Interactions of the dye, Rhodamine B with kaolinite and montmorillonite in water

Two of the most common clay minerals, kaolinite and montmorillonite, were allowed to interact with the cationic dye, Rhodamine B in aqueous solution. The clay minerals were used along with those modified by treatment with 0.25 and 0.50 M H2SO4. The influences of pH, interaction time, clay mineral amount, dye concentration and temperature on the adsorption process were monitored and explained on the basis of physico-chemical characteristics of the clay minerals and the behaviour of the dye molecules. The optimum pH for Rhodamine B adsorption was 4.0. The interactions attained equilibrium in ~ 180 min and the rate processes followed second order kinetics. It was observed that montmorillonite had a Langmuir monolayer capacity of more than eight times that of kaolinite. The modified clay minerals showed slightly higher adsorption capacity than the raw clay minerals. Adsorption was endothermic with enthalpy of adsorption increasing with rise in temperature and the endothermic interactions were driven by increase in entropy and decrease in Gibbs energy. The results suggested that the dye can be separated from aqueous solution by both clay minerals.