Revealing the role of conductive materials on facilitating direct interspecies electron transfer in syntrophic methanogenesis: A thermodynamic analysis

Facilitating the direct interspecies electron transfer with supplying versatile conductive materials to accelerate methane production in anaerobic digestion has received considerable attention, while the stimulatory mechanism of conductive materials on the direct interspecies electrons transfer still remains unclear. Herein, the thermodynamic characteristics of syntrophic methanogenesis induced by different conductive materials in various fermentation conditions were investigated to explore the stimulatory mechanism. The results showed that the supplementation of conductive materials significantly affected the Gibbs free energy changes of both propionate oxidation and electrotrophic methanogenesis. Activated carbon contributed to propionate oxidation as a thermodynamically favorable reaction with Gibbs free energy changes in the range of −310.9 and −399.4 kJ mol−1 and the available energy of propionate oxidation was 4.4 times higher than that with c-type cytochrome at a CO2 partial pressure of 0.4 bar. Conductive material with a low redox potential facilitated electrotrophic methanogenesis as a thermodynamically favorable reaction. The Gibbs free energy change of propionate oxidation increased after elevating CO2 partial pressure or electrons concentration but decreased with an increase in pH, temperature, or the propionate concentration. •Conductive material (CM) greatly affected thermodynamic characteristics of DIET.•CM with a high positive redox potential facilitated propionate oxidation.•CM with a low negative redox potential favored electrotrophic methanogenesis.•Elevating pH or temperature promoted available energy of propionate oxidation.•CM redistributed available energy in the system beneficial for DIET.