A bismuth oxide composite sponge carrier photocatalyst and a preparation method and application thereof

Abstract

This invention discloses a bismuth oxide composite sponge-supported photocatalyst, its preparation method, and its application, belonging to the technical field of photocatalytic wastewater treatment. This invention uses bismuth oxide and melamine sponge as the catalyst and support, respectively, through Ca... 2+ Crosslinking anchors bismuth oxide in a sodium alginate gel network, which is then loaded onto a melamine sponge framework, forming a three-in-one functional interface of "adsorption-enrichment-catalysis". This effectively combines the excellent adsorption performance of melamine sponge, the chelating enrichment capacity of sodium alginate, and the photocatalytic activity of bismuth oxide. Relying on the synergistic coupling effect of adsorption and photocatalysis, the pollutant removal efficiency is significantly improved, thereby achieving the goal of economical, efficient, rapid, thorough, and harmless treatment of methyl orange and hexavalent chromium wastewater.

Description

Technical Field

[0001] This invention belongs to the technical field of photocatalytic wastewater treatment, specifically relating to a bismuth oxide composite sponge carrier photocatalyst, its preparation method, and its application. Background Technology

[0002] Azo dyes and hexavalent chromium are typical recalcitrant pollutants in industrial wastewater. Dyeing and printing wastewater is particularly problematic due to its high color intensity, complex composition, high biotoxicity, and difficulty in degradation. The azo bonds in the molecular structure of methyl orange azo dye are difficult to break under natural conditions. Once it enters water bodies, it significantly reduces light transmittance, inhibits photosynthesis in aquatic plants, and disrupts the ecological balance. Its degradation products, such as aromatic amine compounds, have carcinogenic, mutagenic, and teratogenic effects. Meanwhile, chromium-containing wastewater discharged from industries such as electroplating and metallurgy has been listed as a priority pollutant by the US EPA. Its toxicity is more than 100 times that of trivalent chromium, and it can accumulate in organisms, posing serious ecological and health risks.

[0003] Currently, the main methods for treating dyes and hexavalent chromium are biological methods, physical adsorption methods, and photocatalytic reduction methods. However, neither biological nor physical adsorption methods can fundamentally solve the problem of dye and hexavalent chromium pollution. Chinese patent document CN112079462A discloses a method for adsorbing hexavalent chromium from wastewater using sieved low-rank coal, but adsorption cannot solve the fundamental problem, and hexavalent chromium pollution still exists. Chinese patent document CN112159025A discloses a method for precipitating hexavalent chromium from wastewater and then separating it, but this method is complex, and the hexavalent chromium precipitate pollution still exists. Therefore, reducing hexavalent chromium to lower valent chromium has become the fundamental strategy to avoid hexavalent chromium pollution. This makes photocatalytic reduction the preferred choice for treating hexavalent chromium wastewater pollution. The photocatalytic process uses light energy to activate the catalyst, producing strong oxidizing species that reduce or oxidize heavy metal ions to non-toxic or low-toxic forms. Current reports on the reduction of hexavalent chromium in wastewater suffer from problems such as weak adsorption capacity for metal ions in high-concentration industrial wastewater, poor stability, poor economic efficiency, poor environmental friendliness, and stringent preparation conditions. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a bismuth oxide composite sponge carrier photocatalyst, its preparation method and application, so as to solve the problems mentioned in the background art or achieve better technical effects.

[0005] To solve the above-mentioned technical problems, the inventors derived the technical solution of the present invention through practice and summarization. The present invention discloses a bismuth oxide composite sponge carrier photocatalyst, wherein the photocatalyst is a porous composite material with melamine sponge as a three-dimensional framework, sodium alginate as a cross-linked gel layer, and bismuth oxide as a photocatalytic active component.

[0006] Furthermore, based on the amount of components used, the mass ratio of bismuth oxide to sodium alginate is 2~3:1.

[0007] Furthermore, in the above-described method for preparing the bismuth oxide composite sponge-supported photocatalyst, melamine sponge is used as the support. First, sodium alginate is used to specifically modify the melamine sponge, introducing abundant carboxyl and amino groups; then, Ca... 2+ Crosslinking anchors bismuth oxide in a sodium alginate gel network, which is then loaded onto a melamine sponge framework, forming a three-in-one functional interface of "adsorption-enrichment-catalysis". The carboxyl groups in the sodium alginate gel layer form Bi-OC coordination bonds with bismuth ions on the surface of bismuth oxide, shortening the transport distance of photogenerated carriers.

[0008] Furthermore, the preparation method of the bismuth oxide composite sponge support photocatalyst includes the following steps:

[0009] S1: Disperse bismuth nitrate pentahydrate crystals in deionized water, and add sodium hydroxide solution dropwise while stirring to obtain a suspension;

[0010] The resulting suspension was stirred at 80℃ for 1 hour, then centrifuged at 7500 r / min and washed repeatedly with deionized water until neutral. The final product was dried at 80℃ for 12 hours to obtain bismuth oxide crystals.

