Kinetics and mechanism of Np(IV) oxidation with nitric acid

Article abstract of DOI:10.1007/s11137-005-0082-x

Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions of low acidity (<～0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k ₁[Np(IV)]/[H ⁺]² + k ₂[Np(I

Full text of DOI:10.1007/s11137-005-0082-x

Radiochemistry, Vol. 47, No. 3, 2005, pp. 252 257. Translated from Radiokhimiya, Vol. 47, No. 3, 2005, pp. 228 233.
Original Russian Text Copyright 2005 by Koltunov, Taylor, Marchenko, Savilova, Dvoeglazov, Zhuravleva.
Kinetics and Mechanism of Np(IV) Oxidation with Nitric Acid
V. S. Koltunov*, R. Taylor**, V. I. Marchenko*, O. A. Savilova*,
K. N. Dvoeglazov*, and G. I. Zhuravleva*
*
Bochvar Research Institute of Inorganic Materials, State Scientific Center of the Russian Federation,
Moscow, Russia
*
* British Nuclear Fuels (BNFL), The United Kingdom
Received May 31, 2004
Abstract Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions
+
2
of low acidity (<~0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k [Np(IV)]/[H ] +
1
k [Np(IV)] [U(VI)]/[H ] , in which k = (2.0 0.3) 10 5 mol l min 1 and k2 = (5.50 0.47)
2
+ 3
2
2
2
1
2
mol l 1 min at 50 C and solution ionic strength = 0.5. The activation energies of the two pathways
1
1
0
1
are 148 31 and 122 12 kJ mol . The reaction along the main pathway (with the rate constant k ) is limited
2
by disproportionation of Np(IV) involving NpOH3 and Np(OH) UO2 complex ions.
+
4+
2
It was shown in [1, 2] that Np(IV) in the absence
The Np(IV) stock solution was prepared by dis-
of catalytic impurities is fairly stable in HNO solu-
solution of a weighed portion of NpO in 7 M HNO3
3
2
tions at moderate acidity ( 1 M) even at elevated
temperatures (50 C) and in the presence of HNO2.
Sokhina et al. [3] observed Np(IV) oxidation to
at reflux, followed by reduction of Np(VI) to Np(IV)
with hydrazine at 90 C. Then, Np(IV) was purified
by sorption on an anion-exchange resin, scrubbing
with 7.5 M HNO , and desorption with 0.5 M HNO .
Np(V) at 50 100 C and [HNO ] < 0.3 M; however,
3
3
3
no data on the reaction rate were reported.
The UO (NO ) stock solution was purified by per-
2 3 2
oxide precipitation as in [6]. Recrystallized NaNO
2
In this study we examined the kinetics and mechan-
was used as a source of HNO . Distilled water and
2
ism of Np(IV) oxidation in dilute HNO solutions.
3
distilled HNO were used to prepare both stock and
3
working solutions.
EXPERIMENTAL
The total Np concentration in the stock solution
was determined by -ray spectrometry, and the content
of its valence forms, spectrophotometrically, by
characteristic bands of Np(IV) at 723 and Np(V) at
The kinetics of Np(IV) oxidation with HNO was
3
studied spectrophotometrically as the time dependence
+
of the NpO optical density at 980 nm, taking into
2
4+
9
80 nm. The U(VI) concentration was determined by
account small absorption of Np (extinction coeffi-
cient
+
30 l mol 1 cm ).
1
titration with ammonium vanadate solution. The H
concentration in stock solutions was established by
potentiometric titration, and in working solutions
Preliminary experiments showed that the reaction
is strongly decelerated as the solution acidity is in-
(after reaction completion), pH-metrically with an
creased. For instance, in 0.5 M HNO at 50 C the
3
MP-220 pH meter (Mettler Toledo). The HNO con-
2
reaction does not start, at least in 6 h after mixing
centration in the working solutions was determined by
UV absorption at 372 nm. In the presence of U(VI),
the reactants, while in 0.02 M HNO it is completed
3
within 1 h (at [U(VI)] = 0.042 M). On the contrary,
HNO was analyzed colorimetrically by the Griess
2+
2
U
O
i
o
n
s
a
c
c
e
l
e
r
a
t
e
N
p
(
I
V
)
o
x
i
d
a
t
i
o
n
w
i
t
h
H
N
O
.
