A paa-grafting-based perfluorocompound rejection negatively charged nanofiltration membrane and a preparation method thereof

Abstract

The application discloses a kind of based on PAA grafting perfluorinated compound interception negative electric nanofiltration membrane and its preparation method, including base film pretreatment: base film cleaning and activation, preparation containing acrylic acid and glutaraldehyde pretreatment mixed solution, pregrafting reaction;Interface polymerization: preparation contains 1.3~1.5wt%PIP+0.1~0.14wt%TAP+0.14~0.18wt%PEG+0.8~1.2wt%GA+0.03~0.05wt%SDBS aqueous solution, preparation oil phase solution, polymerization reaction;Surface activation: after interface polymerization, membrane is immersed in 0.04~0.06wt%NaOH solution;Surface layer grafting functional layer: preparation contains acrylic acid and glutaraldehyde grafting mixed solution, surface layer grafting reaction.The application realizes nanofiltration membrane "high selectivity-high flux-high stability-wide adaptability" multidimensional performance promotion, meets complex water quality processing and one, two ion precision separation demand.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane materials technology, and in particular to a perfluorinated compound-based negatively charged nanofiltration membrane grafted with PAA and its preparation method. Background Technology

[0002] The separation performance of nanofiltration membranes relies on the synergistic effect of "pore size sieving + charge effect," with surface negative charge density being key to improving the retention capacity for negatively charged pollutants and divalent cations. Existing technologies for increasing the negative charge on the membrane surface mainly involve introducing functional groups such as carboxyl and sulfonic acid groups, but these methods have certain limitations. Firstly, single-functional monomer modification, which only involves adding carboxyl-containing monomers during interfacial polymerization, results in limited improvement in negative charge density and is prone to performance degradation due to group detachment. Secondly, physical coating modification introduces negatively charged substances through adsorption or deposition, but the interlayer bonding is weak, making it easy to detach under water flow and resulting in insufficient stability.

[0003] Polyacrylic acid (PAA) is a water-soluble anionic polymer with the molecular formula (C3H4O2). n The relative molecular weight can be adjusted as needed. Its linear molecular backbone is densely packed with carboxyl functional groups, making it not only highly water-soluble and capable of covalent cross-linking with aldehydes or amino groups, but also maintaining structural stability over a wide pH range of 3-11. Furthermore, the carboxyl anions that can dissociate under neutral and alkaline conditions provide a stable negative charge to the material. In the field of membrane separation, PAA grafting onto the membrane surface can form a functional layer, which can increase the negative charge density on the membrane surface to enhance electrostatic repulsion against negatively charged pollutants and divalent cations, while also improving the membrane's hydrophilicity, water flux, and antifouling ability. However, traditional PAA-modified nanofiltration membranes often employ physical coating or single-stage grafting processes, resulting in weaknesses such as weak adhesion between PAA and the membrane substrate, easy detachment, and uneven charge distribution, making it difficult to simultaneously achieve ion selectivity and new pollutant retention performance.

[0004] Chinese Patent Publication No. CN113877268A discloses a method for preparing a high-charge nanofiltration membrane, which introduces negative charges by adding sulfonated monomers to the aqueous phase. However, it does not employ a gradient grafting process, resulting in uneven distribution of negative charges and the absence of a through-linked network, leading to insufficient long-term operational stability. Chinese Patent Publication No. CN114524863A proposes a method for preparing a nanofiltration membrane containing polyethylene glycol chains, which focuses on increasing flux through pore-forming agents. However, it does not optimize for negative charge density, resulting in a rejection rate of less than 85% for negatively charged pollutants, which cannot meet the requirements of high-demand treatment scenarios.

[0005] In summary, the existing technology still has the following shortcomings:

[0006] 1. Imbalance in charge regulation: Existing nanofiltration membranes generally suffer from insufficient negative charge density or uneven distribution, resulting in small differences in the sieving of divalent and monovalent cations, low ion selectivity, and difficulty in achieving precise separation; at the same time, charged groups are prone to detachment or unstable ionization, and the separation performance deteriorates significantly after long-term operation, which cannot meet the long-term use requirements of high-demand scenarios.

[0007] 2. Weak interlayer bonding: The bonding between the base membrane and the functional layer, and between the functional layer and the surface modified layer, is mostly physical adsorption or local covalent bonding, lacking a stable cross-linked network. Under actual operating conditions such as water flow scouring and pressure fluctuations, interlayer delamination is prone to occur, leading to simultaneous deterioration of flux and retention performance.

[0008] 3. Insufficient structural stability: The membrane does not form a complete three-dimensional cross-linking system, resulting in poor anti-swelling performance. Under different pH environments, temperature fluctuations, or chemical media, the membrane pore structure is prone to deformation, the pore size distribution becomes wider, and the separation accuracy decreases. At the same time, membrane fouling is prone to accumulate during long-term operation, further aggravating performance degradation.

[0009] 4. Poor process synergy: Existing preparation methods are difficult to achieve synergistic optimization of "high throughput-high selectivity-high stability", and there is often an imbalance problem of "high throughput with poor selectivity, and good selectivity with low throughput". Moreover, the adaptability of preparation process parameters is insufficient, and they are highly sensitive to reaction conditions, making it difficult to guarantee repeatability and stability in large-scale production.

[0010] 5. Limited environmental adaptability: It has a weak ability to adapt to complex water quality and a wide pH range. In acidic or alkaline wastewater treatment scenarios, the separation performance is easily affected by membrane structure damage or deactivation of charged groups, resulting in a large fluctuation in the scope of application. Summary of the Invention

[0011] This invention addresses the problems of charge regulation imbalance, weak interlayer bonding, insufficient structural stability, poor process synergy, and limited environmental adaptability in existing nanofiltration membranes. It proposes a PAA-grafted perfluorinated compound-based negatively charged nanofiltration membrane and its preparation method. By constructing a stable cross-linked network and optimizing the charge regulation mechanism and preparation process synergistically, this invention achieves a multi-dimensional performance improvement in nanofiltration membranes, encompassing high selectivity, high flux, high stability, and wide adaptability. This overcomes the shortcomings of existing nanofiltration membranes in separation accuracy, structural stability, and environmental adaptability, meeting the industrial demands for complex water treatment and precise separation of monovalent and divalent ions.

[0012] This invention is achieved by a method for preparing a PAA-grafted perfluorinated compound-based nanofiltration membrane with a negative charge cutoff, comprising the following steps:

[0013] S1. Base film pretreatment

[0014] This includes base film cleaning and activation, preparation of pretreatment mixture, and pre-grafting reaction;

[0015] The pretreatment mixture contains acrylic acid and glutaraldehyde;

[0016] Pre-grafting reaction: The surface-activated base film is immersed in the pretreatment mixture, so that glutaraldehyde crosslinks the hydroxyl groups and carboxyl groups of the base film at the same time, while reserving some aldehyde groups;

[0017] S2. Interface Aggregation

[0018] This includes the preparation of aqueous solutions, the preparation of oil solutions, and polymerization reactions;

[0019] The aqueous solution was prepared by sequentially adding 1.3wt%~1.5wt% piperazine (PIP), 0.1wt%~0.14wt% triaminopyrimidine (TAP), and 0.14wt%~0.18wt% polyethylene glycol-1000 (PEG-1000) to deionized water and stirring until homogeneous. Just before film formation, 0.8wt%~1.2wt% glutaraldehyde (GA) and 0.03wt%~0.05wt% sodium dodecylbenzenesulfonate (SDBS) were added, and the mixture was stirred until homogeneous.

[0020] Polymerization reaction: The pretreated base film is laid flat, and then an aqueous solution and an oil solution are poured on it successively;

[0021] S3. Surface activation

[0022] Polyamide surface activation: Immerse the interfacially polymerized film in 0.04wt%~0.06wt% NaOH solution and let it stand at 20℃~30℃ for 10~20 min; rinse until neutral.

