Photoelectrocatalysis system for nitrogen removal of nitrogen-containing underground water through cathode reduction coupled with anodic oxidation water production of hydrogen peroxide

Abstract

The invention provides a photoelectrocatalysis system for nitrogen removal of nitrogen-containing underground water through cathode reduction and coupling with anodic oxidation of water to produce hydrogen peroxide. The photoelectrocatalysis system can be used for nitrogen removal of nitrogen-containing underground water through reduction and water oxidation to prepare hydrogen peroxide. An electrolyte solution in the cathode chamber is nitrogen-containing underground water in the northeast agricultural region, an electrolyte in the anode chamber is a sodium sulfate solution, after the photoelectrocatalysis treatment is performed for 3 hours, the nitrate content in the underground water is lower than the national drinking water standard, and the concentration of hydrogen peroxide in the solution in the anode chamber can meet the disinfection requirement in a medical or household environment. The device has the characteristics of small occupied area, modular assembly and easiness in automatic control, and the process has the advantages of low cost, no pollution and remarkable treatment effect, realizes the combination of water pollution control and resource utilization, and is a novel treatment mode capable of realizing household application.

Description

Technical Field

[0001] The present invention belongs to the technical field of collaborative control of water pollution and resource regeneration. Specifically, it relates to a water treatment and resource utilization system for the preparation of Co-Cu2O / CFC electrodes and CuW-C3N4 / CFC electrodes and the electrocatalytic cathodic reduction of nitrate nitrogen in groundwater while anodically oxidizing water to prepare hydrogen peroxide. It can efficiently remove nitrates in groundwater and prepare hydrogen peroxide, which provides important reference value for the integrated technology of photoelectrocatalytic treatment of nitrogen-containing groundwater and in-situ supply of functional chemicals. Background Art

[0002] The accumulation of nitrate nitrogen in the water environment will seriously endanger human health. Long-term drinking of groundwater polluted by NO3 - poses a very high risk of human methemoglobinemia, and the accumulated NO3 - will be converted into highly carcinogenic, teratogenic and mutagenic nitroso compounds under specific conditions, inducing major diseases such as esophageal cancer, gastric cancer, intestinal cancer and thyroid tumors. In industry, physical and chemical methods and biological methods are often used to remove nitrates in water. The defects of adsorption, ion exchange, membrane separation and electrodialysis methods are that they cannot completely eliminate NO3 - , but transfer NO3 - to another medium such as concentrated liquid or regenerated liquid that needs to be treated secondary. The biological method is based on biological denitrification. Without adding a carbon source, it is difficult for the biological method to be applicable to the removal of NO3 - in groundwater with a low C / N ratio. Therefore, solving the problem of NO3 - pollution in groundwater has become an important task in environmental governance.

[0003] Due to its suitable oxidizing property, bleaching property and no secondary pollution during use, hydrogen peroxide is widely used in chemical synthesis, sewage treatment, medical treatment, metallurgy, food processing and other fields. At present, the main production process for large-scale production of hydrogen peroxide in industry is the anthraquinone method. Its process includes steps such as hydrogenation, oxidation, extraction, regeneration, refining and concentration of the anthraquinone method to obtain an aqueous hydrogen peroxide solution. This method is very conducive to the large-scale production of high-concentration hydrogen peroxide. Although the process has been continuously optimized and improved to make the technology increasingly mature, there are still some serious sustainability challenges to be solved. For example, the problems of large factories, high energy consumption, organic waste pollution, and safe storage and transportation of high-concentration hydrogen peroxide need to be solved urgently. Considering these problems, a more green, economic and environmentally friendly solution for small-scale on-site flexible production of hydrogen peroxide is needed.

