Investigation of adsorption efficiency of Cu2+ and Zn2+ by red soil and activated bentonite from acid copper mine drainage

Heavy metals can be transmitted to living creatures and ultimately to humans through various ecosystems due to their accumulative properties. The entry of these compounds through acid mine drainage into the water, soil, and agricultural products has created a condition that is considered always a threat for human health. The research was carried out in order to investigate the removal of Cu2+ and Zn2+ by red soil and activated bentonite from acid copper mine drainage in Rafsanjan. Variables examined in this study were temperature, particle size, contact time, pH, and adsorbent concentration. Both adsorbents were characterized using X-ray fluorescence and Fourier transform infrared. The Cu2+ and Zn2+ concentrations of samples were determined using flame atomic absorption spectroscopy after the adsorbents were added. The results showed that increases in contact time from 5 to 45 min had a notable effect on the removal process. The Cu2+ removal rates by both adsorbents were higher than those of Zn2+. The current study showed that the process of adsorption of Cu2+ and Zn2+ ions by natural soils is a thermal reaction, and adsorption capacity and equilibrium increase with increasing temperature. With reducing adsorbent particle size from the range of 2-6.7 mm to the range of 0.5-0.8 mm, the removal efficiency of Cu2+ and Zn2+ was reduced on average by 13% using activated bentonite and red soil. The results indicated that the Cu2+ and Zn2+ of acid copper mine drainage could be reduced using activated bentonite and red soil as low-cost and available adsorbents. Results indicated that Freundlich adsorption isotherm was the best model for the metal ion adsorption onto the activated bentonite and red soil with higher R-2. The kinetic studies revealed that the adsorption process followed the pseudo-second-order kinetics model. The decrease of the adsorption efficiency in the real sample was notable compared with the synthetic sample, and this reduction was bigger for zinc metal. The results indicated that the activated bentonite and red soil achieved acceptable efficiency in removing copper and zinc metals from the acid copper mine drainage.