Coordination and hydrolysis of plutonium ions in aqueous solution using car-parrinello molecular dynamics free energy simulations

Car-Parrinello molecular dynamics (CPMD) simulations have been used to examine the hydration structures, coordination energetics, and the first hydrolysis constants of Pu³⁺, Pu⁴⁺, PuO₂ ⁺, and PuO₂²⁺ ions in aqueous solution at 300 K. The coordination numbers and structural properties of the first shell of these ions are in good agreement with available experimental estimates. The hexavalent PuO₂²⁺ species is coordinated to five aquo ligands while the pentavalent PuO₂⁺ complex is coordinated to four aquo ligands. The Pu³⁺ and Pu⁴⁺ ions are both coordinated to eight water molecules. The first hydrolysis constants obtained for Pu³⁺ and PuO₂²⁺ are 6.65 and 5.70, respectively, all within 0.3 pH unit of the experimental values (6.90 and 5.50, respectively). The hydrolysis constant of Pu⁴⁺, 0.17, disagrees with the value of -0.60 in the most recent update of the Nuclear Energy Agency Thermochemical Database (NEA-TDB) but supports recent experimental findings. The hydrolysis constant of PuO₂⁺, 9.51, supports the experimental results of Bennett et al. [Radiochim. Acta 1992, 56, 15]. A correlation between the pK_a of the first hydrolysis reaction and the effective charge of the plutonium center was found.