State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, China
Superwetting membrane can significantly improve the flux of emulsion treatment and effectively alleviate serious membrane pollution problems due to its unique wetting properties of water and oil. Therefore, more and more attention has been paid to the field of emulsion wastewater treatment. In this paper, the design and preparation methods of superhydrophilic membrane, Janus membrane and superwetting membrane with functional sites are summarized, the separation effect is evaluated and the mechanism of action is explored. The membrane is designed and prepared mainly by constructing hydrophilic surface and rough structure. The cross-linked fiber membrane synthesized by electrospinning method has a good application prospect in emulsion separation because it can break through the limitation of membrane pore size caused by screening effect. At the same time, depending on the type of oil/water separation mode and the application of membranes, we induce the filtration demulsification mode of superhydrophilic membrane and the oil gathering demulsification mode of Janus membrane and their characteristic, the former has the characteristics of low operating pressure, high filtration flux and good anti pollution performance, while the latter has the characteristics of high purity oil gathering. The effect of membrane structure and surfactant concentration on emulsion separation efficiency is also analyzed. Then, the vertical and tangential migration and transformation of droplets in the process of emulsion treatment by superwetting membrane are summarized, and the mechanism of droplet penetrating through the membrane in the process of Janus membrane oil gathering and demulsification and its mechanical analysis are discussed. At present, this part of research is mainly based on the prediction of droplet morphology and qualitative analysis of mechanics, the empirical and quantitative mechanical analysis of relevant mechanisms still need to be further studied. On this basis, a new idea and direction for the subsequent treatment of emulsion wastewater is proposed.
Contents
1 Introduction
2 Design and preparation of superwetting membrane and its properties
2.1 Superhydrophilic membrane
2.2 Janus membrane
2.3 Superwetting membrane with functional sites
2.4 Characteristic analysis of superwetting membrane
3 Separation effect of superwetting membrane in emulsion demulsification
3.1 Oil-water separation model of superwetting membrane for emulsion wastewater
3.2 Analysis of main influencing factors in emulsion membrane demulsification separation process
4 the mechanism of superwetting membrane in The demulsification process
4.1 the interception and demulsification mechanism of The hydrophilic structure
4.2 the one-way penetration mechanism of The Janus structure
4.3 the mechanism between functional sites on the membrane surface and surfactants at the oil-water interface
The oil in oily wastewater can be divided into floating oil, dispersed oil and emulsified oil. Among them, emulsified oil will be dispersed in water in the form of micro-nano droplets wrapped by surfactants, forming an emulsion with stability for several years. Waste emulsion mainly comes from mechanical processing, crude Oil exploitation, food processing, cosmetics, shipping, leather and other industries. It is mainly Oil in Water (O/W) type, with the characteristics of complex Water composition, difficult treatment and strong stability. It is listed in the HW09 category of the National Hazardous Waste List[1]. In order to realize the efficient oil-water separation of waste emulsion, the key point and difficulty in the treatment process is to destroy the stable structure of surfactant[2⇓~4]. Commonly used technologies include chemical demulsification, evaporation and membrane separation[5⇓~7]. Among them, membrane separation has the advantages of good separation effect, low energy consumption and low dosage of reagents, which has attracted wide attention in the research of waste emulsion treatment[8⇓~10].
Traditional membrane separation of emulsion is based on the interception effect of membrane pores, which intercepts oil droplets and filters out water. Ultrafiltration membrane is often used to achieve micro-nano oil droplets, but there are problems of high filtration pressure and low filtration flux. Fouling of membrane by oil and surfactant also greatly limits the practical application of ultrafiltration membrane in emulsion separation[11,12]. Compared with traditional ultrafiltration membranes, ultra-wetting membrane materials have gradually emerged in the field of oil-water separation because of their unique wettability, strong oil-water selectivity and anti-fouling[13]. The common types include: super-hydrophilic membrane, Janus membrane and super-wetting membrane with functional sites[14][15][16]; At first, the treatment object was simple oil-water mixture and simple emulsion without surfactant. In recent years, the membrane separation research on stable emulsion with different surfactants has also begun to attract attention. The mechanism of demulsification process mainly includes the wetting of oil droplets by the hydrophilic structure of the membrane, the one-way drive of Janus structure and the role of functional sites.
In this paper, the design, preparation, separation effect and mechanism of superwetting membrane materials are reviewed, and the important research results in this field are summarized, and the development of superwetting membrane materials is prospected.
2 Design, Preparation and Analysis of Composition, Structure and Properties of Super-wetting Membrane Material
2.1 Super-hydrophilic film
When the Water Contact Angle (WCA) of the membrane is between 0 ° and 30 °, the membrane is super-hydrophilic. When WCA is between 150 ° and 180 °, it is superhydrophobic. The WCA of the super-hydrophilic membrane materials used in the treatment of emulsion wastewater is mostly 0 °, and the Underwater Oil Contact Angle (UOCA) is mostly above 130 °, and generally required to reach 150 ° and above.
The preparation methods of super-hydrophilic membranes can be generally divided into direct synthesis and modification, in which modification includes modification of hydrophilic substances and modification of micro-nano particles. There are as many as 20 ~ 30 kinds of chemical substances used to construct hydrophilic surfaces, such as Polyacrylonitrile, Polyacrylamide, Polyacrylic Acid, Polyvinyl Alcohol, Polydopamine, etc. These substances contain functional groups such as cyano (-CN), amino (—NH2), carboxyl (-COOH) or hydroxyl (-OH[17][14][18][19][20]. Among them, PDA, as a kind of biological glue, can be firmly combined with the surface of the membrane and the modification method is simple, so it is widely used.
