Method for preparing fulvic acid through Fenton oxidation

Abstract

The invention provides a method for preparing fulvic acid through Fenton oxidation, and belongs to the technical field of fulvic acid preparation. The preparation method comprises the following steps: firstly, mixing a biomass raw material with water, and performing hydrothermal treatment to obtain a solid-liquid mixture; adding a Fenton reagent into the solid-liquid mixture, and then sequentially carrying out oxidation treatment, alkaline extraction and acidic precipitation to obtain a crude fulvic acid solution; and finally, purifying the crude fulvic acid solution to obtain the fulvic acid. According to the method, firstly, the biomass raw material is subjected to hydrothermal treatment, the hydrothermal treatment product is subjected to Fenton oxidation, hydroxyl free radicals (. OH) generated by a Fenton reagent effectively destroy C-C bonds and C-O bonds in the biomass structure, continuous decomposition and conversion of the hydrothermal product can be effectively promoted, generation of the target product fulvic acid is facilitated, the reaction time is shortened, and the yield is increased. And the production efficiency is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of fulvic acid preparation, and particularly relates to a method for preparing fulvic acid by Fenton oxidation. Background Art

[0002] Fulvic acid is a kind of complex natural organic compound widely existing in the ecosystem, with a unique and complex structure, containing various active functional groups, and having excellent properties such as complexation, adsorption, and antioxidant properties. It has wide applications in the fields of agriculture, animal husbandry, environmental protection, industrial production, and medicine. However, due to the scarcity of fulvic acid resources and its complex structure, the existing acquisition of fulvic acid mainly relies on two methods: natural extraction and artificial synthesis. Natural extraction has problems such as low extraction efficiency, long cycle, and serious environmental pollution; while the preparation of fulvic acid by the artificial fermentation method has problems such as demanding fermentation conditions, long fermentation cycle, large yield fluctuations, unstable purity, and environmental pollution, which still pose challenges to the large-scale production of fulvic acid. Therefore, developing a green and efficient method for preparing fulvic acid has become a research hotspot.

[0003] In recent years, biomass, as a renewable resource, has received increasing attention. Lignocellulosic biomass is mainly composed of cellulose, hemicellulose, and lignin. Through thermochemical conversion, a variety of high-value-added products can be produced and prepared, and the preparation of fulvic acid is one of the potential utilization ways of biomass. In the preparation of fulvic acid, the hydrothermal treatment technology decomposes biomass to generate the target product under high temperature and high pressure conditions. However, traditional hydrothermal treatment for producing fulvic acid usually uses strong acids or strong bases as solvents and catalysts to accelerate the reaction and improve the product yield. Such strong acid-base conditions not only cause equipment corrosion and operation complexity but also easily produce toxic wastewater, increasing the subsequent treatment cost and environmental protection pressure.

[0004] However, when solely relying on hydrothermal treatment of biomass with a neutral solvent to obtain fulvic acid, due to the stable structure of biomass, harsh reaction conditions, and low product yield, it affects the actual production and application of fulvic acid. Therefore, it is of great significance to study a method for preparing fulvic acid by Fenton oxidation to improve the yield and purity of fulvic acid. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing fulvic acid by Fenton oxidation to solve the problems of long production cycle, unstable quality, low yield, and low purity in the preparation of fulvic acid in the prior art.

[0006] In order to achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0007] The present invention provides a method for preparing fulvic acid by Fenton oxidation, comprising the following steps:

[0008] (1) Mix the biomass raw material and water, and then carry out hydrothermal treatment to obtain a solid-liquid mixture;

[0009] (2) Add Fenton's reagent to the solid-liquid mixture, and then successively carry out oxidation treatment, alkaline extraction and acid precipitation to obtain a crude fulvic acid solution;

[0010] (3) Purify the crude fulvic acid solution to obtain fulvic acid.

[0011] Preferably, in step (1), the mass-volume ratio of the biomass raw material to water is 1-5 g: 40-60 mL; during the hydrothermal treatment, the heating rate is 5-10 °C / min, the temperature is 150-240 °C, and the time is 1-4 h.

