Method for converting lignin into humic acid-like acid by using DES

Abstract

The invention discloses a method for converting lignin into humic acid by using DES, and belongs to the technical field of humic acid preparation. Comprising the following steps: mixing and dissolving lignin and a deep-eutectic solvent, transferring the obtained solution to a high-pressure reaction kettle for hydrothermal reaction, cooling to room temperature after the reaction is finished, adding water for dilution, standing, centrifuging, and collecting precipitate to obtain the lignin-based humic acid. According to the method, the reaction type DES is utilized, a hydrothermal humification method is combined, the biomass humification process is enhanced, and the humification rate of lignin is greatly increased.

Description

Technical Field

[0001] The invention belongs to the technical field of humic acid preparation, and in particular relates to a method for converting lignin into humic acid-like substances by utilizing DES. Background Art

[0002] Humic acid, a class of biologically active macromolecules, is a key component of soil organic matter and has been widely used in agriculture, environmental protection, and chemical engineering. Traditionally, humic acid is produced primarily through extraction from coal (weathered coal, peat, and lignite). However, coal is a non-renewable resource, and large-scale mining can cause irreversible ecological damage. Furthermore, current extraction methods, primarily alkaline extraction, are complex and produce large amounts of wastewater, leading to serious environmental pollution.

[0003] Lignin is one of the key precursors for humic acid. In recent years, converting lignin-containing waste biomass into humic acid-like substances has attracted widespread attention. Hydrothermal humification of biomass has become a key research area. However, hydrothermal humification methods are still limited by low yields. Therefore, enhancing the hydrothermal humification process and increasing its yield are of great research significance.

[0004] Deep eutectic solvents (DES) are a new type of green solvent with advantages such as low toxicity, biodegradability, and on-demand preparation. A common method for preparing DES is to mix hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) in a certain proportion. Common HBDs include quaternary ammonium salts and choline derivatives, while HBAs include carboxylic acids, metal salts, and polyols. DES exhibits excellent performance in biomass dissolution. Studies have shown that reactive DES can not only dissolve biomass but also react with it. However, there are currently no reports on methods for converting lignin into humic acid-like substances using DES. Summary of the Invention

[0005] The present invention aims to address the current issues of low lignin utilization and added value. To this end, the present invention proposes a method for converting lignin into humic acid-like substances using DES. This method utilizes reactive DES in combination with hydrothermal humification to enhance the biomass humification process.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for converting lignin into humic acid-like substances using DES comprises the following steps: mixing and dissolving lignin with a deep eutectic solvent, transferring the resulting solution to a high-pressure reactor for a hydrothermal reaction, cooling the solution to room temperature after the reaction, diluting the solution with water, allowing the solution to stand, centrifuging the solution, and collecting the precipitate to obtain lignin-based humic acid-like substances.

[0008] The present invention establishes a DES-hydrothermal synergistic reaction system to achieve the targeted conversion of lignin to humic acid-like substances. The hydrogen bond donor (e.g., malic acid) in the DES forms strong hydrogen bonds with the phenolic hydroxyl groups of the lignin, breaking the β-O-4 ether bond and releasing the guaiacyl / syringyl monomers. The hydrogen bond acceptor (e.g., serine) attacks the Cα position of the lignin side chain through the Lewis basic site (-NH2), triggering the cleavage of the β-aryl ether bond (nucleophilic substitution reaction). Under the hydrothermal reaction, the lignin fragments deconstructed by the DES undergo free radical condensation (C-C coupling) and oxidation reactions (e.g., the formation of quinone methide intermediates), generating the characteristic structure of humic acid (aromatic fused ring + carboxyl / phenolic hydroxyl groups).

[0009] Furthermore, the lignin is one of enzymatically hydrolyzed lignin, alkali lignin, lignin sulfonate and organic lignin.

