Environmental Engineering Reference
In-Depth Information
are responsible for the formation of complex with trace elements. Therefore, the
fluorescence intensity is either enhanced or quenched due to the complexation of
FDOM fluorophores with trace elements (Wu et al.
2004a
,
2004b
; Fu et al.
2007
;
Lu and Jaffé
2001
; Cabaniss and Shuman
1988
; Cabaniss
1992
).
The complexation of trace elements with fluorescent substances does not
only affect the fluorescence intensity, but also the fluorescence peak position of
the respective fluorophore (Wu et al.
2004c
). Both excitation and emission wave-
lengths of the respective fluorophore peak in fulvic acid gradually increase with
increasing reaction time (Wu et al.
2004c
). It has been hypothesized that dona-
tion of electrons occurs from functional groups or fluorophores in DOM to empty
d
-orbitals in transition metals or in partially empty
p
-orbitals in metal/metal-
loid (F:
+
M
n
+
→
F:M
n
+
), thereby causing a strong
π
-electron bonding system
between DOM and metals (Mostofa et al.
2009a
,
2011
). Donation of electrons
from functional groups or fluorophores in DOM causes the
d
-orbitals to be either
stabilized or destabilized in the complex compound, thereby causing the fluo-
rescence to either decrease or increase, respectively, in the M-DOM complexes
(Mostofa et al.
2009a
,
2011
).
3.5 Salinity
Salinity can significantly affect the fluorescence properties of FDOM in natural
waters (Dorsch and Bidleman
1982
; Hayase et al.
1987
; Coble
1996
; Determann
et al.
1996
; Parlanti et al.
2000
; Nakajima
2006
; Laane
1980
; Willey and Atkinson
1982
; Berger et al.
1984
; Laane and Kramer
1990
; de Souza Sierra et al.
1997
;
Boyd et al.
2010
). The fluorescence intensity decreases linearly with salinity
(Dorsch and Bidleman
1982
; Hayase et al.
1987
; Laane
1980
; Willey and Atkinson
1982
; Berger et al.
1984
; Laane and Kramer
1990
). It is shown that the fluores-
cence intensities of fulvic acid are quenched significantly with modest saline
mixing (Boyd et al.
2010
). Two types of result are detected during the mixing
of freshwater and seawater (Determann et al.
1996
; de Souza Sierra et al.
1997
).
First, a slow blue-shift of the fluorescence at peak C-region is detected for humic
(fulvic)-like fluorophores during the initial mixing of freshwater to seawater
between salinity 0 and 32 (de Souza Sierra et al.
1997
). Secondly, for higher salin-
ity (>32) a rapid wavelength shift is detected until the salinity reaches the maxi-
mum seawater value (de Souza Sierra et al.
1997
).
On the other hand, salinity is presumably considered to shift the excita-
tion-emission wavelengths of freshwater fulvic acid (peak C) from the shorter
wavelengths found in freshwater rivers (325-340/450-475 nm) and lakes (310-
350/410-464 nm) to longer wavelength regions (350-365/446-465 nm) in marine
environments (Mostofa et al.
2009a
,
2005a
; Coble
1996
; Parlanti et al.
2000
;
Yamashita and Tanoue
2003a
; Nakajima
2006
). The mixing of standard organic
substances with Milli-Q and seawater shows that the excitation-emission wave-
length maxima of SRFA, DAS1, tyrosine, benzoic acid,
p
-hydroxybenzoic acid,
