Preparation method and application of Fe3O4 / BC composite material

Abstract

The invention belongs to the technical field of solid waste resource utilization, and particularly relates to a preparation method and application of a Fe3O4 / BC composite material. By combining the advantages of a carbon-based conductive material and an iron-based conductive material, charcoal-loaded nano ferroferric oxide (Fe3O4 / BC) is prepared to serve as a conductive substance of DIET, and by adding Fe3O4 / BC, the feed organic load (OLR) and the raw material gas production rate in the anaerobic fermentation process of the rotten vegetable leaves are effectively improved, volatile fatty acid and COD are reduced, and the anaerobic fermentation flora structure is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of solid waste resource utilization, and more specifically relates to a preparation method of a Fe3O4 / BC composite material and an application thereof. Background Art

[0002] In recent years, with the vigorous development of China's economy and society, people's demand for vegetables in daily life has increased day by day, and the area and output of vegetable cultivation have also increased year by year. However, while the vegetable cultivation industry has boosted China's economic development, it has also brought huge challenges to the ecological environment in rural areas. In vegetable planting areas and storage and transfer sites, a large number of vegetable wastes have not been properly handled, causing varying degrees of pollution to the local ecological environment. If these wastes are not properly and effectively disposed of, they will not only affect soil quality and damage water resources, but also have a negative effect on the surrounding air and ecological balance.

[0003] Due to the high water content and perishable nature of vegetable waste, the gas production rate is low when using vegetable waste as the sole substrate for anaerobic fermentation, and it is easy to cause the accumulation of VFA and lead to acidification of the fermentation system. Alvarez and LidénandJiang et al. reported that using only vegetable waste as a single substrate for AD caused the accumulation of VFA, resulting in a decrease in system pH and ultimately AD failure. Previous related reports have limited the organic load (OLR) of vegetable waste to 3.0 g.VS.L -1 .d -1 In order to avoid acidification during anaerobic fermentation of vegetable waste, most of the research on anaerobic fermentation of vegetable waste mainly uses animal manure, sludge and other organic matter mixed with vegetable waste for co-digestion. However, in the vicinity of large-scale vegetable planting production areas and storage cold storages, there are few large-scale farms or sewage treatment plants to provide sufficient co-digestion raw materials.

[0004] At the same time, there are complex microbial flora in anaerobic fermentation, and interspecies electron transfer between acetogens and methanogens has an important influence on the methanogenesis process. In recent years, many studies have found that (IET) mainly includes three types: interspecies hydrogen transfer (IHT), interspecies formate transfer (IFT), and interspecies direct electron transfer (DIET). IHT and IFT require hydrogen or formate as carriers to transfer electrons. The IHT process is a process that uses H2 molecules as diffusion electron carriers to transfer electrons from propionic acid and butyric acid to carbon dioxide. Since H2 is produced by hydrogen-producing acetogens using organic substrates such as butyric acid, propionic acid and ethanol as raw materials, it is usually necessary to absorb energy during the metabolic process. To achieve the above reaction, only by using methanogens to consume H2 in the system and reduce the hydrogen partial pressure in the system can it be achieved under the joint metabolism of hydrogen-producing methanogens and methanogens. Once methanogens reduce the utilization rate of H2 molecules, it will also slow down the use of propionic acid and butyric acid, which will lead to the accumulation of VFAs in the anaerobic fermentation system. Baek G et al found that the IFT process is that formate is oxidized to CO2 by formate dehydrogenase, and then CO2 is reduced to methane. Since H2 has advantages over formate in terms of low solubility and high diffusion rate, H2 may be mainly involved in short-range electron transfer, while formate may be involved in long-range electron transfer. However, during the anaerobic fermentation of vegetable tails, the acidification process is fast, and a large amount of volatile fatty acids will accumulate in the early stage, resulting in high hydrogen partial pressure and formic acid concentration, which limits the progress of IHT and IFT.

[0005] Therefore, how to provide a preparation method and application of Fe3O4 / BC composite materials, thereby increasing the feed organic load (OLR) and raw material gas production rate during anaerobic fermentation of vegetable tails, reducing volatile fatty acids and COD, and improving the anaerobic fermentation flora structure, is a problem that technical personnel in this field urgently need to solve. Summary of the invention

[0006] In order to overcome the shortcomings and deficiencies in the prior art, the present invention provides a preparation method of a Fe3O4 / BC composite material and application thereof.

