Preparation method for positively charged nanofiltration membrane for improving "trade-off" effect

Abstract

The present invention relates to the technical field of nanofiltration membranes, and particularly relates to a preparation method for a positively charged nanofiltration membrane for improving a "trade-off" effect. The preparation method comprises the following process: sequentially adding a water phase and an oil phase to the surface of a support layer to perform a reaction, so as to prepare a polyamide layer; pouring in a functionalized graphene dispersion; and sequentially placing same into an N,N-dimethyl propane diamine solution, a sodium borohydride solution and an iodomethane solution to perform a reaction, so as to obtain a nanofiltration membrane, wherein the functionalized graphene is pyrrolidone-based graphene. According to the present invention, a mixture of piperazine and polyethyleneimine is used as a water phase and is subjected to interfacial polymerization with an oil phase 1,3,5-trimesoyl chloride on the surface of the support membrane; and by means of the compounding of micromolecular piperazine and macromolecular polyethyleneimine, the rapid formation of a polyamide layer is promoted, the distance of a system cross-linked network is increased, a positively charged polyamide layer having a large pore diameter and good hydrophilicity is formed, and a good water flux and a good rejection rate are achieved, thereby improving the "trade-off" effect.

Description

A preparation method for positively charged nanofiltration membrane with improved "trade-off" effect Technical Field

[0001] The present invention relates to the technical field of nanofiltration membranes, and in particular to a method for preparing a positively charged nanofiltration membrane capable of improving a "trade-off" effect. Background Art

[0002] Membrane separation technology, one of the most effective techniques in contemporary separation science, has attracted considerable attention due to its simplicity, low cost and energy consumption, and high separation efficiency. Nanofiltration, a type of membrane separation technology, has a pore size of 1.0-2.0 nm, an operating pressure intermediate between reverse osmosis and ultrafiltration, and a molecular weight cutoff range of 100-1000 Da. It is primarily used to filter particles in the 0.001-0.01 μm range, including most organic molecules, viruses, and a range of salts. Existing commercially available nanofiltration membranes are generally negatively charged, resulting in low efficiency in the removal and separation of high-valent cations, limiting their application in many industrial fields. Positively charged membranes, on the other hand, are susceptible to a "trade-off" effect in their permeation and separation performance. While positively charged membranes offer good cation retention, they often exhibit low water flux. Therefore, we propose a method for preparing positively charged nanofiltration membranes that mitigates this "trade-off" effect.

[0003] Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, so as to solve the problems raised in the above background technology.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following processes:

[0007] Step (1) preparing a polyamide layer:

[0008] Take the support layer, add the aqueous solution on its surface, and soak for 2 to 3 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution;

[0009] Add the oil phase solution and react for 10 to 60 seconds; discard the excess oil phase and rinse with n-hexane for 15 to 20 seconds;

[0010] placing the mixture at a temperature of 58 to 62° C. for 3 to 5 minutes to obtain a polyamide layer;

[0011] Step (2) surface modification:

[0012] Pour the functionalized graphene dispersion onto the surface of the polyamide layer and allow it to react for 90 to 120 minutes; pour off the excess liquid and absorb excess water;

[0013] Place it in N,N-dimethylpropylenediamine solution, react for 1 to 4 hours, and take it out; place it in sodium borohydride solution, react for 2 to 24 hours, take it out, and wash it with water; place it in iodomethane solution, react for 8 to 10 hours, take it out, and wash it with ethanol and deionized water; place it at 60 to 70°C for heat treatment for 3 to 5 minutes to obtain a nanofiltration membrane.

[0014] Furthermore, the aqueous phase solution is a mixed aqueous solution of piperazine and polyethyleneimine;

[0015] In the aqueous solution, the concentration of piperazine is 0.8-1.2 wt %; the concentration of polyethyleneimine is 0.8-1.2 wt %.

