Preparation method of composite nanofiltration membrane and composite nanofiltration membrane

Abstract

The present invention designs a preparation method of a composite nanofiltration membrane and the composite membrane prepared by this method to solve the problem of the separation layer shedding of the composite nanofiltration membrane. A preparation method of a composite nanofiltration membrane includes the following steps: Step 1, dissolve an aldehyde crosslinking agent and a polyamine monomer in water to make a pretreatment aqueous solution, and soak the substrate membrane in the pretreatment aqueous solution once to obtain a substrate membrane with a crosslinked layer attached to the surface; Step 2, first soak the substrate membrane with a crosslinked layer attached to the surface in Step 1 in a polyethyleneimine aqueous solution for a second time, then soak it in an organic phase solution containing an acyl chloride monomer for a third time, and finally obtain a highly crosslinked composite nanofiltration membrane. By constructing a crosslinked layer on the substrate membrane, the present invention enables the formation of stable chemical bonds between the separation layer and the crosslinked layer on the substrate membrane, which not only indirectly ensures the connection stability between the separation layer and the substrate membrane, but also can prevent the pores of the substrate membrane from deforming due to excessive water absorption, thereby avoiding the deformation of the separation layer located on the substrate membrane and preventing the desalination stability of the composite nanofiltration membrane from being affected.

Description

Technical Field

[0001] The present invention relates to the field of water purification, and particularly to a method for preparing a composite nanofiltration membrane and a composite nanofiltration membrane. Background Art

[0002] At present, there are the following technical difficulties in composite nanofiltration membranes. The material compatibility between the base membrane and the separation layer is poor, and there is a lack of effective physical or chemical interaction at the interface between the two, so that the separation layer is prone to separate from the base membrane under long-term operation or extreme water quality conditions. This phenomenon is more obvious in hollow fiber composite nanofiltration membranes. In addition, during actual operation (especially during high-speed cleaning), the base membrane is prone to jitter and surface friction, which also increases the possibility of the separation layer falling off, seriously affecting the practicality of the composite nanofiltration membrane. Summary of the Invention

[0003] In order to solve the above problems, the first object of the present invention is to provide a method for preparing a composite nanofiltration membrane, including the following steps: Step 1, dissolve an aldehyde crosslinking agent and a polyamine monomer in water to prepare a pretreatment aqueous solution, and soak the base membrane in the pretreatment aqueous solution once to obtain a base membrane with a crosslinked layer (formed by the crosslinking reaction of the aldehyde crosslinking agent and the polyamine monomer) attached to the surface; Step 2, soak the base membrane with the crosslinked layer attached thereto in Step 1 in a polyethyleneimine aqueous solution for a second time. During the second soaking, the aldehyde groups on the crosslinked layer react with polyethyleneimine, and then soak it in an organic phase solution containing an acyl chloride monomer for a third time. During the third soaking, the reaction between the aldehyde groups and polyethyleneimine continues and an interfacial polymerization reaction (the reaction between polyethyleneimine and acyl chloride) occurs to form a separation layer on the surface of the crosslinked layer, and finally a highly crosslinked composite nanofiltration membrane is obtained. When the third soaking starts in this application, the reaction between the aldehyde groups on the crosslinked layer and polyethyleneimine is very little, and most of the polyethyleneimine is physically attached to the surface of the crosslinked layer; therefore, during the third soaking, there is a situation where the reaction between the aldehyde groups on the crosslinked layer and polyethyleneimine and the reaction between polyethyleneimine and acyl chloride proceed simultaneously.

[0004] The composite nanofiltration membrane of the present invention includes a base membrane layer, a crosslinked layer, and a separation layer; wherein, the crosslinked layer is located between the base membrane layer and the separation layer. The thickness of the crosslinked layer is 20-90 nm, or 30-70 nm, or 40-50 nm. The crosslinked layer is formed by the crosslinking reaction of an aldehyde crosslinking agent and a polyamine monomer. The separation layer is formed by the interfacial polymerization reaction of polyethyleneimine and acyl chloride. In the present invention, polyethyleneimine serves as an intermediate connector, synchronously connecting the crosslinked layer and the separation layer, so that a stable chemical bond is indirectly constructed between the crosslinked layer and the separation layer, achieving the technical effect of stabilizing the separation layer to prevent it from falling off. In addition, the presence of the crosslinked layer not only ensures the connection stability between the separation layer and the base membrane, but also can prevent the pores of the base membrane from deforming (such as excessive swelling) due to excessive water absorption, thereby avoiding the deformation of the separation layer located on the base membrane and preventing the desalination stability of the composite nanofiltration membrane from being affected.

