HERFD-XANES spectroscopy at the U M4-edge applied to the analysis of U oxidation state in a heavily contaminated wetland soil

Abstract

Determining U oxidation state in contaminated (sub)surface soils and sediments is essential to depict the geochemical processes affecting U in natural media. This information is also mandatory to infer the mechanisms governing the mobilization and transfer of this toxic radionuclide to the environment. Here, in attempt to detect U(IV), U(V) and U(VI) in wetland soil samples contaminated by past mining activities, we have performed high-resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) measurements at the U M4-edge. Linear combination fitting (LCF) analysis of the spectra have been conducted using reference samples representative of the wetland geochemistry, in which U occurs as U-phosphate minerals and mononuclear U complexes. Our experimental constraints for HERFD measurements at low energy (3.7 keV) implied to limit the thickness of the Kapton® foil used to protect the samples, which lead to slow oxidation by air during the measurements. In this context, U(IV) appeared to partly oxidize into U(VI) and/or U(V) within a few tens of hours. Nano-crystalline reference samples showed contrasted oxidation pathways for U(IV), transforming into U(V)/U(VI)-uranate in biogenic nano-uraninite, and into U(VI)-uranyl in nano-U(IV)-rhabdophane. In the wetland soils samples, uranium was mainly present as U(IV) and U(VI) with detection of minor U(V) (<13% of total U), possibly pristine and/or resulting from oxidation during the measurements. Our results thus show that U(V) may result from oxidation of mononuclear or nano-crystalline U(IV) after moderate air exposure, which challenges unambiguous detection of U(V) in environmental samples and calls for further U M4-edge HERFD-XANES measurements under strict anoxia.