Acid Mine Drainage

Many ores of metals such as Cu, Zn, and Pb contain pyritic materials. In addition, high-S coals also contain substantial amounts of pyrite and organic S. When the ores or coal are removed, the traditional practice has been to leave the mine spoils in heaps of tailings exposed to air and water. While sulfide material oxidation may be abiotic, the reaction rate is orders of magnitude greater in the presence of oxidizing bacteria such as Ac. ferrooxidans. The result is the accelerated oxidation of pyrite and other sulfidic minerals in the same process described above for acid sulfate soils. Acid mine drainage (AMD) includes the acidified water that percolates through aboveground mine tailings and to a lesser extent the groundwater that becomes contaminated when underground mine workings are abandoned and the water table is allowed to rebound. Uncontrolled leaching of mine tailings and abandoned sites is a major cause of environmental degradation and expensive remediation in numerous areas. In the United States alone, AMD from abandoned mine sites has polluted 75,000 ha of impoundments and lakes, as well as 20,000 km of rivers and streams (Pierzynski et al., 2000), and it continues to be the most important environmental challenge for the mining industry.

Acid mine drainage environments are scientifically interesting as model ecosystems for the analysis of biogeochemical interactions and feedbacks and microbial community structure and function (Baker and Banfield, 2003, Fig. 15.11). Microbial communities in AMD systems tend to contain few distinct taxa, but these taxa are phylogenetically diverse. In regions of AMD systems exposed to sunlight, photosynthesis is an important source of energy; however, below the surface, inputs of externally derived fixed C and N are minimal. The primary metabolic groups detected in AMD systems are lithoautotrophs that oxidize Fe2+ and S~ released by pyrite dissolution, organoheterotrophs that utilize C produced by the lithoautotrophs, lithoheterotrophs that oxidize Fe and S, and anaerobes that couple oxidation of S or organic C to Fe3+ reduction. A subset of these organisms must produce all the fixed C and N required by the community. At lower temperatures (<30°C) and higher pH (>2), the thiobacilli are probably the dominant group responsible for CO2 fixation. At lower pH and higher temperatures, autotrophic taxa include Leptospirillum spp., Ferroplasma spp., Sulfobacillus spp., Ferromi-crobium spp., and Acidimicrobium spp. The supply of N to the system is more problematic because in the largely aerobic AMD environment, N2 fixation by

CO2 CO2

N2, CO2

N2, CO2

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