In situ biomineralization-constructed superhydrophilic and underwater superoleophobic PVDF-TiO2 membranes for superior antifouling separation of oil-in-water emulsions

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.

Tunable Wettability of a Dual-Faced Covalent Organic Framework Membrane for Enhanced Water Filtration

Membrane technology plays a central role in advancing separation processes, particularly in water treatment. Covalent organic frameworks (COFs) have transformative potential in this field due to their adjustable structures and robustness. However, conventional COF membrane synthesis methods are often associated with challenges, such as time-consuming processes and limited control over surface properties. Our study demonstrates a rapid, microwave-assisted method to synthesize self-standing COF membranes within minutes. This approach allows control over the wettability of the surface and achieves superhydrophilic and near-hydrophobic properties. A thorough characterization of the membrane allows a detailed analysis of the membrane properties and the difference in wettability between its two faces. Microwave activation accelerates the self-assembly of the COF nanosheets, whereby the thickness of the membrane can be controlled by adjusting the time of the reaction. The superhydrophilic vapor side of the membrane results from -NH2 reactions with acetic acid, while the nearly hydrophobic dioxane side has terminal aldehyde groups. Leveraging the superhydrophilic face, water filtration at high water flux, complete oil removal, increased rejection with anionic dye size, and resistance to organic fouling were achieved. The TTA-DFP-COF membrane opens new avenues for research to address the urgent need for water purification, distinguished by its synthesis speed, simplicity, and superior separation capabilities.

Biomineralization-Inspired Synthesis of Hybrid COF Nanosheets toward Efficient Desalination Membranes

Synthesis of covalent organic framework nanosheets (CONs) with high aspect ratio is crucial to their assembly into advanced membranes. Nonetheless, the π-π stacking between covalent organic framework (COF) layers often leads to thick CONs. Herein, inspired by biomineralization process, a series of aspect ratio CONs >15 000 is synthesized by multifunctional polyelectrolytes which not only provide the nucleation sites for pre-assembly with COF monomer, but also suppress π-π interaction for anisotropic growth through protonation. The membrane assembled from CONs exhibited water permeance of 341 kg m-2 h-1 and salt rejection of 99.5% in desalination, outperforming ever-reported membranes. This method establishes a platform for the synthesis of crystalline nanosheets.

Fabrication and Functional Regulation of Biomimetic Interfaces and Their Antifouling and Antibacterial Applications: A Review

Biomimetic synthesis provides potential guidance for the synthesis of bio-nanomaterials by mimicking the structure, properties and functions of natural materials. Behavioral studies of biological surfaces with specific micro/nano structures are performed to explore the interactions of various molecules or organisms with biological surfaces. These explorations provide valuable inspiration for the development of biomimetic surfaces with similar effects. This work reviews some conventional preparation methods and functional modulation strategies for biomimetic interfaces. It aims to elucidate the important role of biomimetic interfaces with antifouling and low-pollution properties that can replace non-environmentally friendly coatings. Thus, biomimetic antifouling interfaces can be better applied in the field of marine antifouling and antimicrobial. In this review, the commonly used fabrication methods for biomimetic interfaces as well as some practical strategies for functional modulation is present in detail. These methods and strategies modify the physical structure and chemical properties of the biomimetic interfaces, thus improving the wettability, adsorption, drag reduction, etc. that they exhibit. In addition, practical applications are presented of various biomimetic interfaces for antifouling and look ahead to potential biomedical applications. By continuously discovering functional surfaces with biomimetic properties and studying their microstructure and macroscopic properties, more biomimetic interfaces will be developed.

The antifouling mechanism and application of bio-inspired superwetting surfaces with effective antifouling performance

With the rapid development of industries, the issue of pollution on Earth has become increasingly severe. This has led to the deterioration of various surfaces, rendering them ineffective for their intended purposes. Examples of such surfaces include oil rigs, seawater intakes, and more. A variety of functional surface techniques have been created to address these issues, including superwetting surfaces, antifouling coatings, nano-polymer composite materials, etc. They primarily exploit the membrane's surface properties and hydration layer to improve the antifouling property. In recent years, biomimetic superwetting surfaces with non-toxic and environmental characteristics have garnered massive attention, greatly aiding in solving the problem of pollution. In this work, a detailed presentation of antifouling superwetting materials was made, including superhydrophobic surface, superhydrophilic surface, and superhydrophilic/underwater superoleophobic surface, along with the antifouling mechanisms. Then, the applications of the superwetting antifouling materials in antifouling domain were addressed in depth.

