Hydrogen peroxide production in the radiolysis of water with high radical scavenger concentrations

Article abstract of DOI:10.1021/jp0207578

The radiation chemical yields of hydrogen peroxide formed in the γ-radiolysis of water with scavengers for oxidizing and/or reducing radicals were measured to examine initial water decomposition pathways to oxidizing species. Hydrogen peroxide yields were found to decrease toward zero with increasing concentration of OH radical scavenger in all solutions, suggesting that the OH radical is the sole precursor to hydrogen peroxide. The yields of hydrogen peroxide in nitrate and selenate solutions are closely associated with the scavenging capacity of the precursor to the hydrated electron, suggesting that its reactions have a significant role in hydrogen peroxide formation. Observed hydrogen peroxide yields at high nitrate concentrations coupled with model calculations show that the molecular cation of water, H₂O⁺, is the dominant precursor of the oxidizing species leading to hydrogen peroxide. Proton-transfer reactions of the water molecular cation give 79% of the oxidizing species, whereas other reactions such as dissociative recombination reactions account for the rest. There is a significant additional production of OH radicals in the radiolysis of selenate solutions due to the production of ^·O^- and the scavenging of low-level excited water molecules.

Full text of DOI:10.1021/jp0207578

9
352
J. Phys. Chem. A 2002, 106, 9352-9358
Hydrogen Peroxide Production in the Radiolysis of Water with High Radical Scavenger
Concentrations
Akihiro Hiroki, Simon M. Pimblott, and Jay A. LaVerne*
Radiation Laboratory, UniVersity of Notre Dame, Notre Dame, Indiana 46556
ReceiVed: March 20, 2002; In Final Form: July 11, 2002
The radiation chemical yields of hydrogen peroxide formed in the γ-radiolysis of water with scavengers for
oxidizing and/or reducing radicals were measured to examine initial water decomposition pathways to oxidizing
species. Hydrogen peroxide yields were found to decrease toward zero with increasing concentration of OH
radical scavenger in all solutions, suggesting that the OH radical is the sole precursor to hydrogen peroxide.
The yields of hydrogen peroxide in nitrate and selenate solutions are closely associated with the scavenging
capacity of the precursor to the hydrated electron, suggesting that its reactions have a significant role in
hydrogen peroxide formation. Observed hydrogen peroxide yields at high nitrate concentrations coupled with
+
model calculations show that the molecular cation of water, H
2
O , is the dominant precursor of the oxidizing
species leading to hydrogen peroxide. Proton-transfer reactions of the water molecular cation give 79% of
the oxidizing species, whereas other reactions such as dissociative recombination reactions account for the
rest. There is a significant additional production of OH radicals in the radiolysis of selenate solutions due to
•
-
the production of O and the scavenging of low-level excited water molecules.
Introduction
the reaction mechanism. It is especially important to examine
the dependence of hydrogen peroxide formation on the molec-
ular cation of water, H2O . This species is difficult to probe
Hydrogen peroxide is the main oxidizing molecular product
found in the γ-radiolysis of water and so it is important in both
fundamental science and many applied engineering problems.
It is formed by the combination reactions of OH radicals
produced in the radiolytic decomposition of water. These
reactions occur in the initial nonhomogeneous distribution of
water decomposition products, commonly called a spur, pro-
duced by the deposition of energy by the primary γ-ray. Because
no satisfactory technique exists for measuring the time-resolved
formation of hydrogen peroxide, its temporal dependence has
been probed by varying the concentration of efficient scavengers
for OH radicals.1 On the other hand, it is known that the OH
radical reacts with other species, such as the hydrated electron,
during the decay of the spur. It has been reported that scavenging
of the hydrated electron has an effect on the yield of hydrogen
peroxide.3 In this work, the formation of hydrogen peroxide
has been examined as a function of both electron and OH radical
scavengers to explore the fast chemistry of this oxidizing species
in the γ-radiolysis of water. A special effort was made to
determine the effects due to scavenging of the precursor to the
hydrated electron.
+
directly, but its chemistry can be examined indirectly by
scavenging the precursor to the hydrated electron, which is
10-13
known to be very reactive with the water molecular cation.
In addition to fundamental interest in the role of the oxidizing
species in the radiolysis of water, hydrogen peroxide is important
in many technological applications because of its potential to
induce corrosion. Preventing oxidative damage is especially
critical in the core and the circulation lines of pressurized and
14
boiling water reactor plants. Therefore, the yields of hydrogen
peroxide and its kinetics during the initial stage of water
radiolysis are extremely useful for reactor design and water
chemistry management.
