Reduction of Np(VI) with Carbohydrazide in a Perchloric Acid Solution

Article abstract of DOI:10.1134/S1066362219030044

The stoichiometry of the reaction of Np(VI) with carbohydrazide (NH₂NH)₂CO in a 0.02 M HClO₄ solution was studied by spectrophotometry. With Np(VI) taken in excess, 1 mol of carbohydrazide reduces 5 mol of Np(VI) to Np(V). In 0.1?2.0 M HClO₄ solutions (the ionic strength of 2.0 was supported by adding LiClO₄) containing 3–100 mM (NH₂NH)₂CO, Np(VI) at a concentration of 1 mM at 20-45°С is consumed in accordance with a first-order rate law until less than 0.2 mM Np(VI) remains. Then the reaction decelerates. The reaction is first-order with respect to carbohydrazide and has the order of ?1.45 with respect to Н⁺ ions. The activated complex is formed with the loss of 1 and 2 Н⁺ ions. The activation energy is 86 ± 5 kJ mol^?1.

Full text of DOI:10.1134/S1066362219030044

ISSN 1066-3622, Radiochemistry, 2019, Vol. 61, No. 3, pp. 309–311. © Pleiades Publishing, Inc., 2019.
Russian Text © The Author(s), 2019, published in Radiokhimiya, 2019, Vol. 61, No. 3, pp. 231–233.
Reduction of Np(VI) with Carbohydrazide
in a Perchloric Acid Solution
a
a
V. P. Shilov* and A. M. Fedoseev
a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, korp. 4, Moscow, 119071 Russia
*
e-mail: shilovV@ipc.rssi.ru
Received May 23, 2018; revised May 23, 2018; accepted June 19, 2018
Abstract—The stoichiometry of the reaction of Np(VI) with carbohydrazide (NH
solution was studied by spectrophotometry. With Np(VI) taken in excess, 1 mol of carbohydrazide reduces
mol of Np(VI) to Np(V). In 0.1–2.0 M HClO solutions (the ionic strength of 2.0 was supported by adding
) containing 3–100 mM (NH NH)
2
NH)
2
CO in a 0.02 M HClO
4
5
4
LiClO
4
2
2
CO, Np(VI) at a concentration of 1 mM at 20–45°С is consumed
in accordance with a first-order rate law until less than 0.2 mM Np(VI) remains. Then the reaction decelerates.
+
The reaction is first-order with respect to carbohydrazide and has the order of –1.45 with respect to Н ions.
+
–1
The activated complex is formed with the loss of 1 and 2 Н ions. The activation energy is 86 ± 5 kJ mol .
Keywords: neptunium(VI), carbohydrazide, stoichiometry, perchloric acid, kinetics
DOI: 10.1134/S1066362219030044
The extraction reprocessing of high-burn-up spent
nuclear fuel based on uranium dioxide and on uranium
nitride (for fast reactors) requires that the reagents used
for reducing Pu(IV, VI), Np(VI), and Tc(VII) should
form ultimately the minimal amount of mineral salts.
Carbohydrazide (NH NH) CO was suggested as one of
lence forms were similar to those used previously [3].
The stock aqueous solution of carbohydrazide was pre-
pared from a weighed portion of the compound (Acros
Organics, the United States) containing no less than
97% main substance.
2
2
Perchloric acid and lithium perchlorate were of
chemically pure grade; lithium perchlorate was addi-
tionally purified by recrystallization from water, fol-
lowed by drying in air at 190°C. The stock and work-
ing solutions were prepared using double-distilled
such reagents [1]. It should be borne in mind, however,
that carbohydrazide forms explosive compounds with
some metal ions. For example, the carbohydrazide
complex of iron(II) perchlorate exhibits properties
of a high explosive [2]. Nevertheless, proper organiza-
tion of the process allows accidents to be avoided.
Therefore, it is quite justified to study reactions of acti-
nides(VI, V, IV) and Tc(VII) with carbohydrazide.
Volk et al. [1] studied the reaction of Np(VI) and
water. The HClO concentration was determined by
4
titration with a standard NaOH solution. The LiClO
4
solution was prepared from a weighed portion of the
anhydrous recrystallized compound.
The experiment procedure was as follows. When
studying the reaction stoichiometry, a temperature-
controlled quartz cell (l = 1 cm) was charged with ap-
proximately 3 mL of an Np(VI) solution in 0.02 M
Np(V) with carbohydrazide in HNO solutions. They
3
noted high rate of the Np(VI) reduction to Np(V) at
room temperature. However, the kinetics of this reac-
tion was not studied in detail.
HClO , the absorption spectrum in the range 950–990 nm
4
In this work, we studied the stoichiometry and ki-
netics of the reaction of Np(VI) with carbohydrazide in
was recorded with a Shimadzu PC 3100 (Japan) or
SF-46 (Leningrad Optical and Mechanical Association,
Russia) spectrophotometer, an (NH NH) CO aliquot
HClO solutions and outlined the process mechanism.
4
2
2
was added, and the absorption at a wavelength of
80.8 nm, corresponding to the main absorption band
EXPERIMENTAL
9
The procedures for purifying 237Np, preparing a
stock solution of NpO (ClO ) in 0.02 M HClO , deter-
of Np(V), was measured at regular intervals. When
studying the reaction kinetics, the cell was charged
with a solution containing HClO , LiClO , and
2
4 2
4
mining the Np concentration, and monitoring the va-
4
4
3
09

