Removal of cationic and anionic dyes using purified and surfactant-modified Tunisian clays: Kinetic, isotherm, thermodynamic and adsorption-mechanism studies

Purified and surfactant-modified Tunisian clays were investigated for their capacity to remove cationic and anionic dyes (crystal violet, CV and methyl orange, MO) from aqueous solution. The samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FUR), and potentiometric acid-base titration. Batch-sorption experiments were carried out to evaluate the influence of pH, contact time, initial dye concentration and temperature on the adsorption of dyes. Pseudo-first order, pseudo-second order, intra-particle diffusion and Elovich kinetic models were considered to evaluate the kinetic parameters. To understand the interaction of the dye with the adsorbent, Langmuir, Freundlich, Temkin and Dubinin-Radushkevish isotherms were applied. Thermodynamics studies were conducted to calculate the changes in free energy (Delta degrees G), enthalpy (Delta degrees H) and entropy (Delta degrees S). A difference in the maximum adsorption capacity was observed, suggesting that the retention of dyes was influenced by structure, functional groups of dyes and surface properties of the adsorbents. Moreover, different mechanisms may control the removal of dyes. The purified Tunisian clays are excellent adsorbents for removal of the cationic dye CV and its modified form is suitable for removal of the anionic dye, MO, from aqueous solution.