[0012] Preferably, the Fenton's reagent includes hydrogen peroxide solution and ferrous salt; the mass-volume ratio of the biomass raw material to the hydrogen peroxide solution is 1-5 g: 1-5 mL, and the concentration of the hydrogen peroxide solution is 30 wt.%; the addition amount of the ferrous salt is 10% of the mass of the biomass raw material.

[0013] Preferably, in step (2), the temperature of the oxidation treatment is 20-40 °C, the stirring speed of the oxidation treatment is 400-600 rpm, and the time of the oxidation treatment is 1-3 h.

[0014] Preferably, in step (2), the specific steps of the alkaline extraction are: after the oxidation treatment is completed, add a mixed alkali solution, adjust the pH value to 12-14 for reaction, and after the reaction is completed, wash and filter by suction to obtain a solid and a humic acid solution.

[0015] Preferably, the mixed alkali solution is prepared from sodium hydroxide and sodium pyrophosphate.

[0016] Preferably, in step (2), the specific steps of the acid precipitation are: adjust the pH of the humic acid solution to 1-2 with hydrochloric acid solution for reaction, and after the reaction is completed, filter by suction to separate to obtain humic acid and a crude fulvic acid solution.

[0017] Preferably, in step (3), the specific steps of the purification include:

[0018] S1: Adsorb and desorb the crude fulvic acid solution with XAD-8 resin to obtain a fulvic acid-ethanol-aqueous solution;

[0019] S2: Treat the fulvic acid-ethanol-aqueous solution with strongly acidic H + saturated cation exchange resin to obtain a refined fulvic acid solution;

[0020] S3: Dry the refined fulvic acid solution to obtain fulvic acid.

[0021] Preferably, in step S1, during adsorption, the crude fulvic acid solution flows through the chromatography column at a flow rate of 1-3 BV / h, and after the injection is completed, 1-2 BV of deionized water is used to displace the crude fulvic acid; during desorption, ethanol flows through the chromatography column at a flow rate of 1-2 BV / h until the eluate is colorless.

[0022] In the present invention, BV is the multiple of the resin volume.

[0023] Preferably, in step S2, the fulvic acid-ethanol-aqueous solution flows through the strongly acidic H + saturated cation exchange resin at a flow rate of 1-3 BV / h, and after the injection is completed, 1-2 BV of deionized water is used to displace the fulvic acid-ethanol-aqueous solution to obtain a refined fulvic acid solution.

[0024] Advantages of the present invention:

[0025] (1) The present invention first performs hydrothermal treatment on the biomass raw material. Under neutral and mild conditions, without the catalysis of strong acids or strong bases, the corrosion problem of equipment is avoided, and the production safety risk and maintenance cost are significantly reduced. Fenton oxidation is carried out on the product of hydrothermal treatment. Utilizing the acidity of the hydrothermal reaction product to couple with the hydroxyl radicals generated by the Fenton reaction, the hydroxyl radicals (·OH) effectively break the C-C bonds and C-O bonds in the biomass structure, can effectively promote the further decomposition and transformation of the hydrothermal product, facilitate the formation of the target product fulvic acid, shorten the reaction time, and improve the production efficiency.

[0026] (2) The lignocellulosic biomass (taking straw as an example) in the biomass raw material reacts under neutral hydrothermal conditions to generate a variety of small molecule acids, making the overall reaction product solution weakly acidic, showing the effect of dilute acid catalysis, which is beneficial to the Fenton reaction. Compared with the traditional fermentation method, the present invention does not rely on microbial populations, the process is stable and controllable, avoiding the disadvantage of being easily contaminated, and ensuring the stability of product quality. In addition, the present invention minimizes the use of acids and bases as much as possible, meeting the requirements of green chemistry and sustainable development, and providing a more environmentally friendly and efficient technical approach for the industrial application of fulvic acid.

