PRECIPITATE EVALUATION OF AN ACID MINE DRAINAGE SOLUTION COLLECTED AT THE GREEN VALLEY MINE, INDIANA, USA

Acid mine drainage (AMD) commonly contains elevated concentrations of numerous dissolved trace elements, some of which are hazardous to the environment. The solubility and migration of these elements is impacted by dilution factors associated with increased discharge from rainfall and snow melt that decrease acidity. This study examines the formation of precipitates from an AMD sample that was titrated across a range of pH from 3.0 to 10.0. Three liters of AMD were collected from a constructed channel lined with carbonate rock at the Green Valley abandoned coal mine site in western Indiana. The pH at time of sampling was 3.3. In the laboratory, the sample was separated into three 1-L aliquots so that triplicate titrations could be performed to establish reproducibility. Eh and pH were measured continuously as AMD was titrated with 1 M NaOH to induce precipitation. The solution mixture was continuously stirred to maintain homogeneity. At each 0.5 pH interval, 25 mL of the solution mixture were extracted and pipetted into individual pre-weighed plastic centrifuge tubes. Samples were centrifuged and decanted to separate the precipitate and liquid phases, resulting in 88 liquid and 88 precipitate samples across all three aliquots. Acidity curves for these aliquots indicate three major precipitation events at pH 3.0, 5.0 and 9.0. These events correspond to the removal of Fe, Al, and a third element, possibly Mg. Weight measurements of accumulated precipitates show notable increases in the amount of precipitate formation at about every 2.0 to 3.0 pH increment, with a prominent precipitate color change from orange to greenish between pH 6.0 and 6.5. Eh and pH data were plotted on Pourbaix diagrams to illustrate phase changes with special emphasis given to the pH 6.0 interval. Results suggest a change in precipitate composition from goethite to pyrite at about pH 6.0. The samples are currently being analyzed by inductively coupled plasma mass spectrometry (ICPMS) to define trace element precipitation patterns. These data will ultimately be compared with X-ray fluorescent (XRF) data to evaluate the accuracy of using a hand-held XRF to determine elemental composition of AMD precipitates in the field.