Method for promoting microorganisms to convert CO2 to produce methane by using iron-carbon nano composite material

Abstract

The invention relates to a method for promoting microorganisms to convert CO2 to produce methane by using an iron-carbon nano composite material, which comprises the following steps: (1) preparing the iron-carbon nano composite material: loading nano zero-valent iron on a carbon-based material carrier, so that smooth nano zero-valent iron particles are uniformly distributed on the surface of the carbon-based material; the particle size of the carbon-based material is 20-100 [mu] m; (2) obtaining a target reaction system, and ensuring that the target reaction system contains hydrogen nutrition type methanogens; (2) adding the iron-carbon nano composite material prepared in the step (1) into a target reaction system, introducing CO2 gas, and converting by using hydrogen nutritional methanogens; and after a period of time, methane gas is harvested. Compared with the prior art, the method has the advantages that the CO2 conversion rate and the methane production rate of microorganisms can be increased by using the iron-carbon nano composite material, and the iron-carbon nano composite material has the advantages of mild synthesis conditions, simplicity in operation, greenness, economy, good dispersity and high stability of composite material injection, and has a good application prospect in the fields of energy development and environmental protection.

Description

Technical Field

[0001] The present invention belongs to the fields of environmental engineering and the resource utilization of CO2 in oil reservoirs, and relates to a method for promoting the microbial conversion of CO2 to methane by an iron-carbon nanocomposite material. Background Art

[0002] The large-scale use of fossil energy has led to a large amount of greenhouse gas CO2 emissions, resulting in a series of ecological and environmental problems. Converting CO2 into methane is an effective and promising way of energy regeneration. The use of microbial means to achieve the conversion of CO2 to methane has attracted much attention due to advantages such as no need for external catalysts, mild reaction conditions, low cost, and environmental friendliness.

[0003] The oil reservoir environment refers to the underground geological environment for storing oil, which harbors a rich variety of and functionally diverse microbial communities. Among them, hydrogenotrophic methanogens can convert the sealed CO2 into methane. Hydrogen is a key limiting factor in this biochemical process, but its content in oil reservoirs is very limited. The problems of high cost, safety hazards in transportation, and difficulty in injection limit the external input.

[0004] Nanoscale zero-valent iron, as a substitute for hydrogen, has been widely used by researchers in the anaerobic digestion of organic substrates to produce methane. Suanon et al. (Application of nanoscale zero valent iron and iron powder during sludge anaerobic digestion: Impact on methane yield and pharmaceutical and personal care products degradation, Journal of Hazardous Material, 2017, 321: 47-53) added 100 mg / L of nanoscale zero-valent iron to the sludge wastewater digestion system, and finally the chemical oxygen demand removal rate increased by 10%, and the methane production increased by 25.2%. Although nanoscale zero-valent iron has the advantages of high specific surface area and rich active sites, its application in on-site injection in oil reservoirs is still limited by problems such as easy aggregation, easy sedimentation, poor dispersibility, and short lifespan.

[0005] Due to their rich functional groups and large specific surface area, carbon-based materials are often used as support carriers for nanoparticles, which can effectively inhibit the aggregation of metal nanoparticles. In the anaerobic digestion system, iron-carbon nanocomposites can not only promote direct interspecies electron transfer but also reduce the accumulation of acidic substances, thereby enhancing the activity of methanogens. Zhang et al. (Impact of biochar supported nano zero-valent iron on anaerobic co-digestion of sewage sludge and food waste: Methane production, performance stability and microbial community structure, Bioresource Technology, 2021, 340: 125715) found that biochar-supported nano zero-valent iron materials can promote the anaerobic co-digestion of sewage sludge and food waste, with the final total solid suspension removal rate increasing by 11.44% and the methane production increasing by 42.87%.

[0006] Patent CN114291989A provides a method for increasing the methane production in anaerobic digestion of sludge by using an iron / carbon / bioenzyme coupling technology, which includes the following steps: S1: Weigh Chinese liquor starter and place it in an Erlenmeyer flask, add distilled water, and keep it in a constant temperature water bath for 0.5 - 1.5 h. Then, perform suction filtration with quantitative filter paper to remove large particulate matter and some molds and yeasts, and obtain the leachate for standby; S2: Inject the sludge into an anaerobic reaction vessel, add nano zero-valent iron, activated carbon, and the leachate of Chinese liquor starter prepared in S1, continuously charge inert gas, and after removing oxygen, seal the reaction vessel; S3: Carry out constant temperature anaerobic digestion on the sealed reaction vessel; The intensifiers used in this invention (nano zero-valent iron, activated carbon, and the leachate of Chinese liquor starter) have low costs, are all cheap and easily available, and the addition amounts are small, with low operating costs, having good application prospects, and solving the technical problems of low substrate conversion rate and low biogas production in the existing anaerobic digestion process.

