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

Exceptional anti-fouling, self-cleaning and high-flux ZIF-8@polyacrylonitrile based nanofiber composite membrane via in situ growth of seaweed-like ZnIn2S4 for efficient separation of emulsified oily wastewater

The membrane separation technique encounters the problems of low permeability and weak anti-fouling ability during the large-scale treatment of emulsified oily wastewater. To address this issue, the high-flux and photocatalytic self-cleaning polyacrylonitrile (PAN)-based nanofiber composite membrane (ZnIn2S4/ZIF-8@PAN) was constructed via the electrospinning of PAN@ZIF-8 nanofiber membrane with rivet-like structure and in situ growth of highly porous seaweed-like ZnIn2S4 in the hydrothermal process. The intrinsic hydrophilicity, porous and hierarchical structure of ZnIn2S4 effectively regulated the transport channel, superhydrophilic/underwater superoleophobic feature (WCA: ∼ 0 °, UOCA: up to 155.9 °), and ultra-low oil adhesion behavior of composite membrane, thus achieving superior separation flux of diverse surfactant-stabilized O/W emulsions (up to 5062.7 ± 189.4 L·m-2·h-1) and separation efficiency of 99.11 ± 0.18 %. As evidenced by the results of Hermia model and dynamic oil adhesion experiment, the ZnIn2S4/ZIF-8@PAN composite membrane displayed exceptional oil resistance and anti-fouling ability under harsh conditions, retaining its high separation efficiency (separation flux: 4408.1 L·m-2·h-1, rejection rate: 99.03 %) for O/W emulsions after 10 consecutive separation cycles. Furthermore, we discovered that the composite membrane offered favorable self-cleaning performance towards photocatalytic degradation of various organic dyes under exposure to visible light, with degradation efficiency up to 96.88 % within 120 minutes. The DFT calculation, EIS impedance, and free radical inhibition experiments demonstrated that the well-matched band structure and intimate contact interface between ZIF-8 and ZnIn2S4 facilitated the efficient transfer and separation of photo-induced charge carriers. Such PAN multifunctional composite membrane has great potential in the field of complex oily wastewater treatment.

Preparation of a vegetable-oil film-type emulsion dust suppressant and adsorption performance study on road dust surface in open-pit coal mines

Open-pit mine pavements have a low moisture content, and water is easily evaporated from the pavement surface. In this study, a dust suppressant was developed using an oil film-type emulsion with a high moisturizing effect to enhance the road dust surface moisture content and to reduce environmental hazards associated with the use of conventional heavy oil films. The characteristics and dust suppression properties of the emulsion were analyzed, and the emulsification mechanism and microscopic adsorption mechanism of the dust suppressant were studied through molecular dynamics simulations. The results showed that a mixture containing 0.23 % sodium dodecylbenzene sulfonate (SDBS) and 0.37 % sodium secondary alkyl sulfonate (SAS60), which are anionic with the same polar hydrophilic group, prepared at a mass fraction of 5:8 had a synergistic effect and produced the best emulsification effect, with the interfacial tension being less than 0.01 mN/m. In the emulsion-stable state, a significant prevention of water evaporation from the dust surface could be achieved. At room temperature (25 °C), the dust sample contained a high amount of water even after 70 h, and the contents of the hydrophilic functional groups in the dust molecules increased. Emulsified oil particles were adsorbed at the hydrophobic sites of the dust particles, transforming the hydrophobic sites into hydrophilic sites, thereby increasing the wettability. At the same time, the broken emulsion covered the oil film on the dust surface and inhibited the evaporation of water. Finally, the optimal spraying amount of the dust suppressant and effective dust suppression time under different compaction degrees of the road were determined. This study proposes a new research direction for effectively inhibiting the diffusion of road dust.

