Method for reducing bioavailability of heavy metals in rice field soil and application

Abstract

The invention discloses a method for reducing bio-availability of heavy metals in rice field soil and application, and the method comprises the following steps: taking biogas slurry, adding water for dilution, and fully and uniformly mixing; sequentially adding cyclodextrin, ferric salt and polyacrylamide into the diluted biogas slurry, uniformly stirring and mixing, adjusting the pH value, applying the prepared mixed solution into the soil, applying a phosphate fertilizer after the biogas slurry is applied into the soil, and performing deep ploughing; the biogas slurry-cyclodextrin-ferric salt-polyacrylamide composite solution prepared by the invention has a synergistic effect with a phosphate fertilizer, so that the content of metal cadmium in rice can be remarkably reduced, and the content of cadmium in brown rice is less than 0.1 mg / kg; according to the method, deep and stable fixation of the heavy metal is achieved through the molecular inclusion-surface stabilization-flocculation cooperation effect, the dissociation rate of the heavy metal in the heavy metal organic compound is reduced, the content of metal cadmium in the two-season rice can be at least reduced by applying the composite solution once, the cadmium reduction cost is low, and the good application value is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of remediation of heavy metal contaminated paddy soil, in particular to a method for reducing the bioavailability of heavy metals in paddy soil and application thereof. BACKGROUND

[0002] In the soil with excessive heavy metal content, it is impossible to produce agricultural products for human consumption. In extreme cases, crops cannot grow effectively, and crop yields will gradually decline. Farmland may even be abandoned due to severe soil pollution.

[0003] Heavy metal pollution not only damages the soil and crops, but also has a profound impact on the farmland ecosystem. Due to the continuous existence of heavy metal pollution sources, its impact on river water quality is slow and lasting. Over time, a large amount of heavy metal substances has penetrated into the soil bottom, which has had adverse effects on groundwater and surrounding plants and animals. Therefore, if the source of heavy metal pollution is not effectively controlled, its impact on the soil ecosystem will become increasingly serious.

[0004] The remediation of heavy metal contaminated soil mainly uses physical, chemical and biological remediation methods. In general, it can be divided into two categories: the first category is total removal method, that is, through various engineering or physical and chemical technical means, the total amount of heavy metals in the soil is reduced to below the level allowed by regulations, and the production capacity of the soil is restored; the second method does not seek to reduce the total amount of heavy metals in the soil, but reduces the mobility of heavy metals through technical means, thereby reducing the absorption of crops to heavy metals, suppressing its impact and ecological risk, that is, solidification / stabilization method.

[0005] Since the heavy metals that can be absorbed by crops are the effective heavy metals in the soil, methods for reducing the mobility and bioavailability of pollutants in the soil are particularly important. Due to technical and cost reasons, the solidification materials currently used are mainly inorganic solidification materials, including cement, lime and fly ash or other gel materials. These materials generally have high alkalinity, which can fix heavy metals and their hydroxides or complexes in soil particles by increasing pH, chemical absorption, adsorption, encapsulation and other ways, and inhibit the migration of heavy metals. However, these materials also have some inherent defects in application: since the solidification / stabilization measures are mostly chemical processes, their stabilization capacity is easily affected by environmental conditions, and they have poor acid resistance, heavy metals are easily released, and usually require high technical cost and have poor persistence.

[0006] Therefore, based on the consideration of price and safety, it is urgent to develop a stable, inexpensive and effective method for reducing the bioavailability of heavy metals in paddy soil. SUMMARY

