Environmental Engineering Reference
In-Depth Information
photosynthesis. All oxygenic photosynthetic organisms extract electrons and pro-
tons from water and use them to reduce NADP
+
and plastoquinone for use as energy
sources for metabolism such as the Calvin cycle (CO
2
fixation) and other pathways.
However, oxygenic phototrophs, such as cyanobacteria and microalgae, can tran-
siently produce H
2
under anaerobic conditions
via
proton reduction, catalyzed by
a hydrogenase (or nitrogenase) in competition with other intracellular processes.
In this case the electrons and protons ultimately produced by water oxidation are
redirected at the level of ferredoxin/NADPH into hydrogenase (Kruse et al.
2005
).
Hydrogen producing cyanobacteria may be either nitrogen-fixing or non-nitro-
gen-fixing. The examples of nitrogen-fixing, hydrogen producing cyanobacteria in-
clude non-marine
Anabaena
species, marine species of
Anabaena,
such as
Anabae-
na cylindrica, Anabaena variabilis, Anabaena variabilis
PK84,
Anabaena
AMC41,
marine cyanobacteria in the genera
Calothrix, Oscillatori
a,
Gloebacter
PCC7421,
and
Synechococcus
PCC602, and the marine species
Aphanocapsa montana.
Some hydrogen producing species of
Synechococcus
,
Gloebacter,
and
Anabaena
are non-nitrogen-fixing and produce more hydrogen than nitrogen-fixing cyanobac-
teria. Heterocystous filamentous
Anabaena cylindrica
is a well-known hydrogen
producing cyanobacterium, but
Anabaena variabilis
has received more attention in
recent years, because of higher hydrogen yield (Liu et al.
2006
).
Heterocysts provide an oxygen-free environment for the oxygen-sensitive ni-
trogenase enzyme that reduces molecular nitrogen into NH
3
, and protons to H
2
(Eq. 11.1). In a N
2
-containing atmosphere, nitrogen-fixation is the predominant re-
action while H
2
is a minor byproduct. More H
2
can only be formed in the absence
of molecular nitrogen according to the Eq. 11.2. The reducing power for H
2
evolu-
tion is derived from the energy-rich carbohydrate (CH
2
O) stored in the heterocyst
or transferred from neighbor cells. Because of the high energy demand (4 ATP per
H
2
), the energy conversion efficiency from light to H
2
by nitrogenase is quite low
(< 1 %) (Yoon et al.
2006
).
NHe TP
+++ →++ +
8
+
8
16
2
NH H DP
16
16
Pi
(11.1)
2
3
2
8
He
+
++ →+ +
8
16
ATPHADP i
4
16
16
(11.2)
2
The maximum specific H
2
evolution rate per gram of cell mass or chlorophyll
a
(the
pigment content accounts for 2-3 % of cell mass) for the representative nitrogen-
fixing cyanobacteria varies between 0.21-3.06 mmol g
−1
h
−1
. Volumetric produc-
tivity of H
2
, on the other hand, has been reported to be between 0.084-0.93 mmol
H
2
L
−1
h
−1
. Surface area is one of the major cost factors for photobioreactors. Hy-
drogen production is also compared in terms of energy productivity, a general per-
formance parameter for energy generation based on the energy output per volume
per time. The energy productivity is calculated by multiplying the volumetric pro-
ductivity (mmol H
2
L
−1
h
−1
) by the heat of combustion of hydrogen at 25 °C (ΔHc,
H
2
= − 0.24 kJ mmol
−1
) (Weissmann and Benemann
1977
).
Biological H
2
production is often conducted in sequential mode under different
headspace conditions (argon is preferred during the hydrogen production stage): the
