A loose nanofiltration membrane and a method for preparing the same

Abstract

The application provides a loose nanofiltration membrane and a preparation method thereof, the loose nanofiltration membrane is prepared by twice polystyrene sulfonate (PSS) assisted pyrrole liquid phase in-situ polymerization on the surface of a porous support membrane, PSS is used as a structure directing agent to control the loose growth of polypyrrole, and a polycation intermediate layer and a vacuum filtration process are not needed, the rejection rate of the loose nanofiltration membrane to Congo red dye is above 99%; meanwhile, the loose nanofiltration membrane has a high permeation flux, has excellent dye / inorganic salt separation selectivity, and has a wide application prospect in the fields of dye production and purification and dye wastewater desalination.

Description

Technical Field

[0001] This invention belongs to the field of dye separation technology, and in particular relates to a loose nanofiltration membrane and its preparation method. Background Technology

[0002] The textile dyeing and printing industry is one of the major sources of industrial wastewater, with large discharge volumes and complex compositions. Faced with increasingly stringent environmental protection requirements and the need for resource recycling, there is an urgent need to develop new treatment technologies capable of efficiently separating dyes and salts and recovering them through resource recycling. Membrane separation technology, due to its advantages such as simple operation, high separation efficiency, low energy consumption, and no phase change, shows broad application prospects in the field of wastewater treatment. Among them, loose nanofiltration membranes, with their unique pore size sieving and electrostatic repulsion synergistic effect, provide a feasible path for the selective separation of dyes and salts in dyeing and printing wastewater. Depending on the treatment objectives, the efficient separation and resource recovery of dyes and salts can be achieved by adjusting the pore size and surface charge characteristics of the membrane.

[0003] Polypyrrole (PPy), as a conductive polymer material, has been widely used in recent years to prepare functional separation membranes due to its good chemical stability and biocompatibility. Currently, researchers have made significant progress in the field of polypyrrole-modified separation membranes. For example, patent CN119034516A uses vacuum filtration to disperse oxidants and small-molecule dopants with functional groups onto the surface of the substrate membrane and within the membrane pores. A functionalized polypyrrole layer containing functional groups is then prepared through the gas-phase polymerization of pyrrole monomers. The synergistic effect of the size effect of the functional groups and the membrane pore expansion and contraction enhances the adjustable range of desalination performance.

[0004] Patent CN106512728A uses a polymer ultrafiltration membrane as the base membrane, with a porous carrier layer coated on the surface. After layer-by-layer self-assembly ion treatment with sodium polystyrene sulfonate (PSS) polyanion and polydiallyl dimethyl ammonium chloride (PDADMAC) polycation, a graphene oxide solution is filtered, and then a polyamide functional layer is formed through interfacial polymerization. Finally, a PVA protective layer is coated. This method requires multiple complex processes, including the introduction of a porous carrier layer, graphene oxide, and interfacial polymerization. Furthermore, the layer-by-layer self-assembly of PDADMAC and PSS forms a dense polyelectrolyte multilayer membrane, making it difficult to form a truly loose nanofiltration structure. Additionally, the high cost of graphene oxide hinders large-scale production.

[0005] Layer-by-layer (LBL) self-assembly technology provides an effective approach for the precise control of membrane structure. Based on intermolecular non-bonding forces (such as electrostatic interactions and hydrogen bonds), this technology allows for precise control of membrane composition, structure, and thickness at the molecular level through the alternating deposition of polyelectrolytes with opposite charges on a substrate surface. It offers advantages such as low cost, ease of operation, and high controllability. Patent CN103846010A employs static layer-by-layer self-assembly technology, using a polysulfone ultrafiltration membrane as the base membrane, and prepares a multilayer polyelectrolyte nanofiltration membrane through the alternating deposition of PDADMAC and PSS. This membrane is used for desalination and ammonia nitrogen recovery from high-salinity, high-ammonia-nitrogen wastewater.

