Continuous sulfide supply enhanced autotrophic production of alcohols with Clostridium ragsdalei

Autotrophic syngas fermentation with clostridia enables the conversion of CO, CO₂, and H₂ into organic acids and alcohols. The batch process performance of Clostridium ragsdalei was studied in fully controlled and continuously gassed (600 mbar CO, 200 mbar H₂, 200 mbar CO₂) stirred-tank bioreactors. The final ethanol concentration varied as function of the reaction conditions. Decreasing the pH from pH 6.0-5.5 at a temperature of 37 °C increased the ethanol concentration from 2.33 g L^-1 to 3.95 g L^-1, whereas lowering the temperature from 37 to 32 °C at constant pH 6.0 resulted in a final ethanol concentration of 5.34 g L^-1 after 5 days of batch operation. The sulphur availability was monitored by measuring the cysteine concentration in the medium and the H₂S fraction in the exhaust gas. It was found that most of the initially added sulphur was stripped out within the first day of the batch process (first half of the exponential growth phase). A continuous sodium sulfide feed allowed ethanol concentrations to increase more than threefold to 7.67 g L^-1 and the alcohol-to-acetate ratio to increase 43-fold to 17.71 g g^-1.