A method for synthesizing humic acid and the application of spongy MnFe2O4 catalyst in humic acid synthesis

Abstract

This invention relates to a method for synthesizing humic acid and the application of a sponge-like MnFe₂O₄ catalyst in humic acid synthesis. The method comprises: mixing biomass feedstock with water and adding alkali to obtain a mixture with a pH of 12.5-13.5; adding a sponge-like MnFe₂O₄ catalyst to the mixture and performing a hydrothermal reaction, followed by separation and purification to obtain humic acid; or pretreating the biomass feedstock by acidic hydrolysis in an acidic solution with a pH of 0.5-1.5 to obtain a mixture; adjusting the pH of the mixture to 12.5-13.5 with alkali, adding a sponge-like MnFe₂O₄ catalyst and performing a hydrothermal reaction, followed by separation and purification to obtain humic acid. This invention is the first to use magnetic MnFe₂O₄ with a sponge-like porous structure and an optimized reaction process to synthesize humic acid, significantly improving the yield of humic acid. Furthermore, the sponge-like MnFe₂O₄ catalyst used can be recovered and recycled after humic acid synthesis.

Description

Technical Field

[0001] This invention belongs to the field of environmental catalytic materials and artificial humic synthesis technology, and particularly relates to a method for synthesizing humic acid and the application of sponge-like MnFe2O4 catalyst in humic acid synthesis. Background Technology

[0002] Humic acid (HA), as a core component of soil organic matter, plays an irreplaceable role in carbon cycling, heavy metal sequestration, soil fertility maintenance, and crop growth promotion. Currently, commercially available humic acid is mainly derived from mineral extraction from non-renewable resources such as peat and lignite, which not only depletes natural resources but may also damage the ecological environment. Although traditional composting fermentation methods can convert biomass into humus, this process relies on microbial activity, is time-consuming (weeks to months), requires large land areas, and generates greenhouse gas emissions. To overcome these problems, the rapid preparation of artificial humic acid (AHA) from biomass waste (such as straw and sludge) using non-biological means has become a research hotspot in the environmental and agricultural fields. Hydrothermal humification is currently the mainstream technology for AHA preparation. Studies have shown that in a high-temperature, high-pressure water environment, biomass can undergo a series of hydrolysis, dehydration, decarboxylation, and condensation reactions, simulating and accelerating the lengthy natural humification process within hours to produce AHA products with structures and functions similar to natural humic acid. Recent studies have successfully prepared agrochemically valuable organic humic acid (AHA) from various wastes such as corn stalks, rice stalks, urban sludge, and distiller's grains, demonstrating its significant potential in improving soil structure, promoting plant photosynthesis, and enhancing carbon sequestration. To further improve the yield and / or degree of polymerization of AHA, researchers have begun to introduce exogenous additives, particularly manganese-based materials, which have been shown to significantly promote the condensation of humic precursors.

[0003] Despite significant progress in AHA synthesis technology, key challenges remain. While manganese-based catalysts are highly effective, existing powdered catalysts (especially oxides) are prone to agglomeration in viscous humic acid reaction solutions, leading to masking of active sites and a rapid decline in catalytic efficiency with each cycle. Simultaneously, fine particles are difficult to separate from the product, resulting in catalyst loss and decreased product purity. Current AHA preparation methods largely employ a one-pot hydrothermal process, failing to optimize the two stages of biomass hydrolysis and depolymerization, as well as oxidative polymerization, making it difficult to simultaneously achieve a high degree of humification and yield in the final product.

[0004] Therefore, employing a catalyst with both high catalytic activity and excellent magnetic separation performance, coupled with an optimized reaction process, is key to achieving efficient AHA synthesis. Consequently, it is essential to provide a method for synthesizing humic acid and the application of a sponge-like MnFe2O4 catalyst in humic acid synthesis. Summary of the Invention

[0005] To address one or more technical problems in existing artificial humic acid synthesis reactions, such as low yield, low degree of polymerization, difficulty in separating and recovering the catalyst materials in the liquid-phase reaction system, small specific surface area of ​​the catalyst materials, and limited catalytic activity due to lack of porous structure, this invention provides a method for synthesizing humic acid and the application of a sponge-like MnFe2O4 catalyst in humic acid synthesis. This invention utilizes a magnetic MnFe2O4 catalyst with a sponge-like porous structure and an optimized reaction process to synthesize humic acid.

[0006] The present invention provides a method for synthesizing humic acid in a first aspect, the method comprising the following steps: (1) Mix biomass raw materials with water evenly and add alkali to obtain a mixture with a pH of 12.5~13.5; (2) Add a sponge-like MnFe2O4 catalyst to the mixture and carry out a hydrothermal reaction, then separate and purify to obtain humic acid.

[0007] In a second aspect, the present invention provides a method for synthesizing humic acid, the method comprising the following steps: (a) The biomass feedstock is placed in an acidic solution with a pH of 0.5 to 1.5 for acidic hydrolysis pretreatment to obtain a mixture; (b) After adjusting the pH of the mixture to 12.5-13.5 with alkali, a sponge-like MnFe2O4 catalyst is added and a hydrothermal reaction is carried out. After separation and purification, humic acid is obtained.

[0008] In a third aspect, the present invention provides the application of a sponge-like MnFe2O4 catalyst in the synthesis of humic acid. The sponge-like MnFe2O4 catalyst can be recovered and recycled after the synthesis of humic acid. After three cycles of recycling, the yield reduction rate of the sponge-like MnFe2O4 catalyst in the synthesis of humic acid is no more than 15%.

[0009] Compared with the prior art, the present invention has at least the following beneficial effects: (1) Existing catalytic methods for preparing humic acid mostly involve direct catalytic humification of biomass under alkaline conditions or assisted oxidative polymerization by strong oxidants such as hydrogen peroxide. These methods have several shortcomings: biomass macromolecules (insufficiently hydrolyzed cellulose and lignin) cannot be fully converted into small molecule precursors that can participate in the reaction; direct entry into the alkaline system leads to insufficient precursor supply, resulting in loose structure and low aromaticity of the generated humic acid; and strong oxidants easily cause over-mineralization, making it impossible to recycle. In contrast, this invention innovatively proposes a two-step enhanced humification process of "acidic hydrolysis + alkaline sponge-like MnFe2O4 catalytic condensation". In this two-step process, the acidic hydrolysis stage (pH=0.5~1.5) can significantly promote the chain breaking and hydrolysis of cellulose, hemicellulose and lignin in straw, greatly increasing the concentration of small molecule precursors that can participate in subsequent humification reactions; the alkaline catalytic stage (pH=12.5~13.5, with the addition of sponge-like MnFe2O4) can effectively promote the condensation of cellulose, hemicellulose and lignin in straw, greatly increasing the concentration of small molecule precursors that can participate in subsequent humification reactions. 2+ / Fe 3+ Under the action of reversible redox pairs, condensation, polymerization, and aromatization of small molecules such as phenols, carbonyl groups, and amines are effectively induced, thereby significantly improving the yield and degree of polymerization of humic acid. The sponge-like MnFe2O4 catalyst used in this invention exhibits a sponge-like three-dimensional interconnected mesoporous structure, denoted as MnFe2O4-S, with a specific surface area as high as 120.7 m². 2 With a density of over / g, the pore connectivity is excellent, significantly reducing mass transfer resistance. Its pore structure is mainly mesoporous and macroporous, highly compatible with the generation of humic acid macromolecules, intermediate condensation, product diffusion and desorption. The low proportion of micropores avoids the problems of difficulty in entry of macromolecular intermediate products and easy pore blockage. The interconnected sponge-like pores are more conducive to the continuous occurrence of macromolecular polymerization reactions, and the effective active sites are more likely to contact macromolecular precursors, resulting in high catalytic efficiency. Compared with the existing nanoparticle agglomeration or dense block structure of MnFe2O4, the sponge-like MnFe2O4 catalyst used in this invention achieves a leapfrog improvement in structural hierarchy from two-dimensional stacking to a three-dimensional network, significantly improving the accessibility and utilization rate of active sites.

[0010] (2) The spongy MnFe2O4 used in this invention has significant magnetic properties and high saturation magnetization. It can be quickly separated from the reaction liquid by an external magnetic field, thus realizing the convenient recovery and recycling of the catalyst.