-5
In this work, the formation of hydrogen peroxide has been
examined in the γ-radiolysis of water in the presence of
scavengers for both oxidizing and reducing radicals. Special
care is taken in the selection of scavengers for the hydrated
electron and its precursor. Scavenger concentrations are varied
to probe the temporal dependence of the hydrogen peroxide
yield, especially at high scavenger concentrations (which are
equivalent to short times). A stochastic diffusion-kinetic model,
employing simulated γ-ray track structures, is used to analyze
the data to give detailed explanations of the short time chemistry
in the γ-radiolysis of water.
,6,7
The OH radical is the primary oxidizing species produced in
the radiolysis of water, whereas the hydrated electron is the main
reducing species. Recent studies on the precursor to the hydrated
electron have shown that it is responsible for much of the
molecular hydrogen formation in deaerated water, probably
through a dissociative recombination reaction with the molecular
Experimental Section
8,9
cation of water. This reaction also produces an oxidizing
species that may be an OH radical or something that is rapidly
converted to it. Proton-transfer reactions of the water molecular
cations also lead to OH radicals. Because there are two potential
sources of OH radicals, the formation of hydrogen peroxide at
very short times can have dissimilar responses to different
scavengers for oxidizing and/or reducing species depending on
Sodium nitrate (NaNO , Aldrich, reagent grade), sodium
3
selenate decahydrate (Na SeO , Aldrich, reagent grade), metha-
2
4
nol (MeOH, Aldrich, HPLC grade), and nitrous oxide (N O,
2
99%) were used as received without further purification. The
samples consisted of helium (ultrahigh purity) saturated aqueous
solutions containing MeOH as the OH radical scavenger and
either NaNO3 or Na2SeO4 as the scavenger of the hydrated
1
0.1021/jp0207578 CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/06/2002

γ-Radiolysis of Water
J. Phys. Chem. A, Vol. 106, No. 40, 2002 9353
electron or its precursor. Saturated solutions of N2O were used
as the electron scavenger in some experiments. The concentra-
-
6
tion of MeOH was varied from 1.0 × 10 to 10.0 M; the
-
6
concentration of NaNO3 was varied from 2.5 × 10 to 3.0 M;
-
4
and the concentration of Na2SeO4 was varied from 2.5 × 10
to 1.0 M. All solutions used in this study were prepared using
water from a Millipore Milli-Q UV system.
Ιrradiations were carried out using a Gammacell 220 60Co
γ-ray source at the Radiation Laboratory of the University of
Notre Dame. The dose rate was 10 Gy/min as determined using
the Fricke dosimeter.1 Samples solutions were irradiated to
total a dose of 25 to 600 Gy at room temperature. Density
corrections were applied in the determination of the dose at high
scavenger concentrations.
,15
The concentrations of hydrogen peroxide (H2O2) were
determined by the Ghormley tri-iodide method with KI, in which
Figure 1. Dependence of the absorbance on the irradiation dose with
10 M methanol and (b) 25 mM sodium nitrate or (9) 25 mM sodium
selenate. The dashed lines are the initial rates used for the calculation
of the yield of hydrogen peroxide.
-
-
16,17
-4
I
is oxidized to I3 by the hydrogen peroxide.
Sample
solutions were placed in a 1-cm quartz cell, and the absorbance
was measured at 350 nm (the wavelength of maximum
-
absorption for I3 ), using a diode array spectrophotometer
(Hewlett-Packard HP8453). Reference samples containing the
solutes at the concentrations under study and the reagent were
prepared and measured simultaneously with irradiated samples.
-
The molar extinction coefficient of I3 at 350 nm was taken as
-
1
-1 5
2
5 500 M cm . Error limits for H2O2 yields are estimated
to be about (2%, except in the case of large scavenger
concentrations and low yields, where it was (4%.
Diffusion Kinetic Modeling
Diffusion-kinetic modeling of the experimental systems was
performed using the stochastic independent reaction times (IRT)
approach, employing simulated track structures. This technique
18
has been described extensively in the literature. A track
structure was obtained by modeling the collision-to-collision
attenuation of a 1 MeV electron, and its daughters, using cross-
sections appropriate for liquid water.19 The nonhomogeneous
chemistry was then modeled from the spatial configuration of
the radiation-induced reactants. An ensemble of random reaction
times was generated using the inter-reactant separations employ-
ing the independent pairs approximation. The kinetics of the
track was obtained by repeatedly taking the minimum reaction
time of the ensemble, with a new reaction time being generated
for “reactive products” as they are formed.
