Reduction of Np(VI) with Hexamethylenetetraacetic Acid in HClO4 Solution

Article abstract of DOI:10.1134/S1066362220020034

Abstract: Spectroscopic method was used to examine the stoichiometry of the reaction of No(VI) with hexamethylenediaminetetraacetic acid (HMDTA, H₄hmdta) in a 0.05 M HClO₄ solution. At an excess of Np(VI), 1 mole of complexon reduces about 4 moles of Np(VI) to Np(V). In 0.1–1.0 M HClO₄ solutions (the ionic strength of 1.0 was maintained with LiClO₄) containing 2–20 mM HMDTA, neptunium(VI) with concentration of 0.3–3.0 M decreases at 35–55°C by the first-order rate law until the instant when less than 20% Np(VI) remains. The initial reaction rate has the first order in [HMDTA] and –2 order in [H⁺]. An activated complex is formed with loss of two H⁺ ions. The activation energy is 102 ± 7 kJ/mol.

Full text of DOI:10.1134/S1066362220020034

ISSN 1066-3622, Radiochemistry, 2020, Vol. 62, No. 2, pp. 170–172. © Pleiades Publishing, Inc., 2020.
Russian Text © The Author(s), 2020, published in Radiochemistry, 2020, Vol. 62, No. 2, pp. 123–125.
Reduction of Np(VI) with Hexamethylenetetraacetic Acid
in HClO Solution
4
a,
a
V. P. Shilov * and A. M. Fedoseev
a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, 119071 Russia
e-mail: ShilovV@ipc.rssi.ru
*
Received January 10, 2019; revised January 10, 2019; accepted January 24, 2019
Abstract—Spectroscopic method was used to examine the stoichiometry of the reaction of No(VI) with hexameth-
ylenediaminetetraacetic acid (HMDTA, H hmdta) in a 0.05 M HClO solution. At an excess of Np(VI), 1 mole of
4
4
complexon reduces about 4 moles of Np(VI) to Np(V). In 0.1–1.0 M HClO solutions (the ionic strength of 1.0 was
4
maintained with LiClO ) containing 2–20 mM HMDTA, neptunium(VI) with concentration of 0.3–3.0 M decreases
4
at 35–55°C by the first-order rate law until the instant when less than 20% Np(VI) remains. The initial reaction rate
+
+
has the first order in [HMDTA] and –2 order in [H ]. An activated complex is formed with loss of two H ions.
The activation energy is 102 ± 7 kJ/mol.
Keywords: neptunium(VI), hexamethylenediaminetetraacetic acid, reduction, kinetics
DOI: 10.1134/S1066362220020034
In a systematic study of the stability of Np(VI) in
complexon solutions, attention has been given to the
influence exerted by the number of nitrogen atoms
and carboxylate groups in a complexon molecule and
to the structure of a carbon chain between nitrogen
atoms. In the present study, we examined the behavior
of an analog of EDTA, i.e., the complexon in which
the carbon chain of six methylene groups replaced the
ethylene group in the EDTA molecule. The molecule of
hexamethylenediaminetetraacetic acid (HMDTA) differs
from the EDTAmolecule in its solubility in water and acid
media and also in the value of the dissociation constants.
It is difficult to predict the reducing capacity of HMDTA
in comparison with EDTA. This problem can be only
solved experimentally.
molar ratio. The Li H hmdta solution was quantitatively
2 2
transferred into a measuring flask. Chemically pure
HClO and LiClO of pure grade were used. Lithium
4
4
perchlorate was recrystallized from a bidistillate and dried
in air. All the solutions were prepared with a bidistillate
and normalized by the known procedures.
When studying the reaction stoichiometry, we
introduced an aliquot of the Li H hmdta solution into
2
2
a quartz cuvette (lc1 cm) with a solution of Np(VI) in
HClO and the optical absorption at 981 nm, where
4
the main absorption band of Np(V) is peaked, was
periodically recorded with a Shimadzu PC 3100 (Japan)
or SF-46 LOMO (Russia) spectrophotometer. In kinetic
studies, an aliquot of a Np(VI) solution was added to the
HClO + H hmdta solution in a thermostated cuvette and
4
4
the increase in the Np(V) concentration was monitored.
Each experiment was repeated 2–3 times.
EXPERIMENTAL
A 237Np preparation was used in experiments.
Neptunium was purified and a stock solution of Np(VI)
was prepared and normalized by the procedure reported
in [1]. Hexamethylenediaminetetraacetic acid of pure
grade (main-substance content no less than 99%) was
used without additional purification. A weighed portion
of HMDTA was mixed with a LiOH solution in a 1 : 2
RESULTS AND DISCUSSION
The stoichiometry of the reaction Np(VI) +
H hmdta was examined in 0.01 M HClO at 23°C and
4
4
0.05 M HClO at 45°C. The initial concentrations of
4
Np(VI) and H hmdta were 3 and 0.5 mM, respectively.
4
The Np(V) concentration was calculated by using the
170

