Slow-release biochar-based compound fertilizer and production process thereof

Abstract

The application discloses a slow-release biochar-based compound fertilizer and a production process thereof, and belongs to the technical field of biochar-based fertilizers.The compound fertilizer comprises, in terms of weight parts, modified biochar 30-45 parts, inorganic nutrient raw material 50-65 parts, composite cross-linking binder 4-10 parts and chelated trace elements 1-3 parts;the production process comprises, in sequence, biochar micropore cleaning and activation, free radical oxidation modification, surface grafting polymerization, vacuum impregnation loading and cross-linking embedding granulation;the application increases oxygen-containing functional groups on the surface of biochar and constructs hierarchical pore structures through an oxalic acid-assisted catalytic free radical oxidation system and a maleic anhydride-itaconic acid double monomer grafting technology;through modification of biochar and in combination with a vacuum loading process, nutrients are locked inside the hierarchical pores, and then, a moisture-responsive binder coating is supplemented, so that double synergistic slow release of nutrients is realized, the problems of loose nutrient loading, easy loss and short slow-release period of existing carbon-based fertilizers are solved, and the application is especially suitable for saline-alkali soil improvement and dry farming.

Description

Technical Field

[0001] This invention relates to the field of biochar-based fertilizer technology, specifically to a slow-release biochar-based compound fertilizer and its production process. Background Technology

[0002] Biochar is a solid product obtained by pyrolyzing biomass raw materials, including agricultural and forestry waste, livestock and poultry manure, and kitchen waste, under high temperature and low oxygen conditions. It possesses a loose, porous, sponge-like structure and a stable aromatic ring carbon skeleton, making it an effective conditioner for acidic, compacted soils. Due to its well-developed pore structure, specific surface area, and adsorption capacity, biochar is often used as a carrier for inorganic fertilizers. Adding a certain amount of binder can produce slow-release, environmentally friendly biochar-based compound fertilizers.

[0003] However, existing biochar-based fertilizers still face the following drawbacks in practical applications. Current technologies mostly employ simple mechanical mixing of biochar and chemical fertilizers or atmospheric pressure impregnation. Because biochar often retains tar and has "dead pores" in its natural pores, nutrients can only remain on the surface or in the larger pores. This simple physical adhesion is extremely weak; under mechanical friction during fertilization or leaching by water after application, nutrients quickly fall off, making it difficult to achieve the desired loading effect. Traditional strong acid oxidation or simple high-temperature activation methods, while increasing a small number of oxygen-containing functional groups, often lead to the collapse of the biochar framework. Furthermore, the limited variety of functional groups (mostly randomly distributed hydroxyl groups) results in insufficient charge attraction for cations such as ammonium and potassium, failing to lock nutrients at the molecular level, leading to fertilizer loss upon water entering the biochar.

[0004] Therefore, there is an urgent need to develop a slow-release biochar-based compound fertilizer and its production process, which is of great significance for improving fertilizer utilization and promoting sustainable agricultural development. Summary of the Invention

[0005] Based on the above analysis, the present invention aims to provide a slow-release biochar-based compound fertilizer and its production process to solve the problems of insufficient loading and excessively rapid release of biochar-based fertilizers in the prior art.

[0006] The objective of this invention is mainly achieved through the following technical solutions: This invention provides a slow-release biochar-based compound fertilizer, comprising the following components by weight: 30-45 parts of modified biochar; 50-65 parts of inorganic nutrient raw materials; 4-10 parts of composite crosslinking adhesive; 1-3 parts of trace elements in chelated form; The modified biochar has a total surface oxygen-containing functional group content of 3.5-5.2 mmol / g, a molar ratio of carboxyl to hydroxyl groups of 1:1.2-1.8, and a hierarchical pore structure consisting of micropores of 2-10 nm and macropores of 50-100 nm.

