Sorption characterization of a cationic dye retained by clays and perlite

The adsorption of C.I. Basic Blue 41 onto raw perlite, expanded perlite, bentonite and montmorillonite was characterized. Sorption experiments demonstrated that the clays are more efficient adsorbents compared with the perlites but their dye retention is less increased with pH and temperature. Higher dye aggregates are formed in all aluminosilicates due to the existence of adsorption locations that promote the agglomeration of the dye. The presence of dye monomers in bentonite indicates a second type of adsorption sites impairing the dyestuff aggregation. Seven adsorption models were applied to the experimental data: the obeying of the Langmuir and Harkins-Jura isotherms was better for the clays while the Freundlich, Halsey, Henderson, BET and Smith models fitted more successfully to the perlites. The applicability of the adsorption equations was correlated with the uniformity, homogeneity and distribution of the adsorption sites, the incorporation of self-assembled dye aggregates and the formation of self-organized multilayers of dye into the adsorbents. Thermodynamic quantities that characterize the sorption phenomena, i.e., enthalpy, entropy and free energy change of the adsorption, were calculated leading to conclusions on the efficiency of the adsorbing substrates and the properties of the sorbent-dye systems. (C) 2008 Elsevier Inc. All rights reserved.