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 interception and filtration of emerging micropollutants. The composite nanofiltration membrane comprises a porous base membrane supporting layer, a hydrophobic middle layer and a filtering skin layer, the hydrophobic middle layer is made of hydrophobic organic materials, and the porous base membrane supporting layer and the filtering skin layer are both made of hydrophilic organic materials; 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 hydrophilic porous basement membrane supporting layer; s2, preparing a hydrophobic middle layer on the porous base membrane supporting layer; s3, performing interfacial polymerization reaction on the hydrophobic middle layer to form a hydrophilic 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 micropollutants 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 intercepting and filtering emerging micropollutants. Background Art

[0002] At present, more than one-third of the world's population lives in water-scarce areas. Access to clean and safe water is one of the most severe survival challenges facing all mankind today. Nanofiltration membranes can effectively remove colloids, bacteria, suspended solids, organic matter and some soluble ions in water. They can be used for seawater desalination, drinking water and wastewater treatment. They also have lower energy consumption, relatively loose structure, and relatively high water flux. In addition, water treatment technology based on nanofiltration membranes also has unique advantages in 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 be weakened 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 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 and a preparation method and its application in filtering and intercepting emerging pollutants. Using the composite nanofiltration membrane provided by the present application, a tightly structured hydrophobic intermediate layer 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 in the interception effect on 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 hydrophobic intermediate layer is composed of a hydrophobic organic material, and the porous base membrane support layer and the filtration skin layer are both composed of a hydrophilic organic material; 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 compact 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.

[0007] Preferably, the thickness of the nanofiber middle layer is 100nm-1um.

[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, which floats on the hydrophobic middle layer and has a dense structure. The thickness is 47nm-94nm, preferably 50nm; the pore size is 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

[0010] like Figure 1a The second aspect of the present invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps:

[0011] S1. Obtaining a hydrophilic porous base membrane support layer;

[0012] S2. Preparing a hydrophobic intermediate layer on the porous base membrane support layer, wherein the hydrophobic intermediate layer is configured to be composed of an organic material;

[0013] S3. Performing an interfacial polymerization reaction on the hydrophobic intermediate layer to form a hydrophilic filtration skin layer.

[0014] Preferably, the step S2 is implemented in the following manner: the hydrophobic intermediate layer is a nanofiber intermediate layer with a compact structure, and the nanofiber intermediate layer is prepared on a porous base membrane support layer by electrospinning.

[0015] Preferably, the step S2 further comprises drying and hot pressing the porous base membrane support layer supporting the nanofiber intermediate layer.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] Preferably, the electrospinning method is performed by an electrospinning device, and at a certain rotation speed, the duration of the electrospinning is set to between 5 minutes and 20 minutes.

[0020] 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 100 rpm, and the electrospinning time to 15 minutes.

[0021] Preferably, the step S3 performs interfacial polymerization reaction in the following manner: a certain mass percentage of piperazine (PIP) aqueous solution 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 certain mass percentage of benzyl chloride (TMC) n-hexane solution for a certain period of time to induce a polymerization reaction between piperazine (PIP) and benzyl chloride (TMC) to form the filter cortex.

[0022] 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 piperazine (PIP) and trimesoyl chloride (TMC); and drying the membrane prepared in the above step and then curing it, the curing time being 30 to 90 seconds.

[0023] 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:

[0024] S1-1: First, the polyethersulfone (PES) base membrane is soaked and then placed in the electrospinning device;

[0025] S2-1: preparing the electrospinning solution required for electrospinning;

[0026] S3-1: Preparation of a polyethersulfone (PES) based membrane with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer;

[0027] 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.

[0028] 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.

[0029] 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.

[0030] 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, and the nanofibers dry and bond to each other to form a new base film. After the electrospinning is completed, the membrane is placed in a drying oven at 60°C for 24 hours to remove the residual solvent.

[0031] 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:

[0032] 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;

[0033] S6: performing surface treatment on the pre-treated film;

[0034] 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.

[0035] 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;

[0036] The method also includes the steps of preparing an aqueous phase solution and an organic phase solution respectively.

[0037] 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;

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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;

[0042] 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.

[0043] 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.

[0044] Preferably, in the process of preparing the aqueous phase solution, piperazine (PIP) is dissolved in deionized water in a light-proof brown container.

[0045] 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.

[0046] 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%.

[0047] 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.

[0048] The third aspect of the present invention provides a composite nanofiltration membrane prepared by any preparation method of the second aspect of the present invention.

[0049] In a fourth aspect of the present invention, a nanofiltration membrane is provided for filtering and intercepting emerging pollutants, which is prepared by any preparation method according to the second aspect of the present invention. The emerging pollutants are not limited to paraben compounds, sulfamethoxazole (SMX) and trimethoprim (TMP).

[0050] The beneficial effect of the present invention is that a thin layer of tightly structured hydrophobic intermediate layer is constructed on the hydrophilic porous basement membrane support layer, and the porous basement membrane support layer is modified, which not only improves the formation of the filter cortex and avoids the collapse of the filter cortex, 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 composite nanofiltration membrane of the present invention and the method for preparing the composite nanofiltration membrane have the advantages of simple operation and low material cost by making improvements on the existing commonly used basement membrane support layer, and provide a better performance option for the nanofiltration membrane and its preparation in the field of water treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] 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.

