Method for preparing a composite nanofiltration membrane and composite nanofiltration membrane prepared thereby

Abstract

The present application relates to a preparation method of composite nanofiltration membrane and the composite nanofiltration membrane prepared by the method. The method comprises the following steps: preparing a polyacrylonitrile membrane as a base film, modifying the hydrophilicity of the base film, then sequentially contacting with an aqueous solution of a polymer containing primary amine groups and secondary amine groups, an aqueous solution of a metal salt containing metal ions capable of forming a complex with the polymer containing primary amine groups and secondary amine groups, and an aqueous solution containing an imidazole compound, and obtaining the composite nanofiltration membrane through post-treatment. The method of the present application is simple in process and green in environment, not only reduces the production cost, but also reduces the pollution to the environment in the preparation process of the membrane. The composite nanofiltration membrane prepared by the method of the present application has a high rejection rate for toxic organic dye molecules and a high permeability for metal salts, and realizes the selective separation of metal salts and dye molecules, and has a high flux.

Description

Technical Field

[0001] This invention relates to the technical field of nanofiltration membranes, and particularly to a method for preparing a composite nanofiltration membrane and a composite nanofiltration membrane prepared therefrom. Background Technology

[0002] Wastewater generated by the textile dyeing and printing industry is highly toxic, has high color intensity, and high salt content, seriously threatening the human living environment. Therefore, it is necessary to explore effective methods for separating toxic dyes from wastewater. Traditional methods such as coagulation, oxidation, adsorption, chemical degradation, and ultrafiltration have been used for a long time to separate organic dye molecules from wastewater generated by the textile dyeing and printing industry. However, the treatment of most dye wastewater usually involves the addition of additional chemicals, thus causing secondary pollution problems.

[0003] Nanofiltration technology has advantages such as small footprint, low energy consumption, and environmental friendliness, and is considered a green and effective method for treating dye wastewater. Furthermore, designing nanofiltration membranes with high permeability and selectivity is key to improving their separation efficiency.

[0004] Thin film composite (TFC) membranes consist of an ultrathin selective layer and a highly porous support layer. They are a technologically mature separation membrane structure that has been used in practical applications for decades. However, in recent years, the performance of TFC membranes has clearly reached its limits in terms of separation capacity.

[0005] To overcome this deficiency, thin film nano-composite (TFN) membranes have been developed. However, TFN nanofiltration membranes suffer from several problems, including the tendency of nanoparticles to aggregate in the membrane matrix, leading to the formation of non-selective voids and reduced membrane selectivity. Although numerous researchers have conducted extensive studies on TFN nanofiltration membranes, achieving uniform dispersion of nanoparticles in a polymer matrix remains a highly challenging problem.

[0006] In recent years, the invention of polymer / metal-organic framework (MOF) composite membranes has ingeniously solved the problem of nanoparticle aggregation, greatly improving the separation performance of separation membranes. Numerous studies have demonstrated that, due to the porosity of MOFs, polymer / MOF composite membranes inherently exhibit superior separation performance compared to pure polymer membranes. Furthermore, different MOFs, including ZIF-8 (Zeolitic Imidazolate Framework-8), ZIF-11, ZIF-71, and HKUST-1, have been used to prepare polymer / MOF composite membranes. However, the weak bonding between MOFs and polymer substrates, the low hydrophilicity of the polymer substrates themselves, and the stringent synthesis conditions of the MOF layers still hinder further research and widespread application of polymer / MOF composite membranes.

[0007] Therefore, there is still a need to improve existing polymer / MOF composite films. To improve the adhesion of MOF to the polymer substrate, the surface of the polymer substrate should be modified before the MOF synthesis step. Summary of the Invention

[0008] The problem the invention aims to solve

[0009] To address the problems existing in current polymer / MOF composite membranes, such as weak bonding between MOF and polymer substrate, low hydrophilicity of the polymer substrate itself, easy aggregation during MOF synthesis, and the frequent use of organic solvents in MOF synthesis, the present invention aims to provide a method for preparing a composite nanofiltration membrane and a composite nanofiltration membrane prepared by the method, so as to simultaneously improve the above-mentioned problems existing in the prior art.

