Method for preparing fluorinated polyamide anti-pollution and chlorine-resistant nanofiltration membrane through grafting point location provided by defect engineering and application of fluorinated polyamide anti-pollution and chlorine-resistant nanofiltration membrane

Abstract

The invention discloses a method for preparing a nanofiltration membrane with ultrahigh chemical stability by introducing a fluorinated active group for interfacial polymerization, which comprises the following steps: by taking polyamine and polyacyl chloride as polymeric monomers, regulating the reaction ratio of the monomers, and introducing'structural defects' into a polyamide cross-linked network to form active sites containing free amino groups. A rigid biphenyl monomer containing a strong electron-withdrawing polyfluoroalkyl group and a high-activity sulfonyl chloride group is selected and subjected to an acylation reaction with free amino groups on the surface of polyamide, the grafting density is controlled, and the nanofiltration membrane is obtained. The preparation method has the following advantages: by virtue of extremely low surface energy of the surface-grafted fluorination reagent, more benzene ring-containing structural units are introduced at the same time, so that a conjugated pi electron system is formed, the electron cloud density is effectively expanded, the chlorine oxidation resistance of the nanofiltration membrane is remarkably improved, and the flux recovery rate is up to 96% or above; the implementation of the defect engineering constructs a more efficient mass transfer channel in the polyamide membrane, so that the membrane obtains high permeability and excellent interception capability on divalent salt.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of modification of nanofiltration membranes, and specifically discloses a method for preparing fluorinated polyamide antifouling and chlorine-resistant nanofiltration membranes by providing grafting sites through defect engineering and application thereof. BACKGROUND

[0002] Membrane separation technology is valued in the field of water treatment due to its energy-saving and environmentally friendly characteristics. As a new type of separation medium between ultrafiltration and reverse osmosis, nanofiltration membranes have been widely used in industrial scenarios such as textile wastewater purification, heavy metal recovery, and biological protein purification due to their low operating pressure, large permeation flux, and selective separation advantages. Among them, polyamide-based nanofiltration membranes have become the core component in water treatment processes due to their excellent separation performance.

[0003] Water treatment systems often use chlorination disinfection to control microbial contamination, but residual active chlorine can cause polyamide polymer chain rupture, leading to separation layer structure damage and performance degradation. This chemical degradation problem seriously restricts the long-term operation stability of polyamide nanofiltration membranes. At the same time, the deposition of pollutants on the membrane surface during operation forms a biological / organic fouling layer, which significantly reduces the membrane flux and shortens the service life.

[0004] Currently, there are two technical paths to improve the chlorine resistance of nanofiltration membranes: one is to reconstruct the polyamide network structure through the design of new monomer molecules; the other is to modify the surface of the separation layer with chlorine-resistant functional groups. Among them, the interface engineering strategy of surface grafting antioxidant groups is the most commonly used modification method due to its strong controllability.

[0005] For example, patent CN110064312A discloses a high-flux solvent-resistant interfacial polymerization composite membrane and a preparation method thereof. The method uses a solvent-resistant gel membrane as a base film, and an interfacial polymerization is used to deposit a polyamide functional separation layer on the surface of the base film to obtain a composite membrane. Among them, the poly-p-phenyleneterephthalamine gel membrane is prepared by a phase inversion method, and after the solution is coated on the support layer, it is immersed in a coagulation bath to form; the chitosan or polyvinyl alcohol gel membrane is formed by coating the casting solution on the support layer. During interfacial polymerization, the aqueous solution is a mixture of water-soluble monomers, acid / base adjuster and water, and the organic solution is a mixture of polyacyl chloride monomers and organic solvents. According to whether the coagulation bath contains water-soluble monomers, different interfacial polymerization operation steps are adopted. However, compared with traditional polymer ultrafiltration membranes, its mechanical properties are still insufficient, especially in high-pressure operation scenarios, which affects the service life of the membrane.

