A composite nanofiltration membrane, its preparation method and application

Abstract

This invention provides a composite nanofiltration membrane, its preparation method, and its application, belonging to the field of pollutant removal technology. The invention first coats the surface of a macroporous base membrane with a high molecular weight polyelectrolyte system to cover the macropores, then fills the micropores of the base membrane with a low molecular weight polyelectrolyte. This improves the retention rate of micropollutants in the layer-by-layer self-assembled membrane, reducing pore size while maintaining high permeability. This invention prepares a multilayer composite nanofiltration membrane by coating polyelectrolyte systems of different molecular weights onto a high-porosity base membrane and repeating the coating process. The prepared composite nanofiltration membrane achieves a removal rate of over 90% for micropollutants with a molecular weight of around 200 Da, with a maximum removal rate of 99.8%.

Description

Technical Field

[0001] This invention relates to the field of pollutant removal technology, and in particular to a composite nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Micropollutants (MPs) typically refer to small-molecule organic compounds with molecular weights between 200 and 1000 Da. These are emerging pollutants composed of numerous anthropogenic and natural substances, originating from waste generated by industries such as industry, pharmaceuticals, and agriculture. Examples include pharmaceuticals, pesticides, endocrine disruptors, plasticizers, cosmetics, and other organic pollutants. They are characterized by their trace presence in water bodies, bioaccumulation, and environmental toxicity, seriously endangering human health. In recent years, the concentration of micropollutants in groundwater, surface water, and even drinking water has been increasing, reaching levels in the range of several ng / L. -1 to μg L -1 These pollutants have already threatened aquatic environments and drinking water quality, thus attracting widespread attention and concern globally. Many organic micropollutants have been included in the EU's wastewater discharge testing standards. Therefore, removing micropollutants from surface water and drinking water is an urgent task.

[0003] Currently, the technologies for treating micropollutants include: adsorption, advanced oxidation (AOPs), and membrane separation technology. The main adsorption material is activated carbon, which is low in cost, highly flexible, and economical; however, activated carbon has a low adsorption efficiency and requires a relatively high carbon dosage (up to 20 mg / L) to effectively remove micropollutants; in addition, due to the small size of micropollutants, the removal rate is generally low (<90%). Activated carbon after adsorption saturation becomes solid waste, which is a transfer from liquid pollutants to solid pollutants and still requires further advanced treatment. Advanced oxidation methods (AOPs) include three main technologies: (1) Ozone oxidation technology; it is a mature and effective method for degrading most pharmaceuticals and industrial chemicals and has been widely used in existing water treatment industries; however, O3 has a certain selectivity and is difficult to degrade ozone-resistant micropollutants such as ibuprofen, which has certain limitations. (2) Fenton oxidation: The main material of Fenton oxidation reagent is iron, and it does not contain toxic substances, making it relatively environmentally friendly and green, with no harm to the environment. However, to avoid iron precipitation at high pH values ​​during Fenton oxidation, it is usually required to be used in a low pH environment, which places certain requirements on the quality of the wastewater. Therefore, it is not suitable for large-scale applications. Photolysis under ultraviolet irradiation: For micro-pollutants that are easily decomposed by photolysis, ultraviolet photolysis can achieve good removal. However, this technology is not suitable for treating large-scale wastewater, as its removal efficiency is relatively low.

[0004] Membrane separation technology is a low-energy, high-efficiency separation technique. Compared to traditional separation techniques (such as distillation, crystallization, extraction, chromatography, etc.), membrane separation technology is more sustainable and generally less expensive. Therefore, membrane separation technology has become increasingly important in recent years. Commercial reverse osmosis (RO) membranes can remove more than 90% of micro-pollutants, but due to their low water permeability (1-2 L / (m³)...),... 2The high permeability (·h·bar) makes reverse osmosis a very energy-intensive process. Compared to reverse osmosis membranes, commercial nanofiltration (NF) membranes have slightly larger pore sizes and higher water permeability coefficients, but this also results in a lower rejection rate for micro-pollutants, especially small, uncharged molecules. Furthermore, reverse osmosis and dense nanofiltration membranes, originally designed for desalination, remove both divalent and monovalent ions along with micro-pollutants, leading to a high-salt concentrate that is difficult to process. Additionally, the permeate requires additional salt to make the permeate suitable for agricultural irrigation or human consumption. Therefore, commercial reverse osmosis membranes or dense nanofiltration membranes are not the optimal method for removing micro-pollutants.

[0005] In summary, it is currently believed that research is needed to develop nanofiltration membranes with suitable pore sizes for removing micro-pollutants, achieving both effective removal of micro-pollutants and allowing inorganic salt ions to permeate through the membrane, thus achieving good separation. Layer-by-layer self-assembly (LBLNF) technology offers a promising direction for precisely controlling the pore size of nanofiltration membranes. Unlike RO membranes, LBLNF membranes allow monovalent salt ions such as sodium chloride to permeate, maintaining high water flux while significantly reducing the required operating pressure and energy. By adjusting the pore size, surface charge, and assembly materials of LBLNF membranes, separation designs can be tailored for specific substances, even achieving purposes such as fouling resistance and antibacterial properties. LBLNF membranes are separation membranes formed by the self-assembly of polyanionic and cationic electrolytes in a specific inorganic electrolyte aqueous solution. From a microscopic perspective, the primary factor determining the pore size of an LBL membrane is the average distance between adjacent anion-cation pairs (or physical cross-linking) in the polyelectrolyte complex Pol+Pol-. Membrane pores are the interconnected spaces between the physical cross-linking points within the polyelectrolyte multilayer membrane. The LBL membrane coating process is generally considered to consist of two stages: the first stage is the pore-filling stage, where the polyelectrolyte contacts and fills the pores of the base membrane; the second stage is the layer growth stage, where the polyelectrolyte, after filling the pores of the base membrane, is coated onto the surface of the base membrane. Therefore, the pore structure and chemical structure of the supporting membrane surface significantly affect the permeation and separation performance of the LBL nanofiltration membrane. Currently, the base membranes used in publicly disclosed patents have pore sizes ranging from 10kDa to 100kDa, all of which are small-pore membranes. For example, patent CN 107519765 A uses a PES ultrafiltration membrane with a molecular weight cutoff of 10kDa to prepare a polydopamine amphoteric composite nanofiltration membrane. The resulting membrane has a pure water permeability coefficient of 15.665 LMH / bar, a molecular weight cutoff of 935 Da, and a maximum rejection rate of 92.50% for pharmaceuticals and personal care products (PPCP). Patent CN 102553461 A prepared an inorganic / organic composite nanofiltration membrane on a base membrane with a molecular weight cutoff of 10–100 kDa. The transition layer was coated with a polyelectrolyte, and a composite layer of inorganic soluble calcium salt and soluble carbonate was formed on the surface of the transition layer. A dense layer was formed by coating the transition layer with polyelectrolyte and then cross-linking it. This composite nanofiltration membrane had a MgSO4 rejection rate of 76.5%–95.5% and a low water flux of 1.7–5.3 LMH / bar. Patent CN 113413760A coated an allylated modified polyethersulfone and sulfonated polyethersulfone blended ultrafiltration membrane with a molecular weight cutoff of 65 kDa with a polyelectrolyte coating. The resulting nanofiltration membrane had a MgSO4 rejection rate of 92% and a water flux of 7.4 LMH / bar. Patent CN 111659268 A describes a nanofiltration membrane with a MgSO4 rejection rate of 91% obtained by coating a graphene-modified polyethersulfone hollow fiber ultrafiltration membrane with a molecular weight cutoff of 50 kDa with polypropylene-based ammonium chloride and sodium polystyrene sulfonate, followed by crosslinking treatment.Patent CN 102580550B describes a polysulfone ultrafiltration membrane with a molecular weight cutoff of 50 kDa coated with polycationic polydimethyldiallyl ammonium chloride and polyanionic sodium polystyrene sulfonate. The resulting nanofiltration membrane exhibits a Na₂SO₄ rejection rate of 91% and a permeability coefficient of 5.1 LMH / bar. Patent CN 106310975A describes a polyelectrolyte multilayer nanofiltration membrane prepared using a hollow fiber ultrafiltration membrane with a molecular weight cutoff of 50 kDa, achieving a MgSO₄ rejection rate of 76%–79%. However, neither of these LBL membranes can simultaneously achieve high permeability and separation performance.

