Interactions of uranium with bacteria and kaolinite clay

doi:10.1016/j.chemgeo.2005.03.008

Chemical Geology

Volume 220, Issues 3–4, 5 August 2005, Pages 237-243

https://doi.org/10.1016/j.chemgeo.2005.03.008 Get rights and content

Abstract

We assessed the accumulation of uranium (VI) by a bacterium, Bacillus subtilis, suspended in a slurry of kaolinite clay, to elucidate the role of microbes on the mobility of U(VI). Various mixtures of bacteria and the koalinite were exposed to solutions of 8 × 10^− 6 M- and 4 × 10^− 4 M-U(VI) in 0.01 M NaCl at pH 4.7. After 48 h, the mixtures were separated from the solutions by centrifugation, and treated with a 1 M CH₃COOK for 24 h to determine the associations of U within the mixture. The mixture exposed to 4 × 10^− 4 M U was analyzed by transmission electron microscope (TEM) equipped with EDS. The accumulation of U by the mixture increased with an increase in the amount of B. subtilis cells present at both U concentrations. Treatment of kaolinite with CH₃COOK, removed approximately 80% of the associated uranium. However, in the presence of B. subtilis the amount of U removed was much less. TEM–EDS analysis confirmed that most of the U removed from solution was associated with B. subtilis. XANES analysis of the oxidation state of uranium associated with B. subtilis, kaolinite, and with the mixture containing both revealed that it was present as U(VI). These results suggest that the bacteria have a higher affinity for U than the kaolinite clay mineral under the experimental conditions tested, and that they can immobilize significant amount of uranium.

Introduction

The migration of uranium from uranium-mining operations and the disposal of radioactive wastes are major environmental concerns (Buck et al., 1996, Airey and Ivanovich, 1986). Uranium typically occurs as hexavalent uranyl aqueous complexes in oxic environments (Langmuir, 1978). The mobility of U(VI) is determined by its interactions with soils and subsoils composed of abiotic and biotic components, principally minerals and bacteria, respectively (Dent et al., 1992, Ticknor, 1994, Waite et al., 1994, Sylwester et al., 2000, Fowle et al., 2000, Haas et al., 2001, Francis et al., 2004). There have been extensive studies done on the accumulation of U(VI) by bacteria (Lovley et al., 1991, Fowle et al., 2000, Haas et al., 2001, Brantley et al., 2001, Francis et al., 2004) and by minerals (Dent et al., 1992, Ticknor, 1994, Waite et al., 1994, Sylwester et al., 2000). However, as far as we are aware, little is known about U sorption in a mixture of bacteria and minerals.

Studies of U(VI) interactions with bacteria showed that U(VI) may be associated with the functional groups on the cellular surface (Fowle et al., 2000, Haas et al., 2001, Francis et al., 2004), precipitated to form uranyl-containing minerals (Macaskie et al., 1992, Young and Macaskie, 1995, Jeong et al., 1997), or reduced to insoluble U(IV) (Lovley et al., 1991, Francis et al., 1994, Suzuki et al., 2002). Studies with alumino-silicates minerals revealed that U(VI) is absorbed by the reactive groups of the minerals at pHs between 3 and 5. Therefore, an understanding of the behavior of U(VI) in soils and rocks requires a detailed knowledge of its interactions not only with the bacterial and mineral surfaces, but also within a mixture of bacteria and minerals.

In this study we investigated the accumulation of U by mixtures of Bacillus subtilis and kaolinite. B. subtilis and kaolinite were chosen because (a) both are ubiquitous in the terrestrial environment; and, (b) their surfaces are well characterized. We also determined whether U showed preferential affinity to bacteria, kaolinite clay or in a mixture of both. In addition to carrying out sorption and desorption experiments, we examined the association of U by transmission electron microscopy (TEM).

Section snippets

Microorganism, kaolinite, and U solution

B. subtilis (IAM 1069) was obtained from the Institute of Molecular and Cellular Biosciences, The University of Tokyo. This strain is a Gram-positive, rod-shaped heterotrophic bacterium. The cells were grown for 40–48 h in 500-mL conical flasks at 30 °C in 250 mL sterilized liquid growth medium containing meat extract (3 g L^− 1), polypeptone (5 g L^− 1), and NaCl (5 g L^− 1). Cells at the stationary growth phase were harvested by centrifugation at 2500×g for 10 min, and washed twice by 0.1 M NaCl.

Adsorption and desorption of U by the mixtures of B. subtilis and kaolinite

Fig. 1 shows the accumulated fraction of U by a mixture of B. subtilis and kaolinite as a function of dry weight percent fraction of B. subtilis. Kaolinite adsorbed 45% of the uranium from a U solution of 8 × 10^− 6 M; the amount rose to 95% with an increase in the fraction of B. subtilis up to 5%. In a solution containing of 4 × 10^− 4 M uranium, approximately 10% of U was removed by kaolinite, and about 40% by B. subtilis. Removal of uranium increased to approximately 30% with increasing the fraction

Discussion

TEM and EDS analyses of B. subtilis and kaolinite showed no evidence of precipitation of uranium on the surface. However, rod-shaped uranyl-phosphate minerals (Renninger et al., 2001) and uraninite particles (Suzuki et al., 2002) were observed around bacterial cells by TEM. The inconsistency between our results and those of Renninger et al. (2001) and Suzuki et al. (2002) suggests that U precipitation is not a dominant accumulation mechanism in the mixtures of B. subtilis and kaolinite under

Conclusions

The preferential association of U(VI) with B. subtilis was observed by TEM and EDS analyses of a mixture consisting of the bacterium and kaolinite. Desorption of U(VI) with a 1 M CH₃COOK solution from a mixture of B. subtilis and kaolinite indicated a tighter association with the former. Differences in the functional groups able to bind U(VI) cause the preferential association of U(VI) to B. subtilis rather than kaolinite in the mixtures under the experimental conditions. These results suggest

