A biochar-based coated selenium slow-release fertilizer and preparation method thereof

Abstract

The invention discloses a biochar-based coated selenium slow-release fertilizer and a preparation method thereof, and belongs to the technical field of fertilizers. The fertilizer comprises a core layer and a coating layer coated on the surface of the core layer, and the raw materials of the core layer comprise the following components by mass: 70-80 parts of urea, 60-70 parts of monoammonium phosphate, 50-60 parts of potassium chloride, 30-40 parts of superphosphate, 2-3 parts of sodium silicate, 2-4 parts of magnesium sulfate, 8-15 parts of nano-selenium, and 4-6 parts of tourmaline powder; the raw materials of the coating layer comprise the following components: 120-150 parts of modified biochar powder, and 20-30 parts of modified starch / polyvinyl alcohol composite. The fertilizer has good slow-release effect, high utilization rate of selenium element, good fertilizer efficiency, can effectively improve the yield and quality of crops, and has a simple preparation process, low cost, and no pollution.

Description

Technical Field

[0001] The invention relates to the technical field of fertilizers, and in particular to a biochar-based coated selenium slow-release fertilizer and a preparation method thereof. Background Art

[0002] Selenium is an essential trace element for humans and animals. In recent years, its various biological functions have been gradually discovered by scientists and have attracted people's attention. Selenium is directly involved in the synthesis and metabolism of nucleic acids and enzymes. Selenium can reduce the incidence of cardiovascular and cerebrovascular diseases, slow down aging, and enhance immunity.

[0003] However, selenium is not naturally present in food, and current selenium-enriched products are all produced by applying selenium-enriched fertilizers. Ordinary selenium-enriched fertilizers have a short release period, low fertilizer efficiency, and low absorption and utilization rate of selenium, while slow-release selenium-enriched fertilizers use a special method to process fertilizers, and the effective ingredients are slowly released after application, which prolongs the fertilizer release period, reduces the adsorption or fixation of effective selenium in the soil, and thus improves the utilization rate of selenium. Therefore, selenium slow-release fertilizers are more popular. However, current selenium-enriched slow-release fertilizers still have the problems of limited application, low utilization rate of selenium, unsatisfactory slow-release effect, complex manufacturing process and high cost, and the coating materials of existing selenium slow-release fertilizers mostly use chemically synthesized non-degradable polymers, and these difficult-to-degrade polymer coating materials are very easy to remain in agricultural soils, causing soil degradation and serious pollution, resulting in the phenomenon that selenium-enriched slow-release fertilizers are difficult to effectively promote and utilize. Therefore, it is very necessary to develop a selenium slow-release fertilizer with good slow-release effect, high utilization rate of selenium, simple manufacturing process, and environmentally friendly. Summary of the invention

[0004] The purpose of the present invention is to provide a biochar-based coated selenium slow-release fertilizer and a preparation method thereof, so as to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention: a biochar-based coated selenium slow-release fertilizer, comprising a core layer and a coating layer coated on the surface of the core layer, wherein the raw materials of the core layer comprise the following components by mass: 70-80 parts of urea, 60-70 parts of monoammonium phosphate, 50-60 parts of potassium chloride, 30-40 parts of heavy superphosphate, 2-3 parts of sodium silicate, 2-4 parts of magnesium sulfate, 8-15 parts of nano-selenium, and 4-6 parts of tourmaline powder; the raw materials of the coating layer comprise the following components: 120-150 parts of modified biochar powder and 20-30 parts of modified starch / polyvinyl alcohol composite.

[0007] Furthermore, the modified starch / polyvinyl alcohol complex is used as a binder for the coating layer.

[0008] Furthermore, the preparation method of the modified biochar powder is: crushing peanut shells, first heating and soaking them in a mixed aqueous solution of NaOH and Na2SO3, then heating and soaking them in a H2O2 solution, filtering, drying, and then high-temperature carbonizing and grinding to obtain the modified biochar powder.

[0009] Furthermore, the peanut shells are crushed and passed through a 40-mesh sieve.

[0010] Furthermore, the concentration of NaOH in the mixed aqueous solution of NaOH and Na2SO3 is 1.2-1.4 mol / L, the concentration of Na2SO3 is 0.2-0.4 mol / L, and the concentration of H2O2 solution is 2-4wt.%.

