Electrochemical oxidation of Am(III) and Am(V) ions in HNO3 solutions containing potassium phosphotungstate K10P 2W17O61

Article abstract of DOI:10.1007/s11137-006-0008-2

The kinetics of electrochemical oxidation of Am(III) and Am(V) on Pt electrode in solutions of concentrated nitric acid containing potassium phosphotungstate was studied spectrophotometrically. The processes occurring in the system follow the rate law for reversible reaction. The direct reaction is first-order with respect to the concentration of Am(III) ions, and the reverse process is a zero-order reaction.

Full text of DOI:10.1007/s11137-006-0008-2

Radiochemistry, Vol. 47, No. 6, 2005, pp. 563 566. Translated from Radiokhimiya, Vol. 47, No. 6, 2005, pp. 517 519.
Original Russian Text Copyright 2005 by Erin, Baranov, Volkov, Chistyakov.
Electrochemical Oxidation of Am(III) and Am(V) Ions in HNO3
Solutions Containing Potassium Phosphotungstate K P W O
17 61
1
0 2
E. A. Erin, A. A. Baranov, A. Yu. Volkov, and V. M. Chistyakov
Research Institute of Nuclear Reactors, State Scientific Center of the Russian Federation, Dimitrovgrad,
Ul’yanovsk oblast, Russia
Received June 7, 2004; in final form, July 11, 2005
Abstract The kinetics of electrochemical oxidation of Am(III) and Am(V) on Pt electrode in solutions of
concentrated nitric acid containing potassium phosphotungstate was studied spectrophotometrically. The proc-
esses occurring in the system follow the rate law for reversible reaction. The direct reaction is first-order with
respect to the concentration of Am(III) ions, and the reverse process is a zero-order reaction.
The chemical or electrochemical oxidation of
Am(III) to Am(V) and Am(VI) in solutions of mineral
acids (1 8 M) in the presence of potassium phospho-
tungstate (KPW) was studied previously [1 6]. How-
ever, development of novel procedures of americium
recovery requires more detailed data on the direction
and completeness of the redox reactions and on the
kinetics of oxidation processes.
lated at HNO concentration exceeding 4 M do not
3
pass through the origin. This fact suggests that the
general form of the rate equation of electrochemical
oxidation of Am(III) is as follows:
CAm(III)/dt = keCAm(III)
kc,
(1)
i.e., electrochemical oxidation of Am(III) proceeds
by two parallel pathways: (1) first-order reaction with
In [7, 8] we studied electrochemical oxidation of
Am(III) and Am(V). Additional studies and com-
prehensive analysis of experimental data suggested a
new interpretation of the kinetics of electrochemical
oxidation of Am(III), revealing common features and
differences in the kinetics of Am(III) and Am(V) elec-
trochemical oxidation. This was the subject of the
present study.
EXPERIMENTAL
The procedures of electrochemical oxidation of
Am(III) and Am(V) and spectrophotometric monitor-
ing of various oxidation states of americium are
presented elsewhere [7, 8].
Fig. 1. Kinetics of electrochemical oxidation of Am(III);
3
3
C
5.5 10 M, C 0.92 10 M. C
, M: (1) 1.0,
KPW
0
HNO
3
(
2) 1.9, (3) 3.9, and (4) 5.7.
RESULTS AND DISCUSSION
Kinetics of Electrochemical Oxidation of Am(III)
The kinetic curves of electrochemical oxidation of
Am(III) as influenced by HNO concentration are
3
shown in Fig. 1. As seen from the spectrophotometric
data, in all the cases Am(III) is oxidized to Am(IV)
and
CAm(III)
=
CAm(IV).
After mathematical treatment, these kinetic curves
are easily linearized in the CAm(III)/ t CAm(III)
coordinates (Fig. 2). As seen, the straight lines calcu-
Fig. 2. Linearized kinetic curves of electrochemical oxida-
tion of Am(III). C
(4) 5.7.
, M: (1) 1.0, (2) 1.9, (3) 3.9, and
HNO
3
1
066-3622/05/4706-0563 2005 Pleiades Publishing, Inc.

