Subsurface Uranium Fate and Transport: Integrated Experiments and Modeling of Coupled Biogeochemical Mechanisms of Nanocrystalline Uraninite Oxidation by Fe(III)-(hydr)oxides - Project Final Report

Peyton, Brent M.¹  Timothy, Ginn R.²  Sani, Rajesh K.³ 

[1] Montana State Univ., Bozeman, MT (United States);Univ. of California, Davis, CA (United States);South Dakota School of Mines and Technology, Rapid City, SD (United States)

关键词: Subsurface;    Uranium;    Biogeochemical;    Nanocrystalline;    Uraninite;    Fe(III)-(hydr)oxide;   

DOI  :  10.2172/1089976
RP-ID  :  Final Report--FG02-07ER64366
PID  :  OSTI ID: 1089976

学科分类：环境科学（综合）

美国|英语

来源: SciTech Connect

【 摘 要 】

Subsurface bacteria including sulfate reducing bacteria (SRB) reduce soluble U(VI) to insoluble U(IV) with subsequent precipitation of UO₂. We have shown that SRB reduce U(VI) to nanometer-sized UO₂ particles (1-5 nm) which are both intra- and extracellular, with UO₂ inside the cell likely physically shielded from subsequent oxidation processes. We evaluated the UO₂ nanoparticles produced by Desulfovibrio desulfuricans G20 under growth and non-growth conditions in the presence of lactate or pyruvate and sulfate, thiosulfate, or fumarate, using ultrafiltration and HR-TEM. Results showed that a significant mass fraction of bioreduced U (35-60%) existed as a mobile phase when the initial concentration of U(VI) was 160 ??M. Further experiments with different initial U(VI) concentrations (25 - 900 M) in MTM with PIPES or bicarbonate buffers indicated that aggregation of uraninite depended on the initial concentrations of U(VI) and type of buffer. It is known that under some conditions SRB-mediated UO₂ nanocrystals can be reoxidized (and thus remobilized) by Fe(III)-(hydr)oxides, common constituents of soils and sediments. To elucidate the mechanism of UO₂ reoxidation by Fe(III) (hydr)oxides, we studied the impact of Fe and U chelating compounds (citrate, NTA, and EDTA) on reoxidation rates. Experiments were conducted in anaerobic batch systems in PIPES buffer. Results showed EDTA significantly accelerated UO₂ reoxidation with an initial rate of 9.5 M day-1 for ferrihydrite. In all cases, bicarbonate increased the rate and extent of UO₂ reoxidation with ferrihydrite. The highest rate of UO₂ reoxidation occurred when the chelator promoted UO₂ and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO₂ dissolution did not occur, UO₂ reoxidation likely proceeded through an aqueous Fe(III) intermediate as observed for both NTA and citrate. To complement to these laboratory studies, we collected U-bearing samples from a surface seep at the Rifle field site and have measured elevated U concentrations in oxic iron-rich sediments. To translate experimental results into numerical analysis of U fate and transport, a reaction network was developed based on Sani et al. (2004) to simulate U(VI) bioreduction with concomitant UO₂ reoxidation in the presence of hematite or ferrihydrite. The reduction phase considers SRB reduction (using lactate) with the reductive dissolution of Fe(III) solids, which is set to be microbially mediated as well as abiotically driven by sulfide. Model results show the oxidation of HS??? by Fe(III) directly competes with UO₂ reoxidation as Fe(III) oxidizes HS??? preferentially over UO₂. The majority of Fe reduction is predicted to be abiotic, with ferrihydrite becoming fully consumed by reaction with sulfide. Predicted total dissolved carbonate concentrations from the degradation of lactate are elevated (log(pCO₂) ~ ???1) and, in the hematite system, yield close to two orders-of-magnitude higher U(VI) concentrations than under initial carbonate concentrations of 3 mM. Modeling of U(VI) bioreduction with concomitant reoxidation of UO₂ in the presence of ferrihydrite was also extended to a two-dimensional field-scale groundwater flow and biogeochemically reactive transport model for the South Oyster site in eastern Virginia. This model was developed to simulate the field-scale immobilization and subsequent reoxidation of U by a biologically mediated reaction network.