Removal and recovery of Ni(II) ions from synthetic wastewater using surface modified Strychnos potatorum seeds: experimental optimization and mechanism

Strychnos potatorum seeds have been utilized for the preparation of adsorbent, surface modified S. potatorum seeds (SMSP), by sulfuric acid treatment with 1:2 ratios of precursor to sulfuric acid. The adsorption process depends on the solution pH, adsorbent dose, contact time, initial Ni(II) ions concentration, and temperature. The adsorption kinetics of Ni(II) ions removal by the SMSP was relatively fast and it reaches the equilibrium at 30 min. The maximum removal of Ni(II) ions was observed at an optimum conditions: pH of 5.0, adsorbent dose of 5 g/L, contact time of 30 min, and at temperature of 30 degrees C for an initial Ni(II) ions concentration of 100 mg/L. In order to investigate the adsorption kinetics for the removal of Ni(II) ions by SMSP, pseudo-first-order and pseudo-second-order kinetic models were studied. It was observed that the pseudo-second-order kinetic model fits the experimental data better than the pseudo-first-order kinetic model with good coefficient of determination values. Adsorption mechanism was discussed with different models such as intraparticle diffusion, Boyd kinetic, and shrinking core models. It was observed that the adsorption process was controlled by both film and particle diffusion. The effective diffusivity and diffusivity values were estimated from the Boyd kinetic and shrinking core models, respectively. Adsorption isotherm data were tested with the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich models. It was observed that the Freundlich model fits the isotherm data better than other isotherm models with good coefficient of determination values. The Freundlich constant "n" was found to be of 3.888 g/L which indicates that the adsorption of Ni(II) ions onto the SMSP followed the physical process. The thermodynamic parameters such as change in free energy, enthalpy, and entropy were also calculated. It was found that the adsorption process was spontaneous and exothermic in nature.