Production of hydrogen, ethanol and volatile fatty acids from the seaweed carbohydrate mannitol

Fermentative hydrogen from seaweed is a potential biofuel of the future. Mannitol, which is a typical carbohydrate component of seaweed, was used as a substrate for hydrogen fermentation. The theoretical specific hydrogen yield (SHY) of mannitol was calculated as 5mol H₂/mol mannitol (615.4mL H₂/g mannitol) for acetic acid pathway, 3mol H₂/mol mannitol (369.2mL H₂/g mannitol) for butyric acid pathway and 1mol H₂/mol mannitol (123.1mL H₂/g mannitol) for lactic acid and ethanol pathways. An optimal SHY of 1.82mol H₂/mol mannitol (224.2mL H₂/g mannitol) was obtained by heat pre-treated anaerobic digestion sludge under an initial pH of 8.0, NH₄Cl concentration of 25mM, NaCl concentration of 50mM and mannitol concentration of 10g/L. The overall energy conversion efficiency achieved was 96.1%. The energy was contained in the end products, hydrogen (17.2%), butyric acid (38.3%) and ethanol (34.2%).