A method for preparing a diatomite-supported graphite carbon nitride composite photocatalyst

Abstract

This invention belongs to the field of catalysis technology, specifically relating to a method for preparing a diatomaceous earth-supported graphite carbon nitride composite photocatalyst. The preparation method includes the following steps: Step A, hydrothermally reacting diatoms and an aqueous solution containing ammonia nitrogen at 150–200°C for 5–15 hours, cooling to room temperature, separating the solid and liquid phases, washing and drying the solid product to obtain nitrogen-containing diatomaceous earth; Step B, calcining urea and / or melamine with the nitrogen-containing diatomaceous earth obtained in Step A at 500–600°C for 2–6 hours, cooling to room temperature to obtain the diatomaceous earth-supported graphite carbon nitride composite photocatalyst. This invention combines the characteristics of ammonia nitrogen solidification by living diatoms and the template effect to develop a highly efficient, non-toxic, and green composite photocatalyst. Without harming the aquatic ecosystem, it reduces ammonia nitrogen wastewater discharge and promotes the production of high-value-added photocatalysts, achieving both water quality improvement and commercial value enhancement.

Description

Technical Field

[0001] This invention belongs to the field of catalysis technology, specifically relating to a method for preparing a diatomaceous earth-supported graphite carbon nitride composite photocatalyst. Background Technology

[0002] With economic development, people's demand for aquatic products is increasing. As catches cannot meet market demand, aquaculture has developed rapidly. However, while satisfying people's material needs, it has also brought corresponding environmental problems. The discharge of large amounts of aquaculture wastewater has a huge impact on the surrounding environment, leading to the deterioration of aquatic environments, frequent red tides, and damage to ecological balance and biodiversity. The decline in water quality in aquaculture areas has also caused huge losses to my country's fishery economy. Ammonia nitrogen is the main pollutant in aquaculture wastewater and is also the most difficult substance to remove. Its main sources are residual feed and excrement from farmed organisms. The most prominent harm of ammonia nitrogen to humans is eutrophication, which disrupts the ecological balance of aquatic bodies. Large amounts of ammonia nitrogen-containing wastewater entering water bodies lead to excessive nutrient production in aquatic plants, resulting in the proliferation of aquatic organisms, mainly algae. Some algae are toxic, reducing the living space for fish and affecting and disrupting the ecological balance of aquatic bodies. Moreover, non-ionic ammonia nitrogen can directly produce toxicity to farmed organisms, affecting their growth and development, causing extremely serious consequences and huge economic losses to aquaculture.

[0003] Currently, the main methods for treating ammonia nitrogen wastewater include physicochemical methods, biological methods, and chemical methods. Physicochemical methods are simple to operate and easy to control, offering stable ammonia removal. However, these methods also have drawbacks: expensive equipment, high energy consumption, low ammonia removal efficiency at low temperatures, and the release of free ammonia causing secondary pollution. Chemical methods offer fast reaction speeds, stable treatment effects, and are less affected by temperature. They remove ammonia nitrogen while also removing some organic matter and having a bactericidal effect. However, residual chemicals in the treated water may cause secondary pollution, requiring filtration with activated carbon before discharge. This secondary treatment increases costs and energy consumption, making it suitable only for treating low-concentration ammonia nitrogen wastewater. Biological treatment removes ammonia nitrogen through nitrification-denitrification by microorganisms, ultimately converting ammonia nitrogen into nitrogen gas, or through the transformation and absorption of pollutants like ammonia nitrogen by organisms, ultimately removing it from the water. This method avoids secondary pollution, and the use of algae and aquatic plants for absorption can yield byproducts, providing some economic benefits.

