FIGURE 15.11 A simplified acid mine drainage system showing examples of organisms (1, 2,

3, 4) that control the flow of iron, sulfur, nitrogen, carbon, and energy in different temperature regimes (with permission from Baker and Banfield, 2003).

nitrogenase is inhibited. In moderately acidic AMD, Ac. ferrooxidans may overcome this problem by using tetrathionate as an electron donor and ferric iron (rather than O2) as an electron acceptor when fixing N. N fixation has not yet been observed directly in very low pH environments. Another important symbiosis exists between heterotrophic and certain autotrophic species in the AMD ecosystem: autotrophs may depend on coexisting heterotrophs to remove organic compounds that are toxic to them.

Given that both oxygen and water are required for the formation of AMD, it should be possible to prevent or minimize AMD production by excluding one, or both, of these (Johnson and Hallberg, 2005). Options for preventing the formation of AMD include: (1) flooding or sealing of underground mines, (2) underwater storage of mine tailings, (3) land-based storage of sealed tailings, (4) blending of mineral wastes, (5) total solidification of tailings, (6) application of anion surfactants, and (7) microencapsulation (coating) of wastes. Given the practical difficulties involved in inhibiting the formation of AMD at the source, the strategy more often applied is to minimize the impact that AMD has on receiving streams and rivers using "migration control" measures. These measures have been divided between those remediation technologies that rely on biological activities and those that do not. Abiotic remediation strategies generally involve the addition of alkaline materials such as lime to neutralize the pH of the AMD, causing many of the metals in solution to precipitate. The basis of biological remediation of AMD relies on the abilities of some microorganisms to generate alkalinity and immobilize metals. Microbiological processes that generate net alkalinity are mostly reductive processes and include methanogenesis, sulfate reduction, and iron and manganese reduction. The current state of the art of bioremediation of AMD is to exploit these reactions in constructed aerobic wetlands, compost reactors, or packed-bed iron oxidation bioreactors.

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