A hydrophilic intermediate layer acid-resistant nanofiltration membrane and its preparation method

Abstract

This invention discloses a method for preparing an acid-resistant nanofiltration membrane with a hydrophilic interlayer. An interlayer is obtained by sequentially depositing a hydrocarbon compound solution and a diazonium salt solution on the surface of an ultrafiltration membrane. A nanofiltration membrane is then prepared on this interlayer through interfacial polymerization. This method effectively controls the interfacial polymerization process, altering the structure and morphology of the separation layer, thereby improving the performance of the nanofiltration membrane.

Description

Technical Field

[0001] This invention belongs to the field of water treatment material preparation technology, and specifically relates to a method for preparing an acid-resistant nanofiltration membrane modified with a hydrophilic interlayer. Background Technology

[0002] Membrane separation, as a novel separation technology, is gradually becoming a research hotspot due to its low energy consumption, low cost, and ease of industrialization. Its separation principle involves forcing substances of a certain size through the membrane under pressure, temperature, or an electric field, while retaining substances larger than that size, thus achieving separation. No change in the chemical properties of the materials occurs during this process; it is a physical separation. Based on different membrane pore sizes (or molecular weight cutoffs), membranes can be classified into microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes. Microfiltration membranes, with pore sizes between 100-1000 nm, can retain soil particles, microorganisms, and algae in solution, while most solvents and a small amount of large molecular solutes can permeate through the membrane. Ultrafiltration membranes, with pore sizes between 2-100 nm, are used to remove suspended solids, colloids, particles, bacteria, viruses, and other large molecules from water. Reverse osmosis membranes, with pore sizes below 1 nm, can retain various inorganic ions in water, producing pure water. Nanofiltration technology has received widespread attention and research since its invention in the 1890s. It was initially called "low-pressure reverse osmosis" or "loose reverse osmosis." Nanofiltration membranes have pore sizes of around 1 nm, a molecular rejection rate of 300-500 Da, and separation capabilities between reverse osmosis and ultrafiltration. Similar to reverse osmosis, it has important applications in separating organic salts and small organic molecules. However, compared to reverse osmosis, it has a higher flux, effectively reducing operating pressure and saving operating costs. Simultaneously, it has a lower rejection rate for high-valent salts, exhibiting good selectivity, and can be widely used in water treatment, pharmaceuticals, biotechnology, and food processing.

[0003] Currently, most nanofiltration membranes on the market are composite membrane structures, consisting of a nonwoven fabric, a base membrane, and a separation layer. The separation layer is primarily prepared through interfacial polymerization of an aqueous phase containing multifunctional organic amines and an oil phase containing multifunctional organic acyl chlorides. It is generally believed that the base membrane mainly provides the mechanical strength of the composite membrane, while the separation layer determines the permeability of the nanofiltration membrane. Therefore, many studies have focused on modifying the structure of the separation layer to regulate the performance of the composite membrane. This includes synthesizing new monomers, doping with nanoparticles, and altering membrane fabrication conditions. However, research has found that the structure and properties of the base membrane are crucial to the formation of the separation layer. By changing the pore structure and surface chemical properties of the base membrane surface, the structure of the separation layer can be effectively adjusted, thereby improving the performance of the nanofiltration membrane. Adding additives to the base membrane (Journal of Membrane Science, 2009, 336(1-2): 140-148) can alter the hydrophilicity and surface pore structure of the base membrane, thereby obtaining separation layers with different properties; rebuilding the surface pore structure of the base membrane (Journal of Membrane Science, 2017, 541: 39-52) involves immersing the base membrane material in a good solvent, causing the polymer chains on the base membrane surface to loosen and rearrange, resulting in different pore structures; and surface modification of the base membrane (Journal of Membrane Science, 2006, 286(1-2): 193-201) can introduce different functional groups, including amino, hydroxyl, and carboxyl groups, onto the base membrane surface, thereby altering the interfacial polymerization process.

