Environ. Sci. Technol. 2006, 40, 3782-3786
Microbial Selenate Sorption and Reduction in Nutrient Limited Systems P A U L A . K E N W A R D , * ,†,§ D A V I D A . F O W L E , †,§ A N D N A T H A N Y E E ‡ Great Lakes Institute for Environmental Research Centre, University of Windsor Canada, Department of Earth and Environmental Sciences, Rutgers, The State University of New Jersey, Newark, New Jersey 07102 and Department of Geology, University of Kansas, Lawrence, Kansas, 66044
In this study, batch sorption experiments and X-ray adsorption spectroscopy (XAS) were utilized to investigate selenate sorption onto Shewanella putrefaciens 200R. Selenate sorption was studied as a function of pH (ranging from 3 to 7), ionic strength (ranging from 0.1 to 0.001 M), and initial selenate concentration (ranging from 10 to 5000 µM) in the absence of external electron donors. The results show that the extent of selenate sorption is strongly dependent on pH and ionic strength, with maximum sorption occurring at low pH (pH ) 3) and low ionic strength (0.001 M NaCl) conditions. The strong dependence of Se sorption with ionic strength suggests the formation of outersphere complexes with the cell wall functional groups. Langmuir isotherm plots yielded log Kads values from 2.74 to 3.02. Desorption experiments demonstrated that the binding of selenate onto S. putrefaciens was not completely reversible. XANES analysis of the cells after sorption experiments revealed the presence of elemental selenium, indicating that S. putrefaciens has a capacity to reduce Se(VI) to Se(0) in the absence of external electron donors. We conclude that Se sorption onto S. putrefaciens cell walls is the result of the combination of outer-sphere complexation and cell surface reduction. This sorption process leads to a complex reservoir of bound Se which is not entirely reversible.
Introduction Selenium (Se) is a common contaminant of surface and groundwaters worldwide. A variety of agricultural and industrial processes increase the mass transfer of selenium into aquatic settings beyond the contributions from natural reservoirs such as shales, fossil fuels, and alkaline soils (1). The ubiquity of selenium and its sometimes high concentrations in the environment have been proven to have both acute and chronic toxicity to organisms in aquatic systems (2, 3). Selenium contamination in irrigation waters can cause embryonic abnormalities and death in aquatic birds (4). Acute selenium poisoning in humans can cause respiratory difficulty and gastrointestinal distress, while chronic exposure can lead to lethal liver damage. * Corresponding author phone: (785) 841-0570; fax: (785) 8645276; e-mail:
[email protected]. † Great Lakes Institute for Environmental Research Centre. ‡ Rutgers, The State University of New Jersey. § University of Kansas. 3782
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Selenium has a complex aqueous geochemistry which is dependent on pH, ionic strength, and the redox properties of the system of interest. In low-temperature aquatic environments selenium speciation is dominated by the oxyanions selenate (SeO42-) and selenite (SeO32-). While selenate and selenite are soluble and mobile, the reduced form of Se0 readily precipitates and appears to be stable and perhaps less bioavailable in these settings (5, 6). As such, there is a great interest in the factors controlling the redox states of selenium. Selenate and selenite can be reduced readily in most systems through interactions with organic matter or via microbiological processes that drive Se0 formation by providing reducing conditions and overcoming kinetic barriers to selenium reduction. Many studies have shown that Se-reducing bacteria can effectively catalyze the reduction of selenate into elemental selenium. Other strains of bacteria may further reduce elemental selenium to selenide or oxidize selenide to form elemental selenium (7). Most of these microbe-mediated processes actively reduce selenate to elemental selenium by utilizing selenate as a terminal electron acceptor (TEA) during the respiration of organic carbon. For example, in agricultural drainage water of the San Joaquin Valley, California, selenate was effectively reduced to elemental selenium by Enterobacter taylorae (8) using this pathway. In this study yeast extracts, other nutrients and near-neutral conditions were required to facilitate this reduction. Under similar conditions Bacillus sp. SF-1 was able to reduce selenate to elemental selenium (9, 10). Some bacteria, like Enterobacter cloacae SLD1a-1 (11), have known pathways for the detoxification of selenate which include a series of reductive transformations of the anion and in the presence of an electron donor. Sulfurospirillum barnesii, Bacillus selentiriducens, and Selenihalanaerobacter shriftii reduced toxic selenium oxyanions