Composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a composite nanofiltration membrane as well as a preparation method and application thereof, and belongs to the technical field of nanofiltration membranes. The preparation method comprises the following steps: taking terephthalic acid and amino terephthalic acid as organic ligands, carrying out coordination reaction on the organic ligands and zirconium salt to obtain an NH2-UIO-66 material, and mixing the NH2-UIO-66 material with a polycation electrolyte to obtain the polycation electrolyte containing an amino structure; the preparation method comprises the following steps: respectively mixing a polyanion electrolyte and a polycation electrolyte containing an amino structure with a solvent to obtain a polyelectrolyte solution with opposite charges; a layer-by-layer self-assembly method is adopted, polyelectrolyte solutions with opposite charges are adopted for alternate coating treatment on the flat ultrafiltration membrane, a polyelectrolyte coating is formed, and the composite nanofiltration membrane is obtained. According to the preparation method disclosed by the invention, a composite separation layer with an ordered multi-layer structure, a regular channel structure and ideal interface stability is constructed by utilizing a spontaneous assembly process driven by entropy increase among polyelectrolytes, high throughput is realized while a high rejection rate is guaranteed, and the high-performance composite nanofiltration membrane is obtained.

Description

Technical Field

[0001] This invention relates to the field of nanofiltration membrane technology, and in particular to a composite nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Nanofiltration membranes possess unique separation characteristics, enabling advanced treatment of drinking water. They efficiently remove pesticide residues, emerging pollutants from pharmaceuticals and personal care products, trace organic matter such as endocrine disruptors, and achieve high retention rates for pathogenic microorganisms like Cryptosporidium and viruses. More importantly, nanofiltration membranes exhibit excellent selective separation capabilities, effectively removing harmful substances while selectively retaining beneficial mineral ions, producing safe, healthy, and high-quality drinking water with excellent taste. In industrial water treatment, nanofiltration membranes are widely used for the concentration and recovery of heavy metals in electroplating wastewater, the separation and reuse of dyes in textile wastewater, and as a highly efficient pretreatment unit in seawater desalination systems, significantly reducing the operating load and scaling risk of subsequent reverse osmosis systems. The separation mechanism of nanofiltration membranes is not simply sieving, but a synergistic process dominated by size sieving and the Donnan effect: their nanoscale pore size can block large molecules and divalent ions through size sieving, while the charge on the membrane surface preferentially and efficiently retains divalent ions (such as sulfate, calcium, and magnesium ions) through the Donnan effect and electrostatic repulsion, thus achieving selective separation of different components. It is this synergy between physical sieving and charge action that gives nanofiltration membranes their unique advantages in high-efficiency purification and resource recovery.

[0003] Patent CN111330464A discloses a method for preparing a blended modified polysulfone charged nanofiltration membrane and the resulting membrane. The core of this nanofiltration membrane lies in the preparation of UiO-66-NH3. + Metal-organic framework powder was blended with polysulfone and other materials at a mass ratio of 5% to prepare a casting solution, and finally a nanofiltration membrane was obtained by phase inversion. The modified nanofiltration membrane significantly improved the performance of the traditional polysulfone membrane, possessing both high water flux and high rejection rate, and its surface carries a positive charge (NH3). +This method can achieve highly selective retention (retention rate >97%) of negatively charged dye molecules (such as methyl orange) in dyeing and printing wastewater through electrostatic interaction. Its main drawbacks lie in the balance and stability of membrane performance, the sensitivity of performance to environmental pH, and limitations in application areas, which may restrict its application in a wider range of water treatment scenarios. Patent CN114768547A discloses a method for preparing a UiO-66-doped multifunctional layered composite nanofiltration membrane. The core of this method lies in combining layer-by-layer self-assembly technology with interfacial polymerization: firstly, a cationic polyelectrolyte solution doped with UiO-66 and anionic polyelectrolyte are alternately deposited on a base membrane to construct a multilayer polyelectrolyte pre-coating; then, interfacial polymerization is performed on this pre-coating to form a dense polyamide separation layer, and finally, the composite nanofiltration membrane is obtained through heat treatment. This nanofiltration membrane has a high flux (pure water flux ≥49 L / m³). 2 The nanofiltration membrane exhibits comprehensive performance characteristics such as high flux (≥98% for MgSO4) and excellent long-term stability (≤6% flux decay rate over 30 days), significantly improving its separation efficiency and antifouling ability. However, its complex process and potential cost and stability issues are the main obstacles to its transition from laboratory to large-scale industrial application. Patent CN115382399A discloses a method for preparing a composite nanofiltration membrane based on UiO-66 nanoparticles and its products. The core of this method is to provide a method for preparing a composite nanofiltration membrane based on positively charged UiO-66 nanoparticles. By introducing amino-functionalized UiO-66 into the aqueous phase and polymerizing it at the interface with polyacrylamide chlorides, a polyamide selective layer with a regular wrinkled morphology is successfully constructed. This membrane effectively solves the problems of low flux and insufficient rejection of small molecule organic matter and divalent cations in traditional nanofiltration membranes, exhibiting high flux and high selectivity. However, this technology still has drawbacks such as complex preparation process, high cost, easy agglomeration of nanoparticles affecting stability, and low retention rate of monovalent ions. Its long-term performance and industrial feasibility still need to be verified. Summary of the Invention

