BIOGEOCHEMICAL PHENOMENA INDUCED BY BACTERIA WITHIN SULFIDIC MINE TAILINGS

Mill tailings resulting from mining and metallurgical processes are usually disposed of into open-air impoundments, where they become subjected to chemical or microbial leaching. At the surface of the tailings, where oxic conditions prevail, acidophilic bacteria, such as thiobacilli, can oxidize sulfidic minerals (e.g. pyrite and pyrrhotite) and generate acidic metal-rich leachates as by-products of their metabolism. This, combined with chemical oxidation, leads to acid mine drainage (AMD). Biomineralization, whereby a proportion of the metal leachate is precipitated can also occur in the oxidized tailings, often as a result of a close metal-bacteria interaction. Iron-rich precipitates are usually found on bacterial cell walls, and are thought: to serve as nucleation sites for further mineralization within the tailings impoundments. As depth increases in mine tailings, oxygen depletion and the presence of water-saturated pores usually lead to anoxic conditions. Under such redox and chemical conditions, populations of sulfate-reducing bacteria (SRBs) can colonize the tailings. As a result of their metabolic activity, sulfate is reduced to hydrogen sulfide, which in turn can react with dissolved metals to form metal sulfide precipitates. Microbial sulfate reduction also generates alkalinity, although chemical dissolution of carbonate and oxide minerals probably also play an important role in the generation of alkaline conditions in mine tailings.