Computational Water, Energy, and Environmental Engineering, 2013, 2, 50-55
doi:10.4236/cweee.2013.22B009 Published Online April 2013 (
Copyright © 2013 SciRes. CWEEE
Effect of pH and Dissolved Silicate on the Formation of
Surface Passivation Layers for Reducing Pyrite Oxidation
Shengjia Zeng1, Jun Li1, Russell Schumann2, Roger Smart1
1Miner als and Mat er ials Science & Technology, Mawson Institute
2Levay& Co.; University of South Australia, Adelaide, South Australia, Australia
Email: Shengjia.Z eng@unisa.ed u. au
Received 2013
Acid mine drainage (AMD)and toxic metal release generated by oxidation of sulphide minerals, particularly pyrite, in
mine wastes, are a critical environmental issue worldwide. Currently, there are many options to diminish sulphide
oxida- tion including barrier methods that isolate pyrite from oxygen or water, chemical additives and inhibition of
iron-oxidizing bacteria. This stud y foc uses on underst anding t he role that silicate and p H conditions play in the for ma-
tion and stab ilisation of pyrite surface passivation layers found in lab and field studies. The r esults from pyrite dis solu-
tion tests under various conditions showed that the pyrite oxidation rate has been reduced by up to 60% under neutral
pH with additional soluble silicate. Solution speciation calculation predicted that crystalline goethite is formed in the
experiment without silicate additionbutan amorphous iron hydroxide surface layer is stabilized by the addition of the
silicate, inhibiting goethite formation and continuing pyrite oxidation. This coherent, continuous amorphous layer has
been verified in SEM.
Keywords: Acid Mine Drainage; P yrite; Pa ssivation Coa ting; Silica te
1. Introduction
Acid mine drainage (AMD) is a critical global environ-
mental issue from many mine wastes. The discharge of
AMD with associated toxic metal release has caused
acidification of water environment, lea ding to extermina-
tion of aquatic life and compromised water quality for
agriculture and drinking. Sulphide oxidation of sulfidic
mine rals dominantly contributes to most of AMD issues
at min in g s ite s, a nd p yr ite , t h e most abundant sulphide min-
eral in the earth’s crust [1], is widely found in mining
activities. Itsoxidation process under various conditions
has been therefore widely studied to explore effective
approaches and controlling factors to retard the oxidation
process [2-5]. It is known that the oxidant (ferric ion or
dissolved oxygen), water content and catalyst (i.e. iron
oxidizing bacteria) have significant impact on the pyrite
oxidation rate [6]. Reduction of any of these factors can
contribute a remarkable reduction of the oxidation rate.
The oxida nt can be inhibited a t pyrite partic le surfaces by
a surface barrier through the formation of passivation
laye r s [2,3,7-10] and some coating agents, e.g. acetyl
acetone, humic acids, ammonium lignosulfonates, oxalic
acid and sodium silicate, were reported to be employed-
for pyrite surface treatmentbut only minor reductions in
pyrite oxidation rates resulted [1,3,9,11]. Huminicki and
Rimstidt [12] reported that precipitation of iron hydrox-
ide particle reduced oxidant’s diffusion coefficient by
more than five orders of magnitude. Evangelou [8] in-
troduced a more stableiron hydroxide/silica coating gen-
erated via precipitation of an Fe-Si complex. Smart et al.
[13] and Miller et al. [14] found from their long term
column leaching and field studies at the Grasberg mine
Indonesia that silicate may be responsible for stabilising
the iron oxy-hydroxide passivation layer on pyrite sur-
faces. It was also reported from some of fund amental
studies that silicate in hibits the trans formation ofa-
morphous iron hydroxide Fe(OH)3 to crystalline goet hite
(FeOOH) in synthesis processes [4,15,16]. However,
little has been known on the reaction path ways of the
surface coating associated with silicate or the trans for-
mation process of iron hydroxideprecipitated and coating
stability. In this study, pyrite dissolution tests under
various pH conditions in the presence of low concentra-
tions of Na2SiO3(10 and 20 mg/Lsilicate as Si),were per-
fo rme dto investigate the influence of pH and dissolved
silicate on the stabilit y of iron oxyhydroxide s urface lay-
ers formed during oxidative dissolution of pyrite at
circum-neutral pH.
