A high-permeability, positively charged, chlorine-resistant reverse osmosis membrane and its preparation method

Abstract

This invention discloses a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane and its preparation method. By blending PEI and MPD with TMC through an interfacial polymerization reaction, the inherent electronegativity of the polyamide membrane is altered, causing both the surface and bottom layers to simultaneously become positively charged. Aromatic aldehyde TPA reacts with residual polyamide aminoamine aldehydes on the polyamide membrane to generate Schiff base structures. This prevents active chlorine from damaging the selective layer structure while simultaneously reducing the positive charge through the steric occupancy of the polyamide-aromatic aldehyde, thus producing a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane. Compared to traditional reverse osmosis membranes, this invention's preparation method achieves double-sided chlorine resistance in the positively charged polyamide selective layer through "end-sealing densification + steric hindrance." Furthermore, this membrane also exhibits higher water flux and has significant application value.

Description

Technical Field

[0001] This invention relates to a method for preparing a reverse osmosis membrane, specifically a method for preparing a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane, belonging to the fields of nanofiltration membranes and reverse osmosis membranes. Background Technology

[0002] As a key innovation in the field of water treatment, reverse osmosis membrane technology uses high pressure to drive water molecules to selectively permeate through a semi-permeable membrane, effectively retaining dissolved salts, organic matter, and microorganisms, achieving a desalination rate of over 99%. With its advantages of high-precision filtration capabilities at nanoscale pore size, low energy consumption, and small footprint, it has become the fastest-growing and most widely used water treatment technology in the past 20 years, extensively applied in wastewater reuse, pure water production, and seawater desalination. Compared to nanofiltration and ultrafiltration membrane technologies, reverse osmosis membranes require only slightly higher water pressure to achieve comprehensive improvements in desalination rate, effluent quality, and concentration ratio.

[0003] In widely used nanofiltration and reverse osmosis membranes, due to the inherent properties of the polyamide layer—the dissociation of amino or carboxyl groups on its surface—it typically carries a neutral or negative charge, making most membranes negatively charged. The charge properties of the membrane significantly influence its separation performance. Because of the electrostatic attraction or repulsion that occurs when water permeates the membrane, materials with different charges exhibit different separation properties for substances with different charges. Positively charged membranes can more effectively retain polyvalent cations, selectively separate low-valent cations, and improve the removal rate of negatively charged organic matter. They can be used for hard water softening and heavy metal removal, and have significant application value in biological treatment, the pharmaceutical industry, and wastewater treatment.

[0004] Patent CN105561799B describes a positively charged membrane made from cross-linked cationic guar gum material, which improves antifouling performance, achieves higher organic matter retention, and increases membrane permeation flux.

[0005] Patent CN113289498B describes a positively charged membrane formed by cross-linking polyvinyl alcohol and glutaraldehyde to create a protective layer, which verifies the membrane's high rejection rate for polyvalent cations, especially divalent cations.

[0006] Patent CN115945071B describes a positively charged membrane formed by the polymerization of chitosan and polyanionic polymers to create an intermediate layer. This demonstrates that, in addition to its high water flux, the membrane also exhibits excellent lithium-magnesium separation performance.

[0007] The above methods all demonstrate the unique advantages of positively charged polyamide membranes in the field of water treatment.

[0008] However, at the user's drinking water end, there is residual chlorine in the pipes to disinfect water that may be contaminated after transportation. When water containing residual chlorine passes through the membrane at the drinking water end, it may damage the polyamide structure of the membrane. If the membrane is soaked in a chlorine-containing environment for a long time, its retention function will gradually fail.

