Radiolytic yield of UIV oxidation into UV1: A new mechanism for Uv reactivity in acidic solution

Article abstract of DOI:10.1021/jp9051177

The yields of the radiolytic oxidation of U^IV and of the U ^VI formation, measured by spectrophotometry, are found to be the same (G(-U^IV)_N2o - G(U^VI)N20 = 8.4 x 10 ^-7 mol J^-1) and almost double the H₂ formation yield (G(H₂) = 4.4 x 10^-7 mol J^-1) in the ⁶⁰Co y radiolysis of N₂O-aqueous solutions in the presence of 2 mol L^-1 Cl^- at pH - 0 (HCl). According to the mechanism of U^IV radiolytic oxidation, we show that under the conditions of our experiments the Uv ions do not disproportionate, but undergo a stoichiometric oxidation into U^VI by H^- with forming H₂.

Full text of DOI:10.1021/jp9051177

2
080
J. Phys. Chem. A 2010, 114, 2080–2085
IV
VI
V
Radiolytic Yield of U Oxidation into U : A New Mechanism for U Reactivity in Acidic
Solution
E. Atinault,†,‡ V. De Waele, J. Belloni, C. Le Naour, M. Fattahi, and M. Mostafavi*
†
†
§
‡
,†
Laboratoire de Chimie Physique/ELYSE, CNRS-UniVersit e´ Paris-Sud, B aˆ t. 349, 91405 Orsay, France,
SUBATECH, Ecole des Mines de Nantes, UniVersit e´ de Nantes, 44307 Nantes, France, and Institut de Physique
Nucl e´ aire, CNRS-UniVersit e´ Paris-Sud, B aˆ t. 100, 91406 Orsay, France
ReceiVed: June 1, 2009; ReVised Manuscript ReceiVed: December 9, 2009
The yields of the radiolytic oxidation of U and of the UVI formation, measured by spectrophotometry, are
IV
IV
VI
-7
-1
found to be the same (G(-U )N2O ) G(U )N2O ) 8.4 × 10 mol J ) and almost double the H
2
formation
-
7
-1
60
yield (G(H
2
) ) 4.4 × 10 mol J ) in the Co γ radiolysis of N
2
O-aqueous solutions in the presence of 2
mol L- Cl at pH ) 0 (HCl). According to the mechanism of U radiolytic oxidation, we show that under
1
-
IV
V
the conditions of our experiments the U ions do not disproportionate, but undergo a stoichiometric oxidation
VI
+
2
into U by H with forming H .
V
+
+
V
2+
Introduction
U O + H f U O H
(2)
(3)
2
2
Aqueous solutions of uranium are widely used at different
steps in nuclear power engineering. Uranium is present under
different redox states, and for each of them various complex-
ations are possible. Because the knowledge on redox and
solubility properties of each state constitutes a key point for
the treatment and the storage of uranium, the chemistry of the
different valence states of uranium has been extensively studied
under various conditions. As uranium is radioactive and usually
coexists with other radioactive elements, the effect of irradiation
on the uranium systems was also considered. In particular, the
V
2+
V
+
2
U O H + U O f products
2
V
+
or the formation of a dimer of U reacting with H :
V
V
1
2U f [U ]
2
(4)
(5)
V
+
[
U ] + H f products
2
radiolysis of aqueous solutions of uranium ions and complexes
and also of solid UO
oxidation of U has been extensively considered under acidic
2
2
was widely studied. Specially, the
IV
V
The decay of U (initial concentration at a few millimolar,
HClO acidic medium) was found to obey a second-order law
4
3
–6
IV
conditions (H
2
SO
4
or HCl). The product of oxidation of U
VI
2
-1 -1
is U , which is stable and very well characterized. It is
with a rate constant of (1 to 4) × 10 L mol s , depending
V
+
V
considered that U is a transient valence state, which is unstable
on the [H ] conditions. The lifetime of millimolar U was found
to be in the range of seconds.
