Polyamide reverse osmosis membrane based on quaternary ammonium salt chitosan modification, preparation method of polyamide reverse osmosis membrane and application of polyamide reverse osmosis membrane in seawater desalination

Abstract

The invention discloses a polyamide reverse osmosis membrane based on quaternary ammonium salt chitosan modification and a preparation method and application thereof in seawater desalination, the preparation method comprises the following steps: step 1, sequentially coating a polysulfone support layer with a water phase solution and an oil phase solution, airing, and carrying out a water bath reaction at 50-80 DEG C for 3-30 min to form a polyamide ultrathin separation layer, the nascent state aromatic polyamide reverse osmosis membrane is obtained; the water phase solution is a m-phenylenediamine water solution with the mass fraction of 0.3-1%, and the pH value of the water phase solution is 7-9; the mass fraction of the multifunctional acyl halide in the oil-phase solution is 0.05-0.5%, and the balance is the oil-phase solvent; and 2, activating the nascent-state aromatic polyamide reverse osmosis membrane, coating the membrane surface with the membrane covering solution, drying in the air, carrying out water bath reaction at 50-80 DEG C for 3-30 minutes, and drying in the air to form a coating surface layer, thereby obtaining the target product. The method has the beneficial effects that high flux and high desalination rate can be kept, the service life of the membrane is remarkably prolonged, and the method can be used for seawater desalination and high-salinity wastewater treatment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of reverse osmosis membranes, and relates to a polyamide reverse osmosis membrane modified by quaternary ammonium salt chitosan, a preparation method thereof and application in seawater desalination, especially in seawater desalination and high-salinity wastewater treatment. BACKGROUND

[0002] Reverse osmosis (RO) technology is one of the most popular high-efficiency water treatment technologies at present, and the core component thereof is a composite polyamide reverse osmosis membrane. The membrane is usually prepared by an interfacial polymerization method, that is, piperazine (PIP) and trimesoyl chloride (TMC) are reacted on a porous support layer to form an ultra-thin (about 100-200 nm) polyamide (PA) active separation layer. The principle thereof is that, under the action of a pressure higher than the osmotic pressure of a solution, the solute and the solvent in the solution are separated by the selective interception of a semi-permeable membrane that allows only water to pass through but not other substances. The reverse osmosis membrane is the core of the reverse osmosis technology, the reverse osmosis membrane is the most widely used seawater desalination technology, and the innovation of the polyamide reverse osmosis membrane has always been the hottest research direction in the field of reverse osmosis technology.

[0003] In existing research, Zhang Ninghui (Zhang Ninghui. Preparation and performance of anti-biofouling and chlorine-resistant forward osmosis membranes[D]. Tianjin University of Technology, 2023. DOI: 10.27357 / d.cnki.gtgyu.2023.000100.) prepared a hydrogel by coating citral cross-linked chitosan on a polyamide reverse osmosis membrane to prepare an anti-biofouling polyamide (PA) forward osmosis composite membrane, so that the anti-fouling capacity is greatly improved, and the water flux is slightly reduced, which cannot be directly adapted to the interfacial polymerization process of the reverse osmosis membrane. Similarly, Zhang Ninghui also provides a preparation method of a high-flux high-desalination anti-biofouling forward osmosis membrane, which uses citral cross-linked chitosan to prepare a hydrogel, and then coats the hydrogel on the forward osmosis membrane to resist biofouling and organic fouling, and the water flux decreases by about 8%. However, the anti-bacterial and anti-fouling capacities of the modified forward osmosis membrane are enhanced, and the anti-biofouling and organic fouling reach more than 98%, but the water flux attenuation and process compatibility problems cannot be directly applied to the reverse osmosis membrane.

[0004] Therefore, it is a technical bottleneck to be urgently broken through in the field to develop a polyamide reverse osmosis membrane that can maintain high flux and high desalination rate characteristics and significantly improve the anti-biofouling capacity. SUMMARY

[0005] In view of the shortcomings of the prior art described above, the purpose of the present application is to provide a polyamide reverse osmosis membrane modified by quaternary ammonium salt chitosan, a preparation method thereof and application in seawater desalination, which is used to solve the problems of small water flux and low anti-fouling capacity of the existing reverse osmosis membrane.

