Composite nanofiltration membrane with reactive nanofiber middle layer as well as preparation method and application of composite nanofiltration membrane

Abstract

The invention provides a composite nanofiltration membrane with a reactive nanofiber middle layer and a preparation method and application of the composite nanofiltration membrane, and belongs to the field of nanofiltration membranes, and the composite nanofiltration membrane comprises a base material layer, the reactive nanofiber middle layer loaded on the base material layer and a polyamide separation layer formed by interfacial polymerization with the middle layer. By introducing the reactive nanofiber middle layer and combining with a specific preparation method, firm combination of the base material layer, the middle layer and the separation layer is realized, and the application performance and the separation stability of the composite nanofiltration membrane are improved; meanwhile, the specific nanofiber membrane structure of the middle layer and PEI crosslinked with nanofibers are utilized, the diffusion rate of monomers in the interfacial polymerization process is effectively controlled, and the thickness of the separation layer is changed; and the nanofiber middle layer also provides a rough membrane surface structure, so that a rough surface separation layer is generated, the effective permeation area of the nanofiltration membrane is increased, and the composite nanofiltration membrane material with high permeation flux and high rejection rate is prepared.

Description

Technical Field

[0001] The invention relates to the technical field of nanofiltration membranes, and in particular to a composite nanofiltration membrane with a reactive nanofiber intermediate layer and a preparation method and application thereof. Background Art

[0002] Nanofiltration membranes have attracted more and more attention in the field of water treatment due to their advantages such as low energy consumption, no phase change, high yield and low pollution. However, retention and permeation are two important indicators that nanofiltration membranes often cannot have at the same time. Therefore, under the premise of ensuring high separation efficiency, the pursuit of high permeability is the direction and driving force of the continuous development of nanofiltration membranes. Common commercial membranes are prepared by interfacial polymerization (IP) reaction of water-phase monomer piperazine (PIP) and oil-phase monomer trimethylol chloride (TMC) on porous substrates. Due to the dissociation of amino or carboxyl groups on its surface, it is usually neutral or negatively charged; therefore, this commercial model cannot effectively separate multivalent cations and cationic small molecules commonly seen in the actual environment by electrostatic repulsion. In order to achieve effective separation of these substances, it is necessary to prepare NF membranes with positively charged surfaces; in addition, at the water / oil interface where the water-phase monomer piperazine and the oil-phase monomer trimethylol chloride are mixed, the IP reaction occurs instantly and the rate is difficult to control, so that the generated polyamide (PA) separation layer is often highly cross-linked and thick, which will greatly increase the transport channels of water molecules in the membrane pores, resulting in lower permeability.

[0003] A certain prior art discloses a high-performance polyamide nanofiltration membrane and a preparation method thereof. The nanofiltration membrane comprises an aqueous solution of piperazine, an organic solution comprising trimesoyl chloride and an aqueous solution of polymerizable amphiphilic molecules, wherein the polymerizable amphiphilic molecules can be polymerized by ultraviolet light initiation to construct a monolayer membrane, and the surface of a macroporous polyethersulfone base membrane is used as the interface between the aqueous solution of piperazine and the organic solution, and the polymerizable amphiphilic molecules are caused to form a monolayer membrane by ultraviolet initiation reaction at the interface, and piperazine and trimesoyl chloride are caused to undergo condensation polymerization reaction at the interface, thereby forming an ultrathin active separation layer with rich wrinkled morphology on the surface of the macroporous polyethersulfone base membrane; the preparation method introduces a monolayer membrane composed of polymerizable amphiphilic molecules at the water / oil two-phase interface, and improves the diffusion rate of piperazine from the water phase into the oil phase by controlling the interaction of the polymerizable amphiphilic molecules based on piperazine, thereby promoting the interfacial polymerization condensation reaction, thereby obtaining a high-performance polyamide nanofiltration membrane. However, this method has limited control effect on IP reaction, and the effect of improving the permeability of the separation layer by only using the wrinkled structure of the separation layer is also limited, which is difficult to meet market application requirements.

