A charge-compensated crosslinker constructs graphene oxide nanofiltration membranes and a preparation method thereof

Abstract

The application discloses a kind of charge compensation crosslinking agent constructs graphene oxide nanofiltration membrane and preparation method thereof, the nanofiltration membrane is first with charge compensation crosslinking agent and graphene oxide nanosheet crosslinking, then after dilution, ultrasonic, filtration of the nanosheet dispersion liquid after crosslinking is prepared.The application further discloses the preparation method and application of the above-mentioned nanofiltration membrane.The macroporous polymer membrane is used as support layer in the application, and the charge compensation crosslinked graphene oxide nanosheet is loaded on the support layer by vacuum filtration, and the charge compensation crosslinked graphene oxide nanofiltration membrane is prepared.The method is simple in operation, and the charge compensation crosslinking agent is crosslinked with graphene oxide nanosheet, the interlayer electronegativity is compensated while the interlayer spacing of nanofiltration membrane is stabilized, so that the selectivity of the membrane is improved;Meanwhile, the crosslinking agent with sulfonic acid group improves the permeability of crosslinked graphene oxide nanofiltration membrane due to the hydrophilicity of sulfonic acid group.

Description

Technical Field

[0001] This invention relates to a charge-compensating crosslinking agent for constructing graphene oxide nanofiltration membranes and a method for preparing the same, belonging to the technical field of crosslinked graphene oxide nanofiltration membrane materials and their preparation. Background Technology

[0002] Membrane separation technology is widely used in seawater desalination due to its advantages such as high cost-effectiveness, high efficiency, and environmental friendliness. Graphene oxide (GO) is rich in oxygen-containing functional groups on its surface (such as carboxyl, hydroxyl, carbonyl, and epoxy groups), exhibiting good hydrophilicity, flexibility, and processability. The nanochannels in GO membrane structures typically exhibit a "size sieving effect," while the deprotonation of carboxyl groups at the edges of GO nanosheets generates a negative charge. Due to the synergistic effect of the size sieving effect and the Donnan effect, pure GO membranes can strongly repel negatively charged organic molecules and divalent ions, thereby achieving the retention and filtration of organic molecules and salt ions. However, graphene oxide membranes are prone to swelling in aqueous solutions, leading to low selectivity between metal ions. Simultaneously, graphene oxide sheets readily interact with ions, hindering ion transport and resulting in low ion flux. Therefore, modification and regulation of graphene oxide membranes are necessary to improve ion flux and selectivity.

[0003] To improve the structural stability of graphene-based membranes, molecular crosslinking strategies can be employed to precisely and stably control the interlayer structure and separation performance of GO membranes. For example, the journal *Carbon* (2016, 101, 290-295) reported a method for achieving different sieving of metal ions by constructing crosslinked graphene oxide membranes. Under the action of an acid catalyst, the graphene oxide membrane was sequentially immersed in dicarboxylic acid and diamine solutions to achieve crosslinking. Compared to pure graphene oxide membranes, the crosslinked graphene oxide membranes prepared by this method showed improved sieving performance for metal ions of different valence states. However, during the crosslinking process, a large number of oxygen-containing functional groups on the graphene oxide surface were consumed, weakening the Donnan effect. This resulted in unsatisfactory separation performance of the obtained membrane material for negatively charged salt ions, with a relatively low ion flux of only 10⁻⁶. -2 mol·m -2 ·h -1 Furthermore, the preparation process is complex, the membrane has poor controllability, and it is not conducive to mass production.

[0004] Therefore, it is of great significance to develop a graphene oxide-based membrane material that simultaneously possesses interlayer structural stability and interlayer electronegativity to improve the separation of salt ions and ion flux of nanofiltration membranes. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a method for constructing graphene oxide nanofiltration membranes using charge-compensated crosslinking agents, thereby solving the problems of easy swelling of existing graphene oxide-based membrane materials, which leads to poor structural stability and selectivity of the membranes.

[0006] To achieve the above objectives, the present invention provides a charge-compensating crosslinking agent for constructing a graphene oxide nanofiltration membrane. The nanofiltration membrane is prepared by first crosslinking the charge-compensating crosslinking agent and a graphene oxide dispersion, and then diluting, sonicating, and filtering the crosslinked nanosheet dispersion. The charge-compensating crosslinking agent contains two groups that can react with graphene oxide and has a sulfonic acid group.

