Chromium(VI) removal by maghemite nanoparticles

Maghemite nanoparticles were prepared by a co-precipitation method and characterized by Fourier transform infrared spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, nitrogen adsorption and desorption isotherms. The Brunauer–Emmett–Teller surface area, average particle size, pore volume and porosity of maghemite were 73.8 m2 g−1, 17.2 ± 4.4 nm, 0.246 cm3 g−1, and 56.3%, respectively. Removal of Cr(VI) by the maghemite nanoparticles follows a pseudo-second-order kinetic process. Intraparticle diffusion kinetics implies the adsorption of Cr(VI) onto the maghemite occurs via two distinct phases: the diffusion controlled by external surface followed by an intra-particle diffusion. The equilibrium data was nicely fit to the Langmuir and Langmuir–Freundlich (L–F) models and indicates the adsorption of Cr(VI) is spontaneous and highly favorable. The heterogeneity index, 0.55, implies heterogeneous monolayer adsorption. The adsorption Cr(VI) is favorable under acidic and neutral conditions with maximum removal observed at pH 4. The adsorption of Cr(VI) is modestly inhibited by the presence of ⩾5 ppm humic acid. In summary, the adsorption of Cr(VI) by maghemite nanoparticles is rapid, can be accurately modeled, and is effective under a variety of conditions. Our results indicate these magnetic materials have promising potential to cleanup Cr(VI) contaminated waters to acceptable drinking water standards.

► Co-precipitation synthesis of maghemite nanoparticles.
► The adsorption of Cr(VI) follows a pseudo-second-order kinetic model.
► The adsorption of Cr(VI) occurs in two phases.
► Accurate modeling using Langmuir and Langmuir–Freundlich isotherms.
► Solution pH and presence of humic acid influence adsorption.