Method for efficiently preparing fulvic acid

Abstract

The invention discloses a method for efficiently preparing fulvic acid. The forestry waste is smashed and sieved, the pH is adjusted to be acidic, and a pretreatment solution is obtained after ultrasonic treatment; the preparation method comprises the following steps: dissolving Fe (NO3) 3.9 H2O in deionized water, adding gamma-Al2O3, carrying out ultrasonic treatment, adding EDTA-2Na, stirring until the solution is clear, filtering, and drying to obtain a composite catalyst; adding an H2O2 solution and a composite catalyst into the pretreatment solution, mixing, heating and stirring for reaction, adjusting the pH to be alkaline, standing and filtering to obtain supernate; and drying the supernate to constant weight, grinding and sieving to obtain the fulvic acid. According to the method, the masson pine sawdust is used as a raw material, and the EDTA-Fe / Al2O3 composite catalyst and H2O2 are subjected to oxidation reaction, so that the yield, the purity and the carboxyl content of fulvic acid are remarkably improved. The method can be used for soil improvement, organic fertilizer production and environmental governance, and has remarkable ecological and economic values.

Description

Technical Field

[0001] The present invention relates to the technical fields of environmental protection and resource recycling, and particularly relates to a method for efficiently preparing fulvic acid. Background Art

[0002] Fulvic acid is a water-soluble substance in the form of a reddish-brown or grayish-black powder, an organic compound containing various oxygen functional groups, which can promote plant growth, play an important role in drought resistance, improve the stress resistance of plants, increase production and improve quality, and is widely used in aspects such as soil improvement, plant nutrition, sewage treatment, etc.

[0003] The preparation methods of fulvic acid include the ion exchange method, the sulfuric acid-acetone method, etc. Among them, the ion exchange method is as follows: water and regenerated hydrogen-type strong acid cation exchange resin are pre-placed in a reactor, weathered coal powder ground to less than 100 meshes is added, after the reaction, the material is discharged into a sedimentation tank, so that the coal powder slag and the resin settle, the upper aqueous solution containing fulvic acid, after further deashing by centrifugation, flows into an evaporator for concentration, and is input into a spray dryer for drying to obtain the finished product. The sulfuric acid-acetone method: weathered coal of 40-60 meshes and acetone containing 10%-20% water are added to a reaction tank in a certain proportion, and concentrated sulfuric acid is gradually added under stirring to make fulvic acid beneficial and dissolve in the solvent. After the reaction, the material is discharged into a sedimentation tank and naturally settled for 8 hours, the clarified extract is transferred to a jacketed heating evaporator to evaporate most of the solvent, and the concentrate is also poured into a shallow pan and dried in an oven to obtain the product. The above methods are all traditional fulvic acid preparation methods, mostly relying on coal, peat, etc. or high-energy-consuming and highly polluting chemical methods, causing relatively large environmental pressure.

[0004] Plant-derived fulvic acid can be prepared from forestry waste without using weathered coal and without wasting fossil raw materials. Therefore, the preparation of fulvic acid from plant sources has become a new trend. There are two methods for the production of plant-derived fulvic acid: one is the microbial fermentation method, in which organic matter is converted into humic acid-like active substances including biochemical fulvic acid (BFA) under the combined action of various microorganisms. However, microbial fermentation takes a long time and the production rate is low. The other is the chemical method, in which a catalyst and an oxidant are added for chemical decomposition at high temperature to prepare oxidized fulvic acid (OFA). The preparation rate is relatively fast and it is widely used in production. However, the fulvic acid fertilizer produced by the chemical method has unbalanced nutrients and needs to be mixed with other mineral fertilizers for subsequent use. The patent with application number CN202310796695.8 discloses a method for co-producing fulvic acid fertilizer from agricultural waste, which produces fulvic acid organic fertilizer by combining the microbial fermentation method and the chemical catalytic oxidation method. This method not only has a long preparation time and a complex preparation method, but also the highest content of fulvic acid obtained is 66%. The patent with application number CN202410134857.6 discloses a method for preparing potassium humate and a Fenton-like catalyst by thermal catalysis of bamboo powder. The residue containing metallic iron is mixed with a template agent, ground, and then subjected to high-temperature calcination in a nitrogen atmosphere; the calcined product is soaked in hydrochloric acid, washed with water, and dried to obtain an atomically dispersed iron-carbon Fenton-like catalyst with good performance. The iron-carbon Fenton-like catalyst is used to prepare potassium humate from weathered coal. The highest yield of potassium humate is 58.2%, and the highest biomass conversion rate is 86.25%. Although this method has a high biomass conversion rate, it requires weathered coal as a fossil raw material, and the potassium humate obtained contains fewer carboxyl groups than fulvic acid. When used for saline-alkali soil improvement, the effect is not as good as that of fulvic acid. Therefore, a chemical method for preparing fulvic acid is needed, which can not only quickly prepare fulvic acid, but also improve the product purity and carboxyl content of fulvic acid and reduce resource consumption. Summary of the Invention