[0052] Figure 1a , Figure 1b The preparation process of the composite nanofiltration membrane of different embodiments of the present application;

[0053] Figure 2a-2e A schematic diagram of a scanning electron microscope (SEM) of a PES base membrane and a PES membrane loaded with a nanofiber intermediate layer with different electrospinning times in an embodiment of the present application comparing the base membrane surface;

[0054] Figure 3 Schematic diagram of the water contact angle of the surface of the PES base membrane compared with the PES membrane loaded with nanofiber intermediate layers with different electrospinning times in the embodiments of the present application;

[0055] Figure 4a , Figure 4b Schematic diagram of Na2SO4 retention effect of composite nanofiltration membrane loaded with electrospinning for 15 minutes under different mass percentages of interfacial polymerization monomers in the embodiment of the present application;

[0056] Figure 5a-5j Schematic diagram of the surface scanning electron microscope (SEM) of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after loading the nanofiber intermediate layer with different electrospinning time in the embodiment of the present application;

[0057] Figure 6a-6j Schematic diagram of the cross section scanning electron microscope (SEM) and the roughness atomic force microscope (AFM) of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after the nanofiber intermediate layer with different electrospinning time is loaded in the embodiment of the present application;

[0058] Figure 7 Schematic diagram of ultraviolet spectra of a comparative nanofiltration membrane formed on a PES base membrane and a composite nanofiltration membrane formed after loading a nanofiber intermediate layer with different electrospinning times in an embodiment of the present application;

[0059] Figure 8a-8d Schematic diagram of various surface properties of a comparative nanofiltration membrane formed on a PES base membrane and a composite nanofiltration membrane formed after loading a nanofiber intermediate layer with different electrospinning times in an embodiment of the present application;

[0060] Figure 9a-9b Schematic diagram of PEG molecular weight cutoff and pore size distribution of a comparative nanofiltration membrane formed on a PES-based membrane and a composite nanofiltration membrane formed after loading a polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning times in different embodiments of the present application;

[0061] Fig.10 Schematic diagram of pure water flux and Na2SO4 salt retention of a comparative nanofiltration membrane formed on a PES-based membrane and a composite nanofiltration membrane formed after loading a nanofiber intermediate layer with different electrospinning times in an embodiment of the present application;

[0062] Fig.11a , 11b Schematic diagram of the series resistance model of the nanofiltration membrane formed on the pure PES base membrane and the composite nanofiltration membrane formed after loading the polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning times in different embodiments of the present application: (a) pure water flux; (b) distribution of hydraulic resistance of each layer;

[0063] Fig.12a , 12bSchematic diagram of the permeation flux and retention performance of different salts of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after loading the nanofiber intermediate layer with different electrospinning times in the embodiment of the present application;

[0064] Fig.13a , 13b Schematic diagram of the interception performance of emerging micropollutants (EMPs) for the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after loading the nanofiber intermediate layer with different electrospinning times in the embodiment of the present application;

[0065] Figures 14a-14c Schematic diagram of the stability performance of a composite nanofiltration membrane loaded with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer electrospun for 15 minutes in an embodiment of the present application;

[0066] Figures 15a-15f Schematic diagram of the anti-pollution performance of the composite nanofiltration membrane formed on the PES base membrane and the nanofiber intermediate layer loaded with electrospinning time of 15 minutes in the embodiment of the present application. DETAILED DESCRIPTION

[0067] 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.

[0068] In one aspect of the present application, a composite nanofiltration membrane is provided, comprising a porous base membrane support layer, a hydrophobic intermediate layer and a filtration skin layer, wherein the hydrophobic intermediate layer is composed of a hydrophobic organic material, and the porous base membrane support layer and the filtration skin layer are both composed of a hydrophilic organic material; the thickness of the hydrophobic intermediate layer is less than that of the porous base membrane support layer.

[0069] Furthermore, the hydrophobic middle layer is a tightly structured nanofiber middle layer, 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.

[0070] Furthermore, 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.

[0071] Furthermore, the filter skin layer is a polyamide layer, floating on the hydrophobic middle layer, having a dense structure, a thickness of 47nm-94nm, preferably 50nm; and a pore size of 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

[0072] 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).

[0073] The present invention also provides a method for preparing a composite nanofiltration membrane, such as Figure 1a As shown, the following steps are included:

[0074] S1. Obtaining a hydrophilic porous base membrane support layer;

[0075] S2. Preparing a hydrophobic intermediate layer on the porous base membrane support layer, wherein the hydrophobic intermediate layer is configured to be composed of a hydrophobic organic material;

[0076] S3. An interfacial polymerization reaction is carried out on the hydrophobic middle layer to form a hydrophilic filter skin layer.

[0077] 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 electrostatic spinning method, the thickness of the base membrane is between 100um-300um, so that the nanofiber intermediate layer structure is compact, 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 the composite nanofiltration membrane. In actual use, a hydrophilic porous base membrane support layer is used, a hydrophobic intermediate layer 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.

[0078] In a preferred embodiment, the specific operation steps are as follows:

[0079] First, the polyethersulfone (PES) base membrane was fixed and placed in an electrospinning device;

[0080] 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 vinylidene fluoride (PVDF) polymer is 17%, and the weight fraction 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.

[0081] Next, a hydrophobic, tightly structured nanofiber intermediate layer is prepared on a polyethersulfone (PES) base membrane support layer. In the present embodiment, the degassed polyvinylidene fluoride (PVDF) spinning solution is poured into two 10mL syringes, each 2mL, and the spinning solution is sent to the spinneret of the electrostatic spinning device at a constant rate 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 electrostatically spun into ultrafine nanofibers, and the nanofibers dry and bond to each other to form a tight structure. The surface roughness of the polyethersulfone (PES) base membrane support layer after the load intermediate layer is reduced, and it has a relatively smooth surface structure with suitable porosity and pore size. After electrospinning is completed, the polyethersulfone (PES) base membrane of the load nanofiber intermediate layer is placed at 60°C in a drying oven for 24 hours to remove residual solvent.

