Preparation method of lignin slow-release fertilizer

Abstract

The invention relates to the technical field of agricultural biomass fertilizers, in particular to a preparation method of a straw lignin slow-release nitrogen fertilizer. According to the method, gramineous plants (such as corn straw) are adopted as a natural lignin source, lignin quaternary ammonium salt is prepared through a specific technology, then ion exchange is carried out on the lignin quaternary ammonium salt and bentonite, and the hydrophobic lignin-based nano hybrid material is formed. The material can effectively wrap urea to form a single-layer lignin slow-release nitrogen fertilizer, and a PBAT-starch-coated double-layer lignin slow-release nitrogen fertilizer is further synthesized. The preparation method disclosed by the invention has the advantages of simple process, low cost, environmental friendliness and the like, and the obtained slow release fertilizer product has excellent water retention performance and slow release performance, is particularly suitable for drought and water resource lacking areas, and is beneficial to improving the utilization rate of the fertilizer, reducing the influence of fertilizer loss on the environment and promoting the sustainable development of agriculture.

Description

Technical Field

[0001] The present invention relates to the technical field of agricultural biomass fertilizers, and particularly relates to a preparation method of a lignin slow-release fertilizer. Background Art

[0002] With the advancement of the global sustainable development strategy, ecological environment protection and efficient utilization of resources have received unprecedented attention. In agricultural production, the rational use of fertilizers is crucial for increasing crop yields and ensuring food security. However, the excessive application of traditional chemical fertilizers has led to many environmental problems. According to statistics, the average utilization rate of traditional fertilizers is only about 40%, and a large amount of nutrients are lost through runoff, leaching and other channels, resulting in environmental problems such as water eutrophication and soil structure damage. To address these challenges, researchers are working on developing more environmentally friendly and efficient fertilizer slow-release technologies.

[0003] Lignin, as the second most abundant macromolecular organic substance in nature, not only has a rich chemical structure and biological activity, but also plays a dual role of support and protection in plants. In addition, as an abundant biomass resource, lignin has the characteristics of wide sources, low cost, natural hydrophobicity and renewability. Developing it into a coating material for slow-release fertilizers not only has significant social and economic value, but also is of great significance for promoting the sustainable development of agriculture.

[0004] However, the current industrial application ratio of lignin is relatively low, and a large amount of lignin is treated as waste during the production process, which not only causes resource waste, but also may pose a potential threat to the environment. The existing research on lignin-based slow-release fertilizers mostly focuses on chemical modification or physical compounding, and these methods are often complex in process and high in cost, restricting their application in the field of agricultural slow-release fertilizers.

[0005] In addition, considering the problem of poor water holding capacity commonly existing in agricultural soils, especially dryland soils, developing a fertilizer that not only has slow-release performance but also can improve the water holding capacity of the soil has significant practical significance for improving fertilizer utilization rate, promoting crop growth and improving the soil environment.

[0006] Corn straw, as a natural and renewable plant material, is rich in lignin and has unique growth characteristics and various uses. The annual output of corn straw is increasing, but the recycling rate of straw is very low. Therefore, large-scale extraction of lignin from corn straw is not only feasible but also cost-effective, showing its potential in wide applications.

[0007] At present, the application of lignin in fertilizer coating materials is not yet extensive. However, considering the chemical structure and biological activity of lignin, as well as the sustainable supply of straw lignin, its application potential in the fertilizer field is worthy of further research. By developing slow-release fertilizers based on corn straw lignin, not only can the environmental friendliness and efficiency of fertilizers be improved, but also the sustainable development of agriculture can be promoted.

[0008] Using lignin as a raw material to prepare lignin-based slow-release fertilizers has the potential to realize the high-value utilization of lignin in biomass resources, which is in line with the concepts of environmental protection and sustainable development. The utilization of straw lignin helps to reduce the dependence on fossil energy and thus reduce environmental pollution. In the agricultural field, the development of such slow-release fertilizers can not only improve the environmental performance and use efficiency of fertilizers, but also may have a positive impact on soil improvement and crop growth, with significant application value and research significance. Summary of the Invention

[0009] To solve the problems presented in the above background art, the purpose of the present invention is to provide a lignin slow-release fertilizer with a wide range of raw material sources, good slow-release effects, and no pollution.

