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Polonium (Po) is receiving increasing attention due to its pronounced toxicity, natural presence in uranium-rich environments and its artificial production in nuclear industry activities (e.g. in lead-bismuth cooled fast neutron reactors). This has led to a real need to understand the chemical behavior of Po in various environments, especially since that the aqueous chemistry of Po remains poorly understood. The knowledge about the Po species formed in solution has primarily originated from indirect partition experiments with tracer amounts of Po using e.g., solvent extraction or ion-exchange. These studies often lead to conflicting interpretations of the results due to the lack of a direct structural characterization of Po species present. Notably, the direct characterization of aqueous Po species is very challenging due to its scarcity, and only a limited number of attempts have been reported regarding the structural characterization of the polonium complexes in solution. Consequently, even the nature of Po chlorido complexes remains controversial, this is despite being one of the best studied aqueous species of Po. Chloride is expected to be an important inorganic ligand for Po with regard to e.g., its mobility in highly saline environments and its behavior during industrial processes involving naturally occurring radioactive material. X-ray absorption fine structure spectroscopy (XAFS) is a powerful technique which enables to get speciation information about species formed in solution, nevertheless, it has never been used to study the speciation of Po. Therefore, the aim of the presented study was to carry out the direct experimental characterization of Po chlorido complexes in aqueous media by XAFS.
In this work, microgram amounts of Po (mainly 209Po) were obtained by the irradiation of a bismuth target with a 16 MeV deuterium beam and its further processing with successive solvent extraction and extraction chromatography. The multiple separation steps resulted in the decrease of the Bi content in the Po source low enough to prevent the strong interference of the Bi L3 edge during XAFS measurement, giving access to the Po L3 edge. This enabled to record successfully for the first time the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) at the Po L3 edge. A combination of EXAFS obtained for Po in a highly acidic chloride solution and complementary DFT calculations revealed new structural information about the polonium chloride complexes formed. The developed approach paves the way towards the study of the speciation of Po with XAFS which will help to expand our fundamental knowledge and understanding of the aqueous chemistry of polonium.