[0023] S4. Surface grafting functional layer

[0024] This includes the preparation of the grafting mixture and the surface grafting reaction;

[0025] The grafting mixture contains acrylic acid and glutaraldehyde;

[0026] Surface grafting reaction: The surface-activated film is immersed in a grafting mixture to bridge the amino groups of polyamide and acrylic acid with glutaraldehyde, thereby constructing a PAA grafted functional layer.

[0027] Further, in step S1, the base membrane is cleaned and activated: take a polysulfone ultrafiltration base membrane, ultrasonically clean it with deionized water for 5-15 min; immerse it in 0.4wt%-0.6wt% NaOH solution, let it stand at 20℃-30℃ for 15-25 min to activate the hydroxyl groups on the surface of the base membrane, rinse it until neutral and drain it.

[0028] Further, in step S1, the pretreatment mixture is prepared by adding 1.5wt%~2wt% acrylic acid and 0.3wt%~0.5wt% glutaraldehyde to deionized water and stirring magnetically for 1~10 min.

[0029] Furthermore, in step S1, during the pre-grafting reaction, when the surface-activated base film is immersed in the pretreatment mixture, it is sealed and left to stand at 20℃~30℃ for 1~3 hours, then rinsed with deionized water and allowed to air dry.

[0030] Further, in step S2, the oil phase solution is prepared by preparing a hexane solution containing 0.12wt%~0.15wt% TMC and ultrasonically dispersing it for 1~10 min.

[0031] Further, in step S2, the polymerization reaction is as follows: the pretreated base film is laid flat, an aqueous solution is poured on it, and it is allowed to stand at 20℃~30℃ for 1~5 min. The excess aqueous phase is poured off, and the film is purged with nitrogen and air-dried. The oil phase solution is poured on immediately, and the film is reacted at 20℃~30℃ for 0.5~1.5 min. The oil phase is poured off, and the film is air-dried naturally.

[0032] Further, in step S4, the grafting mixture is prepared by adding 6wt%~7wt% acrylic acid and 1.5wt%~2wt% glutaraldehyde to deionized water and stirring until homogeneous.

[0033] Furthermore, in step S4, during the surface grafting reaction, when the surface-activated membrane is immersed in the grafting mixture, it is sealed and left to stand at 20℃~30℃ for 1~2 hours, then rinsed with clean water and air-dried naturally.

[0034] Furthermore, it also includes:

[0035] S5. Cleaning and Storage

[0036] Immerse the membrane in deionized water to hydrate the PAA grafted functional layer, then remove it and blot off the surface moisture before use. For long-term storage, it needs to be sealed and immersed in deionized water at 2~5℃.

[0037] This invention employs a multi-step synergistic process: pretreatment of the base membrane to construct a bottom charge network, interfacial polymerization to form a polyamide functional layer, surface activation, and surface grafting of functional layers to enhance the negative charge on the surface. Negative charge regulation relies on the synergistic effect of the bottom acrylic acid carboxyl groups, the surface PAA grafted functional layer, and the TAP pyrimidine rings. Crosslinking is achieved through a three-dimensional network constructed with glutaraldehyde. Performance balance is achieved through PEG-induced pore formation, SDBS rate regulation, and optimization of the glutaraldehyde addition sequence. Precise activation is achieved through low-concentration NaOH regulation, thus realizing the synergistic optimization of charge regulation and structural stability. Ultimately, this results in a nanofiltration membrane with high selectivity (average sodium and magnesium ion selectivity ≥60%), practical flux (≥40 LMH), and high stability (no risk of interlayer delamination).

[0038] A perfluorinated compound-based nanofiltration membrane with PAA grafting was prepared using the method described above.

[0039] The advantages and positive effects of this invention are:

[0040] 1. Significantly improved separation selectivity: Through multi-dimensional negative charge regulation, the average selectivity of sodium and magnesium ions can reach 62.05, and the magnesium sulfate rejection rate is ≥98.77%, which is significantly better than the existing technology (selectivity is less than 30), achieving precise separation of monovalent and divalent ions and effectively solving the problem of charge regulation imbalance.

[0041] 2. Enhanced interlayer bonding and structural stability: The through-type three-dimensional cross-linked network significantly improves the interlayer bonding force, eliminating the risk of interlayer delamination; at the same time, it enhances the membrane's anti-swelling performance, and the pore size distribution remains stable in a wide range of environments from pH 3 to 11. After 50 hours of long-term operation, the retention rate is maintained at ≥95%, solving the defects of weak interlayer bonding and insufficient structural stability.

[0042] 3. Synergistic balance between throughput and selectivity: By leveraging PEG pore formation, SDBS reaction rate regulation, and glutaraldehyde dosing sequence optimization, a practical throughput of ≥40 LMH is achieved on the basis of high selectivity, overcoming the bottleneck of the difficulty in achieving both throughput and selectivity, and solving the industry pain point of poor process synergy.

[0043] 4. Significantly broadened environmental adaptability: The synergistic effect of dynamic and permanent negative charges ensures the activity stability of the charged groups under complex water quality and wide pH range; the mild process and stable structural design ensure that the membrane performance fluctuation is ≤5% in acidic and alkaline wastewater treatment scenarios, effectively solving the problem of limited environmental adaptability, and the applicable scope covers multiple scenarios such as industrial wastewater treatment and lithium extraction from salt lakes. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the nanofiltration membrane provided in Embodiment 1 of the present invention;

[0045] Figure 2 The image shows a scanning electron microscope (SEM) image (×10000x) of the surface of the nanofiltration membrane obtained in Example 1 of the present invention.

[0046] Figure 3 The image shows a scanning electron microscope (SEM) image (×20000x) of the surface of the nanofiltration membrane obtained in Example 1 of the present invention.

[0047] Figure 4 This is an AFM plan view of the nanofiltration membrane surface obtained in Embodiment 1 of the present invention;

[0048] Figure 5 This is an AFM 3D image of the nanofiltration membrane surface obtained in Example 1 of the present invention. Detailed Implementation

[0049] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below with reference to embodiments. However, it should be understood that the following embodiments are only preferred embodiments of the present invention, and the scope of protection claimed by the present invention is not limited thereto.

[0050] This invention provides a method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane, comprising the following steps:

[0051] S1. Base film pretreatment

[0052] S11. Base membrane cleaning and activation: Take the polysulfone ultrafiltration base membrane and ultrasonically clean it with deionized water for 5~15 min; immerse it in 0.4wt%~0.6wt% NaOH solution and let it stand at 20℃~30℃ for 15~25 min to activate the hydroxyl groups on the base membrane surface. Rinse until neutral and then drain.

[0053] Preparation of pretreatment mixture: Add 1.5wt%~2wt% acrylic acid and 0.3wt%~0.5wt% glutaraldehyde to 100 mL of deionized water and stir magnetically for 1~10 min.

[0054] S12. Pre-grafting reaction: Immerse the surface-activated base film in the pretreatment mixture, seal and stand at 20℃~30℃ for 1~3h, so that glutaraldehyde crosslinks the hydroxyl groups and carboxyl groups of the base film at the same time, leaving some aldehyde groups; after taking it out, rinse it twice with deionized water and let it air dry.

[0055] The aldehyde group of glutaraldehyde reacts simultaneously with the hydroxyl group (-OH) and the carboxyl group (-COOH) of the base membrane to form a "-O-CH(OH)-GA-CH(OH)-OOC-" crosslinking structure. The remaining aldehyde group does not react and serves as an anchor point for subsequent bonding with the polyamide layer. The reaction time must be strictly controlled. If it is too short, the grafting will be insufficient and an effective bottom charge network cannot be formed. If it is too long, the low concentration of acrylic acid may still block the pores of the base membrane and affect the subsequent flux.

[0056] S2. Interface Aggregation

[0057] S21. Preparation of aqueous solution: Add 1.3wt%~1.5wt% piperazine (PIP), 0.1wt%~0.14wt% triaminopyrimidine (TAP), and 0.14wt%~0.18wt% polyethylene glycol-1000 (PEG-1000) sequentially to 100 mL of deionized water, stir for 15~25 min, and mix evenly; just before film formation, add 0.8wt%~1.2wt% glutaraldehyde (GA) and 0.03wt%~0.05wt% sodium dodecylbenzenesulfonate (SDBS), and continue stirring for 1~3 min, and mix evenly.

