Microbial oxidation of ferrous iron in acid mine water at sulfur and iron-sulfide mine

The microbial oxidation of ferrous iron in the acid mine water from the abandoned Matsuo sulfur and iron-sulfide mine area was investigated. The acid mine water had an extremely low pH value and contained a high concentration of ferrous iron (about 1,000 ppm). The ferrous iron was oxidized rapidly to the ferric form in the acid mine water. The oxidation of ferrous iron in the acid mine water was completely inhibited by millipore filtration, toluene addition or incubation at a low temperature. These facts indicate that the oxidation of ferrous iron is not a spotaneous oxidation by atmospheric oxygen but a biological oxidation. Iron-oxidizing bacteria were ubiquitously found in the acid mine water samples taken from the mine area. The number of iron-oxidizing bacteria and the iron oxidation ratio along the Akagawa River were found to increase gradually as the stream ran down. The degree of iron oxidation was well correlated with the number of iron-oxidizing bacteria. From these results it is concluded that the activities of iron-oxidizing bacteria are responsible for the oxidation of ferrous iron in the acid mine water at the Matsuo Mine.