Environmental Engineering Reference
In-Depth Information
the photoinduced degradation of DOM in waters, although there is evidence that the
formation rate of HO
may be insufficient to account for the DOM mineralization
(Vione et al. 2009 ).
Variations in the spectral irradiance penetration among various waters and in the
effect of radiation wavelength on DOM transformation might be caused by three fac-
tors. First, concentration levels and molecular nature of DOM can modify the absorp-
tion spectrum for a variety of waters. Second, contents of total iron, a major factor of
HO
production through photo-Fenton reaction and probably also of DOM mineraliza-
tion through HO
-independent processes (Vione et al. 2009 ), are greatly varied for a
variety of waters. Third, depletion of the stratospheric ozone layer may greatly increase
the UV-B radiation, thereby enhancing the photoinduced mineralization of DOM by
UV-B (Qian et al. 2001 ; Randall et al. 2005 ). An effort is still needed to accout for
the different results obtained in different studies. On the one hand, a limited increase
(~4 %) in DOM photoinduced mineralization has been observed in Brazil compared
to Sweden, although the dose of UV-B was three-fold higher in Brazil than in Sweden
(Graneli et al. 1998 ). On the other hand, half of the total photoinduced degradation of
DOM was attributable to wavelengths shorter than 360 nm (Vahatalo et al. 2000 ).
3.2 Water Temperature
Air temperature is greatly varied from 0 to approximately 50 °C in different regions,
which might control the water temperature (WT) and its variation in natural waters.
A low WT can reduce the movement of the reactants in the aqueous solution, thereby
causing a decrease in the reaction kinetics of DOM in waters. WT that is driven by
solar intensity is directly related to the photoinduced generation of H 2 O 2 . The H 2 O 2 in
river shows a significantly higher production in summer and lower in winter (Mostofa
and Sakugawa 2003 , 2009 ). The lower production of H 2 O 2 due to low WT and solar
irradiance may subsequently decrease the production rate of HO
(Qian et al. 2001 )
and, as a consequence, the photoinduced degradation rate of DOM in aqueous solution.
3.3 Effects of Total Dissolved Fe and Photo-Fenton Reaction
The concentration of total dissolved Fe is one of the most important factors for the
photoinduced degradation of DOM in waters (Vione et al. 2009 ; Gao and Zepp
1998 ; Wu et al. 2005 ; Gennings et al. 2001 ), through the photo-Fenton reaction
(Zepp et al. 1992 ; Southworth and Voelker 2003 ; McKnight et al. 1988 ; Arakaki and
Faust 1998 ); or via HO
-independent processes (Vione et al. 2009 ). The generation
rate of HO is much higher for elevated Fe levels in acidic waters (McKnight et al.
1988 ; Allen et al. 1996 ). The oxidation of Fe 2 + by photo-generated H 2 O 2 causes the
production of HO
and Fe 3 + , but Fe 2 + is regenerated from Fe 3 + by several pathways
via the process of h υ /H 2 O 2 /O 2
. The regeneration of Fe 2 + greatly enhances the
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