ISSN 1066-3622, Radiochemistry, 2013, Vol. 55, No. 3, pp. 287–290. © Pleiades Publishing, Inc., 2013.

Original Russian Text © V.P. Shilov, A.M. Fedoseev, 2013, published in Radiokhimiya, 2013, Vol. 55, No. 3, pp. 232–235.

287

Oxidation of Np(IV) with Hydrogen Peroxide 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, 119071 Russia; * e-mail: ShilovV@ipc.rssi.ru

Received July 5, 2012

Abstract—Oxidation of Np(IV) with hydrogen peroxide in NaHCO3–Na2CO3 solutions was studied by spec-trophotometry. In NaHCO3 solution, Np(IV) is oxidized to Np(V) and partially to Np(VI). It follows from the electronic absorption spectra that Np(IV) in 1 M Na2CO3 forms with H2O2 a mixed peroxide–carbonate com-plex. Its stability constant β is estimated at 25–30. The Np(IV) bound in the mixed complex disappears in a first-order reaction with respect to [Np(IV)]. The first-order rate constant k' is proportional to [H2O2] in the Н2О2 concentration range 2.5–11 mM, but further increase in [H2O2] leads to a decrease in k'. The bimolecular rate constant k = k'/[H2O2] in solutions containing up to 11 mM Н2О2 increases in going from 1 M NaHCO3 to 1 M Na2CO3 and significantly decreases with a further increase in the carbonate content. The activated complex is formed from Np(IV) peroxide–carbonate and carbonate complexes. Synchronous or successive electron transfer leads to the oxidation of Np(IV) to Np(V). Large excess of Н2О2 oxidizes Np(V) to Np(VI), which is then slowly reduced. As a result, Np(V) is formed in carbonate solutions at any Np(IV) and Н2О2 concentrations.

Keywords: neptunium(IV), carbonate solution, hydrogen peroxide, redox reactions, kinetics

It was suggested recently [1, 2] to use for spent nu-clear fuel (SNF) reprocessing a scheme involving SNF dissolution in carbonate solution in the presence of H2O2 instead of the Purex process (dissolution in HNO3 and extraction of U and Pu with TBP). Along with fission products, SNF contains 237Np. Therefore, it was necessary to study the reaction of Н2О2 with Np ions in various oxidation states, in particular, with Np(IV). Н2О2 should oxidize Np(IV). This can be ex-pected from the following facts. The formal potential of the Np(V)/(IV) couple in 1 M K2СО3 is 100 mV [3]. It is known that H2O2 in 1 M NaHCO3 (pH < 9) oxi-dizes Np(V) to Np(VI) [4] and in 0.02–1 M Na2CO3 (pH ~12) converts Np(VI) to Np(V) [5]. The formal potential of the Np(VI)/(V) couple is 440 mV in 1 M Na2CO3 [6] and 445 mV in 0.05 M Na2CO3 [7]. The dual behavior of Н2О2 depends on pH of the solution and on the properties of the second reaction partici-pant. The composition of the medium will affect the Np(IV) oxidation. In addition, oxidation of Np(IV) to Np(V) involves structural rearrangement, which may affect the reaction kinetics. This study deals with the reaction of Н2О2 with Np(IV) and Np(V) in NaHCO3–Na2CO3 solutions.

We used 237Np purified by anion exchange. Np(V) and Np(IV) were prepared by procedures described in [8], i.e., via precipitation of Np(V) hydroxide and Cs2NpCl6, which were dissolved in appropriate media. We used analytically pure grade NaHCO3 and chemi-cally pure grade Na2CO3; both were additionally puri-fied by recrystallization. Н2О2 and other chemicals were of analytically pure or chemically pure grade. We used two types of Np(IV) stock solutions: in 2 M HCl and in 2 M Na2CO3. All solutions were prepared in double-distilled water. The NaHCO3 solution was used on the day of preparation. All the stock solutions were analyzed by known procedures. The Н2О2 concentra-tion in the stock solution and in experiments on study-ing the kinetics of Н2О2 decomposition in carbonate solutions was determined by titration with a KMnO4 solution after acidifying the sample with sulfuric acid. The neptunium transformations were studied by spec-trophotometry with Shimadzu UV PC 3100 (Japan) and SF-46 (Leningrad Optical and Mechanical Asso-ciation, Russia) devices as follows. A quartz cell (l = 1–5 cm) was charged with a NaHCO3 or Na2CO3 solu-

