Ferrate(VI)-Induced Arsenite and Arsenate Removal by In Situ Structural Incorporation into Magnetic Iron(III) Oxide Nanoparticles

We report the first example of arsenite and arsenate removal from water by incorporation of arsenic into the structure of nanocrystalline iron­(III) oxide. Specifically, we show the capability to trap arsenic into the crystal structure of γ-Fe₂O₃ nanoparticles that are in situ formed during treatment of arsenic-bearing water with ferrate­(VI). In water, decomposition of potassium ferrate­(VI) yields nanoparticles having core–shell nanoarchitecture with a γ-Fe₂O₃ core and a γ-FeOOH shell. High-resolution X-ray photoelectron spectroscopy and in-field ⁵⁷Fe Mössbauer spectroscopy give unambiguous evidence that a significant portion of arsenic is embedded in the tetrahedral sites of the γ-Fe₂O₃ spinel structure. Microscopic observations also demonstrate the principal effect of As doping on crystal growth as reflected by considerably reduced average particle size and narrower size distribution of the “in-situ” sample with the embedded arsenic compared to the “ex-situ” sample with arsenic exclusively sorbed on the iron oxide nanoparticle surface. Generally, presented results highlight ferrate­(VI) as one of the most promising candidates for advanced technologies of arsenic treatment mainly due to its environmentally friendly character, in situ applicability for treatment of both arsenites and arsenates, and contrary to all known competitive technologies, firmly bound part of arsenic preventing its leaching back to the environment. Moreover, As-containing γ-Fe₂O₃ nanoparticles are strongly magnetic allowing their separation from the environment by application of an external magnet

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