Synergy of the mechanical, antifouling and permeation properties of a carbon nanotube nanohybrid membrane for efficient oil/water separation

Surface Segregation Methods toward Molecular Separation Membranes

As a highly promising approach to solving the issues of energy and environment, membrane technology has gained increasing attention in various fields including water treatment, liquid separations, and gas separations, owing to its high energy efficiency and eco-friendliness. Surface segregation, a phenomenon widely found in nature, exhibits irreplaceable advantages in membrane fabrication since it is an in situ method for synchronous modification of membrane and pore surfaces during the membrane forming process. Meanwhile, combined with the development of synthesis chemistry and nanomaterial, the group has developed surface segregation as a versatile membrane fabrication method using diverse surface segregation agents. In this review, the recent breakthroughs in surface segregation methods and their applications in membrane fabrication are first briefly introduced. Then, the surface segregation phenomena and the classification of surface segregation agents are discussed. As the major part of this review, the authors focus on surface segregation methods including free surface segregation, forced surface segregation, synergistic surface segregation, and reaction-enhanced surface segregation. The strategies for regulating the physical and chemical microenvironments of membrane and pore surfaces through the surface segregation method are emphasized. The representative applications of surface segregation membranes are presented. Finally, the current challenges and future perspectives are highlighted.

Review of Artificial Nacre for Oil–Water Separation

Due to their extraordinary prospective uses, particularly in the areas of oil–water separation, underwater superoleophobic materials have gained increasing attention. Thus, artificial nacre has become an attractive candidate for oil–water separation due to its superhydrophilicity and underwater superoleophobicity properties. Synthesized artificial nacre has successfully achieved a high mechanical strength that is close to or even surpasses the mechanical strength of natural nacre. This can be attributed to suitable synthesis methods, the selection of inorganic fillers and polymer matrices, and the enhancement of the mechanical properties through cross-linking, covalent group modification, or mineralization. The utilization of nacre-inspired composite membranes for emerging applications, i.e., is oily wastewater treatment, is highlighted in this review. The membranes show that full separation of oil and water can be achieved, which enables their applications in seawater environments. The self-cleaning mechanism’s basic functioning and antifouling tips are also concluded in this review.

Textured ceramic membranes for desilting and deoiling of produced water in the Permian Basin

Oil production in the Permian Basin gives rise to large volumes of produced water contaminated by silt, emulsified oil, and additives used for enhanced oil recovery. There is intense interest in the design of membrane modules as sustainable alternatives for produced water treatment to enable the reuse of produced water for agricultural applications, injection into aquifers, and redeployment in oil recovery. Here, we report a hierarchically textured cement-based membrane exhibiting orthogonal wettability, specifically, superhydrophilic and underwater superoleophobic characteristics. The in situ formation of ettringite needles accompanied by embedding of glass spheres imbues multiscale texturation to stainless-steel mesh membranes, enabling the separation of silt and oil from produced water at high flux rates (1600 L h⁻¹۰m⁻², at ca. 2.7 bar). Oil concentration is reduced as low as 1 ppb with an overall separation efficiency of 99.7% in single-pass filtration. The membranes show outstanding mechanical resilience and retention of performance across multiple cycles.

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Multiscale 3D texturation of a metal mesh membrane engenders orthogonal wettability

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High separation efficiency of ca. 99% was achieved for produced water

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Oil concentration reduced to ≤1 ppb in a single pass at a high flux rate

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Desiliting and deoiling demonstrated for real produced water streams

Recent Advancements in Polyphenylsulfone Membrane Modification Methods for Separation Applications

Polyphenylsulfone (PPSU) membranes are of fundamental importance for many applications such as water treatment, gas separation, energy, electronics, and biomedicine, due to their low cost, controlled crystallinity, chemical, thermal, and mechanical stability. Numerous research studies have shown that modifying surface properties of PPSU membranes influences their stability and functionality. Therefore, the modification of the PPSU membrane surface is a pressing issue for both research and industrial communities. In this review, various surface modification methods and processes along with their mechanisms and performance are considered starting from 2002. There are three main approaches to the modification of PPSU membranes. The first one is bulk modifications, and it includes functional groups inclusion via sulfonation, amination, and chloromethylation. The second is blending with polymer (for instance, blending nanomaterials and biopolymers). Finally, the third one deals with physical and chemical surface modifications. Obviously, each method has its own limitations and advantages that are outlined below. Generally speaking, modified PPSU membranes demonstrate improved physical and chemical properties and enhanced performance. The advancements in PPSU modification have opened the door for the advance of membrane technology and multiple prospective applications.

