Composite nanofiltration membrane as well as preparation method and application thereof

Abstract

The invention provides a composite nanofiltration membrane, a preparation method and application of the composite nanofiltration membrane in filtering and intercepting emerging pollutants. The composite nanofiltration membrane comprises a porous base membrane supporting layer, a hydrophobic middle layer and a filtering skin layer, and the porous base membrane supporting layer and the filtering skin layer are both made of hydrophilic organic materials; the hydrophobic middle layer is made of a hydrophobic organic material and is of a directional arrangement structure; the thickness of the hydrophobic middle layer is smaller than that of the porous base membrane supporting layer. The preparation method comprises the following steps: S1, obtaining the porous base membrane supporting layer; s2, preparing a hydrophobic middle layer with a directional arrangement structure on the porous base membrane supporting layer; s3, performing interfacial polymerization reaction on the hydrophobic middle layer to form a filtering skin layer. By adopting the composite nanofiltration membrane, the problem of collapse of a filtering cortex layer is effectively prevented, the composite nanofiltration membrane has excellent water flux and pollutant interception performance, and the interception effect on emerging pollutants has remarkable stability and anti-pollution performance.

Description

Technical Field

[0001] The present application relates to the field of water treatment, and in particular to a composite nanofiltration membrane and a preparation method thereof, as well as its application in filtering and intercepting emerging micropollutants. Background Art

[0002] Nanofiltration membranes can effectively remove colloids, bacteria, suspended matter, organic matter and some soluble ions in water. They can be used for seawater desalination, drinking water and wastewater treatment. They have lower energy consumption, a relatively loose structure, and a relatively high water flux. In addition, water treatment technology based on nanofiltration membranes has unique advantages in terms of simple and convenient operation, scalability, small footprint and energy efficiency, and has received widespread attention.

[0003] The traditional composite nanofiltration membrane is composed of a porous substrate support layer and a dense filter cortex formed by interfacial polymerization. However, there is a trade-off effect between permeability and solute selectivity in the composite nanofiltration membrane, especially for the composite nanofiltration membrane with a large-pore substrate support layer, whose filter cortex is easy to collapse on the large-pore substrate support layer, thereby affecting the permeability of the composite nanofiltration membrane. Patent application CN115501753A discloses a method for preparing a nanofiltration membrane based on a nuclear pore membrane support layer, including preparing a nuclear pore support membrane and generating a selective layer on the support layer by interfacial polymerization, the purpose of which is to solve the problem that as the porosity of the support layer increases, the bonding force between the support layer and the selective layer will weaken due to the increase in the pores of the support layer, resulting in the prepared nanofiltration membrane being easily damaged and having poor performance. However, the above patent method still cannot solve the problems such as the collapse of the filter cortex, which leads to a decrease in water flux. In addition, many current studies have also considered improving the flux and selectivity of the membrane at the same time by introducing intermediate layer modifications. However, the intermediate layers of existing composite nanofiltration membranes are mostly deposited or coated with modified materials on the substrate. The prepared composite membranes usually face the problem of monomers easily invading the pores of the matrix, causing interfacial polymerization reactions to occur in the pores of the substrate rather than on the surface, which in turn causes defects in the cortex, greatly reducing the separation performance of the membrane. Summary of the invention

[0004] In order to solve the above technical problems, the embodiments of the present application provide a composite nanofiltration membrane with an organic hydrophobic intermediate layer and a preparation method, and a nanofiltration membrane for filtering and intercepting emerging pollutants. Using the composite nanofiltration membrane provided by the present application, an organic hydrophobic intermediate layer with a directional arrangement structure is formed on the hydrophilic porous base membrane support layer, and the interface polymerization is formed on the intermediate layer to form a filter cortex, which effectively solves the collapse problem of the filter cortex and improves the water flux. At the same time, the preparation method has the advantages of simple operation and low production cost. The nanofiltration membrane for filtering and intercepting emerging pollutants provided by the present application has excellent water flux and pollutant retention performance, and has significant stability and anti-pollution performance for the interception effect of emerging pollutants.

[0005] On the one hand, the present application provides a composite nanofiltration membrane, comprising a porous base membrane support layer, a hydrophobic intermediate layer and a filtration skin layer, wherein the porous base membrane support layer and the filtration skin layer are both composed of hydrophilic organic materials; the hydrophobic intermediate layer is composed of hydrophobic organic materials and is arranged to have a directional arrangement structure; the thickness of the hydrophobic intermediate layer is less than that of the porous base membrane support layer.

[0006] Preferably, the hydrophobic intermediate layer is a nanofiber intermediate layer with a directional arrangement structure, which forms a thin layer on the porous base membrane support layer, and the thin layer forms a completely hydrophobic surface on the porous base membrane support layer; the thickness of the nanofiber intermediate layer is 100nm-1um.

[0007] Preferably, the nanofiber intermediate layer has slit-like pores.

[0008] Preferably, the porous base membrane support layer is a polyethersulfone base membrane, and its average pore size is 0.35nm-0.5nm; the nanofiber intermediate layer is a polyvinylidene fluoride material, and its nanofibers have an average pore size of 0.35nm-0.5nm.

[0009] Preferably, the filter skin layer is a polyamide layer, and the filter skin layer has a dense structure, a thickness of 47nm-94nm, preferably 50nm; and a pore size of 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

[0010] Preferably, the water contact angle of the composite nanofiltration membrane is between 19.6° and 33.9°.

[0011] The second aspect of the present invention, as Figure 1a As shown, a method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane is provided, comprising the following steps:

[0012] S1. Obtaining a hydrophilic porous base membrane support layer;

[0013] S2. Preparing a hydrophobic intermediate layer having an oriented structure on the porous base membrane support layer, wherein the hydrophobic intermediate layer is composed of an organic material;

[0014] S3. Performing an interfacial polymerization reaction on the hydrophobic intermediate layer to form a hydrophilic filtration skin layer.

[0015] Preferably, the step S2 is implemented in the following manner: the organic hydrophobic intermediate layer is a nanofiber intermediate layer, and the nanofiber intermediate layer is prepared on a porous base membrane support layer by electrospinning.

[0016] Preferably, the step S2 further comprises drying and hot pressing the porous base membrane support layer supporting the nanofiber intermediate layer.

[0017] Preferably, the nanofiber middle layer is prepared by the following method: preparing an electrospinning solution, wherein the electrospinning solution is a polyvinylidene fluoride (PVDF) spinning solution, and obtaining the nanofiber middle layer by electrospinning.

[0018] Preferably, the polyvinylidene fluoride (PVDF) spinning solution is prepared by completely dissolving a certain mass percentage of polyvinylidene fluoride (PVDF) polymer and tetrabutylammonium chloride (TBAC) in a certain volume ratio of dimethylformamide (DMF) and acetone solvent to prepare the polyvinylidene fluoride (PVDF) spinning solution.

[0019] Preferably, the preparation method of the polyvinylidene fluoride (PVDF) spinning solution includes: dissolving 17% by mass of polyvinylidene fluoride (PVDF) polymer and 0.01% of tetrabutylammonium chloride in a dimethylformamide (DMF) and acetone solvent in a volume ratio of 1:3, completely dissolving under constant temperature conditions, and standing at room temperature of 15°C to 25°C for degassing for more than 24 hours.

[0020] Preferably, the electrospinning method is performed by an electrospinning device, and the rotation speed of the electrospinning device is set between 100 rpm and 800 rpm.

[0021] Preferably, the conditions for the electrospinning are: applying a voltage of 20 kV, maintaining the ambient humidity at 40% RH, setting the rotation speed to 700 rpm, and the duration of the electrospinning is 5 to 20 minutes.

[0022] Preferably, the step S3 performs interfacial polymerization reaction in the following manner: a piperazine (PIP) aqueous solution with a mass percentage of 0.2%-2% is evenly distributed on the surface of the polyvinylidene fluoride (PVDF) nanofiber middle layer, and after drying, the polyvinylidene fluoride (PVDF) nanofiber middle layer is immersed in a n-hexane solution of trimesoyl chloride (TMC) with a mass percentage of 0.1%-0.2% for a certain period of time to induce a polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC) to form the filter cortex.

