Reduction of Am(IV) with water in KHCO3 + K2CO 3 solutions

Article abstract of DOI:10.1023/A:1026068327516

Reduction of Am(IV) with water in KHCO₃ and K₂CO ₃ solutions (pH 8.5-10.5) was studied spectrophotometrically at 54-70°C. The Am(IV) concentration decreases, following the first-order rate law. The reduction rate increases with pH (Δ log k/ΔpH = 0.4), but decreases with increase in (HCO₃^- + CO₃ ^2-) concentration. It was assumed that the thermally excited Am(IV) ion forms a dimer with unexcited Am(IV). The dimer dissociates into two Am(III) ions and H₂O₂. Hydrogen peroxide reduces two more Am(IV) ions. In this process, the excited Am(III) ion appears, which transfers the excitation to Am(IV) at collision. Thus, a chain process is initiated. This scheme can also explain the kinetics of Am(VI) and Np(VII) reduction in carbonate solutions.

Full text of DOI:10.1023/A:1026068327516

Radiochemistry, Vol. 45, No. 3, 2003, pp. 279 282. Translated from Radiokhimiya, Vol. 45, No. 3, 2003, pp. 256 259.
Original Russian Text Copyright 2003 by Shilov.
Reduction of Am(IV) with Water in KHCO + K CO Solutions
3
2
3
V. P. Shilov
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received May 21, 2002
Abstract Reduction of Am(IV) with water in KHCO and K CO solutions (pH 8.5 10.5) was studied
3
2
3
spectrophotometrically at 54 70 C. The Am(IV) concentration decreases, following the first-order rate law.
2
The reduction rate increases with pH ( logk/ pH = 0.4), but decreases with increase in (HCO + CO ) con-
3
3
centration. It was assumed that the thermally excited Am(IV) ion forms a dimer with unexcited Am(IV).
The dimer dissociates into two Am(III) ions and H O . Hydrogen peroxide reduces two more Am(IV) ions.
2
2
In this process, the excited Am(III) ion appears, which transfers the excitation to Am(IV) at collision. Thus,
a chain process is initiated. This scheme can also explain the kinetics of Am(VI) and Np(VII) reduction in
carbonate solutions.
In bicarbonate carbonate solutions, Am(IV) gradu-
ally disappears because of disproportionation and
reduction with water [1, 2]. Disproportionation is
important at pH higher than 10.The mechanism of
this reaction was studied in [3]. The data on Am(IV)
reduction with water are fragmentary and do not
discover the reaction mechanism. In this work, we
studied the kinetics of Am(IV) reduction under vari-
ous conditions and the reaction mechanism.
quartz focusing lens. Under the action of UV light,
the colorless solution became yellow, suggesting
Am(III)
Am(IV) transformation (confirmed spec-
trophotometrically). The rate of Am(IV) accumulation
depends on the solution composition. To obtain (4
6) 10 5 M Am(IV), 1 3 h is required. In the process,
a part of Am remained in the trivalent state.
The cell with the solution was placed in a brass
holder temperature-controlled with circulated water
from a U-10 thermostat (Germany). In turn, the holder
was placed in the cell compartment of the spectro-
photometer. After 20 30 min temperature stabiliza-
tion, the light absorption was measured at 370 nm and
in the 400 600 nm range. After the measurements,
the temperature in the cell was checked, and after
cooling the cell to 20 C the pH of the solution was
measured with an OP-211/1 pH meter (Hungary) with
a glass electrode.
EXPERIMENTAL
243Am containing 0.15 wt % 241
Accordingly to -ray spectrometry, the contribution of
Am was used.
2
41
244
Am and
Cm to the -activity were 2.6 and
2
.4%, respectively. The Am(ClO ) solutions were
4
3
prepared and purified as in [4, 5]. KHCO and K CO
3
2
3
were twice recrystallized. Double-distilled and, in
some cases, triple-distilled water was used.
RESULTS AND DISCUSSION
The electronic absorption spectrum of Am(IV) in
KHCO3 + K CO3 solutions has a band at 369
2
In 1.5 M KHCO (pH 8.48 8.70), Am(IV) (5
10 M) completely transforms into Am(III) at 22
3
70 nm with the extinction coefficient
=
3
5
840 l mol 1 cm [6], which allows spectrophoto-
1
2
6
8 C. In the 0.75 M KHCO + 0.75 M K CO solu-
metric monitoring of the Am(IV) reduction. For in-
3 2 3
5
stance, in a 2-cm optical cell, 5 10 5 M solutions of
tion (pH 9.96), Am(IV) (n 10 M) is almost com-
pletely reduced at 20 25 C, but at 55 C 25% of
Am(IV) remains unchanged. The other valence states
of Am were not observed. Thus, under the conditions
studied, Am(IV) is mainly consumed in reduction
reactions. The typical kinetic curves of Am(IV) con-
sumption are shown in Fig. 1. Certain distortions of
the initial portions of the curves are due to the tem-
perature change in this period. The middle part of the
curve is a straight line in semilog coordinates, which
indicates the first order of the reaction with respect
Am(IV) can be studied. At such low concentrations,
the disproportionation of Am(IV) is minimal.
To avoid introduction of any additional substances
into the solution, Am(IV) was prepared photochemi-
4
cally [7]. An 1 10 M Am(ClO ) solution contain-
4
3
ing KHCO or KHCO + K CO was added into a
3
3
2
3
cylindrical quartz cell, which was placed under the
UV beam generated by a VIO-1 device equipped with
two super-high-pressure Hg lamps and cylindrical
1
066-3622/03/4503-0279$25.00 2003 MAIK Nauka/Interperiodica

