Chemolithoautotrophic reduction of CO2 to acetic acid in gas and gas-electro fermentation systems: Enrichment, microbial dynamics, and sustainability assessment

Abstract

Microbial-mediated CO2 reduction was evaluated in gas and gas-electro fermentation systems with and without applied potential by selective enriched biocatalyst. Selective enrichment conditions with treated culture (Heat-shock, Acid-shock and 2-Bromoethano sulphonate (BESA)) showed at least one fold increment in acetatic acid production over the control, suggesting the influence of selective enrichment on CO2 conversion. A maximum of 0.84 g/L acetic acid was observed in the Heat+CO2 condition in gas fermentation (GF) stage with 31 mg.L−1.h−1 inorganic carbon fixation rate (ICFR) and this best condition from GF experiments was evaluated in a single-chambered microbial electrochemical system (MES; Gas-electro fermentation) under different applied potentials (-0.4 and -0.6 V vs Ag/AgCl [3.5 M KCl]). The higher acetic acid titre of 1.02 g/L was obtained with -0.6 V applied potential. Higher charge transfer rates (52% faradaic efficiency), and lower corrosion and resistances (Rct -0.4 ohm) improved the CO2 fermentation efficiencies. An abundance of Proteobacteria, Firmicutes, and Actinobacteria, in the enriched biocatalyst, represented the substantial shift of microbiome towards the CO2 feeding microbes on continuous experimental operation. The beta diversity (PCoA) also showed selective enrichment of C1 consumers majorly in the pre-treated conditions. The life cycle assessment (LCA) depicted the hotspots of the process and emphasized the adoption of renewable energy to achieve low-carbon footprints.