阅读说明：本技术 一种木质素基保水双层缓控释肥料的制备方法 (Preparation method of lignin-based water-retaining double-layer controlled-release fertilizer ) 是由 李慧 王宁 张笠 廖勋桦 魏园园 王臻煜 马应霞 张龙 于 2021-10-14 设计创作，主要内容包括：一种木质素基保水双层缓控释肥料的制备方法,步骤(1)将聚乙烯醇和纤维素衍生物共混得到混合液体,备用；步骤(2)将丙烯酸-丙烯酰胺共聚物与木质素衍生物接枝聚合得到木质素基吸水剂；步骤(3)将步骤(1)中的共混溶液均匀地喷洒在肥料颗粒的表面,重复多次包膜后得到单层包膜肥料；步骤(4)将提前研磨好的木质素基吸水剂粉末均匀地包覆在单层包膜肥料的外层,最后使肥料在55℃下烘干即得到双层包膜缓释肥料。本发明所用包覆物均为可降解材料,并采用双层包膜的形式,可弥补单层包膜性能单一、不易控制的不足。本发明工艺简单,成本低廉,工艺过程周期短且不产生废弃物。(A preparation method of a lignin-based water-retention double-layer controlled-release fertilizer comprises the following steps of (1) blending polyvinyl alcohol and a cellulose derivative to obtain a mixed liquid for later use; step (2), carrying out graft polymerization on an acrylic acid-acrylamide copolymer and a lignin derivative to obtain a lignin-based water absorbent; step (3) uniformly spraying the blending solution obtained in the step (1) on the surface of fertilizer particles, and repeatedly coating for multiple times to obtain a single-layer coated fertilizer; and (4) uniformly coating the lignin-based water absorbent powder ground in advance on the outer layer of the single-layer coated fertilizer, and finally drying the fertilizer at 55 ℃ to obtain the double-layer coated slow-release fertilizer. The coating materials used in the invention are all degradable materials, and a double-layer coating mode is adopted, so that the defects that the single-layer coating performance is single and the control is difficult can be overcome. The invention has simple process, low cost, short process period and no waste.)

1. A preparation method of a lignin-based water-retention double-layer controlled-release fertilizer is characterized by comprising the following steps:

step (1) preparation of polyvinyl alcohol and cellulose derivative blend solution: a condensation pipe and a mechanical stirrer are connected into a reaction container, 1-6g of polyvinyl alcohol and 100-200 mL of distilled water are added, the temperature of a water bath kettle is raised to 90-95 ℃, stirring is carried out, and heat preservation is carried out for 1h until the polyvinyl alcohol is completely dissolved, so as to obtain 1-6% polyvinyl alcohol solution;

cooling the system to 50-60 ℃, quickly stirring and adding 0.5-2g of cellulose derivative, stirring for 30min at the temperature to obtain 1-6% polyvinyl alcohol and cellulose derivative blending solution;

comparing the pure polyvinyl alcohol solution prepared in the step (2) with the solution obtained in the step (1);

step (3) preparation of lignin-based solution: preparing 1 mol/L sodium hydroxide solution by using a 50mL volumetric flask, weighing 0.25-1g lignin-based derivative, and adding the lignin-based derivative into the sodium hydroxide solution to obtain a lignin-based solution;

preparing a mixed solution in the step (4): dissolving acrylic acid, acrylamide and N, N' -methylene bisacrylamide in 25-30 mL of self-made distilled water to obtain a mixed solution;

uniformly dispersing the lignin-based solution and the mixed solution under ultrasonic waves, adding attapulgite into the lignin-based solution, and adding the mixed solution into the lignin-based solution after the attapulgite is uniformly dispersed;

when the system is uniformly dispersed, mixing the two solutions, adding the two solutions into a reaction container, adjusting the rotating speed of a magnetic stirring water bath to 30 rpm, adjusting the temperature to 20 ℃, adding 0.1 mol/L diluted hydrochloric acid to adjust the pH value of the system to 4-5, immediately adopting nitrogen protection, adding 0.065 g of potassium persulfate into the system when the temperature is increased to 65 ℃ under the magnetic stirring condition and the nitrogen protection, and reacting at 65 ℃, 76 ℃ and 85 ℃ for 1h respectively, wherein the nitrogen protection is adopted in the reaction process;

