Role of GAC-MnO2 catalyst for triggering the extracellular electron transfer and boosting CH4 production in syntrophic methanogenesis

The addition of conductive materials or metal oxide nanoparticles (NPs) to anaerobic systems is an attractive strategy to enhance the anaerobic digestion and the production of CH4. This study proposed a strategy to boost the CH4 production by adding granular activated carbon (GAC)-MnO2 nanocomposites into an anaerobic methanogenic system. The associated mechanisms and the microbial community structure during anaerobic digestion were investigated systematically. Compared with a control with GAC only, after adding GAC-MnO2 composite the chemical oxygen demand (COD) removal efficiency and CH4 yield were increased by 77% and 36%, respectively. The addition of GAC-MnO2 stimulated the secretion of extracellular polymeric substance (EPS), while the secretion of humic substances was inhibited. The spatial distribution of EPS in the anaerobic sludge affects the extracellular electron transfer efficiency as well. Manganese ions concentrated in the EPS layer facilitated the electron flow and, thus, accelerated the extracellular electron transfer. The enhancement of anaerobic methanogenesis can be mainly attributed to the reduction/oxidation cycle of Mn4+/Mn2+. Electron transfer system activity and Cytochrome C content reached up to 341 and 38 nmol/L, respectively, under optimal conditions. 16S rRNA gene sequencing analysis indicated that Spirochaetaceae, Cloacibacterium, and Treponema were the dominant bacteria. The abundance of the methanogenic archaea Methanobacterium and Methanosaeta were increased with the addition of GAC-MnO2.