State Water Resources Research Institute Program

Project ID: 2007SD104B
Title: Permeable Reactive Bio-Barriers for Uranium Removal: Role of Iron Minerals on Uranium Fate and Transport
Project Type: Research
Start Date: 3/01/2007
End Date: 2/28/2008
Congressional District: First
Focus Categories: Radioactive Substances, Wastewater, Treatment
Keywords: Groundwater, Iron sulfides, Uranium, and Stability
Principal Investigators: Sani, Rajesh Kumar; Bang, Sookie S; Dixon, David J.
Federal Funds: $ 14,855
Non-Federal Matching Funds: $ 29,735
Abstract: In the United States, past activities associated with mining, extraction, and processing of uranium (U) for nuclear fuel and weapons have generated substantial quantities of toxic waste materials contaminated with U. U is a known carcinogen (Miller et al., 2002), and the high solubility of its hexavalent form can result in U transport to sensitive receptors such as drinking water sources. One potential method of treating U contamination is by using natural dissimilatory metal reducing bacteria (DMRB) to reduce soluble U(VI) to insoluble U(IV) (as uraninite, UO2), which can lead to in situ U immobilization and plume stabilization (Gorby and Lovley, 1992; Lovley et al., 1991). Sulfate reducing bacteria (SRB) are one group of such DMRB that are present in many contaminated subsurface sites (Chang et al., 2001; Suzuki et al., 2003). Stimulating SRB for U remediation can be especially advantageous since SRB can reduce U(VI) by direct enzymatic mechanisms as well as indirectly by sulfide production. In addition, in the presence of Fe-bearing minerals, biogenic sulfide can react to form iron sulfides, which can help maintain reducing environments important to uraninite stability.

Before SRB can be applied for bioremediation, however, it is important to understand the fate and transport of uraninite, especially in light of a recent published report, which showed that nanometer-sized particles of uraninite (size 3 - 5 nm) were produced when native SRB were stimulated in batch reactors containing natural U(VI)-contaminated sediments from Midnite Mine, WA (Suzuki et al., 2002). Results from our laboratory confirm the formation of 3 - 5 nm sized uraninite particles during U(VI) reduction by the SRB, Desulfovibrio desulfuricans G20, indicating that this phenomenon might be generic among U(VI)-reducing bacteria (Sani et al. 2006). Furthermore, our data show that most of the reduced uranium can pass though 0.2 µm filters (Sani et al. 2006), indicating that perhaps most of the microbially produced uraninite is sub-micron sized. This new discovery raises very important questions about the stability of biologically reduced U, since these nanoparticles are likely transportable in groundwater and may also be easily re-oxidized due to their large surface area to mass ratio. Thus, for successful application of U bio-stabilization, it is vital to understand the factors that impact the mobility and reactive transport of bioreduced U (uraninite). The proposed research is aimed at elucidating the transport and chemical stability of uraninite formed when U flows through a biologically reactive soil matrix containing U(VI)-reducing SRB. Specifically, we propose to construct permeable reactive bio-barriers with the U(VI)-reducing SRB, Desulfovibrio Desulfuricans G20 in bench-scale soil columns and quantify the removal of U across the barrier. In addition, we will also quantify the stability of uraninite immobilized in the bio-barrier under oxidizing conditions. The proposed experiments will provide data to enable evaluation of factors that may influence the long-term stability and bioremediation potential of sulfate-reducing bio-barriers under geochemical conditions that may be expected in natural environments. In addition, the data obtained can be used to develop mathematical models for predicting stability of bio-reduced U as a function of space and time, based on a mechanistic understanding of the complex interactions between iron minerals, microbes, and U.

Progress/Completion Report, PDF

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