Environmental Engineering Reference
In-Depth Information
dependence observed for reactive intermediate production is consistent with the
decrease in the efficiency of the primary photobleaching: because the efficiency of
reactive intermediate production decreases with increasing wavelength, any indi-
rect photobleaching caused by reactions with these reactive intermediates would
be expected to follow the same trend (del Vecchio and Blough
2002
).
Polychromatic irradiation of SRFA and natural waters can result in the loss of
absorption across the entire spectrum and the bleaching is often more pronounced in
the spectral region that is transmitted by the cut-off filter (Fig.
10
) (del Vecchio and
Blough
2002
), in analogy with the results obtained for monochromatic irradiation.
Coherently, absorption losses increase with decreasing
λ
of the cut-off filters (Fig.
9
)
(del Vecchio and Blough
2002
). The results show that the relative loss of absorption
is higher at longer wavelengths, although the efficiency of direct photobleaching
decreases significantly with increasing wavelength (del Vecchio and Blough
2002
).
This result can be attributed to two factors (del Vecchio and Blough
2002
): (i) the
higher number of longer-wavelength photons produced by the light source; (ii) the
higher rates of indirect absorption loss produced at longer wavelengths by the absorp-
tion of short wavelength photons. The losses of absorption at longer wavelengths lead
to an increase of
S
when the spectral data are fit to a single exponential function using
either linear or non-linear least squares methods (del Vecchio and Blough
2002
).
Using a 320-nm filter, the spectral dependence of a solar simulator is similar to
that of ground-level solar spectrum. The changes in
S
for
λ
irr
> 320 nm (Fig.
10
)
should thus reflect the changes in an optically thin section of surface water. The
results indicate that in waters where the penetration depths of the photolytically
active UV-B and UV-A wavelengths are comparable to the mixed layer depth, the
loss of CDOM absorption and the increase in
S
in the mixed layer will be rel-
atively rapid. If the penetration depths are much shallower than the mixed layer
depth, absorption losses and changes in
S
in the mixed layer will be very small
even over extended periods of time (del Vecchio and Blough
2002
).
4.3.5 Factors Controlling the Photoinduced Degradation
of CDOM Absorption
Photodegradation of CDOM depends on the sources of water, CDOM concentra-
tion, optical-chemical CDOM nature, time, space, sunlight irradiance, water chem-
ical conditions, DOM contents, mixing regime, rain or precipitation and so on (Ma
and Green
2004
; Reche et al.
1999
; Whitehead and Vernet
2000
; Gonsior et al.
2008
). It has been shown that photobleaching varies significantly depending on the
lamp distance from the samples. The decrease of CDOM absorption is 8-19 % at
5 cm lamp distance, but only 2-5 % when the lamp is positioned at 45 cm, dur-
ing a 2-12 h irradiation period using a UV-B lamp (Zhang et al.
2009
). Moreover,
the key factors that affect CDOM photobleaching are: (1) Solar radiation;
(2) Water temperature; (3) Effects of total dissolved Fe and photo-Fenton reac-
tion; (4) Occurrence and quantity of NO
2
−
−
ions; (5) Molecular nature
of DOM; (6) pH and alkalinity of the water; (7) Dissolved oxygen (O
2
; (8) Depth
and NO
3
