A method for preparing a hydrogel-coated modified biochar-red soil composite

Abstract

The application discloses a preparation method of a hydrogel-coated modified biochar-red soil composite, and comprises the following steps: pyrolyzing crop straw after pretreatment to obtain biochar; adding acrylamide, a chain transfer agent, a silane coupling agent, acetic acid and a photoinitiator into deionized water, stirring, and then irradiating the mixture with an ultraviolet lamp to obtain a hydrogel paint; immersing the biochar into the hydrogel paint, filtering, and then waiting for the hydrogel paint to solidify to obtain modified biochar; and mixing the modified biochar with red soil, and performing granulation treatment through a granulator to obtain the hydrogel-coated modified biochar-red soil composite. A stable coating layer is formed on the surface of the biochar through the hydrogel paint, dust flying in the production, transportation and application processes of the biochar is inhibited, the hydrogel is used to bond red soil fine particles, secondary dust flying caused by soil disturbance is reduced, and dust-free treatment is realized in the whole process.

Description

Technical Field

[0001] This invention belongs to the field of biochar modification and soil improvement technology, and particularly relates to a method for preparing a hydrogel-coated modified biochar-red soil composite. Background Technology

[0002] Red soils are generally characterized by strong weathering, a predominantly acidic pH, a compact and heavy soil structure, poor water and air permeability, low organic matter content, and a deficiency of available nitrogen and phosphorus nutrients, severely restricting crop growth. Although nitrogen fertilizer application can increase crop yields in the short term, red soils have low nitrogen utilization efficiency, and long-term excessive application can easily lead to nitrate leaching, resulting in the accumulation of hydrogen ions in the soil and further exacerbating soil acidification. Traditional soil conditioners have many limitations in practical applications, such as acting only on the topsoil with limited impact on the subsoil, potentially causing soil compaction, destroying beneficial soil microbial communities, reducing soil organic carbon content, and causing imbalances in elements such as calcium, potassium, and magnesium. In addition, to maintain the improvement effect, frequent application of agents is often required, increasing usage costs and environmental risks.

[0003] Existing technologies do not focus on improving the "acidic, sticky, and infertile" characteristics of red soil (such as traditional soil conditioners and biochar), but neglect the pollution risks and health hazards they bring; or they only attempt to alleviate a single pollution problem (such as biochar pollution mitigation technologies), which cannot take into account the improvement effect or may have secondary negative impacts.

[0004] The invention patent with publication number CN107955614B discloses a southern red soil improver based on mining tailings and its application method. It uses rare earth mining tailings as the improver material to turn waste into treasure. It systematically and comprehensively improves red soil from the perspectives of sand content, reducing clay particles, optimizing texture, improving aeration, and increasing pH. However, there are risks such as secondary pollution from tailings, limited material sources, high cost, and uncertain ecological impact.

[0005] The invention patent application with publication number CN 120555066A discloses a red soil conditioner and its preparation method and application, which improves the pH, organic matter, permeability and water retention of red soil. However, it uses dry grinding, and the grinding times and time are long, resulting in problems such as high cost and dust pollution. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a method for preparing hydrogel-coated modified biochar-red soil composites.

[0007] The present invention is achieved through the following technical solutions.

[0008] This invention provides a method for preparing a hydrogel-coated modified biochar-red soil composite, comprising the following steps: S1: Biochar is obtained by pyrolyzing crop straw after pretreatment; S2: Acrylamide, chain transfer agent, silane coupling agent, acetic acid and photoinitiator are added to deionized water, stirred and then the mixture is irradiated with an ultraviolet lamp to obtain hydrogel paint; S3: Immerse biochar in hydrogel paint, filter, and wait for the hydrogel paint to cure to obtain modified biochar. S4: The modified biochar is mixed with red soil and granulated using a granulator to obtain a hydrogel-coated modified biochar-red soil composite.

[0009] Preferably, the pretreatment step of the crop straw includes: washing the crop straw with deionized water, drying it at 60-80℃ for 20-28 hours, crushing the crop straw to a particle size of 1-4 mm, and sieving it.

[0010] Preferably, the pyrolysis step of the crop straw includes: placing the pretreated crop straw in a muffle furnace, introducing nitrogen gas and burning it under limited oxygen conditions at 8-13°C·min. -1 The temperature is increased to 420-490℃ and held for 1-4 hours, then cooled to room temperature to obtain biochar.

