Bioleaching of Metal-Contaminated Soil in Semicontinuous Reactor

Recently, microbial leaching of heavy metals from contaminated soils with indigenous iron-oxidizing microflora has been successfully applied in flask experiments. However, the long hydraulic residence time (HRT) required and the batch mode are not well suited for an industrial scale process. Therefore, this research focused on bioleaching of Zn, Cu, and Mn from a contaminated soil in the semicontinuous mode. Metal leaching experiments were carried out in an 8 L semicontinuous stirred tank reactor (SCSTR) with a soil concentration of 100 g/L, as well as in 500 mL shake flasks. It was found that bioleaching in the SCSTR entails a reduction of treatment duration (from 10 to 2 days) and an increase in the solubilization efficiency. Metal solubilization efficiency in the SCSTR was 40% for Zn, 47% for Cu, and 34% for Mn. When the volume of the soil suspension remaining in the SCSTR between transfers was small (20% instead of 50%), corresponding to a shorter HRT (2.5 days instead of 4 days), the solubilization efficiencies were reduced. The volumetric oxygen transfer coefficient and the oxygen uptake rate were calculated in the SCSTR for both HRTs tested. The dissolved oxygen concentration below which the microflora does not grow was found to be 0.2 to 0.3 mg/L, and the concentration below which oxygen is limiting was 0.8 mg/L.