A review on nanotechnological application of magnetic iron oxides for heavy metal removal

Journal of Water Process Engineering 31 (2019) 100845

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A review on nanotechnological application of magnetic iron oxides for heavy metal removal Rachna Bhateria, Rimmy Singh

T

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Maharshi Dayanand University Rohtak India, Type 2University Campus, 124001, Rohtak, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Nanotechnology Nanoparticles Iron oxide Heavy metal and adsorption

With increasing trend in industrialization, heavy metals possess a great threat to the environment due to their discharge in water and wastewater above permissible limits. Heavy metals have toxic effects on human and environment. However, advancement in newly budding and fangled nanotechnology offers better treatment techniques. Development of novel and cost-effective 0D, 1D, 2D and 3D nanomaterials for environmental remediation, pollution detection and other applications has attracted considerable attention. Zero valent iron and iron oxide nanoparticles are found to be the best candidates for heavy metal adsorption and removal. Various mechanical, optical and electrical properties of nanoparticles play important role in nanoparticle formation and interaction. Forms of iron oxide such as hematite (α Fe2O3) and magnetite (Fe3O4) nanoparticles of varied morphology and size (10 nm, 20 nm, 50 nm etc.) were synthesized by various methods like sol-gel, precipitation, hydrothermal processes and magnetic nano-composites with different iron precursors (iron acetate, iron nitrate, ferric chloride, ferrous sulphate etc.). Iron oxide nanoparticles (in a variety of chemical and structural forms) have already exhibited its diversity and potential in many frontiers of environmental area. Present review is focused on the application of iron and iron oxide nanoparticles towards heavy metal removal.

1. Introduction Heavy metals are the metallic elements which have a relatively high density. Heavy metals are capable of inducing toxicity even at lower levels on exposure. Due to the heavy metal contamination in the environment, there has been a progressive concern about ecology and public health. In the last few decades, there is an emerging progress in agricultural, industrial and urban activities leading to the increased pollution up to many folds. Burning of fossil fuels, municipal wastes, fertilizers, mining and smelting of metallic ferrous ores, pesticides and sewage sludge are the primary sources of pollution [1,2]. The Environmental Protection Agency (EPA) has set maximum contaminant level (MCLs) as a standard for drinking water quality. Pollutants present above these levels have adverse health effects [3]. Cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn) are among most common heavy metal contaminants of industrial pollution. Heavy metals are a class of persistent toxic substances (PTSs), have acute toxicity and accumulation effects on living beings [4,5] (Fig. 1). Heavy metals have deleterious effects on the human health and environment by entering into the food chain. Although, heavy metals occur naturally in the earth crust but the anthropogenic activities have increased the heavy metal content and results in the environmental ⁎

contamination. However, these anthropogenic sources are battery production, phosphate fertilizer, alloy production, textiles, sewage irrigation, atmospheric deposition, mining biosolids, improper stacking of industrial solid waste, coating, explosive manufacturing, leather tanning, pesticides, printing pigments, smelting, steel and electroplating industries, photographic materials, dyes and wood preservation [6,7]. Heavy metals such as cobalt (Co), copper(Cu), chromium(Cr), iron(Fe), magnesium(Mg), zinc(Zn), selenium(Se), manganese(Mn), nickel(Ni) and molybdenum(Mo) are considered to be the essential trace elements for the biochemical and physiological functioning. Other than these metals all are considered as non-essential metals. But if these metals are present beyond their tolerance value, they can have deleterious effects on the environment and human health. It was found that large quantity of lead present in potable water can cause cancer, renal kidney disease, anemia, nervous disorders etc. Metals such as arsenic [8–10], cadmium [11], chromium [12,13], lead [14,15] and mercury [16,17] are categorized under the prime concern metals which are important to public health. Hence, due to their high-grade toxicity, they can induce multiple organ damage on exposure to even low concentrations. According to the United States Environmental Protection Agency (U.S. EPA) and the International Agency for Research on Cancer (IARC), the metals like mercury,

Corresponding author. E-mail address: [email protected] (R. Singh).

https://doi.org/10.1016/j.jwpe.2019.100845 Received 17 October 2018; Received in revised form 15 March 2019; Accepted 26 April 2019 2214-7144/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Water Process Engineering 31 (2019) 100845

