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ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 1, pp. 27–30. © Pleiades Publishing, Inc., 2010. Original Russian Text © M.V. Nikonov, I.G. Tananaev, B.F. Myasoedov, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 1, pp. 26–28. 27 Effect of Hydrogen Peroxide on Precipitation of Pu(IV) in Alkaline Solutions M. V. Nikonov*, I. G. Tananaev, and B. F. Myasoedov Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia; * e-mail: [email protected] Received April 14, 2009 Abstract—Formation of peroxy compounds of Pu(IV) is possible in concentrated alkali solutions despite insta- bility of hydrogen peroxide under these conditions. The resulting peroxy compound of Pu(IV) is fairly stable in alkaline solutions. The [Pu] : [O 2 ] ratio in the compound is close to 1 : 2. The formation of the peroxy com- pound favors a decrease in the content of colloidal polymeric Pu(IV) species in solution, thus making the Pu precipitation from solution more complete. Key words: plutonium, peroxides Peroxy compounds are known for Am(III), Np(IV),(V), and Pu(IV). However, although the method for precipitation of Np and Pu with hydrogen peroxide from acidic, neutral, and alkaline solutions is known for a long time [1], available data on the properties of actinide peroxy compounds are scarce. It is known that Np(V) peroxide in alkaline solutions transforms into double peroxy salts of Np(V) and Na. There are also some data on Am peroxy compounds in alkaline solutions [2, 3]. As for Pu, data on precipi- tation of Pu(IV) from alkaline solutions in the pres- ence of H 2 O 2 or on reduction of Pu in higher oxida- tion states with H 2 O 2 under the same conditions are lacking, apparently because of the suggested precipi- tation of Pu(IV) hydroxide in alkaline solutions [because Pu(IV) is strongly susceptible to hydrolysis] and of rapid decomposition of H 2 O 2 in alkaline solu- tions. Therefore, the goal of our study was to examine the behavior of Pu in alkaline solutions in the pres- ence of H 2 O 2 in more detail. We performed experi- ments aimed to (1) study the Pu(VI) reduction in NaOH solutions with hydrogen peroxide, (2) eluci- date the effect of H 2 O 2 on the completeness of Pu(IV) precipitation in alkaline solutions, and (3) examine the behavior of Pu(IV) hydroxide in an alkali in the presence of H 2 O 2 . Here we report the results of these experiments. Reduction of Pu(VI) in NaOH Solutions with Hydrogen Peroxide DOI: 10.1134/S1066362210010066 Experiments were performed in 1–6 M NaOH at a Pu(VI) concentration in the solution of ~10 –3 M at room temperature. The H 2 O 2 concentration in the solu- tions was ~1.6 M. Immediately after adding excess H 2 O 2 to the yellow-green solution of Pu(VI), the solu- tion became red-brown. The electronic absorption spectrum of the resulting solution of Pu in 4 M NaOH is shown in Fig. 1. This spectrum is similar to the known spectrum of the ‘brown” peroxy complex of Pu(IV) [4] containing Fig. 1. Electronic absorption spectrum of the “brown” per- oxy complex of Pu(IV) in 4 M NaOH. Total Pu concentra- tion in solution ~2.5 × 10 –3 M.

Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

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Page 1: Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

ISSN 1066-3622, Radiochemistry, 2010, Vol. 52, No. 1, pp. 27–30. © Pleiades Publishing, Inc., 2010.

Original Russian Text © M.V. Nikonov, I.G. Tananaev, B.F. Myasoedov, 2010, published in Radiokhimiya, 2010, Vol. 52, No. 1, pp. 26–28.

27

Effect of Hydrogen Peroxide on Precipitation of Pu(IV) in Alkaline Solutions

M. V. Nikonov*, I. G. Tananaev, and B. F. Myasoedov

Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991 Russia; * e-mail: [email protected]

Received April 14, 2009

Abstract—Formation of peroxy compounds of Pu(IV) is possible in concentrated alkali solutions despite insta-bility of hydrogen peroxide under these conditions. The resulting peroxy compound of Pu(IV) is fairly stable in alkaline solutions. The [Pu] : [O2

–] ratio in the compound is close to 1 : 2. The formation of the peroxy com-pound favors a decrease in the content of colloidal polymeric Pu(IV) species in solution, thus making the Pu precipitation from solution more complete.