[0005] In view of the above-mentioned prior art, the object of the present invention is to provide a method for efficiently preparing fulvic acid. The present invention uses pine sawdust as a raw material and utilizes the oxidation reaction of the EDTA-Fe / Al2O3 composite catalyst with H2O2 to significantly improve the yield, purity and carboxyl content of fulvic acid. It can not only be used for soil improvement and the production of organic fertilizers, but also be applied to heavy metal pollution remediation and environmental governance, with significant ecological and economic value.

[0006] To achieve the above object, the present invention adopts the following technical solutions: In the first aspect of the present invention, a method for efficiently preparing fulvic acid is provided, comprising the following steps: (1) Crushing and sieving forestry waste containing lignin, adding an acidic solution to adjust the pH to acidic, and performing ultrasonic treatment to obtain a pretreatment solution; (2) Dissolve Fe(NO3)3·9H2O in deionized water, add γ-Al2O3 powder, and then perform ultrasonic treatment. After ultrasonic treatment, add EDTA-2Na and stir until the solution becomes clear. Filter and dry to obtain the composite catalyst. (3) Add the H2O2 solution and the composite catalyst prepared in step (2) to the pretreatment solution prepared in step (1) and mix them. Heat and stir for reaction. After the reaction, adjust the pH to alkaline with an alkali solution, let it stand, and filter to obtain the supernatant. Dry the supernatant to a constant weight, grind it and pass it through a sieve to obtain a brownish powder, which is fulvic acid.

[0007] Preferably, in step (1), the forestry waste containing lignin is plant sawdust; the acidic solution includes sulfuric acid solution and hydrochloric acid solution; adjust the pH to 3.5.

[0008] Preferably, the plant sawdust includes masson pine sawdust; the volume ratio of the sulfuric acid solution to the hydrochloric acid solution is 3:1; the concentration of the sulfuric acid solution is 1 mol / L; the concentration of the hydrochloric acid solution is 0.5 mol / L.

[0009] Preferably, in step (1), the temperature of the ultrasonic treatment is 40 - 50 °C, the time of the ultrasonic treatment is 0.5 - 1.5 h, and the power of the ultrasonic treatment is 100 W.

[0010] Preferably, in step (2), the mass ratio of Fe(NO3)3·9H2O to γ-Al2O3 is 1:2; the molar ratio of EDTA-2Na to Fe(NO3)3·9H2O is 1.2:1.

[0011] Preferably, in step (2), the temperature of the ultrasonic treatment is 40 - 50 °C, the time of the ultrasonic treatment is 1 - 3 h; the temperature of the drying is 80 °C, and the time of the drying is 6 - 10 h.

[0012] Preferably, in step (3), the addition amount of the H2O2 solution is 15% of the total mass of the forestry waste; the addition amount of the composite catalyst is 5% of the total mass of the forestry waste; the mass concentration of the H2O2 solution is 30%.

