Dissolution of actinide oxides in HNO3 using a concentrated ozone-oxygen mixture in the presence of Ce(IV) ions

Article abstract of DOI:10.1134/S1066362211030040

Accumulation of Ce(IV) in HNO₃ solutions as a result of Ce(III) oxidation with an ozone-oxygen mixture (OOM) was studied. An increase in the ozone concentration in OOM from 30 to 180 mg dm^-3 leads to a considerable increase in the rate of Ce(III) oxidation to Ce(IV). With concentrated OOM, the Ce(III) to Ce(IV) oxidation yield of two ions per ozone molecule is attained. An increase in the steady-state concentration of Ce(IV) in the solution leads to considerable acceleration of the dissolution of highly calcined UO₂.

Full text of DOI:10.1134/S1066362211030040

ISSN 1066-3622, Radiochemistry, 2011, Vol. 53, No. 3, pp. 250–255. © Pleiades Publishing, Inc., 2011.
Original Russian Text © V.M. Gelis, Yu.V. Shumilova, B.G. Ershov, A.G. Maslennikov, V.V. Mulyutin, A.F. Seliverstov, 2011, published in Radiokhimiya,
2
011, Vol. 53, No. 3, pp. 214–218.
Dissolution of Actinide Oxides in HNO Using a Concentrated
3
Ozone–Oxygen Mixture in the Presence of Се(IV) Ions
V. M. Gelis, Yu. V. Shumilova, B. G. Ershov, A. G. Maslennikov,
V. V. Mulyutin*, and A. F. Seliverstov
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, block 4, Moscow, 119991 Russia; * e-mail: vmilyutin@mail.ru
Received May 17, 2010
Abstract—Accumulation of Се(IV) in HNO solutions as a result of Се(III) oxidation with an ozone–oxygen
3
–3
mixture (OOM) was studied. An increase in the ozone concentration in OOM from 30 to 180 mg dm leads to
a considerable increase in the rate of Се(III) oxidation to Се(IV). With concentrated OOM, the Се(III) to
Се(IV) oxidation yield of two ions per ozone molecule is attained. An increase in the steady-state concentration
of Се(IV) in the solution leads to considerable acceleration of the dissolution of highly calcined UO .
2
Keywords: actinide oxides, dissolution, oxidation, cerium(III), cerium(IV), ozone
DOI: 10.1134/S1066362211030040
In reprocessing of nuclear materials containing
tial of the dissolution toward positive values relative to
the Се(IV)/Ce(III) couple allowed certain acceleration
of the process. However, an advantage of cerium as
large amounts of PuO , quantitative transfer of Pu into
2
the solution is hindered by the fact that the oxidation
potential of HNO solutions traditionally used in radio-
mediator of the oxidative dissolution of PuO is the
3
2
chemical technology is insufficient for the oxidation of
fact that cerium is present in appreciable amounts in
nuclear materials as a fission product, whereas Ag and
Со are additional salt-forming agents whose presence
in solution can complicate the subsequent Pu reproc-
essing. For the dissolution of other materials based on
Pu(IV) to Pu(VI) [1]. Quantitative dissolution of PuO
2
–
can be attained by introducing F ions into the nitric
acid solution, owing to formation of stable complex
4–x
ions PuF . However, dissolution of the dioxide in the
x
presence of fluoride ions is undesirable, because it
gives rise to problems in the subsequent steps of Pu
refining, associated with increased neutron and γ-ray
PuO , it may be promising to use as mediator the
2
2
41
Am(V)/Am(III) couple, because Am is accumulated
in PuO in the course of its storage. Data on the possi-
2
1
9
background due to (α,nγ) reactions on F nuclei and
bility of preparing nitric acid solutions containing large
amounts of Am(V) are contradictory [4]. Trushima
et al. [5] showed that the yield of Am(VI,V) in ozo-
with lower corrosion resistance of the apparatus in this
medium. The most widely used method of PuO oxida-
2
tion to obtain soluble Pu(VI) complexes is introduction
into the solution of so-called mediators or electron-
transfer agents, i.e., of ion couples capable of fast and
reversible electron transfer at sufficiently high oxida-
tion potentials. Such a process was performed for the
first time by Ryan [2]. Cerium(IV) ions ensuring the
oxidation potential of the solution higher than 1.6 V
vs. SHE were chosen as mediator. Сеruim(IV) was
generated in the solution by galvanostatic electrolysis.