[0007] Patent CN114703235B discloses a method for promoting anaerobic digestion by using iron carbonyl, which relates to the field of biological fermentation technology; the method includes: adding iron carbonyl to the fermentation broth for anaerobic digestion; This invention applies iron carbonyl to anaerobic digestion, which can improve the anaerobic digestion effect and thus increase the methane production; On the one hand, iron carbonyl can accelerate the degradation of complex organic substances by promoting microbial iron respiration, and on the other hand, it can provide the essential iron element supplement for the growth of microorganisms, and has a promoting effect on both hydrolytic acidification bacteria and methanogens.

[0008] Although nano-zero valent iron has certain potential in promoting the bioconversion of CO2 to methane, its practical application still faces many challenges in the complex environment of oil reservoirs. On the one hand, under reservoir conditions, nano-zero valent iron is vulnerable to erosion by complex fluids, resulting in rapid attenuation of its structure and performance. On the other hand, nano-zero valent iron is prone to agglomeration and easy to settle after injection into the reservoir, and the problem of poor mobility has not been effectively solved, which hinders its practical application. Although iron-carbon nanocomposites have shown good performance in other fields, there is no report on their application in reservoir environment, and less attention has been paid to their dispersibility and migration ability in porous media. Summary of the Invention

[0009] The purpose of the present invention is to overcome the above problems existing in the prior art and provide a method for promoting the microbial conversion of CO2 to methane by an iron-carbon nanocomposite. Nano-zero valent iron has strong reducing ability and is convenient for transportation, but it is extremely prone to agglomeration due to size effect. In the present invention, nano-zero valent iron is loaded on activated carbon, which not only improves the dispersibility and migration ability of nano-zero valent iron, but also provides an ideal attachment and growth environment for microorganisms. At the same time, the loaded nano-zero valent iron continuously releases electrons after corrosion, reacts with H + in water to generate H2, drives hydrogenotrophic methanogens to convert CO2 into methane, and can also produce ferrous carbonate minerals for CO2 fixation, thereby improving the methane production rate and CO2 conversion rate.

[0010] The purpose of the present invention can be achieved by the following technical solutions:

[0011] The technical solution of the present invention provides a method for promoting the microbial conversion of CO2 to methane by an iron-carbon nanocomposite, and the method includes the following steps:

[0012] (1) Prepare an iron-carbon nanocomposite: The iron-carbon nanocomposite loads nano-zero valent iron on a carbon-based material carrier, so that smooth nano-zero valent iron particles are evenly distributed on the surface of the carbon-based material; the particle size of the carbon-based material is 20-100 μm;

[0013] The specific method for preparing the iron-carbon nanocomposite is as follows: Under the protection atmosphere of inert gas and continuous stirring, dissolve iron salt in a deoxygenated organic solvent-water mixed solution, add the carbon-based material and mix evenly; then, add a reducing agent, let the reaction system stand and age for a period of time, and then separate to obtain a solid particle product; then wash the solid particle product with a deoxygenated washing solvent; finally, vacuum dry the washed solid particle product to obtain the iron-carbon nanocomposite.

[0014] (2) Obtain the target reaction system, ensuring that hydrogenotrophic methanogens are contained in the target reaction system; add the iron-carbon nanocomposite material prepared in step (1) to the target reaction system containing hydrogenotrophic methanogens, introduce CO2 gas, and carry out transformation using hydrogenotrophic methanogens; after a period of time, harvest methane gas.

[0015] Furthermore, the inert gas in step (1) is at least any one of nitrogen, argon, and helium; the rotation speed of the continuous stirring is 200 - 400 rpm.

[0016] Furthermore, the iron salt in step (1) is any one of ferrous sulfate heptahydrate, ferric chloride hexahydrate, ferric nitrate nonahydrate, or ferrous chloride tetrahydrate.

[0017] Furthermore, the deoxygenated organic solvent - water mixed solution in step (1) is any one of deoxygenated ethanol aqueous solution or deoxygenated methanol aqueous solution.

[0018] Furthermore, the carbon-based material in step (1) is any one of activated carbon, biochar, or graphene oxide; in some specific embodiments of the present invention, before use, the activated carbon is soaked in 1 mol / L hydrochloric acid for more than 24 hours, washed to neutrality, and then dried.

