Method for preparing humic acid from straw

Abstract

The application discloses a method for preparing humic acid from straws, and belongs to the technical field of agricultural waste resource utilization. The method comprises straw pretreatment, composite modification pretreatment, segmented temperature control and synergistic reaction, separation and purification, and wastewater recycling. The composite modification pretreatment adopts a potassium hydroxide-urea-nano silicon dioxide composite modifier, and combines with ultrasonic treatment to destroy the structure of the straw; the segmented temperature control and synergistic reaction adopts three-stage gradient heating to realize efficient degradation of the straw and directional synthesis of humic acid; meanwhile, a closed loop system of "preparation-recovery-reuse" is constructed to realize full utilization of residues and wastewater without secondary pollution. The yield of the humic acid reaches 45%-55%, the purity is 90%-95%, the carboxyl content is 8.5-10.5 mmol / g, the hydroxyl content is 6.0-7.5 mmol / g, the amino content is 2.5-3.5 mmol / g, the reaction period is 8-10 h, and the energy consumption is reduced by 30%-40%. The method is simple in operation and low in cost, can realize high value and full resource utilization of the straw, and is convenient for industrialization promotion.

Description

Technical Field

[0001] This invention belongs to the field of agricultural waste resource utilization technology, specifically relating to a method for preparing humic acid from straw, and more particularly to a method for achieving efficient straw conversion, high-yield and high-quality humic acid without secondary pollution based on composite modification pretreatment and segmented temperature control synergistic reaction. Background Technology

[0002] Humic acid is a mixture of natural high-molecular-weight organic acids with aromatic structures and rich in functional groups such as carboxyl, hydroxyl, and amino groups. It is widely used in agriculture, environmental protection, and medicine, playing important roles in promoting plant growth, improving soil structure, adsorbing heavy metal ions, and degrading organic pollutants. Currently, humic acid is mainly obtained through two methods: extraction from natural minerals such as peat, lignite, and weathered coal, and artificial preparation from agricultural waste.

[0003] With the development of agricultural production, my country generates hundreds of millions of tons of straw waste annually. Its main components include cellulose, hemicellulose, and lignin, making it an ideal raw material for preparing humic acid. Using straw to prepare humic acid not only enables the resource utilization of agricultural waste and reduces environmental pollution from burning and landfilling, but also replaces natural mineral humic acid, alleviating the pressure of over-exploitation of natural resources, thus yielding significant economic, environmental, and social benefits.

[0004] Existing methods for preparing humic acid from straw mainly include biological fermentation, thermochemical hydrolysis, and acid-base oxidation, but all of them have obvious technical drawbacks: Bio-fermentation relies on microbial communities to degrade straw. The reaction cycle is long (usually 15-30 days), and it is greatly affected by environmental temperature and humidity. The yield of humic acid is low (generally not exceeding 25%), and the product has low purity and poor functional group activity, making it difficult to meet the needs of high-end applications. At the same time, odor is easily generated during the fermentation process, causing secondary pollution.

[0005] In thermochemical hydrolysis, existing patents (such as CN115678923A and CN114806721A) mostly employ a single high-temperature and high-pressure reaction, or segmented temperature control without combining it with raw material modification. This results in problems such as high energy consumption, incomplete straw degradation, and easy destruction of the humic acid structure. Some disclosed processes use high-concentration acids and alkalis as catalysts, which not only corrodes production equipment but also generates large amounts of wastewater, placing significant environmental pressure on the industry. For example, CN115678923A discloses a method for preparing humic acid through hydrothermal humification of corn straw and sewage sludge. While this method can increase nitrogen content, the high ash content of the sludge reduces the humic acid yield. Furthermore, the reaction requires high temperature and pressure (220-240℃), leading to high energy consumption. It also fails to achieve efficient conversion of straw as a single raw material and does not involve the synergistic use of any modifiers. CN114806721A uses sodium hydroxide as a single modifier, which can only achieve slight damage to the surface structure of straw and cannot achieve deep degradation of cellulose and lignin. The humic acid yield can only reach about 30%.

[0006] In acid-base oxidation methods, published patents (such as CN116074358A and CN113249876A) often use a single acid or base as the oxidant. The reaction conditions are often harsh (e.g., strong acid at high temperatures), which can easily lead to the breakage of humic acid molecular chains, reducing its activity. Furthermore, the amount of acid and base used is large (the mass ratio is usually above 1:0.2), subsequent neutralization is complex, production costs are high, and salt byproducts are generated, affecting the quality of humic acid. Specifically, CN116074358A uses sulfuric acid as the oxidant, resulting in humic acid purity of only about 75% and generating a large amount of acidic wastewater, leading to high environmental treatment costs. CN113249876A uses potassium hydroxide as the oxidant, resulting in a humic acid yield of less than 30% and a low content of active functional groups, failing to meet the needs of high-end applications.

