Visible light response carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane and preparation method thereof

Abstract

This invention belongs to the field of reverse osmosis membrane technology, and relates to a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane and its preparation method. The method specifically includes preparing porous ultrathin carbon nitride nanosheets; synthesizing zinc oxide nanoparticles rich in oxygen vacancies; combining the two to form a Z-shaped heterojunction photocatalyst; and using an interfacial polymerization process to stably load the heterojunction into a polyamide separation layer to obtain the composite reverse osmosis membrane. The membrane of this invention maintains a high desalination rate while exhibiting significant visible light response characteristics. Its core advantages are: the construction of the heterojunction significantly improves the visible light utilization rate and photogenerated charge separation efficiency, enabling the membrane to actively and efficiently degrade organic pollutants under light irradiation, achieving self-cleaning, thereby greatly improving antifouling ability and extending the chemical cleaning cycle.

Description

Technical Field

[0001] This invention belongs to the field of reverse osmosis membrane technology, and relates to a visible light responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane and its preparation method. Background Technology

[0002] Reverse osmosis technology is currently the core method for seawater desalination, brackish water desalination, and ultrapure water production. Its performance mainly depends on the separation efficiency, fouling resistance, and long-term operational stability of the membrane material. Traditional commercial reverse osmosis membranes are mostly polyamide thin-layer composite membranes, which achieve efficient salt retention by forming a dense polyamide separation layer through interfacial polymerization on a porous support layer such as polysulfone. However, in actual water treatment processes, especially when treating water sources containing complex components such as organic matter, microorganisms, and colloids, the membrane surface and pores are highly susceptible to fouling (such as organic fouling, biological fouling, and inorganic scaling). The adsorption and deposition of pollutants on the membrane surface leads to a significant decrease in membrane flux, an increase in operating pressure, and an increase in energy consumption, requiring frequent physical or chemical cleaning. This not only shortens the membrane's lifespan but also increases operating and maintenance costs.

[0003] To address the membrane fouling problem, existing technologies mainly modify reverse osmosis membranes using the following methods, but significant drawbacks still exist:

[0004] (1) Current mainstream methods focus on reducing the adsorption force between pollutants and the membrane surface by hydrophilizing the membrane surface (such as grafting hydrophilic polymers PEG and PVA), introducing negative charges (such as sulfonation), or constructing smooth surfaces (such as biomimetic coatings). These methods are relatively passive and can only delay the adhesion rate of pollutants to a certain extent, but cannot eliminate the pollutants that have already adhered. Once a fouling layer is formed, the membrane performance will still deteriorate, and external means are still needed to restore flux. Moreover, the process is cumbersome, prone to secondary pollution, and may damage the membrane material.

[0005] (2) Introducing photocatalytic materials (such as TiO2 and ZnO) into reverse osmosis membranes to achieve the purpose of actively degrading pollutants. However, commonly used materials such as TiO2 only respond to ultraviolet light, while the ultraviolet components in natural light and indoor lighting are extremely small, which greatly limits their application efficiency under conventional lighting conditions. The electron-hole pairs generated by a single photocatalyst are very easy to recombine rapidly, resulting in low quantum efficiency and limited actual catalytic degradation ability. If photocatalytic nanoparticles are loaded into the membrane through physical blending or simple dip coating, the binding force between the particles and the polymer matrix is ​​weak, and they are very easy to dissolve or fall off during long-term water flow shear, pressure fluctuations and cleaning processes. This not only causes rapid failure of catalytic function, but the lost nanoparticles may also cause secondary pollution. If the introduction of inorganic nanoparticles fails to be organically integrated with the polyamide layer, it may destroy the compactness of the polyamide network, form defects, and lead to a decrease in desalination rate. Summary of the Invention

[0006] To address the aforementioned problems, this invention provides a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane and its preparation method, specifically including the following steps:

[0007] Step 1: Urea and melamine are used as precursors and mixed in a molar ratio of (2-4):1. The mixture is heated to 500-550℃ at a rate of 2-3℃ / min and calcined for 3-4 hours. It is then naturally cooled to room temperature to obtain bulk carbon nitride (g-C3N4). Carbon vacancies and structural stress are introduced into the interior through the co-condensation of the precursors.

