Visualisation of pyrite leaching by selected thermophilic archaea: Nature of microorganism–ore interactions during bioleaching

In this study, pyrite (FeS2) was leached by Acidianus brierleyi, Metallosphaera sedula and Sulfolobus metallicus during a 60 day experimental period. Leaching occurred over a redox potential range of 800 to 860 mV (S.H.E.) and in the presence of increasing Fe3+ levels. A modified ferrozine assay was developed to detect the increase of iron in solution as bioleaching of the ore progressed. For the first time, the interactions of these extreme thermophiles with the metal sulfide ore particles were extensively documented using SEM and TEM. As the pyrite degraded, there appeared to be a progression of deposited structures forming, ranging from sub-micron precipitates and disc-shaped structures on the ore's surface, which ultimately were similar for all leaching cultures. Furthermore, the residues resulting from the leaching of pyrite by M. sedula, the most active thermophile, were characterised using SEM/EDX, and appeared to be dominated by iron sulfate precipitates. The nature of the deposits formed, together with our other results, indicate that A. brierleyi, M. sedula and S. metallicus acted through the ‘contact’ and ‘non-contact’ sub-mechanisms of the indirect bioleaching mechanism for the dissolution of pyrite. The role of the bioleaching microorganisms is thus to maintain sufficient levels of Fe3+ and acid during pyrite leaching, for maximal mineral dissolution.