Nanofiltration membrane and preparation method thereof

Abstract

The present invention relates to the technical field of water treatment, and in particular to a nanofiltration membrane and a preparation method thereof. The preparation method comprises: soaking a base membrane in an aqueous solution, taking out and drying the surface, coating an oil phase solution on the base membrane for interfacial polymerization reaction, and obtaining a nanofiltration membrane after solidification; the aqueous solution comprises a polyamine monomer, a surfactant, an acid binding agent and water; the acid binding agent is bicarbonate; the oil phase solution comprises an acyl chloride monomer, an organic solvent a and an organic solvent b; the organic solvent a is ethylcyclohexane; the organic solvent b comprises at least one of monochloromethane, monochloroethane, ethyl ether, methyl ethyl ether, dimethyl ether and methyl formate. The present invention improves the membrane flux while improving the acid chemical cleaning resistance of the membrane.

Description

Technical Field

[0001] The present invention relates to the technical field of water treatment, and in particular to a nanofiltration membrane and a preparation method thereof. Background Art

[0002] Nanofiltration membranes can effectively intercept macromolecular substances (substances with a weight average molecular weight of 200-1000) and divalent or higher ions due to their selective separation properties, allowing most small molecules and monovalent ions to pass through. This property also allows them to operate at lower working pressures than reverse osmosis membranes. Nanofiltration membranes in the prior art are mostly synthesized by the reaction of piperazine and polyacyl chlorides. Due to the slow reaction rate, the resulting polyamide structure is thinner and flatter, and its structure causes the following limitations:

[0003] 1) The thinner structure makes the nanofiltration membrane flux higher than the reverse osmosis membrane, but some application scenarios require products with higher flux. The structure obtained by the conventional preparation method is relatively flat, which is not as good as the reverse osmosis membrane polyamide structure with obvious wrinkles and vesicle structure, and the specific surface area is small, so it is impossible to obtain a higher flux;

[0004] 2) In engineering applications, acid-base chemical cleaning is unavoidable for nanofiltration membrane elements. In an acidic environment, the carbon atoms of the amide group in the polypiperazineamide structure are susceptible to nucleophilic attack by water molecules, resulting in hydrolysis (the stronger the acidity, the more severe the hydrolysis), which will directly lead to the destruction of the chemical structure, thereby gradually losing the filtration function. After cleaning, the performance is irreversibly reduced, shortening the service life of the nanofiltration membrane and increasing the cost. The thinner polyamide layer structure further reduces its hydrolysis tolerance in an acidic environment and shortens its life.

[0005] Nanofiltration membrane reactions are generally carried out using polyamine and polyacyl chloride monomers. In order to solve the defect that polyamide structures are not resistant to acid hydrolysis, relevant studies (CN111686592A) have mentioned the use of polysulfonyl chlorides to replace traditional polyacyl chloride monomers to obtain polysulfonamide structural materials. Compared with amide groups, sulfonamide groups have greater steric hindrance, and S atoms are more difficult to be nucleophilically attacked by water molecules in acidic environments, thus showing better acid resistance. However, at the same time, sulfonyl chloride structures are also difficult to react with polyamine monomers. The sulfonamidation reaction rate is slower than the conventional polyacyl chloride and polyamine reaction rate. The formed sulfonamide structure is loose and thick, and the membrane flux is relatively low.

[0006] There are also related studies on improving the flux of nanofiltration membranes, such as adding auxiliary solvents to increase the migration rate of polyamines and increase the reaction speed, thereby reducing the thickness of the polyamide layer, but this will shorten the chemical cleaning life of the product; adding new monomers to change the polyamide structure, but it is not conducive to improving reaction efficiency and cost control; adding nano-hydrophilic materials has limitations on mixing uniformity.

[0007] Therefore, while the existing technology solves the problem of acid resistance of nanofiltration membranes, the membrane flux is relatively low; when the membrane flux is improved by modification, its tolerance, reaction cost and other aspects cannot be taken into account. Summary of the invention

[0008] In view of this, the technical problem to be solved by the present invention is to provide a nanofiltration membrane and a preparation method thereof, which can improve the membrane flux while improving the membrane's resistance to acidic chemical cleaning.

