Environmental Engineering Reference
In-Depth Information
which indicates that dark production pathways of ROOH are operational in natural
waters. (iv) The photoinduced generation of ROOH compounds is typically higher
for low concentration of fulvic acid (FA, 1 mg L
−
1
), and decreases with increasing
FA concentration (3 and 5 mg L
−
1
, Fig.
3
b). This finding suggests that the for-
mation of ROOH compounds does not depend on DOM concentration which, on
the contrary, might favor ROOH decomposition. These results indicate that ROOH
compounds are quickly decomposed, which might be due to their inherently unsta-
ble chemical nature. ROOH compounds are sensitive to acid, alkali, redox and
light in aqueous solution (Mostofa and Sakugawa
2009
).
1.4 Steady State Concentration and Half-Life
of H
2
O
2
and ROOH
The concentration levels of H
2
O
2
and ROOH are often measured in natural
waters or in irradiated aqueous solutions, and they are often in a steady state.
Steady-state concentrations of H
2
O
2
and ROOH compounds in natural waters
are mostly dependent on three major phenomena. First, enzymes (catalase,
peroxidase and superoxide dismutase) in microbes, phytoplankton and algae
present in natural waters are active agents for the rapid decay of peroxides
(Mostofa
2005
; Fujiwara et al.
1993
; Moffett and Zafiriou
1990
; Petasne and
Zika
1997
). These processes limit the occurrence of organic peroxides in natural
waters. Second, the incident solar irradiance may be involved into the produc-
tion of peroxides in waters (Cooper and Zika
1983
; Moore et al.
1993
; Baxter
and Carey
1983
; Mostofa and Sakugawa
2003
). Third, the organic peroxides
may take part to the generation of free radicals (HO
•
•
) by direct pho-
tolysis or photo-Fenton reactions in natural waters (Zepp et al.
1992
; Jeong and
Yoon
2005
; Southworth and Voelker
2003
; Voelker et al.
1997
). The free radi-
cals then cause the photodegradation of DOM (Gao and Zepp
1998
; Brezonik
and Fulkerson-Brekken
1998
; Goldstone et al.
2002
). A general scheme for
the steady-state concentration of H
2
O
2
and ROOH in aqueous media can be
expressed as follows (Fig.
4
):
or RO
M
n+
Oxidation of transition metal ions or
other processes
hυ
hυ
H
2
O
2
+ROOH
DOM
HO
•
/RO
•
DOM
Degraded products
Decay by biological
and any other processes
Fig. 4
A schematic diagram of steady state concentration of photoinduced generation of H
2
O
2
and ROOH from DOM in natural waters