[0058] Glutaraldehyde is added in the later stages before membrane fabrication, which shortens its reaction time with the aqueous amino group, keeping the Schiff base reaction mild and controllable. This avoids excessive reaction with the aqueous amino group in the early mixing stage, which could generate a large amount of prepolymer and cause "local bursts" in interfacial polymerization. This ensures the "orderly growth" of the polyamide chains, forming a functional layer with uniform crosslinking and narrow pore size distribution, guaranteeing the uniformity of polyamide layer crosslinking. SDBS reduces the surface tension of the aqueous phase, regulates the interfacial polymerization rate, inhibits excessive densification of the polyamide layer, and avoids excessively low flux. PEG, as a porogen, optimizes membrane pore uniformity and provides channels for water molecule transport. The three components work synergistically to ensure a balance between flux and retention performance.

[0059] An optimized aqueous phase formulation system of "1.3wt%~1.5wt% PIP + 0.1wt%~0.14wt% TAP + 0.14wt%~0.18wt% PEG-1000 + 0.03wt%~0.05wt% SDBS" was formed to achieve pore size control, reaction rate regulation and charge enhancement.

[0060] Preparation of oil phase solution: Prepare 100 mL of n-hexane solution containing 0.12 wt% to 0.15 wt% TMC and ultrasonically disperse for 1 to 10 min.

[0061] S22. Polymerization reaction: Spread the pretreated base film flat, pour on the aqueous phase solution, let stand at 20℃~30℃ for 1~5 min, pour off the excess aqueous phase, purge with nitrogen and air dry; immediately pour on the oil phase solution, react at 20℃~30℃ for 0.5~1.5 min, pour off the oil phase, and air dry naturally.

[0062] S3. Surface activation

[0063] Polyamide surface activation: Immerse the interfacially polymerized film in 0.04wt%~0.06wt% NaOH solution and let it stand at 20℃~30℃ for 10~20 min; rinse until neutral and then blot dry.

[0064] Precise surface activation using low-concentration NaOH (0.04wt%~0.06wt%) allows for precise control of the hydrolysis degree of amide bonds on the polyamide surface. This ensures sufficient exposure of amino groups (-NH2) to the surface amide bonds, providing ample anchor points for subsequent PAA grafting of functional layers. It also avoids excessive hydrolysis of amide bonds caused by high-concentration NaOH (e.g., 0.5wt%), preventing enlarged pore sizes and uneven charge distribution in the functional layers. Furthermore, a mild reaction temperature of 20℃~30℃ and a reaction time of 10~20 min further prevent runaway hydrolysis, ensuring the structural integrity of the polyamide functional layers.

[0065] S4. Surface grafting functional layer

[0066] S41. Preparation of grafting mixture: Add 6wt%~7wt% acrylic acid and 1.5wt%~2wt% glutaraldehyde to 100 mL of deionized water and stir until homogeneous.

[0067] S42. Surface grafting reaction: Immerse the surface-activated film in the grafting mixture, seal and stand at 20℃~30℃ for 1~2h to allow glutaraldehyde to bridge the amino groups of polyamide and acrylic acid, thus constructing a PAA grafted functional layer; after removal, rinse with water and air dry naturally.

[0068] Glutaraldehyde plays a bridging role in this step, reacting with the amino groups exposed after the polyamide layer is activated on one end and combining with acrylic acid on the other end to form a PAA grafted functional layer. At the same time, it undergoes secondary cross-linking with the aldehyde groups reserved in the base film pretreatment stage, constructing a through-three-dimensional network of "base film layer-polyamide layer-PAA grafted functional layer" to enhance the interlayer bonding force. The PAA grafted functional layer formed by high concentration of acrylic acid can strengthen the negative charge density of the surface layer and enhance the electrostatic repulsion of divalent cations through charge synergy of "bottom layer-intermediate layer-surface layer".

[0069] S5. Cleaning and Storage

[0070] Cleaning and storage: Immerse the membrane in deionized water and let it stand at 20℃~30℃ for 1~3 hours to hydrate the PAA grafted functional layer. Remove it and wipe off the surface moisture before use. For long-term storage, it needs to be sealed and immersed in deionized water at 2~5℃.

[0071] This invention employs a continuous process combination of "base film pretreatment → interfacial polymerization → precise activation → acrylic acid grafting", with each step closely linked to form a structured design of "bottom charge network - polyamide functional layer - surface negative charge layer", constituting a complete performance optimization system.

[0072] Through the synergistic effect of the underlying acrylic carboxyl groups, the surface PAA grafted functional layer, and the TAP pyrimidine ring, a multi-dimensional negative charge regulation system is achieved to balance dynamic and permanent negative charges, thereby increasing the negative charge density and ensuring charge stability.

[0073] Using glutaraldehyde as the crosslinking core, a gradient concentration ratio was adopted in the three stages of base film pretreatment, interfacial polymerization and acrylic acid grafting to construct a through-type three-dimensional crosslinking network of "base film-polyamide layer-PAA grafted functional layer", which strengthened the interlayer bonding force.

[0074] The entire process adopts natural air drying instead of traditional heat curing, avoiding excessive shrinkage of the membrane layer and damage caused by the "pore size-charge synergistic effect", thus further ensuring the integrity of the membrane structure and the stability of its performance.

[0075] To better understand the above embodiments of the present invention, further explanation is provided below with reference to specific examples.

[0076] Example 1

[0077] A method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane includes the following steps:

[0078] S1. Base film pretreatment

[0079] S11. Base membrane cleaning and activation: Take the polysulfone ultrafiltration base membrane, ultrasonically clean it with deionized water for 10 min; immerse it in 0.5wt% NaOH solution, let it stand at 25℃ for 20 min to activate the hydroxyl groups on the base membrane surface, rinse until neutral and drain.

[0080] Preparation of pretreatment mixture: Add 1.8wt% acrylic acid and 0.4wt% glutaraldehyde to 100 mL of deionized water and stir magnetically for 5 min.

[0081] S12. Grafting reaction: Immerse the surface-activated base film in the pretreatment mixture, seal and stand at 25°C for 2 h, so that glutaraldehyde crosslinks the hydroxyl groups and carboxyl groups of the base film at the same time, leaving some aldehyde groups; after taking it out, rinse it twice with deionized water and let it air dry.

[0082] S2. Interface Aggregation

[0083] S21. Preparation of aqueous solution: Add 1.4wt% PIP, 0.12wt% TAP and 0.18wt% PEG-1000 to 100 mL of deionized water in sequence, stir for 20 min and mix evenly; add 1.0wt% GA and 0.04wt% SDBS just before film formation, continue stirring for 2 min and mix evenly.

[0084] Preparation of oil phase solution: Prepare 100 mL of n-hexane solution containing 0.13 wt% TMC and ultrasonically disperse for 5 min.

[0085] S22. Polymerization reaction: Spread the pretreated base film flat, pour on the aqueous phase solution, let stand at 25℃ for 3 min, pour off the excess aqueous phase, purge with nitrogen and air dry; immediately pour on the oil phase solution, react at 25℃ for 1 min, pour off the oil phase, and air dry naturally.

[0086] S3. Surface activation

[0087] Polyamide surface activation: Immerse the interfacially polymerized film in 0.05wt% NaOH solution and let it stand at 30℃ for 15 min; rinse until neutral and then blot dry.

[0088] S4. Surface grafting functional layer

[0089] S41. Preparation of grafting mixture: Add 6.5wt% acrylic acid and 1.8wt% glutaraldehyde to 100 mL of deionized water and stir for 5 min.

[0090] S42. Surface grafting reaction: Immerse the surface-activated film in the grafting mixture, seal and stand at 25°C for 1 hour to allow glutaraldehyde to bridge the amino groups of polyamide and acrylic acid to form a PAA grafted functional layer; after removal, rinse with water and air dry naturally.

