Citation:

Li, D., D. Kaplan, H. Chang, J. Seaman, P. Jaffe, P. Koster van Groos, K. Scheckel, C. Segre, N. Chen, D. Jiang, M. Newville, AND A. Lanzirotti. Spectroscopic Evidence of Uranium Immobilization in Acidic Wetlands by Natural Organic Matter and Plant Roots. David L. Sedlak (ed.), ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 49(5):2823-2832, (2015).

Impact/Purpose:

There were several former U processing facilities at the Savannah River Site (SRS), Aiken, SC. As a result of their operations, uranium has entered the surrounding environments. For example, approximately 45,000 kg of depleted U in an acidic plume (pH 2.6-5.8) was released into Tims Branch stream and its associated wetlands between 1958 and 1980.^1,2 Approximately 70% of this U still remains in the stream and its associated wetland sediments. The biogeochemistry of U in wetlands (e.g., the SRS Tims Branch and its associated wetlands) is profoundly different than in uplands because of the presence of sharp geochemical gradients (e.g., pH, dissolved O₂, and E_h), active vegetation, elevated organic carbon concentrations and microbial activity, and the transient nature of hydraulic regimes (e.g., rainfall-induced flooding events and drought cycles). These conditions are related to the interface between relatively anoxic groundwater and oxic surface water and are known to greatly affect the mobility of U and other redox sensitive metals. The objective of this work was to conduct spectroscopic measurements of wetland sediments and associated plant roots to help understand the biogeochemical factors responsible for the observed strong binding of U to the SRS wetland sediments. State-of-the art analytical methods were used, including U L₃-edge X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) spectroscopy, and X-ray fluorescence (XRF) mapping. The intent of this research was to understand the bonding mechanisms responsible for the observed strong binding of U to the SRS wetland systems, especially the roles of NOM, plant roots and their associated rhizosphere on U immobilization. As such, the samples investigated in this work included: (1) U-contaminated SRS wetland sediments were studied to help understand U oxidation state and bonding environment under field conditions. (2) Noncontaminated SRS sediments were amended with U(VI) over a range of pH values relevant to the SRS systems to provide additional information about the sensitivity of U binding and oxidation state to pH changes. 3) Plant roots were recovered from a greenhouse microcosm study to provide a controlled system for studying the role of plant roots in U immobilization and U biogeochemistry.

Description:

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 process, 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 on the long-term stewardship of U-contaminated wetlands.

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