[0009] Therefore, researchers have adopted various methods to avoid this situation and studied a variety of chlorine-resistant composite films. Researchers have explored various methods to improve chlorine resistance in textiles. One approach involves directly altering the membrane's structure to prevent chlorine attack. For instance, patent CN120204958A uses tannic acid, chlorogenic acid, and rhamnose as an aqueous solution, which are then interfacially polymerized with trimesoyl chloride to form a polyester nanofiltration membrane for separating salt stains in dyeing wastewater. Another method involves surface modification to enhance chlorine resistance. For example, patent CN120305825A uses sodium lignin sulfonate molecules to react with residual acyl chloride groups, anchoring more sodium lignin sulfonate on the polyamide active layer surface to prevent the intrusion of active chlorine. A third method involves modifying the aqueous monomers to avoid residual amine chlorination. For instance, patent CN 120227762A uses piperazine monomers to react with bis(2-chloroethyl)amine to generate piperazine monomers with a helical structure, ensuring that the final interfacially polymerized structure does not contain primary amine (-NH2) and secondary amine (-NH) groups that are easily attacked by chlorine.

[0010] None of the above methods improve the chlorine resistance of positively charged polyamide membranes. Traditional polyamide reverse osmosis membranes inherently contain primary and secondary amine groups that are easily attacked by active chlorine. While the surface modification and chemical reactions used in the above methods are effective in improving the chlorine resistance of negatively charged polyamide membranes, they are ineffective for positively charged polyamide membranes. Taking the PEI method (which involves introducing polyethyleneimine to participate in the reaction of polyacrylamide chloride monomers), widely used in the preparation of positively charged polyamide membranes, as an example, the positive charge of this polyamide membrane comes from the large number of protonable amine groups introduced by PEI. These protonable amine groups are precisely the groups most easily reacted with active chlorine. The positive charge source of the positively charged membrane and the primary attack site of active chlorine are the same target at the molecular level. Through N-chlorination and Orton rearrangement, compared to the chlorine sensitivity of negatively charged membranes, the crosslinking network of positively charged polyamide membranes is more easily destroyed by chlorination.

[0011] Specifically, as in patent CN120204958A, replacing polyamide with polyester removes some of the amides that are easily chlorinated, but it also eliminates the source of the membrane's positive charge. Another example is patent CN120305825A, where anchoring anionic layers such as lignin sulfonate directly pulls the surface potential towards neutral or negative values, weakening the repulsive selectivity against cations. Neutral active chlorine can still penetrate the hydrophilic coating and diffuse into the active layer, attacking the underlying amine-amide bonds. While patent CN 120227762A modifies the aqueous monomer to consume residual amine groups, removing a small amount of unreacted amines, for positively charged polyamide membranes, the protonated amine groups are not residual amines, but rather the main body of the network framework and positive charge function. Although patent CN111434375A also uses polyethyleneimine and aldehydes, the aldehyde glutaraldehyde in this patent is a commonly used crosslinking stabilizer. It is mainly used to crosslink and extend the polyethyleneimine layer on the surface into a stable structure to facilitate subsequent quaternary ammonium salt grafting. It does not react with the exposed amide in the lower polyamide layer, which is easily attacked by active chlorine. Furthermore, the protonable amine groups that have a positive charge effect in the upper grafted quaternary ammonium salt layer will also be attacked by active chlorine and cannot retain their positive charge in a chlorine environment.

[0012] Therefore, it is necessary to provide a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane and its preparation method. Summary of the Invention

[0013] To address the shortcomings of existing technologies, the present invention aims to provide a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane and its preparation method.

[0014] To achieve the above objectives, the present invention adopts the following technical solution:

[0015] A high-permeability, positively charged, chlorine-resistant reverse osmosis membrane, comprising, from top to bottom, a chlorine-resistant end-capped polyamide layer, a positively charged polyamide layer, and a base membrane layer;

[0016] The end-capped chlorine-resistant polyamide layer is formed by reacting a mixed solution of polyamine monomers and monomers containing strong cationic electrolytes with acyl chloride to form a polyamide layer, and then end-capping the upper surface layer with aromatic aldehydes.

[0017] The positively charged polyamide layer polyamine monomer and the mixed solution of monomer containing strong cationic electrolyte react with acyl chloride to form the bottom layer of the polyamide layer;

[0018] The base film layer is a porous support film.

[0019] The aforementioned monomers containing strong cationic electrolytes include polyethyleneimine (PEI) and polydiallyl dimethylammonium (PDADMAC).

