Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane.

Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide (CO2) directly to methane (CH4), promise to be an innovative technology for anaerobic digester biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH4 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 CH4 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 CH4 production in the four prior cycles without ZVI addition. Furthermore, CH4 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 CH4 than in the non-ZVI cycles, respectively. The higher CH4 production could not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H2 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 CH4 production. Thus, ZVI may be used to increase biocathode CH4 production, assist in the start-up of an electromethanogenic biocathode, and/or maintain microbial activity during voltage interruptions.