[0004] The purpose of this invention is to provide a composite nanofiltration membrane, its preparation method, and its application. The preparation method is simple and easy to implement, and the prepared composite nanofiltration membrane has a high divalent salt rejection rate and high flux.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps: Zirconium salt, organic ligand, regulator and solvent are mixed and a coordination reaction is carried out to obtain NH2-UIO-66 material; the organic ligand includes terephthalic acid and aminoterephthalic acid; The NH2-UIO-66 material was mixed with a polycationic electrolyte to obtain an amino-containing polycationic electrolyte. The polyanionic electrolyte is mixed with the first solvent to obtain a polyanionic electrolyte solution; The amino-containing polycationic electrolyte is mixed with a second solvent to obtain a polycationic electrolyte solution; The polyanionic electrolyte solution and the polycationic electrolyte solution are used to alternately coat the flat ultrafiltration membrane to form a polyelectrolyte coating, thereby obtaining a composite nanofiltration membrane.

[0006] Preferably, the zirconium salt comprises zirconium tetrachloride; the molar ratio of zirconium ions to the molar ratio of the organic ligand in the zirconium salt is 1:(1~5).

[0007] Preferably, the molar amount of the aminoterephthalic acid is 10-100% of the molar amount of the organic ligand; The regulator includes acetic acid or formic acid; The solvent includes N,N-dimethylformamide, N,N-dimethylacetamide, or tetrahydrofuran.

[0008] Preferably, the coordination reaction is carried out at a temperature of 100-150°C for 12-24 hours.

[0009] Preferably, the polycationic electrolyte comprises poly(propyleneamine hydrochloride), poly(vinylbenzyltrimethylammonium chloride), polyethyleneamine, or polyacrylamide; The molar amount of the NH2-UIO-66 material is 10 to 50% of the molar amount of the amino-containing polycationic electrolyte.

[0010] Preferably, the NH2-UIO-66 material is mixed with the polycationic electrolyte at a temperature of 15~25℃ for a time of 30~60 min.

[0011] Preferably, the polyanionic electrolyte includes polyacrylic acid, polyaminoacetic acid, polystyrene sulfonic acid, or polysulfate.

[0012] Preferably, the total number of layers of the polyelectrolyte coating is 1 to 20; The temperature for the alternating coating treatment is independently 15~25℃, and the time is independently 2~30 min.

[0013] The present invention provides a composite nanofiltration membrane prepared by the above preparation method.

[0014] This invention provides the application of the above-mentioned composite nanofiltration membrane in drinking water purification or wastewater treatment and resource recovery.