[0091] S5. Cleaning and Storage

[0092] Cleaning and storage: Immerse the membrane in deionized water and let it stand at 25°C for 1 hour to hydrate the PAA grafted functional layer. Remove it and wipe off the surface moisture before use. For long-term storage, it needs to be sealed and immersed in deionized water at 4°C.

[0093] The nanofiltration membrane prepared in Example 1 is named membrane M1, as follows: Figure 1 As shown.

[0094] The test content is as follows:

[0095] Ion rejection test: 2000 mg / L NaCl single salt solution and 2000 mg / L MgSO4 solution were prepared for testing. A self-made membrane testing machine was used, with an operating pressure of 0.69 MPa, a test temperature of 25℃, and cross-flow filtration mode. After pre-pressurization for 1 hour, permeate water was collected. Two parallel samples were set up for each group. The conductivity of the influent and effluent was measured by a conductivity meter, and the rejection rate and sodium and magnesium ion selectivity were calculated. The flux was calculated by the amount of permeate collected per unit time.

[0096] Perfluorinated compound rejection test: 25 μg / L perfluorooctane sulfonic acid (PFOS), 100 μg / L perfluorooctanoic acid (PFOA), and 100 μg / L perfluorodecanoic acid (PFDA) were selected as target pollutants. Water samples were prepared using methanol standard solution. The test conditions were the same as those for ion rejection test. After pre-pressurization for 1 hour, perfluorinated water was collected. Parallel perfluorinated water samples were prepared. The concentration of perfluorinated compounds in the influent and effluent water was accurately detected by solid phase extraction-liquid chromatography-tandem mass spectrometry (SPE-LC-MS / MS) and the rejection rate was calculated.

[0097] The formula for calculating the retention rate is as follows:

[0098]

[0099] In the formula, R The retention rate is expressed as %; The concentration of ionic or perfluorinated pollutants in the influent is expressed in mg / L. This represents the concentration of ionic or perfluorinated pollutants in the produced water, expressed in mg / L.

[0100] The formula for calculating the selectivity of sodium and magnesium ions is shown below:

[0101]

[0102] In the formula, S Selectivity for sodium and magnesium ions; The concentration of sodium ions in the product water is expressed in mg / L. The concentration of sodium ions in the influent is expressed in mg / L. The concentration of magnesium ions in the product water is expressed in mg / L. The concentration of magnesium ions in the influent is expressed in mg / L.

[0103] The test results are as follows:

[0104] Table 1. Sodium and Magnesium Ion Separation Performance

[0105]

[0106] Table 2 Retention performance of perfluorinated compounds

[0107]

[0108] The nanofiltration membrane prepared by this embodiment 1 through a four-step synergistic process achieves a triple performance breakthrough of "high ion selectivity, high efficiency in the retention of new pollutants, and high stability".

[0109] ① In terms of ion separation: The nanofiltration membrane of this invention has an average sodium and magnesium ion selectivity of 62.05, which is an absolute advantage over commercial nanofiltration membranes and reverse osmosis membranes; the average magnesium sulfate rejection rate is 98.77%, which is close to that of commercial reverse osmosis membranes; the flux decay rate after 12 hours of operation is only 4.2%, which is also an advantage over commercial membranes.

[0110] ② Regarding the removal of new pollutants: The nanofiltration membrane of this invention achieves average removal rates of 99.52%, 97.36%, and 99.71% for PFOS, PFOA, and PFDA, respectively, all higher than those of the commercial membrane NF50. Compared to the commercial membrane BWRO, the nanofiltration membrane of this invention has advantages in the removal of PFOA and PFDA organic matter. Compared to the commercial membrane SWRO, the nanofiltration membrane of this invention has advantages in the removal of PFOS and PFDA, with overall levels being similar.

[0111] However, the average flux of membrane M1 in Example 1 (40.56 LMH) is much higher than that of commercial membrane BWRO (conventional flux 15~35 LMH).

[0112] The membrane M1 in this embodiment 1 not only meets the requirements for precise separation of monovalent and divalent ions in scenarios such as lithium extraction from salt lakes and desalination of industrial wastewater, but also efficiently retains perfluorinated compounds, and its application scope covers multiple scenarios such as deep treatment of industrial wastewater and purification of drinking water.

[0113] The membrane M1 of Example 1 was characterized by zeta potential and by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results are shown in Table 3 and 4. Figures 2-5 .

[0114] Table 3. Zeta potential on the surface of membrane M1 in Example 1

[0115]

[0116] As shown in Table 3, in terms of charge regulation, the bottom acrylic carboxyl groups, the surface PAA grafted functional layer, and the TAP pyrimidine ring form a "dynamic + permanent" negative charge synergistic system. Under neutral conditions, the zeta potential reaches as high as -45 mV. This strong negative potential significantly enhances the effect on Mg. 2+ The electrostatic repulsion (directly enhancing ion selectivity) of TAP, combined with the anionic properties of perfluorinated compounds, produces a strong Donnan effect (enhancing pollutant retention), while the pyrimidine ring structure of TAP ensures charge stability over a wide pH range. Figure 4 and Figure 5 AFM characterization revealed a membrane surface roughness Ra of 38 nm. This roughness increases the contact area between the membrane and water and contaminants, providing more exposure sites for charged groups, while avoiding excessive roughness that could lead to contaminant adsorption and accumulation, thus further optimizing the balance between retention and antifouling. Figure 2 and Figure 3 As can be seen from the SEM images, regarding structural stability, the polyamide layer forms a regular "ridge-valley" structure, with a uniform layered material covering the surface. There is no interlayer delamination or structural breakage, confirming that the gradient crosslinking of glutaraldehyde in three stages successfully constructed a covalent network of "base film layer - polyamide layer - PAA grafted functional layer," effectively suppressing membrane swelling and interlayer separation, resulting in significantly excellent performance retention over long-term operation. The moderate roughness and complete crosslinking structure of membrane M1 in Example 1 jointly enhance structural stability and charge distribution uniformity.

[0117] Comparative Example 1: (Comparison Membrane M2: Base membrane pretreatment steps omitted)

[0118] The nanofiltration membrane prepared in Comparative Example 1 is named Comparative Membrane M2. The difference between its membrane preparation steps and those in Example 1 is that step S1 (base membrane pretreatment) is omitted.

[0119] Steps S2 (interface polymerization), S3 (surface activation), S4 (surface grafting functional layer), and S5 (cleaning and preservation) are retained, and the operation and parameters of the retained steps are consistent with those in Example 1.

[0120] Table 4. Comparison of performance test data between membrane M1 and control membrane M2

[0121]

[0122] Therefore, it can be seen that the core separation performance of the control membrane M2 is significantly worse than that of the membrane M1 in Example 1 due to the lack of a base membrane pretreatment step: the average selectivity for sodium and magnesium ions decreased from 62.05 to 43.31, a decrease of 30.2%, and the flux also decreased from 40.56 LMH to 30.68 LMH. The base membrane pretreatment step can construct a bottom carboxyl charge network on the base membrane surface, forming a gradient negative charge synergistic system with the TAP pyrimidine ring added in step S2 and the surface PAA grafted functional layer formed in step S4; the control membrane M2 lacks this bottom network, and its performance is significantly worse for divalent Mg. 2+ The electrostatic repulsion effect is weakened, and the difference in ion sieving is reduced. Simultaneously, base membrane pretreatment optimizes the pore structure of the base membrane and reduces water permeation resistance; in contrast, membrane M2, lacking this step, exhibits decreased pore permeability and consequently reduced flux. This confirms that base membrane pretreatment is the key foundation for achieving a balance between high selectivity and high flux in membrane M1 of Example 1 of this invention.

[0123] Comparative Example 2: (Comparative film M3: Base film pretreatment, surface activation, and surface grafting of functional layers are omitted)

[0124] The nanofiltration membrane prepared in Comparative Example 2 is named Comparative Membrane M3. Its membrane preparation steps differ from those in Example 1: Step S1 (base membrane pretreatment), Step S3 (surface activation), and Step S4 (surface grafting functional layer) are omitted.