[0010] Solution for solving the problem

[0011] Through in-depth research, the inventors of this invention discovered that modifying the polymer base film can improve its hydrophilicity. By chemically bonding the primary or secondary amine groups of a polymer containing positively charged primary and secondary amine groups to hydrophilic groups on the surface of the hydrophilic polymer base film, the polymer base film is modified to exhibit hydrophilicity. Furthermore, the polymer containing positively charged primary and secondary amine groups forms a complex with metal ions through chelation, and the metal ions and imidazole compounds form a zeolitic imidazolate-like framework structure in situ on the surface of the polymer base film. The framework allows the formed zeolite-like imidazole ester (ZIF) framework structure to adhere to the polymer substrate in an ordered, uniform, and firm manner, thereby improving the bonding force between the ZIF framework structure and the polymer substrate. Furthermore, the ZIF formed by this method does not exhibit agglomeration on the polymer substrate surface, resulting in an increased effective filtration area of ​​the prepared polymer / ZIF composite nanofiltration membrane, thus exhibiting high permeability. In addition, the synthesis of ZIF is carried out in aqueous solution without the use of organic solvents.

[0012] One aspect of the present invention relates to a method for preparing a composite nanofiltration membrane, the method comprising the following steps:

[0013] (1) Prepare a polyacrylonitrile film as a base film. The polyacrylonitrile film has a reinforcing material layer and a polymer layer from bottom to top. Modify the base film so that the surface of the base film is rich in hydrophilic groups. The hydrophilic groups are carboxyl groups.

[0014] (2) The base film with hydrophilic groups on its surface obtained in step (1) is sequentially contacted with an aqueous solution of a polymer containing primary amine groups and secondary amine groups, and an aqueous solution of a metal salt containing metal ions that can form complexes with the polymer containing primary amine groups and secondary amine groups.

[0015] (3) Contact the membrane obtained in step (2) with an aqueous solution containing imidazole compounds;

[0016] (4) A composite nanofiltration membrane is obtained after post-processing.

[0017] According to the preparation method of the present invention, the modification is carried out in an aqueous solution of an alkali metal hydroxide, wherein the alkali metal hydroxide is at least one selected from sodium hydroxide and potassium hydroxide.

[0018] According to the preparation method of the present invention, the molar concentration of the alkali metal hydroxide is 1.5-2.0 mol / L, based on the volume of the aqueous solution of the alkali metal hydroxide.

[0019] According to the preparation method of the present invention, the polymer containing primary amine groups and secondary amine groups contains at least polyethyleneimine.

[0020] According to the preparation method of the present invention, the concentration of the polymer containing primary and secondary amine groups is 0.1-1.0 wt%, based on the weight of the aqueous solution of the polymer containing primary and secondary amine groups.

[0021] According to the preparation method of the present invention, the metal ion capable of forming a complex with the polymer containing primary and secondary amine groups contains at least Co. 2+ ion.

[0022] According to the preparation method of the present invention, the molar concentration of the metal ion is 0.05-0.2 mol / L, based on the volume of the aqueous solution containing the metal salt of the metal ion capable of forming a complex with the polymer containing primary and secondary amine groups.

[0023] According to the preparation method of the present invention, the imidazole compound contains at least 2-methylimidazole.

[0024] According to the preparation method of the present invention, the molar concentration of the imidazole compound is 0.1-0.2 mol / L, based on the volume of the aqueous solution containing the imidazole compound.

[0025] Another aspect of the present invention relates to a composite nanofiltration membrane prepared by the preparation method described in the present invention.

[0026] The effects of the invention

[0027] The method for preparing the composite nanofiltration membrane of the present invention: (1) improves the hydrophilicity of the polymer substrate; (2) improves the bonding force between ZIF and the polymer substrate; (3) enables ZIF to grow uniformly and in an orderly manner and adhere firmly to the polymer substrate without agglomeration on the surface of the polymer substrate; (4) the synthesis of ZIF is carried out in aqueous solution without the use of organic solvents. This method can be realized on existing membrane preparation equipment. The process is simple, green and environmentally friendly, which not only reduces production costs, but also reduces the pollution to the environment during the membrane preparation process.

