PH-responsive magnetic biochar sustained-release microspheres as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of biomass resource utilization and anaerobic fermentation, and relates to a pH response type magnetic biochar sustained-release microsphere as well as a preparation method and application thereof. The sustained-release microsphere has a core-shell structure and comprises a functional core and a pH response shell wrapping the functional core, the functional core comprises magnetic charcoal, an alkaline buffer agent and a methanogen accelerant; the pH response shell is polyelectrolyte composite hydrogel formed by ionic crosslinking of sodium alginate and chitosan. Through the design of the core-shell structure, active sensing and intelligent regulation and control of pH abnormity of the anaerobic fermentation system are realized, and rancidity of the system is effectively prevented; and meanwhile, by utilizing multiple synergistic effects of the functional kernel, the operation stability and the methane conversion efficiency of the system are remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of biomass resource utilization and anaerobic fermentation technology, and relates to a pH-responsive magnetic biochar slow-release microsphere, its preparation method and application. Background Technology

[0002] Biomass energy, as a sustainable energy source with abundant reserves, wide distribution, and carbon neutrality, has attracted widespread attention for its efficient conversion technologies. Anaerobic fermentation technology can convert organic waste into high-quality biogas, realizing the energy and resource utilization of waste, and possessing significant environmental and economic benefits.

[0003] However, the practical application of this technology has long been limited by a key technical challenge—system rancidity. This problem stems from the inherent imbalance in the metabolic rate of microorganisms in the anaerobic fermentation system: in the early stages of fermentation, hydrolytic acid-producing bacteria are metabolically active and can rapidly decompose organic matter into acidic intermediates such as volatile fatty acids, which then accumulate quickly. Meanwhile, the slow-growing methanogenic archaea, which are extremely sensitive to acidic environments, cannot consume these acids in time, leading to a sharp drop in the system's pH value. This severely inhibits or even interrupts the methanogenic metabolic pathway, ultimately causing the entire fermentation system to become unbalanced and fail, resulting in huge operating costs and economic losses.

[0004] To address this acid inhibition problem, existing technologies mainly rely on the direct addition of acidic and alkaline chemicals, the use of ordinary biochar, or emerging magnetic biochar. However, these methods all have significant limitations. While the direct addition of chemicals such as sodium hydroxide or sodium bicarbonate can immediately neutralize acidity, its regulatory mechanism is crude and its response is slow, relying entirely on manual intervention and unable to achieve real-time, precise control. More seriously, localized excessive strong alkali can easily cause secondary shocks to methanogenic bacteria, and the accompanying accumulation of inorganic salt ions may also trigger new inhibitory effects. Overall, it is a passive strategy that treats the symptoms but not the root cause, and may even exacerbate system instability. Biochar, with its porous structure and surface properties, can provide microbial habitats and has some buffering capacity, but its passive mode of action and limited slow-release efficiency make it difficult to cope with the rapid, instantaneous accumulation of volatile fatty acids, resulting in minimal regulatory effect under high acid loads. In recent years, magnetic biochar has shown some potential in terms of convenient recycling and promoting interspecies electron transfer. However, its core function is still limited to microenvironment optimization. When dealing with sudden, high-intensity acid shocks, it still lacks an intrinsic pH regulation mechanism that can actively sense and intelligently respond, and therefore cannot fundamentally prevent the risk of system rancidity. Summary of the Invention

[0005] To address the problems in existing technologies, this invention provides a pH-responsive magnetic biochar slow-release microsphere, its preparation method, and its application. Through core-shell structure design, it achieves active sensing and intelligent regulation of pH anomalies in anaerobic fermentation systems, effectively preventing system rancidity. At the same time, by utilizing the multiple synergistic effects of the functional core, it significantly improves the system's operational stability and methane conversion efficiency.

[0006] To achieve the above objectives, the present invention employs the following technical solution: In a first aspect, the present invention provides a pH-responsive magnetic biochar slow-release microsphere, the slow-release microsphere having a core-shell structure, including a functional core and a pH-responsive shell surrounding the functional core; The functional core includes magnetic biochar, alkaline buffer, and methanogen promoter; The pH-responsive shell is a polyelectrolyte composite hydrogel formed by ionic crosslinking of sodium alginate and chitosan.

[0007] Preferably, based on the total mass of the sustained-release microspheres, the functional core comprises the following components by mass percentage: Magnetic biochar 10%~30%; Alkaline buffer 5%~15%; Methanogen promoter: 0.1%~2%.