[0006] However, existing polypyrrole-based separation membranes and layer-by-layer self-assembled membrane technologies mainly suffer from the following problems that urgently need to be addressed: First, gas-phase polymerization processes (such as CN119034516A) require vacuum filtration and gas-phase reaction equipment, which have high equipment requirements and complex operation, limiting large-scale application. Moreover, their invention aims at the dynamic expansion and contraction of electroresponsive membrane pores, not at the loose nanofiltration structure for dye / salt separation. Second, layer-by-layer self-assembly technologies (such as CN103846010A and CN106512728A) rely on the anionic and cation exchange of PDADMAC and PSS. Alternating deposition of ions results in a dense polyelectrolyte membrane structure with low water flux, requiring additional interfacial polymerization, graphene oxide, or PVA protective layers, making the process cumbersome. Third, while in-situ liquid-phase polymerization is simple to operate, it lacks effective control over the polymerization behavior of polypyrrole, which is prone to random aggregation, leading to uneven separation layers and decreased flux. Fourth, existing technologies struggle to simultaneously achieve high dye rejection rates and high permeation flux, particularly in the selective separation of dyes and inorganic salts, lacking a structurally controllable, simple, and high-performance separation solution. Therefore, there is an urgent need to develop a simple, non-vacuum- and gas-phase-independent method for preparing loose nanofiltration membranes that can achieve both high rejection rates and high flux, while being easily industrially scalable. Summary of the Invention

[0007] In view of this, the present invention aims to propose a loose nanofiltration membrane and its preparation method. Through a cyclic process of two dopant adsorption-pyrrole liquid-phase in-situ polymerization, the loose growth of polypyrrole is regulated by the structure guidance of the dopant, and a polypyrrole / PSS composite separation layer with a gradient loose structure is constructed. This invention aims to solve the technical problems of existing polypyrrole separation layers, such as easy agglomeration, uneven structure, low permeation flux, and poor dye / salt selectivity. The prepared loose nanofiltration membrane has the characteristics of high flux, high dye rejection rate, and low salt rejection rate, and is suitable for dye wastewater treatment and dye / salt separation.

[0008] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A method for preparing a loose nanofiltration membrane, the method comprising the following steps: S1. Pretreatment of the porous support membrane; S2. The pretreated porous support membrane is immersed in a dopant aqueous solution for the first adsorption. After removal, it is rinsed with deionized water to obtain a porous support membrane adsorbed with the dopant. The dopant is sodium polystyrene sulfonate (PSS). S3. The support membrane obtained in step S2 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant to carry out the first liquid phase in-situ polymerization reaction. After being taken out, it is rinsed with deionized water to obtain the first layer of polypyrrole-doped composite membrane. S4. The composite membrane obtained in step S3 is immersed again in the dopant aqueous solution for a second adsorption. After removal, it is rinsed with deionized water to allow the dopant molecules to be further adsorbed on the surface and pores of the polypyrrole layer, providing nucleation sites for the second polymerization. The dopant is sodium polystyrene sulfonate. S5. The composite membrane treated in step S4 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant for a second liquid-phase in-situ polymerization reaction to form a second layer of polypyrrole-doped composite membrane. After removal, it is rinsed with deionized water to obtain a loose nanofiltration membrane with a gradient separation layer having a loose porous structure.

[0009] Furthermore, in S1, the porous support membrane is one of the following: polyethersulfone hollow fiber ultrafiltration membrane, polysulfone ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, and polyvinylidene fluoride ultrafiltration membrane, with a molecular weight cutoff of 50-150 kDa; the pretreatment is soaking in deionized water and then draining.

[0010] Furthermore, in S2, the concentration of the dopant aqueous solution is 0.5-1 w / v, the molecular weight of sodium polystyrene sulfonate is 60-100 kDa, and the soaking time is 5-30 min.

[0011] Furthermore, the reaction temperature in S3 is 20-40℃, the reaction time is 5-30min; the concentration of pyrrole monomer is 0.1-1w / v%, and the molar ratio of oxidant to pyrrole monomer is (1-3):1.

[0012] Furthermore, in S4, the concentration of the dopant aqueous solution is 0.1-0.5 w / v%, and the concentration of the dopant aqueous solution in S4 is less than that in S2, and the soaking time is 5-30 min.

[0013] Furthermore, in S5, the reaction temperature is 20-40℃ and the reaction time is 5-30 min.

[0014] Furthermore, the concentration of pyrrole monomer in S5 is 0.5-3 w / v%, and in order to further improve the density of the composite membrane and thus increase the retention rate, the concentration of pyrrole monomer in S5 is greater than that in S3.

[0015] Preferably, the concentration of pyrrole monomer in S3 is 0.5 w / v, and the concentration of pyrrole monomer in S5 is 2 w / v.

[0016] Furthermore, the oxidant in S3 and S5 is any one of ammonium persulfate, ferric chloride, or ferric p-toluenesulfonate, with a concentration of 0.1-5 w / v; the pH of the acidic aqueous solution is 1-3.

