245
Photoinduced Generation of Hydroxyl Radical in Natural Waters
where the activation energy is 5.5 ± 1.6 kJ mol –1 .
These values are essentially constant for H 2 O 2 photolysis at 351 nm (Eqs. 4.16,
4.17):
A recent study has shown that the quantum yield Φ HO = 1.11 ± 0.07 in the
excitation range of 205–280 nm for the photolysis of H 2 O 2 . This is in agreement
with earlier studies (Goldstein et al. 2007). Therefore, the available data suggest
that the photolysis of aqueous H 2 O 2 at wavelengths ≥ 300 nm generates HO
• with
a quantum yield Φ HO ~1 at room temperature (25 °C). Φ OH decreases to approximately 0.82 at 5 °C (278 K). It can be noted that the effective quantum yield of
H 2 O 2 dissociation is approximately 0.5, but the photolysis of H 2 O 2 yields two
(4.16)
Φ HO (298 K) = 0. 96 ± 0. 04
(4.17)
Φ HO (T ) = Φ HO (298 K) exp
(580 ± 160)
1
298
−
1
T
Fig. 6 Quantum yields of
HO
• of H 2 O 2 photolysis with
variation in temperature at
specific wavelengths (308
and 351 nm, respectively) at
pH 7. Data source Zellner et
al. (1990)
0.7
0.8
0.9
1
1.1
275
280
285
290
295
3 00
Wavelength (308 nm)
Wavelength (351 nm)
Temperature (ºK)
Φ at various pHs
HO
Table 6 Quantum yield of HO
• production (Φ HO ) from H 2 O 2 photolysis at pH 7 as a function of
temperature (T)
T (ºK)
Φ HO at pH 7
308 nm
351 nm
278
0.82 ± 0.03
0.83 ± 0.05
283
0.86 ± 0.09
0.88 ± 0.06
288
0.91 ± 0.04
0.90 ± 0.06
293
0.93 ± 0.07
0.93 ± 0.08
298
0.98 ± 0.03
0.96 ± 0.04
Data source Zellner et al. (1990)