[0008] Preferably, the magnetic biochar is prepared by mixing biomass raw materials with iron-containing compounds and carrying out a pyrolysis reaction under an inert atmosphere to obtain the magnetic biochar.

[0009] Preferably, the biomass raw material is corn stalks, wheat stalks, walnut shells, or coconut shells.

[0010] Preferably, the iron-containing compound is one or more of ferric chloride, ferrous sulfate, and ferric nitrate.

[0011] Preferably, the alkaline buffer is sodium bicarbonate.

[0012] Preferably, the methanogen promoter is a nickel source, a cobalt source, or a methanogen signaling molecule.

[0013] Secondly, the present invention provides a method for preparing pH-responsive magnetic biochar sustained-release microspheres, comprising the following steps: Magnetic biochar, alkaline buffer and methanogen promoter are dispersed in sodium alginate aqueous solution and stirred evenly to form a mixed slurry; The mixed slurry is dropped into a coagulation bath containing calcium ions to form gel microspheres; The gel microspheres were placed in a chitosan solution for ionic cross-linking to obtain the pH-responsive magnetic biochar slow-release microspheres.

[0014] Preferably, the coagulation bath is an aqueous solution of calcium chloride.

[0015] Thirdly, this invention provides an application of pH-responsive magnetic biochar slow-release microspheres in the field of anaerobic fermentation of organic waste, comprising the following steps: The slow-release microspheres were added to the anaerobic fermentation system for organic waste. When the pH value of the anaerobic fermentation system for organic waste is lower than a preset critical point, the pH-responsive shell of the slow-release microspheres dissolves and releases the functional core to neutralize excess acidic substances and maintain the pH value of the anaerobic fermentation system for organic waste within the preset range required by methanogens.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention relates to pH-responsive magnetic biochar slow-release microspheres with a core-shell structure design. By integrating multiple functional components into a controllable release system, it achieves intelligent regulation of the anaerobic fermentation process. The functional core acts as the core functional unit, responsible for providing alkaline buffering capacity, promoting microbial metabolic activity, and enhancing interspecies electron transfer. The pH-responsive shell, as a key barrier for environmental sensing and controlled release, can trigger structural changes when the system pH fluctuates abnormally, thereby initiating the release mechanism of the functional components. Detailed Implementation

[0017] To enable those skilled in the art to understand the features and effects of the present invention, the terms and expressions used in the specification and claims are explained and defined in general below. Unless otherwise specified, all technical and scientific terms used herein have the ordinary meaning understood by those skilled in the art regarding the present invention, and in case of conflict, the definitions in this specification shall prevail.

[0018] The theories or mechanisms described and disclosed herein, whether right or wrong, should not in any way limit the scope of the invention, that is, the contents of the invention can be implemented without being limited by any particular theory or mechanism.

[0019] In this document, all features defined by numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are for the sake of brevity and convenience only. Accordingly, descriptions of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0020] In this article, unless otherwise specified, the terms “contains,” “includes,” “containing,” “has,” or similar terms cover the meanings of “composed of” and “mainly composed of”. For example, “A contains a” covers the meanings of “A contains a and others” and “A contains only a”.

[0021] For the sake of brevity, not all possible combinations of the technical features in each implementation scheme or embodiment are described herein. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each implementation scheme or embodiment can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification.

[0022] The first objective of this invention is to provide a pH-responsive magnetic biochar slow-release microsphere, wherein the slow-release microsphere has a core-shell structure, comprising a functional core and a pH-responsive shell surrounding the functional core; The functional core includes magnetic biochar, alkaline buffer, and methanogen promoter; The pH-responsive shell is a polyelectrolyte composite hydrogel formed by ionic crosslinking of sodium alginate and chitosan.

