Effects of sulfur dosage on continuous bioleaching of heavy metals from contaminated sediment

The bioleaching technology has been considered as a promising green technology for remediation of contaminated sediments in recent years. Bioleaching technology was generally conducted in the batch bioreactor; however, the continuous bioreactor should be developed for the application of bioleaching technology in the future. The purposes of this study were to establish a continuous bioleaching process, and to evaluate the effects of sulfur dosage on the efficiency of metal removal during this continuous bioleaching process. The obtained results show that the pH decrease, sulfate production and metal removal efficiency all increased with increasing sulfur dosage in the continuous bioleaching process due to high substrate concentration for sulfur-oxidizing bacteria. After 30 days of operation time, the maximum solubilization efficiencies for Zn, Ni, Cu and Cr were found to be 78%, 90%, 88% and 68%, respectively, at 5% of sulfur dosage. After the bioleaching process, heavy metals bound in the carbonates, Fe-Mn oxides and organics/sulfides in the sediment were effectively removed and the potential ecological and toxic risks of treated sediment were greatly reduced. The results of bacterial community analyses demonstrated that this continuous bioleaching process were dominated by several acidophilic sulfur-oxidizing bacteria; S. thermosulfidooxidans, At. thiooxidans/At. ferrooxidans, S. thermotolerans and At. albertensis, whereas the percentage of less-acidophilic sulfur-oxidizing bacteria (T. thioparus and T. cuprina) was lower than 15% of total bacteria. In addition, the cell numbers of sulfur-oxidizing bacteria increased as the sulfur dosage was increased in the continuous bioleaching process.