Method for preparing humic acid through cotton straw alkali heat one-step humification

Abstract

The invention provides a method for preparing humic acid through cotton straw alkali-heating one-step humification, which comprises the following steps: drying cotton straws, crushing to obtain powder, adding a sodium hydroxide solution, soaking, extracting lignin in the cotton straws into an alkaline solution; the method comprises the following steps: carrying out high-speed centrifugation to realize solid-liquid separation, putting obtained lignin extraction alkali liquor of cotton straws into a reaction kettle for reaction to obtain humic acid liquor, carrying out pH value adjustment, separating precipitated humic acid solids by high-speed centrifugation, washing the humic acid solids by pure water, drying and grinding to obtain humic acid powder. And the extracted humic acid sample is subjected to a plurality of characterizations. Compared with a microbial conversion method, the method provided by the invention has the advantages of quicker process and more controllable humic acid structure; compared with the existing detection standard, the characterization means provided by the invention comprehensively analyzes the structure of the prepared humic acid. Results show that the prepared humic acid has higher organic matter content, has the same fluorescence property as the mineral source humic acid, and has clearer carboxyl, hydroxyl, quinonyl and other characteristic functional group structures.

Description

Technical Field

[0001] The invention belongs to the interdisciplinary field of comprehensive utilization of agricultural resources and environmental science, and relates to a method for preparing humic acid by alkali-thermal one-step humification of cotton straw. Background Art

[0002] Xinjiang, my country's largest cotton-producing region, boasts a massive cotton stalk output. In recent years, Xinjiang has actively explored resource utilization for cotton stalks, but only 40% is used for low-value-added purposes such as feed and fuel, resulting in significant waste of resources. Therefore, achieving high-value utilization of cotton stalks has become a critical challenge that needs to be addressed urgently. Cotton stalks primarily consist of cellulose, hemicellulose, and lignin, with lignin accounting for between 15% and 25%. This provides a rich, low-cost, and high-quality base for the production of humic acid. Therefore, extracting humic acid from cotton stalks offers a dual solution for improving soil structure, enhancing fertility, and promoting green transformation. To date, researchers have continuously optimized the extraction process for humic acid, but its preparation methods still face challenges such as low extraction efficiency and complex processes. More importantly, existing characterization methods for humic acid samples still suffer from incomplete structural elucidation and a lack of characterization tools. In summary, significant challenges remain in extracting and comprehensively characterizing humic acid. Therefore, in the traditional humic acid extraction process, increasing the degree of straw humification is of great significance for the extraction of humic acid. Summary of the Invention

[0003] The present invention aims to provide a one-step alkaline-thermal humification method for producing humic acid from cotton straw. The method involves drying the cotton straw, pulverizing it to obtain a powder, then soaking it in sodium hydroxide solution to extract the lignin from the cotton straw and exposing it to an alkaline solution. High-speed centrifugation is then used to achieve solid-liquid separation. The resulting lignin-extracted alkali solution from the cotton straw is placed in a reactor to react and produce a humic acid solution. The pH is then adjusted, and the precipitated humic acid solid is separated by high-speed centrifugation. The solid is then rinsed with pure water, dried, and ground to obtain a humic acid powder. The extracted humic acid sample is then subjected to various characterization methods. Compared to microbial conversion methods, the proposed method offers the advantages of a faster process and more controllable humic acid structure. Compared to existing testing standards, the proposed method utilizes a more comprehensive range of characterization methods to comprehensively elucidate the structure of the prepared humic acid. Results show that the prepared humic acid has a higher organic matter content, exhibits fluorescence properties comparable to mineral-derived humic acid, and possesses more distinct characteristic functional groups, such as carboxyl, hydroxyl, and quinone groups.

[0004] More importantly, the present invention prepares humic acid through a one-step alkaline-thermal humification of cotton straw. Compared with the plant humic acid and its production process described in the invention patent (CN106478961 A), the raw materials used in the present invention are different, the preparation process is simpler, and no other chemical substances (anthraquinone, concentrated phosphoric acid, etc.) need to be added. This method has advantages in terms of cost, efficiency, and environmental protection.