[0023] Among them, magnetic biochar is a type of biochar that has both a high specific surface area (300m²) and... 2 A multifunctional carbon-based material ( / g) and superparamagnetic, prepared by co-pyrolysis of biomass feedstock and iron-containing compounds, possesses excellent adsorption and conductivity properties, enabling it to promote direct interspecies electron transfer (DIET) among microorganisms. Furthermore, the presence of iron oxides endows the microspheres with magnetic recovery capabilities. An alkaline buffer is used to neutralize excess acidic substances accumulated during fermentation, maintaining the system pH within the suitable range for methanogens (typically 6.8–7.5). Methanogen promoters directly activate the metabolic activity of methanogenic archaea by supplementing key trace elements or signaling molecules, improving their utilization efficiency of substrates such as acetic acid and hydrogen. The pH-responsive shell is a polyelectrolyte composite hydrogel formed by ionic cross-linking of sodium alginate and chitosan. This shell, based on natural polysaccharides, exhibits good biocompatibility and environmental friendliness. Sodium alginate is rich in carboxyl groups and carries a negative charge under neutral or weakly alkaline conditions, enabling it to crosslink with calcium ions to form a stable three-dimensional network structure. Chitosan, on the other hand, is rich in amino groups and becomes protonated and positively charged under acidic conditions, allowing it to interact electrostatically with sodium alginate to form a polyelectrolyte complex. When the system is in the normal pH range (e.g., >6.5), the outer shell structure is dense and stable, effectively preventing the leakage of internal functional components. However, when the system pH drops below the critical value (e.g., <6.5), a large number of amino groups on the chitosan molecular chain are protonated, leading to increased hydrophilicity, intensified swelling, and disruption of the original electrostatic balance. This causes the outer shell pores to expand or even rupture, thereby releasing the encapsulated functional core.

[0024] For example, based on the total mass of the sustained-release microspheres, the functional core comprises the following components by mass percentage: Magnetic biochar 10%~30%; Alkaline buffer 5%~15%; Methanogen promoter: 0.1%~2%.

[0025] Magnetic biochar, serving as a carrier material and electron transport medium, is controlled at a content between 10% and 30%. This provides sufficient specific surface area for adsorbing microorganisms and intermediate metabolites while maintaining good conductivity to promote direct interspecies electron transfer, thereby enhancing the metabolic flux of methanogenesis. The content of an alkaline buffer (such as sodium bicarbonate) is set at 5% to 15%, primarily to neutralize the acidification risk caused by the accumulation of volatile fatty acids. This component is rapidly released after the shell swells in response to pH, effectively alleviating the local acidic environment and preventing irreversible inhibition of methanogens. Choosing this concentration range ensures sufficient neutralization capacity while preventing drastic pH fluctuations or inorganic salt accumulation due to excessive addition, thus reducing the possibility of secondary inhibition. The amount of methanogen promoter added is 0.1% to 2%, a trace functional component whose role is to activate or enhance the metabolic activity of methanogenic archaea. This promoter can be selected from sources such as nickel and cobalt, or from methanogenic signaling molecules. Even low doses can significantly enhance the expression levels of key enzymes (such as hydrogenase and carbon monoxide dehydrogenase). Controlling the content within this range ensures both biological effectiveness and avoids the toxic effects of high concentrations of metal ions on other microbial communities.

[0026] The magnetic biochar is prepared by mixing biomass raw materials with iron-containing compounds and carrying out a pyrolysis reaction under an inert atmosphere to obtain the magnetic biochar.

[0027] By uniformly mixing biomass feedstock with iron-containing compounds and then subjecting it to high-temperature pyrolysis under an inert atmosphere, a carbonization reaction can be induced in the biomass under anaerobic conditions to form a porous carbon framework. Simultaneously, the iron-containing compounds decompose and are reduced to generate magnetic iron-based nanoparticles. These magnetic particles are in situ embedded or attached to the porous structure of the biochar, thus endowing the material with excellent superparamagnetism. The resulting magnetic biochar not only retains the high specific surface area and rich pore structure of the original biomass, which is beneficial for adsorbing volatile fatty acids and serving as a microbial carrier, but also possesses the ability to respond to applied magnetic fields, facilitating efficient recovery and reuse from the fermentation system.

[0028] The biomass raw materials are iron-rich corn stalks, wheat stalks, walnut shells, or coconut shells. This not only realizes the high-value utilization of widely available and low-cost agricultural and forestry waste, effectively reducing the preparation cost of functional core materials, but also, due to their unique composition and structural characteristics (such as straw being rich in cellulose and lignin, which is conducive to the formation of a stable porous carbon skeleton and promotes the dispersion of magnetic particles, while walnut shells and coconut shells have high mechanical strength and dense microstructure, which helps to develop a rich pore network and enhance the catalytic activity of the material), the prepared magnetic biochar has excellent adsorption performance, stable superparamagnetism, and high reactivity.

[0029] The iron-containing compound is one or more of ferric chloride, ferrous sulfate, and ferric nitrate. Ferric chloride, as a Lewis acid, has strong complexing ability and permeability, and can effectively promote the formation of Fe... 3+ It enters the cell wall of biomass and forms a highly dispersed magnetic phase during pyrolysis; ferrous sulfate is inexpensive, has good water solubility, and is easy to mix evenly with biomass raw materials, and can generate Fe3O4 through self-reduction reaction during pyrolysis; ferric nitrate, due to its high oxygen content, helps to regulate the redox environment during pyrolysis and promotes the generation of magnetic particles with complete crystal form.