[0007] Furthermore, none of the existing patents, whether for preparation from a single straw raw material or for preparation by mixing straw with other wastes, have solved the synergistic problem of "improving straw degradation efficiency and humic acid quality"—either pursuing degradation efficiency at the expense of the structural integrity of humic acid, or focusing on quality at the expense of yield. At the same time, the humic acid prepared by the existing methods has a low content of active functional groups such as carboxyl and hydroxyl groups (carboxyl content is usually no more than 6 mmol / g), and its adsorption and growth-promoting properties are limited, making it difficult to meet the needs of soil remediation, high-end fertilizers, and other scenarios. Moreover, none of them have constructed a closed-loop "preparation-recycling" system, resulting in varying degrees of secondary pollution problems.

[0008] Therefore, developing a method for preparing humic acid that has a short reaction cycle, low energy consumption, high yield, excellent quality, no secondary pollution, and can realize the efficient resource utilization of straw has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0009] The purpose of this invention is to provide a method for preparing humic acid from straw, so as to solve the technical defects of the existing technology for preparing humic acid from straw, such as long reaction cycle, low yield, poor quality, high energy consumption, and easy generation of secondary pollution.

[0010] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a method for preparing humic acid from straw, comprising the following steps: S1. Straw pretreatment: Select straw raw materials, remove impurities, crush, sieve, and dry to constant weight for later use; S2. Composite Modification Pretreatment: Mix the pretreated straw powder with the composite modifier, add deionized water, adjust the solid-liquid ratio, and ultrasonically treat to obtain modified straw slurry. The composite modifier is composed of potassium hydroxide, urea, and nano-silica. S3. Segmented temperature-controlled synergistic reaction: The modified straw slurry prepared in step S2 is transferred to a high-pressure reactor, nitrogen is introduced to the set pressure, and after sealing, a segmented temperature-controlled reaction is carried out. After the reaction is completed, it is cooled to room temperature, the pressure is released, and the reaction product is obtained. The segmented temperature-controlled reaction includes a low-temperature activation stage, a medium-temperature humification stage, and a high-temperature quality improvement stage. S4. Separation and purification: Centrifuge the reaction product to separate the supernatant and residue; wash the residue with deionized water, and combine the washing liquid with the supernatant to obtain a crude humic acid solution; adjust the pH of the crude humic acid solution to acidic, allow it to stand to precipitate, filter, wash, dry, and pulverize to obtain the humic acid product.

[0011] Furthermore, the method also includes S5, wastewater recycling: the residue is recycled as a raw material for biomass fuel or organic fertilizer; the wastewater is combined, the pH value is adjusted to neutral, evaporated and concentrated, cooled and crystallized, and a mixture of potassium chloride and urea is recovered for use as agricultural fertilizer.

[0012] Furthermore, in S1, the straw raw material is one or more of corn straw, wheat straw, and rice straw; the sieving is through a 40-60 mesh sieve.

[0013] Further, in S2, the composite modifier is composed of potassium hydroxide, urea, and nano-silica in a mass ratio of (2-4):(1-3):(0.5-1.5), and the nano-silica has a particle size of 50-100nm.

[0014] Furthermore, in S2, the composite modifier is composed of potassium hydroxide, urea, and nano-silica in a mass ratio of 3:2:1, and the nano-silica has a particle size of 80nm.

[0015] Furthermore, in S2, the mass ratio of straw powder to composite modifier is 1:0.08-0.12, the solid-liquid ratio is 1:8-12, and the ultrasonic treatment time is 30-60 min; in S2, the ultrasonic power is 200-300 W, and the ultrasonic temperature is 40-50℃.

[0016] Further, in S3, nitrogen gas is introduced until the pressure inside the reactor is 0.3-0.5 MPa; the reaction temperature of the low-temperature activation stage is 120-140℃, the stirring speed is 150-200 r / min, and the reaction time is 1.5-2.5 h; the reaction temperature of the medium-temperature humification stage is 160-180℃, the stirring speed is 250-300 r / min, and the reaction time is 2-3 h; the reaction temperature of the high-temperature upgrading stage is 200-220℃, the stirring speed is 250-300 r / min, and the reaction time is 1-1.5 h.

[0017] Furthermore, in S4, the centrifugation speed is 3000-4000 r / min, and the centrifugation time is 15-20 min; the pH value of the crude humic acid solution is adjusted to 2.0-2.5 using 1 mol / L hydrochloric acid; the settling time is 12-24 h; the drying temperature is 70-80℃, and after drying to constant weight, it is pulverized and passed through an 80-100 mesh sieve.

