Carriers for nano zerovalent iron (nZVI): synthesis, application and efficiency

Abstract

Nanoparticles refer to very small size elements less than 100 nm. These particles have a large surface area to volume ratio with enhanced reactivity. One nanomaterial very interesting for remediation activities is nano zero valent iron (nZVI) estimated to be 10 to 1000 times more reactive than granular ZVI, thereby creating a greater reductive capacity per gram. Thus smaller amounts of iron would be needed to treat a contaminated plume at a faster rate, which would further reduce costs. nZVI also has the potential to treat recalcitrant pollutants such as chlorinated organic compounds, nitrates and hexavalent chromium. But the high reactivity of nZVI alone is not enough to make it an effective remediation agent, the nano iron particles should also be able to form stable dispersions and migrate to contaminated plumes. These expected additional properties are seen to be missing as the nZVI quickly agglomerates and oxidizes once released into the environment. This study looks at the challenges associated with the use of nZVI and the different transporters or carriers that have sought to overcome these shortfalls. Further attention is focused on how these nZVI particles are synthesized on carriers and the results obtained when used to treat pollutants.

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Junias Adusei-Gyamfi

Junias Adusei-Gyamfi is a Master's student of Sustainable Management of Pollution at Catholic University of Lille (ISA), France. He has a bachelor's degree in Chemistry from KNUST, Ghana. Before his master's, he worked as a project officer for an Environmental non-governmental organization, Learnyoung Africa. He considers himself as a very disciplined and focused man with great interest in sustainable environmental development. He desires to help developing countries overcome their myriad environmental issues both through research and field works.

Victor Acha

Dr. Victor Acha is Associate Professor of Nanotechnologies and Bioprocess Engineering at LaSalle Beauvais, France. He got his B.Sc. in Chemical Engineering at USFX (Bolivia), M.Sc. in Food Engineering and Ph.D. in Bioengineering at UCL (Belgium). He worked in Colorado State University (USA) for 4 years and UTHSC San Antonio (USA) for 2 years. Cofounder of the Research Unit Hydrise (LaSalle), interested in the treatment of polluted waters by nanoparticles, ozonation, model-based estimations (software sensors and adaptive control), and development of nano(bio)sensors for environmental, biomedical, and food safety applications. Peer reviewer Certificate from Publons. Member of different Research Associations.

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Introduction

Nanotechnology is one of the fastest growing fields of scientific research and promises a potential revolution in approaches to remediation of polluted sites.¹ The term nanoparticle is used to classify particles with dimensions less than 100 nanometers (nm) in size. The unique and specific characteristics of different nanomaterials defines their usage and applications in machinery, energy, cosmetics, optics, electronics, drug delivery, and medical diagnostics.^2,3 The nano metals commonly used include iron (Fe⁰), magnesium (Mg⁰), palladium (Pd⁰) and silver (Ag⁰)⁴ with nano zero valent iron being the most used.⁵ In 2000 the first field trial of nano zero valent iron (nZVI) for the treatment of trichloroethylene in groundwater at a manufacturing site in Trenton, New Jersey, USA, was documented.⁶

nZVI is a promising material for groundwater in situ remediation because of its high surface area, high reactivity, fast kinetics, small particle size, and magnetic property that is beneficial for separating and recovering tiny particles from wastewater.⁷ Though the definition of nano particles sets the threshold size at 100 nm, a review of applications in literature indicates the average particles size is approximately 60 nm (>90%)⁸ while the critical diameter below which specific nano effect occurs is 30 nm.⁹ The high specific surface area allows nZVI to be 10 to 1000 times highly reactive than the granular ZVI, and the sorption capacity is also much higher^2,10 thereby creating a greater reductive capacity per gram. Thus smaller amounts of nZVI would be needed to treat a contaminated plume that would further reduce costs.¹¹

The principal potential benefits of nZVI are the speed of contaminant degradation due to its high surface area and reactivity that reduces remediation time.¹² It also has the potential of treating recalcitrant pollutants such as chlorinated organic compounds, nitrates and hexavalent chromium that can be found in aquatic environments.⁴ There is almost complete degradation process eliminating the formation of toxic intermediate products and it has the potential for synergistic application with bioremediation techniques.⁶

Fe⁰ has standard reduction potential (E⁰) of −0.440 V for half-reaction between the Fe²⁺/Fe⁰ couple (eqn (1)) which confirms that it is an effective electron donor regardless of its particle size. Reduction potential measures the probability of a chemical species being reduced. Thus a higher reduction potential means a higher probability of the species to be reduced. This means that theoretically Fe⁰ can reduce any pollutant that has a higher reduction potential than −0.440 V.¹³

Fe²⁺ + 2e⁻ → Fe⁰ (1)

In the subsurface environment, the predominant electron receptors are water and to some extent residual dissolved oxygen as described in eqn (2) and (3)

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

2Fe⁰_(s) + 4H⁺_(aq) + O_2(aq) → 2Fe²⁺_(aq) + 2H₂O_(l) (3)

Since the oxidation of Fe⁰ consumes H⁺ and produces OH⁻ ions (eqn (2) and (3)), there is an increase in solution pH and an associated decline in solution potential (E_h). The high surface free energy of nZVI coupled with the dissociative adsorption of water on the iron surface results in the formation of surface-bound hydroxyl species. Thus before reacting with target pollutants, the nZVI rapidly reacts with water and/or oxygen in its surrounding condition, resulting in the formation of passive oxidation layers.⁷ The structure of the nanoparticle is therefore made up of an inner core which exhibits characteristics of a metallic iron (reductant) and an oxidized shell with characteristics of an iron oxide (sorbent) (Fig. 1).⁸

Fig. 1 A core–shell structure for iron nanoparticles in aqueous solution. This figure has been adapted from ref. 8 with permission from Elsevier.

The principal mechanism of the core is to act as an electron source for the reduction of pollutants. Iron oxides however can strongly adsorb metals in soils thus immobilizing metallic contaminants in soil but not much is known about the efficiency of the treatment in soils that are not saturated with water. It also serves as the barrier for electron passage from the core.¹

The high reactivity of nZVI alone is not enough to make it an effective remediation agent, the nano iron particles should also be able to form stable dispersions and migrate to contaminated plume.^14,15

This study looks at the challenges associated with the use of nZVI and the different transporters or carriers that have sought to overcome these challenges. A further attention is focused on how these nZVI particles are synthesized on carriers and the results obtained when used to treat pollutants.

