A method for enhancing methane production and carbon sequestration in anaerobic digestion by zero-valent iron in cooperation with exogenous CO2

Abstract

The present invention discloses a method for enhancing methane production and carbon sequestration in anaerobic digestion by zero-valent iron in cooperation with exogenous CO2. The method includes: inoculating sludge in a reactor, adding zero-valent iron thereto, and then adding wastewater. The reaction temperature is maintained at 35 ± 1 °C, and CO2 is injected into the liquid surface at a flow rate of 1.5 L / min for 5 - 50 min / L under stirring. CO2 and zero-valent iron jointly regulate the pH of the system, providing a suitable anaerobic digestion environment for microorganisms. Through the corrosion reaction of zero-valent iron under anaerobic conditions, electrons are released and H2 is generated, stimulating the proliferation of hydrogen-consuming microorganisms, while improving the activity of microorganisms and enzymes, increasing the methane production and rate. In addition, some methanogens in the system utilize H2 and CO2 to produce amorphous carbon, and zero-valent iron and exogenous CO2 are used to regulate the growth of amorphous carbon, strengthening the fixation of CO2 in the system.

Description

Technical Field

[0001] The present invention relates to the technical field of anaerobic digestion for methane production, and particularly to a method for enhancing anaerobic digestion for methane production and carbon sequestration by zero-valent iron in combination with exogenous CO2. Background Art

[0002] Global warming is a major global challenge faced by human sustainable development today. Under the goals of "carbon peak" and "carbon neutrality", converting CO2 into value-added products such as biofuels and biochemicals through carbon capture, utilization, and storage (CCUS) technology helps reduce CO2 in the atmosphere and simultaneously reduces people's dependence on fossil fuels, thus playing a key role in sustainable development.

[0003] In recent years, with the rapid economic development and the continuous improvement of people's living standards, the generation amount of organic solid wastes such as crop straws, livestock and poultry manures, sludge, and food waste has been increasing year by year, and the accompanying environmental pollution risks and treatment pressures have also been increasing day by day. Anaerobic digestion is a highly economical and practical method for treating organic solid wastes or wastewater, which can achieve energy recovery and carbon compensation. The biogas produced by anaerobic digestion mainly consists of 50 - 70% methane (CH4) and 30 - 50% CO2. CO2 reduces the calorific value of biogas and limits the utilization range of biogas. Therefore, improving the CH4 production rate in anaerobic digestion and reducing the CO2 content in biogas are the keys to enhancing anaerobic digestion technology at present.

[0004] Zero-valent iron is an active metal with strong reducing ability. It undergoes reduction reactions with oxidizing substances in the anaerobic digestion system. On the one hand, it reacts with pollutants as a reducing agent to reduce the toxicity of pollutants; on the other hand, it can maintain a relatively low redox potential in the digestion system, which is beneficial to the conversion of propionate to acetate. In addition, zero-valent iron corrodes and releases hydrogen under anaerobic conditions, stimulating the proliferation of hydrogen-consuming microorganisms. At the same time, waste iron filings act as electron donors and conductive media, improving the electron transfer efficiency. Zero-valent iron can also improve the microbial and enzyme activities in the system. Fe 2+ can react with S 2- to form FeS precipitation, reducing the inhibition of S 2- on the anaerobic digestion system.

[0005] Therefore, combining CCUS technology with anaerobic digestion and using zero-valent iron to enhance the methane production performance of the anaerobic digestion system while achieving CO2 fixation is of great significance for renewable energy substitution and the realization of the dual-carbon goal. Summary of the Invention

[0006] To achieve the above object, the present invention provides a method for enhancing anaerobic digestion for methane production and carbon sequestration by zero-valent iron in combination with exogenous CO2, and the method includes:

[0007] Step (1): Put inoculated sludge into the reactor. For every gram of volatile solids in the inoculated sludge, add 0.1 - 1.5 g of zero-valent iron, and then introduce wastewater. Control the temperature in the reactor at 35 ± 1 °C.