[0023] Preferably, the step S3 comprises: fixing the porous base membrane support layer loaded with the nanofiber intermediate layer obtained in the step S2 with a frame; pouring a 0.5% by mass piperazine (PIP) aqueous solution on the surface of the nanofiber intermediate layer and letting it stand for 3 to 7 minutes, draining and drying the excess solution; immersing the membrane obtained in the above step in a 0.1% by mass solution of trimesoyl chloride (TMC) in n-hexane for 1 to 6 minutes to induce a polymerization reaction between PIP and TMC; and drying the membrane prepared in the above step and then curing it, the curing time being 30 to 90 seconds.

[0024] Specifically, in a further embodiment, Figure 1b As shown, the specific preparation method of the composite nanofiltration membrane of the present application is obtained by electrospinning, comprising the following steps:

[0025] S1-1: First, soak the purchased polyethersulfone (PES) base membrane and place it in the electrospinning device;

[0026] S2-1: preparing the electrospinning solution required for electrospinning;

[0027] S3-1: Preparation of a polyethersulfone (PES) based membrane with an intermediate layer of polyvinylidene fluoride (PVDF) nanofibers having an oriented structure;

[0028] S4-1: Place the polyethersulfone (PES) base membrane loaded with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer on a flat plate with the polyethersulfone (PES) base membrane layer facing upward, and perform interfacial polymerization on the base membrane polyethersulfone (PES) base membrane to obtain a polyvinylidene fluoride (PVDF) nanofiber intermediate layer composite nanofiltration membrane with selective permeability.

[0029] Preferably, in step S1-1: the polyethersulfone (PES) base membrane is placed and fixed in an electrospinning device according to the experimental steps, so as to facilitate the subsequent electrospinning method to load the polyvinylidene fluoride (PVDF) nanofiber intermediate layer onto the polyethersulfone (PES) base membrane.

[0030] In the step S2-1, 17% of polyvinylidene fluoride (PVDF) polymer and a small amount of 0.01% of tetrabutylammonium chloride (TBAC) were dissolved in a dimethylformamide (DMF) and acetone solvent with a volume ratio of 1:3, and then stirred at a constant temperature of 60°C in a water bath for 10 hours to completely dissolve the polymer. After the solution was stirred evenly, it was allowed to stand at room temperature of 20±5°C for 24 hours for degassing to obtain the electrospinning solution required for the experiment.

[0031] In the step S3-1: pour 2 mL of the electrospinning solution obtained in S2-1 into two 10 mL syringes, feed the spinning solution into the spinneret at a constant rate through a syringe pump, and then apply a DC voltage between the spinneret and the stainless steel drum. Under the influence of the electric field, the polymer solution is continuously electrospun into ultrafine nanofibers, the nanofibers dry and bond to each other to form a new base film, and the electrospinning speed is adjusted between 100 rpm and 800 rpm to give it a directional structure. After the electrospinning is completed, the membrane is placed in a drying oven at 60°C for 24 hours to remove the residual solvent.

[0032] In the step S4-1, an interfacial polymerization reaction is carried out on the polyethersulfone (PES) based membrane loaded with the polyvinylidene fluoride (PVDF) nanofiber intermediate layer to obtain a composite nanofiltration membrane having a polyamide (PA) filter skin layer, comprising the following steps:

[0033] S5: soaking the polyethersulfone (PES)-based membrane loaded with the polyvinylidene fluoride (PVDF) nanofiber intermediate layer in an aqueous solution containing a polyamine compound, and removing excess aqueous solution on the polyethersulfone (PES)-based membrane after a first specified time to obtain a pretreated membrane;

[0034] S6: performing surface treatment on the pre-treated film;

[0035] S7: pouring the organic phase solution containing the polyacyl chloride compound onto the pretreated membrane treated in step (S6) to carry out an interfacial polymerization reaction, and removing excess organic phase solution after a second specified time to obtain a composite nanofiltration membrane having a polyamide (PA) filter skin layer.

[0036] In the step S4-1: the polyethersulfone (PES) base membrane is placed flat on a smooth flat plate with the polyethersulfone (PES) base membrane facing upward. In the embodiment of the present application, the smooth flat plate is a smooth glass plate, and further includes a rubber ring of a custom size and a stainless steel frame; the rubber ring and the stainless steel frame are fixed on the glass plate, and further include a mounting fixture and a leak-proof device;

[0037] The method also includes the steps of preparing an aqueous phase solution and an organic phase solution respectively.

[0038] Preferably, the aqueous phase solution is a polyamine compound aqueous phase solution, and the organic phase solution is a polyacyl chloride compound organic phase solution; in the embodiment of the present application, the aqueous phase solution is a piperazine (PIP) aqueous phase solution, and the organic phase solution is a trimesoyl chloride (TMC) organic phase solution;

[0039] Preferably, step S5 is to pour an aqueous solution into the frame to soak the base film, remove excess aqueous solution after a first specified time, and dismantle the fixing device and the leak-proof device to obtain a pretreated film having an amine monomer aqueous solution on the surface.

[0040] Preferably, the step S6 is to use a collecting roller to roll over the surface of the pre-treated membrane to evenly distribute the aqueous solution containing the amine monomer on the membrane surface, making the membrane smoother. The collecting roller in the embodiment of the present application is made of rubber.

[0041] Preferably, step S7 is to pour the organic phase solution into the frame for reaction, and pour out the excess organic phase solution after the second specified time; since the two-phase solutions are incompatible with each other, the amine monomers in the aqueous phase diffuse into the organic phase solution, and the monomers in the two-phase solution can only contact and react at the interface between the two phases. In addition, the reaction rate between the monomers is extremely fast, and a polyamide (PA) filter cortex can be formed on the surface of the support layer in a very short time.

[0042] Preferably, the method further comprises the following step S8: curing the prepared membrane after naturally drying it in the air, and obtaining a nanofiber intermediate layer composite nanofiltration membrane having a polyamide (PA) filter skin layer after a third specified time;

[0043] S9: The nanofiber middle layer composite nanofiltration membrane with a polyamide (PA) filter skin layer is stored in deionized water until testing; preferably, it is stored in deionized water at a temperature of 4°C.

[0044] Preferably, the process flow for preparing the aqueous phase solution is to dissolve piperazine (PIP) in deionized water in an opaque brown container and then ultrasonicate for 10-50 minutes; in the embodiment of the present application, 0.5 wt% piperazine (PIP) is dissolved in 50 mL of deionized water and ultrasonicate for 30 minutes to completely dissolve it.

[0045] Preferably, in the process of preparing the aqueous phase solution, piperazine (PIP) is dissolved in deionized water in a light-proof brown container.

[0046] Preferably, the process flow for preparing the organic phase is to dissolve 1,3,5-benzenetricarboxylic acid chloride (TMC) in a n-hexane solvent and then ultrasonicate for 10-50 minutes. In the embodiment of the present application, 0.1wt% of 1,3,5-benzenetricarboxylic acid chloride (TMC) is dissolved in 50mL of n-hexane solution and then ultrasonicate for 30 minutes to completely dissolve it.

[0047] Preferably, the mass percentage of piperazine (PIP) in the aqueous phase solution is 0.2 wt%-2 wt%, and the mass percentage of 1,3,5-benzenetricarboxylic acid chloride (TMC) in the organic phase solution is 0.1 wt%-0.2 wt%.

[0048] Preferably, the first designated time is 3-7 minutes, more preferably 5 minutes; the second designated time is 1-6 minutes, more preferably 2 minutes; the third designated time is 30-90 seconds, more preferably 60 seconds.

[0049] The third aspect of the present application provides an organic hydrophobic intermediate layer composite nanofiltration membrane, which is prepared by any preparation method described in the second aspect above.