[0082] Further, interfacial polymerization is carried out on the membrane of the nanofiber intermediate layer. In the present embodiment, the composite nanofiltration membrane obtained is a composite nanofiltration membrane with a polyamide layer obtained by interfacial polymerization on the polyethersulfone (PES) base membrane of the load intermediate layer. The polyethersulfone (PES) base membrane is immersed in an aqueous phase solution containing a polyamine compound, and excess aqueous phase solution is removed in 3-7 minutes, and the time in the present embodiment is 5 minutes. Then, the organic phase solution containing a polyacyl chloride compound is poured on the polyethersulfone (PES) base membrane of the load intermediate layer, and the polymerization reaction between piperazine (PIP) and benzene trimesoyl chloride (TMC) is induced, and the excess organic phase solution is removed after 1 to 6 minutes, and 2 minutes in the present embodiment, and then cured for 30 to 90 seconds, and 60 seconds in the present embodiment, to obtain a composite nanofiltration membrane with a polyamide (PA) filter skin layer. In this embodiment, the above method is used to obtain a filter cortex with a dense structure, the thickness of which is 47nm-94nm, and in a preferred embodiment the thickness is 50nm; the pore size is 0.25nm-0.75nm, and preferably between 0.3nm-0.5nm.

[0083] In this embodiment, the aqueous phase solution is a piperazine (PIP) aqueous phase solution, with a mass percentage of 0.2wt%-2wt%, and 0.5wt% is selected in this embodiment; the organic phase solution is a trimesoyl chloride (TMC) organic phase solution, with a mass percentage of 0.1wt%-0.2wt%, and 0.1wt% of 1,3,5-benzene trimesoyl chloride (TMC) is selected in this embodiment to be dissolved in 50mL of normal hexane. 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 solutions can only contact and react at the interface of the two phases, and the reaction rate between the monomers is extremely fast, so a polyamide layer can be formed on the surface of the polyethersulfone (PES) base membrane support layer in a very short time.

[0084] Further, in one embodiment of the present invention, a voltage of 20 kV is applied, the ambient humidity is maintained at about 40% RH, a rotation speed of 100 rpm is set, and electrospinning is performed according to the above steps, and the electrospinning time is set to 5 minutes, 10 minutes, 15 minutes, and 20 minutes, respectively, and the polyethersulfone (PES) base membrane loaded with the nanofiber intermediate layer is marked as PES-1, PES-2, PES-3, and PES-4; the composite nanofiltration membrane prepared by the interfacial polymerization reaction process is marked as TFC membrane, TFC-1 membrane, TFC-2 membrane, TFC-3 membrane, and TFC-4 membrane. For the convenience of comparison, the polyethersulfone (PES) base membrane that has not been electrospun is marked as PES, and the composite nanofiltration membrane prepared by the interfacial polymerization reaction process on PES is marked as TFC membrane.

[0085] like Figure 2a-2eThe figure shows the scanning electron microscope (SEM) diagram of the polyethersulfone (PES) base membrane support layer under different electrospinning times of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer. Figure 2a It can be observed that the surface of the pure PES support layer has obvious large pores, while Figure 2b-Figure 2e It can be seen that a thin layer of polyvinylidene fluoride (PVDF) nanofibers can be observed on the surface of the polyethersulfone (PES) base membrane support layer with a nanofiber middle layer, and the fiber thickness is relatively uniform. In addition, as the spinning time of the polyvinylidene fluoride (PVDF) middle layer increases, more and more surface fibers appear. When the electrospinning time reaches 20 minutes, the polyethersulfone (PES) base membrane support layer is completely covered and accumulated by the polyvinylidene fluoride (PVDF) nanofiber middle layer. Generally speaking, when a high-voltage power supply is connected to a prepared spinning solution, the introduced current causes the ejected droplets to deform into Taylor cones and form ultrafine nanofibers under high voltage, forming a tightly structured nanofiber middle layer.

[0086] like Figure 3 As shown in the figure, it is a schematic diagram of the water contact angle of each base membrane under different electrospinning time of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer. The change of the hydrophilicity of the membrane can be judged according to the water contact angle. As can be seen from the figure, the contact angle of the pure PES support membrane is 36.7°, which has obvious hydrophilicity (<90°). The contact angle of the composite nanofiltration membrane with a polyvinylidene fluoride (PVDF) intermediate layer electrospun for 5 minutes increased from 36.7° at the beginning to around 40.5°. The increase in the contact angle is due to the strong hydrophobicity of the polyvinylidene fluoride (PVDF) material itself, but the hydrophilicity of the PES-1 surface does not change much and is still relatively hydrophilic. This also shows that a completely hydrophobic surface cannot be formed when the intermediate layer is electrospun for 5 minutes. When the spinning time reaches 10 minutes, the contact angle increases to 113.2°, which is in a hydrophobic (>90°) state. At this time, the hydrophilicity of the membrane surface is dominated by the polyvinylidene fluoride (PVDF) intermediate layer. When the spinning time is 15 minutes and 20 minutes, the contact angles of the membrane surface are 125.1° and 132.3°, respectively, both in a hydrophobic state, satisfying the hydrophobic intermediate layer conditions required in the present application.

