One-step facile fabrication of PVDF/graphene composite nanofibrous membrane with enhanced oil affinity for highly efficient gravity-driven emulsified oil/water separation and selective oil absorption

High-Performance Membranes Based on Spherical-Beaded Nanofibers and Nanoarchitectured Networks for Water-in-Oil Emulsion Separation

High-performance separation materials for oil-water emulsions are crucial to environmental protection and resource recovery; however, most existing fibrous separation materials are subject to large pore size and low porosity, resulting in limited separation performance. Herein, we create high-performance membranes consisting of spherical-beaded nanofibers and nanoarchitectured networks (nano-nets) using electrostatic spinning/netting technology, for water-in-oil emulsion separation. By manipulating the nonequilibrium stretching of jets, spherical-beaded nanofibers capable of generating a robust microelectric field are fabricated as scaffolds, on which charged droplets are induced to eject and phase separate to self-assemble nano-nets with small pores. Benefiting from 3D undulating networks with cavities originating from 2D nano-nets supported by 1D spherical-beaded nanofibers, the membranes exhibit under-oil superhydrophobicity (>152°), a striking separation performance with an efficiency of >99.2% and a flux of 5775 L m-2 h-1, together with wide pressure applicability, antifouling, and reusability. This work may open up new horizons in developing fibrous materials for separation and purification.

Highly Efficient Photocatalytic Degradation of Organic Pollutants Using a Polyvinylidene Fluoride/Polyvinylpyrrolidone-Cuprous Oxide Composite Membrane

Enhancing the efficiency of photocatalysts in the removal of organic pollutants is of vital importance in wastewater treatment. In this work, a set of composite membranes that can be used for efficient removal of the organic dyes, such as methyl orange (MO), methylene blue (MB), and Congo red (CR), were prepared through coblending/electrospinning techniques using polyvinylidene fluoride (PVDF) as the substrate, polyvinylpyrrolidone (PVP) as the dispersing agent and wettability regulator, and cuprous oxide (Cu2O) as the photocatalyst. The results showed that Cu2O particles were well encapsulated in the electrospun PVDF/PVP fibers, and the composite membranes exhibited apparently enhanced hydrophilicity. Furthermore, compared with the pure Cu2O particles, the composite membranes not only showed a higher photocatalytic degradation ratio for MO (93.6%) but also showed a much higher degradation rate (62.4 mg/(mg·h)) in comparison with the other reported Cu2O-based composite photocatalytic materials in the literature. In addition, the membrane sample also had excellent recycling stability, and the retention rate of its removal ability maintained 92.1% after 5 times of recycling. Furthermore, the composite membranes also showed high removal ability toward MB and CR, with photocatalytic degradation ratios of 81.4 and 76.1%, respectively. This work indicates that the prepared PVDF/PVP-Cu2O composite membranes possess promising application prospects in wastewater treatment.

Phase inverted hydrophobic polyethersulfone/iron oxide-oleylamine ultrafiltration membranes for efficient water-in-oil emulsion separation

Exploration and transportation of oil offshore can result in oil spills that cause a wide range of adverse environmental consequences and destroy aquatic life. Membrane technology outperformed the conventional procedures for oil emulsion separation due to its improved performance, reduced cost, removal capacity, and greater eco-friendly. In this study, a hydrophobic iron oxide-oleylamine (Fe-Ol) nanohybrid was synthesized and incorporated into polyethersulfone (PES) to prepare novel PES/Fe-Ol hydrophobic ultrafiltration (UF) mixed matrix membranes (MMMs). Several characterization techniques were performed to characterize the synthesized nanohybrid and fabricated membranes, including scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), contact angle, and zeta potential. The membranes' performance was assessed using a surfactant-stabilized (SS) water-in-hexane emulsion as a feed and a dead-end vacuum filtration setup. The incorporation of the nanohybrid enhanced the hydrophobicity, porosity, and thermal stability of the composite membranes. At 1.5 wt% Fe-Ol nanohybrid, the modified PES/Fe-Ol MMM membranes reported high water rejection efficiency of 97.4% and 1020.4 LMH filtrate flux. The re-usability and antifouling properties of the membrane were examined over five filtration cycles, demonstrating its great potential for use in water-in-oil separation.

