ENVIRONMENTAL AND PUBLIC HEALTH MICROBIOLOGY

Production of Biogenic Mn Oxides by Leptothrix discophora SS-1 in a Chemically Defined Growth Medium and Evaluation of Their Pb Adsorption Characteristics

Yarrow M. Nelson, Leonard W. Lion, William C. Ghiorse, Michael L. Shuler
DOI: 10.1128/AEM.65.1.175-180.1999

ABSTRACT

Biogenic Mn oxides were produced by the bacterium Leptothrix discophora SS-1 (= ATCC 3182) in a chemically defined mineral salts medium, and the Pb binding and specific surface area of these oxides were characterized. Growth of SS-1 in the defined medium with pyruvate as a carbon and energy source required the addition of vitamin B12. Complete oxidation of Mn(II) within 60 h required the addition of ≥0.1 μM FeSO4. Pb adsorption isotherms were determined for the biogenic Mn oxides (and associated cells with their extracellular polymer) and compared to the Pb adsorption isotherms of cells and exopolymer alone, as well as to abiotic Mn oxides. The Pb adsorption to cells and exopolymer with biogenic Mn oxides (0.8 mmol of Mn per g) at pH 6.0 and 25°C was 2 orders of magnitude greater than the Pb adsorption to cells and exopolymer alone (on a dry weight basis). The Pb adsorption to the biogenic Mn oxide was two to five times greater than the Pb adsorption to a chemically precipitated abiotic Mn oxide and several orders of magnitude greater than the Pb adsorption to two commercially available crystalline MnO2 minerals. The N2Brunauer-Emmet-Teller specific surface areas of the biogenic Mn oxide and fresh Mn oxide precipitate (224 and 58 m2/g, respectively) were significantly greater than those of the commercial Mn oxide minerals (0.048 and 4.7 m2/g). The Pb adsorption capacity of the biogenic Mn oxide also exceeded that of a chemically precipitated colloidal hydrous Fe oxide under similar solution conditions. These results show that amorphous biogenic Mn oxides similar to those produced by SS-1 may play a significant role in the control of trace metal phase distribution in aquatic systems.

Fe and Mn oxides have long been recognized as important adsorbing phases governing the cycling of trace metals in aquatic environments (33, 35, 46, 62, 64). Fe oxides are often considered more important than other adsorbing phases because the high specific surface area of amorphous or colloidal Fe oxides is expected to result in high adsorption capacity, and thus trace metal adsorption to Fe oxides has been studied extensively (e.g., see reference 20). Less attention has been given to trace metal adsorption by Mn oxides, even though it is likely that fresh biologically oxidized Mn is poorly crystallized or amorphous (41, 58, 65) and could exhibit much greater trace metal adsorption than crystalline Mn oxides.

The Mn oxidation states and mineral forms of biogenic Mn oxides have been characterized in several prior investigations (3, 23, 24, 41, 57); however, specific surface areas and trace metal binding characteristics of biogenic Mn oxides have not been reported previously. Moffett and Ho (44) reported that Co, Zn, Ce, and trivalent lanthanides could be incorporated into biogenic Mn oxides by being “coprocessed” via the same putative enzymatic pathways as those used for Mn oxidation, but adsorption of these elements to already formed Mn oxides was not described. He and Tebo (32) measured the surface area of Mn-oxidizing Bacillus sp. strain SG-1 spores and Cu adsorption to these spores, but Cu adsorption to Mn oxides formed by the spores was not reported. Tessier et al. (59) observed trace metal adsorption to freshwater sediment extracts containing Mn oxides, Fe oxides, and organic materials, but the specific role of Mn oxides was not elucidated because the extracted Mn oxides were contaminated with Fe oxides and residual organic material. Additional information on the metal adsorption capacity and specific surface area of biogenic Mn oxides under controlled laboratory conditions is needed to assess the relative importance of biogenic Mn oxides in controlling trace metal adsorption in natural aquatic environments.