Figure 2. Schematic for the decomposition of water leading to the
formation of hydrogen peroxide.
hydrogen peroxide. Conventionally, radiation chemical yields,
G values, are given in units of molecules per 100 eV of energy
absorbed in the total solution. G values are proportional to the
initial slopes of the data as presented in Figure 1. Extremely
low doses must be used in the selenate solutions to obtain initial
yields. Accordingly, the G values of hydrogen peroxide forma-
The diffusion and reaction parameters for the diffusion-kinetic
simulations were derived from the compilations of Buxton et
-
2-
tion, G(H2O2), in the γ-radiolysis of NO3 and SeO4 systems
were calculated from doses below 150 and 25 Gy, respectively.
al. Typically, 102 to 103 different track structures were
averaged to give a prediction for the kinetics of an irradiated
solution.
20
The absorption of energy by a water molecule leads to its
decomposition in a scheme shown in Figure 2, which gives the
main reactions leading to the formation of hydrogen peroxide
based on previous studies of molecular hydrogen production.21
Ionization to produce an electron and a molecular cation of water
is quickly followed by hydration of the electron and proton
transfer by the cation, or by their dissociative recombination.
This scheme proposes two very different processes leading to
the OH radical, the precursor to hydrogen peroxide. Scavenging
of the precursor to the hydrated electron should increase the
yield of OH radicals formed by the water molecular cations,
because the combination reaction is prevented. On the other
hand, scavenging the electron precursor decreases the amount
of dissociative recombination reactions giving reactive oxygen
species. The relationship between hydrogen peroxide production
and the scavenging of the precursor to the hydrated electron
will give information about the relative importance of the
pathways of water decomposition when analyzed with the aid
of model calculations.
Results and Discussion
Radiation chemistry experiments are usually performed at
doses as low as possible, to avoid the buildup of products that
may interfere with the chemistry of interest. This interference
can be readily determined by examining the formation of
products as a function of dose. Figure 1 shows that the
absorbance, which is proportional to the hydrogen peroxide
-
4
-
concentration, of irradiated 10 M MeOH and 25 mM NO3
solutions is a linear function of the irradiation dose for a wide
range of doses. Similar results were observed over the entire
range of MeOH and nitrate concentrations used here. However,
the figure also shows that 10-4 M MeOH and 25 mM SeO42-
solutions are nearly independent of dose above about 150 Gy.
Products from the reactions of electrons with selenate are
scavenging the OH radicals and preventing the formation of

9354 J. Phys. Chem. A, Vol. 106, No. 40, 2002
Hiroki et al.
TABLE 1: Yields of Hydrogen Peroxide [G(H O ) (molecules/100 eV)] in the γ-Radiolysis of Aqueous Solutions Containing
2 2
Methanol and Nitrate
NaNO
3
concentration
2.5 mM
MeOH
(M)
2.5 µM
10 µM
25 µM
100 µM
250 µM
25 mM
250 mM
1 M
2.5 M
3 M
-
-
-
-
-
-
0
6
10
10
10
10
10
10
10
10
0.17
0.13
0.06
0.07
0.06
0.03
0.03
0.01
0.30
0.27
0.25
0.21
0.20
0.14
0.08
0.01
0.49
0.50
0.50
0.49
0.47
0.40
0.25
0.05
0.57
0.58
0.57
0.58
0.56
0.49
0.35
0.03
0.68
0.69
0.68
0.66
0.63
0.58
0.37
0.08
0.75
0.73
0.75
0.75
0.73
0.64
0.43
0.09
0.79
0.78
0.79
0.81
0.80
0.72
0.47
0.11
0.78
0.76
0.80
0.80
0.80
0.75
0.56
0.12
0.65
0.63
0.68
0.67
0.67
0.64
0.52
0.20
0.57
0.56
0.59
0.58
0.58
0.56
0.49
0.17
5
4
3
2
1
0.03
0.02
0.02
0.02
0.01
0.02
0.01
1
The dominant reactions of OH radicals immediately following
their formation are with sibling radicals formed in the spur:
•
•
2k ) 1.1 × 10 M s-1 (1)
10
-1
OH + OH f H O
2
2
1
•
OH + eaq f OH-
-
k ) 3.0 × 10 M s-1 (2)
10
-1
2
All rate coefficients are from the compilation of Buxton et al.20
Hydrogen peroxide is mainly formed by the combination
reaction of OH radicals, reaction 1. Reactions of OH radicals
with H atoms are considered to be negligible because of the
low H atom yield in neutral water. The hydroperoxyl radical,
HO2, can lead to hydrogen peroxide formation, but this species
is not important in the γ-radiolysis of deaerated solutions.
