Role of silica mid-layer in thermal and chemical stability of hierarchical Fe3O4-SiO2-TiO2 nanoparticles for improvement of lead adsorption: Kinetics, thermodynamic and deep XPS investigation

A core-shell of Fe3O4-TiO2 (FT) and a core-multishell of Fe3O4-SiO2-TiO2 (FST) were synthesized to remove Pb(II) from water. Removal kinetics and isotherm fitted better with Pseudo-second order and Freundlich models, respectively. Activation energies for FST (55.54 kJ mol−1) and FT (78.775 kJ mol−1) suggested chemical adsorption and also easier adsorption on FST. The thermodynamic parameters disclosed that the adsorption of Pb(II) on nanoparticles was endothermic (ΔH°: 38.27 and 65.43 kJ mol−1) and spontaneous. XPS was done to determine the role of SiO2 by its influence on chemical stability, surface properties and functional groups as well as to explore adsorption mechanism while XRD proved thermal instability for FT. The results suggested that ion exchange between hydroxyl groups and Pb species was accounted for by the adsorption and led to PbTiO3 and PbO formation. However creation of Fe2TiO5/Fe2O3 interface on FT was responsible for its higher e/h recombination and lower adsorption capability.