Environmental Engineering Reference
In-Depth Information
7.3.3
Classification of Hydrogen-Forming Processes
7.3.3.1
Photoautotrophic Hydrogen Production
Representative photoautotrophic organisms include algae such as Scenedesmus
sp. and Chlamydomonas reinhardtii [18]. Interestingly, Melis et al. [18] developed
a two-stage photosynthesis and hydrogen production system based on the green
alga C. reinhardtii. It was shown that lack of sulfur caused a specific but reversible
decline in the rate of the oxygen-producing step of photosynthesis. In enclosed
cultures, this imbalance in the photosynthesis-respiration relationship due to
S-deficiency resulted in the net consumption of oxygen, leading to anaerobiosis in
the culture and increased hydrogen production. The two-step system allows the
culture to grow and maintain an active photosynthesis apparatus in a complete
growth medium. However, if the medium is made S-deficient, hydrogen is evolved.
Recently, Melnicki et al. [19] showed that the poly-β-hydroxybutyrate content of
the algae increased within 24 h of S-starvation. This implies that this process can
be used to produce both hydrogen and biologically important compounds.
7.3.3.2
Photoheterotrophic Hydrogen Production
The photoheterotrophic microorganisms are represented by cyanobacteria such as
Nostoc sp. and Anabaena variabilis. They are capable of photosynthesis using sun-
light and carbon dioxide in a similar manner to the photoautotrophs. In addition,
they are also capable of nitrogen fixation mediated by a nitrogenase enzyme. Organic
carbon is required to enable this process, as shown by the stoichiometric formula:
NHe TP
+++ →++ +
8
+
8
16
2
NH H
16
ADP
16
Pi
2
3
2
Hydrogen production requires energy that is obtained from light according to the
following expression:
CHOHO light energy
+
2
+
2
CO gasHgas
+
4
242
2
2
2
These organisms therefore have two enzymes that produce hydrogen: a nitroge-
nase and a hydrogenase. There is a very wide range of photoheterotrophs that
could be investigated to determine their capacity for hydrogen production as well
as their potential for genetic modification and tailoring for biofuel production.
7.3.3.3
Heterotrophic Production of Hydrogen
Heterotrophic microorganisms generate hydrogen by metabolizing organic com-
pounds via processes such as that represented by the stoichiometric equation below:
CHOHO HCOO
+ → +
4
2
2
HCOH
+
4
6 26 2
3
3
2
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