[0028] The composite nanofiltration membrane prepared by the method of the present invention has a high rejection rate for toxic organic dye molecules and a high permeability for metal salts, which can effectively achieve selective separation of dye molecules and metal salts. At the same time, due to the high porosity and specific surface area of ​​the ZIF structure, the membrane has high permeability and thus high water flux, which can be effectively applied to dye removal and wastewater treatment in the printing and dyeing industry. Detailed Implementation

[0029] This invention relates to a method for preparing a composite nanofiltration membrane, which includes the following steps:

[0030] (1) Prepare a polyacrylonitrile film as a base film. The polyacrylonitrile film has a reinforcing material layer and a polymer layer from bottom to top. Modify the base film so that the surface of the base film is rich in hydrophilic groups. The hydrophilic groups are carboxyl groups.

[0031] (2) The base film with hydrophilic groups on its surface obtained in step (1) is sequentially contacted with an aqueous solution of a polymer containing primary amine groups and secondary amine groups, and an aqueous solution of a metal salt containing metal ions that can form complexes with the polymer containing primary amine groups and secondary amine groups.

[0032] (3) Contact the membrane obtained in step (2) with an aqueous solution containing imidazole compounds;

[0033] (4) A composite nanofiltration membrane is obtained after post-processing.

[0034] The technical concept of the preparation method of this invention lies in improving the hydrophilicity of the polymer base film by modifying it; by chemically bonding the polymer containing positively charged primary and secondary amine groups to the hydrophilic groups and primary or secondary amine groups on the surface of the hydrophilic polymer base film, thereby binding them to the hydrophilic modified polymer base film; furthermore, the polymer containing positively charged primary and secondary amine groups forms a complex with metal ions through chelation, and the metal ions and imidazole compounds form a zeolitic imidazolate-like framework structure in situ on the surface of the polymer base film. The ZIF framework is used to ensure that the formed zeolite-like imidazole ester framework (ZIF) is attached to the polymer substrate in an orderly, uniform, and firm manner, thereby improving the bonding force between the ZIF and the polymer substrate. Furthermore, the ZIF formed in this way does not agglomerate on the surface of the polymer substrate, resulting in an increased effective filtration area and high permeability of the prepared polymer / ZIF composite nanofiltration membrane. In addition, the ZIF is synthesized in aqueous solution without the use of organic solvents.

[0035] In the preparation method of the present invention, a polyacrylonitrile film is prepared as a base film. The polyacrylonitrile film has a reinforcing material layer and a polymer layer from bottom to top. Preferably, the reinforcing material is a non-woven fabric, such as polypropylene non-woven fabric.

[0036] In the preparation method of the present invention, the modification is carried out in an aqueous solution of an alkali metal hydroxide, wherein the alkali metal hydroxide is at least one selected from sodium hydroxide and potassium hydroxide. When the modification is carried out in an aqueous solution of an alkali metal hydroxide, the carbon-nitrogen triple bonds on the surface of the polyacrylonitrile film are hydrolyzed and broken to form carboxyl groups, making the film surface negatively charged. This allows for a more uniform distribution of metal ions on the film surface and a stronger binding force to the film during the process of forming a complex with the polymer containing positively charged primary and secondary amine groups through chelation.

[0037] In the preparation method of this invention, the molar concentration of the alkali metal hydroxide is 1.5-2.0 mol / L, based on the volume of the aqueous solution of the alkali metal hydroxide. When the concentration is below 1.5 mol / L, the ideal hydrolysis effect cannot be achieved; when the concentration is above 2.0 mol / L, the excessive alkalinity will cause the entire polyacrylonitrile film to be decomposed and destroyed.

[0038] In the preparation method of the present invention, there is no particular limitation on the temperature during modification. Preferably, it is carried out at a temperature of 50 to 70°C. When the temperature is within this range, the hydrolysis of the membrane will proceed in a relatively gentle and uniform manner. There is also no particular limitation on the modification time. Preferably, it is carried out within 30 to 60 minutes.

[0039] In the preparation method of the present invention, the polymer containing primary and secondary amine groups contains at least polyethyleneimine. Preferably, the concentration of the polymer containing primary and secondary amine groups is 0.1-1.0 wt% based on the weight of the aqueous solution of the polymer containing primary and secondary amine groups. When the concentration is below 0.1 wt%, the structure of the prepared composite nanofiltration membrane will be too sparse due to the low concentration, and the dye retention effect will not be achieved. When the concentration is above 1.0 wt%, the structure of the prepared composite nanofiltration membrane will be too dense due to the high concentration, and the water flux will be too low.