[0006] To achieve the above and other related objectives, the technical solution adopted is as follows:

[0007] This invention provides a method for preparing a polyamide reverse osmosis membrane based on quaternary ammonium salt chitosan modification, comprising the following steps:

[0008] Step 1: Coat the polysulfone support layer with an aqueous solution and an oil solution sequentially. After drying, react in a water bath at 50-80℃ for 3-30 minutes to form an ultrathin polyamide separation layer, thus obtaining a nascent aromatic polyamide reverse osmosis membrane. The aqueous solution is a 0.3-1% (w / w) aqueous solution of m-phenylenediamine with a pH of 7-9. The oil solution contains 0.05-0.5% (w / w) of polyfunctional acyl halides, with the remainder being the oil solvent.

[0009] Step 2: After activating the nascent aromatic polyamide reverse osmosis membrane obtained in Step 1, soak it or coat the membrane surface with a coating solution, dry it, and then react it in a water bath at 50-80℃ for 3-30 minutes. After drying it again, a coating surface layer is formed, resulting in a high-flux, high-desalination-rate, biofouling-resistant polyamide reverse osmosis membrane.

[0010] Preferably, the aqueous solution is an aqueous solution of m-phenylenediamine with a mass fraction of 0.5-1%.

[0011] More preferably, the aqueous solution is an aqueous solution of m-phenylenediamine with a mass concentration of 0.5%.

[0012] Preferably, in step 1, the water bath reaction temperature is 60℃ and the reaction time is 5 minutes. During the water bath process, the unreacted functional groups such as acyl chloride groups, amino groups, and carboxyl groups in the nascent polyamide membrane undergo further chemical reactions driven by heat energy, thereby forming new covalent bonds between polymer chains. This process makes the polymer network structure more compact and stable, which can improve the desalination rate of the polyamide reverse osmosis membrane.

[0013] Preferably, the polyfunctional acyl halide is pyromellitic trimethylolpropionate chloride.

[0014] Preferably, the mass fraction of the polyfunctional acyl halide in the oil phase solution is 0.1%, with the remainder being the oil phase solvent.

[0015] Preferably, in step 1, the oil phase solvent is selected from one or more of C4-C12 aliphatic hydrocarbons, cyclic aliphatic hydrocarbons, and aromatic hydrocarbons.

[0016] More preferably, the oil phase solvent is an isoparaffin solvent oil or n-hexane. n-Hexane is the most preferred.

[0017] Preferably, in step 1, after coating the aqueous phase solution, the solution is allowed to evaporate to a moisture content of 28-31% in a humidity-controlled environment of 20-30°C and 40-80% relative humidity before coating the oil phase solution.

[0018] Preferably, the thickness of the polyamide ultrathin separation layer is 100-200 nm.

[0019] Preferably, in step 2, the coating solution is a quaternary ammonium salt chitosan solution with a mass fraction of 0.05-1%, and the molecular formula of the quaternary ammonium salt chitosan is [(C8H12H2O)2]. 13 NO5) 1-x (C 14 H 28 ClN2O6) x ] n , where x is the degree of substitution of the quaternary ammonium group, 0 < x < 1.2; n is the degree of polymerization, 0 < n < 4000.

[0020] Preferably, in step 2, the mass concentration of the quaternary ammonium salt chitosan solution is 0.05-0.2%. More preferably, it is 0.1%. By coating or immersing the membrane in a quaternary ammonium salt chitosan solution, this invention can introduce a positively charged and hydrophilic coating surface layer, namely the quaternary ammonium salt chitosan surface layer, through surface coating and chemical crosslinking without damaging the fine structure of the original polyamide (PA) separation layer, thus endowing the membrane surface with "anti-fouling" properties.

