Environmental Engineering Reference
In-Depth Information
species, especially
C. freundii
have been shown to produce H
2
from CO and H
2
O
by the water-gas shift reaction under anaerobic conditions (Jung et al.
1999
). Some
aerobic bacteria like
Alcaligenes eutophus
and
Bacillus licheniformis
are known
to produce H
2
.
B. licheniformis
was reported to produce 0.5 mol H
2
per mole of
glucose. Use of cell immobilization enhanced H
2
yield increasing it up to 1.5 mol
mole
−1
glucose.
Alcaligenes eutophus
, which contains a soluble NAD-reducing hy-
drogenase, can grow heterotrophically on gluconate and fructose and produces H
2
when exposed to anaerobic conditions (Kalia et al.
1994
; Kumar et al.
1995
).
3.1.2
Enzymes Involved
Availability of a hydrogen-producing enzyme is the most crucial aspect of all biohy-
drogen processes. One should note that the catalytic activity of the various enzymes
differs enormously and the quantity or inherent activity of these enzymes could
limit the overall process. However, currently there is no evidence proving that the
quantity of hydrogen-producing enzyme is the limiting factor in any known system.
In contrast, potential catalytic activity far surpasses the amount of hydrogen pro-
duced which means that other metabolic factors might be limiting.
Hydrogen-producing enzymes catalyze the simplest chemical reaction:
2
He
+
+↔
2
H
2
However, it is known that enzymes capable of hydrogen evolution contain com-
plex metallo-clusters as active sites harboring Ni and Fe atoms. At present, three
enzymes carrying out this reaction are known; nitrogenase, Fe-hydrogenase, and
Ni-Fe hydrogenase. Like most metalloenyzmes, hydrogenases are quite sensitive
to oxygen, high temperature and some other environmental factors. Protein matrix
surrounding the metal centers allows hydrogenases to function properly, selectively
and effectively (VoIbeda et al.
1995
).
3.1.3
Feedstock for Dark Fermentative Hydrogen Production
There are two concerns regarding the feedstock that could be utilized i) the range of
organic compounds and ii) quality of the feedstock. Carbohydrates are the preferred
organic carbon source for H
2
production. Glucose or in principle its isomer hexoses
or its polymers starch and cellulose, give maximum yield of 4H
2
per glucose when
acetic acid is the by-product
CHOHO HCOO
+ → +
2
2
−
2
CO
+
4
H
6 26 2
3
2
2
On the other hand, half of this yield per glucose is obtained with butyrate as the
fermentation end product as follows:
CHOHO HCHCHCOOH
+ → +
2
2
CO
+
2
H
6 26 2
3
2
2
2
2
