doi:10.1016/j.gca.2004.04.012
Pyrite dissolution in acidic media
M. DESCOSTES,* P. VITORGE, and C. BEAUCAIRE
†
CEA, DEN Saclay, DPC/SECR/L3MR, CEN, F-91191 Gif-sur-Yvette, France
(Received July 28, 2003; accepted in revised form April 8, 2004)
Abstract—Oxidation of pyrite in aqueous solutions in contact with air (oxygen 20%) was studied at 25°C
using short-term batch experiments. Fe
2+
and SO
4
2-
were the only dissolved Fe and S species detected
in these solutions. After a short period, R = [S]
tot
/[Fe]
tot
stabilized from 1.25 at pH = 1.5 to 1.6 at pH
= 3. These R values were found to be consistent with previously published measurements (as calculated
from the raw published data). This corresponds to a nonstoichiometric dissolution (R 2) resulting from
a deficit in aqueous sulfur. Thermodynamics indicate that S(-I) oxidation can only produce S
(s)
0
and
SO
4
2-
under these equilibrium conditions. However, Pourbaix diagrams assuming the absence of SO
4
2-
indicate that S
2
O
3
2-
and S
4
O
6
2-
can appear in these conditions. Using these species the simplest expected
oxidation mechanism is
FeS
2s
+ 1.5O
2
→ Fe
2+
+ S
2
O
3
2-
followed by
S
2
O
3
2-
+ 1.2H
+
→ 0.4S
s
0
+ 0.4S
4
O
6
2-
+ 0.6H
2
O,
and finally
S
4
O
6
2-
+ 3.5O
2
+ 3H
2
O → 4SO
4
2-
+ 6H
+
possibly in several steps
The overall reaction is
FeS
2
+ 2.9O
2
+ 0.6H
2
O → Fe
2+
+ 0.4S
s
0
+ 1.6SO
4
2-
+ 1.2H
+
,
consistent with R = 1.6. In the most acidic (pH = 1.5) conditions, SO
2
formation is expected as an
intermediate step in the oxidation of S
4
O
6
2-
to SO
4
2-
. Degassing of SO
2(g)
would result in R 1.6, again
consistent with experimental observations. The above multistep mechanism, based on known aqueous redox
chemistry of sulfur species, accounts for the deficit in aqueous sulfur noticed in all published experimental
observations. The intermediate species cannot be detected, and it is consistent with calculated concentrations
being below the detection limits. Under nonacidic conditions, S
2
O
3
2-
can be detected, but evaluation of the
dissolution mechanism is hindered by precipitation of Fe(III) as iron oxyhydroxides. Copyright © 2004
Elsevier Ltd
1. INTRODUCTION
Pyrite (FeS
2
) is one of the major minerals on Earth, partic-
ipating in the sulfur and iron global cycles. Pyrite is known as
a redox buffer in anoxic conditions (Beaucaire et al., 2000), and
therefore as a redox sink for sulfur and iron, since its solubility
is very low (Berner, 1984). Its presence, synonymous of reduc-
ing conditions, is used as an indicator for uranium and other
metal hydrothermal ores in geochemical exploration (Rich et
al., 1977). The surface reactivity of pyrite is often discussed in
the context of the origin of life (McClendon, 1999; Wächter-
shäuser, 2000), sorption of precious metals such as Au and Ag
(Scaini et al., 1997), and pyrite is also mentioned in relation to
solar energy devices (Ennaoui et al., 1986). Finally, pyrite
oxidation by oxygen or another oxidant, according to
FeS
2
+ 7 ∕ 2O
2
+ H
2
O → Fe
2+
+ 2SO
4
2-
+ 2H
+
(1)
leads to the release of two moles of H
+
per mole of oxidized
pyrite. Acidification can be further enhanced by the oxidation
of iron according to
FeS
2
+ 15 ∕ 4O
2
+ 7 ∕ 2H
2
O → FeOH
3s
+ 2SO
4
2-
+ 4H
+
,
(2)
and ferric iron, produced in reaction 2, is also known as a
strong oxidant of pyrite in strongly acidic conditions (Garrels
and Thomson, 1960; Singer and Stumm, 1970; Moses et al.,
1987). This oxidation autocatalysis can be written
FeS
2
+ 14Fe
3+
+ 8H
2
O → 15Fe
2+
+ 2SO
4
2-
+ 16H
+
. (3)
Reactions (1–3) described processes producing acid mine
* Author to whom correspondence should be addressed
(michael.descostes@cea.fr).
†
Present address: IRSN/DPRE/SERGD, F-92265 Fontenay-aux-Roses,
France
Pergamon
Geochimica et Cosmochimica Acta, Vol. 68, No. 22, pp. 4559-4569, 2004
Copyright © 2004 Elsevier Ltd
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