A Sustainable Solution for Mine Waste Water: Developing Application Parameters for the Treatment of Mine Water Bodies with Severe Acid Mine Drainage (pH > 3) to Promote Biological Polishing
Acid Mine Drainage (AMD) or Acid Rock Drainage (ARD) is a particularly insidious, self-perpetuating problem that is produced in three distinct phases. When sulphidic minerals weather, through exposure to oxygen and moisture, sulphuric acid is produced; the acid dissolves metals and sulphides, which exposes new surfaces to weathering, and attracts acidophilic bacteria; the bacteria feed upon the energy released when molecules of sulphide minerals are torn apart and extrude enzymes to hasten the process. This cycle plays itself out in mines, waste rock piles, and in tailings deposits, which form an ideal habitat for the acidophiles.
Different groups of these bacteria specialize in breaking down particular minerals and others convert ferrous iron into ferric iron, an oxidizing agent. Unless the process is interrupted, old mine sites will generate AMD/ARD so long as iron is available. This low pH acidic water is a direct threat to fish and other aquatic life. Depending on the reactivity of the sulphidic mining wastes, acid mine drainage can rapidly decrease to a pH of below three, where iron stays in the water, and treatment costs increase. Iron rich low acid drainage is not amenable to a passive ecological treatment system, which is considered sustainable and economical. To make such drainage suitable to ecological treatment, a treatment, which gradually increases the pH, needs to be found, such that biological polishing biota can start to remove metals and relegate them to the sediment in the water body.