[0011] (3) In the application of this invention in humic acid synthesis, the combination of the MnFe2O4-S catalyst with the two-step process of "acidic hydrolysis + alkaline catalytic polymerization" shows the best effect. Experimental results show that the total humic acid content obtained by acid hydrolysis followed by alkaline catalytic polymerization (in combination with MnFe2O4-S) is significantly higher than that of the untreated group or the experimental group that only underwent alkaline treatment. This indicates that the catalyst used in this invention can effectively promote the condensation and polymerization reaction of small molecule precursors, thereby improving the yield and quality. Attached Figure Description

[0012] Figure 1 These are the XRD patterns of the MnFe2O4 catalysts prepared in Examples 1-5 of this invention; Figure 2 These are SEM images of the MnFe2O4 catalysts prepared in Examples 1-5 of this invention; in the figures, (a1) and (a2) correspond to SEM images of MnFe2O4-S0.5 at different magnifications, (b1) and (b2) correspond to SEM images of MnFe2O4-S1.5 at different magnifications, (c1) and (c2) correspond to SEM images of MnFe2O4-S2.5 at different magnifications, (d1) and (d2) correspond to SEM images of MnFe2O4-S3.5 at different magnifications, and (e1) and (e2) correspond to SEM images of MnFe2O4-S4.5 at different magnifications. Figure 3 These are EDX spectra, SEM images, and mapping test results of the sponge-like MnFe2O4 catalyst prepared in Example 1 of this invention; in the figures, (a) is the EDX spectrum, (b) is the SEM image, and (c) is the mapping test result. Figure 4 The XRD patterns are those of the spongy MnFe2O4 catalyst prepared in Example 1 of this invention (denoted as MnFe2O4-S in the figure) and the MnFe2O4 catalyst prepared in Comparative Example 1 (denoted as MnFe2O4-P in the figure). Figure 5 These are the XPS spectra of the spongy MnFe2O4 catalyst prepared in Example 1 of this invention (denoted as MnFe2O4-S in the figure) and the MnFe2O4 catalyst prepared in Comparative Example 1 (denoted as MnFe2O4-P in the figure); in the figure, (a) is the full spectrum; (b) is the 2p orbital electron binding energy spectrum of Fe element in the sample; (c) is the 2p orbital electron binding energy spectrum of Mn element in the sample; Figure 6 The figures show the nitrogen adsorption-desorption curve and pore size distribution curve of the sponge-like MnFe2O4 catalyst prepared in Example 1 of this invention; in the figures, (a) is the nitrogen adsorption-desorption curve, Adsorbed is the adsorption curve, Desorption is the desorption curve, Quantity Adsorbed is the adsorption amount, and (b) is the pore size distribution curve. Figure 7The figures show the VSM curves of the spongy MnFe2O4 catalyst prepared in Example 1 of this invention (denoted as MnFe2O4-S in the figure) and the MnFe2O4 catalyst prepared in Comparative Example 1 (denoted as MnFe2O4-P in the figure); in the figure, Magnetic Field is the applied magnetic field strength, and Moment is the magnetization intensity of the sample. Figure 8 The graph shows the total humic acid content, dry weight of the solid after hydrothermal reaction, and pH value of the products prepared in Examples 6-11 and Comparative Example 2 of the present invention, measured by volumetric method; (a) shows the total humic acid content determined by volumetric method; (b) shows the dry weight of the solid after hydrothermal reaction and pH value of the product. Figure 9 The total humic acid content of the products obtained in Examples 12-14 was determined by volumetric method; (a) total humic acid content of rice straw as raw material determined by volumetric method; (b) total humic acid content of wheat straw as raw material determined by volumetric method; (c) total humic acid content of peanut shells as raw material determined by volumetric method; Figure 9 In (a), (b), and (c), the Control group represents the blank group, indicating a hydrothermal reaction where biomass raw materials and deionized water were mixed evenly without adding KOH to adjust the pH and without adding a catalyst to the mixture; the MnFe2O4 group in (a), (b), and (c) represents a hydrothermal reaction where biomass raw materials and deionized water were mixed evenly without adding KOH to adjust the pH, but with MnFe2O4 catalyst added to the mixture; the KOH group in (a), (b), and (c) represents a hydrothermal reaction where no catalyst was added to the mixture at pH 13; the KOH+MnFe2O4 group (pH adjusted to 13 and MnFe2O4-S2.5 added) corresponds to the total humic acid content of the products obtained in Examples 12-14, respectively. Figure 10 The graph shows the total humic acid content of the products prepared in Example 15 and Comparative Examples 3-7 of the present invention, as well as the dry weight of the solids at different reaction stages, measured by volumetric method; (a) is the total humic acid content determined by volumetric method; (b) is the dry weight of the solids at different reaction stages. Figure 11 The total humic acid content of the products obtained in Examples 16-18 was determined by volumetric method; (a) total humic acid content of rice straw as raw material determined by volumetric method; (b) total humic acid content of wheat straw as raw material determined by volumetric method; (c) total humic acid content of peanut shells as raw material determined by volumetric method; Figure 11In (a), (b), and (c), the Control group represents the blank group. In this blank group, the biomass feedstock was uniformly mixed with hydrochloric acid at pH 1 and pretreated with acidic hydrolysis at 180°C for 4 hours. After cooling to room temperature, KOH was added to adjust the pH to 7, followed by hydrothermal reaction at 180°C for 4 hours. In (a), (b), and (c), the MnFe2O4 group was formed by uniformly mixing the biomass feedstock with hydrochloric acid at pH 1 and pretreatment with acidic hydrolysis at 180°C for 4 hours. After cooling to room temperature, KOH was added to adjust the pH to 7, followed by the addition of the feedstock prepared in Example 1. MnFe2O4-S2.5 was then hydrothermally reacted at 180℃ for 4 hours; in (a), (b), and (c), the KOH group was prepared by mixing biomass raw materials with hydrochloric acid at pH 1 and performing acidic hydrolysis pretreatment at 180℃ for 4 hours, then cooling to room temperature and adding KOH to adjust the pH to 13 before hydrothermally reacting at 180℃ for 4 hours; the KOH+MnFe2O4 group (pH adjusted to 13 and MnFe2O4-S2.5 prepared in Example 1 was added) correspond to the total humic acid content of the products obtained in Examples 16-18, respectively. Detailed Implementation

[0013] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0014] The present invention provides a method for synthesizing humic acid in a first aspect, the method comprising the following steps: (1) Mix biomass raw materials with water evenly and add alkali to obtain a mixture with a pH of 12.5~13.5; (2) Add a sponge-like MnFe2O4 catalyst to the mixture and carry out a hydrothermal reaction, then separate and purify to obtain humic acid.

[0015] In a second aspect, the present invention provides a method for synthesizing humic acid, the method comprising the following steps: (a) The biomass raw material is placed in an acidic solution with a pH of 0.5 to 1.5 for acidic hydrolysis pretreatment to obtain a mixture; in this invention, the acidic solution is an acidic aqueous solution. This step (a) can hydrolyze the biomass macromolecules into soluble precursor small molecules, and the resulting mixture is a mixture containing solid-phase hydrothermal carbon and liquid-phase products; in steps (1) and (a) of this invention, the biomass raw material is a material that has been washed, cut into blocks, dried, crushed and sieved (through an 80-mesh sieve); (b) After adjusting the pH of the mixture to 12.5-13.5 with alkali, a sponge-like MnFe2O4 catalyst is added and a hydrothermal reaction is carried out. After separation and purification, humic acid is obtained. In this invention, the sponge-like MnFe2O4-S catalyst promotes the condensation of small molecules to generate humic acid. The acidic hydrolysis pretreatment in step (a) and the hydrothermal reaction in steps (2) and (b) are carried out, for example, in a reaction vessel. The reaction vessel is sealed and carried out under high temperature and high pressure conditions. After the reaction is completed, the solid and liquid phases are separated, the liquid phase product is collected, and then acidified, precipitated, washed and dried to obtain the humic acid. In this invention, for example, the acidic hydrolysis pretreatment and the hydrothermal reaction are carried out at a pressure of 1.2-1.6 MPa. This invention does not specifically limit the reaction vessel. Those skilled in the art can choose conventionally. For example, a reaction vessel with a polytetrafluoroethylene liner that can withstand 3 MPa can be used.