In the present work, Pb adsorption and specific surface area were measured for biogenic Mn oxides produced by the bacteriumLeptothrix discophora SS-1 in a chemically defined medium.L. discophora is a well-characterized, model Mn-oxidizing bacterium whose structure, physiology, and phylogeny have been studied extensively (8, 26-28). In pure cultures ofL. discophora, Mn oxidation has been shown to occur extracellularly (1-3, 8, 16, 17, 21, 22, 54, 58). The Mn oxides formed by L. discophora were previously shown to be mixed Mn(III, IV) oxides or oxyhydroxides with an average oxidation state of 3.6 (3). Pb adsorption by the biogenic Mn oxide (with cells and exopolymer) was compared to Pb adsorption byL. discophora cells and exopolymer alone (without Mn oxide). The adsorption properties of the biogenic Mn oxides were also compared to those of Mn oxides of abiotic origin.

To unambiguously assess Pb adsorption to Mn oxides produced byL. discophora, it was necessary to grow the bacterium in a defined medium free of competing trace metals and undefined organic ligands or mineral precipitates that could interfere with Pb adsorption. Previous work with another strain, L. discophora SP-6, had shown that this organism could be grown in a defined mineral salts medium that contained 10 μM FeSO4and a suite of vitamins (3). Therefore, to facilitate our experiments, it was first necessary to determine the minimal vitamin and Fe requirements of strain SS-1. These experiments revealed that vitamin B12 was required for SS-1 growth and also revealed that supplemental Fe enhanced Mn oxidation in the defined medium. Development of a chemically defined medium based on these determinations resulted in more accurate and meaningful measurement of Pb adsorption to the resulting biogenic Mn oxides.

MATERIALS AND METHODS

Culture and medium. L. discophora SS-1 (= ATCC 43182) cultures (1, 29) were maintained at 4°C on plates of peptone-Trypticase-yeast extract-pyruvate medium that contained 0.25 g of peptone (Difco Laboratories, Detroit, Mich.) per liter, 0.25 g of Trypticase (BBL Becton Dickinson Microbiology Systems, Cockeysville, Md.) per liter, 0.50 g of yeast extract (Difco Laboratories) per liter, 0.6 g of MgSO4 · 7H2O (Fisher Scientific, Pittsburgh, Pa.) per liter, 0.07 g of CaCl2 · 2H2O (Fisher Scientific) per liter, 10 μM FeSO4 (Fisher Scientific), 2.38 g of HEPES buffer (U.S. Biochemical Corp., Cleveland, Ohio) per liter, 0.3 g of pyruvate (Aldrich Chemical Co., Milwaukee, Wis.) per liter, and 1.6% agar (Difco Laboratories). Inocula from these plates were first grown in a minimal mineral salts (MMS) liquid medium (Table 1) lacking FeSO4 and vitamin B12 but containing 0.25 mg of peptone per liter and 0.50 mg of yeast extract per liter. Mn oxidation experiments were conducted in MMS liquid medium containing filter-sterilized MnSO4 at a concentration of 50 μM. For the Mn oxidation experiments, L. discophora cultures were inoculated with 1% (vol/vol) inocula containing residual peptone and yeast extract. Thus, the maximum peptone and yeast extract concentrations in final media used for Mn oxidation experiments were 2.5 and 5.0 μg/liter, respectively. Cell dry weights were determined by analyzing the total suspended solids in the broth (standard method 2540 D [5]).

View this table:
Table 1.