[0011] Preferably, the volume ratio of the chain transfer agent to deionized water is 1:800-1100, the volume ratio of the silane coupling agent to deionized water is 5-6.5:1000, and the volume ratio of the photoinitiator to deionized water is 1:800-1100.

[0012] Preferably, the chain transfer agent comprises (3-mercaptopropyl)trimethoxysilane and tetrahydrofuran; the silane coupling agent is propyl 3-(trimethoxysilyl)methacrylate; and the photoinitiator comprises 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone and ethanol.

[0013] Preferably, the volume ratio of (3-mercaptopropyl)trimethoxysilane to tetrahydrofuran is 1:8-12, and the concentration of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone is 0.05-0.2 mol / L.

[0014] Preferably, the concentration of acrylamide is 1-4 mol / L, the volume ratio of acrylamide to deionized water is 1:400-600, the concentration of acetic acid is 0.05-0.2 mol / L, and the volume ratio of acetic acid to deionized water is 1:85-110.

[0015] Preferably, in the step of irradiating the mixture with ultraviolet light, the distance between the ultraviolet light and the mixture is 3-8 cm, and the irradiation time is 20-50 minutes.

[0016] Preferably, the hydrogel paint is cured in a humid environment at a temperature of 50-70°C for 20-28 hours.

[0017] Preferably, the mass ratio of the modified biochar to the red soil is 1:50-150, and the granulation process uses a vertical rotary granulator with a rotation speed of 100-300 rpm and a rotation time of 10-20 minutes.

[0018] The beneficial effects of this invention are as follows: 1. Solve the dust pollution problem in biochar application: Solve the dust problem caused by loose particles during the production, transportation, application and mechanical disturbance of biochar. Through hydrogel coating and granulation process, achieve static and dynamic dust control throughout the process, reduce dust emissions by more than 90%, improve the safety and environmental protection of the operating environment, and break through the environmental bottleneck of large-scale application of biochar.

[0019] 2. Improve core obstacle factors of red soil: Specifically address the key problems of "acidity, drought and erosion" in red soil. Neutralize the acidity of red soil with alkaline biochar to alleviate acid barrier and aluminum toxicity; improve soil water retention by using hydrogel; enhance the water erosion resistance of red soil by using composite aggregate structure to reduce soil particle loss and topsoil damage caused by rainfall.

[0020] 3. Improve biochar utilization efficiency and stability: Reduce the oxidation and loss of biochar carbon components through hydrogel coating, increase the organic carbon retention rate by more than 10%, and enhance its carbon sequestration function; at the same time, improve the hydrophilicity of biochar (contact angle decreases from 104.6° to 78.0°), promote its bonding with red soil particles, avoid the loss of functional components, and improve the long-term effectiveness of the improvement.

[0021] 4. Achieve synergy between ecological improvement and production benefits: Based on optimizing the physical and chemical properties of red soil (acidity adjustment, water retention, carbon fixation, and erosion resistance), promote crop growth and significantly increase crop biomass (e.g., a 29.01% increase in bok choy), thus meeting the actual needs of improving arable land quality.

[0022] 5. Provide simple and versatile technical solutions: Adopt conventional vertical high-speed rotary granulation equipment to simplify the preparation process, reduce energy consumption, and facilitate large-scale production; at the same time, it takes into account diverse application scenarios such as waste disposal and slope protection, providing efficient and environmentally friendly technical support for ecological restoration and sustainable agricultural development in red soil areas. Attached Figure Description

[0023] Figure 1 This is a SEM image of the modified biochar prepared in Example 6 of the present invention; Figure 2 SEM image of the biochar prepared in Comparative Example 1; Figure 3This is a contact angle measurement diagram of the modified biochar prepared in Example 6 of the present invention; Figure 4 The contact angle measurement diagram is for the biochar prepared in Comparative Example 1. Figure 5 This is a comparison chart of the total organic carbon content of soil in the materials prepared in Example 6 and Comparative Example 1 of the present invention; where CK is the original soil, BC is the biochar prepared in Comparative Example 1, and G-BC is the modified biochar prepared in Example 6. Figure 6 The image shows a comparison of the growth of Chinese cabbage using materials prepared in Example 6 and Comparative Example 1 of this invention; where CK is the original soil, BC is the biochar prepared in Comparative Example 1, and G-BC is the modified biochar prepared in Example 6.

