Preparation method and application of magnetic composite electron mediator material for driving anaerobic iron respiration coupling carbon reduction

Abstract

The invention provides a preparation method and application of a magnetic composite electron mediator material for driving anaerobic iron respiration coupling carbon reduction. Researches find that by adding MnFe2O4 into an anaerobic system, the degradation of SCOD and the daily yield of CH4 can be improved, and the anaerobic digestion process is remarkably enhanced. Besides, in combination with a microbial community structure and functional gene analysis, it is shown that MnFe2O4 mediated DIET with VFAs oxidized into an electron sink can be established among Acetoanaerobium, Syntrophomonas and Methanosaeta, so that decomposition of PCOW is promoted. Therefore, it is further shown that proper supplement of MnFe2O4 in the anaerobic digestion process is a feasible way for optimizing the protein organic wastewater treatment process. The MnFe2O4 in the invention is synthesized by a hydrothermal coprecipitation method.

Description

Technical Field

[0001] The invention relates to the technical field of composite material synthesis and clean energy production, and in particular to a preparation method and application of a magnetic composite electron mediator material for driving anaerobic iron respiration coupled carbon reduction. Background Art

[0002] The efficient operation of traditional protein-containing wastewater (PCOW) methanogenesis mainly relies on the interspecies hydrogen transfer (IHT) mechanism of converting long-chain volatile acids into acetic acid. Since IHT is highly sensitive to adverse external environments, there is obvious acid accumulation during anaerobic digestion of protein-containing wastewater and it harms the operation of the system. The mediating effect of metal conductive materials on anaerobic systems comes from the fact that the dissimilatory metal reduction process driven by organic matter mineralization can realize the transfer of electrons accompanied by the release of low-valent ions. Dissimilatory metal reduction can not only ensure the efficient and directional transmission of electrons, but also the low-valent metal ions produced by reduction can stimulate the activity of microbial enzymes. The existence of this electron transfer pathway can reduce the energy consumption of the electron transfer respiratory chain of anaerobic microorganisms and ensure the high efficiency of electrons between microbial populations, thereby achieving the purpose of strengthening PCOW methanogenesis.

[0003] The current mainstream metal conductive materials are mostly single metal-based materials such as iron-based and manganese-based materials. In order to explore the superiority of composite materials compared to single metal-based materials, this scheme proposes to prepare low-cost and biocompatible bimetallic-based materials to solve the acid accumulation problem of PCOW anaerobic fermentation, drive PCOW anaerobic carbon reduction, improve quality and efficiency, and thus provide new ideas for traditional PCOW anaerobic treatment. Summary of the invention

[0004] In view of this, the present invention proposes a preparation method and application of a magnetic composite electron mediator material for driving anaerobic iron respiration coupled carbon reduction to solve the above problems.

[0005] The technical solution of the present invention is achieved in this way:

[0006] An application of a magnetic composite electron mediator material for driving anaerobic iron respiration coupled with carbon reduction, including at least one of the following uses (A) to (D):

[0007] (A) Application in promoting SCOD degradation of protein-organic wastewater;

[0008] (B) Application in promoting acid production by anaerobic digestion of protein organic wastewater;

[0009] (C) Application in promoting methanogenesis by anaerobic digestion of protein-based organic wastewater;

[0010] (D) Application in promoting the change of microbial community in anaerobic digestion of proteinaceous organic wastewater;

[0011] The magnetic composite electron mediator material for driving anaerobic iron respiration coupled with carbon reduction is an iron-manganese bimetallic-based material.

[0012] Preferably, the iron-manganese bimetallic based material is MnFe2O4.

[0013] Preferably, the added amount of MnFe2O4 is 200-800 mg / g·VSS.

[0014] Preferably, the added amount of MnFe2O4 is 600 mg / g·VSS.

[0015] Preferably, the MnFe2O4 is synthesized by a hydrothermal co-precipitation method.

[0016] Preferably, the preparation method of MnFe2O4 is: take manganese salt and iron salt according to the molar ratio of Mn / Fe elements, dissolve them in oxygen-free deionized water, and fully mix them with NaOH solution. The resulting mixed solution is heated at 80-90°C for 2-3h, naturally cooled to room temperature, and the black solid is collected by magnetic separation and washed with deionized water until neutral. Then, the black solid is first dispersed in a dimethyl sulfoxide solution at 70-80°C, ultrasonically treated, and finally the MnFe2O4 is centrifuged and washed with distilled water and anhydrous ethanol.

[0017] Preferably, the manganese salt includes manganese chloride hydrate, manganese nitrate hydrate, and manganese sulfate hydrate.

[0018] Preferably, the iron salt includes ferric chloride hydrate, ferric nitrate hydrate, and ferric sulfate hydrate.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) By adding MnFe2O4 to the anaerobic system, the degradation of SCOD and the daily production of CH4 can be improved, significantly strengthening the anaerobic digestion process. After 90 days of digestion, it was found that the addition of 600 mg / g·VSS MnFe2O4 can promote the degradation of SCOD and the daily production of CH4, with a degradation rate of up to 89.13%, and the daily production of methane was 40.53% higher than that of the control group, showing the best degradation performance and maximization of resource recovery.

[0021] (2) In the experiments under different organic load conditions, when the influent SCOD concentration was the highest, the SCOD removal rate of MnFe2O4 was basically maintained at 66%, which was 10.49% and 15.26% higher than that of the Fe3O4 group and the MnO2 group. Combined with the microbial community structure and functional gene analysis, it was found that the addition of magnetic MnFe2O4 could enrich the dominant populations such as Acetoanaerobium, Syntrophomonas and Methanosaeta, thus establishing an interaction mechanism dominated by Acetoanaerobium, Syntrophomonas and Methanosaeta.

