Layer-by-layer self-assembled nanofiltration membrane, and preparation method and use thereof

Abstract

The application provides a layer-by-layer self-assembly nanofiltration membrane, a preparation method and application thereof. The layer-by-layer self-assembly nanofiltration membrane comprises a base film and a self-assembly coating layer formed on the base film, wherein the self-assembly coating layer comprises at least a starting alternating coating layer and a separation alternating coating layer; the molecular weight cut-off of the base film is not less than 500 kDa; the starting alternating coating layer is alternately assembled by a high-molecular-weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer; the separation alternating coating layer is alternately assembled by a low-molecular-weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer; the starting alternating coating layer is formed on the base film, and the high-molecular-weight cationic polyelectrolyte layer in the starting alternating coating layer adopts a cationic polyelectrolyte with a number-average molecular weight of not less than 500 kDa; and the low-molecular-weight cationic polyelectrolyte layer in the separation alternating coating layer adopts a cationic polyelectrolyte with a number-average molecular weight of not more than 100 kDa. The layer-by-layer self-assembly nanofiltration membrane has more excellent permeability and micro-pollutant rejection rate.

Description

TECHNICAL FIELD

[0001] The present application relates to a nanofiltration membrane, in particular to a layer-by-layer self-assembly nanofiltration membrane and a preparation method and use thereof. BACKGROUND

[0002] Nanofiltration membrane (NF membrane for short) is a new membrane separation technology between reverse osmosis and ultrafiltration. Under the action of pressure difference driving force, nanofiltration membrane allows salt and small molecule substances to pass through the nanofiltration membrane, and retains macromolecular substances. It is a separation membrane that can efficiently remove substances with a number average molecular weight of 200-1000 Da or more, and is mainly used for concentrating and purifying macromolecular substances in a solution. Layer-by-layer self-assembly technology (Layer-by-layer, LBL) is a technology for preparing multilayer nanometer thin films: two or more assembly materials are coated on a substrate in turn, and the coating process can be repeated multiple times as needed. The driving force for assembling the polyelectrolyte multilayer film is the increase in entropy of oppositely charged polyelectrolytes during pairing. The water-soluble linear polyelectrolytes used in LBL can form connected, pore size adjustable, high permeability nanometer pore channels, and the operating pressure is low and the water recovery rate is high, which is one of the most effective technologies for removing micro-pollutants and ion separation.

[0003] The core construction of preparing a layer-by-layer self-assembly NF membrane requires two materials, a base film and a coated polyelectrolyte. For the base film, high surface porosity generally means that the membrane pores are large. For a polyelectrolyte composite nanofiltration membrane, experimental verification shows that small molecular weight polyelectrolytes are more likely to fill the pores of the base film, thereby improving the filling to form thinner and denser separation layers and improving the rejection rate of the NF membrane to the target removal molecules. In an aqueous solution, small molecular weight polyelectrolytes in a spherical shape can improve the rejection rate of the NF membrane to the target removal molecules. Patent CN 102580550 B discloses a method for preparing a nanofiltration membrane by alternately depositing oppositely charged polyelectrolytes on the surface of an ultrafiltration membrane. The base film used has a MWCO of 50 kDa, and the prepared NF membrane has a permeability of 5.1-7.8 L / m 2 ·h·bar, and the rejection rate to Na2SO4 is 91%. Patent CN 102688701 A discloses a method for layer-by-layer self-assembly coating of a polyanion polyelectrolyte containing transition metal ions and a polycation on a base film with a MWCO of 50 kDa. The prepared NF membrane has a permeability of 3.0-7.4 L / m 2 ·h·bar, and the rejection rate to Na2SO4 is 90%. Although some previous research methods can prepare NF membranes with high rejection performance to divalent salt ions, such as PSS / PAH coating on a base film with a MWCO of 65 Da, the prepared NF membrane has a permeability of 5.1-7.8 L / m 2+ ·h·bar, and the rejection rate to Mg 2+ , Ca 2+ , and SrNF membranes with a rejection rate between 97.7% and 99.5% (Desalination 534 (2022) 115793); coating PSS / PAH on a base membrane with a MWCO of 52 kDa, resulting in a permeability of 2.5 L / m 2+ NF membranes with a rejection rate of up to 99.9% (Journal of Membrane Science 663 (2022) 121027), but the permeability of these NF membranes is difficult to improve to 10 L / m 2 ·h·bar above, coating PAH / PSS on a base membrane with a MWCO of 113 kDa improves the permeability to 12 L / m 2 ·h·bar, but the rejection rate of Mg 2+ slightly decreased to 94.1% (Desalination 574 (2024) 117229). These experimental results prove that a base membrane with high porosity and large pore size can improve the permeability of LBL NF membranes, but at the expense of the compactness of the skin layer, resulting in a possible decrease in the rejection rate of NF membranes. Literature reports that when using base membranes with MWCO of 500 kDa and 500 kDa, even after coating 10 bilayers, no rejection performance of Na2SO4 was observed (Langmuir 21 (2005) 10587-10592), indicating that traditional layer-by-layer self-assembly method cannot be used to coat and prepare NF membranes with high rejection performance on base membranes with excessively large pore sizes. SUMMARY

[0004] In view of the above-mentioned disadvantages of the prior art, the purpose of the present application is to provide a layer-by-layer self-assembly nanofiltration membrane and a preparation method and use thereof, to solve the problem that the nanofiltration membrane in the prior art cannot meet the performance requirements.

[0005] To achieve the above-mentioned objects and other related objects, the present application is implemented by including the following technical solutions.