[0010] Specifically, it is achieved through the following technical solutions:

[0011] The present invention provides a preparation method of a lignin slow-release fertilizer, including the following steps:

[0012] A preparation method of a lignin slow-release fertilizer is a slow-release fertilizer formed by coating one to multiple layers of film-structured materials on the outer surface of fertilizer beads, including the following steps:

[0013] S100. Mix lignin with glycidyltrimethylammonium chloride in an alkaline solution and react at 60 - 80 °C until a reddish-brown emulsion is produced, and then dry it to obtain quaternary ammonium salt lignin;

[0014] S200. Perform ion exchange on the quaternary ammonium salt lignin and sodium-based bentonite to form a lignin-bentonite nano-hybrid;

[0015] S300. Mix the lignin-bentonite nano-hybrid with sodium alginate fertilizer beads to form fertilizer beads with a single-layer coating structure, that is, the lignin slow-release fertilizer.

[0016] As a preferred technical solution of the present invention, the alkaline solution is a sodium hydroxide solution.

[0017] As a preferred technical solution of the present invention, the specific method for the ion exchange between quaternary ammonium lignin and sodium bentonite is to vigorously stir the quaternary ammonium lignin and sodium bentonite in deionized water, so that the positively charged quaternary ammonium lignin enters the channels of sodium bentonite, forming a lignin-bentonite nano-hybrid in water.

[0018] 4. As a preferred technical solution of the present invention, the preparation method of the sodium alginate fertilizer beads is to dissolve the fertilizer into a fertilizer solution with deionized water, add a 2% w / v sodium alginate solution to the fertilizer solution and stir until a gel is formed; then slowly drop the gel into a 4% w / v calcium chloride solution for crosslinking to form the sodium alginate fertilizer beads, and take them out for drying and standby.

[0019] 5. As a preferred technical solution of the present invention, the following steps are further included:

[0020] S400: Using glycerol as a lubricant, mix starch and poly(butylene adipate-co-terephthalate) and stir evenly at 130 - 140 °C, then put them into the fertilizer beads with a single-layer coating structure and continue stirring to make them evenly coated, thus obtaining fertilizer beads with a double-layer coating structure.

[0021] 6. As a preferred technical solution of the present invention, it is characterized in that the ratio of glycerol, starch, and poly(butylene adipate-co-terephthalate) in S400 is (70 - 80)∶28∶(40 - 50).

[0022] 7. As a preferred technical solution of the present invention, it is characterized in that the fertilizer includes urea.

[0023] 8. As a preferred technical solution of the present invention, the preparation method of glycidyltrimethylammonium chloride is to mix trimethylamine and epichlorohydrin in a molar ratio of 10∶7 to form a mixture, and stir the mixture in an ice-salt bath and let it stand for reaction; wherein the weight ratio of sodium chloride to ice in the ice-salt bath is 1∶3.

[0024] 9. As a preferred technical solution of the present invention, the preparation method of the lignin includes:

[0025] Crush corn straw, place it in a hydrothermal reactor and add anhydrous ethanol, deionized water, and acetic acid to form a mixture solution, heat it to 200 °C and keep it warm, after the reaction is completed, perform vacuum filtration on the mixture solution to obtain a solid residue and wash it with anhydrous ethanol, remove the solid residue and then obtain a concentrated solution, add distilled water to the concentrated solution and stir, centrifuge, and freeze-dry to form a solid product, thus obtaining the lignin.

[0026] As a preferred technical solution of the present invention, the mixing ratio of absolute ethanol, deionized water and acetic acid added to the hydrothermal reactor is 29.1 ml∶41.6 ml∶6.66 ml.

[0027] As a preferred technical solution of the present invention, starch and polybutylene adipate-co-terephthalate (PBAT) particles are selected as the film materials. By utilizing the water absorption of starch and the biodegradability of PBAT, the slow-release effect and environmental friendliness of the fertilizer can be improved.