[0011] S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, clean and remove impurities, and then dry them;

[0012] S3: Add sodium alginate powder to deionized water, stir, and obtain a homogeneous sodium alginate solution;

[0013] S4: The bismuth oxide prepared in S1 is uniformly dispersed in the sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion.

[0014] S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 to ensure that the pores of the melamine sponge fully adsorb the sodium alginate solution; then the adsorbed sponge is removed and immersed in a 5% calcium chloride solution for crosslinking; Ca 2+ A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework. After crosslinking, the surface is rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate. After drying, the bismuth oxide composite sponge carrier photocatalyst is obtained.

[0015] Furthermore, in S1, the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide is 1:3~6.

[0016] Furthermore, in step S2, the cleaning and impurity removal process is as follows: first, wash with deionized water three times to remove dust and impurities attached to the surface; then wash with anhydrous ethanol twice to remove organic pollutants.

[0017] Furthermore, in step S3, the stirring temperature is 60°C, the stirring speed is 500 r / min, and the stirring time is 2 h.

[0018] Furthermore, in step S3, the mass fraction of the sodium alginate homogeneous solution is 0.25~0.375%.

[0019] Furthermore, the bismuth oxide composite sponge carrier photocatalyst is applied to the photocatalytic degradation of methyl orange in wastewater.

[0020] Furthermore, the bismuth oxide composite sponge carrier photocatalyst is applied in the photocatalytic reduction of hexavalent chromium in wastewater.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] (1) This invention breaks through the conventional simple loading idea of ​​"support + catalyst" and proposes a synergistic design of "confined micro-effect + pH response": Bi on the surface of bismuth oxide 3+ It forms a Bi-OC coordination bond with the carboxylate group of sodium alginate, preventing the catalyst from falling off during recycling; SA-Ca 2+ The nanopores (2-50 nm) of the gel layer match the size of the bismuth oxide nanosheets, allowing pollutants to be pre-enriched before diffusing to the catalytic sites, resulting in increased local concentrations. Under strong acid conditions, sodium alginate protonation densifies and positively charges the gel layer, forcing the enrichment of anionic pollutants. Under neutral conditions, the gel layer swells, the charge density decreases, and the catalytic activity self-shuts off. The electron-rich groups of sodium alginate capture valence band holes, delaying recombination and enabling the efficient reduction of hexavalent chromium by conduction band electrons. Simultaneously, the oxidized sodium alginate free radicals produce ·OH to degrade methyl orange dye.

[0023] (2) The sponge composite material synthesized in this invention has a larger specific surface area than pure bismuth oxide powder, which increases the number of adsorption sites for pollutants. The efficiency of pollutant removal by the composite material is significantly improved. It still has a high photocatalytic removal efficiency after 30 cycles. In addition, pure bismuth oxide powder is difficult to separate and recover from water bodies, which can easily cause secondary pollution. The synthesized sponge composite material has a larger volume and is convenient to recover.

[0024] (3) Compared with pure melamine sponge, the composite material prepared by the present invention has the synergistic effect of bismuth oxide photocatalytic activity and sponge three-dimensional skeleton; compared with bismuth oxide / sodium alginate gel, the composite material has the light trap effect of sponge, which enhances light absorption, and the three-dimensional skeleton of sponge promotes mass transfer, which significantly enhances the removal efficiency of pollutants of composite material; compared with existing titanium dioxide composite sponge, titanium dioxide is easy to dissolve under strong acid conditions, and bismuth oxide has significantly better chemical stability than titanium dioxide, so that bismuth oxide sponge composite material has high efficiency photocatalytic degradation activity.

[0025] (4) This invention specifically modifies melamine sponge with sodium alginate and loads it with bismuth oxide to prepare a bismuth oxide composite sponge carrier photocatalyst. The modified composite material successfully incorporates abundant carboxyl and amino groups, while bismuth oxide provides electrons and holes, thereby increasing the adsorption rate and performance of dye molecules and heavy metal ions in industrial wastewater. Under ultraviolet light irradiation, it can be used for the adsorption-degradation of methyl orange in dyeing and printing wastewater and the adsorption-reduction of hexavalent chromium in electroplating industrial wastewater. Under acidic conditions, the carboxyl and amino groups of sodium alginate can effectively adsorb methyl orange and hexavalent chromium. With increased ultraviolet light, bismuth oxide can be excited to generate electron-hole pairs, driving the adsorption of O2. - It generates reactive oxygen species such as ·OH, promotes the degradation of methyl orange and the reduction of hexavalent chromium, and can also effectively utilize natural polysaccharides and three-dimensional porous materials, which is in line with the concepts of green chemistry and circular economy and has positive significance for environmental protection. Attached Figure Description

[0026] Figure 1 This is a scanning electron microscope image of the surface of the photocatalyst prepared in Example 2 of the present invention;

[0027] Figure 2 The infrared spectra of the photocatalyst prepared in Example 2 of this invention before and after the reaction are shown.

[0028] Figure 3 The ultraviolet diffuse reflectance spectrum of the photocatalyst prepared in Example 2 of this invention;

[0029] Figure 4 The adsorption-desorption isotherm diagram of the photocatalyst prepared in Example 2 of this invention;

[0030] Figure 5 The pore size distribution diagram is shown for the photocatalyst prepared in Example 2 of this invention.

