Adsorption of Cd(II), Ni(II), and Zn(II) by Tourmaline at Acidic Conditions: Kinetics, Thermodynamics, and Mechanisms

Abstract

Batch experiments investigated the effects of time, temperature, and initial concentration of metal ions on heavy metal removal by tourmaline under acidic conditions to evaluate the kinetics, thermodynamics, and mechanisms. The results indicated that the adsorption of Cd(II), Zn(II), and Ni(II) depended significantly on all the above-mentioned parameters. The adsorption kinetics closely followed the pseudo-second-order model. The Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich models well described the equilibrium isotherm at acidic conditions. The maximum adsorption capacity ( q m ) of tourmaline for Cd(II), Zn(II), and Ni(II), as calculated from Langmuir isotherms at an initial pH 4.0, was greater than that obtained for previously reported materials, and followed in the order Cd(II) > Zn(II) > Ni(II) under different temperature conditions. Thermodynamic parameters indicated that adsorption was feasible, spontaneous, and endothermic, and suggested that both physisorption and chemisorption were involved. Furthermore, the pH variation after adsorption, metal ions desorbed and released, ζ-potential, and FT-IR analysis indicated that the physisorption and chemisorption mechanisms of tourmaline for heavy metals included water automatically polarized by tourmaline, the ion exchange process, and electropolar adsorption, Among the mechanisms, the automatic polarization of water caused by tourmaline is a unique adsorption mechanism of tourmaline.