Biochar-based slow-release fertilizer and preparation method thereof

Abstract

The invention relates to the technical field of fertilizers, and discloses a charcoal-based slow-release fertilizer and a preparation method thereof. The biochar-based slow-release fertilizer comprises a fertilizer core and a coating layer formed by in-situ reaction of biochar and a binder on the surface of the fertilizer core. The unique microstructure of the biochar-based slow-release fertilizer can stably control the initial release rate of nutrients to be 15% or below, the slow-release performance is excellent, the fertilization frequency can be effectively reduced, the fertilizer utilization rate is increased, and therefore agricultural non-point source pollution is reduced; the biochar-based slow-release fertilizer also has a soil improvement function, and the biochar coating layer can permanently improve the soil structure, enhance the water and fertilizer retention capability and passivate heavy metals, so that the combination of fertilization and land cultivation is realized, and the ecological benefit is high; the preparation method creatively utilizes the in-situ reaction of the biochar and the binder under the conditions of room temperature and no solvent for coating, only simple mixing is needed, the process is extremely simple, the energy consumption is extremely low, no wastewater or waste residue is discharged, and the preparation method is a whole-process green and environment-friendly production technology.

Description

Technical Field

[0001] This invention relates to the field of fertilizer technology, and in particular to a biochar-based slow-release fertilizer and its preparation method. Background Technology

[0002] Biochar is a carbon-rich material produced by the pyrolysis of biomass under anaerobic conditions. Its raw materials are widely available, including crop straw, livestock manure, sawdust, urban sludge, and food processing residues. Pyrolysis is the most common method for preparing biochar, and it can be classified into slow pyrolysis, fast pyrolysis, and gasification pyrolysis based on the heating rate and temperature. Biochar possesses a porous structure, large specific surface area, and abundant surface functional groups, which give it broad application prospects in environmental remediation, energy storage, and agricultural improvement. In the environmental field, biochar can effectively adsorb heavy metals and organic pollutants in soil and water; in the energy field, it can be used as an electrode material or fuel additive; in industrial applications, it can be used as a catalyst carrier or metallurgical impurity adsorbent; and in agricultural applications, biochar can not only improve soil structure and enhance water and fertilizer retention capacity, but also serve as a microbial carrier to enhance soil ecological functions.

[0003] Biochar itself contains nitrogen, phosphorus, potassium, and various trace elements. Applying it to the soil can effectively replenish nutrients, and it is especially important for improving infertile soils. For example, in the Amazon River basin, local residents bury the carbonized residues from biomass combustion into the soil to form "black soil," which significantly improves soil fertility and organic matter content. As modern agriculture increasingly demands sustainability, biochar's role as an "ecological regulator" is becoming more prominent, and its application has evolved from direct application to compound use with chemical fertilizers and organic fertilizers.

[0004] In recent years, researchers have begun to explore the use of biochar as a carrier material for slow-release fertilizers. Existing technologies show that biochar-based slow-release fertilizers can be prepared through various processes, but most suffer from problems such as complex processes, stringent conditions, high costs, and poor environmental compatibility. For example, CN 108314591 A discloses a biochar-based slow-release fertilizer and its preparation method. This method first uses potassium hydroxide to pyrolyze and activate biochar, followed by a hydrothermal reaction at 130-180℃. After the reaction, multiple steps such as washing and drying are required. This process is not only energy-intensive and requires strict reaction temperatures, but also easily generates a large amount of waste liquid, increasing subsequent treatment costs and environmental burden. CN 120398615 A ​​discloses a method for preparing lanthanum oxide-modified biochar fertilizer. This process involves multiple steps such as microwave radiation activation, alkaline solution impregnation, and complex synthesis. It not only has high raw material costs and expensive modifiers, but also a long production cycle, high energy consumption, and generates alkaline waste liquid, making it difficult to meet the requirements of green production.

[0005] Therefore, there is an urgent need to develop a simple, low-cost, environmentally friendly, and large-scale biochar-based slow-release fertilizer preparation method to fully realize the potential of biochar in sustainable agricultural development. Summary of the Invention

[0006] The present invention aims to at least solve one of the aforementioned technical problems existing in the prior art. Therefore, one objective of the present invention is to provide a biochar-based slow-release fertilizer.

