Oscillatory reactions Am(VI) ? Am(V) under ozonation of Am(OH) 3 suspension in bicarbonate solution

Article abstract of DOI:10.1023/B:RACH.0000031680.78106.98

During prolonged ozonation of Am(III) hydroxide in bicarbonate solutions, oscillatory Am(VI) ? Am(IV) reactions were observed. Substitution of ²⁴¹Am with ²⁴³Am, which is characterized by substantially lower specific radioactivity, does not change the character and parameters of the oscillatory process: the yields of ²⁴¹Am and ²⁴³Am, and the oscillation period of about 2 min do not differ noticeably. The results suggest that the reductants in the system mainly originate from the ozone decomposition products; the arising hydroperoxy radicals and hydrogen peroxide partially reduce Am(VI) in the solution. The Am(VI) yield in ozonation of the Am(OH)₃ suspension in a bicarbonate solution substantially decreases with increasing the initial americium content.

Full text of DOI:10.1023/B:RACH.0000031680.78106.98

Radiochemistry, Vol. 46, No. 3, 2004, pp. 246 248. Translated from Radiokhimiya, Vol. 46, No. 3, 2004, pp. 226 228.
Original Russian Text Copyright 2004 by Nikonov, Gogolev, Tananaev, Myasoedov.
Oscillatory Reactions Am(VI)
Am(V) under Ozonation
of Am(OH)3 Suspension in Bicarbonate Solution
M. V. Nikonov, A. V. Gogolev, I. G. Tananaev, and B. F. Myasoedov
Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received April 11, 2003
Abstract During prolonged ozonation of Am(III) hydroxide in bicarbonate solutions, oscillatory
Am(VI)
Am(IV) reactions were observed. Substitution of 241Am with 243Am, which is characterized
by substantially lower specific radioactivity, does not change the character and parameters of the oscillatory
process: the yields of 241Am and 243Am, and the oscillation period of about 2 min do not differ noticeably.
The results suggest that the reductants in the system mainly originate from the ozone decomposition products;
the arising hydroperoxy radicals and hydrogen peroxide partially reduce Am(VI) in the solution. The Am(VI)
yield in ozonation of the Am(OH)3 suspension in a bicarbonate solution substantially decreases with increas-
ing the initial americium content.
It is known that ozonation of Am(OH) suspension
RESULTS AND DISCUSSION
3
in bicarbonate solutions is the best method of prepar-
ing Am(VI) solutions [1, 2]. However, the kinetic
characteristics, the effect of various factors on the
Am(VI) yield, and the mechanism of this reaction
have not been studied up to now. The goal of this
work was to obtain the detailed information on the
above reaction.
After passing ozone oxygen mixture through a
241
Am(OH) suspension in 0.12 M aqueous NaHCO
3
3
for several minutes, the solution acquires a reddish
tint. In the absorption spectrum, the band at 503 nm
3+
belonging to the hydrated cation Am nH O disap-
2
pears already after oxidation for 4 min, and the pre-
cipitate dissolves. In the process, the optical density
of the solution grows within the 300 600 nm range,
characteristic for Am(IV) in carbonate media [3]
(Fig. 1, curves 1, 2). Further ozonation causes the
optical density in this spectral region to decrease; after
15 min of ozonation, it decreases by a factor of 2
(curve 3). This fact can be attributed to Am(IV) oxi-
dation to Am(V), whose ions do not absorb the light
at 300 600 nm, in agreement with data of [3]. Then,
at further ozozation, the absorption increases again,
and after 60 min only one band at 370 nm character-
istic for Am(VI) [3] is observed (curves 4 7).
EXPERIMENTAL
241
In all the experiments, we used
Am(OH) or
3
243
Am(OH) obtained by addition of 1 M NaOH to
3
neutral Am(NO ) solutions. An Am(OH) precipitate
3 3
3
was coagulated for 10 min, washed with water to
neutral reaction, and then 4 ml of 0.12 M NaHCO
3
was introduced. The resulting aqueous suspension
containing 0.2 1 mg of Am hydroxide was placed
into a glass bubbler, and an ozone oxygen mixture
with 3.5% O content (generated by a high-frequency
3
These data suggest that in bicarbonate solution
Am(III) is oxidized with ozone at room temperature
successively with formation of Am(IV), Am(V), and
then Am(VI). It should be noted that the solution
ozonized for several minutes acquires a strong red-
dish-brown color. The color does not change visually
in time in spite of the above changes in the absorption
spectrum. This fact, similarity of Am(IV) and Am(VI)
spectra in bicarbonate solutions, and rapid dissolution
ozonizer produced by Tekhnozon company, Russia)
1
was passed through it at 20 C with 5 7 l h flow
rate. At regular intervals, ozonation was interrupted,
and Am-containing mixture was transferred into a
plastic tube with a capillary. After 10-min centrifug-
ing at 8000g, the solution was placed into a 1-cm
optical cell, and its absorption spectrum was recorded
within the 300 1200 nm range on a Shimadzu-UV-
160 spectrophotometer (Japan).
of the Am(OH) precipitate can lead to wrong conclu-
3
Ultrapure grade NaOH, NaHCO , and HNO were
sions when such a phenomenon will be considered as
stabilization of americium in the hexavalent state.
3
3
used.
1066-3622/04/4603-0246 2004 MAIK Nauka/Interperiodica