[0005] Among them, a crosslinked layer is obtained through a crosslinking reaction during the first immersion process. In one embodiment, the reactants of the crosslinking reaction are as follows:

[0006] The products of the crosslinking reaction are as follows:

[0007]

[0008] The reactants during the second immersion process are the products of the crosslinking reaction and PEI (where PEI is polyethyleneimine):

[0009]

[0010] The products during the second immersion process are as follows:

[0011]

[0012] An interfacial polymerization reaction occurs during the third immersion process, and its reactants are PEI and TMC (TMC is trimesoyl chloride):

[0013]

[0014] The products of the interfacial polymerization during the third immersion process are as follows:

[0015]

[0016] In the present invention, the base film is first immersed in a pretreatment aqueous solution containing an aldehyde crosslinking agent and a polyamine monomer, and a crosslinking reaction occurs between the two; when the base film is immersed in this solution, an ultrathin crosslinked layer (for example, with a thickness of 20 - 90 nm, or a thickness of 40 - 50 nm) will be constructed on the surface of the base film. This crosslinked layer can prevent the base film from undergoing excessive swelling due to excessive water absorption, thereby preventing the pores of the base film from deforming and affecting the desalination stability. On the other hand, the introduction of the crosslinked layer (usually the crosslinked layer is thinner than the base film and the separation layer, usually the thickness of the separation layer is 100 - 500 nm or 200 - 300 nm) can enhance the adsorption between the crosslinked layer and the base film; in addition, the crosslinked layer can prevent the pores of the base film from deforming due to excessive water absorption, and also further makes the contact between the crosslinked layer and the base film closer, thereby further enhancing this adsorption.

[0017] Subsequently, the base film with a crosslinked layer attached to its surface is immersed in an aqueous solution of polyethyleneimine for a second time. At this time, the aldehyde groups on the crosslinked layer (the aldehyde groups carried on the product after the reaction of the excessive aldehyde crosslinking agent and the polyamine monomer) react with the polyethyleneimine attached to the base film after the second immersion. Then, it is immersed in an organic phase solution containing acyl chloride monomers for a third time. The polyethyleneimine attached to the base film undergoes an interfacial polymerization reaction with the acyl chloride, forming a separation layer on the surface of the crosslinked layer of the base film. Therefore, a stable chemical bond is constructed between the separation layer and the crosslinked layer (polyethyleneimine reacts with both the crosslinked layer and the acyl chloride to generate the separation layer, so that the crosslinked layer and the separation layer are indirectly combined through chemical bonds), significantly improving the problem of peeling between the separation layer and the base film. In summary, due to the presence of the crosslinked layer, the connection stability between the separation layer and the base film is indirectly enhanced, preventing the separation layer from peeling off and ensuring the desalination stability of the separation layer.

[0018] As an implementation manner, the time for the first immersion in step one is 1 - 5 minutes. As an implementation manner, the time for the second immersion in step two is 1 - 10 minutes. As an implementation manner in step two, the time for the third immersion is 1 - 5 minutes.

[0019] As an implementation manner, the mass fraction of the aldehyde crosslinking agent in the pretreatment aqueous solution in step one is 0.20 - 3.00%; or the mass fraction is 0.50 - 3.00%; or the mass fraction is 1.00 - 3.00%.

[0020] As an implementation manner, the aldehyde crosslinking agent in step one is one or more of formaldehyde, glyoxal, and glutaraldehyde.

[0021] As an implementation manner, the mass fraction of the polyamine monomer in the pretreatment aqueous solution in step one is 0.05 - 0.50%; further, the mass fraction is 0.20 - 0.40%. The setting of the above mass fractions can make the crosslinked layer have an appropriate thickness, while attaching to the base film, reducing the impact on the water flux of the base film. In addition, the combination of the selected mass fraction of the polyamine monomer and the mass fraction of the aldehyde crosslinking agent can ensure that the aldehyde crosslinking agent is excessive in the reaction, so as to ensure that the generated crosslinked layer has excessive aldehyde groups.

[0022] As an implementation manner, the polyamine monomer in step one is one or more of ethylenediamine, 1,2 - propanediamine, 1,3 - propanediamine, and hexamethylenediamine.

[0023] As an implementation manner, after the first immersion in step one, the base film is taken out and hung for 5 - 30 minutes, and then the base film with a crosslinked layer attached to its surface is subjected to the first heat treatment at a temperature of 20 - 120°C for 5 - 30 minutes; or the base film with a crosslinked layer attached to its surface is subjected to the first heat treatment at a temperature of 60 - 120°C for 5 - 10 minutes.

[0024] Among them, the first heat treatment can accelerate the reaction between the aldehyde crosslinking agent and the polyamine monomer. When the base film is taken out after being soaked in the pretreatment aqueous solution, a small amount of unreacted aldehyde crosslinking agent and polyamine monomer adhere to its surface. The first heat treatment can promote the complete reaction of the aldehyde crosslinking agent and the polyamine monomer, further ensuring the construction of an ultrathin crosslinked layer on the surface of the base film.

[0025] As an implementation manner, the mass fraction of polyethyleneimine in the polyethyleneimine aqueous solution in step two is 0.10 - 1.00%; or the mass fraction is 0.30 - 0.80%; or the mass fraction is 0.60 - 0.80%.

[0026] As an implementation manner, the weight-average molecular weight of polyethyleneimine in step two is selected from 8000 - 50000.

[0027] As an implementation manner, the acyl chloride monomer in step two is one or more of trimesoyl chloride, terephthaloyl chloride, phthaloyl chloride, isophthaloyl chloride, and biphenyl dicarbonyl chloride. The mass fraction of the acyl chloride monomer is 0.10 - 0.50% or the mass fraction is 0.30 - 0.50%. The solvent of the organic phase solution is one or more of pentane, hexane, cyclohexane, and heptane. The setting of the above mass fraction can make polyethyleneimine in excess in the interfacial polymerization reaction. On the one hand, it can ensure that it can react with the aldehyde groups on the crosslinked layer to form chemical bonds. On the other hand, it can ensure the thickness and separation effect of the separation layer after the interfacial polymerization reaction.

[0028] As an implementation manner, after the three-time soaking in step two, the base film is placed at a temperature of 20 - 120 °C for the second heat treatment, and the treatment time is 5 - 30 minutes. Or the base film is placed at a temperature of 60 - 120 °C for the second heat treatment, and the treatment time is 5 - 20 minutes.

[0029] Among them, the excessive aldehyde crosslinking agent in the pretreatment aqueous solution makes the crosslinked layer on the surface of the base film have excessive active aldehyde groups (aldehyde groups at both ends of the crosslinked layer product). Through the second heat treatment, it can further accelerate the chemical crosslinking of the aldehyde groups with polyethyleneimine to make it completely react, and can also accelerate the interfacial polymerization reaction of polyethyleneimine with acyl chloride to make it completely react. Thus, while forming the separation layer in the interfacial polymerization reaction, a chemical bond is constructed between the separation layer and the crosslinked layer, strengthening the connection between the separation layer and the base film and improving the problem of separation layer peeling. In addition, the temperature of the second heat treatment cannot be too high, otherwise it will affect the progress of the interfacial polymerization reaction.

[0030] As an implementation manner, the material of the base membrane is one or more of polysulfone, polyethersulfone, polyimide, polyamide, and polyacrylonitrile. However, other materials can also be used for the base membrane. In addition, the base membrane can be a hollow fiber membrane or other types of membranes.

[0031] The second object of the present invention is to provide a composite nanofiltration membrane prepared by the above preparation method.