[0004] Photocatalysis is an effective technology for degrading pollutants using inexhaustible and clean solar energy. In recent years, graphitic carbon nitride (g-C3N4), as a non-metallic semiconductor photocatalyst, has attracted widespread attention for its photocatalytic degradation of organic pollutants due to its advantages such as non-toxicity, good visible light responsiveness, and photochemical stability. However, the photocatalytic efficiency of bulk g-C3N4 obtained by thermal polymerization of traditional precursors such as melamine, dicyandiamide, and cyanamide is limited by factors such as rapid recombination of photogenerated electron-void pairs, visible light absorption edge, and small specific surface area. To improve its photocatalytic performance, various methods have been developed, such as semiconductor coupling, elemental doping, and nanostructure engineering. To date, the catalytic activity of g-C3N4 has been reduced due to its tendency to aggregate. Nanocomposite structures with high specific surface area and high porosity typically possess efficient light-harvesting capabilities, abundant exposed photocatalytic active sites, and accessible reactive diffusion adsorption channels. Minerals generally have the advantages of abundant reserves, low cost, high thermal stability, and large adsorption capacity. Diatomaceous earth is a natural porous siliceous rock derived from the remains of ancient diatoms. Due to its advantages such as small pore size (ranging from micrometers to nanometers), high porosity, high adsorption capacity, non-toxicity, chemical stability, and low cost, it has been developed as a novel photocatalyst carrier. The composite of g-C3N4 and diatomaceous earth improves the dispersibility of g-C3N4, increases the active sites for degrading dye pollutants, accelerates the charge separation and migration of photogenerated electron-hole pairs, and enhances the adsorption capacity and adsorption / photocatalytic synergy of the composite. All of these factors contribute to improving the photocatalytic performance of the g-C3N4 / diatomaceous earth composite material.

[0005] However, the current method for diatomaceous earth-supported graphite carbon nitride (g-C3N4) composite materials is to co-fire diatomaceous earth with melamine / urea, so that g-C3N4 is loaded on the surface of diatomaceous earth. However, this method has the problem of poor bonding between g-C3N4 and diatomaceous earth, and g-C3N4 is easy to fall off.

[0006] To address the aforementioned problems, this invention is proposed. Summary of the Invention

[0007] This invention provides a method for preparing a diatomaceous earth-supported graphite carbon nitride composite photocatalyst, the method comprising the following steps:

[0008] Step A: The diatoms and the aqueous solution containing ammonia nitrogen are hydrothermally reacted at 150-200 degrees Celsius for 5-15 hours, cooled to room temperature, and the solid and liquid are separated. The solid product is washed and dried to obtain nitrogen-containing diatomaceous earth.

[0009] Step B: Calcine urea and / or melamine with the nitrogen-containing diatomaceous earth obtained in Step A at 500-600 degrees Celsius for 2-6 hours, and then cool to room temperature to obtain a diatomaceous earth-supported graphite carbon nitride composite photocatalyst.

[0010] Preferably, the diatoms in step A are nitrogen-rich diatoms, which are obtained by culturing live diatoms in an aqueous solution containing ammonia nitrogen.

[0011] Preferably, the concentration of ammonia nitrogen in the aqueous solution containing ammonia nitrogen is 50–400 mg / L.

[0012] Preferably, the mass ratio of urea and / or melamine to the nitrogen-containing diatomaceous earth obtained in step A is 1:100. For example, the mass ratio of the sum of urea and melamine to the nitrogen-containing diatomaceous earth obtained in step A is 1:100.

[0013] The beneficial effects of this invention are as follows:

[0014] 1. The method of this invention solves the problems of long treatment cycles, high costs, and easy secondary pollution associated with current methods for treating ammonia nitrogen wastewater, as well as the tendency of g-C3N4 photocatalysts to aggregate. This invention combines the characteristics of ammonia nitrogen solidification by living diatoms and their template function to develop a highly efficient, non-toxic, and green composite photocatalyst. Without harming the aquatic ecosystem, it reduces ammonia nitrogen wastewater discharge while simultaneously promoting the production of high-value-added photocatalysts, thus achieving water quality improvement and commercial value enhancement.

