Environmental Engineering Reference
In-Depth Information
HO
-
2
O
-
2
H
2
O
2
HO
2
O
2
:
H
2
O
2
reductant
OH
-
H
2
O
2
→
OH +
H
2
O :
↓
H
2
O
2
oxidant
OH
-
H
2
O :
Fig. 1
Electron transfer and proton transfer reactions in the reduction of O
2
from H
2
O
2
to H
2
O,
demonstrating the intermediates involved.
Data source
Moffett and Zafiriou (
1990
)
where
h
is an electron vacancy (hole), generated in the pigment under the effect of
light. The generation of a single molecule of oxygen from water requires at least
four light quanta, each of which generates an 'electron-hole' couple. The electron
is used in the reaction (H
+
+
e
→
H) required for the subsequent fixation of CO
2
.
Hylakoid particle preparations of the filamentous cyanobacterium
Oscillatoria
chalybea
in laboratory experiments using labeled
16
O
2
and
18
O
2
show the occur-
rence of at least three types of oxygen uptake: one is associated with PSII and the
S-state system, whereas the other two types apparently belong to the respiratory
pathway. The S-state system is involved in
18
O
2
production from H
2
O
2
(Bader and
Schmid
1988
,
1989
). Comparison of the relaxation kinetics of the first two flashes of
a sequence with the steady-state signals as well as the detailed analysis of the mass
spectrometric signals reveal that O
2
is evolved by various cyanobacteria through the
decomposition of H
2
O
2
, which requires only two light quanta (Bader
1994
).
The release of O
2
from H
2
O
2
is confirmed by the redox behavior of H
2
O
2
in
water (Moffett and Zafiriou
1990
; Rose and Waite
2003
; Jeong and Yoon
2005
).
When H
2
O
2
acts as a reductant, O from H
2
O
2
is transformed into O
2
(Moffett and
Zafiriou
1990
). When H
2
O
2
acts as an oxidant, O from H
2
O
2
is converted into
H
2
O (Moffett and Zafiriou
1990
). The chain reactions of H
2
O
2
as reductant and
oxidant are schematically depicted below (Fig.
1
) (Moffett and Zafiriou
1990
):
The detailed mechanism for the release of O
2
in the first scheme can be gen-
eralized using the reduction of Fe
3
+
(or Cu
2
+
) by H
2
O
2
in the following ways
(Eqs.
3.11
-
3.15
) (Bielski et al.
1985
; Hardwick
1957
; Moffett and Zika
1987a
,
b
;
Marianne and Sulzberger
1999
):
(3.11)
HOOH
↔
H
+
+
HO
2
−
Fe
3
+
+
HO
2
−
→
Fe
2
+
+
HO
2
•
(3.12)
HO
2
•
↔
H
+
+
O
2
•−
K
=
1. 58
×
10
−
5
M
−
1
S
−
1
(3.13)
FE
3
+
+
O
2
•−
→
FE
2
+
+
O
2
K
=
1. 5
×
10
8
M
−
1
S
−
1
(3.14)
H
2
O
2
+
Fe
2
+
→
Fe
3
+
+
HO
•
+
OH
−
k
=
63 M
−
1
s
−
1
(3.15)
