Disproportionation of Pu(V) in aqueous HCOOH solutions

Article abstract of DOI:10.1134/S106636221001008X

The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH-H₂O system was studied by spectrophotometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO₄ changes on adding HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at 830.6 nm, corresponding to an f-f transition in the Pu ₂ ²⁺ aqua ion, a new band arises with the maximum shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1: 1). The Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO₄ solution, which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately proportional to [Pu(V)]² and grows with an increase in [HCOOH]. The reaction products affect the reaction rate: Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cationcation complex. The disproportionation occurs via formation of a Pu(V)-Pu(V) cation-cation complex whose thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).

Full text of DOI:10.1134/S106636221001008X

ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 1, pp. 35–39. © Pleiades Publishing, Inc., 2010.
Original Russian Text © V.P. Shilov, A.V. Gogolev, A.A. Bessonov, A.V. Anan’ev, M.Yu. Boltoeva, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 1,
pp. 32–35.
Disproportionation of Pu(V) in Aqueous HCOOH Solutions
V. P. Shilov, A. V. Gogolev, A. A. Bessonov, A. V. Anan’ev, and M. Yu. Boltoeva
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, Moscow, 119991 Russia; e-mail: bessonov_ipc@mail.ru
Received June 6, 2009
Abstract—The behavior of Pu(VI), Pu(V), and Pu(IV) in the HCOOH–H
photometry. The Pu(VI) absorption spectrum in solutions containing less than 1 mM HClO
2
O system was studied by spectro-
changes on adding
4
HCOOH to a concentration of 0.53 M. Along with a decrease in the intensity of the absorption maximum at
2
2
+
8
30.6 nm, corresponding to an f–f transition in the PuO
aqua ion, a new band arises with the maximum
shifted to 834.5 nm. These transformations are due to formation of a Pu(VI) formate complex (1 : 1). The
Pu(IV) absorption spectra in HCOOH solutions vary insignificantly in going from 3.0 to 9.0 M HCOOH and
are similar to the spectrum of Pu(IV) in a 0.88 M HCOOH + 0.41 M NaHCOO + 0.88 M NaClO solution,
4
which indicates that the composition of the Pu(IV) formate complexes is constant. Pu(V) is unstable in
HCOOH solutions and disproportionates to form Pu(VI) and Pu(IV). The reaction rate is approximately propor-
2
tional to [Pu(V)] and grows with an increase in [HCOOH]. The reaction products affect the reaction rate:
Pu(IV) accelerates the process, and Pu(VI) decelerates the consumption of Pu(V) by binding Pu(V) in a cation–
cation complex. The disproportionation occurs via formation of a Pu(V)–Pu(V) cation–cation complex whose
thermal excitation yields an activated complex with its subsequent decomposition to Pu(VI) and Pu(IV).
Key words: plutonium(V), disproportionation, formic acid
DOI: 10.1134/S106636221001008X
The Pu(V) disproportionation
plexes. Therefore, in dilute НСООН solutions the
hydrolysis of Pu(IV) is possible, especially at ele-
vated temperatures.
+
2
+
4+
2+
2
2
PuO
+ 4H = Pu + PuO
+ 2H
2
O
(1)
(2)
and the related reaction
EXPERIMENTAL
+
2
4+
3+
2+
2
PuO
+ Pu = Pu + PuO
239
Working solutions of Pu(VI) in 0.01 M HClO4
were prepared by the known procedure [1]. HCOOH,
determine the stability of Pu(V) in solutions of any
composition. The rate of reaction (1) depends on the
content of Pu(V), Н ions, water, and complexing
HCOONa, and Н О were of analytically pure grade.
2
2
All solutions were prepared in double-distilled water.
+
Plutonium(V) was prepared by two procedures. The
first procedure is reduction of Pu(VI) with hydrogen
peroxide in 0.02–0.03 M HCOONa:
ligands. The latter factor is important for the occur-
rence of reaction (2). In aqueous carboxylic acid solu-
tions, the effect of all the above factors can be traced.
In our previous paper [1], we considered the Pu(V)
disproportionation in the СН СООН–Н О system.
+
2
Pu(VI) + H
2
O
2
= 2Pu(V) + 2H .
(3)
3
2
In this study we examined the behavior of Pu(V)
in solutions of formic acid, which is a stronger acid
than acetic acid but forms (as anion) less strong com-
H O was taken in a 10% excess relative to the
2
2
stoichiometry, because Н О is also consumed in side
2
2
reactions.
plexes with Pu(VI) than do acetate ions {logβ at an
1
+
ionic strength of 1.0 for ·PuO (НСОО) and PuO ·
The second procedure is photochemical reduction
of Pu(VI) in 0.2–0.3 M НСООН. For this purpose, a
quartz cell with a Pu(VI) solution was placed in a
2
2
+
(
СН СОО) is 1.03 [2] and 2.05 [3], respectively}.
3
The similar ratio should be observed for Pu(IV) com-
3
5