[0008] The bulk carbon nitride was pulverized and dispersed in a concentrated nitric acid solution with a mass fraction of 60-70% at a ratio of (1-2) mg: 15 mL. The solution was then refluxed in an oil bath at 80-90°C for 5-7 hours. During this process, nitric acid molecules not only intercalate between the layers, but their strong oxidizing properties also selectively etch some carbon-nitrogen bonds, creating more active sites and mesopores on the layers. Simultaneously, a large amount of -NH3 was introduced onto the surface of the carbon nitride through protonation. + Isopositively charged groups. After the reaction was complete, the gel was washed with deionized water by centrifugation until neutral to obtain activated carbon nitride gel.

[0009] Activated carbon nitride gel was dispersed in a 0.4-0.6 mol / L tetramethylammonium hydroxide (TMAOH) aqueous solution, sonicated at 40-50 kHz for 50-70 min, and reacted at 120-150℃ for 4-5 h. Under the synergistic effect of TMAOH intercalation and alkaline heating, the van der Waals forces between carbon nitride layers were effectively weakened, achieving the exfoliation from multiple layers to fewer layers (target 3-5 layers). After naturally cooling to room temperature, the gel was centrifuged at 2500-3500 rpm for 10-15 min to remove the unexfoliated thick sheets and collect the supernatant. The supernatant was centrifuged at 10000-15000 rpm for 15-25 min, and the precipitate was collected to obtain porous ultrathin g-C3N4 nanosheets (pCNNs). These nanosheets were redispersed in deionized water to obtain a colloidal suspension A with a concentration of 1-2 mg / mL.

[0010] Preferably, the ratio of the activated carbon nitride wet gel to the TMAOH aqueous solution, on a dry weight basis, is (1-2) mg:(15-25) mL.

[0011] Step 2: Dissolve zinc acetate dihydrate and citric acid monohydrate in equal volumes of anhydrous ethanol to obtain zinc acetate dihydrate-ethanol solution and citric acid monohydrate-ethanol solution. Under stirring at 400-600 rpm, add citric acid monohydrate-ethanol solution to zinc acetate dihydrate-ethanol solution at a rate of 2-4% total volume / min. Aging at 55-65℃ for 20-25 h yields zinc-citric acid complex sol.

[0012] Preferably, the ratio of zinc acetate dihydrate, citric acid monohydrate, and anhydrous ethanol (total volume) is (0.08-0.12) mol:(0.11-0.13) mol:2 L.

[0013] Under an inert gas atmosphere, the zinc-citric acid complex sol was irradiated under a 35-40W, 254nm UV lamp for 2-3 hours. The UV light excited the sol system and induced partial photolysis of the citric acid ligands, leading to the in-situ formation of oxygen-vacancy-rich ZnO crystal nuclei. The introduction of oxygen vacancies broadened the visible light absorption range of ZnO. Subsequently, the irradiated sol was heat-treated at 110-130℃ for 5-7 hours to allow the crystal nuclei to grow slowly and crystallize. After the reaction was complete, the crystals were collected, washed with anhydrous ethanol, and then dispersed in anhydrous ethanol to obtain a colloidal solution B with a concentration of 2-3 mg / mL.