Directly synthesizing the super-hydrophilic membrane, namely utilizing the hydrophilic substance to form the membrane by methods such as phase inversion, electrostatic spinning and the like[21][22]. Among them, electrostatic spinning refers to the method of adding high-voltage static electricity to polymer solution or melt to accelerate the spraying and stretching of charged polymer droplets under the action of electric field force. He et al. Directly prepared a hydrophilic PAN nanofiber membrane with a cross-linked network structure by electrospinning using only PAN as the raw material, with its WCA approaching 0 ° and UOCA reaching 151 °[23][24⇓~26]. Different from the traditional phase separation membrane forming method, the membrane prepared by the electrostatic spinning method contains interconnected aperture channels, has higher specific surface area and extremely high porosity, and the preparation process is flexible and controllable.Membrane materials with different thicknesses and pore sizes can be synthesized by adjusting working parameters (such as spinning time, working voltage, roller speed, etc.), which makes this method widely concerned in recent years[26].
In recent years, researchers have focused on the modification of hydrophilic substances on membranes because of the wide range of membrane materials and simple methods. The commonly used modification methods are dip-coating, cross-linked grafting, hydrothermal method and vapor deposition[27][28][14][29]. Among them, the cross-grafting method is the most widely used, which connects the modifiers with the functional groups on the membrane surface through stable chemical bonds, strengthens the structural stability, can resist the damage caused by chemical or mechanical erosion, and the prepared materials show good hydrophilic properties[25]. When the hydrophilic material is modified on the membrane, parameters such as the amount of the material, the reaction time and the like are regulated and controlled, and the membrane material with different hydrophilic gradients can be synthesized. Wang et al. soaked PVDF in a mixed solution containing different amounts of γ-glycidoxypropyltrimethoxysilane (KH560) to prepare a super-hydrophilic membrane. With the increase of KH560 content, the initial WCA on the membrane surface decreased from 47 ° to 0 °. Zhu et al. used PMAPS to graft polymerization on a hydrophobic PVDF membrane (initial WCA = 130 °) for 3 H, the WCA on the membrane surface decreased sharply to 50 °, and the WCA gradually decreased with the extension of polymerization time, reaching a minimum of 11 ° at 12 H[30][28]. Although the method of modifying hydrophilic functional materials on the membrane is more convenient, the pore size of the membrane prepared by this method is relatively fixed and lacks flexibility due to the limitation of the pore size of the basement membrane.
The super-hydrophilicity of the membrane can be enhanced by loading micro-nano particles or additives on the membrane to induce nucleation to construct a micron-nano rough structure. The common loading methods include SiO2, CaCO3, TiO2, etc. Deposition and dip-coating are the common loading methods. For example, Yang et al. Immersed a polypropylene/polydopamine/polyethyleneimine (PP/PDA/PEI) composite film into Tetramethoxysilane (TMOS), and deposited SiO2 nanoparticles on the film. The UOCA of the resulting composite film reached 164.9 ° ± 5.0 °, and the Slide Angle (SA) was 2.5 ° ± 1.4 °[20]. Chen et al. Soaked polypropylene/polyacrylic acid (PP/PAA) composite films alternately in Oil Contact Angle aqueous solution and Na2CO3 aqueous solution, respectively, to obtain CaCO3 nanoparticle coatings. The Oil Contact Angle (OCA) of the resulting composite films was greater than 150 °, and the SA was less than 5 °[18]. The additive method is usually salt-induced or alkali-induced to produce cluster spherical particles. Zhang et al. Added NaCl to the coagulation bath, and self-assembled polyacrylic acid grafted polyvinylidene fluoride (PAA-g-PVDF) micelles around it to form spherical particles through salt induction (Fig. 1A). When the NaCl concentration was 35%, WCA decreased from 88 ° to about 0 °, and UOCA increased from 62 ° to about 160 °[31]. Then they added NaOH to the coagulation bath to increase the micro-nano rough structure of the PAN film by alkali induction. When the concentration of NaOH was 10%, the WCA of the PAN film decreased from 79 ° to 0 °, and the UOCA increased from 80 ° to 155 °. As the concentration of NaOH increases from 0% to 10%, the surface roughness increases from 3.7 nm to 6.7 nm[21]. In addition, the structural stability of the membrane after loading should also be considered when loading micro-nanoparticles on the membrane. Too much particle loading will affect the pore size of the membrane, but the use of additive induction method can effectively avoid the above problems.
2.2 Janus membrane
In the study of membrane separation, Janus membrane generally refers to the membrane with "opposite" properties on both sides, one side is hydrophobic, the other side is hydrophilic, and it shows one-way permselectivity when contacting oil or water, that is, "Liquid Diode" characteristics[32]. When the Janus hydrophilic surface contacts the O/W emulsion, the oil droplets can selectively penetrate and migrate to the hydrophobic side under the action of the wetting gradient of the membrane[15]. The preparation methods of Janus membranes include asymmetric synthesis and asymmetric modification.
Asymmetric synthesis is the direct formation of asymmetric structure in the process of membrane preparation. The commonly used methods include: continuous electrospinning, vacuum filtration, mixed casting and so on[25]. A hydrophilic (or hydrophobic) film on one side is electrospun by a continuous electrospinning method, and then polymers with opposite wettability are electrospun on the film to form an asymmetric structure[33]. Vacuum filtration can directly load carbon nanotubes (CNTs), gold nanorods and other substances on membranes with opposite wettability to construct Janus membranes[16,34,35][36]. In addition, the membrane can also be prepared by mixing the two components of the membrane in the casting solution[37]. In order to ensure the stability of the film, the interfacial compatibility between the two layers should be fully considered during the asymmetric synthesis of Janus films.
