METAL SULFIDE OXIDATION - THE IMPORTANCE OF MICROORGANISMS AND SULFUR COMPOUNDS IN DIFFERENT ENVIRONMENTS

Metal sulfide oxidation is an important process in the past and present global biogeochemical sulfur cycle. By this process, various intermediary sulfur compounds, namely elemental sulfur, polysulfides, thiosulfate, polythionates, sulfite and sulfate, are generated in different environments. The formation of the sulfur compounds depends on the mineralogy of the metal sulfide and the geochemical conditions in the environment and can be described by two different pathways, namely the thiosulfate mechanism and the polysulfide mechanism. Microorganisms play a crucial role in the formation and oxidation of intermediary sulfur compounds. Under oxic and acidic conditions, e.g. in sulfidic mine waste, microorganisms control the metal sulfide oxidation kinetic by their ability to oxidize Fe(II) to Fe(III) which serves as oxidant for the metal sufides as well as for most of the intermediary sulfur compounds. Under oxic and pH-neutral conditions, e.g. in carbonate buffered sulfidic mine waste or at the surface of marine sediments, the metal sulfide oxidation kinetic is controlled by the availability of oxygen, and the microorganisms oxidize the intermediary sulfur compounds. Under anoxic and pH-neutral conditions, e.g. in marine sediments, metal sulfides and intermediary sulfur compounds are oxidized either chemically by MnO2 or by microorganisms using nitrate as electron acceptor. High metal sulfide oxidation rates, e.g. for pyrite in oxic sulfidic mine waste, can be measured by microcalorimetry. For the determination of low oxidation rates, e.g. in marine sediments, the degradation of radioactively labeled 55FeS2 can be used. These methods could be used for a quantification of metal sulfide oxidation in different environments to evaluate the importance of this process for the global biogeochemical sulfur cycle.