[0050] In a fourth aspect of the present invention, a nanofiltration membrane is provided for filtering and intercepting emerging micropollutants, which is prepared by any preparation method of the second aspect above, wherein the emerging micropollutants are not limited to paraben compounds, sulfamethoxazole (SMX) and trimethoprim (TMP).

[0051] The beneficial effect of the present invention is that a hydrophobic intermediate layer with a directional arrangement structure is constructed on a porous base membrane support layer, which not only improves the formation of the filter cortex and prevents the filter cortex from collapsing, but also optimizes the path for the transmission of water molecules, enhances the precision separation ability of the membrane, and improves the interception effect of the nanofiltration membrane on emerging pollutants, anti-pollution performance, pressure resistance, salt resistance and long-term test stability. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane of the present invention has the advantages of simple operation and low material cost, and provides a better performance option for nanofiltration membranes and their preparation in the field of water treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0053] Figure 1a , Figure 1b The preparation process of the composite nanofiltration membrane of different embodiments of the present application;

[0054] Figure 2a-2d Schematic diagram of the scanning electron microscope (SEM) image of the surface structure of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) nanofiber intermediate layer at different rotation speeds;

[0055] Figure 3 Schematic diagram of the contact angle of the middle layer of polyvinylidene fluoride (PVDF) nanofibers at different rotation speeds;

[0056] Figure 4a-4d Schematic diagram of scanning electron microscope (SEM) on the surface of composite nanofiltration membrane at different rotation speeds;

[0057] Figure 5a-5d Schematic diagram of atomic force microscopy (AFM) of the roughness of the composite nanofiltration membrane surface at different rotation speeds;

[0058] Figure 6 The schematic diagram of UV-visible spectrum after piperazine (PIP) is diffused in n-hexane through composite nanofiltration membrane at different rotation speeds;

[0059] Figure 7a It is a schematic diagram of the water contact angle of the composite nanofiltration membrane prepared at different rotation speeds;

[0060] Figure 7b It is a schematic diagram of the surface zeta potential of the composite nanofiltration membrane prepared at different rotation speeds;

[0061] Figure 7c is a schematic diagram of the absorption characteristics of the composite nanofiltration membrane prepared at different rotation speeds;

[0062] Figure 7d It is a schematic diagram of the elemental composition of the polyamide (PA) filter cortex of the composite nanofiltration membrane prepared at different rotation speeds;

[0063] Figure 8a-8b It is a schematic diagram of the pure water permeability and interception performance of the intermediate layer composite nanofiltration membrane at different rotation speeds;

[0064] Fig. 9 Schematic diagram of pure water flux and Na2SO4 salt retention of composite nanofiltration membranes at different rotation speeds;

[0065] Figure 10a-Figure 10b It is a schematic diagram of the distribution of hydraulic resistance of each layer of composite nanofiltration membrane at different rotation speeds;

[0066] Figure 11a-Figure 11b Schematic diagram of the permeation flux and retention performance of different salts of composite nanofiltration membranes at different rotation speeds;

[0067] Fig.12 Schematic diagram of the retention performance of PA / PVDF1 and PA / PVDF7 for substances with different molecular weights;

[0068] Figure 13a-13b Schematic diagram of the rejection performance of composite nanofiltration membranes at different rotation speeds for emerging micropollutants (EMPs);

[0069] Figure 14a-Figure 14c Schematic diagram of the stability performance of composite nanofiltration membrane at different rotation speeds;

[0070] Figure 15a-15f Schematic diagram of the anti-fouling performance of PA / PVDF1 and PA / PVDF7 composite nanofiltration membranes. DETAILED DESCRIPTION

[0071] In order to make the technical means, creative features, objectives and effects achieved by this application easy to understand, the application is further explained below with reference to specific diagrams.

[0072] In one aspect of the present application, a composite nanofiltration membrane is provided, comprising a porous base membrane support layer, a hydrophobic middle layer and a filtration skin layer, wherein the hydrophobic middle layer is made of an organic material and is arranged to have a directional arrangement structure.

[0073] Furthermore, the hydrophobic middle layer is a nanofiber middle layer with a directional arrangement structure, which forms a thin layer on the porous base membrane support layer, and the thin layer forms a completely hydrophobic surface on the porous base membrane support layer; the thickness of the nanofiber middle layer is 100nm-1um. In a preferred embodiment, the nanofiber middle layer is a polyvinylidene fluoride material, the average pore size of the nanofiber is 0.35nm-0.5nm, and the nanofiber middle layer has slit-like pores.

[0074] Furthermore, the porous base membrane support layer is a polyethersulfone base membrane with an average pore size of 0.35nm-0.5nm; the filter skin layer is a polyamide layer with a dense structure and a thickness of 47nm-94nm, preferably 50nm; the pore size is 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

[0075] Another aspect of the present invention provides a nanofiltration membrane for intercepting and filtering emerging micropollutants, which include but are not limited to parabens, sulfamethoxazole (SMX) and trimethoprim (TMP).

[0076] The present invention also provides a method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane, such as Figure 1a As shown, the following steps are included:

[0077] S1. Obtaining a hydrophilic porous base membrane support layer;

[0078] S2. Preparing a hydrophobic intermediate layer having a directional arrangement structure on the porous base membrane support layer, wherein the hydrophobic intermediate layer is configured to be composed of an organic material;

[0079] S3. An interfacial polymerization reaction is carried out on the hydrophobic middle layer to form a hydrophilic filter skin layer.

[0080] In one embodiment of the present invention, a polyethersulfone (PES) base membrane support layer is used as a porous base membrane support layer, which has the characteristics of hydrophilicity; a nanofiber intermediate layer is used as a hydrophobic intermediate layer, and the nanofiber intermediate layer is loaded on the polyethersulfone (PES) base membrane by an electrospinning method, and the base membrane thickness is between 100um-300um, so that the nanofiber intermediate layer has a directional arrangement structure, and the thickness of the nanofiber intermediate layer is set at 100nm-1um; then an interfacial polymerization reaction is carried out on the nanofiber intermediate layer to form a hydrophilic filtration cortex. In fact, this embodiment only optimizes the material for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane. In actual use, a hydrophilic porous base membrane support layer is used, and a hydrophobic intermediate layer with a directional arrangement structure is formed thereon, and then a hydrophilic filtration cortex is formed after an interfacial polymerization reaction, which can well solve the problem of the filtration cortex collapsing into the pores of the base membrane support layer in the prior art, optimize the performance of the composite nanofiltration membrane, and improve the water flux.

[0081] In a preferred embodiment, the specific operation steps are as follows:

[0082] A polyethersulfone (PES) base membrane support layer is used as the porous base membrane support layer;

[0083] Then, an electrospinning solution with a hydrophobic organic material is prepared. The electrospinning solution with a hydrophobic organic material used in this embodiment is a polyvinylidene fluoride (PVDF) spinning solution, which is prepared in the following manner: a certain mass percentage of polyvinylidene fluoride (PVDF) polymer and tetrabutylammonium chloride (TBAC) are completely dissolved in a certain volume ratio of dimethylformamide (DMF) and acetone solvent to prepare a polyvinylidene fluoride (PVDF) spinning solution. In this embodiment, the mass percentage of the polyvinylidene fluoride (PVDF) polymer is 17%, and the mass percentage of tetrabutylammonium chloride (TBAC) is 0.01%. In order to achieve better results, the polymer powder is completely dissolved by stirring in a 60°C water bath after dissolution, which can be carried out under constant temperature conditions. After the spinning solution is stirred evenly, it is allowed to stand and degas at room temperature, such as 20±5°C for more than 24 hours.