[0016] This invention innovatively proposes a two-step enhanced humification process of "acidic hydrolysis + alkaline sponge-like MnFe2O4 catalytic condensation," significantly improving the yield of humic acid. In this two-step process, the acidic hydrolysis stage (pH=0.5~1.5) significantly promotes the chain breaking and hydrolysis of cellulose, hemicellulose, and lignin in straw, greatly increasing the concentration of small molecule precursors that can participate in subsequent humification reactions. Furthermore, this invention is the first to use sponge-like MnFe2O4 for humic acid synthesis. Compared with existing processes that use oxidants such as hydrogen peroxide, sodium hypochlorite, nitric acid, or chlorine dioxide, or catalysts such as manganese dioxide or ferric oxide, this invention, in the alkaline catalytic stage (pH=12.5~13.5, with the addition of sponge-like MnFe2O4), can significantly improve the yield of humic acid. 2+ / Fe 3+ Under the dynamic catalytic action of reversible redox pairs, condensation, polymerization, and aromatization of small molecules such as phenols, carbonyl groups, and amines are effectively induced, thereby significantly improving the yield and degree of polymerization of humic acid. The sponge-like MnFe2O4 catalyst used in this invention has a sponge-like three-dimensional interconnected mesoporous-macroporous structure with a specific surface area as high as 120.7 m². 2With a density of over / g, the pore connectivity is excellent, significantly reducing mass transfer resistance. Its pore structure is mainly mesoporous and macroporous, highly compatible with the generation of humic acid macromolecules, intermediate condensation, product diffusion and desorption. The low proportion of micropores avoids the problems of difficulty in entry of macromolecular intermediate products and easy pore blockage. The interconnected sponge-like pores are more conducive to the continuous occurrence of macromolecular polymerization reactions, and the effective active sites are more likely to contact macromolecular precursors, resulting in high catalytic efficiency. Compared with the existing nanoparticle agglomeration or dense block structure of MnFe2O4, the sponge-like MnFe2O4 catalyst used in this invention achieves a leapfrog improvement in structural hierarchy from two-dimensional stacking to a three-dimensional network, significantly improving the accessibility and utilization rate of active sites. The spongy MnFe2O4 used in this invention has significant magnetic properties and high saturation magnetization, which can be rapidly separated from the reaction solution by an external magnetic field, realizing convenient recovery and recycling of the catalyst. Even after three cycles, the yield of humic acid still decreases by no more than 15%. While improving synthesis efficiency and product quality, it significantly reduces catalyst loss and process costs, providing an innovative solution for the green, efficient and sustainable synthesis of humic acid.

[0017] In this invention, precisely controlling the pH of the acidic hydrolysis pretreatment stage to 0.5–1.5 and strictly adjusting the pH of the subsequent catalytic condensation stage to 12.5–13.5 are key to achieving efficient humic acid synthesis. This invention reveals that in the acidic stage, this pH range effectively promotes the chain scission and hydrolysis of cellulose, hemicellulose, and lignin in biomass, generating a large number of small molecule precursors such as phenols and sugars. In the subsequent alkaline stage, the strongly alkaline environment of pH 12.5–13.5 provides optimal working conditions for the spongy MnFe2O4 catalyst, fully activating the Mn in the catalyst. 2+ / Fe 3+ Redox pairs are highly efficient catalysts for the condensation and aromatization of precursors such as phenols. If the pH is too high during the acidic phase, hydrolysis will be insufficient, resulting in inadequate precursor formation; if it is too low, it may lead to excessive degradation or carbonization of biomass, resulting in the loss of effective carbon sources. Conversely, if the pH is too low during the alkaline phase, catalyst activity will be insufficient, the condensation reaction will be slow, and the product will have a low degree of polymerization. If the pH is too high, some active components may dissolve or the catalyst structure may become unstable. It may also cause excessive hydrolysis of humic acid or unnecessary saponification side reactions, thus affecting product yield and quality.

[0018] According to some preferred embodiments, the pH of the acidic solution is 1; and / or the temperature of the acidic hydrolysis pretreatment is 150~200°C (e.g., 150°C, 160°C, 170°C, 180°C, 190°C or 200°C), preferably 180°C, and the time is 2~6h (e.g., 2, 3, 4, 5 or 6h), preferably 4h.

[0019] According to some preferred embodiments, the biomass raw material is one or more of corn stalks, rice stalks, wheat stalks, and peanut shells; the pH of the mixture is 13; and / or in step (1), the solid-liquid ratio of the biomass raw material to the water is 1g:(15~30)mL, preferably 1g:20mL, or in step (a), the solid-liquid ratio of the biomass raw material to the acidic solution is 1g:(15~30)mL, preferably 1g:20mL.

[0020] According to some preferred embodiments, the temperature of the hydrothermal reaction is 150~200℃ (e.g., 150℃, 160℃, 170℃, 180℃, 190℃ or 200℃), preferably 180℃, and the time is 2~8h (e.g. 2, 3, 4, 5, 6, 7 or 8h), preferably 4~8h (e.g. 4, 5, 6, 7 or 8h).

[0021] According to some preferred embodiments, the separation and purification process sequentially includes: solid-liquid separation, acid precipitation, washing, and drying. In this invention, the acid precipitation can be performed, for example, using hydrochloric acid with a concentration of 0.05~0.15 mol / L. In some specific embodiments, for example, the product after the reaction is completed is filtered for solid-liquid separation, the liquid phase product is collected, and hydrochloric acid with a concentration of 0.1 mol / L is slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 0.5~1.5. At this time, the target product precipitates out in the form of flocculent precipitate, obtaining a precipitation system. The precipitation system is allowed to stand at 4~10℃ for 2 hours for aging, and then centrifuged to remove the supernatant, obtaining the precipitate. The precipitate is then washed three times with hydrochloric acid pre-cooled to 4~10℃ with a pH of 3.0~4.0, followed by washing twice or more with anhydrous ethanol. The washed precipitate is transferred to a vacuum drying oven and dried at 50~80℃ for more than 12 hours.

[0022] According to some preferred embodiments, the amount of the sponge-like MnFe2O4 catalyst is 3-8% of the mass of the biomass raw material in step (1); in this invention, it is preferred that the amount of the sponge-like MnFe2O4 catalyst is 3-8% of the mass of the biomass raw material in step (1). This invention has found that this dosage range can ensure that the sponge-like MnFe2O4 catalyst provides sufficient and uniformly distributed active sites in the reaction system, fully utilizing its high specific surface area advantage of three-dimensional interconnected mesoporous-macroporous structure, effectively catalyzing the oxidative condensation and aromatization reactions of small molecule precursors such as phenols, from While ensuring high humic acid yield and degree of polymerization, it is crucial to maintain good dispersibility and mass transfer efficiency of the sponge-like MnFe2O4 catalyst. If the catalyst dosage is too low, there will be insufficient active sites, making it difficult to effectively maintain the catalytic reaction throughout the process, resulting in a slow reaction rate, incomplete conversion, and ultimately a decrease in humic acid yield and degree of polymerization. If the dosage is too high, it will not only waste catalyst and increase costs, but may also cause agglomeration due to excessively high local concentrations, blocking its porous structure and reducing the accessibility of effective active sites. At the same time, excessive catalyst may introduce unnecessary metal ion interference, affecting product purity and subsequent separation.

[0023] According to some preferred embodiments, the preparation of the sponge-like MnFe2O4 catalyst includes: Manganese source, iron source, and complexing agent are placed in a mortar and ground evenly to obtain a paste precursor. In this invention, for example, theoretically measured amounts of manganese source, iron source, and complexing agent are weighed and placed in a mortar and ground thoroughly for at least 20 minutes, for example, continuously grinding and mixing at room temperature for 20-30 minutes. Since the manganese source, iron source, and complexing agent are preferably hydrates, and ferric nitrate has strong deliquescence, the mixture gradually becomes a viscous paste during grinding. Grinding continues until the mixture forms a paste precursor with uniform color and texture. In this invention, the grinding process can, for example, be carried out using a corrosion-resistant mortar. Preferably, an agate mortar (e.g., a natural agate mortar) is used; the paste-like precursor is placed in a crucible and dried in a forced-air drying oven to obtain a precursor; in this invention, excess moisture is removed by the drying process to obtain a dry, relatively fluffy precursor; the precursor is calcined at high temperature in an air atmosphere to obtain a sponge-like MnFe2O4 catalyst; in this invention, the high-temperature calcination is carried out, for example, in a muffle furnace, and the heating program of the muffle furnace is, for example, a heating rate of 1~5℃ / min, preferably 2℃ / min, from room temperature to 300~500℃.

[0024] In this invention, for example, a crucible containing the precursor is placed in a muffle furnace with the lid open and calcined at high temperature in an air atmosphere to form a MnFe2O4 material with a sponge-like porous structure. After cooling, the sponge-like porous MnFe2O4 material is taken out of the crucible. The product is a black, extremely loose, sponge-like solid that can be easily scraped off with a spatula. Preferably, the obtained product can also be ground in a mortar for 5 to 10 minutes.