[0021] Preferably, in step 2, the water bath reaction temperature is 60℃ and the reaction time is 5 minutes. During the water bath process, the unreacted functional groups such as acyl chloride groups, amino groups, and carboxyl groups in the nascent aromatic polyamide reverse osmosis membrane undergo further chemical reactions driven by heat energy, thereby forming new covalent bonds between polymer chains. This process makes the polymer network structure more compact and stable, which can improve the desalination rate of the polyamide reverse osmosis membrane.

[0022] Preferably, in step 2, the activation includes immersing the nascent aromatic polyamide reverse osmosis membrane in deionized water. The purpose of activation is to remove unreacted monomers and oligomers remaining in the polyamide ultrathin separation layer, while simultaneously weakening further cross-linking of the polyamide ultrathin separation layer during heat treatment, and promoting the dissolution of hydrophilic residues in water. This operation can effectively reduce the membrane's separation layer thickness, roughness, and amide group content, thereby significantly improving the membrane's permeability. The activation time is 12-34 hours.

[0023] Preferably, the thickness of the quaternary ammonium salt chitosan surface layer is 50-200 nm.

[0024] Preferably, the drying process involves removing any visible moisture from the film surface.

[0025] The present invention also provides a polyamide reverse osmosis membrane based on quaternary ammonium salt chitosan modification, which is prepared by the aforementioned preparation method.

[0026] The present invention also provides an application of a polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan in seawater desalination.

[0027] Preferably, the application involves flushing the water to be treated at 1.0-5.0 MPa, 25°C, and a membrane flow rate of 1.0-2.0 GPM / min for 20-50 minutes.

[0028] Preferably, the water to be treated is an aqueous solution of seawater and sodium chloride.

[0029] Compared with the prior art, the beneficial effects of the present invention are:

[0030] 1. Synergistic breakthrough in long-lasting anti-biofouling and performance maintenance: Through quaternary ammonium salt chitosan surface modification technology, quaternary ammonium groups are grafted onto the polyamide separation layer to construct a firmly bonded, positively charged, and highly hydrophilic QACS surface layer. This layer utilizes a dual mechanism of electrostatic repulsion / bactericidal action (quaternary ammonium groups destroy microbial cell membranes) and a hydration layer barrier (hydrophilic structures form a hydration protective layer) to achieve effective antifouling. Its "surface post-treatment" characteristics ensure that the fine structure of the polyamide separation layer itself is not damaged. The ultra-thin functional layer not only achieves long-lasting anti-biofouling but also effectively solves the problem of membrane performance degradation caused by traditional modification methods, providing an innovative solution for the application of reverse osmosis membranes in seawater desalination and other fields.

[0031] 2. Structural advantages of the "surface post-treatment" process: The "surface post-treatment" process (non-covalent grafting) ensures that the fine structure of the polyamide separation layer is not damaged, and the ultra-thin functional layer has a negligible impact on the water transport resistance, so that the membrane core separation performance can be fully preserved. This solves the problem of balancing antifouling modification and water flux / salt rejection rate in existing technologies.

[0032] 3. Application innovation in fields such as seawater desalination: This technology provides efficient and stable solutions for reverse osmosis membranes in scenarios such as seawater desalination and high-salt wastewater treatment. While maintaining high flux and high desalination rate, it significantly extends the membrane life and promotes the large-scale application of reverse osmosis technology. Attached Figure Description

[0033] Figure 1 The effect of polyamide reverse osmosis membranes prepared with different quaternary ammonium chitosan concentrations on water flux and salt rejection rate.

[0034] Figure 2 The effect of polyamide reverse osmosis membranes prepared with different concentrations of quaternary ammonium salt chitosan on water flux and flux recovery rate after cleaning.

[0035] Figures 3-9 Electron micrographs of the polyamide reverse osmosis membranes prepared in Examples 1-6 and the reverse osmosis membrane prepared in Comparative Example 1 ( Figure 3 For comparison, Figure 4 Representative Example 1, Figure 5 Representative Example 2, Figure 6 Representative Example 3, Figure 7 Representative Example 4, Figure 8 Representative Example 5, Figure 9 Representative Example 6).

