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

In this study, we employed classical molecular dynamics (CMD) and first-principles molecular dynamics (FPMD) simulations to investigate the speciation of uranyl in carbonate-rich hydrothermal solutions. The association constants (log₁₀K_A) of uranyl carbonate complexes were derived from the potential of mean forces (PMFs) obtained from CMD simulations, and the acid constants (pK_as) of uranyl aqua ions were calculated using the FPMD-based vertical energy gap method. The results showed that uranyl ions could form stable mono- and bi-carbonate complexes at elevated temperatures and that uranyl aqua ions strongly hydrolyzed in neutral solutions at temperatures exceeding 473 K. The speciation of uranyl in hydrothermal solutions was constructed based on the calculated thermodynamics data. It was found that uranyl carbonate complexes were predominant in aqueous solutions at temperatures below 373 K, and at higher temperatures, UO₂(OH)₂/UO₂(OH)⁺ became the predominant species. These findings provide molecular-level insight into the speciation of uranyl in hydrothermal solutions and highlight the role of uranyl hydroxides in the transport and deposition of uranium in hydrothermal processes.