(221b) Biochar-Assisted Acetogenic Conversion of Carbon Dioxide to C2-C6 Alcohols: A Sustainable Solution for Renewable Fuel Production

Globally, carbon dioxide (CO₂) emissions have exceeded 36 billion tons annually, contributing to a rise in greenhouse gas (GHG) emissions. Development of carbon capture and conversion technologies is crucial to reduce the global carbon footprint. Current technologies for the capture and direct conversion of CO₂ into various products have the potential to improve process economics and reduce GHG emissions. However, electrochemical and thermochemical processes are energy-intensive, suffer from catalyst poisoning, and have low conversion efficiencies. In contrast, microbial CO₂ fixation and transformation into biofuels and value-added products using autotrophic microorganisms have gained attention for their ability to occur at near ambient temperature and pressure. Additionally, biochar has emerged as a promising material for carbon sequestration and soil improvement, which can complement the benefits of microbial CO₂ conversion. This carbon-rich material has a high surface area and pH buffering capacity and contains minerals and metals that can replace expensive medium components. In this study, biochar produced from switchgrass and poultry litter at 350°C and 700°C was investigated for its effects on the conversion of CO₂ into C2-C6 alcohols and acids using Clostridium ragsdalei strain P11, C. carboxidivorans strain P7, and C. muellerianum strain P21. Fermentations were performed in 250 ml serum bottles with 50 ml working volume using H₂:CO₂:N₂ (60:20:20) at 37°C and 125 rpm. Results showed that medium supplemented with the switchgrass biochar produced at 350°C (SGB350) resulted in more C2-C6 alcohols with the three strains compared to control and other biochar media. The medium supplemented with poultry litter biochar made at 350°C (PLB350) also enhanced growth and formation of C2-C6 fatty acids with the three strains compared to other media. Strain P11 in the SGB350 medium produced ethanol (6.4 g/L), which was 3- and 2-fold more than with strains P7 and P21 in the same medium. Strain P11 in the PLB350 medium produced acetic acid (9.1 g/L), which was 21% and 55% more than with strains P7 and P21, respectively, in the same medium. Strain P21 produced over 2-fold more butyric acid and butanol in the PLB350 and SGB350 media, respectively, than with strain P7. Only strain P21 produced hexanol. The amounts of CO₂ converted by strain P11 in the SGB350 medium were 35% and 4% higher than with strains P7 and P21, respectively. The type of biochar and the temperature at which it was produced affected the three strains differently. These findings demonstrate the potential for SGB and PLB biochar to be used as a medium substitute for expensive nutrients to enhance the production of C2-C6 alcohols and fatty acids from CO₂, increasing the feasibility of the process. Furthermore, the produced C2-C6 alcohols from CO₂ could serve as intermediates for making renewable jet fuels, which are in high demand.