[0006] Patent CN109351190A discloses a cross-linked fluorine-containing polyamide composite solvent-resistant nanofiltration membrane, a preparation method and applications thereof. The preparation method comprises interface polymerization, chemical cross-linking and solvent activation steps: first, an ultrafiltration membrane such as a polyimide or a polyetherimide is used as a base film, the base film is contacted with a water-phase monomer solution containing a fluorine-containing aromatic polyamine compound and an aromatic diamine compound, and then reacted with an organic-phase solution containing an aromatic ternary acyl chloride, and a fluorine-containing dry polyamide composite membrane is obtained through heat treatment; then the fluorine-containing dry polyamide composite membrane is immersed in an alcohol solution containing a cross-linking agent for cross-linking; finally, the fluorine-containing dry polyamide composite membrane is activated in an activation solvent, and after being replaced with an alcohol solvent, the fluorine-containing dry polyamide composite membrane is stored. The separation skin layer of the composite membrane contains fluorine-containing polyamide repeating structural units, the base film and the skin layer are connected through a covalent bond, and after cross-linking and activation, the solvent resistance and the flux of the membrane are significantly improved. The -CF3 group of the fluorine-containing monomer promotes the diffusion of the water-phase monomer to the oil phase, increases the specific surface area of the membrane, and improves the flux; the high bond energy of the C-F bond enhances the stability of the polymer skeleton and improves the solvent resistance. The cross-linking step enhances the binding force between the base film and the skin layer, and the activation treatment optimizes the pore structure of the membrane. However, this method may cause uneven distribution of fluorine monomers in the skin layer, and the local fluorine content is too high or too low, which affects the overall solvent resistance and flux stability of the membrane.

[0007] The above two patents have innovations in the preparation of solvent-resistant membranes, but there are obstacles to industrialization. CN110064312A uses a gel membrane to prepare a composite membrane, which has poor mechanical properties and a short service life under high pressure; CN109351190A improves the performance of the membrane by using fluorine-containing monomers, but faces the problem of uneven distribution of fluorine monomers, which affects the overall performance of the membrane. Moreover, both methods have many steps in the synthesis of reaction monomers, high cost, and complex process; the support layer treatment requires high equipment, is greatly affected by material and parameters, and has poor batch stability, which limits large-scale industrial production. SUMMARY

[0008] To solve the problems in the prior art, the present application provides a method for preparing a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane by providing grafting points through defect engineering. The present application solves the technical bottleneck of poor chlorine resistance and anti-fouling property of existing nanofiltration membranes, overcomes the problem of flux decline caused by the introduction of fluorine elements, and greatly promotes the wide application of nanofiltration membranes.

[0009] Another object of the present application is to provide a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane prepared by the above method.

[0010] Still another object of the present application is to provide the application of the above fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane in the industrial field.

[0011] The rejection rate of the fluorine-containing nanofiltration membrane prepared by the method reaches 97% or more to magnesium sulfate, and the salt rejection rate decreases very little after 16000ppm·h chlorination treatment. The fluorine groups on the surface of the polyamide act as chlorination sacrifice sites, and the benzene rings introduced by the fluorinated reagent with a biphenyl conjugated skeleton disperse the free radicals generated by the attack of active chlorine through conjugate effect, thereby weakening the chlorination degradation of active chlorine. At the same time, the flux of the grafted nanofiltration membrane can reach 44L·m 2 ·bar -1 ·h -1 The above, compared with the flux of the ungrafted membrane, is significantly improved. In addition, after two pollution cycles, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin is 96% or more. The conjugate system introduced by the fluorine-containing benzene ring group optimizes the pore structure of the membrane through steric hindrance effect, and cooperatively improves the mass transfer efficiency, thereby significantly improving the flux and anti-pollution performance. It shows that the fluorine-containing nanofiltration membrane prepared has excellent chlorine resistance and high flux, and its anti-pollution performance is also enhanced.

[0012] To achieve the above technical purposes, the technical scheme adopted by the present application is:

[0013] A method for preparing a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane by providing grafting points through defect engineering, comprising the following steps:

[0014] S1, using a water phase immersion method, immersing a microporous membrane in an aqueous solution of an amine monomer, and then drying to obtain a primary composite membrane;

[0015] S2, using an oil phase immersion method, immersing the primary composite membrane of step S1 in an organic solvent solution of an acyl chloride monomer, and then drying to obtain a secondary composite membrane;

[0016] S3, immersing the secondary composite membrane of step S2 in a fluorinated reagent, and then drying to obtain the fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane;

[0017] The amine monomer is any one of piperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, ethylenediamine, hexanediamine, triethylamine, o-xylylenediamine, m-xylylenediamine, p-xylylenediamine, or a mixture of two thereof in any ratio; the acyl chloride monomer is any one of trimesoyl chloride, isophthaloyl chloride, terephthaloyl chloride and phthaloyl chloride, or a mixture of two thereof in any ratio;

[0018] The fluorinated reagent monomer is one or a mixture of two thereof in any ratio of 4,4''-bis(4,4,5,5,6,6,6-heptafluoro-1,3-dioxanyl)-o-terphenyl-4-sulfonyl chloride and 4'-(trifluoromethyl)-[1,1'-biphenyl]-3-sulfonyl chloride.

