Non-uraninite Products of Microbial U(VI) Reduction

Abstract

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO4)2], U2O(PO4)2, and U2(PO4)(P3O10) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.

Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation

“…With only FeS (Figure 3), a peak concentration for dissolved Fe(II) of ∼60 μM was rapidly established after ∼5 τ, which then continuously decreased until complete FeS oxidation. This background Fe(II) profile agreed with previous studies at pH 7,23 showing a loss of ∼18% of total Fe(II) to the effluent. In the presence of noncrystalline U(IV), however, dissolved Fe(II) remained at a high concentration (∼60 μM) for an additional ∼30 τ after the peak at ∼6 τ in 10.8 mM DIC solution.…”

“…The final product was freeze-dried under a vacuum and stored in capped glass vials inside the anaerobic chamber until use. The resulting mackinawite was characterized as nanocrystalline particles with mineralogical description as detailed in Jeong et al 28 Biomass-associated noncrystalline U(IV) was produced as previously described in Bernier-Latmani et al 7 Briefly, Shewanella oneidensis MR-1 cultures were grown anaerobically in sterile Luria−Bertani (LB) medium and harvested when they reached mid-exponential phase. Cells were harvested by centrifugation at 8000 g for 10 min and washed in simple BP medium, composed of 30 mM NaHCO 3 and 20 mM 1,4-piperazinediethanesulfonic acid (PIPES buffer) adjusted to pH 6.8.…”

Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism

“…The P coordination and the generalized periplasmic mineralization observed in the PilA − mutant cells suggest that U(VI) permeated deep and fast into the cell envelope, where it formed carboxyl and phosphoryl-coordinated complexes with periplasmic proteins and the peptidoglycan layer (30,31) and membrane phospholipids (33), respectively. The formation of a mononuclear U(IV) phase has also been reported for other bacteria of relevance to U bioremediation (34,35), yet contrasts with earlier reports of uraninite formation by Geobacter spp. (10,36).…”

“…We used a bicarbonate buffer and conditions used in previous studies with G. sulfurreducens (13), whereas studies reporting uraninite formation used PIPES-buffered solutions (36) or bicarbonatebuffered uncontaminated groundwater (10). Evidence for the microbial reduction of U(VI) to nonuraninite U(IV) products is also emerging from field-scale studies (35,37), whereas uraninite formation has been linked to conditions of reduced bioreducing activities (38,39); this suggests that abiotic factors may contribute to the formation of uraninite.…”

Oxidative Dissolution of Biogenic Uraninite in Groundwater at Old Rifle, CO

“…In addition, this material contained a small fraction of monomeric U(IV) complexes associated with the biomass. 13 A uraninite slurry (either with biomass (P103-BIOMASS-slurry, B02-BIOMASS-slurry) or without biomass (P103-CLN-slurry, B02-CLN-slurry)) was placed directly into a sample tube for deployment, used to create a UO 2 -doped gel puck for mass balance determination (described below), or archived for characterization and dissolution rate measurements. The biomass associated with the uraninite was not expected to grow under deployment conditions due to the absence of electron donor.…”