[0007] Furthermore, the composite crosslinking binder is composed of modified starch, polyvinyl alcohol, and nano-bentonite in a mass ratio of 5:2:1; the modified starch is one of oxidized starch, esterified starch, or phosphate starch; the degree of alcoholysis of the polyvinyl alcohol is 88%-99%; the nano-bentonite is sodium-based bentonite modified with organic quaternary ammonium salt; when the binder comes into contact with soil moisture, the swelling rate is regulated by the hydrogen bond network formed by polyvinyl alcohol and modified starch, thereby achieving a water-triggered nutrient release response.

[0008] Furthermore, the inorganic nutrient raw material is selected from at least one of urea, monoammonium phosphate, diammonium phosphate, potassium nitrate, potassium sulfate, potassium chloride, and potassium dihydrogen phosphate; and the particle size of the inorganic nutrient raw material is 0.5-2.0 mm. The chelated trace elements include at least two of chelated zinc, chelated boron, chelated manganese, chelated iron, chelated copper, chelated molybdenum, and chelated magnesium; the chelating agent is selected from one or more of EDTA (ethylenediaminetetraacetic acid), citric acid, amino acids, or humic acid.

[0009] This invention also provides a production process for a slow-release biochar-based compound fertilizer, comprising the following steps: S1. Cleaning and activating the micropores of biochar: The biochar is ultrasonically treated in a dilute acid solution to clean its microporous structure, then washed until neutral and dried. S2, Free radical oxidation modification: The biochar treated in step S1 is added to the oxidation system to react and the activated biochar is catalytically oxidized to introduce oxygen-containing functional groups on its surface. S3. Surface grafting polymerization: Add modification liquid and initiator to the oxidized biochar obtained in step S2 and carry out reflux reaction. Under the action of initiator, organic acid monomers containing double bonds undergo graft copolymerization reaction on the surface of oxidized biochar to form a polymer brush layer with ion exchange function. S4. Vacuum impregnation loading: Inorganic nutrient raw materials are prepared into a mixed molten liquid, and the modified biochar in step S3 is added. The mixture is maintained under negative pressure for 15-25 minutes to allow the inorganic nutrient molten liquid to penetrate into the hierarchical channels of the biochar. Then, the pressure is restored to normal. S5. Cross-linking and embedding granulation: Using the composite cross-linking binder as described in claim 2, chelated trace elements are added to embed and granulate the nutrient-loaded biochar to form particles with semi-permeable membrane properties.

[0010] Further, in step S1, the biochar raw material is obtained by pyrolysis at 500-700℃ and pulverized to 100-150 mesh; the mass fraction of the dilute hydrochloric acid solution is 2%-5%; the ultrasonic treatment temperature is 40-50℃ and the treatment time is 30-50 min.

[0011] Further, in step S2, the oxidation system is composed of hydrogen peroxide, ferrous sulfate and oxalic acid, and the mass ratio of the three to biochar is 5-10:0.2-0.5:0.1-0.2:100; the reaction temperature is 55-65℃ and the reaction time is 2.5-4 hours.

[0012] Further, in step S3, the reflux reaction temperature is 95-110℃, and the reaction time is 3-5 hours; the modified solution is a mixed aqueous solution of maleic anhydride and itaconic acid, with a molar ratio of 1:0.5-1; the initiator is ammonium persulfate, and its addition amount is 0.8%-1.5% of the biochar mass.

[0013] Further, in step S4, the mixed melt is a eutectic or high-concentration suspension formed by urea melt and other inorganic salt raw materials, and the concentration of the mixed melt is 70%-85%; the vacuum degree of the negative pressure condition is -0.092MPa to -0.098MPa.

[0014] Furthermore, in step S5, the drying temperature during granulation is 75-85℃.

[0015] Furthermore, the present invention also provides an application of the above-mentioned slow-release biochar-based compound fertilizer in the preparation of special fertilizer for saline-alkali soil improvement or long-acting fertilizer for dryland agriculture.

