Bioproduction of acetic acid from carbon dioxide as single substrate and zero valent iron (ZVI) by clostridia

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Developing innovative bioprocess strategies for carbon capture and utilization has been a major focus of research over the past decade as a way of creating a more resilient world. In this study, zero-valent iron (ZVI) was utilized to generate hydrogen, which was then utilized as an electron donor for CO2 reduction by either Clostridium aceticum or Clostridium carboxidivorans. For this bioprocess, an early acidic condition in the reactors facilitated more hydrogen production and thereby more acetic acid accumulation, in contrast to later acidification, elucidating the role of pH in ZVI-CO2 bioconversion. Acetic acid was produced as single end product of the process in both batch bottle studies with no pH control as well as in reactor studies with pH regulation. This is, to the best of our knowledge, the first report on selective pure CO2 bioconversion to acetic acid by clostridia, with ZVI. In order to minimize costs, scrap iron rather than pure ZVI can be used for scaling-up. By lowering the pH to 6.6 for C. aceticum, in reactor studies with 75 g/L ZVI, some ethanol production (125 mg/L) was also observed, besides acetic acid. In reactor studies, a maximum acetic acid concentration of about 2 g/L was obtained at ZVI concentrations of 50 and 75 g/L. Thus, ZVI dosage and medium pH have a major effect on the metabolites generated during CO2 utilization.