Magnetic iron-carbon composite material and application thereof in increasing methane yield in anaerobic digestion system

Abstract

The invention provides a magnetic iron-carbon composite material and application thereof to increase the methane yield in an anaerobic digestion system, and relates to the technical field of sewage treatment and resource utilization. Sodium citrate is used as a green precursor, honeycomb porous biochar is prepared through a pyrolysis method, iron oxide is introduced in situ for loading, and the magnetic iron-carbon composite material is prepared. Compared with some existing magnetic porous carbon (such as biomass carbon based on agricultural and forestry wastes), sodium citrate is used as a precursor, the characteristics of low cost and environmental protection are fully exerted, a highly-ordered honeycomb structure is formed, and the material is superior to random pores of traditional magnetic carbon; ideal habitat sites are provided for acid-producing bacteria and methanogens, and the interspecific electron transfer distance is reduced, so that efficient metabolism synergy in an anaerobic system is promoted. Meanwhile, due to the excellent paramagnetism, the problem of accumulation caused by the fact that a traditional powder carbon material is lost along with water or detained in an anaerobic system is solved, and the material loss and the operation cost are reduced to a certain extent.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment and resource utilization technology, specifically to a magnetic iron-carbon composite material and its application in increasing methane production in anaerobic digestion systems. Background Technology

[0002] Anaerobic digestion comprises four stages: hydrolysis, acidification, acetogenesis, and methanogenesis. It mainly involves the hydrolysis of macromolecules, the acidification of small organic molecules, and methanogenesis. The microorganisms at each stage interact to maintain the balance of anaerobic digestion. Various factors, including temperature, pH, organic load, and redox potential, affect the stable operation of anaerobic digestion. High organic loads, in particular, exacerbate the imbalance between acetogenic and methanogenic bacteria, leading to the accumulation of volatile fatty acids and ultimately reducing methanogenesis yield and efficiency.

[0003] Biochar (BC) refers to the solid material produced by high-temperature pyrolysis of various biomass under anaerobic or extremely low oxygen conditions. However, traditional biochar materials suffer from insufficient surface active sites and limited electron transfer efficiency. Therefore, it is necessary to modify certain properties of biochar through chemical methods to improve its performance in practical applications. By utilizing the advantages of biochar, such as its large pore size, large specific surface area, functional groups, and electrical conductivity, and combining it with the positive effects of iron in accelerating biodegradation, promoting electron transfer, and enhancing methanogenesis, iron-carbon composite materials are obtained.

[0004] Common iron oxides used in the preparation of Fe / BC composites include iron oxide (Fe2O3), magnetic iron oxide (Fe3O4), and iron hydroxyl oxide (FeOOH). Iron oxides enhance anaerobic digestion performance by participating in direct interspecies electron transfer processes, i.e., the process in which microorganisms directly obtain electrons from another microorganism, acting as acceptors. This pathway is far more efficient at transporting electrons than interspecies hydrogen transfer.

[0005] Existing research has found that adding conductive materials to anaerobic digestion systems to improve interspecies electron transfer efficiency has become a new approach. For example, patent application CN115991561A discloses a method for increasing methane production by anaerobic digestion of sludge using biochar. This patent application involves pyrolyzing polyaluminum chloride sludge to prepare aluminum salt dehydrated sludge-based biochar, which is then added to an anaerobic digestion system for anaerobic reaction, thereby increasing methane production during anaerobic digestion by 28%, reaching a maximum of 168.8 mL / VS. However, this traditional biochar has problems such as insufficient surface active sites and limited electron transfer efficiency. In addition, patent CN119219286B discloses a conductive material composed of FeS and biochar added to a sludge anaerobic digestion system to improve the efficiency of microbial anaerobic digestion. The introduction of FeS improves the conductivity of the material, but it slowly releases sulfides in an anaerobic environment. Excessive FeS can inhibit the activity of microorganisms and is highly toxic to methanogens, thus reducing methane production. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention introduces magnetic iron oxide onto sodium citrate pyrolysis char to prepare a magnetic iron-carbon composite material. This material not only synergizes with the conductive network of biochar to promote direct interspecies electron transfer, but also allows the loaded magnetic iron oxide (Fe3O4) to be separated from the sludge mixture, making it recyclable and providing the possibility for material recycling and stable process operation.