[0007] In order to achieve the above object, the present invention adopts the following technical solution:

[0008] A method for preparing a Fe3O4 / BC composite material comprises the following steps:

[0009] (1) The biomass raw material is crushed and dried, and then placed in a tube furnace, argon gas is continuously introduced, the temperature is raised and maintained, and the biomass raw material is taken out after cooling to room temperature, and then washed with deionized water and anhydrous alcohol for 3-4 times respectively, dried, ground with a ball mill, passed through a 50-200 mesh sieve, and sealed for storage to obtain biochar BC;

[0010] (2) FeCl3·6H2O and FeCl2·4H2O were uniformly mixed in distilled water, and biochar BC was added. Ultrasonication was performed for 30 min, and strong stirring was performed to precipitate for 30 min. Ar gas was passed through and stirred, and ammonia water was slowly added dropwise. Stirring was continued for 30 min, and then the mixture was transferred to a water bath, and Ar gas was continuously passed through and stirred slowly. The mixture was heated at 90 °C for 3 h, and then cooled to room temperature.

[0011] (3) A strong magnet was placed at the bottom of the cooled solution to precipitate for 10 min to screen out the solid, which was then repeatedly washed with anhydrous ethanol and distilled water until the pH was 6.5-7.5, and dried at 60°C for 12 h to obtain a Fe3O4 / BC composite material.

[0012] Preferably, in step (1), the biomass raw material is selected from one of corn straw, bamboo, wheat straw, and walnut shell, and the biomass raw material is crushed and dried at 60° C. for 12 hours;

[0013] The temperature was raised to 550-600°C at a heating rate of 5-10°C / min and maintained for 3 h.

[0014] Preferably, the molar ratio of FeCl3·6H2O to FeCl2·4H2O is 40-172:20-58, the volume ratio of distilled water to ammonia water is 100-250:40-100, the mass volume ratio of BC to distilled water is 5-15:100-250 g / mL, and the concentration of ammonia water is 25-28%.

[0015] The present invention also provides a method for preparing an anaerobic fermentation inoculum of vegetable tails, using the Fe3O4 / BC composite material obtained above, and the specific operations are as follows:

[0016] 1) Taking anaerobic sludge from a biogas fermentation device, putting it into an anaerobic fermentation tank, adding nutrients and trace elements for the first time, adding Fe3O4 / BC composite materials at the same time, sealing the biogas fermentation device, acclimating and culturing the strains, controlling the fermentation temperature at 25-50°C, starting to produce biogas after 1 day, and the gas production decreases after 7 days;

[0017] 2) After 10 days, nutrients, trace elements and Fe3O4 / BC composite materials were added and the second inoculum was cultured under the same conditions;

[0018] 3) After the second inoculum was cultured for 7 days, nutrient salts, trace elements and Fe3O4 / BC composite materials were added, and the third inoculum was cultured under the same conditions, and the inoculum of anaerobic fermentation of vegetable tails was obtained after 7 days.

[0019] Preferably, the amount of nutrient salt added in step 1) is 1.5-2.0 kg / m 3 In step 2), the amount of nutrient salt added is 3.0-4.0 kg / m3 In step 3), the amount of nutrient salt added is 4.5-5.0 kg / m 3 .

[0020] Preferably, the amount of trace elements added each time is 0.1-0.2% of the total weight of the inoculum, and the amount of Fe3O4 / BC composite material added each time is 100-400 mg / L fermentation volume.

[0021] Preferably, the nutrient salt comprises the following raw materials in parts by weight: 100 parts of calcium acetate, 20 parts of calcium propionate, 10 parts of calcium butyrate, 5 parts of urea, 5 parts of superphosphate, and 1 part of yeast extract.

[0022] Preferably, the trace elements include the following raw materials in parts by weight: 1000 parts of ferrous sulfate, 1 part of zinc chloride, 1 part of manganese chloride, 0.4 parts of nickel chloride, and 0.4 parts of cobalt chloride.

[0023] The present invention also provides the use of the Fe3O4 / BC composite material prepared by the method in promoting the anaerobic fermentation of vegetable waste.