[0019] Furthermore, the reducing agent in step (1) is any one of sodium borohydride solution or potassium borohydride solution; the role of the reducing agent is to reduce iron.

[0020] Furthermore, the deoxygenated washing solvent in step (1) is preferably deoxygenated water and absolute ethanol.

[0021] Furthermore, the mass ratio of the iron salt to the carbon-based material in step (1) is 5:1 - 1:1, preferably 5:1.

[0022] Furthermore, the molar ratio of the reducing agent to the iron salt in step (1) is 5:1 - 3:1, preferably 4:1.

[0023] Furthermore, the standing and aging time in step (1) is 15 - 45 min, preferably 30 min.

[0024] Further, the target reaction system described in step (2) is any one of oilfield formation water, an anaerobic serum bottle containing an inorganic salt medium, or an anaerobic reactor, but is not limited thereto; the main component in the target reaction system is an inorganic salt that can ensure the normal growth and metabolism of hydrogenotrophic methanogens in the system. Among them, oilfield formation water is the actual application scenario of this application. In some specific embodiments of this application, an inorganic salt medium is used to simulate the liquid environment in the oilfield formation water. The preferred formulation of the inorganic salt medium is as follows: 0.34 g / L KCl; 4 g / L MgCl2·6H2O; 1.68 g / L MgSO4; 0.25 g / L NH4Cl; 0.1 g / L CaCl2; 18 g / L NaCl; 0.14 g / L K2HPO4; 1 g / L sodium acetate; 2 g / L yeast powder; 2 g / L tryptone; 0.1 g / L coenzyme; 10 mL / L trace element solution; 0.1 mL / L amino acid; 0.1 mL / L hemin; 1 mL / L vitamin. Among them, the preferred formulation of the trace element solution is as follows: 1.5 g / L nitrilotriacetic acid; 0.5 g / L MnSO4·7H2O; 0.1 g / L FeSO4·7H2O; 0.18 g / L CoSO4·7H2O; 0.18 g / L ZnSO4·7H2O; 0.01 g / L CuSO4·7H2O; 0.02 g / L AlK(SO4)2·12H2O; 0.01 g / L H3BO3; 0.01 g / L Na2MoO4·2H2O; 0.03 g / L NiCl2·6H2O; 0.3 mg / L Na2SeO3·5H2O; 0.4 mg / L Na2WO4·2H2O. In some other embodiments of this application, the formulation of the medium can be adaptively adjusted according to the specific type of hydrogenotrophic methanogens.

[0025] Further, the hydrogenotrophic methanogens described in step (2) include, but are not limited to, Methanomicrobiales, Methanococcales, Methanobacteriales, Methanomicrobiaceae, Methanosarcinales at the "order" level. Among them, Methanococcus is preferably Methanococcus maripaludis. Methanococcus maripaludis is the hydrogenotrophic methanogen used in the inorganic salt medium in laboratory simulation. The inoculation amount of Methanococcus maripaludis in the inorganic salt medium is 5% - 10%, and the culture temperature is 37°C - 55°C. When actually applied in oilfield formation water, the hydrogenotrophic methanogens are preferably Methanothermobacter belonging to Methanobacteriales and / or Methanoculleus belonging to Methanomicrobiales, and the temperature of the target reaction system is 37°C - 55°C.

[0026] Further, in some specific embodiments of the present application, the method of adding the iron-carbon nanocomposite into the target reaction system in step (2) is as follows: First, the iron-carbon nanocomposite is separately packed in a serum bottle, filled with nitrogen and sealed, and then the target reaction system that has been deoxygenated is added to the serum bottle through nitrogen replacement. In some other embodiments, it is also possible to prepare a suspension of the iron-carbon nanocomposite with water or a surfactant solution and then inject it into the target reaction system. The specific injection method can be selected according to the actual application scenario, and it is not used to limit the technical solution of the present application.

[0027] Further, the mass-volume ratio (w / v) of the iron-carbon nanocomposite described in step (2) to the target reaction system is 10-30 g / L. The nano zero-valent iron loaded in the iron-carbon nanocomposite continuously releases electrons after corrosion, reacts with H + in water to generate H2, and H2 is a key limiting factor for hydrogenotrophic methanogens; that is, hydrogenotrophic methanogens use CO2 and the hydrogen provided by nano zero-valent iron to produce methane.

