The experiment started more than five years ago and is still running. Three main conditions are studied at successive time intervals: (a) the initial phase of the complex leaching process, reproducing the average annual rain conditions; (b) the dry-weather conditions phase, with one-third of the average annual rain-water input; (c) the metal-depletion phase of the complex leaching process, showing sulfide depletion of the leached mine waste at two-thirds of the average annual rain-water input. The four microcosms simulate two scenarios: (1) bioleaching within a large waste dump in two replicate microcosms containing only mine waste (M1, M2); and (2) bioleaching in a mine-waste dump in contact with the surrounding soil in two other replicate microcosms, containing mine waste on top of a thin soil layer (T1, T2). The waste material and the soil were analysed for total (aqua regia extract) and mobile metal (NH4OAC + AcOH + EDTA extract) content at the start of the experiment. The quantity and quality (pH, As, Cd, Cu, Pb and Zn content) of the output leachates from each microcosm were measured at regular time intervals. The temperature within the microcosms was monitored. The cell concentration of the sulfide-oxidizing bacteria, Acidithiobacillus ferrooxidans, was measured (Sand et al. 2007). Results of the microcosm experiment were interpreted, based on the following: (a) input leachant and output leachate volumes; (b) cell concentration of the sulfide-oxidizing bacteria and pH of the leachates; (c) metal concentration of the leachates; and (d) metal concentration of the mine-waste and soil samples in the microcosms at the start of the experiment.

The microcosm experiment provided site-specific quantitative parameters for risk assessment and risk-reduction planning, with the aim of reducing metal emissions from the point and diffuse sources in the area (Gruiz et al. 2006b).

DOI 10.2462/09670513.953