Positively charged acid and alkali resistant composite nanofiltration membrane, its preparation method and application

Abstract

This invention relates to the field of membranes, and discloses a positively charged acid- and alkali-resistant composite nanofiltration membrane, its preparation method, and its application. The positively charged acid- and alkali-resistant composite nanofiltration membrane sequentially comprises a bottom layer, a porous intermediate layer, and an acid- and alkali-resistant separation layer; the acid- and alkali-resistant separation layer is grafted with quaternary ammonium salt groups as shown in Formula I and / or quaternary phosphate salt groups as shown in Formula II via N atom grafting; Formula I; Formula II; wherein, R1 is a substituted or unsubstituted alkylene group of C1-C9; R2, R3, and R4 are each independently selected from alkyl or phenyl groups of C1-C3; R5 is an alkylene group, phenyl group of C1-C6, or aralkyl group of C7-C10; R6, R7, and R8 are each independently phenyl or aralkyl group of C7-C10; X is a halogen. This allows the nanofiltration membrane to maintain a high water flux while possessing a high magnesium-lithium separation coefficient.

Description

Technical Field

[0001] This invention relates to the field of membranes, specifically to a positively charged acid and alkali resistant composite nanofiltration membrane, its preparation method, and its application. Background Technology

[0002] Nanofiltration is a pressure-driven membrane separation process. Nanofiltration membranes fall between ultrafiltration and reverse osmosis membranes, with pore sizes of 1-2 nm and molecular weight cutoffs of 200-2000 Da. Due to its advantages such as high separation efficiency, low energy consumption, and small footprint, nanofiltration separation is widely used in many fields, including wastewater treatment, seawater desalination, chemical separation, and food and pharmaceutical processing, playing an important role in modern separation and purification.

[0003] The mechanism of nanofiltration separation is complex. Currently, it is generally believed that nanofiltration performance is the result of the combined effects of size sieving and the Donnan effect. Besides the membrane's density, the nanofiltration process exhibits significant charge selectivity. Polyamide membrane composite nanofiltration membranes are the most widely used commercial nanofiltration membranes. The acyl chloride groups on the membrane surface hydrolyze to form a large number of carboxyl groups, making the membrane surface negatively charged. Due to the Donnan effect, the membrane has a low removal rate of high-valence cations, limiting its application in some specialized fields. A representative example is the removal of Mg from lithium extracted from salt lake brine. 2+ and Li + Applications include the separation of pollutants, softening of drinking water, treatment of heavy metal wastewater, refining of crude salt, and concentration and separation of special substances in the food industry. Furthermore, positively charged nanofiltration membranes exhibit better antifouling properties during the retention of positively charged ions and dye molecules. Based on these findings, high-performance positively charged nanofiltration membranes show promising application prospects.

[0004] Besides the charged properties of the membrane, some applications also place higher demands on the membrane's acid resistance. In hydrometallurgical processes, acidic leachates containing metal cations such as lithium, nickel, cobalt, and manganese are obtained. Positively charged nanofiltration membranes can effectively separate monovalent and hypervalent cations, which can then be concentrated and enriched using reverse osmosis technology, thus achieving comprehensive resource utilization. In adsorption-membrane combined lithium extraction processes, the desorption of the adsorbent needs to be carried out under acidic conditions, making the original solution for nanofiltration acidic. However, under acidic conditions, the C=O bonds in the polyamide structure are easily affected by H+. + Nucleophilic electron attack causes hydrolysis of amide bonds, which damages the membrane separation layer structure and reduces retention performance.

[0005] Therefore, there is an urgent need to develop a positively charged nanofiltration membrane that is simple to prepare and has excellent acid resistance, in order to overcome the limitations of existing nanofiltration membranes in the above-mentioned application scenarios. Summary of the Invention

[0006] The purpose of this invention is to overcome the problems of poor acid and alkali resistance and low magnesium-lithium separation efficiency of existing nanofiltration membranes, and to provide a positively charged acid and alkali resistant composite nanofiltration membrane, its preparation method and application. The specific polymer structure of the separation layer in this positively charged acid and alkali resistant composite nanofiltration membrane has good chemical inertness and high hydrogen bond density, which makes the separation layer have high acid and alkali resistance. Grafting specific quaternary ammonium salt groups and / or quaternary phosphate salt groups can enhance the positive charge of the membrane surface, so that the nanofiltration membrane has a high magnesium-lithium separation coefficient while maintaining a high water flux.

[0007] To achieve the above objectives, the first aspect of the present invention provides a positively charged acid and alkali resistant composite nanofiltration membrane, wherein the positively charged acid and alkali resistant composite nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer and an acid and alkali resistant separation layer.

[0008] The acid and alkali resistant separation layer is constructed by grafting N atoms with quaternary ammonium salt groups as shown in Formula I and / or quaternary phosphate salt groups as shown in Formula II.

[0009]

[0010] Wherein, R1 is a substituted or unsubstituted alkylene group of C1-C9; R2, R3, and R4 are each independently selected from alkyl or phenyl groups of C1-C3; R5 is an alkylene group of C1-C6, a phenyl group, or an aralkyl group of C7-C10; R6, R7, and R8 are each independently a phenyl group or an aralkyl group of C7-C10.

[0011] X is a halogen.

[0012] A second aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the preparation method includes the following steps:

[0013] S1. Under stirring conditions, a first solution containing haloquaternary ammonium salt and / or haloquaternary phosphorus salt and catalyst is added to a second solution containing polyamine to carry out the reaction and obtain modified polyamine;

[0014] S2. Prepare a porous support layer on the bottom layer;

[0015] S3. After the surface of the porous support layer is first contacted with the aqueous phase containing the modified polyamine of step S1, it is then contacted with the organic phase containing multiple polar monomers in a second manner, and then heat-treated to obtain the positively charged acid and alkali resistant composite nanofiltration membrane.

[0016] The haloquaternary ammonium salt has the structure shown in Formula III; the haloquaternary phosphorus salt has the structure shown in Formula IV.

[0017]

[0018] Wherein, R1' is a substituted or unsubstituted alkylene group of C1-C9; R2', R3', and R4' are each independently selected from C1-C3 alkyl or phenyl groups; R5' is a C1-C6 alkylene group, phenyl group, or C7-C10 aralkyl group; R6', R 7’ R8' is independently a phenyl or a C7-C10 aryl group;

[0019] X and X1 are halogens, each independently.

[0020] A third aspect of the present invention provides a positively charged acid and alkali resistant composite nanofiltration membrane prepared by the above-described preparation method.

[0021] The fourth aspect of this invention provides an application of the above-mentioned positively charged acid and alkali resistant composite nanofiltration membrane in the field of water treatment separation.

[0022] Through the above technical solutions, the positively charged acid and alkali resistant composite nanofiltration membrane, its preparation method, and its application provided by the present invention achieve the following beneficial effects:

[0023] The specific polymer structure in the separation layer of the positively charged acid- and alkali-resistant composite nanofiltration membrane provided by this invention endows the separation layer with good chemical inertness and a high hydrogen bond density, thus giving the separation layer excellent acid and alkali resistance. The acid- and alkali-resistant separation layer, through the grafting of specific quaternary ammonium salt groups and / or quaternary phosphate salt groups onto N atoms, significantly increases the positive charge density of the nanofiltration membrane. The repulsion effect between positive charges enhances the nanofiltration membrane's resistance to Mg. 2+ This improves the retention effect, thereby enhancing the magnesium-lithium separation efficiency of the nanofiltration membrane while maintaining its high water flux.

[0024] The present invention provides a method for preparing a positively charged, acid- and alkali-resistant composite nanofiltration membrane. This method involves a chemical reaction between a quaternary ammonium halide and / or a quaternary phosphorus halide and a polyamine under the action of an alkaline catalyst. The polyamine is then modified, and the modified polyamine undergoes a polymerization reaction with a polar monomer, introducing groups with strong conjugation effects and good chemical inertness. All of these significantly improve the acid and alkali resistance of the nanofiltration membrane. Simultaneously, the high positive charge density on the membrane surface enhances the nanofiltration membrane's resistance to Mg2+. 2+ It improves the retention effect, enhances the magnesium-lithium separation efficiency of nanofiltration membranes, and maintains a high water flux of nanofiltration membranes. Attached Figure Description

[0025] Figure 1 The infrared spectrum is that of the positively charged acid and alkali resistant composite nanofiltration membrane prepared in Example 1. Detailed Implementation

[0026] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0027] The first aspect of the present invention provides a positively charged acid and alkali resistant composite nanofiltration membrane, wherein the positively charged acid and alkali resistant composite nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer and an acid and alkali resistant separation layer;

[0028] The acid and alkali resistant separation layer is constructed by grafting N atoms with quaternary ammonium salt groups as shown in Formula I and / or quaternary phosphate salt groups as shown in Formula II.