[0028] Compared with the prior art, the present application has at least the following improvements and beneficial effects:

[0029] (1) In the present invention, nano zero-valent iron is loaded on activated carbon, and the synthesis conditions are normal temperature and pressure, without the need for complex high-temperature and high-pressure equipment and harsh conditions, which greatly simplifies the operation process, effectively improves the dispersion and stability of nano zero-valent iron injected into the reservoir, and increases the conversion rate of CO2 and the rate of CO2 conversion to methane by hydrogenotrophic methanogens. The operation is simple, economical and environmentally friendly, and has good application prospects in the fields of energy development and environmental protection.

[0030] (2) In the prior art, iron-carbon nanocomposites are often used for the removal of pollutants in wastewater or the anaerobic digestion of organic substrates to produce methane; while the present application is applied to the reservoir based on the technology of improving oil recovery by carbon dioxide (Carbon Dioxide-Enhanced Oil Recovery, CO2-EOR) in the carbon capture, utilization and storage system (Carbon Capture, Utilization and Storage, CCUS), and the substrate is CO2 rather than organic matter. At the same time, the nano zero-valent iron continuously releases electrons after corrosion, reacts with H + in water to generate H2, driving hydrogenotrophic methanogens to convert CO2 into methane, effectively solving the problem of extremely low H2 content and difficulty in injecting hydrogen in the reservoir environment, and can also produce ferrous carbonate minerals for CO2 fixation, which is a win-win means for carbon emission reduction and sustainable development.

[0031] (3) Compared with a method for promoting the conversion of CO2 to methane by reservoir microorganisms provided in the inventor's previous patent CN106544369B, the present application uses nanoscale zero-valent iron, and the dispersibility and mobility of the nano zero-valent iron have been significantly improved. The present application enhances the application efficiency of nano zero-valent iron in the field of CO2 resource utilization in oil reservoirs and also provides a more efficient and stable choice of materials and methods for related industries. Description of the Drawings

[0032] Figure 1 It is a scanning electron microscope image, where Figure A is the iron-carbon nanocomposite material; Figure B is the nano zero-valent iron material;

[0033] Figure 2 It is the sedimentation curve of the iron-carbon nanocomposite material and the nano zero-valent iron;

[0034] Figure 3 It is the breakthrough curve of the iron-carbon nanocomposite material and the nano zero-valent iron;

[0035] Figure 4 It is for the system with the addition of the iron-carbon nanocomposite material and the system without addition: (A) the change diagram of the utilization of CO2 by microorganisms; (B) the change diagram of methane production. Detailed Embodiments

[0036] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to specific embodiments. It should be noted that the following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be pointed out that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can be made. These all belong to the protection scope of the present invention.

[0037] There is no particular limitation on the source of all raw materials of the present invention, and those purchased on the market or prepared according to the conventional methods well-known to those skilled in the art are all acceptable.

[0038] Comparative Example 1:

[0039] The synthesis method of the nano zero-valent iron is the same as that in Example 1, except that activated carbon is not added.

[0040] Example 1:

[0041] (1) Dissolve 2.5 g of ferrous sulfate heptahydrate in 100 mL of deoxygenated ethanol aqueous solution, add 0.5 g of activated carbon (the mass ratio of ferrous sulfate heptahydrate to activated carbon is 5:1), and stir evenly. Dissolve 1.5 g of sodium borohydride in 75 mL of deoxygenated water (the molar ratio of sodium borohydride to ferrous sulfate heptahydrate is 4.4:1), and drop it into the reaction vessel. After dropping, age for 30 min. The whole synthesis process is carried out under nitrogen protection and continuous stirring. Wash the product with deoxygenated water and absolute ethanol and then dry it under vacuum to obtain an iron-carbon nanocomposite with an iron-carbon mass ratio of 1:1;

[0042] The scanning electron microscope image of the synthesized iron-carbon nanocomposite provided in Example 1 is shown in Figure 1 A, and the scanning electron microscope image of the nano zero-valent iron material provided in Comparative Example 1 is shown in Figure 1 B. It can be seen that the agglomeration phenomenon of nano zero-valent iron on the iron-carbon nanocomposite is greatly reduced and the dispersibility is improved.

[0043] The optical density is detected by using an ultraviolet spectrophotometer that can be purchased on the market. The changes in the optical density of the synthesized iron-carbon nanocomposite and nano zero-valent iron material in the suspension are shown in Figure 2 , and after standing for two hours, the suspension of the iron-carbon nanocomposite is significantly higher than that of the nano zero-valent iron.

