A method for preparing a citric acid-modified rice hull biomass charcoal

Abstract

This invention discloses a method for preparing citric acid-modified rice husk biochar, belonging to the field of agricultural waste resource utilization and acid soil improvement technology. The method includes washing, drying, crushing, and sieving rice husks; performing oxygen-limited pyrolysis at 450–600℃ under an inert atmosphere to obtain a rice husk biochar precursor; then impregnating it with a 0.2 mol / L to 1.5 mol / L citric acid solution under ultrasonic assistance; followed by hydrothermal reaction at 120–200℃; washing until neutral; and drying to obtain modified biochar. The citric acid-modified rice husk biochar is applied to acidic red soil at 1%–5% of the soil mass, simultaneously inoculated with acid-resistant functional microbial agents. The resulting material has carboxyl functional groups anchored on its surface, enabling it to complex and adsorb Al. 3+ Buffer H + It also provides a carbon source carrier for acid-resistant functional microorganisms, which is suitable for acidity improvement of acid red soil, aluminum toxicity mitigation and microbial agent carrier application.

Description

Technical Field

[0001] This invention belongs to the field of agricultural waste resource utilization and acid soil improvement technology, specifically relating to a method for preparing citric acid-modified rice husk biochar. Background Technology

[0002] The red soil region in southern my country covers approximately 2.18 million square kilometers, accounting for 22.7% of the country's land area, and is an important agricultural production base. However, due to the hot and rainy climate and long-term unreasonable human activities, the problem of red soil acidification is becoming increasingly serious. A national soil pollution survey shows that over 60% of the arable land in the southern red soil region has a pH below 5.5, classifying it as strongly acidic soil. The core hazard of red soil acidification lies in the fact that under low pH conditions, large amounts of solid aluminum dissolve from the soil, primarily Al. 3+ It exists in soil solution and has a strong aluminum toxicity effect on plant roots—inhibiting root tip cell division and elongation, damaging cell membrane structure, and interfering with nutrient (Ca, Mg, P) absorption, ultimately leading to reduced crop yield or even crop failure. Simultaneously, an acidic environment (high concentration of H₂) further exacerbates the problem. + ) and Al 3+ Synergistic stress severely inhibits the growth and activity of beneficial soil microorganisms (such as nitrogen-fixing bacteria, phosphate-solubilizing bacteria, and Bacillus), disrupting the soil microecological balance.

[0003] Traditional methods for improving acidic soils mainly rely on the application of lime (CaO / CaCO3), but lime-based methods have the following shortcomings: (1) The improvement depth is limited, only affecting the top 0-10cm layer, and is ineffective against deep soil acidification; (2) Long-term application leads to soil compaction and damages soil structure; (3) It cannot alleviate aluminum toxicity—although lime can increase pH and reduce some aluminum toxicity, it can still cause some aluminum toxicity. 3+ Precipitation occurs, but contributes very little to the formation of organic complexed aluminum; (4) it cannot provide a carbon source and colonization carrier for soil microorganisms. In recent years, biochar has been widely studied for the improvement of acidic soils due to its alkaline nature, rich pore structure and organic carbon.

[0004] Rice husks are China's largest by-product of rice processing, with an annual output of approximately 40 million tons. Rich in organic carbon (approximately 38%–42% cellulose and hemicellulose, and approximately 20%–25% lignin) and silicon dioxide (approximately 15%–20%), rice husks are an excellent raw material for biochar production. Applying biochar prepared from the pyrolysis of rice husks to acidic red soils can neutralize soil acidity and increase soil pH through its alkaline components, and improve soil physical properties through its well-developed porous structure.

[0005] However, unmodified rice husk biochar still faces the following key bottlenecks in the improvement of acidic red soil: (1) For Al 3+Insufficient adsorption capacity: The surface acidic oxygen-containing functional groups (especially carboxyl-COOH groups) of raw rice husk biochar are low (usually <0.2 mmol / g), which limits its adsorption capacity for Al. 3+ It mainly relies on non-specific physical adsorption and alkali-induced precipitation, with limited adsorption capacity, and the formed Al(OH)3 precipitate may redissolve under acidic conditions.

[0006] (2) Low carbon source availability: Biochar prepared by high-temperature pyrolysis is mainly composed of inert aromatic carbon, with low content of easily decomposable organic carbon that can be directly utilized by microorganisms. When used alone, it has a limited effect on promoting the growth of soil microorganisms.

[0007] (3) Single function: Unmodified biochar mainly acts as an "inert modifier" and lacks the ability to modify Al. 3+ Its specific complexing ability and carbon source supply function to microorganisms make it difficult to achieve the synergistic effect of "acid reduction and detoxification - carbon supply and bacterial promotion".

[0008] Citric acid (C6H8O7) is a natural tricarboxylic acid organic acid widely found in citrus fruits, possessing excellent Al content. 3+ It exhibits strong complexing ability (stability constant logK≈8.0) and good biocompatibility. Citric acid is a core substance in the aluminum tolerance mechanism secreted by plant roots—under aluminum stress, plants actively secrete citric acid into the rhizosphere, where it complexes with Al. 3+ Aluminum toxicity can be deactivated by forming a stable Al-citric acid complex. However, free citric acid degrades too quickly in soil (its half-life is usually <7 days), making it difficult to maintain its effectiveness.

[0009] The study [Effects of Biochar Combined with Acid-Tolerant Bacillus subtilis on Red Soil Acidity and Microbial Diversity](https: / / d.wanfangdata.com.cn / periodical / tr202601021) showed that, in the short term (6 days), the combined use of biochar as a microbial carrier and acid-tolerant bacteria can increase soil pH by 0.36 units, decrease exchangeable acidity by 24.6%, and decrease exchangeable aluminum by 9.6%; in the long term (60 days), it can further reduce exchangeable aluminum by 19.8% and increase bacterial community richness by 11.62%. However, this study used unmodified raw biochar, and its functional potential has not yet been fully explored.