after the reaction in the step (7) is finished, firstly preparing 0.1 mol/L sodium hydroxide solution by using a 200 mL volumetric flask, then soaking the obtained hydrogel in the sodium hydroxide solution for 5 min, and finally washing the hydrogel for 3 times by using self-made distilled water;

step (8), drying the washed hydrogel at 75-80 ℃ for 24 h to obtain a dark brown lignin-based water absorbent;

step (9) a lignin-free water-absorbing agent was prepared according to the above-described steps as a comparative group;

step (10), firstly, placing fertilizer granules in a coating machine to rotate, then uniformly spraying a polyvinyl alcohol and cellulose derivative blending solution on the surfaces of the fertilizer granules, and repeatedly coating for multiple times to obtain a single-layer coated fertilizer;

and (11) spraying 5-10ml of ethanol on the surface of the single-layer coated fertilizer to moisten the single-layer coated fertilizer, uniformly coating the pre-ground lignin-based water absorbent powder on the outer layer of the single-layer coated fertilizer, and finally drying the fertilizer at 55 ℃ to obtain the double-layer coated slow-release fertilizer.

2. The preparation method of the lignin-based water-retention double-layer slow and controlled release fertilizer according to claim 1, characterized in that the stirring speed of the polyvinyl alcohol solution in the step (1) is 280 rpm, after the polyvinyl alcohol solution is completely dissolved, the stirring speed is adjusted to 320 rpm under the condition of adding methyl cellulose, and the polyvinyl alcohol solution is reacted in a water bath to obtain a uniform inner-layer coating with good fluidity.

3. The method for preparing the lignin-based water-retention double-layer slow and controlled release fertilizer according to claim 1, wherein the cellulose derivative in the step (1) is methyl cellulose, or cellulose acetate butyrate, or ethyl cellulose, or carboxymethyl cellulose, or hydroxyethyl cellulose, or hydroxypropyl methyl cellulose.

4. The method for preparing the lignin-based water-retention double-layer controlled-release fertilizer according to claim 1, wherein the pure polyvinyl alcohol solution in the step (2) is a blended solution without adding cellulose derivatives.

5. The method for preparing the lignin-based water-retention double-layer controlled-release fertilizer according to claim 1, wherein the lignin-based derivatives in the step (3) comprise: and adding 0.25-1.0% by mass of lignin into a sodium hydroxide solution to obtain a blending solution.

6. The method for preparing the lignin-based water-retention double-layer slow and controlled release fertilizer according to claim 1, wherein the mass of the acrylic acid in the step (4) is 3.0-5.0 g, the mass of the acrylamide is 3.0-5.0 g, and the mass of the N, N' -methylenebisacrylamide is 0.26-0.50 g and 3.0-5.0 g.

7. The preparation method of the lignin-based water-retention double-layer slow and controlled release fertilizer according to claim 1, wherein the mass of the attapulgite in the step (5) is 0.8 g, and the blending solution is subjected to ultrasonic treatment at a frequency of 80hz at room temperature for 20-30 min.

8. The method for preparing the lignin-based water-retention double-layer slow and controlled release fertilizer as claimed in claim 1, wherein the lignin-based water absorbent in the step (6) is weakly acidic in a dilute hydrochloric acid adjustment system and reacts under nitrogen and magnetic stirring conditions.

Technical Field

The invention relates to a preparation technology of a lignin-based water-retaining double-layer controlled-release fertilizer.

Background

Due to the problems of low fertilizer utilization rate, single fertilizer function, too fast nutrient release and the like, the slow-release fertilizer plays an increasingly important role in the aspects of quality improvement and efficiency improvement of modern agriculture, and is the most widely applied fertilizer with the optimal effect in the prior multifunctional fertilizer. In particular, the degradable, environment-friendly, low-price and recyclable fertilizer coating material adds a strong ink for the development of green agriculture. In recent years, China gives high attention to development and modification of slow/controlled release fertilizers, so that research and development of China are greatly promoted in corn, soybean, flowers and rice, but since the development is later than that of developed countries, the slow/controlled release fertilizer technology still has great problems in film-forming material selection, production process technology and slow/controlled effect.

