Iron-containing nanomaterials: synthesis, properties, and environmental applications

Abstract

Available data on the iron-containing nanomaterials are reviewed. Main attention is paid to the following themes: synthetic methods, structures, composition and properties of the nano zerovalent iron (NZVI), and polymorphic forms of iron oxides and FeOOH. Synthetic methods summarized here include a series of physico-chemical methods such as microwave heating, electrodeposition, laser ablation, radiolytical techniques, arc discharge, metal-membrane incorporation, pyrolysis, combustion, reverse micelle and co-deposition routes. We have also included a few “greener” methods. Coated, doped, supported with polymers or inert inorganic materials, core–shell nanostructures, in particular those of iron and its oxides with gold, are discussed. Studies of remediation involving iron-containing nanomaterials are discussed and special attention is paid to the processes of remediation of organic contaminants (chlorine-containing pollutants, benzoic and formic acids, dyes) and inorganic cations (Zn(II), Cu(II), Cd(II) and Pb(II)) and anions (nitrates, bromates, arsenates). Water disinfection (against viruses and bacteria), toxicity and risks of iron nanomaterials application are also examined.

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Boris I. Kharisov

Dr Boris I. Kharisov (born in Russia in 1964) is currently a Professor and Researcher at the Universidad Autónoma de Nuevo León (UANL). Degrees: An MS in 1986, in radiochemistry and a PhD in inorganic chemistry in 1993, from the Moscow State University, Russia; Dr Hab. in physical chemistry in 2006 from Rostov State University, Russia. He is the co-author of six books, 122 articles, five book chapters, and has two patents. Co-editor: Three invited special issues of international journals. He is the member of the Editorial board of four journals. Specialties: Coordination and inorganic chemistry, phthalocyanines, ultrasound, and nanotechnology.

H. V. Rasika Dias

Dr Rasika Dias is a Professor of Chemistry at the University of Texas at Arlington. Born in Colombo, Sri Lanka, he received his BSc Degree from University of Peradeniya (Sri Lanka) and PhD from University of California, Davis (USA). Dr Dias was a Visiting Research Scientist at the DuPont Central Research & Development, Delaware before joining the University of Texas at Arlington faculty in 1992. Professor Dias specializes in inorganic and organometallic chemistry, and has been the author or co-author of several patents and over 160 publications. He has won several awards including the 2009 Southwest Regional American Chemical Society Award.

Oxana V. Kharissova

Dr Oxana V. Kharissova (born in 1969 in Ucrania, former USSR) is currently a Professor and Researcher at the UANL. Degrees: An MS in 1994, in crystallography from Moscow State University, Russia, and a Ph.D. in Materials from the Universidad Autónoma de Nuevo León, 2001, Mexico. Memberships: National Researchers System (Level I), Materials Research Society. She is the co-author of three books, four book chapters, 60 articles, and has two patents. Specialties: Nanotechnology (carbon nanotubes, nanometals, fullerenes), microwave irradiation and crystallography.

Victor Manuel Jiménez-Pérez

Dr Victor Manuel Jiménez-Pérez was born in Tlaxcala (México, 1974). He received his BSc (1998) in Chemistry at the Universidad Autonóma de Tlaxcala. In 2005, he received his PhD in Chemistry at the CINVESTAV under the supervision of Prof. Rosalinda Contreras in the field of tin chemistry. After postdoctoral work in the Prof. Herbert Roeskýs group at Universität Göttingen (2007), he jointed as a full-professor in Chemistry Faculty at the Universidad Autónoma de Nuevo León. So far, he is the author of more than 20 publications. His research is mainly focused on the design and synthesis of luminescent and sensor organometallic compounds and “green” chemistry.

Betsabee Olvera Pérez

Betsabee Olvera Pérez studied Industrial Chemistry at the Universidad Autonóma de Nuevo León (2011). After working for a short time in a private chemical company in Monterrey, Mexico, she is currently an active MSc student at the same University, working in the area of fabrication of iron nanoparticles by “green” chemical and biological methods. She is co-author of two submitted manuscripts.

Blanca Muñoz Flores

Dr Blanca Muñoz Flores (born in Tlaxcala, México, 1976) studied Industrial Chemistry at the Universidad Autonóma de Tlaxcala (1999). She received her PhD in Organic Chemistry (2008) at the CINVESTAV-IPN under direction of Prof. Norberto Farfán García. She moved to the Chemistry Faculty of the Universidad Autónoma de Nuevo León (2009) as a full professor. Presently, her research group is working on the design, synthesis, characterization and determination nonlinear optical (NLO) properties of organic and organometallic compounds, as well as on nanoalloys. So far, she is the author of more than 10 research publications in leading scientific journals.

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Introduction

According to the classic definition, nanomaterials are those materials whose key physical characteristics are dictated by the nano-objects they contain. Nanomaterials are classified into compact materials and nanodispersions. The first type includes so-called “nanostructured” materials, i.e., materials isotropic in the macroscopic composition and consisting of closely packed nanometer-sized units as repeating structural elements. Among nanomaterials, the iron-containing nanomaterials have attracted a great deal of interest due to their magnetic properties, allowing such applications as targeted drug delivery¹ and other areas of nanomedicine. Core–shell iron (iron oxide)/gold nanoparticles are of special interest due to possibility to be controlled by magnetic fields (iron core) and functionalization (gold shell).² Some applications of iron-containing nanomaterials have been recently reviewed,^3,4 including their usefulness in water treatment (remediation of pollutants and bacteria).^5–7 In this review, we pay main attention to the synthetic methods for obtaining Fe⁰ and Fe_xO_y based nanomaterials and their application in water treatment⁸ (remediation of (in)organic contaminants and disinfection of groundwater).

General data on the iron-containing nanostructures

Iron-containing nanomaterials (Table 1) are mainly consisted of zerovalent iron (ZVI is currently a classic term, as well as NZVI (nano zerovalent iron)), iron nanoalloys or core–shell nanoparticles, and oxides, among others, such as, for example, ferrites. Iron typically exists in the environment as iron(II) and iron(III) oxides.⁹ Iron oxides (iron oxide nanoparticles are also referred in several reports to as superparamagnetic iron-oxide nanoparticles (SPIONs) although SPIONS have inducible magnetic properties),^10,11 are one of the most important transition metal oxides of technological importance. Iron oxides is also a collective term for oxides, hydroxides and oxy-hydroxides composed of Fe(II) and/or Fe(III) cations and O²⁻ and/or OH⁻ anions. Sixteen pure phases of iron oxides, i.e., oxides, hydroxides or oxy-hydroxides are known to date. These are Fe(OH)₃, Fe(OH)₂, Fe₅HO₈·4H₂O, Fe₃O₄, FeO, five polymorphs of FeOOH and four of Fe₂O₃. Characteristics of these oxide compounds include mostly the trivalent state of the iron, low solubility and brilliant colors. The main advantages of nano-iron among other nanomaterials are relatively low toxicity and biodegradability. In addition, iron is a relatively cheap and a widespread material.¹²

Table 1 Main iron-containing nanostructures

Composition	Description
Fe	The preparation of nanoparticles consisting of pure iron is a complicated task, because they always contain oxides, carbides and other impurities.
BCC-Fe (α-Fe)	α-Fe nanoparticles with a body-centered cubic (BCC) lattice can be prepared by (a) grinding a high-purity (99.999%) Fe powder, (b) evaporation of the metal in an Ar or He atmosphere followed by deposition on a substrate, (c) laser vaporization of pure iron.
FCC-Fe (γ-Fe)	In the phase diagram of a bulk Fe sample, this phase exists at the ambient pressure in the temperature range of 1183–1663 K, i.e., above the Curie point (1096 K). In some special alloys, this phase, which exhibits antiferromagnetic properties (the Neel temperature is in the 40–67 K range), was observed at room temperature. These nanoparticles, in several syntheses, can contain substantial amounts of carbon (up to 14 mass%). γ-Fe nanoparticles, synthesized by treatment of Fe(CO)5 with a CO2 laser radiation, may contain, for example, γ-Fe (30 atom%), α-Fe (25 atom%) and iron oxides (45 atom%).
“Amorphous” Fe (metallic glass)	Synthesis by: (a) the ultrasonic treatment of Fe(CO)5 in the gas phase or of a solution of Fe(CO)5 in decane under an inert atmosphere; (b) thermal decomposition of Fe(CO)5 in decalin (460 K) in the presence of surfactants.
Fe2O3	Among several crystalline modifications of Fe2O3, there are two magnetic phases, namely, rhombohedral α-Fe2O3 (hematite) and cubic γ-Fe2O3 (maghemite) phases. The α-Fe2O3 and FeOOH (goethite) nanoparticles are obtained by controlled hydrolysis of Fe^3+ salts. The γ-Fe2O3 nanoparticles are obtained by (a) mild oxidation (on treatment with Me3NO) of pre-formed metallic nanoparticles, (b) by direct introduction of Fe(CO)5 into a heated solution of Me3NO, (c) thermal decomposition of Fe^3+ salts in various media, (d) vaporization of iron(III) oxide in a solar furnace with subsequent condensation, (e) mechanochemical synthesis milling iron powder in a planetary mill with water.
Fe3O4 (magnetite)	The cubic spinel Fe3O4 is ferrimagnetic at temperatures below 858 K. The route to these particles most often involves treatment of a solution of a mixture of iron salts (Fe^2+ and Fe^3+) with a base under an inert atmosphere.
	In some cases, thermal decomposition of compounds containing Fe^3+ ions under oxygen-deficient conditions is accompanied by partial reduction of Fe^3+ to Fe^2+. Fe3O4 nanoparticles can be also prepared by thermal decomposition of Fe2(C2O4)3·5H2O or by the controlled reduction of ultradispersed α-Fe2O3 in a hydrogen stream.
FeO (wustite)	Cubic Fe^2+ oxide is antiferromagnetic (Tc = 185 K) in the bulk state. Synthesis technique: joint milling of Fe and Fe2O3 powders taken in a definite ratio gives nanoparticles consisting of FeO and Fe. On heating of these particles at temperatures of 250-400 °C, the metastable FeO phase disproportionates to Fe3O4 and Fe, while above 550 °C it is again converted into nanocrystalline FeO.
α-FeOOH (goethite)	The orthorhombic α-FeOOH (goethite) is antiferromagnetic in the bulk state and has Tc = 393 K, β-FeOOH (akagenite) is paramagnetic at 300 K, γ-FeOOH (lipidocrokite) is paramagnetic at 300 K and δ-FeOOH (ferroxyhite) is ferrimagnetic. As a rule, α-FeOOH is present in iron nanoparticles as an admixture phase.
FexCy	Iron carbide is often present in Fe-containing nanoparticles. It is formed either upon decomposition of organic ligands present in the starting MCC or upon the reaction of Fe with the organic medium. Thermal decomposition of Fe(CO)5 on a carbon support forms Fe78C22 nanoparticles.
Ferrofluids	Ferrofluids or so-called magnetic liquids are suspensions of colloid magnetic particles stabilized by surfactants in liquid media. The magnetic phase in ferrofluids can be represented by magnetite, ferrites (group of nonmetallic, ceramic like, usually ferromagnetic compounds of ferric oxide with other oxides) and FexCy particles resulting from the thermal decomposition of Fe(CO)5.
Fe–Co alloys	The saturation magnetization of Fe–Co alloys reaches a maximum at a Co content of 35 atom%. Fe, Co and Fe–Co (20, 40, 60, 80 atom%) nanoparticles with a structure similar to the corresponding bulk phases can be prepared in a stream of hydrogen plasma.
Fe–Ni	Bulk samples of iron–nickel alloys are either nonmagnetic or are magnetically soft ferromagnets (for example, permalloys containing >30% of Ni and various doping additives).
Fe–Pt	Fe–Pt nanoparticles can be prepared by joint thermolysis of Fe(CO)5 and Pt(acac)2 in the presence of oleic acid and oleylamine. The reaction of FePt nanoparticles with Fe3O4 followed by heating of the samples at 650 °C in an Ar + 5% H2 stream resulted in a FePt–Fe3Pt nanocomposite with unusual magnetic characteristics.

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Metallic or zerovalent iron (Fe⁰) is a moderate reducing reagent, which can react with dissolved oxygen and to some extent with water (eqn (1) and (2)):

2Fe⁰_(s) + 4H⁺_(aq) + O_2(aq) → 2Fe²⁺_(aq) + 2H₂O_(l) (1)

Fe⁰_(s) + 2H₂O_(aq) → Fe²⁺_(aq) + H_2(g) + 2OH⁻_(aq) (2)

The above equations are the classical electrochemical/corrosion reactions by which iron is oxidized from exposure to oxygen and water. The corrosion reactions can be accelerated or inhibited by manipulating the solution chemistry and/or solid (metal) composition.

NZVI particles range from 10 to 100 nm in diameter.¹³ They exhibit a typical core–shell structure. The core consists primarily of zerovalent or metallic iron whereas the mixed valent [i.e., Fe(II) and Fe(III)] oxide shell is formed as a result of oxidation of the metallic iron (Fig. 1). The presence of excess stabilizers was shown to stabilize the core/shell nanoparticles from further oxidation.¹⁴ Various organic molecules can be used for functionalization of NZVI to improve the stability of their aqueous dispersions (strong inhibition of the agglomeration of the iron nanoparticles), for instance PEG, methoxyethoxyethoxyacetic acid (MEEA), polyacrylic acid and 1,4-butanediphosphonic acid (Fig. 2) have been used for this purpose.¹⁵

Fig. 1 Schematic diagram of zerovalent iron nanoparticle. Reproduced with permission.

Fig. 2 Scheme of a nanoscale iron particle covered by MEEA.

Among iron oxides (SPIONs), magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are of a particular interest. Magnetite is an inverse spinel ferrite. The oxygen ions form a close-packed cubic lattice with the iron ions located at two different interstices between them, tetrahedral (A) sites and octahedral (B) sites. Chemically, magnetite/maghemite can be represented by the formula: Fe³⁺ [Fe²⁺_1−yFe³⁺_1−yFe³⁺_1.67y□_0.33y]O₄, where y = 0 for pure magnetite and y = 1 for pure maghemite (fully oxidized magnetite). In the temperature range from room to Curie temperature (T_c = 860 K) the A sites are populated by Fe³⁺ ions and the B sites are populated equally by Fe³⁺ and Fe²⁺ ions. That way, twice as many sites are populated with Fe³⁺ than with Fe²⁺ ions. Although a simple dehydration of lepidocrocite (γ-FeOOH) topotactically transforms into γ-Fe₂O_3, commercial manufacturing of maghemite follows the multistep process (3):

(α and/or γ)-FeOOH (oxidation) → α-Fe₂O₃ (reduction) → Fe₃O₄ (controlled oxidation) → γ-Fe₂O₃ (3)

SPIONs typically consist of two components, an iron oxide core of one or more magnetic crystallites embedded in a coating. The SPIONs' core can be composed of magnetite (Fe₃O₄) and/or maghemite (γ-Fe₂O₃). The size of SPIONs makes important contribution to their fate in organism. Categories of SPIONs, based on their overall diameter (including iron oxide core and hydrated coating), are noted in the literature as oral or micron-sized SPIONs between 300 nm and 3.5 μm; standard or small SPIONs (SSPIONs) at approximately 60–150 nm; ultrasmall SPIONs (USPIONs) of approximately 10–50 nm; and monocrystalline iron oxide nanoparticles (MION—a subset of USPIONs) of approximately 10–30 nm. MION are so named to underline the single crystal nature of their core. This is in contrast to SPIONs greater than 50 nm that are comprised of multiple iron oxide crystals.¹⁶ SPIONs have much larger magnetic susceptibilities (compared with strictly paramagnetic materials) as the entire crystal aligns with the applied field due to its single crystal nature. Hence SPIONs are useful as contrast agents or for hyperthermic treatment of malignant tumors.¹⁷

Magnetic nanoparticles offer advantages over non-magnetic nanoparticles because they can easily be separated from water using an external magnetic field. Separation using magnetic gradients, the so-called high magnetic gradient separation (HGMS), is a process widely used in medicine and ore processing.¹⁸ This technique allows one to design processes where the particles can not only be used to remove compounds from water but also be separated and then be recycled or regenerated. This approach has been proposed with magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and jacobsite (MnFe₂O₄) nanoparticles for removal of chromium(VI) from wastewater. In addition, the following iron-based compositions are usually considered in the literature as media for magnetic information recording: α-Fe, γ-Fe₂O₃, Fe₂O₃–Fe₃O₄, Co–γ-Fe₂O₃, CrO₂, BaFe₁₂O₁₉. In recent years, nanocrystalline (5–10 nm) CoPr, FePt, CoPt, CoSm, SmFeSiC and SmFeAlC films are regarded as the most promising candidates for magnetic materials.