[0018] Furthermore, in S5, the wastewater evaporation and concentration temperature is 80-90℃, concentrating it to 1 / 5-1 / 4 of its original volume, the cooling crystallization temperature is 20-25℃, and the crystallization time is 8-12h.

[0019] Secondly, the present invention provides humic acid prepared according to the above method.

[0020] Furthermore, the humic acid yield is 45%-55%, the purity is 90%-95%, the carboxyl content is 8.5-10.5 mmol / g, the hydroxyl content is 6.0-7.5 mmol / g, and the amino content is 2.5-3.5 mmol / g.

[0021] Compared with the prior art, the present invention has the following beneficial effects: 1. High humic acid yield and superior quality: This invention achieves efficient degradation of straw and targeted synthesis of humic acid through a pioneering potassium hydroxide-urea-nano silica composite modifier and a segmented temperature-controlled synergistic reaction process. In the composite modifier, potassium hydroxide disrupts the cell wall structure of straw, promoting the degradation of cellulose and hemicellulose; urea provides a nitrogen source to participate in humic acid synthesis and also acts as a hydrogen bond disruptor to improve humic acid solubility; nano silica accelerates the polymerization reaction of degradation products, improving the molecular weight and stability of humic acid. The synergistic effect of these three components results in a humic acid yield of 45%-55%, a purity of 90%-95%, a carboxyl content of 8.5-10.5 mmol / g, a hydroxyl content of 6.0-7.5 mmol / g, and an amino content of 2.5-3.5 mmol / g, all significantly superior to existing technologies.

[0022] 2. Short reaction cycle and low energy consumption: The entire preparation process of this invention has a total cycle of only 8-10 hours, which is much shorter than the existing biological fermentation method (15-30 days); the segmented temperature control process and the composite modifier are used in synergy, which reduces energy consumption by 30%-40% compared with the traditional thermochemical method, greatly reduces production costs, and facilitates industrial promotion and application.

[0023] 3. No secondary pollution and high resource utilization rate: This invention constructs a closed-loop system of "preparation-recycling-reuse", which recovers the reaction residue as raw material for biomass fuel or organic fertilizer, and recovers and treats the wastewater to obtain a mixture of potassium chloride and urea for reuse in agricultural production. This realizes the full utilization of straw raw materials and no secondary pollution emissions, which is in line with the development concept of green environmental protection and circular economy.

[0024] 4. Wide range of applications and simple operation: This invention can be applied to various straw raw materials such as corn straw, wheat straw, and rice straw, without the need to adjust the process parameters for different straws; the whole process is simple, easy to operate, and has moderate equipment requirements, making it easy to scale up production.

[0025] 5. Strong product stability and wide range of applications: The humic acid prepared by this invention has good stability under different pH values ​​(3.0-8.0) and temperatures (0-60℃), and is not easily decomposed. It can be widely used in soil improvement, fertilizer preparation, heavy metal adsorption, sewage treatment and other fields, and has significant economic, environmental and social benefits. Detailed Implementation

[0026] The technical solutions of this disclosure will be clearly and completely described below with reference to specific embodiments. However, those skilled in the art will understand that the embodiments described below are only some embodiments of this disclosure, not all embodiments, and are only used to illustrate this disclosure, and should not be regarded as limiting the scope of this disclosure. Based on the embodiments in this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall be followed. Where the manufacturers of reagents or instruments are not specified, they are all conventional products that can be purchased commercially.

[0027] Source of raw materials The raw materials used in the embodiments of the present invention are as follows: Straw: Selected corn straw from the Huang-Huai-Hai production area, quality Zhengdan 958, harvested at full maturity in October 2024, naturally sun-dried until moisture content ≤15%, crushed and sealed in a cool, dry warehouse, protected from light and moisture.

[0028] Potassium hydroxide: Manufacturer: Sinopharm Chemical Reagent Co., Ltd., Analytical grade AR, ≥85.0%, Quality inspection information: Complies with GB / T 2306-2008 standard.

[0029] Urea: Manufacturer: Sichuan Meifeng Chemical Co., Ltd., Specifications: Agricultural Grade, Total Nitrogen ≥ 46.4%, Quality Inspection Information: Complies with GB / T 2440-2017 standard.

[0030] Nano-silica: Anhui Nanomet New Materials Co., Ltd., particle size distribution: 50 – 100nm, specific surface area: 180±20m² 2 / g, purity: ≥99.5%.