[0004] Some researchers have tried to add non-porous or porous nanoparticles to the PA layer, and use the unique spatial configuration of nanomaterials to generate a large number of interfacial pores in the PA layer to form a loose separation layer; however, the aggregation of nanomaterials will lead to low uniformity of the selection layer. When nanomaterials are introduced as an intermediate layer between the porous substrate and the active layer, they are mostly deposited on the substrate surface through vacuum filtration, which is not only difficult to achieve in large-scale applications, but also difficult to maintain stability in long-term filtration tests because the interaction between the intermediate layer and the substrate is not strong enough. Summary of the invention

[0005] In view of the technical problems existing in the background technology, the present application provides a composite nanofiltration membrane with a reactive nanofiber intermediate layer and its preparation method and application, aiming to solve the technical problems that the existing nanofiltration membrane is difficult to have both retention and permeation performance, the filtration performance is poor in long-term stability, and the preparation method is difficult to apply on a large scale.

[0006] In the first aspect, the embodiment of the present application provides a composite nanofiltration membrane with a reactive nanofiber intermediate layer, comprising a substrate layer, a reactive nanofiber intermediate layer supported on the substrate layer, and a polyamide separation layer formed by interfacial polymerization with the reactive nanofiber intermediate layer. The reactive nanofiber intermediate layer is a PEI / EVOH membrane composed of EVOH nanofibers and polyetherimide co-crosslinked, and the polyamide separation layer is formed by interfacial polymerization of polyetherimide in the reactive nanofiber intermediate layer and tricarbonyl chloride.

[0007] In the technical solution of the embodiment of the present application, by introducing a reactive nanofiber intermediate layer between the substrate layer and the separation layer of the composite nanofiltration membrane, and then forming a polyamide separation layer through interfacial polymerization, a firm combination of the substrate layer, the intermediate layer and the separation layer is achieved, and the application performance and separation stability of the composite nanofiltration membrane are improved. At the same time, the unique nanofiber membrane structure of the formed intermediate layer is utilized to effectively control the diffusion rate of the monomer during the interfacial polymerization process, thereby changing the thickness of the separation layer; and the presence of the nanofiber intermediate layer also provides a rough membrane surface structure, which will induce the instability of the monomer during the diffusion process, and then produce a rough surface separation layer, thereby increasing the effective permeation area of ​​the nanofiltration membrane.

[0008] In some embodiments, the thickness of the polyamide separation layer is 120-240 nm, and the substrate layer is a polyethersulfone membrane.

[0009] In this embodiment, the increase in the thickness of the polyamide separation layer will affect the roughness of the separation layer surface and hinder the transport of water, so the thickness of the composite nanofiltration membrane needs to be limited; the substrate layer adopts one of the ultrafiltration membrane materials with a molecular weight cutoff of 200kDa.

[0010] In a second aspect, the present application provides a method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer, comprising the following steps:

[0011] S1, mixing the EVOH nanofiber suspension with polyetherimide, diluting with pure water under magnetic stirring for 1.5 to 2.5 hours at room temperature, adding the crosslinking agent solution, and continuously stirring for 10 to 14 hours to obtain a mixed solution;

[0012] S2, spraying the mixed solution of step S1 on the polyethersulfone substrate layer by high-pressure airflow method, and drying in air to obtain a composite membrane in which the PEI / EVOH intermediate layer is uniformly loaded on the polyethersulfone substrate layer;

[0013] S3. Fix the composite membrane obtained in step S2 on an interfacial polymerization device, add a surfactant, and then add an organic solution of tricarbonyl chloride to react for 3 to 8 minutes. After taking it out, place it in an oven for heat treatment for 0.5 to 1.5 hours to form a polyamide separation layer on the surface of the PEI / EVOH middle layer of the composite membrane, thereby obtaining a composite nanofiltration membrane with a reactive nanofiber middle layer.

[0014] In the technical solution of the embodiment of the present application, EVOH nanofibers are first cross-linked with PEI, loaded onto the substrate layer, to form a loose and positively charged reactive PEI / EVOH intermediate layer, and then immersed in an organic solution of triformyl chloride, and the interfacial polymerization of PEI and triformyl chloride in the intermediate layer is utilized to construct a polyamide separation layer on the intermediate layer, thereby achieving the purpose of intercepting cations; after PEI and EVOH nanofibers are cross-linked, interfacial polymerization can effectively control the monomer diffusion rate, and avoid excessive cross-linking and thick size of the separation layer due to the difficulty in controlling the instantaneous reaction, resulting in lower permeability. This preparation method can realize the regulation of the structure and performance of the polyamide separation layer, can be applied on a large scale, and prepare composite nanofiltration membrane materials with high permeability flux and high retention rate.