Types of materials containing iron-based magnetic nanoparticles are, in general, as follows: (1) nanoparticles on a substrate surface (for instance, α-Fe or Fe₃O₄ on ZrO₂), (2) iron-containing nanoparticles in matrices, including (a) inorganic matrices (such as zeolites, molecular sieves, glass, xerogels and silica gel, dispersed carbon, highly dispersed SiO₂ and Al₂O₃, magnetic nanoparticles in non-magnetic metals (for example, Fe–Ag or Fe–Hg samples) and organic polymer matrices (ion exchange resins, soluble polymers (such as PVA), polybutadiene, polystyrene and styrene copolymers with butadiene, 4-vinylpyridine, N-vinylpyrrolidone, phenylvinylketoxime, carbochain polymers without heteroatoms and functional groups, polymers containing heteroatoms (such as polyimines or polyvinylpyrrolidone) or block copolymers (for example, [NORCOOH]₃₀[MTD]₃₀₀ block copolymers (NORCOOH is 2-norbornerne-5,6-dicarboxylic acid, MTD is methyltetracyclododecene)).

In addition to the NZVI and SPIONs, a variety of composite inorganic iron-based nanomaterials have been discovered, in particular core–shell Fe (or Fe_xO_y)/Au (see section below) or more complex trimetallic nanoparticles such as (Fe₆₀Co₄₉)core/Au.¹⁹ Figuerola et al.²⁰ classified them based on their levels of compositional and/or structural complexity: (1) nanostructures made of an iron-based magnetic material different from iron oxide; (2) nanostructures whose morphology is not a sphere (e.g. hollow structure); (3) multi-material nanostructures, i.e. each of them is made of two or more domains of different inorganic materials joined together. Some examples of these approaches are as follows. Thus, the strategy for functionalization of CoFe₂O₄ superparamagnetic Nps (nanoparticles) with a mixture of amino and thiol groups that facilitate the electrostatic attraction and further chemisorption of gold Nps, respectively, is shown in Fig. 3.²¹ Related MnFe₂O₄ nanoparticles have surpassed SPIONs as contrast agents for MRI in vivo.²² The enhanced sensitivity of MnFe₂O₄ nanoparticles has been proven in vivo, enabling detection of a tumor mass as small as 50 mg. The composite iron-based nanocrystals can be used for treatment of malignant tumors. Thus, FePt nanoparticles functionalized with luteinizing hormone-releasing hormone (LHRH) peptide have enhanced cytotoxicity against ovarian cancer cells that express LHRH-receptors.²³ In the acidic environment of lysosomes, these nanoparticles release toxic iron species, which catalyze the formation of reactive oxygen. The latter is toxic for cells as it can damage lipid membranes, DNAs and proteins. Nanoshells CoS₂@FePt also possess better antitumor activity than that of cis-platin.²⁴

Fig. 3 Reaction strategy showing the successive steps for gold covering process onto 4 : 1 amino : mercapto functionalized cobalt ferrite Nps. Reproduced with permission from Elsevier Science.

Several groups revealed that ZVI nanoparticles exhibit antimicrobial properties (see section below) against Gram-negative E. coli, Pseudomonas fluorescens and Gram-positive Bacillus subtilis var. niger microorganisms.^25,26 The inactivation of E. coli by ZVI nanoparticles could be because of the penetration of the small particles (size ranging from 10–80 nm) into E. coli membranes. ZVI nanoparticles could then react with intracellular oxygen, leading to oxidative stress and causing disruption of the cell membrane. In addition, iron oxide nanoparticles also possess antimicrobial properties (see the section on water disinfection below). In general, iron-based nanomaterials are thoroughly investigated according to their relatively low toxicity integrated with unique properties in order to exploit them in such biomedical applications as remediation of environment, development of novel diagnostic tools and methods for individualized treatment.

Synthesis techniques for NZVI and nano-iron oxides

There is a series of special techniques for preparation of iron and other nanomaterials, which could be artificially divided into “physical”, “physico-chemical”, “chemical” and “biological” methods. Of course, such a separation is conditional; almost any physical process in these transformations is accompanied in these reactions by a chemical transformation. We consider as “physical” methods those requiring special equipment, for example a ⁶⁰Co source for γ-irradiation of samples or sputtering equipment. Application of different methods has led to formation of iron-containing nanostructures having distinct compositions, iron oxidation states and structures. It is worth mentioning that the use of Mössbauer spectroscopy is almost a strict requirement in working with Fe-nanostructures in order to determine exact iron oxidation number. Tables 2 and 3 contain examples of methods used and the corresponding products.

Table 2 “Physical”, “chemical” and “biological” methods for preparation of iron-containing nanoparticles^148–150

Method	Description	Examples of products
Physical methods
Condensation methods	The method of nanoparticle synthesis from supersaturated metal vapors is based on the classical nucleation theory in which the nascent phase clusters are described by the spherical liquid drop model. Nanoparticles (clusters) are prepared using various means of metal evaporation: laser vaporization, thermal vaporization, arc discharge, plasma vaporization and solar energy-induced evaporation. Special installations are needed. In the classical thermal vaporization method, a metal or alloy sample is heated in a tungsten boat in an argon or helium stream. The atoms of the vaporized metal lose kinetic energy upon collisions with inert gas atoms, gather in clusters and condense on a cooled substrate as a nanodispersed powder.	Heterometallic nanoparticles (∼30 nm) with the composition Fe–M (M = Ni, Mn, Pt, Cr).
Nanodispersion of a compact material	The mechanochemical dispersion of a compact material in mills of various designs is applied, as well as electrolytic erosion and electrochemical generation.	γ-Fe2O3 nanoparticles.
Chemical synthesis of magnetic nanoparticles
Thermolysis of iron-containing compounds	Metal organic chemical vapor deposition (MOCVD) technique.	Nanodispersed Fe oxides from [Fe(O^tBu)3]2 precursor.
		α-Fe nanoparticles from Fe(CO)5 as a precursor.
Cold-plasma chemical vapor deposition	The growth occurs on an Fe catalyst supported by kanthal (iron–chromium–aluminium (FeCrAl) alloys) wires while heating under a hydrocarbon precursor gas and plasma created by a bias. A possible application for such nanotubes on metallic wires is in luminescent tubes.	Growth of CNTs on Fe wires from hydrocarbon precursor.
Aerosol/vapor methods	In spray pyrolysis, a solution of ferric salts and a reducing agent in organic solvent is sprayed into a series of reactors, where the aerosol solute condenses and the solvent evaporates. The resulting dried product consists of particles whose size depends upon the initial size of the original droplets.	Maghemite γ-Fe2O3 nanoparticles.
Combustion synthesis	Reaction process is completed within a few minutes. The method uses no additional fuel and nitrate, which is present in the precursor itself, to drive the reaction.	Iron oxide/iron (γ-Fe2O3, α-Fe2O3, Fe3O4 and Fe) coated carbons (activated carbon, anthracite, cellulose fiber) or silica.
Chemical precipitation	Addition of alkali to iron salt solutions and keeping the suspensions for ageing. A large amount of nanoparticles can be synthesized.	FeOOH, Fe3O4 or γ-Fe2O3.
Coprecipitation	Alkaline coprecipitation of Fe(III) and Fe(II) salts in aqueous media:	Fe3O4
	2Fe^3+ + Fe^2+ + 8OH^− → Fe3O4 + 4H2O
	It is essential to adjust the stoichiometric ratio of Fe(II)/2Fe(III) at the initial stage because it is difficult to control the oxidation kinetics of Fe(II) afterward. Oxygen also plays an important role in the formation of single phase magnetite; usually bubbling nitrogen gas directly into the media prior to reaction is efficient enough for removal of oxygen.
Precipitation by anhydrous solution	Anhydrous solution approaches for the production of metal oxide nanoparticles and in particular iron oxide.	γ-Fe2O3 nanoparticles (direct oxidation of iron pentacarbonyl in the presence of oleic acid with trimethylamine oxide as an oxidant).
Decomposition of iron-containing compounds by ultrasonic treatment	Use of strong sources of ultrasound. At very high temperatures, hot spots generated by the rapid collapse of sonically generated cavities, allows for the conversion of salts into nanoparticles.	Iron nanoparticles from Fe(CO)5 as a precursor, Fe2O3 and Fe3O4 nanoparticles.
The reduction of iron-containing compounds	Magnetic metallic nanoparticles can be prepared from metal salts using strong reducing agents, namely, alkali metal dispersions in ethers or hydrocarbons, alkali metal complexes with organic electron acceptors (e.g., naphthalene), NaBH4 and other complex hydrides.	Using NaBH4 as a reductant, both homo- (Fe, Co, Ni) and heterometallic (Fe–Co, Fe–Cu, Co–Cu) nanoparticles are obtained as amorphous powders containing substantial amounts of boron (20 mass% or more).
Biomimetic mineralization	Bio-mimetic mineralization within protein cages provides an attractive alternative approach for synthesizing monodisperse nanosized particles. Since maximum particle sizes are limited by the cage inner diameter, reactions carried out in cages can lead to highly monodisperse size distributions, particularly if nucleation rather than growth is the rate-limiting stage of the synthesis.	Fe2O3 nanoparticles.
Synthesis in reverse micelles	A microemulsion is defined as the thermodynamically stable isotropic dispersion of two immiscible liquids, stabilized by a monolayer of surfactant at the interface of the two liquids. Synthesis of nanoparticles in nanosized “reactors” as the size of “nanoreactors” can be controlled within certain limits. A micelle is an example of these nanoreactors. Reverse micelles are tiny drops of water stabilized in a hydrophobic liquid phase due to the formation of a surfactant monolayer on their surface.
Microemulsion technique	Microemulsions may consist of oil-in-water or water-in-oil, depending on the concentration of the different components. By varying the concentration of the dispersed phase and the surfactant, it is possible to tailor the size of the droplets in the approximate range of 1–100 nm.	α-FeOOH, magnetite nanoparticles.
Sol–gel method and forced hydrolysis technique	Extremely versatile method since it allows the formation of a large variety of metal oxides at relatively low temperatures via the processing of metal salt or metal alkoxide precursors.	α-FeOOH, γ-FeOOH and Fe3O4 (prepared by forced hydrolysis technique).
		Reduction of Ni^2+ and Fe^2+ ions inserted in silica gel in 3 : 1 ratio with hydrogen at 733–923 K results in Ni3Fe nanoparticles (4–19 nm) within the SiO2 matrix.
Surfactant mediated/template synthesis	This technique uses neutral or charged template molecules.	α-Fe2O3 nanorods and nanotubes (polyisobutylene bissuccinimide or Span80 as surfactants).
Synthesis of magnetic nanoparticles at a gas–liquid interface	Nanoparticles can also be synthesized in the absence of solid substrates or matrices by redox reactions at an interface between two phases, one containing a metal compound (precursor) and the other, the reducing agent.	γ-Fe2O3 nanoparticles from Fe(CO)5 as a precursor.
UV-irradiation	Iron pentacarbonyl is exposed to an UV-irradiation.	γ-Fe2O3 nanoparticles Fe(CO)5 as a precursor.
Hydrothermal technique	Hydrothermal treatment of iron salt could generate iron oxides when the applied conditions are appropriate.	Decomposition of α-FeOOH to α-Fe2O3.
Flow injection	The technique consists of continuous or segmented mixing of reagents under laminar flow regime in a capillary reactor.	Magnetite nanoparticles.
Electrochemical methods	Electrons act as reactant. It is an environmental friendly process with no pollution. However, the costly platinum is used as an electrode and not for reuse in aqueous solution.	Maghemite or γ-Fe2O3 nanoparticles.
Biosynthesis	Biosynthesis of iron oxide nanoparticles with the help of Fe(III)-reducing bacteria (Shewanella sps., Geobacter sps., Thermoanaerobacter ethanolicus etc.), SRB (Archaeoglobus fulgidus, Desulfuromonas acetoxidans) and MTB (Magnetospirillum magnetotacticum, M. gryphiswaldense), are reliable, ecofriendly and economic at ambient temperatures, pressures and neutral pH, that can be used in environmental remediation.	Iron oxides.

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Table 3 Methods for preparation of specific types of iron-containing nanoparticles¹⁵¹

Method	Description	Examples of products
Specific methods for the preparation of particular types of iron-containing nanoparticles
Heterometallic nanoparticles	As a rule, these particles are prepared by simultaneous thermal decomposition of two metal complexes of different compositions (hydrogen is used most often as the reducing agent).	Heterometallic nanoparticles, Fe48Pt52 and Fe70Pt30, from Pt(acac)2 and Fe(CO)5.
		Fe–Co nanoparticles from cobalt ferrite CoFe2O4 nanoparticles or (η^5-C5H5)CoFe2(CO)9.
Ferrites	The key method for the preparation of powders of magnetic hexagonal ferrites with a grain size of more than 1 μm includes heating of a mixture of the starting compounds at temperature above 1000 °C (so-called ceramic method).	MnFe2O4 (40 nm), MgFe2O4 (6–18 nm), Co0.2Zn0.8Fe2O4 (2–45 nm), BaFe12−2xSnxZnxO19 (45 nm), SrFe12O19 (30–80 nm).
Magnetic nanoparticles with anisotropic shapes	(1) Decomposition of metalloorganic compounds, in particular by ultrasound in magnetic fields.	Fe nanoparticles from Fe(CO)5 as a precursor.
	(2) The standard way of synthesis of nanothreads and nanowires composed of anisotropic Fe and Co nanoparticles includes the electrolysis of solutions of the metal salts at an aluminium cathode, which is pre-coated by an Al2O3 layer containing channels with a diameter of 18–35 nm and a depth of up to 500 nm. During the electrolysis, these channels are filled by the reduced metal. After completion of the process, the matrix is dissolved in a mixture of acids to separate the nanoparticles.
	3) Electric arc decomposition of Fe(CO)5 resulted in thread-like (10 to 100 nm in diameter) compounds also consisting of α-Fe and Fe3C nanoparticles.
Methods for the synthesis of stoichiometrically inhomogeneous magnetic nanoparticles
Oxidation of nanoparticles	Oxidation of formed nanoparticles by oxygen-containing gas mixtures.	Iron oxide nanoparticles from Fe nanoparticles (contact with air). Iron passivation is possible. α- and γ-Fe2O3 by oxidation of Fe1−xCx nanoparticles.
Chemisorption of small molecules on a nanoparticle surface	The interaction of α-Fe nanoparticles with small CO, H2 and O2 molecules. Chemisorption of these molecules on a nanoparticle surface induces only minor changes in the parameters of the Mössbauer spectra. Studies of the reaction of α-Fe nanoparticles with nitrogen showed that only the Fe atoms of the surface layer participate in the chemical binding of nitrogen (the process starts at 300 K).	Chemisorption of N2 on α-Fe surface.
Targeted modification of the surface of magnetic nanoparticles	The immobilisation of biological molecules (amino acids, DNA, simple peptides, polysaccharides, lipids) on the surface of magnetic nanoparticles.	γ-Fe2O3 nanoparticles containing enzymes on the surface.

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A host of modern techniques, as described below, are being currently used for obtaining nanoparticles of Fe-containing nanomaterials, although well-known classic wet chemical routes are not forgotten and continue to be applied for production of NZVI²⁷ (in particular, by classic borohydride reduction),²⁸ Fe₂O₃ (sol–gel method²⁹ or electrochemical deposition from solution³⁰), or Fe₃O₄ (hydrolysis of Fe³⁺ and Fe²⁺ salts in the presence of urea and NaOH with the following ultrasonic treatment of FeO(OH)/Fe(OH)₂). Morphology and sizes of the resulting products, produced by various methods, can vary depending on the synthetic method used and conditions. For instance, it was shown that the Fe₃O₄ nanoparticles³¹ produced by radiofrequency nitrogen plasma had regular spherical form while the particles produced by wet chemical synthesis had well shaped cubic form. The size distribution of the plasma prepared oxides was wider, producing small particles with size in the range of 25–80 nm, and larger particles with size above 100 nm. The wider particle size distribution is characteristic of plasma prepared powders and it was explained by the authors by different growth conditions of particles due to temperature and velocity gradients of the plasma flow. This does not take place in the wet chemical route: the liquid phase synthesis provides preparation of Fe₃O₄ nanoparticles with average particles size in the range of 16–26 nm. Similarly, the shape, size and production rate of Fe₂O₃ nanoparticles, obtained by electrodeposition, were strongly influenced by the electrochemical conditions (e.g. FeCl₃ precursor concentration and current density).

NZVI and Fe–M nanoalloys or core–shell nanostructures

Among physical methods, the method of electrical explosion of wires for production of nanopowders was reported.^32,33 Intermetallic species can be also prepared by the arc discharge method, for instance Fe–Sn nanoparticles.³⁴ The synthesized nanoparticles have a shell/core structure with a SnO₂ shell of 5–10 nm in thickness and a core of polycrystalline intermetallic compounds. It was found that the intermetallic compounds FeSn₂ and Fe₃Sn₂ were generated and coexist with the Sn phase as a single nanoparticle. Microwave treatment (used to produce mainly inorganic compounds/composites/materials, and to a lesser extent, organic/organometallic compounds), is an alternative method to common heating of mixtures/precursors. The transformations of metals in MW field are generalized in a comprehensive recent book.³⁵ Metallic nanoparticles in distinct forms can also be produced using MW. For instance, Fe based nanoparticles have been produced using the microwave–polyol process in ethylene glycol at 100 and 150 °C in the presence of poly(vinyl pyrrolidone) and dodecyl amine.³⁶ In addition, pulsed excimer laser radiation at 248 nm wavelength was used to ablate ~2 μm feedstock of permalloy (Ni 81% : Fe 19%) under both normal atmospheric conditions and in other gases and pressures.³⁷ α-Fe particles were also prepared using a modified metal–membrane incorporation technique based on diffusing metal ions through a dialysis membrane.³⁸ The diffusion-time varied up to 15 min.

