Electro-catalytic reduction membrane for high-selectivity electro-catalysis low-concentration nitrate water reductive denitrification and preparation method of electro-catalytic reduction membrane

Abstract

The invention discloses an electrocatalytic reduction membrane for high-selectivity electrocatalytic low-concentration nitrate water reductive denitrification and a preparation method of the electrocatalytic reduction membrane, and belongs to the technical field of water treatment. The method solves the problems that the removal efficiency of the low-concentration nitrate polluted water body is insufficient by adopting a conventional water treatment process, and the cost is high, the economical efficiency is poor and the environment is not protected when ammonia is directly recovered by adopting an electrocatalytic reduction method. The preparation method comprises the following steps: performing high-temperature pyrolysis on a copper-doped ZIF-8 material to obtain a Cu-CN / CB catalyst, mixing the Cu-CN / CB catalyst with PVDF and acidified CNT, and performing suction filtration to form the electrocatalytic reduction membrane. Carbon black is used as a core to grow copper-doped ZIF-8, the catalyst is obtained after carbonization, metal on the catalyst is uniformly distributed, catalytic sites are dense, and a conductive network is constructed by CNT; and by taking the reduction membrane as a cathode, optimizing the mass transfer efficiency of the catalyst and nitrate in a low-concentration nitrate water body through the CNT-Cu-CN / CB network, and fully completing nitrate adsorption-catalytic degradation in a confinement space.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to an electrocatalytic reduction membrane for highly selective electrocatalytic reduction and denitrification of low-concentration nitrate water and its preparation method. Background Technology

[0002] The nitrogen cycle is one of the core biogeochemical cycles maintaining the stability of Earth's ecosystems. Through processes such as nitrogen fixation, nitrification, denitrification, anaerobic ammonium oxidation, and organic nitrogen mineralization, it achieves the continuous transformation of nitrogen among air, water, and organisms. The nitrogen cycle regulates the availability of nitrogen in ecosystems, thereby controlling primary productivity, influencing microbial community structure, and maintaining the nutrient balance of soil and water. Simultaneously, the nitrogen cycle determines the forms and fates of nitrogen in the environment, profoundly impacting agricultural yields, freshwater quality, and global climate (such as N2O emissions). Therefore, the rational regulation and use of nitrogen in nature is of paramount importance.

[0003] Nitrate and ammonia nitrogen are the most common forms of inorganic nitrogen in natural water bodies and pollution sources, and their concentration changes directly determine the nitrogen nutrient level of the aquatic environment. When they accumulate to high concentrations in water bodies, they not only disrupt the ecosystem structure, causing abnormal algal proliferation and eutrophication, but may also generate toxic intermediates such as nitrite, posing a stress to aquatic organisms. Furthermore, high concentrations of nitrate and ammonia nitrogen migrate through surface water and groundwater, further threatening drinking water safety. For example, nitrate can be converted into nitrite in the human body, leading to health risks such as methemoglobinemia; ammonia nitrogen increases the formation of disinfection byproducts, affecting the stability of water treatment processes. Therefore, controlling the release and accumulation of nitrate and ammonia nitrogen in water bodies is a crucial prerequisite for ensuring water quality and drinking water safety. Compared to electrocatalytic nitrate recovery for ammonia resource recovery, lower concentration nitrate methods have low Faraday efficiency and poor economic benefits. Meanwhile, my country's "Standards for Drinking Water Quality (GB5849-2022)" stipulates that nitrate concentrations should be ≤10 mg N / L and ammonia ≤0.5 mg N / L to ensure that drinking water is within safe thresholds for human health. However, conventional water treatment processes (coagulation-sedimentation-filtration-disinfection) have limited nitrate removal capabilities. When the raw water nitrate concentration is between 10-50 mg N / L, relying on conventional water treatment processes or recovering ammonia through nitrate reduction is uneconomical and environmentally unfriendly. Therefore, there is an urgent need to develop a new denitrification method suitable for water bodies with low nitrate pollution, possessing both high efficiency and cost-effectiveness, and being environmentally friendly, to achieve rapid denitrification of nitrates from water bodies. Summary of the Invention

[0004] This invention addresses the shortcomings of conventional water treatment processes for removing low-concentration nitrate contaminated water, which are insufficient, while electrocatalytic reduction for direct ammonia recovery is costly, uneconomical, and environmentally unfriendly. It provides an electrocatalytic reduction membrane for highly selective electrocatalytic reduction and denitrification of low-concentration nitrate water.

