A self-cleaning separation membrane, a preparation method thereof, and a self-cleaning method

Abstract

The present invention provides a preparation method of a self-cleaning separation membrane, which includes preparing graphitic carbon nitride and rod-shaped carbon nitride by using a nitrogen-containing precursor; dispersing the graphitic carbon nitride into a solvent, adding an Fe<supgt;3+< / supgt; solution and ammonium bicarbonate, centrifuging to take out the precipitate and drying after the reaction is completed to obtain an FeOOH / carbon nitride composite with photo-Fenton activity; dispersing the FeOOH / carbon nitride composite in water and adding the rod-shaped carbon nitride for self-assembly to obtain a mixed solution; modifying the mixed solution onto a membrane substrate material by a vacuum filtration method, and then drying to obtain the self-cleaning separation membrane. The preparation method of the present invention is simple, environmentally friendly, low in cost, easy to scale up production, the obtained self-cleaning separation membrane is conducive to realizing self-cleaning after pollution, and the energy driving for self-cleaning is solar energy, which conforms to the national policy of energy conservation and emission reduction.

Description

Technical Field

[0001] The present invention relates to the field of functional membrane materials, and particularly to a self-cleaning separation membrane, a preparation method thereof, and a self-cleaning method. Background Art

[0002] With the rapid increase in human demand for oil resources, a large amount of oily wastewater is generated every day in oil exploitation, accidental oil spills, shipping operations, petrochemical industrial production, etc. If these oily wastewaters are directly discharged into the ecosystem, it will not only waste precious water resources and oil resources, but also have a catastrophic impact on aquatic organisms and human health. According to the form of oil in water, these oil-water mixtures can be divided into stratified oil-water mixtures and emulsified oil-water mixtures. Compared with stratified oil-water mixtures, emulsified oil-water mixtures are thermodynamically more stable. Traditional methods for treating them, such as skimming, gravity separation, coagulation flotation, physical adsorption, etc., often require a large amount of energy, and there are also disadvantages such as low separation accuracy, complex processes, and expensive equipment.

[0003] In recent years, membrane separation technology has been widely used in the separation of oil-water emulsions due to its advantages such as high separation efficiency, low energy consumption, and simple operation. However, membrane fouling will irreversibly reduce the flux and service life of the membrane, severely limiting the practical application of separation membranes in the field of oil-water separation. At present, the coupling of membrane separation technology and photocatalytic degradation can not only efficiently separate oil-water emulsions, but also in-situ degrade the oil stains retained on the membrane, thus realizing the self-cleaning function of the membrane. The entire self-cleaning process can be completed only by relying on solar energy, so it shows great potential in practical applications. In addition, the photo-Fenton reaction is an advanced oxidation process (AOP) that combines photocatalysis and the Fenton reaction. In the presence of light and H2O2, hydroxyl radicals are generated, which can significantly improve the degradation efficiency and shorten the reaction time. For example, Patent CN109569311B and Patent CN112023721B disclose a self-cleaning membrane based on the coupling of carbon nitride-like Fenton reagent and a preparation method thereof. A carbon nitride photocatalyst with high chemical stability, low cost, and non-toxicity is selected, and a separation membrane is prepared by surface modification with a Fenton reagent-like substance. It can achieve in-situ degradation of pollutants retained by nanofiltration under light through photo-Fenton, providing a new way to solve the insurmountable membrane fouling problem of traditional nanofiltration membrane materials.

[0004] Despite the above-mentioned progress, photocatalysis or the photo-Fenton method can only effectively degrade organic dyes (Patent CN109569311B: methylene blue), low-viscosity organic solvents or light oils (Patent CN112023721B: n-hexane) contaminated on the membrane, and still cannot achieve the self-cleaning of the membrane contaminated by crude oil or even heavy oil. With the gradual depletion of light crude oil resources, the production of heavy crude oil has been increasing year by year. Therefore, a large amount of waste emulsified heavy oil generated during the exploitation process urgently needs to be treated. Membrane technology still faces huge challenges in separating emulsified heavy oil: 1) Heavy oil is extremely prone to cause membrane fouling due to its ultra-high viscosity; 2) Heavy oil contains a large amount of heavy components such as asphaltenes and resins, etc., and its structure is very complex, consisting of polycyclic aromatic hydrocarbon or naphthenic aromatic hydrocarbon nuclei containing heteroatoms and alkyl side chains, and it is very difficult to be completely degraded by traditional photocatalysis / photo-Fenton technology. Therefore, there is an urgent need to adopt a new self-cleaning strategy combined with advanced membrane materials to treat stubborn high-viscosity crude oil / water emulsions. Summary of the Invention