3
10
SHILOV, FEDOSEEV
small amount of N O. Hence, a certain step of reac-
2
tions (2)–(5) involves the hydrolysis of the radical with
the elimination of N H and H CO groups.
2
x
2
3
n+
The kinetics of the Np(VI) + H (NH NH) CO
n
2
2
reaction was studied in 0.1–2.0 M HClO solutions at
4
the ionic strength I = 2.0, supported by adding LiClO .
4
The reaction stoichiometry allowed performing studies
at the carbohydrazide concentration exceeding the
Np(VI) concentration by a factor of only 3. Figure 1
shows the kinetic curves in the coordinates log(D –
∞
D)–time at T = 30°C. For a 2 M HClO solution con-
4
Fig. 1. Linearization of the kinetic curves of the Np(VI)
taining 1 mM Np(VI) and 100 mM carbohydrazide, the
kinetic dependence is a straight line up to 95% conver-
sion. In solutions containing 50 and 20 mM carbohy-
drazide, the reaction noticeably decelerates after 86
and 77% conversion, respectively. We failed to ac-
count for this fact.
reduction with carbohydrazide. [HClO
4
] = 2 M, [Np(VI)] =
1
mM, T = 30°C, l = 1 cm; [(NH
2
NH) CO], mM: (1) 20,
2
(
2) 50, and (3) 100.
(
NH NH) CO and was kept for 10–15 min at the preset
2
2
temperature. After introducing an aliquot of the stock
solution of Np(VI), the Np(V) accumulation was moni-
tored. Each experiment was performed no less than in
triplicate.
Linearization of the kinetic curves in the semilog
coordinates shows that the reaction is first-order with
respect to Np(VI). Therefore, the rate equation can be
presented in the form
RESULTS AND DISCUSSION
–
d[Np(VI)]/dt = k'[Np(VI)] = k'([Np(V)] – [Np(V)]), (6)
∞
The reaction stoichiometry was studied at 25–28°С.
where k' is the first-order rate constant. In the integral
form, after the replacement of [Np(V)] by the propor-
tional quantity D, Eq. (6) transforms into the expression
The Np(V) accumulation was observed in a 0.02 M
HClO solution containing 2 mM Np(VI) after adding
4
(
NH NH) CO to a concentration of 0.2 mM. From
2
2
the initial rate, we calculated the bimolecular rate con-
stant and estimated the time of the 99.9% reaction
completion. It should be equal to 10–11 min. Actually
the optical density ceased to increase in 16 min. In the
subsequent 3 h, the optical density increased by only
2
.303log(D
∞
– D) = k't + const.
(7)
Here, D and D are the running and final optical densi-
ties, respectively.
∞
The first-order rate constant increases proportion-
ally to the carbohydrazide concentration. The data ob-
tained in a 2 M HClO
Np(VI) are given in Table 1.
0
.010. The Np(V) concentration was calculated using
the molar extinction coefficient of Np(V), equal to
10 L mol–1 cm [4]. In three experiments, we ob-
–1
4
solution containing 1 mM
4
tained Δ[Np(VI)]/Δ[(NH NH) CO] = 5.
2
2
The fact that the bimolecular rate constant k =
1
Hydrazine exists in acid solutions in the cationic
form. Presumably, carbohydrazide also takes up one or
k'[(NH NH) CO] is independent of the carbohydrazide
2
2
concentration shows that the reaction is first-order
two protons in an HClO solution. Therefore, the fol-
4
with respect to the reductant; here, k includes the rate
1
lowing reactions occur between Np(VI) and carbohy-
drazide:
constant of reaction (1) and of the subsequent fast
reactions of the radicals with Np(VI). The k values ob-
1
NpO2
+
+ H
(NH
NH)
COn+ → NpO
+
2
+ H + R
+
2
n
2
2
1
,
(1)
2)
(3)
(4)
tained under different conditions are given in Table 2.
2
+
+
+
+
NpO
2
+ R
+ R
+ R
1
2
3
→ NpO
→ NpO
2
+ R ,
2
(
As seen from Table 2, the Np(VI) reduction sharply
2
+
+
NpO
NpO
NpO
2
2
2
+ R
3
,
decelerates with increasing H concentration. Figure 2
+
+
shows the log–log dependences of k on [H ]. At 25
→ NpO
2
+ R
4
,
1
2
2
2
+
+
and 30°C, the experimental points fall on straight lines
with a slope of –1.45. Hence, the reduction occurs via
formation of an activated complex with the elimination
of one and two protons. The activated complex is
+ R
4
→ NpO
2
+ products.
(5)
As noted in [5], excess carbohydrazide reacts with
Np(VI) to form a gas phase containing N , CO , and a
2
2
RADIOCHEMISTRY Vol. 61 No. 3 2019