[0020] The aforementioned polyamine monomers include piperazine, m-phenylenediamine, p-phenylenediamine, 1,2-ethylenediamine, or 1,6-hexanediamine.

[0021] The aforementioned polyacryl chlorides include pyromellitic trichloroisocyanurate, isophenyltrichloroisocyanurate, cyclohexanetrichloroisocyanurate, cyclopentanetrichloroisocyanurate, propyltrichloroisocyanurate, or pentanetrichloroisocyanurate; the solvents for the organic solutions include n-hexane, n-heptane, or tetradecane.

[0022] The aforementioned aromatic aldehydes include cinnamaldehyde, benzaldehyde, o-methylbenzaldehyde, m-methylbenzaldehyde, p-methylbenzaldehyde, and salicylaldehyde, which contain a single aldehyde group, or o-phthalaldehyde, terephthalaldehyde, and m-phthalaldehyde, which contain two aldehyde groups.

[0023] The base film is made of materials including polyethersulfone, polysulfone, polyethylene, polyamide imide, polypropylene or polyacrylonitrile, and the molecular weight cutoff of the base film is 100~300kDa.

[0024] The mass ratio of the above-mentioned strong cationic electrolyte monomer and polyamine monomer is 5:1 to 10:1.

[0025] The preparation method of the above-mentioned high-permeability, positively charged, chlorine-resistant reverse osmosis membrane includes the following steps:

[0026] S1. A mixed solution is prepared by ultrasonically mixing a monomer containing a strong cationic electrolyte, a polyamine monomer, and deionized water.

[0027] S2. Mix polyacryl chloride into an organic solvent and heat in a water bath to obtain an oil phase solution;

[0028] S3. Mix aromatic aldehydes into an organic solvent and sonicate to prepare a capped solution;

[0029] S4. Fix the base film in the reaction frame, pour in the mixed solution, wet the film surface and then pour it out; immediately pour in the oil phase solution, carry out the interfacial polymerization reaction and then pour it out, rinse with organic solvent and dry; then pour in the end-capping solution, carry out the end-capping reaction and then pour it out, and heat-cur it.

[0030] The strong cationic electrolyte monomer in step S1 is 1.67-1.82 wt% polyethyleneimine (PEI), and the polyamine monomer is 0.18-0.33 wt% m-phenylenediamine (MPD).

[0031] The polyacryl chloride in step S2 is 0.10 wt% trimesoyl chloride (TMC), and the organic solvent is n-hexane;

[0032] The aromatic aldehyde in step S3 is 0.05-0.15 wt% terephthalaldehyde (TPA).

[0033] The pH value of the mixed solution prepared in step S1 is 7.5.

[0034] The room temperature in step S4 is controlled at 20°C and the humidity at 35-40%.

[0035] The ultrasonic rotation speeds in steps S1 and S3 are 100 rpm and 300 rpm, respectively, and the ultrasonic mixing times are 30 min and 60 min, respectively.

[0036] The thermosetting process in step S4 should be carried out in an oven for 2 minutes.

[0037] The advantages of this invention are:

[0038] This invention discloses a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane and its preparation method. The membrane undergoes an interfacial polymerization reaction between a blend of polyethyleneimine (PEI) and m-phenylenediamine (MPD) and triphenylcarbamate chloride (TMC), altering the inherent electronegativity of the polyamide membrane and simultaneously converting both the surface and bottom layers to positive charge. Aromatic aldehydes—terephthalaldehyde (TPA)—react with residual polyamide aminoamine aldehydes on the polyamide membrane to generate Schiff base structures. This prevents active chlorine from disrupting the selective layer structure while simultaneously reducing the positive charge through the spatial occupancy of the polyamide-aromatic aldehydes, thus producing a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane.

