Proton transfer reinforced by rare earth oxides enables highly efficient methanogenesis during biochar-mediated direct interspecies electron transfer.

Efficient electron/proton transfer between syntrophic microbes is essential for the stable operation of anaerobic digestion (AD) systems. Although biochar can accelerate methanogenesis by promoting direct interspecies electron transfer (DIET), its limited proton-transfer capacity often constrains overall efficiency. Here, a composite material with La/Nd-modified CeO2 (labeled as LNC) decorated on biochar (labeled as LNC@BC) was employed to investigate the association between methanogenesis and electron/proton transfer. Compared with the corresponding control groups in the acidogenic stage and the complete AD process, LNC@BC increased volatile fatty acid production by 240% and methane yield by 250%, respectively. In promoting methanogenesis, mechanistic analyses of electron transport activities, electron/proton transfer components, and methanogenic pathways demonstrated that LNC@BC can promote the conversion of propionate and butyrate to methane via DIET/proton-coupled electron transfer (PCET) and the conversion of acetate to methane via decarboxylation. Complementary assessments of electron transfer capacity, c-type cytochrome C, and the kinetic isotope effect further indicated that biochar in LNC@BC can function as an electron conduit to facilitate DIET, while LNC can primarily promote PCET. Thus, LNC@BC exhibits great potential to simultaneously facilitate electron and proton transfer during AD. These findings provide a reference for the rational design of novel conductive materials.