[0029]

[0030] Wherein, R1 is a substituted or unsubstituted alkylene group of C1-C9; R2, R3, and R4 are each independently selected from alkyl or phenyl groups of C1-C3; R5 is an alkylene group of C1-C6, a phenyl group, or an aralkyl group of C7-C10; R6, R7, and R8 are each independently a phenyl group or an aralkyl group of C7-C10.

[0031] X is a halogen.

[0032] In this invention, * refers to the quaternary ammonium salt group or quaternary phosphate salt group and the -NH- in the acid and alkali resistant separation layer. The location where the connection forms a bond.

[0033] In this invention, specific quaternary ammonium salt groups and / or quaternary phosphate salt groups are grafted into the acid and alkali resistant separation layer, so that the positively charged acid and alkali resistant composite nanofiltration membrane has a high magnesium-lithium separation coefficient while maintaining a high water flux, making it particularly suitable for the field of water treatment separation.

[0034] In this invention, R1 can be a branched alkylene or a straight-chain alkylene.

[0035] Furthermore, R1 is a substituted or unsubstituted alkylene group of C1-C6, and R2, R3, and R4 are each independently CH3; R5 is an alkylene group of C1-C3, a phenyl group, or an aralkyl group of C8-C10; R6, R7, and R8 are each independently phenyl.

[0036] Furthermore, R1 is an unsubstituted alkylene group of C1-C3, wherein the substituent is a hydroxyl or carboxyl group, preferably a hydroxyl group.

[0037] Furthermore, X is Cl, Br, or I.

[0038] Furthermore, the acid and alkali resistant separation layer is selected from at least one of polyurea separation layer, polytriazine amine separation layer, polysulfonamide separation layer, poly(triazine amine-urea) separation layer, poly(sulfonamide-urea) separation layer, poly(triazine amine-sulfonamide) separation layer and poly(triazine amine-sulfonamide-urea) separation layer, preferably at least one of polyurea separation layer, polytriazine amine separation layer and polysulfonamide separation layer.

[0039] In one specific embodiment of the present invention, in formula I, R1 is ethylene, Propylene, butylene, pentylene, or hexylene; R2, R3, and R4 are methyl, ethyl, or phenyl; X is Cl, Br, or I.

[0040] In one specific embodiment of the present invention, in Formula II, R5 is propylidene, ethylidene, methylene, or... R6, R7, and R8 are phenyl groups; X is Cl, Br, or I.

[0041] According to the present invention, the nitrogen atom content in the quaternary ammonium salt groups of the positively charged acid and alkali resistant composite nanofiltration membrane is 10-20 at.%.

[0042] In this invention, when the nitrogen atom content in the quaternary ammonium salt group of the positively charged acid and alkali resistant composite nanofiltration membrane meets the above-mentioned range, the positively charged acid and alkali resistant composite nanofiltration membrane has a high surface electrode potential and a small average pore size, which makes the positively charged acid and alkali resistant composite nanofiltration membrane have a high magnesium-lithium separation coefficient; at the same time, the acid and alkali resistant separation layer does not affect the water flux of the membrane too much by grafting the quaternary ammonium salt group shown by N atom, so that the membrane maintains a good water flux.

[0043] Furthermore, the nitrogen atom content in the quaternary ammonium salt groups of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 10-15 at.%.

[0044] According to the present invention, the phosphorus content in the positively charged acid and alkali resistant composite nanofiltration membrane is 0.4-3 at.%.

[0045] In this invention, when the phosphorus atom content in the positively charged acid-alkali resistant composite nanofiltration membrane meets the above-mentioned range, the positively charged acid-alkali resistant composite nanofiltration membrane has a high surface electrode potential and a small average pore size, which makes the positively charged acid-alkali resistant composite nanofiltration membrane have a high magnesium-lithium separation coefficient; at the same time, the acid-alkali resistant separation layer does not affect the water flux of the membrane too much through the quaternary phosphate salt group represented by N atomic formula II, so that the membrane maintains good separation efficiency.

[0046] Furthermore, the phosphorus content in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-2 at.%.

[0047] According to the present invention, the surface Zeta potential of the positively charged acid and alkali resistant composite nanofiltration membrane is 0-30mV.

[0048] In this invention, when the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above-mentioned range, it indicates that the composite nanofiltration membrane has a high surface electrode potential. When used for lithium-magnesium separation in acidic and alkaline environments, it can better repel Mg. 2+ This causes Mg in the liquid 2+ It is not easy to pass through the separation membrane, and at the same time, it can make Li + To maximize the throughput, the membrane should achieve a high magnesium-lithium separation efficiency.

[0049] In this invention, the surface Zeta potential of the positively charged acid and alkali resistant composite nanofiltration membrane refers to the surface Zeta potential at pH=7.

[0050] Furthermore, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 5-25 mV.

[0051] According to the present invention, the average pore size of the positively charged acid and alkali resistant composite nanofiltration membrane is 0.1-0.4 nm.

[0052] In this invention, when the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above-mentioned range, it indicates that the positively charged acid- and alkali-resistant composite nanofiltration membrane has high density. When used for lithium-magnesium separation in acidic and alkaline environments, it can better repel Mg. 2+ This causes Mg in the liquid 2+ It is not easy to pass through the separation membrane, and at the same time, it can make Li + To achieve the highest possible magnesium-lithium separation efficiency, the material should be allowed to pass through as much as possible.

[0053] Furthermore, the average pore size of the positively charged acid and alkali resistant composite nanofiltration membrane is 0.2-0.3 nm.

[0054] According to the present invention, the contact angle of the positively charged acid and alkali resistant composite nanofiltration membrane is 30-80°.

[0055] In this invention, the positively charged acid and alkali resistant composite nanofiltration membrane has a contact angle within the range described in this invention, thereby indicating that the positively charged acid and alkali resistant composite nanofiltration membrane has good hydrophilicity, enabling the composite nanofiltration membrane to have high water flux.

[0056] Furthermore, the contact angle of the positively charged acid and alkali resistant composite nanofiltration membrane is 30-60°.

[0057] In this invention, the bottom layer and the porous support layer are not specifically limited and can be made of various existing materials with certain strength that can be used for nanofiltration and reverse osmosis membranes.

[0058] In this invention, the bottom layer is a nonwoven fabric, preferably polyester and / or polyethylene.

[0059] In this invention, the porous support layer material is selected from at least one of polyethersulfone, polysulfone, polyaromatic ether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride, and polyaryletherketone.

[0060] In this invention, the acid and alkali resistant separation layer is selected from at least one of polyurea separation layer, polytriazine amine separation layer, polysulfonamide separation layer, poly(triazine amine-urea) separation layer, poly(sulfonamide-urea) separation layer, poly(triazine amine-sulfonamide) separation layer and poly(triazine amine-sulfonamide-urea) separation layer, preferably at least one of polyurea separation layer, polytriazine amine separation layer and polysulfonamide separation layer.

[0061] Furthermore, the acid and alkali resistant separation layer is generated by interfacial polymerization of polyamines modified with haloquaternary ammonium salts and / or haloquaternary phosphorus salts with multiple polar monomers.

[0062] According to the present invention, the multi-polar monomer is selected from at least one of polyisocyanates, triazine compounds containing at least two C-Cl bonds, and polysulfonyl chlorides.

[0063] In this invention, a polyamine reacts with at least one of a polyisocyanate, a triazine compound containing at least two C-Cl bonds, and a polysulfonyl chloride to introduce groups with strong conjugation effect and good chemical inertness into a nanofiltration membrane. The resulting polymer has good structural stability in both acidic and alkaline environments, which greatly improves the acid and alkali resistance of the nanofiltration membrane, making it particularly suitable for acidic environments.

[0064] According to the present invention, the polyamine is at least one selected from polyethyleneimine, polyethyleneamine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylene polyamine, piperazine, m-phenylenediamine, and p-phenylenediamine; preferably polyethyleneimine and / or polyethylene polyamine.