[0006] The present application provides a layer-by-layer self-assembly nanofiltration membrane, which comprises a base membrane and a self-assembly coating layer formed on the base membrane, wherein the self-assembly coating layer comprises at least a starting alternating coating layer and a separation alternating coating layer; the molecular weight cut-off of the base membrane is not less than 500 kDa; the starting alternating coating layer is alternately assembled by a high molecular weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer, and the separation alternating coating layer is alternately assembled by a low molecular weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer; the starting alternating coating layer is formed on the base membrane, and the high molecular weight cationic polyelectrolyte layer in the starting alternating coating layer uses a cationic polyelectrolyte with a number average molecular weight of not less than 500 kDa; and the low molecular weight cationic polyelectrolyte layer in the separation alternating coating layer uses a cationic polyelectrolyte with a number average molecular weight of not more than 100 kDa.

[0007] In the technical solution of the present application, the molecular weight of the polyelectrolyte has a direct impact on the structure of the nanofiltration membrane, such as the pore size and morphology of the separation functional layer, thereby ultimately affecting the performance of the nanofiltration membrane. Specifically, on a high-porosity, large-pore base membrane, a suitable large-molecular-weight polyelectrolyte layer is first formed, and then a suitable small-molecular-weight polyelectrolyte layer is formed. This technical concept not only ensures the successful preparation of the nanofiltration membrane, but also enables it to have high permeability and high retention performance.

[0008] Preferably, the anionic polyelectrolyte layer is a finishing coating. This is because in subsequent application environments, such as salt lake or aqueous solution environments, cationic polyelectrolytes are more prone to swelling and are not resistant to oxidation, while anionic polyelectrolytes have better performance in these aspects.

[0009] Preferably, the starting alternating coating has 1-10 alternating periods. For example, it can be 1 alternating period, 2 alternating periods, 3 alternating periods, 4 alternating periods, 5 alternating periods, 6 alternating periods, 7 alternating periods, 8 alternating periods, 9 alternating periods, or 10 alternating periods.

[0010] Preferably, the separation alternating coating has 1-10 alternating periods. For example, it can be 1 alternating period, 2 alternating periods, 3 alternating periods, 4 alternating periods, 5 alternating periods, 6 alternating periods, 7 alternating periods, 8 alternating periods, 9 alternating periods, or 10 alternating periods. Preferably, in the starting alternating coating, the number-average molecular weight of the large-molecular-weight cationic polyelectrolyte is 500-2000 kDa. For example, it can be 500 kDa, 600 kDa, 700 kDa, 800 kDa, 900 kDa, 1000 kDa, 1100 kDa, 1200 kDa, 1300 kDa, 1400 kDa, 1500 kDa, 1600 kDa, 1700 kDa, 1800 kDa, 1900 kDa, or 2000 kDa.

[0011] Preferably, the number-average molecular weight of the cationic polyelectrolyte and the anionic polyelectrolyte in the alternating coating is 1-100 kDa. For example, it can be 5 kDa, 10 kDa, 15 kDa, 20 kDa, 25 kDa, 30 kDa, 35 kDa, 40 kDa, 45 kDa, 50 kDa, 55 kDa, 60 kDa, 65 kDa, 70 kDa, 75 kDa, 80 kDa, 85 kDa, 90 kDa, 95 kDa, or 100 kDa.

[0012] Preferably, the material of the base membrane comprises one or more of the following polymers: polyether sulfone, polysulfone, sulfonated polyether sulfone, sulfonated polysulfone, polyacrylonitrile, polyimide, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polybutylene, polyolefin elastomer, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, polyetherimide, polyether ether ketone, aminated polysulfone, aminated polyether sulfone, aminated polyacrylonitrile. In this application, the type and specification of the base membrane are mainly selected based on the molecular weight cut-off of the base membrane.

[0013] Generally, the initial alternating coating can be first coated to form a layer of a high-molecular-weight cationic polyelectrolyte and then coated to form a layer of an anionic polyelectrolyte; or first coated to form a layer of an anionic polyelectrolyte and then coated to form a layer of a high-molecular-weight cationic polyelectrolyte. In a specific implementation, considering the interaction between the anion and the cation, in order to better achieve the fitting self-assembly effect between different layers, the coating formed on the base membrane in the initial alternating coating is a coating that can be self-assembled with the base membrane. If the base membrane contains cationic groups, the initial alternating coating can be first coated to form a layer of an anionic polyelectrolyte; if the base membrane contains anionic groups, the initial alternating coating can be first coated to form a layer of a cationic polyelectrolyte.

[0014] The molecular weight cut-off (abbreviated as MWCO) in this application refers to the molecular weight of a substance with a known molecular weight when the base membrane has a 90% retention rate for the substance under specific conditions. The molecular weight cut-off of the base membrane in this application refers to the molecular weight of dextran when the base membrane has a 90% retention rate for dextran in a dextran aqueous solution. The concentration of the dextran aqueous solution is 100 mg / L. The test pressure is 0.5 bar, and the system control temperature is 25°C.

[0015] Preferably, the molecular weight cut-off of the base membrane is 500-2000 kDa. It can be 500 kDa, 600 kDa, 700 kDa, 800 kDa, 900 kDa, 1000 kDa, 1100 kDa, 1200 kDa, 1300 kDa, 1400 kDa, 1500 kDa, 1600 kDa, 1700 kDa, 1800 kDa, 1900 kDa, or 2000 kDa. The base membrane has a large molecular weight cut-off, i.e., a large pore size. In the prior art, small-pore-size base membranes are often used, and it is difficult to successfully prepare a high-performance nanofiltration membrane using a large-pore-size base membrane.

[0016] The base membrane for implementing the technical solution of this application is not limited by its specific processing and molding form. Preferably, the base membrane is selected from one or more of hollow fiber membranes, flat sheet membranes, and tubular membranes.