[0016] Furthermore, the oil phase solution is a 1,3,5-trimethylbenzenecarboxylic acid chloride solution, the solvent includes 70-100v% n-hexane and 0-30v% xylene; the concentration of the oil phase solution is 0.10-0.12wt%;

[0017] Xylene is a mixture of three isomers: 45% to 70% meta-xylene, 15% to 25% para-xylene and 10% to 15% o-xylene.

[0018] Furthermore, the volume ratio of the amount of aqueous solution added to the amount of oily solution added is 2:1.

[0019] In the above technical solution, piperazine and polyethyleneimine are used as the aqueous phase, and 1,3,5-benzenetricarboxylic acid chloride is used as the oil phase to carry out interfacial polymerization on the surface of the support membrane to obtain a positively charged polyamide layer. The small molecule amine monomer piperazine has a high solubility and reactivity in the oil phase solvent, which can promote the rapid formation and densification of the polyamide layer, and is beneficial to the initial construction of the polyamide layer (hereinafter referred to as the primary layer). Polyethyleneimine contains rich amino groups, which has a relatively significant effect on the electrical properties of the prepared polyamide layer, thereby enhancing the positive charge strength of the membrane layer and improving the retention capacity of the polyamide layer. The use of polyethyleneimine with a higher molecular weight has a slower migration and diffusion speed. In the later stage of interfacial polymerization, due to the obstruction of the primary layer and the steric hindrance effect, the amino groups in the polyethyleneimine are difficult to diffuse and are not easy to break through the initial polyamide layer. The spacing of the cross-linked network in the polyamide layer increases, forming a polyamide layer with loose pores and a lower thickness. It also has good surface hydrophilicity, so that the prepared polyamide layer has good water flux. However, as the amount of polyethyleneimine increases, the water flux of the polyamide layer increases, while the retention performance decreases. By controlling the quality of the aqueous phase components piperazine and polyimide, the water flux and retention rate of the polyamide layer can be balanced, thus improving the "trade-off" effect.

[0020] Furthermore, the functionalized graphene dispersion has a concentration of 3-8 wt %, a pH of 8.0-8.6, and a temperature of 30-38° C.

[0021] Furthermore, the functionalized graphene dispersion is prepared by the following process:

[0022] The amino-modified graphene and N-vinyl pyrrolidone were mixed in deionized water, and ascorbic acid and hydrogen peroxide were added under nitrogen atmosphere. The mixture was heated to 58-62°C and stirred for 8-12 hours. The mixture was washed by centrifugation with deionized water and ethanol, and dried to obtain functionalized graphene.

[0023] Functionalized graphene is added into an aqueous solution containing sodium dodecyl sulfate to obtain a functionalized graphene dispersion.

[0024] Furthermore, the ratio of N-vinyl pyrrolidone, amino graphene, ascorbic acid, hydrogen peroxide, and deionized water is (0.2-0.3) g:0.1 g:(0.08-0.09) g:(0.07-0.08) g:100 mL.

[0025] Furthermore, in the functionalized graphene dispersion, the concentration of sodium dodecyl sulfate is 1.0-1.2 g / L, and the concentration of formic acid is 1.2-1.5 g / L.

[0026] Furthermore, in the N,N-dimethylpropylenediamine solution, the solvent is anhydrous ethanol, the solutes are N,N-dimethylpropylenediamine and formic acid, and the temperature is 37-42°C;

[0027] The concentration of N,N-dimethylpropylenediamine was 15 wt %, and the concentration of formic acid was 15 v %.

[0028] Furthermore, the concentration of the sodium borohydride solution is 0.1 wt %, and the pH value is 8-9.

[0029] Furthermore, the concentration of the iodomethane solution is 5.0-7.5 wt %, the pH is 8-9, and the temperature is 30-35° C.