[0014] Step 3: Under stirring conditions of 800-1200 rpm, add colloidal solution B to colloidal suspension A at a rate of 1-2% of total volume / min, and continue stirring in the dark for 10-15 h to ensure sufficient electrostatic interaction and interfacial contact. Then filter, remove the filtrate, and dry the filter residue at 55-65℃ for 8-12 h to obtain Z-type heterojunction nanocomposite material (pCNNs@ZnO-OVs). In this structure, pCNNs and ZnO-OVs are coupled through a tight electrostatic interface, which is conducive to the migration of photogenerated electrons from the conduction band of ZnO to the valence band of pCNNs for recombination, while retaining stronger reducing electrons in the conduction band of pCNNs and stronger oxidizing holes in the valence band of ZnO, laying the foundation for achieving efficient visible light photocatalytic degradation of organic matter.

[0015] Step four involves mixing polysulfone (PSF) and N-methylpyrrolidone (NMP) to form a casting solution with a mass fraction of 10-20%. The casting solution is then scraped onto a flat substrate to form a liquid film with a thickness of 100-200 μm. This film is immediately immersed in deionized water at 1-2°C to convert it into a porous polysulfone substrate. After thorough rinsing with deionized water, the substrate is immersed in a surface modifier for 8-12 minutes, removed, and drained to obtain the pretreated substrate. This step introduces a positively charged PEI molecular layer onto the substrate surface, enhancing the adsorption of negatively charged monomers and the initial interaction with the heterojunction in subsequent steps.

[0016] Preferably, the surface modifier is an aqueous solution of polyethyleneimine (PEI, molecular weight 70,000) with a mass fraction of 0.1-0.2%.

[0017] Step 5: Mix pCNNs@ZnO-OVs, dispersant, and 2-4% (w / w) m-phenylenediamine (MPD) aqueous solution at a mass ratio of (0.2-0.3):(0.4-0.6):1000. Sonicate in a water bath at 35-45℃ and 30-40kHz for 50-70 min, then stir at 300-400 rpm for 5-7 h at room temperature to obtain solution C. SDS can promote the dispersion of the heterojunction and its initial interaction with MPD. Hydrogen bonds and π-π interactions exist between the functional groups on the heterojunction surface and MPD molecules, achieving pre-loading.

[0018] Solution D is prepared by mixing a proton absorber, a coupling agent, and a 0.1-0.2% (w / w) solution of trimesoyl chloride (TMC)-Isopar G at a mass ratio of (0.01-0.03):(0.08-0.12):100. The proton absorber slows down the reaction rate between the amine and the acyl chloride at the interface, resulting in a more complete reaction and a denser polyamide layer. The coupling agent can condense with the hydroxyl / amino groups of the heterojunction or polyamide chain during subsequent hydrolysis.

[0019] Preferably, the proton absorber is camphor sulfonic acid, the coupling agent is γ-aminopropyltriethoxysilane (APTES), and the dispersant is sodium dodecyl sulfate.

[0020] Step 6: Immerse the pretreated substrate in solution C for 3-5 minutes to ensure that MPD molecules and heterojunction nanomaterials are fully adsorbed onto the pores and surface of the substrate. After removal and draining, immerse it in solution D for 70-80 seconds to carry out the interfacial polymerization reaction. During this process, TMC and MPD react rapidly at the interface, while APTES in the oil phase begin to migrate to the interface and initially interact with the hydroxyl or amino groups on the surface of some heterojunction particles. CSA adjusts the pH of the reaction microenvironment. Remove and drain. Heat-treat at 65-75℃ and 80-90%RH for 7-9 minutes. The humid and hot environment promotes further reaction between unreacted acyl chloride groups and amine groups, increasing the crosslinking degree of the polyamide layer and thus improving the desalination rate. It also accelerates the hydrolysis and condensation reaction of APTES, forming strong Si-OC or Si-O-Zn covalent bonds between the heterojunction nanoparticles and the polyamide network, greatly enhancing the long-term stability of the heterojunction and preventing loss. Finally, the humid and hot conditions partially relax and reorganize the polyamide chain segments, embedding the heterojunction particles more tightly beneath the membrane surface, forming an organic-inorganic composite structure rather than simple physical adhesion. Remove and drain again, then soak in an alkaline neutralization solution for 1-3 minutes to neutralize residual acyl chloride groups and acid, yielding a carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane.