[0027] (3) Compared with traditional methods such as natural extraction method, fermentation method, and hydrothermal treatment under strong acid or strong base conditions, the present invention proposes a method for preparing fulvic acid by using Fenton oxidation-assisted biomass hydrothermal method, which has obvious advantages in product purity, production cycle, operation safety, and environmental impact. While efficiently utilizing renewable resources, it improves the preparation method and efficiency of fulvic acid, providing a new idea for the green and sustainable production of fulvic acid. Description of the drawings

[0028] Figure 1 Process flow chart for preparing fulvic acid without using Fenton oxidation

[0029] Figure 2 This is the process flow chart for preparing fulvic acid by Fenton oxidation in the present invention;

[0030] Figure 3 This is the fluorescence excitation-emission matrix spectrum of fulvic acid, where a is the fluorescence excitation-emission matrix spectrum of the fulvic acid prepared in Comparative Example 1, and b is the fluorescence excitation-emission matrix spectrum of the fulvic acid prepared in Example 3. Detailed implementation manners

[0031] The present invention provides a method for preparing fulvic acid by Fenton oxidation, which includes the following steps:

[0032] (1) Mix the biomass raw material and water and then perform hydrothermal treatment to obtain a solid-liquid mixture;

[0033] (2) Add Fenton's reagent to the solid-liquid mixture, and then successively perform oxidation treatment, alkaline extraction, and acid precipitation to obtain a crude fulvic acid solution;

[0034] (3) Purify the crude fulvic acid solution to obtain fulvic acid.

[0035] In the present invention, there is no specific limitation on the specific type of the biomass raw material, as long as it is various organisms formed through photosynthesis.

[0036] The present invention preferably crushes and sieves the biomass raw material, aiming to facilitate its full mixing with the solvent and improve the reaction efficiency. The size of the crushed biomass raw material is 30-200 mesh.

[0037] In the present invention, in step (1), the mass-volume ratio of the biomass raw material to water is 1-5 g: 40-60 mL, preferably 2-4 g: 45-55 mL, and more preferably 2.5-3.5 g: 50 mL; during the hydrothermal treatment, the heating rate is 5-10 °C / min, preferably 5 °C / min, 6 °C / min, 8 °C / min, 10 °C / min; the temperature is 150-240 °C, preferably 150 °C, 180 °C, 210 °C, 220 °C, 240 °C; the time is 1-4 h, preferably 1 h, 2 h, 3 h, 4 h.

[0038] In the present invention, the Fenton's reagent includes a hydrogen peroxide solution and a ferrous salt; the mass-volume ratio of the biomass raw material to the hydrogen peroxide solution is 1-5 g: 1-5 mL, preferably 2.5-3.5 g: 1-3 mL, and the concentration of the hydrogen peroxide solution is 30 wt.%; the addition amount of the ferrous salt is 10% of the mass of the biomass raw material.

[0039] In the present invention, in step (2), the temperature of the oxidation treatment is 20 to 40 °C, preferably 25 to 35 °C; the stirring speed of the oxidation treatment is 400 to 600 rpm, preferably 450 to 550 rpm, and more preferably 500 rpm; the time of the oxidation treatment is 1 to 3 h, preferably 1 h, 2 h, or 3 h.

[0040] In the present invention, in step (2), the specific steps of the alkaline extraction are as follows: after the oxidation treatment is completed, a mixed alkali solution is added, the pH value is adjusted to 12 to 14 for reaction, and after the reaction is completed, washing and suction filtration are carried out to obtain a solid and a humic acid solution.

[0041] In the present invention, the mixed alkali solution is prepared from sodium hydroxide and sodium pyrophosphate. Sodium hydroxide can extract the beneficial fulvic acid in the solution, and sodium pyrophosphate is beneficial to the extraction of bound fulvic acid. The concentration of sodium hydroxide in the mixed alkali solution is preferably 0.1 mol / L, and the concentration of sodium pyrophosphate is preferably 0.1 mol / L.