[0024] The application of the above-mentioned Fe3O4 / BC composite material in promoting anaerobic fermentation of vegetable waste is specifically performed as follows:

[0025] (a) Early stage: adding the above-obtained inoculum to the fermenter, the inoculum accounts for 20%-40% of the effective fermentation volume, the organic load of the vegetable tail added for the first time is 0.688g.VS / Ld, and then the OLR is increased every 3d, each time the OLR is increased by 10%-20%, and at the same time, Fe3O4 / BC composite material is added each time the feed is fed, and the addition amount is 200mg / L feed amount, and no feed is discharged in the early stage until the fermenter reaches the effective fermentation volume;

[0026] (b) When the fermentation tank reaches the effective fermentation volume, the OLR is increased every 5-8 days, by 5%-10% each time, until the OLR reaches 3.715 g.VS / Ld, and then no longer increased;

[0027] (c) When the fermentation tank reaches the effective fermentation volume, the material is discharged first and then fed in every day, and the discharge amount is equal to the feed amount to keep the effective fermentation volume unchanged. Fe3O4 / BC composite material is added once every 15-30 days, and the addition amount is 200-400 mg / L. The discharged biogas slurry is placed in a liquid storage barrel and the Fe3O4 / BC is magnetically separated and recovered. After collecting for 5-10 days, the recovered Fe3O4 / BC is uniformly added to the fermentation tank along with the raw materials.

[0028] It can be seen from the above technical solutions that, compared with the prior art, the present invention provides a system and method for preparing a suture with a developing function, which has the following beneficial effects:

[0029] 1. The present invention combines the advantages of carbon-based conductive materials and iron-based conductive materials to prepare biochar-loaded nano-ferroferric oxide (Fe3O4 / BC) as a conductive material for DIET. By adding Fe3O4 / BC, the feed organic load (OLR) and raw material gas production rate during the anaerobic fermentation of vegetable tails are effectively improved, volatile fatty acids and COD are reduced, and the anaerobic fermentation flora structure is improved.

[0030] 2. In the present invention, the carbon-based conductive material has good electrical conductivity and a porous structure with a high specific surface area, so it can provide sufficient production space and metabolic conditions for microorganisms. Iron-based conductive materials can affect the anaerobic digestion mode by regulating the secretion of extracellular polymers and promote the anaerobic process propionate metabolism to trigger methane production. At the same time, the two materials are easy to combine. The nano-iron tetroxide particles are smaller in size and can be loaded in the pores inside the biochar. The biochar particles are larger in size. After combining, it is easy to recover the catalyst magnetically. Anaerobic microorganisms can attach to the surface and internal pores of biochar, and the biochar itself and nano-iron tetroxide are used as conductive media to transfer electrons between different conductive bacteria. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0032] Figure 1 It is the daily gas production and OLR of anaerobic fermentation of vegetable waste.

[0033] Figure 2 Cumulative gas production.

[0034] Figure 3 is the abundance of bacterial communities at the phylum level.

[0035] Figure 4 is the abundance of bacterial communities at the genus level.

[0036] Figure 5 is the abundance of the archaeal community at the phylum level.

[0037] Figure 6 is the abundance of the archaeal community at the genus level. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present invention are described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] Example 1

[0040] A method for preparing a Fe3O4 / BC composite material comprises the following steps:

[0041] (1) Preparation of biochar (BC): BC was prepared using corn stalks as raw materials. The corn stalks were crushed and placed in an oven and dried at 60 °C for 12 h. The dried and crushed corn stalks were then placed in a tubular furnace and argon was continuously introduced. The temperature was raised to 550 °C at a rate of 5 °C / min and maintained for 3 h. After cooling to room temperature, the stalks were taken out and washed three times with deionized water and anhydrous alcohol respectively. The stalks were placed in an oven for drying, ground with a ball mill, and passed through a 100-mesh sieve and sealed for storage.

[0042] (2) Preparation of Fe3O4 / BC: Fe3O4 nanoparticles were prepared by coprecipitation method. 86mmol of FeCl3·6H2O and 43mmol of FeCl2·4H2O were uniformly mixed in 200mL of distilled water. 10.00g of biochar (BC) was weighed and added to the above solution. Ultrasonication was performed for 30min, and precipitation was performed after strong stirring for 30min. Ar was introduced and stirred, and 80mL of ammonia water was slowly added dropwise, and stirring was continued for 30min. Then, the mixture was transferred to a water bath, Ar gas was continuously introduced and stirred slowly, and the mixture was heated at 90℃ for 3h and cooled to room temperature. Finally, the reaction solution was placed in a beaker, and a strong magnet was placed at the bottom of the beaker to precipitate for 10min to screen out the solid. The solid was then washed repeatedly with anhydrous ethanol and distilled water until the pH was 7.0, and dried at 60℃ for 12h to obtain a solid powder, i.e., Fe3O4 / BC composite material.

