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

In batch tests, the biohydrogen yield from glucose declined gradually from 3.00 mol/mol to 2.00 mol/mol along with the substrate levelincreasing 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 gener-ally 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.