[0004] Meanwhile, Livingston (Science, 2015, 348(6241):1347-1351) found that by adding a uniformly pore-distributed cadmium hydroxide nanowire interlayer between the base membrane and the separation layer, an ultrathin separation layer can be prepared by controlling the diffusion of aqueous monomers, significantly improving the flux of the composite membrane. Therefore, the preparation of the interlayer has received widespread attention. Multi-walled carbon nanotubes (Journal of Membrane Science, 2016, 515:238-244.), single-walled carbon nanotubes (Small, 2016, 12(36):5034-5041.), copper hydroxide nanowires (Applied Materials Today, 2017, 8:54-59.), and nanocellulose crystals (Journal of Materials Chemistry A, 2017, 5(31):16289-16295.) are all used as interlayers. By utilizing their uniform pore structure, the migration rate of aqueous monomers can be controlled. Therefore, ultrathin separation layers were obtained, exhibiting high throughput. However, these methods all involve depositing the material onto the substrate surface via vacuum filtration, making large-scale industrial production difficult.

[0005] This invention discloses a simple method for preparing an intermediate layer, namely, a hydrophilic intermediate layer can be obtained by surface deposition, which improves the separation performance of nanofiltration membranes and shows broad application prospects. Summary of the Invention

[0006] To improve the performance of nanofiltration membranes, this invention employs a simple method to deposit a hydrophilic intermediate layer on the surface of a base membrane, and then prepares a composite membrane via interfacial polymerization. This method allows for effective control of the interfacial polymerization process, altering the structure and morphology of the separation layer, thereby enhancing the performance of the nanofiltration membrane.

[0007] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is disclosed. This method involves sequentially depositing a hydrocarbon compound solution and a diazonium salt solution onto the surface of an ultrafiltration membrane, thereby obtaining an interlayer on the ultrafiltration membrane surface. Furthermore, the nanofiltration membrane prepared by interfacial polymerization on the interlayer exhibits superior performance. Compared to other interlayer preparation methods, this invention offers advantages such as low cost and simple operation.

[0008] A method for preparing an acid-resistant nanofiltration membrane modified with a hydrophilic interlayer includes the following steps:

[0009] 1) An intermediate layer can be obtained on the surface of an ultrafiltration membrane by successively depositing a hydrocarbon compound solution and a diazonium salt solution.

[0010] 2) Nanofiltration membranes are prepared on the intermediate layer using interfacial polymerization;

[0011] The hydrocarbon compound is selected from one or a mixture of several of tannic acid (TA), 2,4-diaminobenzenesulfonic acid (DBSA), 3,5-diaminobenzoic acid (DABA), α-naphthol, and α-naphtholsulfonic acid, with a concentration of 0.3-5 wt%.

[0012] The diazonium salt is selected from one or a mixture of several of 4,4-(9-pyroxene)diphenylamine diazonium salt (DDS), fluoroborate diazonium salt, pyrazole diazonium salt and triterpenoid diazonium salt, with a concentration of 0.01-5 wt%.

[0013] The specific preparation method of the acid-resistant nanofiltration membrane is as follows:

[0014] The ultrafiltration membrane is first soaked and washed with a 5-85 wt% ethanol solution.

[0015] The ultrafiltration membrane material includes polysulfone, polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polyimide, cellulose acetate, etc. The membrane is obtained by coating a nonwoven fabric, specifically through the following process: dissolving one or more polymers in an organic solvent, stirring thoroughly to dissolve; allowing to stand or removing air bubbles under vacuum; coating the polymer solution onto the nonwoven fabric using a doctor blade or spin coater; placing the nonwoven fabric in a coagulation bath and washing thoroughly with deionized water to obtain the polymer membrane.

[0016] The diazonium salt includes one or a mixture of several of the following: 4,4-(9-pyroxene)diphenylamine diazonium salt (DDS), fluoroborate diazonium salt, pyrazole diazonium salt, and triterpenoid diazonium salt, with a concentration of 0.01-5 wt%.

[0017] The hydrocarbon compounds include one or a mixture of several of the following: tannic acid (TA), 2,4-diaminobenzenesulfonic acid (DBSA), 3,5-diaminobenzoic acid (DABA), α-naphthol, α-naphtholsulfonic acid, etc., with a concentration of 0.3-5 wt%.

