Thermodynamic Formation Constants of the Calcium(II)triscarbonatoactinylate(VI) (U, Np, and Pu) Complexes Using Capillary Electrophoresis ICP-MS

The formation constants of CanAnO₂(CO₃)₃(4-2n)- (An = U, Np, and Pu) are determined in 0.1 mol/kgwater NaClO₄ medium at 25 °C via the coupling of capillary electrophoresis and inductively coupled plasma mass spectroscopy (CE-ICP-MS) using a poly(etheretherketone) (PEEK) capillaries. Prior to the calcium titration experiments, the stabilization of Np(VI) and Pu(VI) under the experimental conditions and their complexation behavior with carbonate were studied. The selection of literature data from limited available studies and the impact of minor AnO₂(CO₃)m2-2m species are also discussed. Experimental results indicate that for the three actinides, AnO₂(CO₃)m2-2m complexes form CaAnO₂(CO₃)₃²⁻ complexes first and subsequently Ca₂AnO₂(CO₃)₃(aq) complexes, as already shown for U(VI). The cumulative stability constants at infinite dilution and at 25 °C are log₁₀β°(CaUO₂(CO₃)₃²⁻) = 27.01 ± 0.11, log₁₀β°(Ca₂UO₂(CO₃)₃(aq)) = 30.13 ± 0.16, log₁₀β°(CaNpO₂(CO₃)₃²⁻) = 25.14 ± 0.28, log₁₀β°(Ca₂NpO₂(CO₃)₃(aq)) = 28.64 ± 0.24, log₁₀β°(CaPuO₂(CO₃)₃²⁻) = 25.85 ± 0.69, and log₁₀β°(Ca₂PuO₂(CO₃)₃(aq)) = 29.15 ± 0.66. Theoretical calculations using the obtained data also suggest that the formation of CanAnO₂(CO₃)₃(4-2n)- complexes favors the stabilization of Np(VI) and Pu(VI) in calcium- and carbonate-rich oxidant environments.