[0007] The present application provides a method and application for reducing the bioavailability of heavy metals in paddy field soil, which comprises the following steps: taking biogas slurry, adding water for dilution, and mixing thoroughly; adding cyclodextrin, iron salt and polyacrylamide into the diluted biogas slurry in sequence, stirring and mixing uniformly, adjusting the pH value, applying the prepared mixed solution into the soil, applying phosphorus fertilizer into the soil after the biogas slurry is applied into the soil, and deep ploughing. The prepared biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution can significantly reduce the content of cadmium in rice under the action of deep ploughing and phosphorus fertilizer, the content of cadmium in brown rice is less than 0.1 mg / kg, and the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution only needs to be applied once a year, which can reduce the content of cadmium in rice for at least two seasons, and the cost of reducing cadmium is low; through the application of phosphorus fertilizer, the pH value of the soil is increased, the biogas slurry, cyclodextrin, iron salt and polyacrylamide interact with each other, the cyclodextrin and polyacrylamide realize the stable and deep fixation of heavy metals through "molecular inclusion-surface stabilization-flocculation synergy", and the dissociation rate of heavy metals in the organic complex is reduced, so that the content of cadmium in rice is long-acting reduced.

[0008] In order to achieve the above technical effects, the following technical solutions are adopted: A method for reducing the bioavailability of heavy metals in paddy field soil, comprising the following steps: Step S1: taking biogas slurry, adding water for dilution, and mixing thoroughly; Step S2: adding cyclodextrin, iron salt and polyacrylamide into the diluted biogas slurry in sequence, stirring and mixing uniformly, and adjusting the pH value of the mixed solution; Step S3: applying the mixed solution obtained in step S2 into the soil; Step S4: after the biogas slurry is applied into the soil, applying phosphorus fertilizer into the soil, and deep ploughing; Further, the cyclodextrin is beta-cyclodextrin, and the molecular weight of the polyacrylamide is 5-10 million; Further, the weight ratio of the diluted biogas slurry, cyclodextrin, iron salt and polyacrylamide in step S2 is 1000:0.08-0.2:3-40:0.1-0.3; Further, the content of water-soluble organic matter in the diluted biogas slurry in step S1 is 800-1200 mg / L; Further, the content of water-soluble organic matter in the diluted solution in step S1 is 1000 mg / L; Further, the iron salt is one or more of ferric chloride, ferric sulfate and ferric nitrate; Further, the application amount of the mixed solution in step S3 is 1000-2000 kg / acre; Further, the phosphorus fertilizer is applied 8-12 days after the biogas slurry is applied in step S4, and the application amount of the phosphorus fertilizer in step S4 is 10-30 kg / acre; Further, the phosphorus fertilizer in step S4 is one or more of superphosphate, potassium phosphate, potassium dihydrogen phosphate and dipotassium hydrogen phosphate. Further, in step S2, when the pH value of the soil is less than 5.0, the pH value of the mixed solution is adjusted to 6.0; when the pH value of the soil is 5.0-5.5, the pH value of the mixed solution is adjusted to 5.5; and when the pH value of the soil is 5.51-6.0, the pH value of the mixed solution is adjusted to 5.0. Further, the biogas slurry is prepared by an anaerobic fermentation method or is obtained by purchase.

