Positively charged acid- and alkali-resistant composite nanofiltration membrane and preparation method therefor and use thereof

Abstract

A positively charged acid- and alkali-resistant composite nanofiltration membrane and a preparation method therefor and a use thereof. 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 comprises: (1) the quaternary ammonium salt group shown in formula I grafted to the acid- and alkali-resistant separation layer through a N atom and / or the quaternary phosphonium salt group shown in formula II; formula I; formula II; wherein R1 is substituted or unsubstituted C1-C9 alkylene or phenylene; R2, R3, and R4 are each independently selected from C1-C3 alkyl or phenyl; R5 is C1-C6 alkylene, phenylene, or C7-C10 arylene; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl; X is halogen; or (2) a quaternary ammonium salt structure, wherein the quaternary ammonium salt structure comprises the structural formula shown in formula III and / or the structural formula shown in formula IV; formula III; formula IV, wherein X is halogen; n is an integer of 0 to 10; and m is an integer of 2 to 10. The acid- and alkali-resistant separation layer is selected from a polyurea separation layer, a polytriazine amine separation layer, a polysulfonamide separation layer, and a poly(triazine amine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazine amine-sulfonamide) separation layer, and a poly(triazine amine-sulfonamide-urea) separation layer. The nanofiltration membrane has a high metal rejection rate while maintaining a high water flux.

Description

Positively charged acid- and alkali-resistant composite nanofiltration membrane and its preparation method and application Technical Field

[0001] The present invention relates to the field of membranes, and in particular to a positively charged acid- and alkali-resistant composite nanofiltration membrane and a preparation method and application thereof. Background Art

[0002] Nanofiltration is a pressure-driven membrane separation process. Nanofiltration membranes are intermediate between ultrafiltration and reverse osmosis membranes, with pore sizes of 1-2 nm and a molecular weight cut-off of 200-2000 Da. Due to its advantages, including high separation efficiency, low energy consumption, and minimal footprint, nanofiltration is widely used in wastewater treatment, seawater desalination, chemical separation, food and pharmaceutical processing, and other fields, occupying a key position in modern separation and purification.

[0003] The mechanism of nanofiltration separation is relatively complex. It is generally believed that the nanofiltration separation performance is the result of the combined effect of size screening effect and Donnan effect. In addition to the density of the membrane itself, the nanofiltration separation process has significant charge selectivity. Polyamide thin film composite nanofiltration membrane is the most widely used commercial nanofiltration membrane. The acyl chloride groups on the membrane surface are hydrolyzed to form a large number of carboxyl groups, making the membrane surface negatively charged. Due to the influence of the Donnan effect, the membrane has a low removal rate for high-valent cations, which limits its application in some special fields. The more representative ones include the extraction of Mg from salt lake brine for lithium. 2+ He Li + Positively charged nanofiltration membranes are widely used in the separation of ions, drinking water softening, heavy metal wastewater treatment, crude salt refining, and the concentration and separation of special substances in the food industry. Furthermore, positively charged nanofiltration membranes exhibit superior anti-pollution properties in intercepting positively charged ions and dye molecules. Based on these considerations, high-performance positively charged nanofiltration membranes have promising application prospects.

[0004] In addition to the charge properties of the membrane, some application scenarios also have higher requirements for the acid resistance of the membrane. One application scenario is the wastewater treatment of hydrometallurgical processes. In the hydrometallurgical process, an acidic leachate containing metal cations such as lithium, nickel, cobalt, and manganese is obtained. The positively charged nanofiltration membrane can effectively separate monovalent and high-valent cations, and then use reverse osmosis technology to concentrate and enrich them separately, thereby realizing the comprehensive utilization of resources. In the adsorption-membrane combined lithium extraction process, the desorption process of the adsorbent needs to be carried out under acidic conditions, which makes the original solution of the nanofiltration process acidic. However, under acidic conditions, the C=O bond in the polyamide structure is easily affected by H + The nucleophilic electrons attack the amide bond, causing hydrolysis of the amide bond, which destroys the structure of the membrane separation layer and reduces the retention performance.

[0005] Another application scenario for nanofiltration membranes is the treatment of acidic electroplating wastewater. Electroplating is a process that uses electrochemical technology to control the surface properties of metals and non-metals to achieve ideal surface corrosion resistance, conductivity, and decorative properties. Electroplating is widely used in my country and generates a large amount of wastewater each year, accounting for approximately 20% of total industrial wastewater discharge. Electroplating wastewater contains a large number of toxic and hazardous substances, of which wastewater containing heavy metal ions such as copper, nickel, chromium, and zinc accounts for approximately 40%. The pickling and activation process commonly used in the electroplating process makes this wastewater acidic. The comprehensive treatment of acidic electroplating wastewater is an environmental protection requirement and an effective means of recycling industrial wastewater resources to reduce costs and increase efficiency. Currently, the effective recovery of heavy metal ions in electroplating wastewater has become a key issue for the sustainable development of the electroplating industry.

[0006] Conventional chemical precipitation methods have problems such as large amounts of toxic sludge and low heavy metal resource recovery rates. In comparison, membrane separation technology has the characteristics of high separation efficiency, no secondary pollution, low energy consumption, and easy operation. It has been widely used to recover metal ions such as copper, nickel, chromium, and zinc from electroplating wastewater. Among them, nanofiltration separation has good application prospects in the above process. It is generally believed that nanofiltration separation performance is the result of the combined effects of size screening effect and Donnan effect. In addition to the density of the membrane itself, the nanofiltration separation process has significant charge selectivity. Polyamide thin film composite nanofiltration membrane is the most widely used commercial nanofiltration membrane. The acyl chloride groups on the membrane surface are hydrolyzed to form a large number of carboxyl groups, making the membrane surface negatively charged. Due to the influence of the Donnan effect, the membrane has a low removal rate for high-valent cations, resulting in a low recovery rate for metal ions such as copper, nickel, chromium, and zinc.

[0007] In addition to the charged properties of the membrane, the treatment of acidic electroplating wastewater also has higher requirements for the membrane's acid resistance. Under acidic conditions, the C=O bond in the conventional polyamide nanofiltration membrane structure is easily affected by H + The nucleophilic electrons attack the amide bonds, causing hydrolysis of the amide bonds, which damages the membrane separation layer structure and reduces the retention performance. To ensure the structural stability of the nanofiltration membrane, the influent pH needs to be adjusted, which not only increases the amount of reagents used, but also makes the treatment process more complicated.

[0008] Another application scenario of nanofiltration membranes is the treatment of wastewater from the production of titanium dioxide by the sulfuric acid method. The main component of titanium dioxide is titanium dioxide, which is safe and non-toxic. It is widely used in industrial fields such as coatings, papermaking, printing, chemical fibers, plastics, and rubber, of which coatings account for more than 60%. my country is a major producer of titanium dioxide. Since 2019, the overall scale of titanium dioxide in my country has ranked first in the world, among which the output of titanium dioxide produced by the sulfuric acid method accounts for more than 90% of the total output of titanium dioxide. In the process of producing titanium dioxide by the sulfuric acid method, for every ton of titanium dioxide produced, 8-10 tons of waste sulfuric acid with a concentration of 15-20wt% will be produced. The above-mentioned waste sulfuric acid solution contains about 5wt% of Fe 2+, as well as small amounts of metal sulfates such as aluminum, manganese, calcium, and magnesium. The primary challenge plaguing the development of my country's titanium dioxide industry is how to solve the low-cost treatment and pollution-free discharge of acidic wastewater from the sulfuric acid process.

[0009] At present, domestic titanium dioxide companies mainly use three processes for the comprehensive treatment of waste acid: neutralization, waste acid concentration, and purification. Among them, the alkali neutralization method is the most commonly used waste acid treatment process. However, this process not only cannot separate and recover sulfuric acid and many valuable metals in the waste acid, but also produces titanium gypsum and other solid wastes with calcium sulfate as the main component. The treatment of these solid wastes has become a new problem. With the development of membrane separation technology, nanofiltration has provided a new option for the treatment of titanium dioxide acidic wastewater. Based on size screening and the Donnan effect, nanofiltration membranes can selectively separate monovalent and divalent ions and have good application prospects in the above process. In addition, the membrane treatment process is short, simple to operate, and has low production costs. It can also achieve the recycling of sulfur resources while treating waste acid, which is of great significance to the green and sustainable development of the titanium dioxide industry.

[0010] The problem is that the currently commercialized polyamide nanofiltration membrane is not acid-resistant. Under acidic conditions, the C=O bond in the polyamide structure is easily affected by H + The nucleophilic electrons attack the amide bond, causing hydrolysis of the amide bond, which destroys the structure of the membrane separation layer and significantly reduces the retention performance. In addition, the acyl chloride groups on the surface of the polyamide membrane hydrolyze to form a large number of carboxyl groups, making the membrane surface negatively charged. Due to the influence of the Donnan effect, the membrane is less sensitive to Fe 2+ , Ca 2+ Mg 2+ The removal rate of divalent cations is low. In comparison, positively charged nanofiltration membranes better meet the needs of the above application scenarios.

[0011] Therefore, there is an urgent need to develop a positively charged nanofiltration membrane with simple preparation methods and excellent acid resistance to overcome the limitations of existing nanofiltration membranes in the above-mentioned application scenarios. In particular, there is an urgent need to develop a positively charged nanofiltration membrane with simple preparation methods and excellent acid resistance to enhance the Donnan effect during the nanofiltration separation process and achieve effective separation and recovery of target metal cations.

[0012] Summary of the Invention

[0013] The purpose of the present invention is to overcome the problems of poor acid and alkali tolerance and low metal separation efficiency of existing nanofiltration membranes, and to provide a positively charged acid and alkali resistant composite nanofiltration membrane and its preparation method and application. The specific polymer structure of the separation layer in the positively charged acid and alkali resistant composite nanofiltration membrane has good chemical inertness and a high hydrogen bond density, so that the separation layer has high acid and alkali resistance; and the grafting of specific quaternary ammonium salt groups and / or quaternary phosphonium salt groups can enhance the positive charge of the membrane surface, so that the nanofiltration membrane has excellent retention effect on divalent and high-valent metal ions and a high metal separation coefficient while maintaining a high water flux.

[0014] In order to achieve the above-mentioned object, 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 a bottom layer, a porous middle layer and an acid- and alkali-resistant separation layer in sequence;

[0015] Wherein, the acid-alkali resistant separation layer comprises:

[0016] (1) a quaternary ammonium salt group represented by formula I and / or a quaternary phosphonium salt group represented by formula II grafted onto the acid-base resistant separation layer via a nitrogen atom;

[0017] wherein R1 is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2, R3, and R4 are each independently selected from a C1-C3 alkyl or phenyl group; R5 is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl group; and X is a halogen; or

[0018] (2) a quaternary ammonium salt structure, wherein the quaternary ammonium salt structure comprises the structural formula shown in Formula III and / or the structural formula shown in Formula IV,

[0019] X is a halogen; n is an integer of 0-10; m is an integer of 2-10;

[0020] Wherein, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer.

[0021] 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 comprises the following steps:

[0022] S1. Under stirring, adding a first solution containing a quaternary ammonium halogenate salt and / or a quaternary phosphonium halogenate salt and a catalyst to a second solution containing a polyamine to react and obtain a modified polyamine;

[0023] S2. preparing a porous supporting layer on the bottom layer;

[0024] S3, bringing the surface of the porous support layer into first contact with an aqueous phase containing the modified polyamine of step S1, and then into second contact with an organic phase containing a multi-polar monomer, followed by heat treatment to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane;

[0025] The halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2;

[0026] wherein R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; R5' is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6', R7', and R8' are each independently phenyl or C7-C10 aralkyl group;

[0027] X and X1 are each independently halogen.

[0028] A third aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the method comprises the following steps:

[0029] S1', sequentially preparing a porous support layer and an acid-base resistant separation layer on the bottom layer to obtain a composite membrane;

[0030] S2', contacting the composite membrane with an aqueous solution containing a catalyst, a halogenated quaternary ammonium salt and / or a halogenated quaternary phosphonium salt, and drying to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane;

[0031] The halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2;

[0032] wherein R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; R5' is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6', R7', and R8' are each independently phenyl or C7-C10 aralkyl group;

[0033] X and X1 are each independently halogen.

[0034] A fourth aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the preparation method comprises the following steps:

[0035] S1", preparing a porous supporting layer on the bottom layer;

[0036] S2", the membrane layer obtained in step S1" is first contacted with an aqueous phase of a polyamine containing a tertiary amine group, and second contacted with an organic phase containing a polyvalent polar monomer, and after heat treatment, a composite nanofiltration membrane comprising an acid- and alkali-resistant separation layer is obtained;

[0037] S3″, bringing the composite nanofiltration membrane into a third contact with an organic solution containing a halogenated alkane and a catalyst, and drying to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane.

[0038] The fifth aspect of the present invention provides a positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the above-mentioned preparation method.

[0039] A sixth aspect of the present invention provides an application of the above-mentioned positively charged acid- and alkali-resistant composite nanofiltration membrane in the field of water treatment.

[0040] Through the above technical solution, the positively charged acid- and alkali-resistant composite nanofiltration membrane provided by the present invention and its preparation method and application achieve the following beneficial effects:

[0041] The specific polymer structure in the separation layer of the positively charged acid- and alkali-resistant composite nanofiltration membrane provided by the present invention gives the separation layer good chemical inertness and high hydrogen bond density, so that the separation layer has good acid and alkali resistance; the acid and alkali-resistant separation layer is grafted with specific quaternary ammonium salt groups and / or quaternary phosphonium salt groups by N atoms, or the acid and alkali-resistant separation layer has quaternary ammonium salt groups, so that the positive charge density of the nanofiltration membrane is significantly improved, and the repulsive effect between the positive charges is used to improve the nanofiltration membrane's resistance to Mg. 2+ 、Ni 2+ 、Cu 2+ 、Fe 2+ It can effectively retain divalent and high-valent metal cations, thereby improving the metal separation efficiency of the nanofiltration membrane while maintaining a high water flux of the nanofiltration membrane.

[0042] The preparation method of the positively charged acid- and alkali-resistant composite nanofiltration membrane provided in the second aspect of the present invention comprises the following steps: reacting a quaternary ammonium halogenate and / or a quaternary phosphonium halogenate with a polyamine in a biochemical reaction under the action of an alkaline catalyst to modify the polyamine; the modified polyamine undergoes a polymerization reaction with a polar monomer, and a group with a strong conjugation effect and good chemical inertness is introduced, which greatly improves the acid and alkali resistance of the nanofiltration membrane; at the same time, the positive charge density on the surface of the separation membrane is high, which improves the nanofiltration membrane's resistance to Mg. 2+ The interception effect can improve the magnesium-lithium separation efficiency of the nanofiltration membrane while maintaining a high water flux of the nanofiltration membrane.

[0043] The third aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane. The prepared positively charged acid- and alkali-resistant composite nanofiltration membrane contains good chemically inert groups and has a high hydrogen bond density, so that the nanofiltration membrane has good acid and alkali resistance; and then specific quaternary ammonium salt groups and / or quaternary phosphonium salt groups are grafted to significantly increase the positive charge density on the surface of the nanofiltration membrane, and the repulsive effect between the positive charges is utilized to improve the retention effect of the nanofiltration membrane on divalent and high-valent heavy metal ions while maintaining good water flux.

[0044] The fourth aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, in which halogenated alkanes react biochemically with tertiary amines in the acid- and alkali-resistant separation layer under the action of an alkaline catalyst to convert the tertiary amines into positively charged quaternary ammonium salt groups. The repulsive effect between the positive charges improves the retention capacity of the nanofiltration membrane for divalent and high-valent metal cations, while also enabling the positively charged acid- and alkali-resistant composite nanofiltration membrane to maintain a higher water flux. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG1 is an infrared spectrum of the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in Example A1 of the present invention.

[0046] FIG2 is an XPS spectrum of phosphorus on the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in Example B1 of the present invention.

[0047] FIG3 is an XPS spectrum of nitrogen on the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in Example C1 of the present invention. DETAILED DESCRIPTION

[0048] The endpoints of the ranges and any values ​​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 endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0049] 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 a bottom layer, a porous middle layer, and an acid- and alkali-resistant separation layer in sequence;

[0050] Wherein, the acid-alkali resistant separation layer comprises:

[0051] (1) a quaternary ammonium salt group represented by formula I and / or a quaternary phosphonium salt group represented by formula II grafted onto the acid-base resistant separation layer via a nitrogen atom;

[0052] wherein R1 is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2, R3, and R4 are each independently selected from a C1-C3 alkyl or phenyl group; R5 is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl group; and X is a halogen; or

[0053] (2) a quaternary ammonium salt structure, wherein the quaternary ammonium salt structure comprises the structural formula shown in Formula III and / or the structural formula shown in Formula IV,

[0054] X is a halogen; n is an integer of 0-10; and m is an integer of 2-10.

