Method for regulating iron-carbon quantum dots pH value to enhance methanogenic performance of anaerobic digestion

Abstract

The application discloses a method for regulating and controlling the pH value of iron-carbon quantum dots to strengthen the methanogenic performance of anaerobic digestion, and the method is characterized in that the pH value of Fe-CD suspension is regulated and controlled, the dissociation state of the surface functional groups of Fe-CD is accurately adjusted, the activity of the surface functional groups of Fe-CD is optimized, the surface charge and the redox activity of Fe-CD are adjusted, the interface interaction between Fe-CD and anaerobic microorganisms, substrates and intermediate metabolites is improved, and then the reaction efficiency and the comprehensive methanogenic performance of the anaerobic digestion system are strengthened. The experimental results show that the method can improve the stability and the methanogenic activity of the reaction system, not only promotes the COD degradation and the transformation of volatile fatty acids, but also makes most of the accumulated VFA be used by methanogens and be converged into the methanogenic metabolic pathway, so that the methanogenic rate and the cumulative methane production are significantly improved.

Description

Technical Field

[0001] This invention belongs to the field of application technology of functional carbon materials to enhance the performance of anaerobic digestion and promote methanogenesis, specifically involving a method for regulating the pH value of iron-carbon quantum dots to enhance the performance of anaerobic digestion and promote methanogenesis. Background Technology

[0002] Municipal sludge is a significant byproduct of wastewater treatment. Its complex composition includes not only abundant organic matter but also harmful substances such as pathogenic microorganisms, parasite eggs, heavy metals, and organic pollutants. Improper disposal can easily lead to environmental pollution and resource waste. Anaerobic digestion technology can degrade the organic components in sludge through microbial degradation, producing methane and achieving resource recycling. However, traditional anaerobic digestion processes suffer from problems such as slow hydrolysis rates, insufficient transition between the acidification and methanogenesis stages, easy accumulation of volatile fatty acids, and insufficient system stability. In recent years, adding functional materials to anaerobic digestion systems to improve electron transfer processes has become an important method for enhancing anaerobic digestion. Iron-based materials have good conductivity and electron transfer capabilities, which can promote interspecies electron transfer among microorganisms. However, traditional iron-based conductive materials, such as zero-valent iron, Fe2O3, Fe3O4, and FeS2, may have problems in practical applications, such as easy aggregation, easy passivation, poor dispersibility, large particle size, high mass transfer resistance, and potential biotoxicity, which limits their enhancing effect in anaerobic digestion systems. Summary of the Invention