[0011] Furthermore, the temperature of heating and soaking in the mixed aqueous solution of NaOH and Na2SO3 is 100°C and the time is 6-8h.

[0012] Furthermore, the pH of the H2O2 solution is adjusted to 10 with aqueous ammonia before heating and soaking in the H2O2 solution.

[0013] Furthermore, the temperature of heating and soaking in the H2O2 solution is 70-80°C and the time is 2-4 hours.

[0014] Furthermore, the high temperature carbonization temperature is 500-700°C, the time is 4-6h, and the heating rate is 5-10°C / min.

[0015] Furthermore, the high-temperature carbonization is carried out under a nitrogen protective atmosphere.

[0016] Furthermore, the product is carbonized at high temperature, ground and passed through an 80-mesh sieve.

[0017] Furthermore, the preparation method of the modified starch / polyvinyl alcohol complex is: starch is mixed with NaOH solution, heated for reaction, then polyvinyl alcohol is added, kept warm for 1-2 hours, cooled to 50-60°C, pH is adjusted to 5-6, borax, glycerol and nano-silica are added, reacted for 2-4 hours, and the modified starch / polyvinyl alcohol complex is obtained.

[0018] Furthermore, the heating reaction is carried out at a temperature of 90-100°C and for a time of 20-30 minutes.

[0019] Furthermore, the solid-liquid ratio of the starch to the NaOH solution is 1g:20-40mL.

[0020] Furthermore, the concentration of the NaOH solution is 0.02 g / mL.

[0021] Furthermore, the insulation temperature after adding polyvinyl alcohol is the same as the temperature of the heating reaction of starch and NaOH solution.

[0022] Furthermore, the particle size of the tourmaline powder is 600 mesh.

[0023] Furthermore, the mass ratio of starch to polyvinyl alcohol is 1:0.5-2, the amount of borax is 1-3% of the total mass of starch and polyvinyl alcohol, the amount of glycerol is 10-30% of the total mass of starch and polyvinyl alcohol, and the amount of nano-silicon dioxide is 0.5-2% of the total mass of starch and polyvinyl alcohol.

[0024] The second technical solution of the present invention is a method for preparing the above-mentioned biochar-based coated selenium slow-release fertilizer, comprising the following steps:

[0025] (1) mixing the component raw materials of the core layer uniformly and granulating them to obtain core layer particles;

[0026] (2) Preheating the core layer particles to 60-70° C., spraying the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles, and simultaneously sprinkling the modified biochar powder, rotating for 5-20 minutes to form a coating layer, and drying to obtain the biochar-based coated selenium slow-release fertilizer.

[0027] Furthermore, before mixing the component raw materials of the core layer, urea, monoammonium phosphate, potassium chloride, double superphosphate, sodium silicate and magnesium sulfate are first ground through an 80-mesh sieve and then mixed with nano-selenium and tourmaline powder.

[0028] Furthermore, the modified starch / polyvinyl alcohol complex is sprayed onto the surface of the core layer particles, and the modified biochar powder is sprinkled in and rotated for 5-20 minutes. The process of forming the coating layer is carried out in multiple times, each time 10-20% of the total mass of the modified starch / polyvinyl alcohol complex is sprayed, 10-20% of the total mass of the modified biochar powder is sprinkled in, and the mixture is rotated for 5-20 minutes. After all the coating layer raw materials are coated, the mixture is cooled to room temperature, the coated fertilizer is taken out, and dried to a moisture content of ≤5%.

[0029] The present invention discloses the following technical effects:

