Adsorption of Congo red from aqueous solutions onto Ca-bentonite

The ability of Ca-bentonite to remove Congo red dye from aqueous solutions has been carried out as a function of contact time, temperature (20-50 °C), pH (5-10) and concentration (50-200 mg L−1). An amount of 0.2 g of Ca-bentonite could remove more than 90.0% of the dye from 100 mg L−1 Congo red dye solution for the temperature range studied here. The amount of dye adsorbed per unit weight of Ca-bentonite increased from 23.25 to 85.29 mg g−1 with increasing concentration from 50 to 200 mg L−1, but it had a little change with temperature and decreased slightly with increasing pH. The kinetics of adsorption in view of three kinetic models, i.e., the pseudo-first-order Lagergren model, the pseudo-second-order model and the intraparticle diffusion model, was discussed. The pseudo-second-order kinetic model described the adsorption of Congo red on Ca-bentonite very well. Analysis of adsorption results obtained at 20 °C showed that the adsorption pattern on Ca-bentonite followed the Freundlich isotherms. It was indicative of the heterogeneity of the adsorption sites on the clay particles. From thermodynamic studies, it was seen that the adsorption was spontaneous and endothermic.