7
4-
in solution. Photochemical reduction of UO
2
(CO
)
3 3
in deoxy-
genated aqueous carbonate solutions at pH g 11.2 containing
ethanol or sodium formate showed that the molecular formula
of the uranium(V) complex under basic condition is
The fundamental understanding of the mechanisms of these
reactions remains obscure.10 More recently, a theoretical study
+
reported that a binuclear complex is formed between two UO
2
,
2
- 8
[
(UO
2
)
4
(OH)
6
(H
2
O
9
] . However, because of its short lifetime
and that this binuclear complex can accept two protons by
4+
and its very low absorbency, the knowledge on this transient
2 2
forming [(UO H) ] . According to the authors, this binuclear
V
IV
VI
valence state U is much less rich than on U or U . Although
there is considerable discrepancy in the value of the reported
complex undergoes an inner-sphere disproportionation reac-
tion.11 Recently, we measured the radiolytic yield of U
IV
V
12
IV
rate constants, there is a general agreement that U undergoes
oxidation in O
2
-saturated solutions. The oxidation yield of U
-
7
-1
60
in these acidic solutions a disproportionation reaction:
was found to be 8.7 × 10 mol J in the Co γ radiolysis of
-
3
-1
IV
aqueous solutions containing 4.4 × 10 mol L
U
in the
-
1
-
V
IV
VI
presence of 2 mol L Cl at pH ) 0 (HCl). We presented a
mechanism to explain the yield of oxidation of U , and we
2
U f U + U
(1)
IV
have shown that this rather high value suggested that all primary
radicals formed by water radiolysis were used to eventually
oxidize U in these solutions.
The kinetics and mechanism of the disproportionation of UV
(
initially at millimolar concentration) have been investigated
IV
9
more in detail by Ekstrom using the stop-flow method. He
reported two other different second-order mechanisms catalyzed
by protons involving either the formation of a complex between
However, in O
2
-saturated solution, it was difficult to draw a
strict conclusion on the yields and mechanism of oxidation of
IV
VI
·
U
into U , because under these conditions the radical HO
2
+
+
H and UO
2
:
IV
·
is formed and the mechanism of oxidation of U by HO
complex. Generally, the reactivity of HO
2
is
·
2
toward the metal
*
Corresponding author. E-mail: mehran.mostafavi@lcp.u-psud.fr.
Laboratoire de Chimie Physique/ELYSE, CNRS-Universit e´ Paris-Sud.
Universit e´ de Nantes.
ions needs a deep study because it may react through ligandation
as an oxidizing or a reducing species. To avoid this difficulty,
we study in the present work the results obtained in N O-
2
†
‡
§
Institut de Physique Nucl e´ aire, CNRS-Universit e´ Paris-Sud.
1
0.1021/jp9051177  2010 American Chemical Society
Published on Web 01/15/2010

Radiolytic Yield of UIV Oxidation into UVI
saturated solutions. In fact, under N
O, the radical HO ·
J. Phys. Chem. A, Vol. 114, No. 5, 2010 2081
2
2
is not
formed. Under this condition, we compare the yield of radiolytic
To analyze the gases produced during irradiation, another
IV
series of samples were studied. The U solutions were irradiated
IV
VI
U
oxidation with the yields of radiolytic formation of U and
in another specific polypropylene cell equipped with a valve,
after being filled with nitrous oxide at a pressure equal to 1.5
( 0.15 bar. After each irradiation, the molecular hydrogen and
other gases were measured using an original connection device
between the irradiated sample cell and the mass spectrometer
gases. On the basis of the quantitative results obtained on these
yields, we revisit the mechanism of the oxidation process and
V
discuss also the occurrence of the U disproportionation reaction.
Experimental Section
(
Minilab Residual Gas Analyzer from MKS). Before calibration
The solutions were prepared using ultrapure water from a
Millipore system (18 MΩ cm) and reagent grade chemicals used
without further purification: HCl, 37%, from Normapur; NaCl,
and gas analysis, a cleaning of the setup is performed with pure
argon to remove any trace of atmospheric gases. The calibration
of the gas analysis involves two steps. During the first one, the
signals of the relative gas pressures are calibrated in atmospheric
air by using a Faraday detector and assuming 1% for the relative
pressure of argon. In the second step, high-purity argon is
injected to determine the different proportions of the isotopes:
ACS reagent, with g99.8% purity from Sigma-Aldrich; N
2
and
N
2
O with 99.99% purity from Air Liquide. To study the
IV
radiolytic oxidation of an aqueous solution of U , it is first
necessary to synthesize a pure solution of this oxidation state
of uranium. For that purpose, an aqueous solution of hexavalent
40
36
38
Ar, Ar, and Ar. Three electron-multipliers are used for
uranyl nitrate, UO
uranium metal oxidation with nitric acid, is reduced under N
by applying a constant electric intensity between two electrodes.
2
(NO
3
)
2
(H
2
O)
6
, which has been obtained by
measuring separately the three argon isotopes. For the sample
gas analysis, argon was used as the carrier gas with a flow rate
2
-
1
of about 5 mL min
. Two measurements were performed for
-3
-1
Solutions containing 100 mL of 5 × 10 mol L uranium
each sample. The analysis uncertainty is 10%, and the detection
threshold is 20 ppm.
