Comparison studies of adsorption properties for Hg(II) and Au(III) on polystyrene-supported bis-8-oxyquinoline-terminated open-chain crown ether

Abstract

Differences between adsorption properties for Hg(II) and Au(III) on polystyrene-supported bis-8-oxyquinoline-terminated open-chain crown ether (PS-DB) were studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and the density functional theory (DFT) method. The comparison studies included kinetics, equilibriums and thermodynamics of the adsorption process. Pseudo-first-order and pseudo-second-order kinetic models were applied to test the experimental data. The pseudo-second-order kinetic model provided a better correlation with the experimental data for Hg(II) in comparison with the pseudo-first-order model. While for Au(III), the conclusion was the reverse. Both the adsorption rate and adsorption amount at equilibrium for Hg(II) were greater than those for Au(III) ions, which was probably caused by the differences in form and volume between the two kinds of absorbed ions. Langmuir, Freundlich and Tempkin isotherm models were applied in the adsorption isotherm study. The results showed that the Langmuir and Tempkin isotherm models were more applicable in describing the processes. The thermodynamic parameters for the adsorption process such as free energy of adsorption (ΔG), enthalpy of adsorption (ΔH) and entropy of adsorption (ΔS) were calculated. The density functional theory (DFT) method was used to investigate the coordination geometries and the interactions between the metal ions with PS-DB. These results showed that the adsorption for Hg(II) was physisorption while that for Au(III) was chemisorption, which was also consistent with the data obtained from the Dubinin–Radushkevich (D–R) isotherm model and ΔH values.