[0036] Figure 10 Infrared spectra of the polyamide reverse osmosis membranes prepared in Examples 1-5. Detailed Implementation

[0037] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0038] It should be noted that the process equipment or apparatus not specifically mentioned in the following embodiments are all conventional equipment or apparatus in the art.

[0039] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0040] In the following embodiments of this application, the quaternary ammonium chitosan was purchased from Shandong Keyuan Biochemical Co., Ltd., with molecular weights of 100,000 Da, 50,000 Da, and 150,000 Da, respectively. The chitosan was purchased from Shandong Keyuan Biochemical Co., Ltd., with a molecular weight of 100,000 Da.

[0041] Example 1

[0042] This invention proposes a method for preparing a polyamide reverse osmosis membrane based on quaternary ammonium salt chitosan modification, comprising the following steps:

[0043] Step 1: Coat the polysulfone support layer with an aqueous solution and an oil solution sequentially. After drying, react in a water bath at 60°C for 5 minutes to form an ultrathin polyamide separation layer, thus obtaining a nascent aromatic polyamide reverse osmosis membrane. The aqueous solution is a 0.5% (w / w) aqueous solution of m-phenylenediamine with a pH of 7-9. The oil solution contains 0.1% (w / w) of polyfunctional acyl halide, with the remainder being the oil solvent.

[0044] The aqueous phase solution is prepared as follows: Dissolve 5g of m-phenylenediamine in 995g of pure water, adjust the pH value to 8.5-9 with 0.1M sodium hydroxide, and stir evenly to obtain the aqueous phase solution.

[0045] The oil phase solution is prepared as follows: Dissolve 1g of trimesoyl chloride in 999g of n-hexane, stir until homogeneous to obtain the oil phase solution.

[0046] Specifically, follow these steps:

[0047] An aqueous solution is coated onto the polysulfone support layer. After removing excess solution from the surface, the polysulfone substrate membrane, which has adsorbed the aqueous solution, is passed through a closed space equipped with a heating and ventilation system. The internal temperature is controlled at 20℃-30℃ and the relative humidity at 40%-80%, allowing the moisture on the membrane surface to evaporate further. Then, an oil-phase solution is coated onto it. After removing some of the oil-phase solution from the surface, it is then placed in a 60℃ constant temperature water bath for 5 minutes to form an ultrathin polyamide separation layer, thus obtaining the nascent polyamide reverse osmosis membrane.

[0048] Step 2: After activating the nascent aromatic polyamide reverse osmosis membrane obtained in Step 1 by soaking it in deionized water, a coating solution with a mass fraction of 0.05% is applied to its surface. After drying, it is reacted in a water bath at 60°C for 5 minutes and then dried again to form a quaternary ammonium salt chitosan surface layer, thus obtaining a polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan.

[0049] The preparation method of 0.05% coating solution is as follows: 0.5g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 999.5g of water.

[0050] The polyamide reverse osmosis membrane prepared in this embodiment was characterized and tested as follows:

[0051] (1) Morphological characterization: The polyamide reverse osmosis membrane was characterized by SEM using transmission electron microscopy.

[0052] SEM images of the polyamide reverse osmosis membrane prepared in Example 1 are shown below. Figure 1As shown, its surface morphology is intuitively displayed.

[0053] (2) Composition characterization:

[0054] Infrared spectroscopy is used to determine the characteristic frequencies of chemical bond vibrations and molecular rotations in material molecules, thereby determining the chemical structure, functional group types, and content of the substance.

[0055] The infrared spectrum of the polyamide reverse osmosis membrane prepared in Example 1 is as follows: Figure 10 As shown, the curve represents the infrared spectrum of the sample, recording the change in transmittance of the sample under infrared light. The higher the transmittance, the weaker the absorption of infrared light at the corresponding wavenumber; the lower the transmittance, the stronger the absorption.

