A water treatment method for the controllable conversion of nitrate to ammonia or nitrogen on the same photocatalyst

Abstract

The present invention discloses a water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst, belonging to the technical field of photocatalysis. Add amorphous bismuth oxybromide-bismuth tungstate photocatalyst and hole sacrificial agent to water containing nitrate, adjust the pH value of the system to 7.0-10.0, introduce inert gas, and irradiate with an ultraviolet-visible lamp, so as to achieve highly selective nitrogen production while efficiently removing nitrate; add the same photocatalyst and hole sacrificial agent as described above to sewage containing nitrate, then add a certain amount of hydroxyl radical quencher and adjust the pH value of the system to 1.0-4.0, introduce inert gas, and irradiate with an ultraviolet-visible lamp, so as to achieve highly selective ammonia nitrogen production while efficiently removing nitrate. The method of the present invention is simple to operate and can be manually and freely controlled on the same catalyst according to the desired product; there is no need for additional materials or raw materials to separately prepare photocatalysts suitable for two systems, saving economic costs.

Description

Technical Field

[0001] The present invention relates to a water treatment and waste resource recovery method, and particularly to a water treatment method for realizing controllable and highly selective conversion of nitrate into ammonia or nitrogen on the same photocatalyst, belonging to the field of photocatalysis technology. Background Art

[0002] Due to the discharge of domestic sewage and industrial wastewater, the infiltration of solid waste leachate, and the excessive application of nitrogen fertilizers, the groundwater and surface water in many regions are facing serious nitrate pollution. Nitrate may be converted into nitrite in the human body, which may further cause a series of diseases, such as methemoglobinemia and even cancer.

[0003] The commonly used methods for removing nitrate mainly include: physicochemical method, biological denitrification method, and chemical method. The physical treatment methods include reverse osmosis method, membrane separation method, ion exchange method, etc. The chemical method is mainly chemical reduction technology. These treatment methods and technologies all have some deficiencies. For example, the nitrate conversion rate is low, the selectivity is low, it is easy to cause secondary pollution, the process is complex, the reaction conditions are harsh and difficult to control, and the cost is high and the efficiency is low. Therefore, from the economic and practical application perspectives, the actual effects of the above methods for removing nitrate from water are not ideal. Due to its environmental friendliness, photocatalysis technology has been used in many nitrate removal systems. The products of photocatalytic reduction of nitrate (NO3 - ) in water mainly include nitrite (NO2 - ), nitrogen (N2), and ammonia nitrogen (NH3).

[0004] As an important chemical raw material and potential energy storage medium, ammonia nitrogen is widely used in industry and agriculture. Since the Haber-Bosch process has the characteristic of high energy consumption in ammonia nitrogen production, and the dissociation energy of the N=O double bond in nitrate is much lower than that of the N≡N triple bond in photocatalytic nitrogen fixation, nitrate can be used as an alternative nitrogen source in ammonia nitrogen production. Selectively reducing nitrate to ammonia nitrogen under the ambient conditions of normal temperature and pressure is a powerful green process that integrates nitrate removal and waste resource recovery for ammonia production. Therefore, it is extremely challenging to achieve controllable high nitrogen selectivity or high ammonia nitrogen selectivity on the same photocatalyst through condition conversion.

[0005] Some existing technologies only consider the efficient removal of nitrate without considering the selectivity of the final product. For example, a Chinese patent with the patent application number CN201910499977.5 discloses a nanocomposite for removing nitrate nitrogen in water, its preparation method and application; a Chinese patent with the patent application number CN202210282530.4 discloses a method for photocatalytic removal of nitrate nitrogen in water. There are also some existing technologies that usually can only highly selectively convert nitrate into harmless nitrogen gas or only into ammonia nitrogen under a certain specific condition. For example, a Chinese patent with the patent application number CN202210563213.X provides a preparation method of a catalyst for electrocatalytic reduction of nitrate to produce nitrogen gas; a Chinese patent with the patent application number CN202211263208.3 describes a preparation method of a catalyst for electrocatalytic reduction of nitrate to produce ammonia; a Chinese patent with the patent application number CN202110506976.6 discloses a preparation method of an electrocatalyst for nitrate reduction to ammonia. Although these patents disclose methods for efficiently removing nitrate and highly selectively converting it into ammonia nitrogen or nitrogen gas respectively, they do not involve a water treatment method for controllable conversion of nitrate into ammonia or nitrogen on the same photocatalyst. This method can be regulated according to human needs without changing the catalyst to efficiently convert nitrate into nitrogen gas or ammonia nitrogen. Summary of the Invention

[0006] In order to overcome the above-mentioned existing deficiencies, the following invention is proposed.

