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 CaUO(CO) was previously identified as the main uranium species in natural seawater, together with CaUO(CO). 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 Mg and Ca, using Sr 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 Ca is replaced by Sr, SrUO(CO) is the main complex in solution and it occurs with the presence of at least one monodentate carbonate in the uranyl coordination sphere.