Resolving the kinetics of individual aqueous reaction steps of actinyl (AnO ₂ ⁺ and AnO ₂ ²⁺ ; An=U, Np, and Pu) tricarbonate complexes with ferrous iron and hydrogen sulfide from first principles

Journal Article · Thu Jul 25 00:00:00 EDT 2019 · Radiochimica Acta

DOI:https://doi.org/10.1515/ract-2018-3083· OSTI ID:1800339

Bender, Will M. ^[1]; Becker, Udo ^[1]

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1. Department of Earth and Environmental Sciences , University of Michigan , Room 2534, 1100 N University Ave , Ann Arbor, MI 48109-1005 , USA

The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (An^V,VIO₂^+/2+_(aq)) form strong complexes with available ligands, like carbonate (CO₃^2-), which may inhibit reduction to relatively insoluble An^IVO_2(s). Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO₂(CO₃)₃]^5-/4-) with two different reductants, [Fe(OH)₂(H₂O)₄]⁰and [H₂S(H₂O)₆]⁰. Energetically-favorable outer-sphere complexes (OSC) are found to form rapidly, on the order of milliseconds to seconds over a wide actinyl concentration range (pM to mM). The systems then encounter energy barriers (E_a), some of which are prohibitively high (>100 kJ/mol for some neptunyl and plutonyl reactions with Fe²⁺and H₂S), that define the transition from outer- to inner-sphere complex (ISC; for example, calculatedE_aof ISC formation between UO₂⁺and UO₂²⁺with Fe²⁺are 35 and 74 kJ/mol, respectively). In some reactions, multiple OSCs are observed that represent different hydrogen bonding networks between solvent molecules and carbonate. Even when forming ISCs, electron transfer to reduce An⁶⁺and An⁵⁺is not observed (no change in atomic spin values or lengthening of An–O_axbond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe²⁺to U⁶⁺and Pu⁶⁺. Not all proton transfer reactions yielded reduction of An⁶⁺to An⁵⁺and only a few pathways were energetically-favorable (e. g. H⁺transfer from H₂O to drive Pu⁶⁺reduction to Pu⁵⁺with ΔE = -5 kJ/mol). The results suggest that the tricarbonate complex serves as an effective shield against actinide reduction in the tested reactions and will maintain actinyl solubility at elevated pH conditions. The results highlight reaction steps, such as inner-sphere complex formation and electron transfer, which may be rate-limiting. Thus, this study may serve as the basis for future research on how they can be catalyzed by a mineral surface in a heterogeneous process.

• Research Organization: Univ. of Michigan, Ann Arbor, MI (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• DOE Contract Number: FG02-06ER15783
• OSTI ID: 1800339
• Journal Information: Radiochimica Acta, Vol. 108, Issue 3; ISSN 0033-8230
• Publisher: de Gruyter
• Country of Publication: United States
• Language: English

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