Nanofiltration membrane with positive charges as well as preparation method and application of nanofiltration membrane

Abstract

The invention belongs to the technical field of nanofiltration, and particularly discloses a nanofiltration membrane with positive charges and a preparation method and application thereof. The preparation method comprises the following steps: S1, mixing and reacting an ultrafiltration membrane with a monomer solution, and then sequentially mixing and reacting with an initiator and an oxidizing agent to obtain a treated ultrafiltration membrane; s2, mixing the treated ultrafiltration membrane with a cross-linking agent solution, and carrying out a cross-linking reaction to obtain a pre-cross-linked ultrafiltration membrane; and S3, heating the pre-crosslinked ultrafiltration membrane, and carrying out a crosslinking reaction to obtain the nanofiltration membrane. According to the preparation method, the average pore size of the prepared nanofiltration membrane can be adjusted by adjusting the types of the monomers, the surface of the prepared nanofiltration membrane is positively charged, the prepared nanofiltration membrane has high hydrophilicity, antibiotics such as tetracycline can be efficiently separated, and the rejection rate of tetracycline reaches up to 91.3%. In addition, the preparation method is simple to operate, mild in preparation condition and easy to popularize and apply.

Description

Technical Field

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

[0002] The widespread use of antibiotics in medicine, animal husbandry, aquaculture, and agriculture provides an effective means for the treatment and prevention of bacterial infections. However, the improper use of antibiotics brings serious environmental risks, especially antibiotic pollution in water bodies. The traditional method of antibiotic desalination is mainly chemical precipitation. Although the chemical precipitation method is simple and easy to implement, it requires the addition of a large amount of chemical reagents, which is costly and unstable, and may also cause secondary pollution. In contrast, nanofiltration technology has shown significant advantages in the treatment of antibiotic wastewater. Nanofiltration technology can achieve the selective separation and recovery of liquid molecules, and has the advantages of high separation efficiency, simple operation, energy saving and environmental protection, and is suitable for treating complex wastewater environments.

[0003] Nanofiltration is a pressure-driven separation process that can effectively separate different solutes from solutions (molecular cutoff of 200–2000Da). Nanofiltration membranes usually adopt a thin film composite structure. Through the combination of a polyamide selective performance layer and a substrate, they can maintain good performance and structural stability to achieve the separation of target solutes and particles. The principle of this technology is to use a membrane with a certain pore size to separate the solute molecules in the liquid through size screening and electrostatic repulsion to obtain the desired substance. However, the pore size of existing commercial nanofiltration membranes is difficult to adjust, and the surface is negatively charged. For some positively charged or smaller antibiotics, the separation performance is poor, which seriously restricts the application of nanofiltration membranes in the field of antibiotic desalination. Summary of the Invention

[0004] In order to overcome at least one technical problem existing in the above-mentioned prior art, one of the objectives of the present invention is to provide a method for preparing a nanofiltration membrane.

[0005] A second object of the present invention is to provide a nanofiltration membrane.

[0006] A third object of the present invention is to provide an application of the above-mentioned nanofiltration membrane in the field of antibiotic nanofiltration or sewage treatment.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is:

[0008] A first aspect of the present invention provides a method for preparing a nanofiltration membrane, comprising the following steps:

[0009] S1: mixing and reacting the ultrafiltration membrane with the monomer solution, and then sequentially mixing and reacting with the initiator and the oxidant to obtain a treated ultrafiltration membrane;

[0010] S2: mixing the treated ultrafiltration membrane with a crosslinking agent solution and performing a crosslinking reaction to obtain a pre-crosslinked ultrafiltration membrane;

[0011] S3: heating the pre-crosslinked ultrafiltration membrane to perform a crosslinking reaction to obtain the nanofiltration membrane.