Chinese patent document CN107162747B (201710435632.4) discloses a preparation method of a multifunctional compound water-retaining slow-release fertilizer. The coating material is prepared from a water-soluble cellulose derivative, polymerizable cyclodextrin, a water-soluble monomer, halloysite loaded with urea, a cross-linking agent and an initiator, and is obtained by free radical polymerization at a high temperature, so that the harm of an organic solvent to the environment is overcome, the water retention is improved, but a water-soluble polymer is easy to absorb a large amount of water in a nutrient release stage, swelling and cracking are generated, the burst release of nutrients is caused, the leakage of a film layer is caused, and the slow release performance of the nutrients is damaged. And the coating material is single-layer copolymerization coating, only shows the water retention performance of the fertilizer, has short slow release time and is difficult to control.

In recent years, lignin, the second most natural polymer material to cellulose, is not only widely available and cheap, but also has excellent biocompatibility and biodegradability, making it an outstanding green material (Tianjin Ling. preparation of lignin-based slow/controlled release pesticide and its performance research, northeast forestry university, 2015), and cellulose derivative has wide raw materials, good film-forming properties, and high transparency, and is an ideal nontoxic and harmless film material. In addition, polyvinyl alcohol is used as a medicinal auxiliary material, is easy to dissolve in water, has strong cohesive force, good film forming property and high thermal stability, is compatible with methyl cellulose blending beads, can enable the surface of the fertilizer to become smooth and compact to block the water from entering, thereby reducing the release rate of fertilizer nutrients and improving the utilization rate of the fertilizer (Schroeilong, the selection of modified polyvinyl alcohol coating and the research on the slow release performance of the coated fertilizer, the university of Sanxia, 2017.).

At present, a plurality of problems to be solved exist in the biomass-based water retention material, such as too high cost of the coating material, too hydrophilic material, poor compatibility, fast nutrient release speed and difficult degradation.

Disclosure of Invention

The invention aims to provide a preparation method of a lignin-based water-retaining double-layer controlled-release fertilizer.

The invention relates to a preparation method of a lignin-based water-retaining double-layer controlled-release fertilizer, which comprises the following steps:

step (1) preparation of polyvinyl alcohol and cellulose derivative blend solution: a condensation pipe and a mechanical stirrer are connected into a reaction container, 1-6g of polyvinyl alcohol and 100-200 mL of distilled water are added, the temperature of a water bath kettle is raised to 90-95 ℃, stirring is carried out, and heat preservation is carried out for 1h until the polyvinyl alcohol is completely dissolved, so that the polyvinyl alcohol solution with the concentration of 1-6% is obtained. Cooling the system to 50-60 ℃, quickly stirring and adding 0.5-2g of cellulose derivative, stirring for 30min at the temperature to obtain 1-6% polyvinyl alcohol and cellulose derivative blending solution;

comparing the pure polyvinyl alcohol solution prepared in the step (2) with the solution obtained in the step (1);

step (3) preparation of lignin-based solution: preparing 1 mol/L sodium hydroxide solution by using a 50mL volumetric flask, weighing 0.25-1g lignin-based derivative, and adding the lignin-based derivative into the sodium hydroxide solution to obtain a lignin-based solution;

preparing a mixed solution in the step (4): dissolving acrylic acid, acrylamide and N, N' -methylene bisacrylamide in 25-30 mL of self-made distilled water to obtain a mixed solution;

uniformly dispersing the lignin-based solution and the mixed solution under ultrasonic waves, adding attapulgite into the lignin-based solution, and adding the mixed solution into the lignin-based solution after the attapulgite is uniformly dispersed;

when the system is uniformly dispersed, mixing the two solutions, adding the two solutions into a container, adjusting the rotating speed of a magnetic stirring water bath to 30 rpm, adjusting the temperature to 20 ℃, adding 0.1 mol/L diluted hydrochloric acid to adjust the pH value of the system to 4-5, immediately adopting nitrogen protection, adding 0.065 g of potassium persulfate into the system when the temperature is increased to 65 ℃ under the magnetic stirring condition and the nitrogen protection, and then reacting at 65 ℃, 76 ℃ and 85 ℃ for 1h respectively, wherein the nitrogen protection is adopted in the reaction process;

