257
Photoinduced Generation of Hydroxyl Radical in Natural Waters
(Arakaki et al. 1998, 1999a, b; Arakaki and Faust 1998; Nakatani et al. 2001;
Kobayashi et al. 2002). It is considered that the HO
• formed in the liquid phase
on the needle surfaces of Japanese red pine, which are frequently present in
the dew on sunny mornings in the warm-temperate region, are the cause of
ecophysiological disorders in plants (Kobayashi et al. 2002).These effects subsequently affect to decrease in the maximum CO 2 assimilation rate, stomatal
conductance, minimal fluorescence and needle lifespan (Kume et al. 2000).
7 Summary and Scope of the Future Challenges
The HO
• radical is the most reactive among the many oxygen transient species produced photolytically in natural waters. However, a few studies on HO
•
production have been conducted on fresh- and seawater as a whole. The HO
•
production in acidic lake waters is not well investigated, although it is known
that acidification enhances the formation of HO
• (Vione et al. 2009). The HO
•
production varies with the contents of DOM, which may be a key factor to
understand the photoinduced processes in a variety of natural waters. Moreover,
there is no study conducted about production of alkoxide radicals (RO
•
) and
their role in the photodegradation kinetics of DOM along with HO
•
. An example
of key possible research on free radicals needed for the future challenges are:
(i) Effect of pH and temperature on the production of HO
• for a variation in quality and quantity of DOM in natural waters. (ii) Investigation on the sources of
free radicals (HO
• and RO
• ), particularly from fluorescent dissolved organic
matter (FDOM) for a variety of waters. (iii) Photoinduced generation of alkoxide
radicals (RO
• ) and their relative reaction kinetics with respect to HO
• and other
solution components. (iv) Impacts of free radicals on specific biota in the aquatic
environment.
Problems
(1) List the various sources of HO
• and their role in natural waters.
(2) List three important free radical species except HO
• and their importance in
natural waters.
(3) How is the steady state concentration of HO
• defined, and why does the
steady state concentration vary for a variety of natural waters?
(4) Explain why the potential of HO
• formation is low in surface waters compared to those of deep lakes and the sea.
(5) Mention the important processes of HO
• formation in natural waters and
explain the mechanism of in situ generation of HO
• from DOM via H 2 O 2 .
(6) Explain why the photo-Fenton reaction is more suitable than the Fenton
reaction in the degradation of organic pollutants in aqueous solution.
Photoinduced Generation of Hydroxyl Radical in Natural Waters
(Arakaki et al. 1998, 1999a, b; Arakaki and Faust 1998; Nakatani et al. 2001;
Kobayashi et al. 2002). It is considered that the HO
• formed in the liquid phase
on the needle surfaces of Japanese red pine, which are frequently present in
the dew on sunny mornings in the warm-temperate region, are the cause of
ecophysiological disorders in plants (Kobayashi et al. 2002).These effects subsequently affect to decrease in the maximum CO 2 assimilation rate, stomatal
conductance, minimal fluorescence and needle lifespan (Kume et al. 2000).
7 Summary and Scope of the Future Challenges
The HO
• radical is the most reactive among the many oxygen transient species produced photolytically in natural waters. However, a few studies on HO
•
production have been conducted on fresh- and seawater as a whole. The HO
•
production in acidic lake waters is not well investigated, although it is known
that acidification enhances the formation of HO
• (Vione et al. 2009). The HO
•
production varies with the contents of DOM, which may be a key factor to
understand the photoinduced processes in a variety of natural waters. Moreover,
there is no study conducted about production of alkoxide radicals (RO
•
) and
their role in the photodegradation kinetics of DOM along with HO
•
. An example
of key possible research on free radicals needed for the future challenges are:
(i) Effect of pH and temperature on the production of HO
• for a variation in quality and quantity of DOM in natural waters. (ii) Investigation on the sources of
free radicals (HO
• and RO
• ), particularly from fluorescent dissolved organic
matter (FDOM) for a variety of waters. (iii) Photoinduced generation of alkoxide
radicals (RO
• ) and their relative reaction kinetics with respect to HO
• and other
solution components. (iv) Impacts of free radicals on specific biota in the aquatic
environment.
Problems
(1) List the various sources of HO
• and their role in natural waters.
(2) List three important free radical species except HO
• and their importance in
natural waters.
(3) How is the steady state concentration of HO
• defined, and why does the
steady state concentration vary for a variety of natural waters?
(4) Explain why the potential of HO
• formation is low in surface waters compared to those of deep lakes and the sea.
(5) Mention the important processes of HO
• formation in natural waters and
explain the mechanism of in situ generation of HO
• from DOM via H 2 O 2 .
(6) Explain why the photo-Fenton reaction is more suitable than the Fenton
reaction in the degradation of organic pollutants in aqueous solution.