[0022] (3) Based on the study of operating efficiency and changes in microbial communities, it is shown that the Fe element in MnFe2O4 can induce an iron respiration reaction dominated by dissimilatory iron reduction (DIR), and can form an "electric" syntrophic effect with the final methanogenesis. The specific mechanism is as follows: (1) DIR-type iron-respiring bacteria (Syntrophorhabdus aromatics) produce electrons and Fe when oxidizing volatile fatty acids (VFAs). 2+ ; (2) The generated electrons are efficiently transferred to the final electron acceptor (Methanosaeta bristle methanogens) under the secondary mediation effect of the Mn element; (3) As the iron-respiratory VFAs oxidation reaction continues, a new direct interspeices electron transfer (DIET) mode is finally established, which is driven by MnFe2O4 to drive iron respiration-coupled carbon reduction. The dual-channel electron-mediated effect of MnFe2O4 can optimize the carbon reduction efficiency of PCOW from the aspects of accelerating electron transfer, promoting microbial aggregation, improving the system's ability to resist shock loads, and improving the quality and efficiency of methanogenesis.

[0023] (4) The unique magnetic characteristics of MnFe2O4 can further realize the recovery and secondary recycling of this material, and it has excellent potential for engineering applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The results of characterization of the physical and chemical properties of MnFe2O4 magnetic materials; Figure 1 (a) Figure 1 (b) is the SEM morphology of MnFe2O4. Figure 1 (c) is the EDS spectrum of MnFe2O4. Figure 1 (d) is the magnetic test diagram of MnFe2O4.

[0025] Figure 2is the XPS spectrum of MnFe2O4; Figure 2 (a) is the Mn 2p orbital diagram, Figure 2 (b) is the Fe 2p orbital diagram, Figure 2 (c) is the O1s orbital diagram.

[0026] Figure 3 The electrochemical properties of MnFe2O4; Figure 3 (a) to (c) are the peak current CV curves and logarithmic fitting curves at different scan rates without substrate; Figure 3 (d) to (f) are the changes in electron transfer capacity (EDC and EAC); Figure 3 (g) to (i) are the relevant fitting images of the electron donating capacity (EDC) and electron accepting capacity (EAC) of three different conductive materials (MnO2, Fe3O4, MnFe2O4) at different dosages.

[0027] Figure 4 The effects of different experimental groups on SCOD degradation; Figure 4 (a) is the effluent SCOD concentration, Figure 4 (b) is the effluent SCOD degradation rate.

[0028] Figure 5 The changes of VFAs concentration in each group during the experiment.

[0029] Figure 6 The pH changes of each group during the experiment.

[0030] Figure 7 The effect of different experimental groups on the methanogenic efficiency; Figure 7 (a) is the daily production of CH4, Figure 7 (b) is the daily production of CO2.

[0031] Figure 8 This is the sludge morphology diagram after 90 days of operation; Figure 8 (a) to (c) are sludge surface images. Figure 8 (d) to (e) are sludge observation pictures under optical microscope. Figure 8 (f) to (g) are scanning electron microscope photos of sludge.

[0032] Fig. 9 is the degradation degree of PCOW by each conductive material under different operating conditions; Fig. 9 (a) for different HRT conditions, Fig. 9 (b) Different organic loadings.

[0033] Fig.10 The bacterial and archaeal community structures at the genus level.

[0034] Fig.11 Bacterial and archaeal community structures at the phylum level.

[0035] Fig.12 This is the gene expression pathway related to PCOW anaerobic digestion mediated by MnFe2O4.

[0036] Fig.13 Relative abundance of genes related to PCOW anaerobic digestion mediated by MnFe2O4. DETAILED DESCRIPTION

[0037] In order to better understand the technical content of the present invention, specific embodiments are provided below to further illustrate the present invention.

[0038] Unless otherwise specified, the experimental methods used in the embodiments of the present invention are all conventional methods.

[0039] Unless otherwise specified, the materials, reagents, etc. used in the embodiments of the present invention can be obtained from commercial sources.

[0040] The present invention adopts tryptone and yeast extract in a ratio of about 1:1 to prepare protein-rich organic wastewater, which mainly provides carbon and nitrogen sources for microorganisms. The COD concentration of synthetic wastewater is controlled at about 5000 mg / L, and the pH value is adjusted to 6.5-7.0 with a 1.5 mol / L NaOH solution. The anaerobic sludge used in the experiment was taken from an anaerobic fermentation tank with good performance, and was placed in a serum bottle and stored in a biochemical incubator at 35±2°C before use. Before the experiment, experimental synthetic water was added to the serum bottle containing anaerobic sludge to restore the activity of anaerobic microorganisms. The properties of anaerobic sludge are shown in Table 1.