[0084] Next, a nanofiber intermediate layer with a directional arrangement structure is prepared on the polyethersulfone (PES) base membrane support layer by electrospinning. The spinning solution is fed into the spinneret of the electrospinning device by a syringe pump, and then a DC voltage is applied between the spinneret and the stainless steel drum. Under the influence of the electric field, the polyvinylidene fluoride (PVDF) spinning solution is continuously electrospun into ultrafine nanofibers, which dry and bond to each other to form a new base membrane. The surface roughness of the polyethersulfone (PES) base membrane support layer after loading the intermediate layer is reduced, and it has a relatively smooth surface structure with suitable porosity and pore size. In this embodiment, the degassed polyvinylidene fluoride (PVDF) spinning solution is poured into two 10mL syringes, 2mL each, and a voltage of 20kV is applied. The ambient humidity is maintained at about 40%RH, the rotation speed is set to 100rpm, the syringe pump propulsion rate, round-trip distance and moving speed are set, and electrospinning is performed for a certain period of time. The fibers are deposited on the surface of the collector wrapped with a circle of PES filter membrane. The polyethersulfone (PES) base membrane carrying the nanofiber middle layer is removed from the collector surface and moved into a 60°C oven for drying. Then, the membrane can be hot-pressed to tightly combine the nanofiber middle layer and the base membrane support layer.

[0085] By adjusting the rotation speed of the electrospinning instrument, at a high speed, the spinning solution jet is pulled by the rapidly rotating collector and oriented along the direction of its rotation. The rotation speed of the electrospinning device is between 100 rpm and 800 rpm, and the duration of electrospinning is generally selected from 5 min to 20 min. In this embodiment, the rotation speed is 100 rpm, and the obtained nanofiber intermediate layer is a directional polyvinylidene fluoride (PVDF) nanofiber thin layer.

[0086] Then, an interfacial polymerization reaction is carried out on the nanofiber intermediate layer with a directional arrangement structure to form a filter cortex. It includes: after removing the membrane containing the intermediate layer after electrostatic spinning from the collector surface, the membrane containing the intermediate layer is fixed by a frame after drying, a certain mass percentage of piperazine (PIP) aqueous solution is evenly distributed on the surface of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer, staying for 3 to 7 minutes, draining the excess solution and drying, in this embodiment, staying and standing for 4 minutes, and after drying, the polyvinylidene fluoride (PVDF) nanofiber intermediate layer is immersed in a certain mass percentage of benzene trimecarboxylic acid chloride (TMC) n-hexane solution for 1 to 6 minutes, in this embodiment, immersed for 2 minutes, inducing the polymerization reaction between piperazine (PIP) and benzene trimecarboxylic acid chloride (TMC), and the membrane prepared in the above steps is dried and cured, the curing time is 30 seconds to 90 seconds, and 60 seconds in this embodiment, to form a polyamide (PA) cortex. Through the above method, the thickness of the filter cortex obtained is 47nm-94nm.

[0087] In this embodiment, the mass percentage of the piperazine (PIP) aqueous solution is 0.5%, and the mass percentage of the trimesoyl chloride (TMC) n-hexane solution is 0.1%.

[0088] Furthermore, in another embodiment of the present invention, the rotation speeds of 400 rpm, 700 rpm and 800 rpm are selected to spin the intermediate layer for 20 min, respectively, to prepare polyvinylidene fluoride (PVDF) nanofiber intermediate layers with different orientation degrees.

[0089] According to the different rotation speeds, the polyvinylidene fluoride (PVDF) nanofiber middle layer spun at a rotation speed of 100 rpm for 20 min in the embodiment is marked as PVDF1. Similarly, the PVDF nanofiber middle layers prepared at rotation speeds of 400 rpm, 700 rpm, and 800 rpm are marked as PVDF4, PVDF7, and PVDF8.

[0090] FIG2 is a scanning electron microscope (SEM) image of the surface structure of a polyvinylidene fluoride (PVDF) nanofiber middle layer at different rotation speeds obtained using a cold field scanning electron microscope, from which it can be seen that a directional nanofiber middle layer is formed.

[0091] Figure 3The figure is a schematic diagram of using a contact angle meter to measure the static contact angle of the surface of a polyvinylidene fluoride (PVDF) nanofiber intermediate layer composite nanofiltration membrane at different rotation speeds. Figure 3 As shown, with the increase of rotation speed, the contact angles of PVDF1, PVDF4, PVDF7 and PVDF8 are 148.7°, 125.5°, 103.4° and 109.2° respectively. The contact angles of the nanofiber substrates with obvious oriented arrangement trend (PVDF7 and PVDF8) are significantly lower than those of the randomly oriented nanofiber substrates (PVDF1 and PVDF4), and all are in a hydrophobic state.

[0092] Figure 4a-4d The schematic diagram of the surface scanning electron microscope (SEM) of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) nanofiber intermediate layer at different rotation speeds. As shown in the figure, interfacial polymerization was carried out on the polyvinylidene fluoride (PVDF) intermediate layer at different rotation speeds, and the newly synthesized composite nanofiltration membrane was characterized by scanning electron microscope (SEM) morphology. It can be seen that the relatively dense polyamide (PA) filter cortex is significantly affected by the underlying fibers. Even after the composite nanofiltration membrane is synthesized, the fiber morphology can still be clearly observed, proving that the polyamide (PA) layer is successfully formed on the polyvinylidene fluoride (PVDF) nanofiber intermediate layer. As the rotation speed increases from 100rpm to 800rpm, the nanofibers gradually align in the direction of the collector roller rotation to form a nanofiber intermediate layer with slit-like pores. The morphology of the polyamide (PA) filter cortex gradually changes from the polyamide (PA) layer structure along the fiber nodules to a pleated structure. The surface of the PA / PVDF7 membrane presents a bamboo-like pleated stripe structure on both sides of the fibers in the directional arrangement structure. The nanofiber middle layer with a directional arrangement structure allows the piperazine (PIP) aqueous solution to diffuse evenly along the fiber surface, and limits the diffusion of the piperazine (PIP) aqueous solution into the pores in the slit-like pores of the nanofiber middle layer, so that it will not collapse into the pores to form defects, and form a polyamide (PA) filter skin layer with multiple folds on the top of the support membrane.

[0093] Figure 5a-5dA schematic diagram of atomic force microscopy (AFM) for measuring the roughness of the composite nanofiltration membrane surface at different rotation speeds using an atomic force microscope. The root mean square roughness (Rq) and arithmetic mean roughness (Ra) were obtained through software analysis. As shown in the figure, the roughness images of all composite nanofiltration membranes show typical fiber morphology. When the rotation speed increases from 100rpm to 400rpm, 700rpm, and 800rpm, the randomly arranged nanofiber imprint gradually weakens and begins to gradually show the morphology of oriented fiber morphology. The roughness in Figure 5 corresponds to the morphology in the scanning electron microscope in Figure 4. With the increase of the rotation speed of the electrospinning system, the surface roughness of the nanofiber nanofiltration membrane with different orientations first decreases and then increases. The surface roughness of PA / PVDF7 is the smallest (Ra=62nm). The smooth and wrinkled membrane surface is conducive to increasing the surface area of ​​the membrane and enhancing the permeability of the membrane, making it more advantageous in improving anti-pollution performance.

[0094] Figure 6 UV-visible spectrophotometer was used to detect the change in the diffusion rate of piperazine (PIP) solution. It can be seen that when the rotation speed increases from 100 rpm to 400 rpm and then to 700 rpm, the contact surface structure of piperazine (PIP) gradually tends to a slit-like and uniformly distributed pore surface. The surface chemical properties and directional fiber structure of polyvinylidene fluoride (PVDF) modified membranes at different rotation speeds can make the amine monomer more evenly distributed.

[0095] Furthermore, Figure 6 It shows that as the speed increases, the uniform distribution of slit-like pores leads to strong diffusion drive, and the increase in the absorption peak intensity represents the increase in the diffusion rate of piperazine (PIP), further forming a wrinkled polyamide (PA) filter cortex. However, the too high speed of 800rpm makes the fibers tend to be randomly arranged, making the pores more freely distributed, resulting in a decrease in the diffusion rate.