[0007] In one embodiment of the present invention, the charge-compensating crosslinking agent is selected from any one of 2,5-diaminobenzenesulfonic acid, 2,4-diaminobenzenesulfonic acid, p-phenylenediamine-2,5-disulfonic acid, and 2,5-diaminobenzene-1,4-disulfonic acid.

[0008] In one embodiment of the present invention, the mass ratio of graphene oxide to charge-compensating crosslinking agent is 1:0.1 to 1:5, preferably 1:1 to 1:2.

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

[0010] (1) Preparation of graphene oxide nanosheet dispersion: flake graphite, concentrated sulfuric acid and concentrated phosphoric acid were mixed and stirred, potassium permanganate was slowly added, and the mixture was reacted in a water bath at 50 ℃ for 24 h. Then 0.8 ~ 3 L of deionized water was added, followed by 10 ~ 30 mL of 30% hydrogen peroxide was added dropwise until the solution turned golden yellow. After standing overnight, the mixture was centrifuged at 3000 ~ 3500 r / min for 15 ~ 30 min, for a total of 15 ~ 20 centrifugations. The centrifuged product was washed with 1 ~ 5% dilute hydrochloric acid and deionized water until pH=5 ~ 6. After dialysis, the product was dispersed in ultrapure water to obtain graphene oxide nanosheet dispersion.

[0011] (2) Preparation of charge-compensating crosslinking agent dispersion: Dissolve the charge-compensating crosslinking agent in ultrapure water, heat and stir until dissolved to obtain charge-compensating crosslinking agent dispersion;

[0012] (3) Preparation of charge-compensated cross-linked graphene oxide nanosheet dispersion: The graphene oxide nanosheet dispersion obtained in step (1) and the charge-compensated cross-linking agent dispersion obtained in step (2) are mixed and stirred thoroughly, and then thermal cross-linked to obtain the charge-compensated cross-linked graphene oxide nanosheet dispersion.

[0013] (4) Preparation of charge-compensated cross-linked graphene oxide nanofiltration membrane: The charge-compensated cross-linked graphene oxide nanosheet dispersion obtained in step (3) was diluted, ultrasonically treated, loaded onto the surface of a macroporous polymer support membrane by vacuum filtration, and dried to obtain the charge-compensated cross-linked graphene oxide nanofiltration membrane.

[0014] In one embodiment of the present invention, in step (1), the size of the flake graphite used in the preparation of graphene oxide nanosheets is 100 to 400 mesh, and the concentrations of concentrated sulfuric acid and concentrated phosphoric acid are 80% to 98% and 80 to 85%, respectively, with a volume ratio of 9:1 to 7:1.

[0015] In one embodiment of the present invention, in step (1), the mass-to-volume ratio of the flake graphite to concentrated sulfuric acid is 1 g:(100~120) mL.

[0016] In one embodiment of the present invention, in step (1), the concentration of graphene oxide nanosheets in the graphene oxide nanosheet dispersion is 1 ~ 5 mg / mL.

[0017] In one embodiment of the present invention, in step (2), the concentration of the charge-compensating crosslinking agent in the charge-compensating crosslinking agent dispersion is 1 ~ 5 mg / mL.

[0018] In one embodiment of the present invention, in step (2), the charge-compensating crosslinking agent is a crosslinking agent containing two reactive groups that can react with graphene oxide and having sulfonic acid groups, including at least one of 2,5-diaminobenzenesulfonic acid (DABA), 2,4-diaminobenzenesulfonic acid, p-phenylenediamine-2,5-disulfonic acid, and 2,5-diaminobenzene-1,4-disulfonic acid.

[0019] In one embodiment of the present invention, in step (2), the heating temperature is 60 ~ 100℃, the stirring rate is 300 ~ 500 r / min, and the heating time is 1-3 h.

[0020] In one embodiment of the present invention, the mixing ratio of the reduced graphene oxide nanosheet dispersion and the charge-compensating crosslinking agent dispersion in step (3) is 1:0.1 to 1:5, preferably 1:1 to 1:2, the stirring time is 0.5 to 12 h, the stirring rate is 300 to 500 r / min, the thermal crosslinking temperature is 60 to 80 °C, and the time is 1 to 3 h.

