Comparative Isotherms Studies on Adsorptive Removal of Congo Red from Wastewater by Watermelon Rinds and Neem-Tree Leaves

Equilibrium adsorption studies for detoxification of Congo Red (CR) dye from single component model wastewater by powdered watermelon rinds and neem leaves adsorbents were carried out with the view to test the applicability of the adsorption process to Langmuir, Freundlich, Temkin, Dubinin-Radushkevich and Harkins-Jura isotherm models. The values of correlation coefficient, R2 (0.9359 - 0.9998), showed that all the experimental data fitted the linear plots of the tested isotherm models. Dubinin-Radushkevich’s monolayer maximum adsorption capacity qD (20.72 - 26.06 mg/g) is better than Langmuir’s qm (18.62 - 24.75 mg/g) for both adsorbents with the capacities higher for adsorption on watermelon rind than on neem leaves. Values of Langmuir separation factor (RL) suggest unfavourable adsorption processes (i.e. chemisorption) of the dye on both the adsorbents, while Freundlich constant (nF) indicates unfavourable process only for CR adsorption onto neem leaves. The Dubinin-Radushkevich’s mean free energy of adsorption, E (0.29 - 0.32 kJ/mol), suggests physical adsorption processes. Values for Temkin’s heat of adsorption, bT (-0.95 to 0.74 kJ/mol), also show physical adsorption process.