Direct sulfide oxidation in the solubilization of sulfide ores by Thiobacillus ferrooxidans.

The increased rate of solubilization of sulfide ores of iron and copper caused by Thiobacillus ferrooxidans has been attributed either to direct action of the bacteria on the ore or to an indirect effect whereby the ferric ion is presumed to oxidize the ore and is returned to the active, oxidized state by the iron-oxidizing bacteria. Attempts by M. P. Silverman (3) to demonstrate that microbial action could be duplicated by the presence of ferric ion alone were not definitive and led him to suggest that both processes may occur simultaneously. A similar idea was expressed by Duncan, Landesman, and Walden (2) who, having established the differential susceptibility of the bacterial oxidation of ferrous iron and sulfur to N-ethyl maleimide and sodium azide, determined the effect of these inhibitors on pyrite and chalcopyrite oxidation. Decreased rates of oxidation in the presence of each inhibitory compound suggested that both iron and sulfur (sulfide) were simultaneously oxidized. These results, which support some direct-action mechanism, are in agreement with the following observations made in our laboratory. Heavy suspensions of intact cells of T. ferrooxidans, when studied over extended periods of time, exhibit a gradual decrease in the rate of oxidation of ferrous iron and of sulfur, with the latter decreasing more rapidly. A number of aged suspensions have been studied which, while retaining good action on iron, have lost completely the ability to oxidize elemental sulfur. Each of these was found to be unable to oxidize pyrite or chalcopyrite, and, furthermore, we have never found a suspension that was able to use sulfur that was unable to oxidize sulfide ores. Apparently the sulfur and iron oxidation systems are different and the former appears essential for solubilization of sulfide ores. Additional support for the direct-action theory is noted in Table 1. Cell suspensions of T. ferrooxidans were allowed to oxidize the four substrates, ferrous ion, elemental sulfur, pyrite, and chalcopyrite, in the presence of '4C02. An earlier