2
3
Ilosvay method after dilution of the solution samples
Taking into account these facts, we studied the reac-
to U(VI) concentrations less than 1 10 4 M.
+
tion kinetics within the ranges of [H ] 0.018 0.103 M
The solution spectra were measured on Lambda-40
Perkin Elmer) and UV-1201 (Shimadzu) spectro-
and [U(VI)] 0 0.113 M at 30 50 C (mainly 50 C).
The ionic strength of the solution was kept constant
(
photometers.
with NaNO . Under these conditions, the extinction
3
coefficient of Np(V) does not depend on the concen-
RESULTS AND DISCUSSION
+
2+
trations of H and UO2 ions; however, it decreases
at [U(VI)] > 0.15 M because of formation of a
Np(V) U(VI) cation cation complex [4, 5].
Since the kinetic experiments were performed in
the presence of uranyl, let us consider its effect on the
1
066-3622/05/4703-0252 2005 Pleiades Publishing, Inc.

KINETICS AND MECHANISM OF Np(IV) OXIDATION WITH NITRIC ACID
253
Np(IV) spectrum. We found that the Np(IV) extinc-
tion coefficients at 723 and 960 nm are unchanged
The absence of HNO is confirmed by the negative
results of its determination with the Griess Ilosvay
colorimetric method.
2
within the [HNO ] 0.05 0.2 M range, but decrease at
3
higher solution acidity. This follows from the data
Thus, the actual oxidant species in this reaction re-
mains to be elucidated. We will return to this problem
further.
presented below (at 20 C and
= 723 nm):
, l mol 1 cm 1
IV
[
HNO ], M
3
One of the features of the reaction between Np(IV)
0
0
0
0
0
1
.05
.10
.20
.35
.50
.00
72.0
73.0
72.4
66.2
57.2
43.5
and HNO is its high rate in the initial stage and
3
strong deceleration in the final stage of the reaction.
The treatment of the kinetic data under various experi-
mental conditions showed that they are well described
by the equation
2
The Np(IV) extinction coefficient also decreases
with increasing U(VI) concentration (at 20 C and
d[Np(V)]/dt = k [Np(IV)] + k [Np(IV)] ,
(2)
1
2
[
HNO ] = 0.2 M):
taking into account the occurrence of the process
along two parallel pathways with the first and the sec-
ond reaction orders with respect to Np(IV).
3
U(VI), M
0
, l mol 1 cm 1
IV
72.4
69.1
64.6
55.7
46.5
Denoting the Np(IV) initial concentration as a and
its concentration after time t from the start of the
0
0
0
0
.042
.10
.21
.42
reaction as (a
x), we have
2
d(a
x)/dt = k (a
x) + k (a
x) ,
(3)
1
2
which can be ratonalized by formation of a cation
cation complex Np(IV) U(VI). This suggestion is
and after integration,
confirmed by appearance of a new absorption band
at 977 nm with = 45 l mol 1 cm 1 in the Np(IV)
k2(a
x) + k1
k2a + k1
ln
= ln
+ k1t,
(4)
(5)
spectrum, whose intensity is proportional to the U(VI)
concentration.
a
x
a
or, in another form,
ln[(a
Using the above dependence of the observed
Np(IV) extinction coefficient at 723 nm on U(VI)
concentration and the equation of the balance of the
optical density at this wavelength, we can estimate the
stability constant of this complex, i.e., the equilibrium
constant of the reaction
x) 1 + K] = ln(a 1 + K) + k1t,
where K = k /k .The treatment of the kinetic data
2
1
by Eq. (5) consists in choosing K for each experiment
so as to attain the highest regression coefficient of
linear function (5). In most experiments, Eq. (5)
describes the reaction progress to conversion no less
than 75% with R > 0.999. The adequacy of Eq. (5) to
the experimental data is illustrated by Fig. 1.
