An acid-resistant composite nanofiltration membrane and its preparation method

Abstract

This invention discloses an acid-resistant composite nanofiltration membrane and its preparation method, relating to the field of membrane separation technology. The method includes: cleaning a polysulfone ultrafiltration membrane; co-depositing a polydopamine / tannic acid interlayer in a Tris-HCl buffer solution containing dopamine hydrochloride and tannic acid; sequentially coupling-modifying the interlayer with γ-aminopropyltriethoxysilane and polyethylene glycol diglycidyl ether; preparing a coating solution with sulfonated polyetheretherketone and phytic acid, applying it to the surface of the coupling-modified membrane, and then curing it through thermal crosslinking. This method constructs an acid-resistant interlayer through co-deposition, utilizes a silane coupling agent and a flexible epoxy compound to achieve interlayer covalent bridging, and then forms a charged functional separation layer using a sulfonated polyetheretherketone / phytic acid crosslinking system. The resulting composite nanofiltration membrane exhibits structural stability in a strongly acidic environment, high rejection rate for multivalent metal ions, strong interlayer bonding, and excellent resistance to interfacial micro-exfoliation, significantly improving the membrane's service life and operational stability.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, specifically to an acid-resistant composite nanofiltration membrane and its preparation method. Background Technology

[0002] Nanofiltration membranes, as a pressure-driven membrane intermediate between ultrafiltration and reverse osmosis, have been widely used in water treatment, chemical separation, and food processing due to their unique molecular sieving and charge repulsion mechanisms. In recent years, with the continuous expansion of applications in hydrometallurgy, waste battery recycling, and acidic mine drainage treatment, there has been an urgent need for the long-term operational stability of nanofiltration membranes under strong acid conditions (pH < 2). Currently, most commercial nanofiltration membranes use polyamide as the separation functional layer, and their preparation typically employs interfacial polymerization methods involving piperazine or m-phenylenediamine with trimesoyl chloride. However, the amide bonds in the polyamide structure are highly susceptible to hydrolytic breakage in strong acid media, leading to membrane structure destruction and a sharp decline in retention performance, severely limiting their industrial application in acidic environments.

[0003] In existing technologies, researchers have attempted to crosslink acyl chloride monomers with steric hindrance effects with polyamine compounds, and then regulate the membrane density and flux through subsequent acid dissociation treatment. For example, Chinese Patent Publication No. CN114950135B discloses an acid-resistant nanofiltration membrane and its preparation method, which utilizes cyanuric chloride and polyethyleneimine for interfacial polymerization, followed by secondary crosslinking with glutaraldehyde, and finally dissociates the cyanuric chloride component by acid immersion to obtain an acid-resistant membrane. Although this method improves the acid resistance of the membrane to some extent, it has the following shortcomings: First, the acid dissociation step is extremely sensitive to operating conditions (acid concentration, immersion time), resulting in poor process controllability and easy batch-to-batch performance fluctuations; second, the membrane structure after dissociation is relatively loose, making it prone to compaction deformation under high pressure, resulting in insufficient long-term operational stability; third, the cyanuric chloride used in the preparation process is highly toxic, placing high demands on the operating environment and wastewater treatment. Therefore, developing a simple, environmentally friendly, stable membrane structure and excellent acid resistance method for preparing nanofiltration membranes is of great significance for promoting the large-scale application of nanofiltration technology in acidic systems. Summary of the Invention

[0004] The purpose of this invention is to provide an acid-resistant composite nanofiltration membrane and its preparation method to solve the technical problems mentioned in the background.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: (1) Immerse the polysulfone ultrafiltration membrane in deionized water to remove residual substances, and dry the free water on the membrane surface for later use.

[0006] (2) Dissolve tris(hydroxymethyl)aminomethane in deionized water, add hydrochloric acid solution to adjust the pH value, and make up to volume to obtain Tris-HCl buffer solution.

[0007] (3) Add dopamine hydrochloride and tannic acid to the above Tris-HCl buffer solution in sequence and stir to dissolve, and prepare a co-deposition mother liquor; immerse the pretreated polysulfone ultrafiltration membrane in the co-deposition mother liquor for shaking reaction, and wash it with deionized water and anhydrous ethanol in sequence after the reaction is completed, and then dry it to obtain PDA / TA@PSf membrane.

[0008] (4) Add γ-aminopropyltriethoxysilane to an aqueous ethanol solution and stir to pre-hydrolyze. Immerse the above PDA / TA@PSf membrane in the silane hydrolysate for reaction. After taking it out, wash it with an aqueous ethanol solution. Add polyethylene glycol diglycidyl ether to an aqueous ethanol solution and stir to dissolve. Then immerse the membrane after washing to obtain the coupling modified membrane.

[0009] (5) Slowly add polyether ether ketone powder to concentrated sulfuric acid and stir to carry out sulfonation reaction; after the reaction is completed, pour the reaction solution into an ice-water mixture to precipitate the product, filter it, wash it repeatedly with deionized water until neutral and dry it, and then grind it to obtain sulfonated polyether ether ketone powder.

[0010] (6) The sulfonated polyether ether ketone powder prepared above is added to deionized water and heated and stirred until completely dissolved. After cooling, phytic acid aqueous solution is added and stirred evenly. After standing and degassing treatment, SPEEK / phytic acid coating solution is obtained.

[0011] (7) The above-mentioned coupling modified membrane is laid flat and fixed, and SPEEK / phytic acid coating liquid is applied to its surface to form a uniform liquid film; pre-drying and high-temperature thermal crosslinking treatment are performed in sequence to cure the coating; after cooling, the membrane is peeled off and immersed in deionized water for cleaning to obtain the acid-resistant composite nanofiltration membrane.

