Preparation method and application of a biochar-based coated fertilizer

Abstract

The present application belongs to the technical field of agricultural fertilizer, and particularly relates to a preparation method and application of a biochar-based coated fertilizer. The present application is advantageous for nano-iron modification of starch by mixing ferric chloride, ferrous chloride and starch solution; the swelling property and water retention of the coating liquid are improved by mixing the starch-based ferroferric oxide solution and polyvinyl alcohol, and the obtained coating liquid is wrapped outside the fertilizer core, which is advantageous for improving the slow-release property of the biochar-based coated fertilizer. Experimental results show that the swelling property of the prepared coating material is as high as 189.91%; after the coating material is used to prepare the biochar-based coated fertilizer, the 30-day cumulative release rates of nitrogen, phosphorus and potassium are 22.87%, 34.93% and 84.08% respectively, and the water retention of the soil after 33 days is 38.46%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of agricultural fertilizers, and particularly relates to a preparation method and application of a biochar-based coated fertilizer. BACKGROUND

[0002] Biochar-based slow-release fertilizer is prepared by coupling biochar and traditional fertilizer through a certain process, which can not only provide longer nutrient supply and reduce the frequency of fertilizer application, but also reduce the cost of crop management and labor input. In addition, applying biochar-based slow-release fertilizer to farmland soil also has the potential to increase soil organic matter content, improve soil water and fertilizer retention capacity, and improve soil microbial community structure. Therefore, the application prospect of biochar-based slow-release fertilizer is extremely wide.

[0003] Biochar-based coated fertilizer is a common biochar-based slow-release fertilizer at present, and the selection of coating material and the preparation process are the key to determine the slow-release effect. For example, Chinese patent 201710959416.X discloses a method for preparing coated slow-release fertilizer by using starch and polyvinyl alcohol, although the nutrient slow-release effect of the coated slow-release fertilizer is improved, but its water absorption is strong and its mechanical properties are poor, so it will cause the film to rupture and lose the slow-release performance due to water absorption; for another example, Chinese patent 201910986318.4 discloses a method for preparing coated controlled-release fertilizer by using polyvinyl alcohol / graphene, although the coating of the controlled-release fertilizer has excellent nutrient slow-release effect, but the cost of raw materials for production is high. Therefore, how to prepare a biochar-based coated fertilizer with low economic cost, environmental friendliness and high slow-release performance is a problem to be solved in the field. SUMMARY

[0004] The purpose of the present application is to provide a preparation method and application of a biochar-based coated fertilizer, which has low cost, environmental friendliness and high slow-release performance, and has the characteristics of improving the water retention of the fertilizer and prolonging the nutrient availability of the fertilizer after being used to prepare the biochar-based coated fertilizer.

[0005] To solve the above technical problems, the technical solutions of the present application are as follows:

[0006] The present application provides a preparation method of a coating liquid, comprising the following steps:

[0007] The starch solution is mixed and dissolved with the iron-containing compound under alkaline conditions to obtain a starch-based ferroferric oxide solution;

[0008] The starch-based ferroferric oxide solution is mixed with polyvinyl alcohol and then subjected to crosslinking reaction to obtain a coating liquid;

[0009] The iron-containing compound includes ferric chloride and ferrous chloride.

[0010] Preferably, the mass ratio of ferric iron to ferrous iron in the iron-containing compound is 2:1.

[0011] The mass-volume ratio of the iron-containing compound to the starch solution is 0.489-2.4 g:300 mL.

[0012] Preferably, the alkaline condition comprises a pH value of 10-11.

[0013] Preferably, the mass percentage of starch in the starch solution is 1.3%.

[0014] Preferably, the volume-mass ratio of the starch-based ferroferric oxide solution to polyvinyl alcohol is 300 mL:6 g.

[0015] Preferably, the cross-linking reaction comprises adding a cross-linking agent to a mixed solution containing the starch-based ferroferric oxide solution and polyvinyl alcohol.

[0016] The cross-linking agent comprises borax and glycerol.

[0017] The mass of borax, the volume of glycerol, and the volume of the mixed solution are in a ratio of 0.1 g:2.5-3.5 mL:300 mL.

[0018] Preferably, the temperature of the mixed solution is 55-65℃; the mixed solution is accompanied by stirring, the stirring time is 0.5-1 h, and the stirring speed is 400-700 r / min.

[0019] The mixing of the starch-based ferroferric oxide solution and polyvinyl alcohol is accompanied by stirring; the mixing temperature is 85-95℃; the stirring time is 1-1.5 h, and the stirring speed is 400-700 r / min.

[0020] The application provides an application of the coating liquid prepared by the preparation method in the biochar-based coated fertilizer.

[0021] The application provides a preparation method of a biochar-based coated fertilizer, comprising the following steps:

[0022] The coating liquid prepared by the preparation method is used to coat a fertilizer core, and biochar and paraffin are coated on the outer layer of the fertilizer core to obtain the biochar-based coated fertilizer.

[0023] The raw material of the biochar comprises one or more of rice straw, corn straw, and wheat straw.

[0024] The application of the biochar-based coated fertilizer prepared by the preparation method in improving the water retention of the fertilizer and / or prolonging the nutrient availability of the fertilizer.

