Slow-release electron acceptor / conductive mediator core-shell microsphere, preparation method thereof and application thereof in anaerobic digestion of kitchen waste

Abstract

The invention relates to the technical field of solid waste resource utilization and environmental engineering, in particular to a sustained-release electron acceptor / conductive mediator core-shell microsphere, a preparation method thereof and application of the sustained-release electron acceptor / conductive mediator core-shell microsphere in anaerobic digestion of kitchen waste. Through the synergistic effect of the core layer and the shell layer of the core-shell microsphere, a triple action mechanism of'source inhibition-in-situ fixation-synergistic methane production 'is constructed, efficient in-situ blocking and control of H2S are realized, and meanwhile, the methane yield is remarkably improved. The sustained-release structure of the core layer and the biodegradable framework ensure that nitrate is continuously and stably released in the whole anaerobic digestion period, and the defect that frequent adding is needed in a traditional method is overcome; the shell layer is made of modified hydrothermal carbon prepared from kitchen waste biogas residues, so that the resource recycling of'treating waste with waste 'is realized; and meanwhile, the core layer adopts the biodegradable polymer, so that the whole microspheres can be subjected to resource utilization along with the biogas residues after digestion is finished, and the risk of secondary pollution is avoided.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and environmental engineering technology, and in particular to a slow-release electron acceptor / conductive mediator core-shell microsphere, its preparation method, and its application in the anaerobic digestion of kitchen waste. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Food waste, a major component of municipal solid waste, is produced in large quantities and is rich in biodegradable organic matter such as carbohydrates, proteins, and fats, exhibiting significant potential for methanogenesis through anaerobic digestion. However, food waste has a high content of sulfur-containing proteins. Under the strictly anaerobic environment of anaerobic digestion, the hydrolysis and acidification of sulfur-containing organic matter will produce hydrogen sulfide (H2S). Naturally occurring sulfate-reducing bacteria (SRB) in the anaerobic system will use organic substrates as electron donors to reduce sulfates in food additives to H2S. H2S is not only highly corrosive, damaging reactors, pipelines, and gas treatment equipment, increasing operating and maintenance costs, but it also releases foul odors, pollutes the environment, and is highly toxic to the core microorganisms of the anaerobic digestion system—methanogens—inhibiting methane production efficiency and even causing system instability.

[0004] To address the H2S pollution problem in the anaerobic digestion of kitchen waste, several technical methods have been researched and applied, mainly including: (1) Iron salt precipitation method: by adding iron salts such as FeCl3 and FeSO4, the Fe2S is precipitated. 2+ / Fe 3+ H2S is removed by reacting with H2S to form insoluble sulfide precipitates. This method has the problems of large iron salt dosage and high cost, and the introduced chloride or sulfate ions will increase the salinity of the system, which may cause secondary inhibition of the methanogenesis process. (2) Micro-oxygen desulfurization method: By injecting a small amount of oxygen into the system, aerobic or facultative anaerobic microorganisms are used to oxidize H2S into elemental sulfur or sulfate. The key to this method is the precise control of the amount of oxygen added: insufficient addition will result in limited desulfurization effect; excessive addition will destroy the anaerobic environment, inhibit methanogenic bacteria, and may form an explosive mixture of methane and oxygen, bringing safety risks; at the same time, excessive oxygen will also promote the proliferation of facultative bacteria, consume organic substrates, and reduce methane yield. (3) Nitrate addition method: by adding nitrate, denitrifying bacteria (NRB) and sulfate-reducing bacteria (SRB) compete for electron donors, thereby inhibiting SRB activity and reducing H2S generation. However, nitrates and their reduction intermediates (such as nitrites and NO) are highly toxic to methanogens and can easily cause the accumulation of volatile fatty acids, leading to system acidification or even collapse.

[0005] In general, existing H2S control technologies are mostly "single-function oriented" strategies. Iron salt methods focus on chemical precipitation, micro-oxygen methods rely on oxidation removal, and nitrate methods focus on competitive inhibition of microorganisms. They lack synergistic integration from source inhibition and process purification to system efficiency enhancement. Conventional approaches in this field are often limited to optimizing single materials or methods. For example, when using nitrates, only the dosing method is adjusted, without considering slow release and micro-area control through structural design; when using iron-based materials, only their precipitation and desulfurization functions are emphasized, without integrating their potential to promote microbial symbiosis through conductivity; and when utilizing carbon materials, they are mostly used as adsorbents or microbial carriers, failing to combine the dual objectives of biogas residue resource utilization and electron transfer enhancement. Summary of the Invention

[0006] In view of this, the present invention provides a slow-release electron acceptor / conductive mediator core-shell microsphere, its preparation method, and its application in the anaerobic digestion of food waste. The present invention provides a multifunctional composite system integrating "slow-release electron acceptor regulation + conductive adsorption synergy + microbial metabolic guidance." Through the structural design and synergistic effect of functionalized core-shell microspheres, it achieves synergistic effects of hydrogen sulfide inhibition, methane production enhancement, and resource recycling, ensuring the stable operation of the anaerobic digestion system.

[0007] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the present invention provides a slow-release electron acceptor / conductive mediator core-shell microsphere, wherein the core layer framework material of the core-shell microsphere is a polymer, and nitrate electron acceptors are embedded in the framework material; The shell is prepared by mixing modified hydrothermal carbon powder loaded with magnetic nano-zero valent iron (nZVI) with a porous film-forming agent.

[0008] Furthermore, the polymer is selected from sodium alginate, polyvinyl alcohol, or chitosan.

[0009] Furthermore, the nitrate electron acceptor is selected from calcium nitrate (Ca(NO3)2) or sodium nitrate (NaNO3).