For ZVI,³⁹ the easiest, standard and most popular synthetic method is through the reduction of ferric Fe(III) or ferrous Fe(II) salts with NaBH₄, NaAlH₄ or LiAlH₄ as reducing agents. Thus, NZVI was synthesized (Fig. 4, reaction (4))⁴⁰ in ethanol medium by the method of reduction of FeX_n (X = Cl, OH, OR, CN, OCN, SCN) using sodium borohydride under atmospheric conditions.⁴¹ The resulting iron nanoparticles were found to be mainly in zerovalent oxidation state and remained without significant oxidation for weeks. Among other reductants used for obtaining iron nanoparticles, poly(vinyl alcohol-co-vinyl acetate-coitaconic acid) is an example.⁴²

2FeCl₃ + 6NaBH₄ + 18H₂O → 2Fe⁰ + 6NaCl + 6B(OH)₃ + 21H₂ (4)

Fig. 4 Schematic diagram for synthesis of iron nanoparticles. Reproduced with permission.

A patent⁴³ describes a route to metal nanoparticles by thermal decomposition of iron acetate Fe(OOCCH₃)₂, placed in a reaction vessel with a passivating solvent such as a glycol ether. The contents of the reaction vessel were mixed for a period of time to form a substantially homogenous mixture. The contents of the reaction vessel were then refluxed at a temperature above the melting point of the iron acetate. The desired particle size was achieved by controlling the concentration of metal salt in the passivating solvent and by varying the amount of reflux time. As an application of the pyrolysis technique, we would like to mention obtaining iron nanoparticles embedded in a carbon matrix from metal phthalocyanine,⁴⁴ and carbon-encapsulated Fe nanoparticles (size between 5 and 20 nm) via a picric acid-detonation-induced pyrolysis of ferrocene, which is characterized by a self-heating and extremely fast process.⁴⁵

There are also “green” techniques^46–50 for nanoparticle synthesis. Among these methods, use of plant extracts and other natural products as reductants and capping agents for NZVI and various other Fe-containing nanoparticle preparation is interesting⁵¹ as well. For example, the use of herbal tea extracts to reduce iron(III) chloride to iron nanoparticles (50 nm) has been reported.⁵² This process as well as several others involving Ag and Au salts employs the principle that the tea extracts (green tea, chamomile tea, hibiscus tea and the normal table tea) contain phenolic compounds that can act as reducing and capping agents in their application. A similar green single-step preparation of Fe nanoparticles using tea (Camellia sinensis) polyphenols has been described.⁵³ The expedient reaction between polyphenols and ferric nitrate occurs within a few minutes at room temperature and is indicated by color changes from pale yellow to dark greenish/black in the formation of Fe nanoparticles. The nanoparticles were utilized to catalyze H₂O₂ for treatment of organic contamination. In addition, nanoscale zerovalent iron (NZVI) particles, prepared using tea polyphenols,⁵⁴ were assessed with methyl tetrazolium (MTS, (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium)) and lactate dehydrogenase (LDH) toxicological assays, and some of them were found to be nontoxic when compared with control samples prepared using the conventional borohydride reduction protocols. The authors anticipated the development of various technological and environmental remediation applications for these stable particles including dechlorination of dibenzo-P-dioxins, reduction of chlorinated ethanes, arsenic and hexavalent chromium removal.

A variety of mainly physico-chemical methods have been used for the synthesis of Fe-containing bi- and polymetallic alloys and core–shell nanostructures. For example, high-entropy Nd–Fe–Co–Ni–Mn alloy nanostructure films were synthesized⁵⁵ by electrodeposition at room temperature. It was shown that the surfaces of the films, prepared at −2.2 V on Ti substrates for 5 min, were composed of great deal of close-grained and homogeneous nanoparticles with size about 200 nm. The film becomes more compact with a longer deposition time. The EDS indicated that the five elements were co-deposited. Monodisperse crystalline zerovalent Fe, Fe–Ni, Fe–Pd nanowires were synthesized similarly.⁵⁶ Prior to nanowire fabrication, alumina nanotemplates with controlled pore structure (e.g. pore diameter and porosity) were fabricated by anodizing high purity Al foil in H₂SO₄. After fabrication of alumina nanotemplates, Fe, Fe–Ni and Fe–Pd nanowires were electrodeposited within the pore structure. 200 nm-thick Fe–Pt nanocrystalline magnetic films having planar texture were prepared using magnetron sputtering and crystalline annealing in a magnetic field.⁵⁷ The prepared films were used to manufacture trial samples of magnetic memory devices having open Kittel domain structure for super-high density recording heads. For Fe₃Pt, FePt and FePt₃, heat formation was determined.⁵⁸ X-Ray diffraction, magnetic measurements and Mössbauer spectroscopy were used to study magnetic properties and hyperfine interaction parameters of nanocrystalline (<10 nm) and bulk bcc Fe, Fe₉₀Ge₁₀ and Fe₇₇Al₂₃ alloys.⁵⁹ It was established that the nanocrystalline state does not influence the formation of specific saturation magnetization, Curie temperature, isomer shift and hyperfine magnetic field. A slight increase (∼20%) of the width of the nanocrystalline iron Mössbauer spectral lines was observed.

Iron–gold alloys and core–shell nanoparticles

A special case, which should be discussed apart, corresponds to core–shell nanoparticles on the basis of iron and its oxides with gold due to the following justification. Finely divided iron has long been known to be pyrophoric, which is major reason that Fe nanoparticles have not been more fully studied.⁶⁰ This extreme reactivity has traditionally made Fe Nps difficult to study and problematic for practical applications. However, iron has a great deal to offer at the nanoscale, including very potent magnetic, catalytic and medical applications, so methods for reducing its reactivity with simultaneous retention of magnetic and catalytic properties have been developed. Although pure metal iron Nps are unstable in air, by coating the Nps surface with a noble metal, the formed air-stable Nps are protected from oxidation and retain most of the favorable magnetic properties, which possess the potential for applications mentioned above, as well as in high density memory devices by forming self-assembling nanoarrays.⁶¹

Gold is very promising coating for magnetic and other particles to be functionalized for targeted drug delivery. This metal (Au) as well as iron oxides Fe₂O₃ and Fe₃O₄, are biocompatible and suitable for human use. It is also possible to track within human body, and can be functionalized with organic and bioorganic molecules, such as proteins or enzymes. At the same time, Nps of iron and its oxides possessing magnetic properties can be delivered to an organ, tissue, or cancer tumor by using an external magnetic field. So, the unique combination of the nanoscale magnetic core and the functional shell makes Fe-doped Au Nps ideal for biological and biomedical applications due to their conjugation chemistry, optical properties, and surface chemistry.

During the last decade, considerable experimental efforts have been dedicated to biomedical applications of coated magnetic Nps, such as in magnetic resonance imaging contrast enhancement, tissue repair, immunoassay, detoxification of biological fluids, hyperthermia, targeted drug delivery, and cell separation. Results of the investigations in the area of gold-coated iron (iron oxides) core Nps are detailed in a series of monographs,^62–68 as well as in reviews^69,70 and patents^71,72 describing strategies for the synthesis of these Nps, characterization of the core/shell nanostructures, and exploration of potential applications of the core/shell nanomaterials in terms of biological and catalytic interfacial reactivities.

Types of Fe/Au Nps

Water-soluble Fe/Au alloy Nps (Fe/Au alloy Nps are solid solutions where iron atoms substitute gold sites in the face center cubic lattice) have been reported.⁷³ The diameter of these alloy Nps was 4.9 ± 1.0 and 3.8 ± 1.0 nm for two different precursors of iron, ferrous sulfate heptahydrate and iron pentacarbonyl;⁷⁴ while the estimated particle iron content was 14.8 ± 4.7 mol%. Formation of alloy nanostructure with a narrow distribution of particle sizes and three compositions Au_0.25Fe_0.75, Au_0.5Fe_0.5 and Au_0.75Fe_0.25 by a polyol process while retaining the optical and magnetic properties of the individual components has been discussed.⁷⁵ Fe/Au Nps of L1₀ type (mean composition Fe-22 atom% Au) were fabricated by annealing Fe/Au particles.⁷⁶ Au-rich (>32 atom%) and Au-poor (<16 atom%) regions were formed in most of particles.

The major part of publications has been devoted to Fe cores coated with Au shells.^77–80 For instance, nanosized iron–gold magnetic Nps with an average particle size 5–25 nm were prepared by a reverse micelle method, which is the principal technique in obtaining core–shell Nps, together with the polylol technique and homogenous solution reduction, efficiently used in many reports. The magnetic properties measurements confirmed behavior typical of a superparamagnetic system.^81,82 The XPS studies of the core–shell Fe/Au Nps indicate that besides Fe⁰ inside the cores, small amounts of Fe(II,III), located onto the gold surface, were also formed during the sample preparation. A certain number of reports have mentioned pure iron core, for instance Fe core/Au shell (11 nm Fe core, 2.5 nm Au shell) without iron oxides.⁸³

The general method for obtaining Fe core/Au shell Nps consists of subsequent reduction of iron and Au salts by NaBH₄ and other reductants in aqueous or organic media in the presence of surfactants for preventing agglomeration of the resulting Nps. In a series of reports, the characterized Nps were considered as containing pure metallic iron as a core. However, it should be especially emphasized that the authors of the communications who applied Mössbauer and X-ray absorption (XAS) spectroscopy, together with SEM, TEM and other techniques, noted that only the use of two additional methods mentioned above can give complete information on the state of iron in the core. Thus, gold passivated Fe Nps were prepared in a reverse micelle of cetyltrimethylammonium bromide,⁸⁴ and their studies by a series of methods above, including Mössbauer spectral study (⁵⁷Fe), showed the presence of iron in the forms of α-Fe (major part), unexpected Fe_1-xB_x alloy and several poorly crystallized ordered Fe₂O₃ components, as well as residual paramagnetic Fe(II) and Fe(III), indicating a more complex structure than had been believed, although the powder X-ray diffraction of Fe/Au Nps-containing samples revealed both the presence of crystalline α-Fe and gold and the absence of any crystalline iron oxides or other crystalline products. In another report,⁸⁵ the oxidation state of Fe in Fe/Au core–shell Nps was analyzed showing that, in contrast to many previous reports assuming the metallic state of iron in Fe/Au Nps obtained from reverse micelles, the iron component was found to be fully oxidized, according to X-ray absorption spectroscopy data (this method was considered as “an ideal tool for characterizing iron Nps”).

Additionally to the above mentioned reverse micelle technique, other methods have also been well-developed for obtaining Fe/Au Nps. Thus, Fe/Au Nps (8 nm) were prepared by sequential high temperature decomposition of organometallic compounds in a coordinating solvent and functionalized with alkanethiolate ligands preventing aggregation, enabling solubility in a series of both hydrophilic and hydrophobic solvents and allowing further derivatization via ligand exchange reactions.⁸⁶ Fe/Au Nps, confirmed to be solid solutions, were electrodeposited on an amorphous carbon electrode in a three-compartment glass-made electrochemical cell from aqueous electrolytes at room temperature using 1 mM FeSO₄ and HAuCl₄ solutions as precursors and CsClO₄ as supporting electrolyte in Ar atmosphere.⁸⁷ It has been reported that these Nps were homogeneously alloyed and the composition of the alloyed Nps was dependent on the electrodepositing potential. In comparison with similarly prepared Au/Ni and Au/Co Nps, the chain length for Fe/Au Nps is lower: Fe/Au < Ni/Au < Co/Au (the largest length was observed for pure Au Nps); this suggests that the transition metals in the alloyed Nps may prohibit the agglomeration of the alloyed particles. Radiolytical reduction of mixed Au^III/Fe^II ethylene glycol solutions led to two kinds of particles: 2 nm Fe rich Nps as well as large rods (a few tens of nm) and faceted particles rich in gold and containing a small amount of iron.⁸⁸ The adjunction of iron to gold enhances remarkably its electro-catalytic properties toward oxygen and proton reduction. The authors noted that Au–Fe system is very promising for application in fuel cells. Pulsed laser deposition of the Fe–Au alloy (35% Fe) in a mesoporous alumina membrane template allowed formation of strongly paramagnetic Fe/Au Nps (17% Fe, 46 ± 13 nm), which can be easily functionalized with biological macromolecules.⁸⁹

Current and proposed uses

Fe/Au Nps, in particular those supported onto HY- and NaY-type zeolites have been investigated for use as catalysts or cleaning surfaces.⁹⁰ Thus, interactions of Au with Fe species, introduced into the NaY zeolite by two methods (wet-impregnation and ion exchange), in Fe/Au-imp/NaY zeolite system were studied.⁹¹ 1%Au-0.5%Fe/HY sample, possessing high activity in the CO oxidation, was prepared by coexchange in HY zeolite using Fe(II) ethylenediamine and Au(III) ethylenediamine complex ions.⁹² Photoinduced sulfur desorption from the surfaces of Au Nps loaded on a series on metal oxides, in particular Fe₂O₃, was studied.⁹³ Elemental sulfur S₈ was selectively adsorbed on the Au Nps surfaces of Au/metal oxides in an atomic state. This phenomenon is applicable to the low-temperature cleaning of sulfur-poisoned metal catalysts.

In addition to core–shell Fe_xO_y/Au Nps, Fe/Au Nps have also been used in biomedicine. Thus, the method for synthesizing antibody-magnetic Nps with gold shell and iron core for recognition and separation of tumor cells was elaborated.⁹⁴ Also, their utility as T1 contrast agents in the unoxidized form,⁹⁵ capturers for pathogens,⁹⁶ carriers of streptavidin,⁹⁷etc, have been reported. The 7 nm Fe/Au Nps (1.5% of Fe), obtained⁹⁸ by simultaneous reduction of Fe³⁺ and Au³⁺, can be easily conjugated to thiolated DNA; moreover, and heated in solution to temperatures above 40 °C, indicating suitability for hyperthermia.

Fe/Au Nps can serve as precursors for other Nps or functionalized derivatives. Thus, 200–350 nm C-encapsulated magnetic Nps possessing surfaces with functional groups such as –OH were prepared by hydrothermal heating an aqueous glucose solution of FeAu Nps at 160–180 °C for 2 h.^99,100 An organometallic approach to the synthesis of CO-protected Fe/Au Nps led to colloids with hydrodynamic diameters between 4 and 300 nm, from which [Au₂₁{Fe(CO)₄}₁₀]⁵⁻, [Au₂₂{Fe(CO)₄}₁₂]⁶⁻, [Au₂₈{Fe(CO)₃}₄{Fe(CO)₄}₁₀]⁸⁻ and [Au₃₄{Fe(CO)₃}₆{Fe(CO)₄}₈]⁸⁻ were isolated.¹⁰¹ Chemical oxidation in solution of Au Nps, protected by octanethiol and 12-(N-pyrrolyl)dodecanethiol, with FeCl₃ leads to cross-linked Au Nps with chainlike structures.¹⁰² The authors proposed that this approach may also be applied for Fe/Au.

Coated and supported iron nanoparticles

Fe⁰ composite nanoparticles, coated with inorganic compounds and minerals, are common and prepared by distinct techniques. Thus, iron nanoparticles coated with boron nitride (BN) nanomaterials were synthesized by using Fe₄N and B powders as raw materials.¹⁰³ The Fe₄N was reduced to α-Fe during annealing at 1000 °C for several hours with flowing N₂ gas. NZVI/montmorillonite heterostructures were synthesized and their textural evolution under different Fe loadings was investigated.¹⁰⁴ The hybridized Fe nanoparticles were well dispersed on the montmorillonite surface, size adjustable, and resistant to oxidation under the protection of native Fe-oxide shells. It was revealed that as the Fe loadings increased, the total pore and mesopore volumes remained almost unchanged but the total, micropore and external surface areas as well as the micropore volume decreased and the average pore diameter increased. With increasing Fe loadings, the mesoporous character was enhanced for these heterostructures. Nanosized composite magnetic particles MgO/Fe were in situ combustion-synthesized at 620 °C for the Mg-70.9 wt% Fe₃O₄ system.¹⁰⁵ It was identified that Mg (29.1 wt%) was the suitable reactant ratio, the sintered composite spherical particles with mean diameter 40 nm were distributed evenly, the particles had good soft magnetic properties, and could serve in the future as drug carrier materials. In addition, well-protected, isolated bcc-iron nanoparticles embedded in silicon dioxide were prepared by e-beam evaporation and post-annealing of multilayers in an ultrahigh vacuum system. The spherical shape and isolation of the particles were confirmed by plan-view and cross-sectional transmission electron microscopy.¹⁰⁶ Similarly, an approach using a low-energy electron beam radiation system was investigated to synthesize carbon hybrid structures in amorphous carbon thin films.¹⁰⁷

Large-size 2D nano-TiO₂ and iron-doped nano-TiO₂ thin films were prepared¹⁰⁸ in a low-temperature aqueous system by using a molecular self-assembly method. The photocatalytic activity was evaluated with the degradation of methyl orange solution under UV and visible light radiation. The doped iron could obviously improve photocatalytic activity of TiO₂. The degradation data of methyl orange were respectively 98.62% and 89.24% under illumination by using UV lamp and visible light. Fe-nano ZrO₂ composite coating was prepared¹⁰⁹ by adopting iron electroplating technology, choosing nano ZrO₂ as second phase particle, and using asymmetrical AC–DC electroplating. It was indicated that this method can lead to compact coatings (body-centered cubic α-Fe crystal structure) with small inner stress. Al₂O₃ coated polyhedral Fe nanocapsules were prepared by arc-discharging a Fe–Al (8 atom% Al) alloy.¹¹⁰ In addition, mobilization and deposition of iron nano and sub-micrometer particles (INSMP) in a porous medium were investigated using a water-saturated glass micromodel.¹¹¹ It was shown that there were dense aggregations at the pores as the concentration of INSMP increased. The presence of dissolved humic substances (>1 ppm) significantly reduced deposition of suspended particles and enhanced detachment of the deposited particles.