[0087] Figure 4 is a schematic diagram of the Na2SO4 retention effect of the composite nanofiltration membrane with a load electrospinning time of 15 minutes at different interfacial polymerization monomer concentrations. Figure 4aThe results show that when the mass percentage of trimesoyl chloride (TMC) is 0.1wt%, the rejection rate of the composite nanofiltration membrane for 1000ppm Na2SO4 is 82.33, 93.69, 96.35, 99.15% at different mass percentages of piperazine (PIP) (0.2wt%, 0.5wt%, 1wt%, 2wt%), and the permeation flux is 29.43, 19.45, 14.38, 9.36L·m -2 ·h -1 bar -1 The results show that the performance is optimal when the mass percentage of trimesoyl chloride (TMC) is 0.1wt% and the mass percentage of piperazine (PIP) is 0.5wt%. Figure 4b It shows that when the mass percentage of trimesoyl chloride (TMC) of the composite nanofiltration membrane is 0.2wt%, the rejection rates are 90.25, 94.27, 96.73, and 97.10%, respectively, and the permeation fluxes are 15.48, 15.67, 12.23, and 7.74 L·m -2 ·h -1 bar -1 The results show that when the mass percentage of trimesoyl chloride (TMC) is 0.2wt%, the rejection rate remains at a high level, but the permeation flux is lower than the data in Figure a below with the same amount. Therefore, in the end, the optimal monomer mass percentages selected for interfacial polymerization in this embodiment are 0.1wt% of trimesoyl chloride (TMC) and 0.5wt% of piperazine (PIP).

[0088] like Figure 5a-5j The scanning electron microscope (SEM) images of the surface morphology of the composite nanofiltration membrane with different electrospinning times at different magnifications. Figure 5a-5j A typical nanofiltration membrane structure can be observed in the Figure 5, indicating that the polyamide (PA) filtration skin layer was successfully formed on the nanofiber middle layer. Figure 5a , Figure 5f In the TFC membrane of the blank control group, the appearance of the polyamide (PA) filter cortex that collapsed into the pores can be clearly observed, and some surface pores are not completely filled with piperazine (PIP), thus forming a defective polyamide layer. The nodular structure on the membrane surface indicates that it is not easy to prepare a complete and non-collapsed polyamide (PA) filter cortex on the macroporous hydrophilic polyethersulfone (PES) base membrane support layer. Figure 5b-Figure 5e , Figure 5g-5jIn the figure, it can be seen that the collapse degree of the polyamide (PA) filter cortex gradually decreases due to the increase of polyvinylidene fluoride (PVDF) nanofibers. Since TFC-1 and TFC-2 have less surface fiber coverage and larger pore size, the skeleton structure of the PES membrane can still be clearly seen, and the phenomenon of the polyamide (PA) filter cortex collapsing into the pores of the polyethersulfone (PES) base membrane support layer still occurs; when the nanofiber intermediate layer continues to completely cover the polyethersulfone (PES) base membrane support layer, the skeleton structure of the polyethersulfone (PES) base membrane support layer is no longer obvious, and the morphology of the polyamide (PA) filter cortex formed instead is mainly affected by the surface properties of the polyvinylidene fluoride (PVDF) intermediate layer. It can be observed from the figure that TFC-3 and TFC-4 membranes form a flat, non-collapsed polyamide (PA) filter cortex with many folds, which will help increase the surface area of ​​the membrane and improve the water channel during the permeation process.

[0089] like Figure 6a-6j The following are the cross-sectional scanning electron microscope (SEM) images and the atomic force microscope (AFM) images of the membrane surface of the polyvinylidene fluoride (PVDF) intermediate layer modified nanofiltration membrane. It can be observed from the figure that the polyamide (PA) filter skin layer can be successfully attached to the surface of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer through interfacial polymerization. Figure 6a In the scanning electron microscope (SEM) image of the TFC cross section, it can be more intuitively observed that the polyamide layer collapses into the large pores of the PES support layer, and the thickness distribution is very uneven, with the maximum thickness reaching 156nm. Figure 6b-Figure 6e The thickness of the polyamide (PA) filter skin layer of TFC-1, TFC-2, TFC-3 and TFC-4 are 94nm, 65nm, 50nm and 47nm, respectively.

[0090] Generally speaking, the typical thickness of the polyamide (PA) filter skin layer formed on the composite nanofiltration membrane through the interfacial polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC) is 50nm-200nm, and the polyamide layer generally has a very dense non-porous structure. Figure 6f-6j It can be seen that the TFC-3 and TFC-4 membranes have an ultra-thin and ultra-smooth polyamide (PA) filter cortex, which is conducive to the rapid passage of water molecules and enhances the permeation flux and filtration efficiency of the nanofiltration membrane during actual operation.

[0091] like Figure 7The figure shows the schematic diagram of the UV spectrum after loading different amounts of polyvinylidene fluoride (PVDF) nanofiber intermediate layers on the PES-based membrane. The figure shows that with the increase of the intermediate layer spinning time, the absorption wavelength at 294nm shows a gradual decrease, which indicates that the diffusion rate of piperazine (PIP) monomer becomes slow, resulting in the same change trend in the permeation flux, selective layer thickness and cross-linking degree of the modified membrane loaded with the intermediate layer.

[0092] Figure 8a-8d Schematic diagram of various surface properties of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after loading the nanofiber intermediate layer with different electrospinning time in the embodiment of the present application. Figure 8a The figure shows the water contact angle of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) intermediate layer prepared on the polyethersulfone (PES) base membrane support layer. The water contact angle of the TFC membrane is 52.0°. With the loading of the polyvinylidene fluoride (PVDF) intermediate layer, the water contact angle of TFC-1 increases slightly to 55.1°, the water contact angles of TFC-2 and TFC-3 gradually decrease to 52.6° and 42.4°, and the water contact angle of the TFC-4 membrane increases slightly.

