Contribution of Yeast Extract, Activated Carbon, and an Electrostatic Field to Interspecies Electron Transfer for the Bioelectrochemical Conversion of Coal to Methane

Abstract

The bioelectrochemical conversion of coal to methane was investigated in an anaerobic batch reactor containing yeast extract and activated carbon. In anaerobic degradation of coal, yeast extract was a good stimulant for the growth of anaerobic microorganisms, and activated carbon played a positive role. An electrostatic field of 0.67 V/cm significantly improved methane production from coal by promoting direct and mediated interspecies electron transfers between exoelectrogenic bacteria and electrotrophic methanogenic archaea. However, the accumulation of coal degradation intermediates gradually repressed the conversion of coal to methane, and the methane yield of coal was only 31.2 mL/g lignite, indicating that the intermediates were not completely converted to methane. By supplementing yeast extract and seed sludge into the anaerobic reactor, the intermediate residue could be further converted to methane under an electrostatic field of 0.67 V/cm, and the total methane yield of coal increased to 98.0 mL/g lignite. The repression of the intermediates to the conversion of coal to methane was a kind of irreversible substrate inhibition. The irreversible substrate inhibition in the conversion of coal to methane could be attenuated under the electrostatic field of 0.67 V/cm by ensuring sufficient biomass through biostimulation or bioaugmentation.

Electric Field-Driven Direct Interspecies Electron Transfer for Bioelectrochemical Methane Production from Fermentable and Non-Fermentable Substrates

“…Considering that the percentage of electrons captured in control was 49.2%, the DIET contribution to methane production in AcEF can be estimated as up to 81.2% from the electron balance (96 × x + 49.2 × (100 − x) = 100 × 87.2, x = 81.2). This suggests that the electric field in AcEF improved methane production from acetate by promoting DIET [15,16,30].…”

“…In order to expose the bulk solution to the electric field, the electrodes in the anaerobic reactor were polarized by applying a DC voltage. The electric field exposed to the bulk solution was 0.33 V/cm, which was selected based on previous studies [4,[15][16][17][18]. The anaerobic batch reactors exposed to the electric field were referred to as AcEF, GluEF, and MixEF, depending on the substrate type.…”

“…The cyclic voltammetry (CV) was performed for the bulk solution within the potential range of -1.0 to 1.0 V using a potentiostat (ZIVE SP1, WonA Tech, Korea). The scan rate was 10 mV/sec, which was a value in the range used in previous studies [15,[30][31][32]. For the CV, small pieces of stainless steel mesh (1 cm × 1 cm) were used as the working electrode and the counter electrode, respectively, and Ag/AgCl electrode (ALS Co., Ltd., Japan) was used as the reference electrode.…”

“…In AcEF, the main electron transfer route for methane production is likely to be biological DIET, considering that its ultimate methane production was significantly higher than that of the control. This indicates that the electric field enriches the bulk solution with electroactive microorganisms, and promotes biological DIET for methane production [15][16][17][18]. On the other hand, in anaerobic digestion, acetate can be converted to methane by acetoclastic methanogenesis, or hydrogenotrophic methanogenesis through syntrophic acetate oxidation pathway [1,24].…”

“…However, electrical energy is required in proportion to the amount of methane produced through eDIET [2,4,7]. Interestingly, electroactive microorganisms can also be abundant in the bulk solution of bioelectrochemical anaerobic digesters with polarized electrodes, and improve methane production [4,15,16]. It is worth noting that the bulk solution around the polarized electrode is exposed to an electric field.…”