Acknowledgement

Part of this study was supported by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan to T. Ohnuki and in part by the U.S. Department of Energy's Office of Science Biological and Environmental Research, Natural Accelerated Bioremediation Research (NABIR) program, under Contract DE-AC02-98CH10886. [DR]

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However, microorganisms may be able to survive under elevated metal levels because they possess a variety of mechanisms to resist high concentrations of metals (Bruins et al., 2000). These mechanisms may involve reduction (Lovley et al., 1991), bioaccumulation (Merroun et al., 2006), biosorption (Ohnuki et al., 2005) and/or biomineralization (Martinez et al., 2007) of metals into forms that are less soluble and hence less bioavailable or less toxic. In this way, microorganisms also play an important role in metal speciation, which influences migration and toxicity of metals (Merroun et al., 2006).
Soil microorganisms may respond to metal stress by a shift in the microbial community from metal sensitive to metal resistant microorganisms. We assessed the bacterial community from low (2–20 mg kg⁻¹), medium (200–400 mg kg⁻¹), high (500–900 mg kg⁻¹) and very high (>900 mg kg⁻¹) uranium soils at Ranger Uranium Mine in northern Australia through pyrosequencing. Proteobacteria (28.85%) was the most abundant phylum at these sites, followed by Actinobacteria (9.31%), Acidobacteria (7.33%), Verrucomicrobia (2.11%), Firmicutes (2.02%), Chloroflexi (1.11%), Cyanobacteria (0.93%), Planctomycetes (0.82%), Bacteroidetes (0.46%) and Candidate_division_WS3 (Latescibacteria) (0.21%). However, 46.79% of bacteria were unclassified. Bacteria at low U soils differed from soils with elevated uranium. Bacterial OTUs closely related to Kitasatospora spp., Sphingobacteria spp. and Rhodobium spp. were only present at higher uranium concentrations and the bacterial community also changed with seasonal and temporal changes in soil uranium and physicochemical variables. This study using next generation sequencing in association with environmental variables at a uranium mine has laid a foundation for further studies of soil-microbe-metal interactions which may be useful for developing sustainable management and rehabilitation strategies. Furthermore, bacterial species associated with higher uranium may serve as useful indicators of uranium contamination in the wet-dry tropics.
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This article has been retracted at the request of the Executive Editor-in-Chief. Concern was raised about the spectra shown by Figure 5 for two reasons. First, the variation of these spectra along the x-axis (i.e., their phase) is inconsistent with the known behavior of such spectra. Second, the claimed fits to the data are not reproducible using the fitting parameters shown in Table 2. Consequently, the Editor decided to retract the article.
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In addition, Figure S2 in the supporting document is highly similar to Figure 4B,D of the article previously published by the first author, the last author et al in Geochimica et Cosmochimica Acta 140 (2014) 621 http://dx.doi.org/10.1016/j.gca.2014.06.001.
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As a key part of water management at the Ranger Uranium Mine (Northern Territory, Australia), stockpile (ore and waste) runoff water was applied to natural woodland on the mine lease in accordance with regulatory requirements. Consequently, the soil in these Land Application Areas (LAAs) presents a range of uranium concentrations. Soil samples were collected from LAAs with different concentrations of uranium and extracts were plated onto LB media containing no (0 ppm), low (3 ppm), medium (250 ppm), high (600 ppm) and very high (1500 ppm) uranium concentrations. These concentrations were similar to the range of measured uranium concentrations in the LAAs soils. Bacteria grew on all plates except for the very high uranium concentrations, where only fungi were recovered. Identifications based on bacterial 16S rRNA sequence analysis showed that the dominant cultivable bacteria belonged to the genus Bacillus. Members of the genera Paenibacillus, Lysinibacillus, Klebsiella, Microbacterium and Chryseobacterium were also isolated from the LAAs soil samples. Fungi were identified by sequence analysis of the intergenic spacer region, and members of the genera Aspergillus, Cryptococcus, Penicillium and Curvularia were dominant on plates with very high uranium concentrations. Members of the Paecilomyces and Alternaria were also present but in lower numbers. These findings indicate that fungi can tolerate very high concentrations of uranium and are more resistant than bacteria. Bacteria and fungi isolated at the Ranger LAAs from soils with high concentrations of uranium may have uranium binding capability and hence the potential for uranium bioremediation.

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¹: Present address: Material and Energy Research Center, P.O. Box 14155-4777, Tehran, Iran.

²: Present address: Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CBZ 3QZ, UK.

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Interactions of uranium with bacteria and kaolinite clay

Abstract

Introduction

Section snippets

Microorganism, kaolinite, and U solution

Adsorption and desorption of U by the mixtures of B. subtilis and kaolinite

Discussion

Conclusions

Acknowledgement

Chem. Geol.

Chem. Geol.

Geochim. Cosmochim. Acta

Geochim. Cosmochim. Acta

J. Elec. Spec. Rel. Phen.

Geochim. Cosmochim. Acta

J. Contam. Hydrol.

Geochim. Cosmochim. Acta

Uptake of trace metals and rare earth elements from hornblende by a soil bacterium

Geomicrobiol. J.

Contaminant uranium phases and leaching at the Fernald site in Ohio

Environ. Sci. Technol.

An EXAFS study of uranyl ion in solution and sorbed onto silica and montmorillonite clay colloids

J. Colloid Interface Sci.

Experimental study of uranyl adsorption onto Bacillus subtilis

Environ. Sci. Technol.

XPS and XANES studies of uranium reduction by Clostridium sp.

Environ. Sci. Technol.