A denitrification method of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalysis coupled with anaerobic ammonium oxidation

Abstract

This invention discloses a nitrogen removal method using a magnesium-aluminum layered double hydroxide (MLD) / titanium dioxide composite photocatalyst coupled with anaerobic ammonia oxidation. The invention incorporates magnesium-aluminum MLD into a titanium dioxide-based catalyst, imbuing the composite photocatalyst with alkaline active sites on its surface, enabling the oxidation of ammonia nitrogen in water under slightly alkaline conditions. Polypropylene multifaceted hollow spheres are used as a floating carrier, floating and rotating in water. Relying on light and water flow shear forces, the hollow spheres prevent biofilm growth, achieving self-cleaning. Iron and carbon particles are loaded onto a polyurethane sponge for modification, creating a strictly anaerobic environment within the biofilm carrier, which is conducive to the growth of the anaerobic ammonia oxidation biofilm. A polyurethane sponge filled with the anaerobic ammonia oxidation biofilm is embedded inside the floating carrier, further reacting the nitrite intermediates generated during photocatalytic oxidation of ammonia nitrogen with the ammonia nitrogen to convert them into harmless nitrogen gas, achieving higher nitrogen removal efficiency.

Description

Technical Field

[0001] This invention belongs to the field of water treatment, specifically relating to a denitrification method using magnesium aluminum hydrotalcite / titanium dioxide composite photocatalytic coupling anaerobic ammonia oxidation. Background Technology

[0002] High ammonia nitrogen concentration wastewater mainly originates from industrial production processes, particularly in the fertilizer, chemical, and pharmaceutical industries, and its discharge causes serious harm to the aquatic environment. With the rapid development of industrialization, the problem of industrial ammonia nitrogen wastewater has become increasingly prominent, with massive discharge volumes and a significant impact on the aquatic environment. High ammonia nitrogen concentration wastewater contains high concentrations of ammonia nitrogen compounds and low dissolved oxygen concentrations, exhibiting strong toxicity to aquatic ecosystems and posing potential threats to aquatic life and human health. Currently, technologies for treating high ammonia nitrogen concentration wastewater mainly include biological treatment and physiochemical treatment technologies. Biological treatment technologies rely on the redox reactions of nitrifying and denitrifying bacteria on ammonia nitrogen, but suffer from slow denitrification kinetics and uncontrolled disproportionation reactions. Physicochemical treatment technologies, such as air purging and breakpoint chlorination, can rapidly remove nitrogen, but they suffer from equipment corrosion, scaling, and may generate large amounts of sludge requiring further treatment. In recent years, with technological advancements, emerging technologies such as adsorption, advanced oxidation, and electrochemical oxidation have also been gradually applied to the treatment of high ammonia nitrogen wastewater. Among them, semiconductor photocatalysis technology, with its advantages of strong oxidation capacity, low production cost, small reactor size, and ability to utilize sunlight, is considered a treatment method with deep denitrification potential. Among numerous semiconductor photocatalysts, titanium dioxide has received in-depth research and widespread application due to its strong oxidation capacity, stable physicochemical properties, low cost, and non-toxicity.

[0003] In current photocatalysis research, most semiconductor photocatalysts are dispersed in solution as powders for photocatalysis. This leads to problems in practical water remediation, such as easy deposition of nano-photocatalysts, low light energy utilization, and difficulty in recycling. Furthermore, titanium dioxide photocatalysts cannot directly oxidize ammonia nitrogen in water; they can only oxidize it as free ammonia. Therefore, current photocatalytic oxidation of ammonia nitrogen experiments are mostly conducted in strongly alkaline environments, which significantly increases experimental costs and creates substantial difficulties for subsequent treatment. Moreover, photocatalytic oxidation of ammonia nitrogen produces some nitrite nitrogen, a product that is also a pollutant requiring treatment in wastewater. These problems severely hinder the practical application of photocatalytic oxidation of ammonia nitrogen. Summary of the Invention

[0004] To address the aforementioned problems in existing technologies, this invention provides a nitrogen removal method using a magnesium-aluminum layered double hydroxide (MLD) / titanium dioxide composite photocatalyst coupled with anaerobic ammonia oxidation. This invention modifies the titanium dioxide-based photocatalyst by doping it with magnesium-aluminum MLD, constructing surface alkaline active sites to achieve photocatalytic oxidation of ammonia nitrogen under slightly alkaline conditions, while simultaneously coupling with anaerobic ammonia oxidation, thus achieving highly efficient nitrogen removal under slightly alkaline conditions.

