Cadmium-reducing modifier for acid soil, preparation method, use method and application

Abstract

The invention belongs to the technical field of soil heavy metal pollution remediation, and discloses an acid soil cadmium-reducing modifier, a preparation method, a use method and application, the modifier is formed by mixing sulfydryl minerals, biochar and a compound organic fertilizer according to a certain configuration, the sulfydryl mineral is prepared from calcium carbonate, bentonite and 3-mercaptopropyl triethoxy silane. When the soil conditioner is used, the soil conditioner is uniformly applied to a soil plough layer by 0-20 cm manually or mechanically, and the applied soil conditioner accounts for 1-5% of the mass of soil or is 180-220 kg / mu. The soil cadmium-reducing modifier can obviously reduce the effective state content of heavy metal cadmium in the soil, so that the cadmium content in planted rice grains is obviously reduced, the organic matter content of the soil is obviously increased, the ecological environment of polluted farmland soil is improved, the growth of the rice is promoted, and the yield of the rice is increased. The soil conditioner disclosed by the invention is simple to prepare and environment-friendly to soil, and safe utilization of cadmium-polluted farmland soil can be realized.

Description

Technical Field

[0001] This invention belongs to the field of soil heavy metal pollution remediation technology, and particularly relates to a cadmium-reducing soil conditioner for acidic soil, its preparation method, usage method and application. Background Technology

[0002] Soil is one of the main natural resources upon which humankind depends for survival and an important component of the human ecological environment. In recent years, with the development of modern industrialization and urbanization, soil pollution has become increasingly prominent, the polluted area has continued to expand, and heavy metal pollution in soil has received increasing attention. Therefore, the research and development of new and efficient methods for remediating heavy metal pollution in soil is urgently needed. Currently, the main methods for remediating heavy metal pollution in soil include physical remediation, chemical remediation, and bioremediation. Among them, in-situ passivation technology in chemical remediation is widely used due to its low cost, high efficiency, and good remediation effect on large-scale heavy metal contaminated soils.

[0003] In-situ passivation technology can be achieved by altering the form of heavy metals in the soil environment using soil amendments. These amendments can adsorb, complex, or precipitate heavy metals in the soil, thereby inhibiting their migration and bioavailability. Currently reported fixatives include lime, clay minerals, and biochar. Fixatives can reduce the available form of heavy metals in the soil and the concentration of heavy metals in crops by adjusting pH or having a high binding capacity for heavy metals. Meanwhile, the remediation of farmland contaminated with heavy metals also needs to consider soil use type, improving soil quality while remediating contamination to facilitate subsequent production recovery. However, traditional passivation materials suffer from poor effectiveness, are unsuitable for soils with high concentrations of heavy metal contamination, and often have adverse effects on crop growth. Taking lime, the most commonly used material, as an example, although lime can temporarily alleviate heavy metal problems in the soil, long-term use of lime can cause soil compaction and has a strong destructive effect on the soil ecosystem.

[0004] In view of the above problems, developing a new type of soil conditioner that is environmentally friendly, can effectively treat heavy metals in soil, and can improve soil quality is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a cadmium-reducing soil conditioner for acidic soils, its preparation method, its application method, and its uses. This conditioner effectively treats cadmium pollution in soil while improving soil quality and enhancing soil fertility. To achieve the above objectives, this invention provides the following technical solution: According to one aspect of the present invention, an acidic soil cadmium-reducing amendment is provided, the amendment comprising a mixture of thiol minerals, biochar, and compound organic fertilizer in a certain configuration, wherein the thiol minerals are prepared from calcium carbonate, bentonite, and 3-mercaptopropyltriethoxysilane.

[0006] In a further preferred embodiment of the above scheme, the modifier further includes an adhesive, which is a mixture of the adhesive and thiol minerals, biochar and compound organic fertilizer. The final modifier is prepared by bonding the mixture of thiol minerals, biochar and compound organic fertilizer with the adhesive.

[0007] In a further preferred embodiment of the above scheme, the adhesive solution is a sodium alginate solution with a concentration of 0.5-2%; the organic matter content of the compound organic fertilizer is ≥40%, and the effective viable count of all types of bacteria in the compound organic fertilizer is 2×10⁻⁶. 9 ~8×10 9 CFU / g; The compound organic fertilizer refers to the organic fertilizer obtained by composting and drying pig manure, then crushing it appropriately, and then adding wood vinegar, nitrogen, phosphorus, potassium, trace elements and one or more of various organic substances to it evenly mix, with an organic matter content ≥40%.

[0008] In a further preferred embodiment of the above scheme, the preparation process of the thiol mineral includes the following steps: Step S11: Mix calcium carbonate and bentonite in a mass ratio of (2-4):(1-2) until homogeneous, then microwave-activate at 400-450℃ for 5-10 minutes. After cooling to room temperature, grind through an 80-mesh sieve to obtain mineral particle products for later use. Step S12: Add 3-mercaptopropyltriethoxysilane to an ethanol solution to prepare a 15-25% solution. wt% A solution of 3-mercaptopropyltriethoxysilane was allowed to stand at room temperature for 30 minutes. The ethanol content in the ethanol solution was 85%. Step S13, according to a solid-liquid ratio of 1:2 ( m / v In this manner, the 3-mercaptopropyltriethoxysilane solution from step S12 is added to the mineral particle product from step S11, and the mixture is stirred at 60-70°C for 1.5-4 hours and then filtered. The filter residue obtained is dried at 80-90°C. Step S14: Grind the dried material obtained in step S13 and pass it through an 80-mesh sieve to obtain mercapto minerals.

[0009] In a further preferred embodiment of the above scheme, the biochar preparation process includes the following steps: Step S21: Clean the rice straw and dry it at 105℃~115℃ to kill the green. Step S22: Crush the dried rice straw to 1.5–2 cm and place it in a 20% ( wt% Soak in potassium humate solution for 24 hours, then remove and dry. Step S23: Place the product from step S22 in a muffle furnace and pyrolyze it at 400-550°C for 1.5-3 hours under a nitrogen atmosphere. After cooling to room temperature, grind the product and pass it through an 80-mesh sieve to obtain biochar.

[0010] According to another aspect of the present invention, the present invention provides a method for preparing a cadmium-reducing soil conditioner for acidic soil, the method comprising the following steps: Step 1: Mix thiol minerals, biochar, and compound organic fertilizer in a mass ratio of (1.3~2.2):(0.6~1.2):(0.6~1.2) to prepare a mixture. Step 2: Take 5% to 8% of the mass of the mixture as binder solution. Add the mixture to the disc granulator and rotate for 10 to 20 minutes. During this time, add the binder solution to the granulator by spraying and mix it with the mixture. After the binder solution is sprayed, continue to rotate the granulator for another 15 to 20 minutes to allow the material to gradually aggregate into granules. Step 3: Cool and sieve the particles. After cooling the particles, sieve them to ensure that the particle size of the finished product is within the range of 2-5mm, thus obtaining the final soil conditioner product.

[0011] A further preferred embodiment of the above scheme is to screen the particles after the temperature has been reduced to below 40°C.

[0012] The acidic soil cadmium-reducing amendment of this invention combines the advantages of various commonly used amendments, is rich in multiple remediation factors, and has a rapid and stable significant remediation effect. The raw materials of the soil amendment are widely available, low in cost, environmentally friendly, and do not cause secondary pollution. It has high acceptance and is easy to promote on a large scale. The acidic soil cadmium-reducing amendment of this invention needs to be applied evenly to the topsoil layer of 0-20cm by manual or mechanical means and allowed to age and balance for 7-14 days. The application rate of the amendment can be preferably 1-5% of the soil mass or 180-220kg / mu, depending on the remediation area.

[0013] In summary, compared with existing heavy metal contaminated soil remediation and soil improvement technologies, the acidic soil cadmium-reducing amendment of the present invention has the following beneficial effects: (1) The present invention can significantly change the structural characteristics of the material by organic modification, and soften its surface functional groups, thereby enhancing its adsorption capacity for heavy metals and improving the repair efficiency.

[0014] (2) This invention is made by compounding multiple materials. It can reduce the available content of heavy metal cadmium through multiple effects such as increasing soil pH, complexation / chelation, functional group coordination, and co-precipitation. At the same time, it can also increase soil fertility and improve soil quality. Compared with single materials, it has a more comprehensive improvement effect. Moreover, the raw materials are readily available and easy to prepare, and it has great potential for promotion and application. (3) Acidic paddy fields are mainly distributed in the southern region. Affected by the parent material of red soil and yellow soil, as well as the effects of long-term application of chemical fertilizers and leaching by rainfall, the soil pH value is generally low, which affects the growth and yield of rice. Therefore, the effect of applying the acidic soil cadmium-reducing amendment of this invention to plant rice in cadmium-contaminated farmland soil is shown. Ultimately, the available cadmium content in the soil is significantly reduced, the soil nutrients are significantly increased, and the soil environmental quality is improved. The cadmium content in rice grains is lower than the limit of 0.2 mg / kg in the National Food Safety Standard for Limits of Contaminants in Food (GB 2762-2017), which meets the standard for safe human consumption. Attached Figure Description

[0015] Figure 1 Schematic diagram of available cadmium content in soil under different treatments in indoor cultivation experiments. Figure 2 Schematic diagram of soil organic matter content under different treatments in indoor cultivation experiments Figure 3 Schematic diagram of soil pH under different treatments in indoor cultivation experiments Figure 4 Schematic diagram of soil organic matter content at different locations and under different treatments in field experiments. Figure 5 Schematic diagram of soil pH at different locations and under different treatments in field experiments Figure 6 Schematic diagram of available cadmium content in soil at different locations and under different treatments in a field experiment. Figure 7 Schematic diagram of cadmium content in rice grains at different locations and under different treatments in a field experiment. Detailed Implementation To make the objectives, technical solutions, and advantages of this invention clearer, the following optimal formulation examples are provided for further detailed explanation. Furthermore, unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention are commercially available.

[0016] This invention discloses an acidic soil cadmium-reducing amendment (SBF), mainly composed of thiol minerals, biochar, and compound organic fertilizer mixed in a mass ratio of (1.3~2.2):(0.6~1.2):(0.6~1.2), and prepared by binding with an adhesive. The adhesive solution is sodium alginate solution with a concentration of 0.5~2%. The organic matter content of the compound organic fertilizer is ≥40%. The compound organic fertilizer refers to organic fertilizer obtained by composting pig manure, air-drying it, and then appropriately crushing it, and then uniformly mixing it with wood vinegar, nitrogen, phosphorus, potassium, trace elements, and one or more substances from various organic matter. The effective viable count of various bacteria in the compound organic fertilizer is 2×10⁻⁶. 9 ~8×10 9 CFU / g, the main bacteria in the compound organic fertilizer include Bacillus licheniformis, Bacillus subtilis, Trichoderma longipes, and Streptomyces flavus. The preparation method of the conditioner specifically includes the following steps: Step 1: Mix thiol minerals, biochar, and compound organic fertilizer in a mass ratio of (1.3~2.2):(0.6~1.2):(0.6~1.2) to prepare a mixture. Step 2: Take 5% to 8% of the mass of the mixture as binder solution. Add the mixture to the disc granulator and rotate for 10 to 20 minutes. During this time, add the binder solution to the granulator by spraying and mix it with the mixture. After the binder solution is sprayed, continue to rotate the granulator for another 15 to 20 minutes to allow the material to gradually aggregate into granules. Step 2: Cool and screen the particles. After cooling the particles, screen them to ensure that the particle size of the finished product is within the range of 2-5mm, thus obtaining the final soil conditioner product.