[0040] In the preparation method of the present invention, the molecular weight of the polymer containing primary and secondary amine groups is not particularly limited. Preferably, the weight-average molecular weight of the polymer is in the range of 10,000-70,000. When the weight-average molecular weight is within this range, the prepared film surface is more uniform and easier to coat.

[0041] In the preparation method of the present invention, the metal ion capable of forming a complex with the polymer containing primary and secondary amine groups contains at least Co. 2+ ion.

[0042] In the preparation method of the present invention, preferably, the molar concentration of the metal ions is 0.05-0.2 mol / L, based on the volume of the aqueous solution containing the metal salt of the metal ions capable of forming complexes with the polymer containing primary and secondary amine groups. When the concentration is below 0.05 mol / L, the amount of MOF (more specifically ZIF) formed in situ by the metal ions and imidazole compounds will be too small, resulting in sparse distribution on the membrane surface. This leads to an excessively high flux in the prepared composite nanofiltration membrane, making it unable to achieve the effect of dye retention. When the concentration is above 0.2 mol / L, the amount of MOF formed in situ will be too large, resulting in weak bonding between the polymer containing primary and secondary amine groups and the base membrane. This makes the MOF layer on the surface of the prepared composite nanofiltration membrane prone to detachment.

[0043] In the preparation method of the present invention, there are no particular limitations on the temperature and time for chelation to form a complex. Preferably, it is carried out at a temperature of 40 to 50°C. When the temperature is within this range, the complex formed by chelation is uniform and metal ions are detached. There are also no particular limitations on the time, but preferably, it is carried out within 8 to 10 hours.

[0044] In the preparation method of the present invention, there is no particular limitation on the type of metal salt. For example, nitrates, chlorides, sulfates, etc. can be used. Hydrates of metal salts can also be used, such as Co(NO3)2·6H2O, CoCl2·6H2O and CoSO4·7H2O.

[0045] In the preparation method of the present invention, the imidazole compound contains at least 2-methylimidazole.

[0046] Preferably, the molar concentration of the imidazole compound is 0.1-0.2 mol / L, based on the volume of the aqueous solution containing the imidazole compound. When the concentration is below 0.1 mol / L, the MOFs formed in situ by the metal ions and imidazole compounds will be unstable and few in number, resulting in an unstable composite nanofiltration membrane with excessively high flux and low rejection rate, failing to effectively retain dyes. When the concentration is above 0.2 mol / L, a large number of MOFs will be formed, and the MOFs will easily aggregate, resulting in poor separation performance of the prepared composite nanofiltration membrane.

[0047] As a non-limiting example, the method for preparing the composite nanofiltration membrane of the present invention includes the following steps:

[0048] (1) Modification of the base film

[0049] Polyacrylonitrile (PAN) base film is immersed in NaOH aqueous solution at a certain temperature for hydrolysis modification. After hydrolysis, it is rinsed with deionized water to remove residual alkali on the film surface until the rinsing water is neutral, thus obtaining a base film with carboxyl groups distributed on the surface (also known as HPAN).

[0050] (2) Formation of complexes

[0051] The base film (HPAN) with carboxyl groups distributed on its surface obtained in step (1) is immersed in a solution containing polyethyleneimine (also known as PEI), and an equal volume of a solution containing Co is slowly poured in. 2+ The aqueous solution was placed in an oven at a certain temperature and heated for a certain period of time to allow Co to react. 2+ Ions and PEI chelate to form complexes that are assembled on the surface of HPAN. The complexes are rinsed with deionized water 3-5 times and then post-treated in an oven.

[0052] (3) In-situ formation of ZIF structure

[0053] The membrane obtained in step (2) was immersed in an aqueous solution containing a certain concentration of 2-methylimidazole, and ZIF-67 was grown in situ on the surface of the membrane to prepare a PAN / ZIF-67 composite nanofiltration membrane, which was then post-treated for later use.

[0054] The present invention also relates to a composite nanofiltration membrane prepared by the preparation method according to the present invention.