Facile, green and scalable preparation of low-cost PET-PVDF felts for oil absorption and oil/water separation

3D felt materials with pore structures have the advantages of high absorption performance and recyclability in oily wastewater treatment and chemical leakage. However, most of them were fabricated using either toxic organic solvents or complicated procedures. Herein, we report a facile, green, and scalable route for the fabrication of 3D composite felts with large pore structures by sequentially stirring and heating polyethylene terephthalate (PET) fibers and polyvinylidene fluoride (PVDF). The resulting PET-PVDF felt exhibits high oil absorption capacity to a variety of oil and organic solvents with a maximum saturated absorption capacity of 32 g/g. Additionally, it can be used to separate oil/water mixtures with a separation efficiency of 99.9% and separation flux of 89570 L m^-2 h^-1. Moreover, this felt shows excellent mechanical durability and chemical stability under acid, base, salt solution, and other harsh environments. The current study provides a promising approach for large-scale industrial oily wastewater separation.

Magnetic, self-heating and superhydrophobic sponge for solar-driven high-viscosity oil-water separation

In this work, a novel oil-adsorption sponge with superhydrophobicity was fabricated using polymer-assisted electroless deposition and dip-coating techniques for depositing a rough polydopamine layer, magnetic particles, and low surface energy polydimethylsiloxane onto the surface of a sponge skeleton. The as-prepared superhydrophobic sponge (WCA > 150° and SA < 5°) exhibited rapid adsorption behavior, large adsorption capacity (up to 50.6 times its own dry weight or above 90% of its own volume), excellent durability (above 80% of the adsorption capacity after 80 recycles), and a self-cleaning property owing to sufficient open-cell pores and superelasticity provided by the melamine-formaldehyde host as well as the hierarchical roughness and convenient magnetic recovery enabled by the polymer-assisted electroless deposition approach. The pump-, gravity-, and solar-driven oil-water separation devices based on the fabricated cubic composites were also demonstrated, particularly the separation of high-viscosity oil-water mixtures via the solar-driven mode, demonstrating the broad prospects of such modified sponges in actual applications. This study provides a new avenue for rationally designing novel oil adsorption and separation materials.

Sophorolipid modification enables high reactivity and electron selectivity of nanoscale zerovalent iron toward hexavalent chromium

Nanoscale zero-valent iron is considered to be a promising nanostructure for environmental remediation, while increasing the electron selectivity of nanoscale zerovalent iron (nZVI) during target contaminant removal is still a challenge (electron selectivity, defined as the percentage of electrons transferred to the target contaminants over the number of electrons donated by nZVI). In this study, the strategy for increasing the reactivity and electron selectivity of nZVI via sophorolipid (SL-nZVI) modification was proposed. The results showed that the removal efficiency and electron selectivity of SL-nZVI toward Cr(VI) was 99.99% and 56.30%, which was higher than that of nZVI (61.16%, 25.91%). Meanwhile, the particles were well characterized and the mechanism for enhanced reactivity and electron selectivity was investigated. Specially, both the morphology and BET specific surface area characterization suggested that stability against aggregation was higher in SL-nZVI nanoparticles than in nZVI. Besides, X-ray photoelectron spectroscopy (XPS), Tafel polarization curves, and Electrochemical impedance spectroscopy also indicated that the introduction of sophorolipid successfully prevent the nanoparticles from oxidation and benefit the electron transferring. In addition, a water contact angle test revealed that SL-nZVI nanoparticles were less hydrophilic (contact angle = 34.8°) than nZVI (contact angle = 23.9°). Therefore, in terms of reactivity, sophorolipid modification inhibited the aggregation of the nanoparticles and enhanced the electrical conductivity. For electron selectivity, the introduction of sophorolipid not only benefited Cr(VI) adsorption and the electron transfer from Fe⁰ to the surface-adsorbed Cr(VI) that followed but also reduced the possibility of side reactions between Fe⁰ and H₂O. This study demonstrates that the introduction of sophorolipid is an effective strategy for developing a highly efficient nZVI-based nanocomposite system and highlights the potential role of sophorolipid in improving the electron selectivity of nZVI.