[0096] Figure 7a It is the water contact angle of the composite nanofiltration membrane prepared by the polyvinylidene fluoride (PVDF) nanofiber middle layer at different rotation speeds. The water contact angles of PA / PVDF1, PA / PVDF4, PA / PVDF7 and PA / PVDF8 membranes are 33.9°, 20.5°, 19.6° and 31.3° respectively. The aligned nanofiber middle layer at the bottom of PA / PVDF7 has the function of regulating the distribution of piperazine (PIP) aqueous solution. The PA layer formed has a smooth surface and rich hydrophilic groups, which can help the rapid penetration of water molecules. The hydrophilic surface is conducive to improving water flux, improving the interception of hydrophobic pollutants and anti-pollution.

[0097] Furthermore, the surface zeta potential of the composite nanofiltration membrane prepared with the polyvinylidene fluoride (PVDF) nanofiber intermediate layer at different rotation speeds is shown in Figure 7bAs shown. When the pH value changes in the range of 3 to 11, the charge performance of various composite nanofiltration membranes has the same trend of change, and they all show isoelectric points (IEP) at pH values ​​of 3 to 4; when the pH value is greater than 4, they all show negative surface charge properties. The figure also shows that in a pH environment close to neutral, the composite nanofiltration membranes prepared with polyvinylidene fluoride (PVDF) nanofiber intermediate layers at different rotation speeds have relatively similar negative charge potentials, among which PA / PVDF1 is -16.45mv, PA / PVDF4 is -15.01mv, PA / PVDF7 is -16.77mv and PA / PVDF8 is -16.65mv. The negative charge on the surface of the composite nanofiltration membrane is mainly due to the carboxyl groups generated by the hydrolysis of acyl chloride.

[0098] Further, such as Figure 7c As shown in the figure, the characteristic absorption peaks of the composite nanofiltration membrane exist, that is, polyamide has a vibration absorption peak at 1650-1700 cm -1 and 1620cm -1 The peaks of -NH (amide II) and -C=ON (amide I) respectively confirm that a polyamide (PA) filter layer is formed on the polyvinylidene fluoride (PVDF) nanofiber intermediate layer. -1 、1410cm -1 and 1485cm -1 The characteristic peaks at 1532 cm -1 and 1620cm -1 There is a slight change in the transmittance of the peak.

[0099] Further, such as Figure 7d As shown in the figure, the elemental composition of the polyamide (PA) filter cortex of the composite nanofiltration membrane was analyzed. The characterization in the figure shows that the typical peaks of three elements C, N and O are observed on all polyamide (PA) filter cortexes, and O1s can further analyze the characteristic peaks related to carboxyl groups. The results show that the carboxyl / amide ratio of the PA / PVDF7 membrane prepared on PVDF7 is reduced, which means that the cross-linking degree of the PA selection layer is enhanced, which is beneficial to improve the selectivity of the membrane based on the size screening mechanism. As can be seen from Table 1, compared with the PA / PVDF1 membrane, the O element content of the PA / PVDF7 membrane increased from 14.69% to 15.00%. The O / N ratio decreased from 1.24 to 1.19, and the cross-linking degree increased from 67.87% to 73.97%, confirming that the polyamide (PA) filter cortex on PVDF7 is more dense.

[0100] Table 1 Elemental analysis and cross-linking degree of composite nanofiltration membranes at different rotation speeds

[0101]

[0102] Figure 8a The molecular weight cutoff (MWCO) of PA / PVDF1, PA / PVDF4, PA / PVDF7 and PA / PVDF8 is shown. As shown in Figure 8, when the retention exceeds 90%, the molecular weight corresponding to this moment is the molecular weight cutoff (MWCO) corresponding to the membrane. Therefore, it can be seen that the molecular weight cutoff (MWCO) of PA / PVDF1, PA / PVDF4, PA / PVDF7 and PA / PVDF8 are approximately 554Da, 463Da, 381Da and 418Da, respectively. Further, the pure water permeability and Na2SO4 retention performance of the intermediate layer composite nanofiltration membrane at different rotation speeds are shown. The change in PVDF fiber orientation improves both flux and separation performance. Among them, the permeability of PA / PVDF1, PA / PVDF4, PA / PVDF7 and PA / PVDF8 is 18.79L·m -2 ·h -1 bar -1 、20.47L·m -2 ·h -1 bar -1 、23.64L·m -2 ·h -1 bar -1 and 22.68 L·m -2 ·h -1 bar -1 . PA / PVDF7 has the best permeability because the continuous and interconnected aligned nanofibers can form a network structure, and the water molecule transport through the interface of the aligned fiber interlayer may be faster than that of the random fiber interlayer. The unique interfacial area between the nanofibers and the polyamide (PA) filter cortex allows the creation of "water channels", thereby increasing the permeation flux without affecting the separation capacity of Na2SO4. The pore size distribution of the NF membrane was further detected based on the results of MWCOs, and the pore size distribution curve was obtained by density functional theory, such as Figure 8bAs shown. Among them, the average pore sizes of PA / PVDF1, PA / PVDF4, PA / PVDF7, and PA / PVDF8 membranes are 0.52nm, 0.47nm, 0.41nm, and 0.50nm, respectively. The pore sizes are mainly distributed below 1.5nm, and can be between 0.25nm and 0.75nm. Compared with PA / PVDF1, the pore size distribution of PA / PVDF7 is narrower and more concentrated, which is consistent with the detection results of XPS cross-linking degree. Due to the presence of the hydrophobic PVDF intermediate layer, more aqueous phase solution floats on the membrane surface, reducing the possibility of cortical collapse and invading the substrate, thereby adjusting the corresponding PA layer polymerization network to produce a polyamide layer with concentrated pore size distribution and high cross-linking density. It is worth noting that the reduction in average pore size means a greater particle size screening effect. Therefore, the electrospun PVDF intermediate layer can better improve the separation performance of the composite nanofiltration membrane.

[0103] Fig. 9 The pure water permeability and Na2SO4 retention performance of the intermediate layer composite nanofiltration membrane at different rotation speeds are shown. The change in the orientation of polyvinylidene fluoride (PVDF) fibers simultaneously improves the flux and separation performance. The permeabilities of PA / PVDF1, PA / PVDF4, PA / PVDF7, and PA / PVDF8 are 18.79, 20.47, 23.64, and 22.68 L·m, respectively. -2 ·h -1 bar -1 PA / PVDF7 has the best permeability because continuous and interconnected nanofibers with oriented structures can form a network structure, and the water molecule transport through the interface of the oriented fiber interlayer may be faster than that of the random fiber interlayer. The unique interfacial area between the nanofiber interlayer and the polyamide (PA) layer allows the creation of "water channels", thereby increasing the permeation flux without affecting the separation ability of Na2SO4.

[0104] Figure 10a-Figure 10b The hydraulic resistance distribution of each layer of the composite nanofiltration membrane at different rotation speeds is shown in the figure. The hydraulic resistance of the membrane layer is analyzed using the series resistance model. In the figure, yellow represents the membrane resistance of the polyamide filter skin layer (marked as PALayer in the figure), red represents the membrane resistance of the nanofiber intermediate layer (marked as PVDF Interlayer in the figure), and blue represents the membrane resistance of the polyethersulfone-based membrane support layer (marked as PES Support in the figure). As can be seen from Figure 10, the overall membrane resistance of PA / PVDF7 is the lowest (1.52×1013m -1 ), water flux increases.

[0105] Figure 11 shows the permeation flux and retention performance of different salts of composite nanofiltration membranes at different speeds. Fig.11aAs shown, the polyvinylidene fluoride (PVDF) nanofiber middle layer at 700 rpm can effectively improve the water permeability. Fig.11b As shown, the rejection rates of all membranes for Na2SO4 and MgSO4 are higher than those for MgCl2 and NaCl.