[0010] In the present invention, enzymatically hydrolyzed lignin retains a high concentration of phenolic hydroxyl groups (high active site density), improving its binding efficiency with DES hydrogen bond donors. Alkali lignin undergoes oxidative degradation, generating phenolic free radicals (such as guaiacyl and syringyl units). These free radicals form high-molecular-weight, cross-linked humic acid-like structures through C-C coupling (such as 5-5' and β-5' bonds) or C-O-C cross-linking (such as 4-O-5' bonds). The sulfonic acid groups in the lignin sulfonate form salt bridges with the carboxylic acids in the DES, stabilizing free radical intermediates and reducing excessive condensation. Humic acid yields vary little between different lignins, and the process is highly universal.

[0011] Furthermore, the deep eutectic solvent is prepared by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 1:(1-20).

[0012] In the present invention, amino acids (such as Lys-F) preferentially attack the α-OH in the β-O-4 bond due to their strong nucleophilicity (-NH2) and proton transfer ability, inducing ether bond cleavage to generate low molecular weight phenolic monomers.

[0013] Furthermore, the hydrogen bond donor is one of lactic acid, malic acid, formic acid, oxalic acid and ethylene glycol.

[0014] The present invention provides specific functional groups to participate in the deconstruction or reconstruction of lignin, so as to reduce side reactions (such as tar formation) and improve the selectivity of humic acid.

[0015] Furthermore, the hydrogen bond acceptor is one of serine, lysine, glycine, urea, arginine, serine, threonine, histidine and betaine.

[0016] The present invention introduces nitrogen-containing functional groups to simulate the natural humification process. The high nitrogen content of humic acid can increase the value of fertilizer.

[0017] Furthermore, the temperature of the hydrothermal reaction is 120-200° C., and the time is 2-12 hours.

[0018] The present invention balances the cracking (bond breaking) and condensation (reconstruction) reaction rates by controlling the hydrothermal reaction conditions.

[0019] Furthermore, the mass ratio of the lignin to the deep eutectic solvent is 1:(20-100).

[0020] The present invention optimizes mass transfer and reaction site accessibility by regulating the raw material dosage ratio. Excessive DES reduces mass transfer resistance but may lead to excessive dissolution of lignin (colloidalization); excessive local concentration increases free radical coupling side reactions (forming coke).

[0021] Furthermore, the mass ratio of the deep eutectic solvent to water is 1:(1-20).

[0022] The present invention regulates the viscosity of the reaction system and the stability of DES by regulating the amount of water. Water promotes the interaction between DES and lignin through hydrogen bond reconstruction. Water induces the conversion of secondary amines (-NH-) of amino acids into primary amines (-NH2), thereby improving the hydrogen bond donor capacity of DES.

[0023] Furthermore, the standing time is 2 hours.

[0024] The present invention promotes the aggregation and sedimentation of humic acid colloids through static treatment, thereby preventing the loss of small-particle humic acid.

[0025] Furthermore, the method further includes processing the precipitate using the standard method for determining humic acid established by the International Humic Acid Society (IHSS) to determine whether the prepared substance is humic acid.

[0026] The present invention selectively separates humic acid (HA) and fulvic acid (FA), and sodium pyrophosphate complexes Ca in humic acid. 2+ / Fe 3+ The humic acid is released from the metal cross-linking. Due to its large molecular weight and protonation of the carboxyl groups, the humic acid precipitates, while the fulvic acid remains in solution. The purity of the humic acid reaches 95%, meeting international standards.

[0027] Compared with the prior art, the present invention has the following advantages and technical effects:

[0028] 1) The lignin used in the present invention comes from a wide range of sources;

[0029] 2) The present invention utilizes a reactive DES solvent in combination with a hydrothermal humification process, which greatly increases the humification rate of lignin to over 40%;

[0030] 3) The liquid obtained after separating the humic acid in the present invention can be used as liquid fertilizer;

[0031] 4) This method has the advantages of being green and environmentally friendly and having a high humification rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0033] Figure 1 IR characterization images of humic acid in Example 1, Comparative Example 1 and Comparative Example 2;

[0034] Figure 2 The UV characterization diagram of humic acid in Example 1, Comparative Example 1 and Comparative Example 2;

