Preparation method of composite nanofiltration membrane

Abstract

The invention discloses a preparation method of a composite nanofiltration membrane, and belongs to the technical field of filter membranes, and the preparation method comprises the following steps: synthesizing functionalized chitosan containing carbon-carbon double bonds, dissolving chitosan in an acetic acid solution, adding a double bond-containing epoxy monomer into the solution, mechanically stirring, and washing and precipitating in an ice bath to obtain the functionalized chitosan; preparing a porous supporting layer by adopting a non-solvent induced phase separation method; and carrying out interfacial polymerization-click chemical reaction on the porous support layer to prepare the thin composite layer with a semi-interpenetrating network structure. According to the invention, interfacial polymerization and sulfydryl-olefin click chemistry are combined, an ultraviolet light source is added for irradiation in the interfacial polymerization process, no extra equipment requirement is required, the synthesis method is simple, the cost is low, batch and large-scale production can be realized, sulfydryl compounds with different chain lengths and functionalities are adopted as the cross-linking agent, controllable adjustment of the cross-linking density is realized, and the preparation method is suitable for industrial production. Therefore, composite membranes with different fluxes and molecular weight cutoff are obtained, and more separation requirements are met.

Description

Technical Field

[0001] The present invention belongs to the technical field of separation membranes, and particularly relates to a preparation method of a composite nanofiltration membrane. Background Art

[0002] Most commercial dyes are difficult to degrade due to their complex aromatic structures and are one of the most challenging organic pollutants in the field of industrial wastewater treatment. Compared with traditional separation methods, membrane separation, as a new and efficient water treatment technology, has the advantages of low preparation cost and high separation efficiency and plays an important role in the printing and dyeing wastewater treatment industry. Exploring the relationship between the structure and performance of membrane materials and developing new polymer materials to prepare separation membranes with excellent performance are important theoretical bases for the application and development of membrane separation technology in the field of printing and dyeing wastewater treatment.

[0003] Interfacial polymerization in an immiscible two-liquid phase system is an ingenious method for developing separation membranes. Its reaction area is limited, and the controllable diffusion of reaction monomers results in defect-free membranes. A typical polyamide membrane is prepared by the polycondensation reaction of an amine and an acyl chloride at the water-oil interface. Almost all current technological improvements focus on the performance improvement of polyamide membranes, and other possible reaction types occurring at the interface rarely attract attention.

[0004] Under ultraviolet light irradiation, free radical-initiated thiol-ene click chemistry has become an important way to efficiently and controllably prepare crosslinked polymer networks due to the high reactivity of thiols and mild reaction conditions. Zhu et al. used thiol-ene / yne addition polymerization to react at the water-oil interface on a PES support layer and prepared polyether sulfide ultrathin membranes for ion separation using commercial multi-functional thiols and alkyne monomers as raw materials. These membranes have good separation performance and acid tolerance. The rejection rate of Na2SO4 is 95.09%, and it only decreases by 0.96% after being soaked in acid for 15 days (Dianyu Dong, Yuzhang Zhu, Wangxi Fang, and Jian Jin. “Clickable” Interfacial Polymerization of Polythioether Ultrathin Membranes for Ion Separation [J]. Macromolecules, 2023, 56, 7132-7141). However, the limited availability of commercially available monomers restricts the possibility of further development.

[0005] Chitosan (CS), a natural polymer with abundant amino and hydroxyl functional groups, has been used in the research of composite membranes through physical methods such as blending and coating due to its good film-forming ability, hydrophilicity, easy processability, environmental friendliness, low cost, and availability. However, there are still few methods that focus on functionalizing it by chemical methods and constructing crosslinked network polymers. Summary of the Invention

[0006] To solve the above problems, the present invention aims to provide a method for preparing a composite nanofiltration membrane.

[0007] To achieve the above object, the present invention adopts the following technical solutions: A method for preparing a composite nanofiltration membrane, comprising the following steps:

[0008] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve chitosan in an acetic acid solution, and add an epoxy monomer containing double bonds to the solution. After mechanical stirring, wash the precipitate in an ice bath to obtain functionalized chitosan.

