Enhanced Cr(vi) removal from aqueous solutions using Ni/Fe bimetallic nanoparticles: Characterization, kinetics and mechanism

In this study, Ni/Fe bimetallic nanoparticles were prepared by a liquid-phase chemical reduction method and characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) with image mapping, transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The as-prepared Ni/Fe material was applied to remove Cr(vi) via a coupled adsorption/reduction process. It was found that Cr(vi) removal followed pseudo-second-order reaction kinetics. Acidic pH favored the efficient removal of Cr(vi) due to the abundance of reactive H species that were mediated by the Ni catalyst. XPS studies demonstrated that Cr(vi) removal on the surface of the bimetallic nanoparticles was a synergistic adsorption and reduction process. The introduction of nickel to nZVI not only controls iron passivation but also facilitates the efficient flow of electron transfer between iron and Cr(vi), and thus the efficient reduction of Cr(vi) to Cr(iii). Hydroxylated Cr(OH)₃ and co-precipitation of Cr_xFe_1-x(OH)₃ were the final products of Cr(vi) removal by the Ni/Fe material.