Development and Performance Study of Continuous Oil-Water Separation Device Based on Superhydrophobic/Oleophilic Mesh

Oil-water separation is an important method for treating oily wastewater and recovering oil resources. Based on the different affinities of superhydrophobic surfaces to water and oil, long-term oil-water separation devices with low-energy and high efficiency can be developed through the optimization of structure and process parameters. Superhydrophobic coatings were prepared on stainless-steel mesh surfaces using a spray method to construct single-channel oil-water separation equipment with superhydrophobic/oleophilic meshes, and the effects of structural and process parameters on separation efficiency were systematically investigated. Additionally, a multi-channel oil-water separation device was designed and fabricated to evaluate the feasibility and stability of long-term continuous operations. The optimized single V-shaped channel should be horizontally placed and made from 150-mesh stainless-steel mesh folded at an angle of 38.9°. For the oil-water mixtures containing 20 wt.% oil, the oil-water separation efficiencies for single and two-stage separation were 92.79% and 98.96%, respectively. After 36 h of continuous operation, the multi-channel separation device achieved single-stage and two-stage separation efficiencies of 94.60% and 98.76%, respectively. The maximum processing capacity of the multi-channel device reached 168 L/h. The modified stainless mesh can remain stable with a contact angle (CA) higher than 150° to water for 34 days. The average residence time and contact area during the oil-water separation process significantly affect separation efficiency. By optimizing oil-water separation structures and process parameters, and using a superhydrophobic spray modification method, separation efficiency can be improved while avoiding the generation of secondary pollutants.

Superhydrophilic membrane coupled with hydroxide ion-assisted bubbles for efficient separation of surfactant-stabilized oil/water emulsions

Developing a feasible and efficient membrane for the purification of surfactant-stabilized emulsions is urgently needed but impeded by the issues of membrane fouling and the inherent trade-off between separation efficiency and permeation flux. A superhydrophilic conductive membrane was developed by coating MXene/carbon nanotubes layer and polydopamine-hydrogel molecular layer, which as cathode integrates feasible hydroxide ion-assisted bubbles on its surface by electrolysis of water. These bubbles are more effective than conventional ones in removing surfactant-stabilized oil droplets because the hydroxide ions significantly promote the aggregation of oil droplets and bubbles by reducing their Debye length. In this way, the oil droplets even the small-sized ones assisted by these bubbles are quickly detached from the membrane surface, avoiding the oil accumulation and penetration, significantly mitigating the membrane fouling and trade-off challenges. Therefore, the membrane has outstanding separation efficiency (99.57 %), permeation flux (2065 L m-1 h-1 bar-1), antifouling ability and durability in surfactant-stabilized oil/water emulsion separation. Besides, this membrane coupled with hydroxide ion-assisted bubbles is easily manageable and eco-friendly, which provides a promising solution and valuable insights for efficient emulsion separation and wastewater remediation.

Wetting and Spreading Behaviors of Impacting Metal Droplet Regulated by 2D Ultrasonic Field

The wetting and spreading behaviors of metal droplets on solid substrates are critical aspects of additive manufacturing. However, the inherent characteristics of metal droplets, including high surface tension, elevated viscosity, and extreme temperatures, pose significant challenges for wetting and spreading on nonwetting substrates. Herein, this work proposes a strategy that employs a two-dimensional (2D) orthogonal ultrasonic field to construct a vibration deposition substrate with radial vibration amplitude gradient, thereby enhancing the wettability and adhesive strength of impacting metal droplets ejected by a piezoelectric micro-jet device. First, a 2D ultrasonic vibration device is designed based on the combination of longitudinal vibration modes. Additionally, oblique and circular vibration trajectories are synthesized. The vibration amplitude distributions and trajectories of the deposition substrate are verified utilizing the finite element method. Subsequently, the experimental results demonstrate that the contact angle is decreased by 24.7%, the spreading diameter is increased by 10.3%, and the adhesive strength is enhanced by 5.4 times compared to deposition on a static substrate. The 2D ultrasonic field facilitates the transition of metal droplets from a non-wetting state to a wetting state on the nonwetting substrate, which highlights the versatility and adaptability of ultrasonic strategy for expanding the applications of metal droplets.

Bioinspired interlaced wetting surfaces for continuous on-demand emulsion separation

Maintaining high separation performance during continuous emulsion separation remains a challenge. Herein, based on biomimetic coupling ideas, hole array interlaced wetting surfaces (HAIWSs) and mastoid array interlaced wetting surfaces (MAIWSs) were prepared by laser processing, electroless silver deposition, thiol modification, and spraying for on-demand emulsion separation. When the separation is going on, randomly moving emulsion droplets are prone to being captured by holes or mastoids due to interlaced wettability. Under this unique interface behavior, the occurrence of filter cake and pore clogging is reduced, thus achieving both high efficiency (∼99.5 and ∼99.3 %). Meanwhile, the high flux can also be maintained (∼3212 and ∼3458 L m-2 h-1). Significantly better than surfaces without pores or mastoid structures. Further, the as-prepared surfaces also exhibit excellent recyclability. After 50 separation cycles, optimized HAIWS and MAIWS still maintained high efficiency (∼96.2 and ∼95.8 %) and high flux (∼3042 and ∼3164 L m-2 h-1), exceeding other surfaces without hole or mastoid structure. Notably, complex physical/chemical cleaning processes are avoided. Besides, even in harsh conditions, HAIWS and MAIWS still maintain excellent stability. The above strategy provides a novel mechanism for effective on-demand emulsion separation and is expected to encourage the creation of new-class separation devices for oily wastewater treatment in industry.