[0007] This invention protects a magnetic iron-carbon composite material, which is composed of honeycomb-shaped sodium citrate pyrolysis carbon obtained from sodium citrate pyrolysis, and magnetic iron oxide loaded on its surface by co-precipitation. The magnetic iron oxide contains Fe. 2+ with Fe 3+ The preferred molar ratio is 1:2.

[0008] Specifically, the preparation method of this magnetic iron-carbon composite material includes the following steps: Step 1: Place the dried sodium citrate into a tube furnace and continuously purge with nitrogen for 10 minutes to ensure an oxygen-free environment. Pyrolysis is carried out in an oxygen-free environment. After pyrolysis, cool to room temperature, wash with hydrochloric acid, and then wash repeatedly with deionized water. After drying and grinding, honeycomb-shaped sodium citrate pyrolysis carbon is obtained. Step 2: Dissolve ferrous sulfate heptahydrate and anhydrous ferric chloride in deionized water at a mass ratio of 1:1 to 1:1.5 and stir thoroughly to form a mixed solution. Add sodium citrate pyrolysis carbon to the mixed solution, wherein the mass ratio of the amount of sodium citrate pyrolysis carbon added to the total amount of ferrous sulfate heptahydrate and anhydrous ferric chloride added is 1:36 to 1:50. Step 3: Mix the solution continuously at 65~75℃, and add sodium hydroxide solution dropwise while continuously stirring to adjust the pH to 9~11, and separate the solid substance; Step 4: Wash the solid material with deionized water until neutral pH, centrifuge, filter to obtain a magnetic solid material, and dry the solid material in a vacuum drying oven at 100~120℃ for 20~30h to obtain a magnetic iron-carbon composite material.

[0009] Preferably, in step 1, the temperature is increased to 600°C at a rate of 5°C / min and held for 2 hours.

[0010] Preferably, in step 1 and step 3, the mixed solution is continuously stirred and mixed at 70°C, and the amount of sodium hydroxide solution added is 100 mL with a concentration of 5 mol / L.

[0011] Preferably, in step 4, the solid material is first washed three times with anhydrous ethanol, then washed three times with deionized water until neutral pH, and then dried in a vacuum drying oven at 110°C for 24 hours.

[0012] This invention also protects the application of the above-mentioned magnetic iron-carbon composite material in increasing methane production in anaerobic digestion systems, by adding the above-mentioned magnetic iron-carbon composite material to activated sludge and carrying out the digestion reaction under closed, stirred and anaerobic conditions.

[0013] Preferably, the concentration of the magnetic iron-carbon composite material is 1.2 g / L, and the anaerobic digestion conditions are 35~40℃ and 120~140 rpm.

[0014] This invention uses sodium citrate as a green precursor to prepare honeycomb porous biochar via pyrolysis, and introduces iron oxide loading in situ to impart magnetism to the material. Compared with some existing magnetic porous chars (such as biochar based on agricultural and forestry waste), using sodium citrate as a precursor fully leverages its low cost and environmental friendliness, and forms a highly ordered honeycomb structure with superior random pores compared to traditional magnetic chars. This provides ideal habitat sites for acid-producing and methanogenic bacteria, reduces interspecies electron transfer distance, and thus promotes efficient metabolic synergy within the anaerobic system. Simultaneously, the excellent paramagnetism avoids the accumulation problems caused by the loss of traditional powdered char materials with water or retention within the anaerobic system, reducing material loss and operating costs to a certain extent.

[0015] This invention first involves pyrolyzing sodium citrate to produce a carbon material. Then, iron oxides are precipitated onto the surface of the carbon material via co-precipitation, forming a magnetic iron-carbon composite material (MBC). When added to an anaerobic digestion system, the MBC enhances the degradation rate of organic matter, shortens the methane lag phase, and increases methane production during the anaerobic digestion reaction. Furthermore, the MBC accelerates the direct interspecies electron transfer process, promoting the methanation of various organic compounds during anaerobic digestion. After entering the anaerobic digestion system, the MBC can also be used for subsequent resource utilization along with the anaerobic digestate.