Composition of the MMS liquid medium used for growth of and Mn oxidation byL. discophora SS -1a

Measurement of Mn oxidation. L. discophora SS-1 cultures were grown at 25°C in 500 ml of MMS liquid medium (Table 1) in 1-liter flasks on a rotary shaker (Innova 2000; New Brunswick Scientific, Edison, N.J.) operated at 150 rpm. The medium was not buffered, and the pH increased from 6.0 to 7.8 during growth. Mn(II) oxidation was monitored by aseptically removing 20-ml aliquots of broth, centrifuging them at 13,400 × g for 30 min with a Centra MP4R centrifuge (IEC, Needham Heights, Mass.), and measuring the supernatant Mn concentrations by atomic absorption spectroscopy using an AAnalyst 100 instrument (Perkin-Elmer, Norwalk, Conn.) with an acetylene flame. To distinguish between Mn oxidation and Mn adsorption to cells, the procedure of Bromfield and David (10) was used, as follows. CuSO4 (5 mM) was added to biogenic Mn oxide suspensions at pH 4.2. These mixtures were centrifuged as described above, and each supernatant was analyzed for desorbed Mn2+ by atomic absorption spectroscopy. Since Cu2+ was not observed to displace Mn2+, we concluded that Mn was oxidized and not merely adsorbed.

Preparation of abiotic Mn oxide precipitates.Mn oxide precipitates were prepared by reacting MnCl2 with KMnO4 as described previously (6, 12, 45). KMnO4 (8 g) was dissolved in 200 ml of distilled deionized water (ddH2O) and heated to 90°C, and then 10 ml of 5 N NaOH was added. Fifteen grams of MnCl2 · 4H2O was dissolved in 75 ml of ddH2O, and the resulting solution was added slowly to the basic KMnO4solution. The resulting suspension was heated at 90°C for 1 h. This procedure has been reported to produce an Mn solid phase with a low degree of crystallinity and an X-ray diffraction pattern attributed to δMnO2 (45). After the precipitate was cooled, it was washed several times by centrifugation and resuspension in ddH2O. A portion of the washed precipitate was resuspended in MMS medium and used immediately for Pb adsorption experiments. The remaining precipitate was lyophilized and stored in a desiccator. Pb adsorption was also measured by using suspensions prepared from lyophilized Mn oxide.

Measurement of Pb adsorption to Mn oxides.Pb adsorption to biogenic and abiotic Mn oxide suspensions was measured with a series of jacketed, 500-ml beakers. The inside surfaces of the beakers were treated with (CH3)2SiCl2 (Eastman Kodak Co., Rochester, N.Y.) to produce a hydrophobic surface and minimize Pb adsorption to the glass. Water was circulated through the beaker jackets with a recirculation bath in order to maintain a constant temperature at 25 ± 0.5°C. Glass autoclavable pH probes (Ingold Electrodes, Wilmington, Md.) were installed in each beaker and connected to individual pH controllers (Chemcadet; Cole Parmer, Vernon Hills, Vt.) in order to maintain the pH at 6.00 ± 0.05 by automatic addition of 0.01 N HNO3 (glass distilled; GFS Chemicals, Columbus, Ohio). To each beaker we added 300 ml of an Mn oxide suspension in MMS medium (Table 1) prepared without vitamin B12, FeSO4, or pyruvate. Pb2+ was added to the suspensions from a reference solution containing 1,000 mg of Pb per liter in 2% HNO3 (Fisher Scientific) in order to produce an initial concentration range of 0.1 to 4.0 μM. The speciation of Pb in these solutions was calculated with MINEQL (52, 66), and approximately 89% of the metal was present as the aquo Pb2+ ion (Table 1). Biogenic Mn oxide suspensions (consisting of cells and associated exopolymer and Mn oxide) were diluted 20-fold with MMS medium (which contained no vitamins, FeSO4, or pyruvate) in order to obtain measurable final equilibrium concentrations of dissolved Pb. Both the abiotic Mn oxide and the biogenic Mn oxide mixtures were equilibrated for 24 h with slow magnetic stirring (kinetic experiments indicated that adsorption was complete within 18 h). After equilibration, 20-ml aliquots were centrifuged for 30 min at 13,400 × g in Teflon centrifuge tubes, and 7.5 ml of supernatant was pipetted from each centrifuge tube and acidified by adding 100 μl of 15% HNO3. The Pb concentrations in centrifuged and uncentrifuged samples were measured by graphite furnace atomic absorption spectroscopy with a Perkin-Elmer AAnalyst 100 instrument equipped with a model HGA 800 graphite furnace and a model AS-72 autosampler. The final Pb concentrations in control solutions (no adsorbent) were within 5% of the initial concentrations after 24 h of equilibration. At pH 6.0, a portion (typically about 30%) of the biogenic Mn oxide dissolved during equilibration, and the amount of Mn dissolved was subtracted from the total amount of Mn in order to calculate Pb adsorption on a per mole of Mn basis. Pb adsorption was also measured with two commercially available Mn oxides: a chemically derived granular βMnO2 (Fisher Scientific) and a powdered MnO2 (ICN Pharmaceuticals, K&K Laboratories, Plainview, N.Y.), by the same methods. The observed adsorption data were fitted to Langmuir adsorption isotherms having the form Γ = Γmax · K · [ Pb2+]/(1 + K · [Pb2+]), where Γ is the Pb adsorption (millimoles of Pb per mole of Mn), Γmax is the maximum Pb adsorption, and K is the Langmuir equilibrium constant.

