Influencing mechanism of Fe²⁺ on biomethane production from coal

Highlights

• The addition of Fe²⁺ considerably enhanced the conversion of coal to biomethane.

• The occurrence state of Fe²⁺ is changed from inorganic state to humic acid state.

• Iron in the humic acid state can promote hydrogenase and microbial flora activity.

• The dominant flora in archaea changed from Methanobacterium to Methanosarcina.

Abstract

The conversion of coal to biomethane is gaining considerable attention and can play a significant role in the field of energy research. To study the underlying influencing mechanism of adding exogenous Fe²⁺ on the process of the conversion of coal to biomethane, the biomethane production experiments were conducted using lignite and bituminous coal A as the substrate, and supplemented by different concentrations of ferrous ions. It is found that the optimum concentration of Fe²⁺ in the process of producing biomethane from coal is 10 mg/L. At this time, the cumulative methane production of lignite increased from 192 mL to 246 mL, and that of bituminous coal A increased from 165 mL to 197 mL, which increased by 13.5% and 10.8%. The added Fe²⁺ increased the content of iron in the humic acid state, which was easily absorbed and effectively utilized by microorganisms. This increase promoted the synthesis of hydrogenase and advanced the activity. Iron in the humic acid state also participated in the enzymatic reaction and promoted the activity of sulfate reducing bacteria and methanogens. Fe²⁺ in the free state formed FeS precipitates with the sulfur ion, which reduced the toxicity of soluble sulfides to bacteria. In both treated and untreated samples, the dominant methanogenic archaea changed from Methanobacterium before anaerobic fermentation to Methanosarcina after the reaction, indicating that Methanosarcina was favored under iron-abundant conditions. The experimental results improve understanding of the mechanism through which the addition of Fe²⁺ promotes the conversion of coal to biomethane.

Introduction

As we all know, coalbed methane (CBM) is an important high-quality clean energy and chemical raw material(Bao et al., 2020). It is reported that China is rich in CBM resources, with the total reserves ranking the third in the world. However, due to the complexity of CBM geological reservoir conditions, the extraction rate of CBM is low, and seriously restricts the development of the CBM industry(Xu et al., 2021). How to improve the recovery rate of CBM and realize the increase and stabilization of CBM production have become one of the key bottlenecks in the development of CBM industrialization in China. Based on the formation of biological CBM, the Microbially enhanced coalbed methane (MECBM)(Park and Liang, 2016; Ritter et al., 2015) has the characteristics of green, no pollution, producing new CBM and increasing coal seam permeability, which provides a new path for the large-scale commercial operation of CBM, and has attracted more and more attention.

MECBM technology is to inject anaerobic microbial population and nutrients or active stimulants of anaerobic microorganisms into the coal seam and realize the increase of CBM production by utilizing the characteristics of microbial degradation of coal to produce methane(Park and Liang, 2016; Bao et al., 2016). However, methane production from coal anaerobic fermentation is usually low, which hinders its full application. The main reasons are: 1) complexity and firmness of coal structure; 2) lack of efficient functional flora; 3) lack of sufficient electronic donors to regulate methane production. Although many methods have been used to increase the production of biomethane from coal, most of them are focused on the first two aspects. For example, based on mechanical grinding, ultrasonic treatment, hydraulic fracturing and other physical methods, increasing methane production by increasing contact area with microorganisms(Zhi et al., 2018). Using chemical methods such as HNO₃, H₂O₂, KMnO₄ and some surfactants to increase coal solubility(Guo et al., 2021; Haq et al., 2018; Tamamura et al., 2016). The bioconversion rate of coal is improved by the biological pretreatment of bacteria and fungi(Haider et al., 2013; Xia et al., 2020). The research of high-efficiency functional bacteria is mainly to obtain high-efficiency degradation bacteria from exogenous microorganisms by domestication or improvement and promote the production of biogas together with the native bacteria. By applying an electric field to the coal anaerobic fermentation system and supplying electrons to increase the production of biological methane(Guo et al., 2020). If we can enhance the electron transfer of the reaction system, we can fundamentally accelerate the production of biological methane. However, so far, there are relatively few studies in this field.

Iron is an important part of the key functional proteins of methanogenic bacteria for energy metabolism, which is widely used in anaerobic fermentation(Choong et al., 2016; Ortner et al., 2014). Iron usually exists in the form of zero-valent iron (ZVI), Fe²⁺ or Fe³⁺. Under anaerobic conditions, ZVI can be converted to Fe²⁺ or hydrogen can be generated through Schikorr reaction(Baek et al., 2019; Zhang et al., 2020). After that, hydrogen nutrition methanogenic bacteria used H⁺ as electron donor and transferred electrons to ferredoxin oxidation (Fd_ox) to reduce CO₂ under the action of hydrogenase, increasing of methane production. However, the surface of ZVI in an anaerobic system is easier to form a biological passivation membrane under the action of microorganisms. The existence of passivation film can seriously weaken the reduction of ZVI, making the anaerobic system more complex, and the methane production rate is low, which cannot reach the ideal state. Fe²⁺ is an important component of hydrogenase, which can be used as a nutrient for a microbial activity or an electron shuttle for a redox reaction to promote the conversion of organic matter into methane(Guo et al., 2017; Meng et al., 2013). Another beneficial effect of Fe²⁺ is to control the content of hydrogen sulfide (H₂S) in biogas, remove sulfide by precipitation, and prevent the toxic and side effects of sulfide due to transition accumulation, so as to improve the output of methane(Choi et al., 2018). The presence of Fe³⁺ led to the transfer of electron flow from methane production to iron reduction, which greatly inhibited the growth and activity of methanogenic bacteria(Qu et al., 2004; Zhou et al., 2014).

At present, the addition of exogenous Fe²⁺ is an effective way to improve the methane yield of coal anaerobic fermentation. However, so far, the effect of Fe²⁺ on the preparation of biomethane by anaerobic fermentation of coal has not been studied. How does the addition of Fe²⁺ affect the anaerobic fermentation of coal to produce biomethane? If the addition of Fe²⁺ can promote the anaerobic fermentation of coal, what is the optimal concentration of Fe²⁺? Under the optimum addition concentration, what changes will happen to the occurrence state of the iron element? What are the changes of hydrogenase (proteomics) and related flora (genomics) during fermentation? What is the increasing mechanism of Fe²⁺ on biomethane production by coal anaerobic fermentation? Based on the above assumptions, this paper takes lignite and bituminous coal A with two metamorphic degrees as the research objects, examines the changing trend of adding different concentrations of Fe²⁺ in the process of increasing biogas production, and determines the optimal additive amount of different coal samples. By studying the occurrence state of iron in the reaction system, specific enzyme activities and key microorganisms, the stimulation mechanism of adding the optimal dose of Fe²⁺ in the coal-to-biomethane process were revealed.