Environmental Engineering Reference
In-Depth Information
they remain stable over a period of 100 s (Wu et al.
2004a
). The mixing of stand-
ard fluorescent organic substances with Milli-Q water and seawater shows that the
excitation-emission wavelength maxima of SRFA, DAS1, tyrosine, benzoic acid,
p
-hydroxy benzoic acid,
p
-hydroxy benzaldehyde and
p
-hydroxy acetophenone
in seawater are significantly shifted from shorter to longer wavelength regions
compared to Milli-Q water (Nakajima
2006
). For example, the fluorescence
peak C of SRFA dissolved in seawater is detected Ex/Em
=
345/452 nm whilst
the same peak in Milli-Q water is detected at Ex/Em
=
325/442 nm. The peak A
remains almost the same in both aqueous media (Nakajima
2006
). The fluores-
cence 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 water during the photoinduced and microbial assimilations of algae (Mostofa
et al.
2009b
). In another study, the same 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)
(Zhang et al.
2009
). The autochthonous fulvic acid or marine humic-like material
of algal origin (peak M) at the peak C-region is found to be shifted from shorter
excitation wavelengths (290/400-410 nm in pure Milli-Q water) to a longer wave-
length region (300-310/400-410 nm in seawater) (Parlanti et al.
2000
).
The shift in excitation and emission wavelength maxima with salinity is pre-
sumably caused by the anions and cations present in sea water. Such a shift in
excitation-emission from shorter to longer wavelengths is termed the red shift of
fulvic acid-like fluorescence. The mechanism behind this red shift in sea water
is attributed to the complex formation of the functional groups (or flurophores
at peak C-region) in fulvic acid with trace elements or ions markedly present in
sea water. The complexation of trace elements with the SRFA functional group
(or fluorophore) can significantly enhance the electron transfer of that functional
group bound at peak C from the ground state to the excited state upon absorp-
tion of longer wavelength radiation. The effect is a shift of the excitation-emission
maxima of the peak C to longer wavelengths. This is evidenced by the photoin-
duced formation of aqueous electrons (e
aq
−
) from organic substances, which is
higher in the presence of NaCl than with organic substances alone in aqueous
solution (Gopinath et al.
1972
; Zepp et al.
1987
; Assel et al.
1998
; Richard and
Canonica
2005
; Fujiwara et al.
1993
). Rapid excitation of electrons in ionic (saline)
FDOM solution is susceptible to shift both excitation and emission maxima of
fluorophores (or functional groups) associated to the peak C of allochthonous ful-
vic and humic acids or autochthonous fulvic acids or other autochthonous DOM.
This effect is presumably responsible for the high production of hydrogen perox-
ide in natural waters (Mostofa and Sakugawa
2009
; Fujiwara et al.
1993
). Indeed,
photogeneration of H
2
O
2
from ultrafiltered river DOM is substantially increased
with salinity, from 15 to 368 nM h
−
1
at circumneutral pH (Osburn et al.
2009
).
The mechanism behind this phenomenon apparently can be factors: first, irradiated
CDOM can induce photoinduced production of hydrogen peroxide (H
2
O
2
) that
is a hydroxyl radical (HO
•
) source via photolysis or the Fenton reaction, and the
photoinduced generation of H
2
O
2
is enhanced by salinity. Trace metal ions (M) in
