A harmless treatment method for nitrate wastewater based on microwave-assisted catalytic reduction

Abstract

The application particularly relates to a harmless treatment method of nitrate wastewater based on microwave-assisted catalytic reduction, and the technical scheme is as follows: according to the mass ratio of catalyst to nitrate wastewater of 0.001-0.1:1, the catalyst is added into the nitrate wastewater to obtain a mixed solution; then 1-5wt% of a cavity scavenger in the mixed solution is added into the mixed solution, hydrochloric acid is used to adjust the pH of the mixed solution to 1-6, the mixed solution is stirred in a dark room for 1-2h to obtain a suspension I; then the suspension I is placed into a microwave reaction device, and is reacted under the microwave irradiation of 0.3-20GHz for 60-120min to obtain nitrogen gas based on the harmless treatment of the nitrate wastewater by the microwave-assisted catalytic reduction. The application has the advantages of simple operation, low cost, low energy consumption, and the prepared catalyst can be repeatedly used and easily used in large scale, the method has high conversion rate of nitrate, no secondary pollution, high selectivity of target product, and can realize the harmless removal of nitrogen.

Description

Technical Field

[0001] This invention belongs to the technical field of harmless treatment of nitrate wastewater. Specifically, it relates to a method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction. Background Technology

[0002] Direct discharge of nitrogen-containing wastewater from industries such as chemical, pharmaceutical, and electroplating, along with excessive application of nitrogen fertilizers in agriculture, has led to a large accumulation of reactive nitrogen in the environment, resulting in severe nitrate pollution of surface and groundwater. While nitrate itself has relatively low toxicity, it can be reduced to nitrite (NO2) in the human body. - The latter reacts with amines to form highly carcinogenic nitrosamines, and can also cause diseases such as methemoglobinemia in infants (blue baby syndrome), posing a long-term potential threat to ecosystems and human health. Therefore, how to control high concentrations and high mobility of NO3 in water bodies is a crucial issue. The efficient and economical conversion of nitrates into harmless substances, especially environmentally friendly nitrogen (N2), to achieve complete nitrogen removal has become a key challenge in water pollution control. Among existing technologies, biological reduction, chemical reduction, electrocatalysis, and photocatalysis are the main pathways for nitrate reduction and conversion, but all have significant limitations. For example: The patented technology, "An Anaerobic Biodegradation Bacterium for Dissimilatory Nitrate Reduction to Ammonia Nitrogen by Polycyclic Aromatic Hydrocarbons and Its Application" (CN202510442263.6), discloses a strain of *Mesochrysis rubra* named *PheICNA*. This strain can degrade polycyclic aromatic hydrocarbons such as phenanthrene under anaerobic conditions and simultaneously reduce nitrate to ammonium nitrogen. Although this method can achieve the degradation of organic pollutants and the conversion of nitrate, its essence is the reduction of nitrate to ammonia nitrogen rather than harmless nitrogen gas. The generated ammonia nitrogen is still in the form of biologically active nitrogen, and it does not fundamentally achieve the complete and harmless removal of nitrogen. Furthermore, there is a risk of ammonia nitrogen accumulating in the environment and causing secondary pollution, which cannot meet the requirements for deep denitrification and harmless treatment of high-nitrogen-load wastewater.

[0003] The patented technology, "Preparation Method and Application of Aluminum-Copper-Carbon Composite Material for High-Efficiency Reduction of Nitrate to Nitrogen" (CN202211606860.0), discloses a method for selectively converting nitrate to nitrogen at room temperature using an aluminum-copper-carbon ternary composite as a solid reducing agent. While this method utilizes inexpensive raw materials and has high reduction efficiency, it is a heterogeneous chemical reduction process requiring continuous addition of a solid reducing agent. The reaction produces a large amount of solid residue, resulting in various byproducts. Subsequent solid-liquid separation steps are cumbersome and costly, and may also introduce new solid waste disposal problems.

[0004] The patented technology, "Preparation Method of Single-Atom Transition Metal / Nitrogen-Doped Mesoporous Carbon Material for Electrocatalytic Reduction of Nitrate to Nitrogen," (CN202211147183.6), provides an electrocatalytic technology that uses nitrogen-doped mesoporous carbon material loaded with single atoms of iron / cobalt / nickel as the cathode catalyst to achieve nitrate reduction. While this method exhibits good performance in terms of nitrogen selectivity and catalyst cycle stability, its catalyst synthesis process is complex, requires strict condition control, and the electrocatalytic process demands a continuous power supply and uses high-cost electrode materials, resulting in high equipment investment and operating energy consumption. Furthermore, the presence of competing side reactions such as hydrogen evolution at the cathode reduces the system's current efficiency and the target nitrogen conversion efficiency.