[0017] In this invention, the preparation process of the thiol mineral includes the following steps: Step S11: Mix calcium carbonate and bentonite in a mass ratio of (2-4):(1-2) until homogeneous, then microwave-activate at 400-450℃ for 5-10 minutes. After cooling to room temperature, grind through an 80-mesh sieve to obtain mineral particle products for later use. Step S12: Add 3-mercaptopropyltriethoxysilane to an ethanol solution. The ethanol solution is an ethanol-water solution with an ethanol content of 85%, prepared to a concentration of 15-25%. wt% A solution of 3-mercaptopropyltriethoxysilane was allowed to stand at room temperature for 30 minutes. Step S13, according to a solid-liquid ratio of 1:2 ( m / vIn this manner, the 3-mercaptopropyltriethoxysilane solution from step S12 is added to the mineral particle product from step S11, and the mixture is stirred at 60–70°C for 1.5–4 hours before filtration. The resulting filter residue is then dried at 80–90°C.

[0018] Step S14: Grind the dried material obtained in step S13 and pass it through an 80-mesh sieve to obtain mercapto minerals.

[0019] In this invention, the method for preparing the biochar (BC) is as follows: Step S21: Clean the rice straw and dry it at 105℃~115℃ to kill the green. Step S22: Crush the dried rice straw to 1.5–2 cm and place it in a 20% ( wt% Soak in potassium humate solution for 24 hours, then remove and dry. Step S23: Place the product from step S22 in a muffle furnace and pyrolyze it at 400-550°C for 1.5-3 hours under a nitrogen atmosphere. After cooling to room temperature, grind the product and pass it through an 80-mesh sieve to obtain biochar. To verify the effects of the aforementioned soil conditioner on soil and crops, the following tests were conducted in this program.

[0020] Example 1: I. Material Preparation: The acidic soil cadmium-reducing amendment of the present invention is prepared according to the following steps: Step 1: The amelior, consisting of thiol minerals (SC), biochar (BC), and compound organic fertilizer (FL), is mixed in a mass ratio of 2:1:1. Step 2: Take 6% of the mass of the mixture as a binder solution, wherein the binder solution is a sodium alginate solution with a concentration of 1.2%; while the mixture is added to the disc granulator and rotated for 10 minutes, the binder solution is added to the granulator by spraying to mix and granulate with the mixture. After the binder solution is sprayed, the granulator is kept rotating for another 15 minutes to allow the material to gradually aggregate into granules. Step 3: Cool and sieve the granules. After cooling the granules, sieve them to ensure that the particle size of the finished product is within 3mm, thus obtaining the final soil conditioner product.

[0021] The method for preparing the thiol mineral (SC) includes the following steps: Step S11: Mix calcium carbonate and bentonite at a mass ratio of 3:2 until homogeneous, then microwave-activate at 400℃ for 5 minutes. After cooling to room temperature, grind the mixture through an 80-mesh sieve to obtain mineral particle products for later use. Step S12: Add 3-mercaptopropyltriethoxysilane to an ethanol solution to prepare a 20% ( wt% A solution of 3-mercaptopropyltriethoxysilane was allowed to stand at room temperature for 30 minutes. The ethanol content in the ethanol solution was 85%. Step S13, according to a solid-liquid ratio of 1:2 ( m / v In this manner, the 3-mercaptopropyltriethoxysilane solution from step S12 is added to the mineral particle product from step S11, stirred at 70°C for 3 hours, and then filtered. The filter residue obtained is dried at 80°C. Step S14: Grind the dried material obtained in step S13 and pass it through an 80-mesh sieve to obtain mercapto minerals (SC).