[0012] In some embodiments of the present invention, the monomer in the monomer solution is selected from at least one of pyrrole, 3-aminopyrrolidine, piperazine, aniline, o-methylaniline, m-methylaniline, p-methylaniline, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 3-acetylpyrrole, 2,3-dimethylaniline, methoxyaniline, o-n-propylaniline, ethoxyaniline, 3-valerylpyrrole, p-phenylenediaminesulfonic acid, m-phenylenediaminesulfonic acid, naphthalenediamine, aminobiphenyl, 3,3′-diaminobenzidine, benzyl diamine, m-tolidine, 3,3′-dihydroxybenzidine, and dimethylbenzyl diamine.

[0013] In some embodiments of the present invention, the initiator is selected from at least one of copper chloride, copper sulfate, and copper nitrate.

[0014] In some embodiments of the present invention, the oxidant is selected from at least one of sodium periodate, sodium iodate, hydrogen peroxide, potassium persulfate, ammonium persulfate, and potassium permanganate.

[0015] In some embodiments of the present invention, the crosslinking agent in the crosslinking agent solution is at least one selected from epichlorohydrin, ethylenediamine, diethylenetriamine, glutaraldehyde, nonanedialdehyde, and adipaldehyde.

[0016] In some embodiments of the present invention, the material of the ultrafiltration membrane is selected from at least one of polysulfone, polyethersulfone, sulfonated polysulfone, polyacrylonitrile, and polytetrafluoroethylene.

[0017] In some embodiments of the present invention, the mass percentage of the crosslinker in the crosslinker solution is 1-5%; in some embodiments of the present invention, the mass percentage of the crosslinker in the crosslinker solution is any one of 1%, 2%, 3%, 4%, 5%, or a range formed by any two of them.

[0018] In some embodiments of the present application, the monomer concentration in the monomer solution is 0-30 g / L and not 0; in some embodiments of the present application, the monomer concentration in the monomer solution is 1-30 g / L; in some embodiments of the present application, the monomer concentration in the monomer solution is any one of 1 g / L, 2 g / L, 4 g / L, 6 g / L, 8 g / L, 10 g / L, 12 g / L, 14 g / L, 16 g / L, 18 g / L, 20 g / L, 22 g / L, 24 g / L, 26 g / L, 28 g / L, 30 g / L or a range value formed by any two of them; in some embodiments of the present application, the monomer concentration in the monomer solution is 10-30 g / L.

[0019] In some embodiments of the present application, the temperature of the crosslinking reaction in step S2 is 25-60℃; in some embodiments of the present application, the temperature of the crosslinking reaction in step S2 is any one of 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃, 60℃ or a range value formed by any two of them.

[0020] In some embodiments of the present application, the temperature of the crosslinking reaction in step S3 is 40-80℃; in some embodiments of the present application, the temperature of the crosslinking reaction in step S3 is any one of 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃, 80℃ or a range value formed by any two of them.

[0021] In some embodiments of the present application, the mass ratio of monomer to initiator in the monomer solution is 1:(0.4-0.7); in some embodiments of the present application, the mass ratio of monomer to initiator in the monomer solution is any one of 1:0.4, 1:0.5, 1:0.6, 1:0.7 or a range value formed by any two of them.

[0022] In some embodiments of the present application, the mass ratio of monomer to oxidizing agent in the monomer solution is 1:(1.5-2.5); in some embodiments of the present application, the mass ratio of monomer to oxidizing agent in the monomer solution is any one of 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5 or a range value formed by any two of them.

[0023] In some embodiments of the present application, the time of the crosslinking reaction in step S2 is 0-2h and not 0; in some embodiments of the present application, the time of the crosslinking reaction in step S2 is 0.5-2h.

[0024] In some embodiments of the present invention, in step S3, the cross-linking reaction time is 0 to 30 minutes and is not 0; in some embodiments of the present invention, in step S3, the cross-linking reaction time is 1 to 30 minutes; in some embodiments of the present invention, in step S3, the cross-linking reaction time is 5 to 20 minutes.

[0025] In some embodiments of the present invention, step S1 comprises: allowing the ultrafiltration membrane to react with the monomer solution for 1 to 5 minutes, then allowing the ultrafiltration membrane to react with the initiator for 1 to 5 minutes, and then allowing the ultrafiltration membrane to react with the oxidant for 1 to 10 hours, to obtain a treated ultrafiltration membrane. In some embodiments of the present invention, step S1 comprises: allowing the ultrafiltration membrane to react with the monomer solution for 1 to 5 minutes, then allowing the initiator solution to react with the initiator solution for 1 to 5 minutes, and then allowing the oxidant solution to react with the initiator solution for 1 to 10 hours, to obtain a treated ultrafiltration membrane.

