Modeling of microbial kinetics and mass transfer in bioreactors simulating the natural attenuation of arsenic and iron in acid mine drainage.

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 is developed in order to identify the main controlling factors. It simulates batch and flow-through experiments that reproduce natural attenuation in a high-As AMD. The 2-D model couples second-order microbial kinetics (Fe- and As- oxidation) and geochemical reactions to hydrodynamic transport. Oxidation only occurrs in the biofilm with an oxygen transfer from the air through the water column. The model correctly simulates the Fe(II)-Fe(III) and As(III)-As(V) concentrations in the outlet waters and the precipitates, over hydraulic retention times from 30 min to 800 min. It confirms that the natural attenuation at 20 °C is driven by the fast Fe(II) oxidation and slow As(III) oxidation that favors arsenite trapping by schwertmannite over amorphous ferric arsenate (AFA) formation. The localization of iron oxidation in the biofilm limits the attenuation of arsenic and iron as the water column height increases. 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.