[0006] In addition, numerous patents have applied LBL membranes to the removal of micropollutants. Patent CN 105169962 B, coated with polyethyleneimine and graphene oxide, prepared a nanofiltration membrane with a pure water flux of 6.97–22.96 LMH / bar and a rejection rate of 80.46%–99.99% for Bengal rose red dye. Patent CN 110869108B utilizes poly(diallyldimethylammonium chloride)-co-sulfone and poly(styrene sulfonate) to prepare a layer-by-layer self-assembled nanofiltration membrane, achieving a water flux of 9.4 LMH / bar, a MgCl2 rejection rate of 97.1%, a MgSO4 rejection rate of 97.7%, a Na2SO4 rejection rate of 90.6%, a rejection rate of 98.2% for atenolol, a rejection rate of 59.9% for naproxen, a rejection rate of 33.1% for sulfamethoxazole, and a rejection rate of 30.3% for bisphenol A. Patent CN 107519765 A describes a composite nanofiltration membrane obtained by coating a polycationic 2-hydroxypropyltrimethylammonium chloride chitosan and a polyanionic dopamine onto a polyethersulfone ultrafiltration membrane and crosslinking it with epichlorohydrin. The membrane exhibits a pure water permeability coefficient of 1.6 LMH / bar and a carbamazepine rejection rate of 92.50%. However, neither patent can simultaneously achieve both high membrane permeability and effective removal of micro-pollutants. Summary of the Invention

[0007] The purpose of this invention is to provide a composite nanofiltration membrane, its preparation method, and its application. The composite nanofiltration membrane material prepared using a green and controllable layer-by-layer self-assembly technology has high removal rate and high permeability when removing micro-pollutants, solving the problems of existing nanofiltration membranes in which it is difficult to achieve both removal rate and permeability in the process of removing micro-pollutants, as well as the non-green preparation process.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0009] This invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps:

[0010] Using the impregnation coating method,

[0011] (1) Alternately coat the base film with high molecular weight polyelectrolyte solutions carrying opposite charges, and repeat the alternating coating until the required number of layers is obtained to obtain a macroporous filled polyelectrolyte multilayer film.

[0012] (2) Alternately coat the macroporous polyelectrolyte solutions with opposite charges onto the macroporous polyelectrolyte multilayer membrane, repeating the alternating coating until the required number of polyelectrolyte multilayer membranes are formed, to obtain a composite nanofiltration membrane;

[0013] The high molecular weight polyelectrolyte solution contains a high molecular weight polyelectrolyte with a molecular weight of 200–3000 kDa.

[0014] The molecular weight of the low molecular weight polyelectrolyte in the solution is 1–200 kDa.

[0015] Preferably, the porosity of the base membrane is 50-80%, and the pore size is 10-200 nm; the material of the base membrane includes polyethersulfone, polyacrylonitrile, polysulfone, polyetheretherketone, polyimide, or polyvinyl chloride, and the type of the base membrane is a hollow fiber membrane.

[0016] Preferably, the high molecular weight polyelectrolyte and the low molecular weight polyelectrolyte independently include polycationic electrolytes or polyanionic electrolytes.

[0017] Preferably, the polycationic electrolyte comprises one or more of polyacrylamide hydrochloride, polydimethyldiallylammonium chloride, polyethyleneimine, and polyethyleneamine;

[0018] The polyanionic electrolyte includes one or more of sodium polystyrene sulfonate, sodium polyacrylate, sodium polyvinyl sulfonate, and sulfonated polyether ether ketone.

[0019] Preferably, the concentration of the polyelectrolyte in the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution is 0.01 to 10 g / L.

[0020] Preferably, the background salt solutions used for the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution independently include NaF, NaCl, NaBr or NaNO3, and the salt concentration in the background salt solution is 0.0001 to 2.5 mol / L.

[0021] Preferably, based on obtaining one double layer by alternating coating, the number of alternating coating layers in step (1) is 2 to 10 double layers; the number of alternating coating layers in step (2) is 1 to 10 double layers.

[0022] Preferably, the order of alternating coatings in steps (1) and (2) is independent, namely, coating polycationic electrolyte and polyanionic electrolyte in sequence.

[0023] The present invention provides a composite nanofiltration membrane prepared by the preparation method described in the above technical solution.

[0024] This invention provides the application of the composite nanofiltration membrane described in the above technical solution in drinking water or surface water treatment.

