Release of sorbed sulfate from iron oxyhydroxides precipitated from acid mine drainage associated with coal mining

Batch experiments were used to investigate the release of sulfate sorbed on X-ray amorphous iron oxyhydroxide precipitates that formed in acid mine drainage (AMD) (pH = 2.8-3.2) from the Steams Coal Belt in southeastern Kentucky. The sediments were characterized by high sulfate concentrations (similar to 600-1000 mmol/kg) and iron/sulfate ratios that ranged from 6.6 to 8.6 as determined by dissolution in 10 M hydrochloric acid. The results of a ligand exchange experiment (using separate preparations of 0-0.25 N sodium nitrate, chloride, phosphate, bicarbonate, and oxalate) indicated that similar to 60-70% of the total sulfate will be retained in the presence of monovalent ligands. This may be indicative of a ''bidentate'' bridging mechanism bonding iron and sulfate. Most of this sulfate would likely be stable upon the secondary iron oxyhydroxides associated with acid mine drainage. Sulfate desorption increased directly with pH. At neutral pH, approximately 33-50% of the total sulfate present in these precipitates was released to solution. The bicarbonate ion released similar to 60% of the total sulfate from one of the AMD precipitates. These results imply that acid neutralization methods (e.g., application of crushed limestone) used to stabilize metals in AMD can have the unwanted effect of raising sulfate concentrations within impacted watersheds.