Adsorptive removal of Cr(III) and Fe(III) from aqueous solution by chitosan/attapulgite composites: Equilibrium, thermodynamics and kinetics

Adsorption of Cr(III) and Fe(III) onto chitosan/attapulgite composites (CTS/ATP) was investigated with respect to pH, temperature, initial concentration and binary solution. Equilibrium data, at various temperatures, were described by the Langmuir, Freundlich isotherm models. The Freundlich isotherm model was fitted to the experimental data significantly better than the Langmuir isotherm. In a binary solution, the combined effect of adsorbing one metal ion in the presence of the other metal ion reduced the adsorption capacity of either metal ion. The kinetic data was well fitted to the intraparticle diffusion equation, which indicated that the three adsorption steps belonged to the pseudo-second-order adsorption process. Intraparticle diffusion increased with the increase in adsorbate concentration while film and pore diffusion decreased. The thermodynamics parameters (positive values of ΔH°, ΔS° and negative values of ΔG°) indicated the binding systems between CTS/ATP and adsorbate were endothermic, entropy gained and spontaneous in nature.