Interaction of neptunium and technetium with UO2+x

Article abstract of DOI:10.1134/S1066362207040157

Interaction of uranium dioxide with highly mobile radionuclides ²³⁷Np and ⁹⁹Tc was studied under oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is reduced on the UO_2+x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO_2+x surface corresponds to U₄O₉. In acid solution, U(VI) is leached to form surface UO₂. Although the free surface area of a UO_2+x sample is low, the Np distribution coefficients K _d at pH > 6 are relatively high: log K _d > 2. Unlike Np, Tc(VII) is not reduced on the UO_2+x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.

Full text of DOI:10.1134/S1066362207040157

ISSN 1066-3622, Radiochemistry, 2007, Vol. 49, No. 4, pp. 409 414. Pleiades Publishing, Inc., 2007.
Original Russian Text O.N. Batuk, S.N. Kalmykov, V.G. Petrov, Yu.A. Teterin, E.V. Zakharova, 2007, published in Radiokhimiya, 2007, Vol. 49, No. 4,
pp. 359 363.
Interaction of Neptunium and Technetium with UO2+x
O. N. Batuka, S. N. Kalmykova, V. G. Petrova, Yu. A. Teterinb, and E. V. Zakharovac
a
Chemical Faculty, Lomonosov Moscow State University, Moscow, Russia
b
Russian Research Centre Kurchatov Institute, Moscow, Russia
c
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Moscow, Russia
Received March 24, 2007
Abstract Interaction of uranium dioxide with highly mobile radionuclides 237Np and 99Tc was studied under
oxidative conditions. Sorption of these radionuclides at different pH was measured, and the mechanism of
redox reaction occurring in the course of their sorption were determined. In alkaline solution, Np(V) is
reduced on the UO2 + x surface and is sorbed in the form of tetravalent species. In neutral solutions, Np is
sorbed in the form of Np(V). This is due to the fact that the stoichiometry of the UO2 + x surface corresponds
to U4O9. In acid solution, U(VI) is leached to form surface UO2. Although the free surface area of a UO2 + x
sample is low, the Np distribution coefficients Kd at pH > 6 are relatively high: logKd > 2. Unlike Np, Tc(VII)
is not reduced on the UO2 + x surface. However, the sorption capacity of uranium dioxide for Tc(IV) is high.
PACS numbers: 68.43.-h
DOI: 10.1134/S1066362207040157
Large amounts of radioactive waste (RW) have
been accumulated at nuclear power plants. In many
countries, including Russia, high-level RW is intended
to be disposed of in geological formations after parti-
tioning of radionuclides and their immobilization in
glass or mineral matrices. A concept of underground
repositories of HLRW is based on a multibarrier sys-
tem restricting or preventing release of radionuclides
into the environment. At the same time, depleted ura-
nium (DU) is simultaneously accumulated in produc-
tion of enriched uranium fuel for atomic power plants
(APPs). Depleted uranium is stored in the form of
technetate anion is sorbed only on positively charged,
i.e., protonated surface (low pH for most of metal
oxides and hydroxides). Neptunyl(V) cation has a low
effective charge of +2.2 [4] and hence is poorly sorbed
on minerals.
The aim of this study was to examine sorption of
Tc and Np on uranium dioxide under oxidative condi-
tions and the influence of redox reactions of these
radionuclides on their sorption.
EXPERIMENTAL
highly toxic UF in surface repositories. At the begin-
6
ning of the XXI century, the world’s resources of DU
were estimated [1, 2] to be 1 million tons. Depleted
uranium is used for production of mixed oxide (MOX)
fuel and fabrication of containers for RW transporta-
tion [3]. In this study we examined the possibility of
using DU in the form of uranium dioxide as an engi-
neered barrier in repositories of solidified HLW and
spent nuclear fuel (SNF). This approach allows safe
storage of HLW for a long time and, on the other
hand, utilization of DU. To estimate the possibility of
using DU at repositories of HLW and SNF, its sorp-
tion properties with respect to artificial radionuclides
Determination of the Composition of a UO
Sample
2+x
We studied sorption properties of commercial
UO (TVEL Joint-Stock Company, Elektrostal,
2+x
Russia). UO
was prepared by heat treatment of
2+x
UF in an oxygen flow and subsequent treatment of
6
the product in an Ar/H mixture at 650 C. The U/O
2
ratio in the bulk of the UO
sample was determined
2+x
by X-ray phase analysis (XPA) using the linear depen-
dence of the lattice parameter a on x in the range from
0 (UO ) to 0.25 (U O ) [5]. ThO prepared by calci-
2
4
9
2
nations of thorium oxalate at 500 C in an oxygen flow
99
237
should be studied. Radionuclides Tc and
Np are
[6] was used as a UO analog stable to oxidation.
2
toxic and environmentally hazardous since they are
long-lived and are converted under oxidative condi-
tions into highly mobile water-soluble species. Per-
The phase composition of UO
determined by XPA at room temperature in a three-
and ThO was
2
2+x
409