[0093] like Figure 8b The figure shows the test of the Zeta potential of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) intermediate layer prepared on the PES support layer. As can be seen from the figure, the pH value between 3-10 is negatively charged. The nanofiltration membrane with negative surface charge will have an electrostatic repulsion effect on negatively charged solutes, increasing its retention rate. However, as the polyvinylidene fluoride (PVDF) gradually increases, the negative surface potential of the TFC-4 composite nanofiltration membrane gradually decreases. The Zeta potential of TFC-3 at pH 7 is -27.9±0.8mV, which is the membrane with the lowest negative charge under neutral conditions.

[0094] like Figure 8c The FT-IR spectrum of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) as the intermediate layer prepared on the PES support layer is shown in FIG. From the figure, it can be seen that the FT-IR spectra of the composite nanofiltration membrane prepared in this embodiment all show similar characteristic peaks. -1 The CO stretching vibration of the amide bond (-NH-CO) I band at 1485 cm -1 -CH2- asymmetric stretching vibration appeared, and at 1410cm -1 The appearance of CN stretching band at 1245 cm indicates the formation of polyamide structure. -1The peak at represents the asymmetric stretching vibration of COC. The absorption peak intensity of the polyamide nanofiltration membranes prepared at different polyvinylidene fluoride (PVDF) spinning times did not change significantly, indicating that the polyamide (PA) filter skin layer of all membranes was completely covered on the middle layer.

[0095] like Figure 8d The figure shows the XPS spectrum of the composite nanofiltration membrane with polyvinylidene fluoride (PVDF) intermediate layer prepared on the PES support layer. The C, N and O elements constituting the polyamide (PA) filter skin layer were detected by XPS. There was an obvious N1s peak on the surface of the TFC membrane after interfacial polymerization, proving the successful synthesis of the polyamide layer. In addition, a small F1s peak was found on the TFC-4 membrane, indicating that on the thicker polyvinylidene fluoride (PVDF) intermediate layer, due to being too hydrophobic, less piperazine (PIP) aqueous phase monomer was stored, the thickness of the prepared polyamide (PA) filter skin layer was small, or there may be some small defects, so that the fluorine (F) element in the polyvinylidene fluoride (PVDF) intermediate layer was detected, forming a small peak. The polyamide (PA) filter skin layers of other composite nanofiltration membranes completely covered the polyvinylidene fluoride (PVDF) nanofiber intermediate layer.

[0096] Table 1 shows the crosslinking degree of the polyamide (PA) filter cortex of different membranes calculated based on the atomic percentage of C, O and N. By comparing the characteristics of different element percentages and crosslinking degree of the composite nanofiltration membrane with the polyvinylidene fluoride (PVDF) intermediate layer loaded with different electrospinning times in the nanofiltration membrane and the present embodiment, it can be observed that after the introduction of the polyvinylidene fluoride (PVDF) intermediate layer, the crosslinking degree of the TFC membrane increases from 53.70% to 72.73% in the TFC-3 membrane. However, due to its excessive hydrophobicity, the crosslinking degree of TFC-4 decreases. Therefore, TFC-3 exhibits the characteristics of a relatively hydrophilic, negatively charged and dense defect-free polyamide (PA) filter cortex, thereby achieving excellent separation performance.

[0097] Table 1 Elemental analysis and cross-linking degree of composite nanofiltration membrane

[0098]

[0099]

[0100] Figure 9a-9bSchematic diagram of the polyethylene glycol (PEG) molecular weight cutoff and pore size distribution of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer loaded with different electrospinning times in different embodiments of the present application. As can be seen from the figure, the pore size distribution of the polyamide (PA) filter cortex is an important parameter that dominates its separation performance, and is used to determine the size exclusion effect of the polyamide (PA) filter cortex. In order to determine the changing trend of the surface pore size of the composite nanofiltration membrane of this embodiment, the molecular weight cutoff (MWCO) of the nanofiltration membrane was determined by filtering neutral polyethylene glycol (PEG) solutions of different molecular weights (200, 400, 600, 800 and 1000). From Figure 9a It can be observed that the molecular weight cutoff (MWCO) of TFC-1, TFC-2, TFC-3 and TFC-4 membranes is comparable to that of the original TFC membrane; in addition, 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, as shown in Figure 9b As shown. Among them, the average pore sizes of TFC, TFC-1, TFC-2, TFC-3, and TFC-4 membranes are 0.42nm, 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 TFC, the pore size distribution of TFC-3 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.

[0101] Fig.10 Schematic diagram of pure water flux and Na2SO4 salt interception of composite nanofiltration membrane formed by forming nanofiltration membrane on PES base membrane and loading polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning time in different embodiments of the present application. It can be found from the figure that with the increase of polyvinylidene fluoride (PVDF), the water flux gradually increases and then decreases. Among them, the permeability of TFC, TFC-1, TFC-2, TFC-3 and TFC-4 are 10.1, 14.8, 16.8, 18.8 and 17.6 L·m -2 ·h -1 bar -1 . TFC-3 had the highest permeability.

[0102] See also Fig.11a , Fig.11b, which is a schematic diagram of the series resistance model of a comparative nanofiltration membrane formed on a pure PES-based membrane and a composite nanofiltration membrane formed after loading a polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning times in different embodiments of the present application.

[0103] Fig.11a It is shown in Figure 2 that the pure water permeability of the membrane gradually decreases with the increase of PVDF loading, but the decrease is small. Fig.11b The resistance distribution of the nanofiltration membrane was studied and it was found that the supporting layer played a key role in hydraulic resistance, and the presence of the hydrophobic middle layer played a significant role in reducing the overall resistance of the nanofiltration membrane. The load of the middle layer not only affects the pure water flux of the substrate, but also affects the reaction interface conditions of interfacial polymerization. The presence of the polyvinylidene fluoride (PVDF) nanofiber middle layer makes the supporting layer thinner, and the thickness can be between 100um-300um. The thickness of the nanofiber middle layer is set between 100nm-1um, thereby reducing the resistance of the supporting layer, which is the most critical to the total resistance of the membrane.

