Method for grafting phenolic ammonium zwitterions on the surface of aromatic polyamide composite membrane

Abstract

The invention provides a method for grafting ammonium phenol zwitterions on the surface of an aromatic polyamide composite membrane. Specifically, an isocyanate crosslinking agent containing ammonium phenol units and polyether chains in the molecular structure is used to graft and crosslink the surface of the aromatic polyamide composite membrane, and the modified membrane surface has high hydrophilicity; the intrinsic N-H groups of the aromatic polyamide separation layer are replaced by new aromatic polyamide N-H groups, and the latter replaces the reaction between the aromatic polyamide and the residual chlorine or chlorine oxide in seawater, thereby effectively ensuring the long-term stability of the aromatic polyamide composite membrane structure.

Description

Technical Field

[0001] The present invention relates to a method for grafting modification of the surface of an aromatic polyamide composite membrane with a functional isocyanate crosslinking agent. The functional isocyanate crosslinking agent refers to an isocyanate crosslinking agent containing a phenol ammonium structural unit in its molecular structure, and specifically relates to a method for grafting phenol ammonium zwitterions on the surface of an aromatic polyamide composite membrane with an isocyanate group as a spacer arm. The method is applied to a seawater desalination process and belongs to the field of functional polymer materials. Technical Background

[0002] Since aromatic polyamide reverse osmosis composite membranes became mainstream products for seawater desalination, the improvement of the chlorine oxidation resistance of related aromatic polyamide reverse osmosis composite membranes has always attracted people's attention. In order to prevent marine biofouling on the membrane surface, a simple and inexpensive measure is to use chlorine to thoroughly disinfect and sterilize the seawater before reverse osmosis to achieve the purpose of cleaning water sources and controlling biological pollution. However, aromatic polyamide reverse osmosis composite membranes are sensitive to chlorine oxidation. Residual active chlorine (including chlorine or hypochlorite) in seawater chemically destroys the polymer chain structure of the aromatic polyamide separation layer, causing the membrane separation performance to decline rapidly and the service life to be greatly reduced. Therefore, for decades, people have been continuously investing huge manpower, financial resources and material resources to research and develop aromatic polyamide reverse osmosis composite membranes with high chlorine resistance.

[0003] The important methods to solve the problem of marine biofouling are (1) surface coating, (2) surface deposition, and (3) surface grafting of aromatic polyamide reverse osmosis composite membranes. The most common method for membrane surface grafting modification is chemical methods, such as surface grafting of water-soluble substances, polyethylene glycol, polyacrylic acid, polyacrylamide, quaternary ammonium salts, zwitterionic polymer brushes, betaine, etc. There are also grafting of sulfide, sulfonamide, chloramine, polysaccharide, graphene oxide or carbon nanotubes, which have a positive effect on improving its antifouling performance and anti-chlorine stability. There are many experimental facts that show that zwitterions such as carboxybetaine, sulfobetaine, and phosphorylcholine have strong hydration and sterilization and antibacterial effects, and have excellent anti-protein or microbial surface adhesion properties; and can also significantly improve the hydrophilicity of the membrane surface and reduce the water permeability resistance of the membrane; there are also reports that the flexible accumulation and hydration of the polyether chain of polyethylene glycol on the membrane surface also have significant anti-biological adhesion properties; however, the uniformity of the membrane surface modification using the above methods is poor, the surface smoothness is reduced, the density of the introduced functional groups is small, and the changes in the membrane surface characteristics are not obvious; some are also very easy to cause membrane pore blockage, and the water permeability is reduced; in addition, during the membrane surface graft copolymerization modification process, the monomer raw material waste caused by the homopolymerization of monomers is serious; finally, the outstanding defects of these existing surface modification methods and technologies are low suitability for aromatic polyamide composite membrane varieties and single performance improvement. How to improve the comprehensive performance of the membrane in an all-round way is still a bottleneck problem in the field of reverse osmosis membrane desalination.