[0042] In the present invention, in step (2), the specific steps of the acid precipitation are as follows: the pH of the humic acid solution is adjusted to 1 to 2 with a hydrochloric acid solution for reaction, and after the reaction is completed, suction filtration separation is carried out to obtain humic acid and a crude fulvic acid solution.

[0043] In the present invention, in step (3), the specific steps of the purification include:

[0044] S1: Adsorbing and desorbing the crude fulvic acid solution with XAD-8 resin to obtain a fulvic acid-ethanol-aqueous solution;

[0045] S2: Treating the fulvic acid-ethanol-aqueous solution with a strongly acidic H + saturated cation exchange resin to obtain a refined fulvic acid solution;

[0046] S3: Drying the refined fulvic acid solution to obtain fulvic acid.

[0047] In the present invention, in step S1, during the adsorption, the crude fulvic acid solution flows through the chromatography column at a flow rate of 1 to 3 BV / h, and after the injection is completed, the crude fulvic acid is displaced with 1 to 2 BV of deionized water; during the desorption, ethanol flows through the chromatography column at a flow rate of 1 to 2 BV / h until the eluate is colorless.

[0048] In the present invention, after flowing through the chromatography column with ethanol at a flow rate of 1 to 2 BV / h, there is also a dilution step, specifically: diluting the eluted solution with deionized water so that the ethanol concentration < 10%, to obtain a fulvic acid-ethanol-aqueous solution. Diluting the eluted solution aims to avoid the influence of too high ethanol concentration on the adsorption degree of the strongly acidic H + saturated cation exchange resin.

[0049] In the present invention, in step S2, the fulvic acid-ethanol-aqueous solution flows through a strongly acidic H + saturated cation exchange resin at a flow rate of 1-3 BV / h. After the injection is completed, 1-2 BV of deionized water is used to displace the fulvic acid-ethanol-aqueous solution to obtain a refined fulvic acid solution.

[0050] In the present invention, an XAD-8 resin is used to adsorb and desorb crude fulvic acid, aiming to exclude the influence of the acidic reagent used for adjusting the pH value and the generated inorganic salts on the final product. A strongly acidic H + saturated cation exchange resin is used to treat the fulvic acid-ethanol-aqueous solution, aiming to reduce the content of metal ions in the finally refined fulvic acid.

[0051] The technical solutions provided by the present invention will be described in detail below in conjunction with embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0052] The XAD-8 resin used in the embodiments of the present invention is purchased from ROHMHAAS; the strongly acidic H + saturated cation exchange resin is purchased from HECHEN.

[0053] In the embodiments of the present invention, corn straw (taken from the suburbs of Shenyang) is used as the raw material. The proximate analysis, ultimate analysis and calorific value of corn straw are shown in Table 1:

[0054] Table 1 Proximate analysis, ultimate analysis and calorific value of corn straw

[0055]

[0056]

[0057] Example 1

[0058] The corn straw is screened and crushed, and then passed through a 100-mesh sieve to obtain corn straw powder. 2.5 g of corn straw powder is added to a 100 mL batch high-temperature and high-pressure reactor equipped with a magnetic stirrer, 50 mL of deionized water is added, the reactor is sealed, the air in the reactor is purged and replaced with high-purity argon, the rotation speed of the magnetic stirrer is 500 rpm, the temperature is raised to 180 °C at a heating rate of 10 °C / min, and the hydrothermal treatment is carried out at a temperature of 180 °C for 1 h. After the reaction is completed, the heating insulation jacket of the reactor is removed, and the reactor is cooled by mechanical ventilation. When the temperature in the reactor drops to room temperature, the exhaust valve is opened to collect the gas. When the pressure in the reactor is close to the ambient air pressure, the reactor is opened, the reaction product is poured out, and the reaction product on the reactor wall and pipeline is washed repeatedly with deionized water until there is no residue, and finally a solid-liquid mixture is obtained after mixing.