[0025] This invention provides a method for preparing a composite nanofiltration membrane. The method employs a layer-by-layer self-assembly impregnation coating technique. First, a high molecular weight polyelectrolyte system is alternately coated onto the surface of a high-porosity base membrane to cover and fill the large pores. Then, a low molecular weight polyelectrolyte is alternately coated to fill the small pores of the base membrane. This improves the retention rate of magnesium sulfate in the layer-by-layer self-assembled membrane, reduces the pore size while maintaining high permeability, and achieves both high micropollutant removal rate and high permeability, resulting in a composite nanofiltration membrane with high permeability and high micropollutant removal rate. The prepared composite nanofiltration membrane achieves a removal rate of over 90% for micropollutants with a molecular weight of around 200 Da, with a removal rate as high as 99.8%.

[0026] This invention utilizes a layer-by-layer self-assembly technology to prepare multilayer composite nanofiltration membranes. This process is simple, efficient, highly controllable, environmentally friendly (using non-toxic and harmless water as a solvent), low-cost, and automated. Furthermore, by adjusting the parameters, methods, and assembly materials of the membrane coating process, the pore size and surface characteristics of the membrane can be controlled more precisely, thereby improving the overall performance and service life of the membrane.

[0027] The composite nanofiltration membrane material obtained by this invention exhibits separation performance suitable for the removal of micro-pollutants of varying molecular weights. Nanofiltration membranes prepared using polyelectrolyte coatings of different molecular weights significantly improve the removal efficiency of micro-pollutants. Specifically, low-molecular-weight polyelectrolyte coatings can form a dense membrane structure, enhancing the retention capacity for small-molecule micro-pollutants; while high-molecular-weight polyelectrolyte coatings provide higher permeability, making them suitable for removing larger-molecular-weight micro-pollutants.

[0028] The composite nanofiltration membrane material prepared by this invention is suitable for removing micro-pollutants of different properties. The combined use of polyelectrolytes with different molecular weights can optimize the physicochemical properties of the membrane, such as hydrophilicity and surface charge, thereby achieving highly efficient removal of various micro-pollutants. This method not only improves the selectivity and stability of the nanofiltration membrane but also exhibits excellent durability and mechanical properties under complex water quality conditions, making it suitable for a variety of water treatment applications.

[0029] The nanofiltration membrane prepared by this invention has excellent permeation separation performance, with a permeability coefficient between 10 and 30 LMH / bar. The removal rate of micro-pollutants with a molecular weight of less than 200 Da in drinking water or surface water is consistently between 90.0% and 99.8%. The removal rate and permeability of this membrane for micro-pollutants are far superior to those of existing commercial nanofiltration membranes. Detailed Implementation

[0030] Unless otherwise specified, all raw materials or reagents used in this invention are commercially available products well known in the art.

[0031] This invention provides a method for preparing a composite nanofiltration membrane, comprising the following steps:

[0032] Using the impregnation coating method,

[0033] (1) Alternately coat the base film with high molecular weight polyelectrolyte solutions carrying opposite charges, and repeat the alternating coating until the required number of layers is obtained to obtain a macroporous filled polyelectrolyte multilayer film.

[0034] (2) Alternately coat the macroporous polyelectrolyte solutions with opposite charges onto the macroporous polyelectrolyte multilayer membrane, repeating the alternating coating until the required number of polyelectrolyte multilayer membranes are formed, to obtain a composite nanofiltration membrane;

[0035] The high molecular weight polyelectrolyte solution contains a high molecular weight polyelectrolyte with a molecular weight of 200–3000 kDa.

[0036] The molecular weight of the low molecular weight polyelectrolyte in the solution is 1–200 kDa.

[0037] The present invention does not impose any special limitations on the specific operation of the impregnation coating method; it can be self-assembled layer by layer according to coating methods well known in the art.

[0038] In this invention, polyelectrolyte solutions with opposite charges, namely positively charged polycationic electrolyte solutions and negatively charged polyanionic electrolyte solutions, are obtained by dissolving polyelectrolytes with opposite charges in a background salt solution.

[0039] The coating method described in this invention is a dip-coating method. First, the base film is immersed in a polycationic electrolyte solution for 5–30 minutes, then immersed in a background salt solution without polycationic electrolyte to wash away unbound polycations. Next, it is transferred to a polyanionic electrolyte solution and soaked for 5–30 minutes, followed by cleaning in a background salt solution without polyanionic electrolyte. This completes one polyelectrolyte bilayer. This invention first alternately coats several layers of high molecular weight polycationic electrolyte and polyanionic electrolyte to fill the macropores of the base film, repeating the high molecular weight coating step until the required number of layers is reached. Then, several layers of low molecular weight polycationic electrolyte and polyanionic electrolyte are alternately coated to cover the micropores, repeating the low molecular weight polyelectrolyte coating step until the required number of layers is reached, until a polyelectrolyte multilayer film of the desired thickness is formed.

[0040] In this invention, the porosity of the base membrane is preferably 50-80%, more preferably 55-60%, and the pore size is preferably 10-200 nm, more preferably 14-90 nm. The material of the base membrane includes polyethersulfone, polyacrylonitrile, polysulfone, polyetheretherketone, polyimide, or polyvinyl chloride, and the type of the base membrane is preferably a hollow fiber membrane. In this invention, the molecular weight cutoff of the base membrane is preferably 110-350 kDa, more preferably 120-280 kDa. The nanofiltration membrane formed by the macroporous base membrane of this invention has a smaller pore size, which enhances the size sieving effect of the membrane and can achieve a higher removal rate of micropollutants.

[0041] In this invention, the molecular weight of the high molecular weight polyelectrolyte in the high molecular weight polyelectrolyte solution is 200-3000 kDa, preferably 600-1000 kDa; the molecular weight of the low molecular weight polyelectrolyte in the low molecular weight polyelectrolyte solution is 1-200 kDa, preferably 1.5-100 kDa, and more preferably 15-70 kDa.

[0042] In this invention, the molecular weight of the polyelectrolyte significantly affects the assembly, growth, and performance of the multilayer membrane. First, higher molecular weight polyelectrolyte chains, due to their larger volume and lower mobility, form a looser multilayer structure, leading to increased membrane permeability but potentially reduced selectivity and retention. Conversely, lower molecular weight polyelectrolyte chains have higher mobility and can bind more tightly together, forming a denser multilayer structure. This density helps improve the membrane's selectivity and retention, but may reduce its permeability. During assembly, low molecular weight polyelectrolyte chains are more easily and uniformly distributed on the substrate surface, forming a smooth coating, while high molecular weight polyelectrolyte chains may lead to increased surface roughness. Furthermore, molecular weight also affects the mechanical properties and stability of the multilayer membrane. Low molecular weight polyelectrolyte multilayer membranes generally exhibit higher mechanical strength and chemical stability, while high molecular weight polyelectrolyte multilayer membranes may be more flexible. In summary, the molecular weight of the polyelectrolyte significantly influences the assembly, growth, and final performance of the multilayer membrane by affecting chain mobility, binding tightness, and surface properties. This invention selects a suitable high molecular weight polyelectrolyte as the pore-filling bottom layer and a suitable low molecular weight polyelectrolyte as the dense top layer based on the porosity and pore size of the base membrane.

