Polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane and application thereof

Abstract

The invention provides a polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane and application, and the polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane is prepared by the following steps: stirring and dissolving two polymers, namely polymethyl methacrylate (PMMA) and polyethylene glycol (PEG), in an N, N-dimethylformamide solvent to form a membrane casting solution; the preparation method comprises the following steps: preparing a nanofiltration membrane with a specific thickness through a membrane scraper, then placing the prepared nanofiltration membrane in a fuming cupboard for standing, forming a blended asymmetric nanofiltration membrane with a compact structure based on different volatilization speeds of a solvent on the upper layer and at the bottom of the nanofiltration membrane, completely immersing the blended asymmetric nanofiltration membrane in deionized water, removing the solvent through a phase transfer method, and drying to obtain the asymmetric nanofiltration membrane. Further curing the film; in the membrane preparation process, polymethyl methacrylate is used as a main supporting material, polyethylene glycol is used as a pore-foaming agent and a hydrophilic material, and the membrane realizes efficient separation and removal of neutral chlorinated phenol disinfection byproducts with the molecular weight of less than or equal to 200 in water.

Description

Technical Field

[0001] The present invention belongs to the fields of nanofiltration membranes and their preparation, water treatment with nanofiltration membranes, and research on membrane separation mechanisms; it studies the transport processes of solutes and solvents in membranes; and explores separation mechanisms such as size sieving and charge effects. In particular, it relates to a poly(methyl methacrylate) / polyethylene glycol blended asymmetric nanofiltration membrane and a preparation method thereof. Background Art

[0002] Due to the prevalence of various viruses such as influenza A virus, the usage amount of disinfectants in domestic water remains high, resulting in the large generation of many harmful disinfection by-products (DBPs). The presence of DBPs in drinking water has been proven to have neurotoxicity and is closely related to bladder cancer, colon cancer, rectal cancer, etc. The World Health Organization has strict control standards for aliphatic disinfection by-products. However, chlorophenols are a new type of aromatic DBPs with a molecular weight ≤ 200 and no charge. Although their content in water is low, their toxicity is nearly a thousand times higher than that of common aliphatic DBPs, posing a serious threat to the safety of drinking water quality. Membrane separation technology has the advantages of simple operation, easy scaling up, low energy consumption, stable operation, and no consumption of chemicals. The rapid development of this technology provides a simple and fast method for the separation and removal of organic pollutants in the water environment. Among them, nanofiltration membranes have a long-term and stable effect on the separation and removal of organic pollutants and have advantages such as high flux, good selectivity, and low energy consumption compared with other membrane separation methods. In the sewage treatment system, the separation method with nanofiltration membranes as the core has always been a research hotspot at the intersection of multiple disciplines such as environment, materials, and chemistry.

[0003] The separation and removal mechanisms of nanofiltration membranes mainly include the following two aspects: 1. Pore size sieving: Nanofiltration membranes have a certain pore size distribution. When a fluid passes through the membrane, particles, molecules, etc. larger than the membrane pore size will be intercepted, while substances smaller than the membrane pore size can pass through the membrane, just like a sieve screening particles of different sizes; 2. Charge repulsion: The surface of nanofiltration membranes usually carries a certain charge, which is due to the properties of the membrane material itself or functional groups introduced during the preparation process; ions and charged particles in the solution will interact with the charges on the membrane surface. Like charges repel each other, making ions and charged particles with the same charge as the surface of the nanofiltration membrane difficult to approach the membrane surface and thus being intercepted.

[0004] For neutral chlorophenol disinfection by-products with a molecular weight ≤ 200, their molecular size is relatively small. Conventional nanofiltration membrane systems have good separation and removal effects on organic pollutants with a molecular weight greater than 200. However, their separation and removal performance for organic pollutants with a molecular weight ≤ 200 is limited. Moreover, chlorophenol itself is uncharged and exists in the form of neutral molecules in aqueous solutions, so the ability to separate and remove chlorophenol based on charge repulsion is limited. For example, the retention rate of the commercial nanofiltration membrane NF-90 for neutral organic small molecules with a molecular weight ≤ 200 is less than 40%, which greatly limits its practical application value in the separation and removal of water pollutants. Therefore, how to construct a highly selective nanofiltration membrane to achieve efficient separation and removal of neutral chlorophenol disinfection by-products with a molecular weight ≤ 200 in the water environment is a very challenging task. Summary of the Invention

[0005] The present invention provides a preparation method and application of a poly(methyl methacrylate) and polyethylene glycol blended asymmetric nanofiltration membrane. The nanofiltration membrane has the common characteristics of small pore size, high porosity, and high retention rate, and can selectively separate and remove neutral chlorophenol disinfection by-products (DBPs) with a molecular weight ≤ 200 in the water environment, breaking through the problem that it is difficult for nanofiltration membranes to efficiently separate neutral small molecule organic pollutants, and achieving good separation and removal effects on neutral chlorophenol DBPs.