A host of reports are dedicated to carbon-supported ZVI nanomaterials.¹¹² Such encapsulation of iron nanoparticles in protective carbon cages can lead to unique hybrid core–shell nanomaterials.¹¹³ Thus, special carbon encapsulated Fe core–shell nanoparticles (15–40 nm) with a size range of 15–40 nm were prepared via a confined arc plasma method.¹¹⁴ It was shown that the carbon encapsulated Fe nanoparticles had clear core–shell structure, the core (16 nm diameter) of the particles is bcc-structure Fe, and the shell (thickness 6–8 nm) of the particles is disordered carbon. The related arc discharge technique¹¹⁵ is also common for production of nanomaterials, in particular for iron. Thus, a simple, inexpensive and one-step synthesis method of metal-containing carbon nanocapsules using an arc discharge in aqueous solution is reported.¹¹⁶ It was found that Fe nanoparticles could be in situ encapsulated in carbon shells when the arc was performed respectively in aqueous solutions of FeSO₄. To explain the formation mechanism of metal-containing carbon nanocapsules, a model of discharge in solution was proposed. Iron nanoparticles, coated with graphite nanolayers, were synthesized by annealing mixtures of hematite and carbon under nitrogen atmosphere.¹¹⁷ Hematite was reduced to Fe₃O₄, FeO, and finally to Fe completely at 1200 °C. High-resolution electron micrographs reveal that the Fe particles are ~200 nm in diameter and coated with graphite nanolayers of ~30 nm. Combustion synthesis of iron oxide/iron coated carbons such as activated carbon, anthracite, cellulose fiber and silica using a Panasonic kitchen microwave with inverter technology was described.¹¹⁸ The size of the iron oxide/iron nanoparticle-coated activated carbon, anthracite, cellulose fiber and silica samples were found to be in the nano range (50–400 nm). Iron oxide/iron nano particles existed in four major phases: γ-Fe₂O₃, α-Fe₂O₃, Fe₃O₄ and Fe, some of which showed significant arsenic adsorption. In addition, carbon-coated iron nanoparticles with well-developed quasi-spherical shape were prepared with Fe(NO₃)₃·9H₂O and starch as carbon source (Fig. 5).^119,120 This is an efficient approach for the mass production of nanocage structures under mild conditions, which needs to be further explored for preparing various carbon coated metal nanomaterials. Radiation methods are being applied more widely in the last decade, in particular for iron carbon-supported nanostructures fabrication. Thus, two types of amorphous carbon films, a 15 atom% iron containing film and with column/inter-column structures, were deposited onto Si substrates by a sputtering technique and subsequently exposed to an electron shower of which the energy and dose rate were much smaller compared to an intense electron beam used in transmission electron microscopy. Graphitic structures were formed in an amorphous matrix at a relatively low temperature up to 450 K. It was found that the graphitization progressed more in the electron irradiation than in annealing at 773 K, and it was attributed to thermal and catalytic effects which are strongly related to grain growth of metal clusters.

Fig. 5 HRTEM of carbon-encapsulated iron nanoparticles from starch (scale bar 10 nm) and inset corresponding electron diffraction pattern taken from iron core regions. Reproduced with permission.

Significant attention is paid to ferrocene as a metal precursor for carbon-supported nanoparticle synthesis¹²¹ and the resulting carbon support can exist in distinct forms. For example, carbon nanotubes containing iron nanoparticles were fabricated using microwave heating.^122,123 Carbon-encapsulated iron nanoparticles with uniform diameters were synthesized on a large scale by co-carbonization of an aromatic heavy oil and ferrocene at 480 °C under autogenous pressure (Fig. 6).¹²⁴ It was found that, by increasing the amount of ferrocene added from 2 to 45 wt%, the size of the nanoparticles increased from 15 to 50 nm and the morphologies of the resulting products changed from spherical-type to iron-filled carbon nanorods when the ferrocene loading was higher than 30 wt%. The iron particles pyrolyzed from ferrocene existed mainly in the form of α-Fe and small amounts of Fe₃C were also formed when the ferrocene content was higher than 20 wt%.

Fig. 6 HREM images of carbon-encapsulated iron nanomaterials obtained at 480 °C in the presence of ferrocene contents of 13.0 wt% [(a) and (b)] and 40.0 wt% (c). Reproduced with permission.

Ferric alginate fibers were prepared by wet spinning of sodium alginate into a coagulating bath containing ferric chloride.¹²⁵ Carbon-supported nanoscale zerovalent iron fibers (CSNZVIF, with high surface areas of 352 m² g⁻¹) were obtained through thermal degradation of ferric alginate fibers at 900 °C under an N₂ atmosphere. It was found that zerovalent iron particles were well dispersed in the amorphous carbon fibers. The authors stated that the existence of carboxylic and hydroxyl groups in the ferric alginate structure unit play a key role in the formation of carbon-supported nanoscale zerovalent iron fibers. Fe³⁺ was reduced to Fe⁰ by hydroxyl group and as-formed amorphous carbon during heating under N₂.

Polymer-supported Fe nanoparticles are also widespread. Thus, the procedure developed to produce polymer-supported nano Fe metal particles involved thermal decomposition of Fe(CO)₅ in the presence of a monomer (styrene) (reaction (5), which can be better described as a combination of reactions (6) and (7)) that was allowed to concurrently polymerize.¹²⁶

nH₂C=C(H)Ph + Fe(CO)₅ → (–CH₂–CH(Ph)–)_n + 5CO + Fe⁰ (5)

Fe(CO)₅ → “Fe” + 5CO (6)

nH₂C=C(H)Ph + “Fe” → “Fe”–(–CH₂–CH(Ph)–)_n (7)

The effect of pH on the aggregation of polymer-coated NZVI and its deposition onto sand and clay (kaolinite) surfaces was studied.¹²⁷ NZVI coatings included a high molecular weight (90 kg mol⁻¹) strong polyanion, poly(methylacrylic acid)-b-(methyl methacrylate)-b-(styrenesulfonate) (PMAA-PMMA-PSS) and a low molecular weight (2.5 kg mol⁻¹) weak polyanion, polyaspartate. Enhanced deposition at lower pH was indicated because the elutability of polyaspartate-modified hematite (which did not aggregate) also decreased at lower pH. The greater deposition onto clay minerals compared to similar sized fine silica is attributed to charge heterogeneity on clay mineral surfaces, which is sensitive to pH.

Nano-Fe₂O₃ phases and their composites

Distinct iron oxide nanostructures and composites based upon them have been prepared by a variety of conventional techniques. Thus, iron oxide nanowires were grown by resistive heating of iron wire (with a diameter of 0.25 mm) under ambient laboratory conditions.¹²⁸ The synthesis can be easily controlled by observing the color of the wire and by varying the applied heating power. The wire was completely covered by nanowires, having a sword-like shape, i.e., they are belt-like structures, which are thicker at the base and thinner at the end. Maghemite (γ-Fe₂O₃) nanoparticles were fabricated by microwave heating of (acetylacetonato)iron(III) precursor.¹²⁹ This precursor is very easy to make, and there is no need for controlling the reaction conditions and heat treatments. Upon irradiation by microwaves it decomposes to maghemite nanoparticles. The β-Fe₂O₃ cubic phase, one of the least studied Fe–O systems, was obtained by CVD method using a Fe(II) β-diketonate diamine complex, Fe(hfa)₂TMEDA, as the molecular source (hfa = 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate, TMEDA = N,N,N′,N′-tetramethylethylenediamine).¹³⁰ Also, a Fe(II) 1D coordination polymer, [Fe(pyterpy)₂](SCN)₂·MeOH (pyterpy = 4′-(4-pyridyl)-2,2′:6′,2′-terpyridine), was synthesized by using a branched tube and used as a precursor to fabricate Fe(III) oxide nanoparticles¹³¹ by its thermolysis in oleic acid (surfactant) at 286 °C in air. Magnetic iron oxide nanopowders were synthesized electrochemically,¹³² using a low-carbon steel electrode immersed in a NaCl aqueous solution, at constant temperature of the electrolyte, pH and current density. In the second step, portions of the starting admixture were boiled at ∼360 K during 2 h and autoclaved at various temperatures. In addition, nanostructured iron oxide/hydroxide materials were synthesized by sol–gel technology, starting from the ternary system Fe(NO₃)₃·9H₂O–ethanol–propylene oxide.¹³³ The produced materials were composed by aggregates of nanometric crystallites: ∼1 nm for xerogels and ∼5 nm for aerogels. Their high porosity and surface area (xerogels ∼50% and 150 m² g⁻¹; aerogels ∼90% and 400 m² g⁻¹) make them suitable for surface-dependent processes, with the aerogels far superior. It was shown that the 2-line ferrihydrite is their most probable constituent phase.

As well as in case of Fe⁰-containing nanomaterials (see above), radiation methods have been applied for the generation of oxides and composites. Thus, to prepare nanocrystalline iron(III) oxide at room temperature and ambient pressure without using any kind of catalysts, a 2 MeV 10 mA GJ-2-11 electronic accelerator was used as the radiation source.¹³⁴ The optimal conditions were pH of the solution = 6.54 and a 300 kGy radiation dose. It was shown that the α-Fe₂O₃ particles had hexagonal structures, spherical shape morphology and good dispersity, and average diameter of ∼30–60 nm. Alternatively, Fe₂O₃ was fabricated¹³⁵ from metallic iron under electron beam irradiation and further reaction with oxygen. Iron oxide nanowires were rapidly synthesized in large quantities at room temperature by pulsed-laser ablation (248 nm) of iron powder under methanol.¹³⁶ At high collection rates, a lamellate nanobelt morphology was observed, whereas at low collection rates nanowires dominated. The as-synthesized products had the stoichiometry of the goethite [FeO(OH)] phase (proposal formation reactions are shown by reactions (8) and (9)) which after annealing at temperatures above 400 °C crystallizes into hematite (α-Fe₂O₃).

2Fe + 12CH₃OH→ 6C₂H₅OH + 2Fe(OH)₃ + 3H₂ (8)

Fe(OH)₃ →FeO(OH) + H₂O (9)

Composite nanoparticles consisting of gold and iron oxide were synthesized in aqueous solution systems by using a high-energy electron beam.¹³⁷ The electron irradiation induced radiation-chemical reaction to form metallic gold nanoparticles (reaction mechanism is shown by eqns (10)–(14)), which were firmly immobilized on the surface of the support iron oxide nanoparticles. The size of these gold nanoparticles depended on the concentrations of gold ions, polymers and iron oxide nanoparticles in the solutions before the irradiation. Many other examples of core–shell iron oxide/gold nanoparticles were reviewed,¹³⁸ where many possible synthetic techniques for core–shell structures on the basis of iron or its oxides with gold have been discussed. Among them, we emphasize that, additionally to the mostly represented “standard” iron oxide core/gold shell Nps, some reports concerned interesting unexpected nanostructures. Thus, iron oxide coated gold nanorods were prepared (Fig. 7) starting from gold nanorods, synthesized by a three-step seed mediated protocol and coated with a layer of poly(sodium 4-styrenesulfonate).¹³⁹ Here, on the contrary, the gold metal acts as a core and the iron oxide as a shell. The negatively charged polymer on the nanorod surface electrostatically attracted a mixture of aqueous Fe(II) and Fe(III) ions. Coprecipitation of these iron salts was used to form uniform coatings of iron oxide Nps on the surface of the gold nanorods. The oxidation state of iron in the iron oxide coatings was determined and was consistent with Fe₂O₃ rather than Fe₃O₄. Multifunctional Fe₃O₄/polymer/Au shell Nps¹⁴⁰ with preserved strong magnetization and good NIR absorption display good dispersibility and stability in aqueous solution. The authors supposed that these features should facilitate biomedical applications, combining the benefits of MRI diagnosis, magnetically targeted delivery and photothermal ablation.

H₂O (γ-rays or e-beam) → e⁻_aq, H·, OH·, etc. (10)

(CH₃)₂CHOH + OH· → (CH₃)₂·COH + H₂O (11)

(CH₃)₂·CHOH + H· → (CH₃)₂·COH + H₂ (12)

e⁻_aq + Mⁿ⁺ → M⁽ⁿ⁻¹⁾⁺ (13)

(CH₃)₂·COH + Mⁿ⁺ → (CH₃)₂CO + M⁽ⁿ⁻¹⁾⁺ + H⁺ (14)

Fig. 7 General protocol used to prepare in situ iron oxide coated gold nanorods. Reproduced with permission from ACS (Journal of Electron Microscopy; jmicro.oxfordjournals.org).

Zeolite and related supporting materials serve as an ideal basis for iron oxide composites. Thus, the loading of zeolite with nano-iron oxide (Fig. 8) by a simple chemical route has been described.¹⁴¹ The average crystallite sizes of the doped nanomaterials were 4–6 nm. It was revealed that zeolite acquire magnetic properties after doping with nano-iron oxide. Mesoporous iron oxide silicate nanocomposites Fe₂O₃-SBA-15 (SBA-15 is hexagonally ordered mesoporous silica) with iron loadings in the range of 1.2–35.8 wt% were synthesized by a direct hydrothermal method.¹⁴² It was shown that these mesoporous nanocomposites with well-dispersed nanoclusters of iron oxide in the walls of ordered mesoporous silica structures and high surface area were attained under the condition of adjustment of Fe/Si molar ratios and appropriate Fe loadings.

Fig. 8 Loading of iron oxide nanoparticles by zeolite. Reproduced with permission.

Fe₃O₄ and other iron-containing nanomaterials

A certain number of composite nanomaterials based on Fe₃O₄ is known, for instance core/shell Fe₃O₄ coated gold nanoparticles (diameter 50–100 nm).¹⁴³ Their possible formation mechanism was proposed as follows: pH-sensitive polymer owing to a shrunken or stretched structure of polyethyleneimine (PEI), led to the aggregation of Fe₃O₄–gold seed nanoparticles, then gold reduces onto the surface of Fe₃O₄–gold seed nanoparticles. It was concluded that these core/shell multifunction nanomaterials will not only have external magnetic separation by the core of Fe₃O₄, but also detect large biological molecules using the shell of gold. In addition, iron phthalocyanine prepolymer/Fe₃O₄ nano hybrid magnetic material¹⁴⁴ can be applied as high temperature-resistant polymer magnetic composite material.

Use of “Greener” methods superparamagnetic Fe₃O₄ nanoparticles (blocking temperature (T_B) of 150 K and saturation magnetization of 37.1 emu g⁻¹) have been synthesized via soya bean sprouts (SBS) templates at room temperature and at normal atmospheric pressure.¹⁴⁵ Spherical Fe₃O₄ nanoparticles with an average diameter of 8 nm simultaneously formed on the epidermal surface and the interior stem wall of SBS, which were responsible for size and morphological control during the whole formation of nanoparticles. In a related report,¹⁴⁶ similar Fe₃O₄ superparamagnetic nanoparticles (12.5 nm, inverse spinel structure) were prepared using α-D-glucose as the reducing agent and gluconic acid (a product of glucose oxidation) as the stabilizer and dispersant. In this case, the T_B was found to be higher (190 K) and the magnetic hysteresis loop at 300 K showed a saturation magnetization of 60.5 emu g⁻¹.

Among other iron-containing nanomaterials, we note carbon encapsulated iron carbide nanoparticles (5–40 nm) were synthesized from ferrocene in a quite narrow temperature range (about 600–800 °C) under high (4 GPa) static pressure.¹⁴⁷ The material produced under optimum parameters consists of round carbide particles covered with onion-like carbon shells that are embedded into the carbon matrix. In the presence of an excess of carbon a solid-state Fe₃C → Fe₇C₃ transformation is possible in nanoparticles under high (13 GPa) pressure and temperature (1200 °C). Other examples of Fe-containing nanomaterials are shown in Table 2.

Remediation of groundwater using iron-based nanomaterials

Research over the past two decades has demonstrated the efficiency of Fe for the removal of a wide range of chemical and microbial contaminants (e.g. bacteria, chlorinated organics, dyes, emerging contaminants, heavy metals, radionuclides, viruses) from water.¹⁵² The prevailing concept considers that the mechanism of Fe⁰ remediation varies depending on the contaminant of interest. This concept was recently revisited and Fe was proven as a universal material for water treatment.