[0055] In one embodiment of the present invention, a positively charged acid- and alkali-resistant composite nanofiltration membrane is provided, wherein the positively charged acid- and alkali-resistant composite nanofiltration membrane comprises a bottom layer, a porous middle layer, and an acid- and alkali-resistant separation layer in sequence;

[0056] Wherein, the acid-base resistant separation layer is grafted with the quaternary ammonium salt group shown in formula I and / or the quaternary phosphonium salt group shown in formula II by N atom;

[0057] wherein R1 is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2, R3, and R4 are each independently selected from a C1-C3 alkyl or phenyl group; R5 is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl group;

[0058] X is halogen.

[0059] In the present invention, * refers to the quaternary ammonium salt group or quaternary phosphonium salt group and the -NH-, The location where the bond is connected.

[0060] In the present invention, specific quaternary ammonium salt groups and / or quaternary phosphonium 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, and is particularly suitable for the field of water treatment and separation.

[0061] In the present invention, R1 may be a branched alkylene group or a linear alkylene group.

[0062] Further, R1 is a substituted or unsubstituted alkylene or phenylene of C1-C9, and the substituent is a carboxyl group, a nitro group, an amino group, a silyl group, a carbonyl group, an ether group or an ester group; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C9; preferably, R1 is a substituted or unsubstituted alkylene or phenylene of C1-C6; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C6; preferably, R1 is a substituted or unsubstituted alkylene of C1-C3; preferably, R1 is an unsubstituted alkylene of C1-C3.

[0063] Furthermore, R2, R3, and R4 are each independently selected from a C1-C3 alkyl group; preferably, R2, R3, and R4 are each independently CH3.

[0064] Further, R5 is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5 is a C1-C3 alkylene group or a C8-C10 aralkylene group.

[0065] Furthermore, R6, R7, and R8 are each independently a phenyl group or a C8-C10 aralkyl group; preferably, R6, R7, and R8 are each independently a phenyl group.

[0066] Furthermore, X is Cl, Br or I.

[0067] Furthermore, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer, preferably at least one of a polyurea separation layer, a polytriazinamine separation layer and a polysulfonamide separation layer.

[0068] In one embodiment of the present invention, in formula I, R1 is ethylene, Propylene, butylene, pentylene or hexylene; R2, R3, R4 are methyl, ethyl or phenyl; X is Cl, Br or I.

[0069] In one embodiment of the present invention, in formula II, R5 is propylene, ethylene, methylene or R6, R7, and R8 are phenyl groups; and X is Cl, Br, or I.

[0070] According to the present invention, the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-20 at.%, preferably 2-15 at.%.

[0071] According to the present invention, the content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.4-3 at.%, preferably 0.5-2 at.%.

[0072] In one embodiment, the positively charged acid- and alkali-resistant composite nanofiltration membrane is prepared by the preparation method according to the second aspect.

[0073] According to the present invention, the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 10-20 at.%, such as 10-18 at.%, 11-17 at.%.

[0074] In the present invention, when the content of nitrogen atoms in the quaternary ammonium salt groups in 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, so that the positively charged acid- and alkali-resistant composite nanofiltration membrane has a high magnesium-lithium separation coefficient; at the same time, the acid- and alkali-resistant separation layer does not excessively affect the water flux of the membrane through the quaternary ammonium salt groups shown in I grafted by N atoms, so that the membrane maintains good water flux.

[0075] Furthermore, the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 10-15 at.%.

[0076] According to the present invention, the content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.4-3 at.%, such as 0.4-2.5 at.%, 0.8-2.2 at.%.

[0077] In the present invention, when the content of phosphorus atoms in 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, so that the positively charged acid- and alkali-resistant composite nanofiltration membrane has a high magnesium-lithium separation coefficient; at the same time, the acid- and alkali-resistant separation layer does not excessively affect the water flux of the membrane through the quaternary phosphonium salt group shown in the N-atom grafting formula II, so that the membrane maintains good water flux.

[0078] Furthermore, the content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-2 at.%.

[0079] According to the present invention, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0-30 mV.

[0080] In the present invention, when the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above range, it indicates that the composite nanofiltration membrane has a high surface electrode potential and can better exclude Mg when used for lithium-magnesium separation in an acid- and alkali-resistant environment. 2+ , so that Mg in the liquid 2+ It is not easy to pass through the separation membrane, and it can make Li + As much as possible can pass through the membrane to achieve a higher magnesium-lithium separation efficiency.

[0081] In the present 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.

[0082] Furthermore, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 5-25 mV.

[0083] 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.

[0084] In the present invention, when the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above range, it indicates that the positively charged acid- and alkali-resistant composite nanofiltration membrane has a high density and can better exclude Mg when used for lithium-magnesium separation in an acid- and alkali environment. 2+ , so that Mg in the liquid 2+ It is not easy to pass through the separation membrane, and it can make Li + As much as possible, so as to obtain a higher magnesium-lithium separation efficiency.

[0085] Furthermore, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.2-0.3 nm.

[0086] According to the present invention, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 30-80°.

[0087] In the present invention, the positively charged acid- and alkali-resistant composite nanofiltration membrane has a contact angle within the range of the present invention, which indicates that the positively charged acid- and alkali-resistant composite nanofiltration membrane has good hydrophilicity and can enable the composite nanofiltration membrane to have a high water flux.

[0088] Furthermore, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 30-60°.

[0089] In the present invention, there is no specific limitation on the bottom layer and the porous support layer, and they can be made of various existing materials that have a certain strength and can be used for nanofiltration and reverse osmosis membranes.

[0090] In the present invention, the bottom layer is a non-woven fabric, preferably polyester and / or polyethylene.

[0091] In the present invention, the porous support layer material is at least one selected from polyethersulfone, polysulfone, polyarylether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride and polyaryletherketone.

[0092] In the present invention, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer, preferably at least one of a polyurea separation layer, a polytriazinamine separation layer and a polysulfonamide separation layer.

[0093] Furthermore, the acid-base resistant separation layer is formed by interfacial polymerization of a polyamine modified with a quaternary ammonium halogenate salt and / or a quaternary phosphonium halogenate salt and a polyvalent polar monomer.

[0094] According to the present invention, the polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides.

[0095] In the present invention, polyamine reacts with at least one of polyisocyanate, triazine compound containing at least two C-Cl bonds and polysulfonyl chloride to introduce groups with strong conjugation effect and good chemical inertness into the nanofiltration membrane. The resulting polymer has good structural stability in both acidic and alkaline environments, greatly improving the acid and alkali resistance of the nanofiltration membrane, making it particularly suitable for acidic environments.

[0096] According to the present invention, the polyamine is at least one of polyethyleneimine, polyethyleneamine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylenepolyamine, piperazine, m-phenylenediamine and p-phenylenediamine; preferably polyethyleneimine and / or polyethylenepolyamine.

[0097] According to the present invention, the polyisocyanate is selected from at least one of meta-phenylenediisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylenediisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenylenediisocyanate, 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-meta-phenylenediisocyanate; preferably 1,4-phenylenediisocyanate and / or 1,3-phenylenediisocyanate.

[0098] 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.

[0099] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride.

[0100] In the present invention, the halogenated quaternary ammonium salt has the structure shown in Formula 1;

[0101] Wherein, R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; and X and X1 are each independently halogen.

[0102] Further, R1' is a substituted or unsubstituted C1-C9 alkylene or phenylene, and the substituent is a hydroxyl group, a carboxyl group, a nitro group, an amino group, a silyl group, a carbonyl group, an ether group or an ester group; preferably, R1' is an unsubstituted C1-C9 alkylene or phenylene; preferably, R1 is a substituted or unsubstituted C1-C6 alkylene or phenylene; preferably, R1 is an unsubstituted C1-C6 alkylene or phenylene; preferably, R1 is a substituted or unsubstituted C1-C3 alkylene; preferably, R1 is an unsubstituted C1-C3 alkylene.

[0103] Furthermore, R2', R3', and R4' are each independently selected from a C1-C3 alkyl group; preferably, R2', R3', and R4' are each independently CH3.

[0104] Furthermore, X is Cl, Br or I.

[0105] Furthermore, X1 is Cl, Br or I.

[0106] In one embodiment of the present invention, R1' is a C1-C3 unsubstituted alkylene group.

[0107] In a specific embodiment of the present invention, R1' is a C1-C3 substituted alkylene group, wherein the substituent is a hydroxyl group or a carboxyl group, preferably a hydroxyl group.

[0108] In a specific embodiment of the present invention, the halogenated quaternary 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.

[0109] In the present invention, the halogenated quaternary phosphonium salt has a structure shown in Formula 2;

[0110] wherein R5′ is a C1-C6 alkylene group, a phenylene group, or a C7-C10 aralkylene group; R6′, R7′, and R8′ are each independently a phenyl group or a C7-C10 aralkyl group;

[0111] X and X1 are each independently halogen.

[0112] Further, R5' is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5' is a C1-C3 alkylene group or a C8-C10 aralkylene group.

[0113] Furthermore, R6', R7', and R8' are each independently a phenyl group or a C8-C10 aralkyl group; preferably, R6', R7', and R8' are each independently a phenyl group.

[0114] Furthermore, X is Cl, Br or I.

[0115] Furthermore, X1 is F, Cl, Br or I.

[0116] In a specific embodiment of the present invention, the halogenated quaternary phosphonium salt is selected from 3-bromopropyltriphenylphosphonium bromide, 2-bromoethyltriphenylphosphonium bromide, 1-bromoethyltriphenylphosphonium bromide, bromomethyltriphenylphosphonium bromide, 3-chloropropyltriphenylphosphonium chloride, 2-chloroethyltriphenylphosphonium chloride, 1-chloroethyltriphenylphosphonium chloride, chloromethyltriphenylphosphonium chloride, 3-iodopropyltriphenylphosphonium iodide, 2-iodoethyltriphenylphosphonium iodide, 1-iodoethyltriphenylphosphonium iodide, iodomethyltriphenylphosphonium bromide, 4 At least one of bromomethylbenzyltriphenyl phosphonium bromide, 2-bromomethylbenzyltriphenyl phosphonium bromide, 3-bromomethylbenzyltriphenyl phosphonium bromide, 4-chloromethylbenzyltriphenyl phosphonium chloride, 2-chloromethylbenzyltriphenyl phosphonium chloride, 3-chloromethylbenzyltriphenyl phosphonium chloride, 4-iodomethylbenzyltriphenyl phosphonium iodide, 2-iodomethylbenzyltriphenyl phosphonium iodide and 3-iodomethylbenzyltriphenyl phosphonium iodide, preferably 3-bromopropyltriphenyl phosphonium bromide and / or 4-bromomethylbenzyltriphenyl phosphonium bromide.

[0117] According to the present invention, the present invention does not particularly limit the thickness of the bottom layer, the porous support layer, and the acid- and alkali-resistant separation layer, which can be conventionally selected in the field. However, in order to enable these three layers to play a better synergistic role, the obtained positively charged acid- and alkali-resistant composite nanofiltration membrane can better have an excellent magnesium-lithium separation coefficient and a higher 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; the thickness of the acid- and alkali-resistant separation layer is 10-500 nm, preferably 50-300 nm.

[0118] In one embodiment of the present invention, a positively charged acid- and alkali-resistant composite nanofiltration membrane is provided, wherein the positively charged acid- and alkali-resistant composite nanofiltration membrane comprises a bottom layer, a porous support layer, and an acid- and alkali-resistant separation layer stacked in sequence;

[0119] Wherein, the acid-base resistant separation layer contains a quaternary ammonium salt structure.

[0120] In the present invention, the acid- and alkali-resistant separation layer of the positively charged acid- and alkali-resistant composite nanofiltration membrane contains a quaternary ammonium salt structure, which significantly improves the positive charge density on the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane, and the repulsion effect brought about by the positive charges improves the retention effect of the positively charged acid- and alkali-resistant composite nanofiltration membrane on divalent iron ions, thereby improving the retention capacity of the positively charged acid- and alkali-resistant composite nanofiltration membrane for divalent and high-valent metal cations while maintaining the high water flux of the positively charged acid- and alkali-resistant composite nanofiltration membrane.

[0121] According to the present invention, the quaternary ammonium salt structure comprises the structural formula shown in formula III and / or the structural formula shown in formula IV,

[0122] X is a halogen; n is an integer of 0-10; and m is an integer of 2-10.

[0123] In the present invention, * refers to the position where the quaternary ammonium salt structure is bonded to the non-H structure portion in the acid- and alkali-resistant separation layer.

[0124] Furthermore, X is Cl, Br or I.

[0125] Furthermore, n is an integer of 0-7; m is an integer of 2-8.

[0126] According to the present invention, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer.

[0127] In the present invention, a specific acid- and alkali-resistant separation layer introduces inert chemical groups into the composite nanofiltration membrane, thereby improving the stability of the composite nanofiltration membrane and enabling the composite nanofiltration membrane of the present invention to be used in acidic / alkaline environments, thereby expanding its scope of use.

[0128] Furthermore, the acid- and alkali-resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazineamine separation layer, and a polysulfonamide separation layer.

[0129] In one embodiment, the positively charged acid- and alkali-resistant composite nanofiltration membrane is prepared by the preparation method according to the fourth aspect.

[0130] In formula III and formula IV, the N atom and the structural part connected to it via * are derived from the polymer in the acid-base resistant separation layer, -(CH2) n -CH3 and -(CH2) m - The structural moiety is derived from a haloalkane.

[0131] According to the present invention, the content of nitrogen atoms in the quaternary ammonium salt structure in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-5 at.%, such as 1-4.5 at.%, 1.5-4 at.%.

[0132] In the present invention, when the content of nitrogen atoms in the quaternary ammonium salt structure meets the above range, it indicates that the positively charged acid- and alkali-resistant composite nanofiltration membrane contains more quaternary ammonium salt groups, which can enable the composite nanofiltration membrane to have a higher surface positive potential.

[0133] Furthermore, the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 2-5 at.%.

[0134] In the present invention, the surface Zeta potential of the nanofiltration membrane refers to the surface Zeta potential at pH=7.

[0135] According to the present invention, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0-30 mV.

[0136] In the present invention, when the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above range, it indicates that the composite nanofiltration membrane has a high surface electrode potential. When it is used for the separation of divalent and high-valent metal cations, it can better repel metal cations, making it difficult for metal cations in the liquid to pass through the separation membrane, thereby obtaining a higher retention capacity for divalent and high-valent metal cations.

[0137] Furthermore, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 5-20 mV.

[0138] According to the present invention, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.1-0.5 nm.

[0139] In the present invention, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is tested using a PEG solute transfer method.

[0140] In the present invention, when the average pore size of the nanofiltration membrane meets the above range, the positively charged acid- and alkali-resistant composite nanofiltration membrane has a high density, can better exclude divalent and high-valent metal cations, and makes it difficult for divalent and high-valent metal cations in the liquid to pass through the separation membrane, thereby obtaining a higher retention capacity for divalent and high-valent metal cations.

[0141] Furthermore, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.15-0.3 nm.

[0142] According to the present invention, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 20°-80°.

[0143] In the present invention, when the contact angle of the nanofiltration membrane satisfies the above range, it indicates that the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane has good hydrophilicity, thereby obtaining a higher water flux.

[0144] Furthermore, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 30°-60°.

[0145] In the present invention, there is no specific limitation on the bottom layer and the porous support layer, and they can be made of various existing materials that have a certain strength and can be used for nanofiltration and reverse osmosis membranes.

[0146] In the present invention, the bottom layer is a non-woven fabric material, preferably polyester and / or polyethylene.

[0147] In the present invention, the porous support layer material may be at least one of polyethersulfone, polysulfone, polyarylether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride and polyaryletherketone.

[0148] According to the present invention, the present invention does not particularly limit the thickness of the bottom layer, the porous support layer, and the acid- and alkali-resistant separation layer, which can be conventionally selected in the field. However, in order to enable these three layers to play a better synergistic role, the obtained composite nanofiltration membrane can better have excellent retention capacity for divalent and high-valent metal cations and higher 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; the thickness of the acid- and alkali-resistant separation layer is 10-500 nm, preferably 50-300 nm.

[0149] In a particularly preferred embodiment, the thickness of the acid-base resistant separation layer is 120-180 nm.