[0006] Based on this, the present invention adopts the following technical solutions: A preparation method of a poly(methyl methacrylate) and polyethylene glycol blended asymmetric nanofiltration membrane, characterized in that two polymers, poly(methyl methacrylate) and polyethylene glycol, are dissolved in an N,N-dimethylformamide solvent by stirring to form a casting solution, and a nanofiltration membrane with a specific thickness is prepared by a film scraper. During the membrane preparation process, poly(methyl methacrylate) is the main supporting material, and polyethylene glycol is the pore-forming agent and hydrophilic material. Then, the prepared nanofiltration membrane is placed in a fume hood and left to stand. Based on the different solvent evaporation rates on the upper layer and the bottom of the nanofiltration membrane, a blended asymmetric nanofiltration membrane with a dense structure and a loose structure is formed. The blended asymmetric nanofiltration membrane is completely immersed in deionized water, and the solvent is removed by the phase inversion method to solidify the membrane.

[0007] The above poly(methyl methacrylate) and polyethylene glycol blended asymmetric nanofiltration membrane is prepared through the following steps, and the specific steps are as follows: (1) Based on the calculation of preparing a solution with a solid content of 10 - 30%, prepare an N,N-dimethylformamide solution, weigh poly(methyl methacrylate) and polyethylene glycol in proportion, and the mass ratio of poly(methyl methacrylate) to polyethylene glycol is (7 - 9)∶(1 - 3); then add poly(methyl methacrylate) and polyethylene glycol to the N,N-dimethylformamide solution to prepare a casting solution with a solid content of 10 - 30%. (2) Add the casting solution into the reaction vessel; in a constant-temperature oil bath, at a temperature of 90 - 105 °C, stir for 5 - 8 h, and set up a condensation reflux device to prevent the volatilization of N,N-dimethylformamide, so that polymethyl methacrylate and polyethylene glycol are fully dissolved in the N,N-dimethylformamide solution; (3) Let the solution obtained in step (2) stand and defoam for 0.5 - 1.5 h at a temperature of 75 - 85 °C; use a pipette to transfer the defoamed solution onto a silicon plate, and use a scraper on the silicon plate to prepare a blend membrane with a membrane thickness of 10 - 100 μm. Let the blend membrane on the silicon plate stand in a fume hood for 10 - 30 min. Through solvent evaporation, the surface solvent evaporates quickly, forming a dense structure. The lower surface layer adheres tightly to the glass, and the solvent evaporates slowly, thus continuing to retain a loose and porous structure, forming a blend nanofiltration membrane with an asymmetric structure; (4) Completely immerse the silicon plate with the blend asymmetric nanofiltration membrane in deionized water, and soak it in deionized water for 8 - 15 h. Through the phase inversion method, the solvent gradually dissolves in the aqueous solution, and the solvent is removed to solidify the asymmetric nanofiltration membrane; (5) Take out the blend asymmetric nanofiltration membrane in deionized water, air-dry it naturally, cut it and press it flat to a suitable size for use.

[0008] In step (1), regarding the solid content of the casting solution, research and analysis were carried out. When the polymer content in the casting solution is high, gelation is likely to occur and no film can be formed; when the polymer content is low, there is less cross-linking between polymer molecules, and the performance of the formed film is not high, and it is fragile and has poor mechanical properties. Through comparison, it can be obtained that when the content of the casting solution is 20%, the performance of the prepared blend asymmetric nanofiltration membrane is the best; for the film-making temperature, we respectively selected oil baths at 75 °C, 85 °C, 95 °C, and 105 °C to prepare the casting solution, and found that at 75 °C and 85 °C, polymethyl methacrylate is not fully dissolved, and the film-making performance is not as good as that at 95 °C and 105 °C. However, considering that too high a temperature may form a gel, we selected 95 °C as the optimal film-making temperature.

[0009] In step (2), during the heat stirring treatment, the temperature will affect the diffusion resistance and diffusion rate of monomers in the solution. As the heat treatment temperature increases, the water flux of the blend asymmetric nanofiltration membrane decreases, while the rejection rate first increases and then decreases. The heat stirring treatment time affects the pore size shrinkage and cross-linking degree of the separation layer of the blend asymmetric nanofiltration membrane. As the heat treatment time prolongs, the rejection rate of the prepared blend asymmetric nanofiltration membrane first increases and then decreases, and the optimal stirring time is finally determined to be 5 h.

