A mussel inspired highly stable graphene oxide membrane for efficient oil-in-water emulsions separation

Improved Efficiency and Stability of MXene Membranes via Interlayer Space Tuning for Oily Water Separation

Surfactant-stabilized oil-in-water emulsions are a major environmental concern due to their severe consequences for aquatic organisms and humans. Two-dimensional materials, particularly MXenes, are widely used in various applications and could be used in designing advanced membranes. The narrow interlayer spacing and intrinsic oxidation severely limit mass diffusion and induce poor stability, respectively, of MXene-based separating layers on the membrane support, rendering it challenging to use for oil-water separation. Herein, a high-performing, minimally defective MXene membrane with large d-spacing was fabricated. The d-spacing of the MXene sheets was controlled using Si-based species as the intercalating agents. The modified MXene-based membrane (ultrasonication-assisted exfoliated MXene with Si pillars) (U-MX-Si) exhibited an enlarged interlayer spacing of 11 Å, increased surface energy of 41 mJ·m-2, and less defective separating layer compared to that of pristine MXene, which was due to enhanced interlayer spacing. This phenomenon induced a higher degree of exfoliated sheets that facilitated better MXene sheet self-assembly on the membrane support, thereby resulting in high separation efficiency (99%). An increase in the surface energy of the U-MX-Si membrane caused a constant permeate flux during operation, which demonstrated their practical implications. This study presents an important pathway for designing MXene-based membranes for separation applications.

Applications of graphene oxide (GO) in oily wastewater treatment: Recent developments, challenges, and opportunities

The treatment of oily wastewater has become a serious environmental challenge, for which graphene oxide has emerged as a promising material in solving the problem. The ever-growing utilization of graphene oxide (GO) in the treatment of oily wastewater necessitates a constant review. This review article employs a comprehensive literature survey methodology, systematically examining peer-reviewed articles, focusing on, but not entirely limited to, the last five years. Major databases such as EBSCOhost, Scopus, ScienceDirect, Web of Science and Google Scholar were searched using specific keywords related to GO and oily wastewater treatment. The inclusion criteria focused on studies that specifically address the application, efficiency, and mechanisms of GO in treating oily wastewater. The data extracted from these sources were then synthesized to highlight the most important developments, challenges, and prospects in this field. As far as oily wastewater treatment is concerned, the majority of the studies revolve around the use of GO in mitigating fouling in membrane processes, improving the stability, capacity and reusability of sorbents, and enhancing photodegradation by minimizing charge recombination.

Cellulose nanofiber-poly(ethylene terephthalate) nanocomposite membrane from waste materials for treatment of petroleum industry wastewater

Petroleum industry wastewater contains high level of crude oil and other types of organic substances that can cause immense harm to the agriculture, aquatic as well as terrestrial organisms. Organic solvent resistance of membranes is very important to treat such wastewater that contains high level of organic pollutants. This work reports the designing of a superhydrophilic and organic solvent resistant nanocomposite membrane using waste bottles made of poly(ethylene terephthalate) (PET) and cellulosic papers. Using in-situ synthesized cellulose nanofibers we could successfully fabricate porous membranes which is not possible for bare PET matrix using water as nonsolvent. Thus, we could successfully replace methanol which was used as a suitable non-solvent in earlier reports by distilled water. We successfully used the membrane for separation of synthetic crude oil-water emulsion. The membrane showed permeability up to 98 Lm^-2h^-1 applying pressure of 1.5 bar. The membrane also achieved removal of more than 97 % of organic substances from a crude oil-water emulsion system. The optimum membrane also showed good thermal stability with initial degradation temperature ∼350 °C and tensile strength of 0.86 MPa. The antimicrobial property of the nanocomposite membranes could be achieved by coating its surface with carbon dots rooted graphene oxide.

Impact of Membrane Modification and Surface Immobilization Techniques on the Hemocompatibility of Hemodialysis Membranes: A Critical Review

Despite significant research efforts, hemodialysis patients have poor survival rates and low quality of life. Ultrafiltration (UF) membranes are the core of hemodialysis treatment, acting as a barrier for metabolic waste removal and supplying vital nutrients. So, developing a durable and suitable membrane that may be employed for therapeutic purposes is crucial. Surface modificationis a useful solution to boostmembrane characteristics like roughness, charge neutrality, wettability, hemocompatibility, and functionality, which are important in dialysis efficiency. The modification techniques can be classified as follows: (i) physical modification techniques (thermal treatment, polishing and grinding, blending, and coating), (ii) chemical modification (chemical methods, ozone treatment, ultraviolet-induced grafting, plasma treatment, high energy radiation, and enzymatic treatment); and (iii) combination methods (physicochemical). Despite the fact that each strategy has its own set of benefits and drawbacks, all of these methods yielded noteworthy outcomes, even if quantifying the enhanced performance is difficult. A hemodialysis membrane with outstanding hydrophilicity and hemocompatibility can be achieved by employing the right surface modification and immobilization technique. Modified membranes pave the way for more advancement in hemodialysis membrane hemocompatibility. Therefore, this critical review focused on the impact of the modification method used on the hemocompatibility of dialysis membranes while covering some possible modifications and basic research beyond clinical applications.