[0043] In this invention, the high molecular weight polyelectrolytes and low molecular weight polyelectrolytes independently include polycationic electrolytes or polyanionic electrolytes.

[0044] In this invention, the polycationic electrolyte preferably includes one or more of polyacrylamide hydrochloride (PAH), polydimethyldiallylammonium chloride (PDADMAC), polyethyleneimine (PEI), and polyethyleneamine (PVAm); the polyanionic electrolyte preferably includes one or more of sodium polystyrene sulfonate (PSS), sodium polyacrylate (PAA), sodium polyvinyl sulfonate (PVS), and sulfonated polyether ether ketone.

[0045] In this invention, the concentration of polyelectrolyte in the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution is 0.01-10 g / L, more preferably 0.05-5 g / L, and even more preferably 0.1 g / L.

[0046] Polyelectrolyte concentration has a relatively small impact on nanofiltration membrane pore size, but it does affect its permeability. When polycations and polyanions come into contact and combine, they form aggregates within and on the surface of the porous base membrane. These aggregates are uniformly dispersed within and on the membrane. Excessive aggregates can hinder the entry of subsequent polyelectrolytes into the membrane and their adsorption onto the membrane surface. Higher polyelectrolyte concentrations typically result in thicker coatings because high-concentration polyelectrolyte solutions contain more polymer chains, leading to a thicker coating on the base membrane surface. A thicker coating increases resistance to solution flow through the membrane, reducing the nanofiltration membrane's permeability coefficient. Conversely, excessively low polyelectrolyte concentrations can prevent complete pore filling, resulting in membrane defects and inhomogeneities, hindering effective removal of micro-pollutants.

[0047] In this invention, the salts in the background salt solutions used for the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution independently include NaF, NaCl, NaBr or NaNO3. The salt concentration in the background salt solution is preferably 0.0001 to 2.5 mol / L, more preferably 0.01 to 1.0 mol / L, and even more preferably 0.05 mol / L.

[0048] The present invention does not impose any particular limitation on the preparation method of the high molecular weight polyelectrolyte solution, the low molecular weight polyelectrolyte solution, and the background salt solution; the corresponding solutions can be prepared according to methods well known in the art.

[0049] Polyelectrolytes are dispersed in a background salt solution. The type of background salt solution significantly affects the permeation selectivity of nanofiltration membranes, mainly in terms of coating thickness, uniformity, mechanical properties, permeability, and stability. Different types of background salts alter the adsorption behavior and molecular chain arrangement of the polyelectrolytes, thus affecting the final performance of the coating. The specific effects of different salt ion types are due to the different positions of the ions in the Hofmeister sequence. The Hofmeister sequence describes the influence of ions on the solubility and stability of proteins and other macromolecules. Ions in the sequence are arranged from strongest to weakest according to their influence on macromolecular solubility; for example, sulfate ions (SO42-) are arranged in order of increasing strength. 2- ) usually stabilize the structure of macromolecules, while thiocyanate ions (SCN) - This will disrupt the structure of macromolecules. Ions in the background salt can affect the conformation and adsorption behavior of polymer chains through electrostatic interactions and solvation effects. Ions in the background salt can shield the charges on the polyelectrolyte molecular chains, reducing electrostatic repulsion and making it easier for the molecular chains to approach and adsorb onto the substrate surface, thereby increasing the coating thickness. For example, when NaCl is used as the background salt, Cl... - This will moderately shield the charge on the polycation chains, reducing electrostatic repulsion and making it easier for the polycation chains to approach and adsorb onto the base film surface, potentially increasing the coating thickness. However, when using Na2SO4, because SO4... 2- A strong solvation effect significantly influences the polymer chain alignment, potentially reducing coating thickness. This change in coating thickness affects the permeability and selectivity of the nanofiltration membrane. The size and charge density of different salt ions also affect the conformation of the polyelectrolyte chains and the degree of expansion of the adsorbent layer, thus altering the coating thickness and uniformity. Furthermore, the type of background salt affects the mechanical properties and stability of the coating. For example, Na₂SO₄ can improve the chemical stability and durability of the coating, while other salts such as NaCl may make the coating more flexible and prone to damage.

[0050] Furthermore, the concentration of the polyelectrolyte background salt solution significantly affects the conformation and growth mechanism of polyelectrolyte multilayer films during assembly. The mechanism of polyelectrolyte multilayer growth includes the following steps: First, the substrate surface carries an initial charge, attracting polyelectrolyte molecules with opposite charges. As the first layer of polyelectrolyte is adsorbed, the surface charge reverses, attracting a second layer of polyelectrolyte molecules with opposite charges. This process is repeated continuously to form a multilayer structure. There is a competitive relationship between salt ions and polyelectrolytes: at low salt concentrations, adjacent groups with the same charge on the polyelectrolyte segments repel each other, resulting in a more extended conformation of the segments. At this time, oppositely charged groups on the anionic and cationic polyelectrolyte segments combine with each other, i.e., internal compensation; at high salt concentrations, the charge of the polyelectrolyte is mainly neutralized by oppositely charged salt ions, producing an electrostatic screening effect. At this time, the repulsive force between adjacent groups with the same charge on the segments is weakened, resulting in a more aggregated conformation of the segments; the combination of charged groups on the polyelectrolyte segments and salt ions is called external compensation. It is generally believed that when internal compensation dominates, the membrane thickness is smaller and the membrane is more dense; while when external compensation dominates, the membrane is relatively thicker and more porous. Background salt concentration affects the adsorption efficiency of each layer and the final membrane performance by regulating the electrostatic shielding effect and the flexibility of the polyelectrolyte chains. With increasing background salt concentration, the electrostatic shielding effect between polyelectrolyte chains strengthens, causing the molecular chains to more easily approach and adsorb onto the substrate surface, thus forming a thicker coating. During assembly, background salt concentration also affects the conformation and diffusion behavior of the polyelectrolyte chains. High-concentration salt solutions cause the polyelectrolyte chains to align more tightly, forming a dense multilayer structure. The principle of polyelectrolyte coating is based on electrostatic interactions. Positively and negatively charged polyelectrolyte molecules form a stable multilayer structure through electrostatic attraction during alternating deposition; the deposition of each layer affects the adsorption behavior of the next layer, thus progressively building the desired membrane thickness and performance.

