The essence of separation membrane: the interface form used to separate two parts: solid or liquid characteristics: selective permeation advantages: high efficiency, low energy consumption, small occupied area, etc. Application: Basic overview of separation membrane in food industry and medicine, advantages of membrane separation, research progress of graphene-based separation membrane, preparation of graphene-based basement membrane, application of graphene-based separation membrane, Basic overview of separation membrane A separation membrane refers to an interface that can restrict and transfer fluid substances in a specific form to separate two phases or two parts. The form of the film can be solid or liquid. The fluid separated by the membrane can be liquid or gas. Separation membrane is a kind of special thin-layer substance with selective permeation function, which can make one or several substances in the fluid permeate, while other substances do not, thus playing the role of concentration, separation and purification. Since the advent of membrane technology, microfiltration membrane, ion exchange membrane, reverse osmosis membrane, ultrafiltration membrane and gas membrane separation have been widely used. Because they can achieve separation under the condition of maintaining the original biological system environment, and can efficiently concentrate and enrich products and effectively remove impurities, they have the advantages of convenient operation, compact structure, low energy consumption, simplified process, no secondary pollution and no need to add chemicals, and are gradually becoming the basic unit operation process in food industry and medicine. Advantages of membrane separation Membrane separation has the advantages of high separation efficiency, low energy consumption, small floor space, simple process (easy to scale up and control), convenient operation and no environmental pollution. Research progress of graphene-based separation membranes The rapid development of industrialization has brought convenience to people's lives, but it also faces environmental problems caused by pollution such as wastewater and waste gas. Membrane separation technology, as one of the effective technologies to control the environment, appeared in the early 20th century. In practical application, membrane separation technology faces many challenges, and membrane pollution and low separation efficiency are the main limiting factors. In order to further develop and improve the membrane separation technology, different separation membrane materials have been developed one after another, among which graphene materials with excellent selectivity and stability stand out and become the most potential non-traditional membrane materials. Graphene is a two-dimensional monolayer crystal formed by carbon atoms in the form of six-membered rings, which has excellent mechanical properties and stability. Graphene oxide (GO) has a two-dimensional planar structure similar to graphene, and a large number of polar oxygen-containing functional groups such as hydroxyl, carboxyl and epoxy groups are distributed on its surface. The existence of these groups is beneficial to the functional design of graphene basement membrane, thus changing the surface charge and hydrophobicity of the membrane and adjusting the interlayer size. In addition, the raw material (graphite) for preparing graphene basement membrane is widely available and cheap, which provides a favorable foundation for the large-scale preparation and wide application of graphene basement membrane. Preparation of graphene-based membranes In recent ten years, various graphene-based membranes have been developed and applied in the field of membrane separation. At present, the preparation methods of graphene basement membrane mainly include vacuum filtration, spray/spin coating, layer-by-layer self-assembly and * * * mixing method. 1 vacuum suction vacuum suction method is the most commonly used method to prepare graphene basement membrane, and its main process is as follows: firstly, graphene or graphene oxide dispersion is poured into a suction bottle with a filter membrane, and then vacuum suction is carried out to make the membrane adhere to the basement membrane. Dikin et al. prepared graphene oxide films with a thickness of 1 ~ 30μ m by suction filtration for the first time. Mechanical tests show that the modulus of GO film is as high as 32 GPa, which is much higher than that of traditional films. Subsequently, Li et al. used the chemically reduced graphene () membrane sleeve prepared by vacuum filtration to carry out pressure-driven liquid phase separation. The experiment shows that the water flux reaches 41l m-2h-1bar-1under the reduction condition of 90℃, and the nano-gold and nano-platinum particles are basically trapped. Huang et al. obtained GO ultrafiltration membrane on polycarbonate (PC) membrane by vacuum filtration. It is found that the nano-scale folds between GO sheets are the main channels for ions and molecules. The size of folds can be adjusted by controlling the applied pressure, salt concentration and pH value, thus directly regulating the pore structure and molecular sieve performance of GO separation membrane. The oxygen-containing functional groups on the surface of GO make the interlayer spacing larger, which is not conducive to the interception of small molecules. Therefore, after the surface functional groups are removed by reduction, the interlayer spacing can be further reduced and the interception performance can be improved. The vacuum filtration method is simple to operate, and the basement membrane is diversified, and the film thickness can be adjusted by solution concentration. The membrane prepared by this method has good mechanical properties and excellent separation performance, but it is easy to be damaged in the process of separation from the basement membrane, and its integrity is difficult to maintain. Spin coating is a simple and effective film forming method. Uniformly dispersing the solution on the substrate by adjusting the rotating speed, and