[0003] To address the above problems, this invention proposes a method for enhancing anaerobic digestion and methanogenesis using iron-carbon quantum dots (Fe-CD). By preparing iron-carbon quantum dots and adjusting their pH to 1-7, these quantum dots can be used as bioconductive materials to promote interspecies electron transfer and methane generation among microorganisms in the anaerobic digestion system. A method for enhancing the methanogenic performance of anaerobic digestion by regulating the pH value of iron-carbon quantum dots involves adding iron-carbon quantum dots to an anaerobic digestion system, wherein the pH value of the added iron-carbon quantum dots is 1-7, preferably 1 or 7, and more preferably 7. Preferably, the Fe-CD dosage concentration is 0.1 g / L - 10 g / L, more preferably 5 g / L, based on the working volume of the anaerobic digestion system. Preferably, the anaerobic digestion temperature is 35-45 °C; the anaerobic digestion time is 7-14 days; and the initial pH of the anaerobic digestion system is 6-9. More preferably, the anaerobic digestion temperature is 40 °C, the reaction cycle is 10 days, and the initial pH of the anaerobic digestion system is 7.5. Preferably, the preparation method of the iron-carbon quantum dots is as follows: Fe-CD is prepared by hydrothermal reaction of citric acid and ferric chloride, wherein the molar ratio of citric acid to ferric chloride is 1:1-10:1, the hydrothermal reaction temperature is 200-230 °C, and the hydrothermal reaction time is 2-24 h. Specifically, citric acid and ferric chloride are dissolved in deionized water and stirred for 30-60 min to allow iron ions to combine with citric acid; then the stirred precursor solution is transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner and hydrothermally reacted at 230 °C for 12 h, and Fe-CD is obtained after natural cooling. The Fe-CD exhibits good biocompatibility, not only due to iron doping in its composition but also because its surface contains oxygen-containing functional groups, hydroxyl groups, and nitrogen-containing structures. Fe-CD can promote the conversion of volatile fatty acids, increase the rate of methane production, and improve the cumulative methane production. In addition, the present invention also provides the application of iron-carbon quantum dots in sludge treatment. The application of iron-carbon quantum dots in sludge treatment, wherein iron-carbon quantum dots, as bioconductive materials, promote interspecies electron transfer and methane generation among microorganisms in anaerobic digestion systems, wherein the pH of the iron-carbon quantum dots is 1-7, preferably 1 or 7, and more preferably 7. The preparation method of the iron-carbon quantum dots is as follows: Fe-CD is prepared by hydrothermal reaction of citric acid and ferric chloride, wherein the molar ratio of citric acid to ferric chloride is 1:1-10:1, the temperature of the hydrothermal reaction is 200-230 ℃, and the time of the hydrothermal reaction is 2-24 h. Specifically, citric acid and ferric chloride are dissolved in deionized water and stirred for 30-60 min to allow ferric ions to combine with citric acid. Then, the stirred precursor solution is transferred to a stainless steel high-pressure reactor with a polytetrafluoroethylene liner and hydrothermally reacted at 230 °C for 12 h. After natural cooling, Fe-CD is obtained. Based on the characteristics of carbon quantum dots, such as small size, large specific surface area, good dispersibility and rich surface functional groups, iron is introduced into carbon quantum dots to form iron-carbon quantum dots. This invention combines the advantages of carbon materials and iron-containing active sites to improve the electron transfer efficiency of anaerobic digestion systems. By enhancing interspecies electron transfer among acid-producing bacteria, acetic-producing bacteria and methanogenic bacteria, the conversion efficiency of volatile fatty acids to methane is improved, thereby increasing the rate of methane production and the cumulative methane production in anaerobic digestion systems. This invention uses citric acid and ferric chloride as raw materials to prepare a Fe-CD suspension rich in functional groups such as carboxyl and hydroxyl groups via a hydrothermal method. The Fe-CD is then added as a conductive material to the anaerobic environment of municipal sludge. By precisely controlling the pH of the Fe-CD and optimizing the surface functional groups, particle size distribution, and electron transport properties of iron-carbon quantum dots, direct interspecies electron transport (DIET) and hydrogen nutrient interspecies electron transport (HIT) within the anaerobic digestion system are significantly enhanced. This improves the substrate degradation efficiency, methane yield, and system stability of the anaerobic digestion system, achieving enhanced regulation of the anaerobic digestion process. Experimental results show that this method can improve system stability, promote COD degradation and volatile fatty acid conversion, increase the methanogenesis rate and cumulative methane production. The optimal enhancement effect of Fe-CD at pH 7 is observed, increasing methanogenesis by 27.9%. (1) The present invention uses citric acid and ferric chloride as precursors to prepare Fe-CD through hydrothermal reaction. The preparation process is simple, the raw material cost is low, and the obtained material has small particle size, good water dispersibility and rich surface functional groups. (2) The Fe-CD surface of the present invention contains oxygen-containing functional groups such as carboxyl, hydroxyl, carbonyl and ether bonds, and has Fe-O or Fe-OC related iron active sites, which can build electron transfer channels between anaerobic digestion microbial communities and reduce electron transfer resistance. (3) Compared with traditional granular iron-based materials, the Fe-CD of the present invention has small size and good dispersibility, and is not easy to form large particle agglomeration, which can reduce mass transfer resistance and improve its efficiency in anaerobic digestion system. (4) The method described in this invention can promote the further conversion of volatile fatty acids such as acetic acid and propionic acid while maintaining the stability of the system pH and the continuous reduction of COD, thereby significantly increasing the amount of methane produced. (5) The method described in this invention can significantly improve the methanogenesis rate and cumulative methane production in the anaerobic digestion system. Experimental results show that the Fe-CD-7 group was significantly higher than other groups, with an increase of 27.9% compared to the blank group. Attached Figure Description

