Effect of Sulphur Concentration on Bioleaching of Cr(III) and Other Metals from Tannery Sludge by Indigenous Sulphur-Oxidizing Bacteria

An investigation on the effect of elemental sulphur concentration on bioleaching of Cr(III) and other metals from tannery sludge by sludge indigenous sulphur-oxidizing bacteria was performed. Elemental sulphur concentrations ranging from 5 to 40 g/L were tested during this study. The sludge solids concentration was fixed at 40 g/L. The results showed that the sludge pH decrease was faster and sharper with increased sulphur concentration. The time required to achieve the final pH (the lowest pH) was about 15 days irrespective of sulphur concentration. The leaching efficiencies of Cr(III), Al, and Fe, and SO42- production increased with increasing initial sulphur concentration. However, the leaching efficiencies of Zn, Ca, and Mg were less affected by the initial sulphur concentration. The preferred sulphur concentration for Cr leaching was 20 g/L, from an economic viewpoint. The total and volatile suspended solids decreased during the process. The measurements of sulphur-oxidizing bacteria showed that less-acidophilic thiobacilli as well as acidophilic thiobacilli contributed to the pH reduction in tannery sludge. The sulphur-oxidizing bacteria could survive in tannery sludge at a pH as low as 0.5 at 21°C