A poly(methyl methacrylate) and poly(ethylene glycol) blended asymmetric nanofiltration membrane and application thereof

Abstract

The application provides a kind of poly (methyl methacrylate) and polyethylene glycol blend asymmetric nanofiltration membrane and application, the poly (methyl methacrylate) and polyethylene glycol blend asymmetric nanofiltration membrane, poly (methyl methacrylate) (PMMA) and polyethylene glycol (PEG) two polymers are stirred and dissolved in N, N-dimethylformamide solvent to form casting solution, the nanofiltration membrane of specific thickness is prepared by doctor blade, then, the prepared nanofiltration membrane is placed in fume hood and stands, based on the different solvent evaporation rates of upper layer and bottom of nanofiltration membrane, form a blend asymmetric nanofiltration membrane with dense structure, immerse the blend asymmetric nanofiltration membrane in deionized water, remove the solvent by phase transfer method, and then solidify the membrane; in the process of membrane preparation, poly (methyl methacrylate) is the main support material, polyethylene glycol is the pore former and hydrophilic material, and the membrane realizes efficient separation and removal of neutral chlorophenol disinfection by-products with molecular weight ≤200 in water.

Description

Technical Field

[0001] This invention belongs to the field of nanofiltration membranes and their preparation, the field of nanofiltration membranes in water treatment, and the field of membrane separation mechanism research; it studies the transfer process of solutes and solvents in membranes; and explores separation mechanisms such as size sieving and charge effects. Specifically, it relates to a polymethyl methacrylate / polyethylene glycol blend asymmetric nanofiltration membrane and its preparation method. Background Technology

[0002] Due to the prevalence of viruses such as H1N1 influenza, the use of disinfectants in drinking water remains high, leading to the large-scale generation of many harmful disinfection byproducts (DBPs). The presence of DBPs in drinking water has been proven to be neurotoxic and closely related to bladder cancer, colon cancer, and rectal cancer. The World Health Organization has strict control standards for aliphatic disinfection byproducts. However, chlorophenols are a new type of aromatic DBP with a molecular weight ≤200 and no charge. Although their concentration in water is low, their toxicity is nearly a thousand times higher than common aliphatic DBPs, posing a serious threat to drinking water quality safety. Membrane separation technology has advantages such as simple operation, easy scale-up, low energy consumption, stable operation, and no chemical consumption. Its rapid development provides a simple and rapid method for the separation and removal of organic pollutants in the aquatic environment. Among these, nanofiltration membranes have a long-term stable effect on the separation and removal of organic pollutants and have advantages such as high flux, good selectivity, and low energy consumption compared to other membrane separation methods. In wastewater treatment systems, separation methods based on nanofiltration membranes have always been a research hotspot at the intersection of environmental science, materials science, and chemistry.

[0003] The separation and removal mechanism of nanofiltration membranes mainly includes the following two aspects:

[0004] 1. Pore size sieving: Nanofiltration membranes have a certain pore size distribution. When fluid passes through the membrane, particles and molecules larger than the membrane pore size will be trapped, while substances smaller than the membrane pore size can pass through the membrane, just like a sieve sieving particles of different sizes.

[0005] 2. Charge Repulsion: Nanofiltration membranes typically carry a certain charge on their surface, due to the inherent properties of the membrane material or functional groups introduced during preparation. Ions and charged particles in the solution interact with the charge on the membrane surface. Like charges repel, making it difficult for ions and charged particles with the same charge as the nanofiltration membrane surface to approach the membrane surface, thus resulting in their retention.

[0006] For neutral chlorophenol disinfection byproducts with a molecular weight ≤200, their small molecular size means that while conventional nanofiltration membrane systems are effective at separating and removing organic pollutants with a molecular weight greater than 200, their performance is limited for those with a molecular weight ≤200. Furthermore, chlorophenols are uncharged and exist as neutral molecules in aqueous solutions, limiting the ability to separate and remove them based on charge repulsion. For example, the commercial nanofiltration membrane NF-90 has a rejection rate of less than 40% for neutral small organic molecules with a molecular weight ≤200, significantly limiting its practical application in the separation and removal of pollutants in water. Therefore, constructing highly selective nanofiltration membranes to achieve efficient separation and removal of neutral chlorophenol disinfection byproducts with a molecular weight ≤200 in aquatic environments is a highly challenging task. Summary of the Invention

[0007] This invention proposes a method for preparing and applying a polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane. The nanofiltration membrane has the characteristics of small pore size, high porosity, and high rejection rate, and can effectively separate and remove neutral chlorophenol disinfection byproducts (DBPs) with a molecular weight ≤200 in the water environment. It overcomes the problem that nanofiltration membranes are difficult to efficiently separate neutral small molecule organic pollutants, and can achieve a very good separation and removal effect on neutral chlorophenol DBPs.

