An experimental and theoretical investigation of cationic azine dye adsorption on natural sepiolite in single and multi-component systems

Abstract

Adsorption kinetics and equilibria of the azine dyes onto natural sepiolite are investigated using Methylene Blue, Thionin and Safranin O. Experimental equilibrium results for three azine dyes are well predicted by the Freundlich, Langmuir and Redlich-Peterson isotherm models. The model parameters obtained for single solute systems at 298 K are used for the calculation of adsorption isotherms in binary and ternary dye solutions using multi-component isotherm models. Modified extended Langmuir model is satisfactorily fit to the adsorption in binary and ternary solutions. The diffuse-reflectance-infrared-Fourier-transforms technique was used to characterize the bare and dye loaded sepiolite surface. The kinetic results for single, binary and ternary dye systems have been analyzed according to the pseudo-first-order and Elovich models. Vermeulen approximation was also applied to the data for prediction of the diffusion behavior of the dyes. The molecular chemical reactivity of the dye molecules estimated based on the Density Functional Theory was well correlated to the sorption results. The energy gap between the highest-occupied molecular-orbital and lowest-unoccupied-molecular-orbital energies, chemical potential, chemical hardness, softness, global electrophilicity and global nucleophilicity values prove that Methylene Blue is more reactive than Thionine and Safranine O cationic dyes.