[0104] Fig.12a , 12b The schematic diagram of the permeation flux and interception performance of the composite nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane formed after the polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning times in the embodiment of the present application is shown. The flux and interception performance of different membranes for various salts are tested in the figure. The flux of salt electrolytes is Na2SO4<MgSO4<MgCl2<NaCl from large to small, and the interception performance is Na2SO4>MgSO4>MgCl2>NaCl. This trend shows that the polyamide layer is a typical negatively charged nanofiltration membrane and has a significant trade-off effect. Regardless of the positive or negative charge, the interception capacity of the composite nanofiltration membrane surface for divalent ions is stronger than that for monovalent ions, so it has a higher interception rate for electrolytes composed of large-sized ions. The retention rate of sulfate (0.76nm) is higher than that of chloride (0.66nm) due to the steric hindrance effect and the stronger electrostatic repulsion on the surface of the negatively charged composite nanofiltration membrane.

[0105] like Fig.13a , 13b Shown is a schematic diagram of the retention performance of emerging micropollutants (EMPs) of a composite nanofiltration membrane formed on a PES-based membrane and loaded with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer with different electrospinning times in an embodiment of the present application. Fig.13a The experimental results of the MP, PP and BP retention rates show that as the middle layer spinning time increases, the retention effect of the membrane on parabens compounds gradually increases. As the molecular weight of the pollutant increases, the retention effect of the membrane becomes more and more obvious. It is observed that the TFC-3 composite nanofiltration membrane has the best retention performance for different pollutants. Fig.13b Two antibiotics with different charges were tested, namely the negatively charged sulfamethoxazole (SMX) and the positively charged trimethoprim (TMP). The results showed that TFC-3 still had the best removal effect.

[0106] like Figures 14a-14c The figure shows the stability performance of the composite nanofiltration membrane loaded with a polyvinylidene fluoride (PVDF) nanofiber intermediate layer electrospun for 15 minutes in the embodiment of the present application. Fig.14a and 14b The operational stability of TFC-3 was further studied by changing the salt concentration of the feed solution and the operating pressure in the membrane filtration system; Fig.14c The purpose is to explore the long-term operating stability of the composite nanofiltration membrane. Fig.14a By observing the changing trends of the membrane's permeability and retention performance at different Na2SO4 salt concentrations, it was found that as the ion concentration increased from 1000ppm to 5000ppm, the retention rate increased, which is consistent with the membrane's better retention rate at high ion concentrations. The increase in ion concentration will intensify the concentration polarization on the membrane surface, thereby affecting the retention of solutes. The decrease in the membrane's permeability flux with the increase in salt concentration is due to the increase in feed concentration, which increases the osmotic pressure, resulting in a decrease in transmembrane pressure. Fig.14b In the present invention, as the operating pressure increases, the permeation flux of the TFC-3 membrane increases linearly, and the relatively constant salt flux entering the permeate through the membrane will be diluted by the increased water flux, so the salt retention rate is slightly improved. As one of the important parameters of nanofiltration membrane performance, the stability of the long-term operation of the membrane is crucial. In the examples of the present application, a Na2SO4 aqueous solution with a concentration of 1000ppm was used as the feed solution, and the test was conducted continuously for 50 hours at 5bar to evaluate the feasibility of the long-term operation of the TFC-3 membrane. As can be seen from the figure, during the 50-hour filtration process, the Na2SO4 permeation flux of the newly synthesized nanofiltration membrane fluctuated slightly, and finally maintained at 18L·m -2 ·h -1 bar -1 The rejection rate was stable at about 96%. These results show that highly cross-linked TFC-3 has good selectivity and stability under various operating conditions.

[0107] like Figure 15a-15fThe figure shows the anti-fouling performance of the nanofiltration membrane formed on the PES base membrane and the composite nanofiltration membrane with a polyvinylidene fluoride (PVDF) intermediate layer loaded with electrospinning for 15 minutes in the embodiment of the present application. In the embodiment 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) were used as pollutants, and the anti-fouling performance of TFC and TFC-3 membranes was evaluated. 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 comparison. Fig.15a , 15b 15c and 15d are the normalized flux attenuation diagrams of TFC and TFC-3 after 3 hours of filtration in feed liquids of bovine serum albumin (BSA), sodium alginate (SA) and humic acid (HA). It can be seen from the figure that after 3 hours of pollution recovery, Fig.15a , 15b TFC-3 in 15c still has higher permeability and the best anti-pollution ability. Fig.15d , 15e Figures 15a and 15f are the distribution of the sludge resistance ratio of the membrane after being polluted by different organic substances. The figure evaluates the anti-fouling ability of the membrane in detail. The FRR of TFC-3 for various pollutants is 84.57% for bovine serum albumin (BSA), 87.23% for sodium alginate (SA), and 95% for humic acid (HA), all of which are higher than the FRR of the TFC membrane. The Rir of TFC-3 is 15.43% for bovine serum albumin (BSA), 12.77% for sodium alginate (SA), and 5% for humic acid (HA), indicating that only a few pollutants are difficult to remove by washing the membrane surface, especially sodium alginate (SA) and humic acid (HA). 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.

[0108] In addition, the composite nanofiltration membrane of the present application was tested on water bodies containing emerging micropollutants (EMPs). The removal rates of methyl paraben (MP), propyl paraben (PP), benzyl paraben (BP), sulfamethoxazole (SMX) and trimethoprim (TMP) were stabilized at 33.00%, 37.76%, 94.32%, 82.05% and 63.32%, respectively. The results show that the composite nanofiltration 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.