[0044] The breakthrough curves of the synthesized iron-carbon nanocomposite and nano zero-valent iron material in the simulated porous medium are shown in Figure 3 . The maximum relative effluent concentration of the nano zero-valent iron suspension is close to zero, while the relative maximum effluent concentration of the iron-carbon nanocomposite suspension can reach more than 35%, and the breakthrough ability is significantly higher than that of the nano zero-valent iron.

[0045] (2) Prepare a deoxygenated inorganic salt medium with the following formula: 0.34 g / L KCl; 4 g / L MgCl2·6H2O; 1.68 g / L MgSO4; 0.25 g / L NH4Cl; 0.1 g / L CaCl2; 18 g / L NaCl; 0.14 g / L K2HPO4; 1 g / L sodium acetate; 2 g / L yeast extract; 2 g / L tryptone; 0.1 g / L coenzyme; 10 mL / L trace element solution; 0.1 mL / L amino acids; 0.1 mL / L hemin chloride; 1 mL / L vitamins. Among them, the trace element solution has the following formula: 1.5 g / L nitrilotriacetic acid; 0.5 g / L MnSO4·7H2O; 0.1 g / L FeSO4·7H2O; 0.18 g / L CoSO4·7H2O; 0.18 g / L ZnSO4·7H2O; 0.01 g / L CuSO4·7H2O; 0.02 g / L AlK(SO4)2·12H2O; 0.01 g / L H3BO3; 0.01 g / L Na2MoO4·2H2O; 0.03 g / L NiCl2·6H2O; 0.3 mg / L Na2SeO3·5H2O; 0.4 mg / L Na2WO4·2H2O;

[0046] (3) Weigh the iron-carbon nanocomposite prepared in the above step (1) into a serum bottle, add the deoxygenated inorganic salt medium prepared in step (2), and the mass-volume ratio (w / v) of the iron-carbon nanocomposite to the medium is 10 g / L. Replace the headspace gas in the bottle with a CO2 / N2 mixed gas with a volume ratio of 4:1, and inoculate 10% of the Methanococcus maripaludis bacterial solution, and culture anaerobically in the dark at 37 °C, and detect the composition of the headspace gas. The difference between the control group and the experimental group is that the iron-carbon nanocomposite is not added. Methanococcus maripaludis is a hydrogenotrophic methanogen. In the case of not adding the iron-carbon nanocomposite, since there is no hydrogen in the headspace, methane cannot be produced.

[0047] The preservation number of Methanococcus maripaludis described in step (3) is: JCM 10722, purchased from the Chengdu Institute of Biogas Science, Ministry of Agriculture and Rural Affairs of China. The same applies to the following examples. The detection of the composition of the headspace gas (methane and carbon dioxide) described in step (3) is carried out using a commercially available gas chromatograph or other instruments, and the detection procedures and methods are conventional operations in the art.

[0048] The changes in the contents of methane and carbon dioxide during the microbial culture are shown in Figure 4 . After culturing for 568 h, the methane production in the culture system added with the iron-carbon nanocomposite was 128.37 μmol, the methane production rate was 246.54 μmol / L / day, and the CO2 conversion rate was 90%; while no methane production was detected in the control group without adding the iron-carbon nanocomposite, and the CO2 content hardly changed.

[0049] Example 2:

[0050] (1) Dissolve 2.5 g of ferrous sulfate heptahydrate in 100 mL of deoxygenated ethanol aqueous solution, add 1.5 g of activated carbon (the mass ratio of ferrous sulfate heptahydrate to activated carbon is 1.67:1), and stir evenly. Dissolve 1.7 g of sodium borohydride in 75 mL of deoxygenated water (the molar ratio of sodium borohydride to ferrous sulfate heptahydrate is 5:1), and add it dropwise to the reaction vessel. After the addition is complete, age for 45 min. The entire synthesis process is carried out under nitrogen protection and continuous stirring. Wash the product with deoxygenated water and absolute ethanol and then dry it under vacuum to obtain an iron-carbon nanocomposite with an iron-carbon mass ratio of 1:3;