[0010] Furthermore, the embedding amount of nitrate electron acceptors accounts for 30%-50% of the total mass of the core layer. This embedding amount is quantitatively determined by the differential weighing method (precisely weighing the difference between the total mass of the dried core layer and the dried mass of the polymer). The actual embedding amount deviates from the theoretical design value by ≤ 3%. This embedding amount allows for the continuous and slow release of nitrates during the 20-30 day anaerobic digestion cycle, with the nitrate nitrogen concentration in the digestate always remaining below 10 mg / L, avoiding toxic inhibition of methanogens, and simultaneously meeting the electron donor competition requirements of denitrifying bacteria (NRB) and sulfate-reducing bacteria (SRB).

[0011] Furthermore, the specific surface area of ​​the modified hydrothermal carbon powder can reach 200-600 m². 2 / g.

[0012] Furthermore, magnetic nano-zero valent iron was loaded onto the surface of modified hydrothermal carbon at a mass ratio of 5%-10%.

[0013] Furthermore, the porous film-forming agent is selected from chitosan or polyvinyl alcohol.

[0014] Furthermore, the thickness of the shell is 0.3-0.8 mm.

[0015] The core layer, as the center of a slow-release electron acceptor, functions primarily to achieve the slow, controlled release of electron acceptors, avoiding the toxic inhibitory effect of instantaneous high concentrations on methanogens. The core layer is prepared using biodegradable polymers as the backbone material, preferably biocompatible and biodegradable materials such as sodium alginate, polyvinyl alcohol, or chitosan. High concentrations of nitrate-based electron acceptors are embedded within the backbone material. The biodegradability of the backbone material ensures that the core layer gradually degrades during the anaerobic digestion cycle, providing a channel for the continuous release of nitrates while avoiding secondary contamination caused by microsphere residues.

[0016] The shell layer, as a conductive adsorption layer, is innovative in that it achieves a precise match of three functions: In existing technologies, modified hydrothermal carbon is only used for adsorption or resource recovery, and nano-zero-valent iron is only used for chemical desulfurization. The two have never formed a "supported composite substrate". Furthermore, this invention is the first to discover that the conductivity (conductivity > 500 μS / cm) of "magnetic nano-zero-valent iron supported modified hydrothermal carbon" can be precisely matched with the electron transfer requirements of acid-producing bacteria and methanogens. At the same time, its porous structure can regulate the nitrate release rate of the core layer (making the nitrate nitrogen in the digestion liquid < 10 mg / L). This functional coupling of "conductivity-slow release regulation-chemical desulfurization" is something that those skilled in the art cannot derive through conventional experiments.

[0017] The shell layer uses modified hydrothermal carbon as the base material, preferably prepared from kitchen waste sludge through hydrothermal carbonization and activation treatment (achieving "waste treatment with waste," overcoming the limitation of existing technologies that rely on externally purchased conductive materials). After activation with KOH, the specific surface area of ​​the hydrothermal carbon can reach 200-600 m². 2 The modified hydrothermal carbon (H₂S) exhibits excellent electrical conductivity and adsorption properties. Magnetic nano-zero-valent iron is loaded onto the surface of the modified hydrothermal carbon at a mass ratio of 5%-10%, achieving in-situ chemical fixation of H₂S through the reducing and chemical activity of the nano-zero-valent iron. A porous film-forming agent binds the modified hydrothermal carbon powder loaded with nano-zero-valent iron to form a porous shell, ensuring the shell's permeability and mass transfer performance, while simultaneously enabling precise control of the nitrate release rate from the core layer. The shell thickness is controlled at 0.3-0.8 mm, simultaneously meeting the triple requirements of "nitrate slow release rate," "electron transfer efficiency," and "H₂S adsorption capacity."

[0018] In a second aspect, the present invention provides a method for preparing a sustained-release electron acceptor / conductive mediator core-shell microsphere as described in the first aspect, comprising the following steps: (1) Dissolve the polymer in water and stir until a uniform polymer solution is formed; add nitrate to the polymer solution and stir to obtain the core layer precursor solution; drop the core layer precursor solution into the crosslinking curing solution and stir continuously during the drop process to solidify it into spherical gel beads; let it stand and after curing, the core layer gel beads are obtained. (2) The kitchen waste sludge is dried, crushed and then hydrothermally carbonized to obtain hydrothermal carbon; the hydrothermal carbon is activated with an activator to obtain modified hydrothermal carbon; the modified hydrothermal carbon is mixed with nano-zero valent iron precursor, a reducing agent is added and reacted, and after the reaction is completed, modified hydrothermal carbon powder loaded with magnetic nano-zero valent iron is obtained. (3) Add the modified hydrothermal carbon powder loaded with magnetic nano-zero valent iron prepared in step (2) to the porous film-forming agent solution, stir evenly to obtain the coating slurry, and then immerse the core layer gel beads prepared in step (1) into the coating slurry to obtain core-shell microspheres based on sustained-release electron acceptor / conductive mediator.

[0019] Furthermore, in step (1), the ratio of polymer to water is 1:20-40 g / mL; the water temperature is 60-80℃.

[0020] Furthermore, in step (1), the dripping rate of the nucleus layer precursor solution is 1-3 mL / min.

[0021] Furthermore, in step (1), the crosslinking curing liquid is a calcium chloride solution with a mass fraction of 1-3%.

[0022] Furthermore, in step (1), the mixture is continuously stirred during the dropwise addition process at a stirring rate of 100-200 rpm.

[0023] Furthermore, in step (1), the settling time is 2-4 hours.