Along with Се(IV), Ag(II) and Со(III) ions, also gen-
erated electrochemically, were tested as mediators.
They ensured the solution oxidation potential of 1.7–
nolysis of 0.1–0.5 M HNO solutions can reach 25%.
3
The oxidative dissolution of PuO with Ce(IV) ions
2
generated in HNO solutions by oxidation of Сe(III)
3
with ozone was examined in detail in [6–9]. In these
papers, the dependences of the ozone solubility in
HNO solutions on the HNO concentration, flow rate
3
3
of the gas mixture, and temperature were analyzed [6,
7], the kinetic parameters of the heterogeneous reac-
tion of PuO oxidation with Ce(IV) ions were deter-
2
mined [8], and the effect of various ions on the mecha-
nism of this reaction was examined [9]. It was shown
that the rate of the Pu(VI) accumulation in solution
2
.0 V (vs. SHE) [3]. The shift of the oxidation poten-
2
50

DISSOLUTION OF ACTINIDE OXIDES IN HNO
3
251
could be considerably increased with an increase in the
steady-state concentration of Ce(IV). At the same time,
neither data of [6–8], nor a special study of the kinetics
of Ce(III) oxidation to Ce(IV) in HNO [10] allow un-
3
ambiguous conclusions on how the Ce(IV) accumula-
tion rate depends on the ozone concentration in the
ozone–oxygen mixture (OOM).
In this study we examined the kinetics of the
Се(IV) accumulation in HNO solutions in the course
3
of Сe(III) oxidation with ozone at various concentra-
tions of ozone in OOM. The experimental conditions
ensured the steady-state concentration of ozone in
Fig. 1. Ozonation installation: (1) cylinder with oxygen,
(2) ozonizer, (3) thermostat, (4) reactor with jacket,
НNO solution close to the saturation level (3.6 ×
3
–3
(5) ozone meter, and (6) stirrer.
1
0 M). We also studied the UO oxidation in HNO
2
3
solutions containing Се(IV) prepared by oxidation of
Се(III) with ozone. Highly calcined UO was used as a
periods, solution samples were taken and the Ce(IV)
concentration was determined. Oxidative dissolution of
2
model for studying the influence of the ozone concen-
tration in OOM on the kinetic characteristics of oxida-
tive dissolution of actinide oxides under the action of
ozone in the presence of Се(IV).
UO was performed in the same cell with continuous
2
stirring at 20°С.
The Ce(III) concentration in the initial solution was
determined by complexometric titration of cerium with
a Na EDTA solution in the presence of Xylenol Or-
EXPERIMENTAL
2
ange [12], and the Ce(IV) concentration in a mixture
with Се(III), by potentiometric titration with Mohr’s
All the solutions of acids and alkalis were prepared
using distilled water and chemicals of no less than ana-
lytically pure grade.
salt in 1.0 M H SO [13]. The U(VI) content was de-
2
4
termined by differential pulse polarography on a sta-
tionary mercury electrode in 0.1 M HClO [14].
4
Ce(III) solutions were prepared by dissolving a
weighed portion of chemically pure grade Ce(NO ) ·
3
3
RESULTS AND DISCUSSION
6
H O in HNO of required concentration.
2
3
In the experiments simulating the PuO dissolu-
2
The temperature dependence of the rate of the
tion, we used a UO pellet (d = 10 mm, h = 3 mm) sin-
2
Се(III) oxidation with ozone in HNO solutions of
3
tered in a vacuum at 1800°С and residual pressure no
various concentrations was studied in [7]. The oxida-
tion occurs in accordance with the overall equation
–
6
higher than 5.0 × 10 mm Hg. The pellet weight was
2
.5674 g.
To generate ozone, we used an ozonizer with a cap-
Се3+ + О3(aq) + 2Н → 2Се + О
+
4+
2
2
↑ + Н
2
О.
(1)
illary discharge chamber ensuring preparation of OOM
–
3
with the ozone concentration of up to 200 mg dm .