[0008] Step (2): Keep stirring in the reactor. At the same time, inject CO2 into the reactor below the reaction liquid surface at a flow rate of 1.5 L / min. Based on each liter of reaction liquid volume, the injection time is 5 - 50 min. Close the inlet pipe and open the outlet pipe to collect the methane gas obtained from the reaction.

[0009] Step (3): After the reaction ends, filter to obtain granular sludge. After washing with sodium hydroxide solution, hydrochloric acid, deionized water, and methanol, black particles obtained by fixing exogenous CO2 are obtained.

[0010] Further, in step (1), the inoculated sludge is anaerobic granular sludge from a UASB reactor with brewery wastewater as the influent, with a total solid content of 11.4 - 12.2 wt% and a volatile solid content of 8 - 9 wt%. The inoculated sludge has biological activity to accelerate the initial stage of the biological process and contains anaerobic microorganisms such as methanogenic archaea.

[0011] Further, in step (1), the wastewater is wastewater with a COD of 3000 mg / L, where COD is chemical oxygen demand.

[0012] Further, in step (1), the zero-valent iron is waste iron filings (ZVSI) pretreated to remove rust, preferably spiral iron shavings with dimensions of 10 mm × 5 mm × 1 mm.

[0013] Further, the zero-valent iron is obtained by the following method: Immerse the iron shavings in sodium hydroxide solution (1 mol / L) for 24 h, then rinse three times with dilute hydrochloric acid (0.1 mol / L), and finally wash with deionized water to obtain waste iron filings with a shiny metal surface, which is the zero-valent iron.

[0014] The working principle of zero-valent iron promoting anaerobic digestion is as follows: Through the corrosion reaction of zero-valent iron under anaerobic conditions (Fe + 2H2O → Fe 2+ + H2 + 2OH - , ΔG0 = -79.8 kJ / mol), electrons are released and H2 is generated, stimulating the proliferation of hydrogen-consuming microorganisms. The hydrogenotrophic methanogens in the system use H2 as the electron donor and CO2 as the electron acceptor to convert H2 and CO2 into methane, while improving the activity of microorganisms and enzymes, increasing the methane production and rate.

[0015] Furthermore, in step (2), the pH in the reactor is 6.8 to 7.2. pH is an important influencing factor for the stable operation of the anaerobic digestion system. When the pH fluctuates, adjusting it with strong acids and bases easily affects the microbial activity. Therefore, a mild and effective pH regulation method needs to be found. Using exogenous CO2 and zero-valent iron to stabilize the pH of the anaerobic digestion system can avoid the use of strong acids and achieve the resource utilization of CO2 and waste while maintaining the system stability.

[0016] The specific mechanism is as follows: CO2 dissolves in water to form carbonic acid and releases to lower the system pH; zero-valent iron undergoes a corrosion reaction under anaerobic conditions (Fe + 2H2O → Fe 2+ + H2 + 2OH - ), releasing OH - , which can make the system alkaline. The combined action of the two maintains the pH stability of the anaerobic digestion system, stabilizing at 6.8 to 7.2.

[0017] Furthermore, in step (3), the black particles are amorphous carbon.

[0018] The mechanism of the present invention is that CO2 and zero-valent iron jointly regulate the system pH to provide a suitable anaerobic digestion environment for microorganisms. Through the corrosion reaction of zero-valent iron under anaerobic conditions (Fe + 2H2O → Fe 2+ + H2 + 2OH−, ΔG0 = -79.8 kJ / mol), electrons are released and H2 is generated, stimulating the proliferation of hydrogen-consuming microorganisms. The hydrogenotrophic methanogens in the system use H2 as the electron donor and CO2 as the electron acceptor to convert H2 and CO2 into methane, while improving the microbial and enzyme activities and increasing the methane production and rate. In addition, some methanogens in the system use H2 and CO2 to produce amorphous carbon (CO2 + 2H2 → C + 2H2O ΔG = -79.8 kJ / mol), and the growth of amorphous carbon is regulated by zero-valent iron and exogenous CO2 to strengthen the CO2 fixation in the system.