[0013] Preferably, in step (3), the temperature of the thermal stirring reaction is 70 - 90 °C, the time of the thermal stirring reaction is 1 - 2 h; the alkali solution is 1 mol / L NaOH solution; adjust the pH to 9.0; the standing time is 20 - 40 min; the temperature of the drying is 60 °C.

[0014] In the second aspect of the present invention, there is provided the application of the above method in at least one of the following 1) - 3): 1) Improve the yield of fulvic acid; 2) Improve the purity of fulvic acid; 4) Prepare low-molecular-weight fulvic acid.

[0015] Preferably, the purity of the fulvic acid ≥ 92%, the content of carboxyl groups in the fulvic acid ≥ 12 mmol / g, and the molecular weight of the fulvic acid is 300 - 500 Da.

[0016] Advantages of the present invention: (1) The present invention uses masson pine sawdust (forestry processing waste) as the main raw material, replacing traditional fossil-based raw materials (such as lignite, peat), significantly reducing the raw material cost. Using masson pine sawdust directly for preparation without pretreatment simplifies the preparation process and reduces costs. Through the resource utilization of waste, it reduces the environmental pressure brought by incineration or landfill, meeting the concept of circular economy and sustainable development.

[0017] (2) The present invention uses the green oxidant H2O2, avoiding the pollution caused by traditional strong oxidants. The reaction by-product is only water, meeting the requirements of green chemistry, and the catalyst can be recycled.

[0018] (3) Through precise pH adjustment and stepwise acid-base treatment in the present invention, the purity and yield of the fulvic acid product are greatly improved, and the generation of waste is reduced, meeting the goals of circular economy and sustainable development.

[0019] (4) The preparation method of the present invention is simple, with low and easily obtainable raw material costs, mild reaction conditions, and low equipment requirements, which can greatly reduce the production cost and has broad market application prospects. Description of the Drawings

[0020] Figure 1 : Photograph of the fulvic acid prepared in the example; Figure 2 : Infrared spectrum of the fulvic acid prepared in the example. Detailed Embodiments

[0021] It should be noted that the following detailed description is illustrative and is intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present application belongs.

[0022] As introduced in the background art section, the methods for preparing fulvic acid using forestry waste are mainly biological fermentation method and chemical method. Among them, the chemical method is to add a catalyst and an oxidant to carry out chemical decomposition at high temperature to prepare oxidized fulvic acid (OFA). The preparation rate is relatively fast and it is widely used in production. However, the fulvic acid produced by the chemical method has low purity. There are many degradation products of lignin, which can decompose to produce sugars such as monosaccharides and oligosaccharides, amino acids, polypeptides, phenolic acids, and may also produce humic acid or brown humic acid, etc. Therefore, it is easy to cause low yield and purity of fulvic acid.

[0023] Based on this, the object of the present invention is to provide a method for efficiently preparing fulvic acid. The present invention uses masson pine sawdust as the core raw material, which has a lignin content of 25-30% and is rich in phenylpropane structural units (such as guaiacyl, syringyl), serving as the precursor substance of fulvic acid. Through the design of an iron-based catalyst (a composite support of Fe(NO3)3·9H2O and γ-Al2O3), highly dispersed FeOx active sites (particle size <10 nm) are formed, which have both Lewis acidity and redox properties. Under the synergistic effect of ultrasonic activation (100 W, 1 h) and H2O2 oxidation (30% concentration), the iron-based catalyst triggers a Fenton-like reaction to generate highly active ·OH radicals, which preferentially attack the β-O-4 bond and Cα-Cβ bond in lignin, directionally break the side chains of non-aromatic structures, while retaining the condensed aromatic nucleus and introducing carboxyl groups (-COOH) and phenolic hydroxyl groups (-OH), thereby generating low-molecular-weight fulvic acid (FA). In addition, EDTA-2Na is used as a coordination stabilizer to form a stable complex with Fe 3+ to inhibit its hydrolysis and precipitation and chelate Ca 2+ / Mg 2+ impurities, significantly improving the cyclic stability of the catalyst (>5 times of use).