The following methods are currently considered as conventional treatment technologies for water treatment:¹⁵³ (a) filtration (using ceramic or biosand filters, charcoal and activated carbon filters, granular media and rapid rate filters, fiber and fabric filters), (b) heat and UV-radiation, (c) chemical treatment (coagulation–flocculation, chemical disinfection and flocculant–disinfection), desalination and arsenic removal (reverse osmosis, distillation, adsorptive filter media for arsenic removal). The nanotechnology-based water treatment technologies include: (a) carbon nanotube-based technologies (CNTs membranes, nanomesh), (b) other nanofiltration approaches (nanofiltration membranes and devices, nanofibrous alumina filters and nanofiber gravity-flow devices), (c) nanoporous ceramics, clays and other adsorbents, (d) zeolites, (e) nanocatalyst-based technologies (nanostructured ZVI, TiO₂ and iron oxide adsorbents) and (f) magnetic nanoparticles (magnetoferritin and ZVI).

Starting from the 1990s, inorganic nanoparticles,¹⁵⁴ in particular NZVI and iron oxides have been used for elimination of certain inorganic and organic substances from groundwater,¹⁵⁵ sediments and soils, despite that the NZVI utility in full-scale remediation projects is limited by material costs. ZVI nanoparticles react with a wide range of contaminants, and the effect of particle size on the reactivity of NZVI is as important as other effects such as particle composition, contaminant structure and solution chemistry. It has been established that NZVI is very effective, covering the broadest range of environmental contaminants. Common environmental contaminants (Tables 4–6) that can be transformed by nanoscale iron particles are as follows:¹⁵⁶ (a) chlorinated methanes (CH_nCl_4−n), (b) chlorinated benzenes (C₆H_nCl_6−n), (c) pesticides (DDT, lindane), (d) organic dyes (Orange II, Chrysoidine, Tropaeolin O, Acid Orange, Acid Red), (e) heavy metal ions (Hg²⁺, Ni²⁺, Ag⁺, Cd²⁺), (f) trihalomethanes (CHBr₃, CHBr₂Cl, CHBrCl₂), (g) chlorinated ethenes (C₂Cl₄), C₂HCl₃, cis- or trans-C₂H₂Cl₂, 1,1-dichloroethene C₂H₂Cl₂, vinyl chloride), (h) other polychlorinated hydrocarbons (PCBs, for example hexachlorocyclohexanes¹⁵⁷), dioxines, pentachlorophenol (C₆HCl₅O), (i) other organic contaminants (N-nitrosodimethylamine (NDMA) (C₄H₁₀N₂O), TNT (C₇H₅N₃O₆) and dinitrotoluene,¹⁵⁸ (j) inorganic anions (Cr₂O₇²⁻, AsO₄³⁻, ClO₄⁻, NO₃⁻, SeO₄²⁻), (k) also removing dissolved metals from solution, e.g. Pb,²⁺ Ag⁺ and Ni²⁺. It may also be able to reduce radionuclides.

Nanoscale zerovalent iron (NZVI) is used for both in situ and ex situ treatment of contaminated groundwater. It functions simultaneously as an adsorbent and a reducing agent, causing organic contaminants to break down into less toxic simple carbon compounds and heavy metals to agglomerate and stick to the soil surface. NZVI can be injected directly into the source of contaminated groundwater as slurry for in situ treatment, or it can be used in membranes for ex situ applications. Bimetallic NZVI, in which the iron nanoparticles are coated with a second metal such as palladium to further increase the reactivity of the iron, is also available. NZVI is more reactive and has a large surface area than granular ZVI.

Among many important investigations in this area, we note recent contributions of Tratnyek et al.^159–168 In relation to chlorohydrocarbons, the mechanism of reactivity for ZVI is similar to the mechanism of corrosion (i.e., oxidation of iron) and involves the generation of electrons which in turn reduces the organic species through dechlorination (reaction (15)):

Fe⁰ + RCl + H⁺ → Fe²⁺ + RH + Cl⁻ (15)

Similarly, ZVI acts as a reducing agent for sequestration of metal ions having reduction potential higher than Fe. The chemistry can be understood by an example of interaction of Ni²⁺ with ZVI (Fig. 9). In the beginning of the reaction, Ni²⁺ is physically adsorbed (outer sphere interaction) and then is chemisorbed (inner sphere interaction) and finally reduced to metallic nickel. Similar chemistry has been applied for sequestration of many metal ions and metalloids: Pb, Cd, Cr, Co, Cu, Hg, Ni and Se.

Fig. 9 (a) TEM images of iron nanoparticles, (b) HR-XPS survey on the Ni 2p_3/2 of iron nanoparticles and (c) a conceptual model for nickel deposition on iron nanoparticles.¹⁷⁰

There is an ambiguity regarding the relationship between NZVI and conventional (micron- to millimeter-sized) ZVI.¹⁶⁹ NZVI exhibits unique properties like those of some other nanostructured materials, when the behavior of NZVI under environmental remediation conditions is, in fact, not very much different than conventional ZVI. It was shown that: (a) the NZVI used in groundwater remediation that are much larger than particles that exhibit “true” nanosize effects, (b) the higher reactivity of this NZVI is mainly due to its high specific surface area, (c) one factor that limits the intrinsic reactivity of NZVI is the high degree of passivation by enveloping oxides, and (d) the mobility of NZVI is less than a few meters under almost all relevant conditions.

NZVI and iron oxide magnetic nanoparticles have also been chosen¹⁷¹ as promising adsorbents for the removal of (in)organic contaminants from polluted water, because: (a) they can be produced in large quantities using physicochemical methods, (b) it is expected that their adsorption capacity and affinity for pollutants is higher considering the larger surface area and possibly highly active surface sites, (c) the separation of metal-loaded magnetic nano-adsorbent from treated water can be achieved via an external magnetic field, and (d) nanoparticles might be regenerated and reused. Also, iron oxide nanoparticles can be surface-coated with organic compounds to increase their sorption efficiency and specificity for pollutants. In this case, it is worth noting that the covalent attachment of molecules to the surface of nanoparticles and the stability of the coating are important. Some adsorption processes for wastewater treatment have utilized ferrites and a variety of iron containing minerals, such as akaganeite, feroxyhyte, ferrihydrite, goethite, hematite, lepidocrocite, maghemite and magnetite.¹⁷² Adsorption of organics to the nanoparticle media was extremely rapid. More than 90% of the organics were adsorbed within 30 min. The isotherm studies indicated that, on a surface area basis, the adsorption capacities of the nanoparticle media were significantly (>2 fold) higher than the ferric oxide media typically used in water treatment.

Remediation of metal ions

NZVI (20–80 nm), obtained using the borohydride reduction method, was examined for aqueous Co²⁺ removal over a wide concentration range, 1–1000 mg L⁻¹.¹⁷³ Fe nanoparticles demonstrated very rapid uptake and large removal capacity for Co²⁺. It was indicated that Co²⁺ fixation occurred by the interaction of cobalt ions with oxohydroxyl groups on Fe nanoparticle surfaces as well as spontaneous precipitation formation at high loads. The sorption properties of Zn(II), Cu(II), Cd(II) and Pb(II) to nanohematite in single- and binary-adsorbate systems were investigated.¹⁷⁴ It was indicated that the presence of a secondary metal can affect the sorption process depending upon the molar ratios, such as increased or reduced adsorption. Also, Pb and Cu adsorption to nanohematite is an endothermic, and a physical adsorption process; however, it is only spontaneous at higher temperatures.

Remediation of inorganic anions

The elimination of nitrates¹⁷⁵ and arsenic-containing anions is the main application of the NZVI in respect to inorganic matter; a host of reports are dedicated to this area. Thus, the NZVI in different forms (freshly synthesized, dried, and dried-sonicated) was studied as an effective nitrate reduction material.¹⁷⁶ As a result, different types of NZVIs were found to be able effectively reduce highly concentrated nitrate without requiring pH control. The authors suggested that the aggregate size and catalyst prominently affect the nitrate reduction rate and that the aggregation effect is more important than the catalyst effect as the aggregate size becomes smaller. The final product of the reaction was ammonium, with nitrite being produced as a byproduct; NZVI changed into different shapes of magnetite (Fe₃O₄) after the reaction, depending on the reaction conditions. The mechanism for nitrate reduction by nano-zerovalent iron was also studied in ref. 177. Bimetallic iron-containing nanoparticles react similarly; thus, improved N₂ selectivity of NZVI via NO₃⁻ denitrification using bimetallic Ni/Fe nanoparticles was discussed.¹⁷⁸ NO₃⁻ was first adsorbed on the nano-Ni/Fe surface and reduced to NO₂⁻. With the rapid reduction of NO₂⁻ to NH₄⁺, this NH₄⁺ was desorbed into solution due to its smaller affinity for the particle surface. In addition, microbial reduction of nitrate in the presence of NZVI was evaluated to assess the feasibility of using NZVI in the biological nitrate treatment.¹⁷⁹ Nitrate was completely reduced within 3 days in a nanoscale Fe(0)-cell reactor, while only 50% of the nitrate was abiotically reduced over 7 days at 25 °C. Efficient removal of nitrate by Fe(II)-supported anaerobic culture in 14 days indicated that Fe(II), which was produced during anaerobic Fe corrosion in the Fe⁰-cell system, might act as an electron donor for nitrate. In addition, microbial reduction of nitrate was not significantly affected by low temperature conditions.

For arsenic elimination (this is a very important problem in many countries¹⁸⁰), nanophase Fe₃O₄ (magnetite, 17 nm) and Fe₂O₃ (hematite, 12 nm) were synthesized through a precipitation method¹⁸¹ and were utilized for the removal of either arsenic(III) or (V) from aqueous solution as a possible method for drinking water treatment. The binding was observed to be pH independent from pH 6 through pH 9 and a significant drop in the binding was observed at pH 10. Batch isotherm studies were performed using the Fe₂O₃ and Fe₃O₄ to detect the binding capacity of As(III) and As(V) to the iron oxide nanomaterials. Among other studies, XPS and X-ray absorption spectroscopy were used for NZVI treated with Cr(III) and Cr(VI).¹⁸² It was shown that (a) the crystalline Fe(III) phase is composed of lepidocrocite (γ-FeOOH), (b) Cr(VI) was entirely reduced to Cr(III) by nano Fe⁰ with no residual Cr(VI) after reaction. Cr(III) precipitated as Cr(OH)₃ in the presence of corroding nano Fe⁰ was nearly identical to the Cr(VI)-nano Fe⁰ reaction product and both Cr(III)- and Cr(VI)-treated nano Fe⁰ yielded a predominantly hydroxylated Cr(OH)₃ and/or a mixed phase Cr_xFe_1−x(OH)₃ product, both of which are highly insoluble under environmental conditions. In addition, NNZVI was also evaluated for the reduction of bromate that is a highly persistent and carcinogenic oxyhalide formed as an ozonization byproduct during oxidative disinfection in drinking water treatment.¹⁸³ It was revealed that in a 20 min bromate reduction NZVI mostly converted to Fe₂O₃ and Fe₃O₄ corrosion products mixed with Fe hydroxides. Humic acid was the most influencing factor to decrease NZVI reactivity in bromate reduction. In addition, the effect of sonication pretreatment showed that the bromate reduction efficiency could be enhanced by increasing the actual reactive surface area.

Remediation of organic pollutants

A large variety of organic pollutants, especially chlorinated hydrocarbons, have been used in remediation experiments using NZVI, Fe/M nanoparticles and iron oxides. Thus, the oxidative capability of NZVI was analytically compared via experiments using the model compounds benzoic acid and formic acid.¹⁸⁴ Rapid production of H₂O₂ with concomitant loss of the model compound was observed upon addition of NZVI and bimetallic nanoparticles to oxygenated solutions with eventual loss of formic acid. On the other hand, functionalization of NZVI with polymeric stabilizers starch, carboxylmethylcellulose and alginate slowed down the rate of H₂O₂ production and the consumption of formic acid. Fe nanoparticles were used to remediate PCB-contaminated soil and an attempt was made to maximize PCB destruction in each treatment step.¹⁸⁵ The destruction efficiency during the preliminary treatment (mixing of soil and Fe nanoparticles in water) can be increased by increasing the water temperature. A minimum total PCB destruction efficiency of 95% can be achieved. At 300 °C in air, Fe₂O₃ is also a good catalyst for remediating PCB-contaminated soils. Nanoscale palladized iron (Pd/Fe) bimetallic particles (spherical granules with diameter of 47 ± 11.5 nm connected with one another to form chains and the chains composed nanoscale Pd/Fe bimetallic particles), prepared by reductive deposition method¹⁸⁶ and containing α-Fe⁰, revealed the best dechlorination effect in elimination of monochloroacetic acid by different reductants followed the trend: nanoscale Pd/Fe bimetallic particles of 0.182% Pd/Fe > nanoscale Fe > reductive Fe. The effectiveness of NZVI to dechlorinate atrazine (1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine) in contaminated water and soil was studied, analyzing the influence of iron sources, solution pH, Pd catalyst and presence of Fe or Al sulfate salts.¹⁸⁷ The results indicated nano ZVI can be successfully used to remediate atrazine in water and soil. Atrazine destruction kinetic rates were greatly enhanced in both contaminated water and soil treatments by NZVI when sulfate salts of Fe(II), Fe(III) or Al(III) was added with the following order of removal rates: Al(III) 2.23 > Fe(III) 2.04 > Fe(II). Another example of higher activity of composite Fe nanoparticles in comparison to free Fe⁰ samples is a one-pot method,¹⁸⁸ developed to prepare Fe/FeS nanoparticles using dithionite at r.t. The FeS precipitations on the Fe surface were formed by the interaction between dissolved Fe species and H₂S, one of the decomposition products of dithionite in solution. The resulting Fe/FeS nanoparticles showed a much higher reactivity toward contaminants than the pure Fe nanoparticles and were applied for the rapid removal of TCE from water.

Supporting NZVI or iron oxides on natural materials or compounds having high surface area leads to their longer activity. Thus, NZVI was reported as an effective material for azo dye removal, however, similar to other nanomaterials, ultra-fine powder has a strong tendency to agglomerate into larger particles, resulting in an adverse effect on both effective surface area and catalyst performance. Nanosized Fe⁰ particles dispersed onto the surface of natural bentonites were fabricated¹⁸⁹ and their ability to decolorize Orange II (OII) was evaluated. Spherical individual Fe⁰ particles were observed after dispersion onto bentonites, and these samples were used for Orange II decolorization over a wide pH range. Higher reactivity was attributed to the good dispersion of Fe⁰ particles on clay mineral surface. The same pollutant (Orange II) was also removed by use of non-supported self-assembled Fe₃O₄ hierarchical nanostructures (obtained by hydrothermal approach),¹⁹⁰ which could be easily transformed to γ-Fe₂O₃ and α-Fe₂O₃ without changing its original morphology by calcination in air. Another example is the study of the kinetics of nitrobenzene reduction by Fe(II) sorbed on α-Al₂O₃, measured while simultaneously characterizing the Fe oxidation product with Mössbauer spectroscopy and electron microscopy.¹⁹¹ The onset of nitrobenzene reduction coincided with a change in particle suspension color from white to yellow–ochre due to formation of nanogoethite rods (α-FeOOH) from oxidation of sorbed Fe(II). Formation of nanogoethite on the α-Al₂O₃ particles appears to promote the rapid reduction of nitrobenzene. In addition, an encapsulation approach that relies upon Gum Arabic to stabilize high quantities of NZVI (∼12 g L⁻¹) in the dispersed phase of a soybean oil-in-water emulsion was offered.¹⁹² The formed emulsion is kinetically stable due to substantial repulsive barriers to droplet–droplet induced deformation and subsequent coalescence. Sedimentation time scales were found to be on the order of hours (τ = 4.77 ± 0.02 h). NZVI within the emulsion was shown to be reactive with both trichloroethane degradation and H₂ production observed. A special case is the use of iron oxide/TiO₂ nanoparticles.¹⁹³ Thus, photocatalytic oxidation with TiO₂ nanoparticles (6–20 nm) was investigated as a promising water-treatment process.¹⁹⁴¹⁹⁵ When irradiated with UV light, TiO₂ nanoparticles can adsorb and degrade a wide variety of environmental organic pollutants. For instance, the strong affinity between the surface of TiO₂ nanoparticles for organic arsenic species (monomethylarsonic [MMA] and dimethylarsinic [DMA] acids) leads to covalent bonding between MMA or DMA and the surface of nanoparticles through bidentate (AsMMA-Ti 3.32 Å) and monodentate (AsDMA-Ti 3.37 Å) inner-sphere complexes, respectively. Doping TiO₂ nanoparticles with Fe³⁺ ions at 0.1–0.5% may significantly increase the photocatalytic activity. The doped ions act as charge separators of the photoinduced electron–hole pair and enhanced interfacial charge transfers. In a related work, iron-doped-TiO₂ was prepared by the hydrothermal method.¹⁹⁶ Titanium(IV) tetra-tert-butoxide and FeCl₃ or FeCl₂ dissolved in n-octanol was heated at 230 °C for 2 h in the presence of water. The resulting powders were rinsed, dried and calcined at 560 °C. Photocatalyst doped with FeCl₃ had better photoactivity for degradation of dye in aqueous solution under UV and visible light. It was found that the amount of doped iron ions plays a significant role in affecting its photocatalytic activity.

