The adsorption behavior of Chromium (Cr)(VI) and Nickel (Ni)(II) from aqueous solution onto date pits (DPs) was investigated as a function of initial concentration (5-100 mg/L), contact time (0-70 min), adsorbent dose (2-20 g/L), pH (1-9), and temperature (25-95[Formula: see text] Equilibrium took place after 45 and 55 min for Cr(VI) and Ni(II), respectively. The removal efficiency reached 100% and 95% for Cr(VI) and Ni(II), respectively, at optimal conditions. Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) analyses were performed to characterize the adsorbent. The Langmuir, Freundlich, Tempkin, and Dubinin-Radushkevich (D-R) models were applied to describe the equilibrium isotherms. The values of the free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were 34.599 kJ/mol, 17.5736 kJ/mol K, and -51.58 kJ/mol K, respectively, at pH 3 for Cr(VI) and -25.283 kJ/mol, -14.8525 kJ/mol K, and 31.31 kJ/mol K, respectively, at pH 6 for Ni(II). Kinetics of the adsorption was analyzed. The pseudo-first-order was suitable for Cr(VI) at R2 = 0.9977, and the pseudo-second-order model was suitable for the Ni(II) at R2 = 0.999. The maximum adsorption capacities were 110.02 mg Cr(VI)/g and 10.1 mg Ni(II)/g. A single-stage batch adsorber was designed for the adsorption of Cr(VI) and Ni(II) by DP based on the optimum isotherm.
Keywords: Adsorption isotherm; chromium(VI); date pits; kinetics; nickel(II); thermodynamic parameters.