[0005] Cu2O-Cu 0 @Fe 0 The paper "Selective Reduction of Nitrate to Nitrogen by Composite Material Combined with HCOOH under Ultraviolet Radiation" (CN201811401858.0) proposes a photocatalytic reduction method that uses a specific composite material and reduces nitrate under ultraviolet light irradiation and in the presence of formic acid. Although this method can achieve a high nitrate conversion rate, its reliance on ultraviolet light sources leads to high energy consumption, and the continuous addition of formic acid as a hole sacrificial agent or reduction aid not only increases reagent consumption and operating costs, but the residual formic acid also increases the chemical oxygen demand (COD) of the effluent, causing secondary pollution.

[0006] In summary, existing nitrate catalytic reduction technologies suffer from several drawbacks, including complex processes, difficulties in catalyst preparation or recovery, the need for chemical additives, high energy consumption, secondary pollution, and high equipment investment and maintenance costs. Furthermore, they fail to efficiently reduce NO3-. Directed conversion and deep denitrification and harmless treatment of wastewater with high nitrogen load are insufficient to meet the pollution control requirements of nitrate in large-scale water bodies. Summary of the Invention

[0007] The present invention aims to overcome the shortcomings of the existing technology and provides a simple, low-cost and low-energy-consumption method for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction. The catalyst prepared by this method is easy to use on a large scale and can be reused. The method has a high nitrate conversion rate, no secondary pollution, and high selectivity for target products, and can achieve harmless removal of nitrogen.

[0008] To achieve the above objectives, the technical solution of the present invention comprises the following steps: According to the mass ratio of catalyst to nitrate wastewater of 0.001~0.1:1, the catalyst is added to the nitrate wastewater to obtain a mixed solution; then, 1~5wt% of a cavitation scavenger is added to the mixed solution, and the pH of the mixed solution is adjusted to 1~6 with hydrochloric acid. The solution is stirred in a dark room for 1~2 hours to obtain suspension I; then, suspension I is placed in a microwave reactor and reacted under microwave irradiation conditions of 0.3~20GHz for 60~120 minutes to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0009] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3~5:3~6:70~80:20~30, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene, and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 180~220°C for 10~14 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0010] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3~4:2~3:200~210:90~100, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe2O4 powder in a mass ratio of 1:0.14~1.14 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven for hydrothermal reaction at 160~200°C for 8~12 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe2O4@ZnIn2S4 composite catalyst.

[0011] The nitrate wastewater is one of the following: domestic wastewater, industrial wastewater, or agricultural wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.001~1g / L.

[0012] The cavitation agent is one of formic acid, ethylene glycol, oxalic acid, isopropanol, and ascorbic acid.

[0013] The microwave reaction device is one of a household microwave oven, an industrial microwave oven, and a laboratory microwave reactor.

[0014] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2-4 hours.

[0015] The first washing involves washing the solid reaction product I with anhydrous ethanol 3 to 4 times.

[0016] The temperature for the first drying is 60~80ºC, and the drying time is 30~60min.

[0017] The stirring device used for the second magnetic stirring is the same as the magnetic stirrer used in the first stirring, and the stirring time for the second magnetic stirring is 0.5 to 1 hour.

[0018] The second washing involves washing the solid reaction product II with anhydrous ethanol 3 to 4 times.

[0019] The second drying process is the same as the first drying process.

[0020] Due to the adoption of the above technical solution, the present invention has the following positive effects and outstanding features compared with the prior art: 1. The catalyst raw materials used in this invention are readily available, and costs can be effectively controlled even during large-scale production. Furthermore, the catalyst is magnetic, allowing for rapid separation and recovery from the reaction system via magnetic separation technology, resulting in good reusability and further reducing production costs and process energy consumption.

[0021] 2. The microwave heating method used in this invention has excellent penetration, uniform heating characteristics, and selective heating advantages. Compared with photocatalysis and electrocatalysis technologies, microwaves can selectively heat catalysts, thereby concentrating microwave energy on the catalytic reduction process of nitrates, significantly improving energy utilization efficiency, reducing energy consumption, and thus greatly saving production costs.