[0024] A composite-loaded substance of Fe(NO3)3·9H2O and γ-Al2O3 is adopted, combined with ultrasonic-assisted impregnation technology, to achieve high dispersion of the metal active components. By introducing EDTA-2Na as a coordination stabilizer, the antioxidant capacity and cyclic stability of the Fe-based substance are enhanced. This catalyst can efficiently activate H2O2 to generate active free radicals (such as ·OH), directionally promote the cleavage and oxidation reaction of the benzene ring structure in lignin, and can significantly improve the yield and purity of fulvic acid compared with the single addition of such substances and related processes. Using H2O2, a green oxidant, to replace traditional strong oxidants for preparing fulvic acid (such as concentrated nitric acid, potassium permanganate), the reaction by-product is only water, avoiding pollution by nitro compounds or heavy metal residues. In the preparation of the catalyst, through ultrasonic impregnation at 50 °C for 2 h, the high-frequency cavitation effect is utilized to promote the uniform loading of Fe³+ on the mesoporous structure (pore size 2-5 nm) of γ-Al2O3, forming stable Fe-O-Al bonds, enhancing the interaction between the support and the active components. At the same time, some surface hydroxyl groups (-OH) of the support are removed to expose more active sites, increasing the activation efficiency of H2O2 by 15-20%. The iron-based catalyst can be recycled, and the process waste liquid can meet the discharge standards after simple neutralization treatment, meeting the requirements of green chemistry and clean production In addition, the present invention uses ultrasonic activation (100 W, 1 h) to avoid the decomposition or mineralization of small molecule substances under high temperature and high pressure conditions, ensuring the integrity of low molecular weight active ingredients. The ·OH free radicals generated by the synergistic action of the iron-based catalyst and H2O2 preferentially attack the β-O-4 bond and Cα-Cβ bond in lignin, retain the aromatic nucleus structure and introduce active functional groups such as carboxyl groups and phenolic hydroxyl groups, generating a large amount of small molecule fulvic acid (FA) and its derivatives. Under acidic conditions, H + protonates the lignin ether bond, and Cl⁻ penetrates into the microporous structure of lignin, synergistically promoting the swelling and depolymerization of lignocellulose; at the same time, it inhibits the self-decomposition of H2O2 and prolongs the action time of ·OH free radicals. After the reaction is completed, the pH is adjusted to 9 with 1 mol / L NaOH, so that the carboxyl group (-COOH) of fulvic acid ionizes to -COO⁻, and combines with Fe 3+ and Al 3+ in the system to form an insoluble complex precipitate, realizing the efficient separation from unreacted lignin and humic acid (HA), and the product yield exceeds 90%. Through the step-by-step operations of acid depolymerization (pH = 3.5) and alkaline precipitation (pH = 9), small molecule fulvic acid is selectively separated to avoid the co-precipitation loss with humic acid (HA) or unreacted lignin. Therefore, applying this production process can retain more small molecule active substances, which have a significant promoting effect on soil improvement and plant growth.

[0025] By ultrasonic pretreatment (100 W, 1 h), the harsh conditions (such as high temperature and high pressure) of traditional acid hydrolysis or high temperature pyrolysis processes are reduced. The pH regulation of the reaction system adopts a composite acidification of sulfuric acid and dilute hydrochloric acid (3:1), avoiding the high corrosiveness of a single strong acid to equipment, reducing the consumption of acid and base reagents at the same time, and significantly reducing the overall process energy consumption and operating cost.

[0026] The fulvic acid prepared by the present invention has a high aromatic condensation degree (nC / nO = 0.87) and rich oxygen-containing functional groups (such as carboxyl groups and phenolic hydroxyl groups), and has excellent ion exchange ability and biostimulation activity. The product can be directly used for soil improvement, organic fertilizers or heavy metal pollution remediation, and has a wide range of market application scenarios.

[0027] In order to enable those skilled in the art to more clearly understand the technical solutions of the present application, the technical solutions of the present application will be described in detail below with specific embodiments.

[0028] The test materials used in the embodiments of the present invention are all conventional test materials in the art and can be obtained through commercial channels.