[0028] As a preferred technical solution of the present invention, the mixing ratio of starch, PBAT particles and glycerol is 28:45:75 to optimize the film forming performance and slow-release characteristics of the film. It can be used as a slow-release fertilizer for fertilizing crops in arid areas to improve fertilizer utilization rate and crop growth efficiency; it has good water and fertilizer retention and slow-release characteristics, which not only realizes the resource utilization of waste, but also maintains the soil water and fertilizer nutrition, improves the soil, and has good application and promotion value.

[0029] Through innovative technologies, the present invention can utilize straw lignin, bentonite and sodium alginate to develop a hydrophobic biomass-based composite bead layer for encapsulating fertilizers. On the surface of this bead layer, a composite layer of highly water-absorbent starch and PBAT is added to form a double-layer slow-release fertilizer with good controlled-release and water retention properties. This kind of fertilizer has significant application potential in sandy soil and arid areas. The raw materials are widely sourced, low-cost, the preparation materials are easily degradable, environmentally friendly, can improve fertilizer utilization rate, promote carbon sequestration and emission reduction, and reduce environmental pollution.

[0030] The preparation method of the present invention is simple, green and safe, easy to operate and control, has high stability and broad application prospects, and shows great development potential and market value as a chemical fertilizer. This technology not only has important reference value in the field of agricultural applications, but also opens up a new way for the high-value application of lignin in the biological field. Description of the Drawings

[0031] Figure 1 SEM photos of the surface and cross-section of the sodium alginate urea beads of the present invention;

[0032] Figure 2 SEM photos of the surface and cross-section of the sodium-based bentonite urea beads (B-SRFs) of the present invention;

[0033] Figure 3 SEM photos of the surface and cross-section of the single-layer coated urea beads (LB-SRFs) of the present invention;

[0034] Figure 4 SEM photos of the surface and cross-section of the double-layer film urea beads (PSLB-SRFs) of the present invention;

[0035] Figure 5 Cumulative release rate comparison curves of urea in Example, Comparative Examples 1, 2, 3, and 4 of the present invention in a 30-day static water slow-release experiment

[0036] Figure 6 Water retention rate comparison curves of Example, Comparative Examples 2, 3, and 4 of the present invention in a 48-hour water retention test Detailed implementation manners

[0037] The technical solution of the present invention will be further described in detail below through specific examples

[0038] Preparation of single-layer coated urea beads (LB-SRFs) in Example 1

[0039] The preparation method of single-layer lignin slow-release fertilizer specifically includes the following steps

[0040] 1) Experimental method for straw lignin extraction: Add 5 g of corn straw, 29.1 mL of absolute ethanol, 41.6 mL of deionized water, and 6.66 mL of acetic acid into the inner liner of a 100 mL hydrothermal reactor. Place the reactor in a constant temperature vacuum oven at 200 °C for 9 h, then take it out of the reactor and cool it to room temperature. The solid-liquid mixture is separated by filtration and washing to obtain a solid residue. The liquid is evaporated and concentrated in a rotary evaporator under vacuum (50 °C). The concentrated solution is slowly dropped into distilled water (166.6 mL) with stirring to wash the precipitated lignin. After centrifuging the mixture at a high-speed centrifuge (9000 rpm) for 7 min, the lower layer precipitate is retained. After 24 h of freezing and drying, the obtained lignin is stored in a sealed manner

[0041] 2) Synthesis of epiglycidyltrimethylammonium chloride (ETAC): Trimethylamine (TMA) and epichlorohydrin (ECH) are mixed in a molar ratio of 10:7 in a three-necked flask equipped with a condenser. Then the mixture is stirred in an ice-salt bath (weight ratio of sodium chloride / ice is 1:3), and the reaction is allowed to stand overnight

[0042] 3) Synthesis of quaternary ammonium salt lignin (QAL): According to the existing method, add 2.5 g of lignin into a flask, add 25 mL of 20 wt% NaOH solution in a water bath at 80 °C, mix for 20 min, then add ETAC into the flask, and continuously stir for 5 h until a brown-red emulsion is obtained. The final product is dried under vacuum and stored in a refrigerator at 4 °C

[0043] 4) Synthesis of urea beads

[0044] Preparation of sodium alginate urea beads

[0045] First, dissolve 0.5 g of urea in deionized water to form a urea solution, heat it to 80 °C on a heating plate, then add a 2% w / v sodium alginate solution to the urea solution and stir until a gel is formed. Then, slowly drip the gel into a 4% w / v calcium chloride solution using a syringe to form beads. Crosslinking is achieved by placing these beads in the calcium chloride solution for 20 min. After that, collect all the beads from the bottom of the flask and dry them at room temperature.