[0065] According to the present invention, the polyisocyanate is selected from at least one of isophthalic diisocyanate, isophorone diisocyanate, 1,6-hexanediisocyanate, toluene-2,6-diisocyanate, 1,4-phenyl diisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenyl diisocyanate, 3,3'-dichloro-4,4'-diisocyanate biphenyl, dicyclohexylmethane-4,4'-diisocyanate, trimethylhexamethylene diisocyanate, L-lysine-ethyl ester-diisocyanate, 1,4-cyclohexyl diisocyanate, and 4-chloro-6-methyl isophthalic diisocyanate; preferably 1,4-phenyl diisocyanate and / or 1,3-phenyl diisocyanate.

[0066] According to the present invention, the triazine compound containing at least two C-Cl bonds is selected from at least one of cyanuric chloride, 2,4-dichloro-1,3,5-triazine, 2,5-dichloro-1,3,5-triazine and 2,4-dichloro-6-phenyl-1,3,5-triazine; preferably cyanuric chloride.

[0067] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzene disulfonyl chloride, 1,2-benzene disulfonyl chloride, 1,4-benzene disulfonyl chloride, 2,4-disulfonyl chloride methyltrimethylbenzene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzene disulfonyl chloride, 2,6-naphthalene disulfonyl chloride, 1,3-naphthalene disulfonyl chloride, 2,7-naphthalene disulfonyl chloride, 1,3,5-benzene trisulfonyl chloride, and 1,3,6-naphthalene trisulfonyl chloride; preferably 1,3-benzene disulfonyl chloride.

[0068] In this invention, the haloquaternary ammonium salt has the structure shown in Formula III;

[0069]

[0070] Wherein, R1' is a substituted or unsubstituted alkylene group of C1-C9; R2', R3', and R4' are each independently selected from alkyl or phenyl groups of C1-C3; and X1 and X are each independently halogens.

[0071] Furthermore, R1' is a substituted or unsubstituted alkylene or phenyl group of C1-C6; R2', R3', and R4' are each independently CH3; X is Cl, Br, or I; and X1 is F, Cl, Br, or I.

[0072] In one specific embodiment of the present invention, R1' is an unsubstituted alkylene group of C1-C3, wherein the substituent is a hydroxyl or carboxyl group, preferably a hydroxyl group.

[0073] In one specific embodiment of the present invention, the haloquaternary ammonium salt is selected from at least one of 2-chloroethyltrimethylammonium chloride, 2-chloroethyltriethylammonium chloride, 2-chloroethyltriphenylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, 3-chloropropyltrimethylammonium chloride, 4-chlorobutyltrimethylammonium chloride, 5-chloropentyltrimethylammonium chloride, 6-chlorohexyltrimethylammonium chloride, 2-bromoethyltrimethylammonium bromide, 3-bromopropyltrimethylammonium bromide, 4-bromobutyltrimethylammonium bromide, 5-bromopentyltrimethylammonium bromide, 6-bromohexyltrimethylammonium bromide, iodomethyltrimethylammonium iodide, 2-iodoethyltrimethylammonium iodide, 3-iodopropyltrimethylammonium iodide, 4-iodobutyltrimethylammonium iodide, 5-iodopentyltrimethylammonium iodide, and 6-iodohexyltrimethylammonium iodide.

[0074] In this invention, the haloquaternary phosphate salt has the structure shown in Formula IV;

[0075]

[0076] Wherein, R5' is a C1-C6 alkylene, phenyl, or C7-C10 aralkyl; R6', R7', and R8' are each independently a phenyl or C7-C10 aralkyl.

[0077] X and X1 are halogens, each independently.

[0078] Furthermore, R5' is a phenyl or a C8-C10 aralkyl group; R6', R7', and R8' are each independently a phenyl group; R1' is an unsubstituted C1-C3 alkylene group, wherein the substituent is a hydroxyl or carboxyl group.

[0079] Furthermore, X is Cl, Br, or I.

[0080] Furthermore, X1 can be F, Cl, Br, or I.

[0081] In one specific embodiment of the present invention, the haloquaternary phosphorus salt is selected from 3-bromopropyltriphenylphosphine bromide, 2-bromoethyltriphenylphosphine bromide, 1-bromoethyltriphenylphosphine bromide, bromomethyltriphenylphosphine bromide, 3-chloropropyltriphenylphosphine chloride, 2-chloroethyltriphenylphosphine chloride, 1-chloroethyltriphenylphosphine chloride, chloromethyltriphenylphosphine chloride, 3-iodopropyltriphenylphosphine iodide, 2-iodoethyltriphenylphosphine iodide, 1-iodoethyltriphenylphosphine iodide, iodomethyltriphenylphosphine bromide, 4- At least one of 1-bromomethylbenzyltriphenylphosphine bromide, 2-bromomethylbenzyltriphenylphosphine bromide, 3-bromomethylbenzyltriphenylphosphine bromide, 4-chloromethylbenzyltriphenylphosphine chloride, 2-chloromethylbenzyltriphenylphosphine chloride, 3-chloromethylbenzyltriphenylphosphine chloride, 4-iodomethylbenzyltriphenylphosphine iodide, 2-iodomethylbenzyltriphenylphosphine iodide and 3-iodomethylbenzyltriphenylphosphine iodide, preferably 3-bromopropyltriphenylphosphine bromide and / or 4-bromomethylbenzyltriphenylphosphine bromide.

[0082] According to the present invention, the thickness of the bottom layer, the porous support layer, and the acid and alkali resistant separation layer is not particularly limited and can be a conventional choice in the art. However, in order to enable these three layers to play a better synergistic role and enable the obtained positively charged acid and alkali resistant composite nanofiltration membrane to better combine excellent magnesium-lithium separation coefficient and high water flux, preferably, the thickness of the bottom layer is 30-150 μm, preferably 50-120 μm; the thickness of the porous support layer is 10-100 μm, preferably 30-60 μm; and the thickness of the acid and alkali resistant separation layer is 10-500 nm, preferably 50-300 nm.

[0083] A second aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the preparation method includes the following steps:

[0084] S1. Under stirring conditions, a first solution containing haloquaternary ammonium salt and / or haloquaternary phosphorus salt and catalyst is added to a second solution of polyamine to carry out the reaction and obtain modified polyamine.

[0085] S2. Prepare a porous support layer on the bottom layer;

[0086] S3. After the surface of the porous support layer is first contacted with the aqueous phase containing the modified polyamine of step S1, it is then contacted with the organic phase containing multiple polar monomers in a second manner, and then heat-treated to obtain the positively charged acid and alkali resistant composite nanofiltration membrane.

[0087] The haloquaternary ammonium salt has the structure shown in Formula III; the haloquaternary phosphorus salt has the structure shown in Formula IV.

[0088]

[0089] Wherein, R1' is a substituted or unsubstituted alkylene group of C1-C9; R2', R3', and R4' are each independently selected from alkyl or phenyl groups of C1-C3; R5' is an alkylene group of C1-C6, a phenyl group, or an aralkyl group of C7-C10; R6', R7', and R8' are each independently selected from phenyl groups or aralkyl groups of C7-C10.

[0090] X and X1 are halogens, each independently.

[0091] In this invention, in the presence of an alkaline catalyst, a polyamine is grafted with specific quaternary ammonium salt groups and / or quaternary phosphate salt groups to obtain a positively charged modified polyamine. After the polyamine is interfacially polymerized with a polar monomer, groups with good chemical inertness are introduced, which improves the positive charge density and acid and alkali tolerance of the nanofiltration membrane surface. This allows the nanofiltration membrane to maintain a high water flux while having a high magnesium-lithium separation coefficient. Even in acidic or alkaline environments, it can still maintain a high water flux and have relatively good magnesium-lithium separation performance.

[0092] Furthermore, R1' is a substituted or unsubstituted alkylene group of C1-C6; R2', R3', and R4' are each independently CH3; R5' is a phenyl or an aralkyl group of C8-C10; and R6', R7', and R8' are each independently phenyl.

[0093] Furthermore, R1' is an unsubstituted alkylene group of C1-C3, wherein the substituent is a hydroxyl or carboxyl group, preferably a hydroxyl group.

[0094] Furthermore, X is Cl, Br, or I.

[0095] Furthermore, X1 can be F, Cl, Br, or I.