[0018] The deposition time of the compound solution and diazonium salt solution on the base film surface is 0.1-1.5 h.

[0019] The organic amine containing multifunctional groups is one or a combination of two or more of piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, ethylenediamine, N,N-bis(2-aminoethyl)ethylenediamine, trivinyltetramine, divinyltriamine, and polyethyleneimine, with a concentration of 0.1-5 wt%.

[0020] The organic acyl chloride containing multifunctional groups is one or a mixture of several of the following: trimesoyl chloride, phthaloyl chloride, phthaloyl chloride, pyromellitic acyl chloride, malonyl chloride, glutaryl chloride, fumarate chloride, etc. The solvent is one or a combination of two or more of the following: n-hexane, cyclohexane, n-heptane, toluene, benzene, isopar G, isopar E, isopar H, isopar L, isopar M, and the concentration is 0.01-3 wt%.

[0021] The interfacial polymerization reaction time is 10-240 s.

[0022] The oven has a heating temperature of 30-80℃ and a heating time of 1-60 minutes.

[0023] The preparation method of this hydrophilic intermediate layer modified nanofiltration membrane mainly includes the following steps: First, the ultrafiltration base membrane coated on the nonwoven fabric is thoroughly washed with an ethanol aqueous solution and dried in an oven; the base membrane is fixed with the front side facing up; an aqueous solution of one or more hydrocarbon compounds is prepared and the base membrane surface is covered with the solution and deposited for a period of time; it is taken out, thoroughly washed with deionized water, dried, and the dried base membrane is refixed; then, aromatic primary amines are reacted with nitrous acid in a low temperature and strong acid environment to generate diazonium salts, and a diazonium salt solution is prepared. The diazonium salt solution is poured onto the base membrane for coupling and crosslinking reaction, and the reaction is carried out for a period of time; it is taken out, thoroughly washed with deionized water, dried, and the dried base membrane is refixed; its surface is soaked in an aqueous solution of an organic amine containing multifunctional groups; after a period of time, the aqueous phase is dried with an air knife; then it is soaked in an oil phase of an organic acyl chloride containing multifunctional groups for a period of time to fully react; after the oil phase is discarded, it is placed in an oven for a period of time to obtain a composite membrane.

[0024] This invention also claims protection for the hydrophilic interlayer modified nanofiltration membrane prepared by the above method. The hydrophilic interlayer can enhance the deposition of amine monomers, allowing them to spread uniformly on the base membrane and adjusting their diffusion. The introduction of hydrophilic groups, which protonate under the influence of water, charges the membrane surface, thus modulating the charge properties of the composite membrane surface and affecting the interaction between the membrane and the reactants. The hydrophilic interlayer material typically contains hydrophilic groups such as hydroxyl and carboxyl groups, which can form hydrogen bonds with water molecules, promoting water molecule transfer and providing lateral transport channels for the mass transfer process, thereby improving mass transfer efficiency. Furthermore, the modified interlayer can effectively resist hydrolysis caused by acids and exhibits good salt treatment capabilities in acidic environments.

[0025] Diazo coupling crosslinking refers to the electrophilic substitution reaction that occurs between diazonium salts and phenols, aromatic amines, and compounds containing active methylene groups under weak acid, weak base, and low temperature conditions, ultimately producing azo compounds. In the coupling reaction, the diazonium cation acts as an electrophile, attacking the carbon atom with the highest electron cloud density in the coupling component. Phenols and aromatic amines participate in the reaction as phenoxide anions (ArO₂) in the coupling components. - In a weakly alkaline medium, and in an acidic medium, the amine reacts with a diazonium salt (in an acidic medium) and a free amine. During the reaction, intermediates I and II are initially formed reversibly. Subsequently, the intermediates rapidly lose a proton and irreversibly transform into azo compounds. Additionally, when diazonium salts react with primary or secondary aromatic amines, they may also react directly with the active hydrogen atom on the ammonia atom to form diazonium amino compounds. The coupling reaction mechanism is summarized in the following reaction equation.

