BIOGEOCHEMICAL PHENOMENA INDUCED BY BACTERIA WITHIN SULFIDIC MINE TAILINGS

Mill tailings resulting from mining and metallurgical processes are us ually disposed of into open-air impoundments, where they become subjec ted to chemical or microbial leaching. At the surface of the tailings, where oxic conditions prevail, acidophilic bacteria, such as thiobaci lli, can oxidize sulfidic minerals (e.g. pyrite and pyrrhotite) and ge nerate 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 resu lt of a close metal-bacteria interaction. Iron-rich precipitates are u sually found on bacterial cell walls, and are thought: to serve as nuc leation sites for further mineralization within the tailings impoundme nts. As depth increases in mine tailings, oxygen depletion and the pre sence of water-saturated pores usually lead to anoxic conditions. Unde r such redox and chemical conditions, populations of sulfate-reducing bacteria (SRBs) can colonize the tailings. As a result of their metabo lic activity, sulfate is reduced to hydrogen sulfide, which in turn ca n react with dissolved metals to form metal sulfide precipitates. Micr obial sulfate reduction also generates alkalinity, although chemical d issolution of carbonate and oxide minerals probably also play an impor tant role in the generation of alkaline conditions in mine tailings.