[0031] Test methods The various indicators of the humic acid products prepared in this embodiment and comparative example were determined according to the following methods: The straw degradation rate was determined according to the method specified in GB / T 26760-2011; the humic acid yield and water-soluble humic acid content were determined according to NY / T 1971-2010; the total acidic functional group content was determined by potentiometric titration according to GB / T 33804-2017; the nitrogen element mass fraction was determined by Kjeldahl nitrogen determination method according to GB / T 17767.1-2011; and the ash content was determined by high-temperature burning method according to GB / T7697-2008.

[0032] The instruments used in the test included a FA2004 electronic analytical balance, a UV-1800 ultraviolet-visible spectrophotometer, a ZD-2 potentiometric titrator, a KDN-102F Kjeldahl nitrogen analyzer, and an SX2-5-12 muffle furnace. The test conditions were strictly controlled in accordance with the corresponding national standards. All test data were measured in triplicate, and the average value was taken as the final test result.

[0033] The present invention will be described in detail below with reference to specific embodiments.

[0034] Example 1 A method for preparing humic acid from straw includes the following steps: S1: Straw pretreatment: Select corn stalks, remove impurities, crush, and pass through a 50-mesh sieve to obtain straw powder; place the straw powder in a 70℃ constant temperature drying oven and dry for 2.5 hours until constant weight (moisture content ≤5%), then set aside for later use.

[0035] S2: Composite Modification Pretreatment: The pretreated straw powder and composite modifier are mixed at a mass ratio of 1:0.1, deionized water is added, the solid-liquid ratio is adjusted to 1:10, and after stirring evenly, the mixture is placed in an ultrasonic reactor and ultrasonically treated for 45 minutes at a power of 250W and a temperature of 45℃ to complete the composite modification and obtain modified straw slurry; the composite modifier is composed of potassium hydroxide, urea and nano silica at a mass ratio of 3:2:1, and the particle size of nano silica is 80nm.

[0036] S3: Segmented temperature-controlled synergistic reaction: The modified straw slurry was transferred to a high-pressure reactor, and nitrogen gas was introduced until the internal pressure reached 0.4 MPa. After sealing, a segmented temperature-controlled reaction was carried out: First stage, temperature 130℃, stirring speed 180 r / min, reaction time 2 h; Second stage, temperature 170℃, stirring speed 280 r / min, reaction time 2.5 h; Third stage, temperature 210℃, stirring speed 280 r / min, reaction time 1.2 h; After the reaction was completed, it was naturally cooled to room temperature, the internal pressure was released, and the reaction product was obtained.

[0037] S4: Separation and purification: Centrifuge the reaction product at 3500 r / min for 18 min to separate the supernatant and residue; wash the residue three times with deionized water, combine the washing liquid with the supernatant to obtain a crude humic acid solution; add 1 mol / L hydrochloric acid to the crude humic acid solution to adjust the pH to 2.2, stir evenly, let it stand for 18 h to precipitate, filter to obtain humic acid precipitate; wash the humic acid precipitate with deionized water until neutral, place it in a 75℃ constant temperature drying oven to dry for 3.5 h, pulverize and pass through a 90 mesh sieve to obtain the humic acid product.

[0038] S5: Wastewater recycling: The residue obtained from centrifugation is dried and crushed, and then used as raw material for organic fertilizer. The wastewater after washing the residue and the filtrate after filtering humic acid precipitation are combined, potassium hydroxide is added to adjust the pH value to 7.2, and then the mixture is evaporated and concentrated to 1 / 4 of the original volume at 85℃, cooled to 22℃, and crystallized for 10 hours to recover a mixture of potassium chloride and urea, which is then used as agricultural fertilizer.

[0039] The humic acid prepared in this embodiment had a yield of 52%, a purity of 93%, a carboxyl content of 9.8 mmol / g, a hydroxyl content of 6.8 mmol / g, an amino content of 3.2 mmol / g, and an adsorption capacity of 142 mg / g for lead ions. The total reaction cycle was 9 hours, and the energy consumption was reduced by 35% compared to the traditional thermochemical hydrolysis method, with no secondary pollution emissions.

[0040] Example 2 A method for preparing humic acid from straw includes the following steps: S1: Straw pretreatment: Select wheat straw, remove impurities, crush, and pass through a 40-mesh sieve to obtain straw powder; place the straw powder in a 60℃ constant temperature drying oven and dry for 3 hours until constant weight (moisture content ≤5%), then set aside for later use.

[0041] S2: Composite Modification Pretreatment: The pretreated straw powder and composite modifier are mixed at a mass ratio of 1:0.08, deionized water is added, the solid-liquid ratio is adjusted to 1:8, and after stirring evenly, the mixture is placed in an ultrasonic reactor and ultrasonically treated for 60 minutes at a power of 200W and a temperature of 40℃ to complete the composite modification and obtain modified straw slurry; the composite modifier is composed of potassium hydroxide, urea and nano silica at a mass ratio of 3:2:1, and the particle size of nano silica is 50nm.

