A solid waste-based magnetic biochar and a preparation method and application thereof

Abstract

The application discloses a kind of magnetic biochar based on solid waste and its preparation method and application, preparation method is to red mud excessive nitric acid is fully reacted after solid-liquid separation, obtain the mixed solution containing iron nitrate, aluminum nitrate and calcium nitrate and SiO2 Solid product;Broken straw particle is added to mixed solution and is fully stirred and mixed, impregnation;Appropriate alumina is added to mixed solution, and the Fe / Al ratio in mixed solution is adjusted to (1~2):1;Straw after impregnation is washed and dried, and is fully mixed with calcium-rich compound after high-temperature pyrolysis treatment in anoxic environment, after cooling, grinding and screening, magnetic biochar is obtained.The raw materials required for the preparation of the material are mainly derived from industrial and agricultural solid waste, and the preparation cost is low, which can effectively realize the resource utilization of industrial and agricultural solid waste.The material has excellent adsorption performance for trace elements such as phosphorus, cadmium and arsenic, and can separate and remove pollutants from the medium by magnetic separation.

Description

Technical Field

[0001] This invention belongs to the field of biochar preparation technology, specifically relating to a magnetic biochar based on solid waste, its preparation method, and its application. Background Technology

[0002] The increasing emissions of industrial waste gas, wastewater, and solid waste, coupled with the long-term application of chemical fertilizers and pesticides in agricultural activities, have led to a growing problem of heavy metal and phosphorus pollution in aquatic environments, particularly in water bodies. Cadmium and arsenic, as typical highly toxic heavy metals, are teratogenic and carcinogenic. They can migrate through industrial wastewater or leach into the environment via soil, accumulating in crops and ultimately threatening human health through the food chain. Simultaneously, eutrophication of water bodies caused by phosphorus-containing industrial wastewater and agricultural non-point source pollution continues to worsen. While phosphorus is an essential nutrient for organisms, excessive emissions disrupt the balance of aquatic ecosystems, triggering a chain reaction of explosive algal blooms, dissolved oxygen depletion, and biodiversity degradation.

[0003] Solid wastes generated from industrial production, such as red mud (rich in iron / aluminum / silicon oxides), agricultural waste straw (biomass carbon source), and aquaculture waste shells (calcium carbonate), generally suffer from problems such as large stockpiles, high disposal costs, and significant environmental risks.

[0004] In recent years, biochar has emerged as a novel environmental remediation material, exhibiting excellent adsorption capacity for heavy metals and organic pollutants due to its large specific surface area and abundant pore structure. However, the recyclability and removal capacity of ordinary biochar still need improvement. Therefore, it is essential to explore modifications to biochar to prepare more powerful magnetic biochar. Summary of the Invention

[0005] To address at least one of the aforementioned problems, this invention provides a magnetic biochar based on solid waste, its preparation method, and its applications. The raw materials required for this material preparation mainly originate from industrial and agricultural solid waste, resulting in low preparation costs and effective resource utilization of industrial and agricultural solid waste. Furthermore, this material exhibits excellent adsorption performance for trace elements such as phosphorus and heavy metals such as cadmium and arsenic, and pollutants can be separated and removed from the medium through magnetic separation.

[0006] To achieve the above objectives, the present invention employs the following technical means:

[0007] The first aspect of the present invention provides a method for preparing magnetic biochar based on solid waste, comprising the following steps:

[0008] S1. Add excess nitric acid to the red mud and stir until fully reacted at room temperature;

[0009] S2. The mixture after the reaction is complete is separated into solid and liquid by centrifugation and filtration to obtain a mixed solution containing ferric nitrate, aluminum nitrate and calcium nitrate, as well as SiO2 solid product.

[0010] S3. Thoroughly mix and soak the crushed straw particles with the above-mentioned mixed solution;

[0011] S4. Add an appropriate amount of alumina to the above mixed solution to adjust the Fe / Al ratio in the mixed solution to (1~2):1, while neutralizing the excess acid in the solution and adjusting the pH of the solution to 5~6; the mass ratio of alumina to red mud is (0.5~4):10.