[0055] Example

[0056] The present invention will be further described in detail below with reference to specific embodiments, but the technical solutions of the present invention are by no means limited to the following embodiments. It should be noted that, unless otherwise specified, the reagents, raw materials and equipment used in the embodiments are all commercially available conventional products.

[0057] Example 1

[0058] Preparation of composite nanofiltration membranes:

[0059] (1) First, the polyacrylonitrile (PAN) base film is modified. Specifically, the base film is cut into a rectangular piece with a length of 20cm and a width of 15cm. Then, it is fixed at the bottom of a square plate, immersed in a 2mol / L NaOH aqueous solution, placed in an oven at 60℃ for 40min, and then rinsed multiple times with a large amount of deionized water until the surface of the base film is neutral. Then, it is placed in an oven to dry for later use.

[0060] (2) Prepare a 0.1 wt% solution of polyethyleneimine (PEI, M). WPrepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.1 mol / L at the same time;

[0061] (3) Pour the solution prepared in step (2) onto the surface of the base film obtained in step (1) and place it in an oven at 50°C for 8 hours to allow for crosslinking of PEI with the base film and crosslinking of PEI with the ZIF-67 precursor Co. 2+ After chelation, discard the excess solution, rinse gently three times with deionized water, and place in an oven at 60°C for 30 minutes for further cross-linking and chelation.

[0062] (4) Prepare an aqueous solution of 2-methylimidazole with a concentration of 0.2 mol / L;

[0063] (5) Pour the solution prepared in step (4) onto the surface of the membrane obtained in step (3), and grow ZIF-67 in situ at room temperature for 30 min to obtain PAN / ZIF-67 composite nanofiltration membrane. After the reaction is completed, discard the unreacted solution, rinse with deionized water 3 times, and cut the membrane into round pieces with a diameter of 7 cm and put them into deionized water for later use.

[0064] Example 2

[0065] Preparation of composite nanofiltration membranes:

[0066] (1) First, the polyacrylonitrile (PAN) base film is modified. Specifically, the base film is cut into a rectangular piece with a length of 20cm and a width of 15cm. Then, it is fixed at the bottom of a square plate, immersed in a 2mol / L NaOH aqueous solution, placed in an oven at 60℃ for 40min, and then rinsed multiple times with a large amount of deionized water until the surface of the base film is neutral. Then, it is placed in an oven to dry for later use.

[0067] (2) Prepare a 0.1 wt% solution of polyethyleneimine (PEI, M). W Prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.1 mol / L (=70000) and at the same time prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.1 mol / L.

[0068] (3) Pour the solution prepared in step (2) onto the surface of the base film obtained in step (1) and place it in an oven at 50°C for 8 hours to allow for crosslinking of PEI with the base film and crosslinking of PEI with the ZIF-67 precursor Co. 2+ After chelation, discard the excess solution, rinse gently three times with deionized water, and place in an oven at 60°C for 30 minutes for further cross-linking and chelation.

[0069] (4) Prepare an aqueous solution of 2-methylimidazole with a concentration of 0.2 mol / L;

[0070] (5) Pour the solution prepared in step (4) onto the surface of the membrane obtained in step (3), and grow ZIF-67 in situ at room temperature for 30 min to obtain PAN / ZIF-67 composite nanofiltration membrane. After the reaction is completed, discard the unreacted solution, rinse with deionized water 3 times, and cut the membrane into round pieces with a diameter of 7 cm and put them into deionized water for later use.

[0071] Example 3

[0072] Preparation of composite nanofiltration membranes:

[0073] (1) First, the polyacrylonitrile (PAN) base film is modified. Specifically, the base film is cut into a rectangular piece with a length of 20cm and a width of 15cm. Then, it is fixed at the bottom of a square plate, immersed in a 2mol / L NaOH aqueous solution, placed in an oven at 60℃ for 40min, and then rinsed multiple times with a large amount of deionized water until the surface of the base film is neutral. Then, it is placed in an oven to dry for later use.

[0074] (2) Prepare a 0.1 wt% solution of polyethyleneimine (PEI, M). W Prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.1 mol / L (=100000) and at the same time prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.1 mol / L.