[0004] Photoelectrocatalysis (PEC) can convert solar energy into chemical energy that can be stably accumulated in the long term, and is considered one of the promising options for solving global energy and environmental problems. Photoelectrocatalysis combines the advantages of photocatalysis and electrocatalysis, achieving a higher solar energy conversion efficiency than photocatalysis. At the same time, it significantly reduces the power consumption compared with electrocatalysis and is more sustainable in terms of energy consumption. The PEC reaction to produce H2O2 mainly involves three processes: light absorption, charge separation, and surface reaction. When the semiconductor photoanode absorbs appropriate light energy, electrons are excited from the valence band to the conduction band, leaving holes in the valence band. The photogenerated electrons are transferred to the counter electrode, and water is reduced to hydrogen. Then, under the condition that the valence band is greater than the redox potential of H2O / H2O2, the photogenerated holes will be transferred to the semiconductor / electrolyte interface by the electric field, and the 2e-WOR reaction occurs to generate hydrogen peroxide. Therefore, by simultaneously producing H2 and high-value-added hydrogen peroxide chemicals, the conversion and accumulation efficiency of solar energy can be improved, and the related economic benefits can be enhanced. The photoelectrocatalytic reduction of nitrate does not require the addition of chemical agents. It directly uses the electrons generated by the electric field on the electrode surface as the electron donor for nitrate reduction, without producing secondary pollution and reprocessing costs. The treatment efficiency of the nitrate electrochemical reduction reaction process is high, the reaction rate is fast, and the products are controllable. By optimizing the electrode materials, reactors, and operating parameters, the generation of nitrogen and other valuable products can be effectively regulated. This process is driven by electric energy, and the electric energy can be provided by renewable energy such as solar energy and wind energy. The electrochemical reactor has a small footprint, can be modularly assembled, and is easy to automate control, and has been widely applied in the field of advanced wastewater treatment. Therefore, the photoelectrocatalytic removal of nitrate in water and the production of hydrogen peroxide using water resources is a feasible and promising technology. Summary of the Invention

[0005] The object of the present invention is to combine the photoelectrocatalytic water treatment system of anodic oxidation and cathodic reduction, and successfully build a modular cathode for efficient treatment of nitrogen-containing groundwater denitrification, and at the same time anodic oxidation of water to produce hydrogen peroxide photoelectrocatalytic device. The sequential batch water supply method is used to provide the anode and cathode solutions. The anode is a sodium sulfate solution, and the cathode is the actual nitrate-containing groundwater. Utilizing solar energy and electric energy, coupling the photoelectric response and catalytic ability of the anode and cathode catalysts, efficient treatment of nitrogen-containing groundwater is achieved.

[0006] The technical solution of the present invention:

[0007] A photoelectrocatalytic system for cathodic reduction of nitrogen-containing groundwater for denitrification coupled with anodic oxidation of water to produce hydrogen peroxide. The photoelectrocatalytic system includes an electrode chamber, electrodes, a diaphragm, an electrolyte solution, an external circuit, a visible light source, and auxiliary components. The electrode chamber includes an anodic chamber 9 and a cathodic chamber 4, and the anodic chamber 9 and the cathodic chamber 4 are separated by a diaphragm. The electrolyte solution in the anodic chamber 9 and the cathodic chamber 4 is processed in a sequential batch mode with inlet at the top and outlet at the bottom. The external circuit includes wires and a DC power supply 1. The wires are used to connect the anodic electrode 10 and the cathodic electrode 5, and the DC power supply 1 is arranged between the anodic electrode 10 and the cathodic electrode 5. A visible light source 7 is installed above the electrodes to form the photoelectrocatalytic system.

[0008] The anodic catalytic material is a CuW-C3N4 composite catalyst, and the cathodic catalytic material is a Co-Cu2O composite catalyst.

[0009] Furthermore, in a photoelectrocatalytic system for cathodic reduction of nitrogen-containing groundwater for denitrification coupled with anodic oxidation of water to produce hydrogen peroxide, the inlet water of the anodic chamber 9 and the cathodic chamber 4 is added with the electrolyte solution through a sampling port reserved above the electrode chamber and sealed with a rubber stopper, and the outlet water of the anodic chamber 9 and the cathodic chamber 4 is discharged with the electrolyte solution through an outlet reserved below the electrode chamber and connected with a rubber tube.