It was found in [7] that an increase in the HNO3
concentration, initial Се(III) concentration, and flow
rate of the ozone–oxygen mixture leads to an increase
in the oxidation rate. The process kinetics is described
by a first-order equation with respect to the Се(III)
concentration. The mechanism of the Се oxidation,
described by overall Eq. (1), remains unclear. Ivanov
and Nikitina [6] showed that an increase in the OOM
flow rate did not lead to a proportional increase in the
specific flux of ozone through gas–solution phase
boundary and, correspondingly, to a proportional in-
crease in the dissolved ozone concentration. Data of
The О concentration in OOM was determined with an
3
optical UV analyzer [11].
All the experiments on solution ozonation were
performed on an installation schematically shown in
Fig. 1. The main unit of the installation was a tempera-
3+
3
ture-controlled cell (V = 0.4 dm ) equipped with a me-
chanical stirrer. OOM was passed through the solution
at preset values of temperature and ozone concentra-
tion in the mixture. The ozone concentration in the gas
mixture was measured in the course of the experiment
at the inlet and outlet of the cell. After definite time
RADIOCHEMISTRY Vol. 53 No. 3 2011

2
52
GELIS et al.
from 3 h to 20 min. With OOM containing
–
3
1
80 mg dm ozone, it is possible to fully convert
Се(III) to Ce(IV) at 20 and 90°С. Quantitative inter-
pretation of the kinetic data obtained is difficult be-
cause of the diversity of processes simultaneously oc-
curring in solution, including the mass transfer of
ozone through the phase boundary [6], spontaneous
decomposition of ozone [8], oxidation of Сe(III) with
ozone (1) [7] and its decomposition products [8], and
possible reduction of the Се(IV) formed with hydrogen
peroxide arising in the solution as one of intermediate
products of О decomposition [8].
3
Simultaneous monitoring of the ozone concentra-
tion in OOM at the ozonation cell outlet and of the
Се(IV) accumulation in the solution (Fig. 3) showed
that, in the initial period of the Сe(III) oxidation to
Ce(IV), the solution was rapidly saturated with ozone,
Fig. 2. Curves of Ce(IV) accumulation in solutions at vari-
ous concentrations of ozone in OOM and temperatures:
–
3
–3
(
(
(
1) 180 mg dm
O
3
, 90°C; (2) 180 mg dm O
3
O
3
, 20°C;
, 20°C;
3 3
O , 90°C; (6) 30 mg dm O , 20°C. Solu-
3
–
3
–3
3) 70 mg dm
O
3
, 90°C; (4) 70 mg dm
–
3
–3
5) 30 mg dm
tion volume 0.4 dm , stirring rate 700 rpm, OOM feed rate
after which the О concentration increased relatively
3
3
–1
7
.2 dm h ; the same for Figs. 3–5.
slowly with an increase in the Се(IV) concentration. It
should be noted that saturation of the solution with
ozone is attained in 15–20 min and is independent of
the presence of Ce(III) in the solution. These results
indicate that the rate of the Ce(III) oxidation to Сe(IV)
is considerably lower than the rate of the ozone trans-
port through the gas–solution phase boundary and the
rate of the decomposition of ozone dissolved in HNO
3
–
3
3 –1/2
–1/2 –1
{
k = (0.8–1.0) × 10 (dm ) mol s [7] for HNO
3
concentration of 2.0–4.0 M}. As a result, the steady-
state concentration of the dissolved ozone in the course
of the Сe(III) oxidation remains practically constant
and increases with an increase in the ozone concentra-
tion in OOM [7]. According to the law of mass action,
the Сe(III) oxidation rate in accordance with overall
Eq. (1) should increase with an increase in the dis-
solved ozone concentration and, correspondingly, in
Fig. 3. Variation of the ozone concentration in the ozone–
oxygen mixture at the outlet of the ozonation cell in the
3
course of oxidation of Сe(III) to Ce(IV) in 3.0 M HNO .
[
6, 10] do not allow unambiguous conclusions on the
the О concentration in OOM.
3
influence exerted by the increase in the ozone concen-
tration in OOM on the rate of the Се(IV) accumulation
in the nitric acid solution.