after the reaction in the step (7) is finished, firstly preparing 0.1 mol/L sodium hydroxide solution by using a 200 mL volumetric flask, then soaking the obtained hydrogel in the sodium hydroxide solution for 5 min, and finally washing the hydrogel for 3 times by using self-made distilled water;

step (8), drying the washed hydrogel at 75-80 ℃ for 24 h to obtain a dark brown lignin-based water absorbent;

step (9) a lignin-free water-absorbing agent was prepared according to the above-described steps as a comparative group;

step (10), firstly, placing fertilizer granules in a coating machine to rotate, then uniformly spraying a polyvinyl alcohol and cellulose derivative blending solution on the surfaces of the fertilizer granules, and repeatedly coating for multiple times to obtain a single-layer coated fertilizer;

and (11) spraying 5-10ml of ethanol on the surface of the single-layer coated fertilizer to moisten the single-layer coated fertilizer, uniformly coating the pre-ground lignin-based water absorbent powder on the outer layer of the single-layer coated fertilizer, and finally drying the fertilizer at 55 ℃ to obtain the double-layer coated slow-release fertilizer.

The invention has the beneficial effects that: 1. compared with the existing single-layer water-retaining slow-release fertilizer, the single-layer water-retaining slow-release fertilizer can make up the defects of single performance, short slow-release time and difficult control of the single-layer coated fertilizer. 2. The fertilizer coated on the surface of fertilizer particles in the step (1) is coated with an inner-layer coating, so that the surface of the fertilizer becomes smooth and compact to block moisture from entering, the release rate of fertilizer nutrients is reduced, and the utilization rate of the fertilizer is improved; 3. the coating layer prepared in the step (6) is an outer coating film which is similar to a switch layer, and the water absorption and water retention capacity of the soil is improved by graft polymerization of the acrylic acid-acrylamide copolymer and the lignin derivatives, so that the fertilizer can be slowly released according to the water content of the soil, and the release rate of the fertilizer can be controlled. 4. The method is suitable for preparing various water-soluble slow-release fertilizers, and the materials used for the inner and outer coatings are natural degradable fiber materials, so that the method has no pollution to the soil environment, can promote green sustainable development of agriculture in the future, and can make up the defects that the synthetic polymer is not degradable and pollutes the soil and the environment compared with the existing synthetic polymer. 5. The invention has simple process, clear thought, simple and convenient operation, low cost, short process period, no waste generation, biodegradable coating material and contribution to increasing the yield and income.

Drawings

FIG. 1 is a schematic representation of the present invention wherein PVA and MC are polyvinyl alcohol and methyl cellulose, respectively; FIG. 2 is a fertilizer core of the coated slow release fertilizer; FIG. 3 is a process of water penetration of the controlled release fertilizer; FIG. 4 shows the dissolution process of the fertilizer core of the sustained and controlled release fertilizer; FIG. 5 is a process of releasing nutrients of a controlled release fertilizer; FIG. 6 is a diagram of a comparison sample object of uncoated, single-layer coated and double-layer coated; FIG. 7 is an SEM tissue photograph of the PVA/MC blended monolayer coating obtained in the step (1) of the implementation; FIG. 8 is a SEM photograph of the appearance of the lignin grafted and modified outer coating obtained in the step (2); FIG. 9 is an infrared spectrum of lignin (a), attapulgite (b), lignin-free gel (c) and lignin-based water absorbent (d); FIG. 10 is a differential scanning calorimetry trace of lignin (a), lignin-free gel (b), and lignin-based water absorbent (c); FIG. 11 is a graph showing the effect of the amount of lignin on the water absorption of the outer water absorbing agent; FIG. 12 is a graph showing the influence of the amount of lignin on the water content of the outer water absorbing agent; FIG. 13 is a graph of the effect of methylcellulose usage and polyvinyl alcohol concentration on the viscosity of the inner film solution; FIG. 14 is a graph of the effect of methylcellulose usage and polyvinyl alcohol concentration on water contact angle of inner layer blended films.