Special studies of remediation applying iron-containing nanomaterials

Radiotracer studies of remediation. NZVI synthesized under atmosphere conditions were used for the removal of Ba²⁺ at concentrations 10⁻³–10⁻⁶ M.^133,197 Ba was used as a tracer to study the effects of time, concentration and temperature. Observed thermodynamic parameters showed that the process is exothermic and hence enthalpy driven. Carbon isotope fractionation is of great interest in assessing chlorinated ethene transformation by nanoscale Fe⁰ at contaminated sites, particularly in distinguishing the effectiveness of an implemented abiotic degradation remediation scheme from intrinsic biotic degradation.¹⁹⁸ Thus, transformation of trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl chloride (VC) with two types of nanoscale Fe materials showed different reactivity trends, but relatively consistent carbon isotope enrichment factors of −19.4 ± 1.8‰ (VC), −21.7 ± 1.8‰ (cis-DCE) and −23.5 ± 2.8‰ (TCE) with one type of iron (Fe^BH), and from −20.9‰ ± 1.1‰ to −26.5 ± 1.5‰ (TCE) with the other (Fe^H2, manufactured by a reduction of goethite and hematite particles with H₂ gas at 200–600 °C) (distinct types of iron were purchased with different companies). Related monitoring of the effectiveness of ZVI injection by assessing TCE and 1,1,1-TCA (1,1,1-trichloroethane) degradation was carried out by the same research group.¹⁹⁹ Prior to ZVI injection, carbon isotope measurements demonstrated biodegradation of TCE by native microorganisms, which was quantified by measuring the enrichment of ¹³C in TCE samples downstream of the suspected source. When ZVI was injected through only two injection wells, no changes in TCE and 1,1,1-TCA isotope signatures were detected compared to preinjection values.

NZVI oxidative capacity studies. The addition of electron shuttling molecules was shown to enhance the oxidative capacity of NZVI. It was established that the addition of the polyoxometallates (POM), sodium polyoxotungstate (Na₃PW₁₂O₄₀), enhances the oxidative capacity of NZVI in oxidation of HCOOH and production of H₂O₂.²⁰⁰ Based on these results, the mechanism for the enhancement in oxidative capacity of NZVI was offered as is through two separate processes: (a) the POM out-competes H₂O₂ for electrons from Fe⁰ thereby increasing the H₂O₂ concentration, and (b) the reduced form of the POM, PW₁₂O₄₀⁴⁻, facilitates the cycling of Fe(III) to Fe(II) which enhances the homogeneous Fenton reaction. Elucidating the factors which affect the yield of oxidants, it was established that HCHO production by NZVI was low under acidic conditions, increased with increasing pH until pH 7 and then decreased at higher pH.²⁰¹ The addition of Fe(II) resulted in negligible HCHO generation at pH < 5, as expected based on the pH-dependent rate of Fe(II) oxidation. It was indicated that Fe(s)⁰ oxidation was responsible for H₂O₂ production at pH < 5 and that Fe(II) oxidation produces almost all of the H₂O₂ at pH > 6.5. The decrease in yields above pH 7 may be attributable to limited Fe solubility at elevated pHs.

Stability, air contact, decrease of activity and movility of NZVI. Nanosized particles of pure ZVI are pyrophoric and react spontaneously with atmosphere oxygen. So, a series of methods to stabilize the particles against aerial oxidation were presented.²⁰² Thus, using different chelating agents, an attempt to synthesize air-stable NZVI in the presence of EDTA, diethylenetriamine pentacetic acid (DTPA), nitriloacetic acid (NTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), hydroxyethylenediaminetetraacetic acid (HEDTA), triethylene tetraamine (TRTA) and N-cetyl-N,N,N-trimethyl ammonium bromide (CTAB) chelating agents, was carried out.²⁰³ The chelating effect was the best for EDTA, NTA and HEDTA, but the least for CDTA and CTAB. Hydroxyl groups, lone pair electrons on nitrogen atom and steric effects of the cyclohexane ring and bulky surroundings played the main role to provide air stability towards synthesized NZVI. Trying to stabilize NZVI by another route, nanosized air-stable ZVI particles were produced²⁰⁴ by applying ultrasound to a solution of Fe(CO)₅ in edible oil. The resulting iron nanoparticles were dispersed in a carbon matrix, and coated with a non-crystalline carbon layer of ~2.5 nm. Although these nanoparticles are non-pyrophoric and stable in air, dechlorination of tetrachloroethene was demonstrated in synthetic aqueous medium and in polluted groundwater. Dechlorination reaction products of this NZVI were trichloroethene, cis-dichloroethene, vinyl chloride, ethene and ethane. The effect of anions on the reactivity of final NZVI was also studied, Thus, solutions containing chloride, phosphate, sulfate and nitrate anions and ferric ions were used to generate iron oxide nanoparticles.²⁰⁵ The reactivity of the resulting zerovalent iron nanoparticles was quantified by monitoring the kinetics as well as products of carbon tetrachloride reduction. The reactivity of nanoparticles prepared in the presence of sulfate and phosphate demonstrated the highest reactivity and chloroform yield.

Generally, in soils the NZVI particles remain active towards the contaminants for a period of 6–8 weeks. During this time, the NZVI particles can aggregate and attach to soil grains in the subsurface leading to reduced mobility, thereby limiting their remediation potential. NZVI has been shown to be effective across a broad range of soil pHs, temperatures, and nutrient levels. Competing anions, however, may reduce its effectiveness. The amount of groundwater that NZVI can treat may depend on the quality of the iron, including the number of times it has been reused, the type of substrate used (for ex situ use), and the quality of the water used to make the injectable slurry, including the amount of oxygen and the amounts and types of particulates in contains (for in situ use). The effects of reaction conditions such as reaction time and NZVI concentration on NZVI characteristics and reactivity were studied.²⁰⁶ The particle size was dramatically decreased from 87.4 to 9.5 nm under short reaction time and high reductant concentration. The reactivity of NZVI (evaluated by a nitrate reduction test) was increased in the direction of high reductant concentration and fast synthesis, although deactivation increased in the same direction. Also, column and batch sedimentation studies were conducted²⁰⁷ to study the transport of nanoscale NZVI particles stabilized by three polyelectrolytes: polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A), poly(acrylic acid) (PAA) and soy proteins. It was revealed that the both PV3A and PAA can increase NZVI mobility by reducing particle size and generating negative charged surfaces of NZVI. PV3A stabilized NZVI has the best transport performance among the three materials. Due to the large surface area of NZVI, large amounts of polyelectrolytes are often needed. In addition, to understand more the factors that limit NZVI mobility, several field-scale tests were performed²⁰⁸ using carboxylmethyl cellulose (CMC) stabilized NZVI.

Regeneration of NZVI reactivity. Iron reducing bacteria (IRB) survive through the anaerobic metabolism reducing ferric iron, as electron acceptor, to ferrous iron.²⁰⁹ It helps ZVI remediation treatment to regenerate the reactivity of iron. NZVI was researched to improve the reactivity and the mass transfer while it has been reported that nanoscale particles have toxic effects on human or nature. It was found that the IRB in NZVI system were not harmed. Additionally, in ferrozine test, ferrous iron, which was reduced from ferric iron as electron acceptor for IRB, was increased in the micro-scale ZVI (MZBI) system. Another sustainable and cost-effective strategy to improve the longevity of NZVI treatment zones is through the use of dithionite (S₂O₄²⁻, described above for creation of FeS/Fe nanoparticles) as a chemical regenerant.²¹⁰ It was shown that dithionite can extend, and in some cases enhance, the removal of two model groundwater pollutants (1,1,1,2-tetrachloroethane and hexavalent Cr(VI)) in NZVI treatment systems. The efficiency of the dithionite regeneration process is strongly dependent upon pH and redox state, which dictate the products of NZVI oxidation, as well as the redox active species generated from the reaction between passivated NZVI and dithionite.

Delivery techniques. The delivery of the NZVI to a groundwater aquifer is difficult for a number of reasons:²¹¹ delivery of the particles by way of a slurry injected at multiple injection well sites is difficult due to aggregation of the particles at the elution points, many types of iron particles oxidize quickly in the presence of air, and the nanoparticles are fairly unreactive towards alkyl chlorocarbons. The problem to resolve is to offer coatings to allow the particles to travel passively through the soil while maintaining their reactive integrity. One such approach is the delivery in the form of foams.²¹² Foam delivery of remedial amendments for in situ immobilization of deep vadose zone contaminants can overcome the intrinsic problems associated with solution-based delivery, such as preferential flow and contaminant mobilization.²¹³ To study this, carboxyl-modified polystyrene latex microspheres were used as surrogates for nanoparticles of remediation purposes. It was demonstrated that foam can carry significant fractions of zerovalent Fe nanoparticles at concentrations relevant to field remediation conditions (≤5.3 g L⁻¹).

Scalability. Large-scale applications of NZVI for environmental remediation have been inhibited by the high costs associated with its conventional production technologies. So, the optimization of NZVI production is in a permanent progress. Thus, carbon-supported Fe nanoparticles (C–Fe; 20–100 nm diameter) were synthesized²¹⁴ by reacting Fe salts, adsorbed or impregnated from aqueous solutions on 80 m² g⁻¹ carbon black, at 600–800 °C under Ar. Fe₃O₄ particles formed at 300–500 °C in the initial stage of the reaction, being reduced to a mixture of α- and γ-Fe nanoparticles at >600 °C. At a 10 : 3 Fe : Cr mole ratio, C–Fe reduced a 10 ppm Cr⁶⁺ solution to ∼1 ppm within 3 days. This method can be made in a scalable process from inexpensive starting materials by carbo-thermal reduction. Another approach without any chemical process is as follows. Unlike conventional methods such as chemical synthesis and vapor phase condensation, which typically involve toxic chemicals, sophisticated equipment and extensive labor, the precision milling method relies solely on the mechanical impact forces generated by stainless steel beads in a high-speed rotary chamber to break down the iron microparticles.²¹⁵ The system used nontoxic solvents, was completely scalable to large-scale manufacturing. After 8 h of milling, the feed micro iron was effectively reduced to particles with sizes below 50 nm. A result of application of such techniques is the use of NZVI at industrial scale. Thus, application of NZVI technology for remediation of groundwater at an operating industrial facility, containing chlorinated solvent contaminant, was offered.²¹⁶ As a result, the nonaqueous phase source area in the groundwater was successfully treated and the chlorinated solvent degraded to benign byproducts within a year. Use of the nanoscale iron facilitated distribution of the NZVI into the silty clay soil pores using high pressure injection. The work was performed adjacent to active plant buildings and roadways with no loss of plant production.

Among other studies on NZVI particles, we note their in situ modification techniques²¹⁷ and potentiometric detection.²¹⁸

Tables 4–6 list examples of examples of pollutant remediation using NZVI, supported and alloyed NZVI and iron oxides and FeOOH.

Table 4 Examples of pollutant remediation using NZVI

System	Ion/substance to be eliminated	Description	Ref.
NZVI	Cr(VI)	0.10 g L^−1 NZVI completely reduces Cr(VI) within 120 min following pseudo-first order kinetics. NZVI showed significant Cr(VI) reduction at field also, indicating it an effective tool for managing sites contaminated with Cr(VI).	219
NZVI (size 30–400 nm)	Heavy metals (Cd, Cr, Hg)	Reduction of heavy metals in the heavy metal-polluted soil into lower valent fixed heavy metals.	220
NZVI	Cd(II)	The adsorption of Cd^2+ on NZVI increased significantly with increasing pH. Zn^2+, Co^2+ and Mg^2+ are potential inhibitors to Cd^2+ adsorption by NZVI.	221
NZVI	Chlorinated organic contaminants (e.g., solvents, pesticides) and inorganic anions or metals.	Use of NZVI for remediation of a series of substances.	222
NZVI (10 μm particles)	Orange 6	98% of the dye can be degraded within 30 min of contact.	223
NZVI	Pyrene	NZVI particles were found to be more efficient in removing pyrene than commercially available microscale ZVI (MZVI, <10 μm) particles.	224
NZVI	Lindane (classified by the United States Environment Protection Agency as a potent carcinogen and teratogen).	Lindane (10 μ/g) completely disappeared from spiked soil within 24 h at nZVI concentration of 1.6 g L^−1, indicating its possible use in environmental cleanup.	225
NZVI	Malodorous sulfides	NZVI has high capacity for the removal and sequestration of malodorous sulfides such as hydrogen sulfide and dimethyl disulfide. The core–shell structure of iron nanoparticles plays a key role during the reactions. Both iron in the core and FeOOH in the shell provide rich surface sites for the sorption of sulfides, while the surface associated sulfide can further react with hydrogen sulfide and evolve to iron polysulfide.	226
NZVI	Xanthan	The rheological properties of NZVI-xanthan suspensions were extensively tested under two different flow conditions (simple shear flow and flow through a porous medium), showing a shear thinning behavior that is dependent on iron concentration.	227
NZVI (40 nm)	Arsenite [As(III)]	NZVI demonstrated very rapid adsorption and large capacity for the removal of As(III). The maximum As(III) adsorption capacity in batch experiments calculated by Langmuir adsorption isotherm was 76.3 mg L^−1 of As(III)/g of NZVI.	228
NZVI	Chlorinated ethenes	The concentration of chlorinated ethenes decreased by 30–50%. The oxidizing iron reduces chlorinated hydrocarbons and forms non-chlorinated hydrocarbons. Organic matter in water is thus reduced and treated water is safe for living organisms without health danger.	229
NZVI	Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)	RDX is a common contaminant of soil and water at military facilities. Its degradation with NZVI nanoparticles in water in the presence or absence of a stabilizer additive such as CM-cellulose (CMC) or poly(acrylic acid) (PAA) was studied, showing that the degraded RDX to the corresponding nitroso derivatives prior to completed decomposition.	230
NZVI	Anions (NO3^−, Cl^−, SO4^2−, HCO3^−) and natural organic matter (NOM).	More than 95% of 1.8 mM TCE was removed within 20 h with a NZVI dosage of 25 g L^−1 (k = 0.15 h^−1). TCE degradation reactions were not substantially affected by the presence of each anion with concentrations as high as 100 times the average field concentrations. However, when four anions (NO3^−, Cl^−, SO4^2−, HCO3^−) were present simultaneously, the degradation reactivity was decreased by 60% (k = 0.069 h^−1).	231
NZVI, Fe/Ni or Fe3O4	Uranium ions.	The Fe nanoparticles removed 98% of the total uranium from solution, resulting in a final U-concentration of <4 μg L^−1. In all the systems, uranium was reduced to U(IV) and retained on the surfaces of the nanoparticulate solids for up to 48 h; the U-stability was not affected by annealing the Fe or the FeNi nanoparticles before use.	232, 233
NZVI	Vinyl chloride (VC)	The degradation efficiency of vinyl chloride (VC) detected at most of monitoring wells was 50–99%.	234

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Table 5 Examples of pollutant remediation using supported and alloyed NZVI

