211
Photoinduced Generation of Hydroxyl Radical in Natural Waters
TiO 2 , which plays a key role in the heterogeneous photocatalytic degradation
of organic contaminants (Sun and Bolton 1996; Ullah et al. 1998; Konstantinou
and Albanis 2004). However, an important difference between TiO 2 photocatalysis and the other processes of HO
• generation described before is that
the irradiation of TiO 2 mainly causes the production of surface-bound HO
•
groups, which are somewhat less reactive than homogeneous HO
• (Serpone
and Pelizzetti 1989). The hydroxyl radical has been detected in rainwater,
dew, cloud and fog (Arakaki et al. 1998, 1999a, b; Arakaki and Faust 1998;
Nakatani et al. 2001; Kobayashi et al. 2002), snow (Chu and Anastasio 2005;
Anastasio et al. 2007; Matykiewiczová et al. 2007), aerosols (Anastasio and
Jordan 2004), in aqueous extracts of cigarette tar (Zang et al. 1995), and in living organisms (Buettner et al. 1978; Buettner 1987; Miller et al. 1990; Buettner
and Jurkiewicz 1996; Cadet et al. 1999; Bourdat et al. 2000; Paradies et al.
2000; Blokhina et al. 2003; Li et al. 2008). The HO
• is rapidly consumed in
natural waters by the subsequent reactions with dissolved organic compounds
(Schuchmann and von Sonntag 1979; Neta et al. 1988; Westerhoff et al. 1999;
Goldstone et al. 2002; Miller and Chin 2002; Miller et al. 2002; Ervens et al.
2003) and several inorganic species (Zafiriou et al. 1984, 1987; Brezonik and
Fulkerson-Brekken 1998; Neta et al. 1988; Song et al. 1996).
The generation of HO
• and its interaction with the dynamics of DOM and
nutrient as well as with aquatic organisms are very important in natural waters.
There are a number of factors that can control the production and consumption of
HO
• in that ecosystem. However, there is no general overview published on HO
•
in natural waters. A short review by von Sonntag (2007) covers the formation of
free radicals and their reactions in aqueous solution.
This review will provide a general overview on sources, production mechanisms, steady state concentration and biogeochemical functions of HO
• in water
environment. This paper also discusses the analytical methods that can be adopted
to measure the photoinduced generation of HO
• , the factors controlling its production and decay, as well as the significance and impact of HO
• in the aquatic ecosystems. It is shown how the production of HO
• differs among DOM components,
as well as between freshwaters and marine environments.
2 Hydroxyl Radical (HO
•
) and Other Free Radical Species
The hydroxyl radical (HO
•
) is the most powerful oxidizing agent among the
photolytically generated ones. It is a short-lived, highly reactive and nonselective transient, able to oxidize dissolved organic substances and other
chemical species in natural waters. The oxidation potentials for a series of common oxidants in surface waters is as follows: Fluorine (E = 3.03 V) > HO
•
(2.80 V) > Atomic oxygen (2.42 V) > Ozone (2.07 V) > Peracetic acid (ROOH)
(1.80 V) > H 2 O 2 (1.78 V) > Perhydroxyl radical (1.70 V) > Potassium permanganate (1.68 V) > Chlorine dioxide (1.57 V) > Hypochlorous acid (1.49 V) > Chlorine
Photoinduced Generation of Hydroxyl Radical in Natural Waters
TiO 2 , which plays a key role in the heterogeneous photocatalytic degradation
of organic contaminants (Sun and Bolton 1996; Ullah et al. 1998; Konstantinou
and Albanis 2004). However, an important difference between TiO 2 photocatalysis and the other processes of HO
• generation described before is that
the irradiation of TiO 2 mainly causes the production of surface-bound HO
•
groups, which are somewhat less reactive than homogeneous HO
• (Serpone
and Pelizzetti 1989). The hydroxyl radical has been detected in rainwater,
dew, cloud and fog (Arakaki et al. 1998, 1999a, b; Arakaki and Faust 1998;
Nakatani et al. 2001; Kobayashi et al. 2002), snow (Chu and Anastasio 2005;
Anastasio et al. 2007; Matykiewiczová et al. 2007), aerosols (Anastasio and
Jordan 2004), in aqueous extracts of cigarette tar (Zang et al. 1995), and in living organisms (Buettner et al. 1978; Buettner 1987; Miller et al. 1990; Buettner
and Jurkiewicz 1996; Cadet et al. 1999; Bourdat et al. 2000; Paradies et al.
2000; Blokhina et al. 2003; Li et al. 2008). The HO
• is rapidly consumed in
natural waters by the subsequent reactions with dissolved organic compounds
(Schuchmann and von Sonntag 1979; Neta et al. 1988; Westerhoff et al. 1999;
Goldstone et al. 2002; Miller and Chin 2002; Miller et al. 2002; Ervens et al.
2003) and several inorganic species (Zafiriou et al. 1984, 1987; Brezonik and
Fulkerson-Brekken 1998; Neta et al. 1988; Song et al. 1996).
The generation of HO
• and its interaction with the dynamics of DOM and
nutrient as well as with aquatic organisms are very important in natural waters.
There are a number of factors that can control the production and consumption of
HO
• in that ecosystem. However, there is no general overview published on HO
•
in natural waters. A short review by von Sonntag (2007) covers the formation of
free radicals and their reactions in aqueous solution.
This review will provide a general overview on sources, production mechanisms, steady state concentration and biogeochemical functions of HO
• in water
environment. This paper also discusses the analytical methods that can be adopted
to measure the photoinduced generation of HO
• , the factors controlling its production and decay, as well as the significance and impact of HO
• in the aquatic ecosystems. It is shown how the production of HO
• differs among DOM components,
as well as between freshwaters and marine environments.
2 Hydroxyl Radical (HO
•
) and Other Free Radical Species
The hydroxyl radical (HO
•
) is the most powerful oxidizing agent among the
photolytically generated ones. It is a short-lived, highly reactive and nonselective transient, able to oxidize dissolved organic substances and other
chemical species in natural waters. The oxidation potentials for a series of common oxidants in surface waters is as follows: Fluorine (E = 3.03 V) > HO
•
(2.80 V) > Atomic oxygen (2.42 V) > Ozone (2.07 V) > Peracetic acid (ROOH)
(1.80 V) > H 2 O 2 (1.78 V) > Perhydroxyl radical (1.70 V) > Potassium permanganate (1.68 V) > Chlorine dioxide (1.57 V) > Hypochlorous acid (1.49 V) > Chlorine