[0035] Figure 3 This is the XPS spectrum of humic acid in Comparative Example 2;

[0036] Figure 4 This is the XPS spectrum of humic acid in Comparative Example 1;

[0037] Figure 5 This is the XPS spectrum of humic acid in Example 1;

[0038] Figure 6 3D fluorescence spectra of humic acid in Example 1(c), Comparative Example 1(b) and Comparative Example 2(a);

[0039] Figure 7 This is the infrared characterization image of Example 2;

[0040] Figure 8 This is the UV characterization diagram of Example 2;

[0041] Figure 9 This is the infrared characterization image of Example 3;

[0042] Figure 10 This is the UV characterization diagram of Example 3;

[0043] Figure 11 This is the infrared characterization image of Example 4;

[0044] Figure 12 This is the UV characterization diagram of Example 4. DETAILED DESCRIPTION

[0045] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0046] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0047] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0048] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be illustrative only.

[0049] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0050] The formation of humic acid in nature is a lengthy process, and one of its precursors is lignin. Therefore, the present invention proposes utilizing a reactive DES solvent to perform a hydrothermal humification reaction, thereby converting lignin into a humic acid-like substance.

[0051] An embodiment of the present invention provides a method for converting lignin into humic acid-like substances using DES, comprising the following steps: mixing and dissolving lignin with a deep eutectic solvent, transferring the resulting solution to a high-pressure reactor for a hydrothermal reaction, cooling the solution to room temperature after the reaction, diluting the solution with water, allowing the solution to stand, centrifuging, and collecting the precipitate to obtain lignin-based humic acid-like substances.

[0052] In some optional embodiments of the present invention, the lignin is one of enzymatically hydrolyzed lignin, alkali lignin, lignin sulfonate and organic lignin. For example, in the following preferred embodiments of the present invention, the lignin is alkali lignin or enzymatically hydrolyzed lignin.

[0053] In some optional embodiments of the present invention, the deep eutectic solvent is prepared by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 1:(1-20). For example, in the following preferred embodiments of the present invention, the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:1, 1:2, or 1:9.

[0054] In some optional embodiments of the present invention, the hydrogen bond donor is one of lactic acid, malic acid, formic acid, oxalic acid and ethylene glycol. Exemplarily, in the following preferred embodiments of the present invention, the hydrogen bond donor is malic acid, lactic acid, formic acid, lactic acid or oxalic acid.

[0055] In some optional embodiments of the present invention, the hydrogen bond acceptor is one of serine, lysine, glycine, urea, arginine, serine, threonine, histidine and betaine. Exemplarily, in the following preferred embodiments of the present invention, the hydrogen bond acceptor is serine, glycine, urea or lysine.

[0056] In some optional embodiments of the present invention, the temperature of the hydrothermal reaction is 120-200° C., and the time is 2-12 hours. For example, in the following preferred embodiments of the present invention, the temperature of the hydrothermal reaction is 160° C. or 200° C., and the time of the hydrothermal reaction is 6 hours, 8 hours, or 12 hours.

[0057] In some optional embodiments of the present invention, the mass ratio of lignin to deep eutectic solvent is 1:(20-100). For example, in the following preferred embodiments of the present invention, the mass ratio of lignin to deep eutectic solvent is 1:25, 1:50 or 1:100.

[0058] In some optional embodiments of the present invention, the mass ratio of the deep eutectic solvent to water is 1:(1-20). For example, in the following preferred embodiments of the present invention, the mass ratio of the deep eutectic solvent to water is 1:4 or 1:5.

[0059] In some optional embodiments of the present invention, the standing time is 2 hours.

[0060] In some optional embodiments of the present invention, the precipitate is further treated with the standard method for determining humic acid established by the International Humic Acid Society (IHSS) to determine whether the prepared substance is humic acid. The specific steps include: dissolving the precipitate with a mixed solution of Na4P2O7·10H2O and NaOH, shaking at room temperature for 24 hours, then centrifuging at 8000 rpm for 10 minutes, filtering through a 0.45 μm microporous membrane, then adjusting the pH to 1.5 with HCl, and separating fulvic acid (FA) and humic acid (HA) at 4°C for 12 hours; the supernatant is FA and the precipitate is HA. The HA is separated at 8000 rpm for 10 minutes, and then washed twice with HCl and deionized water, respectively.