[0031] Figure 6 The XPS spectrum of bismuth element in the photocatalyst prepared in Example 2 of this invention;

[0032] Figure 7 The above is the oxygen element XPS spectrum of the photocatalyst prepared in Example 2 of this invention. Detailed Implementation

[0033] To make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to specific examples.

[0034] Unless otherwise specified, all raw materials or reagents used in the following examples are commercially available products.

[0035] The detection method for the photocatalyst prepared in this invention is as follows: the concentration of pollutants in the solution after the adsorption and photocatalysis experiments are detected by ultraviolet spectrophotometer.

[0036] The formula for calculating the removal rate (%) of hexavalent chromium and methyl orange is as follows:

[0037] ;

[0038] in: (mg / L) and (mg / L) represent the initial concentration of the pollutant and the concentration at time t, respectively. This represents the removal rate of pollutants.

[0039] A method for preparing a bismuth oxide composite sponge-supported photocatalyst, comprising the following steps:

[0040] S1: Bismuth nitrate pentahydrate crystals were dispersed in deionized water, and then NaOH solution was added dropwise under stirring to obtain a suspension; the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide was 1:3~6; the obtained suspension was stirred at 80℃ for 1 h, centrifuged at 7500 r / min, and washed repeatedly with deionized water until neutral; the final product was dried at 80℃ for 12 h to obtain bismuth oxide crystals;

[0041] S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, wash it three times with deionized water to remove dust and impurities adhering to the surface; then wash it twice with anhydrous ethanol to remove organic pollutants; finally, dry it in a 60℃ constant temperature forced-air drying oven for 24 hours.

[0042] S3: Add sodium alginate powder to deionized water and stir magnetically at 60℃ and 500r / min for 2h to prepare a homogeneous sodium alginate solution.

[0043] S4: The bismuth oxide powder prepared in S1 is uniformly dispersed in the sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion. The mass ratio of bismuth oxide powder to sodium alginate is 2~3:1. Ultrasonic dispersion can effectively prevent bismuth oxide agglomeration and increase the specific surface area.

[0044] S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 for 30 minutes to ensure that the melamine sponge pores fully adsorb the sodium alginate solution. The high porosity and strong capillary action of the sponge are used to achieve uniform loading of the catalyst on the sponge support. Then, the adsorbed sponge is removed and slowly immersed in a 5% calcium chloride solution for crosslinking for 6 hours. 2+ A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework to prevent catalyst detachment. After crosslinking, the surface is repeatedly rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate, avoiding interference from free ions on subsequent photocatalytic reactions. The bismuth oxide composite sponge carrier photocatalyst is then dried in a 60°C drying oven for 12 hours to obtain the photocatalyst.

[0045] Example 1

[0046] A method for preparing a bismuth oxide composite sponge-supported photocatalyst, comprising the following steps:

[0047] S1: Bismuth nitrate pentahydrate crystals were dispersed in deionized water, and then 1 mol / L NaOH solution was added dropwise under stirring to obtain a suspension; the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide was 1:4; the resulting suspension was stirred at 80℃ for 1 h, centrifuged at 7500 r / min, and repeatedly washed with deionized water until neutral; the final product was dried at 80℃ for 12 h to obtain bismuth oxide crystals;

[0048] S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, wash it three times with deionized water to remove dust and impurities adhering to the surface; then wash it twice with anhydrous ethanol to remove organic pollutants; finally, dry it in a 60℃ constant temperature forced-air drying oven for 24 hours.

[0049] S3: Add 0.375g of sodium alginate powder to deionized water and stir magnetically at 60℃ and 500r / min for 2h to prepare 100mL of homogeneous sodium alginate solution with a mass fraction of 0.375%.

[0050] S4: 0.75g of bismuth oxide powder prepared in S1 is uniformly dispersed in sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion. The mass ratio of bismuth oxide powder to sodium alginate is 2:1.

[0051] S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 for 30 minutes to ensure that the melamine sponge pores fully adsorb the sodium alginate solution. The high porosity and strong capillary action of the sponge are used to achieve uniform loading of the catalyst on the sponge support. Then, the adsorbed sponge is removed and slowly immersed in a 5% calcium chloride solution for crosslinking for 6 hours.2+ A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework to prevent catalyst detachment. After crosslinking, the surface is repeatedly rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate, avoiding interference from free ions on subsequent photocatalytic reactions. The bismuth oxide composite sponge carrier photocatalyst is then dried in a 60°C drying oven for 12 hours to obtain the photocatalyst.

[0052] Example 2

[0053] A method for preparing a bismuth oxide composite sponge-supported photocatalyst, comprising the following steps:

[0054] S1: Bismuth nitrate pentahydrate crystals were dispersed in deionized water, and then 1 mol / L NaOH solution was added dropwise under stirring to obtain a suspension; the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide was 1:4; the resulting suspension was stirred at 80℃ for 1 h, centrifuged at 7500 r / min, and repeatedly washed with deionized water until neutral; the final product was dried at 80℃ for 12 h to obtain bismuth oxide crystals;

[0055] S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, wash it three times with deionized water to remove dust and impurities adhering to the surface; then wash it twice with anhydrous ethanol to remove organic pollutants; finally, dry it in a 60℃ constant temperature forced-air drying oven for 24 hours.

