Spectroscopic Evidence of Uranium Immobilization in Acidic Wetlands by Natural Organic Matter and Plant Roots

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.