Surface area measurements.The surface areas of the biogenic and abiotic Mn oxides were measured by adsorption of N2 gas from a mixture containing 30% N2 and 70% He with a surface area analyzer (Quantasorb, Syosset, N.Y.) and were calculated by a single-point Brunauer-Emmet-Teller (BET) method (11). Biogenic and fresh abiotic Mn oxides were lyophilized (Flexi-Dry μP; FTS Systems, Stone Ridge, N.Y.) before their surface areas were measured. All samples were outgassed under N2 at 110°C for 4 h prior to surface area measurement. The surface area of the biogenic Mn oxide was calculated by subtracting the experimentally determined surface area of L. discophora cells without Mn (approximately 7 m2/g), a value that was very small compared to the surface area of the biogenic Mn oxide.

RESULTS

Vitamin B12 requirement for L. discophora SS-1 growth and Fe requirement for Mn oxidation.Growth of L. discophora SS-1 was not observed in MMS liquid medium (Table 1) without vitamins. However, growth was observed after 2 μg of vitamin B12 per liter was added to the growth medium. Microscopic examination of the white, turbid suspensions confirmed the presence of long filamentous cells containing inclusions of polyhydroxyalkanoate that are characteristic of sheathlessL. discophora SS-1 cells (1).

Although L. discophora SS-1 grew in MMS medium containing vitamin B12, Mn oxidation did not occur during growth in this medium as it did in peptone-Trypticase-yeast extract-pyruvate medium containing Mn. However, addition of Fe as FeSO4 stimulated Mn oxidation. To determine the minimum amount of Fe required for Mn oxidation, L. discophoraSS-1 cultures were amended with FeSO4 at concentrations ranging from 0.01 to 10 μM. Cell growth was observed in all cultures, as indicated by increases in optical density at 600 nm. When 50 μM MnSO4 was added at the time of inoculation along with the added FeSO4, complete Mn oxidation occurred simultaneously with growth within 60 h only in cultures containing 0.1 μM or more FeSO4 (Fig. 1). The average Mn oxidation rate in the presence of 0.1 μM FeSO4and 50 mg of cells (dry weight) per liter was 1.9 μM/h. This rate is about five times higher than the rates reported for freshwater lakes (61). In contrast, the Mn oxidation rate was much lower and oxidation was not complete in 60 h when 0.01 μM FeSO4 or no FeSO4 was added to the medium.

Fig. 1.
Fig. 1.

Fe requirement for Mn oxidation by L. discophora SS-1 in MMS medium (Fe and Mn were added at the time of inoculation).

The biological role in the Mn oxidation was confirmed by inhibiting the cell culture with sodium azide (51). No decrease in the soluble Mn(II) level was observed with controls containingL. discophora SS-1 cells plus 10 μM FeSO4in the presence of 0.15 mM sodium azide. Since Fe oxidation was likely to occur prior to the addition of Mn and the azide inhibitor, the possibility that Fe oxide catalyzed Mn oxidation in these experiments was also excluded by these controls.

