217
Photoinduced Generation of Hydroxyl Radical in Natural Waters
waters (al Housari et al. 2010; Brezonik and Fulkerson-Brekken 1998; Arakaki
et al. 1999b; Hoigné et al. 1989; Schwarzenbach et al. 1993; Nakatani et al. 2004).
DOM and carbonate are the major scavengers or sinks of HO
• in freshwaters
(White et al. 2003; Nakatani et al. 2004), but in seawater the bromide (Br − ) ions
are actually the main scavengers (Song et al. 1996; Nakatani et al. 2004; Zafiriou
et al. 1987). The steady-state concentration of HO
• shows a large variability in
natural waters. Examples of concentration values reported in the literature are
(3.0–8.5) × 10 –16 M in rivers (Brezonik and Fulkerson-Brekken 1998; Arakaki
et al. 1999b; Nakatani et al. 2004), (9.41 ± 0.12) × 10 –17 M to (1.72 ± 0.01) × 10 –
16 M in estuarine waters (al Housari et al. 2010), 12 × 10 –18 M in coastal surface seawater and 1.1 × 10 –18 M in the open ocean (Mopper and Zhou 1990). In
Antarctic waters the steady-state concentrations have been determined as 4.3 × 10 –
19 M in coastal waters and 2.6 × 10 –19 M in the open ocean (Qian et al. 2001).
Elevated HO
• concentration values (from 6.7 × 10 –15 to 4.0 × 10 –12 M) have been
described in surface stream waters contaminated with acidic mine drainage (AMD).
These waters have pH ~2.1–3.4, are highly rich of iron (6–1203 mg L –1 ) and have
a high concentration of NO 3
– (5.9 × 10 –6 –5.8 × 10 –3 M) (Allen et al. 1996). The
reported data suggest that the steady-state concentration of HO
• can be very variable in different water systems. A major caveat that should be considered while comparing different studies is that the irradiation conditions are usually unequal, which
accounts for at least part of the variability. However, variations in the steady-state
HO
• concentration have also been observed with waters of different origin under
the same irradiation conditions. The major factors that account for the variation of
the steady-state concentration of HO
• in the aquatic environments are: (i) presence
of elevated concentrations of NO 2
– and NO 3
– ions; (ii) presence of the elevated
amounts of Fe 3+ -containing complexes; (iii) occurrence of photo Fenton-type reactions that take place between H 2 O 2 and reduced transition metal ions; (iv) amount
and nature of the dissolved organic matter (DOM).
The reciprocal of the consumption rate constant allows the assessment of
the life-time of HO
•
, which is (2.6–6.0) × 10 –6 s in river, dew and cloud water
(Arakaki and Faust 1998; Arakaki et al. 1999b; Nakatani et al. 2004) and several
times higher (3.0–66.0 × 10 –6 s) in remote polluted clouds, as estimated from a
modeling study (Jakob 1986).
2.4 An HPLC Method for Measuring HO • in Irradiated
Natural Waters, Based on Benzene as Probe Molecule
This section reports a detailed description of a possible method that can be
adopted for the determination of HO
• , based on benzene as a probe. The description is very detailed to enable the reader easily reproducing a similar experimental
set-up. Note that other probe molecules can also be used for HO
• determination,
e.g. cumene (isopropylbenzene), pyridine and terephthalic acid (Mill et al. 1980;
Photoinduced Generation of Hydroxyl Radical in Natural Waters
waters (al Housari et al. 2010; Brezonik and Fulkerson-Brekken 1998; Arakaki
et al. 1999b; Hoigné et al. 1989; Schwarzenbach et al. 1993; Nakatani et al. 2004).
DOM and carbonate are the major scavengers or sinks of HO
• in freshwaters
(White et al. 2003; Nakatani et al. 2004), but in seawater the bromide (Br − ) ions
are actually the main scavengers (Song et al. 1996; Nakatani et al. 2004; Zafiriou
et al. 1987). The steady-state concentration of HO
• shows a large variability in
natural waters. Examples of concentration values reported in the literature are
(3.0–8.5) × 10 –16 M in rivers (Brezonik and Fulkerson-Brekken 1998; Arakaki
et al. 1999b; Nakatani et al. 2004), (9.41 ± 0.12) × 10 –17 M to (1.72 ± 0.01) × 10 –
16 M in estuarine waters (al Housari et al. 2010), 12 × 10 –18 M in coastal surface seawater and 1.1 × 10 –18 M in the open ocean (Mopper and Zhou 1990). In
Antarctic waters the steady-state concentrations have been determined as 4.3 × 10 –
19 M in coastal waters and 2.6 × 10 –19 M in the open ocean (Qian et al. 2001).
Elevated HO
• concentration values (from 6.7 × 10 –15 to 4.0 × 10 –12 M) have been
described in surface stream waters contaminated with acidic mine drainage (AMD).
These waters have pH ~2.1–3.4, are highly rich of iron (6–1203 mg L –1 ) and have
a high concentration of NO 3
– (5.9 × 10 –6 –5.8 × 10 –3 M) (Allen et al. 1996). The
reported data suggest that the steady-state concentration of HO
• can be very variable in different water systems. A major caveat that should be considered while comparing different studies is that the irradiation conditions are usually unequal, which
accounts for at least part of the variability. However, variations in the steady-state
HO
• concentration have also been observed with waters of different origin under
the same irradiation conditions. The major factors that account for the variation of
the steady-state concentration of HO
• in the aquatic environments are: (i) presence
of elevated concentrations of NO 2
– and NO 3
– ions; (ii) presence of the elevated
amounts of Fe 3+ -containing complexes; (iii) occurrence of photo Fenton-type reactions that take place between H 2 O 2 and reduced transition metal ions; (iv) amount
and nature of the dissolved organic matter (DOM).
The reciprocal of the consumption rate constant allows the assessment of
the life-time of HO
•
, which is (2.6–6.0) × 10 –6 s in river, dew and cloud water
(Arakaki and Faust 1998; Arakaki et al. 1999b; Nakatani et al. 2004) and several
times higher (3.0–66.0 × 10 –6 s) in remote polluted clouds, as estimated from a
modeling study (Jakob 1986).
2.4 An HPLC Method for Measuring HO • in Irradiated
Natural Waters, Based on Benzene as Probe Molecule
This section reports a detailed description of a possible method that can be
adopted for the determination of HO
• , based on benzene as a probe. The description is very detailed to enable the reader easily reproducing a similar experimental
set-up. Note that other probe molecules can also be used for HO
• determination,
e.g. cumene (isopropylbenzene), pyridine and terephthalic acid (Mill et al. 1980;