[0012] S5. After washing the soaked straw, place it in an oven to dry. Once the straw is dry, mix it thoroughly with the calcium-rich compound.

[0013] S6. The mixture of S5 is subjected to high-temperature pyrolysis in an oxygen-deficient environment. After cooling to room temperature, it is ground through a 0.10~0.20mm sieve to obtain magnetic biochar.

[0014] In some embodiments of the present invention, the main chemical composition of the red mud is 30-60% Fe2O3, 10-20% Al2O3, 10-20% SiO2, 2-10% CaO, and 2-5% Na2O.

[0015] In some embodiments of the present invention, the calcium-rich compound is calcium carbonate, or crushed calcium-rich waste containing calcium oxide, calcium carbonate, and calcium hydroxide, such as crushed aquaculture waste shell powder.

[0016] In some embodiments of the present invention, the SiO2 solid product obtained from the filter residue after acid washing of red mud can be used for subsequent resource utilization.

[0017] In some embodiments of the present invention, the mass-to-volume ratio of red mud and nitric acid in step S1 is 1 g: (12~15) mL.

[0018] In some embodiments of the present invention, the mass-to-volume ratio of straw particles to solution in step S3 is 1g:(10~12)mL, and the soaking time is 1~2h.

[0019] In some embodiments of the present invention, the mass ratio of straw to calcium-rich compound in step S5 is 10:(1~2); the drying temperature in the oven is 60-80℃.

[0020] In some embodiments of the present invention, the conditions for high-temperature pyrolysis treatment in step S6 are pyrolysis at 400~600℃ for 2~3 hours and a heating rate of 15~20℃ / min.

[0021] A second aspect of the present invention provides a magnetic biochar prepared using the method described in the first aspect.

[0022] The third aspect of the present invention provides the application of the magnetic biochar described in the second aspect in the remediation of phosphorus-containing wastewater and cadmium- and arsenic-contaminated wastewater.

[0023] In some embodiments of the present invention, the method is as follows: magnetic biochar as described in the second aspect is added to phosphorus-containing wastewater or cadmium- and arsenic-contaminated wastewater, and after shaking reaction for 6-12 hours, the magnetic biochar is separated from the wastewater by magnetic separation to achieve wastewater purification.

[0024] A fourth aspect of this invention provides the application of magnetic biochar as a slow-release phosphorus fertilizer, the method of which is as follows: The magnetic biochar described in the second aspect is added to phosphorus-containing wastewater, and after shaking reaction for 6-12 hours, the adsorbed magnetic biochar is separated from the wastewater by magnetic separation. The separated adsorbed magnetic biochar is then applied to the soil as a slow-release phosphorus fertilizer. In some embodiments of this invention, the amount of adsorbed magnetic biochar added is 1%-5%.

[0025] The fifth aspect of the present invention provides the application of the magnetic biochar described in the second aspect in the remediation of cadmium-arsenic co-contaminated soil.

[0026] In some embodiments of the present invention, the method is as follows: 3% by mass of magnetic biochar is added to cadmium and arsenic contaminated soil, the mixture is thoroughly mixed, and the soil is incubated at a constant temperature of 25℃-27℃ for 55-60 days.

[0027] Beneficial effects of the present invention

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The raw materials for this scheme are industrial and agricultural solid wastes, which have low preparation costs and simple methods. Using industrial and agricultural solid wastes such as red mud, agricultural waste straw and aquaculture waste shells, the red mud is treated with nitric acid to extract a mixed solution containing ferric nitrate, aluminum nitrate and calcium nitrate, as well as SiO2 solid products. The mixed solution can be used to prepare magnetic biochar for the adsorption of trace element phosphorus and heavy metals cadmium and arsenic. Since the acid solution is extracted with nitric acid, chloride ions will not be introduced. The SiO2 solid products are mainly silicon dioxide, which can be further utilized as resources and mixed with fly ash, cement and other materials to prepare cementitious materials or geopolymers.

