A high-permeability selective strong negative nanofiltration membrane, a preparation method and application thereof

Abstract

The application discloses a high-permeability and high-selectivity strong negative nanofiltration membrane and a preparation method and application thereof, and relates to the technical field of water treatment. The preparation method comprises the following steps: step one, obtaining a sulfonated polyvinyl alcohol solution through an esterification reaction under acid catalysis; step two, blending a monomer aqueous solution of a polyamine and a polymer aqueous solution of the sulfonated polyvinyl alcohol, and then depositing and infiltrating on a macroporous ultrafiltration membrane after pH adjustment; and step three, performing an interfacial polymerization reaction on the membrane surface through a monomer n-hexane solution of a polybasic acid chloride to prepare a polyamide functional layer with a surface potential lower than-45 mV. The nanofiltration membrane provided by the application has a water flux increased by about one time without sacrificing the sodium sulfate rejection rate, has high anion separation performance, has the ability of efficiently rejecting organic pollutants and retaining beneficial mineral ions, and has good anti-pollution capacity. The nanofiltration membrane provided by the application can be widely applied to the field of drinking water deep treatment.

Description

Technical Field

[0001] This invention relates to a highly selective, strongly negatively charged nanofiltration membrane, its preparation method, and its application, belonging to the field of nanofiltration membrane technology. Background Technology

[0002] Traditional drinking water production processes typically rely on coagulation, sedimentation, filtration, and disinfection to remove turbidity, pathogens, and some organic pollutants. However, with industrialization and urbanization, water sources now contain persistent trace organic pollutants, endocrine disruptors, and pesticide residues, posing new challenges to drinking water safety. Nanofiltration membranes, with their excellent retention of divalent and multivalent ions and effective removal of small organic molecules, have gradually become an important technological choice in drinking water production. Currently, nanofiltration technology is widely used in upgrading water treatment plants, deep purification, and the treatment of slightly polluted water sources, providing a reliable solution for ensuring drinking water safety and quality.

[0003] Currently, commercially available nanofiltration membranes perform excellently in ion removal. While this strong desalination capability helps improve water safety, it also retains some beneficial divalent and polyvalent ions and other trace elements in drinking water, resulting in insufficient mineral content in the effluent. In some projects, additional mineralization is required to ensure the taste of drinking water. Long-term consumption of excessively desalinated drinking water may have adverse effects on human health, which is one of the important problems facing nanofiltration technology in drinking water production. To solve this problem, researchers have started by improving the membrane fabrication process to prepare nanofiltration membranes with better sieving performance, aiming to retain organic pollutants while retaining beneficial mineral ions. The main approach to achieving efficient retention of organic pollutants and retaining beneficial mineral ions is to compensate for weakened size sieving by enhancing charge repulsion. Therefore, it is necessary to first enhance the negative charge on the membrane surface. Most current methods reduce the reaction rate of amine monomers and polyacrylamide chlorides, thereby reducing the degree of crosslinking of polyamide and increasing the surface carboxyl group density. However, reducing the degree of crosslinking of polyamide requires increasing the polyamide thickness to ensure sieving properties, which leads to a sacrifice in water transport rate. Even if a high-performance loose nanofiltration membrane is produced, there is an upper limit to the degree of reduction in polyamide crosslinking. The upper limit of surface carboxyl group density determines that it is difficult for the membrane to simultaneously improve water flux and selective sieving properties.