[0015] In some embodiments, in step S1, in the mixed solution, the mass ratio of polyetherimide to EVOH nanofiber is (2.5-10):1, and the mass ratio is preferably 7.5:1. The crosslinking agent solution is a glutaraldehyde solution with a mass percentage of 2-3 wt.%, and the glutaraldehyde solution contains 1 wt.% HCl.

[0016] In this embodiment, during the cross-linking process of polyetherimide and EVOH nanofibers, as the PEI content increases, the formed intermediate layer becomes denser and the surface becomes smoother; and the thickness of the polyamide separation layer after interfacial polymerization increases with the increase of PEI load. Although high PEI load helps to improve water flux, the thicker separation layer hinders water transport, and therefore the mass ratio of polyetherimide to EVOH nanofibers needs to be strictly limited.

[0017] In some embodiments, in step S3, the reaction time of the composite film in the organic solution of triformyl chloride is 5 minutes, and the temperature of the heat treatment is 50-70° C. The surfactant is a sodium dodecyl sulfate solution with a mass percentage of 0.05-0.15 wt.%, and the organic solution of triformyl chloride is a triformyl chloride n-hexane solution with a mass percentage of 0.2-0.4 wt.%.

[0018] In this embodiment, the reaction time of the composite membrane in the organic solvent of trimethylol chloride affects the degree of interfacial polymerization formed, and further affects the thickness of the formed polyamide separation layer. Therefore, it is necessary to limit it to avoid the polyamide separation layer being too thick. In addition, heat treatment after the completion of the interfacial polymerization can not only remove residual solvent and moisture, but also promote the further occurrence of the polymerization reaction, thereby improving the surface formation and structure of the membrane and enhancing the bonding force between the membrane and the substrate.

[0019] In some embodiments, in step S1, the concentration of the EVOH nanofiber suspension is 0.01-0.02 g / mL, and the preparation method of the EVOH nanofiber suspension is: preparing EVOH nanofibers with an average diameter of 200 nm by a melt extrusion phase separation method, dispersing the dried EVOH nanofibers in a mixed solvent of deionized water and isopropanol, and performing high-speed shear dispersion to obtain a uniformly dispersed EVOH nanofiber suspension; the mass ratio of deionized water to isopropanol in the mixed solvent is 1:1.

[0020] In this embodiment, the use of uniformly dispersed EVOH nanofibers is not only conducive to the crosslinking of EVOH nanofibers and PEI to form a middle layer with uniform structure, but also utilizes the fact that the nanofibers can be anchored in the substrate layer under the action of high-pressure airflow, thereby improving their bonding strength with the substrate layer.

[0021] In the third aspect, an embodiment of the present application provides an application of a composite nanofiltration membrane having a reactive nanofiber intermediate layer. The composite nanofiltration membrane having a reactive nanofiber intermediate layer is any one of the above or is prepared by any one of the preparation methods described above. The composite nanofiltration membrane is applied in the field of wastewater treatment or in the field of extracting lithium from waste batteries under acidic conditions.

[0022] When used, the composite nanofiltration membrane with a reactive nanofiber middle layer can achieve a large retention rate for cations under the premise of high flux, and has good market application prospects.

[0023] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solution of the present application, the following is a brief introduction to the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 The surface and cross-sectional morphology of the intermediate layer and the polyamide separation layer of the composite nanofiltration membrane prepared in Examples 1 to 4 were observed using a scanning electron microscope;

[0026] Figure 2 This is a graph showing the surface roughness measurement results of the intermediate layer (a) and the polyamide separation layer (b) of Example 3 of the present application;

[0027] Figure 3 This is a graph of the separation performance test data of the composite nanofiltration membrane of Examples 1 to 4 of the present application. DETAILED DESCRIPTION

[0028] The following embodiments of the technical solution of the present application will be described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0030] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0031] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0032] In the description of the embodiments of the present application, the term "and / or" is merely a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to specific circumstances.