[0009] Step 2: Prepare a porous support layer by non-solvent induced phase separation. Vacuum dry the polymer raw material to remove water, dissolve the dried raw material in an organic solvent, stir at 150 °C for 3 - 5 h until completely dissolved, and let stand to remove air bubbles; coat a slit on the non-woven fabric, and form a base membrane through liquid-solid phase transformation. Immerse the obtained polymer base membrane in deionized water for later use.

[0010] Step 3: Perform interfacial polymerization-click chemical reaction on the porous support layer to prepare a thin composite layer with a semi-interpenetrating network structure.

[0011] Step 31: Dissolve the functionalized chitosan, photoinitiator, and linear polymer from Step 1 in an aqueous solution.

[0012] Step 32: Then immerse the polymer base membrane obtained in Step 2 in an organic solution of a mercapto compound with two or more functional groups, irradiate with ultraviolet light, and wash the preformed membrane with an organic solvent to remove the unreacted mercapto compound.

[0013] Step 33: Finally, perform heat treatment to complete the cross-linking reaction.

[0014] Preferably, Step 1 includes the following steps:

[0015] Step 11: Dissolve chitosan in an acetic acid solution, add a pH regulator to adjust the pH to 4, and add an epoxy monomer containing double bonds to the solution.

[0016] Step 12: Then, under an inert gas atmosphere, mechanically stir and react the mixture obtained in Step 11, and cool in an ice bath.

[0017] Step 13: Pour the cooled solution into an excess of acetonitrile to precipitate, wash the precipitate several times with an organic solvent, and vacuum dry at room temperature to obtain functionalized chitosan.

[0018] Preferably, the concentration of the acetic acid solution in Step 11 is 0.4 M; the concentration of the pH regulator is 0.05 M, and the addition amount is 5% of the total reaction system mixture.

[0019] The double bond-containing epoxy monomer is one or more of allyl glycidyl ether, 4-vinylbenzyl glycidyl ether, 4-hydroxybutyl acrylate glycidyl ether, glycidyl methacrylate, 3,4-epoxycyclohexylmethyl methacrylate, glycidyl acrylate, 1,2-epoxy-5-hexene, and 1,2-epoxy-7-octene.

[0020] Preferably, the inert gas in step 12 is argon or nitrogen, the reaction temperature is 60 °C, the reaction time is ≤2 h, and the ice bath time is ≥15 min.

[0021] Preferably, the volume ratio of acetonitrile to the product-containing solution in step 13 is at least 8:1, and the organic solvent is acetone or tetrahydrofuran.

[0022] Preferably, the polymer in step 2 is any one of polyethersulfone, polyester, polyacrylonitrile, and polyimide, and the mass fraction is 15% - 28%;

[0023] The organic solvent is any one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

[0024] Preferably, in step 2, the phase inversion time is 0.5 - 1 min, the water bath temperature is 15 - 20 °C, the thermal curing water bath temperature is 70 - 85 °C, and the total thickness of the base film including non-woven fabric is 5.0 - 6.0 mil.

[0025] Preferably, after the base film obtained in step 31 is immersed in the aqueous solution for a period of time, the aqueous solution is removed until there are no visible water spots on the surface; the ultraviolet light in step 32 is 365 nm, and the irradiation time is 2 - 20 min.

[0026] Preferably, the photoinitiator in step 3 is any one or two of benzoin and its derivatives, benzil derivatives, dialkoxyacetophenone, ɑ-hydroxyalkylphenone, and acylphosphine oxides;

[0027] The linear polymer is any one of polyethylene oxide (PEO) and polyvinyl alcohol (PVA);

[0028] The difunctional mercapto compound is SH-(CH2)n-SH, where n is any repeating unit number greater than or equal to 2, dithiothreitol, or bis(3-mercaptopropionic acid) ethylene ester, and the polyfunctional mercapto compound is any one of pentaerythritol tetrakis(mercaptoacetate), pentaerythritol tetra-3-mercaptopropionate, and trimethylolpropane tris(3-mercaptopropionate);

[0029] The organic solvent is one of ethanol, ethyl acetate, benzene, xylene, etc.

[0030] Preferably, in step 3, the mass fraction of functionalized chitosan in the aqueous solution is 0.5-2%, the mass fraction of the photoinitiator is 0.2-0.4%, and the mass fraction of the linear polymer is 0.05-0.2%. The treatment time of the base film in the aqueous solution is 0.5-20 min; the mass fraction of the thiol compound in the organic phase solution in the ethyl acetate solution is 0.05-3%; finally, the heat treatment temperature is 50-70 °C and the time is 10-60 min.

