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.
Highlights
Uranium speciation in contaminated soils, groundwater, vadose zones and bottom sediments of waste storage pools governs the migration behavior in plumes of mine tailings and legacy sites of nuclear weapon production (Zachara et al, 2013; Peterson et al, 2018; Kaplan et al, 2020; Stetten et al, 2020)
To correctly interpret the EXAFS and Raman data, all studied compounds had to be free of impurities
U–humate complexation through carboxyl groups cannot be completely excluded in the uranium-contaminated soil (UCS) sample, the formation of uranyl carbonates seems to be more likely, as we found that U distribution correlates with Ca and Cauranyl carbonate species such as libegite Ca2[(UO2)2(CO3)4] are assumed
Summary
Uranium speciation in contaminated soils, groundwater, vadose zones and bottom sediments of waste storage pools governs the migration behavior in plumes of mine tailings and legacy sites of nuclear weapon production (Zachara et al, 2013; Peterson et al, 2018; Kaplan et al, 2020; Stetten et al, 2020). Decommissioning of closed enterprises and decontamination of territories require knowledge of U species to predict U migration in the environment (Maher et al, 2013; Mehta, 2017; Katsenovich et al, 2018). The complexity of U behavior in the environment due to the presence of carbonates, organic matter, Fe oxides, bacterial activity etc. Requires a detailed study of individual U species under controlled laboratory conditions. Along with surface complexation with minerals and incorporation into solids like calcite, an essential mechanism of U immobilization is the formation of intrinsic U(VI) phases. The other type of UO22+ formation in the environment is the oxidation of U(IV) phases
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