Adsorption of CTAB at lignite-aqueous solution interface

Abstract

The adsorption of a cationic surfactant (cetyltrimethylammonium bromide, CTAB) from aqueous solution onto the lignite was investigated. The samples used for the adsorption experiments were: fresh; demineralized; demineralized then oxidized. The oxidation process was conducted by dry method (in air at 150 and 200°C). The UV spectrophotometric techniques were used for the analysis of CTAB in solution. The adsorption isotherms were measured experimentally at 21 and 45°C, and it was found that the adsorption capacities of all samples increased with rising temperature, and the adsorption of CTAB decreased by demineralization and oxidation. The changes in the thermodynamic quantities, enthalpy and entropy, related adsorption process that occured in some steps were calculated. Isosteric heats of adsorption and the corresponding changes in entropy have found to be negative for all of the samples. From the abrupt difference between ΔH and ΔS for the demineralized sample and those of fresh sample, it was concluded that the orientation of CTAB molecules on the surface of the demineralized sample differs than that of fresh sample, and also that the adsorption of CTAB on the demineralized sample takes place with the hydrophobic interactions in addition to the electrostatic interactions, while in the case of fresh sample, the predominant mechanism of adsorption is through electrostatic interactions. Also, the lower values of ΔH for both demineralized and oxidized samples in comparison with the demineralized sample indicates that the oxidation process diminishes the contribution of the hydrophobic interactions to the adsorption of CTAB.