238
K. M. G. Mostofa et al.
Suwannee River Fulvic Acid is (3.8–4.4) × 10 4 (mg C) –1 L s –1 (Southworth
and Voelker 2003; Westerhoff et al. 2007). Rate constants in the same ranges,
(1–7) × 10 4 (mg C) –1 L s –1 , have been determined for the reaction between HO
•
and DOM present in natural water samples or extracted from them (Vione et al.
2006; Westerhoff et al. 1999; Goldstone et al. 2002; Gao and Zepp 1998). The
half-life of a model pollutant can be estimated as t 1/2 = ln 2 (k HO [HO
•
] SS ) –1 ,
where k HO is the second-order reaction rate constant with HO
• , and [HO
•
] ss is
given by Eq. (4.1). Depending on the mixed layer depths that influence [HO
•
] ss ,
and for k HO values of the order of 10 9 –10 10 M –1 s –1 , t 1/2 can vary from some days
to some months.
4.2.1 Fulvic Acid as a Producer and Scavenger of HO
•
in Natural Waters
Fulvic acid (FA) can produce HO
• photolytically in aqueous solution (Table 2)
(Vaughn and Blough 1998; Goldstone et al. 2002). FA can account for approximately 23–70 % of H 2 O 2 production in rivers (Mostofa and Sakugawa 2009).
A general reaction of FA that leads to the formation of H 2 O 2 (Eq. 4.2) can
be depicted on the basis of Eqs. (3.13–3.18) (see chapter “Photoinduced and
Microbial Generation of Hydrogen Peroxide and Organic Peroxides in Natural
Waters”):
The generation of H 2 O 2 from FA can lead to HO
• (H 2 O 2 + hυ → 2HO
•
) that
could further react with FA, at the same time being consumed and causing transformation of FA (Voelker and Sulzberger 1996). Recent experimental studies indicate that at least 50 % of the hydroxylation reactions photosensitized by DOM
isolates would be a result of a pathway that is independent of hydrogen peroxide
(Page et al. 2011). Recently, the photo-degradation of various functional groups
in DOM by HO
• has been observed, and the rates determined in aqueous solution
(Minakata et al. 2009). The results suggest that DOM or FA is important scavengers of photolytically generated HO
• in aqueous solution.
4.3 Other Chemical Species or Processes as HO
•
Sinks
There are several processes that can inhibit HO
• formation or consume these radicals in the aquatic environments, which can be distinguished as:
(1) Decrease in light intensity in deeper waters, which reduces the formation
rate of H 2 O 2 and of Fe(II). Photo-generated H 2 O 2 and Fe(II) at the surface could
be moved downward through vertical mixing processes, thereby reducing their
concentration in the surface layer (Southworth and Voelker 2003; Pullin et al.
2004). Such an effect can greatly decrease the HO
• production in water.
(4.2)
FA + O 2 + H 2 O
hυ
− → FA
•+ + H 2 O 2 + O 2 + OH
−
K. M. G. Mostofa et al.
Suwannee River Fulvic Acid is (3.8–4.4) × 10 4 (mg C) –1 L s –1 (Southworth
and Voelker 2003; Westerhoff et al. 2007). Rate constants in the same ranges,
(1–7) × 10 4 (mg C) –1 L s –1 , have been determined for the reaction between HO
•
and DOM present in natural water samples or extracted from them (Vione et al.
2006; Westerhoff et al. 1999; Goldstone et al. 2002; Gao and Zepp 1998). The
half-life of a model pollutant can be estimated as t 1/2 = ln 2 (k HO [HO
•
] SS ) –1 ,
where k HO is the second-order reaction rate constant with HO
• , and [HO
•
] ss is
given by Eq. (4.1). Depending on the mixed layer depths that influence [HO
•
] ss ,
and for k HO values of the order of 10 9 –10 10 M –1 s –1 , t 1/2 can vary from some days
to some months.
4.2.1 Fulvic Acid as a Producer and Scavenger of HO
•
in Natural Waters
Fulvic acid (FA) can produce HO
• photolytically in aqueous solution (Table 2)
(Vaughn and Blough 1998; Goldstone et al. 2002). FA can account for approximately 23–70 % of H 2 O 2 production in rivers (Mostofa and Sakugawa 2009).
A general reaction of FA that leads to the formation of H 2 O 2 (Eq. 4.2) can
be depicted on the basis of Eqs. (3.13–3.18) (see chapter “Photoinduced and
Microbial Generation of Hydrogen Peroxide and Organic Peroxides in Natural
Waters”):
The generation of H 2 O 2 from FA can lead to HO
• (H 2 O 2 + hυ → 2HO
•
) that
could further react with FA, at the same time being consumed and causing transformation of FA (Voelker and Sulzberger 1996). Recent experimental studies indicate that at least 50 % of the hydroxylation reactions photosensitized by DOM
isolates would be a result of a pathway that is independent of hydrogen peroxide
(Page et al. 2011). Recently, the photo-degradation of various functional groups
in DOM by HO
• has been observed, and the rates determined in aqueous solution
(Minakata et al. 2009). The results suggest that DOM or FA is important scavengers of photolytically generated HO
• in aqueous solution.
4.3 Other Chemical Species or Processes as HO
•
Sinks
There are several processes that can inhibit HO
• formation or consume these radicals in the aquatic environments, which can be distinguished as:
(1) Decrease in light intensity in deeper waters, which reduces the formation
rate of H 2 O 2 and of Fe(II). Photo-generated H 2 O 2 and Fe(II) at the surface could
be moved downward through vertical mixing processes, thereby reducing their
concentration in the surface layer (Southworth and Voelker 2003; Pullin et al.
2004). Such an effect can greatly decrease the HO
• production in water.
(4.2)
FA + O 2 + H 2 O
hυ
− → FA
•+ + H 2 O 2 + O 2 + OH
−