Selective Removal of Heavy Metals from Industrial Wastewater Using Maghemite Nanoparticle: Performance and Mechanisms

Abstract

This study investigated the applicability of maghemite $\left(\gamma \text{-}{\mathrm{Fe}}_2{\mathrm{O}}_3\right)$ nanoparticles for the selective removal of toxic heavy metals from electroplating wastewater. The maghemite nanoparticles of $10\mathrm{nm}$ were synthesized using a sol–gel method and characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The surface area of the nanoparticles was determined to be $198{\mathrm{m}}^2∕\mathrm{g}$ using the Brunauer–Emmett–Teller method. Batch experiments were carried out to determine the adsorption kinetics and mechanisms of $\mathrm{Cr}\left(\mathrm{VI}\right)$ , $\mathrm{Cu}\left(\mathrm{II}\right)$ , and $\mathrm{Ni}\left(\mathrm{II}\right)$ by maghemite nanoparticles. The adsorption process was found to be highly pH dependent, which made the nanoparticles selectively adsorb these three metals from wastewater. The adsorption of heavy metals reached equilibrium rapidly within $10\min$ and the adsorption data were well fitted with the Langmuir isotherm. Regeneration studies indicated that the maghemite nanoparticles undergoing successive adsorption–desorption processes retained original metal removal capacity. Mechanism studies using TEM, XRD, and X-ray photoelectron spectroscopy suggested that the adsorption of $\mathrm{Cr}\left(\mathrm{VI}\right)$ and $\mathrm{Cu}\left(\mathrm{II}\right)$ could be due to electrostatic attraction and ion exchange, and the adsorption of $\mathrm{Ni}\left(\mathrm{II}\right)$ could be as a result of electrostatic attraction only.