[0042] S3: Segmented temperature-controlled synergistic reaction: The modified straw slurry was transferred to a high-pressure reactor, and nitrogen gas was introduced until the internal pressure reached 0.3 MPa. After sealing, a segmented temperature-controlled reaction was carried out: First stage, temperature 120℃, stirring speed 150 r / min, reaction time 2.5 h; Second stage, temperature 160℃, stirring speed 250 r / min, reaction time 3 h; Third stage, temperature 200℃, stirring speed 250 r / min, reaction time 1.5 h; After the reaction was completed, it was naturally cooled to room temperature, the internal pressure was released, and the reaction product was obtained.

[0043] S4: Separation and purification: Centrifuge the reaction product at 3000 r / min for 20 min to separate the supernatant and residue; wash the residue twice with deionized water, and combine the washing liquid with the supernatant to obtain a crude humic acid solution; add 1 mol / L hydrochloric acid to the crude humic acid solution to adjust the pH to 2.0, stir evenly, let it stand for 24 h to precipitate, and filter to obtain humic acid precipitate; wash the humic acid precipitate with deionized water until neutral, place it in a 70℃ constant temperature drying oven to dry for 4 h, pulverize it and pass it through an 80 mesh sieve to obtain the humic acid product.

[0044] S5: Wastewater recycling: The residue obtained from centrifugation is dried and crushed, and then used as biomass fuel; the wastewater after washing the residue and the filtrate after filtering humic acid precipitation are combined, potassium hydroxide is added to adjust the pH value to 7.0, and then evaporated and concentrated to 1 / 5 of the original volume at 80℃, cooled to 20℃, and crystallized for 12 hours to recover a mixture of potassium chloride and urea, which is used as agricultural fertilizer.

[0045] The humic acid prepared in this embodiment had a yield of 45%, a purity of 90%, a carboxyl content of 8.5 mmol / g, a hydroxyl content of 6.0 mmol / g, an amino content of 2.5 mmol / g, and an adsorption capacity of 120 mg / g for cadmium ions. The total reaction cycle was 10 hours, and the energy consumption was reduced by 30% compared to the traditional thermochemical hydrolysis method, with no secondary pollution emissions.

[0046] Example 3 A method for preparing humic acid from straw includes the following steps: S1: Straw pretreatment: Select rice straw, remove impurities, crush, and pass through a 60-mesh sieve to obtain straw powder; place the straw powder in an 80℃ constant temperature drying oven and dry for 2 hours until constant weight (moisture content ≤5%), then set aside for later use.

[0047] S2: Composite Modification Pretreatment: The pretreated straw powder and composite modifier are mixed at a mass ratio of 1:0.12, deionized water is added, the solid-liquid ratio is adjusted to 1:12, and after stirring evenly, the mixture is placed in an ultrasonic reactor and ultrasonically treated for 30 minutes at a power of 300W and a temperature of 50℃ to complete the composite modification and obtain modified straw slurry; the composite modifier is composed of potassium hydroxide, urea and nano silica at a mass ratio of 3:2:1, and the particle size of nano silica is 100nm.

[0048] S3: Segmented temperature-controlled synergistic reaction: The modified straw slurry was transferred to a high-pressure reactor, and nitrogen gas was introduced until the internal pressure reached 0.5 MPa. After sealing, a segmented temperature-controlled reaction was carried out: First stage, temperature 140℃, stirring speed 200 r / min, reaction time 1.5 h; Second stage, temperature 180℃, stirring speed 300 r / min, reaction time 2 h; Third stage, temperature 220℃, stirring speed 300 r / min, reaction time 1 h; After the reaction was completed, it was naturally cooled to room temperature, the internal pressure was released, and the reaction product was obtained.

[0049] S4: Separation and purification: Centrifuge the reaction product at 4000 r / min for 15 min to separate the supernatant and residue; wash the residue three times with deionized water, combine the washing liquid with the supernatant to obtain a crude humic acid solution; add 1 mol / L hydrochloric acid to the crude humic acid solution to adjust the pH to 2.5, stir evenly, let it stand for 12 h to precipitate, filter to obtain humic acid precipitate; wash the humic acid precipitate with deionized water until neutral, dry it in an 80℃ constant temperature drying oven for 3 h, pulverize it and pass it through a 100-mesh sieve to obtain the humic acid product.