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

[0010] The base membrane is immersed in an aqueous solution, taken out and dried, and then an oil phase solution is coated on the base membrane to perform an interfacial polymerization reaction, and after curing, a nanofiltration membrane is obtained;

[0011] The aqueous phase solution comprises a polyamine monomer, a surfactant, an acid binder and water; the acid binder is a bicarbonate;

[0012] The oil phase solution comprises an acyl chloride monomer, an organic solvent a and an organic solvent b; the organic solvent a is ethylcyclohexane; the organic solvent b comprises at least one of methyl chloride, ethyl chloride, ethyl ether, methyl ethyl ether, dimethyl ether and methyl formate.

[0013] Preferably, in the aqueous solution, the mass content of the acid binding agent is 2% to 4%.

[0014] Preferably, the acid binding agent is at least one of sodium bicarbonate and potassium bicarbonate.

[0015] Preferably, the polyamine monomer includes at least one of piperazine, m-phenylenediamine, ethylenediamine and 1,6-hexanediamine; and the mass content of the polyamine monomer in the aqueous phase solution is 1% to 2%.

[0016] Preferably, the surfactant includes at least one of sodium dodecyl sulfonate, sodium dodecyl sulfate and sodium lauryl sulfate; and the mass content of the surfactant in the aqueous phase solution is 0.5% to 1%.

[0017] Preferably, the acyl chloride monomer includes at least one of trimesoyl chloride, phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride and adipoyl chloride;

[0018] In the oil phase solution, the mass content of the acyl chloride monomer is 0.1% to 0.5%.

[0019] Preferably, in the oil phase solution, the mass content of organic solvent a is 49% to 75%;

[0020] In the oil phase solution, the mass content of the organic solvent b is 25% to 50%.

[0021] Preferably, the base film is immersed in the aqueous solution for 10 to 30 seconds.

[0022] Preferably, the thickness of the oil phase solution coated on the base film is 0.2-0.5 mm;

[0023] The interfacial polymerization reaction is carried out at room temperature for 5 to 20 seconds.

[0024] The present invention also provides a nanofiltration membrane prepared by the preparation method described above.

[0025] The present invention provides a method for preparing a nanofiltration membrane, comprising the following steps: soaking a base membrane in an aqueous solution, taking out and drying the surface, coating an oil phase solution on the base membrane for interfacial polymerization reaction, and curing to obtain a nanofiltration membrane; the aqueous solution comprises a polyamine monomer, a surfactant, an acid binding agent and water; the acid binding agent is bicarbonate; the oil phase solution comprises an acyl chloride monomer, an organic solvent a and an organic solvent b; the organic solvent a is ethylcyclohexane; the organic solvent b comprises at least one of monochloromethane, monochloroethane, ether, methyl ethyl ether, dimethyl ether and methyl formate. The present invention adds a specific acid binding agent to the aqueous solution and a specific organic solvent to the oil phase solution, so that the polyamide layer generated by the reaction is thicker and presents a wrinkled vesicle structure like a reverse osmosis membrane structure. The thicker polyamide layer can improve the acid hydrolysis resistance of the membrane product during the chemical cleaning process, and the wrinkled vesicle structure can significantly increase the specific surface area of ​​the polyamide layer and improve the membrane flux. The present invention further limits the amount of the acid binding agent, which is more conducive to improving the above-mentioned effect.

[0026] The present invention creatively changes the physical form of polypiperazineamide without changing its chemical structure, thereby improving the membrane flux while improving the membrane's acid chemical cleaning resistance. DETAILED DESCRIPTION

[0027] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0028] The present invention provides a method for preparing a nanofiltration membrane, comprising the following steps:

[0029] The base membrane is immersed in an aqueous solution, taken out and dried, and then an oil phase solution is coated on the base membrane to perform an interfacial polymerization reaction, and after curing, a nanofiltration membrane is obtained;

[0030] The aqueous phase solution comprises a polyamine monomer, a surfactant, an acid binder and water; the acid binder is a bicarbonate;

[0031] The oil phase solution comprises an acyl chloride monomer, an organic solvent a and an organic solvent b; the organic solvent a is ethylcyclohexane; the organic solvent b comprises at least one of methyl chloride, ethyl chloride, ethyl ether, methyl ethyl ether, dimethyl ether and methyl formate.