[0024] Furthermore, in step (1), after curing is completed, a water washing process is also included, with 3-5 water washings.

[0025] Furthermore, in step (2), the hydrothermal carbonization temperature is 200-240℃ and the time is 3-5h.

[0026] Furthermore, in step (2), after hydrothermal carbonization, there are also cooling, washing and drying steps to obtain hydrothermal carbon.

[0027] Furthermore, in step (2), the activator is KOH. The mass ratio of hydrothermal char to activator is 1:2-1:4; the activation temperature is 700-900℃; and the activation time is 1-2h.

[0028] Furthermore, in step (2), the activation process also includes water washing and drying steps; water washing until neutral.

[0029] Furthermore, in step (2), the nano-zero-valent iron precursor is FeSO4.

[0030] Furthermore, in step (2), the reducing agent is NaBH4.

[0031] Furthermore, in step (2), after the reaction is completed, washing, drying and grinding steps are also included.

[0032] Furthermore, in step (3), the porous film-forming agent solution is a dilute acetic acid solution of the porous film-forming agent; the concentration of the film-forming agent is 2-5%.

[0033] Furthermore, in step (3), the modified hydrothermal carbon powder loaded with magnetic nano-zero valent iron is 10-20% of the mass of the porous film-forming agent solution.

[0034] Furthermore, in step (3), the soaking time is 5-25 min; preferably 10-20 min; and a drying step is also included after soaking.

[0035] Thirdly, the present invention provides the application of the core-shell microspheres described in the first aspect or the core-shell microspheres prepared by the preparation method described in the second aspect in the anaerobic digestion of food waste.

[0036] This invention constructs a triple-action mechanism of "source inhibition - in-situ fixation - enhanced methanogenesis" through the synergistic effect of the core and shell layers of core-shell microspheres, far exceeding the "single mechanism" of existing technologies. This is specifically reflected in the following three groundbreaking innovations, and the synergistic effect between these mechanisms cannot be predicted by existing technologies: (1) Spatial hindrance and slow-release competition mechanism (source inhibition): In the prior art, nitrate slow release is only used for agricultural fertilization or water treatment denitrification, and has never been applied to anaerobic digestion sulfur control; the present invention is the first to combine "nitrate slow release" with "shell microenvironment construction", which allows nitrate to slowly seep to the shell surface through the spatial hindrance of the shell, forming a "local hypoxic micro-region" - this micro-region can selectively enrich NRB rather than methanogens, solving the toxicity problem of "nitrate directly contacting methanogens" in the prior art. This combination design of "slow-release carrier + microenvironment regulation" breaks the cognitive inertia of those skilled in the art that "nitrate addition must inhibit methanogenesis", which is a non-obvious breakthrough in thinking.

[0037] The porous structure of the shell and the resistance to mass transfer create a steric hindrance effect, effectively limiting the rapid release of nitrate from the core layer. This allows nitrate to slowly seep to the shell surface in a "release-on-demand" manner. This slow-release effect keeps the nitrate concentration in the digestive fluid consistently below 10 mg / L, avoiding toxic inhibition of methanogenic bacteria. Simultaneously, the slowly released nitrate creates a localized hypoxic microenvironment on the shell surface, inducing NRB to accumulate and form a biofilm. Denitrifying bacteria use nitrate as an electron acceptor and thermodynamically preferentially compete with SRB for electron donors such as H2 and acetic acid in the system, thereby inhibiting the metabolic activity of SRB at its source and reducing H2S production.

[0038] (2) In-situ chemical fixation mechanism (process purification): For small amounts of H2S generated by breakthrough source inhibition, the magnetic nano-zero-valent iron loaded in the shell can undergo a rapid chemical reaction with it, precipitating H2S in situ into insoluble sulfides such as FeS and FeS2. These sulfides can be adsorbed into the porous structure of the shell, preventing them from diffusing into the digestion liquid or gas phase. Simultaneously, the nano-zero-valent iron releases Fe during the reaction process. 2+ As a trace element essential for microbial growth, it can promote the metabolic activity of acid-producing and methanogenic bacteria, achieving a synergistic effect of "sulfur removal" and "bacterial promotion".

[0039] (3) Conductive mediator-enhanced DIET mechanism (methanogenesis enhancement): This is the core innovation of this invention. In the prior art, when conductive materials are used to enhance anaerobic digestion and methanogenesis, they need to be added separately and are prone to aggregation and failure. This invention integrates conductive materials into the shell of core-shell microspheres for the first time, and solves the problem of conductive material aggregation by utilizing the immobilization effect of the shell. More importantly, it discovers that "the conductivity of the shell can synergize with the nitrate slow-release effect" - the DIET process enhanced by conductive mediators can not only counteract the slight metabolic interference of nitrate denitrification, but also accelerate the conversion of recalcitrant substrates such as propionic acid, thereby increasing the methane yield by 15%-20%. This reverse synergistic effect of "sulfur inhibition and methanogenesis enhancement" completely exceeds the conventional expectations of those skilled in the art (in the prior art, sulfur inhibition technology is generally accompanied by a decrease in methane yield), and is an unexpected technical effect.

[0040] The modified hydrothermal carbon in the shell layer possesses excellent electrical conductivity (>500 μS / cm), serving as a conductive mediator to construct an "electronic bridge" between acid-producing and methanogenic bacteria, significantly enhancing the DIET process. This enhanced DIET mechanism accelerates the conversion efficiency of recalcitrant volatile fatty acids such as propionic acid and butyric acid, preventing the risk of rancidity due to their accumulation within the system. Simultaneously, this mechanism counteracts the slight metabolic interference that may arise from nitrate denitrification, ensuring that methane yield is not only unaffected but significantly increased, achieving the goal of "suppressing sulfur without suppressing gas."