Modified polymer membranes for the removal of pharmaceutical active compounds in wastewater and its mechanism-A review

Membrane technology can play a suitable role in removing pharmaceutical active compounds since it requires low energy and simple operation. Even though membrane technology has progressed for wastewater applications nowadays, modifying membranes to achieve the strong desired membrane performance is still needed. Thus, this study overviews a comprehensive insight into the application of modified polymer membranes to remove pharmaceutical active compounds from wastewater. Biotoxicity of pharmaceutical active compounds is first prescribed to gain deep insight into how membranes can remove pharmaceutical active compounds from wastewater. Then, the behavior of the diffusion mechanism can be concisely determined using mass transfer factor model that represented by β and B with value up to 2.004 g h mg-1 and 1.833 mg g-1 for organic compounds including pharmaceutical active compounds. The model refers to the adsorption of solute to attach onto acceptor sites of the membrane surface, external mass transport of solute materials from the bulk liquid to the membrane surface, and internal mass transfer to diffuse a solute toward acceptor sites of the membrane surface with evidenced up to 0.999. Different pharmaceutical compounds have different solubility and relates to the membrane hydrophilicity properties and mechanisms. Ultimately, challenges and future recommendations have been presented to view the future need to enhance membrane performance regarding fouling mitigation and recovering compounds. Afterwards, the discussion of this study is projected to play a critical role in advance of better-quality membrane technologies for removing pharmaceutical active compounds from wastewater in an eco-friendly strategy and without damaging the ecosystem.

Compressible Separator and Catalyst for Simultaneous Separation and Purification of Emulsions and Aqueous Pollutants

Oily wastewater, a global environmental concern, demands efficient oil/water separation and pollutant removal. Our compressible separator and catalyst (CSC) balls, prepared through sponge etching and metal nanoparticle synthesis, exhibited efficient degradation of dyes of varying sizes, spanning a molecular weight range from 139 to 696 g/mol during the oil/water separation. Control over the distance between catalysts was achieved by incorporating Ag-Pt-Pd catalysts into the sponge skeleton and by adjusting the compression rates. The dispersion of the catalysts improved degradation efficiency for larger dyes, while concentrating the catalysts proved to be more effective for the smaller ones. By optimizing the compression rates of CSC balls, we successfully achieved the effective removal of emulsions of different sizes and precise control of flux. Our CSC ball-loaded system offers efficient and versatile solutions for concurrent separation and purification of emulsions and pollutants with potential environmental benefits.

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.

Preparation and Characterization of PVDF-TiO2 Mixed-Matrix Membrane with PVP and PEG as Pore-Forming Agents for BSA Rejection

Polymeric membranes offer straightforward modification methods that make industry scaling affordable and easy; however, these materials are hydrophobic, prone to fouling, and vulnerable to extreme operating conditions. Various attempts were made in this study to fix the challenges in using polymeric membranes and create mixed-matrix membrane (MMMs) with improved properties and hydrophilicity by adding titanium dioxide (TiO₂) and pore-forming agents to hydrophobic polyvinylidene fluoride (PVDF). The PVDF mixed-matrix ultrafiltration membranes in this study were made using the non-solvent phase inversion approach which is a simple and effective method for increasing the hydrophilic nature of membranes. Polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG) as pore-forming chemicals were created. Pure water flux, BSA flux, and BSA rejection were calculated to evaluate the mixed-matrix membrane's efficiency. Bovine serum albumin (BSA) solution was employed in this study to examine the protein rejection ability. Increases in hydrophilicity, viscosity, and flux in pure water and BSA solution were achieved using PVP and PEG additives. The PVDF membrane's hydrophilicity was raised with the addition of TiO₂, showing an increased contact angle to 71°. The results show that the PVDF-PVP-TiO₂ membrane achieved its optimum water flux of 97 L/(m²h) while the PVDF-PEG-TiO₂ membrane rejected BSA at a rate greater than 97%. The findings demonstrate that use of a support or additive improved filtration performance compared to a pristine polymeric membrane by increasing its hydrophilicity.