[0010] In step (3), when using a film scraper to prepare the film, the film thickness also has a direct impact on the retention performance of the blended asymmetric nanofiltration membrane. The thicker the film, the better the retention rate, the lower the water flux, and the better the film toughness; the thinner the film, the lower the retention rate, the higher the water flux, but the mechanical properties become worse and it is prone to breakage under the drive of a pressure pump. In this patent, blended asymmetric nanofiltration membranes with thicknesses of 10, 25, 50, 75, and 100 μm were respectively prepared. It was found that the blended asymmetric nanofiltration membranes with thicknesses of 10 and 25 μm were fragile under the working pressure of pump filtration, and their retention rates were not as good as those of the blended asymmetric nanofiltration membrane with a thickness of 50 μm. When the film thickness was 50, 75, and 100 μm, the retention rate of the blended asymmetric nanofiltration membrane did not increase significantly, and the water flux decreased. Therefore, considering the balance between the retention rate and the water flux, as well as various factors such as working conditions and costs, it is considered that a film thickness of 50 μm is the most suitable, with the best retention rate and water flux, and good mechanical properties at the same time.

[0011] In step (4), the phase inversion method is an efficient and convenient film preparation method. It mainly uses two immiscible solvents (such as the aqueous phase and the organic phase), and through interfacial reaction or diffusion, transfers the target substance from one phase to another phase, and finally forms a thin film on the substrate. The advantage of this method is that it can prepare ultra-thin and uniform organic polymer films with good controllability. By adjusting the film preparation conditions, the thickness and properties of the organic polymer film can be precisely controlled.

[0012] Therefore, further, in step (1), the mass ratio of polymethyl methacrylate to polyethylene glycol is 8:2; polymethyl methacrylate and polyethylene glycol are added to an N,N-dimethylformamide solution to prepare a casting solution with a solid content of 20%; in step (2), at a temperature of 95 °C, stir for 5 h; in step (3), at a temperature of 80 °C, let it stand and defoam for 1 h; use a pipette to transfer the defoamed solution onto a silicon plate, and use a scraper on the silicon plate to prepare a blended asymmetric nanofiltration membrane with a film thickness of 50 μm, and let the blended membrane on the silicon plate stand in a fume hood for 10 min; in step (4), immerse it in deionized water for 12 h. Through the phase inversion method, the solvent gradually dissolves in the aqueous solution, and the solvent is removed to solidify the asymmetric nanofiltration membrane.

[0013] The above-mentioned blended asymmetric nanofiltration membrane of polymethyl methacrylate and polyethylene glycol can highly selectively separate and remove neutral chlorophenol disinfection by-products with a molecular weight ≤ 200 in water.

[0014] Advantages of the present invention: By comparing with the performance of nanofiltration membranes on the market, it is found that in the present invention, PMMA and PEG organic polymers are selected to prepare a blended asymmetric nanofiltration membrane, whose physical properties are enhanced to a certain extent. And due to the serious phase separation of the two polymers, the internal structure is loose, so that the membrane has a certain water flux even under low pressure; moreover, the prepared blended asymmetric nanofiltration membrane has a high rejection rate and has good rejection performance for neutral organic small molecule chlorophenol DBPs with different structures; generally speaking, the blended asymmetric nanofiltration membrane prepared by the present invention has good physical properties, high rejection rate, and has a certain water flux under low pressure conditions; it has a good rejection effect on chlorophenol DBPs with small diameters and no charges in water; it provides a new nanofiltration membrane material for water resource purification and protection, and also provides certain reference value for the construction of highly selective nanofiltration membranes. Description of the Drawings