[0012] In the technical solution of this invention, the acid resistance of the composite nanofiltration membrane is improved in the following ways: (1) Dopamine hydrochloride and tannic acid are co-deposited in a weakly alkaline buffer system. The core purpose is to construct an intermediate transition layer with strong adhesion and excellent acid resistance on the surface of the polysulfone-based membrane. Dopamine is a naturally occurring small molecule containing a catechol structure. Its polydopamine film formed by oxidative self-polymerization is known for its super strong substrate adhesion and can be firmly attached to the surface of almost all types of materials. This provides a reliable anchoring platform for the subsequent construction of functional layers. At the same time, tannic acid, as a natural polyphenol macromolecule, contains a large number of catechol and pyrogallol structural units. These phenolic hydroxyl groups hardly undergo proton dissociation under strong acid conditions, and the chemical bonds are extremely stable. Therefore, this deposited layer has a much better acid resistance than the traditional amide bond system. The co-deposition process of the two under weakly alkaline conditions is actually a complex process of intermolecular interaction and cross-linking between the quinone intermediates produced by dopamine oxidation and the phenolic hydroxyl groups of tannic acid. In the end, a dense organic thin film layer with carbon-carbon bonds and carbon-oxygen bonds as the main framework and rich in a large number of polar hydroxyl groups and aromatic ring structures is formed. This framework system can still maintain structural integrity in high-concentration acid solutions with pH values ​​as low as 0.5. At the same time, the abundant phenolic hydroxyl and amino functional groups provide ample anchoring sites for the chemical grafting of the upper functional layer, fundamentally solving the problem of the overall structural collapse of traditional nanofiltration membranes due to the inability of the bottom layer to withstand acid. (2) A dense charged functional separation layer is constructed on the surface of the intermediate layer using a thermal cross-linking system of sulfonated polyether ether ketone and phytic acid. The purpose is to achieve efficient retention of target metal ions in acidic solutions through the synergistic effect of Donnan charge repulsion mechanism and size sieving effect. Sulfonated polyether ether ketone is a high-performance engineering plastic derivative that incorporates sulfonic acid groups into its main aromatic ether structure. Its aromatic ether backbone is composed of chemically inert carbon-carbon and carbon-oxygen bonds, which neither hydrolyze nor degrade in strong acid environments, thus ensuring the long-term acid resistance and stability of the separation layer skeleton itself. In addition, the acid dissociation constant of the sulfonic acid group is extremely low, which means that even under extremely acidic conditions with a pH of 0 to 1, the sulfonic acid group still maintains a nearly completely deprotonated state and exhibits a constant negative charge, thereby forming a continuous and strong electrostatic repulsion barrier against divalent and trivalent metal cations on the membrane surface. Phytic acid, a natural organic phosphoric acid molecule containing six phosphate ester groups, can form a strong ionic crosslinking and dense hydrogen bond network with the sulfonic acid groups on the sulfonated polyether ether ketone chain under heating conditions. This weaves the originally linear polymer chain into a three-dimensional network structure, which not only reduces the effective pore size of the separation layer to the sub-nanometer level, thereby enhancing the size interception ability of hydrated ions, but also further increases the negative charge density of the membrane surface through the phosphate groups carried by phytic acid itself, resulting in a double improvement in retention performance and acid resistance.

[0013] Preferably, in step (1), the polysulfone ultrafiltration membrane has a molecular weight cutoff of 50 kDa; the soaking and cleaning conditions are: soaking at room temperature for 24 hours, and replacing the deionized water every 8 hours during this period.

[0014] Preferably, in step (3), the mass ratio of dopamine hydrochloride to tannic acid is 2:(0.5-1.5).

[0015] Preferably, in step (3), the oscillation reaction temperature is 25-30°C and the oscillation reaction time is 10-12h.

[0016] Preferably, in step (4), the mass ratio of γ-aminopropyltriethoxysilane to polyethylene glycol diglycidyl ether is 1.5:(0.5-1).

[0017] In experiments, this invention revealed that the surface of the co-deposited polydopamine-tannic acid interlayer is rich in phenolic hydroxyl groups, while the coated sulfonated polyether ether ketone / phytic acid functional layer is itself a highly hydrophilic charged polymer system. Although both layers are hydrophilic, they lack covalent anchoring. Under the repeated effects of long-term acidic solution permeation and operating pressure, water molecules gradually penetrate along the interface between the two layers, weakening the interfacial bonding force. This leads to micro-delamination between the functional layer and the interlayer, resulting in irreversible degradation of the membrane's rejection rate and flux after continuous operation. To solve this technical problem, this invention introduces γ-aminopropyltriethoxysilane (KH-550) and polyethylene glycol diglycidyl ether (PEGDGE) sequentially for synergistic interfacial bridging. Specifically, firstly, the alkoxysilane end of the KH-550 molecule undergoes hydrolysis in a water-alcohol mixed environment, followed by a condensation reaction with the abundant phenolic hydroxyl groups on the surface of the intermediate layer to form a stable Si-O covalent bond anchoring. Meanwhile, the amino group at the other end is exposed on the membrane surface as an active site for subsequent crosslinking. Then, PEGDGE, a flexible long-chain molecule containing bi-epoxy end groups, undergoes a ring-opening reaction with the exposed amino group of KH-550 and the polar groups in the upper sulfonated polyether ether ketone segment, forming a flexible yet robust molecular bridging segment. This stepwise reaction ensures that the reactants in each step only undergo directional reactions at the membrane interface, completely avoiding the problem of premature crosslinking and consumption of the two additives in solution. The combined effect of these two additives creates a gradient transition region at the interface, which was originally bound by weak physical action, and has both covalent anchoring and flexible buffering characteristics. This fundamentally eliminates the risk of interlayer micro-delamination, and the flexible segments of PEGDGE alleviate the interfacial stress concentration caused by the difference in thermal expansion coefficients between the two layers. This allows the composite membrane to maintain complete interlayer bonding under high pressure and acidic operating conditions, ultimately achieving a simultaneous improvement in membrane performance and service life.

[0018] Preferably, in step (4), the molecular weight of polyethylene glycol diglycidyl ether is 500 Da.

[0019] Preferably, in step (5), the stirring reaction is carried out at a constant temperature of 40-50°C for 3-5 hours.

[0020] Preferably, in step (6), the mass ratio of sulfonated polyether ether ketone powder to phytic acid is 3:(1-2).

[0021] Preferably, in step (7), the thermal crosslinking treatment temperature is 130-140°C and the time is 15-20 min.

[0022] An acid-resistant composite nanofiltration membrane is prepared by the method described above.

[0023] Compared with the prior art, the beneficial effects of the present invention are: 1. The intermediate transition layer constructed by co-deposition of dopamine hydrochloride and tannic acid, and the functional separation layer formed by cross-linking sulfonated polyether ether ketone and phytic acid, both have stable carbon-carbon bonds and carbon-oxygen bonds as the main framework. They do not undergo hydrolysis or degradation in a strong acid environment, which fundamentally solves the problem of structural collapse caused by the poor acid resistance of the bottom layer of traditional nanofiltration membranes.

[0024] 2. The functional separation layer utilizes the sulfonic acid groups of sulfonated polyether ether ketone to maintain a constant negative charge under strong acid conditions. Combined with the sub-nanometer three-dimensional network structure formed by phytic acid crosslinking, it achieves efficient retention of divalent and trivalent metal cations through the synergistic effect of Donnan charge repulsion and size sieving.

[0025] 3. By introducing γ-aminopropyltriethoxysilane and polyethylene glycol diglycidyl ether for synergistic interface bridging, a gradient transition region with covalent anchoring and flexible buffering is constructed between the intermediate layer and the functional layer, which effectively prevents performance degradation caused by interfacial micro-exfoliation during long-term operation and achieves simultaneous improvement in membrane performance and service life. Attached Figure Description

[0026] Figure 1 This is a low-magnification SEM image of the surface of the composite nanofiltration membrane prepared in Example 4 of the present invention.