Properties of nZVI

Iron which is a strong lustrous, ductile, malleable metal is seen as one of the most abundant elements in the earth's crust is found in group 8, period 4 and block D of the periodic table. nZVI is a sub-micrometer particles of iron metal. Iron is highly reactive to both air (oxygen) and water, and this reactivity increases significantly at the nanoscale. A summary of the general properties of the bulk Fe is shown in Table 1. The properties of nZVI are usually different from the bulk Fe particle and this difference is mainly caused by factors such as the history of the sample, its handling and processing which would affect the size, structure and even composition of the nanoparticle. Additionally, the close proximity to other nanoparticles can also affect the electronic and magnetic characteristics of individual nanoparticles. The packing or dispersion of nanoparticles for measurement purposes may in some circumstances also change the results for the properties being measured.¹⁶

Table 1 General properties of iron

Symbol	Fe
Melting point	1538 °C
Boiling point	2861 °C
Density (g cm^−3)	7.87
Relative atomic mass	55.845
Atomic number	26
Key isotopes	56Fe
Electron configuration	[Ar] 3d^64s^2
First ionization energy	761 kJ mol^−1

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Table 2 Summary of nZVI synthesis methods³⁴

Chemical synthesis methods	Liquid-phase reduction or borohydride reduction of ferrous salts
	Gas-phase reduction
	Microemulsion
	Controlled chemical co-precipitation
	Chemical vapor condensation
	Pulse electrodeposition
	Liquid flame spray
	Thermal reduction of ferrous iron
	Electrolysis
Physical synthesis methods	Polyphenolic plant extract
	Inert gas condensation
	Severe plastic deformation
	High-energy ball milling
	Ultrasound shot peening

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Different characterization tools (in situ and ex situ) such as optical spectroscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction have been developed to study detailed characteristics and properties of the nanoparticles such as size, shape, size distribution, surface area, and chemical state. These tools have helped reveal the core–shell structure of nZVI as shown in Fig. 1. This core-structure dictates the chemical properties of the nanoparticle by exhibiting characteristics of both iron oxides (sorption) and metallic iron (electron source). According to the core–shell model, the mixed valence iron oxide shell is largely insoluble under neutral pH conditions and may protect the ZVI core from rapid oxidation.¹⁷

Magnetic properties

nZVI has shown generally to possess some magnetic properties though this properties largely vary on factors such as history (preparation method and storage), size, shape, chemical composition, surface oxidation and dimension of the nanoparticle.^18,19 In fact it is this property that results in the bare nanoparticles forming chain like structure and resulting in agglomeration. The strong magnetic properties of nZVI have been exploited in many recovery processes like using magnetic separation of coated nZVI particles to extract algae from solution. The nonlinear hysteresis loops with nonzero remnant magnetization (M_r) and coercivity (H_c) exhibits ferromagnetic properties like iron oxide core–shell structures with saturation magnetization (M_s) value of about 100 emu g⁻¹ (ref. 20 and 21) confirmed this that it is evident that the nanoparticles behave ferromagnetically with Curie temperature much higher than 76.85 °C further adding that both the squareness ratio (M_r/M_s) ratio and coercive field H_c decreases monotonically with increasing temperature, as expected for small particle systems. Actually in monodisperse systems, when magnetic field strength is strong enough to overcome hysteresis of the particles, larger ferromagnetic particles show higher heating performance than smaller superparamagnetic ones.²²

Metallic ferromagnetic particles show very large coercivity compared with the value predicted by the simple Stoner–Wohlfarth model under the assumption of bulk anisotropy. nZVI are therefore likely to possess uniaxial anisotropy with their effective anisotropy being much larger than that of bulk Fe particles.²³ When the particle size is increased, coercivity and relative remanence increases which corresponds to transition from superparamagnetic to ferrimagnetic behavior. Ferrimagnetic and ferromagnetic share very similar properties but differ in the direction neighboring dipoles are arranged. In ferromagnetic, the dipoles are arranged in the same direction whiles in ferrimagnetic materials, the neighboring dipoles are opposite to each other. Ferromagnetic particles also show higher reversal losses than smaller superparamagnetic particles.²²

Generally due to the increase of surface component of anisotropy (K_s), magnetic anisotropy tends to increase with decreasing of particle size. Magnetic anisotropy is generated by the spin–orbit coupling occurred at magnetic cations, and the anisotropy decreases with decreasing spin–orbit coupling, leading to a decrease of coercivity.²⁴

Catalytic properties

Due to factors such as cost, abundance, stability, recyclability and environmental friendliness, transition metals are considered to play an important role in catalysis and as popular substitutes of platinum metal based catalysts. This role is of more important when the metal particle is on the nanoscale due to the modification in its physical structure such as surface area to volume ratio and surface exposure of atoms. These factors tend to contribute positively towards increasing the efficiency of the catalyst. nZVI in the quantum dot range have been reported to be effective catalysts in many reduction reactions such as being used as a catalysts in the hydrogenation reaction of various substituted aromatic ketones to alcohols with NaBH₄.²⁵

nZVI is also a very desirable catalyst in Fischer–Tropsch (F–T) synthesis though after the reaction the catalyst/waxy product separation is problematic to regenerate the catalyst. Low product selectivity, catalyst agglomeration and sintering are other concerns of the use of this catalyst. However, iron is still desirable because of its high activation which ensures that the hydrocarbons produced are high in olefinic content.^26,27 The synthesis method of the nanoparticles also plays an important role in physical properties and performance of catalysts.²⁸

In the synthesis of both single and multi-walled carbon nanotubes through chemical vapor deposition (CVD) process, nZVI have been used as an effective catalyst. The results indicate that there is an upper limit for the size of the catalyst particles to nucleate the nanotubes and the size of the nanoparticle determines the diameter of the tubes.²⁹ It permits submicron scale patterned nanotube growth that retains a high degree of surface cleanliness^30,31 however found carbon nanotubes synthesized with Fe are not very stable. Thus with increasing temperature (>700 °C), diffusion of carbon in iron is favored forming more amorphous carbon.

In Fenton's reaction, the metallic or zero-valent iron, which is a two-electron donor, can directly reduce dissolved molecular oxygen in aqueous solutions to hydrogen peroxide at its solid surface according to the well-studied iron corrosion theory. The hydrogen peroxide produced can then also heterogeneously oxidize zero-valent iron into ferrous iron.³² One main disadvantage however of using Fenton reagent is that the pH has to be lowered to less than 4 to keep the iron in solution.¹⁷

Synthesis of nZVI

Nanoparticles can occur naturally in air, water, soil and sediments or be produced intentionally for specialized process. It can also be formed as accidental by-products of industrial processes.¹

Although micro particles of iron are inexpensive, reactive nanoparticles can be much more expensive because of the materials and processes needed to make them.³³ There exist several physical and chemical methods for the synthesis of nZVI such as grinding, abrasion, lithography, nucleation from homogeneous solutions or gas, annealing at elevated temperatures and reacting with reducing agents. These synthesis methods are grouped under two broad approaches; the bottom-up and top-down approaches.