DOI: 10.1134/S1066362213030077

EXPERIMENTAL

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RADIOCHEMISTRY Vol. 55 No. 3 2013

RESULTS AND DISCUSSION

tion containing Np(IV) or Np(V), and the absorption spectrum was recorded in the range 350–800 nm. Then Н2О2 was added, and a decrease in the optical density at 704–706 nm [Np(IV) absorption] or an increase in the optical density at 350 nm, followed by its decrease, was monitored. In some cases, the solution from the cell was transferred into a beaker, and a calculated vol-ume of concentrated HCl was added in a fume hood. The solution was returned to the cell, and the absorp-tion spectrum in the range 350–1000 nm was recorded.

In a ~1 M NaHCO3 solution containing Np(IV) and Н2О2 (1 mM each) at 19°С, Np(IV) underwent slow oxidation. In 20 h, Np(IV) was fully oxidized, 0.2 mM Np(VI) accumulated, and a precipitate formed. The precipitate appeared to be a Np(V) compound, as de-termined immediately after dissolving it in 1 M HCl. With an increase in the Н2О2 concentration to 20 mM, a suspension is formed in the Np(IV) solution. Heating of the cell with the suspension to 50°С, with keeping for 16 min, leads to the suspension dissolution. By that time, 0.5 mM Np(IV) disappeared. In 20 h (19°C), Np(IV) was fully oxidized. The suspension is not formed in a 0.96 M NaHCO3 solution containing 11 mM Н2О2, 1 mM Np(IV), and 0.03 M Na2CO3. A decrease in the Np(IV) concentration to 0.2 mM in a 0.98 M NaHCO3 solution with the same Н2О2 concen-tration allows the suspension formation to be avoided even at 0.007 M Na2CO3. In such solutions, Np(IV) disappeared in accordance with a first-order rate law, because in the logD–τ coordinates (τ is time, D = Dτ – D∞, Dτ is the running optical density, and D∞ is the optical density at the end of the reaction) the experi-mental points lie on a straight line. In solutions with the initial Np(IV) concentration of 0.2–1 mM, Np(V) accumulated by the end of the reaction and 0.2 mM Np(VI) appeared irrespective of the initial Np(IV) con-centration.

In 0.5 M NaHCO3 + 0.5 M Na2CO3 or 1 M Na2CO3 solutions containing 1 mM Np(IV) and 0.5 mM Н2О2 at 19°C, Np(IV) was half-oxidized in 200 min and fully disappeared in 20 h. Thus, the following reaction occurs under these conditions:

2Np(IV) + H2O2 = 2Np(V). (1)

We found that Н2О2 at its initial concentration of 200 mM in 1 M Na2CO3 at 19°С disappeared in accor-dance with a first-order rate law up to the end of the

reaction. The first-order rate constant was 7 × 10–5 s–1. Hence follows that the Np(IV) oxidation rate in a car-bonate solution containing 1 mM Np(IV) and 0.5 mM Н2О2 considerably exceeds the Н2О2 decomposition rate.

In the same carbonate solutions containing 1 mM Np(IV) and 1 mM H2O2, the absorption at 350 nm in-creased in 78 s after mixing the reactants, which was followed by a slow decrease in D both at 350 and at 705 nm, i.e., Np(IV) was consumed; 20% of Np(IV) remained in 200 min. An increase in the Н2О2 concen-tration is accompanied by an increase in the absorption at 350 and 704–705 nm, which is caused by the forma-tion of a mixed Np(IV) peroxide–carbonate complex, similar to the formation of a mixed Pu(IV) peroxide–carbonate complex in 2 M Na2CO3 after adding Н2О2 [9]. Np(IV) in aqueous KHCO3–K2CO3–KOH solution exists in the form of Np(CO3)5

6– at low and high car-bonate concentrations and high bicarbonate concentra-tion and in the form of Np(OH)2(CO3)2

2– at a low bicar-bonate concentration [10]. According to [11], in NaHCO3–Na2CO3–NaClO4 solutions with the total carbonate concentration of 0.005 to 0.1 M at an ionic strength of 0.5, 1, and 2 M, the prevalent Np(IV) spe-cies is Np(OH)2(CO3)2