[0029] Example: Preparation of fulvic acid (1) Raw material pretreatment: The masson pine sawdust was crushed and passed through an 80-mesh sieve. 40 g of the sifted material was mixed with 400 ml of deionized water, and 1 mol / L sulfuric acid and 0.5 mol / L hydrochloric acid were mixed in a ratio of 3:1 to adjust the pH to 3.5. It was treated in a 100-W ultrasonic reactor for 1 hour, and the temperature was maintained at 45 °C to obtain a pretreatment solution.

[0030] (2)Catalyst preparation: Fe(NO3)3·9H2O was dissolved in deionized water and mixed with γ-Al2O3 powder. The mass ratio of Fe(NO3)3·9H2O to γ-Al2O3 was 1:2. It was ultrasonically dispersed at 50 °C for 2 hours to ensure that Fe 3+ was evenly loaded on the surface of the carrier. EDTA-2Na with a molar amount 1.2 times that of Fe 3+ was added, and the mixture was stirred until the solution became clear. Subsequently, it was dried in an inert atmosphere (nitrogen) at 80 °C for 8 hours. The dried sample was placed in a hydrothermal reaction kettle, heated to 180 °C, maintained for 2 hours, then naturally cooled, filtered, and dried to obtain a composite catalyst.

[0031] (3)Oxidation reaction and product separation: The pretreated sawdust solution was mixed with 30% H2O2 solution and the composite catalyst. The addition amount of the H2O2 solution accounted for 15% of the mass of the sawdust, and the addition amount of the composite catalyst accounted for 5% of the mass of the sawdust. The reaction was stirred at 80 °C for 90 minutes. After the reaction ended, the pH was adjusted to 9 with 1 mol / L NaOH, allowed to stand for 30 minutes, and the residue and catalyst particles were removed by filtration to obtain the supernatant.

[0032] (4)Product purification: The supernatant was transferred to a vacuum drying oven at 60 °C and dried to a constant weight, ground, and passed through a 100-mesh sieve. Finally, a brownish powder, which is fulvic acid, was obtained, as shown in Figure 1 . The infrared spectrum of the product is as shown in Figure 2 , and the analysis shows that it has typical aromatic structure and oxygen-containing functional group characteristics.

[0033] Comparative Example 1: Prepared according to Example 1 in the preparation method and application of a kind of ore-source fulvic acid with the application number CN202311302644.1 Raw materials: weathered coal (from Changzhi Coal Mine, Shanxi; the total humic acid content in the weathered coal is 53.02%, the fulvic acid content is 35.15%, and the moisture content is 13.28%); Extraction method: Crushing: The weathered coal was air-dried under natural conditions and crushed and sieved to less than 100 meshes; Washing: Weigh 10 g of the sample into a centrifuge tube, add distilled water and shake and wash three times, then perform centrifugation. The mass-volume ratio of the weathered coal to distilled water is 1:8 (w / v). Take out the centrifuge tube, pour off the supernatant and set aside.

[0034] Activation: Add the washed weathered coal into a reaction vessel, add distilled vinegar and citric acid in different ratios, stir and mix evenly. Under the condition of 40 - 60 °C, after reacting for 30 - 60 min, the activated weathered coal is obtained; Acid extraction: Add a hydrochloric acid solution with a mass concentration of 10% into the activated weathered coal, stir well at 40 °C for 1 h, then perform solid - liquid separation with an 800 - mesh sieve, and wash the residue three times with distilled water. Collect and combine the filtrates; Drying: Evaporate with a rotary evaporator and vacuum - dry. The temperature of the rotary evaporator is 65 °C to obtain fulvic acid.

[0035] Comparative Document 2 (1) Raw material pretreatment: Crush the masson pine sawdust and pass it through an 80 - mesh sieve. Take 40 g of the sifted material and mix it with 400 ml of deionized water. Add a mixture of 1 mol / L sulfuric acid and 0.5 mol / L hydrochloric acid in a ratio of 3:1 to adjust the pH to 3.5. Treat it in a 100 - W ultrasonic reactor for 1 hour, and maintain the temperature at 45 °C to obtain a pretreatment solution.

