Efficiency of ball milled South African bentonite clay for remediation of acid mine drainage

The feasibility of using vibratory ball milled South African bentonite clay for neutralization and attenuation of inorganic contaminants from acidic and metalliferous mine effluents has been evaluated. Treatment of acid mine drainage (AMD) with bentonite clay was done using batch laboratory assays. Parameters optimized included contact time, adsorbent dosage and adsorbate concentration. Ball milled bentonite clay was mixed with simulated AMD at specific solid: liquid (S/L) ratios and equilibrated on a table shaker. Contact of AMD with bentonite clay led to an increase in pH and a significant reduction in concentrations of metal species. At constant agitation time of 30 min, the pH increased with the increase in dosage of bentonite clay. Removal of Mn2+, Al3+, and Fe3+ was greatest after 30 min of agitation. The adsorption affinity obeyed the sequence: SO42− (221.8 mg g−1) > Mn (30.7 mg g−1) > Al (30.5 mg g−1) > Fe (30.2 mg g−1). The pH of reacted AMD ranged from ≈3 to 6. Bentonite clay showed high adsorption capacities for Al and Fe at concentration <500 mg/L, while the capacity for Mn was lower. Adsorption capacity for sulphate was >50%. Adsorption kinetics revealed that the suitable kinetic model describing data was pseudo-second-order hence confirming chemisorption. Adsorption isotherms indicated that removal of metals fitted the Langmuir adsorption isotherm for Fe and sulphate and the Freundlich adsorption isotherm for Al and Mn, respectively. Ball-milled bentonite clay showed an excellent capacity in neutralizing acidity and lowering the levels of inorganic contaminants in acidic mine effluents.