[0061] Unless otherwise specified, the "room temperature" in the present invention refers to 25±2°C.

[0062] The raw materials used in the present invention are all purchased from the market.

[0063] The technical solution of the present invention is further illustrated by the following examples.

[0064] Example 1

[0065] A method for converting lignin into humic acid-like substances using DES comprises the following steps:

[0066] 1) Preparation of DES: Malic acid, serine, and water were mixed in a molar ratio of 1:1:3 and stirred vigorously (600 rpm). The resulting mixture was heated to 80°C until a homogeneous colorless liquid was formed, which was the DES solution.

[0067] 2) Hydrothermal synthesis of humic substances: 0.2 g of alkali lignin was mixed and dissolved with 5 g of DES solution, placed in an autoclave, and reacted at 180°C for 6 hours. After the reaction, the mixture was cooled to room temperature, and 20 g of water was added to the resulting liquid for dilution. The mixture was allowed to stand for two hours and then centrifuged to obtain the precipitate.

[0068] According to the International Humic Acids Society (IHSS) method, the precipitate was dissolved in a mixture of 0.1 mol / L Na₄P₂O₇·10H₂O and 0.1 mol / L NaOH (mass ratio of mixed solution to precipitate: 10:1) and shaken at room temperature for 24 hours. The mixture was then centrifuged at 8000 rpm for 10 minutes and filtered through a 0.45 μm microporous membrane. The pH was then adjusted to 1.5 with 6 mol / L HCl, and fulvic acid (FA) and humic acid (HA) were separated at 4°C for 12 hours. The supernatant, containing DES and FA, was dehydrated by rotary evaporation, leaving 3.8 g of DES and the precipitate containing HA. The HA was separated at 8000 rpm for 10 minutes, washed twice with 0.1 mol / L HCl and twice with deionized water, and then dried in a 60°C oven. 0.61 g of HA was weighed.

[0069]

[0070] After calculation according to the formula, the HA yield of this example is 43.57%.

[0071] Example 2

[0072] A method for converting lignin into humic acid-like substances using DES comprises the following steps:

[0073] 1) Preparation of DES: Glycine and lactic acid were mixed in a molar ratio of 1:9, stirred vigorously, and then heated to 80° C. until a homogeneous colorless liquid was formed;

[0074] 2) Hydrothermal synthesis of humic substances: 0.1 g of enzymatically hydrolyzed lignin was mixed and dissolved with 10 g of DES solution, placed in an autoclave, and reacted at 200°C for 12 hours. After the reaction, the mixture was cooled to room temperature, diluted with 40 g of water, allowed to stand for two hours, and then centrifuged to collect the precipitate.

[0075] According to the International Humic Acids Society (IHSS) method, the precipitate was dissolved in a mixture of 0.1 mol / L Na₄P₂O₇·10H₂O and 0.1 mol / L NaOH and shaken at room temperature for 24 hours. The mixture was then centrifuged at 8000 rpm for 10 minutes and filtered through a 0.45 μm microporous membrane. The pH was then adjusted to 1.5 with 6 mol / L HCl, and fulvic acid (FA) and humic acid (HA) were separated at 4°C for 12 hours. The supernatant, containing DES and FA, was dehydrated by rotary evaporation, leaving the precipitate as HA. HA was separated at 8000 rpm for 10 minutes and then washed twice with 0.1 mol / L HCl and deionized water.

[0076] Example 3

[0077] A method for converting lignin into humic acid-like substances using DES comprises the following steps:

[0078] 1) Preparation of DES: Serine and formic acid were mixed in a molar ratio of 1:1, stirred vigorously, and then heated to 80°C until a homogeneous colorless liquid was formed.