[0004] This invention considers introducing sulfonic acid groups into polyamides. Structurally, sulfonic acid groups contain one sulfur atom and three strongly electronegative oxygen atoms, exhibiting stronger electron-withdrawing effects and resonance delocalization capabilities, resulting in stronger electronegativity at the same pH. While researchers have attempted to develop methods to introduce sulfonic acid groups into polyamide layers—for example, Chinese patent number 202210095242.8 describes a method for preparing an electronegatively enhanced nanofiltration membrane—after obtaining the initial polyamide nanofiltration membrane, a modified aqueous solution is applied to the polyamide nanofiltration membrane via vacuum filtration, dispersing the polyelectrolyte containing sulfonic acid groups on the nanofiltration membrane surface. Another example is Chinese patent number 202510280572.8, which describes a method for preparing a membrane using an aminobenzenesulfonic acid blend-modified aqueous solution. By doping with p-aminobenzenesulfonic acid during the preparation of a traditional TFC nanofiltration membrane, the electronegativity of the entire membrane is enhanced, improving its ability to react with Co. 2+ The repulsion ability of these methods is limited. However, compared with the present invention, these methods have more complex preparation processes, making them less stable for large-scale membrane applications and prone to producing defective nanofiltration membranes. For example, Chinese Patent No. 202510341586.6 describes an optimized high negative charge nanofiltration membrane and its preparation process. First, the hydrophilic substrate is modified with an organic solvent, and then the biomass raw materials are subjected to enhanced oxidation treatment to obtain biomass nanofibers carrying oxygen-containing functional groups. These nanofibers are deposited on the surface of the hydrophilic substrate to form a biomass nanofiber intermediate layer. Finally, an interfacial polymerization reaction is carried out on the surface of the biomass nanofiber intermediate layer using polyamine monomers and acyl chloride monomers. The resulting high negative charge nanofiltration membrane has an extremely low Zeta potential and exhibits high ion separation efficiency. However, compared with the present invention, the biomass materials involved in its preparation process are expensive, and the membrane preparation process is complex and time-consuming, which cannot effectively improve the separation selectivity of the membrane while ensuring controllable membrane production costs. For example, Chinese patent number 202010159127.3 introduces a method for modifying nanofiltration membranes with sodium lignosulfonate. The method uses an aqueous solution of sodium lignosulfonate containing abundant functional groups such as phenolic hydroxyl, alcoholic hydroxyl, sulfonic acid group, and carboxyl group and triethylamine as the aqueous phase. A nanofiltration membrane with high separation selectivity is prepared by a simple interfacial polymerization method of wetting-reaction-heat treatment. Although the nanofiltration membrane prepared by this method is relatively stable and has cost advantages, the flux is less than 15 LMH / bar and the operating efficiency is still low. Summary of the Invention

[0005] In order to overcome the shortcomings of the existing technology, the main purpose of this invention is to provide a method for preparing and applying a highly selective and strongly negatively charged nanofiltration membrane. The method mainly utilizes sulfonated polyvinyl alcohol to participate in the reaction of amine monomers and polyacrylamide chlorides to prepare a nanofiltration membrane rich in sulfonic acid groups, with strong negative charge, good hydrophilicity and few defects.

[0006] The present invention discloses a method for preparing and applying a highly permeable and selective strongly negatively charged nanofiltration membrane, comprising the following steps:

[0007] (1) Dissolve 5-sulfosalicylic acid in deionized water at room temperature. The mass fraction of 5-sulfosalicylic acid is 15-25%. After stirring for 20-22 min, add 0.5 wt% concentrated acid and then add 5-7.5 wt% polyvinyl alcohol (PVA) to the solution. Transfer the solution to a water bath at 95-98℃ and stir at 1000-1500 rpm for 30-36 h. After the reaction is completed, transfer the reaction solution to a water bath at 59-61℃ and stir at 100-300 rpm. The sulfonated polyvinyl alcohol solution prepared needs to be continuously stirred for later use.

[0008] (2) Prepare a solution of piperazine (PIP) and deionized water in a brown bottle. The concentration of PIP is 0.15-0.17 wt%. After stirring for 30-32 min, add the sulfonated polyvinyl alcohol solution dropwise to the PIP aqueous solution. Adjust the pH to 7-8 with 1.9-2.1 mol / L NaOH. The concentration of the sulfonated polyvinyl alcohol solution is 0.0459-0.1376 wt%. After adding the solution, sonicate it in an ice bath for 90-95 min to obtain an aqueous solution for later use.

[0009] (3) Weigh the polyacryl chloride and transfer it to a sealed brown bottle. Place it in an oven at 70°C for 20-30 min to liquefy the polyacryl chloride. Add organic solution A of straight-chain saturated alkane to prepare a solution of polyacryl chloride with a mass fraction of 0.20 wt%. The oil phase solution is then ready for use.

[0010] (4) Take out the substrate membrane that has been soaked in deionized water for 24 hours in advance and remove the surface liquid with a rubber roller. Fix it in the reaction tank as a support material. Pour in the aqueous phase solution obtained in step (2) to wet the membrane surface for 1.9-2.1 min. After pouring out the aqueous phase in the reaction tank, tilt the reaction tank and dry the surface quickly with an air knife. Immediately pour in the oil phase solution obtained in step (3) and react on the membrane surface for 0.95-1.05 min. After the reaction is completed, pour out the oil phase and soak the membrane in deionized water for storage.

[0011] (5) After the membrane obtained in step (4) is neatly cut according to the rolling size, it is soaked in a 10-20 wt% glycerol solution to prevent drying and shrinkage. Then, a water inlet mesh is laid on the water inlet side and a water permeable mesh is laid on the water product side. The membrane is sealed with epoxy resin on three sides, leaving only one side for bonding with the central tube. Next, the membrane is tightly bonded and fixed to the central tube. The membrane and mesh structure are wound into a membrane roll on a winding machine, and the ends of the membrane roll are sealed with epoxy resin. Finally, the membrane roll is installed into the pressure vessel shell, and the end caps and sealing rings are installed to obtain the nanofiltration membrane module.

[0012] Preferably, in step (1), sulfonated polyvinyl alcohol is introduced into sulfonic acid groups by esterification of the sulfonated dicarboxylic acid with the hydroxyl groups of the PVA molecular chain under high temperature acid catalysis, and the degree of alcoholysis of the PVA should not be less than 99%.

[0013] Preferably, the concentrated acid is concentrated hydrochloric acid or concentrated sulfuric acid.

[0014] Preferably, the sulfosalicylic acid is one of 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, or p-sulfosalicylic acid.

[0015] Preferably, in step (2), the polyamine monomer is piperazine, m-phenylenediamine, p-phenylenediamine, polyethyleneimine, 1,2-ethylenediamine, or 1,6-hexanediamine.

[0016] Preferably, the polyacryl chloride in step (3) is one of pyromellitic triacryl chloride, isophenyltriacryl chloride, cyclohexanetriacryl chloride, cyclopentanetriacryl chloride, propyltriacryl chloride, or pentanetriacryl chloride.

[0017] Preferably, the solvent of the organic solution A of polyacrylamide chloride in step (3) is one of n-hexane, n-heptane or tetradecane.

[0018] Preferably, in step (4), the material of the base membrane is one of polyethersulfone, polysulfone, polyethylene, polyamide imide, polypropylene or polyacrylonitrile, and the molecular weight cutoff of the base membrane should be between 100 and 300 kDa.

[0019] The preparation method described above yields a highly permeable and selective, strongly negatively charged nanofiltration membrane.

[0020] Application of a highly permeable and selective, strongly negatively charged nanofiltration membrane in the selective separation of monovalent and multivalent ions.

[0021] Application of a highly permeable and selective, strongly negatively charged nanofiltration membrane in the selective separation of mineral ions and organic pollutants.

[0022] The present invention involves continuous phenolic hydroxyl groups on the side chains of polyvinyl alcohol polymer molecules participating in the interfacial polymerization reaction. Sulfonated polyvinyl alcohol, as a novel polymer with sulfonic acid functional groups, participates in the reaction of amine monomers and acyl chloride monomers, effectively improving the ability of sulfonic acid groups to embed into the polymer network. This ensures that the interfacial polymerization reaction remains continuous at the reaction interface, which is beneficial for the large-scale preparation of defect-free nanofiltration membranes.

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0024] (1) The nanofiltration membrane prepared by the present invention has continuous sulfonic acid functional groups in the polyamide layer, which has higher electronegativity than conventional polyamide nanofiltration membranes, thereby significantly improving the membrane's separation effect on mono / divalent anions and mineral ions / organic matter.

[0025] (2) The nanofiltration membrane prepared by the present invention introduces sulfonic acid functional groups through polymer, which significantly changes the membrane structure. The membrane surface has a ring-shaped Turing structure pattern and the interior is rich in nanocavities, thereby significantly increasing the water permeable area of ​​the membrane and increasing the water flux of the nanofiltration membrane.

[0026] (3) The physicochemical properties of the nanofiltration membrane prepared by the present invention change, and the hydrophilicity is improved compared with the conventional polyamide nanofiltration membrane, which is conducive to the formation of a hydration layer on the membrane surface and increases the membrane's ability to retain hydrophobic pollutants in the water.

[0027] (4) In addition, the nanofiltration membrane prepared by the present invention improves the permeability of mineral ions and reduces the concentration polarization effect of minerals such as calcium and magnesium ions on the membrane surface compared with conventional polyamide nanofiltration membranes, thereby significantly inhibiting the tendency of gypsum fouling of the membrane, reducing the frequency of membrane cleaning and improving membrane life. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 The Zeta potentials of the membrane surface in Test Examples 1-3 and Comparative Example 1 of this invention are shown in the figure;

[0030] Figure 2 These are scanning electron microscope images of the membrane surfaces of Experimental Examples 1-3 and Comparative Example 1 of the present invention;

[0031] Figure 3 These are atomic force microscopy images of the membrane surfaces of Experimental Examples 1-3 and Comparative Example 1 of the present invention;

[0032] Figure 4 These are transmission electron microscope images of the membrane cross-sections of Experimental Examples 1-3 and Comparative Example 1 of the present invention;

[0033] Figure 5 These are functional group diagrams of the membrane surface of Experimental Examples 1-3 and Comparative Example 1 of the present invention;

[0034] Figure 6 The membrane water contact angle diagrams are for Experimental Examples 1-3 and Comparative Example 1 of this invention;

[0035] Figure 7The water flux diagrams for Experimental Examples 1-3 and Comparative Example 1 of this invention are shown.

[0036] Figure 8 The retention rates of sodium sulfate and sodium chloride in Experimental Examples 1-3 and Comparative Example 1 of this invention are shown in the graph.

[0037] Figure 9 This is a Zeta potential diagram of the membrane surface in Comparative Example 2 of the present invention;

[0038] Figure 10 This is a water flux diagram for Comparative Example 2 of the present invention;

[0039] Figure 11 This is a scanning electron microscope image of the membrane surface in Comparative Example 3 of the present invention;

[0040] Figure 12 This is a graph showing the water flux and sodium sulfate rejection rate of Comparative Example 3 of the present invention;

[0041] Figure 13 The separation factor diagram of calcium ions and organic pollutants in Application Example 1 of the present invention;

[0042] Figure 14 This is a graph showing the membrane flux change in Application Example 2 of the present invention;

[0043] Figure 15 This is a scanning electron microscope image of the contamination layer on the membrane surface in Application Example 2 of the present invention. Detailed Implementation

[0044] The invention will now be further described with reference to the accompanying drawings.

[0045] A method for preparing a highly permeation-selective, strongly negatively charged nanofiltration membrane includes the following steps:

[0046] (1) Dissolve 5-sulfosalicylic acid in deionized water at room temperature. The mass fraction of 5-sulfosalicylic acid is 15-25%. After stirring for 20-22 min, add 0.5wt% concentrated acid, i.e. concentrated sulfuric acid, to make it account for 0.5wt% of the total mass of the sulfonated PVA solution. Then add 5-7.5wt% polyvinyl alcohol PVA to the solution, i.e. polyvinyl alcohol PVA, to make it account for 5-7.5wt% of the total mass of the sulfonated PVA solution. Transfer the solution to a water bath at 95-98℃ and stir at 1000-1500 rpm for 30-36 h. After the reaction is completed, transfer the reaction solution to a water bath at 59-61℃ and stir at 100-300 rpm. The prepared sulfonated polyvinyl alcohol solution needs to be continuously stirred for later use.

[0047] (2) Prepare a solution of polyamine monomers and deionized water in a brown bottle. The concentration of polyamines is 0.15-0.17 wt%. After stirring for 30-32 min, add sulfonated polyvinyl alcohol solution dropwise to the polyamine aqueous solution. Adjust the pH to 7-8 with 1.9-2.1 mol / L NaOH. The concentration of sulfonated polyvinyl alcohol solution is 0.0459-0.1376 wt%. After adding the solution, sonicate it in an ice bath for 90-95 min to obtain an aqueous solution for later use.

[0048] (3) Weigh the polyacryl chloride and transfer it to a sealed brown bottle. Place it in an oven at 70°C for 20-30 min to liquefy the polyacryl chloride. Add organic solution A to prepare a solution with a mass fraction of 0.20 wt% polyacryl chloride. The oil phase solution is then ready for use.

[0049] (4) Take out the substrate membrane that has been soaked in deionized water for 24 hours in advance and remove the surface liquid with a rubber roller. Fix it in the reaction tank as a support material. Pour in the aqueous phase solution obtained in step (2) to wet the membrane surface for 1.9-2.1 min. After pouring out the aqueous phase in the reaction tank, tilt the reaction tank and dry the surface quickly with an air knife. Immediately pour in the oil phase solution obtained in step (3) and react on the membrane surface for 0.95-1.05 min. After the reaction is completed, pour out the oil phase and soak the membrane in deionized water for storage.

[0050] (5) After cutting the membrane obtained in step (4), soaking it in glycerol, laying the mesh, sealing it with epoxy resin and sticking the central tube, the membrane is wound into a membrane roll, thus obtaining a highly permeable and selective strongly negatively charged nanofiltration membrane. The membrane module is then placed into the outer shell.

[0051] In this invention, in step (1), sulfonated polyvinyl alcohol undergoes an esterification reaction with the hydroxyl groups of the PVA molecular chain via a sulfonated diacid, thereby introducing sulfonic acid groups. The degree of alcoholysis of the PVA should be no less than 99%. The concentrated acid is concentrated hydrochloric acid or concentrated sulfuric acid. The sulfosalicylic acid is one of 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, or p-sulfobenzoic acid.

[0052] In this invention, in step (2), the polyamine monomer is piperazine, m-phenylenediamine, p-phenylenediamine, polyethyleneimine, 1,2-ethylenediamine, or 1,6-hexanediamine. In step (3), the polyacrylamide chloride is one of trimesoyl chloride, iso-trimethylbenzene chloride, cyclohexanetriacryl chloride, cyclopentanetriacryl chloride, triacrylamide chloride, or pentanetriacryl chloride. In step (3), the solvent of the organic solution A of the polyacrylamide chloride is one of hexane, heptane, or tetradecane. In step (4), the material of the base membrane is one of polyethersulfone, polysulfone, polyethylene, polyamideimide, polypropylene, or polyacrylonitrile, and the molecular weight cutoff of the base membrane should be between 100 and 300 kDa.