Because of the possibility that the added Fe stimulated growth rather than Mn oxidation, the experiment was repeated with cultures ofL. discophora SS-1 that were first grown to the stationary phase (75 h at 25°C) with and without 0.1 μM FeSO4 (optical density at 600 nm, 0.060 ± 0.005; Spectronic 20 colorimeter; Bausch and Lomb, Rochester, N.Y.). The final turbidity values with and without Fe added were similar (within 10%). Mn oxidation was rapid and complete within 20 h when 0.1 μM FeSO4 was added, while Mn oxidation was slow and incomplete when Fe was not added. Therefore, the Fe requirement appears to have been primarily for oxidation of Mn rather than for growth.

Based on the results of these experiments, the growth medium used for subsequent experiments contained 2 μg of vitamin B12 per liter and 0.1 μM FeSO4 as shown in Table 1.

Pb adsorption to biogenic and abiotic Mn oxides and surface areas.Pb adsorption to L. discophora cells without Mn followed a linear isotherm (Fig.2A). Pb adsorption to L. discophora cells with biogenic Mn oxide deposits (0.8 mmol of Mn/g) was much greater and followed a Langmuir isotherm (Fig. 2B). Figure 2 shows that Mn oxide on the cell surface increased Pb adsorption at least 2 orders of magnitude compared to cells without Mn (note the difference in scales in Fig. 2A and B).

Fig. 2.
Fig. 2.

Pb adsorption isotherms for L. discophora SS-1 cells. (A) Without Mn. (B) With biogenic Mn oxide (0.8 mmol/g). The temperature was 25°C, the pH was 6.0, and the concentration of total suspended solids was 63 mg/liter.

The Pb adsorption by Mn oxidized by L. discophora was significantly greater than the Pb adsorption observed with fresh abiotic Mn oxide precipitates (referred to here as δMnO2) that were prepared by reaction of MnCl2 with KMnO4 (Fig. 3). At the concentrations used, the Pb adsorption by the biogenic Mn oxide was two to five times greater than the Pb adsorption by the fresh precipitates. The levels of Pb adsorption to both biogenic and fresh abiotic δMnO2 precipitates were similar for several different replicate preparations of material and for freeze-dried and fresh (not dried) precipitates (biogenic replicate 3 and abiotic replicate 3 [Fig. 3] were not freeze-dried prior to measurement of Pb adsorption). TheLeptothrix-oxidized Mn and the fresh abiotic δMnO2 precipitate both adsorbed several orders of magnitude more Pb than commercially obtained Mn(IV) oxides (Fig.4 and Table 2). In all cases, adsorption followed Langmuir isotherms, with r2values ranging from 0.83 to 0.87 (Table 2). The Pb adsorption of the biologically oxidized Mn was also 2 orders of magnitude greater than the Pb adsorption previously determined by Nelson et al. for colloidal Fe oxide deposits under the same solution conditions (49) (Fig. 4).

Fig. 3.
Fig. 3.

Pb adsorption to biogenic Mn oxide and fresh abiotic δMnO2 precipitates. Symbols: •, biogenic replicate 1; ■, biogenic replicate 2; ▴, biogenic replicate 3; ⧫, biogenic replicate 4; ○, abiotic replicate 1; □, abiotic replicate 2; ▵, abiotic replicate 3.

View this table:
Table 2.

Langmuir adsorption isotherm parameters for Pb adsorption to biologically oxidized Mn and abiotic Mn oxides

Fig. 4.
Fig. 4.

Comparison of Pb adsorption by biogenic Mn oxide, by abiotic δMnO2 precipitates, by commercial Mn oxide minerals, and by colloidal Fe oxide (log scale).

