Environmental Engineering Reference
In-Depth Information
The
Φ
HO
of NO
3
-
photolysis at room temperature (298 K) is (Eq.
4.12
)
(4.12)
HO
(
298 K
) =
0. 017
±
0. 03 for 4
≤
pH
≤
9
The temperature dependence of
Φ
HO
is mathematically derived from the results
of NO
3
-
photolysis at 308 nm, in the range of 4
≤
pH
≤
9 (Table
5
). It can be
written as (Eq.
4.13
) (Zellner et al.
1990
):
1
298
−
1
T
(4.13)
Φ
HO
(
T
) = Φ
HO
(
298 K
)
exp
(
1800
±
480
)
where the apparent activation energy
E
A
=
15
±
4 kJ mol
-1
. The effect of tem-
perature on the HO
•
quantum yield is relatively stronger than the effect of pH:
Φ
HO
increases by almost a factor of 5 over the temperature range 278-353 K, for
4
≤
pH
≤
9 (Zellner et al.
1990
). Because of the low molar absorption coefficient
of NO
3
-
at 351 nm (
ε
351
≈
0.05 L mol
-1
cm
-1
) (Meyerstein and Treinin
1961
),
nitrate is a less effective HO
•
source at 351 than at 308 nm. However, the results
•
indicate that the HO
quantum yield and the temperature dependence are constant
at these wavelength (Zellner et al.
1990
). There is an overall consistency between
the results of different studies into the temperature and wavelength dependence of
the HO
•
quantum yield for NO
3
-
photolysis (e.g., obtained
Φ
HO
=
0.015
±
0.003
at 313 nm) (Zepp et al.
1987
).
4.5 Dependence on H
2
O
2
Photolysis: Effect of Wavelength
Spectrum and Temperature
•
•
The photoinduced generation of HO
) depends
on the wavelength spectrum of UV-B radiation and on water temperature (Zellner
et al.
1990
; Hunt and Taube
1952
; Baxendale and Wilson
1956
; Volman and
Chen
1959
; Goldstein and Rabani
2008
; Chu and Anastasio
2005
; Goldstein
et al.
2007
). The quantum yield of HO
from H
2
O
2
(H
2
O
2
+
h
υ
→
2 HO
•
(
Φ
HO
) has been determined for
3
×
10
-3
M H
2
O
2
at pH 7, at the wavelengths of 308 and 351 nm (Zellner et al.
1990
). The H
2
O
2
absorption coefficients for these wavelengths are
ε
308
=
1.10 and
ε
351
=
0.11 L mol
-1
cm
-1
, respectively (Taylor and Cross
1949
). It has also been
shown that the HO
•
production gradually increases with an increase in temperature
at pH 7 (Fig.
6
; Table
6
) (Zellner et al.
1990
). Zellner and his co-workers derived
the equations for the temperature dependence of
Φ
HO
upon H
2
O
2
photolysis.
For the photolysis at 308 nm,
Φ
HO
can be expressed as (Eq.
4.14
):
(4.14)
Φ
HO
(
298 K
) =
0. 98
±
0. 03
The temperature dependence of
Φ
HO
can be expressed as (Eq.
4.15
):
1
298
−
1
T
(4.15)
Φ
HO
(
T
) = Φ
HO
(
298 K
)
EXP
(
660
±
190
)