[0030] A second objective of this invention is to provide a method for preparing pH-responsive magnetic biochar sustained-release microspheres, comprising the following steps: Magnetic biochar, alkaline buffer and methanogen promoter are dispersed in sodium alginate aqueous solution and stirred evenly to form a mixed slurry; The mixed slurry is dropped into a coagulation bath containing calcium ions (such as an aqueous solution of calcium chloride), and solidifies to form gel microspheres after 10-30 minutes. The gel microspheres were placed in a chitosan solution for ionic cross-linking reaction for 10-20 minutes to obtain the pH-responsive magnetic biochar sustained-release microspheres.

[0031] This invention utilizes the rapid ionogelation of calcium ions and sodium alginate to form structurally stable primary gel microspheres, achieving effective loading of functional components. Subsequently, a dense polyelectrolyte composite membrane is formed through electrostatic self-assembly of chitosan and the sodium alginate calcium gel surface. This dual crosslinking strategy ensures the stability of the shell structure and pH-responsiveness. The entire preparation process is carried out in an aqueous phase, requiring no complex equipment or harsh conditions, facilitating large-scale production. Furthermore, all materials used are biocompatible natural polymers, making it environmentally friendly.

[0032] The sodium alginate aqueous solution has a mass-volume concentration of 2% to 4% (w / v), meaning it contains 2 to 4 grams of sodium alginate per 100 ml of solution. This concentration ensures that the mixed slurry has a suitable viscosity, enabling both uniform mixing with the functional components and maintaining the integrity of the droplets during the droplet addition process, forming uniformly sized droplets. The calcium chloride aqueous solution has a mass-volume concentration of 2% to 5% (w / v), meaning it contains 2 to 5 grams of calcium chloride per 100 ml of solution. This concentration allows for sufficient reaction with the sodium alginate. Furthermore, moderate ionic cross-linking rapidly forms a gel microsphere core with good mechanical strength, effectively encapsulating functional substances. The chitosan solution is prepared by dissolving chitosan in an aqueous acetic acid solution. The chitosan solution itself has a mass-volume concentration of 0.5% to 1.5% (w / v), that is, 0.5 g to 1.5 g of chitosan is dissolved in every 100 ml of aqueous acetic acid solution. This concentration allows for effective ionic interaction between the chitosan solution and the surface of the gel microspheres, forming a dense and pH-sensitive intelligent shell.

[0033] Furthermore, by employing a sharp-aperture device with a pore size of 0.5 mm to 1.0 mm for the droplet addition of the mixed slurry, this invention effectively ensures the regularity of the final product's morphology and the uniformity of its performance. This specific pore size range ensures that the slurry enters the coagulation bath as uniformly sized droplets, thereby forming gel microspheres with good sphericity and a concentrated particle size distribution.

[0034] The third objective of this invention is to provide an application of pH-responsive magnetic biochar slow-release microspheres in the field of anaerobic fermentation of organic waste, comprising the following steps: The slow-release microspheres were added to the anaerobic fermentation system for organic waste. When the pH value of the anaerobic fermentation system for organic waste is lower than a preset critical point (e.g., 6.5), the pH-responsive shell of the slow-release microspheres dissolves and releases the functional core to neutralize excess acidic substances and maintain the pH value of the anaerobic fermentation system for organic waste within the preset range required by methanogens. After anaerobic fermentation, the magnetic biochar can be recovered and reused by applying an external magnetic field.

[0035] This invention introduces slow-release microspheres into a fermentation system. The pH-responsive outer shell of these microspheres autonomously senses environmental acidification signals and intelligently releases the functional core before acid inhibition occurs, thus enabling early intervention and continuous stabilization of the system. Furthermore, this invention utilizes the inherent properties of magnetic biochar to facilitate the separation and recycling of functional materials after fermentation using an external magnetic field. This significantly improves resource utilization efficiency, reduces long-term operating costs, and avoids the residual accumulation of added materials in the system, ensuring the sustainability of the process.

[0036] For example, the anaerobic fermentation system includes a substrate represented by wheat straw or corn straw, and inoculum such as feces, sludge, wastewater, or biogas slurry rich in anaerobic microorganisms, with an inoculum amount (based on volatile solids) of 20% to 50%. The slow-release microspheres are added at a rate of 0.5% to 5% of the total mass of the anaerobic fermentation substrate and inoculum. This addition ratio ensures a sufficient number of microspheres distributed in the system to provide comprehensive and effective pH buffering and microbial promotion functions, avoiding regulatory failure due to insufficient addition; at the same time, it prevents material accumulation, resource waste, or potential negative impacts on the physical properties of the fermentation substrate that may result from excessive addition.