280
SHILOV
Fig. 1. Kinetic curves of Am(IV) reduction with water in
KHCO + K CO solutions. Cell thickness l = 2 cm, T =
3
2
3
5
(
(
4.2 C. [KHCO ], M: (1) 0.75 and (2) 1.5; [K CO ], M:
3 2 3
5
1) 0.75 and (2) 1.5. [Am(IV)] 10 , M: (1) 7.5 and
2) 5.4.
Fig. 3. Effect of pH on the rate constant of Am(IV) reduc-
tion with water. T, C: (1) 54.2, (2) 63, and (3) 70.
0
to Am(IV), and hence, to a first approximation, the
kinetic equation is
d[Am(IV)]/dt = k [Am(IV)].
(1)
In the integral form, after substitution of the
Am(IV) concentration by the proportional value (D
D ) (D is light absorption at 370 nm), Eq. (1) trans-
forms into
2
.3log(D
D ) = k t + const,
(2)
Fig. 2. Rate constant of Am(IV) reduction with water as
a function of total KHCO + K CO concentration. pH
3
2
3
where D and D are the optical densities at the
moment and after Am(IV) disappearance, respectively.
The constant k was determined graphically. Its aver-
age values under various conditions are listed in the
1
0.01 9.86, T = 54.2 C.
Rate constants of Am(IV) reduction with water in KHCO +
3
K CO solutions
2
3
2
table. As seen, at [HCO ] : [CO ] = 1 : 1 and con-
3
3
4
[
KHCO3], [K CO ],
k
10 ,
stant temperature Am(IV) becomes more stable with
2
3
T,
C
pH
1
2
M
M
s
an increase in the total HCO + CO3 concentration.
3
The experimental values of k (in the logarithmic
coordinates) fall on a straight line, whose shape n =
5
5
5
5
5
5
5
5
6
6
6
6
6
7
7
7
7
7
4.2
0.375
0.75
1.125
1.5
1.5
1.25
1.0
0.5
1.5
1.25
1.0
0.75
0.5
0.375
0.75
1.125
1.5
9.86
9.96
9.98
10.01
8.50
9.27
9.65
10.26
8.6
19.4
5.24
2.22
1.23
1.6
3.14
4.00
6.67
4.14
7.60
9.9
4.2
4.2
4.2
4.2
4.2
4.2
4.2
3
3
3
3
3
2
logk / log[HCO + CO ] = 2.0. This indicates that
3
3
2
the Am(IV) complex ion eliminates two CO3 ions
(2n
4)
Am(CO )
(H O) 2x n + zH O
Am(CO ) (H O)
3 n 2 x +z
0.25
0.5
1.0
3 n + 2
2
2
2
+
2CO ,
(3)
3
either in the rate determining or preceding stage.
In the solutions with the constant total concentra-
0.25
0.50
0.75
1.0
0.25
0.5
0.75
0.8
1.0
9.3
9.65
9.96
10.26
9.4
9.63
9.96
10.01
10.26
12.3
2
tion [HCO ] + [CO ] = 1.5 M, the rate constant k
3
3
15.2
14.8
15.2
22.9
29.0
28.8
increases with pH. The dependence of logk on pH is
linear (Fig. 3). The shape n = logk / pH equal to
0
0
0
0
1.25
1.0
0.75
0.7
0
.34 0.40 shows that OH ions participate in the
consumption of Am(IV). Firstly, this may be due to
parallel disproportionation of Am(IV), whose rate is
0
0.5
2
proportional to [OH ] . Therefore, the rate of Am(IV)
RADIOCHEMISTRY Vol. 45 No. 3 2003

