New insight into the ternary complexes of uranyl carbonate in seawater. (November 2017) - Beccia, M.R.
Abstract: 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 Ca2 UO2 (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 Mg 2+ and Ca 2+, using Sr 2+ 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 2+ is replaced by Sr 2+, 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. Graphical abstract: Highlights: Ternary carbonate complexes are the main uranyl species in seawater. Experimental proof is provided for alkaline earth ion presence in these complexes. With Ca 2+, Ca2 UO2 (CO3 )3 is the main species in seawater with 3 bidentate carbonates. With Sr 2+, SrUO2 (CO3 )3 2− is the main species, with at least a monodentate carbonate.