The oxidation of either ferrous iron or sulfur by Thiobacillus ferrooxidans was selectively inhibited or controlled by various anions, inhibitors, and osmotic pressure. Iron oxidation was more sensitive than sulfur oxidation to inhibition by chloride, phosphate, and nitrate at low concentrations (below 0.1 M) and also to inhibition by azide and cyanide. Sulfur oxidation was more sensitive than iron oxidation to the inhibitory effect of high osmotic pressure. These differences were evident not only between iron oxidation by iron-grown cells and sulfur oxidation by sulfur-grown cells but also between the iron and sulfur oxidation activities of the same iron-grown cells. Growth experiments with ferrous iron or sulfur as an oxidizable substrate confirmed the higher sensitivity of iron oxidation to inhibition by phosphate, chloride, azide, and cyanide. Sulfur oxidation was actually stimulated by 50 mM phosphate or chloride. Leaching of Fe and Zn from pyrite (FeS 2 ) and sphalerite (ZnS) by T. ferrooxidans was differentially affected by phosphate and chloride, which inhibited the solubilization of Fe without significantly affecting the solubilization of Zn.

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The interaction of chalcopyrite and sphalerite with pyrite during leaching by Thiobacillus ferrooxidans and Thiobacillus thiooxidans was studied in shake-flask leaching experiments and in respirometer experiments with resting cells. Two samples of pyrite (Nos. 1 and 2) were used. In shake-flask experiments in the absence of bacteria, the copper and zinc extraction rates increased in the presence of No. 1 pyrite from 1.6 to 10.2 and 13%, respectively, and in the presence of No. 2 pyrite from 1.6 to 3.5 and 5.8%, respectively. Thus the effect of pyrite was greater with sphalerite than with chalcopyrite and the No. 1 pyrite was more effective than the No. 2 pyrite. Thiobacillus ferrooxidans increased the leaching rates of Cu from chalcopyrite (from 1.6 to 11.1%), of Zn from sphalerite (from 1.6 to 36%), and of Fe from pyrite (from 7.3 to 29.9% (No. 1) and from 5.1 to 49.3% (No. 2)), although a period of adaptation was required for sphalerite leaching. Thiobacillus thiooxidans did not leach Cu form chalcopyrite, but increased leaching of Zn from sphalerite (from 1.6 to 8.3%) and some Fe from pyrite (from 7.3 to 9.5% (No. 1) and from 5.1 to 13.1% (No. 2)). In the chalcopyrite–pyrite mixture, Cu leaching was further stimulated by T. ferrooxidans (from 10.2 to 14.5% (No. 1) and from 3.5 to 12.2% (No. 2)) and by T. thiooxidans (from 3.5 to 5.8% (No. 2)). The Zn leaching from the sphalerite–pyrite combination increased further with T. ferrooxidans (from 13.0 to 61.7% (No. 1) and from 5.8 to 65.8% (No. 2)) and T. thiooxidans (from 13.0 to 22.4% (No. 1) and from 5.8 to 14.7% (No. 2)). The Fe solubilization from pyrite was inhibited by the presence of chalcopyrite or sphalerite with or without bacteria. Short-term respirometer studies generally supported the leaching results. Oxygen consumption was faster for chalcopyrite or sphalerite in the presence of pyrite. The effect was stronger for sphalerite, although the rapid pyrite oxidation by T. ferrooxidans was inhibited by the interaction. Chalcopyrite alone was oxidized by T. ferrooxidans but not by T. thiooxidans, while sphalerite alone was oxidized only by the latter. It is concluded that shake-flask leaching and Warburg respirometer experiments complement each other in elucidating the complex processes involved in bacterial metal leaching of sulfide ores. Key words: Thiobacillus, sulfide ores, oxidation, mineral leaching.

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