[0046] Preparation of Sodium Bentonite Urea Beads (B-SRFs)

[0047] Sodium bentonite is a natural mineral colloid with good adsorption and water retention capabilities.

[0048] Bentonite-urea beads were prepared by adding a 2% w / v bentonite solution to the sodium alginate solution, and the preparation process was the same as that for preparing sodium alginate-urea beads.

[0049] Stir straw lignin quaternary ammonium salt and sodium bentonite vigorously in deionized water to prepare straw lignin-bentonite nano-hybrid urea beads. The QAL with bentonite nano-hybrids enters the bentonite channels and forms lignin-bentonite nano-hybrids in water.

[0050] Example 2 Preparation of Double-Layer Membrane Urea Beads (PSLB-SRFs)

[0051] PBAT has high water absorbency and starch has good water retention. In the present invention, starch / PBAT composite film is used to wrap clay to form a composite material with stronger water retention and slow release properties. This double-layer slow-release fertilizer can better meet the growth needs of crops, reduce the amount of fertilizer applied, improve the water retention rate, reduce agricultural production costs, and the double-layer slow-release fertilizer is easily degradable and has no pollution to the environment.

[0052] Mix starch, PBAT particles and glycerol (lubricant) in a mass ratio of 28:45:75, and use a magnetic stirrer to mix them evenly (at 135 °C) to make a film. Put the prepared LC-SRFs into it and stir until evenly coated to obtain PSLB-SRFs.

[0053] To verify the beneficial effects of the above embodiments of the present invention, the following comparative examples are carried out for comparison with the above embodiments. The comparative examples are:

[0054] Comparative Example 1 is pure urea

[0055] Comparative Example 2 is sodium alginate urea beads

[0056] Comparative Example 3 is sodium bentonite urea beads (B-SRFs)

[0057] Comparative Example 4 is single-layer membrane urea beads (LB-SRFs)

[0058] Then, SRFs made from 0.5 g of urea were separately placed into 100-mesh nylon mesh bags. After tightening, they were placed in PVA bottles containing 200 mL of deionized water. Meanwhile, 500 mg of urea was directly added to PVA bottles under the same conditions. The urea content was measured every 6 h, 12 h, 24 h, 120 h, 240 h, 480 h, and 720 h, using an ultraviolet spectrophotometer. First, urea solutions with different concentrations were prepared, and the absorbance was measured and recorded at a wavelength of 430 nm using an ultraviolet spectrophotometer. A standard curve was plotted, and the absorbance measured for the samples was converted into a concentration value C0. The urea cumulative release rate was calculated as follows, and the experimental results are shown in Table 1.

[0059] SRFs urea release rate = C0 × 0.4 × 100%

[0060]

[0061] Table 1

[0062] As can be seen from Table 1, the sodium alginate urea beads prepared by the present invention itself have good slow-release performance, and the double-layer membrane urea beads (PSLB-SRFs) prepared based on the sodium alginate urea beads have excellent slow-release performance. The slow-release ability of slow-release urea can be further improved.

[0063] Water retention test:

[0064] Test method: In 10 g of sandy soil, 0.5 g of SRFs was added respectively, and then 5 mL of water was added. They were placed in a petri dish. By recording the total weight within 48 h, the water retention rate was calculated as follows, and the experimental results are shown in Table 2.

[0065]

[0066]

[0067] Table 2

[0068] As can be seen from Table 1 and Table 2, the double-layer membrane urea beads (PSLB-SRFs) prepared by the present invention have very good slow-release ability, and based on straw lignin, the water retention ability of the present invention is greatly improved.

