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Updated: Jan 9, 2026

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Published on: June 8, 2022
Thermodynamic Formation Constants of the Calcium(II)triscarbonatoactinylate(VI) (U, Np, and Pu) Complexes Using
Ruopei Sun1, Erwan Dupuis1, Pascal E Reiller1
1Université Paris-Saclay, CEA, Service de Physico-Chimie, Gif-sur-Yvette 91191, France.
Abstract:
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.
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