[0021] In one embodiment of the present invention, in step (4), the concentration range of the diluted charge-compensated cross-linked graphene oxide nanosheet dispersion is 0.001 ~ 0.1 mg / mL, the ultrasonic power is 60 ~ 180 W, and the ultrasonic time is 0.2 ~ 0.5 h.

[0022] In one embodiment of the present invention, the polymer support membrane in step (4) includes any one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), and mixed cellulose (MCEM) microporous filter membrane.

[0023] In one embodiment of the present invention, the polymer support membrane has a pore size of 0.22 ~ 0.44 μm and a diameter of 25 ~ 50 cm.

[0024] In one embodiment of the present invention, the drying time in step (4) is 12 to 24 hours and the drying temperature is 40 to 60 °C.

[0025] The present invention also provides a separation device comprising the above-described charge-compensated cross-linked graphene oxide nanofiltration membrane.

[0026] The present invention also provides the application of the above-mentioned charge-compensated cross-linked graphene oxide nanofiltration membrane in the field of water treatment.

[0027] The beneficial effects of this invention are:

[0028] (1) The present invention crosslinks graphene oxide nanosheets with a charge-compensating crosslinking agent, thereby stabilizing the interlayer spacing of nanofiltration membranes and compensating for interlayer electronegativity, thereby improving the selectivity of the membrane.

[0029] (2) The present invention uses a charge-compensating crosslinking agent to crosslink graphene oxide nanosheets. The crosslinking agent with sulfonic acid groups improves the permeability of the crosslinked graphene oxide nanofiltration membrane due to the hydrophilicity of the sulfonic acid groups.

[0030] (3) The nanofiltration membrane prepared by the method of the present invention has excellent membrane permeability, and its pure water flux can reach up to 1.8 L·m -2 ·h -1 ·bar -1 Meanwhile, the prepared nanofiltration membrane has a Na2SO4 rejection rate of over 92.8%. Attached Figure Description

[0031] Figure 1 The images shown are AFM test results of the charge-compensated cross-linked graphene oxide nanosheets in Example 1, where (a) is the AFM test result of the charge-compensated cross-linked graphene oxide nanosheets and (b) is the height curve of the charge-compensated cross-linked graphene oxide nanosheets after AFM testing.

[0032] Figure 2 The Zeta potential diagrams are for graphene oxide in Comparative Example 1 and the charge-compensated cross-linked graphene oxide nanosheet dispersion in Example 1.

[0033] Figure 3 Scanning electron microscope images of the surface (a) and cross-section (b) of the charge-compensated cross-linked graphene oxide nanofiltration membrane prepared in Example 1. Detailed Implementation

[0034] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0035] Example 1

[0036] A method for preparing a graphene oxide nanofiltration membrane using a charge-compensating crosslinking agent includes the following steps:

[0037] (1) Preparation of graphene oxide nanosheet dispersion: 4 g of 325 mesh graphite flakes, 480 mL of 98% concentrated sulfuric acid and 54 mL of 85% concentrated phosphoric acid were mixed and stirred. 24 g of potassium permanganate was slowly added. After reacting in a water bath at 50℃ for 24 h, 800 mL of deionized water was added. Then, 30% hydrogen peroxide was added dropwise until the solution turned golden yellow. After standing overnight, the solution was centrifuged. The centrifuged product was washed with dilute hydrochloric acid and deionized water until pH=5~6. After dialyzing, the product was dispersed in ultrapure water to obtain a graphene oxide nanosheet dispersion with a concentration of 1 mg / mL.

[0038] (2) Preparation of charge-compensating crosslinking agent dispersion: 0.021 mmol 2,5-diaminobenzenesulfonic acid (4 mg) was dissolved in 4 mL of ultrapure water, and then dissolved at 400 r / min and 80℃ for 3 h to obtain a charge-compensating crosslinking agent dispersion with a concentration of 1 mg / mL.

[0039] (3) Preparation of charge-compensated cross-linked graphene oxide nanosheet dispersion: The graphene oxide nanosheet dispersion obtained in step (1) and the charge-compensated cross-linking agent dispersion obtained in step (2) are mixed in a volume ratio of 1:1 and stirred at 400 r / min for 30 min to make the dispersion uniform. Then, the cross-linking reaction is carried out at 400 r / min and 80℃ for 3 h.