[0010] Furthermore, the electrolyte solution in the anodic chamber 9 is a sodium sulfate solution.

[0011] Furthermore, the diaphragm is a cation exchange membrane 11; the auxiliary components include silica gel gaskets, studs and nuts for sealing.

[0012] Furthermore, the electrolyte solution in the cathodic chamber 4 is a groundwater solution containing nitrate.

[0013] Furthermore, the CuW-C3N4 / CFC electrode is prepared by a method of first high-temperature calcination and then sol coating; the Co-Cu2O / CFC electrode is prepared by a method of first impregnation growth and then high-temperature calcination.

[0014] Furthermore, the preparation method of the CuW-C3N4 / CFC electrode is as follows:

[0015] Step (1) Synthesis of W-C3N4: Dissolve urea and tungstic acid in deionized water and mix well, then dry in an oven; then transfer to a muffle furnace for calcination to obtain the W-C3N4 catalyst.

[0016] Step (2) Preparation of the CuW-C3N4 / CFC electrode: Mix basic copper carbonate and the W-C3N4 catalyst, and add them to a silica sol solution to obtain a coating solution. Brush the coating solution on both sides of the carbon fiber cloth to obtain the CuW-C3N4 / CFC electrode.

[0017] Further, in the step (1), the molar ratio of urea to tungstic acid is 1:0.01 - 0.1; the drying temperature is 50 - 80 °C, and the drying time is 4 - 8 h; the calcination heating rate is 1 - 10 °C / min; the calcination temperature is 500 - 600 °C; the calcination time is 2 - 4 h; in the step (2), the mass ratio of basic copper carbonate to the W-C3N4 catalyst is 1:0.5 - 2; the mass content of the silica sol solution is 10 - 30%; the mass content of basic copper carbonate in the silica sol solution is 25% - 75%.

[0018] Further, the preparation method of the Co-Cu2O / CFC electrode is as follows:

[0019] Dissolve copper sulfate pentahydrate in deionized water to form solution A; dissolve sodium hydroxide in deionized water to form solution B; then slowly pour solution B into solution A while stirring to obtain a catalyst precursor mixed solution, and then ultrasonicate the catalyst precursor mixed solution; place the carrier into the precursor mixed solution for impregnation, take it out and put it into an oven for drying, then dropwise add a solution containing cobalt salt, and then put it into an oven for drying; place the dried electrode in a tube furnace, calcine it under an inert atmosphere, and then naturally cool down to obtain the Co-Cu2O / CFC electrode.

[0020] Further, the concentration of solution A is 1 - 2 mol / L; the concentration of solution B is 2 - 5 mol / L; the molar ratio of copper sulfate pentahydrate to sodium hydroxide is 1:2 - 3; the carrier is carbon fiber cloth, activated carbon, carbon felt; the impregnation time is 10 - 30 min; the drying temperature of the oven is 50 - 80 °C; the drying time is 4 - 8 h; the cobalt salt is cobalt chloride, cobalt bromide, cobalt iodide, cobalt carbonate, cobalt sulfate; the concentration is 0.1 - 0.5 mol / L; the mass ratio of the carrier to the solution containing cobalt salt is 1:2 - 8; the calcination heating rate is 1 - 10 °C / min; the calcination temperature is 400 - 600 °C; the calcination time is 2 - 4 h.

[0021] The beneficial effects of the present invention: The present invention provides a modular cathode photoelectrocatalytic device for efficiently treating nitrogen-containing groundwater for denitrification and simultaneously producing hydrogen peroxide by anodic oxidation of water. Innovatively coupling anodic oxidation, cathodic reduction with photoelectrocatalysis, the nitrate removal rate in groundwater reaches 90.5%, the nitrogen selectivity reaches 81.5%, the Faraday efficiency reaches 91.9%, and the hydrogen peroxide production rate reaches 0.32 mol / (L·h) per gram of catalyst. After 3 hours of photoelectrocatalytic treatment, the nitrate content in groundwater meets the national drinking water standard (NO3 --N ≤ 10 mg / L), the hydrogen peroxide solution reaches the disinfection solution concentration (≥ 0.89 mol / L) in the medical or household environment. At the same time, the device has a small footprint, can be modularly assembled, and is easy to automate control. Therefore, this treatment system has broad and feasible application prospects in the field of removing nitrates in groundwater and producing hydrogen peroxide using water resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is a schematic diagram of a photoelectrocatalytic device for modular cathode reduction and anodic oxidation of the present invention.