R. Bhateria and R. Singh

In order to remediate toxic and harmful substances, engineering offers new opportunities by adapting more advanced nanotechnological tools as compared to conventional processes by developing highly sophisticated methods for wastewater treatment. [31]. 2.1. Nanomaterials- building blocks of nanotechnology In recent times, a range of engineered nanomaterials (e.g. nanoparticles, quantum dots, nanomembranes, nanowires, nanosheets, nanotubes etc.) have been delineated, which has been used in miscellaneous disciplines [32–39]. The first classification idea of NSMs (Nanoscale materials) was given by Gleiter in 1995 [40] and further was explained by Skorokhod in 2000 [41]. But Gleiter and Skorokhod system of classification was not well thought out as 0D, 1D, 2D, and 3D structures such as fullerenes, nanotubes, and nanoflowers were not explained properly. Hence, Pokropivny and Skorokhod [42] formulated an advanced classification system for NSMs, in which 0D, 1D, 2D and 3D NSMs are included. According to them, Zero-dimensional (0-D) nanomaterials are not confined to the nanoscale range (< 100 nm). They can be, one-dimensional (1-D two-dimensional (2-D) (Fig. 3c), and three-dimensional (3-D). The most general representation of zero-dimensional (Fig. 3a) nanomaterials is nanoparticles. These materials have every dimension within the nanoscale (no dimensions, or 0-D, below 100 nm). A one-dimension (Fig. 3b) nanomaterial ranges out of nanoscale. They consist of needle like nanostructures for instance, nanotubes, nanorods, and nanowires. However, in two-dimensional nanomaterials, two of the dimensions are not confined to the nanoscale which includes nanofilms, nanolayers, and nanocoating’s. Three-dimensional nanomaterials or bulk nanomaterials are not restricted to the nanoscale in any of the dimension; thus specified by their three erratic dimensions higher than 100 nm. Nanocrystalline structure or bulk nanomaterials can comprise of a numerous position of nanosized crystals, usually in diverse orientations [43]. Surfaces and interfaces are of great significance to new nanomaterials. As the particle size decreases, fraction of surface atoms multiplies in comparison with inside volume [44]. In addition to size, shape also affects the various properties as it changes the exposed crystal facets by altering the atomic arrangement in each facet which ultimately exerts tremendous impact on their properties [45]. Most of the micro-structured materials have interrelated properties to the corresponding bulk materials while the nanomaterials have significantly different properties from those of atoms and bulk materials. The basis behind such behavior is the nonorange of materials which provides them with high number of surface atoms, spatial confinement, high surface energy and reduced imperfections which is not found in the corresponding bulk materials. Nanomaterials with a sequence of distinctive properties i.e. small size effect, surface effect, and quantum size effect have fascinated the consideration of researchers [46–48]. Due to the small size effect of nanomaterials, they demonstrate exceptional fundamental functions, like low temperature sensing, excellent catalytic activities and high sensitivity [46,38]. Due to their surface effect they possess high reaction efficiency and strong adsorption capability [47–49]. Quantum size effect can endure nanomaterials with exceptional optical and electronic properties [50]. Engineered nanomaterials as the supporter catalysts, comprehend the segregation of the products and the catalysts from the reaction mixture and they play significant role in the field of catalysis, [51,52]. The magnetic nanoparticles (the catalyst supporter in organic synthesis) can be reused by recovering the catalysts with an external magnet without losing their activity [53–55]. Furthermore, other methods such as filtration and centrifugation can also be accepted for the recovery of catalysts [55,56,51]. Metals in nano range, as an imperative area of engineered nanomaterials, demonstrate possible applications [57–60]. For example, metallic nanoparticles can be used as efficient catalysts. Nanoparticles based chemical sensors and nanowires have enhanced the sensitivity and sensor selectivity. It is believed that nanomaterials with their internal structure at

Fig. 1. Heavy metal toxicity.

chromium, cadmium, arsenic and lead are categorized as either “known” or “probable” human carcinogens on the basis of their epidemiological and experimental studies. So, there have been many technologies to cope up with increasing heavy metal pollution by adopting remediation technologies There are many conventional methods adopted for the heavy metal removal in waste water [18]. Some of the techniques for waste removal includes biosorption [19], rubber tire activated carbon (RTAC) [20], activated carbon adsorption [21]. But these methods have some of the limitations. However, in recent years, nanotechnology has fangled its roots in environmental remediation. Size-dependent properties of nanoparticles (NPs) put forward countless opportunities towards miscellaneous applications in the technical era. This field with particle size from micro to nanoscale has received considerable global attention with an increasing demand towards nanomaterials [22]. A series of heavy metals have been scavenging by nanomaterials due to their large surface area and high reactivity. Moreover, trending nano metal oxides such as nanosized ferric oxides, aluminum oxides, manganese oxides, magnesium oxides, titanium oxides and cerium oxides endow with high surface area and unambiguous affinity for absorbing heavy metal from aqueous systems [23]. Nanoscale zero-valent iron (nZVI) has been effectively used for treating water and wastewater polluted with chlorinated organic compounds [24], heavy metals, including chromium [25], cadmium [25], copper [26], silver [26], zinc [26,27], dyes [28] and phenol [29]. This review comes up with the view point of researchers working on nanotechnology to deal with water and waste water problems. It also focuses on magnetic iron oxide nanoparticles and their efficiency in heavy metal elimination. 2. Concept of nanotechnology Nanotechnology is the “art and science of manipulating matter at the nanoscale (Fig. 2) (down to 1/100,000 the width of a human hair) to create new and unique materials and products”. The history of modern nanotechnology begins with Richard Feynman's lecture in 1959, "There's Plenty of Room at the Bottom," in the conference of American Physical Society [30]. Nanotechnology has almost revolutionized the concept of scientific and technological implications in environmental safety. The proficient and perceptive implementation of nanotechnology in environment is for water remediation and treatment. Modern nanotechnology provides nanomaterial application for decontamination of water through various mechanisms such as elimination of heavy metals and related pollutants, eradication and dormancy of pathogens and conversion of highly toxic substances to least toxic. For the development of new adsorbent with unique characteristics nanotechnology played a crucial role and has opened a new area. 2

