WWRC 88-15
Solubility Relationships of Aluminum and Iron Minerals Associated with Acid Mine Drainage
Abstract
The ability to properly manage the oxidation of pyritic minerals and associated acid mine drainage is dependent upon
understanding the chemistry of the disposal environment. One accepted disposal method is placing pyritic-containing
materials in the groundwater environment. the objective of this study was to examine the solubility relationships of Al
and Fe minerals associated with pyritic waste disposed in a low leaching aerobic saturated environment. Two eastern
oil shales were used in this oxidizing equilibration study, a New Albany Shale (unweathered, 4.6 percent pyrite), and a
Chattanooga Shale (weathered, 1.5 percent pyrite). Oil shale samples were equilibrated with distilled-deionized water from
1 to 180 d with a 1:1 solid-to-solution ratio. The suspensions were filtered and the clear filtrates were analyzed for total
cations and anions. Ion activities were calculated from total concentrations. Below pH 6.0, depending upon SO42-
activity, Al3+ solubility was controlled by AlOHSO4 (solid phase) for both shales.
Initially, Al3+ solubility for the New Albany Shale showed equilibrium with amorphous Al(OH)3.
The pH decreased with time, and Al3+ solubility approached equilibrium with AlOHSO4(s).
Below pH 6.0, Fe3+ solubility appeared to be regulated by a basic iron sulfate solid phase with the
stoichiometric composition of FeOHSO4(s). The results of this study indicate that below pH 6.0, Al3+
and Fe3+ solubilities, are limited by basic Al and Fe sulfate solid phases (AlOHSO4(s) and
FeOHSO4(s)). The results from this study further indicate that the acidity in oil shale waters is produced
from the hydrolysis of Al3+ and Fe3+ activities in solution. These results indicate a
fundamental change in the stoichiometric equations used to predict acidity from iron sulfide oxidation. The results of this
study also indicate that water quality predictions associated with acid mine drainage can be based on fundamental
thermodynamic relationships. As a result, waste management decisions can be based on waste-specific/site-specific
test methods.
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