[0106] Fig.12 The retention performance of PA / PVDF1 and PA / PVDF7 for substances with different molecular weights. By increasing the amount of glycerol (MWCO = 92.09 Da, Stokes radius ) and glucose (MWCO = 180.16 Da, Stokes radius ) Two small molecular weight neutral solutes were observed to have a retention rate of PA / PVDF1 and PA / PVDF7. Fig.12 As shown, for glycerol less than 100Da, the retention rate of PA / PVDF1 is only 13.24%, while that of PA / PVDF7 is increased to 19.27%. For glucose with a molecular weight of 180Da, the retention rate of PA / PVDF1 is 48.78%, while that of PA / PVDF7 is over 50%, which is 62.33%. It can be seen that after the arrangement of the nanofiber middle layer tends to be directional, the nanofiltration membrane generated by the interfacial polymerization reaction not only increases the hydrophilicity and cross-linking degree of the polyamide (PA) filter cortex, but also through the filtration experiment of small molecular weight substances, it is found that PA / PVDF7 also has the ability to separate more precise small molecules.

[0107] Figure 13a-13b The interception performance of composite nanofiltration membranes with different rotation speeds for emerging micropollutants (EMPs). As shown in Figure 13a, with the increase of the rotation speed of the intermediate layer, the membrane interception effect gradually increases, and PA / PVDF7 has the best separation effect of parabens, that is, MP = 33.00%, PP = 37.76% and BP = 94.32%. Fig.13b The removal effects of the two tested charged emerging micropollutants (EMPs) were SMX = 82.05% and TMP = 63.32%. Specifically, the removal rates of neutral and positively charged emerging micropollutants (EMPs) were generally lower than those of negatively charged emerging micropollutants (EMPs), and the removal rates of hydrophobic emerging micropollutants (EMPs) were lower than those of hydrophilic emerging micropollutants (EMPs).

[0108] Figure 14a-Figure 14c This is a long-term operation experiment on the stability performance of composite nanofiltration membranes at different rotation speeds. Fig.14a It shows that as the concentration of Na2SO4 in the feed solution increases, the membrane permeability decreases, but the concentration of Na2SO4 in the filtered solution remains almost unchanged. Therefore, the retention rate of Na2SO4 shows an upward trend. Fig.14bAs shown in the figure, due to the compaction under pressure and the concentration polarization effect, the permeation flux of the nanofiber membrane is proportional to the increase in pressure, and it exhibits excellent retention rate stability for Na2SO4 at all pressures. Fig.14c Although the flux of PA / PVDF7 fluctuated slightly, the overall filtration performance began to stabilize after 20 hours, and the water flux increased from the initial 25 to 23 L·m -2 ·h -1 bar -1 Fluctuates to a stable level of 23 L·m -2 ·h -1 bar -1 The slight decrease in flux is due to the membrane being pressed more and more densely under the long-term pressure test, thereby increasing the resistance to water passing through. The retention rate of Na2SO4 has remained at around 97%, and PA / PVDF7 has not broken or fallen off due to long-term filtration. The results show that the directional polyvinylidene fluoride (PVDF) intermediate layer can be stably combined with the polyethersulfone (PES) base membrane support layer and the polyamide (PA) filter skin layer, and has good long-term filtration stability.

[0109] Figure 15a-15f The anti-fouling performance of PA / PVDF1 and PA / PVDF7 composite nanofiltration membranes. Among them, FRR is the recovery rate test result, Rt is the total pollution rate, Rr is the reversible pollution rate, and Rir is the irreversible pollution rate. It can be seen that the downward trend of bovine serum albumin (BSA) and humic acid (HA) is more obvious. However, since bovine serum albumin (BSA) is a macromolecular pollutant and the membrane surface is more hydrophilic, after flushing, the membrane can be restored to a higher level, namely: FRRPA / PVDF1 = 80.95%, FRRPA / PVDF7 = 95.75%. On the contrary, the smaller humic acid (HA) flux recovery rate is not as obvious as bovine serum albumin (BSA), which are: FRRPA / PVDF1 = 70.87%, FRRPA / PVDF7 = 89.58%. Sodium alginate (SA) is an anionic polyelectrolyte, therefore, the electrostatic interaction between sodium alginate (SA) and the polyamide layer can reduce membrane fouling to a great extent, and also has relatively high flux recovery rates FRRPA / PVDF1=77.31% and FRRPA / PVDF7=91.54%.

[0110] like Figure 15a-15f The figure shows the anti-fouling performance of PA / PVDF1 and PA / PVDF7 composite nanofiltration membranes. In the examples of the present application, three different types of organic pollutants were investigated, namely, 1000ppm of bovine serum albumin (BSA), sodium alginate (SA) and humic acid (HA) as pollutants, and the anti-fouling performance of PA / PVDF1 and PA / PVDF7 membranes was evaluated. Fig.15a, 15c 15e and 15e are the normalized flux attenuation diagrams of PA / PVDF1 and PA / PVDF7 after 5 hours of filtration in the feed liquid of bovine serum albumin (BSA), sodium alginate (SA) and humic acid (HA). It can be seen from the figure that after 5 hours of pollution recovery, Fig.15a , 15c PA / PVDF1 in 15e still has higher permeability and the best anti-fouling ability. Fig.15b , 15d 15f and 15f are the distribution of sludge resistance ratio of the membrane after being polluted by different organic matters. The figure evaluates the anti-fouling ability of the membrane in detail. The FRR of PA / PVDF7 for various pollutants is 95.75% for bovine serum albumin (BSA), 91.54% for sodium alginate (SA), and 89.58% for humic acid (HA), all of which are higher than the FRR of PA / PVDF1 membrane. The Rir of PA / PVDF7 is 4.26% for bovine serum albumin (BSA), 8.46% for sodium alginate (SA), and 10.42% for humic acid (HA), indicating that only a few pollutants are difficult to remove by washing the membrane surface. These data show that the anti-fouling performance of the composite nanofiltration membrane with a thin layer of hydrophobic nanofiber intermediate layer synthesized by polyethersulfone (PES)-based membrane support in this application has been significantly improved, which can meet the needs of practical use.

[0111] In addition, this application uses PA / PVDF7 membrane to test water bodies containing emerging pollutants. The removal rates of methylparaben (MP), propylparaben (PP), benzylparaben (BP), sulfamethoxazole (SMX) and trimethoprim (TMP) are stabilized at around 33.00%, 37.76%, 94.32%, 82.05% and 63.32%, respectively. The results show that the PA / PVDF7 membrane has a significant enhancement in the removal rate of emerging micropollutants (EMPs). By testing small molecular weight substances, it was found that the separation accuracy of the membrane has also been improved to a certain extent.

[0112] In the embodiment of the present application, the optimal speed of electrostatic spinning of polyvinylidene fluoride (PVDF) nanofiber intermediate layer is 700rpm, and the duration of electrostatic spinning is set to 20 minutes. The PA / PVDF7 prepared under this duration has a directional arrangement structure of polyvinylidene fluoride (PVDF) nanofiber intermediate layer. In the preparation process, the mass percentage of piperazine (PIP) aqueous solution is 0.5%, and the mass percentage of benzene trimecarboxylic chloride (TMC) n-hexane solution is 0.1%. The composite nanofiltration membrane prepared under the above conditions has excellent permeability and good salt retention effect, and also forms an ultra-thin, flat, non-collapsed polyamide layer with many wrinkles; the optimal speed of electrostatic spinning is 700rpm, and the duration of electrostatic spinning is set to 20 minutes. The prepared TFC7 membrane has the best performance, and now has excellent permeability, retention effect and anti-pollution performance.

[0113] The preparation method and performance of the PA / PVDF1, PA / PVDF4, PA / PVDF7, and PA / PVDF8 membranes of the present application are further described in detail below through specific examples.

[0114] Example 1, specific steps for preparing PA / PVDF1 membrane:

[0115] 1. Soak the polyethersulfone (PES) base membrane and fix it on the surface of the collector;

[0116] 2. Dissolve 17% of polyvinylidene fluoride (PVDF) polymer and a small amount of 0.01% of tetrabutylammonium chloride (TBAC) in a volume ratio of 1:3 dimethylformamide (DMF) and acetone solvent, and then stir at a constant temperature in a 60℃ water bath for 10 hours to completely dissolve the polymer. After the spinning solution is stirred evenly, it is left to degas at room temperature of 20±5℃ for 24 hours to obtain the electrospinning solution required for the experiment. Pour 2mL of the degassed polyvinylidene fluoride (PVDF) spinning solution into two 10mL syringes, apply a high DC voltage of 20KV between the spinneret and the stainless steel drum, and then feed the spinning solution into the spinneret at a constant rate through a syringe pump. Under the influence of the electric field, the polymer solution is continuously electrospun into ultrafine nanofibers, and the nanofibers dry and bond to each other to form a new base membrane. After the electrospinning is completed, the membrane is placed in a drying oven at 60℃ for 24h to remove the residual solvent. In this embodiment, the rotation speed of the polyvinylidene fluoride (PVDF) electrospinning in this step is 100 rpm.

