Solution chemistry during the lag phase and exponential phase of pyrite oxidation by Thiobacillus ferrooxidans

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Pyrite was oxidised by growth of Thiobacillus ferrooxidans aerobically at 32°C by orbital shaking at 90 revolutions per minute (rpm) in the laboratory. The analyses of the experimental solutions showed a long period of adaptation (lag phase) before the onset of rapid bio-oxidation. Lag phase lasted for approximately 400 h. During this period, the dissolved iron and sulphur content increased very slowly compared with a very rapid rise during the exponential phase of growth. The molar ratio of the dissolved Fe to S in solution decreased from 1.3 to approximately 1 during the lag phase. The molar ratio continued to fall during the exponential phase and reached approximately 0.5, which is the ratio defined by the stoichiometry of pyrite. The form of dissolved iron during the lag phase was ferrous, while during the exponential phase, it was mostly ferric. On the other hand, all the dissolved sulphur was in the form of SO 4 during both the lag and exponential phase. These indicate that the Fe is preferentially leached from pyrite, but S is the main source of energy for T. ferrooxidans during the lag phase. The solution chemistry and scanning electron microscope images of the pyrite surfaces reveals that both direct and indirect leaching are important for pyrite oxidation during the exponential phase. During the lag phase, however, direct leaching appears to be the dominant mechanism, which may explain the preferential oxidation of S during the early stages of growth.