[0041] Fourthly, the present invention provides a method for anaerobic digestion of kitchen waste, the method comprising using the core-shell microspheres described in the first aspect.

[0042] Further, the method specifically involves: after the food waste is crushed and homogenized, its total solids (TS) content is adjusted to 10%-15%, and the core-shell microspheres described in the first aspect are added. The operating parameters for anaerobic digestion are controlled as follows: temperature 35-38℃ (mesophilic digestion) or 50-55℃ (hyperthermic digestion), pH value 7.0-7.5, and hydraulic retention time (HRT) 20-30 days.

[0043] Furthermore, the dosage of core-shell microspheres is 3-8 g / L of the digested raw material volume; the core-shell microspheres are added all at once during startup or added in 1-2 supplementary doses according to the H2S concentration monitoring results.

[0044] Compared with the prior art, the present invention has achieved the following beneficial effects: (1) This invention achieves efficient in-situ control of H2S through the triple action mechanism of core-shell microspheres. The H2S concentration in the gas phase can be stably controlled below 50ppm, and the removal rate can reach more than 95%. At the same time, the DIET mechanism enhanced by the conductive shell significantly improves the methane yield. Compared with the control group without microspheres, the methane yield is increased by 15%-20%, truly achieving the precise control target of "suppressing sulfur without suppressing gas".

[0045] (2) The slow-release structure of the core layer and the biodegradable skeleton in the core-shell microspheres provided by the present invention ensure that nitrate is continuously and stably released throughout the entire anaerobic digestion cycle (20-30 days). A single addition can meet the H2S control requirements of the entire cycle, avoiding the drawbacks of the traditional method that requires frequent addition and reducing operation and maintenance costs.

[0046] (3) The shell material of the core-shell microspheres provided by the present invention is modified hydrothermal carbon prepared from kitchen waste sludge, which realizes the resource recycling of "using waste to treat waste" and reduces the preparation cost of microspheres; at the same time, the core layer is made of biodegradable polymer, and the entire microsphere can be recycled along with the sludge after digestion, without the risk of secondary pollution.

[0047] (4) The technical solution of the present invention does not require complex equipment modification. The core-shell microspheres can be directly added to the existing anaerobic digestion device. The slow release of nitrates avoids the impact of high concentration toxicity, and there are no safety hazards such as explosion or corrosion. The operation is simple and easy to carry out, and it is suitable for large-scale promotion and application. Attached Figure Description

[0048] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0049] Figure 1 This is a flowchart illustrating the preparation process of the core-shell microspheres based on the sustained-release electron acceptor / conductive mediator of this invention. Figure 2 This is a schematic diagram of the structure of the core-shell microspheres based on the sustained-release electron acceptor / conductive mediator of the present invention; Figure 3 This is a schematic diagram of the mechanism by which the present invention achieves H2S resistance control based on core-shell microspheres of sustained-release electron acceptor / conductive mediator. Detailed Implementation

[0050] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0051] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0052] Example 1: Preparation of core-shell microspheres The fabrication process flow diagram of core-shell microspheres based on sustained-release electron acceptor / conductive mediator is shown below. Figure 1 The details are as follows: Core layer preparation: 5g of sodium alginate (weighed after drying at 105℃ for 12h, the dried mass is 4.8g) was dissolved in 100mL of 70℃ hot water and stirred for 30min until completely dissolved to obtain a sodium alginate solution; 3.33g of calcium nitrate (theoretical encapsulation amount = 3.33g / (5g+3.33g)×100%≈40%) was added to the sodium alginate solution, and stirring was continued for 45min until the calcium nitrate was completely dissolved to form the core layer precursor solution; the core layer precursor solution was added dropwise to a 2% CaCl2 solution at a rate of 2mL / min using a peristaltic pump, and the stirring rate was controlled at 150rpm. After the droplets formed spherical gel beads, they were allowed to stand in the CaCl2 solution for 3h to complete cross-linking; the gel beads were removed, rinsed 4 times with deionized water, drained, and vacuum dried at 60℃ for 8h. The total mass of the dried core layer was weighed as 7.9g (after deducting the small amount of Ca adsorbed during the cross-linking process). 2+ Including trace evaporation loss, the actual effective core layer mass is calculated as 7.9g. The actual embedding amount is calculated by differential weight method as (7.9g-4.8g) / 7.9g×100%=39.2±1.1%, which deviates from the theoretical design value by ≤1%, which is within the design range of 30%-50%, and core layer gel beads with a diameter of 2-3 mm are obtained.

[0053] Preparation of shell material: Kitchen waste biogas residue was dried at 105℃ for 12 hours and then pulverized to 100 mesh. 20g of the pulverized residue was placed in a hydrothermal reactor, and 80mL of deionized water was added. The mixture was hydrothermally carbonized at 220℃ for 4 hours. After cooling, the carbonized product was removed, washed with deionized water until neutral, and dried at 105℃ for 8 hours to obtain hydrothermal carbon. The hydrothermal carbon was mixed with KOH at a mass ratio of 1:3 and activated at 800℃ for 1.5 hours. After cooling, it was washed until neutral and dried to obtain modified hydrothermal carbon. 10g of the modified hydrothermal carbon was weighed and mixed with 50mL of 0.5mol / L FeSO4 solution. After stirring for 30 minutes, 20mL of solution was slowly added dropwise. In situ reduction was completed by stirring a 1 mol / L NaBH4 solution for 1 h. After the reaction, the powder was washed with deionized water until neutral, dried under vacuum at 60 °C for 6 h, and ground to 200 mesh to obtain magnetic nano-zero valent iron-loaded modified hydrothermal carbon powder (nano-zero valent iron loading was 5%).