[0117] 3. Dissolve piperazine (PIP) at a mass percentage of 0.5wt% in deionized water in a 50mL opaque brown container, and ultrasonicate for 30min to obtain a piperazine (PIP) aqueous phase solution; dissolve 1,3,5-benzenetricarboxylic acid chloride (TMC) at a mass percentage of 0.1wt% in 50mL of n-hexane solvent and ultrasonicate for 30min to completely dissolve it to obtain a 1,3,5-benzenetricarboxylic acid chloride (TMC) organic phase solution.

[0118] 4. Place the prepared modified polyethersulfone (PES) base membrane flat on a clean glass plate, and use a stainless steel plate frame and a rubber ring to fix and prevent leakage. The polyethersulfone (PES) base membrane loaded with the intermediate layer is placed on the glass plate in an upward manner.

[0119] 5. Pour 50 mL of the prepared piperazine (PIP) aqueous solution into the frame to soak the polyethersulfone (PES) base membrane for 5 minutes, then pour out the excess aqueous solution and remove the stainless steel plate frame and rubber ring to obtain the pretreated membrane.

[0120] 6. Roll the rubber roller over the surface of the pre-treated film.

[0121] 7. Pour 50 mL of the prepared 1,3,5-benzenetricarboxylic acid chloride (TMC) organic phase solution into the frame and react for 2 minutes, then pour off the excess organic phase solution; obtain a preliminary nanofiltration membrane with an interfacial polymerization reaction cortex.

[0122] 8. The obtained preliminary nanofiltration membrane was naturally dried in the air for 60 seconds and then placed in an oven for curing for 5 minutes to obtain a composite nanofiltration membrane with a selectively permeable polyamide layer.

[0123] 9. The prepared composite nanofiltration membrane with polyamide layer was stored in deionized water at 4°C until testing.

[0124] The basement membrane pores in the nanofiltration membrane prepared in the above embodiment are as follows: Figure 2a As shown in the figure. Pure water flux and inorganic salt retention tests were performed using a cross-flow filtration device. The measured membrane area of ​​the membrane tester is 3.14 cm 2 , test pressure is 5 bar, cross flow rate is 30 L·H -1 The test was conducted at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 67.87%, the water contact angle was about 33.9°, and the average water flux was 18.79 L·m -2 ·h -1 bar -1 ;like Figure 7b As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -16.45 mv in a neutral pH environment.

[0125] Example 2 is a specific method for preparing PA / PVDF4 membrane.

[0126] The difference between Example 2 and Example 1 is that the electrospinning speed of the PVDF intermediate layer loaded on the polyethersulfone (PES) base membrane is 400 rpm, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as PA / PVDF4 membrane.

[0127] The basement membrane pores in the nanofiltration membrane prepared in Example 2 are as follows: Figure 2b As shown in PES1. Pure water flux and inorganic salt retention tests were performed using a cross-flow filtration device. The measured membrane area of ​​the membrane tester was 3.14 cm 2 , test pressure is 5 bar, cross flow rate is 30 L·H -1 The test was conducted at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 69.06%, the water contact angle was about 20.5°, and the average water flux was 20.47 L·m -2 ·h -1 bar -1 ;like Figure 7b As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -15.01 mV in a neutral pH environment.

[0128] Example 3 is a specific method for preparing PA / PVDF7 membrane.

[0129] The difference between Example 3 and Example 1 is that the electrospinning speed of the PVDF intermediate layer loaded on the polyethersulfone (PES) base membrane is 700 rpm, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as PA / PVDF7 membrane.

[0130] The basement membrane pores in the nanofiltration membrane prepared in Example 3 are as follows: Figure 2c As shown in PES-2. Pure water flux and inorganic salt retention tests were performed using a cross-flow filtration device. The measured membrane area of ​​the membrane tester was 3.14 cm 2 , test pressure is 5 bar, cross flow rate is 30 L·H -1 The test was carried out at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 73.97%, the water contact angle was about 19.6°, and the average water flux was 23.64 L·m -2 ·h -1 bar -1 ;like Figure 7b As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -16.77 mV in a neutral pH environment.

[0131] Example 4 is a specific method for preparing PA / PVDF8 membrane.

[0132] The difference between Example 4 and Example 1 is that the electrospinning speed of the PVDF intermediate layer loaded on the polyethersulfone (PES) base membrane is 800 rpm, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as PA / PVDF8 membrane.

[0133] The basement membrane pores in the nanofiltration membrane prepared in Example 4 are as follows: Figure 2d As shown in PES-3. Pure water flux and inorganic salt retention tests were performed using a cross-flow filtration device. The measured membrane area of ​​the membrane tester was 3.14 cm 2 , test pressure is 5 bar, cross flow rate is 30 L·H -1 The test was carried out at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 71.49%, the water contact angle was about 31.3, and the average water flux was 22.68 L·m -2 ·h -1 bar -1 The flux recovery rates (FRR) of various pollutants are: 84.57% for BSA, 87.23% for SA, 95% for HA, and 96% for salt retention. Figure 7b As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -16.65 mV in a neutral pH environment.

[0134] The present application regulates the pore size and hydrophilicity of the PES-based membrane by electrospinning a polyvinylidene fluoride (PVDF) nanofiber intermediate layer. The pore size of the nanofiber intermediate layer is between 0.35nm and 0.5nm. After the interfacial polymerization (IP) process, a non-collapsed, continuous and wrinkled polyamide (PA) filter cortex is formed, thereby preparing a polyvinylidene fluoride (PVDF) nanofiber intermediate layer composite nanofiltration membrane, and testing its emerging pollutant separation performance. The composite nanofiltration membrane was characterized and analyzed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), solid surface Zeta potential instrument and X-ray photoelectron spectrometer (XPS). The performance and structure of the composite nanofiltration membrane provided by the present application mainly include a cortex thickness of 47nm-156nm; a crosslinking degree of 67.87%-73.97%; a water contact angle of 19.6°-33.9°, and a maximum water flux of the composite nanofiltration membrane of 23.64L·m -2 ·h -1 bar -1 The results showed that the polyvinylidene fluoride (PVDF) nanofiber intermediate layer with an appropriate electrospinning speed under the optimal monomer concentration can well regulate the PES-based membrane and prepare a composite nanofiltration membrane with excellent separation performance.

[0135] In addition, the nanofiltration membrane provided by the present application is prepared by electrospinning a PVDF intermediate layer on a PES base membrane to prepare a composite nanofiltration membrane. The loading and directional arrangement structure of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer compensates for the defects of the rough macropores of the PES base membrane, so that a series of physical and chemical properties such as hydrophilicity, roughness, pore size and electrochemical properties of the base membrane surface have changed, and the new morphology of the intermediate layer will affect the morphology and performance of the filter cortex. With the increase in the electrospinning speed of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer, the PES base membrane forms a new substrate morphology, high hydrophobicity, and reasonable pore size. The uniform roughness provides greater reaction activity and contact area for the reaction of piperazine (PIP) and benzene trimacyl chloride (TMC), thereby producing a higher degree of crosslinking and excellent nanofiltration performance.

