Multiple influences of nitrate on uranium solubility during bioremediation of uranium‐contaminated subsurface sediments

Abstract

Microbiological reduction of soluble U(VI) to insoluble U(IV) has been proposed as a remediation strategy for uranium-contaminated groundwater. Nitrate is a common co-contaminant with uranium. Nitrate inhibited U(VI) reduction in acetate-amended aquifer sediments collected from a uranium-contaminated site in New Mexico. Once nitrate was depleted, both U(VI) and Fe(III) were reduced concurrently. When nitrate was added to sediments in which U(VI) had been reduced, U(VI) reappeared in solution. Parallel studies with the dissimilatory Fe(III)-, U(VI)- and nitrate-reducing microorganism, Geobacter metallireducens, demonstrated that nitrate inhibited reduction of Fe(III) and U(VI) in cell suspensions of cells that had been grown with nitrate as the electron acceptor, but not in Fe(III)-grown cells. Suspensions of nitrate-grown G. metallireducens oxidized Fe(II) and U(IV) with nitrate as the electron acceptor. U(IV) oxidation was accelerated when Fe(II) was also added, presumably due to the Fe(III) being formed abiotically oxidizing U(IV). These studies demonstrate that although the presence of nitrate is not likely to be an impediment to the bioremediation of uranium contamination with microbial U(VI) reduction, it is necessary to reduce nitrate before U(VI) can be reduced. These results also suggest that anaerobic oxidation of U(IV) to U(VI) with nitrate serving as the electron acceptor may provide a novel strategy for solubilizing and extracting microbial U(IV) precipitates from the subsurface.

Effect of Mn(II) on the Structure and Reactivity of Biogenic Uraninite

“…Both factors could accelerate oxidative dissolution rather than retard it, making the observed stability of Mnreacted uraninite all the more remarkable. The conclusion that Mn-reacted uraninite is less soluble and more resistant to dissolution than unreacted uraninite is consistent with the reported increase in stability conferred by impurities to geological uraninites [8,10,11]. A recent study of the effect of Ca 2+ on biogenic uraninite has shown that contrary to Mn 2+ , this cation does not affect the kinetics of uraninite oxidation (25).…”

“…A significant issue associated with uranium bioremediation is the susceptibility of biogenic uraninite, the product of microbial U(VI) reduction (2), to chemical oxidation by O 2 (3)(4)(5), nitrite (6), nitrate (4), and Fe(III) (hydr)oxides (7) and to biological oxidation coupled with nitrate reduction (8).…”

Impact of elevated nitrate on sulfate-reducing bacteria: a comparative Study of Desulfovibrio vulgaris

“…Nitrate inhibition of metal-reducing microbial populations, such as the SRB, hinders bioremediation efforts exploiting these microbial biocatalysts (Abdelouas et al, 1998;Finneran et al, 2002;Istok et al, 2004;Nyman et al, 2006). However, the persistence of sulfate-reducing bacteria at contaminated sites with high nitrate levels suggested the presence of potential resistance mechanisms (Gu et al, 2005;Bagwell et al, 2006;Fields et al, 2006), which were explored in this study using physiological and genomics approaches.…”

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

“…Evidence from both field and laboratory studies also suggest a nexus between iron redox cycling and uranium redox processes (Galloway, 1978;Posey-Dowty et al, 1987). The biostimulation of DMRB will likely lead to biological Fe(III) reduction (Wielinga et al, 2000;Finneran et al, 2002;Anderson et al, 2003;Elias et al, 2004;) and production of sorbed Fe(II) or Fe(II)-bearing minerals as metabolic products. The Fe(II)-bearing phases found include magnetite, siderite, vivianite, ferruginous smectite, and green rust (Bell et al, 1987;Roden and Zachara, 1996;Fredrickson et al, 1998;Zachara et al, 1998;Dong et al, 2000;Roh et al, 2003;O'Loughlin et al, 2007;Komlos et al, 2008;O'Loughlin et al, 2010).…”