Surface Properties of Heat-Treated Clinoptilolites and Their Relationship to Lead Removal from Aqueous Solutions

This study investigated the effects of heat treatment on the surface properties, structure, and Pb(II) sorption capacity of natural clinoptilolite zeolite (Nat-CLI). For this purpose, 10 g samples of Nat-CLI were heated separately at 300, 400, and 500 °C for two hours. The resulting samples were labeled CLI-300, CLI-400, and CLI-500, respectively. The samples were characterized using thermogravimetric analysis (TGA), scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) analysis, and point of zero-charge (pHpzc) measurements. XRD results confirmed that the crystal structure of Nat-CLI remained unchanged after heat treatment, a finding supported by FT-IR and TGA analyses. BET analysis revealed that the heating temperature altered both the specific surface area (SBET) and the mean pore diameter. The values were as follows: SBET = 16.5, 14.1, 14.4, and 14.9 m2/g and mean pore diameter = 38.1, 36.0, 48.6, and 36.0 Å for Nat-CLI, CLI-300, CLI-400, and CLI-500, respectively. The results of the kinetic study showed that the pseudo-second order model best agreed with the experimental data for all adsorbents. The maximum sorption capacities of Pb(II) were 28.26, 32.96, 34.69, and 33.85 mg/g for Nat-CLI and CLI-300, CLI-400, and CLI-500, respectively. These results suggest that the sample treated at 400 °C (CLI-400) achieved the highest sorption capacity due to its larger mean pore diameter.