[0007] The purpose of the present invention is to provide a water treatment method for controllable conversion of nitrate into ammonia or nitrogen on the same photocatalyst to overcome the deficiencies in the existing methods.

[0008] An embodiment of the present invention provides a water treatment method for controllable conversion of nitrate into ammonia or nitrogen on the same photocatalyst, which includes:

[0009] For the water treatment method of controllable conversion of nitrate into ammonia or nitrogen on the same photocatalyst, add ultrathin sheet-shaped amorphous bismuth oxybromide-bismuth tungstate photocatalyst and hole sacrificial agent to the water containing nitrate. The key operation is to adjust the pH value of the mixed system to 7.0 - 10.0, and then introduce a stable inert gas atmosphere, and irradiate the system with an ultraviolet-visible lamp. During the reaction process, there is a photochromic phenomenon of the catalyst, so as to achieve the efficient removal of nitrate and highly selective conversion into nitrogen gas; add the same photocatalyst and hole sacrificial agent as mentioned above to the water containing nitrate. The key operation is to add a certain amount of hydroxyl radical quencher and adjust the pH value of the mixed system to 1.0 - 4.0, and then introduce a stable inert gas atmosphere, and irradiate the system with an ultraviolet-visible lamp, so as to achieve the efficient removal of nitrate and highly selective conversion into ammonia nitrogen.

[0010] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst according to the present invention, the quencher of hydroxyl radicals can be one of tert-butanol, methanol, isopropanol, and sodium thiosulfate.

[0011] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst according to the present invention, the stable inert gas introduced is one of argon and nitrogen.

[0012] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst according to the present invention, the hole sacrificial agent can be one of formic acid, acetic acid, oxalic acid, EDTA-2Na, and humic acid.

[0013] The function of adding the quencher of hydroxyl radicals in the present invention is to consume the strongly oxidizing radical hydroxyl radicals generated by the reaction of OH- and photo-generated holes in the solution, preventing the hydroxyl radicals from oxidizing the generated ammonia nitrogen into nitrogen; introducing a stable inert gas is to keep the reaction system in an anaerobic state throughout the reaction process, which is beneficial to the photocatalytic reduction of nitrate; the function of adding the hole sacrificial agent is to react with the photo-generated holes generated during the photocatalytic reaction, preventing the recombination of photo-generated holes and photo-generated electrons from reducing the efficiency of photocatalytic reduction of nitrate, and the generation of.

[0014] Compared with the prior art, the present invention creatively only needs to simply change the reaction conditions without changing the catalyst, and can efficiently detoxify the toxic waste resource nitrate and efficiently and selectively convert it into harmless nitrogen or further recycle it as ammonia nitrogen for resource reuse; this method is simple to operate, highly operable, and can be manually controlled freely according to the desired product; there is no need to separately prepare photocatalysts suitable for two systems with additional materials or raw materials, saving economic costs; the preparation method of the surface amorphous bismuth oxybromide-bismuth tungstate photocatalyst adopted in the present invention is simple, the raw materials are easy to obtain, and the conditions are easy to control; compared with the prior art, the significant progress is that nitrate has a high removal rate in a wide pH range (1.0 - 10.0), and the high-selectivity production of nitrogen can be changed to high-selectivity production of ammonia nitrogen after regulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0016] Figure 1It is a schematic diagram of the photocatalytic nitrate reaction device in the embodiment of the present invention.

[0017] Figure 2 It is the X-ray diffraction pattern of pure bismuth tungstate and amorphous bismuth oxybromide-bismuth tungstate photocatalysts in Examples 1-2 of the present invention.

[0018] Figure 3 It is the Raman spectrum of pure bismuth tungstate and amorphous bismuth oxybromide-bismuth tungstate photocatalysts in Examples 1-2 of the present invention.

[0019] Figure 4 It is the transmission electron microscopy image of the amorphous bismuth oxybromide-bismuth tungstate photocatalyst in Examples 1-2 of the present invention.

