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14 <br />• high quality. In other tests on the Argo Tunnel drainage <br />it was found that Pb and Ag are removed from the drainages <br />of low pH's comparable to those for the removal of Fe(21). <br />Microbes and Catalysis <br />The pyrite oxidation model shown in reaction 15 requires <br />the attack of gaseous 02 on solid FeS This type of reaction <br />is usually very slow. Indeed, Singer and Stumm (22) have <br />found that Step a in the model is so slow that usually <br />Fe(III) dissolves the pyrite through step d. Nevertheless, <br />02 is needed for the weathering of pyrite to occur and, as <br />Wentz (8) explains, this oxidation has to take place more <br />rapidly than it does under sterile laboratory conditions. <br />• In waters of low pH and high concentrations of Fe and S04 , <br />it is generally accepted that microorganisms accelerate <br />the weathering of FeS2 by 02 (6, 8, 23). The bacteria are <br />aerobic (need oxygen to live), chemoautotropic (derive <br />their nourishment from chemical reactions of inorganic com- <br />pounds), acidophilic (can live only in waters of low pH), <br />and are found in soils throughout the world. There is con- <br />fusion in the literature over the proper designation of the <br />bacteria because different microbiologists found the bacteria <br />operating in various localities. Some of this confusion has <br />been resolved in recent reviews (24, 25). A summary of the <br />important types appears below: <br />A. Thiobacillus ferrooxidans: Gains its primary nourishment <br />. by catalyzing the oxidation of Fe(II) to Fe(III). It can