[0030] In the above technical solution, the amino graphene is modified with 3-aminopropyltriethoxysilane (APTMS), and N-vinyl pyrrolidone is polymerized by the initiation of ascorbic acid / hydrogen peroxide. The amino groups modified on the graphene surface are added and grafted with N-vinyl pyrrolidone to obtain functionalized graphene with pyrrolidone groups on the surface. The functionalized graphene is mixed with sodium dodecyl sulfate and formic acid to prepare a dispersion, which is poured on the surface of the polyamide layer. The carbon-oxygen double bond in the pyrrolidone group can react with the surface amino group of the polyamide layer to load the graphene on the surface of the polyamide layer. It is then placed in an N,N-dimethylpropylenediamine environment, and a large number of amino groups are introduced to the graphene surface through the reaction between the amino groups in N,N-dimethylpropylenediamine and the unreacted pyrrolidone groups on the surface of the functionalized graphene. The nanofiltration membrane is then reduced with sodium borohydride and subjected to nitrogen alkylation with methyl iodide and amino groups on the polyamide layer and graphene surface. This creates a large number of quaternary ammonium groups on the surface of the resulting nanofiltration membrane, strengthening its positive surface charge and enhancing the Donna repulsion effect for multivalent cations, effectively increasing the nanofiltration membrane's retention capacity for multivalent cations. The graphene and pyrrole five-membered rings introduced to the nanofiltration membrane surface enhance its hydrophilicity and cation adsorption capacity, hindering the passage of cations. This helps further improve the nanofiltration membrane's retention rate for multivalent cations while maintaining a high water flux, resulting in a nanofiltration membrane with excellent multivalent cation salt separation capabilities.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. The present invention describes a method for preparing a positively charged nanofiltration membrane with an improved "trade-off" effect. A mixture of piperazine and polyethyleneimine is used as the aqueous phase, and interfacial polymerization is carried out with the oil phase 1,3,5-benzenetricarboxylic acid chloride on the surface of a support membrane. The composite of small molecule piperazine and large molecule polyethyleneimine can promote the rapid formation and densification of a polyamide layer, increase the spacing of the system's cross-linked network, and form a polyamide layer with loose pores and good hydrophilicity. The polyamide layer is positively charged, has good water flux and retention rate, and the "trade-off" effect is improved.

[0033] 2. The present invention describes a method for preparing a positively charged nanofiltration membrane with an improved "trade-off" effect. The method comprises modifying amino-modified graphene with N-vinylpyrrolidone and loading it on the surface of a polyamide layer. Utilizing N,N-dimethylpropylenediamine, sodium borohydride, and iodomethane, amino groups are introduced onto the graphene surface, followed by reduction and quaternization. This strengthens the positive charge on the surface of the prepared nanofiltration membrane, improves its hydrophilicity and cation adsorption capacity, and improves the nanofiltration membrane's ability to retain polyvalent cations while maintaining a relatively high water flux, thereby providing excellent polyvalent cation salt separation capabilities. DETAILED DESCRIPTION

[0034] Based on the embodiments of the present invention, specific implementations are described herein, but the scope of the disclosure is not limited to these specific implementations.

[0035] In the following specific embodiments,

[0036] The supporting layer is a polyacrylonitrile (PAN) ultrafiltration membrane, which was obtained from TCI (Shanghai) Chemical Industry Development Co., Ltd. and has a size of 60 mm × 60 mm.

[0037] Polyethyleneimine: EPOMIN P-1050, from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0038] The oil phase solution is a 1,3,5-trimethylbenzene carboxylic acid chloride solution, and the solvent includes 70v% n-hexane and 30v% mixed xylene;

[0039] Mixed xylene: X139941, from Shanghai Aladdin Biochemical Technology Co., Ltd.;

[0040] Aminated graphene: Aminated graphene (APTMS modified) XF315 was obtained from Jiangsu Xianfeng Nanomaterials Technology Co., Ltd.