Asymmetric modification is the modification opposite to the hydrophilicity and hydrophobicity on one side of the film, or asymmetric modification on both sides of the film, such as unilateral diffusion of the modifier and light radiation. The single-sided diffusion method is to make the modifier contact with only one side of the film under the action of diffusion driving force, so as to form a coating with opposite wettability[38]. In order to prevent the solution penetration caused by capillary action, a dense polymer can be used to protect one side of the membrane in advance, so as to realize the unilateral modification of the membrane[39]. The illumination radiation may provide an energy gradient that causes the photochemical reaction to proceed nonuniformly along the film thickness. The Janus structure can be formed by photolithography, photodegradation, or photoinitiated grafting under directed irradiation of laser or UV light[40][41][15]. For example, Wang et al. irradiated the hydrophilic fiber membrane with light from one side to induce the polymer containing the polydimethylsiloxane hydrophobic block to undergo crosslinking polymerization on the irradiated side, thereby forming an asymmetric structure (fig. 1b)[15]. This approach can obtain a variety of asymmetric membrane structures with high structural stability, but the preparation process is more elaborate and complex.
The structure and thickness of the membrane are important factors affecting the oil/water unidirectional permeability. When the hydrophilic layer of Janus membrane is thicker than a certain thickness, the directional migration of oil droplets will be stopped, and when the layer is too thin, it is not conducive to the aggregation and demulsification of oil droplets[39]. Compared with the single-side or double-side modification of the finished membrane surface, the direct synthesis preparation method, especially the vacuum filtration method, has more research and application values because the thickness of each layer can be effectively controlled[42]. How to design and prepare membrane materials with appropriate wettability and membrane thickness according to the characteristics of emulsion is the key factor to realize emulsion membrane separation.
2.3 Super-wetting membrane with functional sit
Emulsions often contain surfactants to make their emulsion structure more stable. In order to destroy the encapsulation of surfactant on droplets and reduce the pollution of surfactant on membranes, some studies have been carried out to construct functional sites on the surface of membranes that can act on surfactants. Functional sites can be divided into electrostatic sites, adsorption sites and morphological sites according to their modes of action, among which electrostatic sites are the most abundant[43][44][23]. Polyethyleneimine (PEI) is an organic macromolecule with high cationic charge density, which is often used to construct positive charge sites. Lin et al. Modified the cation-rich branched PEI on the electrospun fiber membrane by layer-by-layer self-assembly method.It has positive charge sites, which can repel the stable oil droplets of cationic surfactants through electrostatic force, and reduce the adhesion pollution of surfactants on the membrane[43]. It has also been studied to promote the selective permeation of surfactant-coated droplets based on electrostatic attraction by constructing sites with opposite electrical properties to surfactants. An et al. Carboxylated the CNTs on the hydrophilic side of the Janus membrane to make them negatively charged, which promoted the demulsification of the positively charged emulsion[16]. Conversely, making the membrane positively charged promotes the demulsification of the negatively charged emulsion (fig. 1C).
The construction of adsorption sites, such as the use of multi-walled carbon nanotubes (MWCNTs), is based on the separation of nonionic surfactants from the adsorption sites, thus promoting the coalescence of oil droplets and achieving the membrane demulsification separation of surfactant-stabilized emulsions[44]. Morphological sites break the thermodynamic equilibrium and make the droplets unstable through the extrusion and deformation of the droplets, and demulsification is achieved by removing part of the surfactant or rearranging it[23]. At present, the design and preparation of functional sites for surfactant treatment are still in the initial stage, and the research content is less, which needs to be deepened and expanded urgently.
2.4 Analysis of the Characteristics of Super-wetting Membrane Material
In terms of composition, ultra-wetting membrane materials can be divided into inorganic materials (including metal mesh, carbon nanotubes, ceramic membranes, etc.) And organic materials (including polymer membranes, cotton fabrics, cellulose non-woven fabrics, etc.). For inorganic materials, stainless steel mesh and copper mesh are widely studied in metal mesh materials, which have the advantages of high temperature resistance, strong mechanical properties, easy processing, low cost and adjustable pore size, but their micro-nano rough structure of surface modification is unstable, which leads to the weakening or even loss of super-wetting performance[27,45,46][25]. Carbon nanotubes can not only be used as a super-wetting membrane material, but also be prepared as a substrate membrane to further construct a functional surface with special wettability because of its hydrophobicity, high porosity, high specific surface area, good conductivity and other advantages[47][48]. Ceramic membrane has the ability of strong stability, acid and alkali resistance, high temperature and other harsh environments, but directly using ceramic membrane to treat emulsion will cause serious pollution problems and rapid flux decline, so its surface often needs to be further modified[24]. Among the organic materials, polymer membranes are widely used in the field of membrane separation because of their easy chemical modification, good processability and high porosity. However, there are also some problems such as poor cycle performance and poor antifouling ability in the application process, which also need to be modified and optimized[49,50]. In contrast, inorganic materials have good mechanical strength and are easy to clean, but they are brittle. Organic materials usually have excellent flexibility and easy processing, but their mechanical strength is relatively weak, and their cycle performance is poor.
In terms of structure, different material compositions and synthesis methods will make the super-wetting membrane present different structural characteristics, which can be divided into pore structure, cross-linked fiber structure and particle structure based on the pore formation principle of the membrane[51][52][53]. The membrane with the pore structure comprises a straight pore structure and an irregular structure, wherein the capillary effect of the straight pore structure causes permeated water to be sucked out from the support layer and forces the water to flow upwards, and the upwards permeated water can remove surfactant and oil droplets in the pores of the membrane, so that the service life of the membrane is prolonged, and the membrane with the pore structure has better self-cleaning property than the membrane with an irregular structure[51]. Cross-linked fiber structure refers to the structure of fibers interlaced into a web, which is mainly formed by weaving polymer materials by electrospinning. This cross-linked structure can be based on layer-by-layer deposition of fine fibers, making it difficult for small emulsions to pass through all the voids, thereby capturing droplets smaller than the average pore size[52]. However, due to the large pore size, the penetration pressure of the structure is relatively low, and it often shows low resistance to harsh environments such as acid, alkali and salt corrosion[26]. The particle structure refers to the two-dimensional membrane composed of spherical micro-nano particles, which is mainly formed by the phase transformation of the solution. The surface of these particles is not smooth, and the particles are connected by fibrous structure, which makes the membrane have high mechanical strength[31].