[0010] [Effects of Biochar on Different Aluminum Forms in Red Soil and the Growth of Chinese Cabbage](https: / / opaj.napstic.cn / periodicalArticle / 0120171001843820) Studies have shown that applying 3% biochar can increase the pH of red soil by 0.79 units and reduce exchangeable Al content. 3+It reduces by 75.5%, but this effect mainly relies on the precipitation mechanism of the alkaline components of biochar and lacks specific complexation of aluminum.

[0011] [Assessing the impact of biochar microbes in acidic oils: alleviating the toxicity of aluminum and acidity](https: / / www.sciencedirect.com / science / article / abs / pii / S0301479723015840)(Xia et al., 2023, J. Environ. Manage.) found that the Al on the surface of biochar... 3+ Mainly Al(OH) 2+ Al(OH)2 + And Al 3+ Morphological enrichment, 27 AlNMR revealed the formation of Al-organic functional group complexes on the surface of biochar, suggesting that increasing the number of oxygen-containing functional groups on the biochar surface is crucial for enhancing the aluminum complexation and fixation capacity.

[0012] Stabilizing Active Aluminum (Al) 3+ The study *Toxics* (2026) in *Acidic Soils via Biochar-Induced Microbial Niches* (https: / / doi.org / 10.3390 / toxics14020157) confirmed that rice husk biochar creates a "neutral microdomain" in acidic soils through its alkalinity and porous structure, enriching beneficial microbial communities such as denitrifying bacteria. Within 180 days, Al... 3+ The stability efficiency reached 22.1%. This provides direct evidence for the combined improvement strategy of "biochar carrier + functional microorganisms".

[0013] [The effect of rice husk biochar on the improvement of acidity in red and yellow-brown soils](http: / / qikan.cqvip.com / Qikan / Article / Detail?id=35515145) (Yuan Jinhua et al., 2010) confirmed that rice husk biochar can significantly reduce exchangeable acid and exchangeable aluminum in red soil, but has little effect on active Al. 3+ The selective complexation fixation effect is limited.

[0014] In summary, current technologies do not yet possess the ability to chemically bond citric acid to the surface of rice husk biochar, thereby enabling it to simultaneously possess "Al" properties. 3+ Specific complex adsorption + H +Reports have documented a quadruple function of "neutralizing buffer + microbial available carbon source + inoculant carrier". Therefore, developing a modified biochar material with simple process, controllable cost and multiple functions is of great scientific significance and application value for the ecological restoration and sustainable agricultural use of acidic red soil in southern China. Summary of the Invention

[0015] I. Technical problems to be solved The purpose of this invention is to overcome the above-mentioned shortcomings of the prior art and provide a method for preparing citric acid-modified rice husk biochar. This method prepares rice husk as a biochar precursor, and then modifies it through ultrasonic impregnation and hydrothermal combined modification with citric acid. This anchors a large number of carboxyl (-COOH) functional groups on the surface of the biochar via chemical bonding, giving the resulting modified biochar the following four functions simultaneously: (a) Carbon source function: Provides available organic carbon source and colonization carrier for soil microorganisms (especially exogenously added acid-resistant functional agents); (b)Al 3+ Specific complex adsorption: adsorption of Al by surface carboxyl groups 3+ High affinity complexation to active Al 3+ It is converted into a stable Al-organic complex state, thus eliminating aluminum toxicity; (c)H + Neutralization and buffering: Through the inherent alkalinity of biochar and the proton consumption effect of carboxyl deprotonation, soil acidity is neutralized and pH is increased; (d) Microbial microenvironment construction: Neutral microenvironment is constructed in acidic soil using the porous structure of biochar to protect functional microbial agents from acid and aluminum stress.

[0016] This invention also provides a method for applying the above-mentioned modified biochar in the improvement of severely acidic red soil, especially a synergistic improvement scheme using it in combination with acid-resistant functional bacteria agents.

[0017] II. Technical Solution To achieve the above objectives, the technical solution of the present invention is as follows: A method for preparing citric acid-modified rice husk biochar includes the following steps: (1) Raw material pretreatment: Wash the rice husk raw material with deionized water 2 to 3 times to remove surface dust and impurities, dry it at 60 to 80℃ to constant weight (moisture content <5%), crush it and pass it through a 40 to 100 mesh sieve to obtain rice husk powder; (2) Oxygen-limited pyrolysis carbonization: The rice husk powder obtained in step (1) is placed in a tube furnace or muffle furnace, heated to the pyrolysis temperature under an inert atmosphere, carbonized at the temperature, and then naturally cooled to room temperature to obtain rice husk biochar precursor; the inert atmosphere is nitrogen (N2) or argon (Ar), the gas flow rate is 100-300 mL / min; the pyrolysis temperature is 450-600℃, the heating rate is 5-15℃ / min, and the carbonization time is 1-3 h; (3) Citric acid impregnation modification: The rice husk biochar precursor obtained in step (2) is mixed with citric acid aqueous solution at a solid-liquid ratio of 1:(20-60) g / mL, and impregnated under ultrasonic assistance to obtain an impregnated mixture; the concentration of the citric acid aqueous solution is 0.2-1.5 mol / L; the ultrasonic assistance conditions are: ultrasonic power 100-300W, ultrasonic frequency 20-40kHz, ultrasonic time 1-3h, and temperature controlled at 25-60℃; (4) Hydrothermal reaction: The impregnation mixture obtained in step (3) is transferred to a hydrothermal reactor with a polytetrafluoroethylene liner and subjected to a hydrothermal reaction at 120-200°C for 4-12 hours. After the reaction is completed, it is naturally cooled to room temperature. (5) Washing and drying: The solid product after the hydrothermal reaction in step (4) is repeatedly washed with deionized water until the pH of the washing solution is 6.5-7.5 (neutral), and then dried at 80-105℃ to constant weight to obtain the citric acid modified rice husk biochar.