[0005] The technical solution of the present invention is as follows: One objective of this invention is to provide a method for preparing an electrocatalytic reduction membrane for highly selective electrocatalytic denitrification of low-concentration nitrate solutions, the method comprising: Step 1: Add copper source, zinc source and carbon black to methanol to form dispersion A, dissolve 2-methylimidazole in methanol to form solution B, mix dispersion A and solution B, sonicate reaction, and centrifuge, wash and vacuum dry the product to obtain copper-doped ZIF-8 material (Cu-ZIF-8 / CB) loaded on carbon black. Step 2: The copper-doped ZIF-8 material loaded on carbon black is pyrolyzed at high temperature in an inert gas environment. The product is then acid-washed and vacuum-dried to obtain the Cu-CN / CB catalyst. Step 3: Add PVDF to DMF, heat to dissolve, and obtain DMF solution. After cooling, add Cu-CN / CB catalyst and acidified multi-walled carbon nanotubes (CNTs) to the DMF solution. After ultrasonic dispersion and cell disruption, obtain base film dispersion. Step 4: The base membrane dispersion is filtered in batches onto the inorganic membrane. After the last filtration, a wet membrane is obtained on the inorganic membrane. The wet membrane is rinsed, dried and peeled off to obtain the electrocatalytic reduction membrane (Cu-CN / CB-EM).

[0006] Further specifying, the mass ratio of copper source, zinc source and carbon black in step 1 is (0.1-0.8):(2.0-5.0):(0.05-0.2).

[0007] Further specified, the ultrasonic reaction power in step 1 is 200-300 W, and the reaction time is 30-60 min.

[0008] Furthermore, the ultrasonic response power is 300 W and the response time is 60 min.

[0009] Further specified, the centrifugation intensity in step 1 is 5000-7000 g, and the centrifugation time is 5-15 min.

[0010] Further specified, the centrifugation intensity was 5000 g and the centrifugation time was 10 min.

[0011] Further specifying, the inert gas in step 2 is nitrogen, argon, or helium.

[0012] To be further specified, the inert gas is argon.

[0013] Further specifying, the high-temperature pyrolysis temperature in step 2 is 900-1000 ℃, and the time is 2-6 h.

[0014] Further specifying, the high-temperature pyrolysis temperature is 950 ℃, and the time is 4 h.

[0015] Further specifying, the pickling solution in step 2 is sulfuric acid or hydrochloric acid.

[0016] To be further specified, the pickling solution is sulfuric acid.

[0017] Further specifying, the pickling process involves refluxing the product with a 2 mol / L pickling solution at 80°C for 12 hours.

[0018] Furthermore, the acid washing process involves reflux washing the product with 2 mol / L sulfuric acid at 80°C for 12 h.

[0019] Further specifying, the mass ratio of PVDF, Cu-CN / CB catalyst and multi-walled carbon nanotubes in step 3 is (18.88-75.52):(40-60):(40-60).

[0020] Further specifying, the copper doping content in the Cu-CN / CB catalyst of step 2 is 0.2-2 wt%.

[0021] Further specifying, the solution used in the acidification process in step 3 is at least one of hydrochloric acid, sulfuric acid, and nitric acid.

[0022] Further specifying, the acidification process in step 3 is as follows: multi-walled carbon nanotubes are acidified with an acidification solution at 90 °C for 6 h, then washed with deionized water until neutral, and freeze-dried.

[0023] To further specify, the acidification process is as follows: multi-walled carbon nanotubes are acidified at 90°C for 6 hours with an acidification solution composed of nitric acid and sulfuric acid in a volume ratio of 1:3, then washed with deionized water until neutral, and freeze-dried.

