Capacity of Wood Ash Filters to Remove Iron from Acid Mine Drainage: Assessment of Retention Mechanism

Acid mine drainage (AMD) with high iron concentrations can be a major challenge for passive treatment systems, particularly when sulphate reducing passive bioreactors (SRPBs) are used. The capacity of wood ash filters to act as a polishing step after SRPB treatment of a high-iron AMD (4,000 mg L-1 of Fe) was assessed. Five columns (1.7 L) with different mixtures of wood combustion ash and sand were investigated for their potential to remove metals from an SRPB effluent over 122 days. These materials had a high specific surface area (between 46 and 159 mA(2)A g(-1)), high organic carbon contents (between 12 and 32 %), and a high paste pH (up to 12.2). The Freundlich isotherm model reflects the observed iron sorption behavior on the material surface. Column study results indicate that the wood ash decreased iron concentrations for more than 100 days below 10 mg L-1 (99 % iron removal), mainly due to iron hydroxide precipitation and sorption. The risk of system clogging was negligible since the saturated hydraulic conductivity remained stable, between 5.0 x 10(-3) and 3.1 x 10(-2) cm s(-1). Between 44 and 52 % of the sulphate was also removed due to gypsum precipitation.