Reaction of Np(VI) with H2O2 in carbonate solutions

Article abstract of DOI:10.1134/S1066362210030045

The kinetics and stoichiometry of the reaction of Np(VI) with H ₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1-0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as δ[Np(VI)]/δ[H₂O₂] ≈ 2. In Na ₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na ₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5-0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol^-1. Pleiades Publishing, Inc., 2010.

Full text of DOI:10.1134/S1066362210030045

ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 3, pp. 245–249. © Pleiades Publishing, Inc., 2010.
Original Russian Text © V.P. Shilov, A.M. Fedoseev, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 3, pp. 209–212.
Reaction of Np(VI) with Н2О2 in Carbonate Solutions
V. P. Shilov and A. M. Fedoseev*
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
Leninskii pr. 31, block 4, Moscow, 119991 Russia; * e-mail: fedosseev@ipc.rssi.ru
Received July 22, 2009
Abstract—The kinetics and stoichiometry of the reaction of Np(VI) with Н2О2 in carbonate solutions were
studied by spectrophotometry. In the range 1–0.02 M Na2CO3, the reaction 2Np(VI) + H2O2 = 2Np(V) + O2
occurs, as Δ[Np(VI)]/Δ[H2O2] ≈ 2. In Na2CO3 + NaHCO3 solutions, the stoichiometric coefficient decreases,
which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and Н2О2 follows the
first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide–carbonate com-
plex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An
increase in the Н2О2 concentration leads to the reaction deceleration owing to formation of a complex with two
peroxy groups. In a 1 M Na2CO3 solution containing 1 mM Н2О2, the first-order rate constant k increases with a
decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [Н2О2] = 1 mM, k passes through a
minimum at [Na2CO3] = 0.5–0.1 M. The activation energy in a 0.5 M Na2CO3 solution is 48 kJ mol–1.
Key words: neptunium(VI), hydrogen peroxide, redox reactions
DOI: 10.1134/S1066362210030045
Carbonate and bicarbonate solutions are interesting
in that, firstly, they simulate to a certain extent natural
media. Secondly, these solutions are used for stabiliza-
tion of higher valence states of actinides. Radiation
from radionuclides in aqueous solutions gives rise to
radiolysis products, including Н2О2. This fact makes it
necessary to study the behavior of Np(VI) and Np(V)
in the presence of Н2О2 in Na2CO3–NaHCO3 solutions.
rated by centrifugation and, after washing with double-
distilled water cooled to 0°С, was dissolved in a mini-
mal amount of HClO4. The Np concentration in stock
solutions was determined by complexometric titration
after reduction of Np to the tetravalent state [1]. We
used chemically pure grade Na2CO3 and analytically
pure grade NaHCO3. For additional purification,
Na2CO3 was recrystallized from double-distilled water.
The Na2CO3 concentration was determined by acid
titration. Medical grade Н2О2 stabilized by a NaH2PO4
additive was distilled at reduced pressure. The Н2О2
concentration in the resulting solution was found by
permanganate titration. All the solutions were prepared
in double-distilled water.
EXPERIMENTAL
The 237Np sample was purified by anion exchange.
From the resulting nitric acid solution, we precipitated
Np(IV) oxalate. After centrifugation and washing, the
precipitate was dissolved in 16 M HNO3, concentrated
HClO4 was added, and the mixture was boiled for 1 h
in a quartz beaker covered with a watch glass. After
that, the watch glass was removed and the heating was
continued until white vapors appeared. Concentrated
HClO4 was added to the residue, and the evaporation
was repeated. The wet salts were dissolved in 0.01 M
HClO4. A spectrophotometric analysis showed that
more than 99% of Np was in the hexavalent state. A
part of the Np(VI) solution was reduced with sodium
nitrite. The resulting Np(V) was precipitated with am-
monia. The precipitate of Np(V) hydroxide was sepa-
Experiments were performed using Shimadzu
UVРС-3100 (Japan) and SF-46 (Leningrad Optical
and Mechanical Association, Russia) spectrophotome-
ters as follows. A temperature-controlled quartz cell
was charged with a solution of Na2CO3 or Na2CO3 +
NaHCO3, and the absorption spectrum in the wave-
length range 300–800 nm was recorded. Then we
added a Np(VI) solution and an Н2О2 solution, and
every time we recorded the spectrum or monitored the
loss of the optical density at the chosen wavelength. In
some experiments with an excess of Н2О2, at the end
245