[0051] In this invention, based on the principle that alternating coatings are applied once to obtain a double layer, the number of alternating coating layers in step (1) is preferably 2 to 10 double layers, more preferably 4 to 8 double layers; the number of alternating coating layers in step (2) is preferably 1 to 10 double layers, more preferably 4 to 8 double layers.

[0052] In this invention, the number of polyelectrolyte coating layers has a significant impact on the permeation selectivity of the nanofiltration membrane. The number of high molecular weight polyelectrolyte coating layers must be sufficient to cover most of the large pores of the base membrane to ensure high flux, while the number of low molecular weight polyelectrolyte layers must be sufficient to form a complete and dense skin layer to ensure high micro-pollutant removal rate of the nanofiltration membrane. In embodiments of this invention, at least 2 to 10 bilayer high molecular weight polyelectrolyte layers are coated, allowing the high molecular weight polyelectrolyte to fill the larger pores of the base membrane to ensure high flux, more preferably 3 to 6 bilayers; at least 1 to 10 bilayer low molecular weight polyelectrolyte layers are coated, allowing the low molecular weight polyelectrolyte to fill the smaller pores of the base membrane to ensure a dense top layer, thereby achieving both high flux and high retention, more preferably 3 to 6 bilayers.

[0053] In this invention, the order of alternating coatings in steps (1) and (2) is independent: coating polycationic electrolyte and polyanionic electrolyte in sequence.

[0054] The present invention provides a composite nanofiltration membrane prepared by the preparation method described in the above technical solution.

[0055] This invention provides the application of the composite nanofiltration membrane described above in drinking water or surface water treatment. This invention does not specifically limit the method of application; any method well-known in the art can be used.

[0056] The immersion coating method employed in this invention is simple and direct, requiring only the immersion of the base membrane in the coating solution and then its extraction at a controllable speed. It automates the coating process without the need for complex equipment or specialized skills. Due to its simple equipment and operation, automation capabilities, and wide coverage of the coating solution, the immersion coating method is low-cost and suitable for large-scale production. This process offers high repeatability, achieving consistent coating quality and thickness, making it ideal for mass production of nanofiltration membranes. By adjusting the membrane fiber pulling speed and the type and concentration of the background salt solution, the coating thickness can be precisely controlled to meet the application requirements of nanofiltration membranes under various conditions. Compared to other coating technologies, the immersion coating method does not involve irritating chemicals or high-energy-consuming operations, making it more environmentally friendly and energy-efficient.

[0057] This invention does not impose any particular limitations on the morphology and type of the composite nanofiltration membrane; hollow fiber membranes, flat sheet membranes, spiral wound membranes, and tubular membranes well-known in the art are all applicable. The method of this invention is particularly suitable for the preparation of nanofiltration materials and can be achieved through effective control of the parameters of the polyelectrolyte coating.

[0058] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0059] In this embodiment of the invention, the pure water permeability coefficient (P) of the nanofiltration membrane is defined according to the definition of water flux in GB / T 34242-2017 "Test Methods for Nanofiltration Membranes". The pure water permeability coefficient of a nanofiltration membrane is defined as the amount of water permeating per unit membrane area per unit time per unit pressure under certain operating conditions.

[0060]

[0061] Where P represents the pure water permeability coefficient (L / m 2 hbar), Δm is the sample mass (kg), ρ is the density of water (kg / L), and S is the effective membrane area (m²). 2 ), where Δt is the sample collection time (h) and Δp is the transmembrane pressure (bar).

[0062] The permeability coefficient test conditions for the composite nanofiltration membrane prepared by this invention are as follows: deionized water is used as the feed liquid, the test pressure is 0.1-0.5 MPa, and the test system temperature is 25℃.

[0063] The micro-pollutant removal rate (R) in this invention M (Refer to the definition in J.Membr.Sci.2018,549,120-128.) Micropollutant removal rate is the selective or separating ability of a membrane material for specific micropollutants, driven by a pressure gradient; it is the ratio of the concentration of micropollutants on the permeate side to the concentration on the feed side. Micropollutant removal rate reflects the selectivity and separation efficiency of nanofiltration membranes for micropollutants and is an important indicator for evaluating nanofiltration membrane performance.

[0064] The detection of micropollutant content in water samples was conducted according to the methods in the "Technical Guidelines for Accuracy Assessment of New Pollutant Screening - Liquid Chromatography-Mass Spectrometry (Trial)" (Anal. Chim. Acta 2017, 959, 91) and "Chemosphere 2021, 273, 128524". The testing procedure was as follows: a micropollutant feed solution with a concentration of 200 μg / L was prepared, and after stabilization at constant pressure in a cross-flow NF device for 24 hours, samples were taken. The test pressure range was 3–6 bar. The micropollutant content in the feed solution sample and the NF membrane permeate sample was detected using high-performance liquid chromatography-mass spectrometry.

[0065] Preparation of micro-contaminant solutions: The feed solution for each micro-contaminant was a mixed solution with a concentration of 200 μg / L. The stock solution (concentration 1 g / L) for each micro-contaminant was prepared by dissolving each micro-contaminant (see Table 1) in mass spectrometry-grade methanol and stored in brown reagent bottles. The stock solutions were stored at -8°C. 0.2 mL of each micro-contaminant stock solution was transferred to the feed tank, diluted with ultrapure water to 3 L, and the pH was adjusted to 5.8 to obtain a feed mixture with a concentration of 200 μg / L for each micro-contaminant.

[0066] Table 1. Molecular weight of micropollutants tested in experiments.

[0067]

[0068] Micropollutant filtration conditions: filtration temperature 25℃±3℃, test mode cross-flow mode, solution concentration constant concentration mode, flow rate 0.66m / s, filtration pressure 3~6bar, filtration time 24h.

[0069] Preparation of standard samples for micro-contaminant concentration testing: Take an appropriate amount of 1 g / L of the micro-contaminant stock solution and perform a series of gradient dilutions: 0, 10, 20, 50, 100, 200, and 400 μg / L. Each concentration solution should be 1 mL in volume to facilitate the establishment of liquid chromatography-mass spectrometry (LC-MS) calibration lines. This concentration range covers the feed solution and permeate concentrations used in filtration testing.