The results obtained are expressed as ratios between partial
salt are electrolyzed using a potentiostat (Radiometer Voltalab
2
1) and applying a current of -1 mA during 27 h. The
VI
completion of the process, that is, total reduction of U into
U , is checked by using spectrophotometrical monitoring.
2
and N O detected. They permit one to calculate
pressures of H
the amount of gas molecules from the partial pressure, and from
2O pressure
2
IV
The pH of the medium, in which is dissolved the uranyl salt,
the solution and dead volumes of samples. The N
is in large excess and does not change notably during irradiation
variation less than 5% within 3 kGy).
-
1
-1
is adjusted to 0 with 1 mol L HCl and 1 mol L NaCl to
IV
12
stabilize U under atmosphere containing O
2
.
Actually, the
(
nitrate anions coming from the uranium salt have disappeared
VI
III
to form N
2
during the coulometric reduction of U into U ,
Results
Measurements of the Molar Extinction Coefficients of U
and U . The radiolytic oxidation of U into U is measured
IV
which is then oxidized into U as explained by Kolthoff et
1
3,14
IV
al.
To minimize the possible reactions under irradiation, no
VI
IV
VI
other molecule is added, for example, as stabilizer. The complex
IV
VI
forms of U and U are not clearly established under our
by observing the dose-dependent absorption spectra. As shown
VI
IV
conditions. It was reported that U in acetonitrile is complexed
by 6 Cl forming UCl
reported that U and U in the presence of 3 mol L Cl are
under the forms of U(H
in Figure 1a, U in NaCl/HCl is characterized by four peaks at
-
2- 15,16
6
,
but a recent EXAFS study in water
430, 495, 549, and 648 nm. As the uranium ions are highly
sensitive to complexation, the extinction coefficients of these
spectra should be accurately calibrated under the used conditions
IV
VI
-1
-
3
+
+
2 8 2 2 4
O) Cl and UO (H O) Cl , respec-
tively.17
of high Cl concentration. For each absorption peak, the values
-
IV
For dose-dependent absorbance measurements, when the
of the molar extinction coefficients of U in this medium were
IV
IV
coulometric preparation of U is over, the U solution (5 mM
determined. Experiments were done by using a set of solutions
at pH ) 0 and 2 mol L- Cl ) is inserted in a special cell divided
into two parts: the first one is a reservoir in which the sample
is stored during the irradiation, and the second one with a rubber
plastic septum is a spectrophotometric suprasil cell (1 cm). Both
parts are connected by a bridge in quartz, which allows the
system to be deaerated before irradiation and filled with the
1
-
of U at various concentrations in 1 mol L NaCl and 1 mol
L
IV
-1
-1
HCl. In Figure 1b, the absorbance of these solutions at each
selected wavelength is reported versus the concentration. The
absorbance is strictly proportional to the concentration in the
-1
range 0.5-10 mmol L (Figure 1b and inset). The molar optical
absorptivity was measured at different wavelengths correspond-
ing to the four maxima. The values are equal to (16 ( 1) L
selected atmosphere, the nitrous oxide (N
The concentration of N
O in pure water is 2.4 × 10 mol L ,
but at high ionic strength, such as in the solutions prepared in
2
O) in the present work.
-2
-1
-1
-1
-1
-1
2
mol cm at 430 nm, (29 ( 1) L mol cm at 495 nm, (21
1
-
-1
-1
-1
( 1) L mol cm at 549 nm, and (62 ( 1) L mol cm at
648 nm. During the irradiation, the concentration of U was
deduced from the absorbance at 648 nm where U does not
-
2
-1 18
IV
this work, it is only 1.5 × 10 mol L .
The γ-irradiation was performed using the panoramic 60Co
source at Chimie Physique laboratory, Orsay, with an activity
of 2400 Ci. The dose rate depends on the distance of the sample
to the 60Co source. In this method, the energy is absorbed by
the solvent where radiolytic oxidizing radicals are generated
with a controlled dose rate. The dose rate was measured using
VI
absorb. It is interesting to compare the corresponding extinction
coefficient to other ones, found in the literature. The value of
-1
-1
the extinction coefficient (62 L mol cm at 648 nm) deduced
-
1
-1
from Figure 1 is higher than the value 53 L mol cm as
obtained, for example, by spectrophotometry in the medium 1
mol L HBr aqueous solution. The difference is explained
by the specific complexation, because in the present solution
3
+
-1
19
the Fricke dosimeter with a radiolytic yield G(Fe ) equal to
-7
-1
3+
1
6.2 × 10 mol J and a molar extinction coefficient of Fe
-1 -1
-
3+
-
at 304 nm of 2160 L mol cm . The dose rate was corrected
2 8
the ions are complexed by Cl (U(H O) Cl ) instead of Br
IV
3+
17,20
by taking into account the density of the U solutions, which
is 1.04 kg L . The dose rate was equal to 45 ( 2 Gy min .
