Super-wetting, photoactive TiO2 coating on amino-silane modified PAN nanofiber membranes for high efficient oil-water emulsion separation application

Advanced nanofibers for water treatment: Unveiling the potential of electrospun polyacrylonitrile membranes

The challenges pertaining to the potable water scarcity and pollution motivates us to envision innovative strategies. Industrial wastewater containing hazardous heavy metals, synthetic dyes, and oil exacerbates the pursuit of clean drinking water. Among the array of available technologies, electrospun nanofiber membranes have garnered attention due to their efficiency, high surface-to-volume ratio, cost-effectiveness, scalability, and multifunctionality. These membranes possess distinct physical and chemical attributes that position them as ideal solutions to water purification challenges. Their versatility enables effective contaminant removal through filtration, adsorption, and chemical interactions. Polyacrylonitrile (PAN) emerges as a frontrunner among electrospun polymers due to its affordability, remarkable physical and chemical characteristics, and the ease of production. Research efforts have been dedicated to the study of electrospun PAN membranes, exploring modifications in terms of the functionalization of PAN molecular chain, incorporation of appropriate nanoparticles, and composition with other functional polymers. Parameters such as functional groups, hydrophilicity, mechanical properties, porosity, pore structure, reusability, sustainability, zeta potential, and operational conditions significantly influence the performance of electrospun PAN membranes in treating the contaminated water. Despite progress, challenges surrounding fouling, toxicity, scalability, selectivity, and production costs ought to be addressed strategically to enhance their practicality and real-world viability. This review comprehensively scrutinizes the current landscape of available electrospun PAN membranes in water treatment encompassing diverse range of synthesized entities and experimental outcomes. Additionally, the review delves into various approaches undertaken to optimize the performance of electrospun PAN membranes while proposing potential strategies to overcome the existing hindrances. By carefully analyzing the parameters that impact the performance of these membranes, this overview offers invaluable guidelines for researchers and engineers, thus empowering them to design tailored electrospun nanofiber membranes for specific water purification applications. As the innovative research continues and strategic efforts address the current challenges, these membranes can play a pivotal role in enhancing water quality, mitigating water scarcity, and contributing to environmental sustainability. The widespread application of electrospun nanofiber membranes in water treatment has the potential to create a lasting positive impact on global water resources and the environment. A dedicated effort towards their implementation will undoubtedly mark a crucial step towards a more sustainable and water-secure future.

Construction of biodegradable superhydrophilic/underwater superoleophobic materials with CNF (cellulose nanofiber) fence-like attached on the surface for efficient oil/water emulsion separation

Superhydrophilic/underwater superoleophobic materials for the separation of oil-water emulsions by filtration have received much attention in order to solve the pollution problem of oil-water emulsion. In this paper, a fence-like structure on the surface of CNF/KGM (Konjac Glucomannan) materials by a simple method using CNF instead of metal nanowires was successfully developed based on the hydrogen bonding of KGM and CNF. The resulted organic CNF/KGM materials surface has outstanding superhydrophilic (WCA = 0°) in air and superoleophobicity (OCA≥151°) in water, which could separate oil-water mixtures with high separation efficiency above 99.14 % under the pressure of the emulsion itself. The material shows good mechanical properties because of the addition of CNF and has outstanding anti-fouling property and reusability. More importantly, the material can be completely biodegraded after buried in soil for 4 weeks since both of KGM and CNF are organic substances. Therefore, it may have a broad application prospect in the separation of oil-water emulsion because of its outstanding separation properties, simply preparation method and biodegradability.

Recent advances in membrane technologies applied in oil-water separation

Effective treatment of oily wastewater, which is toxic and harmful and causes serious environmental pollution and health risks, has become an important research field. Membrane separation technology has emerged as a key area of investigation in oil-water separation research due to its high separation efficiency, low costs, and user-friendly operation. This review aims to report on the advances in the research of various types of separation membranes around emulsion permeance, separation efficiency, antifouling efficiency, and stimulus responsiveness. Meanwhile, the challenges encountered in oil-water separation membranes are examined, and potential research avenues are identified.

