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Applied and Environmental Microbiology, December 1999, p. 5285-5292, Vol. 65, No. 12
0099-2240/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Leaching of Zinc Sulfide by Thiobacillus
ferrooxidans: Bacterial Oxidation of the Sulfur Product Layer
Increases the Rate of Zinc Sulfide Dissolution at High Concentrations
of Ferrous Ions
T. A.
Fowler and
F. K.
Crundwell*
Billiton Centre for Bioprocess Modelling,
University of the Witwatersrand, Johannesburg, South Africa
Received 17 June 1999/Accepted 14 September 1999
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ABSTRACT |
This paper reports the results of leaching experiments conducted
with and without Thiobacillus ferrooxidans at the same
conditions in solution. The extent of leaching of ZnS with bacteria is
significantly higher than that without bacteria at high concentrations
of ferrous ions. A porous layer of elemental sulfur is present on the
surfaces of the chemically leached particles, while no sulfur is
present on the surfaces of the bacterially leached particles. The
analysis of the data using the shrinking-core model shows that the
chemical leaching of ZnS is limited by the diffusion of ferrous ions
through the sulfur product layer at high concentrations of ferrous
ions. The analysis of the data shows that diffusion through the product layer does not limit the rate of dissolution when bacteria are present.
This suggests that the action of T. ferrooxidans in
oxidizing the sulfur formed on the particle surface is to remove the
barrier to diffusion by ferrous ions.
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INTRODUCTION |
Thiobacillus ferrooxidans
is the microorganism that is primarily associated with the oxidation of
sulfide minerals. The bacterial interaction with sulfide minerals is a
significant factor in the formation of acid mine drainage, and large
amounts of effort are invested in the remediation of sites of acid mine
drainage (9). On the other hand, this process has been
exploited in the extraction of gold, nickel, copper, and cobalt from
sulfide ores (8).
The dissolution of zinc sulfide (sphalerite) by ferric sulfate has been
well studied. Ferric ions oxidize the sphalerite to form zinc and
ferrous ions in solution and elemental sulfur (5-7, 25,
26). This reaction is illustrated as follows:
|
(1)
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The ferrous ions formed in the leaching reaction can be oxidized
to ferric ions by T. ferrooxidans in the following reaction:
|
(2)
|
Because T. ferrooxidans accelerates the rate of the
oxidation of ferrous ions, given by equation 2, by a factor of
106 (19), the presence of bacteria can
significantly increase the rate of the overall leaching process. In
addition to the above reactions, T. ferrooxidans is capable
of oxidizing the sulfur formed in the dissolution reaction by the
following reaction:
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(3)
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In the set of reactions represented by equations 1 to 3, the role
of the bacteria is to oxidize the products of the dissolution reaction,
that is, the ferrous ions and the sulfur. However, Silverman and
Ehrlich (17) proposed in 1964 that T. ferrooxidans enhances the rate of oxidation of sulfide minerals
above that achieved by a chemical reaction with ferric ions at the same
conditions. They proposed a mechanism of "direct oxidative attack on
metal sulfide minerals independent of the action of the ferric
sulfate" (17). This mechanism was envisaged as
contributing to the overall leaching process by the following reaction:
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(4)
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The distinguishing feature between the two mechanisms is the
substitution of ferric ions as the oxidant at the surface of the zinc
sulfide by an unidentified biological process. Thus, Silverman and
Ehrlich (17) proposed that the rate of dissolution of the
sulfide mineral with bacteria is greater than that without bacteria at
the same conditions in solution. However, there has been no research
work that has directly confirmed that bacteria increase the rate of
leaching above that achieved by chemical leaching at the same solution
conditions. As a result, the debate concerning these mechanisms has not
been settled (2).
In addition, the interpretation of the data from previously reported
experiments on bacterial leaching has proved difficult. This is because
in all previous work the concentrations of ferric and ferrous ions
varied considerably during the experiment (1, 3, 4, 27).
This has made it difficult to compare data from experiments with and
without bacteria at the same solution conditions.
However, by performing experiments in which the solution concentrations
are maintained at a set value, we are able to compare directly the
extents of leaching with and without bacteria. We previously described
an experimental apparatus in which the concentrations of ferrous and
ferric ions are controlled at the initial value for the duration of the
experiment (10, 11). This apparatus is a two-compartment
electrolytic cell, and the redox potential in the compartment in which
leaching occurs is controlled by manipulating the electrolytic current.