[0026]

[0027] The present invention has the following beneficial effects:

[0028] A polymer-based membrane with a hydrophilic interlayer can be obtained by sequentially depositing a prepared hydrocarbon compound solution and a diazonium salt solution on the surface of an ultrafiltration membrane. The hydrophilic interlayer enhances the deposition of amine monomers, allowing them to spread uniformly on the membrane, and also modulates their diffusion. The hydrophilic interlayer typically introduces hydrophilic groups, which protonate under the influence of water, giving the membrane surface a charge. This can modulate the charge properties of the composite membrane surface, thereby affecting the interaction between the membrane and the reactants, which is crucial for improving the desalination performance of the composite membrane. The hydrophilic interlayer material usually contains hydrophilic groups such as hydroxyl and carboxyl groups. These groups can form hydrogen bonds with water molecules, promoting water molecule transport and providing lateral transport channels for the mass transfer process, thus improving mass transfer efficiency. Furthermore, the modified interlayer can effectively resist hydrolysis caused by acids and exhibits good salt removal capabilities even in acidic environments. High-performance nanofiltration membranes can be obtained through this hydrophilic interlayer. Its operation is simple and inexpensive, and it can improve the performance of nanofiltration composite membranes in a short time, showing broad application prospects. Attached Figure Description

[0029] Figure 1 This is an AFM image of the surface of the polyethylene ultrafiltration membrane and the polymer hydrophilic interlayer in Embodiment 1 of the present invention.

[0030] Figure 2 This is a surface SEM image of the polyethylene ultrafiltration membrane and the polymer hydrophilic intermediate layer in Embodiment 1 of the present invention.

[0031] Figure 3 This is an AFM image of the surface of the nanofiltration membrane prepared on the polyethylene ultrafiltration membrane and the polymer hydrophilic intermediate layer in Example 1 of the present invention.

[0032] Figure 4 This is a SEM image of the surface of the nanofiltration membrane prepared on the polyethylene ultrafiltration membrane and the polymer hydrophilic intermediate layer in Example 1 of the present invention. 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] In the following examples, after pre-pressurizing for 2 hours at 1.0 MPa, 25°C, and 7 LPM, the desalination rate and permeate flux of the prepared nanofiltration composite membrane for 2000 mg / L MgSO4 or 2000 mg / L NaCl were tested. The permeate flux of the nanofiltration composite membrane is expressed in liters per square meter per hour.

[0035] Example 1

[0036] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0037] (1) Wash the polyethylene ultrafiltration membrane with a 30wt% ethanol aqueous solution.

[0038] (2) Pour the prepared 2wt% tannic acid (TA) aqueous solution onto the base membrane, spread it over the entire base membrane, remove the excess TA solution after 10 min, wash thoroughly with deionized water, and blow dry.

[0039] (3) Pour the prepared 4,4-(9-pyroxene)diphenylamine diisocyanate (DDS) solution onto the base film obtained in step (2) for coupling and crosslinking reaction. After reacting for 1 minute, pour off the excess solution and rinse the PE film surface with deionized water. Soak the film in deionized water for 24 hours for later use.

[0040] The DDS diazonium salt solution was prepared as follows: First, 1 mmol of 4,4'-(9-pyridyl)diphenylamine was added to 100 ml of deionized water. Then, 2.4 ml of concentrated hydrochloric acid was added while stirring. After the system was completely dissolved, it was placed in an ice-water bath to lower the temperature to 0-5℃. While stirring, 2.05 ml of a 1 mol / L NaNO2 solution was added. After stirring for 30 min, it was stored at 0-5℃ for later use.

[0041]

[0042] DDS Synthesis Steps

[0043]

[0044] Reaction mechanism of TA-DDS

[0045] (4) Under normal pressure, temperature of 20℃ and relative humidity of 40%, a 1wt% trivinyltetramine (TETA) solution was coated onto the surface of the PE film containing the intermediate layer, left to stand for 5 minutes, poured out and dried with an air knife.