3
6
SHILOV et al.
of Pu(VI). Then the Pu(VI) reduction continued for
several hours in the dark with the reductants accumu-
lated in the system.
The Pu(IV) solution was prepared as follows. From
a nitric acid solution of Pu(IV), we precipitated
Pu(OH) with an ammonia solution. The precipitate
4
was separated by centrifugation, washed with ice-cold
double-distilled water, and dissolved in concentrated
HCl. The PuCl obtained after evaporation was dis-
4
solved in 2.75 M HCl.
The progress of Pu(VI) reduction and Pu(V) dispro-
portionation was monitored by the Pu(VI) absorption in
the range 830–850 nm. The disproportionation leads to
the formation of Pu(IV), and its absorption is summed
up with the Pu(VI) absorption. Therefore, initially we
examined the spectra of Pu(VI) and Pu(IV) in HCOOH
solutions at wavelengths of 400–1000 nm with a Shima-
dzu UV-3100 PC spectrophotometer (Japan). The ki-
netic studies were performed with an SF-46 spectropho-
tometer (Leningrad Optical and Mechanical Association,
Russia) by the following procedure. In experiments per-
formed at 18–20°С with solutions with [HCOOH] ≤
Fig. 1. Variation of the Pu(VI) absorption spectrum in
aqueous HCOOH solutions. [Pu(VI)] = 2 × 10 M.
–
3
(
(
1) [HClO
4) 8.00, and (5) 26.10.
4
] = 1.0 M; [HCOOH], M: (2) 0.53, (3) 1.04,
beam of UV and visible light from a VIO-1 installation
with two super-high-pressure mercury lamps DRK-
20. The irradiation was interrupted at regular inter-
vals, and the absorption spectrum in the range 400–
000 nm, where the absorption bands of all the valence
1
1
forms of Pu are located, was recorded. The irradiation
was stopped after disappearance of approximately 80%
6
M, Pu(V) was prepared directly in the spectropho-
tometric cell either by reduction of Pu(VI) with hydro-
gen peroxide or photochemically, after which an aliquot
of anhydrous НСООН was added to the cell. In experi-
ments with [HCOOH] = 7–12 M at 20°C and with
[HCOOН] = 2–6 M at 25–45°С, to 0.05–0.10 ml of the
initial Pu(VI) solution we added 0.015 ml of 5 M
NaHCO and a calculated volume of 0.08–0.1 M H O .
Table 1. Influence of the НСООН concentration on the
major absorption band of Pu(VI)
[
HCOOH],
M
First maximum
Second maximum
–
1
–1
–1
–1
λ, nm ε, l mol cm λ, nm ε, l mol cm
–
830.2
830.2
830.2
830.2
830.2
830.2
830.2
830.2
830.4
830.4
830.4
830.4
555
2
2
2
0
1
2
2
4
6
7
8
.53
.04
.02
.93
.56
.00
.83
.00
356
834.0
834.1
834.1
834.1
834.1
834.1
834.1
834.4
834.4
834.4
834.4
834.5
834.5
834.5
834.5
834.5
834.5
834.5
834.5
210.8
239.1
261.2
266.9
273.6
273 ± 3
273 ± 3
273 ± 5
273 ± 5
273 ± 5
273 ± 5
273 ± 5
273 ± 5
278
After 20–25 min, 0.1–0.2 ml of the Pu(V) solution was
introduced into the cell with an HCOOH solution.
311.7
269.6
247.4
225.3
210.2
198
RESULTS AND DISCUSSION
Spectra of Pu(VI) and Pu(IV) in HCOOH Solutions
Figure 1 shows the spectra of Pu(VI) in 1 M HClO4
solution and in HCOOH solutions containing about
195.8
189.8
188
0
.1 mM HClO . With an increase in the HCOOH con-
1
1
1
1
1
2
2
2
2
2
2
0.0
2.1
4.1
7.8
8.8
0.6
2.6
4.3
5.2
5.7
6.1
4
centration in solution, the intensity of the absorption
maximum at 830.6 nm, corresponding to an f–f transi-
192
2
+
tion in the PuO2 aqua ion, decreases, and a new band
arises with the maximum shifted to 834.5 nm. These
transformations are due to formation of a 1 : 1 Pu(VI)
formate complex:
282.6
292
2
+
–
+
{PuO
2
}·5H
2
O + HCOО = {PuO
2
HCOО }·(5 – n)H
2
O + nH
2
O.
4)
(
301
306
The positions of the maxima and the molar extinc-
tion coefficients ε are given in Table 1.
315
RADIOCHEMISTRY Vol. 52 No. 1 2010