[0024] Further specified, the cell disruption power in step 3 is 9-900 W, and the disruption time is 10-50 min.

[0025] Further specified, the crushing power is 60 W and the crushing time is 30 min.

[0026] Further specifying, the drying process in step 4 is as follows: first, dry at 30-60 ℃ for 1-8 h, then dry at room temperature for 1-12 h, and finally dry at 30-60 ℃ for 2-8 h.

[0027] To further specify, the drying process is as follows: first, dry at 60 ℃ for 1 h, then dry at room temperature for 12 h, and finally dry at 60 ℃ for 1 h.

[0028] The second objective of this invention is to provide an electrocatalytic reduction membrane obtained by the above preparation method for highly selective electrocatalytic reduction and denitrification of low-concentration nitrate water.

[0029] The third objective of this invention is to provide a method for efficient denitrification by electrocatalytic reduction of electrolyte containing low-concentration nitrate. This method uses the aforementioned electrocatalytic reduction membrane as the cathode and a titanium mesh, ruthenium-iridium titanium mesh, foamed titanium, or foamed ruthenium-iridium titanium as the anode. An insulating and water-permeable layer is placed between the anode and the cathode, and electrocatalytic denitrification is completed in an electrolyte solution with low nitrate concentration.

[0030] Further specifying, the mesh size of titanium mesh, ruthenium-iridium titanium mesh, or foamed titanium is 600 mesh.

[0031] Further specifying, the anode material is foamed ruthenium-iridium-titanium.

[0032] Further specifying, the insulating and permeable layer uses a sand core, a PTFE membrane with a pore size of 0.45μm, or a PVDF membrane with a pore size of 0.45μm. Further specifying, the insulating and permeable layer uses a PTFE membrane with a pore size of 0.45μm.

[0033] Further specified, the nitrate concentration in the electrolyte solution is 20 mg N / L.

[0034] Furthermore, the electrolyte solution also includes Na₂SO₄ at a concentration of 20 mmol / L and NaCl at a concentration of 5 mmol / L. Further specifying, the current density for the electrocatalytic denitrification process is 1-3 mA / cm². 2 .

[0035] The beneficial effects of this invention are as follows: This invention involves carbonizing a metal-doped MOF material, specifically copper-doped ZIF-8, and then mixing it with PVDF and acidified CNTs to prepare an electrocatalytic reduction membrane. This membrane is used as the cathode, and a metal mesh or foam-like metal is used as the anode. An insulating, water-permeable layer separates the cathode and anode to prevent short circuits. This allows for highly efficient reduction and denitrification of nitrates in a low-concentration nitrate solution. Compared with existing technologies, this invention also has the following advantages: (1) This invention involves the in-situ growth of copper-doped ZIF-8 using carbon black particles as the core, followed by high-temperature pyrolysis and carbonization to form a Cu-CN / CB catalyst. Carbon black serves as a carrier during the synthesis of copper-doped ZIF-8, while the metal source and 2-methylimidazole form ZIF-8 material on the carbon black surface. After high-temperature pyrolysis of Cu-ZIF-8 / CB, zinc sublimates and detaches, leaving Cu as Cu... x -Ny (x is 1, 2, or s, representing the existence of single-atom, diatomic, or copper nanoclusters, respectively; y is commonly 3, 4, 6, etc., and together with x, describes the copper-nitrogen coordination structure.) The coordination form exists in the framework after ZIF-8 carbonization and remains loaded on the carbon black surface, denoted as Cu-CN / CB, and nitrogen-doped carbon (CB) is formed after ZIF-8 carbonization. Furthermore, during the preparation of the Cu-CN / CB catalyst, the copper doping amount is controlled to be 0.2-2 wt%, resulting in uniform copper distribution on the catalyst, and the catalytic sites on the Cu-CN / CB surface are dense due to the increase in ZIF-8. Therefore, the electrocatalytic reduction membrane exhibits excellent electrochemical characteristics, with highly efficient nitrate adsorption and conversion capabilities. Moreover, the electrocatalytic reduction membrane of this invention is simple to prepare and has stable performance.