[0008] Based on this, the present invention adopts the following technical solution: a method for preparing a polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane, characterized in that 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 membrane scraper. During the membrane preparation process, polymethyl methacrylate is used as the main supporting material, and polyethylene glycol is used as a pore-forming agent and a hydrophilic material. Then, the prepared nanofiltration membrane is placed in a fume hood and left to stand. Based on the different solvent evaporation rates of the upper and lower layers 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 a phase transfer method, thereby solidifying the membrane.

[0009] The aforementioned polymethyl methacrylate and polyethylene glycol blend asymmetric nanofiltration membrane is prepared through the following steps:

[0010] (1) Based on the calculation of preparing a solution with a solid content of 10-30%, prepare N,N-dimethylformamide solution, weigh polymethyl methacrylate and polyethylene glycol in proportion, with the mass ratio of polymethyl methacrylate to polyethylene glycol being (7-9):(1-3); then add polymethyl methacrylate and polyethylene glycol to N,N-dimethylformamide solution to prepare a casting solution with a solid content of 10-30%;

[0011] (2) Add casting solution to the reaction vessel; in a constant temperature oil bath, at a temperature of 90-105℃,

[0012] Stir for 5-8 hours and set up a reflux condenser to prevent N,N-dimethylformamide from volatilizing, so that polymethyl methacrylate and polyethylene glycol are fully dissolved in the N,N-dimethylformamide solution;

[0013] (3) The solution obtained in step (2) is allowed to stand and degas at 75-85℃ for 0.5-1.5 h; the degassed solution is transferred to a silicon plate using a pipette, and a blend membrane with a thickness of 10-100 μm is prepared on the silicon plate using a scraper. The blend membrane on the silicon plate is allowed to stand in a fume hood for 10-30 min. The solvent evaporates quickly on the surface, forming a dense structure. The lower surface layer is in close contact with the glass, and the solvent evaporates slowly, thus retaining a loose and porous structure and forming a blend nanofiltration membrane with an asymmetric structure.

[0014] (4) The silicon plate with the blended asymmetric nanofiltration membrane is completely immersed in deionized water and soaked in deionized water for 8-15 h. The solvent is gradually dissolved in the aqueous solution by phase transfer method, the solvent is removed, and the asymmetric nanofiltration membrane is solidified.

[0015] (5) Take out the deionized water co-blended asymmetric nanofiltration membrane, let it air dry naturally, and then cut and flatten it to the appropriate size for use.

[0016] In step (1), the solid content of the casting solution was studied and analyzed. A high polymer content in the casting solution easily leads to gelation and prevents film formation. A low polymer content results in less cross-linking between polymer molecules, leading to poor membrane performance, fragility, and poor mechanical properties. Through comparison, it was found that the best performance of the blended asymmetric nanofiltration membrane was obtained when the casting solution content was 20%. As for the membrane forming temperature, we selected oil baths at 75℃, 85℃, 95℃, and 105℃ to prepare the casting solution. It was found that polymethyl methacrylate was not fully dissolved at 75℃ and 85℃, and the membrane forming performance was not as good as at 95℃ and 105℃. However, considering that too high a temperature may lead to gelation, we selected 95℃ as the optimal membrane forming temperature.

[0017] In step (2), during the heated stirring process, the temperature affects the diffusion resistance and diffusion rate of the monomers in the solution. As the heat treatment temperature increases, the water flux of the blended asymmetric nanofiltration membrane decreases, while the retention first increases and then decreases. The heat stirring time affects the pore size shrinkage and cross-linking degree of the blended asymmetric nanofiltration membrane separation layer. As the heat treatment time increases, the retention of the prepared blended asymmetric nanofiltration membrane first increases and then decreases. Finally, the optimal stirring time was determined to be 5 h.