[0040] (4) Preparation of charge-compensated cross-linked graphene oxide nanofiltration membrane: Take 2 mL of the charge-compensated cross-linked graphene oxide nanosheet dispersion obtained in step (3) and dilute it into 200 mL of solution. After sonicating at 180 W power for 0.5 h, the uniformly mixed dispersion is filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum-assisted filtration. The membrane is dried at 45 °C for 24 h to obtain the charge-compensated cross-linked graphene oxide nanofiltration membrane, named DABA-GO membrane.

[0041] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 1.8 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 92.8%.

[0042] Performance characterization of charge-compensated cross-linked graphene oxide nanofiltration membranes

[0043] The charge-compensated crosslinked graphene oxide nanofiltration membrane prepared in Example 1 was characterized, and the results are shown in the figure. Figure 1 ~3. From Figure 1 It can be seen that the prepared charge-compensated cross-linked graphene oxide nanosheets exhibit a distinct two-dimensional structure with an undulating surface and a thickness of 10–14 nm. These results indicate that GO and DABA were successfully cross-linked.

[0044] from Figure 2 It can be seen that the charge-compensated cross-linked graphene oxide nanosheet dispersion has a more negative Zeta potential than the graphene oxide dispersion, exhibiting stronger electronegativity.

[0045] from Figure 3 The membrane surface is dense and intact, without defects, and the cross-sectional thickness is about 225 nm, which helps the membrane retain salt ions.

[0046] Example 2

[0047] The difference between Example 2 and Example 1 is that the amount of 2,5-diaminobenzenesulfonic acid added is 2 mg, resulting in a charge-compensating crosslinking agent dispersion with a GO:DABA mass ratio of 1:0.5.

[0048] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 3.6 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 87.8%.

[0049] Example 3

[0050] The difference between Example 3 and Example 1 is that the amount of 2,5-diaminobenzenesulfonic acid added is 6 mg, resulting in a charge-compensating crosslinking agent dispersion with a GO:DABA mass ratio of 1:1:5.

[0051] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 1.4 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 91.3%.

[0052] Example 4

[0053] The difference between Example 4 and Example 1 is that the amount of 2,5-diaminobenzenesulfonic acid added is 8 mg, resulting in a charge-compensating crosslinking agent dispersion with a GO:DABA mass ratio of 1:2.

[0054] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 1.2 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 92.3%.

[0055] As can be seen from Examples 1-4, with the increase of the mass ratio of the charge-compensating crosslinking agent 2,5-diaminobenzenesulfonic acid to graphene oxide nanosheets, the retention performance of the composite membrane for Na2SO4 gradually improves and then stabilizes, but the pure water flux gradually decreases. When the mass ratio of graphene oxide to charge-compensating crosslinking agent is 1:1, the composite membrane exhibits better separation performance.

[0056] Example 5

[0057] The difference between Example 5 and Example 1 is that 4 mg of 2,4-diaminobenzenesulfonic acid was added to obtain a charge-compensating crosslinking agent dispersion with a mass ratio of GO to 2,4-diaminobenzenesulfonic acid of 1:1.

[0058] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 0.9 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 94.4%.

[0059] As can be seen from Examples 1 and 5, compared with 2,5-diaminobenzenesulfonic acid, which uses 2,4-diaminobenzenesulfonic acid with closer intermolecular distance between diamine molecules, the composite membrane has improved Na2SO4 retention performance, but the pure water flux is relatively reduced.

[0060] Comparative Example 1:

[0061] The difference between Comparative Example 1 and Example 1 is that steps (2) and (3) are omitted. The graphene oxide nanosheet dispersion from step (1) is directly diluted to 200 mL. After sonication at 180 W for 0.5 h, the uniformly mixed dispersion is filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm using vacuum-assisted filtration. The membrane is then dried at 45 °C for 24 h to obtain a pure graphene oxide nanofiltration membrane, which is named GO membrane.

[0062] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 6.9 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 68.2%.

[0063] Comparative Example 2:

[0064] Graphene oxide nanosheets were dispersed in deionized water to obtain a graphene oxide nanosheet dispersion with a concentration of 1 mg / mL. The preparation method of the graphene oxide nanosheets was the same as in Example 1. The graphene oxide dispersion was hydrothermally reduced at 400 r / min and 80 °C for 3 h. 1 ml of the hydrothermally reduced graphene oxide nanosheet dispersion was diluted to 200 mL, and after sonication at 180 W for 0.5 h, the uniformly mixed dispersion was filtered through a PES microporous membrane with a pore size of 0.22 μm and a diameter of 50 mm using vacuum-assisted filtration. The membrane was dried at 45 °C for 24 h to obtain a reduced graphene oxide nanofiltration membrane, named rGO membrane. The separation performance of the membrane was tested using a cross-flow filtration device, and its pure water flux was 5.1 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 80.7%.