In addition, in recent years, some researchers have designed the material structure for membrane fouling, and some studies have found intelligent wetting membranes with special wetting response (such as voltage response, light response and pH response).By changing the external conditions, it can adjust its own properties, combined with the physical and chemical properties of pollutants, it can achieve the removal of irreversible pollutants to a certain extent[54,55]. At the same time, it shows excellent anti-fouling and self-cleaning performance in many cycles. However, at present, most of the smart wetting membranes are used to treat pure organic pollutants, such as proteins, polymers and dyes, and there are few studies on emulsion wastewater, but these self-cleaning methods and theories provide new ideas and possibilities for the preparation of anti-fouling super-wetting membranes[56].
3 Separation effect of ultra-wetting membrane in emulsion demulsification
3.1 Oil-water Separation Mode of Emulsion Wastewater by Super-wetting Membrane
Based on the characteristics of super-hydrophilic membrane and Janus membrane, there are two main demulsification separation modes of O/W emulsion in the current study: super-hydrophilic membrane water filtration and Janus membrane oil collection.
3.1.1 Water filtration and demulsification mode of super-hydrophilic membrane
Water filtration demulsification mode means that the continuous water phase rapidly spreads, infiltrates and permeates through the hydrophilic membrane, while the dispersed oil droplets are trapped on the membrane under the physical interception of the pore channel and the barrier effect of the hydrophilic layer, and the droplets are further aggregated to achieve demulsification. Table 1 summarizes the treatment object characteristics, membrane material parameters, operating conditions and treatment effects under this mode.
The treatment objects of water filtration and demulsification are mostly model emulsions prepared in the laboratory. The oil phase is mainly composed of toluene, chloroform and other short-chain hydrocarbons, and the oil concentration is mostly in the range of 1000 ~ 100000 ppm. The surfactants in the model emulsion were Tween, Span and SDS, and the dosage was less than 0. 2 wt%, even no surfactant was added. However, the actual waste emulsions often contain high viscosity and non-single oil phase such as mineral oil and synthetic ester, and the oil concentration can be as high as 10 000 mg/L. The surfactants also contain a variety of non-ionic and anionic emulsifiers, resulting in the droplet size of the emulsion is mostly less than 500 nm, the droplet has strong stability, complex structure and great difficulty in treatment. If the research of oil-water separation wants to expand from the model emulsion in the laboratory to the practical engineering application, it still faces many challenges.
In terms of operating conditions, when emulsion separation is performed using a conventional membrane, in order to quickly complete filtration, it is usually necessary to apply a pressure three times the gravity or even higher.However, when the ultra-wetting membrane is used to separate the emulsion, most of the experiments can obtain higher flux only by gravity self-drive, and the treatment flux can be increased by more than 10 times at the same operating pressure as the traditional membrane[57,58]. In terms of separation effect, the separation rate of traditional membrane to emulsion is generally low, mostly in the range of 70% ~ 95%, but the separation rate of ultra-wetting membrane can generally reach 99% or more because of its wettability to strengthen the demulsification of emulsion. The filtration flux of traditional membrane is usually difficult to exceed the 500 L·m-2·h-1·bar-1, while the super-wetting membrane, due to its unique wettability, enables the water phase to spread rapidly on the membrane surface and reduces the invasion pressure, so it can usually obtain high flux exceeding the 1000 L·m-2·h-1·bar-1 even at low operation pressure[59,60]. It can be seen that the water filtration demulsification mode has the advantages of high treatment flux and low operating pressure.
The method for improving the flux recovery rate of the ultra-wetting membrane is different from complex methods such as an acid-base cleaning method, a strong oxidant cleaning method, a surfactant cleaning method and the like which are frequently used in the traditional membrane,Because of its high wettability and high anti-invasion pressure to oil droplets, the ultra-wetting membrane is not easy to be physically adhered by oil droplets, and its filtration flux can be restored to a greater extent by simple deionized water cleaning[65]. Other methods to further improve the flux recovery rate of the super-wetting membrane include ethanol immersion and ultrasonic deionized water cleaning, which can minimize the damage to the fine structure of the super-wetting membrane, thus maintaining the high super-wetting performance of the membrane[71]. In addition, for the functionalized film designed by electrostatic force, the methods such as acid washing based on the regulation of surface charge of the film are more suitable. For some smart wetting membranes with special stimulus response properties, the method of changing pH and light conditions is also applied[54,55].
3.1.2 Oil Collection and Demulsification Mode of Janus Membrane
The mode of oil-collecting demulsification is that the Janus membrane with wetting gradient separates the oil phase from the water phase by blocking the water phase and filtering and enriching the oil phase. This mode can be used in conjunction with conventional dead-end and cross-flow installations (Figure 2b), and can also be applied to installations of special construction such as H-type, boat-type, etc[15,16][75]. Since the oil phase in the O/W emulsion is mostly light oil (density less than water), the H-type filtration device suitable for filtering light oil in the oil collection mode is widely used (Fig. 2a)[15,16]. The membrane arranged vertically in the device overcomes the defect that the light oil in the common dead end device cannot contact the membrane, and improves the oil filtering efficiency. In addition, the boat-shaped Janus oil collecting film is also widely used (Fig. 2C). This film absorbs the oil phase under the action of wetting gradient force to achieve the effect of separation and collection. At the same time, combined with the boat-shaped structure, it can prevent the oil from leaking back to the original liquid, thus realizing the non-destructive collection and transportation of oil[75]. In terms of separation effect, the oil collection and demulsification mode can achieve high-quality recovery of oil, and the purity of the separated and collected oil is extremely high. For example, a Janus membrane can quickly recover 98.4% of heavy oil and 69.4% of light oil from an oil-in-water emulsion with oil droplet sizes ranging from nanometers to microns, and the purity of crude oil can reach 99.9%[39]. But at the same time, it can be seen that this mode can not separate the oil and water in the emulsion well, and other studies have shown that it takes 3 hours to continuously treat 1 L of emulsion with 20% oil content in this mode, and the flux is only about 10 L·m-2·h-1·bar-1, so it is difficult to achieve the ultra-high separation rate and higher treatment efficiency of the water filtration and demulsification mode at the same time[76].