Detailed Description

The lignin-based water-retention double-layer slow-release fertilizer comprises an inner-layer coating material, a water-retention double-layer coating material and a water-retention double-layer coating material, wherein the inner-layer coating material comprises 1-6g of polyvinyl alcohol, 100ml of distilled water and 0.5-2g of cellulose derivatives; the outer layer coating material consists of lignin derivatives, sodium hydroxide, acrylic acid, acrylamide, N, N '-methylene bisacrylamide, distilled water, attapulgite and potassium persulfate, and the components of the outer layer coating material comprise 0.25-1g of lignin derivatives, 50mL of sodium hydroxide solution with the concentration of 1 mol/L, 3.0 g of acrylic acid, 3.0 g of acrylamide, 0.26 g of N, N' -methylene bisacrylamide, 25 mL of distilled water, 0.8 g of attapulgite and 0.065 g of potassium persulfate.

The invention relates to a preparation method of a lignin-based water-retention controlled-release fertilizer, which comprises the following steps:

step (1) preparation of polyvinyl alcohol and cellulose derivative blend solution: a condensation pipe and a mechanical stirrer are connected into a reaction container, 1-6g of polyvinyl alcohol and 100-200 mL of distilled water are added, the temperature of a water bath kettle is raised to 90-95 ℃, stirring is carried out, and heat preservation is carried out for 1h until the polyvinyl alcohol is completely dissolved, so that the polyvinyl alcohol solution with the concentration of 1-6% is obtained. Cooling the system to 50-60 ℃, quickly stirring and adding 0.5-2g of cellulose derivative, stirring for 30min at the temperature to obtain 1-6% polyvinyl alcohol and cellulose derivative blending solution;

comparing the pure polyvinyl alcohol solution prepared in the step (2) with the solution obtained in the step (1);

step (3) preparation of lignin-based solution: preparing 1 mol/L sodium hydroxide solution by using a 50mL volumetric flask, weighing 0.25-1g lignin-based derivative, and adding the lignin-based derivative into the sodium hydroxide solution to obtain a lignin-based solution;

preparing a mixed solution in the step (4): dissolving acrylic acid, acrylamide and N, N' -methylene bisacrylamide in 25-30 mL of self-made distilled water to obtain a mixed solution;

uniformly dispersing the lignin-based solution and the mixed solution under ultrasonic waves, adding attapulgite into the lignin-based solution, and adding the mixed solution into the lignin-based solution after the attapulgite is uniformly dispersed;

when the system is uniformly dispersed, mixing the two solutions, adding the two solutions into a container, adjusting the rotating speed of a magnetic stirring water bath to 30 rpm, adjusting the temperature to 20 ℃, adding 0.1 mol/L diluted hydrochloric acid to adjust the pH value of the system to 4-5, immediately adopting nitrogen protection, adding 0.065 g of potassium persulfate into the system when the temperature is increased to 65 ℃ under the magnetic stirring condition and the nitrogen protection, and then reacting at 65 ℃, 76 ℃ and 85 ℃ for 1h respectively, wherein the nitrogen protection is adopted in the reaction process;

after the reaction in the step (7) is finished, firstly preparing 0.1 mol/L sodium hydroxide solution by using a 200 mL volumetric flask, then soaking the obtained hydrogel in the sodium hydroxide solution for 5 min, and finally washing the hydrogel for 3 times by using self-made distilled water;

step (8), drying the washed hydrogel at 75-80 ℃ for 24 h to obtain a dark brown lignin-based water absorbent;

step (9) a lignin-free water-absorbing agent was prepared according to the above-described steps as a comparative group;

step (10), firstly, placing fertilizer granules in a coating machine to rotate, then uniformly spraying a polyvinyl alcohol and cellulose derivative blending solution on the surfaces of the fertilizer granules, and repeatedly coating for multiple times to obtain a single-layer coated fertilizer;

and (11) spraying 5-10ml of ethanol on the surface of the single-layer coated fertilizer to moisten the single-layer coated fertilizer, uniformly coating the pre-ground lignin-based water absorbent powder on the outer layer of the single-layer coated fertilizer, and finally drying the fertilizer at 55 ℃ to obtain the double-layer coated slow-release fertilizer.