2. Method and Materials
Pyrite(particle size 38 - 75 μm)obtained from Geo Dis-
coveries (NSW, Australia) was leached under conditions
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shown in Table 1 Dissolved silicate, shown as Si con-
centrations from assays, has been chosen based on con-
centrations in effluent of column leach tests. Experiments
were conducted at four different solution pHs (3.5, 4.5,
5.5 and ~8) and with t hree concentr atio ns of disso lved Si
(none added, 10 mg/L and 20 mg/L added as sodium
silicate).Solution samples were periodically taken for Fe,
S and Si concentrations by ICP analysis as well as for
solution Eh and pH measurements. Solution modelling
software, PHREEQC [17], was used to predict the pre-
cipitation of po ssible secondary minerals based on meas-
ured Eh, pH and the dissolved element concentrations.
This is a thermodynamic prediction of precipitates and
solution species and requires experimental verification
where possible. Scanning electron microscopy (SEM)
was int roduced to inves tigate the surface coating.
3. Results and Discussion
3.1. Effect of pH and Silicate
Concentrationonpyrite Oxidation
Figure 1 shows pyrite oxidation rates plotted as a func-
tion of reaction time for each of the different solution pH
and silicate concentrations examined. Oxidat ion rates were
calculated from the total solution concentration (meas-
ured by ICP) in samples removed from the reactions
throughout 16 0 d ays of dis solution. I nitial o xid ation rate s
were between 3 and 4 × 10-10 mol/ m 2/s for all experi-
ments, which decreased to around 1 × 10-10 mol /m2/s
during about 100 days of oxidation for solution pH be-
tween 3.5 and 5.5 irrespective of how much silicate had
been added. At the higher pH (near 8) of the saturated
calcite solutions there was a difference in pyrite oxida-
tion rates as a result of silicate addition. With no silicate
added, pyrite oxidation rates in calcite saturated solution
were about 60%fasterthan those at lower pH after about
40 days, while in the presence of 10 - 20 mg/L Si the
oxidation rates were slower at the higher pH of the cal-
cite saturated solutions after about 40 days. The pyrite
oxidation rate after about 100 days at circum-neutral pH
and with 20 mg/L Si added, was about half of the rate in
similar solutions at lower pH (3.5 - 5.5).
Table 1. Experi mental deta ils o f pyrite dis solution tests.
Object Description
Pyrite 38-75 μm, BET surface area: 0.539 m2/g , usage: 2 g
for each sample
Calcite 38-75 μm, usage: 1.66 g (mole ratio: 1:1 with pyrite),
Solution A 1L of 0.01 m ol/l KCl, No Si addition
Solution B 1L of 0.0 1 m o l/l KCl, 10 mg/LSi (Na2SiO3)
Solution C 1L of 0.0 1 m o l/l KCl, 20mg/LSi (Na2SiO3)
Manually controlled at 3.5, 4. 5 and 5.5
or buffered around8by calcite
1 .E-11
1 .E-10
1 .E-09
050100 150 200
Pyrite Dissolution (mol/m
/s )
Ti me (days)
pH 3.5
A No Si
C 20 mg/L Si
1 .E-11
1 .E-10
1 .E-09
050100 150 200
Pyrite Dissolution (mol/m
/s )
Ti me (days)
pH 4.5
1 .E-11
1 .E-10
1 .E-09
050100 150 200
Pyrite Dissolution (mol/m
/s )
Ti me (days)
pH 5.5
1 .E-11
1 .E-10
1 .E-09
050100 150 200
Pyrite Dissolution (mol/m
/s )
Ti me (days)
pH 8 (cal cit e)
Figure 1 . Pyrite dissolution rate s as function of time at different solution pH and silicate concentrations.
Copyright © 2013 SciRes. CWEEE
The decreasing oxidation rate as a function of time is
indicative of passivating iron oxy-hydroxide layer forma-
tion found in SEM studies of lab and field samples [13,
14]. These resul ts sugge st that a cross the p H range 3 .5 to
5.5, neither the pH nor the concentration of silicate affect
the nature of the iron oxy-hydroxide layer that forms as
oxidation proceeds. However at the higher pH of calcite
saturated solution there appears to be a clear influence of
silicate on the oxidation rate, suggesting that increasing
solution silicate concentration results in surface layers
that are less permeable to oxygen and more stable.
3.2. Change in Solution Fe concentrations
Figure 2 shows solution iron concentrations as a func-
tion of time for all of the pyrite oxidation experiments.