[0056] S3: Add 0.3g of sodium alginate powder to deionized water and stir magnetically at 60℃ and 500r / min for 2h to prepare 100mL of homogeneous sodium alginate solution with a mass fraction of 0.3%.

[0057] S4: 0.75g of bismuth oxide powder prepared in S1 is uniformly dispersed in sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion. The mass ratio of bismuth oxide powder to sodium alginate is 2.5:1.

[0058] S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 for 30 minutes to ensure that the melamine sponge pores fully adsorb the sodium alginate solution. The high porosity and strong capillary action of the sponge are used to achieve uniform loading of the catalyst on the sponge support. Then, the adsorbed sponge is removed and slowly immersed in a 5% calcium chloride solution for crosslinking for 6 hours. 2+A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework to prevent catalyst detachment. After crosslinking, the surface is repeatedly rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate, avoiding interference from free ions on subsequent photocatalytic reactions. The bismuth oxide composite sponge carrier photocatalyst is then dried in a 60°C drying oven for 12 hours to obtain the photocatalyst.

[0059] Example 3

[0060] A method for preparing a bismuth oxide composite sponge-supported photocatalyst, comprising the following steps:

[0061] S1: Bismuth nitrate pentahydrate crystals were dispersed in deionized water, and then 1 mol / L NaOH solution was added dropwise under stirring to obtain a suspension; the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide was 1:4; the resulting suspension was stirred at 80℃ for 1 h, centrifuged at 7500 r / min, and repeatedly washed with deionized water until neutral; the final product was dried at 80℃ for 12 h to obtain bismuth oxide crystals;

[0062] S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, wash it three times with deionized water to remove dust and impurities adhering to the surface; then wash it twice with anhydrous ethanol to remove organic pollutants; finally, dry it in a 60℃ constant temperature forced-air drying oven for 24 hours.

[0063] S3: Add 0.25g of sodium alginate powder to deionized water and stir magnetically at 60℃ and 500r / min for 2h to prepare 100mL of homogeneous sodium alginate solution with a mass fraction of 0.25%.

[0064] S4: 0.75g of bismuth oxide powder prepared in S1 is uniformly dispersed in sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion. The mass ratio of bismuth oxide powder to sodium alginate is 3:1.

[0065] S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 for 30 minutes to ensure that the melamine sponge pores fully adsorb the sodium alginate solution. The high porosity and strong capillary action of the sponge are used to achieve uniform loading of the catalyst on the sponge support. Then, the adsorbed sponge is removed and slowly immersed in a 5% calcium chloride solution for crosslinking for 6 hours. 2+A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework to prevent catalyst detachment. After crosslinking, the surface is repeatedly rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate, avoiding interference from free ions on subsequent photocatalytic reactions. The bismuth oxide composite sponge carrier photocatalyst is then dried in a 60°C drying oven for 12 hours to obtain the photocatalyst.

[0066] Example 4 (Application Example)

[0067] The bismuth oxide composite sponge-supported photocatalysts prepared in Examples 1-3 were subjected to photocatalytic degradation of methyl orange, as follows:

[0068] Several groups of methyl orange standard solutions with a concentration of 50 mg / L and pH values ​​of 1, 3, 5, and 7 were prepared. 25 mg of each of the three catalysts prepared in Examples 1-3 were placed in 25 mL of methyl orange solutions with different pH values, and the solutions were tested at 360 nm–480 nm and 20–100 mW / cm². 2 Under illumination, the concentration of methyl orange in the solution was measured. The photocatalytic degradation efficiency of methyl orange by the photocatalysts prepared in Examples 1-3 is shown in Table 1 below:

[0069] Table 1. Efficiency of photocatalytic degradation of methyl orange by the photocatalysts prepared in Examples 1-3

[0070]

[0071] As shown in Table 1, the photocatalyst prepared in Example 2 of this invention exhibits the best degradation ability for methyl orange at pH 1, reaching 98.3%. This is because, under strongly acidic conditions, the azo bonds (-N=N-) and sulfonic acid groups (-SO-) in the methyl orange molecule... 3- The ease of protonation makes it easier for dye molecules to be adsorbed and enriched by the positively charged sites on the surface of the composite material, shortening the contact distance between reactive oxygen species and pollutants; at the same time, the low pH environment provides sufficient H₂. + It can combine with photogenerated electrons to generate active hydrogen atoms (H·), or directly participate in photocatalytic reactions to promote the generation of ·OH, thus accelerating the oxidative decomposition of methyl orange molecules; in addition, acidic conditions inhibit the recombination of photogenerated electrons and holes, allowing more charge carriers to participate in the degradation reaction.