[0016] This invention achieves topological reconstruction of biochar through acid washing and ultrasonication in step S1 and free radical catalytic oxidation in step S2. The dilute hydrochloric acid ultrasonic treatment removes ash (minerals) and viscous tar remaining in the pores of the biochar during pyrolysis, fully opening the original 50-100 nm macropores (transport channels). In step S2, the catalytic system formed by oxalic acid chelating iron ions induces a heterogeneous free radical reaction on the biochar surface. The free radicals selectively exfoliate the carbon skeleton, etching numerous 2-10 nm micropores on the macropore walls. This structure enables rapid introduction of molten nutrients through the macropores, while the micropores deeply adsorb and lock in nutrients. The macropores act as a buffer zone for external moisture ingress and nutrient overflow, while the micropores, through their extremely high surface energy and chemical sites, tightly lock in nutrients, forming the first physical slow-release barrier.

[0017] During the drying process in step S5, the composite crosslinking binder (modified starch / PVA / nano-bentonite) forms a three-dimensional network structure. High-density intermolecular hydrogen bonds are formed between the long-chain hydroxyl groups of polyvinyl alcohol (PVA) and the phosphate groups / hydroxyl groups of modified starch (such as phosphate starch). The organically modified nano-bentonite is embedded in the hydrogen bond network in a layered form, acting as physical crosslinking points, which enhances the mechanical strength of the coating. When the fertilizer enters the soil and comes into contact with water, the intrusion of water molecules gradually weakens the hydrogen bonds between PVA and starch. Due to the binding effect of nano-bentonite, the coating does not dissolve immediately, but rather undergoes controlled swelling. The thickness of the gel layer in this swollen state and the crosslinking density together determine the nutrient diffusion rate, thereby achieving water-triggered inductive release.

[0018] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: (1) This invention constructs a hierarchical pore structure with micropores for deep nutrient storage and macropores for buffering transport by modifying biochar in stages. Combined with vacuum impregnation process, the molten nutrient liquid is forced into the depth of the micropores, forming the first physical slow-release barrier. At the same time, the polymer brush layer grafted on the surface chemically binds the nutrients through ion exchange. Finally, the outer composite cross-linked binder coating forms a water-controlled semi-permeable membrane, constituting the second release barrier. This makes the release of fertilizer in still water slow and can match the needs of crop growth. In addition, since the nutrients are deeply fixed inside the biochar and released in a controlled manner, the loss of nutrients such as nitrogen and potassium caused by rainfall runoff and deep leaching is greatly reduced.

[0019] (2) The modified biochar used in this invention has abundant oxygen-containing functional groups and a stable pore structure. It can improve soil aggregate structure, enhance water and fertilizer retention capacity, and regulate pH in soil that exists for a long time. In particular, the composite crosslinking binder can sense changes in soil moisture and intelligently adjust the swelling state through the hydrogen bond network of polyvinyl alcohol and modified starch. This enables water-responsive fertilizer supply that is slow to release when water is scarce and promotes release when water is available. It is especially suitable for dryland agricultural areas with large water fluctuations.

[0020] In this invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and some advantages may become apparent from the description or be learned by practicing the invention. The objectives and other advantages of this invention can be realized and obtained through the specific details pointed out in the embodiments of the description. Attached Figure Description

[0021] Figure 1 This is a process flow diagram of Embodiment 1 of the present invention. Detailed Implementation

[0022] The present application will now be described in further detail with reference to the accompanying drawings. It should be noted that the specific embodiments described below are only for further explanation of the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above-described content. Experimental methods in the embodiments that do not specify specific conditions are generally carried out under conventional conditions or according to the manufacturer's recommendations. Unless otherwise specified, all reagents and instruments used are products that can be obtained through conventional commercial channels.

[0023] Main ingredients: Biochar: Prepared by pyrolysis of rice straw and pine sawdust at 500℃, 600℃ and 700℃ respectively under limited oxygen conditions, and then pulverized for later use.