[0041] Example 1: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following steps:

[0042] Step (1) preparing a polyamide layer:

[0043] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 2 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixed aqueous solution containing 0.8 wt% piperazine and 1.2 wt% polyethyleneimine;

[0044] Add 10 mL of oil phase solution (0.10 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 10 seconds; discard the excess oil phase and rinse with n-hexane for 15 seconds;

[0045] Place at 58°C for 3 minutes to obtain a polyamide layer;

[0046] Step (2) surface modification:

[0047] 0.1 g of amino-modified graphene and 0.2 g of N-vinyl pyrrolidone were mixed in 100 mL of deionized water. Under nitrogen atmosphere, 0.080 g of ascorbic acid and 0.070 g of hydrogen peroxide were added. The mixture was heated to 58°C and stirred for 8 h. The mixture was washed by centrifugation with deionized water and ethanol and dried to obtain functionalized graphene.

[0048] The functionalized graphene is added to an aqueous solution containing sodium dodecyl sulfate to obtain a functionalized graphene dispersion; in the functionalized graphene dispersion, the concentration of sodium dodecyl sulfate is 1.0 g / L, and the concentration of formic acid is 1.2 g / L;

[0049] Pour 10 mL of 3 wt% functionalized graphene dispersion (pH 8.0) onto the surface of the polyamide layer and react at 30°C for 90 min; pour off the excess liquid and absorb excess water;

[0050] Place in a 15 wt% N,N-dimethylpropylenediamine solution, react at 37°C for 1 hour, remove; place in a 0.1 wt% sodium borohydride solution (pH 8), react for 2 hours, remove, and wash with water; the solvent of the N,N-dimethylpropylenediamine solution is anhydrous ethanol, and the solutes are N,N-dimethylpropylenediamine and formic acid, with a formic acid concentration of 15 v%;

[0051] The membrane was placed in a 5.0 wt% iodomethane solution (pH 8), reacted at 30° C. for 8 h, taken out, and washed with ethanol and deionized water; and heat-treated at 60° C. for 3 min to obtain a nanofiltration membrane.

[0052] Example 2: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following steps:

[0053] Step (1) preparing a polyamide layer:

[0054] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 2.5 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixed aqueous solution containing 1.0 wt% piperazine and 1.0 wt% polyethyleneimine;

[0055] Add 10 mL of oil phase solution (0.11 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 35 seconds; discard the excess oil phase and rinse with n-hexane for 18 seconds;

[0056] Place at 60°C for 4 minutes to obtain a polyamide layer;

[0057] Step (2) surface modification:

[0058] 0.1 g of amino-modified graphene and 0.25 g of N-vinyl pyrrolidone were mixed in 100 mL of deionized water. Under nitrogen atmosphere, 0.085 g of ascorbic acid and 0.075 g of hydrogen peroxide were added. The mixture was heated to 60°C and stirred for 10 h. The mixture was washed by centrifugation with deionized water and ethanol and dried to obtain functionalized graphene.

[0059] The functionalized graphene is added to an aqueous solution containing sodium dodecyl sulfate to obtain a functionalized graphene dispersion; the concentration of sodium dodecyl sulfate in the functionalized graphene dispersion is 1.1 g / L, and the concentration of formic acid is 1.3 g / L;

[0060] Pour 10 mL of 5 wt% functionalized graphene dispersion (pH 8.3) onto the surface of the polyamide layer and react at 35°C for 105 min; pour off the excess liquid and absorb excess water;

[0061] Place in a 15wt% N,N-dimethylpropylenediamine solution, react at 40°C for 3 hours, then remove; place in a 0.1wt% sodium borohydride solution (pH 8.5), react for 5 hours, remove, and wash with water; the solvent of the N,N-dimethylpropylenediamine solution is anhydrous ethanol, and the solutes are N,N-dimethylpropylenediamine and formic acid, with a formic acid concentration of 15v%;

[0062] The membrane was placed in a 6.5 wt% iodomethane solution (pH 8.5), reacted at 32° C. for 9 h, taken out, washed with ethanol and deionized water, and heat-treated at 65° C. for 4 min to obtain a nanofiltration membrane.