[0033] Nanofiltration membranes have attracted more and more attention in the field of water treatment due to their advantages such as low energy consumption, no phase change, high yield and low pollution. However, interception and permeation are two important indicators that nanofiltration membranes often cannot meet at the same time. In the prior art, there are studies that introduce a monolayer membrane composed of polymerizable amphiphilic molecules at the water / oil interface, control the interaction of the polymerizable amphiphilic molecular heads based on piperazine, improve the diffusion rate of piperazine from the water phase into the oil phase, and promote the interfacial polymerization condensation reaction, thereby obtaining a high-performance polyamide nanofiltration membrane; however, this method has limited control effect on the IP reaction, and the effect of only using the folded structure of the separation layer to improve its permeability is also limited, which is difficult to meet market application requirements. In addition, some researchers have tried to add non-porous or porous nanoparticles to the PA layer, and use the unique spatial configuration of nanomaterials to generate a large number of interfacial pores in the PA layer to form a loose separation layer; however, the aggregation of nanomaterials will result in low uniformity of the selection layer. When introducing nanomaterials as an intermediate layer between the porous substrate and the active layer, they are mostly deposited on the substrate surface through vacuum filtration. This is not only difficult to achieve in large-scale applications, but also difficult to maintain stability in long-term filtration tests because the interaction force between the intermediate layer and the substrate is not strong enough.

[0034] In order to solve the technical problems that the existing nanofiltration membranes are difficult to have both interception and permeation performance, the filtration performance is poor in long-term stability, and the preparation method is difficult to apply on a large scale, the present application provides a composite nanofiltration membrane with a reactive nanofiber intermediate layer and its preparation method and application, wherein the composite nanofiltration membrane achieves a firm combination of the substrate layer, the intermediate layer and the separation layer by introducing a reactive nanofiber intermediate layer and combining a specific preparation method, thereby improving the application performance and separation stability of the composite nanofiltration membrane. At the same time, the unique nanofiber membrane structure of the formed intermediate layer and the PEI after cross-linking with the EVOH nanofiber are utilized to effectively control the diffusion rate of the monomer during the interfacial polymerization reaction, thereby changing the thickness of the separation layer; and the presence of the nanofiber intermediate layer also provides a rough membrane surface structure, which will induce the instability of the monomer during the diffusion process, and then produce a rough surface separation layer, thereby increasing the effective permeation area of ​​the nanofiltration membrane, and preparing a composite nanofiltration membrane material with high permeation flux and high retention rate.

[0035] For the convenience of description, the following embodiments take a composite nanofiltration membrane with a reactive nanofiber intermediate layer and a preparation method thereof according to an embodiment of the present application as an example for description.

[0036] In the first aspect, the embodiment of the present application provides a composite nanofiltration membrane with a reactive nanofiber intermediate layer, comprising a substrate layer, a reactive nanofiber intermediate layer supported on the substrate layer, and a polyamide (PA) separation layer formed by interfacial polymerization with the reactive nanofiber intermediate layer. The reactive nanofiber intermediate layer is a PEI / EVOH membrane composed of EVOH nanofibers and polyetherimide co-crosslinked, and the polyamide separation layer is formed by interfacial polymerization of polyetherimide in the reactive nanofiber intermediate layer and tricarbonyl chloride.

[0037] In the technical solution of the embodiment of the present application, by introducing a reactive nanofiber intermediate layer between the substrate layer and the separation layer of the composite nanofiltration membrane, and then forming a polyamide separation layer through interfacial polymerization, a firm combination of the substrate layer, the intermediate layer and the separation layer is achieved, and the application performance and separation stability of the composite nanofiltration membrane are improved. At the same time, the unique nanofiber membrane structure of the formed intermediate layer is utilized to effectively control the diffusion rate of the monomer during the interfacial polymerization process, thereby changing the thickness of the separation layer; and the presence of the nanofiber intermediate layer also provides a rough membrane surface structure, which will induce the instability of the monomer during the diffusion process, and then produce a rough surface separation layer, thereby increasing the effective permeation area of ​​the nanofiltration membrane.

[0038] In some embodiments, the thickness of the polyamide separation layer is 120-240 nm, and the substrate layer is a polyethersulfone membrane.

[0039] In this embodiment, the increase in the thickness of the polyamide separation layer will affect the roughness of the separation layer surface and hinder the transport of water, so the thickness of the composite nanofiltration membrane needs to be limited; the substrate layer adopts one of the ultrafiltration membrane materials with a molecular weight cutoff of 200kDa.