[0065] Comparative Example 3:

[0066] The difference between Comparative Example 3 and Example 1 is that the crosslinking agent in step (2) is replaced with p-phenylenediamine, and the nanofiltration membrane is prepared according to the following scheme.

[0067] (1) Preparation of graphene oxide dispersion: Graphene oxide nanosheets were diluted and dispersed in deionized water to obtain a graphene oxide nanosheet dispersion with a concentration of 1 mg / mL. The preparation method of graphene oxide nanosheets was the same as in Example 1.

[0068] (2) Preparation of p-phenylenediamine crosslinking agent dispersion: 0.021 mmol p-phenylenediamine (2.3 mg) was dissolved in 4 mL of ultrapure water, and then dissolved at 400 r / min and 80℃ for 3 h to obtain a p-phenylenediamine crosslinking agent dispersion with a concentration of 2 mg / mL.

[0069] (3) Preparation of p-phenylenediamine crosslinked graphene oxide nanosheet dispersion: The graphene oxide nanosheet dispersion obtained in step (1) and the p-phenylenediamine crosslinking agent dispersion obtained in step (2) were mixed in a volume ratio of 1:1 and stirred at 400 r / min for 30 min to make the dispersion uniform. Then, the crosslinking reaction was carried out at 400 r / min and 80℃ for 3 h.

[0070] (4) Preparation of p-phenylenediamine cross-linked graphene oxide nanofiltration membrane: Take 2 ml of the p-phenylenediamine cross-linked graphene oxide nanosheet dispersion obtained in step (3) and dilute it into 200 mL of solution. After sonicating at 180 W power for 0.5 h, the uniformly mixed dispersion is filtered onto a PES microporous filter membrane with a pore size of 0.22 μm and a diameter of 50 mm by vacuum-assisted filtration. The membrane is dried at 45 °C for 24 h to obtain the p-phenylenediamine cross-linked graphene oxide nanofiltration membrane, named PDD-GO membrane.

[0071] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 0.9 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 61.6%.

[0072] Comparative Example 5:

[0073] The difference between Comparative Example 5 and Example 1 is that the amount of 2,5-diaminobenzenesulfonic acid added is 40 mg, resulting in a charge-compensated crosslinking agent dispersion with a GO:DABA mass ratio of 1:10.

[0074] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 1.5 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 85.9%.

[0075] Comparative Example 6:

[0076] The difference between Comparative Example 6 and Example 1 is that the crosslinking temperature of 2,5-diaminobenzenesulfonic acid and GO nanosheets in step (3) is 40 °C.

[0077] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 5.2 L·m⁻¹. -2 ·h-1 ·bar -1 The rejection rate for Na2SO4 was 78.3%.

[0078] Comparative Example 7:

[0079] The difference between Comparative Example 7 and Example 1 is that in step (4), the ultrasound time is 1 hour.

[0080] The membrane separation performance was tested using a cross-flow filtration device, with a pure water flux of 2.7 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 88.5%.

[0081] Comparative Example 8

[0082] The difference between Comparative Example 8 and Example 1 is that, in step (1), the concentration of the graphene oxide nanosheet dispersion is 8 mg / mL; in step (2), the amount of 2,5-diaminobenzenesulfonic acid added is 32 mg, and the concentration of the charge-compensating crosslinking agent dispersion is 8 mg / mL; in step (4), the volume of the charge-compensating crosslinking agent dispersion dispersed in 200 mL of ultrapure water is 0.25 mL.

[0083] The membrane separation performance was tested using a cross-flow filtration device, and its pure water flux was 16.7 L·m⁻¹. -2 ·h -1 ·bar -1 The rejection rate for Na2SO4 was 34.7%.