[0150] 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 comprises the following steps:

[0151] S1. Adding a first solution containing a quaternary ammonium halogenate and / or a quaternary phosphonium halogenate and a catalyst to a second solution of a polyamine under stirring to react and obtain a modified polyamine;

[0152] S2. preparing a porous supporting layer on the bottom layer;

[0153] S3, bringing the surface of the porous support layer into first contact with an aqueous phase containing the modified polyamine of step S1, and then into second contact with an organic phase containing a multi-polar monomer, followed by heat treatment to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane;

[0154] The halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2;

[0155] wherein R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; R5' is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6', R7', and R8' are each independently phenyl or C7-C10 aralkyl group;

[0156] X and X1 are each independently halogen.

[0157] In the present invention, in the presence of an alkaline catalyst, a polyamine is grafted with specific quaternary ammonium salt groups and / or quaternary phosphonium salt groups to obtain a positively charged polyamine; after the polyamine and the polar monomer are interfacially polymerized, groups with good chemical inertness are introduced, thereby improving the positive charge density and acid-base tolerance of the nanofiltration membrane surface, so that the nanofiltration membrane has a high magnesium-lithium separation coefficient while maintaining a high water flux. In an acidic or alkaline environment, it can still maintain a high water flux and have relatively good magnesium-lithium separation performance.

[0158] Further, R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene, and the substituent is a hydroxyl group, a carboxyl group, a nitro group, an amino group, a silyl group, a carbonyl group, an ether group or an ester group; preferably, R1' is a C1-C9 unsubstituted alkylene or phenylene; preferably, R1' is a C1-C6 substituted or unsubstituted alkylene or phenylene; preferably, R1' is a C1-C6 unsubstituted alkylene or phenylene; preferably, R1' is a C1-C3 substituted or unsubstituted alkylene; preferably, R1' is a C1-C3 unsubstituted alkylene.

[0159] Furthermore, R2', R3', and R4' are each independently selected from a C1-C3 alkyl group; preferably, R2', R3', and R4' are each independently CH3.

[0160] Further, R5' is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5' is a C1-C3 alkylene group or a C8-C10 aralkylene group.

[0161] Furthermore, R6', R7', and R8' are each independently a phenyl group or a C8-C10 aralkyl group; preferably, R6', R7', and R8' are each independently a phenyl group.

[0162] Furthermore, R1' is a C1-C3 unsubstituted alkylene group.

[0163] Furthermore, R1' is a C1-C3 substituted alkylene group, wherein the substituent is a hydroxyl group or a carboxyl group, preferably a hydroxyl group.

[0164] Furthermore, X is Cl, Br or I.

[0165] Furthermore, X1 is F, Cl, Br or I.

[0166] According to the present invention, the halogenated quaternary 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.

[0167] According to the present invention, the halogenated quaternary phosphonium salt is selected from 3-bromopropyltriphenylphosphonium bromide, 2-bromoethyltriphenylphosphonium bromide, 1-bromoethyltriphenylphosphonium bromide, bromomethyltriphenylphosphonium bromide, 3-chloropropyltriphenylphosphonium chloride, 2-chloroethyltriphenylphosphonium chloride, 1-chloroethyltriphenylphosphonium chloride, chloromethyltriphenylphosphonium chloride, 3-iodopropyltriphenylphosphonium iodide, 2-iodoethyltriphenylphosphonium iodide, 1-iodoethyltriphenylphosphonium iodide, iodomethyltriphenylphosphonium bromide, 4-bromomethyltriphenylphosphonium chloride, At least one of benzyltriphenyl phosphonium bromide, 2-bromomethylbenzyltriphenyl phosphonium bromide, 3-bromomethylbenzyltriphenyl phosphonium bromide, 4-chloromethylbenzyltriphenyl phosphonium chloride, 2-chloromethylbenzyltriphenyl phosphonium chloride, 3-chloromethylbenzyltriphenyl phosphonium chloride, 4-iodomethylbenzyltriphenyl phosphonium iodide, 2-iodomethylbenzyltriphenyl phosphonium iodide and 3-iodomethylbenzyltriphenyl phosphonium iodide; preferably 3-bromopropyltriphenyl phosphonium bromide and / or 4-bromomethylbenzyltriphenyl phosphonium bromide.

[0168] According to the present invention, the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the first solution is 0.5 wt % to 20 wt %.

[0169] In the present invention, when the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the first solution in step S1 is controlled to meet the above 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 phosphonium salt groups grafted on the surface of the acid- and alkali-resistant separation layer will not excessively affect the water flux of the membrane, thereby maintaining good water flux.

[0170] In the present invention, if the concentration of the quaternary ammonium halogenate and / or the quaternary phosphonium halogenate in the first solution in step S1 is lower than the above range, the quaternary ammonium halogenate and / or the quaternary phosphonium halogenate in the first solution cannot fully react with the polyamine in the second solution; if the concentration of the quaternary ammonium halogenate and / or the quaternary phosphonium halogenate in the first solution in step S1 is higher than the above range, the quaternary ammonium halogenate and / or the quaternary phosphonium halogenate excessively consumes 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.

[0171] Furthermore, the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the first solution is 1 wt%-10 wt%.

[0172] 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.

[0173] According to the present invention, the concentration of the catalyst in the first solution is 0.01 wt%-5 wt%.

[0174] In the present invention, when the concentration of the catalyst in the first solution in step S1 is controlled to meet the above range, it can be ensured that the halogenated quaternary ammonium salt and / or halogenated quaternary phosphonium salt in the first solution can fully react with the polyamine in the second solution and have a high reaction rate, and the quaternary ammonium salt and / or quaternary phosphonium salt structure is introduced into the molecular structure of the polyamine in a relatively short time.

[0175] Furthermore, the concentration of the catalyst in the first solution is 0.1 wt%-1 wt%.

[0176] According to the present invention, in step S1, the polyamine is selected from at least one of polyethyleneimine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylenepolyamine, piperazine, m-phenylenediamine and p-phenylenediamine; preferably polyethyleneimine and / or polyethylenepolyamine.

[0177] According to the present invention, the concentration of the polyamine in the second solution is 1 wt%-20 wt%, preferably 5 wt%-10 wt%.

[0178] 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 halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt, and the catalyst is 1-1000:1-100:1.

[0179] In the present invention, when the mass ratio of the polyamine, the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt and the catalyst is controlled to meet the above range, it can be ensured that the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium 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 phosphonium salt and / or quaternary ammonium salt structures can be introduced into the polyamine in a relatively short time.

[0180] Furthermore, the amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt, and the catalyst is 1-200:1-50:1.

[0181] According to the present invention, in step S1, the reaction conditions include: reaction temperature of 25-90° C.; reaction time of 1-48 h.

[0182] In the present invention, when the conditions for controlling the reaction in step S1 meet the above range, it can ensure that the halogenated quaternary ammonium salt and / or halogenated quaternary phosphonium 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 phosphonium salt and / or quaternary ammonium salt structures can be introduced into the polyamine in a relatively short time.

[0183] Furthermore, in step S1, the reaction conditions include: reaction temperature of 40-60° C.; reaction time of 6-24 h.

[0184] In a preferred embodiment of the present invention, the first solution is added dropwise to the second solution containing the polyamine. By adding the first solution dropwise to the second solution containing the polyamine, the reaction rate can be controlled and the first solution and the second solution are fully mixed to ensure that the quaternary ammonium halide and / or quaternary phosphonium halide reacts fully with the polyamine.

[0185] According to the present invention, the dropping speed is 0.5-5 mL / min, preferably 1-3 mL / min.

[0186] In the present invention, there is no particular limitation on the method for preparing the porous support layer in the bottom layer in step S2. It can be prepared by conventional methods in the art, preferably by a 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 by phase inversion.

[0187] In the present invention, the phase inversion method can preferably be: dissolving the supporting layer polymer material in a solvent to obtain a polymer solution with a concentration of 10-20wt%, and degassing at 20-40°C for 10-180min; then coating the polymer solution on the bottom layer to obtain an initial membrane, and then soaking it in water at a temperature of 10-30°C for 10-60min to obtain the supporting layer polymer porous membrane through the phase inversion layer.

[0188] The solvent may be at least one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide.

[0189] In the present invention, the acid-base resistant separation layer is formed by interfacial polymerization of a polyamine modified with a quaternary ammonium halogenate salt and / or a quaternary phosphonium halogenate salt and a polyvalent polar monomer.

[0190] According to the present invention, the polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides.

[0191] According to the present invention, the polyisocyanate is selected from at least one of meta-phenylenediisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylenediisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenylenediisocyanate, 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-meta-phenylenediisocyanate; preferably 1,4-phenylenediisocyanate and / or 1,3-phenylenediisocyanate.

[0192] 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.

[0193] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride.

[0194] According to the present invention, the concentration of the modified polyamine in the aqueous phase is 0.1 wt%-10 wt%, preferably 0.5 wt%-2.5 wt%.

[0195] According to the present invention, the concentration of the multi-polar monomer in the organic phase is 0.01 wt%-2 wt%, preferably 0.05-1 wt%.

[0196] In the present invention, there is no particular limitation on the type of solvent of the organic phase, as long as it can dissolve the polyacyl chloride. Preferably, the solvent of the organic phase is one or more of n-hexane, dodecane, n-heptane, and alkane solvent oil (Isopar E, Isopar G, Isopar H, Isopar L, and Isopar M).

[0197] In the present invention, in step S3, there is no special limitation on the conditions for the interfacial polymerization between the modified polyamine and the polypolar monomer, and it can be carried out according to conventional conditions in the art. However, in order to enable the three layers to play a better synergistic role, the obtained positively charged acid- and alkali-resistant composite nanofiltration membrane can better have an excellent magnesium-lithium separation coefficient and a higher water flux. Preferably, the first contact time is 5-100s, preferably 10-60s; the second contact time is 10-200s, preferably 20-120s; the heat treatment conditions include: the heat treatment temperature is 40-150°C, preferably 50-120°C; the heat treatment time is 0.5-10min, preferably 1-5min.

[0198] A third aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the method comprises the following steps:

[0199] S1', sequentially preparing a porous support layer and an acid-base resistant separation layer on the bottom layer to obtain a composite membrane;

[0200] S2', contacting the composite membrane with an aqueous solution containing a catalyst, a halogenated quaternary ammonium salt and / or a halogenated quaternary phosphonium salt, and drying to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane.

[0201] In the present invention, the positively charged acid- and alkali-resistant composite nanofiltration membrane contains chemically inert groups that can be used in acidic / alkaline environments, and is grafted with specific quaternary ammonium salt groups and / or quaternary phosphonium salt groups, so that the nanofiltration membrane has a high retention rate for divalent and high-valent metal ions while maintaining a high water flux.

[0202] In the present invention, the ratio of the amount of the aqueous solution containing the catalyst, the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt to the membrane area of ​​the composite membrane is 0.1-1 g / cm 2 , preferably 0.3-0.5g / cm 2 .

[0203] In the present invention, there is no specific limitation on the bottom layer and the porous support layer, and they can be made of various existing materials that have a certain strength and can be used for nanofiltration and reverse osmosis membranes.

[0204] In the present invention, the bottom layer is a non-woven fabric, preferably polyester and / or polyethylene.

[0205] In the present invention, the porous support layer material is at least one selected from polyethersulfone, polysulfone, polyarylether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride and polyaryletherketone.

[0206] In the present invention, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer, preferably at least one of a polyurea separation layer, a polytriazinamine separation layer and a polysulfonamide separation layer.

[0207] Furthermore, the acid-base resistant separation layer is generated by interfacial polymerization of polyamines and polypolar monomers.

[0208] In the present invention, the interfacial polymerization process includes first contacting the surface of the porous support layer with an aqueous phase containing polyamines, then second contacting the surface of the porous support layer with an organic phase containing polyvalent polar monomers, and then performing heat treatment.

[0209] In one embodiment of the present invention, step S1' includes:

[0210] S1-1', preparing a porous supporting layer on the bottom layer; and

[0211] S1-2', bringing the surface of the porous support layer into first contact with an aqueous phase containing polyamines, and then into second contact with an organic phase containing polyvalent polar monomers, followed by heat treatment to obtain a composite membrane.

[0212] The reaction conditions for the above first contact, second contact and heat treatment are conventional conditions in the art. Preferably, the time of the first contact is 5-100s, preferably 10-60s; the temperature of the first contact is 15-50°C, preferably 20-40°C; the time of the second contact is 10-200s, preferably 20-120s; the temperature of the second contact is 15-40°C, preferably 20-30°C; the conditions of the heat treatment include: the heat treatment time is 0.5-10min, preferably 1-5min; the heat treatment temperature is 40-150°C, preferably 50-120°C.

[0213] According to the present invention, the polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides.

[0214] Polyamine reacts with at least one of polyisocyanate, triazine compound containing at least two C-Cl bonds and polysulfonyl chloride to introduce groups with strong conjugation effect and good chemical inertness into the nanofiltration membrane. The resulting acid-base resistant separation layer contains a polymer with good structural stability in both acidic and alkaline environments, thereby greatly improving the acid-base resistance of the nanofiltration membrane and making it particularly suitable for acidic environments.

[0215] According to the present invention, the polyamine is at least one of polyethyleneimine, polyethyleneamine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylenepolyamine, piperazine, m-phenylenediamine and p-phenylenediamine; preferably polyethyleneimine and / or polyethylenepolyamine.

[0216] According to the present invention, the polyisocyanate is selected from at least one of meta-phenylenediisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylenediisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(isocyanatophenyl), 1,3-phenylenediisocyanate, 3,3'-dichloro-4,4'-diisocyanatebiphenyl, dicyclohexylmethane-4,4'-diisocyanate, trimethylhexamethylene diisocyanate, L-lysine-ethyl ester-diisocyanate, 1,4-cyclohexyl diisocyanate and 4-chloro-6-methyl-meta-phenylenediisocyanate; preferably 1,4-phenylenediisocyanate and / or 1,3-phenylenediisocyanate.

[0217] 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.

[0218] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride.

[0219] According to the present invention, the weight ratio of the polyamine in the aqueous phase to the polyvalent polar monomer in the organic phase is 2-200:1, preferably 5-80:1.

[0220] According to the present invention, the concentration of the polyamine in the aqueous phase is 0.1 wt%-10 wt%, preferably 0.5 wt%-2.5 wt%.

[0221] According to the present invention, the concentration of the multi-polar monomer in the organic phase is 0.01 wt%-2 wt%, preferably 0.05 wt%-1 wt%.

[0222] In the present invention, the ratio of the amount of the aqueous phase containing the polyamine to the membrane area of ​​the porous support layer is 0.05-0.5 g / cm 2 , preferably 0.1-0.3g / cm 2 .

[0223] In the present invention, the ratio of the amount of the organic phase containing the multi-polar monomer to the membrane area of ​​the porous support layer is 0.03-0.3 g / cm 2 , preferably 0.05-0.2g / cm 2 .

[0224] According to the present invention, wherein the halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2;

[0225] wherein R1' is a C1-C9 substituted or unsubstituted alkylene group; preferably, R1' is a C1-C9 unsubstituted alkylene group; R2', R3', and R4' are each independently selected from a C1-C3 alkyl group or a phenyl group; R5' is a C1-C6 alkylene group, a phenylene group, or a C7-C10 aralkylene group; R6', R7', and R8' are each independently selected from a phenyl group or a C7-C10 aralkyl group;

[0226] X and X1 are each independently halogen.

[0227] Further, R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene, and the substituent is a hydroxyl group, a carboxyl group, a nitro group, an amino group, a silyl group, a carbonyl group, an ether group or an ester group; preferably, R1' is a C1-C9 unsubstituted alkylene or phenylene; preferably, R1' is a C1-C6 substituted or unsubstituted alkylene or phenylene; preferably, R1' is a C1-C6 unsubstituted alkylene or phenylene; preferably, R1' is a C1-C3 substituted or unsubstituted alkylene; preferably, R1' is a C1-C3 unsubstituted alkylene.

[0228] Furthermore, R2', R3', and R4' are each independently selected from a C1-C3 alkyl group; preferably, R2', R3', and R4' are each independently CH3.

[0229] Further, R5' is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5' is a C1-C3 alkylene group or a C8-C10 aralkylene group.

[0230] Furthermore, R6', R7', and R8' are each independently a phenyl group or a C8-C10 aralkyl group; preferably, R6', R7', and R8' are each independently a phenyl group.

[0231] Furthermore, R1' is a C1-C3 unsubstituted alkylene group.

[0232] Furthermore, R1' is a C1-C3 substituted alkylene group, wherein the substituent is a hydroxyl group or a carboxyl group, preferably a hydroxyl group.

