Revealing the mechanisms of rhamnolipid enhancing methane production from anaerobic digestion of bituminous coal

Anaerobic digestion (AD) of coal to produce biogas represents a promising strategy for clean coal utilization and the reduction of greenhouse gas emissions. However, the complex structure and resistant organic matter in bituminous coal limit the microbial activity, resulting in low biogas yields. This study evaluates the impact of rhamnolipid biosurfactant on the AD of bituminous coal at concentrations of 0.1 %, 0.3 %, and 0.5 %. The results show that 0.1 % rhamnolipid significantly increases biogas production, yielding 164.66 mL, which represents a 120.10 % increase compared to the untreated control (74.82 mL). The rhamnolipid reduces liquid surface tension, correspondingly improving coal wettability and mitigating particle agglomeration, which facilitates a better microbial interaction with coal surfaces. Parallel factor analysis (PARAFAC) identified protein- and humic-like substances as the primary fluorescent components in the fermentation broth. A decrease in fluorescence intensity indicates an effective degradation of coal's functional groups and aromatic structures. Additionally, rhamnolipid enhanced bacterial diversity, particularly Bacteroidota, Bacillota, and Spirochaetota, which are beneficial to degradation of organic matter. The acetate methanogenic pathway was suppressed, while the CO2 reduction and methylotrophic methanogenic pathways were promoted. Finally, rhamnolipid facilitated transmembrane transport and ATP synthesis. These findings highlight a novel approach to improve biogas production from bituminous coal through an application of biosurfactants.