[0018] In step (3), when using a membrane scraper to form a membrane, the membrane thickness directly affects the retention performance of the blended asymmetric nanofiltration membrane. The thicker the membrane, the better the retention rate, the lower the water flux, and the better the membrane toughness. The thinner the membrane, the lower the retention rate, the higher the water flux, but the worse the mechanical properties, and the easier it is to break under the pressure of the pump. This patent prepared blended asymmetric nanofiltration membranes with thicknesses of 10, 25, 50, 75, and 100 μm, respectively. It was found that the blended asymmetric nanofiltration membranes with thicknesses of 10 and 25 μm were easily broken under the working pressure of the pump filtration, and the retention rate was not as good as that of the blended asymmetric nanofiltration membrane with a thickness of 50 μm. When the membrane thickness was 50, 75, and 100 μm, the retention rate of the blended asymmetric nanofiltration membrane did not increase significantly, and the water flux even decreased. Therefore, considering the balance between rejection rate and water flux, as well as working conditions and cost, a membrane thickness of 50 μm is considered most suitable, as it has the best rejection rate and water flux, while also exhibiting good mechanical properties.

[0019] In step (4), the phase transfer method is an efficient and convenient film-forming method. It mainly utilizes two immiscible solvents (such as an aqueous phase and an organic phase) to transfer the target substance from one phase to another through interfacial reaction or diffusion, ultimately forming a thin film on the substrate. Its advantages are that it can prepare ultrathin and uniform organic polymer films, and it has good controllability. By adjusting the film-forming conditions, the thickness and properties of the organic polymer film can be precisely controlled.

[0020] 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 N,N-dimethylformamide solution to prepare a casting solution with a solid content of 20%; in step (2), the mixture is stirred at 95°C for 5 h; in step (3), the mixture is allowed to stand and degas at 80°C for 1 h; the degassed solution is transferred to a silicon plate using a pipette, and a blended asymmetric nanofiltration membrane with a thickness of 50 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 min; in step (4), the membrane is soaked in deionized water for 12 h, and the solvent is gradually dissolved in the aqueous solution by phase inversion method, the solvent is removed, and the asymmetric nanofiltration membrane is solidified.

[0021] The aforementioned polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane can selectively separate and remove neutral chlorophenol disinfection byproducts with a molecular weight ≤200 in water.

[0022] The beneficial effects of this invention are as follows: Compared with the performance of commercially available nanofiltration membranes, the blended asymmetric nanofiltration membrane prepared by this invention using PMMA and PEG organic polymers exhibits enhanced physical properties. Furthermore, the severe phase separation of the two polymers results in a loose internal structure, allowing the membrane to maintain a certain water flux even at lower pressures. The prepared blended asymmetric nanofiltration membrane also boasts a high rejection rate and good retention performance for neutral small organic chlorophenol DBPs with different structures. Overall, the blended asymmetric nanofiltration membrane prepared by this invention possesses good physical properties, a high rejection rate, and a certain water flux under low pressure conditions. It also demonstrates excellent retention of uncharged, small-diameter chlorophenol DBPs in water. This invention provides a new nanofiltration membrane material for water purification and protection, and offers valuable reference for the construction of highly selective nanofiltration membranes. Attached Figure Description

[0023] Figure 1 The images shown are scanning electron microscope (SEM) images of the PMMA / PEG blend asymmetric nanofiltration membranes from Examples 1-5.

[0024] Figure 2 The images show cross-sectional scanning electron microscope (SEM) images of the PMMA / PEG blend asymmetric nanofiltration membranes from Examples 1-5.

[0025] Figure 3 This is a magnified scanning electron microscope image of a cross-section of the PMMA / PEG blend asymmetric nanofiltration membrane in Example 3. Figure 4 Standard UV absorption curves for 4-phenol, 2,4-dichlorophenol, 2,6-dichlorophenol, and 2,4,6-trichlorophenol;

[0026] Figure 5 UV absorbance plots of PMMA / PEG blended asymmetric nanofiltration membranes before and after 4-phenol retention, and double Y plots of water flux and retention rate of 4-phenol for blended asymmetric nanofiltration membranes with different ratios.

[0027] Figure 6 UV absorbance plots of PMMA / PEG blended asymmetric nanofiltration membranes before and after 2,4-dichlorophenol retention, and double Y plots of water flux and retention rate of PMMA / PEG blended asymmetric nanofiltration membranes for 2,4-dichlorophenol.