[0051] (2) Prepare a deoxygenated inorganic salt medium with the following formula: 0.34 g / L KCl; 4 g / L MgCl2·6H2O; 1.68 g / L MgSO4; 0.25 g / L NH4Cl; 0.1 g / L CaCl2; 18 g / L NaCl; 0.14 g / L K2HPO4; 1 g / L sodium acetate; 2 g / L yeast powder; 2 g / L tryptone; 0.1 g / L coenzyme; 10 mL / L trace element solution; 0.1 mL / L amino acid; 0.1 mL / L hemin chloride; 1 mL / L vitamin. Among them, the trace element solution has the following formula: 1.5 g / L nitrilotriacetic acid; 0.5 g / L MnSO4·7H2O; 0.1 g / L FeSO4·7H2O; 0.18 g / L CoSO4·7H2O; 0.18 g / L ZnSO4·7H2O; 0.01 g / L CuSO4·7H2O; 0.02 g / L AlK(SO4)2·12H2O; 0.01 g / L H3BO3; 0.01 g / L Na2MoO4·2H2O; 0.03 g / L NiCl2·6H2O; 0.3 mg / L Na2SeO3·5H2O; 0.4 mg / L Na2WO4·2H2O;

[0052] (3) Weigh the iron-carbon nanocomposite prepared in the above step (1) into a serum bottle, add the deoxygenated inorganic salt medium prepared in step (2), and the mass-volume ratio (w / v) of the iron-carbon nanocomposite to the medium is 20 g / L. Replace the headspace gas in the bottle with a CO2 / N2 mixed gas with a volume ratio of 4:1, and inoculate with 10% Methanococcus mazei suspension, and culture anaerobically at 37°C in the dark, and detect the composition of the headspace gas. The difference between the control group and the experimental group is that the iron-carbon nanocomposite is not added.

[0053] After 568 h of cultivation, the methane production in the cultivation system with the addition of the iron-carbon nanocomposite was 65.94 μmol, the methane production rate was 126.64 μmol / L / day, and the CO2 conversion rate was 87.35%; while no methane production was detected in the control group without the addition of the iron-carbon nanocomposite, and the CO2 content hardly changed.

[0054] Example 3:

[0055] (1) Dissolve 2.5 g of ferrous sulfate heptahydrate in 100 mL of deoxygenated ethanol aqueous solution, add 2.5 g of activated carbon (the mass ratio of ferrous sulfate heptahydrate to activated carbon is 1:1), and stir evenly. Dissolve 1.36 g of sodium borohydride in 75 mL of deoxygenated water (the molar ratio of sodium borohydride to ferrous sulfate heptahydrate is 4:1), and drop it into the reaction vessel. After dropping, age for 30 min. The whole synthesis process is carried out under nitrogen protection and continuous stirring. Wash the product with deoxygenated water and absolute ethanol and then dry it in vacuum to obtain an iron-carbon nanocomposite with an iron-carbon mass ratio of 1:5;

[0056] (2) Prepare a deoxygenated inorganic salt medium with the following formula: 0.34 g / L KCl; 4 g / L MgCl2·6H2O; 1.68 g / L MgSO4; 0.25 g / L NH4Cl; 0.1 g / L CaCl2; 18 g / L NaCl; 0.14 g / L K2HPO4; 1 g / L sodium acetate; 2 g / L yeast extract; 2 g / L tryptone; 0.1 g / L coenzyme; 10 mL / L trace element solution; 0.1 mL / L amino acid; 0.1 mL / L hemin chloride; 1 mL / L vitamin. Among them, the trace element solution has the following formula: 1.5 g / L nitrilotriacetic acid; 0.5 g / L MnSO4·7H2O; 0.1 g / L FeSO4·7H2O; 0.18 g / L CoSO4·7H2O; 0.18 g / L ZnSO4·7H2O; 0.01 g / L CuSO4·7H2O; 0.02 g / L AlK(SO4)2·12H2O; 0.01 g / L H3BO3; 0.01 g / L Na2MoO4·2H2O; 0.03 g / L NiCl2·6H2O; 0.3 mg / L Na2SeO3·5H2O; 0.4 mg / L Na2WO4·2H2O;

[0057] (3) Weigh the iron-carbon nanocomposite prepared in the above step (1) into a serum bottle, and add the deoxygenated inorganic salt medium prepared in step (2). The mass-volume ratio (w / v) of the iron-carbon nanocomposite to the medium is 30 g / L. Replace the headspace gas in the bottle with a CO2 / N2 mixture with a volume ratio of 4:1, and inoculate with 10% Methanococcus maripaludis bacterial solution. Incubate anaerobically in the dark at 37 °C, and detect the composition of the headspace gas. The difference between the control group and the experimental group is that the iron-carbon nanocomposite is not added.

[0058] After culturing for 568 h, the methane production in the culture system added with the iron-carbon nanocomposite was 40.14 μmol, the methane production rate was 77.62 μmol / L / day, and the CO2 conversion rate was 95.92%; while no methane production was detected in the control group without addition, and the CO2 content hardly changed.