[0233] Furthermore, X is Cl, Br or I.

[0234] Furthermore, X1 is F, Cl, Br or I.

[0235] In one embodiment of the present invention, in Formula 1, R1' is ethylene, propylene, butylene, pentylene or hexylene; R2', R3', R4' are methyl, ethyl or phenyl; X is Cl, Br or I.

[0236] In one embodiment of the present invention, in formula 2, R5' is propylene, ethylene, methylene or R6', R7', and R8' are phenyl groups; and X is Cl, Br, or I.

[0237] According to the present invention, in step S2', the contacting method is a common contacting method in the art, preferably immersion.

[0238] According to the present invention, the contact time is 10s-10min; the contact temperature is 20-80°C.

[0239] In the present invention, when the contact conditions are controlled to meet the above range, it can ensure that the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt react fully on the membrane surface, thereby increasing the positive charge density on the membrane surface, so that the nanofiltration membrane has good retention capacity for divalent and high-valent metal ions while maintaining a high water flux; and the process is compatible with the existing industrial continuous production process and is suitable for scale-up preparation.

[0240] Furthermore, the contact time is 20s-1min; the contact temperature is 30-50°C.

[0241] According to the present invention, the drying time is 0.5-10 minutes; the drying temperature is 40-80°C.

[0242] In the present invention, when the drying conditions are controlled to meet the above range, the grafting reaction of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt on the surface of the acid- and alkali-resistant separation layer can be promoted, and the positive charge density on the membrane surface is further improved, thereby further enhancing the nanofiltration membrane's ability to retain divalent and high-valent metal ions and ensuring a higher water flux; at the same time, the process is compatible with existing industrial continuous production processes and can be scaled up for batch production.

[0243] Furthermore, the drying time is 3-5 minutes; the drying temperature is 50-70°C.

[0244] According to the present invention, in step S2', 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.

[0245] According to the present invention, the halogenated quaternary 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.

[0246] According to the present invention, the halogenated quaternary phosphonium salt is selected from 3-bromopropyltriphenylphosphonium bromide, 2-bromoethyltriphenylphosphonium bromide, 1-bromoethyltriphenylphosphonium bromide, bromomethyltriphenylphosphonium bromide, 3-chloropropyltriphenylphosphonium chloride, 2-chloroethyltriphenylphosphonium chloride, 1-chloroethyltriphenylphosphonium chloride, chloromethyltriphenylphosphonium chloride, 3-iodopropyltriphenylphosphonium iodide, 2-iodoethyltriphenylphosphonium iodide, 1-iodoethyltriphenylphosphonium iodide, iodomethyltriphenylphosphonium bromide, 4-bromomethyltriphenylphosphonium chloride, At least one of benzyltriphenyl phosphonium bromide, 2-bromomethylbenzyltriphenyl phosphonium bromide, 3-bromomethylbenzyltriphenyl phosphonium bromide, 4-chloromethylbenzyltriphenyl phosphonium chloride, 2-chloromethylbenzyltriphenyl phosphonium chloride, 3-chloromethylbenzyltriphenyl phosphonium chloride, 4-iodomethylbenzyltriphenyl phosphonium iodide, 2-iodomethylbenzyltriphenyl phosphonium iodide and 3-iodomethylbenzyltriphenyl phosphonium iodide; preferably 3-bromopropyltriphenyl phosphonium bromide and / or 4-bromomethylbenzyltriphenyl phosphonium bromide.

[0247] According to the present invention, the concentration of the catalyst in the aqueous solution is 0.01 wt%-5 wt%.

[0248] In the present invention, when the concentration of the catalyst in the aqueous solution is controlled to meet the above range, the reaction of the grafted quaternary ammonium halides and / or quaternary phosphonium halides on the membrane surface can proceed more fully without causing waste of the catalyst, thereby improving the economic efficiency of the process.

[0249] Furthermore, the concentration of the catalyst in the aqueous solution is 0.1 wt%-1 wt%.

[0250] According to the present invention, the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the aqueous solution is 1 wt%-20 wt%.

[0251] In the present invention, when the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the aqueous solution is controlled to meet the above range, the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt can be fully grafted on the membrane surface, thereby increasing the surface charge density of the positively charged acid- and alkali-resistant composite nanofiltration membrane, thereby increasing the retention rate of the nanofiltration membrane for divalent and high-valent metal ions, while causing no waste and improving the economic efficiency of the process.

[0252] Furthermore, the concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the aqueous solution is 5 wt%-10 wt%.

[0253] In one embodiment of the present invention, a positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method of the third aspect is provided.

[0254] According to the present invention, the positively charged acid- and alkali-resistant composite nanofiltration membrane comprises, in sequence, a bottom layer, a porous support layer, and an acid- and alkali-resistant separation layer;

[0255] Wherein, the acid-base resistant separation layer is grafted with a quaternary ammonium salt group represented by formula I and / or a quaternary phosphonium salt group represented by formula II via N atoms;

[0256] wherein R1 is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2, R3, and R4 are each independently selected from a C1-C3 alkyl or phenyl group; R5 is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl group;

[0257] X is halogen.

[0258] Further, R1 is a substituted or unsubstituted alkylene or phenylene of C1-C9, and the substituent is a carboxyl group, a nitro group, an amino group, a silyl group, a carbonyl group, an ether group or an ester group; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C9; preferably, R1 is a substituted or unsubstituted alkylene or phenylene of C1-C6; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C6; preferably, R1 is a substituted or unsubstituted alkylene of C1-C3; preferably, R1 is an unsubstituted alkylene of C1-C3.

[0259] Furthermore, R2, R3, and R4 are each independently selected from a C1-C3 alkyl group; preferably, R2, R3, and R4 are each independently CH3.

[0260] Further, R5 is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5 is a C1-C3 alkylene group or a C8-C10 aralkylene group.

[0261] Furthermore, R6, R7, and R8 are each independently a phenyl group or a C8-C10 aralkyl group; preferably, R6, R7, and R8 are each independently a phenyl group.

[0262] Furthermore, X is Cl, Br or I.

[0263] In one embodiment, the positively charged acid- and alkali-resistant composite nanofiltration membrane is prepared by the preparation method according to the third aspect.

[0264] According to the present invention, the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-6 at.%, such as 1-5.5 at.%, 1.5-5 at.%.

[0265] In the present invention, when the content of nitrogen atoms in the quaternary ammonium salt groups in 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, so that the positively charged acid- and alkali-resistant composite nanofiltration membrane has good retention performance for divalent and high-valent heavy metal cations; at the same time, the acid- and alkali-resistant separation layer will not excessively affect the water flux of the membrane through the quaternary ammonium salt groups shown in nitrogen atom grafting formula I.

[0266] Furthermore, the content of nitrogen atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 2-5 at.%.

[0267] According to the present invention, the content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.4-3 at.%, such as 0.4-2.5 at.%, 0.8-2.2 at.%.

[0268] In the present invention, when the content of phosphorus atoms in 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, so that the positively charged acid- and alkali-resistant composite nanofiltration membrane has good retention performance for divalent and high-valent heavy metal cations; at the same time, the acid- and alkali-resistant separation layer will not excessively affect the water flux of the membrane through the quaternary phosphonium salt group shown in nitrogen atom grafting formula II, and the membrane can have good separation efficiency.

[0269] Furthermore, the content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.5-2 at.%.

[0270] According to the present invention, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0-30 mV.

[0271] In the present 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.

[0272] In the present invention, when the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane meets the above range, it indicates that the composite nanofiltration membrane has a high surface electrode potential. When it is used for the separation of high-valent heavy metal ions in an acidic and alkaline environment, it can better repel high-valent heavy metal ions, making it difficult for high-valent heavy metal ions in the liquid to pass through the nanofiltration membrane, thereby achieving effective recovery of high-valent heavy metal cations.

[0273] Furthermore, the surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 5-20 mV.

[0274] According to the present invention, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.1-0.5 nm.

[0275] In the present 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 a high density. When it is used for the separation of high-valent heavy metal ions in an acidic and alkaline environment, it can better repel high-valent heavy metal ions, making it difficult for high-valent ions in the liquid to pass through the nanofiltration membrane, thereby obtaining a higher separation efficiency of high-valent heavy metal ions.

[0276] Furthermore, the average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.15-0.3 nm.

[0277] According to the present invention, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 30-80°.

[0278] In the present invention, the positively charged acid- and alkali-resistant composite nanofiltration membrane has a contact angle within the range described in the present invention, which indicates that the positively charged acid- and alkali-resistant composite nanofiltration membrane has excellent hydrophilicity and can enable the positively charged acid- and alkali-resistant composite nanofiltration membrane to have a high water flux.

[0279] Furthermore, the contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 30-60°.

[0280] In the present invention, there is no specific limitation on the bottom layer and the porous support layer, and they can be made of various existing materials that have a certain strength and can be used for nanofiltration and reverse osmosis membranes.

[0281] In the present invention, the bottom layer is a non-woven fabric, preferably polyester and / or polyethylene.

[0282] In the present invention, the porous support layer material is at least one selected from polyethersulfone, polysulfone, polyarylether, polybenzimidazole, polyetherketone, polyetheretherketone, polyacrylonitrile, polyvinylidene fluoride and polyaryletherketone.

[0283] In the present invention, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazinamine separation layer, a polysulfonamide separation layer, a poly(triazinamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazinamine-sulfonamide) separation layer and a poly(triazinamine-sulfonamide-urea) separation layer, preferably at least one of a polyurea separation layer, a polytriazinamine separation layer and a polysulfonamide separation layer.

[0284] According to the present invention, the present invention does not particularly limit the thickness of the bottom layer, the porous support layer, and the acid- and alkali-resistant separation layer, which is a conventional choice in this field. However, in order to enable these three layers to play a better synergistic role, the obtained positively charged acid- and alkali-resistant composite nanofiltration membrane can better have both excellent high-valent heavy metal ion separation coefficient and higher water flux. Preferably, the thickness of the bottom layer is 30-150μm, preferably 50-120μm; the thickness of the porous support layer is 10-100μm, preferably 30-60μm; the thickness of the acid- and alkali-resistant separation layer is 10-500nm, preferably 50-300nm.

[0285] A fourth aspect of the present invention provides a method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, wherein the preparation method comprises the following steps:

[0286] S1", preparing a porous supporting layer on the bottom layer;

[0287] S2", the membrane layer obtained in step S1" is first contacted with an aqueous phase of a polyamine containing a tertiary amine group, and second contacted with an organic phase containing a polyvalent polar monomer, and after heat treatment, a composite nanofiltration membrane comprising an acid- and alkali-resistant separation layer is obtained;

[0288] S3″, bringing the composite nanofiltration membrane into a third contact with an organic solution containing a halogenated alkane and a catalyst, and drying to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane.

[0289] In the present invention, the composite nanofiltration membrane is brought into contact with an organic solution containing halogenated alkanes and a catalyst. Under the action of the catalyst, the halogenated alkanes react biochemically with the tertiary amine in the acid- and alkali-resistant separation layer, and the tertiary amine is converted into a positively charged quaternary ammonium salt group. The repulsive effect between the positive charges improves the retention capacity of the positively charged acid- and alkali-resistant composite nanofiltration membrane for divalent and high-valent metal cations, and at the same time, the positively charged acid- and alkali-resistant composite nanofiltration membrane can maintain a higher water flux.

[0290] In the present invention, there is no particular limitation on the method for preparing the porous support layer on the bottom layer. It can be prepared by conventional methods in the art, preferably by a 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 by phase inversion.

[0291] In the present invention, the phase inversion method can preferably be: dissolving the supporting layer polymer material in a solvent to obtain a polymer solution with a concentration of 10-20 weight%, and degassing at 20-40°C for 10-180 minutes; then coating the polymer solution on the bottom layer to obtain an initial membrane, and then soaking it in water at a temperature of 10-30°C for 10-60 minutes to obtain the supporting layer polymer porous membrane through the phase inversion layer.

[0292] The solvent may be N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, dimethyl sulfoxide, or the like.

[0293] According to the present invention, in step S2″, the polyamine containing a tertiary amine group is selected from at least one of polyethyleneimine, polyethylene polyamine, 1-aminopiperazine, 1,4-diaminopiperazine, 1,4-piperazinediethylamine and 1,4-bisaminopropylpiperazine, preferably polyethyleneimine and / or polyethylene polyamine.

[0294] According to the present invention, the concentration of the polyamine containing tertiary amine groups in the aqueous phase is 0.1 wt%-10 wt%, preferably 0.5 wt%-2.5 wt%.

[0295] According to the present invention, the polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides.

[0296] According to the present invention, the polyisocyanate is selected from at least one of meta-phenylenediisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylenediisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(isocyanatophenyl), 1,3-phenylenediisocyanate, 3,3'-dichloro-4,4'-diisocyanatebiphenyl, dicyclohexylmethane-4,4'-diisocyanate, trimethylhexamethylene diisocyanate, L-lysine-ethyl ester-diisocyanate, 1,4-cyclohexyl diisocyanate and 4-chloro-6-methyl-meta-phenylenediisocyanate; preferably selected from 1,4-phenylenediisocyanate and / or 1,3-phenylenediisocyanate.

[0297] 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.

[0298] According to the present invention, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride.

[0299] According to the present invention, the concentration of the multi-polar monomer in the organic phase is 0.01 wt%-2 wt%, preferably 0.05-1 wt%.

[0300] According to the present invention, the amount of the aqueous phase of the polyamine containing tertiary amine groups and the organic phase containing multiple polar monomers is such that the weight ratio of the polyamine containing tertiary amine groups to the multiple polar monomers is 2-200:1.

[0301] In the present invention, when the weight ratio of the polyamine containing tertiary amine groups to the multipolar monomers is controlled to meet the above range, the interfacial polymerization reaction between the polyamine containing tertiary amine groups and the multipolar monomers can be ensured to be sufficient, thereby improving the crosslinking density of the obtained acid-base resistant separation layer.

[0302] Furthermore, the amount of the aqueous phase of the polyamine containing tertiary amine groups and the organic phase containing multiple polar monomers is such that the weight ratio of the polyamine containing tertiary amine groups to the multiple polar monomers is 5-80:1.

[0303] In the present invention, there is no particular limitation on the type of solvent in the organic phase containing the multi-polar monomer, as long as it can dissolve the multi-polar monomer. Preferably, the solvent of the organic phase is one or more of n-hexane, dodecane, n-heptane, and alkane solvent oil (Isopar E, Isopar G, Isopar H, Isopar L, and Isopar M).

[0304] According to the present invention, there is no special limitation on the interfacial polymerization conditions of the polyamine containing tertiary amine groups and the polyvalent polar monomers, and the polymerization can be carried out according to conventional conditions in the art. However, in order to enable the three layers to play a better synergistic role and enable the obtained composite nanofiltration membrane to better have excellent retention capacity for divalent and high-valent metal cations and higher water flux, preferably, the first contact time is 5-100s, preferably 10-60s; the second contact time is 10-200s, preferably 20-120s; the heat treatment conditions include: the heat treatment temperature is 40-150°C, preferably 50-120°C; the heat treatment time is 0.5-10min, preferably 1-5min.

[0305] In the present invention, in step S2", the ratio of the volume of the aqueous solution of the polyamine containing tertiary amine groups to the membrane area of ​​the membrane layer obtained in step S1" is 0.05-1 mL / cm 2 , preferably 0.1-0.5 mL / cm 2 .

[0306] In the present invention, in step S2", the ratio of the volume of the organic phase solution containing the multi-polar monomer to the membrane area of ​​the membrane layer obtained in step S1" is 0.25-0.5 mL / cm 2 , preferably 0.05-0.3 mL / cm 2 .

[0307] According to the present invention, in step S3″, the halogenated alkane is selected from the halogenated alkane shown in Formula 3 and / or the halogenated alkane shown in Formula 4;

[0308] X1', X2' and X3' are each independently halogen; n is an integer of 0-10; and m is an integer of 2-10.

[0309] The use of the halogenated alkane having the above-mentioned specific structure can further increase the positive charge density on the surface of the positively charged composite nanofiltration membrane and further reduce the average pore size of the nanofiltration membrane.

[0310] Furthermore, X1', X2' and X3' are each independently Cl, Br or I.

[0311] Furthermore, n is an integer of 0-7; m is an integer of 2-8.