[0024] Figure 7 The modified biochar and red soil composite prepared in Example 6 of this invention; Figure 8 The composite of biochar and red soil prepared in Comparative Example 1. Detailed Implementation

[0025] The technical solution of the present invention is further described below, but the scope of protection is not limited to what is described.

[0026] Example 1: A method for preparing a hydrogel-coated modified biochar-red soil composite includes the following steps: S1: Rice straw is pretreated and then pyrolyzed to obtain biochar; S2: Add 1 mol of acrylamide to 400 ml of deionized water to obtain a monomer solution. Then, add chain transfer agent, silane coupling agent and acetic acid to the monomer solution in sequence. After magnetic stirring for 1 min, add photoinitiator and stir to mix evenly. Then, irradiate the mixture with an ultraviolet lamp to obtain hydrogel paint. S3: Immerse biochar in hydrogel paint, filter, and wait for the hydrogel paint to cure to obtain modified biochar. S4: The modified biochar is mixed with red soil and granulated using a granulator to obtain a hydrogel-coated modified biochar-red soil composite.

[0027] The rice straw pretreatment steps are as follows: after washing the rice straw with deionized water, it is dried at 60℃ for 20 hours, and then the rice straw is crushed to a particle size of 1mm and sieved.

[0028] The pyrolysis step of the rice straw is as follows: the pretreated rice straw is placed in a muffle furnace, nitrogen is introduced, and it is burned under limited oxygen conditions at 8°C·min. -1 The temperature was increased to 420°C and held for 1 hour, then cooled to room temperature to obtain biochar.

[0029] The volume ratio of the chain transfer agent to deionized water is 1:800, the volume ratio of the silane coupling agent to deionized water is 5:1000, and the volume ratio of the photoinitiator to deionized water is 1:800.

[0030] The chain transfer agent is (3-mercaptopropyl)trimethoxysilane and tetrahydrofuran; the silane coupling agent is 3-(trimethoxysilyl)propyl methacrylate; and the photoinitiator is 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone and ethanol.

[0031] The volume ratio of (3-mercaptopropyl)trimethoxysilane to tetrahydrofuran is 1:8, and the concentration of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone is 0.05 mol / L.

[0032] The acrylamide concentration is 1 mol / L, the volume ratio of acrylamide to deionized water is 1:400, the acetic acid concentration is 0.05 mol / L, and the volume ratio of acetic acid to deionized water is 1:85.

[0033] In the step of irradiating the mixture with ultraviolet light, the distance between the ultraviolet light and the mixture is 3 cm, and the irradiation time is 20 minutes.

[0034] The hydrogel paint is cured in a humid environment at a temperature of 50°C for 20 hours.

[0035] The modified biochar and red soil were mixed at a mass ratio of 1:50. The granulation process was performed using a vertical rotary granulator at a rotation speed of 100 rpm for 10 minutes.

[0036] Example 2: A method for preparing a hydrogel-coated modified biochar-red soil composite includes the following steps: S1: Rice straw is pretreated and then pyrolyzed to obtain biochar; S2: Add 1 mol of acrylamide to 600 ml of deionized water to obtain a monomer solution. Then, add chain transfer agent, silane coupling agent and acetic acid to the monomer solution in sequence. After magnetic stirring for 1 min, add photoinitiator and stir to mix evenly. Then, irradiate the mixture with an ultraviolet lamp to obtain hydrogel paint. S3: Immerse biochar in hydrogel paint, filter, and wait for the hydrogel paint to cure to obtain modified biochar. S4: The modified biochar is mixed with red soil and granulated using a granulator to obtain a hydrogel-coated modified biochar-red soil composite.

[0037] The rice straw pretreatment steps are as follows: after washing the rice straw with deionized water, it is dried at 80℃ for 28 hours, and then the rice straw is crushed to a particle size of 4mm and sieved.

[0038] The pyrolysis step of the rice straw is as follows: the pretreated rice straw is placed in a muffle furnace, nitrogen is introduced, and it is burned under limited oxygen conditions at 13°C·min. -1 The temperature was increased to 490°C and held for 4 hours, then cooled to room temperature to obtain biochar.

