Optimum condition in Cr3+ bioleaching of tannery sludge with Thiobacillus ferrooxidans LX5, Thiobacillus thiooxidans TS6 and Rhodotorula sp. R30

The tolerance of Thiobacillus ferrooxidans LX5, Thiobacillus thiooxidans TS6 and Rhodotorula sp. R30 to Cr3+ ions and removal efficiency of metal in sludge by bioleaching through the combination of the above three strains were explored. The effect on the growth and oxidative activity of T. ferrooxidans LX5 and T. thiooxidans TS6 could be neglected when the Cr3+ concentration was no more than 700mg/L, a common concentration range found in tannery sludge bioleaching system. However, over 500mg/L of Cr3+ clearly restrained the growth of Rhodotorula sp. R30. In the batch trial of tannery sludge bioleaching, with the combination of strain LX5, TS6 and R30, the time of bioleaching could be apparently shortened. The optimum inoculation ratio were 2.0% for Rhodotorula sp. R30 and 10% for T. ferrooxidans LX5 and T. thiooxidans TS6. Treating tannery sludge with bio-acidified sludge containing 104/mL and 102/mL yeast R30 as inoculum exhibited that the bioleaching efficiency was not obviously higher in the former treatment than the latter treatment. With inoculation of 2.0% yeast R30 in the beginning of bioleaching, the period of bioleaching could be apparently shortened to 36 hours. It was suggested that for improving bioleaching efficiency inoculating 2.0% of yeast R30 (108 cell/mL) in the treated sludge was important in the initial phase of bioleaching.