[0030] (2) By adding alumina, the Fe / Al ratio in the mixed solution is adjusted, which not only neutralizes the excess acid in the solution and adjusts the pH of the solution, but also helps to improve the adsorption function of magnetic biochar.

[0031] (3) Immerse the straw in the mixed solution of iron nitrate and aluminum nitrate extracted from red mud, so that iron and aluminum elements can be loaded on the surface of the straw, and then prepare magnetic biochar through the process of calcium addition and calcination. During the calcium addition and calcination process, calcium carbonate decomposes into calcium oxide and carbon dioxide, promoting the pore development of biochar. At the same time, iron nitrate, aluminum nitrate and calcium oxide pyrolyze to form iron oxide, aluminum oxide, iron-aluminum bimetallic oxide (such as Fe3O4-Al2O3 composite) and calcium oxide attached to the surface of biochar. On the one hand, the loading of Fe3O4 endows the biochar with magnetism, facilitating the rapid recovery after biochar adsorption. On the other hand, Fe / Al bimetals also form nanoscale oxide particles during pyrolysis, which can greatly increase the specific surface area and mesopore ratio of biochar, providing more adsorption sites for the adsorption of phosphorus, cadmium and arsenic.

[0032] (4) During the adsorption process, magnetic biochar can synchronously release Fe 3+ and Al 3+ , which compete with phosphate (PO4 3- ), arsenate (AsO4 3- ) and arsenite (AsO3 3- ) in the solution to form Fe-Al bimetallic phosphate / arsenate precipitates. The solubility of these precipitates is lower than that of single-metal phosphates / arsenates, and the adsorption stability is stronger. In addition, iron oxide is positively charged under acidic conditions (pH < pHpzc), which can enhance the electrostatic attraction of biochar to PO4 3- , AsO4 3- and AsO3 3- . Aluminum oxide can maintain a relatively high positive charge density under neutral to alkaline conditions. Under the synergistic effect of iron-aluminum bimetallic oxides, biochar can have high adsorption capacity for phosphorus and arsenic within a wide pH range (3 - 12), breaking through the pH limit of single-metal adsorption.

[0033] (5) Since magnetic biochar forms Fe-Al bimetallic phosphate precipitates during the adsorption of phosphorus, and its solubility is lower than that of single-metal phosphates, the phosphorus-rich magnetic biochar can slowly release phosphorus in the forms of Ca-P, Fe-P and Al-P in the soil, having a longer phosphorus slow-release period. At the same time, due to the chemical stability of iron-aluminum bimetallic oxides, it can reduce the dissolution of Fe 3+ and Al 3+ . After 5 cycles, the removal rates of arsenic and cadmium still remain above 80%. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is the electron microscope scanning image of the magnetic biochar prepared in Examples 1 to 4 of the present invention;

[0035] Figure 2This invention provides a comparison of the removal effects of magnetic biochar prepared in Examples 1 to 4 on phosphate in water.

[0036] Figure 3 This is a comparison of the effects of magnetic biochar prepared in Examples 1 to 4 of the present invention on the slow release of phosphorus in soil after adsorbing phosphate.

[0037] Figure 4 This invention compares the removal effects of magnetic biochar prepared in Examples 1 to 4 on Cd and As in water.

[0038] Figure 5 This invention provides a comparison of the removal effects of magnetic biochar prepared in Examples 1 to 4 on Cd and As in soil.

[0039] Figure 6 This invention compares the phosphorus adsorption performance of biochar prepared in Examples 3, 5 to 7 under pH conditions of 2-12.

[0040] Figure 7 This invention compares the adsorption performance of biochar prepared in Examples 3, 5 to 7 of the present invention for arsenic under pH conditions of 2-12.