[0007] The second objective of this invention is to provide a method for preparing this biochar-based slow-release fertilizer.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A first aspect of the present invention provides a biochar-based slow-release fertilizer, comprising a fertilizer core and a coating layer formed by the in-situ reaction of biochar and a binder on the surface of the fertilizer core.

[0009] In some embodiments of the present invention, the fertilizer core is granular with a particle size of 2-5 mm; the thickness of the coating layer is 300-500 μm.

[0010] In some embodiments of the present invention, the biochar-based slow-release fertilizer comprises, by weight percentage, the following raw materials: 55%-60% fertilizer core, 20%-30% biochar, and 12%-20% binder.

[0011] In some preferred embodiments of the present invention, the biochar-based slow-release fertilizer comprises the following raw materials by mass percentage: 55%-58% fertilizer core, 25%-28% biochar, and 16%-18% binder.

[0012] In some embodiments of the present invention, the fertilizer core is selected from at least one of urea, diammonium phosphate, potassium sulfate, ammonium carbonate, ammonium sulfate, potassium chloride, potassium nitrate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium nitrate phosphate compound fertilizer.

[0013] In some preferred embodiments of the present invention, the fertilizer core is urea.

[0014] In some embodiments of the present invention, the raw materials for preparing the biochar are selected from at least one of livestock and poultry manure sludge and agricultural and forestry waste.

[0015] In some embodiments of the present invention, the adhesive comprises a mixture of waste oil and polyisocyanate; the mass ratio of the waste oil and polyisocyanate is (1-5):1.

[0016] In some preferred embodiments of the present invention, the mass ratio of the waste oil to the polyisocyanate is (1-3):1.

[0017] In some embodiments of the present invention, the polyisocyanate is selected from at least one of diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl polyisocyanate.

[0018] In some embodiments of the present invention, the waste oil is selected from at least one of waste animal oil and waste vegetable oil.

[0019] In some preferred embodiments of the present invention, the waste animal oil is selected from at least one of waste lard, tallow, mutton fat, and fish oil.

[0020] In some preferred embodiments of the present invention, the waste vegetable oil is selected from at least one of waste peanut oil, rapeseed oil, soybean oil, castor oil, flaxseed oil, rice bran oil, corn oil, olive oil, and tung oil.

[0021] A second aspect of the present invention provides a method for preparing the biochar-based slow-release fertilizer described in the first aspect of the present invention, comprising the following steps: After mixing the fertilizer core with a portion of the binder, add a portion of biochar and mix well to form a single-layer coating on the outer surface of the fertilizer core; repeat the above steps of adding binder and biochar to obtain the biochar-based slow-release fertilizer.

[0022] In some embodiments of the present invention, the repetition is performed more than 5 times.

[0023] In some preferred embodiments of the present invention, the repetition is performed 6-7 times.

[0024] In some embodiments of the present invention, the amount of the adhesive used in a single application is 5wt%-20wt% of the total amount of adhesive; and the amount of biochar used in a single application is 10wt%-25wt% of the total amount of biochar.

[0025] In some preferred embodiments of the present invention, the amount of the adhesive used in a single application is 7wt%-19wt% of the total amount of adhesive; and the amount of biochar used in a single application is 11wt%-20wt% of the total amount of biochar.