[0019] Technical effects

[0020] CO2 can be fixed through two ways in the anaerobic digestion system. One is that hydrogenotrophic methanogens convert CO2 and H2 into CH4 (CO2 + 4H2 → CH4 + 2H2O). The other is that some methanogens use H2 and CO2 to produce amorphous carbon (CO2 + 2H2 → C + 2H2O ΔG = -79.8 kJ / mol), converting CO2 into carbon elemental through a biological process, which is a new way of microbial anaerobic carbon fixation. The generated amorphous carbon may be similar to biochar in physiological functions, and can drive microbial metabolism by acting as an electron acceptor and an electron donor, providing a good living environment for microorganisms and adsorbing toxic substances in the system, etc., to promote interspecies electron transfer in the system.

[0021] Under the same conditions, the methane production rate of the reactor with CO2 introduced and zero-valent iron added is 4.67% - 35.53% higher than that of the reactor with only wastewater added; compared with the reactor with CO2 introduced, the methane production rate can be increased by 0.16% - 18.26%; compared with the reactor with only iron shavings added, the methane production can be increased by 3.87% - 94.63%. In addition, the content of amorphous carbon extracted from the granular sludge of the reactor with exogenous CO2 or zero-valent iron added is higher than that of the reactor with only wastewater added. It can be seen that the method of the present invention effectively realizes the synergy of exogenous CO2 fixation and anaerobic methanation of organic waste. Description of the Drawings

[0022] Figure 1 It is a schematic structural diagram of the anaerobic digestion device used in Example 1 of the present invention;

[0023] Reference numerals: 1: Anaerobic digestion reactor; 2: Anaerobic granular sludge; 3: Added pretreated iron shavings; 4: Feed pipe; 5: Inlet gas pipe; 6: Agitator; 7: Outlet gas pipe; 8: Outlet liquid pipe;

[0024] Figure 2 It is a graph showing the change of methane production rate with time for each group in Example 2 of the present invention;

[0025] Figure 3 It is a graph showing the changes of VFAs and SCOD for each group in Example 2 of the present invention;

[0026] Figure 4 It is the three-dimensional fluorescence spectrum of the fermentation broth of each group on the 30th day in Example 2 of the present invention;

[0027] Figure 5 It is the CV curve of the sludge after the reaction of C0, C1, R0 and R1 in Example 2 of the present invention;

[0028] Figure 6 It is the DPV curve of the sludge of each group after the reaction in Example 2 of the present invention;

[0029] Figure 7 It is the scanning electron microscope image of the black particles of amorphous carbon in the granular sludge in Example 2 of the present invention;

[0030] Figure 8 It is the first derivative spectra of C1s and CKL at the surface of the black particles and at the etched 20 nm and 50 nm in Example 2 of the present invention;

[0031] Figure 9 It is the Raman spectrum characterization of the black particles in Example 2 of the present invention;

[0032] Figure 10 It is the weight percentage - temperature curve of the black particles in Example 2 of the present invention. Detailed implementation manners

[0033] The following introduces multiple preferred embodiments of the present invention with reference to the accompanying drawings of the specification, making its technical content clearer and easier to understand. The present invention can be embodied in many different forms of embodiments, and the protection scope of the present invention is not limited to the embodiments mentioned in the text.