Progress for Co-Incorporation of Polydopamine and Nanoparticles for Improving Membranes Performance

Incorporating polydopamine has become a viable method for membrane modification due to its universality and versatility. Fillers in their different categories have been confirmed as effective elements to improve the properties of membranes such as hydrophilicity, permeability, mechanical strength, and fouling resistance. Thus, this paper mainly highlights the recent studies that have been carried out using polydopamine and nanomaterial fillers simultaneously in modifying the performance of different membranes such as ultrafiltration, microfiltration, nanofiltration, reverse osmosis, and forward osmosis membranes according to the various modification methods. Graphene oxide nanoparticles have recently attracted a lot of attention among different nanoparticles used with polydopamine, due to their impressive characteristics impacts on enhancing membrane hydrophilicity, mechanical strength, and fouling resistance. Thus, the incorporation techniques of graphene oxide nanoparticles and polydopamine for enhancing membranes have been highlighted in this work. Moreover, different studies carried out on using polydopamine as a nanofiller for optimizing membrane performance have been discussed. Finally, perspectives, and possible paths of further research on mussel-inspired polydopamine and nanoparticles co-incorporation are stated according to the progress made in this field. It is anticipated that this review would provide benefits for the scientific community in designing a new generation of polymeric membranes for the treatment of different feed water and wastewater based on adhesive mussel inspired polydopamine polymer and nanomaterials combinations.

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.

A facile and effective strategy to develop a super-hydrophobic/super-oleophilic fiberglass filter membrane for efficient micron-scale water-in-oil emulsion separation

In order to achieve efficient micron-scale water-in-oil emulsion separation, a facile and effective strategy is developed to prepare a super-hydrophobic/super-oleophilic fiberglass filter membrane (FGm). Methyl-trichlorosilane (MTS) is successfully cross-linked on the surface of the fiberglass filter membrane (FGm) and aggregates into a 3D nanowire array to provide low surface energy. Nano fumed hydrophobic silica (SH-SiO₂) is used to construct the well-defined nanosphere structure on the surface of FGm and enhance the ability of the membrane to resist extreme conditions. The optimally modified membrane displays outstanding super-hydrophobic properties with a contact angle of 156.2°. It is impressive to find that the MTS@SH-SiO₂@FGm not only demonstrates the ability to separate water-in-oil emulsions with a particle size of less than 20 μm, but also the removal efficiency of separation has reached 99.98%. More attractively, the membrane still has stable super-hydrophobic features and reusable water-in-oil emulsion separation performance even under exposure to diverse harsh conditions, including extremely acidic corrosive solutions and ultra-high temperature systems.

In order to achieve efficient micron-scale water-in-oil emulsion separation, a facile and effective strategy is developed to prepare a super-hydrophobic/super-oleophilic fiberglass filter membrane (FGm).

Recent Developments and Advancements in Graphene-Based Technologies for Oil Spill Cleanup and Oil-Water Separation Processes

The vast demand for petroleum industry products led to the increased production of oily wastewaters and has led to many possible separation technologies. In addition to production-related oily wastewater, direct oil spills are associated with detrimental effects on the local ecosystems. Accordingly, this review paper aims to tackle the oil spill cleanup issue as well as water separation by providing a wide range of graphene-based technologies. These include graphene-based membranes; graphene sponges; graphene-decorated meshes; graphene hydrogels; graphene aerogels; graphene foam; and graphene-coated cotton. Sponges and aerogels modified by graphene and reduced graphene oxide demonstrated effective oil water separation owing to their superhydrophobic/superoleophilic properties. In addition, oil particles are intercepted while allowing water molecules to penetrate the graphene-oxide-coated metal meshes and membranes thanks to their superhydrophilic/underwater superoleophobic properties. Finally, we offer future perspectives on oil water separation that are hindering the advancements of such technologies and their large-scale applications.