[0043] The Fe3O4 / BC particles prepared by the present invention have a diameter of about 500-800 μm, BC is irregularly granular, and has a large number of pore structures on the surface. Fe3O4 nanoparticles are attached to the BC surface and inside the pore structure, and the Fe3O4 nanoparticles are consistent with pure Fe3O4 [JCPDS No.88-0315] of cubic inverse spinel structure, and the grain size of the Fe3O4 nanoparticles is calculated to be 18.6 nm. There is a clustering phenomenon between the Fe3O4 nanoparticles, forming nanoclusters with a size of about 200nm-800nm. Using element distribution mapping technology, it can be observed that the Fe element is uniformly distributed on the BC surface, and the surface micro-area is quantitatively analyzed by energy dispersive spectrometer (EDS), and the mass percentage of the Fe element is 68.51%, and the mass percentage of the C element is 11.70%, indicating that a large number of Fe3O4 nanoparticles are loaded on the BC surface.

[0044] The hysteresis loop of Fe3O4 / BC basically passes through the origin, there is no hysteresis phenomenon, no residual magnetization and coercive force are found, indicating that the prepared Fe3O4 / BC has superparamagnetism at room temperature. The saturation magnetization of Fe3O4 / BC is 55.75emu / g.

[0045] A method for preparing anaerobic fermentation inoculum of vegetable tailings, using the Fe3O4 / BC composite material obtained above, the specific operation is as follows:

[0046] Take anaerobic sludge from a biogas fermentation device with good biogas production performance (rural household biogas pool or large biogas tank), put it into an anaerobic fermentation tank, and prepare inoculum by adding nutrients, trace elements and Fe3O4 / BC. The first addition is 1.88kg / m 3 The total weight of nutrient salt and trace elements of 0.1% of the inoculum was added, and 100 mg / L of Fe3O4 / BC was added at the same time. The biogas fermentation device was sealed for bacterial strain acclimatization and the fermentation temperature was controlled at 25°C-50°C. Biogas production began after 1 day, and the gas production decreased after 7 days. After 10 days, nutrient salt, trace elements and Fe3O4 / BC were added, and the nutrient salt was 3.3 kg / m 3 The trace elements were 0.1% of the total weight of the inoculum, and the Fe3O4 / BC was 100 mg / L fermentation volume. The second inoculum was cultured under the same conditions. After the second inoculum was cultured for 7 days, nutrients, trace elements and Fe3O4 / BC were added. The nutrient salt was 4.7 kg / m 3 , trace elements were 0.1% of the total weight of the inoculum, Fe3O4 / BC was 100 mg / L fermentation volume, and the third inoculum was cultured under the same conditions. After 7 days, the inoculum for anaerobic fermentation of vegetable tails was obtained.

[0047] The nutrient salt comprises the following raw materials in parts by weight: 100 parts of calcium acetate, 20 parts of calcium propionate, 10 parts of calcium butyrate, 5 parts of urea, 5 parts of superphosphate, and 1 part of yeast extract.

[0048] The trace elements include the following raw materials in parts by weight: 1000 parts of ferrous sulfate, 1 part of zinc chloride, 1 part of manganese chloride, 0.4 parts of nickel chloride, and 0.4 parts of cobalt chloride.

[0049] The application of Fe3O4 / BC composite material in promoting anaerobic fermentation of vegetable waste is as follows:

[0050] (a) Early stage: When the anaerobic fermentation of vegetable tails was started, an inoculum accounting for 30% of the effective fermentation volume was added to the fermenter. The organic load OLR of the vegetable tails added for the first time was 0.688 g.VS / Ld, and then the OLR was increased by 10% every 3 days. At the same time, Fe3O4 / BC composite material was added each time the feed was added, and the addition amount was 200 mg / L feed amount. No feed was discharged in the early stage until the fermenter reached the effective fermentation volume;

[0051] (b) When the fermenter reaches the effective fermentation volume, the OLR is increased by 5% every 5 days until it reaches the maximum OLR of 3.715 g.VS / Ld, and then it is no longer increased;

[0052] (c) When the fermentation tank reaches the effective fermentation volume, the material is discharged first and then fed in every day. The discharge amount is equal to the feed amount to keep the effective fermentation volume unchanged. Fe3O4 / BC composite material is added once every 20 days, and the addition amount is 200 mg / L. The discharged biogas slurry is placed in a liquid storage barrel and the Fe3O4 / BC is recovered by magnetic separation. After collecting for 7 days, the recovered Fe3O4 / BC is added to the fermentation tank along with the raw materials.