[0034] Embodiment 1

[0035] The structure of the anaerobic digestion device used in this embodiment is as Figure 1 shown. The anaerobic digestion device is an anaerobic digestion reactor 1, and a feed pipe 4, an inlet gas pipe 5, a stirrer 6, an outlet gas pipe 7, and an outlet liquid pipe 8 are arranged above it. Among them, the end of the inlet gas pipe 5 extends below the reaction liquid in the anaerobic digestion reactor 1, the end of the outlet gas pipe 7 is above the reaction liquid in the anaerobic digestion reactor 1, and an outlet liquid pipe 8 is provided at a position flush with the reaction liquid level on the side of the anaerobic digestion reactor 1 to ensure that the liquid volume in the reactor remains unchanged.

[0036] A method for zero-valent iron synergistic exogenous CO2 to enhance anaerobic digestion for methane production and carbon sequestration in the above anaerobic digestion device includes the following steps:

[0037] Step (1), inoculated sludge 2 (TS = 11.4 - 12.2 wt%, VS = 8 - 9 wt%) is put into the anaerobic digestion reactor 1. Corresponding to each g of volatile solids in the inoculated sludge, 0.5 - 1.5 g of pretreated iron shavings 3 with a metallic luster is added, and then wastewater with a COD of 3000 mg / L is introduced, and the temperature in the reactor is controlled at 35 ± 1 °C;

[0038] Step (2), stirring is maintained in the reactor, and at the same time, CO2 is injected into the reactor below the reaction liquid level at a flow rate of 1.5 L / min. Based on each L of the reaction liquid volume, the injection time is 5 - 50 min. The inlet gas pipe is closed, and the outlet gas pipe is opened to collect the methane gas obtained from the reaction;

[0039] Step (3), after the reaction is completed, granular sludge is obtained by filtration, and after being washed with sodium hydroxide solution, hydrochloric acid, deionized water, and methanol, amorphous carbon black particles obtained by fixing exogenous CO2 are obtained.

[0040] Embodiment 2

[0041] The experiment was carried out in a 100 mL reactor with a working volume of 80 mL. The feeding ratios of inoculated sludge, wastewater, and zero-valent iron were the same as in Example 1. A total of 10 groups of experiments were set up, with 3 parallel samples in each group. They were distinguished by different dosages of zero-valent iron and whether CO2 was introduced, including groups C0 - C4 (corresponding to 0, 0.1, 0.5, 1.0, and 1.5 g of pretreated iron shavings with metallic luster per gram of volatile solids in the inoculated sludge), and groups R0 - R4 (on the basis of adding 0, 0.1, 0.5, 1.0, and 1.5 g-ZVSI / g VSS respectively, exogenous CO2 was introduced, and the CO2 introduction amount increased sequentially in four stages of 30 s, 2 min, 5 min, and 10 min). The flow rate was always maintained at 1.5 L / min. All reactors were flushed with N2 for 3 min to maintain an anaerobic environment, sealed with butyl rubber stoppers, and then placed in an incubator at 35 °C. The experiment ran for 39 days, and the changes in indicators such as gas production and composition, system pH, soluble organic matter concentration (sCOD), volatile fatty acids (VFAs), and Fe 2+ content were monitored every three days.

[0042] Experimental results:

[0043] 1. Methane production rate

[0044] The methane production amount in the reaction was monitored, and the results are as Figure 2 shown.

[0045] As can be seen from the figure, the methane production effects of reactors R0, R1, R2, R3, and R4 with different dosages of waste iron shavings and the introduction of exogenous CO2 were significantly higher than those of each control group. In the first stage with a CO2 introduction duration of 30 s, reactor R2 (with a waste iron shaving dosage of 0.5 g-ZVSI / g VSS) had the best methane production effect. Compared with the control group C0 with only glucose added, the methane production rate increased by 24.71% - 35.53%. In the following three stages, compared with the control group C0, the methane production rate improvement effects of groups R0, R1, R2, R3, and R4 were similar, and there was no significant dominant group, with the improvement degree ranging from 4.67% to 34.36%. In addition, the methane production of reactors C1, C2, C3, and C4 with only ZVI added decreased to varying degrees compared with the control group C0. This may be because ZVI only has a significant effect when the system is under adverse conditions (such as high organic load or high ammonia nitrogen inhibition). In this experiment, it may be due to the rapid dissolution of ZVI generating a large amount of H2, and the too high hydrogen partial pressure is not conducive to methane production; or because microorganisms utilize the excess Fe 0 to form iron salts or ferrous salts, and the high osmotic pressure generated by their high concentration dehydrates microbial cells, causing cytoplasmic wall separation, and reducing the dehydrogenase activity due to salting out at high concentrations, thus making NADPH → NADP in methanogens -The process slows down or stops, resulting in a decrease in the methane production of the system. It is worth noting that the introduction of exogenous CO2 effectively alleviates this inhibitory phenomenon and effectively improves the methane production performance of the system.