The bottom-up approach involves assembling individual atoms and molecules to form nano sized structures. It uses a wide range of reductants to convert dissolved iron in solution to nZVI. The top-down approach on the contrary involves the crushing of bulk particles (microscale or granular) of iron to fine nano-sized particles by mechanical or chemical ways.^3,6 The choice of synthesis method influences both the size and shape of the nano particles produced. Table 2 summarizes different physical and chemical synthesis methods used by researchers with some of the mostly used techniques detailed in the next section.

Among the chemical synthesis methods, the borohydride reduction of ferrous salts is more popularly used due to its simplicity as no special instruments or materials are needed; in addition, the products have a homogenous structure and are very reactive.¹² Another reason for the preference of this technique is its lower associated cost of production at laboratory scale³³ estimated to $200 per kg of nZVI.³⁵ Most of other techniques for synthesizing nano materials are not feasible or cost-effective for industrial large scale production.³⁶

Liquid-phase reduction or borohydride reduction

The synthesis of nZVI can be achieved by reacting iron salt or compound with a strong reducing agent such as sodium borohydride (NaBH₄). The borohydride solution is slowly added into the iron (ferric or ferrous) salt while stirring vigorously under anaerobic conditions. The resulting black precipitate formed is vacuumed filtered, washed with deionized water or ethanol and subsequently dried.³⁷ The reaction follows the order:

4Fe³⁺_(aq) + BH₄⁻ + 3H₂O → 4Fe²⁺_(s) + H₂BO₃⁻ + 4H⁺_(aq) + 2H_2(g) (4)

2Fe²⁺_(aq) + BH₄⁻ + 3H₂O → 2Fe⁰_(s) + H₂BO₃⁻ + 4H⁺_(aq) + 2H_2(g) (5)

4Fe³⁺_(aq) + 3BH₄⁻ + 9H₂O → 4Fe⁰_(s) + 3H₂BO₃⁻ + 12H⁺_(aq) + 6H_2(g) (6)

The chemical reduction method includes four steps:³⁸

• Preparation of supersaturated solution.

• Nucleation of the nZVI cluster.

• Growth of nZVI nuclei.

• Agglomeration of nZVI.

Gas-phase reduction

This process uses gases such as H₂, CO₂ or CO acting as reducing agents to reduce iron salt or compound to nano form at higher temperature (>500 °C).³³ The particles produces have an average particle size ranging from 50 to 300 nm with a specific area of about 7.55 m² g⁻¹.³⁴ The reaction proceeds according to eqn (7) and (8):³³

Fe(C₂H₃O₂)_2(aq) + C_(g) → Fe⁰_(s) + 2CH₂CO + CO + H₂O (7)

Fe₃O_4(aq) + 2C_(g) → 3Fe⁰_(s) + 2CO₂ (8)

Thermal decomposition

One synthesis method which is well-known commonly for producing higher quality particles with more narrow size distribution is the thermal decomposition of an iron precursor. This method is quite similar to the gas phase reduction previously explained. The iron precursor (iron oxide or an iron salt) are reduced at high temperatures (>500 °C) in the presence of gaseous reducing agents. The precursor/capping agent ratio, rate of heating, the final temperature of the reaction, and the annealing time are all very important to both the size and size distribution of the nanoparticles.³⁹ The reducing agents used for this synthesis include N₂, H₂, CO, and Ar. If the reducing agent e.g. CO is produced as a result of thermally decomposing carbon-based materials such as biochar or carbon black, the overall synthesis is termed carbothermal decomposition/reduction.⁴⁰ Hydrothermal syntheses of iron nanoparticles is performed in aqueous media in reactors or autoclaves where the pressure can be higher than 2000 psi and the temperature can be above 200 °C. The longer the reaction time, the larger the size of the nanoparticle synthesized, likewise the higher the water content, the greater the precipitation of nanoparticles. The shape of the nanoparticles also depends on the reflux time.⁴¹

Ultrasound assisted method in synthesis of nZVI

This method that was first synthesized by ref. 36 can be used to enhance either the physical or chemical methods. This method like the chemical method uses reducing agents like sodium borohydride and ammonium hydroxide to produce small, uniform and equal-axe grains of iron nanoparticle of average size of 10 nm.¹²

4Fe²⁺_(aq) + BH₄⁻_(aq) + 7NH₄OH → 4Fe⁰ + H₃BO_3(aq) + 7NH₄⁺_(aq) + 4H₂O (9)

The morphology of the nanoparticles depends on the frequency of the ultrasound that is used to assist the chemical reduction method in defining the shape of the particles.³⁸

Precision milling

Precision milling method consists of applying mechanical force to crush micro iron with steel shot in a high speed rotary chamber for about 8 h without any chemical to achieve highly reactive nanoparticles of diameter 10–50 nm and surface area 39 m² g⁻¹. Upon contact with the steel shot, the particles are deformed and cracked producing nanoparticles with irregular shapes and with strong tendency to aggregate³⁵ because of its higher surface energy.³⁴

Electrochemical

This is a cheap and fast method in which cathodes are used to attract Fe²⁺/Fe³⁺ ions from solution by help of electric current. Cationic surfactants are used to act as a stabilizing agent and ultrasonic waves (20 kHz) as a source of energy necessary for fast removal of iron nanoparticles from the cathode:⁴⁰

Cathode: Fe³⁺ + 3e⁻ + stabilizer → nFe⁰ (10)

The electrochemical method can be augmented with ultrasound to produce particles of diameter 1–20 nm and specific surface area of 25.4 m² g⁻¹.⁴²

Green synthesis

Plants. This is an inexpensive and environmentally friendly procedure where highly water soluble, less toxic, and biodegradable plant extracts are used to reduce iron to nanoscale. The extract is heated in water close to the boiling point and then mixed with the iron ion solution causing the irons to be reduced to nZVI in presence of polyphenols.⁴³ The extracts serves two purposes, first as a reducing agent and then as a capping agent for Fe. The capping polyphenols further prevents oxidation as they are potential antioxidants and can also scavenge free radicals.⁴⁴ The produced nZVI are spherical and range from 5 to 15 nm in size.⁴³ However, one of the major concerns of this method is the destruction of plants and its parts.⁴⁰

Bacterial. Green synthesis can also be achieved by using iron reducing bacteria such as Acidiphilium spp. Bacteria of the genus Acidiphilium known to be able to reduce Fe³⁺ to Fe²⁺ under both aerobic and anaerobic conditions as well as in acidic and iron-rich environments.⁴⁵ This reduced species of iron including Fe⁰ have been confirmed by Mössbauer spectroscopy.⁴⁶

Table 3 gives a summary of the different synthesis methods of nZVI that have been described including the pros and cons of each.

