PRECIPITATE FORMATION IN AN ACID MINE DRAINAGE SOLUTION

Acid mine drainage (AMD) serves as a carrier of numerous trace elements, some of which are hazardous to the environment. Consequently, the chemical behavior of AMD as it is exposed to such dilution factors as increased discharge from rainfall and snow melt that decrease acidity have a significant impact on the solubility and migration of these elements in the environment. This preliminary study evaluates precipitation behavior from an AMD sample as it is titrated across a range of pH from 3.00 to 10.00 and characterizes both the resulting solid and liquid phases. Three liters of AMD were collected from a constructed channel lined with carbonate rip-rap at the Green Valley abandoned coal mine site in western Indiana. AMD pH at the time of sampling was 3.30. In the laboratory, the sample was separated into three 1-L aliquots so that triplicate titrations could be performed to establish reproducibility. Both pH and Eh were measured continuously as AMD was titrated with 1 M NaOH to induce precipitation. The solution mixture was continuously stirred to maintain homogeneity. Twenty-five mL samples were taken at each 0.5 pH interval and pipetted into individual pre-weighed plastic centrifuge tubes. Samples were then centrifuged and decanted to separate precipitates and solution, resulting in 15 liquid and 15 precipitate samples. Initial acidity curves for these tests indicate that there are three primary precipitate reactions around pH’s 3.00, 5.00 and 9.00, reflecting removal of Fe, Al, and possibly Mg, respectively, from the AMD solution. Noticeable precipitation increases occurred at about every 2 to 3 pH increments. Eh and pH values will be plotted on known Pourbaix diagrams to demonstrate partitioning of Fe, Al and Mg into liquid versus solid phases. Future work includes analysis of the liquid and precipitate phases by inductively coupled plasma mass spectrometry to determine trace element concentrations and associated coprecipitation.