Np4+ + UO 22 +
4+
NpUO .
6+
(1)
1
2
6
+
2+
K1 = [NpUO ]/[Np ][UO ] = 1.3 0.2 l mol
2
2
+
at 20 C and [H ] = 0.2 M.
However, as follows from the calculation results,
the use of Eq. (5) is incorrect in case when K substan-
tially exceeds 1/(a x), i.e., at small values of the rate
constant k . Therefore, the rate constants k and k we
The stoichiometry of the reaction between Np(IV)
and HNO is of particular interest. The changes in the
3
spectrum of the reaction mixture during the reaction
and after its completion show that Np(V) is one of
the reaction products. Further oxidation of Np(V) to
Np(VI) can be ruled out, taking into account the high
rate of the Np(IV) reaction with Np(VI) [7].
1
1
2
determined by numerical differentiation of Eq. (3)
using the minimal sum of deviation squares as a crite-
rion for choosing k and k . This method of data treat-
1
2
ment allowed satisfactory description of the kinetic
curves to conversion no less than 90%. An example of
data treatment with Eq. (3) is presented in Fig. 2.
Presumably, HNO3 is reduced with Np(IV) to
HNO . However, after reaction completion (and dur-
2
ing the reaction progress), the characteristic peaks of
The results of treatment of the experimental data
showed that the main contribution (more than 80%) to
HNO at 360 385 nm do not appear in the spectrum.
2
RADIOCHEMISTRY Vol. 47 No. 3 2005

254
KOLTUNOV et al.
+
decreases in inverse proportion to H concentration
squared. Thus, the rate of Np(IV) oxidation along the
pathway with the rate constant k is described by
1
the equation
+
2
d[Np(V)]/dt = k [Np(IV)]/[H ] ,
(6)
1
where k = (2.0 0.3) 10 5 mol2 l 2 min 1 at 50 C,
1
[
U(VI)] = 0.042 M, and
= 0.5.
The rate of the reaction along its main pathway
+
also decreases with increasing [H ]. As seen from
the table, the reaction order with respect to hydrogen
ion is approximately 3. Thus, at constant [U(VI)] and
, the Np(IV) oxidation along the pathway with the
Fig. 1. An example of treatment of kinetic curves by
3
Eq. (5). [Np(IV)] = 1 10 M, 50 C, [U(VI)] = 0.041 M,
0
rate constant k obeys the kinetic equation
2
+
=
0.5, [H ], M: (1) 0.033, (2) 0.050, and (3) 0.084.
Points: experiment; lines: calculation.
2
+ 3
d[Np(V)]/dt = k"[Np(IV)] /[H ] ,
(7)
2
where k" = (2.20 0.22) 10 3 mol2 l 2 min 1 at
2
5
0 C, [U(VI)] = 0.042 M, and
= 0.5.
As expected, the rate constant k" is virtually in-
2
dependent of the Np(IV) concentration in the initial
solution (see table), which additionally confirms the
second-order reactin with respect to Np(IV).
The effect of uranyl nitrate on the rate of Np(IV)
oxidation was studied at U(VI) concentration not
exceeding 0.15 M, which is caused by the necessity to
avoid complications in calculation of [Np(V)] from
the optical density at 980 nm, due to formation of
a Np(V) U(VI) complex. It follows from the table
that the rate constant of the main reaction pathway
increases approximately in proportion with the U(VI)
concentration. With regard to this dependence and
Eqs. (6), (7) for both reaction pathways, the kinetic
equation of the overall reaction is
Fig. 2. Kinetic curves of Np(V) formation in solutions con-
+
taining 0.042 M U(VI) at [H ] = 0.02 M, 50 C, = 0.5.
