Surface Characterization, Isotherm Modeling and Thermodynamics of Zn(II) Adsorption onto Modified Corncob

Investigations were conducted to study the adsorption behavior of Zn(II) ions on a lowcost adsorbent. The natural corncob (NCC) was citric acid modified to enhance its adsorption ability. The physical and chemical properties of the modified corncob (MCC) such as surface area, surface functional groups and surface charge determine the adsorption characteristics of the material. The surface areas of NCC and MCC were determined to be 13 m² /g and 9 m² /g, respectively. The values for the pHPZC of the NC and MCC were found to be 5.8 and 3.6, respectively. The effect of various parameters such as pH of the medium, adsorbent dose, initial Zn(II) concentration and temperature were examined. The equilibrium adsorption data were modeled by Langmuir, Freundlich , Temkin and Dubinin-Radushkevich isotherm equations. The constants of each isotherm model were evaluated. The presented values of the normalized standard deviation (Δq) indicate that the Freundlich isotherm provide the best correlation between experimental and predicted data with the lowest value of Δq equal to 3.4 % and appears to be the best fit model for the present Zn(II) adsorption studies. The obtained value for the mean energy of adsorption (Eads = 10.8 kJ mol^-1), shows that the interaction between Zn(II) ions and MCC proceeded principally by ion exchange at all temperatures. The optimum conditions for the maximum Zn(II) adsorption capacity (qm= 26.3 mg g^-1 ) by MCC were pH 5.0±0.1, adsorbent dose 1.0 g, temperature 318 K, and initial Zn(II) concentration 250 mg L^-1. The sorption process was found to be endothermic in nature ( ΔH : 70.7 kJ mol^-1), with increase in entropy ( ΔS : 292.2 J mol^-1 K^-1). The negative value of Gibbs free energy, ΔG indicates that the adsorption occurs via a spontaneous process. The increase in -ΔG from 16.7 to 21.9 kJ mol^-1 with increase of temperature from 298 K to 318 K indicates that the adsorption of Zn(II) onto MCC is more favorable at higher temperatures. The results show that MCC , which has a very low economical value, may be used effectively in removal of Zn(II) from aqueous solution.