[0022] 3. The microwave catalysis method involved in this invention utilizes the unique "hot spot effect" of microwave radiation to induce the generation of highly active "hot electrons," thereby driving the nitrate reduction reaction. This "hot spot effect" originates from the localized high-temperature microregions formed on the catalyst surface due to molecular friction and vibration under microwave irradiation. These microregions can excite "hot electrons" with high energy and catalytic activity, thus increasing the reaction rate of nitrate reduction to environmentally friendly nitrogen gas and achieving a high nitrate conversion rate and nitrogen selectivity. NO3 The conversion rate was 72.71-100%, and the N2 selectivity was 87.31-100%.

[0023] 4. The microwave catalysis process provided by this invention is simple and convenient. The reduction reaction can be completed simply by mixing the catalyst with a nitrate-containing solution and then irradiating it in a microwave field. Currently, industrial-grade microwave reactors are relatively mature, supporting the large-scale application of this technology.

[0024] Therefore, this invention is not only simple to operate, low in cost and energy consumption, but also produces a catalyst that can be reused and is easy to scale up. The method has a high nitrate conversion rate, no secondary pollution, and high selectivity for the target product, and can achieve harmless removal of nitrogen. Attached Figure Description

[0025] Figure 1 X-ray diffraction pattern of a ZnFe2O4@ZnIn2S4 composite microwave catalyst prepared in this invention; Figure 2 for Figure 1 The image shown is a scanning electron microscope image of the ZnFe2O4@ZnIn2S4 composite microwave catalyst. Figure 3 for Figure 1 The image shows the magnetic separation effect of the ZnFe2O4@ZnIn2S4 composite microwave catalyst. Figure 4 for Figure 1 The ZnFe2O4@ZnIn2S4 composite microwave catalyst was used to microwave treat NO3 in a 100 mg / L nitrate solution. Graph showing the changes in conversion rate and N2 selectivity over reaction time. Detailed Implementation

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments are only used to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0027] A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this specific embodiment is as follows: According to the mass ratio of catalyst to nitrate wastewater of 0.001~0.1:1, the catalyst is added to the nitrate wastewater to obtain a mixed solution; then, 1~5wt% of a cavitation scavenger is added to the mixed solution, and the pH of the mixed solution is adjusted to 1~6 with hydrochloric acid. The solution is stirred in a dark room for 1~2 hours to obtain suspension I; then, suspension I is placed in a microwave reactor and reacted under microwave irradiation conditions of 0.3~20GHz for 60~120 minutes to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0028] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3~5:3~6:70~80:20~30, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene, and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 180~220°C for 10~14 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0029] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3~4:2~3:200~210:90~100, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe2O4 powder in a mass ratio of 1:0.14~1.14 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven for hydrothermal reaction at 160~200°C for 8~12 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe2O4@ZnIn2S4 composite catalyst.

[0030] The nitrate wastewater is one of the following: domestic wastewater, industrial wastewater, or agricultural wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.001~1g / L.

[0031] The cavitation agent is one of formic acid, ethylene glycol, oxalic acid, isopropanol, and ascorbic acid.

[0032] The microwave reaction device is one of a household microwave oven, an industrial microwave oven, and a laboratory microwave reactor.

[0033] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2-4 hours.

[0034] The first washing involves washing the solid reaction product I with anhydrous ethanol 3 to 4 times.

[0035] The temperature for the first drying is 60~80ºC, and the drying time is 30~60min.

[0036] The second magnetic stirring time is 0.5~1h.

[0037] The second washing involves washing the solid reaction product II with anhydrous ethanol 3 to 4 times.

[0038] In this specific implementation: The second drying process is the same as the first drying process.

[0039] The stirring device used for the second magnetic stirring is the same as the magnetic stirrer used in the first stirring.

[0040] The zinc chloride, ferric chloride hexahydrate, sodium acetate, ethylene glycol, and polyethylene glycol mentioned in step 1, as well as the zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol mentioned in step 2, are all of industrial grade or higher purity. The zinc chloride mentioned in step 1 and the zinc chloride mentioned in step 2 are the same.

[0041] The details will not be repeated in the examples.