According to the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the stirring speed of the polyvinyl alcohol solution in the step (1) is 280 rpm, the stirring speed is adjusted to 320 rpm under the condition of adding methyl cellulose after the polyvinyl alcohol solution is completely dissolved, and the polyvinyl alcohol solution reacts in a water bath to obtain an inner layer coating with uniformity and good fluidity.

In the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the cellulose derivative in the step (1) is methyl cellulose, or ethyl cellulose, or cellulose acetate butyrate, or carboxymethyl cellulose, or hydroxyethyl cellulose, or hydroxypropyl methyl cellulose.

In the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the pure polyvinyl alcohol solution in the step (2) is a blending solution without adding cellulose derivatives.

In the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the lignin-based derivative in the step (3) comprises the following steps: and adding 0.25-1.0% by mass of lignin into a sodium hydroxide solution to obtain a blending solution.

The preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer comprises the following steps of (4) 3.0-5.0 g of acrylic acid, 3.0-5.0 g of acrylamide and 0.26-0.50 g of N, N' -methylenebisacrylamide, wherein the mass of the acrylic acid is 3.0-5.0 g.

According to the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the mass of the attapulgite in the step (5) is 0.8 g, and the blending solution is subjected to ultrasonic treatment for 20-30min at the room temperature with the frequency of 80 hz.

In the preparation method of the lignin-based water-retaining double-layer controlled-release fertilizer, the lignin-based water absorbent in the step (6) is weakly acidic in a dilute hydrochloric acid adjusting system, and reacts under the conditions of nitrogen and magnetic stirring.

The technical solutions in the embodiments of the present invention will be described clearly and completely below, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Example 1: the preparation method comprises the following steps:

coating the inner layer: according to the mass portion, 1g of polyvinyl alcohol and 100ml of aqueous solution are mechanically stirred at 85 ℃ at the rotating speed of 200r/min and are kept warm for 1 h. Reducing the temperature to 45 ℃, increasing the rotating speed to 300r/min, adding 0.5g of ethyl cellulose, and rapidly stirring for 30min to obtain a blending solution for later use;

secondly, outer layer coating: 0.25g of sodium lignin sulfonate was dissolved in 100ml of sodium hydroxide solution (1 mol/L) to obtain a lignin-based solution. 4.0 g of acrylic acid, 4.0 g of acrylamide and 0.31 g of N, N' -methylenebisacrylamide were dissolved in 30 mL of distilled water to obtain a mixed solution. And (3) ultrasonically oscillating the solution at the frequency of 50MHz for 40min, adding 0.8 g of Attapulgite (ATP) into the lignin-based solution, and adding the mixed solution into the lignin-based solution. After the system is dispersed uniformly, adding HCl to adjust the pH value of the system to 4 under the conditions of magnetic stirring and room temperature. 0.085 g of potassium persulfate (KPS) is added, and the mixture is reacted for 1 hour at 60 ℃, 70 ℃ and 80 ℃ respectively under the protection of nitrogen and magnetic stirring. After the reaction was completed, the hydrogel obtained was immersed in 250 mL of NaOH solution for 10 min, and washed with distilled water 3 times. And finally drying at the temperature of 55 ℃ for 20 h to obtain the dark brown lignin-based water absorbent. Meanwhile, hydrogel containing no lignin was prepared as a control group according to the above method.

Thirdly, uniformly spraying the blending solution prepared in the step one on the surface of fertilizer granules, and repeating the steps for multiple times to prepare a single-layer coated fertilizer;

fourthly, spraying 5ml of ethanol on the surface of the single-layer coated fertilizer obtained in the third step for wetting, adding lignin-based water absorbent powder to uniformly coat the outer layer of the single-layer coated fertilizer, and drying the fertilizer at 70 ℃ to obtain the double-layer coated slow-release fertilizer.