[0079] 2) Hydrothermal synthesis of humic substances: 0.1 g of enzymatically hydrolyzed lignin was mixed and dissolved with 5 g of DES solution, placed in an autoclave, and reacted at 160°C for 8 hours. After the reaction, the mixture was cooled to room temperature, diluted with 20 g of water, and allowed to stand for two hours before centrifugation to collect the precipitate.

[0080] According to the International Humic Acid Society (IHSS) method, the precipitate was dissolved in a mixture of 0.1 mol / L Na₄P₂O₇·10H₂O and 0.1 mol / L NaOH and shaken at room temperature for 24 hours. The mixture was then centrifuged at 8000 rpm for 10 minutes and filtered through a 0.45 μm microporous membrane. The pH was then adjusted to 1.5 with 6 mol / L HCl, and fulvic acid (FA) and humic acid (HA) were separated at 4°C for 12 hours. The supernatant, containing DES and FA, was dehydrated by rotary evaporation. HA was separated at 8000 rpm for 10 minutes and then washed twice with 0.1 mol / L HCl and deionized water.

[0081] Example 4

[0082] A method for converting lignin into humic acid-like substances using DES comprises the following steps:

[0083] 1) Preparation of DES: Urea and lactic acid were mixed in a molar ratio of 1:2, stirred vigorously, and then heated to 80°C until a homogeneous colorless liquid was formed;

[0084] 2) Hydrothermal synthesis of humic substances: 0.1 g of enzymatically hydrolyzed lignin was mixed and dissolved with 5 g of DES solution, placed in an autoclave, set at 200°C for 6 hours, cooled to room temperature, diluted with 20 g of water, allowed to stand for two hours, and then centrifuged to collect the precipitate;

[0085] According to the International Humic Acid Society (IHSS) method, the precipitate was dissolved in a mixture of 0.1 mol / L Na₄P₂O₇·10H₂O and 0.1 mol / L NaOH and shaken at room temperature for 24 hours. The mixture was then centrifuged at 8000 rpm for 10 minutes and filtered through a 0.45 μm microporous membrane. The pH was then adjusted to 1.5 with 6 mol / L HCl, and fulvic acid (FA) and humic acid (HA) were separated at 4°C for 12 hours. The supernatant, containing DES and FA, was dehydrated by rotary evaporation. HA was separated at 8000 rpm for 10 minutes and then washed twice with 0.1 mol / L HCl and deionized water.

[0086] Example 5

[0087] A method for converting lignin into humic acid-like substances using DES comprises the following steps:

[0088] 1) Preparation of DES: Choline chloride and ethylene glycol were mixed in a molar ratio of 1:2, stirred vigorously, and then heated to 80° C. until a homogeneous colorless liquid was formed;

[0089] 2) Hydrothermal synthesis of humic substances: 0.1 g of enzymatically hydrolyzed lignin was mixed and dissolved with 10 g of DES solution, then placed in an autoclave and reacted at 200°C for 6 hours. After the reaction, the mixture was cooled to room temperature and diluted with 40 g of water. After standing for two hours, no precipitation was observed. No precipitation was also observed when the pH was adjusted to 1 with 1 mol / L sulfuric acid.

[0090] Comparative Example 1

[0091] Humic acid was extracted from black soil, and the specific steps included: taking 5g of black soil and placing it in a conical container, adding 0.1mol / L H2SO4 solution, and mixing thoroughly. After stirring for 24 hours, filter with a Buchner funnel, wash the resulting precipitate repeatedly with distilled water until the acid is completely removed, and air-dry the resulting soil at 50°C. Then, add 20mL of 0.5mol / L KOH aqueous solution to the dry soil and keep it at 50°C for 24 hours. The resulting solution was centrifuged at 4000rpm for 20 minutes to remove the coarse suspension. The solution was then vacuumed for 24 hours, 1mol / L HCl was added to precipitate HA from the solution, and the precipitate was separated by centrifugation, followed by a drying step.

[0092] Comparative Example 2

[0093] Humic acid in the prior art was purchased from Adamas Reagent Company.