then drying to obtain the film. In 2008, Becerril et al. used spin coating method to evenly coat graphene oxide solution on the surface of glass and substrate, and prepared GO thin films. Lue et al. found that compared with the drop coating method, the perfluorosulfonic acid/graphene oxide composite membrane constructed by spin coating method is beneficial to produce a well-arranged layered stacking structure, thus reducing fuel permeability and improving fuel cell performance. Kim et al. found that the moisture between GO lamellae will be removed during the spin coating process, thus forming a strong capillary force, which is beneficial to the deposition of GO lamellae and forming a relatively dense structure. The gas flux of the separation membrane is related to the transmembrane pressure and inversely proportional to the relative molecular mass of gases (except CO 2). At 140℃, the selective permeability of H 2/CO 2 can reach 40. Spin coating method requires simple equipment structure, controllable conditions and adjustable film area and thickness to some extent. However, this method is difficult to prepare in a large area, and there is a problem of uneven film formation. Spraying method is to spray graphene oxide or graphene dispersion evenly on the substrate with spraying equipment to form a thin film. 2.3 Layer-by-layer self-assembly method Layer-by-layer self-assembly method (LbL) deposits and self-assembles multilayer films through hydrogen bonding, electrostatic attraction and valence bonding. As an efficient and simple method, LbL method has successfully constructed a variety of ultra-thin composite membrane structures. The rich functional groups and good water solubility on the surface of graphene oxide make it one of the ideal materials for constructing composite membranes by LbL method. Hu et al. took the lead in using dopamine-modified polysulfone membrane as the supporting layer and triphenylmethyl chloride (TMC) as the cross-linking agent, so that GO was self-assembled layer by layer to form multi-layer GO membrane. In this method, a stable graphene separation membrane structure is constructed by * * * valence crosslinking. The membrane can still remain intact under repeated washing and ultrasonic conditions, and its water flux can reach 4 ~ 10 times that of the traditional nanofiltration membrane. Zhao et al. prepared polyethyleneimine/graphene oxide composite films by LbL method under the action of external electric field. The experiment shows that the applied electric field accelerates the deposition speed and amount of GO on the substrate during the composite membrane assembly process, thus shortening the assembly time and reducing the soaking times. In addition, as a uniform external force, the electric field makes the GO layer spread evenly on the substrate, thus forming a tight and orderly gas protective layer. When the applied voltage is 25V, the rejection rate of PEI/GO membrane for hydrogen is 65% higher than that of common composite membrane. The composite film can be used as a protective layer on the metal surface, effectively preventing the invasion of hydrogen, thus inhibiting hydrogen corrosion. Layer-by-layer self-assembly method is simple to operate, and the film-making process is not limited by the shape and size of the substrate. The prepared films have good mechanical properties and are mainly used to construct complex multilayer film structures. 4*** mixed graphene has excellent physical and chemical properties, which can be compounded with specific polymers to form new composite materials and realize functional modification of membranes. Various graphene-modified separation membranes can be constructed by mixing graphene with membrane-making polymer materials or polymer precursors. Due to the stability of polymer materials, graphene modified films obtained by * * * mixing method can exist stably in water and acid-base conditions for a long time. Wang et al. dispersed polyvinylidene fluoride (PVDF) and GO in 2- methylformamide, and then prepared a mixed ultrafiltration membrane by phase inversion method. When the content of GO is 0.20wt%, the permeability increases to 96.4% and the contact angle decreases from 79.2 to 60.7. Subsequently, Fryczkowska and others studied the influence of GO on the properties of PVDF membrane, and confirmed that the addition of GO increased the hydrophilicity of the membrane, decreased the porosity and increased the pore size of the membrane, thus increasing the membrane flux. Ouyang et al. made cobalt oxide/graphene oxide composite and polyethersulfone into ultrafiltration membrane by * * * mixing method. Compared with the original membrane, when the content of Co 3 O4-GO is 65438 0.5%, the water flux of the composite membrane is increased by 344%, and the retention rate of bovine serum protein is still above 94%. After the filtration experiment of activated sludge (SV=30%), the recovery rate of membrane flux is as high as 8 1. 1%, while the unmodified membrane is only 55.7%. The improvement of membrane performance is mainly attributed to the uniform distribution of nanosheets in polymer matrix, which improves the hydrophilicity of membrane surface. Compared with ordinary membranes, the flux, anti-pollution and mechanical properties of graphene hybrid membranes prepared by * * * mixing method have been significantly improved. Application of Graphene-based Separation Membrane Graphene is the thinnest material that can be used as separation membrane at present, and complete graphene is impermeable to all molecules, while the macroscopic membrane formed by stacking graphene nanoplates face to face can be separated through nano-channels between plates. On the other hand, graphene materials obtained by introducing nanopores or artificially designing folds based on molecular sieve separation effect can be used as high-efficiency separation membranes. Graphene-based separation membranes can not only be used for gas separation and CO 2 capture, but also have broad application prospects in emerging fields such as seawater desalination and isotope separation.