[0026] In some embodiments of the present invention, the concentration of the initiator solution is 0 to 20 g / L and is not 0; in some embodiments of the present invention, the concentration of the initiator solution is 1 to 20 g / L; in some embodiments of the present invention, the concentration of the initiator solution is 1 g / L, 2 g / L, 4 g / L, 6 g / L, 8 g / L, 10 g / L, 12 g / L, 14 g / L, 16 g / L, 18 g / L, 20 g / L or any two of them; in some embodiments of the present invention, the concentration of the initiator solution is 10 to 20 g / L.

[0027] In some embodiments of the present invention, the concentration of the oxidant solution is 0-200 g / L and is not 0; in some embodiments of the present invention, the concentration of the oxidant solution is 10-200 g / L; in some embodiments of the present invention, the concentration of the oxidant solution is 10 g / L, 20 g / L, 30 g / L, 40 g / L, 50 g / L, 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, 160 g / L, 170 g / L, 180 g / L, 190 g / L, 200 g / L, or a range formed by any two of the values; in some embodiments of the present invention, the concentration of the oxidant solution is 80-150 g / L.

[0028] In some embodiments of the present invention, the reaction in step S1 and step S2 is respectively carried out in any mixing mode selected from shaking, stirring or ultrasound.

[0029] The second aspect of the present invention provides a nanofiltration membrane, which is prepared using the preparation method described in the first aspect of the present invention.

[0030] In some embodiments of the present invention, the surface of the nanofiltration membrane is positively charged.

[0031] In some embodiments of the present invention, the nanofiltration membrane has a porous structure.

[0032] In some embodiments of the present invention, the average pore size of the nanofiltration membrane is 0.9 to 1.2 nm; in some embodiments of the present invention, the average pore size of the nanofiltration membrane is 0.97 to 1.13 nm.

[0033] In some embodiments of the present invention, the water contact angle of the nanofiltration membrane is 34 to 46°.

[0034] In some embodiments of the present invention, the water flux of the nanofiltration membrane is 10 to 16 L m –2 h –1 bar –1 .

[0035] The third aspect of the present invention provides the use of the nanofiltration membrane described in the second aspect of the present invention in the field of antibiotic nanofiltration or sewage treatment.

[0036] In some embodiments of the invention, the antibiotic comprises tetracycline.

[0037] The present invention has the beneficial effects of adjusting the average pore size of the resulting nanofiltration membrane by adjusting the type of monomers. Furthermore, the resulting nanofiltration membrane has a positively charged surface and high hydrophilicity, enabling efficient separation of antibiotics such as tetracycline, with a tetracycline retention rate as high as 91.3%. Furthermore, the preparation method of the present invention is simple to operate, requires mild preparation conditions, and is readily applicable. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 Schematic diagram of the reaction membrane used in Examples 1 to 3 and Comparative Examples 1 to 3.

[0039] Figure 2 Surface morphologies of the nanofiltration membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 3.

[0040] Figure 3 1 is the membrane surface Zeta potential diagram of the nanofiltration membrane prepared in Examples 1 to 3.

[0041] Figure 4 Graph showing the molecular rejection curves of the nanofiltration membranes prepared in Examples 1 to 3.

[0042] Figure 5 Graph showing the pore size distribution of the nanofiltration membranes prepared in Examples 1 to 3. DETAILED DESCRIPTION

[0043] The specific implementation of the present invention will be further described in detail below in conjunction with the accompanying drawings and examples, but the implementation and protection of the present invention are not limited thereto. It should be noted that if there are any processes that are not particularly described in detail below, they can be implemented or understood by those skilled in the art with reference to the prior art. The reagents or instruments used that do not indicate the manufacturer are all conventional products that can be purchased commercially.

