Adsorptive removal of Pb(II) and Zn(II) from water onto manganese oxide-coated sand:Isotherm, thermodynamic and kinetic studies

Manganese oxide-coated sand, MOCS, was synthesized and characterized by XRD, FTIR, SEM, and EDX techniques, and was used for the adsorption of Pb(II) and Zn(II) from their aqueous solutions. The metal ion concentration, contact time, MOCS dose and pH of the solutions were optimized. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms were studied and the maximum adsorption capacities for Pb(II) and Zn(II) were found to be 147.06 and 116.28 μg/g at 40 °C and 27 °C, respectively. These values are higher than many tested adsorbents. The energy 0.011 and 0.079 kJ/mol for Pb(II) and Zn(II) indicated adsorption as physical process. In thermodynamics the free energy change for Pb(II) ranged between −12.344 and −11.371 kJ/mol and that for Zn(II) ranged −9.638 to −11.155 kJ/mol between 27 and 40 °C that predicted the spontaneity of process. The enthalpy change −39.816 and 25.574 kJ/mol indicated the exothermic and endothermic nature of the adsorption of Pb(II) and Zn(II). Kinetic studies indicated that Pb(II) adsorption followed pseudo-second order, whereas Zn(II) followed partial pseudo-first and pseudo-second order rates. The adsorptions of both metals were controlled by film diffusion step.