[0042] Example 1 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.001:1 to obtain a mixed solution. Then, 5 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 2 with hydrochloric acid. The solution was stirred in a dark room for 1 hour to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under microwave irradiation at 2.45 GHz for 60 minutes to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0043] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3.97:3.61:73.5:24.8, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 200°C for 10 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0044] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.25:2.2:208:92.5, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.2 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 180°C for 10 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0045] The nitrate wastewater is domestic wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.1 g / L.

[0046] The hole scavenger is formic acid.

[0047] The microwave reaction device is a laboratory microwave reactor.

[0048] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2 hours.

[0049] The first wash involves washing the solid reaction product I three times with anhydrous ethanol.

[0050] The temperature for the first drying step was 60ºC, and the drying time was 30 minutes.

[0051] The second magnetic stirring time is 0.5 hours.

[0052] The second washing involves washing the solid reaction product II four times with anhydrous ethanol.

[0053] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate is 100%, and the N2 selectivity is 100%.

[0054] Example 2 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.005:1 to obtain a mixed solution. Then, 3 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 6 with hydrochloric acid. The solution was stirred in a dark room for 1.2 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under microwave irradiation at 2.45 GHz for 70 min to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0055] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3:3:70:20, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 180°C for 10.5 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0056] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.0:2.0:200:90, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.14 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 160°C for 8 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0057] The nitrate wastewater is agricultural wastewater containing nitrates; the nitrate concentration in the nitrate wastewater is 0.08 g / L.

[0058] The hole scavenger is ethylene glycol.

[0059] The microwave reaction device is a household microwave oven.

[0060] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2.2 hours.

[0061] The first wash involves washing the solid reaction product I four times with anhydrous ethanol.

[0062] The temperature for the first drying step was 80ºC, and the drying time was 55 minutes.

[0063] The second magnetic stirring time is 0.6 hours.

[0064] The second washing involves washing the solid reaction product II three times with anhydrous ethanol.

[0065] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 79.36%, and the N2 selectivity was 89.12%.

[0066] Example 3 A method for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction. The method described herein is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.01:1 to obtain a mixed solution. Then, 2 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 5 with hydrochloric acid. The solution was stirred in a dark room for 1.5 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under microwave irradiation at 0.915 GHz for 80 min to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0067] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3.3:3.5:72:22, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 185°C for 11 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0068] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.1:2.1:201:92, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.3 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 165°C for 8.5 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0069] The nitrate wastewater is industrial wastewater containing nitrates; the nitrate concentration in the nitrate wastewater is 0.001 g / L.

[0070] The hole scavenger is isopropanol.

[0071] The microwave reaction device is a laboratory microwave reactor.

[0072] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2.4 hours.

[0073] The first wash involves washing the solid reaction product I three times with anhydrous ethanol.

[0074] The temperature for the first drying step was 66ºC, and the drying time was 40 minutes.

[0075] The second magnetic stirring time is 0.7 hours.

[0076] The second washing involves washing the solid reaction product II three times with anhydrous ethanol.

[0077] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 82.32%, and the N2 selectivity was 90.63%.

[0078] Example 4 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.008:1 to obtain a mixed solution. Then, 5 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 4 with hydrochloric acid. The solution was stirred in a dark room for 1.4 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under 0.3 GHz microwave irradiation for 85 min to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0079] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3.6:4:74:24, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 190°C for 11.5 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0080] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.3:2.3:202:94, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.5 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 170°C for 9 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0081] The nitrate wastewater is domestic wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.02 g / L.

[0082] The cavitation scavenger is oxalic acid.

[0083] The microwave reaction device is an industrial microwave oven.

[0084] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2.6 hours.

[0085] The first wash involves washing the solid reaction product I four times with anhydrous ethanol.

[0086] The temperature for the first drying step was 79ºC, and the drying time was 45 minutes.

[0087] The second magnetic stirring time is 0.8 hours.

[0088] The second washing involves washing the solid reaction product II four times with anhydrous ethanol.

[0089] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 90.61%, and the N2 selectivity was 92.36%.

[0090] Example 5 A method for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction. The method described herein is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.02:1 to obtain a mixed solution. Then, 1 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 2 with hydrochloric acid. The solution was stirred in a dark room for 1.8 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted for 110 min under 15 GHz microwave irradiation to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0091] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3.9:4.5:76:26, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 195°C for 12 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0092] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.4:2.4:203:96, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.47 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 175°C for 9.5 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0093] The nitrate wastewater is agricultural wastewater containing nitrates; the nitrate concentration in the nitrate wastewater is 1 g / L.