[0025] Advantages:

[0026] The application provides a preparation method of a coating liquid, comprising the following steps: mixing and dissolving a starch solution and an iron-containing compound to obtain a starch-based ferroferric oxide solution under an alkaline condition; and mixing the starch-based ferroferric oxide solution and polyvinyl alcohol and then performing a cross-linking reaction to obtain the coating liquid; the iron-containing compound comprises ferric chloride and ferrous chloride. The ferric chloride, the ferrous chloride and the starch solution are mixed, so that the starch is modified by nano-iron; the starch-based ferroferric oxide solution is mixed with the polyvinyl alcohol, so that the swelling property and the water retention property of the coating liquid are improved; and after the obtained coating liquid is wrapped outside the fertilizer core, the slow-release property of the biochar-based coated fertilizer is improved. Experiments prove that the swelling property of the coating material prepared by the application is as high as 189.91%.

[0027] Based on the above technical advantages, the application also provides an application of the coating liquid prepared by the preparation method to a biochar-based coated fertilizer. Experiments prove that after the biochar-based coated fertilizer provided by the application is used, the cumulative release rates of nitrogen, phosphorus and potassium in 30 days are 22.87%, 34.93% and 84.08%, respectively, and the water retention property of the soil after 33 days is 38.46%. BRIEF DESCRIPTION OF DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows.

[0029] Figure 1 The swelling property of the fertilizer film with different treatments in application example 1;

[0030] Figure 2 The degradation property of the fertilizer film with different treatments in application example 1;

[0031] Figure 3 The soil water retention property of the fertilizer with different treatments in application example 2;

[0032] Figure 4 The nitrogen nutrient release curve of the different biochar-based coated fertilizers in the soil column leaching test in application example 3;

[0033] Figure 5 The phosphorus nutrient release curve of the different biochar-based coated fertilizers in the soil column leaching test in application example 3;

[0034] Figure 6 The potassium nutrient release curve of the different biochar-based coated fertilizers in the soil column leaching test in application example 3;

[0035] Figure 7 The nutrient content of the different biochar-based coated fertilizers in application example 4;

[0036] Figure 8For the application example 5, the effects of different fertilization treatments on the height of pepper plants are shown in the following table.

[0037] Figure 9 For the application example 5, the growth of peppers under different fertilization treatments is shown in the following table. DETAILED DESCRIPTION

[0038] The application provides a preparation method of a coating liquid, comprising the following steps: mixing and dissolving a starch solution and an iron-containing compound under alkaline conditions to obtain a starch-based ferroferric oxide solution; and mixing the starch-based ferroferric oxide solution with polyvinyl alcohol and then performing cross-linking reaction to obtain the coating liquid; the iron-containing compound comprises ferric chloride and ferrous chloride.

[0039] In the application, the raw materials and equipment used are conventionally purchased unless otherwise specified.

[0040] The starch solution is preferably prepared in the application. In the application, the preparation method of the starch solution preferably comprises: mixing and dissolving starch and water under stirring to obtain the starch solution; the stirring speed is preferably 400-700 r / min, more preferably 500 r / min; the stirring time is preferably 0.5-1 h, more preferably 0.5 h; the temperature during mixing is preferably 55-65 DEG C, more preferably 60 DEG C; the water is preferably deionized water; during mixing and dissolving, the starch addition amount and the water volume are preferably configured according to the mass percentage of starch in the starch solution; the mass percentage of starch in the starch solution is preferably 1.3%. In the specific embodiment of the application, the starch addition amount is 4 g, the water volume is 300 mL, and a starch solution with a starch content of 1.3% is prepared in this way.

[0041] After obtaining the starch solution, the starch solution is mixed and dissolved with the iron-containing compound under alkaline conditions to obtain a starch-based ferroferric oxide solution. In the application, the iron-containing compound comprises ferric chloride and ferrous chloride; the ferric chloride preferably comprises ferric chloride hexahydrate; the ferrous chloride is preferably ferrous chloride tetrahydrate; the mass ratio of trivalent iron and divalent iron in the ferric chloride and the ferrous chloride in the iron-containing compound is preferably 2:1.

[0042] In the present application, the mass-volume ratio of the iron-containing compound to the starch solution is preferably 0.489-2.4 g: 300 mL, when the iron-containing compound is ferric chloride hexahydrate and ferrous chloride tetrahydrate, the mass-volume ratio of the iron-containing compound to the starch solution is further preferably 0.8-2.4 g: 300 mL, more preferably 0.8 g: 300 mL or 1.6 g: 300 mL or 2.4 g: 300 mL; when the iron-containing compound is ferric chloride and ferrous chloride, the mass-volume ratio of the iron-containing compound to the starch solution is further preferably 0.489-1.465 g: 300 mL, more preferably 0.489 g: 300 mL or 0.978 g: 300 mL or 1.465 g: 300 mL.

[0043] The temperature of the mixing and dissolving is preferably 55-65°C, more preferably 60°C; the mixing and dissolving is accompanied by stirring, the stirring time is preferably 0.5-1 h, more preferably 1 h; the stirring speed is preferably 400-700 r / min, more preferably 500 r / min; the pH value of the mixed solution is adjusted to provide alkaline conditions for the system, the reagent for adjusting the pH value of the mixed solution is preferably NaOH solution; the concentration of the NaOH solution is preferably 1 mol / L; the pH value of the mixed solution is more preferably 10-11, more preferably 11. The solution containing Fe 2+ and Fe 3+ ions is mixed by the above co-precipitation method, and Fe3O4 nanoparticles are co-precipitated, so as to obtain starch-based ferroferric oxide.