[0017] The present invention also provides a loose nanofiltration membrane prepared by the preparation method described above.

[0018] The present invention also provides an application of the loose nanofiltration membrane as described above in the treatment of dyeing and printing wastewater.

[0019] Compared with existing technologies, the loose nanofiltration membrane and its preparation method described in this invention have the following advantages: (1) The loose nanofiltration membrane of the present invention has a high rejection rate of Congo red dye, especially in Example 1, which is above 99%. It also has a high membrane permeation flux and can be applied to wastewater treatment fields such as dye / inorganic salt separation, dye production purification, and dye wastewater desalination.

[0020] (2) The preparation method of the loose nanofiltration membrane described in this invention is simple and can be achieved through a simple soaking-polymerization process. It has low monomer concentration, mild preparation conditions, wide applicability, is easy to scale up and promote, and is easy to achieve industrial production.

[0021] (3) This invention proposes for the first time a stepwise liquid-phase in-situ polymerization strategy assisted by pure PSS, constructing a polypyrrole composite membrane with a loose structure. Specifically: On the one hand, PSS acts as a macromolecular dopant in the first in-situ polymerization, and its sulfonic acid groups combine with the positively charged polypyrrole backbone. Through the steric hindrance effect of the macromolecular chains, the inter-chain spacing of polypyrrole molecules is increased, and the inter-chain packing density is reduced, making the bottom layer structure formed by the first polymerization loose and porous, thereby significantly improving the permeation flux; on the other hand, the PSS molecules exposed on the surface and pores of polypyrrole after the first polymerization act as nucleation templates and dopants for the second polymerization during the second adsorption process, effectively guiding the uniform growth of polypyrrole, avoiding random aggregation, thereby forming a continuous and relatively dense surface separation structure, ensuring a high dye rejection rate. This invention uses room temperature liquid-phase polymerization, which does not require vacuum equipment and gas-phase reaction devices, making the process simpler; moreover, this invention does not use polycationic electrolytes, and there is no alternating assembly process of anions and cations, resulting in a completely different film formation mechanism, and forming a loose polypyrrole composite layer rather than a dense polyelectrolyte multilayer membrane. In addition, the membrane also has good antifouling properties and operational stability. Attached Figure Description

[0022] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 The surface morphology of the loose nanofiltration membranes prepared in each embodiment is shown in (a) for Example 1, (b) for Example 2, and (c) for Example 3. Figure 2 The surface morphology of the loose nanofiltration membranes are shown in the comparative examples. (a) is a polyethersulfone (PES) hollow fiber membrane, (b) is comparative example 1, and (c) is comparative example 2. Figure 3 The antifouling performance and stability of the loose nanofiltration membrane prepared in Example 1 are tested; (a) is the permeation flux under different dye contamination, and (b) is the separation performance change curve of Congo red / NaCl mixed solution after continuous filtration for 24 hours. Detailed Implementation

[0023] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0024] In this document, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0025] In this document, when values ​​are described as ranges, it should be understood that such disclosure includes disclosure of all possible subranges within that range, as well as the specific numerical values ​​falling within that range, regardless of whether the specific numerical value or specific subrange is explicitly specified.

[0026] In this article, the terms "multiple" or "more than" are used unless otherwise specified, referring to a quantity greater than or equal to 2. For example, "one or more" means one or more types.

[0027] In this document, the terms "preferred" and "more preferred" are used only to describe implementation methods or embodiments with better effects, and should be understood as not constituting a limitation on the scope of protection of this invention.

[0028] In this document, terms such as "further" are used for descriptive purposes to indicate differences in content, but should not be construed as limiting the scope of protection of this invention.

[0029] In this article, the term "and / or" describes an association between objects, indicating that three relationships can exist. For example, A and / or B means: A or B, or A and B.

[0030] In this document, the term "about" means a specified value of + / - 10%, preferably + / - 5%, and more preferably + / - 1%.

[0031] In this article, the terms “include,” “including,” “have,” “contain,” etc., are all open-ended terms, meaning that they include but are not limited to.

[0032] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0033] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] Example 1 A method for preparing a loose nanofiltration membrane, comprising the following steps: S1. Pretreatment of polyethersulfone (PES) hollow fiber ultrafiltration membrane (100 kDa): Soak the porous support membrane in deionized water and drain.