[0026] The basic principles of this invention are explained as follows: The biochar-based slow-release fertilizer provided by this invention forms a coating layer through an in-situ reaction of biochar and a binder on the surface of the fertilizer core. 1) Biochar originates from the pyrolysis of livestock and poultry manure sludge or agricultural and forestry waste. It has a rich porous structure and a huge specific surface area. When it is wrapped around the fertilizer core, these micropores form a tortuous path and physical barrier for the outward diffusion of nutrients (such as nitrogen), effectively delaying the dissolution of nutrients. When livestock and poultry manure sludge is used as biochar raw material, biochar itself also contains a certain amount of phosphorus, potassium and trace elements. While slowly releasing the main nutrients in the fertilizer core, it can also slowly release its own nutrients and improve soil structure. The surface of biochar is also rich in active oxygen-containing functional groups such as hydroxyl and carboxyl groups. These groups can not only serve as adsorption sites, but also react chemically with the isocyanate groups in the binder. 2) The binder is obtained by mixing waste oil and polyisocyanate. The waste oil serves as a source of polyols or carboxylic acids, and its long molecular chains provide flexibility and continuity for the final coating layer, preventing the film from becoming brittle. The polyisocyanate serves as an active crosslinking agent, and the -NCO groups on its molecules are the core of the reaction. Part of it is used to react with -OH / -COOH in the oil to form a polyurethane prepolymer (i.e., the binder itself), and the other part is used to react with biochar. 3) In the preparation process of biochar-based slow-release fertilizer, a certain amount of binder is first added to the fertilizer core and mixed evenly to uniformly wet the surface of the fertilizer core. Immediately after the binder layer is wetted, a certain amount of biochar is added and mixed evenly. At this time, the active groups (such as hydroxyl groups) on the surface of the biochar undergo an in-situ chemical reaction with the unreacted -NCO groups in the binder, forming a single-layer coating on the surface of the fertilizer core. The steps of "adding binder - mixing - adding biochar - mixing" are repeated more than 5 times. Each newly added binder not only binds the new biochar, but also further crosslinks with the remaining groups in the previously reacted biochar film, thus layering the coating and gradually increasing the thickness of the coating, making the structure more compact. After multiple cycles, a coating layer with a specific thickness and strength is formed on the outside of the fertilizer core, which is composed of biochar and binder through chemical bonding.

[0027] Compared with the prior art, the beneficial effects of the present invention are: 1) The biochar-based slow-release fertilizer provided by this invention has a unique microstructure that can stably control the initial nutrient release rate to below 15%, exhibiting excellent slow-release performance. This effectively reduces the number of fertilization applications, improves fertilizer utilization, and thus reduces agricultural non-point source pollution. The biochar-based slow-release fertilizer also has soil improvement functions. The biochar coating layer can persistently improve soil structure, enhance water and fertilizer retention capacity, and passivate heavy metals, achieving a combination of fertilization and soil improvement, resulting in high ecological benefits. 2) The preparation method of biochar-based slow-release fertilizer provided by this invention abandons the complex pretreatment and high-temperature and high-pressure reaction conditions such as activation and modification required by traditional biochar fertilizers. It creatively utilizes the in-situ reaction of biochar and binder at room temperature and under solvent-free conditions for coating. Only simple mixing is required, the process is extremely simple, energy consumption is extremely low, and there is no wastewater or waste residue discharge. It is a green and environmentally friendly production technology throughout the entire process. At the same time, the core raw materials such as livestock and poultry manure biochar residue and waste oil are all derived from cheap and readily available waste, realizing the treatment of waste with waste, significantly reducing production costs, and making it easy to scale up for large-scale production. Attached Figure Description

[0028] Figure 1 The infrared characterization results are for the waste soybean oil, polymethylene polyphenyl polyisocyanate, binder, biochar and coating layer in Example 1; Figure 2 This is a scanning electron microscope image of the biochar-based slow-release fertilizer prepared in Example 1. Detailed Implementation

[0029] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments and comparative examples are all available from conventional commercial sources or can be obtained by existing technical methods. Unless otherwise specified, the test or experimental methods are conventional methods in the art.

[0030] Example 1 This embodiment prepares a biochar-based slow-release fertilizer, and the steps are as follows: Waste soybean oil and polymethylene polyphenyl polyisocyanate (PM200) were mixed evenly at a mass ratio of 3:1 to obtain an adhesive; Livestock and poultry manure residue is converted into biochar using conventional pyrolysis methods; Using urea with a particle size of 2-5 mm as the fertilizer core, 40 g of urea was placed in a 250 mL Erlenmeyer flask, and 1.9835 g of binder was added. The flask was hand-shaken to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 2.6380 g of biochar was added while shaking. After the addition was complete, shaking was continued for about 8 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 5 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 1. Table 1. Amounts of binder and biochar added each time in Example 1.

