Environmental Engineering Reference
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fractions (<0.10
μ
m) in surface layers and 8 % in deeper layers (Table
1
). However,
DOM with molecular fractions of <5 kDa is not altered at all under dark incubation
(Table
1
). These results can suggest four important features about microbial degra-
dation of DOM in natural waters. First, molecular fractions of <0.1
μ
m in both lake
surface and deeper waters are labile to microbial degradation, but deeper waters are
much labile than surface waters. It is also suggested that autochthonous DOM in
surface waters, typically produced during the summer stratification period, is less
susceptible to microbial processes. It can be noted that the DOC concentration in
Lake Biwa waters is autochthounously produced (45 %) during the summer strati-
fication period, as estimated from the summer DOC values (135
μ
M C in August)
that are higher than the winter ones (~93
μ
M C in January) during vertical mixing
(Mostofa et al.
2005
). Second, DOM with molecular weight <5 kDa in both surface
and deeper layers is not susceptible to microbial degradation.
2.3 Mechanisms of the Photoinduced Degradation of DOM
in Natural Waters
•
The HO
radical plays a significant role in photoinduced degradation of DOM
in natural waters (Zepp et al.
1992
; Southworth and Voelker
2003
; Zafiriou et al.
1984
; Zika
1981
; Voelker et al.
1997
). Photoinduced degradation of DOM gener-
ally occurs upon direct absorption of UV and visible sunlight by functional groups
in DOM, which are optically detected either as chromophores in CDOM or fluoro-
phores in FDOM. Evidence has been provided that an electron transfer from func-
tional groups on DOM can lead to the photoinduced formation of H
2
O
2
in aqueous
solution (Eqs. 3.13-3.18, see also chapter
“
Dissolved Organic Matter in Natural
the generation of HO
•
, by direct photochemistry or by Fenton/photo-Fenton/photo-
ferrioxalate reaction systems. These processes can be involved into the photo trans-
formation of DOM in natural waters. Therefore, a general mechanistic scheme for
photoinduced degradation of DOM can be depicted as follows (Eqs.
2.1
-
2.4
):
h
υ
(2.1)
DOM + O
2
+
H
2
O+H
+
→
H
2
O
2
+
DOM
•+
+
O
2
+
OH
−
h
υ
(2.2)
→
2HO
•
H
2
O
2
h
υ
→
DOM
•+
+
HO
•
DOM
•+
HO
•
∗
(2.3)
DOM
•+
HO
•
∗
h
→
LMWDOM + DIC + CO
2
+
other byproducts
(2.4)
•
−
ion by the release
of electrons from DOM chromophores or fluorophores, due to solar effects (Eq.
2.1
)
that have been discussed earlier in chapter
“
Photoinduced and Microbial Generation
of Hydrogen Peroxide and Organic Peroxides in Natural Waters
”
. Subsequent irra-
diation converts H
2
O
2
into HO
First, H
2
O
2
is formed photolytically through production of O
2
•
either photolytically (Eq.
2.2
), or via Fenton and