3.2 Analysis of influencing factors in the process of demulsification and separation of emulsion liquid membrane
The membrane properties and the characteristics of the treated object will have a certain impact on the demulsification separation rate, treatment flux and anti-fouling performance of the ultra-wetting membrane. Membrane properties include membrane surface wettability and pore size. Emulsion types include oil-water mixture, emulsion with or without surfactant. The existing ultra-wetting membrane has ideal separation effect on oil-water mixture and most emulsions without surfactants, but for stable emulsions containing surfactants,Especially, the treatment effect of industrial waste emulsion with high oil concentration and complex surfactant composition is still not good, especially in the improvement of treatment flux and membrane fouling control.
3.2.1 Effect of membrane wettability on separation efficiency
The influence of membrane wettability on the separation effect is mainly reflected in the treatment flux. The improvement of membrane treatment flux can be solved by enlarging the pore size of membrane, reducing the thickness of membrane and increasing the wettability of membrane. However, excessive expansion of membrane pore size may lead to the failure of interception of oil droplets with small particle size, resulting in a decrease in separation rate. An excessive reduction in the thickness of the film will result in a reduction in the mechanical properties of the film and a reduction in its durability. Therefore, compared with the first two factors, increasing the membrane wettability is a good choice to increase the treatment flux. Through the analysis of the existing research data (see Table 1), it is found that the wettability of the membranes with higher treatment flux is generally higher, and the UOCA is greater than 150 °.
Secondly, the wettability of the membrane also has a certain impact on the anti-fouling performance of the membrane. Due to the hydrophilic and oleophobic surface properties, the membrane with better wettability has a good ability to resist the physical adhesion of oil droplets and the adhesion of pollutants. After being used for multiple cycles of filtration, the separation rate of emulsion and the recovery rate of membrane flux are higher. When UOCA > 150 °, the separation rate and flux recovery rate of the membrane can reach more than 99%, and the separation rate of the membrane for emulsion fluctuates slightly at a high level of more than 99.9% in multiple circulation experiments[69]. Compared with the first experiment, the flux decrease rate is less than 1%, and the flux recovery rate is close to 100%[73]. However, the membrane with UOCA much lower than 150 ° is difficult to achieve such a high recovery rate, and its recovery rate is generally lower than 50%, and its anti-fouling performance is relatively poor[60].
3.2.2 Effect of Membrane Pore Size on Separation Efficiency
By analyzing the relationship between the membrane pore size and the droplet size of the emulsion in Table 1, it is found that since the droplet size of the emulsion is mostly distributed below 20 μm, the membrane pore size for treating the emulsion is generally below 5 μm. Most of the studies on the treatment of emulsion by ultra-wetting membrane show that the pore size of the membrane is slightly smaller than that of the droplet, that is, the filtration effect is achieved by physical interception[69,77]. Compared with the traditional emulsion liquid membrane separation, the pore size of the ultra-wetting membrane can be larger by intercepting equivalent emulsion; Under the condition of the same retention effect, the flux of ultra-wetting membrane is higher. In addition, in some other studies, the membrane pore size is larger than the droplet size, and the better treatment effect is also obtained. Cai et al. Used a PAN/ZIF-8 composite membrane with a micron-sized membrane pore size to separate emulsions with droplet diameters of 20 ~ 100 nm, and the separation rate reached more than 99.9%, and the treatment flux was about 1000 L·m-2·h-1·bar-1[59]. Li et al. Used a N/S-nm composite membrane to separate emulsions with droplet size in the range of 60 ~ 100 NM. The average membrane pore size was significantly larger than the droplet size to be treated, but the separation rate was still close to 99. 9%, and the treatment flux was 300~500 L·m-2·h-1·bar-1[60]. Different from the through straight channels, part of the ultra-wetting membrane has fine and irregular curved channels, so that more nano-level multi-layer structures are interlaced in the membrane.It enhances the collision and aggregation of droplets, so that small droplets smaller than the pore size of the membrane are aggregated into large droplets and retained. In addition, the unique wettability of the ultra-wetting membrane is also one of the reasons for this phenomenon[77,78]. The spatial structure of ultra-wetting membrane and its high wettability make it possible to break through the sieving effect in the process of emulsion demulsification and separation.
3.2.3 Influence of Surfactant on Separation Effect
The surfactant in the emulsion will affect the droplet size, network structure and electrostatic repulsion between droplets of the emulsion, which will encircle the small droplets.The surface tension of the small droplets in the emulsion is reduced, so that the small droplets are difficult to coalesce to form large droplets, the stability of the emulsion is further improved, and the separation effect of the membrane filtration emulsion is remarkably influenced.
