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

Air Stable Magnetic Bimetallic Fe–Ag Nanoparticles for Advanced Antimicrobial Treatment and Phosphorus Removal

We report on new magnetic bimetallic Fe–Ag nanoparticles (NPs) which exhibit significant antibacterial and antifungal activities against a variety of microorganisms including disease causing pathogens, as well as prolonged action and high efficiency of phosphorus removal. The preparation of these multifunctional hybrids, based on direct reduction of silver ions by commercially available zerovalent iron nanoparticles (nZVI) is fast, simple, feasible in a large scale with a controllable silver NP content and size. The microscopic observations (transmission electron microscopy, scanning electron microscopy/electron diffraction spectroscopy) and phase analyses (X-ray diffraction, Mössbauer spectroscopy) reveal the formation of Fe₃O₄/γ-FeOOH double shell on a “redox” active nZVI surface. This shell is probably responsible for high stability of magnetic bimetallic Fe–Ag NPs during storage in air. Silver NPs, ranging between 10 and 30 nm depending on the initial concentration of AgNO₃, are firmly bound to Fe NPs, which prevents their release even during a long-term sonication. Taking into account the possibility of easy magnetic separation of the novel bimetallic Fe–Ag NPs, they represent a highly promising material for advanced antimicrobial and reductive water treatment technologies

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

Enhanced Oxidative and Adsorptive Removal of Diclofenac in Heterogeneous Fenton-like Reaction with Sulfide Modified Nanoscale Zerovalent Iron

Sulfidation of nanoscale zerovalent iron (nZVI) has shown some fundamental improvements on reactivity and selectivity toward pollutants in dissolved-oxygen (DO)-stimulated Fenton-like reaction systems (DO/S-nZVI system). However, the pristine microstructure of sulfide-modified nanoscale zerovalent iron (S-nZVI) remains uncovered. In addition, the relationship between pollutant removal and the oxidation of the S-nZVI is largely unknown. The present study confirms that sulfidation not only imparts sulfide and sulfate groups onto the surface of the nanoparticle (both on the oxide shell and on flake-like structures) but also introduces sulfur into the Fe(0) core region. Sulfidation greatly inhibits the four-electron transfer pathway between Fe(0) and oxygen but facilitates the electron transfer from Fe(0) to surface-bound Fe­(III) and consecutive single-electron transfer for the generation of H₂O₂ and hydroxyl radical. In the DO/S-nZVI system, slight sulfidation (S/Fe molar ratio = 0.1) is able to nearly double the oxidative removal efficacy of diclofenac (DCF) (from 17.8 to 34.2%), whereas moderate degree of sulfidation (S/Fe molar ratio = 0.3) significantly enhances both oxidation and adsorption of DCF. Furthermore, on the basis of the oxidation model of S-nZVI, the DCF removal process can be divided into two steps, which are well modeled by parabolic and logarithmic law separately. This study bridges the knowledge gap between pollutant removal and the oxidation process of chemically modified iron-based nanomaterials

Zero-Valent Iron Nanoparticles with Unique Spherical 3D Architectures Encode Superior Efficiency in Copper Entrapment

The large-scale preparation of spherical condensed-type superstructures of zero-valent iron (nZVI), obtained by controlled solid-state reaction through a morphologically conserved transformation of a magnetite precursor, is herein reported. The formed 3D nanoarchitectures (S-nZVI) exhibit enhanced entrapment efficiency of heavy metal pollutants, such as copper, compared to all previously tested materials reported in the literature, thus unveiling the relevance in the material’s design of the morphological variable. The superior removal efficiency of these mesoporous S-nZVI superstructures is linked to their extraordinary ability to couple effectively processes such as reduction and sorption of the metal pollutant

Sulfidation of Iron-Based Materials: A Review of Processes and Implications for Water Treatment and Remediation

Iron-based materials used in water treatment and groundwater remediationespecially micro- and nanosized zerovalent iron (nZVI)can be more effective when modified with lower-valent forms of sulfur (i.e., “sulfidated”). Controlled sulfidation for this purpose (using sulfide, dithionite, etc.) is the main topic of this review, but insights are derived by comparison with related and comparatively well-characterized processes such as corrosion of iron in sulfidic waters and abiotic natural attenuation by iron sulfide minerals. Material characterization shows that varying sulfidation protocols (e.g., concerted or sequential) and key operational variables (e.g., S/Fe ratio and sulfidation duration) result in materials with structures and morphologies ranging from core–shell to multiphase. A meta-analysis of available kinetic data for dechlorination under anoxic conditions, shows that sulfidation usually increases dechlorination rates, and simultaneously hydrogen production is suppressed. Therefore, sulfidation can greatly improve the efficiency of utilization of reducing equivalents for contaminant removal. This benefit is most likely due to inhibited corrosion as a result of sulfidation. Sulfidation may also favor desirable pathways of contaminant removal, such as (i) dechlorination by reductive elimination rather than hydrogenolysis and (ii) sequestration of metals as sulfides that could be resistant to reoxidation. Under oxic conditions, sulfidation is shown to enhance heterogeneous catalytic oxidation of contaminants. These net effects of sulfidation on contaminant removal by iron-based materials may substantially improve their practical utility for water treatment and remediation of contaminated groundwater