[0024] Chemical reagents: hydrogen peroxide (30%), ferrous sulfate, oxalic acid, maleic anhydride, itaconic acid, ammonium persulfate, urea, monoammonium phosphate, potassium dihydrogen phosphate, potassium chloride, etc., all of which are analytical grade.

[0025] Binder raw materials: oxidized starch (hydroxypropyl oxidized starch), polyvinyl alcohol (PVA1788, degree of hydrolysis 98%), and organically modified nano-bentonite (modified with hexadecyltrimethylammonium bromide).

[0026] Trace elements: EDTA-Zn, EDTA-Fe, borax, ammonium molybdate, etc.

[0027] Performance testing methods: Biochar structure characterization: Specific surface area and pore size distribution were determined using a nitrogen adsorption-desorption apparatus (BET method); the total amount and proportion of oxygen-containing functional groups (carboxyl groups, hydroxyl groups, etc.) on the surface were determined using the Boehm titration method.

[0028] Nutrient slow-release performance: In accordance with the national standard for slow-release fertilizers (GB / T 23348-2009), fertilizer samples were accurately weighed into nylon mesh bags and immersed in deionized water at a constant temperature of 25℃. Samples were taken on days 1, 3, 7, 14, 21, and 28 to determine the nitrogen (or phosphorus, potassium) content in the aqueous solution and calculate the cumulative release rate.

[0029] Particle strength: Using a particle strength tester, 20 finished fertilizer particles were randomly selected and their average pressure (N) during crushing was measured.

[0030] Nutrient loading: Calculated by measuring the difference in total nitrogen, phosphorus and potassium elements before and after loading of modified biochar.

[0031] Pot experiment: A 60-day pot experiment was conducted using corn seedlings. A blank control, a conventional compound fertilizer control, and the fertilizer treatment group of this invention were set up. The dry weight of the plants, plant height, and apparent utilization rate of nitrogen fertilizer were measured. Example

[0032] S1: Biochar micropore cleaning and activation Take the above biochar, crush it and pass it through a 120-mesh sieve; weigh 100g of biochar powder and place it in a 2L beaker, add 1500mL of 3.5% hydrochloric acid solution; place the beaker in a 45℃ constant temperature water bath and simultaneously perform ultrasonic treatment (power 250W, frequency 40kHz), mechanical stirring speed 200rpm, and continue treatment for 40 minutes; after the reaction is completed, filter it using a vacuum filtration device, wash it with deionized water 4-5 times until the pH of the filtrate is neutral (pH≈7.0); transfer the filter cake to an oven and dry it at 105℃ to constant weight to obtain pretreated biochar A1.

[0033] S2: Free radical oxidation modification All of the pretreated biochar A1 was transferred to a 2000mL three-necked flask, and 1000mL of deionized water was added. The mixture was stirred to form a suspension. 7.5g of hydrogen peroxide (30%), 0.35g of ferrous sulfate and 0.15g of oxalic acid were added to the system in sequence. The mixture was heated to 60℃ in a water bath and stirred continuously at this temperature for 3 hours at a stirring speed of 300 rpm. After the reaction was completed, the mixture was filtered, washed and dried to obtain oxidized biochar B1.

[0034] According to Boehm titration, the total amount of oxygen-containing functional groups on its surface is 4.2 mmol / g, and the molar ratio of carboxyl groups to hydroxyl groups is approximately 1:1.5.

[0035] S3: Surface grafting polymerization Oxidized biochar B1 was resuspended in 800 mL of deionized water and transferred to a three-necked flask equipped with a reflux condenser. 10.0 g of maleic anhydride and 7.4 g of itaconic acid (molar ratio 1:0.75) were added. The mixture was heated to 100 °C in a water bath and reflux was initiated. 20 mL of an aqueous solution containing 1.2 g of ammonium persulfate (1.2% of the biochar mass) was slowly added dropwise using a constant pressure dropping funnel over approximately 30 minutes. After the addition was complete, the mixture was refluxed at 100 °C for 4 hours. After cooling, the mixture was filtered and washed alternately with hot water and ethanol to remove homopolymers and unreacted monomers. After drying, grafted modified biochar C1 was obtained.