[0096] According to the present invention, the haloquaternary ammonium salt is selected from at least one of 2-chloroethyltrimethylammonium chloride, 2-chloroethyltriethylammonium chloride, 2-chloroethyltriphenylammonium chloride, 3-chloro-2-hydroxypropyltrimethylammonium chloride, 3-chloropropyltrimethylammonium chloride, 4-chlorobutyltrimethylammonium chloride, 5-chloropentyltrimethylammonium chloride, 6-chlorohexyltrimethylammonium chloride, 2-bromoethyltrimethylammonium bromide, 3-bromopropyltrimethylammonium bromide, 4-bromobutyltrimethylammonium bromide, 5-bromopentyltrimethylammonium bromide, 6-bromohexyltrimethylammonium bromide, iodomethyltrimethylammonium iodide, 2-iodoethyltrimethylammonium iodide, 3-iodopropyltrimethylammonium iodide, 4-iodobutyltrimethylammonium iodide, 5-iodopentyltrimethylammonium iodide, and 6-iodohexyltrimethylammonium iodide; preferably 2-chloroethyltrimethylammonium chloride and / or 3-bromopropyltrimethylammonium bromide.

[0097] According to the present invention, the haloquaternary phosphorus salt is selected from 3-bromopropyltriphenylphosphine bromide, 2-bromoethyltriphenylphosphine bromide, 1-bromoethyltriphenylphosphine bromide, bromomethyltriphenylphosphine bromide, 3-chloropropyltriphenylphosphine chloride, 2-chloroethyltriphenylphosphine chloride, 1-chloroethyltriphenylphosphine chloride, chloromethyltriphenylphosphine chloride, 3-iodopropyltriphenylphosphine iodide, 2-iodoethyltriphenylphosphine iodide, 1-iodoethyltriphenylphosphine iodide, iodomethyltriphenylphosphine bromide, 4-bromomethylphosphine bromide, etc. At least one of benzyltriphenylphosphine bromide, 2-bromomethylbenzyltriphenylphosphine bromide, 3-bromomethylbenzyltriphenylphosphine bromide, 4-chloromethylbenzyltriphenylphosphine chloride, 2-chloromethylbenzyltriphenylphosphine chloride, 3-chloromethylbenzyltriphenylphosphine chloride, 4-iodomethylbenzyltriphenylphosphine iodide, 2-iodomethylbenzyltriphenylphosphine iodide, and 3-iodomethylbenzyltriphenylphosphine iodide; preferably 3-bromopropyltriphenylphosphine bromide and / or 4-bromomethylbenzyltriphenylphosphine bromide.

[0098] According to the present invention, the concentration of haloquaternary ammonium salt and / or haloquaternary phosphorus salt in the first solution is 0.5wt%-20wt%.

[0099] In this invention, when the concentration of haloquaternary ammonium salt and / or haloquaternary phosphate salt in the first solution in step S1 meets the above-mentioned range, the prepared positively charged acid and alkali resistant composite nanofiltration membrane has a high magnesium-lithium separation coefficient; at the same time, the quaternary ammonium salt groups and / or quaternary phosphate salt groups grafted on the surface of the acid and alkali resistant separation layer will not affect the water flux of the membrane too much, thereby maintaining good water flux.

[0100] In this invention, if the concentration of quaternary ammonium halide and / or quaternary phosphate halide in the first solution in step S1 is lower than the above range, the quaternary ammonium halide and / or quaternary phosphate halide in the first solution cannot react sufficiently with the polyamine in the second solution; if the concentration of quaternary ammonium halide and / or quaternary phosphate halide in the first solution in step S1 is higher than the above range, the quaternary ammonium halide and / or quaternary phosphate halide consumes too much of the amino groups in the polyamine structure in the second solution, resulting in insufficient density of the separation layer formed by interfacial polymerization in step S3.

[0101] Furthermore, the concentration of haloquaternary ammonium salt and / or haloquaternary phosphorus salt in the first solution is 1wt%-10wt%.

[0102] According to the present invention, the catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate, potassium bicarbonate, disodium hydrogen phosphate, and dipotassium hydrogen phosphate, preferably potassium hydroxide.

[0103] According to the present invention, the concentration of the catalyst in the first solution is 0.01 wt% to 5 wt%.

[0104] In this invention, when the concentration of the catalyst in the first solution in step S1 meets the above-mentioned range, it can ensure that the quaternary ammonium salt and / or quaternary phosphate salt in the first solution can fully react with the polyamine in the second solution and have a high reaction rate, so that the quaternary ammonium salt and / or quaternary phosphate salt structure can be introduced into the molecular structure of the polyamine in a short time.

[0105] Furthermore, the concentration of the catalyst in the first solution is 0.1 wt% to 1 wt%.

[0106] According to the present invention, in step S1, the polyamine is selected from at least one of polyethyleneimine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylene polyamine, piperazine, m-phenylenediamine and p-phenylenediamine; preferably polyethyleneimine and / or polyethylene polyamine.

[0107] According to the present invention, the concentration of polyamine in the second solution is 1 wt%-20 wt%, preferably 5 wt%-10 wt%.

[0108] According to the present invention, the amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the haloquaternary ammonium salt and / or haloquaternary phosphorus salt, and the catalyst is 1-1000:1-100:1.

[0109] In this invention, when the mass ratio of polyamine, haloquaternary ammonium salt and / or haloquaternary phosphorus salt and catalyst is controlled to meet the above range, it can be ensured that the haloquaternary ammonium salt and / or haloquaternary phosphorus salt in the first solution can fully react with the polyamine in the second solution, and the reaction rate and reaction efficiency are both high, and more quaternary phosphorus salt and / or quaternary ammonium salt structures can be introduced into the polyamine in a short time.

[0110] Furthermore, the amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the haloquaternary ammonium salt and / or haloquaternary phosphorus salt, and the catalyst is 1-200:1-50:1.

[0111] According to the present invention, in step S1, the reaction conditions include: a reaction temperature of 25-90°C and a reaction time of 1-48h.

[0112] In this invention, when the reaction conditions in step S1 meet the above-mentioned range, it can be ensured that the haloquaternary ammonium salt and / or haloquaternary phosphate salt in the first solution can fully react with the polyamine in the second solution, and the reaction rate and reaction efficiency are both high, which can introduce more quaternary phosphate salt and / or quaternary ammonium salt structures into the polyamine in a short time.

[0113] Furthermore, in step S1, the reaction conditions include: a reaction temperature of 40-60℃ and a reaction time of 6-24h.

[0114] In a preferred embodiment of the present invention, a first solution is added dropwise to a second solution containing a polyamine. Adding the first solution dropwise to the second solution containing the polyamine allows for control of the reaction rate and ensures thorough mixing of the first and second solutions, guaranteeing sufficient reaction between the quaternary ammonium halide and / or quaternary phosphate halide and the polyamine.

[0115] According to the present invention, the dripping rate is 0.5-5 mL / min, preferably 1-3 mL / min.

[0116] In this invention, there is no particular limitation on the method for preparing the porous support layer in the bottom layer in step S2. Conventional methods in the art can be used for preparation, preferably the phase inversion method. Specifically, a polymer solution of the porous support layer material is coated on one surface of the bottom layer, and the porous support layer is obtained through phase inversion.

[0117] In this invention, the phase inversion method is preferably as follows: dissolving the support layer polymer material in a solvent to obtain a polymer solution with a concentration of 10-20 wt%, degassing at 20-40°C for 10-180 min; then coating the polymer solution onto the bottom layer to obtain an initial film, and then immersing it in water at a temperature of 10-30°C for 10-60 min, thus forming the phase inversion layer of the support layer polymer porous membrane.

[0118] The solvent may be at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0119] In this invention, the acid and alkali resistant separation layer is generated by interfacial polymerization of polyamines modified with halogenated quaternary ammonium salts and / or halogenated quaternary phosphorus salts and multi-polar monomers.

[0120] According to the present invention, the multi-polar monomer is selected from at least one of polyisocyanates, triazine compounds containing at least two C-Cl bonds, and polysulfonyl chlorides.

[0121] According to the present invention, the polyisocyanate is selected from at least one of isophthalic diisocyanate, isophorone diisocyanate, 1,6-hexanediisocyanate, toluene-2,6-diisocyanate, 1,4-phenyl diisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenyl diisocyanate, 3,3'-dichloro-4,4'-diisocyanate biphenyl, dicyclohexylmethane-4,4'-diisocyanate, trimethylhexamethylene diisocyanate, L-lysine-ethyl ester-diisocyanate, 1,4-cyclohexyl diisocyanate, and 4-chloro-6-methyl isophthalic diisocyanate; preferably 1,4-phenyl diisocyanate and / or 1,3-phenyl diisocyanate.

[0122] According to the present invention, the triazine compound containing at least two C-Cl bonds is selected from at least one of cyanuric chloride, 2,4-dichloro-1,3,5-triazine, 2,5-dichloro-1,3,5-triazine, and 2,4-dichloro-6-phenyl-1,3,5-triazine; preferably cyanuric chloride.

[0123] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzene disulfonyl chloride, 1,2-benzene disulfonyl chloride, 1,4-benzene disulfonyl chloride, 2,4-disulfonyl chloride methyltrimethylbenzene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzene disulfonyl chloride, 2,6-naphthalene disulfonyl chloride, 1,3-naphthalene disulfonyl chloride, 2,7-naphthalene disulfonyl chloride, 1,3,5-benzene trisulfonyl chloride, and 1,3,6-naphthalene trisulfonyl chloride; preferably 1,3-benzene disulfonyl chloride.

[0124] According to the present invention, the concentration of the modified polyamine in the aqueous phase is 0.1wt%-10wt%, preferably 0.5wt%-2.5wt%.

[0125] According to the present invention, the concentration of the multi-polar monomer in the organic phase is 0.01wt%-2wt%, preferably 0.05-1wt%.

[0126] In this invention, the type of solvent for the organic phase is not particularly limited, as long as it can dissolve the polyacrylamide chloride. Preferably, the solvent for the organic phase is one or more of n-hexane, dodecane, n-heptane, and alkane solvent oils (Isopar E, Isopar G, Isopar H, Isopar L, and Isopar M).

[0127] In this invention, there are no particular limitations on the interfacial polymerization conditions between the modified polyamine and the multi-polar monomer in step S3. These conditions can be carried out according to conventional conditions in the art. However, to ensure better synergistic effects among the three layers and to enable the resulting positively charged acid- and alkali-resistant composite nanofiltration membrane to better combine excellent magnesium-lithium separation coefficients and high water flux, preferably, the first contact time is 5-100 s, more preferably 10-60 s; the second contact time is 10-200 s, more preferably 20-120 s; and the heat treatment conditions include: a heat treatment temperature of 40-150℃, more preferably 50-120℃; and a heat treatment time of 0.5-10 min, more preferably 1-5 min.

[0128] The third aspect of the present invention provides a positively charged acid and alkali resistant composite nanofiltration membrane prepared by the above preparation method.

[0129] The fourth aspect of this invention provides an application of the above-mentioned positively charged acid and alkali resistant composite nanofiltration membrane in the field of water treatment separation.

[0130] In this invention, the positively charged acid and alkali resistant composite nanofiltration membrane has a MgCl2 desalination rate of ≥88%, preferably ≥90%; a LiCl desalination rate of ≥26%, preferably ≥28%; a magnesium-lithium separation coefficient of 10-90, preferably 25-86; and a water flux of ≥22 LMH, preferably ≥25 LMH.

[0131] Other features and advantages of the present invention will be described in detail in the following detailed description section.

[0132] The present invention will be described in detail below through embodiments.

[0133] (1) The water flux of the nanofiltration membrane was tested by the following method: The nanofiltration membrane was loaded into the membrane tank and pre-pressed at 1.5 MPa for 1 hour. The water permeation rate of the positively charged acid and alkali resistant composite nanofiltration membrane was measured at a pressure of 2 MPa and a temperature of 25°C for a certain period of time. The water flux was calculated by the following formula:

[0134] J = Q / (A·t), where J is the water flux, Q is the water permeation rate (L), and A is the effective filtration membrane area (m²) of the positively charged acid- and alkali-resistant composite nanofiltration membrane. 2 ), where t is time (h).

[0135] (2) The desalination rate of the nanofiltration membrane was obtained by the following method: A positively charged acid and alkali resistant composite nanofiltration membrane was loaded into the membrane tank and pre-pressurized at 1.5 MPa for 1 hour. The salt concentration changes in the raw aqueous solution with an initial concentration of 2000 ppm and the permeate were measured within 1 hour under the conditions of pressure of 2 MPa and temperature of 25℃. The desalination rate was calculated by the following formula:

[0136] R = (C f -C p ) / C f ×100%, where R is the desalination rate, and C is the desalination rate. f C represents the concentration of magnesium chloride or lithium chloride in the original solution (measured by a conductivity meter). p This represents the concentration of magnesium chloride or lithium chloride in the permeate.

[0137] (3) The magnesium-lithium separation coefficient of the nanofiltration membrane was obtained by the following method: A positively charged acid- and alkali-resistant composite nanofiltration membrane was loaded into a membrane tank. The original aqueous solution was a mixture of 2000 ppm magnesium chloride and 100 ppm lithium chloride. The mixture was pre-pressurized at 1.5 MPa for 1 h, and then the permeate was obtained under the conditions of 2 MPa and 25 °C. The mass concentrations of magnesium ions and lithium ions in the original solution and the permeate were measured by ion chromatography, and the lithium-magnesium separation coefficient was calculated by the following formula:

[0138]

[0139] Where S is the lithium-magnesium separation coefficient, and C Mg,f and C Li,f These represent the mass concentrations of magnesium and lithium ions in the raw water (measured by ion chromatography); C Mg,p and C Li,p These represent the mass concentrations of magnesium ions and lithium ions in the permeate (measured by ion chromatography).

[0140] (4) Test method for acid resistance of positively charged acid and alkali resistant composite nanofiltration membrane: Immerse the nanofiltration membrane in a 5 wt% HCl solution for 7 days, take it out and clean it thoroughly, and then test the changes in water flux and magnesium-lithium separation coefficient of the nanofiltration membrane.

[0141] (5) Measurement of the surface contact angle of positively charged acid and alkali resistant composite nanofiltration membrane

[0142] The surface contact angle of the composite film sample was tested using a DSA100 surface contact angle meter manufactured by KRUSS GmbH, Germany, via the static drop method. Before the test, the sample was dried in a vacuum oven at 60°C for 30 minutes to remove surface and internal moisture. Then, the dried film was attached to a flat glass slide with double-sided tape. The volume of each water droplet was 2 μL. The water droplet was placed on the film surface for 3 seconds and the test was performed immediately. The final contact angle was determined by taking the average value after multiple measurements.

[0143] (6) Determination of nitrogen atom content in quaternary ammonium salt groups and phosphorus atom content in positively charged acid- and alkali-resistant composite nanofiltration membranes:

[0144] Before measurement, the samples were dried to constant weight in an oven, and the elemental composition of the composite film sample surface was determined using a Sigma Probe X-ray photoelectron spectrometer manufactured by Thermo VG, UK. For samples containing quaternary ammonium salt groups, the molar content of nitrogen atoms in the quaternary ammonium salt groups is equal to the corresponding molar content of halogens, and the content of nitrogen atoms in the quaternary ammonium salt groups can be determined by measuring the halogen content.

[0145] The phosphorus atom content in quaternary phosphate salts in positively charged acid and alkali resistant composite nanofiltration membranes can be directly measured by XPS spectroscopy.

[0146] (7) Positively charged acid and alkali resistant composite nanofiltration membrane pore size test: The pore size was measured using the PEG solute transfer method, and the detailed steps are as follows:

[0147] (i) Test the retention rate of positively charged acid and alkali resistant composite nanofiltration membrane for PEG of different molecular sizes;

[0148] (ii) Linear fitting of PEG size and rejection rate in log-probability coordinate system, the PEG size corresponding to 50% rejection rate is the average pore size of positively charged acid and alkali resistant composite nanofiltration membrane.

[0149] (8) Zeta potential test of positively charged acid and alkali resistant composite nanofiltration membrane surface: The test was performed using a Surpass electric analyzer (AntonPaar), the circulating solution was a dilute aqueous solution of KCl, and the pH of the test solution was 7.