[0125] Steps S2 (interface aggregation) and S5 (cleaning and saving) are retained, and the operation and parameters of the retained steps are consistent with those in Example 1.

[0126] Table 5. Comparison of performance test data between membrane M1 and control membrane M3

[0127]

[0128] Therefore, it can be seen that, due to the absence of several key steps, the performance of membrane M3 is comprehensively inferior to that of membrane M1 in Example 1: the average selectivity for sodium and magnesium ions drops to 42.46, and the flux is only 29.12 LMH. On the one hand, without base membrane pretreatment, there is no underlying charge network; without surface activation and PAA functional layer grafting, there is no enhancement of surface negative charge; relying solely on the self-charge and pore size sieving of the polyamide layer formed in step S2, the ability to distinguish between monovalent and divalent ions is significantly reduced. On the other hand, the surface hydrophilicity of the membrane without PAA functional layer grafting is insufficient, water permeation resistance increases, and flux is further reduced. This verifies that the four-step core process of membrane M1 in Example 1 of this invention (base membrane pretreatment - interfacial polymerization - surface activation - surface grafting functional layer) is inseparable, and each step jointly supports the core separation performance of the membrane.

[0129] Comparative Example 3: (Comparative film M4: surface activation and surface grafting of functional layers are omitted)

[0130] The nanofiltration membrane prepared in Comparative Example 3 is named Comparative Membrane M4. The difference between its membrane preparation steps and those in Example 1 is that steps S3 (surface activation) and S4 (surface grafting functional layer) are omitted.

[0131] Steps S1 (base film pretreatment), S2 (interfacial polymerization), and S5 (cleaning and storage) are retained, and the operation and parameters of the retained steps are consistent with those in Example 1.

[0132] Table 6. Comparison of performance test data between membrane M1 and control membrane M4

[0133]

[0134] Therefore, compared to membrane M4, the average sodium and magnesium ion selectivity decreased to 41.71, the flux plummeted to 24.02 LMH, and the sodium chloride rejection rate abnormally increased to 52.94%. This is because the surface activation step S3 was omitted, resulting in insufficient amino anchors in the polyamide layer, making it impossible to complete the PAA functional layer grafting in step S4, thus losing the surface negative charge enhancement and hydrophilicity improvement effects. The membrane surface structure without the PAA grafted functional layer is more dense, although it is more resistant to monovalent sodium ions... + The retention rate increased, but the synergistic mechanism of "pore size sieving + charge repulsion" was disrupted, affecting the retention of divalent Mg. 2+ The selectivity decreases significantly, and the dense structure leads to a sharp reduction in water flux. This demonstrates that surface activation and PAA functional layer grafting are the core steps in ensuring high flux and high selectivity in membrane M1 of Embodiment 1 of this invention.

[0135] Comparative Example 4: (Comparative membrane M5: base film pretreatment, surface activation, and surface grafting of functional layers are omitted, plus the initial addition of glutaraldehyde during interfacial polymerization)

[0136] The nanofiltration membrane prepared in Comparative Example 4 is named Comparative Membrane M5. Its membrane preparation steps differ from those of Example 1 in the following ways: First, steps S1 (base membrane pretreatment), S3 (surface activation), and S4 (surface grafting of functional layers) are omitted. Second, in steps S2 (interfacial polymerization)-S21, the addition of 1.0 wt% glutaraldehyde during aqueous solution preparation is changed from "adding it just before membrane preparation" to "mixing and stirring with PIP, TAP, and other reagents for 30 min in advance." Specifically, the aqueous solution preparation involves sequentially adding 1.4 wt% PIP, 0.12 wt% TAP, 0.18 wt% PEG-1000, and 1.0 wt% GA to 100 mL of deionized water, stirring for 30 min until homogeneous; then adding 0.04 wt% SDBS just before membrane preparation, and continuing stirring for 2 min until homogeneous.

[0137] The remaining operations of step S2 and step S5 (cleaning and saving) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0138] Table 7 Comparison of performance test data between membrane M1 and control membrane M3

[0139]

[0140] Therefore, it can be seen that the comparison membrane M5 exhibits the extreme characteristic of "increased flux but drastically reduced selectivity": the average selectivity of sodium and magnesium ions is only 27.20, less than half that of the membrane M1 in Example 1 of this invention, while the flux increases to 50.96 LMH. There are two main reasons for this: First, the lack of multiple steps results in the absence of a bottom and surface charge network, relying solely on the basic separation function of the polyamide layer, leading to a significant decrease in ion sieving capacity; Second, the premature addition of glutaraldehyde in step S2 causes excessive reaction with the amino groups in the aqueous phase to generate a large amount of prepolymer, resulting in excessive cross-linking in some areas and insufficient cross-linking in others during interfacial polymerization, leading to uneven distribution of membrane pores. The increased proportion of large-pore areas increases water flux, but sacrifices the accuracy of ion retention. This highlights the process advantage of the membrane M1 in Example 1 of this invention, which features "glutaraldehyde added before membrane fabrication + synergistic effect across all steps".

[0141] Comparative Example 5: (Comparison membrane M6: No glutaraldehyde was added during interfacial polymerization)

[0142] The nanofiltration membrane prepared in Comparative Example 5 was named Control Membrane M6. Its membrane preparation steps differed from those of Example 1 in that: in steps S2 (interfacial polymerization) - S21, glutaraldehyde was not added during the preparation of the aqueous solution. Specifically, the aqueous solution was prepared as follows: 1.4 wt% PIP, 0.12 wt% TAP, and 0.18 wt% PEG-1000 were added sequentially to 100 mL of deionized water, and the mixture was stirred for 20 min until homogeneous. Just before membrane preparation, 0.04 wt% SDBS was added, and the mixture was stirred for another 2 min until homogeneous.

[0143] The steps S1 (base film pretreatment), the remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer), and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0144] Table 8. Comparison of performance test data between membrane M1 and control membrane M6

[0145]

[0146] Therefore, it can be seen that the retention performance of the comparison membrane M6 is completely ineffective due to the lack of glutaraldehyde: the magnesium sulfate retention rate drops from 98.77% to 46.65%, and the sodium magnesium ion selectivity is only 1.54, almost losing its ion separation ability. Glutaraldehyde is the core agent for constructing the cross-linked network of membrane M1 in Example 1 of this invention. It undergoes a Schiff base reaction with the aqueous phase amino group, which can promote the uniform cross-linking of the polyamide layer and form a stable, uniformly pore-sized retention layer. Without glutaraldehyde, the degree of cross-linking of the polyamide layer is insufficient, the structure is loose, and the pore size distribution is wide, making it impossible to screen and retain divalent ions through the pore size, nor can it form a stable negatively charged network. This proves that glutaraldehyde is the key agent to ensure the retention performance of membrane M1 in Example 1 of this invention.

[0147] Comparative Example 6: (Comparison membrane M7: No SDBS or glutaraldehyde was added during the initial stage of interfacial polymerization)

[0148] The nanofiltration membrane prepared in Comparative Example 6 is named Comparative Membrane M7. The difference between its membrane preparation steps and those in Example 1 is as follows: In steps S2 (interfacial polymerization) - S21, 0.04wt% SDBS is removed during the preparation of the aqueous solution, and the addition of 1.0wt% glutaraldehyde is changed from "adding it just before membrane preparation" to "mixing and stirring it with PIP, TAP and other reagents for 30 min in advance". That is, the aqueous solution preparation is as follows: 1.4wt% PIP, 0.12wt% TAP, 0.18wt% PEG-1000 and 1.0wt% GA are added sequentially to 100 mL of deionized water and stirred for 30 min until they are mixed evenly.

