Kinetics and Equilibrium Thermodynamic Studies of the Adsorption of Phenolphthalein and Methyl Orange onto Muscovite Clay

A batch adsorption method for the removal of methyl orange and phenolphthalein from aqueous media onto muscovite clay has been assessed and proven to be successful. The adsorption studies were performed at 303, 323 and 343 K. Factors such as temperature and pH were evaluated. Equilibrium adsorption for all the adsorbates was attained after 30 minutes. Investigation of the adsorption isotherm at 303 K using the Langmuir and Freundlich isotherm models showed that the adsorption of both indicators obey the Langmuir isotherm model with monolayer adsorptive capacities of 13.00 and 2.48 mg/g for methyl orange and phenolphthalein, respectively. The pseudo-second-order kinetic model best describes both adsorption processes with R2 > 0.99 and negative activation energies indicating physisorption processes. Assessment of the thermodynamic parameters showed that although the adsorption processes were endothermic (ΔH values of +8.77 kJ/mol and +15.62 kJ/mol for methyl orange and phenolphthalein respectively) over the range of temperatures studied, the relatively high entropy changes (+38.05 kJ/(molK) and +52.52 kJ/(molK) for methyl orange and phenolphthalein respectively) gave an overall negative change in Gibbs free energy making the processes spontaneous. Generally, the adsorption of both dyes was found to increase steadily within the pH range of 3.3 to 7.0 but decreased drastically from pH = 8.0 to 10.0, a phenomenon which can be attributed to electrostatic repulsion between anionic sites on the dyes and negatively charged active sites on the surface of the adsorbent.