Bioleaching of heavy metals from municipal sludge by the co-inoculation of two acidophilic Thiobacillus

The effect of the co-inoculation of two acidophilic Thiobacillus(Acidithiobacillus ferrooxidans LX5 and Acidithiobacillus thiooxidans TS6)on the solubilization efficiency of sludge-born heavy metals by bioleaching was studied in batch experiments.The correlation between sludge-born metal solubilization and pH and/or oxidation-reduction potential(ORP)arising from bioleaching is also discussed.The results show that bioleaching with the two acidophilic Thiobacillus greatly improved the heavy metal solubilization efficiency from a sludge matrix.Removal efficiencies of 98% for Cu,99% for Zn,and 90% for Ni were obtained after 10 days of bioleaching.The greater the inoculum percentage,the shorter the bioleaching time.For example,the bioleaching reaction time for different inoculum treatments were in the order 20% inoculum 10%5%0%(control).The decrease of pH and the increase of ORP from bioleaching were two important factors resulting in metal solubilization,especially for sludge-born Cu.However,the solubilization of Zn and Ni derived from sludge was attributed to the pH decrease only.The effect of ORP on Zn and Ni solubilization was not observed.It was found that the threshold values of pH for metal solubilization were about pH 4 for Cu,pH 5 for Zn,and pH 5 for Ni.In addition,the contents of organic matter,nitrogen,phosphorus,and potassium in bioleached sludge were,to a certain extent,decreased due to bioleaching treatment,but this did not affect the application of bioleached sludge to farmland.