Hydrogen peroxide will react with radicals to re-form water,
unless it is protected by suitable scavengers.
Figure 3. Hydrogen peroxide yields in the γ-radiolysis of water as a
-
•
-
10
-1 -1
function of the scavenging capacity for OH radical with different
H O + e f OH + OH
k ) 1.1 × 10 M
s
2
2
aq
3
-
concentrations of NO
3
: (9) 0.25 mM, (b) 2.5 mM, (2) 25.0 mM,
(3)
(
2
1) 0.25 M, ([) 1.0 M, and (+) 2.5 M, this work; (O), 2.5 mM, (4)
5.0 mM, (3) 0.25 M, and (]) 1.0 M, ref 3; (×) 25.0 mM, ref 4. The
•
•
k ) 9.0 × 10 M s-1
7
-1
H O + H f OH + H O
2
2
2
4
lines are from the model calculations.
(
4)
slower, but sufficient to compete with hydrogen peroxide at
reasonable concentrations of solute.
•
•
7
-1 -1
H O + OH f HO + H O
k ) 2.7 × 10 M
s
2
2
2
2
5
(5)
NO3- + eaq f NO
-
•
2-
k ) 9.7 × 10 M s-1 (7)
9
-1
3
7
In this work, sodium nitrate or sodium selenate was added to
the system to minimize the attack of hydrogen peroxide by the
NO3- + H f HNO
•
•
-
k ) 1.4 × 10 M s-1 (8)
6
-1
3
8
-
eaq and H atom.
Experimentally measured yields of hydrogen peroxide in the
γ-radiolysis of solutions containing nitrate and MeOH are
presented in Table 1. Nitrate concentrations above a few
millimolar are sufficient to shield hydrogen peroxide from
reactions with the reducing radicals produced in radiolysis. At
higher concentrations, the nitrate begins to scavenge the pre-
cursor to the hydrated electron, thereby influencing hydrogen
peroxide formation.
Methanol was used as an OH radical scavenger to protect
the hydrogen peroxide, but more importantly to compete with
the production of hydrogen peroxide in reaction 1.
•
•
CH OH + OH f CH OH + H O
3
2
2
k ) 9.7 × 10 M s-1 (6)
8
-1
6
With increasing MeOH concentration reaction 6 will compete
more favorably with reaction 1, resulting in a decrease in
hydrogen peroxide formation. The lifetime of the OH radical,
and therefore hydrogen peroxide production, with respect to
reaction 6 is related to the inverse of the scavenging capacity,
s (equal to the product of the scavenger concentration and the
Figure 3 shows G(H2O2) values for some of the higher
concentrations of nitrate as a function of the scavenging capacity
for the OH radical. With increasing scavenging capacity for the
OH radical, G(H2O2) decreases toward zero in all solutions.
These results suggest that the OH radical is the only precursor
to hydrogen peroxide. The present data are in good agreement
-
1
2,3
rate coefficient for the scavenging reaction, in units of s ).
Previous studies have focused on increasing OH radical
scavenger concentrations using sufficient concentrations of
scavengers for the reducing species to protect the hydrogen
with the previous results of Draganic and Draganic. However,
the present work covers a much wider range of both electron
and OH radical scavenging capacities. At low scavenging
capacity for the OH radical, G(H2O2) reaches a constant value
at between 0.6 and 0.7 molecules/100 eV. These yields are only
slightly dependent on the nitrate concentration and represent
the limiting or escape yield after the spatial relaxation of the
spur. The escape yield of hydrogen peroxide in the γ-radiolysis
of 25 mM nitrate solutions is 0.75 in this work, which agrees
4,5
peroxide formed. The present work will examine the outcome
of increasing the concentrations of scavengers for both the
reducing and oxidizing species with the goal of studying the
oxidizing precursors to hydrogen peroxide.
Nitrate Scavenger. The nitrate anion reacts very rapidly with
the hydrated electron. Its reaction with the H atom is much
4
,5
well with previous results.

