Disproportionation of Am (IV)

Зайцев, А. А.; Kosyakov, V.N.; Rykov, A.G.; Sobolev, Yu.P.; Yakovlev, G.N.

doi:10.1007/bf01683116

Cited by 5 publications

(14 citation statements)

References 3 publications

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“…This leads to additional formation of HO 2 • , H 2 O 2 , and HNO 2 , which supports previous observations of enhanced Am(V) reduction in the presence of HO 2 • and H 2 O 2 , ,, and a reduced rate of Am(V) reduction in HNO 3 due to the intimate relationship between HNO 2 and H 2 O 2 (reaction ). , However, our multiscale calculations disagree with this interpretation, as they show that the radiolytic reduction of Am(V) by these processes leads to significant disagreement with experimental data, especially with respect to the changes in radiation quality. For example, even if very low rate coefficients (<1 M –1 s –1 ) are employed by calculation for reactions –, significantly more Am(V) is consumed than measured, leading to ingrowth of Am(VI), which is not observed in Figure A.…”

Section: Resultssupporting

confidence: 90%

“…A combination of radical and molecular processes have been postulated to explain the slow radiation-induced autoreduction of Am(V). In addition to reactions –, the following reactions can also occur: − ,,,,, …”

Section: Resultsmentioning

confidence: 99%

“…Interest in the high oxidation states of americium has seen a resurgence due to the opportunity to explore new americium chemistries and the potential need for facilitating the separation of americium from the trivalent lanthanides, which remains a key challenge for recycling used nuclear fuel. − ,− The lifetimes of these americyl states are beholden to a combination of chemical and radiolytic processes, the latter of which are critical to understanding these states. To date, their properties have only been empirically determined, which limits our fundamental mechanistic understanding and ability to predict their redox behavior. ,− With respect to pentavalent americium (Am(V)/AmO 2 + ), radiolysis is believed to be responsible for its reduction in a variety of media, e.g., perchloric, ,,, sulfuric, and nitric acid, ,, generally attributed to reactions involving the hydrated electron (e aq – , E ° = −2.9 V), hydrogen atom (H • , E ° = −2.3 V), hydrogen peroxide (H 2 O 2 , E ° = 1.8 V), and the hydroperoxyl radical (HO 2 • , E ° = 1.46 V): ,,, Am(V) is also susceptible to oxidation, for example, by the hydroxyl radical ( • OH, E ° = 2.7 V): Radiolysis of hexavalent americium (Am(VI)/AmO 2 2+ ) is constrained to reductive processes: ,,,,, Clearly, the lifetimes of Am(V) and Am(VI), and their subsequent oxidation state distributions, are strongly dependent upon irradiation conditions. However, the water radiolysis processes presented above are compounded by radiolytic effects/species specific to a given medium, which has led to conflicting results and mechanistic interpretations.…”

Section: Introductionmentioning

confidence: 99%

“…Zaitsev et al found that the rate of Am(V) reduction in nitric acid is slower than that in other acids . They attributed this observation to consumption of H 2 O 2 by nitric acid radiolysis products, in particular nitrous acid (HNO 2 , E ° = 0.984 V): − In contrast, Frolov et al found that Am(V) reduction was accelerated in concentrated nitric acid, the effect being attributed to the presence of HNO 2 . These conflicting observations are further exacerbated by two studies that showed nitrite (HNO 2 ⇌ NO 2 – + H aq + , p K a = 3.2) and H 2 O 2 to have little effect on Am(V) reduction in dilute nitric acid.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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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.

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Section: Resultssupporting

confidence: 90%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

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“…Reduction of Am(V) by H 2 O 2 From studies of the reduction of AmO þ 2 to Am 3þ by H 2 O 2 in 0.1 M HClO 4 ,Zaitsev et al (1960a) deduced the rate law:…”

mentioning

confidence: 99%