[0005] The present invention discloses a method for preparing humic acid by alkali-thermal one-step humification of cotton straw, which converts lignin in cotton straw into humic acid. The specific operation is carried out according to the following steps: a. Dry the fresh cotton straw at 40°C, and crush the dried cotton straw into brown-yellow cotton straw powder with a diameter of 48 μm and a mesh size of 300; b. Add 30 g of the cotton straw powder obtained in step a to 400 mL of 0.1 mol / L sodium hydroxide solution (pH = 13), stir for 3 h, and extract the lignin in the cotton straw by soaking in alkaline solution; c. Use high-speed centrifugation at 10,000 r / min for 10 min to achieve solid-liquid separation to obtain lignin extraction alkali solution from cotton straw, and store the extracted alkali solution in the dark at room temperature; d. Place the alkali solution extracted from the cotton straw lignin in step c in a reactor at 180° C. for 5 h to obtain humic acid solution after lignin humification; e. The pH value of the humic acid solution obtained in step d was adjusted with hydrochloric acid solution. When the pH was 3, a large amount of humic acid precipitated in the liquid. The precipitated humic acid solid was separated by high-speed centrifugation at 10,000 r / min for 8 min. The humic acid solid was then rinsed 2-3 times with pure water at 65°C and dried and ground for 6 h to obtain humic acid powder.

[0006] The obtained humic acid was characterized in various ways: The obtained humic acid powder was dissolved in methanol solvent to obtain a methanol solution of humic acid, which exhibited yellow-blue fluorescence when the excitation light wavelength was 365 nm; the excitation-emission fluorescence spectrum was measured within the excitation wavelength range of 250-550 nm and the emission wavelength range of 350-750 nm; and the absorption spectrum was measured within the wavelength range of 200-800 nm.

[0007] The obtained humic acid powder was tested for elements, organic matter content, molecular weight and functional groups in the sample by X-ray photoelectron spectroscopy, organic element analysis, gel permeation chromatography, thermogravimetry and infrared spectroscopy.

[0008] The present invention provides a method for preparing humic acid by alkali-thermal one-step humification of cotton straw, and a method for performing multiple characterizations on the extracted humic acid samples, which solves the problems of low extraction efficiency and lack of characterization methods for humic acid samples in current humic acid preparation methods.

[0009] The present invention discloses a method for preparing humic acid by alkali-thermal one-step humification of cotton straw. The method comprises the following steps: extracting lignin from cotton straw powder using solutions with different pH values by screening reaction conditions: 0.01 mol / L, 0.02 mol / L, 0.10 mol / L, 0.25 mol / L, 0.50 mol / L, 0.75 mol / L, and 1.00 mol / L sodium hydroxide solutions, wherein the pH values of the solutions are 12.0, 12.3, 13.0, 13.4, 13.7, and 14.0, respectively. Based on the lignin extraction rate and reaction cost, a sodium hydroxide solution with a pH of 13 and a 0.10 mol / L solution is selected as the optimal alkali solution for cotton straw extraction. The reaction temperature of cotton straw lignin humification was regulated: 0.10 mol / L sodium hydroxide solution (pH=13.0) was transferred into the reactor and the reaction was carried out at temperatures of 120 ℃, 140 ℃, 160 ℃, 180 ℃, 200 ℃, and 220 ℃ for 5 h. The spectra of the final products at different temperatures were compared, and the critical temperature of 180 ℃ at which the spectrum was stable was taken as the optimal reaction temperature.

[0010] Compared with the prior art, the present invention has the following beneficial effects: 1. The method for preparing humic acid by alkali-heat one-step humification of cotton straw used in the present invention has the advantages of higher efficiency and more controllable process and product structure compared with the microbial method.

[0011] 2. Compared with traditional mineral-source humic acid, the humic acid product prepared by the present invention has a higher organic matter content and more abundant highly active carboxyl and hydroxyl groups.

[0012] 3. The humic acid sample obtained by the method of the present invention has complete characterization methods, which solves the problem of lack of characterization methods for humic acid samples in existing reports and improves the structural analysis of humic acid samples. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is a graph showing the content of various elements in organic matter measured in humic acid powder according to the present invention; Figure 2 This is the oxygen-permeable thermogravimetric spectrum measured in humic acid powder according to the present invention; Figure 3 This is the molecular weight distribution diagram measured in humic acid powder according to the present invention; Figure 4(a) The excitation-emission fluorescence spectrum analysis diagram obtained in the humic acid methanol solution of the present invention, wherein the horizontal axis is the fluorescence emission wavelength range and the vertical axis is the fluorescence excitation wavelength range; (b) The ultraviolet absorption spectrum, wherein the horizontal axis is the spectrum wavelength and the vertical axis is the absorption intensity; Figure 5 This is the infrared spectrum measured in humic acid powder of the present invention; Figure 6 The X-ray photoelectron spectrum measured in humic acid powder of the present invention, wherein (a) is the 1s electron orbital spectrum of carbon element, and (b) is the 1s electron orbital spectrum of oxygen element. DETAILED DESCRIPTION