[0018] Preferably, the pyrolysis temperature in step (2) is 500–550℃, the heating rate is 8–12℃ / min, and the holding time for carbonization is 1.5–2.5h. The basis for selecting this temperature range is that: if the temperature is too low (<450℃), the pyrolysis is insufficient, the specific surface area and pore structure of the biochar are not developed enough, which is not conducive to the subsequent impregnation and loading of citric acid and the colonization of microorganisms; if the temperature is too high (>600℃), a large amount of the original oxygen-containing functional groups on the surface of the biochar will be lost due to pyrolysis, and the carbon structure will be excessively aromatic and the surface hydrophobicity will be enhanced, which is not conducive to the penetration and chemical anchoring of citric acid molecules.

[0019] Preferably, the concentration of the citric acid aqueous solution in step (3) is 0.5 to 1.0 mol / L, and the solid-liquid ratio is 1:(30 to 50) g / mL.

[0020] Preferably, the ultrasonic auxiliary conditions in step (3) are: ultrasonic power 150-250W, ultrasonic frequency 28-40kHz, ultrasonic time 1.5-2.5h, and temperature controlled at 30-50℃.

[0021] Preferably, the hydrothermal reaction temperature in step (4) is 150-180°C and the hydrothermal reaction time is 6-10 hours.

[0022] Preferably, the rice husk powder in step (1) has a particle size that passes through a 60-80 mesh sieve.

[0023] The present invention also provides citric acid-modified rice husk biochar prepared by any of the above preparation methods.

[0024] This invention also provides the application of the above-mentioned citric acid-modified rice husk biochar in the improvement of acidic red soil. The preferred application is to apply the modified biochar to the acidic red soil at a concentration of 1% to 5% of the soil mass. More preferably, the modified biochar is used as a carbon source carrier and colonization substrate for acid-resistant functional microbial agents, and is applied in combination with the agents to the acidic red soil.

[0025] Preferably, the acidic red soil has a pH below 5.5 and an exchangeable aluminum ion content greater than 2.0 cmol / kg; more preferably, the acidic red soil is a strongly acidic red soil from the south.

[0026] This invention also provides a combined method for improving acidic red soil, comprising: applying citric acid-modified rice husk biochar at 1%–5% of soil mass into the acidic red soil, simultaneously inoculating with an acid-resistant functional microbial agent, thoroughly mixing, and then curing for 30–90 days under field water holding capacity of 60%–80%; wherein the acid-resistant functional microbial agent includes at least one of acid-resistant Bacillus subtilis, acid-resistant phosphate-solubilizing bacteria, and acid-resistant nitrogen-fixing bacteria, and the preferred inoculation amount of the acid-resistant functional microbial agent is 1×10⁻⁶. 6 ~1×10 8 CFU / g soil.

[0027] III. Beneficial Effects Compared with the prior art, the core beneficial effects of the present invention are reflected in the following aspects: (1)Al 3+ Strong specific complexation adsorption capacity: Citric acid molecules are anchored to the surface of rice husk biochar via hydrothermal reaction through ester and hydrogen bonds, introducing a large number of carboxyl (-COOH) functional groups. The carboxyl groups have a strong affinity for Al... 3+ It has extremely high complexing affinity (Al) 3+ - The stability constant of the citric acid complex is approximately logK ≈ 8.0, which allows it to specifically complex active Al in soil solution under low pH conditions (pH 3.5–5.5). 3+ This process transforms aluminum into a stable Al-organic carboxyl complex, fundamentally eliminating the toxicity of aluminum to plant roots and microorganisms. Experimental verification shows that the modified biochar prepared in this invention effectively neutralizes Al in solution. 3+ Its adsorption capacity can reach 35-65 mg / g, which is 5-10 times higher than that of unmodified biochar precursors; after being applied to severely acidic red soil with a pH of 4.2, the soil's exchangeable Al content... 3+The content is reduced by 65% ​​to 85%, and the effect is significantly better than the same amount of lime and unmodified biochar.

[0028] (2) Continuous neutralization of soil acidity: Modified biochar plays a "dual acid-reducing" role - firstly, the inherent alkaline components of biochar (basic ions such as K, Ca, and Mg) neutralize H+. + Firstly, it rapidly increases soil pH; secondly, the surface carboxyl groups (-COOH) deprotonate under weakly acidic to neutral conditions, releasing OH groups. - (-COOH+OH) - →-COO - (+H2O) forms a continuous proton consumption buffer system, avoiding the "sharp rise and fall" effect of lime improvement. Soil incubation experiments show that when 3% modified biochar is applied to red soil with pH 4.2, the soil pH steadily increases by 1.2 to 1.8 units after 60 days, reaching the suitable range of 5.4 to 6.0.

[0029] (3) Providing carbon source and colonization carrier for functional microbial agents: The oxygen-containing functional groups (carboxyl, hydroxyl, lactone, etc.) introduced on the surface of biochar by citric acid modification, as well as some soluble organic carbon components generated during hydrothermal processes, can serve as readily available carbon sources for microorganisms, promoting the rapid colonization and proliferation of exogenous functional microbial agents in the soil. Simultaneously, the porous structure of biochar (specific surface area 80–200 m²) 2 / g) provides a physically protected microdomain for the functional bacteria, shielding them from the high concentrations of H in acidic soil. + And Al 3+ The original stress. Experiments using modified biochar carrier in combination with acid-resistant Bacillus subtilis showed that the 60-day survival rate of the inoculum in acidic red soil was increased by 80% to 150%.

[0030] (4) Constructing a virtuous cycle of "acid reduction and detoxification - carbon supply and bacterial promotion": modified biochar adsorbs Al 3+ He Zhonghe H + Subsequently, the acidity of the soil microenvironment and aluminum toxicity stress were significantly reduced, creating suitable conditions for the growth and metabolism of functional bacteria. The metabolic activities of functional bacteria (alkali production, extracellular polysaccharide secretion, organic acid decomposition, etc.) further stabilized soil pH, promoted organic carbon turnover and nutrient release, forming a positive feedback ecological restoration cycle. This effect cannot be achieved by traditional lime amelioration or single biochar amelioration.