[0109] In the embodiment of the present application, the electrospinning time of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer is preferably 15 minutes. The TFC-3 prepared under this time has a smooth surface structure, good uniformity and hydrophobicity. The monomer concentrations are: the mass percentage of the aqueous phase monomer is preferably 0.2-2wt%, and more preferably 0.5wt%; the mass percentage of the organic phase monomer is preferably 0.1wt%-0.2wt%, and more preferably 0.1wt%. 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; when the monomer mass percentage PIP used in the intermediate layer is 0.5wt% and TMC is 0.1wt%, the electrospinning time is preferably 15min, and the prepared TFC-3 membrane has the best performance, and the water flux at this time is 18.8L·m -2 ·h -1 bar -1 , the interception rate of the emerging pollutant BP reached 97.25%, the long-term stability performance salt interception rate was stable at about 96%, and the FRR of various pollutants reached more than 80%, among which humic acid (HA) reached 95%. Therefore, the embodiment of the present application has excellent permeability, interception effect and anti-pollution performance.

[0110] The preparation methods and properties of the TFC, TFC-1, TFC-2, TFC-3 and TFC-4 membranes of the present application are further described in detail below through specific examples.

[0111] Example 1, specific steps for preparing TFC membrane:

[0112] 1. Soak the polyethersulfone (PES) base membrane and fix it on the surface of the collector;

[0113] 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 electrospinning time of polyvinylidene fluoride (PVDF) in this step is 0 min.

[0114] 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.

[0115] 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.

[0116] 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.

[0117] 6. Roll the rubber roller over the surface of the pre-treated film.

[0118] 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.

[0119] 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.

[0120] 9. The prepared composite nanofiltration membrane with polyamide layer was stored in deionized water at 4°C until testing.

[0121] The basement membrane pores in the nanofiltration membrane prepared in the above embodiment are as follows: Figure 2a The PES membrane is shown in Figure 2. The pure water flux and inorganic salt retention test were performed using a cross-flow filtration device. The 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 carried out at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 53.70%, the water contact angle was about 52.0°, and the average water flux was 10.1 L·m -2 ·h -1 bar -1 The flux recovery rates (FRR) of various pollutants are: 66.67% for BSA, 80.63% for SA, 80.77% for HA, and the salt retention rate is 95%. Figure 8c As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -2 to -18 mV in an environment with a pH value of 3 to 10.

[0122] Example 2 is a specific method for preparing a TFC-1 membrane.

[0123] The difference from Example 1 is that the electrospinning time of the PVDF intermediate layer loaded on the polyethersulfone (PES) base membrane is 5 minutes, and the other conditions are consistent with Example 1. The nanofiltration membrane thus prepared is denoted as TFC-1 membrane.

[0124] The basement membrane pores in the nanofiltration membrane prepared in Example 2 are as follows: Figure 2b As shown in PES-1. 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 59.40%, the water contact angle was about 55.1°, and the average water flux was 14.8 L·m -2 ·h -1 bar -1 , the salt retention rate is 94%; Figure 8c As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -11 to -24 mV in an environment with a pH value of 3 to 10.

[0125] Example 3 is a specific method for preparing a TFC-2 membrane.

[0126] The difference between Example 3 and Example 1 is that the electrospinning time of the PVDF intermediate layer loaded on the polyethersulfone (PES) base membrane is 10 minutes, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as TFC-2 membrane.

[0127] 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 conducted at room temperature. The crosslinking degree of the nanofiltration membrane prepared in the above embodiment was tested to be 66.55%, the water contact angle was about 52.6°, and the average water flux was 16.8 L·m -2 ·h -1 bar -1 , the salt retention rate is 98.8%; Figure 8c As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -13 to -27 mV in an environment with a pH value of 3 to 10.

[0128] Example 4 is a specific method for preparing a TFC-3 membrane.

[0129] The difference between Example 4 and Example 1 is that the electrospinning time of the PVDF intermediate layer loaded on the PES base membrane is 15 minutes, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as TFC-3 membrane.

[0130] 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 72.73%, the water contact angle was about 42.4°, and the average water flux was 18.8 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 8c As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -22 to -34 mV in an environment with a pH value of 3 to 10.

[0131] Example 5 is a specific method for preparing a TFC-4 membrane.

[0132] The difference between Example 5 and Example 1 is that the electrospinning time of the PVDF intermediate layer loaded on the PES base membrane is 20 minutes, and the other conditions are the same as those of Example 1. The nanofiltration membrane thus prepared is denoted as TFC-4 membrane.

[0133] The basement membrane pores in the nanofiltration membrane prepared in Example 5 are as follows: Figure 2e As shown in PES-4. 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 55.32%, the water contact angle was about 55.6°, and the average water flux was 17.6 L·m -2 ·h -1 bar -1 , the salt retention rate is 92%; Figure 8c As shown, the Zeta potential of the nanofiltration membrane prepared in the above embodiment is about -11mv to -27mV in an environment with a pH value of 3 to 10.

[0134] This application regulates the pore size and hydrophilicity of the PES-based membrane by electrospinning a polyvinylidene fluoride (PVDF) nanofiber intermediate layer, and forms a non-collapsed, continuous and wrinkled polyamide (PA) filter cortex through an interfacial polymerization (IP) process, thereby preparing a polyvinylidene fluoride (PVDF) nanofiber intermediate layer composite nanofiltration membrane, and tests 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 skin thickness of 47nm-156nm; a cross-linking degree of 53.70%-72.73%; a water contact angle of 42.4°-55.6°, wherein the water contact angle of the base membrane portion is 36.7°-132.3°; a Zeta potential of -34mV-0mV; and a maximum water flux of the composite nanofiltration membrane of 18.8L·m -2 ·h -1 bar -1 The salt rejection rate can reach 96%-98.6%. The results show that the polyvinylidene fluoride (PVDF) nanofiber intermediate layer with the appropriate electrospinning time under the optimal monomer concentration can well regulate the PES base membrane and prepare a composite nanofiltration membrane with excellent separation performance.

