Effect of different inorganic iron compounds on the biological methanation of CO2 sequestered in coal seams
Introduction
Coal is a heterocyclic macromolecular organic compound, which contains C, H, O and variable amounts of other elements. The hydrocarbon compounds and lignin derivatives of coal can be degraded by microorganisms by serving as carbon source for their growth and as a H2 source for methanogenesis [1,2]. Scott put forward the concept of microbial enhanced coalbed methane (MECBM) for the first time, which mainly uses microbial action to degrade coal by injecting methanogenic archaea and nutrients into coal seams [3]. Biogenic coalbed methane (CBM) is an important component of natural gas resources worldwide. Indeed, approximately 20% of global CBM is biogenic gas. CO2 reduced biogas is one of the main types of biogenic CBM. At present, most biogenic CBM reservoirs are produced by CO2 reduction [[4], [5], [6]]. Moreover, methanogens have been shown to degrade coal to CH4 under suitable environmental conditions by using exogenous CO2, and almost all types of methanogens can produce methane by reducing CO2 [7,8]. Since the CO2 adsorption capacity of coal is greater than that of methane, CO2 can be used to replace methane in the coal seam, so that methane changes from an adsorbed state to a free state and enhanced coalbed methane [9]. Therefore, by combining the microbial method and the CO2 displacement CBM method, injecting carbon dioxide into coal seams can achieve both CO2 storage and CBM production [10,11]. However, injecting CO2 into coal seams has been shown to increase the concentration of water-soluble CO2 and carbonate, as well as to affect the energy and metabolic pathways of microbial metabolism. These factors affect the occurrence and metabolic activity of microorganisms, change the structure and composition of endogenous microbial communities in coal seams, and thus affect methane production [[12], [13], [14], [15]]. In our previous work, sodium bicarbonate was used to simulate CO2 sequestered in coal seams and conducted biogas production experiment, proved the feasibility of CO2 bio-methanation. It was found that ethers are preferentially utilized by microorganisms, improving the hydrolytic ability and hydrogenotrophic methanogenesis. However, the methane production (<0.14 mmol/g) and the production rate were low in the process of biological methanation [16]. In addition, the acidity of coal seam increases as the CO2 concentration increases, which greatly affect the metabolic activity of endogenous microorganisms; and these conditions are very unfavorable for the production of biomethane by CO2 reduction.
As a necessary trace element in the biochemical reaction of microorganisms, Fe has various forms and valency, which can promote the enzyme synthesis, electron transfer and proton transfer process, and change the microbial communities’ structure. Among these forms, proper Fe0 can improve the activity of lactate dehydrogenase, can be used as an electron donor to produce hydrogen, and can also be used as nutrient for microbial growth [17]. However, when the Fe0 concentration is too high, it inhibits the microbial activity [18]. Fe2+ can reduce the oxidation-reduction potential (ORP) and buffer organic acids, and also promote the production of coenzyme F420, to affect the methane production rate [19,20]. Fe2+ is easily oxidized to Fe3+ to improve the methanogens activity. However, it was also reported that Fe3+ induced the competition between reducing bacteria and methanogens for acetate and hydrogen consumption, and might inhibit temporarily the methanogens activity [21]. Available electron donors are necessary and critical in the two types of methane generation (CO2 reduction and acetate fermentation) [22]. Elemental iron is considered an alternative electron donor for methanogens and can be used as an electron donor during the reduction in CO2 to methane production, thereby improving the methanogenesis performance of various methanogens [23,24]. In recent years, scrap iron and various forms of iron compounds have been extensively studied in anaerobic digestion and remediation of environmental pollutants, and adding rusty waste iron to sludge anaerobic digestion systems has been shown to increase the diversity of bacteria, increase the abundance of iron reducing bacteria and facilitate the degradation of complex substrates [25,26]. Adding goethite and FeCl3 to continuous stirred-tank reactors (CSTR) can promote anaerobic digestion of algae to produce methane [27]. High sulfur coal is an important coal resource in China, accounting for approximately 8% of China’s coal reserves. Currently, high sulfur coal is mainly used after desulfurization by various desulfurization technologies, but several problems still exist, such as low desulfurization efficiency, secondary pollution and low-cost performance [28]. Whether Fe added as an alternative electron donor for methanogens in anaerobic environments can stimulate the bioconversion of CO2 to methane in coal seam environments has not been verified, and existing knowledge regarding the effects of different inorganic iron compounds on biomethanation of CO2 is very limited. In addition, the in situ utilization of underground high sulfur coal can be realized by biological methods remains to be studied.
Using bituminous coal D from Qianqiu Coal Mine in Yima, Henan Province as the substrate in the laboratory, methanogenic archaea enriched by indigenous microorganism in mine water were used as the microorganisms’ source to simulate biogas production from coal. Bicarbonate, along with different inorganic iron compounds, were added to samples to simulate CO2 sequestered in coal seams, after which changes in liquid-phase test indexes such as the biogas production efficiency, fermentation broth pH, COD, ammonia nitrogen and microbial content during anaerobic reaction were evaluated based on comparison with a control group without added iron compounds. The changes in the coal surface morphology and structure before and after the biological reaction were also discussed to analyze the effects of the iron compounds on the biomethanation of CO2 in coal seams to provide a feasible method for promoting this process and the development and utilization of high sulfur coal.
Section snippets
Sample collection
Fresh coal samples were collected from the underground face of the Qianqiu Mine in Yima, Henan Province. To maintain a strict anaerobic environment, samples were sealed and stored and carefully transported back to the laboratory. Fresh mine water collected from the same location using sterile plastic barrels was used as the source of microorganisms for subsequent experiments and stored in the laboratory at a low temperature.
The collected coal samples were processed into coal particles sized
Biomethane production
During biomethane production, the gas production was recorded every 3 days, and the gas remaining after gas production was analyzed using a gas chromatograph. The total gas production, gas composition and concentration are shown in Table 3.
Variations in the cumulative gas production of coal samples under different conditions through time are shown in Fig. 1. The CYM2 had a higher gas production advantage, and the gas production rate was significantly higher than that of other experimental
Significance of biological methanation of CO2 sequestered in pyrite-enhanced coal seams
China is the largest coal producer and consumer in the world, and sulfur has attracted a great deal of attention as an environmental hazard associated with the development and utilization of coal. High sulfur coal, which has a sulfur content greater than 3%, causes a series of environmental problems during its direct utilization such as acid rain pollution produced by the release of large amounts of SO2. Therefore, the use of high sulfur coal has been restricted by environmental protection
Conclusions
Different inorganic iron compounds have different effects on the biological methanation of CO2. In general, FeCl2 and FeS2 can promote this process; however, the effects of FeCl3 and FeOOH were not obvious. The addition of inorganic iron compounds has a significant effect on the liquid phase index and the solid phase structure of the anaerobic reaction system; the pH value of the system increased, then decreased and increased again. Both the COD concentration and the ammonia nitrogen content
CRediT authorship contribution statement
Daping Xia: Conceptualization, Methodology, Formal analysis. Song Huang: Data curation, Resources, Software. Zhixiang Gao: Writing - original draft, Writing - review & editing. Xianbo Su: Visualization, Funding acquisition, Validation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was funded by the National Natural Science Foundation of China (Grant no. 41472127, 41472129 and 41502158), and Scientific and Technological Research Projects of Henan Province (Grant no. 182102310845 and 192102310196). Moreover, the authors are grateful to the editor and anonymous reviewers of this paper.
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