Experimental study on the adsorption of Cr⁺⁶ and Ni⁺² from aqueous solution using low-cost natural material

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$°C).$ 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.