246
SHILOV, FEDOSEEV
Table 1. Influence of component concentrations (M) on the stoichiometry of the reaction Np(VI) + H2O2 (α =
Δ[Np(VI)]/Δ[H2O2])
[Na2CO3]
1.0
[NaHCO3]
[Np(VI)] × 103
[Np(V)] × 103
[H2O2] × 103
0.47
α
1.0
1.0
1.0
1.0
1.0
1.1
1.0
1.0
1.0
1.97 ± 0.03
1.93 ± 0.03
1.95 ± 0.10
1.95 ± 0.05
1.82
1.0
1.0
0.1
0.1
0.02
0.75
0.5
1.0
0.47
1.0
0.47
1.2
0.35
0.47
0.47
1.0
1.97 ± 0.03
1.80
0.25
0.5
0.5
1.71
0.94
0.5
of the reaction, when Np(VI) disappeared virtually
completely, the solution was transferred into a beaker
and acidified with perchloric acid with vigorous stir-
ring (this was done in an exhaust cabinet because of
evolution of gas bubbles), and the sum of Н2О2 and
Np(V) was determined by permanganate titration. The
Н2О2 consumption was calculated as Δ[H2O2] =
[H2O2]0 – (V1N/V0 – 2[Np(VI)]0), where V1 is the vol-
ume of the KMnO4 solution spent for the titration; N,
its concentration, g-equiv l–1; and V0, solution volume
in the cell.
solutions at 19–20°С. The data obtained for solutions
of various compositions are given in Table 1.
In a 1 M Na2CO3 solution with an excess of Np(VI)
relative to Н2О2, and also with an excess of Н2О2, the
stoichiometric coefficient is close to 2. Thus, there are
good grounds to believe that the following reaction
occurs in the system:
2Np(VI) + H2O2 = 2Np(V) + O2.
(1)
Additional introduction of 1 mM Np(V) does not
affect the reaction stoichiometry. A decrease in the
Na2CO3 concentration does not alter the stoichiometric
coefficient at 2[Np(VI)] > [H2O2]. However, with an
excess of Н2О2, the coefficient decreases, i.e., Н2О2 is
consumed in side reactions. In the case of Na2CO3 +
NaHCO3 solutions, an increase in the NaHCO3 con-
centration leads to a decrease in the stoichiometric co-
efficient, especially with an excess of Н2О2. For exam-
ple, in a 0.5 M Na2CO3 + 0.5 M NaHCO3 solution con-
taining 1 mM Np(VI) and 1 mM Н2О2, 0.04 mM
Np(VI) remained in 22 min, but then the optical ab-
sorption associated with Np(VI) started to slowly in-
crease, and by the 90th minute of the experiment the
Np(VI) concentration was 0.063 mM. It did not change
significantly during the subsequent 24 h. By that time,
Н2О2 was fully consumed. Similar phenomenon was
observed previously in 1 M NaHCO3 solution [3],
when after adding Н2О2 the Np(VI) concentration first
decreased and then increased.
From the amount of Np(VI) introduced into the car-
bonate solution, we calculated the extinction coeffi-
cient ε required for estimating the Np(VI) concentra-
tion in the course of the reduction. To make sure that
the carbonate solution contained no reductants, we
checked the Np(VI) content. To a cell with a 0.02 M
Na2CO3 solution containing 0.5–1 mM of Np(VI), we
added 10–20 mg of solid K4Fe(CN)6·3H2O and re-
corded the Fe(CN)63– absorption. At this concentration
of Na2CO3, the following reaction occurs:
Np(VI) + Fe(CN)64– = Np(V) + Fe(CN)63–.
The Fe(CN)36– and, correspondingly, Np(VI) con-
centrations were calculated using the Fe(CN)63– extinc-
tion coefficient ε420 = 1030 l mol–1 cm–1 [2]. The ε val-
ues found for Np(VI) slightly increase with an increase
in the Na2CO3 concentration, e.g., ε350 increases from
1620 to 1690–1700 l mol–1 cm–1 in going from 0.02 to
1.0 M Na2CO3.
Kinetics of the Np(VI) + H2O2 Reaction
RESULTS AND DISCUSSION
The semilog plots in the figure demonstrate the
Np(VI) loss with time. In a 1 M Na2CO3 solution con-
taining 1 mM Np(VI) and 1 mM Н2О2, the experimen-
The stoichiometry of the reaction of Np(VI) with
H2O2 was studied in Na2CO3 and Na2CO3 + NaHCO3
RADIOCHEMISTRY Vol. 52 No. 3 2010