[0021] Preferably, the mass ratio of the pretreated substrate, solution C, solution D and alkali neutralization solution is (1-2):(100-150):(70-90):(350-450).

[0022] Most preferably, the alkali neutralizing solution is a sodium carbonate aqueous solution with a mass fraction of 1-2%.

[0023] The present invention has the following advantages:

[0024] (1) Unlike existing passive antifouling membranes or photocatalytic membranes that rely on ultraviolet light, this invention constructs a Z-type heterojunction (pCNNs@ZnO-OVs). This structure, through bandgap engineering, not only effectively broadens the photoresponse range to the visible light region, allowing the membrane to be activated under sunlight or indoor light, but more importantly, the Z-type electron migration pathway greatly suppresses the recombination of photogenerated electrons and holes. The strong reducing electrons retained in the carbon nitride conduction band and the strong oxidizing holes in the zinc oxide valence band can efficiently generate reactive oxygen species (such as ·OH and ·O2). - This process enables reverse osmosis membranes to possess powerful active cleaning capabilities. Organic pollutants, microorganisms, and their metabolites adsorbed on the membrane surface can be continuously and in situ catalytically degraded into smaller molecules or mineralized, fundamentally preventing the accumulation and solidification of the fouling layer. This keeps the membrane surface relatively clean, significantly slows down the flux decline rate, significantly extends the chemical cleaning cycle, and markedly improves operational stability and energy efficiency.

[0025] (2) The heterojunction nanomaterials (pCNNs@ZnO-OVs) of the present invention are not simple physical doping, but rather, through preloading and precise control during the interfacial polymerization process, multiple interactions (hydrogen bonds, π-π interactions) occur between the heterojunction particles, MPD monomers and polyamide networks. In particular, the hydrolysis and condensation of the coupling agent are promoted by wet heat treatment, forming a strong Si-OC or Si-O-Zn covalent bond bridge between the heterojunction and the polyamide chain. This process acts as a crosslinking point, promoting the polyamide chain segments to be more tightly arranged and crosslinked around the heterojunction, thereby optimizing and enhancing the network density of the polyamide separation layer. It not only does not introduce defects, but also further improves the membrane's ability to retain salt ions, while enhancing its anti-fouling and high desalination properties.

[0026] (3) This invention pretreats the substrate film with PEI to form a positively charged layer, which enhances the initial adsorption with the negatively charged heterojunction and MPD. Most importantly, during the wet heat treatment stage after interfacial polymerization, the coupling agent APTES undergoes hydrolysis and covalent condensation reaction with the hydroxyl / amino groups on the surface of the heterojunction and the relevant groups in the polyamide chain, constructing a strong covalent bond bridge between the inorganic nanoparticles and the organic polymer network. This chemical bonding method deeply embeds and firmly locks the heterojunction particles under the polyamide surface, forming an organic-inorganic integrated composite structure. Even under long-term high-pressure operation and periodic cleaning, the heterojunction catalyst is difficult to detach, ensuring the long-term durability and stability of the photocatalytic self-cleaning function and avoiding the performance degradation and secondary pollution risks caused by catalyst loss.

[0027] (4) This invention provides an innovative pretreatment for carbon nitride and zinc oxide precursors. For carbon nitride, a specific ratio of urea / melamine co-condensation combined with concentrated nitric acid oxidation activation and TMAOH alkaline-thermal synergistic exfoliation is used. This not only successfully prepares few-layer porous ultrathin nanosheets, significantly increasing the specific surface area and active sites, but also introduces abundant -NH3 on its surface. + The presence of isopositively charged groups is crucial for its subsequent electrostatic assembly with ZnO. For zinc oxide, an in-situ induced synthesis technique using ultraviolet light irradiation was employed to directly generate oxygen-vacancy-rich ZnO nuclei within a citric acid complex system. The introduction of oxygen vacancies not only expands the visible light absorption of ZnO but also enhances its surface activity and charge separation efficiency. These two pretreated materials then form a tightly bound Z-shaped heterojunction through controllable electrostatic self-assembly, ensuring the composite photocatalyst's superior visible light absorption, charge separation efficiency, and interfacial coupling strength from the source, laying the material foundation for the preparation of high-performance modified films. Detailed Implementation