[0041] Table 1 Main properties of experimental sludge

[0042]

[0043] Example 1 Synthesis and Characterization of MnFe2O4

[0044] Synthesis method of MnFe2O4:

[0045] The MnFe2O4 material was synthesized by a hydrothermal coprecipitation method, comprising the following steps: first, 1.97 g of MnCl2·4H2O and 5.4 g of FeCl3·6H2O were taken at a molar ratio of 1:2, dissolved in 40 mL of oxygen-free deionized water that had been stripped of nitrogen for 30 minutes, and fully mixed with 160 mL of 1.5 M NaOH solution under magnetic stirring at 5000 r / min. The resulting mixed solution was heated at 85°C for 2 hours, naturally cooled to room temperature, black solids were collected by magnetic separation, and washed with deionized water until neutral, and then the black solids were first dispersed in 500 ml of dimethyl sulfoxide solution at 75°C for 3 hours, and then ultrasonically treated for 10 minutes to reduce the aggregation of the material, and finally the MnFe2O4 was centrifuged and washed multiple times with distilled water and anhydrous ethanol.

[0046] Characterization of the physicochemical properties of MnFe2O4 magnetic materials

[0047] The physical and chemical properties of MnFe2O4 prepared by coprecipitation were analyzed by SEM-EDS and XPS. The surface morphology of MnFe2O4 was observed by SEM, and the mineral composition of MnFe2O4 was analyzed by EDS. Figure 1 As shown in (a) and (b), cubic mechanisms and spherical structures are formed, and the cluster sizes are mostly between 20nm and 100nm. Figure 1 As shown in (c), the data from the energy spectrometer can confirm that manganese, iron and oxygen elements do exist in the MnFe2O4 particles, and the content of each element can basically be determined by EDS. The composition contents of Fe, Mn and O are 54.0%, 26.4% and 19.6% respectively, indicating that iron and manganese have been successfully incorporated into the synthetic material and MnFe2O4 has been successfully synthesized. Figure 1 (d) It can be concluded that the prepared MnFe2O4 has strong magnetism.

[0048] In order to further determine the elemental composition and oxidation state of the prepared MnFe2O4 particles, XPS was used to detect MnFe2O4. Figure 2 In (a), in Mn 2p 3 / 2 and Mn 2p l / 2 Mn was observed in the oxidation state 2+ The presence of Mn at 643.8 eV corresponds to the binding energies of 641.3 eV and 653.0 eV. 2+ Satellite peaks. Figure 2 The Fe 2p spectrum in (b) shows that Fe 2p 3 / 2 and Fe 2p 1 / 2 The binding energies of Fe and Fe are 710.9 and 726.4 eV, respectively. In addition, Fe3+ The characteristic satellite peaks indicate the presence of Fe 3+ .like Figure 2 (c) shows the O1s spectrum of MnFe2O4 biochar, with two O1s peaks at binding energies of 530.0eV and 531.1eV. The peak at 531.1eV is OH, and the peak at 530.0eV is the formation of metal oxygen bonds such as Fe-O and Mn-O in MnFe2O4. These results indicate that the synthesis of MnFe2O4 was successful.

[0049] Characterization of electrochemical properties of MnFe2O4 magnetic materials

[0050] The electrochemical properties of MnFe2O4 were studied by cyclic voltammetry (CV). Figure 3 As shown in (c), at different scan rates, a pair of symmetrical peaks appeared on the CV curve, which were -0.3V (relative to SCE, oxidation peak) and -0.5V (relative to SCE, reduction peak). The addition of conductive particles enhanced the electrochemical response. When the scan rate was 100mV / s, the maximum peak current of MnFe2O4 at -0.3V was 1.26mA, and the maximum peak current at -0.5V was 1.44mA.

[0051] Electrochemical redox methods (MEO and MER) were used to quantitatively determine the electron donating and accepting capacities (EDC and EAC) of MnFe2O4. Figure 3 (a) to (c) show the oxidation current peaks and reduction current peaks of the three materials in MEO and MER. The peak area increases proportionally with the sampling dose (from 1.0 mg to 5.0 mg) and is proportional to the electron transfer number (Q). For all samples, Q is linearly proportional to the sample mass analyzed in MER and MEO. Quantitative electrochemical analysis methods show that the performance of EDC and EAC is improved after the synthesis of MnFe2O4. The EAC of MnFe2O4 is 0.528 mmol e - ·g -1 , EDCs is 0.269mmole - ·g -1 , the results show that the prepared MnFe2O4 has good electron exchange ability, conductivity and redox ability. Therefore, MnFe2O4 is more likely to accept electrons from electron donors.

[0052] Example 2 Study on the effect of MnFe2O4 magnetic material on PCOW anaerobic digestion

[0053] In order to explore the degradation of organic matter in PCOW and the impact resistance of organic wastewater by different addition amounts of MnFe2O4, the reaction device used in this study was 5 UASB reactors with a working volume of about 900mL. The anaerobic digestion (AD) experiment was carried out in 5 identical 900mL UASB reactors. The effective reaction zone volume of the reactor was 500mL, and a gas collection bag was installed on the top of the reactor. In order to ensure the absolute anaerobic setting of UASB, the reactor was flushed with N2 for 30min before startup. 300mL of anaerobic sludge was injected into the UASB reactor. The influent was laboratory synthetic simulated PCOW, COD was basically maintained at 5000mg / L, pH=6.5~7. During the whole experiment, the reactor temperature was maintained at 35±2℃. The AD experiment was divided into 5 groups, namely the control group without MnFe2O4 (R0), 200 mg / g·VSS MnFe2O4 (R1), 400 mg / g·VSS MnFe2O4 (R2), 600 mg / g·VSS MnFe2O4 (R3) and 800 mg / g·VSS MnFe2O4 (R4). According to the size of the reactor and the pump, the reflux pump and the water inlet pump with the corresponding speed were selected. The whole test period was about 90 days. The gas production of the reactor was collected by the gas collection bag and measured every day. The time interval for taking water samples was 2 days. The water samples can be used to determine pH, SCOD (soluble chemical oxygen demand), VFAs (volatile acid), Mn 2+ , Fe 2+ When the reaction was running for 90 days, the anaerobic sludge was stored in a -8℃ refrigerator for microbial community analysis and ETS activity determination.