[0015] The beneficial effects of this invention are: This invention uses terephthalic acid and aminoterephthalic acid as organic ligands to synthesize NH2-UIO-66 material through a coordination reaction, achieving the introduction of amino functional groups without destroying its crystal structure. On the one hand, the in-situ introduction of amino groups effectively enhances the hydrophilicity of the UIO-66 material itself, thereby significantly improving the overall hydrophilicity of the composite nanofiltration membrane, which is beneficial for increasing water flux and reducing membrane fouling. On the other hand, the amino groups can be protonated to -NH3 in an aqueous environment. + This significantly enhances the positive charge density on the surface of the polycationic electrolyte layer and the final nanofiltration membrane, thereby strengthening the protection against divalent cations (such as Ca). 2+ Mg 2+ The electrostatic repulsion of NH2-UIO-66 enhances its rejection rate while maintaining a high pure water flux. Secondly, the prepared NH2-UIO-66 material is blended with a polycationic electrolyte to obtain a polycationic electrolyte containing NH2-UIO-66, forming a stable functionalized polyelectrolyte composite solution. Finally, using an ultrafiltration flat sheet membrane as the base membrane, a layer-by-layer self-assembly technique is employed. By alternately depositing solutions containing NH2-UIO-66 polycationic electrolyte and solutions containing polyanionic electrolyte, and utilizing the entropy increase-driven spontaneous assembly process between the polyelectrolytes, a composite separation layer with an ordered multilayer structure, regular channel structure, and ideal interfacial stability is constructed. This achieves high throughput while maintaining a high rejection rate, resulting in a high-performance composite nanofiltration membrane. This composite nanofiltration membrane shows promising application prospects in surface water purification, high-hardness wastewater softening, biopharmaceutical separation, and heavy metal pollution control.

[0016] In summary, this invention not only yields a composite nanofiltration membrane product with controllable structure and excellent performance, but also establishes a green preparation route that is mild, process-controllable, and suitable for scale-up. Attached Figure Description

[0017] Figure 1 The images show the XRD patterns of UIO-66 in Comparative Example 1 and NH2-UIO-66 nanoparticles in Example 1 of this invention. Figure 2 The images are SEM images of UIO-66 in Comparative Example 1 and NH2-UIO-66(25) nanoparticles in Example 1 of this invention, where a is UIO-66 and b is NH2-UIO-66(25). Figure 3 This is a cross-sectional view of the polyelectrolyte composite nanofiltration membrane in Embodiment 1 of the present invention; Figure 4 The diagram shows the contact angles of the composite nanofiltration membrane in Comparative Example 1 and the polyelectrolyte composite nanofiltration membrane in Example 1 of the present invention, where a represents Comparative Example 1 and b represents Example 1. Figure 5This is a graph showing the water flux of the composite nanofiltration membrane under different molar percentages of aminoterephthalic acid in Comparative Example 1 and Examples 1-3 of the present invention; Figure 6 The diagram shows the salt rejection rate of the composite nanofiltration membrane under different molar percentages of aminoterephthalic acid in Comparative Example 1 and Examples 1-3 of this invention. Detailed Implementation

[0018] This invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps: Zirconium salt, organic ligand, regulator and solvent are mixed and a coordination reaction is carried out to obtain NH2-UIO-66 material; the organic ligand includes terephthalic acid and aminoterephthalic acid; The NH2-UIO-66 material was mixed with a polycationic electrolyte to obtain an amino-containing polycationic electrolyte. The polyanionic electrolyte is mixed with the first solvent to obtain a polyanionic electrolyte solution; The amino-containing polycationic electrolyte is mixed with a second solvent to obtain a polycationic electrolyte solution; The polyanionic electrolyte solution and the polycationic electrolyte solution are used to alternately coat the flat ultrafiltration membrane to form a polyelectrolyte coating, thereby obtaining a composite nanofiltration membrane.

[0019] In this invention, unless otherwise specified, all raw materials required for preparation are commercially available products well known to those skilled in the art.

[0020] In this invention, zirconium salt, organic ligand, regulator and solvent are mixed to carry out a coordination reaction. After the coordination reaction is completed, all precipitates are transferred to 15 mL centrifuge tubes and centrifuged, washed and dried in sequence to obtain NH2-UIO-66 material.

[0021] In this invention, the zirconium salt preferably includes zirconium tetrachloride (ZrCl4); the molar ratio of zirconium ions to organic ligands in the zirconium salt is preferably 1:(1~5), and more preferably 1:1.