[0053] Comparative experiment:

[0054] The anaerobic fermentation of vegetable tails was carried out in four groups of CSTR reactors, named BG, VEG.100mg, VEG.200mg, and VEG.300mg, respectively. BG was the blank control group without adding Fe3O4 / BC, and VEG.100mg, VEG.200mg, and VEG.300mg were the experimental groups with the added concentrations of Fe3O4 / BC being 100mg.L -1 , 200mg.L -1 , 300mg.L -1 The CSTR adopts a continuous feeding fermentation process, the fermentation temperature is constant at 35°C, the stirring speed is 60r / min, stirring is once every 4h, and each stirring time is 20min.

[0055] The process of adding Fe3O4 / BC is as follows: when the fermentation volume of the CSTR reactor does not reach 4.5L, Fe3O4 / BC is added according to the feed amount during daily feeding, and the addition is stopped after reaching 4.5L on the 26th day. In the later stage, in order to make up for the loss of Fe3O4 / BC caused by discharging, Fe3O4 / BC is supplemented every 12 days, and the amount of each supplement is calculated based on 1L of feed amount.

[0056] The main steps of AD are as follows: At the start, add 1000ml of inoculum to the fermenter, and continue fermentation for 7 days until the gas production ends. In order to prevent the rapid accumulation of VFAs in the early stage of fermentation, the first tail vegetable OLR is 0.688g.VS / Ld (calculated according to the fermentation volume of 4.5L), and the OLR is increased by 20% every 3 days in the early stage. No material is discharged in the early stage until the fermentation volume reaches 4.5L before discharging. When the fermentation volume reaches 4.5L, increase the OLR by 20% every 5 days, observe the gas production rate, and when the gas production rate increases slowly, reduce the OLR increase rate to 10% until the gas production rate reaches the maximum value and then begins to decline. At this time, it means that the fermentation system exceeds the maximum OLR. After that, the OLR begins to decrease according to the change in gas production until the system is stable. After the gas production begins to rise, the OLR is gradually increased. When the OLR is increased to 95% of the maximum OLR before acidification, the increase of OLR is stopped, and the stable operation is more than 10 days. If the gas production rate does not change much, the optimal OLR of the system is reached.

[0057] from Figure 1 It can be seen that the fermentation cycle is divided into three stages. Stage I is the early stage of AD. With the gradual increase of OLR, the volumetric gas production rate of the experimental group increased more than that of BG. On the 26th day, the anaerobic fermentation system reached a maximum fermentation volume of 4.5L. After that, the same volume of biogas slurry needed to be discharged after daily feeding. On the 27th day, the volumetric gas production rates of the experimental group and BG reached their maximum values, which were 2.951LL and 2.5LL, respectively. -1 .d -1 (VEG.200mg), 2.840LL -1 .d -1 (VEG.300mg), 2.450LL -1 .d -1 (VEG.100mg) and 2.421LL -1 .d -1(BG), at this time the OLR is 2.945g.VS / Ld. Stage II is the AD instability recovery period. As the OLR increases again, the gas production rate begins to decline. Interestingly, on the 42nd and 43rd days, when the OLR reaches the maximum value of 3.887g.VS / Ld, the gas production rate suddenly increases and then begins to decline sharply, indicating that VFAs have accumulated inside the fermentation system, and the OLR exceeds the maximum value that the system can withstand. The activity of methanogenic archaea has been inhibited, resulting in a significant reduction in the gas production rate. Subsequently, the OLR began to decrease rapidly, and the rate of gas production slowed down until the 53rd to 56th day, when the OLR was about to reach 1.183g.VS / Ld, and the gas production rate tended to stabilize. During this process, VEG.100mg decreased more, and VEG.200mg decreased less, indicating that Fe3O4 / BC has the best resistance to OLR shock at an addition amount of 200mg / L. Stage III is the recovery period of AD instability. From the 57th day, OLR gradually increased. Similar to the early stage of fermentation, the gas production rate of the experimental group was still significantly higher than that of BG. When OLR reached 3.715 g.VS / Ld, the gas production rate was basically close to the highest value in the early stage. On the 70th day, the gas production rate reached the maximum value of 3.009LL. -1 .d -1 (VEG.200mg), 2.956LL -1 .d -1 (VEG.300mg), 2.757LL -1 .d -1 (VEG.100mg) and 2.844LL -1 .d -1 (BG), after running for 12 days under this OLR condition, the gas production rate did not show a significant decrease, and the optimal OLR was achieved.

