Preparation method and application of biochar-coated LDH-oyster shell-coated microorganism composite material

Abstract

The invention discloses a preparation method and application of a biochar-coated LDH-oyster shell-microbe composite material, and belongs to the technical field of soil remediation. The preparation method comprises the following steps: preparing a biochar-CaFe LDH composite material by a hydrothermal coprecipitation method, modifying an oyster shell passivation material, using the modified oyster shell passivation material as a bacillus subtilis immobilization carrier, and preparing the biochar-LDH-oyster shell-microorganism composite material. According to the method, a combined remediation technology is adopted, the excellent characteristics of the biochar are fully utilized, and more point locations are provided for microorganism loading by changing the properties of the oyster shells. The prepared composite material has a good passivation effect on different ion types of heavy metals such as cadmium, lead, arsenic and chromium in soil, the defects of a single remediation method or a remediation material are overcome, multiple heavy metals in the soil are treated at the same time, and the remediation efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of soil remediation, and particularly relates to a preparation method and application of a biochar@LDH-oyster shell@microorganism composite material. Background Art

[0002] The purpose of remediating heavy metal contaminated soil is to strongly remove or immobilize heavy metals in the soil to reduce their mobility and biological activity, thereby reducing the risks to health and the environment. Currently, the methods for remediating heavy metal contaminated soil are mainly based on two strategies: one is to separate and remove heavy metals from the soil to reduce the heavy metal concentration in the soil; the other is to change the migration performance of heavy metals in the soil or change their existing forms into stable chemical forms to reduce potential risks. In order to remediate heavy metal contaminated soil, many amendments have been applied, including lime, sepiolite, red mud, phosphate fertilizer, silica, hydroxyapatite, bentonite, and organic solidifying agents, etc. These amendments have a certain remediation effect on cadmium contaminated soil to a certain extent, but their stability and cost still restrict their large-scale application. For example, lime-based passivating agents will be reactivated due to the self-purification ability of the soil when remediating contaminated soil by changing the pH; the solidification effect of organic solidifying agents is average, and there may be secondary pollution at the same time.

[0003] Patent CN108906872A, a method for treating wastewater and soil heavy metal pollution with calcined oyster shell powder, discloses a method for treating wastewater and soil heavy metal pollution with calcined oyster shell powder. The material used in this method is a composite passivating and remediating agent, and its components are: calcined oyster shell powder, biochar powder, and hydroxyapatite powder. The above three traditional passivating and remediating agents are mixed in certain mass fractions to obtain the composite passivating and remediating agent. The material prepared by this method only enables different passivating and remediating materials to play their respective roles through mechanical mixing, but does not involve the role of microorganisms.

[0004] Patent CN109852395A, oyster shell powder and its preparation method, soil heavy metal passivating agent and organic fertilizer, discloses a preparation method of a soil heavy metal passivating agent based on oyster shell powder. Calculated by mass fraction, the soil heavy metal passivating agent of this method includes 80 parts to 95 parts of the oyster shell powder, 1 part to 10 parts of organic matter raw materials, and 0.2 parts to 2 parts of microbial inoculum. This method utilizes the properties of oyster shell powder having a honeycomb sponge-like microporous structure and strong adsorption force for heavy metals, and can adsorb heavy metals in the soil, etc., but does not fully utilize the function of the porous structure of oyster shell powder as a microbial carrier; this method also needs to add exogenous organic matter, which may have some impacts on the original soil environment.

[0005] Patent CN116199492A discloses an injectable microbial repair material based on oyster shell and its preparation method and application, which includes an injectable microbial repair material based on oyster shell and its preparation method. The material of this method includes oyster shell, water and a separately packaged repair microorganism that can utilize oyster shell as a substrate to synthesize calcite. The application scenarios are concrete and rock repair materials, rather than the field of soil repair technology.

[0006] Patent CN110184065A discloses a preparation method of a layered double hydroxide and alkali-activated biochar composite material, which discloses a preparation method of a Mg / Fe-type layered double hydroxide (LDH) and alkali-activated biochar composite material. In this method, a biochar material that has been pretreated by alkali activation is prepared, and then the Mg / Fe-type layered double hydroxide is in-situ loaded onto the biochar material and aged to obtain a Mg / Fe-type layered double hydroxide and alkali-activated biochar composite material. This composite material can be used for soil improvement and repair and / or non-point source pollution control. However, in this method, the raw materials for synthesizing LDH are MgCl2 and FeCl3, and the finally prepared LDH is chloride ion type, which will activate heavy metals in the soil and has now been prohibited from use by the Ministry of Agriculture and Rural Affairs.

