On the highest oxidation states of plutonium in alkali solutions in the presence of ozone

Article abstract of DOI:10.1023/B:RACH.0000039108.13731.5b

During ozonation of Pu(VI) alkaline solutions, the highest oxidation state of Pu is formed in an oscillatory reaction. This plutonium species is reduced with Pu(VI) or Fe(III). Ferrate ion is also reduced with Pu(VI). It was assumed that in alkali solutio

Full text of DOI:10.1023/B:RACH.0000039108.13731.5b

Radiochemistry, Vol. 46, No. 4, 2004, pp. 340 342. Translated from Radiokhimiya, Vol. 46, No. 4, 2004, pp. 312 314.
Original Russian Text Copyright 2004 by Nikonov, Gogolev, Tananaev, Myasoedov.
On the Highest Oxidation States of Plutonium
in Alkali Solutions in the Presence of Ozone
M. V. Nikonov*, A. V. Gogolev**, I. G. Tananaev*, and B. F. Myasoedov**
*
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
*
* Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences,
Moscow, Russia
Received February 17, 2004
Abstract During ozonation of Pu(VI) alkaline solutions, the highest oxidation state of Pu is formed in
an oscillatory reaction. This plutonium species is reduced with Pu(VI) or Fe(III). Ferrate ion is also reduced
with Pu(VI). It was assumed that in alkali solution plutonium is partially oxidized with ozone to Pu(VIII).
Ozonation of a Pu(VI) hydroxide suspension or
Pu(VI) solution in 1 3 M NaOH is the best known
method for preparing Pu(VII) [1, 2]. In this process,
blue-black solutions having a characteristic absorption
spectrum with a maximum at 635 nm are formed. In
accordance with [1 4], the extinction coefficient
reduce the highest valence forms of Pu and Am in
the solution.
The fact of oscillations of the plutonium solution
optical density during oxidation with ozone (with the
amplitude reaching 30% of the maximal value) was
not noted earlier and casts doubt on the kinetic param-
eters of this reaction reported in [3] and, first of all,
on data on 100% yield of oxidized Pu form in the
above process. To clarify the nature of this phenome-
non, we performed a series of additional experiments.
635
1
1
varies from 530 to 600 l mol cm .
In this work, we performed experiments on plutoni-
um(VI) oxidation in NaOH solutions with ozone
(
Tekhnozon high-frequency ozonizer, Russia; the O3
1
content in O was about 3.5%, flow rate 5 7 l h
Reaction of plutonium obtained by ozonation in
NaOH solutions with Pu(VI). Aliquots of 1.3 mM
Pu(VI) in 1.5 M NaOH (0.05 ml) were added in suc-
cession to a 2.9 mM Pu(VI) solution in 1.5 M NaOH
(1 ml), through which ozone was preliminarily
bubbled for 60 min at room temperature to obtain
blue-black color, and then the absorption spectrum of
the solution was recorded within the 400 800 nm
range (Fig. 2). Another part of the blue-black solution
was left as a reference.
2
at 20 C). Ultrapure grade NaOH was used. Its solu-
tions were ozonized just before the experiment to
eliminate possible reducing impurities. The Pu(VI)
alkaline solutions were prepared by the known method
of ozonation of freshly precipitated Pu(IV) hydroxide
to Pu(VI)O nH O, followed by dissolution in
3
2
ozonized alkali with preset concentration, which ex-
cluded the presence of reducing impurities in them.
We found that, during oxidation of 2.9 mM Pu(VI)
in 3.5 M NaOH with ozone for more than 30 min,
the optical density of the ozonized solution undergoes
oscillations, their amplitude reaching 30% of the
maximal value (Fig. 1), which substantially exceeded
the probable error in optical density determination
(on a Shimadzu UV-160 spectrophotometer). Similar
results were obtained also during ozonation of Pu(VI)
hydroxide suspension at [NaOH] 0.7 and 1.5 M.
Similar effect was observed also during prolonged
ozonation of Am(III) hydroxide in bicarbonate solu-
tions, in which [Am(VI)] [Am(V)] oscillations were
observed [5]. In both cases, the effect found is prob-
ably the result of ozone decomposition in the sys-
tems studied, followed by formation of hydroperoxy
radicals and hydrogen peroxide, which partially
Fig. 1. Absorption spectra of Pu(VI) solution in 3.5 M
NaOH ozonated for (1) 42, (2) 45, and (3) 47 min.
1
066-3622/04/4604-0340 2004 MAIK Nauka/Interperiodica