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.

A comprehensive review on the behavior and evolution of oil droplets during oil/water separation by membranes

Membrane separation technology has significant advantages for treating oil-in-water emulsions. Understanding the evolution of oil droplets could reveal the interfacial and colloidal interactions, facilitate the design of advanced membranes, and improve the separation performances. This review on the characteristic behavior and evolution of oil droplets focuses on the advanced analytical techniques, and the subsequent fouling as well as demulsification effects during membrane separation. A detailed introduction is provided on microscopic observations and numerical simulations of the dynamic evolution of oil droplets, featuring real-time in-situ visualization and accurate reconstruction, respectively. Characteristic behaviors of these oil droplets include attachment, pinning, wetting, spreading, blockage, intrusion, coalescence, and detachment, which have been quantified by specific proposed parameters and criteria. The fouling process can be evaluated using Hermia and resistance models. The related adhesion force and intrusion pressure as well as droplet-droplet/membrane interfacial interactions can be accurately quantified using various force analysis methods and advanced force measurement techniques. It is encouraging to note that oil coalescence has been achieved through various effects such as electrostatic interactions, mechanical actions, Laplace pressure/surface free energy gradients, and synergistic effects on functional membranes. When oil droplets become destabilized and coalesce into larger ones, the functional membranes can overcome the limitations of size-sieving effect to attain higher separation efficiency. This not only bypasses the trade-off between permeability and rejection, but also significantly reduces membrane fouling. Finally, the challenges and potential research directions in membrane separation are proposed. We hope this review will support the engineering of advanced materials for oil/water separation and research on interface science in general.

Efficacy of Electrospun Nanofiber Membranes on Fouling Mitigation: A Review

Despite the advantages of high contaminant removal, operational flexibility, and technical advancements offered, the undesirable fouling property of membranes limits their durability, thus posing restrictions on their usage. An enormous struggle is underway to conquer this major challenge. Most of the earlier reviews include the basic concepts of fouling and antifouling, with respect to particular separation processes such as ultrafiltration, nanofiltration, reverse osmosis and membrane bioreactors, graphene-based membranes, zwitterionic membranes, and so on. As per our knowledge, the importance of nanofiber membranes in challenging the fouling process has not been included in any record to date. Nanofibers with the ability to be embedded in any medium with a high surface to volume ratio play a key role in mitigating the fouling of membranes, and it is important for these studies to be critically analyzed and reported. Our Review hence intends to focus on nanofiber membranes developed with enhanced antifouling and biofouling properties with a brief introduction on fabrication processes and surface and chemical modifications. A summary on surface modifications of preformed nanofibers is given along with different nanofiller combinations used and blend fabrication with efficacy in wastewater treatment and antifouling abilities. In addition, future prospects and advancements are discussed.

An Overview of Recent Progress in Nanofiber Membranes for Oily Wastewater Treatment

Oil separation from water becomes a challenging issue in industries, especially when large volumes of stable oil/water emulsion are discharged. The present short review offers an overview of the recent developments in the nanofiber membranes used in oily wastewater treatment. This review notes that nanofiber membranes can efficiently separate the free-floating oil, dispersed oil and emulsified oil droplets. The highly interconnected pore structure nanofiber membrane and its modified wettability can enhance the permeation flux and reduce the fouling. The nanofiber membrane is an efficient separator for liquid-liquid with different densities, which can act as a rejector of either oil or water and a coalescer of oil droplets. The present paper focuses on nanofiber membranes' production techniques, nanofiber membranes' modification for flux and separation efficiency improvement, and the future direction of research, especially for practical developments.

