A method of stabilizing an oxidized graphene skin layer with an ultrafiltration base membrane

Abstract

The application discloses a method for stabilizing an oxidized graphene skin layer and an ultrafiltration base film, and belongs to the field of membrane separation technology. The method for stabilizing the oxidized graphene skin layer and the ultrafiltration base film comprises the following steps: performing functional treatment on a PAN membrane to obtain a functionalized PAN membrane, then coating oxidized graphene on the functionalized PAN membrane, and performing heat treatment to obtain an oxidized graphene composite membrane. Through optimization of membrane preparation parameters (EDA adsorption time, heat treatment temperature after adsorption, GO thickness and the like), the oxidized graphene composite membrane (GO composite membrane) with improved screening performance and more stable pore structure is obtained.

Description

Technical Field

[0001] This invention relates to the field of membrane separation technology, and in particular to a method for stabilizing a graphene oxide skin layer and an ultrafiltration membrane. Background Technology

[0002] Graphene oxide (GO), as a novel two-dimensional material, not only has the advantages of strong mechanical properties, good dispersibility and large-scale preparation, but also has excellent hydrophilicity, antibacterial and chlorine resistance. It is an excellent surface material for water treatment and is expected to be applied to separation processes such as ultrafiltration, nanofiltration, reverse osmosis and pervaporation.

[0003] There are various methods for constructing graphene oxide (GO) separation layers, primarily through a self-assembly process to form a two-dimensional stacked structure. Specific assembly-driven methods include pressure-driven and dispersion evaporation-driven methods. Pressure-driven methods, specifically pressure-assisted self-assembly, can be further divided into "pressure filtration" and "negative pressure filtration." The key feature is the use of a GO dispersion of a certain concentration, where, under pressure, the GO skin layer in the dispersion is deposited onto the base film, resulting in a GO composite membrane. The advantages of this method are its convenience, speed, controllable separation layer thickness, and adjustable film formation rate and GO layer microstructure.

[0004] Establishing stable interactions between the functional layers and skin layers of GO composite membranes can prevent the separation layer from detaching during actual use due to fluid convection and other operations. In constructing this stable interaction, the base membrane is often surface-functionalized to impart functional groups that can strongly interact with the GO sheets. In the construction of GO composite membranes, the base membrane modification methods used are mostly polydopamine-based surface modifications, which have broad compatibility with material surfaces. However, this method itself has certain limitations. Polydopamine surface modification achieves surface functionalization by depositing small polydopamine particles onto the modified surface. When dealing with base membranes with high requirements for surface morphology, especially those with pores only 10 nm in size, such as ultrafiltration base membranes, pore blockage may occur during modification, reducing membrane separation flux. For base membranes with easily blocked surfaces, effectively functionalizing the surface while maintaining the surface pore structure is a direction that needs further research. Summary of the Invention

[0005] The purpose of this invention is to provide a method for stabilizing graphene oxide skin and ultrafiltration membrane to solve the problems existing in the prior art. The PAN membrane used in this invention is a common microfiltration membrane construction material. By controlling the hydrolysis of PAN on the membrane surface, the surface structure can be carboxylated, and then covalently bonded with oxygen-containing groups on the GO surface. Alternatively, it can undergo secondary functionalization with a diamine crosslinking agent and then act with the GO separation layer. This can effectively maintain the microstructure of the small pores while improving the retention performance (as can be proven by surface electron microscopy (SEM) images, specifically: after hydrolysis and carboxylation, the surface morphology of the PAN membrane after secondary functionalization of amino groups is similar to that of PAN, and it still maintains obvious surface porosity).

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] One of the technical solutions of the present invention: a method for stabilizing graphene oxide skin and ultrafiltration membrane, comprising the following steps:

[0008] The PAN film (PAN base film) is functionalized to obtain a functionalized PAN film. Then, graphene oxide is coated onto the functionalized PAN film, and after heat treatment, a graphene oxide composite film is obtained.

[0009] Furthermore, the functionalized PAN film includes a carboxyl-functionalized PAN film or an amino-functionalized PAN film; the heat treatment temperature is 25–100°C, and the time is 0.5 h.