[0028] The technical solutions in the embodiments of the invention are described clearly and completely below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] Step 1: Urea and melamine were used as precursors and mixed in a molar ratio of 3:1. The mixture was heated to 530℃ at a rate of 2.5℃ / min and calcined for 3.5h to obtain bulk carbon nitride. The bulk carbon nitride was pulverized and dispersed in a 65% (w / w) concentrated nitric acid solution at a ratio of 1.5mg:15mL. The mixture was refluxed in an oil bath at 85℃ for 6h. After the reaction was completed, the mixture was washed with deionized water by centrifugation until neutral to obtain activated carbon nitride gel. The activated carbon nitride gel was dispersed in a 0.5mol / L TMAOH aqueous solution, sonicated at 45kHz for 60min, and reacted at 135℃ for 4.5h. After naturally cooling to room temperature, the mixture was centrifuged at 300rpm for 12min to remove the precipitate and collect the supernatant. The supernatant was centrifuged at 13000rpm for 20min, and the precipitate was collected to obtain pCNNs. These pCNNs were redispersed in deionized water to obtain a colloidal suspension A with a concentration of 1.5mg / mL. The ratio of the activated carbon nitride wet gel to the TMAOH aqueous solution, based on dry weight, is 1.5 mg: 20 mL.

[0031] Step two, the ratio of zinc acetate dihydrate, citric acid monohydrate and anhydrous ethanol (total volume) is 0.1 mol: 0.12 mol: 2 L.

[0032] Zinc acetate dihydrate and citric acid monohydrate were dissolved in equal volumes of anhydrous ethanol to obtain zinc acetate dihydrate-ethanol solution and citric acid monohydrate-ethanol solution, respectively. Under stirring at 500 rpm, citric acid monohydrate-ethanol solution was added to zinc acetate dihydrate-ethanol solution at a rate of 3% total volume / min. The mixture was aged at 60℃ for 24 h to obtain zinc-citric acid complex sol.

[0033] Under a nitrogen atmosphere, the zinc-citric acid complex sol was irradiated under a 36W, 254nm UV lamp for 2.5h and then heat-treated at 120℃ for 6h. The crystals were collected, washed with anhydrous ethanol, and then dispersed in anhydrous ethanol to obtain a colloidal solution B with a concentration of 2.5mg / mL.

[0034] Step 3: Under stirring at 1000 rpm, add colloidal solution B to colloidal suspension A at a rate of 1.5% of total volume / min, continue stirring in the dark for 12 h, filter, remove the filtrate, and dry the filter residue at 60℃ for 10 h to obtain pCNNs@ZnO-OVs.

[0035] Step 4: Mix PSF and NMP to form a casting solution with a mass fraction of 15%. Scrape the casting solution onto a flat substrate to form a liquid film with a thickness of 150 μm. Immediately immerse the film in deionized water at 1.5 °C to convert it into a membrane, resulting in a porous polysulfone substrate. After rinsing with deionized water, soak the substrate in a 0.15% (mass fraction) aqueous solution of polyethyleneimine (PEI, molecular weight 70,000) for 10 min. Remove the substrate and drain it to obtain the pretreated substrate.

[0036] Step 5: Mix pCNNs@ZnO-OVs, sodium dodecyl sulfate, and 3% MPD aqueous solution at a mass ratio of 0.25:0.5:1000, sonicate at 40℃ and 35kHz in a water bath for 60 min, and then stir at 350rpm for 6 h at room temperature to obtain solution C.