[0007] Based on this, it is urgent to develop a new repair technology to make up for the deficiencies of single repair methods or repair materials and achieve the simultaneous treatment of multiple heavy metals in the soil. Summary of the Invention

[0008] The purpose of the present invention is to provide a preparation method of a biochar@LDH-oyster shell@microorganism composite material, which adopts a combined repair technology, combines multiple soil repair methods, makes up for the deficiencies of single repair methods or repair materials, realizes the simultaneous treatment of multiple heavy metals in the soil, and improves the repair efficiency.

[0009] To achieve the above purpose, the present invention adopts the following technical solutions: A preparation method of a biochar@LDH-oyster shell@microorganism composite material includes the following steps: (1) Preparation of biochar@LDH: Dispersing bamboo-based biochar in an aqueous solution of Ca(OH)2, adding an aqueous solution of Fe2(SO4)3 dropwise in a water bath environment, stirring and reacting for 4-5 h, filtering, washing and drying after aging to obtain biochar@LDH; (2) Modified oyster shell@microorganism composite material: Crushing and sieving oyster shell to obtain oyster shell powder, after modification with dipotassium hydrogen phosphate or amino acid, adding the modified oyster shell to the Bacillus subtilis bacterial solution, stirring to obtain a bacterial suspension, and drying to obtain a modified oyster shell@microorganism composite material; (3) The products obtained in steps (1) and (2) are mixed evenly at a mass ratio of 1:1 to obtain a biochar@LDH-oyster shell@microorganism composite material.

[0010] Furthermore, the molar ratio of Ca(OH)2 to Fe2(SO4)3 in step (1) is 3:1.

[0011] Furthermore, in step (1), the temperature of the water bath is 80°C, the aging temperature is 80°C, and the aging time is 18 - 24 h.

[0012] Furthermore, the steps for modifying with dipotassium hydrogen phosphate in step (2) are as follows: Oyster shell powder and dipotassium hydrogen phosphate are mixed and ground evenly and then calcined. The mass ratio of oyster shell powder to dipotassium hydrogen phosphate is 1:0.15, the calcination temperature is 200°C, and the calcination time is 2 h.

[0013] Furthermore, the amino acid in step (2) is polyaspartic acid, and the modification step is to add 1 g of calcined oyster shell powder to an aqueous solution of 5 wt.% polyaspartic acid and stir; the calcination temperature is 200°C, and the calcination time is 2 h.

[0014] Furthermore, the dosage ratio of the Bacillus subtilis bacterial solution to the modified oyster shell in step (2) is: 4 mL:8 g.

[0015] The present invention also provides the application of the biochar@LDH-oyster shell@microorganism composite material in the remediation of heavy metal contaminated soil, and the heavy metals include Cd, As, Cr, and Pb.

[0016] The advantages of the present invention are as follows: ① Make full use of various excellent properties of biochar: 1) As a carrier of LDH; 2) Add it to the soil to improve the physical and chemical properties of the soil (pH value, CEC, water holding capacity, air permeability, fertilizer retention, etc.); 3) Provide nutrients such as carbon and nitrogen sources for the growth of subsequent microorganisms; 4) Provide a habitat for soil microorganisms, avoid the death of microorganisms caused by drastic changes in the soil environment, and play a buffering role; 5) Treating the soil with biochar will stimulate the "priming effect" of microorganisms in the soil environment (a strong short-term change in SOC turnover caused by relatively mild soil treatment), and enhance their life activities.

[0017] ② In view of the fact that the Ministry of Agriculture and Rural Affairs prohibits the use of chloride-type LDH materials as soil passivation and remediation agents, in the selection of materials in step one, the raw materials are selected as Ca(OH)2 and Fe2(SO4)3, and the synthesized LDH is sulfate-type. It avoids the activation of heavy metals in the soil caused by the release of Cl - from the traditional LDH synthesized with CaCl2 and FeCl3 as raw materials.

