Comparison of the oxidation kinetics of different pyrites in the presence of Thiobacillus ferrooxidans or Leptospirillum ferrooxidans

The question whether the overall bacterial oxidation rate of a pyrite is determined by both the chemical reactivity of the pyrite as well as the affinity of a bacterial strain to the ferrous to ferric iron ratio, was examined. Bioleaching studies were carried out with a pure culture of Thiobacillus ferrooxidans and an enrichment culture of Leptospirillum bacteria on a framboidal pyrite from Germany and a euhedral pyrite from Prieska, South Africa. It was found that the Leptospirillum bacteria were able to oxidise euhedral pyrite and framboidal pyrite, whereas T. ferrooxidans was able to oxidise framboidal pyrite but not euhedral pyrite. The German, framboidal pyrite has a granular and irregular surface structure, which is probably more chemically reactive than the highly crystalline surface structure of euhedral, Prieska pyrite. In previous work it has been shown that the oxidation of pyrite by Leptospirillum bacteria is determined by a mechanism which consists of two sub-processes: (i) the chemical oxidation of pyrite with ferric iron to ferrous iron and sulphate, and (ii) the bacterial oxidation of ferrous iron. By using roughly estimated kinetic constants from a batch culture of T. ferrooxidans on framboidal pyrite, and the independently determined ferrous iron oxidation kinetics of T. ferrooxidans, it was possible to explain why this strain is not able to oxidise the refractory euhedral pyrite. The results are explained in terms of the relative affinities of T. ferrooxidans and the Leptospirillum bacteria for the ferrous/ferric-iron ratio, and susceptibility of the two types of pyrite to ferric leaching.