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Fig. 2. Nanoscale [134].

Fig. 3. (a) Zero dimensional nanoparticles (b) one dimensional nanoparticles (c) two dimensional nanoparticles.

Fig. 4. Properties of Nanomaterials.

catalysts, increased strength, tunable photoactivity and many other interesting characteristics [61,62].

nanoscale dimensions are hardly something new to science. Perhaps, the particle size is not the only feature of a nanomaterial, nanocrystal, or nanoparticle. However, the vital and precise property of various nanomaterials is the bulk of atoms localized on the surface of a particle, in addition to commonplace materials where atoms are dispersed around majority of a particle. Alternatively, the process of formation and aggregation of nanomaterials is mainly depending upon the various magnetic, electrical, optical and mechanical properties (Fig. 4). At the nanoscale, extremely attractive properties are created due to quantum effects, dominance of interfacial phenomena, and size confinement. These novel and exceptional properties of nanostructured materials, nanoparticles and other related nanotechnologies results in improved

2.2. Nanoparticles Particles can be segregated on the basis of their size in requisites of diameter; fine particles are in between 100 and 2500 nm. Conversely, ultrafine particles have size between 1 and 100 nm. Similarly, particles within the range of 1–100 nanometers are categorized as nanoparticles. They are unique as their physical behavior shifts from classical physics to quantum physics with declining particle size. Nanoparticles can exist in solid, liquid or gaseous state. A nanoparticle when assumed like a 3

Journal of Water Process Engineering 31 (2019) 100845

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Fig. 5. Top down and bottom up approach.

ball in shape which has a radius of 2.5 nm, density of 5 g/cm3 and surface area 240 m2/g. It means that about 20% of the particle atoms are localized on the surface. However, the surface of a nanoparticle is never "naked". Due to high energetic adhesive forces close to the surface, the particles are either aggregated to their neighbors, glued to the next available surface or work like an activated charcoal filter towards other small molecules [44]. Nanoparticles could show size-related properties that vary considerably from those of fine particles or bulk materials [63]. Worldwide, metal nanoparticles have also fascinated the interest of the researchers due to their unique electronic, optical, and catalytic and several other structural features. At nanoscale materials shows elevated catalytic properties, high surface area, and greater active sites to intermingle with metallic species [64,65]. Various methods are used for the synthesis of nanoparticles, such as physical, chemical and biological methods. Top down and bottom up approaches also the methods of nanoparticle synthesis (Fig. 5). Previously, top–down approach was used widely which employed the use of physical and chemical means to reduce the bulk material into nanosized [66]. But such methodologies are associated with large amount of materials and energy consumption, hence, are not preferred. Now days, bottom–up approach has become more popular and is used at laboratory scales. The “top–down’’ approach fundamentally deals with bulk material, then reduced to nanoscale by specialized ablations e.g. sputtering, thermal decomposition, mechanical milling, laser ablation, etching, and lithography [67]. Contrarily, second one is the “bottomup” approach most preferably considered for nanoparticles synthesis, which involves a homogeneous system where catalysts (e.g. reducing agent and enzymes) produce nanostructures that are restricted by catalyst properties, reaction media, and conditions (e.g., solvents, stabilizers, and temperature). The above mentioned two methods differs in a matter that top-down method usually leads to crystalline samples, for instance small single crystals or polycrystalline material formed from previous “thermodynamically” starting material. In a crystalline solid, a “thermodynamically” formed “product usually complies with the ideal crystal structure of the specific compound (e.g. NaCl, CsCl, rutile, and corundum structures)”. Alternatively, bottom-up route outcomes with the formation of minute particles from crystalline areas that do not correspond to the ideal lattice (defect structure). These structures are difficult to illustrate and are typical “kinetic” products i.e. products that don’t have sufficient time for an ideal crystal growth [68]. Therefore, a “kinetic” product leads to a “defect structure”, which is also referred to as a real structure in literature. Such defect or real structures are classified according to different defect classes: 0-dimensional defects (nonstoichiometry), 1-dimensional defects (dislocations), 2-dimensional defects (grain boundaries) and 3-dimensional defects (pores) [69]. There are various techniques which are used to characterize nanomaterials. Synthesis of NPs is confirmed by employing UV–vis Spectrophotometry which is then followed by centrifugation of mixture and drying the pellet in a hot air oven to get the crystal NPs [70]. Synthesized nanoparticles are further characterized using X-ray diffractometer