[0037] In addition, the system operating conditions are controlled as follows: total solids concentration of 10% to 12%, carbon-nitrogen ratio of 20:1 to 30:1, fermentation temperature of 35℃ to 45℃, and the upper space of the system needs to be replaced with nitrogen to ensure a strict anaerobic environment.

[0038] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0039] The following examples use instruments and equipment conventional in the art. Experimental methods in the following examples, unless otherwise specified, are generally performed under conventional conditions or as recommended by the manufacturer. All raw materials used in the following examples are conventional commercially available products with specifications conventional in the art. In this specification and the following examples, unless otherwise specified, "%" refers to weight percentage, "parts" refers to parts by weight, and "ratio" refers to weight proportion.

[0040] Example 1: Preparation of pH-responsive magnetic biochar slow-release microspheres and their application in anaerobic fermentation of corn straw S1. Preparation of magnetic biochar Corn stalks collected from Lianyungang, Jiangsu Province, were used as raw material. After washing and drying, the stalks were crushed into 2-5 cm pieces. The crushed stalks were then soaked in a 0.5 mol / L ferric chloride solution (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then pyrolyzed at 500℃ for 2 hours under nitrogen protection with a programmed heating rate of 5℃ / min, followed by natural cooling, grinding, and sieving through a 100-mesh sieve to obtain magnetic biochar. Material characterization showed that it had a specific surface area of ​​360 m² / g and exhibited superparamagnetism.

[0041] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0042] 1.0 g of magnetic biochar prepared by S1, 0.5 g of sodium bicarbonate (as an alkaline buffer) and 0.02 g of nickel chloride (as a methanogen promoter) were added to the above sodium alginate solution and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0043] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0044] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0045] After cross-linking, the microspheres were rinsed with deionized water until neutral and stored at 4°C for later use.

[0046] S3, Anaerobic Fermentation Experiment Crush the corn stalks to 2-5 cm and adjust their moisture content to 45%.

[0047] Weigh 200 g (based on total solids) of pretreated straw and place it in a 1 L fermentation bottle. Inoculate it with acclimatized anaerobic sludge (total solids content of 8.01% ± 0.06 and volatile solids content of 4.39% ± 0.04) at an inoculation rate of 30% (based on volatile solids).

[0048] The carbon-to-nitrogen ratio of the system was adjusted to 25:1 using ammonium chloride, and deionized water was added to maintain the total solids concentration of the system at 10%.

[0049] S2-prepared slow-release microspheres were added to the fermentation system at a rate of 2% of the total mass of the substrate and inoculum.

[0050] Nitrogen gas was introduced into the system for 2 minutes to replace the air in the bottle and ensure a strictly anaerobic environment. Then, it was placed at a constant temperature of 45°C for fermentation.

[0051] During fermentation, mechanical stirring was performed three times a day, and gas samples were collected every two days to determine the methane content. At the same time, the system pH value and volatile fatty acid concentration were monitored.

[0052] Comparative Example 1 No microspheres were added, and all other experimental conditions were exactly the same as in Example 1.

[0053] Comparative Example 2 An equal amount of ordinary biochar was added (i.e., the preparation conditions were exactly the same as those of the magnetic biochar in Example 1, except that it was not modified with ferric chloride), and all other experimental conditions were exactly the same as those in Example 1.

[0054] Example 1 verified that the pH-responsive magnetic biochar slow-release microspheres can intelligently regulate the anaerobic fermentation process of corn straw in situ. For a normally operating anaerobic fermentation system, the pH value should typically be stable between 6.5 and 7.5, and the total volatile fatty acid concentration should be below 2000 mg / L. The experimental results showed that the pH value of the system in Example 1 remained stable within the excellent range of 6.8-7.2, and the highest concentration of volatile fatty acids was only 1850 mg / L; Comparative Example 1 (blank control) showed severe rancidity on day 5, with the pH value dropping to 5.8 and the volatile fatty acid concentration reaching as high as 7500 mg / L; Comparative Example 2 (ordinary biochar) showed mild acid inhibition on day 8, with the pH value dropping to a minimum of 6.2 and the peak volatile fatty acid concentration reaching 4800 mg / L.

