U(VI) Sequestration in Hydroxyapatite Produced by Microbial Glycerol 3-Phosphate Metabolism

Abstract

Previous studies have demonstrated the potential for removal of U(VI) from solution via precipitation of U(VI)-bearing calcium-phosphate (Ca-P) minerals coupled to microbial hydrolysis of glycerol phosphate compounds. We evaluated this process in circumneutral-pH groundwater from Area 2 of the U.S. Department of Energy Field Research Center at Oak Ridge National Laboratory. Area 2 groundwater contains high concentrations of dissolved calcium (ca. 4 mM), and thus, release of phosphate during glycerol phosphate metabolism has the potential to create conditions favorable for U(VI) sequestration in Ca-P minerals. Microbial enumeration and isolation studies verified the presence of aerobic and nitrate-reducing glycerol 3-phosphate (G3P)-metabolizing microorganisms in Area 2 sediments. Coprecipitation of U(VI) with Ca-P minerals coupled to microbial G3P hydrolysis was demonstrated in artificial groundwater under aerobic and nitratereducing conditions. Transmission electron microscopy analysis and mineral-washing experiments demonstrated that U(VI) was incorporated into the structure of the insoluble Ca-P mineral hydroxyapatite [Ca 5 (PO 4 ) 3 OH]. Our results support the idea that U(VI) can be effectively removed from solution in contaminated aquifers through stimulation of microbial organophosphate metabolism.Reductive precipitation of U(VI) through stimulation of anaerobic, dissimilatory metal-reducing microbial activity is a demonstrated method for in situ immobilization of uranium in subsurface environments (3,41). This is an attractive approach for aquifers that are naturally low in dissolved O 2 and NO 3 Ϫ , e.g., the uranium-contaminated aquifer adjacent to the Colorado River in Rifle, CO (3). Alternative uranium remediation approaches may be called for in uranium-contaminated subsurface environments that contain significant concentrations of O 2 and NO 3 Ϫ entering through vertical recharge or lateral transport. One such potential strategy is immobilization of uranium through precipitation of U(VI)-bearing calcium-phosphate (Ca-P) phases (6, 39). Uranium incorporation into Ca-P minerals is a naturally occurring process. For example, U(VI)-phosphate phases at a weathered portion of the Coles Hill, VA, uranium ore deposit maintain groundwater U(VI) concentrations at ca. 0.06 M (11, 12). The shallow portion of this deposit exhibits a sharp Fe redox front with U(IV) assemblages (containing uraninite, UO 2 ) located on the reduced side and U(VI) assemblages [containing autunite, Ca(UO 2 ) 2 (PO 4 ) 3 , and other uranium-bearing Ca-P phases] on the weathered oxidized side. Interestingly, uranium concentrations on both oxidized and reduced sides of the redox front are similar, suggesting no significant uranium loss as a result of oxidation and weathering.A major limitation for engineering uranium immobilization through coprecipitation with Ca-P minerals is the potential decrease in hydraulic conductivity of sediments due to rapid mineral precipitation at or near the point of P i injection. In order to avoid this, P i...

Evidence for multiple modes of uranium immobilization by an anaerobic bacterium

“…However, a major challenge for phosphate-based remediation approaches is the distribution of phosphate in the subsurface; its extremely high reactivity can lead to clogging of the injection well and consequently a limited treatment radius (Wellman et al, 2006). Other researchers have proposed the use of precursor compounds such as glycerol-3-phosphate (Beazley et al, 2007;Shelobolina et al, 2009) or polyphosphate (Wellman et al, 2006) that can be transformed in situ to release phosphate in the subsurface. UFO1 has shown the ability to mediate removal of U(VI) from solution via both bioreduction and phosphate mineral precipitation.…”

The effect of pH and natural microbial phosphatase activity on the speciation of uranium in subsurface soils

“…Unfortunately, phosphate concentrations are too low in groundwater, and direct phosphate addition to a contaminated aquifer has proven less than effective, as phosphate precipitation near the injection wells prevented distribution/transport of phosphorus source across the subsurface (Wellman et al, 2006). The biomineralization of U(VI) as a result of microbial phosphatase activity is an alternative biologically-induced process by both naturally-occurring and genetically-engineered metal-resistant bacteria that can reduce the solubility of U(VI) by forming uranium phosphate precipitates (Macaskie et al, 1992;Powers et al, 2002;Appukuttan et al, 2006;Martinez et al, 2007;Shelobolina et al, 2009) over a wide pH range . In this process, bacterial phosphatase enzymes hydrolyze phosphate from organophosphate substrates (Macaskie et al, 1994) either to acquire inorganic phosphorus as essential nutrient or to precipitate U(VI) as a detoxification mechanism (Macaskie et al, 1992).…”

Phosphate-Mediated Remediation of Metals and Radionuclides

“…To further highlight such adaptation, recent investigations of terrestrial and marine bacterial isolates belonging to the genera Aeromonas, Bacillus, Myxococcus, Pantoea, Pseudomonas, Rahnella, and Vibrio demonstrated that Cr, Pb, and U were removed from solution as phosphate minerals under both oxic and anoxic growth conditions [57,60,74,152,[154][155][156][157][158]. Our recent work further examined lead and 6 Advances in Ecology uranium precipitates produced by Rahnella sp.…”

“…Studies examining phosphatase activities (e.g., alkaline or acid) of genetically engineered and naturally occurring strains of Gram-positive and Gram-negative bacteria were shown to promote U immobilization (>90% precipitation of soluble U) via precipitation and coprecipitation reactions [50,57,60,74,[149][150][151][152][153]. Interestingly, soil bacterial isolates demonstrated constitutive phosphatase activities that liberated comparable, if not greater concentrations, of reactive phosphate when compared to the phosphatase activities of genetically modified strains [57,149].…”

“…With recent studies identifying autuniteand hydroxyapatite-precipitating capabilities of Aeromonas, Bacillus, Pantoea, Pseudomonas, and Rahnella spp. in both oxic and anoxic growth conditions, the synergistic properties of these minerals (i.e., ion-exchange reactions that sequester cooccurring metals) highlight an important role in not only stabilizing U contamination but also cooccurring metals [57,60,74,157]. Furthermore, contaminated sites that are characterized by acidic to circumneutral porewater pH represent environments that can support stable mineral formation (Figures 2(a) and 2(b)), provided that carbonates are not present in significant concentrations (i.e.,P CO 2 < 10 −3.5 atm) [176,177].…”