[0009] The present application has the following beneficial effects: The application provides a method for reducing the bioavailability of heavy metals in paddy field soil, which can significantly reduce the content of metal cadmium in rice by applying biogas slurry, cyclodextrin, iron salt and polyacrylamide mixed solution to the soil, and the content of cadmium in brown rice is less than 0.1 mg / kg, which meets the food safety standards; and the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution is applied before planting early rice, and the above-mentioned composite solution does not need to be applied again before planting late rice in the same year, and the content of cadmium in early and late rice brown rice is less than 0.1 mg / kg, that is, the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution only needs to be applied once a year, and at least the content of metal cadmium in two seasons of rice can be reduced, and the cost of reducing cadmium is low. The mechanism of action of the application is as follows: the biogas slurry pre-complexes the heavy metals in the soil to provide heavy metal-humic acid complexes, the cyclodextrin can include the heavy metal-humic acid complexes to form the inclusion compound of the heavy metal-organic complex, the biogas slurry can improve the inclusion efficiency of the cyclodextrin through the pre-complexing effect, the polyacrylamide can stabilize the inclusion compound of the heavy metal-organic complex through the hydrogen bonding effect, and the polyacrylamide can further adsorb the heavy metal through the amide group and the carboxyl group. The cyclodextrin screen inner cavity further includes the hydrophobic chain segment of the polyacrylamide through the hydrophobic effect, and the long molecular chain of the polyacrylamide bridges multiple cyclodextrin-heavy metal complexes to form a super large network flocculation, so as to further settle the inclusion compound of the heavy metal-organic complex. Meanwhile, the iron salt and the polyacrylamide form a Fe-PAM flocculation network to further fix the heavy metal organic complex, and the water-soluble small molecule organic matter in the biogas slurry interacts with the iron salt to wrap and precipitate the heavy metal. By applying phosphorus fertilizer, the cyclodextrin and the polyacrylamide can realize the stable deep fixation of the heavy metal through the "molecular inclusion-surface stabilization-flocculation synergy" in the process of increasing the pH value of the soil, and reduce the dissociation rate of the heavy metal in the heavy metal organic complex. In addition, the biogas slurry is rich in organic matter and humic acid, and the combined application of the biogas slurry, the soluble iron salt and the phosphorus fertilizer can improve the soil fertility, improve the water retention and air permeability of the soil, and alleviate the hardening problem. And the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution only needs to be applied once a year, and at least the content of metal cadmium in two seasons of rice can be reduced, which has excellent long-acting performance. DETAILED DESCRIPTION

[0010] In order to make the objects, technical solutions and advantages of the present application clearer, the following further describes the present application with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application.

[0011] It should be pointed out that the following detailed description is exemplary and is intended to provide further description of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as generally understood by those skilled in the art to which the present application belongs.

[0012] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the exemplary embodiments of the present invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, and / or combinations thereof.

[0013] In the following examples, unless otherwise specified, all reagents and consumables were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used are conventional methods and techniques in the art.

[0014] The preparation method of biogas slurry is as follows: Select corn stalks, crushed to 2 cm, with a carbon-to-nitrogen ratio of 70:1; collect chicken manure, pre-dry until the moisture content is 70%, remove impurities, and maintain a carbon-to-nitrogen ratio of 18:1; mix the corn stalks and chicken manure evenly at a carbon-to-nitrogen ratio of 30:1 and add to the biogas digester, adding appropriate moisture until the total solids content of the system is 10%, and add 20% of the total wet weight of the fermentation material as old biogas slurry. After 24 hours, add 0.2% of the dry weight of the fermentation material as EM bacteria, and carry out anaerobic fermentation at 40℃; monitor the pH once a day for 3-5 days before fermentation, adjusting the system pH to 7.0-7.5; stir once a day for 15 minutes each time for the first 10 days of fermentation; after 10 days of fermentation, stir once every 3 days, stopping stirring after gas production; after fermentation is complete (gas production rate <0.1m³ / min). 3 / (m 3 •d) Collect the biogas slurry, filter out impurities, and store it in a sealed container for 30 days; take 1000 kg of biogas slurry and add 10000 kg of water to dilute it. After dilution, the water-soluble organic matter content in the biogas slurry is 1000 mg / L.

[0015] Example 1: Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.1 kg of β-cyclodextrin, 30 kg of ferric sulfate, and 0.2 kg of polyacrylamide. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a biogas slurry-cyclodextrin-ferric salt-polyacrylamide composite solution.

[0016] Example 2 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.08 kg of β-cyclodextrin, 10 kg of ferric chloride, and 0.1 kg of polyacrylamide. The polyacrylamide has a molecular weight of 10 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a biogas slurry-cyclodextrin-ferric salt-polyacrylamide composite solution.

[0017] Example 3 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.2 kg of β-cyclodextrin, 24 kg of ferric nitrate, and 0.3 kg of polyacrylamide. The polyacrylamide has a molecular weight of 8 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a biogas slurry-cyclodextrin-ferric salt-polyacrylamide composite solution.