and produced selenium nanospheres that accumulated outside the cell wall (12). These experiments were conducted at a high pH, 7 to 9.8, and the bacteria were supplied lactate as an electron donor. These processes require a specialized metabolic guild of bacteria, dissolved organic carbon and an available source of nutrients. There is a significant fraction of inactive or dead biomass in soils and sediments residing under organic carbon and nutrient limitation. The question remains whether these organisms and their cell wall components may retain enough reductive potential to reduce selenate in these environments. While the mechanisms by which bacteria sorb and nonmetabolically reduce selenate are poorly understood, there are many studies that have investigated and quantified the sorption of other metals and metalloids onto bacterial cell walls (13-16). These studies have shown that reactive functional groups associated with the structural components of the cell wall framework or lipopolysaccharides are the active sites responsible for metal binding. Furthermore, recent work provides evidence that under acidic conditions there are reactive sites that carry a positive charge that may control anion binding (17). In this study, we examine selenate sorption onto the Gram negative bacteria S. putrefaciens 200R in the absence of an electron donor. S. putrefaciens 200 was first discovered in Canadian oil pipeline by C. O. Obuekwe in 1980 (18) and the 200R strain, a facultative anaerobe and rifamycin-resistant strain, was later isolated by DiChristina and DeLong in 1994 (19). We have chosen S. putrefaciens as our experimental organism because it is common in many lacustrine and seawater sedimentary environments and has well character10.1021/es052210n CCC: $33.50
2006 American Chemical Society Published on Web 05/13/2006
ized cell wall structure and surface chemistry (20). It has also been demonstrated that S. putrefaciens is a model strain capable of catalyzing the redox transformations of many metal oxides such as technetium(VII), iron(III), and manganese(IV) (21-23). The objectives of this study were to quantify the affinity of selenate for sorption onto the S. putrefaciens cell wall and determine if this process leads to irreversible changes in selenium speciation via redox reactions.
Materials and Methods Culture Conditions. All media, reagents, and electrolytes were made using reverse osmosis deionized (18 MΩ-cm) water. The sorption experiments were performed using Shewanella putrefaciens 200R as the bacterial substrate. The cells were cultivated in a nutrient-rich broth comprised of 3% tryptic soy broth and 0.5% yeast extracts. They were grown at 30 °C aerobically until they reached the mid-stationary phase (16 h). The cells were then washed by centrifugation at 6000 rpm for 10 min in order to pellet the bacteria, which were then re-suspended in 0.001 M NaCl. This process was repeated five times, and a final suspension was made with 0.001 M NaCl. The washing step ensures that no growth media remained on the cell wall prior to experimentation. However, some trace amounts of organic carbon may remain in the system and allow the bacteria to retain a small amount of metabolic activity. Bacteria utilized in the experiments were in stationary phase and viable but with no external electron donors or source of carbon. We provide dry weights of bacteria in our experiments by retrieving three sub-samples, of known volume, from the experimental suspension and drying at 30 °C until the mass remained constant. Selenate Removal Experiments. Batch experiments were performed in order to test the ability of Shewanella putrefaciens to remove selenate from solution as a function of pH, ionic strength, and initial aqueous selenate concentration. All these experiments were performed on the benchtop at room temperature (25 °C). Known volumes of mineral salts media containing approximately 6 g/L washed cells of bacteria were spiked with stock selenate solution (0.1 M). Control experiments followed similar methodology in the presence and absence of selenium and bacteria. The background electrolyte was made using varying amounts of reagent grade NaCl and pH was held constant using small amounts of 0.1 M NaOH and HCl. Stock solutions of selenate were made using reagent grade Na2SeO4‚5H2O. We studied the effects of selenate concentration, pH, ionic strength, and time. Experiments designed to test the effects of initial aqueous concentration on selenate sorption were first adjusted to the appropriate pH and electrolyte strength, and then divided among separate vials. Afterward, these aliquots were spiked with different volumes of stock sodium selenate to bring them to the desired concentrations. It was during these experiments that the effects of varying ionic strength was tested by suspending the bacteria in