[0015] Figure 1 It is the planar scanning electron micrograph of the PMMA / PEG blended asymmetric nanofiltration membrane in Examples 1-5; Figure 2 It is the cross-sectional scanning electron micrograph of the PMMA / PEG blended asymmetric nanofiltration membrane in Examples 1-5; Figure 3 It is the partial enlarged cross-sectional scanning electron micrograph of the PMMA / PEG blended asymmetric nanofiltration membrane in Example 3; Figure 4 It is the standard ultraviolet absorption curve of 4-phenol, 2,4-dichlorophenol, 2,6-dichlorophenol and 2,4,6-trichlorophenol; Figure 5 It is the ultraviolet absorbance diagram of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios before and after the rejection of 4-phenol, and the double Y diagram of the water flux and rejection rate of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios for 4-phenol; Figure 6 It is the ultraviolet absorbance diagram of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios before and after the rejection of 2,4-dichlorophenol, and the double Y diagram of the water flux and rejection rate of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios for 2,4-dichlorophenol; Figure 7 It is the ultraviolet absorbance diagram of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios before and after the rejection of 2,6-dichlorophenol, and the double Y diagram of the water flux and rejection rate of the PMMA / PEG with different ratios for 2,6-dichlorophenol; Figure 8 It is the ultraviolet absorbance diagram of the PMMA / PEG blended asymmetric nanofiltration membrane with different ratios before and after the rejection of 2,4,6-trichlorophenol, and the double Y diagram of the water flux and rejection rate of the PMMA / PEG with different ratios for 2,4,6-trichlorophenol; Figure 9 I-V current diagrams for Examples 1-5; Figure 10 I-V current diagrams for Examples 2-3; Figure 11 I-V current diagrams for Examples 1-2. Detailed implementation manners

[0016] The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

[0017] The reagents used in Examples 1-5 of the present invention are as follows: Polymethylmethacrylate (PMMA) with an average molecular weight Mw = 200000, Anyij Chemical (China); Polyvinylalcohol (PEG) with a polymerization degree of 600, Sinopharm Chemical Reagent Co., Ltd.; N,N-dimethylformamide, analytical grade, Sinopharm Chemical Reagent Co., Ltd.; 4-chlorophenol, 98% (4-chlorophenol), Anyij Chemical (China); 2,4-dichlorophenol, 98% (2,4-Dichlorophenol), Anyij Chemical (China); 2,6-dichlorophenol 99% (2,6-Dichlorophenol), Anyij Chemical (China); 2,4,6-trichlorophenol, 97% (2,4,6-trichlorophenol), Anyij Chemical (China).

[0018] The prepared blend asymmetric nanofiltration membrane of the present invention is used for the separation and removal of chlorophenol DBPs in water. The separation and removal performance of the nanofiltration membrane for chlorophenol DBPs is represented by the rejection rate. The rejection rate and water flux are two important parameters for evaluating the blend asymmetric nanofiltration membrane. The parameters are obtained by testing and calculating the nanofiltration performance in the way of cross-flow filtration. A raw liquid with a concentration of 0.6 mmol / L of chlorophenol DBPs is configured, and the filtration performance is tested by a peristaltic pump on a blend asymmetric nanofiltration membrane with a diameter of 25 mm under a certain external pressure. The time and volume when the blend asymmetric nanofiltration membrane permeates the permeate are respectively recorded, and when the working pressure is stable, the pressure of the membrane at this time is recorded, the water flux is calculated, and its average value is taken. The rejection rate and water flux are obtained by formula calculation.

[0019] Among them, the rejection rate is defined as:

[0020] Among them,C f represents the concentration of chlorophenol DBPs in water before passing through the membrane; C p represents the concentration of chlorophenol DBPs in the solution after passing through the membrane. The rejection rate can be calculated by substituting the concentrations of chlorophenol DBPs before and after retention into the formula.

[0021] The water flux is defined as: under certain operating pressure conditions, the volume of water passing through a unit membrane area per unit time, and its unit is L / (h·m 2 ·bar). The formula is:

[0022] where V represents the volume of the permeated solution, with the unit of L; A represents the effective membrane area, with the unit of m 2 ; T represents the time, with the unit of h; P represents the operating pressure, with the unit of bar.

[0023] Example 1 (1) First, weigh 1.8 g of polymethyl methacrylate and 0.2 g of polyethylene glycol (PMMA / PEG is 9:1) respectively with an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare a 10 mL casting solution with a solid content of 20%; during this process, use a 10 mL pipette to transfer.

[0024] After weighing, put it into a 50 mL round-bottom flask (reaction vessel), add a magnetic stirrer bar, then place it in a constant temperature oil bath at 95 °C and heat with a magnetic stirrer for 5 h (rotation speed 1200 r / min), and set up a condensation reflux device to prevent the volatilization of N,N-dimethylformamide.

[0025] (3) After stirring, let it stand for 1 h to remove bubbles, transfer it to a silicon plate with a 1 mL pipette, and use a scraper to scrape a membrane with a thickness of 50 μm. Let the membrane on the silicon plate stand in a fume hood for 10 min. During the process of solvent volatilization, make the membrane grow branches to form a denser blend asymmetric nanofiltration membrane.