[0015] 2. Under the same conditions, by adjusting the concentration of ammonia nitrogen wastewater, the amount of urea added, the hydrothermal time, and the calcination temperature, a diatomaceous earth-supported g-C3N4 composite material with high binding strength can be prepared. This composite material has a stable structure and controllable dimensions. Furthermore, the safety of the diatomaceous earth-supported g-C3N4 composite material and its wide range of applications in health and water treatment are widely recognized, therefore it will not cause secondary pollution to the environment.

[0016] 3. Another advantage of the photocatalyst prepared using this method is its low cost, which will provide a safe reference for the concentration of photocatalysts for the commercial growth of active algae.

[0017] 4. This method successfully prepared diatomaceous earth-supported g-C3N4 composite materials using ammonia nitrogen wastewater, diatoms cultivated in ammonia nitrogen wastewater, and inexpensive urea / melamine as raw materials via hydrothermal annealing. The microstructure and physicochemical properties of the diatomaceous earth-supported g-C3N4 composite material were optimized by controlling the reaction conditions, resulting in improved nanostructure and photocatalytic performance. Compared with traditional diatomaceous earth-derived g-C3N4, the prepared composite material exhibits highly efficient visible light photocatalytic degradation capabilities. This invention provides a new strategy for ammonia nitrogen wastewater treatment, activated diatom conversion, and low-cost preparation of high-performance g-C3N4 / diatomaceous earth composite materials.

[0018] 5. This method effectively reduces the amount of urea / melamine input in the entire process by using ammonia nitrogen wastewater, thereby reducing the actual cost of ammonia nitrogen wastewater treatment.

[0019] 6. The reason for the good bonding between diatomaceous earth and g-C3N4 in this invention is that nitrogen-containing diatomaceous earth is prepared hydrothermally from diatoms and ammonia nitrogen wastewater, allowing nitrogen to accumulate in the cavities of the diatomaceous earth. During the subsequent calcination process with urea and / or melamine, g-C3N4 begins to grow in the cavities of the diatomaceous earth, resulting in better bonding between the diatomaceous earth and g-C3N4, and more uniform growth of g-C3N4. Attached Figure Description

[0020] Figure 1 (a) Nitrogen-containing diatomite precursor (ammonia nitrogen concentration of 5 mg / L) obtained in step (2), (b) pure g-C3N4, and (ch) electron micrographs of diatomite-supported g-C3N4 composite materials prepared from wastewater with different concentrations of ammonia nitrogen.

[0021] Figure 2 XRD patterns of diatomaceous earth-supported g-C3N4 composite materials prepared for wastewater with different concentrations of ammonia nitrogen.

[0022] Figure 3 The graph shows the photocatalytic removal performance of NO by seven diatomaceous earth-supported g-C3N4 composite materials.

[0023] Figure 4 A schematic diagram illustrating the photocatalytic NO performance of diatomaceous earth-loaded g-C3N4 samples prepared using diatoms cultured with and without ammonia nitrogen wastewater as precursors under sunlight for 3 hours.

[0024] Figure 5 Photographs of g-C3N4 / diatomite hybrids prepared in the article "Immobilization of g-C3N4 nanosheets on diatomite via electrostatic adsorption and their photocatalytic activity" (DOI:10.1039 / C8RA05408H(Paper)RSC Adv.,2018,8,28032-28040). Detailed Implementation

[0025] The present invention will be further described below through embodiments, but is not limited to these embodiments. Experimental methods not specifically described in the embodiments generally use conventional conditions and conditions described in manuals, or conditions recommended by the manufacturer. The general equipment, materials, reagents, etc., used are all commercially available unless otherwise specified. The raw materials used in the following embodiments and comparative examples are all commercially available.

[0026] The ammonia nitrogen wastewater in this article is all simulated ammonia nitrogen wastewater, and its preparation method is as follows:

[0027] Weigh 381.9 g of ammonium chloride (NH4Cl) dried at 100℃, dissolve it in water, transfer it to a 1000 mL volumetric flask, and dilute to the mark. Each mL of this solution contains 100 mg of ammonia nitrogen ammonium standard stock solution. For other ammonia nitrogen concentrations, transfer a certain amount of ammonium standard stock solution to a 500 mL volumetric flask and dilute with water to the corresponding concentration.