[0036] (2) The Cu-CN / CB particles formed by the size difference of the carbon black support in this invention have a rich variety of sizes. This variety fills the gaps between the catalyst and CNTs during the preparation of the electrocatalytic membrane, resulting in smaller pores and a denser structure in the Cu-CN / CB-EM carbon-based conductive membrane formed by multiple filtrations after blending the catalyst and CNTs. The carbonized porous carbon framework has a strong adsorption capacity, allowing wastewater containing low concentrations of nitrates to have a longer residence time after filtration through Cu-CN / CB-EM. Simultaneously, the porous carbon framework has channels or cavities, forming confined spaces within which Cu... x -N y The active site enables highly efficient catalytic reduction and denitrification of nitrates.

[0037] (3) The cathode catalytic membrane prepared in this invention is based on a conductive network constructed from carbon nanotubes, with a uniformly distributed and dense catalytic core. In low-concentration nitrate water, the carbon nanotube-Cu-CN / CB network optimizes the mass transfer efficiency of the catalyst and nitrate in the water, fully completing adsorption-catalytic degradation within the confined space. The flow field assists in avoiding excessive hydrogenation to produce ammonia while simultaneously carrying away the nitrogen product to avoid interfering with the electric field, thus achieving continuous denitrification. The foamed ruthenium-iridium-titanium anode oxidizes chloride ions in the solution to free chlorine and oxidizes the excessive hydrogenation product ammonia to nitrogen, ensuring denitrification efficiency. (4) This invention proposes a scheme for the continuous catalytic reduction of nitrate and highly selective denitrification of copper-doped ZIF-8 material after carbonization, based on a carbon-based conductive framework (a conductive network formed by multi-walled carbon nanotubes). After carbonization, copper-doped ZIF-8 material achieves efficient adsorption of nitrate in a confined space. The catalytic sites achieve rapid nitrate reduction in response to the electric field. At the same time, the flow field further mediates and regulates the adsorption residence time of nitrogen intermediates, controlling the nitrogen intermediates to achieve nitrogen-nitrogen coupling denitrification in the confined space, avoiding excessive hydrogenation reduction to produce ammonia. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the electrocatalytic nitrate denitrification device in Examples 1-3; Figure 2 The image shows a SEM image of the Cu2-CN / CB catalyst from Example 1. Figure 3 The image shows the Raman spectrum of the Cu2-CN / CB catalyst in Example 1. Figure 4 The XRD pattern of the Cu2-CN / CB catalyst in Example 1 is shown below. Figure 5 The graph shows the electrocatalytic reduction denitrification efficiency of Cu1-CN / CB-EM using Example 2; Figure 6 The graph shows the electrocatalytic reduction denitrification efficiency of Cu2-CN / CB-EM using Example 1. Figure 7 Cu using Example 3 s -CN / CB-EM electrocatalytic reduction denitrification efficiency diagram; Figure 8 The figure shows the electrocatalytic denitrification efficiency of CN / CB-EM using Comparative Example 1. Detailed Implementation

[0039] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the examples in the specification.

[0040] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0041] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0042] Unless otherwise specified, the experimental methods used in the following examples are conventional methods. Unless otherwise specified, the materials, reagents, methods, and instruments used are all conventional materials, reagents, methods, and instruments in the art, and can be obtained commercially by those skilled in the art.

[0043] In the following examples, M represents mol / L.