[0031] Compared with the prior art, the present invention has the following advantages:

[0032] 1. Due to the strong hydrogen bond interaction between chitosan derivative molecules and between chitosan derivatives and polyvinyl alcohol molecules, physical entanglement can be achieved between polymer chains through non-covalent bonds, further preventing the dissolution of the polymer and inhibiting swelling.

[0033] 2. Due to the unreacted protonated amino groups of chitosan, the surface of the prepared composite film is positively charged and can adsorb anionic dyes with negative charges.

[0034] 3. The present invention uses thiol compounds with different chain lengths and functionalities as crosslinking agents to achieve controllable adjustment of the crosslinking density, thereby obtaining composite membranes with different fluxes and cut-off molecular weights to meet more separation requirements.

[0035] 4. The present invention combines interfacial polymerization and thiol-ene click chemistry. Except for adding an ultraviolet light source for irradiation during the interfacial polymerization process, there are no additional equipment requirements, and the synthesis method is simple, with low cost, and can be mass-produced in batches. Description of the Drawings

[0036] In order to more clearly illustrate the technical solutions in the specific embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following described drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0037] Figure 1 It is a schematic diagram of the semi-interpenetrating network structure prepared in Example 1 of the present invention. Detailed Embodiments

[0038] The following will further illustrate the present invention in conjunction with the drawings and specific embodiments. However, it should not be understood that the scope of the subject matter of the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, all modifications, substitutions, and changes made according to ordinary technical knowledge and customary means in the art are included in the scope of the present invention.

[0039] The present invention relates to a method for preparing a composite nanofiltration membrane, and the method comprises the following steps:

[0040] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve chitosan in an acetic acid solution, add a pH regulator to adjust the pH of the chitosan acetic acid solution to be approximately equal to 4, and add an epoxy monomer containing double bonds to the solution. Then, react the mixture in an inert gas atmosphere under mechanical stirring at 60 °C for a period of time, and cool it in an ice bath for a period of time to prevent further reaction. Pour the solution into an excessive amount of acetonitrile for precipitation, wash the precipitate several times with an organic solvent, and dry it under vacuum at room temperature to obtain functionalized chitosan.

[0041] The concentration of the acetic acid solution is 0.4 M; the concentration of the pH regulator is 0.05 M, and the addition amount is approximately 5% of the total reaction system mixture.

[0042] In this step, the epoxy monomer containing double bonds is one or more of allyl glycidyl ether, 4-vinylbenzyl glycidyl ether, 4-hydroxybutyl acrylate glycidyl ether, glycidyl methacrylate, 3,4-epoxycyclohexylmethyl methacrylate, glycidyl acrylate, 1,2-epoxy-5-hexene, 1,2-epoxy-7-octene. The molar ratio of chitosan to the epoxy monomer containing double bonds is 1:1 to 1:4.

[0043] The above-mentioned inert gas is argon or nitrogen, the reaction temperature is generally 60 °C, the reaction time is generally not less than 2 hours, the ice bath time is generally not less than 15 min, and the volume ratio of acetonitrile to the solution containing the product is at least 8:1.

[0044] The above-mentioned organic solvent is acetone or tetrahydrofuran; the pH regulator is an aqueous solution of any one of sodium hydroxide, potassium hydroxide, weak base triethylamine, pyridine, sodium carbonate, sodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, sodium carbonate, sodium bicarbonate, potassium carbonate, sodium citrate, potassium citrate, etc.

[0045] Step 2: Prepare a porous support layer by non-solvent induced phase separation method. First, vacuum dry the polymer raw material to remove water, and then dissolve the dried raw material in an organic solvent, stir at 150 °C for 3 - 5 h until completely dissolved, and let it stand to remove bubbles; perform slit coating on the non-woven fabric, and form a base film through liquid-solid phase transformation, and soak the obtained polymer base film in deionized water for use.

[0046] The above-mentioned polymer can be any one of polyethersulfone, polyester, polyacrylonitrile, polyimide, and the mass fraction is 15% - 28%. The organic solvent used is any one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), N-methylpyrrolidone (NMP).

