Ray and six co-authors reported in 2011 that microbial reduction of hexavalent uranium has been studied widely for its potential role in bioremediation and immobilization of soluble U(VI) in contaminated groundwater.
Figures a, b and c - Typical TEM cross-sections of cells
treated with 100 lM U(VI) in the absence of AQDS showing
intracellular uranium deposition along the plasma membrane
and cytoplasm; in (b), x denotes structures characteristic of
U(IV) deposition, and y denotes needlelike structures
characteristic of U(VI) phosphate precipitates; (d) shows
the cross-section of a cell containing sheets of needle-like
material; (e) shows magnification of the inset shown in (d);
(f) EDS spectrum of the precipitates circled in (e). Copper
peaks at 8–9 keV are characteristic of the grid. The spectrum
shows clear U, P, and O signals.
More recently, some microorganisms have been examined for their role in immobilization of U(VI) via precipitation of uranyl phosphate minerals mediated by microbial phosphate release, alleviating the requirement for long-term redox control. Here, we investigated the mechanism of U(VI) removal mediated by an environmental isolate, strain UFO1, that is indigenous to the Field Research Center (FRC) in Oak Ridge, TN and has been detected in U(VI)-contaminated sediments.
Changes in U(VI) speciation were examined in the presence and absence of the electron-shuttling moiety, anthraquinone-2,6-disulfonate (AQDS). Cell suspensions were capable of nearly complete removal of 100 lM U(VI) from solution within 48 h; U(VI) removal was not dependent on the presence of an exogenous electron donor or AQDS, although AQDS increased the rate of U(VI) removal. X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic measurements indicated that U(IV) was the predominant oxidation state of uranium in cell suspensions in both the absence and presence of 100 lM AQDS.
Interestingly, 17% of the cell-associated precipitates in a U(VI)-treated suspension that lacked AQDS had spectral characteristics consistent with a uranyl phosphate solid phase. The potential involvement of phosphate was consistent with observed increases in soluble phosphate concentrations over time in
UFO1 cell suspensions, which suggested phosphate liberation from the cells. TEM-EDS confirmed the presence of uranyl phosphate with a U:P ratio consistent with autunite (1:1).
EXAFS analyses further suggested that U(IV) was bound to low-Z neighbors such as C or P, inferred to be present as functional groups on biomass. These results suggest that strain UFO1 has the ability to facilitate U(VI) removal from solution via reductive and phosphate precipitation mechanisms. Both mechanisms offer potential for the remediation of U-contaminated sediments at the FRC or elsewhere.
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