2– in the pH range 8.5–10.5 and Np(OH)4(CO3)2

4– at pH > 12. In view of the equilib-rium

(3)

(5)

(4)

Np(OH)2(CO3)22– + H2O2 = Np(O2)(CO3)2

2– + 2H2O,

Np(OH)2(CO3)22– + HO2

– = Np(O2)(CO3)22– + OH– + H2O,

Np(CO3)5

6– + HO2– = Np(O2)(CO3)2

2– + HCO3– + 2CO3

2–.

for which pK = 11.58 [12], the Np(IV) peroxide–carbonate complex is formed in the reactions

Н2О2 = НО2– + Н+,

The formation constant of the mixed complex,

β = [Np(O2)(CO3)22–][Np(IV)]–1[H2O2]

–1, (6)

can be estimated from the dependence of the molar extinction coefficient of Np(IV), ε, on [H2O2]. In 1 M Na2CO3, the following data were obtained for ε:

[H2O2], mM 0 10 18 40 82 120 163 210

ε350, L mol–1 cm–1 480 860 1120 1500 1700 1650

ε705, L mol–1 cm–1 46 52 56 100 140 140 136 148

From the common relationship ε = (ε0 + ε1β[H2O2])/

(2)

OXIDATION OF Np(IV) WITH HYDROGEN PEROXIDE 289

RADIOCHEMISTRY Vol. 55 No. 3 2013

(1 + β[H2O2]), where ε is the mean molar extinction coefficient of Np(IV) and ε0 and ε1 are the molar ex-tinction coefficients of the Np(IV) carbonate and per-oxide–carbonate complexes, we found β = 25–30.

In a Na2CO3 solution, an increase in [H2O2] acceler-ates the disappearance of Np(IV). It follows a first-order rate law

–d[Np(IV)]/dτ = k'[Np(IV)] + const, (7)

where k' is the first-order rate constant, s–1. Integration of Eq. (7) and replacement of [Np(IV)] by the propor-tional quantity, D, lead to the expression

(8) 2.3logD = k'τ + const.

The first-order rate constant k' was estimated from the slope of the kinetic curves in the coordinates logD705–τ.

Below we present the values of k' in 1 M Na2CO3 solution containing 1 mM Np(IV) and 2.54–210 mM Н2О2 at 20°С [we used a stock solution of Np(IV) in 2 M Na2CO3].

[H2O2], mM 2.54 5.08 11.5 18.4 40 210 k' × 103, s–1 1.16 2.3 5.2 5.0 0.68 0.21

k, L mol–1 s–1 0.455 0.445 0.451 0.270 0.017 0.001

With an increase in [Н2О2] in the range 2.54– 11.5 mM, k' increases in direct proportion. Hence, the Np(IV) oxidation follows a first-order rate law with respect to [H2O2]. The bimolecular rate constant k = k'/[H2O2] does not vary in this range of Н2О2 concen-trations. At these Н2О2 concentrations, the fraction of the Np(IV) peroxide–carbonate complex relative to the

total Np(IV) amount is less than 50%. In solutions with [H2O2] = 18–210 mM, k' decreases significantly. The fraction of the complex Np(O2)(CO3)2

2– exceeds 50%, and the fraction of the complex Np(OH)2(CO3)2

2– or Np(CO3)5

6– decreases. The possible cause of a de-crease in k' is participation of Np(IV) peroxide–carbonate and carbonate complex ions in the rate-determining step. An increase in temperature to 34–45°С alters the shape of the kinetic curves. In the se-milog coordinates, the initial portions (0 to 35–45 s) have steeper slope than the subsequent main portion. The values of k in 2.5–11.5 mM Н2О2 solutions under different conditions are given in Table 1. For the inter-val 34–45°С, we took the k values corresponding to the main portions of the kinetic curves.