[0027] Figure 2 This is a medium-magnification SEM image of the surface of the composite nanofiltration membrane prepared in Example 4 of the present invention.

[0028] Figure 3 This is a high-magnification SEM image of the surface of the composite nanofiltration membrane prepared in Example 4 of the present invention.

[0029] Figure 4 The image shows the XRD pattern of the composite nanofiltration membrane prepared in Example 4 of this invention. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1 A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: Step (1): Take one polysulfone (PSf) ultrafiltration flat sheet membrane with a molecular weight cutoff of 50 kDa (effective membrane area approximately 28.3 cm²). 2 Immerse the membrane completely in 500 mL of deionized water and let it stand at room temperature for 24 hours. Change the deionized water every 8 hours to thoroughly remove any residual glycerol or other protective solutions inside the membrane. After soaking, remove the membrane and gently press the membrane surface and back with clean qualitative filter paper to absorb the free water adhering to the surface. Place the dried membrane flat in a clean petri dish, cover it to prevent dust, and set it aside for later use.

[0032] Step (2): Weigh 0.605 g of tris(hydroxymethyl)aminomethane (Tris) into a 250 mL beaker, add about 80 mL of deionized water and stir until completely dissolved. Then, add 1 mol / L hydrochloric acid solution dropwise with a pipette while monitoring the pH with a pH meter until the pH of the solution drops precisely to 8.5. Then, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water and shake well to obtain a Tris-HCl buffer solution with a concentration of 50 mmol / L and a pH of 8.5.

[0033] Step (3): Weigh 0.2g of dopamine hydrochloride and 0.13g of tannic acid using an analytical balance, and add them to 100mL of Tris-HCl buffer solution prepared in step (2) and stir to dissolve, thus obtaining a co-deposition mother liquor; fix the polysulfone membrane treated in step (1) on the membrane holder with a stainless steel clamp and immerse it vertically in the co-deposition mother liquor, and continuously shake it at a frequency of 60rpm in a constant temperature shaking box at 28℃ for 11h. After the reaction is completed, take out the membrane and wash it in three beakers containing 200mL of deionized water for 1min each, and then wash it in three beakers containing about 200mL of anhydrous ethanol for 1min each. After cleaning, place the membrane in a 50℃ forced-air drying oven and dry it for 10min to obtain a PDA / TA@PSf membrane with a uniform brown surface.

[0034] Step (4): Mix 50 mL of anhydrous ethanol and 50 mL of deionized water to obtain an ethanol aqueous solution with a volume fraction of 50%. Add 1.5 g of γ-aminopropyltriethoxysilane (KH-550) and perform pre-hydrolysis by magnetic stirring at 300 rpm for 30 min at room temperature to obtain a uniform and transparent silane hydrolysate. Immerse the PDA / TA@PSf membrane obtained in step (3) flat into the hydrolysate, seal it, and place it in a constant temperature water bath at 50℃ for 1 h. After the reaction is completed, take out the membrane and place it in three containers of 200 mL of 50% ethanol. The membrane was rinsed in an aqueous solution for 2 minutes to remove uncovalently grafted free silane. Separately, 50 mL of anhydrous ethanol and 50 mL of deionized water were mixed evenly, and 0.9 g of polyethylene glycol diglycidyl ether (PEGDGE, molecular weight 500 Da) was added. The mixture was magnetically stirred for 10 minutes until it was evenly dissolved. The cleaned membrane was then immersed in the PEGDGE treatment solution, sealed, and placed in a 50°C constant temperature water bath for 2 hours to react. After the reaction was completed, the membrane was removed and rinsed 3 times with deionized water. It was then dried in a 50°C forced-air drying oven for 5 minutes to obtain the coupling modified membrane.

[0035] Step (5): In a fume hood, add 50 mL of concentrated sulfuric acid (98 wt%) to a 250 mL three-necked flask, and then slowly add 5.0 g of polyether ether ketone (PEEK) powder in three portions under stirring. After all the powder has been added, turn on the oil bath and heat to 45 °C. React at a constant temperature for 4 h under continuous stirring (200 rpm). After the reaction is completed, wait for the reaction solution to cool to about 35 °C, and slowly pour it into a 1000 mL mixture of crushed ice and water through a glass rod. The sulfonated polyether ether ketone will then precipitate out as a light yellow flocculent. Collect the precipitate by filtration with a Buchner funnel, and wash it repeatedly with deionized water until the pH of the filtrate is 6-7 and there is no white turbidity when BaCl2 solution is added. Transfer the washed filter cake to a vacuum drying oven at 60 °C and dry for 24 h. Take it out and grind it into a fine powder to obtain sulfonated polyether ether ketone (SPEEK) powder with a sulfonation degree of about 55%.

[0036] Step (6): Weigh 3.0g of SPEEK powder prepared in step (5) and place it in a 150mL conical flask. Add 97.0g of deionized water and stir at 300rpm for 50min in a 70℃ constant temperature water bath until completely dissolved and an amber transparent liquid is formed. After naturally cooling to room temperature, add 5.0g of phytic acid aqueous solution (containing 1.8g of phytic acid) and continue stirring for 30min until uniform. Then let it stand for 1h to remove bubbles and obtain SPEEK / phytic acid coating solution.

[0037] Step (7): Lay the coupling modified membrane obtained in step (4) flat on a clean, thick glass plate. Use the surface tension of the coating solution to allow the membrane to adhere naturally. Gently press to remove air bubbles and make the membrane surface flat and wrinkle-free. Take 5 mL of the SPEEK / phytic acid coating solution prepared in step (6) and use a doctor blade coater with a gap of 100 μm to evenly coat the membrane surface once to form a uniform liquid film layer with a wet film thickness of about 100 μm. Place the glass plate carrying the membrane horizontally into a 50℃ forced-air drying oven for pre-drying for 10 minutes. The membrane surface was left to dry until no obvious liquid luster was observed. Then, the entire glass plate was transferred into a preheated drying oven at 135°C for constant temperature thermal crosslinking treatment for 18 minutes. The oven door was kept closed and the glass plate was slowly cooled to below 60°C. The glass plate was then removed and cooled to room temperature. The fully cured membrane was peeled off from the edge with tweezers. The membrane was then immersed in 500mL of deionized water and left to stand at room temperature for 12 hours, with the deionized water being changed once during the process. After cleaning, the membrane was removed and stored in deionized water in a wet state to obtain the acid-resistant composite nanofiltration membrane.

[0038] Example 2 A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: Step (1): Take one polysulfone (PSf) ultrafiltration flat sheet membrane with a molecular weight cutoff of 50 kDa (effective membrane area approximately 28.3 cm²). 2 Immerse the membrane completely in 500 mL of deionized water and let it stand at room temperature for 24 hours. Change the deionized water every 8 hours to thoroughly remove any residual glycerol or other protective solutions inside the membrane. After soaking, remove the membrane and gently press the membrane surface and back with clean qualitative filter paper to absorb the free water adhering to the surface. Place the dried membrane flat in a clean petri dish, cover it to prevent dust, and set it aside for later use.