[0028] Figure 7 UV absorbance spectra of PMMA / PEG blended asymmetric nanofiltration membranes before and after 2,6-dichlorophenol retention, and double Y plots of water flux and retention rate of PMMA / PEG for 2,6-dichlorophenol at different ratios.

[0029] Figure 8UV absorbance plots of PMMA / PEG blended asymmetric nanofiltration membranes before and after 2,4,6-trichlorophenol rejection, and double Y plots of water flux and rejection rate of PMMA / PEG for 2,4,6-trichlorophenol for different ratios.

[0030] Figure 9 The IV current diagrams for Examples 1-5 are shown.

[0031] Figure 10 The IV current diagrams for Examples 2 and 3 are shown.

[0032] Figure 11 The IV current diagrams are from Examples 1 and 2. Detailed Implementation

[0033] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0034] The reagents used in Examples 1-5 of this invention are as follows: Polymethyl methacrylate (PMMA) with an average molecular weight (Mw) of 200,000, from Anaiji Chemical (China); Polyvinyl alcohol (PEG) with a degree of polymerization of 600, from Sinopharm Chemical Reagent Co., Ltd.; N,N-dimethylformamide, analytical grade, from Sinopharm Chemical Pharmaceutical Co., Ltd.; 4-chlorophenol, 98%, from Anaiji Chemical (China); 2,4-dichlorophenol, 98%, from Anaiji Chemical (China); 2,6-dichlorophenol, 99%, from Anaiji Chemical (China); 2,4,6-trichlorophenol, 97%, from Anaiji Chemical (China).

[0035] The blended asymmetric nanofiltration membrane prepared in this 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, and the rejection rate and water flux are two important parameters for evaluating the blended asymmetric nanofiltration membrane. These parameters are obtained by testing and calculating the nanofiltration performance using a cross-flow filtration method. A feed solution with a chlorophenol DBP concentration of 0.6 mmol / L was prepared, and the filtration performance was tested on a 25 mm diameter blended asymmetric nanofiltration membrane under a certain external pressure using a peristaltic pump. The time and volume of permeate through the blended asymmetric nanofiltration membrane were recorded, and the membrane pressure was recorded when the operating pressure was stable. The water flux was calculated, and the average value was taken. The rejection rate and water flux were then calculated using formulas.

[0036] The retention rate is defined as follows:

[0037]

[0038] in, C f This indicates the concentration of chlorophenol DBPs in the water before membrane treatment; C p This indicates the concentration of chlorophenol DBPs in the solution after membrane filtration. The rejection rate can be calculated by substituting the concentrations of chlorophenol DBPs before and after rejection into the formula.

[0039] Water flux is defined as the volume of water passing through a unit membrane area per unit time under a certain operating pressure, and its unit is L / (h·m²). 2 The formula for bar is:

[0040]

[0041] Where V represents the volume of the permeated solution, in L; and A represents the effective membrane area, in m². 2 T represents time, in hours (h); P represents operating pressure, in bars (bar).

[0042] Example 1

[0043] (1) First, weigh 1.8 g of polymethyl methacrylate and 0.2 g of polyethylene glycol (PMMA / PEG ratio 9:1) using an electronic analytical balance, measure 8.4 mL of N,N-dimethylformamide solution, and prepare 10 mL of casting solution with a solid content of 20%; transfer the solution using a 10 mL pipette during this process.

[0044] (2) After weighing, put it into a 50 mL round bottom flask (reaction vessel), add a magnetic stir bar, and then heat it in a constant temperature oil bath at 95℃. Stir with a magnetic stirrer for 5 h (1200 r / min), and build a reflux condenser to prevent N,N-dimethylformamide from volatilizing.

[0045] (3) After stirring, let it stand for 1 hour to remove bubbles. Use a 1 mL pipette to transfer the solution onto a silicon plate. 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 minutes. During the solvent evaporation process, the membrane will undergo branching growth to form a denser blended asymmetric nanofiltration membrane.

[0046] (4) The silicon plate is completely immersed in deionized water to remove the organic solvent. As the solvent is removed, the organic polymer gradually solidifies the membrane. The membrane is then soaked in deionized water for 12 hours to completely remove the organic solvent, forming a more stable organic polymer blended asymmetric nanofiltration membrane.

[0047] (5) Remove the blended asymmetric nanofiltration membrane from the deionized water, air dry it naturally, and then cut it to the appropriate size using a 25 mm membrane cutter. Flatten it and keep it for later use. A blended asymmetric nanofiltration membrane of polymethyl methacrylate and polyethylene glycol is prepared.