[0032] Advantages of the present invention:

[0033] This application uses a pretreatment aqueous solution containing an aldehyde crosslinking agent and a polyamine monomer, and immerses the base membrane in the pretreatment aqueous solution for pretreatment. Thus, on the one hand, an ultrathin crosslinked layer is constructed on the surface of the base membrane to prevent the base membrane from deforming due to water absorption; on the other hand, the aldehyde crosslinking agent in the pretreatment aqueous solution is in excess, so the crosslinked layer formed after the reaction has a large number of active aldehyde groups. While the separation layer is generated in the interfacial polymerization reaction, the aldehyde groups on the crosslinked layer react with polyethyleneimine. Polyethyleneimine serves as an intermediate connector to synchronously connect the crosslinked layer and the separation layer, enabling the crosslinked layer and the separation layer to indirectly establish stable chemical bonds for connection, achieving the technical effect of stabilizing the shedding of the separation layer. In summary, the presence of the crosslinked layer not only indirectly ensures the connection stability between the separation layer and the base membrane, but also can prevent the pores of the base membrane from deforming (such as excessive swelling) due to excessive water absorption, thereby avoiding the deformation of the separation layer located on the base membrane and preventing the desalination stability of the composite nanofiltration membrane from being affected. Specific embodiments

[0034] The following specific examples describe the present invention in detail. However, the present invention is not limited to the following examples.

[0035] The test methods for the flux and desalination rate of the composite nanofiltration membrane of the present invention are as follows:

[0036] 1. Water flux and desalination performance test

[0037] The prepared composite nanofiltration membrane is pre-pressed with an electrolyte solution of 0.20% mass concentration of MgSO4 at 0.31 MPa for half an hour to test the desalination performance and water flux of the composite nanofiltration membrane.

[0038] The calculation formula for water flux is as shown in (1):

[0039]

[0040] Among them, A = πDL (A - effective membrane area, m 2 ; D - average diameter (outer diameter) of the membrane filament, m; L - effective length of the membrane filament, m); t - time required to collect the Q volume of the produced liquid, h; Q - volume of the produced liquid collected within t time, L).

[0041] The calculation method of the desalination performance of the membrane is shown in (2):

[0042]

[0043] Wherein, R is the desalination rate of the membrane, and C f is the conductivity of the raw solution, μS / cm; C p is the conductivity of the produced water, μS / cm.

[0044] 2. Test on the binding stability of the separation layer

[0045] Binding stability test: In order to test the binding force between the separation layer and the base membrane, the above-mentioned composite nanofiltration membrane after the test is immersed in an ethanol solution for 24 hours, and after washing with water, the separation performance test is carried out on it by the same method. The ethanol solution is a pure ethanol solution, that is, absolute ethanol.

[0046] The principle of the ethanol immersion method is that due to the difference in the swelling coefficients of various materials, the base membrane and the separation layer will swell to different degrees after being treated with an ethanol solvent, resulting in a weakened stability at the interface. Therefore, this method can be used to test the magnitude of the interfacial binding force. The test results are shown in Table 1.

[0047] Comparative Example 1

[0048] To evaluate the effect of the pretreatment process on the stability of the separation layer of the enhanced composite nanofiltration membrane, an example without using the pretreatment process is designed as a blank control. The specific process is as follows:

[0049] (1) Use a base membrane made of polyamide. First, immerse the base membrane in a blank pure aqueous solution for 5 minutes, then take the base membrane out of the solution, and then place the base membrane in a blast drying oven at 120 °C for 5 minutes.

[0050] (2) Then place the base membrane in an aqueous solution of polyethyleneimine (weight average molecular weight 30,000) with a mass fraction of 1.00% for 10 minutes. Take the base membrane out of the polyethyleneimine aqueous solution. Then immerse the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of trimesoyl chloride with a mass fraction of 0.50% for 2 minutes. The organic solvent of the trimesoyl chloride organic phase solution is pentane. Take the above base membrane out of the organic phase (trimesoyl chloride organic phase solution) and place it in a blast drying oven at 120 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0051] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane filament is tested. The obtained results are as follows: the desalination rate of the composite nanofiltration membrane is 94.3%, and the water flux is 18.4 L / m 2 h.