[0039] Compared to traditional reverse osmosis membranes, the preparation method of this invention achieves double-sided chlorine resistance of the positively charged polyamide selective layer through "end-sealing densification + steric hindrance". At the same time, the membrane also has higher water flux and better application value. It provides a more feasible molecular design and process scheme for the engineering application of chlorine resistance of positively charged polyamide composite membranes, and has guiding significance for the development of new nanofiltration membrane production processes. It has strong practicality and wide applicability. Attached Figure Description

[0040] Figure 1 The graph shows the sodium chloride retention rate of Experimental Example 2, Comparative Example 1, and Comparative Example 2 after chlorine treatment time.

[0041] Figure 2 The graph shows the water flux of Experimental Example 2 and Comparative Examples 1 and 2 after chlorination treatment time.

[0042] Figure 3 The zeta potential diagrams of the membrane surface of Experimental Example 2 and Comparative Examples 1 and 2 are shown.

[0043] Figure 4 The water contact angle diagrams are for Experimental Example 2 and Comparative Examples 1 and 2.

[0044] Figure 5 The images are scanning electron microscope images of the membrane surfaces of Experimental Example 2 and Comparative Examples 1 and 2. Detailed Implementation

[0045] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0046] All chemical reagents used in this invention are commercially available.

[0047] Appendix Figure 1 , 2 The performance testing conditions were as follows: the water pressure during membrane operation was 20 bar; pre-pressurization for 1 hour was performed before measuring water flux and salt rejection rate; and the salt concentration in the feed solution was 2 g / L. For chlorine resistance testing, a sodium hypochlorite solution containing 6–14 wt% active chlorine was prepared to a 1000 ppm chlorine solution, and the pH of the immersion solution was adjusted to 7.

[0048] Appendix Figure 3 The ZETA potential was measured using a solid surface ZETA potential meter from Anton Paar Surpass 3 of Austria. The pH was 6.9, the temperature was 22°C, the conductivity was 15 mS / m, and the Gap Height was adjusted to 106-108 μm.

[0049] Appendix Figure 4 The water contact angle was measured using a DSA30S water contact angle measuring instrument from Krüz GmbH, Germany, employing the seated drop method, with an ambient temperature of 20℃.

[0050] Appendix Figure 5 Topographic images were taken using a Hitachi Regulus 8100 scanning electron microscope (SEM) from Hitachi, Japan. The ambient temperature was 22.5℃ and the relative humidity was 54%. Gold was sputtered for 45 seconds using a Quorum SC7620 sputtering system at a speed of 10mA. The accelerating voltage for topographic images was 3kV, and the accelerating voltage for energy dispersive spectroscopy (EDS) mapping images was 20kV. The detector was an SE2 secondary electron detector.

[0051] Experimental Example 1

[0052] A method for preparing a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane includes the following steps:

[0053] S1. Take a 30.00 wt% polyethyleneimine (PEI) aqueous solution at room temperature, dilute it with deionized water to a 5 wt% solution, sonicate for 30 min, and store the resulting aqueous solution in a light-proof brown bottle for later use.

[0054] Prepare a mixed solution of polyethyleneimine (PEI), m-phenylenediamine (MPD), and deionized water in a brown bottle, wherein the concentration of MPD is 0.18-0.33 wt% and the concentration of PEI is 1.67-1.82 wt%. Sonicate for 30 minutes to obtain the mixed solution for later use, and use it within two hours to avoid oxidation.

[0055] S2. Weigh out trimesoyl chloride (TMC) powder, transfer it to a sealed brown bottle, add n-hexane solution to prepare an organic solution of 0.10 wt% TMC, heat it in a water bath to 30-40℃ and sonicate it for more than 30 minutes to obtain an oil phase solution for later use.

[0056] S3. Weigh terephthalaldehyde (TPA) powder, transfer it to a sealed brown bottle, add n-hexane solution to prepare an organic solution of 0.05wt% TPA, sonicate for 60 min to obtain a cross-linked end-capped organic solution (end-capped solution) for later use.

[0057] S4. Soak the PES membrane in deionized water for more than one day, take it out and dry it, and use it as a support membrane to fix it in the circular reaction frame.