Key words: plutonium, peroxides

Peroxy compounds are known for Am(III), Np(IV),(V), and Pu(IV). However, although the method for precipitation of Np and Pu with hydrogen peroxide from acidic, neutral, and alkaline solutions is known for a long time [1], available data on the properties of actinide peroxy compounds are scarce. It is known that Np(V) peroxide in alkaline solutions transforms into double peroxy salts of Np(V) and Na. There are also some data on Am peroxy compounds in alkaline solutions [2, 3]. As for Pu, data on precipi-tation of Pu(IV) from alkaline solutions in the pres-ence of H2O2 or on reduction of Pu in higher oxida-tion states with H2O2 under the same conditions are lacking, apparently because of the suggested precipi-tation of Pu(IV) hydroxide in alkaline solutions [because Pu(IV) is strongly susceptible to hydrolysis] and of rapid decomposition of H2O2 in alkaline solu-tions. Therefore, the goal of our study was to examine the behavior of Pu in alkaline solutions in the pres-ence of H2O2 in more detail. We performed experi-ments aimed to (1) study the Pu(VI) reduction in NaOH solutions with hydrogen peroxide, (2) eluci-date the effect of H2O2 on the completeness of Pu(IV) precipitation in alkaline solutions, and (3) examine the behavior of Pu(IV) hydroxide in an alkali in the presence of H2O2.

Here we report the results of these experiments.

Reduction of Pu(VI) in NaOH Solutions with Hydrogen Peroxide

DOI: 10.1134/S1066362210010066

Experiments were performed in 1–6 M NaOH at a Pu(VI) concentration in the solution of ~10–3 M at room temperature. The H2O2 concentration in the solu-tions was ~1.6 M. Immediately after adding excess H2O2 to the yellow-green solution of Pu(VI), the solu-tion became red-brown. The electronic absorption spectrum of the resulting solution of Pu in 4 M NaOH is shown in Fig. 1.

This spectrum is similar to the known spectrum of the ‘brown” peroxy complex of Pu(IV) [4] containing

Fig. 1. Electronic absorption spectrum of the “brown” per-oxy complex of Pu(IV) in 4 M NaOH. Total Pu concentra-tion in solution ~2.5 × 10–3 M.

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NIKONOV et al. 28

RADIOCHEMISTRY Vol. 52 No. 1 2010

Fig. 2. Electronic absorption spectrum of the perchloric acid solution prepared by dissolving the precipitate that formed in the course of the experiment [Pu(IV), “red” com-plex].

Fig. 3. Electronic absorption spectrum of Pu(III) prepared by reduction of Pu(IV) with peroxy groups incorporated in the plutonium peroxy complex.

Fig. 4. Electronic absorption spectrum of (1) solid Pu(IV) peroxide and (2) Pu(IV) hydroxide.

two Pu atoms per peroxy group. Then the solution gradually decolorized with the formation of a green precipitate. The transformation was complete within ~30 min after the start of the reaction. The transforma-

tion of the “brown” Pu(IV) complex into the “red” per-oxy complex of the composition [Pu] : [O2

2–] = 1 : 1, characteristic of acid solutions, was not observed in our case, apparently, because of rapid transformation of the “red” complex into a compound with higher content of peroxy groups. The same pattern was ob-served at other alkali concentrations in the range 1– 6 M NaOH. The precipitate obtained was washed with water to negative reaction of wash waters with VO2+ ions forming an intensely colored complex with H2O2. One portion of the precipitate was dissolved in 2 M HClO4. The electronic absorption spectrum of the re-sulting solution corresponds to the spectrum of the “red” peroxy complex of Pu(IV) in acidic solutions [4] (Fig. 2).

