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

The solubility and mobility of actinides (An), like uranium, neptunium, and plutonium, in the environment largely depends on their oxidation states. Actinyls (AnO) form strong complexes with available ligands, like carbonate (CO), which may inhibit reduction to relatively insoluble AnO. Here we use quantum-mechanical calculations to explore the kinetics of aqueous homogeneous reaction paths of actinyl tricarbonate complexes ([AnO(CO)]) with two different reductants, [Fe(OH)(HO)] and [HS(HO)]. 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), some of which are prohibitively high (>100 kJ/mol for some neptunyl and plutonyl reactions with Fe and HS), that define the transition from outer-to inner-sphere complex (ISC; for example, calculated E of ISC formation between UO and UO with Fe 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 An and An is not observed (no change in atomic spin values or lengthening of An-O bond distances). Proton transfer from bicarbonate and water to actinyl O was tested as a mechanism for electron transfer from Fe to U and Pu. Not all proton transfer reactions yielded reduction of An to An and only a few pathways were energetically-favorable (e. g. H transfer from HO to drive Pu reduction to Pu 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.