[0053] Example 1

[0054] This embodiment describes a method for preparing a highly selective, strongly negatively charged nanofiltration membrane, specifically carried out according to the following steps:

[0055] Step 1: Dissolve 20 g of 5-sulfosalicylic acid in 74.5 mL of deionized water at room temperature. After stirring for 20 min, add 0.27 mL of 98 wt% concentrated sulfuric acid and then add 5 g of type 1799 polyvinyl alcohol. Transfer the solution to a water bath at 95-98℃ and stir at 2000 rpm for 36 h. After the reaction is complete, transfer the reaction solution to a water bath at 60℃ to obtain 100 g of sulfonated PVA solution. The solution needs to be continuously stirred at 100 rpm for later use.

[0056] Step 2: Dissolve 0.16 g of PIP in 999.84 mL of deionized water to obtain a PIP aqueous solution with a concentration of 0.16 wt%. After stirring for 30 min, add 5.520 mL of the above sulfonated PVA solution dropwise to the PIP aqueous solution so that the sulfonated PVA accounts for 0.1376 wt% of the total mass of the PIP aqueous solution. Then adjust the pH to 8 with 2 mol / L NaOH, and prepare a 0.20 wt% hexane solution of trimesoyl chloride (TMC).

[0057] Step 3: Use a pre-wetted polyethersulfone membrane with a molecular weight cutoff of 300 kDa as a substrate for interfacial polymerization. The aqueous phase wetting time is 2 min. Remove excess aqueous solution with an air knife. The sulfonated polyvinyl alcohol polymer participates in the polymerization reaction of amine monomers and acyl chloride monomers for 1 min. After the reaction, the membrane is stored in deionized water to obtain a highly permeable and selective strongly negatively charged nanofiltration membrane.

[0058] Example 2

[0059] The difference between this embodiment and Example 1 is that 5-sulfosalicylic acid in step one is replaced with p-sulfosalicylic acid. Everything else is the same as in Example 1.

[0060] Example 3

[0061] The difference between this embodiment and Example 1 is that the acid catalysis in step one is replaced with 0.5 wt% concentrated hydrochloric acid. Everything else is the same as in Example 1.

[0062] Example 4

[0063] The difference between this embodiment and Examples 1-3 is that in step two, m-phenylenediamine (MPD) is used instead of PIP. Specific process parameters are as follows: prepare a 2.0 wt% MPD aqueous solution, then prepare a 0.10 wt% TMC hexane solution; the substrate membrane is a 100 kDa polysulfone membrane. Everything else is the same as in Examples 1-3.

[0064] Example 5

[0065] This embodiment differs from Example 1 in that: in step three, the membrane after the interfacial polymerization reaction is heat-treated in an oven at 60 °C for 90 seconds before being stored in deionized water. Everything else is the same as in Examples 1-3. A strongly negatively charged nanofiltration membrane with a moderately reduced pore size can be obtained.

[0066] Example 6

[0067] This embodiment differs from Example 1 in that, in step three, before immersing the membrane in deionized water after the interfacial polymerization reaction, the membrane surface is rinsed with n-hexane organic solvent for 10 seconds. Everything else is the same as in Examples 1-3. A strongly negatively charged nanofiltration membrane with a moderately enlarged pore size can be obtained.

[0068] Example 7

[0069] This embodiment differs from Example 1 in that it incorporates sulfonated polyvinyl alcohol into the nanofiltration membrane interlayer. Specific process parameters are as follows: A 0.1 wt% sulfonated polyvinyl alcohol solution is first impregnated onto a 300 kDa polyethersulfone membrane substrate. Then, a 0.0001 wt% glutaraldehyde aqueous solution is added for crosslinking to form the interlayer. Excess interlayer solution is removed using an air knife. A 0.16 wt% PIP aqueous solution is prepared and impregnated for 2 minutes. Excess aqueous phase solution is then removed again using an air knife. A 0.20 wt% TMC solution in n-hexane is added, and the polymerization reaction time is 1 minute. The resulting membrane is stored immersed in deionized water. Other steps are the same as in Examples 1-3. This method yields a highly permeable and selective nanofiltration membrane with a moderately reduced pore size and a moderately weakened negative charge.