[0058] from Figure 2 It can be seen from the cumulative total gas production that there was no obvious difference between the experimental group and the control group in the early stage, but with the continuous improvement of OLR, VEG.200mg and VEG.300mg in the experimental group were significantly higher than BG, and the difference between VEG.100mg and BG was not obvious. When OLR reached the optimal level in the later stage, the trend of cumulative total gas production was VEG.200mg>VEG300mg>VEG.100mg>BG, indicating that the addition of Fe3O4 / BC can indeed increase the gas production of anaerobic fermentation of vegetable tails. When the addition amount is 100mg / L, it has little effect on the gas production. When it reaches 300mg / L, it will inhibit the increase in gas production, so the optimal addition amount is 200mg / L.

[0059] Diversity index (Simpson, Shannon), richness index (Chao1, Ace) and coverage index (Coverage) were used to evaluate the species diversity, richness, coverage and other information of the four groups of microbial communities. For bacterial and archaeal communities, the coverage of the four groups of samples was 99%, indicating that the test results can truly reflect the structure of bacterial and archaeal communities in the samples. As shown in Table 1, the Shannon and Simpson values ​​of bacteria VEG.200mg and VEG.300mg increased compared with BG after adding Fe3O4 / BC, indicating that the bacterial communities in the VEG.200mg and VEG.300mg experimental groups have higher diversity, and the diversity of bacterial communities in the VEG.100mg experimental group with a small amount of Fe3O4 / BC decreased. From the values ​​of chao1 and Ace, it can be seen that except for the VEG.100mg experimental group, the values ​​of the other experimental groups have increased compared with the BG group, indicating that the bacterial communities in the VEG.200mg and VEG.300mg experimental groups have higher abundance. The above data show that the appropriate addition of Fe3O4 / BC is beneficial to improve the diversity and abundance of bacteria in the anaerobic fermentation system. As shown in Table 2, the diversity of archaea after adding Fe3O4 / BC is VEG.100mg>VEG.200mg>BG>VEG.300mg, indicating that the addition of appropriate Fe3O4 / BC is beneficial to improve the diversity of archaea, but excessive Fe3O4 / BC will also inhibit the growth and reproduction of archaea and reduce diversity. Chao1 and Ace of VEG.200mg are significantly higher than those of other groups, indicating that the addition concentration of Fe3O4 / BC of 200mg / L is beneficial to improve the abundance of archaea.

[0060] Table 1 Bacterial diversity index statistics

[0061]

[0062] Table 2. Archaeal diversity index statistics

[0063]

[0064] Changes in bacterial community composition and abundance at the phylum and genus levels in the BG and different concentrations of Fe3O4 / BC experimental groups Figure 3 and Figure 4As shown. Based on the Top10 phylum level, Bacteroidota, Firmicutes, Cloacimonadota, Patescibacteria, Synergistota and Spirochaetota are the main phyla, and these six phyla account for more than 90% of the total abundance. The main function of Bacteroidota is to degrade macromolecular organic matter such as sugar, cellulose, amino acids and organic acids into acetic acid, providing substrates for acetic acid-trophic methanogens. Bacteroidota (27.53%) in the BG group and Bacteroidota (16.14%) in the VEG.200mg group. The addition of Fe3O4 / BC inhibited the growth and reproduction of Bacteroidota, reduced the rate of cellulose hydrolysis and acidification, and prevented the rapid accumulation of VFAs during the fermentation of vegetable tails. Cloacimonadota is very important in the process of cellulose hydrolysis, and studies have shown that the abundance of this bacterium has a significant relationship with biogas production. Wei et al. found in the study of corn straw and chicken manure co-digestion that although the abundance of Cloacimonadota was less than 1.5%, it played an important role in the hydrolysis and acidification of the substrate. Yang found in the study of sludge straw co-digestion system that there is a positive correlation between the abundance of Cloacimonadota and the gas production of the system. The higher the abundance of Cloacimonadota in the system, the higher the cumulative methane production of the system. The addition of Fe3O4 / BC significantly increased the abundance of Cloacimonadota in the experimental group, which is related to Figure 2 From the perspective of cumulative gas production, it is once again proved that there is a positive correlation between the abundance of Cloacimonadota and the gas production of the system. Based on the Top10 genus level, the top three species in abundance are Thermovirga, Christensenellaceae_R-7_group and Sphaerochaeta. Thermovirga has strong hydrolysis ability and sulfate reduction ability. Christensenellaceae_R-7_group can promote the degradation of cellulose, thereby providing a carbon source for the growth of microorganisms. Sphaerochaeta plays a certain role in methanogenesis and also has cellulose degradation function. The addition of Fe3O4 / BC increased the abundance of these three species, among which the VEG.200mg group was most obvious for Christensenellaceae_R-7_group and Sphaerochaeta.