The N2 BET specific surface area ofLeptothrix-oxidized Mn was about four times the BET specific surface area of fresh abiotic δMnO2 precipitates, and the surface area of the fresh δMnO2 was 1 to 3 orders of magnitude greater than the surface area of the more crystalline Mn oxide minerals (Table 3). Pb adsorption increased with increasing specific surface area (Fig. 4and Table 3).

View this table:
Table 3.

BET surface areas of biologically oxidized Mn and abiotic Mn oxides

DISCUSSION

L. discophora SS-1 requires vitamin B12 for growth and 0.1 μM Fe for complete Mn oxidation under the conditions used in this study. When vitamin B12 was added, SS-1 grew in a defined medium containing no additional trace metals. Any cellular requirements for trace metals were probably satisfied by trace impurities in the reagents used for medium preparation. Although the previous work of Emerson and Ghiorse (21) showed that a suite of vitamins and 10 μM Fe were required for growth of L. discophoraSP-6 (a sheathed strain) in a mineral salts medium, nutritional requirements have not been reported previously for growth of SS-1 or for biological Mn oxidation by either SS-1 (1, 2) or SP-6 (21). The requirement for 0.1 μM Fe for Mn oxidation suggests that Fe may be a cofactor in the extracellular Mn-oxidizing system of SS-1, but further research is needed to elucidate the exact role of Fe in Mn oxidation. Copper (a contaminant in the FeSO4 reagent) may have been responsible for the observed stimulation of Mn oxidation, but we believe that this is unlikely because the calculated concentration of copper in 0.1 μM FeSO4 reagent-grade salt would have been less than 0.5 nM. The contribution of 0.1 μM Fe to the Pb adsorption observed should be negligible compared to the contribution of the 50 μM Mn used in the investigation of Pb binding to Leptothrix-oxidized Mn. Indeed, cells grown with 0.1 μM Fe but without added Mn exhibited minimal Pb adsorption (Fig. 2A).

Pb adsorption to Mn oxide deposits on Leptothrix cells overshadowed direct Pb adsorption to cells alone by 2 orders of magnitude. These results underscored previous reports which revealed that hydrous metal oxides played a greater role than cellular surfaces in governing Pb phase distribution. These reports were based on both laboratory adsorption experiments (33, 35, 62) and selective extraction of natural sediments (4, 7, 9, 36, 38). However, the biological catalysis of Mn oxidation has an indirect but very significant influence on Pb adsorption. This role should be large in circumneutral aquatic environments in which Mn oxidation is biologically mediated. Abiotic oxidation of Mn at circumneutral pH values is thermodynamically favorable but kinetically unfavorable (55), and thus it is thought that Mn oxidation in natural aquatic environments requires active enzymatic catalysis by microorganisms (8, 19, 30, 48, 58). In some cases, phototrophic organisms have been shown to facilitate the oxidation of Mn by increasing the local pH and the dissolved oxygen concentration (50). However, studies of the biogeochemical mechanisms and molecular genetics of Mn-oxidizing microorganisms (15, 47, 54, 58, 63) have indicated that enzymatically linked microbial processes are extremely influential in controlling global Mn cycles. Field studies have provided evidence that biological oxidation of Mn occurs in marine environments (43, 48, 60), estuarine environments (42), and freshwater environments (40, 48). Tessier et al. (59) found that Mn oxides in freshwater sediments were poorly crystallized and had diagenetic origins. Cycling of Mn between oxidizing aquatic environments and sediments or bottom waters under reducing conditions occurs over short time scales, making it likely that suspended Mn oxides in the water column are freshly oxidized (48, 56, 65) and therefore poorly crystalline or amorphous.

A wide range of N2 BET surface areas have been reported for δMnO2 prepared by methods similar to those used in this study (Table 4). Since some researchers have reported surface areas for δMnO2 that are as large as the surface area observed for the biogenic MnO2 in this study, it is possible that the trace metal adsorption by synthetic δMnO2 precipitates could approach the trace metal adsorption by the biogenic MnO2. However, it is not known if the range of reported surface areas is due to actual differences in properties of the oxides or to difficulties in interpreting N2 BET surface area measurements (6). In either case, the surface areas of both the biogenic Mn oxide and the fresh δMnO2 are much greater than the surface area of the more crystalline Mn oxides, supporting the conclusion that biogenic Mn oxides in aquatic environments are much more surface active than crystalline Mn oxides are.