[0035] S2. The pretreated porous support membrane is immersed in an aqueous solution of sodium polystyrene sulfonate (PSS) for the first adsorption. The immersion time is 5 min. After removal, it is rinsed with deionized water to remove the unadsorbed dopant on the surface, and a support membrane with adsorbed dopant is obtained. The concentration of PSS is 0.5 w / v% and the molecular weight is 70 kDa.

[0036] S3. The support membrane obtained in step S2 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant for the first liquid-phase in-situ polymerization reaction. The reaction temperature is 30℃ and the reaction time is 5min. After removal, it is rinsed with deionized water to obtain the first layer of polypyrrole-doped composite membrane. The concentration of pyrrole monomer is 0.5w / v%, and the molar ratio of oxidant to pyrrole monomer is 1:1.

[0037] S4. The composite membrane obtained in step S3 is immersed again in an aqueous solution of sodium polystyrene sulfonate (PSS) for a second adsorption. The immersion time is 10 min. After removal, it is rinsed with deionized water to allow the dopant molecules to be further adsorbed on the surface and pores of the polypyrrole layer, providing nucleation sites for the second polymerization. The concentration of the PSS aqueous solution is 0.1 w / v%, and the molecular weight is 70 kDa.

[0038] S5. The composite membrane treated in step S4 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant for a second liquid-phase in-situ polymerization reaction. The concentration of pyrrole monomer is 2 w / v%, the reaction temperature is 30℃, and the reaction time is 10 min to form a second layer of polypyrrole-doped composite membrane. After removal, it is rinsed with deionized water to obtain a loose nanofiltration membrane with a gradient separation layer of loose porous structure.

[0039] Example 2 The difference from Example 1 is that the concentration of pyrrole monomer in S3 is 0.1 w / v and the concentration of pyrrole monomer in S5 is 1 w / v.

[0040] Example 3 The difference from Example 1 is that the concentration of pyrrole monomer in S3 is 1 w / v and the concentration of pyrrole monomer in S5 is 3 w / v.

[0041] Figure 1 The images show scanning electron microscope (SEM) images of the loose nanofiltration membranes prepared in Examples 1-3. It can be seen that the membrane surface gradually becomes denser with increasing pyrrole concentration. This is because as the pyrrole concentration increases, the density of pyrrole monomers on the membrane surface and within the membrane pores increases, allowing more monomers to participate in the polymerization reaction, thus making the separation layer more compact.

[0042] Comparative Example 1 The difference from Example 1 is that PSS adsorption in S2 and S4 is not performed, but only two in-situ polymerizations of pyrrole monomers in S3 and S5 are performed.

[0043] Comparative Example 2 The difference from Example 1 is that the two in-situ polymerizations of pyrrole monomers in S3 and S5 are not performed, and only the PSS adsorption in S2 and S4 is performed.

[0044] Comparative Example 3 The difference from Example 1 is that only one adsorption and in-situ polymerization are performed, that is, only steps S1-S3 are performed, and steps S4-S5 are not performed.

[0045] Comparative Example 4 The difference from Example 1 is that the dopant is replaced with Congo red.

[0046] Comparative Example 5 The difference from Example 1 is that the dopant is replaced with sodium dodecyl sulfonate.

[0047] Comparative Example 6 The difference from Example 1 is that the concentration of pyrrole monomer in S5 is equal to the concentration of pyrrole monomer in S3, that is, the concentration of pyrrole monomer in both cases is 2w / v.

[0048] The separation performance of the loose nanofiltration membranes prepared in each example and comparative example was tested using a membrane performance evaluation instrument. The test conditions were: room temperature; feed pressure 1 bar; sodium chloride concentration: 1 g / L; dye concentration: 0.1 g / L; and the dye was Congo red.

[0049] Table 1. Test results of the separation performance of the loose nanofiltration membranes prepared in each embodiment and comparative example.

[0050] As shown in Table 1, in Examples 1-3, the membrane permeation flux increased from 164.89 L•m with increasing pyrrole monomer concentration. -2 •h -1 •bar -1 It gradually decreased to 4.93 L•m -2 •h -1 •bar -1 The rejection rate for Congo red increased from 96.29% to 100%. Considering both separation performance and permeation efficiency, the optimal pyrrole concentration was determined to be the concentration in Example 1, at which point the rejection rate for Congo red was 99.99% and the flux was 62.68 L·m⁻¹. -2 •h -1 •bar -1 .