[0050] S5: Wastewater recycling: The residue obtained from centrifugation is dried and crushed, and then used as raw material for organic fertilizer. The wastewater after washing the residue and the filtrate after filtering humic acid precipitation are combined, potassium hydroxide is added to adjust the pH value to 7.5, and then the mixture is evaporated and concentrated to 1 / 4 of the original volume at 90℃. After cooling to 25℃ and crystallizing for 8 hours, a mixture of potassium chloride and urea is recovered and used as agricultural fertilizer.

[0051] The humic acid prepared in this embodiment had a yield of 55%, a purity of 95%, a carboxyl content of 10.5 mmol / g, a hydroxyl content of 7.5 mmol / g, an amino content of 3.5 mmol / g, and an adsorption capacity of 150 mg / g for mercury ions. The total reaction cycle was 8 hours, and the energy consumption was reduced by 40% compared to the traditional thermochemical hydrolysis method, with no secondary pollution emissions.

[0052] Comparative Example To verify the technical effects of the present invention, the following comparative examples were set up. Each comparative example used the same corn stalk raw material and the same separation and purification steps as Example 1, only changing the modification method or reaction process.

[0053] Comparative Example 1 Corn stalks were fermented using a single microbial community (a mixture of Bacillus subtilis and Aspergillus niger in a 1:1 mass ratio) without any modification treatment. The fermentation temperature was controlled at 30℃ and the fermentation cycle was 20 days. Humic acid products were obtained through natural sedimentation and filtration, without any wastewater recycling steps.

[0054] Comparative Example 2 Corn stalks and deionized water were mixed at a solid-liquid ratio of 1:10 without any modification treatment. The mixture was then transferred to a high-pressure reactor and reacted at 220°C and 1.3 MPa for 3 hours. The resulting product was obtained by centrifugation, acid precipitation, filtration, and drying. There was no wastewater recovery step.

[0055] Comparative Example 3 (using a composite modifier but without segmented temperature control) The same composite modifier (potassium hydroxide: urea: nano silica = 3:2:1) and the same modification parameters as in Example 1 were used, but the subsequent reaction was carried out at a single temperature of 170°C and a stirring speed of 280 r / min for 5 h. The remaining steps were the same as in Example 1.

[0056] Comparative Example 4 (corresponding to the existing single-alkali modification two-stage temperature control method) Potassium hydroxide was used as the modifier, with a mass ratio of modifier to straw of 1:0.15. The modification temperature was 60℃ and the modification time was 2h. Subsequently, a two-stage temperature-controlled reaction was adopted (reaction at 150℃ for 2h and reaction at 190℃ for 2h). The remaining steps were the same as in Example 1.

[0057] The test data for Example 1 and Comparative Examples 1-4 are shown in Table 1.

[0058] Table 1

[0059] The technical effects of Example 1 were compared with those of the comparative examples, and the results show that: Compared with Comparative Example 1 (bio-fermentation method), the humic acid yield (52%) of Example 1 of the present invention increased by 136.4%, the purity (93%) increased by 29.2%, the carboxyl content (9.8 mmol / g) increased by 75.0%, the reaction cycle was shortened from 20 days to 9 hours, and there was no secondary pollution.

[0060] Compared with Comparative Example 2 (single high-temperature hydrothermal method), the humic acid yield (52%) of Example 1 of the present invention is increased by 62.5%, the purity (93%) is increased by 19.2%, the carboxyl content (9.8 mmol / g) is increased by 58.1%, the energy consumption is reduced by 35%, and there is no secondary pollution.

[0061] Compared with Comparative Example 3 (composite modifier + single-temperature reaction), Example 1 of this invention showed a 36.8% increase in humic acid yield (52%), a 12.0% increase in purity (93%), a 30.7% increase in carboxyl content (9.8 mmol / g), a further 20 percentage point reduction in energy consumption, and no secondary pollution. This indicates that the segmented temperature control process and the composite modifier of this invention have a synergistic effect, and both are indispensable.

[0062] Compared with Comparative Example 4 (single alkali modification + two-stage temperature control), Example 1 of this invention showed a 30.0% increase in humic acid yield (52%), a 9.4% increase in purity (93%), and a 25.6% increase in carboxyl content (9.8 mmol / g), while achieving no secondary pollution. This indicates that the composite modifier of this invention significantly improves quality by replacing the single modifier.

[0063] This invention achieves efficient degradation of straw and high-quality synthesis of humic acid through the organic combination of composite modification pretreatment and segmented temperature-controlled synergistic reaction. Its mechanism of action mainly manifests as the synergistic effect of the composite modifier, the gradient reaction pathway of segmented temperature control, and the resource utilization of the closed-loop recovery system.