[0017] Preferably, the anionic polyelectrolyte has a number average molecular weight of 50-2000 kDa. In the present application, the anionic polyelectrolyte in the anionic polyelectrolyte layer of the initial alternating coating and the anionic polyelectrolyte in the anionic polyelectrolyte layer of the separation alternating coating can both adopt the number average molecular weight range. Generally, the number average molecular weight of the anionic polyelectrolyte in the present application can be 50 kDa, 100 kDa, 200 kDa, 300 kDa, 400 kDa, 500 kDa, 600 kDa, 700 kDa, 800 kDa, 900 kDa, 1000 kDa, 1100 kDa, 1200 kDa, 1300 kDa, 1400 kDa, 1500 kDa, 1600 kDa, 1700 kDa, 1800 kDa, 1900 kDa or 2000 kDa.

[0018] Preferably, the cationic polyelectrolyte in the cationic polyelectrolyte layer is a polymer selected from at least one of a primary amine, a secondary amine, a tertiary amine or a quaternary amine salt group.

[0019] Preferably, the anionic polyelectrolyte in the anionic polyelectrolyte layer is a polymer selected from at least one of a sulfonate, a carboxylate, a phosphate or a nitrate.

[0020] Preferably, the cationic polyelectrolyte is selected from one or more of polyallylamine hydrochloride (PAH for short), polydimethyl diallyl ammonium chloride (PDADMAC for short), polyvinyl benzyl trimethyl ammonium chloride (PVBTMAC for short), polyethyleneimine (PEI for short), polydichloroethyl ether tetramethyl ethylenediamine, poly methacryloyloxyethyl trimethyl ammonium chloride (CPF for short), chitosan and polyacrylamide (PAM for short).

[0021] Preferably, the anionic polyelectrolyte is selected from one or more of sodium polystyrene sulfonate, sodium dextran sulfate, sodium carboxymethyl cellulose, sodium polyacrylate, sodium vinyl sulfonate, sulfonated polyether ether ketone and hyaluronic acid.

[0022] Preferably, the layer-by-layer self-assembly nanofiltration membrane comprises one or more of the following characteristics:

[0023] The pure water permeability coefficient is not less than 15 L / m 2 hbar;

[0024] The rejection rate of MgSO4 is not less than 95%.

[0025] The rejection rate The second aspect of the present application also discloses a preparation method of the layer-by-layer self-assembly nanofiltration membrane according to any one of the above, which comprises the following steps: coating an initial alternating coating on a base film, and then coating a separation alternating coating.

[0026] In the present application, the concentration of the polyelectrolyte solution has little effect on the structure and performance of the nanofiltration membrane, such as little effect on the pore size. Once the cationic polyelectrolyte and the anionic polyelectrolyte are combined, the film formed will hinder the subsequent combination of polyelectrolytes, and the steric hindrance generated can limit the addition of more polyelectrolytes to the polyelectrolyte multilayer film.

[0027] Preferably, the concentration of the macromolecular weight cationic polyelectrolyte solution is 0.1-20 g / L when forming the initial alternating coating. It can be, for example, 1 g / L, 1.5 g / L, 2 g / L, 2.5 g / L, 3 g / L, 3.5 g / L, 4 g / L, 4.5 g / L, 5 g / L, 5.5 g / L, 6 g / L, 6.5 g / L, 7 g / L, 7.5 g / L, 8 g / L, 8.5 g / L, 9 g / L, 9.5 g / L, 10 g / L, 10.5 g / L, 11 g / L, 11.5 g / L, 12 g / L, 12.5 g / L, 13 g / L, 13.5 g / L, 14 g / L, 14.5 g / L, 15 g / L, 15.5 g / L, 16 g / L, 16.5 g / L, 17 g / L, 17.5 g / L, 18 g / L, 18.5 g / L, 19 g / L, 19.5 g / L, or 20 g / L.

[0028] Preferably, the concentration of the small molecular weight cationic polyelectrolyte solution is 1-20 g / L in the solution for forming the separate alternating coating. It can be, for example, 1 g / L, 2 g / L, 3 g / L, 4 g / L, 4.5 g / L, 5 g / L, 5.5 g / L, 6 g / L, 6.5 g / L, 7 g / L, 7.5 g / L, 8 g / L, 8.5 g / L, 9 g / L, 9.5 g / L, 10 g / L, 10.5 g / L, 11 g / L, 11.5 g / L, 12 g / L, 12.5 g / L, 13 g / L, 13.5 g / L, 14 g / L, 14.5 g / L, 15 g / L, 16 g / L, 17 g / L, 18 g / L, 19 g / L, or 20 g / L.

[0029] Preferably, the concentration of the anionic polyelectrolyte in the solution for forming the separate alternating coating or the initial alternating coating is 2-10 g / L. It can be, for example, 2 g / L, 2.5 g / L, 3 g / L, 3.5 g / L, 4 g / L, 4.5 g / L, 5 g / L, 5.5 g / L, 6 g / L, 6.5 g / L, 7 g / L, 7.5 g / L, 8 g / L, 8.5 g / L, 9 g / L, 9.5 g / L, or 10 g / L.

[0030] Preferably, the solvent in the solution is a salt solution, and the concentration of the salt solution is 0.001-3.0 mol / L. For example, the concentration can be 0.05 mol / L, 0.1 mol / L, 0.15 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.35 mol / L, 0.4 mol / L, 0.45 mol / L, 0.5 mol / L, 0.55 mol / L, 0.6 mol / L, 0.65 mol / L, 0.7 mol / L, 0.75 mol / L, 0.8 mol / L, 0.9 mol / L, 0.95 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L or 3 mol / L. Preferably, the concentration of the salt solution is 0.05-1.0 mol / L. The salt can be one or more of NaCl, NaNO3, NaBr, NaAc, NaF, NaBr, KCl, KBr, Na2SO4 and MgCl2.

[0031] The third aspect of the application also discloses a use of the layer-by-layer self-assembly nanofiltration membrane as described in any of the above for removing small organic molecule pollutants in water.

[0032] The fourth aspect of the application also discloses a use of the layer-by-layer self-assembly nanofiltration membrane as described in any of the above for extracting lithium from salt lakes.