[0019] Preferably, in the step S1, the mass concentration of the amine monomer in the aqueous solution of the amine monomer is 0.01% to 5.0%, and the immersion time is 1 min to 30 min.

[0020] Preferably, in the step S2, the mass concentration of the acyl chloride monomer in the organic solvent solution of the acyl chloride monomer is 0.01% to 5.0%, and the immersion time is 1 min to 30 min.

[0021] Preferably, in the step S3, the mass concentration of the acyl chloride monomer in the fluorination reagent is 0.01% to 5.0%, and the immersion time is 1 min to 30 min.

[0022] Preferably, in the step S3, the temperature for drying is 20℃ to 70℃, the relative humidity is 20% to 80%, and the drying time is 0.5h to 3h.

[0023] Preferably, the material of the microporous membrane is one or more of polyvinylidene fluoride, polytetrafluoroethylene, polysulfone, and polyethersulfone.

[0024] Preferably, the organic solvent is one or a mixture of two in any ratio of tetrahydrofuran, dichloromethane, chloroform, N,N-dimethylformamide, dimethyl sulfoxide, toluene, and ethyl acetate.

[0025] Preferably, the solvent of the fluorination reagent is one or a mixture of two in any ratio of trichloromethane, carbon tetrachloride, benzene, toluene, ethyl acetate, cyclohexane, n-heptane, and dioxane.

[0026] The present application adopts polyamines and polyacyl chlorides as polymerization monomers, introduces controllable "structural defects" in the polyamide crosslinked network by controlling the monomer reaction ratio in the process of interfacial polymerization, forms active sites containing free amino groups. Then, a high-activity fluorination reagent containing a biphenyl rigid skeleton and a strong electron-withdrawing polyfluoroalkyl-sulfonyl chloride bifunctional group is selected to undergo acylation reaction with the free amino groups on the surface of the polyamide, and the grafting density is controlled by adjusting the temperature and the concentration of the fluorination reagent, to obtain the fluorinated polyamide anti-fouling, chlorine-resistant nanofiltration membrane. The prepared fluorinated sodium filter membrane has a magnesium sulfate rejection rate of 97.9%, and after 16000ppm·h of chlorine treatment, the salt rejection rate only decreases by 0.2%. At the same time, the flux of the fluorinated sodium filter membrane can reach 44L·m 2 ·bar -1 ·h -1 above. In addition, after two cycles of pollution, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin is more than 96%. These results show that the prepared grafted nanofiltration membrane has excellent chlorine resistance and high flux, and its anti-pollution performance is also enhanced.

[0027] Compared with the background art, the present application has the beneficial effects that:

[0028] Currently, most of the modification strategies for nanofiltration membranes often have difficulty in achieving high antifouling property and high mass transfer performance at the same time, and the improvement of rejection capacity is often accompanied by the decrease of permeation flux. The fluorinated polyamide nanofiltration membrane prepared by the present application breaks through the performance contradiction bottleneck: firstly, the fluorinated reagent grafted on the surface builds a physical barrier to repel pollutants by virtue of extremely low surface energy, significantly weakens the interaction between the pollutants and the membrane surface, and introduces more benzene ring structure units, which realizes the effective expansion of electron cloud density by forming a conjugated π electron system, and significantly improves the chlorine oxidation resistance of the nanofiltration membrane by virtue of the electron delocalization mediated by the conjugation effect, and the flux recovery rate is more than 96%; secondly, the clever use of defect engineering builds an efficient mass transfer network in the polyamide membrane, which not only optimizes the water molecule transmission path across the membrane and improves the permeation performance of the membrane, but also realizes the excellent rejection effect of organic dyes by precisely regulating the pore size and surface charge distribution. Through the dual synergy of surface modification and internal structure regulation, the traditional modification limitations are broken, the antifouling, chlorine resistance and flux are synergistically optimized, and the comprehensive performance of the nanofiltration membrane is greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 Morphology change diagram of the composite nanofiltration membrane prepared in Example 1 before and after chlorine erosion;

