The purpose of this study was to evaluate the efficiency of chemically treated (NaOH or HCl) cactus for the removal of Zn2+ and Mn2+ ions from aqueous solution via biosorption. The characteristics of the adsorbents were studied using Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and Brunauer-Emmett-Teller analyses. In addition, the effects of biosorbent dose, initial metal ion concentration, contact time, initial solution pH, and temperature were investigated. Adsorption kinetics, equilibrium, and isotherm studies were also conducted. The FTIR results indicated that chemical treatment changed the stretching vibration. The elemental analysis showed that NaOH treatment increased the oxygen content by 56.91% and decreased the carbon content by 26.44%. The highest removal efficiencies of Zn2+ and Mn2+ were found with the NaOH-treated cactus, followed by the HCl-treated cactus. The biosorption efficiency of Zn2+ increased from 36.0% to 95.9% and from 36.0% to 84.6% when the raw biosorbent was treated with NaOH and HCl, respectively. For Mn2+, the biosorption efficiency increased from 33.2% to 88.6% and from 33.2% to 71.0% when the raw biosorbent was treated with NaOH and HCl, respectively. Moreover, a rapid increase in binding efficiency was observed under conditions of pH up to 5, with Zn2+ adsorption greater than that of Mn2+. Metal ion biosorption increased with increasing contact time and initial metal ion concentration. The pseudo-second-order model was more appropriate than the pseudo-first-order model for explaining the biosorption processes of Zn2+ and Mn2+ by cactus due to its higher R-2 value. From this study, it can be concluded that chemical pretreatment of raw cactus (Opuntia) with NaOH strongly enhanced its biosorption potential for the studied metals.
