Environmental Engineering Reference
In-Depth Information
2004
; Vahatalo et al.
2000
; Mostofa et al.
2000
,
2007
; Gao and Zepp
1998
;
Graneli et al.
1996
,
1998
; Wu et al.
2005
; Mopper et al.
1991
; Miller and Zepp
1995
; Miller and Moran
1997
; Rosenstock et al.
2005
; Nieto-Cid et al.
2006
). The
Photoinduced degradation rate is mostly dependent on several major factors that
are greatly related to variable local conditions and to the concentration levels and
molecular nature of DOM. The key factors are: (1) Sunlight or solar radiation,
(2) Water temperature, (3) Effects of total dissolved Fe and photo-Fenton reaction,
(4) Occurrence and quantity of NO
2
−
and NO
3
−
ions, (5) Molecular nature of
DOM, (6) pH and alkalinity of the waters, (7) Dissolved oxygen (O
2
), (8) Depth
of the water, (9) Physical mixing in the surface mixing zone, (10) Increasing
UV-radiation during ozone hole events, (11) Global warming and (12) Salinity.
3.1 Sunlight or Solar Radiation
Solar radiation is the key factor for photoinduced degradation of DOM or organic
contaminants in water (Morris and Hargreaves
1997
; Reche et al.
1999
; Mostofa and
Sakugawa
2009
; Vahatalo et al.
2000
; Mostofa et al.
2007
; Moran et al.
2000
; Wu
et al.
2005
; Molot and Dillon
1997
; Dobrovi ´ et al.
2007
). Photoinduced degrada-
tion of DOM depends on the spectral wavelengths of solar radiation such as UV-A
(315-400 nm), UV-B (280-315 nm), and visible light (400-700 nm) as well as
their significant variations after penetration in the water column (Scully et al.
1996
;
Morris and Hargreaves
1997
; Reche et al.
1999
; Vahatalo et al.
2000
; Graneli et al.
1996
,
1998
; Lindell et al.
1996
; Kieber et al.
1990
; Molot and Dillon
1997
; de Haan
1993
; Herndl et al.
1993
; Valentine and Zepp
1993
). DOM is typically suscepti-
ble to absorb UV radiation in sea and lake waters (Kirk
1994
; Morris et al.
1995
).
The penetration of UV radiation in natural waters is greatly variable, with typical
penetration depths in clear ocean water of ~20 m for UV-B and ~50 m for UV-A
radiation, 5-10 m for UV-B radiation in oligotrophic oceans and 0.5-3 m in fresh-
waters (Kirk
1994
; Smith and Baker
1981
; Waiser and Robarts
2000
). It can be
expected that the photoinduced degradation of DOM is significantly dependent
on the attenuation of downward irradiance in natural waters. It has been shown
that the contribution of solar intensity to total photoinduced degradation of DOM
in lakes is 39-69 % by UV-A, 9-17 % by UV-B, and 23-44 % by visible light
radiation (Vahatalo et al.
2000
). Photoinduced mineralization of natural DOC is
increased <9 % when the UV-B radiation is doubled in humic lakes (Vahatalo
et al.
2000
). Control irradiation by wavelengths 254 nm (hereafter UV) and 185 nm
(hereafter VUV) on DOM demonstrates that the DOM degradation rate at 185 nm
increases approximately ten-fold compared to those at 254 nm. An increase in fluxes
of the UV radiation can substantially increase the quantity of the reactive free radi-
cals such as HO
•
and H
2
O
2
in waters (Qian et al.
2001
; Rex et al.
1997
; Yocis et al.
2000
). Rex et al. 1997; Yocis et al. 2000; During an ozone hole event, the produc-
tion rates of HO
•
are greatly enhanced in Antarctic waters (Qian et al.
2001
; Rex
et al.
1997
). The HO
•
is the most powerful oxidizing agent that can be involved into
