Biogas from anaerobic digestion consists of carbon dioxide (CO2), methane (CH4) and other trace gases (e.g., H2S). Biogas upgrading is the process of increasing the energy (i.e., CH4) content of the gas and is one promising technique for energy recovery in water resource recovery facilities. However, traditional methods of biogas upgrading separate or sequester CO2, producing a waste product. Instead, bioelectrochemical systems (BESs) may be utilized to directly convert CO2 to CH4. Methanogenic BESs, which pair an oxidizing bioanode with a CO2-reducing biocathode, have been successfully demonstrated. Iron is an important trace metal for methanogens and is a central molecule in NiFe hydrogenases, ferredoxin and other iron-sulfur proteins. Iron, in the form of zero valent iron (ZVI), may affect methanogenesis by supplying H2 and/or electrons as it undergoes anaerobic corrosion. Furthermore, ZVI may remove hydrogen sulfide (H2S) through the precipitation of iron sulfide complexes. However, the use of ZVI to simultaneously upgrade biogas and remove H2S in a methanogenic biocathode has not previously been explored. Thus, the objective of this study was to systematically evaluate the effects of ZVI and H2S on BES biogas upgrading. Four BESs were developed: BES1 (control, no ZVI and no H2S), BES2 (H2S amended), BES3 (ZVI amended), and BES4 (H2S and ZVI amended). Prior to H2S addition, the ZVI-amended BES4 biocathode removed 86.9% of the initial CO2 within 1 d. In contrast, following an initial 3% v/v H2S addition to the biocathode headspace, 99.7% CO2 removal was achieved within 1 d. Thus, with the addition of 3% H2S, the 3-d mean rate of CH4 production increased by 61% to 0.870±0.057 mmol/d (R2=0.960). In addition, biocathode H2 production was significantly greater during the first day of the feeding cycle after H2S was added, likely due to accelerated ZVI corrosion in the presence of H2S. In the BES4 catholyte, the concentration of Fe2+ was significantly lower than in the BES3 catholyte throughout the feeding cycle, likely due to the formation of iron sulfides. Indeed, the BES4 biocathode removed H2S from the gas to below the odor threshold (&lt; 0.01–1.54 ppm). In contrast, the BES2 biocathode removed H2S to below the detection limit (&lt;1% H2S v/v) but not below the odor threshold. Thus, the addition of ZVI to a biogas upgrading BES both enhanced CH4 production and H2S removal.

Zero valent iron enhances bioelectrochemical biogas upgrading and hydrogen sulfide removal

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.