Preparation method of multifunctional organic fertilizer coupled with heavy metal passivation by straw carbonization

Abstract

The present application relates to the technical field of organic fertilizer, and discloses a preparation method of a multifunctional organic fertilizer for straw carbonization and heavy metal passivation, which comprises the following components: in-situ phosphorus-calcium doped biochar, plant-derived organic material, composite functional bacterial agent and wood ash, wherein the in-situ phosphorus-calcium doped biochar is prepared by co-pyrolysis of crop straw and animal aggregate. The method forms a stable structure for fixing phosphorus and calcium nutrients and passivation functional sites in the form of chemical bonding and physical coating in the biochar matrix through in-situ co-pyrolysis. The fertilizer prepared by the present application has the functions of long-acting passivation of soil heavy metals and sustained release of nutrients, can realize multiple effects of reducing crop heavy metal absorption, promoting crop growth and improving soil microbial activity, and the fertilizer particles have high compressive strength and water dispersion stability, which are suitable for mechanized application.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of organic fertilizer, in particular to a preparation method of a multifunctional organic fertilizer with straw carbonization coupling heavy metal passivation. BACKGROUND

[0002] In view of the increasingly serious problem of soil heavy metal pollution, using biochar-based modifier for remediation has become an important technical means. Currently, the technical practice in this field usually relies on physically mixing pre-prepared biochar carriers with heavy metal passivation agents such as phosphates, organic nutrients, and functional microorganisms, etc., in order to obtain a composite product with multiple functions.

[0003] However, this preparation path based on physical mixing fundamentally limits the final performance of the product. Since the passivation functional components are only combined with the biochar carrier through physical adsorption or mechanical force, the combination strength is limited, and in complex soil environments, the passivation agent is prone to leaching due to rainwater or ion exchange, resulting in insufficient durability and reliability of heavy metal passivation effect, and even secondary environmental risks caused by the loss of passivation agent.

[0004] In addition, there is a lack of internal synergy between the functional components. Simple physical blending cannot provide a stable colonization microenvironment for functional microorganisms, making it difficult for them to survive and continuously function in the soil. At the same time, the nutrients in the mixture tend to be released quickly, which cannot effectively match the crop growth cycle. At the preparation process level, the separate preparation of each component and subsequent mixing increases the complexity of the process, and the granulation process often requires the introduction of additional inorganic binders, which not only increases the cost, but also the particles produced usually have low mechanical strength and poor water dispersion stability, which cannot guarantee that the functional components achieve the expected long-term effect in the soil. Therefore, the existing technology urgently needs a technical solution that can build a stable chemical structure inside the product through an integrated process, thereby organically integrating heavy metal long-term passivation, nutrient sustained release, and microbial synergistic effects. SUMMARY

[0005] The present application aims to solve the technical problems of weak combination of passivation agent and carrier, unstable passivation effect, and easy secondary pollution of biochar-based heavy metal passivation fertilizer prepared by physical mixing in the prior art, as well as the complexity of the preparation process and the lack of synergy between components.

[0006] To solve the above technical problems, the present application provides a multifunctional organic fertilizer and a preparation method thereof.

[0007] In a first aspect, the present application provides a multifunctional organic fertilizer, which adopts the following technical solution:

[0008] A multifunctional organic fertilizer, comprising the following components by dry weight basis:

[0009] Phosphorus-calcium doped biochar 60-70%;

[0010] Plant-derived organic matter 15-25%;

[0011] Compound functional microbial agent 5-10%;

[0012] Wood ash 5-10%;

[0013] The phosphorus-calcium doped biochar is made of crop straw and animal bone materials through co-pyrolysis process.

[0014] The core function of the fertilizer is borne by the phosphorus-calcium doped biochar. The structural characteristics of the biochar are derived from its specific preparation process: in the co-pyrolysis process, the calcium phosphate in the animal bone materials and the carbon precursor in the straw undergo solid-phase reaction at high temperature to form stable P-C (phosphorus-carbon) and Ca-O-C (calcium-oxygen-carbon) chemical bonds. This chemical bonding structure fixes the phosphorus and calcium passivation functional sites in the form of atomic or molecular clusters in the carbon skeleton, and its bond energy is much higher than physical adsorption or electrostatic attraction, so that the passivation functional sites have the ability to resist soil pH fluctuations and ion exchange leaching, fundamentally avoiding the risk of loss of passivation agent and desorption of heavy metals.

[0015] In addition, this process makes the phosphorus and calcium sites highly dispersed in the three-dimensional porous carbon matrix, greatly increasing the number and accessibility of effective passivation sites. The porous structure of the biochar also provides physical colonization space for the compound functional microbial agent, and cooperates with other organic materials to form a composite functional system that integrates heavy metal passivation, soil structure improvement, microbial activity, and nutrient supply.

[0016] Preferably, the compound functional microbial agent contains one or more of phosphorus-solubilizing bacteria, nitrogen-fixing bacteria, and organic matter-decomposing bacteria.

[0017] More specifically, the phosphorus-solubilizing bacteria are Bacillus megaterium, the nitrogen-fixing bacteria are Azotobacter chroococcum, and the organic matter-decomposing bacteria are Bacillus subtilis.

[0018] By adopting the above technical solution, the specific functional bacteria introduced can colonize and carry out life activities in the microenvironment provided by the phosphorus-calcium doped biochar after the fertilizer is applied to the soil.

[0019] Phosphorus-solubilizing bacteria can secrete organic acids or phosphatases to convert insoluble phosphate in the soil into soluble phosphorus that can be absorbed by plants; nitrogen-fixing bacteria can convert atmospheric nitrogen into ammonia nitrogen; and organic matter-decomposing bacteria can accelerate the mineralization of organic matter. These biological conversion processes provide non-chemically added, continuous biological source nutrient supply for crop growth.

[0020] Preferably, the plant-derived organic material is spent mushroom substrate and / or soybean meal.

[0021] By adopting the above technical solution, edible fungus waste bran or soybean meal is selected as organic material. It is rich in carbohydrates and proteins that are easily decomposed and utilized by microorganisms, providing a direct carbon and nitrogen source for the survival and reproduction of compound functional microbial agents in the early stage of fertilizer storage and application, thus ensuring the biological activity of the microbial agents.

[0022] Secondly, this application provides a method for preparing a multifunctional organic fertilizer, which adopts the following technical solution:

[0023] A method for preparing a multifunctional organic fertilizer includes the following steps:

[0024] (1) Mix crop straw with animal bone meal to obtain a mixture;

[0025] (2) The mixture is subjected to staged programmed heating co-pyrolysis under oxygen-limited conditions to obtain in-situ phosphorus-calcium doped biochar.

