Environmental Engineering Reference
In-Depth Information
The value of standard electrode potential of the reaction of O
2
formation from
H
2
O
2
(Eq.
3.19
) is significantly lower than for H
2
O (Eqs.
3.24
,
3.25
) (Komissarov
2003
):
H
2
O
2
=
O
2
+
2H
+
+
2e
−
,
E
0
=−
0. 69 V
(3.24)
H
2
O
=
1
/
2O
2
+
2H
+
+
2e
−
,
E
0
=−
1. 23 V
(3.25)
Therefore, in vivo formation of oxygen would be preferable from hydrogen
peroxide than from water.
3.3 Generation of H
2
O
2
from DOM and POM
The most important source of H
2
O
2
is the photoinduced generation from DOM
and POM (e.g. algae) under solar illumination in natural waters. The mecha-
nism has been discussed in earlier chapters (see
“
Photoinduced and Microbial
“
Chlorophylls and Their Degradation in Nature
”). In addition, DOM can also pro-
duce H
2
O
2
under dark incubation. Algae or phytoplankton can produce H
2
O
2
from
superoxide radical anion
O
2
•−
,
which can be formed either by photoinduced
generation of electrons from Chlorophyll bound in microorganisms, or via autoch-
thonous DOM. In the latter case, H
2
O
2
generation can take place under photo- and
microbial respiration (assimilations) of phytoplankton (see chapter
“
Photoinduced
Waters
”
and
“
Chlorophylls and Their Degradation in Nature
”
). Overall, production
of H
2
O
2
from various sources can be depicted as follows (Fig.
3
).
DOM + POM
DOM + POM
hυ
dark condition (microbially)
H
2
O
2
+ by-products
-
22
hυ + dark condition
Microbial respiration
Phytoplankton
Photo-
respiration
Autochthonous DOM
Phytoplankton
Fig. 3
Production of H
2
O
2
from various sources in natural waters
3.4 Endogenous H
2
O
2
in the Photosynthetic Cell and Effects
of Exogenous H
2
O
2
Endogenous H
2
O
2
is formed in photosynthetic cells of organisms through produc-
tion of superoxide radical ion
O
2
•−
from whole bacteria of several species, from
phagocytic cells, from spermatozoa as well as peroxisomes, mitochondria and
