Leaching of zinc sulfide by Thiobacillus ferrooxidans: Bacterial oxidationof the sulfur product layer increases the rate of zinc sulfide dissolutionat high concentrations of ferrous ions

This paper reports the results of leaching experiments conducted with and w ithout Thiobacillus ferrooxidans at the same conditions in solution. The ex tent of leaching of ZnS with bacteria is significantly higher than that wit hout bacteria at high concentrations of ferrous ions. A porous layer of ele mental sulfur is present on the surfaces of the chemically leached particle s, while no sulfur is present on the surfaces of the bacterially Leached pa rticles. The analysis of the data using the shrinking-core model shows that the chemical leaching of ZnS is limited by the diffusion of ferrous ions t hrough the sulfur product layer at high concentrations of ferrous ions. The analysis of the data shows that diffusion through the product layer does n ot limit the rate of dissolution when bacteria are present. This suggests t hat the action of T. ferrooxidans in oxidizing the sulfur formed on the par ticle surface is to remove the barrier to diffusion by ferrous ions.