[0023] Figure 1 In the figure: 1 DC power supply; 2 external circuit; 3 rubber stopper of the cathode chamber; 4 cathode chamber; 5 cathode electrode; 6 valve of the cathode chamber; 7 visible light source; 8 rubber stopper of the anode chamber; 9 anode chamber; 10 anode electrode; 11 cation exchange membrane; 12 valve of the anode chamber.

[0024] Figure 2 is the experimental effect diagram of different cathode electrodes and anode electrodes in Example 4. Among them, (a) is the change diagram of nitrate concentration of different cathode electrodes, (b) is the change diagram of nitrogen production of different cathode electrodes, and the cathode is the Co-Cu2O / CFC electrode with different cobalt contents; (c) is the change diagram of hydrogen peroxide production of different anodes, and the anode is the CuW-C3N4 / CFC electrode with different tungsten contents.

[0025] Figure 3 is the experimental effect diagram of Example 5. Among them, (a) is the change diagram of cathode nitrate concentration, (b) is the change diagram of cathode nitrogen production, and (c) is the diagram of cathode nitrate removal rate, nitrogen selectivity and Faraday efficiency. The cathode is the Co-Cu2O / CFC electrode with a cobalt content of 0.25 mmol.

[0026] Figure 4 is the experimental effect diagram of Example 6. Among them, (a) is the production of hydrogen peroxide by anodic water oxidation, and (b) is the hydrogen peroxide yield of the anode. The anode is the CuW-C3N4 / CFC electrode with a tungsten content of 0.02 mol. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following further illustrates the specific implementation manners of the present invention in combination with the technical solutions and the drawings, which does not limit the protection scope of the present invention.

[0028] As Figure 1 shown, the present invention provides a photoelectrocatalytic system for modular cathode high-efficiency treatment of nitrogen-containing groundwater denitrification and simultaneous anodic oxidation of water to produce hydrogen peroxide. This coupling system has the following modular characteristics, and its treatment design composition includes:

[0029] The described DC power supply 1 is connected to the catalytic electrode through an external circuit 2 to provide a variable external voltage and current for the photoelectrocatalytic system; the visible light source 7 provides the light energy required for photocatalysis for the electrode.

[0030] The rubber stoppers 3 in the cathode chamber and 8 in the anode chamber are both used to isolate the air. The rubber stopper 3 in the cathode chamber is to prevent the oxygen in the air from affecting the reduction of nitrate, and the rubber stopper 8 in the anode chamber is to prevent the hydrogen peroxide generated at the anode from volatilizing and decomposing.

[0031] The cathode chamber 4 is used to hold the cathode electrolyte and the cathode electrode 5, and the anode chamber 9 is used to hold the anode electrolyte and the anode electrode 10, which are separated by a cation exchange membrane 11.

[0032] The cathode electrolyte is nitrogen-containing groundwater, which flows into the cathode chamber 4 through the opening under the rubber stopper 3 in the cathode chamber; the anode electrolyte is a 0.5 mol / L sodium sulfate solution, which flows into the anode chamber 9 through the opening under the rubber stopper 8 in the anode chamber.

[0033] There are two drain valves, the cathode chamber valve 6 and the anode chamber valve 12, respectively, under the cathode chamber 4 and the anode chamber 9, which are used to collect the treated groundwater solution and hydrogen peroxide solution respectively.

[0034] Preparation of the anode: Weigh a certain mass of basic copper carbonate and the W-C3N4 catalyst respectively, drop the silica sol solution in a certain proportion, and coat them on both sides of the carbon fiber cloth.

