Microbial electrolysis enhanced bioconversion of coal to methane compared with anaerobic digestion: Insights into differences in metabolic pathways

Coal seams are considered occurrence sites for coalbed methane (CBM) accumulation and bio-methanation and sequestration of CO 2 . Biological approaches for the enhancement of the methanation rate of CO 2 for CBM production have attracted increasing attention. The microbial electrolysis cell (MEC) is a technology with strong potential to improve the performance of the traditional anaerobic digestion (AD) system. In this study, AD and MEC-AD systems were developed to produce biomethane from coal. The results showed that the cumulative methane production of MEC-AD was 6.05 mL/g coal, 39.08% higher compared to AD alone (4.35 mL/g coal). The CO 2 concentration was lower in the MEC-AD reactor, suggesting that the CO 2 from coal degradation was further converted to biomethane. Metagenomics sequencing results showed that Geobacter.sp and Methanosarcina.sp were enriched in the anodic biofilm to the greatest extent, implying that direct interspecies electron transfer at the anode may promote biomethanation. Moreover, there was significant upregulation in the gene abundances of key enzymes involved in the degradation of aromatic compounds and methanogenic metabolism in hydrolytic bacteria. In particular, the gene pilA , which controls conductive pili, was significantly upregulated. The key intermediate metabolites (benzoic acid, phenol, pentadecane, etc.) showed a higher concentration and conversion rate in MEC-AD hydrolysis compared to AD. These results suggest that the efficiency of hydrolysis and interspecies electron transfer during coal digestion are facilitated by MEC technology, achieving a significant increase in biomethane production. This study provides insights into the development of efficient and low or negative carbon technologies for enhancing biomethane production and CO 2 emission reduction.