[0005] The present invention first aims to provide a preparation method of a self-cleaning separation membrane, which obtains a superhydrophilic / underwater superoleophobic separation membrane with photo-Fenton activity and anti-oil fouling performance, and can efficiently separate high-viscosity heavy oil / water emulsions.

[0006] The present invention also provides a new, sunlight-driven membrane self-cleaning method, which overcomes the problem that the existing technology cannot self-clean the membrane fouling caused after separating high-viscosity heavy oil / water emulsions, and for the first time realizes the self-cleaning of the membrane contaminated by heavy oil.

[0007] The basic concept of the technical solution of the present invention is as follows:

[0008] A preparation method of a self-cleaning separation membrane includes preparing graphitic carbon nitride and rod-shaped carbon nitride using a nitrogen-containing precursor; dispersing the graphitic carbon nitride into a solvent, adding an Fe 3+ solution and ammonium bicarbonate, centrifuging to remove the precipitate and drying after the reaction is completed to obtain an FeOOH / carbon nitride composite with photo-Fenton activity; dispersing the FeOOH / carbon nitride composite in water and adding the rod-shaped carbon nitride, and self-assembling to obtain a mixed solution;

[0009] Modifying the mixed solution onto a membrane substrate material, and then drying to obtain the self-cleaning separation membrane.

[0010] As one mode, the mass ratio of the FeOOH / carbon nitride composite to the rod-shaped carbon nitride is 5:1 - 1:1.

[0011] As one mode, graphitic carbon nitride or rod-shaped carbon nitride is prepared by performing a thermal polymerization reaction at 400 - 600 °C for 2 - 5 hours using a nitrogen-containing precursor.

[0012] As a method, urea is used as a nitrogen-containing precursor to prepare the graphitic carbon nitride, and melamine is used as a nitrogen-containing precursor to prepare the rod-like structure carbon nitride.

[0013] As a method, the mixed solution is modified onto the membrane substrate material by vacuum filtration.

[0014] As a method, the membrane substrate material is one of a polyvinylidene fluoride membrane, a polytetrafluoroethylene membrane, a polycarbonate membrane, or a polyethersulfone membrane.

[0015] As a method, the Fe 3+ solution is a ferric chloride solution, a ferric sulfate solution, or a ferric nitrate solution.

[0016] The present invention also provides a self-cleaning separation membrane obtained according to the preparation method described in any one of the above.

[0017] The present invention also provides a self-cleaning method for self-cleaning the above self-cleaning separation membrane after being contaminated by ultra-high viscosity heavy oil with a viscosity > 60000 mPa·s, including the following steps:

[0018] Under the illumination condition of visible light, the contaminated self-cleaning separation membrane is placed in an H2O2 solution for photo-Fenton degradation;

[0019] Then the self-cleaning separation membrane is taken out of the H2O2 solution and directly illuminated for photo-driven volatilization.

[0020] As a method, the concentration of the H2O2 solution is 10 - 50 mM.

[0021] The present invention has the following beneficial effects compared with the prior art:

[0022] 1. The preparation method of the self-cleaning separation membrane (abbreviated as FPCN-RCN separation membrane) of the present invention is simple, green and environmentally friendly, low in cost, easy to scale up production, the prepared self-cleaning separation membrane is conducive to realizing self-cleaning after contamination, and the energy drive for self-cleaning is solar energy, which conforms to the national policy of energy conservation and emission reduction.