[0048] Example 2

[0049] First, weigh 1.7 g of polymethyl methacrylate and 0.3 g of polyethylene glycol (PMMA / PEG ratio 8.5:1.5) using an electronic analytical balance. Measure 8.4 mL of N,N-dimethylformamide solution and prepare 10 mL of casting solution with a solid content of 20%. During this process, use a 10 mL pipette to transfer the solution.

[0050] The remaining steps are the same as in Example 1.

[0051] Example 3

[0052] First, weigh 1.6 g of polymethyl methacrylate and 0.4 g of polyethylene glycol (PMMA / PEG ratio 8:2) using an electronic analytical balance. Measure 8.4 mL of N,N-dimethylformamide solution and prepare 10 mL of casting solution with a solid content of 20%. During this process, use a 10 mL pipette to transfer the solution.

[0053] The remaining steps are the same as in Example 1.

[0054] Example 4

[0055] First, weigh 1.5 g of polymethyl methacrylate and 0.5 g of polyethylene glycol (PMMA / PEG ratio 7.5:2.5) using an electronic analytical balance. Measure 8.4 mL of N,N-dimethylformamide solution and prepare 10 mL of casting solution with a solid content of 20%. During this process, use a 10 mL pipette to transfer the solution.

[0056] The remaining steps are the same as in Example 1.

[0057] Example 5

[0058] First, weigh 1.4 g of polymethyl methacrylate and 0.6 g of polyethylene glycol (PMMA / PEG ratio 7:3) using an electronic analytical balance. Measure 8.4 mL of N,N-dimethylformamide solution and prepare 10 mL of casting solution with a solid content of 20%. During this process, transfer the solution using a 10 mL pipette.

[0059] The remaining steps are the same as in Example 1.

[0060] Figure 1 These are scanning electron microscope (SEM) images of the PMMA and PEG blended asymmetric nanofiltration membranes from Examples 1-5. Figure (a) (Example 1) shows the membrane with the smallest and densest surface pores. Figure (b) (Example 2) shows the membrane with the largest and most numerous pores. This is likely because during static growth after membrane scraping, the surface organic solvents readily evaporate, leading to faster polymer precipitation and more physical cross-linking, thus forming a dense layer. Conversely, the bottom layer, with less organic solvent evaporation, results in less physical cross-linking of the polymer in the solvent, leading to larger pores. Figure (b) (Example 2) should be the porous layer of the membrane, i.e., the bottom surface of the membrane. The membranes in Figure (c) (Example 3) have relatively large pore sizes and high porosity; the pores on the membrane surface are small, with high porosity and dispersed pore size distribution, but uniform pore size. Figure (d) (Example 4) has pore sizes similar to Figure (c) (Example 3), but lower porosity, inconsistent pore size, and cracks, indicating low membrane stability and susceptibility to breakage. Figure (e) (Example 5) shows obvious cracks and aggregation of PMMA and PEG, resulting in a large accumulation of PMMA particles on the surface. The increased interaction between PEG and PMMA leads to the formation of individual micelles. In summary, increasing PEG content directly leads to a significant increase in pore size and a decrease in porosity; nanofiltration membranes develop cracks, resulting in reduced physical properties and stability.

[0061] Figure 2The images show cross-sectional scanning electron microscope (SEM) images of the PMMA / PEG blend asymmetric nanofiltration membranes in Examples 1-5. Figure (a) (Example 1) shows that the PMMA / PEG blend asymmetric nanofiltration membrane with a ratio of 9:1 has obvious finger-like pores. Figures (b) (Example 2) and (c) (Example 3) show obvious honeycomb-like pores. In terms of 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. Figures (d) (Example 4) and (e) (Example 5) have more obvious pore structures. This is because as the PEG content increases, the internal hydroxyl group effect increases, resulting in severe phase separation and larger pores appearing in the membrane. As the pore size gradually increases, the rejection rate decreases and the water flux increases.