[0005] The present invention discloses a nitrogen removal method using a magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalytic coupled anaerobic ammonia oxidation, comprising the following steps:

[0006] Step 1: Preparation of magnesium aluminum hydrotalcite powder

[0007] Under constant temperature of 80℃, 200 mL of 0.8 mol / L sodium hydroxide and 0.02 mol / L sodium carbonate were slowly added dropwise to 100 mL of a mixed solution of 0.5 mol / L magnesium chloride hexahydrate and 0.2 mol / L aluminum chloride hexahydrate. During this process, vigorous stirring was required to promote the homogeneity of the reaction, while ensuring that the pH value of the solution was maintained between 8 and 10 to optimize the co-precipitation conditions. Then, the solution was heated at 60℃ for 15 h to promote the growth and crystallization of magnesium aluminum hydrotalcite crystals. After cooling to room temperature, the precipitate was washed, dried, and ground to obtain magnesium aluminum hydrotalcite powder with high crystallinity, obvious lamellar structure, good thermal stability, and regular morphology, with a magnesium, aluminum, and carbonate element ratio of 6:2:1.

[0008] Step 2: Preparation of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst

[0009] Add 0.5 mL of hydrochloric acid to 40 mL of ethanol solution and stir at room temperature for 10 min. Then, slowly add 3.0 mL of tetrabutyl titanate and 0.072 g of magnesium aluminum hydrotalcite to the solution and continue stirring for 20 min to form a mixed solution. Place the solution in an ultrasonic machine and sonicate for 20-30 min to enhance dispersibility and form a slurry. Then, use a reaction vessel to perform a hydrothermal reaction at 150-180℃ for 20-24 h to allow TiO2 to grow and crystallize uniformly on the surface of magnesium aluminum hydrotalcite. After cooling to room temperature, wash, dry, and grind the precipitate to finally obtain a magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst with a magnesium aluminum hydrotalcite doping ratio of 15 w%.

[0010] Step 3: Loading the composite photocatalyst on the floating support

[0011] First, the polyamide resin coating is heated to a molten state in a 150℃ oil bath using a magnetically stirred water bath. Then, magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder is added to the molten polyamide resin at a weight ratio of 5-15%. The mixture is stirred with a rotor for 20-30 minutes to ensure uniform dispersion, resulting in a magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder coating. This powder coating is then evenly applied to polypropylene multifaceted hollow spheres using a brush. The coating is allowed to cool and then re-solidify, completing the loading of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder onto the polypropylene multifaceted hollow spheres. The photocatalyst powder accounts for 3%-5% of the total mass.

[0012] Step 4: Modification of polyurethane foam

[0013] First, the iron particles were pretreated by soaking them in a 0.1 mol / L NaOH solution for 2 hours, then drying them in a vacuum oven at 105°C for 2 hours. Next, they were soaked in a 0.1 mol / L HCl solution for 2 hours, and then dried in a vacuum oven at 105°C for 2 hours. The activated carbon particles were washed three times with deionized water and then dried in a vacuum oven at 105°C for 2 hours. The dried iron and activated carbon particles were then pulverized into powder and sieved through 80-mesh and 200-mesh screens, respectively, to obtain the required iron and activated carbon particles. A medium-pore polyurethane sponge with a porosity of 20-40 ppi was selected. Iron and carbon particles were mixed at a mass ratio of 1:1 and then loaded onto the surface of the polyurethane sponge using water-based polyurethane adhesive. Successful loading was indicated by the sponge surface being uniformly covered with particles that were not washed away by water, thus completing the preparation of the iron-carbon-polyurethane sponge.