[0040] In a second aspect, the present application provides a method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer, comprising the following steps:

[0041] S1, mixing EVOH (ethylene-vinyl alcohol copolymer) nanofiber suspension with polyetherimide (PEI), diluting with pure water under magnetic stirring at room temperature for 1.5 to 2.5 hours, adding crosslinking agent solution, and continuously stirring for 10 to 14 hours to obtain a mixed solution;

[0042] S2, spraying the mixed solution of step S1 on the polyethersulfone (PES) substrate layer by high-pressure airflow method, and drying in air to obtain a composite membrane in which the PEI / EVOH intermediate layer is uniformly loaded on the polyethersulfone substrate layer;

[0043] S3. Fix the composite membrane obtained in step S2 on an interfacial polymerization device, add a surfactant, and then add an organic solution of trimethylol chloride (TMC) to react for 3 to 8 minutes. After taking it out, place it in an oven for heat treatment for 0.5 to 1.5 hours to form a polyamide separation layer on the surface of the PEI / EVOH middle layer of the composite membrane, thereby obtaining a composite nanofiltration membrane with a reactive nanofiber middle layer.

[0044] The present application first cross-links EVOH nanofibers with PEI, loads them onto a substrate layer, forms a loose and positively charged reactive PEI / EVOH intermediate layer, and then immerses it in an organic solution of triformyl chloride, utilizes the interfacial polymerization of PEI and triformyl chloride in the intermediate layer, and constructs a polyamide separation layer on the intermediate layer, thereby achieving the purpose of intercepting cations; after PEI and EVOH nanofibers are cross-linked, interfacial polymerization can effectively control the monomer diffusion rate, avoid excessive cross-linking of the separation layer and thick size due to the difficulty in controlling the instantaneous reaction, resulting in lower permeability. This preparation method can achieve the regulation of the structure and performance of the polyamide separation layer, can be applied on a large scale, and prepare composite nanofiltration membrane materials with high permeability flux and high retention rate.

[0045] Further, in some embodiments, in step S1, in the mixed solution, the mass ratio of polyetherimide to EVOH nanofiber is (2.5-10):1, preferably 7.5:1. The crosslinking agent solution is a glutaraldehyde (GA) solution with a mass percentage of 2-3 wt.%, and the glutaraldehyde solution contains 1 wt.% HCl.

[0046] In the technical solution of this embodiment, during the cross-linking process of polyetherimide and EVOH nanofibers, as the PEI content increases, the formed intermediate layer becomes denser and the surface becomes smoother; and the thickness of the polyamide separation layer after interfacial polymerization increases with the increase of PEI load. Although high PEI load helps to improve water flux, the thicker separation layer hinders water transport, and therefore the mass ratio of polyetherimide to EVOH nanofibers needs to be strictly limited.

[0047] Further, in some embodiments, in step S3, the reaction time of the composite film in the organic solution of triformyl chloride is 5 minutes, and the temperature of the heat treatment is 50-70° C. The surfactant is a sodium dodecyl sulfate (SDS) solution with a mass percentage of 0.05-0.15 wt.%, and the organic solution of triformyl chloride is a triformyl chloride n-hexane solution with a mass percentage of 0.2-0.4 wt.%.

[0048] In the technical solution of this embodiment, the reaction time of the composite membrane in the organic solvent of tricarbonyl chloride affects the degree of interfacial polymerization formed, and further affects the thickness of the formed polyamide separation layer. Therefore, it is necessary to limit it to avoid the polyamide separation layer being too thick. In addition, heat treatment is performed after the completion of the interfacial polymerization, which can not only remove residual solvent and moisture, but also promote the further occurrence of the polymerization reaction, thereby improving the surface formation and structure of the membrane and enhancing the bonding force between the membrane and the substrate.

[0049] Furthermore, in some embodiments, in step S1, the concentration of the EVOH nanofiber suspension is 0.01-0.02 g / mL, and the method for preparing the EVOH nanofiber suspension is as follows: EVOH nanofibers with an average diameter of 200 nm are prepared by a melt extrusion phase separation method, and the dried EVOH nanofibers are dispersed in a mixed solvent of deionized water and isopropanol, and high-speed shear dispersion is performed to obtain a uniformly dispersed EVOH nanofiber suspension; the mass ratio of deionized water to isopropanol in the mixed solvent is 1:1.

[0050] In the technical solution of this embodiment, the use of uniformly dispersed EVOH nanofibers is not only conducive to the cross-linking of EVOH nanofibers and PEI to form a middle layer with uniform structure, but also utilizes the fact that the nanofibers can be anchored in the substrate layer under the action of high-pressure airflow, thereby improving their bonding strength with the substrate layer.