This apparatus is shown in Fig. 1.
Previous experiments using this apparatus showed that the rate of
growth of T. ferrooxidans is unaffected by the small
electrolytic current (<2 A) (11).
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FIG. 1.
Schematic diagram of the experimental apparatus. The
working compartment is that in which leaching experiments are
conducted. The flow of current is regulated by adjusting the variable
resistor so that the redox potential in the working compartment remains
at the setpoint value.
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In this way, we can directly test the proposal of Silverman and Ehrlich
(17). In this paper, we report results on the leaching of
zinc sulfide with and without T. ferrooxidans at high
concentrations of ferrous ions.
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MATERIALS AND METHODS |
Apparatus.
The electrolysis cell was made of Plexiglas and
was divided into two sections by an anion-exchange membrane (Sybron
Chemicals Inc., Birmingham, N.J.). The electrolysis cell was fitted
with a Plexiglas lid to minimize evaporation of the solution. The
working volume of the cell was 2 liters. The contents of the working
compartment were stirred by a three-bladed impeller driven by an
overhead motor, and the compartment was sparged with air. Electrodes to measure the redox potential and the concentration of oxygen were suspended in the solution of the working compartment.
Redox potential measurements were made with a platinum electrode and an
Ag-AgCl reference electrode by using a high-impedance
galvanically
isolated differential amplifier and an analog-to-digital
control card
(type PC30; Eagle Technology, Cape Town, South Africa)
and were
recorded by computer. A computer program determined the
values of the
output signals from the PC30 card to the relay switch
and to the
variable resistor that varied the direction and magnitude
of the
current. The current was measured by determining the potential
difference across a precision
resistor.
The redox potential was controlled within 0.1% of the setpoint for the
period of the experiment. All the solution samples
were analyzed for
ferrous ions in order to confirm that the control
of the redox
potential maintained the concentration of ferrous
ions at a constant
value. Typical results showed that the concentration
of the ferrous
ions differed from the initial value by less than
0.8%.
The pH of the solution in the working compartment was 1.6. The pH was
measured throughout each experiment and remained within
0.05 pH unit of
the initial value. The concentration of dissolved
oxygen was measured
(Hanna Instruments) and was maintained manually
at a value of 5.9 mg/liter. The electrolysis cell was placed in
a water bath, and the
temperature was maintained at 35 ± 0.1°C.
Bacterial culture.
A pure strain of T. ferrooxidans (strain FC1) was used. This organism, which was
supplied by D. Rawlings, University of Cape Town, Cape Town, South
Africa, has been thoroughly characterized (16). The bacteria
were cultured on a medium which contained (per liter) 1.5 g of
(NH4)2SO4, 0.5 g of
K2HPO4, 0.5 g of MgSO4 · 7H2O, and 45 g of FeSO4 · 7H2O (18). The pH of the medium was adjusted to
1.6 by adding H2SO4. The bacteria were
maintained in the exponential growth phase by subculturing one-third of
the culture volume on a daily basis.
Preparation and characterization of the ore.
The sphalerite
concentrate, which was from the Gamsberg deposit, was supplied by Gold
Fields Research Laboratories. This sample was milled and wet screened
to a size fraction of between 53 and 45 µm. The ore contained 53.3%
Zn, 7.84% Fe, 32.5% S, 1.19% Mn, and 0.24% Pb. The metallic element
contents were determined by analyzing the solution by atomic adsorption
spectrophotometery with a Varian Spectra AA30 spectrophotometer after
acid digestion. The sulfur content was determined with a LECO model
SC32 DB64 sulfur determinator. X-ray diffraction at a scan rate of
0.05°/s indicated that the sample contained 98% sphalerite, which
supported the conclusion that the iron was present in solid solution in ZnS rather than as a separate mineral (6). The ore sample
was washed with a 0.5 M solution of sodium sulfide to remove flotation agents and to sulfidize the mineral surface (5, 12, 20, 22).
Analytical techniques.
The number of bacterial cells in a
solution was determined by counting with a hemacytometer (depth, 0.1 mm; area, 0.0025 mm2). The cells were stained by using
crystal violet in a citric acid solution as the stain. The standard
deviation for the cell number determinations was 1.2% of the mean (10 replicates).
