Iron precipitation kinetics in synthetic acid mine drainage

To evaluate the design and operation of a new active treatment system for acid mine drainage (AMD), the behavior of ferric iron solutions after the addition of bicarbonate ions was investigated. The effects of various other factors common to AMD on the precipitation rate of iron were also studied. It was found that the rate of Fe III precipitation in synthetic AMD was not affected by the presence of Al or Mn, within the concentration ranges investigated (for Al, 0-0.01 M, for Mn, 0-0.002 M). Our experiments showed that the induction time (t_md),i.e., the time elapsed between the addition of base ions and the detection of iron precipitation, decreased with increasing iron concentration and pH but increased with increasing sulfate concentration: log t_ind=6.7(±0.30)?1.29 (±0.10) pH+0.94(±0.07) log SO₄]?0.36 (±0.05) log Fe] Our results suggested that sulfate sorbed to the surface of growing iron oxyhydroxides, inhibiting their growth. This effect offers an important tool that can be used to control the precipitation of iron in AMD treatment facilities.