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I~ <br /> <br /> <br />i ' <br /> <br /> <br /> <br /> <br />~I <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />INTRODUCTION <br />The uncontrolled release of acid mine drainage (AMD) is perhaps <br />the most serious impact mining can have on the environment. in addition to <br />low pH and high acidity, acid mine drainage often contains dissolved heavy <br />metals in toxic concentrations. The processes that produce acid mine <br />drainage are natural but they are accelerated by mining to produce large <br />volumes of contaminated leachates and effluents. <br />Kim et. al. (I) identified three stages in the generation of acid <br />mine drainage. In the first stage, chemical and, or, biological oxidation <br />of pyrite and other sulfide minerals slowly produces acid. This acid may <br />be neutralized by carbonate minerals in the rock. If the acid is not <br />neutralized the process enters the second stage. As the pH drops in the <br />microenvironment around the sulfide minerals, populations of acid loving <br />(acidophilic) bacteria such as Thiobacillus ferrooxidans begin to multiply. <br />These bacteria cause a significant increase in the oxidation rate and the <br />process enters the third stage. When pH around the pyrite decreases to <br />below 3, ferric iron remains in solution. Ferrous iron is converted by the <br />bacteria to ferric iron which in turn oxidizes pyrite. The rate of acid <br />formation is rapid in the last stage and is limited by the concentration of <br />ferric iron. The reactions are described in Figure 1. <br />Once in the third stage, the acid generation processes acre <br />extremely difficult to arrest. As such, prediction and prevention of acid <br />mine drainage formation are key elements of a strategy to control pollution <br />from mining operations. Prediction of AMD requires a thorough understand- <br />ing of the Stage I initiation process. Unfortunately, this step has <br />received less study than the bacterially catalyzed step 3. The role of <br />microorganisms, such as Thiobacillus ferrooxidans in the initial step is <br />still subject to controversy. <br />For example, Kleinmann and Crerar (2) found that Thiobacillus <br />ferrooxidans was capable of colonizing and acidifying natural pyritic <br />environments at an initial pH of up to 6.9. They suggested that this <br />