Transport, fate and speciation of heavy metals (Pb, Zn, Cu, Cd) in mine drainage: geochemical modeling and anodic stripping voltammetric analysis.
The maximum concentrations (ppb) of heavy metals in the mine drainage (pH: down to 3.3) of Chonam-ri creek in the abandoned Kwangyang gold-silver mine, South Korea, are 22600 Zn, 2810 Cu, 182 Cd, and 109 Pb. A small, limestone-infused retention pond, about 440 meters downstream from the waste dump, plays an important role in the removal of heavy metals: the factors of reduction for Zn, Cu, Cd, and Pb are 12, 24, 14, and 14, respectively. This is due to the pH increase (up to >5.4) accompanying adsorption onto and/or coprecipitation with Fe- and Al-hydroxides (goethite and gibbsite). From the waste dump to the pond, heavy metal concentrations also progressively decrease due to pH increase. Geochemical modeling (using the computer code WATEQ4F) predicts that free aqueous metal ions are dominant (mostly >70% for Cu and Zn, and >60% for Pb and Cd) in samples collected upstream from the pond, whereas complexing with sulfate, carbonate and hydroxyl ions becomes important in the samples collected downstream. The comparison between the concentrations of electrochemically labile species (determined by Anodic Stripping Voltammetry) and the result of computer modeling shows that Cd and Zn are present predominantly as labile inorganic species throughout the whole range of the creek. However, Cu and Pb in the samples collected downstream from the pond largely form electrochemically inert species (possibly, metal-organic complexes). The above results indicate that the retention pond is effective in reducing the toxicity of heavy metals, especially Cu and Pb.