WWRC 88-22
Solubility Relationships of Zinc Associated with Acid Mine Drainage
Abstract
Acid mine drainage, produced from the oxidation of iron-sulfides, often contains elevated levels of dissolved Zn.
To predict environmental hazards, it is important to understand the solubility relationships of Zn associated
with acid mine drainage. Two eastern oil shales were leached under aerobic unsaturated environments using
the humidity cell method. This study used a New Albany Shale (unweathered, 4.6% pyrite) and a
Chattanooga Shale (weathered, 1.5% pyrite). The leachates from the humidity cells were filtered, and the
filtrates were analyzed for total concentrations of cations and anions. From the total concentrations, ion
activities were calculated, after correcting for significant solution complexes and ion pairs. The results show
that the pH of the leachates decreased due to oxidation of pyrite. Furthermore, oxidation of pyrite increased
the solubility of Fe3+ and ZN2+ in leachates. Above pH 7.00, the Zn2+
activity was supported byZn2SiO4 (willemite) in equilibrium with measured
H4SiO40 activity. Below pH 5.00, Zn2+
activity for both shales appeared to be saturated by ZnFe2O4 (franklinite)
in equilibrium with measured Fe3+ activity. The results in this study suggest that Zn2+
solubility in acid mine drainage, produced in an aerobic unsaturated environment, may be limited by
ZnFe2O4 solid phase.
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