[0004] Figure 1 The image shows a transmission electron microscope (TEM) characterization of Fe-CD; where, Figure 1 In the image, 'a' is a high-resolution transmission electron microscope image. Figure 1 In the diagram, b represents the transmission electron microscope image and particle size distribution map. Figure 2 X-ray diffraction patterns of CD and Fe-CD. Figure 3 The image shows the Fourier transform infrared spectra of CD and Fe-CD. Figure 4 High-resolution X-ray photoelectron spectra of CD and Fe-CD; among which, Figure 4 In the diagram, 'a' represents the total spectrum of CD and Fe-CD. Figure 4 The bd spectra in the figure are the C1s, N1s, and O1s spectra of CD and Fe-CD, respectively. Figure 4 In the figure, 'e' represents the Fe2p spectrum of Fe-CD. Figure 5 This is a graph showing the pH change over time during anaerobic digestion in different treatment groups. Figure 6 This is a graph showing the change of COD over time during anaerobic digestion in different treatment groups. Figure 7 The graph shows the methane production performance during anaerobic digestion in different treatment groups; among them, Figure 7 In the graph, 'a' represents the rate of methanogenesis as a function of operating time. Figure 7 In the graph, b represents the cumulative methane production over time. Figure 8 This is a graph showing the changes in the composition and concentration of volatile fatty acids over time during anaerobic digestion in different treatment groups; among them, Figure 8 The values ​​of a and e represent the composition and concentration of VFA on days 2, 4, 6, 8, and 10 of the reaction, respectively. Detailed Implementation

