Environmental Engineering Reference
In-Depth Information
fluorescence of peak C into the photo-bleached fluorescence peak (peak M
p
),
which can show the highest fluorescence intensity in some natural waters (Mostofa
et al.
2007a
,
2005b
; Moran et al.
2000
; Komada et al.
2002
; Burdige et al.
2004
).
Due to photodegradation, the new photo-bleached peak M
p
is shifted at shorter
excitation-emission wavelengths (Fig.
3
a, d, e). Such a change in the fluorescence
peak caused by photodegradation is termed as `blue-shift`. Blue-shift is com-
monly observed in surface lake or seawaters where photodegradation is important
due to exposure to natural sunlight (Fig.
3
d, e). Photoinduced effects can decrease
the fluorescence intensity (FI) of fulvic acid-like (peak C), FWAs-like (peak
W), and tryptophan-like (peak T) compounds, which are commonly observed in
natural waters in field and experimental observations (Fig.
5
; Table
4
) (Hayase
and Shinozuka
1995
; Mostofa et al.
2005a
;
2005b
,
2010
,
2007a
,
2011
; Stedmon
et al.
2007a
; Brooks et al.
2007
; Garcia et al.
2005
; Winter et al.
2007
; Mostofa
KMG et al., unpublished data; Skoog et al.
1996
; Moran et al.
2000
; Osburn et al.
2009
; Lepane et al.
2003
; Abboudi et al.
2008
; Poiger et al.
1999
; Fu et al.
2010
;
Borisover et al.
2009
; Yamashita and Tanoue
2008
; Vodacek et al.
1997
; Yamashita
et al.
2007
; Shank et al.
2010
).
Fulvic Acid-like Components in Natural Waters
Fluorescence intensity losses of fulvic acid-like substances by photoin-
duced degradation are 1-84 % in rivers, 16-83 % in lakes, 19-67 % in estu-
aries, and 9-84 % in sea waters studied experimentally in the course of short
(hours) to long-term (days or months) irradiation (Table
4
). In lake water
after 12 days irradiation, the losses of fulvic acid-like fluorescence inten-
sity have been 36 % at the surface (2.5 m) and 48 % in deeper waters (70 m)
for DOM fractions of <0.1
μ
m. In the case of DOM molecular-weight frac-
tions below 5 kDa, the corresponding losses have been 16 % in surface waters
(2.5 m) and 50 % in deeper waters (70 m). The low fluorescence intensity
decrease in the case of surface-water DOM with molecular weight below 5 kDa
may be explained by the fact that the corresponding samples have been col-
lected during an ongoing summer stratification period (September). Therefore,
the photosensitive DOM fractions had probably already undergone photoin-
duced decomposition before sample collection. The higher fluorescence inten-
sity decrease observed for deep-water DOM may be accounted for by the fact
that deep waters undergo photoinduced degradation processes to a lesser extent
because of the reduced sunlight irradiance compared to surface waters (Laurion
et al.
2000
). As a consequence, deep-water samples may contain significant
amounts of photosensitive DOM components, which have not been degraded in the
natural environment and can undergo photoinduced decomposition when irradi-
ated in the laboratory (Table
4
). For similar reasons, photoinduced DOM miner-
alization is very difficult to be observed in surface lake water samples and is much
easier to be detected upon irradiation of groundwater (Vione et al.
2009
). In estua-
rine water it has been observed a fluorescence intensity decrease in fulvic acid-like