View this table:
Table 4.

Reported N2 BET surface areas of synthetic δMnO2

The maximum binding capacity of the biogenic Mn oxide (Γmax) was 550 mmol of Pb/mol of Mn, and this value can be compared to the metal binding capacities of abiotic MnO2reviewed by Luoma and Davis (39). The majority of workers have reported metal binding capacities of MnO2 that are between 150 and 250 mmol/mol (39), while one group reported a Pb binding capacity as high as 520 mmol/mol (25). Thus, a comparison with previously observed metal binding capacities indicates that the Pb binding capacity of the biogenic Mn oxide described here is at the upper limit of the reported range. The greater adsorption observed for biogenic Mn oxide is likely due to the amorphous nature of the surface compared to the surfaces of the more crystalline abiotic Mn minerals.

The Pb adsorption by Leptothrix-oxidized Mn was especially greater than the Pb adsorption by the fresh abiotic Mn oxide precipitate at low Pb concentrations. This high affinity for Pb at low Pb concentrations is reflected by the high K value of the Langmuir isotherm for biogenic Mn oxide (Table 2). Since dissolved Pb concentrations are typically very low in aquatic environments, these results suggest that consideration of the metal binding properties of biogenic Mn oxides is particularly important for the development of phase distribution models.

Based on the observations made in this work, the importance of Mn oxides (compared to the importance of Fe oxides) in controlling Pb adsorption may be greater than was previously thought. Fe oxides would be expected to bind more trace metal than crystalline Mn oxides based on the high specific surface area and observed trace metal adsorption of Fe oxides compared to Mn oxide minerals. However, the present work shows that the Pb adsorption of biologically oxidized Mn is much greater than the Pb adsorption of Mn oxide minerals and colloidal hydrous Fe oxide, and thus trace metal scavenging by Mn oxides in aquatic environments could be more significant under some conditions. It is possible that biogenic Fe oxides also exhibit greater trace metal binding capacity than abiotic Fe oxides exhibit. However, Fe oxides in aquatic environments are likely to contain a smaller fraction that has a biogenic origin since Fe is easily oxidized abiotically. We are currently developing experiments to generate biologically oxidized Fe under controlled laboratory conditions in order to measure its trace metal binding capacity.

Further research is needed to fully characterize the trace metal adsorption and other physical and chemical properties of biologically oxidized Mn and to establish the importance of biologically oxidized Mn in natural aquatic systems. The pH was fixed at 6.0 in this research to facilitate comparisons of different Mn minerals and to allow comparisons to prior measurements obtained with Fe oxides. It would be interesting to evaluate Pb adsorption by biogenic Mn oxides at a range of pH values and to similarly examine the adsorption of other trace metals. Experiments of this type could be used to determine parameters for surface complexation modeling of biogenic Mn oxides in order to provide more realistic simulations of trace metal adsorption to natural suspended particulate material (18, 20, 53). It is also interesting to speculate that the high trace metal adsorption capacity of biologically oxidized Mn may have a number of engineering applications. For example, fixed-film bioreactors containing Mn-oxidizing bacteria could be designed to remove trace metals from wastewater. Trace metal adsorption systems based on biogenic Mn oxides could be regenerated by dissolving Mn oxides and the associated trace metals by using a reduction in pH or redox potential.

ACKNOWLEDGMENTS

We are grateful for the technical assistance of Paul Koster van Groos, Barbara Eaglesham, and Cameron Willkens.

This research was supported by grants CHE-9708093 and BES-9706715 from the National Science Foundation.

FOOTNOTES

    • Received 21 May 1998.
    • Accepted 15 October 1998.

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