Not unexpectedly, there is a strong correlation between
the solution pH and solution iron concentration, with
higher concentrations in the lower pH solutions. During
the first 10 days of pyrite oxidation at pH 3.5 and 4.5
there was stoichiometric dissolution of iron and sulphur.
After this time, the ratio of iron to sulphur decreased be-
low two, indicative of iron oxy-hydroxide precipitation
[12]. At pH 5.5 and in saturat ed calcite solution the ratio
of iron to sulphur in solut ion was less than two fro m the
start, indicating formation of passivating layers from the
beginning. Despite these differences, as discussed above,
there appears to be no difference in pyrite oxidation rates
with silicate addition excep t for oxidatio n in calcite sat u-
rated solution after about 40 days. While these results
may suggest differences in the amount of precipitated
iron oxy-hydroxide, this does not appear to strongly in-
fluence the oxidation rate in solutions with pH between
3.5 and 5.5.
After about 100 days, there was a small increase in the
concentration of iron in the solutions at pH 3.5 and 4.5.
This corresponds with a slight increase in the oxidation
rate and may suggest some dissolution of the iron oxy-
hydroxide coating at lower pH values [12].
The data sho wn in Figure 2 also indicate a corr elation
between the concentration of iron in solution and that of
silicate. They suggest that iron may be stabilized by
complexation with silicate enhancing its solubility at the
lower pH values. Other investigations on the influence of
silicate on pyrite oxidation have shown that, at higher
silicate concentrations than those used here and at lower
pH, pyrite oxidation is actually enhanced in the presence
of silicate, possibly due to stabilization of Fe (III) by
complexation with silicate [4].
050100 150 200
Fe concentration (mg/L)
Time (day s)
pH 3.5
A No Si
B 10 mg/ L Si
C 20 mg/ L Si
050100 150 200
Fe concentration (mg/L)
Time (day s)
pH 4.5
050100 150 200
Fe concentration (mg/L)
Time (day s)
pH 5.5
050100 150 200
Fe concentration (mg/L)
Time (day s)
pH 8 (calcite)
Figure 2 . Iron conc entration a s function of ti me and pH.
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3.3. Change in Solution Si Concentrations
Figure 3 shows solution Si concentrations as a function
of time for all of the pyrite ox idation experiments except
series A where no silicate was added and silicon concen-
trations were generally non-detectable (<0.05 mg/L).
There appears to be a clear correlation between solution
pH and the concentration of silicon in solution. At pH 3.5
there is little decrease in the concentration of Si during
the 160 days of the experiments. At the circum-neutral
pH conditions of oxidation in calcite saturated solution
there is close to a 50% decrease in the concentration of
silicate, suggesting adsorption and/or precipitation of
silicate with iron oxy-hydroxide. These results suggest
that as the pH increases there is likely to be an increase
in the amount of silicate in the iron oxy-hydroxide coat-
ing passivating the pyrite surface. However, as discussed
above this does not appear to have influenced oxidation
rates except in the case of oxidation at circum-neutr al p H.
It therefore appears that a combination of both relatively
high silicate concentration and neutral to high pH is re-
quir ed to have a si gni fica nt e ffec t on the o xyge n p er mea-
bility of iron oxy-hydroxide layers formed during pyrite
3.4. Speciation Calculations
Speciation calculations for the pyrite dissolution experi-
ments conducted in calcite saturated solutions predict
that goethite is saturated both in the presence and ab-
sence of added silicate (Table 2). In contrast, amorphous
iron hydroxide is predicted to be unsaturated in the ab-
sence of added silicate suggesting that conversion to
goethite via dissolution and re-precipitation is thermo-
dynamica lly li kely. Ho wever , in the solut ion to which 2 0
mg/L silicate (as Si) has been added, speciation calcula-
tions predict that amorphous iron hydroxide is saturated
indicating increased stability of the amorphous phase in
the presence of silicate. These results are consistent with
the observations of silicate -stabilized amorphous iron
hydroxide and retarded the crystallization of goethite
repor te d in literature [4,15,18,19].