[0039] Step (2): Weigh 0.605 g of tris(hydroxymethyl)aminomethane (Tris) into a 250 mL beaker, add about 80 mL of deionized water and stir until completely dissolved. Then, add 1 mol / L hydrochloric acid solution dropwise with a pipette while monitoring the pH with a pH meter until the pH of the solution drops precisely to 8.5. Then, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water and shake well to obtain a Tris-HCl buffer solution with a concentration of 50 mmol / L and a pH of 8.5.

[0040] Step (3): Weigh 0.2g of dopamine hydrochloride and 0.08g of tannic acid using an analytical balance, and add them to 100mL of Tris-HCl buffer solution prepared in step (2) and stir to dissolve, thus obtaining a co-deposition mother liquor; fix the polysulfone membrane treated in step (1) on a membrane holder with a stainless steel clamp and immerse it vertically in the co-deposition mother liquor, and continuously shake it at a frequency of 60rpm in a constant temperature shaking box at 28℃ for 11h. After the reaction is completed, take out the membrane and wash it in three beakers containing 200mL of deionized water for 1min each, and then wash it in three beakers containing about 200mL of anhydrous ethanol for 1min each. After cleaning, place the membrane in a 50℃ forced-air drying oven and dry it for 10min to obtain a PDA / TA@PSf membrane with a uniform brown surface.

[0041] Step (4): Mix 50 mL of anhydrous ethanol and 50 mL of deionized water to obtain an ethanol aqueous solution with a volume fraction of 50%. Add 1.5 g of γ-aminopropyltriethoxysilane (KH-550) and perform pre-hydrolysis by magnetic stirring at 300 rpm for 30 min at room temperature to obtain a uniform and transparent silane hydrolysate. Immerse the PDA / TA@PSf membrane obtained in step (3) flat into the hydrolysate, seal it, and place it in a constant temperature water bath at 50℃ for 1 h. After the reaction is completed, take out the membrane and place it in three containers of 200 mL of 50% ethanol. The membrane was rinsed in an aqueous solution for 2 minutes to remove uncovalently grafted free silane. Separately, 50 mL of anhydrous ethanol and 50 mL of deionized water were mixed evenly, and 0.6 g of polyethylene glycol diglycidyl ether (PEGDGE, molecular weight 500 Da) was added. The mixture was magnetically stirred for 10 minutes until it was evenly dissolved. The cleaned membrane was then immersed in the PEGDGE treatment solution, sealed, and placed in a 50°C constant temperature water bath for 2 hours to react. After the reaction was completed, the membrane was removed and rinsed three times with deionized water. It was then dried in a 50°C forced-air drying oven for 5 minutes to obtain the coupling modified membrane.

[0042] Step (5): In a fume hood, add 50 mL of concentrated sulfuric acid (98 wt%) to a 250 mL three-necked flask, and then slowly add 5.0 g of polyether ether ketone (PEEK) powder in three portions under stirring. After all the powder has been added, turn on the oil bath and heat to 45 °C. React at a constant temperature for 4 h under continuous stirring (200 rpm). After the reaction is completed, wait for the reaction solution to cool to about 35 °C, and slowly pour it into a 1000 mL mixture of crushed ice and water through a glass rod. The sulfonated polyether ether ketone will then precipitate out as a light yellow flocculent. Collect the precipitate by filtration with a Buchner funnel, and wash it repeatedly with deionized water until the pH of the filtrate is 6-7 and there is no white turbidity when BaCl2 solution is added. Transfer the washed filter cake to a vacuum drying oven at 60 °C and dry for 24 h. Take it out and grind it into a fine powder to obtain sulfonated polyether ether ketone (SPEEK) powder with a sulfonation degree of about 55%.

[0043] Step (6): Weigh 3.0g of SPEEK powder prepared in step (5) and place it in a 150mL conical flask. Add 97.0g of deionized water and stir at 300rpm for 50min in a 70℃ constant temperature water bath until completely dissolved and an amber transparent liquid is formed. After naturally cooling to room temperature, add 5.0g of phytic acid aqueous solution (containing 1.3g of phytic acid) and continue stirring for 30min until uniform. Then let it stand for 1h to remove bubbles and obtain SPEEK / phytic acid coating solution.

[0044] Step (7): Lay the coupling modified membrane obtained in step (4) flat on a clean, thick glass plate. Use the surface tension of the coating solution to allow the membrane to adhere naturally. Gently press to remove air bubbles and make the membrane surface flat and wrinkle-free. Take 5 mL of the SPEEK / phytic acid coating solution prepared in step (6) and use a doctor blade coater with a gap of 100 μm to evenly coat the membrane surface once to form a uniform liquid film layer with a wet film thickness of about 100 μm. Place the glass plate carrying the membrane horizontally into a 50℃ forced-air drying oven for pre-drying for 10 minutes. The membrane surface was left to dry until no obvious liquid luster was observed. Then, the entire glass plate was transferred into a preheated drying oven at 135°C for constant temperature thermal crosslinking treatment for 18 minutes. The oven door was kept closed and the glass plate was slowly cooled to below 60°C. The glass plate was then removed and cooled to room temperature. The fully cured membrane was peeled off from the edge with tweezers. The membrane was then immersed in 500mL of deionized water and left to stand at room temperature for 12 hours, with the deionized water being changed once during the process. After cleaning, the membrane was removed and stored in deionized water in a wet state to obtain the acid-resistant composite nanofiltration membrane.

[0045] Example 3 A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: Step (1): Take one polysulfone (PSf) ultrafiltration flat sheet membrane with a molecular weight cutoff of 50 kDa (effective membrane area approximately 28.3 cm²). 2 Immerse the membrane completely in 500 mL of deionized water and let it stand at room temperature for 24 hours. Change the deionized water every 8 hours to thoroughly remove any residual glycerol or other protective solutions inside the membrane. After soaking, remove the membrane and gently press the membrane surface and back with clean qualitative filter paper to absorb the free water adhering to the surface. Place the dried membrane flat in a clean petri dish, cover it to prevent dust, and set it aside for later use.

[0046] Step (2): Weigh 0.605 g of tris(hydroxymethyl)aminomethane (Tris) into a 250 mL beaker, add about 80 mL of deionized water and stir until completely dissolved. Then, add 1 mol / L hydrochloric acid solution dropwise with a pipette while monitoring the pH with a pH meter until the pH of the solution drops precisely to 8.5. Then, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water and shake well to obtain a Tris-HCl buffer solution with a concentration of 50 mmol / L and a pH of 8.5.