3
[
(
Np(IV)] 10 , M: (1) 1.24, (2) 11.0, (3) 0.87, and
0
4) 0.51. Points: experiment; lines: calculation by Eq. (3).
the Np(IV) oxidation rate is made by the reaction
pathway with the rate constant k . As for the second
2
reaction pathway with the rate constant k , it becomes
1
noticeable in the final stage of the process, where the
change of the solution optical density is very small,
+ 2
d[Np(V)]/dt = k [Np(V)]/[H ]
1
and correspondingly the error of k is high. Therefore,
2
+ 3
1
+ k [Np(IV)] [U(VI)]/[H ] ,
(8)
2
we could not reliably find k at low solution acidity
1
+
(
[H ] = 0.018 0.043 M, as in the majority of kinetic
where k = (2.0 0.3) 10 mol l min 1 and k =
5
2
2
1
2
experiments).
5.50 0.47) 10 mol l 1 min at 50 C and = 0.5.
2
1
(
As seen from the data listed below (at 50 C,
To determine the activation energies for these reac-
tion pathways, we studied the temperature depen-
3
[
0
U(VI)] = 0.042, [Np(IV)] = 1 10 M, and
.5):
=
0
dences of k and k . The results are presented below.
1
2
+
k1 10 , min 1
3
[
H ], M
T, C k1 10 , mol l min 1 k2 10 , mol l min 1
5
2
2
2
1
0
0
0
0
.050
.065
.084
.103
8.0
5.2
2.6
30
35
40
0.047
0.175
0.450
0.875
2.00
0.262
0.548
1.048
2.500
5.24
1.75
4
5
+
at higher concentration of H ions, the rate constant k
50
1
RADIOCHEMISTRY Vol. 47 No. 3 2005

KINETICS AND MECHANISM OF Np(IV) OXIDATION WITH NITRIC ACID
255
+
2+
The dependence of the second-order rate constant k on the concentrations of H and UO ions and Np(IV) initial
2
2
concentration at 50 C and
= 0.5
H ], M [U(VI)], M [Np(IV)]0 10 , M k , l mol 1 min 1 k" 10 ,* mol l min 1 k2 10 ,** mol l min 1
+
3
3
2
2
2
1
[
2
2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.033
.027
.029
.033
.023
.019
.023
.018
.033
.043
.050
.065
.084
.103
.019
.018
.019
.021
0
1.0
1.0
1.0
1.1
1.0
1.0
1.0
1.0
1.1
1.0
0.95
1.1
1.1
2.0
37.6
37.0
62.6
0.07
0.74
0.90
2.25
3.65
5.70
7.66
1.97
2.23
2.54
2.50
1.92
1.63
2.19
2.54
2.33
1.90
2.45
0.0105
0.021
0.042
0.063
0.084
0.113
0.042
0.042
0.042
0.042
0.042
0.042
0.042
0.042
0.042
0.042
0.042
7.05
4.30
5.33
5.79
6.78
6.78
4.45
5.30
6.01
5.95
4.58
3.90
5.20
6.04
5.55
4.52
5.84
300
830
630
320
62.0
32.0
20.0
7.0
2.75
2.00
370
400
275
265
1.0
0.48
0.51
0.87
1.24
Average
5.50 0.47
+
3
*
k" = k [H ] . ** k = k"/[U(VI)].
2 2 2 2
The activation energies calculated from these data
by the least-squares method are as follows: E = 148
There are no published data on the kinetic features
of Np(IV) disproportionation. Nevertheless, we can
make some conclusions on the kinetic equation of this
reaction considering the expression for the constant
of equilibrium (9)
1
1
3
1 and E = 122 12 kJ mol . As for the probable
2
reaction mechanism, it should be noted that HNO3
(nitrate ions) cannot be the actual oxidant species,
since (as follows from special experiments) Np(IV) is
oxidized also in HClO solutions. HNO (nitrite ions)
+
4
2
Keq = [Np(V)][Np(III)][H ] /[Np(IV)]
(10)
4
2
is not the oxidant for Np(IV) either, since the reaction
rate is independent of its concentration and, further-
more, the oxidation is observed in the presence of
and kinetic equation for reverse reaction between
Np(V) and Np(III) [9]
hydrazine, a reagent rapidly reacting with HNO [8].