[0054] Shell coating: Dissolve 3g of chitosan in 100mL of 1% dilute acetic acid solution and stir for 2 hours until completely dissolved to obtain a 3% chitosan solution; add 15g of the above shell material powder to the chitosan solution and stir for 1 hour to form a uniform coating slurry; immerse the core-shell gel beads in the coating slurry, soak for 15 minutes, then remove and air dry in a ventilated place for 24 hours to form a conductive black shell with a thickness of about 0.5mm, thus obtaining core-shell microspheres. A schematic diagram of the structure of core-shell microspheres based on a sustained-release electron acceptor / conductive mediator is shown below. Figure 2 .

[0055] Example 2: Application effect of core-shell microspheres in anaerobic digestion of food waste Experimental setup: A 5L anaerobic digestion reactor with an effective volume of 4L was used, equipped with a temperature control system, a stirring device, and a gas collection device.

[0056] Experimental materials: Kitchen waste was taken from a local canteen, and after crushing and homogenization, the total sulfide (TS) content was adjusted to 12%. Its main components were: carbohydrates 45%, protein 28%, fat 18%, and ash 9%. Inoculated sludge was taken from the anaerobic digester of a municipal wastewater treatment plant, and the volume ratio of inoculated sludge to raw materials was 1:1.

[0057] Experimental grouping: An experimental group and a control group were set up, with 3 replicates in each group. The experimental group was given the core-shell microspheres prepared in Example 1 at a dose of 5 g / L, while the control group was given no desulfurizing agent. All other conditions were kept the same.

[0058] Operating parameters: mesophilic digestion, temperature controlled at 37±0.5℃, stirring 3 times a day for 10 minutes each time, pH value naturally maintained (7.2-7.4), hydraulic retention time 30 days.

[0059] Monitoring indicators and results: The concentration of H2S and the volume fraction of methane in the gas phase were monitored daily, and the concentration of VFA and nitrate nitrogen in the digestate were monitored every 3 days.

[0060] Experimental Results: In the control group, H2S peaked between days 5 and 15 of digestion, with the highest H2S concentration in the gas phase reaching 2500 ppm and the lowest methane volume fraction dropping to 52%. VFA accumulation occurred on day 8, reaching a concentration of 3800 mg / L. In the experimental group, the H2S concentration in the gas phase remained below 50 ppm throughout the entire digestion cycle, with an H2S removal rate of 98.2%. The nitrate nitrogen concentration in the digestate remained below 8 mg / L, with no VFA accumulation and the concentration maintained below 500 mg / L. The methane volume fraction remained stable at 65%-68%, and the average methane yield increased by 17.6% compared to the control group.

[0061] Example 3: The Influence of Different Shell Materials on Resistance Control Effect Using the same core layer preparation process as in Example 1, three types of core-shell microspheres with different shell layers were prepared: modified hydrothermal carbon without nano-zero ferric iron (Sample A), modified hydrothermal carbon loaded with 10% nano-zero ferric iron (Sample B), and ordinary activated carbon loaded with nano-zero ferric iron (Sample C). These microspheres were added to the anaerobic digestion system of food waste at a dosage of 5 g / L, with all other conditions consistent with Example 2. The experimental results are shown in Table 1. Table 1. Influence of different shell materials on resistance control effect

[0062] As shown in Table 1, modified hydrothermal carbon loaded with nano-zero valent iron has the best sulfur control effect and methanogenesis enhancement effect when used as shell material, and the increase of nano-zero valent iron loading can further enhance the sulfur control effect. Compared with ordinary activated carbon, modified hydrothermal carbon prepared from kitchen waste sludge has a better synergistic effect due to its excellent conductivity and adsorption properties.

[0063] Experimental results: Sample A achieved an H2S removal rate of 72% and a methane yield increase of 8%; Sample B achieved an H2S removal rate of 99.1% and a methane yield increase of 19.3%; Sample C achieved an H2S removal rate of 85% and a methane yield increase of 10%. The results indicate that modified hydrothermal carbon loaded with nano-zero valent iron, when used as a shell material, exhibits the best sulfur control and methane production enhancement effects, and increasing the nano-zero valent iron loading can further enhance the sulfur control effect.

[0064] Example 4: Effect of different core framework materials on sustained-release effect Using the same core layer preparation process as in Example 1, the core layer framework material was replaced with polyvinyl alcohol (sample D) and chitosan (sample E), respectively. Sodium nitrate (40% encapsulation) was used as the electron acceptor in both samples. The shell material was uniformly the magnetic nano-zero-valent iron-supported modified hydrothermal carbon from Example 1. Two types of core-shell microspheres were prepared and added to the anaerobic digestion system of food waste at a dosage of 5 g / L. All other conditions were the same as in Example 2. The experimental results are shown in Table 2. Table 2. Effects of different core layer framework materials on sustained-release effect

[0065] As shown in Table 2, all three biodegradable framework materials can achieve stable and sustained release of nitrates, with a release cycle (28-32 days) matching the anaerobic digestion cycle. The average release rate is controlled at 0.29-0.32 mg / (L·d), ensuring that the nitrate nitrogen concentration in the digestate remains below 10 mg / L. The H2S removal rate of different framework materials all exceed 96%, and the methane yield is increased by more than 15%, demonstrating that the core framework materials have good versatility and adaptability.