[0047] Step (3): Weigh 0.2g of dopamine hydrochloride and 0.10g of tannic acid using an analytical balance, and add them to 100mL of Tris-HCl buffer solution prepared in step (2) and stir to dissolve, thus obtaining a co-deposition mother liquor; fix the polysulfone membrane treated in step (1) on the membrane holder with a stainless steel clamp and immerse it vertically in the co-deposition mother liquor, and continuously shake it at a frequency of 60rpm in a constant temperature shaking box at 28℃ for 11h. After the reaction is completed, take out the membrane and wash it in three beakers containing 200mL of deionized water for 1min each, and then wash it in three beakers containing about 200mL of anhydrous ethanol for 1min each. After cleaning, place the membrane in a 50℃ forced-air drying oven and dry it for 10min to obtain a PDA / TA@PSf membrane with a uniform brown surface.

[0048] Step (4): Mix 50 mL of anhydrous ethanol and 50 mL of deionized water to obtain an ethanol aqueous solution with a volume fraction of 50%. Add 1.5 g of γ-aminopropyltriethoxysilane (KH-550) and perform pre-hydrolysis by magnetic stirring at 300 rpm for 30 min at room temperature to obtain a uniform and transparent silane hydrolysate. Immerse the PDA / TA@PSf membrane obtained in step (3) flat into the hydrolysate, seal it, and place it in a constant temperature water bath at 50℃ for 1 h. After the reaction is completed, take out the membrane and place it in three containers of 200 mL of 50% ethanol. The membrane was rinsed in an aqueous solution for 2 minutes to remove uncovalently grafted free silane. Separately, 50 mL of anhydrous ethanol and 50 mL of deionized water were mixed evenly, and 0.8 g of polyethylene glycol diglycidyl ether (PEGDGE, molecular weight 500 Da) was added. The mixture was magnetically stirred for 10 minutes until it was evenly dissolved. The cleaned membrane was then immersed in the PEGDGE treatment solution, sealed, and placed in a 50°C constant temperature water bath for 2 hours to react. After the reaction was completed, the membrane was removed and rinsed 3 times with deionized water. It was then dried in a 50°C forced-air drying oven for 5 minutes to obtain the coupling modified membrane.

[0049] Step (5): In a fume hood, add 50 mL of concentrated sulfuric acid (98 wt%) to a 250 mL three-necked flask, and then slowly add 5.0 g of polyether ether ketone (PEEK) powder in three portions under stirring. After all the powder has been added, turn on the oil bath and heat to 45 °C. React at a constant temperature for 4 h under continuous stirring (200 rpm). After the reaction is completed, wait for the reaction solution to cool to about 35 °C, and slowly pour it into a 1000 mL mixture of crushed ice and water through a glass rod. The sulfonated polyether ether ketone will then precipitate out as a light yellow flocculent. Collect the precipitate by filtration with a Buchner funnel, and wash it repeatedly with deionized water until the pH of the filtrate is 6-7 and there is no white turbidity when BaCl2 solution is added. Transfer the washed filter cake to a vacuum drying oven at 60 °C and dry for 24 h. Take it out and grind it into a fine powder to obtain sulfonated polyether ether ketone (SPEEK) powder with a sulfonation degree of about 55%.

[0050] Step (6): Weigh 3.0g of SPEEK powder prepared in step (5) and place it in a 150mL conical flask. Add 97.0g of deionized water and stir at 300rpm for 50min in a 70℃ constant temperature water bath until completely dissolved and an amber transparent liquid is formed. After naturally cooling to room temperature, add 5.0g of phytic acid aqueous solution (containing 1.5g of phytic acid) and continue stirring for 30min until uniform. Then let it stand for 1h to remove bubbles and obtain SPEEK / phytic acid coating solution.

[0051] Step (7): Lay the coupling modified membrane obtained in step (4) flat on a clean, thick glass plate. Use the surface tension of the coating solution to allow the membrane to adhere naturally. Gently press to remove air bubbles and make the membrane surface flat and wrinkle-free. Take 5 mL of the SPEEK / phytic acid coating solution prepared in step (6) and use a doctor blade coater with a gap of 100 μm to evenly coat the membrane surface once to form a uniform liquid film layer with a wet film thickness of about 100 μm. Place the glass plate carrying the membrane horizontally into a 50℃ forced-air drying oven for pre-drying for 10 minutes. The membrane surface was left to dry until no obvious liquid luster was observed. Then, the entire glass plate was transferred into a preheated drying oven at 135°C for constant temperature thermal crosslinking treatment for 18 minutes. The oven door was kept closed and the glass plate was slowly cooled to below 60°C. The glass plate was then removed and cooled to room temperature. The fully cured membrane was peeled off from the edge with tweezers. The membrane was then immersed in 500mL of deionized water and left to stand at room temperature for 12 hours, with the deionized water being changed once during the process. After cleaning, the membrane was removed and stored in deionized water in a wet state to obtain the acid-resistant composite nanofiltration membrane.

[0052] Example 4 A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: Step (1): Take one polysulfone (PSf) ultrafiltration flat sheet membrane with a molecular weight cutoff of 50 kDa (effective membrane area approximately 28.3 cm²). 2 Immerse the membrane completely in 500 mL of deionized water and let it stand at room temperature for 24 hours. Change the deionized water every 8 hours to thoroughly remove any residual glycerol or other protective solutions inside the membrane. After soaking, remove the membrane and gently press the membrane surface and back with clean qualitative filter paper to absorb the free water adhering to the surface. Place the dried membrane flat in a clean petri dish, cover it to prevent dust, and set it aside for later use.

[0053] Step (2): Weigh 0.605 g of tris(hydroxymethyl)aminomethane (Tris) into a 250 mL beaker, add about 80 mL of deionized water and stir until completely dissolved. Then, add 1 mol / L hydrochloric acid solution dropwise with a pipette while monitoring the pH with a pH meter until the pH of the solution drops precisely to 8.5. Then, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water and shake well to obtain a Tris-HCl buffer solution with a concentration of 50 mmol / L and a pH of 8.5.

[0054] Step (3): Weigh 0.2g of dopamine hydrochloride and 0.15g of tannic acid using an analytical balance, and add them to 100mL of Tris-HCl buffer solution prepared in step (2) and stir to dissolve, thus obtaining a co-deposition mother liquor; fix the polysulfone membrane treated in step (1) on the membrane holder with a stainless steel clamp and immerse it vertically in the co-deposition mother liquor, and continuously shake it at a frequency of 60rpm in a constant temperature shaking box at 30℃ for 12h. After the reaction is completed, take out the membrane and wash it in three beakers containing 200mL of deionized water for 1min each, and then wash it in three beakers containing about 200mL of anhydrous ethanol for 1min each. After cleaning, place the membrane in a 50℃ forced-air drying oven and dry it for 10min to obtain a PDA / TA@PSf membrane with a uniform brown surface.