[0065] During the anaerobic fermentation process, the final methanogenic stage is entirely participated by the archaeal community. Figure 5 and Figure 6The abundance of the top 10 archaeal communities at the phylum and genus levels shows that based on the phylum level, Synergistota, Halobacterota and Bacteroidota account for more than 99% of the abundance, and are the dominant bacterial phyla in the anaerobic fermentation of vegetable tails. Synergistota (53.77%) in the BG group and Synergistota (68.23%) in the VEG.200mg group. Synergistota is a hydrogen-producing and acetic acid-producing bacterium that can produce hydrogen and acetic acid using amino acid substances, which is important for balancing the acid production and methanogenesis process. Zhao et al. found that in the system with granular activated carbon added, Synergistaceae uses amino acids as substrates, and granular activated carbon can be used as an electrical connection for the DIET of methanogens. The addition of Fe3O4 / BC greatly increased the abundance of Synergistota in the anaerobic fermentation system of vegetable tails, which can indicate that Fe3O4 / BC can be used as an electrical connection to effectively promote the DIET process of methanogens. Based on the genus level, Thermovirga, Methanosarcina, DMER64 and Methanosaeta accounted for more than 99% of the abundance. Thermovirga can adapt to high-salt environments and has sulfate reduction ability. Liang et al. added magnetite to sulfate organic wastewater and found that the addition of Fe3O4 could enrich Thermovirga. Yang et al. studied the effect of biochar on anaerobic digestion of pig manure and found that Thermovirga was dominant with biochar addition and might participate in DIET together with Methanothrix. The addition of Fe3O4 / BC significantly increased the abundance of Thermovirga, which is beneficial to the DIET process of vegetable tails.

[0066] The results of the anaerobic fermentation experiment of vegetable waste showed that 3.009LL can be obtained under the conditions of Fe3O4 / BC addition of 200mg / L and organic loading rate (OLR) of 3.715g.VS / Ld. -1 d -1 The maximum volumetric gas production rate and 0.658Lg -1 VS -1The study found that at high organic loading rates, the accumulation of volatile fatty acids (VFAs) was effectively inhibited, especially the accumulation of butyrate was greatly reduced. The diversity and abundance of bacteria and archaea increased. At the genus level, the abundance of bacteria Christensenellaceae_R-7group and Sphaerochaeta, as well as the archaea Thermovirga, increased.

[0067] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a Fe3O4 / BC composite material, characterized in that: The following steps are involved: (1) The biomass raw material is crushed and dried, and then placed in a tube furnace, argon gas is continuously introduced, the temperature is raised and maintained, and the biomass raw material is taken out after cooling to room temperature, and then washed with deionized water and anhydrous alcohol for 3-4 times respectively, dried, ground with a ball mill, passed through a 50-200 mesh sieve, and sealed for storage to obtain biochar BC; (2) FeCl3·6H2O and FeCl2·4H2O were uniformly mixed in distilled water, and biochar BC was added. Ultrasonication was performed for 30 min, and strong stirring was performed to precipitate for 30 min. Ar gas was passed through and stirred, and ammonia water was slowly added dropwise. Stirring was continued for 30 min, and then the mixture was transferred to a water bath, and Ar gas was continuously passed through and stirred slowly. The mixture was heated at 90 °C for 3 h, and then cooled to room temperature. (3) A strong magnet was placed at the bottom of the cooled solution to precipitate for 10 min to screen out the solid, which was then repeatedly washed with anhydrous ethanol and distilled water until the pH was 6.5-7.5, and dried at 60°C for 12 h to obtain a Fe3O4 / BC composite material.