(XRD), Energy Dispersion Analysis of X-ray (EDAX), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Field Emission Scanning Electron Microscopy (FESEM), Atomic Force Microscopy (AFM), Thermal-gravimetric Differential Thermal Analysis (TG-DTA), Photoluminescence Analysis (PL), X-ray Photoelectron Microscopy (XPS), Raman Spectroscopy, Attenuated total reflection (ATR), UV–vis Diffuse Reflectance Spectroscopy (UV-DRS), and Dynamic Light Scattering (DLS) [70,71]. Nanoparticles are composed of assemblies of atoms and molecules in nanometer scale and are more advantageous than their bulk counterparts because of their high surface-to-volume ratio. So, their application in varied sectors can play a counter role to improve the current situation of health and environment. 2.3. Iron nanoparticles Due to the high reduction and adsorption capacity of nanoscale zero-valent metals (NZVMs) it represents the cutting edge of technologies which is thought to be the potential materials for environmental remediation and antimicrobial effect. Since 1980s, nanoscale zero-valent metals have fascinated science as a category of efficient reducers for removing contaminants in water. Iron has a standard redox potential of -0.44 V (E0 = -0.44 V). It is thus an effective reductant when reacting with oxidized pollutants in water [72]. Its core-shell structures (Fig. 6) are advantageous for heavy metal elimination and transformation. The metallic core acts as an electron source and acts as the reducing character, whereas the oxide shell can absorb the pollutants via electrostatic interactions as well as surface complexation, and the proficient electron can transfer from the metallic core to the surface [73]. The core mainly consists of zero-valent iron and provides the reducing power for reactions with environmental contaminants. The shell is comprising of iron oxides/hydroxides as result of oxidation of zero-valent iron. The shell forms the platform for chemical complex formation (e.g., chemisorption) [74]. The thickness of outer iron oxide

Fig. 6. The core-shell model of zero-valent iron nanoparticles [74]. 4

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Fig. 7. Crystal structure and crystallographic data of the hematite, magnetite and maghemite (the black ball is Fe2+, the green ball is Fe3+ and the red ball is O2−) [86]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and inner core of Fe0 was observed to be 10 and 20 nm, respectively [75]. The shell mainly consists of magnetite Fe3O4 and often with maghemite (γ-Fe2O3) or lepidocrocite (γ-FeOOH) [76–78]. Nano iron is highly reactive when comes in contact with air [79] due to which its surface reactivity becomes low. Iron NPs show dual characteristics of zerovalent iron (reduction) and iron hydroxides (complex formation). The synthesis mechanism of Iron nanoparticles involves the reduction of Fe (II) or Fe (III) salt with sodium borohydride in an aqueous medium.

it contains both of divalent and trivalent iron. Fe3O4 consists of cubic inverse spinal configuration which has cubic close packed arrangement of oxide ions, wherein Fe2+ ions reside in half of the octahedral sites and the Fe3+ are divided uniformly across the left-over octahedral sites and the tetrahedral sites. Fe3O4 NPs of size 10–15 nm were synthesized and used for ultra-sonic assisted adsorption [84,85]. Maghemite (γ-Fe2O3) - Maghemite has cubic structure which consists of 32 O2− ions, 21⅓ Fe3+ ions and 2⅓ vacancies. Oxygen anions form cubic close-packed arrangement while ferric ions are scattered on tetrahedral sites (eight Fe ions per unit cell) and octahedral sites (the remaining Fe ions and vacancies). Therefore, the maghemite can be well thought-out of fully oxidized magnetite with n-type semiconductor characteristics and band gap of 2.0 eV [86]. In nanotechnology, an iron oxide nanoparticle is defined as a “small object that behaves as a whole unit in terms of its transport and properties”. Various studies on magnetic nanoparticles of different iron and iron oxides have been carried out. The bare iron or iron oxide NPs are chemically vigorous and can be oxidized easily with air (especially magnetite), due to this their dispersibility and magnetism vanishes. Therefore, it is necessary to maintain their stability by budding some efficient protection tactics and to coat their surface (or grafting) with nonmetal elementary substances polymers, biomolecules or inorganic layer such as silica, metal, metal sulfide, surfactants, metal oxide or organic molecules [87].