[0037] Camphor sulfonic acid, APTES, and 0.15% TMC-Isopar G solution were mixed at a mass ratio of 0.02:0.1:100 to obtain solution D.

[0038] Step 6: Immerse the pretreated membrane in solution C for 4 minutes, drain it, then immerse it in solution D for 75 seconds to carry out interfacial polymerization reaction, drain it, heat treat it at 70℃ and 85%RH for 8 minutes, drain it, and then immerse it in a 1.5% sodium carbonate aqueous solution for 2 minutes to obtain the carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane.

[0039] Experimental Example 1

[0040] Membrane sample preparation: The experimental group used the carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane (CN / ZnO-RO membrane) prepared in Example 1. The control group used the polyamide composite reverse osmosis membrane (PA-RO membrane) purchased from Suzhou Tangrun Environmental Protection Technology Co., Ltd.

[0041] Test setup: A cross-flow reverse osmosis test setup was used, with an effective membrane area of ​​20 cm². 2 The operating pressure was set to 1.55 MPa, the temperature to 25 ± 1 °C, and the flow rate to 1.0 L / min.

[0042] Test solution preparation: Simulated seawater: NaCl solution, concentration 2000 mg / L. Simulated brackish water: NaCl solution, concentration 500 mg / L.

[0043] Test procedure: The membrane was pre-pressurized in deionized water for 30 minutes until it stabilized before the test. Permeate was collected every 10 minutes and the test was conducted continuously for 60 minutes (average value was taken). The results are shown in Table 1.

[0044] Water flux: J (L·m -2 ·h -1 =V / (A×t).

[0045] Desalination rate: R = [1 - (C p / C f )]×100%. C p The permeate conductivity C f The conductivity of the feed liquid.

[0046] Experimental Example 2

[0047] Preparation of pollutant solution: Methylene blue (MB), a typical organic pollutant, was selected with an initial concentration of 10 mg / L.

[0048] Experimental setup: A circulating photocatalytic membrane reactor was used, equipped with a visible light source (λ≥420nm, light intensity 100mW / cm²). 2 The effective area of ​​the membrane is 15 cm². 2 .

[0049] Experimental procedure: The membrane was irradiated in the dark for 30 min to allow MB to reach adsorption equilibrium on the membrane surface. Then, a visible light source was turned on, and samples were taken every 20 min for a total of 120 min. The absorbance of MB at 664 nm was measured using a UV-Vis spectrophotometer, and the degradation rate was calculated. The results are shown in Table 2.

[0050] Experimental Example 3

[0051] Preparation of contaminant solution: Bovine serum albumin (BSA) was selected as the model contaminant at a concentration of 500 mg / L and dissolved in 2000 mg / L NaCl solution.

[0052] Contamination-cleaning cycle experiment: Each cycle includes a contamination phase (60 min, dark state) → light self-cleaning phase (60 min, visible light) → physical cleaning (rinsing with deionized water for 5 min), and 5 cycles are performed continuously. After each cycle, the water flux recovery rate is tested.

[0053] Flux recovery rate calculation: FRR=J2 / J1×100%, where J1 is the initial water flux and J2 is the stable flux after each cycle of cleaning.

[0054] Table 1. Comparison of basic performance of CN / ZnO-RO membrane and PA-RO membrane

[0055]

[0056] Table 2. Degradation performance of different films on MB under visible light.

[0057]

[0058] Table 3. Comparison of flux recovery rates of different membranes in the BSA fouling-cleaning cycle.

[0059]

[0060] As shown in Tables 1-3, the CN / ZnO-RO membrane prepared in this invention maintains a high desalination rate while exhibiting a significantly higher water flux than the traditional PA-RO membrane, increasing it by approximately 14.3% under seawater conditions. The CN / ZnO-RO membrane possesses excellent visible light photocatalytic degradation capabilities, achieving a degradation rate of up to 96.8% for MB within 120 minutes, while the control membrane showed almost no degradation effect. The CN / ZnO-RO membrane demonstrates good self-cleaning and antifouling durability; after five fouling-light cycles, the flux recovery rate remains above 93%, significantly better than the approximately 64.5% of the control membrane.