After each irradiation dose absorbed by the U solution, an
(UBr ) in the reported reference.
The intensity of the
-1
-1
VI
absorption band of U is known to be very low. Because of
the difficulty to estimate accurately the extinction coefficient
of U in such a low-concentration uranium solution, it was
IV
VI
absorption spectrum of the sample was registered. On the basis
IV
VI
IV
of the calibration of the absorbencies of U and U , the dose-
dependent concentrations of the ions are obtained.
evaluated by using the isosbestic point at 423 nm between U
VI
and U spectra (see below).

2
082 J. Phys. Chem. A, Vol. 114, No. 5, 2010
Atinault et al.
Figure 1. Absorption spectra of aqueous solutions containing 1 mol L of NaCl and 1 mol L-1 of HCl with (a) 4.3 × 10 mol L-1 of U . Graph
-
1
-3
IV
(
b) represents the measurements of the molar absorptivity of UIV at different wavelengths. The correspondence with the symbols is as follows: (9)
-
1
-1
4
1
30 nm, (b) 495 nm, (2) 549 nm, and ([) 648 nm. The results for the different molar extinction coefficients are (16 ( 1) L mol cm , (29 (
-1 -1 -1 -1 -1 -1
) L mol cm , (21 ( 1) L mol cm , and (62 ( 1) L mol cm , respectively. Inset of (b): Linear variation of the absorbance at low
concentrations.
2
Figure 2. Absorption spectrum of N O-saturated aqueous solutions
-
3
-1
IV
-1
-1
(
4.3 × 10 mol L of U + 1 mol L NaCl + 1 mol L HCl)
60
during the Co γ-irradiation. The arrows show the decrease of the peaks
at 430, 495, 549, and 648 nm and the increase at 420 nm, with
increasing irradiation dose (0-5.5 kGy).
Figure 3. Concentration of UIV (closed symbols) and UVI (open
symbols) as a function of irradiation dose obtained for N O-saturated
aqueous solution (9) and O -saturated aqueous solution (b). Average
values of three series of experimental data.
2
2
IV
Determination of the Radiolytic Oxidation Yield G(-U )
VI
2
and of the Formation Yields G(U ) and G(H ). The disap-
pearance of U and the formation of U were followed using
the absorption spectrum scanned after each irradiation of N O-
IV
VI
VI
observation of the isosbestic points, the U concentration is
IV
derived from the difference with initial [U ]. Figure 3 includes
2
saturated solutions (Figure 2). With increasing irradiation dose,
the intensities of the four peaks at 430, 495, 549, and 648 nm,
corresponding to U , decrease, whereas the intensity of UV
the average values of three different series of experiments
performed with initial concentrations of U at around 5 mM.
IV
IV
The concentration follows a linear variation versus the irradiation
IV
band centered at 416 nm increases, characterizing the formation
dose up to the exhaustion of U and complete formation of
VI
VI
of U . It is worth noting that two isosbestic points independent
U . The total irradiation time to achieve the oxidation of all
IV
of the irradiation dose are present at 423 and 448 nm, so
U
ions is around 3 h. The radiolytic yields of disappearance
IV
VI
IV
IV
VI
VI
indicating a stoichiometric oxidation of each U into U during
the radiolysis and excluding the presence of other valence states
as stable products on the time scale of analysis (Figure 2). In
of U , G(-U ), and of formation of U , G(U ), which are
equal, were obtained by linear fitting of the experimental data.
IV
VI
In N
2
O-saturated solutions, G(-U )N2O ) G(U )N2O ) 8.4 ×
V
-7
-1
particular, the concentration of transient U is negligible. The
10 mol J . It is worth noting that we checked that no
postirradiation process occurred. Therefore, at the end of
IV
VI
extinction coefficient found at 423 nm of U or U is 8 L
-
1
-1
mol cm , and it is close to that reported in the literature. We
irradiation the system is in equilibrium. In addition, the H
2
evolution for each irradiation dose was measured (Figure 4).
VI
deduce that the extinction coefficient of U at the maximum
4
16 nm is 10 ( 1 L mol- cm . According to literature data,
1
-1
Unexpectedly, as compared to water radiolysis, we found large
VI
IV
the extinction coefficient of perchloric acidic U solutions at
2
H volumes under irradiation. Like the concentrations of U
4
16 nm is also 10 L mol- cm .
1
-1
or U , the pressure of H
VI
2
versus irradiation dose presents also
is G(H N2O ) (4.4 ( 0.4)
IV
The concentration of U is calculated for each irradiation
a linear evolution. The yield of H
× 10 mol J (Figure 4).