[0056] (3) Test the flux and salt rejection rate of the polyamide reverse osmosis membrane:

[0057] ① Test the initial water flux and salt rejection rate of the polyamide reverse osmosis membrane:

[0058] The polyamide reverse osmosis membrane was rinsed for 30 min after filtering 2000 ppm of water (0.0342 M sodium chloride aqueous solution) at 1.0-5.0 MPa, 25℃ and membrane surface flow rate of 1.0-2.0 GPM / min. The initial water flux and salt rejection rate of the membrane were tested. The test results are shown in Table 1.

[0059] ② Test the water flux of the polyamide reverse osmosis membrane after fouling:

[0060] The water to be treated was replaced with a mixed solution of electronegative sodium dodecyl sulfate (50 g / L), electronegative dodecyltrimethylammonium bromide (50 g / L), and electronegative bovine serum albumin (100 g / L). The solution was filtered at 1.0-5.0 MPa, 25 ℃, and a membrane flow rate of 1.0-20 GPM / min. The solution was then rinsed with 2000 ppm of the mixed solution for 30 min. The water flux of the polyamide reverse osmosis membrane after fouling was tested. The test results are shown in Table 1.

[0061] ③ Test the water flux and flux recovery rate after cleaning of the polyamide reverse osmosis membrane after fouling treatment:

[0062] After the fouling test was completed, the pressure was released, and the fouled membrane was cleaned with deionized water as the feed solution. Then, a 2000 ppm 0.0342M sodium chloride aqueous solution was filtered for 30 min at 1.0-5.0 MPa, 25 ℃ and a membrane surface flow rate of 1.0-20 GPM / min. The water flux and flux recovery rate after cleaning were tested after the polyamide reverse osmosis membrane fouling treatment. The test results are shown in Table 1.

[0063] Flux recovery rate after cleaning = (flux after cleaning / initial flux) * 100%.

[0064] Example 2

[0065] The difference between this embodiment and Embodiment 2 lies in the mass fraction of the coating solution. In this embodiment, the mass fraction of the coating solution is 0.1%. The preparation method of the coating solution is as follows: 1g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 999g of water. All other process conditions are exactly the same.

[0066] Example 3

[0067] The difference between this embodiment and Embodiment 2 lies in the mass fraction of the coating solution. In this embodiment, the mass fraction of the coating solution is 0.2%. The coating solution is prepared as follows: 2g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 998g of water. All other process conditions are exactly the same.

[0068] Example 4

[0069] The difference between this embodiment and Embodiment 2 lies in the mass fraction of the coating solution. In this embodiment, the mass fraction of the coating solution is 0.3%. The preparation method of the coating solution is as follows: 3g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 997g of water. All other process conditions are exactly the same.

[0070] Example 5

[0071] The difference between this embodiment and Embodiment 2 lies in the mass fraction of the coating solution. In this embodiment, the mass fraction of the coating solution is 0.4%. The coating solution is prepared as follows: 4g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 996g of water. All other process conditions are exactly the same.

[0072] Example 6

[0073] The difference between this embodiment and Embodiment 2 lies in the mass fraction of the coating solution. In this embodiment, the mass fraction of the coating solution is 0.5%, and the preparation method of the coating solution is as follows: 5g of quaternary ammonium salt chitosan with a molecular weight of 100,000 Da is dissolved in 995g of water. All other process conditions are exactly the same.

[0074] Example 7

[0075] The difference between this embodiment and Embodiment 2 is that the molecular weight of the quaternary ammonium chitosan is different; the molecular weight of the quaternary ammonium chitosan is 50,000 Da. All other process conditions are exactly the same.

[0076] Example 8

[0077] The difference between this embodiment and Embodiment 2 is that the molecular weight of the quaternary ammonium chitosan is different; the molecular weight of the quaternary ammonium chitosan is 150,000 Da. All other process conditions are exactly the same.

[0078] Example 9

[0079] The difference between this embodiment and Embodiment 1 is that the water bath reaction conditions in step 2 are different; the water bath reaction temperature is 80°C and the reaction time is 3 minutes. All other process conditions are exactly the same.