[0059] Add 0.25 g of ferrous chloride to the solid-liquid mixture. Under the condition of 35 °C, stir at a constant speed of 500 rpm for 2 h for oxidation treatment. Add the product obtained after oxidation treatment to the mixed alkaline solution of sodium hydroxide and sodium pyrophosphate, adjust the pH value to 13, and stir at a constant speed of 500 rpm for 2 h for alkaline extraction at 35 °C. Wash the reaction mixture with deionized water and filter by suction until the washing effluent is neutral. Dry the residue washed to neutrality at 105 °C to constant weight to obtain a solid (carbon), and the liquid phase is a humic acid solution. Use HCl hydrochloric acid solution to adjust the pH value of the humic acid solution to 1.5, and stir continuously at room temperature at a speed of 500 rmp for 1 h for acid precipitation. Filter and separate the reaction mixture to obtain solid and liquid phase products. Dry the solid phase at 105 °C to constant weight to obtain humic acid, and the liquid phase is the crude fulvic acid solution extracted.

[0060] Pack 20 mL of the treated XAD-8 resin into a column, place absorbent cotton at the top, and inject the crude fulvic acid solution through the chromatography column at a flow rate of 40 mL / h. After the injection is completed, replace the crude fulvic acid solution in the resin layer with 20 mL of deionized water. After washing the resin layer with deionized water, pass 40 mL of absolute ethanol and deionized water through the resin layer at a flow rate of 20 mL / h to elute fulvic acid until the eluate becomes colorless and stop elution. Collect the eluted fulvic acid solution, add 400 mL of deionized water, and then pass through the treated strongly acidic H + Saturated cation exchange resin chromatography column. After the injection is completed, replace the fulvic acid-ethanol-aqueous solution in the resin layer with 20 mL of deionized water to obtain a refined fulvic acid solution. Finally, dry it to constant weight at a temperature of 105 °C to obtain fulvic acid.

[0061] Example 2

[0062] Sieve and crush the corn straw, and then pass it through a 100-mesh sieve to obtain corn straw powder. Add 2.5 g of corn straw powder to a 100 mL batch-type high-temperature and high-pressure reactor equipped with a magnetic stirrer, add 50 mL of deionized water, seal the reactor, purge and replace the air in the reactor with high-purity argon. The speed of the magnetic stirrer is 500 rpm. Raise the temperature to 180 °C at a heating rate of 10 °C / min, and keep the temperature at 180 °C for 1 h for hydrothermal treatment. After the reaction is completed, remove the heating insulation sleeve of the reactor, and cool the reactor by mechanical ventilation. When the temperature in the reactor drops to room temperature, open the exhaust valve to collect the gas. When the pressure in the reactor is close to the ambient air pressure, open the reactor and pour out the reaction product. Wash the reaction product on the reactor wall and pipeline with deionized water repeatedly until there is no residue, and finally obtain a solid-liquid mixture after mixing.

[0063] Add 1 mL of hydrogen peroxide solution with a concentration of 30 wt% to the solid-liquid mixture, add 0.25 g of ferrous chloride, and under the condition of 35 °C, stir at a constant speed of 500 rpm for 2 h for oxidation treatment. Add the product obtained after oxidation treatment to the mixed alkali solution of sodium hydroxide and sodium pyrophosphate, adjust the pH value to 13, and under the condition of 35 °C, stir at a constant speed of 500 rpm for 2 h for alkaline extraction. Wash the reaction mixture with deionized water and filter by suction until the washing effluent is neutral. Dry the residue washed to neutral at 105 °C to constant weight to obtain solid (carbon), and the liquid phase is humic acid solution. Use HCl hydrochloric acid solution to adjust the pH value of the humic acid solution to 1.5, and stir continuously at room temperature at a speed of 500 rmp for 1 h for acid precipitation. Filter and separate the reaction mixture to obtain solid and liquid phase products. Dry the solid at 105 °C to constant weight to obtain humic acid, and the liquid phase is the extracted crude fulvic acid solution.

