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The solubility, migration behavior and bioavailability of radionuclides in the marine environment strongly depend on their speciation. This focus article reviews the state-of-the-art and recent advances in the determination of radionuclide speciation in seawater, which is still challenging because of the very high ionic strength of the medium associated with ultra-trace concentrations of these elements in the oceans. In particular, we have highlighted the contribution and usefulness of synchrotron-based techniques such as X-ray absorption spectroscopy. Within this scope, we overview some major radionuclides in seawater, their natural or anthropogenic origin, and their reactivity and natural concentrations. We outline the theoretical speciation models currently used, based on thermodynamic stability constants, and compare them to published experimental data recently obtained from spectroscopic investigation of radionuclides in natural seawater samples. Finally, we discuss some leading perspectives on radionuclide speciation using X-ray absorption spectroscopy in environmental samples at concentrations that must deal with spectroscopy detection limits.

International audience; 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 Me2+ 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 SO2+, SrUO2(CO3)(3)(2-) is the main complex in solution and it occurs with the presence of at least one mono dentate carbonate in the uranyl coordination sphere.

In the ocean, complex interactions between natural and anthropogenic radionuclides, seawater, and diverse marine biota provide a unique window through which to examine ecosystem and trophic transfer mechanisms in cases of accidental dissemination. The nature of interaction between radionuclides, the marine environment, and marine species is therefore essential for better understanding transfer mechanisms from the hydrosphere to the biosphere. Although data pertaining to the rate of global transfer are often available, little is known regarding the mechanism of environmental transport and uptake of heavy radionuclides by marine species. Among marine species, sponges are immobile active filter feeders and have been identified as hyperaccumulators of several heavy metals. We have selected the Mediterranean sponge Aplysina cavernicola as a model species for this study. Actinide elements are not the only source of radioactive release in cases of civilian nuclear events; however, their physicochemical transfer mechanisms to marine species remain largely unknown. We have targeted europium(III) as a representative of the trivalent actinides such as americium or curium. To unravel biological uptake mechanisms of europium in A. cavernicola, we have combined radiometric (γ) measurements with spectroscopic (time-resolved laser-induced fluorescence spectroscopy, TRLIFS, and X-ray absorption near-edge structure, XANES) and imaging (transmission electron microscopy, TEM, and scanning transmission X-ray microscopy, STXM) techniques. We have observed that the colloids of NaEu(CO