Anaerobic bioremediation of hexavalent uranium in groundwater by reductive precipitation with methanogenic granular sludge (original) (raw)

Optimization of a bioremediation system of soluble uranium based on the biostimulation of an indigenous bacterial community

High concentrations of uranium(VI) in the Witwatersrand Basin, South Africa from mining leachate is a serious environmental concern. Treatment systems are often ineffective. Therefore, optimization of a bioremediation system that facilitates the bioreduction of U(VI) based on biostimulation of indigenous bacterial communities can be a viable alternative. Tolerance of the indigenous bacteria to high concentrations of U and the amount of citric acid required for U removal was optimized. Two bioreactor studies which showed effective U(VI) removal more than 99 % from low (0.0037 mg L(-1)) and high (10 mg L(-1)) concentrations of U to below the limit allowed by South African National Standards for drinking water (0.0015 mg L(-1)). The second bioreactor was able to successfully adapt even with increasing levels of U(VI) feed water up to 10 mg L(-1), provided that enough electron donor was available. Molecular biology analyses identified Desulfovibrio sp. and Geobacter sp. among known species, which are known to reduce U(VI). The mineralogical analysis determined that part of the uranium precipitated intracellularly, which meant that the remaining U(VI) was precipitated as U(IV) oxides and TEM-EDS also confirmed this analysis. This was predicted with the geochemical model from the chemical data, which demonstrated that the treated drainage was supersaturated with respect to uraninite > U4O9 > U3O8 > UO2(am). Therefore, the tolerance of the indigenous bacterial community could be optimized to remediate up to 10 mg L(-1), and the system can thus be upscaled and employed for remediation of U(VI) impacted sites.

Donor-dependent Extent of Uranium Reduction for Bioremediation of Contaminated Sediment Microcosms

Journal of Environmental Quality, 2009

Bioremediation of uranium was investigated in microcosm experiments containing contaminated sediments from Oak Ridge, Tennessee to explore the importance of electron donor selection for uranium reduction rate and extent. In these experiments, all of the electron donors, including ethanol, glucose, methanol, and methanol with added humic acids, stimulated the reduction and immobilization of aqueous uranium by the indigenous microbial community. Uranium loss from solution began after the completion of nitrate reduction but essentially concurrent with sulfate reduction. When electron donor concentrations were normalized for their equivalent electron donor potential yield, the rates of uranium reduction were nearly equivalent for all treatments (0.55-0.95 μmol L −1 d −1 ). Uranium reduction with methanol proceeded after a 15-d longer lag time relative to that of ethanol or glucose. Signifi cant diff erences were not found with the inclusion of humic acids. Th e extent of U reduction in sediment slurries measured by XANES at various time periods after the start of the experiment increased in the order of ethanol (5-7% reduced at 77 and 153 d), glucose (49% reduced at 53 d), and methanol (93% reduced at 90 d). Th e microbial diversity of ethanol-and methanol-amended microcosms in their late stage of U reduction was analyzed with 16S rRNA gene amplifi cation. Members of the Geobacteraceae were found in all microcosms as well as other potential uranium-reducing organisms, such as Clostridium and Desulfosporosinus. Th e eff ectiveness of methanol relative to ethanol at reducing aqueous and sediment-hosted uranium suggests that bioremediation strategies that encourage fermentative poising of the subsurface to a lower redox potential may be more eff ective for long-term uranium immobilization as compared with selecting an electron donor that is effi ciently metabolized by known uranium-reducing microorganisms.

Aerobic granular biomass: a novel biomaterial for efficient uranium removal

Current Science, 2006

Aerobic microbial granules, self-immobilized microbial consortia cultured in aerobically operated bioreactors, primarily consist of mixed species of bacteria ensconced in an extracellular polymeric matrix of their own creation. Such aerobically grown microbial granules have attracted considerable research interest in environmental biotechnology. In recent times, it has been demonstrated that the granules could be used for efficient degradation of recalcitrant organic compounds and for the treatment of a growing number of wastes. The objective of this study was to investigate whether aerobic granules could be used as novel biomass material for biosorption of uranium from aqueous solutions. The granular biomass for biosorption experiments was cultivated in a laboratory-scale sequencing batch reactor by feeding with synthetic wastewater. Biosorption of uranium [U(VI)] was studied at different initial pH values (1 to 8) and different initial uranium concentrations (6 to 750 mg l–1). Biosorption was observed to be rapid (<1 h) in acidic pH range (1 to 6) compared to that at pH 7.0 or above. Almost complete removal of uranium was observed in the range 6–100 mg l–1 in less than 1 h. Redlich–Peterson model gave the best fit when the experimental data were analysed using different adsorption isotherm equations. The maximum biosorption capacity of U(VI) was determined to be 218 ± 2 mg g–1 dry granular biomass. Further, it wasobserved that cations such as Na+, K+, Mg+2, and Ca+2 were simultaneously released into the bulk solution during U(VI) biosorption, indicating the involvement of ion exchange mechanism in radionuclide uptake. Live and dead biomass did not show significant difference in U removal, indicating the involvement of a passive sorption process. The study suggests that aerobic granular biomass has the potential to be employed as an effective biosorbent material for recovering/removing uranium (and probably other radionuclides) from dilute nuclear wastes.