[0064] Pack 20 mL of treated XAD-8 resin into a column, place absorbent cotton at the top, and inject the crude fulvic acid solution into the chromatography column at a flow rate of 40 mL / h. After the injection is completed, displace the crude fulvic acid solution in the resin layer with 20 mL of deionized water. After the resin layer is washed with deionized water, pass 40 mL of anhydrous ethanol and deionized water through the resin layer at a flow rate of 20 mL / h to elute the fulvic acid until the eluate becomes colorless and terminate the elution. Collect the eluted fulvic acid solution, add 400 mL of deionized water, and then pass through the treated strongly acidic H + Saturated cation exchange resin chromatography column. After the injection is completed, displace the fulvic acid-ethanol-aqueous solution in the resin layer with 20 mL of deionized water to obtain a refined fulvic acid solution. Finally, dry it to constant weight at a temperature of 105 °C to obtain fulvic acid.

[0065] Example 3

[0066] The difference from Example 1 is that the addition amount of the hydrogen peroxide solution with a concentration of 30 wt% is 3 mL, and other conditions are the same.

[0067] Example 4

[0068] The difference from Example 1 is that the addition amount of the hydrogen peroxide solution with a concentration of 30 wt% is 5 mL, and other conditions are the same.

[0069] Comparative Example 1

[0070] The corn straw was screened and crushed, and then passed through a 100-mesh sieve to obtain corn straw powder. 2.5 g of the corn straw powder was added to a 100 mL batch high-temperature and high-pressure reactor equipped with a magnetic stirrer, 50 mL of deionized water was added, the reactor was sealed, the air in the reactor was purged and replaced with high-purity argon, the rotational speed of the magnetic stirrer was 500 rpm, the temperature was raised to 180 °C at a heating rate of 10 °C / min, and the hydrothermal treatment was carried out at 180 °C for 1 h. After the reaction ended, the heating insulation jacket of the reactor was removed, and the reactor was cooled by mechanical ventilation. When the temperature in the reactor dropped to room temperature, the exhaust valve was opened to collect the gas. When the pressure in the reactor was close to the ambient air pressure, the reactor was opened, the reaction product was poured out, and the reaction product on the reactor wall and pipeline was repeatedly washed with deionized water until there was no residue. Finally, a solid-liquid mixture was obtained after mixing.

[0071] A mixed alkali solution of sodium hydroxide and sodium pyrophosphate was added to the solid-liquid mixture to adjust the pH value to 13. Under the condition of 35 °C, the mixture was continuously stirred at a rotational speed of 500 rpm for 2 h for alkaline extraction. After the reaction, the mixture was washed with deionized water and filtered by suction until the washing effluent was neutral. The residue washed to neutral was dried to constant weight at 105 °C to obtain a solid (carbon), the liquid phase was a humic acid solution, and the pH value of the humic acid solution was adjusted to 1.5 with HCl hydrochloric acid solution. The mixture was continuously stirred at room temperature at a rotational speed of 500 rmp for 1 h for acid precipitation. After the reaction, the mixture was filtered by suction to separate the solid phase and the liquid phase products. The solid phase was dried to constant weight at 105 °C to obtain humic acid, and the liquid phase was the extracted crude fulvic acid solution.

[0072] 20 mL of the treated XAD-8 resin was packed into a column, and absorbent cotton was placed at the top. The crude fulvic acid solution was injected into the chromatography column at a flow rate of 40 mL / h. After the injection ended, 20 mL of deionized water was used to displace the crude fulvic acid solution in the resin layer. After the resin layer was washed with deionized water, 40 mL of absolute ethanol and deionized water were passed through the resin layer at a flow rate of 20 mL / h to elute the fulvic acid until the eluate became colorless and the elution was terminated. The eluted fulvic acid solution was collected, 400 mL of deionized water was added, and then it passed through the treated strongly acidic H + saturated cation exchange resin chromatography column at a flow rate of 40 mL / h. After the injection ended, 20 mL of deionized water was used to displace the fulvic acid-ethanol-aqueous solution in the resin layer to obtain a refined fulvic acid solution. Finally, it was dried to constant weight at a temperature of 105 °C to obtain fulvic acid.