[0020] Figure 5 It is the change curves of nitrate concentration, total nitrogen concentration and nitrogen selectivity in the high-selectivity nitrogen production system in Example 1 of the present invention during the reaction. The inset is the ammonia nitrogen color development result using the Nessler's reagent colorimetric method.

[0021] Figure 6 It is the change curves of nitrate concentration, total nitrogen concentration and ammonia nitrogen selectivity in the high-selectivity ammonia nitrogen production system in Example 2 of the present invention during the reaction. The inset is the ammonia nitrogen color development result using the Nessler's reagent colorimetric method. Detailed Embodiments

[0022] To make the objectives, technical solutions and advantages of the present invention clearer, the following will describe the detailed embodiments of the present invention with reference to the accompanying drawings. The embodiments of the present invention shown in the drawings and described according to the drawings are only exemplary and are not limited to these described embodiments.

[0023] Here, only the structures or processing steps closely related to the solution of the present invention are shown in the drawings, while other details less related to the present invention are ignored.

[0024] In an embodiment of the present invention, a water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst is provided, and the characteristics include: adding an ultrathin flake amorphous bismuth oxybromide-bismuth tungstate photocatalyst and a hole sacrificial agent into water containing nitrate. The key operation is to adjust the pH value of the mixed system to 7.0 - 10.0, and then introduce a stable inert gas atmosphere, and irradiate the system with an ultraviolet-visible light lamp. During the reaction process, there is a photochromic phenomenon of the catalyst, thereby achieving the efficient removal of nitrate and the highly selective conversion into nitrogen; adding the same photocatalyst and hole sacrificial agent as described above into water containing nitrate. The key operation is to add a certain amount of hydroxyl radical quencher and adjust the pH value of the mixed system to 1.0 - 4.0, and then introduce a stable inert gas atmosphere, and irradiate the system with an ultraviolet-visible light lamp, thereby achieving the efficient removal of nitrate and the highly selective conversion into ammonia nitrogen.

[0025] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst of the present invention, the hydroxyl radical quencher can be one of tert-butanol, methanol, isopropanol, and sodium thiosulfate.

[0026] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst of the present invention, the stable inert gas introduced is one of argon and nitrogen.

[0027] In the water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst of the present invention, the hole sacrificial agent can be one of formic acid, acetic acid, oxalic acid, EDTA-2Na, and humic acid.

[0028] In some more specific typical embodiments, the water treatment method for controllably converting into ammonia or nitrogen specifically includes:

[0029] Take a certain amount of the aforementioned amorphous bismuth oxybromide-bismuth tungstate photocatalyst, place it in a nitrate solution, and add a certain amount of formic acid to consume the photogenerated holes produced during the reaction; use sodium hydroxide solution and dilute hydrochloric acid to adjust the pH value of the mixed system to 9.0; then, first introduce a stable argon flow in the reaction device for 15 minutes to make the reaction system under anaerobic conditions; finally, carry out a photocatalytic reaction under a stable argon gas flow and ultraviolet-visible light irradiation. Through ultraviolet-visible light irradiation of the catalyst, the removal rate of nitrate can reach 97.4%, and the nitrogen selectivity can reach 90.0%.

[0030] Take a certain amount of the aforementioned surface amorphous bismuth oxybromide-bismuth tungstate photocatalyst, place it in a nitrate solution, add a certain amount of formic acid solution to consume the photogenerated holes generated during the reaction; then add a certain amount of tert-butanol to quench the hydroxyl radicals generated during the reaction; use sodium hydroxide solution and dilute hydrochloric acid to adjust the pH value of the mixed system to 3.0; then, first introduce a stable argon gas flow into the reaction device for 15 min to make the reaction system under anaerobic conditions; finally, carry out the photocatalytic reaction under a stable argon gas flow and ultraviolet-visible light irradiation. Through ultraviolet-visible light irradiation, the removal rate of nitrate can reach 82.1%, and the ammonia nitrogen selectivity can reach 91.7% for the catalyst.

[0031] In some typical embodiments, the preparation method of the surface amorphous bismuth oxybromide-bismuth tungstate photocatalyst includes:

[0032] Dissolve soluble bismuth salt, tungstate and cetyltrimethylammonium bromide in a solvent and mix evenly, then adjust the mixed solution to make its pH ≤ 1.0, and then react the mixed solution at 160 °C for 12 h to obtain the surface amorphous bismuth oxybromide-bismuth tungstate photocatalyst.