[0061] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a visible light responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane, characterized in that, Includes the following steps: Step 1: Mix urea and melamine in a molar ratio of (2-4):1, calcine to obtain carbon nitride, pulverize and disperse in concentrated nitric acid solution, reflux in an oil bath at 80-90℃ to obtain activated carbon nitride gel, disperse the activated carbon nitride gel in 0.4-0.6 mol / L TMAOH aqueous solution, react at 120-150℃ for 4-5 h, collect the supernatant by low-speed centrifugation, collect the precipitate by high-speed centrifugation, redisperse it in deionized water to obtain colloidal suspension A with a concentration of 1-2 mg / mL; Step 2: Mix zinc acetate dihydrate, citric acid monohydrate and anhydrous ethanol at a ratio of (0.08-0.12) mol: (0.11-0.13) mol: 2 L, irradiate with ultraviolet light under an inert gas atmosphere, react at 110-130 °C for 5-7 h, collect the crystals, wash them, and disperse them in anhydrous ethanol to obtain colloidal solution B with a concentration of 2-3 mg / mL. Step 3: Under stirring conditions, add colloidal solution B to colloidal suspension A, stir in the dark, filter, and dry the filter residue to obtain pCNNs@ZnO-OVs; Step 4: Mix PSF and NMP to form a casting solution with a mass fraction of 10-20%, scrape it into a liquid film, and soak it in deionized water and surface modifier at 1-2℃ in sequence to obtain a pretreated substrate film. Step 5: Mix pCNNs@ZnO-OVs, dispersant, and 2-4% (w / w) MPD aqueous solution at a mass ratio of (0.2-0.3):(0.4-0.6):1000, sonicate in a water bath, and stir to obtain solution C; mix proton absorber, coupling agent, and 0.1-0.2% (w / w) TMC-Isopar G solution at a mass ratio of (0.01-0.03):(0.08-0.12):100 to obtain solution D; Step 6: Immerse the pretreated membrane in solution C and solution D in sequence, remove and drain, heat treat at 65-75℃ and 80-90%RH for 7-9 minutes, and then immerse in alkaline neutralization solution to obtain carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane.

2. The method for preparing a carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane with visible light response according to claim 1, characterized in that, In step one, the temperature is increased to 500-550℃ at a rate of 2-3℃ / min, and calcined for 3-4 hours.

3. The method of claim 1, wherein the method is characterized by: In step one, the ratio of activated carbon nitride wet gel to TMAOH aqueous solution, based on dry weight, is (1-2) mg:(15-25) mL.

4. The method of claim 1, wherein the method is characterized by: The surface modifier mentioned in step four is a PEI aqueous solution with a mass fraction of 0.1-0.2%.

5. The method of claim 1, wherein the method is characterized by: The dispersant mentioned in step five is sodium dodecyl sulfate.

6. The method for preparing a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane according to claim 1, characterized in that, The proton absorber mentioned in step five is camphor sulfonic acid.

7. The method for preparing a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane according to claim 1, characterized in that, The coupling agent mentioned in step five is APTES.

8. The method for preparing a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane according to claim 1, characterized in that, The mass ratio of the pretreatment substrate, solution C, solution D and alkali neutralization solution in step six is ​​(1-2):(100-150):(70-90):(350-450).

9. The method for preparing a visible light-responsive carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane according to claim 1, characterized in that, The alkali neutralization solution mentioned in step six is ​​a sodium carbonate aqueous solution with a mass fraction of 1-2%.

10. The carbon nitride / zinc oxide heterojunction modified reverse osmosis membrane prepared by the method of any one of claims 1-9.

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