[0023] 2. Further, the method of the present invention increases the interlayer spacing of FeOOH / carbon nitride nanosheets by adding rod-like carbon nitride, thereby improving the membrane flux. With the help of the hydrophilicity of carbon nitride itself, the prepared FeOOH / carbon nitride composite (FPCN) and RCN (rod-like structure carbon nitride) with photo-Fenton activity have natural superhydrophilicity and underwater superoleophobicity, and have excellent anti-crude oil pollution performance, without the need to further modify other hydrophilic materials.

[0024] 3. Further, the present invention also provides a self-cleaning method that combines photo-Fenton degradation and photo-driven evaporation, achieving in-situ degradation of ultra-high viscosity crude oil retained on the separation membrane, demonstrating excellent oil removal ability on the membrane surface, and for the first time realizing self-cleaning of the separation membrane after being contaminated by ultra-high viscosity heavy oil (viscosity > 60,000 mPa·s). The flux recovery rate after self-cleaning treatment is close to 100%. It provides a brand-new approach for efficient separation of emulsified crude oil, which is an advantage not possessed by the photocatalytic and photo-Fenton separation membranes reported in the prior art.

[0025] Other features and advantages of the present invention will be described in the following specification, and part of them will become obvious from the specification or be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the structures specifically pointed out in the specification, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings are used to provide a further understanding of the technical solution of the present invention, and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the technical solution of the present invention and do not constitute a limitation to the technical solution of the present invention.

[0027] Figure 1 It is a surface scanning electron microscope photograph of the FPCN-RCN separation membrane in Examples 1-4 of the present invention and the FPCN membrane in Comparative Example 1.

[0028] Figure 2 It is a surface EDS element distribution photograph of the FPCN-RCN-1.5 separation membrane in Example 3 of the present invention.

[0029] Figure 3 It is a graph of the water contact angle in air and the underwater oil contact angle of each test separation membrane in Example 5 of the present invention; where a is the water contact angle in air and b is the underwater oil contact angle.

[0030] Figure 4 It is a graph of the anti-oil pollution performance test of the FPCN-RCN-1.5 separation membrane in Example 3 of the present invention and the unmodified PVDF membrane; where a is that of the FPCN-RCN-1.5 separation membrane and b is that of the unmodified PVDF membrane.

[0031] Figure 5 a is a comparison graph of the underwater oil contact angle of the FPCN-RCN-1.5 separation membrane before and after being contaminated by heavy oil and after self-cleaning through Example 6 and Comparative Examples 2-4 of the present invention respectively; Figure 5 b is a graph of the anti-oil pollution performance test of the FPCN-RCN separation membrane after being treated by the combined method of photo-Fenton degradation and photo-driven evaporation.

[0032] Figure 6 a andFigure 6 b is a graph showing the water flux and heavy oil emulsion flux of the separation membranes prepared in Examples 1-4 and Comparative Example 1; Figure 6 c shows the water flux of water after the FPCN-RCN-1.5 separation membrane is rinsed in water for 2 minutes 5 minutes after filtering the heavy oil emulsion, and the water flux of the membrane after treatment by combined photo-Fenton degradation and photo-driven evaporation; Figure 6 d shows the water flux and heavy oil emulsion flux of the FPCN-RCN-1.5 separation membrane after 5 cycles of use. Detailed implementation manners

[0033] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0034] The experimental methods used in the following embodiments are all conventional methods unless otherwise specified. The reagents, materials, etc. used in the following embodiments can all be obtained from commercial channels unless otherwise specified.

[0035] Example 1. Preparation method of self-cleaning separation membrane

[0036] (1) Using urea as a nitrogen-containing precursor, it is heated in a muffle furnace from room temperature to 550 °C at a heating rate of 20 °C / min and maintained for 4 hours to prepare g-C3N4 nanosheets (graphitic carbon nitride);

[0037] (2) Using melamine as a nitrogen-containing precursor, it is heated in a muffle furnace from room temperature to 500 °C at a heating rate of 20 °C / min and maintained for 4 hours to prepare rod-like carbon nitride (RCN);

[0038] (3) Take 1 gram of the g-C3N4 nanosheets prepared in step (1) and dissolve it in 50 milliliters of anhydrous ethanol reagent, ultrasonicate for 20 minutes, add 1 mM ferric chloride hexahydrate as a photo-Fenton reaction precursor, and stir at 500 revolutions per minute for 10 minutes;