[0014] Step 5: Formation of anaerobic ammonia oxidation biofilm

[0015] Iron-carbon-polyurethane sponges were placed in an anaerobic ammonia oxidation reactor, which was a stable laboratory-run USBR reactor. After 30 days of enrichment, acclimatization, and biofilm formation, the sponges were finally removed to obtain an iron-carbon-polyurethane sponge carrier with a large amount of anaerobic ammonia oxidation biofilm attached inside.

[0016] Step 6: Coupling of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst and anaerobic ammonium oxidation biofilm

[0017] The iron-carbon polyurethane sponge with an anaerobic ammonia oxidation biofilm attached inside is cut into a suitable size with scissors so that its side length is consistent with the inner diameter of the polypropylene multifaceted hollow sphere. It is then inserted into the polypropylene multifaceted hollow sphere with a magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst attached to its surface, thereby achieving the coupling of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst and the anaerobic ammonia oxidation biofilm.

[0018] Step 7: Catalytic Degradation

[0019] The composite carrier obtained in step 6 is added to the ammonia nitrogen-polluted water body at a dosage of 1~1.5 g / L based on the composite photocatalyst. Then, the pH of the water environment is controlled within 8.0~8.5 using alkali to provide a slightly alkaline environment. Then, a photochemical reaction is carried out to oxidize ammonia nitrogen into nitrite. At the same time, anaerobic ammonia oxidation reaction is used to remove ammonia nitrogen and nitrite, thus achieving efficient removal of ammonia nitrogen.

[0020] Magnesium aluminum hydrotalcite is a type of mineral composed of magnesium oxide on its surface. 2+ and Al 3+ Hydrogen layer and interlayer CO3 2- Layered clay composed of anions exhibits a strong alkaline surface due to the presence of a surface hydroxide layer, while magnesium aluminum hydrotalcite provides alkaline active sites. This invention first utilizes a magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst on a floating carrier surface to oxidize ammonia nitrogen to nitrite under slightly alkaline conditions. Then, an anaerobic ammonia oxidation reaction is used to remove both ammonia nitrogen and nitrite, achieving in-situ removal of ammonia nitrogen from water.

[0021] Compared with existing technologies, the beneficial effects of this invention are as follows:

[0022] (1) The magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst provided by this invention has surface alkaline active sites. Magnesium-aluminum hydrotalcite is a type of photocatalyst composed of surface Mg... 2+ and Al 3+ Hydrogen layer and interlayer CO3 2- The anionic layered clay, due to the presence of a hydroxide layer on its surface, exhibits a strong alkaline surface on magnesium aluminum hydrotalcite, which can provide alkaline active sites. By modifying the surface of TiO2 with magnesium aluminum hydrotalcite, an alkaline microenvironment can be constructed. Under weakly alkaline conditions, ammonia nitrogen in the solution can be converted into free ammonia on the surface of the photocatalyst and carried out subsequent oxidation reactions without the need to add a large amount of alkali to the solution to obtain a strong alkaline environment, thus significantly saving processing costs.

[0023] (2) The present invention uses polypropylene multifaceted hollow spheres as floating carriers to load photocatalysts. With a large specific surface area, the system can float in water, which not only maximizes the utilization of light energy, but also makes it easy to recycle, breaking through the limitations of traditional powdered photocatalysts that are easy to settle and cannot be recycled.

[0024] (3) The present invention uses polypropylene multifaceted hollow spheres as floating carriers to load photocatalysts, presenting a multifaceted hollow shape. When water flows over the multifaceted spheres, it can drive the composite photocatalytic system to spin in the water. It utilizes the combined effects of water shear force, its own photocatalytic oxidation ability and external light to remove the biofilm that grows on the outer surface of the photocatalytic system due to long-term use, thereby achieving self-cleaning of the system surface.

[0025] (4) In this invention, modified iron-carbon-polyurethane sponge is used as a biofilm carrier for anaerobic ammonia oxidation. Iron-carbon consumes dissolved oxygen in the water in the early stage of the reaction. In the later stage, the high mass transfer resistance of the iron-carbon-polyurethane sponge carrier blocks the entry of oxygen. In addition, the pore size of the medium-pore polyurethane sponge with a porosity of 20~40ppi is small, thus forming a strict anaerobic environment inside the carrier, creating a suitable growth condition for anaerobic ammonia oxidizing bacteria.