[0026] (4) Immerse the silicon plate completely in deionized water to remove the organic solvent. As the solvent is removed, the organic polymer gradually solidifies the membrane. And soak it in deionized water for 12 h to completely remove the organic solvent, forming a more stable organic polymer blend asymmetric nanofiltration membrane.

[0027] (5) Take out the blend asymmetric nanofiltration membrane in deionized water, air-dry it naturally, then use a 25 mm membrane cutter to cut it to the appropriate size, and flatten it for use. A blend asymmetric nanofiltration membrane of polymethyl methacrylate and polyethylene glycol is prepared.

[0028] Example 2 First, weigh 1.7 g of polymethyl methacrylate and 0.3 g of polyethylene glycol (PMMA / PEG is 8.5:1.5) respectively with an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare a 10 mL casting solution with a solid content of 20%; during this process, use a 10 mL pipette to transfer.

[0029] The remaining steps are the same as those in Example 1.

[0030] Example 3 First, weigh 1.6 g of polymethyl methacrylate and 0.4 g of polyethylene glycol (PMMA / PEG is 8:2) respectively with an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare a 10 mL casting solution with a solid content of 20%; during this process, use a 10 mL pipette to transfer.

[0031] The remaining steps are the same as those in Example 1.

[0032] Example 4 First, weigh 1.5 g of polymethyl methacrylate and 0.5 g of polyethylene glycol (PMMA / PEG is 7.5:2.5) respectively with an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare a 10 mL casting solution with a solid content of 20%; during this process, use a 10 mL pipette to transfer.

[0033] The remaining steps are the same as those in Example 1.

[0034] Example 5 First, weigh 1.4 g of polymethyl methacrylate and 0.6 g of polyethylene glycol (PMMA / PEG is 7:3) respectively with an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare a 10 mL casting solution with a solid content of 20%; during this process, use a 10 mL pipette to transfer.

[0035] The remaining steps are the same as those in Example 1.

[0036] Figure 1It is the scanning electron microscopy image of the surface of the PMMA and PEG blended asymmetric nanofiltration membrane in Examples 1-5; in Figure (a) (Example 1), the surface pore size of the membrane is the smallest and the most dense; in Figure (b) (Example 2), the generated pores are the largest and the most numerous. This may be because when standing and growing after film scraping, since the surface organic solvent is easy to volatilize, the precipitation rate of the polymer is faster, generating more physical crosslinks to form a dense layer. While at the bottom, due to less volatilization of the organic solvent, less physical crosslinking of the polymer occurs in the solvent, thus forming larger pore sizes; Figure (b) (Example 2) should be the loose layer of the membrane, that is, the bottom surface of the membrane has larger pore sizes and porosity; in Figure (c) (Example 3), the pore size on the membrane surface is small, the porosity is high, and the pore size distribution is dispersed and the pore sizes are uniform; in Figure (d) (Example 4), the pore size is similar to that in Figure (c) (Example 3), but the porosity is less, the pore sizes are not uniform, and cracks are generated. From this, it can also be seen that the stability of the membrane is not high and it is easy to rupture; in Figure (e) (Example 5), obvious cracks can be observed, as well as the agglomeration of PMMA and PEG respectively, enriching a large number of PMMA particles on the surface. Due to the obvious increase in the interaction force between PEG and PMMA with the increase of PEG, finally, respective micelles are formed. In short, with the increase of PEG content, it will directly lead to an obvious increase in pore size and a decrease in porosity; cracks will occur in the nanofiltration membrane, resulting in a reduction in the physical properties and stability of the membrane.

[0037] Figure 2 It is the scanning electron microscopy image of the cross-section of the PMMA / PEG blended asymmetric nanofiltration membrane in Examples 1-5; Figure (a) (Example 1) shows that the PMMA / PEG = 9:1 blended asymmetric nanofiltration membrane has obvious finger-like pores; in Figure (b) (Example 2) and Figure (c) (Example 3), obvious honeycomb-like pores can be seen. From the pore size, the pore size in Figure (b) (Example 2) is smaller than that in Figure c (Example 3), and the pore size distribution is also more uniform; Figure (d) (Example 4) and Figure (e) (Example 5) have more obvious pore structures. This is because with the increase of PEG content, the internal hydroxyl group effect increases, resulting in serious phase separation, leading to larger pores appearing in the membrane. When the pore size gradually becomes larger, the rejection rate decreases and the water flux increases.