[0030] Figure 2 Morphology change diagram of the composite nanofiltration membrane prepared in Comparative Example 1 before and after chlorine erosion. DETAILED DESCRIPTION

[0031] The technical solutions of the present application will be described clearly and completely below in combination with the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0032] The test methods used in the embodiments of the present application are conventional methods unless otherwise specified; the materials, reagents, etc. used are reagents and materials available from commercial channels unless otherwise specified.

[0033] Example 1

[0034] (1) Water phase immersion: immerse the polytetrafluoroethylene flat membrane in a 0.5% mass concentration of piperazine aqueous solution, soak for 10 min, take out and dry in air, to obtain a first intermediate membrane;

[0035] (2) Oil phase immersion: immerse the first intermediate membrane in step (1) in a 0.01% mass concentration of trimesyl chloride in n-hexane solution, soak for 3 min, take out and dry in air, to obtain a second intermediate membrane;

[0036] (3) Graft modification: the second intermediate film of step (2) was immersed in a 0.025% mass concentration of 4,4"-bis(4,4,5,5,6,6,6-heptafluoro-1,3-dioxanyl)-o-terphenyl-4-sulfonyl chloride cyclohexane solution for 10 min, taken out and air dried to obtain a third intermediate film;

[0037] (4) Heat treatment: the third intermediate film of step (3) was washed with deionized water and then placed in an oven with a temperature of 80°C and a relative humidity of 80% for 15 min to obtain a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane prepared by providing grafting sites through defect engineering.

[0038] The prepared grafted nanofiltration membrane had a flux of 44.3 L·m 2 ·bar -1 ·h -1 at 25°C and 0.5 MPa. The rejection rate for MgS04was 97.5%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, taken out, and subjected to cross-flow filtration testing at 25°C and 0.4 MPa. The rejection rate for MgS04was 97.2%. In addition, the flux recovery rate of the grafted nanofiltration membrane for bovine serum albumin after two cycles of pollution was 99%.

[0039] Example 2:

[0040] (1) Water phase immersion: a polyvinylidene fluoride flat membrane was immersed in a 0.3% mass concentration of m-phenylenediamine aqueous solution for 3 min, taken out and air dried to obtain a first intermediate film.

[0041] (2) Oil phase immersion: the first intermediate film of step (1) was immersed in a 0.05% mass concentration of trimesoyl chloride in n-hexane solution for 2 min, taken out and air dried to obtain a second intermediate film.

[0042] (3) Graft modification: the second intermediate film of step (2) was immersed in a 0.035% mass concentration of 4'-(trifluoromethyl)-[1,1'-biphenyl]-3-sulfonyl chloride in benzene solution for 5 min, taken out and air dried to obtain a third intermediate film.

[0043] (4) Heat treatment: the third intermediate film of step (3) was washed with deionized water and then placed in an oven with a temperature of 50°C and a relative humidity of 50% for 10 min to obtain a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane prepared by providing grafting sites through defect engineering.

[0044] Cross-flow filtration test was carried out at 25℃, 0.5 MPa, and the prepared grafted nanofiltration membrane had a flux of 36.7 L·m 2 ·bar -1 ·h -1 The rejection rate of MgSO4 was 99.1%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, and then taken out, and cross-flow filtration test was carried out at 25℃, 0.4 MPa, and the rejection rate of MgSO4 was 98.5%, which was mainly due to the buffering effect of the conjugated system formed by the benzene ring in the fluorinated reagent containing a biphenyl conjugated skeleton and the polyamide on the chlorine attack. In addition, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin after two pollution cycles was 98.7%.

