Browsing by Author "Stewart, B. D."

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Detection of biological uranium reduction using magnetic resonance
(2012-04) Vogt, Sarah J.; Stewart, B. D.; Seymour, Joseph D.; Peyton, Brent M.; Codd, Sarah L.
The conversion of soluble uranyl ions (UO22+) by bacterial reduction to sparingly soluble uraninite (UO2(s)) is being studied as a way of immobilizing subsurface uranium contamination. Under anaerobic conditions, several known types of bacteria including iron and sulfate reducing bacteria have been shown to reduce U (VI) to U (IV). Experiments using a suspension of uraninite (UO2(s)) particles produced by Shewanella putrefaciens CN32 bacteria show a dependence of both longitudinal (T1) and transverse (T2) magnetic resonance (MR) relaxation times on the oxidation state and solubility of the uranium. Gradient echo and spin echo MR images were compared to quantify the effect caused by the magnetic field fluctuations (T*2 ) of the uraninite particles and soluble uranyl ions. Since the precipitate studied was suspended in liquid water, the effects of concentration and particle aggregation were explored. A suspension of uraninite particles was injected into a polysaccharide gel, which simulates the precipitation environment of uraninite in the extracellular biofilm matrix. A reduction in the T2 of the gel surrounding the particles was observed. Tests done in situ using three bioreactors under different mixing conditions, continuously stirred, intermittently stirred, and not stirred, showed a quantifiable T2 magnetic relaxation effect over the extent of the reaction.
Influence of chelating agents on biogenic uraninite reoxidation by Fe(III) (Hydr)oxides
(2013-01) Stewart, B. D.; Giradot, Crystal L.; Spycher, Nicolas; Sani, Rajesh K.; Peyton, Brent M.
Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO2(S), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO2 may be susceptible to reoxidation by environmental factors, with Fe(III) (hydr)oxides playing a significant role. Little is known about the role that organic compounds such as Fe(III) chelators play in the stability of reduced U. Here, we investigate the impact of citrate, DFB, EDTA, and NTA on biogenic UO2 reoxidation with ferrihydrite, goethite, and hematite. Experiments were conducted in anaerobic batch systems in PIPES buffer (10 mM, pH 7) with bicarbonate for approximately 80 days. Results showed EDTA accelerated UO2 reoxidation the most at an initial rate of 9.5 Î¼M dayâˆ’1 with ferrihydrite, 8.6 Î¼M dayâˆ’1 with goethite, and 8.8 Î¼M dayâˆ’1 with hematite. NTA accelerated UO2 reoxidation with ferrihydrite at a rate of 4.8 Î¼M dayâˆ’1; rates were less with goethite and hematite (0.66 and 0.71 Î¼M dayâˆ’1, respectively). Citrate increased UO2 reoxidation with ferrihydrite at a rate of 1.8 Î¼M dayâˆ’1, but did not increase the extent of reaction with goethite or hematite, with no reoxidation in this case. In all cases, bicarbonate increased the rate and extent of UO2 reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO2 reoxidation occurred when the chelator promoted both 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 with lower reoxidation rates observed. Reaction modeling suggests that strong Fe(II) chelators promote reoxidation whereas strong Fe(III) chelators impede it. These results indicate that chelators found in U contaminated sites may play a significant role in mobilizing U, potentially affecting bioremediation efforts.
Reactivity of uranium and ferrous iron with natural iron oxyhydroxides
(2015-07) Stewart, B. D.; Cismasu, A. C.; Williams, Kenneth H.; Peyton, Brent M.; Nico, P. S.
Determining key reaction pathways involving uranium and iron oxyhydroxides under oxic and anoxic conditions is essential for understanding uranium mobility as well as other iron oxyhydroxide mediated processes, particularly near redox boundaries where redox conditions change rapidly in time and space. Here we examine the reactivity of a ferrihydrite-rich sediment from a surface seep adjacent to a redox boundary at the Rifle, Colorado field site. Iron(II)â€“sediment incubation experiments indicate that the natural ferrihydrite fraction of the sediment is not susceptible to reductive transformation under conditions that trigger significant mineralogical transformations of synthetic ferrihydrite. No measurable Fe(II)-promoted transformation was observed when the Rifle sediment was exposed to 30 mM Fe(II) for up to 2 weeks. Incubation of the Rifle sediment with 3 mM Fe(II) and 0.2 mM U(VI) for 15 days shows no measurable incorporation of U(VI) into the mineral structure or reduction of U(VI) to U(IV). Results indicate a significantly decreased reactivity of naturally occurring Fe oxyhydroxides as compared to synthetic minerals, likely due to the association of impurities (e.g., Si, organic matter), with implications for the mobility and bioavailability of uranium and other associated species in field environments.