[0055] Step (4): Mix 50 mL of anhydrous ethanol and 50 mL of deionized water to obtain an ethanol aqueous solution with a volume fraction of 50%. Add 1.5 g of γ-aminopropyltriethoxysilane (KH-550) and perform pre-hydrolysis by magnetic stirring at 300 rpm for 30 min at room temperature to obtain a uniform and transparent silane hydrolysate. Immerse the PDA / TA@PSf membrane obtained in step (3) flat into the hydrolysate, seal it, and place it in a constant temperature water bath at 50℃ for 1 h. After the reaction is completed, take out the membrane and place it in three containers containing 200 mL of 50% ethanol. The membrane was rinsed in an alcohol-water solution for 2 minutes to remove uncovalently grafted free silane. Separately, 50 mL of anhydrous ethanol and 50 mL of deionized water were mixed evenly, and 1 g of polyethylene glycol diglycidyl ether (PEGDGE, molecular weight 500 Da) was added. The mixture was magnetically stirred for 10 minutes until it was evenly dissolved. The cleaned membrane was then immersed in the PEGDGE treatment solution, sealed, and placed in a 50°C constant temperature water bath for 2 hours to react. After the reaction was completed, the membrane was removed and rinsed 3 times with deionized water. It was then dried in a 50°C forced-air drying oven for 5 minutes to obtain the coupling modified membrane.

[0056] Step (5): In a fume hood, add 50 mL of concentrated sulfuric acid (98 wt%) to a 250 mL three-necked flask, and then slowly add 5.0 g of polyether ether ketone (PEEK) powder in three portions under stirring. After all the powder has been added, turn on the oil bath and heat to 50 °C. React at a constant temperature for 5 h under continuous stirring (200 rpm). After the reaction is completed, wait for the reaction solution to cool to about 35 °C, and slowly pour it into a 1000 mL mixture of crushed ice and water through a glass rod. The sulfonated polyether ether ketone will then precipitate out as a light yellow flocculent. Collect the precipitate by filtration with a Buchner funnel, and wash it repeatedly with deionized water until the pH of the filtrate is 6-7 and there is no white turbidity when BaCl2 solution is added. Transfer the washed filter cake to a vacuum drying oven at 60 °C and dry for 24 h. Take it out and grind it into a fine powder to obtain sulfonated polyether ether ketone (SPEEK) powder with a sulfonation degree of about 55%.

[0057] Step (6): Weigh 3.0g of SPEEK powder prepared in step (5) and place it in a 150mL conical flask. Add 97.0g of deionized water and stir at 300rpm for 50min in a 70℃ constant temperature water bath until completely dissolved and an amber transparent liquid is formed. After naturally cooling to room temperature, add 5.0g of phytic acid aqueous solution (containing 2g of phytic acid) and continue stirring for 30min until uniform. Then let it stand for 1h to remove bubbles and obtain SPEEK / phytic acid coating solution.

[0058] Step (7): Lay the coupling modified membrane obtained in step (4) flat on a clean, thick glass plate. Use the surface tension of the coating solution to allow the membrane to adhere naturally. Gently press to remove air bubbles and make the membrane surface flat and wrinkle-free. Take 5 mL of the SPEEK / phytic acid coating solution prepared in step (6) and use a doctor blade coater with a gap of 100 μm to evenly coat the membrane surface once to form a uniform liquid film layer with a wet film thickness of about 100 μm. Place the glass plate carrying the membrane horizontally into a 50℃ forced-air drying oven for pre-drying for 10 minutes. The membrane surface was dried until no obvious liquid luster was observed. Then, the entire glass plate was transferred into a preheated drying oven at 140°C for constant temperature thermal crosslinking treatment for 20 minutes. The oven door was kept closed and the glass plate was slowly cooled to below 60°C. The glass plate was then removed and cooled to room temperature. The fully cured membrane was peeled off from the edge with tweezers. The membrane was then immersed in 500mL of deionized water and left to stand at room temperature for 12 hours, with the deionized water being changed once during the process. After cleaning, the membrane was removed and stored in deionized water in a wet state to obtain the acid-resistant composite nanofiltration membrane.

[0059] Example 5 A method for preparing an acid-resistant composite nanofiltration membrane includes the following steps: Step (1): Take one polysulfone (PSf) ultrafiltration flat sheet membrane with a molecular weight cutoff of 50 kDa (effective membrane area approximately 28.3 cm²). 2 Immerse the membrane completely in 500 mL of deionized water and let it stand at room temperature for 24 hours. Change the deionized water every 8 hours to thoroughly remove any residual glycerol or other protective solutions inside the membrane. After soaking, remove the membrane and gently press the membrane surface and back with clean qualitative filter paper to absorb the free water adhering to the surface. Place the dried membrane flat in a clean petri dish, cover it to prevent dust, and set it aside for later use.

[0060] Step (2): Weigh 0.605 g of tris(hydroxymethyl)aminomethane (Tris) into a 250 mL beaker, add about 80 mL of deionized water and stir until completely dissolved. Then, add 1 mol / L hydrochloric acid solution dropwise with a pipette while monitoring the pH with a pH meter until the pH of the solution drops precisely to 8.5. Then, transfer the solution to a 100 mL volumetric flask, dilute to the mark with deionized water and shake well to obtain a Tris-HCl buffer solution with a concentration of 50 mmol / L and a pH of 8.5.

[0061] Step (3): Weigh 0.2g of dopamine hydrochloride and 0.05g of tannic acid using an analytical balance, and add them to 100mL of Tris-HCl buffer solution prepared in step (2) and stir to dissolve, thus obtaining a co-deposition mother liquor; fix the polysulfone membrane treated in step (1) on the membrane holder with a stainless steel clamp and immerse it vertically in the co-deposition mother liquor, and continuously shake it at a frequency of 60rpm in a constant temperature shaking box at 25℃ for 10h. After the reaction is completed, take out the membrane and wash it in three beakers containing 200mL of deionized water for 1min each, and then wash it in three beakers containing about 200mL of anhydrous ethanol for 1min each. After cleaning, place the membrane in a 50℃ forced-air drying oven and dry it for 10min to obtain a PDA / TA@PSf membrane with a uniform brown surface.

[0062] Step (4): Mix 50 mL of anhydrous ethanol and 50 mL of deionized water to obtain an ethanol aqueous solution with a volume fraction of 50%. Add 1.5 g of γ-aminopropyltriethoxysilane (KH-550) and perform pre-hydrolysis by magnetic stirring at 300 rpm for 30 min at room temperature to obtain a uniform and transparent silane hydrolysate. Immerse the PDA / TA@PSf membrane obtained in step (3) flat into the hydrolysate, seal it, and place it in a constant temperature water bath at 50℃ for 1 h. After the reaction is completed, take out the membrane and place it in three containers of 200 mL of 50% ethanol. The membrane was rinsed in an aqueous solution for 2 minutes to remove uncovalently grafted free silane. Separately, 50 mL of anhydrous ethanol and 50 mL of deionized water were mixed evenly, and 0.5 g of polyethylene glycol diglycidyl ether (PEGDGE, molecular weight 500 Da) was added. The mixture was magnetically stirred for 10 minutes until it was evenly dissolved. The cleaned membrane was then immersed in the PEGDGE treatment solution, sealed, and placed in a 50°C constant temperature water bath for 2 hours to react. After the reaction was completed, the membrane was removed and rinsed 3 times with deionized water. It was then dried in a 50°C forced-air drying oven for 5 minutes to obtain the coupling modified membrane.