[0054] (1) Effect of different dosages of MnFe2O4 on the degradation of SCOD in PCOW

[0055] The effluent SCOD changes as follows Figure 4As shown. Each reactor showed excellent SCOD removal rate at the beginning of operation, and the SCOD removal rate in the first 10 days was about 60.0%. This is mainly due to the high biodegradability of the substrate, as well as the long HRT (hydraulic retention time) and high activity of the inoculated sludge. On the 16th day of the reactor operation, different doses of MnFe2O4 particles were added to different systems. After 15 days of acclimatization, the SCOD concentration in the effluent of each reactor dropped below 2000 mg / L and stabilized at around 1800 mg / L. Starting from the 16th day, after the addition of MnFe2O4 particles, the SCOD concentration in the effluent of each MnFe2O4-mediated reactor decreased. Thereafter, the removal rates of the R1~R4 reactors gradually maintained at 70.72%~89.13%, and the effluent SCOD concentration stabilized between 759.65~1515.85 mg / L. In the final period of the reaction operation, the SCOD removal rates of R1 to R4 were basically maintained at 72.98±0.04% to 93.50±0.19%, achieving a stable effluent SCOD concentration between 324.78±9.34 and 1351.13±1.95 mg / L, and the removal rate was increased by 7.82±0.30% to 27.92±0.17% compared with R0.

[0056] (2) Changes in VFA composition and content during anaerobic digestion of MnFe2O4 at different dosages

[0057] Volatile acids (VFAs) are important intermediates of hydrolysis and substrates for methanogenesis. The trends of VFAs content in each group were similar, but there were certain differences in specific content (e.g. Figure 5As shown). Overall, the VFAs content increased sharply from the 0th day to the 2nd day, reaching a peak on the 2nd day. Then it was consumed by methanogens, and the VFAs content gradually decreased until the end of the AD process. Correspondingly, due to the addition of MnFe2O4 on the 16th day, the dynamic distribution of the five groups of VFAs showed different reduction patterns from the 16th day to the 90th day after the hydrolysis and acidogenesis process. In addition, on the 2nd day, acetic acid accounted for about 35.5% of the total VFAs concentration in the five groups of VFAs composition, followed by propionic acid, butyric acid and valeric acid. Obviously, the acidogenesis products include H2 / CO2, acetic acid, propionic acid, butyric acid and valeric acid as necessary intermediates, which can participate in the subsequent acetone production process and the final methanogenesis reaction. It can be clearly observed that on the 16th day, the cumulative VFAs concentration of the MnFe2O4-mediated group was lower than that of the control group, which in turn proved that the reason why the control group had the highest cumulative VFAs concentration was the low hydrolysis efficiency. Subsequently, with the continuous consumption of acetic acid, the conversion of propionic acid and butyric acid seemed to gradually become the rate-limiting step. Especially in the control group, we noticed that the concentrations of acetate, propionate, butyrate, and valerate still accumulated significantly at the end of the operation, which were 1687.68±0.89, 1189.67±0.63, 991.53±0.52, and 883.16±0.47 mg / L, respectively, indicating that the consumption of VFAs was blocked due to the low microbial synergy between hydrolysis, acidogenesis, acetogenesis, and final methanogenesis. The degradation efficiency of acetate in the optimal MnFe2O4-mediated group (R3) from the 2nd to the 90th day was as high as 87.34±0.32%, which was more than 3.11 times that of the control group (28.11±0.02%). The addition of MnFe2O4 was beneficial to AD, promoted the activity of the methanogenic community, and improved the conversion efficiency of VFAs. In contrast, when 800 mg / g·VSS MnFe2O4 was added, the degradation rate of acetate was lower (55.04±0.17%), indicating a lower methanogenic capacity during AD. Increasing the amount of MnFe2O4 added reduced the methanogenic activity and led to the accumulation of VFAs. High VFAs concentrations in the AD system easily caused methanogenic inhibition. These results indicate that low concentrations of MnFe2O4 can effectively stimulate the microbial activity of VFAs conversion, but high concentrations of MnFe2O4 will antagonize methanogenesis.

[0058] like Figure 6 As shown in the figure, the pH changes in each group coexisted with VFAs. On the second day, due to the high content of VFAs, the pH slowly increased from the initial pH 6.03 to 7.1-7.2. After that, the pH gradually increased, and the pH of each group ended at 7.59-7.70. Studies have reported that the optimal pH range for the growth of methanogens is 7.0-8.0. It can be seen that during most of the time of the methanogenesis process, the pH values ​​of each group varied within this range.