[0004] Secondly, reducing the presence of carbamate NH in the separation layer of aromatic polyamide reverse osmosis composite membrane is also an effective measure to improve its chlorine resistance and antifouling performance. CN103349922B and CN105251372B once used chemically active groups on the surface of aromatic polyamide composite membranes to "plant" quaternary ammonium salts and salicylaldehyde. Due to the complex process and single modification object, the application prospects are not good. Subsequently, CN201811305536.9 and CN201811011240.6 proposed that the terminal isocyanate polyurethane prepolymer containing quaternary ammonium cations, polyethers and salicylaldehyde units in the molecular structure is subjected to addition reaction with the carbamate NH of the aromatic polyamide separation layer to achieve graft modification of the surface of the aromatic polyamide composite membrane, and the hydrophilicity and chlorine resistance of the membrane surface are greatly improved, but the interchain structure of the aromatic polyamide separation layer is destroyed, which affects the desalination separation efficiency of the membrane.

[0005] To this end, the present invention proposes a method for surface grafting modification of an aromatic polyamide composite membrane using a functional isocyanate crosslinking agent containing a phenol ammonium structural unit in its molecular structure. The functional isocyanate crosslinking agent can not only chemically react with intrinsic aromatic amide NH on the surface of the aromatic polyamide composite membrane, eradicate the parts that are easily attacked by active chlorine, and improve the chlorine resistance of the membrane; and through the crosslinking of the aromatic polyamide separation layer chains by the functional isocyanate crosslinking agent, the interchain structural firmness of the separation layer of the aromatic polyamide composite membrane is strengthened, and its desalination separation efficiency and the efficient and long-lasting operation of the membrane are guaranteed. In addition, the newly formed aromatic amide NH units generated by the reaction of the functional aromatic isocyanate crosslinking agent with the residual intrinsic primary amino group or intrinsic aromatic amide NH on the surface of the membrane can replace the intrinsic aromatic amide NH of the membrane to participate in the chemical reaction of active chlorine in seawater, and protect the membrane itself from the attack of active chlorine in seawater. In summary, the present invention provides an isocyanate crosslinking agent containing a phenol ammonium structural unit in its molecular structure, which uses isocyanate as a spacer arm to bond phenol ammonium zwitterions and polyether chains to the surface of the aromatic polyamide composite membrane, thereby achieving comprehensive improvement of membrane performance. Summary of the invention

[0006] The method for grafting phenol ammonium zwitterions on the surface of the aromatic polyamide composite membrane of the present invention is achieved according to the following steps: the washed and dried aromatic polyamide composite membrane is immersed in a functional isocyanate crosslinking agent solution, or the functional isocyanate crosslinking agent solution is sprayed, brushed or rolled on the surface of the aromatic polyamide composite membrane, and then heat-treated at 60 to 130° C. for 0.2 to 20 hours. After cooling, the membrane is washed with ethanol and deionized water and dried to obtain an aromatic polyamide composite membrane with phenol ammonium zwitterions grafted on the surface. The structure of the aromatic polyamide composite membrane is shown in the following formula:

[0007]

[0008] The aromatic polyamide composite membrane comprises one of a nascent membrane, a hydrolyzed membrane, a modified membrane or a regenerated membrane; the nascent membrane refers to an aromatic polyamide composite membrane directly produced by a polycondensation reaction on a porous polysulfone support membrane or a porous polyethersulfone support membrane using m-phenylenediamine and trimesoyl chloride as raw materials; the hydrolyzed membrane refers to an aromatic polyamide composite membrane obtained by hydrolysis of the nascent membrane; the modified membrane refers to an aromatic polyamide composite membrane obtained by surface modification of the nascent membrane, the hydrolyzed membrane or the commercially available aromatic polyamide composite membrane; the regenerated membrane refers to an aromatic polyamide composite membrane that has been used and cleaned by physical or chemical methods.

[0009] The mass percentage concentration of the functional isocyanate crosslinking agent in the functional isocyanate crosslinking agent solution is 0.5-50%; the dosage of the functional isocyanate crosslinking agent solution is 5-50% of the mass of the aromatic polyamide composite film.

[0010] The functional isocyanate cross-linking agent has a structure shown in general formula (A):

[0011]

[0012] Wherein R in the general formula (A) is selected from H or methyl, n is selected from a natural number between 1 and 2000, and R1 and R2 are selected from C1 to C 18 Hydrocarbon, Selected from C1~C 18 Hydrocarbylene, the Selected from C1~C 18 Alkylene or One of the above, where p is selected from a natural number between 1 and 2000.

