 |
INTRODUCTION |
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 bacterium Leptothrix 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 of
L. 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 by
L. 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 by
L. 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 FeSO4
and 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]).
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 KMnO4
solution. 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 
MnO2 (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 
MnO2 (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 
= max · K · [Pb2+]/(1 + K · [Pb2+]), where is the Pb adsorption (millimoles
of Pb per mole of Mn), max 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 sheathless
L. 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. discophora
SS-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 FeSO4
and 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.
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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).
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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 containing L. discophora SS-1 cells plus 10 µM FeSO4
in 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 of
L. 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).
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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.
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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 MnO2)
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 MnO2 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). The
Leptothrix-oxidized Mn and the fresh abiotic
MnO2 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 r2
values 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).
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FIG. 3.
Pb adsorption to biogenic Mn oxide and fresh abiotic
MnO2 precipitates. Symbols:  , biogenic replicate 1;
 , biogenic replicate 2;  , biogenic replicate 3;  , biogenic
replicate 4;  , abiotic replicate 1;  , abiotic replicate 2;  ,
abiotic replicate 3.
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TABLE 2.
Langmuir adsorption isotherm parameters for Pb adsorption
to biologically oxidized Mn and abiotic Mn oxides
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FIG. 4.
Comparison of Pb adsorption by biogenic Mn oxide, by
abiotic MnO2 precipitates, by commercial Mn oxide
minerals, and by colloidal Fe oxide (log scale).
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The N2 BET specific surface area of
Leptothrix-oxidized Mn was about four times the BET specific
surface area of fresh abiotic MnO2 precipitates, and the
surface area of the fresh MnO2 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. 4
and Table 3).
| |
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. discophora
SP-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
MnO2 prepared by methods similar to those used in this
study (Table 4). Since some researchers
have reported surface areas for MnO2 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
MnO2 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
MnO2 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.
The maximum binding capacity of the biogenic Mn oxide
(max) was 550 mmol of Pb/mol of Mn, and this value can
be compared to the metal binding capacities of abiotic MnO2
reviewed 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.
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.
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Abstract
Introduction
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 N2
Brunauer-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.
