Role of the bacterium Thiobacillus in the formation of silicates in acidic mine tailings

Acidophilic chemolithotrophic bacteria, such as Thiobacillus, are commonly found in sulfidic mine tailings. They oxidize Fe(II) and reduced sulfur species from sulfide minerals left in tailings and produce acidic leachates loaded with heavy metals. Our study of the Kam Kotia mine tailings (CuZn ore), an abandoned mine located near Timmins, Ontario, Canada, showed that Thiobacillus played an important role in the in-situ formation of silicates. Transmission electron microscopic observations of ultrathin sections of tailings revealed the presence of bacteria with fine-grained silica (SiO2) associated with their cell walls. Under laboratory conditions mimicking the chemistry of the pore waters of the tailings (i.e., identical pH, [Fe], [Si] and [SO4]), Thiobacillus promoted the rapid formation (48 h) of amorphous silica at low pH values (pH 2.2–2.3), while an abiotic system with the same chemical conditions did not show silica precipitation. However, in the presence of Fe-oxides, silica formation was not observed because dissolved Si was preferentially sorbed to the surface of the oxides. In addition, when the cells were grown in the presence of micas (biotite and muscovite) under acidic conditions, mixed (Mg, Al, Si, Fe)-silicate precipitates were formed on the surface of Thiobacillus. These results indicate that Si cycling and solubility in mine tailings could be strongly influenced by bacteria, especially in sulfide-depleted tailings like the Kam Kotia ones, where the formation of Fe-oxide is negligible.