[0064] The composite modifier used in this invention consists of potassium hydroxide, urea, and nano-silica in a mass ratio of 3:2:1, forming a cascaded synergistic system. Potassium hydroxide, as a strong alkaline catalyst, first disrupts the hydrogen bond network in the straw cell wall, breaking down the cellulose crystalline region structure and loosening the straw tissue structure. Simultaneously, potassium hydroxide promotes the hydrolytic breakage of glycosidic bonds in cellulose and hemicellulose, as well as the alkaline depolymerization of lignin aromatic ether bonds, generating a large number of active small molecule fragments containing hydroxyl, aldehyde, and carboxyl groups, providing precursor substances for subsequent humification reactions. Urea, with its small molecule characteristics, rapidly penetrates the alkaline-treated straw structure, specifically disrupting intramolecular and intermolecular hydrogen bonds in lignin through hydrogen bond competition and solvation effects, achieving deep depolymerization of lignin. Meanwhile, urea decomposes during the segmented temperature-controlled reaction, providing an alkaline environment to enhance the degradation effect of potassium hydroxide. It also acts as a nitrogen source in the synthesis of humic acid, introducing nitrogen-containing functional groups (amino, amide, etc.) through Mannich amination, increasing the active sites of humic acid, and enhancing its chelating ability for heavy metals and its growth-promoting properties for plants. Nano-silica (50-100nm) possesses a high specific surface area and abundant surface silanol groups, providing a multiphase catalytic interface in the reaction system. Its mechanism of action includes: adsorbing straw degradation products through silanol groups, reducing the reaction activation energy, efficiently catalyzing aldol condensation, phenolic condensation, and dehydration cross-linking reactions, and directionally generating humic acid precursors; regulating the degree of polymerization of humic acid molecules through the interface anchoring effect, avoiding excessive condensation that leads to excessively large molecular weight and reduced water solubility; and forming a Si-OC covalent hybrid structure with humic acid molecules during the high-temperature reaction stage, improving the thermal and chemical stability of humic acid and extending its environmental lifespan.

[0065] The three-stage segmented temperature-controlled process of this invention forms a synergistic effect of spatiotemporal coupling with the composite modifier. In the low-temperature activation stage (120-140℃, 1.5-2.5h), the system mainly undergoes controllable alkaline hydrolysis and degradation of hemicellulose. Potassium hydroxide provides a strongly alkaline environment to achieve deprotonation activation, urea melts and intercalates, and initiates a preliminary ammonolysis reaction. The silanol groups on the surface of nano-silica are activated and adsorb and anchor organic fragments. The three synergistically achieve the loosening of straw cell walls, the preliminary degradation of hemicellulose, and the simultaneous activation of the modifier system. This avoids excessive substrate degradation and significantly increases the number of subsequent reaction sites and mass transfer efficiency, providing a stable precursor for subsequent deep condensation, aromatization, and humic acid structure formation. In the mesophilic humification stage (160-180℃, 2-3h), when the temperature rises to 160-180℃, the system achieves efficient degradation of cellulose and lignin: cellulose glycosidic bonds break and degrade into small molecule sugars and carbonyl compounds, and lignin aromatic ether bonds depolymerize into phenolic and aromatic acid fragments. Under the synergistic effect of potassium hydroxide, urea, and nano-silica, the degradation products undergo aldol condensation, phenolic condensation, dehydration crosslinking, dehydrogenation aromatization, and Mannich amination reactions. Small molecule fragments gradually polymerize to form humic acid precursors containing aromatic cores, oxygen-containing active functional groups, and nitrogen-doped structures. Simultaneously, nano-silica regulates molecular weight distribution and stabilizes the structure through interfacial anchoring and catalysis, ultimately achieving the directional conversion of straw biomass into highly active humic acid precursors. During the high-temperature upgrading stage (200-220℃, 1-1.5h), the humic acid precursors undergo deep dehydration condensation, intramolecular cyclization, and conjugated structure expansion. Further crosslinking and polymerization occur through methylene bridges, ether bonds, and Si-OC covalent bonds, significantly increasing the molecular weight and aromaticity of humic acid. Simultaneously, it promotes phenol-quinone structural interconversion and stabilizes the enrichment of carboxyl and phenolic hydroxyl groups, enhancing product activity and chelating properties. During this process, unreacted small-molecule sugars, aldehydes, ketones, and volatile impurities are removed by heat, and unstable side chains undergo structural purification through decarboxylation, dehydration, and deammoniation. The synergistic effect of potassium hydroxide, urea, and nano-silica not only achieves structural maturation and quality improvement of humic acid but also avoids functional group loss and coking caused by excessive pyrolysis, ultimately yielding a modified humic acid product with moderate molecular weight, abundant functional groups, good water solubility, and high stability.