[0062] Figure 3 The scanning electron microscope (SEM) images of the cross-section of the PMMA / PEG blended asymmetric nanofiltration membrane in Example 3 (8:2) show a distinct asymmetric structure. The tomographic cross-section in Figure (a) clearly shows a dense upper layer and a porous lower layer. While the dense layer constitutes a small proportion in terms of volume distribution, it plays a crucial role in improving the retention rate. The majority of the porous layer provides support and increases the water flux of the blended asymmetric nanofiltration membrane, which is beneficial for low-pressure filtration. Figure (b) shows the dense layer; Figure (c) shows the transition layer between the dense and porous layers, also known as the intermediate layer; and Figure (d) shows the porous layer.

[0063] Test Example 1

[0064] The retention rates of chlorophenol DBPs in Example 1 were tested. 4-phenol, 2,4-dichlorophenol, 2,6-dichlorophenol, and 2,4,6-trichlorophenol were used as test samples. Standard curves were plotted by measuring the UV absorbance at different concentrations to obtain the correlation between concentration and absorbance. The absorbance of the molecular solution before and after membrane filtration was further measured to obtain the concentration of the molecular solution, thereby calculating the retention rates before and after membrane filtration. The standard curves are shown below. Figure 4 As shown, standard curves were plotted for the UV absorbance of different concentrations of 4-chlorophenol (a), 2,4-dichlorophenol (b), 2,6-dichlorophenol (c), and 2,4,6-trichlorophenol (d) solutions. The fitted R-values ​​were then analyzed. 2 The distribution of values ​​and regression curve points shows that the fitting effect is good, and the calculated retention rate value is also relatively accurate.

[0065] Test Example 2

[0066] Water flux was tested in Examples 1-5. The solution volume, flow time, and pressure passing through the membrane were recorded under the same membrane area. The water flux was then calculated as follows: Figures 5-8 As shown.

[0067] Figure 5 Figure 1 shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane before and after 4-chlorophenol retention, and the double Y-plots showing the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 4-chlorophenol. Figure (a) shows the UV absorbance of Example 1 before and after 4-chlorophenol retention, Figure (b) shows the 4-chlorophenol retention of Example 2, Figure (c) shows the UV absorbance of Example 3 before and after 4-chlorophenol retention, Figure (d) shows the UV absorbance of Example 4 before and after 4-chlorophenol retention, Figure (e) shows the UV absorbance of Example 5 before and after 4-chlorophenol retention, and Figure (f) shows the UV absorbance of Example 1... 5. A double Y-plot showing the water flux and rejection rate of 4-chlorophenol for the nanofiltration membrane; it can be seen from the plot that the blended asymmetric nanofiltration membrane of Example 3 (PMMA / PEG ratio 8:2) has a rejection rate of 76.75% for 4-chlorophenol and a water flux of 1.23 L / (h·m). 2 (bar), and the water flux increases significantly with the increase of PEG content.

[0068] Figure 6 Figure 1 shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after 2,4-dichlorophenol retention, and a double Y-plot of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,4-dichlorophenol. Figure (a) shows the UV absorbance of 2,4-dichlorophenol before and after retention in Example 1; Figure (b) shows the UV absorbance of 2,4-dichlorophenol before and after retention in Example 2; and Figure (c) shows the UV absorbance of 2,4-dichlorophenol before and after retention in Example 3. Figure (d) shows the UV absorbance before and after 2,4-dichlorophenol retention in Example 4; Figure (e) shows the UV absorbance before and after 2,4-dichlorophenol retention in Example 5; Figure (f) shows the double Y-plot of water flux and retention rate of 2,4-dichlorophenol by nanofiltration membranes in Examples 1-5. From the figures, it can be seen that Examples 1, 2, and 3 have better retention effects, with Example 3 (solid content 8:2) being the best. As shown in Figure (c), the nanofiltration membrane retention rate is 91.70%, and the water flux is 1.52 L / (h·m). 2 ·bar).

[0069] Figure 7Figure 1 shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after 2,6-dichlorophenol retention, as well as a double Y-plot of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,6-dichlorophenol. Figure (a) shows the UV absorbance of Example 1 before and after 2,6-dichlorophenol retention; Figure (b) shows the UV absorbance of Example 2 before and after 2,6-dichlorophenol retention; Figure (c) shows the UV absorbance of Example 3 before and after 2,6-dichlorophenol retention; Figure (d) shows the UV absorbance of Example 4 before and after 2,6-dichlorophenol retention; Figure (e) shows the UV absorbance of Example 5 before and after 2,6-dichlorophenol retention; Figure (f) shows the UV absorbance of Example 1... 5. Double Y-plot of water flux and rejection rate of nanofiltration membrane for 2,6-dichlorophenol; preferably Example 3, as shown in Figure (c), the nanofiltration membrane rejection rate is 88.15% and the water flux is 1.24 L / (h·m). 2 ·bar).