[0047] The phase inversion time is controlled within 0.5 - 1 min, the water bath temperature is 15 - 20 °C, the thermal curing water bath temperature is 70 - 85 °C, and the total thickness of the base film including the non-woven fabric is controlled within 5.0 - 6.0 mil.

[0048] Step 3: Perform interfacial polymerization-click chemistry reaction on the porous support layer to prepare a thin composite layer with a semi-interpenetrating network structure. Dissolve functionalized chitosan, photoinitiator, and linear polymer in an aqueous solution. After the base film is immersed in the aqueous solution for a period of time, remove the aqueous solution until there are no visible water spots on the surface, and then immerse the base film in an organic solution of a mercapto compound with two or more functional groups, while exposing it to ultraviolet light (365 nm) for a certain period of time. Gently wash the preformed film with an organic solvent to remove the unreacted mercapto compound, and then perform heat treatment to complete the cross-linking reaction. Finally, store the obtained film in deionized water for the next step.

[0049] In this step, the photoinitiator is any one or two of benzoin and its derivatives, benzil derivatives, dialkoxyacetophenone, ɑ-hydroxyalkylphenone, acylphosphine oxides; the linear polymer is any one of polyethylene oxide (PEO) and polyvinyl alcohol (PVA); the difunctional mercapto compound is SH-(CH2)n-SH (where n is any repeating unit number greater than or equal to 2), dithiothreitol, ethylene glycol bis(3-mercaptopropionate); the polyfunctional mercapto compound is any one of pentaerythritol tetrakis(mercaptoacetate), pentaerythritol tetra-3-mercaptopropionate, trimethylolpropane tris(3-mercaptopropionate); the organic solvent is one of ethanol, ethyl acetate, benzene, xylene, etc.

[0050] In the above aqueous solution, the mass fraction of functionalized chitosan is 0.5 - 2%, the mass fraction of the photoinitiator is 0.2 - 0.4%, and the mass fraction of the linear polymer is 0.05 - 0.2%. The treatment time of the base film in the aqueous solution can be 0.5 - 20 min, and the optimal is 3 - 10 min.

[0051] In the above organic phase solution, the mass fraction of the mercapto compound in the ethyl acetate solution is preferably 0.05 - 3%. The exposure time under ultraviolet light is 2 - 20 min, the heat treatment temperature is 50 - 70 °C, preferably 60 °C, and the time is 10 - 60 min, preferably 30 min.

[0052] Example 1:

[0053] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve 5 g of chitosan in an aqueous solution of 0.4 M acetic acid, add an aqueous solution of sodium hydroxide to adjust the pH to approximately 4, and add 18.6 g of allyl glycidyl ether to this solution. Then react the mixture under mechanical stirring at 60 °C in a nitrogen atmosphere for 8 h. After the reaction is completed, pour the solution into an excess of acetonitrile to obtain a light yellow precipitate, wash the precipitate three times with acetone and vacuum dry it at room temperature for 4 h to obtain functionalized chitosan.

[0054] Step 2: Prepare a porous support layer by non-solvent induced phase separation method. First, vacuum dry the polyethersulfone raw material to remove water. Then, dissolve the dried polyethersulfone in DMF and stir at 150 °C for 3 - 5 h until completely dissolved, and let it stand to remove air bubbles; perform slit coating on the non-woven fabric, and form a base film through liquid-solid phase transformation. The phase transformation time is controlled at 0.5 min, the water bath temperature is 18 °C, the heat curing water bath temperature is 80 °C, and the film thickness is controlled at 5.5 mil; soak the obtained polymer base film in deionized water for later use.

[0055] Step 3: Perform interfacial polymerization-click chemical reaction on the porous support layer to prepare a thin composite layer with a semi-interpenetrating network structure. Dissolve functionalized chitosan, photoinitiator 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone), and polyvinyl alcohol in an aqueous solution. After the base film is immersed in the aqueous solution for a period of time, remove the aqueous solution until there are no visible water spots on the surface. Then immerse the base film in an ethyl acetate solution of 1,2-ethanedithiol and expose it to ultraviolet light (365 nm) for 1 min. Gently wash the preformed film with ethyl acetate to remove unreacted 1,2-ethanedithiol, and then heat treat it at 60 °C for 30 min to complete the cross-linking reaction. Finally, store the obtained film in deionized water for the next step of use, denoted as CS-2C2SH.

