Uranyl Speciation in Carbonate-Rich Hydrothermal Solutions:\nA Molecular Dynamics Study

In this study, we employed classical\nmolecular dynamics (CMD) and\nfirst-principles molecular dynamics (FPMD) simulations to investigate\nthe speciation of uranyl in carbonate-rich hydrothermal solutions.\nThe association constants (log<sub>10</sub><i>K</i><sub>A</sub>) of uranyl carbonate complexes were derived from the potential\nof mean forces (PMFs) obtained from CMD simulations, and the acid\nconstants (p<i>K</i><sub>a</sub>s) of uranyl aqua ions were\ncalculated using the FPMD-based vertical energy gap method. The results\nshowed that uranyl ions could form stable mono- and bi-carbonate complexes\nat elevated temperatures and that uranyl aqua ions strongly hydrolyzed\nin neutral solutions at temperatures exceeding 473 K. The speciation\nof uranyl in hydrothermal solutions was constructed based on the calculated\nthermodynamics data. It was found that uranyl carbonate complexes\nwere predominant in aqueous solutions at temperatures below 373 K,\nand at higher temperatures, UO<sub>2</sub>(OH)<sub>2</sub>/UO<sub>2</sub>(OH)<sup>+</sup> became the predominant species. These findings\nprovide molecular-level insight into the speciation of uranyl in hydrothermal\nsolutions and highlight the role of uranyl hydroxides in the transport\nand deposition of uranium in hydrothermal processes.