A method for treating aquaculture wastewater

Abstract

The present application relates to sewage treatment technical field, specifically for a kind of aquaculture sewage treatment method, aquaculture sewage is reacted with catalyst under illumination, can;The catalyst is composed of sodium bismuth titanate with tubular structure and alkali metal doped g-C3N4, sodium bismuth titanate and alkali metal doped g-C3N4 form the heterojunction interface of close combination by electrostatic attraction, the synergistic effect of both is fully played, the defects of single titanate material light absorption range limited and single g-C3N4 carrier recombination fast are overcome, the method of the present application is mild, without additional chemical oxidizing reagent, no secondary pollution is generated, can directly utilize sunlight to drive reaction, operating cost is low, in the field of livestock and poultry aquaculture sewage advanced treatment has wide application prospect.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically a method for treating aquaculture wastewater. Background Technology

[0002] Livestock wastewater typically contains high concentrations of organic pollutants, ammonia nitrogen, phosphorus, and residual antibiotics. Direct discharge of such wastewater will lead to eutrophication of water bodies, groundwater pollution, and damage to soil ecosystems, posing a serious threat to the environment and human health.

[0003] Currently, wastewater treatment methods for aquaculture mainly fall into three categories: physical, chemical, and biological methods. Physical methods primarily include technologies such as screen filtration, sedimentation, flotation, and membrane separation. Sedimentation and flotation can effectively remove suspended solids and some organic matter from wastewater, but they are inefficient at removing dissolved pollutants. While membrane separation technology offers good treatment results, it suffers from membrane fouling, high investment and operating costs, limiting its application in large-scale aquaculture wastewater treatment. Chemical methods include flocculation sedimentation, chemical oxidation, and electrochemical methods. Flocculation sedimentation uses flocculants such as aluminum and iron salts to cause pollutants to precipitate, but it produces large amounts of chemical sludge and is ineffective at removing ammonia nitrogen. Advanced oxidation technologies such as Fenton oxidation and ozone oxidation have high degradation efficiency, but the reagents are expensive, the operation is complex, and secondary pollution may be introduced. Biological methods are currently the most widely used aquaculture wastewater treatment technologies, such as anaerobic digestion, aerobic activated sludge processes, and biofilm processes. However, the high concentrations of ammonia nitrogen and antibiotics in aquaculture wastewater strongly inhibit microbial activity, leading to slow start-up and unstable operation of traditional biological treatment systems, especially with a sharp decline in treatment efficiency during cold seasons. Although anaerobic digestion can produce biogas, the effluent still contains high concentrations of ammonia nitrogen and recalcitrant organic matter, requiring subsequent aerobic treatment. This process is lengthy and requires a large area.

[0004] Photocatalysis, as an emerging advanced oxidation technology, utilizes photocatalysts to generate highly oxidizing reactive oxygen species under light, which can non-selectively mineralize organic pollutants into carbon dioxide and water, and at the same time oxidize ammonia nitrogen into nitrogen gas or nitrates. It has significant advantages such as mild reaction conditions, no secondary pollution, and the ability to utilize sunlight, and shows great potential in the treatment of recalcitrant aquaculture wastewater.

[0005] Titanates, as a class of inorganic compounds with unique layered or tunnel structures, have wide applications in photocatalysis, ion exchange, and other fields. Sodium bismuth titanate, a multi-component metal oxide composed of titanium-oxygen octahedrons, bismuth ions, and sodium ions, can effectively separate photogenerated carriers under the influence of a built-in electric field, demonstrating excellent photocatalytic potential. However, sodium bismuth titanate alone may suffer from insufficient specific surface area or limited light absorption range, restricting its practical application. Summary of the Invention

[0006] Purpose of the invention: To address the above-mentioned technical problems, this invention proposes a method for treating aquaculture wastewater.

[0007] The technical solution adopted is as follows: A method for treating aquaculture wastewater involves reacting the wastewater with a catalyst under sunlight. The catalyst consists of sodium bismuth titanate with a tubular structure and alkali metal-doped g-C3N4.

[0008] Furthermore, the preparation method of the sodium bismuth titanate is as follows: To prepare H2Ti3O7 nanorods, bismuth salt, sodium hydroxide, H2Ti3O7 nanorods, ethylene glycol, and water were mixed and then sealed for hydrothermal reaction at 170-190℃ for 36-72 hours. The product was collected, washed, dried, and then calcined to obtain the final product.

