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Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to Methane Christy M. Dykstra, and Spyros G. Pavlostathis Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02777 • Publication Date (Web): 10 Oct 2017 Downloaded from http://pubs.acs.org on October 10, 2017
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Environmental Science & Technology
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Zero-Valent Iron Enhances Biocathodic Carbon Dioxide Reduction to
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Methane
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Christy M. Dykstra1 and Spyros G. Pavlostathis1,*
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School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta,
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Georgia 30332-0512, United States
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*Corresponding author. Phone: 404-894-9367; fax: +404-894-8266;
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E-mail address:
[email protected] (S. G. Pavlostathis)
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ABSTRACT: Methanogenic bioelectrochemical systems (BESs), which convert carbon dioxide
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(CO2) directly to methane (CH4), promise to be an innovative technology for anaerobic digester
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biogas upgrading. Zero-valent iron (ZVI), which has previously been used to improve CH4
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production in anaerobic digesters, has not been explored in methanogenic biocathodes. Thus, the
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objective of this study was to assess the effect of biocathode ZVI on BES performance at 1 and 2
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g/L initial ZVI concentrations and at various cathode potentials (-0.65 to -0.80 V versus SHE).
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The total CH4 produced during a 7-day feeding cycle with 1 and 2 g/L initial ZVI was 2.8 and
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2.9 fold higher, respectively, than the mean CH4 production in the four prior cycles without ZVI
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addition. Furthermore, CH4 production by the ZVI-amended biocathodes remained elevated
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throughout three subsequent feeding cycles, despite catholyte replacement and no new ZVI
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addition. The fourth cycle following a single ZVI addition of 1 g/L and 2 g/L yielded 123% and
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231% more total CH4 than in the non-ZVI cycles, respectively. The higher CH4 production could
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not be fully explained by complete anaerobic oxidation of the ZVI and utilization of produced H2
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by hydrogenotrophic methanogens. Microbial community analysis showed that the same
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phylotype, most closely related to Methanobrevibacter arboriphilus, dominated the archaeal
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community in the ZVI-free and ZVI-amended biocathodes. However, the bacterial community
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experienced substantial changes following ZVI exposure, with more Proteobacteria and fewer
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Bacteroidetes in the ZVI-amended biocathode. Furthermore, it is likely that a redox-active
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precipitate formed in the ZVI-amended biocathode, which sorbed to the electrode and/or biofilm,
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acted as a redox mediator, and enhanced electron transfer and CH4 production. Thus, ZVI may
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be used to increase biocathode CH4 production, assist in the start-up of an electromethanogenic
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biocathode and/or maintain microbial activity during voltage interruptions.
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INTRODUCTION Biogas produced by anaerobic digestion contains a mixture of carbon dioxide (CO2) and
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methane (CH4), along with other trace gases.1, 2 Current methods of biogas upgrading separate or
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sequester CO2, but a methanogenic bioelectrochemical system (BES) is capable of directly
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converting CO2 to CH4, improving both biogas energy content and CH4 yield.2, 3 In a
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methanogenic BES, a low applied cathode potential (