[0033] Further, the molar ratio of the soluble bismuth salt to the tungstate is 2:1.

[0034] Further, the soluble bismuth salt includes bismuth nitrate, but is not limited thereto.

[0035] Further, the tungstate includes sodium tungstate, but is not limited thereto.

[0036] Further, the solvent includes a mixed solution of dilute nitric acid and water, but is not limited thereto.

[0037] Further, the substance for adjusting the mixed solution to make its pH ≤ 1.0 includes sodium hydroxide, but is not limited thereto.

[0038] Further, the preparation method further includes: after reacting the mixed solution at 160 °C for 12 h, washing and filtering it alternately with deionized water and absolute ethanol, and then drying it in an oven at 40-80 °C for 12 h to obtain the surface amorphous bismuth oxybromide-bismuth tungstate photocatalyst.

[0039] With the above technical solutions, the present invention creatively enables the efficient detoxification of the toxic waste resource nitrate and its efficient and selective conversion into harmless nitrogen gas or further recovery as ammonia nitrogen for resource reuse by simply changing the reaction conditions without changing the catalyst; this method is simple to operate, highly operable, and can be manually and freely controlled according to the desired product; compared with the traditional denitrification process flow, there is no need for additional materials or raw materials to separately prepare photocatalysts suitable for two systems, saving economic costs; it has significant progress compared with the prior art.

[0040] The following further explains and illustrates the technology of the present invention in conjunction with the accompanying drawings and embodiments.

[0041] Example 1

[0042] Weigh 1.9403 g of bismuth nitrate pentahydrate and place it in a 100 mL beaker. Add 15 mL of 1 mol·L -1 dilute nitric acid, place it on a magnetic stirrer at room temperature and stir until the solution is transparent, and label it as A. Weigh 1 g of cetyltrimethylammonium bromide and 0.6597 g of sodium tungstate dihydrate and place them in another 100 mL beaker. Then add 45 mL of deionized water, and ultrasonically treat it at room temperature until the solution is transparent, and label it as B. After that, slowly pour B into A, and at this time, the pH value of the mixed reaction solution is <1.0. Place it on a magnetic stirrer at room temperature and stir for 1 h, then transfer it to a 100 mL autoclave with a polytetrafluoroethylene liner, and then heat it in a forced-air drying oven at 160 °C for 12 h. After the sample cools to room temperature, wash and filter it alternately with deionized water and absolute ethanol. Then place the sample in an oven at a temperature of 40 - 80 °C and dry it for 12 h to obtain the amorphous bismuth oxybromide-bismuth tungstate photocatalyst on the surface.

[0043] Weigh 0.08 g of the amorphous bismuth oxybromide-bismuth tungstate photocatalyst prepared by the above method and put it into a 200 mL reaction solution containing 50 mg·L -1 (0.8 mmol·L -1 ) nitrate concentration, and stir vigorously for 5 minutes to obtain a homogeneous mixture. Then, add 2.5 mmol·L -1 formic acid solution to the suspension to consume the photogenerated holes generated during the reaction, and use 0.1 mol·L -1 NaOH or HCl solution to adjust the pH value of the solution to 9.0. After that, transfer the adjusted mixed solution to a 250 mL photocatalytic reaction device with a quartz cold trap (closed reaction device, see Figure 1) First, a steady argon stream was introduced into the reaction device for 15 min to make the reaction system under anaerobic conditions. Immediately afterwards, condensed water was introduced to keep all subsequent reactions at room temperature. Finally, the photocatalytic reaction was carried out under a steady argon stream, ultraviolet-visible light irradiation (100 W high-pressure mercury lamp), and stirring with a magnetic stirrer. Water samples were taken at different time intervals (0, 20, 40, 60, 80, 100, and 120 min), and the solid samples in the water samples were filtered off with a 0.22 μm cellulose acetate membrane. The subsequent clarified water samples were used to measure the concentrations of nitrate, nitrite, ammonia nitrogen, and total nitrogen to check the photocatalytic reaction situation. After 2 h of illumination, the nitrate removal rate could reach 97.4%, and the nitrogen selectivity could reach 90.0% (see Figure 5 ).

