Thermodynamic Parameters of Iron–Cyanide Adsorption onto γ-Al2O3

Abstract

In this study, we describe the adsorption of ferricyanide (Fe(CN)63−) on γ-Al2O3by a modified Langmuir adsorption isotherm. The modified Langmuir isotherm developed herein can accurately predict the apparent equilibrium constant (Kapp) at different pH values and temperatures. In the experimental pH range of 5–6.5, adsorption capacity and apparent equilibrium constants decrease as the temperature increases. Because surface protonation increases as pH decrease, a more positively charged site on the surface increases the attraction force that exists between γ-Al2O3and the adsorbate. According to the Gibbs–Helmholtz equation, these apparent equilibrium constants at various temperatures and pH conditions can be applied to determine enthalpy changes (ΔH0) for the adsorption at different pH conditions. The result indicates that enthalpy change at different pH values is near −5 kcal/mol which indicates that the enthalpy change is in the order of hydrogen bonding and confirms the exothermic nature of the adsorption process. The results also indicate that other thermodynamic parameters change of ferricyanide adsorption. The positive entropy change (ΔS0) has been explained from the viewpoint of exchange reaction in this study. The negative free energy change (ΔGads0) for ferricyanide adsorption shows the spontaneous nature of the adsorption process.