[0031] (5) Green and environmentally friendly and low cost: It uses natural organic acid (citric acid) as the only modifying agent, which can be completely biodegraded and has no risk of secondary pollution; it uses agricultural waste rice husks as raw materials, which are widely available and inexpensive (about 200-500 yuan / ton), and the process is simple to operate, without the need for strong acids and alkalis, making it suitable for large-scale production. Attached Figure Description

[0032] Figure 1 This is a process flow diagram for preparing citric acid-modified rice husk biochar according to the present invention.

[0033] Figure 2 The Fourier transform infrared (FTIR) spectrum comparison of the citric acid modified rice husk biochar (CA-BC) prepared in Example 1 and the unmodified rice husk biochar (BC) in Comparative Example 1 is shown.

[0034] Figure 3 The effect of rice husk biochar on Al in solution before and after citric acid modification under different initial pH conditions. 3+ Adsorption capacity comparison chart (initial Al) 3+ Concentration 100 mg / L, adsorbent dosage 1 g / L, 25℃, 12 h).

[0035] Figure 4 After CA-BC prepared in Example 1 and BC prepared in Comparative Example 1 were applied at a 3% application rate to acidic red soil (initial pH 4.2), the soil pH and exchangeable Al2O3 were compared. 3+ Comparison chart of content changes over culture time.

[0036] Figure 5 Comparative graph showing the effects of CA-BC and BC as microbial carriers on the survival rate of acid-resistant Bacillus subtilis in acidic red soil (60 days of culture, CFU counting method).

[0037] Figure 6 Biochar prepared under different citric acid concentrations (0.2, 0.5, 1.0, 1.5 mol / L) for Al modification 3+ A comparison of adsorption capacity and surface carboxyl group content along two Y axes. Detailed Implementation

[0038] The technical solution of the present invention will be further described in detail below through specific embodiments, but the scope of protection of the present invention is not limited to the following embodiments. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods; the raw materials and reagents used can be purchased from commercial channels unless otherwise specified.

[0039] Main raw materials and reagent sources: Rice husks: Taken from a rice processing plant in Jiangxi Province, with a moisture content of approximately 9.2%, a total carbon content of approximately 38.5%, and an ash content of approximately 17.2% (of which SiO2 content is approximately 93.8%). Citric acid (C6H8O7·H2O): analytical grade, Sinopharm Chemical Reagent Co., Ltd. Nitrogen (N2): Purity ≥ 99.99%; Deionized water: prepared in the laboratory, resistivity ≥18.2 MΩ·cm; The tested acidic red soil was collected from the Red Soil Ecological Experimental Station of the Chinese Academy of Sciences in Yingtan City, Jiangxi Province. It is a typical red soil developed from Quaternary red clay. Its basic physicochemical properties are shown in Table 1.

[0040] Table 1: Basic Physicochemical Properties of the Tested Acidic Red Soils

[0041] Example 1

[0042] This embodiment provides a method for preparing citric acid-modified rice husk biochar, including the following steps: (1) Raw material pretreatment: 500g of rice husk raw material was washed three times with deionized water to remove surface dust and impurities, and then placed in an electric heating drying oven at 80℃ for 12 hours to constant weight (moisture content 3.5%). The dried rice husk was then pulverized using a high-speed pulverizer and passed through a 60-mesh standard sieve to obtain approximately 415g of rice husk powder.

[0043] (2) Oxygen-limited pyrolysis carbonization: 200g of rice husk powder obtained in step (1) was placed in a corundum boat and then placed in a tube furnace. High-purity N2 (purity ≥ 99.99%) was introduced as a protective atmosphere, with a gas flow rate controlled at 200mL / min. The temperature was increased from room temperature to 550℃ at a rate of 10℃ / min, and carbonized at 550℃ for 2 hours. After carbonization, the mixture was naturally cooled to room temperature under the protection of the N2 atmosphere. The solid product was then removed, yielding approximately 62g of rice husk biochar precursor (denoted as BC), with a yield of approximately 31.0%. The BET specific surface area of ​​BC was 95.6m². 2 / g, with a surface carboxyl content of 0.15mmol / g.

[0044] (3) Citric acid impregnation modification: Weigh 20g of the rice husk biochar precursor (BC) obtained in step (2) and mix it with 800mL of 0.8mol / L citric acid aqueous solution (solid-liquid ratio 1:40g / mL) in a beaker. Place the beaker in an ultrasonic cleaner (ultrasonic power 200W, ultrasonic frequency 40kHz) and ultrasonically impregnate it at 40℃ for 2h, stirring with a glass rod every 30min during the ultrasonic process. Obtain the impregnated mixture.

[0045] (4) Hydrothermal reaction: The impregnation mixture obtained in step (3) was transferred to a 1.5L hydrothermal reactor with a polytetrafluoroethylene liner. After sealing, the reactor was placed in an oven and subjected to a hydrothermal reaction at 160°C for 8 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature.

[0046] (5) Washing and drying: The solid-liquid mixture after the hydrothermal reaction in step (4) was vacuum filtered to obtain a solid product. The solid product was repeatedly washed with deionized water, and the pH value of the washing solution was measured with a pH meter after each wash until the pH value of the washing solution reached 7.0±0.3. The washed solid product was placed in an electric heating drying oven and dried at 105℃ for 12 hours to constant weight to obtain citric acid modified rice husk biochar, denoted as CA-BC (yield approximately 22.8g).

[0047] Example 2

[0048] The only difference between this embodiment and Embodiment 1 is that the pyrolysis temperature in step (2) is 450°C, while the other steps and conditions are exactly the same as in Embodiment 1. The resulting precursor is denoted as BC-450, and the final product is denoted as CA-BC-450.

[0049] Example 3

[0050] The only difference between this embodiment and Embodiment 1 is that the pyrolysis temperature in step (2) is 600℃, while the other steps and conditions are exactly the same as in Embodiment 1. The resulting precursor is denoted as BC-600, and the final product is denoted as CA-BC-600.

[0051] Example 4

[0052] The only difference between this embodiment and Example 1 is that the concentration of the citric acid aqueous solution in step (3) is 0.5 mol / L, and the solid-liquid ratio is 1:30 g / mL. All other steps and conditions are exactly the same as in Example 1. The final product is denoted as CA-BC-0.5.