[0025] According to some specific embodiments, the preparation of the sponge-like MnFe2O4 catalyst includes the following steps: I. Raw materials: Manganese acetate and ferric nitrate are used as metal sources; citric acid is used as both a complexing agent and fuel. II. Proportioning and Mixing: Mix Mn and Fe according to the stoichiometric ratio, add citric acid to make the citric acid / metal molar ratio within a certain range, and grind thoroughly until a paste-like precursor is formed; III. Drying pretreatment: Dry the paste precursor (e.g., 80~120℃) to obtain the precursor; IV. Calcination: The precursor is calcined at high temperature in an air atmosphere; V. Post-processing: The product obtained after calcination can be ground and / or sieved, and if necessary, washed and dried to obtain sponge-like MnFe2O4 (abbreviated as MnFe2O4-S). In this invention, if there are incompletely decomposed organic residues, unreacted metal salts, or other soluble impurities in the product obtained after calcination, these byproducts need to be removed by a washing step (such as using deionized water), followed by drying to ensure product purity.

[0026] The present invention preferably prepares the sponge-like MnFe2O4 catalyst using a solvent-free method assisted by citric acid. Citric acid is used as fuel, and the large amount of gas instantaneously released during combustion acts as an "endogenous pore-forming source," eliminating the need for external templates or supports. This successfully prepares MnFe2O4-S with a sponge-like three-dimensional interconnected mesoporous-macroporous structure. The prepared catalyst has a specific surface area as high as 120.7 m². 2 With a density of over / g, the pore connectivity is excellent, and the mass transfer resistance is significantly reduced. Compared with the nanoparticle agglomeration or dense block structure obtained by common co-precipitation, hydrothermal or sol-gel methods for Fe-Mn catalysts, this invention achieves a leapfrog improvement in structural hierarchy from two-dimensional stacking to a three-dimensional network, significantly improving the accessibility and utilization of active sites. The MnFe2O4 catalyst used in this invention is a MnFe2O4-S material with a specific sponge-like porous morphology, mesoporous-macroporous structure and high specific surface area.

[0027] According to some preferred embodiments, the manganese source is manganese acetate, preferably manganese acetate tetrahydrate; the iron source is ferric nitrate, preferably ferric nitrate nonahydrate; the complexing agent is citric acid, preferably citric acid monohydrate (i.e., citric acid monohydrate); preferably, the molar ratio of manganese acetate to ferric nitrate is 1:(1.5~2.5), preferably 1:2; preferably, the molar ratio of citric acid to the sum of the amounts of manganese acetate and ferric nitrate is (0.5~4.5):1 (e.g., 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1 or 4.5:1), preferably 2.5:1; in this invention, the molar ratio of citric acid to the sum of the amounts of manganese acetate and ferric nitrate is (0.5~4.5):1, that is, the molar ratio of the sum of the amounts of manganese acetate and ferric nitrate to... The molar ratio of citric acid is 1:(0.5~4.5); in this invention, manganese acetate and ferric nitrate are preferably used as metal sources, and citric acid is used as a complexing agent, while citric acid can also be used as fuel; when preparing the spongy MnFe2O4 catalyst, the key ratio of the molar ratio of citric acid to the sum of the amounts of manganese acetate and ferric nitrate, i.e., the molar ratio of citric acid to the metal source (CA / Metal), is more preferably 2.5:1. This invention has found that at this ratio, the amount and rate of gas released during combustion reach equilibrium, which can construct an optimal three-dimensional interconnected porous network and avoid pore collapse or product densification; the drying temperature is 80~100℃, and the time is 10~12h; and / or the high-temperature calcination temperature is 300~500℃, preferably 400℃, and the time is 1~3h, preferably 2h.

[0028] According to some preferred embodiments, in step (2) or step (b), sodium anthraquinone-2-sulfonate is also added, wherein the amount of sodium anthraquinone-2-sulfonate is 0.1 to 1% of the mass of the biomass raw material in step (1) (e.g., 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9% or 1%).

[0029] In this invention, it is preferable to also add 0.1% to 1% (by mass) of the anthraquinone-2-sulfonate sodium salt of the biomass raw material. This invention has found that the addition of anthraquinone-2-sulfonate sodium salt can further improve the yield and degree of polymerization of humic acid. The possible reason is that adding 0.1% to 1% (by mass) of anthraquinone-2-sulfonate sodium salt (AQS) as a redox mediator can form a synergistic catalytic system with the spongy MnFe2O4 catalyst, significantly enhancing the electron transfer efficiency in the humic acid synthesis process. Furthermore, anthraquinone-2-sulfonate sodium salt, through its reversible redox properties of quinone / hydroquinone, can enhance the redox efficiency of MnFe2O4 in the Mn... 2+ / Fe 3+Redox pairs construct electron transport bridges between themselves and organic precursors, accelerating the activation and free radical generation of small molecule precursors such as phenols, and promoting the directional condensation and aromatization of these active intermediates. This simultaneously increases the yield and aromatization degree of humic acid. If the amount of sodium anthraquinone-2-sulfonate is too low, the concentration of redox mediators will be too low, and an effective electron transport network cannot be established, resulting in limited promotion of the reaction. On the other hand, if the amount is too high, excessive AQS may undergo irreversible oxidation or compete with the catalyst for active sites, thus inhibiting the normal catalytic cycle of MnFe2O4.

[0030] In a third aspect, this invention provides the application of a sponge-like MnFe2O4 catalyst in the synthesis of humic acid, particularly its application in the hydrothermal humification reaction for the synthesis of humic acid. For example, the sponge-like MnFe2O4 catalyst can promote the conversion of biomass into humic acid. After the synthesis of humic acid, the sponge-like MnFe2O4 catalyst can be recovered and recycled. After three cycles of recycling, the yield reduction of the sponge-like MnFe2O4 catalyst for the synthesis of humic acid is no more than 15%.

[0031] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Example 1 The preparation of sponge-like MnFe2O4 catalyst includes the following steps: ① Place manganese source, iron source and citric acid in a mortar and grind for 30 minutes to obtain a paste precursor; wherein, the manganese source is manganese acetate tetrahydrate, the iron source is ferric nitrate nonahydrate, the citric acid is citric acid monohydrate, the molar ratio of the manganese source to the iron source is 1:2; the molar ratio of the citric acid to the sum of the amounts of the manganese source and the iron source is 2.5:1, that is, the molar ratio of the citric acid to the metal source is 2.5:1.

[0033] ② The paste precursor was placed in a crucible and dried in an 80°C forced-air drying oven for 12 hours to obtain the precursor.

[0034] ③ The precursor is placed in a muffle furnace and calcined at high temperature in an air atmosphere to obtain a sponge-like MnFe2O4 catalyst (denoted as MnFe2O4-S2.5); the high-temperature calcination is to raise the temperature from room temperature to 400℃ at a heating rate of 2℃ / min and calcine at 400℃ for 2h.

[0035] Example 2 Example 2 is basically the same as Example 1, except that: The molar ratio of the citric acid to the sum of the manganese source and the iron source is 0.5:1, to obtain a MnFe2O4 catalyst (denoted as MnFe2O4-S0.5).

[0036] Example 3 Example 3 is basically the same as Example 1, except that: The molar ratio of the citric acid to the sum of the manganese source and the iron source is 1.5:1, to obtain a MnFe2O4 catalyst (denoted as MnFe2O4-S1.5).

[0037] Example 4 Example 4 is basically the same as Example 1, except that: The molar ratio of the citric acid to the sum of the manganese source and the iron source is 3.5:1, resulting in a sponge-like MnFe2O4 catalyst (denoted as MnFe2O4-S3.5).

[0038] Example 5 Example 5 is basically the same as Example 1, except that: The molar ratio of the citric acid to the sum of the manganese source and the iron source is 4.5:1, thus obtaining a MnFe2O4 catalyst (denoted as MnFe2O4-S4.5).

[0039] The MnFe2O4-S obtained in Examples 1-5 were characterized by XRD, SEM, and EDS. The test results are as follows: Figure 1 , Figure 2 and Figure 3 As shown.

[0040] from Figure 1It can be seen that the diffraction peaks of all samples are completely consistent with the standard MnFe2O4 cubic spinel structure (JCPDS#10-0319), corresponding to the (111), (220), (311), (400), (422), (511), and (440) crystal planes. No impurity peaks were observed, indicating that pure phase MnFe2O4 was successfully synthesized at different CA / Metal ratios. As the CA / Metal ratio increases, the intensity of the diffraction peaks of the samples gradually increases and the peak shape becomes sharper, indicating that the crystallinity of the crystal is improved. This phenomenon is attributed to the more intense combustion reaction caused by the higher fuel content, which promotes more complete crystal growth. At the same time, the increase in the amount of combustion-released gas also provides conditions for the formation of a more developed pore structure. Conversely, at low CA / Metal ratios (such as 0.5), the XRD peaks are wider and the crystallinity is lower, indicating that the combustion reaction is incomplete, the product grains are smaller, and the pores are not uniform enough. Therefore, the CA / Metal ratio not only affects the intensity of the combustion reaction and the crystallinity of the products, but also determines the pore structure characteristics of MnFe2O4 by adjusting the amount and rate of gas release.

