Abstract
Natural attenuation in acid mine drainage (AMD) due to biological iron and arsenic oxidation offers a promising strategy to treat As-rich AMD in passive bioreactors. A reactive transport model was developed in order to identify the main controlling factors. It simulated batch and flow-through experiments that reproduced natural attenuation in a high-As AMD. The 2-D model coupled secondorder microbial kinetics (Fe-and As-oxidation) and geochemical reactions to hydrodynamic transport. Oxidation only occurred in the biofilm with an oxygen transfer from the air through water column. The model correctly simulated the Fe(II)-Fe(III) and As(III)-As(V) concentrations in the outlet waters and the precipitates, over hydraulic retention times from 50 min to 800 min. It confirmed that the natural attenuation at 20 °C was driven by the fast Fe(II) oxidation and slow As(III) oxidation that favored arsenite trapping by schwertmannite over amorphous ferric arsenate (AFA) formation. The localization of iron oxidation in the biofilm limited the attenuation of arsenic and iron as the water column height increased. The change in the composition of the bacterial iron-oxidizer community of the biofilm at the lowest pH boundary seems to control the Fe(II) oxidation kinetic rate besides the bacterial concentration.