[0033] As described above, the layer-by-layer self-assembly nanofiltration membrane, the preparation method and the use thereof have the following beneficial effects:

[0034] 1. The applicant found in the practice that for a base membrane with a MWCO of 500 kDa or more, a small molecular weight polyelectrolyte with a molecular weight of less than 5 kDa cannot be used alone to prepare a NF membrane with a rejection performance, but a large molecular weight polyelectrolyte with a molecular weight of not less than 500 kDa can be used to prepare a NF membrane with excellent rejection performance, that is, a large molecular weight (number average molecular weight not less than 500 kDa) cationic polyelectrolyte is first coated on a base membrane with a large pore size to fill the pores, and then a small molecular weight polyelectrolyte (number average molecular weight not more than 100 kDa) is used to fill and reduce the pore size of the coating, thereby the technical solution claimed in the application is proposed.

[0035] 2. The layer-by-layer self-assembly nanofiltration membrane provided by the application has a permeability much higher than that of the polyelectrolyte layer-by-layer self-assembly NF membrane reported in the current patents and documents, and the layer-by-layer self-assembly polyelectrolyte NF membrane has both high porosity and small pore size.

[0036] 3. Compared with existing commercial nanofiltration membranes and the current mainstream layer-by-layer self-assembled nanofiltration membrane preparation technology, it has better permeability and micro-pollutant rejection rate. Moreover, the process operation method is simple, and the membrane structure and properties are easy to control. Compared with conventional processes for preparing nanofiltration membranes and reverse osmosis membranes, such as interfacial polymerization, this technology is more flexible, non-toxic, harmless, green and environmentally friendly, and has a lower cost. Attached Figure Description

[0037] Figure 1 The nanofiltration membranes prepared in Examples 1-4 have pure water permeability coefficients and 500 ppm MgSO4 rejection rates.

[0038] Figure 2 The pure water permeability coefficient and the rejection rate of 500 ppm MgSO4 of the nanofiltration membranes prepared in Comparative Examples 1-4.

[0039] Figure 3 Magnesium-lithium separation coefficients of nanofiltration membranes prepared in Examples 1-4 and Comparative Examples 1-4.

[0040] Figure 4 The retention rates of micropollutants by the nanofiltration membranes prepared in Examples 1-4 and Comparative Examples 1-4. Detailed Implementation

[0041] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0042] The term "coating" as used herein should be interpreted broadly, encompassing any means that enables at least two substances to be coated, bonded, or joined together by means of a physical or chemical reaction. This could include, for example, applying one layer onto another under suitable conditions, or immersing one layer in another. Coating in this application can be by brushing, rolling, spraying, dipping, spin coating, screen printing, pressure filtration, or curtain coating, etc.

[0043] In this document, the term "base membrane" refers to a self-supporting or externally supported membrane material, which can be a hollow fiber membrane, a flat sheet, or a tubular membrane. Anyone with basic knowledge of membrane preparation can prepare membrane materials of any shape and type according to the methods provided in this invention.

[0044] In this application, the term "cationic polyelectrolyte layer" refers to a coating formed containing a cationic polyelectrolyte material, wherein the cationic polyelectrolyte is a high molecular weight polymer that can ionize into cations in an aqueous solution, and the polymer is a water-soluble polymer with positive ionic groups on the main chain or side chain.

[0045] In the present application, the term "anionic polyelectrolyte layer" refers to a coating layer formed by an anionic polyelectrolyte material, which is a water-soluble polymer that can dissociate into a large number of small ions (counterions) and a macromolecular ion when dissolved in water. The anionic polyelectrolyte has a proton-accepting group, and common anionic groups include -COO", -SO3", -O-CS2", -O-PO3 ^2 , etc.

[0046] These polyelectrolytes have properties similar to electrolytes (salts) and polymers (high-molecular-weight compounds), and are sometimes also referred to as polyelectrolytes. Like salts, their solutions are electrically conductive, and like polymers, their solutions are usually viscous.

[0047] In the present application, the rejection rate (R) is defined as follows: the difference between the concentration of the raw material liquid (C f ) and the concentration of the permeate liquid (C p ) under certain conditions, divided by the concentration of the raw material liquid (C f ), R = (C f -C p ) / C f . The test conditions for the rejection rate of the layer-by-layer self-assembled nanofiltration membrane of the present application are as follows: A) for the rejection rate of magnesium sulfate: a 500 mg / L (corresponding to 500 ppm) magnesium sulfate salt solution concentration, a test pressure of 3 bar,

[0048] , and a system control temperature of 25°C;

[0049] B) for organic small molecule substances: a water solution containing six micro-pollutants of testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole, and norfloxacin, and the content of each specific organic small molecule substance is 200 ppb;

[0050] , a test pressure of 3 bar, and a system control temperature of 25°C.

[0051] In the present application, the magnesium-lithium separation coefficient (S) is defined as follows: the ratio of the concentrations of Mg 2+ and Li + in the permeate (p) under certain conditions, divided by the ratio of the concentrations of Mg 2+ and Li + in the feed liquid side (f). S Mg,Li = (C Li+ / C Mg 2+ ) p / (C Li+ / C Mg 2+ ) f = (1-R Li+ ) / (1-R Mg 2+ ) The test condition for magnesium-lithium separation factor of the layer-by-layer self-assembly nanofiltration membrane of the present application is that the concentration of the mixed salt solution is 2000-6000 mg / L (corresponding to 2000-6000 ppm), the mass ratio of magnesium to lithium is 20-60, the test pressure is 2.0-4.0 bar, and the system control temperature is 25°C. In a more specific test example, the test pressure is 4 bar, the mass ratio of magnesium ion to lithium ion is 20, the mixed solution of magnesium chloride and lithium chloride is selected, and the concentration of the mixed salt solution is 2000 ppm.