[0041] from Figure 2 It can be seen that when CA / Metal = 0.5 and 1.5, the gas production is insufficient, and the product structure is dense. When CA / Metal = 2.5, the gas production and release rate reach the optimal balance, successfully constructing a three-dimensional interconnected uniform porous network. When CA / Metal = 3.5 and 4.5, a large number of agglomerated particles appear on the surface of MnFe2O4, and the pore structure becomes irregular. This may be because excess CA leads to intense combustion, which further causes the pore structure and irregular particles to collapse too violently, resulting in pore structure collapse and sintering. Residual carbon also blocks the pores, leading to a decrease in specific surface area.

[0042] from Figure 3 It can be seen that the distribution of Mn and Fe is synchronous throughout the entire spongy framework, whether in the main trunk or branches, with no obvious elemental agglomeration or enrichment regions observed. This indicates that manganese and iron ions have achieved a highly uniform mixture at the atomic scale, successfully forming a single spinel phase, rather than a mixture of independent oxides. Further semi-quantitative analysis of the atomic percentages of the elements revealed that the measured atomic ratio of Mn to Fe is close to 1:2, which is highly consistent with the theoretical stoichiometry of MnFe₂O₄.

[0043] Comparative Example 1 ①Prepare manganese and iron salt solutions Weigh 2.0 mmol of MnCl2·4H2O and 4.0 mmol of FeCl3·6H2O, dissolve them together in 80 mL of deionized water, place the solution in a three-necked flask, and preheat to 80 °C with stirring.

[0044] ②Preparation of precipitant solution Weigh 3.2g of NaOH, dissolve it in deionized water to prepare a 2mol / L NaOH solution, and transfer it to a constant pressure dropping funnel.

[0045] ③ Preparation of nanoparticle MnFe2O4 catalyst by coprecipitation method Place the three-necked flask in an 80°C constant-temperature water bath and maintain vigorous stirring (800 rpm). Add NaOH solution dropwise (approximately 1-2 drops / second) to the vigorously stirred mixed manganese and iron salt solution through a constant-pressure dropping funnel. A brownish-black precipitate will immediately form upon the addition of NaOH solution. Continue adding until the solution pH reaches 11.5. After the addition is complete, maintain the reaction system at 80°C with vigorous stirring (800 rpm) for 2 hours to allow the precipitate to age and promote crystal growth. After the reaction is complete, stop heating and allow it to cool naturally to room temperature. Transfer the reaction mixture to a centrifuge tube and centrifuge at 10,000 rpm for 10 minutes, discarding the supernatant. Resuspend the precipitate in deionized water, sonicate for 5 minutes, and centrifuge again. Repeat this process 4 times until the supernatant pH is close to neutral to remove excess Na. + and Cl - Ions. To further remove impurities, wash twice with anhydrous ethanol and centrifuge.

[0046] ④ Drying The resulting precipitate was transferred to a petri dish and dried in a vacuum drying oven at 60°C for 12 hours to obtain black nanoparticle-like MnFe2O4 powder, which was named MnFe2O4-P.

[0047] ⑤ Repeat Repeat steps ① to ④ above to prepare three batches in total, thereby obtaining a sufficient amount of nanoparticle MnFe2O4 catalyst.

[0048] The spongy MnFe2O4 catalyst prepared in Example 1 of this invention and the MnFe2O4-P prepared in Comparative Example 1 were characterized by XRD, XPS, BET, and VSM. The test results are as follows: Figure 4 , Figure 5 , Figure 6 and Figure 7 As shown.

[0049] from Figure 4It can be seen that the diffraction peak positions of the two samples (MnFe2O4-P and MnFe2O4-S) match the main peak positions on the JCPDS#10-0319 standard card. The strongest peak is usually the (311) crystal plane diffraction peak, located around 35°. This indicates that both samples are MnFe2O4 and have a spinel structure. No obvious impurity peaks were observed, indicating that the samples have high purity. The diffraction peaks of MnFe2O4-P are very sharp and clear, with a high signal-to-noise ratio. This indicates that it has very good crystallinity and a large grain size. The sharp peak shape means that the internal structure of the crystal is regular and the long-range order is good, which is consistent with the characteristics of co-precipitation method. The diffraction peaks of MnFe2O4-S are significantly broadened, and the peak intensity is relatively weak, indicating that its grain size is small and its crystallinity is low.

[0050] from Figure 5 As can be seen, the XPS full spectrum test results show that the sample contains Fe, Mn, O, and C elements, further indicating the successful synthesis of MnFe₂O₄-P and MnFe₂O₄-S. In MnFe₂O₄, iron is mainly produced as Fe. 3+ Manganese exists primarily in the form of Mn. 2+ exist.

[0051] from Figure 6 It can be seen that the shape of this isotherm belongs to type IV(a). The initial stage curve bulges upwards, indicating a strong interaction between the adsorbate (nitrogen) and MnFe₂O₄-S. After a rapid increase in adsorption in the low-pressure region, an inflection point appears, marking the completion of monolayer adsorption and the beginning of multilayer adsorption. In the intermediate stage, when the relative pressure reaches approximately 0.45, the adsorption amount rises sharply. This is attributed to capillary condensation occurring in the pores of the material. This is a typical characteristic of mesoporous materials (pore sizes between 2 and 50 nm). Specific surface area and pore size analysis of the MnFe₂O₄-S material revealed a specific surface area of ​​120.7 m². 2 The pore volume is 0.3799 mL / g. This further indicates that MnFe2O4-S is a mesoporous-macroporous material with high porosity, which is beneficial for contact with reactants during the catalytic process.

[0052] from Figure 7As can be seen, the entire curve exhibits a typical "S" shape, and there is almost no area enclosed by the hysteresis loop near the origin, meaning the hysteresis loop is very narrow, a typical characteristic of superparamagnetic materials. In the figure, when the magnetic moment is zero, the intersection of the hysteresis loop and the magnetic field is very close to zero. This indicates that only a very small reverse magnetic field is needed to eliminate its remanent magnetism. When the external magnetic field is zero, the value of the magnetic moment is also very close to zero. This indicates that MnFe₂O₄-S has extremely low remanence. Without an external magnetic field, due to the lack of magnetism, MnFe₂O₄-S is not prone to magnetic agglomeration and can maintain good dispersion stability in solution. Figure 7 As can be seen, with the increase of the applied magnetic field, the magnetic moment of the material increases rapidly and gradually approaches saturation at a magnetic field of approximately 10 kOe. The saturation magnetization of MnFe2O4-S is approximately 35 emu / g, and that of MnFe2O4-P is approximately 42 emu / g. It can be concluded that both can be rapidly attracted and separated when an external magnetic field is applied. Under an applied external field, the MnFe2O4-S prepared in this embodiment of the invention has a saturation magnetization of approximately 35 emu / g, which fully meets the requirements for efficient magnetic recovery. In contrast, MnFe2O4-P (with a measured specific surface area of ​​68.7 m²) exhibits a significantly higher magnetization. 2 The sponge-like MnFe2O4-S catalyst of this invention has a greater overall performance in catalytic synthesis of humic acid. The sponge-like porous morphology of the MnFe2O4-S catalyst in this invention endows the material with a larger specific surface area and more active sites, which significantly improves the mass transfer efficiency and catalytic activity of the reactants, thereby directly promoting the improvement of humic acid yield and quality.

[0053] Example 6 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of deionized water and add them to the reaction vessel. Mix them evenly and add potassium hydroxide (KOH) to adjust the pH of the system to 13 to obtain a mixture with a pH of 13.

[0054] ② Add 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 to the mixture obtained in step ①. Seal the reactor and perform a hydrothermal reaction for 8 hours at a temperature of 180℃ and a pressure of 1.5MPa. After the reaction, the product (i.e., hydrothermal liquid, denoted as sample S4) is obtained. The product is separated and purified as follows: after cooling the product to room temperature, solid-liquid separation is performed by filtration. The liquid phase product is collected. Hydrochloric acid with a concentration of 0.1mol / L is slowly added dropwise to the liquid phase product under stirring to adjust the pH value of the system to 1.0, resulting in a precipitation system. The precipitation system is allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant is discarded to obtain the precipitate. The precipitate is then washed three times with hydrochloric acid pre-cooled to 4℃ and pH=3.0, followed by washing twice with anhydrous ethanol. The washed precipitate is transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0055] Example 7 Example 7 is basically the same as Example 6, except that: In step ②, no sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) was added. After the reaction was completed, the product (sample S1) was obtained.