[0028] Example 1

[0029] The preparation method of the diatomaceous earth-supported g-C3N4 composite material photocatalyst is as follows:

[0030] (1) Seven different concentrations of simulated ammonia nitrogen wastewater containing diatoms (the ammonia nitrogen concentrations in the simulated ammonia nitrogen wastewater were 5 mg / L, 10 mg / L, 15 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, and 45 mg / L, respectively, and the original concentration of diatoms was 0.3 g / L (dry weight)) were induced to grow under light (simulated sunlight). After 2 weeks, seven diatom-rich waste liquids were obtained. The purpose was to solidify nitrogen into the diatom cells.

[0031] (2) The seven suspensions obtained in step (1) were stirred for 10 minutes, and then 70 mL of the suspensions were transferred to a 100 mL polytetrafluoroethylene-lined stainless steel reactor. The reactor was heated at 170 °C for 10 hours for hydrothermal reaction. After cooling to room temperature, the hydrothermal products were vacuum filtered, washed with deionized water and ethanol, and dried at 60 °C for 10 hours to obtain seven nitrogen-containing diatomaceous earths as precursors. The nitrogen in the nitrogen-containing diatomaceous earths comes from two sources: nitrogen within the diatom cells and nitrogen from the wastewater adsorbed by the diatomaceous earths.

[0032] (3) 0.01g of the seven precursors obtained in step (2) were calcined with 1g of urea in a covered quartz crucible at 560°C for 4 hours and then naturally cooled to room temperature to obtain seven diatomaceous earth-supported g-C3N4 composite materials.

[0033] Figure 1 (a) Nitrogen-containing diatomite precursor obtained in step (2) (ammonia nitrogen concentration of 5 mg / L), (b) pure g-C3N4, and (ch) electron micrographs of seven diatomite-supported g-C3N4 composite materials obtained in step (3) (ammonia nitrogen concentrations in ch are 5 mg / L, 10 mg / L, 15 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, and 45 mg / L, respectively).

[0034] Figure 1 It can be seen that: after hydrothermal treatment, the living diatoms maintained the integrity of the diatomaceous earth shell and exhibited a porous morphology.

[0035] Figure 1b. It can be seen that pure g-C3N4 exhibits obvious agglomeration. The preparation method of pure g-C3N4 is as follows: 1g of urea is calcined at 560℃ for 4 hours in a covered quartz crucible and then naturally cooled to room temperature.

[0036] Figure 1 As observed in c-1h, the loading of g-C3N4 significantly increased with increasing ammonia nitrogen concentration, indicating a favorable environment for its formation. Furthermore, the uniformity of loading on diatomaceous earth improved, suggesting that treatment of high-concentration ammonia nitrogen wastewater is beneficial for the formation of high-quality composite materials.

[0037] Figure 2 XRD patterns of diatomaceous earth-supported g-C3N4 composite materials prepared for four different concentrations of ammonia nitrogen wastewater. Figure 2 This demonstrates the successful loading of g-C3N4 onto diatomaceous earth.

[0038] Application Example 1

[0039] The seven diatomaceous earth-supported g-C3N4 composite materials obtained in Example 1 were tested to assess their NO removal efficiency as catalysts. The test methods are as follows:

[0040] The concentration of 100 ppm (1 ppm = 1 mg / m³) is transferred through a three-way valve. 3 NO standard gas (adjusted flow rate to 24 mL / min) and standard air (adjusted flow rate to 2.4 mL / min) are introduced into a mixing bottle and mixed to prepare a low-concentration NO solution of approximately 520 ppb (1 ppb = 1 μg / mL). 3 The catalyst (diatomaceous earth-supported g-C3N4 composite material) was introduced into the reactor. After the adsorption and desorption of NO by the catalyst reached equilibrium (i.e., the NO concentration stabilized), the lamp was immediately turned on (simulating sunlight performance testing) for testing. The concentration at the reactor outlet was measured by a NOx analyzer. This instrument sampled and recorded the concentrations of NO, NO2, and NOx (NO+NO2) every minute. The irradiation lamp used a full-spectrum light.