[0045] Example 3:

[0046] (1) Water phase immersion: immerse the polysulfone flat membrane in a 0.3% mass concentration p-phenylenediamine aqueous solution, soak for 20 min, take out and air dry to obtain a first intermediate membrane;

[0047] (2) Oil phase immersion: immerse the first intermediate membrane of step (1) in a 0.2% mass concentration p-phthaloyl chloride and trimesoyl chloride n-hexane solution with a mass ratio of 2:8, soak for 3 min, take out and air dry to obtain a second intermediate membrane;

[0048] (3) Graft modification: immerse the second intermediate membrane of step (2) in a 0.01% mass concentration 4'-(trifluoromethyl)-[1,1'-biphenyl]-3-sulfonyl chloride and 4,4"-bis(4,4,5,5,6,6,6-heptafluoro-1,3-dioxanyl)-o-terphenyl-4-sulfonyl chloride toluene solution, soak for 10 min, take out and dry to obtain a third intermediate membrane;

[0049] (4) Heat treatment: rinse the third intermediate membrane of step (3) with deionized water, then place it in an oven with a temperature of 80℃ and a relative humidity of 40%, and stay for 10 min to obtain a fluorinated polyamide antifouling and chlorine-resistant nanofiltration membrane prepared by providing grafting sites through defect engineering.

[0050] Cross-flow filtration test was carried out at 25℃, 0.5 MPa, and the prepared grafted nanofiltration membrane had a flux of 40.7 L·m 2 ·bar -1 ·h -1 The rejection rate of MgSO4 was 97.5%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, and then taken out, and cross-flow filtration test was carried out at 25℃, 0.4 MPa, and the rejection rate of MgSO4 was 97.0%. In addition, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin after three pollution cycles was 96.8%.

[0051] Comparative Example 1:

[0052] (1) Water phase immersion: polyvinylidene fluoride flat membrane was immersed in a 0.5% mass concentration of piperazine aqueous solution for 10 min, taken out and dried in air to obtain a first intermediate membrane;

[0053] (2) Oil phase immersion: the first intermediate membrane of step (1) was immersed in a 0.01% mass concentration of trimesyl chloride in n-hexane solution for 3 min, taken out and dried in air to obtain a second intermediate membrane;

[0054] (3) Heat treatment: the second intermediate membrane of step (2) was washed with deionized water, and then placed in an oven at 80°C and 80% relative humidity for 15 min to obtain a control membrane;

[0055] The prepared control nanofiltration membrane had a flux of 33.2 L·m 2 ·bar -1 ·h -1 , and the rejection rate of MgS04was 97.2%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, and then taken out and subjected to cross-flow filtration test at 25°C and 0.4 MPa. The rejection rate of MgS04was 77.1%. In addition, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin after three cycles of pollution was 65.3%.

[0056] Comparative Example 2:

[0057] (1) Water phase immersion: polyether sulfone flat membrane was immersed in a 0.3% mass concentration of ethylenediamine aqueous solution for 5 min, taken out and dried in air to obtain a first intermediate membrane;

[0058] (2) Oil phase immersion: the first intermediate membrane of step (1) was immersed in a 0.05% mass concentration of isophthaloyl chloride in n-hexane solution for 3 min, taken out and dried to obtain a second intermediate membrane;

[0059] (3) Modification treatment: the second intermediate membrane of step (2) was immersed in a 0.02% mass concentration of benzenesulfonyl chloride in toluene solution for 5 min, taken out and dried to obtain a modified membrane;

[0060] (4) Heat treatment: the modified membrane was washed with deionized water, and then placed in an oven at 70°C and 50% relative humidity for 15 min to obtain a nanofiltration membrane of Comparative Example 2.

[0061] The cross-flow filtration test was carried out at 25°C and 0.5 MPa, and the flux of the prepared control nanofiltration membrane was 30.5 L·m2·bar-1·h-1, and the rejection rate to MgSO4 was 95.0%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, and then taken out, and the cross-flow filtration test was carried out at 25°C and 0.4 MPa, and the rejection rate to MgSO4 was 65.2%. In addition, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin was 72.0% after three pollution cycles.

[0062] Comparative Example 3:

[0063] (1) Water phase immersion: immerse the polyvinylidene fluoride flat membrane in a 0.5% mass concentration of triethylamine aqueous solution, soak for 10 min, take out and dry, to obtain a first intermediate membrane;

[0064] (2) Oil phase immersion: immerse the first intermediate membrane in step (1) in a 0.1% mass concentration of terephthaloyl chloride in n-hexane solution, soak for 2 min, take out and dry, to obtain a second intermediate membrane;

[0065] (3) Modification treatment: after washing the second intermediate membrane in step (2) with deionized water, immerse it in a 0.03% mass concentration of 4-chlorobiphenylsulfonyl chloride in dichloromethane solution, stay for 10 min, take out and dry, to obtain a modified membrane;

[0066] (4) Heat treatment: after washing the modified membrane with deionized water, place it in an oven at 80°C and 60% relative humidity, dry for 20 min, to obtain the nanofiltration membrane of Comparative Example 3.