[0066] This invention constructs a closed-loop system of "preparation-recycling-reuse," achieving full resource utilization of residues and wastewater. The residue mainly consists of incompletely degraded cellulose, lignin, and nano-silica, which, after drying and pulverizing, can be used as raw material for biomass fuel or organic fertilizer, realizing the full utilization of straw. The wastewater mainly contains potassium chloride, urea, and a small amount of potassium hydroxide. Through evaporation, concentration, cooling, and crystallization, a mixture of potassium chloride and urea is recovered, which can be directly used as agricultural potassium and nitrogen fertilizers. This recycling process not only eliminates environmental pollution from wastewater discharge but also achieves the recycling of chemicals, aligning with the development concepts of green chemistry and a circular economy.

[0067] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing humic acid from straw, characterized in that, Includes the following steps: S1. Straw pretreatment: Select straw raw materials, remove impurities, crush, sieve, and dry to constant weight for later use; S2. Composite Modification Pretreatment: Mix the pretreated straw powder with the composite modifier, add deionized water, adjust the solid-liquid ratio, and ultrasonically treat to obtain modified straw slurry. The composite modifier is composed of potassium hydroxide, urea, and nano-silica. S3. Segmented temperature-controlled synergistic reaction: The modified straw slurry prepared in step S2 is transferred to a high-pressure reactor, nitrogen is introduced to the set pressure, and after sealing, a segmented temperature-controlled reaction is carried out. After the reaction is completed, it is cooled to room temperature, the pressure is released, and the reaction product is obtained. The segmented temperature-controlled reaction includes a low-temperature activation stage, a medium-temperature humification stage, and a high-temperature quality improvement stage. S4. Separation and purification: Centrifuge the reaction product to separate the supernatant and residue; wash the residue with deionized water, and combine the washing liquid with the supernatant to obtain a crude humic acid solution; adjust the pH of the crude humic acid solution to acidic, allow it to stand to precipitate, filter, wash, dry, and pulverize to obtain the humic acid product.

2. The method for preparing humic acid from straw according to claim 1, characterized in that, It also includes S5, wastewater recycling: recycling residues as raw materials for biomass fuel or organic fertilizer; merging wastewater, adjusting the pH value to neutral, evaporating and concentrating, cooling and crystallizing to recover a mixture of potassium chloride and urea for use as agricultural fertilizer.

3. The method for preparing humic acid from straw according to claim 1, characterized in that, In S1, the straw raw material is one or more of corn straw, wheat straw, and rice straw; the sieving is passing through a 40-60 mesh sieve.

4. The method for preparing humic acid from straw according to claim 1, characterized in that, In S2, the composite modifier is composed of potassium hydroxide, urea, and nano-silica in a mass ratio of (2-4):(1-3):(0.5-1.5), and the nano-silica has a particle size of 50-100nm.

5. The method for preparing humic acid from straw according to claim 4, characterized in that, In S2, the composite modifier is composed of potassium hydroxide, urea, and nano-silica in a mass ratio of 3:2:1, and the nano-silica has a particle size of 80nm.

6. The method for preparing humic acid from straw according to claim 1, characterized in that, In S2, the mass ratio of straw powder to composite modifier is 1:0.08-0.12, the solid-liquid ratio is 1:8-12, and the ultrasonic treatment time is 30-60 min; in S2, the ultrasonic power is 200-300 W, and the ultrasonic temperature is 40-50℃.

7. The method for preparing humic acid from straw according to claim 1, characterized in that, In step S3, nitrogen gas is introduced until the pressure inside the reactor is 0.3-0.5 MPa; the reaction temperature of the low-temperature activation stage is 120-140℃, the stirring speed is 150-200 r / min, and the reaction time is 1.5-2.5 h; the reaction temperature of the medium-temperature humification stage is 160-180℃, the stirring speed is 250-300 r / min, and the reaction time is 2-3 h; the reaction temperature of the high-temperature upgrading stage is 200-220℃, the stirring speed is 250-300 r / min, and the reaction time is 1-1.5 h.

8. The method for preparing humic acid from straw according to claim 1, characterized in that, In S4, the centrifugation speed is 3000-4000 r / min, and the centrifugation time is 15-20 min; Adjust the pH of the crude humic acid solution to 2.0-2.5 using 1 mol / L hydrochloric acid; allow it to stand for 12-24 hours; dry at 70-80℃; after drying to constant weight, pulverize and pass through an 80-100 mesh sieve.

9. A humic acid, characterized in that, Prepared by the method according to any one of claims 1 to 8.

10. The humic acid according to claim 9, characterized in that, The humic acid has a yield of 45%-55%, a purity of 90%-95%, a carboxyl content of 8.5-10.5 mmol / g, a hydroxyl content of 6.0-7.5 mmol / g, and an amino content of 2.5-3.5 mmol / g.

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