[0010] Furthermore, the heat treatment method is either forced-air drying oven heating treatment, vacuum drying oven heating treatment, electrothermal treatment, microwave heating treatment, light radiation heating treatment, or air heat treatment.

[0011] Furthermore, the heat treatment temperature is 80°C and the time is 0.5 hours.

[0012] Furthermore, the graphene oxide is coated in the form of a dispersion; the concentration of the dispersion is 0.4 mg / mL and the volume is 20–40 μL.

[0013] Furthermore, the preparation method of the carboxyl-functionalized PAN membrane (functionalized at the molecular level, hydrolyzed) includes the following steps:

[0014] The PAN membrane was placed in an alkaline solution, shaken in a water bath, and then immersed in an acidic solution to obtain the carboxyl-functionalized PAN membrane.

[0015] PAN membranes are a must-have, as they provide the material basis for hydrolysis.

[0016] Furthermore, the alkaline solution is a NaOH solution with a concentration of 1.5 mol / L; the water bath shaking temperature is 50°C and the time is 1 hour; the acidic solution is a hydrochloric acid solution with a concentration of 1 mol / L; and the soaking time is 24 hours.

[0017] Furthermore, the preparation method of the amino-functionalized PAN membrane (functionalized at the molecular level through hydrolysis and EDA functionalization) includes the following steps:

[0018] The carboxyl-functionalized PAN film was placed in an ethylenediamine solution (EDA solution), shaken in a water bath, and then dried to obtain an amino-functionalized PAN film.

[0019] Furthermore, the concentration of the ethylenediamine solution is 10 mg / mL; the temperature of the water bath oscillation is 25°C, and the time is 1–30 min; the drying temperature is 80°C, and the time is 0.5 h.

[0020] PAN membranes are functionalized at the molecular level to ensure the preservation of their microporous structure (microfiltration membranes often have pores of about 10 nm on their surface).

[0021] Furthermore, the preparation method of the graphene oxide specifically includes: mixing acid, graphene and potassium permanganate and heating to react, then pouring the reacted solution onto ice and mixing evenly, and adding hydrogen peroxide dropwise. After the addition is completed, centrifuging is performed, and the precipitate is the graphene oxide.

[0022] Furthermore, the acid solution is a mixed solution of concentrated sulfuric acid (18.4 mol / L) and concentrated phosphoric acid (14.6 mol / L) in a volume ratio of 5:1; the heating reaction is carried out at a temperature of 40°C for 6 hours.

[0023] Furthermore, the coating method includes vacuum filtration, spin coating, pressure assembly, or gas-liquid interface assembly.

[0024] The second technical solution of the present invention: a graphene oxide composite film prepared by the above method.

[0025] The third technical solution of the present invention: the application of the above-mentioned graphene oxide composite membrane in nanofiltration, reverse osmosis, forward osmosis, ultrafiltration, pervaporation or oil-water separation.

[0026] Furthermore, the nanofiltration process retains inorganic divalent anions, inorganic trivalent anions, negatively charged small organic molecules, or neutral small molecules with a molecular weight of 200 or higher.

[0027] The present invention discloses the following technical effects:

[0028] (1) The method of the present invention solves the problem of unstable interaction between the functional layer and the skin layer of the GO composite film, and at the same time can make the graphene oxide composite film tightly connected to the substrate, avoid skin layer shedding, and improve the retention performance.

[0029] (2) The graphene oxide composite membrane prepared by the method of the present invention has excellent nanofiltration performance.

[0030] (3) By optimizing the membrane-making parameters (EDA adsorption time, heat treatment temperature after adsorption, GO thickness, etc.), the present invention obtained a graphene oxide composite membrane (GO composite membrane) with improved sieving performance and more stable pore structure.

[0031] (4) The present invention performs two functionalization treatments on the PAN membrane, namely hydrolysis carboxylation and ethylenediamine, and the resulting GO composite membrane has better retention performance (the parameters of the second ethylenediamine functionalization and the parameters of the composite membrane heat treatment are very important in the preparation of composite membrane performance during the base membrane treatment process).