γ-Radiolysis of Water
J. Phys. Chem. A, Vol. 106, No. 40, 2002 9355
2 2
TABLE 2: Yields of Hydrogen Peroxide [G(H O )
(molecules/100 eV)] in the γ-Radiolysis of Aqueous Solutions
Containing Methanol and Selenate
Na SeO
25 µM 250 µM 2.5 mM 25 mM 250 mM 1 M
2
4
MeOH
(M)
-
-
-
-
-
5
1
1
1
1
1
1
1
0
0
0
0
0
0.27
0.27
0.28
0.30
0.23
0.13
0.07
0.57
0.59
0.62
0.57
0.50
0.30
0.09
0.76
0.82
0.81
0.74
0.63
0.44
0.14
0.86
0.92
0.93
0.87
0.64
0.42
0.11
1.03
1.19
1.19
1.09
0.74
0.43
0.18
1.12
1.47
1.49
1.29
0.99
0.60
4
3
2
1
0
At high scavenging capacities for the OH radical, the G(H2O2)
values increase with increasing concentration of nitrate. For
instance, the hydrogen peroxide yield doubles from 2.5 mM to
3
M nitrate in 10 M MeOH solutions. One explanation for this
increase in hydrogen peroxide yields at such early times is that
the precursor to the hydrated electron is being scavenged. The
water molecular cation is then less likely to neutralize and more
probable to undergo a proton-transfer reaction to give OH
radicals, which will ultimately give hydrogen peroxide. How-
ever, it has also been reported that the nitrate can scavenge the
Figure 4. Yields of hydrogen peroxide in the γ-radiolysis of water as
a function of the scavenging capacity for OH radical with different
2-
concentrations of SeO
4
: (9) 2.5 mM, (b) 25.0 mM, (2) 0.25 M,
O, this
and (1) 1.0 M, this work; (O) 25.0 mM, ref 4; (4) saturated N
work. The lines are from the model calculations. Lines for 1 M SeO
are (a, solid line) normal scavenging reactions and (b, dashed line)
2
2-
4
2
-
2
2
scavenging of the lowest excited states of water by SeO
additional oxidizing species (see text).
4
to give
molecular cation of water by an electron-transfer reaction:
NO3- + H O f NO + H O
+
•
k ) ∼1 × 10 M
12
-1 -1
s
Figure 4 shows the G(H2O2) values measured as a function
of the scavenging capacity for OH radical with the higher
concentrations of selenate. The G(H2O2) in all the selenate
systems decreases toward zero with increasing scavenging
capacity for the OH radical. This result confirms that the OH
radical is the precursor to hydrogen peroxide. At very high OH
radical scavenging capacities, the yields of hydrogen peroxide
in selenate solutions are comparable to those found in solutions
with an equivalent concentration of nitrate. Both solutes have
about the same reactivity toward the precursor to the hydrated
electron, whereas selenate is a relatively poor scavenger of the
hydrated electron.
2
3
2
9
(9)
This reaction will lead to a decrease in OH radical production
at high nitrate concentrations, as discussed later.
Selenate Scavenger. Other scavengers for electrons were
desired to verify the results with nitrate and to give a different
relative scavenging of the hydrated electron and its precursor.
Unfortunately, most of the other common scavengers for the
electron interfered with the detection method used for hydrogen
peroxide. Selenate was found to be compatible with the
hydrogen peroxide analysis up to very high concentrations, and
it is a poor scavenger of the hydrated electron compared to
nitrate. The major scavenging reactions of selenate for the
hydrated electron and H atom were thought to be very similar
to the reactions of nitrate.
The G(H2O2) values of about 1.2 measured at high selenate
concentrations and low MeOH concentration are larger than
those of the nitrate system, which has a maximum G value of
0.8 molecules/100 eV. Selenate solutions should give a slightly
larger yield of OH radicals (and therefore H2O2) than the
corresponding concentrations of nitrate solutions. Selenate is a
poor scavenger of hydrated electrons compared to nitrate, but
both solutes are similar in their scavenging of the precursor to
the hydrated electron. At a given scavenging capacity for
hydrated electrons, selenate leaves fewer hydrated electrons to
react with OH radicals because it scavenges more of the
precursors to hydrated electron than does nitrate. However,
diffusion kinetic calculations show that this increase in the yield
of OH radical is not sufficient to give the relatively large yields
of hydrogen peroxide measured with selenate. In fact, the
magnitude of the increased H2O2 yield suggests that there is an
increased production of OH radicals in the radiolysis of selenate
solutions.