[0026] (3) The in-situ phosphorus and calcium doped biochar is mixed with plant-derived organic materials, compound functional microbial agents and wood ash, and the moisture content is adjusted before aerobic fermentation is carried out to obtain fermented materials.

[0027] (4) The fermented material is granulated and dried to obtain the final product.

[0028] By adopting the above technical solution, the core of this method lies in the staged programmed temperature co-pyrolysis in step (2). This process integrates the traditionally separate steps of biochar preparation and modifier loading into a single, controlled chemical synthesis process. By programmatically controlling the temperature and time of the pyrolysis process, the physicochemical transformations and solid-phase reactions at different stages are precisely induced and completed, directly generating functional materials with the target chemical bonding structure, simplifying the process flow, and realizing the intrinsic regulation of the structure and performance of the final product.

[0029] Preferably, in step (1), before mixing, a pretreatment step of acid activation treatment of the animal bone material is included.

[0030] By adopting the above technical solution, the animal bone material can be pre-activated by acid treatment, which can remove some inorganic impurities on its surface and increase its specific surface area and the number of surface hydroxyl groups, providing more initial contact reaction sites for the solid-phase reaction in the subsequent pyrolysis process.

[0031] Preferably, the staged programmed temperature rise co-pyrolysis in step (2) includes:

[0032] First stage: Heat the mixture to a temperature of 200-300°C and keep it at that temperature for 30-60 minutes;

[0033] Second stage: After completing the first stage, the material is further heated to a temperature of 450-550℃ and kept at that temperature for 60-90 minutes.

[0034] Furthermore, the heating rate in the first stage is 5–10 °C / min, and the heating rate in the second stage is 10–20 °C / min.

[0035] By adopting the above technical solution, the inherent mechanism of this process is as follows:

[0036] In the first low-temperature stage, the hemicellulose in the straw preferentially undergoes pyrolysis, generating organic acid vapors such as acetic acid and formic acid. These acidic gases generated in situ in the reaction system permeate evenly into the mixture, gently etching the surface of the calcium phosphate particles in the animal bone, thus achieving uniform "in-situ activation".

[0037] After activation, the process enters the second high-temperature stage, where the cellulose and lignin in the straw undergo intense carbonization to form highly reactive carbon free radicals. These carbon free radicals then react chemically with the highly reactive phosphorus and calcium sites activated in the first stage, forming stable chemical bonds. This pre-designed, staged reaction pathway ensures that the activation step is completed before the main carbonization and chemical bonding steps, which is a crucial condition for achieving efficient in-situ doping.

[0038] Preferably, in step (3), the conditions for aerobic fermentation are: the moisture content of the material is adjusted to 50-60%, and the fermentation time is 7-10 days.

[0039] By adopting the above technical solution, the set material moisture content is the balance point between the water required for the metabolic activities of aerobic microorganisms and the sufficient oxygen in the pores of the material; the set fermentation time ensures that the microorganisms have enough time to complete the proliferation cycle and to carry out preliminary decomposition and maturation of the organic material, so that the biological components and chemical components in the final product can reach a stable coexistence state.

[0040] Preferably, step (2) further includes collecting the liquid byproduct wood vinegar produced during the pyrolysis process; and in the granulation process of step (4), the wood vinegar is used as a binder.

[0041] By adopting the above technical solution, the pyrolysis byproduct wood vinegar is directly recycled into the granulation process, achieving closed-loop utilization of this byproduct. During granulation, the organic components in the wood vinegar act as a binder, increasing the granule formation rate and mechanical strength. Simultaneously, the organic acids and phenolic compounds in the wood vinegar themselves have the function of regulating plant growth and inhibiting the activity of some microorganisms. Through this method, they are solidified together in the final fertilizer granules, not only avoiding the need for secondary treatment of waste liquid but also transforming the original byproduct into one of the functional components of the fertilizer product.

[0042] In summary, the present invention has at least one of the following beneficial technical effects:

[0043] 1. This invention employs a staged programmed temperature co-pyrolysis process, achieving chemical bonding of phosphorus and calcium elements during biochar formation, resulting in stable PC and Ca-OC structures. Compared to traditional methods of physically mixing passivating agents with biochar, this chemically bonded structure firmly anchors the passivation functional sites onto the carbon skeleton, significantly enhancing its stability in the soil environment. This effectively prevents passivating agent loss due to rainwater leaching or changes in soil chemical conditions, thus ensuring the long-term effectiveness and reliability of heavy metal passivation.

[0044] 2. This invention tightly integrates in-situ phosphorus-calcium-doped biochar, functional microbial agents, and organic materials through a subsequent fermentation process, forming a multifunctional synergistic system. The porous biochar, prepared using a special process, is not only a highly efficient heavy metal passivating agent but also provides an excellent colonization carrier and survival microenvironment for the composite functional microbial agents. This structure allows the chemical passivation function and the biological functions of microbial phosphorus solubilization, nitrogen fixation, and soil fertility improvement to coexist and mutually promote each other within a single particle, achieving simultaneous remediation of contaminated soil and improvement of soil fertility.

[0045] 3. The preparation method of this invention collects the wood vinegar byproduct generated during pyrolysis and reuses it in the final granulation process as a functional binder. This design not only recycles waste that would otherwise require treatment, achieving a closed-loop material flow within the process and reducing environmental impact, but also eliminates the cost of purchasing chemical binders. This improves the atom economy and greenness of the entire preparation process, making the production process more integrated and efficient. Attached Figure Description

[0046] Figure 1 The Cd content in the leachate of each sample in Test Example 1 of this invention 2+ Box plot showing the concentration distribution;

[0047] Figure 2 The Cd samples in Test Example 1 of this invention 2+ A graph showing the cumulative leaching concentration over time;

[0048] Figure 3 This is a heatmap showing the dynamic process of the effective phosphorus concentration in the leachate of each sample in Test Example 2 of the present invention changing over time.

[0049] Figure 4 This is a two-dimensional scatter plot showing the relationship between the aboveground dry weight of the plant and the Cd content in the plant in Test Example 3 of this invention.

[0050] Figure 5 This is a box plot comparing the radial compressive strength and water dispersion stability of the sample particles in Test Example 4 of this invention.