[0032] (5) In this invention, the common PAN microfiltration membrane is selected as the substrate, and the effects of single-carboxyl functionalized base membrane and double-functionalized base membrane on the performance of composite membrane are studied respectively. Specifically: (1) The surface of PAN base membrane is hydrolyzed to achieve carboxyl functionalization, and then the GO skin layer is directly connected to obtain a stable GO composite membrane (PAN-COOH-GO composite membrane); (2) The surface of PAN base membrane is hydrolyzed to achieve carboxyl functionalization, and then the carboxylated base membrane is double-functionalized with ethylenediamine to obtain an ultrafiltration membrane with amino functionalization at the molecular level. Using this double-functionalized ultrafiltration membrane as the substrate, the GO separation layer is stabilized on the upper surface of the base membrane by covalent bonding to obtain a GO composite membrane with skin layer and base membrane stabilization (PAN-COOH-EDA-GO composite membrane). The ion retention performance of the composite membrane was tested using a laboratory cross-flow membrane performance evaluation instrument. The results showed that the GO composite membrane (PAN-COOH-EDA-GO) prepared by the present invention with secondary functionalized PAN ultrafiltration membrane as substrate has improved retention performance compared with the substrate carboxyl-bonded GO composite membrane (PAN-COOH-GO) and the substrate non-strongly interacting GO composite membrane (PAN-GO).

[0033] (6) Conventional surface deposition methods involve depositing particles with diameters ranging from a few nanometers to tens of nanometers onto the surface of microporous membranes, which easily clogs the pores. However, this invention utilizes molecular-level functionalization, with particles smaller than 1 nm, which does not clog the pores. When ultrafiltration membranes are treated using the method of this invention, the pore structure and PAN (nanofiltration membrane) show little change (virtually no impact on the pore morphology of the membrane surface, preventing pore clogging of the ultrafiltration base membrane), as evidenced by SEM image comparisons. If a GO separation layer is added, the pore structure changes from ultrafiltration to nanofiltration, and the pore size changes from the 10 nm level to the 1 nm level (GO dense layer). Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 SEM images of PAN, PAN-COOH, and PAN-COOH-EDA membranes;

[0036] Figure 2 Surface infrared spectra of PAN, PAN-COOH, and PAN-COOH-EDA films;

[0037] Figure 3 The figures show the nanofiltration performance results of the PAN-COOH-EDA-GO membranes prepared at different adsorption times in Examples 5 and 6 of this invention.

[0038] Figure 4 The figures show the nanofiltration performance results of the PAN-COOH-EDA-GO composite membranes prepared at different heat treatment temperatures in Examples 5 and 7 of this invention.

[0039] Figure 5 The figures show the nanofiltration performance results of PAN-COOH-EDA-GO composite membranes prepared using different volumes of graphene oxide standard dispersions in Examples 5 and 8 of this invention.

[0040] Figure 6 The graph shows the pure water flux test results of the PAN-GO composite membrane prepared in Comparative Example 2 of this invention.

[0041] Figure 7 The graph shows the pure water flux test results of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 of this invention.

[0042] Figure 8 The graph shows the nanofiltration performance of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 of this invention under different filtration targets.

[0043] Figure 9 The figures show the nanofiltration performance of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 of this invention and the PAN-COOH-diamine-GO composite membrane prepared in Comparative Example 1 of this invention under different diamines.

[0044] Figure 10 The graph shows the nanofiltration performance of the PAN-GO composite membrane prepared in Comparative Example 2 of this invention under different soaking times.

[0045] Figure 11 The graph shows the nanofiltration performance of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 of this invention under different soaking times. Detailed Implementation

[0046] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0047] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0048] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0049] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0050] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0051] Example 1

[0052] The preparation of graphene oxide (GO) followed the classic Hummers method, and the specific steps are as follows:

[0053] (1) Add 240 mL of a mixture of concentrated sulfuric acid (18.4 mol / L) and concentrated phosphoric acid (14.6 mol / L) in a volume ratio of 5:1, 5.0 g of graphene solid, and 25.0 g of potassium permanganate solid to a 500 mL round-bottom flask. After reacting at 40 °C for 6 h, stop heating and cool to room temperature. Pour the solution onto about 400 g of ice and mix well. Then slowly add 20 mL of hydrogen peroxide. After the addition is complete, centrifuge at 4000 r / min for 4 h and discard the supernatant. The solid is the crude GO product.