Nanofibrous/biopolymeric membrane a sustainable approach to remove organic micropollutants: A review

Aquifers are severely polluted with organic and inorganic pollutants, posing a serious threat to the global ecological system's balance. While various traditional methods are available, the development of innovative methods for effluent treatment and reuse is critical. Polymers have recently been widely used in a variety of industry sectors due to their unique properties. Biopolymers are a biodegradable material that is also a viable alternative to synthetic polymers. Biopolymers are preferably obtained from cellulose and carrageenan molecules from various biological sources. While compared with conventional non-biodegradable polymeric materials, the biopolymer possesses unique characteristics such as renewability, cost-effectiveness, biodegradability, and biocompatibility. The improvements towards the biopolymeric (natural) membranes have also been thoroughly discussed. The use of nanofillers to stabilise and improve the effectiveness of biopolymeric membranes in the elimination of organic pollutants is one of the most recent developments. This was discovered that the majority of biopolymeric membranes technology consolidated on organic pollutants. More research should be directed toward against emerging organic/persistent organic pollutants (POP) and micropollutants. Furthermore, processes for regenerating and reusing utilized biopolymer-based carbon - based materials are emphasized.

Fabrication of Superhydrophobic Coating Based on Waterborne Silicone-Modified Polyurethane Dispersion and Silica Nanoparticles

In this work, eco-friendly superhydrophobic coatings were prepared by dispersing hydrophobic silica nanoparticles and a waterborne silicone-modified polyurethane dispersion into an ethanol solution, which was free of fluorine and volatile toxic solvents. The effects of the silica content on the hydrophobicity and scratch resistance of the hydrophobic surfaces were investigated by WCA measurements and a sandpaper abrasion test, respectively. The experimental results indicated that when the silica content exceeded 30% by mass, the silica/silicone-modified polyurethane coatings had superhydrophobicity. Meanwhile, the superhydrophobic coatings with a silica content of 30% by mass simultaneously had the optimal mechanical stability. We studied the morphology and roughness of the hydrophobic surfaces with different silica content and attempted to briefly explain the influence mechanism of silica content. Furthermore, anti-icing and oil-water separation experiments were carried out on the superhydrophobic coatings, which exhibited good anti-icing performance and high separation efficiency. The eco-friendly superhydrophobic coating is expected to be applied in the fields of oil-water separation, anti-icing, and self-cleaning, etc.

Electro-Enhanced Separation of Microsized Oil-in-Water Emulsions via Metallic Membranes: Performance and Mechanistic Insights

Conventional separation membranes suffer from evitable fouling and flux decrease for water treatment applications. Herein, a novel protocol of electro-enhanced membrane separation is proposed for the efficient treatment of microsized emulsions (∼1 μm) by rationally designing robust electroresponsive copper metallic membranes, which could mitigate oil fouling and coenhance permeance (from ∼1026 to ∼2516 L·m^-2·h^-1·bar^-1) and rejection (from ∼87 to ∼98%). High-flux Cu membranes exhibit superior ductility and electrical conductivity, enabling promising electroactivity. Separation performance and the fouling mechanism were studied under different electrical potentials and ionic strengths. Application of negative polarization into a large-pore (∼2.1 μm) Cu membrane is favorable to not only almost completely reject smaller-sized oil droplets (∼1 μm) but also achieve antifouling and anticorrosion functions. Moreover, surfactants around oil droplets might be redistributed due to electrostatic repulsion, which effectively enhances the steric hindrance effect between neighboring oil droplets, mitigating oil coalescence and consequently membrane fouling. Furthermore, due to the screening effect of surfactants, the presence of low-concentration salts increases the adsorption of surfactants at the oil-water interface, thus preventing oil coalescence via decreasing oil-water interfacial tension. However, under high ionic strengths, the fouling mechanism converts from cake filtration to a complete blocking model due to the reduced electrostatic repulsion between the Cu membrane and oil droplets. This work would provide mechanistic insights into electro-enhanced antifouling for not only oil emulsion separation but also more water treatment applications using rationally designed novel electroresponsive membranes.

