ISSN 1066-3622, Radiochemistry, 2013, Vol. 55, No. 4, pp. 357–359. © Pleiades Publishing, Inc., 2013. Original Russian Text © V.P. Shilov, A.M. Fedoseev, 2013, published in Radiokhimiya, 2013, Vol. 55, No. 4, pp. 292–293.

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Reaction of Ozone with Np(V) and Np(IV) 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—A spectrophotometric study showed that ozone in concentrated carbonate solutions forms com-plexes with СО3

2– ions, which inhibits the ozone decomposition. Free ozone oxidizes Np(V) at high rate. The bound ozone reacts with Np(V) at moderate rate. Np(IV) reacts with О3 slowly, with Np(VI) formed in NaHCO3 solution and only Np(V) formed in Na2CO3 solution.

Keywords: ozone, neptunium(V) and (IV), carbonate solutions, oxidation kinetics

Ozone is used for oxidation of Np(IV) and (V) in acid and carbonate solutions and of Np(IV–VI) in al-kali solutions. The kinetics of some of these reactions has been studied in acid [1] and alkali [2] solutions but has not been studied in carbonate solutions. In alkali and carbonate solutions, the ozone stability is the ma-jor factor governing the course of these reactions. The ozone decomposition in carbonate solutions was con-sidered in [3, 4]. It was found that СО3

2– ions inhibit the ozone decomposition. For example, at 25°С in solu-tions with I = 0.5 M NaClO4, [СО3

2–] = 0.004 M, [O3]0 = 5.8 × 10–5–1.1 × 10–4 M, and pH 12.5 and 10.7, the ozone half-life τ1/2 is 0.05 and 1.6 s, respectively [3]. Mizuno et al. [4] determined the ozone stability in phosphate buffer solutions (1 mM) with pH 6.8–8.1, containing up to 0.0025 M СО3

2– and approximately 2 × 10–4 M О3 (T = 20°C). The ozone half-life ex-ceeded 1000 s. The inhibiting effect was due to the joint action of phosphate and carbonate (or bicarbon-ate) ions. However, the extrapolation of the τ1/2 values obtained to concentrated carbonate solutions seems doubtful. This study is aimed to determine the ozone stability and elucidate the specific features of the reac-tions of ozone with Np(V) and (IV) in concentrated NaHCO3 and Na2CO3 solutions.

stock solution of NpO2ClO4 was prepared by the stan-dard procedure. Solutions of Np(IV) in 2 M HCl and in 2 M Na2CO3 were prepared from dry Cs2NpCl6. The Np solutions were analyzed by known procedures, i.e., the Np concentration in stock solutions was deter-mined by EDTA titration with Xylenol Orange as indi-cator, Np(V) was preliminarily reduced to Np(IV) with hydroxylamine in hydrochloric acid solution [5], and the content of valence forms was determined by spec-trophotometry. Analytically pure grade NaHCO3 and chemically pure grade Na2CO3 were additionally puri-fied by recrystallization. The other chemicals were of chemically pure or analytically pure grade. All solu-tions were prepared in double-distilled water. Ozone was generated from technical-grade oxygen. The ex-perimental procedure was as follows. A quartz cell with a ground-quartz stopper (l = 5 cm, volume 25 mL; l = 2.2 cm, volume 10.5 mL; l = 1 cm, volume 4 mL) was charged with water or a carbonate solution, and the О2 + О3 mixture was bubbled for 15–20 min. Immediately after the bubbling completion or after the lapse of some time, an aliqout of a solution of K4Fe(CN)6, Np(V), or Np(IV) was added to the cell. The cell was stoppered, the solution was stirred several times, and the absorption spectrum was recorded in the range 260–800 nm with a Shimadzu PC 3100 (Japan) or SF-46 (Leningrad Optical and Mechanical Association, Russia) spectrophotometer. In most cases, we monitored the variation of the optical density at a chosen wavelength.

DOI: 10.1134/S1066362213040024

EXPERIMENTAL

Experiments were performed with 237Np, which was purified by anion exchange. The weakly acidic

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RADIOCHEMISTRY Vol. 55 No. 4 2013

RESULTS AND DISCUSSION

First, we examined the stability of ozone in carbon-ate solutions. In 1 M NaHCO3, the О3 loss was moni-tored by a decrease in the absorption intensity at 260 nm. The decrease in the optical density approxi-mately followed the first-order rate law. The first-order rate constant was k = 3.68 × 10–3 s–1 (T = 22°C), i.e., τ1/2 = 188 s. In a 1 M Na2CO3 solution containing ozone, the absorption at 260 nm was insignificant. However, on adding K4Fe(CN)6 to a concentration of 10 mM, the absorption band with a maximum at 420 nm appeared, suggesting the formation of Fe(CN)6