[0054] (2) The crude GO product was washed 8 times with 1 mol / L HCl solution, and the solid was collected. Then it was washed 5 times with RO water (reverse osmosis water) (the conductivity of the supernatant in the last wash was measured; if it was 0 mS / cm, it indicated that the ions attached to its surface had been washed away), and the solid was collected. The washed solid was dispersed in 200 mL of RO water and filtered through a PVDF membrane with a pore size of 0.45 μm. The resulting filter cake was vacuum dried at room temperature to obtain graphene oxide paper.

[0055] (3) A certain mass of graphene oxide paper was dispersed in deionized water and sonicated at room temperature for 5 min to promote uniform dispersion, resulting in a standard dispersion of graphene oxide (GO-ss) with a concentration of 0.4 mg / mL.

[0056] Example 2

[0057] The specific steps for preparing PAN-COOH membranes (carboxyl-functionalized PAN membranes) are as follows:

[0058] Add 200 mL of ethanol to a 500 mL beaker, then place the PAN membrane in the ethanol and place the beaker in a water bath shaker to shake for 1 hour (temperature 25℃). Replace with 300 mL of fresh ethanol and let it soak for 12 hours. Rinse twice with RO water. Remove the membrane and place it in 300 mL of RO water for 24 hours to fully replace any residual ethanol. After rinsing, remove the surface RO water from the membrane and place it in a 400 mL... In a beaker containing 1.5 mol / L NaOH solution, the beaker was placed in a 50°C constant temperature water bath shaker for 1 hour (temperature 50°C). The membrane was then removed, washed three times with RO water, and immersed in 200 mL of 1 mol / L HCl solution. It was soaked at room temperature for 24 hours to achieve full protonation. The membrane was then removed and washed with RO water for 30 minutes (temperature 25°C). This washing process was repeated three times. The membrane was then immersed in 300 mL of RO water and shaken for 24 hours. The resulting PAN-COOH membrane was then immersed in RO water and stored under cold for later use.

[0059] Example 3

[0060] The specific steps for preparing PAN-COOH-EDA membranes at different adsorption times are as follows:

[0061] 20 mL of 10 mg / mL ethylenediamine solution was transferred into five clean weighing bottles. Five PAN-COOH membranes prepared in Example 2 were taken out, dried, and placed in weighing bottles. They were then placed in a water bath shaker for adsorption at 25°C for 1 min, 3 min, 5 min, 15 min, and 30 min, respectively. After adsorption, the membranes were removed, the ethylenediamine solution on the surface was dried, and then the membranes were heated in an 80°C oven for 0.5 h. The membranes were then removed, cooled to room temperature, immersed in ethanol, and washed in a water bath shaker for 0.5 h (25°C). This process was repeated four times. The membranes were then removed, dried, immersed in RO water, and washed in a water bath shaker (25°C) to obtain PAN-COOH-EDA membranes, which were stored in RO water for later use.

[0062] Example 4

[0063] The specific steps for preparing the PAN-COOH-GO composite membrane are as follows:

[0064] (1) In a beaker, 30 μL of the graphene oxide standard dispersion (GO standard dispersion) prepared in Example 1 was mixed with 15 mL of deionized water to obtain a film-forming solution; the mouth of the beaker was covered with plastic wrap and sealed with a rubber band, and the film-forming solution was placed in an ultrasonic cleaner and sonicated for 5 min to obtain film-forming solution G.

[0065] (2) Take out the glass frit filter device and clean it three times with tap water and RO water respectively. After cleaning the glass frit with RO water, place the PAN-COOH membrane prepared in Example 2 on the glass frit, use paper to remove the air bubbles between the membrane and the frit and assemble the device. Take out the membrane preparation solution G and pour it into the glass frit filter device. After the liquid surface is stable, connect the vacuum circulating water pump. Use the vacuum-assisted filtration assembly method to coat GO onto the PAN-COOH membrane. After drying at room temperature, heat treat it in an oven at 80°C for 1 hour to obtain the PAN-COOH-GO membrane with carboxyl linked skin (PAN-COOH-GO composite membrane or graphene oxide composite membrane). Take it out and keep it for use.

