Influence of the alkyl chain length of cyanine dyes on their adsorption by Na+-montmorillonite from aqueous solutions

Adsorption of cyanine dyes from aqueous solution onto the surface of Na+-montmorillonite (Na+-MMT) was investigated spectrophotometrically. A set of four linear alkyl chain cyanine dyes (D-C-5, D-C-7, D-C-10, and D-C-18), were used in this work to evaluate the effect of carbon chain length on the efficiency of dye adsorption. The loading of the dye onto Na+-MMT surface was increased with increasing the length of alkyl chain. The effect of the contact time, initial concentration of the dye, pH of solution and temperature on the adsorption process were studied. Adsorption isotherms were established and found to correlate well with the Freundlich, Langmuir, and Dubinin-Radushkevich adsorption models. Values of the constants K-f and 1/n determined from the Freundlich isotherm decreased with the increase in the tail length of the dye (D-C-5 -> D-C-18). The same trend was observed for the Langmuir adsorption constant, K-L. The E values calculated from Dubinin-Radushkevich isotherm increased with increasing the alkyl chain length of the cyanine dyes but still less than 16 kJ mol(-1), suggesting physical adsorption process. Further, the thermodynamic parameters (the enthalpy, Delta H, Gibbs free energy, Delta G, and entropy, Delta S) characteristics of Na+-MMT/dye interaction were evaluated. The adsorption of cyanine dyes onto Na+-MMT was not spontaneous and endothermic. The magnitudes of Delta H and Delta S have dropped on going from D-C-5 to D-C-18. In contrast, as the size of the alkyl tail on the dye increased, Gibbs free energy Delta G was decreased. As far as the adsorption kinetics is concerned, the present adsorption regime followed pseudo-second-order kinetic model rather than pseudo-first-order model. The intraparticle diffusion model resulted in the existence of both the bulk and intraparticle diffusions of the dye molecules into the interior pore surface of Na+-MMT particles. (C) 2012 Elsevier B.V. All rights reserved.