Adsorption characteristics and the kinetics of the cation exchange of rhodamine-6G with Na+-montmorillonite

The adsorption and the kinetics of the cation exchange of rhodamine-6G (Rh-6G) with Na+-montmorillonite (Na+-MMT) have been studied. The binding parameters of Rh-6G have been determined by applying Freundlich and D-R isotherms. The enthalpy and the entropy of adsorption have been determined. The isosteric heat of adsorption has also been determined and decreases with increasing the concentration of Rh-6G. Increasing the concentration of Rh-6G led to a decrease in the adsorption capacity, which attributed to the formation of Rh-6G aggregates. Kinetic measurements of the cation exchange were followed up using a stopped-flow electrical conductivity detection unit. The cation-exchange process exhibited first-order kinetics with respect to the dye concentration and inversely proportional to the clay concentration. The measurements were accomplished at different temperatures and the activation parameters were determined. Increasing the Na+-MMT concentration led to a decrease in the rate constant. The latter is also affected by changing the exchangeable cation. (C) 2002 Elsevier Science (USA).