Treatment of acid mine drainage by an combined chemical/biological column apparatus: mechanisms of heavy metal removal

Natural oxidation of sulphide minerals, exposed to the combined action of oxygenand water, results in the worst environmental problem associated with mining activities,i.e. acid mine drainage (AMD). Waters polluted by AMD are often characterised by lowpH, elevated concentrations of iron, sulphates and toxic metals.Biological remediation options in passive systems (permeable reactive barriers,PRB) usually exploit sulphur production by sulphate reducing bacteria, SRB.In this report a combined chemical-biological treatment was tested fordecontamination of synthetic AMD containing iron, arsenic, copper, manganese and zinc.Particular attention was paid to the investigation of the mechanisms involved inpollutant removal (chemical precipitation, sorption, bioprecipitation and biosorption) as afundamental preliminary step for permeable reactive barrier design and long termperformance estimation.Experimental tests were performed both in batch reactors and in a two-columnapparatus for sequential treatment by chemical precipitation (first column filled with natural limestone) followed by bioprecipitation/biosorption (second column filled with anatural organic mixture inoculated by sulphate reducing bacteria).Distinct mechanisms of removal for each metal were identified by combiningtheoretical data of metal solution chemistry, and results obtained from independentexperimental tests: batch and column tests, blank tests using natural organic mixture asbiosorbing materials, acid digestions, and selective extractions of metals using solidsamples of filling material after column dismantlement.This analysis allowed isolating metal-specific mechanism of abatement and denotedthe relevant contribution of biosorption phenomena in metal removal in biologicalcolumn. This contribution, generally neglected in biological PRB design with respect tobioprecipitation, should be taken into account in order to avoid misleading estimation ofSRB performance and also to better estimate PRB duration. © 2008 Nova Science Publishers, Inc. All rights reserved.