[0094] The cavitation scavenger is ascorbic acid.

[0095] The microwave reaction device is a laboratory microwave reactor.

[0096] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2.8 hours.

[0097] The first wash involves washing the solid reaction product I four times with anhydrous ethanol.

[0098] The temperature for the first drying step was 70ºC, and the drying time was 36 minutes.

[0099] The second magnetic stirring time is 0.9 hours.

[0100] The second washing involves washing the solid reaction product II three times with anhydrous ethanol.

[0101] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 88.52%, and the N2 selectivity was 95.67%.

[0102] Example 6 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.004:1 to obtain a mixed solution. Then, 5 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 3 with hydrochloric acid. The solution was stirred in a dark room for 1.9 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under 20 GHz microwave irradiation for 120 min to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0103] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:4.2:5:78:28, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven for hydrothermal reaction at 200°C for 12.5 h, followed by natural cooling to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I, which was washed and dried to obtain ZnFe2O4 powder.

[0104] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.5:2.5:205:97, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:0.9 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 185°C for 10.5 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0105] The nitrate wastewater is domestic wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.5 g / L.

[0106] The hole scavenger is formic acid.

[0107] The microwave reaction device is a household microwave oven.

[0108] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 3 hours.

[0109] The first wash involves washing the solid reaction product I three times with anhydrous ethanol.

[0110] The temperature for the first drying step was 76ºC, and the drying time was 60 minutes.

[0111] The second magnetic stirring time is 1 hour.

[0112] The second washing involves washing the solid reaction product II four times with anhydrous ethanol.

[0113] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 79.64%, and the N2 selectivity was 94.68%.

[0114] Example 7 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.006:1 to obtain a mixed solution. Then, 4 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 3 with hydrochloric acid. The solution was stirred in a dark room for 1.7 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted under 0.3 GHz microwave irradiation for 90 min to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0115] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:4.5:5.5:79:29, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 205°C for 13 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0116] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.6:2.7:207:98, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:1.0 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 190°C for 11 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0117] The nitrate wastewater is agricultural wastewater containing nitrates; the nitrate concentration in the nitrate wastewater is 0.2 g / L.

[0118] The hole scavenger is ethylene glycol.

[0119] The microwave reaction device is an industrial microwave oven.

[0120] The stirring device for the first magnetic stirring was a magnetic stirrer, and the first magnetic stirring time was 3.3 hours.

[0121] The first wash involves washing the solid reaction product I four times with anhydrous ethanol.

[0122] The temperature for the first drying step was 69ºC, and the drying time was 33 minutes.

[0123] The second magnetic stirring time is 0.6 hours.

[0124] The second washing involves washing the solid reaction product II three times with anhydrous ethanol.

[0125] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 82.68%, and the N2 selectivity was 92.61%.

[0126] Example 8 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.01:1 to obtain a mixed solution. Then, 3 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 5 with hydrochloric acid. The solution was stirred in a dark room for 1.1 h to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted for 100 min under microwave irradiation at 2.45 GHz to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0127] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:4.7:6:71:21, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 210°C for 13.5 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0128] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3.8:2.9:209:99, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:1.10 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 195°C for 11.5 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed a second time and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0129] The nitrate wastewater is industrial wastewater containing nitrates; the nitrate concentration in the nitrate wastewater is 0.01 g / L.

[0130] The cavitation scavenger is oxalic acid.

[0131] The microwave reaction device is a laboratory microwave reactor.

[0132] The stirring device for the first magnetic stirring was a magnetic stirrer, and the first magnetic stirring time was 3.6 hours.

[0133] The first wash involves washing the solid reaction product I three times with anhydrous ethanol.

[0134] The temperature for the first drying step was 73ºC, and the drying time was 38 minutes.

[0135] The second magnetic stirring time is 0.8 hours.

[0136] The second washing involves washing the solid reaction product II three times with anhydrous ethanol.

[0137] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 89.67%, and the N2 selectivity was 97.51%.

