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 (pH_pzc), 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 (q_max) 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.