[0150] Additionally, in the following embodiments and comparative examples:

[0151] Branched polyethyleneimine (weight average molecular weight 25000 g / mol), polyethylene polyamine, 1,4-phenylene diisocyanate, cyanuric chloride, 1,3-benzene disulfonyl chloride, 2-chloroethyltrimethylammonium chloride, 3-bromopropyltriphenylphosphine bromide, 4-bromomethylbenzyltriphenylphosphine bromide, chloromethyltriphenylphosphine chloride, 2-chloroethyltrimethylammonium chloride, 3-bromopropyltrimethylammonium bromide, and iodomethyltrimethylammonium iodide were all purchased from Bailingwei Technology Co., Ltd. Other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

[0152] The support layer is prepared using a phase transformation method, and the specific steps are as follows:

[0153] A certain amount of polysulfone (number average molecular weight of 80,000 g / mol) was dissolved in N,N-dimethylformamide to prepare a polysulfone solution with a concentration of 18 wt%. The solution was degassed at 25 °C for 120 min. Then, the polysulfone solution was coated onto a polyester nonwoven fabric (75 μm thick) using a doctor blade to obtain an initial film. The film was then immersed in water at 25 °C for 60 min, which allowed the polysulfone layer on the surface of the polyester nonwoven fabric to undergo phase transformation into a porous film. Finally, after three water washes, a support layer with a total thickness of 115 μm was obtained.

[0154] Example 1

[0155] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water and heat to 60℃ under stirring. Add 10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide (X1 and X are Br, R5' is propylene, and R6', R7', and R8' are phenyl) and 1wt% potassium hydroxide to the solution at a rate of 2mL / min. After the addition is complete, continue the reaction at 60℃ for 24h. The mass ratio of polyamine, haloquaternary phosphate salt and catalyst is 5:0.5:0.1 = 50:5:1.

[0156] (2) The obtained quaternary phosphate modified polyethyleneimine aqueous solution prepared above was diluted with water to adjust its concentration to 0.5 wt% quaternary phosphate modified polyethyleneimine aqueous solution. A 400 cm² area of ​​this solution was then used as the solution. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% of the quaternary phosphate modified polyethyleneimine prepared above, and the solution was drained after 60 s at 25 °C. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenyl diisocyanate, and the solution was drained after 60 s at 25 °C. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N1.

[0157] The infrared spectrum of the positively charged acid and alkali resistant composite nanofiltration membrane N1 is as follows: Figure 1 As shown. By Figure 1 It can be seen that it is located at 1113cm -1 The characteristic peak of the CP bond indicates that the quaternary phosphorus group in the 3-bromopropyltriphenylphosphine bromide structure was successfully modified on the surface of the nanofiltration membrane, thereby enhancing the positive charge of the positively charged acid and alkali resistant composite nanofiltration membrane surface and improving the magnesium-lithium separation performance of the membrane.

[0158] Example 2

[0159] The preparation method is the same as in Example 1, except that "polyethylene polyamine" is used instead of "polyethyleneimine" to obtain a positively charged acid and alkali resistant composite nanofiltration membrane N2.

[0160] Example 3

[0161] The preparation method is the same as in Example 1, except that “4-bromomethylbenzyltriphenylphosphine bromide (X1 and X are Br, R5' is C1 arylmethyl, R6', R7', and R8' are phenyl)” is used to replace “3-bromopropyltriphenylphosphine bromide” to obtain a positively charged acid and alkali resistant composite nanofiltration membrane N3.

[0162] Example 4

[0163] The preparation method is the same as in Example 1, except that “chloromethyltriphenylphosphine chloride (R5' is methylene, R6', R7', R8' are phenyl, X1 and X are Cl)” is used instead of “3-bromopropyltriphenylphosphine bromide” to obtain a positively charged acid and alkali resistant composite nanofiltration membrane N4.

[0164] Example 5

[0165] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water and heat to 60℃ under stirring. Add 10g of an aqueous solution containing 5wt% 2-chloroethyltrimethylammonium chloride (X and X1 are Cl, R1' is ethylene, and R2', R3', and R4' are methyl) and 1wt% potassium hydroxide to the solution at a rate of 2mL / min. After the addition is complete, continue the reaction at 60℃ for 24h. The mass ratio of polyamine, haloquaternary ammonium salt to catalyst is 5:0.5:0.1 = 50:5:1.

[0166] (2) The obtained quaternary ammonium salt modified polyethyleneimine aqueous solution was diluted with water to adjust its concentration to 0.5 wt% quaternary ammonium salt modified polyethyleneimine aqueous solution. A 400 cm² area was then prepared. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% of the quaternary ammonium salt modified polyethyleneimine prepared above, and the solution was drained after 60 s at 25 °C. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenyl diisocyanate, and the solution was drained after 60 s at 25 °C. Then, the membrane was placed in an oven and heated at 70 °C for 3 min to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N5.

[0167] Example 6

[0168] The preparation method is the same as in Example 5, except that "3-bromopropyltrimethylammonium bromide (X and X1 are Br, R1' is propylene, and R2', R3', and R4' are methyl)" is used instead of "2-chloroethyltrimethylammonium chloride" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N6.

[0169] Example 7

[0170] The preparation method is the same as in Example 5, except that "2-chloroethyltrimethylammonium chloride" is replaced with "iodomethyltrimethylammonium iodide (X and X1 are I, R1' is methylene, R2', R3', and R4' are methyl)" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N7.

[0171] Example 8

[0172] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water and heat to 60°C under stirring. Add 10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 1wt% potassium hydroxide dropwise to the solution at a rate of 2mL / min. After the addition is complete, continue the reaction at 60°C for 24h.

[0173] (2) The obtained quaternary phosphate modified polyethyleneimine aqueous solution prepared above was diluted with water to adjust its concentration to 2wt% quaternary phosphate modified polyethyleneimine aqueous solution. A 400cm² area was then prepared. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% of the quaternary phosphate modified polyethyleneimine prepared above, and the solution was drained after 60 s at 25 °C. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.05 wt% cyanuric chloride, and the solution was drained after 60 s at 25 °C. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N8.

[0174] Example 9

[0175] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water and heat to 60°C under stirring. Add 10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 1wt% potassium hydroxide dropwise to the solution at a rate of 2mL / min. After the addition is complete, continue the reaction at 60°C for 24h.

[0176] (2) The obtained quaternary phosphate modified polyethyleneimine aqueous solution prepared above was diluted with water to adjust its concentration to 2wt% quaternary phosphate modified polyethyleneimine aqueous solution. A 400cm² area was then prepared. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% of the quaternary phosphate modified polyethyleneimine prepared above, and the solution was drained after 60 s at 25 °C. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.2 wt% 1,3-benzenedisulfonyl chloride, and the solution was drained after 60 s at 25 °C. Then, the membrane was placed in an oven and heated at 70 °C for 3 min to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N9.

[0177] Example 10

[0178] The preparation method is the same as in Example 1, except that "10g of 3-bromopropyltriphenylphosphine bromide containing 0.5wt% is added dropwise to the solution at a rate of 2mL / min" instead of "10g of 3-bromopropyltriphenylphosphine bromide containing 5wt% is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N10.

[0179] Example 11

[0180] The preparation method is the same as in Example 1, except that "10g of 3-bromopropyltriphenylphosphine bromide containing 8wt% is added dropwise to the solution at a rate of 2mL / min" instead of "10g of 3-bromopropyltriphenylphosphine bromide containing 5wt% is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N11.

[0181] Example 12

[0182] The preparation method is the same as in Example 1, except that "10g of 3-bromopropyltriphenylphosphine bromide containing 0.2wt% is added dropwise to the solution at a rate of 2mL / min" instead of "10g of 3-bromopropyltriphenylphosphine bromide containing 5wt% is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N12.

[0183] Example 13

[0184] The preparation method is the same as in Example 1, except that "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 0.05wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" instead of "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 1wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N13; the mass ratio of polyamine, haloquaternary phosphate salt and catalyst is 5:0.5:0.005 = 1000:100:1.

[0185] Example 14

[0186] The preparation method is the same as in Example 1, except that "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 0.5wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" instead of "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 1wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N14; the mass ratio of polyamine, haloquaternary phosphate salt and catalyst is 5:0.5:0.05 = 100:10:1.

[0187] Example 15

[0188] The preparation method is the same as in Example 1, except that "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 0.005wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" instead of "10g of an aqueous solution containing 5wt% 3-bromopropyltriphenylphosphine bromide and 1wt% potassium hydroxide is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N15; the mass ratio of polyamine, haloquaternary phosphate salt and catalyst is 5:0.5:0.0005 = 10000:1000:1.