[0026] (5) This invention utilizes an anaerobic ammonia oxidation biofilm as a coupling between a biological system and a photocatalytic system. The intermediate product of nitrite nitrogen generated by photocatalytic oxidation of ammonia nitrogen can be further reacted with ammonia nitrogen in wastewater to convert it into harmless nitrogen gas, thereby achieving higher efficiency in removing ammonia nitrogen from wastewater while removing nitrite nitrogen.

[0027] (6) The synthesis method of the present invention is simple, the raw materials are inexpensive, and there is no need to use highly toxic and polluting drugs. It conforms to the principles of green chemistry, has higher photocatalytic applicability and lower treatment cost, and can be widely used in wastewater treatment and other fields. Attached Figure Description

[0028] Figure 1 This paper presents the microstructure of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst prepared in this invention. Among them, (a) is the SEM and TEM image of the surface morphology of magnesium-aluminum hydrotalcite doped with titanium dioxide; (b) is the magnesium element mapping image of the composite photocatalyst surface; and (c) is the aluminum element mapping image of the composite photocatalyst surface.

[0029] Figure 2 shows the photocatalytic oxidation efficiency of ammonium chloride solution by the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst with different doping ratios in Example 1 of the present invention.

[0030] Figure 3 shows the composite carrier of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalytic coupling anaerobic ammonia oxidation in Example 2 of the present invention.

[0031] Figure 4 This is a diagram illustrating the denitrification mechanism of magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalytic coupled anaerobic ammonia oxidation, as implemented in this invention.

[0032] Figure 5 This describes the photocatalytic degradation effect of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst of Example 3 of the present invention on ammonia nitrogen. Detailed Implementation

[0033] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0034] The present invention discloses a nitrogen removal method using magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalysis coupled with anaerobic ammonium oxidation, the specific method of which is as follows:

[0035] (1) Purchased polypropylene hollow spheres as a floating carrier.

[0036] (2) Under constant temperature of 80℃, 200 mL of 0.8 mol / L sodium hydroxide and 0.02 mol / L sodium carbonate were slowly added dropwise to 100 mL of a mixed solution of 0.5 mol / L magnesium chloride hexahydrate and 0.2 mol / L aluminum chloride hexahydrate. During this process, vigorous stirring was required to promote the uniformity of the reaction, and the pH value of the solution was kept between 8 and 10 to optimize the co-precipitation conditions. Then, the solution was heated at 60℃ for 15 h to promote the growth and crystallization of magnesium aluminum hydrotalcite crystals. After cooling to room temperature, the precipitate was washed, dried, and ground to obtain magnesium aluminum hydrotalcite powder with high crystallinity, obvious lamellar structure, good thermal stability, and regular morphology, with a magnesium, aluminum, and carbonate element ratio of 6:2:1.

[0037] (3) Add 0.5 mL of hydrochloric acid to 40 mL of ethanol solution, stir at room temperature for 10 min, then slowly add 3.0 mL of tetrabutyl titanate and 0.072 g of magnesium aluminum layered double hydroxide to the solution, and continue stirring for 20 min to form a mixed solution. Then place it in an ultrasonic machine for ultrasonic treatment for 20-30 min to enhance dispersibility and form a slurry. Afterwards, use a reaction vessel to perform a hydrothermal reaction of the mixed solution at 150-180℃ for 20-24 h to allow TiO2 to grow and crystallize uniformly on the surface of magnesium aluminum layered double hydroxide. After cooling to room temperature, wash, dry, and grind the precipitate to finally obtain a magnesium aluminum layered double hydroxide / titanium dioxide composite photocatalyst with a doping ratio of 15 w%.

[0038] (4) Heat the polyamide resin coating in the beaker in a water bath at 150°C with a magnetic stirrer until it is completely melted. Then, add magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder to the molten polyamide resin at a weight ratio of 5-15%. Stir with a rotor for 20 minutes to disperse it evenly and obtain magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder coating. Then, use a brush to evenly coat the powder coating onto the polypropylene hollow polyhedral sphere. Wait for the powder coating to cool and then solidify again to complete the loading of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst onto the polypropylene hollow polyhedral sphere.