[0041] The blank group consists of samples without any added catalyst.

[0042] Figure 3 The graph shows the photocatalytic removal performance of NO by seven diatomaceous earth-supported g-C3N4 composite materials.

[0043] Figure 3 As can be seen:

[0044] In an empty chamber, the gas concentration remained stable at around 520-530 ppb. The test samples (seven kinds of diatomaceous earth-supported g-C3N4 composite materials) all showed a certain degree of adsorption, and the adsorption tended to increase with the increase of ammonia nitrogen wastewater concentration. This should be due to the increase in mesoporous channels formed by g-C3N4 nanosheets and diatomaceous earth.

[0045] Under simulated sunlight irradiation, the diatomaceous earth-supported g-C3N4 composite material of Example 1 effectively removed low concentrations of NO without producing the toxic byproduct NO2. No signs of inactivation were observed even after prolonged light exposure.

[0046] Under simulated sunlight irradiation, diatomaceous earth-loaded g-C3N4 prepared from ammonia nitrogen wastewater with a concentration of 5 mg / L exhibited approximately 30% activity, while the material prepared from ammonia nitrogen wastewater with a concentration of 10 mg / L did not show an improvement over the 30% activity level. Diatomaceous earth-loaded g-C3N4 prepared from ammonia nitrogen wastewater with concentrations of 15 mg / L, 25 mg / L, 30 mg / L, 35 mg / L, and 45 mg / L showed a 10% improvement in activity compared to the 5 mg / L concentration. Therefore, considering the influence of actual wastewater concentrations, this material is more suitable for preparation using high-concentration ammonia nitrogen wastewater.

[0047] Comparative Example 1

[0048] The preparation method of diatomaceous earth-loaded C3N4 samples prepared by culturing diatoms in ammonia nitrogen-infused wastewater under simulated sunlight as a precursor is as follows:

[0049] (1) The ammonia nitrogen wastewater containing diatoms (the ammonia nitrogen concentration in the wastewater is 5 mg / L and the original concentration of diatoms is 0.3 g / L (dry weight)) was induced to grow under light, and after 2 weeks, a diatom-rich waste liquid was obtained.

[0050] (2) Stir the suspension obtained in step (1) for 10 minutes, then transfer 70 mL of the suspension to a 100 mL polytetrafluoroethylene-lined reactor and heat at 170 °C for 10 hours for hydrothermal reaction. After cooling to room temperature, filter the hydrothermal product under vacuum, wash with deionized water and ethanol, and dry at 60 °C for 10 hours to obtain nitrogen-containing diatomaceous earth as a precursor.

[0051] (3) 0.01g of the precursor obtained in step (2) and 1g of urea were calcined at 560°C for 4 hours in a covered quartz crucible and then naturally cooled to room temperature to obtain diatomaceous earth-supported g-C3N4 composite material, which was used as sample 1.

[0052] The preparation method of diatomaceous earth-loaded C3N4 samples prepared by culturing diatoms in simulated sunlight-treated wastewater without ammonia nitrogen as a precursor is as follows:

[0053] The only difference between this sample and the one cultured with ammonia nitrogen wastewater is that in step (1), the ammonia nitrogen wastewater is replaced with tap water, resulting in a diatomaceous earth-supported g-C3N4 composite material, which is used as sample 2.

[0054] Figure 4 A schematic diagram illustrating the photocatalytic NO performance of diatomaceous earth-loaded g-C3N4 samples prepared using diatoms cultured with and without ammonia nitrogen wastewater as precursors under sunlight for 3 hours.

