TRACE ELEMENT DISTRIBUTION IN PRECIPITATES FORMED AT VARIOUS PH VALUES: GREEN VALLEY COAL MINE, INDIANA

This study evaluates the distribution of trace elements in precipitates formed from acid mine drainage (AMD) adjusted to various pH levels. AMD was collected from the Green Valley abandoned coal mine site in western Indiana from three locations (upper, mid-, and lower sections) in a constructed channel lined with carbonate rip-rap. Effluent pH at the sampling sites was 3.8, 3.5, and 3.4, respectively. The AMD was transported on ice to the laboratory and initially treated with hydrogen peroxide to oxidize ferrous Fe to ferric Fe. Sodium hydroxide was then titrated into the AMD solutions from each site to incrementally increase pH to induce precipitation. Samples were centrifuged to separate precipitates and aggregated into 1 pH increments. Dried, powered precipitate samples were analyzed using a hand-held X-ray florescence (XRF) analyzer. Elements detected above the analytical detection limit in order of abundance were S>Fe>Al>Ca>Mg>Mn>Si>Cl>Zn>K>Cr>Ba>Cu>W>Ti>V>Te>Cs>Co>Sc>Nb>Sn>Bi>Sr>Ni>Cd>Mo>Sb>Zr>Rb>Pb>U. Data indicate that S, Al, Ca, Si, Cl, K, Cr, Cu, Ti, V, Sr, and Rb were detected in precipitates that formed over the entire tested pH range from 3 to 12. Iron, Ba, Te, Cs, Sn, Bi, and Sb showed peak co-precipitation/adsorption in precipitates forming at pH<4. Scandium, Nb, Cd, Mo, Zr, and Pb appeared in precipitates forming between pH 5 to 12, with higher concentrations occurring in precipitates forming at higher pH levels; whereas, Mn and Zn formed in precipitates over the same pH range but showed peak concentration at pH 8. Cobalt and Ni were detected in precipitates formed at pH 8; whereas, Mg and W were detected in precipitates forming at pH 12. Uranium was restricted to precipitates forming between pH 4 to 8. Analysis of these precipitates by inductively coupled plasma optical emission spectroscopy for comparison with the XRF data is pending.