[0135] In addition, the nanofiltration membrane provided in the present application is prepared by electrospinning a PVDF intermediate layer on a PES base membrane to prepare a TFC-X membrane, the pore size of the nanofiber intermediate layer is between 0.35nm-0.5nm, and the loading of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer makes up 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 of the electrospinning time of the polyvinylidene fluoride (PVDF) nanofiber intermediate layer, the PES base membrane forms a new substrate morphology, high hydrophobicity, and reasonable pore size, and 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 covers the hydrophobic middle layer, which reduces the pore size of the filter cortex sinking 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 floats on the hydrophobic middle layer and has a dense structure. The pore size is uniform and the pore size is 0.25nm-0.75nm, 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: the water flux and salt retention rate are: 18.8L·m -2 ·h -1 bar -1 and 99.15%. Because the filtration cortex when a concave liquid surface is formed on the pores in the water transport layer, the whole floats on the surface of the basement membrane and will not penetrate into the basement 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 nanofiltration membrane provided in the present application has a high removal rate for emerging pollutants such as methylparaben (MP), propylparaben (PP), benzylparaben (BP), sulfamethoxazole (SMX) and trimethoprim (TMP) in water treatment, wherein the removal rate of benzylparaben (BP) reaches 97.25%. In the anti-pollution performance test of organic pollutants, the flux recovery rate of the nanofiltration membrane provided in the present application is above 84%, and can reach up to 95%. Therefore, the nanofiltration membrane provided in the present application not only has a high permeability, but also shows excellent emerging pollutant removal effect and anti-pollution performance.

[0139] In fact, this embodiment only optimizes the materials and preparation methods for preparing the composite nanofiltration membrane. In actual use, a hydrophilic porous base membrane support layer is used, and a hydrophobic intermediate layer is formed thereon. A hydrophilic filter cortex is formed after an interfacial polymerization reaction. This can solve the problem of the filter cortex collapsing into the pores of the base membrane support layer in the prior art, thereby optimizing the performance of the composite nanofiltration membrane, increasing the water flux, and improving the separation accuracy, stability and anti-fouling performance of the membrane to a certain extent.

[0140] 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 hydrophobic middle layer is composed of a hydrophobic organic material, and the porous base membrane support layer and the filter skin layer are both composed of a hydrophilic organic material; 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 compact 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.

3. The composite nanofiltration membrane according to claim 2, characterized in that The thickness of the nanofiber middle layer is 100nm-1um.

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, which floats on the hydrophobic middle layer and has a dense structure. The thickness is 47nm-94nm, preferably 50nm; the pore size is 0.25nm-0.75nm, preferably 0.3nm-0.5nm.

6. A method for preparing a 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 on the porous base membrane support layer, wherein the hydrophobic intermediate layer is configured to be composed of an organic material; S3. Performing an interfacial polymerization reaction on the hydrophobic intermediate layer to form a hydrophilic filtration skin layer.

7. The method for preparing a composite nanofiltration membrane according to claim 6, characterized in that: The step S2 is achieved in the following manner: the hydrophobic middle layer is a nanofiber middle layer with a compact structure, and the nanofiber middle layer is prepared on a porous base membrane support layer by electrospinning.

8. The method for preparing the composite nanofiltration membrane according to claim 7, characterized in that: The step S2 further comprises drying and hot pressing the porous base membrane support layer supporting the nanofiber intermediate layer.

9. The method for preparing a composite nanofiltration membrane according to claim 7, 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.

10. The method for preparing a composite nanofiltration membrane according to claim 9, 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.

11. The method for preparing a composite nanofiltration membrane according to claim 10, 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.

12. The method for preparing a composite nanofiltration membrane according to claim 11, characterized in that: The electrospinning method is completed by an electrospinning device. At a certain rotation speed, the duration of the electrospinning is set to be between 5 minutes and 20 minutes.

13. The method for preparing a composite nanofiltration membrane according to claim 12, characterized in that: The conditions for the electrospinning are: applying a voltage of 20 kV, maintaining the ambient humidity at 40% RH, setting the rotation speed to 100 rpm, and the electrospinning time to 15 minutes.

14. The method for preparing a composite nanofiltration membrane according to claim 13, characterized in that: The step S3 performs interfacial polymerization reaction in the following manner: a certain mass percentage of piperazine (PIP) aqueous solution 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 certain mass percentage of benzyl chloride (TMC) n-hexane solution for a certain period of time to induce a polymerization reaction between piperazine (PIP) and benzyl chloride (TMC) to form the filter cortex.

15. The method for preparing a composite nanofiltration membrane according to claim 14, 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 solution of trimesoyl chloride (TMC) in n-hexane for 1 to 6 minutes to induce a polymerization reaction between piperazine (PIP) and trimesoyl chloride (TMC); and drying the membrane prepared in the above step and then curing it, the curing time being 30 to 90 seconds.

16. A composite nanofiltration membrane, characterized in that: The composite nanofiltration membrane is prepared by any one of the preparation methods 6-15 above.

17. A nanofiltration membrane for filtering and intercepting emerging pollutants, characterized in that: The nanofiltration membrane is prepared by any one of the preparation methods 6-15 above, and the emerging pollutants are not limited to paraben compounds, sulfamethoxazole (SMX) and trimethoprim (TMP).

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