[0031] Infrared scanning of waste soybean oil, polymethylene polyphenyl polyisocyanate, binder, biochar, and coating layer was performed using the KBr smear method at a resolution of 4 cm⁻¹.-1 The cumulative number of scans is 8. Figure 1 The infrared characterization results of waste soybean oil, polymethylene polyphenyl polyisocyanate, binder, biochar, and coating layer in Example 1 are provided by... Figure 1 It can be seen that the waste soybean oil is at 3008cm. -1 2924cm -1 and 2855cm -1 A carbon chain CH stretching vibration peak is present at 1739 cm⁻¹, with a relatively broad peak shape. -1 The presence of a C=O stretching vibration peak at this location indicates the presence of carboxylic acids in fatty acids, suggesting the abundance of polycarboxylic acid groups in waste soybean oil. The stretching vibration of the -NCO group in polymethylene polyphenyl polyisocyanate (PM200) produces a peak at 2279 cm⁻¹. -1 The cumulative double bond absorption peak is at 3687 cm⁻¹. -1 and 3399cm -1 The absorption peaks appearing at this location are all -OH stretching vibrations; after being mixed with waste soybean oil to make an adhesive, PM200 shows an absorption peak at 2279 cm⁻¹. -1 The absorption peak at 3687 cm⁻¹ decreased significantly. -1 and 3399cm -1 The disappearance of the absorption peak at 1603 cm⁻¹ proves that the -NCO group participated in the reaction. The binder showed an absorption peak at 1603 cm⁻¹. -1 The presence of an absorption peak for -NH groups at 3702 cm⁻¹ indicates that urethane esters were produced during the synthesis of the polyurethane adhesive; biochar showed an absorption peak at 3702 cm⁻¹. -1 The absorption peak appearing at 1630 cm⁻¹ belongs to the -OH stretching vibration absorption peak. -1 The absorption peak appearing at 3345 cm⁻¹ is the C=C stretching vibration absorption peak. After the coating layer is formed by in-situ reaction with the adhesive, the stretching vibration of the -NCO groups on the adhesive completely disappears, and the coating layer spectrum shows a peak at 3345 cm⁻¹. -1 and 3205cm -1 The absence of two NH stretching vibration peaks and the disappearance of the -OH stretching vibration peak on the biochar indicates that the free -NCO groups in the binder react completely with the -OH groups on the biochar. Infrared characterization results show that, in the process of forming the final coating layer through layering, each newly added binder not only binds the new biochar but also further crosslinks with the remaining groups in the previously reacted biochar membrane. This layering process gradually increases the coating thickness and strength, resulting in a denser structure. The covalent bonds formed by the chemical reaction between the biochar and the binder enhance membrane stability and improve the slow-release effect of the fertilizer.

[0032] Scanning electron microscopy (SEM) was used to test the biochar-based slow-release fertilizer. The fertilizer was cut into semi-circles using a scalpel, placed in a vacuum coating machine for gold sputtering, and then observed and photographed under a scanning electron microscope at different magnifications. The thickness was measured to observe the microstructure of the biochar-based slow-release fertilizer. Figure 2 The image shown is a scanning electron microscope (SEM) image of the biochar-based slow-release fertilizer prepared in Example 1. Figure 2 Images (a), (b), (c), (d), (e), and (f) in the image are scanning electron microscope (SEM) images at different magnifications. Figure 2 It is known that the coating layer of biochar-based slow-release fertilizer is 300-500μm thick, and there are a few pores between the layers of coating, indicating that biochar and binder have participated in some reactions and adhered to the outside of urea. These pores facilitate the slow entry of water molecules into the membrane, allowing nutrients to be released outward. Biochar has a rich porous structure. After forming a coating layer with the binder, the pores of biochar gradually decrease. There are small protrusions and depressions on the outer surface of the coating layer, which may be caused by uneven shaking during granulation. However, the coating layer of biochar-based slow-release fertilizer is basically intact.

[0033] Example 2 This embodiment prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 2.3307g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 3.1359g of biochar was added while shaking. After adding the binder, the shaking was continued for about 10 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 5 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 2. Table 2. Amounts of binder and biochar added each time in Example 2.

