Environmental Engineering Reference
In-Depth Information
the presence of dissolved oxygen under natural sunlight (Eqs.
3.13
-
3.18
) (Mostofa
and Sakugawa
2009
; Moore et al.
1993
; Richard et al.
2007
; O'Sullivan et al.
2005
; Cooper et al.
1988
; Clark et al.
2009
; Fischer et al.
1985
; Fischer et al.
1987
; Power et al.
1987
; Cabelli
1997
). In these chain reactions, the functional
groups of DOM absorb photons and are promoted to the singlet excited states
(
1
DOM
*
). The latter can undergo intersystem crossing (ISC) and be converted into
the triplet states (
3
DOM
*
) (Eq.
3.13
). The reaction of oxygen with photo-excited
DOM might generate the superoxide radical anion (O
2
•
−
) (Eq.
3.14
) in equilib-
•
•
−
rium with its conjugate acid perhydroxyl radical (HO
2
) (Eq.
3.15
). Both O
2
•
and HO
2
disproportionate to form H
2
O
2
(Eqs.
3.17
and
3.18
, respectively). The
scheme of the reaction chain is reported below:
1
DOM
∗
IS
→
3
DOM
∗
(3.13)
DOM
+
h
ν →
3
DOM
∗
+
O
2
→
DOM
•
+
O
2
•−
(3.14)
O
2
•−
+
H
+
→
HO
2
•
pK
a
=
4. 8
(3.15)
2O
2
•−
→
O
2
2
−
+
O
2
pK
a
=<
0. 35 M
−
1
s
−
1
(3.16)
HO
2
•
+
HO
2
•
→
H
2
O
2
+
O
2
k
=
8. 6
×
10
5
M
−
1
s
−
1
(3.17)
HO
2
•
+
O
2
•−
+
H
2
O
→
H
2
O
2
+
O
2
+
OH
−
k
=
1. 0
×
10
8
M
−
1
s
−
1
(3.18)
•
•
−
•
The reaction of HO
2
with O
2
(Eq.
3.28
) is faster than that of HO
2
with
•
•
−
HO
2
radicals is too slow to
be significant (Clark et al.
2009
). The acidic constant of HO
2
(Eq.
3.17
), and the termination reaction of two O
2
•
(pK
a
=
4.8) sup-
•
ports the generation of the perhydroxyl radical (HO
2
) in coastal waters (Clark et
al.
2009
; Cabelli
1997
). Therefore, the steady-state concentrations of O
2
•
−
and
H
2
O
2
(Eq.
3.18
) are the result of the photoinduced activity of DOM components
in sunlit surface freshwater and oceanic environments, as well as in other aque-
ous media (Inoue et al.
1982
; Cooper et al.
1994
; Millington and Maurdev
2004
).
DOM
•
+
is susceptible to further photoinduced degradation by photoinduced gen-
eration of hydroxyl radical, and the relevant pathways are depicted in the DOM
degradation chapter (see chapter
“
Photoinduced and Microbial Degradation of
Dissolved Organic Matter in Natural Waters
”
). It can be noted that the excitation
of DOM would involve its functional groups (chromophores or fluorophores) that
are the easiest to be excited. Therefore, the reactivity of DOM toward H
2
O
2
pro-
duction will often resemble that of simple photoactive organic molecules. Recent
evidence highlights that DOM can form complexes with trace elements by a strong
π
-electron bonding system (Mostofa et al.
2009a
,
b
). The metal-DOM complexes
are susceptible to undergoing rapid photoinduced excitation that would finally
result into the production of H
2
O
2
.
In studies mimicking the process of intracellular H
2
O
2
formation, it has been
found that the synthetic analogues of chlorophyll, metal complexes of porphy-
rins and phthalocyanines, act as photoactive species that produce H
2
O
2
under
irradiation in aqueous solutions saturated with dioxygen (Lobanov et al.
2008
;
