Chemistry Reference
In-Depth Information
The stoichiometry coefficients are called
G
values, which indicate that every
100 eV of energy absorbed generates 2.8
•
OH,
2 . e
a
−
, 0.61 H
•
, and
0 03
. HO
•
(Eq. 4.49). H
2
O
2
is the main stable oxidizing species in the radiolysis of water
[269]. The hydrated electron is converted into
•
OH in N
2
O saturated solutions
(24 mM N
2
O, 1 atm, room temperature) to increase the yield of
•
OH, doubled
through reaction (4.50). The results indicate a system contains 90%
•
OH and
10% H
•
:
2
e
−
+
N O
→
•
OH N OH
+
+
−
.
(4.50)
aq
2
2
In synchrotron radiolysis of water, photons in the kilovolt x-ray range are
used [270]. An electron pulse of 0.15-µs duration, using a Varian linear accel-
erator, provides doses of ≈4 gy that could produce ≈2 µmol/L
•
OH.
The induced homolysis of H
2
O
2
by UV light in aqueous solution forms two
•
OH radicals (Eq. 4.51):
+ →
•
.
(4.51)
H O
h
ν
2
OH
2
2
The hemolytic fission of the peroxynitrous acid also yields
•
OH (Eq. 4.52)
[271]:
ONOOH NO
→
•
+
•
OH
.
(4.52)
2
Reaction (4.52) has a half-life of 1 second at pH 7.0. The primary yield of
reaction (4.53) is 0.4-0.5. Subsequently,
•
OH proceeds through the Haber-
Weiss chain reactions (Eqs. 4.53 and 4.54) [272]:
H O
+
•
OH HO H O
→
•
+
k
=
2 7 10
.
×
7
/M/s
(4.53)
2
2
2
2
53
H O HO
+
•
→
H O O
+
+
•
OH
k
= ×
7 10
9
/M/s
.
(4.54)
2
2
2
2
2
54
The Fenton reagent refers to a mixture of ferrous salts and H
2
O
2
, which
generates
•
OH through the reaction (Eq. 4.55)
2
+
3
+
•
−
.
(4.55)
Fe
+
H O
→
Fe
+
OH OH
+
2
2
The
•
OH spectrum has a broad absorption band with a maximum around
230 nm and a wavelength tail that extends beyond 320 nm [273]. The spectra
of
•
OH, measured at different temperatures, are presented in Figure 4.19 [274].
The primary band at 230 nm may be due to hydrogen-bonded OH, while the
band at 310 nm corresponds to “free” OH [274]. An increase in temperature
in water-cooled nuclear power plants resulted in a decrease at 230 nm and a
growth at 310 nm (Fig. 4.19). The isosbestic point appears at ∼305 nm. The
•
OH
radicals self-combine at a nearly diffusion-controlled rate constant, yielding a
hydrogen peroxide product (Eq. 4.56) [275]:
•
•
OH OH H O
2
+
→
2
.
(4.56)
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