[0072] Because a sodium alginate concentration of 0.3% achieves the optimal balance between gel coating thickness, active site density, and mass transfer efficiency, the composite material exhibits the best removal effect on methyl orange at this concentration. The 0.3% SA solution has moderate viscosity, and the Ca... 2+After cross-linking, a dense but not too thick gel network is formed. This network can not only form stable Bi-OC coordination bonds with the hydroxyl groups on the Bi2O3 surface through sufficient carboxyl groups, firmly anchoring nanoparticles and inhibiting their aggregation and loss, but also provide a complete coating layer for the MS framework, allowing amino and carboxyl groups to synergistically exert electrostatic adsorption and pre-enrichment effects. At the same time, the moderate thickness of the gel layer avoids excessively obscuring the active sites of Bi2O3, ensuring that pollutants can quickly diffuse to the photocatalytic interface, and the photogenerated electron-hole pairs can efficiently separate and drive the oxidative degradation of methyl orange.

[0073] At a SA concentration of 0.3%, the amino groups on the MS framework surface are fully protonated to -NH3 at pH=1. + The SA gel layer adsorbs methyl orange anions through strong electrostatic interactions. The carboxyl groups in the SA gel layer further enhance the pre-enrichment effect through hydrogen bonding and hydrophobic interactions, enabling dye molecules to rapidly migrate to the Bi₂O₃ surface. A suitable gel thickness ensures the accessibility of adsorption sites while avoiding excessive scattering and absorption of ultraviolet light by an overly thick coating. This ensures that Bi₂O₃ is fully excited to generate holes and ·OH radicals, efficiently breaking azo bonds and achieving mineralization and degradation. If the SA concentration is too low (0.2%), insufficient adsorption sites result in poor pre-enrichment; if it is too high (0.4%), an excessively thick gel layer hinders the diffusion of dye molecules to the Bi₂O₃ active sites and may shield some photocatalytic sites, leading to a decrease in oxidation efficiency.

[0074] Example 5

[0075] The bismuth oxide composite sponge-supported photocatalysts prepared in Examples 1-3 were subjected to photocatalytic reduction of hexavalent chromium, as follows:

[0076] Several groups of potassium dichromate standard solutions with a concentration of 50 mg / L and pH values ​​of 1, 3, 5, and 7 were prepared. 25 mg of each of the three catalysts prepared in Examples 1-3 were placed in 25 mL hexavalent chromium solutions of different pH values. The solutions were then tested at 360 nm–480 nm and 20–100 mW / cm². 2 Under illumination, the concentration of hexavalent chromium in the solution was measured. The photocatalytic reduction efficiency of hexavalent chromium by the photocatalysts prepared in Examples 1-3 is shown in Table 2 below:

[0077] Table 2. Efficiency of photocatalytic reduction of hexavalent chromium by the photocatalysts prepared in Examples 1-3

[0078]

[0079] As shown in Table 2, the photocatalyst composite material prepared in Example 2 of this invention exhibits the best photocatalytic reduction efficiency for hexavalent chromium at pH 3, reaching 98.5%. Under acidic conditions, hexavalent chromium mainly exists as Cr2O7. 2- and HCrO 4-These anionic forms are readily adsorbed and enriched by protonated amino and carboxyl groups on the surface of the composite material via electrostatic interactions, shortening the transport distance between photogenerated electrons and Cr(VI); simultaneously, the low pH environment provides ample H₂. + It can act as an acceptor of photogenerated electrons in the reduction reaction of Cr(VI); in addition, acidic conditions inhibit the recombination of photogenerated electrons with dissolved oxygen, allowing more electrons to participate in the reduction of Cr(VI) and promoting the reduction of ·O2. - The formation of active species, etc., occurs. As pH increases, Cr(VI) gradually transforms into CrO4. 2- The H⁺ concentration generates electrostatic repulsion with the negatively charged composite material surface, and the decrease in H⁺ concentration weakens the reduction driving force, thus significantly reducing the photocatalytic efficiency.

[0080] The removal of Cr(VI) is primarily achieved through photocatalytic reduction, the efficiency of which depends on the accessibility of electron donors and the transport efficiency of photogenerated electrons. At a SA concentration of 0.3%, the carboxyl groups in the gel layer can directly participate in the reduction reaction of Cr(VI) as electron donors, and the Bi-OC coordination bonds shorten the transport distance of photogenerated electrons from the Bi₂O₃ conduction band to Cr(VI); simultaneously, the appropriate coating thickness allows HCrO₄ to... - / Cr2O7 2- Anions can be electrostatically adsorbed and enriched by protonated amino groups, and can also rapidly contact the Bi₂O₃ surface to gain photogenerated electrons and be reduced to Cr(III). If the SA concentration is too low, there are insufficient electron donors and the Bi₂O₃ is not firmly anchored and easily leaches out; if it is too high, the excessively thick gel layer increases the diffusion resistance of Cr(VI), and some Bi₂O₃ is deeply buried inside the gel and cannot effectively participate in electron transfer, resulting in a weakened reduction driving force. Therefore, a SA concentration of 0.3% achieves optimal synergy among the three stages of "adsorption enrichment-electron transport-catalytic reduction".