[0044] Example 1 In this embodiment, the electrocatalytic reduction membrane is Cu2-CN / CB-EM, and the preparation method includes the following steps: Step 1: Dissolve 0.45 g Cu(NO3)2·3H2O and 3 g Zn(NO3)·6H2O in 40 mL of methanol, and simultaneously add 50 mg of carbon black with a particle size of 300-500 nm to prepare dispersion A. Dissolve 6 g 2-methylimidazole in 80 mL of methanol to prepare solution B. Mix dispersion A and solution B rapidly at 500 rpm to obtain a mixed dispersion. Sonicate the mixed solution at 300 W for 60 min to obtain a suspension. Centrifuge the suspension at 5000 g for 10 min, take the precipitate, add 10 mL of methanol, vortex for 3 min, and then continue to centrifuge at 5000 g for 10 min. Repeat the process of taking the precipitate, adding methanol, and continuing to centrifuge for 10 min three times. Place the washed substrate in a vacuum dryer at 60 ℃ for 6 h to obtain copper-doped ZIF-8 material. Step 2: Take 200 mg of copper-doped ZIF-8 material, grind it thoroughly in a mortar, and heat it to 950℃ at a rate of 5℃ / min under argon protection, hold it at that temperature for 240 min, then cool it to 400℃ at a rate of 5℃ / min, and allow it to cool naturally to room temperature. Grind the sintered material in a mortar, acidify it with 50 mL of 2M sulfuric acid at 80℃ for 12 h, filter it, and vacuum dry the obtained solid material. Grind it in a mortar to obtain Cu2-CN / CB catalyst. The copper doping content in the catalyst was determined to be 0.89 wt% by microwave digestion and inductively coupled plasma mass spectrometry (ICP-MS). Step 3: Take 100 mg of multi-walled carbon nanotubes (length 10-30 μm, diameter 5-15 nm), acidify and reflux at 90 °C for 6 h with a solution of concentrated nitric acid (8 M) and concentrated sulfuric acid (18.4 M) in a volume ratio of 1:3, wash with pure water until the carbon nanotubes are neutral (pH=7.0±0.5), and freeze dry (cold trap temperature -53 °C) for 24 h; PVDF dried under vacuum for 6 h was dissolved in 40 mL of DMF to prepare a DMF solution with a mass fraction of 0.3%. 50 mg of the acidified CNT and 50 mg of Cu2-CN / CB were dispersed in the above DMF solution and ultrasonically dispersed at 300 W for 10 min. Then, the dispersion was obtained by ultrasonic cell disruptor at 60 W for 30 min. Step 4: Using a PTFE filtration assembly, the base membrane dispersion was vacuum filtered multiple times (5 mL / time) onto the surface of a 4 cm diameter circular ceramic membrane. After all the base membrane dispersion was filtered, the wet membrane formed on the ceramic membrane surface was rinsed with ultrapure water, and then the wet membrane along with the ceramic membrane was soaked in pure water for 1 h to complete the precipitation of residual DMF solvent and the transformation of PVDF from liquid to solid phase. Subsequently, it was dried in a 60 ℃ oven for 1 h, dried at room temperature for 12 h, and dried at 60 ℃ for 1 h in sequence, and then peeled off from the ceramic membrane to obtain the electrocatalytic reduction membrane, denoted as Cu2-CN / CB-EM.

[0045] The Cu2-CN / CB-EM obtained in this embodiment was used as a cathode in an electrocatalytic nitrate denitrification device. The specific process is as follows: (1) Place the ruthenium-iridium-titanium foam electrode in 30 mL of acetone, 30 mL of isopropanol and 30 mL of ultrapure water in sequence, and ultrasonically clean it in each solvent for 30 min. After the final cleaning, vacuum dry it at 40 °C for 3 h to remove the residual organic matter on the electrode surface. (2) Using ruthenium-iridium-titanium foam electrodes and Cu2-CN / CB-EM catalytic membranes for Figure 1 In the device shown, according to Figure 1 In the device, Cu2-CN / CB-EM, 0.45 μm PTFE filter membrane, and ruthenium-iridium-titanium foam electrode are arranged sequentially along the water flow direction (in the direction indicated by the three vertical arrows), with Cu2-CN / CB-EM and ruthenium-iridium-titanium foam electrode serving as the cathode and anode, respectively.