With time, the “red” complex is virtually com-pletely reduced to Pu(III) (Fig. 3) exclusively with the O2

2– groups incorporated in the complex, without any other reductants in solution. Significant amounts of Pu(IV) in the solution were not detected, taking into account fairly close values of the extinction coeffi-cients of Pu(IV) at 470 nm (~50 l mol–1 cm–1) and of Pu(III) at 560 and 603 nm (~35 l mol–1 cm–1). Red col-oration of the solution upon dissolution of the green precipitate may be due to neutralization of OH groups probably incorporated in the solid compounds with hydrogen ions. Also, the action of hydrogen ions re-sults in elimination of H2O2 molecules.

Another portion of the precipitate isolated from the alkaline solution was used for recording the electronic absorption spectrum of a thin layer of solid Pu(IV) per-oxide, shown in Fig. 4 (spectrum 1). The electronic absorption spectrum of approximately the same layer of Pu(IV) hydroxide on the cell walls (the spectrum has no well-defined absorption peaks) is also shown in Fig. 4 (spectrum 2). In spectrum 1 in Fig. 4, there is a well-defined band with a maximum at 495 nm, charac-teristic also of the spectrum of the “red” peroxy com-plex of Pu(IV) in acid solutions (Fig. 2). Therefore, it can be assumed that the forming green precipitate is a polymer containing fragments structurally similar to complex ions of the “red” peroxy complex of Pu in acidic solutions, which, in turn, indirectly suggests its linear rather than cyclic structure (these alternatives were considered as equally probable in [6]). The ratio of the numbers of the peroxy groups and Pu atoms was determined for the precipitate washed to remove the unbound peroxide, by dissolving the precipitate in 4 M HClO4 and reducing Pu(IV) to Pu(III) exclusively with peroxy groups incorporated in the Pu(IV) complex.

Page 3: Effect of hydrogen peroxide on precipitation of Pu(IV) in alkaline solutions

EFFECT OF HYDROGEN PEROXIDE ON PRECIPITATION OF Pu(IV) 29

RADIOCHEMISTRY Vol. 52 No. 1 2010

The degree of reduction was monitored spectropho-tometrically. Thus, it becomes possible to determine the amount of H2O2 that was present in the Pu(IV) complex and was consumed in the reaction

2Pu(IV) + H2O2 → 2Pu(III) + O2 + 2H+. (1)

Excess H2O2 was determined from the amount of I3–

formed by the reactions

H2O2 + 2I– + 2H+ → I2 + 2H2O,

I2 + I– → I3–. (3)

(2)

The electronic absorption spectra of I3– solutions

have a characteristic absorption band with a maximum at 350 nm. From these data, the [Pu] : [O2

2–] ratio was found to be close to 1 : 2.

The second series of the experiments was per-formed as in the previous case, but the compounds ob-tained were kept in the mother liquor (1–6 M NaOH) for 5 days to evaluate their stability in alkaline media. Then the precipitates washed as in the previous experi-ments were dissolved in 2 M HClO4, and the mother liquors were checked for the presence of H2O2 with vanadyl ions. In all the cases, in dissolution we ob-served the formation of the “red” complex of Pu(IV) and its subsequent reduction to Pu(III), and in the mother liquors we detected only traces of free hydro-gen peroxide. Thus, it can be concluded that plutonium peroxy complxes are fairly stable in alkaline solutions even at a high NaOH concentration (~6 M).

