Speciation of iron and sulfate in acid waters: Aqueous clusters to mineral precipitates

Acid mine drainage (AMD) contaminates surface water bodies, groundwater, soils, and sediments at innumerable locations around the world. AMD usually originates by weathering of pyrite (FeS2) and is rich in Fe and sulfate. In this study, we investigated speciation of Fe-II, Fe-III, and SO4 in acid waters by Fourier transform infrared and X-ray absorption spectroscopy. The molalities of sulfate (15 mmol/kg) and iron (10, 20, and 50 mmol/kg), and pH (1, 2, and 3) were chosen to mimic the concentration of ions in AMD waters. Sulfate and Fell either associate in outer-sphere complexes or do not associate at all. In contrast, sulfate interacts strongly with Fe-III. The predominating species in Fe-III-SO4 solutions are hydrogen-bonded complexes; inner-sphere complexes account only for 10 +/- 10% of the total sulfate. Our results show that the mode of interaction between Fe-III and sulfate is similar in aqueous phase and in nanocrystalline precipitate schwertmannite (similar toFeO(OH)(3/4)(SO4)(1/8)). Because of this similarity, schwertmannite should be the phase that controls solubility and availability of Fe-III, SO4, and indirectly also other components in the AMD solutions.