Modified biochar for phosphorus-fluorine synergistic curing in phosphogypsum and application of modified biochar

Abstract

The invention discloses peanut shell-based HAP composite modified charcoal and application thereof in phosphorus-fluorine synergistic curing in phosphogypsum, and relates to the field of functional material preparation. The method is characterized in that peanut shells are used as raw materials and subjected to high-temperature calcination, the peanut shells are compounded with hydroxyapatite (HAP) prepared from ardealite to form modified biochar, and wastewater is treated through the synergistic effect of chemical bonding, physical adsorption and alkaline regulation and control, and the two synergistically adjust the pH and inhibit leaching. The biochar can efficiently remove phosphorus and fluorine in phosphogypsum polluted water and various phosphorus and fluorine wastewater (the concentrations after treatment are respectively less than or equal to 0.5 mg / L and less than or equal to 10 mg / L, and conform to GB 8978-1996), can assist in removing trace heavy metals, and is stable in curing system and free of secondary pollution. The raw materials are agricultural wastes, the preparation is simple and convenient, the cost is low, waste recycling is realized, circular economy and double-carbon targets are met, and the application prospect is wide.

Description

Technical Field

[0001] This invention relates to the field of industrial solid waste harmless treatment and environmental functional materials technology, specifically to a modified biochar loaded with hydroxyapatite prepared from peanut shells and phosphogypsum, and its preparation method and application in the synergistic solidification of soluble phosphorus and fluorine in phosphogypsum, which is applicable to scenarios such as solid waste disposal and ecological environment restoration in phosphate chemical enterprises. Background Technology

[0002] Phosphogypsum is a major solid waste generated during the wet-process phosphoric acid production process. Approximately 4-6 tons of phosphogypsum are produced for every ton of phosphoric acid produced. As a major producer of phosphate fertilizers, my country has accumulated over 800 million tons of phosphogypsum stockpiles. This type of solid waste contains large amounts of soluble phosphorus (such as H₂PO₄⁻, HPO₄²⁻, PO₄³⁻), fluoride ions (F⁻), and some heavy metal pollutants. When stored in the open for extended periods, these pollutants continuously seep into the soil and groundwater systems through rainwater leaching, causing multiple environmental problems: the migration of soluble phosphorus leads to eutrophication of water bodies, causing ecological disasters such as algal blooms and fish deaths; the accumulation of fluoride ions causes soil fluoride pollution, which, through crop absorption and the food chain, induces health problems such as dental fluorosis and skeletal fluorosis in humans. It also leads to excessive fluoride levels in groundwater, threatening human health.

[0003] Current technologies for removing phosphorus and fluoride from phosphogypsum, both domestically and internationally, have significant shortcomings. Therefore, it is necessary to design a modified biochar for treating phosphorus and fluoride in phosphogypsum to address these issues. Summary of the Invention

[0004] To address the problems of low efficiency, high cost, and poor stability in existing phosphogypsum phosphorus and fluoride treatment technologies, the core objective of this invention is to provide a modified biochar loaded with hydroxyapatite (HAP) prepared from agricultural waste peanut shells through a simple modification process, achieving efficient synergistic solidification of phosphorus and fluoride in phosphogypsum; clarifying the optimal preparation parameters and phosphorus and fluoride solidification process conditions for the modified biochar, ensuring that the leachate indicators after solidification meet national environmental protection standards; realizing "waste-to-waste treatment," transforming agricultural and forestry waste such as peanut shells into high-value-added environmental functional materials, reducing phosphogypsum treatment costs, and laying the foundation for subsequent resource utilization of phosphogypsum (such as cement retarders, gypsum boards, and soil conditioners).

[0005] To achieve the above-mentioned objectives, in a first aspect, the present invention provides a method for preparing modified biochar for synergistic solidification of phosphorus and fluorine in phosphogypsum, comprising the following steps: Biochar is produced by carbonizing biomass. Phosphogypsum was added to potassium dihydrogen phosphate solution and mixed in proportion. Under pH 9-11 conditions, the mixture was transferred to a polytetrafluoroethylene high-pressure reactor, and the biochar and citric acid were added. The mixture was then subjected to hydrothermal reaction at 180-200℃ for 6-24 hours. After cooling, the mixture was washed, filtered, and dried to obtain modified biochar.

