New insight into the ternary complexes of uranyl carbonate in seawater

Uranium is naturally present in seawater at trace levels and may in some cases be present at higher concentrations, due to anthropogenic nuclear activities. Understanding uranium speciation in seawater is thus essential for predicting and controlling its behavior in this specific environmental compartment and consequently, its possible impact on living organisms. The carbonato calcic complex Ca₂UO₂(CO₃)₃ was previously identified as the main uranium species in natural seawater, together with CaUO₂(CO₃)₃^2-. In this work, we further investigate the role of the alkaline earth cation in the structure of the ternary uranyl-carbonate complexes. For this purpose, artificial seawater, free of Mg²⁺ and Ca²⁺, using Sr²⁺ as a spectroscopic probe was prepared. Combining TRLIF and EXAFS spectroscopy, together with DFT and theoretical thermodynamic calculations, evidence for the presence of Sr alkaline earth counter ion in the complex structure can be asserted. Furthermore, data suggest that when Ca²⁺ is replaced by Sr²⁺, SrUO₂(CO₃)₃^2- is the main complex in solution and it occurs with the presence of at least one monodentate carbonate in the uranyl coordination sphere.