[0022] In this invention, the organic ligand preferably includes terephthalic acid and aminoterephthalic acid; the molar amount of aminoterephthalic acid is preferably 10-100% of the molar amount of the organic ligand, more preferably 25-75%, and even more preferably 50-75%.

[0023] In this invention, the regulator preferably includes acetic acid or formic acid, and more preferably acetic acid; the volume of the regulator is not particularly limited in this invention, and can be adjusted according to actual needs. In the embodiments of this invention, the volume of the regulator is preferably 12~20 mL.

[0024] In this invention, the solvent preferably includes N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and tetrahydrofuran (THF), and more preferably DMF; the volume of the solvent is not particularly limited in this invention, as long as it can dissolve the substance, and the volume of the solvent in the embodiments of this invention is preferably 80 mL.

[0025] In this invention, the temperature of the coordination reaction is preferably 100~150℃, more preferably 120℃, and the time is preferably 12~24 h, more preferably 12 h.

[0026] In this invention, the centrifugation speed is preferably 5000~8000 rpm, more preferably 6000~8000 rpm, and the centrifugation time is preferably 10~30 min.

[0027] In this invention, the solvent used for washing is preferably DMF and anhydrous methanol. The specific washing operation is preferably as follows: after centrifugation, the supernatant is discarded, then DMF is added to the precipitate, the precipitate is dispersed by ultrasonication, and centrifuged at 5000~8000 rpm for 10~30 min. The above steps of discarding the supernatant-adding DMF-ultrasonication-centrifugation are repeated 3~5 times to thoroughly remove unreacted monomers and impurities remaining in the solvent. Then, anhydrous methanol is added to the product washed with DMF, and after thorough dispersion, the centrifuge tube is sealed and placed in a 60℃ constant temperature water bath or oven for 12~24 h to soak. The sample is centrifuged and the old methanol is discarded and replaced with an equal amount of new methanol. The above steps of discarding the old methanol-adding an equal amount of new methanol-dispersion-centrifugation are repeated 3~5 times.

[0028] In this invention, the preferred specific operation of the drying is as follows: the white solid product obtained after the last washing with anhydrous methanol is transferred to a petri dish, spread evenly, and placed in an oven for drying; the drying temperature is preferably 60°C, and the drying time is preferably 12~24 h, more preferably 24 h.

[0029] The present invention preferably mixes the above-mentioned NH2-UIO-66 material with a polycationic electrolyte to obtain an amino-containing polycationic electrolyte.

[0030] In this invention, the polycationic electrolyte preferably includes poly(acrylamine hydrochloride) (PAH), poly(vinylbenzyltrimethylammonium chloride), polyethyleneamine or polyacrylamide, and more preferably poly(acrylamine hydrochloride).

[0031] In this invention, the molar amount of the NH2-UIO-66 material is preferably 10-50% of the molar amount of the amino-containing polycationic electrolyte, and more preferably 30%.

[0032] In this invention, the mixing temperature of the NH2-UIO-66 material with the polycationic electrolyte is preferably 15~25℃, more preferably 20℃, and the mixing time is preferably 30~60 min, more preferably 30 min.

[0033] The present invention preferably involves mixing a polyanionic electrolyte with a first solvent to obtain a polyanionic electrolyte solution; mixing the aforementioned amino-containing polycationic electrolyte with a second solvent to obtain a polycationic electrolyte solution; and using a layer-by-layer self-assembly method, alternately coating the polyanionic electrolyte solution and the polycationic electrolyte solution with opposite charges onto a flat sheet ultrafiltration membrane to form a polyelectrolyte coating, thereby obtaining a composite nanofiltration membrane; the present invention does not have a particular limitation on the order of the polyanionic electrolyte solution and the polycationic electrolyte solution during the alternating coating process.

[0034] In this invention, the polyanionic electrolyte preferably includes polyacrylic acid, polyaminoacetic acid, polystyrene sulfonic acid (PSS), or polysulfate, and more preferably polystyrene sulfonic acid.

[0035] In this invention, the first solvent preferably includes an aqueous sodium chloride solution. The concentration of the first solvent is not particularly limited and can be adjusted according to actual needs. The concentration of the polyanionic electrolyte solution is preferably 0.1~20 g / L, and more preferably 4 g / L.

