Kinetics of the Autoreduction of Hexavalent Americium in Aqueous Nitric Acid

Generation and study of Am(IV) by temperature-controlled electron pulse radiolysis

First-of-a-kind temperature-controlled electron pulse radiolysis experiments facilitated the radiation-induced formation of Am(IV) in concentrated (6.0 M) HNO3, and enabled the derivation of Arrhenius and Eyring activation parameters for instigating the radical reaction between NO3˙ and Am(III).

Multiscale Modeling of Plutonium Radiation Chemistry in Nitric Acid Solutions. 1. Cobalt-60 Gamma Irradiation of Pu(IV)

Careful manipulation of the plutonium oxidation states is essential in the study and utilization of its rich redox chemistry. To achieve this level of control, a comprehensive mechanistic understanding of radiation-induced plutonium redox chemistry is critical due to the unavoidable exposure of plutonium to ionizing radiation fields, both inherent and from in-process applications. To this end, we have developed an experimentally evaluated multiscale computer model for the prediction of gamma radiation-induced Pu(IV) redox chemistry in concentrated nitric acid solutions (1.0, 3.0, and 6.0 M). Under these acidic, aqueous solution conditions, cobalt-60 gamma irradiation afforded marginal net conversion of Pu(IV) to Pu(VI), the extent of which was dependent on the concentration of HNO3 and absorbed gamma dose. Multiscale calculations, which are in excellent agreement with experimental data, indicate that this observation is due to a combination of inherent plutonium disproportionation reactions and several radiation-induced processes, including redox cycling between Pu(IV) and Pu(III), as achieved by the reduction of Pu(IV) by nitrous acid and hydrogen peroxide, the oxidation of Pu(III) by nitrate and hydroxyl radicals, and the sequential oxidation of Pu(IV) to Pu(V) and Pu(VI) by the remaining available yield of nitrate radicals.

Radiation-Induced Defects in Uranyl Trinitrate Solids

Actinides are inherently radioactive; thus, ionizing radiation is emitted by these elements can have profound effects on its surrounding chemical environment through the formation of free radical species. While previous work has noted that the presence of free radicals in the system impacts the redox state of the actinides, there is little atomistic understanding of how these metal cations interact with free radicals. Herein, we explore the effects of radiation (UV and γ) on three U(VI) trinitrate complexes, M[UO2(NO3)3] (where M=K+, Rb+, Cs+), and their respective nitrate salts in the solid state via electron paramagnetic resonance (EPR) and Raman spectroscopy paired with Density Functional Theory (DFT) methods. We find that the alkali salts form nitrate radicals under UV and γ irradiation, but also note the presence of additional degradation products. M[UO2(NO3)3] solids also form nitrate radicals and additional DFT calculations indicate the species corresponds to a change from the bidentate bound nitrate anion into a monodentate NO3 • radical. Computational studies also highlight the need to include the second sphere coordination environment around the [UO2(NO3)3]0,1 species to gain agreement between the experimental and predicted EPR signatures.

Chemical Kinetics for the Oxidation of Californium(III) Ions with Select Radiation-Induced Inorganic Radicals (Cl2•- and SO4•-)

Despite the availability of transuranic elements increasing in recent years, our understanding of their most basic and inherent radiation chemistry is limited and yet essential for the accurate interpretation of their physical and chemical properties. Here, we explore the transient interactions between trivalent californium ions (Cf 3 + ) and select inorganic radicals arising from the radiolytic decomposition of common anions and functional group constituents, specifically the dichlorine (Cl2•-) and sulfate (SO4•-) radical anions. Chemical kinetics, as measured using integrated electron pulse radiolysis and transient absorption spectroscopy techniques, are presented for the reactions of these two oxidizing radicals with Cf 3 + ions. The derived and ionic strength-corrected second-order rate coefficients (k) for these radiation-induced processes are k(Cf 3 + + Cl2•-) = (8.28 ± 0.61) × 105 M-1 s-1 and k(Cf 3 + + SO4•-) = (9.50 ± 0.43) × 108 M-1 s-1 under ambient temperature conditions (22 ± 1 °C).

