Improving the Kinetics of H2-Fueled Biological Methanation with Quinone-Based Redox Mediators

The biomethanation process involves the conversion of CO2 into a valuable energy carrier (i.e., methane) by methanogenic archaea. Since it can be operated at mild conditions, it is more sustainable than traditional chemical approaches. Nevertheless, the efficacy of biomethanation is limited by the low kinetics of the microbiological reaction and the poor solubility of H2 in water. Herein, the effect of soluble (i.e., AQDS) and insoluble (i.e., biochar) quinone-based redox mediators on the kinetics of H2-fueled biological methanation in bench-scale microcosms was investigated. Microcosms were set up in 120 mL serum bottles and were initially inoculated with a methanogenic sludge deriving from a lab-scale anaerobic digester treating food waste. As a result, the kinetics of H2 consumption and CH4 generation were greatly increased (p < 0.05) in presence of AQDS as compared to the control, accounting for up to +160% and +125% in the last experimental cycle, respectively. These findings could be explained by a two-step mechanism, whereby microbes used H2 to quickly reduce AQDS into the highly soluble AH2QDS, which in turn served as a more efficient electron donor for methanogenesis. In contrast, the used biochar had apparently an adverse effect on the biomethanation process.