[0044] The material information used in the following examples and comparative examples is as follows:

[0045] Polyethersulfone ultrafiltration membrane with a molecular weight cutoff of 150 kDa was purchased from Guochu Technology (Xiamen) Co., Ltd.

[0046] Example 1

[0047] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0048] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1 As shown), the reaction mold is combined with a shaker, and the shaker speed is 150 rpm, so that the liquid in the reaction mold can be evenly mixed and reacted under the action of the shaker. First, 80 mL of a 25 g / L p-phenylenediamine (PPD) solution is added to the reaction membrane and reacted for 2 minutes; then 80 mL of a 15.75 g / L CuCl2·2H2O solution is added and reacted for 2 minutes; then 40 mL of a 100 g / L NaIO4 solution is added. After reacting for 5 hours, the remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% by mass glutaraldehyde solution at 60°C and a volume of 500 mL for crosslinking for 1 hour. Then, the membrane is placed in an oven at 60°C for further crosslinking for 10 minutes to obtain a positively charged nanofiltration membrane in this example, recorded as dynamic PPD, which is placed in deionized water for storage.

[0049] Example 2

[0050] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0051] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1As shown), the reaction mold is combined with a shaker, and the shaker speed is 150 rpm, so that the liquid in the reaction mold can be evenly mixed and reacted under the action of the shaker. First, 80 mL of a 25 g / L m-phenylenediamine (MPD) solution is added to the reaction membrane. After reacting for 2 minutes, 80 mL of a 15.75 g / L CuCl2·2H2O solution is added. After reacting for 2 minutes, 40 mL of a 100 g / L NaIO4 solution is added. After reacting for 5 hours, the remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% mass percentage glutaraldehyde solution at 60°C with a volume of 500 mL for crosslinking for 1 hour. Then, the membrane is placed in an oven at 60°C for further crosslinking for 10 minutes to obtain a positively charged nanofiltration membrane in this example, recorded as dynamic MPD, which is placed in deionized water for storage.

[0052] Example 3

[0053] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0054] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1 As shown in the figure, the reaction mold is combined with a shaker, and the shaker speed is 150 rpm, so that the liquid in the reaction mold can be evenly mixed and reacted under the action of the shaker. First, 80 mL of 25 g / L o-phenylenediamine (OPD) solution is added to the reaction membrane. After reacting for 2 minutes, 80 mL of 15.75 g / L CuCl2·2H2O solution is added. After reacting for 2 minutes, 40 mL of 100 g / L NaIO4 solution is added. After reacting for 5 hours, the remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% mass percentage glutaraldehyde solution of 500 mL at 60°C for crosslinking for 1 hour. Then, the membrane is placed in an oven at 60°C for further crosslinking for 10 minutes to obtain the positively charged nanofiltration membrane in this example, recorded as dynamic OPD, which is placed in deionized water for storage.

[0055] Comparative Example 1

[0056] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0057] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1As shown in the figure, the reaction mold is not used in conjunction with a shaker, that is, the reaction liquid in the reaction mold is in a static state during the reaction. First, 80 mL of a 25 g / L p-phenylenediamine (PPD) solution is added to the reaction membrane and reacted for 2 minutes; then 80 mL of a 15.75 g / L CuCl2·2H2O solution is added and reacted for 2 minutes; then 40 mL of a 100 g / L NaIO4 solution is added and reacted for 5 hours. The remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% by mass glutaraldehyde solution at 60°C for cross-linking for 1 hour. The membrane is then placed in an oven for further cross-linking for 10 minutes to obtain a positively charged nanofiltration membrane in this example, recorded as static PPD, which is placed in deionized water for storage.

[0058] Comparative Example 2

[0059] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0060] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1 As shown in FIG, the reaction mold is not used in conjunction with a shaker, that is, the reaction liquid in the reaction mold is in a static state during the reaction. First, 80 mL of a 25 g / L m-phenylenediamine (MPD) solution is added to the reaction membrane. After reacting for 2 minutes, 80 mL of a 15.75 g / L CuCl2·2H2O solution is added. After reacting for 2 minutes, 40 mL of a 100 g / L NaIO4 solution is added. After reacting for 5 hours, the remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% by mass glutaraldehyde solution at 60° C. for cross-linking for 1 hour. The membrane is then placed in an oven for further cross-linking for 10 minutes to obtain a positively charged nanofiltration membrane in this example, recorded as static MPD, which is placed in deionized water for storage.