Fe (H2O)63+ + 3BH4− + 3H2O Fe0 + 3B (OH)3 + 10.5H2

2.4. Iron oxide nanoparticles Among eight known iron oxides [80], hematite (α-Fe2O3), magnetite (Fe3O4) and maghemite (γ- Fe2O3) (Fig. 7) are found to be proficient and most accepted candidates as they show polymorphism that involves temperature-induced phase transition. However, these iron oxides have unique biochemical, magnetic, catalytic and other properties which make them popular candidates for technical and biomedical applications. Hematite (α-Fe2O3) - Hematite is the most stable form of iron oxide. It is eco-friendly, nontoxic, biocompatible and it is economical and highly resistant to corrosion. Hematite (Fe2O3) is crystallized in the rhombohedral system space group R-3c with n-type semiconducting properties (2.1 eV band gap). In bulk hematite, the Neel temperature is at TN ≈ 960 K, whereas the Morin transition takes place at TM ≈ 263 K. Exceeding TM the material is weakly ferromagnetic while lower than TM the material is anti-ferromagnetic. The Morin temperature declines with reduction in particle size and disappear for particles which are 10 nm in size or smaller sizes [81,82]. Facile chemical route was used to coat and synthesize hematite nanoparticles with octyl ether and oleic acid. It was observed that presence and absence of morin transition is affected by the alteration in magnetic characteristics due to small particle size i.e. 7–25 nm [83]. Magnetite (Fe3O4) - Fe3O4 differs from most of other iron oxides as

3. Synthesis of magnetic nanoparticles Magnetic nanoparticles (NPs) of diverse morphologies and structures have been currently used in countless fields such as biomedical applications, agricultural and environmental applications [88]. The significance of magnetic nanoparticles (NPs) is primarily due to their chemical stability, biological compatibility but above all their troublefree production and recyclable process for a range of applications [89]. There are various methods which are used for the fabrication of nanoparticles (Table 1). A brief description of the methods most widely used for preparing

Table 1 Fabrication of iron oxide NPs: comparison of different methods [112]. Characteristics of the iron oxide

Size and size distribution Morphology Magnetization values Advantages

Methods of synthesis Aerosol/vapor (pyrolysis) method

Gas deposition method

Bulk solution method

Sol–gel method

Micro-emulsion method

Approximately 5–60 nm with broad distribution Spherical 10–50 emu/g High production rate

Approximately 5–50 nm with narrow size distribution Spherical > 20 emu/g Useful for protective coatings and thin film deposition

10–50 nm

20–200 nm

4–15 nm

Spherical 20–50 emu/g Synthesis in bulk

Spherical 10–40 emu/g Desired shape and length and hybrid NPs

Spherical or cubic > 30 emu/g Uniform properties

5

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of metal cations. Reducing agents takes many forms, the most common of which are gaseous H2, solvated ABH4 (A = alkali metal), hydrazine hydrate (N2H4H2O), and hydrazine dihydrochloride (N2H42HCl). Type of metals which are precipitating from the solution includes not only first row of transition metals ions, such as Fe2+, Fe3+, Co2+, Ni2+, and Cu2+, but also many second and third row transition metals, as well as most post-transition elements and a few nonmetals.

materials with applications in metal removals is given below: 3.1. Sol–gel deposition Ultrafine particles, Nano thickness films, and Nano porous membranes are all synthesized by sol–gel processing. The initial point is a solution of precursors in a suitable solvent. The precursors are generally inorganic metal salts or metal–organic compounds such as alkoxides metal ions with an organic ligand such as Ti(OC4H9)4. The ultrasound induces cavitation and the cavity collapse causes the reagents to react. Nano porous membranes, nanostructured layers as well as coatings all can be synthesized by Sol–gel processing. The primary process involves a polymerization reaction that forms a colloidal suspension, or “sol,’’ of separated, finely dispersed particles kept in suspension by adding a surfactant. For further processing i.e. casting or spin-coating onto a substrate, the suspension can be treated to extract the particles. There it is converted to a gel by chemical treatment to disable the surfactant to create an extended network of linked particles all through the solution, building a kind of super polymer, one gigantic molecule in the form of an open three-dimensional (3D or, on a surface, a 2D) complex—the “gel.’’ Evaporation of the solvent leads to the dense or Nano porous film. Sol–gel thin film process provides various advantages i.e. lowtemperature processing, effortlessness fabrication, and accurate microstructural and chemical control [90]. Sol–gel methods are the foundation for a broad range of materials like cells, cosmetics, ceramics, detergents etc. [91,92]. This technique is based on hydroxylation and condensation of molecular precursors in solution. Obtained “sol” from nanometric particles is then dried or “gelled’’ either by solvent removal or by chemical reaction to obtain three-dimensional metal oxide network. The solvent used is water, but the precursors can be hydrolyzed using an acid or a base. Basic catalysis yields a colloidal gel, whereas acid catalysis formulates a polymeric gel [93].