[0070] Figure 8 Figure 1 shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after 2,4,6-trichlorophenol retention, and a double Y-plot of the water flux and retention rate of the PMMA / PEG blend asymmetric nanofiltration membrane for 2,4,6-trichlorophenol. Figure (a) shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 before and after 2,4,6-trichlorophenol retention, Figure (b) shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 2 before and after 2,4,6-trichlorophenol retention, and Figure (c) shows the UV absorbance of the PMMA / PEG blend asymmetric nanofiltration membrane prepared in Example 1 ... Figure 3 shows the UV absorbance before and after 2,4,6-trichlorophenol retention. Figure (d) shows the UV absorbance before and after 2,4,6-trichlorophenol retention in Example 4. Figure (e) shows the UV absorbance before and after 2,4,6-trichlorophenol retention in Example 5. Figure (f) shows the double Y-plot of water flux and retention rate of 2,4,6-trichlorophenol for the nanofiltration membranes of Examples 1-5. Example 3 is preferred, as shown in Figure (c), with a membrane retention rate of 88.36% and a water flux of 1.03 L / (h·m). 2 ·bar).

[0071] Depend on Figures 5-8 It can be seen that the nanofiltration membrane prepared by the present invention has the best retention effect on 2,4-dichlorophenol, and the retention rate of Example 3 is the highest and the effect is the most obvious. It can be seen that the blended asymmetric nanofiltration membrane with a solid content ratio of 8:2 has the best retention rate.

[0072] Analysis shows that as the content of polyethylene glycol increases, the water flux of the membrane increases significantly, but the retention rate decreases because the pore size of the channels formed after phase separation also increases.

[0073] Test Example 3

[0074] Picoammeter current pore size tests were performed on Examples 1-5. The change in current measured by the picoammeter can provide a preliminary assessment of the nanofiltration membrane pore size. An AC voltage of -2 V to 2 V was used, with a 0.1 mol / L potassium chloride solution as the electrolyte. Tests were conducted on Examples 1-5, and the current values ​​as a function of the voltage were recorded. Five sets of data were measured, and the average value was taken. Figure 9 As shown, the IV current diagrams for Examples 1 to 5 are presented. It is found that the current values ​​of Examples 2 and 3 are the smallest, which also confirms that their channel pore size is small, the ion conduction performance is poor, and the water flux is low. On the other hand, the current values ​​of Examples 4 and 5 are larger, indicating that the nanofiltration membrane channel pore size is large, the ion conduction performance is good, and the water flux is high.

[0075] Comparative Example 1

[0076] The implementation conditions and steps are the same as in Test Example 3, such as... Figure 10 The figure shows the IV current diagrams for Examples 2 and 3. A comparison reveals that the current in Example 3 is larger, indicating that the nanofiltration membrane in Example 3 has a larger porosity than that in Example 2. Figure 2 As can be seen, Example 3 has a thicker, denser layer, which results in a slightly higher retention rate than Example 2.

[0077] Comparative Example 2

[0078] The implementation conditions and steps are the same as in Test Example 3, such as... Figure 11 As shown, the IV current diagrams for Examples 1 and 2 are presented. The comparison shows that the current in Example 1 is larger, indicating that the aperture of Example 1 is still larger than that in Example 2, mainly due to the presence of the finger-shaped aperture.

[0079] Therefore, through comprehensive analysis of rejection rate and water flux, Example 3 represents the optimal condition set, exhibiting the highest rejection rate and good water flux, thus meeting the requirements for a high-quality nanofiltration membrane. Under optimal conditions, Example 3 achieves a rejection rate of 91.71% for 2,4-dichlorophenol and a water flux of 1.52 L / (h·m). 2 (Bar), indicating that the membrane has superior performance and a high rejection rate.

[0080] The aforementioned Figures 9-11 The correspondence with Examples 1-5 is expressed by indicating the ratio of polymethyl methacrylate (PMMA) to polyethylene glycol (PEG).