2
+
In a separate experiment we found that Np(IV) is oxi-
dized in an argon atmosphere at the same rate as in
air, and hence oxygen cannot be the actual Np(IV)
oxidant.
d[Np(V)]/dt = k [Np(V)][Np(III)][H ].
(11)
r
Since the equilibrium constant is the ratio of the
rates of the reverse and direct reactions, we can obtain
the kinetic equation for reaction (9) by dividing
Eq. (11) by Eq. (10), i.e.,
The second order with respect to Np(IV) found for
the main reaction pathway suggests that, in this path-
way, Np(IV) is oxidized by disproportionation:
2
+ 3
d[Np(V)]/dt = k [Np(IV)] /[H ] .
(12)
f
2
Np4+ + 2H O
Np3+ + NpO+ + 4H+.
(9)
2
2
This equation formally coincides with Eq. (7)
describing the Np(IV) oxidation rate for the main
pathway. This can be considered as an indirect con-
firmation that the Np(IV) disproportionation is the
slow stage of this reaction pathway.
From the thermodynamic point of view, reac-
tion (9) is more probable in solutions of low acidity,
since the redox potential of the Np(V)/Np(IV) couple
rapidly decreases with increasing pH.
+
2+
This study, apparently, gave the first experimental
confirmation for the above hypothesis.
The fact that H ions inhibit this reaction and UO2
ions accelerate it suggests the participation of hy-
RADIOCHEMISTRY Vol. 47 No. 3 2005

256
KOLTUNOV et al.
drolyzed Np(IV) ions and complex Np(IV) U(VI)
ions in the slow stage of the reaction. Taking into
account the reaction order of 3 with respect to H
we obtain after transformations:
1[Np(IV)][H+]
+
3
+
[
NpOH ] =
, (21)
. (22)
ions, we can suggest that both reacting species contain
+
2
+ 2
2+
+
[
H ] + K [H ] [UO ] + [H ] +
hydroxy groups.1 Correspondingly, the scheme of
1
2
1
2
Np(IV) oxidation for the main reaction pathway can
be presented as follows:
2[
Np(IV)]
2
+
[Np(OH) ] =
2
+
2
+ 2
2+
+
[
H ] + K [H ] [UO ] + [H ] +
1 2 1 2
Np4+ + H O
NpOH3+ + H+,
(13)
(14)
(15)
2
Substitution of these expressions in Eq. (19) and
then in (18) gives the overall kinetic equation of the
reaction
Np4+ + 2H O
Np(OH) 22 + + 2H+,
2
Np(OH)2+ + UO 22 +
Np(OH) UO ,
4+
2
2
2
2
2+
+
d[Np(V)]
K [Np(IV)] [UO ][H ]
2 2
1
2
Np(OH) UO4 + NpOH3+
+
NpO H + UO 22 +
2+
=
. (23)
2
2
2
{[H ] (1 + K [UO ]) + [H ] + }2
+
2
2+
+
dt
1 2 1 2
+
Np3+ + H O,
(16)
(17)
2
This equation can be simplified taking into con-
sideration that the constant of equilibrium (1) is small
2
+
NpO +2 + H+.
NpO H
2
1
2+
(K1
1.3 l mol ) and the term (1 + k [UO ]) in
1 2
The charge transfer in the reaction slow stage (16)
proceeds as transfer of a hydrogen atom from the
the denominator exceeds unity by only 15% at the
highest U(VI) concentration used and in the most of
experiments it amounts to 1.055. The constant of the
first stage of Np(IV) hydrolysis is known at the solu-
4
+
3+
Np(OH) UO2 complex ion to the NpOH ion. The
2
rate of this stage and, correspondingly, the rate of
Np(IV) oxidation is
tion ionic strength
10 M). The values of
= 2 and 25 C [11] ( 1 = 5
3
4
+
3+
under other conditions are
1
d[Np(V)]/dt = [Np(OH) UO ][NpOH ].