[0038] Figure 3 It is the scanning electron microscopy image of the cross-section of the PMMA / PEG = 8:2 blended asymmetric nanofiltration membrane in Example 3. An obvious asymmetric structure can be seen; in the fault cross-section diagram in Figure (a), it can be clearly seen that the upper surface layer of the membrane is a dense layer and the lower surface layer is a loose layer. From the volume distribution, the proportion of the dense layer is very small, but it plays a key role in improving the rejection rate. While most of the loose layer provides a supporting role and also increases the water flux of the blended asymmetric nanofiltration membrane, which is beneficial for low-pressure filtration. Figure (b) is the dense layer; Figure (c) is the transition layer between the dense layer and the loose layer, which can also be called the intermediate layer; Figure (d) is the loose and porous layer.

[0039] Test Example 1 Perform a test on the rejection rate of chlorophenol DBPs for Example 1. Use 4-chlorophenol, 2,4-dichlorophenol, 2,6-dichlorophenol, and 2,4,6-trichlorophenol as test samples respectively. Draw a standard curve equation by measuring the ultraviolet absorbance at different concentrations, obtain the corresponding relationship between concentration and absorbance, further measure the absorbance of the molecular solution before and after passing through the membrane, obtain the concentration of the molecular solution, and thus calculate the rejection rate before and after passing through the membrane; the standard curve is as Figure 4 shown. Standard curves drawn from the ultraviolet absorbance of solutions with different concentrations of 4-chlorophenol (a), 2,4-dichlorophenol (b), 2,6-dichlorophenol (c), and 2,4,6-trichlorophenol (d). From the R 2 value of the fitting and the distribution of the regression curve points, it can be seen that the fitting effect is good, and the calculated rejection rate value is also relatively accurate.

[0040] Test Example 2 Perform a test on the water flux of Examples 1 to 5. Record the volume of the solution passing through the membrane, the time of passing through the membrane, and the pressure under the same membrane area, and finally calculate the water flux as Figures 5 - 8 shown.

[0041] Figure 5 It is the ultraviolet absorbance diagram of PMMA / PEG blend asymmetric nanofiltration membrane before and after the rejection of 4-chlorophenol, and the double Y diagram of the water flux and rejection rate of PMMA / PEG blend asymmetric nanofiltration membrane for 4-chlorophenol; Figure (a) is the ultraviolet absorbance diagram of 4-chlorophenol before and after rejection in Example 1, Figure (b) is the 4-chlorophenol rejection diagram in Example 2, Figure (c) is the ultraviolet absorbance diagram of 4-chlorophenol before and after rejection in Example 3, Figure (d) is the ultraviolet absorbance diagram of 4-chlorophenol before and after rejection in Example 4, Figure (e) is the ultraviolet absorbance diagram of 4-chlorophenol before and after rejection in Example 5, Figure (f) is the double Y diagram of the water flux and rejection rate of nanofiltration membranes of Examples 1 to 5 for 4-chlorophenol; it can be seen from it that the rejection rate of the blend asymmetric nanofiltration membrane of Example 3 (PMMA / PEG is 8:2) for 4-chlorophenol is 76.75%, and the water flux is 1.23 L / (h·m 2 ·bar), and, with the increase of the PEG content, the water flux increases significantly.

[0042] Figure 6UV absorbance diagrams of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after the retention of 2,4-dichlorophenol, and double Y diagrams of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,4-dichlorophenol; Figure (a) is the UV absorbance diagram of 2,4-dichlorophenol before and after retention in Example 1, Figure (b) is the UV absorbance diagram of 2,4-dichlorophenol before and after retention in Example 2, Figure (c) is the UV absorbance diagram of 2,4-dichlorophenol before and after retention in Example 3, Figure (d) is the UV absorbance diagram of 2,4-dichlorophenol before and after retention in Example 4, Figure (e) is the UV absorbance diagram of 2,4-dichlorophenol before and after retention in Example 5, Figure (f) is the double Y diagram of the water flux and retention rate of the nanofiltration membranes of Examples 1 to 5 for 2,4-dichlorophenol; It can be seen from the figure that the retention effects of Examples 1, 2, and 3 are better, and preferably Example 3 (solid content 8:2). As shown in Figure (c), the retention rate of the nanofiltration membrane is 91.70%, and the water flux is 1.52 L / (h·m 2 ·bar).