[0009] H₂Ti₃O₇ itself is a layered titanate, with exchangeable H₂ between the layers. + In an ethylene glycol-water mixture containing NaOH and bismuth salt, OH - Na seizes interlayer protons + and Bi 3+ Subsequently, it inserts into the interlayer, generating an amorphous Na-Bi-Ti-O precursor shell in situ on the surface of the nanorods. Ethylene glycol then reacts with Bi... 3+ The complexation effect makes ion exchange more uniform. Once the surface shell becomes dense, the internal H₂Ti₃O₇ core begins to dissolve, because the outward migration rate of titanate ions is higher than that of Na₂Ti₃O₇. + and Bi 3+ Inward diffusion leads to the Kirkendall effect, causing net outflow of matter and gradually hollowing out the core, forming a hollow tubular profile. Simultaneously, Ostwald ripening promotes the dissolution and recrystallization of small inner wall grains onto the outer shell, making the tube wall denser and smoother. A tubular hydrated precursor can be obtained by hydrothermal reaction at 170-190℃ for 36-72 hours. Subsequent calcination removes moisture and organic matter, transforming the precursor from an amorphous state to the perovskite phase Na. 0.5 Bi 0.5 TiO3 undergoes a crystallization transformation, and its tubular morphology is fully preserved due to its thermal stability, ultimately yielding tubular sodium titanate with high crystallinity and good photocatalytic activity.

[0010] Furthermore, the mass ratio of bismuth salt, sodium hydroxide and H2Ti3O7 nanorods, ethylene glycol and water is 5-10:0.5-1:1-5:80-120:40-60.

[0011] Furthermore, the calcination temperature is 700-800℃.

[0012] Furthermore, the calcination time is 1-3 hours.

[0013] Furthermore, the preparation method of the H2Ti3O7 nanorods is as follows: Titanium dioxide and sodium hydroxide solution were mixed and poured into a reaction vessel, sealed, and subjected to hydrothermal reaction to obtain Na2Ti3O7 nanorods. The Na2Ti3O7 nanorods were then subjected to ion exchange in hydrochloric acid solution to obtain the final product.

[0014] Furthermore, the concentration of the sodium hydroxide solution is 9-11 mol / L.

[0015] Furthermore, the hydrothermal reaction is carried out at a temperature of 180-200℃ for 36-72 hours.

[0016] Furthermore, the concentration of the hydrochloric acid solution is 0.5-1.5 mol / L.

[0017] Furthermore, the preparation method of the alkali metal-doped g-C3N4 is as follows: Thiourea and alkali metal iodide salts are dissolved in water, and the water is evaporated to obtain a solid precursor. The solid precursor is placed in a sealed crucible and heat-treated at 500-600℃ for 1-5 hours to obtain the final product.

[0018] Furthermore, the amount of the alkali metal iodide salt used is 1%-10% of the mass of thiourea.

[0019] Furthermore, the mass ratio of sodium bismuth titanate to alkali metal-doped g-C3N4 is 1-4:1-4.

[0020] Furthermore, the catalyst is prepared as follows: Add sodium bismuth titanate to water to adjust the pH to 2-3, add alkali metal-doped g-C3N4 to water to adjust the pH to 9-10, then mix the two, separate the solid and dry it.

[0021] This invention provides a method for treating aquaculture wastewater, employing a composite catalyst composed of sodium bismuth titanate with a tubular structure and alkali metal-doped g-C3N4, which can efficiently degrade organic pollutants and ammonia nitrogen in aquaculture wastewater under light irradiation. Compared with existing technologies, this invention has the following significant advantages: This invention achieves efficient spatial separation of photogenerated electron-hole pairs by constructing a heterojunction composite catalyst of tubular sodium bismuth titanate and alkali metal-doped g-C3N4, which significantly enhances the redox capability of the system.

[0022] The sodium bismuth titanate synthesized in this invention possesses a unique nanotube structure with a larger specific surface area and more abundant active sites. Simultaneously, the one-dimensional nanotube structure facilitates the rapid axial directional transport of photogenerated carriers, effectively suppressing the recombination of electrons and holes in the bulk phase, thereby significantly improving the photocatalytic quantum efficiency.

