Bioconversion of carbon dioxide and zero valent iron to methane by anaerobic sludge: Kinetics and archaeal consortium

Using carbon dioxide (CO2) as a carbon source for renewable energy production has potential applications for CO2 sequestration and greenhouse gas (GHG) emission reduction. In biological conversion, CO2 can be transformed into methane (CH4) by hydrogenotrophic methanogens with hydrogen (H2) as an energy source. In this study, zero-valent iron (ZVI) of 16, 32, 64, and 96 g/L was used as the H2 energy source for a bioconversion of CO2 to CH4. When the ZVI dosage was increased, a decrease in CO2 in the headspace occurred simultaneously with the increase in CH4. The presence of CH4 in both CO2/H2 and CO2/ZVI indicates that hydrogenotrophic methanogens can utilize both ZVI and H2 as electron donors and convert CO2 to CH4. The highest methane yield of 1.728 mmol CH4/mmol CO2 was observed for the CO2/ZVI 96 g/L. The modified Gompertz equation fitted the cumulative CH4 production curves of CO2/H2 and CO2/ZVI very well, where R2 was 0.9915 and 0.9903-0.9968, respectively. 16S rDNA high-throughput sequencing results revealed that ZVI addition facilitated the increase of the family Methanobacteriaceae, which became the most abundant among other archaea. It points out that this family favors ZVI and utilizes electrons more effectively from ZVI than H2.