[0055] like Figure 4 As shown, after a long test of 3 hours, it was found that the photocatalytic NO performance of diatomaceous earth loaded with g-C3N4 prepared by incorporating ammonia nitrogen wastewater into culture could be stabilized at about 65%, while the performance of diatomaceous earth loaded with g-C3N4 prepared by incorporating diatoms without ammonia nitrogen wastewater as a precursor could also be stabilized at about 50%.

[0056] Figure 4 prove:

[0057] 1. Compared with sample 2, sample 1 has a better binding force between diatomite and g-C3N4, thus avoiding the aggregation of g-C3N4 and enhancing the catalytic effect.

[0058] 2. From the perspective of long-term catalytic effect, although the short-term catalytic efficiency of sample 2 is higher than that of sample 1, the lack of ammonia nitrogen solution added during the cultivation of sample 2 resulted in poorer binding force between diatomaceous earth and g-C3N4. After prolonged participation in the reaction, g-C3N4 in the composite material of sample 2 would detach. Therefore, the catalytic performance of sample 2 decreased in the later stages.

[0059] In summary, the material prepared by this invention has good bonding strength.

[0060] Comparative Example 2

[0061] Figure 5 Photographs of g-C3N4 / diatomite hybrids prepared in the article "Immobilization of g-C3N4 nanosheets on diatomite via electrostatic adsorption and their photocatalytic activity" (DOI:10.1039 / C8RA05408H(Paper)RSC Adv.,2018,8,28032-28040). Figure 5 (a)-(b) are original diatomaceous earth photographs. Figure 5 (c)-(d) are photographs of the obtained g-C3N4 / diatomite hybrid.

[0062] The method for preparing the g-C3N4 / diatomite hybrid described in this article involves weighing pretreated g-C3N4 nanosheets and diatomite, and first mixing them in 100 mL of solution. Then, the mixture is sonicated for 30 min and stirred for 60 min to allow the two components to be tightly attracted together by electrostatic adsorption. After filtration, the mixture is dried overnight in an oven at 100 °C and then heated at 500 °C for 2 hours at a heating rate of 2.5 °C / min. -1 Then, the final g-C3N4 / diatomite hybrid is obtained.

[0063] Figure 5 It can be seen that compared with the diatomaceous earth-supported C3N4 composite material prepared in this invention ( Figure 1 (ch)), Figure 5 In (c)-(d), the g-C3N4 on the surface of the g-C3N4 / diatomite hybrid exhibits obvious aggregation. However, in the diatomite-supported g-C3N4 composite material of this invention, the C3N4 distribution on the surface is more uniform, and there is no aggregation.

[0064] Therefore, the method provided by the present invention can achieve uniform loading of g-C3N4 on the surface of diatomaceous earth.

Claims

1. A method for preparing a diatomaceous earth-supported graphite carbon nitride composite photocatalyst, characterized in that, The preparation method includes the following steps: Step A: The diatoms and the aqueous solution containing ammonia nitrogen are hydrothermally reacted at 150-200 degrees Celsius for 5-15 hours, cooled to room temperature, and the solid and liquid are separated. The solid product is washed and dried to obtain nitrogen-containing diatomaceous earth. Step B: Calcine urea and / or melamine with the nitrogen-containing diatomaceous earth obtained in Step A at 500-600 degrees Celsius for 2-6 hours, and cool to room temperature to obtain a diatomaceous earth-supported graphite carbon nitride composite photocatalyst. The diatoms in step A are nitrogen-rich diatoms, which are obtained by culturing live diatoms in an aqueous solution containing ammonia nitrogen.

2. The preparation method according to claim 1, characterized in that, The concentration of ammonia nitrogen in the aqueous solution is 50~400 mg / L.

3. The preparation method according to claim 1, characterized in that, The mass ratio of urea and / or melamine to the nitrogen-containing diatomaceous earth obtained in step A is 1:0.01.

Images