The concentration of ferrous ion in solution was determined by
titration with potassium dichromate, with sodium diphenylamine
sulfonate as the indicator (
28). The standard deviation for
the determinations of ferrous ion concentrations of was 1.1% of
the
mean (10 replicates). The total concentration of iron in solution
was
determined by using the titration for ferrous ions once the
iron had
been reduced to the ferrous state with stannous chloride.
The
concentration of ferric ions was calculated by determining
the
difference. It must be emphasized that the measurement of
the redox
potential was used for control purposes only and not
for chemical
analysis.
The concentration of zinc in solution was determined by atomic
adsorption spectrophotometery with the Varian Spectra AA30
spectrophotometer. The standard deviation for the determinations
of the
concentration of zinc in solution was 0.3% of the mean
(10
replicates).
Procedure.
All experiments were conducted in the same medium
but with different ferrous ion concentrations. The concentration of
ferric ions was 1 g/liter for all experiments. The loading
concentration of solids was 5 g of sphalerite per liter. The
bacterial leaching experiment preparations were inoculated with a
number of T. ferrooxidans cells equivalent to 10% (by
volume) of the reactor size. Samples were withdrawn from the working
compartment at the same time intervals for every leaching experiment performed.
Part of the sample was used to determine the bacterial cell population
in suspension. The remaining part of each sample was
immediately
filtered by using a Millipore filter (Sterifil aseptic
system with a
sterile, individually sealed, 0.45-µm-pore-size
filter membrane). The
filtrate was used to determine the concentrations
of ferrous and ferric
ions and the concentration of zinc ions
in solution. The solid residues
on the filter membrane were immersed
in a 95% solution of ethanol to
fix the bacteria (in bacterial
leaching experiments) on the surfaces of
the mineral particles.
These preparations were dried at the critical
point and coated
for investigation with a scanning electron microscope
(SEM). Samples
were withdrawn at regular intervals from the counter
compartment.
These samples were also analyzed to determine the
concentration
of zinc ions in solution. Analyses revealed that no zinc
was transferred
from the working compartment to the counter compartment
through
the anion-exchange
membrane.
The solution samples obtained from the sterile (chemical leaching)
experiments were checked for contamination by
T. ferrooxidans.
No bacteria were detected by a microscopic
investigation. Between
successive experiments, the electrolysis cell
was soaked in hydrochloric
acid, rinsed with water, cleaned with an
ammonia-based cleaning
solution (pH 10), and finally rinsed with
distilled
water.
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RESULTS |
The results for the leaching of sphalerite with and without
bacteria in the controlled redox potential apparatus are shown in Fig.
2. These experiments were conducted at
various concentrations of ferrous ions. The concentration of ferric
ions was the same for each experiment. The concentrations of ferrous
and ferric ions in solution were constant to within 1% of their
initial concentrations for the duration of the experiment. Because the
solution conditions were controlled in the leaching experiments, the
only difference between these two sets of experiments is the presence
or absence of bacteria.
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FIG. 2.
Effect of the concentration of ferrous ions in solution
on the bacterial and chemical leaching of sphalerite. (A) Results in
the presence of 10% (vol/vol) T. ferrooxidans inoculum; (B)
results in absence of bacteria (i.e., chemical leaching). The solution
conditions were as follows: Fe3+ concentration, 1.0 g/liter; density of solids, 5 g/liter; temperature, 35°C; pH, 1.6;
O2 concentration, 5.9 mg/liter.
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It is clear from the experiments that an increase in the concentration
of ferrous ions resulted in a decrease in the rate of leaching. It is
also apparent that the extent of leaching is higher in the presence of bacteria.
A comparison of the amounts of zinc dissolved with and without bacteria
is shown in Fig. 3. This figure shows
that the extent of leaching with bacteria was the same as that without
bacteria during the early stages of the reaction, for the first 8 to
12 h. At longer reaction times, the rate of dissolution of
sphalerite with bacteria was higher than that without bacteria. It is
clear from Fig. 3 that the extent to which the bacteria enhance the extent of dissolution is dependent on the concentration of ferrous ions. The bacteria have a greater effect on the extent of dissolution at higher concentrations of ferrous ions. Because all other conditions were controlled, an increase in the extent of dissolution in the presence of bacteria indicates a contribution to the dissolution of the
mineral by the bacteria beyond their ability to oxidize ferrous ions.