[0059] (3) Effect of MnFe2O4 on PCOW gas production during anaerobic digestion and Mn 2+ , Fe 2+ Concentration changes

[0060] The daily gas production and the ratio of methane to carbon dioxide of PCOW AD under different MnFe2O4 dosages are shown in Figure 2. Figure 7 As shown. After 90 days of operation, the daily methane production of the control group and the MnFe2O4 addition group reached a stable level, and there was no significant increase afterwards. The average daily methane production after adding MnFe2O4 was 1132.22-1428.39 mL / d, which was 11.39%-40.53% higher than that of the control group (1016.44 mL / d). In the reactor with 600 mg / g·VSS MnFe2O4 added, the daily methane yield was the highest, which was 11.84%-40.53% higher than that of the control group and other MnFe2O4 intermediate groups. It shows that adding an appropriate amount of MnFe2O4 can promote the formation of methane. MnFe2O4 mainly improves methane yield in three aspects. First, MnFe2O4 does not stimulate the activity of methanogens, but improves digestion performance by helping to establish DIET in the AD system, thereby increasing methane production. Secondly, the supplementation of trace elements can biostimulate the growth of methanogenic bacteria and enhance the hydrolysis and acidification of substrates during AD. Finally, MnFe2O4 biochar increased methane production by reducing the inhibitory effects of intermediate metabolites such as ammonium in the AD system. However, when the MnFe2O4 dosage was further increased to 800 mg / g·VSS (R4), an antagonistic effect was found, and the daily methane production decreased by 20.25% compared with the optimal group R3. This was mainly due to the high content of MnFe2O4, which inhibited the occurrence of methane.

[0061] The average daily carbon dioxide production of the MnFe2O4 group was 589.28-780.35 mL / d, which was 18.68%-38.59% lower than that of the control group (959.63 mL / d). The reactor with 600 mg / g·VSS MnFe2O4 added had the lowest daily CO2 production, which was 18.55%-62.85% lower than that of the control group and other MnFe2O4-mediated groups. Compared with the MnFe2O4-mediated group, from the 16th day to the 90th day, the control group had the lowest average daily CH4 production (1016.44±0.84 mL / d) and the highest average daily CO2 production (959.63±2.04 mL / d) ( Figure 7(b)), indicating that the low CH4 production in the control group was mainly due to the ineffective bioconversion of CO2 and the limited hydrolysis, acid production and acetone production caused by CO2 accumulation. In contrast, R3 with the addition of 600 mg / g·VSS MnFe2O4 particles had the highest CH4 production and the lowest CO2 content. Specifically, at the optimal MnFe2O4 dosage, as the continuous operation proceeds, the generated CO2 can be consumed in large quantities, resulting in significant CH4 production (1428.39±0.30 mL / d) ( Figure 7 (a)). In addition, in the MnFe2O4-mediated group, starting from the 16th day, as the cumulative CO2 production decreased, the Mn 2+ Ions and Fe 2+ The concentration of Mn ions showed a downward trend and tended to be stable, because the factors related to sludge absorption and abiotic oxidation could be excluded in multiple cycles, which indicated that the consumed Mn 2+ Ions and Fe 2+ ions may be involved in CO2-dependent methane production.

[0062] (4) Response of anaerobic sludge physical and chemical indicators to MnFe2O4

[0063] At the beginning of the operation of the reactor, the sludge exists in the reactor in the form of flocs. With the operation of the reactor and the increase of biomass concentration, the anaerobic flocculated sludge gradually forms anaerobic granular sludge aggregates under the appropriate hydraulic shearing action. After 3 months of continuous operation, specific anaerobic granular sludge aggregates were observed in the reactor. After one month of operation, sludge aggregation can be observed, and small granular sludge is initially formed. At the same time, in the second month of the operation of the reactor, large anaerobic sludge particles can be observed in the reactor.

[0064] The loading degree of anaerobic granular sludge and MnFe2O4 particles on the sludge was observed by optical microscope and scanning electron microscope (SEM). Figure 8 As shown. The initial color of anaerobic sludge is brown ( Figure 8 (a)). The dense structure of the sludge indicates a high content of extracellular polymer substrate (EPS). Since EPS can build a physical bridge between two adjacent cells and other particles, aggregates are formed in the reactor. EPS can accelerate granulation, enhance sedimentation and long-term stability. Under the action of EPS, the various parts of the flocculated sludge are tightly combined with almost no gaps between them, which is conducive to the formation, tight combination and stable existence of granular sludge. This structure can be formed under appropriate hydraulic shear force, such as Figure 8 (b)~(c). Due to the anaerobic reaction, the anaerobic sludge tends to darken and granulate as the reactor operates. At the same time, the sludge morphology and MnFe2O4 particles were observed in more detail ( Figure 8(d)~(g)). MnFe2O4 particles and anaerobic bacteria and archaea involved in methanogenesis were observed on the sludge surface ( Figure 8 (f)). This indicates that the MnFe2O4 particles were successfully loaded on the sludge. Since MnFe2O4 has good electrical conductivity, it is speculated that its distribution on the surface of anaerobic microorganisms can act as an electron conductor, improve the efficiency of electron transfer, and thus increase the CH4 production during anaerobic fermentation.

[0065] (5) Operation performance test of different conductive materials under different operating conditions

