Earthworm-driven pretreatment enhances methanogenesis in coal anaerobic fermentation: Synergistic mechanisms of structural modification and carbon-nitrogen coupling

This study introduces a novel earthworm-driven pretreatment a strategy to enhance coal anaerobic fermentation for methane bioconversion. Through comparative analyses of gas production dynamics, physicochemical properties, and microbial metabolic pathways between earthworm-pretreated coal (EPC) and untreated coal (UC), the mechanisms underlying earthworm-mediated enhancement of coal anaerobic fermentation were systematically elucidated. Results demonstrated that earthworm pretreatment significantly accelerated the fermentation process (gas production peak advanced by 3 days) and increased cumulative methane yield by 61.59 % (121.82 ± 8.61 μmol g−1). The initial rise and subsequent decline in enzymatic activity indicated coal-induced oxidative stress in earthworms, followed by effective scavenging of reactive intermediates. Earthworms promote coal biodegradation through fragmentation and enzymatic hydrolysis (e.g., cellulase and ligninase), which destabilize aromatic structures, liberate alkanes, and catalyze structural reorganization of coal matrix. These processes significantly modify the physicochemical architecture of coal, enhancing its specific surface area and porosity to promote microbial substrate accessibility. Furthermore, earthworm mucus secretions and excretions optimise carbon-nitrogen coupling within the system, amplifying resource utilization efficiency. FTIR and XPS analyses demonstrated that EPC retained aliphatic carbon chains while exhibiting a significant increase in oxygen-containing functional groups (C=O, COO-), indicating preferential degradation of macromolecular organics by earthworms and the subsequent release of bioavailable small-molecule carbon sources. This study systematically elucidates the dual physical-biochemical mechanisms by which earthworms enhance coal anaerobic fermentation, providing theoretical foundations and practical guidance for developing efficient and sustainable coal biomethanation technologies.