Ultrafiltration separation of Am(VI)-polyoxometalate from lanthanides

Partitioning of americium from lanthanides (Ln) present in used nuclear fuel plays a key role in the sustainable development of nuclear energy1-3. This task is extremely challenging because thermodynamically stable Am(III) and Ln(III) ions have nearly identical ionic radii and coordination chemistry. Oxidization of Am(III) to Am(VI) produces AmO22+ ions distinct with Ln(III) ions, which has the potential to facilitate separations in principle. However, the rapid reduction of Am(VI) back to Am(III) by radiolysis products and organic reagents required for the traditional separation protocols including solvent and solid extractions hampers practical redox-based separations. Herein, we report a nanoscale polyoxometalate (POM) cluster with a vacancy site compatible with the selective coordination of hexavalent actinides (238U, 237Np, 242Pu and 243Am) over trivalent lanthanides in nitric acid media. To our knowledge, this cluster is the most stable Am(VI) species in aqueous media observed so far. Ultrafiltration-based separation of nanoscale Am(VI)-POM clusters from hydrated lanthanide ions by commercially available, fine-pored membranes enables the development of a once-through americium/lanthanide separation strategy that is highly efficient and rapid, does not involve any organic components and requires minimal energy input.

Ion sieving in graphene oxide membrane enables efficient actinides/lanthanides separation

Separation of actinides from lanthanides is of great importance for the safe management of nuclear waste and sustainable development of nuclear energy, but it represents a huge challenge due to the chemical complexity of these f-elements. Herein, we report an efficient separation strategy based on ion sieving in graphene oxide membrane. In the presence of a strong oxidizing reagent, the actinides (U, Np, Pu, Am) in a nitric acid solution exist in the high valent and linear dioxo form of actinyl ions while the lanthanides (Ce, Nd, Eu, Gd, etc.) remain as trivalent/tetravalent spheric ions. A task-specific graphene oxide membrane with an interlayer nanochannel spacing between the sizes of hydrated actinyl ions and lanthanides ions is tailored and used as an ionic cut-off filter, which blocks the larger and linear actinyl ions but allows the smaller and spheric lanthanides ions to penetrate through, affording lanthanides/actinides separation factors up to ~400. This work realizes the group separation of actinides from lanthanides under highly acidic conditions by a simple ion sieving strategy and highlights the great potential of utilizing graphene oxide membrane for nuclear waste treatment.

The Speciation of Americium Cations in Neat Water Implicated from DFT Studies

The recent observed manipulatable redox potential of trivalent americium ion in the aqueous phase by modifying an electrode offers an alternative to accomplish the separation. In order to understand extensively the speciation of Am, which is the prerequisite to understanding the mechanism of the oxidation of Am, we conducted a density functional study to identify the potential species of Am in its tri-, tetra-, and pentavalent states in aqueous phase. Based on the speciation analysis, the calculations implicate a stepwise mechanism for the oxidation of hydrated Am(III), which predominantly exists in its hydrated monatomic cationic form (Am³⁺(aq)). The two sequential one-electron oxidation processes first produce AmO²⁺(aq), which may establish an equilibrium with Am⁴⁺(aq), and the AmO²⁺(aq) may then evolve to the dioxo americyl(V) ion. These results suggest the copresence of Am⁴⁺(aq) and AmO²⁺(aq), which builds a bridge for the conversion of americium ion from a monatomic ion to dioxo americyl(V).

Oxidizing Americium(III) with Sodium Bismuthate in Acidic Aqueous Solutions

Historic perspectives describing f-elements as being redox "inactive" are fading. Researchers continue to discover new oxidation states that are not as inaccessible as once assumed for actinides and lanthanides. Inspired by those contributions, we studied americium(III) oxidation in aqueous media under air using NaBiO_3(s). We identified selective oxidation of Am³⁺_(aq) to AmO₂²⁺_(aq) or AmO₂¹⁺_(aq) could be achieved by changing the aqueous matrix identity. AmO₂²⁺_(aq) formed in H₃PO_4(aq) (1 M) and AmO₂¹⁺_(aq) formed in dilute HCl_(aq) (0.1 M). These americyl products were stable for weeks in solution. Also included is a method to recover ²⁴³Am from the americium and bismuth mixtures generated during these studies.

Transient Radiation-Induced Berkelium(III) and Californium(III) Redox Chemistry in Aqueous Solution

Despite the significant impact of radiation-induced redox reactions on the accessibility and lifetimes of actinide oxidation states, fundamental knowledge of aqueous actinide metal ion radiation chemistry is limited, especially for the late actinides. A quantitative understanding of these intrinsic radiation-induced processes is essential for investigating the fundamental properties of these actinides. We present here a picosecond electron pulse reaction kinetics study into the radiation-induced redox chemistry of trivalent berkelium (Bk(III)) and californium (Cf(III)) ions in acidic aqueous solutions at ambient temperature. New and first-of-a-kind, second-order rate coefficients are reported for the transient radical-induced reduction of Bk(III) and Cf(III) by the hydrated electron (e_aq^-) and hydrogen atom (H^•), demonstrating a significant reactivity (up to 10¹¹ M^-1 s^-1) indicative of a preference of these metals to adopt divalent states. Additionally, we report the first-ever second-order rate coefficients for the transient radical-induced oxidation of these elements by a reaction with hydroxyl (^•OH) and nitrate (NO₃^•) radicals, which also exhibited fast reactivity (ca. 10⁸ M^-1 s^-1). Transient Cf(II), Cf(IV), and Bk(IV) absorption spectra are also reported. Overall, the presented data highlight the existence of rich, complex, intrinsic late actinide radiation-induced redox chemistry that has the potential to influence the findings of other areas of actinide science.