410
BATUK et al.
frame focusing monochromator camera (CuK radia-
tion, effective diameter 228 mm). A Johansson bent
atmospheric CO on the sorption, the experiments
were performed in a hermetically sealed box filled
2
1
quartz single crystal was used as monochromator.
Germanium was added to the sample, and its lines
were used as an internal reference [a = 5.6574(9) ].
The X-ray patterns were measured on an IZA-2 com-
parator with an accuracy of 0.002 mm; the intensity
was visually estimated by a 100-point scale [7]. The
X-ray pattern was indexed and the lattice parameters
were refined using the Powder2 software.
with N . The total radionuclide concentration in all
2
10
the sorption experiments was 1 10
M; the solid-
1
to-liquid ratio was 2.5 g l . The solid phase was sep-
arated by microfiltration on filters with 170-nm pores.
To determine a time required to attain the sorption
equilibrium, aliquots of the examined suspensions
were taken at regular intervals and the concentration
of the radionuclides in the filtrate was measured.
The sorption equilibrium with all the examined radio-
nuclides was attained within a time shorter than 24 h.
Once the sorption equilibrium had been attained,
the solid phase was separated and the U/O ratio on
the surface of the samples kept at different pH was
determined by XPS.
The surface stoichiometry of a UO
sample
2+x
was determined by X-ray photoelectron spectroscopy
(XPS) on an MK II VG Scientific electrostatic spec-
trometer (AlK radiation, h = 1486.6 eV) in a high
7
vacuum (1.3 10 Pa) at room temperature. The
resolution of the spectrometer, determined by the
halfwidth of the Au4f line, was 1.2 eV. The binding
7/2
Technetium(IV) was prepared by reduction of
Tc(VII) with a Sn(II) solution in 1 M HCl [8]. The
oxidation state of the examined elements in the work-
ing solutions was determined by liquid extraction.
Neptunium(IV) was extracted with 0.5 M solution of
1-(2-thenoyl)-3,3,3-trifluoroacetone in toluene [9].
energies of U and O electrons were determined rela-
tive to the C1s line (285.0 eV). Carbon sorbed in
small amount on the sample surface during drying in
a vacuum was used to calibrate the line energy.
The half-width of the lines was measured relative to
that of the C1s line taken as 1.3 eV. The determina-
tion error of binding energies and line width was
0.1 eV. The relative intensity of the line was deter-
mined with an accuracy of 10%.
3
Technetium(VII) was extracted with 1 10 M solu-
tion of methyltrioctylammonium bromide in toluene.
Under these conditions, Tc(IV) is retained in the aque-
ous phase [10].
Samples of UO
and ThO were also charac-
2
2+x
terized by nitrogen sorption at 77 K. The free surface
area and porosity of the samples were calculated by
the BET equation. The particle size and the surface
morphology were determined by dynamic light scat-
tering and scanning electron microscopy, respectively.
To increase the sensitivity of radionuclide detec-
99m
99
tion, we used mixtures of
Tc (T = 6.01 h)/ Tc
Np (T = 2.36 days)/
1/2
1/2
5
239
(T = 2.11 10 years) and
1/2
237
6
Np (T
= 2.14 10 years). A solution with re-
1/2
99
quired TcO concentration was prepared by dissolu-
4
We found that the free surface area of UO
sample was low (1.5 m g ) and its structure was
nonporous. As determined by XPA, the stoichiometry
2+x
2
1
tion of metallic Tc in concentrated HNO on heating.
3
99m
99
Tc was isolated form its mixture with Mo by
elution of a technetium generator (Institute of Energy
of the sample is close to UO [a = 5.4638 ]; the
2
237
+
Physics, Obninsk). Nitric acid solution of
NpO
sample surface is partially oxidized (XPS data) and its
2
was obtained from the Research Institute of Atomic
composition is close to UO
particle size of the sample is 1.5 m. The properties
(U O ). The average
2.25
4 9
239
Reactors (Dimitrovgrad).
equilibrium mixture with
Np was isolated from its
243
Am by extraction with
of crystalline ThO sample are close to those of
2
99m
2
1
trioctylamine [11]. -Ray radiation of
Tc (E
=
UO : low specific surface area (1 m g ), non-
max
2+x
239
porous structure, average particle size 3.5 m, lattice
140.5 keV, p = 89%) and
Np (E
= 106.1 keV,
max
parameter a 5.5997(8)
(calculated from X-ray dif-
p = 27%) was registered using an ultrapure germani-
um semiconducting detector (GC-3020, Canberra).
fraction patterns).
We used Th(IV) to simulate the sorption behavior
Sorption Experiments
234
of Np(IV). For this purpose we used
Th (T
=
1/2
232
10
24.1 days) and
Th (T
= 1.41 10 years).
Sorption of Np(V), Th(IV) and Tc(VII, IV) was
performed from deionized water on UO and ThO
in plastic vessels to prevent sorption of the radio-
nuclides on the walls. To exclude the influence of
1/2
234
Th was recovered from its equilibrium mixture
2+x
2
238
with
U on a column packed with KU-2 cation
exchanger by the procedure described in [12].
RADIOCHEMISTRY Vol. 49 No. 4 2007

