Grantee Research Project Results

Kinetic and Thermodynamic Controls of Enzymatic Uranium Reduction in the Presence of Iron Oxide as a Competitive Terminal Electron Acceptor

EPA Grant Number: F13E31016
Title: Kinetic and Thermodynamic Controls of Enzymatic Uranium Reduction in the Presence of Iron Oxide as a Competitive Terminal Electron Acceptor
Investigators: Belli, Keaton Michael
Institution: Georgia Institute of Technology
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2014 through September 1, 2016
Project Amount: $84,000
RFA: STAR Graduate Fellowships (2013) RFA Text |  Recipients Lists
Research Category: Academic Fellowships , Fellowship - Geochemistry

Objective:

Uranium bioreduction—a bioremediation strategy that utilizes native metal-reducing bacteria in the subsurface to sequester and immobilize uranium as insoluble, reduced uranium minerals—is a cost-effective remediation strategy to address uranium-contaminated groundwater associated with nuclear facilities. The ability to predict the fate of uranium and the success of uranium bioreduction, however, is complicated by multiple reduction mechanisms and a lack of understanding regarding the specific geochemical conditions that promote either chemical or biological uranium reduction. This research will identify the kinetic and thermodynamic constraints that control uranium bioreduction and clarify the contribution of chemical and biological uranium reduction mechanisms across a wide range of geochemical conditions.

Approach:

Shewanella putrefaciens, a model metal-reducing bacteria capable of respiration on uranium and iron oxides, will be used in laboratory pureculture incubations to identify the kinetic and thermodynamic constraints that favor either uranium or iron reduction across a range of geochemical conditions (e.g., pH, concentration of carbonate, calcium and ferrous iron). A mutant strain of S. putrefaciens, which is capable of iron reduction but is unable to reduce uranium, will be used to distinguish between biological uranium reduction (bioreduction) and abiotic reduction of uranium by ferrous iron, a product of iron oxide respiration. Because traditional analytical techniques used to measure uranium provide limited insight to the reduction mechanism, a novel electrochemical technique developed as part of this research will be used to quantify aqueous uranium speciation in incubations and delineate the roles of chemical and biological reduction pathways during uranium immobilization.

Expected Results:

Uranium bioreduction is observed concomitantly with microbial iron reduction during large-scale field studies and laboratory incubations. Although the decrease in dissolved uranium concentrations is often attributed to biological uranium reduction, abiotic uranium reduction pathways likely also play an important role in uranium sequestration and immobilization, especially when respiration of iron oxides is more thermodynamically favorable than uranium respiration. Pure culture incubations with both wild-type and mutant metal-reducing bacteria combined with nontraditional analytical techniques will provide several lines of evidence to support the significance of multiple mechanisms of uranium reduction.

Potential to Further Environmental/Human Health Protection

Uranium represents a significant threat to both environmental and human health; therefore, cost-effective, efficient remediation strategies are needed to deal with existing uranium contamination from the Cold War Era. Additionally, as governments recognize the environmental and economic consequences of a fossil fuel-based energy sector and look to nuclear energy as an alternative energy source, reliable remediation strategies are necessary to ensure safe energy production for posterity. This research will further understanding of the biogeochemistry of uranium, which is necessary to accurately model the mobility of uranium in dynamic subsurface environments.

Supplemental Keywords:

bioremediation, metal-reducing bacteria, uranium bioreduction

Progress and Final Reports:

2015

Final