[0022] The biochar preparation process includes the following steps: Step S21: Clean the rice straw and dry it at 105℃ to kill the green. Step S22: Crush the dried rice straw to 2cm and place it in a 20% ( wt% Soak in potassium humate solution for 24 hours, then remove and dry. Step S23: Place the product from step S22 in a muffle furnace and pyrolyze it at 400°C for 2 hours under a nitrogen atmosphere. After cooling to room temperature, grind the product and pass it through an 80-mesh sieve to obtain biochar. II. Indoor Soil Culture Experiment 1. Test method: Weigh 100g of air-dried soil (passed through a 20-mesh sieve) and place it in a 500mL wide-mouth polyethylene bottle. Add the aforementioned soil conditioner (SBF) and single raw materials (SC, BC, and FL) at 3% of the soil mass. Add a certain amount of deionized water at 70% of the field capacity. After mixing thoroughly, seal the bottle with kraft paper and incubate at 25℃ in a constant temperature incubator. Set up three replicates, and replenish water every two days by weighing. After 30 days of incubation, take samples, air-dry them under natural conditions, and pass them through a 20-mesh sieve to determine the available content of heavy metal cadmium, soil pH, and soil organic matter content.

[0023] 2. Evaluation of experimental results: The effectiveness was evaluated by determining the available cadmium content in soil. The determination method followed the standard procedure "Determination of Eight Available Elements in Soil: Diethylenetriaminepentaacetic Acid Extraction-Inductively Coupled Plasma Optical Emission Spectrometry" (HJ804-2016). The specific procedure was as follows: 10.0 g of air-dried soil sample was weighed and placed in a 100 mL Erlenmeyer flask. 20.0 mL of DTPA-CaCl2-TEA extraction solution was added, and the flask was tightly sealed. The sample was shaken at 180 r / min for 2 h at 20℃. The extract was collected in a centrifuge tube and centrifuged for 10 min. The supernatant was filtered, and the cadmium content was determined using inductively coupled plasma optical emission spectrometry (ICP-OES).

[0024] The results of the indoor soil culture experiment showed that: like Figure 1 As shown, after applying the soil conditioner (SBF) of this invention, the content of available cadmium in the soil was significantly reduced by 49.8% compared with the control (CK); Figure 2 As shown, after SBF application, soil organic matter increased significantly by 3.2 g / kg compared to the control (CK); Figure 3 As shown, after SBF application, the soil pH increased significantly by 0.33 units compared to the control group (CK).

[0025] III. Comparison of Effects Examples The difference from Example 1 of the present invention is that, in this comparative example, thiol minerals (SC), biochar (BC), or compound organic fertilizer (FL) were applied individually, while the experimental methods and procedures were the same as in Example 1. The results are as follows: Figure 1 , Figure 2 and Figure 3 As shown, the application of thiol minerals (SC), biochar (BC), or compound organic fertilizer (FL) alone can also reduce the content of available cadmium in the soil, but the effect is lower than that of the soil conditioner (SBF) of this invention. Although the application of thiol minerals (SC) alone can effectively reduce the content of available cadmium in the soil, it cannot improve soil fertility and soil organic matter, while the application of biochar (BC) or compound organic fertilizer (FL) alone can significantly increase the content of soil organic matter. Similarly, the application of biochar (BC) or compound organic fertilizer (FL) alone can significantly increase soil pH, while the application of thiol minerals (SC) alone has no significant effect on soil pH.

[0026] Example 2 Field trials verification As shown in Example 1, under indoor test conditions, soil conditioner (SBF) can significantly reduce the available content of heavy metal cadmium in soil and increase soil organic matter and pH. Based on this, field trials in two regions were conducted to verify the experimental effect under actual crop planting conditions. The crop planted was rice, and the application rate of soil conditioner was 200 kg / mu.

[0027] Test location: Experimental site A was selected in a cadmium-contaminated paddy field in Shilong Town, Guiping City, Guangxi Zhuang Autonomous Region. The total cadmium content of the soil was 1.32 mg / kg, the available cadmium content was 0.53 mg / kg, the soil pH was 5.73, and the soil organic matter content was 45.4 g / kg.