[0016] The preparation method of this invention is simple, the raw materials are inexpensive and readily available, and the composite material can be efficiently recovered by magnetic separation after anaerobic digestion, which can effectively reduce operating costs and provide a new technology for the resource utilization and sustainable development of sewage and sludge treatment and disposal. Attached Figure Description

[0017] Figure 1(a) shows the elemental composition of sodium citrate pyrolytic carbon BC; Figure 1(b) shows the elemental composition of the magnetic iron-carbon composite material MBC; Figure 2(a) shows the XPS spectrum of the C 1s orbital of BC; Figure 2(b) shows the XPS spectrum of the C 1s orbital of MBC; Figure 2(c) shows the XPS spectrum of the O 1s orbital of MBC; Figure 2(d) shows the XPS energy spectrum of the Fe 2p orbital of MBC; Figure 3 FTIR spectra of BC, Fe3O4, and MBC; Figure 4 Cyclic voltammetry curves for BC, Fe3O4, and MBC; Figure 5 This is a graph showing the cumulative methane production of each experimental group in the anaerobic digestion system of Example 2; Figure 6 This is a graph showing the cumulative methane production of different concentrations of MBC in the anaerobic digestion system of Example 3; Figure 7 This is a graph showing the cumulative methane production of each experimental group in the anaerobic digestion system of Example 4; Figure 8 This is a graph showing the cumulative methane production of each experimental group in the anaerobic digestion system of Example 5. Detailed Implementation

[0018] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. The embodiments of the present invention are given for illustrative and descriptive purposes only, and are not intended to be exhaustive or to limit the invention to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art. The embodiments were chosen and described to better illustrate the principles and practical application of the invention, and to enable those skilled in the art to understand the invention and design various embodiments with various modifications suitable for a particular purpose.

[0019] Example 1

[0020] A magnetic iron-carbon composite material, the preparation method includes the following steps: 1. Sodium citrate was dried in an oven at 120℃ for 12 hours. The dried sodium citrate particles were then placed in a tube furnace and nitrogen gas was continuously introduced for 10 minutes to ensure an oxygen-free environment. The heating rate was set to 5℃ / min and the temperature was raised to 600℃. This temperature was maintained for 2 hours. The temperature was then lowered to room temperature. The particles were washed with 1mol / L hydrochloric acid and then repeatedly washed with deionized water. After drying and grinding, sodium citrate pyrolytic carbon BC was obtained.

[0021] 2. Dissolve 3.89 g FeSO4·7H2O (ferrous sulfate heptahydrate) and 4.54 g FeCl3 (anhydrous ferric chloride) in 400 mL of deionized water (the molar ratio of the iron source is Fe...). 2+ :Fe 3+ =1:2) and mix thoroughly to form a mixed solution. Then, weigh 200mg of sodium citrate pyrolytic carbon BC obtained in step 1 and add it to the mixed solution.

[0022] 3. Set the temperature of the magnetic stirrer to 70℃, and under this condition, stir the mixed solution obtained in step 2 evenly. While stirring, add 100mL of 5mol / L NaOH (sodium hydroxide) solution, then adjust the pH value to 10, and use a strong magnet to separate the solid substances.

[0023] 4. The solid material was washed three times with anhydrous ethanol and then three times with deionized water. After centrifugation and filtration, a magnetic solid material was obtained. Finally, it was dried in a vacuum drying oven at 110°C for 24 hours to obtain the magnetic iron-carbon composite material MBC.

[0024] The elemental contents of sodium citrate pyrolytic carbon BC and magnetic iron-carbon composite material MBC are shown in Figure 1(a) and Figure 1(b), respectively. It can be clearly seen that, compared with sodium citrate pyrolytic carbon BC, magnetic iron-carbon composite material MBC contains Fe, Al and Na elements in addition to C and O elements.

[0025] The X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectra of each element are shown in Figures 2(a)~(d) and 3, respectively. From Figures 2(b) and (d), it can be seen that Fe... 2+ and Fe 3+ Successfully loaded on BC. For example... Figure 3 The FTIR spectrum shows that at 3400 cm⁻¹ -1 The broad band at 1381 cm⁻¹ can be attributed to the OH stretching vibration of adsorbed water molecules. -1 and 1576cm -1 The peak at 574 cm⁻¹ corresponds to the bending absorption of the carboxyl OCO functional group, i.e., the bending vibration, and it does not overlap with the Fe-O bond. The FTIR spectrum shows the presence of Fe₃O₄ on the surface of the pyrolytic carbon (peak at 574 cm⁻¹). -1 (At the location), Fe3O4 was successfully loaded onto BC.