[0039] (5) First, the iron particles were pretreated by soaking them in 0.1 mol / L NaOH solution for 2 hours, then drying them in a vacuum drying oven at 105°C for 2 hours, then soaking them in 0.1 mol / L HCl solution for 2 hours, and then drying them in a vacuum drying oven at 105°C for 2 hours. The activated carbon particles were then washed three times with deionized water and then dried in a vacuum drying oven at 105°C for 2 hours. The dried iron and activated carbon particles were crushed into powder and sieved through 80-mesh and 200-mesh sieves, respectively, to obtain iron and activated carbon particles that met the requirements. A medium-pore polyurethane sponge with a porosity of 20-40 ppi was selected. The iron and carbon particles were mixed at a mass ratio of 1:1 and then loaded onto the surface of the polyurethane sponge using water-based polyurethane adhesive. The loading was successful when the sponge surface was uniformly covered with particles and they were not washed away by water. The preparation of the iron-carbon-polyurethane sponge was completed.

[0040] (6) The iron-carbon-polyurethane sponge was placed in a laboratory anaerobic ammonia oxidation reactor that was operating stably. After 30 days of enrichment, acclimatization and biofilm formation, it was finally taken out to obtain an iron-carbon-polyurethane sponge with an anaerobic ammonia oxidation biofilm growing inside.

[0041] (7) Cut the iron-carbon-polyurethane sponge with anaerobic ammonia oxidation biofilm inside into a suitable size and stuff it into the polypropylene multifaceted hollow sphere with magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst attached, so as to complete the coupling of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst and anaerobic ammonia oxidation biofilm.

[0042] (8) The obtained composite carrier is added to the ammonia nitrogen polluted water body. The dosage is 1~1.5g / L based on the composite photocatalyst. Then, the pH of the water body environment is controlled within 8.0~8.5 by using alkali, that is, a slightly alkaline environment is provided. Then, the photo-reaction is carried out to oxidize ammonia nitrogen into nitrite. At the same time, the anaerobic ammonia oxidation reaction is used to remove ammonia nitrogen and nitrite, so as to achieve efficient removal of ammonia nitrogen. Example 1:

[0043] This invention investigated the optimal doping amount of magnesium-aluminum layered double hydroxide (Mg-Al LDH) in titanium dioxide for this denitrification method. TiO2 was modified with different doping amounts (2.0 w%, 5.0 w%, 10.0 w%, 15.0 w%, and 20.0 w%), and the photocatalytic performance was compared to determine the optimal doping amount. Pure TiO2 photocatalytic oxidation of NH4Cl was used as a control. The photocatalytic experiment was conducted in a 100 mL beaker containing 50 mL of NH4Cl solution (25.0 mg / L) at room temperature (25 ± 2 °C) without stirring to simulate a real-world environment. The initial pH of the solution was adjusted to 8.3 using potassium monohydrogen phosphate, potassium dihydrogen phosphate, and sodium hydroxide solution. Mg-Al LDH / titanium dioxide composite photocatalyst powders with different doping ratios were added to the solution at a concentration of 1 g / L. After reaching adsorption / desorption equilibrium through a 1-hour dark adsorption reaction, the sample was irradiated with a xenon lamp for 4 hours at a light intensity of 500±10 uW / cm². 2 (230 nm < λ < 1100 nm). 1 mL of solution was extracted at fixed time intervals, filtered through a 0.45 μm filter membrane, and the ammonia nitrogen concentration in the sample was measured using a spectrophotometer to analyze its concentration changes. Experimental results are as follows: Figure 2 As shown: With the gradual increase of magnesium aluminum hydrotalcite doping content, the composite photocatalyst oxidizes NH4+. + The -N ability gradually increases, and finally, when the magnesium aluminum layered double hydroxide doping content is 15.0 w%, the highest oxidation capacity level is reached, which can oxidize about 45% of NH4 within 4 hours. + -N, this is because the magnesium aluminum hydrotalcite doped on the TiO2 surface is strongly alkaline, providing surface alkaline sites, allowing NH4+ in the solution to be neutralized. + It is converted into NH3, which can be further oxidized by TiO2. With the further increase of magnesium aluminum layered double hydroxide doping (20.0 w%), the photocatalytic oxidation of NH4... + The -N efficiency drops sharply, possibly because excess magnesium aluminum hydrotalcite blocks the reactive sites on the TiO2 surface, hindering photons from reaching the photocatalyst surface and also occupying NH4+. + -N is the active site required for the oxidation reaction. Therefore, the optimal magnesium aluminum layered double hydroxide doping concentration in this method is 15.0 wt%. Example 2:

