Kinetic and Equilibrium Studies on the Adsorption of Crystal Violet Dye using Kaolin as an Adsorbent

Abstract

Experimental investigations are carried out to adsorb toxic crystal violet dye from aqueous medium using kaolin as an adsorbent. Characterization of kaolin is done by measuring

1. particle size distribution using particle size analyzer,

2. BET surface area using BET surface analyzer,

3. structural analysis using X ray diffractometer, and

4. microscopic analysis using scanning electron microscope.

The effects of initial dye concentration, contact time, kaolin dose, stirring speed, pH, and temperature are studied for the adsorption of crystal violet in batch mode. Adsorption experiments indicate that the extent of adsorption is strongly dependent on the pH of the solution. Free energy of adsorption (ΔG ^o ), enthalpy (ΔH ^o ), and entropy (ΔS ^o ) changes are calculated to know the nature of adsorption. The calculated values of ΔG ^o are −4.11 and −4.48 kJ/mol at 295 K and 323 K, respectively, for 20 mg/L of dye concentration, which indicates that the adsorption process is spontaneous. The estimated values of ΔH ^o and ΔS ^o show the negative and positive sign, respectively, which indicate that the adsorption process is exothermic and the dye molecules are organized on the kaolin surface in more random fashion than in solution. The adsorption kinetic has been described by pseudo first order, pseudo second order and intra‐particle diffusion models. It is observed that the rate of dye adsorption follows pseudo second order model for the dye concentration range studied in the present case. Standard adsorption isotherms are used to fit the experimental equilibrium data. It is found that the adsorption of crystal violet on kaolin follows the Langmuir adsorption isotherm.

Keywords:

• Crystal violet
• kaolin
• adsorption isotherm
• zero point charge
• SEM

ACKNOWLEDGMENTS

This works is partially supported by a grant from the Defense Research Laboratory, Tezpur, India under the scheme no. DRLT‐P1‐2006/Task 10. Any opinions, findings and conclusions expressed in this paper are those of the authors and do not necessarily reflect the views of Defense Research Laboratory, Tezpur, India.