CE-ICP-MS Investigation of the Carbonate Complexation of Am(III), Th(IV), Np(V), and U(VI) and the Influence of Alkali Cations

Capillary electrophoresis inductively coupled plasma mass spectrometry (CE-ICP-MS) was used to investigate the carbonate complexation of Am(III), Th(IV), Np(V), and U(VI) at environmentally relevant concentrations. Experiments were conducted at an ionic strength of 0.33 M in different alkali chloride solutions (Li⁺, Na⁺, and K⁺). Formation constants for the three successive actinide carbonate complexes were determined for Am(III), Np(V), and U(VI). For Th(IV), the CE-ICP-MS measurements posed some challenges. The complexation of U(VI) with carbonate was determined to be stronger than previously assumed. A strong influence of alkali cations on the electrophoretic mobility μ of highly negatively charged actinide carbonate complexes was observed and attributed to the association of cations to those complexes. The electrophoretic mobility of negatively charged An-CO₃ complexes was found to follow the trend |μ_Li| ≤ |μ_Na| < |μ_K|. Furthermore, the reaction of UO₂(CO₃)₃^4- with different alkali cations (Me⁺ = Li⁺-Cs⁺) was studied. Complex formation constants for MeUO₂(CO₃)₃^3- and Me₂UO₂(CO₃)₃^2- for all investigated cations were determined.