[0066] Experimental results: Sample D (polyvinyl alcohol backbone) had a nitrate nitrogen release period of 28 days, an average release rate of 0.32 mg / (L·d), an H2S removal rate of 97.5%, and a methane yield increase of 16.8%; Sample E (chitosan backbone) had a nitrate nitrogen release period of 32 days, an average release rate of 0.29 mg / (L·d), an H2S removal rate of 96.9%, and a methane yield increase of 15.3%; Sample 1 (sodium alginate backbone) had a release period of 30 days and an average release rate of 0.30 mg / (L·d). The results indicate that all three biodegradable backbones can achieve stable sustained release, and the release period and rate meet the requirements of anaerobic digestion, demonstrating the good versatility of the core-layer backbone material.

[0067] Example 5: The effect of different shell thicknesses on synergistic effect Using the core layer preparation process of Example 1, three core-shell microspheres with different shell thicknesses were prepared by adjusting the soaking time (5 min, 15 min, 25 min) during shell coating: 0.3 mm (sample F), 0.5 mm (sample of Example 1, control group), and 0.8 mm (sample G). The shell material and core layer parameters were kept consistent and added to the experimental system of Example 2 at a dose of 5 g / L.

[0068] Experimental results: Sample F (0.3 mm shell) had a peak nitrate nitrogen concentration of 12.3 mg / L, an H2S removal rate of 92.0%, and a methane yield increase of 12.5% ​​(due to slight inhibition caused by slightly higher local nitrate concentrations); Sample G (0.8 mm shell) had a nitrate nitrogen concentration consistently below 6 mg / L, an H2S removal rate of 99.5%, but a methane yield increase of only 14.2% (due to the excessively thick shell affecting electron transfer efficiency); The control group (0.5 mm shell) showed the best overall performance, proving that a shell thickness of 0.3-0.8 mm is a reasonable range, and 0.5 mm is the optimal parameter for balancing sulfur control and efficiency improvement.

[0069] Example 6: Effect of different dosages on the control effect The core-shell microspheres prepared in Example 1 were used to set up three dosage groups: 3 g / L (sample H), 5 g / L (sample from Example 1, control group), and 8 g / L (sample I). These were added to the experimental system of Example 2, with all other conditions kept consistent. The H2S removal rate, methane yield, and operating cost were the key monitoring parameters. The experimental results are shown in Table 3. Table 3. Effects of different dosages on the control effect

[0070] As shown in Table 3, dosages of 3-8 g / L can effectively control H2S. Among them, 5 g / L is the optimal dosage that balances cost and effect and can meet the needs of most kitchen waste anaerobic digestion. The dosage of 3 g / L is suitable for the treatment of kitchen waste with low sulfur content, while the dosage of 8 g / L can be used for the enhanced treatment of kitchen waste with high sulfur content, which reflects the flexibility and adaptability of the present invention.

[0071] Experimental results: The H2S removal rate of sample H (3 g / L) was 90.1%, the methane yield increased by 11.8%, and the unit treatment cost was 0.85 yuan / m³. 3 Biogas; the control group (5 g / L) showed a 98.2% H2S removal rate, a 17.6% increase in methane yield, and a unit treatment cost of 1.42 yuan / m³. 3 Biogas; Sample I (8 g / L) achieved a 99.7% H2S removal rate and an 18.3% increase in methane yield, but the unit treatment cost was 2.27 yuan / m³. 3 Biogas (diminishing marginal returns). Results showed that dosages of 3-8 g / L were effective, with 5 g / L being the optimal dosage balancing cost and effectiveness.

[0072] Example 7: Compatibility verification of high-solids and low-solids raw materials The total sulfide (TS) content of the food waste raw materials was adjusted, and a low solids group (TS=10%, sample J) and a high solids group (TS=15%, sample K) were set up. The core-shell microspheres prepared in Example 1 (5 g / L) were added to both groups. At the same time, a corresponding blank control group (no microspheres were added) was set up. The operating parameters were the same as in Example 2 (mesothermal digestion, HRT=30 days).

[0073] Experimental results: In the low-solids blank group, the peak H2S concentration was 1800 ppm, and the methane yield was 280 mL / g VS. Sample J consistently had an H2S concentration below 45 ppm, with an H2S removal rate of 97.5% and a methane yield of 328 mL / g VS (an improvement of 17.1%). In the high-solids blank group, the peak H2S concentration was 3200 ppm, but due to acid accumulation, the methane yield was only 220 mL / g VS. Sample K consistently had an H2S concentration below 55 ppm, with an H2S removal rate of 98.3%, no acid accumulation, and a methane yield of 258 mL / g VS (an improvement of 17.3%). These results demonstrate that this invention is well-suited for food waste with a TS content of 10%-15%, and is particularly effective in mitigating the risk of rancidity in high-solids systems.

[0074] Example 8: Application effect under high temperature anaerobic digestion conditions Using the raw material and core-shell microsphere addition scheme of Example 2 (5 g / L), the digestion temperature was adjusted to 55 ± 0.5℃ (high temperature digestion). An experimental group and a blank control group were set up, with HRT = 20 days (the high temperature digestion cycle is shorter). The other conditions were the same as in Example 2.

[0075] Experimental results: In the blank control group, the peak H2S concentration was 2100 ppm, the lowest methane volume fraction was 50%, and the peak VFA concentration was 3200 mg / L. In the experimental group, the H2S concentration remained below 52 ppm, the H2S removal rate was 97.5%, the VFA concentration was maintained below 450 mg / L, the methane volume fraction was stable at 63%-66%, and the methane yield was increased by 16.9% compared with the blank group. The results indicate that this invention can still achieve efficient sulfur control and enhanced methane production under high-temperature anaerobic digestion conditions, thus broadening the application scenarios of the technology.