OSCILLATORY REACTIONS Am(VI)
Am(V) UNDER OZONATION
247
In prolonged ozonation of the Am(VI) solution
(>60 min), oscillations of intensity of the absorption
band at 370 nm are observed, suggesting that the
Am(VI) concentration periodically changes between
the upper and lower extremal points (curves 1 12).
The oscillation period found by us in special experi-
ments amounts to 2 min. These oscillations occur
only at ozone bubbling through the solution. After O
3
bubbling was stopped at any instant, the Am(VI)
concentration did not change, and the system was
stabilized for a long time. The maximal Am(VI) yield
in the upper point was 95 96% from the initial amer-
icium content in the reaction system.
The mechanism of the oscillation process may be
represented as follows. Dissolved ozone reacts with
the lower valence state of americium to form Am(VI)
until a sufficient amount of reductants is accumulated
in the solution, and the rate of their reaction with
Am(VI) becomes higher than the rate of its formation.
Then, the reaction occurs in the reverse direction, and
Am(VI) reduction to Am(V) is observed. In the course
of reductant (Red) consumption in the reaction with
Am(VI), its concentration decreases to a value corre-
sponding to the point when the rate of the reaction of
reduced americium species with ozone again becomes
higher than the rate of Am(VI) reduction to Am(V):
Fig. 1. Changes in absorption spectra of Am bicarbonate
solutions during ozonation. O content in the ozone oxygen
3
mixture 3.5%; 20 C; l = 1 cm. [NaHCO ] 0.12 M. (0) Spec-
3
trum of the initial Am(III) solution before Am(OH) pre-
241Am(V) + O3
241Am(VI) + Red.
(1)
3
cipitation. Ozonation time, min: (1) 4, (2) 8, (3) 15, (4) 30,
(5) 40, (6) 50, (7) 60, (8) 90, (9) 105, (10) 120, (11) 135,
and (12) 165.
Hydrogen peroxide, H O , which is a product of
2
2
radiolysis of aqueous solution under the action of
241
Am -particles, can be possible reductant in reac-
centration oscillations between upper and lower ex-
tremal points remained unchanged. As in the case of
.
tion (1). H O and hydroperoxy radical HO can also
arise from ozone decomposition by the scheme pro-
posed in [4]:
2
2
2
241
Am, the oscillations were observed only at ozone
bubbling through the solution, and when ozone bub-
bling was stopped, no changes in the reaction medium
241
O3 + OH = O2 + HO2,
O3 + HO2 = O3 + HO2,
2HO2 = H2O2 + O2,
(2)
(3)
(4)
(5)
(6)
were observed. The maximal yields of both
Am
243
and
Am in the hexavalent form were practically
the same. Thus, the data obtained suggest that the
main source of reductant in the system is the ozone
decomposition followed by formation of hydroperoxy
radicals and hydrogen peroxide, which reduce Am(VI)
in the solution.
241Am(VI) + H2O2
241Am(VI) + HO2
241Am(V),
241Am(V).
It should be noted that, in the above experiments,
we used solutions with relatively low americium con-
4
243
centration, not exceeding 8 10 M. At
Am con-
To find the causes of reductant accumulation in the
solution, we estimated the contribution of the radia-
tion factor to the reaction studied. For this purpose,
we repeated the above experiment with
specific radioactivity is by a factor of 15 lower than
Am, and found that this substitution did
not affect the process: the amplitude of Am(VI) con-
3
centration 10 M, the pattern somewhat changes.
243
In this case, after
Am oxidation to Am(VI) and
243
start of the oscillatory process, a brown precipitate is
formed in the system. Small portions of the precipitate
were taken after stopping ozonation. It was soluble
in 0.1 M NaHCO and in 1 M HNO . The spectra of
Am, whose
241
that of
3
3
RADIOCHEMISTRY Vol. 46 No. 3 2004