[0036] In this invention, the second solvent preferably includes an aqueous sodium chloride solution. The concentration of the second solvent is not particularly limited and can be adjusted according to actual needs. The concentration of the polycationic electrolyte solution is preferably 0.1~20 g / L, and more preferably 10 g / L.

[0037] In this invention, the total number of layers of the polyelectrolyte coating is preferably 1 to 20, and more preferably 6 to 20.

[0038] The present invention does not have any particular limitation on the model or specifications of the flat sheet ultrafiltration membrane, and any commercially available product well known in the art is acceptable. The flat sheet ultrafiltration membrane used in the embodiments of the present invention is preferably a polysulfone flat sheet ultrafiltration membrane (Psf-based membrane).

[0039] In this invention, the alternating coating treatment method is preferably a static coating method, and specifically preferably includes the following steps: (1) Before use, rinse the Psf base film with water, place it in the customized plate frame, and blow it with N2 until there are no water droplets on the surface; (2) Pour 50 mL of polyanionic electrolyte solution evenly into the horizontally placed flat plate frame, let stand for 15 min, pour off the excess polyanionic electrolyte solution, wash with 1000 mL of water in one direction, and purge with N2. This is the 0.5th cycle, that is, a single layer coating is completed. (3) Pour 50 mL of polycationic electrolyte solution evenly into the flat plate frame, let stand for 15 min, pour off the excess polycationic electrolyte solution, wash with 1000 mL of water in one direction, and purge with N2. This is the 1.0 cycle, that is, one double coating is completed. (4) Repeat steps (2) and (3) until the desired coating is obtained.

[0040] In this invention, the temperature of the alternating coating treatment is preferably 15~25℃, more preferably 20~22℃, and the time is preferably 2~30min, more preferably 15min.

[0041] The present invention also provides a composite nanofiltration membrane prepared by the above preparation method.

[0042] The present invention also provides the application of the above-mentioned composite nanofiltration membrane in drinking water purification or wastewater treatment and resource recycling.

[0043] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0044] Example 1

[0045] 8 mmol ZrCl4, 6 mmol terephthalic acid and 2 mmol aminoterephthalic acid were dissolved in 80 mL DMF solution containing 12 mL acetic acid at 120 °C for coordination reaction. The reaction was carried out for 12 h, centrifuged at 6000 r / min for 30 min, ultrasonically mixed for 50 min, washed 5 times with DMF and 5 times with methanol, and dried at 60 °C for 24 h to obtain NH2-UIO-66 material, denoted as NH2-UIO-66(25); 0.3 mol NH2-UIO-66(25) and 0.7 mol poly(acrylamine hydrochloride) powder were mixed at 20°C for 30 min to obtain a polycationic electrolyte containing NH2-UIO-66(25), with the molar percentage of NH2-UIO-66(25) being 30%. Polystyrene sulfonic acid was mixed with 30 g / L sodium chloride solution to obtain a 4 g / L polyanionic electrolyte solution; a polycationic electrolyte containing NH2-UIO-66(25) was mixed with 150 g / L sodium chloride solution to obtain a 10 g / L polycationic electrolyte solution. The above-mentioned polyanionic electrolyte solution and polycationic electrolyte solution were used to perform alternating coating treatment on the Psf base film. Specifically: (1) The Psf base film was rinsed clean with water and placed in a customized plate frame. N2 was blown until there were no water droplets on the surface; (2) At 20°C, 50 mL of polyanionic electrolyte solution was evenly poured into the horizontally placed flat plate film frame, left to stand for 15 min, the excess polyanionic electrolyte solution was poured off, and 1000 mL of water was used to clean in one direction. N2 was blown. This was the 0.5th cycle, that is, one single layer coating was completed; (3) At 20°C, 50 mL of polycationic electrolyte solution was evenly poured into the flat plate film frame, left to stand for 15 min, the excess polycationic electrolyte solution was poured off, and 1000 mL of water was used to clean in one direction. N2 was blown. mL of water is used to clean in one direction, and N2 is used to purge. This is the 1.0 cycle, which means that a double coating is completed. (4) Repeat steps (2) and (3) until 6 polyelectrolyte coating monolayers are formed to obtain a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(25).