[0081] Therefore, the optimal conditions for preparing a polymethyl methacrylate (PMMA) and polyethylene glycol (PEG) blended asymmetric nanofiltration membrane, as described in Example 3, were as follows: a solid content ratio of PMMA to PEG of 8:2 at 95°C; a casting solution with a solid content of 20% was optimal; and the study found that the dense layer structure of the blended asymmetric nanofiltration membrane greatly improved the retention rate, while the loose layer increased the water flux, which is beneficial for low-pressure filtration. The prepared blended asymmetric nanofiltration membrane could be driven to retain water at a pressure of 2 bar.

Claims

1. A method for preparing a poly(methyl methacrylate) and poly(ethylene glycol) blended asymmetric nanofiltration membrane, characterized by: The two polymers of polymethyl methacrylate and polyethylene glycol are dissolved in N,N-dimethylformamide solvent by stirring to form a casting solution, and a nanofiltration membrane with a specific thickness is prepared by a doctor blade. During the membrane preparation process, polymethyl methacrylate is the main support material, and polyethylene glycol is the pore former and hydrophilic material. Then, the prepared nanofiltration membrane is placed in a fume hood for standing. 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 phase transfer method, and then the membrane is solidified. The specific preparation steps are as follows: (1) Prepare N,N-dimethylformamide solution according to the calculation basis of a solution with a solid content of 10-30%, and weigh polymethyl methacrylate and polyethylene glycol in proportion. The mass ratio of polymethyl methacrylate to polyethylene glycol is (8-9):(1-2). Then, the polymethyl methacrylate and polyethylene glycol are added to the N,N-dimethylformamide solution to prepare a casting solution with a solid content of 10-30%; (2) Add the casting solution to the reaction container. In the constant temperature oil bath, stir at a temperature of 90-105°C for 5-8 hours, and build a condensation reflux device to prevent N,N-dimethylformamide from evaporating, so that the polymethyl methacrylate and polyethylene glycol are fully dissolved in the N,N-dimethylformamide solution; (3) The solution obtained in step (2) is placed at a temperature of 75-85°C for 0.5-1.5 hours for standing and degassing. The degassed solution is removed from the silicon plate using a syringe, and a blended membrane with a thickness of 10-100 μm is prepared on the silicon plate using a doctor blade. The blended membrane on the silicon plate is placed in a fume hood for 10-30 minutes. Through solvent evaporation, the surface solvent evaporates quickly, forming a dense structure. The lower layer is tightly attached to the glass, and the solvent evaporates slowly, thereby continuing to retain a loose and porous structure, forming a blended asymmetric nanofiltration membrane with an asymmetric structure; (4) The silicon plate with the blended asymmetric nanofiltration membrane is completely immersed in deionized water, and the solvent is gradually dissolved in the aqueous solution by phase inversion method, and the solvent is removed, so that the blended asymmetric nanofiltration membrane is solidified; (5) Take the blended asymmetric nanofiltration membrane out of the deionized water, air dry naturally, and cut and press to the appropriate size for use.

2. The method for preparing a polymethyl methacrylate and polyethylene glycol blended asymmetric nanofiltration membrane according to claim 1, characterized in that: The blended asymmetric nanofiltration membrane has a dense upper layer and a loose lower layer.

3. The preparation method of the blended asymmetric nanofiltration membrane according to claim 1, characterized in that: In step (1), the mass ratio of polymethyl methacrylate to polyethylene glycol is 8:

2. The polymethyl methacrylate and polyethylene glycol are added to the N,N-dimethylformamide solution to prepare a casting solution with a solid content of 20% as the optimal condition; In step (2), the optimal condition is to stir at a temperature of 95°C for 5 hours; In step (3), the solution is placed at a temperature of 80°C for 1 hour for standing and degassing. The degassed solution is removed from the silicon plate using a syringe, and a blended membrane with a thickness of 50 μm is prepared on the silicon plate using a doctor blade. The blended membrane on the silicon plate is placed in a fume hood for 10 minutes. In step (4), the asymmetric nanofiltration membrane is immersed in deionized water for 12 h, gradually dissolved in the water solution by phase inversion method, and solidified by removing the solvent.

4. A poly(methyl methacrylate) and poly(ethylene glycol) blended asymmetric nanofiltration membrane, characterized by: is prepared by the preparation method of any one of claims 1 to 3.

5. Use of a blended asymmetric nanofiltration membrane according to claim 4, characterized in that: The blended asymmetric nanofiltration membrane can selectively separate and remove neutral chlorophenol disinfection by-products with a molecular weight of ≤200 in water.

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