Effect of Ionizing Radiation on the Redox Chemistry of Penta- and Hexavalent Americium

Abstract

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium’s inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am­(VI) to Am­(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO3 radiolysis, i.e., HNO2, H2O2, and HO2 •, which dictated its practical lifetime under acidic conditions. In contrast, Am­(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO3 •, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am­(IV). Our multiscale modeling calculations predict that the lifetime of Am­(V) is dictated by its rate of disproportionation, 2AmO2 + + 4Haq + → AmO2 2+ + Am4+ + 2H2O, with a fourth-order dependence on [Haq +] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10–6 M–5 s–1. This disproportionation initially produces Am­(IV) and Am­(VI) species, but the lack of any spectroscopic evidence in our study for Am­(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am­(VI)/Am­(V) irradiations is Am­(III), which shows great stability in an irradiation field.

Thermodynamics of CeSiO4: Implications for Actinide Orthosilicates

“…The heavier transuranic elements (Am, Bk, and Cf) show more lanthanide-like characteristics and, thus, prefer having a trivalent oxidation state. − The trivalent oxidation would result in different ionic radii and preferred coordination chemistry. , For such trivalent cations to be introduced into the zircon structure, a charge-coupled substitution would be required. Nonetheless, the tetravalent states of the majority of the aforementioned elements are possible through either a transient state as a result of radiation-induced radiolysis − or a steady state being stabilized by either ligands or electrochemical methods. ,− Both, transient-state and steady-state conditions can be achieved in the unlikely event of a failure of the geological repository and breach in radiological waste canister. Each of these states could be observed under such a hypothetical scenario, if said geological repository is located in a geologic setting where the host lithology is also rich with mineral halides (i.e., halite), such as what is observed at the Waste Isolation Pilot Plant (WIPP) located in New Mexico, , and then there would be a supply of radiation, heat, high-ionic strength aqueous solutions, and the silica source from the natural barriers .…”

“…Nonetheless, the tetravalent states of all the aforementioned elements are possible through either a transient state as a result of radiation-induced radiolysis [91][92][93] or a steady state being stabilized by either ligands or electrochemical methods 88,[94][95][96] . Each of these states, the transient state and steady state conditions, are achievable in the unlikely event of a failure of the geological repository and breach in radiological waste canister.…”

Large-Scale Models of Radiation Chemistry and the Principle of Detailed Balancing

“…Horne and co-workers developed a multiscale model of the radiation chemistry of nitrate solutions and applied it to the behavior of neptunium and americium solutions. − This multiscale model treats radiolysis in four stages: radiation track structure, physicochemical track processes, nonhomogeneous intratrack reactions, and homogeneous bulk reactions. The last stage includes a core set of 109 aqueous reaction steps, some of which are treated as reversible equilibria such as the autoionization of water, and others that are represented as irreversible steps .…”

“…In any case it is clear that both the forward and reverse rate constants for eq are not well-established and that the model results are highly sensitive to the values selected. As this model was used to determine some of the model parameters describing americium and neptunium solutions, − we conclude that those model parameters need to be reassessed.…”

Chemical bonding in actinyl(v/vi) dipyriamethyrin complexes for the actinide series from americium to californium: a computational investigation

“…13 Compared to the pentavalent OS, hexavalent americyl (Am VI O 2 + ) with a 5f 3 occupied valence shell is energetically favoured 14 and more common. [15][16][17][18][19] Dau et al reported the first synthesis of bare Bk(V) and Cf(V) in their dioxide cation forms. They performed CCSD(T) calculations to show that linear actinyl(V) ions (AnO 2 + ) are local energy minima structures through MdO 2 + and all An from Pa through Es (except Cm) formed stable dioxide cations.…”