[0069] Through the above experiments, the present invention successfully prepared a bio-based composite bead layer for coated urea fertilizer using corn straw lignin, low-cost bentonite, and environmentally friendly sodium alginate salt. Then, a layer of highly water-absorbent starch and PBAT (degradable plastic) thin layer was coated on the surface of the lignin-bentonite nanohybrid / alginate urea fertilizer, and a double-layer slow-release fertilizer with excellent controlled-release performance and water retention performance was successfully prepared. It has reference value for the application of lignin-bentonite nanomaterials in the agricultural field and also provides new ideas for the high-value utilization of lignin in the biological field.

[0070] The purpose, technical solution, and beneficial effects of the present invention have been further described in detail. It should be understood that the above is only the specific implementation manner of the present invention and is not used to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a lignin slow-release fertilizer, characterized in that: It is a slow-release fertilizer formed by coating one or more layers of film structure on the surface of fertilizer beads, including the following steps: S100, mixing lignin with epoxypropyltrimethylammonium chloride in an alkaline solution, reacting at 60-80° C. until a reddish brown emulsion is produced, and drying the emulsion to obtain quaternary ammonium salt lignin; S200, performing ion exchange on the quaternary ammonium salt lignin and sodium bentonite to form a lignin-bentonite nanohybrid; S300, mixing the lignin-bentonite nano-hybrid with sodium alginate fertilizer beads to form fertilizer beads with a single-layer coating structure, namely, the lignin slow-release fertilizer.

2. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The alkaline solution is a sodium hydroxide solution.

3. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The specific method of ion exchange between the quaternary ammonium salt lignin and the sodium bentonite is to vigorously stir the quaternary ammonium salt lignin and the sodium bentonite in deionized water, so that the positively charged quaternary ammonium salt lignin enters the channel of the sodium bentonite to form a lignin-bentonite nano hybrid in water.

4. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The preparation method of the sodium alginate fertilizer beads is to dissolve the fertilizer into a fertilizer solution with deionized water, add a 2% w / v sodium alginate solution into the fertilizer solution and stir until a gel is formed; then slowly drip the gel into a 4% w / v calcium chloride solution for cross-linking to form the sodium alginate fertilizer beads, and take out and dry for later use.

5. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The following steps are also included: S400, using glycerol as a lubricant, starch and polybutylene adipate-terephthalate are mixed at 130-140° C., stirred evenly, and then the fertilizer beads with a single-layer coating structure are put into the mixture and stirred continuously to make them evenly coated, thereby obtaining fertilizer beads with a double-layer coating structure.

6. The method for preparing the lignin slow-release fertilizer according to claim 5, characterized in that: The ratio of glycerol, starch and polybutylene adipate-terephthalate in S400 is (70-80):28:(40-50).

7. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The fertilizer includes urea.

8. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The preparation method of the epoxypropyl trimethylammonium chloride is to mix trimethylamine and epichlorohydrin in a molar ratio of 10:7 to form a mixture, place the mixture in an ice-salt bath, stir, and stand for reaction; wherein the weight ratio of sodium chloride to ice in the ice-salt bath is 1:

3.

9. The method for preparing the lignin slow-release fertilizer according to claim 1, characterized in that: The preparation method of the lignin comprises: The corn stalks are crushed, placed in a hydrothermal kettle, and anhydrous ethanol, deionized water, and acetic acid are added to form a mixture solution, which is heated to 200° C. for heat preservation. After the reaction is completed, the mixture solution is vacuum filtered to obtain a solid residue, which is washed with anhydrous ethanol. After the solid residue is removed, a concentrated solution is obtained, distilled water is added to the concentrated solution, and the solution is stirred, centrifuged, and freeze-dried to form a solid product, namely, the lignin.

10. The method for preparing the lignin slow-release fertilizer according to claim 9, characterized in that: The mixing ratio of the anhydrous ethanol, deionized water and acetic acid added to the hydrothermal kettle is 29.1 ml: 41.6 ml: 6.66 ml.

Images