[0056] Example 2:

[0057] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve 5 g of chitosan in an aqueous solution of 0.4 M acetic acid, add an aqueous solution of sodium hydroxide to adjust the pH to approximately 4, and add 18.6 g of allyl glycidyl ether to this solution. Then react the mixture under mechanical stirring at 60 °C in a nitrogen atmosphere for 8 h. After the reaction is completed, pour the solution into an excess of acetonitrile to obtain a light yellow precipitate, wash the precipitate three times with acetone and vacuum dry it at room temperature for 4 h to obtain functionalized chitosan.

[0058] Step 2: Preparation of the porous support layer by non-solvent induced phase separation. First, the polyethersulfone raw material was dried under vacuum to remove water. Then, the dried polyethersulfone was dissolved in DMF and stirred at 150 °C for 3 - 5 h until completely dissolved, and left standing to remove air bubbles. It was slit-coated on the non-woven fabric and a base membrane was formed through liquid-solid phase transformation. The phase transformation time was controlled at 0.5 min, the water bath temperature was 18 °C, the thermal curing water bath temperature was 80 °C, and the membrane thickness was controlled at 5.5 mil. The obtained polymer base membrane was immersed in deionized water for later use.

[0059] Step 3: Interfacial polymerization-click chemical reaction was carried out on the porous support layer to prepare a thin-film composite layer with a semi-interpenetrating network structure. Functionalized chitosan, photoinitiator 1173, and polyvinyl alcohol were dissolved in an aqueous solution. After the base membrane was immersed in the aqueous solution for a period of time, the aqueous solution was removed until there were no visible water spots on the surface. Then the base membrane was immersed in an ethyl acetate solution of 1,4-butanedithiol and exposed to ultraviolet light (365 nm) for irradiation for 15 min. The preformed membrane was gently washed with ethyl acetate to remove unreacted 1,4-butanedithiol, and then heat-treated at 60 °C for 30 min to complete the cross-linking reaction. Finally, the obtained membrane was stored in deionized water for the next step of use, denoted as CS-4C2SH.

[0060] Example 3:

[0061] Step 1: Synthesis of functionalized chitosan containing carbon-carbon double bonds. 5 g of chitosan was dissolved in 0.4 M aqueous acetic acid solution, and sodium hydroxide aqueous solution was added to adjust the pH to about 4. Then 18.6 g of allyl glycidyl ether was added to the solution. Then the mixture was reacted under a nitrogen atmosphere with mechanical stirring at 60 °C for 8 h. After the reaction was completed, the solution was poured into an excess of acetonitrile to obtain a light yellow precipitate. The precipitate was washed three times with acetone and vacuum dried at room temperature for 4 h to obtain functionalized chitosan.

[0062] Step 2: Preparation of the porous support layer by non-solvent induced phase separation. First, the polyethersulfone raw material was dried under vacuum to remove water. Then, the dried polyethersulfone was dissolved in DMF and stirred at 150 °C for 3 - 5 h until completely dissolved, and left standing to remove air bubbles. It was slit-coated on the non-woven fabric and a base membrane was formed through liquid-solid phase transformation. The phase transformation time was controlled at 0.5 min, the water bath temperature was 18 °C, the thermal curing water bath temperature was 80 °C, and the membrane thickness was controlled at 5.5 mil. The obtained polymer base membrane was immersed in deionized water for later use.

[0063] Step 3: Perform interfacial polymerization-click chemistry on the porous support layer to prepare a thin-film composite layer with a semi-interpenetrating network structure. Dissolve functionalized chitosan, photoinitiator 1173, and polyvinyl alcohol in an aqueous solution. After the substrate membrane is immersed in the aqueous solution for a period of time, remove the aqueous solution until there are no visible water spots on the surface. Then immerse the substrate membrane in an ethyl acetate solution of 1,6-hexanedithiol and expose it to ultraviolet light (365 nm) for 15 min. Gently wash the preformed membrane with ethyl acetate to remove unreacted 1,6-hexanedithiol, and then heat-treat it at 60 °C for 30 min to complete the crosslinking reaction. Finally, store the obtained membrane in deionized water for the next step, denoted as CS-6C2SH.