INTERACTION OF NEPTUNIUM AND TECHNETIUM WITH UO2 + x
RESULTS AND DISCUSSION
411
Np(V) Sorption
The pH dependence of Np(V) sorption on UO
2+x
is shown in Fig. 1a. The shape of this dependence is
atypical for cation sorption which is probably due to
redox reactions in the solution or on the UO
sur-
2+x
face. The sorption is high in acid solution (pH 2 3.5),
decreases to the minimal values in the pH range 3.5
7, and then increases at pH > 7. In acid solutions,
Np(V) is probably reduced by the UO
surface to
2+x
Np(IV) which is sorbed. It is known [13, 14] that
sorption of tetravalent actinides substantially differs
from that of pentavalent actinides. The reactivity and
sorption of a cation are determined by its charge
which is equal to +4 and +2.2 for An(IV) and An(V),
respectively [4]. The oxidation state of Th(IV) does
not change in the whole pH range. The fact that
Th(IV) is quantitatively sorbed at pH 2 indirectly
confirm the assumption on sorption of Np(IV) species
Fig. 1. (a) Fraction of (1) Np(V) and (2) Th(IV) sorbed on
UO and (3) fraction of Np(V) sorbed on ThO , and
(b) logarithm of distribution coefficients of these radionu-
clides, as functions of pH.
2 + x
2
on UO
at pH < 3.5 (Fig. 1a). The results of liquid
2+x
extraction show that only 6% of Np being in the con-
tact with UO is in the tetravalent form at pH 3.0.
2+x
These results are consistent with thermodynamic cal-
culations (Fig. 2) which also show the presence of
pentavalent Np in the solution under conditions of
the sorption experiment. The Pourbaix diagram shown
in Fig. 2 was constructed using the thermodynamic
data for Np recommended by NEA (Nuclear Energy
Agency) [15, 16] and the thermodynamic data for U,
taken from [17]. We suggest that, in acid solutions,
Np(V) is reduced to Np(IV) in the electrical double
layer of UO
and is stable in the solution. Under
2+x
these conditions, U(VI) is leached and the surface
stoichiometry of UO kept in acid solutions be-
comes close to UO . As the pH increases, the U(VI)
solubility decreases [18] and the U(VI)/U(IV) ratio on
the sample surface increases.
2+x
Fig. 2. Pourbaix diagram for Np (continuous line) and U
[17] (dashed line). The Eh potential is given vs. standard
hydrogen electrode. Points denote the conditions of sorp-
tion experiments.
2
The change in surface stoichiometry of UO
influenced by pH of the solution was confirmed by
XPS. The O2s, U6p, and U5f spectra of a UO
as
2+x
20]. The oxygen coefficients of UO
mined by XPS are presented below. The stoichiometry
of uranium dioxide kept at pH 7.04 is close to U O .
samples deter-
2+x
2+x
4
9
sample kept in solutions with pH 2.54 and 7.04 are
shown in Fig. 3. The XPS spectrum of U5f electrons
negligibly involved in chemical bonding contains
a relatively sharp band on the low-energy side of the
valence band (Figs. 3c, 3d). The intensity of this band
determined by the contribution of the U5f electrons of
U(IV) can be used to separate spectral states of U(IV),
U(V), and U(VI) in the U4f spectrum (Figs. 3a, 3b).
The oxygen coefficient of uranium oxides can be
determined from the ratio of the intensities of bands
corresponding to different oxidation states of U [19,
The surface stoichiometry of the samples approaches
UO with decreasing pH of the solution.
2
pH of solution
Stoichiometry of UO2 + x surface
2.54
4.35
7.04
UO
UO
2.17
2.14
UO
2.22
At pH 4 the U solubility decreases [21] and
the reduction potential of the UO surface is
insufficient to reduce Np(V) to Np(IV). As a result,
2+x
RADIOCHEMISTRY Vol. 49 No. 4 2007

