According to Frontier molecular orbital (FMO) theory, the
surface-exposed sulfur atom of pyrite possesses an
unshared electron pair which produces a slightly
negatively
charged pyrite surface that can attract cations such as
Fe2+. Because of surface electroneutrality and pH
considerations, however, the pyrite surface Fe2+
coordinates
OH. We proposed that this surface Fe2+ OH when in
the
presence of CO2 is converted to −FeCO3 or
−FeHCO3,
depending on pH. In this study, using Fourier
transform
infrared spectroscopy (FT-IR) we demonstrated that such
complexes form on the surface of pyrite and continue to
persist even after a significant fraction of the surface
Fe2+ was oxidized to Fe3+. FT-IR
spectra also showed the
presence of two carbonyl absorption bands (1682 and
1653 cm-1) on the surface of pyrite upon
exposure to CO2
which suggested that pyrite surface carbonate complexes
existed in two different surface chemical environments,
pointing out two potential mechanisms of pyrite surface−CO2 interactions. One potential mechanism involved
formation
of a pyrite surface−Fe(II)HCO3 complex, whereas
a second
potential mechanism involved formation of a pyrite
surface−carboxylic acid group complex
[−Fe(II)SSCOOFe(II)]. We hypothesized that these pyrite
surface−CO2
complexes could promote abiotic oxidation of pyrite by
accelerating the abiotic oxidation of Fe2+. Iron
(III) would
oxidize the disulfide (−S2) by accepting its
electrons.
Using a miscible displacement technique, oxidation of
FeS2
with H2O2 was carried out in the absence or
presence
of 10 or 100 mM NaHCO3. The data show that 100
mM
NaHCO3 significantly increased the oxidation rate of
FeS2.
Furthermore, the data show that FeS2 oxidation
kinetics were
more dependent on HCO3
- but were less
dependent on
H2O2 for the range of
HCO3
- and H2O2
concentrations tested.
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