[0070] The invention was verified using the following experiments:

[0071] Experimental Example 1

[0072] This experiment demonstrates a method for preparing and testing the performance of a highly selective, strongly negatively charged nanofiltration membrane, specifically carried out according to the following steps:

[0073] Step 1: Dissolve 20 g of 5-sulfosalicylic acid in 74.5 mL of deionized water at room temperature, add 0.27 mL of 98 wt% concentrated sulfuric acid, and then add 5 g of type 1799 polyvinyl alcohol (PVA). Transfer the solution to a water bath at 95-98℃, stir and continue to react for 36 h. After the reaction is completed, transfer the reaction solution to a water bath at 60℃ to obtain 100 g of sulfonated PVA solution.

[0074] Step 2: Dissolve 0.16 g of PIP in 999.84 mL of deionized water to obtain a 0.16 wt% PIP aqueous solution. After stirring for 30 min, add 1.835 mL of the above sulfonated PVA solution dropwise to the PIP aqueous solution, making the sulfonated PVA account for 0.0459 wt% of the total mass of the PIP aqueous solution. Adjust the pH to 8 and prepare a 0.20 wt% TMC solution in n-hexane. Use a 300 kDa polyethersulfone membrane as the substrate. The aqueous phase wetting time is 2 min, the polymerization reaction time is 1 min, and after the reaction is completed, store in pure water.

[0075] Step 3: Cut the membrane sheet and place it in a flat plate clamping cell. Pre-wet the membrane surface and equilibrate it in a 10 mmol / L KCl solution for 60 min. Apply a certain pressure difference across the membrane sample to allow the electrolyte solution to flow on the membrane surface or within the membrane pores. As the ions in the solution flow, they drag the anti-ion layer at the interface, thus generating a potential difference between the upstream and downstream electrodes. Calculate the surface zeta potential of the membrane. Then, process the membrane sheet with embedded sections and take transmission electron microscope (TEM) images of the membrane cross-section. Finally, cut the membrane sheet and take scanning electron microscope (SEM) and atomic force microscope (AFM) images of the membrane surface.

[0076] Step 4: Cut the membrane sheet and place it in the cross-flow nanofiltration membrane cell. The cross-flow velocity is 0.47 m / s. First, pre-compress the membrane with a pressure of 7.5 bar. After pre-compressing, the test pressure is still 7.5 bar. Record the volume of pure water that permeates through the membrane using a timer and an electronic balance. Test the pure water flux with pure water. Test the salt rejection rate with 2 g / L Na2SO4 solution and NaCl solution.

[0077] Experimental Example 2

[0078] The difference between this experiment and Experiment Example 1 is that the concentration of sulfonated PVA in the PIP solution in step two is 0.0917 wt%.

[0079] Experimental Example 3

[0080] The difference between this experiment and Experiment Example 1 is that the concentration of sulfonated PVA in the PIP solution in step two is 0.1376 wt%.

[0081] This comparative example aims to investigate the effect of introducing sulfonated polyvinyl alcohol into the interfacial polymerization reaction on membrane structure and membrane properties.

[0082] Comparative Example 1

[0083] The method is the same as in Example 1, except that sulfonated PVA is not introduced during the interfacial polymerization process. Specifically, it is carried out in the following steps:

[0084] Step 1: Prepare a 0.16 wt% PIP aqueous solution, and then prepare a 0.20 wt% TMC hexane solution;

[0085] Step 2: Use a pre-wetted polyethersulfone membrane with a molecular weight cutoff of 300 kDa as a substrate for interfacial polymerization. The aqueous phase wetting time is 2 min. Remove excess aqueous phase solution with an air knife. The polymerization reaction time is 1 min. After the reaction, the membrane is stored in deionized water.

[0086] The zeta potentials of the membrane surface in Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 1 As shown, the scanning electron microscope images of the membrane surfaces of Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 2 As shown, the atomic force microscopy of the film surface in Experimental Examples 1-3 and Comparative Example 1 is as follows: Figure 3 As shown, the transmission electron microscope images of the membrane cross-sections of Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 4 As shown, the functional groups on the membrane surface of Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 5 As shown, the membrane water contact angle diagrams for Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 6 As shown, the water flux diagrams for Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 7 As shown, the retention rates of sodium sulfate and sodium chloride in Experimental Examples 1-3 and Comparative Example 1 are as follows: Figure 8 As shown.

[0087] The advantages of Experimental Examples 1-3 compared to Comparative Example 1 are as follows: Experimental Examples 1-3 have higher electronegativity, better selective separation characteristics of monovalent and multivalent ions, the polyamide layer has a ring-shaped Turing pattern, is rich in nanocavities, has higher water flux, and stronger hydrophilicity.

[0088] Comparative Example 2

[0089] The method is the same as in Example 1, except that sulfonated PVA is not introduced during the interfacial polymerization process; instead, p-aminobenzenesulfonic acid is introduced. Specifically, the process is carried out as follows:

[0090] Step 1: Mix p-aminobenzenesulfonic acid and piperazine in water. The concentration of PIP in the aqueous solution is 18 g / L, and the concentration of p-aminobenzenesulfonic acid is 5.5 g / L.