[0039] (4) Then add 3 mM ammonium bicarbonate, continue stirring for 8 hours, centrifuge at 10,000 g for 10 minutes, and dry the obtained precipitate at 60 °C to obtain an FeOOH / carbon nitride composite (FPCN) with photo-Fenton activity;

[0040] (5) Add the FPCN prepared in step (4) and the RCN prepared in step 2 to 200 milliliters of deionized water in a mass ratio of 4:1, ultrasonicate for 10 minutes to obtain a mixed membrane-forming solution;

[0041] (6) Using a PVDF nanofiltration membrane as the support substrate, the mixed membrane-forming solution obtained in step (5) is vacuum filtered through a pressure-reducing suction filtration onto the surface of the PVDF substrate membrane to obtain the self-cleaning separation membrane, denoted as the FPCN-RCN-0.5 membrane.

[0042] Example 2. Preparation method of self-cleaning separation membrane

[0043] (1) Using urea as the nitrogen-containing precursor, it is heated from room temperature to 550 °C at a heating rate of 20 °C / min in a muffle furnace and maintained for 4 hours to prepare g-C3N4 nanosheets (graphitic carbon nitride);

[0044] (2) Using melamine as the nitrogen-containing precursor, it is heated from room temperature to 500 °C at a heating rate of 20 °C / min in a muffle furnace and maintained for 4 hours to prepare rod-like structured carbon nitride (RCN);

[0045] (3) Take 1 gram of the g-C3N4 nanosheets prepared in step (1) and dissolve it in 50 milliliters of anhydrous ethanol reagent, ultrasonicate for 20 minutes, add 1 mM ferric chloride hexahydrate as the Fenton-like reaction precursor, and stir at 500 revolutions per minute for 10 minutes;

[0046] (4) Then add 3 mM ammonium bicarbonate, continue stirring for 8 hours, centrifuge at 10,000 g for 10 minutes, and dry the obtained precipitate at 60 °C to obtain the FeOOH / carbon nitride composite (FPCN) with photo-Fenton activity;

[0047] (5) Add the FPCN prepared in step (4) and the RCN prepared in step 2 in a mass ratio of 4:2 to 200 milliliters of deionized water and ultrasonicate for 10 minutes to obtain a mixed membrane-forming solution;

[0048] (6) Using a PVDF nanofiltration membrane as the support substrate, the mixed membrane-forming solution obtained in step (5) is vacuum filtered through a pressure-reducing suction filtration onto the surface of the PVDF substrate membrane to obtain the self-cleaning separation membrane, denoted as the FPCN-RCN-1 membrane.

[0049] Example 3. Preparation method of self-cleaning separation membrane

[0050] (1) Using urea as the nitrogen-containing precursor, it is heated from room temperature to 550 °C at a heating rate of 20 °C / min in a muffle furnace and maintained for 4 hours to prepare g-C3N4 nanosheets (graphitic carbon nitride);

[0051] (2) Using melamine as the nitrogen-containing precursor, it is heated from room temperature to 500 °C at a heating rate of 20 °C / min in a muffle furnace and maintained for 4 hours to prepare rod-like structured carbon nitride (RCN);

[0052] (3) Dissolve 1 g of the g-C3N4 nanosheets prepared in step (1) in 50 mL of anhydrous ethanol reagent, sonicate for 20 minutes, add 1 mM ferric chloride hexahydrate as the Fenton-like reaction precursor, and stir at 500 rpm for 10 minutes;

[0053] (4) Then add 3 mM ammonium bicarbonate, continue stirring for 8 hours, centrifuge at 10000 g for 10 minutes, and dry the obtained precipitate at 60 °C to obtain the FeOOH / graphitic carbon nitride composite (FPCN) with photo-Fenton activity;

[0054] (5) Add the FPCN prepared in step (4) and the RCN prepared in step 2 in a mass ratio of 4:3 to 200 mL of deionized water, sonicate for 10 minutes to obtain a mixed membrane-forming solution;

[0055] (6) Using a PVDF nanofiltration membrane as the support substrate, use a vacuum filtration device to draw the mixed membrane-forming solution obtained in step (5) onto the surface of the PVDF substrate membrane by vacuum filtration to obtain the self-cleaning separation membrane, denoted as the FPCN-RCN-1.5 membrane.