[0066] Example 5

[0067] The specific steps for preparing the PAN-COOH-EDA-GO composite membrane are as follows:

[0068] (1) In a beaker, 30 μL of the graphene oxide standard dispersion (GO standard dispersion) prepared in Example 1 was mixed with 15 mL of deionized water to obtain a film-forming solution; the mouth of the beaker was covered with plastic wrap and sealed with a rubber band, and the film-forming solution was placed in an ultrasonic cleaner and sonicated for 5 min to obtain film-forming solution G.

[0069] (2) Take out the glass frit filter device and clean it three times with tap water and RO water respectively. After cleaning the glass frit with RO water, place the PAN-COOH-EDA membrane with an adsorption time of 3 min in Example 3 on the glass frit. Use paper to remove the air bubbles between the membrane and the frit and assemble the device. Take out the membrane preparation solution G and pour it into the glass frit filter device. After the liquid surface is stable, connect the vacuum circulating water pump. Use the vacuum-assisted filtration assembly method (vacuum filtration method) to coat GO onto the PAN-COOH-EDA membrane. After drying at room temperature, heat treat it in an 80℃ oven (forced air drying oven) for 1 h to obtain the PAN-COOH-EDA-GO membrane with amino-linked skin (PAN-COOH-EDA-GO composite membrane or graphene oxide composite membrane). Take it out and keep it for use.

[0070] Example 6

[0071] Same as Example 5, except that the PAN-COOH-EDA membrane in step (1) is replaced with the PAN-COOH-EDA membranes in Example 3 with adsorption times of 1 min, 5 min, 15 min and 30 min respectively, to obtain 4 different PAN-COOH-EDA-GO composite membranes (graphene oxide composite membranes).

[0072] Example 7

[0073] Same as Example 5, except that the heat treatment temperature in step (2) was replaced with 25°C, 60°C and 100°C respectively, resulting in 3 different PAN-COOH-EDA-GO composite films (graphene oxide composite films).

[0074] Example 8

[0075] Same as Example 5, except that the volume of the graphene oxide standard dispersion in step (1) was replaced with 20 μL, 25 μL, 35 μL and 40 μL respectively, to prepare four different PAN-COOH-EDA-GO composite membranes (graphene oxide composite membranes).

[0076] Comparative Example 1

[0077] GO composite membranes with different diamine crosslinking were prepared. The preparation of each composite membrane was the same as in Example 5, except that the ethylenediamine crosslinking agent of the PAN-COOH-EDA-GO composite membrane in step (2) was replaced with other diamines, namely tris(2-aminoethyl)amine and diethylenetriamine, to prepare two different PAN-COOH-diamine-GO composite membranes (graphene oxide composite membranes).

[0078] Comparative Example 2

[0079] Same as Example 4, except that the PAN-COOH base membrane in step (2) is replaced with a PAN ultrafiltration membrane to prepare a PAN-GO composite membrane.

[0080] Example of effect 1

[0081] The surface morphology of the PAN film, PAN-COOH film, and PAN-COOH-EDA film was characterized using field emission scanning electron microscopy (SEM). The results are shown in the figure. Figure 1 .

[0082] from Figure 1 As can be seen, the membranes prepared by hydrolysis (PAN-COOH membrane) and subsequent hydrolysis and EDA functionalization (PAN-COOH-EDA membrane) have clear pore structures on their upper surfaces, and no obvious pore blockage is observed.

[0083] Example 2

[0084] The surface chemical structures of PAN film, PAN-COOH film, and PAN-COOH-EDA film (adsorption time 3 min) were characterized (surface ATR-IR), and the results are shown in the figure. Figure 2 .

[0085] from Figure 2 As can be seen from this, compared to PAN film, PAN-COOH film has a lower viscosity at 1696 cm⁻¹. -1 A new peak appeared at 1682 cm⁻¹, corresponding to the carbonyl group elution in the carboxylate group, verifying the hydrolysis results on the PAN membrane surface; the PAN-COOH-EDA membrane showed a peak at 1682 cm⁻¹. -1 The appearance of new peaks at 1566 and 1566 corresponds to the carbonyl group in the carboxyl group and amide bond, respectively, indicating that some carboxyl groups were converted into amide groups, confirming that EDA was functionalized on the PAN-COOH surface in a covalent manner.

