Adsorption kinetics, isotherm, and thermodynamic studies of adsorption of pollutant from aqueous solutions onto humic acid

In the present study, humic acid was used as an adsorbent for the investigation of the adsorption kinetics, isotherms, and thermodynamic parameters of hexavalent chromium from aqueous solution at varying pH, temperatures, and concentrations. Adsorption isotherms and equilibrium adsorption capacities were determined by the fittings of the experimental data to three well-known isotherm models: Langmuir, Freundlich, and Redlich-Peterson. The results showed that the Langmuir and Redlich-Peterson models appear to fit the adsorption better than did the Freundlich adsorption model for the adsorption of chromium onto humic acid. The equilibrium constants were used to calculate thermodynamic parameters such as the change of free energy, enthalpy, and entropy. The derived adsorption constants (logaL) and their temperature dependencies from Langmuir isotherm have been used to calculate the corresponding thermodynamic quantities such as the free energy of adsorption, heat, and entropy of adsorption. The thermodynamic data indicate that Cr (VI) adsorption onto humic acid is entropically driven and characterized by physical adsorption.