[0032] About basement membrane:

[0033] In certain embodiments of the present invention, the base membrane is a polysulfone base membrane. The polysulfone base membrane comprises:

[0034] Non-woven fabrics;

[0035] and a coating layer formed on the nonwoven fabric;

[0036] The coating layer is prepared from raw materials including N,N-dimethylformamide, polysulfone and polyvinyl pyrrolidone.

[0037] The nonwoven fabric has a thickness of 90-110 µm, such as 100 µm; and an air permeability of 2.3-2.7 cc / cm 2 / s, such as 2.5 cc / cm 2 / s.

[0038] The mass ratio of the N,N-dimethylformamide, polysulfone and polyvinyl pyrrolidone is 82-86:13-17:0.8-1.2, such as 84:15:1.

[0039] The number average molecular weight of the polyvinyl pyrrolidone is 24,000-130,000.

[0040] In certain embodiments of the present invention, the method for preparing the polysulfone comprises the following steps:

[0041] N,N-dimethylformamide, polysulfone and polyvinyl pyrrolidone are mixed, and the obtained transparent solution is evenly coated on one side of the non-woven fabric, and after curing, a polysulfone-based membrane is obtained.

[0042] The mixing is performed at 65-75° C. (eg, 70° C.), the mixing time is 3-5 h, eg, 4 h, and the mixing is stirring.

[0043] The curing method may be: immersing in 15° C. pure water for phase inversion curing.

[0044] About aqueous solution:

[0045] The aqueous phase solution comprises polyamine monomers, surfactants, acid binding agents and water.

[0046] The polyamine monomer is selected from at least one of piperazine, m-phenylenediamine, ethylenediamine and 1,6-hexanediamine. In the aqueous phase solution, the mass content of the polyamine monomer is 1% to 2%, such as 1%.

[0047] The surfactant is selected from at least one of sodium dodecyl sulfonate, sodium dodecyl sulfate and sodium lauryl sulfate. In the aqueous phase solution, the mass content of the surfactant is 0.5% to 1%, such as 0.5%.

[0048] The acid-binding agent is selected from bicarbonate, and specifically, is selected from at least one of sodium bicarbonate and potassium bicarbonate. In the aqueous solution, the mass content of the acid-binding agent is 2% to 4%, such as 2%.

[0049] In certain embodiments of the present invention, the method for preparing the aqueous solution comprises the following steps:

[0050] The polyamine monomer, surfactant, acid binding agent and water are mixed to obtain a homogeneous transparent solution, which is the aqueous phase solution.

[0051] The mixing is performed at room temperature, the mixing time is 0.3 to 0.7 h, such as 0.5 h, and the mixing is stirring.

[0052] About oil phase solution:

[0053] The oil phase solution comprises an acyl chloride monomer, an organic solvent a and an organic solvent b; the organic solvent a is ethylcyclohexane; the organic solvent b comprises at least one of methyl chloride, ethyl chloride, ethyl ether, methyl ethyl ether, dimethyl ether and methyl formate.

[0054] The acyl chloride monomer is selected from at least one of trimesoyl chloride, phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride and adipoyl chloride. In the oil phase solution, the mass content of the acyl chloride monomer is 0.1% to 0.5%, such as 0.2%.

[0055] In the oil phase solution, the mass content of the organic solvent a is 49% to 75%, for example 74.8%.

[0056] The organic solvent b is selected from at least one of methyl chloride, ethyl chloride, ether, methyl ethyl ether, dimethyl ether and methyl formate. In the oil phase solution, the mass content of the organic solvent b is 25% to 50%, such as 25%.

[0057] In certain embodiments of the present invention, the method for preparing the oil phase solution comprises the following steps:

[0058] The acyl chloride monomer, organic solvent a and organic solvent b are mixed to obtain a homogeneous solution, which is the oil phase solution.