Table 3 Comparison of different synthesis methods of nZVI

Method	Advantages	Disadvantages
Liquid-phase reduction or borohydride reduction	Nanoparticle produced are very reactive	The reducing agents used can be toxic
	Synthesis procedure and methodology is easy to replicate
	Cost effective
Ultrasound assisted method	Simultaneous oxidation reaction occurring during synthesis	The reducing agents used can be toxic
	Possible for producing very small sized particles	Extra reagents are required to prevent oxidation of the nanoparticles
		Particles produced are of different morphologies
Precision milling	No toxic reagent needed	Long milling time is required
	Ability to produce easily on large scale	Nanoparticle size and shape produced are irregular
		Higher tendency of particle aggregating
Electrochemical	Short duration required	Higher tendency of particle agglomerating
	Cost effective method
	Adjusting the current density can control the particle size
Green synthesis	Environmentally friendly method	Possible competition with and destruction of food crops
	Easier to replicate on large scale	Long duration required for bacterial based synthesis
	Cost effective	Greater expertise required
Thermal decomposition	Possible to control particle size and morphology	Higher energy requirement
	Increased yield	Long duration

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The use of nZVI particles have proven to be an effective remediation technique for the reduction of chlorinated hydrocarbons including PCE, TCE, and PCBs using the dehalogenation pathway,^2,47 dense non-aqueous phase liquid (DNAPL) contaminants in aquifers,⁴⁸ metallic and metalloid contaminants, including Pb²⁺, Ni²⁺, Co²⁺, Cu²⁺, AsO₄³⁻ and AsO₃³⁻.² However, like any technique there exist some challenges to their application.

Challenges of nZVI

The application of nZVI works best under anaerobic conditions since it is otherwise quickly transformed to iron oxides under aerobic conditions.⁴⁹ The large surface area of the nZVI is the strength of this technique as the degradation reaction occurs on the surface of the particles. Any modification to the surface such as oxidation would therefore affect the performance of the particles. Some of the major challenges identified with the use of nZVI are the rapid aggregation of the particles, passivation (quick oxidation by non-target compounds), sorption to aquifer materials and rapid sedimentation that consequently limits the mobility of nanoparticles in the aquatic environment.^6,37 This also makes long term storage of nano iron particles materials not feasible as they must be used soon after production unless they are stabilized on a support to inhibit oxidation.⁵⁰

One reason for its rapid sedimentation and high settling velocity would be the density of the iron particle which is 7800 kg m⁻³ coupled with the extremely short settling distances in aquifers.³⁷ For instance in groundwater, they migrate a few inches to a few feet.⁵¹ The surface charge or zeta (ζ) potential of the iron nanoparticles influences their suspension stability and mobility in a particular matrix. Aquifer materials mostly have negative charges on their surface under typical groundwater conditions of neutral pH, thus iron nanoparticles with positive charges at pH lower than 8.3 are attracted to them which reduces their mobility.^8,52

Also, once the particle concentration is increased, there is increase in intrinsic magnetic moment leading to strong magnetization which eventually reduces its surface area and reactivity. Aside the magnetic forces, dipole–dipole attraction may occur between the magnetic moments of the particles thus affecting their size and dispersion stability and so settle into aquifer media pores.^14,15,53 When suspensions are highly concentrated with low viscosity, the kinetics of agglomeration is very high and Brownian motion is responsible for bringing particles within the small range of the interaction forces.⁵² The relatively high ionic strength of groundwater makes it favorable for the reduction of electrostatic repulsion between particles and increase of colloidal aggregation in water.^6,37,54

Carriers used for nZVI and their synthesis

To overcome the challenges outlined in the section above, there would be the need to alter the surface charge of the iron particles.¹⁰ Two solutions proposed are electrostatic repulsion (imparting or increasing the surface charge) and steric stabilization (adsorption of long chain organic molecules).^6,37,54 Electrostatic repulsion introduces strong long ranged repulsive forces needed to overcome the magnetic attraction between the iron nanoparticles. Steric stabilization on the other hand pre-coats the iron nanoparticles with hydrophilic polymers whose long loops and tails extend out into solution.⁵⁵ The coats used can be organic or inorganic molecules but practically it is worthy that the protection shells do not only stabilize the nano particle but also serve as functionalization.⁵⁶ The stabilization of the metal on a support thus prevents aggregation of the metal and further leaching.^47,57 Another solution proposed is the coupling of the nZVI with other metals like palladium or nickel to form a bimetallic system.⁵⁸

Organic substances

Monomers, surfactants and polymers. Organic compounds may be used as a support or carrier to passivate the surface of the nano iron particles during or after the synthesis procedure to limit their agglomeration. The carriers used are linked to the nano particles by chemical bond and may not only provide sufficient repulsive interactions to prevent agglomeration but also provides reactive functional groups such as aldehyde groups, hydroxyl groups, carboxyl groups, amino groups, etc. The organic substances used can be grouped under three categories; hydrophobic, hydrophilic and amphiphilic. Hydrophobic organic substances such as fatty acids and alkyl phenol have little affinity for water while hydrophilic substances like ammonium salt, polyol and lysine have strong attraction for water. Amphiphilic substances such as sulfuric lysine on the other hand exhibit both hydrophobic and hydrophilic properties.⁵⁶

Iron particles of hydrophobic supported functional groups form long-chains. Those supported on hydrophilic groups however have a better dispersion. The oil-soluble support can be transformed into water-soluble type functionalized form by ligand-exchange reaction.⁵⁹

Polymers used can either be natural or synthetic. It may be used as a support to compensate or augment functional group deficit provided by monomers. Coating the surface of the iron particle with a polymer increases the repulsive forces to balance the magnetic and the van der Waals attractive forces acting on the nano particles. The functional groups introduced by the polymer would also decrease the saturation magnetization value of iron particle.⁵⁶ Some examples of monomers, surfactant and polymers that have been used as carriers, their respective method of synthesis and the characterization results obtained are presented below:

Polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A). PV3A is a nontoxic and biodegradable surfactant with molecular weight within the range of 4300–4400 (Fig. 2).

Fig. 2 Molecular structure of polyvinyl alcohol-co-vinyl acetate-co-itaconic. This figure has been adapted from ref. 37 with permission from Elsevier.