REDUCTION OF Am(IV) WITH WATER IN KHCO3 + K CO3 SOLUTIONS
281
2
disappearance should be presented as
required to form H O . Nevertheless, the reaction
2
2
order should remain first, as follows from the experi-
ment. The mechanism similar to the mechanism of
excimer formation (i.e., when the photoexcited species
reacts with unexcited species to form a dimer termed
excimer) meets the above conditions. The mechanism
of excimer formation under the action of light quanta
considered in [11] assumes that the excited species
can be also generated thermally. The fraction of
2
V = k [Am(IV)] = k [Am(IV)] + 2k [Am(IV)] or
1
2
k = k1 + 2k [Am(IV)],
(4)
2
where k is the rate constant of Am(IV) reduction
1
with water and k is the constant of Am(IV) dis-
2
proportionation. To estimate k , we subtracted from k
1
the 2k values taken from [3] and recalculated to
excited molecules *n is ln(*n/n) =
E/(RT), where
2
higher temperatures using the activation energy of
E is the activation energy. Therefore, in our case the
Am(IV) excitation proceeds thermally. It should be
noted that the number of excited ions is small.
1
00 kJ mol 1 [2]. The resulting k values increase
1
with pH; in the logk pH coordinates, the shape of
2
the straight line is 0.44. Thus, Am(IV) is reduced by
two pathways; in one of them the reactive species is
The thermally excited Am(IV) complex ion forms
with unexcited Am(IV) a dimer (bound by coor-
dinated water molecules)
2
n 4
Am(CO ) (H O)
, and in the second it is the
3
n
2
x
(
2n 3)
hydrolyzed ion AmOH(CO ) (H O)
. These
3
n
2
x 1
species are formed in the stages preceding the charge
transfer.
heating
Am(IV)
*Am(IV),
dimer.
(7)
(8)
The activation energy of Am(IV) reduction within
the 54 70 C range (pH 9.96) was estimated at
*
Am(IV) + Am(IV)
1
1
8
6 kJ mol , which is close to 96 kJ mol obtained
The dimer decomposes with the liberation of H O2
and two Am(III) ions. Then, the following reactions
occur:
2
in [2].
Let us consider the mechanism of the charge trans-
fer. It is usually suggested that the oxidant reacts with
water or OH ions to form OH radicals, which then
recombine. Then, in our case, the following reactions
would proceed:
2
CO
3
Am(IV) + H O2
*Am(III) + HO + HCO , (9)
2
3
2
HO2 + CO2
O2 + HCO3,
(10)
(11)
3
2
CO
3
Am(IV) + O2
*Am(III) + O2.
Am(IV) + H O
Am(III) + OH + HCO , (5)
3
2
H O , 2k = 1 10 l mol 1 s 1 [8]. (6)
1
0
The comparison of the redox potentials of
Am(IV)/Am(III), H O /H O, and O /O couples (0.20
OH + OH
2 2
2
2
2
2
2
and 0.33 V for the two latter, respectively [10])
shows that, in reactions (9) and (11), the substantial
free energy is released, which can be localized on
Am(III) and transfer it into the excited state.
Let us estimate the validity of this scheme. The
redox potential of the Am(IV)/Am(III) couple in
bicarbonate carbonate solution is 0.9 V [2, 6]. The
standard potential of the OH/OH couple is 1.77 V
[
9], and at pH 10 the redox potential of the OH radical
The reaction
reaches 2 V. Thus, the concentration of OH radicals
corresponding to equilibrium (5) is 20 orders of
magnitude lower than the Am(IV) concentration. So
low concentration of OH radicals cannot provide the
rate of H O generation corresponding to the observed
*Am(III) + Am(IV)
Am(III) + *Am(IV)
(12)
proceeds with the diffusion-controlled rate.
2
2
The occurrence of reaction (12) is confirmed by the
fact that Am(III) is oxidized under irradiation of the
Am(III) and Am(IV) mixture in spite of superposition
of the Am(IV) and Am(III) charge-transfer bands [7].
Due to reaction (12), the chain process is developed,
in which the number of excited Am(IV) species sub-
stantially exceeds the number of Am(IV) ions excited
thermally. A part of *Am(III) is deactivated in other
reactions.
rate of Am(IV) consumption.
The mechanism of H O oxidation to H O in one
2
2 2
stage seems more acceptable. The standard potential
of the H O /H O couple is 1.76 V [10], in the solu-
tion with pH 10 the potential of this couple decreases
to approximately 1.2 V, i.e., it does not strongly
exceed the potential of the Am(IV)/Am(III) couple.
However, the participation of two Am(IV) ions is
2
2
2
RADIOCHEMISTRY Vol. 45 No. 3 2003