[0075] (3) Pour the solution prepared in step (2) onto the surface of the base film obtained in step (1) and place it in an oven at 50°C for 8 hours to allow for crosslinking of PEI with the base film and crosslinking of PEI with the ZIF-67 precursor Co. 2+ After chelation, discard the excess solution, rinse gently three times with deionized water, and place in an oven at 60°C for 30 minutes for further cross-linking and chelation.

[0076] (4) Prepare an aqueous solution of 2-methylimidazole with a concentration of 0.2 mol / L;

[0077] (5) Pour the solution prepared in step (4) onto the surface of the membrane obtained in step (3), and grow ZIF-67 in situ at room temperature for 30 min to obtain PAN / ZIF-67 composite nanofiltration membrane. After the reaction is completed, discard the unreacted solution, rinse with deionized water 3 times, and cut the membrane into round pieces with a diameter of 7 cm and put them into deionized water for later use.

[0078] Example 4

[0079] Preparation of composite nanofiltration membranes:

[0080] (1) First, the polyacrylonitrile (PAN) base film is modified. Specifically, the base film is cut into a rectangular piece with a length of 20cm and a width of 15cm. Then, it is fixed at the bottom of a square plate, immersed in a 2mol / L NaOH aqueous solution, placed in an oven at 60℃ for 40min, and then rinsed multiple times with a large amount of deionized water until the surface of the base film is neutral. Then, it is placed in an oven to dry for later use.

[0081] (2) Prepare a 0.1 wt% solution of polyethyleneimine (PEI, M). W Prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.05 mol / L (=70000) and simultaneously prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.05 mol / L.

[0082] (3) Pour the solution prepared in step (2) onto the surface of the base film obtained in step (1) and place it in an oven at 50°C for 8 hours to allow for crosslinking of PEI with the base film and crosslinking of PEI with the ZIF-67 precursor Co. 2+ After chelation, discard the excess solution, rinse gently three times with deionized water, and place in an oven at 60°C for 30 minutes for further cross-linking and chelation.

[0083] (4) Prepare an aqueous solution of 2-methylimidazole with a concentration of 0.1 mol / L;

[0084] (5) Pour the solution prepared in step (4) onto the surface of the membrane obtained in step (3), and grow ZIF-67 in situ at room temperature for 30 min to obtain PAN / ZIF-67 composite nanofiltration membrane. After the reaction is completed, discard the unreacted solution, rinse with deionized water 3 times, and cut the membrane into round pieces with a diameter of 7 cm and put them into deionized water for later use.

[0085] Example 5

[0086] Preparation of composite nanofiltration membranes:

[0087] (1) First, the polyacrylonitrile (PAN) base film is modified. Specifically, the base film is cut into a rectangular piece with a length of 20cm and a width of 15cm. Then, it is fixed at the bottom of a square plate, immersed in a 2mol / L NaOH aqueous solution, placed in an oven at 60℃ for 40min, and then rinsed multiple times with a large amount of deionized water until the surface of the base film is neutral. Then, it is placed in an oven to dry for later use.

[0088] (2) Prepare a 0.1 wt% polyethyleneimine (PEI) solution, M W Prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.075 mol / L (=70000) and at the same time prepare an aqueous solution of Co(NO3)2·6H2O with a concentration of 0.075 mol / L;

[0089] (3) Pour the solution prepared in step (2) onto the surface of the base film obtained in step (1) and place it in an oven at 50°C for 8 hours to allow for crosslinking of PEI with the base film and crosslinking of PEI with the ZIF-67 precursor Co. 2+ After chelation, discard the excess solution, rinse gently three times with deionized water, and place in an oven at 60°C for 30 minutes for further cross-linking and chelation.

[0090] (4) Prepare an aqueous solution of 2-methylimidazole with a concentration of 0.15 mol / L;

[0091] (5) Pour the solution prepared in step (4) onto the surface of the membrane obtained in step (3), and grow ZIF-67 in situ at room temperature for 30 min to obtain PAN / ZIF-67 composite nanofiltration membrane. After the reaction is completed, discard the unreacted solution, rinse with deionized water 3 times, and cut the membrane into round pieces with a diameter of 7 cm and put them into deionized water for later use.