The outer surface of the oil droplet wrapped by the surfactant is hydrophilic, and the oil droplet is easier to adhere to the hydrophilic film; The droplet size of the stable emulsion containing surfactant is smaller, and it is easier to diffuse into the membrane pores, resulting in the blockage of the membrane pores. The hydrophilicity, hydrophobicity and chargeability of surfactants can also drive them to adhere to the membrane surface, resulting in membrane contamination, and also change the wettability of the membrane, causing the adhesion of oil droplets. Therefore, the presence of surfactant will further aggravate the membrane fouling[79⇓~81]. When the prepared super-wetting membrane has strong electrification, the surfactant with the same electrification as the prepared super-wetting membrane can be repelled due to electrostatic repulsion, but the surfactant with the opposite electrification can adhere to the surface of the membrane during contact, and even enter the pores of the membrane to cause serious pollution[81]. In addition, when the surfactant molecules stay or adhere to the surface of the super-wetting membrane, the hydrophilicity and hydrophobicity of the membrane surface will be reduced, which will further increase the risk of subsequent membrane fouling[78]. Surfactants not only cause membrane fouling, but also hinder the coalescence of oil droplets on the membrane, slow down the filtration rate, and make the flux drop sharply, which makes the demulsification and separation of emulsion more difficult[22]. In the study of Yuan et al., the addition of 1 mg/L Tween-80 to the O/W emulsion decreased the filtration flux by more than 50%[82]. The SWCNT/TiO2 nanoparticle composite membrane prepared by Gao et al. Has a treatment flux of 32 540 L·m-2·h-1·bar-1 for the O/W emulsion without surfactant, while it has a treatment flux of 15 690 L·m-2·h-1·bar-1 for the same O/W emulsion with surfactant (sodium dodecyl sulfonate), with a decrease of more than 50%[83]. Deng et al. Also studied the effect of surfactant concentration on flux, and found that when the surfactant concentration increased by 2.5 times, the treatment flux decreased by about one third[84]. To sum up, the surfactant contained in the emulsion can significantly reduce the filtration flux, and the effect becomes more significant with the increase of surfactant concentration.
In recent years, a demulsification separation method by attaching functional sites such as PDA microspheres and CNTs to super-wetting membrane materials has been paid attention to in order to reduce the membrane separation efficiency and aggravate the membrane fouling caused by surfactants.And can improve the filtration flux to different degrees according to different additional functional sites, wherein the highest filtration flux can reach more than 30 times of that before the additional functional sites. In electrostatic interaction sites, multilayer polyelectrolyte was used to modify the positive and negative sites on the electrospun membrane, which were used to filter the emulsion containing anionic surfactant and the emulsion containing cationic surfactant, respectively. The filtration flux was 2.5 times and 3.3 times higher than that before adding functional sites, respectively[43]. CNTs can be used as a material for constructing functional sites on the membrane to repel surfactants by utilizing the electric property of the CNTs, so that the demulsification efficiency of the membrane is enhanced, and the membrane fouling is reduced[16]; Among the adsorption sites, MWCNTs as functional sites attached to the collagen fiber membrane had a flux difference of nearly 30 times compared with the same kind of polyamide and polytetrafluoroethylene porous membranes without functional sites[44]. Among the morphological sites, the addition of PDA microspheres as functional sites on the membrane can increase the flux value by 23.3% on the original basis[39].
At present, most of the studies on the influence of surfactant on the separation effect are macroscopic qualitative descriptions, and the quantitative laws of surfactant concentration on the separation effect such as separation rate and treatment flux need to be further explored.
4 Mechanism of Super-wetting Membrane in Demulsification
The research on the preparation and demulsification effect of ultra-wetting membrane separation materials is increasingly rich, but the research on the mechanism of demulsification is not deep enough. The existing studies mainly include the mechanism of oil retention and demulsification by the hydrophilic structure of the membrane, the driving mechanism of oil unidirectional permeation by the Janus structure of the membrane, and the interaction mechanism between the functional sites on the membrane surface and the surfactant at the oil-water interface. The research angle is mostly the law of droplet migration and transformation and the qualitative analysis of mechanics.
4.1 Mechanism of oil droplet retention and demulsification in emulsion by hydrophilic structure of membrane
The important index of O/W emulsion separation by super-hydrophilic membrane is the anti-invasion pressure of membrane material to oil or water. When the external pressure is greater than the water invasion pressure and less than the oil invasion pressure in the filtration process, the external phase water can continuously permeate through the membrane under the combined action of gravity and capillary force, and the oil droplets are blocked, so that the oil-water separation is realized. The unique wettability of the super-hydrophilic membrane is beneficial to increasing the oil invasion pressure and reducing the water invasion pressure, so as to realize the superior separation effect of low pressure and high drive[60,85]. Among them, the interception of oil droplets is the key to the study of separation mechanism, and researchers mainly carry out research from vertical and tangential directions (Fig. 3).
In the process of separation, there are two main speculations about the behavior evolution of oil droplets in the direction perpendicular to the membrane: one is that oil droplets gather at the attachment sites provided by the membrane surface, and gradually increase in size according to Stokes' law, which leads to the demulsification of oil droplets, and eventually leave the membrane surface by buoyancy, forming a continuous oil layer to achieve oil-water separation; Secondly, the oil droplets wrapped by surfactant will gradually form a cake layer on the membrane surface with the filtration process, and the floating ability of oil droplets will be hindered. However, due to the presence of the invasion pressure, the oil droplets can still remain above the membrane surface and will not penetrate through the membrane[82,85]. At the same time, the study of the dynamic water permeation characteristics of the membrane and the dynamic process of droplets contacting and leaving the underwater surface of the membrane by means of a high-speed camera system can prove the rationality of this mechanism, but the specific process and reasons of its behavior evolution need to be further studied[85].
The change of oil droplets in the tangential direction of the membrane surface will be affected by the cross-flow rate, permeate flux and other factors, and then will affect the membrane fouling. It is speculated that in the process of cross-flow filtration, the oil droplets "nailed" in the orifice will be subjected to resistance caused by cross-flow velocity, resistance caused by permeation flux, lift, gravity and buoyancy caused by asymmetric flow field around the droplets, etc[54]. In which the cross-flow field creates a shear stress on the oil droplets at the membrane surface, promoting the detachment of accumulated droplets from the membrane surface. Fux et al. Used a confocal microscope to directly observe the deposition and deformation of oil droplets, and found that with the increase of permeation flux and the decrease of cross-flow velocity, the droplets would deform significantly on the membrane, and then destroy the thin water layer formed on the hydrophilic membrane[86]. The larger the droplet size is, the more intense the deformation of the droplet is, which will aggravate the adhesion and deposition of the oil droplet at the membrane pore. It can be seen that controlling the permeation flux and crossflow rate within a reasonable range is helpful to alleviate the deposition of oil droplets in the membrane pores, and then achieve superior demulsification effect, which has guiding significance for further design of surface modified membranes. However, the research in this direction is still in the stage of theoretical qualitative analysis, and the analysis of the complex deposition and deformation process of droplets in the process of oil-water separation and the quantitative characterization of the interaction relationship are still unclear.