Figure 2 shows that an increase in the temperature
from 20 to 90°C does not appreciably affect the rate of
the Ce(III) oxidation to Ce(IV). This is due to the fact
that, despite an increase in the oxidation rate in accor-
dance with the Arrhenius law, the steady-state concen-
In this connection, we examined low the О con-
3
centration in OOM affects the rate of the Се(IV) accu-
mulation in a 3.0 M HNO solution. Figure 2 shows
3
tration of О in solution decreases with increasing tem-
3
the curves of the Ce(IV) accumulation in solutions
containing 0.023 M Се(III) in relation to the ozone
concentration in OOM and temperature. Analysis of
perature because of a decrease in the O solubility [7].
3
Therefore, an increase in the reaction rate with tem-
perature is compensated by a decrease by a factor of 8–
the kinetic curves shows that an increase in the О con-
3
1
0 in the steady-state concentration of the oxidant.
centration in OOM leads to an increase in the rate of
the Сe(III) oxidation to Ce(IV). As the ozone concen-
Data on the ozone consumption for the Сe(III) oxi-
–
3
tration in OOM is increased from 70 to 180 mg dm ,
dation at various values of the ozone concentration in
the Се(III)/Ce(IV) half-conversion time decreases
OOM, temperature, and HNO concentration, obtained
3
RADIOCHEMISTRY Vol. 53 No. 3 2011

DISSOLUTION OF ACTINIDE OXIDES IN HNO
3
253
by numerical integration of the experimental curves
Figs. 2, 3), are given in Table 1. Experimental condi-
Table 1. Molar consumption Y of ozone for Сe(III) oxida-
tion at various values of the ozone concentration in OOM
(
3
C(O
3
), solution temperature T, and HNO concentration
3
tions: solution volume 0.4 dm , stirring rate 700 rpm,
3
–1
C(HNO ) (run nos. 1–6) and in the course of UO dissolu-
3
2
OOM feeding rate 7.2 dm h . Also presented are data
on the ozone consumption for the Сe(III) oxidation in
the course of UO dissolution in 1.0 M HNO at 20°C,
tion (run nos. 7, 8)
Run C(О ),
3
Y,
3
2
3
–3
T, °C C(HNO ), M
3
–
3
no. mg dm
mol О
/mol Се
at О concentrations in OOM of 30 and 180 mg dm
3
(
run nos. 7, 8).
1
2
3
4
5
6
7
8
30
30
70
180
180
180
30
20
90
90
20
90
20
20
20
3.0
0.80
1.00
0.60
0.50
0.51
0.55
1.05
0.54
3.0
3.0
3.0
3.0
1.0
1.0
1.0
Table 1 shows that an increase in the ozone concen-
tration in OOM leads to a decrease in its consumption
for Сe(III) oxidation. At the ozone concentration of
–
3
1
80 mg dm , about 0.5 mol О /mol Се(IV) was con-
3
sumed for the Ce(III) oxidation, suggesting the occur-
rence of the oxidation in accordance with Eq. (1). The
solution temperature and HNO concentration do not
180
3
appreciably affect the ozone consumption. Similar
trends are observed in dissolution of a UO pellet.
2
Table 2. Rate of UO
2
dissolution in 1 M HNO
3
V (20°C) in
relation to the concentrations of ozone in OOM C(O
Ce(III) in solution C(Ce)
3
) and
Based on the results obtained, we suggested the
following mechanism of the oxidation of Ce(III) ions
with ozone in nitric acid aqueous solution. The first
step of the reaction of ozone with cerium ion is proba-
bly generation of a metastable oxyl radical ion in the
reaction
Run no. C(О
), mg dm–3 C(Ce), mol dm–3 V, mg cm h
–2
–1
3
1
2
3
4
0
180
30
0
0
0.023
0.023
2.7 ± 0.5
23.2 ± 0.5
28.8 ± 0.5
88.4 ± 0.5
Сe3+ + О
→ CeО•3+ + О
(
2)
180
3
2
Different mechanisms are suggested for the reac-
tion of ozone with metal ions in aqueous solutions
solution should be reaction (2). This assumption ac-
counts for the fact found in [10] that the oxidation ki-
netics is described by a first-order equation with re-
spect to the Се(III) concentration. In accordance with
the mechanism under discussion and with overall
Eq. (1), one molecule of ozone should be spent for oxi-
dation of two Ce(III) ions, which is indeed the case
under the optimal conditions at the ozone concentra-
[
15]. However, when direct methods for recording in-
termediates of fast reactions (pulse radiolysis and/or
stopped-flow methods) are used, short-lived products
of the addition of the oxygen atom to the metal ion are
2
+
detected. For example, MnO is generated in the reac-
2+
2
+
2+
tion of ozone with Mn [16], and FeO , with Fe
17]. Therefore, we prefer reaction (2) with the forma-
[
•3+
–3
tion of CeО , which, apparently, decomposes in
a strongly acidic solution to Ce(IV) and hydroxyl
radical:
tion in OOM equal to 180 mg dm (Table 1).