[0059] Example 4:

[0060] (1) Dissolve 5 g of ferrous sulfate heptahydrate in 200 mL of deoxygenated ethanol aqueous solution, add 1 g of activated carbon (the mass ratio of ferrous sulfate heptahydrate to activated carbon is 5:1), and stir evenly. Dissolve 2 g of sodium borohydride in 75 mL of deoxygenated water (the molar ratio of sodium borohydride to ferrous sulfate heptahydrate is 3:1), and add it dropwise to the reaction vessel. After the addition, age for 15 min. The whole synthesis process is carried out under nitrogen protection and continuous stirring. Wash the product with deoxygenated water and absolute ethanol and then dry it in vacuum to obtain an iron-carbon nanocomposite with an iron-carbon mass ratio of 1:1;

[0061] (2) Prepare a deoxygenated inorganic salt medium, and the formula is as follows: 0.34 g / L KCl; 4 g / L MgCl2·6H2O; 1.68 g / L MgSO4; 0.25 g / L NH4Cl; 0.1 g / L CaCl2; 18 g / L NaCl; 0.14 g / L K2HPO4; 1 g / L sodium acetate; 2 g / L yeast powder; 2 g / L tryptone; 0.1 g / L coenzyme; 10 mL / L trace element solution; 0.1 mL / L amino acid; 0.1 mL / L hemin chloride; 1 mL / L vitamin. Among them, the formula of the trace element solution is as follows: 1.5 g / L nitrilotriacetic acid; 0.5 g / L MnSO4·7H2O; 0.1 g / L FeSO4·7H2O; 0.18 g / L CoSO4·7H2O; 0.18 g / L ZnSO4·7H2O; 0.01 g / L CuSO4·7H2O; 0.02 g / L AlK(SO4)2·12H2O; 0.01 g / L H3BO3; 0.01 g / L Na2MoO4·2H2O; 0.03 g / L NiCl2·6H2O; 0.3 mg / L Na2SeO3·5H2O; 0.4 mg / L Na2WO4·2H2O;

[0062] (3) Weigh the iron-carbon nanocomposite prepared in the above step (1) into a serum bottle, and add the deoxygenated inorganic salt medium prepared in step (2). The mass-volume ratio (w / v) of the iron-carbon nanocomposite to the medium is 12.5 g / L. Replace the headspace gas in the bottle with a CO2 / N2 mixture with a volume ratio of 4:1, and inoculate with 5% Methanococcus maripaludis liquid. Cultivate anaerobically in the dark at 37 °C, and detect the composition of the headspace gas. The difference between the control group and the experimental group is that the iron-carbon nanocomposite is not added.

[0063] After culturing for 190 h, the methane production in the culture system with the added iron-carbon nanocomposite is 95.85 μmol, the methane production rate is 155.21 μmol / L / day, and the CO2 conversion rate is 83.64%; while no methane production is detected in the control group without addition, and the CO2 content hardly changes.

[0064] Example 5:

[0065] (1) Dissolve 5 g of ferrous sulfate heptahydrate in 200 mL of deoxygenated ethanol aqueous solution, add 1 g of activated carbon (the mass ratio of ferrous sulfate heptahydrate to activated carbon is 5:1), and stir evenly. Dissolve 2.72 g of sodium borohydride in 75 mL of deoxygenated water (the molar ratio of sodium borohydride to ferrous sulfate heptahydrate is 4:1), and add it dropwise to the reaction vessel. After the addition, age for 30 min. The whole synthesis process is carried out under nitrogen protection and continuous stirring. Wash the product with deoxygenated water and absolute ethanol and then dry it in vacuum to obtain an iron-carbon nanocomposite with an iron-carbon mass ratio of 1:1;

[0066] (2) Weigh the iron-carbon nanocomposite prepared in the above step (1) into a serum bottle, and add the formation water from a certain block of Shengli Oilfield, so that the content of the iron-carbon nanocomposite is 15 g / L. Replace the headspace gas in the bottle with a CO2 / N2 mixture with a volume ratio of 4:1, and cultivate anaerobically in the dark at 55 °C, and regularly detect the composition of the headspace gas. The hydrogenotrophic methanogens in the oilfield water sample are mainly Methanothermus and Methanoculleus. The difference between the control group and the experimental group is that the iron-carbon nanocomposite is not added.