[0036] S4: Vacuum impregnation load Weigh out a total of 200g of urea, potassium dihydrogen phosphate, and potassium sulfate. The mixture (in a ratio of 18-12-15) was heated and melted in an oil bath at 130°C to form a homogeneous molten liquid with a concentration of approximately 82%. Grafted modified biochar C1 (equivalent to 60 parts) was quickly added to the molten liquid and mechanically stirred to ensure uniform mixing. The mixture was then quickly transferred to a vacuum drying oven, and after closing the oven door, a vacuum was rapidly drawn to -0.095 MPa and maintained at this vacuum level for 20 minutes. The pressure relief valve was then instantly opened to restore atmospheric pressure, and the nutrients were forced into the graded channels using the instantaneous pressure difference. The mixture was then cooled to room temperature to obtain a dark black, uniformly textured biochar-based material D1 loaded with nutrients.

[0037] BET testing showed that the nutrient-loaded biochar substrate D1 still clearly exhibited a hierarchical structure consisting of micropores of ~5nm and macropores of ~70nm.

[0038] S5: Cross-linking and embedding granulation Adhesive preparation: Mix oxidized starch, polyvinyl alcohol (PVA1788), and organic nano-bentonite in a mass ratio of 5:2:1, add an appropriate amount of water, and stir at 90°C to prepare a uniform colloid with a solid content of about 12%.

[0039] Mixed granulation: Weigh D1 (100 parts), commercially available granular monoammonium phosphate (particle size about 1.5 mm, equivalent to 40 parts), and chelated trace elements (EDTA-Zn and EDTA-Fe, a total of 2 parts) and place them in a disc granulator; during the rolling process, evenly spray the above-mentioned composite binder colloid (a total of 8 parts); by controlling the disc angle, rotation speed and spraying rate, form wet granules with a particle size of 2-4 mm; transfer the wet granules to a fluidized bed dryer and dry them under hot air at 80℃ until the moisture content is less than 3%, to obtain the final finished fertilizer E1.

[0040] Test results: The finished fertilizer E1 was tested and found to have a 24-hour cumulative nutrient release rate of 8.2%, a 28-day cumulative release rate of 64.5%, a granular compressive strength of 31.2N, and a nutrient loading of 582 mg / g. Example

[0041] The difference from Example 1 is as follows: S2 free radical oxidation modification: The amount of hydrogen peroxide was taken as the lower limit, and the mass ratio of the three to biochar was adjusted to 5:0.2:0.1:100. 5g of hydrogen peroxide (30%), 0.2g of ferrous sulfate and 0.1g of oxalic acid were added. The reaction temperature was 55℃ and the reaction time was 4 hours. The total amount of B2 functional groups in the resulting oxidized biochar was 3.7 mmol / g, and the ratio of carboxyl to hydroxyl groups was ≈1:1.2.

[0042] S3 surface grafting polymerization: the molar ratio of maleic anhydride to itaconic acid is 1:0.5, the amount of initiator is 0.8%, the reaction temperature is 95℃, and the reaction time is 5 hours.

[0043] S4 vacuum impregnation load: melt concentration 70%, vacuum degree -0.092 MPa, maintained for 25 minutes.

[0044] S5 cross-linking and embedding granulation: drying temperature 75℃.

[0045] Test results: The 24-hour release rate of the finished product E2 was 12.5%, the 28-day release rate was 72.3%, the particle compressive strength was 26.8 N, and the nutrient loading was 515 mg / g. The results show that even when the lower limit of the parameters is used, the product's sustained-release performance still meets the requirements. Example

[0046] The difference from Example 1 is as follows: Raw material: 700℃ pyrolysis of pine wood chips biochar.