[0018] Comparative Example 1 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 30 kg of ferric sulfate and 0.3 kg of polyacrylamide. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a biogas slurry-ferric salt-polyacrylamide composite solution.

[0019] Comparative Example 2 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.3 kg of β-cyclodextrin and 30 kg of ferric sulfate, stir and mix evenly, and adjust the pH of the mixture to 6 to obtain a biogas slurry-cyclodextrin-ferric salt composite solution.

[0020] Comparative Example 3 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.1 kg of β-cyclodextrin and 0.2 kg of polyacrylamide. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a biogas slurry-cyclodextrin-polyacrylamide composite solution.

[0021] Comparative Example 4 Take 1000 kg of water and add 0.1 kg of β-cyclodextrin, 30 kg of ferric sulfate and 0.2 kg of polyacrylamide. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a cyclodextrin-ferric salt-polyacrylamide composite solution.

[0022] Comparative Example 5 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 30.3 kg of ferric sulfate, stir and mix evenly, and adjust the pH of the mixture to 6 to obtain a biogas slurry-iron salt composite solution.

[0023] Comparative Example 6 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.3 kg of polyacrylamide, the polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%, stir and mix evenly, and adjust the pH of the mixture to 6 to obtain biogas slurry-polyacrylamide composite solution.

[0024] Comparative Example 7 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), add 0.3 kg of β-cyclodextrin, stir and mix evenly, and adjust the pH of the mixture to 6 to obtain biogas slurry-cyclodextrin composite solution.

[0025] Comparative Example 8 Take 1000 kg of water and add 0.1 kg of β-cyclodextrin and 0.2 kg of polyacrylamide. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain a cyclodextrin-polyacrylamide composite solution.

[0026] Comparative Example 9 Add 30 kg of ferric sulfate and 0.3 kg of polyacrylamide to 1000 kg of water. The polyacrylamide has a molecular weight of 6 million and a degree of hydrolysis of 30%. After stirring and mixing evenly, adjust the pH of the mixture to 6 to obtain an iron salt-polyacrylamide composite solution.

[0027] Comparative Example 10 Take 1000 kg of diluted biogas slurry (water-soluble organic matter content is 1000 mg / L), adjust the pH of the mixture to 6, and obtain diluted biogas slurry.

[0028] Comparative Example 11 The blank control group was replaced with an equal amount of water instead of the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution in Example 1.

[0029] Note: In practical applications, biogas slurry can be prepared by conventional anaerobic fermentation or purchased. The biogas preparation process is not limited by the examples.

[0030] Application examples The following field plot comparison tests were conducted on the mixtures obtained in Examples 1-3 and Comparative Examples 1-11. Each case was repeated in 3 plots. Experimental Site and Field: Yongxin County, Ji'an City, Jiangxi Province. The total area of ​​the experimental field was approximately 2 mu (about 0.33 hectares). According to the experimental design, the field was divided into 42 plots, each 6m x 5m in length and width, forming 14 treatment zones with 3 replicates per treatment zone. Ridges were constructed between plots and covered with plastic film to prevent cross-contamination. All plots were irrigated individually to avoid the impact of irrigation. Basic soil sample testing results showed that the total cadmium content was 0.86 mg / kg, the pH was 4.62, and the available cadmium content was 0.48 mg / kg.

[0031] The rice varieties tested were Xiangzaoxian 45 (early rice) and Yexiangyoulis (late rice).

[0032] Trial period: March-November 2024 The experimental procedure is as follows: Twenty days before early rice planting (March 26), the compound solutions prepared in Examples 1-3 and Comparative Examples 1-11 were applied to the soil of their respective plots by flood irrigation at a rate of 2000 kg / mu. Ten days after the biogas slurry flood irrigation (April 5), superphosphate was applied to the soil at a rate of 20 kg / mu, along with 15 kg of urea per mu, and deep plowing was carried out. Early rice was transplanted using a rice transplanter on April 15 (seedlings aged 20 days), and harvested on July 10. Late rice was not irrigated with biogas slurry and was planted in the same plots as the early rice. Base fertilizer was applied at a rate of 20 kg / mu of superphosphate and 20 kg / mu of urea. Late rice was transplanted normally (July 22) and harvested on November 10.