4.2 Driving mechanism of Janus structure of membrane for unidirectional permeation of oil droplets in emulsion
Janus membrane is mainly based on the wetting gradient structure to drive the oil droplets of the dispersed phase in the emulsion to pass through the membrane unidirectionally, so as to achieve the purpose of filtering out the oil phase and separating the water phase. By means of Direct Observation Through the Membrane (DOTM) and Confocal Laser Scanning Microscope (CLSM), the behaviors of adhesion, coalescence, detachment or permeation of droplets on the Membrane with time during demulsification can be investigated[87]. There are three main viewpoints on the unidirectional permeation process of oil droplets: First, oil droplets diffuse to the vicinity of the membrane surface, first coalesce in the hydrophilic layer of the Janus membrane, then gradually fill the hydrophobic pores of the membrane, and finally permeate through the hydrophobic layer driven by the wetting gradient[87,88]. The other is that the oil droplets do not coalesce, but first contact the hydrophilic layer, then pass through the hydrophilic layer by one-way transport, and then pass through the membrane pores of the hydrophobic layer, directly driven by the wetting gradient force to achieve osmotic separation[39]. Other studies suggest that water will gradually replace the "guiding agent" (acetone) on the membrane to form a continuous water layer during the infiltration process, providing a stable traction force between the water phases on both sides of the membrane and continuously guiding the water phase through the membrane[16]. At present, the behavior of droplets in the process of Janus membrane demulsification and separation is still in the stage of speculation, and there is no unified conclusion.
The migration of an oil droplet on the Janus membrane is affected by hydrostatic pressure, the capillary attraction of the hydrophobic layer of the membrane and the capillary repulsion of the hydrophilic layer of the membrane. When an oil droplet touches the Janus membrane, it is affected by both the capillary repulsion of the hydrophilic layer and the capillary attraction of the hydrophobic layer[89]. When the capillary attraction force is greater than the capillary repulsion force, the oil droplet will be pulled into the hydrophobic and hydrophilic layer, and then penetrate (Figure 4)[90⇓~92]. For a single droplet, the hydrostatic pressure caused by gravity is 4 orders of magnitude lower than the capillary pressure, which is usually negligible[93]. Capillary force can be characterized by Young's Laplace equation (Equation 1), which is usually affected by the pore size of each layer of Janus membrane[39]. In addition, forces such as hydrophilic-hydrophobic forces and Laplace forces for the formation of material morphology have been mentioned in some studies.
The Pc is the capillary force, γ is the liquid surface tension, θ is the droplet contact angle, and d is the membrane pore diameter. Capillary pressure P1 and P2 are induced by hydrophilic-hydrophobic layers, respectively, and ΔG is the sum of Gibbs free energies of hydrophilic-hydrophobic layers. γ represents the water/oil and air/oil interfacial tension, respectively, θ represents the contact angle of the oil at the water/oil/solid and air/oil/solid three-phase boundaries, respectively, and H1 and R1 represent the thickness and average pore size on the hydrophilic film side, while H2 and R2 represent the corresponding parameters on the hydrophobic side.
Combined with equations (2 ~ 4), it can be seen that the main factors affecting the droplet migration in the Janus membrane include the contact angle of the droplet, the surface tension of the liquid, and the different thicknesses and pore sizes of the hydrophilic and hydrophobic membrane layers[39]. Among them, the thickness and relative proportion of the hydrophilic and hydrophobic layer of Janus membrane have a significant effect on the oil droplets driven by the membrane. According to the formula, only when a thick hydrophobic layer and a thin hydrophilic layer are constructed within a certain range of parameters, the oil droplets can penetrate the hydrophilic layer, reach the hydrophobic layer and continue to infiltrate, showing one-way permeability. On the contrary, a thin hydrophobic layer and a thick hydrophilic layer can realize the unidirectional transport capability of water[39]. The Janus membrane prepared by Liu et al. Has a thicker hydrophobic layer and a thinner hydrophilic layer. After preparation, the thickness of the hydrophobic layer is 142.9 μm, while the thickness of the hydrophilic layer is 11.9 μm. The driving force generated by the membrane can realize oil-water separation, and the separation efficiency reaches 99.75%. However, when the hydrophilic layer thickness is greater than 22.4 μm and the hydrophobic layer thickness is less than 136.1 μm, the flux decreases significantly[89]. Thus, the size of the pores in the membrane can adjust the size of the emulsion droplet diameter, which is beneficial to improving the permeation flux of oil[87].
To sum up, the research on the force of Janus film driving droplets mainly focuses on theoretical assumptions and qualitative analysis, focusing on the relationship between the critical force and the material structure.The process of demulsification and separation is explained, but the quantitative study on the measurement and calculation of droplet force in the actual process and the influence mechanism of complex emulsion on droplet driving force still need to be further studied. In addition, the effects of guiding agent, pH value and membrane charge on membrane separation are also worthy of further study.
4.3 Functional Sites on Membrane Surface and Interaction Mechanism of Surfactants at the Oil-Water Interface
Oil-in-water emulsions containing surfactants are more difficult to treat than ordinary emulsions due to the steric hindrance effect caused by surfactant wrapping oil droplets and the existence of electrostatic repulsion, and the use of functional materials to construct sites interacting with surfactants on the membrane can play a good demulsification effect[94]. Although the current research on the interaction mechanism between functional materials and surfactants is insufficient, the following three directions are worth further exploring.
