Environmental Engineering Reference
In-Depth Information
This steady-state approach has been successfully applied to examine the ther-
mal production of HO
•
in ozone-treated natural waters, as well as the photoin-
duced 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 photoin-
duced production rate of the reactive species (Schwarzenbach et al.
1993
) can be
expressed as (Eq.
4.42
):
R
P
=
2. 3
I
(λ) × ε(λ) × Φ(λ) ×
C
(4.42)
λ
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 appar-
ent quantum yield,
and
C
are the absorption coefficient and the concentra-
tion 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
Є
•
from NO
3
-
photolysis from Eq. (
4.39
), from which one gets (Eq.
4.43
)
(Southworth and Voelker
2003
):
[
NO
3
]
2
×
10
−
7
(4.43)
R
NO3
=
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
), adopt-
ing 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 com-
pared 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-Fen-
ton system enhances degradation, compared to the corresponding dark Fenton
system at equal temperature. However, the effect of irradiation is decreased dra-
matically 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 degra-
dation of specific organic compounds (Jeong and Yoon
2005
; Balmer and
Sulzberger
1999
). The pH effect is thought to involve two phenomena (Jeong