[0033] During the rice growing season, irrigation, fertilization, and pest and disease control were carried out according to soil water and fertilizer conditions and seedling growth. All planting methods were the same as conventional rice cultivation techniques, and the planting conditions, water and fertilizer management methods, and pest and disease control methods were identical in all experimental areas. After the rice matured, rice samples were collected, the husks were removed, and the brown rice portion was crushed. The cadmium content in the brown rice was determined using mixed acid digestion-graphite furnace atomic absorption spectrometry, referring to the method in GB 5009.15-2023 National Food Safety Standard for the Determination of Cadmium in Food, and the cadmium reduction effect was evaluated.

[0034] Method for determining cadmium content in brown rice: Take 0.1g of brown rice powder sample, add 5mL of nitric acid and 3.0mL of hydrogen peroxide solution, cover and digest overnight, digest using a graphite digester for 2h and bring the volume to 25mL, the cadmium concentration in the digestion solution is analyzed and determined by inductively coupled plasma mass spectrometry.

[0035] The experimental results are shown in Table 1.

[0036] Table 1. Cadmium content in brown rice in each experimental area

[0037] Table 1 shows that the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution prepared in Examples 1-3 can significantly reduce the cadmium content in brown rice. Furthermore, the cadmium content in both early and late-season brown rice in experimental areas 1-3 is less than 0.1 mg / kg, meeting the national food safety standards. In experimental areas 4-14, when the composite solution prepared in Comparative Examples 1-11 or an equal amount of water was used, the cadmium content in both early and late-season brown rice was significantly higher than in experimental areas 1-3. This indicates that the composite solution prepared using biogas slurry-cyclodextrin-iron salt-polyacrylamide in this invention can significantly reduce the cadmium content in rice. Moreover, since this invention applies the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution before planting early rice, it is not necessary to apply the same composite solution again before planting late rice in the same year. The cadmium content in both early and late-season brown rice was below 0.1 mg / kg. This means that the present invention only requires one application of a biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution per year, which can reduce the cadmium content in rice for at least two seasons. Because the biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution achieves deep and high-intensity stable fixation of heavy metals, soil heavy metal passivation is not even necessary before the second year's planting. Even with direct repeated planting on the same plot in the second year without soil heavy metal passivation, the cadmium content in both early and late-season brown rice was below 0.15 mg / kg, meeting national standards. The cost of cadmium reduction is low, and the rice yield is no lower than that of the control group. In Examples 1-3, the rice yield was above 550 kg / mu. The mechanism of action of this invention... The process is as follows: Biogas slurry pre-complexes heavy metals in the soil, providing heavy metal-humic acid complexes. Cyclodextrin can encapsulate these complexes, forming heavy metal-organic compound inclusions. The pre-complexing process enhances the inclusion efficiency of cyclodextrin. Polyacrylamide stabilizes the heavy metal-organic compound inclusions through hydrogen bonding. Simultaneously, polyacrylamide further adsorbs heavy metals through its amide and carboxyl groups. The inner cavity of the cyclodextrin sieve further encapsulates the hydrophobic segments of polyacrylamide through hydrophobic interactions. Furthermore, the long molecular chains of polyacrylamide bridge multiple cyclodextrin-heavy metal complexes, forming a large network of flocs, which then precipitates the heavy metal-organic compound inclusions. Simultaneously, iron salts react with polyacrylamide to form Fe-PA... The M-shaped flocculent network further immobilizes heavy metal organic compounds, and the interaction between water-soluble small-molecule organic matter in the biogas slurry and iron salts can encapsulate and precipitate heavy metals. By applying phosphate fertilizer, cyclodextrin and polyacrylamide achieve deep stabilization and immobilization of heavy metals through "molecular inclusion-surface stabilization-flocculation synergy" during the process of soil pH increase, reducing the dissociation rate of heavy metals in heavy metal organic compounds. In addition, biogas slurry is rich in organic matter and humic acid, and its combined application with soluble iron salts and phosphate fertilizer can improve soil fertility, improve soil water retention and aeration, and alleviate the problem of compaction. Moreover, only one application of biogas slurry-cyclodextrin-iron salt-polyacrylamide composite solution per year is needed to reduce the content of cadmium in rice for at least two seasons, demonstrating excellent long-term performance.