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FIG. 3.
Comparison of the amounts of zinc dissolved from
sphalerite with and without bacteria at various concentrations of
ferrous ions. The data is that presented in Fig. 2. The experiments
with and without bacteria were conducted at constant solution
conditions throughout each experiment. The solution conditions are
given in the legend for Fig. 2.
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The concentration of bacterial cells in solution during the experiment
increased typically from 3 × 108 to 11 × 108 cells/ml. These measurements underestimate the total
number of cells in the leaching reactor because of cells attached to
the zinc sulfide particles.
The currents that are applied to the electrolysis cell in order to
maintain conditions are a direct measure of the rates of the processes
occurring in the reactor. These currents can be used to estimate the
bacterial growth rates in the reactor. Figure 4 shows the currents that were passed
through the electrolytic cell in these experiments. Positive currents
represent the electrolytic oxidation of ferrous ions, and negative
currents represent the electrolytic reduction of ferric ions. Thus,
positive currents indicate that the leaching reaction is dominant,
since ferrous ions, which are the product of the leaching reaction,
must be oxidized to maintain the solution conditions. Negative
currents, on the other hand, mean that the bacterial oxidation of
ferrous ions is dominant, since ferric ions, which are the product of the bacterial reaction, must be reduced to maintain the solution conditions. In the chemical leaching experiment, the current is a
measure of the rate of leaching by ferric ions. In the bacterial leaching experiment, the current is the difference between the rates of
leaching by ferric ions and the rate of oxidation of ferrous ions by
bacteria. Thus, subtracting the current for the chemical leaching
experiment from that for the bacterial experiment gives the rate of
bacterial oxidation of ferrous ions.
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FIG. 4.
Current data required to maintain the redox potential at
the initial value. Solution conditions are given in the legend for Fig.
2. A 10% (vol/vol) T. ferrooxidans inoculum was used.
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Since the logarithm of the current passed through the electrolysis cell
during the experiments with bacteria is a straight line with time, the
rate of bacterial oxidation increases exponentially with time during
these experiments. Estimates of the bacterial growth rate from this
data and the yield coefficient give doubling times of between 6.1 and
9.6 h. These values compare favorably with the doubling times of
between 8.8 and 10.3 h obtained from batch growth studies of the
same sample of T. ferrooxidans with ferrous ions as the
energy source. Therefore, estimates of the bacterial growth from the
cell numbers in solution are moderate in comparison with those obtained
by calculation from the electrolysis current. This indicates that
substantial growth of cells attached to the mineral surface occurs.
The bacteria attached to the mineral particles were examined by SEM.
The SEM analysis indicated that the bacteria attached rapidly to the
mineral particles, forming large amounts of exopolymer. Typical results
are shown in Fig. 5 and
6. SEM photographs of bacterially treated
and chemically treated particles obtained without critical-point drying
are shown in Fig. 5A and B. The attached bacteria and exopolymer are
dehydrated in Fig. 5A due to the sample preparation. The dried remains
of the exopolymer material present on the surfaces of the bacterially
leached particles are apparent in Fig. 5A.
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FIG. 5.
SEM photographs of bacterially and chemically leached
zinc sulfide after 100 h. (A) Surface of bacterially leached zinc
sulfide. Note that the biofilm has been dehydrated due to the SEM
preparation technique. (B) Surface of chemically leached zinc sulfide.
Note the presence of porous sulfur on the surface. (C) EDAX scan of the
surface in panel A showing that the surface is fresh zinc sulfide. (D)
EDAX scan of the surface in panel B showing that the surface is coated
with sulfur. Conditions: Fe3+ concentration, 1.0 g/liter;
Fe2+ concentration, 4.0 g/L; density of solids, 5 g/liter;
temperature, 35°C; pH, 1.6; O2 concentration, 5.9 mg/liter.
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FIG. 6.