[0013] There are three types of functional groups in the molecular structure of the functional isocyanate crosslinking agent described in the present invention. The first is the isocyanate group (-NCO), which is a spacer arm that chemically reacts with the residual NH2 or intrinsic aromatic amide NH on the surface of the aromatic polyamide composite membrane; the newly generated nascent aromatic amide NH can replace the intrinsic aromatic amide NH on the surface of the aromatic polyamide composite membrane to react with chlorine and oxygen; secondly, the bisphenol ammonium betaine and polyether chain have hydrophilicity, conductivity, antibacterial, antifogging, antifouling or anticoagulant properties, and can produce a hydration layer, a bactericidal and antibacterial layer, and a soft and elastic layer on the membrane surface, all of which will produce anti-marine biofouling performance; furthermore, the functional isocyanate crosslinking agent described in the present invention can chemically crosslink the intrinsic NH in the membrane chain structure to improve its physical, chemical or mechanical stability.

[0014] The specific preparation process of the functional isocyanate crosslinking agent of the general formula (A) of the present invention is as follows: at room temperature, an organic diamine is dissolved in an organic solvent, and polyethylene glycol monoacrylate is slowly added with stirring, wherein the amount of polyethylene glycol monoacrylate is 2.0 to 2.5 times the molar amount of the organic diamine; after the polyethylene glycol monoacrylate is added, the reaction temperature is increased to 50 to 70° C., and the reaction is continued for 4 to 40 hours, and then the Michael addition reaction process is terminated; the reaction temperature is maintained, and a catalyst, a diluent and a polyisocyanate are added to the reaction system in sequence, and the reaction is continued with stirring and heat preservation. After the NCO content in the material in the reactor is detected to be consistent with the predetermined value, the temperature of the reaction product system is lowered to room temperature, and then a 5-chloromethyl salicylaldehyde solution is added to the reaction system, and the reaction is continued with stirring for 0.2 to 20 hours to obtain a functional isocyanate crosslinking agent solution with a structure shown in the general formula (A) for standby use.

[0015] The organic diamine has a general formula (B) as shown below:

[0016]

[0017] Wherein R1 and R2 in the general formula (B) are selected from substituted or unsubstituted hydrocarbon groups, Selected from C1~C 18 Alkylene or One of the above, where p is selected from a natural number between 1 and 2000.

[0018] The organic solvent refers to one or more of tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, chlorobenzene, chloroform, 1,2-dichloroethane, hexane, cyclohexane, toluene, xylene, decahydronaphthalene, ethyl acetate, N-methylpyrrolidone, acetone, butanone, cyclohexanone, N,N-dimethylaniline, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, N,N-dimethylformamide, N,N-dimethylacetamide or dimethyl sulfoxide; the amount of the organic solvent used is 1 to 10 times the mass of the general formula (B).

[0019] The polyethylene glycol monoacrylate has a structure shown in general formula (C):

[0020]

[0021] Wherein R in the general formula (C) is selected from H or methyl, and n is selected from a natural number between 1 and 2000.

[0022] The catalyst refers to a composite of organic tin and organic amine; wherein the organic tin refers to one of dibutyltin dilaurate, stannous octoate, stannous oxalate, dibutyltin dimaleate, di(dodecylsulfide) dibutyltin or dibutyltin diacetate; the organic amine refers to one of triethylamine, p-dimethylaminopyridine, N,N-dimethylformamide, triethylenediamine, 1,4-dialkylpiperazine, 1-alkylimidazole, 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylenediamine carboxylate, 1,4-dialkylpiperazine carboxylate or 1-alkylimidazole carboxylate; the mass ratio of the organic tin to the organic amine is 1:0-1.5; the amount of the catalyst is 0.01-5% of the mass of the polyisocyanate of the general formula (D).

[0023] The polyisocyanate has a structure shown in general formula (D):

[0024]

[0025] Wherein q in the general formula (D) is selected from a positive integer of 1 to 5, Selected from C1~C 18 alkylene;

[0026] The amount of the polyisocyanate used is 2.02 to 2.25 times the molar amount of the organic diamine of the general formula (B).

[0027] The diluent refers to one or more of acetone, butanone, cyclohexanone, methyl acetate, ethyl acetate, ether, tetrahydrofuran, 1,4-dioxane, toluene, xylene or dimethyl sulfoxide; the amount of the diluent is 1 to 10 times the mass of the polyisocyanate of general formula (D).

[0028] The mass percentage concentration of 5-chloromethyl salicylaldehyde in the 5-chloromethyl salicylaldehyde solution is 5-50%, and the amount of 5-chloromethyl salicylaldehyde used is 2.0-2.5 times the molar amount of the organic diamine of the general formula (B).