[0076] Example 9: Long-term continuous operation stability verification A 10L continuous flow anaerobic digester with an effective volume of 8L was used. The raw material of food waste had a TS of 12%. Core-shell microspheres were added at a dose of 5g / L in a single operation. The system was operated continuously for 90 days (3 digestion cycles). The operating parameters were: mesophilic temperature 37±0.5℃, daily feed 0.27L (HRT=30 days), and key indicators were monitored daily.

[0077] Experimental results: In the first cycle, the H2S removal rate was 98.2%, and the methane yield increased by 17.6%; in the second cycle, the H2S removal rate was 97.8%, and the methane yield increased by 16.9%; in the third cycle, the H2S removal rate was 96.5%, and the methane yield increased by 15.8%. Throughout all three cycles, the nitrate nitrogen concentration in the digestate remained below 10 mg / L, with no VFA accumulation. The microspheres in the biogas residue gradually degraded without any residual accumulation. These results demonstrate that this invention can maintain stable performance for three digestion cycles with a single addition, demonstrating its feasibility for long-term operation.

[0078] Example 10: Optimization Experiment of Nitrate Electron Acceptor Embedding Amount Experimental objective: To verify the sustained-release effect, toxicity control, and sulfur barrier performance of nitrate encapsulation, and to determine the optimal encapsulation range.

[0079] Experimental materials and methods: (1) Core layer preparation: The same process as in Example 1 was used, only the amount of calcium nitrate added was adjusted, and 5 groups of different theoretical embedding amounts were set up, with 3 replicates for each group: Group 1: Theoretical encapsulation amount 20% (sodium alginate 5g + calcium nitrate 1.25g); Group 2: Theoretical encapsulation amount 30% (sodium alginate 5g + calcium nitrate 2.14g); Group 3: Theoretical encapsulation amount 40% (sodium alginate 5g + calcium nitrate 3.33g, consistent with Example 1); Group 4: Theoretical encapsulation amount 50% (sodium alginate 5g + calcium nitrate 5g); Group 5: Theoretical encapsulation amount 60% (sodium alginate 5g + calcium nitrate 7.5g).

[0080] (2) Shell preparation and coating: All groups used the shell material (modified hydrothermal carbon loaded with 5% magnetic nano zero-valent iron) and coating process of Example 1 to prepare core-shell microspheres (shell thickness 0.5 mm).

[0081] (3) Characterization and performance testing: Encapsulation amount determination: differential gravimetric method (total mass of core layer after drying - mass of polymer after drying); Sustained-release performance: The core-shell microspheres were placed in a simulated anaerobic digestion solution (pH=7.2, containing 0.1mol / L NaCl) and kept at a constant temperature of 37℃. The nitrate nitrogen concentration at different time points was determined by ion chromatography (ICS-600, Thermo Fisher Scientific) and continuously monitored for 35 days. Anaerobic digestion performance: Added to the anaerobic digestion system of food waste at a dose of 5 g / L (same as in Example 2), and monitor H2S removal rate, methane yield and VFA accumulation.

[0082] Experimental results: The actual encapsulation amount, sustained-release performance and digestion effect of each group are shown in Table 4.

[0083] Table 4. Effects of different encapsulation amounts on sustained-release performance and digestion efficiency.

[0084] Table 4 shows that a nitrate electron acceptor encapsulation amount of 30-50% achieved superior H2S removal and methane yield improvement. At an encapsulation amount of 60%, although the H2S removal rate remained high (99%), the methane yield decreased compared to 50%. When the encapsulation amount was below 30%, both the H2S removal rate and methane yield decreased.

[0085] Example 11 Optimization Experiment of Specific Surface Area of ​​Modified Hydrothermal Carbon Experimental objective: To verify the specific surface area (200-600 m²) of modified hydrothermal carbon. 2 The method for regulating H2S adsorption and conductivity was investigated, and the optimal preparation process parameters were determined.

[0086] Experimental Materials and Methods: (1) Preparation of hydrothermal char: Using the same biogas residue raw material as in Example 1 (dried and pulverized to 100 mesh), the hydrothermal carbonization conditions were fixed at 220℃ for 4 hours to prepare basic hydrothermal char (BET specific surface area 65.8 m²). 2 / g).

[0087] (2) Preparation of modified hydrothermal carbon: By adjusting the ratio of activator (KOH to hydrothermal carbon mass ratio) and activation temperature, five groups of modified hydrothermal carbon with different specific surface areas were prepared, with three replicates for each group: Group 1: KOH / hydrothermal carbon = 1:2, activation temperature 700℃; Group 2: KOH / hydrothermal carbon = 1:3, activation temperature 700℃; Group 3: KOH / hydrothermal carbon = 1:3, activation temperature 800℃ (same as in Example 1); Group 4: KOH / hydrothermal carbon = 1:4, activation temperature 800℃; Group 5: KOH / hydrothermal carbon = 1:4, activation temperature 900℃.

[0088] (3) Characterization and performance testing: Specific surface area and pore structure: BET nitrogen adsorption-desorption method (ASAP2460), test liquid nitrogen temperature -196℃, degassing 105℃ / 3h; H2S adsorption capacity: Static adsorption experiment (25℃, initial H2S concentration 5000ppm, adsorption time 24h). Conductivity: Four-probe method (RTS-9 four-probe tester). Practical application performance: The modified hydrothermal carbon of each group was loaded with 5% nano-zero valent iron according to the process of Example 1 to prepare core-shell microspheres (5g / L addition). In the anaerobic digestion system of kitchen waste (same as Example 2), the H2S removal rate was monitored.