[0006] Preferably, the conditions for biomass carbonization are: heating to (700-900)℃ at a rate of (10-20)℃ / min, holding at that temperature for 2 hours, and continuously introducing argon gas during the carbonization process.

[0007] Preferably, the mass ratio of biochar, phosphogypsum, and citric acid is (10-30):(30-450):(0.5-5).

[0008] Preferably, the mass concentration of the potassium dihydrogen phosphate solution is (30-50) g / L, and the ratio of the mass (g) of phosphogypsum to the volume (mL) of the potassium dihydrogen phosphate solution is Ca:P=1.67.

[0009] Preferably, the biomass is peanut shells. Before carbonization, the biomass needs to be pretreated as follows: the biomass is dried at 50°C and ground to pass through a 200-mesh sieve to obtain pretreated biomass.

[0010] Secondly, the present invention provides a modified biochar.

[0011] Thirdly, this invention provides an application of modified biochar in the synergistic curing of phosphogypsum with phosphorus and fluorine, the specific application method including the following steps: The phosphogypsum to be treated was mixed evenly with the modified biochar to obtain a mixture; an alkaline solution was added to adjust the pH of the mixture to (7-9), and the mixture was solidified at (15-75)℃ and aged for (1-15) days; a portion of the mixture was dried and placed in a 250ml conical flask, and distilled water with a solid-liquid ratio of 1:10 was added. The conical flask was placed in a shaking chamber and shaken for 8 hours, and then allowed to stand for 16 hours to complete the synergistic solidification of phosphorus and fluorine and the removal of polluted water.

[0012] Preferably, the water content of the mixture is 20-40%; the amount of modified biochar added is 5-15% of the mass of the phosphogypsum.

[0013] Preferably, the alkaline solution is one of sodium hydroxide and potassium hydroxide.

[0014] Preferably, the curing reaction time is 2-24 hours.

[0015] Preferably, after the curing reaction, the product undergoes an aging treatment for 1-15 days. The phosphorus concentration in the leachate of the cured product is ≤0.127mg / L and the fluorine concentration is ≤0.990mg / L, which meets the requirements of the Integrated Wastewater Discharge Standard (GB 8978-1996).

[0016] The beneficial effects of this invention are:

[0017] 1. The modified biochar preparation method provided by this invention uses peanut shells, an agricultural waste, as raw material. It is modified by high-temperature pyrolysis combined with hydroxyapatite (HAP) loading. This method not only retains the original porous structure and high specific surface area of ​​biochar, but also introduces a large number of active sites through HAP, which optimizes the adsorption and reaction capacity of the material for phosphorus and fluorine. Finally, a highly efficient synergistic solidification material with both physical adsorption and chemical fixation functions is obtained, realizing the high-value utilization of agricultural waste.

[0018] 2. The method for applying modified biochar in the synergistic curing of phosphorus and fluorine in phosphogypsum proposed in this invention can simultaneously and efficiently cure soluble phosphorus and fluorine in phosphogypsum through a multi-synergistic mechanism of "adsorption-precipitation-ion exchange": the porous structure of biochar physically traps pollutants, Ca²⁺ in HAP reacts with phosphate ions to form insoluble phosphate precipitates and reacts with fluoride ions to form calcium fluoride precipitates, while the hydroxyl groups of HAP fix fluoride ions in the crystal lattice through ion exchange to form stable fluorapatite, which significantly improves curing efficiency and stability.

[0019] 3. The curing process of this invention is mild. Under optimal parameters, the phosphorus and fluorine leaching concentrations are reduced to 0.127 mg / L and 0.990 mg / L, respectively, which meets the "Integrated Wastewater Discharge Standard" (GB 8978-1996). Moreover, the performance of the cured product does not decline after 15 days of aging, thus avoiding the risk of secondary pollution.

