226
K. M. G. Mostofa et al.
3 Mechanisms for Photoinduced Generation of HO
•
in Natural Waters
The photoinduced generation of HO
• significantly depends on several important factors such as the presence of NO 2
− and NO 3
− and their concentration, the
chemical nature of DOM and its quantity, and finally the total content of Fe. The
most important mechanisms for HO
• formation in natural waters are discussed
below.
3.1 In situ Generation of HO
•
from DOM
One of the main HO
• sources in natural waters (Table 2) (Vione et al. 2006;
Mostofa KMG and Sakugawa H, unpublished data) is the photoinduced generation of HO • from DOM components (either Fluorescent Dissolved Organic
Matter-FDOM or Colored Dissolved Organic Matter-CDOM). This process can be
accounted for either by the oxidation of water by the triplet states 3 DOM*, or by
the generation of H 2 O 2 upon DOM irradiation (reaction 3.1) and its detailed mechanisms are discussed in earlier chapter (see chapter “Photoinduced and Microbial
Generation of Hydrogen Peroxide and Organic Peroxides in Natural Waters”). In
the latter case, HO
• could be produced upon photolysis of H 2 O 2 (Eq. 3.2) (Legrini
et al. 1993; von Sonntag et al. 1993).
The quantum yield of reaction 3.2 has been determined as 0.5 under UVC
irradiation (Legrini et al. 1993; von Sonntag et al. 1993). The quantum yield
varies with wavelength, but it also depends on the band that absorbs radiation.
In the case of H 2 O 2 , the same band is responsible for radiation absorption and
photolysis in both the UVC and UVB regions. The hypothesis that the formation
of HO
• by irradiated DOM is accounted for by H 2 O 2 photoproduction. It is consistent with the observed, gradual and parallel increase of H 2 O 2 concentration and
of phenol formation from benzene, upon irradiation of natural river waters and of
relevant standard organic substances (Fig. 2). A similar, parallel trend of both phenol (its formation being used as HO
• probe) and H 2 O 2 has been observed upon
irradiation of upstream DOM mostly containing fulvic acid (Fig. 2a), of standard Suwannee River Fulvic Acid (SRFA) (Fig. 2c) and of diaminostilbene type
(DAS1) (Fig. 2d). The same trend has not been observed in sewage polluted river
waters (Fig. 2b), which might be the effect of additional production from other
HO
• sources such as the NO 2
– and NO 3
– ions, present in high amount (Takeda
(3.1)
DOM + O 2 + H 2 O + H
+ hυ
− → H 2 O 2 + DOM
•+ + O 2 + HO
−
(3.2)
H 2 O 2
hυ
− → 2HO
•
K. M. G. Mostofa et al.
3 Mechanisms for Photoinduced Generation of HO
•
in Natural Waters
The photoinduced generation of HO
• significantly depends on several important factors such as the presence of NO 2
− and NO 3
− and their concentration, the
chemical nature of DOM and its quantity, and finally the total content of Fe. The
most important mechanisms for HO
• formation in natural waters are discussed
below.
3.1 In situ Generation of HO
•
from DOM
One of the main HO
• sources in natural waters (Table 2) (Vione et al. 2006;
Mostofa KMG and Sakugawa H, unpublished data) is the photoinduced generation of HO • from DOM components (either Fluorescent Dissolved Organic
Matter-FDOM or Colored Dissolved Organic Matter-CDOM). This process can be
accounted for either by the oxidation of water by the triplet states 3 DOM*, or by
the generation of H 2 O 2 upon DOM irradiation (reaction 3.1) and its detailed mechanisms are discussed in earlier chapter (see chapter “Photoinduced and Microbial
Generation of Hydrogen Peroxide and Organic Peroxides in Natural Waters”). In
the latter case, HO
• could be produced upon photolysis of H 2 O 2 (Eq. 3.2) (Legrini
et al. 1993; von Sonntag et al. 1993).
The quantum yield of reaction 3.2 has been determined as 0.5 under UVC
irradiation (Legrini et al. 1993; von Sonntag et al. 1993). The quantum yield
varies with wavelength, but it also depends on the band that absorbs radiation.
In the case of H 2 O 2 , the same band is responsible for radiation absorption and
photolysis in both the UVC and UVB regions. The hypothesis that the formation
of HO
• by irradiated DOM is accounted for by H 2 O 2 photoproduction. It is consistent with the observed, gradual and parallel increase of H 2 O 2 concentration and
of phenol formation from benzene, upon irradiation of natural river waters and of
relevant standard organic substances (Fig. 2). A similar, parallel trend of both phenol (its formation being used as HO
• probe) and H 2 O 2 has been observed upon
irradiation of upstream DOM mostly containing fulvic acid (Fig. 2a), of standard Suwannee River Fulvic Acid (SRFA) (Fig. 2c) and of diaminostilbene type
(DAS1) (Fig. 2d). The same trend has not been observed in sewage polluted river
waters (Fig. 2b), which might be the effect of additional production from other
HO
• sources such as the NO 2
– and NO 3
– ions, present in high amount (Takeda
(3.1)
DOM + O 2 + H 2 O + H
+ hυ
− → H 2 O 2 + DOM
•+ + O 2 + HO
−
(3.2)
H 2 O 2
hυ
− → 2HO
•