[0030] (1) The present invention rationally uses nitrogen fertilizer, potassium fertilizer, phosphorus fertilizer and medium and trace elements (heavy superphosphate, sodium silicate, magnesium sulfate) to prepare the core layer, which provides comprehensive nutrition for the growth of crops; uses nano-selenium as a selenium source to provide selenium elements for crops, which is easier for plants to absorb than inorganic selenium sources and has no toxicity; uses tourmaline powder as an absorption promoter, which has unique infrared radiation characteristics and can release a large amount of negative ions, which can effectively enhance the plant's ability to absorb and utilize nutrients, especially selenium, and enhance the utilization effect of selenium fertilizer; uses modified biochar powder with a large specific surface area as the main material of the coating layer, which can make more Fertilizer nutrients, especially selenium, are adsorbed in the membrane structure to achieve a better slow-release effect, and the modified biochar powder has a large porosity, which can not only supplement the organic matter content of the soil, but also effectively preserve water and nutrients. Long-term use can improve the physical and chemical properties of the soil, increase soil fertility, reduce the amount of fertilizer, ensure sustainable agricultural development, and improve the environment; using a modified starch / polyvinyl alcohol complex as a binder for the coating layer can improve the hydrophobicity of the coating layer, inhibit the rapid degradation of the coating layer in the soil, and improve the protective effect and slow-release effect of the coating layer on the core layer, and the modified starch / polyvinyl alcohol complex is biodegradable and pollution-free to the environment. The biochar-based coated selenium controlled-release fertilizer of the present invention has a good slow-release effect, a high utilization rate of selenium, and a good fertilizer efficiency, can effectively increase crop yield and quality, and has a simple preparation process, low cost, and no pollution.

[0031] (2) The present invention uses peanut shells as raw materials, and sequentially heats and soaks them in NaOH / Na2SO3 solution and H2O2 solution, and then carbonizes them at high temperature to obtain modified biochar powder. Heating and soaking them in NaOH / Na2SO3 solution and H2O2 solution can remove lignin and hemicellulose in peanut shells, making their structure loose, and greatly improving their specific surface area and micropore volume. The larger the specific surface area and micropore volume, the more complex the pore structure of biochar, the stronger the adsorption capacity, and the better the adsorption and slow release effect of nutrients.

[0032] (3) In the preparation of the modified starch / polyvinyl alcohol composite, boric acid is used as a cross-linking agent and glycerol is used as a plasticizer, which can reduce the water absorption of the starch / polyvinyl alcohol composite after film formation. Adding nano-silicon dioxide for modification can further improve the mechanical properties and hydrophobicity of the starch / polyvinyl alcohol composite after film formation, and effectively inhibit its rapid degradation in the soil. DETAILED DESCRIPTION

[0033] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0034] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0035] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0036] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present application description and examples are exemplary only.

[0037] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0038] The experimental methods described in the following examples and comparative examples are conventional methods unless otherwise specified; the reagents and raw materials are commercially available unless otherwise specified; the parts are parts by mass; the particle size of the tourmaline powder used is 600 mesh.

[0039] The preparation method of the modified biochar powder used in the following examples and comparative examples is as follows:

[0040] Peanut shells were crushed through a 40-mesh sieve and then mixed with 1.2 mol / L NaOH and 0.2 mol / L The mixture was heated and soaked in a mixed aqueous solution of Na2SO3 at a temperature of 100°C for 6h (the solid-liquid ratio of peanut shell to the mixed solution was 1g:10mL), thoroughly washed with deionized water for 3 times, and then placed in a 3wt.% H2O2 solution with ammonia water adjusted to pH 10 (the solid-liquid ratio of peanut shell to the mixed solution was 1g:10mL, and the peanut shell mentioned here refers to the crushed peanut shell), soaked at 80°C for 2h, thoroughly washed with deionized water for 3 times, filtered, and dried at 80°C for 2h; the dried product was then placed in a porcelain boat, placed in a tubular atmosphere furnace, sealed, evacuated, the air in the tubular atmosphere furnace was discharged, and protective gas nitrogen was introduced at a flow rate of 100sccm. The heating program was set as follows: heating to 700°C at a heating rate of 5°C / min, thermally pyrolyzed for 4h, cooled naturally to room temperature, the sample was taken out, ground through an 80-mesh sieve, and modified biochar powder was obtained. The specific surface area of ​​the modified biochar powder is 221.31 m 2 / g, micropore volume is 0.23cm 3 / g.

[0041] The preparation method of the modified starch / polyvinyl alcohol composite used in the following examples and comparative examples is as follows:

[0042] Weigh 4 parts of starch and 6 parts of polyvinyl alcohol, mix the starch with 0.02g / mL NaOH solution (solid-liquid ratio 1g:20mL), heat at 100°C for 30min to gelatinize the starch. Then add polyvinyl alcohol, keep at 100°C for 1h, cool to 60°C, adjust the pH to 6, then add 0.1 parts of borax (cross-linking agent), 3 parts of glycerol (plasticizer) and 0.15 parts of nano-silicon dioxide (modifier) ​​in sequence, cross-link at 60°C for 2h to obtain a modified starch / polyvinyl alcohol composite.