Example 2: the preparation method comprises the following steps:

coating the inner layer: 3.5g of polyhydroxybutyrate and 50ml of chloroform are mechanically stirred at 95 ℃ at a rotation speed of 500r/min and are kept warm for 3 hours in parts by mass. Reducing the temperature to 55 ℃, reducing the rotating speed to 330r/min, adding 0.75g of cellulose acetate butyrate, and rapidly stirring for 40min to obtain a blended solution for later use;

secondly, outer layer coating: 0.75g of calcium lignosulfonate was dissolved in 150ml of sodium hydroxide solution (1 mol/L) to obtain a lignin-based solution. 5.0 g of acrylic acid, 5.0 g of acrylamide and 0.45 g of N, N' -methylenebisacrylamide were dissolved in 25 mL of distilled water to obtain a mixed solution. Subjecting the above solution to ultrasonic oscillation at 60MHz for 30min, adding 1.0 g of Attapulgite (ATP) into lignin-based solution, and adding the mixed solution into lignin-based solution. After the system is dispersed uniformly, adding HCl to adjust the pH value of the system to 5 under the conditions of magnetic stirring and room temperature. 0.075 g of potassium persulfate (KPS) was added and reacted at 55 deg.C, 65 deg.C and 75 deg.C, respectively, under nitrogen and magnetic stirring for 1 h. After the reaction was completed, the hydrogel obtained was immersed in 150mL of NaOH solution for 20min, and washed with distilled water 3 times. And finally drying for 24 h at 50 ℃ to obtain the dark brown lignin-based water absorbent. Meanwhile, hydrogel without lignin is prepared as a control group according to the method;

thirdly, uniformly spraying the blending solution prepared in the step one on the surface of fertilizer granules, and repeating the steps for multiple times to prepare a single-layer coated fertilizer;

fourthly, spraying 8ml of ethanol on the surface of the single-layer coated fertilizer obtained in the third step for wetting, adding lignin-based water absorbent powder to uniformly coat the outer layer of the single-layer coated fertilizer, and drying the fertilizer at 75 ℃ to obtain the double-layer coated slow-release fertilizer.

Example 3: the preparation method comprises the following steps:

coating the inner layer: 2.5g of polyhydroxybutyrate and 100ml of chloroform are mechanically stirred at 90 ℃ at a rotation speed of 500r/min and kept warm for 3 hours in parts by mass. Reducing the temperature to 60 ℃, increasing the rotating speed to 600r/min, adding 0.75g of hydroxyethyl cellulose, and rapidly stirring for 1h to obtain a blending solution for later use;

secondly, outer layer coating: 0.25g of acetyllignin was dissolved in 50ml of sodium hydroxide solution (1 mol/L) to obtain a lignin-based solution. 2.0 g of acrylic acid, 2.0 g of acrylamide and 0.18 g of N, N' -methylenebisacrylamide were dissolved in 30 mL of distilled water to obtain a mixed solution. Subjecting the above solution to ultrasonic oscillation at 80MHz for 20min, adding 0.5g of Attapulgite (ATP) into lignin-based solution, and adding the mixed solution into lignin-based solution. After the system is dispersed uniformly, HCl is added to adjust the pH value of the system to 4.5 under the conditions of magnetic stirring and room temperature. 0.065 g of potassium persulfate (KPS) is added, and the mixture is reacted for 1h at 65 ℃, 75 ℃ and 85 ℃ respectively under the protection of nitrogen and magnetic stirring. After the reaction was completed, the hydrogel obtained was immersed in 200 mL of NaOH solution for 30min, and washed with distilled water 3 times. And finally drying for 24 h at 60 ℃ to obtain the dark brown lignin-based water absorbent. Meanwhile, hydrogel without lignin is prepared as a control group according to the method; thirdly, uniformly spraying the blending solution prepared in the step one on the surface of fertilizer granules, and repeating the steps for multiple times to prepare a single-layer coated fertilizer;

fourthly, spraying 10ml of ethanol on the surface of the single-layer coated fertilizer obtained in the third step for wetting, adding lignin-based water absorbent powder to uniformly coat the outer layer of the single-layer coated fertilizer, and drying the fertilizer at 85 ℃ to obtain the double-layer coated slow-release fertilizer.