(18)
2
2
unknown. However, we may suggest that in our case
is substantially lower. This conclusion is indirectly
confirmed by the fact that the Np extinction coef-
ficient at 723 nm remains unchanged with increasing
Obviously, because of low concentration of
1
4+
Np(OH)2+ ions, equilibrium (15) is virtually com-
2
pletely shifted to the left. Therefore, the concentration
4+
HNO concentration from 0.05 to 0.2 M. It is known
of Np(OH) UO2 species is low and equal to
3
2
that the hydrolysis constant of metal ions [12] and,
in particular, actinides [13] for the second step is ap-
proximately an order of magnitude lower than the
those for the first step. Thus, we can believe that
4
+
2+
2+
[
Np(OH) UO ] = K [Np(OH) ][UO ],
(19)
2
2
2
2
2
where K is the constant of equilibrium (15).
2
+
2
+
[
H ] >> ( [H ] + 2), and then Eq. (23) becomes
Let us express the concentrations of hydrolyzed
Np(IV) species through the Np(IV) analytical concen-
1
2
+ 3
d[Np(V)]/dt = k [Np(IV)] [U(VI)]/[H ] ,
(24)
tration ([Np(IV)]) and constants of the first ( ) and
2
1
4+
second ( ) steps of Np hydrolysis. It should be
2
2+
4+
where k =
K and [U(VI)] = [UO ].
1 2 2 2
taken into account also that a part of Np ions are
bound into the cation cation complex NpUO [reac-
tion (1)], which does not participate in the slow step
of the reaction. We can neglect small decrease in the
UO2 concentration because of formation of this
complex, since U(VI) occurs in the reaction mixtures
in a large excess as compared to Np(IV).
2
6
+
2
As seen, the calculated equation of Np(IV) oxida-
tion along its main pathway (24) is identical to
Eq. (8).
2+
As regards the minor reaction pathway with the
rate constant k , the known experimental data are
1
insufficient for understanding the mechanism of this
pathway. The negative second order with respect to
Further, using the equation of Np(IV) balance
+
H ions suggests that in the slow stage of the reaction
4
+
3+
2+
6+
[
Np(IV)] = [Np ] + [NpOH ] + [Np(OH) ] + [NpUO ],
2 2
along this pathway Np(IV) reacts probably in the form
of Np(OH)2 ions. However, the actual oxidant
+
(
20)
2
1
species remains to be elucidated. The reaction mech-
It is interesting that the slow stage of Pu(IV) disproportiona-
tion involves the reaction between the hydrolyzed ions
anism including the Np(OH)2 reaction with NO3
+
2
3
+
2+
PuOH
and Pu(OH)
.
ions in the slow stage formally corresponds to kinetic
2
RADIOCHEMISTRY Vol. 47 No. 3 2005

KINETICS AND MECHANISM OF Np(IV) OXIDATION WITH NITRIC ACID
257
equation (6). Regretfully, we cannot check this hypo-
thesis since the concentration of nitrate ions in all
the experiments was constant (0.5 M) because of the
constancy of the solution ionic strength ( = 0.5).
Moulin, J.P., J. Inorg. Nucl. Chem., 1979, vol. 41,
p. 1027.
6. Koltunov, V.S., Taylor, R.J., Marchenko, V.I. et al.,
Radiochim. Acta, 1999, vol. 86, pp. 41 49.
7
8
9
. Hudman, J.C., Sullivan, J.C., and Coner, D., J. Am.
Chem. Soc., 1954, vol. 76, p. 3278.
. Koltunov, V.S., and Marchenko, V.I., Kinet. Katal.,
Probably, a special study of the kinetics of the
reaction between Np(IV) and NO ions in HClO4
3
at high pH and various NO concentrations is required
3
1
966, vol. 7, no. 2, pp. 224 229.
to answer this question.
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