[0058] Pour in the mixed (aqueous phase) solution prepared in step S1, wet the membrane surface for 2 minutes and then pour it out. After pouring out the solution, take out the membrane, remove the remaining liquid with a rubber roller, fix the membrane back in its original position, and absorb the liquid through the non-woven fabric.

[0059] Then pour in the oil phase solution prepared in step S2, carry out the interfacial polymerization reaction for 1 minute, pour it out, rinse with n-hexane solution for 30 seconds, and let it air dry.

[0060] After drying, pour in the sealing solution prepared in step S3 and carry out the sealing reaction for 2 minutes. After pouring out, place it in a 60°C oven for 2 minutes. After taking it out, trim off the membrane perimeter error and store it in deionized water.

[0061] Experimental Example 2

[0062] The procedure is the same as in Experiment 1, except that the concentration of the terephthalaldehyde (TPA) solution prepared in this experiment is 0.10 wt%.

[0063] Experimental Example 3

[0064] The procedure is the same as in Test Example 1, except that the concentration of the terephthalaldehyde (TPA) solution prepared in this test example is 0.15 wt%.

[0065] Comparative Example 1

[0066] This comparative example differs from Experimental Example 1 in that neither the blend monomer PEI nor the subsequent end-capping reaction is added. The specific steps are as follows:

[0067] S1. Prepare a 2.00wt% solution of MPD and deionized water in a brown bottle, sonicate for at least 30 minutes to obtain an aqueous solution for later use, and use within two hours to avoid oxidation.

[0068] S2. Weigh TMC powder, transfer it to a sealed brown bottle, add n-hexane solution, and prepare an organic solution with a mass fraction of 0.10 wt% TMC. Heat the solution in a water bath to 30-40℃ and sonicate for more than 30 minutes to obtain an oil phase solution for later use.

[0069] S3: Membrane preparation: Soak the PES membrane in deionized water for more than one day, remove it and dry it. Use it as a support membrane to fix it in a circular reaction frame. Pour in the prepared MPD aqueous phase solution, wet the membrane surface for 2 minutes and then pour it out. Use a non-woven fabric to absorb the liquid. Then pour in the prepared TMC oil phase solution for interfacial polymerization reaction for 1 minute. After pouring it out, rinse it with n-hexane solution for 30 seconds and let it air dry. After removing it, trim the membrane perimeter error and store it in deionized water.

[0070] Comparative Example 2

[0071] This comparative example differs from Experimental Example 1 in that it incorporates the blend monomer PEI, but does not undergo subsequent end-capping reactions. The specific steps are as follows:

[0072] S1. Take a 30.00 wt% PEI aqueous solution at room temperature, dilute it with deionized water to a 5.00 wt% solution, sonicate for at least 20 minutes, and store the resulting aqueous solution in a light-proof brown bottle for later use; in the brown bottle, prepare a mixed aqueous solution with PEI, MPD and deionized water, with MPD concentration of 0.18-0.33 wt% and PEI concentration of 1.67-1.82 wt%, sonicate for at least 30 minutes to obtain a mixed (aqueous) solution for later use, and use it within two hours to avoid oxidation;

[0073] S2. Weigh TMC powder, transfer it to a sealed brown bottle, add n-hexane solution, and prepare an organic solution with a mass fraction of 0.10 wt% TMC. Heat the solution in a water bath to 30-40℃ and sonicate for more than 30 minutes to obtain an oil phase solution for later use.

[0074] S3. Membrane preparation: Soak the PES membrane in deionized water for more than one day, remove it and dry it. Use it as a support membrane to fix it in a circular reaction frame. Pour in the mixed (aqueous) solution prepared in step S1, wet the membrane surface for 2 minutes and then pour it out. After pouring out the solution, remove the membrane and remove the remaining liquid with a rubber roller. Re-fix the membrane in its original position and absorb the liquid with a non-woven fabric. Then pour in the oil phase solution prepared in step S2 to carry out the interfacial polymerization reaction for 1 minute. After pouring out, rinse with n-hexane solution for 30 seconds and store it in deionized water.

