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09:23
Removal of Trace Elements by Cupric Oxide Nanoparticles from Uranium In Situ Recovery Bleed Water and Its Effect on Cell Viability
Published on: June 21, 2015
Speciation-dependent microbial reduction of uranium within iron-coated sands
Jim Neiss1, Brandy D Stewart, Peter S Nico
1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA.
Environmental Science & Technology
|November 30, 2007
Summary
Calcium presence significantly hinders uranium (U(VI)) reduction by bacteria in mineral columns, leading to increased uranium transport. This highlights the critical role of uranyl speciation in environmental uranium mobility.
Area of Science:
- Environmental Geochemistry
- Microbial Biogeochemistry
- Environmental Science
Background:
- Uranium transport in subsurface environments is primarily governed by its redox state, with U(VI) reduction to U(IV) forming insoluble UO2 precipitates.
- Biological U(VI) reduction is influenced by competing electron acceptors and uranyl speciation, particularly Ca-UO2-CO3 ternary complexes, which can inhibit reduction rates.
- The combined effects of uranyl speciation, mineral matrices, and hydrodynamics on U(VI) reduction remain incompletely understood.
Purpose of the Study:
- To investigate the impact of Ca-UO2-CO3 ternary complex formation on uranyl (U(VI)) reduction by Shewanella putrefaciens.
- To evaluate the influence of uranyl speciation on U(VI) transport under simulated subsurface conditions using packed mineral columns.
Main Methods:
- Experiments were conducted using packed mineral columns containing ferrihydrite-coated quartz sand.
- Uranyl reduction was monitored under conditions with and without calcium (Ca) in the influent solution.
- Hydrodynamic conditions and U(VI) speciation were controlled to mimic natural environments.
Main Results:
- In the absence of Ca, U(VI) reduction was rapid and complete within the first 2.5 cm of the column, with over 2 g of uranium deposited over 54 days.
- Despite ferrihydrite acting as a competing electron acceptor, uranium reduction remained unabated in Ca-free conditions.
- In the presence of 4 mM Ca, U(VI) reduction was effectively absent, leading to breakthrough of uranyl within 18 days at a flow rate of 3 pore volumes per day.
Conclusions:
- Uranyl speciation, specifically the formation of ternary Ca-UO2-CO3 complexes, profoundly impacts U(VI) reduction rates.
- The presence of calcium significantly enhances uranium mobility in anaerobic subsurface systems by inhibiting microbial reduction.
- Understanding uranyl speciation is crucial for predicting and managing uranium transport in contaminated environments.
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