Comparison of bacterial and chemical surfaces after
69 h of leaching. A SEM preparation technique was used to minimize
sample dehydration. (A) Surface of bacterially leached sample,
indicating large amounts of attachment of rod-shaped T. ferrooxidans. (B) Surface of chemically leached sample, indicating
the presence of sulfur, as seen in Fig. 5B. Conditions:
Fe3+ concentration, 1.0 g/liter; Fe2+
concentration, 8.0 g/liter; density of solids, 5 g/liter; temperature,
35°C; pH, 1.6; O2 concentration, 5.9 mg/liter.
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A comparison of Fig. 5A and B indicates that the mineral surface of the
bacterially leached sample is clear, while that of the chemically
leached sample is covered with a porous layer of sulfur. The EDAX
analysis of the surfaces is given in Fig. 5C and D. The EDAX analysis
confirmed that sulfur is present as a reaction product on the surface
of the chemically leached sample and that the surface of the
bacterially leached sample is clear of sulfur coating. This suggests
that T. ferrooxidans oxidized the sulfur formed on the
surface of the sphalerite to sulfate (15, 21, 24).
Figure 6 and shows typical SEM results for bacterially and chemically
leached surfaces obtained with critical-point drying. The surface of
the bacterially leached sample, shown in Fig. 6A, indicates the
presence of large quantities of attached bacteria. The bacteria appear
to be embedded in large quantities of exopolymer material. (This
material is not sulfur, since it is dehydrated in the SEM preparation
process without critical-point drying. In addition, the bacteria cannot
be embedded in the mineral surface, because no indentations caused by
the bacteria were detected in the SEM study in which the samples were
dehydrated, shown in Fig. 5A.) The surface of the chemically leached
sample is similar to that of the sample shown in Fig. 5B, showing the
presence of a thick sulfur layer.
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DISCUSSION |
The results shown in Fig. 3 differ from our previous finding that
the bacteria do not enhance the rate of dissolution of sphalerite (10). The results given in Fig. 3 were obtained at high
concentrations of ferrous ions, while those reported previously were
obtained at low concentrations of ferrous ions. However, it is clear
from Fig. 5 that the chemically leached particles are coated with the porous sulfur that is formed during dissolution. This sulfur is not
present on the particle surface in the leaching experiments with
bacteria. Thus, in interpreting the data, we need to account for the
following observations: (i) T. ferrooxidans does not enhance the rate of dissolution at low concentrations of ferrous ions (10), (ii) T. ferrooxidans does enhance the rate
of dissolution at high concentrations of ferrous ions, (iii) the extent
to which T. ferrooxidans enhances the rate of dissolution at
high concentrations of ferrous ions is dependent on the concentration
of ferrous ions, (iv) at low reaction times (less than 12 h), the
rates of chemical and bacterial leaching are similar, and (v) T. ferrooxidans removes the sulfur product from the particle surface.
In order to build an understanding of these observations, the chemical
dissolution of zinc sulfide, given by equation 1, was examined first,
and then the leaching of zinc sulfide in the presence of bacteria was examined.
Analysis of the chemical leaching of zinc sulfide.
The
chemical leaching of sphalerite by ferric ions has been studied in
detail (5-7, 12). This is a heterogeneous reaction in which
the size of the unreacted sphalerite diminishes with reaction time. As
the sphalerite dissolves, a porous product layer of sulfur forms on the
core of unreacted sphalerite. This is shown schematically in Fig.
7. It is well known that the formation of product layers on the surfaces of particles may limit the rate of
reaction by hindering the diffusion of soluble reactants or products
through this layer (13, 23), and it is known that this is
true for the chemical leaching of sphalerite by ferric ions (5-7,
12). The processes that govern the rate of dissolution are the
transport of ferric, ferrous, and zinc ions through the porous sulfur
layer and the intrinsic reaction, equation 1, at the surface of
unreacted sphalerite.
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FIG. 7.
Schematic diagram of a reacting sphalerite particle. (A)
Chemical leaching mechanism, showing the formation of a porous layer of
sulfur on the core of unreacted sphalerite. The reaction given by
equation 1 occurs at the surface of the unreacted core of zinc sulfide.
Diffusion of soluble reactant and products through the sulfur may
control the rate of dissolution under some conditions. (B) Leaching
mechanism in the presence of bacteria. T. ferrooxidans
oxidizes sulfur, removing any barrier to diffusion.
