Adsorption mechanism of Pb2+ ions by Fe3O4, SnO2, and TiO2 nanoparticles.

Nanosized sorbents for the removal of heavy metal ions are preferred due to high surface area, smaller size, and enhanced reactivity during adsorbate/adsorbent interactions. In the present study, Fe3O4, SnO2, and TiO2 nanoparticles were prepared by microemulsion-assisted precipitation method. The particles were characterized by BET surface area, X-rays diffraction (XRD), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, transmittance electron microscopy (TEM), and X-ray photoelectron (XPS) spectroscopy. The respective particle sizes calculated from TEM were 7 nm (± 2), 10 nm (± 2), and 20 nm (± 3) for Fe3O4, SnO2, and TiO2. The adsorbents were employed for the adsorption of Pb2+ ions from the aqueous solutions. The respective maximum adsorption capacity for Fe3O4, SnO2, and TiO2 nanoparticles was 53.33, 47.21, and 65.65 mg/g at 313 K. Based on the exchange reaction taking place on the surfaces of Fe3O4, SnO2, and TiO2, it is concluded that Pb2+ ions are adsorbed in hydrated form. The X-ray photoelectron spectroscopy (XPS) studies also support the exchange mechanism and confirmed the presence of elements like Fe, Sn, Ti, Pb, and O and their oxidation states. Both Langmuir and Freundlich models in non-linear form were applied, however, based on RL values, the Langmuir model fits well to the sorption data. Moreover, adsorption parameters were also determined by using non-linear form of the Langmuir model along with statistical approaches to remove error. The qm and Kb values confirm better adsorption capacity and binding strength for Pb2+ ions as compared to the values reported in the literature.