[0055] In terms of gas production performance, Example 1 achieved a cumulative methane yield of 458 mL / g VS, representing a 71.54% increase compared to Comparative Example 1 (267 mL / g VS) and a 34.77% increase compared to Comparative Example 2 (345 mL / g VS). Furthermore, after fermentation, the total solids and volatile solids degradation rates of Example 1 reached 68.5% and 71.2%, respectively, significantly higher than the two comparative examples. With the application of an external magnetic field, the recovery rate of magnetic biochar in Example 1 reached as high as 92.3%.

[0056] In summary, the slow-release microspheres prepared in Example 1 effectively solved the problem of easy acidification during anaerobic fermentation of corn straw, and achieved real-time sensing of system pH, active intervention in acid inhibition risk and synergistic enhancement of microbial metabolic activity, thereby significantly improving the system's operational stability and methane yield.

[0057] Example 2: Effects of different microsphere addition amounts on anaerobic fermentation of wheat straw S1. Preparation of magnetic biochar Corn stalks collected from Lianyungang, Jiangsu Province, were used as raw material. After washing and drying, the stalks were crushed into 2-5 cm pieces. The crushed stalks were then soaked in a 0.5 mol / L ferric chloride solution (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then pyrolyzed at 500℃ for 2 hours under nitrogen protection with a programmed heating rate of 5℃ / min, followed by natural cooling, grinding, and sieving through a 100-mesh sieve to obtain magnetic biochar. Material characterization showed that it had a specific surface area of ​​360 m² / g and exhibited superparamagnetism.

[0058] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0059] 1.0 g of magnetic biochar prepared by S1, 0.5 g of sodium bicarbonate (as an alkaline buffer) and 0.02 g of nickel chloride (as a methanogen promoter) were added to the above sodium alginate solution and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0060] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0061] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0062] After cross-linking, the microspheres were rinsed with deionized water until neutral and stored at 4°C for later use.

[0063] S3, Anaerobic Fermentation Experiment Crush the wheat straw to 2-5 cm and adjust the moisture content to 45%.

[0064] Take 200 g (based on total solids) of pretreated wheat straw and place it in a 1 L fermentation bottle. Inoculate it with the same acclimatized anaerobic sludge as in Example 1, with an inoculation amount (based on volatile solids) of 30%.

[0065] Adjust the carbon-to-nitrogen ratio to 25:1 with ammonium chloride, and add water to bring the total solids concentration of the system to 10%.

[0066] S2-prepared sustained-release microspheres were added separately, with three addition gradients: 0.5%, 2%, and 5% of the total mass of substrate and inoculum, respectively.

[0067] Introduce nitrogen into the system for 2 minutes to ensure an anaerobic environment, and then place it at a constant temperature of 45°C for fermentation.

[0068] The system was mechanically stirred three times a day, and gas samples were collected every two days to determine the methane content. The system pH and volatile fatty acid concentrations were also monitored.

[0069] Comparative Example 3 No microspheres were added, and all other experimental conditions were exactly the same as in Example 2.

[0070] Comparative Example 4 An equal amount of ordinary biochar was added (i.e., the preparation conditions were exactly the same as those of the magnetic biochar in Example 2, except that it was not modified with ferric chloride), and all other experimental conditions were exactly the same as those in Example 2.

[0071] Example 2 investigated the regulatory effect of microsphere addition on the anaerobic fermentation of wheat straw. The results showed that: In all experimental groups with added slow-release microspheres, the system pH remained stable between 6.8 and 7.2, and the highest concentration of volatile fatty acids was only 1570 mg / L, with no system rancidity observed. The optimal effect was achieved with a microsphere addition of 2%, resulting in a methane yield of 375 mL / g VS. At a addition of 0.5%, the regulatory effect was insufficient, with a peak volatile fatty acid concentration of 4200 mg / L, indicating that the number of microspheres was insufficient to completely buffer the acidification stress. At an addition of 5%, the effect on system stabilization and methane yield enhancement was comparable to that of 2%, but the economic efficiency was lower.

[0072] Example 3: Applicability verification of microspheres in rice straw fermentation S1. Preparation of magnetic biochar Corn stalks collected from Lianyungang, Jiangsu Province, were used as raw material. After washing and drying, the stalks were crushed into 2-5 cm pieces. The crushed stalks were then soaked in a 0.5 mol / L ferric chloride solution (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then pyrolyzed at 500℃ for 2 hours under nitrogen protection with a programmed heating rate of 5℃ / min, followed by natural cooling, grinding, and sieving through a 100-mesh sieve to obtain magnetic biochar. Material characterization showed that it had a specific surface area of ​​360 m² / g and exhibited superparamagnetism.