[0063] Example 3: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following steps:

[0064] Step (1) preparing a polyamide layer:

[0065] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 3 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixed aqueous solution containing 1.2 wt% piperazine and 0.8 wt% polyethyleneimine;

[0066] Add 10 mL of oil phase solution (0.12 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 60 seconds; discard the excess oil phase and rinse with n-hexane for 20 seconds;

[0067] Place at 62°C for heat treatment for 5 minutes to obtain a polyamide layer;

[0068] Step (2) surface modification:

[0069] 0.1 g of amino-modified graphene and 0.3 g of N-vinyl pyrrolidone were mixed in 100 mL of deionized water. Under nitrogen atmosphere, 0.09 g of ascorbic acid and 0.08 g of hydrogen peroxide were added. The mixture was heated to 62°C and stirred for 12 h. The mixture was washed by centrifugation with deionized water and ethanol and dried to obtain functionalized graphene.

[0070] The functionalized graphene is added to an aqueous solution containing sodium dodecyl sulfate to obtain a functionalized graphene dispersion; the concentration of sodium dodecyl sulfate in the functionalized graphene dispersion is 1.2 g / L, and the concentration of formic acid is 1.5 g / L;

[0071] Pour 10 mL of 8 wt% functionalized graphene dispersion (pH 8.6) onto the surface of the polyamide layer and react at 38°C for 120 min; pour off the excess liquid and absorb excess water;

[0072] Place in a 15 wt% N,N-dimethylpropylenediamine solution, react at 42°C for 4 hours, then remove; place in a 0.1 wt% sodium borohydride solution (pH 9), react for 10 hours, remove, and wash with water; the solvent of the N,N-dimethylpropylenediamine solution is anhydrous ethanol, and the solutes are N,N-dimethylpropylenediamine and formic acid, with a formic acid concentration of 15 v%;

[0073] The membrane was placed in a 7.5 wt% iodomethane solution (pH 9), reacted at 35° C. for 10 h, taken out, and washed with ethanol and deionized water; and heat-treated at 70° C. for 5 min to obtain a nanofiltration membrane.

[0074] Comparative Example 1: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following processes:

[0075] Step (1) preparing a polyamide layer:

[0076] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 2 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixture containing 0.8 wt% piperazine and 1.2 wt% polyethyleneimine;

[0077] Add 10 mL of oil phase solution (0.10 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 10 seconds; discard the excess oil phase and rinse with n-hexane for 15 seconds;

[0078] Place at 58°C for 3 minutes to obtain a polyamide layer;

[0079] Step (2) surface modification:

[0080] Pour 10 mL of 3 wt% amination-modified graphene dispersion (pH 8.0) onto the surface of the polyamide layer and react at 30°C for 90 min; pour off the excess liquid and absorb excess water;

[0081] The polyamide layer was placed in a 5.0 wt% iodomethane solution (pH 8), reacted at 30° C. for 8 h, taken out, washed with ethanol and deionized water, and heat-treated at 60° C. for 3 min to obtain a nanofiltration membrane.

[0082] Comparative Example 2: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following processes:

[0083] Step (1) preparing a polyamide layer:

[0084] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 2 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixed aqueous solution containing 0.8 wt% piperazine and 1.2 wt% polyethyleneimine;

[0085] Add 10 mL of oil phase solution (0.10 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 10 seconds; discard the excess oil phase and rinse with n-hexane for 15 seconds;

[0086] Place at 58°C for 3 minutes to obtain a polyamide layer;

[0087] Step (2) surface modification:

[0088] 10 mL of 3 wt % methacryloyloxyethyl trimethylammonium chloride solution (pH 8) was added to the surface of the polyamide layer and reacted at 35° C. for 2 h; excess liquid was poured off, the membrane was washed with water, and dried to obtain a nanofiltration membrane.