[0075] Comparative Example 3

[0076] The difference between this comparative example and Experimental Example 1 is that, after adding the blend monomer PEI, the interfacial polymerization did not involve a cross-linking and end-capping reaction with terephthalaldehyde (TPA). Instead, a surface grafting treatment using peptide amino acids was performed (the grafting treatment in this comparative example is based on patent CN 112892235A). The specific steps are as follows:

[0077] S1: Take a 30.00 wt% PEI aqueous solution at room temperature, dilute it with deionized water to a 5 wt% solution, sonicate for at least 20 minutes, and store the resulting aqueous solution in a light-proof brown bottle for later use; in the brown bottle, prepare a solution of PEI, MPD and deionized water with an MPD concentration of 0.18-0.33 wt% and a PEI concentration of 1.67-1.82 wt%, sonicate for at least 30 minutes to obtain a mixed (aqueous) solution for later use, and use it within two hours to avoid oxidation;

[0078] S2. Weigh TMC powder, transfer it to a sealed brown bottle, add n-hexane solution, and prepare an organic solution with a mass fraction of 0.10 wt% TMC. Heat the solution in a water bath to 30-40℃ and sonicate for more than 30 minutes to obtain an oil phase solution for later use.

[0079] S3. Weigh out L-polyarginine hydrochloride white powder. The molecular weight of L-polyarginine is 3000 Da. Prepare a 10 wt% L-polyarginine aqueous solution and sonicate for 30 min to obtain the grafting solution for later use.

[0080] S4. Membrane Fabrication: The PES membrane was soaked in deionized water for more than one day, then removed and dried. It was then fixed as a support membrane in a circular reaction frame. A prepared mixed aqueous solution was poured in, wetting the membrane surface for 2 minutes, then poured out. The liquid was absorbed through a non-woven fabric surface. Immediately afterward, a prepared trimesoyl chloride oil phase solution was poured in for interfacial polymerization for 1 minute. The membrane was then rinsed with n-hexane solution for 30 seconds and allowed to air dry. The membrane was then immersed in a prepared polypeptide and amino acid aqueous solution (grafting solution) for surface grafting treatment for 10 minutes. The treated nanofiltration membrane was washed with water, removed, and the perimeter error portion was trimmed. It was then stored in deionized water.

[0081] Comparative Example 4

[0082] This comparative example references the positively charged film method described in patent CN 111434375A, using glutaraldehyde-crosslinked polyethyleneimine grafted onto a polyamide layer. The specific steps are as follows:

[0083] S1: Take a 30.00wt% PEI aqueous solution at room temperature, dilute it with deionized water to a 1.50wt% solution, sonicate for more than 20 minutes, and store the obtained PEI aqueous solution in a light-proof brown bottle for later use.

[0084] Prepare a solution of MPD and deionized water in a brown bottle with a concentration of 2.00 wt%. Sonicate the solution for at least 30 minutes to obtain an MPD aqueous solution for later use. Use the solution within two hours to avoid oxidation.

[0085] S2. Weigh TMC powder, transfer it to a sealed brown bottle, add n-hexane solution, prepare an organic solution with a mass fraction of 0.10 wt%, heat it in a water bath to 30-40℃ and sonicate it for more than 30 minutes to obtain an oil phase solution for later use.

[0086] S3. Prepare a 0.1 wt% glutaraldehyde solution, sonicate it, and set it aside.

[0087] S4. Membrane preparation: Soak the PES membrane in deionized water for more than one day, take it out and dry it. Use it as a support membrane to fix it in a circular reaction frame. Pour in the prepared MPD aqueous solution, wet the membrane surface for 2 minutes and then pour it out. Use a non-woven fabric to absorb the liquid.

[0088] Then pour in the prepared TMC oil phase solution to carry out the interfacial polymerization reaction for 1 minute. After pouring it out, rinse with n-hexane solution for 30 seconds and let it air dry.

[0089] Under light-proof conditions, pour in PEI aqueous solution and react for four hours. After pouring out, rinse with pure water for 1 minute.

