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WWRC 88-26
Chemistry of Aluminum and Iron Minerals in Oil Shales Containing Pyrite


The ability to properly manage acid mine drainage is dependent upon understanding the chemistry of Al and Fe associated with the oxidation of pyritic materials. The objective of this study was to examine the solubility relationships of Al and Fe minerals in oil shales containing pyrite under oxidizing equilibrium conditions. Two eastern oil shales, A New Albany shale (unweathered, 4.6% pyrite) and a Chattanooga shale (wea- thered, 1.5% pyrite), were used in this study. Oil shale samples were equilibrated with distilled-deionized water from 1 to 180 d(l:l solid:solution ratio). The equilibrium solutions were filtered and the clear solutions were analyzed for total cations and anions. Ion activities were calculated from total concentrations. Below pH 6.0, depending upon SO4 activity, Al3+ solubility was controlled by Al(SO4(OH).5H24O for both shales. Initially, Al3+ solubility for the New Albany shale showed equilibrium with amorphous Al(OH)3(s). The pH decreased with time, and Al3+ solubility approached equilibrium with Al(SO4)-(OH).5H2O. Below pH 6.0, Fe3+ solubility appeared to be regulated by a basic iron sulfate solid phase with the stoichlometric composition of FeOHSO4(s). The results from this study further indicate that the acidity, in oil shale waters may be produced from the hydrolysis of Al3+ and Fe3+ in solution.

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