Fabrication of self-assembled 0D-2D Bi2MoO6-g-C3N4 photocatalytic composite membrane based on PDA intermediate coating with visible light self-cleaning performance

A Self-cleaning surface can efficaciously solve the problem of irreversible contamination buildup on filtration membranes. Photocatalytic membranes were fabricated via vacuum assisted layer-by-layer (LBL) self-assembly of 0D-2D Bi₂MoO₆-g-C₃N₄ on a PDA coated thin-film composite PVDF substrate by Schiff base reaction. The rejection rate of the simulated polysaccharide was more than 90%, and that of the simulated protein was more than 80%. The combination of the membrane and the photocatalyst promoted the degradation of tetracycline hydrochloride by the composite membrane to 67.85% when original membranes had minor effect. Under visible light, reversible radiation pollutants (R_r) gradually replaced irreversible pollutants (R_ir) as the main pollutants. The flux recovery ratio (FRR) of 0D-2D Bi₂MoO₆-g-C₃N₄/PVDF membrane was 85% after being irradiated with visible light for 30 min. The flux recovery rate of contaminated photocatalytic membrane remained 75%, and the rejection was maintained in a stable range after four cycles of the cleaning operation under visible light. The results indicated that the excellent photocatalytic performance of 0D-2D Bi₂MoO₆-g-C₃N₄ photocatalysis material and the increase of multi-dimensional functional layer morphology on pollutant contact area improved the mechanical stability, interception performance and self-cleaning performance of the composite membrane. This work not only builds a new type of composite coating membranes, but also help us to further understand the relationship between the dimensions of photocatalytic materials and the improvement of photocatalytic membrane performance.

Graphene Oxide Membranes for Trace Hydrocarbon Contaminant Removal from Aqueous Solution

The aim of this paper is to shed light on the application of graphene oxide (GO) membranes for the selective removal of benzene, toluene, and xylene (BTX) from wastewater. These molecules are present in traces in the water produced from oil and gas plants and are treated now with complex filtration systems. GO membranes are obtained by a simple, fast, and scalable method. The focus of this work is to prove the possibility of employing GO membranes for the filtration of organic contaminants present in traces in oil and gas wastewater, which has never been reported. The stability of GO membranes is analyzed in water solutions with different pH and salinity. Details of the membrane preparation are provided, resulting in a crucial step to achieve a good filtration performance. Material characterization techniques such as electron microscopy, x-ray diffraction, and infrared spectroscopy are employed to study the physical and chemical structure of GO membranes, while gas chromatography, UV-visible spectroscopy, and gravimetric techniques allow the quantification of their filtration performance. An impressive rejection of about 90% was achieved for 1 ppm of toluene and other pollutants in water, demonstrating the excellent performance of GO membranes in the oil and gas field.

Amino Acid Cross-Linked Graphene Oxide Membranes for Metal Ions Permeation, Insertion and Antibacterial Properties

Graphene oxide (GO) and its composite membranes have exhibited great potential for application in water purification and desalination. This article reports that a novel graphene oxide membrane (GOM) of ~5 µm thickness was fabricated onto a nylon membrane by vacuum filtration and cross-linked by amino acids (L-alanine, L-phenylalanine, and serine). The GOM cross-linked by amino acids (GOM-A) exhibits excellent stability, high water flux, and high rejection to metal ions. The rejection coefficients to alkali and alkaline earth metal ions through GOM-A were over 94% and 96%, respectively. The rejection coefficients decreased with an increasing H⁺ concentration. Metal ions (K⁺, Ca²⁺, and Fe³⁺) can be inserted into GOM-A layers, which enlarges the interlayer spacing of GOM-A and neutralizes the electronegativity of the membrane, resulting in the decease in the rejection coefficients to metal ions. Meanwhile, GOM-A showed quite high antibacterial efficiency against E. coli. With the excellent performance as described above, GOM-A could be used to purify and desalt water.

The change from hydrophilicity to hydrophobicity of HEC/PAA complex membrane for water-in-oil emulsion separation: Thermal versus chemical treatment

Recently, the growing environmental concerns and economic demands drive the need to develop effective solutions for the treatment of oily wastewater, especially for oil/water emulsions. In this work, hydroxyethyl cellulose (HEC) and poly(acrylic acid) (PAA) are selected to form a complex membrane on the surface of poly(ethylene terephthalate) (PET) nonwoven via layer-by-layer assembly for separation of water-in-oil emulsions. In order to obtain a hydrophobic surface, two post-treatment methods, thermally and chemically induced cross-linking, are applied to modify the hydrogen-bonded HEC/PAA complex membrane. The properties of the two treated HEC/PAA-PET membranes, including surface morphology, chemical structure, chemical composition, thermal stability, mechanical property, and membrane wettability are systematically studied and compared to each other. When the membranes are applied as oil filters to treat water-in-oil emulsions with different concentrations, both of the modified membranes show excellent separation efficiencies with a more than 99.4% rejection for all tested water-in-oil emulsions.