[0059] The mixing is performed at room temperature, the mixing time is 0.3 to 0.7 h, such as 0.5 h, and the mixing is stirring.

[0060] About the preparation of nanofiltration membrane:

[0061] In certain embodiments of the present invention, the base film is immersed in the aqueous solution for 10 to 30 seconds, such as 15 seconds.

[0062] The surface drying method comprises:

[0063] The excess solution on the surface of the base film is rolled off with a rubber roller, and the residual aqueous solution on the front and back of the film is removed by hot air and vacuum absorption, so that there is no aqueous solution visible on the base film.

[0064] Afterwards, the oil phase solution is coated on one side of the base film, with a coating thickness of 0.2-0.5 mm, such as 0.3 mm.

[0065] The interfacial polymerization reaction is carried out at room temperature for 5 to 20 seconds, such as 10 seconds.

[0066] After the interfacial polymerization reaction, the method further comprises: removing the excess oil phase solution. Specifically, the method comprises: removing the excess oil phase solution with a rubber roller and an air knife.

[0067] The curing method is drying. The drying temperature is 50-60° C., such as 60° C. The drying is carried out in an oven.

[0068] The present invention also provides a nanofiltration membrane prepared by the preparation method described above.

[0069] The present invention has no particular limitation on the sources of the raw materials used above, and they can be generally commercially available.

[0070] In the present invention, water-soluble bicarbonate reacts with hydrogen chloride molecules generated in the interfacial polymerization reaction to generate a large number of carbon dioxide nanobubbles at the interface. These bubbles are formed between the polyamide layer and the substrate. Under certain external conditions, carbon dioxide causes the polyamide layer to form a vesicle structure, thereby increasing the specific surface area and thus increasing the membrane flux.

[0071] When a large amount of polar solvent is added to the oil phase solution, piperazine can form hydrogen bonds with the polar solution, reducing the obstacles for piperazine to enter the oil phase medium and react with polyacyl chlorides, intensifying the interfacial polymerization reaction, increasing the thickness of the polyamide layer, and improving its chemical cleaning tolerance. At the same time, unlike the mild reaction of ordinary polypiperazineamide, a large amount of reaction heat is generated under the violent reaction, which can increase the activity of carbon dioxide nanobubbles generated by the reaction of hydrogen chloride and bicarbonate. During the rapid discharge process, the flat polyamide layer forms a wrinkled vesicle structure, increasing the specific surface area, increasing the pure water permeation channel, resisting the flux decline caused by the increase in the thickness of the polyamide layer, and maintaining a high flux level.

[0072] In order to further illustrate the present invention, a nanofiltration membrane and a preparation method thereof provided by the present invention are described in detail below in conjunction with embodiments, but it should not be construed as limiting the scope of protection of the present invention.

[0073] The reagents used in the following examples and comparative examples are all commercially available.

[0074] Preparation of polysulfone-based membranes in Examples and Comparative Examples:

[0075] 84 parts by weight of N,N-dimethylformamide, 15 parts by weight of polysulfone (BASF, Germany), and 1 part by weight of polyvinyl pyrrolidone (number average molecular weight of 24000-130000) were stirred at 70°C for 4 h to obtain a uniform transparent solution; the solution was evenly coated on a non-woven fabric (thickness 100 µm, air permeability 2.5 cc / cm 2 / s) was immersed in 15°C pure water for phase inversion curing to obtain a polysulfone-based membrane.

[0076] Example 1

[0077] 1) Preparation of aqueous solution:

[0078] Add 1 part by weight of piperazine (polyamine monomer), 0.5 part by weight of sodium dodecyl sulfate (surfactant), 2 parts by weight of sodium bicarbonate (acid binding agent), and 96.5 parts by weight of pure water, and stir at room temperature for 0.5 h to obtain a homogeneous transparent solution, which is the aqueous phase solution.

[0079] 2) Preparation of oil phase solution:

[0080] Add 0.2 parts by weight of trimesoyl chloride (acyl chloride monomer), 74.8 parts by weight of ethylcyclohexane (organic solvent a), and 25 parts by weight of ethyl chloride (organic solvent b), and stir at room temperature for 0.5 h to obtain a homogeneous solution, which is the oil phase solution.