From Transmission Electron Microscopy (TEM) images, the bare nanoparticles are spherical and form a chain-like aggregate with median and mean size of 59.4 nm and 105.7 nm respectively (Fig. 3a). In contrast, PV3A-stabilized iron particles (Fig. 3b) show an even distribution with little aggregation and median and mean size of 7.9 nm and 15.5 nm respectively. Addition of PV3A alters the surface charge on the iron nanoparticle to predominately negative charges over a much wider pH range probably due to the dissociation of the carboxylic acid groups on the PV3A molecules, and as a result increases the surface potential and stability.

Fig. 3 TEM images of: (a) iron particles prepared with the sodium borohydride method and (b) iron particles stabilized with PV3A. The scale bar for both images is 200 nm. This figure has been adapted from ref. 37 with permission from Elsevier.

Polyvinylpyrrolidone (PVP). The synthesis of PVP-nZVI is done by chemical process using NaBH₄ aqueous solution. The average diameter of the PVP-nZVI particles is 50–80 nm (Fig. 4). These particles tend to aggregate to form chain structures, as a result of both their magnetic properties and their tendency to remain in the most thermodynamically favorable state.¹⁵

Fig. 4 TEM image of PVP-nZVI. This figure has been adapted from ref. 15 with permission from Springer.

Polyacrylic acid (PAA). The PAA-nZVI synthesis is achieved by chemical processes using NaBH₄ aqueous solution by adding 8% PAA (v/v) to iron precursors so that iron complexes would form through carboxylate ligand interaction. The particles produced have a diameter of about 9–15 nm with an average of 12 nm.⁶⁰
Ethyl acrylate & 1,7-octadiene copolymer. A copolymer in the form of beads is prepared by mixing ethyl acrylate and 1,7-octadiene before adding to benzoyl peroxide. The mixture is then suspended in an aqueous phase of gum-Arabic and gelatin under mechanical stirring at a temperature of 80 °C. The subsequent copolymer-nZVI synthesis is done by sonication. The particles sizes of the product ranges from 25–75 nm with some overlaps (Fig. 5).

Fig. 5 Scanning electron microscope image of synthesized iron nanoparticles (a) at 10 μm and (b) at 0.5 μm. This figure has been adapted from ref. 61, open access (Hindawi Publishing corporation).

Guar gum. Guar gum is a nontoxic biodegradable green polymer polysaccharide of high molecular weight. Iron nanoparticles that are stabilized on guar gum are much smaller with no aggregation even at very high ionic strength. Guar gum effectively reduces the hydrodynamic radius of bare nanoparticles from 500 nm to less than 200 nm and prevents aggregation of nanoparticles even at very high salt concentrations (0.5 M NaCl and 3 mM CaCl₂).⁵⁵
Xanthan gum. Xanthan gum is a polymer with the ability to increase the velocity of a liquid and thus attain kinetic stabilization. The xanthan gum-nZVI synthesis is done by ultra-sonication. 6 g L⁻¹ of xanthan gum gels is able to avoid the sedimentation of 30 g L⁻¹ and the aggregation of 15 g L⁻¹ iron nanoparticles over a period of 10 days.⁶²
Polystyrene. Polystyrene is a porous polymer that may be able to remain stable under reaction conditions of higher temperatures. The polystyrene-nZVI synthesis is achieved by thermal reduction process. TEM images show a well dispersed nano Fe particles with average size of 44 nm.²⁶
Polyvinylpyrrolidone (PVP-K30). Polyvinylpyrrolidone (PVP-K30) is water-soluble, low-cost, environmentally friendly and used commonly in food processing. The PVP-K30-nZVI synthesis is done following the chemical reduction process using NaBH₄. The composite particles has a surface area of 36.90 m² g⁻¹ and size range of 10–40 nm in diameter.⁶³
Poly(methylmethacrylate) (PMMA)/anisole co-modified nZVI (PnZVI). The PMMA/anisole support is prepared by dissolving PMMA particles in anisole. The PMMA/anisole-nZVI synthesis is done by chemical reduction process using NaBH₄. The particle specific surface area of the composite is 42.32 m² g⁻¹.⁷
Polystyrene resins. nZVI was encapsulated within porous polystyrene resins functionalized with –CH₂Cl and –H₂N⁺(CH₃)₃ respectively. The synthesis was done by the chemical reduction process using NaBH₄. The particle size was in the range of 5–20 nm (ref. 64 and 65) using the similar procedure synthesized nZVI on PolyFlo resin.
Carboxymethyl cellulose (CMC). CMC is a biodegradable food-grade ingredient which is formed by modifying cellulose by replacing the native CH₂OH group in the glucose unit with a carboxymethyl group (Fig. 6). CMC can accelerate nucleation of Fe atoms during the formation of nZVI. The composite products have a bulky negatively charged layer which provides electrosteric stabilization, thus a combination of electrostatic stabilization and steric stabilization.⁶⁶

Fig. 6 The molecular structure of CMC. This figure has been adapted from ref. 67, open access (AUTEX RJ).

The CMC-nZVI synthesis is done following the chemical processing using NaBH₄. The well dispersed composites are spherical in shape and with sizes ranging from 20 to 100 nm (Fig. 7).⁶⁶

Fig. 7 TEM images of Fe⁰ nanoparticles prepared (a) without CMC, and (b) with CMC. This figure has been adapted from ref. 66 with permission from Elsevier.

Inorganic substances

Alternatively, the use of inorganic compounds as support or carrier can greatly reduce the oxidation rate of nano iron particles. Some of the most common inorganic substances used include silica, metal, nonmetal, metal oxides, and sulfides. Other advantages of the use of inorganic support are:⁵⁶

• The coating provides both stability and serves as a barrier to limit inter-particle interactions which can result in agglomeration.

• This composite nano particles possess an improved biocompatibility, hydrophilicity and stability.

• The technology of preparation for size tunable composite nano particles is already mature, and it is quiet easy to control the variation of the shell.

Some examples of inorganic supports include:

Clay. Materials that become plastic when mixed with a small amount of water are often referred to as clay. The two most important and common clay minerals are kaolin and montmorillonite. Their structural configuration, which helps them to hold exchangeable cations and anions on their surfaces, makes them good materials for immobilization of pollutants either by ion exchange mechanism or adsorption. The ions held on their surfaces include Ca²⁺, Mg²⁺, H⁺, K⁺, SO₄²⁻, Cl⁻, PO₄³⁻. Other reaction mechanisms for holding pollutants may include H-bonding, van der Waals interactions or hydrophobic bonding arising from either strong or weak interactions.⁶⁸

Kaolin. Kaolin is a 1 : 1 structured clay consisting of a tetrahedral sheet of SiO₄ and an octahedral sheet with Al³⁺ as the octahedral cation.⁶⁸ Kaolin clay is formed as a result of the alteration of alumosilicates (feldspar, feldspathoid, spodumene, sillimanite) and volcanic glasses, sometimes altered by acidic hydrothermal solutions.⁶⁹ The kaolin-nZVI synthesis is done following the chemical reduction process using NaBH₄. The produced nZVI has an average size of 44.3 nm and a mean specific surface area of 26.11 m² g⁻¹. The mean surface area value obtained is 7.5 times larger than kaolin alone (3.67 m² g⁻¹) (Fig. 8).⁷⁰ Using the same support material and synthesis procedure,⁷¹ obtained a BET and Langmuir surface areas of 9.6 and 34.8 m² g⁻¹ respectively and sizes between 10 and 80 nm.

