Environmental Engineering Reference
In-Depth Information
p
-hydroxybenzaldehyde and
p
-hydroxyacetophenone are significantly shifted from
shorter to longer wavelength regions in seawater (Table
1
) (Nakajima
2006
). For
example, the fluorescence peak C of SRFA dissolved in seawater is detected at
Ex/Em
=
345/452 nm, whilst the same peak in Milli-Q water is detected at Ex/
Em
=
325/442 nm. Peak A remains almost the same in both aqueous media
(Table
1
) (Nakajima
2006
). The fluorescence peak C of autochthonous fulvic acid
(C-like) of algal origin is detected at Ex/Em
=
340/442-448 nm in Milli-Q water,
and at Ex/Em
=
340/454-455 nm in river waters during the photo- and micro-
bial assimilations of algae (Table
2
) (Mostofa KMG et al., unpublished data). In
another study, the same fluorescence peak C of autochthonous fulvic acid (C-like)
of algal origin has been detected at Ex/Em
=
365/453 nm and 270/453 nm in an
isotonic solution during the microbial assimilation of lake phytoplankton (0.5 ‰
salinity) (Table
2
) (Zhang et al.
2009a
). The autochthonous fulvic acid or marine
humic-like of algal origin (peak M) at peak C-region has been found to shift from
290/400-410 nm in Milli-Q water to 300-310/400-410 nm in seawater (Table
2
)
(Parlanti et al.
2000
)). Such a shift in excitation and emission wavelength maxima
is presumably caused by the anions and cations present in sea water and is termed
the red shift of fulvic acid-like fluorescence. The mechanism behind the red shift
in sea water is attributed to complex formation between the functional groups (or
flurophore at peak C- and A-regions) in fulvic acid and trace elements or ions.
The complexation of trace elements with the functional groups (or fluorophores)
bound at peak C or peak A in SRFA can significantly enhance the electron trans-
fer from the ground state to the excited state by longer wavelength energy. This
effect shifts the excitation-emission maxima of the peak C or peak A to longer
wavelength regions. Such a shift in both excitation-emission wavelengths takes
place during the initial complexation processes and increases with time (Wu et al.
2004a
,
2004c
). This is evidenced by the photoinduced formation of aqueous elec-
trons (e
aq
−
) from organic substances and by their high production in NaCl-mixed
solutions compared to Milli-Q water (Gopinathan et al.
1972
; Zepp et al.
1987
;
Fujiwara et al.
1993
; Assel et al.
1998
; Richard and Canonica
2005
).
On the other hand, the mixing of some standard FDOM (e.g. DSBP, phenol,
and tryptophan) with seawater shows that the fluorescence excitation-emission
wavelength maxima (peak C-region and peak T-region) are shifted to shorter
wavelengths compared to Milli-Q waters (Nakajima
2006
). Such changes in fluo-
rescence excitation-emission maxima are termed as blue-shift of the fluorophores
in FDOM. In some cases the blue-shift of the fluorescence peaks could be caused
by the loss of high molecular weight fluorescent components by physicochemical
modifications such as flocculation, aggregation or precipitation during the initial
mixing (de Souza Sierra et al.
1997
; Sholkovitz
1976
; Carlson and Mayer
1983
;
McCarthy et al.
1996
; Van Heemst et al.
2000
; Benner and Opsahl
2001
). In the
case of smaller molecules, the blue-shift phenomenon is presumably caused by
complex formation between anions or cations and the fluorophores (or functional
groups) of few fluorescent organic components. This may increase the excitation
energy of the fluorophores bound to peak C or peak A-region and change the exci-
tation-emission wavelengths from longer to shorter wavelength regions.