[0064] Example 4:

[0065] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve 5 g of chitosan in 0.4 M aqueous acetic acid solution, add aqueous sodium hydroxide solution to adjust the pH to approximately 4, and add 18.6 g of allyl glycidyl ether to the solution. Then react the mixture under mechanical stirring at 60 °C in a nitrogen atmosphere for 8 h. After the reaction is completed, pour the solution into an excess of acetonitrile to obtain a light yellow precipitate. Wash the precipitate three times with acetone and vacuum dry it at room temperature for 4 h to obtain functionalized chitosan.

[0066] Step 2: Prepare a porous support layer by non-solvent induced phase separation method. First, vacuum dry the polyethersulfone raw material to remove water. Then, dissolve the dried polyethersulfone in DMF and stir it at 150 °C for 3 - 5 h until completely dissolved. Let it stand to remove air bubbles; perform slit coating on the non-woven fabric and form a substrate membrane through liquid-solid phase transformation. Control the phase transformation time to be 0.5 min, the water bath temperature to be 18 °C, the heat curing water bath temperature to be 80 °C, and the membrane thickness to be controlled at 5.5 mil; soak the obtained polymer substrate membrane in deionized water for later use.

[0067] Step 3: Perform interfacial polymerization-click chemistry on the porous support layer to prepare a thin-film composite layer with a semi-interpenetrating network structure. Dissolve functionalized chitosan, photoinitiator 1173, and polyvinyl alcohol in an aqueous solution. After the substrate membrane is immersed in the aqueous solution for a period of time, remove the aqueous solution until there are no visible water spots on the surface. Then immerse the substrate membrane in an ethyl acetate solution of bis(3-mercaptopropionic acid) ethylene ester and expose it to ultraviolet light (365 nm) for 15 min. Gently wash the preformed membrane with ethyl acetate to remove unreacted bis(3-mercaptopropionic acid) ethylene ester, and then heat-treat it at 60 °C for 30 min to complete the crosslinking reaction. Finally, store the obtained membrane in deionized water for the next step, denoted as CS-2ESH.

[0068] Example 5:

[0069] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve 5 g of chitosan in an aqueous solution of 0.4 M acetic acid, add an aqueous sodium hydroxide solution to adjust the pH to approximately 4, and add 18.6 g of allyl glycidyl ether to this solution. Then react the mixture under mechanical stirring at 60 °C in a nitrogen atmosphere for 8 h. After the reaction is completed, pour the solution into an excess of acetonitrile to obtain a light yellow precipitate, wash the precipitate three times with acetone and vacuum dry it at room temperature for 4 h to obtain functionalized chitosan.

[0070] Step 2: Prepare a porous support layer by non-solvent induced phase separation method. First, vacuum dry the polyethersulfone raw material to remove water. Then, dissolve the dried polyethersulfone in DMF and stir at 150 °C for 3 - 5 h until completely dissolved, and let it stand to remove bubbles; perform slot coating on the non-woven fabric, and form a base membrane through liquid-solid phase transformation. Control the phase transformation time to be 0.5 min, the water bath temperature to be 18 °C, the heat curing water bath temperature to be 80 °C, and control the membrane thickness to be 5.5 mil; soak the obtained polymer base membrane in deionized water for later use.

[0071] Step 3: Perform interfacial polymerization-click chemical reaction on the porous support layer to prepare a thin-film composite layer with a semi-interpenetrating network structure. Dissolve functionalized chitosan, photoinitiator 1173, and polyvinyl alcohol in an aqueous solution. After the base membrane is immersed in the aqueous solution for a period of time, remove the aqueous solution until there are no visible water spots on the surface. Then immerse the base membrane in an ethyl acetate solution of trimethylolpropane tris(3-mercaptopropionate) and expose it to ultraviolet light (365 nm) for irradiation for 15 min. Gently wash the preformed membrane with ethyl acetate to remove the unreacted trimethylolpropane tris(3-mercaptopropionate), and then perform heat treatment at 60 °C for 30 min to complete the cross-linking reaction. Finally, store the obtained membrane in deionized water for the next step of use, denoted as CS-3ESH.