[0046] (5) A 0.15 wt% solution of 1,3,6-naphthalenetrisulfonyl chloride (NTSC) cyclohexane was uniformly coated onto the surface of the membrane obtained in step (4), and left for 3 min to carry out interfacial polymerization.

[0047] (6) Place the obtained composite membrane in an 80℃ oven for 5 minutes to complete the reaction.

[0048] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 55.6 LMH, a MgSO4 rejection rate of 21.3%, and a NaCl rejection rate of 18.3%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 35.6 LMH, a MgSO4 rejection rate of 97.99%, and a NaCl rejection rate of 95.60%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 70.2 LMH, a MgSO4 rejection rate of 10.9%, and a NaCl rejection rate of 8.56%, while the modified membrane with the intermediate layer had a flux of 40.3 LMH, a MgSO4 rejection rate of 96.32%, and a NaCl rejection rate of 93.84%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0049] Example 2

[0050] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0051] Replace 1 wt% of trivinyltetramine (TETA) in step (4) of Example 1 with 1 wt% piperazine (PIP), and the remaining steps are the same as in Example 1.

[0052] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 49.3 LMH, a MgSO4 rejection rate of 40.2%, and a NaCl rejection rate of 35.6%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 34.3 LMH, a MgSO4 rejection rate of 98.23%, and a NaCl rejection rate of 96.35%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 59.3 LMH, a MgSO4 rejection rate of 32.6%, and a NaCl rejection rate of 28.6%, while the modified membrane with the intermediate layer had a flux of 42.3 LMH, a MgSO4 rejection rate of 95.12%, and a NaCl rejection rate of 94.36%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0053] Example 3

[0054] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0055] Replace the 1 wt% trivinyltetramine (TETA) in step (4) of Example 1 with polyethyleneimine (PEI) with a concentration of 1 wt% and a molecular weight of 10,000, and the remaining steps are the same as in Example 1.

[0056] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 35.2 LMH, a MgSO4 rejection rate of 80.69%, and a NaCl rejection rate of 75.32%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 24.3 LMH, a MgSO4 rejection rate of 97.35%, and a NaCl rejection rate of 95.31%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 42.36 LMH, a MgSO4 rejection rate of 70.26%, and a NaCl rejection rate of 66.35%, while the modified membrane with the intermediate layer had a flux of 33.5 LMH, a MgSO4 rejection rate of 93.65%, and a NaCl rejection rate of 92.26%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0057] Example 4

[0058] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0059] Replace 0.15 wt% of 1,3,6-naphthalenetrisulfonyl chloride (NTSC) in step (5) of Example 1 with 0.15 wt% of trimesoyl chloride (TMC), and the remaining steps are the same as in Example 1.

[0060] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 47.3 LMH, a MgSO4 rejection rate of 42.2%, and a NaCl rejection rate of 39.6%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 33.9 LMH, a MgSO4 rejection rate of 98.23%, and a NaCl rejection rate of 96.35%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 59.3 LMH, a MgSO4 rejection rate of 32.6%, and a NaCl rejection rate of 28.6%, while the modified membrane with the intermediate layer had a flux of 42.3 LMH, a MgSO4 rejection rate of 95.12%, and a NaCl rejection rate of 94.36%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0061] Example 5

[0062] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0063] The 1 wt% trivinyltetramine (TETA) in step (4) of Example 1 was replaced with 2 wt% m-phenylenediamine (MPD), and the 0.15 wt% 1,3,6-naphthalenetrisulfonyl chloride (NTSC) in step (5) was replaced with 0.15 wt% trimesoyl chloride (TMC). The remaining steps were the same as in Example 1.

[0064] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 16.3 LMH, a MgSO4 rejection rate of 99.13%, and a NaCl rejection rate of 98.86%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 15.4 LMH, a MgSO4 rejection rate of 99.26%, and a NaCl rejection rate of 99.01%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 20.4 LMH, a MgSO4 rejection rate of 93.26%, and a NaCl rejection rate of 92.16%, while the modified membrane with the intermediate layer had a flux of 18.5 LMH, a MgSO4 rejection rate of 96.32%, and a NaCl rejection rate of 95.18%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0065] Example 6

[0066] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0067] Replace the 2 wt% tannic acid (TA) in step (2) of Example 1 with 2 wt% 2,4-diaminobenzenesulfonic acid (DBSA), and the remaining steps are the same as in Example 1.

