213
Photoinduced Generation of Hydroxyl Radical in Natural Waters
peroxide radicals with organic substances in aqueous solution (Mill et al. 1980;
Mageli and Kolczynski 1966; Faust and Hoigne 1987; Blough 1988; Kieber and
Blough 1990; Sakugawa et al. 1990; Faust and Allen 1992; Mostofa and Sakugawa
2009). Furthermore, the thermal decomposition of organic peroxides can initiate
the polymerization of vinyl monomers or induce cross-linking of a polymeric substrate upon formation of free radical sites on the polymer (Mageli and Kolczynski
1966). The overall (unspeciated) photostationary-state concentration of peroxyl
radicals in sunlit cloud and fog waters is around 1–30 nM (Faust and Allen 1992).
The superoxide radical anion (O 2
•– ) is the one-electron reduction product
of molecular oxygen. It is an early photoinduced and short-lived intermediate
that is formed in chemical reactions occurring in natural waters, where oxygen acts as the ultimate electron acceptor (Jeong and Yoon 2005; Bielski et al.
1985; Petasne and Zika 1987; Zafiriou 1990; Micinski et al. 1993; Zafiriou et al.
1998; Millington and Maurdev 2004). It has been shown that the photoinduced
superoxide production rates are 0.1–6.0 nM min –1 under full-sun irradiation in
spring, and 0.2–8.0 nM min –1 in fall in a variety of Eastern Caribbean waters
(Micinski et al. 1993). A key reaction of O 2
•– is the production of H 2 O 2 by dismutation; hydrogen peroxide is then able to generate HO • by direct photolysis or
upon photo-Fenton type reactions in sunlit aqueous solutions (Cooper et al. 1988;
Micinski et al. 1993; Fischer et al. 1985). Interestingly, the organic complexes of
Cu as well as the copper-catalyzed dismutation (involving Cu + and Cu 2+ ) can be
significant sinks of photoproduced O 2
•– in seawater (Zafiriou et al. 1998; Voelker
et al. 2000).
The carbon dioxide radical anion (CO 2
•–
) is a short-lived and highly reactive intermediate that is photolytically formed in the ferrioxalate reaction system. It is a strong oxidizing agent that is able to oxidize metals and other
chemical species in aqueous solution. The CO 2
•– is formed photolytically (C 2 O 4
•–
→ CO 2
•− + CO 2 ; k = 2 × 10 6 s –1 ) upon decomposition of the oxalyl radical
anion (C 2 O 4
•–
). The latter is produced by the photoinduced decomposition of the
highly photosensitive ferrioxalate complex [Fe(C 2 O 4 ) 3 ] 3– in aqueous solution
(Hislop and Bolton 1999; Jeong and Yoon 2004, 2005; Mulazzani et al. 1986). A
key role played by CO 2
•– is its ability to oxidize the metal ions, therefore affecting
the biogeochemical cycling of metal-containing species. These processes can have
an impact on the generation of HO
• and of the superoxide radical anion in natural
waters (Hislop and Bolton 1999; Jeong and Yoon 2004, 2005; Wang et al. 2010).
Another potentially important process is the transformation of organic substances
induced by CO 2
•–
, which is formed photolytically from ferrioxalate complexes in
the aqueous solution (Huston and Pignatello 1996).
In addition, it has been reported that quinones photolytically produce species capable of hydroxylation (Alegria et al. 1997; Pochon et al. 2002; Gan et al. 2008; Maurino
et al. 2008; Maddigapu et al. 2010; Page et al. 2011). Some of these quinone-derived
hydroxylating species exhibit reactivity that is around one order of magnitude lower
than free HO
• (Pochon et al. 2002; Gan et al. 2008). It is hypothesized that quinonederived hydroxylating species may contribute at least in part to the photoinduced HO
•
production by DOM (Vaughn and Blough 1998; Page et al. 2011).
Photoinduced Generation of Hydroxyl Radical in Natural Waters
peroxide radicals with organic substances in aqueous solution (Mill et al. 1980;
Mageli and Kolczynski 1966; Faust and Hoigne 1987; Blough 1988; Kieber and
Blough 1990; Sakugawa et al. 1990; Faust and Allen 1992; Mostofa and Sakugawa
2009). Furthermore, the thermal decomposition of organic peroxides can initiate
the polymerization of vinyl monomers or induce cross-linking of a polymeric substrate upon formation of free radical sites on the polymer (Mageli and Kolczynski
1966). The overall (unspeciated) photostationary-state concentration of peroxyl
radicals in sunlit cloud and fog waters is around 1–30 nM (Faust and Allen 1992).
The superoxide radical anion (O 2
•– ) is the one-electron reduction product
of molecular oxygen. It is an early photoinduced and short-lived intermediate
that is formed in chemical reactions occurring in natural waters, where oxygen acts as the ultimate electron acceptor (Jeong and Yoon 2005; Bielski et al.
1985; Petasne and Zika 1987; Zafiriou 1990; Micinski et al. 1993; Zafiriou et al.
1998; Millington and Maurdev 2004). It has been shown that the photoinduced
superoxide production rates are 0.1–6.0 nM min –1 under full-sun irradiation in
spring, and 0.2–8.0 nM min –1 in fall in a variety of Eastern Caribbean waters
(Micinski et al. 1993). A key reaction of O 2
•– is the production of H 2 O 2 by dismutation; hydrogen peroxide is then able to generate HO • by direct photolysis or
upon photo-Fenton type reactions in sunlit aqueous solutions (Cooper et al. 1988;
Micinski et al. 1993; Fischer et al. 1985). Interestingly, the organic complexes of
Cu as well as the copper-catalyzed dismutation (involving Cu + and Cu 2+ ) can be
significant sinks of photoproduced O 2
•– in seawater (Zafiriou et al. 1998; Voelker
et al. 2000).
The carbon dioxide radical anion (CO 2
•–
) is a short-lived and highly reactive intermediate that is photolytically formed in the ferrioxalate reaction system. It is a strong oxidizing agent that is able to oxidize metals and other
chemical species in aqueous solution. The CO 2
•– is formed photolytically (C 2 O 4
•–
→ CO 2
•− + CO 2 ; k = 2 × 10 6 s –1 ) upon decomposition of the oxalyl radical
anion (C 2 O 4
•–
). The latter is produced by the photoinduced decomposition of the
highly photosensitive ferrioxalate complex [Fe(C 2 O 4 ) 3 ] 3– in aqueous solution
(Hislop and Bolton 1999; Jeong and Yoon 2004, 2005; Mulazzani et al. 1986). A
key role played by CO 2
•– is its ability to oxidize the metal ions, therefore affecting
the biogeochemical cycling of metal-containing species. These processes can have
an impact on the generation of HO
• and of the superoxide radical anion in natural
waters (Hislop and Bolton 1999; Jeong and Yoon 2004, 2005; Wang et al. 2010).
Another potentially important process is the transformation of organic substances
induced by CO 2
•–
, which is formed photolytically from ferrioxalate complexes in
the aqueous solution (Huston and Pignatello 1996).
In addition, it has been reported that quinones photolytically produce species capable of hydroxylation (Alegria et al. 1997; Pochon et al. 2002; Gan et al. 2008; Maurino
et al. 2008; Maddigapu et al. 2010; Page et al. 2011). Some of these quinone-derived
hydroxylating species exhibit reactivity that is around one order of magnitude lower
than free HO
• (Pochon et al. 2002; Gan et al. 2008). It is hypothesized that quinonederived hydroxylating species may contribute at least in part to the photoinduced HO
•
production by DOM (Vaughn and Blough 1998; Page et al. 2011).