[0051] Figure 6 This is a radar chart showing the impact of Test Example 5 of the present invention on multiple microbial and enzyme activity indicators in the soil. Detailed Implementation

[0052] In existing technologies, biochar is often used as a carrier, and heavy metal passivating agents (such as hydroxyapatite or bone meal) are added through physical mixing to prepare soil remediation fertilizers. However, this physical mixing relies on weak physical adsorption or electrostatic attraction, and the bond between the passivating agent and the biochar carrier is not strong. In the soil environment, factors such as rainwater leaching, pH changes, or ion exchange can easily cause the passivating agent to detach and be lost from the carrier, resulting in a lack of long-lasting passivation effects and potentially even triggering the secondary release of heavy metals.

[0053] To address this technical problem, the inventors conducted in-depth research on the combination of biochar and passivating agents, aiming to find a method to stably fix passivating functional sites within the biochar framework. During the research, the inventors discovered that simply mixing straw and aggregate and then performing conventional isothermal pyrolysis did not effectively promote chemical bonding between the two. Further research revealed that the reaction pathway during pyrolysis is controllable, leading to a key discovery: by employing a staged, programmed temperature-controlled co-pyrolysis method, synergistic effects of "in-situ activation" and "chemical bonding" can be achieved within an integrated reactor. Specifically, at lower temperatures (200–300°C), the decomposition of hemicellulose in the straw generates organic acid vapors in situ, which uniformly activate the aggregate surface. Subsequently, at higher temperatures (450–550°C), the straw matrix carbonizes to form a highly active carbon matrix, which then undergoes a solid-phase reaction with the highly active phosphorus and calcium sites activated in the first stage, forming stable chemical bonds.

[0054] Based on the above findings, this application prepares a novel in-situ phosphorus-calcium doped biochar in which a passivating agent is chemically bonded to a carbon skeleton by controlling the pyrolysis process. Based on this, the application develops the aforementioned multifunctional organic fertilizer and its preparation method, thereby fundamentally solving the technical problems of easy loss of passivating agent and short-lasting effect.

[0055] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0056] Unless otherwise stated, all raw materials and reagents used in the embodiments of the present invention are commercially available or prepared by conventional methods in the art.

[0057] The corn stalks, rice stalks, or wheat stalks used in the examples were all collected from agricultural production areas. The raw materials were naturally air-dried to a moisture content of less than 15% (w / w), then crushed using a hammer mill, and the material with a particle size of 1-5 mm was collected by sieving for later use.

[0058] The animal bone used in this example is derived from food processing byproducts. Its main chemical component is hydroxyapatite. The raw material is initially cleaned to remove surface grease and residual tissue, dried in an oven at 105°C for 4 hours, then pulverized and sieved to collect bone meal with a particle size of less than 1 mm for later use.

[0059] The microorganisms and organic materials used in the examples are as follows:

[0060] Compound functional microbial agent: prepared from pure cultures of the following three bacterial species: Bacillus megaterium, Azotobacter chroococcum, and Bacillus subtilis.

[0061] Plant-derived organic materials: pre-composted edible fungus waste (mushroom residue) or soybean meal.

[0062] Wood ash: derived from the ash after burning rice husks, its main component is potassium carbonate.

[0063] Examples 1-3,

[0064] Example 1

[0065] This embodiment provides a method for preparing a multifunctional organic fertilizer, the specific steps of which are as follows:

[0066] (1) Raw material pretreatment: Take 400.0 g of air-dried corn stalks and 100.0 g of animal bones. Immerse the animal bones in a 5% citric acid solution and stir at 25°C for 2 hours. Then remove them and wash them with deionized water until neutral. Dry them at 105°C to obtain acid-activated aggregate. Physically mix the corn stalks and acid-activated aggregate evenly to obtain a mixture.

[0067] (2) Preparation of in-situ phosphorus-calcium doped biochar: The mixture obtained in step (1) was placed in a tubular pyrolysis furnace and subjected to staged programmed temperature co-pyrolysis under a nitrogen atmosphere (flow rate of 200 mL / min). The pyrolysis program was set as follows: the temperature was increased from room temperature to 300℃ at a heating rate of 5℃ / min and held at this temperature for 45 minutes; then, the temperature was increased to 500℃ at a heating rate of 15℃ / min and held at this temperature for 75 minutes. The liquid byproducts generated during the pyrolysis process were collected by a condenser to obtain wood vinegar. After the pyrolysis was completed, the mixture was naturally cooled to room temperature, and the solid product was removed to obtain in-situ phosphorus-calcium doped biochar, denoted as SCB-1.

[0068] (3) Fermentation and compounding: Take 155.0 g of SCB-1 biochar obtained in step (2), 47.5 g of pre-composted edible fungus waste, 24.0 g of wood ash, and compound functional microbial agent (Bacillus megaterium: Azotobacter chrysogenum: Bacillus subtilis = 1:1:1, total viable count ≥ 10 9 12.0 g (CFU / g) was placed in a mixer and mixed thoroughly. During the mixing process, deionized water was slowly sprayed in to adjust the total moisture content of the material to 55%. The mixed material was then subjected to windrow aerobic fermentation for 8 days, during which the pile was turned over once a day.

[0069] (4) Granulation and drying: The fermented and decomposed material is fed into a roller extrusion granulator, and the wood vinegar collected in step (2) is used as a liquid binder for granulation. The resulting granules are screened by a vibrating screener to collect finished granules with a particle size of 2-5 mm. The finished granules are dried with hot air at 60°C until the moisture content is less than 10%, which is a multifunctional organic fertilizer, denoted as F-1.

[0070] Example 2

[0071] This embodiment provides a method for preparing a multifunctional organic fertilizer, which differs from Embodiment 1 mainly in the raw material ratio, the absence of acid activation treatment, and the adjustment of pyrolysis parameters. The specific steps are as follows:

[0072] (1) Raw material pretreatment: Take 300.0 g of air-dried rice straw and 100.0 g of animal bone, and mix them directly and evenly to obtain a mixture. In this embodiment, the animal bone is not acid activated.

[0073] (2) Preparation of in-situ phosphorus-calcium doped biochar: The mixture obtained in step (1) was placed in a tubular pyrolysis furnace and subjected to staged programmed temperature co-pyrolysis under a nitrogen atmosphere (flow rate of 200 mL / min). The pyrolysis program was set as follows: the temperature was increased from room temperature to 250℃ at a rate of 10℃ / min and held at this temperature for 60 minutes; then, the temperature was increased to 450℃ at a rate of 10℃ / min and held at this temperature for 90 minutes. The pyrolysis byproduct, wood vinegar, was collected. After cooling, in-situ phosphorus-calcium doped biochar was obtained, denoted as SCB-2.

