Engineering highly effective nanofibrous membranes to demulsify surfactant-stabilized oil-in-water emulsions

Design of carboxymethyl cellulose/alginate aerogels with anti-fouling and light-driven self-cleaning for enhanced oily wastewater remediation

With the increase of oily wastewater discharge and the growing demand for clean water supply, high throughput green materials for oil-water separation with anti-pollution and self-cleaning ability are urgently needed. Herein, the polysaccharide-based composite aerogels of CMC/SA@TiO2-MWCNTs (CSTM) with fast photo-driven self-cleaning ability have been prepared by a simple freeze-drying and ionic cross-linking strategy. The introduction of TiO2 /MWCNTs nanocomposites effectively improves the underwater oleophobic and mechanical properties of polysaccharide aerogels and enables their photo-driven self-cleaning ability for efficient oil-water separation and purification of complex oily wastewater. For immiscible oil-water mixtures, a high separation flux of about 7650 L m-2 h-1 and a separation efficiency of up to 99.9 % was obtained. For surfactant-stabilized oil-in-water emulsion, a flux of 3952 L m-2 h-1 was achieved with a separation efficiency of up to 99.3 %. More importantly, the excellent photoluminescent self-cleaning ability and low oil adhesion contribute to the high contamination resistance, excellent reusability, and robust durability of CSTM aerogel. With the advantages of simple preparation, remarkable performance, and recyclability, this aerogel is expected to provide a green, economical, and scalable solution for the purification of oily wastewater.

Zwitterionic Tröger's Base Microfiltration Membrane Prepared via Vapor-Induced Phase Separation with Improved Demulsification and Antifouling Performance

The fouling of separation membranes has consistently been a primary factor contributing to the decline in membrane performance. Enhancing the surface hydrophilicity of the membrane proves to be an effective strategy in mitigating membrane fouling in water treatment processes. Zwitterionic polymers (containing an equimolar number of homogeneously distributed anionic and cationic groups on the polymer chains) have been used extensively as one of the best antifouling materials for surface modification. The conventional application of zwitterionic compounds as surface modifiers is intricate and inefficient, adding complexity and length to the membrane preparation process, particularly on an industrial scale. To overcome these limitations, zwitterionic polymer, directly used as a main material, is an effective method. In this work, a novel zwitterionic polymer (TB)-zwitterionic Tröger's base (ZTB)-was synthesized by quaternizing Tröger's base (TB) with 1,3-propane sultone. The obtained ZTB is blended with TB to fabricate microfiltration (MF) membranes via the vapor-induced phase separation (VIPS) process, offering a strategic solution for separating emulsified oily wastewater. Atomic force microscopy (AFM), scanning electron microscopy (SEM), water contact angle, and zeta potential measurements were employed to characterize the surface of ZTB/TB blended membranes, assessing surface morphology, charge, and hydrophilic/hydrophobic properties. The impact of varying ZTB levels on membrane surface morphology, hydrophilicity, water flux, and rejection were investigated. The results showed that an increase in ZTB content improved hydrophilicity and surface roughness, consequently enhancing water permeability. Due to the attraction of water vapor, the enrichment of zwitterionic segments was enriched, and a stable hydration layer was formed on the membrane surface. The hydration layer formed by zwitterions endowed the membrane with good antifouling properties. The proposed mechanism elucidates the membrane's proficiency in demulsification and the reduction in irreversible fouling through the synergistic regulation of surface charge and hydrophilicity, facilitated by electrostatic repulsion and the formation of a hydration layer. The ZTB/TB blended membranes demonstrated superior efficiency in oil-water separation, achieving a maximum flux of 1897.63 LMH bar-1 and an oil rejection rate as high as 99% in the oil-water emulsion separation process. This study reveals the migration behavior of the zwitterionic polymer in the membrane during the VIPS process. It enhances our comprehension of the antifouling mechanism of zwitterionic membranes and provides guidance for designing novel materials for antifouling membranes.

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.

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.

