Removal of Methylene Blue by adsorption onto natural and purified clays: Kinetic and thermodynamic study

The present work explores the optimization of experimental conditions for the adsorption of the organic cationic dye methylene blue (MB) by natural and purified clays. The adsorption capacities of samples were determinate to evaluate the efficiency of the purification process. At first, we performed mineralogical and textural analysis of the clay minerals, namely, smooth clay (SC), rigorous clay (RC) and bentonite clay (BC), using various techniques, particularly, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and X-ray fluorescence (XRF). The characterizations data showed that SC and RC are mainly composed of smectite associated with kaolinite, illite and chlorite. While BC is considered a mesoporous and swelling clay with smectite as the major component. The cation exchange capacity (CEC) was determinate as a 6.30 meq/100 g, 10.69 meq/100 g and 32.64 meq/100 g for SC, RC and RC natural samples respectively. It was noticed that the CEC as well as the specific surface area increase once the impurities associated with the clay are removed. A preliminary study was carried out to elucidate the influence of different parameters; such as contact time, exchangeable cation, solute concentration, temperature and pH on MB adsorption in aqueous solution by clays. The adsorption equilibrium data were analyzed by the linear and nonlinear forms of the Langmuir and Freundlich isotherm models. The Langmuir isotherm fitted well with the experimental data, and the pseudo-second-order kinetic model represented appropriately the adsorption kinetics of MB dye. Moreover, purified clays showed improved adsorption capacity for MB compared to pristine ones: 147.64; 257.69 and 1383 mg/g versus 128.75; 193.4 and 681.85 mg/g for SC, RC and BC respectively. This is attributed to a suitable specific surface, ion exchange capacity and electrostatic interactions as result of a more adequate texture and composition of our treated clays. The thermodynamic studies reveled negative Gibbs free energy change (ΔG) values (from -6.293 to -18.608 KJ.mol−1), positive enthalpy change (ΔH) values, exceeding 40 kJ/mol and positive entropy change (ΔS) values confirming the spontaneous, endothermic and chemisorption nature of the MB adsorption process. The results show that the tested purified clays are efficient in MB removal and can be used as an alternative to the commercial adsorbents, which are often relatively expensive.