[0051] In a third aspect, the present application provides an application of a composite nanofiltration membrane having a reactive nanofiber intermediate layer, and the composite nanofiltration membrane is applied to the field of wastewater treatment or the field of extracting lithium from waste batteries under acidic conditions. When applied, the composite nanofiltration membrane having a reactive nanofiber intermediate layer can achieve a high retention rate for cations under the premise of high flux, and has good market application prospects.

[0052] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If specific techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used without specifying the manufacturer are all conventional products that can be obtained commercially.

[0053] Example 1

[0054] This embodiment provides a method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer, comprising the following steps:

[0055] S1. EVOH nanofibers with an average diameter of about 200 nm were prepared by melt extrusion phase separation method, and the obtained dry nanofibers (2 g) were dispersed in a mixture of 200 mL of deionized water and IPA (isopropyl alcohol) (mass ratio of 1:1), and sheared and dispersed by an ultra-high-speed shearing machine to obtain a uniform EVOH nanofiber suspension;

[0056] EVOH nanofiber suspension (30 mL) was mixed with 0.75 g polyetherimide (the mass ratio of polyetherimide to EVOH nanofiber was 2.5:1), diluted with 10 mL pure water under magnetic stirring at room temperature for 2 h, and 3 mL of 2.5 wt.% GA solution (containing 1 wt.% HCl) was added, and stirring was continued for 12 h to obtain a mixed solution;

[0057] S2, spraying the mixed solution of step S1 on the polyethersulfone substrate layer by high-pressure airflow method, and drying in air to obtain a composite membrane in which the PEI / EVOH intermediate layer is uniformly loaded on the polyethersulfone substrate layer;

[0058] S3. Fix the composite membrane obtained in step S2 on an interfacial polymerization device, add 0.1wt.% SDS solution and drip it after 3 minutes, then add 30mL of triformyl chloride n-hexane solution (mass percentage is 0.3wt.%) and react for 5 minutes. After taking out, place it in a 60°C oven for heat treatment for 1h to form a polyamide separation layer on the surface of the PEI / EVOH middle layer of the composite membrane, so as to obtain a composite nanofiltration membrane with a reactive nanofiber middle layer.

[0059] Example 2

[0060] This embodiment provides a method for preparing a composite nanofiltration membrane with a reactive nanofiber intermediate layer. Compared with Example 1, the difference is that in step S1, the mass of polyetherimide is 1.5g, that is, the mass ratio of polyetherimide to EVOH nanofiber is 5:1. The rest is roughly the same as Example 1 and will not be repeated here.

[0061] Example 3

[0062] This embodiment provides a method for preparing a composite nanofiltration membrane with a reactive nanofiber intermediate layer. Compared with Example 1, the difference is that in step S1, the mass of polyetherimide is 2.25g, that is, the mass ratio of polyetherimide to EVOH nanofiber is 7.5:1. The rest is roughly the same as Example 1 and will not be repeated here.

[0063] Example 4

[0064] This embodiment provides a method for preparing a composite nanofiltration membrane with a reactive nanofiber intermediate layer. Compared with Example 1, the difference is that in step S1, the mass of polyetherimide is 3g, that is, the mass ratio of polyetherimide to EVOH nanofiber is 10:1. The rest is roughly the same as Example 1 and will not be repeated here.

[0065] See also Figure 1 The following are the surface and cross-sectional morphology images of the intermediate layer and polyamide separation layer of the composite nanofiltration membranes prepared in Examples 1 to 4 observed by scanning electron microscopy. Among them, (a)-(d) are the surface morphologies of the intermediate layer of Examples 1 to 4, (e)-(h) are the surface morphologies of the polyamide separation layer of Examples 1 to 4, and (i)-(l) are the cross-sectional morphologies of the polyamide separation layer of Examples 1 to 4. Figure 1 It can be seen that after EVOH nanofibers are co-crosslinked with PEI, the layers are denser and the surface is smoother, and as the PEI content increases, the membrane surface becomes smoother; the thickness of the polyamide separation layer increases with the increase of PEI loading. Although high PEI loading helps to increase water flux, a thicker polyamide separation layer will hinder water transport.

[0066] Please refer to Figure 2 The figure shows the surface roughness measurement results of the intermediate layer (a) and the polyamide separation layer (b) of Example 3. The specific characterization method is to use an atomic force microscope (AFM) to measure the surface roughness of the nanofiltration membrane. Each sample is measured repeatedly at least at 5 locations and the average value is calculated. Figure 2 It can be seen that the roughness of the intermediate layer and the polyamide separation layer surface are 155.3nm and 134.1nm, respectively; the high roughness value is mainly due to the ridge and valley structure of the stacked nanofibers, and after interfacial polymerization, the roughness decreases slightly due to the formation of a PA separation layer on the surface of the intermediate layer.