[0056] Example 8 Example 8 is basically the same as Example 6, except that: In step ②, the MnFe2O4 catalyst (MnFe2O4-S0.5) prepared in Example 2 was added. After the reaction was completed, the product (sample S2) was obtained.

[0057] Example 9 Example 9 is basically the same as Example 6, except that: In step ②, the MnFe2O4 catalyst (MnFe2O4-S1.5) prepared in Example 3 was added. After the reaction was completed, the product (sample S3) was obtained.

[0058] Example 10 Example 10 is basically the same as Example 6, except that: In step ②, the MnFe2O4 catalyst (MnFe2O4-S3.5) prepared in Example 4 was added. After the reaction was completed, the product (sample S5) was obtained.

[0059] Example 11 Example 11 is basically the same as Example 6, except that: In step ②, the MnFe2O4 catalyst (MnFe2O4-S4.5) prepared in Example 5 was added. After the reaction was completed, the product (sample S6) was obtained.

[0060] Comparative Example 2 Comparative Example 2 is basically the same as Example 6, except that: In step ②, the MnFe2O4-P catalyst prepared in Comparative Example 1 was added. After the reaction was completed, the product (sample S7) was obtained.

[0061] The total humic acid content, dry weight of the solid after hydrothermal reaction, and pH value of the products obtained in Examples 6-11 and Comparative Example 2, determined by volumetric method, are shown in the following figures. Figure 8 As shown, this invention uses the volumetric method to determine the total humic acid content in the product before separation and purification, according to the HG / T 3276-2019 standard.

[0062] from Figure 8 It can be seen that the MnFe2O4-S experimental groups with CA / Metal ratios of 2.5 and 3.5 had higher total humic acid content and a pH value closer to neutral after the reaction. The dry weight of the remaining solids after the hydrothermal reaction showed no significant change among the groups, indicating that the degree of dissolution and depolymerization of biomass in the liquid phase remained relatively constant, and the catalyst did not significantly promote further degradation of the solid framework. Combined with the significant differences in humic acid yield, it can be concluded that the MnFe2O4-S catalyst mainly acts on the liquid phase system, promoting the formation of humic acid substances through the directional catalytic condensation and aromatization reactions of dissolved intermediates. In other words, the MnFe2O4-S catalyst mainly acts on the polymerization of humic acid in the liquid phase. In this invention, the measured total humic acid content is the percentage by mass of the obtained total humic acid relative to the total biomass feedstock, which represents the humic acid yield.

[0063] Example 12 Example 12 is basically the same as Example 6, except that: In this embodiment, rice straw is used instead of corn straw.

[0064] Example 13 Example 13 is basically the same as Example 6, except that: In this embodiment, wheat straw is used instead of corn straw.

[0065] Example 14 Example 14 is basically the same as Example 6, except that: In this embodiment, peanut shells are used instead of corn stalks.

[0066] The total humic acid content of the products obtained in Examples 12-14 was determined in this invention, and the results are as follows: Figure 9 As shown.

[0067] from Figure 9It can be seen that among the three raw materials (rice straw, wheat straw, and peanut shells), the humic acid yields of the Control group and the MnFe2O4 treatment group were relatively low (approximately 12%-17%). The yield of the KOH group was improved (approximately 22%-26%), indicating that alkali treatment is the basis for humic acid extraction. After adding KOH and MnFe2O4-S catalyst, the total humic acid yield of all three raw materials reached the highest value, which means that there is a significant synergistic effect between the MnFe2O4-S catalyst and KOH. It is not merely a physical mixture, but rather a catalytic effect that further promotes the conversion of biomass (lignin, etc.) into humic acid. Whether it is straw (rice, wheat) or shells (peanut shells), the yield after adding MnFe2O4-S catalyst is higher than that of KOH treatment alone. This shows that the catalyst has good versatility, is not strictly limited by the type of biomass, and can improve the conversion efficiency.

[0068] Example 15 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of hydrochloric acid with a pH of 1 and add them to a reaction vessel. Mix them evenly, seal the reaction vessel, and place it under acidic hydrolysis pretreatment at a temperature of 180℃ and a pressure of 1.5MPa for 4 hours to obtain a mixture.

[0069] ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, and then 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 was added. The reactor was sealed and placed under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product (i.e., the hydrothermal liquid, denoted as sample S2-1) was obtained. The product was separated and purified by filtration to separate the solid and liquid phases. Separate and collect the liquid product. Slowly add 0.1 mol / L hydrochloric acid to the liquid product with stirring to adjust the pH of the system to 1.0 to obtain a precipitate system. Let the precipitate system stand at 4℃ for 2 hours for aging, and then centrifuge to separate it. Discard the supernatant to obtain the precipitate. Wash the precipitate three times with hydrochloric acid pre-cooled to 4℃ and pH=3.0, and then wash it twice with anhydrous ethanol. Transfer the washed precipitate to a vacuum drying oven and vacuum dry it at 60℃ for 12 hours to obtain humic acid.

[0070] Comparative Example 3 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of hydrochloric acid with a pH of 1 and add them to a reaction vessel. Mix them evenly, seal the reaction vessel, and place it under acidic hydrolysis pretreatment at a temperature of 180℃ and a pressure of 1.5MPa for 4 hours to obtain a mixture.

[0071] ② After cooling the mixture obtained in step ① to room temperature, add 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1. Seal the reactor and place it under hydrothermal reaction conditions of 180℃ and 1.5MPa for 4 hours. After the reaction, the product (i.e., hydrothermal liquid, denoted as sample S1-1) is obtained. The product is separated and purified as follows: after cooling the product to room temperature, solid-liquid separation is performed by filtration, and the liquid phase product is collected. Hydrochloric acid with a concentration of 0.1mol / L is slowly added dropwise to the liquid phase product under stirring to adjust the pH value of the system to 1.0, and a precipitation system is obtained. The precipitation system is allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant is discarded to obtain the precipitate. The precipitate is then washed three times with hydrochloric acid pre-cooled to 4℃ and pH=3.0, and then washed twice with anhydrous ethanol. The washed precipitate is transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0072] Comparative Example 4 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of hydrochloric acid with a pH of 1 and add them to a reaction vessel. Mix them evenly, seal the reaction vessel, and place it under acidic hydrolysis pretreatment at a temperature of 180℃ and a pressure of 1.5MPa for 4 hours to obtain a mixture.

[0073] ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 7, and then 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 was added. The reactor was sealed and placed under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product (i.e., the hydrothermal liquid, denoted as sample S3-1) was obtained. The product was separated and purified by cooling the product to room temperature and then filtering it. Solid-liquid separation was performed, and the liquid product was collected. Hydrochloric acid with a concentration of 0.1 mol / L was slowly added dropwise to the liquid product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4℃ and pH=3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0074] Comparative Example 5 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of deionized water and add them to the reaction vessel and mix them evenly to obtain a mixture.

[0075] ② The mixture obtained in step ① was placed under hydrothermal reaction conditions of 180℃ and 1.5MPa for 8 hours. After the reaction was completed, the product (i.e., hydrothermal liquid, denoted as sample S4-1) was obtained. The product was separated and purified as follows: after cooling the product to room temperature, solid-liquid separation was performed by filtration, and the liquid phase product was collected. Hydrochloric acid with a concentration of 0.1mol / L was slowly added dropwise to the liquid phase product under stirring to adjust the pH value of the system to 1.0, and a precipitation system was obtained. The precipitation system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded to obtain the precipitate. The precipitate was then washed three times with hydrochloric acid with pH=3.0 pre-cooled to 4℃, followed by washing twice with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0076] Comparative Example 6 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of KOH aqueous solution with pH 13 and add them to the reaction vessel. Mix them evenly, seal the reaction vessel, and place it under the conditions of 180℃ and 1.5MPa for the first hydrothermal reaction for 4 hours to obtain a mixture.

[0077] ② After cooling the mixture obtained in step ① to room temperature, add 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1. Seal the reactor and place it under conditions of 180℃ and 1.5MPa for a second hydrothermal reaction for 4 hours. After the reaction is completed, the product (i.e., hydrothermal liquid, denoted as sample S5-1) is obtained. The product is then separated and purified by: cooling the product to room temperature and then performing solid-liquid separation by filtration. The liquid product was collected, and 0.1 mol / L hydrochloric acid was slowly added dropwise to the liquid product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was aged at 4°C for 2 hours, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4°C and pH=3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60°C for 12 hours to obtain humic acid.

