric superwetting surfaces. The challenges and opportunities that exist in design and applications of special wettability in interfacial delivery and enrichment for detection of a few molecules are also discussed.

Recent advances in eco-friendly fabrics with special wettability for oil/water separation

Considering the serious damage to aquatic ecosystems and marine life caused by oil spills and oily wastewater discharge, efficient, environment-friendly and sustainable oil/water separation technology has become an inevitable trend for current development. Herein, fabrics are recognized as eco-friendly materials for water treatment due to their good degradability and low cost. Particularly, fabrics with rough structures and natural hydrophilicity/oleophilicity enable the construction of superwetting surfaces for the selective separation of oil/water mixtures and even complex emulsions. Therefore, superwetting fabrics for efficiently solving oil spills and purifying oily wastewater have received extensive attention. Especially, Janus and smart fabrics are highly anticipated to enable the on-demand and sustainable treatment of oil spills and oily wastewater due to their changeable wettability. Moreover, the fabrication of superwetting fabrics with multifunctional performances for oily wastewater purification can further promote their practical industrial applications, such as photocatalytic, self-cleaning, and self-healing characteristics. However, some potential challenges still exist, which urgently need to be systematically summarized to guide the future development of this research field. In this review, firstly, the fundamental theories of wettability and the separation mechanisms based on special wettability are discussed. Then, superwetting fabrics for efficient oil/water separation are systematically reviewed, such as superhydrophobic/superoleophilic (SHB/SOL), superhydrophilic/superoleophobic (SHL/SOB), SHL/underwater superoleophobic (SHL/UWSOB), and UWSOB/underoil superoleophobic (UWSOB/UOSHB) fabrics. Most importantly, we highlight Janus, smart, and multifunctional fabrics based on their superwetting property. Correspondingly, the advantages and disadvantages of each superwetting fabric are comprehensively analyzed. Besides, super-antiwetting fabrics with superhydrophobic/superoleophobic (SHB/SOB) property are also introduced. Finally, the challenges and future research directions are explained.

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.

Fabrication and Characterization of Polyelectrolyte Coatings by Polymerization and Co-Deposition of Acrylic Acid Using the Dopamine in Weak Acid Solution

Existing medical materials (such as silicone rubber, glass slides, etc.) fail to meet the functional requirements of biosensing, cell culture, and drug delivery due to their poor wettability. The preparation of polyelectrolyte coatings with excellent wettability and protein adsorption helps broaden the application of medical materials. Poly(acrylic acid) (PAA) is a common polyelectrolyte with stronger protein adsorption, but the existing methods for obtaining PAA coating have certain shortcomings to limit their industrial applications. In this study, dopamine (DA) was used to polymerize and co-deposit acrylic acid (AA) in weak acid solution to functionalize the surface of materials, and the effects of different mass ratios of DA/AA on the wettability and protein adsorption of the coating were deeply investigated. The results demonstrate that PDA/PAA coating is successfully prepared on the surface of four substrates and greatly reduces the water contact angle of these surfaces. Moreover, these coatings show excellent protein adsorption, and the amount of adsorbed protein on the coated QCM chip is increased by 57.74% than the uncoated QCM chip. In addition, the coating has a certain pH responsiveness, and its wettability and protein adsorption are closely related to the pH of the solution. The preparation strategy proposed is simple and substrate-independent, which provides valuable insights into the application of the one-step polymerization and co-deposition strategy under weak acid conditions.

Bi-functional super-hydrophilic/underwater super-oleophobic 2D lamellar Ti3C2Tx MXene/poly (arylene ether nitrile) fibrous composite membrane for the fast purification of emulsified oil and photodegradation of hazardous organics

Developing the multi-functional membranes including oil/water emulsion separation and removal of hazardous organic pollutants is essential to the purification of the complicated wastewater. However, it remains a daunting challenge to combine these intended functions while maintaining high separation efficiency. Herein, we developed a new 2D lamellar MXene/poly (arylene ether nitrile) (PEN) fibrous composite membrane through the self-assembly of TiO₂ nanoparticles intercalated MXene nanosheets onto the porous PEN nanofibrous mats and bioinspired polydopamine triggered chemical-crosslinking with polyethyleneimine (PEI). Such nano-intercalation and mussel-inspired crosslinking could effectively regulate the interlayer spacing of the MXene nanosheet skin layer and surface wettability of the composite membrane, which also further contributed to the fast separation and unique bifunctional feature. It was found that the MXene@TiO₂/PEN fibrous composite membrane exhibited low oil-adhesion and superhydrophilic (WCA = 0°)/underwater superoleophobic (UOCA > 155°) properties, which could efficiently separate various surfactant-stabilized oil-in-water emulsions under low pressure of 0.04 MPa while keeping good stability (Under 1 M HCl and 2 M NaOH solutions) and recyclability. Interestingly, the fibrous composite membrane achieved favorable permeation flux of 908-1003 Lm^-2h^-1 (2270-2507.5 Lm^-2h^-1bar^-1) in comparison to other reported MXene based multifunctional composite membranes. Moreover, owing to the synergistic effect of MXene nanosheets and TiO₂ nanoparticles, the MXene@TiO₂/PEN membrane showed excellent photocatalytic degradation performance for various dyes under visible light, i.e. the photocatalytic degradation efficiency for 15 ppm MB, MO, CV, and MeB solutions achieved 92.31%, 93.50%, 98.06%, and 99.30% within 60 min, respectively. Such 2D MXene bio-functional composite membranes with outstanding oil/water emulsions separation and photocatalytic degradation of dyes pave an avenue for treating complicated oily wastewater.