[0047] S2 free radical oxidation modification: The amount of hydrogen peroxide was taken to the maximum limit, and the mass ratio of the three to biochar was adjusted to 10:0.5:0.2:100. 10g of hydrogen peroxide (30%), 0.5g of ferrous sulfate and 0.2g of oxalic acid were added. The reaction temperature was 65℃ and the reaction time was 2.5 hours. The total amount of B3 functional groups in the resulting oxidized biochar was as high as 5.1 mmol / g, and the ratio of carboxyl to hydroxyl groups was ≈1:1.8.

[0048] S3 surface grafting polymerization: maleic anhydride to itaconic acid molar ratio of 1:1, initiator dosage of 1.5%, reaction temperature of 110℃, reaction time of 3 hours.

[0049] S4 vacuum impregnation load: melt concentration 85%, vacuum degree -0.098 MPa, maintained for 15 minutes.

[0050] S5 cross-linking and embedding granulation: drying temperature 85℃.

[0051] Test results: The 24-hour release rate of the finished product E3 was 9.4%, the 28-day release rate was 68.1%, the particle compressive strength was 29.5 N, and the nutrient loading was 594 mg / g. The results show that high-parameter combinations and different raw materials can also produce high-quality products, demonstrating the universality of the process.

[0052] Comparative Example 1 Steps S2 and S3 are omitted, and the biochar A1 that has only been acid-washed and activated in S1 is directly used for vacuum loading in S4 and granulation in S5. The other steps are the same as in Example 1.

[0053] Results: Due to the lack of abundant oxygen-containing functional groups and polymer brush layers, biochar had weak nutrient adsorption; the 24-hour release rate of the finished product was as high as 48.6%, and almost complete release (99.2%) was achieved after 28 days, with no sustained-release effect; the nutrient loading (224 mg / g) and strength (15.4 N) were also the lowest.

[0054] Comparative Example 2 In step S2, only hydrogen peroxide and ferrous sulfate are used, and oxalic acid is not added. The other steps are the same as in Example 1.

[0055] Results: The oxidation reaction was uneven, with some biochar surface over-oxidized; the functional group ratio of the obtained product was unbalanced (carboxyl:hydroxyl ≈ 1: 0.8), and the sustained-release performance was significantly reduced (24h: 25.4%, 28d: 88.6%), indicating that oxalic acid is crucial for regulating the functional group ratio.

[0056] Comparative Example 3 In step S3, only maleic anhydride is used, and itaconic acid is not used; the other steps are the same as in Example 1.

[0057] Results: The resulting polymer brush layer had a simple structure with insufficient steric hindrance and ion exchange sites; although the sustained-release performance of the product (24h: 18.5%, 28d: 81.2%) was better than that of Comparative Examples 1 and 2, it was significantly worse than that of Example 1 with dual monomer grafting, proving that dual monomer copolymerization produced synergistic effects.

[0058] Comparative Example 4 In step S4, stirring and impregnation are carried out at atmospheric pressure (0 MPa) for 20 minutes, and the other steps are the same as in Example 1.

[0059] Results: The molten nutrient solution could not effectively penetrate the micropores (2-10 nm) of biochar, adhering only to the surface and macropores. This resulted in an extremely low nutrient loading (295 mg / g) and, due to the nutrients being exposed on the surface, their release was extremely rapid (24h: 22.8%, 28d: 84.3%). This indicates that the pressure difference provided by the vacuum was the key to driving nutrients into the deeper layers of the hierarchical pores.

[0060] Based on the test results of Examples 1-3 and Comparative Examples 1-4, the following table 1 is obtained.