[0041] Figure 8 This is a comparison of the cyclic adsorption performance of the biochar prepared in Examples 3, 6 and 7 of the present invention. Detailed Implementation

[0042] The following examples are used to illustrate preferred embodiments of the invention. Those skilled in the art will understand that the techniques disclosed in the examples represent techniques discovered by the inventors that can be used to implement the invention, and therefore can be considered preferred embodiments for implementing the invention. However, those skilled in the art should understand from this specification that many modifications can be made to the specific embodiments disclosed herein, still yielding the same or similar results, without departing from the spirit or scope of the invention.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and all materials disclosed herein and cited therein are incorporated herein by reference. Many equivalent techniques of specific embodiments of the invention described herein will be recognized or can be understood by ordinary experimentation by those skilled in the art. These equivalents will be included in the claims.

[0044] The technical solution of this application will be further described in detail below with reference to specific embodiments.

[0045] Example 1

[0046] Prepare magnetic biochar using the following steps:

[0047] Step 1: Using 10g of red mud as raw material, place the red mud in a stirrer and add 150 ml of 3.0 mol / L dilute nitric acid. Stir and react fully for 3 hours at room temperature. Obtain a mixed solution of ferric nitrate, aluminum nitrate and calcium nitrate by centrifugation and filtration.

[0048] Step 2: Crush the straw and pass it through a 1mm sieve. Take 15g of straw particles and add them to the above mixed solution. Adjust the pH of the solution to 6, stir thoroughly and let it stand for 24 hours. Filter out the solid and dry it to obtain biochar raw material.

[0049] Step 3: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Take it out and mix it thoroughly with 2g of calcium carbonate. Place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain magnetic biochar (MBC).

[0050] Example 2

[0051] Prepare magnetic biochar using the following steps:

[0052] Step 1: Using 10g of red mud as raw material, place the red mud in a stirrer and add 150 ml of dilute nitric acid (3.0 mol / L). Stir and react fully for 3 hours at room temperature or under heating conditions. Obtain a mixed solution of ferric nitrate, aluminum nitrate and calcium nitrate by centrifugation and filtration.

[0053] Step 2: Crush the straw to pass through a 1mm sieve, add 15g of straw particles to the above mixed solution, add 1.0g of alumina and adjust the pH of the solution to 6, stir thoroughly and let stand for 24 hours, filter out the solid, dry it to obtain biochar raw material;

[0054] Step 3: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Take it out and mix it thoroughly with 2g of calcium carbonate. Place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain magnetic biochar (MBC1).

[0055] Example 3

[0056] Prepare magnetic biochar using the following steps:

[0057] Step 1: Using 10g of red mud as raw material, place the red mud in a stirrer and add 150 ml of dilute nitric acid (3.0 mol / L). Stir and react fully for 3 hours at room temperature or under heating conditions. Obtain a mixed solution of ferric nitrate, aluminum nitrate and calcium nitrate by centrifugation and filtration.

[0058] Step 2: Crush the straw to pass through a 1mm sieve, add 15g of straw particles to the above mixed solution, add 2.0g of alumina and adjust the pH of the solution to 6, stir thoroughly and let stand for 24 hours, filter out the solid, dry it to obtain biochar raw material;

[0059] Step 3: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Take it out and mix it thoroughly with 2g of calcium carbonate. Place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain magnetic biochar (MBC2).

[0060] Example 4

[0061] Prepare magnetic biochar using the following steps:

[0062] Step 1: Using 10g of red mud as raw material, place the red mud in a stirrer and add 150 ml of dilute nitric acid (3.0 mol / L). Stir and react fully for 3 hours at room temperature or under heating conditions. Obtain a mixed solution of ferric nitrate, aluminum nitrate and calcium nitrate by centrifugation and filtration.

[0063] Step 2: Crush the straw to pass through a 1mm sieve, add 15g of straw particles to the above mixed solution, add 3.0g of alumina and adjust the pH of the solution to 6, stir thoroughly and let stand for 24 hours, filter out the solid, dry it to obtain biochar raw material;

[0064] Step 3: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Take it out and mix it thoroughly with 2g of carbonic acid. Place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10mm sieve to obtain magnetic biochar (MBC3).