[0018] ③Disperse the biochar in the Ca(OH)2 solution. In fact, alkali modification is also carried out in this step, which is beneficial to increasing the specific surface area of the biochar and improving the pore structure. At the same time, it can make the biochar evenly dispersed in the Ca 2+ system, improve the formation rate of subsequent CaFeLDH, and make it more evenly loaded on the surface of the biochar.

[0019] ④There are two major advantages in modifying oyster shells with K2HPO4 and aspartic acid: 1) To change the properties of oyster shells themselves and provide more sites for microbial loading; 2) Phosphorus and potassium elements can be used as nutrient sources for microbial growth.

[0020] ⑤In terms of the passivation mechanism, after the final product is added to the soil, the biochar@LDH can undergo an isomorphic substitution reaction with heavy metal cations to exchange out Ca 2+ and the biochar@LDH itself will also precipitate Ca due to its raw material characteristics 2+ , and these free Ca 2+ can further induce the mineralization of heavy metals by bacteria loaded on the modified oyster shells; in addition, in the environment of anion-type heavy metal contaminated soil, the biochar@LDH can also passivate anion-type heavy metals through the intercalation reaction of LDH.

[0021] ⑥In terms of the application scope, the composite material prepared in this patent has good passivation effects on heavy metals of different ionic types such as cadmium, lead, arsenic, and chromium in the soil, and can be applied to heavy metal composite contaminated soil. Description of the Drawings

[0022] Figure 1 is the XRD spectrum of biochar@LDH.

[0023] Figure 2 is the FT-IR spectrum of biochar@LDH.

[0024] Figure 3 is the SEM spectrum of biochar@LDH.

[0025] Figure 4 is the XRD spectrum of K2HPO4-modified oyster shell@microbial composite material and polyaspartic acid-modified oyster shell@microbial composite material.

[0026] Figure 5 is the SEM spectrum of K2HPO4-modified oyster shell (left) and K2HPO4-modified oyster shell@microbial composite material (right).

[0027] Figure 6 is the SEM spectrum of polyaspartic acid-modified oyster shell (left) and polyaspartic acid-modified oyster shell@microbial composite material (right).

[0028] Figure 7Is the adsorption isotherm fitting curve of biochar@LDH.

[0029] Figure 8 Is the adsorption kinetics fitting curve of biochar@LDH.

[0030] Figure 9 Is the adsorption kinetics fitting curve of K2HPO4 modified oyster shell@microbial composite.

[0031] Figure 10 Is the adsorption isotherm fitting curve of polyaspartic acid modified oyster shell@microbial composite.

[0032] Figure 11 Is the physical picture of biochar@LDH (left), K2HPO4 modified oyster shell@microbial composite or polyaspartic acid modified oyster shell@microbial composite (right).

[0033] Figure 12 Is the physical picture of biochar@LDH-modified oyster shell@microbial composite (the appearance of the two modified oyster shells is the same). Detailed implementation manners

[0034] In order to make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific implementation manners, but the present invention is not limited thereto.

[0035] Example 1: A preparation method of a biochar@LDH-oyster shell@microbial composite: (1) Preparation of biochar@LDH: First, dissolve 0.03 mol of Ca(OH)2 in 50 mL of deionized water, which not only disperses the raw materials but also creates an alkaline environment; then add 1 g of bamboo-based biochar (BC) and stir to disperse to form a suspension; dissolve 0.005 mol of Fe2(SO4)3 in 50 mL of deionized water, and after it is completely dissolved, add it dropwise to the suspension. This process is completed in an 80 °C water bath environment, where the molar ratio of Ca 2+ to Fe 3+ is 3:1; after the raw materials are added, keep the 80 °C water bath environment and stir continuously for 4 h. After stopping stirring, place the product at 80 °C for aging for 20 h; collect the product by suction filtration, wash it with ultrapure water to remove impurities, and finally dry it under vacuum at 80 °C to obtain biochar@LDH (BC@CaFe LDH).