[0061] Based on Table 1 above, as well as the various embodiments and comparative examples, the following conclusions can be drawn: Comparing Example 1 and Comparative Example 1, it can be seen that the nutrient loading of the unmodified biochar (Comparative Example 1) is only 224 mg / g, and it basically has no slow-release capacity (the release rate is nearly 50% in 24 hours). This proves that steps S2 and S3 of the present invention are not simple surface treatments, but rather that by constructing a high-density oxygen-containing functional group and polymer brush layer on the surface of the biochar, physical adsorption is improved to the level of chemical chelation. The nitrogen fertilizer utilization rate of Example 1 is as high as 52.3%, which is nearly double that of Comparative Example 1, demonstrating the potential of the present invention in improving fertilizer utilization.

[0062] Comparing Example 1 and Comparative Example 2, it can be seen that the addition of oxalic acid resulted in a more harmonious ratio of surface functional groups (1:1.5) and a significant improvement in sustained-release performance. This is because the complexation system formed by oxalic acid and iron ions regulates the generation rate of hydroxyl radicals, preventing excessive carbonization of the biochar surface, thereby precisely controlling the ratio of carboxyl groups to hydroxyl groups, making it more conducive to subsequent grafting. In Comparative Example 2, after omitting oxalic acid, the molar ratio of carboxyl groups to hydroxyl groups became unbalanced (1:0.8), leading to a significant decrease in sustained-release performance. This indicates that oxalic acid can slow down the disordered attack of free radicals on the carbon skeleton and induce the generation of a specific ratio of oxygen-containing functional groups. Experiments have shown that when the ratio of carboxyl groups to hydroxyl groups is controlled within the range of 1:1.2-1.8, the biochar has the strongest binding force on inorganic ions and the most stable sustained-release curve.

[0063] Comparative Example 3, using only a single monomer, showed a significantly higher nutrient release rate than the example grafted with two monomers. This is because the copolymerization of maleic anhydride and itaconic acid formed a polymer brush layer with greater steric hindrance, which significantly enhanced the charge attraction and physical retention capacity for cations such as ammonium and potassium. The results of Comparative Example 3 (single monomer grafting) and Comparative Example 4 (atmospheric pressure impregnation) indicate that single modification or conventional loading cannot achieve the technical level of this invention. The copolymerization of maleic anhydride and itaconic acid not only increased the charge density but also slowed down the nutrient dissolution rate through steric hindrance. The instantaneous pressure difference provided by vacuum impregnation (S4) is key to overcoming the capillary resistance of micropores. BET testing confirmed that nutrients were forced into deep micropores of about 5 nm. This internally sealed structure is the basis for achieving 28-day long-term controlled release.

[0064] Compared with Comparative Example 1 and Comparative Example 4, under the same modification conditions, the nutrient loading under atmospheric pressure load is only 295 mg / g. Moreover, since the nutrients only remain on the surface, they are released very quickly. A vacuum degree of -0.09 MPa or higher is the key to driving the molten liquid into the 2-10 nm micropores.

[0065] Examples 2 and 3 respectively verified the production under the lower limit of parameters and different biochar raw materials (pine wood chips charcoal). The results showed that even under lower process parameters, the 24h and 28d release rates still met the requirements. This indicates that the production process described in this invention has extremely high stability and can adapt to various biomass sources and different industrial production fluctuations.

[0066] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A slow-release biochar-based compound fertilizer, characterized in that, By weight, it includes the following components: 30-45 parts of modified biochar; 50-65 parts of inorganic nutrient raw materials; 4-10 parts of composite crosslinking adhesive; 1-3 parts of trace elements in chelated form; The modified biochar has a total surface oxygen-containing functional group content of 3.5-5.2 mmol / g, a molar ratio of carboxyl to hydroxyl groups of 1:1.2-1.8, and a hierarchical pore structure consisting of micropores of 2-10 nm and macropores of 50-100 nm.