[0053] Example 5

[0054] The only difference between this embodiment and Example 1 is that the concentration of the citric acid aqueous solution in step (3) is 1.0 mol / L, and the solid-liquid ratio is 1:50 g / mL. All other steps and conditions are exactly the same as in Example 1. The final product is denoted as CA-BC-1.0.

[0055] Example 6

[0056] The only difference between this embodiment and Embodiment 1 is that the hydrothermal reaction temperature in step (4) is 180°C and the hydrothermal reaction time is 6 hours. The other steps and conditions are exactly the same as in Embodiment 1.

[0057] Example 7

[0058] The only difference between this embodiment and Embodiment 1 is that the rice husk powder is passed through a 100-mesh standard sieve in step (1); and the drying temperature is 90°C in step (5). The other steps and conditions are basically the same as in Embodiment 1.

[0059] Comparative Example 1 Unmodified rice husk biochar – prepared only according to steps (1) and (2) of Example 1, without any treatment in steps (3) to (5), is denoted as BC.

[0060] Comparative Example 2 Ultrasonic impregnation only, without hydrothermal treatment—the only difference from Example 1 is that the hydrothermal reaction in step (4) is omitted, that is, after ultrasonic impregnation in step (3), the washing and drying in step (5) are carried out directly. The final product is denoted as CA-BC-NH (NoHydrothermal).

[0061] Comparative Example 3 Hydrothermal treatment only, no ultrasound -- The only difference from Example 1 is that step (3) is not ultrasound-assisted; the biochar precursor is simply immersed in citric acid solution at room temperature with stirring for 2 hours (magnetic stirring, 150 rpm). The final product is designated CA-BC-NS (NoSonication).

[0062] Comparative Example 4 Traditional lime amendment – ​​using analytical grade CaO as an amendment for acidic red soil, with an application rate equivalent to the molar calcium content (approximately 3.6 g / kg soil based on CaO) of biochar treatment.

[0063] Comparative Example 5 Free citric acid – An aqueous solution of free citric acid, equivalent to the citric acid loading in Example 1 (measured to be approximately 8.5% of the total mass of citric acid anchored on the CA-BC surface), was directly applied to the acidic red soil.

[0064] Performance testing and characterization I. Physicochemical Properties Characterization (1) BET specific surface area and pore structure analysis (N2 adsorption-desorption method) Table 2: BET specific surface area and pore structure parameters of different samples

[0065] Analysis: The specific surface area of ​​CA-BC in Example 1 is 118.3 m². 2 / g) compared to BC (95.6m) in Comparative Example 1 2 The specific surface area (S / g) increased by approximately 23.7%. The mild acid etching and partial decomposition and volatilization of citric acid under hydrothermal conditions help to unclog blocked micropores and increase the specific surface area, which is beneficial for providing more colonization surface area for microorganisms. The specific surface area increases in Comparative Example 2 (without hydrothermal treatment) and Comparative Example 3 (without ultrasound) were significantly smaller, indicating that both ultrasound-assisted penetration of citric acid molecules into the pores and hydrothermal reaction promoting the chemical reaction and pore formation of citric acid on the carbon surface are indispensable steps.

[0066] (2) Qualitative analysis of surface functional groups - Fourier transform infrared spectroscopy (FTIR) The FTIR (Thermo-Scientific Nicoletti S50, KBr) tableting method was used, 4000~4000cm -1 4cm resolution -1 CA-BC and BC were characterized by 32 scans.

[0067] Table 3: Comparison of characteristic peaks in the Fourier transform infrared spectra of CA-BC and BC

[0068] Analysis: CA-BC at 1712cm -1 A strong C=O stretching vibration peak appears at 1585 cm⁻¹. -1 1380cm -1 COO appears at [location] - The characteristic peaks directly prove that citric acid molecules were successfully anchored to the biochar surface through chemical bonding (esterification reaction: -OH + citric acid -COOH → -OC(=O)- + H2O), introducing a large number of carboxyl groups and carboxylate functional groups. These functional groups are complexes with Al. 3+ He Zhonghe H + The core active site.

[0069] (3) Quantitative analysis of oxygen-containing functional groups on the surface - Boehm titration method Table 4: Surface oxygen functional group content of different samples

[0070] Analysis: The CA-BC carboxyl content in Example 1 reached 0.78 mmol / g, which was 420% higher than that of BC (0.15 mmol / g); the total acidic functional group content increased from 0.46 to 1.32 mmol / g, an increase of 187%. The carboxyl content of Example 1 was 2.44 times that of Comparative Example 2 (without hydrothermal treatment, 0.32 mmol / g) and 1.90 times that of Comparative Example 3 (without ultrasound, 0.41 mmol / g), indicating that both ultrasound-assisted impregnation and hydrothermal reaction are indispensable for promoting the chemical anchoring of citric acid on the biochar surface—ultrasound promotes the diffusion and pre-adsorption of citric acid molecules into the depth of the pores, while hydrothermal treatment provides the activation energy required for the esterification reaction.

[0071] (4) Zeta potential and isoelectric point analysis Table 5: Zeta potential and isoelectric point of CA-BC and BC under different pH conditions

[0072] Analysis: The isoelectric point of CA-BC drops significantly from 3.6 of BC to 2.5, and its surface zeta potential is significantly more negative than that of BC at all pH values. This means that within the typical pH range of acidic red soil (3.5–5.5), the CA-BC surface carries a strong negative charge and can strongly adsorb positively charged Al atoms through electrostatic attraction. 3+ and Al(OH) 2+ Al(OH)2 + The surface contains aluminum hydroxyl cations, while simultaneously repelling negatively charged anions such as nitrate. The significant increase in surface negative charge is directly attributed to the deprotonation of the carboxyl group introduced by citric acid under neutral / weakly acidic conditions (-COOH → -COO). - +H + ).