3.3. Hydrothermal and Solvo-thermal synthesis In a well packed vessel (bumb, autoclave etc.), let the solvents to heat up in order to attain a temperature above their boiling point. Under such conditions of temperature and pressure, when a chemical reaction takes place then it is referred as Solvo-thermal processing while it is hydrothermal when water is used as solvent. Water above critical temperature and pressure is referred as supercritical and as a fluid it demonstrates characteristics of both liquid and gas. The interfaces of solids and supercritical fluid lack surface tension. Hence, it shows exceptionally less solubilities under ambient conditions [91]. The hydrothermal reactions are performed in a reactor or autoclave in an aqueous media, where the pressure of > 2000 psi and temperature of > 200 °C are maintained. The dehydration of metal salts and low solubility of oxides in aqueous phase supersaturate the medium [96]. Moreover, the hydrothermal and solvothermal route is a facile and conventional method for obtaining hollow IONPs. In a typical procedure, using Fe3+ as the iron resource, acetate, urea, and sodium citrate are mixed in ethylene glycol under stirring, and then the resultant homogeneous dispersion is transferred to a Teflon-lined stainless-steel autoclave and sealed to heat at about 200 °C for 8–24 h [57,97–99]. In Solvo thermal process, an organic solvent is used as a reaction medium instead of water. Normally, a facile hydrothermal method for synthesizing Fe3O4 includes adding FeCl2, FeCl3 and NaOH with a molar ratio of 1:2:8 into an autoclave and a heat treatment at elevated temperature [100]

3.2. Co-precipitation This process is most likely the effortless and most proficient chemical pathway to synthesize magnetic nanoparticles. Magnetite is normally synthesized by an aging stoichiometric mixture of ferrous and ferric salts in aqueous medium. The precipitation of Fe3O4 is expected at a pH between 8 and 14. The size and shape of the nanoparticles can be restricted by adjusting pH, ionic strength, temperature and nature of the salts [94,95]. Co-precipitation reaction involves the concurrent events of nucleation, growth coarsening, and/or agglomeration processes [91]. Precipitated from aqueous solution continue to be a thoroughly investigated subject. The precipitation of the metals from aqueous or non-aqueous solution typically requires the chemical reduction

3.4. Magnetic nanocomposites Inorganic materials like silver, silica or gold can be used to coat Fe3O4 nanoparticles [101–103]. These coatings on the nanoparticle surface provide sites for covalent binding with a particular ligand and also improves the stability of nanoparticles. These magnetic nanocomposites have structure such that it has outer shell of inorganic material while inside core is composed of iron oxide. For instance, variation of electrolytes and pH is conquered by silica coating and hence it offers a high stability to nanoparticle dispersion against this change [104] (Fig. 8).

Fig. 8. schematic representation of three-layered NP synthesis at every step of synthesis (left); TEM images each step of synthesis. a b SiO2 particles covered with silica-primed Fe3O4 NPs (SiO2–Fe3O4). c d SiO2 particles covered with silica-primed Fe3O4 NPs and heavily loaded with Au nanoparticle seeds (SiO2–Fe3O4–Au seeds). e Three-layer magnetic NPs synthesized in a single-step process from particles presented in (c) and (d). Note the uniformity of the gold shell. The inset shows the three-layer magnetic NPs drawn to the wall with a magnet. [87,137]. 6

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4. Potential application of nanoparticles for Heavy metal removal