[0052] In the present application, the permeation coefficient J is defined as the mass flowing out per unit area per unit time under certain conditions. J = Δm / (ρAΔt P), wherein Δm (kg) is the mass increment of the filtrate within the separation time Δt (h), ρ is the density of the filtrate (1 kg / L -1 ), A (m 2 ) is the effective filtration area, and P (bar) is the transmembrane pressure. The system control temperature is room temperature such as 25°C (the same below).

[0053] In the more specific examples and comparative examples shown below in the present application, the specific coating method is as follows: the hollow fiber membrane is formed into a membrane module, and then the relevant solution for forming the initial alternating coating is introduced into the through holes of the hollow fiber membrane, followed by the introduction of the relevant solution for forming the separation alternating coating.

[0054] In a more specific example, the steps include the following: after the hollow fiber membrane base film is pre-cleaned by backwashing with pure water, the macromolecular cationic polyelectrolyte solution is first introduced into the hollow lumen of the hollow fiber membrane, and then left to stand; the inside of the base film is washed with pure water, and the inside of the membrane filament is blown with 0.5 bar nitrogen; the anionic polyelectrolyte solution is introduced into the hollow lumen in the same way, and then washed with pure water and blown with nitrogen, thus ending the initial alternating coating; then the separation alternating coating is formed in the same way. More specifically, the separation alternating coating has two periods, and thus a total of 3 alternating periods of coating are completed.

[0055] Example 1

[0056] In the present specific example, the specific layer-by-layer self-assembly nanofiltration membrane includes the following:

[0057] Base film: polyether sulfone hollow fiber membrane with a molecular weight cut-off (MWCO) of 500 kDa;

[0058] Initial alternating coating: cationic polyelectrolyte polydimethyl diallyl ammonium chloride (PDADMAC) with a number average molecular weight of 600 kDa and anionic polyelectrolyte sodium polystyrene sulfonate (PSS) with a number average molecular weight of 1000 kDa;

[0059] Separation alternating coating: an anionic polyelectrolyte sodium polystyrene sulfonate (PSS) with a number average molecular weight of 1000 kDa and a cationic polyelectrolyte polydimethyl diallyl ammonium chloride (PDADMAC) with a number average molecular weight of 5 kDa.

[0060] The polyelectrolytes in the above were respectively dissolved in a 0.5 mol / L sodium chloride background solution to prepare a coating solution with a polyelectrolyte concentration of 0.02 mol / L.

[0061] After the base membrane was backwashed with pure water for pre-cleaning, a PDADMAC solution with a number average molecular weight of 600 kDa was first input into the hollow lumen of the hollow fiber membrane, and then left to stand. The inside of the base membrane was washed with pure water. In the same way, a PSS solution was input into the hollow lumen, and then washed with pure water and blown with nitrogen. At this point, the initial alternating coating was completed. Then, in the same way, a separation alternating coating was formed. In the separation alternating coating, the cationic polyelectrolyte solution was replaced by a PDADMAC solution with a number average molecular weight of 5 kDa, and the PSS solution remained unchanged. The separation alternating coating had two periods, and thus a total of 3 alternating periods of coating were completed.

[0062] A nanofiltration membrane was formed by the above. The following is a test of the nanofiltration membrane.

[0063] A 500 ppm MgSO4 solution was prepared, and whether the separation layer of the NF membrane was complete was detected at 3 bar.

[0064] Six micro-pollutant solutions containing testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin at a content of 200 ppb were prepared, and the MP retention rate of the NF membrane in Example 1 was tested at 3 bar.

[0065] The test results were: a pure water permeability coefficient of 16.8 L / m 2 ·h·bar, a testosterone retention rate of 100%, a tetracycline retention rate of 100%, an oxytetracycline retention rate of 100%, a sulfadiazine retention rate of 99.3%, a sulfamethoxazole retention rate of 99.7%, and a norfloxacin retention rate of 99.7%.

[0066] A simulated salt lake brine solution was prepared: a 2000 ppm MgCl2 / LiCl mixed solution was used as a raw liquid, wherein the mass ratio of Mg 2+ / Li + was 20. The nanofiltration membrane in Example 1 was tested, and the test pressure was 4 bar. The test results were: a permeability coefficient of 17.1 L / m 2 ·h·bar, a Mg 2+ retention rate of 99.1%, a Li + retention rate of 8.6%, and a magnesium-lithium separation coefficient S of 102.

[0067] Example 2

[0068] In this embodiment, the specific layer-by-layer self-assembly nanofiltration membrane is as follows:

[0069] Base membrane: polyethersulfone hollow fiber membrane with MWCO of 500 kDa is selected;

[0070] Initial alternating coating: the cationic polyelectrolyte is PDADMAC with molecular weight of 600 kDa, and the anionic polyelectrolyte is PSS with molecular weight of 1000 kDa;

[0071] Separation alternating coating: the cationic polyelectrolyte is PDADMAC with molecular weight of 100 kDa, and the anionic polyelectrolyte is PSS with molecular weight of 100 kDa.

[0072] The coating method and the test method are the same as those in Example 1.

[0073] The results show that the nanofiltration membrane prepared in Example 2 has a pure water permeability coefficient of 16.2 L / m 2 ·h·bar, a MgSO4 rejection rate of 97.6%, and a testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin rejection rate of 100%. The rejection rate of Mg 2+ is 99.2%, the rejection rate of Li + is 8.8%, and the magnesium-lithium separation coefficient S is 114.

[0074] Example 3

[0075] In this embodiment, the specific layer-by-layer self-assembly nanofiltration membrane is as follows:

[0076] Base membrane: polyethersulfone hollow fiber membrane with MWCO of 500 kDa is selected;

[0077] Initial alternating coating: the cationic polyelectrolyte is PDADMAC with molecular weight of 600 kDa, and the anionic polyelectrolyte is PSS with molecular weight of 1000 kDa;

[0078] Separation alternating coating: the cationic polyelectrolyte is PAH with molecular weight of 20 kDa, and the anionic polyelectrolyte is PSS with molecular weight of 1000 kDa.