Wettability Improvement in Oil-Water Separation by Nano-Pillar ZnO Texturing

The nanostructure-based surface texturing can be used to improve the materials wettability. Regarding oil−water separation, designing a surface with special wettability is as an important approach to improve the separation efficiency. Herein, a ZnO nanostructure was prepared by a two-step process for sol−gel process and crystal growth from the liquid phase to achieve both a superhydrophobicity in oil and a superoleophobic property in water. It is found that the filter material with nanostructures presented an excellent wettability. ZnO-coated stainless-steel metal fiber felt had a static underwater oil contact angle of 151.4° ± 0.8° and an underoil water contact angle of 152.7° ± 0.6°. Furthermore, to achieve water/oil separation, the emulsified impurities in both water-in-oil and oil-in-water emulsion were effectively intercepted. Our filter materials with a small pore (~5 μm diameter) could separate diverse water-in-oil and oil-in-water emulsions with a high efficiency (>98%). Finally, the efficacy of filtering quantity on separation performance was also investigated. Our preliminary results showed that the filtration flux decreased with the collection of emulsified impurities. However, the filtration flux could restore after cleaning and drying, suggesting the recyclable nature of our method. Our nanostructured filter material is a promising candidate for both water-in-oil and oil-in-water separation in industry.

Recent Progress in Microfiltration/Ultrafiltration Membranes for Separation of Oil and Water Emulsions

Oily wastewater has become one of the leading causes of environmental pollution. A massive quantity of oily wastewater is released from industries, oil spills, and routine activities, endangering the ecosystem's sustainability. Due to the enormous negative impact, researchers put strenuous efforts into developing a sustainable solution to treat oily wastewater. Microfiltration/ultrafiltration membranes are considered an efficient solution to treat oily wastewater due to their low cost, small footprint, facile operation, and high separation efficiencies. However, membranes severely fouled during the separation process due to oil's adsorption and cake layer formation, which shortens the membranes' life. This review has critically discussed the microfiltration/ultrafiltration membrane synthesizing methods and their emulsion's separation performance. In the end, key challenges and their possible solutions are highlighted to provide future direction to synthesize next-generation membranes.

Experimental Study on the Synergistic Effect between Evaporation Weathering and Emulsification of Oil Spills

First, a range of experiments using a stainless steel shallow plate in a thermostatic oscillator were carried out to simulate evaporation weathering of oil spills under different temperatures, wind velocities, oil film thicknesses, and wave conditions. The information on influencing factors of evaporation weathering could be obtained. Then, evaporation experiments of four oil samples with three emulsification states were conducted, and the effect of emulsification on evaporation of oil spills was investigated. The characteristics of each operation were described, mass loss characteristics of oil spills with time were plotted, and the effects of evaporative weathering processes under different conditions were quantitatively compared. A high-precision visualization system was utilized to simulate experimentally oil spill emulsification processes, and the effect of evaporative weathering on emulsification weathering of oil spills was investigated. The results of evaporation experiments showed that the increase of temperature could promote the evaporative weathering. The thin film thickness was beneficial to evaporation of oil spills. The increment of the wind speed could promote evaporation behavior when the wind velocity was small, but the increase of velocity had little effect on oil evaporation when the wind speed was large. Wave conditions had little effect on oil evaporation under the conditions of this experiment. The effect of different emulsification states on oil evaporation was not consistent. Unstable or semistable water-in-oil emulsions inhibited oil evaporation at the initial stage of evaporation, but water evaporation would increase oil-phase evaporation with the destruction of the emulsion structure. Stable water-in-oil emulsions inhibited evaporation weathering. The evaporation weathering of oil was conducive to the emulsification of oil.