[0094] Figure 1 1 is the infrared characterization diagram of humic acid in Example 1, Comparative Example 1 and Comparative Example 2. Figure 1 It can be seen that at 3330-3400cm -1 The broad absorption band is caused by the stretching vibration of hydroxyl groups; 2800-3000cm -1 The peaks in the range reflect the aliphatic stretching vibrations of -CH3 and -CH2; 1600-1700 cm -1 The peaks at 1230-1030 cm-1 may be caused by the stretching vibration of -COOH and C=O bond of ketone; -1 Three bands within the range and 813cm -1 The peaks can be attributed to the CO stretching and CH bending of the -COOH group. It can be seen that the humic acid prepared by using DES in the present invention has functional groups similar to those of commercial humic acid and humic acid extracted from black soil.

[0095] Figure 2 The UV characterization diagram of humic acid in Example 1, Comparative Example 1 and Comparative Example 2. Since humic acid is insoluble in water and easily soluble in alkaline solution, 0.01 mol / L NaOH was used to dissolve humic acid during UV spectrum measurement. Figure 2 As can be seen in the results, all three humic acids exhibit strong absorption in the UV region, with the absorption peaks extending into the visible region. This suggests similarities in molecular structure (e.g., aromatic rings, functional groups, etc.), consistent with the infrared results. Furthermore, the absorption peak of the purchased humic acid has a longer tail, extending to 700 nm, possibly due to its more conjugated structure.

[0096] Figure 3 is the XPS spectrum of humic acid in Comparative Example 2, Figure 4 This is the XPS spectrum of humic acid in Comparative Example 1; Figure 5It is the XPS spectrum of humic acid in Example 1. Figure 3-Figure 5 As can be seen from the data, the CC peak of the purchased humic acid accounts for over 60% (sharp peak shape), while the OC=O peak accounts for less than 15%, indicating that the carbon skeleton is dominated by rigid aromatic and aliphatic structures. The CO peak of the black soil humic acid increases to 35% (peak width increases), indicating a large number of alcohol and ether oxygen-containing functional groups inserted into the carbon skeleton and significant structural branching. The OC=O peak of the DES humic acid accounts for 25%, while the CC peak accounts for 50%, indicating an enrichment of carboxylic acid and ester functional groups. The purchased humic acid is dominated by Amid / pyrrole and Amid (pyrimidine / peptide N) (symmetrical peak shape). This indicates that nitrogen is mainly present in heterocyclic (pyrrole) and biogenic peptide bonds / pyrimidines, resulting in a high degree of humification and difficulty in direct microbial utilization. The Amid / pyrrole peak of the black soil humic acid increases, while the Amid (pyrimidine / peptide N) peak narrows. This reflects the enrichment of heterocyclic nitrogen (pyrrole) and the more concentrated peptide bond / pyrimidine structures, possibly derived from lignin-protein co-degradation products. A new Primaryamine peak (accounting for approximately 15%) was found in DES humic acid, and the Amid peak split into multiple sub-peaks. This indicates that the presence of free amino groups (such as putrescine and cadaverine derivatives) has the highest reactivity of nitrogen functional groups (e.g., Mannich reactions with phenols), which can accelerate soil nitrogen cycling.

[0097] Table 1 Elemental analysis of different humic acids

[0098]

[0099] As can be seen from Table 1, the C / N value of humic acid purchased is 40, which is much higher than the C / N value 10.41 of black soil extraction and the C / N value 6.74 of DES humic acid (Example 1), indicating that its nitrogenous functional group is few, and nitrogen is relatively scarce. Black soil extraction and DES humic acid nitrogen content are high, and may contain more nitrogenous groups (such as amino, amide groups). DES humic acid C / N is the lowest, and nitrogen is the most abundant. The C / O ratio can reflect humification degree and molecular polarity, and the high humification degree of the ratio is high and the polarity is weak. The C / O value of humic acid purchased is about 1.04, and the C / O value of black soil extraction and DES humic acid is about 1.31, indicating that the humification degree of humic acid purchased is low and polarity is strong. Black soil extraction and DES humic acid humification degree are relatively high, and molecular polarity is weak, and the two are close on C / O, and humification degree and polarity characteristics are similar. The C / H value is relevant to the aromatic compound content in the molecule, and the high aromaticity of the ratio is strong. The C / H ratio of purchased humic acid is the highest (about 11.74), and the aromaticity is relatively strong; the C / H ratio of black soil extract (about 9.23) and DES humic acid (about 9.09) are lower, and the aromaticity is weaker. The two are close and have similarities in the content and structure of aromatic compounds. Purchased humic acid has more aromatic characteristics in structure.