ON THE HIGHEST OXIDATION STATES OF PLUTONIUM
It was found that the optical density of the Pu solu-
341
tion decreases in proportion to the amount of the
Pu(VI) added (with regard to dilution of the initial
volume). The results obtained (Fig. 2) show that the
Pu oxidized with ozone in NaOH solution reacts with
the added Pu(VI). At the same time, the spectral char-
acteristics of the reference solution remained constant
in time.
Similar results were obtained in experiments on
reaction of the Pu(VI) solution ( 10 3 M) in 1 3 M
NaOH, oxidized with ozone, with freshly precipitated
Pu(VI) hydroxide taken in excess. At contact of this
solution with the precipitate, the blue-black color
rapidly becomes grayish-blue with a sharp decrease
in the solution optical density within the 500 700 nm
range.
Fig. 2. Absorption spectra of (1) 2.9 mM Pu(VI) solution
in 1.5 M NaOH (1 ml) ozonized for 60 min and (2 8) its
mixtures with 1.3 mM Pu(VI) solution in 1.5 M NaOH.
Additions of Pu(VI) solution, ml: (2) 0.05, (3) 0.1, (4) 0.15,
It should be noted that, during ozonation for more
than 5 h of the Pu(VI) hydroxide suspension taken in
substantial excess as compared to its solubility in
(
5) 0.2, and (6) 0.25; (7, 8) the same as (6), after storage
for 30 and 90 min, respectively.
1
3 M NaOH, neither the solution nor the precipitate
acquire a blue-black color. The spectra of both liquid
and solid ozonation products are smooth, similar to
the spectrum of Pu(VI), and do not show oxidized
plutonium species.
Reaction of Pu(VI) with Fe(VI) in 1 M NaOH.
We found that on adding of 0.2 ml of 2 mM Pu(VI) in
1
M NaOH to 1 ml of 0.3 mM Fe(VI) in 1 M NaOH
at room temperature, the optical density of the solu-
tion decreases by 50% within the time of mixing
(
Fig. 3, spectra 1, 2). Addition of the second aliquot
of 2 mM Pu(VI) solution in 1 M NaOH (0.2 ml) leads
to a further decrease in the optical density of the solu-
tion (spectrum 3), and the solution becomes colorless.
Addition of the next portion of Pu(VI) (0.2 ml) does
not further change the absorption spectrum of the
solution (spectrum 4), which obviously indicates
complete reduction of the ferrate ions. No changes in
the spectrum of a reference Fe(VI) solution were
observed within the same time. These results suggest
Fig. 3. Absorption spectra of (1) 0.3 mM Fe(VI) solution in
1 M NaOH (1 ml) and its mixtures with 2 mM Pu(VI) solu-
tion in 1 M NaOH. Additions of Pu(VI) solution, ml:
(
2) 0.2, (3) 0.4, and (4) 0.6.
(
(
Fig. 4, spectrum 1). Then, to this solution, aliquots
0.05 ml) of weakly acidic 1 mM Fe(III) solution
that in 1 M NaOH Pu(VI) is oxidized with FeO2 ions,
4
were added in succession. On adding the very first
Fe(III) aliquot, the optical density of the solution
decreases (spectrum 2). Addition of the next Fe(III)
aliquot leads to a further decrease in the solution
optical density. At the same time, the band character-
istic of ferrate ion appears (spectrum 3). This solution
after storage for 40 min at room temperature becomes
colorless, obviously due to Fe(VI) reduction with
water molecules.
which follows from decrease in the optical density of
Fe(VI) solution. This fact contradicts the known data
[1] that the redox potential of the Pu(VII)/Pu(VI)
couple in 1 M NaOH amounts to 0.85 V, which is sub-
stantially higher than the potential of the Fe(VI)/Fe(III)
couple, 0.72 V (relative to standard hydrogen elec-
trode) under the same conditions [6, 7].
Reaction of Fe(III) with Pu ozonized in 1 M
NaOH. Ozonation of Pu(VI) in 1 M NaOH was per-
formed for 40 min up to formation of a blue-black
solution with the well-known absorption spectrum [1]
If during ozonation of Pu(VI) alkaline solution plu-
tonium is oxidized to Pu(VII) only, we should expect
the following reaction with Fe(III):
RADIOCHEMISTRY Vol. 46 No. 4 2004