Bioinspired Superhydrophobic/Superhydrophilic Janus Copper Foam for On-Demand Oil/Water Separation

Superwettable Janus membranes with unique interfacial characteristics have versatile applications in oil/water separation, microfluid transportation, and membrane distillation. However, it remains a significant challenge to simply fabricate three-dimensional (3D) metallic foams with Janus superwettability using a facile and environment-friendly method. In this study, a novel method is present to construct a Janus copper foam (CF) by combining superhydrophobicity and superhydrophilicity into CF. Based on gravity, the water in the light oil (LO)/water mixture can be transported from the superhydrophilic (SHL) side to the superhydrophobic (SHB) side, while the heavy oil (HO) in the HO/water/mixture can be transported from the SHB side to the SHL side. Therefore, cylindrical Janus oil/water separation devices with superior separation efficiency and excellent repeatability can achieve on-demand oil/water separation effortlessly. This design and fabrication method offers a novel avenue for the preparation of Janus interface materials for practical applications in liquid transportation, sensor devices, energy materials, and oil spills.

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.

Polymeric Membranes for Oil-Water Separation: A Review

This review is devoted to the application of bulk synthetic polymers such as polysulfone (PSf), polyethersulfone (PES), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF) for the separation of oil-water emulsions. Due to the high hydrophobicity of the presented polymers and their tendency to be contaminated with water-oil emulsions, methods for the hydrophilization of membranes based on them were analyzed: the mixing of polymers, the introduction of inorganic additives, and surface modification. In addition, membranes based on natural hydrophilic materials (cellulose and its derivatives) are given as a comparison.

One-step fabrication of transparent Barite colloid with dual superhydrophilicity for anti-crude oil fouling and anti-fogging

HYPOTHESIS

Transparent superhydrophilic coatings are very promising in various scenarios. Appropriate fabrication of colloid coatings with superhydrophilicity both in air and under oil would enlarge their application potential in anti-oil fouling and facilitate anti-fogging of transparent surfaces.

EXPERIMENTS

The Barite colloid was obtained from a one-step precipitation method and was transferred onto glasses to prepare transparent coatings with different thicknesses simply by dip-coating. Then, the impact of thickness on wettability and property was studied through the investigation of wettability in various phase, anti-crude oil fouling performance and anti-fogging ability.

FINDINGS

Similar surface morphology and roughness of these coatings were achieved and all the coated surfaces showed ultra-hydrophilicity both in air and under oil. Moreover, the hydrophilicity in air and under oil was found to deteriorate with the decrease of coatings' thickness and dual superhydrophilicity could be achieved on thick coatings. More importantly, excellent anti-crude oil fouling property and durable anti-fogging ability were realized on these transparent coatings with dual superhydrophilicity.

A multifunctional and environmentally safe superhydrophobic membrane with superior oil/water separation, photocatalytic degradation and anti-biofouling performance

Wastewater is typically complicated with spilled oil, water soluble toxic dyes and microorganisms, making it hard to be processed and causing a significant threat to the environmental safety and human health. In this paper, we demonstrate a simple solution immersion method to obtain a multifunctional cellulose-based membrane (CBM) that possesses both superhydrophobicity with a water contact angle of 163° and superior functionalities including self-cleaning, oil-water separation, anti-biofouling, and photocatalytic degradation capabilities. The achievement of separation efficiency (96%), comparatively high flux (141 L·m^-2·h^-1) and recyclable (7 times) oil/water separation performance is attributed to the robust superhydrophobicity enabled by the synergy of metal oxide (i.e., CuO) nanostructure coating and stearic acid (SA) modification. The superhydrophobic CBM also preferentially adsorbs organic dyes in aqueous solution, e.g., methylene blue (MB), promoting their efficient decomposition (about 70.3% of MB decomposed in 3 h) with high recyclability under UV irradiation. Most remarkably, the CBM exhibits superior anti-biofouling capability and persistently resists the algae adhesion in long duration (over 20 days), as a result of the self-cleaning ability as well as the antimicrobial property of CuO nanoparticles. Our finding here paves the way to use simple, cost-effective, environmentally safe, and reliable method to fabricate multifunctional materials for wastewater treatment in complex environments.