Adsorption of methylene blue onto natural Saudi Red Clay: isotherms, kinetics and thermodynamic studies

The current work investigates the adsorption behavior of methylene blue (MB) from aqueous solution using natural Saudi Red Clay (SRC) adsorbent. The surface characteristics of the adsorbent were investigated using surface area measurements (S_BET), Scanning Electron Microscopy (SEM), X-ray florescence (XRF) and Fourier-Transform Infrared Spectroscopy (FTIR) and x-ray diffraction (XRD). Various process parameters such as pH, adsorbent dosage, initial dye concentration, contact time and temperature were investigated. MB removal efficiency was noticed to increase with increasing adsorbent dosage while the effect of pH was noticed to be insignificant for the pH range studied. Applying the Langmuir isotherm fitting to the experimental data, a maximum adsorption capacity was found to be 50.25 mg g⁻¹ of clay at 298 K. Pseudo-first-order and pseudo-second-order models were applied to the experimental data to study the kinetics of the process. Pseudo-second-order model fitted well with the experimental as compared to the pseudo-first order model. The spontaneous and endothermic nature of the adsorption process was revealed by studying the thermodynamic parameters of the system. The positive ΔS° value indicates the affinity of MB molecules to the adsorbent surface. Re-usability/recycling study showed that this material can be successfully reused at least four times in this study without significant loss in the adsorption capabilities. (paper)