[0046] Figure 2 This is a SEM image of the Cu2-CN / CB catalyst in this embodiment. Figure 3 The image shows a Raman spectrum. Figure 4 The XRD patterns show the microstructure and composition of the Cu2-CN / CB material. The SEM images show that Cu-ZIF-8 retains a polyhedral structure after carbonization, with monomer sizes ranging from 100 to 200 nm. The Raman spectra clearly show broad D and G bands, typical of carbon black, and combined with the SEM images, confirm that carbon black acts as a core encapsulated within Cu-CN. The XRD patterns do not show typical crystalline peaks for Cu(0), CuO, and Cu2O, indicating that copper in Cu2-CN / CB does not exist as nanoclusters or in an oxidized or crystalline state, but rather in a copper-nitrogen coordinated state.

[0047] Example 2 The difference between this embodiment and Example 1 is that the prepared electrocatalytic reduction membrane is Cu1-CN / CB-EM, the amount of Cu(NO3)2·3H2O in step 1 is 0.10 g, the remaining process operations and parameter settings are the same as in Example 1, and the amount of copper doping in the Cu1-CN / CB catalyst is 0.26%.

[0048] Example 3 The difference between this embodiment and Example 1 is that the prepared electrocatalytic reduction membrane is Cu. s -CN / CB-EM, in step 1, Cu(NO3)2·3H2O is 0.8 g, and the remaining process operations and parameter settings are the same as in Example 1. s The copper doping content in the -CN / CB catalyst is 1.77%.

[0049] Comparative Example 1 The difference between this embodiment and Embodiment 1 is that Cu(NO3)2·3H2O is 0 in step 1, while the remaining process operations and parameter settings are the same as in Embodiment 1, and the prepared reduction film is CN / CB-EM.

[0050] Application examples In actual production and daily life, low concentrations of nitrate pollute water bodies while also containing Cl. This invention uses a 20 mg N / L NaNO3 + 20 mM Na2SO4 + 5 mM NaCl solution as raw water to simulate an actual water body for an electrocatalytic nitrate reduction and denitrification experiment. The raw water is placed in a raw water tank, and the effective filtration area of ​​the membrane electrode is 5.725 cm². 2 The catalytic membrane effluent flux was maintained at a constant pressure of 80 kPa at 0.5 mL / min, and a cross-flow of 50 mL / min was achieved on the cathode membrane surface via a pump. A pressure of 1-3 mA / cm² was applied between the cathode and anode. 2 The current density was used to investigate the nitrate conversion efficiency and denitrification capacity at different current densities. To detect different nitrogen products, nitrate concentration was determined by ion chromatography, and ammonia concentration was determined by Nessler's reagent spectrophotometry. The nitrate conversion efficiency, ammonia, and nitrogen yield at different current densities in Examples 1-3 and Comparative Example 1 are shown below. Figure 6 , 5 7 and 8, as well as Table 1.

[0051] Table 1

[0052] From Table 1 and Figure 5-8The data shows that when using the method of this invention to achieve denitrification in water bodies polluted with low concentrations of chlorine and nitrate, nitrate ions in the water first undergo adsorption-catalytic reduction-desorption by the catalyst in the cathode membrane, forming nitrogen, nitrite, and ammonia as products. Following the water flow direction, when the nitrate reaches the ruthenium-iridium-titanium foam anode, the nitrate ions in the water... Oxidized to a form with a higher redox potential, ammonia formed on the cathode catalytic membrane can be oxidized to nitrogen gas, further enhancing the denitrification effect. Comparative Example 1, without copper doping, exhibits low nitrate conversion rate and nitrogen production efficiency, indicating that copper doping can significantly improve the electrocatalytic reduction efficiency of nitrate. Furthermore, compared to Examples 2 and 3, Example 1 has the most suitable copper doping amount, which is beneficial for the electrocatalytic denitrification of nitrate. At 3 mA / cm²... 2 At this current density, the effluent exhibits a nitrate removal rate of over 95%, indicating that the cathode achieves the highest nitrate conversion rate. The ruthenium-iridium-titanium foam anode can effectively remove Cl from the water. Oxidation converts any ammonia that may be produced into nitrogen gas, achieving more efficient denitrification, ensuring that no further ammonia pollution is generated in the effluent, and ensuring that the effluent meets the "Standards for Drinking Water Quality (GB5849-2022)".