[0138] Example 9 A method for harmlessly treating nitrate wastewater based on microwave-assisted catalytic reduction. The method described in this embodiment is as follows: The catalyst was added to the nitrate wastewater at a mass ratio of 0.009:1 to obtain a mixed solution. Then, 2 wt% of a vaccinator was added to the mixed solution, and the pH of the mixed solution was adjusted to 6 with hydrochloric acid. The solution was stirred in a dark room for 2 hours to obtain suspension I. Suspension I was then placed in a microwave reactor and reacted for 120 minutes under microwave irradiation at 2.45 GHz to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction.

[0139] The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:5:5.8:80:30, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor lined with polytetrafluoroethylene and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 220°C for 14 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained.

[0140] Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:4:3:210:100, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe₂O₄ powder in a mass ratio of 1:1.14 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 200°C for 12 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed and dried to obtain the ZnFe₂O₄@ZnIn₂S₄ composite catalyst.

[0141] The nitrate wastewater is domestic wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.05 g / L.

[0142] The hole scavenger is isopropanol.

[0143] The microwave reaction device is an industrial microwave oven.

[0144] The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 4 hours.

[0145] The first wash involves washing the solid reaction product I four times with anhydrous ethanol.

[0146] The temperature for the first drying step was 63ºC, and the drying time was 50 minutes.

[0147] The second magnetic stirring time is 0.5 hours.

[0148] The second washing involves washing the solid reaction product II four times with anhydrous ethanol.

[0149] The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction described in this embodiment: NO3 The conversion rate was 72.71%, and the N2 selectivity was 87.31%.

[0150] This specific implementation method has the following advantages compared with the prior art: 1. The catalyst raw materials used in this specific embodiment are readily available, and costs can still be effectively controlled during large-scale preparation. The ZnFe2O4@ZnIn2S4 catalyst is shown in the attached figure: Figure 1 X-ray diffraction pattern of the ZnFe2O4@ZnIn2S4 composite microwave catalyst prepared in Example 1; Figure 2 for Figure 1The image shown is a scanning electron microscope image of the ZnFe2O4@ZnIn2S4 composite microwave catalyst. Figure 3 for Figure 1 The image shows the magnetic separation effect of the ZnFe2O4@ZnIn2S4 composite microwave catalyst. (From...) Figure 1 It can be seen that the prepared ZnFe2O4@ZnIn2S4 composite catalyst is mainly composed of two crystal phases, ZnFe2O4 and ZnIn2S4, which is beneficial for separation and recovery and obtaining good microwave catalytic activity; from Figure 2 It can be seen that the average particle size of the prepared ZnFe2O4@ZnIn2S4 composite catalyst is approximately 500 nanometers; from Figure 3 It can be seen that the ZnFe2O4@ZnIn2S4 composite catalyst can be instantly separated and recovered from the nitrate solution by the magnetic attraction of the magnet (as shown in the black part inside the bottle in the figure); that is, it can be rapidly separated and recovered from the reaction system by magnetic separation technology, with good reusability, which is conducive to further reducing production costs and process energy consumption.

[0151] 2. The microwave heating method used in this specific embodiment has the advantages of good penetration, uniform heating characteristics, and selective heating. Compared with photocatalysis and electrocatalysis technologies, microwaves can selectively heat the catalyst, thereby concentrating microwave energy on the catalytic reduction process of nitrates, significantly improving energy utilization efficiency, reducing energy consumption, and thus greatly saving production costs.

[0152] 3. The microwave catalysis method described in this specific embodiment utilizes the unique "hot spot effect" of microwave radiation to induce the generation of highly active "hot electrons," thereby driving the nitrate reduction reaction. This "hot spot effect" originates from the localized high-temperature microregions formed on the catalyst surface due to molecular friction and vibration under microwave irradiation. These microregions can generate "hot electrons" with high energy and catalytic activity, thus increasing the reaction rate of nitrate reduction to environmentally friendly nitrogen gas and achieving a high nitrate conversion rate and nitrogen selectivity. NO3 The conversion rate was 72.71%–100%, and the selectivity of N2 was 87.31%–100%. Conversion rates are attached. Figure 4 As shown, Figure 4 for Figure 1 The ZnFe2O4@ZnIn2S4 composite microwave catalyst was used to microwave treat NO3 in a 100 mg / L nitrate solution. Graphs showing the changes in conversion rate and N2 selectivity over reaction time; from Figure 4 It can be seen that the ZnFe2O4@ZnIn2S4 composite catalyst can reduce NO3 in nitrate solution (100 mg / L) by microwave treatment. Both conversion rate and N2 selectivity can reach 100%.