2. The method for preparing a Fe3O4 / BC composite material according to claim 1, characterized in that: In step (1), the biomass raw material is selected from one of corn straw, bamboo, wheat straw, and walnut shell, and the biomass raw material is crushed and dried at 60° C. for 12 hours; The temperature was raised to 550-600°C at a heating rate of 5-10°C / min and maintained for 3 h.

3. The method for preparing a Fe3O4 / BC composite material according to claim 1, characterized in that: The molar ratio of FeCl3·6H2O to FeCl2·4H2O is 40-172:20-58, the volume ratio of distilled water to ammonia water is 100-250:40-100, the mass volume ratio of BC to distilled water is 5-15:100-250 g / mL, and the concentration of ammonia water is 25-28%.

4. A method for preparing an anaerobic fermentation inoculum of vegetable tailings, characterized in that: Using the Fe3O4 / BC composite material obtained in claim 1, the specific operation is as follows: 1) Taking anaerobic sludge from a biogas fermentation device, putting it into an anaerobic fermentation tank, adding nutrients and trace elements for the first time, adding Fe3O4 / BC composite materials at the same time, sealing the anaerobic fermentation tank, acclimating and culturing the strains, controlling the fermentation temperature at 25-50°C, starting to produce biogas after 1 day, and the gas production decreases after 7 days; 2) After 10 days, nutrients, trace elements and Fe3O4 / BC composite materials were added and the second inoculum was cultured under the same conditions; 3) After the second inoculum was cultured for 7 days, nutrient salts, trace elements and Fe3O4 / BC composite materials were added, and the third inoculum was cultured under the same conditions, and the inoculum of anaerobic fermentation of vegetable tails was obtained after 7 days.

5. The method for preparing the anaerobic fermentation inoculum of vegetable tail vegetables according to claim 4, characterized in that: The amount of nutrient salt added in step 1) is 1.5-2.0 kg / m 3 In step 2), the amount of nutrient salt added is 3.0-4.0 kg / m 3 In step 3), the amount of nutrient salt added is 4.5-5.0 kg / m 3 .

6. The method for preparing an anaerobic fermentation inoculum of vegetable tail vegetables according to claim 4, characterized in that: The amount of trace elements added each time is 0.1-0.2% of the total weight of the inoculum, and the amount of Fe3O4 / BC composite material added each time is 100-400 mg / L fermentation volume.

7. The method for preparing an anaerobic fermentation inoculum of vegetable tail vegetables according to claim 4, characterized in that: The nutrient salt comprises the following raw materials in parts by weight: 100 parts of calcium acetate, 20 parts of calcium propionate, 10 parts of calcium butyrate, 5 parts of urea, 5 parts of superphosphate, and 1 part of yeast extract.

8. The method for preparing an anaerobic fermentation inoculum of vegetable tail vegetables according to claim 4, characterized in that: The trace elements include the following raw materials in parts by weight: 1000 parts of ferrous sulfate, 1 part of zinc chloride, 1 part of manganese chloride, 0.4 parts of nickel chloride, and 0.4 parts of cobalt chloride.

9. Application of the Fe3O4 / BC composite material prepared according to the method according to any one of claims 1 to 3 in promoting anaerobic fermentation of vegetable waste.

10. The use of the Fe3O4 / BC composite material according to claim 9 in promoting anaerobic fermentation of vegetable waste, characterized in that: The specific operations are as follows: (a) Early stage: adding the inoculum obtained in claim 4 to the fermenter, the inoculum accounts for 20%-40% of the effective fermentation volume, the organic load OLR of the first addition of vegetable tail vegetables is 0.688g.VS / Ld, and then the OLR is increased every 3 days, each time the OLR is increased by 10%-20%, and at the same time, Fe3O4 / BC composite material is added each time the feed is fed, and the addition amount is 200mg / L feed amount, and no feed is discharged in the early stage until the fermenter reaches the effective fermentation volume; (b) When the fermentation tank reaches the effective fermentation volume, the OLR is increased every 5-8 days, by 5%-10% each time, until the OLR reaches 3.715 g.VS / Ld, and then no longer increased; (c) When the fermentation tank reaches the effective fermentation volume, the material is discharged first and then fed in every day, and the discharge amount is equal to the feed amount. Fe3O4 / BC composite material is added every 15-30 days, and the addition amount is 200-400 mg / L. The discharged biogas slurry is placed in a liquid storage barrel and the Fe3O4 / BC is magnetically separated and recovered. After collecting for 5-10 days, the recovered Fe3O4 / BC is uniformly added to the fermentation tank along with the raw materials.

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