[0089] Comparative Example 3: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following processes:

[0090] Take the support layer, add 20 mL of aqueous solution on its surface, and soak for 2 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; the aqueous solution is a mixed aqueous solution containing 0.8 wt% piperazine and 1.2 wt% polyethyleneimine;

[0091] Add 10 mL of oil phase solution (0.10 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 10 seconds; discard the excess oil phase and rinse with n-hexane for 15 seconds;

[0092] The nanofiltration membrane was obtained by heat treatment at 58°C for 3 minutes.

[0093] Comparative Example 4: A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, comprising the following processes:

[0094] Take the support layer, add 20 mL of aqueous solution (2.0 wt% piperazine) on its surface, and soak for 2 min; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove residual solution;

[0095] Add 10 mL of oil phase solution (0.10 wt% 1,3,5-trimethylbenzenecarboxylic acid chloride solution) and react for 10 seconds; discard the excess oil phase and rinse with n-hexane for 15 seconds;

[0096] The nanofiltration membrane was obtained by heat treatment at 58°C for 3 minutes.

[0097] Experiment: Take the nanofiltration membranes obtained in Examples 1-3 and Comparative Examples 1-4, prepare samples, test their performance and record the test results:

[0098] Contact angle test: The contact angle of the nanofiltration membrane sample was tested using a contact angle meter. The experimental method was as follows: the dry sample was placed flat on a glass slide and 5 μL of deionized water was added;

[0099] Water flux test: Place the sample in the membrane pool, and allow the raw material liquid to pass through the membrane pool under the pressure of the gear pump. The pressure is set to 0.5 MPa, and the pre-pressure is run for 60 minutes. The water flux is calculated according to the formula J = V / (At), where J is the membrane flux (L / (m 2 ·h)), V is the volume of water permeating the membrane during a single test (L), and A is the effective water flow area of ​​the membrane (m 2 ), t is the single test time (min);

[0100] Retention rate test: The sample was operated at a constant pressure of 0.5 MPa, and the retention rate was calculated according to the formula R = (1-Cp / Cf) × 100%, where R is the retention rate (%), Cp is the concentration of contaminants in the permeate (mg / L), and Cf is the concentration of contaminants in the feed solution (mg / L); the feed solution was a 2000 ppm magnesium chloride solution with a pH of 8.0.

[0101] According to the data in the above table, we can clearly draw the following conclusions:

[0102] The nanofiltration membranes obtained in Examples 1-3 were compared with those obtained in Comparative Examples 1-4. The test results showed that

[0103] Compared with the comparative example, the water contact angle data of the nanofiltration membranes obtained in Examples 1-3 were reduced, and the comprehensive data of membrane flux and retention rate were improved, which fully demonstrated that the present application achieved an improvement in the "trade-off" effect of the prepared nanofiltration membranes.

[0104] Compared with Example 1, in the surface modification of Comparative Example 1, the amino graphene is loaded on the surface of the polyamide layer and then alkylated with iodomethane nitrogen. N-vinylpyrrolidone and N,N-dimethylpropylenediamine are not introduced, which reduces the number of quaternary ammonium salts on the surface of the nanofiltration membrane, reduces its hydrophilicity, increases the contact angle data, and decreases the water flux data; the Donna rejection effect on multivalent cations is weakened, the retention rate decreases, and the nanofiltration membrane is more affected by the "trade-off" effect.

[0105] In Comparative Example 2, quaternization modification was performed by reacting methacryloyloxyethyl trimethylammonium chloride with amino groups on the surface of the polyamide layer. N-vinyl pyrrolidone, N,N-dimethylpropylenediamine, and graphene were not introduced. The number of quaternary ammonium salts on the surface of the nanofiltration membrane was greatly reduced, the surface roughness was reduced, the number of pores was reduced, the hydrophilicity was greatly reduced, and the ability to repel and hinder multivalent cations was weakened, resulting in deterioration of various data of the resulting nanofiltration membrane.