Fig. 8 SEM scanning images of samples. (a) Kaolin (k); (b) nZVI; (c) and (d) k-nZVI. This figure has been adapted from ref. 70 with permission from Elsevier.

Hexadecyl trimethylammonium modified montmorillonite (HDTMA-Mont). The support is prepared by dissolving known grams of montmorillonite in deionized water by ultrasound for 15 min. A solution of HDTMA is subsequently added under magnetic stirring for 3 h at 60 °C. The synthesis of the HDTMA-Mont-nZVI is by chemical process using NaBH₄ aqueous solution. The composite product has specific surface area of 38.1 m² g⁻¹ (Fig. 9).

Fig. 9 TEM images of: (a) unsupported iron nanoparticles, (b) HDTMA-Mont/iron particles: iron nanoparticles supported by HDTMA-Mont, (c) Mont/iron: iron nanoparticles supported by montmorillonite and (d) Mont: montmorillonite particles. This figure has been adapted from ref. 72 with permission from Elsevier.

Bentonite. Bentonite is a traditional low-cost efficient adsorbent, with chemical and mechanical stability, high adsorption capability and unique structural properties. The bentonite-nZVI synthesis is done following the chemical reduction process using NaBH₄. The synthesized particles has diameters in the range of 20 and 90 nm and surface area of 39.94 m² g⁻¹ (Fig. 10).⁷³

Fig. 10 SEM images of laboratory synthesized iron particles with and without a support material. (a) nZVI (b) B-nZVI before reaction with Cr(VI) solution (c) B-nZVI after reaction with Cr(VI) solution. The scale bar for all images is 500 nm. This figure has been adapted from ref. 73 with permission from Elsevier.

Zeolite. Zeolites are microporous aluminosilicate minerals commonly used as adsorbents for removing several pollutants. The zeolite-nZVI synthesis is done following the chemical reduction process using NaBH₄. The mean surface area of the composite is 80.37 m² g⁻¹, compared to 12.25 m² g⁻¹ for nZVI and 1.03 m² g⁻¹ for the zeolite alone (Fig. 11).⁷⁴

Fig. 11 SEM images of (a) nZVI particles and (b) zeolite. This figure has been adapted from ref. 74 with permission from Elsevier.

In a similar synthesis using NaY zeolite as the support,⁷⁵ produced spherical composite particles with diameters in the range of 50 to 100 nm.

Sepiolite. Sepiolite is a soft white clay mineral which is stable in air; it has a good dispersibility in water, cost-effective and with good adsorption properties. The sepiolite-nZVI synthesis was done following the chemical reduction process using NaBH₄. The TEM images of the product show nZVI with particle size range of 20 to 60 nm and with little aggregation (Fig. 12).⁷⁶

Fig. 12 TEM images of (a) nZVI, (b) sepiolite-supported nZVI.⁷⁶ This figure has been adapted from ref. 66, free access (Atlantis press).

Marine clay. Marine clay is found in coastal regions around the world. The marine clay-nZVI synthesis is done following the chemical reduction process using NaBH₄. The product obtained is composed of homogenous rounded particles with size range from 29 to 57 nm.⁷⁷

Corals. Coral collected from Persian Gulf coast is prepared by cleaning, crushing and drying at 100 °C before being sieved through a 100 mesh. The synthesis of the nZVI is by chemical process using NaBH₄ aqueous solution. The sieved coral is added before NaBH₄ during the synthesis procedure. A scanning electron microscope (SEM) image (Fig. 13) shows that most of the sheet structure is changed to irregular small particles with size range of 53–94 nm and a surface area of 16.85 m² g⁻¹.⁷⁸

Fig. 13 SEM of nano iron particles synthesized on coral. The scale bar is 500 nm. This figure has been adapted from ref. 78, open access (CEST).

Oyster shells. Oyster shells collected from the coast of the Persian Gulf is prepared by washing in deionized water, crushed and dried in 100 °C oven before sieving with a mesh of 100. The oyster shell-nZVI synthesis is done following the chemical processing using NaBH₄.

Biochar (BC). Biochar is derived from a variety of cost effective and readily available biomass materials such as wood, leaves, manure and agricultural wastes through the pyrolysis. In addition, it does not need a treatment for further activation like active carbon.^79,80 The preparation of the material is done by mixing biochar (<100 mesh) with 1 M HCl (1/20, v/v) and shaken overnight at room temperature for demineralization. It is subsequently purified using dialysis method until the solution pH is close to neutral, and dried at 80 °C in an oven. The BC-nZVI synthesis is done by chemical process using NaBH₄ aqueous solution. A Brunauer, Emmett and Teller (BET) images after synthesis show a surface area of 142.80 m² g⁻¹. TEM images shows nZVI nanoparticles appeared to be dispersed on the surface and the boundary sites of biochar (Fig. 14)⁸¹ used chitosan, a renewable transformed polysaccharide that can be obtained from natural chitin as an ‘organic glue’ to cement nZVI on BC. Chitosan is an abundant biopolymer obtained by the alkaline deacetylation of chitin (a polymer made up of acetylglucosamine units).⁵⁷ The BC-nZVI synthesis is done by chemical process using 1.2% NaOH aqueous solution.

Fig. 14 (a) Transmission electron microscope and (b) X-ray diffraction images of nZVI, BC and nZVI/BC. This figure has been adapted from ref. 79, open access (Plos One).

Carbon black. Carbon black is a low cost and abundant waste product from the fossil fuel industry. The carbon black-nZVI synthesis is achieved by carbothermal synthesis at 700 °C for 4 h. The produced particles are of surface area 108.67 m² g⁻¹ and size range of approximate 20–150 nm.⁸² Composite of carbon functionalized nano iron particles have higher chemical and thermal stability as well as biocompatibility.⁵⁶

Cationic exchange membrane (CEM). The CEM used consists of hydrophilic polysulfone on polyester support with a pore size of 0.45 μm. The CEM is first prepared by soaking it in 0.2 M Fe(III) solution overnight before washing with distilled water. The subsequent CEM-nZVI synthesis is done by chemical process using NaBH₄ aqueous solution. The produced nZVI are spherical in shape with size range of 30–40 nm and surface area of 1.90 m² g⁻¹ (Fig. 15).

