EQUILIBRIUM AND THEMODYNAMIC STUDIES OF ANIONIC DYES REMOVAL BY AN ANIONIC CLAY-LAYERED DOUBLE HYDROXIDE

Abstract

Adsorption isotherm describes the interaction of adsorbates with adsorbent in equilibrium. Equilibrium data was examined using Langmuir and Freundlich isotherm models. Thermodynamic studies were used to evaluate the thermodynamic parameters; heat of enthalpy change (ΔH°), Gibbs free energy change (ΔG°) and heat of entropy change (ΔS°) in order to gain information regarding the nature of adsorption (exothermic or endothermic). Four reactive dyes of anionic type, Acid Blue 29 (AB29), Reactive Black 5 (RB5), Reactive Orange 16 (RO16) and Reactive Red 120 (RR120) were used to obtain equilibrium isotherms at 25 °C, 35 °C, 45 °C and 55 °C. Based on Giles' classification, the isotherm produced were of L2-type, indicating strong dye affinity towards the adsorbent, and with weak competition with the solvent molecules for active adsorption sites. Equilibrium data fitted both Langmuir and Freundlich isotherm models with high correlation coefficient (R > 0.91) indicating the possibility of both homogeneity and heterogeneous nature of adsorption. The negative values of ΔG° indicate the adsorption processes were spontaneous and feasible. The negative values of ΔH° lie between -20 to -75 kJ/mol, suggesting these processes were exothermic and physical in nature. The negative values of ΔS° are indication of decreased disorder and randomness of spontaneous adsorption of reactive dyes on layered double hydroxide as adsorbent.