[0056] Example 4. Preparation method of self-cleaning separation membrane

[0057] (1) Using urea as the nitrogen-containing precursor, heat it from room temperature to 550 °C at a heating rate of 20 °C / min in a muffle furnace, and maintain for 4 hours to prepare g-C3N4 nanosheets (graphitic carbon nitride);

[0058] (2) Using melamine as the nitrogen-containing precursor, heat it from room temperature to 500 °C at a heating rate of 20 °C / min in a muffle furnace, and maintain for 4 hours to prepare rod-like structured carbon nitride (RCN);

[0059] (3) Dissolve 1 g of the g-C3N4 nanosheets prepared in step (1) in 50 mL of anhydrous ethanol reagent, sonicate for 20 minutes, add 1 mM ferric chloride hexahydrate as the Fenton-like reaction precursor, and stir at 500 rpm for 10 minutes;

[0060] (4) Then add 3 mM ammonium bicarbonate, continue stirring for 8 hours, centrifuge at 10000 g for 10 minutes, and dry the obtained precipitate at 60 °C to obtain the FeOOH / graphitic carbon nitride composite (FPCN) with photo-Fenton activity;

[0061] (5) Add the FPCN prepared in step (4) and the RCN prepared in step 2 in a mass ratio of 4:4 to 200 mL of deionized water, sonicate for 10 minutes to obtain a mixed membrane-forming solution;

[0062] (6) Using a PVDF nanofiltration membrane as the support substrate, the mixed membrane-forming solution obtained in step (5) is filtered onto the surface of the PVDF substrate membrane by vacuum filtration under reduced pressure to obtain the self-cleaning separation membrane, denoted as the FPCN-RCN-2 membrane.

[0063] Comparative Example 1, Preparation method of self-cleaning separation membrane

[0064] (1) Using urea as the nitrogen-containing precursor, it is heated from room temperature to 550 °C at a heating rate of 20 °C / min in a muffle furnace and maintained for 4 hours to prepare g-C3N4 nanosheets (graphitic carbon nitride);

[0065] (2) Take 1 gram of the g-C3N4 nanosheets prepared in step (1) and dissolve it in 50 ml of anhydrous ethanol reagent, sonicate for 20 minutes, add 1 mM ferric chloride hexahydrate as the Fenton-like reaction precursor, and stir at 500 rpm for 10 minutes;

[0066] (3) Then add 3 mM ammonium bicarbonate, continue to stir for 8 hours, centrifuge at 10000 g for 10 minutes, and dry the obtained precipitate at 60 °C to obtain the FeOOH / carbon nitride composite (FPCN) with photo-Fenton activity;

[0067] (4) Add the FPCN prepared in step (3) to 200 ml of deionized water and sonicate for 10 minutes to obtain a mixed membrane-forming solution;

[0068] (6) Using a PVDF nanofiltration membrane as the support substrate, the mixed membrane-forming solution obtained in step (4) is filtered onto the surface of the PVDF substrate membrane by vacuum filtration under reduced pressure to obtain the self-cleaning separation membrane, denoted as the FPCN membrane.

[0069] The surface scanning electron microscope photos of the separation membranes prepared in the above Examples 1-4 and Comparative Example 1 are shown in Figure 1 . It can be Figure 1 found that a dense, uniform and rough structure is formed on the surface of the FPCN membrane. When RCN is added to form the FPCN-RCN membrane, the surface of the FPCN-RCN membrane is rougher than that of the FPCN membrane, which is beneficial to enhancing the hydrophilicity of the membrane.

[0070] The EDS element scanning distribution on the surface of the FPCN-RCN-1.5 separation membrane prepared in Example 3 is shown in Figure 2 . It can be Figure 2 seen that the four elements of C, N, Fe, and O are evenly distributed on the FPCN-RCN-1.5 separation membrane, proving that FPCN and RCN are successfully modified on the surface of the PVDF substrate membrane.