REDUCTION OF Np(VI) WITH CARBOHYDRAZIDE
311
Table 1. Results of kinetic experiments in a 2 M HClO
solution containing 1 mM Np(VI)
4
[
(NH
2
NH)
mM
2
CO],
3
–1
–1 –1
T, °C
k' × 10 , s
k , L mol s
1
3
6
0
0
0
00
3
0
0.157
0.32
0.51
1.02
2.04
5.97
0.312
1.057
2.034
5.5
0.0523
0.0533
0.051
0.051
0.051
0.060
0.104
0.106
0.102
0.11
1
2
4
2
3
5
0
1
1
Fig. 3. logk
1
of the Np(VI) reduction with carbohydrazide
1
as a function of reciprocal temperature. [HClO ] = 2 M and
4
20
0
2 2
[(NH NH) CO] = 3 mM.
5
00
9.91
0.099
This is followed by the intramolecular charge trans-
fer with the formation of NpO and R.
+
2
Table 2. Influence of conditions on the bimolecular rate
constant of the Np(VI) + (NH NH) CO reaction. I = 2 M,
Np(VI)] = 1 mM, [(NH NH) CO] = 3–100 mM
The rate equation taking into account all the partici-
pants of the Np(VI) reduction with carbohydrazide in
2
2
[
2
2
–1
–1
an HClO
4
+ LiClO solution can be written in the form
4
T, °C
[HClO
4
], M
k
1
, L mol s
0.052
0.12
d[Np(VI)]/dt = k'[NpO2+
][(NH
CO][H+]–1.45,
2
1
–
2
2
NH)
2
–
0.45
mol0.45 s–1 at [H ] = 1 M and
+
2
5
0.5
.2
.1
0.54
1.76
4.06
0.104
0.345
0.857
2.79
where k = 0.12 L
T = 25°C.
0
0
2
1
From the temperature dependence of k in the coor-
1
3
–1
dinates 10 /T (K )–logk (Fig. 3), we found the activa-
1
–1
tion energy: 86 ± 5 kJ mol .
3
0
0.5
.2
.1
0
0
2
2
2
CONFLICT OF INTEREST
8.03
3
4
4
5
0
5
0.176
0.296
0.495
The authors declare that they have no conflict of
interest.
REFERENCES
formed by the reactions
NpO2
+
+
CO2+ + H+,
1. Volk, V.I., Marchenko, V.I., Dvoeglazov, K.N., et al.,
2
+ H(NH
2
NH)
2
CO ҙ NpO
2
(NH
2
NH)
2
Radiochemistry, 2012, vol. 54, no. 2, pp. 143–148.
NpO2
+
+ Н
NH)
CO2+ ҙ NpO
(NH
NH)
CO2+ + 2H+.
2
3
4
5
. Riu Liu, Zunning Zhou, Shuykan Qi, et al., Centr. Eur.
J. Energet. Mater., 2013, vol. 10, no. 1, pp. 17–36.
. Shilov, V.P. and Fedoseev, A.M., Radiochemistry,
2
2
(NH
2
2
2
2
2
2
015, vol. 57, no. 3, pp. 255–258.
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diochemistry, 2017, vol. 59, no. 3, pp. 229–232.
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and Mitricas, P.V., Study of the gas phase composition
formed during oxidation–reduction reactions of hydra-
zine organic derivatives, Sixth Int. Conf. on Radiation
and Applications in Various Fields of Research, Ohrid,
Macedonia (FYROM), June 18–22, 2018, http://
rad2018.rad-conference.org/.
+
Fig. 2. Influence of the concentration of H ions on the
bimolecular rate constant of the Np(VI) reduction with
carbohydrazide in a solution with the ionic strength I =
2
M. T, °C: (1) 25 and (2) 30.
Translated by G. Sidorenko
RADIOCHEMISTRY Vol. 61 No. 3 2019