magnetite and maghemite nanoparticles from aqueous solution. At optimal pH conditions, 96–99% uptake of arsenic and chromium was observed under controlled pH conditions. Accordingly, at pH 2 and initial concentration of 1.5 mg/L for both arsenic species, maximum arsenic adsorption was observed at the values 3.69 mg/g for As(III) and 3.71 mg/g for As(V), whereas at the same pH and initial Cr(VI) concentration of 1 mg/L adsorption was found to be 2.4 mg/g. Their results also showed the limitation of arsenic and chromium up‐take by the nano-size magnetite-maghemite mixture in the presence of a competing anion such as phosphate. In an experiment by Sahu et al. [124], Fe3O4 nanoparticles were prepared in aqueous solution by microemulsion process by utilizing Extran (biodegradable surfactant) to enhance the removal efficiency of As(III). The maximum adsorption of As (III) at optimized condition was obtained at adsorbent dose of 0.70 mg/g, solution pH of 7.7, and initial As(III) concentration of 33.32 mg/L. In another study, borohydride reduction method was used to synthesize ZVINs in the presence of PAA (as a stabilizer) for the removal of lead. The optimization variables of Pb(II) removal were initial solution pH, ZVINs concentration (g/L), and initial concentration of Pb(II) (mg/L). It was showed that a significant correlation between predicted values obtained from second-order polynomial model and experimental values (R2 = 93.19 and adjR2 = 87.07) with maximum removal of Pb(II) [125]. Sol-gel method was used for the preparation of magnetite nanoparticles of around 10 nm for Cr(VI) removal. The characterization techniques used were TEM and XRD. 198m2/g was the analyzed surface area. Physico-chemical adsorption governs the Cr(VI) uptake. The adsorption process varies with pH and temperature. Preliminary result indicates that Cr uptake was governed by physio-chemical adsorption and Freundlich isotherm fits well in the data [126]. Modified Fe3O4 magnetic nanoparticles (MNPs) of size 15–20 nm with 3-aminopropyltriethoxysilane and 80 copolymers of acrylic acid and crotonic acid in the removal of Cd2+, Zn2+, Pb2+, Cu2+ from metal contaminated water. It was experimented that Magnetic nanoparticles can be used to eliminate metal ions efficiently. The maximum adsorption capacity of removal was found at pH 5.5. It is used as a recyclable adsorbent with suitable conditions [127]. A list of functionalized iron oxide NMs with their adsorption capacities is summarized in Table 2. Due to the colloidal nature of super paramagnetic nanoparticles, their synthesis is complex process. For metal scavenging applications, an apt nanoparticle surface alteration is an important behavior towards aqueous stability as well as selectivity of these materials. To the ending note, it is important to mention that synthesis methods of organic and inorganic functionalized Fe3O4 nanoparticles has been developed and modified in the last decade. Meanwhile, it is emphasized that reduction in size of iron to nanoscale has been proved to enhance the rate of reaction and reduce the byproduct formation efficiently.

The rapid growth in nanotechnology has a great deal of attention in the environmental applications of nanomaterials. Mitigating water and air pollutants is a big challenge to the society and nanomaterials were found to be efficient for environmental remediation by many researchers. The highly complicated task is the selection of appropriate methods and materials for wastewater treatment, where number of factors is under consideration such as efficiency, quality standards to be met as well as the cost. [105,106]. Therefore, following are the conditions which must be considered while deciding wastewater treatment technologies: (1) treatment flexibility and final efficiency, (2) reuse of treatment agents, (3) environmental security and friendliness, and (4) low cost [106,107]. Recently, engineered nanomaterials are found to have increased environmental applicability’s [108–110]. Nanomaterials have potential reaction and adsorption properties which have been used in the environmental remediation primarily in treating hazardous waste, air and water purification. This development drifts to a substantial increment in the pace of environmental remediation through nanomaterials. Hence, adsorption surfaces show unique and advanced characteristics [111,112]. According to current scenario, nanoscale iron has been considered as the apt option for scavenging various sectors of pollutants like nitrate, tetracycline, methyl orange, and heavy metals, including Cr6+ [113], As5+ [114], Cd2+ [115], and Pb2+ [116] from aquatic ecosystems. Zhang et al. [116] synthesized kaolinite supported Zerovalent iron NPs which eliminate Pb(II) efficiently by reducing Pb (II) to Pb(0). Nano metal oxides were also applied to water and wastewater for eliminating heavy metals [117] for example, magnetic iron oxide (Fe3O4) nanoparticles were observed not only as efficient adsorbing materials but also used in biotechnological processes [118] as well as medical [119] applications. Laboratory findings showed that iron oxide NMs can eliminate a series of heavy metals, such as Pb2+, Hg2+, Cd2+, Cu2+etc. In an experiment conducted by Nassar [120], it was observed that Fe3O4 nanoparticles shows maximum adsorption for Pb(II) ions at 36.0 mg g−1 as compared to the low cost adsorbents. He found that small sized Fe3O4 Nanosorbents was efficient for the diffusion of metal ions from solution onto the active sites of the adsorbents surface. However, for the recovery of heavy metal ions from waste water effluents, Fe3O4 Nanosorbents were recommended as economically viable. Mayo et al. [121] studied the potential of nanoscale iron oxides for the arsenic adsorption focusing on magnetite (Fe3O4) nanoparticles. They studied the influence of Fe3O4 particle size on the adsorption and desorption behavior of As(III) and As(V). They evaluated that as the particle size decline from 300 to 12 nm the adsorption capacities incline to about 200 times for both As(III) and As(V). Convincingly, this increase is higher than expected surface area. They also suggested arsenic adsorption differs in both nanoscale iron oxide and bulk systems. Teng et al. [122] synthesized NZVI stabilized by sodium dodecyl sulfate (SDSNZVI) which were found to efficient in Cr removal [4,5]. studied adsorption capacity of CHI-stabilized NZVI (CNZVI) composites for cadmium ion (Cd2+). They observed that the maximum adsorption capacity (99.9%) of Cd2+ is 124.74 mg/g with 0.08 mass ratio of CHI to iron ions under the condition of pH 6. The adsorption isotherms followed Langmuir model and the kinetic data was well fitted to pseudosecond-order kinetic model. Wu et al. [87] synthesized Fe3O4 which were used to disperse nanoFe0 for Cr (VI) mitigation. Fe0 nanoparticles can attach to the surface of Fe3O4 by addition of large Fe3O4 NPs into the reaction solution during the preparation of Fe0 nanoparticles. When Fe3O4 is introduced it prevents the agglomeration of Fe0 nanoparticles and maintains the high efficiency of the nano composite for Cr (VI) reduction. The results showed that high amount of Fe3O4 in the nano composites leads to the rise in Cr (VI) reduction. 40:1was found to be the optimal ratio of Fe3O4. Fe0 for Cr(VI) reduction. Chowdhury and Yanful [123] experimented for adsorption of arsenic and chromium by using mixed