[0089] Experimental results: The structural parameters and properties of each group of modified hydrothermal carbon are shown in Table 5. Table 5. Effects of each group of modified hydrothermal carbon on resistance control effect

[0090] As can be seen from Table 5, the specific surface area of ​​the modified hydrothermal carbon is 200-600 m². 2 / g (Groups 2-5) showed good H2S control effect, all above 95%.

[0091] The above description is merely a preferred embodiment of the present invention and is not intended to limit the 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 slow release electron acceptor / conductive mediator core-shell microsphere, characterized in that, The core layer skeleton material of the core-shell microsphere is a high molecular polymer, and a nitrate salt type electron acceptor is embedded in the skeleton material; The shell layer is prepared by mixing modified hydrothermal carbon powder loaded with magnetic nano zero-valent iron and a porous film-forming agent; The thickness of the shell layer is 0.3-0.8 mm.

2. The core-shell microspheres of claim 1, wherein, The high molecular polymer is selected from sodium alginate, polyvinyl alcohol or chitosan; And / or, the nitrate salt type electron acceptor is selected from calcium nitrate or sodium nitrate; And / or, the embedding amount of the nitrate salt type electron acceptor accounts for 30%-50% of the total mass of the core layer; And / or, the specific surface area of the modified hydrochar powder can be up to 200-600 m 2 / g; And / or, the magnetic nano zero-valent iron is loaded on the surface of the modified hydrothermal carbon at a mass ratio of 5%-10%; And / or, the porous film-forming agent is selected from chitosan or polyvinyl alcohol.

3. The method for preparing sustained-release electron acceptor / conductive mediator core-shell microspheres as described in claim 1, characterized in that, The method comprises the following steps: (1) Dissolve the high molecular polymer in water and stir until a uniform polymer solution is formed; add a nitrate salt to the polymer solution and stir to obtain a core layer precursor solution; drop the core layer precursor solution into a crosslinking and solidification solution, continuously stir during the dropping process, and allow the solution to solidify to form spherical gel beads; allow the solution to stand, and after solidification is completed, obtain core layer gel beads; (2) Dry and crush kitchen waste sludge, and then perform hydrothermal carbonization to obtain hydrothermal carbon; activate the hydrothermal carbon using an activating agent to obtain modified hydrothermal carbon; Mix the modified hydrothermal carbon with a nano zero-valent iron precursor, add a reducing agent to react, and after the reaction is completed, obtain modified hydrothermal carbon powder loaded with magnetic nano zero-valent iron; (3) Add the modified hydrothermal carbon powder loaded with magnetic nano zero-valent iron prepared in step (2) to a porous film-forming agent solution, stir until uniform to obtain a coating slurry, and then immerse the core layer gel beads prepared in step (1) in the coating slurry to obtain a slow-release type electron acceptor / conductive mediator core-shell microsphere.

4. The production method according to claim 3, wherein In step (1), the mass ratio of the high molecular polymer to water is 1:20-40 g / mL; and the water temperature is 60-80°C; And / or, in step (1), the dropping rate of the core layer precursor solution is 1-3 mL / min; And / or, in step (1), the crosslinking and solidification solution is a calcium chloride solution, and the mass fraction of the calcium chloride is 1-3%; And / or, in step (1), continuously stir during the dropping process, and the stirring rate is 100-200 rpm; And / or, in step (1), the standing time is 2-4 h; And / or, in step (1), after solidification is completed, the process further comprises a water washing process, and the water washing is performed 3-5 times.

5. The production method according to claim 3, wherein In step (2), the hydrothermal carbonization temperature is 200-240°C, and the time is 3-5 h; And / or, in step (2), the activating agent is KOH; the mass ratio of the hydrothermal carbon to the activating agent is 1:2-1:4; the activation temperature is 700-900°C, and the activation time is 1-2 h; And / or, in step (2), after activation, the process further comprises a water washing and drying step; And / or, in step (2), the nano zero-valent iron precursor is FeSO4; And / or, in step (2), the reducing agent is NaBH4; And / or, in step (2), after the reaction is completed, the process further comprises a washing, drying and grinding step.

6. The production method according to claim 3, wherein In step (3), the porous film-forming agent solution is a dilute acetic acid solution of the porous film-forming agent; and the concentration of the film-forming agent is 2-5%. And / or, in step (3), the modified hydrothermal carbon powder loaded with magnetic nano zero-valent iron is 10-20% of the mass of the porous film-forming agent solution; And / or, in step (3), the soaking time is 5-25 min; preferably 10-20 min; and a drying step is further included after soaking.

7. The use of the core-shell microspheres of claim 1 or the core-shell microspheres prepared by the preparation method of any one of claims 3-6 in anaerobic digestion of kitchen waste.

8. A method for anaerobic digestion of kitchen waste, characterized by, The method comprises the use of the core-shell microspheres of any one of claims 1-2.

9. The kitchen waste anaerobic digestion method according to claim 8, wherein, The method specifically comprises: after the kitchen waste is crushed and homogenized, the total solid content is adjusted to 10%-15%, the core-shell microspheres of any one of claims 1-2 are added, and the operation parameters of anaerobic digestion are controlled as follows: temperature 35-38 ℃ or 50-55 ℃, pH value 7.0-7.5, and hydraulic retention time 20-30 days.

10. The kitchen waste anaerobic digestion method according to claim 9, wherein, The dosage of the core-shell microspheres is 3-8 g / L of the volume of the raw material for digestion; and the core-shell microspheres are added at one time during start-up or are supplemented and added in 1-2 times according to the monitoring results of H2S concentration.

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