Bioleaching of heavy metals contaminated sediment by pure and mixed cultures of Acidithiobacillus spp.

The aim of this study was to investigate the effect of bacterial strains on the bioleaching of Cr, Cu, Pb, and Zn from contaminated sediment samples taken from a stationary point in Izmir Inner Bay. Single and mixed cultures of Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans were used separately to achieve metal solubilization in suspension. Of the three trials tested, single culture of A. thiooxidans decreased the pH to 0.7, resulting in the highest metal solubilization ratios. Solubilization efficiencies of Cr, Cu, and Zn were high (> 80%) with A. thiooxidans but Pb could be solubilized to a ratio of 63%. The efficiency of metal solubilization from contaminated sediment in decreasing order is: Zn > Cu > Cr > Pb. In order to check for the mass balance, metals remaining in the residual sediment were also determined. The effect of bioleaching on sequentially extracted (chemically distributed) metals was investigated in addition to the metal solubilization efficiencies. For most of the trials, it is observed that, the stable forms of heavy metals were transferred and accumulated in more soluble forms during bioleaching and they directly solubilized in the water when they were finally transferred to the exchangeable and/or reducible form.

Research highlights
► Acidithiobacillus thiooxidans is found to be the most effective bacteria for the bioleaching efficiency in comparison to Acidithiobacillus ferrooxidans and the mixed culture depending on its highest cell concentrations.
► The efficiency of metal solubilization from contaminated sediments in decreasing order was found to be: Zn > Cu > Cr > Pb.
► During bioleaching, the stable forms of heavy metals are transferred and accumulated in more soluble forms and they directly solubilize in the water when they are finally transferred to the exchangeable and/or reducible form.
► Choosing the right thiobacilli, and providing optimum conditions for the sufficient growth of the bacteria are essential for the bioleaching medium to achieve satisfactory solubilization and removal efficiencies of heavy metals.