[0051] Comparative Example 1, consisting only of a polypyrrole layer without PSS doping, achieved a 100% rejection rate for Congo red, but exhibited extremely low membrane permeation flux and a high NaCl rejection rate of 19.6%, resulting in poor dye / salt separation selectivity. Comparative Example 2, with only PSS / PSS electrostatic adsorption and no polypyrrole polymerization, while possessing the highest membrane permeation flux, had a Congo red rejection rate of only 85%, failing to achieve effective separation. In contrast, Example 1, through the synergistic effect of PSS doping and the bilayer polypyrrole structure, achieved an excellent balance between high dye rejection and low salt rejection while maintaining a high permeation flux.

[0052] Figure 2 Scanning electron microscopy (SEM) images of the surfaces of polyethersulfone (PES) hollow fiber ultrafiltration membranes, and the loose nanofiltration membranes prepared in Comparative Examples 1 and 2, are presented. It can be seen that the PES-based membrane surface exhibits a loose, porous structure. The surface of Comparative Example 1 (polypyrrole layer only) is dense after polymerization, while the surface structure of Comparative Example 2 (PSS / PSS adsorption layer only) is relatively loose. Figure 1 The nanofiltration membrane prepared in the Chinese example exhibits a significantly looser structure compared to Comparative Example 1. This is because PSS was successfully doped as a macromolecular dopant, and its sulfonic acid groups combined with the positively charged polypyrrole backbone. The PSS molecular chains interpenetrate between the polypyrrole backbone, increasing the average distance between the polypyrrole molecular chains through steric hindrance and reducing the inter-chain packing density, thereby forming a looser separation layer structure.

[0053] Comparative Example 3, which underwent only one adsorption and pyrrole polymerization, achieved the highest membrane permeation flux, but its Congo red rejection rate was only 93.21%, failing to achieve effective separation. Compared to Example 1, this demonstrates that performing two in-situ polymerizations can effectively improve the rejection rate, but the permeation flux decreases due to the aggregation problem of polypyrrole.

[0054] Comparative Examples 4 and 5 used different dopants, and compared with Example 1, the permeation flux was much higher than that of Comparative Examples 4 and 5. This is because the differences in molecular structure of the different dopants affected the permeation performance of the polypyrrole separation layer. The three dopants differed fundamentally in molecular weight and structural characteristics: PSS is a high-molecular-weight polymer composed of long-chain macromolecules polymerized from a large number of sodium styrene sulfonate monomers, with a molecular weight as high as about 70 kDa; Congo red is a medium-sized organic dye molecule with a rigid planar structure of two naphthalene rings and two sulfonic acid groups, with a molecular weight of about 696.66 Da; sodium dodecylbenzene sulfonate is a small-molecule surfactant containing only one long-chain alkyl group (12 carbons) and one benzene ring, with a molecular weight of only 348.48 Da. The above differences in molecular structure directly determine the strength of the steric hindrance effect provided by each dopant between the polypyrrole chains. PSS, with its large molecular size and massive polymer chain structure, provides the strongest steric hindrance between polypyrrole molecular chains, effectively increasing interchain spacing and reducing interchain packing density, thereby forming a loose and porous separation layer structure that significantly reduces the resistance to water molecule permeation. While Congo red has a rigid planar structure with two naphthalene rings that can partially open up the polypyrrole chains, its molecular weight is much smaller than PSS, limiting its steric hindrance effect. SDBS has the smallest molecular size, and even with the presence of long-chain alkyl groups, its steric hindrance effect is the weakest.

[0055] Comparative Example 6 used the same concentration of pyrrole polymerization, achieving a Congo red rejection rate of 99.99%, but with a lower permeation flux. Compared to Example 1, this demonstrates that a strategy of two-step polymerization steps, with the second polymerization using a higher pyrrole concentration, can effectively increase permeation flux while maintaining a high rejection rate. However, without the loose substrate formed by the first low-concentration polymerization as support, simply increasing the polymerization concentration can only improve rejection performance and cannot simultaneously meet permeation flux requirements.

[0056] Anti-pollution and stability tests: 1. Anti-fouling: The loose nanofiltration membrane prepared in Example 1 was used to conduct a filtration experiment with Congo red and methylene blue as pollution sources. Each cycle lasted 1 hour, and the membrane was washed with deionized water for 0.5 hours between cycles. The washing process was not included in the cycle count.