[0074] (3) Fermentation and compounding: The subsequent fermentation and compounding steps are the same as step (3) in Example 1.

[0075] (4) Granulation and drying: The subsequent granulation and drying steps are the same as step (4) in Example 1. The final product is a multifunctional organic fertilizer, denoted as F-2.

[0076] Example 3

[0077] This embodiment provides a method for preparing a multifunctional organic fertilizer, which differs from Example 1 mainly in the adjustment of raw material ratios and pyrolysis parameters. The specific steps are as follows:

[0078] (1) Raw material pretreatment: Take 500.0 g of air-dried wheat straw and 100.0 g of animal bone. The acid activation treatment of the animal bone is the same as step (1) in Example 1. The wheat straw and the treated acid-activated bone are physically mixed evenly to obtain a mixture.

[0079] (2) Preparation of in-situ phosphorus-calcium doped biochar: The mixture obtained in step (1) was placed in a tubular pyrolysis furnace and subjected to staged programmed temperature co-pyrolysis under a nitrogen atmosphere (flow rate of 200 mL / min). The pyrolysis program was set as follows: the temperature was increased from room temperature to 200℃ at a heating rate of 8℃ / min and held at this temperature for 30 minutes; then, the temperature was increased to 550℃ at a heating rate of 20℃ / min and held at this temperature for 60 minutes. The pyrolysis byproduct, wood vinegar, was collected. After cooling, in-situ phosphorus-calcium doped biochar was obtained, denoted as SCB-3.

[0080] (3) Fermentation and compounding: The subsequent fermentation and compounding steps are the same as step (3) in Example 1.

[0081] (4) Granulation and drying: The subsequent granulation and drying steps are the same as step (4) in Example 1. The final product is a multifunctional organic fertilizer, denoted as F-3.

[0082] Comparative Examples 1-9:

[0083] Comparative Example 1:

[0084] Compared with Example 1, the difference is that in the preparation of in-situ phosphorus-calcium doped biochar in step (2), a staged programmed heating is not used. Instead, the mixture is directly heated to 500°C at a heating rate of 15°C / min and held at that temperature for 75 minutes. All other aspects are the same.

[0085] Comparative Example 2:

[0086] Compared with Example 1, the difference is that in step (2), 400.0 g of corn stalks were pyrolyzed separately (the pyrolysis procedure was the same as in Example 1) to obtain pure biochar. Then, the pure biochar was physically mixed with 100.0 g of acid-activated aggregate treated by the method in step (1) of Example 1 to replace the SCB-1 prepared in Example 1. All other aspects are the same.

[0087] Comparative Example 3:

[0088] Compared to Example 1, the difference lies in step (2): 400.0 g of corn stalks and 100.0 g of acid-activated aggregate treated by step (1) of Example 1 were placed in two separate pyrolysis furnaces, and both were subjected to the same staged programmed heating program as in Example 1. After pyrolysis, the resulting carbonized straw and pyrolyzed aggregate were mixed in proportion to replace the SCB-1 prepared in Example 1. Everything else remained the same.

[0089] Comparative Example 4:

[0090] The difference from Example 1 is that in step (2) of the preparation of in-situ phosphorus-calcium doped biochar, the target temperature for the second stage of heating is set to 350°C. All other aspects are the same.

[0091] Comparative Example 5:

[0092] The difference from Example 1 is that in step (2) of the preparation of in-situ phosphorus-calcium doped biochar, the target temperature for the second stage of heating is set to 700°C. All other aspects are the same.

[0093] Comparative Example 6:

[0094] Compared with Example 1, the difference is that in the preparation of in-situ phosphorus-calcium doped biochar in step (2), the pyrolysis procedure is reversed, that is, the temperature is first raised to 500°C at a rate of 20°C / min and held at that temperature for 75 minutes, and then cooled to 300°C and held at that temperature for 45 minutes. All other steps are the same.

[0095] Comparative Example 7:

[0096] Compared with Example 1, the difference is that in step (1), the ratio of raw materials is changed to 200.0 g of corn stalks and 200.0 g of animal bone. All other aspects are the same.

[0097] Comparative Example 8:

[0098] Compared with Example 1, the difference is that in step (4) granulation, an equal amount of 5% bentonite aqueous solution is used instead of wood vinegar as a binder. All other aspects are the same.

[0099] Comparative Example 9:

[0100] Compared with Example 1, the difference is that the fermentation and compounding process in step (3) is omitted. After step (2) is completed, the obtained SCB-1 biochar is directly physically mixed with the remaining dry powder raw materials (bacterial bran, wood ash, and inoculant) in step (3) of Example 1, and then directly enters the granulation and drying process in step (4). Everything else is the same.

[0101] Test Examples 1-5:

[0102] Test Example 1: Heavy Metal Curing Stability Test

[0103] The experimental steps are as follows:

[0104] (1) Preparation of contaminated soil: Take uncontaminated topsoil, air-dry it, and then sieve it through a 2mm sieve. Weigh 10kg of the sieved soil and spray it evenly in batches with a solution containing 250 mg / L Cd. 2+ Prepare a cadmium chloride solution and mix thoroughly to achieve a final Cd concentration of 50 mg / kg in the soil. Stabilize the prepared contaminated soil at room temperature for 15 days.

[0105] (2) Sample application: Prepare several polyvinyl chloride (PVC) leaching columns (10 cm inner diameter, 40 cm high) with outlets at the bottom. Lay a 2 cm thick layer of quartz sand and a layer of nylon filter screen at the bottom of each leaching column. Mix the contaminated soil prepared in step (1) with the fertilizers (F-1, F-2, F-3) prepared in Examples 1-3 and the samples prepared in Comparative Examples 1-7 at a ratio of 1% (w / w). Set up a blank control group (CK) without any sample application. Fill the leaching columns with the mixed soil from each group to a height of 30 cm, and control the filling density to be consistent.

[0106] (3) Leaching experiment: Prepare a simulated acid rain solution, which is a mixed solution of sulfuric acid and nitric acid with pH=4.5 (SO4). 2- NO3 -(Molar ratio 5:2). After the experiment began, 500 mL of simulated acid rain solution was slowly added to the top of each leaching column weekly to simulate a rainfall event.

