Bioleaching of Cr from tannery sludge: The effects of initial acid addition and recycling of acidified bioleached sludge

A technologically feasible process called bioleaching was used to remove Cr from tannery sludge with Acidithiobacilli TS6 bacteria in this study. The buffering capacity of tannery sludge and municipal sewage sludge were compared. The effects of initial sulfuric acid addition on bioleaching process and influences of recycle rate of acidified bioleached sludge on subsequent bioleaching reaction were examined. The quantities of initial sulfuric acid addition ranged from 0 to 0.185 ml g^-1 dry sludge. Recycle rate(V^bioleached / V^total) was controlled at 1/3, 1/2, 3/5 and 2/3. We found that tannery sludge had a higher buffering capacity than sewage sludge. The results showed that there was an increase in the rates of pH reduction and Cr solubilization with the increase of initial sulfuric acid addition. The suitable quantity of sulfuric acid addition was 0.185 ml g^-1 dry sludge at the beginning of the first bioleaching process. After 10 days of bioleaching, pH reached 1.5 and almost 100% of Cr could be leached in this case. Besides, an evidently dividing point pH 2.0 was found to be the threshold value for considerable Cr solubilization during the leaching reaction. Recycle experiments confirmed that the application of the method by recycling acidified sludge to launch subsequent bioleaching is feasible. The optimal recycle rate of acidified sludge for consecutive leaching of Cr from tannery sludge is 2/3.