[0136] In addition, the present application has a base membrane part of a support layer that is a hydrophilic support layer and a hydrophobic middle layer, and the filter cortex is covered on the hydrophobic middle layer, which reduces the pore size of the filter cortex that sinks into the base membrane and reduces the increase in the thickness of the base membrane. The base membrane thickness is set between 100nm-300nm to obtain a thinner filter cortex. The filter cortex has a dense structure with a thickness of 47nm-94nm, preferably 50nm; the pore size is 0.25nm-0.75nm, preferably 0.3nm-0.5nm, which reduces the mass transfer resistance of water and ensures high water flux. The TFC membrane prepared by the provided nanofiltration membrane under the preferred conditions has the best permeability and the lowest membrane resistance, and the nanofiltration performance also reaches the best data. Because the filter cortex when a concave liquid surface is formed on the pores in the water transfer layer, it floats on the surface of the base membrane as a whole and will not penetrate into the base membrane.

[0137] In addition, the present application constructs a thin hydrophobic nanofiber intermediate layer on a large-pore hydrophilic base membrane through electrospinning technology, and provides a smooth and flat reaction plane for the interfacial polymerization reaction by regulating the intermediate hydrophobic layer, thereby reducing the hydrophobic materials in the aqueous solution from being trapped in the base membrane, especially the hydrophilic substrate, and supporting the interfacial reaction to form a complete polyamide filtration cortex, thereby forming a complete, continuous and wrinkled nanofiltration membrane cortex, thereby improving the permeability and selectivity of the nanofiltration membrane.

[0138] In addition, the present application changes the pore shape of the fiber middle layer by adjusting the rotation speed of the electrospinning device and changing the orientation degree of the polyvinylidene fluoride (PVDF) nanofibers, thereby causing the area of ​​the piperazine (PIP) aqueous phase contacting the hydrophobic polyvinylidene fluoride (PVDF) fiber to change, making the diffusion rate of the piperazine (PIP) molecules and the intensity of the interfacial polymerization reaction more uniform in the directional slit-shaped pores, thereby achieving controllable regulation of the reaction rate. While avoiding the collapse of the polyamide (PA) filter cortex into the pores, the morphology of the surface of the polyamide (PA) filter cortex membrane changes from an irregular pleated structure to a more densely arranged bamboo-like pleated structure along the fibers, further increasing the surface area of ​​the polyamide (PA) filter cortex, which is beneficial to the improvement of water flux.

[0139] The above is a detailed introduction to a composite nanofiltration membrane and a preparation method thereof provided in the present application. For a person skilled in the art, according to the ideas of the embodiments of the present application, there may be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A composite nanofiltration membrane, characterized in that: It includes a porous base membrane support layer, a hydrophobic middle layer and a filter skin layer, wherein the porous base membrane support layer and the filter skin layer are both composed of hydrophilic organic materials; the hydrophobic middle layer is composed of hydrophobic organic materials and is arranged to have a directional arrangement structure; the thickness of the hydrophobic middle layer is smaller than that of the porous base membrane support layer.

2. The composite nanofiltration membrane according to claim 1, characterized in that The hydrophobic middle layer is a nanofiber middle layer with a directional arrangement structure, which forms a thin layer on the porous base membrane support layer, and the thin layer forms a completely hydrophobic surface on the porous base membrane support layer; the thickness of the nanofiber middle layer is 100nm-1um.

3. The composite nanofiltration membrane according to claim 2, characterized in that The nanofiber middle layer has slit-like pores.

4. The composite nanofiltration membrane according to claim 3, characterized in that The porous base membrane support layer is a polyethersulfone base membrane, and its average pore size is 0.35nm-0.5nm; the nanofiber intermediate layer is a polyvinylidene fluoride material, and its nanofiber has an average pore size of 0.35nm-0.5nm.

5. The composite nanofiltration membrane according to claim 4, characterized in that The filter skin layer is a polyamide layer, and the filter skin layer has a dense structure, a thickness of 47nm-94nm, preferably 50nm; and a pore size of 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

6. The composite nanofiltration membrane according to claim 5, characterized in that The water contact angle of the composite nanofiltration membrane is between 19.6° and 33.9°.

7. A method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane, characterized in that: The following steps are involved: S1. Obtaining a hydrophilic porous base membrane support layer; S2. Preparing a hydrophobic intermediate layer having an oriented structure on the porous base membrane support layer, wherein the hydrophobic intermediate layer is composed of an organic material; S3. Performing an interfacial polymerization reaction on the hydrophobic intermediate layer to form a hydrophilic filtration skin layer.

8. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 7, characterized in that: The step S2 is achieved in the following manner: the hydrophobic middle layer is a nanofiber middle layer, and the nanofiber middle layer is prepared on a porous base membrane support layer by electrospinning.

9. The method for preparing a composite nanofiltration membrane according to claim 8, characterized in that: The step S2 further comprises drying and hot pressing the porous base membrane support layer supporting the nanofiber intermediate layer.

10. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 8, characterized in that: The nanofiber middle layer is prepared by the following method: an electrostatic spinning solution is prepared, wherein the electrostatic spinning solution is a polyvinylidene fluoride (PVDF) spinning solution, and the nanofiber middle layer is obtained by electrostatic spinning.

11. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 10, characterized in that: The polyvinylidene fluoride (PVDF) spinning solution is prepared in the following manner: a certain mass percentage of polyvinylidene fluoride (PVDF) polymer and tetrabutylammonium chloride (TBAC) are completely dissolved in a certain volume ratio of dimethylformamide (DMF) and acetone solvent to prepare the polyvinylidene fluoride (PVDF) spinning solution.

12. The method for preparing a composite nanofiltration membrane according to claim 11, characterized in that: The preparation method of the polyvinylidene fluoride (PVDF) spinning solution comprises: dissolving 17% by mass of a polyvinylidene fluoride (PVDF) polymer and 0.01% of tetrabutylammonium chloride (TBAC) in a dimethylformamide (DMF) and acetone solvent in a volume ratio of 1:3, completely dissolving under a constant temperature condition, and standing and degassing at a room temperature of 15° C. to 25° C. for more than 24 hours.

13. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 12, characterized in that: The electrospinning method is completed by an electrospinning device, and the rotation speed of the electrospinning device is set between 100 rpm and 800 rpm.

14. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 13, characterized in that: The conditions of the electrospinning are: applying a voltage of 20 kV, maintaining the ambient humidity at 40% RH, setting the rotation speed to 700 rpm, and the duration of the electrospinning is 5 to 20 minutes.

15. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 14, characterized in that: The step S3 performs interfacial polymerization reaction in the following manner: a piperazine (PIP) aqueous solution with a mass percentage of 0.2%-2% is evenly distributed on the surface of the polyvinylidene fluoride (PVDF) nanofiber middle layer, and after drying, the polyvinylidene fluoride (PVDF) nanofiber middle layer is immersed in a n-hexane solution of benzyl chloride (TMC) with a mass percentage of 0.1%-0.2% for a certain period of time to induce a polymerization reaction between piperazine (PIP) and benzyl chloride (TMC) to form the filter cortex.

16. The method for preparing an organic hydrophobic intermediate layer composite nanofiltration membrane according to claim 15, characterized in that: The step S3 comprises: fixing the porous base membrane support layer loaded with the nanofiber intermediate layer obtained in the step S2 with a frame; pouring a 0.5% by mass piperazine (PIP) aqueous solution on the surface of the nanofiber intermediate layer and letting it stand for 3 to 7 minutes, draining the excess solution and drying; immersing the membrane obtained in the above step in a 0.1% by mass benzyl chloride (TMC) n-hexane solution for 1 to 6 minutes to induce a polymerization reaction between PIP and TMC; and drying the membrane prepared in the above step and then curing it, the curing time being 30 to 90 seconds.

17. An organic hydrophobic intermediate layer composite nanofiltration membrane, characterized in that: The organic hydrophobic intermediate layer composite nanofiltration membrane is prepared by any one of the preparation methods 7-16 above.

18. A nanofiltration membrane for filtering and intercepting emerging pollutants, characterized in that: The nanofiltration membrane is prepared by any one of the preparation methods 7-16 above, and the emerging pollutants are not limited to paraben compounds, sulfamethoxazole (SMX) and trimethoprim (TMP).

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