[0086] Membrane separation performance characterization (nanofiltration performance)

[0087] The nanofiltration performance of the composite membrane was tested using a laboratory cross-flow filtration apparatus. Each membrane was pre-pressurized at 0.7 MPa for 2 hours, and all membrane data were obtained under parallel testing of three membranes. The test solution was a 1 g / L 1800 mL Na₂SO₄ solution. The liquid was collected in weighed plastic centrifuge tubes (its mass recorded as m1), and the time was recorded from the moment the first drop fell until approximately 3 mL was collected, at which point the time was stopped and recorded as t2. Finally, 3 mL of the original solution was collected in plastic centrifuge tubes and placed on a centrifuge tube rack for testing. The total mass of the plastic centrifuge tubes and the collected liquid was weighed, m2. The conductivity of the solution in each plastic centrifuge tube was measured.

[0088] The permeation flux and solute rejection rate are calculated using Equations 1 and 2:

[0089] Flux (flux rate) = 84.87 × (m2 - m1) / Δt (L·m -2 ·h -1 ) Formula 1

[0090] Rej (retention rate) = (1-C p / C f )×100% Formula 2

[0091] C p : Osmotic solubility; C f : Solubility of the liquid.

[0092] The nanofiltration performance of PAN-COOH-EDA-GO composite membranes prepared under different membrane preparation parameters was compared. The specific membrane preparation parameters included: EDA adsorption time, EDA treatment temperature, and volume of graphene oxide standard dispersion.

[0093] Example 3

[0094] The nanofiltration performance of PAN-COOH-EDA-GO composite membranes prepared using PAN-COOH-EDA membranes prepared at different adsorption times in Examples 5 and 6 was determined, and the results are shown in [Figure 1]. Figure 3 .

[0095] Test conditions: Na2SO4, salt concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH.

[0096] from Figure 3 As can be seen, the PAN-COOH-GO composite membrane without EDA adsorption (adsorption time of 0 min), while maintaining a good rejection rate, has a very low flux, making it almost impossible to apply in practice. After EDA adsorption, the flux is significantly improved; the flux of the PAN-COOH-EDA-GO composite membrane prepared using a PAN-COOH-EDA membrane with an adsorption time of 3 min increases to 14 L·m⁻¹. 2- ·h -1The rejection rate for sodium sulfate was also slightly improved (reaching 91%). Compared with the PAN-COOH-GO membrane, the flux increased significantly, and the rejection rate improved by 3%.

[0097] Example of effect 4

[0098] The nanofiltration performance of the PAN-COOH-EDA-GO composite membranes prepared at different heat treatment temperatures in Examples 5 and 7 was determined, and the results are shown in [Figure number missing]. Figure 4 .

[0099] Test conditions: Na2SO4, salt concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH.

[0100] from Figure 4 As can be seen, the heat treatment at 80℃ has the best effect, indicating that the dehydration condensation of carboxyl and amino groups has a significant effect after the temperature reaches 80℃.

[0101] Example 5

[0102] The nanofiltration performance of the PAN-COOH-EDA-GO composite membranes prepared using different volumes of graphene oxide standard dispersions in Examples 5 and 8 was determined, and the results are shown in [Figure 1]. Figure 5 .

[0103] Test conditions: Na2SO4, salt concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH.

[0104] from Figure 5 As can be seen, with the increase of graphene oxide (GO) loading, the retention of sodium sulfate by the PAN-COOH-EDA-GO composite membrane gradually increases, and the retention rate reaches a certain level when the amount of graphene oxide standard dispersion is 30 μL (GO loading is 0.96 μg·cm³). -2 The rejection rate reached its maximum. As the amount of graphene oxide standard dispersion increased, the water flux gradually decreased. Considering both rejection rate and flux, 30 μL was selected as the optimal film-forming volume for the GO standard dispersion (GO loading of 0.96 μg·cm³). -2 ).