[0067] The cross-flow filtration test was carried out at 25°C and 0.5 MPa, and the flux of the prepared control nanofiltration membrane was 24.1 L·m 2 ·bar -1 ·h -1 , and the rejection rate to MgSO4 was 96.1%. Then, the membrane was immersed in a 1000 ppm NaClO solution for 16 h, and then taken out, and the cross-flow filtration test was carried out at 25°C and 0.4 MPa, and the rejection rate to MgSO4 was 81.5%. In addition, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin (BSA) was 68.4% after three pollution cycles.

[0068] Comparative Example 4:

[0069] (1) Water phase immersion: immerse the polysulfone flat membrane in a 0.2% mass concentration of o-phenylenediamine aqueous solution, soak for 15 min, take out and dry, to obtain a first intermediate membrane;

[0070] (2) Oil phase immersion: the first intermediate film described in step (1) was immersed in a 0.05% mass concentration of isophthaloyl dichloride n-hexane solution for 3 min, taken out and dried to obtain a second intermediate film;

[0071] (3) Modification treatment: after the second intermediate film described in step (2) was washed with deionized water, it was immersed in a 0.02% mass concentration of perfluorooctyltriethoxysilane (FAS-17) ethanol solution at room temperature for 20 min to obtain a modified film;

[0072] (4) Heat treatment: after the modified film was washed with deionized water, it was placed in an oven at 60°C and 40% relative humidity for drying for 30 min to obtain the nanofiltration membrane of Comparative Example 4.

[0073] The prepared control nanofiltration membrane had a flux of 17.8 L·m 2 ·bar -1 ·h -1 , and a rejection rate of 95.5% for MgSO4. Then, after the membrane was immersed in a 1000 ppm NaClO solution for 16 h and taken out, cross-flow filtration test was carried out at 25°C and 0.4 MPa, and the rejection rate of MgSO4 was 89.0%. In addition, after three cycles of pollution, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin (BSA) was 75.5%.

[0074] Comparative Example 5:

[0075] (1) Water phase immersion: a polytetrafluoroethylene flat film was immersed in a 0.3% mass concentration of p-phenylenediamine aqueous solution for 20 min, taken out and dried to obtain a first intermediate film;

[0076] (2) Oil phase immersion: the first intermediate film described in step (1) was immersed in a 0.05% mass concentration of isophthaloyl dichloride n-hexane solution for 3 min, taken out and dried to obtain a second intermediate film;

[0077] (3) Modification treatment: after the second intermediate film described in step (2) was washed with deionized water, it was immersed in a 0.02% mass concentration of perfluorooctyltriethoxysilane (FAS-17) ethanol solution at room temperature for 20 min to obtain a modified film;

[0078] (4) Heat treatment: after the modified film was washed with deionized water, it was placed in an oven at 60°C and 40% relative humidity for drying for 30 min to obtain the nanofiltration membrane of Comparative Example 4.

[0079] The prepared control nanofiltration membrane has a flux of 35.4 L·m-2·h-1 under cross-flow filtration test at 25 DEG C and 0.5 MPa 2 ·bar -1 ·h -1 The rejection rate of MgSO4 is 92.0%. Then, the membrane is taken out after being soaked in a 1000 ppm NaClO solution for 16 h, and the cross-flow filtration test is carried out at 25 DEG C and 0.4 MPa, and the rejection rate of MgSO4 is 87.3%. In addition, after three cycles of pollution, the flux recovery rate of the grafted nanofiltration membrane to bovine serum albumin (BSA) is 78.2%.