[0080] Example 10

[0081] The difference between this embodiment and Embodiment 2 is that the water bath reaction conditions in step 2 are different; the water bath reaction temperature is 50°C and the reaction time is 30 minutes. All other process conditions are exactly the same.

[0082] Example 11

[0083] The difference between this embodiment and Embodiment 2 is that the concentration of the aqueous solution in step 1 is 0.3%. All other process conditions are exactly the same.

[0084] Example 12

[0085] The difference between this embodiment and Embodiment 2 is that the concentration of the aqueous solution in step 1 is 1%. All other process conditions are exactly the same.

[0086] Example 13

[0087] The difference between this embodiment and Embodiment 2 is that the concentration of the oil phase solution in step 1 is 0.05%. All other process conditions are exactly the same.

[0088] Example 14

[0089] The difference between this embodiment and Embodiment 2 is that the concentration of the oil phase solution in step 1 is 0.5%. All other process conditions are exactly the same.

[0090] Comparative Example 1

[0091] The difference between Comparative Example 1 and Example 1 is that step 2 is different. All other process conditions are exactly the same.

[0092] In this comparative example, step 2 is as follows: the nascent aromatic polyamide reverse osmosis membrane obtained in step 1 is activated by soaking in deionized water, dried, reacted in a water bath at 60°C for 5 minutes, and dried again to form a quaternary ammonium salt chitosan surface layer, thus obtaining a polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan.

[0093] Comparative Example 2

[0094] The difference between Comparative Example 1 and Example 1 is that chitosan (molecular weight 100,000 Da) was used instead of quaternary ammonium salt chitosan. All other process conditions were exactly the same.