[0073] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0074] 1.0 g of magnetic biochar prepared by S1, 0.5 g of sodium bicarbonate (as an alkaline buffer) and 0.02 g of nickel chloride (as a methanogen promoter) were added to the above sodium alginate solution and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0075] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0076] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0077] After cross-linking, the microspheres were rinsed with deionized water until neutral and stored at 4°C for later use.

[0078] S3, Anaerobic Fermentation Experiment Crush the rice straw to 2-5 cm and adjust the moisture content to 45%.

[0079] Take 200 g (based on total solids) of pretreated rice straw and place it in a 1 L fermentation bottle. Inoculate it with the same acclimatized anaerobic sludge as in Example 1, with an inoculation amount (based on volatile solids) of 30%.

[0080] Adjust the carbon-to-nitrogen ratio to 25:1 with ammonium chloride, and add water to bring the total solids concentration of the system to 10%.

[0081] S2-prepared sustained-release microspheres were added at an amount equal to 2% of the total mass of the substrate and inoculum.

[0082] Introduce nitrogen into the system for 2 minutes to ensure an anaerobic environment, and then place it at a constant temperature of 45°C for fermentation.

[0083] The system was mechanically stirred three times a day, and gas samples were collected every two days to determine the methane content. The system pH and volatile fatty acid concentrations were also monitored.

[0084] Comparative Example 5 No microspheres were added, and all other experimental conditions were exactly the same as in Example 3.

[0085] Comparative Example 6 An equal amount of ordinary biochar was added (i.e., the preparation conditions were exactly the same as those of the magnetic biochar in Example 3, except that it was not modified with ferric chloride), and all other experimental conditions were exactly the same as those in Example 3.

[0086] Example 3 verified the applicability of the slow-release microspheres to the anaerobic fermentation of rice straw. The results showed that: In Example 3, the system operated stably, with the pH consistently maintained between 6.9 and 7.3, and the highest concentration of volatile fatty acids only reaching 1740 mg / L. The cumulative methane production reached 376 mL / g VS, and the total solids and volatile solids degradation rates were 65.8% and 69.4%, respectively. After fermentation, a magnetic biochar recovery rate of 90.7% was achieved by applying an external magnetic field.

[0087] Example 4 S1. Preparation of magnetic biochar Wheat straw was used as raw material. After washing and drying, it was crushed into 2-5 cm pieces. The crushed straw was soaked in a 0.5 mol / L ferrous sulfate solution (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then pyrolyzed at 500℃ for 2 hours under nitrogen protection with a programmed heating rate of 5℃ / min. After natural cooling, it was ground and passed through a 100-mesh sieve to obtain magnetic biochar.

[0088] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0089] The magnetic biochar prepared by S1, sodium bicarbonate (as an alkaline buffer), and cobalt chloride (as a methanogen promoter) were added to the sodium alginate solution in a set ratio and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0090] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0091] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0092] After cross-linking, the microspheres were rinsed with deionized water until neutral to obtain sustained-release microspheres, which were stored at 4°C for later use. The sustained-release microspheres contained 10% magnetic biochar, 5% sodium bicarbonate, and 2% cobalt chloride by mass.

[0093] Example 5 S1. Preparation of magnetic biochar Walnut shells were used as raw material. After washing and drying, they were crushed into 2-5 cm pieces. The crushed walnut shells were immersed in a 0.5 mol / L ferric nitrate solution (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then heated to 500℃ under nitrogen protection at a programmed heating rate of 5℃ / min and pyrolyzed for 2 hours. After natural cooling, the sample was ground and passed through a 100-mesh sieve to obtain magnetic biochar.

[0094] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0095] The magnetic biochar prepared by S1, sodium bicarbonate (as an alkaline buffer), and methanogenic signal molecules (as methanogenic promoters) were added to the sodium alginate solution in a set ratio and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0096] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0097] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0098] After cross-linking, the microspheres were rinsed with deionized water until neutral to obtain sustained-release microspheres, which were stored at 4°C for later use. The sustained-release microspheres contained 30% magnetic biochar, 15% sodium bicarbonate, and 0.1% methanogenic signal molecules.