[0092] Performance testing of composite nanofiltration membranes :

[0093] The composite nanofiltration membrane obtained in the above embodiments was tested for dye removal performance in a cross-flow device. The test system was an aqueous solution of 100 mg / L methylene blue (MW = 799.8), the test pressure was 0.2 MPa, and the test temperature was room temperature.

[0094] Flux (L·m) -2 ·h -1 ·bar -1 )=V / (A×t),

[0095] Within time t(h), the membrane area is A(m²). 2 The volume of pure water V (L) of the composite membrane;

[0096] Removal rate (R, %) = (C f -C p ) / C f ×100%

[0097] In the formula C f The concentration of methylene blue in the feed solution is 100 mg / L (mol / L), C p The concentration of methylene blue in the permeate (mol / L)

[0098] For convenience, the test results are summarized in Table 1 below.

[0099] Table 1

[0100] 1 71.7 91.5 2 51.2 99.1 3 19.1 99.2 4 62.3 92.7 5 50.7 98.7

[0101] The above embodiments disclose specific methods of the present invention, but these embodiments are merely examples and are not intended to be limiting. Various modifications that will be obvious to those skilled in the art are of course included within the scope of the present invention.

[0102] Industrial availability

[0103] The method for preparing the composite nanofiltration membrane of the present invention: (1) improves the hydrophilicity of the polymer substrate; (2) improves the bonding force between ZIF and the polymer substrate; (3) enables ZIF to grow uniformly and in an orderly manner and adhere firmly to the polymer substrate without agglomeration on the surface of the polymer substrate; (4) the synthesis of ZIF is carried out in aqueous solution without the use of organic solvents. This method can be realized on existing membrane preparation equipment. The process is simple, green and environmentally friendly, which not only reduces production costs, but also reduces the pollution to the environment during the membrane preparation process.

[0104] The composite nanofiltration membrane prepared by the method of the present invention has a high rejection rate for toxic organic dye molecules and a high permeability for metal salts, which can effectively achieve selective separation of dye molecules and metal salts. At the same time, due to the high porosity and specific surface area of ​​the ZIF structure, the membrane has high permeability and thus high water flux, which can be effectively applied to dye removal and wastewater treatment in the printing and dyeing industry.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that... Includes the following steps: (1) Prepare a polyacrylonitrile membrane as a base membrane. The polyacrylonitrile membrane has a reinforcing material layer and a polymer layer from bottom to top. Modify the base membrane so that the surface of the base membrane is rich in hydrophilic groups, and the hydrophilic groups are carboxyl groups. (2) An aqueous solution of a polymer containing primary and secondary amine groups, and an aqueous solution of a metal salt containing a metal ion capable of forming a complex with the polymer containing primary and secondary amine groups, are sequentially poured onto the surface of the base film with hydrophilic groups obtained in step (1). Then, crosslinking and chelation are performed sequentially at 50°C and 60°C, respectively. The polymer containing primary and secondary amine groups is polyethyleneimine, and the concentration of polyethyleneimine is 0.1-1.0 wt% based on the weight of the aqueous solution of the polymer containing primary and secondary amine groups. The weight-average molecular weight of polyethyleneimine is in the range of 50,000-70,000. The metal ion is Co. 2+ Ions, based on the volume of the aqueous solution containing the metal salt of the metal ions capable of forming complexes with the polymer containing primary and secondary amine groups, Co 2+ The molar concentration of the ions is 0.05-0.2 mol / L; (3) The membrane obtained in step (2) is contacted with an aqueous solution containing an imidazole compound, wherein the imidazole compound is 2-methylimidazolium and the molar concentration of 2-methylimidazolium is 0.1-0.2 mol / L based on the volume of the aqueous solution containing the imidazole compound; (4) A composite nanofiltration membrane is obtained after post-processing.

2. The preparation method according to claim 1, wherein the modification is carried out in an aqueous solution of an alkali metal hydroxide, wherein the alkali metal hydroxide is at least one selected from sodium hydroxide and potassium hydroxide.

3. The preparation method according to claim 2, wherein the molar concentration of the alkali metal hydroxide is 1.5-2.0 mol / L based on the volume of the aqueous solution of the alkali metal hydroxide.

4. A composite nanofiltration membrane prepared by the preparation method according to any one of claims 1-3.