Thermodynamic evaluation on H2 production in glucose fermentation

Area of Science:

• Biochemistry
• Microbiology
• Chemical Engineering

Background:

• Biohydrogen (bioH2) fermentation typically yields a maximum of 2 mol H2/(mol glucose).
• Proton reduction is energy-intensive, making thermodynamics a key factor in bioH2 formation.
• Glucose metabolism often results in organic acids and alcohols rather than accumulating electrons in H2.

Purpose of the Study:

• To investigate the thermodynamic control of biohydrogen production during fermentation.
• To evaluate the complete stoichiometry of fermentation under varying pH conditions.
• To identify the relationship between fermentation products and H2 yield.

Main Methods:

• Conducted batch biohydrogen fermentation experiments across a range of pH values.
• Measured all electron sinks to construct complete electron equivalent balances.
• Analyzed the correlation between specific organic acid/alcohol accumulation and H2 yield.

Main Results:

• H2 yields varied from 0 to approximately 2 mol H2/(mol glucose) depending on pH.
• The highest H2 yield (pH ~4) coincided with significant butyrate accumulation.
• Ethanol and lactate accumulation at pH 7 and 10, respectively, correlated with reduced H2 yields.
• Thermodynamics favored the ferredoxin-coupled H2-producing reaction at acidic pH but blocked it at pH 10.
• Butyrate formation was identified as the most thermodynamically favorable ATP-producing reaction post-glycolysis.

Conclusions:

• Thermodynamics fundamentally control biohydrogen yield by influencing key enzymatic reactions.
• pH is a critical factor, with acidic conditions favoring higher H2 production.
• Metabolic pathways, particularly butyrate formation, are thermodynamically optimized for energy production, impacting H2 yields.