[0036] (2) Oxidation reaction and product separation: Mix the pretreated sawdust solution with 30% H2O2 solution and Fe2O3. The addition amount of the H2O2 solution accounts for 15% of the mass of the sawdust, and the addition amount of Fe2O3 accounts for 5% of the mass of the sawdust. Stir and react at 80 °C for 90 minutes. After the reaction, adjust the pH to 9 with 1 mol / L NaOH, let it stand for 30 minutes, filter to remove the residue and catalyst particles to obtain the supernatant.

[0037] (4)Product purification: Transfer the supernatant to a vacuum drying oven at 60 °C and dry it to a constant weight, grind it through a 100 - mesh sieve. Finally, the obtained brown - colored powder is fulvic acid.

[0038] Comparative Example 3 The difference from the example is that in step (2), Fe(NO3)3·9H2O is dissolved in deionized water and mixed with γ - Al2O3 powder. The mass ratio of Fe(NO3)3·9H2O to γ - Al2O3 is 1:2, and it is ultrasonically dispersed at 50 °C for 2 hours to ensure that Fe 3+ is evenly loaded on the surface of the carrier. Add 1.2 times the molar amount of Fe 3+ of EDTA - 2Na, stir until the solution is clear, then dry at 80 °C for 8 hours and calcine at 500 °C for 4 hours with a heating rate of 5 °C / min to obtain a composite catalyst.

[0039] Finally, the obtained brown - colored powder is fulvic acid.

[0040] Test Example 1 The purity of the product was calculated by the volumetric method, the biomass conversion rate was calculated by the organic matter content method, and the carboxyl group content was calculated by the acid-base titration method. The results are shown in Table 1.

[0041] Table 1 As can be seen from Table 1, the fulvic acid prepared in the examples not only has higher yield, biomass conversion rate and purity than those in Comparative Examples 1-3, but also has a higher carboxyl group content than those in Comparative Examples 1-3, and the molecular weight range is lower than that in Comparative Examples 1-3. It shows that more fulvic acid with low molecular weight can be prepared by using the method of the present invention.

[0042] Test Example: Improvement of saline-alkali land In Huanghekou Town, Kenli District, Dongying City, Shandong Province, at a latitude of 37°45'N and a longitude of 118°30'E, a test on improving saline-alkali land with fulvic acid was carried out. The local saline-alkali land is moderately saline-alkali land with a salt content of 3.0‰.

[0043] The fulvic acid prepared in the examples and Comparative Examples 1-3 was used for the test of improving saline-alkali land. The test was divided into four groups: blank control group, example group, Comparative Example 1 group, Comparative Example 2 group, and Comparative Example 3 group. Each group improved 1 mu of saline-alkali land. Except for the blank control group without adding a modifier, the example group and Comparative Examples 1-3 groups were added with the fulvic acid prepared in the examples and Comparative Examples 1-3 as modifiers respectively. The specific test process is as follows: Land ploughing: Before the test started, all the test plots were ploughed with a ploughing depth of 20 cm to ensure that the modifier could be evenly distributed in the soil. Seven days before crop planting, the fulvic acid prepared in the example group and Comparative Examples 1-3 groups was evenly spread on the soil surface, and then mixed into the soil by ploughing. 200 kg of fulvic acid was applied per mu of land. The salt-tolerant crop Suaeda glauca was selected as the test crop, and 8500 holes were planted per mu of land, with a plant spacing of 20 cm and a row spacing of 30 cm. The blank control group did not apply fulvic acid. During the test, the water and fertilizer management of all groups was kept consistent to ensure the reliability of the test results. After 60 days of crop growth, the crop physiological characteristic data of each group were measured.

[0044] Before the test, 50 points were sampled at the same position in each plot of each group, and the pH value in the soil was detected, and the obtained results were averaged. After the improvement, 50 points were sampled at the same position in each plot of each group, and the pH value in the soil was detected, and the obtained results were averaged. Before and after the improvement, the pH of the soil is shown in Table 2.