[0072] Example 6:

[0073] Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve 5 g of chitosan in an aqueous solution of 0.4 M acetic acid, add an aqueous sodium hydroxide solution to adjust the pH to approximately 4, and add 18.6 g of allyl glycidyl ether to this solution. Then react the mixture under mechanical stirring at 60 °C in a nitrogen atmosphere for 8 h. After the reaction is completed, pour the solution into an excess of acetonitrile to obtain a light yellow precipitate, wash the precipitate three times with acetone and vacuum dry it at room temperature for 4 h to obtain functionalized chitosan.

[0074] Step 2: Preparation of the porous support layer by non-solvent induced phase separation. First, the polyethersulfone raw material is dried under vacuum to remove water. Then, the dried polyethersulfone is dissolved in DMF and stirred at 150 °C for 3 - 5 h until completely dissolved, and left standing to remove air bubbles. It is slit-coated on the non-woven fabric and undergoes liquid-solid phase transformation to form a base membrane. The phase transformation time is controlled at 0.5 min, the water bath temperature is 18 °C, the thermal curing water bath temperature is 80 °C, and the membrane thickness is controlled at 5.5 mil. The obtained polymer base membrane is immersed in deionized water for later use.

[0075] Step 3: Interface polymerization-click chemical reaction is carried out on the porous support layer to prepare a thin composite layer with a semi-interpenetrating network structure. Functionalized chitosan, photoinitiator 1173 (2-hydroxy-2-methyl-1-phenyl-1-propanone), and polyvinyl alcohol are dissolved in an aqueous solution. After the base membrane is immersed in the aqueous solution for a period of time, the aqueous solution is removed until there are no visible water spots on the surface. Then, the base membrane is immersed in an ethyl acetate solution of pentaerythritol tetra(3-mercaptopropionate) and exposed to ultraviolet light (365 nm) for 15 min. The preformed membrane is gently washed with ethyl acetate to remove the unreacted pentaerythritol tetra(3-mercaptopropionate), and then heat-treated at 60 °C for 30 min to complete the cross-linking reaction. Finally, the obtained membrane is stored in deionized water for the next step of use, denoted as CS-4ESH.

[0076] The properties of the chitosan-based composite membranes obtained from the above six examples are as follows in the table:

[0077]

[0078] It can be seen that the water permeability coefficient, methylene blue rejection rate, and methylene blue rejection rate of the chitosan-based composite membrane obtained by the present invention have been effectively improved compared with general composite membranes. Due to the strong hydrogen bond interaction between functional chitosan molecules and between functionalized chitosan and linear polymers, physical entanglement can be achieved between polymer chains through non-covalent bonds, further preventing polymer dissolution and inhibiting swelling. Due to the unreacted protonated amino groups of chitosan, the surface of the prepared composite membrane is positively charged and can adsorb anionic dyes with negative charges.

[0079] The present invention uses thiol compounds with different chain lengths and functionalities as cross-linking agents to achieve controllable adjustment of the cross-linking density, thereby obtaining composite membranes with different fluxes and cut-off molecular weights to meet more separation requirements. At the same time, combining interface polymerization and thiol-ene click chemistry, except for adding an ultraviolet light source for irradiation during the interface polymerization process, there are no additional equipment requirements, and the synthesis method is simple, with low cost, and can be mass-produced in batches.

[0080] The above has introduced in detail a preparation method of a composite nanofiltration membrane provided by the present invention. Specific examples are used in this article to elaborate on the structure and working principle of the present invention. The description of the above embodiments is only used to help understand the method and core idea of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the scope of protection of the claims of the present invention.

Claims

1. A preparation method of a composite nanofiltration membrane, characterized in that: it includes the following steps: Step 1: Synthesize functionalized chitosan containing carbon-carbon double bonds. Dissolve chitosan in an acetic acid solution, and add a double-bond-containing epoxy monomer to this solution. After mechanical stirring, wash the precipitate in an ice bath to obtain functionalized chitosan; Step 2: Prepare a porous support layer by the non-solvent induced phase separation method. Vacuum dry the polymer raw material to remove water, dissolve the dried raw material in an organic solvent, stir at 150 °C for 3 - 5 h until completely dissolved, and let it stand to remove air bubbles; coat it in a slit on a non-woven fabric, and form a base membrane through liquid-solid phase transformation. Immerse the obtained polymer base membrane in deionized water for later use. Step 3: Conduct interfacial polymerization-click chemical reaction on the porous support layer to prepare a thin composite layer with a semi-interpenetrating network structure, Step 31: Dissolve the functionalized chitosan, photoinitiator, and linear polymer in Step 1 in an aqueous solution; Step 32: Then immerse the polymer base membrane obtained in Step 2 in an organic solution of a mercapto compound with two or more functional groups, irradiate with ultraviolet light, and wash the preformed membrane with an organic solvent to remove unreacted mercapto compounds, Step 33: Finally, perform heat treatment to make the cross-linking reaction complete.

2. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that: Step 1 includes the following steps: Step 11: Dissolve chitosan in an acetic acid solution, add a pH regulator to adjust the pH to 4, and add a double-bond-containing epoxy monomer to this solution; Step 12: Then carry out mechanical stirring and reaction on the mixture obtained in Step 11 in an inert gas atmosphere, and cool it in an ice bath; Step 13: Pour the cooled solution into an excess of acetonitrile to precipitate, wash the precipitate several times with an organic solvent, and vacuum dry it at room temperature to obtain functionalized chitosan.

3. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that: the concentration of the acetic acid solution in Step 11 is 0.4 M; the concentration of the pH regulator is 0.05 M, and the addition amount is 5% of the total reaction system mixture; the double-bond-containing epoxy monomer is one or more of allyl glycidyl ether, 4-vinylbenzyl glycidyl ether, 4-hydroxybutyl acrylate glycidyl ether, glycidyl methacrylate, 3,4-epoxycyclohexylmethyl methacrylate, glycidyl acrylate, 1,2-epoxy-5-hexene, 1,2-epoxy-7-octene.

4. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that: the inert gas in Step 12 is argon or nitrogen, the reaction temperature is 60 °C, the reaction time ≤ 2 h, and the ice bath time ≥ 15 min.

5. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that: the volume ratio of acetonitrile to the solution containing the product in Step 13 is at least 8:1, and the organic solvent is acetone or tetrahydrofuran.

6. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that: the polymer in Step 2 is any one of polyethersulfone, polyester, polyacrylonitrile, and polyimide, and the mass fraction is 15% - 28%; The organic solvent is any one or more of N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAc), dimethyl sulfoxide (DMSO), and N-methylpyrrolidone (NMP).

7. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: in step 2, the phase inversion time is 0.5 - 1 min, the water bath temperature is 15 - 20 °C, the thermal curing water bath temperature is 70 - 85 °C, and the total thickness of the base membrane including the non-woven fabric is 5.0 - 6.0 mil.

8. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: after the base membrane obtained in step 31 is immersed in the aqueous solution for a period of time, the aqueous solution is removed until there are no visible water spots on the surface; the ultraviolet ray in step 32 is 365 nm, and the irradiation time is 2 - 20 min.

9. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: the photoinitiator in step 3 is any one or two of benzoin and its derivatives, benzil derivatives, dialkoxyacetophenone, ɑ-hydroxyalkylphenone, and acylphosphine oxides; the linear polymer is any one of polyethylene oxide (PEO) and polyvinyl alcohol (PVA); The bifunctional mercapto compound is SH-(CH 2 )n-SH, where n is any repeating unit number greater than or equal to 2, dithiothreitol, bis(3-mercaptopropionic acid) ethylene ester, and the polyfunctional mercapto compound is any one of pentaerythritol tetrakis(mercaptoacetate), pentaerythritol tetra-3-mercaptopropionate, and trimethylolpropane tris(3-mercaptopropionate); the organic solvent is one of ethanol, ethyl acetate, benzene, xylene, etc.

10. The method for preparing a composite nanofiltration membrane according to claim 1, characterized in that: in step 3, the mass fraction of functionalized chitosan in the aqueous solution is 0.5 - 2%, the mass fraction of the photoinitiator is 0.2 - 0.4%, the mass fraction of the linear polymer is 0.05 - 0.2%, and the treatment time of the base membrane in the aqueous solution is 0.5 - 20 min; the mass fraction of the mercapto compound in the ethyl acetate solution in the organic phase solution is 0.05 - 3%; the final heat treatment temperature is 50 - 70 °C, and the time is 10 - 60 min.

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