[0149] The steps S1 (base film pretreatment), the remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer), and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0150] Table 9 Comparison of performance test data between membrane M1 and control membrane M7

[0151]

[0152] Therefore, it can be seen that the comparison membrane M7 exhibits the extreme characteristic of "extremely high flux but poor retention": the flux reaches 179.04 LMH, which is 4.7 times that of the membrane M1 in Example 1 of this invention, but the magnesium sulfate rejection rate is only 46.26%, and the selectivity is insufficient. The reasons are twofold: firstly, SDBS can regulate the interfacial polymerization reaction rate, avoiding excessive porosity in the polyamide layer. Without SDBS, the membrane structure is disordered, resulting in minimal water permeation resistance but loss of retention capacity; secondly, the excessive reaction between glutaraldehyde added in the early stage and the amino groups in the aqueous phase generates a large amount of prepolymer, leading to a "local burst" of interfacial polymerization and uneven cross-linking of the polyamide layer. This proves that the design of the membrane M1 in Example 1 of this invention, which involves "synergistic addition of SDBS and glutaraldehyde + addition of glutaraldehyde before membrane fabrication," is the core of balancing flux and retention performance, effectively avoiding the industry pain point of "flux and selectivity imbalance."

[0153] Comparative Example 7: (Comparison membrane M8: SDBS not added during interface aggregation)

[0154] The nanofiltration membrane prepared in Comparative Example 7 was named Control Membrane M8. Its membrane preparation steps differed from those in Example 1: In steps S2 (interfacial polymerization) - S21, 0.04 wt% SDBS was removed during the preparation of the aqueous solution. Specifically, the aqueous solution was prepared as follows: 1.4 wt% PIP, 0.12 wt% TAP, and 0.18 wt% PEG-1000 were added sequentially to 100 mL of deionized water, and the mixture was stirred for 20 min until homogeneous. Just before membrane preparation, 1.0 wt% GA was added, and the mixture was stirred for another 2 min until homogeneous.

[0155] The steps S1 (base film pretreatment), the remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer), and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0156] Table 10 Comparison of performance test data between membrane M1 and control membrane M8

[0157]

[0158] Therefore, it can be seen that the retention performance of the control membrane M8 is more severely degraded than that of the control membrane M7, and the flux is also further increased: the magnesium sulfate rejection rate drops to 40.89%, the selectivity is only 1.49, and the flux reaches 208.85 LMH. Although step S2 retains the timing of glutaraldehyde addition before membrane preparation to avoid the large-scale generation of prepolymer, the lack of SDBS still leads to the inability to control the interfacial polymerization rate, resulting in a loose and porous structure in the polyamide layer with extremely low water permeation resistance, making it impossible to achieve precise ion sieving. Comparing the control membranes M7 and M8, it can be seen that the regulatory role of SDBS in the polyamide layer structure is irreplaceable. It can reduce the surface tension of the aqueous phase and ensure the uniform diffusion of TMC in the oil phase. It is the key additive for achieving the "high flux-high selectivity" balance in membrane M1 of Example 1 of this invention.

[0159] Comparative Example 8: (Comparative membrane M9: Base membrane pretreatment step omitted + glutaraldehyde added in the early stage of interfacial polymerization)

[0160] The nanofiltration membrane prepared in Comparative Example 8 is named Control Membrane M9. Its membrane preparation steps differ from those of Example 1 in the following ways: First, step S1 (base membrane pretreatment) is omitted. Second, in steps S2 (interfacial polymerization) - S21, the addition of 1.0 wt% glutaraldehyde in the aqueous solution preparation is changed from "adding it just before membrane preparation" to "mixing and stirring it with PIP, TAP, and other reagents for 30 min in advance." Specifically, the aqueous solution preparation is as follows: 1.4 wt% PIP, 0.12 wt% TAP, 0.18 wt% PEG-1000, and 1.0 wt% GA are added sequentially to 100 mL of deionized water and stirred for 30 min until homogeneous; 0.04 wt% SDBS is added just before membrane preparation, and stirring is continued for 2 min until homogeneous.

[0161] The remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer) and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0162] Table 11 Comparison of performance test data between membrane M1 and control membrane M9

[0163]

[0164] Therefore, it can be seen that the comparison membrane M9 exhibits a double degradation characteristic of "selectivity plummeting and flux dropping sharply": the sodium and magnesium ion selectivity drops to 22.67, and the flux is only 15.60 LMH, less than 40% of that of membrane M1 in Example 1 of this invention. On the one hand, omitting the base membrane pretreatment results in the absence of the underlying charge network, making it impossible to form a gradient charge synergistic system, and significantly weakening the electrostatic repulsion of divalent magnesium ions; on the other hand, the premature addition of glutaraldehyde causes a "local burst" of interfacial polymerization, resulting in uneven cross-linking of the polyamide layer. The increased proportion of dense areas causes a sharp drop in water flux, while the loose areas reduce the ion sieving accuracy. Even retaining SDBS cannot compensate for the dual process defects. This further verifies that the process combination of "base membrane pretreatment + glutaraldehyde addition just before membrane fabrication" for membrane M1 in Example 1 of this invention is inseparable.

[0165] Comparative Example 9: (Comparison membrane M10: Glutaraldehyde added in the early stage of interfacial polymerization)

[0166] The nanofiltration membrane prepared in Comparative Example 9 was named Control Membrane M10. Its membrane preparation steps differed from those of Example 1 in that, in steps S2 (interfacial polymerization) - S21, the addition of 1.0 wt% glutaraldehyde during aqueous solution preparation was changed from "adding it just before membrane preparation" to "mixing and stirring it with PIP, TAP, and other reagents for 30 min in advance." Specifically, the aqueous solution preparation involved sequentially adding 1.4 wt% PIP, 0.12 wt% TAP, 0.18 wt% PEG-1000, and 1.0 wt% GA to 100 mL of deionized water, stirring for 30 min until homogeneous; then adding 0.04 wt% SDBS just before membrane preparation, and continuing stirring for 2 min until homogeneous.

[0167] The steps S1 (base film pretreatment), the remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer), and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0168] Table 12 Comparison of performance test data between membrane M1 and control membrane M10

[0169]

[0170] Therefore, the flux of membrane M10 is basically the same as that of membrane M1 in Example 1 of this invention, but the selectivity drops sharply to 9.39, and the magnesium sulfate rejection rate decreases by nearly 6 percentage points. This is because glutaraldehyde is added prematurely in step S2, causing excessive reaction with the aqueous amino group to generate a large amount of Schiff base prepolymer. This leads to "disordered growth" of the polyamide chains during interfacial polymerization, resulting in a wider pore size distribution and uneven charge distribution. Although the base membrane pretreatment and PAA functional layer grafting can provide some charge support, the core rejection structure of the polyamide layer has been destroyed, resulting in a significant reduction in the sieving difference between monovalent and divalent ions. This highlights the decisive influence of the timing of glutaraldehyde addition on the core performance of membrane M1 in Example 1 of this invention.

[0171] Comparative Example 10: (Comparative film M11: base film pretreatment, surface activation, and surface grafting of functional layers are omitted, and a thermosetting process is used after interfacial polymerization)

[0172] The nanofiltration membrane prepared in Comparative Example 10 is named Comparative Membrane M11. Its membrane preparation steps differ from those of Example 1 in the following ways: First, steps S1 (base membrane pretreatment), S3 (surface activation), and S4 (surface grafting of functional layers) are omitted. Second, in steps S2 (interfacial polymerization) to S22, the polymerization reaction is changed from "natural air drying" to "heat curing in a 65°C oven for 10 min." Specifically, the polymerization reaction is as follows: the pretreated base membrane is laid flat, an aqueous solution is poured on it, it is allowed to stand at 25°C for 3 min, excess aqueous phase is poured off, and the membrane is purged with nitrogen and air-dried; immediately, an oil phase solution is poured on it, the reaction is carried out at 25°C for 1 min, the oil phase is poured off, and the membrane is heat-cured in a 65°C oven for 10 min.