[0095] Table 1 Performance data of polyamide composite reverse osmosis membrane

[0096] From the data in Table 1, it can be seen that as the concentration of quaternary ammonium chitosan increases (0.05-0.5%), the initial water flux first increases and then decreases, reaching its highest value (70.4 L·m⁻¹) at a concentration of 0.1%. -2 ·h -1 This indicates that the membrane exhibits optimal hydrophilicity and minimal water molecule permeation resistance at this concentration. However, at concentrations ≥0.3%, the flux decreases significantly (e.g., only 39.7 L·m⁻¹ at 0.5%). -2 ·h -1 The high concentration of quaternary ammonium salts may form a dense coating on the membrane surface, increasing mass transfer resistance. The flux retention rate after fouling (Jw after fouling / Jw initial) improved with increasing concentration: Comparative Example 1 (without quaternary ammonium salt chitosan) showed a 33.2% decrease in flux after fouling, while Examples 1-6 showed decreases of 18.0%-9.5%, indicating that quaternary ammonium salt chitosan significantly inhibited pollutant adsorption. At concentrations ≥0.4%, the flux retention rate after fouling exceeded 90% (e.g., 94.5% at 0.5%), indicating that high concentrations of quaternary ammonium salt chitosan effectively reduced membrane fouling through electrostatic repulsion and hydrophilic interactions. The flux recovery rate after cleaning (Jw after cleaning / Jw initial) was generally higher than 90% and positively correlated with concentration: Comparative Example 1 (0%) showed a recovery rate of only 79.6%, while Examples 1-6 all exceeded 90%, indicating that the quaternary ammonium salt coating can reduce irreversible fouling. When the concentration is ≥0.2%, the recovery rate stabilizes at around 95% (reaching 96.0% at 0.5%), indicating that quaternary ammonium salts can facilitate the removal of pollutants through hydrophilicity and electrostatic interactions. Considering all factors, the optimal concentration range for quaternary ammonium salt chitosan is 0.1%-0.3%; the initial flux is highest at 0.1% (70.4 L·m⁻¹). -2 ·h -1The membrane exhibits a good balance between antifouling and recovery performance (81.3% retention and 91.8% recovery after fouling). Flux decreases significantly after 0.3%, but the improvement in antifouling performance is limited; a trade-off between flux and antifouling requirements is necessary. For high-concentration applications: if antifouling is the priority (e.g., treating highly polluted wastewater), a concentration of 0.4%-0.5% can be used, but flux loss must be accepted. The concentration of quaternary ammonium chitosan significantly affects membrane performance: a concentration of 0.1% achieves the best balance between flux and antifouling, while a concentration of 0.4%-0.5% is more suitable for highly polluted environments. Further research could optimize the coating process (e.g., degree of crosslinking) to improve flux performance at high concentrations. As can be seen from Examples 1, 7, 8, and Comparative Example 2, the molecular weight of quaternary ammonium chitosan directly affects the core performance of the composite reverse osmosis membrane, such as initial water flux and rejection rate, through dimensions such as molecular chain length, solubility, and interaction with pollutants / membrane matrix. The smaller the molecular weight (e.g., 50,000 Da in Example 7), the shorter the molecular chain, the higher the solubility, and the easier it is to form smaller dispersed particles in water. Small molecular weight (Example 7): high solubility, small chitosan dispersed particles in water, less clogging of membrane pores, and relatively high initial water flux (e.g., initial water flux of 67.3 L·m in Example 7). -2 ·h -1 It is close to 66.4 L·m in Example 1. -2 ·h -1 The larger the molecular weight (e.g., 150,000 Da in Example 8), the longer the molecular chain, the lower the solubility, and the easier it is to form large particles. Large molecular weight (Example 8): low solubility, chitosan easily forms large particles, causing more severe blockage of membrane pores, and reducing the initial water flux (e.g., initial water flux of 56.3 L·m⁻¹ in Example 8). -2 ·h -1 (Lower than Example 1) Molecular chain length and adsorption sites: The larger the molecular weight, the longer the molecular chain, and the more quaternary ammonium salt groups (key sites for adsorbing pollutants) exposed on the surface, resulting in stronger adsorption capacity for pollutants; conversely, the smaller the molecular weight, the fewer adsorption sites and the weaker the adsorption capacity. The molecular weight of quaternary ammonium salt chitosan has a differentiated impact on the initial water flux and rejection rate of the composite reverse osmosis membrane through mechanisms such as solubility, number of adsorption sites, and membrane matrix interaction. The performance differences of Example 1 (100,000 Da), Example 7 (50,000 Da), and Example 8 (150,000 Da) are a direct reflection of how molecular weight regulates the above mechanisms. However, when quaternary ammonium salt chitosan is replaced with chitosan, the performance of the prepared product decreases. Although the initial performance of chitosan's ability to adsorb pollutants is better than that of quaternary ammonium salt chitosan, the performance after washing decreases significantly, the recovery rate is significantly reduced, and the prepared product has a serious performance degradation problem, which is not conducive to long-term use.

[0097] Examples 1, 9, and 10 show that the water bath conditions have a certain impact on product performance, with the optimal temperature being 60℃ and a reaction time of 5 minutes. Examples 1, 11, and 12 show that as the concentration of the aqueous solution increases, the initial performance of the product does not differ significantly, but the water flux and flux recovery rate after cleaning show a change of first increasing and then decreasing. The concentration of the aqueous solution leads to the "first increasing and then decreasing" change in flux recovery rate after cleaning through mechanisms such as pollutant adsorption / deposition mode, interaction between cleaning agent and pollutants, and membrane surface structural stability. Examples 1, 13, and 14 show that as the concentration of the oil solution increases, the initial performance and post-use performance of the product are lower than the optimal example, especially the decrease in water flux and flux recovery rate after cleaning is most significant.