[0005] The present invention will be further described below with reference to the accompanying drawings and embodiments. Unless otherwise specified, the equipment and testing methods used are conventional equipment and methods in the art. For ease of description, carbon quantum dots are abbreviated as CD and iron-carbon quantum dots are abbreviated as Fe-CD in this specification. Example 1 Citric acid and ferric chloride were dissolved in deionized water at a molar ratio of 10:1 to prepare a precursor solution. The precursor solution was then stirred on a magnetic stirrer for 30-60 min to allow the ferric ions to fully complex with the carboxyl and hydroxyl groups in the citric acid, forming a homogeneous complex system. The complexation system was transferred to a stainless steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). The reactor was then placed in an oven for hydrothermal reaction at 230 °C for 12 h. After the reaction, the reactor was allowed to cool naturally to room temperature, and the resulting solution was a suspension of iron-carbon quantum dots (Fe-CD). For comparison, carbon quantum dots (CD) without the addition of ferric chloride were also prepared. According to the experimental protocol, Fe-CD and CD were adjusted to pH 1 or pH 7 before being added to the anaerobic digestion system. Example 2 An anaerobic digestion reaction system was set up, with the anaerobic reactor being a 1 L serum bottle, having an effective working volume of approximately 0.5 L. Anaerobic sludge from a wastewater treatment plant was added to each reactor as anaerobic inoculum, along with 2 g of glucose as the organic matter to be treated or an organic carbon source. The initial pH was adjusted to approximately 7.5, and an anaerobic environment was established using nitrogen purging or other deoxygenation methods. The anaerobic digestion temperature was 40 °C, and the reaction cycle was 10 days. The experimental group without any conductive agent was designated as the blank group. Example 3 An anaerobic digestion reaction system was set up, with the anaerobic reactor being a 1 L serum bottle, having an effective working volume of approximately 0.5 L. Anaerobic sludge from a wastewater treatment plant was added to each reactor as anaerobic inoculum, along with 2 g of glucose as the organic matter to be treated or an organic carbon source. The initial pH was adjusted to approximately 7.5, and an anaerobic environment was established using nitrogen purging or other deoxygenation methods. The anaerobic digestion temperature was 40 ℃, and the reaction cycle was 10 days. CD (dihydrotestosterone) at pH 1 was added to the anaerobic digestion system at a concentration of 5 g / L. The experimental group with 5 g / L of pH 1 CD was labeled CD-1. Example 4 An anaerobic digestion reaction system was set up, with the anaerobic reactor being a 1 L serum bottle, having an effective working volume of approximately 0.5 L. Anaerobic sludge from a wastewater treatment plant was added to each reactor as anaerobic inoculum, along with 2 g of glucose as the organic matter to be treated or an organic carbon source. The initial pH was adjusted to approximately 7.5, and an anaerobic environment was established using nitrogen purging or other deoxygenation methods. The anaerobic digestion temperature was 40 ℃, and the reaction cycle was 10 days. CD (dihydrotestosterone) at pH 7 was added to the anaerobic digestion system at a concentration of 5 g / L. The experimental group with 5 g / L of pH 7 CD was labeled CD-7. Example 5 An anaerobic digestion system was set up, with the anaerobic reactor being a 1 L serum bottle, having an effective working volume of approximately 0.5 L. Anaerobic sludge from a wastewater treatment plant was added to each reactor as anaerobic inoculum, along with 2 g of glucose as the organic matter or organic carbon source to be treated. The initial pH was adjusted to approximately 7.5, and an anaerobic environment was established using nitrogen purging or other deoxygenation methods. The anaerobic digestion temperature was 40 ℃, and the reaction cycle was 10 days. Fe-CD at pH 1 was added to the anaerobic digestion system at a concentration of 5 g / L. The experimental group with 5 g / L of pH 1 Fe-CD added was labeled Fe-CD-1. Example 6 An anaerobic digestion system was set up, with the anaerobic reactor being a 1 L serum bottle, having an effective working volume of approximately 0.5 L. Anaerobic sludge from a wastewater treatment plant was added to each reactor as anaerobic inoculum, along with 2 g of glucose as the organic matter to be treated or an organic carbon source. The initial pH was adjusted to approximately 7.5, and an anaerobic environment was established using nitrogen purging or other deoxygenation methods. The anaerobic digestion temperature was 40 ℃, and the reaction cycle was 10 days. Fe-CD at pH 7 was added to the anaerobic digestion system at a concentration of 5 g / L. The experimental group with 5 g / L of Fe-CD at pH 7 was labeled Fe-CD-7. Example 7 To compare the effects of different additives on the methanogenic performance of anaerobic digestion, pH, COD, VFA, methanogenic rate, and cumulative methane production were measured as key technical indicators in five experimental groups. Referring to the accompanying drawings, the performance of this invention in promoting methanogenesis through anaerobic digestion can be fully demonstrated. Figure 1 Fe-CD was characterized by transmission electron microscopy. Figure 1 As can be seen in a, Fe-CD exhibits a dotted structure with relatively clear lattice fringes, and its interplanar spacing is approximately 0.28 nm, indicating that the Fe-CD particles possess an ordered lattice. Figure 1 As shown in b, Fe-CD exhibits a dispersed particle morphology in the field of view, with no obvious large-sized aggregates observed. The particle size distribution results indicate that the Fe-CD particle size is mainly concentrated in the range of approximately 