SREL Reprint #3312

Spectroscopic evidence of uranium immobilization in acidic wetlands by natural organic matter and plant roots

Dien Li1, Daniel I. Kaplan1, Hyun-Shik Chang2, John C. Seaman2, Peter R. Jaffé3, Paul Koster van Groos3, Kirk G. Scheckel4, Carlo U. Segre5, Ning Chen6, De-Tong Jiang7, Matthew Newville8,
and Antonio Lanzirotti8

1Savannah River National Laboratory, Aiken, South Carolina 29808, United States
2Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 29802, United States
3Department of Civil and Environmental Engineering, Princeton University,
Princeton, New Jersey 08544, United States
4U.S. Environmental Protection Agency, National Risk Management Research Laboratory,
Cincinnati, Ohio 45224, United States
5Physics Department & CSRRI, Illinois Institute of Technology, Chicago, Illinois 60616, United States
6Canadian Light Sources Inc., Saskatoon, Saskatchewan S7N 2V3, Canada
7Department of Physics, University of Guelph, Guelph, Ontario N1G 2W1, Canada
8CARS, University of Chicago, Argonne, Illinois 60439, United States

Abstract: Biogeochemistry of uranium in wetlands plays important roles in U immobilization in storage ponds of U mining and processing facilities but has not been well understood. The objective of this work was to study molecular mechanisms responsible for high U retention by Savannah River Site (SRS) wetland sediments under varying redox and acidic (pH = 2.6–5.8) conditions using U L3-edge X-ray absorption spectroscopy. Uranium in the SRS wetland sediments existed primarily as U(VI) bonded as a bidentate to carboxylic sites (U–C bond distance at ~2.88 Å), rather than phenolic or other sites of natural organic matter (NOM). In microcosms simulating the SRS wetland processes, U immobilization on roots was 2 orders of magnitude higher than on the adjacent brown or more distant white sands in which U was U(VI). Uranium on the roots were both U(IV) and U(VI), which were bonded as a bidentate to carbon, but the U(VI) may also form a U phosphate mineral. After 140 days of air exposure, all U(IV) was reoxidized to U(VI) but remained as a bidentate bonding to carbon. This study demonstrated NOM and plant roots can highly immobilize U(VI) in the SRS acidic sediments, which has significant implication for the long-term stewardship of U-contaminated wetlands.

SREL Reprint #3312

Li, D., D. I. Kaplan, H. Chang, J. C. Seaman, P. R. Jaffé, P. Koster van Groos, K. G. Scheckel, C. U. Segre, N. Chen, D. Jiang, M. Newville, and A. Lanzirotti. 2015. Spectroscopic evidence of uranium immobilization in acidic wetlands by natural organic matter and plant roots. Environmental Science and Technology 49(5): 2823-2832.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).