[0078] Comparative Example 7 First, wash the corn stalks to remove excess mud and sand. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, and then crush them using a pulverizer. Pass the crushed material through an 80-mesh sieve and air dry to obtain biomass raw material. Add 3g of biomass raw material and 60mL of a KOH aqueous solution with pH 13 to a reaction vessel and mix thoroughly. Seal the reaction vessel and place it under a hydrothermal reaction at 180℃ and 1.5MPa for 8 hours. After the reaction, the product (i.e., the hydrothermal liquid, denoted as sample S6-1) is obtained. The product is then separated and purified. The separation and purification process is as follows: After cooling the product to room temperature, solid-liquid separation is performed by filtration, and the liquid phase product is collected. Hydrochloric acid with a concentration of 0.1 mol / L is slowly added dropwise to the liquid phase product under stirring to adjust the pH value of the system to 1.0, resulting in a precipitation system. The precipitation system is allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant is discarded to obtain the precipitate. The precipitate is then washed three times with hydrochloric acid pre-cooled to 4℃ with a pH of 3.0, followed by washing twice with anhydrous ethanol. The washed precipitate is transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0079] The total humic acid content of the products obtained in Example 15 and Comparative Examples 3-7 of this invention, as determined by volumetric method, and the dry weight of solids at different reaction stages are shown in the following graph. Figure 10 As shown.

[0080] from Figure 10 It can be seen that, compared with different treatment methods, the raw material treated with acid first had a significantly lower final dry weight of residual solids than the untreated or alkaline-treated groups, proving that acidic conditions can degrade insoluble substances in straw into water-soluble small molecules. Among them, the total humic acid content of the S2-1 group (acid hydrolysis + alkaline catalysis) in Example 15 was the highest as determined by volumetric method, proving that under alkaline conditions, the MnFe2O4-S2.5 catalyst can promote the condensation of precursor small molecules generated by acid hydrolysis into humic acid.

[0081] Example 16 Example 16 is basically the same as Example 15, except that: In this embodiment, rice straw was used instead of corn straw for the experiment.

[0082] Example 17 Example 17 is basically the same as Example 15, except that: In this embodiment, wheat straw was used instead of corn straw for the experiment.

[0083] Example 18 Example 18 is basically the same as Example 15, except that: In this embodiment, peanut shells were used instead of corn stalks for the experiment.

[0084] The total humic acid content of the products obtained in Examples 16-18, determined by volumetric method in this invention, is as follows: Figure 11 As shown.

[0085] from Figure 11It can be seen that the humic acid yields in the blank group (using rice straw and wheat straw as raw materials) and the MnFe2O4 group were both low. The KOH group showed a significant increase in yield; the total humic acid yield reached its highest value after the addition of KOH and MnFe2O4-S catalysts. Furthermore, the yield of the acid-base two-step method was higher than that of the alkali treatment, indicating that acidic conditions better promoted the hydrolysis of straw-based biomass, thus providing more intermediate products for humic acid formation. However, for peanut shells, the total humic acid yield did not reach its highest value after the addition of KOH and MnFe2O4-S catalysts. This may be because peanut shells contain a large amount of tannins and polyphenols, which can effectively catalyze condensation reactions even under neutral conditions (pH=7). After acidic hydrolysis, the dense structure of the peanut shell is opened, and under the action of KOH and MnFe2O4-S catalysts, the highly reactive polyphenols and precursors in the peanut shell undergo condensation reactions. The polymerized molecules become very large, forming insoluble black humic acid. A large amount of macromolecular humic acid remains in the precipitate, resulting in a low humic acid content in the aqueous phase.

[0086] Example 19 Example 19 is basically the same as Example 6, except that: ② Add 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 and 0.015g of sodium anthraquinone-2-sulfonate (the amount of sodium anthraquinone-2-sulfonate is 0.5% of the mass of corn stalks in step ①) to the mixture obtained in step ①. Seal the reactor and place it under hydrothermal conditions of 180℃ and 1.5MPa for 8 hours. After the reaction is completed, the product is obtained. The product is then separated and purified by cooling the product to room temperature. Solid-liquid separation was performed by filtration, and the liquid product was collected. Hydrochloric acid with a concentration of 0.1 mol / L was slowly added dropwise to the liquid product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4℃ and pH=3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0087] Example 20 Example 20 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, followed by the addition of 0.15g of the spongy MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 and 0.015g of sodium anthraquinone-2-sulfonate (the amount of sodium anthraquinone-2-sulfonate is 0.5% of the mass of corn stalks in step ①). The reaction vessel was sealed and subjected to hydrothermal reaction at 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product was obtained. The product was then separated and purified. The process involved cooling the product to room temperature, then separating the solid and liquid phases by filtration. The liquid phase was collected, and 0.1 mol / L hydrochloric acid was slowly added dropwise to the liquid phase with stirring to adjust the pH to 1.0, resulting in a precipitate. The precipitate was aged at 4°C for 2 hours, then centrifuged, and the supernatant was discarded to obtain the precipitate. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4°C and at pH 3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and dried under vacuum at 60°C for 12 hours to obtain humic acid.

[0088] Example 21 Example 21 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, followed by the addition of 0.15g of the spongy MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 and sodium anthraquinone-2-sulfonate (the amount of sodium anthraquinone-2-sulfonate was 2% of the mass of corn stalks in step ①). The reaction vessel was sealed and subjected to hydrothermal reaction at 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product was obtained. The product was then separated and purified. The pure product was prepared by solid-liquid separation by filtration, collecting the liquid phase product, and slowly adding 0.1 mol / L hydrochloric acid to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was aged at 4°C for 2 hours, and then centrifuged to remove the supernatant, obtaining the precipitate. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4°C and at pH=3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60°C for 12 hours to obtain humic acid.

[0089] Example 22 Example 22 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, and then 0.3g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 was added. The reaction vessel was sealed and placed under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product was obtained. The product was separated and purified as follows: after cooling the product to room temperature, solid-liquid separation was performed by filtration, and the liquid phase product was collected. Hydrochloric acid with a concentration of 0.1mol / L was slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitation system. The precipitation system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded to obtain the precipitate. The precipitate was then washed three times with hydrochloric acid with pH=3.0 pre-cooled to 4℃, followed by washing twice with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0090] Comparative Example 8 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of hydrochloric acid with a pH of 3 and add them to a reaction vessel. Mix them evenly, seal the reaction vessel, and place it under acidic hydrolysis pretreatment at a temperature of 180℃ and a pressure of 1.5MPa for 4 hours to obtain a mixture.

[0091] ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 11, and then 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 was added. The reaction vessel was sealed and placed under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product was obtained. The product was separated and purified as follows: after cooling the product to room temperature, solid-liquid separation was performed by filtration, and the liquid phase product was collected. Hydrochloric acid with a concentration of 0.1mol / L was slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitation system. The precipitation system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded to obtain the precipitate. The precipitate was then washed three times with hydrochloric acid with pH=3.0 pre-cooled to 4℃, followed by washing twice with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0092] Comparative Example 9 ① First, wash the corn stalks to remove excess mud and sand from the surface. Then, cut the corn stalks into small pieces, dry them to remove most of the moisture, crush them using a pulverizer, pass them through an 80-mesh sieve, and air dry them naturally to obtain biomass raw materials. Take 3g of biomass raw materials and 60mL of hydrochloric acid with a pH of 0.1 and add them to a reaction vessel. Mix them evenly, seal the reaction vessel, and place it under acidic hydrolysis pretreatment at a temperature of 180℃ and a pressure of 1.5MPa for 4 hours to obtain a mixture.

[0093] ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 14, and then 0.15g of the sponge-like MnFe2O4 catalyst (MnFe2O4-S2.5) prepared in Example 1 was added. The reactor was sealed and placed under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction was completed, the product was obtained. The product was separated and purified as follows: after cooling the product to room temperature, solid-liquid separation was performed by filtration, and the liquid phase product was collected. Hydrochloric acid with a concentration of 0.1mol / L was slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitation system. The precipitation system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded to obtain the precipitate. The precipitate was then washed three times with hydrochloric acid with pH=3.0 pre-cooled to 4℃, followed by washing twice with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60℃ for 12 hours to obtain humic acid.

