Low adsorption capacity of natural clay minerals is a key technical and economical issue which limits their industrial application as low cost absorbents for removal of hazardous contaminants from wastewater. Herein, natural bentonite was modified by thermal activation (TA), acid activation (AA) and combined acid and thermal activation (ATA). In this study, we studied how the key operational parameters: contact time, dye concentration, bentonite dosage, pH and temperature could affect the adsorption performance for the removal of Congo red (CR). The thermodynamic parameters revealed that adsorption reaction using the modified clays is spontaneous and exothermic. The experimental data were analyzed by Langmuir and Freundlich isotherm, Pseudo-kinetic and particle diffusion models. The results revealed that Freundlich isotherm provided a better fit to the experimental data. The adsorption kinetics followed both the pseudo first and second-order rate equations, while the second order giving a better fit. The adsorption process also showed a intra-particle diffusion mechanism. The ATA bentonite demonstrated the highest adsorption capacity by removing over 95% of CR and its adsorption fitted best to all models. (C) 2012 Elsevier B.V. All rights reserved.