[0046] Example 2

[0047] The only difference from Example 1 is: The molar amount of terephthalic acid is 4 mmol, the molar amount of aminoterephthalic acid is 4 mmol, and the resulting NH2-UIO-66 material is denoted as NH2-UIO-66(50); With all other conditions unchanged, a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(50) was obtained.

[0048] Example 3

[0049] The only difference from Example 1 is: The molar amount of terephthalic acid is 2 mmol, the molar amount of aminoterephthalic acid is 6 mmol, and the resulting NH2-UIO-66 material is denoted as NH2-UIO-66(75); With all other conditions unchanged, a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(75) was obtained.

[0050] Example 4

[0051] The only difference from Example 1 is: The molar amount of terephthalic acid is 4 mmol, the molar amount of aminoterephthalic acid is 4 mmol, and the resulting NH2-UIO-66 material is denoted as NH2-UIO-66(50); Two polyelectrolyte coating monolayers are formed; With all other conditions unchanged, a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(50) was obtained.

[0052] Example 5

[0053] The only difference from Example 1 is: The molar amount of terephthalic acid is 4 mmol, the molar amount of aminoterephthalic acid is 4 mmol, and the resulting NH2-UIO-66 material is denoted as NH2-UIO-66(50); Repeat steps (2) and (3) above until four polyelectrolyte coating monolayers are formed; With all other conditions unchanged, a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(50) was obtained.

[0054] Example 6

[0055] The only difference from Example 1 is: The molar amount of terephthalic acid is 4 mmol, the molar amount of aminoterephthalic acid is 4 mmol, and the resulting NH2-UIO-66 material is denoted as NH2-UIO-66(50); The above-mentioned polyanionic electrolyte solution and polycationic electrolyte solution were used to perform alternating coating treatment on the Psf base film. Specifically: (1) Rinse the Psf base film with water, place it in a customized plate frame, and blow N2 until there are no water droplets on the surface; (2) Pour 50 mL of polycationic electrolyte solution evenly into the horizontally placed flat plate frame, let it stand for 15 min, pour off the excess polycationic electrolyte solution, wash with 1000 mL of water in one direction, and blow N2. This is the 0.5th cycle, that is, one single layer coating is completed; (3) Pour 50 mL of polyanionic electrolyte solution evenly into the flat plate frame, let it stand for 15 min, pour off the excess polyanionic electrolyte solution, wash with 1000 mL of water in one direction, and blow N2. This is the 1.0th cycle, that is, one double layer coating is completed; (4) Repeat the above steps (2) and (3) until 4 polyelectrolyte coating single layers are formed. With all other conditions unchanged, a polyelectrolyte composite nanofiltration membrane containing NH2-UIO-66(50) was obtained.

[0056] Comparative Example 1

[0057] The only difference from Example 1 is: The molar amount of terephthalic acid was 8 mmol. Without the addition of aminoterephthalic acid, UIO-66 material was obtained. With all other conditions unchanged, a composite nanofiltration membrane was obtained.

[0058] Characterization and performance testing

[0059] Characterization and testing

[0060] 1. (1) Figure 1 The XRD patterns of UIO-66 in Comparative Example 1 and NH2-UIO-66 nanoparticles in Example 1 show that both UIO-66 and NH2-UIO-66 exhibit obvious characteristic diffraction peaks around 7° and 8°, respectively, which are attributed to the (111) and (200) crystal planes of the UIO-66 structure, indicating that both have successfully formed a typical UIO-66 crystal structure. The diffraction peak positions of NH2-UIO-66 are basically consistent with those of UIO-66, indicating that amino functionalization has not changed its basic framework. The intensity of some diffraction peaks has changed, which may be related to the fine-tuning of the crystal surface structure or electronic environment after the introduction of amino groups. Overall, the crystallinity remains good.

[0061] (2) Figure 2 The images show SEM images of UIO-66 in Comparative Example 1 and NH2-UIO-66(25) nanoparticles in Example 1 of this invention, where a is UIO-66 and b is NH2-UIO-66(25); the size of UIO-66 is 100 nm to 160 nm; due to the introduction of amino groups, the nucleation rate may be slightly accelerated, resulting in the generation of more crystals, but smaller in size. The size of NH2-UIO-66 is slightly smaller than that of UIO-66, in the range of 80 nm to 140 nm.