Fig. 15 SEM images of ZVI-immobilized Fe⁰-CEM. This figure has been adapted from ref. 58 with permission from Elsevier.

Calcium (Ca)–alginate beads. Calcium (Ca)–alginate is one of the most common cost effective materials for immobilizing living cells in food and for drug delivery. The Ca–alginate-nZVI synthesis is done by chemical process using NaBH₄ aqueous solution. TEM images indicate nZVI of particle size ranges from 10 to 100 nm with an average size of 35 nm. A higher magnification TEM image shows a 2.5 nm of oxide shell around the nZVI core (Fig. 16).

Fig. 16 (a) An alginate bead with entrapped nZVI, (b) SEM image of alginate bead. (c) SEM image of alginate bead surface after nZVI entrapment. (d) Higher magnification of SEM image (b), (e) TEM image of Ca–alginate bead section shown in (d), (f) blow-up image of the circled area in image (e). This figure has been adapted from ref. 53 with permission from Elsevier.

Silica fume. Silica fume is a cheap, non-toxic and abundant materials produced during silicon metal production from a blast. The silica fume-nZVI synthesis is done following the chemical reduction process using NaBH₄. The iron particles produced are spherical with size ranging from 20 to 110 nm.⁸³

Graphene oxide (GO). GO obtained from cheap natural graphite is strongly hydrophilic due to the presence of carboxyl, hydroxyl and epoxide groups on its nano sheets. The GO-nZVI synthesis is done following the chemical reduction process using NaBH₄. The produced nano composites have particle size of about 5 nm (Fig. 17).⁸⁴

Fig. 17 TEM images of (a and b) GO, (c and d) Fe and (e and f) composite ration 1 : 5. This figure has been adapted from ref. 84 with permission from Elsevier.

Multi-walled carbon nanotubes (MWCNTs). Multi-walled carbon nanotubes (MWCNTs) used as a porous-based support material for the nZVI nanoparticles are synthesized by chemical reduction process using NaBH₄. The composite particle has a polygon shape with diameter range between 20–80 nm. However, aggregation of nZVI particles is still observed, but much decreased relative to the unsupported nZVI nanoparticles⁸⁵ (Fig. 18).

Fig. 18 (a) SEM and TEM images of MWCNTs, (b) SEM and TEM images of nZVI supported on MWCNTs before reaction, (c) SEM images of nZVI supported on MWCNTs after reaction. This figure has been adapted from ref. 85 with permission from Elsevier.

Other carriers like poly(styrene sulfonate), carboxymethyl, polyaspartate⁸⁶ in addition to those already mentioned have proven their efficiency in reducing aggregation and enhancing nZVI mobility. Those that have been used for depollution purposes have also shown to be more effective compared to the bare nZVI.

In as much as it may be advisable to synthesize nZVI on a support in order to reduce aggregation and in effect improve its effectiveness, care must be taken not to add up more pollution and risk to the environment. The reason for a choice of a carrier should be well defined. Table 4 compares several of these carriers and accesses the advantages and disadvantages they carry when used during nZVI synthesis.

Table 4 Comparison of the different nZVI carriers

Carrier	Method of synthesis	Advantage	Disadvantage	Functional groups	Reference
Polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A)	Chemical reduction	Non toxic	Carrier covers large fraction of reactive surface of the nZVI and can thus affect reactivity especially if there is no evidence of sorption or partitioning between nZVI and the pollutant(s)	–OH, –CO–, and –COOH	37
		Biodegradable surfactant mainly due to the presence of functional groups such as –OH, –CO–, and –COOH
		86% reduction in average size of nZVI particles
		Reduction in the zeta (ζ)-potential at neutral pH
		A shift of the isoelectric point (IEP)
		No observed sedimentation of stabilized nZVI
		Presence of carrier presents both steric and electrostatic repulsions
Polyvinylpyrrolidone (PVP)	Chemical reduction	Cost effective material	PVP-nZVI has an amorphous structure	C=O	15
		80% reduction in average size of nZVI particles	The presence of excess PVP can serve as a bridge between discrete PVP-nZVI particles and increase the sedimentation time
		Reduced sedimentation time
		Effective even in porous soil media treatment
		Reduced zeta potential than other carriers such as CMC and PAA
Polyacrylic acid (PAA)	Chemical reduction	Aside the partial bidentate bridging of PAA to nZVI, it also forms a gel network through hydrogen bonding to trap colloidal particles to stabilize dispersions. However the presence of this gel can be destroyed with increase in concentration of cations like Ca	PAA-nZVI forms poorly ordered and amorphous structures	–OH, –COO–, and C–H	60
Ethyl acrylate & 1,7-octadiene copolymer	Chemical reduction and sonication	Acts as a cation exchanger and thus enriches the cationic byproduct of the aqueous media	The carrier being a weak acid support has a lower reactivity than strong acid supports	C=O and C–H	61
		Capable of adsorbing trivalent ions
Guar gum	Sonication	Cost effective material	Sedimentation occurs after few hours	O–H	55
		Non-hazardous green (of natural origin) material
		Water soluble and biodegradable
		60% reduction in average size of nZVI particles
		Higher molecular weight polymers provide strong long-ranged steric repulsion required for stabilization
Xanthan gum	Ultrasonication	Non-hazardous and biodegradable	N.A	C=O	62
		Possibility of stabilizing for long time (more than 10 days)
		It has wide stable velocity making it simple to dose even on the field
		The polymer is arranged in a network structure making it possible for it to trap colloidal particles and increase stabilization
Polystyrene	Thermal decomposition	Ability to remain stable even at higher temperatures (250–300 °C)	N.A	C=O and C–H	26
Polyvinylpyrrolidone (PVP-K30)	Chemical reduction	Higher adsorption capacity to adsorb both pollutant and degraded by-products	Adsorption of by-products means that toxicity may be spread from the plume site if the by-products are toxic	C=O, C–N and C–H	63
		Cost effective material
		Non-hazardous and environmentally friendly
		Water soluble and biodegradable
Poly(methylmethacrylate) (PMMA)/anisole co-modified nZVI (PnZVI)	Chemical reduction	Reduce significantly surface oxidation of nZVI	Reduced reactivity of the nZVI due to blocking of reactive site by carrier	C=O, –COO– and C–H	7
Polystyrene resins (–CH2Cl and –H2N^+(CH3)3)	Chemical reduction	The presence of ammonium group helps in reducing nZVI particle size	Excess layers of carrier can reduce reactivity	C–N and C–H	64
Carboxymethyl cellulose (CMC)	Chemical reduction	Provides steric stabilisation Environmentally friendly material	The formation of surface passivation layer on the nZVI reduces the reactivity	–CO–, –COOH and OH	66
		Non-toxic and water soluble
Kaolin	Chemical reduction	Acts as natural scavengers	Reduces the specific surface area of nZVI		70 and 71
		Higher mechanical and chemical stability	Edge sites contain more nZVI particles than the surface sites of the clay mineral
		Higher cation exchange capacity
		Increases the reactivity of nZVI
Hexadecyl trimethylammonium modified montmorillonite (HDTMA-Mont)	Chemical reduction	Abundant, cost effective and environmentally friendly material	An additional surfactant like hexadecyl trimethylammonium may be needed to increase reactivity		72
		Has an increased specific surface area and average pore width
Bentonite	Chemical reduction	Traditionally abundant and cost effective absorbent	Reaction products are deposited on the surface of B-nZVI which consequently decreased the activity of the nZVI		73
		Higher mechanical and chemical stability
		Unique structural properties
Zeolite	Chemical reduction	Higher sorption capacity for inorganic pollutants	N.A		74
		85% increase in mean surface area
		Metallic pollutants removed are incorporated into the internal matrix of the composite making them non exchangeable and non-bioavailable
Sepiolite	Chemical reduction	Cost effective material	N.A		76
		Good stability in air
		Good dispersion in water and with equally good adsorption properties
Corals, oyster shells	Chemical reduction and wet impregnation	Cost effective and abundant material	May not be ideal for treating hydrophilic pollutants		78 and 87
		Good stability in air
Biochar (BC)	Chemical reduction	Cost effective and readily available material	Excess biochar loading on the nZVI particles may block reactive sites thereby reducing reactivity		79
		Higher adsorption capacity and stable structure
		It does not need a treatment for further activation
		85% increase in surface area
Biochar + chitosan (organic glue)	Chemical reduction	Polysaccharide is biodegradable and non-toxic	Chitosan decomposes at much lower temperature than pyrogenic carbon		81
		Chitosan has the tendency to dissolve in acid solution and precipitate from basic solution
		Chitosan is nontoxic, hydrophilic, biocompatible and biodegradable polymer
Carbon black	Thermal decomposition	Low cost and abundant material	Higher temperature of about 700 °C for over 4 h is required to have a higher surface area. This may increase production cost		56 and 82
		Higher chemical and thermal stability
Cationic exchange membrane (CEM)	Chemical reduction	Release of metallic pollutants from the support is very minimal	N.A		58
Calcium (Ca)–alginate beads	Chemical reduction	The porosity of the composite allows solutes to diffuse into the beads and come in contact with the entrapped cells	Heterogeneity of pore size results in nanoparticle being entrapped more at some places (agglomeration) than others		53
			Slight reduction in the reactivity of the nZVI
		Non-toxic and biodegradable
Silica fume	Chemical reduction	Non-toxic, abundant and cost effective material	N.A		83
		Higher surface area to homogenously disperse nZVI
		The dominant negatively surface is useful for migration through the soil
Graphene oxide (GO)	Chemical reduction	90% reduction in particle size	N.A		84
		Good thermal and mechanical properties
Multi-walled carbon nanotubes (MWCNTs)	Chemical reduction	nZVI acts as a micro-anode and MWCNTs as a micro-cathode which prevents the formation of oxide film on the surface of the nZVI	N.A		85