2
2
)
-
7
-1
dose from the absorption spectra at 648 nm. Because of the

Radiolytic Yield of UIV Oxidation into UVI
J. Phys. Chem. A, Vol. 114, No. 5, 2010 2083
·
Next, H radicals produced by the water radiolysis and by
reaction 7 are scavenged by nitrous oxide at the saturated
-2
-1
· 18
concentration of 1.5 × 10 mol L and form the radical OH .
H + N O f N + HO 2.1 × 10 mol L s-1 22 (8)
.
.
6
-1
2
2
According to the low rate constant of reaction 8 between H·
-
and N
2
O, the scavenging half-time is about 30 µs. As Cl is
-
1
·
present in solutions at 2 mol L , the radicals OH react
-
·-
predominantly with Cl to form ClOH .
·
-
·-
9
-1
-1
HO + Cl f ClOH
4.3 × 10 mol L s
(9)
+
·-
·-
Next, H reacts with ClOH and produces Cl
species via the following reactions:23
2
as transient
Figure 4. Average of two series of measurements of the concentration
of H
2 2
versus irradiation dose obtained for N O-saturated aqueous
solution. Conditions of Figure 2.
ClOH + H f Cl + H O 2.1 × 10 mol L s-1
·
-
+
·
10
-1
2
(
10)
(11)
Nevertheless, in the spurs a very small quantity of hydrogen
·
-
·-
2
10
-1
-1
Cl + Cl f Cl
2.2 × 10 mol L s
peroxide H
2
2
O can also be formed in competition with reaction
9
:
·
·
9
-1
-1
OH + OH f H O 4.2 × 10 mol L s
(12)
2
2
Consequently, in the N
of microseconds, the oxidizing species are Cl
2
O-saturated solutions, after a few tens
·
-
2
and at a much
is partially replaced by
·
-
2 2 2
lower extent H O . The transient Cl
-
3
Cl :
Cl2 · f Cl + Cl 2 × 10 mol L s
-
-
-
10
-1
-1
(13)
2
3
-
can also oxidize UIV with 2 equivalents.
Cl
According to the oxidation mechanism, the Cl
3
·
-
2
species
IV
3
-1 -1 6
reacts with U with a rate constant of 8.3 × 10 L mol s :
Figure 5. Schematic process during the radiolysis of UIV under the
-
2
·
IV
V
-
same conditions as in Figure 2. The uranium ions are complexed by
Cl + U f U + 2Cl
(14)
-
Cl .
The Cl ·
-
radical is also able to oxidize U with a higher rate
V
2
9
-1 -1 24
Discussion
constant of 6.5 × 10 L mol s .
Under γ-irradiation, water molecules are either excited or
-
·
V
VI
-
ionized to produce at very short times the primary radiolytic
Cl + U f U + 2Cl
(15)
and
2
-
·
products, that is, hydrated electron e
s
, hydroxyl radical OH ,
·
+
hydrogen radical H , proton H , and very small amounts of
2
+
Photochemical studies of solutions containing UO
inorganic anions showed indeed that halide radical anions such
oxidize U to U .
much lower concentration of U as compared to the U
2
molecular products H
2
2 2
and H O .
-
-
V
VI 25,26
as Br
2
and I
2
However, because of the
V
IV
·
-
s
·
+
H O vv f OH , e , H , H , H O , H
(6)
2
2
2
2
concentration during the γ-radiolysis, this oxidation pathway
(
15), in competition with reaction 14, is negligible. During the
At pH ) 0, the hydrated electron reacts with H to form H·
+
radiolysis process, H O can be formed through reaction 12.
The products of the slow reaction between H
U and the radical OH :
2
2
IV
O
2
and U are
atoms within 100 ps (Figure 5):
2
V
·
-
s
+
·
10
-1
-1 21
e + H f H 1.3 × 10 mol L s
(7)
IV
V
·
U + H O f U + OH
(16)
2
2

2
084 J. Phys. Chem. A, Vol. 114, No. 5, 2010
Atinault et al.
-
+
The latter is scavenged by Cl and H of the solution to produce
high as compared to the yield expected in the radiolysis of
·
-
IV
·
the radical Cl
2
, which can oxidize a second U ion, or OH
may, in a second step, oxidize U and form U directly.
Consequently, one molecule of hydrogen peroxide can oxidize
two U ions into two U or more probably oxidize directly
one U ion into U . However, this contribution is negligible
because in our case the solutions contain 2 M Cl , and almost
all OH radicals are scavenged by Cl at around 100 ps and
(reaction 10, solid line in Figure 5). Hence,
a very low amount of OH contributes to form H
line in Figure 5).