1-3 nm, with a small number of particles distributed in the range of approximately 3-5 nm. The fine and uniform particle size and good dispersion are beneficial to the significant improvement of interspecies electron transfer and methanogenesis performance in microorganisms. like Figure 2As shown, X-ray diffraction characterization was performed on CD and Fe-CD. The results show that both CD and Fe-CD exhibit a major diffraction peak around 2θ (approximately 19°), indicating that both possess the structural characteristics of carbon quantum dot materials. Compared to CD, Fe-CD shows a significantly enhanced diffraction peak intensity at this position and exhibits a weak diffraction signal in the higher angle region, suggesting that the introduction of an iron source may affect the formation of carbon nuclei and the construction of locally ordered structures during hydrothermal carbonization. like Figure 3 As shown, Fourier transform infrared spectroscopy was performed to characterize CD and Fe-CD. Both CD and Fe-CD show values ​​around 2988 cm⁻¹. -1 1698 cm -1 1390 cm -1 1165 cm -1 1032 cm -1 And 898 cm -1 816 cm -1 724 cm -1 624 cm -1 588 cm -1 Characteristic absorption peaks appear. Among them, 2988 cm⁻¹... -1 The absorption peak corresponds to the CH stretching vibration; 1698 cm⁻¹ -1 The strong absorption peak originates from the stretching vibration of C=O in the carboxyl or carbonyl group; 1390 cm⁻¹ -1 The absorption peak is attributed to the symmetric stretching vibration of CO in the carboxylate group or the bending vibration of C-OH; 1165 cm⁻¹ -1 and 1032 cm -1 The two absorption peaks correspond to the stretching vibrations of the COC or CO bonds, respectively. These results indicate that both CD and Fe-CD surfaces contain oxygen-containing functional groups such as carboxyl, hydroxyl, carbonyl, and ether bonds. However, while retaining the characteristic absorption peaks of oxygen-containing functional groups on the carbon quantum dot surface, Fe-CD exhibits different absorption peaks in the low wavenumber region, especially at 588 cm⁻¹. -1 and 540 cm -1 It exhibits a relatively obvious absorption characteristic, and this region can be attributed to the stretching vibrations of the Fe-O or Fe-OC bonds. (Combined with 1698 cm⁻¹) -1 and 1390 cm -1 The changes in the absorption peaks related to carboxyl groups / carboxylate salts nearby indicate that iron ions can complex or bond with functional groups such as carboxyl groups and hydroxyl groups on the surface of carbon quantum dots formed by citric acid carbonization during the hydrothermal reaction, thereby forming iron-carbon quantum dots containing iron active sites. like Figure 4As shown, X-ray photoelectron spectroscopy (XPS) characterization was performed on CD and Fe-CD. The overall spectrum of Fe-CD showed characteristic peaks for four elements: C1s, N1s, O1s, and Fe2p, further demonstrating the successful introduction of iron into the carbon dots. In the C1s, N1s, and O1s PPS spectra, compared to CD, Fe-CD also contained CN, C=O, CO, and CC functional groups, as well as different forms of nitrogen species, indicating that the carbon dot surface retained abundant polar functional groups and heteroatom structures after the hydrothermal reaction. The introduction of Fe did not disrupt the original carbon dot structure and maintained the overall carbon skeleton and conjugated structure, which is beneficial for its dispersion in the anaerobic digestion aqueous system and its interfacial interactions with microbial surfaces or extracellular polymers. Meanwhile, satellite peaks at 709.85 eV for Fe2p3 / 2, 723.20 eV for Fe2p1 / 2, and near 714.20 eV and 727.80 eV were observed in the Fe2p spectrum of Fe-CD, indicating the presence of an iron-related coordination environment in Fe-CD. This result is corroborated by the absence of strong peaks in large-sized crystalline iron oxides in XRD and the presence of Fe-O or Fe-OC related vibrational peaks in FTIR, suggesting that iron species do not primarily exist in the form of large-particle iron oxides, but are more likely bound to carbon quantum dot structures or surface functional groups in the form of highly dispersed iron-related active sites, ferro-oxygen coordination structures, or Fe-OC bonds. Therefore, XPS results further demonstrate that Fe-CD not only possesses the carbon, nitrogen, and oxygen surface functional groups of CD, but also introduces additional iron-containing active sites. More importantly, the introduction of iron-containing active sites can effectively promote interspecies electron transfer, enhance interspecies electron transfer between microorganisms, and thus improve the methanogenic performance of the entire anaerobic digestion system. The combined results of TEM, XRD, FTIR, and XPS indicate that the Fe-CD prepared in this invention possesses a small-size structure, good dispersibility and biocompatibility, abundant oxygen / nitrogen-containing functional groups on its surface, and iron-containing active sites. TEM confirms its dispersed point-like structure, XRD rules out the possibility of a large-scale formation of large-size crystalline iron oxide particles, FTIR confirms Fe-O or Fe-OC bonding characteristics, and XPS further confirms the presence of C, N, and O functional groups and Fe2p chemical signals. These structural and surface chemical characteristics collectively constitute the material basis for Fe-CD as a bioconductive material for anaerobic digestion, promoting interspecies electron transfer and methane generation in microorganisms. Based on this, Fe-CD, through its biocompatibility and the action of multiple active sites, enhances the electron transfer capacity of microorganisms in the anaerobic digestion system, promotes the further conversion of acidification products such as acetic acid and propionic acid, mitigates VFA accumulation and pH fluctuations, and this is corroborated by the results of a continuous decrease in COD, an increase in methanogenesis rate, and