WWRC 88-13
Iron Sulfide Oxidation and the Chemistry of Acid Generation
Abstract
Acid mine drainage, produced from the oxidation of iron sulfides,
often contains elevated levels of dissolved aluminum (Al),
iron (Fe), and sulfate (S)4) and low
pH. Understanding the interactions of these elements associated
with acid mine drainage is necessary for proper solid
waste management planning. Two eastern oil shales were
leached using humidity cell methods. This study used a New
Albany Shale (4.6 percent pyrite) and a Chattanooga Shale
(1.5 percent pyrite). The leachates from the humidity cells
were filtered, and the filtrates were analyzed for total
concentrations of cations and anions. After correcting for
significant solution species and complexes, ion activities were calculated
from total concentrations. The results show that the
activities of Fe3+, Fe2+, Al3+, and SO42- increased due to the
oxidation of pyrite. Furthermore, the oxidation of pyrite resulted
in a decreased pH and an increased pe + pH
(redox-potential). The Fe3+ and Fe2+ activities appeared to
be controlled by amorphous Fe(OH)3 solid phase above a
pH of 6.0 and below pe + pH 11.0. The Fe3+, Fe2+, and
SO42- activities reached saturation with respect to FeOHSO4
solid phase between pH 3.0 and 6.0 and below pe + pH
11.0. Below a pH of 3.0 and above ape + pH of. 11.0, Fe2+,
Fe3+, and SO42- activities are supported by FeSO4.7H2O
solid phase. Above a pH of 6.0, the Al3+ activity showed an
equilibrium with amorphous Al(OH)3 solid phase. Below pH
6.0, Al3+ and S042- activities are regulated by the AlOHSO4
solid phase, irrespective of pe + pH. The results of this
study suggest that under oxidizing conditions with low to high
leaching potential, activities of Al and Fe can be predicted on
the basis of secondary mineral formation over a wide range
of pH and redox. As a result, the long-term chemistry associated
with disposal environments can be largely predicted
(including trace elements).
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