Environmental Engineering Reference
In-Depth Information
The variation of
S
suggests two important characteristics of natural waters.
The first is that CDOM chromophores are decomposed photolytically and their
decomposition rates are variable depending on the chemical nature (allochthonous
or autochthonous) of CDOM and on its molecular structure. The second issue is
that the decomposition rates of the different CDOM chromophores affect in dif-
ferent ways the values of
S
in different spectral ranges. The overall effect depends
also in this case on the CDOM origin and composition. It has been shown for
instance that decomposition of CDOM fractions with higher-than-average concen-
trations of carboxyl-, hydroxyl- and ester-substituted aromatic rings, upon either
photoinduced oxidation or chlorine addition, decreases the intensity and width of
the electron-transfer and benzenoid bands (Korshin et al.
1997
). Such a finding
suggests a correlation between the spectral slope (
S
) alterations by photoinduced
degradation and the modification of mean molecular size and molecular structure
of CDOM. The latter have generally been found to decrease from rivers to lakes
and oceans (Moran et al.
2000
; Moran and Zepp
1997
; Corin et al.
1996
; Allard
et al.
1994
; Amador et al.
1989
; Wu et al.
2005
; Yoshioka et al.
2007
; Clark et al.
2008
). Moreover, the spectral slope
S
(Jerlov
1968
) as well as the carbon-specific
absorptivity could be useful indicators to examine photodegradation processes in
natural waters (Vodacek et al.
1997
; Twardowski and Donaghay
2001
; Morris and
Hargreaves
1997
; Whitehead et al.
2000
).
4.3.4 Effect of Monochromatic and Polychromatic Irradiation
on CDOM Absorption
Monochromatic irradiation of Suwannee River Fulvic Acid (SRFA) and natural
waters can result in the loss of absorption across the entire spectrum and the larg-
est absorption losses are often observed at the irradiation wavelength,
λ
irr
(Fig.
9
)
(del Vecchio and Blough
2002
). Outside the band of direct bleaching, the loss of
absorption appears to be fairly uniform across the examined spectral range. Major
secondary bands of absorption loss outside
λ
irr
are not evident in the difference
spectra (Fig.
9
b, d, f). The high losses of absorption at
λ
irr
can mostly be attrib-
uted to the direct photoinduced destruction of the chromophore(s) absorbing at
this wavelength. The kinetics of absorption loss at both
λ
irr
and at the wavelengths
outside of this band exhibit excellent fits to either a single exponential or a sum of
two exponentials functions. The overall rate of absorption loss is always higher at
the
λ
irr
. The smaller, indirect absorption losses observed outside
λ
irr
could be pro-
duced by two effects (del Vecchio and Blough
2002
): (i) the direct photoinduced
destruction of chromophore(s) having absorption bands both at
λ
irr
and outside
λ
irr
; (ii) the production of reactive oxygen species from primary photochemistry
at
λ
irr
that react secondarily to destroy chromophores absorbing at wavelengths
outside
λ
irr
. The uniform loss of absorption outside the primary bleaching band
(e.g., away from
λ
irr
) suggests that indirect photobleaching could result from
the indiscriminate destruction of chromophores by reactive oxygen species pro-
duced by the primary photochemistry. It is shown that the reactive oxygen species