At this point, it is important to know the values of the
radiolytic yields of secondary oxidizing species under the
conditions of the experiments and to express them versus the
radiolytic yields of the primary radicals arising from the water
, H , and OH . In fact, Cl
the OH scavenging issued from reaction 6 or indirectly from
reactions 7 and 8. Recent picosecond pulse radiolysis studies
2
aqueous solutions (the primary yield of H formation in water
V
VI
-7
-1
is only 0.45 × 10 mol J ), but the authors did not explain
2,4
the origin of this high H evolution. It is also worthy to note
2
IV
V
V
that in studies where the disproportionation of U was suggested,
a possible formation of H was not checked.
IV
VI
9
2
-
Under the present conditions, all of the uranium ions are
·
-
-
expected to be complexed by Cl anions, as was also suggested
·
-
replaced by Cl
2
IV
at least by the optical absorption spectrum of U . From literature
·
O
2
(dotted
2+
+
2
data, the reduction potential of UO
/UO
is +0.1 VNHE, which
2
2
+
1
is close to that of H /1/2H
2
. However, the complexation process
has an influence on the reduction potential of the ions, which
depends on the ligand nature. The reduction value of the uranium
-
ions complexed by Cl is, however, unknown.
As explained above, the oxidation of UIV results from an
efficient scavenging of primary radicals. However, quantitatively
-
·
·
·-
radiolysis, that is, e
s
2
is issued from
·
VI
the oxidation results correspond to the formation of U with a
VI
IV
-7
-1
yield of G(U ) ) G(-U ) ) 8.4 × 10 mol J , which implies
·
-7
-1 21
showed that G100ps(OH ) is (4.4 ( 0.2) × 10 mol J . At
IV
-7
an oxidation yield of G(Ox)exp ) 2G(-U ) ) 16.8 × 10 equiv
·
pH 0, all hydrated electrons are replaced by H atoms and after
-
1
J , being almost twice as high as G(Ox)total of the radiolysis in
·
-
-
reactions 7-11 by Cl
2 s
. G(e ) has been measured at pico-
VI
the N
2
O system. Actually, if U were formed exclusively by
second range by Muroya et al. in 2005, and it was found to be
the scavenging of the oxidizing species issued from the
-7
-1 27
(
4.2 ( 0.2) × 10 mol J . The simulations showed that at
-
radiolysis of (N
2
O and Cl ) water solutions, followed by the
·
picosecond range the yields of H
2
O
2
and H are very low
V
VI
-1
disproportionation of the transient U state, the yields G(-U )
-7
·
-7
-1 10
(
G(H
2
O
2
) ) 0.2 × 10 and G(H ) ) 0.4 × 10 mol J ). In
VI
-7
)
G(U ) would be only G(Ox)total/2 ) 4.5 × 10 mol J ,
·
the present experiments, HO
2
is not formed in N
2
O-saturated
and no molecular hydrogen would be found.
·
-
solutions; the single oxidizing species present is finally Cl
or Cl ). Consequently, we can consider that, at most, the total
3
radiolytic yield of oxidizing species should be equal to G(Ox)total
2
Let us consider other possible reactions of H· radicals
-
(
·
producing H
2
, although H atoms are in principle scavenged by
.
N
2
O.
IV
(
i) With the hypothesis of a possible reduction of U into
by H (the rate constant of reaction is known to be lower
-
2
·
-
s
·
G(Cl ) ) G(e ) + G(H ) +
III
·
U
·
-7
-1
6
-1 -1
than 10 L mol s ), the overall uranium oxidation and the
yields would be lower than half of the experimental data.
G(OH ) ≈ G(Ox)
) 9 × 10 mol J
(17)
total
H
2
·
VI
V
(
ii) If H were scavenged by U and U disproportionate,
IV
The earliest work on the radiolytic oxidation of U in acidic
media was published by Ha ¨ı ssinsky et al. They found that, in
sulfuric acid solutions, G(-U ) depends on the dose rate. At
high dose rate, close to that used in the present work (45 Gy
-saturated solutions that G(-U ) )
.4 × 10 mol J , and we found recently a close value in
hydrochloric O -saturated solutions. When the dose rate is 100
VI
2
the H yield would be zero and G(U ) would be less than
4
G(Ox)total. Therefore, our results preclude a possible reduction
IV
·
of uranium ions by H in competition with reaction 8.
·
(
iii) Moreover, H
2
production by H dimerization in the
-1
IV
min ), they found in O
9
2
absence of any other reaction would yield half of the uranium
oxidation and H yields.