[0052] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was retested for performance under the above conditions, and the results were as follows: the salt rejection rate was 80.4%, and the water flux was 30.4 L / m 2 h.

[0053] Example 1

[0054] (1) Formaldehyde and hexamethylenediamine were dissolved in water to prepare a pretreatment aqueous solution, where the mass fraction of formaldehyde was 0.20% and the mass fraction of hexamethylenediamine was 0.05%.

[0055] (2) A base membrane made of polyamide was used. The base membrane was immersed in the pretreatment aqueous solution (containing 0.20% mass fraction of formaldehyde and 0.05% mass fraction of hexamethylenediamine) for 1 minute; the base membrane was taken out of the pretreatment aqueous solution and then placed in a blast drying oven at 60 °C for 5 minutes.

[0056] (3) The pretreated base membrane was placed in an aqueous solution of polyethyleneimine (weight-average molecular weight 8000) with a mass fraction of 0.70% for 1 minute. The base membrane was taken out of the polyethyleneimine aqueous solution. Then, the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) was immersed in an organic phase solution of trimesoyl chloride with a mass fraction of 0.30% for 2 minutes, where the organic solvent of the trimesoyl chloride organic phase solution was pentane. Then, the above base membrane was taken out of the organic phase (trimesoyl chloride organic phase solution) and placed in a blast drying oven at 60 °C for 20 minutes. After washing with water, a composite nanofiltration membrane was obtained.

[0057] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane was tested, and the results were as follows: the salt rejection rate of the composite nanofiltration membrane was 94.7%, and the water flux was 15.8 L / m 2 h.

[0058] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was retested for performance under the above conditions, and the results were as follows: the salt rejection rate was 94.4%, and the water flux was 15.1 L / m 2 h.

[0059] Example 2

[0060] (1) Glyoxal and ethylenediamine were dissolved in water to prepare a pretreatment aqueous solution, where the mass fraction of glyoxal was 3.00% and the mass fraction of ethylenediamine was 0.50%.

[0061] (2) Use a base film made of polyamide. Immerse the base film in a pre-treatment aqueous solution (containing 3.00% by mass of glyoxal and 0.50% by mass of ethylenediamine) and soak for 2 minutes. Take the base film out of the pre-treatment solution and then place it in a forced-air drying oven at 80 °C for 7 minutes.

[0062] (3) Place the pre-treated base film in an aqueous solution of polyethyleneimine (weight-average molecular weight 10,000) with a mass fraction of 0.10% and hold for 5 minutes. Take the base film out of the polyethyleneimine aqueous solution. Then immerse the base film adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of terephthaloyl chloride with a mass fraction of 0.10%, submerge for 5 minutes, where the organic solvent of the terephthaloyl chloride organic phase solution is pentane. Then lift the above base film out of the organic phase (terephthaloyl chloride organic phase solution) and place it in a forced-air drying oven at 120 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0063] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane is tested, and the results are as follows: the desalination rate of the composite nanofiltration membrane is 91.4%, and the water flux is 25.8 L / m 2 h.

[0064] To evaluate the binding stability between the base film and the separation layer in the composite nanofiltration membrane, soak the above composite nanofiltration membrane in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane is re-tested for performance under the above conditions, and the results are as follows: the desalination rate is 91.2%, and the water flux is 26.3 L / m 2 h.

[0065] Example 3

[0066] (1) Dissolve glutaraldehyde and 1,2-propanediamine in water to prepare a pre-treatment aqueous solution, where the mass fraction of glutaraldehyde is 2.50% and the mass fraction of 1,2-propanediamine is 0.10%.

[0067] (2) Use a base film made of polyamide. Immerse the base film in the pre-treatment aqueous solution (containing 2.50% by mass of glutaraldehyde and 0.10% by mass of 1,2-propanediamine) and soak for 5 minutes. Take the base film out of the pre-treatment aqueous solution and then place it in a forced-air drying oven at 120 °C for 10 minutes.

