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 sorb ed on X-ray amorphous iron oxyhydroxide precipitates that formed in ac id mine drainage (AMD) (pH = 2.8-3.2) from the Steams Coal Belt in sou theastern Kentucky. The sediments were characterized by high sulfate c oncentrations (similar to 600-1000 mmol/kg) and iron/sulfate ratios th at ranged from 6.6 to 8.6 as determined by dissolution in 10 M hydroch loric acid. The results of a ligand exchange experiment (using separat e preparations of 0-0.25 N sodium nitrate, chloride, phosphate, bicarb onate, and oxalate) indicated that similar to 60-70% of the total sulf ate will be retained in the presence of monovalent ligands. This may b e indicative of a ''bidentate'' bridging mechanism bonding iron and su lfate. Most of this sulfate would likely be stable upon the secondary iron oxyhydroxides associated with acid mine drainage. Sulfate desorpt ion increased directly with pH. At neutral pH, approximately 33-50% of the total sulfate present in these precipitates was released to solut ion. The bicarbonate ion released similar to 60% of the total sulfate from one of the AMD precipitates. These results imply that acid neutra lization methods (e.g., application of crushed limestone) used to stab ilize metals in AMD can have the unwanted effect of raising sulfate co ncentrations within impacted watersheds.