Atomistic Simulations of Calcium Uranyl(VI) Carbonate Adsorption on Calcite and Stepped-Calcite Surfaces

Adsorption of actinyl ions onto mineral surfaces is one of the main mechanisms that control the migration of these ions in environmental systems. Here, we present computational classical molecular dynamics (MD) simulations to investigate the behavior of U­(VI) in contact with different calcite surfaces. The calcium-uranyl-carbonate [Ca₂UO₂(CO₃)₃] species is shown to display both inner- and outer-sphere adsorption to the flat {101̅4} and the stepped {314̅8} and {31̅2̅16} planes of calcite. Free energy calculations, using the umbrella sampling method, are employed to simulate adsorption paths of the same uranyl species on the different calcite surfaces under aqueous condition. Outer-sphere adsorption is found to dominate over inner-sphere adsorption because of the high free energy barrier of removing a uranyl–carbonate interaction and replacing it with a new uranyl–surface interaction. An important binding mode is proposed involving a single vicinal water monolayer between the surface and the sorbed complex. From the free energy profiles of the different calcite surfaces, the uranyl complex was also found to adsorb preferentially on the acute-stepped {314̅8} face of calcite, in agreement with experiment

Microanalytical X‑ray Imaging of Depleted Uranium Speciation in Environmentally Aged Munitions Residues

Use of depleted uranium (DU) munitions has resulted in contamination of the near-surface environment with penetrator residues. Uncertainty in the long-term environmental fate of particles produced by impact of DU penetrators with hard targets is a specific concern. In this study DU particles produced in this way and exposed to the surface terrestrial environment for longer than 30 years at a U.K. firing range were characterized using synchrotron X-ray chemical imaging. Two sites were sampled: a surface soil and a disposal area for DU-contaminated wood, and the U speciation was different between the two areas. Surface soil particles showed little extent of alteration, with U speciated as oxides U₃O₇ and U₃O₈. Uranium oxidation state and crystalline phase mapping revealed these oxides occur as separate particles, reflecting heterogeneous formation conditions. Particles recovered from the disposal area were substantially weathered, and U­(VI) phosphate phases such as meta-ankoleite (K­(UO₂)­(PO₄)·3H₂O) were dominant. Chemical imaging revealed domains of contrasting U oxidation state linked to the presence of both U₃O₇ and meta-ankoleite, indicating growth of a particle alteration layer. This study demonstrates that substantial alteration of DU residues can occur, which directly influences the health and environmental hazards posed by this contamination