[0091] Step 2: Clean the polyethersulfone membrane with anhydrous ethanol, immerse the dried polyethersulfone membrane in a mixed solution of aminobenzenesulfonic acid and piperazine for 3 minutes, pour out the liquid, and let the membrane air dry naturally.

[0092] Step 3: Immerse the membrane in a 1.5 g / L TMC n-hexane solution and react for 100 seconds. After pouring out the liquid, place the membrane in a drying oven at 50°C for 27 min to obtain the p-aminobenzenesulfonic acid blended nanofiltration membrane of Comparative Example 2.

[0093] The Zeta potential at the membrane surface in Comparative Example 2 is as follows Figure 9 As shown, the water flux of the membrane is as follows Figure 10 As shown.

[0094] The advantages of Experimental Examples 1-3 compared to Comparative Example 2 are: Experimental Examples 1-3 have higher electronegativity and higher water flux.

[0095] Comparative Example 3

[0096] The method is the same as in Example 1, except that the pH is not adjusted after introducing sulfonated PVA during the interfacial polymerization process. Specifically, it is carried out according to the following steps:

[0097] Step 1: Dissolve 5-sulfosalicylic acid in deionized water at room temperature to prepare a 20 wt% solution. Add 0.5 wt% concentrated sulfuric acid and then add 5.0 wt% type 1799 polyvinyl alcohol (PVA). Transfer the solution to a water bath at 95-98℃, stir and continue to react for 36 h. After the reaction is completed, transfer the reaction solution to a water bath at 60℃ to obtain a sulfonated PVA solution.

[0098] Step 2: Prepare a 0.16 wt% PIP aqueous solution, stir for 30 min, then add the sulfonated PVA solution dropwise to the PIP aqueous solution. The concentration of the sulfonated polyvinyl alcohol solution is 0.1376 wt%. Then prepare a 0.20 wt% TMC solution in n-hexane.

[0099] Step 3: Use a pre-wetted polyethersulfone membrane with a molecular weight cutoff of 300 kDa as a substrate for interfacial polymerization. The aqueous phase wetting time is 2 min. Remove excess aqueous phase solution with an air knife. The sulfonated polyvinyl alcohol polymer participates in the polymerization reaction of amine monomers and acyl chloride monomers for 1 min. After the reaction, the membrane is stored by immersing it in deionized water.

[0100] The scanning electron microscope image of the membrane surface in Comparative Example 3 is shown below. Figure 11 As shown, the membrane water flux and sodium sulfate rejection rate are as follows: Figure 12 As shown.

[0101] The advantage of Experimental Examples 1-3 over Comparative Example 3 is that the membrane surface of Experimental Examples 1-3 has no defects, while the membrane surface of Comparative Example 3 has larger defects and cannot effectively retain salt ions.

[0102]

[0103] Application Example 1

[0104] The nanofiltration membrane sheets prepared in Experimental Example 2 and Comparative Example 1 were neatly cut to the required rolling dimensions and then immersed in a 10wt% glycerol solution to prevent drying and shrinkage. A feed water separator was then laid on the feed water side and a permeable separator on the product water side. The three sides of the membrane sheet were sealed with epoxy resin. The membrane sheet was then tightly bonded to the central tube. The membrane sheet and separator structure were wound into a membrane roll on a winding machine, and the ends of the membrane roll were sealed with epoxy resin. Finally, the membrane roll was installed into the pressure vessel shell, and end caps and sealing rings were installed to obtain a nanofiltration membrane module of model 1812, with a diameter of 4.57 cm, a length of 30.48 cm, and an effective membrane area of ​​0.37 m². 2 Actual surface water was collected, and after removing suspended solids through a 0.45 μm filter, the COD concentration was measured to be 16.5 mg / L, while the concentrations of calcium and magnesium ions were 53.54 mg / L and 8.16 mg / L, respectively. 200 μg / L of pollutants (PFHxA, PFHpA, PFHxS, PFOA, EPAG, and NPEO-NP7) were added, and the separation factors for calcium ions and organic pollutants were as follows: Figure 13 As shown.

[0105] In Application Example 1, the core advantage of the membrane prepared in Experimental Example 2 compared to Comparative Example 1 is that, taking PFOA as an example, the separation factor of calcium ions and organic pollutants increased from 4.28 to 18.84.