[0026] Figure 4 The cyclic voltammetry (CV) curves of BC, Fe3O4, and MBC show that MBC combines the adsorption properties of BC and the magnetic / conductive properties of Fe3O4. The current response is between that of BC and Fe3O4. This indicates that the introduction of magnetism effectively enhances the electrochemical activity of the material and may promote direct interspecies electron transfer by increasing active sites or improving charge transport.

[0027] Example 2

[0028] The application of magnetic iron-carbon composite materials in increasing methane production in anaerobic digestion systems includes the following steps: 1. The MBC prepared in Example 1 was added to the anaerobic sludge digestion system. The anaerobic digestion system used serum bottles as the anaerobic environment, and the reaction substrate was artificial simulated wastewater with a COD concentration of 3000 mg / L. The anaerobic sludge was taken from the COFCO wastewater treatment plant in Bengbu City, Anhui Province. The sludge suspended solids (SS) content was 1.8 ± 0.2 g / L, the volatile suspended solids (VSS) content was 1 ± 0.12 g / L, and the sludge moisture content was 84.85%.

[0029] 2. The material comparison experiment was divided into four groups: blank control group, BC group, Fe3O4 group and MBC group. Without changing other variables, three parallel samples were set up in each group. 8.25g of the above-mentioned inoculated sludge and 250mL of artificial simulated wastewater were added to each parallel sample. The material dosage was 0.6g / L.

[0030] Dissolved oxygen in wastewater was removed by nitrogen stripping. The mixture was then sealed with butyl rubber stoppers and placed in a constant-temperature shaking incubator at 35°C and 120 rpm. The entire reaction system was kept in an anaerobic environment.

[0031] The biogas produced during the reaction was collected using a gas bag, and the proportions of methane (CH4) and carbon dioxide (CO2) in the biogas were analyzed using gas chromatography. At each time point, 5 mL of water sample was taken from the bottle, and 5 mL of wastewater of the same concentration was injected into the bottle simultaneously. The water sample was filtered through a 0.45 μm filter membrane, and the COD concentration after the reaction was measured.

[0032] Cumulative methane production Figure 5 As shown, after the reaction, the cumulative methane production in the MBC group reached 163 mL, which was 40% higher than that in the blank control group. Moreover, the effect of increasing methane production by anaerobic digestion was more significant compared with the groups with added BC and Fe3O4.

[0033] Example 3

[0034] Based on Example 2, this embodiment sets up a concentration gradient experiment of magnetic iron-carbon composite material to evaluate the effect of MBC addition concentration on methane production.

[0035] The concentration gradient experiment was divided into four groups, with added MBC concentrations of 0 g / L, 0.6 g / L, 1.2 g / L, 1.8 g / L, and 2.4 g / L, respectively. Without changing other experimental conditions, three parallel samples were set up in each group, and 8.25 g of the above-mentioned inoculated sludge and 250 mL of artificial simulated wastewater were added to each parallel sample.

[0036] Dissolved oxygen in wastewater was removed by nitrogen stripping. The mixture was then sealed with butyl rubber stoppers and placed in a constant-temperature shaking incubator at 35°C and 120 rpm. The entire reaction system was kept in an anaerobic environment.

[0037] The biogas produced during the reaction was collected using a gas bag, and the proportions of methane (CH4) and carbon dioxide (CO2) in the biogas were analyzed using gas chromatography. At each time point, 5 mL of water sample was taken from the bottle, and 5 mL of wastewater of the same concentration was injected into the bottle simultaneously. The water sample was filtered through a 0.45 μm filter membrane, and the COD concentration after the reaction was measured.

[0038] MBC concentration gradient methane production as follows Figure 6 As shown, in the initial stage of the reaction, i.e., within 0-24 hours, the experimental group with an addition dose of 1.2 g / L showed a 29.2% increase in cumulative methane production compared to the blank control group. As the reaction progressed, the cumulative methane production increased to 36.9% of the blank control group at 72 hours, reaching its maximum increase. The results indicate that different concentrations of MBC significantly affect methane production in the anaerobic digestion system, and the optimal addition concentration should be in the range of 1.2-1.8 g / L.

