Geoscience Reference
In-Depth Information
400
1000
Optical
Brighteners
T
B
T
A
T
200
0
280
500
Emission Wavelength (nm)
Figure 3.14. Fluorescence excitation-emission matrices (EEMs) for raw sewage where B = tyrosine-
like, T and A
T
= tryptophan-like, A and C
1
= fulvic-like, C = humic-like fluorescence. (Adapted from
Henderson,
2009
, with permission from Elsevier.) (See Plate 7.)
absorption and quantum yield maxima, the optimum value of △
λ
is set by the difference in
wavelength of the emission and excitation maxima, which is known as Stoke's shift. This
technique has the potential to resolve spectral components arising from different species,
especially when typical offsets for △
λ
are between 20 and 60 nm (Ahmad and Reynolds,
1995
; Wu et al.,
2006
). SFS has been used for tracing the detection of sewage impacted riv-
ers (Galapate,
1998
; Hur and Kong,
2008
; Hur et al.,
2008
) and fingerprinting wastewaters
(Wu et al.,
2006
). Typical synchronous fluorescence spectra are shown in
Figure 3.14
.
In addition to the T, A
C
, B, and C peaks, EEMs acquired from sewage-derived DOM
commonly exhibit additional peaks at
λ
ex
= 375 nm, 350 nm, and 330 nm, and correspond-
ing
λ
em
= 410-450 nm (see
Figure 3.15
). These are now known to be characteristic of opti-
cal brighteners such as those used in detergents (Westerhoff et al.,
2001
; Hayashi et al.,
2002
; Takahashi and Kawamura,
2007
; Hartel et al.,
2008
). As much as 80% of these
optical brighteners are removed during wastewater treatment but they can still be detected
at low concentrations of approximately 0.5 mg L
-1
in river water (Poiger et al.,
1998
).
Studies of EEMs of untreated wastewaters show that they commonly comprise a broad
humic-type peak C with intense T peaks and B peaks which occur at the same position in
optical space as standard solutions of tryptophan and tyrosine, respectively (Baker et al.,
2004
). Peak T generally contributes the highest intensity peaks in wastewaters (Reynolds
and Ahmad,
1997
), and previous researchers (Galapate et al.,
1998
; Baker et al.,
2003
,
2004
; Reynolds,
2003
) have demonstrated that it can be considered as a tracer and relic of
anthropogenic material in natural waters owing to its intensity. Rivers that are impacted
by sewage wastes do not typically show optical brightening agents in their EEMs because
of the high level of background fluorescence (Baker,
2001
). This background is often high
because sewage-derived DOM is rich in proteinaceous material which is associated with
intense tryptophan-like (T and A
T
) fluorescence. Sewage-derived DOM is dominated by
organic matter originating from microbial activity (Hudson et al.,
2008
). In contrast, the