The physical extrusion deformation based on the sieving effect causes the surfactant to fall off from the oil droplets to promote the demulsification. During the emulsion filtration process, the oil droplets accumulate on the membrane surface and deform on the membrane surface under the action of physical extrusion and shear[23,86]. The surfactant layer wrapping the oil droplet undergoes molecular rearrangement or partial separation, which breaks the original thermodynamic equilibrium of the droplet, promotes its coalescence on the membrane, and is repelled and separated from the membrane surface based on the underwater super-oleophobic characteristics of the membrane surface, thus realizing oil-water separation (Fig. 5a-c)[23,60]. In addition, some studies have speculated that the "nanofiber scissors" effect of nanofiber membranes can cause droplet deformation and surfactant rearrangement, resulting in droplet instability and demulsification[23]. In the process of demulsification caused by physical extrusion deformation, the sieving effect and fiber cutting play a significant role. However, these mechanisms are mostly speculative and have not been directly confirmed by experiments.
The demulsification is promoted by the structural adsorption of hydrophilic and hydrophobic functional sites on the interfacial pores of the membrane surface: some membrane materials construct active sites with adsorption function on the hydrophilic membrane, and the adsorption of surfactants is realized by hydrophilic and hydrophobic forces to promote demulsification and separation[87,88]. For example, Ye et al. Used membranes with different MWCNT contents to treat stable emulsions containing surfactants, and found that MWCNTs played an adsorption role on surfactants and promoted the instability of emulsions, and gave a graphical explanation for speculation (as shown in Figure 5d ~ f), but lacked direct proof, which was a follow-up research direction[44]. In addition, too much pressure on the membrane and too many times of membrane separation may lead to the blockage, destruction and collapse of the special membrane structure, so how to construct a stable special membrane structure containing hydrophilic and hydrophobic functional sites deserves further exploration[95].
There are two main methods to promote the demulsification of emulsions containing surfactants based on electrostatic force: one is to physically modify PEI on electrospun fibrous membranes by Lin et al. Using the electrostatic effect of the inherent functional groups on the membranes.Electrostatic repulsion is generated between the formed positive charge sites and cationic surfactant molecules in the emulsion, so that the adhesion pollution of the surfactant on the membrane is reduced, and the permeation flux is improved[54][43]. Another method is to cause the rearrangement of charged surfactant molecules at the oil-water interface by an applied electric field. When an external electric field is applied, the charged surfactant molecules will respond and redistribute at the oil-water interface. When the membrane is used as a cathode, it will repel the surfactant molecules with the same charge based on the principle of mutual repulsion of like charges. When used as an anode, it will cause the aggregation of surfactant molecules. When Zhu et al studied the role of external electric field in membrane separation and membrane cleaning, combined with experiments, they proved that electrostatic force was conducive to reducing the membrane fouling of surfactant molecules on conductive carbon nanotube ultrafiltration membrane, and significantly improved the separation efficiency of the membrane[54]. Regulation of electrostatic force is one of the most commonly used methods of functional site modification, which has good separation effect and can restore membrane flux well in the face of membrane fouling, providing an effective way to control the negative effects of surfactants. In addition, compared with the physical modification of charged functional groups, the electric field control method has the advantages of timely response, flexibility and so on, and has a broader application prospect.
5 Conclusion and prospect
In this paper, the design and preparation methods of ultra-wetting membrane materials and the composition, structure and properties of the membrane are summarized, and two different demulsification separation modes of water filtration and oil collection are summarized.The influence of surfactant concentration on the demulsification efficiency of emulsion was analyzed, and the research progress of the mechanism of ultra-wetting membrane in the process of emulsion demulsification was summarized. The current conclusions are as follows: (1) There are two kinds of super-wetting membrane materials for emulsion separation: super-hydrophilic membrane and Janus membrane, which are usually prepared by constructing hydrophilic surface and rough structure. The main methods include direct synthesis and loading modification, and the latter has become a hot research topic because of its wide range of suitable membrane materials and simple method. In the aspect of direct synthesis, the cross-linked fibrous membrane prepared by electrospinning has a good prospect in emulsion separation because of its interlaced fibrous structure, which makes it difficult for small emulsions to pass through all the gaps, thus capturing droplets smaller than the average pore size, which can break through the limitation of sieving effect on the pore size of the membrane. (2) Emulsion separation modes include super-hydrophilic membrane water filtration demulsification mode and Janus membrane oil collection demulsification mode. The former has the characteristics of low operating pressure, high filtration flux and good anti-fouling performance, while the latter has the characteristics of high purity oil collection; (3) At present, the object of ultra-wetting membrane treatment is mainly emulsion without surfactant or emulsion with low concentration of surfactant. The existence of surfactant in emulsion will increase membrane fouling and membrane pore plugging, and the higher the concentration is, the more significant the effect on filtration flux is. (4) The current research mainly focuses on the behavior and force of droplets on the membrane surface and in the membrane pores: in the filtration process, oil droplets are trapped on the hydrophilic membrane, and then high flux separation is achieved by rapid filtration; For the Janus membrane, the unidirectional permeation from the hydrophilic layer to the hydrophobic layer is driven by the Laplace force. However, the relevant research is mostly based on speculation, which is not deep enough, and the details of its filtering are still controversial.
Based on the literature and related research progress, the research prospects of ultra-wetting membrane are as follows: (1) The design and preparation of functional sites for surfactant treatment are still in the initial stage, and it is an important research direction to explore the influence mechanism of surfactants on membrane demulsification separation process and design functional sites based on mechanism analysis. (2) At present, the study of demulsification and separation of emulsion by ultra-wetting membrane mostly stays in the laboratory conditions, and the practical application of engineering needs to be studied. (3) It is hoped that researchers will build more microscopic platforms to explore droplet behavior in depth, and make empirical and quantitative mechanical analysis of the hypothesis of membrane demulsification mechanism.
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