[0106] Application Example 2

[0107] The 1812 nanofiltration membrane modules prepared in Experimental Example 2 and Comparative Example 1 were used to conduct long-term performance tests on actual surface water after suspended solids were removed by filtration at 0.45 μm. Membrane fouling led to a decrease in membrane performance, as shown in the membrane flux change graph. Figure 14 As shown in the figure, the scanning electron microscope image of the contamination layer on the membrane surface after the test is as follows. Figure 15 As shown.

[0108] In Application Example 2, the core advantage of the membrane prepared in Experimental Example 2 compared to Comparative Example 1 is that the membrane's antifouling performance is significantly improved. After 34 hours of operation, the water flux of Experimental Example 2 was 13.25 LMH / bar, which is significantly better than the 6.43 LMH / bar of Comparative Example 1.

[0109] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a highly permeable and selectively charged nanofiltration membrane, characterized in that, Includes the following steps: (1) Dissolve 5-sulfosalicylic acid in deionized water at room temperature. The mass fraction of 5-sulfosalicylic acid is 15-25%. After stirring for 20-22 min, add 0.5wt% concentrated acid and then add 5-7.5wt% polyvinyl alcohol (PVA) to the solution. Transfer the solution to a water bath at 95-98℃ and stir at 1000-1500 rpm for 30-36 h. After the reaction is completed, transfer the reaction solution to a water bath at 59-61℃ and stir at 100-300 rpm for 30-36 h. The resulting sulfonated polyvinyl alcohol solution is stirred continuously. (2) Prepare a solution of polyamine monomers and deionized water in a brown bottle. The concentration of polyamines is 0.15-0.17 wt%. After stirring for 30-32 min, add sulfonated polyvinyl alcohol solution dropwise to the polyamine aqueous solution. Adjust the pH to 7-8 with 1.9-2.1 mol / L NaOH. The concentration of sulfonated polyvinyl alcohol solution is 0.0459-0.1376 wt%. After adding the solution, sonicate it in an ice bath for 90-95 min to obtain an aqueous solution. (3) Weigh the polyacryl chloride and transfer it to a sealed brown bottle. Place it in an oven at 70°C for 20-30 min to liquefy the polyacryl chloride. Add organic solution A of straight-chain saturated alkane to prepare a solution of polyacryl chloride with a mass fraction of 0.20 wt% to obtain an oil phase solution. (4) Take out the substrate membrane that has been soaked in deionized water for 24 hours in advance and remove the surface liquid with a rubber roller. Fix it in the reaction tank as a support material. Pour in the aqueous phase solution obtained in step (2) to wet the membrane surface for 1.9-2.1 min. After pouring out the aqueous phase in the reaction tank, tilt the reaction tank and dry the surface quickly with an air knife. Immediately pour in the oil phase solution obtained in step (3) and react on the membrane surface for 0.95-1.05 min. After the reaction is completed, pour out the oil phase and soak the membrane in deionized water for storage. (5) After cutting the membrane obtained in step (4), soaking it in glycerol, laying the separator, sealing it with epoxy resin and bonding the central tube, the membrane is wound into a roll to obtain a highly permeable and selectively negatively charged nanofiltration membrane.

2. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, In step (1), sulfonated polyvinyl alcohol is introduced into sulfonic acid groups by esterification of the sulfonated dicarboxylic acid with the hydroxyl groups of the PVA molecular chain under high temperature acid catalysis. The degree of alcoholysis of the PVA should not be less than 99%.

3. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, The concentrated acid is either concentrated hydrochloric acid or concentrated sulfuric acid.

4. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, The sulfosalicylic acid is one of 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, or p-sulfobenzoic acid.

5. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, In step (2), the polyamine monomer is piperazine, m-phenylenediamine, p-phenylenediamine, polyethyleneimine, 1,2-ethylenediamine, or 1,6-hexanediamine.

6. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, In step (3), the polyacryl chloride is one of pyromellitic triacryl chloride, isophenyltriacryl chloride, cyclohexanetriacryl chloride, cyclopentanetriacryl chloride, propionyl triacryl chloride or pentanetriacryl chloride; the solvent of the organic solution A of the polyacryl chloride in step (3) is one of n-hexane, n-heptane or tetradecane.

7. The method for preparing a highly permeable and selectively charged nanofiltration membrane according to claim 1, characterized in that, In step (4), the base membrane is made of one of polyethersulfone, polysulfone, polyethylene, polyamide imide, polypropylene or polyacrylonitrile, and the molecular weight cutoff of the base membrane should be between 100 and 300 kDa.

8. A highly permeable and selectively negatively charged nanofiltration membrane prepared by any one of claims 1-7.

9. The application of the highly permeable and selectively negatively charged nanofiltration membrane according to claim 8 in the selective separation of monovalent and multivalent ions.

10. The application of the highly permeable and selectively negatively charged nanofiltration membrane according to claim 8 in the selective separation of mineral ion organic pollutants.

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