[0044] After obtaining the starch-based ferroferric oxide solution, the starch-based ferroferric oxide solution is mixed with polyvinyl alcohol in the present application. In the present application, the mixing of the starch-based ferroferric oxide solution and polyvinyl alcohol is accompanied by stirring; the temperature of the mixing is preferably 85-95°C, more preferably 90°C; the stirring time is preferably 1-1.5 h, more preferably 1 h; the stirring speed is preferably 400-700 r / min, more preferably 500 r / min; the volume-mass ratio of the starch-based ferroferric oxide solution to polyvinyl alcohol is preferably 300 mL: 6 g, so as to ensure that the added mass of polyvinyl alcohol is 2% of the ferroferric oxide solution.

[0045] After the mixing, the application carries out a cross-linking reaction on the mixed solution. The cross-linking reaction of the application preferably comprises: adding a cross-linking agent in the mixed solution containing the starch-based ferroferric oxide solution and the polyvinyl alcohol, the cross-linking agent preferably comprises borax and glycerol, the mass of the borax, the volume of the glycerol and the volume ratio of the mixed solution are preferably 0.1 g: 2.5-3.5 mL: 300 mL, more preferably 0.1 g: 3 mL: 300 mL, stirring is carried out during the cross-linking reaction, the stirring time is preferably 1.5-3 h, more preferably 2 h, the stirring speed is preferably 400-700 r / min, more preferably 500 r / min, and the stirring temperature is preferably 55-65℃, more preferably 60℃, so that the obtained coating liquid is a nano-iron modified starch / polyvinyl alcohol-based coating material. During the cross-linking, the negative charge of the hydroxyl group on the surface of the starch and the positive charge on the surface of the iron oxide combine, preventing the surface organic ions from being chelated into a nucleus, and thus forming the nano-iron modified starch / polyvinyl alcohol-based coating material.

[0046] The application further provides a preparation method of the biochar-based coated fertilizer, which comprises the following steps: wrapping a fertilizer core with the coating liquid obtained by the preparation method in the above technical solution, and wrapping biochar and paraffin on the outer layer of the fertilizer core to obtain the biochar-based coated fertilizer; and the raw material of the biochar comprises one or more of rice straw, corn straw and wheat straw.

[0047] The application preferably prepares biochar powder. In the application, the preparation method of the biochar powder preferably comprises: crushing the raw material of the biochar, first sieving the crushed raw material to obtain a raw material undersize, pyrolyzing the raw material undersize to obtain biochar, and grinding and second sieving the biochar to obtain biochar powder.

[0048] In the application, the raw material of the biochar is preferably rice straw, and the crushing method has no special requirements and can use the technology well known in the art.

[0049] After the crushing, the application preferably first sieves the crushed raw material to obtain a raw material undersize. In the application, the mesh number of the first sieving is preferably 80-100 mesh, more preferably 100 mesh, which is conducive to uniform pyrolysis of the biochar.

[0050] After the first sieving, the application preferably pyrolyzes the raw material undersize to obtain biochar. The pyrolysis of the application preferably comprises pyrolysis under anaerobic conditions, more preferably pyrolysis under an N2 atmosphere, the pyrolysis temperature is preferably 500-550℃, more preferably 500℃, and the pyrolysis time is preferably 2-3 h, more preferably 2 h, so that the biochar is obtained.

[0051] After the pyrolysis, the present application preferably grinds and secondly screens the biochar. The grinding of the present application is preferably grinding the cooled biochar; the temperature of the cooling is preferably 25-35℃, more preferably 25℃; the mode of the grinding has no special requirements, and the technology well known in the art can be adopted; the mesh number of the second screening is preferably 80-100 mesh, more preferably 100 mesh, so as to facilitate the uniform and appropriate size of the biochar powder coated on the fertilizer.

[0052] The present application utilizes the preparation method of the technical scheme to obtain the coated liquid wrapped fertilizer core. In the present application, the fertilizer core preferably comprises a granular compound fertilizer; the wrapping is preferably spraying and wrapping by using a coating machine; when spraying, the speed of the rotation of the coating machine is preferably 20-25 r / min, more preferably 20 r / min, and the temperature of the heating of the coating machine is preferably 50-70℃, more preferably 50℃. The present application has no special requirements for the model of the coating machine and the flow of the spraying, and the technology well known in the art can be adopted. In this way, it is beneficial to ensure that the surface of the fertilizer core is uniformly and completely wrapped with the coated liquid.

[0053] After obtaining the coated liquid wrapped fertilizer core, the present application wraps biochar on the outer layer of the coated liquid wrapped fertilizer core to obtain a biochar wrapped fertilizer core. When wrapping the biochar, the present application preferably utilizes a coating machine to rotate and wrap; the speed of the rotation is preferably 20-25 r / min, more preferably 20 r / min; the amount of the biochar is preferably 2.2%-3.9% of the mass of the fertilizer core, more preferably 2.2%; the preparation method of the biochar has been described in detail above, and will not be repeated here; the model of the coating machine and the flow of the spraying have no special requirements, and the technology well known in the art can be adopted. In the specific embodiments of the present application, the model of the coating machine is BY-600.