[0046] Figure 3 Shows the changes in VFAs and sCOD of ten treatments. There is little difference in SCOD among the treatment groups, and SCOD is stable between 200 - 500 mg / L during the reactor operation, and almost all of the added organic carbon sources are consumed. According to the theoretical methane production calculation, among the methane produced by the treatment groups R0, R1, R2, R3, and R4 with exogenous CO2 added, approximately 4.04% - 19.76% comes from inorganic carbon sources. Taking the reactor R2 as an example, in the first stage with a CO2 introduction duration of 30 s, approximately 18.5% - 19.76% of the total methane production comes from CO2; in the second stage with a 2 - minute introduction duration, exogenous CO2 contributes 11.21% - 12.31% of the methane production; in the third stage (CO2 introduction duration of 5 minutes), the methane produced by inorganic carbon sources accounts for 6.06% - 12.81% of the total methane production; in the fourth stage (CO2 introduction duration of 10 minutes), approximately 9.57% - 11.97% of the methane comes from CO2. The contribution ratio of inorganic carbon sources to the total methane production is significantly higher in the first stage than in the other three stages, probably because as the hydrogen evolution corrosion of ZVI continues, the concentrations of iron salts and ferrous salts in the system continue to rise, and the excessive concentrations instead affect the microbial and enzyme activities and inhibit the directional methanation of CO2. After the 18th day, the proportion of VFAs in SCOD in the reactors with ZVI and exogenous CO2 is higher than that in each control group, indicating that the combined action of exogenous CO2 and ZVI promotes the conversion of glucose into volatile fatty acids, which is more conducive to the utilization by the system microorganisms.

[0047] The dissolved organic matter (DOM) composed of intermediate products of the microbial process was characterized by three - dimensional fluorescence, Figure 4 Shows the overview on the 30th day, which is respectively related to three peaks located at Ex / Em = 200 - 250 nm / 280 - 320 nm (Region I), Ex / Em = 200 - 250 nm / 380 - 500 nm (Region II), and Ex / Em = 250 - 450 nm / 380 - 500 nm (Region III). Region I, Region II, and Region III are respectively composed of protein - like substances such as tyrosine, fulvic acid - like substances, and humic acid - like substances. The fluorescence intensity is proportional to the DOM component content, thus indicating that the DOM concentration decreases in R0, R1, R2, R3 compared with C0, C1, C2, C3, and C4. The introduction of exogenous CO2 may promote the degradation of organic matter and accelerate the methanation process.

[0048] 2. Electron transfer

[0049] The cyclic voltammetry and differential pulse voltammetry tests were carried out on each group of sludge using an electrochemical workstation.

[0050] Cyclic Voltammetry is a common method for studying electrochemical properties, which can dynamically measure the potential. The peak position represents the type of redox active substances; the peak current characterizes the electron transfer ability of the redox active substances. As Figure 5 shown, compared with the control group C0 with only glucose as the substrate and the treatment group R0 with glucose added and CO2 introduced, the control group C1 and treatment group R1 with ZVI added showed a larger integral area on the CV curve, indicating that the addition of ZVI can increase the electron transfer ability and strengthen the electron transfer process.

