Metals and Mining Processes

Metals constitute an important source of pollution in soils. As discussed in Chapters 5 and 6, they can bind to soils through humic substances, surface complex-ation, or ion exchange. In some cases more than one type of interaction can occur, as in clay minerals (e.g., montmorillonite and vermiculite) that bind metals through ion exchange as well as surface com- 

Metal contamination of soils is primarily due to the application of sewage sludge, manure, phosphate fertilizers, atmospheric deposition, and traffic emissions. The most common heavy metal ions found in soils are Zn, Cu, Ni, Pb, Cr, and Cd. As mentioned earlier (see Section 6.3.1.4), sequential extraction techniques can differentiate among the metal forms in a soil, typically the acid soluble fraction (e.g., carbonates), the reducible fraction (e.g., iron/manganese oxides), and the ox-idizable fraction (i.e., metals in low oxidation states). 

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 Fe2+ thus produced oxidizes to Fe3+ in air at low pH, albeit very slowly. Then, Fe3+ can either hydrolyze and form insoluble Fe(III) hydroxide, or act as a secondary oxidant to FeS2  

Effects of Pollutants on the Chemistry of the Atmosphere, Hydrosphere, and Lithosphere 

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