Bioleaching of pyrite at low pH and low redox potentials by novel mesophilic Gram-positive bacteria

Bioleaching of ground rock pyrite by two novel strains of Sulfobacillus-like acidophilic bacteria was examined in shake flask and bioreactor cultures. The Gram-positive prokaryotes differed from known species of mineral-oxidising Sulfobacillus in being mesophilic and in their tolerance to extreme acidity (pH<1). The oxidative dissolution of pyrite by one of the novel Sulfobacillus strains was stimulated by yeast extract, though the second strain (L-15) was equally effective in organic-free medium. Pyrite dissolution by Sulfobacillus L-15 was compared with that by the strain type of Acidithiobacillus ferrooxidans and a Leptospirillum ferrooxidans isolate, in media set at different initial pH values. All three bacteria leached the mineral in media set at pH 2.5 and pH 1.5, but at pH 1.2, mineral oxidation (at a rate similar to the maximum recorded for At. ferrooxidans) was observed only in cultures of Sulfobacillus L-15. Pyrite leaching by Sulfobacillus L-15 and At. ferrooxidans was compared in bioreactor cultures, at different and controlled pH values. Lowering bioreactor pH from 1.5 to 1.0 resulted in the cessation of biologically accelerated pyrite oxidation by the At. ferrooxidans culture and rapid mortality of the bacteria. In contrast, Sulfobacillus L-15 continued to leach the mineral when the pH was lowered to 1.0 and (after a short lag period) to pH 0.8. In contrast to the At. ferrooxidans culture, large numbers of viable Sulfobacillus L-15 cells were recovered, regardless of bioreactor pH. Redox potentials and ratios of soluble ferric/ferrous iron in the Sulfobacillus L-15 culture were significantly lower (by about 100–150 mV) than values found typically in Gram-negative bacterial leaching cultures, and that recorded in the (active) At. ferrooxidans culture in the present work. The potential advantages of using microorganisms that can leach sulfidic minerals effectively at pH<1 and at low redox potential are discussed.