Slippery hydrogel surface on PTFE hollow fiber membranes for sustainable emulsion separation

Establishing an efficient and sustainable membrane module is of great significance for practical oil/water emulsion separation. Superwetting membranes have been extensively studied but cannot meet long lasting separation owing to inevitable membrane fouling. Herein, we constructed a hydrogel-mediated slippery surface on polytetrafluoroethylene (PTFE) hollow fibers and then designed a flexible and swing hollow fiber membrane module inspired by fish gill respiration, which achieved sustainable emulsion separation. A vinyl silane-crosslinked polyvinylpyrrolidone (PVP) hydrogel was interpenetrated with nano-fibrils of the PTFE hollow fibers, thus facilitating fast water permeance while resisting oil intrusion. Liquid-like polydimethylsiloxane (PDMS) brushes were then grafted to promote oil aggregation-release from the membrane surface. Owing to the heterogeneous surface and gill-like structure, the designed PTFE hollow fiber membrane module could separate emulsion in a long-term filtration process, maintaining a high water permeability of 500 L m-2 h-1 bar-1 with a separation efficiency of over 99.9% for 5000 min. This novel technique shows its great potential to realize practical emulsion separation by solving the persistent problem of membrane fouling and permeance decay.

Durably Superhydrophobic Magnetic Cobalt Ferrites for Highly Efficient Oil-Water Separation and Fast Microplastic Removal

Microplastic pollution has become a primary global concern in the 21st century. Recyclable magnetic particles with micro-nanostructures are considered an efficient and economical way to remove microplastics from water. In this study, superhydrophobic magnetic cobalt ferrite particles were prepared by using a simple coprecipitation method combined with surface functionalization. The micromorphology, chemical composition, hysteresis loop, and surface contact angle of the functionalized cobalt ferrite were characterized. The separation efficiency and absorption capacity of cobalt ferrite particles in water-oil separation and microplastic removal were investigated. The results showed that the saturation magnetic field intensity of cobalt ferrite was 65.52 emu/g, the residual magnetization intensity (Mr) was 18.79 emu/g, and the low coercivity was 799.83 Oe. Cobalt ferrites had stable superhydrophobicity in the pH range of 1-13. The separation efficiency of cobalt ferrite powder for four oil-water mixture separations was higher than 94.2%. The separation efficiency was as high as 99.6% in the separation of the hexane and water mixtures. Due to the synergistic effect of the hydrophobic effect and van der Waals force, the functionalized magnetic cobalt ferrite had a high and stable microplastic removal efficiency and capture capacity. The removal efficiency of microplastics was close to 100%, and the capture capacity was 2.56 g/g. After ten microplastic removal cycles, the removal efficiency reached more than 98%, and the surface contact angle was still greater than 150°.

Bioinspired superwetting oil-water separation strategy: toward the era of openness

Bioinspired superwetting oil-water separation strategies have received significant attention for their potential in addressing global water scarcity and aquatic pollution challenges. Over the past two decades, the field has rapidly developed, reaching a pivotal phase of innovation in the oil-water separation process. However, many groundbreaking studies have not received extensive scientific recognition. In this review, we systematically examine the application of bioinspired superwetting materials for complex multiscale oil-water separation. We discuss the development of 2D membrane filtration and 3D sponge adsorption materials in confined spaces, summarizing the core separation mechanisms, key research findings, and the evolutionary logic of these materials. Additionally, we highlight emerging open-space separation strategies, emphasizing several novel dynamic separation devices of significant importance. We evaluate and compare the design concepts, separation principles, materials used, comprehensive performance, and existing challenges of these diverse strategies. Finally, we summarize these advantages, critical bottlenecks, and prospects of this field and propose potential solutions for real oil-water separation processes from a general perspective.