[0081] Example 6

[0082] Prepare a methyl orange standard solution with pH 1 and a hexavalent chromium standard solution with pH 3, both with a concentration of 50 mg / L. Take 100 mg of each of the catalysts prepared in Examples 1-3 above and place them in the two standard solutions, respectively, with a volume of 100 mL. [The following appears to be a separate, unrelated instruction:] At 360 nm–480 nm and 20–100 mW / cm² [the following appears to be a separate, unrelated instruction:] 2 Under illumination, photocatalytic degradation and reduction were performed. Samples were taken every 5 minutes and 20 minutes to measure the concentrations of methyl orange and Cr(VI), and the removal efficiency was calculated. The results are shown in Table 3 below.

[0083] Table 3. Efficiency of the photocatalysts prepared in Examples 1-3 in removing methyl orange and hexavalent chromium at different time points.

[0084]

[0085] As shown in Table 3, the Bi2O3@MS-SA composite material prepared in this invention exhibits a rapid increase in the degradation efficiency of methyl orange and Cr(VI) in the initial stage of the photocatalytic reaction, indicating that the composite material possesses good photocatalytic activity under ultraviolet light excitation. Subsequently, the degradation rate slows down slightly but still maintains a stable upward trend; it gradually reaches equilibrium with increasing reaction time. This is because in the initial stage of the reaction, the surface of the composite material contains a large number of reaction sites, which can provide a basis for methyl orange and hexavalent chromium to adhere to the surface of the composite material, which is beneficial to the adsorption-photocatalytic reaction, thereby significantly improving the removal efficiency of pollutants.

[0086] Example 7

[0087] A methyl orange standard solution with pH 1 and a hexavalent chromium standard solution with pH 3 were prepared at a concentration of 50 mg / L. The catalyst prepared according to the optimal preparation scheme described above (Example 2) was tested in the dark and at 360 nm–480 nm and 20–100 mW / cm². 2 Catalytic degradation or reduction experiments were conducted under light irradiation, and the results are shown in Table 4 below:

[0088] Table 4. The photocatalysts prepared in Example 2 were respectively used in...

[0089] Comparison of degradation efficiencies of methyl orange and hexavalent chromium under dark and ultraviolet light conditions

[0090]

[0091] As shown in Table 4, the photocatalyst prepared in Example 2 of this invention exhibits low efficiency in degrading methyl orange and adsorbing hexavalent chromium under dark conditions, but achieves removal rates of 98.3% and 98.5% respectively under ultraviolet light irradiation. This comparison demonstrates that increasing ultraviolet light significantly improves the efficiency of pollutant removal. This indicates that the composite material's removal of pollutants is primarily driven by photocatalysis, rather than purely by physical adsorption. The limited removal efficiency under dark conditions mainly stems from the physical adsorption and complexation of the melamine sponge and sodium alginate gel; once adsorption saturation occurs, further removal of pollutants becomes difficult. Under ultraviolet light irradiation, bismuth oxide is excited to generate electron-hole pairs. Photogenerated electrons reduce hexavalent chromium to trivalent chromium, while holes react with water / hydroxyl radicals to generate reactive oxygen species such as ·OH, which oxidize and degrade methyl orange. Simultaneously, the three-dimensional porous framework of the sponge enhances light absorption efficiency through a light trapping effect, and the Bi-OC coordination bonds in the gel layer promote interfacial transport of photogenerated carriers, achieving a synergistic enhancement of adsorption enrichment and photocatalytic degradation, thereby significantly improving pollutant removal efficiency.

[0092] Example 8

[0093] A methyl orange standard solution with pH 1 and a hexavalent chromium standard solution with pH 3, at a concentration of 50 mg / L, were prepared. The catalyst prepared according to the optimal preparation scheme described above (Example 2) was then subjected to a repeatability test. The test results are shown in Table 6 below.

[0094] Table 6. Catalytic efficiency of the photocatalyst prepared in Example 2 of this invention after a certain number of cycles.

[0095]

[0096] As shown in Table 6, after 30 cycles of photocatalytic testing, the degradation of methyl orange and the reduction of hexavalent chromium by the composite material remained at 82.8% and 80.1%, respectively, indicating that the photocatalyst composite material prepared in this invention has good performance for multiple cycles.

[0097] Comparative Example 1

[0098] A methyl orange standard solution with pH 1 and a hexavalent chromium standard solution with pH 3 were prepared at a concentration of 50 mg / L. The catalyst prepared using the optimal preparation scheme described above (Example 2) was selected. The same mass of pure bismuth oxide powder and bismuth oxide-sodium alginate gel spheres synthesized from sodium alginate were taken, along with the same mass of pure melamine sponge used in the composite material preparation of Example 2. All were tested under ultraviolet light irradiation. The test results are shown in Table 5 below.

[0099] Table 5. Comparison of removal efficiency of the photocatalyst prepared in this invention with that of raw materials with different compositions.

