Microscopic and spectroscopic insights into uranium speciation and mobilization pathways associated with carnotite weathering in sediments

Abstract

The speciation and mobility of uranium (U) in sediments are influenced by biogeochemical factors such as redox conditions, microbial activity, and the presence of organic matter (OM) and mineral phases. This study investigates the mechanisms controlling U behavior in sediments from the Punilla Valley (Córdoba, Argentina), using an integrated suite of analytical techniques including pXRF, XRD, SEM-EDS, XPS, and XAS. Field observations, together with geochemical and mineralogical analyses, indicate that U–V mineralization (i.e., carnotite) is heterogeneously distributed within reddish pelitic sediments and occurs as coatings on carbonates, micas, and plant roots. Grain-size and compositional data show U enrichment in fine sediment fractions, reflecting remobilization and secondary concentration under supergene conditions.Uranium redistribution appears to occur through the dissolution of carnotite, followed by interactions with plant roots and authigenic carbonates, as well as partial reduction to U(IV) mediated by Fe-bearing micas and dissolved organic matter. Synchrotron-based XAS analyses indicate that U(VI) is associated with calcite, whereas both U(IV) and U(VI) are associated with Fe-bearing micas, organic matter, and plant roots. These results demonstrate active U mobilization from carnotite, likely due to meteoric water infiltration, and highlight the important role of inorganic and organic sediment components as secondary sinks controlling U dynamics in sedimentary basins.