[0080] Figure 1 The surface morphology of the composite nanofiltration membrane prepared in Example 1 is basically consistent before and after chlorine erosion, and the particle structure is clear and arranged densely, which indicates that the membrane structure is stable. It can be found from the comparison of the electron microscope photos that the morphology is basically unchanged after the attack of strong active chlorine. The strong chlorine resistance is exhibited. Figure 2 After the composite nanofiltration membrane prepared in Comparative Example 1 is eroded by chlorine, the surface is obviously smoothed, the particle boundary is blurred, and some areas are collapsed or damaged, which indicates that the chlorine resistance is poor.

[0081] It can be known from Comparative Examples 2 to 5 that after the replacement of different fluorinated reagents, the initial nanofiltration membrane flux and the rejection rate of MgSO4 are affected to a certain extent, especially after the treatment of active chlorine, the stability is greatly affected, and the rejection rate of MgSO4 and the flux recovery rate of bovine serum albumin (BSA) are significantly adversely affected after the soaking of active chlorine. It can be seen that the composite nanofiltration membrane prepared in the present application can still maintain a stable structure in a strong chlorine environment, and has more excellent chlorine resistance and use stability than the membrane of the comparative example.

[0082] In summary, it can be seen from the analysis of the above examples and comparative examples that the fluorinated grafted nanofiltration membrane prepared in the present application has excellent chlorine resistance and high flux, and its anti-pollution performance is also enhanced. The present application solves the technical bottleneck of poor chlorine resistance and anti-pollution of the existing nanofiltration membrane, and overcomes the problem of flux decline caused by the addition of fluorine element, which greatly promotes the wide application of nanofiltration membrane. In addition, the present application is simple to prepare, easy to operate, and has high production repeatability.

[0083] The above specific embodiments only further illustrate the purpose, technical solutions and beneficial effects of the present application, and it should be understood that the above description is only a specific example of the present application and is not used to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering, characterized in that, The method comprises the following steps: S1, using water phase immersion method, immersing the microporous membrane in the aqueous solution of amine monomer, and then drying to obtain a primary composite membrane; S2, using oil phase immersion method, immersing the primary composite membrane in the organic solvent solution of acyl chloride monomer, and then drying to obtain a secondary composite membrane; S3, immersing the secondary composite membrane in a fluorination reagent, and then drying to obtain the fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane. The fluorination reagent is one or a mixture of two in any proportion of 4,4''-bis(4,4,5,5,6,6,6-heptafluoro-1,3-dioxanyl)-o-terphenyl-4-sulfonyl chloride and 4'-(trifluoromethyl)-[1,1'-biphenyl]-3-sulfonyl chloride.

2. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, In step S1, the mass concentration of the amine monomer in the aqueous solution of the amine monomer is 0.01% to 5.0%, and the immersion time is 1 min to 30 min.

3. The method for preparing a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering according to claim 2, wherein the amine monomer is any one or a mixture of two in any proportion of piperazine, p-phenylenediamine, m-phenylenediamine, o-phenylenediamine, ethylenediamine, hexanediamine, triethylamine, o-xylylenediamine, m-xylylenediamine, and p-xylylenediamine.

4. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, In step S2, the mass concentration of the acyl chloride monomer in the organic solvent solution of the acyl chloride monomer is 0.01% to 5.0%, the acyl chloride monomer is any one or a mixture of two in any proportion of trimesoyl chloride, isophthaloyl chloride, terephthaloyl chloride, and phthaloyl chloride, and the immersion time is 1 min to 30 min.

5. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, In step S3, the drying temperature is 20°C to 70°C, the relative humidity is 20% to 80%, and the drying time is 0.5 h to 3 h.

6. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, The material of the microporous membrane is one or more of polyvinylidene fluoride, polytetrafluoroethylene, polysulfone, and polyethersulfone.

7. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, In step S2, the organic solvent is one or a mixture of two in any proportion of tetrahydrofuran, dichloromethane, chloroform, N,N-dimethylformamide, dimethyl sulfoxide, toluene, and ethyl acetate.

8. The method of claim 1, wherein the method of preparing a fluorinated polyamide antifouling, chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering is characterized by, In step S3, the solvent of the fluorination reagent is one or a mixture of two in any proportion of trichloromethane, carbon tetrachloride, benzene, toluene, ethyl acetate, cyclohexane, n-heptane, and dioxane.

9. A nanofiltration membrane prepared by the method for preparing a fluorinated polyamide anti-fouling and chlorine-resistant nanofiltration membrane by providing grafting sites through defect engineering according to claim 1.

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