DISPROPORTIONATION OF Pu(V) IN AQUEOUS HCOOH SOLUTIONS
37
In the range 5–22 M HCOOH, variation of the solu-
tion composition does not significantly affect ε. How-
ever, in 24–26 M HCOOH, ε increases. This is due to
2
2
+
replacement of H O molecules in the PuO equatorial
2
sphere by НСООН molecules.
Figure 2 shows the spectra of Pu(IV) in HCOOH
solutions containing ~0.05 M NaCl. The positions and
heights of the absorption maxima change insignifi-
cantly in going from 3.0 to 9.0 M HCOOH and to a
0
.88 M HCOOH + 0.41 M NaHCO + 0.88 M NaClO
2 4
Fig. 2. Electronic absorption spectra of aqueous Pu(IV)
solutions: (1) [HCOOH] = 25.7, [Pu(IV)] = 1.66 × 10 M;
solution, suggesting constant composition of the
Pu(IV) formate complexes.
–
3
(
[
1
2) [HCOOH] = 0.88, [HCOONa] = 0.41, [NaClO
4
] = 0.88,
–3
Pu(IV)] = 1.17 × 10 M; (3) [HClO
4
] = 1.14, [Pu(IV)] =
–
3
.58 × 10 M.
Kinetics of Pu(V) Disproportionation
In 4–12 M HCOOH solutions at 18–20°C, by the
end of the reaction the sum of the Pu(VI) and Pu(IV)
concentrations corresponded to the amount of Pu(VI)
taken for the reduction to Pu(V). Only the absorption
bands of Pu(IV) and Pu(VI) were present in the spec-
trum, whereas the Pu(III) bands were absent. It should
be noted that Pu(V) slowly disproportionated in 0.5–
2
M HCOOH solutions, but the arising Pu(IV) was in
the hydrolyzed form and gave a suspension undergoing
sedimentation with time.
Fig. 3. Kinetic curves of Pu(VI) accumulation in Pu(V)
disproportionation. T = 20°С, [HCOOH] = 6 M; [Pu(V)]
M: (1) 1.52 and (2) 3.26.
0
,
In 4 M HCOOH at 25°С, in one experiment Pu(IV)
underwent hydrolysis and polymerization, and in the
other experiment it remained nonpolymerized, but
at 27°С the Pu(IV) hydrolysis was observed in all
the experiments. At the same temperature in several
experiments Pu(IV) underwent hydrolysis in 6 M
HCOOH also.
tionation in solutions containing 3–4 mM Pu(VI) or
mM Pu(IV). The results of these experiments are
given in Table 2.
4
It can be readily seen that Pu(VI) decelerates the
loss of Pu(V). This is caused by formation of cation–
cation complexes (CCCs):
Figure 3 shows the kinetic curves of the Pu(VI)
accumulation. These curves have induction periods
observed in the majority of experiments. some curves
are linearized in the semilog coordinates, which could
suggest the first order of the reaction with respect to
Pu(V). However, the slope of the linearized kinetic
curves changes with an increase in the initial Pu(V)
concentration. In a 6 M HCOOH solution containing
by the end of the induction period 1.21, 1.52, and
Pu(V) + Pu(VI) қ CCC(I).
(5)
Table 2. Effect of Pu(VI) and Pu(IV) on the Pu(V) dispro-
portionation
[
Pu], mM
[HCOOH], M
τ1/2, s
Pu(VI) Pu(IV)
Pu(V)
1.45
1.47
1.52
1.75
0.6
4
4
6
6
6
600
1860
130
372
50
3
.26 mM Pu(V) (T = 20°С), the half-conversion times
4
τ1/2 are 156, 130, and 65 s, respectively. It follows
from these values that the reaction rate follows a sec-
ond-order law, i.e., in the initial period reaction (1)
prevails. The apparent first order is due to participation
of the reaction products in the transformation of Pu(V)
into Pu(VI) and Pu(IV).
4
4.3
1
1
0
0
1.1–1.2
0.62
0.8
24–44
117
80
3
3
We performed experiments on Pu(V) dispropor-
10
RADIOCHEMISTRY Vol. 52 No. 1 2010