[0029] Compared with the prior art, the modified aromatic polyamide composite membrane prepared by the present invention has the following advantages:

[0030] ① After the functional isocyanate cross-linking agent of the general formula (A) provided by the present invention is applied to the surface modification of the aromatic polyamide composite membrane, the surface of the obtained modified polyaromatic amide composite membrane has super hydrophilicity and high anti-fouling and chlorine resistance.

[0031] ② The functional isocyanate cross-linking agent of the general formula (A) provided by the present invention is applied to the post-chemical modification of the surface of the aromatic polyamide composite membrane, which has the characteristics of wide modification adaptability, multi-directional modification, simple process, and adjustable and controllable operation. DETAILED DESCRIPTION

[0032] Example 1 Application of the functional isocyanate cross-linking agent of formula (A1) in the grafting modification of aromatic polyamide composite membrane Preparation of the functional isocyanate cross-linking agent of formula (A1)

[0033] 25 g of N,N'-dibenzylethylenediamine was dissolved in 80 g of 1,4-dioxane and added to a reactor. Stirring was started and 30 g of hydroxyethyl methacrylate was slowly added at room temperature. After the addition of hydroxyethyl methacrylate was completed, the reaction temperature was raised to 55-60°C. After the reaction was continued for 4 hours, the temperature of the reactants in the reactor was maintained, 0.22 g of dibutyltin dilaurate, 0.15 g of triethylenediamine, 120 g of acetone and 40 g of toluene diisocyanate were added to the reactor, and the reaction was continued with insulation and stirring. When the NCO value in the reaction product system was detected to be 3.76-3.83%, the temperature of the reaction product system was lowered to room temperature, and 80 g of a toluene solution containing 35 g of 5-chloromethyl salicylaldehyde was added. After stirring for 2 hours, a solution of a functional isocyanate crosslinking agent of formula (A1) was prepared for use.

[0034]

[0035] Surface Graft Modification of New Aromatic Polyamide Composite Membranes

[0036] According to the methods and steps disclosed in Journal Membrane Science 428 (2013) 403-409, Journal Membrane Science 457 (2014) 88-97, CN103349922B or CN105251372B, m-phenylenediamine and trimesoyl chloride are used as monomers, and a nascent aromatic polyamide composite membrane with a shear area of ​​1*1 cm and containing acyl chloride groups on the surface is prepared by interfacial polymerization on the surface of a porous polysulfone support membrane. The nascent aromatic polyamide composite membrane is immersed in a solution of a functional isocyanate crosslinking agent of formula (A1) with a mass percentage concentration of 4-6%. After 3 minutes, the membrane is taken out and placed in a press plate at 120° C. for heat treatment for 30 minutes, and then cooled and washed with ethanol and deionized water until neutral, to obtain a modified membrane (A1-1) of a nascent aromatic polyamide composite membrane modified by a functional isocyanate crosslinking agent of formula (A1).

[0037] Surface grafting modification of commercial aromatic polyamide composite membranes

[0038] A commercial aromatic polyamide composite membrane (BW-30, DuPont FilmTec RO membrane, USA) was cut into 1*1 cm sample membranes, and the commercial membrane samples were treated with a functional isocyanate cross-linking agent solution of formula (A1) according to the modification procedure and process of the above-mentioned new aromatic polyamide composite membrane to obtain a modified membrane (A1-2) of the commercial aromatic polyamide composite membrane grafted with the functional isocyanate cross-linking agent of formula (A1).

[0039] According to the modification procedure and process of the above-mentioned new aromatic polyamide composite membrane, the modified membrane (A1-2) is treated for a second time with a functional isocyanate crosslinking agent solution of formula (A1) to obtain a second modified commercial membrane (A1-3).

[0040] Example 2: Application of the functional isocyanate crosslinking agent of formula (A2) in the modification of aromatic polyamide composite membrane

[0041] According to the method and operation steps of Example 1, the hydroxyethyl methacrylate in Example 1 is replaced by polyethylene glycol-600 monomethacrylate, and the toluene diisocyanate is replaced by diphenylmethane diisocyanate to prepare a solution of a functional isocyanate crosslinking agent of formula (A2) for later use.