[0066] In order to verify that multi-metal CM has more advantages than single metal-based CM in enhancing the performance of AD degradation of PCOW, according to the previous experimental results, the optimal dosage groups of Fe3O4, MnO2, and MnFe2O4 were selected to carry out verification experiments under different operating conditions. The results are as follows Fig. 9 As shown. In the experiment with different hydraulic retention times, three gradient hydraulic retention times were set, namely 24h, 12h, and 6h. Fig. 9 As shown in (a), with the change of operating HRT, both the Fe3O4 group and the MnO2 group showed low SCOD removal rates. Among them, when HRT = 6h, the effluent SCOD concentrations of the MnO2 group and the Fe3O4 group were basically maintained at 3301.48±8.54mg / L and 2556.3±4.35mg / L. On the contrary, the MnFe2O4 group still maintained a good SCOD removal rate (66.45±0.06%) when the HRT was 6h. Therefore, it can be explained that MnFe2O4 has more advantages and stability in dealing with HRT in a short period of time. In the experiment under different organic load conditions, three different SCOD concentration gradients were set, namely 5000mg / L, 7000mg / L, and 9000mg / L. The results are shown in Fig. 9 As shown in (b), with the increase of influent SCOD concentration, the three CMs showed different degradation degrees. It can be seen that with the increase of SCOD concentration, each group showed different degradation trends. Among the three groups of CM, MnO2 has the worst ability to cope with changes in organic load, followed by Fe3O4, and MnFe2O4 has the best performance. When the influent SCOD concentration is the highest, the effluent SCOD concentration of MnFe2O4 is basically maintained in the range of 3010.41±7.84 mg / L, which is 10.49% and 15.26% higher than that of the Fe3O4 group and the MnO2 group. This shows that the presence of MnFe2O4 in the anaerobic system can better cope with the impact of different organic loads and has higher stability. Based on the above results, it can be concluded that MnFe2O4 can show excellent performance when dealing with different operating conditions.

[0067] (6) Changes in microbial communities mediated by MnFe2O4

[0068] Changes in microbial communities after adding different doses of MnFe2O4 Fig.10 , Fig.11 As shown. The results showed that the dominant bacterial species at the phylum level were Firmicutes and Synergistota, accounting for 8.45%-23.56% and 5.94%-17.98%, respectively. Firmicutes is an important acetic acid bacterium that contributes to the production of acetic acid-type syntrophic oxidation methane. Synergistota is the main active bacterial community in the anaerobic fermentation degradation of amino acids, and the addition of CM can promote the degradation of suspended organic matter. At the phylum level, these two bacterial species showed an increasing trend in the MnFe2O4-mediated group and reached a peak in R3 compared with the control group.

[0069] To comprehensively understand the distribution of microbial community structure in the five AD systems, the sequences of the dominant bacterial communities were analyzed at the gene level. The main bacterial species at the gene level were JGI-0000079-D21, Acetoanaerobium, norank_f_bacteroidetes_vadinha17, Proteiniclasticum, Trichococcus, Syntrophorhabdus, Syntrophobacter, syntrophomonas, and Tissierella. The abundances of JGI-0000079-D21, Acetoanaerobium, Norank_f_bacteroidetes_vadinha17, Proteiniclasticum, Trichococcus, Syntrophorhabdus, Syntrophobacter, syntrophomonas, and Tissierella were 2.08%-13.71%, 2.31%-19.25%, 0.86%-9.34%, 1.06%-20.32%, 0.31%-4.19%, 1.28%-8.65%, 0.68%-8.71%, 0.39%-22.77%, and 0.74%-1.76%, respectively. JGI-0000079-D21 is a functional bacterial genus associated with hydrolysis and acidification, and they have the potential to degrade inhibitory compounds such as phenols and n-heterocyclic compounds. Acetoanaerobium produces acetic acid as a primary anaerobic fermentation product using H2 and CO2 as substrates. It has been reported that Norank_f__Bacteroidetes_vadinHA17 can degrade glucose into acetic acid, propionic acid, and H2 / CO2. Proteiniclasticum can utilize protein as a substrate and mainly produce acetic acid as an anaerobic fermentation product. Norank_f__Bacteroidetes_vadinHA17 is a common acidogen that mainly produces lactic acid, formic acid, acetic acid, and CO2 using glucose. Syntrophorhabdus and Syntrophobacter have the ability to degrade phenolic compounds associated with hydrogenotrophic methanogens. In addition, Syntrophorhabdus, also known as DIET-associated microorganisms, has the ability to degrade phenol into benzoate, which can be further exposed to the open ring to produce acetic acid and hydrogen, and then the acetic acid may be oxidized by methanogens to produce methane. It has been reported that Syntrophomonas not only participates in the degradation of long-chain fatty acids, but also can establish syntrophic metabolism with methanogens. Tissierella belongs to the phylum Firmicutes, produces lactic acid and is adapted to alkaline conditions.Tissierella is also present in bioelectroactive biofilms and is suspected to be electroactive. Tissierella belongs to the phylum Firmicutes and is able to secrete extracellular enzymes such as proteases and lipases, playing an important role in the anaerobic degradation of organic matter and the production of VFAs. Norank_f_bacteroidetes_vadinha17 and Proteiniclasticum can be classified as hydrolytic bacteria, which can degrade proteins and some macromolecules into amino acids and small molecules. JGI-0000079-D21, Acetoanaerobium and Trichococcus can be classified as acidogenic bacteria, which can convert small molecules into VFAs, CO2, etc. In addition, Tissierella, Syntrophorhabdus, Syntrophobacter and Syntrophomonas can be defined as general anaerobic bacteria, which can work closely with methanogenic archaea to complete the anaerobic metabolic process of VFAs. Among them, the content of acetic anaerobes and Tissierella in the R3 group with 600 mg / g·VSS MnFe2O4 added was richer than that in other groups, which may indicate that the appropriate addition of MnFe2O4 promoted the production of acetic acid, thereby promoting the production of CH4. In contrast, the control group (R0) was rich in Norank_f__Bacteroidetes_vadinHA17, while the abundance of Proteiniclasticum was low, which may indicate that the addition of MnFe2O4 changed the hydrolysis pathway of proteins and could directly use proteins as substrates. Syntrophorhabdus was enriched in the MnFe2O4-mediated group, which may indicate that MnFe2O4 changed the CH4 production pathway, strengthened the occurrence of DIET, and led to an increase in CH4 production. While in. Fig.10 In the results, there was a positive correlation between JGI-0000079-D21 and Acetoanaerobium and Syntrophomonas, Tissierella, and Syntrophorhabus, which may indicate that with the participation of MnFe2O4, there was a synergistic ability between acidogenic bacteria and Syntrophorhabus during AD. At the same time, Norank_f__Bacteroidetes_vadinHA17 was negatively correlated with Tissierella or Acetoanaerobium, indicating that in the final stage of R3 operation, the hydrolysis stage was no longer the dominant stage, and the reaction between microorganisms and methanogens had moved forward. Therefore, during the AD process, microorganisms with different functions gathered together to degrade organic matter into CH4 in a more periodic manner with the participation of MnFe2O4.