5
64
ERIN et al.
Table 1. Apparent rate constants of the reaction Am(III)
Am(IV) + e; C0 0.92 10 M, S/V* 1.87 cm ml
of electrochemical oxidation of Am(III). We studied
self-reduction of Am(IV) at various concentrations of
3
2
1
HNO and KPW and different dose rates (J) [7]. We
3
CKPW 103,
k
c
IV
10 ,
7
CHNO3,
III
4
1
found that Am(IV) converts into Am(III) by first- and
zero-order reactions with respect to the Am(IV) con-
centration:
k
10 , s
e
1
1
M
M
mol l s
1
1
2
3
5
5
5
5
.0
.9
.9
.9
.7
.7
.7
.7
5.5
5.5
5.5
5.5
5.5
7.9
10.6
13.6
33
14
7.9
6.3
5.2
4.7
4.4
4.0
CAm(IV)/dt = k C
+ k0.
(2)
1
Am(IV)
0.5
2.66
0.8
0.7
0.6
Based on the data or self-reduction of Am(IV), we
can interpret the kinetic curves of electrochemical
oxidation of Am(III). Since the second term of Eq. (1)
describes reduction of Am(IV), the following expres-
sion should be valid:
2
*
S is the electrode area (cm ) and V is the solution volume
(
kc = k0 + k CAm(IV).
(3)
1
ml); the same for Table 3.
Table 2. Comparison of apparent constants k and k C
1 0
The current concentration of Am(IV) can be expressed
c
through the total (C ) and current concentration of
0
CHNO3, k1 105, k C 107,
kc 10 ,
7
Am(III):
1
0
k /(k C )
s 1
1
1
mol l s 1
1
c
1
0
M
mol l
s
kc = k0 + k C0
k1CAm(III).
(4)
1
2
3
5
.9
.9
.7
1.36
1.53
2.1
1.25
0.14
0.19
After substitution of expression (4) in Eq. (1) and
appropriate transformations, we obtain:
0.5
2.6
3.7
13.6
dCAm(III)/dt = (ke + k )C
k C
1 0
k0. (5)
1
Am(III)
respect to Am(III), whose apparent rate constant (ke)
is determined as the slope of the above straight line,
and (2) zero-order reaction, whose apparent rate con-
Since the two first terms in the right part of Eq. (5)
are greater than k by more than an order of magni-
0
stant (k ) is determined by the intercept.
tude, the contribution of radiation-chemical reduction
of Am(IV) (zero-order reaction) to the total electro-
chemical reduction of Am(III) can be neglected. As
a result, Eq. (6) is obtained:
c
Up to HNO concentration of 4 M, the electro-
3
chemical oxidation follows the kinetic law involving
the first term of Eq. (1) (Fig. 2, lines 1, 2). At HNO3
concentrations exceeding 4 M, the contribution of
the zero-order reaction with respect to the Am(III)
concentration to the total oxidation rate becomes
noticeable. The minus sign at the second term of
Eq. (1) indicates that the chemical oxidation of
Am(III) is complicated by the reverse process, i.e.,
reduction of Am(IV).
dCAm(III)/dt = (ke + k )C
k C .
(6)
1
Am(III)
1 0
If our assumption on self-reduction of Am(IV) in
the course of electrochemical oxidation of Am(III) is
correct, then Eq. (6) becomes equivalent to the experi-
mentally found rate law (1) at
The apparent rate constants k Ie II and k cI V determined
kc = k C .
(7)
1
0
at various concentrations of HNO and KPW are listed
3
III
However, actually Eq. (7) is not observed (Table 2),
and the deviations are the greater, the higher the con-
centration of HNO . These data suggest that the con-
tribution of self-reduction of Am(IV) to the overall
process is insignificant and decreases with increasing
CHNO3.
in Table 1. As seen, ke decreases with increasing
CHNO3 and only slightly changes with varying C
KPW
at CKPW/CAm > 6. At the same time, k Ic V increases
with increasing CHNO3 and is almost independent of
3
CKPW. Hence, the contribution of Am(IV) chemical
reduction to the total process of electrochemical oxi-
dation of Am(III) increases with increasing CHNO
3
Kinetics of Electrochemical Oxidation of Am(V)
and is almost independent of CKPW
.
Self-reduction of Am(IV) is one of the reactions
responsible for the reduction of Am(IV) in the course
Electrochemical oxidation of Am(V) was studied at
various concentrations of HNO (1 7 M) and KPW
3
RADIOCHEMISTRY Vol. 47 No. 6 2005