[0042]

[0043] According to the method and operation steps of Example 1, the surface of the new membrane and the commercial membrane were modified using the functional isocyanate crosslinker solution of formula (A2) to obtain the modified new modified membrane (A2-1) and BW-30 modified membrane (A2-2), respectively.

[0044] Example 3: Application of the functional isocyanate crosslinking agent of formula (A3) in the modification of aromatic polyamide composite membrane

[0045] According to the method and operation steps of Example 1, the N,N'-dibenzylethylenediamine in Example 1 is replaced with N,N'-didodecylhexanediamine, hydroxyethyl methacrylate is replaced with polyethylene glycol-600 monomethacrylate, and toluene diisocyanate is replaced with diphenylmethane diisocyanate to prepare a solution of a functional isocyanate crosslinking agent of formula (A3) for later use.

[0046]

[0047] According to the method and operation steps of Example 1, the surface of the new membrane and the commercial membrane were modified using the functional isocyanate crosslinker solution of formula (A3) to obtain the modified new modified membrane (A3-1) and BW-30 modified membrane (A3-2), respectively.

[0048] Example 4: Application of the functional isocyanate crosslinking agent of formula (A4) in the modification of aromatic polyamide composite membrane

[0049] According to the method and operation steps of Example 1, the N,N'-dibenzylethylenediamine in Example 1 is replaced with α,ω-bis(benzylamino)polyether-200, and the hydroxyethyl methacrylate is replaced with polyethylene glycol-600 monomethacrylate to prepare a solution of a functional isocyanate crosslinking agent of formula (A4) for later use.

[0050]

[0051] According to the method and operating steps of Example 1, the surface of the newly formed aromatic polyamide composite membrane and the commercial aromatic polyamide composite membrane of BW-30 were modified using the functional isocyanate crosslinker solution of formula (A4) to obtain the modified newly formed modified membrane (A4-1) and the modified BW-30 membrane (A4-2), respectively.

[0052] Example 5 Surface properties of membrane before and after modification

[0053] The nascent membrane modified membrane, the hydrolyzed membrane of the nascent membrane, the BW-30 commercial membrane and the modified membrane in Examples 1 to 4 were taken, and the water contact angles of these membranes were measured according to the methods disclosed in Journal of Membrane Science 428 (2013) 403-409 or Journal of Membrane Science 457 (2014) 88-97, CN103349922B or CN105251372B. The results are shown in Table 1.

[0054] Table 1 Surface properties of aromatic polyamide composite membranes

[0055]

[0056] The experimental results in Table 1 clearly show that no matter whether it is a newly prepared aromatic polyamide composite membrane or a commercial aromatic polyamide composite membrane, after being treated with the functional isocyanate cross-linking agent of the present invention, its surface hydrophilicity is greatly improved, and both exhibit water-wetting phenomenon, which is caused by the strong hydration of bisphenol ammonium zwitter ions and polyether chains, thereby confirming that bisphenol ammonium zwitter ions and polyether chains have been grafted on the surface of the aromatic polyamide composite membrane.

Claims

1. A method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane, characterized in that The following steps are involved: The washed and dried aromatic polyamide composite membrane is immersed in a functional isocyanate crosslinking agent solution, or the functional isocyanate crosslinking agent solution is sprayed, brushed or rolled on the surface of the aromatic polyamide composite membrane, and then heat-treated at 60 to 130° C. for 0.2 to 20 hours, and after cooling, washed with ethanol and deionized water, and dried to obtain an aromatic polyamide composite membrane with phenol ammonium zwitterions grafted on the surface; The functional isocyanate crosslinking agent in the functional isocyanate crosslinking agent solution has a mass percentage concentration of 0.5 to 50%; the amount of the functional isocyanate crosslinking agent solution is 5 to 50% of the mass of the aromatic polyamide composite membrane; The functional isocyanate cross-linking agent has a structure shown in general formula (A): Wherein R in the general formula (A) is selected from H or methyl, n is selected from a natural number between 1 and 2000, and R1 and R2 are selected from C1 to C 18 Hydrocarbon, Selected from C1~C 18 Hydrocarbylene, the Selected from C1~C 18 Alkylene or One of the above, where p is selected from a natural number between 1 and 2000.

2. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 1, characterized in that The aromatic polyamide composite membrane includes one of a new membrane, a hydrolyzed membrane, a modified membrane or a regenerated membrane; The nascent membrane refers to an aromatic polyamide composite membrane produced by a condensation reaction on a porous polysulfone support membrane or a porous polyethersulfone support membrane using m-phenylenediamine and trimesoyl chloride as raw materials; the hydrolyzed membrane refers to an aromatic polyamide composite membrane obtained by hydrolysis of the nascent membrane; the modified membrane refers to an aromatic polyamide composite membrane obtained by surface modification of the nascent membrane, hydrolyzed membrane or commercially available aromatic polyamide composite membrane; the regenerated membrane refers to a used aromatic polyamide composite membrane cleaned by physical or chemical methods.

3. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 1, characterized in that The specific preparation process of the functional isocyanate crosslinking agent of the general formula (A) is as follows: at room temperature, an organic diamine is dissolved in an organic solvent, and polyethylene glycol monoacrylate is slowly added with stirring, wherein the amount of polyethylene glycol monoacrylate is 2.0 to 2.5 times the molar amount of the organic diamine; after the polyethylene glycol monoacrylate is added, the reaction temperature is increased to 50 to 70° C., and the reaction is continued for 4 to 40 hours, and the Michael addition reaction process is terminated; the reaction temperature is maintained, and a catalyst, a diluent and a polyisocyanate are added to the reaction system in sequence, and the reaction is continued with stirring and heat preservation. After the NCO content in the material in the reactor is detected to be consistent with the predetermined value, the temperature of the reaction product system is lowered to room temperature, and then a 5-chloromethyl salicylaldehyde solution is added to the reaction system, and the reaction is continued with stirring for 0.2 to 20 hours, to obtain a functional isocyanate crosslinking agent solution with a structure represented by the general formula (A) for standby use.

4. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The organic diamine has a general formula (B) as shown below: Wherein R1 and R2 in the general formula (B) are selected from substituted or unsubstituted hydrocarbon groups, Selected from C1~C 18 Alkylene or One of the above, where p is selected from a natural number between 1 and 2000.

5. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The polyethylene glycol monoacrylate has a structure shown in general formula (C): Wherein R in the general formula (C) is selected from H or methyl, and n is selected from a natural number between 1 and 2000.

6. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The polyisocyanate has a structure shown in general formula (D): Wherein q in the general formula (D) is selected from a positive integer of 1 to 5, Selected from C1~C 18 alkylene; The amount of the polyisocyanate used is 2.02 to 2.25 times the molar amount of the general formula (B).

7. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The organic solvent is selected from one or more of tetrahydrofuran, 1,4-dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, chlorobenzene, chloroform, 1,2-dichloroethane, hexane, cyclohexane, toluene, xylene, decalin, ethyl acetate, N-methylpyrrolidone, N,N-dimethylaniline, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, N,N-dimethylformamide, N,N-dimethylacetamide or dimethyl sulfoxide; the amount of the organic solvent is 1 to 10 times the mass of the organic diamine of general formula (B).

8. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The catalyst refers to a composite of organic tin and organic amine; wherein the organic tin refers to one of dibutyltin dilaurate, stannous octoate, stannous oxalate, dibutyltin dimaleate, di(dodecylsulfide) dibutyltin or dibutyltin diacetate; the organic amine refers to one of triethylamine, p-dimethylaminopyridine, N,N-dimethylformamide, triethylenediamine, 1,4-dialkylpiperazine, 1-alkylimidazole, 1,8-diazabicyclo[5.4.0]undec-7-ene, triethylenediamine carboxylate, 1,4-dialkylpiperazine carboxylate or 1-alkylimidazole carboxylate; the mass ratio of the organic tin to the organic amine is 1:0-1.5; the amount of the catalyst used is 0.01-5% of the mass of the polyisocyanate of the general formula (D).

9. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The diluent refers to one or more of acetone, butanone, cyclohexanone, methyl acetate, ethyl acetate, ether, tetrahydrofuran, 1,4-dioxane, toluene, xylene or dimethyl sulfoxide; the amount of the diluent is 1 to 10 times the mass of the polyisocyanate of general formula (D).

10. The method for grafting phenol ammonium zwitterions onto the surface of an aromatic polyamide composite membrane according to claim 3, characterized in that The mass percentage concentration of 5-chloromethyl salicylaldehyde in the 5-chloromethyl salicylaldehyde solution is 5-50%, and the amount of 5-chloromethyl salicylaldehyde used is 2.0-2.5 times the molar amount of the organic diamine of the general formula (B).

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