[0079] The coating method and the test method are the same as those in Example 1.

[0080] The results show that the nanofiltration membrane prepared in Example 3 has a pure water permeability coefficient of 15.6 L / m 2 ·h·bar, a MgSO4 rejection rate of 99.1%, and a testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin rejection rate of 100%. The rejection rate of Mg 2+The retention rate was 99.3%, Li + The retention rate was 8.2%, and the magnesium-lithium separation coefficient S was 131.

[0081] Example 4

[0082] In this specific embodiment, in the specific layer-by-layer self-assembled nanofiltration membrane:

[0083] Base membrane: A polyethersulfone hollow fiber membrane with a MWCO of 500 kDa was selected;

[0084] Initial alternating coating: The cationic polyelectrolyte is PDADMAC with a molecular weight of 600kDa, and the anionic polyelectrolyte is PSS with a molecular weight of 1000kDa.

[0085] Alternating coatings for separation: the cationic polyelectrolyte is PAH with a molecular weight of 20kDa, and the anionic polyelectrolyte is PSS with a molecular weight of 100kDa.

[0086] The coating method and testing method are the same as in Example 1.

[0087] The results showed that the nanofiltration membrane prepared in Example 4 had a pure water permeability coefficient of 15.2 L / m. 2 At a concentration of ·h·bar, the retention rate of MgSO4 reached 99.4%, and the retention rates of testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole, and norfloxacin all reached 100%. 2+ The retention rate was 99.4%, Li + The retention rate was 12.4%, and the magnesium-lithium separation coefficient S was 146.

[0088] Example 5

[0089] The only difference between this embodiment and Example 1 is the molecular weight cutoff of the base membrane. In this embodiment, the base membrane is a polyethersulfone hollow fiber membrane with a MWCO of 1200 kDa. Everything else is the same as in Example 1.

[0090] Test results show that the permeability of pure water is 18.4 L / m³. 2 At ·h·bar, the retention rate of MgSO4 reached 95.4%;

[0091] The removal rates for tetracycline, oxytetracycline, and sulfadiazine reached 100%, the removal rates for testosterone were 99.1%, sulfamethoxazole was 98.6%, and norfloxacin was 99.2%.

[0092] Mg 2+ The retention rate was 98.9%, Li + The retention rate was 4.3%, and the magnesium-lithium separation coefficient S was 87.

[0093] Example 6

[0094] The difference between this embodiment and embodiment 1 is only that the molecular weight cut-off of the base membrane is different, in this embodiment, the base membrane is a polyether sulfone hollow fiber membrane with MWCO of 1800 kDa, and the others are the same as those in embodiment 1.

[0095] The test results show that:

[0096] The pure water permeability coefficient is 19.2 L / m 2 ·h·bar, the rejection rate of MgSO4 is 95.1%;

[0097] The removal rates of tetracycline and oxytetracycline are 100%, the removal rate of testosterone is 98.8%, the removal rate of sulfamethoxazole is 98.1%, the removal rate of sulfadiazine is 99.3%, and the removal rate of norfloxacin is 98.7%;

[0098] Mg 2+ The rejection rate of Li + is 2.1%, and the magnesium-lithium separation coefficient S is 75.

[0099] Embodiment 7

[0100] The difference between this embodiment and embodiment 1 is only that the material of the base membrane is different, in this embodiment, the base membrane is a polysulfone hollow fiber membrane with MWCO of 600 Da, and the others are the same as those in embodiment 1.

[0101] The test results show that:

[0102] The pure water permeability coefficient is 15.7 L / m 2 ·h·bar, the rejection rate of MgSO4 is 97.1%;

[0103] The removal rates of tetracycline, oxytetracycline and sulfadiazine are 100%, the removal rate of testosterone is 99.4%, the removal rate of sulfamethoxazole is 98.7%, and the removal rate of norfloxacin is 99.2%;

[0104] Mg 2+ The rejection rate of Li + is 7.8%, and the magnesium-lithium separation coefficient S is 102.

[0105] Embodiment 8

[0106] The difference between this embodiment and embodiment 1 is only that the material of the base membrane is different, in this embodiment, the base membrane is a polyacrylonitrile hollow fiber membrane with MWCO of 800 Da, and the others are the same as those in embodiment 1.

[0107] The test results show that:

[0108] The pure water permeability coefficient is 16.2 L / m2 • h·bar, the rejection rate of MgSO4 reached 97.1%,

[0109] The removal rates of tetracycline and terramycin were 100%, the removal rate of testosterone was 99.0%, the removal rate of sulfamethoxazole was 98.9%, the removal rate of sulfadiazine was 99.2%, and the removal rate of norfloxacin was 98.7%;

[0110] Mg 2+ The rejection rate of Li was 99.2%, and the magnesium-lithium separation coefficient S was 117. + The rejection rate of Li was 6.4%, and the magnesium-lithium separation coefficient S was 117.

[0111] Example 9

[0112] The difference between this example and Example 2 is only that the molecular weight of the cationic polyelectrolyte in the starting alternating coating is changed, 1000kDa PDADMAC is used, and the others are the same as Example 2.

[0113] The test results show that:

[0114] The pure water permeability coefficient is 16.4L / m 2 • h·bar, the rejection rate of MgSO4 reached 97.0%,

[0115] The removal rates of testosterone, tetracycline, terramycin, sulfadiazine, sulfamethoxazole and norfloxacin were all 100%;

[0116] Mg 2+ The rejection rate of Li was 99.1%, and the magnesium-lithium separation coefficient S was 105. + The rejection rate of Li was 5.7%, and the magnesium-lithium separation coefficient S was 105.