Preparation of Nano-TiO2-Modified PVDF Membranes with Enhanced Antifouling Behaviors via Phase Inversion: Implications of Nanoparticle Dispersion Status in Casting Solutions

Titanium dioxide (TiO2) nanoparticles have been applied in membrane antifouling performance modification for years. However, the influence of TiO2 nanoparticle dispersion status during the blending process on membrane properties and the inner mechanism has seldom been focused on. Herein, we investigated the influence of the various dispersing statuses of TiO2 nanoparticles on membrane properties and antifouling performance by exploring various blending processes without changing the original recipe. Polyethylene glycol (PEG) was employed as a pore-forming agent during the membrane preparation process, and also as a pre-dispersing agent for the TiO2 nanoparticles via the steric hindrance effect. Compared to the original preparation process of the PVDF/TiO2 composite membrane, the pre-dispersing of TiO2 via PEG ensured a modified membrane with uniform surface pores and structures on cross-sectional morphologies, larger porosity and water permeability, and more negative zeta potential. The contact angle was decreased by 6.0%, implying better hydrophilicity. The improved antifouling performance was corroborated by the increasing free energy of cohesion and adhesion, the interaction energy barrier (0.43 KT) between the membrane surfaces and approaching foulants assessed by classic XDLVO theory and the low flux decline in the filtration experiment. A kinetics mechanism analysis of the casting solutions, which found a low TSI value (<1.0), substantiated that the pre-dispersion of TiO2 with PEG contributed to the high stability and ultimately favorable antifouling behaviors. This study provides an optimized approach to the preparation of excellent nano-TiO2/polymeric composite membranes applied in the municipal sewage treatment field.

Synthesis of a biomimetic zwitterionic pentapolymer to fabricate high-performance PVDF membranes for efficient separation of oil-in-water nano-emulsions

Oily wastewater from industries has an adverse impact on the environment, human and aquatic life. Poly(vinylidene fluoride) (PVDF) membrane modified with a zwitterionic/hydrophobic pentapolymer (PP) with controlled pore size has been utilized to separate oil from water from their nano-emulsions. The PP has been synthesized in 91% yield via pentapolymerization of four different diallylamine salts [(CH2=CHCH2)2NH+(CH2)x A-], bearing CO2-, PO3H-, SO3-, (CH2)12NH2 pendants, and SO2 in a respective mol ratio of 25:36:25:14:100. Incorporating PP into PVDF has shown a substantially reduced membrane hydrophobicity; the contact angle decreased from 92.5° to 47.4°. The PP-PVDF membranes have demonstrated an excellent capability to deal with the high concentrations of nano-emulsions with a separation efficiency of greater than 97.5%. The flux recovery ratio (FRR) of PP-5 incorporated PVDF membrane was about 82%, which was substantially higher than the pristine PVDF.

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.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.

Anti-fouling and protein separation of PVDF-g-PMAA@MnO2 filtration membrane with in-situ grown MnO2 nanorods

MnO₂ nanorods with controllable scale were grown in the PVDF-g-PMAA modified membrane to form PVDF-g-PMAA@ MnO₂ membrane through the in situ redox reaction of KMnO₄ solution, which is confirmed by scanning electron microscopy (SEM) and X-ray energy-dispersion spectroscopy (EDX). The pore size of the membrane decreased with the increase of KMnO₄ solution concentration. The thermodynamic stability and the hydrophilicity of the membrane were also enhanced by the MnO₂ nanorods. The water flux, bovine serum albumin (BSA)/Lysozyme protein solution flux and rejection, flux recovery, etc. showed effective improvement of the anti-fouling performance of the PVDF-g-PMAA@ MnO₂ membrane. More importantly, it can effectively separate BSA from lysozyme, which provided a potential application in the field of biology, food, and other industrial fields for the requirement of separation and purification.