Enhancing Metal Removal by Coaddition of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{S}}^0$ as Substrates of Acidithiobacillus Ferrooxidans for Sewage Sludge Bioleaching

Bioleaching of heavy metals from sewage sludge has been shown to be a promising technique for sludge decontamination in such a complex matrix. The effect of two types of substrates ( ${\mathrm{Fe}}^{2+}$ and ${\mathrm{S}}^0$ ) and their combination on metal removal efficiencies was studied in a batch system using Acidithiobacillus ferrooxidans. The results showed that the inoculation of A. ferrooxidans and coaddition of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{S}}^0$ accelerated pH reduction and metal solubilization. After 4–10 days of bioleaching, the following removal efficiencies were obtained: Cr 80%; Cu 100%; and Zn 100%, which were the maximum that have been reported. The time required for Cr to reach the maximum was 10 days, whereas for Cu and Zn the time needed was only 4–6 days. Three different patterns of solubilization for Cr, Cu, and Zn were established as a function of pH and ${\mathrm{Fe}}^{3+}$ concentration. Cr required a threshold pH of around 2.0 to initiate its solubilization, whereas Cu solubilization was controlled simultaneously by pH and ${\mathrm{Fe}}^{3+}$ concentration. The pH appeared to be the sole factor responsible for the solubilization of Zn that was initiated at 4.0–4.5. Fundamental strategy for enhancing metal removal efficiencies is to lower the pH and to sustain high ${\mathrm{Fe}}^{3+}$ concentration, which can be achieved by increasing sulfate concentration, facilitating ${\mathrm{Fe}}^{2+}$ biooxidation, and decreasing the precipitation of jarosite and schwermannite.