[0071] Example 5, Tests on wetting performance and anti-oil fouling performance

[0072] Figure 3 Underwater oil contact angle diagrams of the separation membranes prepared in Examples 1-4 and Comparative Example 1. As can be seen from the figure, all the separation membranes exhibit good underwater oleophobicity. As the content of RCN increases, the underwater oil contact angle of the membrane slightly increases. Among them, the underwater oil contact angles of the FPCN-RCN-1.5 and FPCN-RCN-2 membranes both exceed 157°.

[0073] Figure 4 a shows the anti-oil fouling performance of the FPCN-RCN-1.5 separation membrane prepared in Example 3. Figure 4 b shows the anti-oil fouling performance of the PVDF substrate membrane. By comparison, the following conclusions can be drawn:

[0074] When the wetted FPCN-RCN-1.5 separation membrane is first immersed in crude oil and then placed in water, the crude oil on the FPCN-RCN separation membrane instantly leaves the surface of the membrane, indicating that the FPCN-RCN-1.5 separation membrane has good anti-oil fouling ability. This is mainly because the FPCN and RCN modified on the surface of the FPCN-RCN-1.5 separation membrane have abundant hydrophilic groups, and these groups adsorb a large number of water molecules to form a water layer to isolate the direct contact between the crude oil and the membrane, so it shows good anti-oil fouling ability. For the PVDF substrate membrane that has been contaminated by crude oil first, although it is immersed in water, the crude oil on the membrane surface still adheres tightly to the membrane surface, indicating that the PVDF substrate membrane itself does not have underwater anti-oil fouling ability.

[0075] Self-cleaning method of the FPCN-RCN separation membrane in Example 6

[0076] (1) Take 2 ml of heavy oil from Liaohe Oilfield (viscosity about 62441 mPas) and add it to 1 L of deionized water, then add sodium dodecyl sulfate with a concentration of 50 mg / L, and stir for 20 minutes with a high-speed stirrer to prepare a high-viscosity heavy oil emulsion with stable surface activity;

[0077] (2) Use the FPCN-RCN-1.5 separation membrane obtained in Example 3 to filter the high-viscosity heavy oil emulsion in step (1) under a negative pressure of 0.2 bar for 5 minutes to obtain the FPCN-RCN separation membrane contaminated by high-viscosity heavy oil;

[0078] (3) Place the contaminated FPCN-RCN-1.5 separation membrane in step (2) in a 40 mM hydrogen peroxide solution and irradiate it with a solar simulator with a light intensity of 1.5 kW / m 2 for 1 hour for photo-Fenton degradation;

[0079] (4) Then take out the FPCN-RCN separation membrane from the hydrogen peroxide solution and directly place it under a light intensity of 1.5 kW / m 2Irradiate under a solar simulator for 0.5 hours to perform light-driven crude oil evaporation.

[0080] Comparative Example 2: Self-cleaning of the FPCN-RCN separation membrane by photo-Fenton degradation

[0081] (1) Take 2 mL of heavy oil from Liaohe Oilfield (viscosity about 62441 mPas), add it to 1 L of deionized water, and then add sodium dodecyl sulfate with a concentration of 50 mg / L. Stir for 20 minutes with a high-speed stirrer to prepare a highly viscous heavy oil emulsion with stable surface activity.

[0082] (2) Use the FPCN-RCN-1.5 separation membrane obtained in Example 3 to filter the highly viscous heavy oil emulsion in step (1) under a negative pressure of 0.2 bar for 5 minutes to obtain the FPCN-RCN separation membrane contaminated with highly viscous heavy oil.

[0083] (3) Place the contaminated FPCN-RCN-1.5 separation membrane in step (2) in a 40 mM hydrogen peroxide solution and irradiate it under a solar simulator with a light intensity of 1.5 kW / m 2 for 1.5 hours to perform photo-Fenton degradation.

