Kinetic and equilibrium studies on the adsorption of crystal violet dye using kaolin as an adsorbent

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 i. particle size distribution using particle size analyzer, ii. BET surface area using BET surface analyzer, iii. structural analysis using X ray diffractometer, and iv. 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 (Delta G degrees), enthalpy (Delta H degrees), and entropy (Delta S degrees) changes are calculated to know the nature of adsorption. The calculated values of Delta G degrees are -4.11 and -4.48kJ/mol at 295K and 323 K, respectively, for 20mg/L of dye concentration, which indicates that the adsorption process is spontaneous. The estimated values of Ho and So 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.