Fluorescence spectroscopy was used to study the Ca 2+ /UO 2 2+ /CO 3 2− solution system at different ionic strengths. Fluorescent titrations confirmed formation of Ca 2 UO 2 (CO 3 ) 3 complex for which the values of log β 213 were measured at 0.1, 0.3, 0.7, 1.0 and 3.0 m (NaClO 4 ) at pcH = 8.1. Specific Interaction Theory (SIT) was used to correlate the log β 213 values as a function of ionic strength. The value obtained for log β 0 213 (I = 0) was 29.8 ± 0.7, indicating that Ca 2 UO 2 (CO 3 ) 3 was the predominant specie at the pH 8 in calcium rich, oxic waters.
The structuralc haracterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3nm. By combining high-energyX-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis,w eh ave characterised for the first time both the short-and mediumrange order of ThO 2 NPs obtained by chemicalp recipitation. By using this methodology,anovel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution functionr evealed ah igh concentration of ThO 2 small units similart ot horium hexamer clusters mixed with1nm ThO 2 NPs in the initial steps of formation. Drying the precipitates at around1 50 8Cp romoted the recrystallisation of the smallest units into more thermodynamically stable ThO 2 NPs. HERFD XANES analysis at the thorium M 4 edge, ad irect probe for fs tates, showed variations that we have correlated with the breakdown of the local symmetry aroundt he thorium atoms, which most likely concerns surface atoms. To gether,H EXS and HERFD XANES are ap owerful methodology for investigatinga ctinideN Ps and their formation mechanism.
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