ELECTROCHEMICAL OXIDATION OF Am(III) AND Am(V) IONS
565
the CAm(V)/ t CAm(V) coordinates (Fig. 4). Thus,
the electrochemical oxidation of Am(V) proceeds by
the same kinetic law as does the electrochemical oxi-
dation of Am(III) [Eq. (1)]. Electrochemical oxidation
of Am(V) is accompanied by the reverse process
yielding Am(V). In contrast to the oxidation of
Am(III), the contribution of this process in the course
of electrochemical oxidation of Am(V) is noticeable
in the entire range of experimental conditions studied.
The experimental data show that the reverse process in
electrochemical oxidation of Am(V) is not related to
self-reduction of Am(VI); kc (1.88 10 7 mol l 1 s )
VI
1
Fig. 3. Kinetics of electrochemical oxidation of Am(V);
3
3
C
4.6 10 M, C 0.91 10 M. C
, M: (1) 1.0,
KPW
0
HNO
is greater by two orders of magnitude than the zero-
3
(
2) 3.0, (3) 4.0, (4) 5.0, and (5) 7.0.
order rate constant k (1.2 10 mol l 1 s ) obtained
9
1
0
in studying self-reduction of Am(VI) [9].
The apparent rate constants of electrochemical oxi-
V
VI
dation of Am(V) (k ) and reduction of Am(VI) (k )
e
c
as influenced by the concentrations of HNO and
3
V
VI
KPW are listed in Table 3. As seen, k and k de-
crease with increasing CHNO and are almost indepen-
e
c
3
dent of CKPW
.
Several conclusions follow from the data on elec-
trochemical oxidation of Am(III) and Am(V). The
III
rates of electrochemical oxidation of Am(III) (k ) and
e
Fig. 4. Linearized kinetic curves of electrochemical oxida-
V
Am(V) (k ) decrease with increasing HNO concen-
e
3
tion of Am(V). C
and (5) 7.0.
, M: (1) 1.0, (2) 3.0, (3) 4.0, (4) 5.0,
HNO
3
tration; this is probably due to a decrease in degree of
complexation of the resulting Am(IV) and Am(VI)
ions, leading to a decrease in their stability. The in-
(
1.8 9.3 M). The kinetic curves of electrochemical
crease in the KPW concentration at CKPW/CAm(III)
>
oxidation of Am(V) at various HNO concentrations
3
6
and CKPW/CAm(V) > 4.6 only slightly affects the
are shown in Fig. 3. The kinetic dependences of the
concentration of Am(V) at varied concentration of
KPW are principally similar to those shown in Fig. 3.
oxidation rate of Am(III) and Am(V). Probably, under
the experimental conditions studied the degree of
complexation of Am(IV) and Am(VI) remains virtual-
ly constant, i.e., certain similarity is observed in the
electrochemical oxidation of Am(III) and Am(V).
The mathematical treatment of all the experimental
kinetic curves showed that they can be linearized in
At the same time, the dependences of the rates of
chemical reduction of Am(IV) and Am(VI) on the
HNO3 concentration are different. With increasing
Table 3. Apparent rate constants of the reaction Am(V)
Am(VI) + e; C0 0.91 10 M, S/V* 4.36 cm ml
3
2
1
IV
VI
VI
7
HNO concentration, kc increases, whereas kc de-
k
c
10 ,
3
3
V
3
1
CHNO3, M CKPW 10 , M
k
10 , s
e
1
1
creases. This indicates that the mechanisms of
Am(IV) and Am(VI) reduction are different. Probably,
reduction of Am(IV) predominantly proceeds by its
mol l
s
1
3
4
5
7
3
3
3
3
1
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
4.6
4.6
4.6
4.6
4.6
1.8
3.8
5.6
9.3
1.76
1.56
1.40
1.17
0.97
1.51
1.71
1.70
1.64
1.90
1.88
1.25
1.10
0.90
0.32
1.08
1.07
1.05
1.17
1.96
reaction with the products of HNO decomposition
3
forming during electrolysis at high oxidation potential
(1.9 V). In the case of reduction of Am(VI), along
with the above process, other redox reactions generat-
ing Am(VI) are also possible: disproportionation of
Am(V) and reversible reaction Am(V) + Am(IV)
Am(VI) + Am(III). The rate ratio of these reactions
would determine the total rate of Am(VI) reduction.
Our experimental data do not allow final conclu-
RADIOCHEMISTRY Vol. 47 No. 6 2005

5
66
ERIN et al.
sion on the mechanism of electrochemical oxidation
of Am(III) and Am(V). However, they can be used in
preparation and stabilization of Am(IV) and Am(VI).
This opens prospects for studying redox reactions of
americium in higher oxidation states, whose occur-
rence in solutions of mineral acids in the absence
of a complexing agent was only assumed.
4. Adnet, J.M., Donnet, L., Brossard, P., and Bourges, J.,
in Int. Conf. Global-95 , France, September 11 14,
1
995, vol. 1, pp. 1040 1049.
5. Adnet, J.M., Donnet, L., Brossard, P., and Bourges, J.,
Abstracts of Papers, 4 Int. Conf. on Nuclear and Radio-
chemistry, St. Malo (France), September 8 13, 1996,
vol. 1, p. C-03.
6
. Adnet, J.M., Donnet, L., Chartier, D., et al., in Int.
Conf. Global-97, Yokohama (Japan), October 5 10,
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