BACTERIALLY INDUCED FERRIC MINERAL PRECIPITATION IN ACID MINE WATERS

Ferric minerals are typically found in acid mine waters. These waters are characterized by low pH and high concentration of heavy metals e.g. iron. Acidophilic bacteria play an important role during mineral precipitation since chemical iron oxidation is negligible under acidic conditions. In addition to the supply of ferric ions for mineral precipitation iron oxidizing bacteria might function actively in mineral precipitation.

We investigated two different species of iron oxidizing bacteria: first the acidophilic iron oxidizer Leptospirillum ferrooxidans, and second the moderately acidophilic iron oxidizer Thiomonas sp. and we analyzed their role during ferric mineral precipitation in acid mine waters. Both species differ in their way of iron oxidation and mineral precipitation. The analyses of chemical parameters during cultivation experiments of acidophilic and moderately acidophilic iron oxidizing bacteria suggested that iron oxidation and mineral formation occurred nearly simultaneously in Leptospirillum ferrooxidans and consecutively in Thiomonas sp.. Electron microscopic investigations revealed no direct interactions between Leptospirillum ferrooxidans cells and the precipitated mineral, whereas a partial incrustation of Thiomonas sp. cells could be observed. XRD indicated that goethite and lepidocrocite were produced in presence of Thiomonas sp.. The precipitated mineral in experiments with Leptospirillum ferrooxidans was mainly composed of schwertmannite, a secondary metastable iron(III)-oxyhydroxysulfate with a hedgehog like structure. Schwertmannite hedgehogs were often found in environmental samples. So far we suggested that the hedgehogs are incrusted bacteria cells. The present data demonstrate that the hedgehog like structure is the result of the oxidation rate and consequently of the supply of ferric ion and independent from the presence of bacterial cells.