[0044] The detailed implementation steps of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalytic coupled anaerobic ammonium oxidation composite support of the present invention are as follows, and the resulting physical image is shown below. Figure 3 As shown:

[0045] 1. Preparation of floating carrier: Purchase commercially available polyethylene multifaceted hollow spheres with a diameter of 50mm and an internal hollow cylinder diameter of 25mm.

[0046] 2. Preparation of magnesium-aluminum hydrotalcite powder: Using a separatory funnel, 200 mL of 0.8 mol / L sodium hydroxide and 0.02 mol / L sodium carbonate were slowly added dropwise to 100 mL of a mixed solution of 0.5 mol / L magnesium chloride hexahydrate and 0.2 mol / L aluminum chloride hexahydrate. The mixture was vigorously stirred in a water bath at 80°C for 2 h to complete co-precipitation, while maintaining the pH of the solution between 8 and 10. The solution was then heated at 60°C for 15 h. After cooling to room temperature, the resulting suspension was centrifuged, filtered, and the precipitate was washed with deionized water and ethanol. Finally, the precipitate was dried in a vacuum drying oven at 70°C. The solid was then ground to obtain magnesium-aluminum hydrotalcite powder with a magnesium, aluminum, and carbonate element ratio of 6:2:1.

[0047] 3. Preparation of magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst: 0.5 mL of hydrochloric acid was added to 40 mL of ethanol solution, and the mixture was stirred with a magnetic stirrer for 10 min at room temperature to form solution A. 3.0 mL of tetrabutyl titanate and 0.072 g of magnesium-aluminum hydrotalcite were slowly added to the solution, and the mixture was stirred for 20 min to form a mixed solution. The solution was then ultrasonically treated for 25 min to enhance dispersibility and form slurry B. Slurry B was transferred to a polytetrafluoroethylene reactor liner, and the liner was placed in a high-pressure reactor. The mixture was subjected to hydrothermal reaction at 180 °C in a vacuum drying oven for 24 h. After cooling to room temperature, the precipitate was centrifuged and washed with ethanol and deionized water. The precipitate was dried in a 70 °C oven to obtain powder C. Powder C was ground to obtain a 15 w% magnesium-aluminum hydrotalcite / titanium dioxide photocatalyst.

[0048] 4. Adhesion of magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst on a floating support:

[0049] (1) Place the polyamide resin in a beaker, pour oil into a magnetically stirred water bath, and heat the beaker at 150°C in an oil bath to completely melt the polyamide resin.

[0050] (2) Add 10% by mass of polyamide resin magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder to a beaker, stir with a rotor for 20 min to disperse it evenly, and obtain magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder coating.

[0051] (3) Apply the powder coating evenly to the polypropylene hollow sphere with a brush, wait for the powder coating to cool and then solidify again, so that the magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst is firmly adhered to the surface of the polypropylene hollow sphere.

[0052] 5. Modification of polyurethane foam:

[0053] First, the iron particles were pretreated by soaking them in 0.1 mol / L NaOH solution for 2 hours, then drying them in a vacuum drying oven at 105°C for 2 hours, followed by soaking them in 0.1 mol / L HCl solution for 2 hours and drying them again in a vacuum drying oven at 105°C for 2 hours. Activated carbon particles were washed three times with deionized water and dried at room temperature before use. The dried iron and activated carbon particles were then pulverized into powder using a mortar and pestle and sieved through 80-mesh and 200-mesh screens respectively to obtain iron and activated carbon particles that met the requirements. A medium-pore polyurethane sponge with a porosity of 25 ppi was selected. The iron and carbon particles were mixed at a mass ratio of 1:1 and then loaded onto the surface of the polyurethane sponge using water-based polyurethane adhesive. Successful loading was indicated by the sponge surface being uniformly covered with particles that were not washed away by water, thus completing the preparation of the iron-carbon-polyurethane sponge.