[0107] (4) Sample Collection and Measurement: At the designated time points (days 1, 3, 7, 14, 21, and 30), all the eluent seeping from the bottom of the leaching column was collected, and its volume was recorded. The Cd concentration in the collected eluent was determined using inductively coupled plasma mass spectrometry (ICP-MS). 2+ The concentration was determined. Three replicates were set up for each treatment group.

[0108] Based on the above experimental procedures, the Cd concentration in the leachate of each treatment group was recorded at different time points. 2+ The concentrations are shown in Table 1.

[0109] Table 1: Cd content in leachate of each sample 2+ Record of concentration (mg / L) changes over time

[0110] Sample group Day 1 Day 3 Day 7 Day 14 Day 21 Day 30 Example 1 0.019 0.023 0.021 0.028 0.035 0.031 Example 2 0.025 0.028 0.033 0.039 0.041 0.038 Example 3 0.021 0.018 0.024 0.026 0.032 0.036 Comparative Example 1 0.153 0.188 0.201 0.245 0.231 0.217 Comparative Example 2 0.512 0.466 0.389 0.315 0.267 0.243 Comparative Example 3 0.136 0.151 0.169 0.198 0.205 0.188 Comparative Example 4 0.201 0.244 0.287 0.311 0.302 0.294 Comparative Example 5 0.098 0.115 0.107 0.126 0.134 0.119 Comparative Example 6 0.162 0.178 0.192 0.213 0.224 0.209 Comparative Example 7 0.354 0.321 0.298 0.251 0.233 0.215 Blank control 0.623 0.655 0.681 0.692 0.679 0.668

[0111] Figure 1 and Figure 2 The results of the heavy metal curing stability test are shown, such as Figure 1 As shown, the samples derived from Examples 1, 2, and 3 have Cd content in their leachate. 2+ The concentration data points are densely distributed within an extremely low numerical range, and the distribution pattern is narrow. Throughout the entire 30-day acidic leaching cycle, the Cd concentration in the leachate... 2+ The concentration remained consistently at extremely low levels. For example, in Example 1, the highest concentration measured was only 0.035 mg / L (day 21). In stark contrast, the initial leaching concentration of Comparative Example 2 (physical mixing) was as high as 0.512 mg / L, more than 14 times the highest concentration in Example 1. This difference in values ​​is significant. Figure 1 The results are visually presented in box plots: the boxes in Examples 1, 2, and 3 are compact and positioned very low, while the boxes in most comparative examples and the blank control are positioned significantly higher and wider. This result directly demonstrates that the material prepared by this technical solution has a fundamentally different ability to restrict heavy metal migration compared to materials prepared by conventional methods.

[0112] The root cause of this performance difference lies in the different material preparation processes. The staged programmed temperature co-pyrolysis process in this scheme induces in-situ solid-phase reactions in the raw materials under a specific temperature program, forming a stable chemically bonded structure. Data from the comparative examples provide direct evidence for this. As shown in Table 1, the leaching concentration of Comparative Example 1 (isothermal pyrolysis) consistently remained above 0.150 mg / L, and the concentration in Comparative Example 6 (reverse temperature program) also remained at a similarly high level. This demonstrates the necessity of the specific procedural sequence of "low-temperature activation followed by high-temperature bonding" in this scheme. The results of Comparative Example 4 (low-temperature pyrolysis) and Comparative Example 5 (high-temperature pyrolysis) define the temperature range necessary for the formation of this effective structure. When the pyrolysis temperature deviates from this range (e.g., the highest concentration in Comparative Example 4 reaches 0.311 mg / L), the material's ability to solidify heavy metals significantly decreases.

[0113] Figure 2 The long-term stability of the material was demonstrated from the perspective of cumulative release. By summing the data in Table 1, it can be calculated that at the end of the 30-day experimental period, the total cumulative leaching concentration in Example 1 was approximately 0.16 mg / L, while the cumulative total in the blank control group was as high as approximately 4.0 mg / L. This is... Figure 2 The curves for Examples 1, 2, and 3 show extremely low slopes and are consistently located at the bottom of the graph. The curves for Examples 1, 2, and 3 are close in position, indicating that within the process parameters of this technical solution, adjusting conditions such as straw type, raw material ratio, or pyrolysis temperature can yield products with consistent heavy metal solidification performance. Conversely, the curves for Comparative Example 2 (physical mixing) and Comparative Example 7 (excess aggregate) show a rapid increase in the initial stage of the experiment, with their cumulative leaching concentrations reaching approximately 2.2 mg / L and 1.6 mg / L, respectively. This reflects the rapid loss of the passivating agent physically adsorbed on their surfaces under acidic leaching. (See Table 1 and...) Figure 2 The data proves that the material prepared by this technical solution has long-term heavy metal solidification stability, and its effect is due to the intrinsic chemical bonding structure constructed by this specific preparation method, which can resist continuous acid leaching.

[0114] Test Example 2: Nutrient Slow-Release Performance Test

[0115] The experimental steps are as follows:

[0116] (1) Preparation of experimental setup: Take several chromatography columns (inner diameter 5 cm, height 30 cm) with water outlets at the bottom. Lay 2 cm thick quartz sand and a layer of nylon filter screen at the bottom of each chromatography column as support and filtration layer.

[0117] (2) Sample packing: Weigh 50.0 g of quartz sand that has been rinsed three times in deionized water and dried, and mix it thoroughly with 5.0 g of the sample to be tested (fertilizers prepared in Examples 1-3 and samples prepared in Comparative Examples 1-9). Pack this mixture into the corresponding chromatography column. Set up a blank control group filled only with quartz sand. Cover the top of the packed layer with a nylon filter to evenly disperse the eluent.

[0118] (3) Leaching experiment: At the beginning of the experiment (day 0), slowly add 100 mL of deionized water to the top of each chromatography column until saturated, and let stand for 24 hours. Starting from day 1, add 50 mL of deionized water to the top of each chromatography column daily for leaching.

[0119] (4) Sample collection and determination: At the set time points (day 1, day 3, day 7, day 14, day 21, and day 28), all the eluent from the bottom of the chromatography column was collected. The concentration of available phosphorus (as P2O5) in the eluent was determined using the molybdenum-antimony spectrophotometric method. Three replicates were set up for each treatment group.

[0120] Based on the above experimental steps, the concentration of available phosphorus in the leachate of each treatment group at different time points was recorded, as shown in Table 2.

