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.

Delamination-Free In-Air and Underwater Oil-Repellent Filters for Oil-Water Separation: Gravity-Driven and Cross-Flow Operations

Separating oil-water mixtures is critical in a variety of practical applications, including the treatment of industrial wastewater, oil spill cleanups, as well as the purification of petroleum products. Among various methodologies that have been utilized, membranes are the most attractive technology for separating oil-water emulsions. In recent years, selective wettability membranes have attracted particular attention for oil-water separations. The membrane surfaces with hydrophilic and in-air oleophobic wettability have demonstrated enhanced effectiveness for oil-water separations in comparison with underwater oleophobic membranes. However, developing a hydrophilic and in-air oleophobic surface for a membrane is not a trivial task. The coating delamination process is a critical challenge when applying these membranes for separations. Inspired by the above, in this study we utilize poly(ethylene glycol)diacrylate (PEGDA) and 1H,1H,2H,2H-heptadecafluorodecyl acrylate (F-acrylate) to fabricate a hydrophilic and in-air oleophobic coating on a filter. We utilize methacryloxypropyl trimethoxysilane (MEMO) as an adhesion promoter to enhance the adhesion of the coating to the filter. The filter demonstrates robust oil repellency preventing oil adhesion and oil fouling. Utilizing the filter, gravity-driven and continuous separations of surfactant-stabilized oil-water emulsions are demonstrated. Finally, we demonstrate that the filter can be reused multiple times upon rinsing for further oil-water separations.

Predicting kinetics of water-rich permeate flux through photocatalytic mesh under visible light illumination

Membrane-based separation technologies are attractive to remediating unconventional water sources, including brackish, industrial, and municipal wastewater, due to their versatility and relatively high energy efficiency. However, membrane fouling by dissolved or suspended organic substances remains a primary challenge which can result in an irreversible decline of the permeate flux. To overcome this, membranes have been incorporated with photocatalytic materials that can degrade these organic substances deposited on the surface upon light illumination. While such photocatalytic membranes have demonstrated that they can recover their inherent permeability, less information is known about the effect of photocatalysis on the kinetics of the permeate flux. In this work, a photocatalytic mesh that can selectively permeate water while repelling oil was fabricated by coating a mixture of nitrogen-doped TiO2 (N-TiO2) and perfluorosilane-grafted SiO2 (F-SiO2) nanoparticles on a stainless steel mesh. Utilizing the photocatalytic mesh, the time-dependent evolution of the water-rich permeate flux as a result of photocatalytic degradation of the oil was studied under the visible light illumination. A mathematical model was developed that can relate the photocatalytic degradation of the organic substances deposited on a mesh surface to the evolution of the permeate flux. This model was established by integrating the Langmuir-Hinshelwood kinetics for photocatalysis and the Cassie-Baxter wettability analysis on a chemically heterogeneous mesh surface into a permeate flux relation. Consequently, the time-dependent water-rich permeate flux values are compared with those predicted by using the model. It is found that the model can predict the evolution of the water-rich permeate flux with a goodness of fit of 0.92.

Surface modification of a PES membrane by corona air plasma-assisted grafting of HB-PEG for separation of oil-in-water emulsions

The main goal of this study is to modify a polyethersulfone (PES) membrane by grafting with hyperbranched polyethylene glycol (HB-PEG) using corona air plasma to intensify the anti-fouling properties of the prepared membrane. The separation efficiency and fouling tendency of the modified membranes were evaluated for the treatment of a synthetic oily wastewater. A mechanism was proposed for the HB-PEG grafting on the surface of the corona treated PES membranes and all steps of the grafting were described in detail. The effects of corona treatment operating conditions on the morphology, surface properties, separation performance and anti-fouling efficiency of the modified PES membranes were investigated. Also, the HB-PEG grafted PES membranes were characterized by FTIR, AFM and contact angle analysis. Finally, the HB-PEG grafting on the surface of the PES membranes altered the surface hydrophilicity and led to the improvement of the anti-fouling property and oil–water permeation flux of all modified membranes without any remarkable changes in oil rejection.

The main goal of this study is to modify a polyethersulfone (PES) membrane by grafting with hyperbranched polyethylene glycol (HB-PEG) using corona air plasma to intensify the anti-fouling properties of the prepared membrane.

R. R, A. S. Tailored design of polyurethane based fouling-tolerant nanofibrous membrane for water treatment

Polyurethane (PU) nanofibers have gained attention due to their good mechanical properties and water resistance.

One-Step Coating toward Multifunctional Applications: Oil/Water Mixtures and Emulsions Separation and Contaminants Adsorption

Here, a method that can simultaneously separate oil/water mixtures and remove water-soluble contaminants has been developed. Various substrates with different pore size were coated by polydopamine and polyethylenepolyamine codeposition films. The as-prepared materials were superhydrophilic and under-water superoleophobic. The materials can separate a range of different oil/water mixtures (including immiscible oil/water mixtures and surfactant-stabilized emulsions) in a single unit operation, with >99.6% separation efficiency and high fluxes. Copper ion and methyl blue can be effectively absorbed from water when it permeates through the materials. This method can be applied on organic and inorganic substrates and used in preparing large-scale product. Therefore, the simple and facile method has excellent potential in practical application and creates a new field for oil/water separation materials with multifunctional applications.

Polymeric membranes for produced water treatment: an overview of fouling behavior and its control

Produced water (PW) from the oil/gas field is an important waste stream. Due to its highly pollutant nature and large volume of generation, the management of PW is a significant challenge for the petrochemical industry. The treatment of PW can improve the economic viability of oil and gas exploration, and the treated water can provide a new source of water in the water-scarce region for some beneficial uses. The reverse osmosis (RO) and selective nanofiltration (NF) membrane treatment of PW can reduce the salt and organic contents to acceptable levels for some beneficial uses, such as irrigation, and different industrial reuses. However, membrane fouling is a major obstacle for the membrane-based treatment of PW. In this review, the author discusses the polymeric membrane (mainly RO/NF) fouling during PW treatment. Membrane fouling mechanisms by various types of foulants, such as organic, inorganic, colloidal, and biological matters, are discussed. The review concludes with some of the measures to control fouling by membrane surface modification approaches.

Synergy of graphene oxide–silver nanocomposite and amphiphilic co-polymer F127 on antibacterial properties and permeability of PVDF membrane

Modified GO–Ag/PVDF membranes were successfully prepared based on an in situ reduction with excellent permeability, hydrophilicity, bactericidal activity.

Improved antifouling and antimicrobial efficiency of ultrafiltration membranes with functional carbon nanotubes

In this study, functional polymer brush grafted carbon nanotubes (p-CNTs) were developed as multifunctional modifiers for PES membrane modification.

Enhanced hydrophilicity of a thermo-responsive PVDF/palygorskite-g-PNIPAAM hybrid ultrafiltration membrane via surface segregation induced by temperature

The hydrophilicity of a thermo-responsive PVDF/palygorskite-g-PNIPAAM hybrid ultrafiltration membrane was enhanced via surface segregation induced by coagulation bath temperature (CBT).

Characterization and antifouling properties of polyethylene glycol doped PAN–CAP blend membrane

The effects of polyethylene glycol (PEG) as an additive to a cellulose acetate phthalate–polyacrylonitrile blend membrane in the ultrafiltration range were investigated.

Sensitivity of coalescence separation of oil–water emulsions using stainless steel felt enabled by LBL self-assembly and CVD

Coalescence sensitivity to surface wettability and pore size of roughened stainless steel felt was revealed for oil-in-water emulsion separation.