Oxidation of Pyrite
4FeS2(s) + 14O2(g) + 4H2O(l) 4Fe 2+(aq) + 8SO42-(aq) + 8H+
The ferrous & hydrogen ions are released into the waters that runoff from mine drainage tunnels or tailings piles.
The ferrous ions are oxidized to form ferric ions
4Fe 2+(aq) + O2(g) + 4H+(aq) 4Fe3+(aq) + 2H2O(l)