Environmental Engineering Reference
In-Depth Information
3 Mechanisms for Photoinduced Generation of HO
•
in Natural Waters
•
The photoinduced generation of HO
significantly depends on several impor-
tant factors such as the presence of NO
2
−
−
and their concentration, the
chemical nature of DOM and its quantity, and finally the total content of Fe. The
most important mechanisms for HO
and NO
3
•
formation in natural waters are discussed
below.
3.1 In situ Generation of HO
•
from DOM
•
One of the main HO
sources in natural waters (Table
2
) (Vione et al.
2006
;
Mostofa KMG and Sakugawa H, unpublished data) is the photoinduced gen-
eration of HO
•
from DOM components (either Fluorescent Dissolved Organic
Matter-FDOM or Colored Dissolved Organic Matter-CDOM). This process can be
accounted for either by the oxidation of water by the triplet states
3
DOM*, or by
the generation of H
2
O
2
upon DOM irradiation (reaction
3.1
) and its detailed mech-
anisms are discussed in earlier chapter (see chapter
“
Photoinduced and Microbial
the latter case, HO
•
could be produced upon photolysis of H
2
O
2
(Eq.
3.2
) (Legrini
et al.
1993
; von Sonntag et al.
1993
).
h
υ
−→
(3.1)
DOM + O
2
+
H
2
O+H
+
H
2
O
2
+
DOM
•+
+
O
2
+
HO
−
h
υ
(3.2)
−→
2HO
•
H
2
O
2
The quantum yield of reaction
3.2
has been determined as 0.5 under UVC
irradiation (Legrini et al.
1993
; von Sonntag et al.
1993
). The quantum yield
varies with wavelength, but it also depends on the band that absorbs radiation.
In the case of H
2
O
2
, the same band is responsible for radiation absorption and
photolysis in both the UVC and UVB regions. The hypothesis that the formation
of HO
•
by irradiated DOM is accounted for by H
2
O
2
photoproduction. It is con-
sistent with the observed, gradual and parallel increase of H
2
O
2
concentration and
of phenol formation from benzene, upon irradiation of natural river waters and of
relevant standard organic substances (Fig.
2
). A similar, parallel trend of both phe-
nol (its formation being used as HO
•
probe) and H
2
O
2
has been observed upon
irradiation of upstream DOM mostly containing fulvic acid (Fig.
2
a), of stand-
ard Suwannee River Fulvic Acid (SRFA) (Fig.
2
c) and of diaminostilbene type
(DAS1) (Fig.
2
d). The same trend has not been observed in sewage polluted river
waters (Fig.
2
b), which might be the effect of additional production from other
HO
•
sources such as the NO
2
-
and NO
3
-
ions, present in high amount (Takeda
