Environmental Engineering Reference
In-Depth Information
Mechanism for Photodegradation of Fluorophores in FDOM in Aqueous
Media
Sequential photodegradation is observed for FDOM fluorophores or functional
groups in FDOM macromolecules, particularly fulvic and humic acids. The
sequential degradation of fluorophores is generally caused by the presence of
diverse functional groups in their molecular structure (Mostofa et al.
2009a
; Senesi
1990a
; Leenheer and Croué
2003
; Malcolm
1985
; Corin et al.
1996
; Peña-Méndez
et al.
2005
; Seitzinger et al.
2005
; Zhang et al.
2005
). This phenomenon can be
understood from the sequential decrease in the fluorescence intensity of fluvic
acid-like components (peak C- and A-regions) with irradiation time (Mostofa et
al.
2005a
,
2007a
; Moran et al.
2000
). The sequential degradation of various func-
tional groups bound to fulvic and humic acids has also been observed in natural
waters (Shriner et al.
1943
; Amador et al.
1989
; Allard et al.
1994
; Xie et al.
2004
;
Li and Crittenden
2009
; Minakata et al.
2009
).
Absorption of photon or light by a fluorophore (or functional group) is gener-
ally caused by its lowest energy excitation, then by the next lowest energy exci-
tation caused by another fluorophore in the molecule and so on (Mostofa et al.
2009a
; Senesi
1990a
). Fluorophore excitation is the first step for the generation
of H
2
O
2
in aqueous media according to (Eq.
3.1
). Photoirradiation converts H
2
O
2
into HO
•
(Eq.
3.2
), photolytically or by Fenton and photo-Fenton reactions (see
The HO
•
radical can then react with the initial excited fluorophore and decom-
pose it (Eq.
3.3
). Therefore, a scheme for the photoinduced degradation of fluro-
phores in macromolecules can be depicted as below (Eqs.
3.1
-
3.4
; see chapters
H
ν
FDOM
+
O
2
+
HO
2
+
H
+
→
H
2
O
2
+
FDOM
•+
+
O
2
+
OH
−
(3.1)
H
ν
→
2HO
•
H
2
O
2
(3.2)
FDOM
•+
HO
•
∗
H
ν
→
FDOM
•+
+
HO
•
(3.3)
H
ν
→
LMWDOM + DIC + CO
2
+
other end products
[
FDOM
•+
HO
•
]
∗
(3.4)
One of the pathways that can lead to H
2
O
2
formation is the production of
•
−
O
2
from O
2
upon release of electrons from irradiated FDOM fluorophores or
chromophores (Eq.
3.1
, see chapter
“
Photoinduced and Microbial Generation of
Hydrogen Peroxide and Organic Peroxides in Natural Waters
”
). Reaction (
3.4
)
produces low molecular weight DOM (LMWDOM), dissolved inorganic carbon