412
BATUK et al.
Fig. 3. X-ray photoelectron spectra of (a, b) U4f electrons and (c, d) valence electrons of UO
and (b, d) 7.04. The bands were resolved in the approximation that the samples contain only U(IV) and U(VI).
samples kept at pH (a, c) 2.54
2 + x
Fig. 4. (a) Fraction of (1) Tc(IV) and (2) Tc(VII) sorbed on UO
as functions of pH.
and (b) logarithm of distribution coefficients of these species
2 + x
the total Np sorption decreases. At pH > 5.5 the Np
sorption increases and is probably caused by forma-
Although the free surface energy of the UO
2+x
sample is low, the distribution coefficients K of Np
d
tion of Np(V) hydroxo complexes on the UO
sur-
at pH > 6 are relatively high ( logK > 2) (Fig. 1b).
2+x
d
face. The portion of the pH dependence of the Np
sorption (Fig. 1a) at pH > 5.5 can be described by
an S-shaped curve typical for cation sorption via
formation of complexes with surface hydroxy groups
of metal oxides or hydroxides. The assumption that
in this pH range Np(V) is sorbed to form complexes
with the surface hydroxy groups is confirmed by
For comparison, logK for sorption of quartz, mi-
d
crocline, and kaolinite is 1.3, 1.5, and 1.0, respec-
tively [22].
Sorption of Tc(VII) and Tc(IV)
The pH dependence of Tc(VII) sorption on UO
2+x
the shape of the curve of Np(V) sorption on ThO
is shown in Fig. 4a. High Tc(VII) sorption at low pH
is due to interation of the TcO anion with the UO
2
(Fig. 1a) as an oxidation-resistant analog of UO .
2
4
2+x
RADIOCHEMISTRY Vol. 49 No. 4 2007

INTERACTION OF NEPTUNIUM AND TECHNETIUM WITH UO2 + x
ACKNOWLEDGMENTS
413
This study was financially supported by Interna-
tional Science and Technology Center (project
no. 2694) and the Russian Foundation for Basic
Research (project no. 05-03-33028a).
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is an efficient sorbent of Np(V) from
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RADIOCHEMISTRY Vol. 49 No. 4 2007