[0121] Table 2: Record of changes in available phosphorus concentration (mg / L) in leachate of each sample over time

[0122] Sample group Day 1 Day 3 Day 7 Day 14 Day 21 Day 28 Example 1 15.8 16.1 14.5 12.9 11.5 10.8 Example 2 17.2 16.4 15.1 13.5 12.1 11.2 Example 3 16.5 15.7 14.9 13.1 11.8 10.5 Comparative Example 1 51.6 35.2 22.8 13.1 8.3 5.4 Comparative Example 2 102.3 58.1 21.5 11.2 7.9 6.1 Comparative Example 3 55.1 38.9 24 14.2 8.8 6 Comparative Example 4 68.3 41.1 23.9 12.4 7.8 5.1 Comparative Example 5 21.8 19.1 16.5 13.9 12 11.3 Comparative Example 6 50.2 33.7 21.5 12.9 8.1 5.8 Comparative Example 7 88.2 45.6 25.1 14.8 9.1 7.7 Comparative Example 8 91.5 51.3 19.8 10.1 7.2 5.9 Comparative Example 9 19.2 17.5 16.3 13.8 11.9 10.6 Blank control 0.3 0.2 0.1 0.3 0.2 0.1

[0123] Figure 3 The dynamics of effective phosphorus release for each sample over a 28-day period are presented in the form of a heatmap. For example... Figure 3 As shown, the rows corresponding to Examples 1, 2, and 3 exhibit a high color intensity and a gradual change over the entire time axis. Specifically, referring to the data in Table 2, the effective phosphorus concentration in Example 2 gradually decreased from 17.2 mg / L on day 1 to 11.2 mg / L on day 28, maintaining a concentration above 11 mg / L throughout the entire experimental period. This data indicates that the material prepared using this technical solution maintains a stable and effective phosphorus release rate over a relatively long period.

[0124] In contrast, most comparative samples were... Figure 3The materials exhibit a color distribution pattern of "dark initially, light later." This pattern indicates that the nutrient release behavior is characterized by an initial concentrated burst followed by rapid decay. For example, the data in Table 2 shows that Comparative Example 2 (physical mixing) had an initial release concentration as high as 102.3 mg / L, but rapidly decreased to 21.5 mg / L by day 7, far lower than the levels of the examples during the same period. Similarly, Comparative Example 7 (imbalanced raw material ratio) and Comparative Example 8 (conventional binder) also exhibited a similar pattern of rapid release followed by decay. This difference in nutrient release patterns directly proves that the intrinsic structure of the material prepared by this technical solution is fundamentally different from the products of conventional physical mixing or simple granulation.

[0125] The long-lasting, sustained-release nutrient performance stems from the material structure formed by the specific preparation process of this scheme. During the staged programmed temperature co-pyrolysis process, phosphorus species derived from animal bone react in situ with straw carbon precursors, forming a stable structure chemically bonded and physically coated by a carbon skeleton. This structure effectively limits the contact area and dissolution rate of phosphorus nutrients with water molecules. The results of the comparative series verified the specific process conditions required to form this structure. As shown in Table 2, Comparative Example 1 (isothermal pyrolysis), Comparative Example 4 (low-temperature pyrolysis), and Comparative Example 6 (reverse temperature program) failed to form an effective sustained-release structure, resulting in rapid nutrient loss. Although Comparative Example 5 (high-temperature pyrolysis) also exhibited some sustained-release characteristics, its overall release concentration (11.3 mg / L to 21.8 mg / L) was significantly lower than that of the examples, which is consistent with the mechanism that high temperature may cause some phosphorus species to undergo crystal transformation and reduce bioavailability. (See Table 2 and...) Figure 3 The data proves that the specific process combination of this technical solution is a necessary condition for obtaining the long-term stable release performance of the nutrients.

[0126] Test Example 3: Evaluation of the Comprehensive Effects of Potted Plants

[0127] The experimental steps are as follows:

[0128] (1) Experimental preparation: Take the contaminated soil with a Cd concentration of 50 mg / kg prepared in Test Example 1, divide it into several plastic flower pots, and fill each pot with 2.0 kg of soil.

[0129] (2) Sample application: The fertilizers prepared in Examples 1-3 and the samples prepared in Comparative Examples 1, 2, 8, and 9 were applied to the corresponding flowerpots at a ratio of 1% of the dry soil weight, and thoroughly mixed with the topsoil (0-15 cm). A blank control group (CK) was set up without any sample application. Each treatment group was replicated four times. All flowerpots were equilibrated in a greenhouse for one week.

[0130] (3) Planting: Select uniformly growing Chinese cabbage seedlings and transplant 3 seedlings into each pot. Carry out conventional cultivation in the greenhouse, control the light, temperature and humidity to be consistent, and irrigate regularly with deionized water to maintain the soil moisture content at 70% of field capacity.

[0131] (4) Sample collection and determination: After 45 days of cultivation, the aboveground parts of all plants were harvested. The samples were washed with deionized water, blanched at 105℃ for 30 minutes, and then dried at 70℃ to constant weight. The dry weight was measured to calculate the biomass per pot. The dried plant samples were pulverized and digested using a mixed acid of HNO3-HClO4. The concentration of Cd in the digestate was determined using inductively coupled plasma mass spectrometry (ICP-MS), and the Cd content of the aboveground parts of the plants was calculated.

[0132] Based on the above experimental steps, the aboveground dry weight and Cd content of plants in each treatment group were recorded, and typical data are shown in Table 3.

[0133] Table 3: Records of plant growth and Cd absorption in each treatment group

[0134] Sample group Aboveground dry weight (g / pot) Aboveground Cd content (mg / kg) Example 1 24.8 0.41 Example 2 25.5 0.38 Example 3 24.2 0.45 Comparative Example 1 18.1 1.88 Comparative Example 2 11.5 4.95 Comparative Example 8 21.6 0.53 Comparative Example 9 19.5 1.12 Blank control 13.2 4.21

[0135] Figure 4 The distribution of plant dry weight (X-axis) and Cd content in the plant (Y-axis) for each treatment group is shown in a two-dimensional scatter plot. As can be seen from the figure, the data points for all treatment groups (Examples 1, 2, and 3) are concentrated in the lower right corner of the plot. Specifically, referring to the data in Table 3, the plant dry weight of the Example 1 treatment group was 24.8 g / pot, while its aboveground Cd content was only 0.41 mg / kg. In contrast, the dry weight of the blank control group was only 13.2 g / pot, while its aboveground Cd content was as high as 4.21 mg / kg. This data comparison shows that the material prepared by this technical solution can significantly promote the accumulation of plant biomass while reducing the absorption of heavy metals by plants.