Claims

1. A method for preparing a graphene oxide nanofiltration membrane using a charge-compensating crosslinking agent, characterized in that, Includes the following steps: (1) Preparation of graphene oxide nanosheet dispersion: flake graphite, concentrated sulfuric acid and concentrated phosphoric acid were mixed and stirred, potassium permanganate was slowly added, and the mixture was reacted in a water bath at 50 ℃ for 24 h. Then 0.8 ~ 3 L of deionized water was added, followed by 10 ~ 30 mL of 30% hydrogen peroxide was added dropwise until the solution turned golden yellow. After standing overnight, the mixture was centrifuged at 3000 ~ 3500 r / min for 15 ~ 30 min, for a total of 15 ~ 20 centrifugations. The centrifuged product was washed with 1 ~ 5% dilute hydrochloric acid and deionized water until pH=5 ~ 6. After dialysis, the product was dispersed in ultrapure water to obtain graphene oxide nanosheet dispersion. (2) Preparation of charge-compensating crosslinking agent dispersion: Dissolve the charge-compensating crosslinking agent in ultrapure water, heat and stir until dissolved to obtain charge-compensating crosslinking agent dispersion; (3) Preparation of charge-compensated crosslinked graphene oxide nanosheet dispersion: The graphene oxide nanosheet dispersion obtained in step (1) and the charge-compensated crosslinking agent dispersion obtained in step (2) are mixed and stirred thoroughly, and then thermally crosslinked to obtain the charge-compensated crosslinked graphene oxide nanosheet dispersion; wherein, the charge-compensated crosslinking agent is a crosslinking agent containing two reactive groups with graphene oxide and having sulfonic acid groups, including at least one of 2,5-diaminobenzenesulfonic acid, 2,4-diaminobenzenesulfonic acid, and 2,5-diaminophenyl-1,4-disulfonic acid, and the mixing ratio of the graphene oxide nanosheet dispersion and the charge-compensated crosslinking agent dispersion is 1:0.1 ~ 1:5; (4) Preparation of charge-compensated cross-linked graphene oxide nanofiltration membrane: The charge-compensated cross-linked graphene oxide nanosheet dispersion obtained in step (3) was diluted, ultrasonically treated, loaded onto the surface of a macroporous polymer support membrane by vacuum filtration, and dried to obtain the charge-compensated cross-linked graphene oxide nanofiltration membrane.

2. The preparation method according to claim 1, characterized in that, In step (1), the size of the flake graphite used in the preparation of graphene oxide nanosheets is 100-400 mesh, the concentrations of concentrated sulfuric acid and concentrated phosphoric acid are 80%-98% and 80-85% respectively, the volume ratio of the two is 9:1-7:1, the mass-volume ratio of the flake graphite to the concentrated sulfuric acid is 1g:(100-120)mL, and the concentration of graphene oxide nanosheets in the graphene oxide nanosheet dispersion is 1-5 mg / mL.

3. The preparation method according to claim 1, characterized in that, In step (2), the concentration of the charge-compensating crosslinking agent in the charge-compensating crosslinking agent dispersion is 1 ~ 5 mg / mL.

4. The preparation method according to claim 1, characterized in that, In step (2), the heating temperature is 60~100℃, the stirring rate is 300~500 r / min, and the heating time is 1-3 h.

5. The preparation method according to claim 1, characterized in that, In step (3), the stirring time is 0.5 to 12 hours, the stirring rate is 300 to 500 r / min, the thermal crosslinking temperature is 60 to 80℃, and the time is 1 to 3 hours.

6. The preparation method according to claim 1, characterized in that, In step (4), the concentration range of the diluted charge-compensated cross-linked graphene oxide nanosheet dispersion is 0.001 ~ 0.1 mg / mL, the ultrasonic power is 60 ~ 180 W, and the ultrasonic time is 0.2 ~ 0.5 h.

7. The preparation method according to claim 1, characterized in that, The polymer support membrane mentioned in step (4) includes any one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethersulfone (PES), and mixed cellulose (MCEM) microporous filter membrane. The pore size of the polymer support membrane is 0.22 ~ 0.44 μm and the diameter is 25 ~ 50 cm. The drying time is 12 ~ 24 h and the drying temperature is 40 ~ 60 °C.

8. The charge-compensated crosslinked graphene oxide nanofiltration membrane prepared by the preparation method according to any one of claims 1 to 7.

9. A separation device comprising the charge-compensated cross-linked graphene oxide nanofiltration membrane of claim 8.

10. The application of the charge-compensated cross-linked graphene oxide nanofiltration membrane according to claim 8 in the field of water treatment.

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