5. Conclusion In the above sections, the current progress of nanotechnology with a view on synthesis, characterization and applications of iron oxide nanoparticle has been reviewed. Nanomaterials have great potential for removal of contaminants due to their unique physical and chemical properties. In this review, nanomaterials prioritization and its further application prospects takes the nanomaterials development a level ahead. Unbeaten rational design of nanomaterials can be harnessed by considering the elementary atomic and molecular properties of the material at the nanoscale. Consequently, it is also important to find solutions for improving the stability and accessibility of functionalized iron oxide nanoparticles in the severe environmental circumstances and the large-scale industrial production. Moreover, iron oxide nanoparticles are found to be the efficient Nanosorbents and their implication in heavy metal adsorption is one of the most proficient and flourishing applications. Although various applications of nanomaterials were successful due to their unique physical and chemical 7

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Table 2 Functionalized iron oxide magnetic nanomaterials in heavy metal adsorption. Nanosorbents

Ligands

Heavy metals

Adsorption capacity

Reference

Mesostructured silica magnetite Magnetic iron–nickel oxide Montmorillonite-supported MNPs PEI-coated Fe3O4 MNPs δ-FeOOH-coated γ-Fe2O3 MNPs Flower-like iron oxides Hydrous iron oxide MNPs Fe3O4–silica Amino-modified Fe3O4 MNPs

–NH2 – –AlO; –SiO –NH2 – – – Si–OH –NH2

Cu(II) Cr(VI) Cr(VI) Cr(VI) Cr(VI) As(V), Cr(VI) As(V), Cr(VI) Pb(II), Hg(II) Cu(II), Cr(VI)

[131] [138] [140] [135] [129] [132] [136] [128,130] [133,139]

m-PAA-Na-coated MNPs

–COO

Cu(II), Pb(II)

Poly-L-cysteine coated Fe2O3 MNPs

–Si–O; –NH2

Ni(II), Pb(II)

Fe2O3 nanoparticles Extran – Fe3O4 nanoparticles

–COO –Si–O; –NH2

Pb (II) As(III)

Adsorbent capacity is 0.5 mmol/g for Cu(II). 30 mg/g uptake capacity of adsorbent for Cr(VI). The maximum adsorption capacity was 15.3 mg/g for Cr(VI). The maximum adsorption capacity for Cr(VI) was 83.3 mg/g. The Cr(VI) adsorption capacity determined to be 25.8 mg/g The As(V) adsorption capacity was 5.3 mg/g. 8 mg of arsenic per g of adsorbent. The removal efficiency was 97.34% and 90% for Pb(II) and Hg(II), respectively. The maximum adsorption capacity was 12.43 mg/g for Cu(II) ions and 11.24 mg/g for Cr(VI) ions, respectively. Adsorption capacity: Cd(II) (5.0 mg g−1); Pb(II) (40.0 mg g−1); Ni(II) (27.0 mg g−1) and Cu(II) (30.0 mg g−1). The recovery of the tested metals were almost all above 50%, even the removal efficiency of Ni (II) reached 89% The maximum adsorption capacity of Fe3O4 nanoparticles for Pb(II) ions was36.0mg/g About 90.5% of As(III) was adsorbed

properties, inspite of this, the application of NMs in wastewater treatment are still limited in the early stage. As exemplified in this review, a series of nano iron oxide-based technologies have been anticipated for the waste water treatment and heavy metal removal. Probable hurdles may be confronted in application of studies related to in situ studies of iron oxide nanomaterials. In the concluding lines, it is summarized that there is much recent interest in the utilization of nano iron oxide which is relatively a non-invasive tool in varied applications including waste water treatment. But it is emphasized that qualms over the health impacts and environmental fate of these nanomaterials need to be addressed before their widespread application. Increasingly, study of their fate and impact in the environment is becoming important due to the discharges already occurring to the environment.

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