[0153] 4. The microwave catalysis process provided in this specific embodiment is simple and convenient. The reduction reaction can be completed simply by mixing the catalyst with a nitrate-containing solution and then irradiating it in a microwave field. Currently, industrial-grade microwave reactors are relatively mature, supporting the large-scale application of this technology.

[0154] Therefore, this specific embodiment is not only simple to operate, low in cost, and low in energy consumption, but also the catalyst prepared can be reused and is easy to use on a large scale. The method has a high nitrate conversion rate, no secondary pollution, and high selectivity for the target product, and can achieve harmless removal of nitrogen.

Claims

1. A method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction, characterized in that, The steps of the method are as follows: According to the mass ratio of catalyst to nitrate wastewater of 0.001~0.1:1, the catalyst is added to the nitrate wastewater to obtain a mixed solution; then, 1~5wt% of a cavitation scavenger is added to the mixed solution, and the pH of the mixed solution is adjusted to 1~6 with hydrochloric acid. The solution is stirred in a dark room for 1~2 hours to obtain suspension I; then, suspension I is placed in a microwave reactor and reacted under microwave irradiation conditions of 0.3~20GHz for 60~120 minutes to obtain nitrogen gas for the harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction. The catalyst is a ZnFe2O4@ZnIn2S4 composite catalyst, and the preparation method of the ZnFe2O4@ZnIn2S4 composite catalyst is as follows: Step 1: Preparation of ZnFe2O4 The ingredients were prepared according to the mass ratio of zinc chloride: ferric chloride hexahydrate: sodium acetate: ethylene glycol: polyethylene glycol of 1:3~5:3~6:70~80:20~30, mixed, and magnetically stirred for the first time to obtain solution I. Solution I was transferred to a stainless steel reactor with a polytetrafluoroethylene liner and then placed in a hydrothermal oven. The reactor was hydrothermally reacted at 180~220°C for 10~14 hours and then naturally cooled to room temperature. The solid and liquid components of solution I after the hydrothermal reaction were separated to obtain solid reaction product I. After the first washing and drying, ZnFe2O4 powder was obtained. Step 2, Preparation of ZnFe2O4@ZnIn2S4 The following mixtures were prepared by mixing zinc chloride, indium chloride tetrahydrate, thioacetamide, N,N-dimethylformamide, and glycerol in a mass ratio of 1:3~4:2~3:200~210:90~100, followed by a second magnetic stirring to obtain solution II. The following mixtures were prepared by mixing solution II and ZnFe2O4 powder in a mass ratio of 1:0.14~1.14 to obtain suspension II. Suspension II was transferred to a stainless steel reactor lined with polytetrafluoroethylene and placed in a hydrothermal oven for hydrothermal reaction at 160~200°C for 8~12 hours. After natural cooling to room temperature, the suspension II was separated into solid and liquid components to obtain solid reaction product II. This product was then washed and dried to obtain the ZnFe2O4@ZnIn2S4 composite catalyst.

2. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The nitrate wastewater is one of the following: domestic wastewater, industrial wastewater, or agricultural wastewater containing nitrate; the nitrate concentration in the nitrate wastewater is 0.001~1g / L.

3. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The cavitation agent is one of formic acid, ethylene glycol, oxalic acid, isopropanol, and ascorbic acid.

4. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The microwave reaction device is one of a household microwave oven, an industrial microwave oven, and a laboratory microwave reactor.

5. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The stirring device for the first magnetic stirring is a magnetic stirrer, and the first magnetic stirring time is 2-4 hours.

6. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The first washing involves washing the solid reaction product I with anhydrous ethanol 3 to 4 times.

7. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The temperature for the first drying is 60~80ºC, and the drying time is 30~60min.

8. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The stirring device used for the second magnetic stirring is the same as the magnetic stirrer used in the first stirring, and the stirring time for the second magnetic stirring is 0.5 to 1 hour.

9. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The second washing involves washing the solid reaction product II with anhydrous ethanol 3 to 4 times.

10. The method for harmless treatment of nitrate wastewater based on microwave-assisted catalytic reduction according to claim 1, characterized in that: The second drying process is the same as the first drying process.

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