[0054] 6. Formation of anaerobic ammonia oxidation biofilm:

[0055] Iron-carbon-polyurethane sponge was placed in an anaerobic ammonia oxidation reactor, which was a stable laboratory USBR reactor with a working volume of 7L, a hydraulic retention time of 48h, and an internal temperature of 32~35℃. After 30 days of enrichment, acclimatization, and biofilm formation, the sponge was finally removed to obtain an iron-carbon-polyurethane sponge carrier with an anaerobic ammonia oxidation biofilm attached inside.

[0056] 7. Coupling of magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst and anaerobic ammonia oxidation biofilm:

[0057] A polyurethane sponge carrier with an anaerobic ammonia oxidation biofilm attached inside was cut into cubes with a side length of 25 mm and stuffed into a polypropylene multifaceted hollow sphere with a magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst attached to its surface. This achieved the coupling of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst and the anaerobic ammonia oxidation biofilm, resulting in a composite carrier for magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalysis coupled with anaerobic ammonia oxidation. Example 3:

[0058] The nitrogen removal method of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalysis coupled with anaerobic ammonia oxidation of the present invention is used to treat ammonia nitrogen in water under slightly alkaline conditions:

[0059] This photocatalytic experiment was conducted in a 100 mL beaker containing 50 mL of NH4Cl solution (25.0 mg / L). The experiment was carried out at room temperature (25 ± 2 °C) without stirring to simulate the actual environment. The initial pH of the solution was adjusted to 8.3 using potassium monohydrogen phosphate, potassium dihydrogen phosphate acid-base pair, and sodium hydroxide solution. One modified photocatalytic coupled anaerobic ammonia oxidation floating carrier prepared in this invention was placed in the solution, with a mass of 1.89 g. Based on the estimate that the photocatalyst accounts for 3% of the total mass of the floating carrier, the photocatalyst concentration in this experiment was 1.134 g / L. After reaching adsorption / desorption equilibrium through a 1 h dark adsorption reaction, the photocatalytic system solution was irradiated with a xenon lamp for 4 h at a light intensity of 500 ± 10 uW / cm². 2 (230 nm < λ < 1100 nm). 1 mL of solution was extracted at fixed time intervals, filtered through a 0.45 μm filter membrane, and the concentrations of ammonia nitrogen, nitrate nitrogen, and nitrite nitrogen in the sample were measured using a spectrophotometer. The concentration changes and experimental results were analyzed. Experimental results show that the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalytic coupled anaerobic ammonia oxidation denitrification method of this invention has a significant advantage over the pure titanium dioxide photocatalytic method in degrading ammonia nitrogen under low alkaline conditions. After 4 hours of light irradiation, more than 40% of the ammonia nitrogen in the water can be oxidized, while previous studies have shown that pure titanium dioxide has no degradation effect under low alkaline conditions. Simultaneously, most of the oxidized ammonia nitrogen is converted into harmless nitrogen gas, and a small portion is converted into nitrite and nitrate. The anaerobic ammonia oxidation biofilm can further react nitrite and ammonia nitrogen to generate harmless nitrogen gas (…). Figure 5 ).

