Biohydrogen Production from Anaerobic Fermentation - Biotechnology in China: Biofuels and Bioenergy - page 147

2.2 Key Factors Affecting Hydrogen Fermentation
Hydrogen production is influenced by various parameters, including products
accumulated such as acetate, lactate, and butyrate, temperature, pH, and metal ions
[ 26 ].
In fermentative hydrogen production, ethanol, acetate, propionate, and buty-
rate are produced as the soluble metabolites, which can cause a sharp drop in the
pH value and affect the corresponding metabolic balance, subsequently inhibiting
the bacterial hydrogen production [ 11 , 12 ]. Several studies have reported the
inhibitory effects of added ethanol, butyrate, and acetate on fermentative
hydrogen production. Zheng and Yu [ 27 ] reported that the specific hydrogen
production rate and the substrate degradation efficiency both decreased on
increasing the added sodium butyrate concentration from 0 to 228 mmol/L.
Wang et al. [ 28 ] illustrated that added acetate had a significant influence on both
substrate degradation and hydrogen production as well as the profile of soluble
products, and the C I,50 values for the specific hydrogen production rate and
hydrogen yield were 11.05 and 31.90 g/L. Wang et al. [ 29 ] found that substrate
degradation efficiency, hydrogen yield, and hydrogen production rate decreased
as ethanol, acetate, propionate, and butyrate addition increased from 0 to
300 mmol/L, but ethanol inhibition was not significant compared with that for
organic acids. Tang et al. [ 30 ] also found that acetate exhibited severer inhibition
than ethanol in E. harbinese B49.
Temperature is a very important parameter because it affects the activity of
essential enzymes (e.g., hydrogenases), which consequently affects hydrogen
production by fermentative bacteria [ 31 , 32 ]. In most cases, hydrogen production
was tested under mesophilic conditions. Although, mesophilic hydrogen produc-
tion is preferred to prevent the need for external heating, hydrogen fermentation at
higher temperatures has a higher hydrogen yield, owing to the suppression of
hydrogen-consuming bacteria and the capability to utilize various substrates. Luo
et al. [ 33 ] studied fermentative hydrogen production from cassava at initial pH 7
and 37 and 60 C, representing mesophilic and thermophilic conditions, corre-
spondingly, and found a maximum hydrogen yield of 53.8 mL H 2 /g volatile solids
under the thermophilic condition, 53.5% higher than that achieved under the
mesophilic condition.
Metal ions as cofactors of many enzymes are essential for cell growth and
function during hydrogen production. Several studies have been conducted on
how the external concentrations of various ions affect the fermentative hydrogen
production using different kinds of inocula [ 17 , 34 - 38 ]. However, the results
obtained are quite different from each other, confirming that biohydrogen pro-
duction and the optimal dose (as well as its inhibiting or stimulating effect) of
Cu 2+ ,Mg 2+ ,Fe 2+ , and Ni 2+ are strictly correlated to the inoculum and the
operating condition applied. Thus, to improve the fermentative hydrogen pro-
duction process for each specific inoculum, dedicated research for addition of
each metal iron is needed.