We also examined the influence of the О concen-
3
tration on OOM on the rate of dissolution of a UO
2
CeО•3+ + H → Ce + OH.
+
4+
•
pellet in 1 M HNO in the presence of Ce(IV) at 20°С.
3
(3)
The dissolution duration was 2 h. The kinetic curves of
•
3+
It is known that the OH radical reacts with Сe
the UO dissolution are shown in Fig. 4. The rate of
2
with a very high rate constant equal to 5 ×
U(VI) accumulation in the solution follows the zero-
order rate equation throughout the examined interval,
in good agreement with published data [19]. In 1.0 M
8
–1 –1
1
0 l mol s [18],
Сe3+ + OH → Ce + OH–.
•
4+
(4)
HNO in the absence of ozone, the rate of the oxida-
3
–2
–1
tion of UО to U(VI) did not exceed 3.0 mg cm h
2
Taking into account extremely high protonation
rate, we can assume that the rate constant of reac-
(
Table 2).
The results obtained show that the rate of dissolu-
tion of UO in 1 M HNO at 20°C with OOM bubbling
increases by a factor of approximately 10.
1
0
3
–1 –1
tion (3) should be of the order of 10 dm mol s .
Therefore, we can conclude that the limiting step of the
Ce(III) oxidation with ozone in strongly acidic aqueous
2
3
RADIOCHEMISTRY Vol. 53 No. 3 2011

2
54
GELIS et al.
than the rate of the oxidation of Се(III) to Ce(IV) with
ozone. This is also confirmed by the fact that an in-
crease in the ozone concentration in OOM leads to a
considerable increase in the steady-state concentration
of Се(IV) and, correspondingly, to an increase in the
UO dissolution rate (Table 1).
2
Thus, in the presence of Се(IV), the UO dissolu-
2
tion rate increases further (by a factor of more than 3).
This acceleration is apparently due to the fact that
Се(IV) ions, when adsorbed on the UO surface, react
2
with UO faster than neutral О molecules do, and the
2
3
Fig. 4. Kinetic curves of the oxidative dissolution of a UO
pellet in 1.0 M HNO in the presence (or in the absence) of
.023 M Се(III) at various О concentrations in the ozone–
oxygen mixture: (1) without Се and О ; (2) without Се,
2
Сe(III) ions formed in the process are sufficiently rap-
idly oxidized both with ozone and with products of its
spontaneous decomposition.
3
0
3
3
–
3
–3
1
80 mg dm
O ; (3) in the presence of Ce, 30 mg dm O ;
3
3
The results of our study furnished the first experi-
mental confirmation for the fact that the rate of the
Сe(IV) accumulation and the rate of the dissolution of
–3
and (4) in the presence of Ce, 180 mg dm
3
O .
highly calcined UO in nitric acid solutions under the
2
conditions of oxidation of Се(III) to Се(IV) with ozone
strongly depend on the content of О in OOM. With
3
–
1
OOM containing 170–180 mg l О , oxidation of
3
Ce(III) to Ce(IV) in the course of ozonation of nitric
acid solutions of Ce(III) occurs with the formation of
approximately 2.0 mol of Се(IV) per mole of the con-
sumed ozone, i.e., in accordance with Eq. (1). This
result can be used when developing procedures for
intensifying the dissolution of PuO -containing materi-
2
als, e.g., of high-level PuO .
2
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3
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