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

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

A Self-Cleaning Heterostructured Membrane for Efficient Oil-in-Water Emulsion Separation with Stable Flux

Lack of clean water is a major global challenge. Membrane separation technology is an ideal choice for the treatment of industrial, domestic sewage owing to its low energy consumption and cost. However, membranes are highly susceptible to contamination, particularly during wastewater treatment, which has limited their practical applications in this field. Similarly, the flux of the membrane decreases with prolonged use due to its reduced interlayer spacing. Preparation of membranes with anticontamination properties and stable flux is the key to addressing this problem. In this study, a 2D heterostructure membrane with visible-light-driven self-cleaning performance is prepared via a self-assembly process. Notably, the addition of palygorskite increases the interlayer spacing of the graphene and heterojunction structures, which increases the flux of the membrane and avoids a decrease of the interlayer spacing of the membrane under pressure. The presence of a heterojunction with visible light catalytic properties effectively avoids membrane fouling and avoids a sharp decrease of the permeation flux. Importantly, the prepared 2D membrane has excellent separation performance for oil-water emulsions with both high flux and efficiency. These features suggest great potential for the prepared 2D membrane in wastewater treatment applications.

The preparation of a modified PVDF hollow fiber membrane by coating with multiwalled carbon nanotubes for high antifouling performance

In this study, an outer surface modified polyvinylidene fluoride (PVDF) hollow fiber membrane (HF-PVDF-CNT) was prepared by coating with dopamine (PD) and multiwalled carbon nanotubes (CNTs), to solve the problems of the instability of pure CNT mats fabricated by filter coating methods and membrane fouling in wastewater treatment. The modified membrane was assessed and characterized by various methods, including studies of its top surface and cross-sectional morphology, wettability, functional groups and electrical conductivity. The CNT material stability was evaluated during backwashing. The antifouling and filtering abilities of the unmodified and modified membranes were tested by monitoring the change in TMP and the rejection performance for different contaminants during filtration in bovine serum albumin solution (BSA), sodium alginate solution (SA) and humic acid solution (HA). Furthermore, HF-PVDF-CNT and electro-assisted HF-PVDF-CNT membranes were employed as the basic separation units in an anaerobic membrane bioreactor (AnMBR) system and an anaerobic electrochemical membrane bioreactor (AnEMBR) system, respectively. Characterization of the HF-PVDF-CNT membrane indicated that the CNT mats exhibited good stability, electrical conductivity and wettability. In filtration experiments using BSA, SA and HA solutions, the HF-PVDF-CNT membrane showed an obvious improvement compared with the HF-PVDF membrane in antifouling performance. During its application in the AnMBR and AnEMBR systems, the electro-assisted HF-PVDF-CNT membrane had greater effects than the HF-PVDF-CNT membrane on reducing fouling.

Asymmetric Aerogel Membranes with Ultrafast Water Permeation for the Separation of Oil-in-Water Emulsion

Owing to highly porous and low density attributes, aerogels have been actively utilized in catalysis and adsorption processes, but their great potential in filtration requires exploitation. In this study, an asymmetric aerogel membrane is fabricated via one-pot hydrothermal reaction-induced self-cross-linking of poly(vinyl alcohol) (PVA), which exhibits ultrafast permeation for the separation of oil-in-water emulsion. Meanwhile, carbon nanotubes are added to improve the mechanical strength of the aerogel membranes. The self-cross-linking of PVA forms the supporting layer, and the exchange of water and vapor at the interface of PVA solution and air generates the separating layer as well as abundant hydroxyl groups on the membrane surface. The density, porosity, pore size, and wettability of the aerogel membrane can be tuned by the PVA concentration. Owing to high porosity (>95%) and suitable pore size (<85 nm), the aerogel membrane exhibits high rejection (99.0%) for surfactant-stabilized oil-in-water emulsion with an ultrahigh permeation flux of 135.5 × 10³ L m^-2 h^-1 bar^-1 under gravity-driven flow, which is 2 orders of magnitude higher than commercial filtration membranes with similar rejection. Meanwhile, the aerogel membrane exhibits superhydrophilicity, superoleophobicity underwater, and excellent antifouling properties for various surfactant-stabilized oil-in-water emulsions, as indicated by the fact that the flux recovery ratio maintains more than 93% after five cycles of the filtration experiment. The findings in this study may offer a novel idea to fabricate high-throughput filtration membranes.

