Attenuation of metal species in acidic solutions using bentonite clay: implications for acid mine drainage remediation

A laboratory batch experimental study has been carried out to evaluate the adsorption capacity of selected metal species in acid mine drainage (AMD) by bentonite clay. Bentonite clay was mixed with simulated AMD at specific solid-liquid (S/L) ratios and agitated in a reciprocating shaker and adsorption of selected toxic metals assessed over time. Cation exchange capacity varied from 1140 to 1290 meq kg(-1). Contact of AMD with bentonite leads to increase in pH and a possible reduction in electrical conductivity and total dissolved solids. At constant agitation time of 60 min, the pH increased with dosage of bentonite. Removal of Mn2+, Al3+, and Fe3+ was observed to be greatest at 60 min of agitation. Bentonite clay exhibits high adsorption for Al3+ and Fe3+ at concentration less than 300 mg L-1, while the capacity for Mn2+ was observed to be lower. Adsorption capacity for SO42- was low with a great percentage of the SO42- remaining in solution. Adsorption capacity of bentonite with more complex formulated AMD and gold tailing leachates was low for Fe3+, Al3+, and Mn2+. This indicates that optimum adsorption of bentonite clay is dependent on the chemistry of the AMD and its application might be site specific.