[0023] In-situ doping of g-C3N4 with alkali metal iodides effectively modulated its electronic structure and band structure, enhancing its absorption of visible light. The introduced alkali metal ions also act as charge trapping centers, further suppressing carrier recombination, improving the photocatalytic activity of g-C3N4, broadening the photoresponse range of the composite material, and enhancing the light energy utilization rate in the visible light region.

[0024] Alkali metal ions can also serve as atomic-level charge trapping / releasing sites, lowering the energy barrier for charge migration at the interface and promoting the directional transfer of photogenerated electrons from g-C3N4 to the sodium bismuth titanate interface. Simultaneously, these sites can instantaneously trap and release electrons, effectively suppressing electron-hole recombination at the interface and allowing more charge carriers to survive and participate in redox reactions.

[0025] This invention employs a self-assembly method to composite two materials, which is simple to operate and easily scalable. The two materials form a tightly bonded heterojunction interface through electrostatic attraction, fully leveraging their synergistic enhancement effect and overcoming the limitations of single titanate materials in terms of limited light absorption range and rapid carrier recombination in single g-C3N4. The method described in this invention operates under mild conditions, requires no external chemical oxidants, produces no secondary pollution, can be directly driven by sunlight, has low operating costs, and shows broad application prospects in the field of advanced treatment of livestock and poultry wastewater. Attached Figure Description

[0026] Figure 1 The image shows the XRD pattern of sodium bismuth titanate prepared in Example 1 of this invention.

[0027] Figure 2 This is a TEM image of sodium bismuth titanate prepared in Example 1 of the present invention. Detailed Implementation

[0028] Unless otherwise specified in the examples, the conditions were performed under standard conditions or as recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products. Techniques not mentioned in this invention refer to existing technologies. Unless otherwise specified, the following examples and comparative examples are parallel experiments, using the same processing steps and parameters.

[0029] Example 1:

[0030] A method for preparing a catalyst is as follows: 20g of titanium dioxide and 500mL of 10mol / L sodium hydroxide solution were mixed and poured into a reaction vessel. The vessel was sealed and heated to 190℃ for hydrothermal reaction for 48 hours. After returning to room temperature, the mixture was centrifuged, the precipitate was collected, washed with deionized water, and dried to obtain Na₂Ti₃O₇ nanorods. The Na₂Ti₃O₇ nanorods were then added to 500mL of 1mol / L hydrochloric acid for ion exchange for 72 hours, with the hydrochloric acid replaced every 12 hours. The mixture was then centrifuged, the precipitate was collected, washed with deionized water, and dried to obtain H₂Ti₃O₇ nanorods. 7.28g of bismuth nitrate pentahydrate was dissolved in 100g of ethylene glycol, and 50g of deionized water, 0.6g of sodium hydroxide, and 2.58g of sodium hydroxide were added. H₂Ti₃O₇ nanorods were ultrasonically dispersed and then subjected to a hydrothermal reaction at 180°C for 48 hours in a sealed container. After cooling, the product was centrifuged and washed with deionized water and anhydrous ethanol, respectively. After drying, the product was calcined at 750°C for 2 hours in air to obtain sodium bismuth titanate with a nanotubular structure. Its XRD pattern is shown in [reference needed]. Figure 1 The XRD pattern shows characteristic peaks corresponding to the (110), (101), (012), and (220) lattice planes of sodium bismuth titanate, which correspond to PDF#46-0001 in the PDF card. The TEM image is available in [link to PDF]. Figure 2 This confirms the formation of nanotube structures.

[0031] Dissolve 10g of thiourea and 0.5g of sodium iodide in 100mL of deionized water, evaporate to dryness under reduced pressure to obtain a solid precursor, place the solid precursor in a sealed crucible, heat it in a muffle furnace to 550℃ for 2 hours to obtain sodium-doped g-C3N4.

[0032] Add 0.25g of sodium bismuth titanate to 10mL of deionized water and adjust the pH to 2-3 with dilute hydrochloric acid. Add 0.25g of sodium-doped g-C3N4 to the water and adjust the pH to 9-10 with ammonia. Then mix the two together, stir for 1 hour, let stand for 10 hours, centrifuge to separate the solid, wash with water and dry.