an increase in cumulative methane production, thereby achieving the effect of enhancing the methanogenesis performance of anaerobic digestion. like Figure 5 As shown, during the anaerobic digestion reaction, the pH of each treatment group generally showed a trend of gradually increasing from neutral to slightly alkaline and then stabilizing. From day 2 to day 4 of the reaction, the pH of each group remained in the range of approximately 7.5-7.8, indicating that no significant acidification occurred in the initial stage of the system. As the reaction progressed to day 6 and day 8, the pH of each group increased, indicating that the acidic intermediates in the system were gradually consumed and the methanogenesis stage gradually intensified. By day 10 of the reaction, the pH of the Fe-CD-1 and Fe-CD-7 groups remained within the slightly alkaline range suitable for anaerobic digestion, without any significant acidification or alkalization imbalance. like Figure 6 As shown, the COD of the blank group, CD-1 group, CD-7 group, Fe-CD-1 group, and Fe-CD-7 group all gradually decreased with the extension of reaction time, indicating that each treatment system can effectively degrade and transform organic matter in the reaction liquid. On the second day of the reaction, the COD of all groups was at a high level, but by the tenth day, the COD of all groups had significantly decreased, indicating that the organic matter in the system was continuously consumed and converted into products such as biogas. The Fe-CD-1 and Fe-CD-7 groups showed a stable COD decreasing trend throughout the reaction process, indicating that the addition of Fe-CD did not inhibit the organic matter degradation process in the anaerobic digestion system and has good microbial compatibility. The results of stable pH and continuous COD reduction jointly demonstrate that Fe-CD can promote organic matter conversion without disrupting the acid-base balance of the anaerobic digestion system, providing a stable reaction environment for subsequent consumption of volatile fatty acids and methane generation. like Figure 7 a and Figure 7 As shown in b, the methanogenesis rate and cumulative methane production of the blank group, CD-1 group, CD-7 group, Fe-CD-1 group, and Fe-CD-7 group during anaerobic digestion were measured. The results showed that each treatment group exhibited a significant peak methanogenesis rate in the initial stage of operation, indicating that readily degradable organic matter could be rapidly utilized and converted into methane after the anaerobic digestion system was started. Subsequently, the methanogenesis rate of each group generally decreased with the extension of operating time, indicating that the reaction system gradually transitioned from a rapid methanogenesis stage to a stable methanogenesis stage. Table 1 shows the cumulative methane production under different additive conditions during anaerobic digestion. The changes in methane production rate indicate that both the Fe-CD-1 and Fe-CD-7 groups exhibited high methane production rates in the initial and middle stages of operation. Specifically, the Fe-CD-7 group showed the highest methane production rate in the initial stage, significantly higher than the control group, CD-1 group, and CD-7 group. The methane production rate of the Fe-CD-1 group was also higher than that of the CD treatment group and the control group. These results demonstrate that Fe-CD can accelerate the methane generation process in the start-up and middle stages of the anaerobic digestion system, increasing the methane production capacity per unit time. Table 1. Cumulative methane production from anaerobic digestion under different additive conditions experimental group conductive materials pH of CD Dosage concentration Cumulative methane production / mL Improve efficiency Blank group none — 0 970 — CD-1 group CD 1 5 g / L 1118 15.3% CD-7 group CD 7 5 g / L 1102 13.6% Fe-CD-1 group Fe-CD 1 5 g / L 1204 24.1% Fe-CD-7 group Fe-CD 7 5 g / L 1241 27.9% The changes in cumulative methane production show that the cumulative methane production of each group continuously increased with the extension of operating time. The control group had the lowest cumulative methane production, while the CD-1 and CD-7 groups showed some improvement compared to the control group. The Fe-CD-1 and Fe-CD-7 groups showed further increases in cumulative methane production. Towards the later stage of operation, the Fe-CD-7 group had the highest cumulative methane production, approximately 1241 mL, an increase of about 27.9% compared to the control group; the Fe-CD-1 group had a cumulative methane production of approximately 1204 mL, an increase of about 24.1% compared to the control group. This indicates that Fe-CD has a better promoting effect on anaerobic methanogenesis than CD, with the Fe-CD-7 group performing best under neutral conditions. like Figure 8 As shown, the composition and concentration of volatile fatty acids (VFAs) in the anaerobic digestate of different treatment groups were detected. The VFAs included acetic acid, propionic acid, butyric acid, isobutyric acid, valeric acid, and isovaleric acid. The results showed that during the anaerobic digestion process, the VFAs in each treatment group were mainly composed of acetic acid and propionic acid, while the proportions of butyric acid, isobutyric acid, valeric acid, and isovaleric acid were relatively low, indicating that acetic acid and propionic acid were the main acidification intermediates in this system. On days 2 and 4 of the reaction, the total VFA concentration in all groups remained at a high level, indicating that the organic matter underwent hydrolysis and acidification in the early stages of the system, producing a large amount of short-chain volatile fatty acids, and the acidification phase was relatively active. As the reaction progressed to day 6, the total VFA concentration in all groups decreased overall, indicating that the accumulated volatile