[0070] Micropollutant concentration testing conditions: Target analytes were analyzed using an ultra-high performance liquid chromatography-triple quadrupole mass spectrometer (UHLC-TQ-MS 30A / Sciex quadrupole 5500, Waters, USA). Target analytes and their internal standards (ILISs) were separated and quantified using an Agilent Poroshell 120EC-C18 (3.0 × 100 mm, 2.7 μm) column. All target analytes and their internal standards were separated using a gradient program with the optimized mobile phase composition as follows: 0.1% formic acid in Milli-Q water (mobile phase A) and 0.1% formic acid in a 50:50 (v / v) mixture of methanol and acetonitrile (mobile phase B). The gradient program started at 10% mobile phase B, held isocratically for 1.5 min, then increased to 90% mobile phase B at 15 min and held for 22 min. At the end of the chromatographic run, the column was reequilibrated to initial conditions within 0.5 min and stabilized for 3 min. The total mobile phase flow rate was 0.45 mL / min. Column temperature and autosampler temperature were maintained at room temperature (24 ± 2 °C and 6 °C, respectively). All analyses used an injection volume of 10 μL. The first 1.35 mL of eluent, which may contain salts / weakly bound impurities, was diverted to waste using a post-column switch and switched to tandem mass spectrometry after 3.0 min. Tandem mass spectrometry was performed on a triple quadrupole with iFunnel technology, which incorporates high-efficiency electron spray ionization (ESI). Tandem mass spectrometry was performed simultaneously in positive ESI(+) and negative ESI(-) modes. Nitrogen was used as both the drying and collision gas. The optimal conditions for the mass spectrometry system were: drying gas temperature 200 °C; drying gas flow rate 14 L / min; nebulizer pressure 20 psi; sheath gas heater temperature 250 °C; sheath gas flow rate 11 L / min; nozzle voltage 1500 V; capillary voltage 3000 V; fragment voltage 380 V.

[0071] The removal rate of micropollutants by nanofiltration membranes is calculated using the following formula:

[0072]

[0073] Among them, R M C represents the membrane's removal rate of micropollutants. f C represents the concentration of micro-pollutants in the feed liquid. p This indicates the concentration of micro-pollutants on the permeate side.

[0074] Example 1

[0075] A polyethersulfone hollow fiber membrane (obtained according to the existing technology "Desalination.2024,574,117229") was selected as the base membrane (porosity 55%, pore size 14nm, molecular weight cutoff MWCO = 120kDa, inner diameter 0.72mm, outer diameter 1.25mm) for polyelectrolyte coating.

[0076] First, a 0.1 g / L PSS polyelectrolyte solution was prepared by dissolving high molecular weight polyanionic polystyrene sulfonate sodium (PSS, Mw = 1,000 kDa, purchased from Anhui Cool Biotechnology Co., Ltd.) in a 0.05 mol / L sodium chloride solution. A 0.1 g / L PSS polyelectrolyte solution was also prepared by dissolving low molecular weight polyanionic polystyrene sulfonate sodium (Mw = 70 kDa, purchased from Anhui Cool Biotechnology Co., Ltd.) in a 0.05 mol / L sodium chloride solution. Then, a 0.1 g / L PDADMAC polyelectrolyte solution was prepared by dissolving high molecular weight polycationic polydimethyldiallyl ammonium chloride (PDADMAC, Mw = 600 kDa, purchased from Shandong Yousuo Chemical Technology Co., Ltd.) in a 0.05 mol / L sodium chloride solution. Finally, a 0.1 g / L PVAm polyelectrolyte solution was prepared by dissolving low molecular weight polycationic polyethyleneamine (PVAm, Mw = 100 kDa, purchased from Wuhan Kemike Biomedical Technology Co., Ltd.).

[0077] Using a high molecular weight cationic poly(PDADMAC) (600 kDa) as the first layer, the polyethersulfone-based membrane was immersed in a PDADMAC (600 kDa) solution for 15 minutes for coating, followed by rinsing with a 0.05 mol / L sodium chloride solution. Next, the membrane fibers were immersed in a high molecular weight anionic poly(PSS) (1000 kDa) solution for 15 minutes for coating, followed by rinsing with a 0.05 mol / L sodium chloride solution, thus completing one polyelectrolyte bilayer. This high molecular weight coating process was repeated until four PDADMAC / PSS (600 kDa / 1000 kDa) bilayers were formed. The membrane fibers were then immersed in a low molecular weight cationic poly(PVAm) (100 kDa) solution for 15 minutes for coating, followed by rinsing with a 0.05 mol / L sodium chloride solution. This low molecular weight coating process was repeated until four PVAm / PSS (100 kDa / 70 kDa) bilayers were formed. The final composite nanofiltration membrane was coated with four bilayers of the high molecular weight system PDADMAC / PSS (600kDa / 1000kDa) and four bilayers of the low molecular weight system PVAm / PSS (100kDa / 70kDa). After coating, it was moisturized with glycerin and dried to obtain the composite nanofiltration membrane.

[0078] Following the testing method described above, the micro-pollutant removal rate test results are as follows: the pure water permeability coefficient is 12.3 L / m². -2 h - 1 bar -1 The removal rates were 98.6% for ibuprofen (Mw = 206 Da), 99.5% for carbamazepine (Mw = 236 Da), and 98.0% for atenolol (Mw = 266 Da).

[0079] Example 2

[0080] The only difference from Example 1 is that the low molecular weight polycation is replaced with polyacrylamide hydrochloride (PAH, Mw = 15 kDa, purchased from Hubei Yamed Biomedical Co., Ltd.), which is dissolved in 0.05 mol / L sodium chloride solution to prepare a 0.1 g / L polyelectrolyte solution. The high molecular weight polyelectrolyte system is coated with four bilayers, and the low molecular weight polyelectrolyte system is coated with four bilayers; the remaining steps are the same as in Example 1.

[0081] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 10.9 L / m. -2 h -1 bar -1The removal rates were 95.4% for ibuprofen (Mw = 206 Da), 98.7% for carbamazepine (Mw = 236 Da), and 99.0% for atenolol (Mw = 266 Da).

[0082] Example 3

[0083] The only difference from Example 1 is that the background salt of the coating is replaced with NaNO3 instead of NaCl in Example 1, and the coating consists of 4 bilayers of the high molecular weight PDADMAC / PSS system and 4 bilayers of the low molecular weight PVAm / PSS system, for a total of 8 bilayers.

[0084] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 11.1 Lm. -2 h -1 bar -1 The removal rates were 98.9% for ibuprofen (Mw = 206 Da), 99.5% for carbamazepine (Mw = 236 Da), and 99.8% for atenolol (Mw = 266 Da).