[0062] (3) Figure 3 This is a cross-sectional view of the polyelectrolyte composite nanofiltration membrane in Embodiment 1 of the present invention; as shown Figure 3 As shown, a separation layer with a thickness of 20 nm is formed between the polyelectrolyte layer and the hollow fiber base membrane.

[0063] 2. (1) Figure 4 The diagram shows the contact angles of the composite nanofiltration membrane in Comparative Example 1 and the polyelectrolyte composite nanofiltration membrane in Example 1, where a represents Comparative Example 1 and b represents Example 1. After coating the polyethersulfone ultrafiltration membrane with polyelectrolyte, the contact angle of the nanofiltration membrane decreased from 65° to 24°, indicating that the polyelectrolyte coating can significantly reduce the contact angle of the membrane surface, making it easier for water molecules to wet the membrane surface, thereby improving the hydrophilicity of the composite nanofiltration membrane.

[0064] (2) The water flux of the composite nanofiltration membranes under different molar percentages of aminoterephthalic acid in Comparative Example 1 and Examples 1-3 was tested according to the following method. The results are shown in the figure. Figure 5 ( Figure 5 (NH2-UIO-66(0) represents the nanofiltration membrane prepared in Comparative Example 1): Pure aqueous solution was used as the feed liquid, the temperature was 20±1℃, the test pressure was 3.0 bar, and the flow rate was 0.1 L / min; the concentration of the feed liquid was kept constant during the test, and the permeate was collected after the device had been running stably for 1 hour; the mass of solution passing through the membrane per unit time and per unit area was measured using an electronic balance and a conductivity meter, and the permeate flux of the membrane was calculated. Figure 5 This is a water flux diagram of the composite nanofiltration membrane under different molar percentages of aminoterephthalic acid in Comparative Example 1 and Examples 1-3 of the present invention. Figure 5 It can be seen that, under the test conditions of 3 bar, the nanofiltration membrane of Comparative Example 1 with only unaminated UIO-66 added has a flux of 25 L·m⁻¹ for MgCl₂, MgSO₄, Na₂SO₄, and LiCl, all at a concentration of 500 mg / L. -2 ·h -1 Left and right; In Example 1, after adding NH2-UIO-66(25), an amino group was introduced into the separation layer of the nanofiltration membrane, thereby enhancing the hydrophilicity of the nanofiltration membrane, and thus the permeability coefficient of the NH2-UIO-66(25) nanofiltration membrane increased to 36 L·m -2 ·h -1 Example 2: NH2-UIO-66(50) was added, increasing the amino ratio from 25% to 50%, thereby increasing the permeability coefficient of the NH2-UIO-66(50) nanofiltration membrane to 38 L·m. -2 ·h -1 Example 3: NH2-UIO-66(75) was added, increasing the amino ratio from 25% to 75%, thereby increasing the permeability coefficient of the NH2-UIO-66(75) nanofiltration membrane to 41 L·m. -2 ·h -1 .

[0065] (3) The separation performance of the composite nanofiltration membranes prepared in Comparative Example 1 and Examples 1-6 was tested using a cross-flow mode. The concentration of neutral organic matter in the solution was 100 mg / L, the operating pressure was 3.0 bar, and the flow rate was 0.65 m / s. After stabilization for 1 h, samples were taken and the concentration of organic matter in the feed liquid and permeate was measured using a conductivity meter to calculate the permeation and retention performance of the nanofiltration membrane for inorganic salts. The results are shown in Table 1 and 2. Figure 6 ( Figure 6 In the middle, NH2-UIO-66(0) represents the nanofiltration membrane prepared in Comparative Example 1; Table 1. Retention rate of metal salts by the composite nanofiltration membranes of Comparative Example 1 and Examples 1-6