[0117] Example 10

[0118] The difference between this example and Example 2 is only that the molecular weight of the cationic polyelectrolyte in the starting alternating coating is changed, 1500kDa PDADMAC is used, and the others are the same as Example 2.

[0119] The test results show that:

[0120] The pure water permeability coefficient is 16.0L / m 2 • h·bar, the rejection rate of MgSO4 reached 96.6%,

[0121] The removal rates of testosterone, tetracycline, terramycin, sulfadiazine, sulfamethoxazole and norfloxacin were all 100%;

[0122] Mg 2+ The rejection rate of Li was 99.0%, and the magnesium-lithium separation coefficient S was 94.8. + The rejection rate of Li was 5.2%, and the magnesium-lithium separation coefficient S was 94.8.

[0123] Example 11

[0124] The difference between this example and Example 2 is the change of cationic polyelectrolyte in the starting and separation alternating coating, specifically: 1) 800 kDa PAH is used in the starting alternating coating; 2) 100 kDa PAH is used in the separation alternating coating; the others are the same as Example 2.

[0125] The test results show that:

[0126] The permeability coefficient of pure water is 15.1 L / m 2 ·h·bar, the rejection rate of MgSO4 is 99.2%,

[0127] The removal rates of testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin are all 100%.

[0128] The rejection rate of Mg 2+ is 99.4%, the rejection rate of Li + is 10.2%, and the magnesium-lithium separation coefficient S is 150.

[0129] Example 12

[0130] The difference between this example and Example 2 is the change of cationic polyelectrolyte in the starting and separation alternating coating, specifically: 1) 800 kDa PAH is used in the starting alternating coating; 2) 100 kDa PAH is used in the separation alternating coating; the others are the same as Example 2.

[0131] The test results show that:

[0132] The permeability coefficient of pure water is 20.6 L / m 2 ·h·bar, the rejection rate of MgSO4 is 95.1%,

[0133] The removal rates of testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin are all 100%.

[0134] The rejection rate of Mg 2+ is 99.1%, the rejection rate of Li + is 6.7%, and the magnesium-lithium separation coefficient S is 104.

[0135] Comparative Example 1

[0136] This comparative example is a comparative example of Example 1.

[0137] The coating method and testing method were the same as in Example 1, except that the initial alternating coating of Example 1 was not used, and instead a separate alternating coating was used in place of the initial alternating coating in the example to form three cycles of alternating coating. Specifically, a 5 kDa PDADMAC and 1000 kDa PSS were used on a polyethersulfone hollow fiber porous support membrane with a MWCO of 500 kDa.

[0138] The results show that the prepared nanofiltration membrane has a pure water permeability coefficient of 51.8 L / m 2 The rejection of MgSO4 was 9.8%, and the rejections of testosterone, tetracycline, oxytetracycline, sulfadiazine, sulfamethoxazole and norfloxacin were all less than 20%. The rejection of Mg 2+ was 10.3%, and the rejection of Li + was 2.8%. The magnesium-lithium separation coefficient S was 1.08.

[0139] Comparative Example 2

[0140] This comparative example is a comparative example of Example 1.

[0141] The coating method and testing method were the same as in Example 1, except that the separate alternating coating was not used in this comparative example, and instead the initial alternating coating of Example 1 was used. Specifically, a 600 kDa PDADMAC and 1000 kDa PSS were used on a polyethersulfone hollow fiber porous support membrane with a MWCO of 500 kDa for three cycles of layer coating.

[0142] The results show that the prepared nanofiltration membrane has a pure water permeability coefficient of 16.6 L / m 2 The rejection of MgSO4 was 90.6%, and the rejections of tetracycline and oxytetracycline were 100%, the rejection of testosterone was 91.4%, the rejection of norfloxacin was 83.7%, the rejection of sulfadiazine was 72.6%, and the rejection of sulfamethoxazole was 68.9%. The rejection of Mg 2+ was 97.9%, and the rejection of Li + was 7.4%. The magnesium-lithium separation coefficient S was 44.10.

[0143] Comparative Example 3

[0144] The coating method and testing method were the same as in Example 1, and a 5 kDa PDADMAC and 1000 kDa PSS were used on a polyethersulfone hollow fiber porous support membrane with a MWCO of 100 kDa for three cycles of layer coating.

[0145] The results show that the prepared nanofiltration membrane has a pure water permeability coefficient of 8.7 L / m 2·h·bar, the rejection rate of MgSO4 reached 93.0%, the rejection rate of testosterone, tetracycline, oxytetracycline and norfloxacin reached 100%, the rejection rate of sulfadiazine reached 99.2%, and the rejection rate of sulfamethoxazole reached 99.5%.Mg 2+ The rejection rate of Li reached 12.4%. The magnesium-lithium separation coefficient S was 109.5. + The rejection rate of Li reached 12.4%. The magnesium-lithium separation coefficient S was 109.5.

[0146] Comparative Example 4

[0147] The coating method and test method are the same as Example 1, and three cycles of layer coating are carried out on the polyether sulfone hollow fiber porous support membrane with a MWCO of 100 kDa using 600 kDa PDADMAC and 1000 kDa PSS.

[0148] The results show that the water permeability coefficient of the prepared nanofiltration membrane is 9.1 L / m 2 ·h·bar, the rejection rate of MgSO4 reached 92.3%, the rejection rate of tetracycline, oxytetracycline and norfloxacin reached 100%, the rejection rate of testosterone reached 98.2%, the rejection rate of sulfadiazine reached 97.4%, and the rejection rate of sulfamethoxazole reached 95.6%.Mg 2+ The rejection rate of Li reached 12.4%. The magnesium-lithium separation coefficient S was 109.5. + The rejection rate of Li reached 8.2%. The magnesium-lithium separation coefficient S was 48.32.