[0034] Example 3 This embodiment prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: Utilizing the principle of an industrial disc granulator, 2 kg of urea and 100 g of binder were added to a self-designed adjustable-speed open coating machine. The rotation speed was appropriately increased, and after the binder was evenly mixed on the urea surface, 132 g of biochar was added and mixed for approximately 10 minutes to form the first coating layer. This process of adding binder and biochar was repeated five times to gradually increase the coating thickness until a coating layer was formed, resulting in a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 3. Table 3. Amounts of binder and biochar added each time in Example 3.

[0035] Comparative Example 1 This comparative example prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 1.7000g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 2.6698g of biochar was added while shaking. After the addition was complete, shaking was continued for about 10 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 4 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 4. Table 4. Amounts of binder and biochar added in each instance in Comparative Example 1

[0036] Comparative Example 2 This comparative example prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 2.1451g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 3.4802g of biochar was added while shaking. After the addition was complete, shaking was continued for about 10 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 4 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 5. Table 5. Amounts of binder and biochar added in each instance in Comparative Example 2

[0037] Comparative Example 3 This comparative example prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 2.7161g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 4.8616g of biochar was added while shaking. After the addition was complete, shaking was continued for about 10 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 4 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 6. Table 6. Amounts of binder and biochar added in each instance in Comparative Example 3

[0038] Comparative Example 4 This comparative example prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 2.0658g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 4.1763g of biochar was added while shaking. After the addition was complete, shaking was continued for about 10 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 4 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 7. Table 7. Amounts of binder and biochar added in each instance in Comparative Example 4

[0039] Comparative Example 5 This comparative example prepares a biochar-based slow-release fertilizer. The fertilizer core, binder, and biochar are the same as in Example 1, with the amounts of urea 40g, binder 5.77g, and biochar 20g, respectively. The preparation steps are as follows: 40g of urea was placed in a 250mL Erlenmeyer flask, and 0.9617g of binder was added. The flask was shaken by hand to allow the urea particles to move in a circular motion at the bottom of the flask. After the binder was evenly distributed on the surface of the urea, 5g of biochar was added while shaking. After adding the binder, shaking was continued for about 8 minutes to form the first coating layer. The above steps of adding binder and biochar were repeated 5 times to gradually increase the coating thickness until a coating layer was formed, thus obtaining a biochar-based slow-release fertilizer. The amounts of binder and biochar added each time are shown in Table 8. Table 8. Amounts of binder and biochar added in each instance in Comparative Example 5

[0040] Referring to HG / T 4215-2011 Controlled-Release Fertilizers, the initial nutrient release rate of the biochar-based slow-release fertilizers prepared in Examples 1-3 and Comparative Examples 1-5 was tested, specifically as follows: 1) Dissolve 2.00 g of p-dimethylaminobenzaldehyde colorimetric reagent in 100 mL of anhydrous ethanol, add 10 mL of concentrated hydrochloric acid, shake well, and let stand for 10 min. Take 5 colorimetric tubes and prepare 0, 2, 4, 6, 8, 10, and 12 mL of 0.5 g / L urea standard solution of known concentration. Add 10 mL of colorimetric reagent to each colorimetric tube, and dilute the 7 colorimetric tubes to 25 mL with distilled water. After standing for 20 min, measure the absorbance value at 430 nm using a UV-Vis spectrophotometer. Plot a standard curve with concentration as the x-axis and absorbance as the y-axis. The regression equation of the curve is A = 3.9857C + 0.0004, R 2 =0.9999, where C is the urea concentration (g / L), A is the absorbance, and R represents the correlation coefficient. The initial nutrient release rate of the coated fertilizer is calculated using this standard curve equation.

[0041] 2) Mix biochar-based slow-release fertilizer with water at a mass ratio of 1:20, let stand for 24 hours, and use the p-dimethylaminobenzaldehyde colorimetric spectrophotometric method to test the absorbance value in the still water at a wavelength of 430 nm. Substitute the absorbance value into the pre-established standard curve equation to calculate the urea concentration in the still water. Then, calculate the initial nutrient release rate according to the formula: Initial nutrient release rate = (m1 / m0) × 100%, where m1 is the mass of nitrogen released after 24 hours, m1 = urea concentration in still water × volume, and m0 = mass of biochar-based slow-release fertilizer × total nitrogen content in biochar-based slow-release fertilizer, where total nitrogen content = urea content (mass fraction) in biochar-based slow-release fertilizer × theoretical nitrogen content of urea.