[0045] Table 2 Note: Different letters represent significant differences in different rice physiological indexes under different treatments (P<0.05).

[0046] As can be seen from Table 2, the crop growth conditions in the example group were significantly better than those in other groups, and the plant height, chlorophyll content, above-ground fresh weight, and root fresh weight were all significantly higher than those in the blank control group and Comparative Example Groups 1 to 3. The crop growth conditions in Comparative Example Group 1 and Comparative Example Group 2 were also improved, but the effect was not as significant as that in the example group. In terms of salt content, the soil salt content in the example group was the lowest, indicating that the fulvic acid prepared in the example had the best improvement effect on saline-alkali land.

[0047] The above are only the preferred embodiments of the present application and are not used to limit the present application. For those skilled in the art, various changes and modifications can be made to the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for efficiently preparing fulvic acid, characterized in that: The following steps are involved: (1) Crush and sieve forestry waste containing lignin, add an acidic solution to adjust the pH to acidic, and obtain a pretreated solution after ultrasonic treatment; (2) Fe(NO3)3·9H2O was dissolved in deionized water, γ-Al2O3 powder was added and ultrasonic treatment was performed, EDTA-2Na was added after ultrasonic treatment, and the solution was stirred until the solution was clear, filtered and dried to obtain a composite catalyst; (3) Adding H2O2 solution and the composite catalyst prepared in step (2) to the pretreatment solution prepared in step (1) and mixing, heating and stirring to react, adjusting the pH to alkaline with alkali solution after the reaction is completed, letting it stand, and filtering to obtain a supernatant; drying the supernatant to constant weight, grinding and sieving to obtain a brown powder, which is fulvic acid.

2. The method according to claim 1, characterized in that In step (1), the forestry waste containing lignin is plant sawdust; the acidic solution includes sulfuric acid solution and hydrochloric acid solution; and the pH is adjusted to 3.

5.

3. The method according to claim 2, characterized in that The plant sawdust includes Masson pine sawdust; the volume ratio of the sulfuric acid solution to the hydrochloric acid solution is 3:1; the concentration of the sulfuric acid solution is 1 mol / L; and the concentration of the hydrochloric acid solution is 0.5 mol / L.

4. The method according to claim 1, characterized in that In step (1), the temperature of the ultrasonic treatment is 40-50°C, the time of the ultrasonic treatment is 0.5-1.5h, and the power of the ultrasonic treatment is 100W.

5. The method according to claim 1, characterized in that In step (2), the mass ratio of Fe(NO3)3·9H2O and γ-Al2O3 is 1:2; and the molar ratio of EDTA-2Na to Fe(NO3)3·9H2O is 1.2:

1.

6. The method according to claim 1, characterized in that In step (2), the temperature of the ultrasonic treatment is 40-50°C, and the time of the ultrasonic treatment is 1-3 hours; the temperature of the drying is 80°C, and the time of the drying is 6-10 hours.

7. The method according to claim 1, characterized in that In step (3), the amount of the H2O2 solution added is 15% of the total mass of the forestry waste; the amount of the composite catalyst added is 5% of the total mass of the forestry waste; and the mass concentration of the H2O2 solution is 30%.

8. The method according to claim 1, characterized in that In step (3), the temperature of the hot stirring reaction is 70-90°C, and the time of the hot stirring reaction is 1-2 hours; the alkali solution is 1 mol / L NaOH solution; The pH value is adjusted to 9.0; the standing time is 20-40 minutes; and the drying temperature is 60°C.

9. Use of the method according to any one of claims 1 to 8 in at least one of the following 1) to 3): 1) Improve the yield of fulvic acid; 2) Improve the purity of fulvic acid; 3) Increase the carboxyl content in fulvic acid; 4) Preparation of low molecular weight fulvic acid.

10. The use according to claim 9, characterized in that: The purity of the fulvic acid is ≥92%, the content of carboxyl groups in the fulvic acid is ≥12 mmol / g, and the molecular weight of the fulvic acid is 300-500 Da.

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