[0068] (3) Immerse the pretreated base membrane in an aqueous solution of polyethyleneimine (weight-average molecular weight 10,000) with a mass fraction of 0.40% for 10 minutes. Take the base membrane out of the polyethyleneimine aqueous solution. Then immerse the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of phthaloyl chloride with a mass fraction of 0.20% for 5 minutes. The organic solvent of the phthaloyl chloride organic phase solution is cyclohexane. Then take out the above base membrane from the organic phase (phthaloyl chloride organic phase solution) and place it in a blast drying oven at 100 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0069] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane was tested, and the results are as follows: the desalination rate of the composite nanofiltration membrane is 96.4%, and the water flux is 12.8 L / m 2 h.

[0070] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was tested for performance under the above conditions again, and the results are as follows: the desalination rate is 95.9%, and the water flux is 13.5 L / m 2 h.

[0071] Example 4

[0072] (1) Dissolve glyoxal and 1,3-propanediamine in water to prepare a pretreated aqueous solution, where the mass fraction of glyoxal is 1.00% and the mass fraction of 1,3-propanediamine is 0.20%.

[0073] (2) Use a base membrane made of polyamide. Immerse the base membrane in the pretreated aqueous solution (containing glyoxal with a mass fraction of 1.00% and 1,3-propanediamine with a mass fraction of 0.20%) for 4 minutes; take the base membrane out of the pretreated aqueous solution and then place it in a blast drying oven at 90 °C for 10 minutes.

[0074] (3) Immerse the pretreated base membrane in an aqueous solution of polyethyleneimine (weight-average molecular weight 30,000) with a mass fraction of 1.00% for 1 minute. Take the base membrane out of the polyethyleneimine aqueous solution. Then immerse the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of isophthaloyl chloride with a mass fraction of 0.50% for 5 minutes. The organic solvent of the isophthaloyl chloride organic phase solution is heptane. Then take out the above base membrane from the organic phase (isophthaloyl chloride organic phase solution) and place it in a blast drying oven at 120 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0075] At 25°C and a pressure of 0.31 MPa, an aqueous solution of MgSO4 with a mass fraction of 0.20% was used as the test water sample to test the separation performance of the composite nanofiltration membrane. The results are as follows: the desalination rate of the composite nanofiltration membrane is 93.4%, and the water flux is 20.3 L / m 2 h.

[0076] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was retested under the above conditions. The results are as follows: the desalination rate is 93.1%, and the water flux is 21.5 L / m 2 h.

[0077] Example 5

[0078] (1) Formaldehyde and hexamethylenediamine were dissolved in water to prepare a pretreatment aqueous solution, where the mass fraction of formaldehyde was 1.00% and the mass fraction of hexamethylenediamine was 0.30%.

[0079] (2) A base membrane made of polyamide was used. The base membrane was immersed in the pretreatment aqueous solution (containing 1.00% mass fraction of formaldehyde and 0.30% mass fraction of hexamethylenediamine) for 2 minutes; the base membrane was taken out of the pretreatment solution and then placed in a forced-air drying oven at 80°C for 6 minutes.

[0080] (3) The pretreated base membrane was placed in an aqueous solution of polyethyleneimine (weight-average molecular weight 50000) with a mass fraction of 0.80% for 10 minutes. The base membrane was taken out of the polyethyleneimine aqueous solution. Then, the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) was immersed in an organic phase solution of 0.5% phthaloyl chloride for 5 minutes, where the organic solvent of the phthaloyl chloride organic phase solution was cyclohexane. Then, the above base membrane was taken out of the organic phase (phthaloyl chloride organic phase solution) and placed in a forced-air drying oven at 120°C for 20 minutes. After washing with water, a composite nanofiltration membrane was obtained.

[0081] At 25°C and a pressure of 0.31 MPa, an aqueous solution of MgSO4 with a mass fraction of 0.20% was used as the test water sample to test the separation performance of the composite nanofiltration membrane. The results are as follows: the desalination rate of the composite nanofiltration membrane is 95.3%, and the water flux is 20.1 L / m 2 h.

[0082] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was retested under the above conditions. The results are as follows: the desalination rate is 94.7%, and the water flux is 21.8 L / m 2 h.