The dependence of hydrogen peroxide yields as a function
of the OH radical scavenging capacity is shown in Figure 5 at
the same scavenging capacity for the precursor to the hydrated
electron. If reaction 10a was dominant in the reaction mecha-
nism, a selenate solution should give a slightly smaller yield of
OH radicals (and thereby H2O2) than the corresponding nitrate
solution. The selenate and nitrate solutions allow the same
percentage conversion of the hydrated electron precursor to the
hydrated electron, but the nitrate solution then scavenges more
of the resulting hydrated electrons. This effect should leave
SeO4 - + eaq f SeO4 f SeO3- + 2OH
2
-
•
3-
•
-
k ) 1.1 × 10 M s-1 (10a)
9
-1
10
2
-
•
•
2-
4
k ) 1.1 × 10 M s-1 (11)
6
-1
SeO4 + H f HSeO
11
The yields of hydrogen peroxide measured with all the selenate
concentrations used here are given in Table 2. As observed with
nitrate, concentrations of selenate above a few millimolar are
necessary to minimize the reactions of the hydrogen peroxide
with radicals produced in the radiolysis. The escape yield of
hydrogen peroxide is 0.76 molecules/100 eV for 2.5 mM
selenate, which is in excellent agreement with the nitrate studies.
At the highest selenate concentration, the yield of hydrogen
peroxide passes through a maximum with decreasing MeOH
concentration. This decrease in hydrogen peroxide yield is
probably due to depletion of the MeOH at the doses used here,
because higher doses result in a shift of the maximum to
appropriately higher MeOH scavenging capacity. The results
presented here are for the lowest possible dose, 25 Gy, using
the present techniques. Higher order reactions such as OH
radical reaction with hydrogen peroxide occur upon depletion
of the MeOH.

9356 J. Phys. Chem. A, Vol. 106, No. 40, 2002
Hiroki et al.
Figure 5. Yields of hydrogen peroxide in the γ-radiolysis of water as
a function of the scavenging capacity for OH radicals at the same
scavenging capacity for the precursor to the hydrated electron: (b)
Figure 6. Yields of hydrogen peroxide in the γ-radiolysis of water as
a function of the scavenging capacity for OH radicals at the same
2
-
2
-
-
scavenging capacity for hydrated electrons: (b) 250 mM SeO
2
4
, (9)
-
3 2
5 mM NO , and (1) 25 mM N O, this work. The solid lines are
2
50 mM SeO
4
and (9) 250 mM NO , this work. The solid and
3
dashed lines are from the model calculation, and the dotted lines are
where the scavenging capacity for the OH radical is equal to that for
from the model calculation, and the dashed line is where the scavenging
capacity for the OH radical is equal to that for the hydrated electron.
2
-
-
3
the hydrated electron using (a) SeO
4
and (b) NO .
-
•
-
•
-
fewer hydrated electrons to react with OH radicals and so give
a higher yield of H2O2 compared to selenate. Exactly the
opposite effect is observed to be true.
N
2
O + eaq f N
2
O f N
2
+ O
k ) 9.1 × 10 M s-1 (14)
9
-1
1
4
The radiation chemistry of Se(IV)/Se(V)/Se(VI) systems has
The results of a series of experiments with solutions saturated
with nitrous oxide are included in Figure 6. The dashed line
shows where the scavenging capacity for OH radicals is
equivalent in value to the scavenging capacity for the hydrated
electron. At high MeOH concentrations (to the right of the
dashed line in Figure 6) where the scavenging of OH radical
occurs before the hydrated electron, G(H2O2) from saturated
N2O solution is the same as that measured from selenate and
from nitrate solutions with the same scavenging capacity for
hydrated electrons. This agreement shows that the precursor to
H2O2, i.e., OH radical, and its yield are the same in the three
systems. At low MeOH concentrations, the yields measured in
the solutions of the three electron scavengers differ. The selenate
solution gives the highest G(H2O2) and the nitrate solution the
lowest. Nitrate is a good scavenger of the hydrated electron
and of its precursor, selenate is a relatively poor scavenger of
the hydrated electron and a very efficient scavenger of the
precursor to the hydrated electron, and nitrous oxide is a good
scavenger of the hydrated electron and a relatively poor
scavenger of the precursor to the hydrated electron.
23-26
received only limited attention.
