Environmental Engineering Reference
In-Depth Information
Radiation absorption by the next lowest energy fluorophore would then produce
further reactive species and cause sequential degradation of the fluorophores, and
so on till the entire degradation of the parent molecule. Photoinduced degrada-
tion of organic substances can also occur by other processes. For instance, phos-
gene (COCl
2
) is highly photosensitive and highly reactive (Shriner et al.
1943
) as
explained previously. The sequential photodegradation mechanism is applicable to
various FDOM such as fulvic acid and humic acid of plant origin, autochthonous
fulvic acid of algal origin, proteins and aromatic amino acids.
Controlling Factors for Photodegradation of FDOM in Natural Waters
Photodegradation of FDOM depends on several key factors that are similar to the
photodegradation of DOM (Mostofa et al.
2011
). The photoinduced degradation of
FDOM depends on the several factors in the aquatic environments: (i) The nature or
the quality of the organic components of DOM; (ii) The concentration or the quantity
of the organic DOM components; (iii) The pH of the sample solution that may affect
the photo-induced generation of HO
•
, a strong oxidizing agent that is involved in
the photodegradation of DOM (Bertilsson and Tranvik
2000
; Wu et al.
2005
; Kwan
and Voelker
2002
). pH also influences the photoactivity of Fe species that take part
to DOM photomineralization (Vione et al.
2009
); (iv) The presence and quantity of
Fe in the water samples that may provide HO
•
through photo-Fenton reaction (H
2
•
O
2
+
Fe
2
+
→
Fe
3
+
+
HO
+
OH
−
) or induce DOM transformation though irra-
diated Fe-DOM complexes (Wu et al.
2005
; Miles and Brezonik
1981
; Zepp et al.
1992
; Southworth and Voelker
2003
);
(v)
The concentration of O
2
that can assist
in the production of HO
•
or H
2
O
2
(Miles and Brezonik
1981
); (vi) The occurrence
, further sources of HO
•
that may enhance the photoinduced
decrease of DOM fluorescence (Table
4
) (Zinder
1993
). For example, irradiation
experiments using a solar simulator have shown that addition of NO
2
−
−
of NO
2
and NO
3
−
to standard
SRFA can slightly enhance the decrease of fluorescence, which reaches approxi-
mately 22 % with 1 mg L
−
1
SRFA
+
50
μ
M NO
2
−
upon 3 h irradiation compared to
19 % with 1 mg L
−
1
SRFA after 3 h (Table
4
). (vii) The light intensity (UV-B, UV-A
and PAR: photosynthetically active radiation) is a key factor in the photoinduced
reactions and controls the production of reactive transients that correspondingly
enhance the photodegradation processes (Garcia et al.
2005
; Bertilsson and Tranvik
2000
; Granéli et al.
1998
; Qian et al.
2001
; Randall et al.
2005
). Interestingly, the
decrease of fluorescence upon addition of NO
2
that is a major HO
•
source (Mack
and Bolton
1999
) was relatively limited (3 %). This finding would be compatible
with SRFA photooxidation primarily occurring because of the photo-induced gen-
eration of HO
−
•
produced photolytically by SRFA itself, or through other processes.
It can be noted that the production rate of H
2
O
2
from SRFA is 69
×
10
−
12
M s
−
1
(Mostofa and Sakugawa
2009
) and a relatively low level of H
2
O
2
can accelerate the
photoinduced degradation of humic acid in aqueous media (Wang et al.
2001
). This
hypothesis is in agreement with the assumption that part of the production of HO
•
by
DOM under irradiation derives from H
2
O
2
photogeneration. It is also in agreement