Claims

1. A nitrogen removal method using magnesium aluminum hydrotalcite / titanium dioxide composite photocatalysis coupled with anaerobic ammonium oxidation, characterized in that... Includes the following steps: Step 1: Preparation of magnesium aluminum hydrotalcite powder Under constant temperature of 80℃, sodium hydroxide and sodium carbonate solutions were added dropwise to a mixed solution of magnesium chloride hexahydrate and aluminum chloride hexahydrate to adjust the pH value of the system to 8~10; then the system was heated at 60℃ to promote the growth and crystallization of magnesium aluminum hydrotalcite crystals. After cooling to room temperature, the precipitate was washed, dried and ground to obtain magnesium aluminum hydrotalcite powder. Step 2: Preparation of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst Hydrochloric acid was added to the ethanol solution and stirred at room temperature. Then, tetrabutyl titanate and magnesium aluminum hydrotalcite were added to the solution and stirred continuously to form a mixed solution. Ultrasonic treatment was used to enhance dispersibility and form a slurry. Then, the temperature was raised to 150~180℃ for hydrothermal reaction for 20~24h to allow TiO2 to grow and crystallize uniformly on the surface of magnesium aluminum hydrotalcite. After cooling to room temperature, the precipitate was washed, dried and ground to obtain magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst. Step 3: Loading the composite photocatalyst on the floating support The polyamide resin coating was heated to 150℃ until it was completely melted and in a molten state. Then, the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder was added into the molten polyamide resin and stirred and dispersed evenly to obtain the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder coating. Subsequently, the powder coating was evenly coated on the polypropylene multifaceted hollow spheres and allowed to cool and solidify, thus completing the loading of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst powder on the polypropylene multifaceted hollow spheres. Step 4: Modification of polyurethane foam First, the iron particles were pretreated by soaking them in 0.1 mol / L NaOH solution and 0.1 mol / L HCl solution for 2 hours in sequence, and then vacuum dried. The activated carbon particles were washed with deionized water and vacuum dried. The dried iron particles and activated carbon particles were pulverized into powder and sieved through 80 mesh and 200 mesh screens, respectively. After mixing, they were loaded onto the surface of polyurethane sponge with water-based polyurethane adhesive to complete the preparation of iron-carbon-polyurethane sponge. Step 5: Formation of anaerobic ammonia oxidation biofilm Iron-carbon-polyurethane sponge was placed in an anaerobic ammonia oxidation reactor. After 30 days of enrichment, acclimatization, and biofilm formation, it was finally removed to obtain an iron-carbon-polyurethane sponge carrier with a large amount of anaerobic ammonia oxidation biofilm attached inside. Step 6: Coupling of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst and anaerobic ammonium oxidation biofilm The iron-carbon polyurethane sponge with an anaerobic ammonia oxidation biofilm attached inside is cut into a suitable size with scissors so that its size is consistent with the inner diameter of the polypropylene multifaceted hollow sphere. It is then inserted into the polypropylene multifaceted hollow sphere with a magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst attached to its surface, thereby achieving the coupling of the magnesium-aluminum hydrotalcite / titanium dioxide composite photocatalyst and the anaerobic ammonia oxidation biofilm. Step 7: Catalytic Degradation The composite carrier obtained in step 6 is added to the ammonia nitrogen-polluted water body. The dosage is 1~1.5g / L based on the composite photocatalyst. The system is controlled to be a slightly alkaline environment. Then, a photo-irradiation reaction is carried out to oxidize ammonia nitrogen into nitrite. At the same time, anaerobic ammonia oxidation reaction is used to remove ammonia nitrogen and nitrite, thus achieving efficient removal of ammonia nitrogen.

2. The denitrification method according to claim 1, characterized in that: In step 1, the magnesium aluminum hydrotalcite powder has a regular hexagonal flake shape, and the ratio of magnesium, aluminum and carbonate is 6:2:

1.

3. The denitrification method according to claim 1, characterized in that: In step 2, the doping ratio of magnesium aluminum hydrotalcite in the magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst is 15 wt%.

4. The denitrification method according to claim 1, characterized in that: In step 3, the amount of magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder added is 5-15% of the mass of polyamide resin.

5. The denitrification method according to claim 1, characterized in that: In step 3, after being loaded onto polypropylene multifaceted hollow spheres, the magnesium aluminum hydrotalcite / titanium dioxide composite photocatalyst powder accounts for 3%-5% of the total mass.

6. The denitrification method according to claim 1, characterized in that: In step 4, iron and carbon particles are mixed in a mass ratio of 1:

1.

7. The denitrification method according to claim 1, characterized in that: In step 4, the porosity of the polyurethane sponge is 20~40ppi.

8. The denitrification method according to claim 1, characterized in that: In step 7, the slightly alkaline environment refers to a system pH value of 8.0~8.5.

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