either, 0.001, 0.01, or 0.1 M NaCl and keeping pH constant. The experiment designed to test desorption of selenate from the cell wall was accomplished by allowing selenate sorption to equilibrate at pH 3 for 2 h. After this equilibration period the bacterial solution was divided into separate aliquots which were titrated upward in pH over the range of 3.5 to 7.5 and allowed to equilibrate for another 2 h. Desorption is expressed as a percentage of the selenate originally sorbed to S. putrefaciens during the initial 2 h sorption at pH 3. Chromatography Analysis. In all experiments samples were centrifuged at the end of the desired reaction period and syringed-filtered through 0.45 micron nylon filters. The supernatant was then measured for soluble selenium via ion chromatography (IC) using a CD25 conductivity detector (Dionex Corp., CA) with an IonPac AS11-HC separation
FIGURE 1. Selenate sorption onto S. putrefaciens (6 g/L) as a function of time at a constant pH (3) and ionic strength (0.001 M NaCl). column and AG11-HC guard column and ASRS-Ultra 4 mm self-regenerating suppressor. Selenate in solution was subtracted from the original concentration to determine the selenate removed by either adsorption or reduction and precipitation or both. XANES Experiments. To determine the valence state of selenium bound to the cell wall, subset samples were collected and analyzed using X-ray absorption near edge structures (XANES). The XANES data were collected at the National Synchrotron Light Source (Brookhaven National Laboratory). The samples were run on beamline ×11B which is capable of an energy range of 5.0-23 keV. The experimental samples were prepared using the same procedure described for the selenate removal experiments only with a 10 times greater concentration of selenate in solution. Bacterial concentrations were approximately 6 g/L and the pH and background electrolyte concentrations were adjusted to observe different conditions. Powdered samples of sodium selenate, sodium selenite, and elemental selenium were used as standards to define the adsorption edge energies for the various oxidation states observed by the XANES spectroscopy. Analysis of the XANES data was performed using WinXAS version 3.0. To determine whether samples were subject to artificial shifts in Se reduction as a result of beam-induced damage, all samples were run repeatedly throughout the experiments in sequence for extended periods of time. No detectable beam induced effects on the valence state of selenium were found either in the standards or the experimental samples.
Results and Discussion Factors Controlling Sorption. To study the kinetics of selenate sorption by S. putrefaciens, experiments were performed at pH 3 (via pH stat) in 0.001 M NaCl as a function of time. The data for these experiments are presented in Figure 1. After 2 h, selenate sorption had reached a steady state of 30.8% + 2.3% selenate removed from the system. The rapid kinetics of removal within the first 2 h is consistent with observed cation sorption data (e.g., 15, 24). Figure 2 shows selenate oyxanion sorption onto S. putrefaciens as a function of pH (3-5) for 2 cell concentrations (1 and 5 g/L) and one control with no cells. In the absence of cells, no selenate sorption was detected across the pH range studied. As cell concentration increased, an increasing mass of selenate was sorbed from solution at all pH values. The highest amount of selenate sorption was observed at pH 3 for all solid solute ratios. At 1 g/L and 5 g/L cell concentrations, the maximum selenate removal measured was 25 and 39% respectively. Interestingly, the increase in percent selenate sorbed was not directly proportional to the increase in biomass. Controls show no removal of selenate as a function of pH. Figure 3 is a Langmuir sorption isotherm depicting the effect of pH and aqueous selenate concentration on selenate VOL. 40, NO. 12, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Selenate sorbed onto S. putrefaciens as a function of pH in the presence of two bacterial concentrations (1 g/L and 5 g/L) and a control with no bacterial cells. Ionic strength was fixed at 0.001 M NaCl.
FIGURE 4. Moles of selenate sorbed onto S. putrefaciens (6 g/L) as a function of aqueous Se (VI) concentration and ionic strength at pH 3. Solid lines represent Langmuir model fits.
TABLE 1. Langmuir Parameters for the Sorption of Se (IV) onto Shewanella Putrefaciens at 25 0Ca experimental condition
Γmax (mmol/g)
pH 3, 0.001 M NaCl pH 3, 0.01 M NaCl pH 3, 0.1 M NaCl
0.17 0.08 insufficient adsorption under these conditions pH 4.5, 0.001 M NaCl 0.12 pH 6, 0.001 M NaCl insufficient adsorption under these conditions
Log Kads
r2
2.74 3.02
0.99 0.88
2.75
0.95
a Γ max is the maximum concentration of surface sites per gram of Shewanella putrefaciens and log Kads is the equilibrium constant.