[0061] Comparative Example 3

[0062] This example provides a method for preparing a positively charged nanofiltration membrane. The specific steps are as follows:

[0063] The cleaned polyethersulfone ultrafiltration membrane is installed and fixed to the reaction membrane (its structural diagram is shown in Figure 1As shown in the figure, the reaction mold is not used in conjunction with a shaker, that is, the reaction liquid in the reaction mold is in a static state during the reaction. First, 80 mL of 25 g / L o-phenylenediamine (OPD) solution is added to the reaction membrane. After reacting for 2 minutes, 80 mL of 15.75 g / L CuCl2·2H2O solution is added. After reacting for 2 minutes, 40 mL of 100 g / L NaIO4 solution is added. After reacting for 5 hours, the remaining solution is poured out, and the reacted membrane is taken out and placed in a 2% by mass glutaraldehyde solution at 60°C for cross-linking for 1 hour. Then, the membrane is placed in an oven for further cross-linking for 10 minutes to obtain the positively charged nanofiltration membrane in this example, which is recorded as static OPD and stored in deionized water for later use.

[0064] Performance testing:

[0065] (1) Surface morphology test

[0066] The surface morphologies of the positively charged nanofiltration membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were measured using a field emission scanning electron microscope (FE-SEM, SU8220, Hitachi, Japan). Figure 2 As shown. Figure 2 It can be seen that the surface density of the nanofiltration membranes prepared in Examples 1 to 3 is better, which further shows that dynamic mixing during the polymerization and cross-linking process of polyphenylenediamine can effectively enhance the degree of reaction and improve the structural uniformity of the nanofiltration membrane, while the surface of the nanofiltration membranes prepared in Comparative Examples 1 to 3 is loose and porous with a larger pore size.

[0067] (2) Zeta potential test

[0068] The surface Zeta potential of the nanofiltration membranes prepared in Examples 1 to 3 was measured at different pH values ​​(pH 3 to 9) using Anton Paar's solid surface Zeta potential analyzer SurPASS 3. Figure 3 As shown. Figure 3 It can be seen that as the pH value decreases, the positive charge density on the membrane surface gradually increases. Among them, the dynamic MPD membrane in Example 2 shows the strongest positive charge, while the dynamic OPD membrane in Example 3 has the lowest positive charge density.

[0069] (3) Molecular retention rate and pore size distribution test

[0070] The molecular weight cut-off (MWCO) curve and pore size distribution curve of the nanofiltration membranes prepared in Examples 1 to 3 were tested respectively. The test method is as follows: the MWCO and pore size distribution of the nanofiltration membranes were evaluated for the cut-off of a series of neutral molecules, including glucose (Mw = 180), sucrose (Mw = 342), dextran (Mw = 1000) and dextran (Mw = 2000). The concentration of the solution was determined by the phenol-sulfuric acid method. The Stokes radius (rs The average effective pore size (d

[0071] logr s = -1.3363 + 0.395 log Mw

[0072] The average effective pore size (d p ,nm) of the nanofiltration membrane corresponds to the Stokes diameter (d s ,nm) of the neutral solute at 50% retention, and the geometric standard deviation (σ p ,nm) of d p is calculated according to the ratio of the solute diameters at 84.13% and 50% retention.

[0073] The pore size (d p ,nm) of the nanofiltration membrane can be calculated by the following probability density function.

[0074]

[0075] The results measured according to the above test methods are shown in Figure 4 and Figure 5 respectively. As can be seen from Figure 4 to Figure 5 , the nanofiltration membranes prepared by using the three different monomers (i.e. MPD, PPD and OPD) in Examples 1-3 respectively have different pore sizes and molecular retention rates, wherein the average pore size of the dynamic MPD membrane in Example 2 is the smallest (its value is 0.97 nm), the average pore size of the dynamic OPD membrane in Example 3 is the largest (its value is 1.13 nm), and the average pore size of the dynamic PPD membrane in Example 1 is 1.03 nm.