Development of Highly Efficient Oil-Water Separation Carbon Nanotube Membranes with Stimuli-Switchable Fluxes

In this work, a carbon nanotube (CNT)-based membrane [(4-((4-((11-ferroceneundecyl)oxy)phenyl)diazenyl)phenoxy)-diethylene triamine (FADETA)/polyethyleneimine (PEI)-decorated CNT membrane] with stimuli-switchable separation fluxes was developed. The multiwalled CNTs were modified by a pH-, light-, and redox stimuli-responsive surfactant FADETA initially, and then the FADETA-decorated CNTs were further cross-linked by PEI and finally coated on the polypropylene membrane. Interestingly, the particular membrane was successfully applied in emulsion systems to separate oil and water with high efficiency. First, the FADETA-/PEI-decorated CNT membrane showed highly porous microstructural characteristics owing to the overlapped and cross-linked CNTs as confirmed by the scanning electron microscopy observation. Then, it showed strong hydrophilicity to water in the air and high oleophobicity to oil underwater, thereby endowing the membrane with the potential to separate oil and water. Owing to the modified multiple stimuli-responsive FADETA on CNTs, the separation fluxes were stimuli-switchable, which could be adjusted reversibly by environmental factors including pH, light, and redox.

Janus Membranes with Charged Carbon Nanotube Coatings for Deemulsification and Separation of Oil-in-Water Emulsions

Oil/water separation, especially for those surfactant-stabilized oil-in-water (O/W) emulsions, is required to protect our ecological environment from destruction. Janus membranes with a function of deemulsification appear as a kind of efficient materials for the separation of O/W emulsions because of a precise adjustment of the surface nature for the hydrophilic and hydrophobic layers. However, existing strategies of membrane preparation suffer from complicated multisteps, leading to uncontrolled thickness of the hydrophilic deemulsification layer. Herein, we present a facile and tunable method to prepare a series of Janus membranes consisting of negatively or positively charged carbon nanotubes (CNTs) and hydrophobic microfiltration membranes by vacuum filtration. The thickness of the hydrophilic CNT coating is thus well-controlled by engineering the amount of CNTs deposited on the substrate membrane. The prepared Janus membranes are effective for the separation of both heavy oil and light oil from O/W emulsions through deemulsification owing to the charge-screening effect. It is very interesting that those membranes displaying a combination of water contact angle and underwater oil contact angle both above 90° have a unique oil delivery behavior and thus high separation performance of oil from O/W emulsions. Such Janus membranes can retrieve 89% of oil in 40 min from the 1,2-dichloroethane/water emulsions with the droplet size of 19 μm. This easy-to-prepare and easy-to-tune strategy provides feasibilities for practical applications of Janus membranes to the deemulsification and separation of O/W emulsions.

Balsam-Pear-Skin-Like Porous Polyacrylonitrile Nanofibrous Membranes Grafted with Polyethyleneimine for Postcombustion CO2 Capture

Amine-containing sorbents have been extensively studied for postcombustion carbon dioxide (CO₂) capture because of their ability to chemisorb CO₂ from the flue gas. However, most sorbents are in the form of powders currently, which is not the ideal configuration for the flue gas separation because of the fragile nature and poor mechanical properties, resulting in blocking of the flow pipes and difficult recycling. Herein, we present a novel approach for the facile fabrication of flexible, robust, and polyethyleneimine-grafted (PEI-grafted) hydrolyzed porous PAN nanofibrous membranes (HPPAN-PEI NFMs) through the combination of electrospinning, pore-forming process, hydrolysis reaction, and the subsequent grafting technique. Excitingly, we find that all the resultant porous PAN (PPAN) fibers exhibit a balsam-pear-skin-like porous structure due to the selective removal of poly(vinylpyrrolidone) (PVP) from PAN/PVP fibers by water extraction. Significantly, the HPPAN-PEI NFMs retain their mesoporosity, as well as exhibit good thermal stability and prominent tensile strength (11.1 MPa) after grafting, guaranteeing their application in CO₂ trapping from the flue gas. When exposed to CO₂ at 40 °C, the HPPAN-PEI NFMs show an enhanced CO₂ adsorption capacity of 1.23 mmol g^-1 (based on the overall quantity of the sample) or 6.15 mmol g^-1 (based on the quantity of grafted PEI). Moreover, the developed HPPAN-PEI NFMs display significantly selective capture for CO₂ over N₂ and excellent recyclability. The CO₂ capacity retains 92% of the initial value after 20 adsorption-desorption cycle tests, indicating that the resultant HPPAN-PEI NFMs have good long-term stability. This work paves the way for fabricating NFM-based solid adsorption materials endowed with a porous structure applied to efficient postcombustion CO₂ capture.