-7
-1
2
2
4
IV
The comparison between our experimental data and the total
yield of oxidizing species thus raises the important point of the
times lower, Ha ¨ı ssinsky et al. found that G(-U ) ) 18.6 ×
-
7
-1
1
0
mol J . This very high value of the oxidation yield was
VI
·
origin of the supplementary oxidation into U . The bieletronic
explained not only by a mechanism in which each H atom gives
equiv of oxidizing species (through the formation of HO ,
2
like in the Fricke dosimeter system), but also by the occurrence
of a chain reaction favored at very low dose rate. Later,
IV
-7
-1
·
U
H
oxidation yield corresponding to 16.8 × 10 mol J and
-7 -1
2
3
2
evolution G(H
) ) 4.4 × 10 mol J are clearly not
achieved simply by the scavenging yield of the radiolytic
-
7
-1
oxidizing species (G(Ox)total ) 9 × 10 mol J ). The
experimental results exclude the hypothesis of the dispropor-
tionation under our conditions. The high H2 and uranium
oxidation yields should then originate from an additional
chemical reaction involving H .
Because UIV and UVI are stable in acidic medium, the species
that is the most probable direct reductant of 2H into H
dimer ion U in the form of (UO
2 2
(UO H ) . We are compelled to conclude that, at least under
2
Bhattacharyya et al. revisited similar experiments in O -saturated
solution in the presence of 0.8 mol L- HCl. The chain
1
IV
mechanism was excluded, and it was found that G(-U ) )
-7
-1 6
7
.8 × 10 mol J . The authors reported a mechanism in which
· · ·
2 2 2
+
H atoms react with O
U
, giving HO
, and then HO oxidizes
IV
according to the Fricke mechanism with 3 equivalents. Both
V
+
Ha ¨ı ssinsky et al. and Bhattacharyya et al. considered that U
ions disproportionate. It is currently admitted in the literature
2
is the
V
+
2
)
2
or of the complex
V
2+
that the uranium ions U , which are produced from the
IV
V
+
scavenging of oxidizing radicals, disproportionate into U and
U . This mechanism was suggested by earlier electrochemical
hydrochloric conditions, after complexation of U with H
(reaction 2) and dimerization (reaction 3) the complex dimer
VI
V
V
+
+
VI
or stopped-flow experiments, where U is first produced very
[U O
2 2 2
,H ] produces directly H and U via inner-sphere
VI
7,9
V
VI
quickly via U reduction.
electron transfer. This integral oxidation process of U into U
However, in this respect, it is important to note that according
to Ha ¨ı ssinsky et al. and Pikaev et al., who were alone in
markedly differs from a disproportionation reaction. Indeed, a
V
small shift of the reduction potential of U due to the
-
measuring the amount of hydrogen produced during the radi-
complexation by Cl would suffice to favor, instead of a
IV
olytic oxidation of U in acidic media, G(H
2
) was exceptionally
disproportionation reaction, a stoichiometric oxidation reaction

Radiolytic Yield of UIV Oxidation into UVI
J. Phys. Chem. A, Vol. 114, No. 5, 2010 2085
+
V
VI
by H of a U dimer into 2U . Therefore, the overall reaction
that we propose is:
performed in the present work, would be helpful to check the
oxidation mechanism in different conditions of acidity and
ligandation.
V
+
+
VI 2+
2
[U O ,H ]
Cl
-
f H + 2[U O ]
-
Cl
(18)
2
2
2
Acknowledgment. We are grateful to Prof. J. Laverne from
Notre Dame University for helpful discussions. Furthermore,
we thank G. Blain and F. Crumi e` re for their contribution in the
experiments of gas measurements.
Both experimental observations of the radiolytic oxidation
IV
IV
of U , that is, the equality of the high yields G(-U ) and
VI
·-
G(U ) with G(Cl
2
) (and not half as with disproportionation),
formation, strongly support this
and the high yield of H
2
References and Notes
mechanism. Hence, the formation and oxidation yields of
V
(1) Morss, L. R.; Edelstein, N. M.; Fuger, J. The Chemistry of the
Actinide & Transactinide Elements; 2006; Vol. 1.
transient U are:
(
2) Pikaev, A. K.; Shilov, V. P.; Gogolev, A. V. Russ. Chem. ReV.
IV
V
·-
1997, 66, 763.
G(-U ) ) G(U ) ) G(Cl )
(19)
(20)
2
(
(
(
(
3) Halpern, J.; Smith, J. G. Can. J. Chem. 1956, 34, 1419.
4) Haissinsky, M.; Duflo, M. J. Chim.-Phys. 1956, 53, 970.
5) Sobkowski, J.; Zalkind, T. I. Zh. Fiz. Khim. 1965, 39, 1388.