[0044] Example 2

[0045] Weigh 1.9403 g of bismuth nitrate pentahydrate and place it in a 100 mL beaker. Add 15 mL of 1 mol·L -1 dilute nitric acid, place it on a magnetic stirrer at room temperature and stir until the solution becomes transparent, and label it as A. Weigh 1 g of cetyltrimethylammonium bromide and 0.6597 g of sodium tungstate dihydrate and place them in another 100 mL beaker. Then add 45 mL of deionized water, and ultrasonically treat it at room temperature until the solution becomes transparent, and label it as B. Then, slowly pour B into A. At this time, the pH value of the mixed reaction solution is <1.0. Place it on a magnetic stirrer at room temperature and stir for 1 h, then transfer it to a 100 mL high-pressure reaction kettle with a PTFE lining, and then heat it in a forced-air drying oven at 160 °C for 12 h. Wait for the sample to cool to room temperature, and wash and filter it alternately with deionized water and absolute ethanol. Then place the sample in an oven at 40 - 80 °C and dry it for 12 h to obtain the amorphous bismuth oxybromide-bismuth tungstate photocatalyst on the surface.

[0046] Weigh 0.08 g of the amorphous bismuth oxybromide-bismuth tungstate photocatalyst prepared by the above method and put it into a 200 mL reaction solution containing 50 mg·L -1 (0.8 mmol·L -1 ) nitrate concentration, and stir vigorously for 5 minutes to obtain a homogeneous mixture. Then, add formic acid solution (2.5 mmol·L -1 ) to consume the photogenerated holes generated during the reaction, and then add tert-butanol (10 mmol·L -1 ) to quench the hydroxyl radicals generated during the reaction, and use 0.1 mol·L -1 NaOH or HCl solution to adjust the pH value of the solution to 3.0. Then, transfer the adjusted mixed solution to a 250 mL photocatalytic reaction device with a quartz cold trap (closed reaction device, seeFigure 1 ) First, a stable argon stream was introduced into the reaction apparatus for 15 min to make the reaction system under anaerobic conditions. Immediately afterwards, condensed water was introduced to keep all subsequent reactions at room temperature. Finally, the photocatalytic reaction was carried out under a stable argon stream, ultraviolet-visible light irradiation (100 W high-pressure mercury lamp), and stirring with a magnetic stirrer. Water samples were taken at different time intervals (0, 20, 40, 60, 80, 100, and 120 min), and the solid samples in the water samples were filtered off with a 0.22 μm cellulose acetate membrane. The subsequent clarified water samples were used to measure the concentrations of nitrate, nitrite, ammonia nitrogen, and total nitrogen to check the reaction situation of the photocatalysis. After 2 h of illumination, the removal rate of nitrate could reach 82.1%, and the selectivity for ammonia nitrogen could reach 91.7% (see Figure 6 ).

[0047] In Examples 1-2, during the reaction process, the concentrations of nitrate and nitrite were detected using an ion chromatograph; ammonia nitrogen was color-developed using the Nessler's reagent colorimetric method and then its concentration was detected using an ultraviolet-visible spectrophotometer; the total nitrogen was determined using a total nitrogen analyzer.

[0048] Among them, the degradation rate of nitrate = (1 - C t / C0) × 100%

[0049] In the formula: C t is the concentration of nitrate in the reaction solution after t hours of reaction, and C0 is the initial concentration of nitrate in the reaction solution.

[0050]

[0051] In the formula: [X]0 and [X] t are the concentrations of various nitrogen species at the initial and after t hours of reaction, respectively.

[0052]

[0053] In the formula: [X]0 and [X] t are the concentrations of various nitrogen species at the initial and after t hours of reaction, respectively.

[0054] Ammonia nitrogen was determined using the Nessler's reagent colorimetric method:

[0055] Specific steps: Pipette 1.0 mL of the water sample and transfer it into a 10 mL colorimetric tube. Dilute it to 10 mL with deionized water and shake well. Add 0.2 mL of potassium sodium tartrate and shake well. Then add 0.3 mL of Nessler's reagent and shake well. Let it stand for 10 min, and measure the absorbance at a wavelength of 420 nm. From this, the nitrogen selectivity and ammonia nitrogen selectivity were calculated using the above formula.

[0056] Preparation of potassium sodium tartrate: Accurately weigh 50 g of potassium sodium tartrate, dissolve it with deionized water, and make up the volume to 100 mL in a volumetric flask for storage.

