Enhanced Cr bioleaching efficiency from tannery sludge with Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans

Bioleaching is a successful and cost-effective strategy with which to remove heavy metals from contaminated sludge. Most of the current studies on bioleaching are concentrated on municipal sludge, and bioleaching of tannery sludge has not been studied extensively. The present study investigated the bioleaching of chromium (Cr) from tannery sludge using the mixture of iron-oxidizing bacteria (Acidithiobacillus ferrooxidans) and sulfur-oxidizing bacteria (Acidithiobacillus thiooxidans) in batch flasks. Changes in the pH, ORP, Fe2+ and SO42- concentrations and the efficiency of bioleaching of Cr from tannery sludge were analyzed. The results showed that the bioleaching efficiency of co-inoculation of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans was better than any of them. The co-inoculation with a mixture of iron-oxidizing and sulfur-oxidizing bacteria together with the addition of Fe2+ and elemental sulfur accelerated acid production and an increase in the oxidation-reduction potential originating from the bio-oxidation of Fe2+ and elemental sulfur. The bioleaching efficiency of co-inoculation of Cr increased as the pH of sludge decreased, reaching a maximum removal of 97.1%. The residual Cr concentration which is 531 mg/kg in bioleached sludge is below the thresholds for agriculture, according to China National Standard GB4284-2018 (<1000 mg/kg dry weight) for pollutants in sludges from agricultural use. The bioleaching efficiency was distinctly correlated with pH or ORP. The data presented showed that the bioleaching efficiency was negatively correlated (R-2 = 0.9864) with pH and positively (R-2 = 0.9821) with ORP, which implied the existence of a substantial indirect leaching mechanism responsible for Cr solubilization.