[0105] Membrane pure water flux-pressure relationship test

[0106] The nanofiltration performance of the composite membranes was tested using a laboratory cross-flow filtration system. The operating pressure of each membrane was gradually varied from 0.1 to 0.7 to 0.1 MPa, with adjustments made every 10 minutes. Membrane data were obtained from parallel testing of three membranes using pure water as the test solution. The liquid was collected in weighed plastic centrifuge tubes (its mass recorded as m1), and the time was recorded from the moment the first drop fell until 10 minutes had elapsed. The total mass of liquid in the centrifuge tube was then recorded as m2. The operating pressure was adjusted, and once stable, the next collection cycle began immediately.

[0107] The permeation flux is calculated using Equation 3:

[0108] Flux (flux rate) = 84.87 × 6 × (m² - m₁) (L·m⁻¹) 2 ·h -1 ·bar -1 ) Formula 3

[0109] Example 5

[0110] (1) The pure water flux of the PAN-GO composite membrane prepared in Comparative Example 2 was measured, and the results are shown in the figure. Figure 6 .

[0111] Test conditions: RO water, 0.1→0.7→0.1MPa, cross-flow rate 30LPH.

[0112] (2) The pure water flux of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 was measured, and the results are shown in [the table below]. Figure 7 .

[0113] Test conditions: RO water, 0.1→0.7→0.1MPa, cross-flow rate 30LPH.

[0114] from Figure 6 and Figure 7 It can be seen that the flux-pressure curves of the two composite membranes show similar patterns, both having two stages: (1) In the initial stage of pressurization, the water flux decreases significantly with the increase of operating pressure, indicating that the channel structure between the GO layers decreases significantly with the increase of pressure; (2) When the pressure reaches 0.7 MPa and then decreases further, the water flux no longer changes with the change of operating pressure, indicating that the membrane structure can be stabilized after high-pressure compaction. There are also some differences between the PAN-GO membrane and the PAN-COOH-EDA-GO membrane. The PAN-COOH-EDA-GO membrane is lower than the PAN-GO membrane in the initial stage of pressurization, but when the pressure is reduced and the flux stabilizes, the water flux is higher than the PAN-GO membrane. This indicates that the addition of -COOH-EDA- has a stabilizing effect on the channels of the GO skin layer, without obvious super-large pores; on the other hand, it is also conducive to the maintenance of the pores of the GO skin layer. Since the flux of the PAN-COOH-GO membrane is too low, the change effect cannot be measured under the existing conditions, so this test was not performed.

[0115] Example 6

[0116] The nanofiltration performance of the PAN-COOH-EDA-GO composite membrane in Example 5 for different filtration targets was determined, and the results are shown in [Figure Number]. Figure 8 .

[0117] Test conditions: Target concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH, Targets: Sodium sulfate, C6H 12 O6 (glucose), magnesium sulfate, magnesium chloride, sodium chloride.

[0118] from Figure 8 As can be seen, the composite membrane has the best retention effect on sodium sulfate, and is most effective at retaining sodium sulfate and C6H. 12 The rejection rates of O6, magnesium sulfate, magnesium chloride, and sodium chloride decrease in that order, similar to the pattern observed in the GO separation layer. This indicates that the treatment of the cross-linked ultrafiltration base membrane and separation layer here mainly serves to stabilize the separation layer and the base membrane, without affecting the structure and surface properties of the separation layer itself.

[0119] Example 7

[0120] The nanofiltration performance of the PAN-COOH-EDA-GO composite membrane prepared in Example 5 of this invention (adsorption for 3 min) and the PAN-COOH-diamine-GO composite membrane prepared in Comparative Example 1 using different diamine crosslinking agents were measured. The results are shown in the figure. Figure 9 .

[0121] Test conditions: Sodium sulfate concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH.

[0122] from Figure 9 As can be seen from the data, among different diamine crosslinking agents, the composite membrane crosslinked with ethylenediamine has the best retention effect on sodium sulfate. The other two longer-chain diamines have significantly reduced effects, and the degree of reduction increases with the increase of chain length, indicating that short-chain diamines are more suitable for linking PAN-COOH base membrane and GO separation layer.

