Thermodynamics and kinetics of adsorption of a cationic dye onto sepiolite

Abstract

The adsorption kinetics and thermodynamics of Maxilon Red GRL (MR-GRL), a cationic textile dye, onto sepiolite were investigated in aqueous solution in a batch system. The effect of contact time, stirring speed, initial dye concentration, initial solution pH, ionic strength, and temperature was studied. The initial tested concentrations of MR-GRL solution were 1.5 x 10(-3), 2.0 x 10(-3) and 2.5 x 10(-3) mol/L. The effect of pH was analyzed in the pH range from 5 to 10. The effect of ionic strength was investigated at 0.001-0.100 mol/L NaCl salt concentrations. The experiments were carried out at 288, 298, 308, and 318 K set with a thermostat. The all parameters significantly influenced the adsorption rate and capacity. Experimental data were fitted to pseudo-first order, second-order, Elovich equation, mass transfer, and intra-particle diffusion models. The adsorption kinetics data showed that this process can be described by the pseudo-second-order model, from which the rate constant and the adsorption capacity were determined. The thermodynamic activation parameters such as the activation energy, enthalpy, entropy, and Gibbs free energy were determined. The value of the standard enthalpy change (19.92 kJ/mol) indicates that the adsorption is physical in nature involving weak forces of attraction and is also endothermic. The results showed the applicability of this clay as an efficient adsorbent for cationic dyes.