Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane

Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide (CO 2 ) directly to methane (CH 4 ), promise to be an innovative technology for anaerobic digester biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH 4 production in anaerobic digesters, has not been explored in methanogenic biocathodes. Thus, the objective of this study was to assess the effect of biocathode ZVI on BES performance at 1 and 2 g/L initial ZVI concentrations and at various cathode potentials (−0.65 to −0.80 V versus SHE). The total CH 4 produced during a 7-day feeding cycle with 1 and 2 g/L initial ZVI was 2.8- and 2.9-fold higher, respectively, than the mean CH 4 production in the four prior cycles without ZVI addition. Furthermore, CH 4 production by the ZVI-amended biocathodes remained elevated throughout three subsequent feeding cycles, despite catholyte replacement and no new ZVI addition. The fourth cycle following a single ZVI addition of 1 g/L and 2 g/L yielded 123% and 231% more total CH 4 than in the non-ZVI cycles, respectively. The higher CH 4 production could not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H 2 by hydrogenotrophic methanogens. Microbial community analysis showed that the same phylotype, most closely related to Methanobrevibacter arboriphilus, dominated the archaeal community in the ZVI-free and ZVI-amended biocathodes. However, the bacterial community experienced substantial changes following ZVI exposure, with more Proteobacteria and fewer Bacteroidetes in the ZVI-amended biocathode. Furthermore, it is likely that a redox-active precipitate formed in the ZVI-amended biocathode, which sorbed to the electrode and/or biofilm, acted as a redox mediator, and enhanced electron transfer and CH 4 production. Thus, ZVI may be used to increase biocathode CH 4 production, assist in the start-up of an electromethanogenic biocathode, and/or maintain microbial activity during voltage interruptions.