Aqua-Chloro Complexes of Plutonyl in Hydrochloric Aqueous Solutions: Theoretical Study of Individual Contributions to XAS and UV-Vis Spectra.

Plutonium speciation in high-salinity media remains difficult to characterize, yet it largely governs the geochemical fate of PuO22+ in marine environments. EXAFS and UV-vis measurements established that the evolution of specific spectral features correlates with the formation of successive aqua-chloro complexes. These signals represent a superposition of coexisting species in equilibrium, masking the individual contributions of each complex. In this work, we address this limitation with a combination of the nuclear ensemble approach (NEA) and high-level quantum-mechanical calculations, accounting for the relativistic Hamiltonian, spin-orbit coupling, and electron correlation, to isolate these overlapping contributions of the [PuO2(H2O)mCln]2-n series and clarify the observed plutonyl speciation. The sensitivity of our methodology is quantified by comparing Wigner sampling against classical molecular dynamics (MD) trajectories. The results establish UV-vis spectroscopy as a high-resolution probe of the equatorial ligand field and provide a molecular basis for Pu(VI) speciation in saline environments.