[0189] Example 16

[0190] The preparation method is the same as in Example 1, except that "10g of 3-bromopropyltriphenylphosphine bromide containing 2.5wt% and 2-chloroethyltrimethylammonium chloride containing 2.5wt% is added dropwise to the solution at a rate of 2mL / min" instead of "10g of 3-bromopropyltriphenylphosphine bromide containing 5wt% is added dropwise to the solution at a rate of 2mL / min" to obtain the positively charged acid and alkali resistant composite nanofiltration membrane N16; the mass ratio of polyamine, haloquaternary ammonium salt and haloquaternary phosphorus salt to catalyst is 5:0.5:0.1 = 50:5:1.

[0191] Example 17

[0192] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water, and then dissolve the above-mentioned area of ​​400cm². 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenyl diisocyanate at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain the composite membrane.

[0193] (2) The obtained composite nanofiltration membrane was immersed in 150g of an aqueous solution containing 10wt% 3-bromopropyltriphenylphosphine bromide and 0.5wt% potassium hydroxide. After 1min, it was taken out and dried at 70℃ for 5min to obtain the quaternary phosphorus salt modified nanofiltration membrane N17.

[0194] Example 18

[0195] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water, and then dissolve the above-mentioned area of ​​400cm². 2The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.05 wt% cyanuric chloride at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain the composite membrane.

[0196] (2) The obtained composite nanofiltration membrane was immersed in 150g of an aqueous solution containing 10wt% 3-bromopropyltriphenylphosphine bromide and 0.5wt% potassium hydroxide. After 1min, it was taken out and dried at 70℃ for 5min to obtain the quaternary phosphorus salt modified nanofiltration membrane N18.

[0197] Example 19

[0198] (1) Dissolve 5g of polyethyleneimine in 100g of deionized water, and then dissolve the above-mentioned area of ​​400cm². 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.2 wt% 1,3-benzenedisulfonyl chloride at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain the composite membrane.

[0199] (2) The obtained composite nanofiltration membrane was immersed in 150g of an aqueous solution containing 10wt% 3-bromopropyltriphenylphosphine bromide and 0.5wt% potassium hydroxide. After 1min, it was taken out and dried at 70℃ for 5min to obtain the quaternary phosphorus salt modified nanofiltration membrane N19.

[0200] Comparative Example 1

[0201] Dissolve 5g of polyethyleneimine in 100g of deionized water. The above-mentioned area is 400cm². 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenyl diisocyanate at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain nanofiltration membrane D1.

[0202] Comparative Example 2

[0203] Dissolve 5g of polyethyleneimine in 100g of deionized water. The above-mentioned area is 400cm². 2The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.05 wt% cyanuric chloride at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain nanofiltration membrane D2.

[0204] Comparative Example 3

[0205] Dissolve 5g of polyethyleneimine in 100g of deionized water. The above-mentioned area is 400cm². 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine at 25 °C for 60 s, and then drained. Then, the upper surface of the support layer was contacted with 30 mL of Isopar E solution containing 0.2 wt% 1,3-benzenedisulfonyl chloride at 25 °C for 60 s, and then drained. The membrane was then placed in an oven and heated at 70 °C for 3 min to obtain nanofiltration membrane D3.

[0206] The thickness of the positively charged acid and alkali resistant composite nanofiltration membrane prepared in the examples and the nanofiltration membrane prepared in the comparative examples were tested, and the results are shown in Table 1.

[0207] The surface zeta potential, average pore size, nitrogen atom content in the quaternary ammonium salt group of the positively charged acid and alkali resistant composite nanofiltration membrane obtained in the examples and the nanofiltration membrane obtained in the comparative examples were tested, and the results are shown in Table 2.

[0208] The water flux, desalination rate of magnesium chloride and lithium chloride, and magnesium-lithium separation coefficient of the positively charged acid-alkali resistant composite nanofiltration membrane obtained in the examples and the nanofiltration membrane obtained in the comparative examples were compared. After acid treatment, the magnesium-lithium separation coefficient and water flux of the positively charged acid-alkali resistant composite nanofiltration membrane obtained in the examples and the nanofiltration membrane obtained in the comparative examples were tested. The results are shown in Table 3.

[0209] Table 1

[0210]

[0211] Note: The thickness of the acid and alkali resistant separation layer here refers to the acid and alkali resistant separation layer grafted with quaternary ammonium salt groups / quaternary phosphate salt groups.

[0212] Table 2

[0213]

[0214] Note: a - The nitrogen atom content in the quaternary ammonium salt group;

[0215] b- The phosphorus content in the positively charged acid and alkali resistant composite nanofiltration membrane;

[0216] "-" indicates that it cannot be measured.

[0217] Table 3

[0218]

[0219]

[0220] As shown in Tables 1-3, the positively charged acid and alkali resistant composite nanofiltration membrane of the present invention can better combine excellent magnesium-lithium separation coefficient and high water flux.

[0221] As can be seen from Table 2, compared with the comparative example, in the positively charged acid and alkali resistant composite nanofiltration membrane prepared in the examples, due to the chemical reaction between the halogenated quaternary ammonium salt and / or quaternary phosphate salt and the polyamine under the action of an alkaline catalyst, the positively charged quaternary ammonium salt groups and / or quaternary phosphate salt groups are modified into the polyamine molecules. After interfacial polymerization, the surface potential of the resulting positively charged acid and alkali resistant composite nanofiltration membrane is significantly improved.

[0222] On the other hand, the introduction of quaternary ammonium salt groups or quaternary phosphate salt groups has a regulatory effect on interfacial polymerization, and the polymerization reaction proceeds more fully. This results in a more compact separation layer and smaller pore size in the final positively charged acid and alkali resistant composite nanofiltration membrane. The positively charged acid and alkali resistant composite nanofiltration membrane has a higher magnesium-lithium separation coefficient while maintaining high water flux.

[0223] As can be seen from the results in Table 3, compared with Examples 17, 18 and 19, Examples 1-16 first modified the polyamine with a positive charge and then carried out interfacial polymerization. The retention rate of magnesium chloride in the membrane obtained after interfacial polymerization was significantly improved and the magnesium-lithium separation coefficient was significantly increased. This is mainly attributed to the enhanced positive charge on the membrane surface and the increased density of the separation layer (density is characterized by average pore size).

[0224] On the other hand, the magnesium-lithium separation coefficient after treatment with 5wt% HCl for 7 days shows that the magnesium-lithium separation coefficient and water flux of the membrane remain basically unchanged after strong acid treatment, indicating that the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by positively charged modified polyamine has better acid / alkali resistance.

[0225] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A positively charged, acid- and alkali-resistant composite nanofiltration membrane, characterized in that, The positively charged acid and alkali resistant composite nanofiltration membrane comprises, in sequence, a bottom layer, a porous intermediate layer, and an acid and alkali resistant separation layer; The acid and alkali resistant separation layer is made by grafting N atoms with quaternary phosphate groups of Formula II. Formula II; Wherein, R5 is a C1-C6 alkylene, phenyl, or C7-C10 aralkyl; R6, R7, and R8 are each independently a phenyl or C7-C10 aralkyl. X is a halogen.

2. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1, wherein, R5 is a C1-C3 alkylene group, a phenyl group, or a C8-C10 aralkyl group; R6, R7, and R8 are each independently a phenyl group; And / or, X is Cl, Br, or I; And / or, the acid and alkali resistant separation layer is selected from at least one of polyurea separation layer, polytriazine amine separation layer, polysulfonamide separation layer, poly(triazine amine-urea) separation layer, poly(sulfonamide-urea) separation layer, poly(triazine amine-sulfonamide) separation layer and poly(triazine amine-sulfonamide-urea) separation layer.

3. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 2, wherein, The acid and alkali resistant separation layer is at least one of polyurea separation layer, polytriazine amine separation layer and polysulfonamide separation layer.

4. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1 or 2, wherein, The phosphorus content in the positively charged acid and alkali resistant composite nanofiltration membrane is 0.4-3 at.%.

5. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 4, wherein, The phosphorus content in the positively charged acid and alkali resistant composite nanofiltration membrane is 0.5-2 at.%.

6. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1 or 2, wherein, The surface zeta potential of the positively charged acid and alkali resistant composite nanofiltration membrane is 0-30mV.

7. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 6, wherein, The surface zeta potential of the positively charged acid and alkali resistant composite nanofiltration membrane is 5-25mV.

8. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1 or 2, wherein, The average pore size of the positively charged acid and alkali resistant composite nanofiltration membrane is 0.1-0.4 nm.

9. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 8, wherein, The average pore size of the positively charged acid and alkali resistant composite nanofiltration membrane is 0.2-0.3 nm.

10. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1 or 2, wherein, The contact angle of the positively charged acid and alkali resistant composite nanofiltration membrane is 30-80°.

11. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 10, wherein, The contact angle of the positively charged acid and alkali resistant composite nanofiltration membrane is 30-60°.

12. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1 or 2, wherein, The thickness of the bottom layer is 30-150 μm; And / or, the thickness of the porous intermediate layer is 10-100 μm; And / or, the thickness of the acid and alkali resistant separation layer is 10-500 nm.

13. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 12, wherein, The thickness of the bottom layer is 50-120 μm; And / or, the thickness of the porous intermediate layer is 30-60 μm; And / or, the thickness of the acid and alkali resistant separation layer is 50-300 nm.

14. A method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1-13, characterized in that, The preparation method includes the following steps: S1. Under stirring conditions, a first solution containing a haloquaternary phosphate salt and a catalyst is added to a second solution containing a polyamine to carry out the reaction and obtain a modified polyamine. S2. Prepare a porous intermediate layer on the bottom layer; S3. After the surface of the porous intermediate layer is first contacted with the aqueous phase containing the modified polyamine of step S1, it is then contacted with the organic phase containing multiple polar monomers for a second time, and then heat-treated to obtain the positively charged acid and alkali resistant composite nanofiltration membrane. The haloquaternary phosphate salt has the structure shown in Formula IV; Formula IV; Wherein, R5' is a C1-C6 alkylene, phenyl, or C7-C10 aralkyl; R6', R 7’ R8' is independently a phenyl or a C7-C10 aryl group; X and X1 are halogens, each independently.

15. The preparation method according to claim 14, wherein, R5' is a phenyl or a C8-C10 aryl group; R6', R7', and R8' are each independently a phenyl group; And / or, X is Cl, Br, or I; And / or, X1 is F, Cl, Br or I.

16. The preparation method according to claim 14 or 15, wherein, In step S1, the haloquaternary phosphorus salt is selected from at least one of 3-bromopropyltriphenylphosphine bromide, 2-bromoethyltriphenylphosphine bromide, 1-bromoethyltriphenylphosphine bromide, bromomethyltriphenylphosphine bromide, 3-chloropropyltriphenylphosphine chloride, 2-chloroethyltriphenylphosphine chloride, 1-chloroethyltriphenylphosphine chloride, chloromethyltriphenylphosphine chloride, 3-iodopropyltriphenylphosphine iodide, 2-iodoethyltriphenylphosphine iodide, 1-iodoethyltriphenylphosphine iodide, iodomethyltriphenylphosphine bromide, 4-bromomethylbenzyltriphenylphosphine bromide, 2-bromomethylbenzyltriphenylphosphine bromide, 3-bromomethylbenzyltriphenylphosphine bromide, 4-chloromethylbenzyltriphenylphosphine chloride, 2-chloromethylbenzyltriphenylphosphine chloride, 3-chloromethylbenzyltriphenylphosphine chloride, 4-iodomethylbenzyltriphenylphosphine iodide, 2-iodomethylbenzyltriphenylphosphine iodide, and 3-iodomethylbenzyltriphenylphosphine iodide. And / or, the catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate, potassium bicarbonate, disodium hydrogen phosphate, and dipotassium hydrogen phosphate; And / or, the concentration of the haloquatriphosphite in the first solution is 0.5wt%-20wt%; And / or, the concentration of the catalyst in the first solution is 0.01wt%-5wt%.

17. The preparation method according to claim 16, wherein, In step S1, the haloquaternary phosphorus salt is 3-bromopropyltriphenylphosphine bromide and / or 4-bromomethylbenzyltriphenylphosphine bromide; And / or, the catalyst is potassium hydroxide; And / or, the concentration of the haloquaternary phosphate salt in the first solution is 1wt%-10wt%; And / or, the concentration of the catalyst in the first solution is 0.1wt%-1wt%.

18. The preparation method according to claim 14 or 15, wherein, In step S1, the polyamine is selected from at least one of polyethyleneimine, polyethyleneamine, polyethylene polyamine, piperazine, m-phenylenediamine, and p-phenylenediamine; And / or, the concentration of the polyamine in the second solution is 1wt%-20wt%; And / or, the amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the haloquaternary phosphate salt, and the catalyst is 1-1000:1-100:

1.

19. The preparation method according to claim 18, wherein, In step S1, the polyamine is polyethyleneimine and / or polyethylene polyamine; And / or, the concentration of the polyamine in the second solution is 5wt%-10wt%; And / or, the amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the haloquaternary phosphate salt, and the catalyst is 1-200:1-50:

1.

20. The preparation method according to claim 18, wherein, In step S1, the polyethylene polyamine is selected from at least one of triethylenetetramine, tetraethylenepentamine, and diethylenetriamine.

21. The preparation method according to claim 14 or 15, wherein, In step S1, the reaction conditions include: a reaction temperature of 25-90℃ and a reaction time of 1-48h.

22. The preparation method according to claim 21, wherein, In step S1, the reaction conditions include: a reaction temperature of 40-60℃ and a reaction time of 6-24h.

23. The preparation method according to claim 14 or 15, wherein, In step S3, the duration of the first contact is 5-100 seconds; And / or, the duration of the second contact is 10-200 s; And / or, the conditions for the heat treatment include: a heat treatment temperature of 40-150℃; and a heat treatment time of 0.5-10 min.

24. The preparation method according to claim 23, wherein, In step S3, the duration of the first contact is 10-60 seconds; And / or, the duration of the second contact is 20-120 seconds; And / or, the conditions for the heat treatment include: a heat treatment temperature of 50-120°C; and a heat treatment time of 1-5 min.

25. The preparation method according to claim 14 or 15, wherein, In step S3, the multi-polar monomer is selected from at least one of polyisocyanates, triazine compounds containing at least two C-Cl bonds, and polysulfonyl chlorides.

26. The preparation method according to claim 25, wherein, The polyisocyanate is selected from at least one of the following: isophthalic diisocyanate, isophorone diisocyanate, 1,6-hexanediisocyanate, toluene-2,6-diisocyanate, 1,4-phenyl diisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenyl diisocyanate, 3,3'-dichloro-4,4'-diisocyanate biphenyl, dicyclohexylmethane-4,4'-diisocyanate, trimethylhexamethylene diisocyanate, L-lysine-ethyl ester-diisocyanate, 1,4-cyclohexyl diisocyanate, and 4-chloro-6-methyl isophthalic diisocyanate. And / or, the triazine compound containing at least two C-Cl bonds is selected from at least one of cyanuric chloride, 2,4-dichloro-1,3,5-triazine, 2,5-dichloro-1,3,5-triazine and 2,4-dichloro-6-phenyl-1,3,5-triazine; And / or, the polysulfonyl chloride is selected from at least one of 1,3-benzene disulfonyl chloride, 1,2-benzene disulfonyl chloride, 1,4-benzene disulfonyl chloride, 2,4-disulfonyl chloride methyltrimethylbenzene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzene disulfonyl chloride, 2,6-naphthalene disulfonyl chloride, 1,3-naphthalene disulfonyl chloride, 2,7-naphthalene disulfonyl chloride, 1,3,5-benzene trisulfonyl chloride, and 1,3,6-naphthalene trisulfonyl chloride; And / or, the concentration of the modified polyamine in the aqueous phase is 0.1wt%-10wt%; And / or, the concentration of the multi-polar monomers in the organic phase is 0.01wt%-2wt%.

27. The preparation method according to claim 26, wherein, The polyisocyanate is 1,4-phenyl diisocyanate and / or 1,3-phenyl diisocyanate; And / or, the triazine compound containing at least two C-Cl bonds is cyanuric chloride; And / or, the polysulfonyl chloride is 1,3-benzenedisulfonyl chloride; And / or, the concentration of the modified polyamine in the aqueous phase is 0.5wt%-2.5wt%; And / or, the concentration of the multi-polar monomers in the organic phase is 0.05-1 wt%.

28. A positively charged, acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method according to any one of claims 14-27.

29. The application of the positively charged acid and alkali resistant composite nanofiltration membrane according to any one of claims 1-13 and 28 in the field of water treatment separation.

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