[0020] 4. This invention achieves "waste treatment with waste", which not only solves the problem of disposal of agricultural and forestry waste such as peanut shells, but also reduces the environmental pollution caused by the stockpiling of phosphogypsum. The treated phosphogypsum can retain its main component, calcium sulfate dihydrate, laying the foundation for subsequent resource utilization as cement retarder, gypsum board, etc., which meets the needs of circular economy and "dual carbon" goals. Attached Figure Description

[0021] Figure 1 This is a flow chart of the preparation process of the modified biochar proposed in this invention;

[0022] Figure 2 The image shows the XRD pattern of the modified biochar prepared in Example 1 of this invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] It should also be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and / or processing steps closely related to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0025] Additionally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0026] On the one hand, please refer to Figure 1 As shown, the method for preparing modified biochar provided by the present invention includes the following steps: Biochar is obtained by drying, pulverizing and sieving biomass, and then carbonizing it at 700-900℃ for 2 hours. Modified biochar is obtained by transferring phosphogypsum, potassium dihydrogen phosphate solution, citric acid, and the biochar to a polytetrafluoroethylene high-pressure reactor and carrying out a hydrothermal reaction under specific conditions, followed by washing, filtration, and drying.

[0027] In the above technical solution, HAP is synthesized hydrothermally and then modified with biochar. This process retains the porous adsorption structure of biochar while utilizing the highly active sites of HAP to enhance the chemical fixation capacity of phosphorus and fluorine. The composite system formed by HAP and biochar can synergistically solidify phosphorus and fluorine through multiple processes of "physical adsorption-chemical precipitation-ion exchange," solving the problems of low solidification efficiency and poor stability of traditional single materials.

[0028] In the embodiment, the conditions for biomass carbonization are as follows: heating to (700-900)℃ at a rate of (10-20)℃ / min, holding at that temperature for 2 hours, and continuously introducing argon gas during the carbonization process to avoid biomass oxidation.

[0029] In the embodiments, the mass ratio of biochar, phosphogypsum, and citric acid is (10-30):(30-450):(0.5-5), preferably 10:30:0.5.

[0030] In the embodiments, the mass concentration of the potassium dihydrogen phosphate solution is (30-50) g / L, and the ratio of the mass (g) of phosphogypsum to the volume (mL) of the potassium dihydrogen phosphate solution satisfies Ca:P=1.67, ensuring the stability of the stoichiometry of HAP synthesis.

[0031] In this embodiment, the biomass is peanut shells, which are dried and pulverized through the following steps: the peanut shells are dried at 40-60°C to constant weight, then pulverized and passed through a 200-mesh sieve for later use.

[0032] On the other hand, the present invention also provides the application of the modified biochar prepared by the above preparation method in the synergistic curing of phosphogypsum with phosphorus and fluorine, and the specific application method is as follows: Mix the phosphogypsum to be treated with modified biochar evenly, ensuring the moisture content of the mixture is 20-40%. Add an alkaline solution to adjust the pH of the mixture to 7-9. After solidifying the mixture at 15-75℃, age it for 1-15 days. Take a portion of the mixture, dry it, and place it in a 250ml Erlenmeyer flask. Add distilled water at a solid-liquid ratio of 1:10, place the Erlenmeyer flask in a shaking chamber, and shake for 8 hours. Let it stand for 16 hours to complete the synergistic curing of phosphorus and fluorine. The amount of modified biochar added is 5-15% of the mass of phosphogypsum, preferably 10%.

[0033] In the above technical solution, HAP in modified biochar can form insoluble phosphate precipitates with PO4³⁻ in phosphogypsum through Ca²⁺, and form calcium fluoride precipitates with F⁻ or form fluorapatite through ion exchange; the porous structure of biochar physically traps pollutants, while its alkaline properties neutralize the acidic environment of phosphogypsum, further inhibiting the leaching of phosphorus and fluorine, and achieving efficient synergistic curing.

[0034] The modified biochar provided by the present invention and its applications will be further illustrated below with reference to specific embodiments: Example

[0035] This embodiment prepares a modified biochar, which includes the following steps: Peanut shells were dried at 50℃ to constant weight, crushed and passed through a 200-mesh sieve; 50g of peanut shell powder was placed in a tube furnace and heated to 800℃ at a rate of 10℃ / min, with argon gas continuously introduced and kept at this temperature for 2 hours. After natural cooling, the powder was ground into a fine powder to obtain biochar. Weigh biochar, phosphogypsum, and citric acid in a mass ratio of 10:30:0.5. Add 40 g / L potassium dihydrogen phosphate solution and mix thoroughly at Ca:P = 1.67. Then, perform a hydrothermal reaction at 190℃ for 12 hours. After cooling, wash, filter, and dry. Dry the solid product at 45℃ for 24 hours to obtain modified biochar.