[0065] Example 5

[0066] Prepare conventional biochar using the following steps:

[0067] Step 1: Wash the straw with deionized water, dry it in an oven at 80°C, crush it and pass it through a 1mm sieve to obtain biochar raw material.

[0068] Step 2: Place the biochar raw material in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain biochar (BC).

[0069] Example 6

[0070] Prepare magnetic biochar using the following steps:

[0071] Step 1: Crush the straw until it passes through a 1mm sieve, then add 15g of straw particles to 200mL of FeSO4·7H2O solution (Fe2+ The concentration was 0.2 mol / L. The pH was adjusted to 6, and the mixture was stirred and reacted for 3 hours at room temperature or under heating conditions. The iron-based biochar raw material was obtained by centrifugation and filtration.

[0072] Step 2: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Then, place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain iron-based modified biochar.

[0073] Example 7

[0074] Prepare magnetic biochar using the following steps:

[0075] Step 1: Crush the straw to pass through a 1mm sieve, take 15g of straw particles and add them to 200mL of AlCl3·6H2O solution (0.5mol / L), adjust the pH to 4.5, stir and react for 3 hours at room temperature or under heating conditions, and obtain aluminum-based biochar raw material by centrifugation and filtration.

[0076] Step 2: Wash the biochar raw material with deionized water and dry it in an oven at 80°C. Then, place it in a muffle furnace and pyrolyze it at 600°C for 3 hours in an oxygen-deficient environment. After cooling to room temperature, grind the material through a 0.10 mm sieve to obtain aluminum-based modified biochar.

[0077] Performance comparison:

[0078] (1) The morphological and structural characteristics of the magnetic biochar prepared in Examples 1 to 4 were analyzed by scanning electron microscopy (SEM). The image results are shown in the figure. Figure 1 .

[0079] The results showed that the magnetic biochar all exhibited a porous structure with their surfaces covered by crystals.

[0080] By impregnating straw in a mixed solution of ferric nitrate and aluminum nitrate extracted from red mud, iron and aluminum elements can be loaded onto the straw surface. Magnetic biochar is then prepared through a calcium-enrichment-calcination process. During calcium-enrichment-calcination, calcium carbonate decomposes into calcium oxide and carbon dioxide, promoting the development of biochar pores. Simultaneously, ferric nitrate, aluminum nitrate, and calcium oxide are pyrolyzed to form iron oxides, aluminum oxides, iron-aluminum bimetallic oxides (such as Fe3O4-Al2O3 complexes), and calcium oxide, which adhere to the biochar surface. The loading of Fe3O4 imbues the biochar with magnetism, allowing for rapid recovery of the adsorbed biochar material using a magnet.

[0081] (2) The elemental composition of the surface of the magnetic biochar materials prepared in Examples 1 to 4 was analyzed by EDS, and the results are shown in Table 1 below.

[0082] Table 1 Surface element content of magnetic biochar

[0083]

[0084] (3) At room temperature (25℃), 0.05 g of the magnetic biochar material prepared in Examples 1 to 4 was weighed into a 50 mL centrifuge tube, and 30 mL of phosphate solution with a phosphate concentration of 100 mg / L was added. The centrifuge tube was placed in a constant temperature air bath shaker and shaken at 250 rpm for 24 h. After centrifugation, the solution was filtered through a 0.45 μm microporous membrane, and the phosphate concentration in the solution was determined by ultraviolet spectrophotometer. The test results are as follows: Figure 2 As shown.

[0085] The results showed that the magnetic biochar material prepared by using red mud extract with Fe / Al ratio control significantly improved the phosphate removal rate compared with the method without Fe / Al ratio control.

[0086] In the preparation of biochar materials, the Fe / Al ratio needs to be strictly controlled to achieve a synergistic effect. Adjusting the Fe / Al ratio in the mixed solution to between 1:1 and 2:1 yields the best results.