[0076] The retention rate data of the nanofiltration membranes in Examples 1-3 for glucose, sucrose and dextran are shown in Table 1 below.

[0077] Table 1 Molecular retention rate data of the nanofiltration membranes in Examples 1-3

[0078] Example 1 Example 2 Example 3 <![CDATA[葡萄糖(M w =180)]]> 19.8％ 23.5％ 13.6％ <![CDATA[蔗糖(M w =342)]]> 36.6％ 44.4％ 26.3％ <![CDATA[葡聚糖(M w =1000)]]> 84.5％ 91.2％ 75.5％ <![CDATA[葡聚糖(M w =2000)]]> 94.6％ 98.3％ 91.3％

[0079] As can be seen from Table 1, the dynamic MPD membrane in Example 2 exhibits the lowest MWCO value, about 1000 Da, indicating that the membrane has a higher retention capacity and a denser membrane structure. In contrast, the dynamic OPD membrane in Example 3 has the highest MWCO, about 2000 Da, meaning that the retention capacity of the membrane is weaker, the pore size of the membrane is relatively larger, and the structure is relatively looser.

[0080] (4) Water contact angle test

[0081] The water contact angles of the nanofiltration membranes and polyethersulfone ultrafiltration membranes prepared in Examples 1-3 were measured using a contact angle analyzer (Biolin Scientific, Attension Theta Lite) in sitting drop mode. The freeze-dried nanofiltration membrane material to be tested was first fixed to a glass slide and placed on a flat stage. A microsyringe was used to drip 3 μL of ultrapure water onto the sample surface. The water was then stabilized for 10 seconds before the water was read. The test results are shown in Table 2 below.

[0082] Table 2 Water contact angle test data of nanofiltration membrane in Examples 1 to 3

[0083] Polyethersulfone ultrafiltration membrane Example 1 Example 2 Example 3 Water contact angle (°) 54.42 39.87 34.75 45.62

[0084] As shown in Table 2, compared with the polyethersulfone ultrafiltration membrane, the nanofiltration membranes in Examples 1 to 3 all introduced the hydrophilic group -NH2, so the hydrophilicity of the nanofiltration membranes in Examples 1 to 3 was improved. The reasons for the difference in hydrophilicity of the nanofiltration membranes in Examples 1 to 3 are: (1) the structures of the three nanofiltration membrane materials are different, and the content of hydrophilic groups exposed on the surface of the nanofiltration membrane is different; (2) the surface roughness of the nanofiltration membrane is different (due to the difference in the surface roughness of the nanofiltration membrane). Figure 2 It can be seen that the roughness of the dynamic MPD membrane in Example 2 is the largest, and the roughness of the dynamic OPD membrane in Example 3 is the smallest. Increasing the surface roughness of the nanofiltration membrane will improve the hydrophilicity of the membrane.

[0085] (5) Chemical structure test

[0086] The chemical structures of the nanofiltration membranes prepared in Examples 1 to 3 were respectively tested using an X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher, UK). The specific test results are shown in Table 3 below.

[0087] Table 3 Chemical structure test results of nanofiltration membranes in Examples 1 to 3

[0088] C-C / C-H (%) C=C (%) C-N (%) C=N (%) Example 1 42.24 27.91 23.65 6.21 Example 2 46.03 12.19 29.27 12.51 Example 3 47.26 24.71 22.87 5.16

[0089] As can be seen from Table 3, the nanofiltration membranes in Examples 1 to 3 are made of three different monomers, the resulting nanofiltration membranes have different structures, and the contents of each bond in the X-ray photoelectron spectroscopy C1s fine spectrum are also different.

