In situ reduction of graphene oxide during anaerobic digestion shifts its role from inhibitor to potential promoter of methanogenesis☆

Abstract

Graphene oxide (GO) has attracted attention for its environmental applications and role as a conductive mediator in anaerobic digestion (AD). However, its effects on volatile fatty acid degradation remain unclear. Serial batch experiments using acetate, propionate, and butyrate evaluated the role of GO in methane production and microbial interactions. Initially, GO slightly inhibited methane production, likely due to suppression of coenzyme F420 activity. From the second batch, GO was microbially reduced to treated GO with reduced GO-like features, enhancing methane production by up to 58.4 %. Acetate and butyrate degradation rates increased by 44.2 % and 54.1 %, respectively, whereas propionate showed no improvement. Microbial analyses revealed strong correlations between GO concentration and taxa related to direct interspecies electron transfer (DIET), including Methanothrix and Syntrophomonadaceae. Thus, microbially reduced GO selectively promotes acetate- and butyrate-driven methanogenesis via DIET, highlighting its pathway-specific potential in AD systems and its promise for improving bioenergy recovery efficiency.