[0073] Comparative Example 2

[0074] The difference from Comparative Example 1 is that the temperature of the hydrothermal treatment is 150 °C, and other conditions are the same.

[0075] Comparative Example 3

[0076] The difference from Comparative Example 1 is that the temperature of hydrothermal treatment is 210 °C, and other conditions are the same.

[0077] Comparative Example 4

[0078] The difference from Comparative Example 1 is that the temperature of hydrothermal treatment is 240 °C, and other conditions are the same.

[0079] Comparative Example 5

[0080] The difference from Comparative Example 1 is that the time of hydrothermal treatment is 4 h, and other conditions are the same.

[0081] Comparative Example 6

[0082] The difference from Comparative Example 2 is that the time of hydrothermal treatment is 4 h, and other conditions are the same.

[0083] Comparative Example 7

[0084] The difference from Comparative Example 3 is that the time of hydrothermal treatment is 4 h, and other conditions are the same.

[0085] Comparative Example 7

[0086] The difference from Comparative Example 4 is that the time of hydrothermal treatment is 4 h, and other conditions are the same.

[0087] The obtained fulvic acid was "authenticated" using fluorescence excitation-emission matrix spectroscopy (EEM), measured in the range of excitation spectrum 200 - 450 nm and emission spectrum 250 - 550 nm, with an interval of 2 nm; py-GC / MS was used for pyrolysis composition analysis of the obtained fulvic acid, setting an inlet temperature of 280 °C and a sample pyrolysis temperature of 600 °C, using split injection, with helium as the carrier gas and a flow rate of 1.0 mL / min. The analysis results are as follows:

[0088] The calculation formulas for raw material conversion rate, fulvic acid yield, humic acid yield, total humic acid yield, and solid (carbon) yield are as follows:

[0089]

[0090] Among them, M 原料 、M 固体 、M 黄腐酸 and M 腐殖酸 are the masses of raw material, solid dry basis substance, fulvic acid dry basis substance, and humic acid dry basis substance respectively; Q 原料 is the raw material conversion rate; Y 黄腐酸 、Y 腐殖酸 、Y 总腐殖酸 and Y 固体(碳) are the yields of fulvic acid dry basis substance, humic acid dry basis substance, total humic acid dry basis substance, and solid dry basis substance respectively.

[0091] Statistics were conducted on the distribution of each product during the production of fulvic acid in Examples 1-4, as shown in Table 2:

[0092] Table 2 Distribution of Each Product during the Production of Fulvic Acid in Examples 1-4

[0093]

[0094] As can be seen from Table 2, the raw material conversion rate was 61.60 - 69.68%, and the fulvic acid yield was 4.35 - 9.92%. When the volume of the 30 wt% H2O2 solution used was 3 mL, the fulvic acid yield was the highest, at 9.92%.

[0095] Statistics were conducted on the distribution of each product during the production of fulvic acid by only hydrothermal treatment in Comparative Examples 1-8, as shown in Table 3:

[0096] Table 3 Distribution of Each Product during the Production of Fulvic Acid in Comparative Examples 1-8

[0097]

[0098] As can be seen from Table 3, the range of the raw material conversion rate was 58.64 - 72.20%. When the hydrothermal reaction time was 1 h, the fulvic acid yield was 5.74 - 7.22%, and when the hydrothermal reaction time was 4 h, the fulvic acid yield was 4.21 - 7.06%. Among them, when the reaction temperature was 180 °C and the residence time was 1 h, the fulvic acid yield was the highest, reaching 7.22%.

