Environmental Engineering Reference
In-Depth Information
FI (4.9
×
10
−
3
MJ
−
1
m
2
) is quite similar to that observed in the second step of
fulvic acid (5.6-8.5
×
10
−
3
MJ
−
1
m
2
). From the results one can hypothesize sev-
eral important photoinduced characteristics of fulvic acid and tryptophan com-
ponents in waters. The fast first-step photodegradation of fulvic acid suggests
the existence of highly sensitive fluorophores (and probably of a discrete class of
them) that might undergo quick photoinduced decomposition (del Vecchio and
Blough
2002
). Schiff-base derivatives (-N
=
C-C
=
C-N-) are highly sensitive fluo-
rophores that are commonly detected in DOM humic substances (fulvic and humic
acids). They show a fluorescence peak at Ex/Em
=
360-390/450-470 nm (Laane
1984
) and could well be involved in the initial high losses of FI by fulvic acid
in DOM in the aquatic environment. The almost linear reaction rate observed in
the second step of fulvic acid photodegradation suggests the presence of homo-
geneous fluorophores which might be photolytically decomposed, although less
quickly than the former ones. The linear photodegradation of tryptophan-like FI
could be due to the presence of merely one type of fluorophore (-CH
2
-CH(NH
2
)-
COOH), which would undergo gradual photoinduced decomposition. It is finally
possible (unless it is a mere coincidence) that the similarity of the reaction rate
constants of tryptophan-like FI and of fulvic acid in the second step are due to the
presence of similar fluorophores (Mostofa et al.
2007a
).
5 Ecological Significance of Photoinduced and Microbial
Degradation of FDOM in Natural Waters
5.1 Ecological Significance of Photoinduced Degradation
of FDOM
The decrease in fluorescence intensity of various FDOM samples reflects the
sequential degradation and mineralization of the corresponding fluorophores or
functional groups that are present in the chemical structure of FDOM (Corin
et al.
1996
; Mostofa et al.
2011
; Amador et al.
1989
; Bertilsson and Tranvik
1998
).
Photoinduced degradation modifies the fluorescence properties, and in particular the
excitation-emission wavelengths (peaks A and C) of fulvic acid in the aquatic envi-
ronments (Mostofa et al.
2007a
,
2007b
; Moran et al.
2000
). From the photoinduced
degradation of bog DOM and of International Humic Substances Society Nordic
fulvic acid, it has been highlighted the losses of carbohydrates, secondary alcohols,
protonated and substituted aromatic compounds, carboxyl, amide, ester, ketone and
quinones, with no changes in aliphatic carbon (Osburn et al.
2001
). Photoinduced
changes in FDOM correspond to a decrease in the dissolved organic carbon (DOC)
concentration (Brooks et al.
2007
; Garcia et al.
2005
; Moran et al.
2000
; Osburn
et al.
2009
; Mostofa et al.
2007b
; Vähätalo and Wetzel
2004
) and to the generation
of photoproducts. These processes can be summarized as: (i) Conversion of high-
molecular weight into low-molecular weight DOM, which is generally observed in
