ALUMINUM CHEMISTRY OF ACID LIMESTONE SPRINGS: A NATURAL ANALOG FOR ACID MINE WATER REMEDIATION SYSTEMS

Aluminum hydroxide precipitation is often blamed for the coating of limestone gravel and the clogging of remediation systems at acid mine drainage (AMD) treatment sites. Successive alkalinity producing systems (SAPS), which rely on the vertical percolation of water through layers of organic material and limstone, have particularly high rates of fouling due to aluminum deposition. A natural analog to SAPS systems has been discovered in the form of a series of limestone springs on the western edge of the Cumberland Plateau of Tennessee. These karst springs rise from the Mississippian Monteagle limestone at the base of the Cumberland Plateau Escarpment on the West Fork of the Obey River. They produce acidic (pH~4.5) water and a thick white precipitate.

Characterization of the white precipitate by differential thermal analysis (DTA), x-ray diffraction (XRD), and Fourier transform infrared (FTIR) spectroscopy yielded none of the common characteristics of either aluminum hydroxide or gypsum. The material is x-ray amorphous, and both wet chemical and EDX analysis show a Si/Al molar ratio similar to that of allophane. Magic angle spinning nuclear magnetic resonance (MAS NMR) indicates a composition of primarily 6-coordinate aluminum, but also a distinct 4- and 5- coordinate composition. The FTIR spectra of the white precipitate are similar to spectra in the literature for allophane.

The ultimate source of aluminum is thought to be abandoned mines of the Wilder coal seam, approximately 5-10 km from the springs. Acidic water is transported laterally and vertically substantial distances through a karstic drainage system without neutralization. The structure and composition of the precipitate raises major questions about the design of AMD treatment systems and the roles precipitates play in the geochemistry of the final effluent.