[0099] Table 4 and Table 5 are respectively the py-GC-MS analysis results of the fulvic acid prepared in Example 3 and Comparative Example 1 (py-GC-MS is helpful for analyzing the fragile and volatile structural components in the organic structure of fulvic acid, which is the product after the pyrolysis of fulvic acid and does not represent that such substances must be contained in fulvic acid, and is only used to analyze its structural composition and source), and the results are as follows:

[0100] Table 4 Analysis Results of the Fulvic Acid in Example 3

[0101]

[0102]

[0103]

[0104] Table 5 Analysis Results of the Fulvic Acid in Comparative Example 1

[0105]

[0106]

[0107]

[0108] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing fulvic acid by Fenton oxidation, characterized in that: The steps include: (1) mixing a biomass raw material and water and performing a hydrothermal treatment to obtain a solid-liquid mixture; (2) adding Fenton's reagent to the solid-liquid mixture, followed by oxidation treatment, alkaline extraction and acid precipitation in sequence to obtain a crude fulvic acid solution; (3) Purifying the crude fulvic acid solution to obtain fulvic acid.

2. The method for preparing fulvic acid by Fenton oxidation according to claim 1, characterized in that: In step (1), the mass volume ratio of the biomass raw material and water is 1-5 g:40-60 mL; during the hydrothermal treatment, the heating rate is 5-10° C. / min, the temperature is 150-240° C., and the time is 1-4 h.

3. The method for preparing fulvic acid by Fenton oxidation according to claim 1 or 2, characterized in that: The Fenton reagent comprises a hydrogen peroxide solution and a ferrous salt; the mass volume ratio of the biomass raw material to the hydrogen peroxide solution is 1-5 g:1-5 mL, wherein the concentration of the hydrogen peroxide solution is 30 wt.%; and the addition amount of the ferrous salt is 10% of the mass of the biomass raw material.

4. The method for preparing fulvic acid by Fenton oxidation according to claim 3, characterized in that: In step (2), the temperature of the oxidation treatment is 20 to 40° C., the stirring speed of the oxidation treatment is 400 to 600 rpm, and the time of the oxidation treatment is 1 to 3 hours.

5. The method for preparing fulvic acid by Fenton oxidation according to claim 2 or 4, characterized in that: In step (2), the specific steps of the alkaline extraction are: after the oxidation treatment is completed, a mixed alkali solution is added, and the pH value is adjusted to 12-14 for reaction, and after the reaction is completed, the solid and humic acid solution are obtained by washing and filtering.

6. The method for preparing fulvic acid by Fenton oxidation according to claim 5, characterized in that: The mixed alkali solution is prepared from sodium hydroxide and sodium pyrophosphate.

7. The method for preparing fulvic acid by Fenton oxidation according to claim 6, characterized in that: In step (2), the specific steps of the acid precipitation are: adjusting the pH of the humic acid solution to 1-2 with a hydrochloric acid solution for reaction, and filtering and separating after the reaction to obtain humic acid and crude fulvic acid solution.

8. The method for preparing fulvic acid by Fenton oxidation according to claim 2, 4, 6 or 7, characterized in that: In step (3), the specific steps of purification include: S1: using XAD-8 resin to adsorb and desorb the crude fulvic acid solution to obtain a fulvic acid-ethanol-water solution; S2: Using strong acid H + The fulvic acid-ethanol-water solution is treated with a saturated cation exchange resin to obtain a refined fulvic acid solution; S3: Drying the refined fulvic acid solution to obtain fulvic acid.

9. The method for preparing fulvic acid by Fenton oxidation according to claim 8, characterized in that: In step S1, during the adsorption, the crude humic acid solution is passed through the chromatography column at a flow rate of 1 to 3 BV / h, and after the injection is completed, the crude humic acid is replaced with 1 to 2 BV of deionized water; during the desorption, ethanol is passed through the chromatography column at a flow rate of 1 to 2 BV / h until the eluent is colorless.

10. The method for preparing fulvic acid by Fenton oxidation according to claim 9, characterized in that: In step S2, the fulvic acid-ethanol-water solution flows through a strongly acidic H + After the injection of the saturated cation exchange resin, 1-2 BV of deionized water was used to replace the fulvic acid-ethanol-water solution to obtain a refined fulvic acid solution.

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