[0014] The present invention will be further described below with reference to specific examples. Example 1

[0015] The present invention discloses a method for preparing humic acid by alkali-thermal one-step humification of cotton straw, which converts lignin in cotton straw into humic acid. The specific operation is carried out according to the following steps: a. Dry the fresh cotton straw at 40°C, and crush the dried cotton straw into brown-yellow cotton straw powder with a diameter of 48 μm and a mesh size of 300; b. Add 30 g of the cotton straw powder obtained in step a to 400 mL of 0.1 mol / L sodium hydroxide solution (pH = 13), stir for 3 h, and extract the lignin in the cotton straw by soaking in alkaline solution; c. Use high-speed centrifugation at 10,000 r / min for 10 min to achieve solid-liquid separation to obtain lignin extraction alkali solution from cotton straw, and store the extracted alkali solution in the dark at room temperature; d. Place the alkali solution extracted from the cotton straw lignin in step c in a reactor at 180° C. for 5 h to obtain humic acid solution after lignin humification; e. The pH value of the humic acid solution obtained in step d was adjusted with hydrochloric acid solution. When the pH was 3, a large amount of humic acid precipitated in the liquid. The precipitated humic acid solid was separated by high-speed centrifugation at 10,000 r / min for 8 min. The humic acid solid was then rinsed 2-3 times with pure water at 65°C and dried and ground for 6 h to obtain humic acid powder. Example 2

[0016] The humic acid powder obtained in Example 1 was characterized by various methods: The humic acid powder obtained in Example 1 was dissolved in a methanol solution. When the excitation wavelength was 365 nm, yellow-blue fluorescence was exhibited. The excitation-emission fluorescence distribution spectrum was measured within the excitation wavelength range of 250-550 nm and the emission wavelength range of 350-750 nm. In the methanol solution, the absorption spectrum was obtained within the wavelength range of 200-800 nm. The humic acid powder obtained in Example 1 was tested for elements, organic matter content, molecular weight and functional groups in the sample by X-ray photoelectron spectroscopy, organic element analysis, gel permeation chromatography, thermogravimetric analysis and infrared spectroscopy.

[0017] Characterization revealed that the humic acid prepared by the present invention mainly contains elements such as C, O, N, and H (in descending order of element content); has an organic matter content of 99.6%; has an average molecular weight of 1939 and a weighted molecular weight of 2806; and, at an excitation wavelength of 450-460 nm, exhibits an emission wavelength concentrated in the range of 530-540 nm, consistent with that of mineral-derived humic acid. Its chemical structure primarily includes characteristic groups such as carboxyl, phenolic hydroxyl, and quinone.

Claims

1. A method for preparing humic acid by alkali-thermal one-step humification of cotton straw, characterized in that: The lignin in cotton straw is converted into humic acid. The specific operation is carried out according to the following steps: a. Dry the fresh cotton straw at 40°C, and crush the dried cotton straw into brown-yellow cotton straw powder with a diameter of 48 μm and a mesh size of 300; b. Add 30 g of the cotton straw powder obtained in step a to 400 mL of 0.1 mol / L sodium hydroxide solution (pH = 13), stir for 3 h, and extract the lignin in the cotton straw by soaking in alkaline solution; c. Use high-speed centrifugation at 10,000 r / min for 10 min to achieve solid-liquid separation to obtain lignin extraction alkali solution from cotton straw, and store the extracted alkali solution in the dark at room temperature; d. Place the alkali solution extracted from the cotton straw lignin in step c in a reactor at 180° C. for 5 h to obtain humic acid solution after lignin humification; e. The pH value of the humic acid solution obtained in step d was adjusted with hydrochloric acid solution. When the pH was 3, a large amount of humic acid precipitated in the liquid. The precipitated humic acid solid was separated by high-speed centrifugation at 10,000 r / min for 8 min. The humic acid solid was then rinsed 2-3 times with pure water at 65°C and dried and ground for 6 h to obtain humic acid powder.

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