fatty acids in the system began to be further converted and consumed, and the acidification and methanogenesis phases gradually established a connection. On day 8, some treatment groups still showed some fluctuations in VFA, indicating that the hydrolysis, acidification, and subsequent conversion processes of organic matter were still ongoing. By day 10, the total VFA concentration in all treatment groups was significantly lower than in the early stages of the reaction, with most of the accumulated VFA being utilized by methanogenic bacteria and incorporated into the methanogenic metabolic pathway, ultimately effectively increasing the system's methane production. Compared to the CD-treated groups, the Fe-CD-1 and Fe-CD-7 groups showed relatively stable changes in VFA during the later stages of the reaction, without a sustained increase in volatile fatty acids, indicating that the addition of Fe-CD does not lead to abnormal accumulation of acidic intermediates. Combined with the fact that the pH of the reaction system was maintained within the suitable weakly alkaline range for anaerobic digestion, this demonstrates that Fe-CD has good microbial compatibility and can promote the conversion of intermediates such as acetic acid and propionic acid formed during the acidification stage to the methane generation stage. The results, combined with those of pH, COD, and VFA, indicate that adjusting the pH of Fe-CD not only increases the methanogenesis rate of the anaerobic digestion system but also enhances the cumulative methane production throughout the entire operating cycle. This is because Fe-CD possesses excellent conductivity and biocompatibility, abundant oxygen-containing functional groups on its surface, and iron-related active sites. It can act as a conductive medium to strengthen interspecies electron transfer among acid-producing, acetic-producing, and methanogenic bacteria, promoting the conversion of intermediate products such as volatile fatty acids into methane, thereby improving the methanogenesis efficiency of the anaerobic digestion system. The above examples and comparative examples demonstrate that Fe-CD possesses small size, good water dispersibility, and abundant oxygen-containing functional groups on its surface. During the hydrothermal reaction, the iron ions provided by ferric chloride can complex with functional groups such as carboxyl and hydroxyl groups in citric acid, forming carbon quantum dot structures containing iron active sites during carbonization. TEM results confirm that Fe-CD has a small particle morphology and approximately 0.28 nm lattice fringes; XRD results show that Fe-CD retains the structural characteristics of carbon quantum dots and does not form obvious large-particle crystalline iron oxides; FTIR results show that the Fe-CD surface contains carboxyl, hydroxyl, carbonyl, ether bonds, and Fe-O or Fe-OC related structures. These structural features collectively constitute the material basis for Fe-CD as an anaerobic digestion conductive additive. Experimental results show that pH optimization of Fe-CD can effectively avoid the inhibitory effect of acid-base imbalance on the anaerobic system, significantly improving the operational stability of the anaerobic digestion system. Simultaneously, it can greatly activate microbial activity, promote the efficient degradation of organic COD in sludge, accelerate the rapid conversion and accumulation of volatile fatty acids, intermediate products of anaerobic digestion, effectively alleviate the system inhibition problem caused by volatile fatty acid accumulation, and ultimately significantly improve the methanogenesis rate of anaerobic digestion, greatly increasing the cumulative methane production of the system, achieving comprehensive enhancement of the methanogenesis performance of municipal sludge anaerobic digestion. Furthermore, after adding Fe-CD to the anaerobic digestion system, its small size structure and oxygen-containing functional groups on the surface facilitate its dispersion in the aqueous system and allow for sufficient contact with microorganisms, extracellular polymers, and soluble intermediate metabolites. The iron-related sites and carbon point conductive structure facilitate the construction of electron transfer channels between microorganisms, promoting electron flow among acid-producing bacteria, acetic-producing bacteria, and methanogens, reducing electron transfer resistance, and improving the conversion efficiency of methanogenic microorganisms for substrates and intermediate metabolites. Performance data show that the Fe-CD described in this invention can maintain a suitable pH range in the anaerobic digestion system, continuously reduce COD, and further convert and consume volatile fatty acids such as acetic acid and propionic acid in the later stages of the reaction, ultimately resulting in an increased methanogenesis rate and a cumulative increase in methane production. Process experiments confirm that the addition of Fe-CD coupled with iron within a suitable pH range not only significantly accelerates the degradation rate of VFAs in the system but also significantly enhances the electron transfer rate between microorganisms. Within the experimentally set process range, the cumulative methane production of the system can be increased by up to 27.9%. Therefore, pH-synergistic iron-carbon quantum dot technology is an efficient and feasible method for enhancing anaerobic digestion, providing a new approach for improving the quality and efficiency of anaerobic fermentation of organic solid waste such as kitchen waste and sludge. In summary, the process conditions of this invention are mild and the operation is simple. By adjusting the pH of Fe-CD, it can stably improve the degradation efficiency of anaerobic digestion substrates and the efficiency of methane production, demonstrating good application value and promising prospects for the resource utilization of organic waste through anaerobic digestion.