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The theory of heterogeneous reactions describes the decrease in
particle size based on the geometry of the particles and the
rate
processes contributing to the overall reaction (
13,
23).
Two
limiting forms of this unreacted shrinking-core theory are
used here to
analyze the results given in Fig.
2. These are the
case in which the
reaction at the sphalerite surface controls
the overall rate of
dissolution and the case in which diffusion
through the porous product
layer of sulfur controls the overall
rate of
dissolution.
If the reaction at the surface of the unreacted core of sphalerite
controls the overall rate of dissolution, then the extent
of reaction,
X, is given by (
13,
23):
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(5)
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where
rZnS is the intrinsic rate of
dissolution of sphalerite (in moles per meter squared per hour),
d is the initial particle
size (48.8 × 10
6 m),
ZnS is the molar density (42,098 mol/m
3), and
t is the reaction time (in hours).
The extent of reaction,
or conversion, is the amount of sphalerite
dissolved divided by
the total amount of sphalerite initially in the
reactor. This
equation is valid only if the concentrations of reactants
in solution
are constant and the particles are of uniform initial size.
The
controlled leaching experiments reported here meet these
criteria.
Equation
5 indicates that if the surface reaction controls the overall
rate of dissolution, then a plot of 1
(1
X)
1/3 against
t should be a straight
line through the origin with slope
proportional to
rZnS. Plots of 1
(1
X)
1/3 versus time are shown in Fig.
8B for the experimental results
reported
here for the chemical leaching of zinc sulfide at various
concentrations of ferrous ions. This figure shows that the plot
is
linear only for the first 8 to 12 h of the reaction time. After
this time, the data deviates from the linear relationship expected
from
equation 5. This result suggests that the reaction at the
surface
controls the overall rate of dissolution only in the early
stages of
the reaction. As the thickness of the sulfur product
layer increases,
the diffusion resistance caused by the sulfur
product layer plays an
increasingly important role. This deviation
from equation 5 because of
product layer diffusion is a well-known
phenomenon in the literature on
the chemical leaching of zinc
sulfide (
5,
6,
12).
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FIG. 8.
Plot of 1 (1 X)1/3 versus time for the bacterial and chemical
leaching data. (A) Plot of the bacterial leaching data from Fig. 2A,
confirming that the shrinking-particle model with surface reaction
control describes the leaching of sphalerite in the presence of
bacteria at high concentrations of ferrous ions. (B) Plot of chemical
leaching data from Fig. 2B showing that the shrinking-particle model
with surface reaction control does not describe the leaching of
sphalerite in the absence of bacteria at high concentrations of ferrous
ions, except for the first 8 to 10 h.
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If the diffusion of ferric and ferrous ions through the porous sulfur
layer controls the overall rate of chemical leaching,
then
X
is described by the following equation (
13,
23):
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(6)
|
where
K is the equilibrium constant,
De is the effective diffusion coefficient in the
porous product layer (in meters squared
per hour), and
[Fe
3+] and [Fe
2+] are the concentrations of
ferric and ferrous ions (in moles
per cubic meter), respectively.
Equation
6 is valid only if the
concentrations of reactants in solution
are constant and the particles
are of uniform initial size. The
controlled leaching experiments
described here meet these
criteria.
Equation
6 indicates that if diffusion through the sulfur layer
controls the overall rate of dissolution, then a plot of 1
3(1
X)
2/3 + 2(1
X) versus time should be a straight line. The plots of
1
3(1
X)
2/3 + 2(1
X) versus time for the chemical leaching results, shown
in
Fig.
9, are linear after about 10 h.
This result indicates
that the controlling mechanism in the chemical
leaching of sphalerite
changes from surface reaction control to product
layer diffusion
as the thickness of the sulfur product layer increases.
The slopes
of the lines in Fig.
9 are linearly proportional to the
concentration
of ferrous ions with a slope of
1, in accordance with
equation
6. This indicates that ferrous ions provide the resistance to
diffusion.
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FIG. 9.
Plot of 1 3(1 X)2/3 + 2(1 X) versus
time showing that the shrinking-core model with diffusion through the
product layer as the rate-controlling step describes the leaching of
sphalerite at high concentrations of ferrous ions in the absence of
bacteria after about 10 h of reaction time. X is
calculated from the data presented in Fig. 2B.
