ABSTRACT
Sulfate-reducing bacteria play an important role in some sedimentary environments in the formation of certain sul de minerals, especially iron pyrite (FeS2). Other microbes play an even more pervasive role in the oxidation of a wide range of metal sul des in some soils and sediments or exposed at rock surfaces, regardless of the mode of origin of these minerals. The oxidative microbial activity is being industrially exploited in the extraction of metals from some metal sul de ores. Currently, the bioextractable sul dic ores of commercial interest include those of copper, nickel, zinc, and cobalt. At least one of the kinds of bacteria capable of leaching the metal in sul dic ores is also capable of leaching uranium from the nonsul dic ore uraninite (UO2). Although gold in sul dic ores is not commercially bioextracted, microbial pretreatment (biobenefi ciation) of such gold ores to remove interfering pyrite and arsenopyrite impurities is now being practiced on a commercial scale. The pyrites in these ores encapsulate the gold, making it inaccessible to a chemical extractant such as aqueous cyanide or thiourea. When cyanide is used as extractant, the pyrites cause excessive consumption of it, resulting in the formation of cyanide complexes with the iron and sulfur components of pyrite, that is, ferro-and ferricyanide and thiocyanate, from none of which the cyanide is readily recoverable. A great potential exists for industrial bioextraction of a variety of other metal sul de ores.
