Confined Channels Induced Coalescence Demulsification and Slippery Interfaces Constructed Fouling Resist-Release for Long-Lasting Oil/Water Separation

Biomimetic materials for efficient emulsion separation: Based on the perspective of energy

Purifying emulsified oily wastewater is particularly crucial for solving environmental pollution and water scarcity. Membrane separation shows great potential for emulsified wastewater treatment. However, realizing continued effective emulsion separation remains a significant challenge. Fortunately, various kinds of creative schemes have been proposed to overcome the current dilemma. In this paper, biomimetic emulsion separation materials with unique wettability are introduced. Besides, This article summarizes the recently advanced emulsion separation strategies. First, we analyze the typical wettability theory and explore the trade-off between separation flux and efficiency. After that, based on emulsion types, the current common emulsion separation materials are summarized and analyzed. Notably, the integration of natural biological inspiration has made separation materials full of potential. Further, from the perspective of external energy input or no-external energy input, this article provides an overview of advanced emulsion separation materials and analyzes the potential separation mechanism. Encouragingly, efficient emulsion separation can be realized by membrane characteristics (microstructure, superwettability, electrostatic interaction) or the appropriate external stimulus (photo, electricity, magnetic). Finally, the challenges and trends are summarized. We hope that this article will provide inspiration for the advancement of novel generations of separation materials.

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.

Eco-friendly, highly interpenetrated and slightly swollen pHEMA hydrogel foam for durable underwater superoleophobicity and emulsion separation

Despite the emergence of hydrogels as ideal candidates for preparing the superhydrophilic materials for emulsion separation, their structural stability and swelling still hinder their long-term use, mainly due to structure defects after swelling. Herein, differing from the common modification, the eco-friendly poly 2-hydroxyethyl methacrylate (pHEMA) hydrogel foam was designed and synthesized via a one-step strategy by using the high internal phase emulsion (HIPE) template method, which endowed it with a highly interpenetrated porous structure. Unlike the normal swellable hydrogels such as poly(N-isoproplyacrylamide) (PNIPAM) hydrogel, or modified hydrogel coatings, the pHEMA hydrogel foam displayed stable structure and underwater superoleophobicity after 20 d of immersion in water. The pHEMA hydrogel foam could separate different kinds of highly surfactant-stabilized oil-in-water (O/W) emulsions with a high separation efficiency of 99.3% for liquid paraffin emulsion obtained solely under gravity-driven. Additionally, it exhibited excellent antifouling performance and long-term acid/alkali tolerance over 100 h without decrease in emulsion separation efficiency (98.0%, oil/water ratio of 99:1) and permeation flux (over 2000 L·m-2·h-1) attributed to its stable bulky structure. Moreover, the pHEMA hydrogel foam demonstrated high cell viability of 96.87% and 95.96% after culturing the 3T3 clone A31 cells in the pHEMA hydrogel foam for 24 h and 48 h, respectively, indicating good biocompatibility. Hence, our work provides a new design to develop an eco-friendly bulk hydrogel foam that achieves stable structure and performance for emulsion separation.

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.

Sequential Demulsification through the Hydrophobic-Hydrophilic-Hydrophobic Filtration Layer toward High-Performing Oil Recovery

Demulsification using membranes is a promising method to coalesce highly stable emulsified oil droplets for oil recovery. Nevertheless, a structure of the current filtration medium that is not efficient for oil droplet coalescence impedes rapid permeability, thereby inevitably restricting their practical applications. Herein, we report a hydrophobic-hydrophilic-hydrophobic (3H) demulsification medium that exhibits a benchmark permeability of ∼2.1 × 104 L m-2 h-1 with a demulsification efficiency of >98.0%. Remarkably, this 3H demulsification medium maintains over 90% demulsification efficiency in the oil-in-water (O/W) emulsions with a wide range of surfactant concentrations, which shows excellent applicability. Based on the combined results of quasi situ microscope images and molecular dynamics simulations, we show that the polydimethylsiloxane-modified hydrophobic layer facilitates the capture and coalescence of oil droplets, the hydrophilic inner layer assists in squeezing the coalescence of enlarged droplets, and the third hydrophobic layer accelerates the discharge of demulsified oil to sustain permeability. The sequential demulsification mechanism between this 3H filtration layer provides a general guide for designing a demulsifying membrane with high demulsification efficiency and high flux toward oil recovery.