REDUCTION OF Np(VI) WITH HEXAMETHYLENETETRAACETIC ACID
171
‒
1
‒1
molar absorption coefficient of 403 M cm . At room
temperature, 1.58 mM of Np(V) was formed in 70 h,
and 1.9 mM of Np(V) accumulated at 45°C in 8.7 h, i.e.,
Δ[Np(V)]/Δ[H4hmdta] is close to 4. Below, an example
is presented of one of the runs.
Time, h
0.014
0.033 0.211 4.17
0.82 1.44 1.70
8.67
1.90
[Np(V)], M 0.54
Presumably, the reaction between Np(VI) and
H hmdta occurs at a high rate because the stoichiometric
4
coefficient reached a value of 1.6 already in 2 min. The
further accumulation of Np(V) is due to the slow reaction
of Np(VI) with decomposition products of HMDTA. The
data obtained suggest that the following reactions occur
in solutions containing HClO and H hmdta.
Fig. 1. Kinetic curves for the reduction of Np(VI) by H hmdta
4
molecules at 45°C. 0.25 M HClO , 0.75 M LiClO , 10 mM
4
4
H4hmdta. [Np(V)], mM: (1) 0.3, (2) 1.0, (3) 3.0.
4
4
The first-order rate constant grows with increasing
H hmdta concentration. This can be seen for the example
of 0.25 M HClO4 and 0.75 M LiClO4 solutions containing
Np

VI

+ H hmdta  Np

V

+ R1

slowly

,
(1)
(2)
(3)
4
4
Np

VI
VI
+ R  Np

+ R  Np

V

+ R2

rapidly


,
,
1
1
mM Np(VI) and 2–20 mM H hmdta at 45°C.
4
Np


+ R  Np

V

+ R3

slowly
2
The bimolecular rate constant k = k'/[H hmdta]
4
Np

VI


V + low-activity products. (4)

3
remains nearly constant in the range of H hmdta
4
concentrations under study, which is indicative of the
first order in the reducing agent. Table 1 lists k values
obtained in different conditions.
The kinetics of the reaction Np(VI) + H hmdta was
4
examined in 0.1–1.0 M HClO solutions at a ionic
4
strength I = 1.0 M, maintained by addition of LiClO4.
Figure 1 shows kinetic dependences plotted in the time–
log(D – D) for 0.25 M HClO + 0.75 M LiClO solutions
It follows from the values listed in Table 1 that
the rate of Np(VI) reduction sharply decreases with
increasing concentration of perchloric acid. Figure 2
∞
4
4
containing 10 mM of H4hmdta and 0.3, 1.0, and 3.0 mM
of Np(VI) at 45°C.
+
shows the dependence of k on [H ], plotted in the
logarithmic coordinates. At 45°C, experimental
points fall on the straight line with slope ratio of –2.0.
Therefore, a conclusion can be made that the reduction
occurs via formation of an activated complex with
[
H hmdta], mM
2
3
5
10
20
40
4
4
–1
k’×10 , s
3.60 4.80 9.50 19
–
1
–1
k, M ·s
0.18 0.16 0.19 0.19 0.20
The kinetic dependences are linear until half or more
of Np(VI) has reacted.
Table 1. Effect of conditions on the bimolecular rate constant
of the reaction Np)VI) + H hmdta. I = 1.0 M. [H4hmdta] =
4
The linearity of the initial portions of the kinetic
curves plotted in semilog coordinates is indicative of the
first order of the reaction in Np(VI). The reaction rate is
described by the equation
3
–10 mM, [Np(VI) = 1 mM.
T, °C
[HClO ], M
k, M–1 s–1
1.520
0.190
0.050
0.012
0.060
0.120
0.38
4
45.0
0.10
0
0
1
.25
.50
.00
(5)
where k’ is the first-order rate constant. In the integral
form, equation (5) passes on replacing [Np(V)] with D
proportional to the concentration into the equation
35.0
0.25
0.25
0.25
0.25
4
4
5
0.0
9.5
5.0
2
.3log

D –D

= –k't+ const.
(6)

0.68
RADIOCHEMISTRY Vol. 62 No. 2 2020

172
SHILOV, FEDOSEEV
+
Fig. 2. Effect of the concentration of H ions on the rate constant
Fig. 3. Dependence of the logarithm of the rate constant of
of Np(VI) reduction by H hmdta molecules in a solution with
Np(VI) reduction on inverse temperature in 0.25 M HClO +
4
4
ionic strength of 1.0 at 45°C.
0.75 M LiClO solutions.
4
splitting of two protons, but it is not improbable that
protons are separated in equilibrium reactions before
the appearance of an activated complex.
The radical R rapidly reacts with Np(VI), but it is not
impossible that the radical activates the molecule or the
ion of the reducing agent.
1
Changing the ionic strength affects the rate of the
Figure 3 shows the temperature dependence of k in
the Arrhenius coordinates. The activation energy found
from this dependence 102 ± 7 kJ/mol.
reaction Np(VI) + H hmdta. In a 0.1 M HClO solution,
4
4
k = 3 M–1 s , and in a 0.1 M HClO + 0.9 M LiClO
–1
4
4
–
1 –1
solution, k = 1.52 M s . The decrease in the reaction
rate with increasing ionic strength is characteristic of the
interaction of oppositely charged species.
Comparison of the kinetic patterns for the reactions
of Np(VI) with H edta and H hmdta shows that the
4
4
rate constants are markedly different under identical
conditions, the reaction mechanisms involving these
compounds being also different.
In solutions, there exist the equilibria
–
+
H hmdta = H hmdta + H ,
4
3
–
2–
+
H hmdta = H hmdta + H .
3
2
CONFLICT OF INTEREST
The authors state that they have no conflict of interest.
REFERENCES
The activated complex (AC) is formed by the reaction
2

2–
NpO + H hmdta = AC.
2
2
Further, charge transfer occurs in the activated
complex
1
. Shilov, V.P. and Fedoseev,A.M., Radiochemistry, 2015,
vol. 57, no. 3, p. 255.
https://doi.org/ 10.1134/S1066362215030054
+
AC  NpO + R .
2
1
RADIOCHEMISTRY Vol. 62 No. 2 2020