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If this mechanism is correct, then the chemical dissolution of zinc
sulfide should not be affected by the diffusion of ferrous
ions through
the product layer at low concentrations of ferrous
ions. In this case,
the chemical leaching results should be described
by equation 5 over
the entire leaching period. Plots of 1
(1
X)
1/3 versus time are shown in Fig.
10B for the experimental results
reported previously at a low concentration of ferrous ions
(
10).
These plots are straight lines through the origin,
with no deviation,
even at high conversions. This indicates that the
chemical dissolution
of sphalerite at low concentrations of ferrous
ions is controlled
by the intrinsic reaction at the sphalerite surface.
Under these
conditions, diffusion through the product layer does not
play
a role in controlling the overall rate of reaction in the chemical
leaching of sphalerite.
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FIG. 10.
Plot of 1 (1 X)1/3 versus time showing that the
shrinking-particle model with surface reaction control describes the
leaching of sphalerite at high concentrations of ferric ions both in
the presence and absence of bacteria. The original data was presented
in Fig. 4 of our previous study (10). (A) Bacterial leaching
of zinc sulfide. (B) Chemical leaching of zinc sulfide. The solution
conditions were as follows: Fe2+ concentration, 1.0 g/liter; density of solids, 5 g/liter; temperature, 35°C; pH, 1.6;
O2 concentration, 5.9 mg/liter.
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From this analysis, it is clear that the chemical dissolution of zinc
sulfide is controlled by two linked processes: reaction
at the surface
of the zinc sulfide and diffusion of ferrous ions
through a product
layer of sulfur present on the surface of the
zinc sulfide. It must be
emphasized that the species whose rate
of transport is limited by the
formation of the sulfur layer is
ferrous ions. At low concentrations of
ferrous ions in the bulk
solution, the product layer diffusion does not
influence the rate
of the overall process. At high concentrations of
ferrous ions,
there is a change in the controlling mechanism. In the
initial
stages of the reaction, the intrinsic chemical reaction at the
surface controls the rate of dissolution. However, with time a
porous
sulfur product accumulates on the particle surface, and
this hinders
the rate of diffusion of ferrous ions from the reaction
surface.
Eventually, this becomes the controlling
mechanism.
Analysis of the bacterial leaching of zinc sulfide.
The
leaching of zinc sulfide in the presence of T. ferrooxidans
was examined in the light of the understanding of the chemical dissolution of zinc sulfide. SEM photographs and the EDAX analysis indicate that there is no sulfur present on the surface of the zinc
sulfide leached in the presence of T. ferrooxidans. This is
because T. ferrooxidans oxidizes the sulfur formed at the
surface. From this observation, it is expected that T. ferrooxidans will enhance the rate of leaching above that of
chemical leaching only when diffusion through the sulfur layer controls
the rate of dissolution. It was shown above that product layer
diffusion controls the rate of chemical leaching when the ferrous ion
concentration is higher than 1 g/liter, and only once the sulfur layer
is of sufficient thickness to provide a barrier to diffusion.
Therefore, if the contribution of T. ferrooxidans is to
oxidize sulfur, the rate of dissolution of zinc sulfide will be
enhanced only when the ferrous ion concentration is above 1 g/liter and
once the sulfur product layer is sufficiently thick for it to control
the rate of chemical leaching. These are exactly the observations made in points (i) to (v) listed at the beginning of Discussion.
During the first 8 to 12 h of chemical leaching, the sulfur
product is not sufficiently thick to hinder the rate of reaction
and
the reaction at the surface controls the overall rate of dissolution.
Removal of the sulfur product would have no effect on the rate
of
chemical leaching during this initial period. This accounts
for the
observation that
T. ferrooxidans does not enhance the
rate
during the first 12 h. After this period, the sulfur layer
hinders
the rate of dissolution of the chemical leaching reaction,
and since
the bacteria remove the sulfur, the bacterial leaching
reaction occurs
at a greater rate than the chemical reaction.
This explains the results
shown in Fig.
3.
The conclusions that the sulfur hinders the rate of chemical leaching
after 12 h and that it is removed by
T. ferrooxidans in
the experiments with bacteria indicate that the ferrous ion-grown
T. ferrooxidans shows very little lag phase for growth on
sulfur.