3– ions by the reaction

For Fe(CN)63–, ε420 = 103 L mol–1 cm–1; the ozone

concentration determined from the stoichiometry of reaction (1) was 1.7 × 10–4 M. The ozone decomposi-tion kinetics was studied as follows. The gas flow rate and the ozonizer voltage were kept constant to ensure constant ozone content of the gas mixture. The cell with a fresh 1 M Na2CO3 solution was saturated with ozone for 15 min and kept closed for several minutes, after which an aliquot of the K4Fe(CN)6 solution was added. The kinetic curve obtained from several experi-ments was linearized in semilog coordinates. Thus, the ozone decomposition followed the first-order rate law with the rate constant k = 4.33 × 10–4 s–1 and τ1/2 = 1600 s. In going from 0.004 to 1 M Na2CO3, τ1/2 increases from 0.05 to 1600 s, i.e., almost in pro-portion with [CO3

2–]2. Thus, it can be assumed that О3 forms a complex with one and two СО3

2– ions. The first complex prevails at low СО3

2– concentrations, and the second complex, at high concentrations. Binding in a complex enhances the stability of ozone and, at the same time, decreases its oxidizing power.

Oxidation of Np(V) with ozone was studied in three series of experiments. In the first series, in a 4– 10.5-mL cell, water was saturated with ozone, and 0.5 mL of a 0.4–2 M Na2CO3 solution containing up to 8 mM Np(V) was added. By the measurement time (within 25–35 s), the reaction

2Fe(CN)64– + O3 + H2O = 2Fe(CN)6

3– + O2 + 2OH–. (1)

(2) 2Np(V) + O3 + H2O = 2Np(VI) + O2 + 2OH–

was complete. The second-order rate constant k in 0.1 and 0.5 M Na2CO3 solution can be estimated at 104 and 5 × 102 L mol–1 s–1, respectively.

In the second series, to a cell containing a 1 M NaHCO3 solution saturated with ozone we added 0.5 mL of a 0.4 M Na2CO3 solution containing 8 mM Np(V). By the measurement time, 0.14 mM Np(VI) formed [about 70% of the final Np(VI) amount]. Then the Np(VI) accumulation rate appreciably decreased. The second-order rate constant k was estimated from the relationship V = k[O3][Np(V)] = k([O3]0 – 2[Np(VI)])([Np(V)]0 – [Np(VI)]) = Δ[Np(VI)]/Δτ. To a first approximation, the ozone decomposition was neglected. The rate constant k in the first and second stages of the reaction was estimated at 160 and 10 L mol–1 s–1, respectively. In the third series, to a cell containing a 0.1–1 M Na2CO3 solution saturated with ozone, we added an aliquot of a 2 M Na2CO3 solution containing Np(V). The reaction also occurred in two stages, but the first stage was considerably shorter than in 1 M NaHCO3. The rate constant k in the first and second stages was 100–200 and 10–30 L mol–1 s–1, respectively. The data obtained allow certain conclu-sions. In the first case, the species reacting with Np(V) was ozone dissolved in water and weakly bound with water molecules. The reaction of ozone with Np(V) was considerably faster than the formation of the com-plexes of ozone with СО3

2– ions. In the second series, the NaHCO3 solution contained free О3 and its com-plexes with СО3

2–. In the third case, the Na2CO3 solu-tion contained О3 only in the form of complexes.

Oxidation of Np(IV) with ozone was studied in 1 M NaHCO3 and 1 M Na2CO3 solutions. The solutions were saturated with ozone, and aliquots of solutions of Np(IV) in 2 M Na2CO3 were added. The Np concentra-tion was 0.1–0.15 mM. In NaHCO3 solution, Np(IV) was oxidized to Np(VI), as judged from the fact that the absorption in the near-UV range arose simultane-ously with the decrease in the Np(IV) absorption at 705 nm. In Na2CO3 solution, Np(IV) was oxidized to Np(V). This is due to the fact that in NaHCO3 solu-tion Np(IV) exists in the form of Np(OH)2(CO3)2

2–, and in Na2CO3 solution, in the form of Np(CO3)5

6– by anal-ogy with the speciation in KHCO3–K2CO3 solutions [6]. In a NaHCO3 solution, the ozone molecule ex-changes with the hydroxy groups and enters into the coordination sphere of Np(IV). This is followed by the transfer of the О atom. In a Na2CO3 solution, ozone is bound in a complex, and its incorporation into the coordination sphere of the Np(IV) carbonate com-plex is complicated. The Np(IV) oxidation also occurs in two stages. The constant k in NaHCO3 and Na2CO3 solutions is 30 and 110 in the first stage, and 4–5

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and 25–35 L mol–1 s–1 in the second stage, respec-tively.