REACTION OF Np(VI) WITH Н2О2 IN CARBONATE SOLUTIONS
247
tal points are fitted by a straight line (line 1), which
indicates that the reaction is first-order with respect to
Np(VI).
The rate equation has the form
(2)
–d[Np(VI)]/dt = k[Np(VI)].
In the integral form, after replacement of the
Np(VI) concentration by the proportional quantity D –
D∞, the equation takes the form
Decrease in the Np(VI) concentration with time. [Na2CO3] =
1 M; [Np(VI)]0 = 1 mM; 20°C. [H2O2], mM: (1) 1, (2) 0.5,
and (3) 5.
2.3log(D – D∞) = kt + const,
(3)
where k is the first-order rate constant; D and D∞ are
the optical densities of the solution at the given time
and after the reduction completion. The constant k
was found graphically. Its mean values from experi-
ments performed under various conditions are given in
Table 2. The extreme values for solutions with the
Np(VI) concentration below 0.5 mM differed by a fac-
tor of 3.
NpVIO2(O2)(CO3)24– → NpVO2(CO3)23– + O2–.
(5)
With an excess of Np(VI), fast reactions occur:
NpО2(СО3)34– + O2– → NpО2(СО3)23– + O2 + СО32– (6)
Table 2. Influence of conditions on the rate of the Np(VI) +
H2O2 reaction
A decrease in the Н2О2 concentration to 0.5 mM in
a 1 M Na2CO3 solution affects the reaction kinetics
insignificantly up to 70% conversion, after which the
reaction decelerates (line 2 in the figure). With an in-
crease in the Н2О2 concentration to 5 mM, the reaction
follows the first-order rate law up to consumption of
60% of Np(VI), after which the decelerated reaction
also follows a first-order rate law (line 3 in the figure).
It should be noted that, in most cases, the reaction de-
celeration occurred in 40–100 s, and the rate constants
k corresponding to this period are given in Table 2 in
parentheses.
[Na2CO3], [Np(VI)], [Np(V)], [H2O2],
T, °C
k × 103, s–1a
M
1
mM
1
mM
–
mM
1
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
35
45
17.3
1
1
–
0.5 16.5 (11.3)
1
1
1
1
1.88
0.5
0.1
0.05
0.02
0.1
1
1
1
1
1
1
1
1
0.5
0.5
1
1
1
1
1
1
1
1
1
1
2
0.5
0.2
0.1
1
1
1
–
–
–
–
–
–
–
2
3
4
5
1
1
1
1
1
4
1
1
1
1
18 (14.6)
18 (13.5)
20 (11.7)
17.9 (6.6)
19.3
12.6 (7.6)
12.4 (7.2)
35 (11)
The independence of the reaction rate from the re-
ductant concentration (in the first half of the process)
indicates that Np(VI) and Н2О2 form a complex in
which an intramolecular charge transfer leads to the
Np(VI) reduction. In a 1 M Na2CO3 solution, pH is 12.
Under these conditions, a considerable amount of hy-
drogen peroxide exists in the form of НО2–, because for
the equilibrium
>100
–
–
–
–
–
0.7
1
1
1
–
11.8 (3.4)
14.9 (12.8)
18.8 (13.9)
32.8
42
Н2О2 ҙ Н+ + НО2–
0.46 12.2 (8.8)
pK = 11.58 at 30°С [4].
1
2
1
1
1
8.7 (5.8)
10.5 (7)
8 (5.5)
32.7 (15.5)
50 (34)
Neptunium(VI) in carbonate solutions exists in the
form of NpO2(CO3)34– [5]. Therefore, the first step of
the process is the reaction
0.14
1
1
–
NpO2(CO3)34– + HO2– = NpO2(O2)(CO3)24– + HCO3–. (4)
The second step is the charge transfer
a
In parentheses are the k values found after performing the
reaction to 50% and higher conversion.
RADIOCHEMISTRY Vol. 52 No. 3 2010