[0067] See also Figure 3 As shown in the figure, the separation performance test data of the composite nanofiltration membrane of Examples 1 to 4 are shown, wherein (a) is the LiCl aqueous solution flux and Li +The retention rate (b) is the FeCl3 aqueous solution flux and Fe 3+ Retention rate. PA-2.5, PA-5, PA-7.5, and PA-10 represent the composite nanofiltration membranes of Examples 1 to 4, respectively. The test method for membrane separation performance is as follows:

[0068] The flux of the salt solution was measured using a nanofiltration membrane performance tester produced by Suzhou Xinwang Membrane Technology Co., Ltd. The effective area of ​​the test was 7.07 cm 2 , the test pressure was maintained at 6 bar; in addition, all membranes were pre-pressed at 6 bar for 0.5 h before the test, and the test was carried out after the flux remained stable. The flux of the membrane was calculated by the following formula:

[0069]

[0070] Where V(L) represents the permeate volume, A(m -2 ) represents the effective test area, and t(h) represents the actual test time;

[0071] In order to evaluate the removal performance of the membrane for different inorganic salts, a filtration test was carried out using a target solution with a concentration of 1000 ppm, and the retention rate R was calculated by the following formula:

[0072]

[0073] where Cp and Cf represent the concentrations in the permeate and feed solution, respectively.

[0074] Depend on Figure 3 It can be seen that with the increase of PEI content, Li + , Fe 3+ The aqueous solution flux increases first and then decreases. + The interception first decreased and then slightly increased, which was opposite to the change trend of water flux. 3+ The retention rate was always maintained above 99%, indicating that increasing the PEI content within a certain range is beneficial to improving the flux of the composite nanofiltration membrane.

[0075] Comparative Example 1

[0076] Comparative Example 1 provides a method for preparing a composite nanofiltration membrane. Compared with Example 3, the difference is that in step S1, EVOH nanofiber suspension is not introduced, and in step S2, the polyetherimide solution is directly sprayed on the substrate layer. The rest is basically the same as Example 3 and will not be repeated here.

[0077] Comparative Example 2

[0078] Comparative Example 2 provides a method for preparing a composite nanofiltration membrane. Compared with Example 3, the difference is that in step S1, the EVOH nanofiber suspension and polyetherimide are not cross-linked, and in step S2, the EVOH nanofiber suspension and the polyetherimide solution are sprayed on the substrate layer in sequence. The rest is roughly the same as Example 3 and will not be repeated here.

[0079] Comparative Example 3

[0080] Comparative Example 3 provides a method for preparing a composite nanofiltration membrane. Compared with Example 3, the difference is that in step S3, no heat treatment is performed, and the rest is roughly the same as Example 3, which will not be repeated here.

[0081] Comparative Example 4

[0082] Comparative Example 4 provides a method for preparing a composite nanofiltration membrane. Compared with Example 3, the difference is that in step S3, the reaction time of the composite membrane in the organic solution of triformyl chloride is 1 min. The rest is roughly the same as Example 3 and will not be repeated here.

[0083] Comparative Example 5

[0084] Comparative Example 5 provides a method for preparing a composite nanofiltration membrane. Compared with Example 3, the difference is that in step S3, the reaction time of the composite membrane in the organic solution of triformyl chloride is 10 minutes, and the rest is roughly the same as Example 3, which will not be repeated here.

[0085] The membrane separation performance of the composite nanofiltration membranes prepared in Comparative Examples 1 to 5 was tested, and the results obtained are shown in the following table.

[0086] Table 1 Characterization of separation performance of composite nanofiltration membranes of Example 3 and Comparative Examples 1 to 5

[0087]

[0088]

[0089] As shown in Table 1, the introduction of the nanofiber intermediate layer and the interfacial polymerization reaction resulted in a thicker PA separation layer, but it still effectively increased the permeation flux and had the effect of resisting Fe 3+ High retention rate and Li + The low rejection rate of the nanofibers was achieved, achieving a high efficiency separation of the two. In Comparative Example 1, nanofibers were not introduced as an intermediate layer, but amino groups were directly introduced by infiltration, and the permeation flux of the prepared nanofiltration membrane was significantly reduced. In Comparative Example 2, PEI and EVOH were not cross-linked, resulting in a decrease in the number of amine groups on the membrane surface, an unstable fiber structure, and a significant reduction in the nanofiltration membrane's ability to resist Fe 3+The retention rate of comparative examples 3 to 4 is reduced or even omitted, resulting in insufficient interfacial reaction and low separation layer thickness, which seriously affects the nanofiltration membrane for Fe 3+ In Comparative Example 5, the interfacial reaction time was too long, resulting in an increase in the thickness of the PA separation layer, which reduced the solution flux of the nanofiltration membrane.