System	Ion/substance to be eliminated	Description	Ref.
Fe ^0/M nanoparticles and their composites with supports
Pd/Fe and Fe (NZVI and MZVI (microscale ZVI))	Water disinfection byproducts; trihalomethanes (THMs) including chloroform, bromoform, dichlorobromomethane, dibromomethane and dibromochloromethane	Complete dehalogenation of THMs by nanoscale Pd/Fe particles was achieved in less than one hour. Methane, accounting for 60–90% of the THMs lost, was the major product. Nanoscale Pd/Fe particles lasted over 700 h without exhausting their ability to dehalogenate THMs in a closed batch system.	235
Pd/Fe	Pentachlorophenol (PCP)	Degradation of PCP by Pd/Fe nanoparticles was carried out in aqueous solutions containing different cations in sulfate form, Na2SO4, CuSO4, NiSO4 and Fe2(SO4)3, respectively. The observed inhibitory effect of Na2SO4 on degradation of PCP was contributed to the existence of SO4^2−. Overcoming the inhibitory effect of SO4^2−, Cu^2+, Ni^2+ and Fe^3+ could facilitate the degradation kinetics and efficiencies of PCP by Pd/Fe nanoparticles. The enhancement effect of Cu^2+ and Ni^2+ result from the presence of reduced forms of Cu and Ni on Pd/Fe surfaces.	236
FePd nanoparticle suspensions in water the presence of various stabilizers (i.e., CM-cellulose (CMC), polyvinylpyrrolidone (PVP) and guar gum).	Trichloroethylene	A dramatic improvement in FePd suspension stability was observed when the stabilizer is present in the matrix during the nanoparticle synthesis step. In case of guar gum, gelation during synthesis significantly increased suspension viscosity, enhancing suspension stability.	237
NZVI, bimetallic nanoparticles (NZVI/Pd), and NZVI/Pd impregnated activated carbon (NZVI/Pd-AC) composite particles	Polybrominated di-Ph ethers (PBDEs) and/or polychlorinated biphenyls (PCBs).	Compared to NZVI, the Fe-normalized rate constants for NZVI/Pd were about 2-, 3- and 4-orders of magnitude greater for tri-, di- and mono-BDEs, respectively, with di-Ph ether as a main reaction product. Pd mainly deposits on the outer part of particles, while Fe was present throughout the AC particles. It was suggested that ∼7% of the total Fe within the AC was zerovalent, which shows the difficulty with in situ synthesis of a significant fraction of zerovalent Fe in the microporous material.	238
Fe/Cu (Cu amended zerovalent iron bimetallic nanoparticles), supported in activated carbon (AC).	γ-HCH (γ-hexachlorocyclohexane)	Cu amended zerovalent iron bimetallic nanoparticles were synthesized by doping Cu on the surface of iron and incorporated with granular activated carbon to prepare supported particles (AC–Fe^0–Cu), which were used to remove γ-HCH. Cu on the surface of iron enhanced the dechlorination activity of Fe^0. The dechlorination rate constant increased with the Cu loading on the surface of iron and the maximum was achieved with 6.073% Cu. AC could adsorb the degradation products.	239
FexNi1−x nanostructures (0 < x < 1.0)	1,1,1,2-Tetrachloroethane	Fe1.0, Fe0.71Ni0.29 and Fe0.55Ni0.45 exhibited reactivity toward 1,1,1,2-tetrachloroethane, with Fe1.0 yielding the greatest rate of reductive dechlorination.	240
Na CM-cellulose (CMC)-stabilized Pd/Fe nanoparticles	2,4-Dichlorophenoxyacetic acid (2,4-D), 2-chlorophenoxyacetic acid (2-CPA)	Considerable enhancements in particle stability and chemical reactivity with the addition of CMC to Pd/Fe nanoparticles were established. CMC-stabilized Pd/Fe nanoparticles (0.6 g Fe L^−1) were able to remove much higher levels of 2,4-D with only one intermediate 2-chlorophenoxyacetic acid (2-CPA) and the final organic product phenoxyacetic acid (PA), than non-stabilized Pd/Fe nanoparticles or microsized Pd/Fe particles.	241
Fe–carbon composites
Fe–C nanocomposites, in particular Fe NPs on the external surface of carbon microspheres (synthesis see Fig. 10)	Chlorinated hydrocarbons	Spherical Fe–C nanocomposites were synthesized through a facile aerosol-based process and a subsequent carbothermal reduction. The distribution and immobilization of Fe particles throughout the C microspheres prevents nano-Fe aggregation, allowing the maintenance of particle reactivity. The carbon microspheres allow adsorption of trichloroethylene (TCE), thus removing dissolved TCE rapidly and facilitating reaction by increasing the local concentration of TCE in the vicinity of Fe particles (Fig. 11).	242–244
Carbon-coated Fe nanoparticles	Cr(VI)	Ferromagnetic carbon-coated Fe nanoparticles (core size of 15 nm) demonstrate strong ability to effectively remove more than 95 wt% of Cr(VI) in wastewater via carbon shell physical adsorption, which is much higher than the commercial available Fe NPs.	245
Poly(acrylic acid)/poly(vinyl alcohol)/multiwalled carbon nanotube (MWCNT) composite	Dyes of acid fuchsine, acridine orange, and methyl blue.	The produced composite nanofibrous mats exhibited a superior capability to decolorize the dyes of acid fuchsine, acridine orange and methyl blue, which are model dyes in wastewater of the printing and dyeing industry.	246
Encapsulated and supported NZVI
NZVI particles (10–90 nm) were encapsulated in biodegradable calcium-alginate capsules	Trichloroethylene	Trichloroethylene (TCE) degradation was 89–91% in 2 h. TCE degradation reaction rates for encapsulated and bare NZVI were similar indicating no adverse affects of encapsulation on degradation kinetics. The shelf-life of encapsulated NZVI was found to be four months with little decrease in TCE removal efficiency.	247
NZVI/chitosan (CS) composite	Cr(VI)	There is no significant difference between the reaction rates of bare NZVI and entrapped NZVI. The reduction capacity for Cr(VI) increases with increasing temperature and NZVI dosage but decreases with the increase in initial concentration of Cr(VI) and pH.	248
NZVI/CTMA-Bent composite (CTMA-Bent = organobentonite)	Atrazine	The removal efficiency of atrazine by the composite was compared with that by commercial Fe powder and NZVI itself. For both treatments by NZVI and NZVI/CTMA-Bent, the removal efficiency increased as the pH of the solution decreased, and the removal percentage of atrazine by NZVI/CTMA-Bent reached 63.5% at initial pH 5.0 after 120 min.	249
NZVI supported on montmorillonite (Mont) and hexadecyl trimethylammonium modified montmorillonite (HDTMA-Mont)	Cr(VI)	The presence of organo-montmorillonite apparently decreased the extent of aggregation and the size of the iron particles. Iron nanoparticles had a core–shell structure with the core being iron, and the shell consisting of iron (hydr)oxides. In contact with Cr(VI), the reduction of Cr(VI) was highest with HDTMA-Mont/iron particles, followed by Mont/iron particles and free iron nanoparticles.	250
NZVI-kaolinite composite (K-NZVI)	Pb^2+	More than 96% of Pb^2+ was removed from aqueous solution using K-NZVI at an initial condition of 500 mg L^−1 Pb^2+ within 30 min under the conditions of 10 g L^−1 of K-NZVI, pH 5.10 and a temperature of 30 °C.	251
ZVI/TiO2 nanocomposite (+UV light)	Phenol and CCl4	Nanosized ZVI was synthesized and immobilized on the surface of a TiO2 nanotube, with carbon tetrachloride (CT) and phenol as the target compounds. The coupled degradation of CT and phenol by the NZVI combined with the titanate nanotube under illumination with UV light could be used for the simultaneous degradation of mixed priority pollutants.	252
Fe ^0–silica composites
Fe^0 nanoparticles (20–110 nm diameter) supported on silica fume (SF–Fe^0)	Cr(VI)	Attachment of Fe^0 nanoparticles on the commercially available sub-micrometer silica fume prevented them from aggregation while maintaining the particle reactivity. When the Fe^0 concentration was 0.4 g L^−1, 88.00% of 40 mg L^−1 Cr(VI) was removed in 120 min, 22.55% higher than unsupported Fe^0. Almost all unsupported Fe^0 was retained, whereas 51.50% and 38.29% of SF–Fe^0 were eluted from the vertical and horizontal sand column, respectively.	253
Nanoscale zerovalent iron entrapped in porous silica particles.	Trichloroethylene (TCE).	Nanoscale zerovalent iron entrapped in porous silica particles and prepared through an aerosol-assisted process were used for TCE remediation. The entrapment of iron nanoparticles into the silica matrix prevents their aggregation while maintaining the particles' reactivity. Furthermore, the silica particles are functionalized with alkyl groups and are extremely efficient in adsorbing dissolved TCE.	254

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Table 6 Examples of pollutant remediation using iron oxides and FeOOH

System	Ion/substance to be eliminated	Description	Ref.
Fe 2 O 3 and Fe 3 O 4
Alumina–zirconia–titania ceramic membranes coated with iron oxide nanoparticles	Disinfection byproduct precursors (in particular, ozonation byproducts: aldehydes, ketones and ketoacids)	Commercial available ceramic membranes (sintered in air at 900 °C for 30 min) with a nominal molecular weight cut-off of 5 kDa were coated 20, 30, 40 or 45 times with sol suspension-processed Fe2O3 nanoparticles (average diameter of 4–6 nm). Optimum water quality was achieved at 40 layers, which corresponds to a surface coating morphology consisting of a uniform, coarse-grained structure with open, nanosized interconnected pores.	255, 256
Iron oxide nanoparticles (19.3 nm magnetite and 37.0 nm hematite)	Arsenic (arsenate and arsenite) and metals (V, Cr, Co, Mn, Se, Mo, Cd, Pb, Sb, Tl, Th, U)	Strong adsorption of arsenic to the magnetite nanoparticles in the column. Arsenic and 12 other metals (V, Cr, Co, Mn, Se, Mo, Cd, Pb, Sb, Tl, Th, U) could be simultaneously removed by the iron oxide nanoparticles in soil. Irreversible sorption of arsenic to the iron oxide nanoparticle surface.	257
Nanomagnetites produced by the bacterial reduction of schwertmannite powder.	Tc(VII) and Cr(VI)	Schwertmannite-derived biomagnetite proved capable of retaining more (∼20%) ^99mTc(VII) than ferrihydrite-derived biomagnetite.	258
Fe3O4	Nitrates	Residual NO3^− concentration of 1.35 mg L^−1 is lower than international standards.	259
FeOOH
Iron (oxyhydr)oxide nanocrystallines	As(III) and heavy metals	Various phases of iron (oxyhydr)oxide nanocrystallites were synthesized from the phase-controlled transformation of amorphous hydrous ferric- or ferrous-oxide in thermal solution with a certain ethanol–water ratio and with the presence of oleic acid (goethite nanorods, hematite nanocubes and magnetite nanoparticles). The as-synthesized goethite nanorods and magnetite nanoparticles demonstrated extremely strong As(III) affinity, with 5.8 and 54 times of As(III) adsorption, respectively, higher than the micron-sized relatives.	260
α-FeOOH	Fluorides	Fluoride from contaminated H2O sample could be successfully brought down from 10.25 to 0.5 mg L^−1.	261

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Fig. 10 (a) Schematic of aerosol-based process and (b) schematic of synthesis route to synthesize iron–carbon composites. Reproduced with permission.

Fig. 11 Conceptual schematic showing potential applicability of Fe–C composites for the adsorption and simultaneous dechlorination of chlorinated hydrocarbon contaminants. Reproduced with permission.

Water disinfection using NZVI and iron oxides

In general, both zerovalent iron (ZVI) and nanoscale ZVI (NZVI) were found to effectively remove many contaminants relevant to drinking water, including viruses, bacteria, chlorine, disinfection byproducts (DBPs), and other chemicals.²⁶² It was proposed that NZVI can be applied to decentralized drinking water systems to improve the performance of point-of-use devices. However, in comparison with use of iron-containing nanomaterials in remediation of chemical pollutants from water, their use for water disinfection is represented by a considerably lesser number of examples.

The bactericidal effect of nanoFe is a unique property of nanoFe, which was considerably less observed in other types of Fe-based compounds. An observed exception corresponds to iron coated sands and micro-scale iron powder, which were reported to adsorb viruses via electrostatic attraction and cause viruses to disintegrate or become noninfective. In any case, nanosized iron particles may provide higher efficiency compared to their bulk or micro-scale counterparts.²⁶³ It was established that NZVI in aqueous solution is able to rapidly inactivate E. coli.²⁶⁴ A strong bactericidal effect of nanoFe was found under deaerated conditions, with a linear correlation between log inactivation and nanoFe dose (0.82 log inactivation/mg L⁻¹ nanoFe-h). The inactivation of E. coli under air saturation required much higher nanoFe doses due to the corrosion and surface oxidation of nanoFe by dissolved oxygen. The effect that adsorbed synthetic polymers and natural organic matter and aging (partial oxidation) have on the bactericidal properties of NZVI to the Gram-negative bacterium, E. coli, was elucidated.²⁶⁵ Exposure to 100 mg L⁻¹ of bare NZVI with 28% Fe⁰ content resulted in a 2.2-log inactivation after 10 min and a 5.2-log inactivation after 60 min. In addition to E. coli, iron oxides showed a high inactivation capacity of MS2 virus.²⁶⁶ Thus, the inactivation of MS2 coliphage (MS2) by nanoparticulate NZVI and ferrous ion (Fe(II)) in aqueous solutions was demonstrated.²⁶⁷ It was established that for NZVI, the inactivation efficiency of MS2 under air-saturated conditions was greater than that observed under deaerated conditions, indicating that reactions (16)–(21) associated with the oxidation of NZVI were mainly responsible for the MS2 inactivation. Under air-saturated conditions, the inactivation efficiency increased with decreasing pH for both NZVI and Fe(II), associated with the pH-dependent stability of Fe(II). The ·OH and Fe(IV) (e.g., FeO²⁺) are mainly responsible for the oxidation of organic compounds and inactivation of pathogenic microorganisms. The authors suggested that the NZVI surfaces interacted directly with the MS2 phages, leading to their inactivation. Also, it was indicated that NZVI caused more capsid damage than Fe(II).

Fe⁰_s + O₂ + 2H⁺ → Fe(II) + H₂O₂ (16)

Fe⁰_s + O₂ + 2H⁺ → Fe(II) + H₂O (17)

Fe(II) + H₂O₂ → Fe(III) + ·OH + OH⁻ (18)

Fe(II) + H₂O₂ → Fe(VI) + H₂O (19)

Fe(II) + O₂ → Fe(III) + ·O₂⁻ (20)

Fe(II) + ·O₂⁻ + 2H⁺ → Fe(III) + H₂O₂ (21)

Other types of bacteria are also available to eliminate using NZVI. Thus, cyanobacteria pose a serious threat to water resources around the world, compounded by the fact that they are extremely resilient, having evolved numerous protective mechanisms to ensure their dominant position in their ecosystem. It was just shown (Fig. 12)²⁶⁸ that treatment with NZVI is an effective and environmentally benign method for destroying and preventing the formation of cyanobacterial water blooms. These nanoparticles have multiple modes of action, including the removal of bioavailable phosphorus, the destruction of cyanobacterial cells, and the immobilization of microcystins, preventing their release into the water column. The primary product of NZVI treatment is nontoxic and highly aggregated Fe(OH)₃, which promotes flocculation and gradual settling of the decomposed cyanobacterial biomass. As another example, the bio-toxicity of Fe⁰ nanoparticles on the denitrifying bacteria Alcaligenes eutrophus was detected by the two methods of detecting inhibition of the growth of microorganisms and nitrification inhibition rate.²⁶⁹ It was shown that Fe⁰ nanoparticles had (a) obvious toxicity on the growth and nitrification inhibition rate and (b) distinct relationship between dose and toxicity of Fe⁰ nanoparticles. Fe⁰ nanoparticles had visible toxicity on the denitrifying bacterial, which could be decreased by surface coating and decoration of nanometal materials.

Fig. 12 (a) SEM images of cyanobacteria before treatment, (b) unused NZVI particles, (c) highly deformed cells after brief exposure to NZVI, and (d) completely destroyed cells surrounded by ferric oxide aggregates. Reproduced with permission.

For the case of supported NZVI, we note first of all Fe-doped TiO₂. It is known that TiO₂ itself possess antimicrobial properties under UV-irradiation. Nonwoven nanofibers of pure and iron-doped titanium dioxide (TiO₂) were tested for evaluation of their antimicrobial attributes for using them as disinfectant gauze for wound healing upon brief activation by UV/IR illumination.²⁷⁰ It was found that the fibers exhibited superior bactericidal affinity when exposed briefly (3–12 s) to either multiphoton laser or IR radiations. On the other hand, exposure to a UV beam for up to 20 min was not effective in mitigating the bacterial colonization of the E. coli. In a related report,²⁷¹ the Fe/Ce codoped nanometer titanic hydrosol, which could be applied to indoor antisepsis, was prepared under atmosphere pressure by a microwave-assisted peptization process and resulted good antibacterial performance. It was shown that the Fe/Ce codoped nano-TiO₂ hydrosol had better antibacterial performance than the pure nano-TiO₂ hydrosol. The relative antibacterial rate exceeded 95% when the illumination of natural light irradiation lasted for 6 h. Finally, yellowish (Fe, N)-doped nanocrystalline TiO₂ powders (anatase in phase, 10 nm size) were prepared using TiOSO₄, CO(NH₂)₂, Fe(NO₃)₃·9H₂O and CN₃H₅·HCl as precursors by a hydrothermal method.²⁷² TiO₂ powders were mixed with organic silicon and acrylic syrup to test their antibacterial performance by the colony counting method. It was shown that the sterilization ratio of E. coli by the heat-treated (Fe, N)-doped nanocrystalline TiO₂ powders was reached up to 94.5% while that of the powders without any heat treatment was 91.1%.

