From EXAFS of reference compounds to U(VI) speciation in contaminated environments

Abstract

Understanding the speciation of technogenic uranium in natural systems is crucial for estimating U migration and bioavailability and for developing remediation strategies for contaminated territories. Reference EXAFS data of model laboratory-prepared uranium compounds (`standards') are necessary to analyze U-contaminated samples from nuclear legacy sites. To minimize errors associated with measurements on different synchrotrons, it is important not only to compare data obtained on environmentally contaminated samples with the literature but also with `standards' collected at the same beamline. Before recording the EXAFS spectra, all reference compounds were thoroughly characterized by Raman spectroscopy and powder X-ray diffraction. The U(VI) local molecular environments in the reference compounds, i.e. uranyl oxyhydroxides, phosphates, carbonates and uranates, were examined using XAFS. Based on the EXAFS fitting results obtained, including the nature of the bonding, interatomic distances and coordination numbers, parameters that are typical for a particular U compound were differentiated. Using data for `standards', U speciation in the sample of radioactively contaminated soil was determined to be a mixture of U oxyhydroxide and carbonate phases.

Uranium oxides structural transformation in human body liquids

“…It should be noted that the Debye-Waller parameter increases with the increase of oxides defectiveness, which is a natural consequence of structure disordering as the number of defects increases. The distance to U coordination sphere is the same for all oxides of the series and coincides with the value known for the stoichiometric UO 2 (3.87 ± 0.01 Å) from earlier EXAFS calculations 60 . A shift of 0.01 Å appears only in UO 2.20 .…”

Crystal Structure of Mixed Np(V)-Ammonium Carbonate

“…As shown in Figure 4A, a similar set of spectral lines is observed for these compounds; this is consistent with related local structures. The Raman spectrum of (NH 4 )[NpO 2 CO 3 ] can be divided into three parts: low-frequency region up to 200 cm −1 , containing bending vibrations of NpO 2 + and CO 3 2− ; stretching vibrations of the NpO 2 + cation at 766 cm −1 [31,32], and peaks of CO 3 2− bending and stretching vibrations at 718 and 1071 cm −1 , respectively [33]. The positions of the peaks of the carbonate group vibrations are consistent with those in ammonium uranyl carbonate and are within the characteristic range of the carbonate ion vibrations [34].…”

Uranium hydroxide/oxide deposits on uranyl reduction

“…[9][10][11][12] Also, uranium speciation in natural waters and in the environment has been studied by various methods, including anion exchange, laser spectroscopy, X-ray absorption spectroscopy, and Raman spectroscopy. [13][14][15][16][17] Most of those studies were in equilibrium and do not include dynamic reaction processes. As the deposition of U includes changes in the oxidation state nanoparticulation following reduction, it was reported that the reduction of U VI to U IV by Fe II and green rust leads to the formation of U IV O 2 nanoparticles.…”