[0036] (2)Preparation of K2HPO4-modified oyster shell@microbial composite: First, the oyster shell was processed, crushed, and sieved through a 100-mesh sieve. 0.15 g of K2HPO4 was added to 1 g of oyster shell powder and ground. After uniform grinding, it was calcined at 200 °C for 2 h to obtain the modified oyster shell. Bacillus subtilis (from Shanghai Microbial Preservation Center: WB600) was activated in a calcium-containing nitrogen-free solid medium and inoculated into a calcium-containing nitrogen-free liquid medium for seed culture. It was shake-cultured at 37 °C and 150 rpm for 48 h to obtain a seed solution. The seed solution was inoculated into a nitrogen-containing liquid medium at an inoculation amount of 5% (V / V) and shake-cultured at 37 °C and 150 rpm for 72 h to obtain a fermentation broth. Under sterile conditions, 8 g of the modified oyster shell material was added to 4 mL of the broth and magnetically stirred for 20 min to obtain a bacterial suspension, and a Bacillus mucilaginosus agent was prepared using freeze-drying technology.

[0037] The composition of the calcium-containing nitrogen-free liquid medium was: 10 g of glucose, 2 g of NaCl, 2 g of MgSO4·7H2O, 2 g of KH2PO4, 5 g of CaCO3, 1000 mL of distilled water, and the pH value was 7.2.

[0038] The composition of the nitrogen-containing liquid medium was: 10 g of sucrose, 1 g of yeast extract powder, 1 g of NaCl, 0.2 g of MgSO4, 0.5 g of K2HPO4, 1000 mL of distilled water, and the pH value was 7.2.

[0039] (3)Preparation of biochar@LDH-K2HPO4-modified oyster shell@microbial composite: The biochar@LDH and the K2HPO4-modified oyster shell@microbial composite were compounded at a mass ratio of 1:1 and mixed evenly to obtain the biochar@LDH-K2HPO4-modified oyster shell@microbial composite.

[0040] A series of physical characterizations were performed on the composite obtained in this example by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, etc.

[0041] Figure 1 This is the XRD pattern of the prepared biochar@LDH (scanning range: 5 - 80°, step size: 0.02°, scanning rate: 8° / min). As shown in the figure, the obtained biochar@LDH has a good lamellar structure, and characteristic peaks of LDH such as 10.5° (003), 22.1° (006), and 34° (012) appear in the diffraction pattern.

[0042] Figure 2 This is the FT-IR spectrum of the prepared biochar@LDH. The enhanced stretching vibration at 3345 cm -1 indicates the generation of lattice water and -OH groups between the LDH layers. The stretching vibration at 1422 cm-1 The stretching vibration at this position represents CO3 2- Embedded in the LDH lamellae, it belongs to the ν3 asymmetric contraction of the carbonate group in the calcite phase, indicating that during the preparation process, CaFe-LDH can react with CO2 to form CaCO3. Therefore, CaFe-LDH needs to be stored in a vacuum. Located at 869 cm -1 and 707 cm -1 The vibration peaks represent the metal-oxygen (Me-O, Me = Ca or Fe) lattice.

[0043] Figure 3 This is the SEM spectrum of the prepared biochar@LDH. It can be clearly observed that the lamellar nanostructure of LDH is present. The sample morphology is good, the particle size is small and evenly distributed, but there is agglomeration, indicating that it has been successfully loaded onto the surface structure of biochar.

[0044] Figure 4 This is the XRD spectrum of the prepared K2HPO4-modified oyster shell@microbial composite (scanning range: 5 - 80°, step size: 0.02°, scanning rate: 8° / min). It can be seen that after loading Bacillus subtilis, the crystal structure of the K2HPO4-modified oyster shell@microbial composite has not changed, indicating that the modification with dipotassium hydrogen phosphate does not damage the calcium carbonate structure of the oyster shell. Its main role is to change the specific surface area and provide more loading sites for microorganisms.

[0045] Figure 5 This is the SEM spectrum of the K2HPO4-modified oyster shell@microbial composite. Under the electron microscope, it can be clearly seen that Bacillus subtilis has been successfully loaded onto the surface of the modified oyster shell.

[0046] Figure 7 This is the adsorption isotherm fitting curve of biochar@LDH, which is more in line with the Freundlich equation, indicating non-uniform monolayer adsorption.

[0047] Figure 8 This is the adsorption kinetics fitting curve of biochar@LDH. It reaches the adsorption equilibrium within about 30 min, and its kinetic process conforms to the pseudo-first-order kinetic model, indicating that the adsorption process is mainly physical adsorption.

[0048] Example 2: (1) The preparation steps of biochar@LDH are the same as those in Example 1.