[0090] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A composite nanofiltration membrane having a reactive nanofiber intermediate layer, characterized in that: The invention comprises a substrate layer, a reactive nanofiber middle layer loaded on the substrate layer, and a polyamide separation layer formed by interface polymerization with the reactive nanofiber middle layer.

2. The composite nanofiltration membrane with a reactive nanofiber intermediate layer according to claim 1, characterized in that: The reactive nanofiber intermediate layer is a PEI / EVOH film formed by co-crosslinking EVOH nanofibers and polyetherimide, and the polyamide separation layer is formed by interfacial polymerization of polyetherimide in the reactive nanofiber intermediate layer and triformyl chloride.

3. The composite nanofiltration membrane with a reactive nanofiber intermediate layer according to claim 1, characterized in that: The thickness of the polyamide separation layer is 120-240 nm, and the substrate layer is a polyethersulfone membrane.

4. A method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1, mixing the EVOH nanofiber suspension with polyetherimide, diluting with pure water under magnetic stirring for 1.5 to 2.5 hours at room temperature, adding the crosslinking agent solution, and continuously stirring for 10 to 14 hours to obtain a mixed solution; S2, spraying the mixed solution of step S1 on the polyethersulfone substrate layer by high-pressure air flow method, and drying in air to obtain a composite membrane in which the PEI / EVOH intermediate layer is uniformly loaded on the polyethersulfone substrate layer; S3. Fix the composite membrane obtained in step S2 on an interfacial polymerization device, add a surfactant, and then add an organic solution of tricarbonyl chloride to react for 3 to 8 minutes. After taking it out, place it in an oven for heat treatment for 0.5 to 1.5 hours to form a polyamide separation layer on the surface of the PEI / EVOH middle layer of the composite membrane, thereby obtaining a composite nanofiltration membrane with a reactive nanofiber middle layer.

5. The method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to claim 4, characterized in that: In step S1, in the mixed solution, the mass ratio of polyetherimide to EVOH nanofiber is (2.5-10):1, and the mass ratio is preferably 7.5:

1.

6. The method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to claim 4, characterized in that: In step S3, the reaction time of the composite film in the organic solution of triformyl chloride is 5 minutes, and the temperature of the heat treatment is 50-70°C.

7. The method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to claim 4, characterized in that: In step S1, the cross-linking agent solution is a glutaraldehyde solution with a mass percentage of 2-3 wt.%, and the glutaraldehyde solution contains 1 wt.% of HCl.

8. The method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to claim 4, characterized in that: In step S3, the surfactant is a sodium dodecyl sulfate solution with a mass percentage of 0.05-0.15 wt.%, and the organic solution of triformyl chloride is a n-hexane solution of triformyl chloride with a mass percentage of 0.2-0.4 wt.%.

9. The method for preparing a composite nanofiltration membrane having a reactive nanofiber intermediate layer according to claim 4, characterized in that: In step S1, the concentration of the EVOH nanofiber suspension is 0.01-0.02 g / mL, and the preparation method of the EVOH nanofiber suspension is as follows: EVOH nanofibers with an average diameter of 200 nm are prepared by a melt extrusion phase separation method, and the dried EVOH nanofibers are dispersed in a mixed solvent of deionized water and isopropanol, and high-speed shear dispersion is performed to obtain a uniformly dispersed EVOH nanofiber suspension; the mass ratio of deionized water to isopropanol in the mixed solvent is 1:

1.

10. An application of a composite nanofiltration membrane having a reactive nanofiber intermediate layer, characterized in that: The composite nanofiltration membrane with a reactive nanofiber intermediate layer is prepared by the preparation method described in any one of claims 1 to 3 or by any one of claims 4 to 9, and the composite nanofiltration membrane is used in the field of wastewater treatment or in the field of extracting lithium from waste batteries under acidic conditions.

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