[0042] Table 9 Initial nutrient release rates of biochar-based slow-release fertilizers in Examples 1-3 and Comparative Examples 1-5

[0043] Table 9 shows the initial nutrient release rates of biochar-based slow-release fertilizers in Examples 1-3 and Comparative Examples 1-5. As can be seen from Table 9, in Examples 1-3, by adding binder and biochar in six separate additions, the coating layers formed each time are stacked to form a coating layer. Regardless of the slight adjustments in the amount of biochar and binder, the release rate remains consistently at an excellent level of 12%-13%. In Example 3, under scale-up conditions, a six-layer coating was used, and the amount of raw materials was controlled. The final measured release rate was similar to that of Example 1, indicating that the preparation method is stable and reliable and has the potential for industrial-scale production. In Comparative Examples 1-4, the binder and biochar were added in five separate additions. Even when the amount of raw materials was within the range provided by this invention, the release rate was consistently higher than 24%, and the slow-release effect significantly deteriorated. This indicates that a five-layer coating cannot form a complete and dense coating layer, and is either defective or too thin. A six-layer coating is the critical point for achieving effective slow release (release rate <15%). In Comparative Example 5, the system failed due to insufficient binder (8.77 wt%), which prevented proper coating. This indicates that sufficient binder is a necessary condition for forming a continuous and robust coating network. In Comparative Example 3, the binder content was the highest (18.72 wt%), but the release rate was the highest due to insufficient layers. This suggests that increasing the binder content further when the number of layers is insufficient may cause uneven coating or internal stress, which is not conducive to sustained release.

Claims

1. A biochar-based slow-release fertilizer, characterized in that, It includes the fertilizer core and a coating layer formed by the in-situ reaction of biochar and binder on the surface of the fertilizer core.

2. The biochar-based slow-release fertilizer according to claim 1, characterized in that, The fertilizer core is granular with a particle size of 2-5 mm; the coating layer has a thickness of 300-500 μm.

3. The biochar-based slow-release fertilizer according to claim 1 or 2, characterized in that, The biochar-based slow-release fertilizer comprises the following raw materials by weight percentage: 55%-60% fertilizer core, 20%-30% biochar, and 12%-20% binder.

4. The biochar-based slow-release fertilizer according to claim 3, characterized in that, The fertilizer core is selected from at least one of urea, diammonium phosphate, potassium sulfate, ammonium carbonate, ammonium sulfate, potassium chloride, potassium nitrate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium nitrate phosphate compound fertilizer.

5. The biochar-based slow-release fertilizer according to claim 3, characterized in that, The raw materials for preparing the biochar are selected from at least one of livestock and poultry manure sludge and agricultural and forestry waste.

6. The biochar-based slow-release fertilizer according to claim 3, characterized in that, The adhesive is prepared by mixing waste oil and polyisocyanate; the mass ratio of the waste oil and polyisocyanate is (1-5):

1.

7. The biochar-based slow-release fertilizer according to claim 6, characterized in that, The polyisocyanate is selected from at least one of diphenylmethane-4,4'-diisocyanate and polymethylene polyphenyl polyisocyanate.

8. The method for preparing the biochar-based slow-release fertilizer according to any one of claims 1-7, characterized in that, Includes the following steps: After mixing the fertilizer core with a portion of the binder, add a portion of biochar and mix well to form a single-layer coating on the outer surface of the fertilizer core; repeat the above steps of adding binder and biochar to obtain the biochar-based slow-release fertilizer.

9. The preparation method according to claim 8, characterized in that, The repetition is repeated more than 5 times.

10. The preparation method according to claim 9, characterized in that, The amount of the binder used in a single application is 5wt%-20wt% of the total binder amount; the amount of the biochar used in a single application is 10wt%-25wt% of the total biochar amount.

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