Behavior of Pu(IV) Hydroxide in Alkaline Solutions in the Presence of H2O2

It was also interesting to determine whether Pu(IV) hydroxide can transform into peroxide in alkaline solu-tions. For this purpose, freshly precipitated Pu(IV) hy-droxide was teated with a 3.2 M H2O2 solution in 2 M NaOH. After 40 min, the precipitate was washed to remove excess peroxide to negative reaction with va-nadyl ions and was dissolved in 2 M HClO4. The solu-tion initially was reddish-brown, as in the above-described cases. The electronic absorption spectrum of the resulting solution, shown in Fig. 5, was recorded 1 h after dissolving the precipitate and did not change further. Because the solution contained Pu(III) and Pu(IV) ions, it can be concluded that a part of Pu(IV) hydroxide in the course of the reaction transformed

into the peroxy compound, because the solution con-tained no reductants other than peroxy groups incorpo-rated in the complex. The degree of conversion to Pu(IV) hydroxide will apparently depend mainly on the time and the H2O2 concentration in the solution, and also on other factors (temperature, alkali concen-tration). In this study we did not examine the effect of these factors in detail and only demonstrated the prin-cipal possibility of transformation of Pu(IV) hydroxide into a peroxy compound in an alkaline medium under the conditions of a strong competing effect of hydro-lytic processes.

Fig. 5. Electronic absorption spectrum of the solution pre-pared by dissolving in 2 M HClO4 the precipitate of Pu(IV) hydroxide, pretreated with H2O2 in 2 M NaOH.

Effect of H2O2 on the Completeness of Pu(IV) Precipitation in Alkaline Medium

It is known that the precipitation of Pu(IV) hydrox-ide from acidic solutions with an alkali can be accom-panied by formation of colloidal and polymeric forms of Pu, making the precipitation incomplete. Therefore, we attempted to transform these species into a peroxy complex. Precipitation was performed from two por-tions of a 2.6 × 10–3 M Pu(IV) solution in 2 M HNO3 with equal volumes of 4 M NaOH. The alkali concen-tration in solutions was ~0.5 M, and the total Pu con-centration, ~1.5 × 10–3 M in a volume of 1.3 ml. Then we added to one solution 0.1 ml of 9.8 M H2O2 and to the other solution, the same volume of water. Both so-lutions were allowed to stand for 24 h at room tem-perature in closed vessels. Then, after centrifugation at 8000 rpm, the α-activity (239Pu) of both solutions was measured. The results obtained are given below. It is seen that the presence of H2O2 in the solutions de-creases the content of residual Pu upon precipitation by a factor of almost 100.

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NIKONOV et al. 30

RADIOCHEMISTRY Vol. 52 No. 1 2010

The results of this study show that a Pu(IV) peroxy compound can be formed even in relatively concen-trated alkali solutions, despite strong competing effect of hydrolytic processes. The resulting peroxy com-pound is stable in alkaline medium for at least several days, and the peroxy groups bound in the complex, in contrast to free H2O2, interact with hydroxide ions weakly. The formation of the Pu(IV) peroxy com-pound in alkaline solution considerably decreases the concentration of colloidal and polymeric forms in solu-tion, which is a possible way to improve processes based on precipitation of Pu(IV) hydroxide.

Here we report only the most general data describ-ing the Pu(IV) behavior in alkaline solutions in the presence of H2O2. Determination of the structure of the

[H2O2], M

∑[Pu(IV)] in solu-tion, M

[Pu] in solution after precipi-tation, M (%)

0 1.5 × 10–3 3.32 × 10–5 (2.2) ~0.7 1.5 × 10–3 4.8 × 10–7 (0.03)

REFERENCES

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2. Buijs, K. and Louwrier, K.P., J. Inorg. Nucl. Chem., 1966, vol. 28, no. 10, pp. 2463–2464.

3. Gogolev, A.V., Nikonov, M.V., Tananaev, I.G., and Myasoedov, B.F., Radiokhimiya, 2004, vol. 46, no. 6, pp. 524–526.

4. Plutonium Handbook: A Guide to the Technology, Wick, O.J., Ed., New York: Gordon and Breach, 1967, vol. 1.

5. Schwartz, H.A. and Bielski, B.H., J. Phys. Chem., 1986, vol. 90, no. 7, pp. 1445–1448.

6. Connick, R.E. and McVey, W.H., J. Am. Chem. Soc., 1949, vol. 71, p. 1534.

peroxy compound formed and examination of addi-tional factors affecting its precipitation are subjects of a separate study.