Synergistic Enhancement of CO₂ Biomethanation by Fe/Co/Ni‑Loaded Biochar from Cattle Manure

Abstract

Carbon dioxide biomethanation offers a promising route for energy storage via biological conversion of renewable surplus electricity and CO₂ into methane. To improve the gas production efficiency and instability, bioaugmentation strategies are commonly employed by using biochar and trace elements individually. However, their coupled effect remains underexplored. In this study, the CM400 (biochar produced from cattle manure under 400 °C) showed superior performance in cumulative methane yield (138 mL·gVS⁻¹). Response surface optimization identified an optimal trace element combination (Fe: 15.1 mg·L⁻¹, Co: 1.5 mg·L⁻¹, Ni: 0.6 mg·L⁻¹), increasing methane yield by 12.9%. Subsequently, a novel modified biochar (CM400 pre-soaked in the optimal Fe/Co/Ni solution and pyrolyzed) was synthesized and significantly enhanced the volumetric methane production (VMP) by 19.8%. Experimentally, it enhanced the removal of total solids (TS), volatile solids (VS), and total chemical oxygen demand (TCOD) by 21.1%, 20.9%, and 3.4%, respectively, while strengthening pH buffering and preventing volatile fatty acid (VFA) accumulation. The modified biochar enriched electroactive bacteria (e.g., Synergistota, Chloroflexota) and key methanogens (Methanothrix). Collectively, these physiological and compositional changes are consistent with improved syntrophic metabolism and suggest the possible involvement of direct interspecies electron transfer.