[0035] Preparation of the cathode: Prepare a precursor solution of the Co-Cu2O catalyst according to a certain ratio, put the carbon fiber cloth into the solution, impregnate it for a specific time, and then dry and calcine it.

[0036] Example 1

[0037] Combined with Figure 1 , first place the cation exchange membrane 11 between the anode chamber 9 and the cathode chamber 4, then place the anode electrode 5 in the anode chamber 9 and the cathode electrode 10 in the cathode chamber 4, and then detect the device sealing performance.

[0038] The anode electrolyte flows into the anode chamber 9 from below the anode rubber stopper, and the cathode electrolyte flows into the anode chamber 9 from below the anode rubber stopper, and then tighten the rubber stopper.

[0039] Connect the external circuit 3 to the two electrodes through the rubber stopper, ensure that the connection part does not contact the electrolyte solution, then connect the other end of the external circuit 3 to the DC power supply 1, connect the anode electrode 5 to the positive pole of the DC power supply 1, and connect the cathode electrode 10 to the negative pole of the DC power supply 1.

[0040] Keep the visible light source 7 turned off, set the DC power supply 1 to a voltage of 3 V, take samples from the rubber stopper every 1 hour to detect the electrocatalytic effect, electrolyze for a total of 3 hours, and after electrolysis is completed, open the anode chamber valve 6 to collect the hydrogen peroxide solution, and open the cathode chamber valve 12 to collect the groundwater solution.

[0041] Keep the visible light source 7 turned on, set the DC power supply 1 to a voltage of 3 V, take samples from the rubber stopper every 1 hour to detect the electrocatalytic effect, electrolyze for a total of 3 hours, and after electrolysis is completed, open the anode chamber valve 6 to collect the hydrogen peroxide solution, and open the cathode chamber valve 12 to collect the groundwater solution.

[0042] Example 2

[0043] Prepare the CuW-C3N4 / CFC electrode as the anode electrode by the method of first high-temperature calcination and then sol coating.

[0044] Preparation of basic copper carbonate: Prepare 100 mL each of 0.5 mol / L CuSO4 solution and 0.5 mol / L Na2CO3 solution respectively. Under the condition of water bath heating at 65 °C, slowly pour the sodium carbonate solution into the copper sulfate solution while stirring. Observe the formation of blue-green precipitate, continue stirring for 15 minutes to make the reaction proceed fully. Filter with a Buchner funnel to collect the precipitate. Wash the precipitate with distilled water multiple times, and then place the precipitate in an oven at 65 °C for drying for 6 h to obtain blue-green basic copper carbonate powder.

[0045] Synthesis of W-C3N4: Take 0.5 mol of urea and 0.02 mol of tungstic acid and dissolve them in a 100 ml beaker of deionized water, stir evenly, and place it in an oven at 65 °C for drying for 6 h. Then transfer it to a muffle furnace and calcine it at a heating rate of 5 °C / min and a temperature of 550 °C for 3 h to obtain W-C3N4.

[0046] Preparation of CuW-C3N4 / CFC electrode: Weigh 0.5 g of basic copper carbonate and W-C3N4 mixed catalyst according to the mass ratio of 1:1, mix them evenly with a 15% mass concentration of silica sol solution according to the mass ratio of 1:1, and then brush them on both sides of the carbon fiber cloth to obtain the CuW-C3N4 / CFC electrode. The mass content of basic copper carbonate in the silica sol solution is 50%.

[0047] Prepare the Co-Cu2O / CFC electrode as the cathode electrode by the method of first impregnation growth and then high-temperature calcination.

[0048] Preparation of Co-Cu2O / CFC electrode: Dissolve 0.04 mol of copper sulfate pentahydrate in 25 ml of deionized water to form solution A. Dissolve 0.1 mol of sodium hydroxide in 25 ml of deionized water to form solution B. Then slowly pour solution B into solution A with stirring to obtain a mixed solution of catalyst precursor, and ultrasonicate the mixed solution for 15 minutes. Put 0.5 g of carbon fiber cloth into the mixed solution and impregnate it for 20 minutes, then place it in an oven at 65 °C and dry for 6 h. After taking it out, drop 1 ml of cobalt chloride solution with a concentration of 0.25 mol / L, and place it in an oven at 65 °C and dry for 6 h. Place the dried electrode in a programmable tube furnace and calcine it under a nitrogen atmosphere with a heating rate of 5 °C / min. After the temperature rises to 500 °C, calcine it at a high temperature for 3 h, and then let it cool naturally to obtain the Co-Cu2O / CFC electrode.