248
NIKONOV et al.
Am(V) + H2O2
[Am(V) H2O2]
[Am(V) H2O2],
(7)
(8)
AmO2 xH2O.
Since the period of oscillations of Am(VI) concen-
trations in the solution was relatively short, only a
part of Am(IV) precipitate formed had time to dis-
solve in the bicarbonate solution before formation of a
new precipitate portion. As a result, a steady state was
attained, and the americium distribution between the
solution and precipitate remained virtually constant
during the experiment (up to 1 h from the start of the
oscillatory process). About 15% of Am from the total
3
243
content of 1.3 10
M
Am in the solution passed
into the precipitate. Thus, as the initial Am content in
the bicarbonate solution to be ozonized is increased,
the Am(VI) yield, under equal other conditions,
decreases.
From the data obtained, we can conclude that the
method of Am(VI) preparation by ozonation of
Am(III) hydroxide suspension in bicarbonate solutions
is a complex multistep process, transforming with
time into oscillatory mode, in which the Am(VI) yield
depends of the initial americium content. Therefore, to
obtain pure Am(VI) solution, continuous spectro-
photometric monitoring of the Am valence forms in
the solution is required.
Fig. 2. Absorption spectra of the solutions obtained by dis-
solution of the precipitate formed during Am(OH) ozona-
3
tion in 0.1 M NaHCO for 90 min (a) in 0.1 M NaHCO
3
3
and (b) in 1 M HNO .
3
these solutions are shown in Fig. 2. The spectrum in
Fig. 2a is identical to the spectrum of Am(IV) carbo-
nate complex described in [3]; no Am(III) was found
in the solution.
REFERENCES
1. Shilov, V.P. and Yusov, A.B. Usp. Khim., 2002, vol. 71,
no. 6, pp. 533 558.
In the spectrum of the precipitate in 1 M HNO ,
2. Penneman, R.A., Coleman, J.S., and Keenan, T.K.,
3
3+
+
bands characteristic for hydrated Am and AmO
J. Inorg. Nucl. Chem., 1961, vol. 17, pp. 138 142.
2
cations are observed (Fig. 2b). We can suggest that
both Am(III) and Am(V) are formed by Am(IV) dis-
proportionation in the stage of the precipitate dissolu-
3. Bourge, J.Y., Guillaume, B., Koehly, G., et al., Inorg.
Chem., 1983, vol. 22, pp. 1179 1184.
4. Shilov, V.P., Nikolaevskii, V.B., and Krot, N.N., Ra-
diokhimiya, 1995, vol. 37, no. 1, p. 32.
tion in HNO . Am(IV), in turn, arises from Am(V)
3
reduction with hydrogen peroxide formed by reactions
(2) (4), in accordance with the reactions [5]
5. Musikas, C., Radiochem. Radioanal. Lett., 1973,
vol. 13, no. 4, pp. 255 258.
RADIOCHEMISTRY Vol. 46 No. 3 2004