[0043] Example 1

[0044] The raw material composition of biochar-based coated selenium slow-release fertilizer is as follows: core layer raw materials: 70 parts of urea, 60 parts of monoammonium phosphate, 50 parts of potassium chloride, 30 parts of double superphosphate, 2 parts of sodium silicate, 2 parts of magnesium sulfate, 8 parts of nano-selenium, 4 parts of tourmaline powder; coating layer raw materials: 120 parts of modified biochar powder, 20 parts of modified starch / polyvinyl alcohol composite (all parts by mass).

[0045] The preparation steps are as follows:

[0046] (1) First, urea, monoammonium phosphate, potassium chloride, double superphosphate, sodium silicate and magnesium sulfate in the raw materials of the core layer components are ground through an 80-mesh sieve, and then the sieved raw materials are mixed with nano-selenium and tourmaline powder and then put into a rotary drum granulator and steam is introduced for rolling humidification granulation. The granulation particle size is 3 mm, and the core layer particles are obtained;

[0047] (2) Add the core layer particles to the water chestnut-type sugar coating machine, start the heating function, set the speed to 30r / min, wait until the surface temperature of the core layer particles is about 65°C, use a high-pressure spray gun to spray the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles in small amounts and multiple times (20% of the total weight each time), and at the same time, slowly sprinkle the modified biochar powder in small amounts and multiple times (20% of the total weight each time), rotate for 5 minutes (rotate for 5 minutes each time the modified biochar powder is sprinkled), thereby forming a tight coating layer, repeat this process 5 times until all the coating layer materials are coated, cool to room temperature, take out the coated fertilizer, and dry it to a moisture content of 5% to obtain a biochar-based coated selenium slow-release fertilizer.

[0048] Example 2

[0049] The raw material composition of biochar-based coated selenium slow-release fertilizer is as follows: core layer raw materials: 75 parts of urea, 65 parts of monoammonium phosphate, 55 parts of potassium chloride, 35 parts of double superphosphate, 2.5 parts of sodium silicate, 3 parts of magnesium sulfate, 12 parts of nano-selenium, and 5 parts of tourmaline powder; coating layer raw materials: 130 parts of modified biochar powder, 25 parts of modified starch / polyvinyl alcohol complex (all parts by mass).

[0050] The preparation steps are as follows:

[0051] (1) First, urea, monoammonium phosphate, potassium chloride, double superphosphate, sodium silicate and magnesium sulfate in the raw materials of the core layer components are ground through an 80-mesh sieve, and then the sieved raw materials are mixed with nano-selenium and tourmaline powder and then put into a rotary drum granulator and steam is introduced for rolling humidification granulation. The granulation particle size is 3 mm, and the core layer particles are obtained;

[0052] (2) Add the core layer particles to the water chestnut-type sugar coating machine, start the heating function, set the speed to 30r / min, wait until the surface temperature of the core layer particles is about 65°C, use a high-pressure spray gun to spray the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles in small amounts and multiple times (20% of the total weight each time), and at the same time, slowly sprinkle the modified biochar powder in small amounts and multiple times (20% of the total weight each time), rotate for about 5 minutes (rotate for 5 minutes each time the modified biochar powder is sprinkled), thereby forming a tight coating layer, repeat this process 5 times until all the coating layer materials are coated, cool to room temperature, take out the coated fertilizer, and dry it to a moisture content of 5% to obtain a biochar-based coated selenium slow-release fertilizer.

[0053] Example 3

[0054] The raw material composition of biochar-based coated selenium slow-release fertilizer is as follows: core layer raw materials: 80 parts of urea, 70 parts of monoammonium phosphate, 60 parts of potassium chloride, 40 parts of double superphosphate, 3 parts of sodium silicate, 4 parts of magnesium sulfate, 15 parts of nano-selenium, and 6 parts of tourmaline powder; coating layer raw materials: 150 parts of modified biochar powder, 30 parts of modified starch / polyvinyl alcohol composite (all parts by mass).