[0083] Example 6

[0084] (1) Dissolve glyoxal and hexamethylenediamine in water to prepare a pretreatment aqueous solution, where the mass fraction of glyoxal is 1.50% and the mass fraction of hexamethylenediamine is 0.30%.

[0085] (2) Use a base film made of polyacrylonitrile. Immerse the base film in the pretreatment aqueous solution (containing 1.50% glyoxal by mass fraction and 0.30% hexamethylenediamine by mass fraction) for 3 minutes of immersion treatment; take out the base film from the pretreatment solution and then place it in a blast drying oven at 70 °C for 6 minutes.

[0086] (3) Place the pretreated base film in an aqueous solution of polyethyleneimine (weight-average molecular weight 8000) with a mass fraction of 0.60% for 3 minutes. Take out the base film from the polyethyleneimine aqueous solution. Then immerse the base film adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of trimesoyl chloride with a mass fraction of 0.20% for 2 minutes of immersion, where the organic solvent of the trimesoyl chloride organic phase solution is pentane. Then take out the above base film from the organic phase (trimesoyl chloride organic phase solution) and place it in a blast drying oven at 90 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0087] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane is tested, and the results are as follows: the desalination rate of the composite nanofiltration membrane is 95.3%, and the water flux is 18.9 L / m 2 h.

[0088] To evaluate the binding stability between the base film and the separation layer in the composite nanofiltration membrane, soak the above composite nanofiltration membrane in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane is re-tested for performance under the above conditions, and the results are as follows: the desalination rate is 94.6%, and the water flux is 19.8 L / m 2 h.

[0089] Example 7

[0090] (1) Dissolve glutaraldehyde and 1,2-propanediamine in water to prepare a pretreatment aqueous solution, where the mass fraction of glutaraldehyde is 2.00% and the mass fraction of 1,2-propanediamine is 0.50%.

[0091] (1) Use a base film made of polyamide. Immerse the base film in the pretreatment aqueous solution (containing 2.00% glutaraldehyde by mass fraction and 0.50% 1,2-propanediamine by mass fraction) for 5 minutes of immersion treatment; take out the base film from the pretreatment solution and then place it in a blast drying oven at 60 °C for 5 minutes.

[0092] (2) Immerse the pretreated base membrane in an aqueous solution of polyethyleneimine (weight-average molecular weight 50,000) with a mass fraction of 0.90% for 9 minutes. Take out the base membrane from the polyethyleneimine aqueous solution. Then immerse the base membrane adsorbed with the aqueous phase (polyethyleneimine aqueous solution) in an organic phase solution of terephthaloyl chloride with a mass fraction of 0.50% for 5 minutes. The organic solvent of the terephthaloyl chloride organic phase solution is heptane. Next, take out the above base membrane from the organic phase (terephthaloyl chloride organic phase solution) and place it in a blast drying oven at 120 °C for 20 minutes. After washing with water, a composite nanofiltration membrane is obtained.

[0093] At 25 °C and a pressure of 0.31 MPa, using an aqueous solution of MgSO4 with a mass fraction of 0.20% as the test water sample, the separation performance of the composite nanofiltration membrane was tested, and the results are as follows: The desalination rate of the composite nanofiltration membrane is 97.5%, and the water flux is 16.7 L / m 2 h.

[0094] To evaluate the binding stability between the base membrane and the separation layer in the composite nanofiltration membrane, the above composite nanofiltration membrane was immersed in an ethanol solution for 24 h. After washing with water, the composite nanofiltration membrane was re-tested for performance under the above conditions, and the results are as follows: The desalination rate is 97.1%, and the water flux is 17.1 L / m 2 h.

