Uranium Speciation in the Rhizosphere and Sediment Compartments in a Riparian Wetland

Authors

Maxim Boyanov^1,2*, (mboyanov@anl.gov), Edward O’Loughlin¹, Pamela Weisenhorn¹, Peng Lin³, Daniel Kaplan³, Kenneth Kemner¹

Institutions

¹Argonne National Laboratory, Argonne, IL; ²Bulgarian Academy of Sciences, Sofia, Bulgaria; ³Savannah River Ecology Laboratory, University of Georgia, Aiken, SC

URLs

Abstract

Savannah River Site manufactured nuclear fuel and target assemblies between 1954 and 1989, resulting in significant contamination of Tims Branch (a small stream and its associated wetlands) with uranium (U) and co-contaminant metals (Ni, Cr, Zn, Pb). The Argonne Science Focus Area (SFA; Wetland Hydrobiogeo-chemistry) focuses on understanding the molecular-scale processes that control elemental dynamics in complex environments, such as adsorption, precipitation, and redox transformations. Researchers examined the behavior of contaminant U in the rhizosphere (Kaplan et al. In revision), a region of high reactivity in the sediment near the roots of plants, where visible iron oxides form due to Fe(II) oxidation. U and Fe were found to be significantly more concentrated in the rhizosphere (R) samples compared to the non-rhizosphere (NR) controls. Fe K-edge EXAFS spectroscopy determined goethite and ferrihydrite as the dominant forms of Fe, with the latter present in greater proportions in the R than in the NR samples. U L_III-edge EXAFS spectroscopy showed U present as U(VI) bound to organic ligands and iron oxides in both the R and NR samples; however, a larger contribution of U(VI) bound to iron oxides was determined in the spectra of the R samples. These results are part of a broader effort by the Argonne SFA to understand U speciation and mobility at Tims Branch, in which approximately 30 cores were collected from the field and analyzed with depth using U LIII-edge EXAFS spectroscopy.

Saturated, organic-rich sediments at about 5 to 15 cm depth showed the predominance of non- uraninite U(IV) species, whereas drier, near-surface sediments showed the predominance of U(VI) that was bound to the mineral surfaces and to OM entities in the sediment. These studies highlight the intricate interdependencies between the constituents of contaminated sediments and provide speciation information that is necessary for the inclusion of appropriate reactions in reactive transport models.

References

Kaplan, D. I. et al. In revision. “Uranium biogeochemistry in the rhizosphere of a riparian wetland,” Environmental Science & Technology.