Environmental Engineering Reference
In-Depth Information
H
2
O
2
HO
2
•—
HO
2
O
2
—
O
2
: Reductant
H
2
O
2
→ OH + OH
—
H
2
O
↓
Oxid ant
OH
—
H
2
O
Fig. 5
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
)
More simply, “Peroxides
SSC
=
produced peroxides—(decay by microbles
and any other processes
+
contribution to DOM photo degradation)”, where
SSC
=
Steady-State Concentration. Therefore, enzymes might be an important fac-
tor in regulating the occurrence of H
2
O
2
and ROOH compounds in natural waters.
The decay rates of H
2
O
2
and ROOH, expressed as half-life times (t
1/2
), are
hours to days depending on the presence of enzymes in natural waters (Harvey
1925
; Mostofa
2005
; Richard et al.
2007
; Cooper and Zepp
1990
; Cooper and
Lean
1992
). For example, the half-life of H
2
O
2
is gradually increased from unfil-
tered to filtered lake waters, from 4.4 h for unfiltered water to 4.7 h for 64
μ
m
filtered water (zooplakton removed), 6.4 h for 12
μ
m filtered water (large algae
removed), 19.1 h for 1.0
μ
m filtered water (small algae removed), and 58.7 h for
0.2
μ
m filtered water (bacteria removed) (Cooper and Lean
1992
). Similarly, the
half-lives are approximately 3 h or less for highly biologically productive coastal
waters or freshwaters, and hundreds of hours for oligotrophic unfiltered waters
(Mostofa
2005
; Fujiwara et al.
1993
; Moore et al.
1993
; Richard et al.
2007
).
1.5 H
2
O
2
Acts as a Reductant and Oxidant-REDOX
H
2
O
2
acts as a reductant and oxidant (REDOX) in many reactions occurring in
natural waters (Moffett and Zika
1987a
;
b
; Moffett and Zafiriou
1990
; Zepp
et al.
1992
; Jeong and Yoon
2005
). When H
2
O
2
acts as a reductant, O from H
2
O
2
is transformed into O
2
. 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.
5
) (Moffett and Zafiriou
1990
).
1.6 Concentration Levels of H
2
O
2
and ROOH Compounds
in Natural Water
The levels of H
2
O
2
and ROOH are greatly variable for a variety of natural waters
(Table
1
) (van Baalen and Marler
1966
; Sinel'nikov
1971
; Sinel'nikov and
Demina
1974
; Cooper and Zika
1983
; Helz and Kieber
1985
; Lazrus et al.
1985
;