A facile and green route to fabricate fiber-reinforced membrane for removing oil from water and extracting water under slick oil

Except the good separation performance, the membranes used for oil-water mixture separation should be fabricated with as little wastewater produced as possible. Thus, we proposed a green tactic--water vapor induced phase inversion to prepare the high-strength and superhydrophilic/underwater superoleophobic nonwoven fabric-based cotton/PA6/PAN membranes which is based on the polymer/solvent/nonsolvent ternary system analysis. Differing from adding additives in polymer solution or coagulation bath, above proposed strategy has an "subtractive effect" with the advantages of constructing three-dimensional porous structure and greatly reducing the organic wastewater produced during preparation process. Moreover, the obtained cotton/PA6/PAN membranes exhibited unexpected performances for separating oil-in-water emulsions. An ultrahigh permeation flux of up to 478,000 L m^-2 h^-1 bar^-1 with a separation efficiency of > 99.9% was obtained under the driving pressure of 1.6 KPa, which was one order of magnitude higher than the conventional separation membranes with similar properties. In addition, it is surprising that the cotton/PA6/PAN membranes can also extract water from the slick oil/water immiscible mixture. Therefore, it is expected that the cotton/PA6/PAN membranes can be used in practical oily wastewater purification.

Extraordinary Superhydrophobic Polycaprolactone-Based Composite Membrane with an Alternated Micro-Nano Hierarchical Structure as an Eco-friendly Oil/Water Separator

Extraordinary superhydrophobic polycaprolactone (PCL) composite membranes with an alternated hierarchical micro-nano structure were designed by addition of SiO₂ aerogel. The highest water contact angle (WCA) of 166.8 ± 1.5° was obtained when SiO₂ aerogel content was 0.5% (PCL/SiO₂-a0.5) in the PCL composite membrane, which was higher than other reported polymer-based membranes. SiO₂ aerogel lowered PCL composite membrane's surface energy. The triple curvature structure composed of microspheres, nanospheres, and nanofibers produced on PCL/SiO₂-a0.5 membranes endowed the excellent roughness of the surface. Also, the inner structure of the PCL/SiO₂-a0.5 composite membrane composed of micro-nano spheres, nanofibers, and microfibers increased the porosity of the separation membrane, which would provide more adsorption space. The PCL/SiO₂-a0.5 composite membrane as a separator for surfactant-stabilized emulsions of water-in-oil showed ultrahigh separation flux and efficiency. Meanwhile, the PCL/SiO₂-a0.5 composite membrane had an outstanding chemical resistance, self-cleaning ability, and good reusability. The composite membranes reported in this work as eco-friendly separation materials possessed all these characters in oil/water separation. This research proposed a very simple method to design eco-friendly high-efficiency separators through the construction of the alternated micro-nano hierarchical structure.

Simulation Study on the Dynamic Behaviors of Water-in-Oil Emulsified Droplets on Coalescing Fibers

Although increasing superwetting membranes have been developed for separating oil-water emulsions based on the "size-sieving" mechanism, their pores are easily blocked and fouled by the intercepted emulsified droplets, which would result in a severe membrane fouling issue and a sharp decline in flux. Instead of droplet interception, a fiber-based coalescer separates oil/water emulsions by inducing the emulsified droplets to coalesce and transform into layered oil/water mixtures, exhibiting an ability to work continuously for a long time with high throughput, which makes it a promising technology for emulsion treatment. However, the underlying mechanism of the separation process is not well understood, which makes it difficult to further improve the separation performance. Hence, in this work, the dynamic behaviors of water-in-oil emulsified droplets on the surface of the coalescing fiber were numerically investigated based on the phase-field model. The attachment, transport, and detachment behaviors of droplets on fibers were directly observed, and the effects of fiber wettability, orientation, arrangement, and fluid speed were studied in detail. First, it was observed that the droplets will move downstream along the fiber surface under the effect of fluid shear, and the large droplets tend to coalesce with their downstream small droplets on the same fiber surface because they move faster compared to the small droplets. Second, it was found that the emulsified droplet will spontaneously transport to the intersection of two angled fibers under the drive of asymmetric Laplace pressure, which demonstrated that the emulsified droplets tend to gather at the intersection of fibers when permeating through a coalescing medium. Third, it was found that the detachment behaviors of droplets from the fiber surface are strongly affected by their size, fiber wettability, and fluid velocity. In addition, the results of our simulation show that the backside of two closely attached fibers can further inhibit the detachment of droplets. We truly believe that our research results are of significance to optimize the parameters of a fiber-based coalescer for separating oil-water emulsions and to develop novel oil/water separators.