[0123] Example 8

[0124] Stability experiments were conducted on the ultrafiltration base membrane and separation layer with and without crosslinking. The PAN-COOH-EDA-GO composite membrane (prepared in Example 5 with adsorption for 3 min) and the PAN-GO composite membrane (prepared in Comparative Example 2) were immersed in RO water for a certain period of time for subsequent sodium sulfate retention experiments. The immersion times were 0 h, 0.5 h, 1 h, and 24 h, respectively. 0 h represents the composite membrane without immersion. The nanofiltration performance of the PAN-GO composite membrane and the PAN-COOH-EDA-GO composite membrane in Comparative Example 2 after immersion in water was measured, and the results are shown below. Figures 10-11 .

[0125] Test conditions: Sodium sulfate concentration: 1 g / L, 0.7 MPa, cross-flow rate: 30 LPH.

[0126] from Figure 10As can be seen, the PAN-GO composite membrane's retention effect on sodium sulfate decreased significantly after soaking in water, while the PAN-COOH-EDA-GO composite membrane remained almost unchanged. Figure 11 The rejection rate decreased by less than 2%, indicating that the PAN-COOH base membrane and the GO separation layer are cross-linked by the -COO-EDA- chains, resulting in a stable underwater membrane structure, which is expected to be used in complex practical nanofiltration separation.

[0127] This invention designs a method for covalently linking a PAN ultrafiltration membrane and a GO separation layer. By optimizing parameters such as EDA adsorption time, post-adsorption heat treatment temperature, and GO skin thickness (amount of graphene oxide standard dispersion), a PAN-COOH-EDA-GO composite membrane with improved retention performance is obtained. Experiments on sodium sulfate retention, pure water flux-operating pressure, and membrane performance after immersion in water show that the PAN-COOH-EDA-GO composite membrane has a more stable membrane structure than the PAN-GO composite membrane, while maintaining abundant water passages (avoiding a sharp drop in flux). This invention introduces a new processing method for PAN membranes, resulting in a composite membrane with stable structure and improved sieving performance, which is expected to be a promising candidate for practical applications.

[0128] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for stabilizing a graphene oxide skin layer and an ultrafiltration membrane, characterized in that, Includes the following steps: The PAN film is functionalized to obtain a functionalized PAN film. Then, graphene oxide is coated onto the functionalized PAN film, dried at room temperature, and then heat-treated to obtain a graphene oxide composite film. The coating method includes vacuum filtration, spin coating, pressure assembly, or gas-liquid interface assembly. The functionalized PAN film is an amino-functionalized PAN film; the heat treatment temperature is 80℃ and the time is 1 hour; The method for preparing the amino-functionalized PAN film includes the following steps: The PAN membrane was placed in an alkaline solution, shaken in a water bath, and then immersed in an acidic solution to obtain a carboxyl-functionalized PAN membrane. The alkaline solution is a NaOH solution with a concentration of 1.5 mol / L; the water bath shaking temperature is 50℃ and the time is 1 hour; the acidic solution is a hydrochloric acid solution with a concentration of 1 mol / L; the soaking time is 24 hours. The carboxyl-functionalized PAN membrane was placed in an ethylenediamine solution, shaken in a water bath, and then dried to obtain an amino-functionalized PAN membrane. The concentration of the ethylenediamine solution is 10 mg / mL; the temperature of the water bath oscillation is 25°C and the time is 3 min; the drying temperature is 80°C and the time is 0.5 h.

2. The method for stabilizing the graphene oxide skin and the ultrafiltration membrane according to claim 1, characterized in that, The specific preparation method of the graphene oxide includes: mixing acid, graphene and potassium permanganate and heating to react, then pouring the reacted solution onto ice and mixing evenly, adding hydrogen peroxide dropwise, centrifuging after the addition is complete, and the precipitate is the graphene oxide.

3. The method for stabilizing the graphene oxide skin and the ultrafiltration membrane according to claim 2, characterized in that, The acid solution is a mixed solution of 18.4 mol / L concentrated sulfuric acid and 14.6 mol / L concentrated phosphoric acid in a volume ratio of 5:1; the heating reaction is carried out at a temperature of 40°C for 6 hours.

4. A graphene oxide composite film prepared by the method according to any one of claims 1 to 3.

5. The application of the graphene oxide composite membrane according to claim 4 in nanofiltration, reverse osmosis, forward osmosis, ultrafiltration or oil-water separation.

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