[0099] Example 6 S1. Preparation of magnetic biochar Coconut shells were washed, dried, and then pulverized into 2-5 cm pieces. The pulverized coconut shells were immersed in a 0.5 mol / L mixed solution of ferric chloride and ferric nitrate (solid-liquid ratio 1:10), shaken for 24 hours, filtered, and dried at 105℃. The dried sample was then pyrolyzed at 500℃ for 2 hours under nitrogen protection with a programmed heating rate of 5℃ / min, followed by natural cooling, grinding, and sieving through a 100-mesh sieve to obtain magnetic biochar.

[0100] Preparation of S2 pH-responsive magnetic biochar sustained-release microspheres Weigh 2.0 g of sodium alginate and dissolve it in 100 mL of deionized water to prepare a sodium alginate solution with a mass-volume concentration of 2%.

[0101] The magnetic biochar prepared by S1, sodium bicarbonate (as an alkaline buffer), and methanogenic signal molecules (as methanogenic promoters) were added to the sodium alginate solution in a set ratio and mechanically stirred at 300 rpm for 2 hours to form a uniform slurry.

[0102] Using a syringe pump (flow rate set to 10 mL / min), the mixed slurry was dripped into a calcium chloride coagulation bath with a mass-volume concentration of 3% through a needle with a 0.7 mm aperture, forming gel microspheres with a diameter of about 2 mm, and then solidified for 20 minutes.

[0103] After curing, the gel microspheres were removed, rinsed with deionized water, and then placed in a 1.0% (w / v) chitosan solution (dissolved in 1% acetic acid solution) for 15 minutes to carry out an ionic crosslinking reaction.

[0104] After cross-linking, the microspheres were rinsed with deionized water until neutral to obtain sustained-release microspheres, which were stored at 4°C for later use. The sustained-release microspheres contained 20% magnetic biochar, 10% sodium bicarbonate, and 1% methanogenic signal molecules.

[0105] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pH-responsive magnetic biochar slow-release microsphere, characterized in that, The sustained-release microspheres have a core-shell structure, including a functional core and a pH-responsive shell enclosing the functional core; The functional core includes magnetic biochar, alkaline buffer, and methanogen promoter; The pH-responsive shell is a polyelectrolyte composite hydrogel formed by ionic crosslinking of sodium alginate and chitosan.

2. The pH-responsive magnetic biochar slow-release microspheres according to claim 1, characterized in that, Based on the total mass of the sustained-release microspheres, the functional core comprises the following components by mass percentage: Magnetic biochar 10%~30%; Alkaline buffer 5%~15%; Methanogen promoter: 0.1%~2%.

3. The pH-responsive magnetic biochar slow-release microspheres according to claim 1, characterized in that, The magnetic biochar is prepared by mixing biomass raw materials with iron-containing compounds and carrying out a pyrolysis reaction under an inert atmosphere to obtain the magnetic biochar.

4. The pH-responsive magnetic biochar slow-release microspheres according to claim 3, characterized in that, The biomass raw materials are corn stalks, wheat stalks, walnut shells, or coconut shells.

5. The pH-responsive magnetic biochar slow-release microspheres according to claim 3, characterized in that, The iron-containing compound is one or more of ferric chloride, ferrous sulfate, and ferric nitrate.

6. The pH-responsive magnetic biochar slow-release microspheres according to claim 1, characterized in that, The alkaline buffer is sodium bicarbonate.

7. The pH-responsive magnetic biochar slow-release microspheres according to claim 1, characterized in that, The methanogen promoter is a nickel source, a cobalt source, or a methanogen signaling molecule.

8. A method for preparing pH-responsive magnetic biochar sustained-release microspheres according to any one of claims 1 to 7, characterized in that, Includes the following steps: Magnetic biochar, alkaline buffer and methanogen promoter are dispersed in sodium alginate aqueous solution and stirred evenly to form a mixed slurry; The mixed slurry is dropped into a coagulation bath containing calcium ions to form gel microspheres; The gel microspheres were placed in a chitosan solution for ionic cross-linking to obtain the pH-responsive magnetic biochar slow-release microspheres.

9. The method for preparing pH-responsive magnetic biochar sustained-release microspheres according to claim 8, characterized in that, The coagulation bath is an aqueous solution of calcium chloride.

10. The application of pH-responsive magnetic biochar slow-release microspheres according to claims 1-7 in the field of anaerobic fermentation of organic waste, characterized in that, Includes the following steps: The slow-release microspheres were added to the anaerobic fermentation system for organic waste. When the pH value of the anaerobic fermentation system for organic waste is lower than a preset critical point, the pH-responsive shell of the slow-release microspheres dissolves and releases the functional core to neutralize excess acidic substances and maintain the pH value of the anaerobic fermentation system for organic waste within the preset range required by methanogens.