[0028] Experimental site B was selected in a cadmium-contaminated paddy field in Beishan Village, Huanjiang County, Hechi City, Guangxi Zhuang Autonomous Region. The total cadmium content of the soil was 1.12 mg / kg, the available cadmium content was 0.47 mg / kg, the soil pH was 5.66, and the soil organic matter content was 39.4 g / kg.

[0029] Experimental Design: The experimental plots were arranged in the same field, separated by raised ridges and covered with agricultural film. Single-row irrigation was implemented, with protective rows in place. Specific fertilizer application rates, application frequencies, and water management were implemented according to local practices. Each treatment was replicated three times. Each plot was 20 m². 2 (4m×5m rectangle), randomized block arrangement. Before rice transplanting, soil conditioner (SBF) is evenly spread on the soil surface, and the soil and conditioner are mixed evenly using a rotary tiller. After equilibration for 7 days, rice can be transplanted to ensure that the number of rice plants in each plot is the same.

[0030] Sample collection: Soil sample collection: Collect soil after planting, air dry it naturally, and then pass it through a 20-mesh sieve before testing.

[0031] Plant sample collection: During harvest, rice in each plot was harvested individually and threshed using a threshing machine. The grains were dried, weighed, hulled, and the brown rice was collected, then crushed and passed through a 60-mesh sieve for later use.

[0032] Testing items: available cadmium content in soil, soil pH and soil organic matter content, rice yield, and cadmium content in rice grains.

[0033] Main experimental results: As shown in Table 1, after applying the acidic soil cadmium-reducing amendment (SBF) of this invention, the rice growth indicators in the Shilong Town experimental plot were significantly improved. Specifically, the rice grain yield in the experimental plot increased significantly by 4.04 kg compared to the control group. Similarly, after applying the SBF of this invention, the rice growth indicators in the Beishan Village experimental plot were also significantly improved, with the rice grain yield in the experimental plot increasing significantly by 3.21 kg compared to the control group. Therefore, it is evident that the application of this invention can effectively improve soil quality and increase crop yield.

[0034] Table 1 like Figure 4 , Figure 5 , Figure 6 and Figure 7 As shown, the application of the acidic soil cadmium-reducing amendment (SBF) of this invention increased the organic matter content of soil in a polluted farmland in Shilong Town by 18.33% (e.g., Figure 4 Soil pH increased by 0.31 units (e.g.) Figure 5 The available cadmium content in the soil decreased by 38.94% (e.g. Figure 6 The cadmium content in rice decreased by 29.63% (e.g.) Figure 7 Similarly, the application of the acidic soil cadmium-reducing amendment (SBF) of this invention increased the soil organic matter in a polluted farmland in Beishan Village by 13.16% (e.g., Figure 4 Soil pH increased by 0.27 units (e.g.) Figure 5 The available cadmium content in the soil decreased by 66.01% (e.g. Figure 6 The cadmium content in rice decreased by 27.79%. Figure 7 Furthermore, the cadmium content in rice from field trials in both regions met the national food safety standard limit for contaminants in food (GB 2762-2017) (0.2 mg / kg), satisfying human consumption standards (e.g., ...). Figure 7 ).

[0035] The above description is merely a preferred embodiment of the present invention, but it should not be construed as limiting the embodiments of the present invention to these examples. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A cadmium-reducing soil conditioner for acidic soils, characterized in that, The improver comprises a mixture of thiol minerals, biochar, and compound organic fertilizer in a certain configuration. The thiol minerals are prepared from calcium carbonate, bentonite, and 3-mercaptopropyltriethoxysilane.

2. The cadmium-reducing soil conditioner for acidic soil according to claim 1, characterized in that, The modifier also includes an adhesive, which is a mixture of thiol minerals, biochar and compound organic fertilizer. The final modifier is prepared by bonding the mixture of thiol minerals, biochar and compound organic fertilizer with the adhesive.