[0043] Figure 7 UV absorbance diagrams of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after the retention of 2,6-dichlorophenol, and double Y diagrams of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,6-dichlorophenol; Figure (a) is the UV absorbance diagram of 2,6-dichlorophenol before and after retention in Example 1, Figure (b) is the UV absorbance diagram of 2,6-dichlorophenol before and after retention in Example 2, Figure (c) is the UV absorbance diagram of 2,6-dichlorophenol before and after retention in Example 3, Figure (d) is the UV absorbance diagram of 2,6-dichlorophenol before and after retention in Example 4, Figure (e) is the UV absorbance diagram of 2,6-dichlorophenol before and after retention in Example 5; Figure (f) is the double Y diagram of the water flux and retention rate of the nanofiltration membranes of Examples 1 to 5 for 2,6-dichlorophenol; Preferably Example 3. As shown in Figure (c), the retention rate of the nanofiltration membrane is 88.15%, and the water flux is 1.24 L / (h·m 2 ·bar).

[0044] Figure 8UV absorbance diagrams of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after the retention of 2,4,6-trichlorophenol, and double Y diagrams of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,4,6-trichlorophenol; Figure (a) is the UV absorbance diagram of 2,4,6-trichlorophenol before and after retention in Example 1, Figure (b) is the UV absorbance diagram of 2,4,6-trichlorophenol before and after retention in Example 2, Figure (c) is the UV absorbance diagram of 2,4,6-trichlorophenol before and after retention in Example 3, Figure (d) is the UV absorbance diagram of 2,4,6-trichlorophenol before and after retention in Example 4, Figure (e) is the UV absorbance diagram of 2,4,6-trichlorophenol before and after retention in Example 5, Figure (f) is the double Y diagram of the water flux and retention rate of the nanofiltration membranes of Examples 1-5 for 2,4,6-trichlorophenol; Preferably Example 3, as shown in Figure (c), the retention rate of the membrane is 88.36%, and the water flux is 1.03 L / (h·m 2 ·bar).

[0045] It can be seen from Figures 5 - 8 that the nanofiltration membrane prepared by the present invention has the best retention effect on 2,4-dichlorophenol, and from the numerical value of the retention rate, Example 3 has the highest retention rate and the most obvious effect; it can be obtained that the blend asymmetric nanofiltration membrane with a solid content ratio of 8:2 has the best retention rate.

[0046] Analysis shows that: as the content of polyethylene glycol increases, the water flux of the membrane increases significantly, but since the channel pore size formed after phase separation also increases, the retention rate decreases.

[0047] Test Example 3 The current pore size of Examples 1-5 was tested with a picoammeter. The pore size of the nanofiltration membrane can be preliminarily judged by the change in the current of the picoammeter; an AC voltage of -2 V to 2 V was used, and a 0.1 mol / L potassium chloride solution was used as the electrolyte; Examples 1-5 were tested, and the current values with the change of voltage were recorded. Five groups of data were taken and averaged. As Figure 9 shown, it is the I-V current diagram of Examples 1-5. It was found that the current values of Examples 2 and 3 were the smallest, which also confirmed that their channel pore sizes were smaller, the ion conduction performance was poor, and the water flux was lower; while the current values of Examples 4 and 5 were larger, indicating that the channel pore sizes of the nanofiltration membranes were larger, the ion conduction performance was good, and the water flux was high.

[0048] Comparative Example 1 The implementation conditions were the same as the steps of Test Example 3. As Figure 10 shown, it is the I-V current diagram of Examples 2-3. By comparison, it can be obtained that the current of Example 3 is larger, indicating that the porosity of the nanofiltration membrane of Example 3 is larger than that of Example 2; from Figure 2It can be seen that Example 3 has a thicker dense layer, so that the rejection of Example 3 is slightly higher than that of Example 2.

[0049] Comparative Example 2 The implementation conditions are the same as those in Step of Test Example 3. As Figure 11 shown, it is the I-V current diagram of Examples 1-2. By comparison, it can be obtained that the current of Example 1 is larger, indicating that the pore size of Example 1 is still larger than that of Example 2, mainly due to the existence of finger holes.

[0050] Therefore, through the comprehensive analysis of the rejection rate and water flux, Example 3 is the optimal condition group, with the highest rejection rate and good water flux, meeting the condition requirements of high-quality nanofiltration membranes; under the best conditions, the rejection rate of Example 3 for 2,4-dichlorophenol can reach 91.71%, and the water flux can reach 1.52 L / (h·m 2 ·Bar). It can be seen that the membrane has excellent performance and high rejection rate.

[0051] In the described Figures 9 - 11 , the corresponding relationship with Examples 1-5 is expressed by marking the ratio relationship between polymethyl methacrylate (PMMA) and polyethylene glycol (PEG).