Enhanced oil removal from a real polymer production plant by cellulose nanocrystals-serine incorporated polyethersulfone ultrafiltration membrane

As discharging oily wastewater from industries to the environment is a potential threat for the aquatic ecosystem, in this research, oil removal from a real case of Kermanshah polymer production plant wastewater was investigated. The focus of this study was on improving the oil rejection performance of polyethersulfone (PES) ultrafiltration membrane due to adding cellulose nanocrystals (CNC) and modified CNC with serine amino acid (CNC-Ser) in PES mix matrix. From the results, the membranes embedded with CNC-Ser showed better performance in terms of water flux, flux recovery ratio, and oil rejection (higher than 97%) compared to the modified membranes with CNC. The lowest water contact angle (41.37°), smoother surface, and higher negative surface potential (- 24 mV) were achieved for the optimum loading of CNC-Ser. Besides, long-term performance of the membranes with optimum loading of CNC and CNC-Ser were compared in both dead-end and cross-flow setups.

Fundamentals and utilization of solid/ liquid phase boundary interactions on functional surfaces

The affinity of macroscopic solid surfaces or dispersed nano- and bioparticles towards liquids plays a key role in many areas from fluid transport to interactions of the cells with phase boundaries. Forces between solid interfaces in water become especially important when the surface texture or particles are in the colloidal size range. Although, solid-liquid interactions are still prioritized subjects of materials science and therefore are extensively studied, the related literature still lacks in conclusive approaches, which involve as much information on fundamental aspects as on recent experimental findings related to influencing the wetting and other wetting-related properties and applications of different surfaces. The aim of this review is to fill this gap by shedding light on the mechanism-of-action and design principles of different, state-of-the-art functional macroscopic surfaces, ranging from self-cleaning, photoreactive or antimicrobial coatings to emulsion separation membranes, as these surfaces are gaining distinguished attention during the ongoing global environmental and epidemic crises. As there are increasing numbers of examples for stimulus-responsive surfaces and their interactions with liquids in the literature, as well, this overview also covers different external stimulus-responsive systems, regarding their mechanistic principles and application possibilities.

A dual-functional layer modified GO@SiO2 membrane with excellent anti-fouling performance for continuous separation of oil-in-water emulsion

As the most significant target of membrane separation, the inadequacy of permeability and anti-fouling frequently constrain the application of the membrane in actual oily wastewater. Herein, a novel concept of membrane surface construction was proposed to mitigate this intractable problem, using SiO₂ as the support layer and graphene oxide (GO) as the isolation layer. The best co-localization proportion of the support layer (56 mg/L) and isolation layer (3.5 mg/L) was determined by the separation performance of the modified membranes for the simulated emulsion. The thin GO layer could effectively prevent contaminants from entering the membrane pores without affecting its roughness. Based on the synergistic action of the isolation layer and support layer, the GO@SiO₂ membrane could well implement emulsion purification with a stable permeability (654.11 LMHB) and high separation efficiency (99.41%). The superior anti-fouling performance of the membrane ensures its long-term cycling stability, with the permeability recovery rate of 89.75% (low-density oil) and 90.41% (high-density oil) after 10 repeated cycles. The storage stability also indirectly increases its value in practical applications. More importantly, the GO@SiO₂ membrane also shows great potential for industrial emulsion treatment with excellent purification and cycling stability (permeability recovery rate of 84.01%).

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.

One-step in-situ fabrication of carbon nanotube/stainless steel mesh membrane with excellent anti-fouling properties for effective gravity-driven filtration of oil-in-water emulsions

The occurrence of membrane fouling has resulted in limited wastewater treatment applications. The development of superhydrophilic-underwater superoleophobic materials has received significant attention owing to their good anti-fouling properties. However, to fabricate such materials need costly regents and tedious steps. Thus, developing a one-step process to prepare a low-cost material for oil/water separation is still desired. In this study, bio-inspired from an arachnid, inorganic carbon nanotube stainless steel meshes (CNT@SSMs) having superhydrophilic-underwater superoleophobic and excellent anti-fouling properties and a unique fiber structure were fabricated via a one-step thermal chemical vapor deposition method. The CNT@SSMs had a small pore size enabling a high water flux of 10,639 L m^-2h^-1 and the separation of oily wastewater, including various emulsions, at a high rejection ratio of >98.89%. As a result of its excellent chemical stability under high temperatures, a broad pH range, and saline environments, the CNT@SSM has the potential to be used in extreme conditions. In summary, these CNT@SSMs are easy to fabricate and are low-cost as a result of inexpensive reagents involved. Moreover, these novel superwetting membranes are promising candidates for treatment of hazardous oily wastewater.