[0049] (2)Preparation of polyaspartic acid-modified oyster shell @ microorganism composite material: First, the oyster shell was processed, crushed, and passed through a 100-mesh sieve. 1 g of oyster shell powder was calcined at 200 °C for 2 h and then added to an aqueous solution of polyaspartic acid containing 5%. It was magnetically stirred at room temperature for 1 h. Bacillus subtilis was activated in a calcium-containing nitrogen-free solid medium and inoculated into a calcium-containing nitrogen-free liquid medium for seed culture. It was shaken and cultured at 37 °C and 150 rpm for 48 h to obtain a seed solution; the seed solution was inoculated into a nitrogen-containing liquid medium at an inoculation amount of 5% (V / V) and shaken and cultured at 37 °C and 150 rpm for 72 h to obtain a fermentation broth; under sterile conditions, 8 g of the modified oyster shell material was added to 4 mL of the broth and magnetically stirred for 20 min to obtain a bacterial suspension, and a Bacillus mucilaginosus agent was prepared using freeze-drying technology.

[0050] (3)The biochar @ LDH and the polyaspartic acid-modified oyster shell @ microorganism composite material were compounded at a mass ratio of 1:1 and mixed evenly to obtain a biochar @ LDH - polyaspartic acid-modified oyster shell @ microorganism composite material.

[0051] Figure 6 is the SEM spectrum of the polyaspartic acid-modified oyster shell @ microorganism composite material. Under the electron microscope, it can be clearly seen that Bacillus subtilis was successfully loaded on the surface of the modified oyster shell.

[0052] Figure 9 is the adsorption kinetic fitting curve of the K2HPO4-modified oyster shell @ microorganism composite material. The adsorption equilibrium was reached within about 100 min, and its kinetic process conforms to the pseudo-first-order kinetic model, indicating that the adsorption process is mainly physical adsorption.

[0053] Figure 10 is the adsorption isotherm fitting curve of the polyaspartic acid-modified oyster shell @ microorganism composite material, which is more in line with the Langmuir equation, indicating that there is no interaction between the adsorbed molecules, and the adsorption is a monolayer adsorption that occurs only at specific positions on the surface of the adsorbent and proceeds uniformly on the surface.

[0054] Application Example 1 Cd(NO3)2·4H2O was dissolved in ultrapure water to prepare a simulated Cd-polluted wastewater with a concentration of 114.2 mg / L. Using unmodified oyster shell, oyster shell modified with dipotassium hydrogen phosphate, oyster shell modified with amino acid, and Bacillus agent with modified oyster shell as the carrier as adsorbents, a water body adsorption experiment was carried out. The specific results are shown in Table 1.

[0055] Table 1 Effects of different materials as adsorbents in the water body adsorption experiment

[0056] Application Example 2 Similar to Application Example 1, using the composite materials obtained in Examples 1 and 2 as adsorbents respectively, a water body adsorption experiment was carried out. The following are the experimental results (Table 2): Table 2 Effects of different composite materials as adsorbents in water body adsorption experiments

[0057] Application Example 3 (1) Collect the surface soil samples of 0-5 cm of heavy metal contaminated red soil in Fuzhou area, air-dry, grind, and pass through a 10-mesh sieve for repair, and measure the cadmium and arsenic contents. After measurement, the total amounts of Cd and As in the soil are 7.32 mg / kg and 5.28 mg / kg respectively. Among them, the proportions of active heavy metals (including F1: exchangeable state, F2: carbonate-bound state) are 78.2% and 53.5% respectively.

[0058] (2) Crush the composite materials prepared in Examples 1 and 2 and pass through a 10-mesh sieve, then add them to the contaminated soil according to a mass ratio of 1:10, and mix well; at the same time, a comparative experiment of unmodified oyster shells and biochar materials was carried out.

[0059] (3) Keep the soil moisture content at about 30%, stir once every 2 days, and carry out a 3-month passivation repair experiment.

[0060] (4) Collect the repaired soil, grind it, and pass through a 100-mesh sieve for use.

[0061] (5) Use the Wenzel or Tessier sequential extraction method to measure the arsenic and cadmium contents in each occurrence form, and calculate its passivation rate (the ratio of heavy metals changing from the active state to the inactive state). The results are as follows:

[0062] Application Example 4 (1) Collect the surface soil samples of 0-5 cm of lead-contaminated brown soil in the Yangshuo lead-zinc mine basin in the northern Guangxi region of China, air-dry, grind, and pass through a 10-mesh sieve for repair, and measure its lead content. After measurement, the total amount of Pb in the soil is 106.85 mg·kg -1 , among which, the proportion of active heavy metals (including F1: exchangeable lead, F2: carbonate-bound lead) is 41.05%.

