252
K. M. G. Mostofa et al.
This steady-state approach has been successfully applied to examine the thermal production of HO
• in ozone-treated natural waters, as well as the photoinduced generation of HO
• upon irradiation of natural waters and of nitrate ions
(Zepp et al. 1987, 1992; Haag and Hoigné 1985).
In natural waters for a thin layer at the surface of a water body, the photoinduced production rate of the reactive species (Schwarzenbach et al. 1993) can be
expressed as (Eq. 4.42):
where r P is the production rate (M s –1 ), I is the incident light intensity
(mEinstein cm –2 s –1 ), ϕ is either the quantum yield (mol Einstein –1 ) or the apparent quantum yield, Є and C are the absorption coefficient and the concentration of the relevant light-absorbing reactive species, respectively, and λ is the
wavelength. Thus, it is possible to determine the near-surface production rate
of HO
• from NO 3
– photolysis from Eq. (4.39), from which one gets (Eq. 4.43)
(Southworth and Voelker 2003):
To obtain (Eq. 4.43), the light intensity values were integrated over wavelength
for a solar declination of 20° (24-h averaged) (Schwarzenbach et al. 1993), adopting a quantum yield of 0.015 for the HO
• photoproduction upon nitrate irradiation
at 25 °C (Zepp et al. 1987).
The degradation rate of formic acid in the photo-Fenton reaction increases
with temperature (Fig. 7) (Farias et al. 2007). From the cited results it can be
estimated that the conversion of HCOOH after 20 min of reaction time is
increased approximately by 70–120 % at 313 K and 160–202 % at 328 K compared to the initial temperature of 298 K, with H 2 O 2 :HCOOH molar ratios in the
range from 3 to 8 (Fig. 7). It is also observed that irradiation in the photo-Fenton system enhances degradation, compared to the corresponding dark Fenton
system at equal temperature. However, the effect of irradiation is decreased dramatically as temperature increases, so that at 328 K there is little advantage in
irradiating the system.
4.8 Photo-Ferrioxalate/H 2 O 2 Reaction: Dependence
on pH and Reactants
Without addition of H 2 O 2 to the photo-ferrioxalate system, the reaction rate
gradually increases with increasing pH as can be measured from the degradation of specific organic compounds (Jeong and Yoon 2005; Balmer and
Sulzberger 1999). The pH effect is thought to involve two phenomena (Jeong
(4.42)
r p = 2. 3
λ
I (λ) × ε(λ) × Φ(λ) × C
(4.43)
r NO3 =
2 × 10
−7
[NO
−
3 ]
K. M. G. Mostofa et al.
This steady-state approach has been successfully applied to examine the thermal production of HO
• in ozone-treated natural waters, as well as the photoinduced generation of HO
• upon irradiation of natural waters and of nitrate ions
(Zepp et al. 1987, 1992; Haag and Hoigné 1985).
In natural waters for a thin layer at the surface of a water body, the photoinduced production rate of the reactive species (Schwarzenbach et al. 1993) can be
expressed as (Eq. 4.42):
where r P is the production rate (M s –1 ), I is the incident light intensity
(mEinstein cm –2 s –1 ), ϕ is either the quantum yield (mol Einstein –1 ) or the apparent quantum yield, Є and C are the absorption coefficient and the concentration of the relevant light-absorbing reactive species, respectively, and λ is the
wavelength. Thus, it is possible to determine the near-surface production rate
of HO
• from NO 3
– photolysis from Eq. (4.39), from which one gets (Eq. 4.43)
(Southworth and Voelker 2003):
To obtain (Eq. 4.43), the light intensity values were integrated over wavelength
for a solar declination of 20° (24-h averaged) (Schwarzenbach et al. 1993), adopting a quantum yield of 0.015 for the HO
• photoproduction upon nitrate irradiation
at 25 °C (Zepp et al. 1987).
The degradation rate of formic acid in the photo-Fenton reaction increases
with temperature (Fig. 7) (Farias et al. 2007). From the cited results it can be
estimated that the conversion of HCOOH after 20 min of reaction time is
increased approximately by 70–120 % at 313 K and 160–202 % at 328 K compared to the initial temperature of 298 K, with H 2 O 2 :HCOOH molar ratios in the
range from 3 to 8 (Fig. 7). It is also observed that irradiation in the photo-Fenton system enhances degradation, compared to the corresponding dark Fenton
system at equal temperature. However, the effect of irradiation is decreased dramatically as temperature increases, so that at 328 K there is little advantage in
irradiating the system.
4.8 Photo-Ferrioxalate/H 2 O 2 Reaction: Dependence
on pH and Reactants
Without addition of H 2 O 2 to the photo-ferrioxalate system, the reaction rate
gradually increases with increasing pH as can be measured from the degradation of specific organic compounds (Jeong and Yoon 2005; Balmer and
Sulzberger 1999). The pH effect is thought to involve two phenomena (Jeong
(4.42)
r p = 2. 3
λ
I (λ) × ε(λ) × Φ(λ) × C
(4.43)
r NO3 =
2 × 10
−7
[NO
−
3 ]