[0087] (4) The magnetic biochar materials from Examples 1 to 4 were separated from the adsorbed wastewater by magnetic separation to obtain magnetic biochar (MCB1-P, MCB2-P, and MCB3-P) after phosphate adsorption. MCB1-P, MCB2-P, and MCB3-P were applied to the soil at an addition rate of 1%. The results of soil available phosphorus content analysis are as follows: Figure 3 As shown.

[0088] The results showed that magnetic biochar after phosphate adsorption significantly increased the available phosphorus content in the soil. This is because magnetic biochar forms Fe-Al bimetallic phosphate precipitate during phosphorus adsorption, which has lower solubility than single-metal phosphate. This allows the phosphorus-rich magnetic biochar to slowly release phosphorus in the forms of Ca-P, Fe-P, and Al-P in the soil, resulting in a longer phosphorus release period.

[0089] (5) At room temperature (25℃), 0.05 g of magnetic biochar material was weighed into a 50 mL centrifuge tube, and Cd and As solutions were added, with concentrations of 40 mg / L and 20 mg / L, respectively. The centrifuge tube was placed in a constant temperature air bath shaker and shaken at 250 rpm for 24 h. After centrifugation, the solution was filtered through a 0.45 μm microporous membrane, and the concentrations of Cd and As in the solution were determined by ICP-MS. The test results are as follows: Figure 4 As shown.

[0090] The results showed that the magnetic biochar material prepared by using red mud extract with Fe / Al ratio control significantly improved the removal rate of cadmium and arsenic in water compared with the method without Fe / Al ratio control.

[0091] (6) Weigh 600 g of cadmium and arsenic contaminated soil and place it in a plastic culture bottle. Add deionized water to adjust the soil moisture to 60% of field capacity. Add MBC, MBC1, MBC2, and MBC3 to the soil at a mass fraction of 3% and mix thoroughly. Place the culture bottle in a 25℃ constant temperature incubator for 60 days. After 60 days of soil culture, weigh 50 g of air-dried soil samples from each treatment group, perform magnetic separation, collect the magnetically separated soil samples, and analyze the removal rates of Cd and As in the soil by the four materials. The test results are as follows: Figure 5 As shown.

[0092] The results showed that the magnetic biochar material prepared by using red mud extract with Fe / Al ratio control significantly improved the removal rate of cadmium and arsenic in soil compared with the method without Fe / Al ratio control.

[0093] (7) The adsorption performance of the biochar prepared in Examples 3, 5 to 7 was investigated under pH conditions of 2-12 for phosphorus and arsenic. The results are as follows: Figure 6 and Figure 7 As shown.

[0094] The results showed that the catalyst MBC2 prepared in Example 3 had significantly better adsorption performance for phosphorus and arsenic than biochar, iron-based modified biochar, and aluminum-based modified biochar. This is because impregnating straw in a mixed solution of ferric nitrate and aluminum nitrate extracted from red mud allows iron-aluminum elements to be loaded onto the straw surface, followed by a calcium-addition-calcination process to prepare magnetic biochar. During the calcium-addition-calcination process, calcium carbonate decomposes into calcium oxide and carbon dioxide, promoting the development of biochar pores. Simultaneously, ferric nitrate, aluminum nitrate, and calcium oxide are pyrolyzed to form iron oxides, aluminum oxides, iron-aluminum bimetallic oxides (such as Fe3O4-Al2O3 complexes), and calcium oxide, which adhere to the biochar surface. The specific surface area and mesopore ratio of the biochar are significantly increased, providing more adsorption sites for phosphorus, cadmium, and arsenic adsorption. During the adsorption process, the magnetic biochar can simultaneously release Fe. 3+ And Al 3+ , with phosphates (PO4) in the solution 3- Arsenate (AsO4) 3- ) and arsenite (AsO3) 3- They compete and combine to form Fe-Al bimetallic phosphate / arsenate precipitates, which have lower solubility than single metal phosphate / arsenate precipitates and stronger adsorption stability.

