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K. M. G. Mostofa et al.
2.1 Sources of HO • in Natural Waters
The HO
• radical is formed photolytically from various sources in natural waters. In
rivers, contributions to HO
• photoproduction are 1–89 % from NO 2
– , 2–70 % from
NO 3
– , 1–50 % from H 2 O 2 , and 2–70 % from the photo-Fenton reaction and/or irradiated CDOM (Takeda et al. 2004; Vione et al. 2006; White et al. 2003; Page et al.
2011; Nakatani et al. 2007; Mostofa KMG and Sakugawa H, unpublished data).
Experimental studies show that DOM isolates from rivers may contribute up to 50 %
of the hydroxylation through production of H 2 O 2 (Page et al. 2011). The results demonstrate that NO 2
– is a key contributor (48–80 %) for HO
• production in seweragepolluted river waters, but NO 3
– can be a major contributor (16–49 %) in clean river
waters. In seawater the major sources of HO
• are 7–75 % from NO 2
– , 1–8 % from
NO 3
– , 0–1 % from H 2 O 2 , and 24–93 % from unknown sources. These data were
obtained from a study carried out in Seto Inland and the Yellow Sea (Takeda et al.
2004). The formation of HO
• from different sources in natural waters can be distinguished as: (i) the photolysis of nitrite and nitrate in the aqueous solution (Mopper
and Zhou 1990; Takeda et al. 2004; Zepp et al. 1987); (ii) the irradiation of CDOM
components via formation of H 2 O 2 in the aqueous solution. In this case the production of HO
• depends on the nature of the CDOM components (Fig. 1) (White
et al. 2003; Mostofa and Sakugawa 2009; Mostofa KMG and Sakugawa H,
unpublished data), but a useful correlation has been found between the formation rate of HO
• and the content of dissolved organic carbon in different lake
water samples (Vione et al. 2006); (iii) the Fenton reaction (Fenton 1894; Walling
1975; Kang et al. 2002), the photo-Fenton reaction (Zepp et al. 1992; Arakaki
et al. 1998; Southworth and Voelker 2003) as well as the photo-ferrioxalate/H 2 O 2
system in natural waters (Southworth and Voelker 2003; Safazadeh-Amiri et al. 1997;
Hislop and Bolton 1999); (iv) the direct photolysis of hydrogen peroxide, i.e. UV/
H 2 O 2 processes in aqueous solution (Draper and Crosby 1981; Wang et al. 2001). The
UV irradiation of natural waters can produce H 2 O 2 that further yields HO
• (Gjessing
and Källqvist 1991; Cooper et al. 1996); (v) the reaction of hydroperoxide radical
(HO 2
• ) with NO (HO 2
• + NO → HO • + NO 2 ) (Sakugawa et al. 1990); (vi) the
photolysis of dimeric
Fe 2 (OH) 2 (H 2 O) 8
4+ species in aqueous solution (Langford
and Carey 1975); (vii) the photolysis of Fe III (OH) 2+ in aqueous solution. The generation of HO
• upon photolysis of Fe III (OH) 2+ is very efficient (quantum yield ~0.2),
but the Fe(III) hydroxocomplex is present in significant concentration only at strongly
acidic pH values that have little environmental significance(Jeong and Yoon 2005;
Pozdnyakov et al. 2000); (viii) the generation of singlet states of oxygen atoms ( 1 O 1 )
by ozonolysis, followed by reaction with H 2 O to form HO
• (Hoigné and Bader 1978,
1979; Staehelin and Hoigné 1985; Takahashi et al. 1995); (ix) the reaction of O 3 with
H 2 O 2 (peroxone process), which generates HO
• (H 2 O 2 + 2O 3 → 2HO
• + 3O 2 )
(Hoigné 1998); (x) the production of HO
• by auto-oxidation of cytotoxic agents
(Cohen and Heikkila 1974); (xi) chemical effects of ultrasound, which can generate
HO
• in aqueous solution (Makino et al. 1983); (xii) ultrasound-induced cavitation
in aqueous solution, yielding HO
• upon water splitting (H 2 O + ultrasound → HO
• ,
K. M. G. Mostofa et al.
2.1 Sources of HO • in Natural Waters
The HO
• radical is formed photolytically from various sources in natural waters. In
rivers, contributions to HO
• photoproduction are 1–89 % from NO 2
– , 2–70 % from
NO 3
– , 1–50 % from H 2 O 2 , and 2–70 % from the photo-Fenton reaction and/or irradiated CDOM (Takeda et al. 2004; Vione et al. 2006; White et al. 2003; Page et al.
