Chemical and biological effects of zero-valent iron (ZVI) concentration on in-situ production of H from ZVI and bioconversion of CO into CH under anaerobic conditions.

The conversion of carbon dioxide (CO) to methane (CH) is a strategy for sequestering CO. Zero-valent iron (ZVI) has been proposed as an alternative electron donor for the CO reduction to CH. In this study, the effects of ZVI concentrations on the abiotic production of H (without the action of microorganisms) in the first part and on the biological conversion of CO to CH using ZVI as a direct electron donor in the second part were examined. In the abiotic H production, the increase in the ZVI concentration from 16 to 32, 64, and 96 g/L was found to have positive effects on both the amounts of H generated and the rates of H production because the extent of ZVI oxidation positively correlates with increasing surface area. Nevertheless, the increase in ZVI concentration from 96 to 224 g/L did not benefit the H production because the ZVI dissolution was suppressed by the increasing aqueous pH above 10. In the bioconversion of CO to CH using ZVI as an electron donor, the main methanogenesis pathway occurred via hydrogenotrophic methanogenesis at pH 8.7-9.5 driven by the genus Methanobacterium of the class Methanobacteria. At ZVI concentrations of 64 g/L and above, the production of volatile fatty acid (VFA) became clear. Acetate was the main VFA, indicating the induction of homoacetogenesis at ZVI concentrations of 64 g/L and above. In addition, the presence of propionate as the second major VFA suggests the production of propionate from CO and acetate under conditions with high H partial pressure. The results indicated that the pathway for ZVI/CO conversion to CH was competitive between hydrogenotrophic methanogenesis and homoacetogenesis.