[0051] Differential Pulse Voltammetry is also a commonly used electrochemical measurement method in recent years. It is a derivative of cyclic voltammetry and has higher sensitivity than cyclic voltammetry. Figure 6 are the differential pulse voltammograms of the sludge after the reactions of C0, C1, C2, C3, C4, R0, R1, R2, R3, and R4 ended. Figure 6 a and Figure 6 b are the oxidation peak obtained from the positive scan and the reduction peak obtained from the negative scan respectively. The DPV peak around -0.4V is attributed to riboflavin, while the DPV peak around -0.1V is attributed to cytochrome C. These results indicate that ZVI in combination with exogenous CO2 can increase the contents of riboflavin and cytochrome C in the reaction system. These substances can participate in the electron transfer process and play an important role in the anaerobic digestion process.

[0052] 3. Amorphous carbon

[0053] The granular sludge of Example 2 after the reaction was first dispersed by an ultrasonic crusher, and the precipitate was taken after centrifugation. It was washed with NaOH, HCl, etc. at 100°C, and then washed with deionized water and methanol and dried at 100°C to obtain black particles. The morphology of the black particles was observed using a scanning electron microscope (SEM), as Figure 7 shown.

[0054] Elemental content analysis of the surface of the black particles and at the etched depths of 20 nm and 50 nm was carried out using X-ray photoelectron spectroscopy (XPS). The results are shown in Table 1. The C element content on the particle surface is 82.79%, the O element content is 11.89%, and the N element content is 3.72%. The contents of Cl, S, Si, etc. are negligible. After the sample was etched, the C element content was above 90%. At the etched depth of 50 nm, the C element content is 90.85%, the O element content is 4.81%, and the N element content is 2.60%, indicating that the main body of the sample is composed of carbon.

[0055] Table 1 Elemental contents at the surface of black particles and at the etched 20 nm and 50 nm depths

[0056] Elemental composition C(％) O(％) N(％) Cl(％) S(％) Si (%) Surface 82.79 11.89 3.72 0.44 0.48 0.68 At 20 nm of etching 90.38 5.15 2.76 0.50 0.76 0.45 At 50 nm of etching 90.85 4.81 2.60 0.48 0.87 0.37

[0057] C1s spectrum of the particle surface ( Figure 8 a) The results of peak deconvolution show that the contents of C-C and C═C are 84.67%, the contents of C-O and C-N are 6.17%, and the content of C═O is 7.36%. This spectral feature is mainly due to the residual binding proteins on the surface of the black particles. The main resolved peak of the etched sample at 285 eV ( Figure 8 b, 8c) lacks evidence of carbon bonding to heteroatoms, indicating the presence of elemental carbon in the sample. In some cases, sp 2 and sp 3 carbons can be distinguished by the C1s spectrum, but the difference in peak positions is usually too small to reliably resolve different carbon bond types. Therefore, to determine whether the carbon is sp 2 or sp 3 hybridized, the first derivative spectra of CKL at the surface of the sample and at the etched 20 nm and 50 nm depths can be further analyzed. It can be seen from the figure that the surface of the black particles is mainly sp 3 carbon ( Figure 8 d, D parameter 14 eV), while the first derivative spectra of CKL of the etched samples ( Figure 8 e, 8f) show that most of the carbon is sp 2 hybridized, with D parameters of 18 eV and 17 eV, respectively. Among them, the D parameter represents the distance between the most positive maximum and the most negative minimum of the first derivative of the CKL spectrum. The degree of sp Figure 1 hybridization is linearly approximated to the magnitude of the D parameter. The D parameters of diamond and graphite are 14.2 eV and 22.5 eV, respectively. The closer the D parameter value is to that of graphite, the higher the degree of sp 2 hybridization. 2

[0058] The black particles were analyzed by Raman spectroscopy, which showed a broad Raman peak at ~1350 cm -1 and an insignificant Raman peak at ~1580 cm -1 ( Figure 9 ). These two peaks represent the D (disorder / defect) and G (graphite) bands, respectively. The relative intensities and shapes of the D and G bands provide a measure for evaluating the levels of order (crystalline carbon) and disorder (amorphous carbon). This indicates that the black particles have the characteristics of elemental carbon materials but a low degree of order, being more similar to amorphous carbon.