Synergistic Construction of Superhydrophilic PVDF Membranes by Dual Modification Strategies for Efficient Emulsion Separation

Solving the problem of oil and water pollution is an important topic in environmental protection. The separation of oil-water emulsion with high efficiency and low consumption has been the direction of social efforts. Membrane separation technology combined with surface wettability and pore size screening is considered to be one of the most promising ways to separate oil-water emulsions. In this paper, the polyvinylidene difluoride (PVDF) membrane is prepared by combining the two methods of blending and coating modification as a double barrier. The prepared PVDF membrane can completely wet water, achieve superhydrophilic in air, and superoleophobic underwater. The separation efficiency and flux are 99.57% and 678 L h-1 m-2 bar-1, respectively, for toluene emulsions containing surfactants with an average particle size of 1.7 µm. At the same time, it can also effectively separate different kinds of light/heavy oils. After three cycles of testing still maintain high efficiency of separation. The results show that the prepared PVDF membrane can effectively separate the emulsion containing surfactant with smaller particle size distribution of oil droplets. This method provides a new strategy for the separation of oil-water emulsions and has broad application prospects.

Unstable Coalescence Mechanism and Influencing Factors of Heterogeneous Oil Droplets

The use of a surfactant solution during oil and gas field development might improve the recovery rate of oil reservoirs. However, the serious emulsification of the produced liquid will bring challenges to the subsequent treatment process and storage and transportation. It is urgent to understand the coalescence mechanism of crude oil under the action of surfactant solution. This research investigates the coalescence mechanism of numerous oil droplets under liquid flow perturbation. The model was established to study the coalescence process of multiple oil droplets. The effects of the number of oil droplets under homogeneous conditions, the size of oil droplets, and the distance between oil droplets under non-homogeneous conditions on the coalescence process were analyzed. Meanwhile, the change rules of the completion time of oil droplet coalescence were drawn. The results show that the smaller the size of individual oil droplets under non-homogeneous conditions, the longer the coalescence completion time is, and when the size of individual oil droplets reaches the nanometer scale, the time for coalescence of oil droplets is dramatically prolonged. Compared to static circumstances, the time it takes for oil droplets to coalesce is somewhat shorter under gravity. In the fluid flow process, in the laminar flow zone, the coalescence time of oil droplets decreases with the increase of the liquid flow rate. However, in the turbulent flow zone, the coalescence time of oil droplets increases with the increase in the liquid flow rate. The coalescence time is in the range of 600~1000 ms in the flow rate of 0.05~0.2 m/s. In the presence of surfactants, the oil content in the emulsion system increases under the influence of pumping flow. The change in oil content rate with various surfactants is less impacted by flow rate, owing to the stable emulsion structure created by the extracted fluid within the reservoir. The study findings presented in this research provide technical assistance for effective crude oil storage and transportation.

Robust PVA/GO@MOF membrane with fast photothermal self-cleaning property for oily wastewater purification

The poor mechanical durability and weak fouling resistance of oil/water separation membranes severely restrict their applications in industry. Herein, a robust PVA/GO@MOF membrane with fast photothermal self-cleaning capability was developed through facile chemical crosslinking and suction-filtration strategies. Attributed to the powerful underwater superoleophobicity, the PVA/GO@MOF membrane exhibited extraordinary anti-oil adhesion even for high-viscosity crude oil and continuous crude oil emulsion purification capability with stable flux (1020 L m-2 h-1 bar-1) and exceptional efficiency (> 99.3%) even after 60 min. Most importantly, in comparison to reported photocatalytic self-cleaning oil/water separation membranes, the PVA/GO@MOF membrane can degrade organic contaminants more rapidly with a higher degradation rate (99.9%) in 50 min due to the superior photothermal conversion capacity. The synergistic photothermal and photocatalytic effects significantly enhanced photodegradation efficiency, which created opportunities for in-depth treatment of complex oily wastewater. Besides, the obtained membrane displayed excellent chemical and mechanical durability with underwater oil contact angle (UWOCA) above 150° even in harsh environments, such as corrosive solutions, UV irradiation, ultrasound treatment, abrasion experiment and bending test. Therefore, the developed PVA/GO@MOF membrane with robust durability and fast photocatalytic self-cleaning property is highly expected to purify oily wastewater and degrade organic pollutants.