Effect of alkalinity source on mechanisms of iron, manganese and zinc removal from acid mine drainage by sulfate-reducing bioreactors

Abstract

The purpose of this study was to investigate the effect of two different sources of alkalinity source on the mechanisms of metal removal in sulfate-reducing bioreactors. Four upward-flow sulfate-reducing bioreactors each containing a 23 L mixture of organic waste materials and either waste mussel shells or limestone as an alkaline amendment were tested at hydraulic retentions of 3.3 and 10 days to treat acidic mine drainage (pH 2.9, 30 mg/L Fe, 16 mg/L Mn, 5 mg/L Zn) for ten months. A combination of methods was used to examine the effect of alkalinity source on the fate of these metals. Consistent with the monitoring data of the effluent that showed circumneutral pH and low metal concentrations, higher concentrations of Fe, Zn and Mn were found in the spent than the initial substrate, with greater metal and acidity removal in l reactors containing mussel shells (at similar residence times). Sequential extraction procedures found that Fe was mainly in the oxidizable and the residual fractions, Zn in the reducible and residual, and Mn in the exchangeable, reducible and acid extractable fractions. SEM analyses confirmed the presence of pyrite in the substrate, and the use of PHREEQC supported the interpretation that precipitation of iron sulfide and oxyhydroxide minerals, manganese carbonates and zinc sulfide occurred within the substrate for both alkalinity sources. Adsorption edge experiments on the initial substrates confirmed the potential for Zn and Mn to adsorb onto organic materials. Alkalinity source greatly affected system performance with the mussel shell reactors outperforming limestone on a volumetric basis, with the inner surfaces of the mussel shells appearing to be important for greater ongoing alkalinity release, and the outer shells important as metal sorption sites not available in limestone reactors.