As can be seen, with the stock solution of Np(IV) in 2 M Na2CO3 the k values are higher than with the stock solution in 2 M HCl. In going from the bicarbon-ate solution to 1 M Na2CO3, k increases. Further in-crease in the СО3

2– concentration decreases k in propor-tion to [CO3

2–]–2, which is quite consistent with Eq. (5). The activation energy is estimated at 64 kJ mol–1. The process mechanism includes the step of thermal excita-tion of the peroxide–carbonate complex:

Table 1. Influence of conditions on the rate constant of the reaction of Np(IV) with H2O2

T, °C [NaHCO3] [Na2CO3] Np stock solution [Np(IV)], mM k, L mol–1 s–1

M 20 0.90 0.07 HCl 1.0 0.17 20 0.90 0.09 HCl 0.2 0.31 20 0.03 0.94 HCl 1.0 0.33 20 0.006 0.994 HCl 0.2 0.30 20 0.963 0.035 Na2CO3 1.0 0.37 20 0.993 0.007 Na2CO3 0.15 0.38 20 1.0 Na2CO3 1.0 0.45 20 1.0 Na2CO3 0.15 0.45 20 1.91 Na2CO3 1.0 0.12 28 1.0 Na2CO3 1.0 1.0 34 1.0 Na2CO3 1.0 1.78 40 1.0 Na2CO3 1.0 2.83 45 1.0 Na2CO3 1.0 4.0

Np(O2)(CO3)22– → *Np(O2)(CO3)2

2–. (9)

The intramolecular charge transfer results in the formation of Np(V) and elimination of О– radical ion in carbonate solution or of ОН radical in bicarbonate solution:

(10) *Np(O2)(CO3)22– → Np(V) + O– (OH).

This is followed by the reactions

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Table 2. Influence of conditions on the reaction 2Np(V) + H2O2 = 2Np(VI) + 2OH–

[Na2CO3], M

[Np(V)] [H2O2] V × 107, L mol–1 s–1

[Np(VI)]max, mM mM

0.01 0.1 300 0

0.10 0.14 80 0.04

0.10 1 40 78 0.5

0.10 1 80 116 0.5

0.20 0.28 20 0.06

1 0.1 40 0

1 0.1 80 0.02

1 0.28 80 0.02

1 1 20 0.6 0.03

1 1 40 2.0 0.12

1 1 80 6.0 0.25

1 1.4 83 10.8 0.36

1 1 166 15.2 0.5

1 1.42 210 47 0.93 1 1.4 420 86 1.36

The ОН radicals can be detected using alcohols with which the radicals react at high rate. In some ex-periments, we studied the Np(IV) oxidation with hy-drogen peroxide in the presence of 1 M tert-butanol or 1 M ethanol. We expected that binding of OH radicals would decrease the Np(IV) oxidation rate by half. However, the rate did not change noticeably. There-fore, it can be concluded that the reaction of Np(IV) with H2O2 does not involve ОН radical.

Another pathway of Np(IV) oxidation seems more probable. The excited Np(IV) peroxide–carbonate complex forms with the Np(IV) carbonate complex an activated complex in which two electrons are trans-ferred synchronously or consecutively, which results in oxidation of two Np(IV) ions:

Monitoring of the Np(IV) loss by a decrease in the absorption at 350 nm also allows estimation of k. How-ever, it appeared to be lower than the value obtained from the kinetic measurements at 705 nm. The possi-ble cause is associated with the appearance of the ab-sorption from Np(VI) formed by the reaction

(13) *Np(O2)(CO3)22– + Np(OH)2(CO3)2

2– → 2Np(V)

or

*Np(O2)(CO3)22– + Np(CO3)5

6– → 2Np(V). (14)

2Np(V) + H2O2 → 2Np(VI) + 2OH–. (15)

This reaction was studied in a wide range of Na2CO3, Np(V), and Н2О2 concentrations. The condi-tions of reaction (15), the initial rate of Np(VI) accu-mulation V, and the maximal Np(VI) concentration are given in Table 2.

The results we obtained allow a conclusion that oxidation of Np(V) with hydrogen peroxide is possible in 0.1–1 M Na2CO3 solutions containing more than 0.1 M Np(V). With an increase in [Np(V)], oxidation starts at lower H2О2 concentrations. The Np(IV) accu-mulation rate and the maximal concentration of Np(VI) increase almost in proportion to the [H2O2] concentration. However, after reaching the maximum, the Np(VI) concentration decreases. The mechanism of the Np(V) oxidation with hydrogen peroxide in Na2CO3 solutions is similar to the mechanism of the Np(V) oxidation in 1 M NaHCO3 [4]. It should be noted in conclusion that Np(V) will be the final prod-uct at any initial concentrations of Н2О2 and Np(IV).

(11)

(12)

CO32– + O– + H2O → CO3

– + 2OH–,

Np(IV) + CO3– → Np(V) + CO3

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