Evaluation of self-cleaning performance of the modified g-C3N4 and GO based PVDF membrane toward oil-in-water separation under visible-light

Photocatalytic membranes (PMs), coupling of membrane filtration and photocatalysis, have exhibited the potential for application in the wastewater treatment. In this study, we firstly adopted the supramolecular aggregates of melamine (M), cyanuric acid (C), and urea (U) in specific dimethyl sulfoxide (DMSO) as precursors to prepare carbon nitride MCU-C₃N₄ with high photocatalytic performance, and a kind of novel-designed photocatalytic membrane was prepared via filtrating the mixture of graphene oxide (GO) nanosheets and MCU-C₃N₄ on PVDF membrane supports, and then crosslinked using glutaraldehyde (GA) to construct a steady coating on the GO/MCU-C₃N₄/PVDF membrane. GO/MCU-C₃N₄/PVDF composite membrane exhibited higher permeation flux than that of GO/PVDF membrane and exhibited excellent separation performance for oil-in-water emulsion. A visible light-driven self-cleaning four-stage filtration by a self-built dead-end filtration system was carried out to evaluate membrane antifouling property, and GO/MCU-C₃N₄/PVDF membrane (M2) possessed higher flux recovery ratio (FRR) (∼92.36%) and lower irreversible fouling resistance (R_ir) ratio (∼8%) under 30min visible-light irradiation, maintaining relatively higher FRR (>72%) during 4 cycling of four-stage filtrating experiments. GO/MCU-C₃N₄/PVDF PMs are equipped with high permeation flux, separation performance, anti-fouling property and stability, indicating potential application in water treatment.

Ion Transport Behavior through Thermally Reduced Graphene Oxide Membrane for Precise Ion Separation

The cation transport behavior of thermally treated reduced graphene oxide membranes (GOMs) is reported. The GOMs were reduced by heat treatment at 25, 80, and 120 °C and then characterized by Fourier transform infrared spectroscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy to determine oxygen group content, C/O ratio, and layer spacing. The permeation rates of various cations with different sizes and charge numbers through these membranes were measured to understand the effect of the cations on transport behavior. The results indicated that the cation transport through the membranes depended on the layer spacing of the membrane and ion size and charge. Cations of the same valence permeating through the same GOM could be differentiated by their hydration radius, whereas the same type of cation passing through different GOMs could be determined by the spacing of the GOM layers. The cation valence strongly affected permeation behavior. The GOM that was prepared at 120 °C exhibited a narrow layer spacing and high separation factors for Mg/Ca, Mg/Sr, K/Ca, and K/Fe. The cations moving through the membrane could insert into the membrane lamellas, which neutralized the negative charge of the membrane, enlarged the layer spacing of the GOMs, and affected cation permeation.

Multifunctional negatively-charged poly (ether sulfone) nanofibrous membrane for water remediation

Multifunctional materials, which can effectively and simultaneously remove various water-soluble contaminants like dyes and heavy metal ions, and separate oil from water, are urgent to meet increasing challenges on wastewater remediation. Herein, a cross-linked poly (acrylic acid) (PAA) modified poly (ether sulfone) nanofibrous membrane (NFM) was fabricated by a facile in-situ pre-reaction followed by electrospinning. The as-prepared NFM showed excellent hydrophilicity and underwater lipophobicity, therefore expressed excellent water permeability with high water flux (about 5142 L m² h^-1). As a result, under solely driven by gravity, the NFM was capable to separate emulsified oil/water emulsion and a wide range of oil/water mixtures. Moreover, repeating separation tests indicated that the NFM had great long-term sustainability even after ten separation cycles. In addition, due to the introduction of PAA and the large surface-to-volume ratio, the NFM also expressed rapid adsorption capacity for cationic dyes as well as heavy metal ions; thus could simultaneously remove these contaminants during the oil/water separation process. Furthermore, the NFM could be also decorated by Ag NPs to endow the membranes with remarkable antibacterial ability against both E. coli and S. aureus. Our findings strongly suggested that the multifunctional NFM may have great potential in treating complicated wastewater.

Separation and purification using GO and r-GO membranes

Many materials with varied characteristics have been used for water purification and separation applications. Recently discovered graphene oxide (GO), a two-dimensional derivative of graphene has been considered as a promising membrane material for water purification due to its excellent hydrophilicity, high water permeability, and excellent ionic/molecular separation properties. This review is focussed on the possible versatile applicability of GO membranes. It is also known that selective reduction of GO results in membranes with a pore size of ∼0.35 nm, ideally suited for desalination applications. This article presents the applicability of graphene-based membranes for multiple separation applications. This is indeed the first review article outlining a comparison of GO and r-GO membranes and discussing the suitability for applications based on the porosity of the membranes.

This review article outlines a comparison of GO and r-GO membranes for separation and purification applications.