Marking actinides for separation: Resonance-enhanced multiphoton charge transfer in actinide complexes

Precise separation and purification of f-block elements are important and challenging especially for the reduction of nuclear waste and the recycling of rare metals but are practically difficult mainly because of their chemical similarity. A promising way to overcome this difficulty is controlling their oxidation state by nonchemical processes. Here, we show resonance-enhanced multiphoton charge transfer in actinide complexes, which leads to element-specific control of their oxidation states owing to the distinct electronic spectra arising from resonant transitions between f orbitals. We observed oxidation of trivalent americium in nitric acid. In addition, we found that the coordination of nitrates is essential for promoting the oxidation reaction, which is the first finding ever relevant to the primary process of photoexcitation via resonant transitions of f-block elements. The resonance-enhanced photochemical process could be used in the nuclear waste management, as it would facilitate the mutual separation of actinides, such as americium and curium.

Curium(iii) radiation-induced reaction kinetics in aqueous media

Insight into the effects of radiolytic processes on the actinides is critical for advancing our understanding of their solution chemistry because the behaviour of these elements cannot be easily separated from the influence of their inherent radiation field. However, minimal information exists on the radiation-induced redox behaviour of curium (Cm), a key trivalent transuranic element present in used nuclear fuel and frequently used as an alpha radiation source. Here we present a kinetic study on the aqueous redox reactions of Cm(iii) with radicals generated through the radiolysis of aqueous media. In particular, we probe reaction kinetics in nitric acid solutions that are used as the aqueous phase component of used nuclear fuel reprocessing solvent systems. Second-order rate coefficients (k) were measured for the reaction of Cm(iii) with the hydrated electron (e_aq^-, k = (1.25 ± 0.03) × 10¹⁰ M^-1 s^-1), hydrogen atom (H˙, k = (5.16 ± 0.37) × 10⁸ M^-1 s^-1), hydroxyl radical (˙OH, k = (1.69 ± 0.24) × 10⁹ M^-1 s^-1), and nitrate radical (NO₃˙, k = (4.83 ± 0.09) × 10⁷ M^-1 s^-1). Furthermore, the first-ever Cm(ii) absorption spectrum (300-700 nm) is also reported. These kinetic data dispel the status quo notion of Cm(iii) possessing little to no redox chemistry in aqueous solution, and suggest that the resulting Cm(ii) and Cm(iv) transients could exist in irradiated aqueous solutions and be available to undergo subsequent redox chemistry with other solutes.

New insight on the simultaneous H2 and HNO2 production in concentrated HNO3 aqueous solutions under alpha radiation

Knowledge of hydrogen and nitrous acid yields (G(H₂) and G(HNO₂)) from α radiolysis of nitric acid solutions is of critical importance for the technological aspects of reprocessing of spent nuclear fuel (SNF). This study provides critical information on the G values for external alpha irradiation of concentrated HNO₃ solutions. An investigation-specifically developed experimental setup allows performing this investigation without encountering issues related to extreme high local doses. In situ monitoring of the UV-visible induced absorption in irradiated HNO₃ solutions permitted quantification of HNO₂ production, and mass spectrometry was used to quantify H₂. The influence of the dose rate and HNO₃ concentration was investigated, and the primary yields of these two species were determined. It was found that dose rate increase leads to diminished production of HNO₂ and H₂, while HNO₃ concentration increase leads to increased HNO₂ formation and reduced H₂ production. The values of the primary yields of these two species were determined and compared to the literature reported values. While the determined values show similar trends as those reported, this study provides accurate radiolytic yields for H₂ and HNO₂ that are radioelement-independent compared to the α radiolysis using radioisotope/HNO₃ mixtures and provides the basis for perfecting numerical codes used for simulating the radiolytic processes associated with SNF reprocessing.

