Kinetics, isotherm and thermodynamics for Cr(III) and Cr(VI) adsorption from aqueous solutions by crystalline hydrous titanium oxide

The synthetic crystalline hydrous titanium(IV) oxide (CHTO), an anatase variety and thermally stable up to 300 °C, has been used for adsorption of Cr(III) and Cr(VI) from the aqueous solutions, the optimum pH-values of which are 5.0 and 1.5, respectively. The kinetic data correspond very well to the pseudo-second order equation. The rates of adsorption are controlled by the film (boundary layer) diffusion, and increase with increasing temperature. The equilibrium data describe very well the Langmuir, Redlich–Peterson, and Toth isotherms. The monolayer adsorption capacities are high, and increased with increasing temperature. The evaluated ΔG∘ (kJ · mol−1) and ΔH∘ (kJ · mol−1) indicate the spontaneous and endothermic nature of the reactions. The adsorptions occur with increase in entropy (ΔS∘ = positive), and the mean free energy (EDR) values obtained by analysis of equilibrium data with Dubinin–Radushkevick equation indicate the ion-exchange mechanism for Cr(III) and Cr(VI)-adsorptions.