[0052] Therefore, the optimal choice is that Example 3 is the condition for preparing the blend asymmetric nanofiltration membrane of polymethyl methacrylate and polyethylene glycol. At 95 °C, the solid content ratio of polymethyl methacrylate and polyethylene glycol is 8:2; the casting solution with a solid content of 20% is the best. And it is found in the research that for the blend asymmetric nanofiltration membrane, the dense layer structure greatly improves the rejection rate; the loose layer increases the water flux, which is beneficial to low-pressure filtration. The prepared blend asymmetric nanofiltration membrane can be driven for rejection under a pressure of 2 bar.

Claims

1. A polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane, characterized in that: Two polymers, polymethyl methacrylate and polyethylene glycol, are dissolved in N,N-dimethylformamide solvent by stirring to form a casting solution, and a nanofiltration membrane of a specific thickness is prepared by a scraper. During the membrane preparation process, polymethyl methacrylate is the main supporting material, and polyethylene glycol is a porogen and a hydrophilic material; then, the prepared nanofiltration membrane is placed in a fume hood and allowed to stand. Based on the different evaporation rates of the solvents on the upper and bottom layers of the nanofiltration membrane, a blended asymmetric nanofiltration membrane with a dense structure is formed. The blended asymmetric nanofiltration membrane is completely immersed in deionized water, and the solvent is removed by a phase transfer method, thereby solidifying the membrane.

2. The polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane according to claim 1, characterized in that: The specific preparation steps are as follows: (1) Based on a solution having a solid content of 10-30%, an N,N-dimethylformamide solution is prepared, and polymethyl methacrylate and polyethylene glycol are weighed in proportion, wherein the mass ratio of polymethyl methacrylate to polyethylene glycol is (7-9): (1-3); then, the polymethyl methacrylate and polyethylene glycol are added to the N,N-dimethylformamide solution to prepare a casting solution having a solid content of 10-30%; (2) Add casting solution into the reaction vessel; in a constant temperature oil bath at 90-105°C, Stir for 5-8 h, and set up a condensation reflux device to prevent N,N-dimethylformamide from volatilizing, so that polymethyl methacrylate and polyethylene glycol are fully dissolved in the N,N-dimethylformamide solution; (3) The solution obtained in step (2) is allowed to stand at a temperature of 75-85° C. and degassed for 0.5-1.5 h; the degassed solution is transferred onto a silicon plate using a pipette, and a blended membrane having a thickness of 10-100 μm is prepared on the silicon plate using a scraper, and the blended membrane on the silicon plate is allowed to stand in a fume hood for 10-30 min. The solvent evaporates quickly on the surface to form a dense structure, while the lower surface is close to the glass and the solvent evaporates slowly, thereby continuing to retain a loose and porous structure, thereby forming a blended asymmetric nanofiltration membrane having an asymmetric structure; (4) completely immersing the silicon plate with the blended asymmetric nanofiltration membrane in deionized water, and gradually dissolving the solvent in the aqueous solution through a phase inversion method, removing the solvent, and solidifying the blended asymmetric nanofiltration membrane; (5) Take out the blended asymmetric nanofiltration membrane from the deionized water, air dry it naturally, and then cut and flatten it to a suitable size for use.

3. The polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane according to claim 2, characterized in that: The blended asymmetric nanofiltration membrane is a nanofiltration membrane with a dense upper surface layer and a loose lower surface layer.

4. The polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane according to claim 2, characterized in that: In step (1), the mass ratio of polymethyl methacrylate to polyethylene glycol is 8:2; adding polymethyl methacrylate and polyethylene glycol to N,N-dimethylformamide solution to prepare a casting solution with a solid content of 20% is the optimal condition; In step (2), stirring at 95°C for 5 h is the optimal condition; In step (3), the mixture was allowed to stand and degas at 80°C for 1 h; the degassing solution was transferred onto a silicon plate using a pipette, and a blended film with a thickness of 50 μm was prepared on the silicon plate using a scraper, and the blended film on the silicon plate was allowed to stand in a fume hood for 10 min; In step (4), the membrane is immersed in deionized water for 12 h. The solvent is gradually dissolved in the aqueous solution by a phase inversion method, and the solvent is removed to solidify the asymmetric nanofiltration membrane.

5. The use of the blended asymmetric nanofiltration membrane prepared according to any one of claims 1, 2 or 4, characterized in that: The blended asymmetric nanofiltration membrane can highly selectively separate and remove neutral chlorinated phenol disinfection by-products with a molecular weight of ≤200 in water.

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