[0136] This dual effect stems from two independent yet synergistic functions of the material prepared in this study. First, the stable chemical bonding structure formed through a specific pyrolysis process effectively immobilizes Cd ions in the soil, significantly reducing their bioavailability, as verified in the leaching experiment of Test Example 1. This reduction in Cd bioavailability directly leads to a decrease in Cd absorption by plant roots, ultimately resulting in extremely low Cd content in the aboveground parts. Second, the phosphorus, potassium, and other nutrients encapsulated within the biochar framework can be released in a long-term, gradual manner (verified in Test Example 2), matching the nutrient requirements of plants throughout their growth cycle. This avoids stress caused by instantaneous nutrient loss or excessively high local concentrations, thereby promoting healthy plant growth and biomass accumulation.

[0137] The results of the comparative series further confirmed the effectiveness of the above mechanism. The data point for Comparative Example 2 (physical mixing) is located in the upper left corner of the graph (dry weight 11.5 g / pot, Cd content 4.95 mg / kg), and its effect is close to that of the blank control group. This proves that without the in-situ co-pyrolysis process described in this scheme, the products cannot effectively fix Cd, nor can nutrient slow release be achieved, leading to inhibited plant growth and the absorption of large amounts of Cd. The effect of Comparative Example 9 (unfermented) was also inferior to the examples; its plant dry weight (19.5 g / pot) and Cd content (1.12 mg / kg) were both between those of the examples and the blank control, indicating that the fermentation step plays an important role in activating functional strains, improving nutrient cycling efficiency, and synergistically fixing Cd. In summary... Figure 4 The data in Table 3 demonstrate that this technical solution, through its specific end-to-end process, integrates the two functions of stable solidification of heavy metals and long-term supply of plant nutrients.

[0138] Test Example 4: Physical Properties Test of Fertilizer Pellets

[0139] The experimental steps are as follows:

[0140] (1) Sample preparation: Select spherical fertilizer particles with a particle size between 3 and 5 mm prepared in Example 1 and Comparative Example 8 for later use.

[0141] (2) Radial compressive strength test: The radial compressive strength of individual fertilizer granules was tested using a texture analyzer. A single granule was placed on the test platform, and pressure was applied downwards at a speed of 1.0 mm / s through a planar probe until the granule broke. The maximum pressure (N) applied by the instrument when the granule broke was recorded. Twenty granules were randomly selected from each of the samples from Example 1 and Comparative Example 8 for repeated testing.

[0142] (3) Determination of water dispersion stability: Accurately weigh 10.0 g of sample particles and place them in a beaker containing 250 mL of deionized water. Let stand for 10 minutes. Then, carefully transfer the contents of the beaker onto a standard sieve with a pore size of 2.0 mm and rinse with a small amount of deionized water. Collect the undisintegrated intact particles on the sieve, dry them at 70 °C to constant weight, and weigh them. Water dispersion stability is expressed as the percentage (%) of the mass of particles on the sieve after drying relative to the initial sample mass. This test was repeated 5 times for each sample group.

[0143] Based on the above experimental steps, the radial compressive strength and water dispersion stability of each treatment group were recorded, as shown in Table 4.

[0144] Table 4: Test Data Record of Physical Properties of Sample Particles

[0145] Sample group Radial compressive strength (N) Water dispersion stability (%) Example 1 35.8，39.1，33.7，41.5，36.6，38.2，40.1，34.9，37.5，39.8 91.5，93.2，90.8，92.5，91.9 Comparative Example 8 15.2，21.8，11.5，18.9，9.8，23.1，14.6，17.2，12.5，19.9 35.6，41.2，29.8，38.1，33.5

[0146] Figure 5 The performance distribution of the two samples, Example 1 and Comparative Example 8, in terms of radial compressive strength and water dispersion stability is shown in the form of a box plot. Figure 5 As shown in Table 4, the median radial compressive strength of the sample in Example 1 is approximately 38 N, with a concentrated data distribution (small interquartile range), while the median value of the sample in Comparative Example 8 is approximately 16 N, with a dispersed data distribution (large interquartile range). Regarding water dispersion stability, the stability percentage of the sample in Example 1 is all above 90%, while the stability percentage of the sample in Comparative Example 8 is below 42%. This data indicates that the particles prepared using the wood vinegar solution generated by pyrolysis in this technical solution as a binder have significantly higher physical strength and structural stability than particles using conventional bentonite as a binder.

[0147] This difference in physical properties stems from the different mechanisms of interaction between the two adhesives and the matrix materials. In this solution, the wood vinegar produced during pyrolysis is rich in various active organic compounds such as phenols, aldehydes, and organic acids. During subsequent mixing and drying, these organic compounds undergo polymerization and cross-linking reactions within and between the particles, forming a continuous, rigid three-dimensional network structure. This structure chemically bonds the biochar matrix particles together, thereby endowing the final product with high compressive strength and excellent water resistance.

[0148] In contrast, the bentonite used in Comparative Example 8 is an inorganic mineral clay whose binding effect mainly relies on the physical filling and van der Waals forces between particles. When exposed to water, bentonite undergoes significant water absorption and swelling, disrupting the original physical bonding forces of the particles and causing rapid disintegration of the particle structure. This directly explains its extremely low water dispersion stability. Excellent physical properties are fundamental to ensuring the functionality of fertilizer products. The high strength and high stability of the particles in Example 1 ensure that they maintain structural integrity during transportation, storage, and application. This is a prerequisite for achieving the long-term solidification of heavy metals described in Test Example 1 and the sustained slow-release of nutrients described in Test Example 2.

[0149] Test Example 5: Evaluation of Soil Microbial Activity

[0150] The experimental steps are as follows:

[0151] (1) Experimental design: The soil used in this experiment was the soil after the pot experiment in Test Example 3. Soil that had been treated with the three treatment groups of Example 1, Comparative Example 9 (unfermented) and blank control (CK) was selected as the research object.

[0152] (2) Soil sampling: Topsoil samples from a depth of 0–15 cm were collected from each replicate pot in each treatment group using a multi-point mixing method. Plant roots and gravel were removed from the samples, which were then sieved through a 2 mm sieve and divided into two portions. One portion was stored at 4°C for determining microbial abundance and soil enzyme activity, while the other portion was air-dried for determining basic physicochemical properties.