Development of Red Clay Ultrafiltration Membranes for Oil-Water Separation

In this study, a red clay/nano-activated carbon membrane was investigated for the removal of oil from industrial wastewater. The sintering temperature was minimized using CaF2 powder as a binder. The fabricated membrane was characterized by its mechanical properties, average pore size, and hydrophilicity. A contact angle of 67.3° and membrane spore size of 95.46 nm were obtained. The prepared membrane was tested by a cross-flow filtration process using an oil-water emulsion, and showed a promising permeate flux and oil rejection results. During the separation of oil from water, the flux increased from 191.38 to 284.99 L/m2 on increasing the applied pressure from 3 to 6 bar. In addition, high water permeability was obtained for the fabricated membrane at low operating pressure. However, the membrane flux decreased from 490.28 to 367.32 L/m2·h due to oil deposition on the membrane surface; regardless, the maximum oil rejection was 99.96% at an oil concentration of 80 NTU and a pressure of 5 bar. The fabricated membrane was negatively charged, as were the oil droplets, thereby facilitating membrane purification through backwashing. The obtained ceramic membrane functioned well as a hydrophilic membrane and showed potential for use in oil wastewater treatment.

Hydrophilic Surface-Modified PAN Nanofibrous Membranes for Efficient Oil-Water Emulsion Separation

In order to protect the environment, it is important that oily industrial wastewater is degreased before discharging. Membrane filtration is generally preferred for separation of oily wastewater as it does not require any specialised chemical knowledge, and also for its ease of processing, energy efficiency and low maintenance costs. In the present work, hybrid polyacrylonitrile (PAN) nanofibrous membranes were developed for oily wastewater filtration. Membrane surface modification changed nitrile groups on the surface into carboxylic groups, which improve membrane wettability. Subsequently, TiO₂ nanoparticles were grafted onto the modified membranes to increase flux and permeability. Following alkaline treatment (NaOH, KOH) of the hydrolysed PAN nanofibres, membrane water permeability increased two- to eight-fold, while TiO₂ grafted membrane permeability increase two- to thirteen-fold, compared to unmodified membranes. TiO₂ grafted membranes also displayed amphiphilic properties and a decrease in water contact angle from 78.86° to 0°. Our results indicate that modified PAN nanofibrous membranes represent a promising alternative for oily wastewater filtration.

High efficient reduction of 4-nitrophenol and dye by filtration through Ag NPs coated PAN-Si catalytic membrane

Catalytic membrane plays an important role in environmental remedy. In this study, we reported an Ag coated membrane (PAN-Si-Cu-Ag) with a high catalytic activity to reduce 4-nitrophenol (4-NP) and methyl orange (MO) from water. The best performance is 99% reduction degree and 280 L m^-2.h^-1.bar^-1 flux for (4-NP) reduction at 4-NP: NaBH₄ = 1:50 (mM) during a 12-h filtration. The reduction degree for MO is above 90% and the flux is about 230 L m^-2·h^-1·bar^-1, which is almost the best report till now. The Ag coated membrane was prepared by metal displacement-epitaxial growth on silica covalent grafted PAN membrane (PAN-Si). Silica atoms were used as linker to ensure the good adhesion between polymer and metal NPs, the loss amount of Ag NPs from the coated catalytic membrane is loss about 2 μg/cm² after one month storage. Cheap metal NPs were firstly reduced on the surface of PAN-Si membrane and then used to displace Ag ions. Thus the obtained catalytic membrane showed a very high loading (28%). Finally, the catalytic filtration mechanism of 4-NP was distinguished by Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and adsorption measurement.