[0055] The preparation steps are as follows:

[0056] (1) First, urea, monoammonium phosphate, potassium chloride, double superphosphate, sodium silicate and magnesium sulfate in the raw materials of the core layer components are ground through an 80-mesh sieve, and then the sieved raw materials are mixed with nano-selenium and tourmaline powder and then put into a rotary drum granulator and steam is introduced for rolling humidification granulation. The granulation particle size is 3 mm, and the core layer particles are obtained;

[0057] (2) Add the core layer particles to the water chestnut-type sugar coating machine, start the heating function, set the speed to 30r / min, wait until the surface temperature of the core layer particles is about 65°C, use a high-pressure spray gun to spray the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles in small amounts and multiple times (20% of the total weight each time), and at the same time, slowly sprinkle the modified biochar powder in small amounts and multiple times (20% of the total weight each time), rotate for about 5 minutes (rotate for 5 minutes each time the modified biochar powder is sprinkled), thereby forming a tight coating layer, repeat this process 5 times until all the coating layer materials are coated, cool to room temperature, take out the coated fertilizer, and dry it to a moisture content of 5% to obtain a biochar-based coated selenium slow-release fertilizer.

[0058] Comparative Example 1

[0059] The raw material composition of the biochar-based coated selenium slow-release fertilizer is as follows: core layer raw materials: 70 parts of urea, 60 parts of monoammonium phosphate, 50 parts of potassium chloride, 30 parts of double superphosphate, 2 parts of sodium silicate, 2 parts of magnesium sulfate, 8 parts of nano-selenium, and 4 parts of tourmaline powder; coating layer raw materials: 120 parts of peanut shell biochar powder, 20 parts of modified starch / polyvinyl alcohol composite (all parts by mass).

[0060] The preparation steps are as follows:

[0061] (1) Preparation of peanut shell biochar powder: Peanut shells were crushed and passed through a 40-mesh sieve, placed in a porcelain boat, and placed in a tubular atmosphere furnace. After sealing, the furnace was evacuated, the air in the tubular atmosphere furnace was exhausted, and protective nitrogen was introduced at a flow rate of 100 sccm. The heating program was set as follows: heating to 700°C at a heating rate of 5°C / min, pyrolysis was maintained for 2 h, and the sample was taken out and ground through an 80-mesh sieve to obtain peanut shell biochar powder. After testing, the specific surface area of ​​the modified biochar powder was 55.22 m 2 / g, micropore volume is 0.036cm 3 / g;

[0062] (2) First, urea, monoammonium phosphate, potassium chloride, double superphosphate, sodium silicate and magnesium sulfate in the core layer component raw materials are ground through an 80-mesh sieve, and then the sieved raw materials are mixed with nano-selenium and tourmaline powder and then put into a rotary drum granulator and steam is introduced for rolling humidification granulation. The granulation particle size is 3 mm to obtain core layer particles;

[0063] (3) Add the core layer particles to the water chestnut-type sugar coating machine, start the heating function, set the speed to 30r / min, wait until the surface temperature of the core layer particles is about 65°C, use a high-pressure spray gun to spray the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles in small amounts and multiple times (20% of the total weight each time), and slowly sprinkle peanut shell biochar powder in small amounts and multiple times (20% of the total weight each time), rotate for about 5 minutes (rotate for 5 minutes each time the modified biochar powder is sprinkled), thereby forming a tight coating layer, repeat this process 5 times until all the coating layer materials are coated, cool to room temperature, take out the coated fertilizer, and dry it to a moisture content of 5% to obtain a biochar-based coated selenium slow-release fertilizer.

[0064] Comparative Example 2

[0065] The same as Example 1, except that an equal mass portion of polyvinyl alcohol solution (concentration of 3 wt.%) is used to replace the modified starch / polyvinyl alcohol complex.

[0066] Comparative Example 3

[0067] The same as comparative example 1, except that an equal mass portion of polyvinyl alcohol solution (concentration of 3 wt.%) is used to replace the modified starch / polyvinyl alcohol complex.

[0068] Comparative Example 4

[0069] The same as Example 1, the only difference is that sodium selenate is used instead of nano-selenium.

[0070] Comparative Example 5

[0071] Same as Example 1, except that the use of tourmaline powder is omitted.

