Vermiculite enriched by Fe(III) oxides as a novel adsorbent for toxic metals removal

Abstract

Expanded vermiculite (EV) was modified by deposition of different spinel ferrite composites on the outer surface of EV 2:1 layers in order to improve its adsorptive properties. Modifications were made by deposition of: magnetite, manganese ferrite, cobalt ferrite and chromium oxide/hematite. The characterization of modified materials was performed by: scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) methodology, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), magnetization measurements, as well as determination of cation exchange capacity (CEC) and pH point of zero charge (pHPZC). Obtained samples were used as adsorbents for Pb2+, Ni2+and Cd2+ ions from an aqueous solution in a batch system. Results showed that adsorption capacity was strongly dependent on physical and chemical changes induced by specific chemical modification. Hydrothermally produced manganese and cobalt ferrites caused significant surface changes and altered the interlayer cation balance. Among the others, EV-Mn/Co-ferrite(s) samples possessed the highest adsorption capacity towards Ni2+ (33.06 mg g-1), along with an increase of the CEC. Freundlich's adsorption isotherm model provided the best fit of obtained experimental data, while kinetic studies showed that the adsorption rate follows the pseudo second-order model, implying heterogeneous adsorbents surface. Thermodynamic and kinetic parameters indicated that the mechanism of cations removal efficacy was dominantly followed by the ion exchange. This study confirmed that doped ferrites, produced by solvothermal method, improve surface properties of EV and increase adsorption potentials towards heavy metals.