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

Abstract

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 (eaq –) 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.

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

“…• radicals: k = (2.07 ± 0.43) × 10 8 and (1.78 ± 0.19) × 10 8 M −1 s −1 , respectively. 12 Consequently, oxidation by the Cl 2…”

“…This additional species was not evident in the absence of Cf 3+ ions, nor can it be attributed to the Cf 4+ oxidation product, as this species has only minimal absorption at 450 nm. 12 A potential candidate for this species is the oxygen adduct of the persulfate peroxyl radical anion,…”

“…These primary radiolysis products have the capacity to interact with any remaining actinide ions to drive changes in their redox distribution and the potential formation of transient nonequilibrium actinide oxidation states. For example, trivalent californium ions ( Cf .1em 3 + ) can be transiently reduced and oxidized to the corresponding di- ( Cf .1em 2 + ) and tetravalent states ( Cf .1em 4 + ) by reaction with the hydrated electron (e aq – ) and hydroxyl radical ( • OH) arising from the direct radiolysis (⇝) of water H 2 normalO ⇝ H 2 normalO ∗ , normale aq − , H • , O • normalH , H 2 O 2 , H 2 , normalH aq + Cf .1em 3 + + normale aq − → Cf .1em 2 + newline0em3em⁣ k = ( 3.63 ± 0.14 ) × 10 10 normalM − 1 normals − 1 …”

Radiolytic Evaluation of 3,4,3-LI(1,2-HOPO) in Aqueous Solutions

“…The presented findings are expected to apply for other trivalent minor actinides (e.g., curium, berkelium, and californium) and lanthanides, whose di-and tetravalent states are typically transient without additional stabilization. 51,52 That said, ongoing research is investigating the combined impact of radiolysis and redox chemistry by using earlier actinides, such as uranium, neptunium, and plutonium. 53 These elements have access to a wider range of stable oxidation states that may facilitate additional inner vs outer sphere chemistry in the corresponding HOPO complex.…”

Co-Crystallization of Plutonium(III) and Plutonium(IV) Diglycolamides with Pu(III) and Pu(IV) Hexanitrato Anions: A Route to Redox Variants of [PuIII,IV(DGA)3][PuIII,IV(NO3)6]x

“…However, this would entail the formation of thermodynamically unfavorable di- and tetravalent lanthanide and transplutonium oxidation states. Concerning the latter, electron pulse irradiation studies have demonstrated the transient formation of some of these nontraditional valence states seen in the noncomplexed metal ion. ,− For example, trivalent curium has been shown to undergo radiation-induced redox processes to yield the corresponding di- and tetravalent states, both of which exhibited lifetimes on the order of tens of microseconds. , This is more than sufficient time to promote changes in their coordination environments and propagate the radiation chemistry.…”