Ferrate(VI)-Prompted Removal of Metals in Aqueous Media: Mechanistic Delineation of Enhanced Efficiency via Metal Entrenchment in Magnetic Oxides

The removal efficiency of heavy metal ions (cadmium­(II), Cd­(II); cobalt­(II), Co­(II); nickel­(II), Ni­(II); copper­(II), Cu­(II)) by potassium ferrate­(VI) (K₂FeO₄, Fe­(VI)) was studied as a function of added amount of Fe­(VI) (or Fe) and varying pH. At pH = 6.6, the effective removal of Co­(II), Ni­(II), and Cu­(II) from water was observed at a low Fe-to-heavy metal ion ratio (Fe/M­(II) = 2:1) while a removal efficiency of 70% was seen for Cd­(II) ions at a high Fe/Cd­(II) weight ratio of 15:1. The role of ionic radius and metal valence state was explored by conducting similar removal experiments using Al­(III) ions. The unique combination of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), in-field Mössbauer spectroscopy, and magnetization measurements enabled the delineation of several distinct mechanisms for the Fe­(VI)-prompted removal of metal ions. Under a Fe/M weight ratio of 5:1, Co­(II), Ni­(II), and Cu­(II) were removed by the formation of MFe₂O₄ spinel phase and partially through their structural incorporation into octahedral positions of γ-Fe₂O₃ (maghemite) nanoparticles. In comparison, smaller sized Al­(III) ions got incorporated easily into the tetrahedral positions of γ-Fe₂O₃ nanoparticles. In contrast, Cd­(II) ions either did not form the spinel ferrite structure or were not incorporated into the lattic of iron­(III) oxide phase due to the distinct electronic structure and ionic radius. Environmentally friendly removal of heavy metal ions at a much smaller dosage of Fe than those of commonly applied iron-containing coagulants and the formation of ferrimagnetic species preventing metal ions leaching back into the environment and allowing their magnetic separation are highlighted