The objective of the present study was to evaluate the application of bioleaching technique to recover heavy metals (Zn, Cu, Ni, Pb, Cd and Cr) in dewatered metal plating sludge not containing sulphide or sulphate compounds. The effects of pH, pulp density of the sludge and agitation time were investigated in both shake flask and completely mixed batch reactor experiments. The leaching of heavy metals increased with decrease of pH and pulp density and increase of agitation time for both bioleaching experiments. The maximum metal leaching efficiencies of 93 and 97% Zn, 86 and 96% Cu, 85 and 93% Ni, 70 and 84% Pb, 65 and 67% Cd and 30 and 34% Cr were attained with the shake flask and the completely mixed batch reactor experiments, respectively, at a pH 2, a solid content of 2% (w/v) and a reaction temperature of 25 ± 2°C during the bioleaching process (20 days). The rates of metal leaching for both bioleaching systems are well described by a kinetic equation related to time. Although the rates of metal leaching were in the decreasing order: Zn > Cu > Ni > Pb > Cd > Cr for both leaching experiments, bioleaching using the completely mixed batch reactor was slightly more efficient than the shake flask due to aeration of system. These results suggest that bioleaching may be an alternative or adjunct to conventional physicochemical treatment of dewatered metal plating sludge for the removal of hazardous heavy metals.
Key words: Bioleaching, waste sludge, heavy metals, Acidithiobacillusferrooxidans
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