3
8
SHILOV et al.
Table 3. Influence of the solution composition on kobs
As a first approximation, Eq. (8) transforms into the
expression
–1 –1
T, °C
[HCOOH], M
kobs, l mol s
1
1
2
2
2
2
2
2
2
3
3
8
9
0
5
0
0
0
0
0
0
5
4
4.6
6
1.15 ± 0.2
1.36 ± 0.2
5 ± 1
2
–
d[Pu(V)]/dt = 2d[Pu(VI)]/dt = kobs[Pu(V)] ,
(9)
where kobs is the observed second-order rate constant.
6
4.35 ± 0.5
6.5 ± 1.5
10.6 ± 3
20 ± 10
24.4 ± 10
~48
To evaluate the effect of the HCOOH concentration
on the stability of Pu(V), we used the second-order rate
constants found by the expression
7
8
9
10
11
6
k
obs = 1/([Pu(V)]τ1/2),
(10)
5.7 ± 0.5
5.2 ± 0.5
where [Pu(V)] = 2{[Pu(VI)] – [Pu(VI)]} = 2ΔD/(εl),
ΔD = D – D , D and D are the solution optical den-
∞
6
∞
1
1
∞
sities by the end of the induction period and after the
reaction completion, respectively; τ1/2 = τ – τ , τ is
2
1
1
The Pu(V) bound in the complex reacts more
slowly or does not react at all with another Pu(V) ion.
Similar phenomenon was observed in Pu(V) dispropor-
the time of the end of the induction period and τ , time
2
IV
VI
at which D = D + ΔD/2; ε = ε + ε . The k values
2
1
obs
are given in Table 3.
tionation in 15 M CH COOH but was not observed in
3
6
M CH COOH [1].
The above data show that the kobs values obtained
under similar conditions exhibit strong scatter. Most
probably, this is due to the hydrolysis of Pu(IV). The
point is that the components were mixed manually, and
therefore it was impossible to standardize the mixing
rate and avoid local effects, which could give rise to
conditions for the formation of Pu(IV) hydroxide oli-
gomers. In cases when these oligomers had no time to
form, Pu(IV) participated in reaction (2) and increased
the rate of Pu(VI) accumulation.
3
Pu(IV) accelerates the Pu(VI) accumulation in
HCOOH in the same manner as it does in CH COOH
3
solutions [1]. Therefore, it can be assumed that, in the
HCOOH–Pu(V) system, reactions (1), (2), (6), and (7),
postulated for acetic acid solutions, occur:
Pu(VI) + Pu(III) → Pu(V) + Pu(IV),
Pu(V) + Pu(III) → 2Pu(IV).
(6)
(7)
The kobs values significantly increase with the
HCOOH concentration. Figure 4 shows in the log–log
coordinates the dependence of kobs on [HCOOH], al-
lowing determination of the reaction order with respect
to НСООН. It appeared to be equal to 3.7. In an aque-
The rate of the Pu(V) disappearance in reac-
tions (1), (2), and (7) and the rate of its accumulation
in reaction (6) are described by the equation
2
–
d[Pu(V)]/dt = 2k
1
[Pu(V)] + k
2
[Pu(V)][Pu(IV)]
ous HClO solution, the rate of the Pu(V) dispropor-
4
+
tionation is directly proportional to [H ] [4]. If this
+
k
7
[Pu(V)][Pu(III)] – k
6
[Pu(VI)][Pu(III)].
(8)
trend were preserved in an aqueous НСООН solution,
+
0.5
where [H ] = (K [HCOOH]) and K is the НСООН
a
a
dissociation constant, k would be proportional to
obs
0
.5
[
HCOOH] . The stronger dependence observed
should be attributed to participation of HCOOH mole-
cules in the formation of an activated complex or its
precursor, cation–cation complex of Pu(V) ions:
Pu(V) + Pu(V) → CCC(II).
(11)
Thermal excitation of the cation–cation complex
yields an activated complex, in which charge transfer
occurs:
Fig. 4. log–log plot of kobs vs. НСООН concentration.
RADIOCHEMISTRY Vol. 52 No. 1 2010

DISPROPORTIONATION OF Pu(V) IN AQUEOUS HCOOH SOLUTIONS
39
CCC(II) қ activated complex,
activated complex → Pu(IV) + Pu(VI).
(12)
(13)
ACKNOWLEDGMENTS
The study was financially supported by the Divi-
sion of Chemistry and Materials Science, Russian
Academy of Sciences (Program no. 1).
Thus, we found that Pu(V) is unstable in HCOOH
solutions and disproportionates to form Pu(VI) and
Pu(IV). The disproportionation rate is approximately
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reaction products affect the disproportionation rate:
Pu(IV) accelerates the process, whereas Pu(VI) decel-
erates the Pu(V) consumption by binding Pu(V) in a
cation–cation complex.
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