ABSTRACT
Sulfate-reducing bacteria play an important role in some sedimentary environments in the formation of certain sulde minerals, especially iron pyrite (FeS2). Other microbes play an even more pervasive role in the oxidation of a wide range of metal suldes 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 sulde ores. Currently, the bioextractable suldic 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 nonsuldic ore uraninite (UO2). Although gold in suldic ores is not commercially bioextracted, microbial pretreatment (biobeneficiation) of
21.1 Introduction / 517 21.2 Natural Origin of Metal Suldes / 518
21.2.1Hydrothermal Origin (Abiotic) / 518 21.2.2Sedimentary Metal Suldes of Biogenic Origin / 519
21.3Principles of Metal Sulde Formation / 520 21.4Laboratory Evidence in Support of Biogenesis of Metal Suldes / 521
21.4.1Batch Cultures / 521 21.4.2Column Experiment: A Model for Biogenesis of Sedimentary Metal Suldes / 522
21.5Biooxidation of Metal Suldes / 523 21.5.1Organisms Involved in Biooxidation of Metal Suldes / 523 21.5.2Direct Oxidation / 525 21.5.3Indirect Oxidation / 528 21.5.4Pyrite Oxidation / 529
21.6Bioleaching of Metal Sulde and Uraninite Ores / 531 21.6.1Metal Sulde Ores / 531 21.6.2Uraninite Leaching / 535 21.6.3Mobilization of Uranium in Granitic Rocks by Heterotrophs / 536 21.6.4Study of Bioleaching Kinetics / 537 21.6.5Industrial versus Natural Bioleaching / 537
21.7Bioextraction of Metal Sulde Ores by Complexation / 537 21.8Formation of Acid Coal-Mine Drainage / 538
21.8.1New Discoveries Relating to Acid Mine Drainage / 539 21.9Summary / 541 References / 541
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 formation of cyanide complexes with the iron and sulfur components of pyrite, i.e., ferroand 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 sulde ores.