[0084] Comparative Example 3: Self-cleaning of the FPCN-RCN separation membrane by photo-Fenton degradation and evaporation in air

[0085] (1) Take 2 mL of heavy oil from Liaohe Oilfield (viscosity about 62441 mPas), add it to 1 L of deionized water, and then add sodium dodecyl sulfate with a concentration of 50 mg / L. Stir for 20 minutes with a high-speed stirrer to prepare a highly viscous heavy oil emulsion with stable surface activity.

[0086] (2) Use the FPCN-RCN-1.5 separation membrane obtained in Example 3 to filter the highly viscous heavy oil emulsion in step (1) under a negative pressure of 0.2 bar for 5 minutes to obtain the FPCN-RCN separation membrane contaminated with highly viscous heavy oil.

[0087] (3) Place the contaminated FPCN-RCN-1.5 separation membrane in step (2) in a 40 mM hydrogen peroxide solution and irradiate it under a solar simulator with a light intensity of 1.5 kW / m 2 for 1 hour to perform photo-Fenton degradation.

[0088] (4) Take out the FPCN-RCN-1.5 separation membrane in step (3) from the hydrogen peroxide solution and directly expose it to air for 8 hours. Evaporate the crude oil by exposing the membrane to air at a room temperature of 25°C.

[0089] Comparative Example 4: The FPCN-RCN photocatalytic membrane realizes self-cleaning of the membrane through photo-driven evaporation

[0090] (1) Take 2 mL of heavy oil from Liaohe Oilfield (viscosity about 62441 mPas) and add it to 1 L of deionized water. Then add sodium dodecyl sulfate with a concentration of 50 mg / L and stir for 20 minutes with a high-speed stirrer to prepare a high-viscosity heavy oil emulsion with stable surface activity;

[0091] (2) Use the FPCN-RCN-1.5 separation membrane obtained in Example 3 to vacuum filter the high-viscosity heavy oil emulsion in step (1) for 5 minutes to obtain the FPCN-RCN-1.5 separation membrane contaminated by high-viscosity heavy oil;

[0092] (3) Place the contaminated FPCN-RCN separation membrane in step (2) under a solar simulator with a light intensity of 1.5 kW / m 2 and irradiate for 1.5 hours for photo-driven evaporation.

[0093] Figure 5 Figure a shows the comparison of the underwater oil contact angles of the FPCN-RCN-1.5 separation membrane before and after being contaminated by heavy oil and after self-cleaning through Example 3 and Comparative Examples 2-4 of the present invention respectively. It can be seen from this figure that the underwater oil contact angle of the FPCN-RCN-1.5 separation membrane drops from 158 ± 5° at the beginning to 113 ± 4° after filtering the heavy oil emulsion, indicating that the FPCN-RCN-1.5 separation membrane has been contaminated by heavy oil. When it is self-cleaned by the method of Example 6, the measured underwater contact angle of the membrane is restored to 157 ± 2°, indicating that the thorough self-cleaning of the FPCN-RCN-1.5 separation membrane is achieved by combining photo-Fenton degradation and photo-driven evaporation.

[0094] When the membrane is treated by the methods of Comparative Examples 2-4, the underwater oil contact angle of the FPCN-RCN-1.5 separation membrane cannot be fully restored, indicating that neither simple photo-Fenton degradation, photo-Fenton degradation combined with evaporation in air, nor simple photo-driven evaporation can achieve good self-cleaning of the membrane.

[0095] Figure 5 Figure b shows the anti-oil pollution performance test diagram of the FPCN-RCN-1.5 separation membrane after being treated by combining photo-Fenton degradation and photo-driven evaporation. The results show that after the underwater oil droplet touches the membrane surface, it can leave the membrane smoothly again and does not deform itself, indicating that the heavy oil on the membrane has been completely removed, further proving the self-cleaning effect of Example 6.

[0096] Figure 6 a and Figure 6Figure b shows the water flux and heavy oil emulsion flux of the separation membranes prepared in Examples 1-4 and Comparative Example 1. As can be seen from the figure, as the RCN content gradually increases, the flux of the FPCN-RCN membrane first increases and then decreases. Among them, the water flux of the FPCN-RCN-1.5 separation membrane is the highest at 4896 L m -2 h -1 bar -1 , and the heavy oil emulsion flux is 778.7 L m -2 h -1 bar -1 . This shows that the intercalation of rod-shaped RCN in the FPCN nanosheets increases the interlayer distance between the nanosheets, thereby increasing the membrane flux. And the separation efficiency of all the prepared membranes for heavy oil emulsions is greater than 99%.

