Radiochemistry, Vol. 47, No. 5, 2005, pp. 482! 483. Translated from Radiokhimiya, Vol. 47, No. 5, 2005, pp. 441! 442. Original Russian Text Copyright + 2005 by Nikonov, Gogolev, Tananaev, Myasoedov.
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Reaction of Am(VI) with Na4XeO6 in Alkaline Solutions in the Presence of Ozone M. V. Nikonov*, A. V. Gogolev**, I. G. Tananaev*,**, and B. F. Myasoedov*,** * Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia ** Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia Received August 10, 2004
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Abstract The reaction of Am(VI) with perxenate ions XeO46! in 1 M NaOH solutions was studied. A solid compound [Am(VI) : Na4XeO6 = 1 : 1] is formed in reaction of 1 mM Am(VI) with solid Na4XeO6; its ozonation in a thin layer yields an Am(VII) compound.
Owing to high reactivity of xenon derivatives, they are often used as strong oxidants in inorganic synthesis, in particular, in preparation of the solid compounds of transuranium elements (TUEs). Xenon anions are strong complexing agents for TUEs in oxidation states III and VI. At high pH they can displace even carbonate anions from the coordination spheres of An3+ and AnO2+ 2 . Owing to extremely high oxidation potential, Xe(VIII) compounds rapidly decompose in neutral and acidic solutions. Among TUE(III) perxenates, Am4(XeO6)3 . 40H2O was prepared [1]. On adding sodium perxenate to a Np(VI) solution (2 : 1 molar ratio) in 0.013 0.25 M K2CO3 or 0.25 M Na2CO3, a brown precipitate is formed; its composition is characterized by the ratio Xe : Np ~1.25. Under similar conditions, a yellow solid compound of Pu(VI) was prepared (Xe : Pu ~1.25) [2]. The resulting precipitates, washed with water and dried in an argon flow or in a vacuum, decompose with time with gas evolution and changes in color. We expected that in an alkaline medium an analogous perxenate complex of Am(VI) could be prepared. Oxidation of Am(VI) perxenate complex in which Am atom is surrounded by six oxygen atoms could stabilize americium in oxidation state VII. This idea was checked in our study.
a total flow rate of 8310 l h31. The oxidation state of americium was monitored with a Shimadzu UV-160A spectrophotometer. RESULTS AND DISCUSSION On addition of a neutral solution of Na4XeO6 (5 mM, 0.5 ml) to a solution of Am(VI) (2 ml, 1 mM) in 0.1 M NaOH, an alkali-soluble Am(VI) perxenate complex is formed, which is confirmed by changes in color from yellow to dark violet. The spectrum of the initial Am(VI) compound and that of the americium solution treated with Na4XeO6 are shown in Fig. 1. On storage of the final solution for ~30 min or on adding solid Na4XeO6 to a 1 mM solution of Am(VI) in 1 M NaOH, a yellow solid is formed; it is insoluble in aqueous alkaline solutions. According to chemical analysis, this compound is characterized by the ratio Xe : Am ~1. The absorption spectra of the solid compound deposited on the quartz cell wall is shown in
EXPERIMENTAL As initial reagent we used an alkaline solution of prepared by ozonation of Am(OH)3 in bicarbonate solution; chemically pure grade NaOH and Na4XeO6 were used without additional purification. Ozone was obtained on a Tekhnozon ozonizer; its content in the oxygen flow was 3.535.0 vol % at 243Am(VI)
Fig. 1. Electronic absorption spectra of the (1) initial Am(VI) solution in 0.1 M NaOH and (2) product of its reaction with Na4XeO6 at 20oC.
1066-3622/05/4705-0482 C 2005 Pleiades Publishing, Inc.
REACTION OF Am(VI) WITH Na4XeO6 IN ALKALINE SOLUTIONS
Fig. 2. Electronic absorption spectra of a solid precipitate formed in the reaction of a 1.94 mM solution of Am(VI) in 1 M NaOH with solid Na4XeO6 (a) before and (b) after drying in a flow of an ozone3oxygen mixture (3.5 vol % ozone) for 5 min at 20oC.
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ilar to that observed in the UV spectrum of Am(VII) (Fig. 3) in 3.5 M NaOH [3]. It is known that, owing to rapid reduction of Am(VII) with water [3], its maximal yield in ozonation of Am(VI) in aqueous alkaline solutions does not exceed 60% even at low temperatures. Therefore, the bands at 440 and 520 nm, observed in the electronic absorption spectra of the solid product of ozonation of Am(VI) perxenate complex [Fig. 2b and Fig. 3, spectrum 2] were not detected in the solution spectrum (Fig. 3, spectrum 1) because of strong absorption of Am(VI) in this range. Presumably, Am(VI) is quantitatively oxidized to Am(VII) in the course of ozonation of the solid Am(VI) perxenate compound; i.e., the yield of Am(VII) is close to 100%. The resulting solid compound is unstable even in the ozone atmosphere. The spectrophotometric data show that the half-time of reduction of Am(VII) to Am(VI) is ~10 min. Additional studies are required to elucidate the process mechanism. Our experimental data show that ozonation of the Am(VI) perxenate compound yields a solid Am(VII) compound, which is relatively unstable even in the strongly oxidizing atmosphere. ACKNOWLEDGMENTS
Fig. 3. Electronic absorption spectrum (1) of a mixture of 1.9 mM Am(VII) and Am(VI) in 3.5 M NaOH [3] and (2) of a solid compound formed in ozonation of the Am(VI) perxenate compound with an ozone3oxygen mixture (3.4 vol % ozone) for 5 min at 20oC.
Fig. 2a. Our results suggest that, similarly to neptunium(VI) and plutonium(VI), Am(VI) in alkaline perxenate solutions forms solid compounds of the general composition Am(VI) : Xe = 131.25. The solid Am(VI) perxenate complex was treated with ozone. The precipitate recovered from the solution and washed with water was dried in a flow of an ozone3oxygen mixture (3.5 vol % ozone) for 5 min. The electronic absorption spectrum of the ozonized precipitate is shown in Fig. 2b. As seen, this spectrum strongly differs from that of the initial compound. A broad absorption band in the range 650 3900 nm with maxima at 712 and 7483750 nm (Fig. 2b) is sim-
RADIOCHEMISTRY
Vol. 47
No. 5
2005
The authors are grateful to D.L. Clark (Seaborg Institute, Los Alamos National Laboratory, the United States) for helpful discussion. The study was financially supported by the Integratsiya Federal Target Program, Charitable Foundation of the Support of Domestic Science, and US Department of Energy (grant no. RCO-20 004-SC14). REFERENCES 1. Marcus, Y. and Cohen, D., Inorg. Chem., 1966, vol. 5, no. 10, pp. 1740 !1743. 2. Gusev, Yu.K., Mefod’eva, M.P., and Kirin, I.S., Radiokhimiya, 1973, vol. 15, no. 6, pp. 801!804. 3. Krot, N.N., Gel’man, A.D., Mefod’eva, M.P., et al., Semivalentnoe sostoyanie neptuniya, plutoniya, ameritsiya (Heptavalent State of Neptunium, Plutonium, and Americium), Moscow: Nauka, 1977.