[0054] After obtaining the biochar wrapped fertilizer core, the present application preferably wraps paraffin on the outer layer of the biochar wrapped fertilizer core to obtain a biochar-based coated fertilizer. When wrapping the paraffin, the present application preferably utilizes a coating machine to rotate and wrap; the speed of the rotation is preferably 20-25 r / min, more preferably 20 r / min; the amount of the paraffin is preferably 2.5%-2.8% of the mass of the fertilizer core, more preferably 2.5%; the model of the coating machine and the flow of the spraying have no special requirements, and the technology well known in the art can be adopted. In the specific embodiments of the present application, the model of the coating machine is BY-600. After wrapping by the above method, a coated fertilizer with an effective content of about 80% is obtained.

[0055] The application further provides application of the coating liquid prepared by the preparation method in the biochar-based coated fertilizer, and application of the biochar-based coated fertilizer in improving water retention of the fertilizer and / or prolonging nutrient availability of the fertilizer.

[0056] Experiments prove that the swelling of the prepared coating material is up to 189.91%; after the coating material is used to prepare the biochar-based coated fertilizer, the cumulative release rates of nitrogen, phosphorus and potassium are 22.87%, 34.93% and 84.08% respectively after 30 days, and the water retention of soil is 38.46% after 33 days.

[0057] In order to further illustrate the application, the preparation method and application of the biochar-based coated fertilizer are described in detail below in combination with the drawings and examples, but they should not be understood as limiting the protection scope of the application.

[0058] Example 1

[0059] A preparation method of the biochar-based coated fertilizer, comprising the following steps:

[0060] Preparation before experiment: starch, ferric chloride, ferrous chloride and polyvinyl alcohol are used as raw materials

[0061] 1) rice straw is used as raw material, which is crushed and sieved through a 100-mesh sieve, and then pyrolyzed under anaerobic conditions at 500 DEG C for 2h, and after cooling, grinding and sieving through a 100-mesh sieve, biochar powder is obtained.

[0062] 2) 4g of starch is dissolved in 300mL of deionized water, and stirred at 500r / min for 0.5h at 60 DEG C to obtain a starch solution with a starch content of 1.3wt.%;

[0063] 3) 0.5856g of ferric chloride hexahydrate (i.e., ferric chloride) and 0.2144g of ferrous chloride tetrahydrate (i.e., ferrous chloride) are weighed respectively and mixed with the starch solution in step 2), and the pH value of the mixed solution is adjusted to 11 with 1mol / L NaOH solution, and stirred at 500r / min for 1h at 60 DEG C to obtain a starch-based ferroferric oxide solution;

[0064] 4) 6g of polyvinyl alcohol is mixed with the starch-based ferroferric oxide solution in step 3), heated and stirred at 500r / min for 1h at 90 DEG C, and 0.1g of borax and 3mL of glycerol are added as crosslinking agents, and the stirring is continued at 500r / min for 2h at 60 DEG C, so as to obtain a nano-iron modified starch / polyvinyl alcohol-based coating material containing 0.25% ferroferric oxide by mass ratio;

[0065] 5) Use a coating machine to spray the nano-iron modified starch / polyvinyl alcohol-based coating material in step 4), and the fertilizer core is a granular compound fertilizer with a mass ratio of nitrogen: phosphorus: potassium of 15:15:15, 100g. The coating machine is BY-600, and the surface of the fertilizer core is completely coated with the coating material. During spraying, the fertilizer core is rotated and dried at 50°C at a speed of 20r / min to ensure uniform and complete coating of the fertilizer surface.

[0066] 6) Add the biochar in step 1) to the coating machine in step 5), and the addition amount of biochar is 2.2% of the mass of the fertilizer core (i.e. 2.2g). Rotate and coat, and the rotation speed is 20r / min. After coating the biochar, add paraffin to the coating machine, and the addition amount of paraffin is 2.5% of the mass of the fertilizer core (i.e. 2.5g). Rotate and coat, and the rotation speed is 20r / min. In this way, a biochar-based coated fertilizer is obtained.

[0067] Example 2

[0068] A method for preparing a biochar-based coated fertilizer, the steps are:

[0069] Preparation before experiment: starch, ferric chloride, ferrous chloride and polyvinyl alcohol as raw materials

[0070] 1) The same as step 1) of Example 1;

[0071] 2) The same as step 2) of Example 1;

[0072] 3) Mix 1.1712g of ferric chloride hexahydrate (i.e. ferric chloride) and 0.4288g of ferrous chloride tetrahydrate (i.e. ferrous chloride) with the starch solution in step 2), and adjust the pH value of the mixed solution to 11 with 1mol / L NaOH solution. Stir at 500r / min for 1h at 60°C to obtain a starch-based ferroferric oxide solution;

[0073] 4) The same as step 4) of Example 1, and a nano-iron modified starch / polyvinyl alcohol-based coating material containing 0.5% ferroferric oxide by mass ratio is obtained;

[0074] 5) The same as step 5) of Example 1;

[0075] 6) The same as step 6) of Example 1.