[0081] 3) Preparation of nanofiltration membrane:

[0082] The polysulfone-based membrane was immersed in the aqueous solution for 15 s, and the excess solution on the surface of the base membrane was removed by rolling with a rubber roller. The residual aqueous solution on the front and back of the membrane was then removed by hot air and vacuum absorption, so that there was no aqueous solution visible on the base membrane. Then, the oil solution was coated on the front of the polysulfone-based membrane with a coating thickness of 0.3 mm. After reacting at room temperature for 10 s, the excess oil solution was removed with a rubber roller and an air knife. The obtained membrane was placed in a 60°C oven for drying to obtain a nanofiltration membrane.

[0083] Example 2

[0084] The difference from Example 1 is:

[0085] In step 2), ethyl chloride (organic solvent b) is replaced by methyl chloride.

[0086] A nanofiltration membrane was prepared.

[0087] Comparative Example 1

[0088] The difference from Example 1 is:

[0089] In step 1), sodium bicarbonate (acid binding agent) is replaced by sodium carbonate.

[0090] A nanofiltration membrane was prepared.

[0091] Comparative Example 2

[0092] The difference from Example 1 is:

[0093] In step 1), sodium bicarbonate (acid binding agent) is replaced with sodium carbonate;

[0094] In step 2), ethyl chloride (organic solvent b) is replaced by methyl chloride.

[0095] A nanofiltration membrane was prepared.

[0096] Example 3

[0097] The difference from Example 1 is:

[0098] In step 2), ethyl chloride (organic solvent b) is replaced by methyl ethyl ether.

[0099] A nanofiltration membrane was prepared.

[0100] Example 4

[0101] The difference from Example 1 is:

[0102] In step 2), ethyl chloride (organic solvent b) is replaced by dimethyl ether.

[0103] A nanofiltration membrane was prepared.

[0104] Example 5

[0105] The difference from Example 1 is:

[0106] In step 2), ethyl chloride (organic solvent b) is replaced by methyl formate.

[0107] A nanofiltration membrane was prepared.

[0108] Example 6

[0109] The difference from Example 1 is:

[0110] In step 1), the aqueous solution is prepared:

[0111] Add 1 part by weight of piperazine (polyamine monomer), 0.5 part by weight of sodium dodecyl sulfate (surfactant), 4 parts by weight of sodium bicarbonate (acid binding agent), and 94.5 parts by weight of pure water, and stir at room temperature for 0.5 h to obtain a homogeneous transparent solution, which is the aqueous phase solution.

[0112] A nanofiltration membrane was prepared.

[0113] Example 7

[0114] The difference from Example 6 is:

[0115] In step 2), the oil phase solution is prepared:

[0116] Add 0.2 parts by weight of trimesoyl chloride (acyl chloride monomer), 49.8 parts by weight of ethylcyclohexane (organic solvent a), and 50 parts by weight of ethyl chloride (organic solvent b), and stir at room temperature for 0.5 h to obtain a homogeneous solution, which is the oil phase solution.

[0117] A nanofiltration membrane was prepared.

[0118] Comparative Example 3

[0119] The difference from Example 1 is:

[0120] In step 1), the aqueous solution is prepared:

[0121] Add 1 part by weight of piperazine (polyamine monomer), 0.5 part by weight of sodium dodecyl sulfate (surfactant), 2 parts by weight of sodium hydroxide (acid binding agent), and 96.5 parts by weight of pure water, and stir at room temperature for 0.5 h to obtain a homogeneous transparent solution, which is the aqueous phase solution.

[0122] In step 2), the oil phase solution is prepared:

[0123] 0.2 parts by weight of trimesoyl chloride (acyl chloride monomer) and 99.8 parts by weight of ethylcyclohexane (organic solvent a) were added and stirred at room temperature for 0.5 h to obtain a homogeneous solution, which is the oil phase solution.

[0124] A nanofiltration membrane was prepared.

