Simultaneous intensification of direct acetate cleavage and CO2 reduction to generate methane by bioaugmentation and increased electron transfer

Abstract

Direct interspecies electron transfer coupled to CO2 reduction, DIET-CO2 reduction, to generate methane is proposed and prosperous in 2010s. It is well known that bioaugmentation and increased electron transfer benefit DIET-CO2 reduction. Herein, we studied whether other methanogenic pathways, such as H2-mediated methanogenic progress and direct acetate cleavage (acetoclastic methanogenesis), are simultaneously favorable in the presence of conductive materials (CMs). If so, contribution of DIET-CO2 reduction may be overestimated because overwhelming studies just considered this pathway. Detailed researches on whether and how Clostridium pasteurianum coupled with CMs, granular activated carbon, biochar, nano-magnetite and grapheme, influenced methanogenic progresses were conducted. Overall, C. pasteurianum enhanced methane production rate, which was further improved by some CMs. Combined with metabolism, kinetic and electrochemical analysis, experimental results showed that hydrogenotrophic methanogenesis occurred and bioaugmentation strengthened this progress, which was further motivated by CMs, such as biochar and magnetite. 16S rRNA gene analysis suggested that Methanobacteriaceae was potentially responsible for methane production. Whereafter, DIET-CO2 reduction may become prosperous according to electrochemical and thermomechanical analysis. Acetoclastic methanogenesis was also triggered by bioaugmentation based on experiments by using inhibitor, CH3F, for acetoclastic methanogenesis and carbon isotope fractionation. More importantly, magnetite and graphene, which significantly accelerated electron transfer based on electrochemical analysis, further stimulated acetoclastic methanogenesis. Methanothrix dominated and may play an important role in this stage. This work suggests that diverse methanogenic pathways may be benefited from an increase of electron transfer other than DIET-CO2 reduction. Consequently, the long-standing view that only DIET-CO2 reduction was stimulated by conductive materials may need to be reevaluated. Our research provides potential guides to increase methane production during anaerobic digestion by the enhancement of acetoclastic methanogenesis.