[0076] Example 3

[0077] A method for preparing a biochar-based coated fertilizer, the steps are:

[0078] Preparation before experiment: starch, ferric chloride, ferrous chloride and polyvinyl alcohol as raw materials

[0079] 1) the same as step 1) of Example 1;

[0080] 2) the same as step 2) of Example 1;

[0081] 3) 1.7568 g of ferric chloride hexahydrate (i.e. ferric chloride) and 0.6432 g of ferrous chloride tetrahydrate (i.e. ferrous chloride) were weighed out respectively and mixed with the starch solution in step 2), and the pH of the mixture was adjusted to 11 with 1 mol / L NaOH solution, and the mixture was stirred at 500 r / min for 1 h at 60°C to obtain a starch-based ferroferric oxide solution;

[0082] 4) the same as step 4) of Example 1, and thus a nanometer iron modified starch / polyvinyl alcohol-based coating material containing 0.75% ferroferric oxide by mass ratio was obtained;

[0083] 5) the same as step 5) of Example 1;

[0084] 6) the same as step 6) of Example 1.

[0085] Comparative Example 1

[0086] A preparation method of a biochar-based coating fertilizer without modification, the steps are as follows:

[0087] Preparation before experiment: starch and polyvinyl alcohol as raw materials

[0088] 1) rice straw as raw material, crushed and passed through a 100 mesh sieve, then pyrolyzed under anaerobic conditions at 500°C for 2 h, cooled, ground, and passed through a 100 mesh sieve to obtain biochar powder;

[0089] 2) starch and polyvinyl alcohol as raw materials, 4 g of starch was dissolved in 300 mL of deionized water, stirred at 500 r / min for 0.5 h at 60°C, and the pH was adjusted to 11 with 1 mol / L NaOH solution, and stirred at 500 r / min for 1 h at 60°C to obtain a starch solution;

[0090] 3) 6 g of polyvinyl alcohol was mixed with the starch solution in step 2), heated and stirred at 500 r / min for 1 h at 90°C, and 0.1 g of borax and 3 mL of glycerol were added as crosslinking agents and continued to stir for 2 h to obtain an unmodified starch / polyvinyl alcohol-based coating material;

[0091] 4) using a coating machine to spray the unmodified starch / polyvinyl alcohol-based coating material obtained in step 3) to coat the fertilizer core, the fertilizer core being a granular compound fertilizer with a mass ratio of nitrogen: phosphorus: potassium of 15: 15: 15, 100 g, and the coating machine is used to completely coat the surface of the fertilizer core with the coating material, and the fertilizer core is rotated and dried at 50°C during spraying at a rotation speed of 20 r / min to ensure that the coating on the surface of the fertilizer is uniform and complete;

[0092] 5) adding the biochar in step 1) to the coating machine in step 4), the amount of biochar added being 2.2 g, and performing rotation coating at a rotation speed of 20 r / min, and then adding paraffin to the coating machine after the biochar is coated, the amount of paraffin added being 2.5 g, and performing rotation coating at a rotation speed of 20 r / min, to obtain the unmodified starch / polyvinyl alcohol-based coated fertilizer product.

[0093] Comparative Example 2

[0094] Common fertilizer: compound fertilizer with a mass ratio of nitrogen: phosphorus: potassium of 15: 15: 15.

[0095] Comparative Example 3

[0096] A method for preparing a coated fertilizer without adding biochar, the steps being:

[0097] 1) using starch, ferric chloride, ferrous chloride and polyvinyl alcohol as raw materials, dissolving 4 g of starch in 300 mL of deionized water, and stirring at a speed of 500 r / min at 60°C for 0.5 h to obtain a starch solution;

[0098] 2) mixing 1.7568 g of ferric chloride hexahydrate (i.e. ferric chloride) and 0.6432 g of ferrous chloride tetrahydrate (i.e. ferrous chloride) with the starch solution in step 1), and adjusting the pH to 11 with a 1 mol / L NaOH solution to obtain a starch-based ferroferric oxide solution, and stirring at a speed of 500 r / min at 60°C for 1 h to obtain a starch-based ferroferric oxide solution;

[0099] 3) mixing 6 g of polyvinyl alcohol with the starch-based ferroferric oxide solution in step 2), heating and stirring at a speed of 500 r / min at 90°C for 1 h, and adding 0.1 g of borax and 3 mL of glycerol as a crosslinking agent to continue stirring for 2 h to obtain a nano-iron modified starch / polyvinyl alcohol-based coating material;

[0100] 4) Use a coating machine to spray and coat the fertilizer core with the nano-iron modified starch / polyvinyl alcohol-based coating material obtained in step 3). The fertilizer core is 100g of granular compound fertilizer with a nitrogen:phosphorus:potassium mass ratio of 15:15:15. The coating material should be completely wrapped around the surface of the fertilizer core. When spraying, the fertilizer core is dried by rotation at 50°C and the rotation speed is 20r / min to ensure that the coating on the fertilizer surface is uniform and complete.

[0101] 5) In step 4), only paraffin wax is added to the coating machine, with an addition amount of 2.5g. The machine is then rotated and coated at a speed of 20r / min to obtain a coated fertilizer product without added biochar.