[0125] Comparative Example 4

[0126] The difference from Example 1 is:

[0127] In step 2), the oil phase solution is prepared:

[0128] Add 0.2 parts by weight of trimesoyl chloride (acyl chloride monomer), 94.8 parts by weight of ethylcyclohexane (organic solvent a), and 5 parts by weight of dichloromethane (organic solvent b), and stir at room temperature for 0.5 h to obtain a homogeneous solution, which is the oil phase solution.

[0129] A nanofiltration membrane was prepared.

[0130] Comparative Example 5

[0131] The difference from Example 1 is:

[0132] In step 1), the aqueous solution is prepared:

[0133] Add 1 part by weight of piperazine (polyamine monomer), 0.5 part by weight of sodium dodecyl sulfate (surfactant), 0.5 part by weight of sodium bicarbonate (acid binding agent), and 98 parts by weight of pure water, and stir at room temperature for 0.5 h to obtain a homogeneous transparent solution, which is the aqueous phase solution.

[0134] Comparative Example 6

[0135] The difference from Example 1 is:

[0136] In step 2), the oil phase solution is prepared:

[0137] 0.2 parts by weight of trimesoyl chloride (acyl chloride monomer) and 99.8 parts by weight of ethylcyclohexane (organic solvent a) were added and stirred at room temperature for 0.5 h to obtain a homogeneous solution, which is the oil phase solution.

[0138] A nanofiltration membrane was prepared.

[0139] Comparative Example 7

[0140] The difference from Example 1 is:

[0141] In step 1), sodium bicarbonate (acid binding agent) is replaced by triethylamine.

[0142] A nanofiltration membrane was prepared.

[0143] Comparative Example 8

[0144] The difference from Example 1 is:

[0145] In step 2), ethyl chloride (organic solvent b) is replaced with ethyl acetate.

[0146] A nanofiltration membrane was prepared.

[0147] Comparative Example 9

[0148] The difference from Example 1 is:

[0149] In step 2), ethyl chloride (organic solvent b) is replaced by toluene.

[0150] A nanofiltration membrane was prepared.

[0151] The nanofiltration membranes obtained in the above examples and comparative examples were tested for performance on a standard membrane test bench. Specifically, the test was performed using a 2000 mg / L magnesium sulfate aqueous solution at a pressure of 70 psi and 25±1°C.

[0152] Where: The calculation formula of membrane flux F is shown in formula (1):

[0153] Formula (1);

[0154] In formula (1), V is the permeate volume (mL), S is the effective membrane area (cm 2 ), t is time (min).

[0155] The calculation formula of the desalination rate R is shown in formula (2):

[0156] Formula (2);

[0157] In formula (2), C f is the concentration of magnesium sulfate in the stock solution (ppm), C p is the concentration of magnesium sulfate in the permeate (ppm).

[0158] The calculation results are shown in Table 1.

[0159] Table 1 Performance test results of nanofiltration membrane

[0160]

[0161] From the membrane flux in Table 1, the membrane flux of Comparative Examples 1 and 2 using sodium carbonate as the aqueous phase additive is smaller than that of other embodiments, because the amount of carbon dioxide nanobubbles generated by absorbing hydrogen chloride molecules is small, and the polyamide vesicles formed under the promotion of the heat of reaction are weakly wrinkled and have a small specific surface area. The electron cloud density of the chlorine atoms of ethyl chloride is large, and the ability to form hydrogen bonds with the hydrogen atoms on the nitrogen in the piperazine structure is stronger than other substances, so there is less obstacle for piperazine to enter the oil phase to participate in the reaction, and it has a greater chance of participating in the reaction. The formed polypiperazineamide layer is denser and thicker, and the desalination rate is relatively high.

[0162] Comparative Example 4 uses dichloromethane organic solvent, which can make piperazine enter the oil phase more quickly to participate in the reaction, but its content is small, which is not enough to support more piperazine to enter the oil phase liquid through the water-oil interface to participate in the reaction, so the reaction is fast, and the polypiperazineamide structure formed is thinner and denser than that of Comparative Example 3, the membrane flux is larger, and the desalination rate is better. However, the hydrogen chloride molecules generated by the reaction are less, and the carbon dioxide molecules obtained by the reaction with the acid binding agent are less; at the same time, the reaction is relatively mild, and the heat of reaction is not enough to cause the carbon dioxide molecules to physically deform the polypiperazineamide structure, so the flux improvement is limited.