[0070] To further explore the effect of MnFe2O4 additives on the diversity of microbial communities after AD, the composition of the archaeal community was analyzed at the gene level. Fig.10 (b). At the gene level, Methanosaeta and Methanobacterium were the two main species, with the total proportions of 25.31% to 52.67% and 10.98% to 61.67%, respectively. A small amount of Methanosarcina could be observed in all samples except the two species involved. The abundance of methanogens in the reactors with added MnFe2O4 was 14.02% to 20.02% higher than that in the control group. Methanosaeta and Methanosarcina are two typical species that utilize acetate to produce methanogens. In contrast, after adding 200, 400, 600 and 800 mg / g·VSS MnFe2O4, the abundance of Methanobacterium decreased significantly from 61.76%, 25.05%, 10.98% and 26.29% in the control group, respectively. Methanobacterium is a typical methanogen that utilizes H2. Therefore, compared with hydrogenotrophic methanogens, MnFe2O4 treatment is beneficial to the activity of acetogenic methanogens. The proportion of methanogens in the MnFe2O4-mediated group was significantly higher than that in the control group, and the abundance of methanogens in the MnFe2O4-mediated group was significantly lower than that in the control group. This phenomenon indicates that the supplementation of MnFe2O4 activated the pathway of acetic methanogens, allowing acetic methanogens to cooperate with hydrogenotrophic methanogens. Methanobacterium can participate in DIET by directly accepting electrons to produce methane. The abundance of Methanosarsina in the MnFe2O4-mediated group increased, indicating that MnFe2O4 may have enhanced the DIET of PCOW. However, the proportion of Methanosaeta in each treatment group was much greater than that of Methanosarsina, indicating that Methanosaeta is more suitable for methanogenesis under PCOW conditions.

[0071] (7) Response mechanism of functional genes mediated by MnFe2O4

[0072] To further reveal the metabolic responses of different doses of MnFe2O4 to acidogenesis, acetogenesis, and methanogenesis, the genes encoding functional enzymes in the five AD systems were evaluated based on functional prediction analysis ( Fig.13 ).

[0073] The corresponding genes of lyase (EC:4.1.19), transferase (EC:2.6.1.1) and oxidoreductase (EC:1.4.1.1) expressed in JGI-0000079-D21 were enriched at a dose of 600 mg / g·VSS(R3)MnFe2O4. Due to the presence of almost complete metabolic genes for glucose and amino acids, pyruvate can become an important metabolite in the catabolism of organic macromolecules. At the same time, amino acids hydrolyzed by proteins can be converted into pyruvate through transamination, deamination and decarboxylation. Pyruvate can be converted into different VFAs under the action of different functional enzymes. It is worth noting that the abundance of genes involved in the expression of functional enzymes involved in acidogenesis in the MnFe2O4-mediated group was higher than that in the control group. This phenomenon shows that the mediation of MnFe2O4 can promote the expression of genes related to acidogenesis.

[0074] About collaborative processes Fig.12 ), it is well known that the conversion pathway of propionate can be divided into important reactions such as the oxidation of succinate to fumarate, the conversion of malate to oxaloacetate, and the generation of acetyl-CoA from pyruvate. Meanwhile, the conversion pathway of butyrate may be related to the oxidation of butyl-CoA to crotonyl-CoA. Symbiotic bacteria such as Syntrophomonas or Syntrophobacter can use magnetite to establish a DIET-based symbiotic process with methanogens. Notably, 3-hydroxybutyl-CoA dehydrogenase (EC:4.2.1.55) was derived from Syntrophomonas and DMER64, while the gene encoding propionyl-CoA carboxylase (EC:6.4.1.3) was assigned to Syntrophobacter. More significantly, at 600 mg / g·VSS MnFe2O4, genes encoding methylenetetrahydrofolate dehydrogenase (EC:1.5.1.5), methyltetrahydrofolate cyclohydrolase (EC:3.5.4.9), and tetrahydrofolate formylases (EC:6.3.4.3), which are involved in acetate oxidation metabolism, were more abundant and mainly assigned to Acetoanaerobium, further confirming that Acetoanaerobium can use acetate oxidation as an electron pool for dissimilated manganese reduction (DMnR) process. In addition, cytochrome-c oxidase (EC:1.9.3.1), a gene encoding cytochrome c, was strongly expressed at 600 mg / g·VSS MnFe2O4, mainly from Acetoanaerobium and Syntrophomonas. This further confirms that DIET is more likely to occur in the acetate oxidation stage and co-culture process, with the participation of Acetoanaerobium and Syntrophomonas.