[0057] Preparation of Nessler's reagent: Accurately weigh 16 g of solid sodium hydroxide pellets, dissolve them in 50 mL of deionized water and cool them down to room temperature thoroughly. Then accurately weigh 10 g of mercuric iodide and 7 g of potassium iodide, dissolve them in 30 mL of deionized water, and label it as solution A. Then add solution A dropwise to the sodium hydroxide solution that is continuously stirred and cooled to room temperature, add water to make up the volume to 100 mL in a volumetric flask, and then transfer it to a plastic bottle for storage.

[0058] Test results:

[0059] See Figure 2 , which shows the X-ray diffraction patterns of pure bismuth tungstate and amorphous bismuth oxybromide-bismuth tungstate photocatalysts in Examples 1-2 of the present invention. All the diffraction peaks of the two curves are basically consistent with the standard pattern of bismuth tungstate (JCPDS: 73-2020). Since the content of amorphous bismuth oxybromide on the surface is small, no obvious diffraction peaks of bismuth oxybromide are detected.

[0060] See Figure 3 , which shows the Raman spectra of pure bismuth tungstate and amorphous bismuth oxybromide-bismuth tungstate photocatalysts in Examples 1-2 of the present invention.

[0061] See Figure 4 , which shows the transmission electron microscope images of the amorphous bismuth oxybromide-bismuth tungstate photocatalyst in Examples 1-2 of the present invention. There are amorphous regions on the surface of bismuth tungstate, and the ordered lattice fringes of bismuth oxybromide appear on the surface of bismuth tungstate after irradiation with high-energy electron beams.

[0062] See Figure 5 , which shows the change curves of nitrate concentration, total nitrogen concentration and nitrogen selectivity in the high-selectivity nitrogen production system in Example 1 of the present invention during the reaction process. The inset is the color development result of ammonia nitrogen by Nessler's reagent colorimetric method. The darker the color development, the higher the ammonia nitrogen concentration.

[0063] See Figure 6 , which shows the change curves of nitrate concentration, total nitrogen concentration and ammonia nitrogen selectivity in the high-selectivity ammonia nitrogen production system in Example 2 of the present invention during the reaction process. The inset is the color development result of ammonia nitrogen by Nessler's reagent colorimetric method. The darker the color development, the higher the ammonia nitrogen concentration.

[0064] It should be understood that the above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the present invention should be covered within the protection scope of the present invention.

Claims

1. A water treatment method for controllably converting nitrate into ammonia or nitrogen on the same photocatalyst, characterized by including: An ultrathin flaky amorphous bismuth oxybromide-bismuth tungstate photocatalyst and a hole sacrificial agent are added to water containing nitrate. The key operation is to adjust the pH value of the mixed system to 7.0 - 10.

0. Then, a stable inert gas atmosphere is introduced, and the system is irradiated with an ultraviolet-visible light lamp. During the reaction process, a photochromic phenomenon of the catalyst occurs, thereby achieving the efficient removal of nitrate and the highly selective conversion of nitrate to nitrogen. An identical photocatalyst and a hole sacrificial agent as described above are added to water containing nitrate. The key operation is to add a certain amount of hydroxyl radical quencher and adjust the pH value of the mixed system to 1.0 - 4.

0. Then, a stable inert gas atmosphere is introduced, and the system is irradiated with an ultraviolet-visible light lamp, thereby achieving the efficient removal of nitrate and the highly selective conversion of nitrate to ammonia nitrogen. The preparation method of the ultrathin flaky amorphous bismuth oxybromide-bismuth tungstate photocatalyst includes: dissolving a soluble bismuth salt, a tungstate, and cetyltrimethylammonium bromide in a solvent and mixing them evenly. Then, the pH of the mixed solution is adjusted to ≤1.0, and the mixed solution is reacted at 160 °C for 12 h to obtain the ultrathin flaky amorphous bismuth oxybromide-bismuth tungstate photocatalyst.

2. The water treatment method capable of being controllably converted into ammonia or nitrogen according to claim 1, wherein The molar ratio of the soluble bismuth salt to the tungstate is 2:

1.

3. The water treatment method capable of being controllably converted into ammonia or nitrogen according to claim 2, characterized in that: The preparation method further includes: after reacting the mixed solution at 160 °C for 12 h, washing and filtering it alternately with deionized water and absolute ethanol, and then drying it in an oven at a temperature of 40 - 80 °C for 12 h to obtain the amorphous bismuth oxybromide-bismuth tungstate photocatalyst.

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