The potential of ball-milled South African bentonite clay for attenuation of heavy metals from acidic wastewaters: Simultaneous sorption of Co²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ ions

Adsorption of Co2+, Cu2+, Ni2+, Pb2+, and Zn2+ by bentonite clay from polycationic solution was investigated. Mineralogical composition of clay was done using X-ray diffraction (XRD), elemental composition using X-ray fluorescence (XRF) and morphology using scanning electron microscopy (SEM) and electron dispersion X-ray (EDX). Optimum adsorption conditions were evaluated using batch experimental procedures. Parameters optimized included: adsorbent dosage, shaking time, ion's concentration and pH. Interaction of bentonite clay with acidic wastewaters led to an increase in pH and significant reduction in metal species concentration. Optimization experiments revealed that 60 min of equilibration, 1 g of clay dosage, 50 mg L-1 of polycations, pH 6, 250 rpm and 26 °C were the optimum condition for removal of heavy metals from the acidic wastewaters. Removal of metals was >99% for all chemical species in solution. The metal species sorption affinity varied as follows: Co > Cu > Ni = Zn > Pb. Kinetic studies showed that adsorption by bentonite clay fitted to the pseudo-second-order model with pore diffusion also acting as a major rate governing step. The adsorption data fitted well to Freundlich adsorption isotherm than Langmuir adsorption isotherm hence confirming multilayer adsorption. This comparative study proved that bentonite clay can increase the pH and effectively remove metal species from acidic and metalliferous wastewaters.