Reaction 10a has been
suggested as the dominant reducing reaction of the hydrated
2-
electron on SeO4 ; however, the following equilibria involving
the HSeO42 radical anion are documented:
-
25
•HSeO
2-
2-
2-
/ SeO32- + OH
•
5 -1
k12a ) 7.3 × 10 s
4
9
-1 -1
k-12a ) 3.5 × 10 M
s
(12a)
•HSeO
•
-
-
8
-1
/ SeO + OH
k12b ) 6.4 × 10 s
4
3
8
-1 -1
k-12b ) 5 × 10 M
s
(
12b)
•HSeO + OH / SeO3 + O + H O
-
2-
•
-
4
2
k ) 3.9 × 10 M s-1
5
-1
13
7
-1 -1
k-13 ) 1.1 × 10 M
s
(
13)
NO3- + epre- f NO
•
2-
k ) 1 × 10 M s-1 (15)
13
-1
Figure 6 shows G(H2O2) for equivalent nitrate and selenate
scavenging capacities for the hydrated electron as a function of
the scavenging capacity of the OH radical. The clear implication
of the experimental data is that some of the scavenging reactions
3
15
2
-
-
•
3-
k ) 1 × 10 M s-1
13
-1
SeO4 + epre f SeO
4
16
(16)
2-
of the hydrated electron, and its precursor, by SeO4 result in
-
•
-
k17 < 10 M s-1 (17)
12
-1
•
-
•
the production of O or OH by reaction 10b.
N O + e
f N O
2
pre
2
SeO4 + eaq (or epre-) f SeO4 f SeO32- + O
2
-
-
•
3-
•
-
The rate coefficients for the reactions of the precursor to the
hydrated electron have been derived from experimental C37
data.
(10b)
1
1,27,28
In the absence of MeOH as an OH radical scavenger, this
reaction would still ultimately result in the production of the
Precursors to OH Radicals. The production of OH radicals,
the precursor to hydrogen peroxide, is clearly dependent on the
scavenging of the electrons produced in radiolysis. A series of
diffusion-kinetic calculations were used to investigate the
reaction mechanism at high nitrate concentrations. The results
of calculations including both reactions 8 and 9 are shown in
•
-
SeO3 radical by the equilibria (12).
•
-
•
The production of O (or OH) in the scavenging of hydrated
electrons by selenate according to reaction 10b is equivalent to
the scavenging of hydrated electrons by nitrous oxide.

γ-Radiolysis of Water
J. Phys. Chem. A, Vol. 106, No. 40, 2002 9357
Figure 3. The agreement with the experimental data is excellent;
however, when reaction 9 is ignored, there is an overestimate
of the yield of H2O2 at nitrate concentrations greater than 0.25
M. Clearly, the reaction of nitrate with the water molecular
cation, reaction 9, at high nitrate concentrations is of importance.
For instance, the simulations suggest that about 31% of the
molecular cations of water are scavenged in 2.5 M nitrate
solution compared to only 12% in 1.0 M solution and 3% in
0
.25 M solution.
Diffusion-kinetic calculations of the radiation chemistry of
aqueous solutions of selenate and MeOH were made using either
reaction 10a or reaction 10b in the mechanism. When reaction
10a is used in the calculations, the predictions significantly
underestimate the experimental yields of H2O2. Figures 4-6
show excellent agreement for selenate concentrations up to 0.25
M between experimental yields and the predictions of calcula-
tions, in which the hydrated electron and its precursor were both
scavenged by selenate according to reaction 10b. The fact that
electron scavenging by selenate occurs via reaction 10b is not
surprising. It is well-known that the irradiation of K2SO4 crystals
Figure 7. Yields of hydrogen peroxide in the γ-radiolysis of water as
a function of the scavenging capacity for hydrated electrons in 1 mM
-
-
methanol solutions: (b) NO , this work; (O) NO , ref 4. The solid
3
3
•
-
•
- 29
produces O and not SO3 . In addition, perbromate reacts
with the hydrated electron,
line is from the model calculation. The dashed line is where the
scavenging capacity for the precursor to the hydrated electron is
equivalent to its solvation lifetime, and the dotted line is where the
scavenging capacity of the water molecular cation is equivalent to the
inverse lifetime of the protonation reaction.
BrO4- + eaq f BrO - + O
-
•
-
10
-1 -1
k ) 2.4 × 10 M
s
3
18
(18)
to give reactive oxygen species.30
At ∼0.3 M, the scavenging capacity of the solution for the
precursor to the hydrated electron is the same as the pseudo-
first-order rate coefficient for its solvation. The scavenging of
the precursor to the hydrated electron results in a decrease in
the number of dissociative recombination reactions, which yield
H2 and two OH radicals. It does, however, allow the sibling
molecular cation to undergo proton transfer to give an OH
radical. The net result is a small reduction in the number of
OH radical equivalents, which causes the yield of hydrogen
peroxide to plateau. At a nitrate concentration of about 1.3 M,
the pseudo-first-order rate coefficient for the molecular cation
reaction with nitrate is about the same as the pseudo-first-order
The yield of hydrogen peroxide is underestimated in selenate
solutions of concentration greater than 0.25 M, implying that
either the molecular cation of water is not scavenged by selenate,
or that scavenging of excited states may also result in the
production of an OH radical or its precursor.