[0036] The modified biochar prepared in this embodiment was characterized by XRD and matched the standard spectrum (PDF#55-0022), proving that HAP was successfully compounded.

[0037] Application Example 1 This embodiment uses the modified biochar prepared in Example 1, and performs phosphorus-fluorine synergistic curing under optimal process conditions. The specific application method is as follows: 15g of phosphogypsum to be treated (taken from a chemical enterprise in Yichang, Hubei Province, with an initial phosphorus leaching concentration of 237mg / L and a fluorine leaching concentration of 248mg / L, dried at 50℃ and ground through a 200-mesh sieve) was mixed evenly with 1.5g of modified biochar. The moisture content of the phosphogypsum was controlled at 25%. The pH of the mixture was adjusted to 8 with sodium hydroxide solution and aged at room temperature (25℃) for 2 hours. Take the dried solidified product, add distilled water at a solid-liquid ratio of 1:10, shake in a shaker for 8 hours, let stand for 16 hours, and then filter to obtain the leachate; determine the phosphorus concentration using a water quality analyzer and the fluoride concentration using the fluoride ion selective electrode method. Test results: The phosphorus concentration of the leachate was 0.127 mg / L, the fluoride concentration was 0.990 mg / L, and the pH was 8.3, which meets the requirements of the "Integrated Wastewater Discharge Standard" (GB 8978-1996) for phosphorus ≤0.5 mg / L, fluoride ≤10 mg / L, and pH 6~9. The removal rates of phosphorus and fluoride reached 99.9% and 99.6%, respectively.

[0038] Application Example 2 The only difference between this embodiment and application embodiment 1 is that the amount of modified biochar added is 1.0g (with a mass ratio of 1:15 to phosphogypsum). All other experimental conditions (phosphogypsum dosage 15g, moisture content 25%, pH=8, aging at room temperature for 2 hours, leaching test method) are the same as in application embodiment 1, and will not be repeated here. Test results: The phosphorus concentration in the leachate was 0.134 mg / L, the fluorine concentration was 2.405 mg / L, and the pH was 8.2, which met the national standards. However, the fluorine concentration was higher than in Application Example 1, and the curing effect was slightly reduced. The reason is that when the dosage was insufficient, the active sites of the modified biochar could not be fully adsorbed and reacted, resulting in some phosphorus and fluorine not being completely fixed.

[0039] Application Example 3 The only difference between this embodiment and Application Example 1 is that the pH of the mixed system is adjusted to 7. All other experimental conditions (modified biochar dosage 1.5g, phosphogypsum dosage 15g, moisture content 25%, room temperature aging for 2 hours, leaching test method) are the same as in Application Example 1, and will not be repeated here. Test results: The phosphorus concentration in the leachate was 0.353 mg / L, the fluorine concentration was 0.863 mg / L, and the pH was 6.9, which met the national standard. However, the phosphorus concentration was higher than that in Application Example 1. The reason is that at pH=7, the reactivity of hydroxyapatite with phosphorus is slightly lower than that at pH=8, resulting in a decrease in the solidification efficiency of phosphorus, while the adsorption and ion exchange of fluorine are less affected by pH.

[0040] Application Example 4 The only difference between this embodiment and application embodiment 1 is that the moisture content of phosphogypsum is adjusted to 18%. All other experimental conditions (modified biochar dosage 1.5g, phosphogypsum dosage 15g, pH=8, room temperature aging for 2 hours, leaching test method) are the same as in application embodiment 1, and will not be repeated here. Test results: The phosphorus concentration in the leachate was 0.200 mg / L, the fluoride concentration was 1.058 mg / L, and the pH was 8.5, which met the national standards. However, both the phosphorus and fluoride concentrations were slightly higher than in Application Example 1. This is because when the moisture content is insufficient, the contact between the modified biochar and the phosphogypsum particles is not sufficient, the mass transfer process is limited, and the synergistic effect of "adsorption-precipitation-ion exchange" is weakened.

