Adsorption of Pb2+ and Zn2+from aqueous solutions using dried powder of cactus Opuntia: characterization, adsorption capacity and kinetics

Dried powder of the cactus Opuntia was used as an adsorbent to remove Pb2+ and Zn2+ from aqueous solution. The characteristics of the adsorbent were studied using Brunauer–Emmett–Teller (BET), point of zero charge (pHpzc), Fourier transform infrared spectroscopy (FTIR),and energy dispersive spectroscopy (EDS) analyses. Adsorption efficiency experiments were conducted on single-metal aqueous systems, including variations in dose, contact time, initial adsorbate concentration, initial solution pH, and temperature. The maximum removal efficiencies of Pb2+and Zn2+ were found to be 90.6% and 37.6%, respectively, with adsorbent doses of 0.5 and 2.0 g. The maximum amount adsorbed onto Opuntia adsorbent for Pb2+ (119.0 mg/g) was found to be higher than that of Zn2+ (32.3 mg/g) at the 300 and 200 mg/L initial concentrations of Pb2+and Zn2+ ions, respectively. A rapid rise in binding efficiency was observed with increasing pH up to 4, and Pb+2 ions adsorbed preferentially compared to Zn2+. The pseudo-second-order and Elovich models best fit the experimental data on the adsorption of Pb2+and Zn2+ onto Opuntia powder. The energy dispersive spectroscopy (EDS) analysis was employed to confirm the adsorption process. In isotherm studies, the maximum adsorption capacity (qmax) was calculated to be 62.9 mg/g for Pb2+ and 21.6 mg/g for Zn2+. The adsorption data for both Pb2+ and Zn2+obeyed the Freundlich model. A thermodynamic study showed that the adsorption process was spontaneous and endothermic. The results of the present study indicate that dried Opuntia powder as low-cost adsorbent could be useful and effective for the removal of heavy metals, especially in developing countries.