[0073] 2. Al 3+ Adsorption performance test (1) Al 3+ Adsorption isotherm AlCl3 solutions with different initial concentrations (10, 20, 50, 80, 100, 150, 200, 300, 400, 500 mg / L) were prepared (pH adjusted to 4.0 ± 0.1 with dilute HCl / NaOH). 50 mL of each solution was placed in a 100 mL Erlenmeyer flask, and 50 mg of the sample (dosage 1 g / L) was added. The solutions were shaken at 150 rpm for 12 h at 25℃ to reach adsorption equilibrium. After filtration through a 0.22 μm microporous membrane, the Al concentration in the filtrate was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). 3+ Concentration. Fitted using the Langmuir model.

[0074] Langmuir isothermal adsorption fitting results: Table 6: Effects of different samples on Al 3+ Langmuir isotherm adsorption fitting results

[0075] Analysis: CA-BC in Example 1 versus Al 3+ The saturated adsorption capacity reached 52.3 mg / g, which was approximately 669% (7.7 times) higher than that of unmodified BC (6.8 mg / g). Example 5 (1.0 mol / L citric acid) yielded the highest adsorption capacity (56.5 mg / g). Langmuir model R 2 All values ​​> 0.98, indicating that Al 3+ On the surface of modified biochar, monolayer chemisorption is dominant, i.e., Al 3+ Stable complex bonds are formed between the adsorption capacity and the surface carboxyl groups. The adsorption capacities of Comparative Examples 2 and 3 were only about 43% and 53% of those of Example 1, respectively, further confirming the importance of ultrasound and hydrothermal methods for carboxyl group anchoring.

[0076] (2) Effects of different initial pH values ​​on Al 3+ Effect of adsorption In the initial Al 3+ Under conditions of 100 mg / L concentration and 1 g / L adsorbent dosage, the effect of initial solution pH (3.0, 3.5, 4.0, 4.5, 5.0, 5.5) on the adsorption capacity of CA-BC and BC was investigated (Al > 5.5). 3+ It begins to hydrolyze significantly to form Al(OH)3 colloidal precipitate, which interferes with the calculation of adsorption capacity, so it was not included.

[0077] Table 7: Effects of CA-BC and BC on Al under different initial pH conditions 3+ adsorption capacity

[0078] Analysis: CA-BC showed significantly better Al content than BC within the characteristic pH range (3.5–5.5) of acidic red soil. 3+ Adsorption capacity. Specifically, under strongly acidic conditions (pH 3.0–3.5), BC adsorption is almost ineffective (2.5–4.2 mg / g), while CA-BC still maintains an adsorption capacity of 18.6–32.5 mg / g. This is because the surface carboxyl groups attract Al... 3+ The complexation effect is relatively small due to pH – even at pH=3.0, some carboxyl groups (pKa≈3.0~4.5) still exist as -COO groups. - It exists in form and can be associated with Al 3+ Complexation occurs. This property is crucial for improving severely acidified red soils (pH often as low as 3.5–4.5).

[0079] (3) Al 3+ Adsorption Mechanism Discussion Based on FTIR, Zeta potential, and adsorption experimental data, CA-BC for Al 3+ The adsorption mechanisms include: Surface carboxyl complexation (dominant mechanism, contributing approximately 60%–70%): Surface -COOH is deprotonated to -COO. - , with Al 3+ Formation of stable bidentate or monodentate complexes (R-COO-Al) 2+ and (R-COO)2-Al + ). COO in FTIR - The appearance of the characteristic peak and its shift after adsorption directly support this mechanism. Electrostatic attraction (contributing approximately 15%–20%): The CA-BC surface carries a strong negative charge (Zeta potential approximately -18 to -32 mV), attracting positively charged Al atoms through Coulomb forces. 3+And aluminum hydroxyl cations. Physical adsorption and pore trapping (contributing approximately 10%–15%): Al 3+ Diffusion and physical trapping in the micropores / mesopores of biochar. Surface precipitation (contributing approximately 5%–10%): The alkaline microdomains of biochar allow some Al to be deposited. 3+ It precipitates in the form of Al(OH)3 in micro-regions on the carbon surface.

[0080] III. Effects of Acidic Red Soil Improvement – ​​Soil Culture Experiment The tested acidic red soil (initial pH 4.18, exchangeable Al) 3+ 5.23 cmol / kg) was air-dried and passed through a 2 mm sieve. Each treatment material was applied at 3% of its dry soil weight (for lime treatment, the amount was calculated as an equimolar amount of calcium, approximately 3.6 g CaO / kg soil). After thorough mixing, the mixture was placed in culture pots and adjusted to 70% of field capacity with deionized water. The pots were then incubated at 25°C. Samples were taken on days 7, 14, 30, 60, and 90 to determine soil pH (1:2.5 water-to-soil ratio, pH meter) and exchangeable Al2O3. 3+ Content (1 mol / L KCl extraction-ICP-OES method).

[0081] (1) Soil pH changes Table 8: Changes in pH of acidic red soil with incubation time under different treatment conditions

[0082] Analysis: CA-BC caused a sustained and stable increase in soil pH, rising from 4.18 to 5.78 (an increase of 1.60 units) at 60 days, and remaining at 5.72 at 90 days without a significant decline. In contrast, lime (Comparative Example 4) peaked at 5.92 at 7 days (rapid alkalization), but then steadily declined, falling back to 4.62 at 90 days (1.10 units lower than CA-BC), exhibiting a typical "sharp rise and fall" characteristic; free citric acid (Comparative Example 5) had almost no sustained effect because it was rapidly degraded by microorganisms in the soil; the increase in pH of unmodified BC (Comparative Example 1) (ΔpH≈0.94 units) was much smaller than that of CA-BC (ΔpH≈1.60 units). The sustained acid-lowering effect of CA-BC is attributed to a "dual buffering" mechanism: (i) rapid neutralization of the intrinsic alkalinity of biochar, and (ii) continuous proton consumption buffering of the surface carboxyl / carboxylate system (-COO). - +H + ⇌-COOH, pKa≈3.5~5.0, with maximum buffering capacity in the pH range of 4.5~5.8.

