Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials

This study was to evaluate the adsorption capability of clay minerals of bentonite, kaolin and zeolite to remove Congo Red from aqueous solution. The experiments were carried out in a batch system to optimise operation variables: adsorbent dosage, Congo Red concentration, pH and temperature. Adsorption kinetic and equilibrium isotherm of the clay materials were studied using pseudo-first order and second order kinetic equations, and Freundlich and Langmuir models. The equilibrium data of kaolin was found to best fit to the Langmuir model, while bentonite and zeolite were best explained by the Freundlich model. The adsorption kinetic followed the pseudo-second order equation for the three adsorbents. Intra-particle diffusion studies revealed that the adsorption rates were not solely controlled by the diffusion step. Further thermodynamic investigations showed that the adsorption is an exothermic and spontaneous process. Sodium bentonite demonstrated the best adsorptive capacity followed by kaolin clay, and they can be employed as low-cost alternatives for recalcitrant dye removal from industrial wastewater.