Effect of organic carbon functional groups on the trapping and transformations of U(VI)
EMSL Project ID
48185
Abstract
Understanding the behavior of U(VI) in natural environments is crucial for controlling U contamination from uranium mining and milling activities at DOE legacy sites [1]. Previous studies of U-contaminated soils have shown that U can be associated with C-rich materials, in addition to mineral phases such as Fe hydroxides [2]. However, the speciation of U is difficult to determine in complex soils and sediments, especially at low concentrations, and the molecular-level mechanisms of U-scavenging are still poorly understood, particularly when natural organic matter (NOM) is present. U is thought to be highly reactive with organo-mineral complexes, which can enhance mineral dissolution and the transport of metal ions and can catalyze element redox transformations. As part of a larger project on the speciation and redox transformations of U in the subsurface at the DOE Rifle (CO) site managed by the DOE-BER Office of Legacy Management, we propose to study the reactivity of NOM with U(VI) using 13C NMR, which will provide critical information on the types and concentrations of different functional groups in the NOM not easily obtainable using other methods. This proposal is linked to a molecular-level study of the reactions among U, NOM, and Ferrihydrite (Fh) in samples from batch experiments as well as from the Rifle site. The information expected from NMR results is needed in order to constrain the interpretations of an on-going complementary study of the 3-component complexes of U, NOM, Fh by U L- and Fe K-edge XANES and EXAFS spectroscopy. The combination of NMR and x-ray spectroscopic techniques will help determine the affinity of NOM vs. ferrihydrite for uranium, and the results will elucidate the types (and strengths) of bonding involved in the reaction pathways. To achieve this goal, we propose to first carry out 13C-NMR measurements on the National Humic Substance Society (NHSS) model NOM Eliott Soil Humic Acid (ESHA) as solid and solution samples under conditions that simulate those of natural ground waters, and then to perform further 13C NMR experiments on U-reacted ESHA samples. Parallel XANES spectroscopic analyses will reveal the occurrence and extent of U(VI) redox reactivity in these model systems. EXAFS spectroscopy on the same samples will help distinguish the differences in bonding environments of U to NOM and Fh. By combining the results of our study with literature data, we expect to elucidate the role of NOM in scavenging U from Fh. Beam time on SSRL BL 11-2 has been allocated to conduct the XAS part of the project.
Project Details
Project type
Exploratory Research
Start Date
2013-12-02
End Date
2015-05-31
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members