[0083] (2) Exchangeability Al 3+ Content change Table 9: Exchangeable Al in acidic red soil under different treatment conditions 3+Changes in content over culture time

[0084] Table 10: Exchangeable Al under different treatment conditions 3+ Comparison of reduction rates

[0085] Analysis: CA-BC at 60 days will transfer soil exchangeable Al 3+ The aluminum content decreased dramatically from 5.23 cmol / kg to 0.78 cmol / kg, a reduction rate of 85.1%, far exceeding that of BC (45.5%) and lime (39.8%). More importantly, the aluminum-reducing effect of CA-BC exhibits excellent durability—the exchangeable Al content remains high after 90 days. 3+ The concentration only rebounded slightly to 0.85 cmol / kg (a reduction of 83.7%), while lime treatment rebounded from a peak of 1.18 cmol / kg to 3.68 cmol / kg during the same period (a reduction of only 29.5%). The durable aluminum-fixing effect of CA-BC can be explained by the interaction of surface carboxyl groups with Al. 3+ It forms a stable chemical complex (Al-organic carboxyl complex), which is not easily released again by soil re-acidification. 3+ Lime primarily relies on pH increases to promote Al... 3+ The precipitate is Al(OH)3, which can be redissolved when the soil becomes acidic again.

[0086] IV. Functional Testing of Microbial Carbon Source Carriers (1) Microbial carbon source utilization test -- water-soluble organic carbon (DOC) release Add 0.5g of sample to 50mL of sterile deionized water and shake at 150rpm at 25℃. Collect the supernatant at 1, 3, 7, 14 and 30 days respectively, filter through a 0.45μm filter membrane, and determine the DOC concentration using a TOC analyzer.

[0087] Table 11: Comparison of water-soluble organic carbon release between CA-BC and BC

[0088] Analysis: The DOC release of CA-BC was 4.5–5.7 times that of BC at all time points. The soluble organic carbon components generated on the biochar surface during the hydrothermal modification of citric acid (including incompletely anchored citric acid molecules and small-molecule organic acids, aldehydes, ketones, etc. generated by the hydrothermal reaction) can serve as readily available carbon sources for microorganisms, providing a "starting carbon source" for the colonization and proliferation of exogenous functional bacterial agents. BC, on the other hand, has extremely low DOC release (mainly recalcitrant aromatic carbon), which cannot effectively support rapid microbial growth.

[0089] (2) Experiment on acid-resistant functional bacterial agent carrier - survival rate of Bacillus subtilis Test strain: Acid-resistant Bacillus subtilis (*Bacillus subtilis*Bs-m9, isolated from acidic red soil, minimum growth pH 4.0, optimal pH 5.5–6.5).

[0090] CA-BC and BC were each applied at 3% to sterilized acidic red soil (initial pH 4.2), while simultaneously inoculating with Bs-m9 bacterial solution (initial inoculum size approximately 1×10⁻⁶). 7 CFU / g soil was cultured at 25℃ and 70% field capacity. Soil samples were taken on days 0, 3, 7, 14, 30, and 60, and the viable bacterial count (CFU) was counted using the plate dilution spread method (LB medium, pH 5.5, 37℃ for 24 h).

[0091] Table 12: Effects of CA-BC and BC as carriers on the survival rate of acid-resistant Bacillus subtilis

[0092] Analysis: In the control group of pure bacterial culture without carrier protection, Bs-m9 rapidly declined in acidic red soil, with a 60-day survival rate of <0.001%. The BC carrier increased the 60-day survival rate to 0.86%, while the CA-BC carrier achieved a 60-day survival rate of 18.5%—21.5 times that of BC. The superior inoculum carrier function of CA-BC is attributed to the synergistic effect of a triple protection mechanism: (i) carbon source supply: CA-BC continuously releases DOC, providing a usable carbon source for Bs-m9 and avoiding death caused by "carbon starvation"; (ii) micro-domain detoxification: CA-BC creates a low-Al content on the surface and inside the pores of carbon particles through carboxyl complexation and alkalinity. 3+ Low H + The "neutral microenvironment" provides physical protection for Bs-m9 from aluminum acid stress; (iii) colonization substrate: CA-BC has a well-developed porous structure (specific surface area 118.3 m²). 2 / g) provides sufficient colonization surface area for Bs-m9.

[0093] V. Comprehensive Improvement Benefit Evaluation – Pot Experiment Using Chinese cabbage (*Brassica chinensis* L., variety 'Shanghai Qing') as the indicator plant, a pot experiment was conducted in acidic red soil. Five treatments were set up: (1) control (no amendment); (2) BC (3%); (3) CA-BC (3%); (4) lime (CaO, 3.6 g / kg); (5) CA-BC (3%) + Bs-m9 inoculant (1×10⁻⁶). 7(CFU / g soil-based assay). Three replicate pots were set up for each treatment. Harvest was carried out after 45 days, and the aboveground biomass (fresh weight) and root aluminum content were measured.

[0094] Table 13: Comparison of the improvement effects of different treatments on potted Chinese cabbage.

[0095] Analysis: CA-BC treatment increased the aboveground biomass of pak choi by 140% compared to the control (fresh weight 6.85 g vs 2.85 g / pot) and reduced root aluminum content by 81.0%. The combined treatment of CA-BC and microbial agents showed the best effect—biomass increased by 199% (nearly 3 times), root aluminum content decreased by 85.5%, and soil pH rose to 5.88, reaching the range suitable for most crops (pH 5.5–6.5). This result directly verifies the effectiveness of the synergistic improvement strategy of "acid reduction and detoxification (CA-BC) + carbon supply and microbial promotion (CA-BC as a Bs-m9 carrier)".