[0102] Application Example 1

[0103] The swelling and degradation properties of different fertilizer films in Examples 1-3 and Comparative Example 1 were verified. Each fertilizer was used for one treatment, and each treatment was repeated three times. The results of the swelling properties of different fertilizer films are shown in [the table below]. Figure 1 The results of the degradability of different fertilizer films are shown in Figure 2 .(exist Figure 1 and Figure 2 In this context, PVA / Starch represents the experimental results of the fertilizer in Comparative Example 1, PVA / Starch@0.25Fe / BC represents the experimental results of the fertilizer in Example 1, PVA / Starch@0.5Fe / BC represents the experimental results of the fertilizer in Example 2, and PVA / Starch@0.75Fe / BC represents the experimental results of the fertilizer in Example 1.

[0104] Depend on Figure 1 and Figure 2 As can be seen, the swelling capacity of the nano-iron modified starch / polyvinyl alcohol-based coating material provided by this invention is 189.91%, which is much higher than that of the unmodified coating. This proves that the modified membrane has better water absorption and swelling properties, and its mechanical properties are much higher than those of the unmodified coating, enabling it to release fertilizer more slowly. In addition, its degradation performance in soil after 30 days is better than that of the unmodified membrane, making it environmentally friendly to the soil.

[0105] Application Example 2

[0106] The water retention capacity of the fertilizers obtained in Examples 1-3 and Comparative Examples 1-3 was verified. Each fertilizer corresponded to one treatment, and each treatment was repeated three times. The water retention capacity of different fertilizers at 0d, 5d, 10d, 15d, 19d, 21d, 25d, 31d, and 33d were statistically analyzed. The results are shown in Table 1 and Table 2. Figure 3 (exist Figure 3In the table, PVA / Starch@BC represents the experimental results of the fertilizer in Comparative Example 1, NPK represents the experimental results of the fertilizer in Comparative Example 2, CK represents the experimental results of the blank group without adding any fertilizer, PVA / Starch@Fe represents the experimental results of the fertilizer in Comparative Example 3, PVA / Starch@0.25Fe / BC represents the experimental results of the fertilizer in Example 1, PVA / Starch@0.5Fe / BC represents the experimental results of the fertilizer in Example 2, and PVA / Starch@0.75Fe / BC represents the experimental results of the fertilizer in Example 3.

[0107] Table 1 Soil water retention of different fertilizers (unit: %)

[0108] 0d 5d 10d 15d 19d 21d 25d 31d 33d Example 1 100 83.26 72.07 61.08 55.51 52.41 47.79 40.41 38.46 Example 2 100 84.62 73.37 61.70 55.86 52.67 48.06 40.41 38.30 Example 3 100 83.26 70.95 59.27 53.50 50.97 45.90 38.24 36.16 Comparative Example 1 100 81.69 69.94 59.05 53.42 50.71 46.25 38.86 36.78 Comparative Example 2 100 81.90 68.43 56.05 50.14 46.82 42.13 34.21 31.88 Comparative Example 3 100 82.63 71.26 59.84 54.16 51.27 46.76 39.24 37.22 Blank group 100 80.16 66.06 53.53 47.87 44.61 39.81 31.87 29.72

[0109] From Table 1 and Figure 3 It can be seen that the water retention of the biochar-based coated fertilizer provided by the present application to the soil still remains 38.46% after 33 days, which is much higher than that of ordinary chemical fertilizer. In actual use of the fertilizer, the good water retention performance of the fertilizer to the soil improves the water holding capacity of the soil and reduces the loss of soil water.

[0110] Application Example 3

[0111] The soil column leaching test of the fertilizers obtained in Examples 1-3 and Comparative Examples 1-3 was carried out with reference to the existing literatures ① ②, (Reference literatures: ① Bakshi S, Banik C, Laird DA, et al. Enhancing biochar as scaffolding for slow release of nitrogen fertilizer [J]. ACS Sustainable Chemistry & Engineering, 2021, 9(24): 8222-8231. ② Luo W, Qian L, Liu W, et al. A potential Mg-enriched biochar fertilizer: Excellent slow-release performance and release mechanism of nutrients [J]. Science of the Total Environment, 2021, 768: 144454), specifically: 100 g of vermiculite containing 40% water was put into an acrylic column containing a filter screen, the fertilizer was buried below the surface of the vermiculite by five centimeters, and the simulated rainwater was poured regularly, the leachate was collected, the nitrogen, phosphorus and potassium contents in the leachate were determined, each fertilizer corresponded to one treatment, each treatment was repeated three times, and the experimental results of 0 d, 6 d, 12 d, 18 d, 24 d and 30 d were statistically analyzed, see Tables 2-4 and Figures 4-6 (At Figures 4-6 In the tables, PVA / Starch@BC all represent the experimental results of the fertilizer in Comparative Example 1, NPK all represent the experimental results of the fertilizer in Comparative Example 2, PVA / Starch@Fe all represent the experimental results of the fertilizer in Comparative Example 3, PVA / Starch@0.25Fe / BC all represent the experimental results of the fertilizer in Example 1, PVA / Starch@0.5Fe / BC all represent the experimental results of the fertilizer in Example 2, and PVA / Starch@0.75Fe / BC all represent the experimental results of the fertilizer in Example 3, Table 2 is the release amount of nutrient nitrogen in different fertilizers, Table 3 is the release amount of nutrient phosphorus in different fertilizers, and Table 4 is the release amount of nutrient potassium in different fertilizers.