282
SHILOV
The decrease in the HCO and CO2 concentrations
causes the appearance of the low-charged Am(IV)
complexes, which favors dimer formation and ac-
celerates the reaction.
Np(VI) + O2
can also proceed.
*Np(V) + O2
(29)
3
3
In conclusion, we should note that the proposed
scheme of reduction of the metal ions with water can
be applicable not only to actinides but also to thermo-
dynamically unstable ions (or molecules) of other
elements.
Ii is found in [1, 2] that Am(IV) reduction in car-
bonate solutions with water obeys the first-order rate
law. Probably, in this case the above dimers are also
formed, i.e., reactions (13) (18) and (10) proceed:
heating
Am(VI)
*Am(VI),
dimer,
(13)
(14)
(15)
ACKNOWLEDGMENTS
*
Am(VI) + Am(VI)
The author is indebted to Doctor of Chemistry
A.B. Yusov for help in the experimental work and
participation in discussion.
dimer
Am(VI) + H O + CO
2Am(V) + H O ,
2 2
2
*Am(V) + HO + HCO , (16)
2
2
3
2
3
REFERENCES
Am(VI) + O2
*Am(V) + O2,
(17)
(18)
1
2
. Frenkel’, V.Ya., Lebedev, I.A., Khizhnyak, P.L., and
Myasoedov, B.F., Radiokhimiya, 1985, vol. 27, no. 4,
pp. 405 407.
*
Am(V) + Am(VI)
Am(V) + *Am(VI).
The excimer mechanism can also explain the
Np(VII) behavior in bicarbonate carbonate solutions.
At pH 9 11, Np(VII) transforms into Np(VI) by the
first-order rate law; increase in pH decelerates the
reaction [12]. In this case, reactions (19) (24) and
. Frenkel’, V.Ya., Khizhnyak, P.L., Lebedev, I.A., and
Myasoedov, B.F., Radiokhimiya, 1985, vol. 27, no. 5,
pp. 576 581.
3
4
5
6
7
8
. Shilov, V.P. and Yusov, A.B., Radiokhimiya, 1993,
vol. 35, no. 5, pp. 61 67.
(
10) proceed:
. Yusov, A.B. and Shilov, V.P., Radiokhimiya, 1992,
vol. 34, no. 6, pp. 21 32.
heating
Np(VII)
*Np(VII),
dimer I,
(19)
(20)
(21)
(22)
(23)
(24)
. Shilov, V.P. and Yusov, A.B., Radiokhimiya, 1993,
vol. 35, no. 1, pp. 38 45.
*Np(VII) + Np(VII)
. Berger, P., Blank, P., and Bourges, J., Radiochim.
Acta, 1988, vol. 43, no. 4, pp. 217 228.
dimer I
Np(VII) + H O
2Np(VI) + H O ,
2 2
. Shilov, V.P. and Yusov, A.B., Khim. Vys. Energ.,
1995, vol. 29, no. 5, pp. 358 362.
2
2
*Np(VI) + HO2,
. Pikaev, A.K., and Kabakchi, S.A., Reaktsionnaya
sposobnost’ pervichnykh produktov radioliza vody:
Spravochnik (Reactivity of Primary Products of Water
Radiolysis: Handbook), Moscow: Energoizdat, 1982.
Np(VII) + O2
*Np(VI) + O2,
*
Np(VI) + Np(VII)
Np(VI) + *Np(VII).
9
. Koppenol, W.H. and Liebman, J.F., J. Phys. Chem.,
After Np(VI) accumulation, reactions
Np(VI) + Np(VI) dimer II,
1
984, vol. 88, no. 1, pp. 99 101.
*
(25)
(26)
(27)
(28)
10. Bratsch, S.G., J. Phys. Chem. Ref. Data, 1989, vol. 18,
no. 1, pp. 1 21.
dimer II
Np(VII) + Np(V)
Np(VI) + H O
2Np(V) + H O ,
2 2
1
1
1. Baltrop, J.A. and Coyle, J.D., Excited States in Or-
ganic Chemistry, London: Wiley, 1975.
2Np(VI),
2. Shilov, V.P., Stepanova, E.S., and Krot, N.N., Radio-
khimiya, 1976, vol. 18, no. 3, pp. 350 354.
Np(V) + HO2,
2
2
RADIOCHEMISTRY Vol. 45 No. 3 2003