[0095] Summary

[0096] The binding stability of the separation layer can be reflected by the stability of the desalination rate. The less the desalination rate decreases, the better the binding stability between the separation layer and the composite nanofiltration membrane (that is, the fewer problems of separation layer shedding or partial shedding exist, and the better the binding condition between the separation layer and the composite nanofiltration membrane). According to the scheme in Table 1, the binding stability of the separation layer was tested. During the test, the average desalination rate of the composite nanofiltration membrane (Examples 1-7) decreased from 94.9% to 94.4%, and the decrease rate was 0.5%, indicating that the binding stability of the separation layer was basically not affected; the test data of Comparative Example 1 showed that the desalination rate decreased from 94.3% to 80.4%, and the decrease rate was 13.9%, indicating that the binding stability of the separation layer was significantly lower than that of the examples of the present application. Therefore, the present application can effectively solve the problem of separation layer shedding of the composite nanofiltration membrane through pretreatment, ensure the binding stability of the separation layer of the composite nanofiltration membrane, and thus ensure the desalination stability of the composite nanofiltration membrane.

[0097] Table 1: Influence of pre-crosslinking treatment of base membrane on the performance and stability of composite nanofiltration membrane

[0098]

[0099] The above are only some specific embodiments of the present invention. The protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. Preparation method of composite nanofiltration membrane, characterized in that, It includes the following steps: Step 1: Dissolve an aldehyde crosslinking agent and a polyamine monomer in water to prepare a pretreatment aqueous solution, and immerse the base film in the pretreatment aqueous solution once to obtain a base film with a crosslinked layer attached to its surface; Step 2: Immerse the base film with a crosslinked layer attached to its surface in a polyethyleneimine aqueous solution for a second time, and then immerse it in an organic phase solution containing an acyl chloride monomer for a third time to obtain a highly crosslinked composite nanofiltration membrane. Among them, in the pretreatment aqueous solution of Step 1, the mass fraction of the aldehyde crosslinking agent is 0.20 - 3.00%, and the mass fraction of the polyamine monomer is 0.05 - 0.50%. The combination of the selected mass fraction of the polyamine monomer and the mass fraction of the aldehyde crosslinking agent ensures that the aldehyde crosslinking agent is in excess in the reaction to ensure that the generated crosslinked layer has an excess of aldehyde groups; In Step 2, the mass fraction of polyethyleneimine in the polyethyleneimine aqueous solution is 0.10 - 1.00%. Polyethyleneimine reacts with both the aldehyde groups on the crosslinked layer and acyl chloride to generate a separation layer. Among them, polyethyleneimine serves as an intermediate linker to synchronously connect the crosslinked layer and the separation layer.

2. The method for preparing a composite nanofiltration membrane according to claim 1, wherein, The aldehyde crosslinking agent in Step 1 is one or more of formaldehyde, glyoxal, and glutaraldehyde.

3. The method for preparing a composite nanofiltration membrane according to claim 1, wherein The polyamine monomer in Step 1 is one or more of ethylenediamine, 1,2 - propanediamine, 1,3 - propanediamine, and hexamethylenediamine.

4. The preparation method of the composite nanofiltration membrane according to claim 1, characterized in that, After the first immersion in Step 1, place the base film with a crosslinked layer attached to its surface at a temperature of 20 - 120 °C for the first heat treatment, and the treatment time is 5 - 30 minutes.

5. The preparation method of the composite nanofiltration membrane according to claim 1, wherein The weight - average molecular weight of the polyethyleneimine in Step 2 is selected from 8000 - 50000.

6. The method for preparing the composite nanofiltration membrane according to claim 1, wherein The acyl chloride monomer in Step 2 is one or more of trimesoyl chloride, terephthaloyl chloride, phthaloyl chloride, isophthaloyl chloride, and biphenyl - 4,4'-dicarbonyl chloride.

7. The method for preparing the composite nanofiltration membrane according to claim 1, wherein In the organic phase solution of Step 2, the mass fraction of the acyl chloride monomer is 0.10 - 0.50%, and the solvent of the organic phase solution is one or more of pentane, hexane, cyclohexane, and heptane.

8. The method for preparing a composite nanofiltration membrane according to claim 1, wherein After the third immersion in Step 2, place the base film at a temperature of 20 - 120 °C for the second heat treatment, and the treatment time is 5 - 30 minutes.

9. A composite nanofiltration membrane, characterized in that, It is prepared by using the preparation method described in any one of claims 1 to 8.

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