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

Pyrite was oxidised by growth of Thiobacillus ferrooxidans aerobically at 3 2 degreesC by orbital shaking at 90 revolutions per minute (rpm) in the lab oratory. 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 durin g 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. Th e molar ratio continued to fall during the exponential phase and reached ap proximately 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 dis solved sulphur was in the form of SO4 during both the lag and exponential p hase. 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 pyrit e surfaces reveals that both direct and indirect leaching are important for pyrite oxidation during the exponential phase. During the lag phase, howev er, direct leaching appears to be the dominant mechanism, which may explain the preferential oxidation of S during the early stages of growth. (C) 200 1 Elsevier Science B.V. All rights reserved.