[0100] Figure 6 3D fluorescence spectra of humic acid in Example 1(c), Comparative Example 1(b) and Comparative Example 2(a). Figure 6 It can be seen that the humic acid fluorescence peak λ Em At 400-550nm, the λ of black soil extracted humic acid and DES humic acid Em Located in λ Em 350-550nm, indicating that the humic acid extracted from black soil and DES humic acid may have more small molecules and more complex structures, and the main fluorescent active components of humic acid extracted from black soil and DES humic acid are closer in type and more similar in structure.

[0101] Figure 7 This is the infrared characterization diagram of Example 2, Figure 9 This is the infrared characterization diagram of Example 3, Figure 11 This is the infrared characterization diagram of Example 4. Figure 7 、 Figure 9 and Figure 11 It can be seen that at 3330-3400cm -1 The broad absorption band is caused by the stretching vibration of hydroxyl groups; 2800-3000cm -1 The peaks in the range reflect the aliphatic stretching vibrations of -CH3 and -CH2; 1600-1700 cm -1 The peaks at 1230-1030 cm-1 may be caused by the stretching vibration of -COOH and C=O bond of ketone; -1 Three bands within the range and 813cm -1 The peaks can be attributed to the C-O stretching and C-H bending of the -COOH groups.

[0102] Figure 8 is the UV characterization diagram of Example 2, Figure 10 is the UV characterization diagram of Example 3, Figure 12 It is the UV characterization diagram of Example 4. Figure 8 、 Figure 10 and Figure 12 As can be seen from the figure, due to the characteristics of humic acid being insoluble in water but easily soluble in alkaline solution, 0.01 mol / L NaOH was used to dissolve humic acid during UV spectrum measurement. Figure 2 As can be seen from the results, all three humic acids have strong absorption in the UV region, and the absorption peaks are tailed into the visible region. This suggests that they may have similarities in molecular structure (such as aromatic rings, functional groups, etc.), which is consistent with the infrared results.

[0103] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. A method for converting lignin into humic acid-like substances using DES, characterized in that: The method comprises the following steps: mixing and dissolving lignin with a low eutectic solvent, subjecting the obtained solution to a hydrothermal reaction, cooling the solution to room temperature after the reaction is completed, adding water to dilute the solution, allowing the solution to stand, centrifuging the solution, and collecting the precipitate to obtain lignin-based humic acid.

2. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The lignin is one of enzymatic lignin, alkali lignin, lignin sulfonate and organic lignin.

3. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The deep eutectic solvent is prepared by mixing a hydrogen bond acceptor and a hydrogen bond donor in a molar ratio of 1:(1-20).

4. The method for converting lignin into humic acid-like substances using DES according to claim 3, wherein: The hydrogen bond donor is one of lactic acid, malic acid, formic acid, oxalic acid and ethylene glycol.

5. The method for converting lignin into humic acid-like substances using DES according to claim 3, wherein: The hydrogen bond acceptor is one of serine, lysine, glycine, urea, arginine, serine, threonine, histidine and betaine.

6. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The temperature of the hydrothermal reaction is 120-200° C., and the time is 2-12 hours.

7. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The mass ratio of the lignin to the deep eutectic solvent is 1:(20-100).

8. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The mass ratio of the deep eutectic solvent to water is 1:(1-20).

9. The method for converting lignin into humic acid-like substances using DES according to claim 1, wherein: The standing time is 2 hours.

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