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

摘要

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 L_3-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 A), 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 bom 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.