[0059] ​The relative quantification of the amorphous carbon content in the sample was carried out by thermogravimetric analysis (TGA). The decomposition of amorphous carbon occurs between 200 - 400 °C. When the sample is heated to 600 °C or higher, all carbon-containing components are removed, and the remaining mass consists of catalytic metals and their oxidation products. According to Figure 10 , the weight loss of the sample between 200 - 400 °C is about 36.54%, and the weight loss between 400 - 600 °C is about 30.48%. It can be estimated that the amorphous carbon content in the sample is about 36.54%, and the graphite-structured carbon content is about 30.48%.

[0060] After the reaction, 5 g of granular sludge was taken from each treatment group for the extraction of amorphous carbon. The weight and percentage of the extracted amorphous carbon in each treatment group are shown in Table 2. The amorphous carbon content in the granular sludge of reactor C0 is 0.88%. Among all the reactors with the simultaneous addition of ZVI and exogenous CO2, the amorphous carbon content in R3 is the highest, which is 0.98%. Compared with the control group with only glucose as the substrate, the amorphous carbon content extracted from the remaining treatment groups is greater than that of the control group, and the increase ratio is between 3.55% - 23.78%. This indicates that the addition of ZVI and exogenous CO2 promotes the growth of amorphous carbon.

[0061] Table 2 Amorphous carbon content extracted from granular sludge after the reaction in each treatment group

[0062]

[0063] Based on the above conclusions, it can be obtained that the addition of zero-valent iron and CO2 increases the methane production in the anaerobic digestion system, and at the same time accelerates the growth of amorphous carbon. The combined action of the two promotes the fixation of CO2 in the anaerobic digestion system.

Claims

1. A method for enhancing methane production and carbon sequestration in anaerobic digestion by zero-valent iron synergistically with exogenous CO2, the method comprising: Step (1): Put inoculated sludge into the reactor. For every gram of volatile solids in the inoculated sludge, add 0.1 - 1.5 g of zero-valent iron, and then introduce wastewater with a COD of 3000 mg / L. Control the temperature in the reactor at 35 ± 1 °C; the inoculated sludge is anaerobic granular sludge from a UASB reactor with brewery wastewater as the influent; the zero-valent iron is waste iron filings pretreated to remove rust. Step (2): Keep stirring in the reactor, and at the same time inject CO2 into the reactor below the reaction liquid surface at a flow rate of 1.5 L / min. Based on each liter of reaction liquid volume, the injection time is 5 - 50 min. Close the inlet pipe and open the outlet pipe to collect the methane gas obtained from the reaction. Step (3): After the reaction ends, filter to obtain granular sludge. After washing with sodium hydroxide solution, hydrochloric acid, deionized water and methanol, obtain black particles obtained by fixing exogenous CO2. The black particles are amorphous carbon.

2. The method according to claim 1, wherein, In step (1), the total solid content of the inoculated sludge is 11.4 - 12.2 wt%.

3. The method according to claim 1, wherein, In step (1), the volatile solid content of the inoculated sludge is 8 - 9 wt%.

4. The method according to claim 1, wherein, In step (1), the inoculated sludge has biological activity and contains methanogenic archaea.

5. The method according to claim 1, wherein, In step (1), the zero-valent iron is obtained by the following method: soak iron shavings in sodium hydroxide solution for 24 h, then rinse with dilute hydrochloric acid three times, and finally wash with deionized water to obtain waste iron filings with a metal-luster surface, that is, zero-valent iron.

6. The method according to claim 1, wherein In step (2), the pH in the reactor is 6.8 to 7.2.

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