Effects of graphene oxide and reduced graphene oxide on acetoclastic, hydrogenotrophic and methylotrophic methanogenesis

This study describes the effects of graphene oxide (GO) and reduced graphene oxide (rGO) on the acetoclastic, hydrogenotrophic and methylotrophic pathways of methanogenesis by an anaerobic consortium. The results showed that GO negatively affected the hydrogenotrophic and acetoclastic pathways at a concentration of 300 mg/L, causing a decrease of ~ 38% on the maximum specific methanogenic activity (MMA) with respect to the controls lacking GO. However, the presence of rGO (300 mg/L) promoted an improvement of the MMA (> 45%) achieved with all substrates, except for the hydrogenotrophic pathway, which was relatively insensitive to rGO. The presence of either rGO or GO enhanced the methylotrophic pathway and resulted in an increase of the MMA of up to 55%. X-ray photoelectron spectroscopy (XPS) analysis revealed that GO underwent microbial reduction during the incubation period. Electrons derived from substrates oxidation were deviated from methanogenesis towards the reduction of GO, which may explain the MMA decreased observed in the presence of GO. Furthermore, XPS evidence indicated that the extent of GO reduction depended on the metabolic pathway triggered by a given substrate.