[0106] The nanofiltration membrane in Comparative Example 3 was not subjected to surface modification treatment, and its surface hydrophilicity was further reduced, and its positive charge was weak, so that the various data of the nanofiltration membrane obtained were deteriorated.

[0107] In Comparative Example 4, the aqueous phase of the nanofiltration membrane is piperazine, the spacing of the cross-linked network of the system is reduced, the pore size of the formed polyamide layer is reduced, the hydrophilicity is reduced, and the positive charge strength is reduced, thereby reducing its contact angle, water flux and retention rate data.

[0108] Finally, it should be noted that the above is only a preferred embodiment of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect, characterized in that: Including the following processes: Step (1) preparing a polyamide layer: Take the support layer, add the aqueous solution on its surface, and soak for 2 to 3 minutes; pour out the aqueous phase, absorb excess water, and purge with nitrogen to remove the residual solution; Add the oil phase solution and react for 10 to 60 seconds; discard the excess oil phase and rinse with n-hexane for 15 to 20 seconds; placing the mixture at a temperature of 58 to 62° C. for a heat treatment period of 3 to 5 minutes to obtain a polyamide layer; Step (2) surface modification: pouring a functionalized graphene dispersion onto the surface of the polyamide layer and reacting for 90 to 120 minutes; pouring off excess liquid and absorbing excess water; Place in N,N-dimethylpropylenediamine solution, react for 1 to 4 hours, take out; place in sodium borohydride solution, react for 2 to 24 hours, take out, and wash with water; Place in iodomethane solution, react for 8-10 hours, take out, wash with ethanol and deionized water; place at 60-70°C for heat treatment for 3-5 minutes to obtain a nanofiltration membrane; The functionalized graphene is pyrrolidone-based graphene.

2. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The functionalized graphene dispersion is prepared by the following process: The amino graphene and N-vinyl pyrrolidone are mixed in deionized water and stirred for 60 to 90 minutes; the graphene is washed by centrifugation with deionized water and ethanol, and dried to obtain functionalized graphene; The functionalized graphene is added into an aqueous solution containing sodium dodecyl sulfate to obtain a functionalized graphene dispersion.

3. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 2, characterized in that: The ratio of N-vinyl pyrrolidone, amino graphene and deionized water is (0.2-0.3) g:0.1 g:100 mL.

4. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 2, characterized in that: In the functionalized graphene dispersion, the concentration of sodium dodecyl sulfate is 1.0-1.2 g / L, and the concentration of formic acid is 1.2-1.5 g / L.

5. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The aqueous phase solution is a mixed aqueous solution of piperazine and polyethyleneimine; In the aqueous phase solution, the concentration of piperazine is 0.8-1.2 wt %; the concentration of polyethyleneimine is 0.8-1.2 wt %.

6. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The oil phase solution is a 1,3,5-trimethylbenzene chloride solution, and the solvent includes 70-100% by volume of normal hexane and 0-30% by volume of xylene; the concentration of the oil phase solution is 0.10-0.12% by weight.

7. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The volume ratio of the amount of aqueous solution added to the amount of oily solution added is 2:

1.

8. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The concentration of the functionalized graphene dispersion is 3-8wt%, the pH is 8.0-8.6, and the temperature is 30-38°C.

9. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: In the N,N-dimethylpropylenediamine solution, the solvent is anhydrous ethanol, the solutes are N,N-dimethylpropylenediamine and formic acid, and the temperature is 37-42°C; The concentration of N,N-dimethylpropylenediamine was 15 wt %, and the concentration of formic acid was 15 v %.

10. The method for preparing a positively charged nanofiltration membrane with improved "trade-off" effect according to claim 1, characterized in that: The concentration of the sodium borohydride solution is 0.1wt%, and the pH is 8-9; The concentration of the iodomethane solution is 5.0-7.5 wt %, the pH is 8-9, and the temperature is 30-35° C.

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