Water-oil dual-channels enabled exceptional anti-fouling performances for separation of emulsified oil pollutant

Oil contamination has been an increasingly concerned environmental issue due to the large quantity of oily wastewater discharged by the industry. The extreme wettability-enabled single-channel separation strategy guarantees efficient separation of oil pollutant from wastewater. However, the ultra-high selective permeability forces the intercepted oil pollutant to form a blocking layer, which weakens the separation capability and slows the kinetics of permeable phase. As a consequence, the single-channel separation strategy fails to maintain a stable flux for a long-term separation process. Herein, we reported a brand-new water-oil dual-channels strategy for accomplishing an ultra-stable long-term separation of emulsified oil pollutant from oil-in-water nano-emulsion by engineering two drastically opposite extreme wettabilities (i.e. superhydrophilicity and superhydrophobicity) to build the water-oil dual-channels. The strategy established the superwetting transport channels to permit water and oil pollutant to permeate through their own channel. In this way, the generation of intercepted oil pollutant was prevented, which guaranteed an exceptional long-lasting (20 h) anti-fouling performance for successful achievement of an ultra-stable separation of oil contamination from oil-in-water nano-emulsion with high flux retention and high separation efficiency. Therefore, our investigations provided a new route for realizing ultra-stable long-term separation of emulsified oil pollutant from wastewater.

Rigid, eco-friendly and superhydrophobic SiO2-Polyvinyl alcohol composite sponge for durable oil remediation

Development of durable and eco-friendly adsorbents for oil remediation is in great demands. However, most of adsorbents were designed to pursue large capabilities while ignored their strength after adsorbing oil, which might cause secondary oil spilling during complex salvage process. Herein, an eco-friendly and superhydrophobic SiO₂-modified polyvinyl alcohol composite (H-SiO₂-G-PVA) sponge with extraordinary rigid structure after oil adsorption is designed for durable oil remediation. Through a two-step hydrolysis-condensation process including deposition of silica microparticles and introduction of hexadecyltrimethoxysilane (HDTMS), a superhydrophobic H-SiO₂-G-PVA sponge has been successfully constructed. The sponge presents stable superhydrophobicity in various complex environments，therefore it efficiently adsorbs oil from water (up to 6 g g^-1) and separate surfactant-stabilized water/oil emulsion with high efficiency (>99%). Noticeably, the H-SiO₂-G-PVA sponge maintains tough strength (3.5 MPa) after oil adsorption, which ideally overcomes secondary oil spilling problem and endows the sponge with excellent recycling performances (>20 cycles). Meanwhile, the excellent biocompatibility of the sponge (high cell viability of 91.85%) ensures the potential for practical applications. This rigid, eco-friendly oil-adsorbing sponge that achieves stable superhydrophobicity and recyclability, fulfills the application needs for durable oil remediation.

Universal Rapid Demulsification by Vacuum Suction Using Superamphiphilic and Underliquid Superamphiphobic Polyurethane/Diatomite Composites

A process for universal rapid demulsification by vacuum suction using an as-prepared superamphiphilic and underliquid superamphiphobic polyurethane (PU)/diatomite composite has been developed and is used to demulsify kerosene-in-water and water-in-kerosene emulsions with and without a surfactant. The results show that the demulsification rate of all the emulsions exceeds 98.5% in long-term operation, with a stable demulsification speed exceeding 0.303 L/m² min. When a superhydrophobic channel for separation is added, the oil/water separation efficiency exceeds 99.0%, and the final products are qualified oil and water. This attractive universal demulsification capability of PU/diatomite originates from its underliquid superamphiphobicity, which attracts a continuous phase to form a stable liquid film and thus repels dispersed phase droplets, which have a similar interaction with the surface but are much less abundant. The vacuum forces emulsion droplets into the microstructure of the PU/diatomite cake, where they are compressed, coalesce, and finally demulsified. This observed mechanism suggests a promising strategy to avoid the negative effects of oil fouling in demulsification and achieve large-scale universal continuous rapid demulsification.

Based On Confined Polymerization: In Situ Synthesis of PANI/PEEK Composite Film in One-Step

Confined polymerization is an effective method for precise synthesis, which can further control the micro-nano structure inside the composite material. Polyaniline (PANI)-based composites are usually prepared by blending and original growth methods. However, due to the strong rigidity and hydrogen bonding of PANI, the content of PANI composites is low and easy to agglomerate. Here, based on confined polymerization, it is reported that polyaniline /polyether ether ketone (PANI/PEEK) film with high PANI content is synthesized in situ by a one-step method. The micro-nano structure of the two polymers in the confined space is further explored and it is found that PANI grows in the free volume of the PEEK chain, making the arrangement of the PEEK chain more orderly. Under the best experimental conditions, the prepared 16 µm-PANI/PEEK film has a dielectric constant of 205.4 (dielectric loss 0.401), the 75 µm-PANI/PEEK film has a conductivity of 3.01×10-4 S m-1 . The prepared PANI/PEEK composite film can be further used as electronic packaging materials, conductive materials, and other fields, which has potential application prospects in anti-static, electromagnetic shielding materials, corrosion resistance, and other fields.