[0068] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 55.6 LMH, a MgSO4 rejection rate of 21.3%, and a NaCl rejection rate of 18.3%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 45.3 LMH, a MgSO4 rejection rate of 97.65%, and a NaCl rejection rate of 95.23%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 70.2 LMH, a MgSO4 rejection rate of 10.9%, and a NaCl rejection rate of 8.56%, while the modified membrane with the intermediate layer had a flux of 53.26 LMH, a MgSO4 rejection rate of 96.32%, and a NaCl rejection rate of 93.23%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0069] Example 7

[0070] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0071] Replace the 2 wt% tannic acid (TA) in step (2) of Example 1 with 2 wt% 1-amino-2-naphthol-4-sulfonic acid, and the remaining steps are the same as in Example 1.

[0072] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 55.6 LMH, a MgSO4 rejection rate of 21.3%, and a NaCl rejection rate of 18.3%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 45.3 LMH, a MgSO4 rejection rate of 97.36%, and a NaCl rejection rate of 94.23%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 70.2 LMH, a MgSO4 rejection rate of 10.9%, and a NaCl rejection rate of 8.56%, while the modified membrane with the intermediate layer had a flux of 50.31 LMH, a MgSO4 rejection rate of 95.98%, and a NaCl rejection rate of 93.65%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0073] Example 8

[0074] A method for preparing a nanofiltration membrane with a hydrophilic interlayer is as follows:

[0075] In Example 1, the 2 wt% tannic acid (TA) in step (2) was replaced with 2 wt% 1-amino-2-naphthol-4-sulfonic acid, and the DDS diazonium salt in step (3) was replaced with a diazonium salt diazotized from p-nitroaniline. The remaining steps were the same as in Example 1.

[0076] The p-nitroaniline diazonium salt solution was prepared as follows: First, 1 mmol of p-nitroaniline was added to 50 ml of deionized water. Then, 3 ml of concentrated hydrochloric acid was added while stirring. After the system was completely dissolved, it was placed in an ice-water bath to lower the temperature to 0-5℃. While stirring, 6.5 ml of a 1 mol / L NaNO2 solution was added. After stirring for 30 min, it was stored at 0-5℃ for later use.

[0077]

[0078] Diazotization reaction of p-nitroaniline

[0079] The separation performance of the prepared nanofiltration membranes was tested. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane had a flux of 55.6 LMH, a MgSO4 rejection rate of 21.3%, and a NaCl rejection rate of 18.3%. The nanofiltration membrane prepared from the polyethylene ultrafiltration membrane with the intermediate layer had a flux of 40.2 LMH, a MgSO4 rejection rate of 95.21%, and a NaCl rejection rate of 92.31%. After soaking in 20 wt% H2SO4 at 80 °C for 24 h, the unmodified membrane had a flux of 70.2 LMH, a MgSO4 rejection rate of 10.9%, and a NaCl rejection rate of 8.56%, while the modified membrane with the intermediate layer had a flux of 49.2 LMH, a MgSO4 rejection rate of 93.21%, and a NaCl rejection rate of 90.63%. Therefore, the intermediate layer can effectively improve the separation and acid resistance performance of the nanofiltration composite membrane.

[0080] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing an acid-resistant nanofiltration membrane modified with a hydrophilic interlayer, comprising the following steps: 1) An intermediate layer can be obtained on the surface of an ultrafiltration membrane by successively depositing a hydrocarbon compound solution and a diazonium salt solution. 2) Nanofiltration membranes are prepared on the intermediate layer using interfacial polymerization; The hydrocarbon compound is selected from one or a mixture of two of 2,4-diaminobenzenesulfonic acid and 3,5-diaminobenzyl, with a concentration of 0.3-5 wt%. In step 2), interfacial polymerization refers to soaking the surface of the intermediate layer obtained in step 1) in an aqueous solution of an organic amine containing multifunctional groups; and then soaking it in an oil phase of an organic acyl chloride containing multifunctional groups for further reaction. The diazonium salt is selected from one or a mixture of several of 4,4-(9-enzyme)diphenylamine diazonium salt, fluoroborate diazonium salt, pyrazole diazonium salt and triterpenoid diazonium salt, with a concentration of 0.01-5 wt%.

