Precipitation of Fe and Al compounds from the acid mine waters in the dogyae area, Korea: A qualitative measure of equilibrium modeling applicability and neutralization capacity?

To investigate the applicability of equilibrium modeling for the estimation of the chemical changes of acid mine waters, the phases predicted to precipitate by equilibrium calculation were compared with what actually precipitates from the stream and acid mine waters in the Dogyae area, Korea. The computer program MINTEQA2 was used for the equilibrium calculations based on the chemical compositional data of the water samples collected in the study area. XRD, IR, thermal and chemical analyses of the collected precipitates were performed to identify their phases.The results of the identification of the collected precipitates are inconsistent with what the equilibrium calculations predict. The equilibrium calculations indicate that ferrihydrite, FeOHSO₄, gibbsite, and AlOHSO₄ should precipitate from the stream and acid mine waters in the study area. However, the experimental analyses show that only ferrihydrite and Al₄(OH)₁₀SO₄ are the recognizable precipitates on the bottom of the stream and mine drainage channels. Comparing the stability relations among the possible precipitates with the field occurrence of the precipitates in the study area suggests that FeOHSO₄ and AIOHSO₄ are kinetically inhibited to precipitate and metastable ferrihydrite and Al₄(OH)₁₀SO₄ appear in their stability field instead. It indicates that the chemical compositional change of the waters due to the solid phase precipitation in the study area must be interpreted and predicted in terms of the precipitation of not the phases predicted by the equilibrium calculation but the actually identified ones.Assuming that the dissolved species in the aqueous phase are in equilibrium with respect to the currently precipitating solid phases in the study area, the water chemistries are attempted to interpret based on the plot of the theoretically calculated activities of the dissolved species on the stability diagram for the identified precipitates and gibbsite. The plot reveals a few evolution paths of the chemical composition of the acid mine water as the acid generation and neutralization progress. The evolution path producing ferrihydrite and then Al₄(OH)₁₀SO₄ precipitation suggests that the system including acid producing pyrite has lost significant amounts of its neutralizing capacity and thus, become intolerable to the impacts from acid mine water.