[0173] The remaining operations of step S2 and step S5 (cleaning and saving) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0174] Table 13 Comparison of performance test data between membrane M1 and control membrane M11

[0175]

[0176] Therefore, the flux of the comparative membrane M11 is close to that of the membrane M1 in Example 1 of this invention, but the selectivity is reduced to 20.40, and the magnesium sulfate rejection rate is also slightly lower. This is because the 65°C thermosetting causes excessive shrinkage of the polyamide layer, resulting in a narrower and more uneven pore size distribution. Furthermore, omitting the base membrane pretreatment and PAA functional layer grafting leaves no gradient charge network support, relying solely on the polyamide layer's own charge for ion separation, leading to a significant decrease in sieving accuracy. In addition, thermosetting exacerbates the interlayer stress between the polyamide layer and the base membrane, increasing the risk of delamination during long-term operation. This demonstrates that the "natural air drying + full-step synergy" method used for membrane M1 in Example 1 of this invention is the optimal choice for ensuring structural stability and balanced performance.

[0177] Comparative Example 11: (Comparison film M12: thermosetting process after interfacial polymerization)

[0178] The nanofiltration membrane prepared in Comparative Example 11 is named Comparative Membrane M12. Its membrane preparation steps differ from those of Example 1: In steps S2 (interfacial polymerization) - S22, the polymerization reaction process changes "natural air drying" to "heat curing in a 65°C oven for 10 min". Specifically, the polymerization reaction involves: laying the pretreated base membrane flat, pouring on the aqueous phase solution, allowing it to stand at 25°C for 3 min, discarding the excess aqueous phase, and purging with nitrogen to air dry; immediately pouring on the oil phase solution, reacting at 25°C for 1 min, discarding the oil phase, and then heat curing in a 65°C oven for 10 min.

[0179] The steps S1 (base film pretreatment), the remaining operations of step S2, step S3 (surface activation), step S4 (surface grafting functional layer), and step S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0180] Table 14 Comparison of performance test data between membrane M1 and control membrane M12

[0181]

[0182] Therefore, it can be seen that the core separation performance of the comparison membrane M12, due to its thermosetting process, deteriorates drastically: the sodium and magnesium ion selectivity drops sharply from 62.05 to 4.56, and the magnesium sulfate rejection rate decreases by 14.44 percentage points. This is because the high temperature of 65°C causes excessive shrinkage of the hydroxyl crosslinking network and polyamide layer of the base membrane, disrupting the "pore size-charge synergistic effect," resulting in a wider pore size distribution and uneven charge distribution. Although the flux is slightly improved, the core function of precise separation of monovalent and divalent ions is lost. In contrast, the choice of natural air drying rather than thermosetting for the membrane M1 in Example 1 of this invention is a key process choice to ensure the integrity of the membrane structure and high selectivity.

[0183] Comparative Example 12: (Comparative film M13: Base film pretreatment step omitted + surface activation using 0.5wt% NaOH)

[0184] The nanofiltration membrane prepared in Comparative Example 12 is named Comparative Membrane M13. Its membrane preparation steps differ from those of Example 1 in two ways: First, step S1 (base membrane pretreatment) is omitted. Second, in step S3 (surface activation), the concentration of the NaOH solution is increased from 0.05 wt% to 0.5 wt%. Specifically, the polyamide surface activation involves immersing the interfacially polymerized membrane in a 0.5 wt% NaOH solution and allowing it to stand at 30°C for 15 min; then rinsing until neutral and drying.

[0185] The steps S2 (interface aggregation), the remaining operations of step S3, step S4 (surface grafting functional layer), and step S5 (cleaning and saving) are retained, and the operations and parameters of the retained steps are consistent with those of Example 1.

[0186] Table 15 Comparison of performance test data between membrane M1 and control membrane M13

[0187]

[0188] Therefore, the selectivity of membrane M13 drops sharply to 5.58, and the magnesium sulfate rejection rate decreases by 10.64 percentage points. On the one hand, the excessive hydrolysis of the amide bonds in the polyamide layer by 0.5 wt% high-concentration NaOH leads to the destruction of the original separation layer structure, resulting in uneven pore size and charge distribution, thus damaging the core retention structure. On the other hand, omitting the base membrane pretreatment leaves no underlying charge network, and the charge of the surface PAA grafted functional layer alone cannot form an effective synergy, significantly weakening the electrostatic repulsion of divalent magnesium ions. Even retaining the PAA functional layer graft cannot compensate for the dual defects of uncontrolled activation concentration and lack of underlying charge. This demonstrates that the "base membrane pretreatment + 0.05% low-concentration precise surface activation" of membrane M1 in Example 1 of this invention is the key to ensuring the synergy between charge and pore size.

[0189] Comparative Example 13: (Comparative film M4: 0.5 wt% NaOH was used for surface activation)

[0190] The nanofiltration membrane prepared in Comparative Example 13 was named Comparative Membrane M14. The difference between its membrane preparation steps and those in Example 1 is as follows: In step S3 (surface activation), the concentration of NaOH solution was increased from 0.05 wt% to 0.5 wt%. That is, polyamide surface activation: the interfacially polymerized membrane was immersed in 0.5 wt% NaOH solution and allowed to stand at 30°C for 15 min; after rinsing until neutral, the water was absorbed.

[0191] The steps S1 (base film pretreatment), S2 (interfacial polymerization), S3 (other operations), S4 (surface grafting functional layer), and S5 (cleaning and preservation) are retained, and the operations and parameters of the retained steps are consistent with those in Example 1.

[0192] Table 16 Comparison of performance test data between membrane M1 and control membrane M14

[0193]

[0194] This indicates that, in contrast, the sodium chloride rejection rate of membrane M14 abnormally increased to 42.12%, but the flux plummeted to 26.00 LMH, and the selectivity also decreased to 43.64. The reason is that while the high concentration of 0.5 wt% NaOH exposes more amino groups for PAA functional layer grafting, it excessively hydrolyzes the polyamide layer, generating a large number of carboxyl and amino groups. Simultaneously, it disrupts the originally regular cross-linked network, causing some membrane pores to collapse, shrink, or form irregular hydrolysis product accumulation layers. This results in a smaller and unevenly distributed effective pore size, an increased proportion of dense regions leading to a sharp decrease in water flux, and a passively increased rejection rate of monovalent sodium ions, thus disrupting the precise separation target of "high rejection of divalent ions and low rejection of monovalent ions." This demonstrates that the "0.05 wt% low concentration activation" of membrane M1 in Example 1 of this invention is the optimal parameter for balancing grafting site exposure and the structural integrity of the polyamide layer.

[0195] Comparative Example 14: (Comparison film M15: Base film pretreatment + surface activation steps omitted)

[0196] The nanofiltration membrane prepared in Comparative Example 14 is named Comparative Membrane M15. The differences between its membrane preparation steps and those in Example 1 are: firstly, step S1 (base membrane pretreatment) is omitted; secondly, step S3 (surface activation) is omitted.

[0197] Steps S2 (interface aggregation), S4 (surface grafting functional layer), and S5 (cleaning and saving) are retained, and the operation and parameters of the retained steps are consistent with those in Example 1.

[0198] Table 17 Comparison of performance test data between membrane M1 and control membrane M15

[0199]

[0200] Therefore, it can be seen that the flux of the comparison membrane M15 is less than 50% of that of the membrane M1 in Example 1 of this invention, and the selectivity is also reduced to 19.98. The lack of NaOH surface activation leads to insufficient exposure of the amino groups in the polyamide layer, resulting in extremely low grafting density of the PAA grafted functional layer in step S4, and almost complete loss of the surface negative charge enhancement effect. Simultaneously, omitting the base membrane pretreatment eliminates the underlying charge network, relying solely on the polyamide layer's own charge for separation, significantly reducing sieving accuracy. Furthermore, the insufficient density of the PAA grafted functional layer results in low surface hydrophilicity, a sharp increase in water permeation resistance, and a further reduction in flux. This demonstrates that "base membrane pretreatment + precise NaOH surface activation" is a necessary prerequisite for effective grafting of the PAA functional layer.