[0072] Effect verification

[0073] Fertilizer application test

[0074] 27 experimental plots were randomly divided in a wheat planting field with uniform soil properties and medium fertility. Each experimental plot was 3m×4m. The experimental groups were respectively applied with the biochar-coated selenium slow-release fertilizers prepared in Examples 1-3 and the biochar-coated selenium slow-release fertilizers prepared in Comparative Examples 1-5. The experimental plots without any fertilizer were used as blank control groups. Three parallel experiments were set up for each group, and the final result was the average of the three groups. Basal fertilizer was applied before wheat sowing, with an application rate of 10kg / mu. No fertilizer was applied during the entire growth period. Other field management activities were the same as the ordinary wheat planting process. During the harvest period, wheat selenium content, yield, flour yield and other parameters were statistically analyzed. The specific results are shown in Table 1.

[0075] Table 1

[0076]

[0077]

[0078] From the results in Table 1, it can be seen that the wheat treated with the biochar-based coated selenium slow-release fertilizer prepared in Examples 1-3 of the present invention has significantly improved yield, wheat selenium content, protein content, etc., which is significantly better than other comparative examples and the blank control group. This is mainly due to the synergistic effect of the components of the biochar-based coated selenium slow-release fertilizer of the present invention, which promotes the absorption and utilization of selenium by crops, improves soil, and increases crop yield and quality.

[0079] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A biochar-based coated selenium slow-release fertilizer, characterized in that: The invention comprises a core layer and a coating layer coated on the surface of the core layer. The raw materials of the core layer comprise the following components by weight: 70-80 parts of urea, 60-70 parts of monoammonium phosphate, 50-60 parts of potassium chloride, 30-40 parts of double superphosphate, 2-3 parts of sodium silicate, 2-4 parts of magnesium sulfate, 8-15 parts of nano-selenium, and 4-6 parts of tourmaline powder; the raw materials of the coating layer comprise the following components: 120-150 parts of modified biochar powder and 20-30 parts of modified starch / polyvinyl alcohol compound; The preparation method of the modified biochar powder is as follows: crushing peanut shells, first heating and soaking them in a mixed aqueous solution of NaOH and Na2SO3, then heating and soaking them in a H2O2 solution, filtering, drying, and then high-temperature carbonizing and grinding to obtain the modified biochar powder; The preparation method of the modified starch / polyvinyl alcohol complex is as follows: starch is mixed with a NaOH solution, heated for reaction, polyvinyl alcohol is added, the mixture is kept warm for 1-2 hours, the temperature is lowered to 50-60° C., the pH value is adjusted to 5-6, borax, glycerol and nano-silicon dioxide are added, and the mixture is reacted for 2-4 hours to obtain the modified starch / polyvinyl alcohol complex; The heating reaction temperature is 90-100°C and the time is 20-30min; The mass ratio of starch to polyvinyl alcohol is 1:0.5-2, the amount of borax is 1-3% of the total mass of starch and polyvinyl alcohol, the amount of glycerol is 10-30% of the total mass of starch and polyvinyl alcohol, and the amount of nano silicon dioxide is 0.5-2% of the total mass of starch and polyvinyl alcohol.

2. The biochar-based coated selenium slow-release fertilizer according to claim 1, characterized in that: The concentration of NaOH in the mixed aqueous solution of NaOH and Na2SO3 is 1.2-1.4 mol / L, the concentration of Na2SO3 is 0.2-0.4 mol / L, and the concentration of H2O2 solution is 2-4 wt.%.

3. The biochar-based coated selenium slow-release fertilizer according to claim 1, characterized in that: The temperature of heating and soaking in the mixed aqueous solution of NaOH and Na2SO3 is 100°C and the time is 6-8h.

4. The biochar-based coated selenium slow-release fertilizer according to claim 1, characterized in that: The heating and soaking temperature in the H2O2 solution is 70-80°C and the time is 2-4h.

5. The biochar-based coated selenium slow-release fertilizer according to claim 1, characterized in that: The high temperature carbonization temperature is 500-700°C, the time is 4-6h, and the heating rate is 5-10°C / min.

6. A method for preparing the biochar-based coated selenium slow-release fertilizer according to any one of claims 1 to 5, characterized in that: The following steps are involved: (1) mixing the component raw materials of the core layer uniformly and granulating them to obtain core layer particles; (2) Preheating the core layer particles to 60-70° C., spraying the modified starch / polyvinyl alcohol complex onto the surface of the core layer particles, and simultaneously sprinkling the modified biochar powder, rotating for 5-20 minutes to form a coating layer, and drying to obtain the biochar-based coated selenium slow-release fertilizer.

Images