[0112] Table 2 Release amount of nutrient nitrogen in different fertilizers (unit: %)

[0113] 0d 6d 12d 18d 24d 30d Example 1 0 0.75 5.95 15.09 20.36 23.36 Example 2 0 0.53 2.77 11.83 19.57 24.99 Example 3 0 1.07 3.84 11.13 18.03 22.87 Comparative Example 1 0 2.83 12.62 20.90 26.99 31.03 Comparative Example 2 0 9.71 21.72 29.94 35.13 38.49 Comparative Example 3 0 4.26 16.86 25.44 30.61 33.82

[0114] Table 3 Release amount of nutrient phosphorus in different fertilizers (unit: %)

[0115] 0d 6d 12d 18d 24d 30d Example 1 0 5.01 21.77 31.47 37.30 41.43 Example 2 0 3.01 12.74 23.36 31.19 36.74 Example 3 0 3.99 15.43 23.70 30.29 34.93 Comparative Example 1 0 8.18 19.88 28.15 33.81 37.92 Comparative Example 2 0 15.16 30.19 39.63 46.17 50.48 Comparative Example 3 0 9.80 23.35 31.42 36.56 40.14

[0116] Table 4: Nutrient potassium release amount in different fertilizers (unit: %)

[0117] 0d 6d 12d 18d 24d 30d Example 1 0 12.12 41.28 63.42 78.40 87.99 Example 2 0 9.95 29.37 52.19 72.12 86.95 Example 3 0 10.29 29.82 50.61 69.51 84.08 Comparative Example 1 0 14.62 41.22 63.10 80.53 93.08 Comparative Example 2 0 25.94 54.35 73.45 86.70 96.08 Comparative Example 3 0 18.61 48.66 68.81 83.02 92.24

[0118] From Tables 2-4 and Figure 4 It can be seen that the cumulative nitrogen, phosphorus and potassium release rates of the biochar-based coated fertilizer provided by the present application are 22.87%, 34.93% and 84.08% respectively after 30 days, which are much lower than those of chemical fertilizers and starch / polyvinyl alcohol-based coated fertilizers without nano-iron modification. The excellent nutrient slow-release performance of the biochar-based coated fertilizer helps to improve the fertilizer utilization rate, reduce the fertilizer dosage and nutrient loss, thereby achieving good economic and environmental benefits.

[0119] Application Example 4

[0120] The fertilizers in Examples 1-3 and Comparative Examples 1-3 were respectively subjected to digestion and cooking, each fertilizer corresponding to one treatment, and each treatment being repeated three times. The nutrient contents in the different treated fertilizers were statistically analyzed, and the results are shown in Table 5 and Figure 7 (In Figure 7 , PVA / Starch@BC represents the experimental results of the fertilizer in Comparative Example 1, NPK represents the experimental results of the fertilizer in Comparative Example 2, PVA / Starch@Fe represents the experimental results of the fertilizer in Comparative Example 3, PVA / Starch@0.25Fe / BC represents the experimental results of the fertilizer in Example 1, PVA / Starch@0.5Fe / BC represents the experimental results of the fertilizer in Example 2, and PVA / Starch@0.75Fe / BC represents the experimental results of the fertilizer in Example 3).

[0121] Table 5: Nutrient content in different fertilizers (unit: %)

[0122] Replicate 1 Replicate 2 Replicate 3 Average Example 1 74.45 76.39 89.54 80.12 Example 2 84.91 80.82 81.99 82.57 Example 3 80.29 79.31 91.00 83.53 Comparative Example 1 84.67 76.64 80.29 80.53 Comparative Example 2 100 100 100 100 Comparative Example 3 88.32 75.66 87.34 83.78

[0123] From Tables 5 and Figure 7 It can be seen that the effective content of fertilizer nutrients in the coated fertilizers is more than 80%, which makes the coated fertilizers have a more significant nutrient slow-release effect with less fertilizer.

[0124] Application Example 5

[0125] Effect of different fertilization treatments on the height and growth of pepper plants

[0126] Pot experiment: each pot contains 1 kg of soil, and one pepper (Sujiang No. 5) is planted in each pot, each pot being one treatment, and each treatment being repeated three times;

[0127] The coated fertilizers prepared from Examples 1-3, Comparative Example 1 and Comparative Example 3, and the ordinary chemical fertilizer in Comparative Example 2 were respectively applied to the pots in which the peppers were planted, and the application amount in each pot was 1 g;

[0128] The treatment without applying any fertilizer is taken as a blank control, and other times are managed conventionally.