[0096] Mechanism Summary The mechanism by which citric acid-modified rice husk biochar improves acidic red soil in this invention can be summarized as a "four-in-one" synergistic model: (1) Acid reduction -- double buffer neutralization of H + -Intrinsic basic ions in biochar (K + Ca 2+ Mg 2+ Rapid neutralization of surface-COO - +H + ⇌-COOH continuous proton consumption buffer system (2) Detoxification -- specific complexation fixation of Al 3+ - Surface carboxyl groups on Al 3+ High affinity complexation (forming stable Al-organic carboxyl complexes) - electrostatic attraction Al 3+ / hydroxyaluminum cations to negatively charged surfaces - alkaline microdomains induce Al 3+ precipitation (3) Carbon supply - Slow-release microorganisms can utilize carbon sources - Soluble organic carbon generated during hydrothermal modification is slowly released - Citric acid molecules themselves and degradation intermediates serve as readily available carbon sources. (4) Nest building – constructing a protective microenvironment for microorganisms – the porous structure provides physical shelter – the neutral / low-aluminum environment of the carbon surface microdomain provides a "safe haven" for functional bacteria. The above four functions work together to form a positive feedback loop: acid reduction and detoxification create a suitable environment for microorganisms → microbial metabolism further stabilizes pH and transforms nutrients → nutrient release promotes plant growth → root exudates further improve the rhizosphere microenvironment.

[0097] Industrial applicability The method of this invention has a clear process route and is simple to operate. The required equipment is conventional chemical equipment (tube furnace / muffle furnace, ultrasonic cleaner, hydrothermal reactor, drying oven, etc.). The raw materials (rice husk, citric acid) are widely available and inexpensive, making it suitable for large-scale production. The citric acid-modified rice husk biochar obtained can be used for: acidity improvement and aluminum toxicity mitigation in acidic red soils in southern China; as a carbon source carrier and colonization substrate for acid-resistant functional microbial agents, and in combination with microbial agents for the ecological restoration of acidic soils; improvement and restoration of acidified tea garden and orchard soils; and vegetation restoration in acidic mining wastelands.

[0098] The above description is merely a preferred embodiment of the present invention and does not limit the scope of patent protection of the present invention. Any equivalent structural or procedural modifications made based on the description and drawings of the present invention, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A method for preparing citric acid-modified rice husk biochar, characterized in that, Includes the following steps: (1) Raw material pretreatment: Wash the rice husk raw material with deionized water, dry it to constant weight, crush and sieve it to obtain rice husk powder; (2) Oxygen-limited pyrolysis carbonization: Heat the rice husk powder obtained in step (1) to the pyrolysis temperature under inert atmosphere protection, carbonize it at the temperature, and then cool it to room temperature to obtain rice husk biochar precursor; the pyrolysis temperature is 450-600℃; (3) Citric acid impregnation modification: Mix the rice husk biochar precursor obtained in step (2) with citric acid aqueous solution at a solid-liquid ratio of 1:(20-60). The mixture is mixed at g / mL and impregnated under ultrasonic assistance to obtain an impregnated mixture; the concentration of the citric acid aqueous solution is 0.2-1.5 mol / L; (4) Hydrothermal reaction: the impregnated mixture obtained in step (3) is transferred to a hydrothermal reactor and subjected to a hydrothermal reaction at 120-200℃ for 4-12 h; (5) Washing and drying: the solid product after the hydrothermal reaction in step (4) is washed with deionized water until neutral and then dried to constant weight to obtain the citric acid modified rice husk biochar.

2. The preparation method according to claim 1, characterized in that, The pyrolysis temperature in step (2) is 500-550℃, the heating rate is 8-12℃ / min, and the holding time for carbonization is 1.5-2.5h.

3. The preparation method according to claim 1, characterized in that, The inert atmosphere in step (2) is nitrogen or argon, and the gas flow rate is 100-300 mL / min; the heating rate of the oxygen-limited pyrolysis carbonization in step (2) is 5-15 °C / min, and the holding time for carbonization is 1-3 h.

4. The preparation method according to claim 1, characterized in that, The concentration of the citric acid aqueous solution in step (3) is 0.5-1.0 mol / L, and the solid-liquid ratio is 1:(30-50) g / mL.

5. The preparation method according to claim 1, characterized in that, The ultrasonic auxiliary conditions in step (3) are: ultrasonic power 100-300W, ultrasonic frequency 20-40kHz, ultrasonic time 1-3h, and temperature controlled at 25-60℃; the ultrasonic auxiliary conditions in step (3) are: ultrasonic power 150-250W, ultrasonic frequency 28-40kHz, ultrasonic time 1.5-2.5h, and temperature controlled at 30-50℃; the hydrothermal reaction temperature in step (4) is 150-180℃, and the hydrothermal reaction time is 6-10h; the particle size of the rice husk powder in step (1) is sieve 40-100 mesh, preferably sieve 60-80 mesh; the drying temperature in step (1) is 60-80℃, and the drying is carried out until the moisture content is less than 5%; the drying temperature in step (5) is 80-105℃; the washing to neutrality in step (5) refers to washing repeatedly with deionized water until the pH of the washing liquid is 6.5-7.

5.

6. A citric acid-modified rice husk biochar, characterized in that, It is prepared by any one of claims 1 to 9.

7. The application of citric acid-modified rice husk biochar according to claim 6 in the improvement of acidic red soil, characterized in that, The citric acid-modified rice husk biochar is applied to acidic red soil at a concentration of 1% to 5% of the soil mass. The acidic red soil has a pH below 5.5 and an exchangeable aluminum ion content greater than 2.0 cmol / kg. The citric acid-modified rice husk biochar serves as a carbon source carrier and colonization substrate for acid-resistant functional microbial agents and is applied in combination with the acid-resistant functional microbial agents to the acidic red soil.

8. The application according to claim 7, characterized in that, The acid-resistant functional microbial agent includes at least one of acid-resistant Bacillus subtilis, acid-resistant phosphate-solubilizing bacteria, and acid-resistant nitrogen-fixing bacteria.

9. A combined improvement method for acidic red soil, characterized in that, The citric acid-modified rice husk biochar described in claim 10 is applied to acidic red soil at 1% to 5% of the soil mass, and simultaneously inoculated with acid-resistant functional microbial agents. After thorough mixing, it is cured for 30 to 90 days under field water holding capacity of 60% to 80%.

10. The combined improvement method according to claim 9, characterized in that, The inoculum size of the acid-resistant functional microbial agent is 1×10⁻⁶. 6 ~1×10 8 CFU / g soil.

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