[0097] Figure 6 Figure c shows that when the heavy oil emulsion is filtered for 5 minutes, the water flux of the FPCN-RCN-1.5 separation membrane after rinsing in water for 2 minutes is 2294.4 L m -2 h -1 bar -1 . It is calculated that the flux recovery rate FRR after water washing is only 46.9%, indicating that clean water cannot achieve the self-cleaning effect of the membrane. After treatment by the combined photo-Fenton degradation and photo-driven evaporation method of Example 6, the water flux of the membrane is 4901.3 Lm -2 h -1 bar -1 , and the FRR is close to 100%. The results prove that the self-cleaning method provided by the present invention has obvious effects. As shown in Figure 6 Figure d, the water flux and heavy oil emulsion flux of the FPCN-RCN-1.5 separation membrane remain basically stable after 5 cycles of use, indicating that the membrane has good stability.

[0098] Although the embodiments disclosed in the present invention are as above, the content described is only the embodiments adopted for the convenience of understanding the present invention and is not used to limit the present invention. Any person skilled in the art within the scope of the present invention can make any modifications and changes in the form and details of the implementation without departing from the spirit and scope disclosed by the present invention. However, the scope of patent protection of the present invention shall still be subject to the scope defined by the appended claims.

Claims

1. A preparation method of a self-cleaning separation membrane, comprising preparing graphitic carbon nitride and rod-like structure carbon nitride by using a nitrogen-containing precursor; dispersing the graphitic carbon nitride in a solvent, adding an Fe 3+ solution and ammonium bicarbonate, centrifuging to take out the precipitate and drying after the reaction is completed to obtain an FeOOH / carbon nitride composite with photo-Fenton activity; dispersing the FeOOH / carbon nitride composite in water and adding the rod-like structure carbon nitride, and self-assembling to obtain a mixed solution; Modify the mixture solution onto the membrane substrate material, and then dry it to obtain the self-cleaning separation membrane; Among them, Use a nitrogen-containing precursor to carry out a thermal polymerization reaction at 400 - 600 °C for 2 - 5 hours to prepare graphitic carbon nitride or rod-like structured carbon nitride; use urea as the nitrogen-containing precursor to prepare the graphitic carbon nitride, and use melamine as the nitrogen-containing precursor to prepare the rod-like structured carbon nitride.

2. The preparation method of the self-cleaning separation membrane according to claim 1, wherein, The mass ratio of the FeOOH / carbon nitride composite to the rod-like structured carbon nitride is 5:1 - 1:

1.

3. The preparation method of the self-cleaning separation membrane according to claim 1, wherein, The mixture solution is modified onto the membrane substrate material by means of vacuum filtration.

4. The preparation method of the self-cleaning separation membrane according to claim 1, wherein The membrane substrate material is one of a polyvinylidene fluoride membrane, a polytetrafluoroethylene membrane, a polycarbonate membrane, or a polyethersulfone membrane.

5. The preparation method of the self-cleaning separation membrane according to claim 1, wherein The Fe-containing 3+ solution is a ferric chloride solution, a ferric sulfate solution or a ferric nitrate solution.

6. A self-cleaning separation membrane, characterized in that, Obtained according to the preparation method of any one of claims 1 - 5.

7. A self-cleaning method, characterized in that, For the self-cleaning of the self-cleaning separation membrane described in claim 6 after being contaminated by ultra-high viscosity heavy oil with a viscosity > 60000 mPa·s, it includes the following steps: Under the illumination condition of visible light, place the contaminated self-cleaning separation membrane in an H2O2 solution for photo-Fenton degradation; Then take out the self-cleaning separation membrane from the H2O2 solution and directly irradiate it for photo-driven volatilization.

8. The self-cleaning method according to claim 7, wherein The concentration of the H2O2 solution is 10 - 50 mM.

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