[0100]

[0101] As shown in Table 5, in the photocatalytic experiment, the synthesized composite material exhibited significantly higher removal efficiency for methyl orange and reduction efficiency for hexavalent chromium than either bismuth oxide powder or melamine sponge alone. Pure melamine sponge showed no catalytic activity under ultraviolet light; 15.5% and 14.8% of the catalytic activity were attributed to physical adsorption.

[0102] This invention involves targeted modification of melamine sponge with sodium alginate and loading it with bismuth oxide to prepare a bismuth oxide composite sponge carrier photocatalyst. The modified composite material successfully incorporates abundant carboxyl and amino groups, while bismuth oxide provides electrons and holes, thereby increasing the adsorption rate and performance for dye molecules and heavy metal ions in industrial wastewater. Under ultraviolet light irradiation, it can be used for the adsorption-degradation of methyl orange in dyeing and printing wastewater and the adsorption-reduction of hexavalent chromium in electroplating industrial wastewater. Under acidic conditions, the carboxyl and amino groups of sodium alginate can effectively adsorb methyl orange and hexavalent chromium. With increased ultraviolet light, bismuth oxide can be excited to generate electron-hole pairs, driving the adsorption of O2. -It generates reactive oxygen species such as ·OH, promotes the degradation of methyl orange and the reduction of hexavalent chromium, and can also effectively utilize natural polysaccharides and three-dimensional porous materials, which is in line with the concepts of green chemistry and circular economy and has positive significance for environmental protection.

[0103] Compared with pure bismuth oxide powder, powdered photocatalysts are prone to agglomeration, resulting in a decrease in specific surface area and a reduction in active sites. The sponge composite material synthesized in this invention has a larger specific surface area than pure bismuth oxide powder, which increases the number of pollutant adsorption sites. The efficiency of pollutant removal by the composite material is significantly improved. It still has a high photocatalytic removal efficiency after 30 cycles. Furthermore, pure bismuth oxide powder is difficult to separate and recover from water bodies, which can easily cause secondary pollution. The synthesized sponge composite material has a larger volume, which makes it easier to recover.

[0104] Compared with pure melamine sponge, the composite material prepared by this invention has the synergistic effect of bismuth oxide photocatalytic activity and sponge three-dimensional framework;

[0105] Compared with bismuth oxide / sodium alginate gel, the composite material has the light trapping effect of a sponge, which enhances light absorption. At the same time, the three-dimensional skeleton of the sponge promotes mass transfer, which significantly enhances the removal efficiency of the composite material for pollutants.

[0106] Compared to existing titanium dioxide composite sponges, titanium dioxide is easily soluble under strong acid conditions, and bismuth oxide exhibits significantly better chemical stability than titanium dioxide. This results in bismuth oxide sponge composites possessing highly efficient photocatalytic degradation activity. Furthermore, the composite material preparation process of this invention is simple, making it an ideal functional material for treating industrial wastewater containing methyl orange and hexavalent chromium. Its advantages of convenient operation, low preparation cost, and high treatment efficiency have been fully demonstrated in practical treatment processes.

[0107] The catalyst prepared in Example 2 was characterized by SEM, FT-IR, UV-vis DRS, BET, XPS, etc.; the SEM results showed that ( Figure 1 Bismuth oxide was highly dispersed on the surface of the MS framework in the form of two-dimensional nanosheets. The framework remained intact after the reaction, exhibiting good mechanical stability. FT-IR analysis ( Figure 2 The reaction confirmed that the main characteristic peaks of the material before and after the reaction remained highly similar, and no significant shifts were observed in the Bi-O bonds, amide groups, and CO groups, indicating that the composite material possesses excellent structural stability. The surface functional groups participated in the degradation of methyl orange and the reduction of hexavalent chromium, respectively. UV-vis DRS ( Figure 3 The results showed that the composite material exhibited strong light absorption in the 200-400 nm range, demonstrating good ultraviolet light response and photocatalytic activity potential. BET analysis ( Figure 4 , Figure 5The results indicate that the material exhibits a typical type IV adsorption isotherm, with pore sizes primarily ranging from 2 to 50 nm, demonstrating mesoporous structure characteristics that are beneficial for pollutant adsorption and mass transfer. XPS ( Figure 6 , Figure 7 The results show that before the reaction, Bi₄f₇ / ₂ is located at 159.0 eV, and the O 1s phase exhibits a Bi-O peak at 529.7 eV and a CO peak at 532.4 eV, indicating that bismuth ions on the bismuth oxide surface form Bi-OC coordination bonds with sodium alginate carboxylate ions. After the degradation of methyl orange, Bi₄f shifts to a higher binding energy by 0.7 eV, and a -SO₃ peak appears at 534.3 eV in the O 1s phase. - Peak. This is due to Bi 3+ Bi is oxidized to a higher valence state by holes and then reduced and reset by the electron-rich groups of SA. After the reduction of Cr(VI), Bi 4f shifts by 0.4 eV and a low-energy shoulder peak appears, and the Bi-O signal in O 1s is weakened. The Bi-OC coordination bond stabilizes the bismuth active center, enabling it to efficiently remove methyl orange and reduce hexavalent chromium under ultraviolet light.