248
and
SHILOV, FEDOSEEV
crease in the reaction rate constant by a factor of 2.
However, at 2 mM Н2О2, the same amount of Np(V)
decreases k to a lesser extent. The effect of Np(V) in
the solution with the initial Np(VI) concentration of
0.1 mM is more pronounced.
NpVIO2(O2)(CO3)24– + O2– + H2O → NpVO2(CO3)23– + O2
+ HO2– + OH–.
(7)
In solutions with [Np(VI)] < [H2O2], the О2– radical
ion participates in reactions (7) and (8):
An increase in the Np(VI) content from 0.1 to
2 mM leads to the reaction deceleration, which is due
to an increase in the Np(V) accumulation rate.
Н2О2 + О2– → ОН + ОН– + О2.
(8)
The arising OH radical reacts with Н2О2 (НО–2) and
СО32–:
In going from 1 to 1.88 M Na2CO3, the pH, fraction
of НО–2, and, correspondingly, the percentage of the
Np(VI) peroxide–carbonate complex increase, which
leads to a certain increase in the rate constant. In 0.5–
0.1 M Na2CO3 solutions, the pH and НО2– fraction are
lower. As a result, the concentration of the Np(VI) per-
oxide–carbonate complex and, correspondingly, the
rate constant decrease. In 0.05 and 0.02 M Na2CO3, the
rate increases. In the latter case, within the time from
reactant mixing to the start of measurements (22–25 s),
≥90% of Np(VI) disappeared. In solutions with a lower
concentration of carbonate or bicarbonate, higher rates
of Np(VI) reduction are expected, similarly to the re-
sults of the reaction of Pu(VI) with H2O2 in 0.005–
0.1 M NaHCO3 [7].
НО2– + ОН → О2– + Н2О,
СО32– + ОН → СО3– + ОН–.
Then the following reactions are possible:
НО2– + СО3– → О2– + НСО3–,
Np(V) + CО3– → Np(VI) + CO32–.
At the [Np(VI)] : [H2O2] ratio equal to 1 : 1, almost
the whole amount of Np(VI) is bonded in a peroxide–
carbonate complex, because an increase in the Н2О2
concentration leads to only a slight increase in the rate
of the Np(VI) consumption. Reaction (7) results in the
release of НО2–, owing to which its concentration in-
creases, and virtually constant fraction of NpO2(O2)·
(CO3)24– is provided. At [Np(VI)] : [H2O2] > 2, the НО2–
concentration remains low, and starting from a certain
moment the relative content of the peroxide–carbonate
complex decreases, which leads to the process decel-
eration (line 2 deviates upwards).
The activation energy calculated from the initial
values of k in 0.5 M Na2CO3 solution is about
48 kJ mol–1.
Partial replacement of carbonate by bicarbonate
affects the Np(VI) reduction insignificantly. Below are
the results of separate (not averaged) experiments with
solutions containing 1 mM Np(VI).
Deviations from the straight line in the case of 2–
5 mM Н2О2 solutions are caused by another factor,
namely, by the formation of a complex of Np(VI) with
two peroxy groups, similarly to the complexation with
uranyl ion [6]. The rate of formation of the second
peroxide–carbonate complex is considerably lower
than the rate of formation of the first complex, but
the second complex is more stable to intramolecular
reduction.
[NaHCO3], M [Na2CO3], M [H2O2], mM k × 103, s–1
0.25
0.25
0.5
0.75
0.75
0.5
0.46
1
0.46
17
26
24
0.5
0.5
1
17.5
The constants are close to those in 1 M Na2CO3 at
the same concentrations of Np(VI) and H2O2. Certain
data scatter is, most probably, due to local effects, be-
cause the stirring was manual.
Interesting effect is exerted by the reaction product,
Np(V). In solutions with Np(VI) and Н2О2 concentra-
tions of 1 and 0.46 mM, respectively, in the presence
of 0.7 mM Np(V) the initial rate constant is lower
by approximately 25% than the rate constant in the
solution without Np(V) added. In solutions with
[Np(VI)] = [Н2О2] = 1 mM, preliminary introduction
of Np(V) to a concentration of 1 mM leads to a de-
It should be noted in conclusion that, similarly to
the reaction of Np(VI) with H2O2, the reaction of
Pu(VI) with H2O2 will decelerate in 0.5–2 M Na2CO3
solutions. However, the reduction will be complicated
by the disproportionation of Pu(V).
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REACTION OF Np(VI) WITH Н2О2 IN CARBONATE SOLUTIONS
249
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