[0075] This is of great significance for determining the functional genes involved in biological methanogenesis, and provides strong evidence for revealing specific methanogenic archaea and specific methane production pathways. Fig.12 It can be seen that there are two main methanogenic pathways in all AD systems, namely the hydrogenotrophic pathway and the acetate oxidation pathway. In the hydrogenotrophic pathway, CO2 is converted to formylmethane furan, 5,10-methyl-THMPT, and finally to CH4. In the acetyl cleavage pathway, the genes encoding enzymes related to acetyl cleavage methanogenesis are acetyl-CoA carboxylase (EC:6.4.1.2), phosphate acetyltransferase (EC:2.3.1.8), acetate kinase (EC:2.7.2.1), and acetate-CoA ligase (EC:6.2.1.1). Functional genes involved in the hydrogenotrophic and acetate cleavage pathways were detected at the same time, which means that the production of methane by CO2 reduction and acetate decarboxylation is coexistent. Compared with R0, the genes encoding acetate kinase (EC:2.7.2.1), phosphate acetyltransferase (EC:2.3.1.8), and acetyl-CoA carboxylase (EC:6.4.1.2) mediated by MnFe2O4 showed a strong response. These corresponding genes may be derived from the enrichment of Methanosaeta. Meanwhile, genes encoding formate dehydrogenase (EC:1.2.1.2) and methyltetrahydromethanophenanthrene cyclohydrolase (EC:3.5.4.27) showed high intensity responses in the control group ( Fig.13 ), which may be related to the high abundance of Methanobacteria. In addition, the genes encoding formylmethanofuran (EC: 2.3.1.101), coenzyme F 420 Genes for hydrogenase (EC:1.12.98.1) and methylenetetrahydromethane opterin dehydrogenase (EC:1.5.98.1) were enriched in the MnFe2O4-mediated group ( Fig.13 ), which may come from Methanosaeta due to its abundant content in the MnFe2O4-mediated group. Obviously, formylmethanofuran (EC:2.3.1.101), coenzyme F 420 Hydrogenase (EC:1.12.98.1) and methylenetetrahydromethane dehydrogenase (EC:1.5.98.1) showed higher intensities in the MnFe2O4-mediated group than genes encoding enzymes related to acetate oxidation methanogenesis (EC:2.7.2.1, EC:2.3.1.8, and EC:6.4.1.2), although the abundance of Methanobacterium corresponding genes (EC:1.2.1.2 and EC:3.5.4.27) was lower. This suggests that Methanosaeta may accept electrons from specific electron-donating bacteria through DIET under the mediation of MnFe2O4, thereby triggering methanotrophic bacteria to reduce CO2 to CH4.

[0076] In the control group dominated by the IHT mechanism, genes encoding the CO2 reduction pathway were expressed in Methanobacterium (EC:2.3.1.101, EC:1.12.98.1 and EC:1.5.98.1), while enzymes related to acetone cleavage and methanogenesis were expressed in Methanobacterium (EC:2.7.2.1, EC:2.3.1.8 and EC:6.4.1.2). In the MnFe2O4-mediated group, genes encoding the CO2 reduction pathway were expressed in Methanosaeta (EC:2.3.1.101, EC:1.12.98.1 and EC:1.5.98.1), while a small number of genes encoding enzymes related to acetone cleavage and methanogenesis were expressed in Methanosaeta (EC:2.7.2.1, EC:2.3.1.8 and EC:6.4.1.2), proving that Methanosaeta has the ability to participate in CO2 reduction and DIET processes.

[0077] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. An application of a magnetic composite electron mediator material for driving anaerobic iron respiration coupled with carbon reduction, characterized in that: Including at least one of the following uses (A) to (D): (A) Application in promoting SCOD degradation of protein-organic wastewater; (B) Application in promoting acid production by anaerobic digestion of protein organic wastewater; (C) Application in promoting methanogenesis by anaerobic digestion of protein-based organic wastewater; (D) Application in promoting the change of microbial community in anaerobic digestion of proteinaceous organic wastewater; The magnetic composite electron mediator material for driving anaerobic iron respiration coupled with carbon reduction is an iron-manganese bimetallic-based material.

2. The use according to claim 1, characterized in that: The iron-manganese bimetallic material is MnFe2O4.

3. The use according to claim 2, characterized in that: The added amount of MnFe2O4 is 200-800 mg / g·VSS.

4. The use according to claim 3, characterized in that: The added amount of MnFe2O4 is 600 mg / g·VSS.

5. The use according to claim 2, characterized in that: The MnFe2O4 is synthesized by a hydrothermal co-precipitation method.

6. The use according to claim 2, characterized in that: The preparation method of MnFe2O4 is as follows: manganese salt and iron salt are taken according to the molar ratio of Mn / Fe elements, dissolved in oxygen-free deionized water, and fully mixed with NaOH solution. The obtained mixed solution is heated at 80-90°C for 2-3h, naturally cooled to room temperature, and the black solid is collected by magnetic separation, and washed with deionized water to neutrality, and then the black solid is first dispersed in a dimethyl sulfoxide solution at 70-80°C, ultrasonically treated, and finally the MnFe2O4 is centrifuged and washed with distilled water and anhydrous ethanol.

7. The use according to claim 6, characterized in that: The manganese salts include manganese chloride hydrate, manganese nitrate hydrate and manganese sulfate hydrate.

8. The use according to claim 6, characterized in that: The iron salts include ferric chloride hydrate, ferric nitrate hydrate and ferric sulfate hydrate.

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