[0033] Example 2:

[0034] The process is basically the same as in Example 1, except that 10g of thiourea and 0.1g of sodium iodide are dissolved in 100mL of deionized water, evaporated under reduced pressure to remove water to obtain a solid precursor, and the solid precursor is placed in a sealed crucible and heated in a muffle furnace to 550°C for 2 hours to obtain sodium-doped g-C3N4.

[0035] Example 3:

[0036] The process is basically the same as in Example 1, except that 10g of thiourea and 1g of sodium iodide are dissolved in 100mL of deionized water, evaporated under reduced pressure to remove water to obtain a solid precursor, and the solid precursor is placed in a sealed crucible and heated in a muffle furnace to 550°C for 2 hours to obtain sodium-doped g-C3N4.

[0037] Example 4:

[0038] The process is basically the same as in Example 1, except that 0.1g of sodium bismuth titanate is added to 10mL of deionized water, and the pH is adjusted to 2-3 with dilute hydrochloric acid. 0.4g of sodium-doped g-C3N4 is added to the water, and the pH is adjusted to 9-10 with ammonia. The two are then mixed, stirred for 1 hour, allowed to stand for 10 hours, centrifuged to separate the solid, washed with water and dried.

[0039] Example 5:

[0040] The process is basically the same as in Example 1, except that 0.4g of sodium bismuth titanate is added to 10mL of deionized water, and the pH is adjusted to 2-3 with dilute hydrochloric acid. 0.1g of sodium-doped g-C3N4 is added to the water, and the pH is adjusted to 9-10 with ammonia. The two are then mixed, stirred for 1 hour, allowed to stand for 10 hours, centrifuged to separate the solid, washed with water and dried.

[0041] Comparative Example 1: The method is basically the same as in Example 1, except that commercially available sodium bismuth titanate (average particle size 5 μm) is used instead of the self-made sodium bismuth titanate with a nanotube structure.

[0042] Comparative Example 2: It is basically the same as Example 1, except that sodium iodide is not added.

[0043] Comparative Example 3: It is basically the same as Example 1, except that sodium-doped g-C3N4 is used alone as a catalyst.

[0044] Comparative Example 4: The process is essentially the same as in Example 1, except that sodium bismuth titanate with a nanotube structure is used alone as a catalyst.

[0045] Performance testing: The experimental water was taken from wastewater from a large dairy farm. After simple settling, the supernatant was used as the target wastewater. Its initial water quality indicators were: Chemical Oxygen Demand (COD) 1860 mg / L, ammonia nitrogen (NH4+)... + -N) 450 mg / L, pH 7.8. The photocatalytic reaction was carried out in a self-made photoreactor with a circulating cooling water jacket, using a 300W xenon lamp (equipped with an AM1.5 filter) as a simulated solar source, with the light intensity controlled at 100 mW / cm². 2Take 100 mL of the above-mentioned aquaculture wastewater and add 0.1 g of the catalyst from Examples 1-5 and Comparative Examples 1-4, respectively. Stir in the dark for 30 minutes to reach adsorption-desorption equilibrium. Then turn on the light source to start the photocatalytic reaction. After 60 minutes, take a sample, filter it through a microfiltration membrane, and determine the concentrations of COD and ammonia nitrogen using the dichromate method (HJ 828-2017) and Nessler's reagent spectrophotometry (HJ 535-2009), respectively, and calculate the removal rate.

[0046] The test results are shown in Table 1 below.

[0047]

[0048] As shown in Table 1 above, the method of the present invention can effectively remove COD and ammonia nitrogen from aquaculture wastewater.

[0049] Comparative Example 1 used commercially available micron-sized sodium bismuth titanate, and its performance was inferior to that of Example 1.

[0050] The reason lies in the fact that the sodium bismuth titanate synthesized by the hydrothermal method in Example 1 possesses a unique nanotube structure with a specific surface area far greater than that of commercially available micron-sized particles, providing abundant reactive sites. Simultaneously, the one-dimensional nanotube structure facilitates the rapid directional transport of photogenerated carriers along the axial direction, effectively suppressing the recombination of electron-hole pairs in the bulk phase. Furthermore, the heterojunction interface formed between the nanotubes and g-C3N4 is more compact and uniform, resulting in higher charge separation efficiency. In contrast, the micron-sized particles in Comparative Example 1 are large, prone to aggregation, have a small effective interface area, and experience severe recombination of photogenerated carriers in the bulk phase, leading to weakened photocatalytic activity.

