Quantitative Prediction of Uranium Speciation and Amidoxime Binding in Seawater from Advanced Simulation Techniques

Technical Report · Sat Dec 29 00:00:00 EST 2018

DOI:https://doi.org/10.2172/1490188· OSTI ID:1490188

Jiang, De-en ^[1]

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1. Univ. of California, Riverside, CA (United States)

The goal of this project was to quantitatively predict stability of uranium species and thermochemistry of uranium binding with amidoxime ligands in realistic seawater conditions. The specific aims of the project were: (i) determine the stable species of uranium and its solvation structure in realistic seawater conditions; also speciation and solvation of uranium's major competing species in seawater, vanadium; (ii) accurately predict the free energies (and thereby stability constants) of binding between uranium and amidoxime; (iii) accurately predict the reaction enthalpies and entropies of binding between uranium and amidoxime as well as between vanadium and amidoxime. Main results and conclusions are as follows. First-principles molecular dynamic simulations found that the structure of the Ca₂UO₂(CO₃)₃ complex is very stable and that one Ca ion binds to the center UO₂(CO₃)₃ ^4- anion stronger than the other Ca ion. This finding suggests that using time-resolved EXAFS spectra may confirm the asymmetry in binding of the two Ca ions in the aqueous Ca₂UO₂(CO₃)₃ complex. To consider the common ions, solvation of the Ca₂UO₂(CO₃)₃ complex in seawater was simulated by classical molecular dynamics simulation. It was found that the structure of the Ca₂UO₂(CO₃)₃ complex is very stable in the model seawater. A Na+ ion was found to be closely associated with the Ca₂UO₂(CO₃)₃ complex by indirectly interacting with one axial oxygen atom of the UO₂ group bridged by a water molecule. In addition, the Na⁺ ion interacts closely with one Ca²⁺ ion than the other. The present simulations revealed the key role of common ions such as Na⁺ in impacting the solvation, structure, and apparent charge of the Ca₂UO₂(CO₃)₃ complex in seawater. The sequential processes of UO₂ ²⁺ binding with the three CO₃ ^2- groups and the two Ca²⁺ ions in pure water and in 0.1 M NaCl were simulated by classical molecular dynamics with both the non-constrained brute-force approach and umbrella sampling. The simulated free energies show excellent agreement with the experiment. The coordination structures of UO₂(CO₃)₃ ^4- and CaUO₂(CO₃)₃ ^2- were found to be significantly affected by the presence of Na⁺ ions, leading to a monodentate binding of a carbonate group to U and a water molecule entering the first coordination shell of U. To shed light on the binding between uranyl and glutardiamidoxime (H₂B), an important model and state-of-the-art ligand for seawater uranium extraction, a suite of computational methods was employed. From molecular dynamics (MD) simulations, it was found that the binding configurations of B^2- with uranyl favor the twofold distorted η₂ binding between the oximate ends (C=N-O-) and U, while HB^- prefers the chelating mode for the oximate end with the neutral end being solvated by water. The free energies of sequential ligand binding to form UO₂B, [UO₂B₂]^2-, and [UO₂(HB)B]^- were simulated with umbrella sampling and very good agreement with the experimental values was achieved, which corroborates the structural insights into the binding mode. State-of-the-art polymeric sorbents employ both amidoximate and carboxylate groups on the side chains to achieve optimal U uptake and selectivity, so we simulated the binding of a model amidoximate–carboxylate bifunctional ligand with uranyl. Classical MD and free-energy simulations in 0.5 M NaCl showed that the carboxylate group binds first to uranyl, leading to a loose intermediate state, and then, the amidoximate group binds, resulting in a more stable and tight chelate state. Binding of the second bifunctional ligand follows a similar process, and the two ligands prefer a trans configuration around the uranyl group. The simulated free energies indicate that the two bifunctional ligands bind with uranyl 14 kcal/mol stronger than the two ligands with only amidoximate groups, confirming an important synergy between amidoximate and carboxylate groups in binding uranyl.

• Research Organization: Univ. of California, Riverside, CA (United States)
• Sponsoring Organization: USDOE Office of Nuclear Energy (NE)
• DOE Contract Number: NE0008397
• OSTI ID: 1490188
• Report Number(s): 15-8536; 15-8536
• Country of Publication: United States
• Language: English

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37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY