GEOLOGICA CARPATHICA, 53, 2, BRATISLAVA, APRIL 2002
79 — 85
REACTIONS BETWEEN Cr(VI) SOLUTIONS AND PYRITE:
CHEMICAL AND SURFACE STUDIES
EMANUELE BENINCASA
1
, MARIA FRANCA BRIGATTI
1*
, GIANCARLO FRANCHINI
3
,
DANIELE MALFERRARI
1
, LUCA MEDICI
2
, LUCIANO POPPI
1
and MASSIMO TONELLI
4
1
Dipartimento di Scienze della Terra Dell’Università di Modena e Reggio Emilia, Modena, Italy
2
IRA-CNR, Tito Scalo, Potenza, Italy
3
Dipartimento di Chimica dell’Università di Modena e Reggio Emilia, Modena, Italy
4
Centro Interdipartimentale Grandi Strumenti dell’Università di Modena e Reggio Emilia, Modena, Italy
(Manuscript received October 4, 2001; accepted in revised form December 13, 2001)
Abstract: Geochemical processes that result in the reduction of hexavalent chromium in natural waters with pyrite
(FeS
2
) have been studied at varying degrees of pH (from 1.2 to 12.6) and solution concentration (from 0.001 to 0.3 M of
Cr(VI)) in order to demonstrate the differences in the proportions of elements between the aqueous and solid phases and
to infer mechanisms that limit the Cr(VI) concentration in pore-waters in iron sulphide-rich environments. The experi-
ments were carried out in the absence of oxygen and on pyrite grains previously treated to remove any oxide or sulphur
layer at the mineral surface. The methods used to characterize reacting solutions and mineral surface comprised: chemi-
cal analyses (microprobe analyses and inductively coupled plasma analyses), scanning electron microscopy, atomic
force microscopy and X-ray single crystal analysis. The results suggest that: 1) mineral dissolution increases with de-
creasing pH, whereas it is close to zero at pH > 7; at alkaline pH, the Cr(VI) reduction is very low and the decrease in total
Cr probably indicates the formation of precipitated phases, like FeCrO
4
, on the pyrite surface; 2) Cr(VI) reduction is
significant at pH < 2.3. Cr(VI) to Cr(III) reduction involves the oxidation of Fe(II) and S
2—
on the pyrite surfaces,
following the reaction 2 FeS
2
+ 5 Cr
2
O
2—
+ 32 H
+
→
2 Fe(OH)
3
+ 4 (SO
4
)
2—
+ 10 Cr
3+
+ 13 H
2
O; 3) at acidic pH all the pyrite
crystals show a variable Cr content on the surfaces.
Key words: chemical and surface analyses, redox reactions, pH, Cr(VI)
solutions, pyrite crystals.
corrensite) depends on the Fe(II) mineral content and is more
sensitive to pH than to temperature (Brigatti et al. 2000a). Fur-
thermore, corrensite, which features both a high Fe(II)/Fe(III)
ratio and a good exchange capacity, adsorbs the greatest
amount of reduced Cr(III). X-ray absorption spectrometry
studies (XAS) indicate changes at the Fe-K edge after the
treatments (i.e., the decrease in Fe(II)/Fe(III) ratio) confirming
that the mineral structures of both chlorite and corrensite are
involved in the reactions (Brigatti et al. 2000b).
Fe-rich clays are commonly associated with Fe(II)-rich ox-
ides and sulphides, which can participate to Cr(VI) reduction.
Among them pyrite (FeS
2
) is quite common. Owing to the
high Fe(II) and S
2—
contents, pyrite in soils can be a potential
reducing agent for hexavalent chromium species in aqueous
solutions, accounting for the overall equation:
2 FeS
2
+ 5 Cr
2
O
2—
+ 32 H
+
→
2 Fe(OH)
3
+ 4 (SO
4
)
2—
+ 10 Cr
3+
+ 13 H
2
O
(1)
which promotes the reduction of Cr(VI). Several authors have
studied reactions between metals in solution and sulphide min-
erals to assess: 1. their potential use in the treatment of metal-
contaminated wastewaters deriving from the mineral extrac-
tion industry (Brown et al. 1979; Jean & Bancroft 1986;
Zouboulis et al. 1992); 2. their role in the removal of metals
from hydrothermal fluids to form ore deposits (Brookins 1976;
Jean & Bancroft 1985); 3. their importance in governing the
transport and reduction of transition metals in soils of contami-
Introduction
Chromium can exist in several valence states, with trivalent
Cr(III) and hexavalent Cr(VI) being the most common. These
two oxidation states exhibit different chemical, biological, and
environmental properties (Felter & Dourson 1997). Cr(III),
which, at low concentrations, is considered to be a nutrient for
humans (Felter & Dourson 1997), is thermodynamically stable
in the environment over the pH range of most natural ground-
waters, as a sorbed surface complex or in a solid phase
(Anderson 1994; Gan et al. 1996). Cr(VI), on the other hand, a
well known toxic contaminant (Xu et al. 1996), is stable as
chromate (CrO
2—
) or dichromate (Cr
2
O
2—
) anionic forms
(Felter & Dourson 1997). Thus, reactions that reduce Cr(VI)
to Cr(III) are important because they can promote the transi-
tion of a toxic, mobile element into a less toxic, immobile
form.
The association of Fe(II)-bearing minerals (Fe-rich oxides,
ferroan phlogopite and some clay minerals such as chlorite
and corrensite) with Cr(VI) reduction has been documented by
several authors. In particular, they have found that: i) at low
pH, Fe(II) structurally linked to mineral structures is a stronger
reducing agent than aqueous Fe(II) (White & Peterson 1996);
ii) mixed-valence Cr(III)/(VI) effluents are reduced to Cr(III)
when magnetite is present, whereas mixed valence Cr(III)/(VI)
adsorbates or precipitated phases occur in soils without Fe(II)
phases (White & Peterson 1996; Peterson et al. 1997); iii) the
rate of reduction of Cr(VI) by Fe-rich phyllosilicates (chlorite,
*
Maria Franca Brigatti, Department of Earth Sciences, University of Modena and Reggio Emilia Via S. Eufemia 19, 41100 Modena,
Italy; Telephone number: +39-0592055805; Fax number: +39-0592055887; E-mail: brigatti@unimo.it
MECC ‘01
2
7
4
7
2
7
80 BENINCASA et al.
nated industrial waste landfill (Loyaux-Lawniczak et al.
2001).
However, few data describing the interaction between pyrite
and Cr(VI) solution at different degrees of Cr(VI) concentra-
tion and pH are available. The present work increases the
knowledge of the Cr(VI) to Cr(III) reduction by pyrite, in the
absence of oxygen, at different Cr(VI) solution concentrations
and in the pH range from 1.2 to 12.6, describing the modifica-
tion of the pyrite surface after each treatment, the precipitated
phases on pyrite surfaces, the mineral structure before and af-
ter treatments.
To achieve these aims we reacted pyrite crystals with solu-
tions containing Cr(VI) in closed Teflon reactors and studied
products (solution and pyrite) by a variety of methods, includ-
ing chemical analyses, scanning electron microscopy, atomic
force microscopy and X-ray single crystal analysis.
Materials and methods
Crystals of pyrite from Elba Island, part of the mineral col-
lection of the Department of Earth Sciences, Modena and Reg-
gio Emilia University, were selected for the experiments. Hand-
picked crystals were ground to a grain size of about 0.15 mm
×
0.15 mm
×
0.05 mm in a nitrogen atmosphere to avoid Fe(II)
oxidation on the surface. The mineral sample was ultrasonical-
ly treated in ethanol for 30 min to free any small adhering par-
ticles. To remove any oxide layer that may have been formed
on the mineral’s surface in air, the mineral samples were treat-
ed in a 5% solution of HCl for several hours. Finally, the sam-
ples were put in carbon disulphide for several hours, dried and
directly put in the Teflon reactors under a dry nitrogen atmo-
sphere. This procedure was applied because several previous
investigators had demonstrated that surface oxidation takes
place as soon the surface comes into contact with oxygen or
air (Raichur et al. 2000) and also to remove any elemental sul-
phur already present on the mineral surface (McGuire et al.
2001).
Untreated crystals were chemically and structurally charac-
terized. The chemical composition of some natural crystals
was determined using: 1) wavelength dispersive microprobe
analyses (EPMA) performed with an ARL-SEMQ instrument
(operating conditions: 20 kV accelerating voltage, 20 nA sam-
ple current and electron beam of about 3
µ
m spot size); 2) in-
ductively coupled plasma Atomic-Emission Spectrometry
(ICP-AES, Varian Liberty 200). ICP analysis was performed
after a microwave digestion of 200 mg of sample with a
HNO
3
/HF mixture in closed Teflon crucibles. Structural deter-
mination was obtained on a crystal (0.10
×
0.15
×
0.04 mm)
mounted on a Siemens P4P (Siemens 1993) rotating-anode
single crystal diffractometer (graphite-monochromatized
MoK
α
radiation, operating at 52 kV and 140 mA). Crystal
structure refinement was performed using the SHELX-97 pro-
gram (Sheldrick 1997) on selected (I/
σ
> 3) reflections.
To summarize, untreated pyrite features: 1) an Fe/S ratio of
0.87 in weight (0.50 in moles), which is very close to the sto-
ichiometric value; 2) space group Pa3; 3) unit cell parameters
a = b = c = 5.4170(3)
×
10
—1
nm; 4) atomic positions and the re-
fined bond distances (Fe-S = 2.2634(3)
×
10
—1
nm; S-S =
2.1623(8)
×
10
—1
nm) very close to those found by Fuji et al.
(1986). We obtained the agreement factor at the end of three
cycles of anisotropic refinement on 245 reflections obtained
by averaging the collected 920 reflections for the half sphere
(i.e. considering P1 symmetry, sometimes indicated for pyrite,
Bayliss 1977) was R = 0.026.
The Cr(VI) treating solutions were prepared using CrO
3
and
K
2
Cr
2
O
7
. The pH was adjusted by the addition of H
2
SO
4
and
NaOH solutions. Analytical-grade reagents were added in
deionized water and then the solutions were filtered through a
0.2-
µ
m membrane filter prior to use. These solutions were
mixed to obtain the Cr(VI) concentrations and pH values re-
ported in Table 1. The amount of mineral to be used in each
experiment, together with 100 ml of each solution, was 3.6 g.
All suspensions were further purged with nitrogen for ~30 min
and then the reactors were closed, protected from light, and
shaken continuously at room temperature for 42 days. Details
for each experiment are reported in Table 1. The experimental
work mainly concerns the behaviour of the acid systems in re-
sponse to slight variations in both pH and Cr(VI) concentra-
tions. According to previous reports (Brigatti et al. 2000a,b),
Cr(VI) to Cr(III) reduction in the presence of Fe(II) in solution
is enhanced at pH < 3.
At the end of the reaction time (t = 42 days) the solid and the
supernatant solution were immediately separated by centrifu-
gation and analyzed for total Cr and Fe, and for Cr(VI). For
each supernatant, total Cr and Fe were determined by ICP-
AES. To evaluate Cr(VI)—Cr(III) reduction, the Cr(VI) content
of the solution was determined by the lead nitrate method
Experiment
number
Cr(VI) M
t=0
pH
t=0
Cr(VI)
t=0
Cr
total (t=42 days)
Cr(III)
t = 42 days
Fe
total (t=42 days)
pH
t = 42 days
1
0.01
2.12
520
435.6
21.3
86.2
2.18
2
0.1
1.36
5200
4499.0
152.7
128.6
1.57
3
0.03
2.05
1560
1320.0
38.5
98.5
2.18
4
0.3
1.23
15600
14100.8
556
166.4
1.59
5
0.02
2.26
1040
839.3
28.3
72.3
2.40
6
0.2
1.29
10400
9168.6
453.8
147.1
1.57
7
0.001
7.16
52
43.9
0.6
0
7.13
8
0.01
7.47
520
420.5
1.8
0
7.44
9
0.01
10.13
520
399.5
2.2
0
9.90
10
0.001
12.56
52
26.7
0.1
0.31
12.12
Table 1: Number of the experiments, Cr(VI) solution concentration (M) used in each experiment, pH values of starting solution (pH
t=0
), Cr(VI)
(ppm) content in the starting solution (Cr(VI)
t=0
), total Cr (ppm) (Cr
total (t=42 days)
), Cr(III)(ppm) (Cr(III)
(t=42 days)
), total Fe (ppm) (Fe
total (t=42 days)
)
and pH values determined on the final solution (after 42 days of each treatment).
REACTIONS BETWEEN Cr(VI) SOLUTIONS AND PYRITE 81
0
2
4
6
8
10
12
Fe
tot
al
,
t
=
42
day
s
(pp
m
)
0
40
80
120
160
0
100
200
300
400
(SO
4
)
2-
t
=
42
da
y
s
(pp
m
)
pH
a
b
Figure 2
(APHA 1985). The SO
2—
amount in solution was analyzed by
ion chromatography, as described by Violante et al. (1991).
Calculations for Cr(VI) speciation at 0.1 M solution concentra-
tion were carried out using the MINTEQA2 program (Allison
et al. 1991).
The pyrite crystals were studied before and after the experi-
ments using a Scanning Electron Microscopy (SEM, Philips
XL 40/604) device for their morphological features. The chem-
ical composition of precipitates and treated-pyrite surfaces was
obtained by an energy dispersive X-ray spectrometer (EDS,
EDAX equipped with SEM). Mineral precipitates were identi-
fied by X-ray powder diffraction, using a Debye-Sherrer cam-
era and CuK
α
radiation. Owing to their small size, precipitated
phases were gently removed from several pyrite crystals from
the same experiment using an optical microscope. In some cas-
es, the variety of precipitates prevented phase identification.
Single crystal X-ray data collection and structure refine-
ments were carried out on a treated pyrite crystal obtained
from experiment 4 (0.3 M Cr(VI) solution concentration, pH =
1.23) to test structural variation after the experiments, follow-
ing the same analytical methods adopted for untreated crystals.
Atomic scale images of the crystal surface’s microtopography
were made on a Park Autoprobe CP Atomic Force Microscopy
(AFM) in contact-mode using standard silicon nitride tips. Py-
rite crystals were mounted on sample stubs with colloidal car-
bon suspended in alcohol and analyzed at room temperature.
All AFM images were recorded in height mode, so that the
quantitative measurement of the surface relief was possible. In
the configuration used, AFM has angström-scale both for verti-
cal and horizontal resolution. All images were collected at a
variety of scan speeds and angles to reduce the possibility of
image artefacts being created.
Results and discussion
The MINTEQA2 simulation results are shown in Fig. 1. At
lower pH (0 < pH < 6) Cr
2
O
2—
, which is involved in the redox
reaction (1), prevails in the solution. As the solution’s pH in-
creases, MINTEQA2 predicts that Cr
2
O
2—
is substituted by
CrO
2—
, species that promotes the precipitation of mixed Cr(VI)
and Fe(II) phases. The most likely forms are FeCrO
4
, FeO
Fig. 1. Relevant Cr(VI) species vs. pH determined using the MINT-
EQA2 program (concentration of the Cr(VI) solution: 0.1 M).
100
x
[Cr
6+
t
=
0
-
(Cr
6+
+
Cr
3+
)
t
=
42
day
s
]
/
Cr
6+
t
=
0
10
20
30
40
50
60
pH
0
2
4
6
8
10
12
100
x
Cr
3+
t
=
42
day
s
/
Cr
6+
t
=
0
0.0
1.0
2.0
3.0
4.0
a
b
Figure 3
Fig. 3. a) Difference (%) between the Cr amount present in start-
ing solution (t = 0) and the Cr amount at the end of the experi-
ments (t = 42 days) vs. starting pH values; b) percentage of
Cr(VI) to Cr(III) reduction at t = 42 days vs. starting pH values.
(wustite), Fe
2+
Fe
3+
O
4
(magnetite). Precipitated sulphur can oc-
cur over the whole pH range.
Analyses of starting and final solution composition indicat-
ed moderate changes for experiments 7 and 8, carried out in
REACTIONS BETWEEN Cr(VI) SOLUTIONS AND PYRITE
Fig. 2. a) Total (SO
4
)
2—
in solution at the end of each experiment
(t = 42 days) vs. the pH values of the starting solution; b) total Fe in
solution at the end of each experiment (t = 42 days) vs. the pH val-
ues of the starting solution.
4
7
7
4
2
82 BENINCASA et al.
In aqueous acid conditions Cr(VI) reduction can be ex-
plained by the following chemical processes related both to
iron and sulphur oxidation:
6 Fe
2+
+ Cr
2
O
2—
+ 14 H
+
→
6 Fe
3+
+ 2 Cr
3+
+ 7 H
2
O (3)
and
3 H
2
S + Cr
2
O
2—
+ H
2
O
→
3 S
0
+ 2 Cr
3+
+ OH
—
(4).
S
0
can further contribute to Cr(VI) reduction following the re-
action:
S
0
+ Cr
2
O
2—
+ 5 H
2
O
→
(SO
4
)
2—
+ 2 Cr
3+
+ 6 (OH)
—
(5).
Fe(II) oxidation on pyrite surfaces in acid aqueous media
depends on the reaction:
FeS
2
+ 3.75 O
2
+ 2.5 H
2
O
→
2 (SO
4
)
2—
+ FeOOH + 4 H
+
(6)
which produces ferric iron. Although, at low pH values,
Fe(III) is considered a much more efficient oxidant for pyrite
than oxygen (McKibben & Barnes 1986), the oxidative effi-
ciency of the Cr(VI)/Cr(III) couple is higher than that of the
Fe(III)/Fe(II) couple. Thus, we can assume that Cr(VI) to
Cr(III) reduction proceeds and we can also justify the presence
Fig. 4. SEM images and semiquantitative EDS-EDAX spectra for the pyrite crystals at the end of the experiments. a) Experiment 1; the
spectrum refers to the grain at the center of the image. b) Experiment 6; the spectrum refers to elongated crystal. c) Experiment 8; the
spectrum refers to the microcrystalline precipitate on the pyrite surface. d) Experiment 10; the spectrum refers to white aggregates.
nearly neutral environments, whereas significant changes were
detected for experiments in acid environments. In particular:
1) acid systems show a small increase in pH value, whereas
basic systems behave in an opposite manner (Table 1); 2) min-
eral dissolution increases with decreasing pH whereas it is
close to zero in neutral and basic environments (Figs. 2a and
2b); 3) the decrease in final total Cr, mostly in neutral and ba-
sic environments, probably indicates the formation of precipi-
tated phases on the pyrite surface (Fig. 3a); 4) the Cr(VI) re-
duction is very low in neutral and basic environments, whereas
it is significant at pH < 2.3 (Fig. 3b); 5) iron and sulphur on
pyrite surfaces may be chemically oxidized during Cr(VI) to
Cr(III) reduction following the reaction (1).
However, to elucidate the mechanisms involved in pyrite—
chromium-bearing aqueous solution systems, the mineral-wa-
ter interface and the pathways of Fe and S oxidation must be
considered in detail. The pyrite surface shows metal hydroxyl
groups and thiol groups which protonate and deprotonate de-
pending on pH changes (Park & Huang 1987; Dzombak &
Morel 1990; Herbert et al. 2000):
Fe
2+
S + H
2
O
→
Fe
2+
S H
2
O
→
Fe
2+
OHSH
.
(2).
Thus, the pH of the solution not only drives the main redox
processes, but also affects the features at the mineral-water in-
terface.
7
7
7
REACTIONS BETWEEN Cr(VI) SOLUTIONS AND PYRITE 83
Fig. 5. Morphology of pyrite surface obtained by AFM; a) natural
pyrite crystal; b) and c) pyrite surface after the acid treatment with
0.3 M Cr(VI) solution at pH = 1.23 (Experiment 4); d—e) pyrite
surface after the acid treatment with 0.02 M Cr(VI) solution at pH
= 2.26 (Experiment 5); f) pyrite surface after the treatment with
0.01 M Cr(VI) solution at pH = 7.47 (Experiment 8); g) pyrite sur-
face after the basic treatment with 0.001 M Cr(VI) solution at pH
= 12.56 (Experiment 10).
84 BENINCASA et al.
of a considerable amount of Fe(III) in solution. However,
Fe(III) can reduce Fe(II) availability and it can therefore com-
pete with Cr(VI) to promote Cr(VI)—Cr(III) reduction. At more
basic pH, Cr(VI)—Cr(III) reduction is inhibited and the forma-
tion of Fe oxide coating on the grains may stop the pyrite oxi-
dizing as well.
Reactions between the mineral surface and reduced Cr in so-
lution (Cr(III)) may occur as: 1) precipitation reactions at the
mineral-water interface to form pure Cr(III) or Fe(III)-phases
or, most likely, solid—solution precipitates:
(1—x) Fe
3+
+ x Cr
3+
+ 3 H
2
O
→
(Fe
1—x
Cr
x
)(OH)
3
+ 3 H
+
(7)
2) chemical adsorption involving surface functional groups
and relatively stable inner sphere complexes in solution:
Fe
3+
OH + Cr
3+
(OH)
→
Fe
3+
OCr
3+
(OH) + H
+
(8)
(where Fe
3+
represents structural iron on mineral surfaces,
whereas Cr
3+
represents reduced chromium in solution); 3)
electrostatic or physical adsorption involving charged hydrat-
ed ions in solution and oppositely charged mineral surfaces
(Parks 1990; Davis & Kent 1990). All these mechanisms ac-
count for the decrease in total Cr content during the experi-
ments; however, the complexity of the system prevents its in-
terpretation by general and stoichiometric equations.
SEM/EDS and X-ray analysis of the reacted crystals re-
vealed that the distribution of precipitates depends on the pH
of the treatment solution. The precipitated phases and their
chemical composition are shown in Figures 4a—4d. At acidic
pH (pH < 2.3), small sulphur crystals were found on the min-
eral surface (Fig. 4a), while at neutral or slightly basic pH, Cr-
Fe mixed phases were found. They can be well crystallized
(Fig. 4b) or microcrystalline (Fig. 4c). X-ray powder patterns
of microcrystalline deposits indicate a mixture of sulphur and
FeCrO
4
. At basic pH (pH = 12.56), precipitated FeO (wustite)
and Fe
2+
Fe
3+
O
4
(magnetite) cover the mineral surface almost
completely (Fig. 4d).
Figure 5 displays the AFM morphological images for unre-
acted and reacted pyrite surfaces. Here only seven selected im-
ages are presented from a larger series. Figure 5a shows the
image of the unreacted pyrite surface which presents the typi-
cal streaking. At acid pH (pH = 1.23 and pH = 1.36; Figs. 5b—
c), no precipitation was observed on the mineral surfaces. At
slightly higher pH (Figs. 5d—e), it is evident that reaction oc-
curs at the mineral-water interface surfaces. The size of the re-
action products increases with increasing pH. The aggregate
morphological features increase in height from ten nm to one
hundred nm as the pH increases from 7.16 to 12.56. Figures 5f
and 5g present the precipitate images at pH 10.13 and 12.56.
The quite different morphological features of the precipitates
account for the differences in their structure and chemistry.
The reaction with the Cr(VI)-bearing solution seems to af-
fect only the mineral surface. The structural refinement on a
treated crystal does not show any structural variation. These
results point to the involvement of just the mineral surface in
the reaction process. Thus, to regenerate the mineral’s reaction
capability it has to be returned to its untreated condition.
Concluding remarks
The concentration of Cr(VI) in water can be limited by min-
eral solubility controls or by adsorption on the mineral’s sur-
face. In all acid experiments, traces of adsorbed Cr were found
on the pyrite surface, and both the reduction and dissolution of
Cr(VI) were enhanced at acid pH. The abundance of iron ox-
ides that cover the pyrite surface in basic environments limits
Cr(VI) to Cr(III) reduction, as does basic pH itself. Unlike
iron-rich phyllosilicates, such as corrensite, which, after re-
duction, sorb Cr(III) in structural sites (Brigatti et al. 2000a),
the mechanisms involving pyrite can mostly be ascribed to
mineral dissolution and the reaction of dissolved iron with
Cr(VI). However, because Fe(II)-bearing sulphides can reduce
Cr(VI), they may be useful in reducing Cr toxicity in contami-
nated environments.
Pyrite reveals a low efficiency in Cr(VI) to Cr(III) reduction
respect to magnetite and ilmenite (White & Peterson 1996).
At the end of the experiments, Cr(III) amount is always lower
than 4.5 % of the total Cr starting amount (Table 1) and the
Cr(VI) reduction is constrained in acidic environments.
Acknowledgments: The Italian MURST and CNR supported
our research program.
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