3. The cadmium-reducing soil conditioner for acidic soil according to claim 1, characterized in that, The adhesive solution is a sodium alginate solution with a concentration of 0.5-2%; the organic matter content of the compound organic fertilizer is ≥40%, and the effective viable count of all types of bacteria in the compound organic fertilizer is 2×10⁻⁶. 9 ~8×10 9 CFU / g.

4. A cadmium-reducing soil conditioner for acidic soil according to claim 1, 2, or 3, characterized in that, The preparation process of the thiol mineral includes the following steps: Step S11: Mix calcium carbonate and bentonite in a mass ratio of (2-4):(1-2) until homogeneous, then microwave-activate at 400-450℃ for 5-10 minutes. After cooling to room temperature, grind through an 80-mesh sieve to obtain mineral particle products for later use. Step S12: Add 3-mercaptopropyltriethoxysilane to an ethanol solution to prepare a 15-25% solution. wt% A solution of 3-mercaptopropyltriethoxysilane was allowed to stand at room temperature for 30 minutes. The ethanol content in the ethanol solution was 85%. Step S13, according to a solid-liquid ratio of 1:2 ( m / v In this manner, the 3-mercaptopropyltriethoxysilane solution from step S12 is added to the mineral particle product from step S11, and the mixture is stirred at 60-70°C for 1.5-4 hours and then filtered. The filter residue obtained is dried at 80-90°C. Step S14: Grind the dried material obtained in step S13 and pass it through an 80-mesh sieve to obtain mercapto minerals.

5. A method for preparing a cadmium-reducing soil conditioner for acidic soil according to claim 1, 2, or 3, characterized in that, The biochar preparation process includes the following steps: Step S21: Clean the rice straw and dry it at 105℃~115℃ to kill the green. Step S22: Crush the dried rice straw to 1.5–2 cm and place it in a 20% ( wt% Soak in potassium humate solution for 24 hours, then remove and dry. Step S23: Place the product from step S22 in a muffle furnace and pyrolyze it at 400-550°C for 1.5-3 hours under a nitrogen atmosphere. After cooling to room temperature, grind the product and pass it through an 80-mesh sieve to obtain biochar.

6. A method for preparing a cadmium-reducing soil conditioner for acidic soil according to any one of claims 1 to 5, characterized in that, The preparation method of the modifier includes the following steps: Step 1: Mix thiol minerals, biochar, and compound organic fertilizer in a mass ratio of (1.3~2.2):(0.6~1.2):(0.6~1.2) to prepare a mixture. Step 2: Take 5% to 8% of the mass of the mixture of materials and add the mixture to the disc granulator. While the mixture is rotating for 10 to 20 minutes, add the binder solution to the granulator by spraying it to mix and granulate with the mixture. After the binder solution is sprayed, keep the granulator rotating for another 15 to 20 minutes to allow the material to gradually aggregate into granules. Step 3: Cool and sieve the particles. After cooling the particles, sieve them to ensure that the particle size of the finished product is within the range of 2-5mm, thus obtaining the final soil conditioner product.

7. The method for preparing a cadmium-reducing soil conditioner for acidic soil according to claim 6, characterized in that, The particles are screened after the temperature is reduced to below 40°C.

8. A method for using a cadmium-reducing soil conditioner for acidic soil, characterized in that, The soil conditioner obtained by the preparation method described in claim 6 or 7 is evenly applied into the soil tillage layer at a depth of 0 to 20 cm at a rate of 1 to 5% of the soil mass or 180 to 220 kg / mu, and then allowed to age and balance for 7 to 14 days.

9. The application of a cadmium-reducing soil conditioner for acidic soils, characterized in that, The amendment obtained by the preparation method according to claim 6 or 7 is applied to soil with an available cadmium content in the range of 0.5 to 1.0 mg / kg and a pH value in the range of 5 to 6.5.

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