[0094] Comparative Example 10 Comparative Example 10 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, and then 0.15g of manganese-based catalyst was added. The reactor was sealed and subjected to hydrothermal reaction at 180℃ and 1.5MPa for 4 hours. After the reaction, the product was obtained. The product was then separated and purified as follows: after cooling the product to room temperature, solid-liquid separation was performed by filtration, and the liquid phase product was collected. Hydrochloric acid with a concentration of 0.1mol / L was slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitation system. The precipitation system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then treated with hydrochloric acid pre-cooled to 4℃ at pH=3.0. The precipitate was washed three times, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and dried under vacuum at 60°C for 12 hours to obtain humic acid. The preparation of the manganese-based catalyst involved grinding a manganese source and citric acid in a mortar for 30 minutes to obtain a precursor. The manganese source was manganese acetate tetrahydrate, and the citric acid was citric acid monohydrate, with a molar ratio of citric acid to manganese source of 2.5:1. The precursor was first dried in a crucible in a forced-air drying oven at 80°C for 12 hours, and then calcined in a muffle furnace under air atmosphere to obtain the manganese-based catalyst. The high-temperature calcination involved heating from room temperature to 400°C at a rate of 2°C / min and calcining at 400°C for 2 hours.

[0095] Comparative Example 11 Comparative Example 11 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, KOH was added to adjust the pH of the mixture to 13, and then 0.15g of iron-based catalyst was added. The reactor was sealed and subjected to hydrothermal reaction at 180℃ and 1.5MPa for 4 hours. After the reaction, the product was obtained. The product was then separated and purified as follows: After cooling the product to room temperature, solid-liquid separation was performed by filtration. The liquid phase product was collected, and 0.1mol / L hydrochloric acid was slowly added dropwise to the liquid phase product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was allowed to stand at 4℃ for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then treated with hydrochloric acid pre-cooled to 4℃ at pH 3.0. The precipitate was washed three times, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and dried under vacuum at 60°C for 12 hours to obtain humic acid. The iron-based catalyst was prepared as follows: ① Iron source and citric acid were ground in a mortar for 30 minutes to obtain a precursor. The iron source was ferric nitrate nonahydrate, and the citric acid was citric acid monohydrate. The molar ratio of citric acid to iron source was 2.5:1. The precursor was first dried in a crucible in a forced-air drying oven at 80°C for 12 hours, and then calcined in a muffle furnace under air atmosphere to obtain the iron-based catalyst. The high-temperature calcination was carried out by heating from room temperature to 400°C at a heating rate of 2°C / min and calcining at 400°C for 2 hours.

[0096] Comparative Example 12 Comparative Example 12 is basically the same as Example 15, except that: ② After cooling the mixture obtained in step ① to room temperature, add KOH to adjust the pH of the mixture to 13, then add 28wt% hydrogen peroxide (the total amount of hydrogen peroxide is such that the mass of H2O2 in the hydrogen peroxide is 5% of the total mass of corn stalks). Seal the reactor and place it under hydrothermal conditions of 180℃ and 1.5MPa for 4 hours. After the reaction is completed, the product is obtained. The product is then separated and purified. The separation and purification are as follows: after cooling the product to room temperature, it is filtered to remove solids. Liquid-liquid separation was performed, and the liquid product was collected. Hydrochloric acid with a concentration of 0.1 mol / L was slowly added dropwise to the liquid product under stirring to adjust the pH of the system to 1.0, resulting in a precipitate system. The precipitate system was allowed to stand at 4°C for 2 hours for aging, and then centrifuged to separate the precipitate. The supernatant was discarded, and the precipitate was obtained. The precipitate was then washed three times with hydrochloric acid pre-cooled to 4°C and pH=3.0, followed by two washes with anhydrous ethanol. The washed precipitate was transferred to a vacuum drying oven and vacuum dried at 60°C for 12 hours to obtain humic acid.

[0097] The total humic acid content and degree of polymerization of the products from Examples 6, 15, 19-22, and Comparative Examples 8-12 were determined by volumetric method, as shown in Table 1 below. The degree of polymerization of humic acid was characterized by ultraviolet-visible spectrophotometry, using the E4 / E6 ratio as the evaluation index. The humic acid sample was dissolved in a 0.05 mol / L NaHCO3 solution to prepare a 0.2 mg / mL test solution. The absorbance at 465 nm (E4) and 665 nm (E6) was measured using an ultraviolet-visible spectrophotometer, and the ratio E4 / E6 was calculated. The E4 / E6 value is negatively correlated with the molecular weight and aromatic condensation degree of humic acid: a smaller ratio indicates a larger molecular weight and a higher degree of aromatic condensation, i.e., a higher degree of polymerization; conversely, a larger ratio indicates a lower degree of polymerization.

[0098] Table 1 In Table 1, the symbol " / " indicates that the performance metric was not tested.

[0099] After the reactions in Examples 15 and 20 were completed, the sponge-like MnFe2O4 catalyst was recovered using an external magnetic field and reused for humic acid synthesis under the same conditions. After three cycles, the humic acid yields in Examples 15 and 20 decreased by 14.2% and 13.9% respectively compared to the initial yield, with yields of 31.23% and 36.68% respectively. This result indicates that the sponge-like MnFe2O4 catalyst used in this invention maintained high stability during recycling; while the manganese-based catalyst used in Comparative Example 10, the iron-based catalyst used in Comparative Example 11, and the hydrogen peroxide oxidant used in Comparative Example 12 could not be recovered and recycled.

[0100] The parts of this invention not described in detail are techniques known to those skilled in the art.

[0101] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for synthesizing humic acid, characterized in that, The synthesis method includes the following steps: (1) Mix biomass raw materials with water evenly and add alkali to obtain a mixture with a pH of 12.5~13.5; (2) Add a sponge-like MnFe2O4 catalyst to the mixture and carry out a hydrothermal reaction, then separate and purify to obtain humic acid.

2. A method for synthesizing humic acid, characterized in that, The synthesis method includes the following steps: (a) The biomass feedstock is placed in an acidic solution with a pH of 0.5 to 1.5 for acidic hydrolysis pretreatment to obtain a mixture; (b) After adjusting the pH of the mixture to 12.5-13.5 with alkali, a sponge-like MnFe2O4 catalyst is added and a hydrothermal reaction is carried out. After separation and purification, humic acid is obtained.

3. The synthesis method according to claim 2, characterized in that: The pH of the acidic solution is 1; and / or The acidic hydrolysis pretreatment is performed at a temperature of 150-200℃, preferably 180℃, for a time of 2-6 hours, preferably 4 hours.

4. The synthesis method according to claim 1 or 2, characterized in that: The biomass raw material is one or more of the following: corn stalks, rice stalks, wheat stalks, and peanut shells; The pH of the mixture is 13; and / or In step (1), the solid-liquid ratio of the biomass raw material to the water is 1g:(15~30)mL, preferably 1g:20mL, or in step (a), the solid-liquid ratio of the biomass raw material to the acidic solution is 1g:(15~30)mL, preferably 1g:20mL.

5. The synthesis method according to claim 1 or 2, characterized in that: The temperature of the hydrothermal reaction is 150~200℃, preferably 180℃, and the time is 2~8h, preferably 4~8h.

6. The synthesis method according to claim 1 or 2, characterized in that: The separation and purification process includes, in sequence: solid-liquid separation, acidification precipitation, washing and drying.

7. The synthesis method according to claim 1 or 2, characterized in that, The amount of the sponge-like MnFe2O4 catalyst used is 3-8% of the mass of the biomass raw material in step (1); and / or the preparation of the sponge-like MnFe2O4 catalyst includes: The manganese source, iron source and complexing agent were placed in a mortar and ground evenly to obtain a paste-like precursor. The paste-like precursor was placed in a crucible and dried in a forced-air drying oven to obtain the precursor. The precursor was calcined at high temperature in air to obtain a sponge-like MnFe2O4 catalyst.

8. The synthesis method according to claim 7, characterized in that: The manganese source is manganese acetate, preferably manganese acetate tetrahydrate; The iron source is ferric nitrate, preferably ferric nitrate nonahydrate; The complexing agent is citric acid, preferably citric acid monohydrate; Preferably, the molar ratio of the manganese source to the iron source is 1:(1.5~2.5), and more preferably 1:2; Preferably, the molar ratio of the citric acid to the sum of the amounts of the manganese source and the iron source is (0.5~4.5):1, more preferably 2.5:1; The drying temperature is 80~100℃, and the time is 10~12h; and / or The high-temperature calcination temperature is 300~500℃, preferably 400℃, and the time is 1~3h, preferably 2h.

9. The synthesis method according to claim 1 or 2, characterized in that: In step (2) or step (b), sodium anthraquinone-2-sulfonate is also added, wherein the amount of sodium anthraquinone-2-sulfonate is 0.1 to 1% of the mass of the biomass raw material in step (1).

10. The application of a sponge-like MnFe2O4 catalyst in the synthesis of humic acid, characterized in that: The sponge-like MnFe2O4 catalyst can be recovered and recycled after humic acid synthesis. After three cycles of recycling, the yield reduction rate of the sponge-like MnFe2O4 catalyst for humic acid synthesis is no more than 15%.

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