[0090] Pour in the prepared glutaraldehyde solution, allow the cross-linking reaction to proceed for 5 minutes, then pour out the solution, allow it to dry naturally, and then store the resulting film in pure water.

[0091] Experimental data:

[0092]

[0093] Experimental Example 2 represents the optimal choice of this invention. A comparison with Comparative Examples 1 and 2 demonstrates the advantages of this invention, as shown in the appendix. Figure 1-5 As shown:

[0094] Appendix Figure 1 The graph shows the sodium chloride retention rate as a function of chlorine treatment time. It indicates that after chlorine treatment for 48 hours, the NaCl retention rate of Example 2 was significantly better than that of Comparative Example 1 and Comparative Example 2, and the advantage became more pronounced with increasing chlorine treatment time.

[0095] Appendix Figure 2 The graph shows the water flux as a function of chlorination treatment time. It indicates that after 120 hours of chlorination treatment, the polyamide membrane structure of Comparative Example 1 and Comparative Example 2 was damaged, and the water flux increased significantly, while the water flux of Experimental Example 2 remained almost unchanged.

[0096] Appendix Figure 3The diagram shows the Zeta potential of the membrane surface. Note: The modifications in Comparative Example 2 and Experimental Example 2 resulted in a positively charged membrane surface, while Comparative Example 1 showed a negatively charged surface.

[0097] Appendix Figure 4 The image shows a scanning electron microscope image of the membrane surface. Note: Experimental Example 2 and Comparative Examples 1 and 2 all exhibit rough wrinkles, with no significant difference in morphology.

[0098] Appendix Figure 5 The diagram shows the water contact angle. Note: The water contact angle of Comparative Example 1 is greater than that of Comparative Example 2 and Test Example 2. All three are hydrophilic membranes, but Comparative Example 2 and Test Example 2 are more hydrophilic and therefore have higher water flux.

[0099] Comparative Example 3 demonstrates that, in a positively charged film, the chlorine resistance of the present invention is superior to the method of patent CN112892235A.

[0100] Comparative Example 4 demonstrates that the cross-linked polyethyleneimine grafted polyamide layer structure in comparative patent CN 111434375A does not have chlorine resistance.

[0101] This invention employs a novel densification and space-occupying protection method centered on aromatic aldehydes to end-cap and cure positively charged reverse osmosis membranes, significantly improving chlorine resistance while maintaining positive electrical properties. During the interfacial polymerization reaction, PEI and MPD simultaneously react with TMC to form a polyamide layer. Since PEI is a long linear polymer containing multiple branches with protonable amine groups, its polyamide layer itself carries a positive charge on both its surface and bottom.

[0102] The mechanism by which TPA-terminated polyamide layers improve chlorine resistance is as follows: Most of the residual amide on the surface of the polyamide layer reacts with TPA to form an amine-aldehyde reaction, generating a Schiff base structure, thus preventing active chlorine from damaging the structure of the polyamide layer. At the same time, due to the abundance of branched protonable amine groups, the surface positive charge is only slightly lower than that on the bottom of the polyamide layer. The spatial occupancy and blocking effect of the TPA benzene ring makes it difficult for active chlorine to enter and destroy the positive charge. For the protonable amine groups on the bottom of the polyamide layer, compared with the structure of aliphatic aldehydes, aromatic aldehydes (TPA) tend to accumulate on the surface of the polyamide layer, making it difficult for them to penetrate the dense polyamide layer and react with the protonable amine groups on the bottom side chains. Meanwhile, the dense functional layer formed by the polyamide-aromatic aldehyde reaction strongly blocks the diffusion of active chlorine, completely preserving the protonable amine groups on the bottom surface.