6) Bhattacharyya, B. K.; Saini, R. D. Radiat. Phys. Chem. 1979, 13,
and
5
7.
(
7) Kern, D. M. H.; Orlemann, E. F. Inorg. Chem. 1949, 71, 2102.
V
VI
G(-U ) ) G(U ) ) 2G(H )
2
(8) Saini, R. D.; Bhattacharyya, P. K.; Iyer, R. M. J. Photochem.
Photobiol., A 1989, 47, ????.
9) Ekstrom, A. Inorg. Chem. 1974, 13, 2237.
(10) Weigl, F. The Chemistry of the Actinide Elements; 1986; Vol. 1.
(
The mechanism would also account for the high hydrogen
yields observed in N -saturated aqueous solutions (HCl) found
2
(
11) Steele, H.; Taylor, R. J. Inorg. Chem. 2007, 46, 6311.
(12) Atinault, E.; Waele, V. D.; Fattahi, M.; LaVerne, J. A.; Pimblott,
in previous studies.2
,4
S. M.; Mostafavi, M. J. Phys. Chem. A 2009, 113, 949.
In Figure 3, the dose dependence of the concentrations is
also represented for comparison for a sample saturated by O
Actually, the yields of G(-U )O2 and G(U )O2 are close to
those in N O-saturated solution. The whole mechanism, includ-
ing (2) and (18), so explains that the experimental results on
G(-U ) ) G(U ) are identical in the presence of N O or O
2 2
and indirectly supports the previous hypothesis on the role of
. In both cases, the transient U complex dimer is oxidized
by H (reaction 18) and yields molecular hydrogen. The only
difference between both systems is that the oxidation yield of
(
13) Kolthoff, I. M.; Harris, W. E.; Matsuyama, G. J. Am. Chem. Soc.
12
2
.
1944, 66, 1782.
(14) The Analyst 1945, 71, 91.
15) Beattie, W. H.; W. B. M. II; Holland, R. F. Spectrochim. Acta 1984,
0A, 897.
(16) Preez, J. G. H. D.; Rohwer, H. E.; Morris, K. B. Inorg. Chim. Acta
992, 191, 203.
17) Hennig, C.; Tutschku, J.; Rossberg, A.; Bernhard, G.; Scheinost,
A. C. Inorg. Chem. 2005, 44, 6655.
18) Janata, E.; Kelm, M.; Ershov, B. G. Radiat. Phys. Chem. 2002,
63, 157.
19) Auzel, F.; Hubert, S.; Delamoye, P. J. Lumin. 1982, 26, 251.
20) Grenthe, I.; Fuger, J.; Konings, R. J. M. Chemical Thermodynamics
of Uranium; 2004; Vol. 1.
(21) Atinault, E.; Waele, V. D.; Schmidhammer, U.; Fattahi, M.;
Mostafavi, M. Chem. Phys. Lett. 2008, 460, 461.
(22) Czapski, G.; Peled, E. Isr. J. Chem. 1968, 6, 421.
IV
VI
(
2
4
IV
VI
1
(
·
V
HO
2
(
+
(
(
IV
·
·
U
in O
2
-saturated solutions occurs via OH and HO
2
due to
·
·-
due to OH·
the H scavenging by O
scavenging in N
2
instead of via only Cl
2
2
O-saturated solutions (Figure 5).
V
The mechanism of the oxidation of U has also an important
consequence on the chemistry of U in more general cases. A
evolution during uranium oxidation should be
examined. Obviously, U ions would constitute as well an
intermediate in the slow chemical oxidation of U by air in
acidic media, where the oxidation of a dimeric complex
(
23) Jayson, G. G.; Parsons, B. J.; Swallow, A. J. J. Chem. Soc., Faraday
Trans. 1973, 69, 1597.
(24) Lierse, C.; Sullivan, J. C.; Schmidt, K. H. Inorg. Chem. 1987, 26,
IV
possible H
2
V
1408.
(
25) Condorelli, G.; Costanzo, L. L.; Pistar a` , S.; Tondello, E. Inorg.
IV
Chim. Acta 1974, 10, 115.
(26) Burrows, H. D.; Jesus, J. D. P. D. J. Photochem. 1976, 5, 265.
V
+
+
(
27) Muroya, Y.; Lin, M.; Wu, G.; Iijima, H.; Yoshii, K.; Ueda, T.;
[
U O
In conclusion, new experiments on chemical oxidation of U
with a complete product analysis, in particular of the gases as
2 2
,H ] should also be considered.
IV
Kudo, H.; Katsumura, Y. Radiat. Phys. Chem. 2005, 72, 169.
JP9051177