FIGURE 3. Moles of selenate sorbed onto S. putrefaciens (6 g/L) as a function of aqueous Se (VI) concentration and pH in 0.001 M NaCl. Solid lines represent Langmuir model fits. removal. With increasing concentrations of aqueous selenate added to the system, there was an increase in total selenate sorbed. As aqueous selenate levels approached the 5 mM concentration, sorption sites on the cell wall of the bacteria appeared to be approaching saturation. At all concentrations of selenate in solution, selenate removal was observed to be the greatest under acidic conditions (pH 3). Significantly less selenate was removed at pH 4.5 and selenate removal was nearly undetectable at pH 6. This relationship with pH is in sharp contrast with the metabolic reduction of selenate by previously studied Se-reducing bacteria where the optimum Se removal is between pH 6 and 8 (9, 11, 12). The influence of pH on sorption appears to have a greater effect on the uptake of selenium with increasing aqueous selenium concentrations. The observed increase in sorption of selenate with decreasing pH is likely due to the protonation of cell wall function groups. At low pH, the functional groups on the cell wall (carboxyl, phosphoryl, and amine groups) are fully protonated and the approach of a negatively charged aqueous species from bulk solution will be less inhibited. Evidence from recent electrophoretic mobility studies suggests the presence of positively charged sites at these pH values which, in turn, would be available to bind negatively charged species (25). The effect of ionic strength and aqueous selenate concentration on selenate removal is illustrated in Figure 4. Increasing concentrations of aqueous selenate lead to an increase in total selenate sorbed. As aqueous selenate concentration increased, saturation of the available surface sites appeared to be occurring as sorption reaches a maximum at 5 mM. At all concentrations of selenate in 3784
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solution, selenate removal was observed to be the greatest at low ionic strengths (0.001 M NaCl). As with pH, we see a greater effect of ionic strength on sorption at higher initial aqueous selenate concentrations. This strong dependence of selenate sorption on ionic strength is characteristic of electrostatically dominated bonding where ions are forming weak outer-sphere complexes with the bacterial surface (25). In solutions with an elevated ionic strength, the cell surface electric field is compressed, decreasing the electrostatic attraction between the positively charged bacterial surface and negatively charged selenate oxyanions. This, in concert with increasing anion competition at the bacteria water interface with electrolyte counterions, will suppress both the approach and retention of selenate on the cell wall of S. putrefaciens. Conversely, at low ionic strength, the cell surface electric field is expanded and competition is decreased, thereby increasing selenate sorption. Sorption Modeling and Mechanism. We utilize a Langmuir model generated using the following equation to fit our experimental data:
Γ ) Γmax
Kads[SeO42-] 1 + Kads[SeO42- ]
Where [SeO42-] is the concentration of selenate in solution, Kads is the equilibrium constant, Γ is the amount of selenate sorbed and Γmax represents the maximum sorption capacity. Plotted on Figures 3 and 4 (solid lines) are the model fits generated from eq 1. The Langmuir models were constrained by the high-concentration data points. The model fits generated in this way accurately describe the sorption isotherms. These model parameters for Figures 3 and 4, are provided in Table 1. At 0.001 M NaCl, increasing pH from 3 to 4.5 does not significantly change the apparent log Kads values but does decrease surface sites (those responsible for
FIGURE 5. Desorption of Se (VI) from the surface of S. putrefaciens (6 g/L) as a function of pH in 0.001 M NaCl. Squares represent the percentage of the selenate desorbed, after initial adsorption at pH 3. Se sorption) from 0.17 mmol/g to 0.12 mmol/g. Comparison of the pH 3 data indicates that increasing ionic strength from 0.001 to 0.01 M NaCl increases the apparent log Kads values from 2.74 to 3.02 and decreases surface sites from 0.17 to 0.08 mmol/g, respectively. These apparent surface site concentrations are within the range of available surface sites determined by other studies (26, 27). As insufficient amounts of Se removal were measured at high ionic strength and high pH, these data could not be used to constrain the log Kads constants or surface site concentrations. To elucidate the mechanisms by which selenate sorbs onto S. putrefaciens, we conducted reversibility and spectroscopic studies. Figure 5 depicts the results of a reversibility study as a function