[0129] Results and analysis:

[0130] After applying the fertilizer for 30 days, the height of the pepper plants in different treatments was counted and analyzed, and the results are shown in Table 6 and Figure 8 (In Figure 8 , PVA / Starch@BC represents the experimental results of the fertilizer in Comparative Example 1, NPK represents the experimental results of the fertilizer in Comparative Example 2, PVA / Starch@Fe represents the experimental results of the fertilizer in Comparative Example 3, PVA / Starch@0.25Fe / BC represents the experimental results of the fertilizer in Example 1, PVA / Starch@0.5Fe / BC represents the experimental results of the fertilizer in Example 2, and PVA / Starch@0.75Fe / BC represents the experimental results of the fertilizer in Example 3); the growth of the pepper plants in different treatments was observed, and the results are shown in Figure 9 (In Figure 9 , the upper figure represents the top view of the plants in different treatments, and the lower figure represents the side view of the plants in different treatments, PVA / Starch@BC represents the plant growth in Comparative Example 1, NPK represents the plant growth in Comparative Example 2, PVA / Starch@Fe represents the plant growth in Comparative Example 3, PVA / Starch@0.25Fe / BC represents the plant growth in Example 1, PVA / Starch@0.5Fe / BC represents the plant growth in Example 2, and PVA / Starch@0.75Fe / BC represents the plant growth in Example 3).

[0131] Table 6 Effect of different fertilizer treatments on the height of pepper plants (unit: cm)

[0132] Replicate 1 Replicate 2 Replicate 3 Average Example 1 12.5 13 13 12.83 Example 2 18 16 14.5 16.17 Example 3 17 16.5 19 17.50 Comparative Example 1 14 13.5 12 13.17 Comparative Example 2 13.5 12.1 12.5 12.70 Comparative Example 3 16 15.5 15.5 15.67 Blank control 12.5 12.4 12.3 12.40

[0133] As can be seen from Table 6, Figure 8 and Figure 9 , the growth of the pepper plants of the nano-iron modified biochar-based coated fertilizer is significantly different from that of the unmodified fertilizer and the ordinary NPK fertilizer. Compared with Comparative Example 3 which only adds biochar, the pepper plants applied with the nano-iron modified coated fertilizer in Example 3 are much higher than the blank group and the ordinary fertilizer, and the growth is more vigorous, including the degree of leaf density and the effect of stem thickness.

[0134] Therefore, the biochar-based coated fertilizer with water-retention and slow-release functions is obtained, the fertilizer has good slow-release effect and is beneficial to promoting growth of plants, can reduce nutrient loss, improve nutrient utilization rate of plants, realize long-acting of nutrient supply, has advantages of simplicity, economy, environmental protection and the like, and has wide market application prospect.

[0135] Although the above embodiment has made a detailed description of the present application, it is only a part of the embodiment of the present application, not all the embodiments, and other embodiments can be obtained according to the embodiment without creativity, and these embodiments all belong to the protection scope of the present application.

Claims

1. A method for preparing a coating solution, characterized in that, It consists of the following steps: A starch-based iron oxide solution was obtained by mixing and dissolving a starch solution with an iron-containing compound under alkaline conditions. The starch-based iron oxide solution was mixed with polyvinyl alcohol and then subjected to a cross-linking reaction to obtain a coating solution. The iron-containing compounds include ferric chloride and ferrous chloride; The method for preparing the starch solution includes: mixing and dissolving starch with water under stirring conditions to obtain a starch solution; wherein the temperature during the mixing and dissolution of starch and water is 55~65℃. The temperature during which the starch solution is mixed and dissolved with the iron-containing compound is 55-65°C; the mixing and dissolution process is accompanied by stirring. The alkaline conditions include conditions with a pH value of 10 to 11; The starch-based ferric oxide solution is mixed with polyvinyl alcohol under stirring conditions; the mixing temperature is 85~95℃. The crosslinking reaction includes adding a crosslinking agent to a mixed solution containing a starch-based iron oxide solution and polyvinyl alcohol; the crosslinking agent includes borax and glycerin.

2. The preparation method according to claim 1, characterized in that, The mass ratio of ferric iron to ferrous iron in the iron-containing compound is 2:1; The mass-to-volume ratio of the iron-containing compound to the starch solution is 0.489~2.4g:300mL.

3. The preparation method according to claim 1 or 2, characterized in that, The starch in the starch solution has a starch mass percentage of 1.3%.

4. The preparation method according to claim 1, characterized in that, The volume-to-mass ratio of the starch-based ferric oxide solution to polyvinyl alcohol is 300 mL: 6 g.

5. The preparation method according to claim 1, characterized in that, The ratio of the mass of borax, the volume of glycerol, and the volume of the mixed solution is 0.1g:2.5~3.5mL:300mL.

6. The preparation method according to claim 1, characterized in that, When mixing and dissolving starch solution with iron-containing compounds, the stirring time is 0.5~1h and the stirring speed is 400~700r / min; When mixing the starch-based ferric oxide solution with polyvinyl alcohol, the stirring time is 1~1.5h and the stirring speed is 400~700r / min.

7. The application of the coating solution obtained by the preparation method according to any one of claims 1 to 6 in biochar-based coated fertilizer.

8. A method for preparing a biochar-based coated fertilizer, characterized in that, Includes the following steps: The fertilizer core is coated with the coating liquid obtained by the preparation method according to any one of claims 1 to 6 to obtain the fertilizer core coated with the coating liquid; biochar is coated on the outer layer of the fertilizer core coated with the coating liquid to obtain the fertilizer core coated with biochar; paraffin is coated on the outer layer of the fertilizer core coated with biochar to obtain the biochar-based coated fertilizer. The raw materials for the biochar include one or more of rice straw, corn straw, and wheat straw.

9. The application of the biochar-based coated fertilizer prepared by the method of claim 8 in improving fertilizer water retention and / or prolonging fertilizer nutrient availability.

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