Improved electrochemical properties of graphite electrodes incubated with iron powders in rice-paddy fields boost power outputs from microbial fuel cells.

Studies have shown that the supplementation of anode-surrounding soil with zero-valent iron (ZVI) boosts power outputs from rice paddy-field microbial fuel cells (RP-MFCs). In order to understand mechanisms by which ZVI boosts outputs from RP-MFCs, the present study operated RP-MFCs with and without ZVI, and compositions of anode-associated bacteria and electrochemical properties of graphite anodes were analyzed after 3-month operation. Metabarcoding using 16S rRNA gene fragments showed that bacterial compositions did not largely differ among these RP-MFCs. Cyclic voltammetry showed improved electrochemical properties of anodes recovered from ZVI-supplemented RP-MFCs, and this was attributed to the adhesion of iron-oxide films onto graphite surfaces. Bioelectrochemical devices equipped with graphite anodes recovered from ZVI-supplemented RP-MFCs generated higher currents than those with fresh graphite anodes. These results suggest that ZVI is oxidized to iron oxides in paddy-field soil and adheres onto graphite anodes, resulting in the boost of power outputs from RP-MFCs.