High efficient dye removal with hydrolyzed ethanolamine-Polyacrylonitrile UF membrane: Rejection of anionic dye and selective adsorption of cationic dye

The dye-water treatment using UF membrane is still a challenge. In the present study, the optimized PAN-ETA ultrafiltration membrane was hydrolyzed and subsequently characterized by SEM, IR, CA, XPS, NMR, mechanic measurement, etc. The obtained membrane (H-PAN-ETA) was used for dye removal and it showed both an excellent anti-dye fouling and a good rejection property for anionic dyes. I.e. 96% rejection for methyl blue (MB), 99% for congo red (CR), 94% for acid fuchsin (AF) with no sign of contamination by dye. The flux of H-PAN-ETA membrane maintained at 50-53 L m^-2⋅ h^-1 during a 10-h filtration, which is higher than that of tight UF membranes reported. Meanwhile, H-PAN-ETA membrane was able to selectively remove cationic dyes, such as methylene blue (MEB), rhodamine B (RB) and, crystal violet (CV), or the mixture of anionic dye/cationic dye by adsorption process. Its adsorption capacity remained unchanged after 20 cycles. Finally, the immobile electrical double layer (EDL) theory combined with electrostatic force was introduced to explain the separation mechanism of charged UF membrane, which is helpful to instruct the preparation of UF membrane for dye removal.

Membrane processes

This literature review provides a review for publications in 2018 and 2019 and includes information membrane processes findings for municipal and industrial applications. This review is a subsection of the annual Water Environment Federation literature review for Treatment Systems section. The following topics are covered in this literature review: industrial wastewater and membrane. Bioreactor (MBR) configuration, membrane fouling, design, reuse, nutrient removal, operation, anaerobic membrane systems, microconstituents removal, membrane technology advances, and modeling. Other sub-sections of the Treatment Systems section that might relate to this literature review include the following: Biological Fixed-Film Systems, Activated Sludge, and Other Aerobic Suspended Culture Processes, Anaerobic Processes, and Water Reclamation and Reuse. This publication might also have related information on membrane processes: Industrial Wastes, Hazardous Wastes, and Fate and Effects of Pollutants.

2D/1D V2O5 Nanoplates Anchored Carbon Nanofibers as Efficient Separator Interlayer for Highly Stable Lithium-Sulfur Battery

Quick capacity loss due to the polysulfide shuttle effects is a critical challenge for high-performance lithium-sulfur (Li-S) batteries. Herein, a novel 2D/1D V2O5 nanoplates anchored carbon nanofiber (V-CF) interlayer coated on standard polypropylene (PP) separator is constructed, and a stabilization mechanism derived from a quasi-confined cushion space (QCCS) that can flexibly accommodate the polysulfide utilization is demonstrated. The incorporation of the V-CF interlayer ensures stable electron and ion pathway, and significantly enhanced long-term cycling performances are obtained. A Li-S battery assembled with the V-CF membrane exhibited a high initial capacity of 1140.8 mAh·g-1 and a reversed capacitance of 1110.2 mAh·g-1 after 100 cycles at 0.2 C. A high reversible capacity of 887.2 mAh·g-1 is also maintained after 500 cycles at 1 C, reaching an ultra-low decay rate of 0.0093% per cycle. The excellent electrochemical properties, especially the long-term cycling stability, can offer a promising designer protocol for developing highly stable Li-S batteries by introducing well-designed fine architectures to the separator.

Fabrication of a PAN–PA6/PANI membrane using dual spinneret electrospinning followed by in situ polymerization for separation of oil-in-water emulsions

A polyacrylonitrile–polyamide 6/polyaniline (PAN–PA6/PANI) doped membrane was prepared using dual spinneret simultaneous electrospinning of PAN and PA6 and in situ polymerization of aniline at low temperature.