[0039] The volume ratio of the chain transfer agent to deionized water is 1:1100, the volume ratio of the silane coupling agent to deionized water is 6.5:1000, and the volume ratio of the photoinitiator to deionized water is 1:1100.

[0040] The chain transfer agent is (3-mercaptopropyl)trimethoxysilane and tetrahydrofuran; the silane coupling agent is 3-(trimethoxysilyl)propyl methacrylate; and the photoinitiator is 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone and ethanol.

[0041] The volume ratio of (3-mercaptopropyl)trimethoxysilane to tetrahydrofuran is 1:12, and the concentration of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone is 0.2 mol / L.

[0042] The acrylamide concentration is 4 mol / L, the volume ratio of acrylamide to deionized water is 1:600, the acetic acid concentration is 0.2 mol / L, and the volume ratio of acetic acid to deionized water is 1:110.

[0043] In the step of irradiating the mixture with ultraviolet light, the distance between the ultraviolet light and the mixture is 8 cm, and the irradiation time is 50 minutes.

[0044] The hydrogel paint is cured in a humid environment at a temperature of 70°C for 28 hours.

[0045] The modified biochar and red soil were mixed at a mass ratio of 1:150. The granulation process was performed using a vertical rotary granulator at a rotation speed of 300 rpm for 20 minutes.

[0046] Example 3: A method for preparing a hydrogel-coated modified biochar-red soil composite includes the following steps: S1: Rice straw is pretreated and then pyrolyzed to obtain biochar; S2: Add 1 mol of acrylamide to 500 ml of deionized water to obtain a monomer solution. Then, add 0.5 ml of chain transfer agent, 2.85 ml of silane coupling agent, and 5 ml of acetic acid to the monomer solution in sequence. After stirring magnetically for 1 min, add 1 ml of photoinitiator and stir to mix evenly. Then, irradiate the mixture with an ultraviolet lamp to obtain a hydrogel paint. S3: Immerse biochar in hydrogel paint, filter, and wait for the hydrogel paint to cure to obtain modified biochar. S4: The modified biochar is mixed with red soil and granulated using a granulator to obtain a hydrogel-coated modified biochar-red soil composite.

[0047] The rice straw pretreatment steps are as follows: after washing the rice straw with deionized water, it is dried at 70℃ for 24 hours, and then the rice straw is crushed to a particle size of 2mm and sieved.

[0048] The pyrolysis step of the rice straw is as follows: the pretreated rice straw is placed in a muffle furnace, nitrogen is introduced, and it is burned under limited oxygen conditions at 10℃·min. -1 The temperature was increased to 450°C and held for 2 hours, then cooled to room temperature to obtain biochar.

[0049] The chain transfer agent is (3-mercaptopropyl)trimethoxysilane and tetrahydrofuran; the silane coupling agent is 3-(trimethoxysilyl)propyl methacrylate; and the photoinitiator is 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone and ethanol.

[0050] The volume ratio of (3-mercaptopropyl)trimethoxysilane to tetrahydrofuran is 1:10, and the concentration of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone is 0.1 mol / L.

[0051] The concentration of acrylamide is 2 mol / L, and the concentration of acetic acid is 0.1 mol / L.

[0052] In the step of irradiating the mixture with ultraviolet light, the distance between the ultraviolet light and the mixture is 5 cm, and the irradiation time is 30 minutes.

[0053] The hydrogel paint is cured in a humid environment at a temperature of 60°C for 24 hours.

[0054] The modified biochar and red soil were mixed at a mass ratio of 1:100. The granulation process was performed using a vertical rotary granulator at a rotation speed of 200 rpm for 15 minutes.

[0055] Example 4: A method for preparing hydrogel-coated modified biochar-red soil composite, based on Example 3, except that the amount of chain transfer agent added is 1 ml.

[0056] Example 5: A method for preparing hydrogel-coated modified biochar-red soil composite, based on Example 3, except that the amount of silane coupling agent added is 5.7 ml.

[0057] Example 6: A method for preparing hydrogel-coated modified biochar-red soil composite, based on Example 3, except that the amount of photoinitiator added is 2 ml.

[0058] Comparative Example 1: A method for preparing a biochar-red soil composite includes the following steps: S1: Rice straw is pretreated and then pyrolyzed to obtain biochar; S2: Biochar is mixed with red soil and granulated using a granulator to obtain a biochar-red soil composite.