All other known examples of use of iron-containing nanomaterials in water disinfection correspond to iron oxides, supported on distinct materials. Some differences in comparison with NZVI use have been observed. Thus, a material system composed of iron oxide (Fe₂O₃) nanoparticles loaded onto a fiberglass support displayed excellent antiviral properties against the model virus, MS2 phage, but is ineffective against bacteria, specifically E. coli.^273,274 To increase the antibacterial properties and still maintain antiviral activity, silver nanoparticles were added to this system through an aqueous hydrothermal reduction process with 0.25 M silver nitrate (AgNO₃). It was revealed that a 0.05 mg mL⁻¹ loading of the Ag modified oligodynamic nanoparticle impregnated fiberglass system consisting of Fe₂O₃ (9.1 wt%) and Ag (0.1 wt%)/g-fiber, displayed robust antibacterial activity by achieving a 2 log removal of 106 CFU/mL E. coli in 1 min. Site competition between the Ag and other species on the surface of the fiberglass was observed. The fiberglass impregnated with CuO/Ag and Fe_xO_y/Ag, both exhibited synergistic disinfection ability greater than that of fiberglass impregnated with only Ag, the latter displaying the highest bactericidal effect. Silver-doped iron oxides were also used in a series of other related reports due to well-known antibacterial properties of this metal both in bulk, nanoforms and Ag⁺ ion. For instance, silver (10 nm) nanoparticles inlaid Fe₃O₄-SiO₂ magnetic composite (Fe₃O₄–SiO₂–Ag) was synthesized and its potential application as an antibacterial material in water disinfection was investigated.²⁷⁵ The minimum inhibitory concentrations of Fe₃O₄–SiO₂–Ag magnetic composite to E-coli and Staphylococcus aureus were 15.625 and 31.25 mg L⁻¹, respectively, and the minimum bactericidal concentrations were 250 and 500 mg L⁻¹, respectively. This disinfectant in normal saline solution could kill 99.9% of the tested bacteria within 60 min. In addition, the effects of silver(I) and iron(III) doping contents on photocatalytic performance of the titania thin film were studied.²⁷⁶ Ag and Fe doping and co-doping contents on nanotitania photocatalytic bactericidal films were prepared by sol–gel method, thus combining three active antibacterial species (Ag, Fe and TiO₂). The photocatalytic activity of titania films was evaluated by the sterilizing rate of the E. coli. For fluorescent light irradiation, optimal doping amounts for silver(I)/titania and iron(III)/titania are 0.05, 0.1%, respectively. More complex Fe–Ag containing systems (a nano-Nd/Fe/B pipe containing ZnO and Ag₂O) are also known.²⁷⁷

Metal oxides, zeolites and such classic objects of nanotechnology as carbon nanotubes have been applied as supports for iron oxides. Among metal oxide nanoparticles, zinc oxide demonstrates significant bacterial growth inhibition on a broad spectrum of bacteria, mainly by catalysis of reactive oxygen species formation from water and oxygen. Zinc oxide was combined²⁷⁸ with iron oxide to produce magnetic composite nanoparticles with improved colloidal aqueous stability, together with adequate antibacterial activity. The Zn/Fe oxide composite nanoparticles (containing iron oxide, zinc oxide and zinc ferrite phases) were synthesized by basic hydrolysis of Fe²⁺ and Zn²⁺ ions in aqueous continuous phase containing gelatin. Their antibacterial activity, tested against Staphylococcus aureus and E. coli, was found to be dependent on the weight ratio [Zn]/[Fe], i.e., the higher the ratio, the higher the antibacterial activity. The activity against Staphylococcus aureus was significantly higher than that observed against E. coli. The environmental safety effect of nanosized Fe materials including Fe₃O₄, Fe₃O₄-zeolite composite and Fe-doped zeolite was studied using a Tetrahymena thermophile model.²⁷⁹ The magnetic nanosized Fe₃O₄ is most sensitive to the suppression of the growth of Tetrahymena thermophile, as compared with the Fe₃O₄–zeolite composite and Fe-doped zeolite. In addition, a photocatalytic technique using visible light and carbon nanotubes and nanosized Fe₂O₃ powder was used to inhibit pathogenic bacterial growth in water.²⁸⁰ It was suggested that after careful design, this system can be used to disinfect drinking water, making it free of pathogenic bacteria.

A special case is the use of polymer supports (membranes) containing the NZVI and iron oxides, whose use precisely for water disinfection has not yet been well developed, in a difference with remediation of chemical pollutants. In contrast to conventional, passive membrane technologies, an approach, reported in ref. 281 utilized two independently controlled, nanostructured membranes in stacked configuration for the generation of the necessary oxidants to purify water (Fig. 13). These include biocatalytic and organic/inorganic (polymer/iron) nanocomposite membranes. The bioactive (top) membrane contains an electrostatically immobilized enzyme for the catalytic production of one of the main reactants, hydrogen peroxide (H₂O₂), from glucose. The bottom membrane contains either immobilized iron ions or ferrihydrite/iron oxide nanoparticles for the decomposition of hydrogen peroxide to form powerful free radical oxidants. By permeating a solution containing a model organic contaminant, such as trichlorophenol, with glucose in oxygen-saturated water through the membrane stack, significant contaminant degradation was realized. According to the authors, other applications including disinfection and/or virus inactivation are possible.

Fig. 13 Schematic of reactive nanostructured stacked membrane system. (A) Setup of stacked membrane system consisting of two membranes of different functionality operated via convective flow. (B) Pore of top membrane with layer-by-layer polycation/polyanion assembly containing electrostatically immobilized GOx for the conversion of reactants A + B → C. (C) Pore of bottom membrane consisting of pH-responsive PAA gel with immobilized iron species in collapsed state. (D) Pore of bottom membrane after exposure to increased pH causing gel to swell; reactive iron species catalyzes conversion of C → D. Reproduced with permission.

Toxicity and risks of iron nanomaterials application

The conventional methods for in situ remediation of chlorinated organic solvents, such as trichloroethylene, tend to produce undesirable byproducts, whereas the use of nanoscale bimetallic particles according to the techniques described above has succeeded in eliminating some of these byproducts.²⁸² However, at present the potential environmental risks of NZVI in in situ field scale applications are largely unknown at the present and traditional environmental risk assessment approaches are not yet able to be completed.²⁸³ Therefore, it may not yet be fully clear how to consider the environmental benefits and risks of NZVI for in situ applications. At present, there are no significant grounds on which to form the basis that NZVI currently possess a significant, apparent risk to the environment, although the majority of the most serious criteria (i.e. potential for persistency, bioaccumulation, toxicity) are generally unknown. Similar discussion could be applied to SPIONs, including magnetite (Fe₃O₄), which are widely used in uses such as hyperthermic malignant cell treatment, magnetic resonance imaging, targeted drug delivery, tissue engineering, gene therapy, and cell membrane manipulation.

This is rather new area of research. Several reviews^284–289 and reports^290–294 (some of them described below) on iron-containing nanoparticles could provide a background to this problem. However, as noted in a review²⁹⁵ superparamagnetic iron oxide nanoparticles (SPIONs) are promising materials for various biomedical applications including targeted drug delivery and imaging, hyperthermia, magneto-transfections, gene therapy, stem cell tracking, molecular/cellular tracking, magnetic separation technologies (e.g. rapid DNA sequencing), and detection of liver and lymph node metastases. The most recent applications for SPIONs for early detection of inflammatory, cancer, diabetes and atherosclerosis have also increased their popularity in academia. Another fundamental discussion of SPIONs concerns potential toxicity. Neenu Singh et al.,²⁹⁶ have investigated such aspects as cytotoxicity, protein–SPION interaction, changes in gene expression, impact on cell proliferation, among others.

NZVI (bare and supported)

Several research groups have studied the factors (size, surface, supporting material, synthesis conditions, etc.) influence the possible damage to living cells caused by zerovalent iron particles. The overall conclusion is that the NZVI do possess certain toxicity for animal cells (for example iron nanoparticles showed perturbations in the expression of a set of functional genes²⁹⁷). However, this toxicity is much lower in comparison with nanoparticles of some other metals. Among other studies, the size-dependent properties of inorganic metal (in particular, Fe⁰) and metal oxide (boehmite) nanoparticles in relation to environmental, health and safety perspectives were defined.²⁹⁸ Major risks concern the ignition, combustion and explosion of the metal particles in reactive atmospheres, like hydrogen and predominantly oxygen and air. So, basic mechanisms of oxidation and reactions at small scale, classification of combustibility, ignition by when initiated by low-energy ignition sources, rates of flame propagation, intensity of hydrogen generation at interaction with water, inflammability initiated by electrostatic discharge were investigated²⁹⁹ for a series of metals, in particular iron. In addition, oxidative stress markers with SiO₂-Fe induced cytotoxicity in human endothelial cells were evaluated using iron-doped nanosilica (16 nm).³⁰⁰ Significant modifications for all parameters in cells treated with these nanoparticles were found. Related SiO₂/Nd–Fe–B nanoparticles had mild toxicity, and could meet the requirement of the national standard for medical implantation materials.³⁰¹ Iron-doped TiO₂ nanoparticles were tested³⁰² as photosensitizers to kill tumor cells by studying the HL60 cells activity with different nanoparticles dosages, different magnetic field strengths and different conditions of receiving the visible radiation and no radiation. The nanoparticles exhibited toxic/inhibiting effect on leukemia cells, the greater the concentration of nanoparticles was, the more the toxic/inhibiting effect was; while the toxic/inhibiting effect of magnetic field on cells was dependent on the concentration of iron-doped and magnetic induction intensity. In addition, it is necessary to mention that toxicological impacts of iron nanoparticles on the aquatic ecosystem remain poorly understood. In a study the larvae of medaka fish (Oryzias latipes) were treated³⁰³ with thoroughly characterized solutions containing CM-cellulose (CMC)-stabilized NZVI, aged nanoscale iron oxides (nFe-oxides) or ferrous ion Fe(II) for 12–14 days aqueous exposure to assess the causal toxic effect(s) of iron NPs on the fish. The authors established that with the CMC-NZVI solution the dissolved oxygen level decreased, and a burst of reactive oxygen species (ROS) was generated as Fe(II) oxidized to ferric ion (Fe(III)); with the other two iron solutions, these parameters did not significantly change.

In case of iron oxides (SPIONs), much more research has been carried out, especially on doped and supported iron oxides in distinct nanostructural forms. Thus, to test the hypothesis about nanoparticle-triggered endothelial dysfunction, iron oxide nanoparticles (Fe₂O₃ and Fe₃O₄), as two widely used nanomaterials and the main metallic components in particulate matter, were selected to assess their potential risks on human endothelial system.³⁰⁴ The direct effects of iron oxide nanoparticles on human aortic endothelial cells (HAECs) and the possible effects mediated by monocyte (U937 cells) phagocytosis and activation were investigated. It was revealed (Fig. 14) that intravascular iron oxide nanoparticles may induce endothelial system inflammation and dysfunction in three ways: (a) nanoparticles may escape from phagocytosis that interact directly with the endothelial monolayer; (b) nanoparticles are phagocytized by monocytes and then dissolved, thus impact the endothelial cells as free iron ions; or (c) nanoparticles are phagocytized by monocytes to provoke oxidative stress responses. Similarly, the SPIONs (6–12 nm diameter and aggregated clusters of these 6–12 nm nanoparticles), produced using a flame synthesis method, were tested in respect of their biocompatibility with endothelial cells for 24 h.³⁰⁵ It was suggested that flame synthesized iron oxide nanoparticles are comparable to commercially available nanoparticles for biological applications.

Fig. 14 Influence of iron oxides on endothelial cells. Reproduced with permission.

Multiply-replicated cytotoxicity (in vitro) assays utilizing a human epithelial (lung model) cell line (A549) consistently demonstrated varying degrees of cell death for essentially all particulate matter (PM) which was characterized as aggregates of nanoparticulates or primary nanoparticles.³⁰⁶ Cytokine release was detected, in particular, for Fe₂O₃, as well as reactive oxygen species (ROS) production. Nanoparticulate materials in the indoor and outdoor environments appear to be variously cytotoxic. Cytotoxicity of nanoparticles with iron oxide as core and a biocompatible polymer as “first layer” was analyzed,³⁰⁷ presenting a broad overview of currently available in vitro and in vivo toxicity data. It was indicated that the toxicity data obtained vary significantly depending on size, size distribution, surface (including coating), and subsequent surface derivatization. The acute toxicity and the anti-phagocytosis of folic acid-O-carboxymethyl chitosans superparamagnetic Fe oxide nanoparticles (FA-OCMCS-SPIO-NPs) and O-carboxymethyl chitosans superparamagnetic Fe oxide nanoparticles (OCMCS-SPIO-NPs) were evaluated.³⁰⁸ Apparent toxic reaction and tissue injury did not occur among the animals. Prussian blue stain showed that FA-OCMCS-SPIO-NPs could escape from phagocytosis of liver and spleen completely and most of OCMCS-SPIO-NPs could while dextran-SPIO-NPs could not.

The toxicities (at both cellular and molecular levels) of three forms of SPIONs of various surface chemistries (COOH, plain, and NH₂) through the comparison with gene expression patterns of three cell types (i.e., human heart, brain, and kidney) were evaluated.³⁰⁹ It was revealed that SPIONs-COOH altered genes associated with cell proliferative responses due to their reactive oxygen species (ROS) properties. Uptake, toxicity and degradation of magnetic nanowires (200 nm diameter and lengths comprised between 1 and 40 μm, fabricated by controlled assembly of γ-Fe₂O₃ nanoparticles) by NIH/3T3 mouse fibroblasts were studied.³¹⁰ It was revealed that (a) the wires do not display acute short-term (<100 h) toxicity towards the cells and (b) the cells are able to degrade the wires and to transform them into smaller aggregates, even in short time periods (days). The in vitro cytotoxicity of 100 nm iron oxide particles (Fe₂O₃) was evaluated in the human embryonic kidney cell line HEK293.³¹¹ Cell viability assays demonstrated that 100 μg mL⁻¹ Fe₂O₃ exhibited 20% reduction in HEK293 cell viability in 24 h. Comparing the results with those for other targets, at both the cellular and molecular levels, the toxicity was observed in the following order: ZnO > Fe₂O₃-NPs > MCM-41.

Combination of microinjection techniques and Raman spectroscopy was used to investigate the effects of Ag and Fe₃O₄ nanoparticles on Hela cells.³¹² The nanoparticles were microinjected inside the cells and these latter ones were probed by means of Raman spectroscopy after a short incubation time, in order to highlight the first and impulsive mechanisms developed by the cells to counteract the presence of the nanoparticles. A different behavior of the cells treated with nanoparticles in comparison with the control cells was observed. These differences were supposed to be generated by an emerging oxidative stress due to the nanoparticles. γ-Fe₂O₃ can cause cell death within 24 h of exposure, most likely through oxidative stress.³¹³In vivo exploration suggested that although γ-Fe₂O₃ nanoparticles are rapidly cleared through the urine, they can lead to toxicity in the liver, kidneys and lungs, while the brain and heart remain unaffected. γ-Fe₂O₃ could exhibit harmful properties and therefore surface coating, cellular targeting, and local exposure should be considered before developing clinical applications. Acute toxicity tests were selected according to their extensive use in toxicological studies of iron oxide nanoparticles and included phytotoxicity using several seeds, Daphnia magna and a bioluminescent test (Microtox), revealing low toxicity.³¹⁴ The toxic effects of inhalation exposure to Fe₂O₃ nanoparticles (together with ZnO) in rats were investigated.³¹⁵ Iron content in liver and lung tissues was significantly increased at 36 h. The levels of serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), total protein (TP), creatine kinase (CK), and lactate dehydrogenase (LDH) were significantly decreased compared to the unexposed controls. It was shown that both types of nanoparticles caused severe damage in liver and lung tissues.

Zeta potential measurements, common in nanotoxicology, were applied to Fe₂O₃, among other oxides in respect of to acquaint the effects of pH and time on nanoparticle zeta potential, agglomerate size and cellular viability.³¹⁶ Fe₂O₃ increased in zeta potential and agglomerate size over time. Cytotoxicity studies revealed that Fe₂O₃ caused decreasing cellular viability over 48 h. It was indicated that alterations in the pH have a large effect on zeta potential and agglomerate size which may be used as a predictive measure of nanotoxicity. Cytotoxicity of SPIONs (bare and poly(ethylene glycol)-co-fumarate (PEGF)-coated SPION with narrow size distributions) and their ability to change cell medium components was investigated³¹⁷ in respect with Dulbecco's modified Eagle's medium (DMEM) and primary mouse fibroblast (L929) cell lines. The potential toxic effects of iron(II,III) oxide nanoparticles were studied.³¹⁸ While in vitro MTT assay showed a moderate cytotoxic effect, the Fe(II,III) nanoparticles proved to be devoid of mutagenic effect in the bacterial systems tested. In addition, peptide–SPION complexes were proved to be biocompatible and are localized around the cells due to their peptide coating.³¹⁹

In case of other iron-containing nanostructures, there is little available information on their toxicity. Thus, the toxicity of Mn_0.5Zn_0.5Fe₂O₄ nanoparticles in vitro on L-02 cell was studied,³²⁰ showing that (a) the activity of L-02 cell decreased with the increasing of Mn_0.5Zn_0.5Fe₂O₄ nanoparticle concentration and prolonging of poisoning time and (b) the activity of L-02 cell became stable after poisoning for 48 h. The toxicity of the related compound, stoichiometric Mn_0.2Zn_0.8Fe₂O₄ monodisperse nanoparticles, was evaluated by viability assays on human umbilical vein endothelial cells.³²¹

Conclusions

Currently, the use of the NZVI, iron oxides and other Fe-contained nanomaterials for remediation of both organic and inorganic pollutants from groundwater and contaminated soils could be considered as a relatively hot topic in the nanotechnology. During the last decade, a series of distinct methods have been offered to synthesize these nanostructures as by traditional wet chemistry routes and more sophisticated modern techniques as well as “greener” methods using plant extracts. Due to relatively low toxicity of iron-containing nanoparticles, it has been allowed to apply them in free and supported forms to reach a considerable decontamination of the environment.

Evaluation of toxicity of the Fe-containing nanomaterials remains an object of permanent research and polemics. Since real long-time effects of the presence of iron-containing nanoparticles in rivers, groundwater, lakes and oceans are still unknown, the idea to introduce iron-containing nanoparticles into oceans,³²² in order to stimulate growth of phytoplankton and better adsorption of CO₂, was further rejected.

The research field, dedicated to core–shell iron (iron oxide)/gold nanoparticles, is of a special interest due to possibility to be controlled by magnetic fields (iron core) and functionalization (gold shell). These air-stable Nps are protected from the oxidation and retain most of the favorable magnetic properties, which possess the potential for applications in drug delivery, high density memory devices by forming self-assembling nanoarrays, and a series of other applications.

Acknowledgements

BIK is grateful to the Paicyt-UANL-2012 project for financial support.

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