[0163] The acid-binding agent content in Comparative Example 5 is relatively small, which is insufficient to generate a large number of carbon dioxide molecules, and the polypiperazineamide structure deformation is relatively small, and the flux improvement is limited.

[0164] In Comparative Example 6, no polar organic solvent was added, the number of piperazine molecules participating in the reaction was limited, the molecular weight of hydrogen chloride generated by the reaction was small, and it was not enough to produce a large number of carbon dioxide molecules. The structural deformation of polypiperazineamide was small, and the flux was low.

[0165] In order to verify the chemical cleaning resistance of the nanofiltration membrane prepared by the present invention, an acid solution was used for immersion, and the performance change of the membrane before and after immersion was evaluated. A hydrochloric acid solution with a mass concentration of 0.1% was prepared, heated and maintained at 40±1°C, and the membrane was immersed for 24 hours, and then a magnesium sulfate standard test was performed to obtain the test results of some embodiments, as shown in Table 2.

[0166] Table 2 Chemical cleaning resistance of nanofiltration membrane

[0167]

[0168] From the performance after acid treatment in Table 2, Example 7 has the best acid resistance, which may be related to the high content of ethyl chloride in its oil phase liquid and the high degree of piperazine participating in the reaction. The polyamide layer generated by the reaction is thicker and has a higher degree of resistance to acid hydrolysis. The polypiperazineamide layer generated by Comparative Examples 3, 4 and 5 is thinner, especially Comparative Example 4, which has a faster reaction, and has a weaker acid resistance. Comparative Example 5 has more piperazine molecules participating in the reaction, but the generated carbon dioxide molecules are not enough to change the structure of the polypiperazine. Although the acid resistance is good, the membrane flux is low.

[0169] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a nanofiltration membrane, comprising the following steps: The base membrane is immersed in an aqueous solution, taken out and dried, and then an oil phase solution is coated on the base membrane to perform an interfacial polymerization reaction, and after curing, a nanofiltration membrane is obtained; The aqueous phase solution comprises a polyamine monomer, a surfactant, an acid binder and water; the acid binder is bicarbonate; the polyamine monomer comprises at least one of piperazine, m-phenylenediamine, ethylenediamine and 1,6-hexanediamine; in the aqueous phase solution, the mass content of the polyamine monomer is 1% to 2%; The oil phase solution is composed of an acyl chloride monomer, an organic solvent a and an organic solvent b; the acyl chloride monomer includes at least one of trimesoyl chloride, phthaloyl chloride, isophthaloyl chloride, terephthaloyl chloride and adipoyl chloride; in the oil phase solution, the mass content of the acyl chloride monomer is 0.1% to 0.5%; The organic solvent a is ethylcyclohexane; the organic solvent b is at least one of methyl chloride, ethyl chloride, ethyl ether, methyl ethyl ether, dimethyl ether and methyl formate; In the oil phase solution, the mass content of organic solvent a is 49% to 75%; In the oil phase solution, the mass content of the organic solvent b is 25% to 50%.

2. The preparation method according to claim 1, characterized in that: In the aqueous phase solution, the mass content of the acid binding agent is 2% to 4%.

3. The preparation method according to claim 1, characterized in that: The acid binding agent is at least one of sodium bicarbonate and potassium bicarbonate.

4. The preparation method according to claim 1, characterized in that: The surfactant includes at least one of sodium dodecyl sulfonate, sodium dodecyl sulfate and sodium lauryl sulfate; in the aqueous phase solution, the mass content of the surfactant is 0.5% to 1%.

5. The preparation method according to claim 1, characterized in that: The base film is immersed in the aqueous solution for 10 to 30 seconds.

6. The preparation method according to claim 1, characterized in that: The thickness of the oil phase solution coated on the base film is 0.2~0.5 mm; The interfacial polymerization reaction is carried out at room temperature for 5 to 20 seconds.

7. A nanofiltration membrane obtained by the preparation method according to any one of claims 1 to 6.

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