Characterization of the structural properties and Pb(II) adsorption behavior of iron oxide coated sepiolite

This paper presents the adsorption of Pb(II) from aqueous solution by raw sepiolite (RS) and iron oxide-coated sepiolite (ICS) samples. Adsorption of Pb(II) by RS and ICS sample was investigated as a function of the initial Pb(II) concentration, solution pH, ionic strength, temperature and inorganic ligand (Cl−). Changes in the surfaces and structure of sepiolite samples were characterized by means of IR and thermal analysis (TG-DTA). The perturbation observed on the zeolitic and bound water vibrations of ICS indicates that some of iron oxide particles enter the interior channels and replace zeolitic water molecules. The exothermal peak at 834 °C for RS and at 774 °C for ICS in the thermal analysis curves is assigned to the phase transformation of sepiolite to enstatite (MgSiO3). The Langmuir monolayer adsorption capacities of RS and ICS in 0.1 M NaNO3 solution at 298 K were estimated as 5.36 and 75.79 mg/g, respectively. Thermodynamic parameters such as change in free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were evaluated for RS and ICS to be − 15.46 kJ/mol (at 298 K), 47.78 kJ/mol and 212 J/mol K, and − 16.03 kJ/mol (at 298 K), 47.48 kJ/mol and 213 J/mol K, respectively.

Research Highlights
► The works related to Pb(II) adsorption on raw sepiolite are scarce.
► IR results revealed the formation of iron oxide/sepiolite covalent bonds.
► Latice irregularities were shown with low enstatite crystallization temperature.
► Ion Exchange/outer-sphere complexation contributes to Pb(II) adsorption on RS.
► Inner-sphere complexation contributes to Pb(II) adsorption on ICS.