of pH. Briefly, the desorption experiment consisted of allowing the cells to equilibrate with selenate at pH 3 for 2 h (based on our previous sorption and kinetics experiments) before desorption via upward adjustment of pH, followed by another 2 h equilibration period. On average, 40.5% of the selenate sorbed initially at pH 3. Over the range of pH between 3.5 and 5.5, selenate remained mostly sorbed. It is not until the pH is raised to 6 and above, that a significant portion of the selenate desorbs (nearly 70% above pH 7). This desorption behavior suggests that sorption is not reversible over the time scales studied and that a process beyond surface complexation is affecting Se sorption. To determine whether the irreversible nature of the binding of selenate to S. putrefaciens was due to reduction and subsequent precipitation of elemental Se0, we collected XANES spectra of the Se sorbed cells. These data, along with reference selenium standards, are depicted in Figure 6. The energy position of the XANES edge for selenate, selenite, and elemental selenium standards were 12.669, 12.664, and 12.659 keV respectively. The peak of the XANES edge measured for the Se sorbed cells was 12.659 keV, which is identical to the energy value detected for Se(0). This indicates that a majority of the selenium associated with the cells was reduced from Se(VI) to elemental selenium after sorption and without the introduction of an external carbon source. These results suggest a mechanism whereby selenate sorbs onto S. putrefaciens via outer-sphere-complexation which is then followed within 2 h by the reduction to elemental Se. Once bound to the cell wall, there are a number of potential candidates for the reduction of selenate to Se0. Many S. putrefaciens strains contain both ubiquinones and naphthoquinones (28) which could act as electron donors for the reduction of selenate. Naphthoquinones have standard potentials ranging from -89 to -300 mV (29). The standard potential of selenate reducing to elemental selenium is equal to 740 mV (30). Considering the significantly greater standard potential of selenate, it is thermodynamically favorable for
FIGURE 6. XANES data showing the comparison of the normalized absorbances of three standards, representing Se (VI), Se (IV), and elemental selenium, and the selenium bound to S. putrefaciens (6 g/L cells, pH 3 and in 0.001 M NaCl). quinones to donate the electrons necessary to reduce selenate to elemental selenium. Ubiquinones are abundant in the electron transport chains (ETC), serving to transfer electrons and are lipid soluble. Only a small portion of the ubiquinones are tightly bound to ETC proteins, and a large fraction of these molecules are free to diffuse around in the membrane, as a ubiquinone pool. This abundance combined with their ability to pass through the cell membrane, make ubiquinones a possible electron donor for selenate reduction by metabolically inactive bacteria. Quinones can also reduce selenate indirectly by reducing peripheral membrane proteins, like Cytochrome c which has a standard potential of approximately 254 mV. These reduced cell wall proteins still retain a lower standard potential than that of selenate which would then act as an electron acceptor once bound to the cell wall. Another study of nonmetabolic reduction by bacteria by Fein et al. (31) has also suggested that, in the absence of external electron donors, cell wall constituents could contribute the electrons necessary to reduce chromium, for example, organic molecules from the cell wall, such as metal reducing enzymes or cytochrome-like molecules associated with metal-reducing bacterial nanowires (32). Chromium, in this case, serves as an excellent oxidant for organic molecules, and it is possible that selenium does the same. Given the number of potential electron donors found within the cell wall, it is possible that the cell wall will be sufficient to reduce selenium, even in the absence of active metabolism. In natural subsurface environments, bacteria experience nutrient poor conditions and are often found as nonmetabolizing cells and cell fragments. Therefore, the impact of bacteria on the toxicity, mobility, and cycling of selenium may extend beyond that exerted by Se-reducing bacteria that employ Se as terminal electron acceptors.
Acknowledgments We thank the Associate Editor Dr. Janet Hering and four anonymous reviewers whose reviews greatly improved this manuscript. This project was supported in part by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant number no. 2005-35107-16230, and the Canada Research Chairs program.
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Received for review November 3, 2005. Revised manuscript received March 29, 2006. Accepted March 30, 2006. ES052210N