[0067] After culturing for 483 h, the methane production in the culture system with the added iron-carbon nanocomposite is 165.79 μmol, the methane production rate is 205.96 μmol / L / day, and the CO2 conversion rate is 86.04%; while the methane production in the control group without addition is 4.3 μmol, the methane production rate is 5.59 μmol / L / day, and the CO2 conversion rate is 18.37%.

[0068] The above description of the embodiments is provided to enable those of ordinary skill in the art to understand and use the invention. It is obvious that those skilled in the art can easily make various modifications to these embodiments and apply the general principles described herein to other embodiments without creative efforts. Therefore, the present invention is not limited to the above embodiments, and all improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A method for promoting microbial conversion of CO2 to methane by an iron-carbon nanocomposite material, characterized in that, It includes the following steps: (1) Prepare an iron-carbon nanocomposite: The iron-carbon nanocomposite loads nano-zero-valent iron on a carbon-based material carrier, so that smooth nano-zero-valent iron particles are evenly distributed on the surface of the carbon-based material; the particle size of the carbon-based material is 20-100 μm; (2) Obtain a target reaction system, ensure that hydrogenotrophic methanogens are contained in the target reaction system; add the iron-carbon nanocomposite prepared in step (1) to the target reaction system containing hydrogenotrophic methanogens, introduce CO2 gas, and carry out conversion using hydrogenotrophic methanogens; after a period of time, harvest methane gas.

2. The method for promoting the conversion of CO2 to methane by microorganisms using the iron-carbon nanocomposite material according to claim 1, characterized in that The method for preparing the iron-carbon nanocomposite in step (1) is as follows: Under the protection atmosphere of inert gas and continuous stirring, dissolve the iron salt in a deoxygenated organic solvent-water mixed solution, add the carbon-based material and mix evenly; then, add a reducing agent, let the reaction system stand and age for a period of time, and then separate to obtain a solid particle product; then wash the solid particle product with a deoxygenated washing solvent; finally, carry out vacuum drying on the washed solid particle product to obtain the iron-carbon nanocomposite.

3. A method for promoting microbial conversion of CO2 to methane by an iron-carbon nanocomposite according to claim 2, characterized in that The inert gas in step (1) is at least any one of nitrogen, argon, and helium; the rotation speed of the continuous stirring is 200-400 rpm.

4. A method for promoting the conversion of CO2 to methane by microorganisms using the iron-carbon nanocomposite according to claim 2, characterized in that, The iron salt in step (1) is any one of ferrous sulfate heptahydrate, ferric chloride hexahydrate, ferric nitrate nonahydrate, or ferrous chloride tetrahydrate; the deoxygenated organic solvent-water mixed solution is any one of deoxygenated ethanol aqueous solution or deoxygenated methanol aqueous solution; the carbon-based material is any one of activated carbon, biochar, or graphene oxide; the reducing agent is any one of sodium borohydride solution or potassium borohydride solution; the deoxygenated washing solvent is deoxygenated water and absolute ethanol.

5. The method for promoting the microbial conversion of CO2 to methane by using the iron-carbon nanocomposite material according to claim 2, wherein, The mass ratio of the iron salt to the carbon-based material in step (1) is 5:1-1:1; the molar ratio of the reducing agent to the iron salt is 5:1-3:

1.

6. A method for promoting microbial conversion of CO2 to methane using the iron-carbon nanocomposite according to claim 2, characterized in that, The standing and aging time in step (1) is 15-45 min.

7. The method for promoting microbial conversion of CO2 to methane by using the iron-carbon nanocomposite material according to claim 1, characterized in that The target reaction system in step (2) is any one of oilfield formation water, an anaerobic serum bottle containing an inorganic salt medium, or an anaerobic reactor.

8. A method for promoting the conversion of CO2 to methane by microorganisms using the iron-carbon nanocomposite material according to claim 1, characterized in that, The hydrogenotrophic methanogens in step (2) include at least one of Methanomicrobiales, Methanococcales, Methanobacteriales, Methanomicrobiaceae, or Methanosarcinaceae at the "order" level.

9. A method for promoting the conversion of CO2 to methane by microorganisms using the iron-carbon nanocomposite according to claim 1, characterized in that, The temperature of the target reaction system containing hydrogenotrophic methanogens is 37°C-55°C.

10. A method for promoting microbial conversion of CO2 to methane using an iron-carbon nanocomposite according to claim 1, characterized in that, The mass-volume ratio of the iron-carbon nanocomposite to the target reaction system in step (2) is 10-30 g / L.

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