[0103] Furthermore, the bridging effect of its dialdehyde groups on adjacent amine groups improves the effective crosslinking degree and segment rigidity of the surface layer, repairing micro-defects on the film surface, thereby achieving outer layer densification and enhanced chemical robustness. Simultaneously, the aromatic Schiff base skeleton also exhibits higher hydrolytic and thermal stability compared to aliphatic Schiff bases. Unlike traditional chlorine-resistant strategies such as polyester substitution or strong hydrophilic coatings, this method retains the positive potential imparted by the PEI interlayer, reducing the exposure and permeation pathways of chlorinated groups solely through "space occupation + densification." Therefore, while maintaining cation selectivity, it suppresses the structural fragility common in electrochemical modifications and alleviates the decrease in ion rejection rate caused by electrochemical modification, leading to a recovery in rejection rate.

[0104] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A high-permeability, positively charged, chlorine-resistant reverse osmosis membrane, characterized in that, From top to bottom, the layers are: a chlorine-resistant end-capped polyamide layer, a positively charged polyamide layer, and a base film layer. The end-capped chlorine-resistant polyamide layer is formed by reacting a mixed solution of polyamine monomers and monomers containing strong cationic electrolytes with acyl chloride to form a polyamide layer, and then end-capping the upper surface layer with aromatic aldehydes. The positively charged polyamide layer is formed by reacting a mixed solution of polyamine monomers and monomers containing strong cationic electrolytes with acyl chloride to form the bottom layer of the polyamide layer; The base membrane layer is a porous support membrane; the monomer containing strong cationic electrolytes includes polyethyleneimine and polydiallyl dimethylammonium; the polyamine monomers include piperazine, m-phenylenediamine, p-phenylenediamine, 1,2-ethylenediamine, or 1,6-hexanediamine.

2. The reverse osmosis membrane according to claim 1, characterized in that, The aromatic aldehydes include cinnamaldehyde, benzaldehyde, o-methylbenzaldehyde, m-methylbenzaldehyde, p-methylbenzaldehyde, and salicylaldehyde, which contain a single aldehyde group, or o-phthalaldehyde, terephthalaldehyde, and m-phthalaldehyde, which contain two aldehyde groups.

3. The reverse osmosis membrane according to claim 1, characterized in that, The base film material of the base film layer includes polyethersulfone, polysulfone, polyethylene, polyamide imide, polypropylene or polyacrylonitrile, and the molecular weight cutoff of the base film is 100~300kDa.

4. The reverse osmosis membrane according to claim 1, characterized in that, The mass ratio of the strong cationic electrolyte monomer to the polyamine monomer is 5:1 to 10:

1.

5. The method for preparing a high-permeability, positively charged, chlorine-resistant reverse osmosis membrane according to any one of claims 1-4, characterized in that, Includes the following steps: S1. A mixed solution is prepared by ultrasonically mixing a monomer containing a strong cationic electrolyte, a polyamine monomer, and deionized water. S2. Mix polyacryl chloride into an organic solvent and heat in a water bath to obtain an oil phase solution; S3. Mix aromatic aldehydes into an organic solvent and sonicate to prepare a capped solution; S4. Fix the base film in the reaction frame, pour in the mixed solution, wet the film surface and then pour it out; immediately pour in the oil phase solution, carry out the interfacial polymerization reaction and then pour it out, rinse with organic solvent and dry; then pour in the end-capping solution, carry out the end-capping reaction and then pour it out, and heat-cur it.

6. The preparation method according to claim 5, characterized in that, In step S1, the strong cationic electrolyte monomer is 1.67-1.82 wt% polyethyleneimine, and the polyamine monomer is 0.18-0.33 wt% m-phenylenediamine; in step S2, the polyacrylamide chloride is 0.10 wt% trimesoyl chloride, and the organic solvent is n-hexane; in step S3, the aromatic aldehyde is 0.05-0.15 wt% terephthalaldehyde.

7. The reverse osmosis membrane according to claim 5, characterized in that, The polyacrylic chlorides include pyromellitic trichloroisocyanurate, isophenyltrichloroisocyanurate, cyclohexanetrichloroisocyanurate, cyclopentanetrichloroisocyanurate, propyltrichloroisocyanurate, or pentanetrichloroisocyanurate; the solvents of the organic solutions include n-hexane, n-heptane, or tetradecane.

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