[0057] The results are as follows Figure 3In (a), the membrane's permeate flux can be largely recovered. The flux recovery rates for Congo red and methylene blue were 91.51% and 85.56%, respectively, indicating that the membrane has good antifouling performance. This good antifouling performance can be attributed to two aspects: firstly, the surface membrane structure is complete and uniform, effectively preventing dye contaminants from entering the membrane pores and avoiding irreversible decreases in permeate flux due to pore blockage; secondly, the doping of PSS introduces sulfonic acid groups onto the composite membrane surface, enhancing the negative charge and hydrophilicity of the membrane surface. Through electrostatic repulsion and hydration, this weakens the adsorption of anionic dye molecules on the membrane surface, thereby reducing membrane fouling.

[0058] 2. Stability: The loose nanofiltration membrane prepared in Example 1 was used as the feed solution and a 24-hour continuous filtration experiment was conducted at an operating pressure of 1 bar to evaluate the operational stability of the composite membrane.

[0059] Stability is one of the important indicators for evaluating the practical application value of membranes. For example... Figure 3 As shown in (b), throughout the test, the membrane's rejection rate for Congo red remained stable, consistently greater than 99%; the rejection rate for NaCl remained around 10%, maintaining a low level. The membrane's permeate flux showed a significant decreasing trend in the first 3 hours. This is because dye molecules, as contaminants, can diffuse into the membrane pores or adsorb onto the membrane surface, leading to a decrease in pore size and an increase in the negative charge density on the membrane surface, thus decreasing the permeate flux and slightly increasing the NaCl rejection rate. Subsequently, the system reached dynamic equilibrium, and the decreasing trend in permeate flux slowed and stabilized.

[0060] The above results demonstrate that the membrane prepared by this invention exhibits good operational stability and excellent dye / salt separation performance.

[0061] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a loose nanofiltration membrane, characterized in that: The method includes the following steps: S1. Pretreatment of the porous support membrane; S2. The pretreated porous support membrane is immersed in a dopant aqueous solution for the first adsorption. After removal, it is rinsed with deionized water to obtain a porous support membrane adsorbed with dopant. The dopant is sodium polystyrene sulfonate. S3. The support membrane obtained in step S2 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant to carry out the first liquid phase in-situ polymerization reaction. After being taken out, it is rinsed with deionized water to obtain the first layer of polypyrrole-doped composite membrane. S4. The composite film obtained in step S3 is immersed again in the dopant aqueous solution for a second adsorption. After removal, it is rinsed with deionized water to further adsorb the dopant molecules on the surface and pores of the polypyrrole layer. The dopant is sodium polystyrene sulfonate. S5. The composite membrane treated in step S4 is placed in an acidic aqueous solution containing pyrrole monomer and oxidant for a second liquid-phase in-situ polymerization reaction to form a second layer of polypyrrole-doped composite membrane. After removal, it is rinsed with deionized water to obtain the loose nanofiltration membrane.

2. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: In S1, the porous support membrane is one of the following: polyethersulfone hollow fiber ultrafiltration membrane, polysulfone ultrafiltration membrane, polyacrylonitrile ultrafiltration membrane, and polyvinylidene fluoride ultrafiltration membrane, with a molecular weight cutoff of 50-150 kDa; the pretreatment is soaking in deionized water and then draining.

3. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: In S2, the concentration of the dopant aqueous solution is 0.5-1 w / v, and the molecular weight of sodium polystyrene sulfonate is 60-100 kDa; the soaking time is 5-30 min.

4. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: The reaction temperature in S3 is 20-40℃, and the reaction time is 5-30 min; the concentration of pyrrole monomer is 0.1-1 w / v%, and the molar ratio of oxidant to pyrrole monomer is (1-3):

1.

5. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: In S4, the concentration of the dopant aqueous solution is 0.1-0.5 w / v%, and the concentration of the dopant aqueous solution in S4 is less than that in S2. The soaking time is 5-30 min.

6. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: In S5, the reaction temperature is 20-40℃ and the reaction time is 5-30 min.

7. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: The concentration of pyrrole monomer in S5 is 0.5-3 w / v%, and the concentration of pyrrole monomer in S5 is greater than that in S3.

8. The method for preparing the loose nanofiltration membrane according to claim 1, characterized in that: The oxidant in S3 and S5 is any one of ammonium persulfate, ferric chloride, or ferric p-toluenesulfonate, with a concentration of 0.1-5 w / v; the pH of the acidic aqueous solution is 1-3.

9. A loose nanofiltration membrane prepared by the preparation method according to any one of claims 1-8.

10. The application of the loose nanofiltration membrane as described in claim 9 in the treatment of dyeing and printing wastewater.

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