[0153] (3) Microbial count determination: The dilution plate count method was used. 10.0 g of fresh soil was weighed and added to 90 mL of sterile water, then shaken on a shaker for 30 minutes. A series of dilutions were prepared and spread onto Pikovskaya's (phosphate-solubilizing bacteria), Ashby's (nitrogen-fixing bacteria), and beef extract peptone (total bacterial count) solid media, respectively. After incubation at a suitable temperature, colonies were counted, and the colony-forming units (CFU) per gram of dry soil were calculated.

[0154] (4) Soil enzyme activity determination: Spectrophotometry was used. Soil phosphatase activity was characterized by measuring the amount of phenol released after incubation of sodium phenyl phosphate substrate at 37°C for 24 hours. Soil urease activity was characterized by measuring the amount of ammonia nitrogen generated after incubation of urea substrate at 37°C for 24 hours. Soil dehydrogenase activity was characterized by measuring the amount of red triphenylmethanehydrazone (TPF) generated by the reduction of 2,3,5-triphenyltetrazolium chloride (TTC) substrate under anaerobic conditions.

[0155] Based on the above experimental steps, the number of soil microorganisms and enzyme activities of each treatment group were recorded, and typical data are shown in Table 5.

[0156] Table 5: Effects of different treatments on soil microorganisms and enzyme activity

[0157] Sample group Phosphorus solubilizing bacteria (10 5 CFU / g)]]> Azotobacter (10 4 CFU / g)]]> Total bacteria (10 7 CFU / g) Phosphatase activity (mg phenol / g / 24h) urease activity (mg NH4 + -N / g / 24h) Dehydrogenase activity (μg TPF / g / 24h) Example 1 25.8 15.1 8.8 3.15 1.82 45.7 Comparative Example 9 8.1 4.9 4.5 1.21 0.79 21.3 Blank control 3.2 1.8 3.1 0.88 0.51 15.6

[0158] Figure 6 Soil biological activity indicators for each treatment group in Table 5 were visualized in a multi-dimensional manner using radar charts. Each vertex in the chart represents an indicator, and the area and shape of the enclosed region visually reflect the overall impact of the treatment group on soil biological activity. Figure 6 It can be seen that the closed polygon formed by the treatment group of Example 1 has the largest area, and is significantly longer than the other two groups in all six index axes. Specifically, referring to the data in Table 5, the number of phosphate-solubilizing bacteria in the treatment group of Example 1 (25.8 × 10⁻⁶) 5 CFU / g) is relative to Example 9 (8.1×10⁻⁶). 5 The material's activity was more than three times that of control group 9 (CFU / g), and its phosphatase activity (3.15 mg phenol / g / 24h) was also significantly higher than that of control group 9 (1.21 mg phenol / g / 24h). These results indicate that the application of the material prepared in this formulation can systematically increase the number of functional microorganisms and the activity of key enzymes in the soil.

[0159] The effect stems from specific steps in the process flow of this solution. The preparation process in Example 1 included the activation and fermentation of functional microorganisms (such as Bacillus subtilis), while Comparative Example 9 lacked this step. The fermentation process not only led to a large proliferation of functional microorganisms, but their metabolic activities also generated various organic acids and active substances. When this product was applied to the soil, it directly introduced a high concentration of beneficial microorganisms. Furthermore, the porous structure of the biochar in the pyrolysis products provided these microorganisms with physical colonization space, while the organic matter in the fermentation products provided initial nutrients. The combination of these two factors jointly promoted the colonization and reproduction of the functional microbial community in the soil, leading to a systematic increase in the activity of downstream related soil enzymes.

[0160] In contrast, Comparative Example 9, lacking a fermentation process, could not introduce pre-proliferated functional microbial communities into the overlying culture medium. Its soil microbial improvement effect primarily relied on the physical aggregation of native microorganisms by the biochar itself; therefore, the improvement in various indicators was limited. Figure 6 The area represented is a polygon much smaller than that of Example 1. The blank control group represents the baseline biological activity level of the untreated soil. (See Table 5 and...) Figure 6 The results demonstrate that the combination of fermentation steps and pyrolysis products in this scheme is the direct reason for effectively regulating soil microbial flora and enzyme activity, thereby enhancing soil biological functions.

[0161] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a multifunctional organic fertilizer with straw carbonization coupled with heavy metal passivation, characterized in that, Includes the following steps: (1) Mix crop straw and animal bone at a weight ratio of 3:1 to 5:1 to obtain a mixture; (2) The mixture is subjected to staged programmed temperature co-pyrolysis under limited oxygen conditions to obtain in-situ phosphorus-calcium doped biochar, and the liquid byproduct wood vinegar produced during the pyrolysis process is collected; wherein, the staged programmed temperature co-pyrolysis includes: First stage: Heat the mixture to a temperature of 200-300°C and keep it at that temperature for 30-60 minutes; Second stage: After completing the first stage, the material is further heated to a temperature of 450-550℃ and kept at that temperature for 60-90 minutes; (3) The in-situ phosphorus and calcium doped biochar is mixed with plant-derived organic materials, compound functional microbial agents and wood ash, and the moisture content is adjusted before aerobic fermentation is carried out to obtain fermented materials. (4) The fermented material is granulated and dried to obtain the multifunctional organic fertilizer; wherein, in the granulation process, the wood vinegar collected in step (2) is used as a binder.

2. The preparation method according to claim 1, characterized in that, In step (1), before mixing, a pretreatment step of acid activation treatment of the animal bone material is also included.

3. The preparation method according to claim 1, characterized in that, The heating rate in the first stage is 5–10 °C / min, and the heating rate in the second stage is 10–20 °C / min.

4. The preparation method according to claim 1, characterized in that, In step (3), the conditions for aerobic fermentation are: the moisture content of the material is adjusted to 50-60%, and the fermentation time is 7-10 days.

5. The preparation method according to claim 1, characterized in that, In step (3), the dry weight ratio of each component in the mixing step is as follows: In-situ phosphorus and calcium doped biochar: 60-70%; Plant-derived organic materials: 15-25%; Compound functional microbial agent 5-10%; Wood ash 5-10%.

6. The preparation method according to claim 1 or 5, characterized in that, The compound functional microbial agent contains one or more of phosphate-solubilizing bacteria, nitrogen-fixing bacteria, and organic matter-decomposing bacteria.

7. The preparation method according to claim 6, characterized in that, The phosphate-solubilizing bacteria are Bacillus megaterium, the nitrogen-fixing bacteria are Azotobacter chrysogenum, and the organic matter-decomposing bacteria are Bacillus subtilis.

8. The preparation method according to claim 1 or 5, characterized in that, The plant-derived organic material is waste mushroom substrate and / or soybean meal.

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