Brandy D. Stewart

Impact of Uranyl−Calcium−Carbonato Complexes on Uranium(VI) Adsorption to Synthetic and Natural Sediments

Adsorption on soil and sediment solids may decrease aqueous uranium concentrations and limit its propensity for migration in natural and contaminated settings. Uranium adsorption will be controlled in large part by its aqueous speciation, with a particular dependence on the presence of dissolved calcium and carbonate. Here we quantify the impact of uranyl speciation on adsorption to both goethite and sediments from the Hanford Clastic Dike and Oak Ridge Melton Branch Ridgetop formations. Hanford sediments were preconditioned with sodium acetate and acetic acid to remove carbonate grains, and Ca and carbonate were reintroduced at defined levels to provide a range of aqueous uranyl species. U(VI) adsorption is directly linked to UO(2)(2+) speciation, with the extent of retention decreasing with formation of ternary uranyl-calcium-carbonato species. Adsorption isotherms under the conditions studied are linear, and K(d) values decrease from 48 to 17 L kg(-1) for goethite, from 64 to 29 L kg (-1) for Hanford sediments, and from 95 to 51 L kg(-1) for Melton Branch sediments as the Ca concentration increases from 0 to 1 mM at pH 7. Our observations reveal that, in carbonate-bearing waters, neutral to slightly acidic pH values ( approximately 5) and limited dissolved calcium are optimal for uranium adsorption.

Kinetic and mechanistic constraints on the oxidation of biogenic uraninite by ferrihydrite.

The oxidation state of uranium plays a major role in determining uranium mobility in the environment. Under anaerobic conditions, common metal respiring bacteria enzymatically reduce soluble U(VI) to U(IV), resulting in the formation of sparingly soluble UO(2(bio)) (biogenic uraninite). The stability of biologically precipitated uraninite is critical for determining the long-term fate of uranium and is not well characterized within soils and sediments. Here, we demonstrate that biogenic uraninite oxidation by ferrihydrite, an environmentally ubiquitous, disordered Fe(III) (hydr)oxide, appears to proceed through a soluble U(IV) intermediate and results in the concomitant production of Fe(II) and dissolved U(VI). Uraninite oxidation rates are accelerated under conditions that increase its solubility and decrease uraninite surface passivation, which include high bicarbonate concentration and pH values deviating from neutrality. Thus, our results demonstrate that UO(2(bio)) oxidation by Fe(III) (hydr)oxides is controlled by the rate of uraninite dissolution and that this process may limit uranium(IV) sequestration in the presence of Fe(III) (hydr)oxides.

Influence of uranyl speciation and iron oxides on uranium biogeochemical redox reactions

Uranium is a pollutant of concern to both human and ecosystem health. Uraniums redox state often dictates its partitioning between the aqueous- and solid-phases, and thus controls its dissolved concentration and, coupled with groundwater flow, its migration within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO2 2+ and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO2. However, various factors within soils and sediments may limit biological reduction of U(VI), inclusive of alterations in U(VI) speciation and competitive electron acceptors. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction with specific emphasis on speciation changes induced by dissolved Ca, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite and hematite) varying in free energies of formation on U reduction. The amount of uranium removed from solution during 100 h of incubation with S. putrefaciens was 77% with no Ca or ferrihydrite present but only 24% (with ferrihydrite) and 14% (no ferrihydrite) were removed for systems with 0.8 mM Ca. Imparting an important criterion on uranium reduction, goethite and hematite decrease the dissolved concentration of calcium through adsorption and thus tend to diminish the effect of calcium on uranium reduction. Dissimilatory reduction of Fe(III) and U(VI) can proceed through different enzyme pathways, even within a single organism, thus providing a potential second means by which Fe(III) bearing minerals may impact U(VI) reduction. We quantify rate coefficients for simultaneous dissimilatory reduction of Fe(III) and U(VI) in systems varying in Ca concentration (0 to 0.8 mM), and using a mathematical construct implemented with the reactive transport code MIN3P, we reveal the predominant influence of uranyl speciation, specifically the formation of uranyl-calcium-carbonato complexes, and ferrihydrite on the rate and extent of uranium reduction in complex geochemical systems.

Reoxidation of Biogenic Reduced Uranium: A Challenge Toward Bioremediation

Uraninite (UO2) is the most desirable end product of in situ bioreduction because of its low solubility under reducing conditions. For effective long-term immobilization of uranium (U), there should be no biotic or abiotic reoxidation of the insoluble biogenic U(IV). It is therefore critical to understand the long-term stability of U(IV) under oxic- and nutrient-limited conditions at U-contaminated subsurface sites. It has now been established that following in situ bioremediation of U(VI) via nutrient addition in the subsurface, a range of physical, chemical, and biological factors control the rate and extent of long-term stability of U(IV). Some of these factors are tied to site specific conditions including existence of oxidants such as Fe(III)(hydr)oxides, Mn(IV) oxides, oxygen, and nitrate; the presence of organic carbon and the reduced forms of U (e.g., mononuclear U(IV) or nanometer-sized uraninite particles); and the carbonate concentration and pH of groundwater. This review analyzes the contribution of these factors in controlling U(IV)-reoxidation, and highlights the competition among U(IV) and other electron acceptors and possible mechanisms of reoxidation of various forms of U(IV).

Influence of chelating agents on biogenic uraninite reoxidation by Fe(III) (Hydr)oxides.

Microbially mediated reduction of soluble U(VI) to U(IV) with subsequent precipitation of uraninite, UO(2(S)), has been proposed as a method for limiting uranium (U) migration. However, microbially reduced UO(2) 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 UO(2) 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 UO(2) reoxidation the most at an initial rate of 9.5 μM day(-1) with ferrihydrite, 8.6 μM day(-1) with goethite, and 8.8 μM day(-1) with hematite. NTA accelerated UO(2) reoxidation with ferrihydrite at a rate of 4.8 μM day(-1); rates were less with goethite and hematite (0.66 and 0.71 μM day(-1), respectively). Citrate increased UO(2) reoxidation with ferrihydrite at a rate of 1.8 μM day(-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 UO(2) reoxidation with ferrihydrite in the presence and absence of chelators. The highest rate of UO(2) reoxidation occurred when the chelator promoted both UO(2) and Fe(III) (hydr)oxide dissolution as demonstrated with EDTA. When UO(2) dissolution did not occur, UO(2) 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.

Detection of biological uranium reduction using magnetic resonance.

The conversion of soluble uranyl ions (UO  22+ ) 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 (

Reactivity of Uranium and Ferrous Iron with Natural Iron Oxyhydroxides.

T_{2}^{*}

Incorporation of oxidized uranium into Fe (Hydr)oxides during Fe(II) catalyzed remineralization.

) 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. Biotechnol. Bioeng. 2012; 109:877–883.

Stability of uranium incorporated into Fe(hydr)oxides under fluctuating redox conditions

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)-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.