[0090] (6) Water flux test

[0091] The water flux and water flux error of the nanofiltration membranes prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were tested respectively. The specific test method was as follows: the nanofiltration membrane was placed in a membrane pool, pre-pressed at 4 bar (1 bar = 0.1 MPa) for 1 hour, and then the water permeation flux was measured at the same pressure and temperature of 25 ° C., and calculated by the following formula:

[0092]

[0093] Among them, J w is the water permeation flux (L m –2 h –1 bar –1 ), V is the volume of water (L), A is the effective area of ​​the nanofiltration membrane (m 2 ), t is time (h), ΔP is the filtration pressure difference (bar).

[0094] The water flux data of the nanofiltration membranes in Examples 1 to 3 measured according to the above test method are shown in Table 4 below, and the water flux error data of the nanofiltration membranes in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 5 below.

[0095] Table 4 Water flux test results of nanofiltration membranes in Examples 1 to 3

[0096]

[0097] Table 5 Water flux error test results of nanofiltration membranes in Examples 1 to 3 and Comparative Examples 1 to 3

[0098]

[0099]

[0100] As shown in Table 4, the nanofiltration membranes in Examples 1 to 3 have different cross-linking densities, pore sizes, and surface morphologies due to the different molecular structures of the monomers used in their preparation, which in turn lead to different water fluxes (MPD <PPD<OPD)

[0101] As shown in Table 5, compared with Comparative Examples 1 to 3, the nanofiltration membranes in Examples 1 to 3 can evenly distribute the polymer on the surface of the membrane, thereby improving the oxidation effect and the performance stability of the membrane material, thereby significantly reducing the water flux error of Examples 1 to 3.

[0102] (7) Antibiotic and salt retention rate test

[0103] The antibiotic and salt retention rates of the nanofiltration membranes prepared in Examples 1 to 3 were tested respectively. The specific test method was as follows: the nanofiltration membrane was placed in a membrane pool, pre-pressed at 4 bar for 1 hour, and then tested at the same pressure, temperature of 25°C, and pH = 3 with an initial concentration of 50 mg L –1 Tetracycline and 1g L –1 The retention rate of tetracycline and sodium chloride in a mixed solution of sodium chloride is calculated by the following formula:

[0104]

[0105] Where, R is the retention rate (%), C f is the concentration of tetracycline or sodium chloride in the stock solution (g L –1 ),C p is the concentration of tetracycline or sodium chloride in the permeate (g L –1 ).

[0106] The separation effect of antibiotics and sodium chloride is determined by the separation factor SF. The calculation formula of the separation factor SF is as follows:

[0107]

[0108] Among them, R NaCl is the sodium chloride retention rate (%), R N is the tetracycline retention rate (%).

[0109] The antibiotic and salt rejection rate data of the nanofiltration membrane obtained according to the above test method and calculation formula are shown in Table 6 below.

[0110] Table 6 Antibiotic and salt retention data of nanofiltration membrane

[0111] Example 1 Example 2 Example 3 Tetracycline rejection rate (%) 82.7 91.3 66.5 Sodium chloride rejection rate (%) 13.0 26.1 21.0 Separation factor (SF) 5.0 8.5 2.4

[0112] As shown in Table 6, the tetracycline retention rates of the three nanofiltration membranes in Examples 1-3 follow a trend of MPD membrane (91.3%) > PPD membrane (82.7%) > OPD membrane (66.5%). This is likely due to the MPD membrane having the smallest pore size and the strongest positive charge, resulting in the strongest screening and electrostatic repulsion for tetracycline (negatively charged at pH 3), and the highest retention rate. The PPD membrane has a slightly larger pore size and weaker positive charge, resulting in a slightly lower retention rate. The OPD membrane, due to steric hindrance from the ortho-substituents, has a larger pore size and the lowest charge density, resulting in the worst retention rate. Overall, the MPD membrane has the highest SF (8.5) for tetracycline / sodium chloride, resulting in the best selectivity; the PPD membrane has a lower SF (5.0) due to its reduced charge; and the OPD membrane, with weaker charge and pore size screening effects, has the lowest SF (2.4) and the worst selectivity.