[0063] Step (5): In a fume hood, add 50 mL of concentrated sulfuric acid (98 wt%) to a 250 mL three-necked flask, and then slowly add 5.0 g of polyether ether ketone (PEEK) powder in three portions under stirring. After all the powder has been added, turn on the oil bath and heat to 40 °C. React at a constant temperature for 3 h under continuous stirring (200 rpm). After the reaction is complete, wait for the reaction solution to cool to about 35 °C, and slowly pour it into a 1000 mL mixture of crushed ice and water through a glass rod. The sulfonated polyether ether ketone will then precipitate out as a light yellow flocculent. Collect the precipitate by filtration with a Buchner funnel, and wash it repeatedly with deionized water until the pH of the filtrate is 6-7 and there is no white turbidity when BaCl2 solution is added. Transfer the washed filter cake to a vacuum drying oven at 60 °C and dry for 24 h. Take it out and grind it into a fine powder to obtain sulfonated polyether ether ketone (SPEEK) powder with a sulfonation degree of about 55%.

[0064] Step (6): Weigh 3.0g of SPEEK powder prepared in step (5) and place it in a 150mL conical flask. Add 97.0g of deionized water and stir at 300rpm for 50min in a 70℃ constant temperature water bath until completely dissolved and an amber transparent liquid is formed. After naturally cooling to room temperature, add 5.0g of phytic acid aqueous solution (containing 1g of phytic acid) and continue stirring for 30min until uniform. Then let stand for 1h to remove bubbles and obtain SPEEK / phytic acid coating solution.

[0065] Step (7): Lay the coupling modified membrane obtained in step (4) flat on a clean, thick glass plate. Use the surface tension of the coating solution to allow the membrane to adhere naturally. Gently press to remove air bubbles and make the membrane surface flat and wrinkle-free. Take 5 mL of the SPEEK / phytic acid coating solution prepared in step (6) and use a doctor blade coater with a gap of 100 μm to evenly coat the membrane surface once to form a uniform liquid film layer with a wet film thickness of about 100 μm. Place the glass plate carrying the membrane horizontally into a 50℃ forced-air drying oven for pre-drying for 10 minutes. The glass plate was heated until the membrane surface no longer showed obvious liquid luster. Then, the entire glass plate was transferred into a preheated drying oven at 130°C for constant temperature thermal crosslinking treatment for 15 minutes. The oven door was kept closed and the glass plate was slowly cooled to below 60°C. The glass plate was then removed and cooled to room temperature. The fully cured membrane was peeled off from the edge with tweezers. The membrane was then immersed in 500mL of deionized water and left to stand at room temperature for 12 hours, with the deionized water being changed once during the process. After cleaning, the membrane was removed and stored in deionized water in a wet state to obtain the acid-resistant composite nanofiltration membrane.

[0066] Comparative Example 1: The difference from Example 4 is that steps 2 and 3 are omitted, and the PDA / TA@PSf membrane used in step 4 is replaced with a polysulfone-based membrane, that is, no polydopamine-tannic acid intermediate layer is formed on the ultrafiltration membrane.

[0067] Comparative Example 2: The difference from Example 4 is that steps 5-7 are omitted, that is, the sulfonated polyether ether ketone / phytic acid functional layer is not formed on the ultrafiltration membrane.

[0068] Comparative Example 3: The difference from Example 4 is that step 4 is omitted, that is, the ultrafiltration membrane is not treated with γ-aminopropyltriethoxysilane and polyethylene glycol diglycidyl ether.

[0069] Comparative Example 4: The difference from Example 4 is that γ-aminopropyltriethoxysilane is not added in step 4.

[0070] Comparative Example 5: The difference from Example 4 is that polyethylene glycol diglycidyl ether is not added in step 4.

[0071] Performance testing: (1) Pure water flux test: The membranes prepared in each embodiment and comparative example were cut and installed in the cross-flow filtration test device (effective membrane area 7.1 cm²). 2Using deionized water as the feed liquid, the membrane was first pre-compacted at an operating pressure of 8 bar for 30 minutes at room temperature to achieve a stable compaction state. Then, the operating pressure was reduced to 6 bar. After the flux stabilized, the permeate was collected within a fixed time and its mass was weighed using an electronic balance. The pure water permeability coefficient was calculated according to the formula J=V / (A·t·ΔP), where J is the pure water flux (L·m³). -2 ·h -1 ·bar -1 V is the permeate volume (L), and A is the effective membrane area (m²). 2 ), t is the collection time (h), ΔP is the operating pressure (bar), and at least 3 membranes in each group were tested in parallel and the average value was taken. The test results are shown in Table 1.

[0072] (2) MgSO4 Retention Rate Test: In the above-mentioned cross-flow filtration device, the feed liquid was replaced with a 1000 mg / L MgSO4 aqueous solution (pH value approximately 6.5). After stable operation at 6 bar for 30 minutes, samples of the feed liquid and permeate were taken simultaneously. The conductivity values ​​of the feed liquid and permeate were measured using a conductivity meter. The corresponding ion concentration was calculated based on the standard curve of conductivity versus concentration. The retention rate was calculated using the formula R = (1 - Cp / Cf) × 100%, where Cp is the permeate concentration and Cf is the feed liquid concentration. The test results are shown in Table 1.

[0073] (3) NaCl Retention Rate Test: The test method is the same as that for MgSO4 retention rate test. The feed solution is replaced with a 1000 mg / L NaCl aqueous solution (pH value of approximately 6.8). After stable operation at 6 bar for 30 min, samples are taken. The conductivity values ​​of the feed solution and permeate are measured using a conductivity meter and the concentration is converted. The NaCl retention rate is calculated according to the above formula. The test results are shown in Table 1.

[0074] (4) Acid stability test (MgSO4 rejection rate and flux change rate after 30 days of acid immersion): Take one membrane from each of the examples and comparative examples, and immerse each membrane in a sulfuric acid solution with a concentration of about 0.09 mol / L (corresponding to pH≈1). Let it stand for 30 days at room temperature. After immersion, take out the membrane and rinse it thoroughly with deionized water. Install it in the cross-flow filtration device mentioned above, and re-determine the pure water flux and MgSO4 rejection rate under the same test conditions as above (6 bar, 1000 mg / L MgSO4 feed solution). Record the MgSO4 rejection rate after acid immersion directly, and calculate it according to the formula ΔJ=(|J 30 The pure water flux change rate is calculated as -J0| / J0)×100%, where J is... 30 J0 represents the pure water flux after 30 days of acid immersion, and J0 represents the initial pure water flux before acid immersion. The test results are shown in Table 1.