[0085] Example 4

[0086] The only difference from Example 1 is that the background salt for the coating is replaced with NaF instead of NaCl in Example 1. The coating consists of 4 bilayers of the high molecular weight PDADMAC / PSS system and 4 bilayers of the low molecular weight PVAm / PSS system, for a total of 8 bilayers.

[0087] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 13.7 L / m. -2 h -1 bar -1 The removal rates were 93.8% for ibuprofen (Mw = 206 Da), 94.5% for carbamazepine (Mw = 236 Da), and 95.0% for atenolol (Mw = 266 Da). Examples 1, 3, and 4 demonstrate that the type of background salt in the coating can affect LBL membrane performance, with the chaotropic anion (NO3) salt being the most effective. - Membranes prepared using background salts are denser and have higher retention rates for micro-pollutants, but have lower flux.

[0088] Example 5

[0089] Polyethersulfone hollow fiber membrane (porosity 60%, pore size 90nm, molecular weight cutoff MWCO = 280kDa, inner diameter 0.83mm, outer diameter 1.29mm, obtained according to existing technology "Desalination.2023,557,116596") was selected as the base membrane for polyelectrolyte coating.

[0090] The following were used: 0.1 g / L high molecular weight polyanionic PSS (Mw = 1000 kDa, purchased from Anhui Cool Biotechnology Co., Ltd., 0.05 mol / L NaCl), 0.1 g / L low molecular weight polyanionic PAA (Mw = 1.5 kDa, 0.05 mol / L NaCl), 0.1 g / L high molecular weight polycationic PDADMAC (Mw = 600 kDa, purchased from Shandong Yousuo Chemical Technology Co., Ltd., 0.05 mol / L NaCl), and 0.1 g / L low molecular weight polycationic PAH (Mw = 15 kDa, purchased from Hubei Yamed Biomedical Co., Ltd., 0.05 mol / L NaCl).

[0091] Using a high molecular weight cationic poly(PDADMAC) (600 kDa) as the first layer, the polyethersulfone-based membrane was immersed in a PDADMAC (600 kDa) solution for 15 minutes and then cleaned with a 0.05 mol / L NaCl solution. Next, the membrane fibers were immersed in a high molecular weight anionic poly(PSS) (1000 kDa) solution for 15 minutes and then cleaned with a 0.05 mol / L NaCl solution, thus completing one polyelectrolyte bilayer. This high molecular weight system coating process was repeated until five PDADMAC / PSS (600 kDa / 1000 kDa) bilayers were formed. The membrane fibers were then immersed in a low molecular weight cationic poly(PAH) (15 kDa) solution for 15 minutes and then cleaned with a 0.05 mol / L sodium chloride solution. Finally, the membrane fibers were immersed in a low molecular weight anionic poly(PAA) (1.5 kDa) solution for 15 minutes and then cleaned with a 0.05 mol / L sodium chloride solution. Repeat the coating steps for the low molecular weight system until five PAH / PAA (150kDa / 1.5kDa) bilayers are formed. The final composite nanofiltration membrane consists of five bilayers of the high molecular weight PDADMAC / PSS (600kDa / 1000kDa) system and five bilayers of the low molecular weight PAH / PAA (15kDa / 1.5kDa) system.

[0092] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 15.2 L / m. -2 h -1 bar -1 The removal rates were 97.0% for ibuprofen (Mw = 206 Da), 98.2% for carbamazepine (Mw = 236 Da), and 99.1% for atenolol (Mw = 266 Da).

[0093] Comparative Example 1

[0094] The only difference from Example 1 is that the number of coating layers of high molecular weight polycationic PDADMAC and high molecular weight polyanionic PSS is increased from 4 bilayers to 8 bilayers, and the number of coating layers of low molecular weight PVAm and low molecular weight PSS is reduced from 4 bilayers to 0 layers. The prepared composite nanofiltration membrane contains only the high molecular weight PDADMAC / PSS system.

[0095] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 19.5 L / m². -2 h -1 bar -1 The removal rates were 82.7% for ibuprofen (Mw = 206 Da), 89.3% for carbamazepine (Mw = 236 Da), and 78.2% for atenolol (Mw = 266 Da).

[0096] Comparative Example 2

[0097] The only difference from Example 1 is that the number of coating layers of high molecular weight cationic PDADMAC and high molecular weight anionic PSS is reduced from 4 bilayers to 0 layers, while the number of coating layers of low molecular weight PVAm and low molecular weight PSS is increased from 4 bilayers to 8 bilayers. This film contains only the low molecular weight PVAm / PSS system.

[0098] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 4.7 Lm. -2 h -1 bar -1 The removal rates were 98.5% for ibuprofen (Mw = 206 Da), 98.5% for carbamazepine (Mw = 236 Da), and 99.3% for atenolol (Mw = 266 Da).

[0099] The results of Examples 1, 1, and 2 show that: high molecular weight polyelectrolyte non-composite coated membranes can achieve high flux but have low micro-pollutant removal rates; low molecular weight polyelectrolyte non-composite coated membranes can achieve high micro-pollutant removal rates but have low flux. Composite membranes, by filling large pores with high molecular weight polyelectrolyte and covering small pores with low molecular weight polyelectrolyte, can achieve high micro-pollutant removal rates while maintaining high flux.

[0100] Comparative Example 3

[0101] The same base film as in Example 5 was used for polyelectrolyte coating.

[0102] A 0.1 g / L PAA polyelectrolyte solution was prepared by dissolving low molecular weight polyanionic sodium polyacrylate (PAA, Mw = 1.5 kDa, purchased from Shanghai Maclean Biochemical Technology Co., Ltd.) in a 0.05 mol / L sodium chloride solution. Then, a 0.1 g / L PAH polyelectrolyte solution was prepared by dissolving low molecular weight polycationic polyacrylamide hydrochloride (PAH, Mw = 15 kDa, purchased from Hubei Yamed Biomedical Co., Ltd.) in a 0.05 mol / L sodium chloride solution.

[0103] Using a low molecular weight polycationic PAH (15kDa) as the first layer, the above-mentioned polyethersulfone-based membrane was immersed in a PAH (15kDa) solution for 15 minutes and then cleaned with a 0.05mol / L sodium chloride solution. Next, the membrane fibers were immersed in a low molecular weight polyanionic PAA (1.5kDa) solution for 15 minutes and then cleaned with a 0.05mol / L sodium chloride solution. This completes a low molecular weight PAH / PAA polyelectrolyte bilayer. The above coating steps were repeated until 10 PAH / PAA (15kDa / 1.5kDa) bilayers were formed.