[0312] According to the present invention, the halogenated alkane is selected from iodomethane, iodoethane, iodopropane, iodobutane, iodopentane, iodohexane, iodoheptane, iodooctane, iodononane, iododecane, iodocyclopropane, iodocyclobutane, iodocyclopentane, iodocycloheptane, iodocyclooctane, 1,2-diiodoethane, 1,3-diiodopropane, 1,4-diiodobutane, 1,5-diiodopentane, 1,6-diiodohexane, 1,7-diiodoheptane, 1,8-diiodooctane, chloromethane, chloroethane, chloropropane, chlorobutane, chloropentane, chlorohexane, chloroheptane, Chlorooctane, chlorononane, chloroquine, chlorocyclopropane, chlorocyclobutane, chlorocyclopentane, chlorocycloheptane, chlorocyclooctane, 1,2-dichloroethane, 1,3-dichloropropane, 1,4-dichlorobutane, 1,5-dichloropentane, 1,6-dichlorohexane, 1,7-dichloroheptane, 1,8-dichlorooctane, methyl bromide, ethyl bromide, bromopropane, bromobutane, bromopentane, bromohexane, bromoheptane , at least one of bromooctane, bromononane, bromodecane, bromocyclopropane, bromocyclobutane, bromocyclopentane, bromocycloheptane, bromocyclooctane, 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromohexane, 1,7-dibromoheptane and 1,8-dibromooctane, preferably at least one selected from bromoethane, chloropropane and iodomethane.

[0313] In the present invention, to ensure that the halogenated alkane can react with the tertiary amine groups remaining in the acid- and alkali-resistant separation layer, the catalyst is a basic catalyst. Furthermore, the catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium tert-butoxide, and sodium tert-butoxide, preferably selected from sodium hydroxide and / or sodium tert-butoxide.

[0314] According to the present invention, the organic solution is an alcohol solvent. There is no particular limitation on the specific type of the alcohol solvent, and the alcohol solvent may be, for example, one of methanol, ethanol, n-butanol, and isopropanol, preferably methanol or ethanol.

[0315] According to the present invention, in step S3″, the concentration of the halogenated alkane in the organic solution is 1 wt%-20 wt%.

[0316] In the present invention, when the concentration of the halogenated alkane in the organic solution in step S3" is controlled to meet the above range, it can ensure that the halogenated alkane is in full contact with the acid- and alkali-resistant separation layer in the composite nanofiltration membrane, so that the tertiary amine groups remaining on the surface of the acid- and alkali-resistant separation layer can fully react with the halogenated alkane, and have a high reaction efficiency, and ultimately the surface of the obtained positively charged acid- and alkali-resistant composite nanofiltration membrane contains more quaternary ammonium salt groups.

[0317] Furthermore, in step S3", the concentration of the halogenated alkane in the organic solution is 5wt%-10wt%.

[0318] According to the present invention, the concentration of the catalyst is 0.01 wt%-5 wt%.

[0319] In the present invention, when the concentration of the catalyst meets the above range, the halogenated alkane can react more fully with the tertiary amine group on the surface of the composite nanofiltration membrane, introducing a specific content of quaternary ammonium salt groups without wasting the catalyst and improving economic benefits.

[0320] Furthermore, the concentration of the catalyst is 0.1 wt%-1 wt%.

[0321] According to the present invention, the weight ratio of the halogenated alkane to the catalyst is 2-50:1.

[0322] Furthermore, the weight ratio of the halogenated alkane to the catalyst is 5-30:1.

[0323] According to the present invention, the ratio of the volume of the organic solution to the membrane area of ​​the composite nanofiltration membrane is 0.1-1 mL / cm 2 .

[0324] In the present invention, by controlling the volume of the organic solution and the membrane area of ​​the composite nanofiltration membrane to meet the above-mentioned range, it is possible to ensure that the halogenated alkanes are in full contact with the acid- and alkali-resistant separation layer in the composite nanofiltration membrane, and to ensure that the halogenated alkanes can have a high reaction efficiency with the tertiary amine groups in the acid- and alkali-resistant separation layer, and ultimately the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane obtained contains more quaternary ammonium salt groups.

[0325] Furthermore, the ratio of the volume of the organic solution to the membrane area of ​​the composite nanofiltration membrane is 0.3-0.5 mL / cm 2 .

[0326] According to the present invention, the conditions for the third contact include: the third contact time is 1-120 minutes.

[0327] In the present invention, when the contact time between the composite nanofiltration membrane and the organic solution containing the halogenated alkane and the catalyst is controlled to meet the above-mentioned range, it can ensure that the halogenated alkane reacts fully with the tertiary amine groups remaining on the surface of the acid- and alkali-resistant separation layer, so that the surface of the positively charged composite nanofiltration membrane obtained contains more quaternary ammonium salt groups.

[0328] Furthermore, the conditions for the third contact include: the third contact time is 5-60 minutes.

[0329] In the present invention, the temperatures for the first contact, the second contact, and the third contact are not particularly limited, and may be, for example, room temperature (20-30° C.).

[0330] According to the present invention, the drying conditions include: drying temperature of 30-80° C.; and drying time of 0.5-5 min.

[0331] Furthermore, the drying conditions include: drying temperature of 40-60° C.; drying time of 1-3 min.

[0332] The fifth aspect of the present invention provides a positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the above-mentioned preparation method.

[0333] A sixth aspect of the present invention provides an application of the above-mentioned positively charged acid- and alkali-resistant composite nanofiltration membrane in water treatment, particularly in the field of water treatment and separation.

[0334] In the present invention, the metal cation rejection rate of the positively charged acid- and alkali-resistant composite nanofiltration membrane is ≥85%, preferably ≥87%, preferably ≥89%, preferably ≥91%, preferably ≥93%, preferably ≥95%; the water flux is ≥20LMH, preferably ≥22LMH, preferably ≥24LMH, preferably ≥26LMH.

[0335] In the present invention, in particular, the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method in the second aspect has a MgCl2 rejection rate of ≥88%, preferably ≥90%; a LiCl rejection rate of ≥26%, preferably ≥28%; a magnesium-lithium separation coefficient of 10-90, preferably 25-86; and a water flux of ≥22LMH, preferably ≥25LMH.

[0336] In the present invention, especially the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method in the third aspect has a NiCl2 retention rate ≥85%, preferably ≥89%; a CuCl2 retention rate ≥88%, preferably ≥88.5%; and a water flux ≥22LMH, preferably ≥24LMH.

[0337] In the present invention, especially the Fe of the positively charged acid and alkali resistant composite nanofiltration membrane prepared by the preparation method in the fourth aspect 2+ Retention rate ≥90%, preferably ≥93%; H + The transmittance is 84-93%, preferably 88-91%, and the water flux is ≥20LMH, preferably ≥22LMH.

[0338] Other features and advantages of the present invention will be described in detail in the following detailed description.

[0339] The present invention will be described in detail below through examples.

[0340] (1) The water flux of the nanofiltration membrane was tested by the following method: the nanofiltration membrane was placed in a membrane pool and pre-pressed at 1.5 MPa for 1 h. The water permeation of the nanofiltration membrane was measured under the conditions of a pressure of 2 MPa and a temperature of 25°C within a certain period of time. The water flux was calculated by the following formula:

[0341] J = Q / (A·t), where J is the water flux, Q is the water permeation (L), and A is the effective filtration membrane area of ​​the nanofiltration membrane (m 2 ), t is time (h).

[0342] (2) The salt rejection rate of the nanofiltration membrane for magnesium chloride or lithium chloride was obtained by the following test method: the nanofiltration membrane was placed in the membrane pool and pre-pressed at 1.5 MPa for 1 hour. The salt concentration change in the raw water solution with an initial concentration of 2000 ppm and the permeate was tested under the conditions of pressure of 2 MPa and temperature of 25°C within 1 hour. The salt rejection rate was calculated by the following formula:

[0343] R=(C f -C p ) / C f ×100%, where R is the desalination rate, C f is the concentration of magnesium chloride or lithium chloride in the original solution (measured by conductivity meter), C p is the concentration of magnesium chloride or lithium chloride in the permeate.

[0344] The retention rate of the nanofiltration membrane for NiCl2 or CuCl2 is obtained by the following test method: the nanofiltration membrane is placed in the membrane pool and pre-pressed at 1.5 MPa for 1 hour. The concentration change of the salt in the raw water solution with an initial concentration of 2000 ppm and the permeate is tested under the conditions of pressure of 2 MPa and temperature of 25°C within 1 hour. The retention rate is calculated by the following formula:

[0345] R=(C f -C p ) / C f ×100%, where R is the retention rate, C f is the concentration of NiCl2 or CuCl2 in the original aqueous solution (measured by conductivity meter), C p is the concentration of NiCl2 or CuCl2 in the permeate.

[0346] Nanofiltration membrane for divalent Fe 2+ The retention rate and H + The permeability was obtained by the following method: the composite nanofiltration membrane was placed in the membrane pool, the original aqueous solution was a mixed solution of 0.2wt% FeSO4 and 5% H2SO4, pre-pressed at 1.5MPa for 1h, and then tested at a pressure of 2MPa and a temperature of 25℃. The Fe content in the original aqueous solution and the permeate after 1h was tested. 2+ and H+ concentration.

[0347] Membrane for divalent Fe 2+ The retention rate is R = (C f -C p ) / C f ×100%, where R is the retention rate, C f Fe in the original aqueous solution 2+ The concentration, C p Fe in the permeate 2+ The concentration of H + The transmittance is T=C p ' / C f '×100%, where C p ' is H in the permeate + The concentration, C f ' is H in the original aqueous solution + concentration.

[0348] (3) The magnesium-lithium separation coefficient of the nanofiltration membrane was obtained by testing the following method: the nanofiltration membrane was placed in a membrane pool, the original aqueous solution was a mixture of 2000 ppm magnesium chloride and 100 ppm lithium chloride, and pre-pressed at 1.5 MPa for 1 hour. Then, the permeate was obtained under the conditions of a pressure of 2 MPa and a temperature of 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:

[0349] Where S is the lithium-magnesium separation coefficient, C Mg,f and C Li,f are the mass concentrations of magnesium ions and lithium ions in the raw water (measured by ion chromatography); C Mg,p and C Li,p are the mass concentrations of magnesium ions and lithium ions in the permeate, respectively (measured by ion chromatography).

[0350] (4) Acid resistance test method of nanofiltration membrane: Soak 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 the water flux of the nanofiltration membrane and the magnesium-lithium separation coefficient or the retention rate of high-valent heavy metal ions.

[0351] Alkali resistance test method of nanofiltration membrane: soak the nanofiltration membrane in a 10wt% NaOH solution for 30 days, take it out and rinse it thoroughly with deionized water, and then test the changes in the water flux of the nanofiltration membrane and the magnesium-lithium separation coefficient or the retention rate of high-valent heavy metal ions.

[0352] (5) Nanofiltration membrane surface contact angle measurement

[0353] The surface contact angle of the composite film sample was tested using the DSA100 surface contact angle meter produced by KRUSS, Germany, using the sessile 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. The dried film was then attached to a flat glass slide with double-sided tape. The volume of each water droplet during the test was 2 μL. The water droplet was dropped on the film surface for 3 seconds and then tested immediately. The average value of multiple measurements was used to determine the final contact angle.

[0354] (6) Determination of the content of nitrogen atoms in quaternary ammonium salt groups and phosphorus atoms in nanofiltration membranes:

[0355] Before measurement, samples were oven-dried to constant weight. The elemental composition of the composite membrane surface samples was determined using a Sigma Probe X-ray photoelectron spectrometer manufactured by Thermo VG, UK. For samples containing quaternary ammonium groups, the molar content of nitrogen atoms in the quaternary ammonium groups is equal to the corresponding molar content of halogen atoms. The measured halogen content can be used to determine the nitrogen content of the quaternary ammonium groups. The phosphorus content of the quaternary phosphates in the nanofiltration membrane can be directly measured using XPS spectroscopy.

[0356] (7) The nitrogen atom content in the quaternary ammonium salt structure in the nanofiltration membrane is measured by the following method:

[0357] Before measurement, the nanofiltration membrane was dried in an oven to constant weight. The elemental composition of the nanofiltration membrane surface was determined using a Sigma Probe X-ray photoelectron spectrometer manufactured by Thermo VG, UK. The nitrogen content of the quaternary ammonium salt structure in the acid- and alkali-resistant separation layer was calculated using the following formula:

[0358] Among them, A N + represents the peak area of ​​the nitrogen atom in the quaternary ammonium salt structure in the XPS spectrum (the peak area at the bond energy of 403 eV in Figure 3); A N represents the peak area of ​​nitrogen atoms in the separation layer in the XPS spectrum (the sum of the peak areas at 403 eV and 399 eV in Figure 3); S N It represents the atomic content (at.%) of nitrogen atoms in the quaternary ammonium salt group in the acid- and alkali-resistant positively charged composite nanofiltration membrane.

[0359] (8) Nanofiltration membrane pore size test: PEG solute transfer method was used for measurement. The detailed steps are as follows:

[0360] (i) Testing the rejection rate of nanofiltration membranes for PEG with different molecular sizes;

[0361] (ii) Linear fitting was performed between PEG size and retention rate in a log-probability coordinate system, and the PEG size corresponding to 50% retention rate was the average pore size of the nanofiltration membrane.

[0362] (9) Zeta potential test of nanofiltration membrane surface: The test was performed using a Surpass electrokinetic analyzer (Anton Paar). The circulating fluid was a dilute aqueous solution of KCl and the pH of the test fluid was 7.

[0363] (10) The thickness of each layer in the nanofiltration membrane is measured by a screw micrometer.

[0364] In addition, in the following examples and comparative examples:

[0365] Branched polyethyleneimine (weight-average molecular weight of 25,000 g / mol), polyethylene polyamine, 1,4-phenylenediisocyanate, cyanuric chloride, 1,3-phenylenedisulfonyl chloride, 2-chloroethyltrimethylammonium chloride, 3-bromopropyltriphenylphosphonium bromide, 4-bromomethylbenzyltriphenylphosphonium bromide, chloromethyltriphenylphosphonium chloride, 2-chloroethyltrimethylammonium chloride, 3-bromopropyltrimethylammonium bromide, iodomethyltrimethylammonium iodide, 1,4-diaminopiperazine, 1,4-cyclohexanediamine, ethyl bromide, propyl chloride, methyl iodide, hexyl bromide, hexyl iodide, 1,3-dibromopropane, and tetradecane bromide were purchased from J&K Technology Co., Ltd. Other chemical reagents were purchased from Sinopharm Chemical Reagent Co., Ltd.

[0366] The support layer is prepared by a phase inversion method, and the specific steps are as follows:

[0367] 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%, and degassed at 25°C for 120 min; then, the polysulfone solution was coated on a polyester non-woven fabric (75 μm thick) using a scraper to obtain an initial membrane, which was then immersed in water at 25°C for 60 min to allow the polysulfone layer on the surface of the polyester non-woven fabric to undergo phase transformation into a porous membrane. Finally, after three water washes, a base layer-porous support layer with a total thickness of 115 μm was obtained.

[0368] Example A1

[0369] (1) 5 g of polyethyleneimine was dissolved in 100 g of deionized water and heated to 60° C. under stirring. 10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenylphosphonium bromide (X1 and X are Br, R5' is a propylene group, and R6', R7', and R8' are phenyl groups) and 1 wt% of potassium hydroxide was added dropwise to the solution at a rate of 2 mL / min. After the addition was complete, the temperature was maintained at 60° C. and the reaction was continued for 24 h. The mass ratio of the polyamine, the halogenated quaternary phosphonium salt, and the catalyst was 5:0.5:0.1 = 50:5:1.

[0370] (2) The obtained quaternary phosphonium salt modified polyethyleneimine aqueous solution was diluted with water to adjust its concentration to 0.5 wt % of the quaternary phosphonium salt modified polyethyleneimine aqueous solution, and the area of ​​400 cm 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 phosphonium salt-modified polyethyleneimine prepared above, and the solution was discharged after the contact time 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% of 1,4-phenylenediisocyanate, and the solution was discharged after the contact time at 25° C.; then, the membrane was placed in an oven and heated at 70° C. for 3 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN1;

[0371] The infrared spectrum of the positively charged acid- and alkali-resistant composite nanofiltration membrane AN1 is shown in Figure 1. As can be seen from Figure 1, the infrared spectrum at 1113 cm -1 The characteristic peak of the CP bond at , which shows that the quaternary phosphonium group in the structure of 3-bromopropyltriphenylphosphonium bromide is successfully modified on the surface of the nanofiltration membrane, thereby enhancing the positive charge of the surface of the positively charged acid-base resistant composite nanofiltration membrane and improving the magnesium-lithium separation performance of the membrane.