[0059] The rice straw pretreatment steps are as follows: after washing the rice straw with deionized water, it is dried at 70℃ for 24 hours, and then the rice straw is crushed to a particle size of 2mm and sieved.

[0060] The pyrolysis step of the rice straw is as follows: the pretreated rice straw is placed in a muffle furnace, nitrogen is introduced, and it is burned under limited oxygen conditions at 10℃·min. -1 The temperature was increased to 450°C and held for 2 hours, then cooled to room temperature to obtain biochar.

[0061] The biochar to red soil mass ratio is 1:100, and the granulation process uses a vertical rotary granulator with a rotation speed of 200 rpm and a rotation time of 15 minutes.

[0062] Figure 1 , 2 SEM test results for modified and original biochar are presented. The SEM images of both materials show a porous structure. In the original biochar, the fine particles are loosely arranged, and the overall surface is relatively smooth. In contrast, the fine particles in the modified biochar are more adhered to the surface of larger particles, resulting in a rougher overall surface. These observations suggest that: 1) the hydrogel coating modification treatment did not significantly affect the pore structure of the biochar; 2) the fine particles in the modified biochar adhere to larger particles, reducing the risk of dust emissions; and 3) the modified biochar has a larger specific surface area, increasing the adhesion sites with soil particles, allowing for better adhesion and promoting aggregate formation.

[0063] Figure 3 , 4 The contact angle measurements are presented, showing that the contact angle of the original biochar is 104.6°, while that of the modified biochar is 78.0°. The contact angle reflects the hydrophilicity or hydrophobicity of the material; a smaller contact angle indicates stronger hydrophilicity, and a larger contact angle indicates stronger hydrophobicity. The original biochar exhibits strong hydrophobicity, reducing the initial hydraulic conductivity of the soil and hindering water infiltration after application. Hydrogel coating can enhance the hydrophilicity of the biochar surface. Furthermore, in actual experiments, we found that due to its low density, strong hydrophobicity, and difficulty in soil water infiltration, some of the original biochar mixed with the soil floats and remains on the soil surface, contributing to biochar dust emissions. Hydrogel coating can enhance the hydrophilicity of the biochar surface. Appropriately enhancing the hydrophilicity of biochar surface can improve soil water infiltration and retention capacity, thereby reducing the risk of biochar dust emissions.

[0064] Dust emission tests were conducted using the modified biochar-red soil composites prepared in Examples 1-6 and the biochar-red soil composites prepared in Comparative Example 1 as samples.

[0065] The experiment was conducted as follows: 200g of sample was placed in a lead box, water was added until the sample reached saturation, and the box was placed in a forced-air drying oven at 40℃ for 24 hours. Afterward, the dried and compacted sample was passed through a 2 mm sieve. The forced-air drying and sieving process simulated natural wind and mechanical agitation in agricultural planting. This wet-dry cycle was repeated three times. The soil weight was measured, and the soil organic carbon content was determined using the potassium dichromate external heating method. Soil organic carbon content was used to characterize the content of primary and modified biochar. The results are shown in the table below, with the control group being red soil.

[0066] As shown in the table above, the total organic carbon content of the red soil composites in Examples 1-6 did not change significantly, while the total organic carbon content of the red soil composite in Comparative Example 1 decreased significantly. The modified biochar prepared in Examples 1-6 effectively reduced its own dust emissions.

[0067] Pot experiments were conducted using the modified biochar-red soil composite prepared in Example 6 and the biochar-red soil composite prepared in Comparative Example 1 as research subjects.

[0068] Ten seeds of pak choi were sown in each pot containing 3 kg of soil. When the seeds reached the one-leaf, one-heart stage, seedlings were thinned to three plants per pot. The pots were placed randomly in a well-ventilated greenhouse. During the crop growth period, no additional fertilizer was applied to any treatment except for water management. The pot experiment lasted 30 days. Every 5 days after planting, three plants from each treatment were randomly selected for destructive sampling to measure plant biomass and soil organic carbon content.