[0038] Therefore, those skilled in the art will recognize that although embodiments of the present invention have been shown and described in detail herein, many other variations or modifications conforming to the principles of the present invention can be directly determined or derived from the disclosure of the present invention without departing from the spirit and scope of the invention. Therefore, the scope of the present invention should be understood and recognized as covering all such other variations or modifications.

Claims

1. A method for reducing the bioavailability of heavy metals in paddy soil, characterized by, The method comprises the following steps: Step S1: Take the biogas slurry and add 5-15 times of water to dilute, and mix well; Step S2: Add cyclodextrin, iron salt and polyacrylamide to the diluted biogas slurry in step S1 in sequence, mix well after stirring, and adjust the pH value of the mixed solution; Step S3: Apply the mixed solution obtained in step S2 to the soil; Step S4: After the biogas slurry is applied to the soil, apply phosphate fertilizer to the soil and plough deeply.

2. The method of reducing the bioavailability of heavy metals in paddy soil according to claim 1, characterized in that, The cyclodextrin is β-cyclodextrin, and the molecular weight of the polyacrylamide is 5-10 million.

3. The method of reducing the bioavailability of heavy metals in paddy soil according to claim 1, characterized in that, The weight ratio of the diluted biogas slurry, cyclodextrin, iron salt and polyacrylamide in step S2 is 1000:0.08-0.2:3-40:0.1-0.

3.

4. The method of reducing the bioavailability of heavy metals in paddy soil according to claim 1, characterized in that, The content of water-soluble organic matter in the diluted biogas slurry in step S1 is 800-1200 mg / L.

5. The method of reducing the bioavailability of heavy metals in paddy soil according to claim 4, characterized in that, The content of water-soluble organic matter in the diluted solution in step S1 is 1000 mg / L.

6. The method of reducing the bioavailability of heavy metals in paddy soil according to claim 1, wherein, The iron salt is one or more of ferric chloride, ferric sulfate and ferric nitrate.

7. A method for rapid and efficient magnesium removal from a zinc hydrometallurgical leach solution as claimed in claim 1, characterized in that, The application amount of the mixed solution in step S3 is 1000-2000 kg / acre.

8. The method for reducing the bioavailability of heavy metals in paddy soil according to claim 1, wherein The phosphate fertilizer is applied to the soil 8-12 days after the biogas slurry is applied to the soil in step S4, and the application amount of the phosphate fertilizer in step S4 is 10-30 kg / acre.

9. The method for reducing the bioavailability of heavy metals in paddy field soil as claimed in claim 1, characterized by, The phosphate fertilizer in step S4 is one or more of superphosphate, potassium phosphate, potassium dihydrogen phosphate and dipotassium hydrogen phosphate.

10. The method for reducing the bioavailability of heavy metals in paddy soil according to claim 1, wherein In step S2, when the pH value of the soil is less than 5.0, the pH value of the mixed solution is adjusted to 6.0; when the pH value of the soil is 5.0-5.5, the pH value of the mixed solution is adjusted to 5.5; and when the pH value of the soil is 5.51-6.0, the pH value of the mixed solution is adjusted to 5.0.

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