[0372] Example A2

[0373] The preparation method is the same as that of Example A1, except that "polyethylene polyamine" is used instead of "polyethylene imine" to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN2.

[0374] Example A3

[0375] The preparation method is the same as that of Example A1, except that "4-bromomethylbenzyltriphenylphosphonium bromide (X1 and X are Br, R5' is the aromatic methyl group of C1, and R6', R7', and R8' are phenyl)" is used instead of "3-bromopropyltriphenylphosphonium bromide" to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN3.

[0376] Example A4

[0377] The preparation method is the same as that of Example A1, except that "chloromethyltriphenylphosphonium chloride (R5' is methylene, R6', R7', R8' are phenyl, X1 and X are Cl)" is used to replace "3-bromopropyltriphenylphosphonium bromide" to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN4.

[0378] Example A5

[0379] (1) 5 g of polyethyleneimine was dissolved in 100 g of deionized water and heated to 60° C. under stirring. 10 g of an aqueous solution containing 5 wt% of 2-chloroethyltrimethylammonium chloride (X and X1 are Cl, R1' is ethylene, R2', R3', and R4' are methyl) and 1 wt% of potassium hydroxide was added dropwise to the solution at a rate of 2 mL / min. After the addition was complete, the temperature was maintained at 60° C. and the reaction was continued for 24 h. The mass ratio of the polyamine, the quaternary amine halide, and the catalyst was 5:0.5:0.1=50:5:1.

[0380] (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, and an area of ​​400 cm 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 liquid was discharged after contacting at 25°C for 60 seconds; then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% of 1,4-phenylenediisocyanate, and the liquid was discharged after contacting at 25°C for 60 seconds; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN5.

[0381] Example A6

[0382] The preparation method is the same as that of Example A5, except that "3-bromopropyltrimethylammonium bromide (X and X1 are Br, R1' is propylene, R2', R3', R4' are methyl)" is used to replace "2-chloroethyltrimethylammonium chloride" to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN6.

[0383] Example A7

[0384] The preparation method is the same as that of Example A5, except that "iodomethyltrimethylammonium iodide (X and X1 are I, R1' is methylene, R2', R3', R4' are methyl)" is used to replace "2-chloroethyltrimethylammonium chloride" to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN7.

[0385] Example A8

[0386] (1) Dissolve 5 g of polyethyleneimine in 100 g of deionized water and heat to 60° C. with stirring. Add 10 g of an aqueous solution containing 5 wt% 3-bromopropyltriphenylphosphonium bromide and 1 wt% potassium hydroxide dropwise to the solution at a rate of 2 mL / min. After the addition is complete, maintain the temperature at 60° C. and continue the reaction for 24 h.

[0387] (2) The obtained quaternary phosphonium salt modified polyethyleneimine aqueous solution was diluted with water to adjust its concentration to 2 wt % of the quaternary phosphonium salt modified polyethyleneimine aqueous solution, and the area of ​​400 cm 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% of the quaternary phosphonium salt modified polyethyleneimine prepared above, and the liquid was discharged after the contact period at 25°C for 60 seconds; then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.05 wt% of cyanuric chloride, and the liquid was discharged after the contact period at 25°C for 60 seconds; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN8.

[0388] Example A9

[0389] (1) Dissolve 5 g of polyethyleneimine in 100 g of deionized water and heat to 60° C. with stirring. Add 10 g of an aqueous solution containing 5 wt% 3-bromopropyltriphenylphosphonium bromide and 1 wt% potassium hydroxide dropwise to the solution at a rate of 2 mL / min. After the addition is complete, maintain the temperature at 60° C. and continue the reaction for 24 h.

[0390] (2) The obtained quaternary phosphonium salt modified polyethyleneimine aqueous solution was diluted with water to adjust its concentration to 2 wt % of the quaternary phosphonium salt modified polyethyleneimine aqueous solution, and the area of ​​400 cm 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 2 wt% of the quaternary phosphonium salt modified polyethyleneimine prepared above, and the liquid was discharged after contacting at 25°C for 60 seconds; then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.2 wt% of 1,3-benzenedisulfonyl chloride, and the liquid was discharged after contacting at 25°C for 60 seconds; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN9.

[0391] Example A10

[0392] The preparation method is the same as that in Example A1, except that “10 g of 0.5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” is used instead of “10 g of 5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane AN10.

[0393] Example A11

[0394] The preparation method is the same as that in Example A1, except that “10 g of 8 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” is used instead of “10 g of 5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane AN11.

[0395] Example A12

[0396] The preparation method is the same as that in Example A1, except that “10 g of 0.2 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” is used instead of “10 g of 5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane AN12.

[0397] Example A13

[0398] The preparation method is the same as that in Example A1, except that “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 0.05 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” instead of “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 1 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN13; the mass ratio of the polyamine, the halogenated quaternary phosphonium salt and the catalyst is 5:0.5:0.005=1000:100:1.

[0399] Example A14

[0400] The preparation method is the same as that in Example A1, except that “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 0.5 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” instead of “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 1 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN14; the mass ratio of the polyamine, the halogenated quaternary phosphonium salt and the catalyst is 5:0.5:0.05=100:10:1.

[0401] Example A15

[0402] The preparation method is the same as that in Example A1, except that “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 0.005 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” instead of “10 g of an aqueous solution containing 5 wt% of 3-bromopropyltriphenyl phosphonium bromide and 1 wt% of potassium hydroxide is added dropwise to the solution at a rate of 2 mL / min” to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN15; the mass ratio of the polyamine, the halogenated quaternary phosphonium salt and the catalyst is 5:0.5:0.0005=10000:1000:1.

[0403] Example A16

[0404] The preparation method is the same as that in Example A1, except that “10 g of 5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” instead of “10 g of 5 wt% 3-bromopropyltriphenyl phosphonium bromide is added dropwise to the solution at a rate of 2 mL / min” to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AN16; the mass ratio of the polyamine, the halogenated quaternary ammonium salt and the halogenated quaternary phosphonium salt to the catalyst is 5:0.5:0.1=50:5:1.

[0405] Comparative Example A1

[0406] 5 g of polyethyleneimine was dissolved in 100 g of deionized water. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethyleneimine, and the solution was drained after contacting at 25°C for 60 seconds; then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenylene diisocyanate, and the solution was drained after contacting at 25°C for 60 seconds; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a nanofiltration membrane AD1.

[0407] Comparative Example A2

[0408] 5 g of polyethyleneimine was dissolved in 100 g of deionized water. 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 seconds, and then the liquid was 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 seconds, and then the liquid was drained; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a nanofiltration membrane AD2.

[0409] Comparative Example A3

[0410] 5 g of polyethyleneimine was dissolved in 100 g of deionized water.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 seconds, and then the liquid was 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 seconds, and then the liquid was drained; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a nanofiltration membrane AD3.

[0411] Comparative Example A4

[0412] 400cm 2 The upper surface of the polysulfone support layer was contacted with 50 g of an aqueous solution containing 0.5 wt% polyethyleneimine at 25° C. for 60 seconds, after which the liquid was drained. The upper surface of the support layer was then contacted with 30 g of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate at 25° C. for 60 seconds, after which the liquid was drained. The membrane was then placed in an oven and heated at 70° C. for 3 minutes to obtain a composite membrane A1.

[0413] At 30°C, the above-mentioned composite membrane A1 was immersed in an aqueous solution containing 150g of 10wt% 2,3-epoxypropyltrimethylammonium chloride (epoxy quaternary ammonium salt) and 5wt% 2,4,6-tris(dimethylaminomethyl)phenol (catalyst). After 1 minute, it was taken out and dried at 70°C for 5 minutes to obtain an acid- and alkali-resistant positively charged nanofiltration membrane AD4 modified with epoxy quaternary ammonium salt.

[0414] Comparative Example A5

[0415] Dissolve 5 g of polyethyleneimine in 100 g of deionized water and heat to 60°C with stirring. Add 10 g of an ethanol solution containing 5 wt% benzyl chloride dropwise to the solution at a rate of 2 mL / min. After the addition is complete, maintain the temperature at 60°C and continue the reaction for 24 hours.

[0416] The quaternized modified polyethyleneimine aqueous solution prepared above was diluted with water to adjust its concentration to 0.5 wt %. 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% of the quaternized modified polyethyleneimine prepared above, and the liquid was discharged after contacting at 25°C for 60 seconds; then, the upper surface of the support layer was contacted with 30 mL of an Isopar E solution containing 0.1 wt% of 1,4-phenylenediisocyanate, and the liquid was discharged after contacting at 25°C for 60 seconds; then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane AD5.

[0417] The thickness of the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in the example and the nanofiltration membrane prepared in the comparative example were tested, and the results are shown in Table 1.

[0418] The surface Zeta potential, average pore size, nitrogen atom content in quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane obtained in the embodiment and the nanofiltration membrane obtained in the comparative example, the phosphorus atom content in the positively charged acid- and alkali-resistant composite nanofiltration membrane and the contact angle were tested, and the results are shown in Table 2.

[0419] The water flux, the desalination rate of magnesium chloride and lithium chloride, and the magnesium-lithium separation coefficient of the positively charged acid-resistant composite nanofiltration membrane obtained in the embodiment and the nanofiltration membrane obtained in the comparative example were compared; the magnesium-lithium separation coefficient and the water flux of the positively charged acid-resistant composite nanofiltration membrane obtained in the embodiment and the nanofiltration membrane obtained in the comparative example were tested after acid treatment, and the results are shown in Table 3.

[0420] Table 1 Note: The thickness of the acid-base resistant separation layer here refers to the acid-base resistant separation layer grafted with quaternary ammonium salt groups / quaternary phosphonium salt groups.

[0421] Table 2 Note: a - the content of nitrogen atoms in the quaternary ammonium salt group; b -The content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane; “-” means it cannot be measured.

[0422] Table 3

[0423] It can be seen from Tables 1-3 that the positively charged acid- and alkali-resistant composite nanofiltration membrane of the present invention can better achieve both an excellent magnesium-lithium separation coefficient and a higher water flux.

[0424] It can be seen from Table 2 that compared with the comparative example, in the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in the embodiment, due to the biochemical reaction of the halogenated quaternary ammonium salt and / or quaternary phosphonium salt with the polyamine under the action of an alkaline catalyst, the positively charged quaternary ammonium salt group and / or quaternary phosphonium salt group is modified into the polyamine molecule. After interfacial polymerization, the membrane surface potential of the obtained positively charged acid- and alkali-resistant composite nanofiltration membrane is significantly improved.

[0425] On the other hand, due to the introduction of quaternary ammonium salt groups or quaternary phosphonium salt groups, the interfacial polymerization is regulated, and the polymerization reaction proceeds more fully, so that the separation layer of the positively charged acid- and alkali-resistant composite nanofiltration membrane finally prepared is denser and has a smaller pore size. The positively charged acid- and alkali-resistant composite nanofiltration membrane has a higher magnesium-lithium separation coefficient while maintaining a high water flux.

[0426] On the other hand, the data after treatment with 5wt% HCl for 7 days / 10wt% NaOH for 30 days show that the magnesium-lithium separation coefficient and water flux of the membrane remain basically unchanged after strong acid / strong base treatment, indicating that the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by positively charged modified polyamine has better acid / alkali resistance.

[0427] Comparative Example A5 has a high rejection rate for MgCl2, but also a high rejection rate for LiCl, which is very unfavorable for magnesium-lithium separation because both are retained by the membrane, preventing effective separation. Furthermore, the low water flux of Comparative Example A5 indicates that the membrane is too dense, resulting in slow water flow and low efficiency.

[0428] Example B1

[0429] 400cm 2 The upper surface of the polysulfone support layer was contacted with 50 g of an aqueous solution containing 0.5 wt% polyethyleneimine at 25° C. for 60 seconds, after which the liquid was drained. The upper surface of the support layer was then contacted with 30 g of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate at 25° C. for 60 seconds, after which the liquid was drained. The membrane was then placed in an oven and heated at 70° C. for 3 minutes to obtain a composite membrane A1.

[0430] At 30°C, the composite membrane A1 was immersed in an aqueous solution containing 150 g of 10 wt% 3-bromopropyltriphenylphosphonium bromide and 0.5 wt% potassium hydroxide for 1 minute, then taken out and dried at 70°C for 5 minutes to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN1;

[0431] The XPS phosphorus atomic spectrum of the positively charged acid- and alkali-resistant composite nanofiltration membrane BN1 is shown in Figure 2. It can be seen from Figure 2 that the signal of the phosphorus element on the membrane surface is obvious (a prominent single peak in the figure), indicating that the acid- and alkali-resistant separation layer successfully grafts the quaternary phosphine salt group shown in Formula I through N atoms, thereby enhancing the positive charge of the surface of the positively charged acid- and alkali-resistant composite nanofiltration membrane and the retention effect of divalent and high-valent heavy metal ions.

[0432] Example B2

[0433] 400cm 2 The upper surface of the polysulfone support layer was contacted with 50 g of an aqueous solution containing 0.5 wt% polyethylene polyamine at 25° C. for 60 seconds, after which the liquid was drained. The upper surface of the support layer was then contacted with 30 g of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate at 25° C. for 60 seconds, after which the liquid was drained. The membrane was then placed in an oven and heated at 70° C. for 3 minutes to obtain a composite membrane A2.

[0434] At 30°C, the above-mentioned composite membrane A2 was immersed in an aqueous solution containing 150g of 5wt% 3-bromopropyltriphenylphosphonium bromide and 0.5wt% potassium hydroxide. After 1 minute, it was taken out and dried at 70°C for 5 minutes to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN2.

[0435] Example B3

[0436] The method of Example B1 was followed, except that "chloromethyltriphenylphosphonium chloride" was used instead of "3-bromopropyltriphenylphosphonium bromide" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN3.

[0437] Example B4

[0438] The method of Example B1 was followed, except that "2-chloroethyltrimethylammonium chloride" was used instead of "3-bromopropyltriphenylphosphonium bromide" to obtain a quaternary ammonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN4.

[0439] Example B5

[0440] The method of Example B1 was followed, except that "3-bromopropyltrimethylammonium bromide" was used instead of "3-bromopropyltriphenylphosphonium bromide" to obtain a quaternary ammonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN5.

[0441] Example B6

[0442] The method of Example B1 was followed, except that “2 wt % aqueous solution of polyethyleneimine” was used instead of “0.5 wt % aqueous solution of polyethyleneimine”, and “0.05 wt % Isopar E solution of cyanuric chloride” was used instead of “0.1 wt % Isopar E solution of 1,4-phenylenediisocyanate” to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN6.

[0443] Example B7

[0444] The method of Example B1 was followed, except that "2 wt% aqueous solution of polyethyleneimine" was used instead of "0.5 wt% aqueous solution of polyethyleneimine", and "0.2 wt% Isopar E solution of 1,3-benzenedisulfonyl chloride" was used instead of "0.1 wt% Isopar E solution of 1,4-phenylenediisocyanate" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN7.

[0445] Example B8

[0446] The method of Example B1 was followed, except that "20 wt % of 3-bromopropyltriphenyl phosphonium bromide" was used instead of "10 wt % of 3-bromopropyltriphenyl phosphonium bromide" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN8.

[0447] Example B9

[0448] The method of Example B1 was followed, except that "0.1 wt % of 3-bromopropyltriphenyl phosphonium bromide" was used instead of "10 wt % of 3-bromopropyltriphenyl phosphonium bromide" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN9.

[0449] Example B10

[0450] The method of Example B1 was followed, except that "0.01 wt % potassium hydroxide" was used instead of "0.5 wt % potassium hydroxide" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN10.

[0451] Example B11

[0452] The method of Example B1 was followed, except that "0.005 wt % potassium hydroxide" was used instead of "0.5 wt % potassium hydroxide" to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN11.

[0453] Example B12

[0454] The method of Example B1 was followed, except that the contact time was 10 s, to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN12.

[0455] Example B13

[0456] The method of Example B1 was followed, except that the contact temperature was 20° C., to obtain a quaternary phosphonium salt-modified positively charged acid- and alkali-resistant composite nanofiltration membrane BN13.

[0457] Comparative Example B1

[0458] The upper surface of the polysulfone support layer was contacted with 50 g of an aqueous solution containing 0.5 wt% polyethyleneimine at 25°C for 60 seconds, and then the liquid was drained. Then, the upper surface of the support layer was contacted with 30 g of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate at 25°C for 60 seconds, and then the liquid was drained. Then, the membrane was placed in an oven and heated at 70°C for 3 minutes to obtain a composite membrane BD1.

[0459] Comparative Example B2

[0460] The process was carried out in the same manner as in Example B1, except that no catalyst was added, to obtain a composite membrane BD2.