2. The method for preparing the acid-resistant nanofiltration membrane with a hydrophilic interlayer according to claim 1, characterized in that, The ultrafiltration membrane material is selected from one or more of polysulfone, polyethersulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polyimide and cellulose acetate.

3. The method for preparing the acid-resistant nanofiltration membrane with a hydrophilic interlayer according to claim 1, characterized in that, Before step 1), the ultrafiltration membrane coated on the nonwoven fabric is thoroughly washed with an ethanol aqueous solution with a concentration of 5-85 wt.

4. The method for preparing the acid-resistant nanofiltration membrane with a hydrophilic interlayer according to claim 1, characterized in that, The deposition time of the hydrocarbon compound solution and the diazonium salt solution on the base film surface is 0.1-1.5 h.

5. The method for preparing the acid-resistant nanofiltration membrane modified with a hydrophilic interlayer according to claim 1, characterized in that, The organic amine containing multifunctional groups is one or a combination of two or more of piperazine, 2-methylpiperazine, 2,5-dimethylpiperazine, 2,6-dimethylpiperazine, 1,2-diaminocyclohexane, 1,4-diaminocyclohexane, trivinyltetramine, ethylenediamine, N,N-bis(2-aminoethyl)ethylenediamine, divinyltriamine, and polyethyleneimine, with a concentration of 0.1-5 wt%.

6. The method for preparing the acid-resistant nanofiltration membrane modified with a hydrophilic interlayer according to claim 1, characterized in that, The organic acyl chloride containing multifunctional groups is one or a mixture of several of the following: trimesoyl chloride, terephthaloyl chloride, phthaloyl chloride, pyromellitic acyl chloride, malonyl chloride, glutaryl chloride, and fumarate chloride. The solvent is one or a combination of two or more of the following: n-hexane, cyclohexane, n-heptane, toluene, benzene, isopar G, isopar E, isopar H, isopar L, and isopar M. The concentration is 0.01-3 wt%.

7. The method for preparing the acid-resistant nanofiltration membrane modified with a hydrophilic interlayer according to any one of claims 1-6, characterized in that, The steps are as follows: First, the ultrafiltration membrane coated on the nonwoven fabric is thoroughly washed with an ethanol-water solution and dried in an oven. The ultrafiltration membrane is then fixed with the front side facing up. One or two hydrocarbon compounds are prepared into an aqueous solution, and the membrane surface is covered with the solution for deposition. The membrane is then removed, thoroughly washed with deionized water, dried, and refixed. Next, aromatic primary amines are reacted with nitrous acid in a low-temperature and strong acid environment to generate diazonium salts, and a diazonium salt solution is prepared. This solution is then poured onto the membrane for coupling and crosslinking. The membrane is then removed, thoroughly washed with deionized water, dried, and refixed. The surface is then soaked in an aqueous solution of an organic amine containing multifunctional groups. The aqueous phase is dried with an air knife. The membrane is then soaked in an oil phase of an organic acyl chloride containing multifunctional groups for a complete reaction. Finally, the membrane is dried to obtain a composite membrane.

8. The method for preparing the acid-resistant nanofiltration membrane modified with a hydrophilic interlayer according to claim 7, characterized in that, The coupling and crosslinking reaction refers to the electrophilic substitution reaction that occurs between diazonium salts and phenols, aromatic amines, and compounds containing active methylene groups under weak acid, weak base, and low temperature conditions, ultimately producing azo compounds.

9. The nanofiltration membrane prepared by the method of preparing the modified acid-resistant nanofiltration membrane with hydrophilic interlayer according to any one of claims 1-8.

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