[0149] By Examples 1-4, the water permeability and micro-pollutant rejection rate data of the obtained nanofiltration membranes are compared. The water permeability coefficient obtained by the technical scheme of the present application is as high as 15 L / m 2 ·h·bar, the rejection rate of micro-pollutants with a molecular weight of 200 Da or more reached more than 99%, and the magnesium-lithium separation coefficient was not less than 100. Further proving that by alternately coating macromolecular polyelectrolyte solutions on the surface of a large-pore, high-porosity base membrane, and then coating a small-molecular polyelectrolyte, the water permeability coefficient of the nanofiltration membrane can be significantly improved, while ensuring a high micro-pollutant rejection rate, providing a good choice for the efficient removal of micro-pollutants in surface water by nanofiltration membranes.

[0150] Therefore, the present application effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0151] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes completed by those skilled in the art without departing from the spirit and technical thought disclosed by the present application should be covered by the claims of the present application.

Claims

1. A layer-by-layer self-assembled nanofiltration membrane, characterized in that, The layer-by-layer self-assembly nanofiltration membrane comprises a base membrane and a self-assembly coating layer formed on the base membrane, the self-assembly coating layer at least comprises a starting alternating coating layer and a separate alternating coating layer; the base membrane has a molecular weight cut-off of not less than 500 kDa; the starting alternating coating layer is alternately assembled by a macromolecular weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer, and the separate alternating coating layer is alternately assembled by a small molecular weight cationic polyelectrolyte layer and an anionic polyelectrolyte layer; the starting alternating coating layer is formed on the base membrane, and the macromolecular weight cationic polyelectrolyte layer in the starting alternating coating layer is a cationic polyelectrolyte with a number average molecular weight of not less than 500 kDa; the small molecular weight cationic polyelectrolyte layer in the separate alternating coating layer uses a cationic polyelectrolyte with a number average molecular weight of not more than 100 kDa.

2. The layer-by-layer self-assembled nanofiltration membrane according to claim 1, wherein, The anionic polyelectrolyte layer is a terminal coating layer; And / or, the separate alternating coating layer has 1-10 alternating periods; And / or, in the starting alternating coating layer, the number average molecular weight of the macromolecular weight cationic polyelectrolyte is 500-2000 kDa; And / or, in the separate alternating coating layer, the number average molecular weight of the small molecular weight cationic polyelectrolyte is 1-100 kDa; And / or, the number average molecular weight of the anionic polyelectrolyte is 50-2000 kDa.

3. The layer-by-layer self-assembled nanofiltration membrane according to claim 1, wherein, The material of the base membrane comprises one or more of the following polymers: polyether sulfone, polysulfone, sulfonated polyether sulfone, sulfonated polysulfone, polyacrylonitrile, polyimide, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polybutylene, polyolefin elastomer, ethylene-vinyl acetate copolymer, ethylene-vinyl alcohol copolymer, polyetherimide, polyether ether ketone, aminated polysulfone, aminated polyether sulfone, and aminated polyacrylonitrile.

4. The layer-by-layer self-assembled nanofiltration membrane of claim 1, wherein, The base membrane has a molecular weight cut-off of 500-2000 kDa; And / or, the base membrane is selected from one of hollow fiber membrane, flat sheet membrane and tubular membrane; And / or, the cationic polyelectrolyte in the cationic polyelectrolyte layer is a polymer containing at least one of primary amine, secondary amine, tertiary amine or quaternary amine salt group; And / or, the anionic polyelectrolyte in the anionic polyelectrolyte layer is a polymer containing at least one of sulfonate, carboxylate or phosphate.

5. The layer-by-layer self-assembled nanofiltration membrane according to claim 4, wherein, The cationic polyelectrolyte is selected from one or more of the following: polyacrylamide hydrochloride, polydimethyl diallyl ammonium chloride, polyvinyl benzyl trimethyl ammonium chloride, polyethyleneimine, polydichloroethyl ether tetramethyl ethylenediamine, methacryloyloxyethyl trimethyl ammonium chloride, chitosan and polyacrylamide; And / or, the anionic polyelectrolyte is selected from one or more of the following: sodium polystyrene sulfonate, sodium dextran sulfate, sodium carboxymethyl cellulose, sodium polyacrylate, sodium polyvinyl sulfonate, sulfonated polyether ether ketone and hyaluronic acid.

6. The layer-by-layer self-assembled nanofiltration membrane according to claim 1, wherein, The layer-by-layer self-assembly nanofiltration membrane comprises one or more of the following characteristics: a) the pure water permeability coefficient is not less than 15 L / m 2 • h • bar; b) The rejection rate for MgSO4 is not less than 95%.

7. A method for preparing a layer-by-layer self-assembly nanofiltration membrane according to any one of claims 1 to 6, characterized in that, The starting alternating coating layer is formed on the base membrane, and then the separate alternating coating layer is formed.

8. The preparation method according to claim 7, characterized in that, When forming the macromolecular weight cationic polyelectrolyte layer in the starting alternating coating layer, the concentration of the macromolecular weight cationic polyelectrolyte solution is 0.1-20 g / L; and / or, the concentration of the small molecular weight cationic polyelectrolyte solution is 1-20 g / L when forming the small molecular weight cationic polyelectrolyte layer in the separation alternate coating; and / or, the concentration of the anionic polyelectrolyte solution is 2-10 g / L when forming the anionic polyelectrolyte layer in the starting alternate coating or the separation alternate coating; and / or, the solvent used for forming the starting alternate coating or the separation alternate coating is a salt solution, the concentration of the salt solution is 0.001 mol / L-3.0 mol / L, and the salt is selected from one or more of NaCl, NaNO3, NaBr, NaAc, NaF, NaBr, KCl, KBr, Na2SO4 and MgCl2.

9. Use of the layer-by-layer self-assembly nanofiltration membrane according to any one of claims 1-6 for removing small organic molecule pollutants in water.

10. Use of the layer-by-layer self-assembly nanofiltration membrane according to any one of claims 1-6 for extracting lithium from salt lakes.

Images