[0053] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing an electrocatalytic reduction membrane for highly selective electrocatalytic denitrification of low-concentration nitrate solutions, characterized in that, The method includes: Step 1: Add copper source, zinc source and carbon black to methanol to form dispersion A, dissolve 2-methylimidazole in methanol to form solution B, mix dispersion A and solution B, sonicate reaction, and centrifuge, wash and vacuum dry the product to obtain copper-doped ZIF-8 material loaded on carbon black. Step 2: The copper-doped ZIF-8 material loaded on carbon black is pyrolyzed at high temperature in an inert gas environment. The product is then acid-washed and vacuum-dried to obtain the Cu-CN / CB catalyst. Step 3: Add PVDF to DMF, heat to dissolve, and obtain DMF solution. After cooling, add Cu-CN / CB catalyst and acidified multi-walled carbon nanotubes to the DMF solution. After ultrasonic dispersion and cell disruption, obtain base film dispersion. Step 4: The base membrane dispersion is filtered in batches onto the inorganic membrane. After the last filtration, a wet membrane is obtained on the inorganic membrane. The wet membrane is washed, dried and peeled off to obtain the electrocatalytic reduction membrane.

2. The preparation method according to claim 1, characterized in that, In step 1, the mass ratio of copper source, zinc source and carbon black is (0.1-0.8):(2.0-5.0):(0.05-0.2).

3. The preparation method according to claim 2, characterized in that, In step 1, the ultrasonic reaction power is 200-300 W, the reaction time is 30-60 min, the centrifugation intensity is 5000-7000 g, and the centrifugation time is 5-15 min.

4. The preparation method according to claim 1, characterized in that, In step 2, the high-temperature pyrolysis temperature is 900-1000℃ and the time is 2-6 h; the pickling solution is sulfuric acid or hydrochloric acid, and the pickling process is to reflux the product with a 2 M pickling solution at 80℃ for 12 h.

5. The preparation method according to claim 1, characterized in that, In step 3, the mass ratio of PVDF, Cu-CN / CB catalyst and multi-walled carbon nanotubes is (18.88-75.52):(40-60):(40-60); the solution used in the acidification process is at least one of hydrochloric acid, sulfuric acid, and nitric acid, and the acidification process is as follows: the multi-walled carbon nanotubes are acidified in the acidification solution at 90 °C for 6 h, then washed with deionized water until neutral, and freeze-dried; the cell disruption power is 9-900 W, and the disruption time is 10-50 min.

6. The preparation method according to claim 1, characterized in that, In step 4, 5 mL of each batch is filtered; the drying process is as follows: first, dry at 30-60 ℃ for 1-8 h, then dry at room temperature for 1-12 h, and finally dry at 30-60 ℃ for 2-8 h.

7. An electrocatalytic reduction membrane for highly selective electrocatalytic denitrification of low-concentration nitrate water by the preparation method according to any one of claims 1-6.

8. A method for efficient nitrogen removal by electrocatalytic reduction of a solution containing low-concentration nitrate, characterized in that, The method uses the electrocatalytic reduction membrane prepared by the method described in any one of claims 1-6 as the cathode, and titanium mesh, ruthenium-iridium titanium mesh, foamed titanium or foamed ruthenium-iridium titanium as the anode. An insulating and water-permeable layer is placed between the anode and the cathode, and electrocatalytic denitrification is completed in an electrolyte solution with a low nitrate concentration.

9. The method according to claim 8, characterized in that, The nitrate concentration in the electrolyte solution was 20 mg N / L, and the current density for the electrocatalytic denitrification process was 1-3 mA / cm². 2 .

10. The method according to claim 8 or 9, characterized in that, The electrolyte solution also includes Na₂SO₄ at a concentration of 20 mmol / L and NaCl at a concentration of 5 mmol / L.

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