[0063] (2) Crush the composite materials prepared in Examples 1 and 2 and pass through a 10-mesh sieve, then add them to the contaminated soil according to a mass ratio of 1:10, and mix well; at the same time, a comparative experiment of unmodified oyster shells and biochar materials was carried out.

[0064] (3) Keep the soil moisture content at about 30%, stir once every 2 days, and carry out a 3-month passivation repair experiment.

[0065] (4) Collect the repaired soil, grind it, and sieve it through a 100-mesh sieve for later use.

[0066] (5) Use the Tessier sequential extraction method to determine the lead content in each occurrence form, and the results are as follows:

[0067] Application Example 5 (1) Collect the surface soil samples of 0-5 cm of chromium-contaminated cinnamon soil in Beijing area of China, air-dry, grind, and sieve them through a 10-mesh sieve for later repair, and determine its chromium content. After determination, the total amount of Cr in the soil is 47.84 mg·kg -1 , among which, the proportion of active heavy metals (including F1: exchangeable chromium, F2: carbonate-bound chromium) is 26.35%.

[0068] (2) Crush the composite materials prepared in Examples 1 and 2 and sieve them through a 10-mesh sieve, and then add them to the contaminated soil according to a mass ratio of 1:10, and mix well; at the same time, conduct a comparative experiment on unmodified oyster shells and biochar materials.

[0069] (3) Keep the soil moisture content at about 30%, stir it every 2 days, and conduct a passivation repair experiment for 3 months.

[0070] (4) Collect the repaired soil, grind it, and sieve it through a 100-mesh sieve for later use.

[0071] (5) Use the Tessier sequential extraction method to determine the lead content in each occurrence form, and the results are as follows:

[0072] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. Preparation method of a biochar@LDH-oyster shell@microbial composite material, characterized in that: It includes the following steps: (1) Preparation of biochar@LDH: Dispersing bamboo-based biochar in an aqueous solution of Ca(OH)2, adding an aqueous solution of Fe2(SO4)3 dropwise in a water bath environment, stirring and reacting for 4 - 5 h, filtering, washing, and drying after aging to obtain biochar@LDH; (2) Modified oyster shell@microbial composite: Crushing and sieving oyster shells to obtain oyster shell powder, after modification with dipotassium hydrogen phosphate or amino acid, adding the modified oyster shell to the Bacillus subtilis bacterial solution, stirring to obtain a bacterial suspension, and drying to obtain the modified oyster shell@microbial composite; (3) Mixing the products obtained in step (1) and step (2) evenly at a mass ratio of 1:1 to obtain the biochar@LDH - oyster shell@microbial composite.

2. The preparation method according to claim 1, wherein: In step (1), the molar ratio of Ca(OH)2 to Fe2(SO4)3 is 3:

1.

3. The preparation method according to claim 1, wherein: In step (1), the temperature of the water bath is 80 °C, the temperature of aging is 80 °C, and the aging time is 18 - 24 h.

4. The preparation method according to claim 1, characterized in that: In step (2), the steps of modification with dipotassium hydrogen phosphate are: Mixing and grinding oyster shell powder and dipotassium hydrogen phosphate evenly and then calcining, the mass ratio of oyster shell powder to dipotassium hydrogen phosphate is 1:0.15, the calcination temperature is 200 °C, and the calcination time is 2 h.

5. The preparation method according to claim 1, wherein: In step (2), the amino acid is polyaspartic acid, and the modification steps are adding 1 g of calcined oyster shell powder to an aqueous solution of 5 wt.% polyaspartic acid and stirring; the calcination temperature is 200 °C, and the calcination time is 2 h.

6. The preparation method according to claim 1, characterized in that: In step (2), the dosage ratio of the Bacillus subtilis bacterial solution to the modified oyster shell is: 4 mL:8 g.

7. The biochar@LDH - oyster shell@microbial composite prepared by the preparation method according to any one of claims 1 - 6.

8. The application of the biochar@LDH - oyster shell@microbial composite according to claim 7 in the remediation of heavy metal - contaminated soil.

9. The application according to claim 8, wherein: The heavy metals include Cd, As, Cr, Pb.

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