2011; Nakatani et al. 2007; Mostofa KMG and Sakugawa H, unpublished data).
Experimental studies show that DOM isolates from rivers may contribute up to 50 %
of the hydroxylation through production of H 2 O 2 (Page et al. 2011). The results demonstrate that NO 2
– is a key contributor (48–80 %) for HO
• production in seweragepolluted river waters, but NO 3
– can be a major contributor (16–49 %) in clean river
waters. In seawater the major sources of HO
• are 7–75 % from NO 2
– , 1–8 % from
NO 3
– , 0–1 % from H 2 O 2 , and 24–93 % from unknown sources. These data were
obtained from a study carried out in Seto Inland and the Yellow Sea (Takeda et al.
2004). The formation of HO
• from different sources in natural waters can be distinguished as: (i) the photolysis of nitrite and nitrate in the aqueous solution (Mopper
and Zhou 1990; Takeda et al. 2004; Zepp et al. 1987); (ii) the irradiation of CDOM
components via formation of H 2 O 2 in the aqueous solution. In this case the production of HO
• depends on the nature of the CDOM components (Fig. 1) (White
et al. 2003; Mostofa and Sakugawa 2009; Mostofa KMG and Sakugawa H,
unpublished data), but a useful correlation has been found between the formation rate of HO
• and the content of dissolved organic carbon in different lake
water samples (Vione et al. 2006); (iii) the Fenton reaction (Fenton 1894; Walling
1975; Kang et al. 2002), the photo-Fenton reaction (Zepp et al. 1992; Arakaki
et al. 1998; Southworth and Voelker 2003) as well as the photo-ferrioxalate/H 2 O 2
system in natural waters (Southworth and Voelker 2003; Safazadeh-Amiri et al. 1997;
Hislop and Bolton 1999); (iv) the direct photolysis of hydrogen peroxide, i.e. UV/
H 2 O 2 processes in aqueous solution (Draper and Crosby 1981; Wang et al. 2001). The
UV irradiation of natural waters can produce H 2 O 2 that further yields HO
• (Gjessing
and Källqvist 1991; Cooper et al. 1996); (v) the reaction of hydroperoxide radical
(HO 2
• ) with NO (HO 2
• + NO → HO • + NO 2 ) (Sakugawa et al. 1990); (vi) the
photolysis of dimeric
Fe 2 (OH) 2 (H 2 O) 8
4+ species in aqueous solution (Langford
and Carey 1975); (vii) the photolysis of Fe III (OH) 2+ in aqueous solution. The generation of HO
• upon photolysis of Fe III (OH) 2+ is very efficient (quantum yield ~0.2),
but the Fe(III) hydroxocomplex is present in significant concentration only at strongly
acidic pH values that have little environmental significance(Jeong and Yoon 2005;
Pozdnyakov et al. 2000); (viii) the generation of singlet states of oxygen atoms ( 1 O 1 )
by ozonolysis, followed by reaction with H 2 O to form HO
• (Hoigné and Bader 1978,
1979; Staehelin and Hoigné 1985; Takahashi et al. 1995); (ix) the reaction of O 3 with
H 2 O 2 (peroxone process), which generates HO
• (H 2 O 2 + 2O 3 → 2HO
• + 3O 2 )
(Hoigné 1998); (x) the production of HO
• by auto-oxidation of cytotoxic agents
(Cohen and Heikkila 1974); (xi) chemical effects of ultrasound, which can generate
HO
• in aqueous solution (Makino et al. 1983); (xii) ultrasound-induced cavitation
in aqueous solution, yielding HO
• upon water splitting (H 2 O + ultrasound → HO
• ,