2.1 Interaction of Am(VI) with water

The reduction rate of ²⁴³Am(VI) in basic aqueous solution (~20% per hour) was found to be of the same order than for ²⁴¹Am(VI). The values of the apparent rate constant (k_a) of the first-order reaction in 3 M and 1 M NaOH are the same, i.e. ~(5.3 ± 0.3) × 10^–4 s^–1. In 0,1 M NaOH, k_a is increased to 1.7 × 10^–3 s^–1. These data show that the reduction of Am(VI) by water molecules is the main contribution to the process and considerable contribution of Am alpha-radiolysis effects into the americium reduction in these conditions can be neglected.

2.2 Am(VII) species

Interaction of ~10^–3 M ²⁴³Am(VI) solution in 1 M NaOH under mixing at 20 °C with sodium perxenate leads to a precipitate. The ozonization of this precipitate leads to the formation of Am green soluble species with specific optical spectrum (Fig. 1). It is much alike the spectra of Np(VII) and Pu(VII) in alkaline media [6], but shifted to the IR range. It is supposed that, in this experiment, an Am(VII) compound has been formed.

2.3 Americium hydroxides interactions

The mixing of equivalent amounts of ²⁴³Am(III) and ²⁴³Am(V) hydroxides in 1 M NaOH at 50–70 °C, or in 7M NaOH at 25 °C during a few minutes led to the appearance of pure Am(IV) species via the reproportionation reaction Am(III) + Am(V) = 2 Am(IV). The absorption spectra of initial and final mixture are given in Figs. 2 and 3, respectively. The reaction between Am(VI) and Am(III) hydroxides in equivalent amounts brings the same result.

2.4 Heterogeneous reversible Am redox reaction in alkaline media

In 0.006 M NaOH, an excess of K₃Fe(CN)₆ does not oxidize Am(III) hydroxide at room temperature. An increase of [NaOH] to 0.087 M leads to the reaction of 0.067M K₃Fe(CN)₆ and 1.5 mM of ²⁴³Am(OH)₃ at 20 °C, with the formation of Am(OH)₄. In 2 M NaOH solution, K₃Fe(CN)₆ (0.04M) oxidizes Am(OH)₃ to Am(V). The redox reaction of Am(VI) and RuO₄^2– ions is reversible in basic solutions. Based on the literature data on extinction coefficient of ruthenate and perruthenate ions and redox potential of RuO₄^–/RuO₄^2– pairs (0,6 V/NHE) [6], we obtained an approximate value of the redox potential of the Am(VI)/Am(V) pair. In 1 M NaOH, E^f(Am(VI)/Am(V)) was estimated at about 0,7 V/NHE.

2.5 Pu(VI) disproportionation in concentrated alkaline solutions

At present, it is considered that Pu(VI) (in contrast to Np(VI) and Am(VI)) is stable in alkaline solutions. The disproportionation of Pu(VI) to Pu(V) and Pu(VII) does not take place in a wide interval of NaOH concentrations. This is connected with the high oxidation potential of the Pu(VII)/Pu(VI) pair in alkaline media. It should be noted that the increase of alkali concentration in solution leads to the decrease of the formal potential of the Pu(VII)/Pu(VI) pair, as well as for Np(VII)/Np(VI) [7] and that the difference of the above-mentioned potentials is constant and equal to 270 mV. Besides, the dependence of the E^f(Pu(VII)/Pu(VI)) value to [NaOH] in the 2–12 M interval can be expressed by an empirical equation with the same coefficient as in the case of neptunium(VII): E^f(Pu(VII)/Pu(VI)) = 0,874 – 0,0355 [NaOH] [7]. The assumption that the above-mentioned dependence may be used for calculations of E^f(Pu(VII)/Pu(VI)) in more concentrated alkaline solutions makes it possible to conclude that E^f(Pu(VII)/Pu(VI)) may be close to E^f(Pu(VI)/Pu(V)) in ~20–22 M NaOH. According to our opinion, the disproportionation reaction 2 Pu(VI) = Pu(VII) + Pu(V) is possible under certain conditions, but it requires experimental confirmation.

In our work, the precipitate of PuO₃·H₂O (~1,5 mg), washed two times with bidistillate water, was treated with hot saturated NaOH solution in the minimum volume by stirring. The contact time of PuO₃·H₂O with alkali was ~1–2 min. Then, the precipitate was isolated from the alkaline solution and was treated with 3 ml of 5 M LiOH. After centrifugation, the lithium alkali solution turned to a slight blue color. As it is shown in Fig. 4, the spectrum of this solution has a wide absorption band at 550–700 nm, with maximums at 605 and 630 nm, characteristic of Pu(VII). The absorbance of the spectrum of solution in the interval 400–800 nm decreases for more than two times during 2 h. The maximums at 605 and 630 nm became slightly distinguished, that is the evidence of the Pu(VII) reducing in the initial solution. Calculation of Pu(VII) yield leads to a value equal to 20% of the total amount of plutonium, transferred in 5 M LiOH solution. However, the real Pu(VII) yield may be significantly higher than the value obtained, as it is known that very fast Pu(VII) to Pu(VI) reduction occurs under dissolution of solid Pu(VII) compounds in aqueous solutions of alkalis because of large local Pu(VII) concentration at the interface between phases. During the first 2–3 min, up to 50% of Pu(VII) can reduce and then the reduction significantly slows down. Subsequently, it is planed to develop a technique for the determination of the yield of the products of Pu(VI) disproportionation in alkaline media. The appliance of the technique will allow us to establish the dependence of Pu(VII) yield on the alkali concentration in the reaction under investigation.