If the only contribution of
T. ferrooxidans in these
experiments is to oxidize the sulfur formed in accordance with equation
1, then the rate of dissolution of sphalerite in the presence
of
T. ferrooxidans should be described by equation 5 for all
sets
of experiments. Plots of 1
(1
X)
1/3 versus time are shown in Fig.
8A and
10A
for the experimental
results at various concentrations with
T. ferrooxidans reported
here. These plots are straight lines through
the origin, indicating
that the dissolution of sphalerite in the
presence of
T. ferrooxidans at high concentrations of
ferrous ions is controlled by the reaction
at the sphalerite surface.
This further confirms that the reason
for the difference between the
rates of chemical and bacterial
leaching shown in Fig.
3 is the removal
of the sulfur product
from the surface of the zinc
sulfide.
The rates of reaction,
rZnS, may be calculated
from the slopes of the lines in Fig.
10. The effect of the
concentration of
ferric ions on the rate of reaction is expressed in
terms of the
order of reaction. The orders of reaction for ferric ions
with
and without bacteria are 0.47 and 0.50, respectively. This result
is consistent with results previously reported (
5-7,
12,
25,
26). In addition, these orders of reaction are consistent with
the electrochemical theory of dissolution, which predicts values
between 0.4 and 0.6 (
14). The orders of reaction with
respect
to the ferrous ions are obtained from the data presented in
Fig.
8. The orders of reaction with and without bacteria are
0.41
and
0.40, respectively. Again, this result is consistent with
the
electrochemical theory, which predicts values of between
0.4
and
0.6 (
14).
Thus, it is clear from this analysis of the orders of reaction that
T. ferrooxidans does not affect the mechanism of the
intrinsic
reaction at the zinc sulfide surface. This provides
additional
evidence for the view that the role of
T. ferrooxidans in enhancing
the rate of dissolution of zinc sulfide
is the removal of sulfur
from the particle
surface.
Thus, all of the observations are accounted for by the model of
dissolution suggested by the above analysis. This model may
be
summarized as follows: (i) at low concentrations of ferrous
ions, the
rate of dissolution of sphalerite is controlled by the
reaction at the
mineral surface, (ii) at high concentrations of
ferrous ions in the
bulk solution, the rate of chemical leaching
of sphalerite is limited
by the diffusion of ferrous ions through
a porous sulfur layer, and
(iii)
T. ferrooxidans enhances the
rate of leaching above
that achieved without bacteria when diffusion
through the sulfur layer
is limiting by removing the sulfur
layer.
In conclusion, we have shown that at high concentrations of ferrous
ions, the rate of dissolution of zinc sulfide is higher
in the presence
of
T. ferrooxidans than it is in the absence of
T. ferrooxidans. We showed that the particle surface of chemically
leached zinc sulfide is covered in sulfur, while sulfur is not
present
on the bacterially leached zinc sulfide. We have argued
that the
chemical leaching of zinc sulfide is controlled by two
mechanisms,
reaction at the surface and diffusion of ferrous ions
through a growing
layer of sulfur on the surface of the zinc sulfide.
We have shown that
T. ferrooxidans enhances the rate of leaching
of sphalerite
at conditions under which chemical leaching is controlled
by diffusion
through the sulfur layer. Thus, the observations
and the analysis of
the data lead to the conclusion that the bacteria
enhance the rate of
dissolution of sphalerite only under conditions
in which diffusion
through the sulfur layer controls the overall
rate of chemical
leaching. This is achieved by the oxidation of
the sulfur by the
bacteria.
| |
ACKNOWLEDGMENTS |
We thank Billiton Process Research and the Foundation for
Research Development for funding this project.
We also thank D. Rawlings (University of Cape Town) and E. Lawson
(University of the Witwatersrand, Johannesburg) for supplying the
bacterial culture and their valuable assistance.
| |
FOOTNOTES |
*
Corresponding author. Mailing address: Billiton Process
Research, Private Bag X10014, Randburg 2125, South Africa. Phone: 27 11 792-7090. Fax: 27 11 792-7097. E-mail:
fcrundwell{at}billiton.co.za.
| |
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Applied and Environmental Microbiology, December 1999, p. 5285-5292, Vol. 65, No. 12
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[Abstract]
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Top
Abstract
Introduction
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