The safety analysis of deep geological repositories for the final disposal of high-level nuclear waste, requires the behavior of the actinides under environmentally relevant conditions to be known. Diffusion and sorption processes dependent strongly on the actinide species present. Knowledge of the speciation of actinides, especially plutonium, under repository-relevant conditions is therefore of great importance.
Plutonium (Pu) shows a vast redox chemistry with the ability to form a wide range of oxidation states between +III and +VI in aqueous solution. It is not unusual for different oxidation states of Pu to exist simultaneously, which complicates the investigation of complex equilibria for Pu. [1]
Current repository concepts envisage the use of cementitious materials as technical barriers. By using such materials, organic molecules can be introduced into the repository system for example in the form of concrete admixtures. Gluconate (Glu) is used as a retarder and has a high potential for complexation and the associated mobilization of radiotoxic elements. [2]
The combination of the sensitive separation of capillary electrophoresis (CE) based on charge to radius ratio and the element selectivity and low detection limits of ICP-MS provides a valuable tool for thermodynamic data collection at low actinide concentration. [3]
In the work presented, the gluconate complexation of the major Pu oxidation states +III, +IV, +V and +VI as well as the redox analogous actinides Th(IV), U(VI), Np(V) and Am(III) was investigated at low concentrations (1x10^-8 - 1x10^-7 M) using CE-ICP-MS. The comparison with the redox-stable actinides combined with the separation capability of the CE-ICP-MS made it possible to determine the complexation constants of the Pu-Glu complexes even with several oxidation states of Pu being present simultaneously.

References
[1] D. L. Clark et al. Plutonium. In: The Chemistry of the Actinide and Transactinide Elements. 2010, 813-1264
[2] J. Tits et al. Appl. Geochem. 2005, 20, 2082–2096.
[3] C. Willberger et al. Inorg. Chem. 2019, 58, 8, 4851–4858