[0098] As can be seen from the above embodiments, the method provided by the present invention, by coating the polyamide separation layer with quaternary ammonium salt chitosan after its formation to induce a cross-linking reaction, can improve the cross-linking degree of the polyamide separation layer, thereby increasing the salt rejection rate of the polyamide reverse osmosis membrane. Simultaneously, it exhibits excellent antifouling properties, constructing a firmly bonded, positively charged, and highly hydrophilic QACS surface layer. This not only prevents a significant decrease in the flux of the polyamide reverse osmosis membrane but also effectively resists fouling through a dual mechanism of electrostatic repulsion / sterilization and a hydration layer barrier. Its "surface post-treatment" characteristic ensures that the fine structure of the polyamide separation layer itself is not damaged. The ultra-thin functional layer has negligible impact on water transport resistance and membrane screening capacity, thus miraculously maintaining the separation performance (water flux and salt rejection rate) of the membrane core while significantly improving antifouling performance. By employing quaternary ammonium salt chitosan surface modification technology, a positively charged, highly hydrophilic ultrathin functional layer is constructed on the polyamide separation layer. This layer utilizes a dual mechanism of electrostatic repulsion and hydration barrier to achieve long-term resistance to biofouling. Simultaneously, a "post-surface treatment" process ensures that the membrane's structure is not damaged, solving the problem of performance degradation caused by traditional modification methods. This technology maintains high flux and high desalination rate while significantly extending membrane lifespan, providing an efficient and stable solution for seawater desalination, high-salinity wastewater treatment, and other fields.

[0099] The above embodiments are for illustrating the implementation schemes disclosed in this invention and should not be construed as limiting the invention. Furthermore, various modifications listed herein, as well as variations in the methods and compositions of the invention, will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been specifically described in conjunction with various specific preferred embodiments, it should be understood that the invention should not be limited to these specific embodiments. In fact, various modifications as described above that are obvious to those skilled in the art to obtain the invention should be included within the scope of this invention.

Claims

1. A polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan and its preparation method, characterized in that, Includes the following steps: Step 1: Coat the polysulfone support layer with an aqueous solution and an oil solution sequentially. After drying, react in a water bath at 50-80℃ for 3-30 minutes to form an ultrathin polyamide separation layer, thus obtaining a nascent aromatic polyamide reverse osmosis membrane. The aqueous solution is a 0.3-1% (w / w) aqueous solution of m-phenylenediamine with a pH of 7-9. The oil solution contains 0.05-0.5% (w / w) of polyfunctional acyl halides, with the remainder being the oil solvent. Step 2: After activating the nascent aromatic polyamide reverse osmosis membrane obtained in Step 1, soak it or coat its surface with a coating solution, dry it, and then react it in a water bath at 50-80℃ for 3-30 minutes. After drying it again, a polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan is obtained.

2. The preparation method according to claim 1, characterized in that: The aqueous solution is a 0.5% (w / w) aqueous solution of m-phenylenediamine.

3. The preparation method according to claim 1, characterized in that: In steps 1 and 2, the water bath reaction temperature is 60℃ and the reaction time is 5 minutes.

4. The preparation method according to claim 1, characterized in that: The polyfunctional acyl halide is pyromellitic trimethylolpropionate chloride.

5. The preparation method according to claim 1, characterized in that: In step 1, the oil phase solvent is selected from one or more of C4-C12 aliphatic hydrocarbons, cyclic aliphatic hydrocarbons, and aromatic hydrocarbons.

6. The preparation method according to claim 1, characterized in that: In step 1, after coating the aqueous phase solution, the solution is evaporated at 20-30°C and 40-80% relative humidity until the water content is 28-31% by mass, and then coated with the oil phase solution.

7. The preparation method according to claim 1, characterized in that: In step 2, the coating solution is a quaternary ammonium salt chitosan solution with a mass fraction of 0.05-1%, and the molecular formula of the quaternary ammonium salt chitosan is [(C8H12H2O)2]. 13 NO5) 1-x (C 14 H 28 ClN2O6) x ] n , where x is the degree of substitution of the quaternary ammonium group, 0 < x < 1.2; n is the degree of polymerization, 0 < n < 4000.

8. The preparation method according to claim 1, characterized in that: In step 2, the activation includes: soaking the nascent aromatic polyamide reverse osmosis membrane in deionized water.

9. A polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan, characterized in that, It is prepared by the preparation method described in any one of claims 1-8.

10. Application of a polyamide reverse osmosis membrane modified with quaternary ammonium salt chitosan in seawater desalination.

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