2. The slow-release biochar-based compound fertilizer according to claim 1, characterized in that, The composite crosslinking binder is composed of modified starch, polyvinyl alcohol, and nano-bentonite in a mass ratio of 5:2:1; the modified starch is one of oxidized starch, esterified starch, or phosphate starch; the degree of alcoholysis of the polyvinyl alcohol is 88%-99%; and the nano-bentonite is sodium-based bentonite modified with organic quaternary ammonium salt.

3. The slow-release biochar-based compound fertilizer according to claim 1 or 2, characterized in that, The inorganic nutrient raw material is selected from at least one of urea, monoammonium phosphate, diammonium phosphate, potassium nitrate, potassium sulfate, potassium chloride, and potassium dihydrogen phosphate; and the particle size of the inorganic nutrient raw material is 0.5-2.0 mm. The chelated trace elements include at least two of chelated zinc, chelated boron, chelated manganese, chelated iron, chelated copper, chelated molybdenum, and chelated magnesium; the chelating agent is selected from one or more of ethylenediaminetetraacetic acid, citric acid, amino acids, or humic acid.

4. A production process for preparing the slow-release biochar-based compound fertilizer as described in any one of claims 1-3, characterized in that, Includes the following steps: S1. Cleaning and activating the micropores of biochar: The biochar is ultrasonically treated in a dilute acid solution to clean its microporous structure, then washed until neutral and dried. S2, Free radical oxidation modification: The biochar treated in step S1 is added to the oxidation system to react and the activated biochar is catalytically oxidized to introduce oxygen-containing functional groups on its surface. S3. Surface grafting polymerization: Add modification liquid and initiator to the oxidized biochar obtained in step S2 and carry out reflux reaction. Under the action of initiator, organic acid monomers containing double bonds undergo graft copolymerization reaction on the surface of oxidized biochar to form a polymer brush layer with ion exchange function. S4. Vacuum impregnation loading: Inorganic nutrient raw materials are prepared into a mixed molten liquid, and the modified biochar in step S3 is added. The mixture is maintained under negative pressure for 15-25 minutes to allow the inorganic nutrient molten liquid to penetrate into the hierarchical channels of the biochar. Then, the pressure is restored to normal. S5. Cross-linking and embedding granulation: Using the composite cross-linking binder as described in claim 2, chelated trace elements are added to embed and granulate the nutrient-loaded biochar to form particles with semi-permeable membrane properties.

5. The production process according to claim 4, characterized in that, In step S1, the biochar raw material is obtained by pyrolysis at 500-700℃ and pulverized to 100-150 mesh; the mass fraction of the dilute hydrochloric acid solution is 2%-5%; the ultrasonic treatment temperature is 40-50℃ and the treatment time is 30-50 min.

6. The production process according to claim 4, characterized in that, In step S2, the oxidation system consists of hydrogen peroxide, ferrous sulfate and oxalic acid, with the mass ratio of the three to biochar being 5-10:0.2-0.5:0.1-0.2:100; the reaction temperature is 55-65℃, and the reaction time is 2.5-4 hours.

7. The production process according to claim 4, characterized in that, In step S3, the reflux reaction temperature is 95-110℃ and the reaction time is 3-5 hours; the modified solution is a mixed aqueous solution of maleic anhydride and itaconic acid, with a molar ratio of 1:0.5-1; the initiator is ammonium persulfate, and its addition amount is 0.8%-1.5% of the biochar mass.

8. The production process according to claim 4, characterized in that, In step S4, the mixed melt is a eutectic or high-concentration suspension formed by urea melt and other inorganic salt raw materials, and the concentration of the mixed melt is 70%-85%; the vacuum degree of the negative pressure condition is -0.092MPa to -0.098MPa.

9. The production process according to claim 4, characterized in that, In step S5, the drying temperature during granulation is 75-85℃.

10. The application of the slow-release biochar-based compound fertilizer according to any one of claims 1-3 in the preparation of special fertilizer for improving saline-alkali soil or long-acting fertilizer for dryland agriculture.

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