Bioreactors for low-pH iron(ii) oxidation remove considerable amounts of total iron

Abstract

Low-pH Fe(II) oxidation occurs naturally in certain acid mine drainage (AMD) systems and can be incorporated into passive treatments by enhancing the development of terraced iron formations (TIFs). For extremely difficult-to-treat AMD (very low pH, high concentrations of Fe(II) and associated metals), an active treatment bioreactor may be required. Based on field studies of eight low-pH TIF sites in the Appalachian Bituminous Coal Basin, US, two sites that displayed the fastest and regional-average rates of Fe(II) oxidation were selected to enrich for Fe(II)-oxidizing microbes in chemostatic bioreactors with controlled geochemistry. After 74 to 128 days of fed-batch enrichment periods and a series of hydraulic residence time (HRT) experiments, four bioreactors (two for each site) were operated in flow-through mode through a series of pH set-points (pH 2.1 to 4.2; fixed [Fe(II)]_in) or influent Fe(II) concentrations (80 to 2400 mg L⁻¹; fixed pH) for an additional 52 to 138 days using site-specific anoxic AMD as influent. Fe(II) oxidation kinetics in bioreactors were remarkably similar from two sites that displayed significantly different field rates. Fe(II) oxidation rates were faster at high flow rate, low pH and high Fe(II) concentrations, consistent with field results. A three-parameter dual-Monod rate law was developed to describe Fe(II) oxidation kinetics solely based on pH and Fe(II) concentration, and agreed well with some other bioreactor and field studies. Importantly, these bioreactors also effectively removed total Fe at rates 7 to 20 times better than passive treatment settling ponds and TIFs sites, at optimal pH between 2.9 to 3.3. All of these results point to the promise of bioreactors enriched by natural-occurring acidophilic Fe(II)-oxidizing microbes for AMD treatment.