[0201] Comparative Example 15: (Comparison film M16: Surface activation step omitted)

[0202] The nanofiltration membrane prepared in Comparative Example 15 is named Comparative Membrane M16. The difference between its membrane preparation steps and those in Example 1 is that step S3 (surface activation) is omitted.

[0203] Steps S1 (base film pretreatment), S2 (interfacial polymerization), S4 (surface grafting functional layer), and S5 (cleaning and preservation) are retained, and the operation and parameters of the retained steps are consistent with those in Example 1.

[0204] Table 18 Comparison of performance test data between membrane M1 and control membrane M16

[0205]

[0206] Therefore, compared to membrane M16, the selectivity is only 16.93, the flux drops to 26.52 LMH, and the sodium chloride rejection rate increases to 40.87%. By omitting the surface activation step S3, the polyamide layer has insufficient amino exposure, the PAA grafted functional layer has low grafting density, and the surface negative charge density drops sharply, significantly weakening the electrostatic repulsion of divalent magnesium ions. Simultaneously, the unactivated polyamide layer surface is denser, increasing the monovalent sodium ion rejection rate and completely destroying ion selectivity. This demonstrates that precise surface activation in step S3 is the core step in achieving efficient PAA grafted functional layer grafting and charge synergy in membrane M1 of Example 1 of this invention.

[0207] In summary, based on the charge characteristics and crosslinking properties of PAA, this invention innovatively proposes a synergistic strategy of "gradient negative charge construction + through-linking network." Through a multi-step process involving base film pretreatment, interfacial polymerization, surface activation, and precise surface grafting, PAA is covalently anchored to the membrane surface to form a gradient charge distribution. Simultaneously, combined with the permanent negative charge centers of triaminopyrimidine (TAP), a breakthrough improvement in negative charge density and structural stability is achieved. Furthermore, the process is compatible with existing production lines, solving the problems of insufficient charge stability and weak interlayer bonding in traditional PAA-modified membranes. This achieves the dual objectives of precise separation of monovalent and divalent ions and efficient retention of perfluorinated compounds. Specifically, this manifests in:

[0208] 1. Solve the problem of charge regulation imbalance: Optimize the density and uniformity of negative charge distribution on the membrane surface, enhance the stability of charged groups, improve the sieving difference between divalent and monovalent cations, achieve a significant improvement in the ideal selectivity of sodium and magnesium ions, and avoid performance degradation caused by the shedding of charged groups during long-term operation.

[0209] 2. Solve the problem of weak interlayer bonding: Construct a through covalent cross-linked network of "base membrane-functional layer-surface modified layer" (i.e., base membrane layer-polyamide layer-PAA grafted functional layer) to strengthen the bonding force between layers, avoid interlayer delamination caused by water flow scouring and pressure fluctuations during actual operation, and ensure the integrity of the membrane structure.

[0210] 3. Addressing insufficient structural stability: Enhancing the three-dimensional cross-linking degree and anti-swelling performance of the membrane ensures stable pore structure across a wide pH range, temperature fluctuations, and chemical media, reduces membrane fouling accumulation, and maintains separation accuracy and flux stability during long-term operation.

[0211] 4. Solve the problem of poor process synergy: Optimize the compatibility of preparation process parameters and reagent ratios, break the imbalance between "throughput and selectivity", achieve synergistic optimization of high throughput and high selectivity, and improve the process repeatability and stability in large-scale production.

[0212] 5. Addressing the issue of limited environmental adaptability: Expanding the membrane's adaptability to complex water qualities and a wide pH range, avoiding performance fluctuations caused by membrane structure damage or deactivation of charged groups in acidic and alkaline wastewater treatment scenarios, and broadening the range of applicable scenarios for the membrane.

[0213] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a PAA-grafted, perfluorocompound-rejecting, negatively charged nanofiltration membrane, characterized in that, Includes the following steps: S1. Base film pretreatment This includes base film cleaning and activation, preparation of pretreatment mixture, and pre-grafting reaction; The pretreatment mixture contains acrylic acid and glutaraldehyde; Pre-grafting reaction: The surface-activated base film is immersed in the pretreatment mixture, so that glutaraldehyde crosslinks the hydroxyl groups and carboxyl groups of the base film at the same time, while reserving some aldehyde groups; S2. Interface Aggregation This includes the preparation of aqueous solutions, the preparation of oil solutions, and polymerization reactions; The aqueous solution was prepared by sequentially adding 1.3wt%~1.5wt% piperazine, 0.1wt%~0.14wt% triaminopyrimidine, and 0.14wt%~0.18wt% polyethylene glycol-1000 to deionized water and stirring until homogeneous. Just before film formation, 0.8wt%~1.2wt% glutaraldehyde and 0.03wt%~0.05wt% sodium dodecylbenzenesulfonate were added, and the mixture was stirred until homogeneous. Polymerization reaction: The pretreated base film is laid flat, and then an aqueous solution and an oil solution are poured on it successively; S3. Surface activation Polyamide surface activation: Immerse the interfacially polymerized film in 0.04wt%~0.06wt% NaOH solution and let it stand at 20℃~30℃ for 10~20 min; rinse until neutral. S4. Surface grafting functional layer This includes the preparation of the grafting mixture and the surface grafting reaction; The grafting mixture contains acrylic acid and glutaraldehyde; Surface grafting reaction: The surface-activated film is immersed in a grafting mixture to bridge the amino groups of polyamide and acrylic acid with glutaraldehyde, thereby constructing a PAA grafted functional layer.

2. The method for preparing PAA grafting based perfluorocompound rejection negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S1, the base membrane is cleaned and activated: take a polysulfone ultrafiltration base membrane, ultrasonically clean it with deionized water for 5-15 minutes; immerse it in 0.4wt%-0.6wt% NaOH solution, let it stand at 20℃-30℃ for 15-25 minutes to activate the hydroxyl groups on the surface of the base membrane, rinse it until neutral and drain it.

3. The method for preparing PAA-grafted-based perfluorocompound rejection negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S1, the pretreatment mixture is prepared by adding 1.5wt%~2wt% acrylic acid and 0.3wt%~0.5wt% glutaraldehyde to deionized water and stirring magnetically for 1~10 minutes.

4. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S1, during the pre-grafting reaction, when the surface-activated base film is immersed in the pretreatment mixture, it is sealed and left to stand at 20℃~30℃ for 1~3 hours. After being taken out, it is rinsed with deionized water and then naturally dried.

5. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S2, the oil phase solution is prepared by preparing a hexane solution containing 0.12wt%~0.15wt% TMC and ultrasonically dispersing it for 1~10 min.

6. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S2, the polymerization reaction is as follows: the pretreated base film is laid flat, an aqueous solution is poured on it, and it is allowed to stand at 20℃~30℃ for 1~5 min. The excess aqueous phase is poured off, and the film is purged with nitrogen and dried. The oil phase solution is poured on immediately, and the film is reacted at 20℃~30℃ for 0.5~1.5 min. The oil phase is poured off, and the film is allowed to air dry naturally.

7. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S4, the grafting mixture is prepared by adding 6wt%~7wt% acrylic acid and 1.5wt%~2wt% glutaraldehyde to deionized water and stirring until homogeneous.

8. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, In step S4, during the surface grafting reaction, when the surface-activated membrane is immersed in the grafting mixture, it is sealed and left to stand at 20℃~30℃ for 1~2 hours. After removal, it is rinsed with clean water and air-dried naturally.

9. The method for preparing a PAA-grafted perfluorinated compound-retaining negatively charged nanofiltration membrane according to claim 1, characterized in that, It also includes the following steps: S5. Cleaning and Storage Immerse the membrane in deionized water to hydrate the PAA grafted functional layer, then remove it and blot off the surface moisture before use. For long-term storage, it needs to be sealed and immersed in deionized water at 2~5℃.

10. A perfluorinated compound-retaining negatively charged nanofiltration membrane based on PAA grafting, characterized in that, The nanofiltration membrane is prepared by the method described in any one of claims 1 to 9 for preparing a PAA-grafted perfluorinated compound-based negatively charged nanofiltration membrane.

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