[0075] Table 1:

[0076] As shown in the table above, the acid-resistant composite nanofiltration membranes prepared in Examples 1-4 all exhibit excellent comprehensive performance: the initial pure water flux ranges from 15.8 to 20.3 L·m⁻¹. -2 ·h -1 ·bar -1 Within the specified range, the MgSO4 rejection rate was above 94.5%, and after continuous immersion in sulfuric acid solution at pH=1 for 30 days, the MgSO4 rejection rate remained above 92.7%, with flux change rates all less than 10%, indicating that the membrane structure has excellent long-term stability under strong acid conditions. Example 4 exhibited the best overall performance, with an initial MgSO4 rejection rate of 97.5%, which decreased by only 1.3 percentage points after 30 days of acid immersion, and a flux change rate of only 5.3%. Example 5, due to its lower tannic acid content, less phytic acid crosslinking agent, and lower thermal crosslinking temperature, resulted in relatively insufficient coating density, leading to a slightly lower initial rejection rate and acid resistance stability compared to other examples, but still significantly better than the comparative examples.

[0077] Comparative Example 1 omitted the dopamine-tannic acid co-deposition step. Although the initial separation performance was similar to that of the example, after 30 days of acid immersion, the MgSO4 rejection rate plummeted to 55.8%, and the flux change rate reached as high as 68.7%. This indicates that without the anchoring protection of the polyphenol intermediate layer, the functional layer cannot maintain stable bonding with the base membrane in an acidic environment, resulting in large-area peeling. Comparative Example 2 omitted the sulfonated polyether ether ketone / phytic acid functional layer, achieving a pure water flux of 48.5 L·m⁻¹. -2 ·h -1 ·bar -1However, the MgSO4 rejection rate was only 32.4%, indicating that although the simple dopamine-tannic acid interlayer has good acid resistance, it does not possess nanofiltration-level ion separation capabilities. Comparative Example 3 omitted the interfacial coupling treatment step, and while the initial performance was good, the rejection rate dropped to 75.3% and the flux change rate reached 35.6% after 30 days of acid immersion, confirming the necessity of covalent interfacial bridging for maintaining long-term stable interlayer bonding. Comparative Example 4 used only PEGDGE without KH-550 for silane anchoring. Due to the lack of Si-O covalent bond anchoring at the bottom layer, PEGDGE could not be effectively fixed to the surface of the co-deposited layer, and the rejection rate dropped to 79.5% after acid immersion. Comparative Example 5 used only KH-550 without PEGDGE for flexible bridging. Due to the excessive rigidity of the interface layer and the lack of elastic buffering capacity, interface stress cracks easily appeared during long-term acid immersion and thermo-mechanical cycling caused by operating pressure. After acid immersion, the rejection rate dropped to 87.6% and the flux change rate was 16.8%. Although it was better than Comparative Example 3, which had no coupling treatment at all, it was significantly worse than Example 4, which used KH-550 and PEGDGE in combination (rejection rate 96.2% and flux change rate 5.3%). This fully verified the technical necessity of the synergistic combination of the two coupling agents.

[0078] Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the essence and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an acid-resistant composite nanofiltration membrane, characterized in that, Includes the following steps: (1) Immerse the polysulfone ultrafiltration membrane in deionized water to remove residual substances, and dry the free water on the membrane surface for later use; (2) Dissolve tris(hydroxymethyl)aminomethane in deionized water, add hydrochloric acid solution to adjust the pH value, and make up to volume to obtain Tris-HCl buffer solution; (3) Add dopamine hydrochloride and tannic acid to the above Tris-HCl buffer solution in sequence and stir to dissolve, and prepare a co-deposition mother liquor; immerse the pretreated polysulfone ultrafiltration membrane in the co-deposition mother liquor for shaking reaction, and wash it with deionized water and anhydrous ethanol in sequence after the reaction is completed, and dry it to obtain PDA / TA@PSf membrane; (4) Add γ-aminopropyltriethoxysilane to an aqueous ethanol solution and stir to pre-hydrolyze. Immerse the above PDA / TA@PSf membrane in the γ-aminopropyltriethoxysilane hydrolysate for reaction. After taking it out, wash it with an aqueous ethanol solution. Add polyethylene glycol diglycidyl ether to an aqueous ethanol solution and stir to dissolve. Then immerse the washed membrane in the solution to obtain the coupling modified membrane. (5) Slowly add polyether ether ketone powder to concentrated sulfuric acid and stir to carry out sulfonation reaction; after the reaction is completed, pour the reaction solution into an ice-water mixture to precipitate the product, filter it, wash it repeatedly with deionized water until neutral and dry it, and then grind it to obtain sulfonated polyether ether ketone powder. (6) The sulfonated polyether ether ketone powder prepared above is added to deionized water and heated and stirred until completely dissolved. After cooling, phytic acid aqueous solution is added and stirred evenly. After standing and degassing treatment, SPEEK / phytic acid coating solution is obtained. (7) Lay the above-mentioned coupling modified membrane flat and fix it, take SPEEK / phytic acid coating liquid and scrape it on its surface to form a uniform liquid film; perform pre-drying and high-temperature thermal crosslinking treatment in sequence to cure the coating. After cooling, the membrane is peeled off and immersed in deionized water for cleaning to obtain the acid-resistant composite nanofiltration membrane.

2. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (1), the polysulfone ultrafiltration membrane has a molecular weight cutoff of 50 kDa; the soaking and cleaning conditions are: soaking at room temperature for 24 hours, and replacing the deionized water every 8 hours during this period.

3. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (3), the mass ratio of dopamine hydrochloride to tannic acid is 2:(0.5-1.5).

4. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (3), the oscillation reaction temperature is 25-30℃ and the oscillation reaction time is 10-12h.

5. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (4), the mass ratio of γ-aminopropyltriethoxysilane to polyethylene glycol diglycidyl ether is 1.5:(0.5-1).

6. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (4), the molecular weight of polyethylene glycol diglycidyl ether is 500 Da.

7. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (5), the stirring reaction is carried out at a constant temperature of 40-50°C for 3-5 hours.

8. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (6), the mass ratio of sulfonated polyether ether ketone powder to phytic acid is 3:(1-2).

9. The method for preparing an acid-resistant composite nanofiltration membrane according to claim 1, characterized in that, In step (7), the thermal crosslinking treatment temperature is 130-140℃ and the time is 15-20min.

10. An acid-resistant composite nanofiltration membrane, characterized in that, It is prepared by the method described in any one of claims 1 to 9 above.

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