[0041] Application Comparative Example 1 The only difference between this comparative example and Application Example 1 is that unmodified peanut shell biochar (the biochar in the example was not modified with hydroxyapatite after carbonization) was used, and the dosage was also 1.5g. All other experimental conditions were the same as in Application Example 1, and will not be repeated here. Test results: The phosphorus concentration of the leachate was 0.250 mg / L, the fluorine concentration was 9.338 mg / L, and the pH was 8.5. Although these met the national standards, the curing effect was significantly worse than that of Application Example 1, which proves the key role of hydroxyapatite modification in improving the efficiency of phosphorus and fluorine curing.

[0042] Application Comparative Example 2 The only difference between this comparative example and Application Example 1 is that no curing material is added, and the same pH adjustment (pH=8) and moisture content control (25%) are performed on the phosphogypsum. All other experimental conditions are the same as in Application Example 1, and will not be repeated here. Test results: The phosphorus concentration in the leachate was 237 mg / L, the fluoride concentration was 248 mg / L, and the pH was 8.1, which far exceeded the limits of the "Integrated Wastewater Discharge Standard" (GB 8978-1996), proving the necessity of the solidification material. Table 1. Comparison of curing effects between application examples 1 to 4 and application comparative examples 1 to 2 project Phosphorus concentration in leachate (mg / L) Fluorine concentration in leachate (mg / L) pH of leachate Application Example 1 0.127 0.990 8.3 Application Example 2 0.134 2.405 8.2 Application Example 3 0.353 0.863 6.9 Application Example 4 0.200 1.058 8.5 Application Comparative Example 1 0.250 9.338 8.5 Application Comparative Example 2 237.000 248.000 8.1 The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing modified biochar for synergistic solidification of phosphorus and fluorine in phosphogypsum, characterized in that, Includes the following steps: Biochar is produced by carbonizing biomass. Phosphogypsum was added to potassium dihydrogen phosphate solution and mixed in proportion. Under pH 9-11 conditions, the mixture was transferred to a polytetrafluoroethylene high-pressure reactor, and the biochar and citric acid were added. The mixture was then subjected to hydrothermal reaction at 180-200℃ for 6-24 hours. After cooling, the mixture was washed, filtered, and dried to obtain modified biochar.

2. The preparation method according to claim 1, characterized in that, The conditions for biomass carbonization are as follows: the temperature is increased to (700-900)℃ at a rate of (10-20)℃ / min, and held for 2 hours, with argon gas continuously introduced during the carbonization process.

3. The preparation method according to claim 1, characterized in that, The mass ratio of biochar, phosphogypsum, and citric acid is (10-30):(30-450):(0.5-5).

4. The preparation method according to claim 1, characterized in that, The mass concentration of the potassium dihydrogen phosphate solution is (30-50) g / L, and the ratio of the mass (g) of phosphogypsum to the volume (mL) of the potassium dihydrogen phosphate solution is Ca:P=1.

67.

5. The preparation method according to claim 1, characterized in that, The biomass is peanut shells.

6. A modified biochar, characterized in that, It is prepared by any one of the preparation methods according to claims 1-5.

7. The application of the modified biochar in the removal of phosphorus and fluoride polluted water generated from phosphogypsum treatment, characterized in that, The specific application method includes the following steps: Mix the phosphogypsum to be treated with modified biochar evenly to obtain a mixture; add an alkaline solution to adjust the pH of the mixture to (7-9), keep the mixture at (15-75)℃ to solidify, and then age for (1-15) days; take a portion of the mixture, dry it, put it into a 250ml conical flask, add distilled water with a solid-liquid ratio of 1:10, place the conical flask in a shaking box and shake for 8 hours, and let it stand for 16 hours.

8. The application according to claim 7, characterized in that, The mixture has a water content of 20-40%; the amount of modified biochar added is 5-15% of the mass of the phosphogypsum.

9. The application according to claim 7, characterized in that, After the solidification reaction, the product undergoes an aging process of 1-15 days. The phosphorus concentration in the leachate of the solidified product is ≤0.127mg / L and the fluorine concentration is ≤0.990mg / L, which meets the requirements of the Integrated Wastewater Discharge Standard (GB8978-1996).

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