[0095] In addition, iron oxides are positively charged under acidic conditions (pH < pHpzc), which can enhance the electrostatic attraction of biochar to PO4 3- , AsO4 3- and AsO3 3- . Aluminum oxides can maintain a relatively high positive charge density under neutral to alkaline conditions. Under the synergistic effect of iron-aluminum bimetallic oxides, biochar can have high adsorption capacity for both phosphorus and arsenic within a wide pH range (3 - 12), breaking through the pH limitation of single-metal adsorption.

[0096] (8) The cyclic adsorption performance of the biochars prepared in Example 3, Example 6 and Example 7 was tested, and the results are as Figure 8 shown.

[0097] The results show that for the catalyst prepared by the method of the present application, due to the chemical stability of the iron-aluminum bimetallic oxides, the dissolution of Fe 3+ and Al 3+ can be reduced. After 5 cycles, the removal rates of arsenic and cadmium still remain above 80%, while those of the modified biochar in the single-metal system are usually lower than 70%.

[0098] All documents mentioned in the present invention are cited herein as references, as if each document was cited separately as a reference. In addition, it should be understood that after reading the above teachings of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the present application.

Claims

1. A method for preparing magnetic biochar based on solid waste, characterized in that, Includes the following steps: S1. Add excess nitric acid to the red mud and stir until fully reacted at room temperature; S2. The mixture after the reaction is complete is separated into solid and liquid by centrifugation and filtration to obtain a mixed solution containing ferric nitrate, aluminum nitrate and calcium nitrate, as well as SiO2 solid product. S3. Thoroughly mix and soak the crushed straw particles with the above-mentioned mixed solution; S4. Add an appropriate amount of aluminum oxide to the above mixed solution to adjust the Fe / Al ratio in the mixed solution to (1~2):1, while neutralizing the excess acid in the solution and adjusting the pH of the solution to 5~6. S5. After washing the soaked straw, place it in an oven to dry. Once the straw is dry, mix it thoroughly with the calcium-rich compound. S6. The mixture in S5 is subjected to high-temperature pyrolysis in an oxygen-deficient environment. After cooling to room temperature, it is ground and passed through a 0.10~0.20 mm sieve to obtain magnetic biochar. In step S1, the mass-to-volume ratio of red mud to dilute nitric acid is 1 g : (12~15) mL; In step S3, the mass-to-volume ratio of straw particles to solution is 1g:(10~12)mL, and the soaking time is 1~2h; In step S5, the mass ratio of straw to calcium-rich compound is 10:(1~2). The conditions for high-temperature pyrolysis in step S6 are pyrolysis at 400~600℃ for 2~3 hours, with a heating rate of 15~20℃ / min; The calcium-rich compound is calcium carbonate.

2. A magnetic biochar prepared using the method of claim 1.

3. The application of the magnetic biochar according to claim 2 in the remediation of wastewater polluted with trace element phosphorus and heavy metals cadmium and arsenic.

4. The application according to claim 3, characterized in that, The method is as follows: Add the magnetic biochar described in claim 2 to phosphorus-containing wastewater or cadmium- and arsenic-contaminated wastewater, and after shaking the reaction for 6-12 hours, separate the magnetic biochar from the wastewater through magnetic separation to achieve wastewater purification.

5. The application of magnetic biochar as a slow-release phosphorus fertilizer, characterized in that, The method is as follows: Add the magnetic biochar described in claim 2 to phosphorus-containing wastewater, and after shaking the reaction for 6-12 hours, separate the adsorbed magnetic biochar from the wastewater by magnetic separation, and apply the separated adsorbed magnetic biochar to the soil as a slow-release phosphorus fertilizer.

6. The application of the magnetic biochar according to claim 2 in the remediation of soil contaminated with heavy metals cadmium and arsenic.

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