Effects of aqueous uranyl speciation on the kinetics of microbial uranium reduction

Geochimica et Cosmochimica Acta, 2015

The ability to predict the success of the microbial reduction of soluble U(VI) to highly insoluble U(IV) as an in situ bioremediation strategy is complicated by the wide range of geochemical conditions at contaminated sites and the strong influence of aqueous uranyl speciation on the bioavailability and toxicity of U(VI) to metal-reducing bacteria. To determine the effects of aqueous uranyl speciation on uranium bioreduction kinetics, incubations and viability assays with Shewanella putrefaciens strain 200 were conducted over a range of pH and dissolved inorganic carbon (DIC), Ca 2+ , and Mg 2+ concentrations. A speciation-dependent kinetic model was developed to reproduce the observed time series of total dissolved uranium concentration over the range of geochemical conditions tested. The kinetic model yielded the highest rate constant for the reduction of uranyl non-carbonate species (i.e., the 'free' hydrated uranyl ion, uranyl hydroxides, and other minor uranyl complexes), indicating that they represent the most readily reducible fraction of U(VI) despite being the least abundant uranyl species in solution. The presence of DIC, Ca 2+ , and Mg 2+ suppressed the formation of more bioavailable uranyl non-carbonate species and resulted in slower bioreduction rates. At high concentrations of bioavailable U(VI), however, uranium toxicity to S. putrefaciens inhibited bioreduction, and viability assays confirmed that the concentration of non-carbonate uranyl species best predicts the degree of toxicity. The effect of uranium toxicity was accounted for by incorporating the free ion activity model of metal toxicity into the bioreduction rate law. Overall, these results demonstrate that, in the absence of competing terminal electron acceptors, uranium bioreduction kinetics can be predicted over a wide range of geochemical conditions based on the bioavailability and toxicity imparted on U(VI) by solution composition. These findings also imply that the concentration of uranyl non-carbonate species, despite being extremely low, is a determining factor controlling uranium bioreduction at contaminated sites.

Removal of uranium(VI) under aerobic and anaerobic conditions using an indigenous mine consortium

Minerals Engineering, 2010

Biological uranium removal was investigated using bacteria sourced from a uranium mine in Limpopo, South Africa. Background uranium concentration in the soil from the mine was determined to be 168.1 mg/kg using the ICP-OES calibrated against the uranium atomic absorption standard solution. Thus the bacteria isolated from the site were expected to be resistant to uranium-6 [U(VI)] toxicity. Preliminary studies suggest that uranium reduction occurs under anaerobic conditions in most cases. U(VI) reduction by obligate aerobes isolated from the soil consortium was poor. The pure cultures mentioned above showed a high reduction rate at pH 5 to 6. The initial U(VI) reduction rate determined at the 50% point was highest in the Pseudomonas sp. at 30 mg/L. Enterobacter sp. outperformed the other two species at 200 mg/L and 400 mg/L. Rapid reduction was observed in all cultures during the first 4-6 hours of incubation with equilibrium conditions obtained only after incubation for 24 hours. However, Enterobacter sp. proved to be the most efficient U(VI) reducer among the cultures with the highest U(VI) reduction rate observed at high concentrations, 200 mg/L and 400 mg/L. A high percentage of uranium recovery occurred at these concentrations. The results demonstrate the potential of microbial U(VI) reduction as an alternative technology to currently used physical/chemical processes for treatment and recovery of uranium in the nuclear industry.

A procedure for quantitation of total oxidized uranium for bioremediation studies

Journal of Microbiological Methods, 2003

A procedure was developed for the quantitation of complexed U(VI) during studies on U(VI) bioremediation. These studies typically involve conversion of soluble or complexed U(VI) (oxidized) to U(IV) (the reduced form which is much less soluble). Since U(VI) freely exchanges between material adsorbed to the solid phase and the dissolved phase, uranium bioremediation experiments require a mass balance of U in both its soluble and adsorbed forms as well as in the reduced sediment bound phase. We set out to optimize a procedure for extraction and quantitation of sediment bound U(VI). Various extractant volumes to sediment ratios were tested and it was found that between 1:1 to 8:1 ratios (v/w) there was a steady increase in U(VI) recovered, but no change with further increases in v/w ratio.

Modeling in-situ uranium (VI) bioreduction by sulfate-reducing bacteria

Journal of Contaminant Hydrology, 2007

We present a travel-time based reactive transport model to simulate an in-situ bioremediation experiment for demonstrating enhanced bioreduction of uranium(VI). The model considers aquatic equilibrium chemistry of uranium and other groundwater constituents, uranium sorption and precipitation, and the microbial reduction of nitrate, sulfate and U(VI). Kinetic sorption/desorption of U(VI) is characterized by mass transfer between stagnant micro-pores and mobile flow zones. The model describes the succession of terminal electron accepting processes and the growth and decay of sulfate-reducing bacteria, concurrent with the enzymatic reduction of aqueous U(VI) species. The effective U(VI) reduction rate and sorption site distributions are determined by fitting the model simulation to an in-situ experiment at Oak Ridge, TN. Results show that (1) the presence of nitrate inhibits U(VI) reduction at the site; (2) the fitted effective rate of in-situ U(VI) reduction is much smaller than the values reported for laboratory experiments; (3) U(VI) sorption/desorption, which affects U(VI) bioavailability at the site, is strongly controlled by kinetics; (4) both pH and bicarbonate concentration significantly influence the sorption/desorption of U(VI), which therefore cannot be characterized by empirical isotherms; and (5) calciumuranyl-carbonate complexes significantly influence the model performance of U(VI) reduction.