[0039] Example 4

[0040] Based on Example 2, an additional material comparison experiment was conducted, in which straw biochar (SBC) and sodium citrate pyrolysis char (BC) were compared. The reaction conditions and reaction matrix were exactly the same as in Example 2. The experiment was divided into three groups: blank group, BC group and SBC group.

[0041] Cumulative methane production such as Figure 7 As shown, compared with straw biochar (SBC), the addition of sodium citrate pyrolysis char (BC) increased methane production by 20-25% overall, indicating that pyrolysis char prepared through precursor regulation has superior degradation performance compared with traditional biochar.

[0042] Example 5

[0043] Based on Example 2, an additional set of comparative experiments was added, namely, comparing the magnetic iron-carbon composite material MBC prepared in Example 1 with straw biochar loaded with Fe3O4 (SBC / Fe3O4). The reaction conditions and reaction matrix were exactly the same as in Example 2. The experiment was divided into three groups: blank group, MBC group and SBC / Fe3O4 group.

[0044] Cumulative methane production such as Figure 8 As shown, after the reaction, the MBC group had a 28-33% higher methane yield compared to the SBC / Fe3O4 group, indicating that sodium citrate-supported Fe3O4 had a more significant effect on improving anaerobic digestion and methanogenesis compared to ordinary biochar-supported Fe3O4.

[0045] Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art and related fields based on the embodiments of the present invention without inventive effort should fall within the scope of protection of the present invention.

Claims

1. A magnetic iron-carbon composite material, characterized in that, This material is composed of honeycomb-shaped sodium citrate pyrolysis carbon obtained from sodium citrate pyrolysis, and magnetic iron oxide supported on its surface by co-precipitation. The magnetic iron oxide contains Fe... 2+ with Fe 3 + The molar ratio is 1:

2. The preparation method of this material includes the following steps: Step 1: Place the dried sodium citrate into a tube furnace and continuously purge with nitrogen for 10 minutes to ensure an oxygen-free environment. Pyrolysis is carried out in an oxygen-free environment. After pyrolysis, cool to room temperature, wash with hydrochloric acid, and then wash repeatedly with deionized water. After drying and grinding, honeycomb-shaped sodium citrate pyrolysis carbon is obtained. Step 2: Dissolve ferrous sulfate heptahydrate and anhydrous ferric chloride in deionized water at a mass ratio of 1:1 to 1:1.5 and stir thoroughly to form a mixed solution. Add sodium citrate pyrolysis carbon to the mixed solution, wherein the mass ratio of the amount of sodium citrate pyrolysis carbon added to the total amount of ferrous sulfate heptahydrate and anhydrous ferric chloride added is 1:36 to 1:

50. Step 3: Mix the solution continuously at 65~75℃, and add sodium hydroxide solution dropwise while continuously stirring to adjust the pH to 9~11, and separate the solid substance; Step 4: Wash the solid material with deionized water until neutral pH, centrifuge, filter to obtain a magnetic solid material, and dry the solid material in a vacuum drying oven at 100~120℃ for 20~30h to obtain a magnetic iron-carbon composite material.

2. The magnetic iron-carbon composite material according to claim 1, characterized in that, In step 1, the temperature is increased to 600℃ at a rate of 5℃ / min and held for 2 hours.

3. The magnetic iron-carbon composite material according to claim 1, characterized in that, In step 3, the mixed solution is continuously stirred and mixed at 70°C, and the amount of sodium hydroxide solution added is 100 mL with a concentration of 5 mol / L.

4. The magnetic iron-carbon composite material according to claim 1, characterized in that, In step 4, the solid material is first washed three times with anhydrous ethanol, then washed three times with deionized water until neutral pH, and then dried in a vacuum drying oven at 110°C for 24 hours.

5. The application of the magnetic iron-carbon composite material according to any one of claims 1-4 in increasing methane production in an anaerobic digestion system.

6. The application according to claim 5, characterized in that, The magnetic iron-carbon composite material according to any one of claims 1-4 is added to activated sludge, and the digestion reaction is carried out under closed, stirred and anaerobic conditions.

7. In the application according to claim 6, the concentration of the magnetic iron-carbon composite material is 1.2 g / L.

8. The application according to claim 6, wherein the anaerobic digestion conditions are 35-40°C and 120-140 rpm.

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