[0104] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 86.5 L / m. -2 h -1 bar -1 The removal rates were as follows: ibuprofen (Mw = 206 Da) 12.6%, carbamazepine (Mw = 236 Da) 17.5%, and atenolol (Mw = 266 Da) 26.8%. This indicates that polyelectrolyte coatings with excessively small molecular weights cannot form a complete skin layer when used with a base membrane with large pores, resulting in lower removal rates for micro-pollutants.

[0105] Comparative Example 4

[0106] The only difference from Comparative Example 3 is that the number of PAH / PAA coating layers was increased from 10 double layers to 20 double layers.

[0107] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 5.6 Lm. -2 h -1 bar -1 The removal rates were 97.5% for ibuprofen (Mw = 206 Da), 97.9% for carbamazepine (Mw = 236 Da), and 99.8% for atenolol (Mw = 266 Da). This example illustrates that when polyelectrolytes with small molecular weights are coated with base membranes with large pores, the number of coating layers needs to be increased to form a complete skin layer, but the membrane permeability coefficient is low.

[0108] Comparative Example 5

[0109] The same base film as in Example 5 was used for polyelectrolyte coating.

[0110] A 0.1 g / L PSS polyelectrolyte solution was prepared by dissolving high molecular weight polyanionic PSS (Mw = 1000 kDa, purchased from Anhui Cool Biotechnology Co., Ltd.) in 0.05 mol / L sodium chloride solution. Then, a 0.1 g / L PDADMAC polyelectrolyte solution was prepared by dissolving high molecular weight polycationic PDADMAC (Mw = 600 kDa, purchased from Shandong Yousuo Chemical Technology Co., Ltd.) in 0.05 mol / L sodium chloride solution.

[0111] Using a high molecular weight polycationic PDADMAC (600kDa) as the first layer, the above-mentioned polyethersulfone-based membrane was immersed in a PDADMAC (600kDa) solution for 15 minutes for coating, and then cleaned with a 0.05mol / L sodium chloride solution. Next, the membrane fibers were immersed in a high molecular weight polyanionic PSS (1000kDa) solution for 15 minutes for coating, and then cleaned with a 0.05mol / L sodium chloride solution. This completes a high molecular weight system PDADMAC / PSS polyelectrolyte bilayer. The above coating steps were repeated until 10 PDADMAC / PSS (600kDa / 1000kDa) bilayers were formed.

[0112] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 28.2 L / m². -2 h -1 bar -1 The removal rates were 56.2% for ibuprofen (Mw = 206 Da), 72.0% for carbamazepine (Mw = 236 Da), and 78.1% for atenolol (Mw = 266 Da). This indicates that high-molecular-weight polyelectrolyte coatings with larger pores can produce high-flux nanofiltration membranes, but with lower removal rates for micro-pollutants.

[0113] Comparative Example 6

[0114] The only difference from Comparative Example 5 is that the number of PDADMAC / PSS coating layers was increased from 10 double layers to 20 double layers.

[0115] The results of the micro-pollutant removal rate test are as follows: the pure water permeability coefficient is 19.2 L / m. -2 h -1 bar -1 The removal rates were 62.7% for ibuprofen (Mw = 206 Da), 75.8% for carbamazepine (Mw = 236 Da), and 81.4% for atenolol (Mw = 266 Da). This indicates that even with increased coating layers, high molecular weight polyelectrolyte coatings with larger pores cannot achieve high micro-pollutant removal rates.

[0116] The results of Examples 5 and Comparative Examples 3, 4, 5, and 6 demonstrate that when coating a macroporous membrane, a smaller molecular weight polyelectrolyte requires more layers to obtain a defect-free nanofiltration membrane, but the flux is low. When coating a larger molecular weight polyelectrolyte, a smaller number of layers can obtain a defect-free nanofiltration membrane, but the removal rate of micropollutants is low. However, a defect-free nanofiltration membrane obtained by filling macropores with a larger molecular weight polyelectrolyte system and small pores with a smaller molecular weight polyelectrolyte system can achieve a high removal rate of micropollutants while ensuring a high water flux.

[0117] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a composite nanofiltration membrane, characterized in that, Includes the following steps: Using the impregnation coating method, (1) Alternately coat the base film with high molecular weight polyelectrolyte solutions carrying opposite charges, and repeat the alternating coating until the required number of layers is obtained to obtain a macroporous filled polyelectrolyte multilayer film. (2) Alternately coat the macroporous polyelectrolyte solutions with opposite charges onto the macroporous polyelectrolyte multilayer membrane, repeating the alternating coating until the required number of polyelectrolyte multilayer membranes are formed, to obtain a composite nanofiltration membrane; The high molecular weight polyelectrolyte solution contains a high molecular weight polyelectrolyte with a molecular weight of 600~1000kDa. The molecular weight of the low molecular weight polyelectrolyte in the solution is 1.5~100kDa; The porosity of the base film is 50-80%, and the pore size is 10-200 nm; The concentration of polyelectrolyte in the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution is independently 0.01~10 g / L; The background salt solutions used for the high molecular weight polyelectrolyte solution and the low molecular weight polyelectrolyte solution independently include NaF, NaCl, NaBr or NaNO3, and the salt concentration in the background salt solution is 0.0001~2.5 mol / L; Based on the principle that alternating coatings are applied once to obtain a double layer, the number of alternating coatings in step (1) is 2 to 10 double layers; the number of alternating coatings in step (2) is 1 to 10 double layers. The high molecular weight polyelectrolytes and low molecular weight polyelectrolytes independently include polycationic electrolytes and polyanionic electrolytes; The polycationic electrolyte includes one or more of polyacrylamide hydrochloride, polydimethyldiallylammonium chloride, polyethyleneimine, and polyethyleneamine. The polyanionic electrolyte includes one or more of sodium polystyrene sulfonate, sodium polyacrylate, sodium polyvinyl sulfonate, and sulfonated polyether ether ketone. The order of alternating coatings in steps (1) and (2) is independent: coating polycationic electrolyte and polyanionic electrolyte in sequence.

2. The preparation method according to claim 1, characterized in that, The base membrane is made of polyethersulfone, polyacrylonitrile, polysulfone, polyetheretherketone, polyimide, or polyvinyl chloride, and the base membrane is a hollow fiber membrane.

3. The composite nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 2.

4. The application of the composite nanofiltration membrane according to claim 3 in drinking water or surface water treatment.

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