[0461] The positively charged acid- and alkali-resistant composite nanofiltration membrane prepared in the example and the composite membrane prepared in the comparative example were tested for thickness, surface Zeta potential, contact angle and average pore size. The results are shown in Tables 4 and 5.

[0462] Table 4 Note: The thickness of the acid-base resistant separation layer here refers to the acid-base resistant separation layer modified with quaternary ammonium salt and / or halogenated quaternary phosphonium salt.

[0463] Table 5 Note: a -The content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid- and alkali-resistant composite nanofiltration membrane; b -The content of phosphorus atoms in the positively charged acid- and alkali-resistant composite nanofiltration membrane; “-” means it cannot be measured.

[0464] The positively charged acid-resistant and alkali-resistant composite nanofiltration membrane prepared in the embodiment and the composite membrane prepared in the comparative example were tested for the retention rate and water flux of NiCl2 and CuCl2, respectively; after the positively charged acid-resistant and alkali-resistant composite nanofiltration membrane prepared in the embodiment and the composite membrane prepared in the comparative example were treated with acid, the retention rate and water flux of NiCl2 and CuCl2 were tested, respectively, and the results are shown in Table 6.

[0465] Table 6

[0466] It can be seen from Examples B1-B7 that the positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method of the present invention has good acid and alkali resistance, and takes into account both the retention effect of divalent and high-valent heavy metal ions and a high water flux.

[0467] It can be seen from Example B1, Example B8, and Example B9 that the concentration of the halogenated quaternary phosphonium salt is fully grafted on the membrane surface, which increases the surface charge density of the positively charged acid- and alkali-resistant composite nanofiltration membrane, thereby increasing the retention rate of the nanofiltration membrane for divalent and high-valent metal ions, while also having a higher water flux; when the concentration of the halogenated quaternary phosphonium salt is too low, the retention rate decreases; when the concentration of the halogenated quaternary phosphonium salt is too high, it will cause a decrease in water flux and waste of raw materials.

[0468] It can be seen from Example B1, Example B10, and Example B11 that the appropriate catalyst concentration of the present invention can make the reaction of the grafted quaternary ammonium halides and / or quaternary phosphonium halides on the membrane surface proceed more fully, thereby improving the retention rate of the nanofiltration membrane for divalent and high-valent metal ions while also having a higher water flux.

[0469] Comparative Example B1 was not modified with halogenated quaternary ammonium salt and / or halogenated quaternary phosphonium salt, and the resulting nanofiltration membrane had a decreased rejection rate for divalent and high-valent metal ions, failing to achieve effective retention of divalent and high-valent metal ions.

[0470] No catalyst was added to Comparative Example B2. From the data in Table 4, it can be seen that the surface Zeta potential of the composite membrane is lower and the phosphorus atom content is lower, which can prove that the acid-base resistant separation layer is not sufficiently grafted. From the data in Table 5, it can also be seen that the retention rate of the composite membrane for NiCl2 and CuCl2 is reduced.

[0471] After treatment with 5wt% HCl for 7 days and 10wt% NaOH for 30 days, the retention rate of NiCl2 and CuCl2 and the water flux of the positively charged acid- and alkali-resistant composite nanofiltration membrane did not change significantly, indicating that the positively charged acid- and alkali-resistant composite nanofiltration membrane has good acid / alkali resistance.

[0472] Example C1

[0473] (1) At 25°C, place the above-mentioned area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethyleneimine, and the liquid was drained after the contact for 60 seconds; the upper surface of the support layer after the first contact was then second contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate, and the liquid was drained after the contact for 60 seconds; finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite nanofiltration membrane A1;

[0474] (2) At 25°C, the obtained composite nanofiltration membrane A1 was immersed in 200 mL of a methanol solution containing 10 wt% bromoethane and 0.5 wt% potassium hydroxide (the weight ratio of bromoethane to potassium hydroxide was 20:1) for a third contact. After 5 minutes, the membrane was taken out and dried at 70°C for 5 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN1.

[0475] From the XPS spectrum of the surface nitrogen atoms of the positively charged acid- and alkali-resistant composite nanofiltration membrane in Figure 3, it can be seen that a new signal peak appears at 402.4 eV, which corresponds to the signal of the quaternary ammonium salt nitrogen atom, proving that ethyl bromide reacts with the tertiary amine in the acid- and alkali-resistant separation layer to form a quaternary ammonium salt structure.

[0476] Example C2

[0477] (1) At 25°C, place the above-mentioned area of ​​400 cm 2 The upper surface of the polysulfone support layer was contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethylene polyamine for a first contact, and the liquid was drained after the contact for 60 seconds; the upper surface of the support layer after the first contact was then contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate for a second contact, and the liquid was drained after the contact for 60 seconds; finally, the membrane after the two contacts was placed in an oven and heated at 70° C. for 3 minutes to obtain a composite nanofiltration membrane A2;

[0478] (2) At 25°C, the obtained composite nanofiltration membrane A2 was immersed in 200 mL of a methanol solution containing 5 wt% of chloropropane and 0.5 wt% of potassium hydroxide (the weight ratio of chloropropane to potassium hydroxide was 10:1) for a third contact. After 5 minutes, the membrane was taken out and dried at 70°C for 5 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN2.

[0479] Example C3

[0480] (1) At 25°C, place the above-mentioned area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 0.5 wt% of 1,4-diaminopiperazine, and the liquid was drained after 60 seconds; the upper surface of the support layer after the first contact was then second contacted with 30 mL of an Isopar E solution containing 0.1 wt% of 1,4-phenylenediisocyanate, and the liquid was drained after 60 seconds; finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite nanofiltration membrane A3;

[0481] (2) At 25°C, the obtained composite nanofiltration membrane A3 was immersed in 80 mL of a methanol solution containing 10 wt% iodomethane and 0.5 wt% potassium hydroxide (the weight ratio of iodomethane to potassium hydroxide was 20:1) for a third contact. After 5 minutes, the membrane was taken out and dried at 70°C for 5 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN3.

[0482] Example C4

[0483] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of 10 wt% bromohexane" is used instead of "200 mL of 10 wt% bromoethane" (the weight ratio of bromohexane to potassium hydroxide is 20:1) to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN4.

[0484] Example C5

[0485] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of 10 wt% iodine hexane" is used instead of "200 mL of 10 wt% bromoethane" (the weight ratio of iodine hexane to potassium hydroxide is 20:1) to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN5.

[0486] Example C6

[0487] (1) At 25°C, place the above-mentioned area of ​​400 cm 2The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine, and the liquid was drained after the contact for 60 seconds. The upper surface of the support layer after the first contact was then second contacted with 30 mL of an Isopar E solution containing 0.05 wt% cyanuric chloride, and the liquid was drained after the contact for 60 seconds. Finally, the membrane after the two contactings was placed in an oven and heated at 70°C for 3 minutes to obtain a composite nanofiltration membrane A6.

[0488] (2) At 25°C, the obtained composite nanofiltration membrane A6 was immersed in 200 mL of a methanol solution containing 10 wt% bromoethane and 0.5 wt% potassium hydroxide (the weight ratio of bromoethane to potassium hydroxide was 20:1) for a third contact. After 5 minutes, the membrane was taken out and dried at 70°C for 5 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN6.

[0489] Example C7

[0490] (1) At 25°C, place the above-mentioned area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine, and the liquid was drained after the contact for 60 seconds. The upper surface of the support layer after the first contact was then second contacted with 30 mL of an Isopar E solution containing 0.2 wt% 1,3-benzenedisulfonyl chloride, and the liquid was drained after the contact for 60 seconds. Finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite nanofiltration membrane A7.

[0491] (2) At 25°C, the obtained composite nanofiltration membrane A7 was immersed in 200 mL of a methanol solution containing 10 wt% bromoethane and 0.5 wt% potassium hydroxide (the weight ratio of bromoethane to potassium hydroxide was 20:1) for a third contact. After 5 minutes, the membrane was taken out and dried at 70°C for 5 minutes to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN7.

[0492] Example C8

[0493] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of a mixed solution containing 5 wt% bromoethane and 5 wt% 1,3-dibromopropane" is used instead of "200 mL of 10 wt% bromoethane" (the ratio of the total weight of bromoethane and 1,3-dibromopropane to the weight of potassium hydroxide is 20:1) to obtain a positively charged acid- and alkali-resistant composite nanofiltration membrane CN8.

[0494] Example C9

[0495] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of 15 wt% bromoethane" is used instead of "200 mL of 10 wt% bromoethane" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN9.

[0496] Example C10

[0497] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of 25 wt% bromoethane" is used instead of "200 mL of 10 wt% bromoethane" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN10.

[0498] Example C11

[0499] The preparation method is the same as that of Example C1, except that in step (2), "take out after 0.5 minutes" is used instead of "take out after 5 minutes" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN11.

[0500] Example C12

[0501] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of potassium hydroxide containing 0.01 wt %" is used instead of "200 mL of potassium hydroxide containing 0.5 wt %" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN12.

[0502] Example C13

[0503] The preparation method is the same as that of Example C1, except that in step (2), "200 mL of potassium hydroxide containing 0.005 wt %" is used instead of "200 mL of potassium hydroxide containing 0.5 wt %" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN13.

[0504] Example C14

[0505] The preparation method is the same as that of Example C1, except that in step (1), "50 mL of 0.5 wt% 1,4-cyclohexanediamine" is used instead of "50 mL of 0.5 wt% polyethyleneimine" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN14.

[0506] Example C15

[0507] The preparation method is the same as that of Example C1, except that: in step (2), "200 mL of 10 wt% bromotetradecane" is used instead of "200 mL of 10 wt% bromoethane" to prepare the positively charged acid- and alkali-resistant composite nanofiltration membrane CN15.

[0508] Comparative Example C1

[0509] At 25°C, the above area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethyleneimine, and the liquid was drained after 60 seconds. The upper surface of the support layer after the first contact was then secondly contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate, and the liquid was drained after 60 seconds. Finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite membrane CD1.

[0510] Comparative Example C2

[0511] At 25°C, the above area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 0.5 wt% polyethylene polyamine, and the liquid was drained after 60 seconds. The upper surface of the support layer after the first contact was then secondly contacted with 30 mL of an Isopar E solution containing 0.1 wt% 1,4-phenylenediisocyanate, and the liquid was drained after 60 seconds. Finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite membrane CD2.

[0512] Comparative Example C3

[0513] At 25°C, the above area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine, and the liquid was drained after 60 seconds. The upper surface of the support layer after the first contact was then second contacted with 30 mL of an Isopar E solution containing 0.05 wt% cyanuric chloride, and the liquid was drained after 60 seconds. Finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite membrane CD3.

[0514] Comparative Example C4

[0515] At 25°C, the above area of ​​400 cm 2 The upper surface of the polysulfone support layer was first contacted with 50 mL of an aqueous solution containing 2 wt% polyethyleneimine, and the liquid was drained after contacting at 25°C for 60 seconds. The upper surface of the support layer after the first contact was then secondly contacted with 30 mL of an Isopar E solution containing 0.2 wt% 1,3-benzenedisulfonyl chloride, and the liquid was drained after 60 seconds. Finally, the membrane after the two contacts was placed in an oven and heated at 70°C for 3 minutes to obtain a composite membrane CD4.

[0516] The thickness, surface Zeta potential, average pore size, nitrogen atom content in the quaternary ammonium salt structure in the acid- and alkali-resistant separation layer, and contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane of the embodiment and the composite membrane of the comparative example are shown in Table 7.

[0517] Table 7

[0518] The separation layer refers to an acid and alkali resistant separation layer;

[0519] a It refers to the content of nitrogen atoms in the quaternary ammonium salt structure in the acid and alkali resistant separation layer.

[0520] The positively charged acid- and alkali-resistant composite nanofiltration membrane of the embodiment and the composite membrane of the comparative example were placed in a membrane pool and pre-pressed at 1.5 MPa for 1 h. Subsequently, the water flux, Fe 2+ The retention rate was tested; and the positively charged acid-base resistant composite nanofiltration membrane of the embodiment and the composite membrane of the comparative example were immersed in a 5wt% HCl aqueous solution for 7 days, and then pre-pressed at 1.5MPa for 1h, and then the water flux, Fe 2+ The retention rate was tested and the results are shown in Table 8.

[0521] Table 8

[0522] As can be seen from Table 8, under the action of alkaline catalyst, the composite nanofiltration membrane is post-treated with halogenated alkanes to react with the tertiary amine groups in the acid-base resistant separation layer structure and convert them into quaternary ammonium salt groups with positive charges. The enhancement of the positive charge of the membrane surface increases the affinity with divalent Fe 2+ The electrostatic repulsion between the two significantly improves the membrane's resistance to divalent Fe 2+ The retention rate.

[0523] Comparing Example C6 and Example C7 shows that different types of multi-polar monomers affect their initial retention rate and initial water flux. Therefore, in the present invention, it is beneficial to select positively charged acid- and alkali-resistant composite nanofiltration membranes prepared with the same multi-polar monomers for vertical comparison. However, the positively charged acid- and alkali-resistant composite nanofiltration membranes prepared in the examples that meet the requirements of the multi-polar monomer type performed better than the comparative examples.

[0524] Comparing Example C1, Example C9, and Example C10, it can be seen that the amount of halogenated alkane used can affect the degree of reaction between the halogenated alkane and the tertiary amine group remaining on the acid- and alkali-resistant separation layer, and ultimately affect the content of nitrogen atoms in the quaternary ammonium salt structure. When the amount of halogenated alkane used is excessive, the retention rate is basically constant and the growth rate is small, but the water flux will be greatly reduced.

[0525] Comparing Example C1 with Example C11, it can be seen that when the third contact time is too short, the reaction cannot be fully carried out, and the retention rate and water flux both decrease.

[0526] Comparing Example C1, Example C12 and Example C13, it can be seen that the amount of catalyst used can promote the reaction of halogenated alkanes with tertiary amine groups on the surface of the composite nanofiltration membrane to be more complete, thereby improving the retention rate and water flux of the positively charged acid- and alkali-resistant composite nanofiltration membrane.

[0527] Comparing Example C1, Comparative Example C1 and Comparative Example C2, it can be seen that when no halogenated alkane is added, the composite nanofiltration membrane cannot achieve a good retention rate and water flux, and has no use value in acid / alkaline environments.

[0528] After 7 days of treatment with 5wt% HCl and 30 days of treatment with 10wt% NaOH, the positively charged acid-base resistant composite nanofiltration membrane had a significant effect on Fe 2+ The changes in the rejection rate and water flux of the membrane are not obvious, indicating that the positively charged acid-alkali resistant composite nanofiltration membrane has good acid / alkali resistance. Therefore, the positively charged composite membrane can achieve the concentration of divalent and high-valent ions and high H under acidic conditions. + Permeability, high H + The permeability shows that the acid can pass through the positively charged acid- and alkali-resistant composite nanofiltration membrane, and the acid can be recycled and reused.

[0529] Comparative Example A4 uses epoxy quaternary ammonium salt to modify the nanofiltration membrane. From the data in Table 1, Table 2, Table 3, Table 6 and Table 8, it can be seen that compared with the quaternary ammonium salt of the present invention, the nanofiltration membrane modified with epoxy quaternary ammonium salt has a lower surface potential and a significantly reduced membrane pore size, resulting in a low rejection rate of the nanofiltration membrane for divalent ions, poor permeability to monovalent ions, and water flux loss.

[0530] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection 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 a bottom layer, a porous middle layer and an acid- and alkali-resistant separation layer in sequence; Wherein, the acid-base resistant separation layer comprises: (1) a quaternary ammonium salt group represented by formula I and / or a quaternary phosphonium salt group represented by formula II grafted onto the acid-base resistant separation layer via a N atom; wherein R1 is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2, R3, and R4 are each independently selected from a C1-C3 alkyl or phenyl group; R5 is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6, R7, and R8 are each independently phenyl or C7-C10 aralkyl group; X is a halogen; or (2) a quaternary ammonium salt structure, wherein the quaternary ammonium salt structure comprises the structural formula shown in formula III and / or the structural formula shown in formula IV, X is a halogen; n is an integer of 0-10; m is an integer of 2-10; Among them, the acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazineamine separation layer, a polysulfonamide separation layer, a poly(triazineamine-urea) separation layer, a poly(sulfonamide-urea) separation layer, a poly(triazineamine-sulfonamide) separation layer and a poly(triazineamine-sulfonamide-urea) separation layer.

2. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1, wherein: For (1), R1 is a substituted or unsubstituted alkylene or phenylene of C1-C9, and the substituent is a carboxyl group, a nitro group, an amino group, a silane group, a carbonyl group, an ether group or an ester group; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C9; preferably, R1 is an unsubstituted alkylene or phenylene of C1-C6; preferably, R1 is an unsubstituted alkylene of C1-C3; and / or R2, R3, R4 are each independently selected from C1-C3 alkyl; preferably, R2, R3, R4 are each independently CH3; and / or R5 is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5 is a C1-C3 alkylene group or a C8-C10 aralkylene group; and / or R6, R7, R8 are each independently phenyl or C8-C10 aralkyl; preferably, R6, R7, R8 are each independently phenyl; and / or X is Cl, Br or I.

3. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to claim 1, wherein: For (2), X is Cl, Br or I; and / or n is an integer from 0 to 7; and / or m is an integer from 2 to 8.

4. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 3, wherein: The acid-base resistant separation layer is selected from at least one of a polyurea separation layer, a polytriazineamine separation layer and a polysulfonamide separation layer.

5. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 4, wherein: The nitrogen atom in the quaternary ammonium salt group or the quaternary ammonium salt structure in the positively charged acid-base resistant composite nanofiltration membrane The content of is 0.5-20at.%, preferably 2-15at.%; and / or The content of phosphorus atoms in the quaternary phosphine salt groups in the positively charged acid-base resistant composite nanofiltration membrane is 0.4-3 at.%, preferably 0.5-2 at.%.

6. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 5, wherein: For (1), the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid-base resistant composite nanofiltration membrane is 10-20 at.%, or 10-15 at.%; or For (1), the content of nitrogen atoms in the quaternary ammonium salt groups in the positively charged acid-base resistant composite nanofiltration membrane is 0.5-6at.%, or 2-5at.%; and / or For (2), the content of nitrogen atoms in the quaternary ammonium salt structure in the positively charged acid-base resistant composite nanofiltration membrane is 0.5-5at.%, preferably 2-5at.%.

7. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 6, wherein: The surface Zeta potential of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0-30 mV, preferably 5-25 mV, and more preferably 5-20 mV.

8. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 7, wherein: The average pore size of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 0.1-0.5 nm, preferably 0.1-0.4 nm, preferably 0.15-0.3 nm, and preferably 0.2-0.3 nm.

9. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 8, wherein: The contact angle of the positively charged acid- and alkali-resistant composite nanofiltration membrane is 20-80°, preferably 30-80°, and preferably 30-60°.

10. The positively charged acid- and alkali-resistant composite nanofiltration membrane according to any one of claims 1 to 9, wherein: The thickness of the bottom layer is 30-150 μm, preferably 50-120 μm; and / or The thickness of the porous support layer is 10-100 μm, preferably 30-60 μm; and / or The thickness of the acid-base resistant separation layer is 10-500 nm, preferably 50-300 nm.

11. A method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, characterized in that: The preparation method comprises the following steps: S1. Under stirring conditions, adding a first solution containing a quaternary ammonium halogenate salt and / or a quaternary phosphonium halogenate salt and a catalyst to a second solution containing a polyamine to react and obtain a modified polyamine; S2, preparing a porous supporting layer on the bottom layer; S3, after the surface of the porous support layer is first contacted with the aqueous phase containing the modified polyamine in step S1, and then secondly contacted with the organic phase containing the polyvalent polar monomer, heat-treated to obtain the positively charged acid-base resistant composite nanofiltration membrane; The halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2; Wherein, R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; R5' is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6', R7', and R8' are each independently phenyl or C7-C10 aralkyl group; X and X1 are each independently halogen.

12. A method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, characterized in that: The method comprises the following steps: S1', sequentially preparing a porous support layer and an acid-base resistant separation layer on the bottom layer to obtain a composite membrane; S2', contacting the composite membrane with an aqueous solution containing a catalyst, a halogenated quaternary ammonium salt and / or a halogenated quaternary phosphonium salt, and drying to obtain the positively charged acid-base resistant composite nanofiltration membrane; The halogenated quaternary ammonium salt has a structure shown in Formula 1; the halogenated quaternary phosphonium salt has a structure shown in Formula 2; Wherein, R1' is a C1-C9 substituted or unsubstituted alkylene or phenylene; R2', R3', and R4' are each independently selected from a C1-C3 alkyl or phenyl group; R5' is a C1-C6 alkylene, phenylene, or C7-C10 aralkylene group; R6', R7', and R8' are each independently phenyl or C7-C10 aralkyl group; X and X1 are each independently halogen.

13. The preparation method according to claim 11 or 12, wherein: R1' is a substituted or unsubstituted C1-C9 alkylene or phenylene, the substituent being a hydroxyl, carboxyl, nitro, amino, silane, carbonyl, ether or ester group; preferably, R1' is an unsubstituted C1-C9 alkylene or phenylene; preferably, R1' is an unsubstituted C1-C6 alkylene or phenylene; preferably, R1' is an unsubstituted C1-C3 alkylene; and / or R2', R3', R4' are each independently selected from C1-C3 alkyl; preferably, R2', R3', R4' are each independently CH3; and / or R5' is a C1-C3 alkylene group, a phenylene group or a C8-C10 aralkylene group; preferably, R5' is a C1-C3 alkylene group or a C8-C10 aralkylene group; R6', R7', R8' are each independently phenyl or C8-C10 aralkyl; preferably, R6', R7', R8' are each independently phenyl; and / or X is Cl, Br or I; and / or X1 is F, Cl, Br or I.

14. The preparation method according to any one of claims 11 to 13, wherein: The halogenated quaternary 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; and / or The halogenated quaternary phosphonium salt is selected from 3-bromopropyl triphenyl phosphonium bromide, 2-bromoethyl triphenyl phosphonium bromide, 1-bromoethyl triphenyl phosphonium bromide, bromomethyl triphenyl phosphonium bromide, 3-chloropropyl triphenyl phosphonium chloride, 2-chloroethyl triphenyl phosphonium chloride, 1-chloroethyl triphenyl phosphonium chloride, chloromethyl triphenyl phosphonium chloride, 3-iodopropyl triphenyl phosphonium iodide, 2-iodoethyl triphenyl phosphonium iodide, 1-iodoethyl triphenyl phosphonium iodide, iodomethyl triphenyl phosphonium bromide, 4-bromomethylbenzyl triphenyl At least one of 2-bromomethylbenzyltriphenylphosphonium bromide, 2-bromomethylbenzyltriphenylphosphonium bromide, 3-bromomethylbenzyltriphenylphosphonium bromide, 4-chloromethylbenzyltriphenylphosphonium chloride, 2-chloromethylbenzyltriphenylphosphonium chloride, 3-chloromethylbenzyltriphenylphosphonium chloride, 4-iodomethylbenzyltriphenylphosphonium iodide, 2-iodomethylbenzyltriphenylphosphonium iodide and 3-iodomethylbenzyltriphenylphosphonium iodide; preferably 3-bromopropyltriphenylphosphonium bromide and / or 4-bromomethylbenzyltriphenylphosphonium bromide; 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; preferably potassium hydroxide.

15. The preparation method according to claim 12, wherein: Step S1' comprises: S1-1', preparing a porous supporting layer on the bottom layer; and S1-2', after the surface of the porous support layer is first contacted with an aqueous phase containing polyamines, and then secondly contacted with an organic phase containing polyvalent polar monomers, heat treatment is performed to obtain a composite membrane.

16. The preparation method according to any one of claims 11 and 15, wherein: The polyamine is selected from at least one of polyethyleneimine, polyethyleneamine, triethylenetetramine, tetraethylenepentamine, diethylenetriamine, polyethylenepolyamine, piperazine, m-phenylenediamine and p-phenylenediamine; preferably polyethyleneimine and / or polyethylenepolyamine; and / or The polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides; Preferably, the polyisocyanate is selected from at least one of meta-phenylenediisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylenediisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenylenediisocyanate, 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 meta-phenylenediisocyanate; preferably 1,4-phenylenediisocyanate and / or 1,3-phenylenediisocyanate; Preferably, 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; Preferably, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride.

17. The preparation method according to any one of claims 11 to 14 and 16, wherein: In step S1, The concentration of the quaternary ammonium halogenate and / or the quaternary phosphonium halogenate in the first solution is 0.5wt%-20wt%, preferably 1wt%-10wt%; and / or The concentration of the catalyst in the first solution is 0.01wt%-5wt%, preferably 0.1wt%-1wt%; and / or The concentration of the polyamine in the second solution is 1wt%-20wt%, preferably 5wt%-10wt%; and / or The amounts of the first solution and the second solution are such that the mass ratio of the polyamine, the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt, and the catalyst is 1-1000:1-100:1, preferably 1-200:1-50:1; and / or The reaction conditions include: reaction temperature of 25-90°C, preferably 40-60°C; reaction time of 1-48h, preferably 6-24h; and / or Among them, in step S3, The concentration of the modified polyamine in the aqueous phase is 0.1wt%-10wt%, preferably 0.5wt%-2.5wt%; and / or The concentration of the multi-polar monomer in the organic phase is 0.01wt%-2wt%, preferably 0.05-1wt%; and / or The first contacting time is 5-100 s, preferably 10-60 s; and / or The second contacting time is 10-200 s, preferably 20-120 s; and / or The heat treatment conditions include: heat treatment temperature of 40-150° C., preferably 50-120° C.; heat treatment time of 0.5-10 min, preferably 1-5 min.

18. The preparation method according to any one of claims 12 to 16, wherein: In step S2', the contacting method is immersion; and / or The contact time is 10s-10min, preferably 20s-1min; the contact temperature is 20-80°C, preferably 30-50°C; and / or The drying time is 0.5-10 min, preferably 3-5 min; the drying temperature is 40-80° C., preferably 50-70° C.; and / or The concentration of the catalyst in the aqueous solution is 0.01wt%-5wt%, preferably 0.1wt%-1wt%; and / or The concentration of the halogenated quaternary ammonium salt and / or the halogenated quaternary phosphonium salt in the aqueous solution is 1 wt%-20 wt%, preferably 5 wt%-10 wt%.

19. A method for preparing a positively charged acid- and alkali-resistant composite nanofiltration membrane, characterized in that: The preparation method comprises the following steps: S1”, preparing a porous supporting layer on the bottom layer; S2", the membrane layer obtained in step S1" is first contacted with an aqueous phase containing a polyamine having a tertiary amine group, and second contacted with an organic phase containing a polyamine polar monomer, and after heat treatment, a composite nanofiltration membrane including an acid-base resistant separation layer is obtained; S3", bringing the composite nanofiltration membrane into a third contact with an organic solution containing halogenated alkanes and a catalyst, and drying to obtain the positively charged acid- and alkali-resistant composite nanofiltration membrane.

20. The preparation method according to claim 19, wherein: In step S2", the polyamine containing a tertiary amine group is selected from at least one of polyethyleneimine, polyethylene polyamine, 1-aminopiperazine, 1,4-diaminopiperazine, 1,4-piperazine diethylamine and 1,4-bisaminopropylpiperazine, preferably polyethyleneimine and / or polyethylene polyamine; and / or In the aqueous phase of the polyamine containing tertiary amine groups, the concentration of the polyamine containing tertiary amine groups is 0.1wt%-10wt%, preferably 0.5wt%-2.5wt%; and / or The polyvalent polar monomer is selected from at least one of polyvalent isocyanates, triazine compounds containing at least two C-Cl bonds, and polyvalent sulfonyl chlorides; Preferably, the polyisocyanate is selected from at least one of meta-phenylene diisocyanate, isophorone diisocyanate, 1,6-hexamethylene diisocyanate, toluene-2,6-diisocyanate, 1,4-phenylene diisocyanate, toluene-2,4-diisocyanate, 4,4'-methylenebis(phenyl isocyanate), 1,3-phenylene 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 meta-phenylene diisocyanate; preferably selected from 1,4-phenylene diisocyanate and / or 1,3-phenylenediisocyanate; Preferably, 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; Preferably, the polysulfonyl chloride is selected from at least one of 1,3-benzenedisulfonyl chloride, 1,2-benzenedisulfonyl chloride, 1,4-benzenedisulfonyl chloride, 2,4-disulfonyl chloride-mesitylene, biphenyl-4,4'-disulfonyl chloride, 4,5-dichloro-1,3-benzenedisulfonyl chloride, 2,6-naphthalenedisulfonyl chloride, 1,3-naphthalenedisulfonyl chloride, 2,7-naphthalenedisulfonyl chloride, 1,3,5-benzenetrisulfonyl chloride and 1,3,6-naphthalenetrisulfonyl chloride; preferably 1,3-benzenedisulfonyl chloride; and / or In the organic phase, the concentration of the multi-polar monomer is 0.01wt%-2wt%, preferably 0.05-1wt%; and / or The amount of the aqueous phase of the polyamine containing tertiary amine groups and the organic phase containing the multi-polar monomer is such that the weight ratio of the polyamine containing tertiary amine groups to the multi-polar monomer is 2-200:1, preferably 5-80:1; and / or The first contacting time is 5-100 s, preferably 10-60 s; and / or The second contacting time is 10-200 s, preferably 20-120 s; and / or The heat treatment conditions include: the heat treatment temperature is 40-150°C, preferably 50-120°C; The heat treatment time is 0.5-10 min, preferably 1-5 min.

21. The preparation method according to claim 19 or 20, wherein: In step S3", the halogenated alkane is selected from the halogenated alkane shown in formula 3 and / or the halogenated alkane shown in formula 4; X1', X2' and X3' are each independently halogen; n is an integer of 0-10; m is an integer of 2-10; Preferably, X1', X2' and X3' are each independently Cl, Br or I; Preferably, n is an integer of 0-7; m is an integer of 2-8; Preferably, the halogenated alkane is selected from methyl iodide, ethyl iodide, propyl iodide, butyl iodide, pentane iodide, iodine, iodine hexane, iodine heptane, iodine octane, iodine nonane, iodine oxalane, iodine cyclopropane, iodine cyclobutane, iodine cyclopentane, iodine cycloheptane, iodine cyclooctane, 1,2-diiodoethane, 1,3-diiodopropane, 1,4-diiodobutane, 1,5-diiodopentane, 1,6-diiodohexane, 1,7-diiodoheptane, 1,8-diiodooctane, methyl chloride, ethyl chloride, chloropropane, chlorobutane, chloropentane, hexyl chloride, chloroheptane, chlorooctane, chlorononane, chlorooxalane, chlorocyclopropane, chlorocyclobutane, chlorocyclopentane, chlorocycloheptane, chlorocyclooctane, 1,2-dichloroethane, 1,3-dichloropropane, 1, At least one of 4-dichlorobutane, 1,5-dichloropentane, 1,6-dichlorohexane, 1,7-dichloroheptane, 1,8-dichlorooctane, methyl bromide, ethyl bromide, propyl bromide, butyl bromide, pentane bromide, hexane bromide, heptane bromide, octane bromide, nonane bromide, cyclopropane bromide, cyclobutane bromide, cyclopentane bromide, cycloheptane bromide, cyclooctane bromide, 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromobutane, 1,5-dibromopentane, 1,6-dibromohexane, 1,7-dibromoheptane and 1,8-dibromooctane, preferably at least one selected from ethyl bromide, propyl chloride and methyl iodide; and / or The catalyst is selected from at least one of sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate, potassium bicarbonate, potassium tert-butoxide and sodium tert-butoxide, preferably selected from sodium hydroxide and / or sodium tert-butoxide; and / or The organic solvent is an alcohol solvent.

22. The preparation method according to any one of claims 19 to 21, wherein: In step S3", in the organic solution, the concentration of the halogenated alkane is 1wt%-20wt%, preferably 5-10wt%; and / or In the organic solution, the concentration of the catalyst is 0.01wt%-5wt%, preferably 0.1wt%-1wt%; and / or The weight ratio of the halogenated alkane to the catalyst is 2-50:1, preferably 5-30:1; and / or The ratio of the volume of the organic solution to the membrane area of ​​the composite nanofiltration membrane is 0.1-1 mL / cm 2 , preferably 0.3-0.5 mL / cm 2 ; and / or The conditions of the third contact include: the third contact time is 1-120 min, preferably 5-60 min; and / or The drying conditions include: drying temperature of 30-80° C., preferably 40-60° C.; drying time of 0.5-5 min, preferably 1-3 min.

23. A positively charged acid- and alkali-resistant composite nanofiltration membrane prepared by the preparation method described in any one of claims 11 to 22.

24. Use of the positively charged acid- and alkali-resistant composite nanofiltration membrane described in any one of claims 1-10 and 23 in the field of water treatment.

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