[0108] The experimental results above show that under strongly acidic conditions (pH=1), the carboxyl (-COOH) and amino (-NH2) groups of sodium alginate undergo complete protonation, resulting in a strongly positively charged surface on the composite material. Simultaneously, the sulfonic acid groups (-SO3) in the methyl orange molecule... - ) and hexavalent chromium HCrO4 - / Cr2O7 2- Both are negatively charged, achieving efficient adsorption through electrostatic attraction. Under ultraviolet light excitation, bismuth oxide generates electron-hole pairs. Conduction band electrons directly reduce hexavalent chromium to trivalent chromium, while valence band holes oxidize H₂O / OH. - The formation of ·OH groups attacks the azo bonds (-N=N-) of methyl orange, causing them to break and degrade. The synergistic effect of adsorption pre-enrichment and photocatalytic oxidation leads to a localized increase in the concentration of pollutants on the material surface, significantly enhancing the apparent reaction rate. The sodium alginate cross-linked bismuth oxide-supported melamine sponge photocatalyst of this invention exhibits highly efficient adsorption and photocatalytic performance for both methyl orange and hexavalent chromium. This composite material also possesses good structural stability and recyclability, thus showing broad application prospects and significant importance in treating organic dyes and heavy metal pollution in industrial wastewater.

Claims

1. A bismuth oxide composite sponge-supported photocatalyst, characterized in that, The photocatalyst is a porous composite material with melamine sponge as a three-dimensional framework, sodium alginate as a cross-linked gel layer, and bismuth oxide as a photocatalytic active component.

2. The bismuth oxide composite sponge-supported photocatalyst according to claim 1, characterized in that, The mass ratio of bismuth oxide to sodium alginate is 2-3:1, based on the amount of each component.

3. A method for preparing the bismuth oxide composite sponge support photocatalyst as described in claim 1 or 2, characterized in that, Using melamine sponge as a carrier, sodium alginate was first used to specifically modify the melamine sponge, introducing abundant carboxyl and amino groups; then, Ca... 2+ Crosslinking anchors bismuth oxide in a sodium alginate gel network, which is then loaded onto a melamine sponge framework, forming a three-in-one functional interface of "adsorption-enrichment-catalysis". The carboxyl groups in the sodium alginate gel layer form Bi-OC coordination bonds with bismuth ions on the surface of bismuth oxide, shortening the transport distance of photogenerated carriers.

4. The method for preparing the bismuth oxide composite sponge support photocatalyst according to claim 3, characterized in that, The steps are as follows: S1: Disperse bismuth nitrate pentahydrate crystals in deionized water, and add sodium hydroxide solution dropwise while stirring to obtain a suspension; The resulting suspension was stirred at 80℃ for 1 hour, then centrifuged at 7500 r / min and washed repeatedly with deionized water until neutral. The final product was dried at 80℃ for 12 hours to obtain bismuth oxide crystals. S2: Cut the melamine sponge into 1cm×1cm×1cm cubes, clean and remove impurities, and then dry them; S3: Add sodium alginate powder to deionized water, stir, and obtain a homogeneous sodium alginate solution; S4: The bismuth oxide prepared in S1 is uniformly dispersed in the sodium alginate solution prepared in S3 by ultrasound to obtain a dispersion. S5: The pretreated melamine sponge from S2 is completely immersed in the dispersion obtained in S4 to ensure that the pores of the melamine sponge fully adsorb the sodium alginate solution; then the adsorbed sponge is removed and immersed in a 5% calcium chloride solution for crosslinking; Ca 2+ A stable gel network is formed by ionic crosslinking with the carboxyl groups of sodium alginate, anchoring bismuth oxide nanosheets on the surface of the sponge framework. After crosslinking, the surface is rinsed with deionized water to remove residual calcium chloride and uncrosslinked sodium alginate. After drying, the bismuth oxide composite sponge carrier photocatalyst is obtained.

5. The method for preparing the bismuth oxide composite sponge support photocatalyst according to claim 4, characterized in that, In S1, the molar ratio of bismuth nitrate pentahydrate to sodium hydroxide is 1:3~6.

6. The method for preparing the bismuth oxide composite sponge support photocatalyst according to claim 4, characterized in that, In step S2, the cleaning and impurity removal process is as follows: first, wash with deionized water three times to remove dust and impurities attached to the surface; then wash with anhydrous ethanol twice to remove organic pollutants.

7. The method for preparing the bismuth oxide composite sponge-supported photocatalyst according to claim 4, characterized in that, In step S3, the stirring temperature is 60℃, the stirring speed is 500 r / min, and the stirring time is 2 h.

8. The method for preparing the bismuth oxide composite sponge support photocatalyst according to claim 4, characterized in that, In S3, the mass fraction of the sodium alginate homogeneous solution is 0.25~0.375%.

9. The application of the bismuth oxide composite sponge carrier photocatalyst according to claim 1 in the photocatalytic degradation of methyl orange in wastewater.

10. The application of the bismuth oxide composite sponge carrier photocatalyst of claim 1 in the photocatalytic reduction of hexavalent chromium in wastewater.

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