342
NIKONOV et al.
plutonium oxidized by ozone is reduced by Fe(III)
with formation of Pu(VI). These results cannot be
explained by formation of Pu(VII) as the only species
in ozonation of Pu(VI) in alkaline solutions, as it was
assumed in earlier works.
Thus, it follows from the experimental data ob-
tained that, during Pu(VI) oxidation with ozone in
alkali solutions, not only Pu(VII) but also (partially)
Pu(VIII) is formed. We estimated the extinction coef-
ficient of Pu(VIII)
and the Pu(VIII) content under the experimental con-
at 2600 400 l mol 1 cm 1
635
ditions, at 12% from the amount of Pu(VI) intro-
5
duced (0.5 ml 1.3 mM = 6.5 10 mmol) into the
Fig. 4. Absorption spectra of (1) Pu(VI) solution in 1 M
NaOH and (2 4) its mixtures with 1 mM Fe(III) solution.
Addition of Fe(III) solution, ml: (2) 0.05 and (3) 0.10;
solution of ozonized plutonium (Fig. 2, spectra 1, 2)
and taking into account a decrease in the optical den-
sity at 635 nm ( 0.180).
(
4) the same as (3), after storage for 40 min.
Further studies aimed to confirm more reliably the
possibility of Pu(VI) oxidation with ozone to Pu(VIII)
in alkali solutions are now in progress.
3
Pu(VII) + Fe(III) = 3Pu(VI) + Fe(VI).
(1)
However, our above data (Fig. 3) show that Pu(VI) is
readily oxidized with ferrate ion.
ACKNOWLEDGMENTS
Thus, the following conclusions arise from our
study.
The work was financially supported by the Integra-
tsiya Federal Target Program, Charitable Founda-
tion for Support of Domestic Science, and US DOE
(grant RCO-20004-SC14).
During oxidation of Pu(VI) with ozone (> 30 min)
in 0.7 3.5 M NaOH, oscillations of the optical density
with the amplitude reaching 30% of the maximal den-
sity are observed. Therefore, discrepancies in the
extinction coefficient 35, which ranges from 530 to
6
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6
00 l mol 1 cm according to different papers [1 4],
1
may be caused by this phenomenon.
1
. Krot, N.N. and Gel’man, A.D., Dokl. Akad. Nauk SSSR,
967, vol. 177, p. 124.
1
Plutonium oxidized with ozone in alkaline solu-
tions reacts with the added Pu(VI). Since in our ex-
periments only ozonized solutions were used (which
excludes the presence of any reductants), the phe-
nomenon observed can be explained by the formation
of Pu(VIII) upon oxidation with ozone. In this case,
the Pu(VI) added reacts with Pu(VIII) (reproportiona-
tion reaction). In addition, it follows from Fig. 2 that
approximately equimolar amount of Pu(VI) reacts
with Pu(VIII). Hence, taking into account the con-
sumption of Pu oxidized with ozone in the experi-
ment, the content of probable reducing impurities (if
they are the reason of this fact) should be close to
the Pu(VI) content, which is hardly possible.
2. Krot, N.N., Gel’man, A.D., Mefod’eva, M.P., et al.,
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RADIOCHEMISTRY Vol. 46 No. 4 2004