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

Large-scale kinetic models containing more than 50 homogeneous reaction steps have been developed to help understand the behavior of actinide solutions in various circumstances. One specific objective is to understand the behaviors in nitrate solutions as such solutions are used in actinide separations. A challenge arises in developing these large-scale models while ensuring compliance with the principle of detailed balancing because it can be very difficult to identify all embedded reaction loops. These loops can violate the principle of detailed balancing either by consisting of reversible steps that do not meet Wegscheider's condition or by having unopposed irreversible steps that cause illegality. Here we report the development of DETBAL, which is a software code that systematically identifies a basis of reversible loops and a basis of all loops within the model, and then automatically checks for illegal loops. We apply DETBAL to two recent large-scale models of the radiation of nitrate solutions, show that these models violate the principle of detailed balancing in many ways, provide potential solutions to these violations, and demonstrate the effect of some of these modifications.

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

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⁺ + 4H_aq⁺ → AmO₂²⁺ + Am⁴⁺ + 2H₂O, with a fourth-order dependence on [H_aq⁺] 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.

Electrochemical oxidation of trivalent americium using a dipyrazinylpyridine modified ITO electrode

We present here the electrochemical oxidation of Am(iii) to AmVO2+ and AmVIO22+ in pH 1 nitric acid using a mesoporous tin-doped indium oxide electrode modified with a covalently attached dipyrazinylpyridine ligand. The applied potential affects the distribution of Am oxidation products. At potential 1.8 V, only Am(v) is observed, while increasing the potential to as much as 2.0 V, results in oxidation of Am(iii) to Am(v) and subsequent oxidation of Am(v) to Am(vi). At applied potentials >2.0 V, Am(iii) is oxidized to Am(v), while Am(vi) is reduced to Am(v). The latter reduction reaction is likely due to the increased rate of hydrogen peroxide formation from the 2-electron oxidation of water at the electrode at these high potentials. The development of future ligand modified electrodes for actinide oxidations must consider how they facilitate Am oxidations while disfavoring unwanted or competing reactions.

Neptunium(iv)-hydroxamate complexes: their speciation, and kinetics and mechanism of hydrolysis

First time determination of rate parameters for hydrolysis of mono- and bis-acetohydroxamatoneptunium(iv) complexes under conditions relevant to nuclear reprocessing.

Solubility and Complexation of the Bismuthate Ion in Nitric Acid Systems

The dissolution rate and solubility of NaBiO₃ have been investigated in nitric acid systems ranging from 4 to 6 M HNO₃ and were found to be 58-76 μg/cm²·d and 490-830 mM, respectively. The presence of 50 mM U(VI) drastically increased the solubility to 540-1200 mM, while rates of dissolution were relatively unchanged. The solubility of NaBiO₃ increased with an increase in U(VI) concentrations at 4 M HNO₃, with log-log analysis indicating a one-to-one complex between Bi and U and infrared spectroscopic evidence monitoring uranyl stretching, suggesting complex formation. Absorbance spectra were obtained experimentally and computationally with an absorbance band in the range of 450-600 nm that has been attributed to Bi(V). The ingrowth and decay of Bi(V) in solution was also studied as a function of mass of solid NaBiO₃ present, acidity, and temperature. The activation energies of dissolution and decomposition were calculated to be 39 ± 4 and 61 ± 6 kJ/mol, respectively. These results indicate that dissolution of NaBiO₃ into the respective Na⁺ and BiO₃^-occurs prior to undergoing reduction, a process which conventionally has been believed to occur in the reverse order.

Molecular Hydrogen Yields from the α-Self-Radiolysis of Nitric Acid Solutions Containing Plutonium or Americium

The yield of molecular hydrogen, as a function of nitric acid concentration, from the α-radiolysis of aerated nitric acid and its mixtures with sulfuric acid containing plutonium or americium has been investigated. Comparison of experimental measurements with predictions of a Monte Carlo radiation track chemistry model shows that, in addition to scavenging of the hydrated electron, its precursor, and the hydrogen atom, the quenching of excited state water is important in controlling the yield of molecular hydrogen. In addition, increases in solution acidity cause a significant change in the track reactions, which can be explained as resulting from scavenging of e_aq^- by H_aq⁺ to form H^•. Although plutonium has been shown to be an effective scavenger of precursors of molecular hydrogen below 0.1 mol dm^-3 nitrate, previously reported effects of plutonium on G(H₂)_α between 1 and 10 mol dm^-3 nitric acid were not reproduced. Modeling results suggest that plutonium is unlikely to effectively compete with nitrate ions in scavenging the precursors of molecular hydrogen at higher nitric acid concentrations, and this was confirmed by comparing molecular hydrogen yields from plutonium solutions with those from americium solutions. Finally, comparison between radionuclide, ion accelerator experiments, and model predictions leads to the conclusion that the high dose rate of accelerator studies does not significantly affect the measured molecular hydrogen yield. These reactions provide insight into the important processes for liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.