[0066] From Table 1 and Figure 6It can be seen that, under the test conditions of 3 bar, the metal salt rejection capacity of the composite nanofiltration membrane with only unaminated UIO-66 added in Comparative Example 1 is MgSO4>MgCl2>NaCl>Na2SO4, with the composite nanofiltration membrane having rejection rates of 78% and 80% for MgCl2 and MgSO4, respectively; while the rejection rate for Na2SO4 is relatively low, at 20%. The composite nanofiltration membrane in Example 1 contains NH2-UIO-66 (25) material, that is, amino groups are introduced into the separation layer of the composite nanofiltration membrane, thereby enhancing the positive charge of the composite nanofiltration membrane and thus improving the rejection capacity of Mg2SO4. 2+ The retention rates of MgCl2 and MgSO4 were 88% and 92%, respectively. In Example 2, the composite nanofiltration membrane contained NH2-UIO-66 (50) material, and the amino ratio was increased from 25% to 50%, with a retention rate of up to 94% for magnesium sulfate. In Example 3, the composite nanofiltration membrane contained NH2-UIO-66 (75) material, and the amino ratio was increased from 25% to 75%, with a retention rate of up to 97% for magnesium sulfate.

[0067] As can be seen from the above embodiments, the present invention provides a composite nanofiltration membrane. Using terephthalic acid and aminoterephthalic acid as organic ligands, an amino-containing UIO-66 material is obtained. This material is then mixed with a polycationic electrolyte to obtain an amino-containing polycationic electrolyte. The polyanionic electrolyte and the amino-containing polycationic electrolyte are respectively mixed with a solvent. The resulting polyelectrolyte solutions with opposite charges are then subjected to alternating coating treatment on a flat-sheet ultrafiltration membrane using a layer-by-layer self-assembly method to obtain the composite nanofiltration membrane. The present invention utilizes the spontaneous assembly process driven by entropy increase between polyelectrolytes to construct a composite separation layer with an ordered multilayer structure, a regular channel structure, and ideal interface stability. This achieves high throughput while ensuring high rejection rate, resulting in a high-performance composite nanofiltration membrane.

[0068] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that, Includes the following steps: Zirconium salt, organic ligand, regulator and solvent are mixed and a coordination reaction is carried out to obtain NH2-UIO-66 material; the organic ligand includes terephthalic acid and aminoterephthalic acid; The NH2-UIO-66 material was mixed with a polycationic electrolyte to obtain an amino-containing polycationic electrolyte. The polyanionic electrolyte is mixed with the first solvent to obtain a polyanionic electrolyte solution; The amino-containing polycationic electrolyte is mixed with a second solvent to obtain a polycationic electrolyte solution; The polyanionic electrolyte solution and the polycationic electrolyte solution are used to alternately coat the flat ultrafiltration membrane to form a polyelectrolyte coating, thereby obtaining a composite nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, The zirconium salt includes zirconium tetrachloride; the molar ratio of zirconium ions to organic ligands in the zirconium salt is 1:(1~5).

3. The preparation method according to claim 2, characterized in that, The molar amount of the aminoterephthalic acid is 10-100% of the molar amount of the organic ligand; The regulator includes acetic acid or formic acid; The solvent includes N,N-dimethylformamide, N,N-dimethylacetamide, or tetrahydrofuran.

4. The preparation method according to claim 1 or 3, characterized in that, The coordination reaction is carried out at a temperature of 100-150℃ for 12-24 hours.

5. The preparation method according to claim 1, characterized in that, The polycationic electrolyte includes poly(acrylamine hydrochloride), poly(vinylbenzyltrimethylammonium chloride), polyethyleneamine, or polyacrylamide; The molar amount of the NH2-UIO-66 material is 10 to 50% of the molar amount of the amino-containing polycationic electrolyte.

6. The preparation method according to claim 5, characterized in that, The NH2-UIO-66 material is mixed with the polycationic electrolyte at a temperature of 15~25℃ for a time of 30~60 min.

7. The preparation method according to claim 1, characterized in that, The polyanionic electrolyte includes polyacrylic acid, polyaminoacetic acid, polystyrene sulfonic acid, or polysulfate.

8. The preparation method according to claim 7, characterized in that, The total number of layers of the polyelectrolyte coating is 1 to 20; The temperature for the alternating coating treatment is independently 15~25℃, and the time is independently 2~30 min.

9. The composite nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the composite nanofiltration membrane according to claim 9 in drinking water purification or wastewater treatment and resource recovery.

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