Acid mine drainage, sulfide mineral

Effluents emerging from sulfide-rich waste-dumps have special characteristics, such as very low pH (< 4), high metal solubility and presence of iron colloids, which provokes water turbidity and precipitation of ochre-products. These effluents are generically named acid mine drainage (AMD), since they result, primarily, from mineral-water interactions involving some sulfide minerals that typically produce acidity upon oxidative dissolution. 

Acid mine drainage Water runoff from coal or ore mines that is rich in sulfuric acid and possibly toxic metals (such as lead, arsenic, and mercury). The sulfuric acid and metals result from the chemical weathering of pyrite (FeS2) and other sulfide minerals in the mined rocks. 

Mine tailings Rocks, minerals, sediments, soils, and other wastes that result from the mining of ore deposits or coal. Mine tailings often contain pyrite and other sulfide minerals, which oxidize in the presence of oxygen and water to form acid mine drainage. 

Among other minerals, metal sulfides are nearly ubiquitous in the environment. When they become exposed to air as a result of mining processes, they become oxidized. Microorganisms typically mediate this process, although pure chemical oxidation is also possible. After coming into contact with water (mainly rainwater), these minerals form aqueous solutions that are notably acidic and are generically called acid mine drainage AMD). Because Fe is often the main metal present, AMD can be represented by the equation... 

The sulfur-bearing minerals that predominate in coal seams are the iron sulfide ores pyrite and marcasite. Both have the same ratio of sulfur to iron, but their crystallographic properties are quite different. Marcasite has an orthorhombic structure, while pyrite is isometric. Marcasite is less stable and more easily decomposed than pyrite. The latter is the most widespread of all sulfide minerals and, as a result of its greater abundance in the eastern United States, pyrite is recognized as the major source of acid mine drainage. FeS2(s) is used here as a symbolic representation of the crystalline pyritic agglomerates found in coal mines. 

Acid mine drainage (AMD) from abandoned coal and metal mining is a serious environmental problem that has affected thousands of miles of streams and waterways. AMD is created via the oxidation of sulfide minerals, such as pyrite see reactions (10.13) and (10.14) ... 

The most common source of pollutant acid in water is acid mine drainage. The sulfuric acid in such drainage arises from the microbial oxidation of pyrite or other sulfide minerals as described in Section 3.5. The values of pH encountered in acid-polluted water may fall below 3, a condition deadly to most forms of aquatic life except the culprit bacteria mediating the pyrite and iron(II) oxidation, which thrive under very low pH conditions. Industrial wastes frequently have the potential to contribute strong acid to water. Sulfuric acid produced by the air oxidation of pollutant sulfur dioxide (see Chapter 7) enters natural waters as acidic rainfall. In cases where the water does not have contact with a basic mineral, such as limestone, the water pH may become dangerously low. This condition occurs in some Canadian and Scandinavian lakes, for example. 

Sulfur/Sulfide Oxidizing Bacteria This broad family of aerobic bacteria derives energy from the oxidation of sulfide or elemental sulfur to sulfate (Fig. 10.10). Some types of aerobes can oxidize sulfur to sulfuric acid, with pH values as low as one reported. These Thiobacillus strains are most commonly found in mineral deposits, and are largely responsible for acid mine drainage, which has become an environmental concern. They proliferate inside sewer lines and can cause rapid deterioration of concrete mains and the reinforcing steel therein. 

Sulfuric acid is formed naturally by the oxidation of sulfide minerals, such as iron sulfide. The resulting water can be highly acidic and is called acid mine drainage (AMD) or acid rock drainage (ARD). This acidic water is capable of dissolving metals present in sulfide ores, which results in brightly colored, toxic streams. The oxidation of pyrite (iron sulfide) by molecular oxygen produces iron(II), or Fe ... 

In Short Course Notes, Sulfide Oxidation and the Generation of Acid Mine Drainage, Proceedings of the Mineral Deposit Study Group Annual Meeting. London, 13-18/12/93. 

Besides their economic relevance, sulfides are also important from the environmental point of view. These minerals can be exposed during the mining activity and their oxidation, in the presence of oxygen and humidity, will produce sulfuric acid. This phenomenon, illustrated in eq. (1) for pyrite, is called Acid Mine Drainage (AMD) or Acid Rock Drainage (ARD) and it results in dramatic consequences to the biological and chemical systems in the environment. 

Acid drainage is a persistent environmental problem in many mineralized areas. The problem is especially pronounced in areas that host or have hosted mining activity (e.g., Lind and Hem, 1993), but it also occurs naturally in unmined areas. The acid drainage results from weathering of sulfide minerals that oxidize to produce hydrogen ions and contribute dissolved metals to solution (e.g., Blowes et al., 2005). 

The most important control on the chemistry of drainage from mineralized areas (once we assume access of oxygen to the sulfide minerals) is the nature of the nonsulfide minerals available to react with the drainage before it discharges to the surface (e.g., Sherlock et al., 1995). These minerals include gangue minerals in the ore, the minerals making up the country rock, and the minerals found in mine dumps. The drainage chemistry of areas in which these minerals have the ability to neutralize acid differs sharply from that of areas in which they do not. 

Sulfur dioxide is a serious atmospheric pollutant, especially serious when it 1 settles in undisturbed pockets oxidized to HjS04 widespread cause of acid rain/Sulfide minerals causeadd mine drainage" -... 

Since selenium substimtes for sulfur in the stmcture of sulfide minerals, drainage from mineralized and mined areas may have high dissolved selenium concentrations. Acid seeps derived from oxidation of sulfide minerals draining the Moreno Shale in the Coast Ranges, USA, have selenium concentrations up to 420pgL with concentrations of aluminum, manganese, zinc, and nickel in the mg L range (Presser, 1994). 

Primary minerals formed in the ore deposit prior to weathering and erosion, including a wide variety of metal sulfides and sulfosalts, metal oxides, metal- and alkaline-earth carbonates, sulfates, crystalline silica, clays, and other silicates. Many metal sulfides (especially iron sulfides such as pyrite), when exposed by erosion or mining to atmospheric oxygen and water, can form acid-rock drainage (ARD). 

In addition to dilution of acid rock/mine drainage under oxidizing conditions, neutralization can occur under mildly or highly anaerobic conditions. This will create distinctive environments in which microorganisms thrive and nanoparticles form as a result of their activity. We describe two examples of such subsurface systems below. However, before turning to these topics, we note that Fe-based microbial ecosystems are not only found in association with metal sulfide deposits, but may be broadly relevant in the subsurface where Fe-rich minerals (biotite, olivine, pyroxenes, etc.) are present in reasonable abundance and dissolve, releasing aqueous ferrous iron. 

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