Results
3.1. Hydrogen production via fermentation
Using cellobiose (0.25% or 14.6 mM glucose for 2500 mg/L of
cellobiose) as the substrate for fermentation, hydrogen and
CO2 production was observed almost immediately after
reactor inoculation (Fig. 1). The rate of hydrogen production
during the linear phase was 1.65 L H2/L-day, with all the
cellobiose consumed at the end of 40 h. Using lignocellulose
(0.25% or 9.1 mM glucose) as a substrate, however, there was
a longer lag phase of 10 h (Fig. 1). The subsequent hydrogen
production rate during the linear phase was 0.25 L H2/L-day
which was much slower than that of cellobiose. Analysis of
the residual solids at the end of 90 h indicated that the cellulose
component of the added lignocellulose was totally
consumed. Based on the consumption of either cellulose or
cellobiose, the hydrogen molar yield was 1.67 mol/molglucose
for the corn stover lignocellulose, and 1.64 mol/molglucose
for the cellobiose. The ratio of H2 to CO2 was slightly
larger for cellobiose (0.93) than for lignocellulose (0.85), but
both results are within the range reported for fermentation of
a-cellulose and delignified wood [23].
Both fermentation reactions yielded acetic acid and
ethanol as the major by products along with minor amounts
of formic, lactic, and succinic acids (Table 1). However, the
original lignocellulose feed contained sugars and acetic acid
derived from hemicellulose (see Materials and Methods),
although C. thermocellum was unable to metabolize them [21].
As a result, 17% (2.4 mM) of the 14 mM acetic acid in the
fermentation effluent was present in the fermentation
influent for the lignocellulose substrate.