3.5. Pyrite Surface Analysis
Scanning electron microscopy (SEM) was applied to
identify whether iron ox-hydroxide coatings formed on
pyrite partic le s which have slowed pyrite oxidation as t he
reaction has proceeded, suggested by the oxidation rate
data and solution analysis. Figure 4 shows SEM images
of pyrite particles sampled from dissolution experiments
conducted in calcite-saturated solution. The top images
display the surface of pyrite after 160 days dissolution in
a solution where no silicate has been added. An iron
ox-hydroxide coating is clearly visible with a need le-like
mor p holo gy sug gestive of a cr ystal line goethite str ucture.
The bottom images show pyrite particles taken after 160
days from a saturated calcite solution to which 20 mg/L
silicate (as Si) had been added. Again an iron
ox-hydroxide coating is obvious on the pyrite surface,
however, with very different morphology to that on py-
rite from the solution in which no silicate had been added.
In this instance the coating appears to be of a more
amorphous nature. The Energy dispersed spectroscopy
(EDS) spectra of the pyrite particles shown in Figure 5
confirms that
Table 2. Saturation indices (SI) of various mineral
phases calc ulated using P HREEQC fo r saturated ca l-
cite solutions from pyrite dissolution wit h and without
added silicate.
Species SI
No added silicate
20 mg/L (as Si)
silicate add ed
Fe (OH)
(amorphous) -0.2 0.5
Goethite 4.9 5.6
050100 150 200
Si con cent r at ion (mg/L)
Time (day s)
20 mg/L Si
C pH 3.5
C pH 4.5
C pH 5.5
C pH 8 cal cit e
050100 150 200
Si con cent r at ion (mg/L)
Time (day s)
10 mg/L Si
B pH 3. 5
B pH 4. 5
B pH 5. 5
B pH 8 cal cit e
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Figure 3 . Si licate c oncentrations as f unction of time and pH .
2 μm
500 nm
5 μm
1 µm
Figure 4. SEM images of pyrite surface coatings after dissolution in saturated calcite solution for 160 days with no added
silicate (top images) and 20 mg/L (as Si ) added silicate (bottom images).
0 1 2 3 4 5
Figure 5. EDS spectra of pyrite surface coatings after dissolution in saturated calcite solution for 160 days with no added
silicate (left) and 20 mg/L (as Si ) added silicate (right).
both coatings contain Fe, S and O. Si is only found in the
less crystalline coating (amorphous-like coating) formed
in the presence silicate. A number of studies examining
the transformation from ferrihydrite (an amorphous
phase) to goethite (a crystal phase) suggest that the addi-
tion of soluble si licate inhibits the cr ystallizatio n process
[4,15, 16,19]. Our experimental results are consistent
with these s tudies indicating that in solut ions with added
silicate, the conversion of the initially formed amorphous
iron hydroxide is i nhibite d, while in the silic ate free so lu-
tion transformation to goethite is more pronounced. It
appears that the presence of silicate inhibits transforma-
tion of amorphous iron oxy-hydroxide to a more crystal-
line goethite-like phase. The former phase appears to be
less pe r- meable to oxygen and therefore pyrite oxidation
is reduced more at circum-neutral pH when the solution
concentratio n of sil ic a te is higher.
4. Conclusions
Based on the results from the pyrite disso lution test s and
the surface analyses, a possible mechanism for the for-
mation and stabilization of iron oxy-hydroxide layers on
pyrite during oxidation at pH > 5.5 was proposed. In the
presence of dissolved silicate an amorphous layer with
low oxygen permeability may form via:
Ferrous ion is oxidized into ferric ion by dissolved
Ferric ion hydrolyses forming colloidal ferric hy-
droxide. Colloidal particles are aggregated onto the
pyrite surface via adsorption eventually re sulting in
complete surface coverage;
Silicate is ab so rbed into the collo idal iron hydr oxide
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particles displacing hydroxyl groups and forming a
Si-O-Fe bond during the formation of colloid par-
A layer with low oxygen permeability is established
which reduces oxidation rates significantly. The
transformation to goethite via dissolution and
re-precipitation is re ta r ded.
5. Acknowledgements
This research has been funded by an Australian Postgra-
duate Award Industr y (APAI) through an Australian Re-
search Council Linkage Project Grant with AMIRA In-
ternational (Mr Gray Bailey). Sponsors of the Savage
River Rehab ilitation Pro ject (SRRP), Hidden Valley Ser-
vices, BHP Billiton Iron Ore, Rio Tinto Ltd. and Teck
Ltd. to support the Project are gratefully acknowledged.
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