[0069] Analysis of total organic carbon content in the soil after 30 days of pot experiment, such as Figure 5 As shown in the figure, the organic carbon content in the BC treatment decreased only slightly from day 15 to 20, with decreases of 4.7% and 5.3% from day 20 to 25 and day 25 to 30, respectively. The significant difference between the initial and post-experiment organic carbon content indicates that biochar was lost through dust emissions during the pot experiment, leading to a decrease in soil organic carbon content. In contrast, the organic carbon content in the CK and G-BC treatments remained stable, with no significant difference before and after the experiment. The significant differences between the BC and G-BC treatments over time further validate the dust emission problem inherent in biochar itself, and demonstrate that the modification of biochar using this invention can significantly reduce biochar dust emissions.

[0070] Figure 6 The changes in pak choi biomass over time are shown. It can be seen that, compared with the control group (CK), the BC and G-BC treatment groups increased pak choi biomass by 24.45% and 29.01%, respectively. Modified biochar has a significant promoting effect on the growth of pak choi.

[0071] Figure 7 , 8 The results, based on comparative experiments, provide a clear indication that the G-BC treatment significantly enhanced the soil's resistance to water erosion. For example... Figure 7 , 8 As shown, the BC treatment group exhibited numerous fine and extensive cracks on the soil surface, with a loose structure and significant particle detachment, indicating poor aggregation and susceptibility to erosion. In contrast, the G-BC treatment group had a smooth soil surface with very few cracks, intact aggregates, and a denser structure. This demonstrates that hydrogel paint modification effectively enhanced the bonding between biochar and soil particles, improved soil aggregate structure, and thus significantly improved the soil's resistance to water erosion and washout, significantly inhibiting soil structure damage and particle loss caused by water erosion.

Claims

1. A method for preparing a hydrogel-coated modified biochar-red soil composite, characterized in that, Includes the following steps: S1: Biochar is obtained by pyrolyzing crop straw after pretreatment; S2: Acrylamide, chain transfer agent, silane coupling agent, acetic acid and photoinitiator are added to deionized water, stirred and then the mixture is irradiated with an ultraviolet lamp to obtain hydrogel paint; S3: Immerse biochar in hydrogel paint, filter, and wait for the hydrogel paint to cure to obtain modified biochar. S4: The modified biochar is mixed with red soil and granulated using a granulator to obtain a hydrogel-coated modified biochar-red soil composite.

2. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that, The pretreatment steps for crop straw include: washing the crop straw with deionized water, drying it at 60-80℃ for 20-28 hours, crushing the crop straw to a particle size of 1-4 mm, and sieving it.

3. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that, The pyrolysis step of the crop straw includes: placing the pretreated crop straw in a muffle furnace, introducing nitrogen gas and burning it under limited oxygen conditions at 8-13℃·min. -1 The temperature is increased to 420-490℃ and held for 1-4 hours, then cooled to room temperature to obtain biochar.

4. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: The volume ratio of the chain transfer agent to deionized water is 1:800-1100, the volume ratio of the silane coupling agent to deionized water is 5-6.5:1000, and the volume ratio of the photoinitiator to deionized water is 1:800-1100.

5. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: The chain transfer agent comprises (3-mercaptopropyl)trimethoxysilane and tetrahydrofuran; the silane coupling agent is propyl 3-(trimethoxysilyl)methacrylate; the photoinitiator comprises 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone and ethanol.

6. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 5, characterized in that: The volume ratio of (3-mercaptopropyl)trimethoxysilane to tetrahydrofuran is 1:8-12, and the concentration of 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylphenylacetone is 0.05-0.2 mol / L.

7. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: The acrylamide concentration is 1-4 mol / L, the volume ratio of acrylamide to deionized water is 1:400-600, the acetic acid concentration is 0.05-0.2 mol / L, and the volume ratio of acetic acid to deionized water is 1:85-110.

8. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: In the step of irradiating the mixture with ultraviolet light, the distance between the ultraviolet light and the mixture is 3-8 cm, and the irradiation time is 20-50 minutes.

9. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: The hydrogel paint is cured in a humid environment at a temperature of 50-70℃ for 20-28 hours.

10. The method for preparing a hydrogel-coated modified biochar-red soil composite as described in claim 1, characterized in that: The mass ratio of the modified biochar to the red soil is 1:50-150, and the granulation process uses a vertical rotary granulator with a rotation speed of 100-300 rpm and a rotation time of 10-20 minutes.

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