Claims

1. A method for enhancing the methanogenic performance of anaerobic digestion by regulating the pH value of iron-carbon quantum dots, characterized in that: Iron-carbon quantum dots are added to the anaerobic digestion system at a pH of 1-7.

2. The method according to claim 1, characterized in that: The pH value at which the iron-carbon quantum dots were added was 7.

3. The method according to claim 1, characterized in that: The concentration of the iron-carbon quantum dots added is 0.1 g / L - 10 g / L.

4. The method according to claim 3, characterized in that: The concentration of the iron-carbon quantum dots added was 5 g / L.

5. The method according to claim 1, characterized in that: The anaerobic digestion temperature is 35-45 ℃; the anaerobic digestion time is 7-14 days; and the initial pH of the anaerobic digestion system is 6-9.

6. The method according to claim 5, characterized in that: The anaerobic digestion temperature was 40 °C, the anaerobic digestion time was 10 days, and the initial pH of the anaerobic digestion system was 7.

5.

7. The method according to claim 1, characterized in that: The method for preparing the iron-carbon quantum dots is as follows: iron-carbon quantum dots are prepared by hydrothermal reaction of citric acid and ferric chloride, wherein the molar ratio of citric acid to ferric chloride is 1:1-10:1, the temperature of the hydrothermal reaction is 200-230 ℃, and the time of the hydrothermal reaction is 2-24 h.

8. The application of iron-carbon quantum dots in sludge treatment, characterized by: Iron-carbon quantum dots, as bioconductive materials, promote interspecies electron transfer and methane generation among microorganisms in anaerobic digestion systems. The pH of the iron-carbon quantum dots is 1-7.

9. The application according to claim 8, characterized in that: The pH of the iron-carbon quantum dots is 7.

10. The application according to claim 9, characterized in that: The method for preparing the iron-carbon quantum dots is as follows: iron-carbon quantum dots are prepared by hydrothermal reaction of citric acid and ferric chloride, wherein the molar ratio of citric acid to ferric chloride is 1:1-10:1, the temperature of the hydrothermal reaction is 200-230 ℃, and the time of the hydrothermal reaction is 2-24 h.

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