[0051] Comparative Example 2 did not add sodium iodide during the preparation process, that is, it used undoped pure g-C3N4 combined with sodium bismuth titanate nanotubes, and its performance was inferior to that of Example 1.

[0052] In Example 1, the addition of sodium iodide enabled in-situ doping of g-C3N4 with sodium. The introduction of sodium effectively modulates the electronic structure of g-C3N4, optimizes its band structure, enhances its absorption of visible light, and improves the material's conductivity. Furthermore, the introduced sodium ions further suppress the recombination of photogenerated carriers. Without this doping step, g-C3N4 exhibits lower activity, the constructed heterojunction lacks sufficient charge separation capability, and the synergistic catalytic effect of the composite material is significantly reduced.

[0053] Comparative Example 3 used sodium-doped g-C3N4 alone, and Comparative Example 4 used nanotube bismuth titanate sodium alone. The photocatalytic degradation efficiency of both was at a low level, far less than that of the composite catalyst in Example 1.

[0054] This stark contrast clearly demonstrates that the significant performance improvement in Example 1 is not a simple superposition of the activities of the two components, but rather stems from the synergistic enhancement effect produced by the highly efficient heterojunction constructed between them. Under illumination, both semiconductors are simultaneously excited, and the built-in electric field at the heterojunction interface drives the efficient spatial separation of photogenerated electrons and holes, maximizing the retention of photogenerated electrons with strong reducing properties and photogenerated holes with strong oxidizing properties, enabling them to effectively participate in the degradation reaction of pollutants. When either component is used alone, photogenerated carriers recombine in large quantities within a very short time, failing to generate sufficient active free radicals to drive the catalytic reaction, thus resulting in low degradation efficiency.

[0055] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for treating aquaculture wastewater, characterized in that, Wastewater from aquaculture can react with a catalyst under sunlight; The catalyst consists of sodium bismuth titanate with a tubular structure and alkali metal-doped g-C3N4.

2. The method for treating aquaculture wastewater as described in claim 1, characterized in that, The preparation method of the sodium bismuth titanate is as follows: To prepare H2Ti3O7 nanorods, bismuth salt, sodium hydroxide, H2Ti3O7 nanorods, ethylene glycol, and water were mixed and then sealed for hydrothermal reaction at 170-190℃ for 36-72 hours. The product was collected, washed, dried, and then calcined to obtain the final product.

3. The method for treating aquaculture wastewater as described in claim 2, characterized in that, The calcination temperature is 700-800℃.

4. The method for treating aquaculture wastewater as described in claim 2, characterized in that, The preparation method of the H2Ti3O7 nanorods is as follows: Titanium dioxide and sodium hydroxide solution were mixed and poured into a reaction vessel, sealed, and subjected to hydrothermal reaction to obtain Na2Ti3O7 nanorods. The Na2Ti3O7 nanorods were then subjected to ion exchange in hydrochloric acid solution to obtain the final product.

5. The method for treating aquaculture wastewater as described in claim 4, characterized in that, The concentration of the sodium hydroxide solution is 9-11 mol / L.

6. The method for treating aquaculture wastewater as described in claim 4, characterized in that, The hydrothermal reaction takes place at a temperature of 180-200℃ for 36-72 hours.

7. The method for treating aquaculture wastewater as described in claim 1, characterized in that, The preparation method of the alkali metal-doped g-C3N4 is as follows: Thiourea and alkali metal iodide salts are dissolved in water, and the water is evaporated to obtain a solid precursor. The solid precursor is placed in a sealed crucible and heat-treated at 500-600℃ for 1-5 hours to obtain the final product.

8. The method for treating aquaculture wastewater as described in claim 7, characterized in that, The amount of alkali metal iodide used is 1%-10% of the mass of thiourea.

9. The method for treating aquaculture wastewater as described in claim 1, characterized in that, The mass ratio of sodium bismuth titanate to alkali metal-doped g-C3N4 is 1-4:1-4.

10. The method for treating aquaculture wastewater as described in claim 1, characterized in that, The catalyst is prepared as follows: Add sodium bismuth titanate to water to adjust the pH to 2-3, add alkali metal-doped g-C3N4 to water to adjust the pH to 9-10, then mix the two, separate the solid and dry it.

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