[0113] In summary, the present invention addresses the problem that the pore size of current commercial nanofiltration membranes is difficult to adjust and they are negatively charged, thus failing to meet the required antibiotic wastewater separation efficiency. By using the self-polymerization method of conjugated molecules (MPD, PPD, OPD), the present invention utilizes phenylenediamine monomers with different structures to prepare high-performance nanofiltration membranes with different pore sizes and positively charged surfaces, thereby improving the water flux of the nanofiltration membranes and the antibiotic wastewater separation efficiency.

[0114] While the embodiments of the present invention have been described in detail above, the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof may be combined with one another unless there is a conflict.

Claims

1. A method for preparing a nanofiltration membrane, characterized in that: The following steps are involved: S1: mixing and reacting the ultrafiltration membrane with the monomer solution, and then sequentially mixing and reacting with the initiator and the oxidant to obtain a treated ultrafiltration membrane; S2: mixing the treated ultrafiltration membrane with a crosslinking agent solution and performing a crosslinking reaction to obtain a pre-crosslinked ultrafiltration membrane; S3: heating the pre-crosslinked ultrafiltration membrane to perform a crosslinking reaction to obtain the nanofiltration membrane.

2. The method for preparing a nanofiltration membrane according to claim 1, wherein: The monomer in the monomer solution is selected from at least one of pyrrole, 3-aminopyrrolidine, piperazine, aniline, o-methylaniline, m-methylaniline, p-methylaniline, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, 3-acetylpyrrole, 2,3-dimethylaniline, methoxyaniline, o-n-propylaniline, ethoxyaniline, 3-valerylpyrrole, p-phenylenediaminesulfonic acid, m-phenylenediaminesulfonic acid, naphthalenediamine, aminobiphenyl, 3,3′-diaminobenzidine, benzyldiamine, m-tolidine, 3,3′-dihydroxybenzidine, and dimethylbenzyldiamine; And / or, the initiator is selected from at least one of copper chloride, copper sulfate, and copper nitrate; and / or, the oxidant is at least one selected from sodium periodate, sodium iodate, hydrogen peroxide, potassium persulfate, ammonium persulfate, and potassium permanganate; and / or, the crosslinking agent in the crosslinking agent solution is at least one selected from epichlorohydrin, ethylenediamine, diethylenetriamine, glutaraldehyde, nonanedialdehyde, and adipaldehyde; And / or, the material of the ultrafiltration membrane is selected from at least one of polysulfone, polyethersulfone, sulfonated polysulfone, polyacrylonitrile and polytetrafluoroethylene.

3. The method for preparing a nanofiltration membrane according to claim 1, wherein: The monomer concentration in the monomer solution is 0 to 30 g / L and is not zero.

4. The method for preparing a nanofiltration membrane according to claim 1, wherein: In step S2, the temperature of the cross-linking reaction is 25-60°C; And / or, in step S3, the temperature of the cross-linking reaction is 40-80°C.

5. The method for preparing a nanofiltration membrane according to claim 1, wherein: The mass ratio of the monomer to the initiator in the monomer solution is 1:(0.4-0.7); And / or, the mass ratio of the monomer to the oxidant in the monomer solution is 1:(1.5-2.5).

6. The method for preparing a nanofiltration membrane according to claim 1, wherein: In step S2, the cross-linking reaction time is 0 to 2 hours and is not 0; And / or, in step S3, the cross-linking reaction time is 0 to 30 minutes and is not 0.

7. The method for preparing a nanofiltration membrane according to claim 1, wherein: The step S1 comprises: mixing the ultrafiltration membrane with the monomer solution for reaction for 1 to 5 minutes, then mixing with the initiator for reaction for 1 to 5 minutes, and then mixing with the oxidant for reaction for 1 to 10 hours to obtain the treated ultrafiltration membrane.

8. The method for preparing a nanofiltration membrane according to any one of claims 1 to 7, characterized in that: The step S1 and the step S2 are respectively reacted under any mixing mode selected from shaking, stirring or ultrasound.

9. A nanofiltration membrane, characterized in that: The preparation method according to any one of claims 1 to 8 is used.

10. Use of the nanofiltration membrane according to claim 9 in the field of antibiotic nanofiltration or sewage treatment.

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