Environmental Engineering Reference
In-Depth Information
•
−
further oxidized M
(n
+
2)
+
ion (Eq.
3.27
). Therefore, formation of O
2
is an impor-
tant step in the generation of organic peroxides in natural waters.
M
n
+
+
O
2
→
M
(
n + 1)
+
O
2
•−
(
M
=
Fe
2
+
, Cu
+
, etc
)
(3.25)
O
2
•−
+
R
+
→
RO
2
•
(
R
=
H
+
/
CH
3
, etc
)
(3.26)
RO
2
•
+
M
(
n
+
1
)+
+
H
+
→
ROOH
+
M
(
n
+
2
)+
(
R=H
+
/
CH
3
)
(3.27)
3.2 Microbial Formation of H
2
O
2
and ROOH
H
2
O
2
and ROOH compounds are typically produced under dark incubation by microbial
activity in natural waters (Fig.
2
) (Palenic and Morel
1988
; Moffett and Zafiriou
1990
;
Vermilyea et al.
2010a
,
b
). They are susceptible to be formed by several biological pro-
cesses. Biota is thought to be the main source of dark H
2
O
2
and ROOH production in
natural waters (Fig.
2
b) (Paradies et al.
2000
; Forman and Boveris
1982
). For instance,
dark production of H
2
O
2
in seawater is particle-dependent and the production rates are in
the range of 0.8-2.4 nM h
−
1
(Moffett and Zafiriou
1990
). Recent studies demonstrate the
high dark production rate (29-122 nM h
−
1
) of H
2
O
2
in several lake waters (Vermilyea
et al.
2010
). Moreover, H
2
O
2
and ROOH may be formed extracellularly by marine phy-
toplankton or cyanobacteria (Palenic and Morel
1988
; Zepp et al.
1986
). Extracellular
H
2
O
2
can be produced under dark conditions by enzymatic reduction of oxygen at the
cell surface (Palenic et al.
1987
) and upon oxygen reduction by other electron trans-
port chains. The latter include the mitochondrial reduction of oxygen followed by H
2
O
2
diffusion out of the cell (Forman and Boveris
1982
; Frimer et al.
1983
). Also the auto-
oxidation of organic material may produce H
2
O
2
and ROOH in the aquatic environment
(Stevens et al.
1973
). In seawater, H
2
O
2
may be produced by particle-dependent and
light-independent microbial processes (Moffett and Zafiriou
1990
). For example, a net
H
2
O
2
production (dark production minus dark consumption) of 1-3 nM h
−
1
has been
observed at 40-60 m in an in situ experiment conducted in the Sargasso Sea (Palenic
and Morel
1988
). Finally, ROOH compounds are produced in bulk natural-water DOM
by light-independent microbial processes (Fig.
2
) (Sakugawa et al.
2000
). For example,
net ROOH production has been observed in both filtered and unfiltered river waters (2b),
while H
2
O
2
is merely produced in filtered river waters (Fig.
2
a). ROOH compounds are
typically more concentrated in deep seawaters than in surface waters (Sakugawa et al.
2000
).
4 Factors Controlling the Production and Decay of H
2
O
2
and ROOH in Natural Waters
Concentration levels of H
2
O
2
and ROOH as well as production rates of H
2
O
2
dif-
fer in a variety of natural waters (Table
1
). The magnitude of the H
2
O
2
production
decreases from coastal waters to open oceans (Zika et al.
1985a
,
b
; Fujiwara et al.