Hemin-Modified Multi-Walled Carbon Nanotube-Incorporated PVDF Membranes: Computational and Experimental Studies on Oil-Water Emulsion Separations

The separation of oil/water emulsions has attracted considerable attention for decades due to the negative environmental impacts brought by wastewater. Among the various membranes investigated for separation, polyvinylidene fluoride (PVDF) membranes have shown significant advantages of ease of fabrication, high selectivity, and fair pore distribution. However, PVDF membranes are hydrophobic and suffer from severe fouling resulting in substantial flux decline. Meanwhile, the incorporation of wettable substrates during fabrication has significantly impacted the membrane performance by lowering the fouling propensity. Herein, we report the fabrication of an iron-containing porphyrin (hemin)-modified multi-walled carbon nanotube incorporated PVDF membrane (HA-MWCNT) to enhance fouling resistance and the effective separation of oil-in-water emulsions. The fabricated membrane was thoroughly investigated using the FTIR, SEM, EDX, AFM, and contact angle (CA) analysis. The HA-MWCNT membrane exhibited a water CA of 62° ± 0.5 and excellent pure water permeance of 300.5 L/m2h at 3.0 bar (400% increment), in contrast to the pristine PVDF, which recorded a CA of 82° ± 0.8 and water permeance of 59.9 L/m2h. The hydrophilic HA-MWCNT membrane further showed an excellent oil rejection of >99% in the transmembrane pressure range of 0.5−2.5 bar and a superb flux recovery ratio (FRR) of 82%. Meanwhile, the classical molecular dynamics (MD) simulations revealed that the HA-MWCNT membrane had greater solvent-accessible pores, which enhanced water permeance while blocking the hydrocarbons. The incorporation of the hemin-modified MWCNT is thus an excellent strategy and could be adopted in the design of advanced membranes for oil/water separation.

Simple Fabrication of Polyvinylidene Fluoride/Graphene Composite Membrane with Good Lipophilicity for Oil Treatment

Graphene (GE) is an emerging type of two-dimensional functional nanoparticle with a tunable passageway for oil molecules. Herein, polyvinylidene fluoride (PVDF)/GE composite membranes with controllable pore structure were fabricated with a simple non-solvent-induced phase separation method. The change of crystallinity and crystal structure (α, β, γ, etc.) generated is due to the addition of GE, which benefits the design of a suitable pore structure for oil channels. Meanwhile, the hydrophobicity and thermal stability of the composite membrane were obviously enhanced. With 3 wt % GE, the contact angle was 124.6°, which was increased greatly compared to that of the GE-0 sample. Moreover, the rate of the phase transition process was affected by the concentration of casting solution, temperature, and composition of the coagulation bath. For example, the composite membrane showed better oil-water separation properties when the coagulation bath was dioctyl phthalate. In particular, the oil flux and separation efficiencies were up to 2484.08 L/m²·h and 99.24%, respectively. Consequently, PVDF/GE composite membranes with excellent lipophilicity may have good prospects for oily wastewater treatment.

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.

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%).

Gravity-Driven Separation of Oil/Water Mixture by Porous Ceramic Membranes with Desired Surface Wettability

Porous Al₂O₃ membranes were prepared through a phase-inversion tape casting/sintering method. The alumina membranes were embedded with finger-like pores perpendicular to the membrane surface. Bare alumina membranes are naturally hydrophilic and underwater oleophobic, while fluoroalkylsilane (FAS)-grafted membranes are hydrophobic and oleophilic. The coupling of FAS molecules on alumina surfaces was confirmed by Thermogravimetric Analysis and X-ray Photoelectron Spectroscopy measurements. The hydrophobic membranes exhibited desired thermal stability and were super durable when exposed to air. Both membranes can be used for gravity-driven oil/water separation, which is highly cost-effective. The as-calculated separation efficiency (R) was above 99% for the FAS-grafted alumina membrane. Due to the excellent oil/water separation performance and good chemical stability, the porous ceramic membranes display potential for practical applications.