[0049] Example 3

[0050] The preparation method is the same as that of Example 2, only changing 0.02 mol of tungstic acid to 0 mol, 0.01 mol, 0.03 mol, 0.04 mol to prepare CuW-C3N4 / CFC electrodes with different tungsten contents; changing 1 ml of cobalt chloride solution with a concentration of 0.25 mol / L to 0 ml, 2 ml, 3 ml, 4 ml to prepare Co-Cu2O / CFC electrodes with different cobalt contents. Among them, when tungstic acid is changed to 0 mol and cobalt chloride solution is changed to 0 ml, it is used as a comparative example.

[0051] Example 4

[0052] Combined with Figure 1 , using Figure 1 in the photoelectrocatalytic device, nitrate in groundwater is reduced at the cathode while hydrogen peroxide is produced at the anode. The cathode is a Co-Cu2O / CFC electrode with different cobalt contents, and the anode is a CuW-C3N4 / CFC electrode with different tungsten contents. The reaction system is treated for 3 h, and samples are taken for detection every 1 h. The nitrate concentration of different cathode electrodes is as shown in Figure 2 in (a); the nitrogen production of different cathode electrodes is as shown in Figure 2 in (b); the hydrogen peroxide production of different anode electrodes is as shown in Figure 2 in (c).

[0053] Example 5

[0054] Combined with Figure 1 , using Figure 1 in the photoelectrocatalytic device to reduce nitrate in groundwater to nitrogen. The cathode is a Co-Cu2O / CFC electrode with a cobalt content of 0.25 mmol. The reaction system is treated for 3 h, and samples are taken for detection every 1 h. The change in nitrate concentration at the cathode is as shown in Figure 3 in (a); the change in nitrogen production at the cathode is as shown in Figure 3in (b); the nitrate removal rate, nitrogen selectivity and Faraday efficiency of the cathode, as shown in Figure 3 in (c).

[0055] Example 6

[0056] Combined with Figure 1 , using Figure 1 in the photoelectrocatalytic device to oxidize water to produce hydrogen peroxide, the anode is a CuW-C3N4 / CFC electrode with a tungsten content of 0.02 mol, the reaction system is treated for 3 h, and samples are taken for detection every 1 h. The hydrogen peroxide production of the anode is as shown in Figure 4 in (a); the hydrogen peroxide production rate of the anode, as shown in Figure 4 in (b).

Claims

1. A photoelectrocatalytic system for cathode reduction of nitrogen-containing groundwater denitrification coupled with anode oxidation of water to produce hydrogen peroxide, characterized in that: The photoelectrocatalytic system comprises an electrode chamber, an electrode, a diaphragm, an electrolyte solution, an external circuit, a visible light source, and auxiliary components; the electrode chamber comprises an anode chamber (9) and a cathode chamber (4), and the anode chamber (9) and the cathode chamber (4) are separated by a diaphragm; the electrolyte solution of the anode chamber (9) and the cathode chamber (4) enters and exits in a top-in-bottom-out sequential batch processing mode; the external circuit comprises a wire and a direct current power supply (1); the wire is used to connect the anode electrode (10) and the cathode electrode (5), the direct current power supply (1) is arranged between the anode electrode (10) and the cathode electrode (5), and a visible light source (7) is added above the electrodes to form a photoelectrocatalytic system; The anode catalyst material is a CuW-C3N4 composite catalyst, and the cathode catalyst material is a Co-Cu2O composite catalyst.

2. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater denitrification coupled with anode oxidation of water to produce hydrogen peroxide according to claim 1, characterized in that: The water inlet of the anode chamber (9) and the cathode chamber (4) is realized by adding electrolyte solution through a sampling port reserved above the electrode chamber and blocked by a rubber stopper, and the water outlet of the anode chamber (9) and the cathode chamber (4) is realized by discharging electrolyte solution through a water outlet connected by a rubber tube reserved below the electrode chamber.

3. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 1, characterized in that: The electrolyte solution in the anode chamber (9) is a sodium sulfate solution.

4. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 1, characterized in that: The diaphragm is a cation exchange membrane (11); the auxiliary components include a silicone gasket and a stud nut for sealing.

5. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 1, characterized in that: The electrolyte solution in the cathode chamber (4) is a groundwater solution containing nitrate ions.

6. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 1, characterized in that: The CuW-C3N4 / CFC electrode is prepared by a method of first high-temperature calcination and then sol coating; the Co-Cu2O / CFC electrode is prepared by a method of first immersion growth and then high-temperature calcination.

7. A photoelectrocatalytic system for cathode reduction of nitrogen-containing groundwater denitrification coupled with anode oxidation of water to produce hydrogen peroxide according to claim 6, characterized in that: The preparation method of CuW-C3N4 / CFC electrode is as follows: Step (1) W-C3N4 synthesis: urea and tungstic acid are dissolved in deionized water and mixed evenly, and then dried in an oven; then transferred to a muffle furnace for calcination to obtain a W-C3N4 catalyst; Step (2) Preparation of CuW-C3N4 / CFC electrode: basic copper carbonate and W-C3N4 catalyst are mixed and added to a silica sol solution to obtain a coating liquid, and the coating liquid is applied to both sides of the carbon fiber cloth to obtain a CuW-C3N4 / CFC electrode.

8. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 7, characterized in that: In the step (1), the molar ratio of urea to tungstic acid is 1:0.01-0.1; the drying temperature is 50-80°C, and the drying time is 4-8h; the calcination heating rate is 1-10°C / min; the calcination temperature is 500-600°C; the calcination time is 2-4h; in the step (2), the mass ratio of basic copper carbonate to W-C3N4 catalyst is 1:0.5-2; the mass content of the silica sol solution is 10-30%; the mass content of basic copper carbonate in the silica sol solution is 25%-75%.

9. The photoelectrocatalytic system of cathode reduction of nitrogen-containing groundwater for denitrification coupled with anode oxidation of water for hydrogen peroxide production according to claim 6, characterized in that: The preparation method of Co-Cu2O / CFC electrode is as follows: Dissolve copper sulfate pentahydrate in deionized water to form solution A; dissolve sodium hydroxide in deionized water to form solution B; then slowly pour solution B into solution A while stirring to obtain a catalyst precursor mixed solution, and then ultrasonicate the catalyst precursor mixed solution; immerse the carrier in the precursor mixed solution, take it out and put it in an oven to dry, then drop a solution containing cobalt salt, and then put it in an oven to dry; place the dried electrode in a tubular furnace, calcine it in an inert atmosphere, and then cool it naturally to obtain a Co-Cu2O / CFC electrode.

10. A photoelectrocatalytic system for cathode reduction of nitrogen-containing groundwater denitrification coupled with anode oxidation of water to produce hydrogen peroxide according to claim 9, characterized in that: The concentration of solution A is 1-2 mol / L; the concentration of solution B is 2-5 mol / L; the molar ratio of copper sulfate pentahydrate to sodium hydroxide is 1:2-3; the carrier is carbon fiber cloth, activated carbon, or carbon felt; the impregnation time is 10-30 min; the oven drying temperature is 50-80° C.; the drying time is 4-8 h; the cobalt salt is cobalt chloride, cobalt bromide, cobalt iodide, cobalt carbonate, or cobalt sulfate; the concentration is 0.1-0.5 mol / L; the mass ratio of the carrier to the solution containing the cobalt salt is 1:2-8; the calcination heating rate is 1-10° C. / min; the calcination temperature is 400-600° C.; and the calcination time is 2-4 h.

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