Metabolic Flow Analysis of Hydrogen Production from Glucose by Dark Fermentation Based on Stoichiometric and Thermodynamic Evaluation

Abstract

In batch tests, the biohydrogen yield from glucose declined gradually from 3.00 mol/mol to 2.00 mol/mol along with the substrate level increasing from 15 g/L to 60 g/L, respectively. The mechanism was revealed through stoichiometric and thermodynamic evaluation. In accordance with quantitative evaluation of the e− equivalent (e−-eq) balances, the decline of hydrogen yields generally accompanied with more e−-eq converting directly from acetate formation to butyrate and ethanol formation. Meanwhile, the acetate/butyrate ratio decreased from 2.1 to 0.6 with the increment of substrate concentration. Likewise, improved yield of the byproduct ethanol (from 0.16 mol/mol to 0.61 mol/mol) was obtained with the rising substrate concentration. It could be attributable to the decrease of Gibbs free energy from −3.9 KJ/e−-eq to −5.6 KJ/e−-eq, which indicated that the higher substrate concentration was more favorable for the fermentation.