SeO4 - + H O f SeO + H O
2
+
•
-
(19)
(20)
2
4
2
SeO4 - + H O* f SeO + H O
2
•
3-
+
2
4
2
When diffusion kinetic calculations are performed ignoring the
22
possibility of the electron transfer from SeO42- to H2O+, the
rate coefficient for proton transfer to the solvent. In this regime,
+
the H2O is efficiently scavenged by the nitrate. This scavenging
significant increase in H2O2 yield observed experimentally is
not reproduced. In fact, there is good reason to expect the
electron-transfer reaction does occur. Kim and Hamill have
decimates the production of OH radicals and consequently
significantly reduces the production of hydrogen peroxide.
Model calculations predict that 79% of the precursors to
hydrogen peroxide, i.e., OH radicals, are produced by the proton-
transfer reactions of the molecular cation of water, whereas other
reactions such as the dissociative recombination reaction account
for the rest.
shown that SO42 is a good donor to H2O , even though it does
-
+
31
not react effectively with the OH radical. If an excited-state
charge-transfer reaction is included in the calculations, then the
experimental yields are reproduced, see curve b in Figure 4.
This mechanism is clearly feasible. The transfer of energy from
excited states of water to anionic solutes has been studied in
depth.32 Furthermore, Bartels and Crowell have suggested that
low-energy photoionization of water takes place via a dissocia-
Conclusions
The radiation chemical yields of hydrogen peroxide formed
in the γ-radiolysis of water with scavengers for oxidizing and/
or reducing radicals were determined. Hydrogen peroxide yields
were found to decrease toward zero with increasing concentra-
tion of OH radical scavenger in all solutions, suggesting that
the OH radical is the sole precursor to hydrogen peroxide. The
scavenging of the precursor to the hydrated electron can have
a significant role in hydrogen peroxide formation, probably
because one of its main reactions is with the molecular cation
3
3
tive proton-coupled electron transfer to a preexisting trap and
the scavenging ability of the selenate anion for the precursor to
the hydrated electron implies that it would be a suitable trap.
The effect of nitrate concentration on the radiation-induced
yield of hydrogen peroxide from 1 mM MeOH solution is shown
in Figure 7. As the nitrate concentration increases, the yield of
hydrogen peroxide first increases and then decreases at high
nitrate concentrations. The plateau region of the hydrogen
peroxide occurs between about 0.3 and 1.3 M nitrate. The initial
increase in the yield of hydrogen peroxide with increasing nitrate
concentration is due to the scavenging of hydrated electrons by
nitrate. Removal of the hydrated electrons reduces the amount
+
of water, H2O . Hydrogen peroxide yields at high nitrate
concentrations coupled with model calculations show that the
molecular cation of water is the dominant precursor of the
oxidizing species leading to hydrogen peroxide. Proton-transfer
reactions of the water molecular cation give 79% of the
oxidizing species, and other reactions such as dissociative
recombination reactions account for the rest. There is a
-
of (eaq + OH) reaction and leaves more OH radicals within
the spur and available to produce hydrogen peroxide. Nitrate is
an effective scavenger of the precursor to the hydrated electron.

9358 J. Phys. Chem. A, Vol. 106, No. 40, 2002
Hiroki et al.
significant additional production of OH radicals in the radiolysis
of selenate solutions due to the production of O and the
scavenging of low-level excited water molecules.
(13) Jonah, C. D.; Miller, J. R.; Matheson, M. S. J. Phys. Chem. 1977,
•
-
81, 1618.
14) Cowan, R. L. Water Chemistry of Nuclear Reactor System 7; British
Nuclear Energy Society: London, 1996.
15) Allen, A. O. The Radiation Chemistry of Water and Aqueous
Solutions; Van Nostrand-Reinhold: Princeton, NJ, 1961.
(
(
Acknowledgment. The work described herein was supported
by Grant DE-FG03-99SF21923 of the Nuclear Energy Research
Initiative Program of the U. S. Department of Energy. This
contribution is NDRL-4369 from the Notre Dame Radiation
Laboratory, which is supported by the Office of Basic Energy
Sciences of the U.S. Department of Energy.
(
(
(
(
16) Hochanadel, C. J. J. Phys. Chem. 1952, 56, 587.
17) Ghormley, J. A.; Stewart, A. C. J. Am. Chem. Soc. 1956, 78, 2934.
18) Pimblott, S. M.; LaVerne, J. A. J. Phys. Chem. A 1997, 101, 5828.
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996, 100, 8595.
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(
(
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