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Generally, the choice of a carrier whether organic or inorganic is influence by;

(1) Availability of the product.

(2) Cost of the carrier.

(3) Environmental friendliness of the carrier (biodetgradability).

(4) The intended use or target pollutant to be treated.

Aside the above mention general factors that affect the choice of carriers, the physical and chemical properties of the carrier are also considered. Organic carriers for example are chosen to alter the surface charge of nZVI particles. These surface charges provide both electrostatic stabilization and steric stabilization and thus reduce aggregation. On the other hand, inorganic carriers have naturally good adsorption properties, surface ions, and consequently exhibit good ion exchange mechanisms.

The TEM images show that nZVI are well dispersed on their respective carriers compared to their bare forms. This then confirms that the use of carriers reduces aggregation and preserves the smaller size and large surface areas of nZVI which is the heart of their performance. However the kind of carrier used and the pollutant to be treated would affect the output efficiency during remediation. Table 5 summarizes different carriers, the pollutants they have been used to treat and the results obtained. It wasn't very clear the reasons that informed these studies to choose one particular carrier over the other. Since the carrier would modify the properties of the nZVI including its toxicity in the environment, it is very important to access both advantages and disadvantages of the intended carrier in relation to the pollutant to be treated before selection.

Table 5 Composite products used for pollution remediation

Support material	Optimum ratio (support–nZVI)	Pollutant	Initial concentration of pollutants (mg L^−1)	Duration (min)	Efficiency (%)	References
Coral	NA	Humic acid	2	90	84.5	78
Biochar	5 : 1	Methyl orange	60	30	98.5	79
Biochar–chitosan	1 : 1 : 2	Pb	NA	NA	93	81
		Cr			38
		As			95
Granular activated carbon (GAC)–Pd	NA	2-Chlorobiphenyl (2-ClBP)	4	1152	90	88
Green tea (Camellia sinensis) polyphenols	NA	Bromothymol blue	580	60	34	89
Cationic exchange membrane	NA	Trichloroethylene (TCE)	59.9	1080	80	58
Calcium (Ca)–alginate beads	NA	Nitrate (NO3^−)	100	120	73	53
Magnesium-aminoclay (MgAC)	7.5 : 1	Nitrate (NO3^−N)	100	30	75	90
Carboxymethyl cellulose (CMC)	NA	Perchloroethylene (PCE)	9.6	360	80	47
		Trichloroethylene (TCE)	15	180	100
Kaolin	5 : 1	Pb^2+	500	20	96	70
Kaolin	5 : 1	Pb^2+	12.75	60	98.8	13
		Total Cr	71		99.8
		Cu^2+	27.25		69.5
		Zn^2+	216.45		28.1
		Ni^2+	8		23.0
Kaolin	0.2 : 1	Cu^2+	100	1440	99	71
		Co^2+			98
Oyster shell	NA	Humic acid	5	120	94.6	87
Cellulose acetate	NA	Trichloroethylene	80	2880	53	50
Chitosan	NA	Cr(VI)	40	180	74.03	91
Bentonite	1 : 1	Cr(VI)	75	240	100	73
		Pb^2+	13		100
		Cu^2+	34		92.7
		Zn^2+	288		59.4
Zeolite	NA	Pb^2+	100	140	96.2	74
NaY zeolite	NA	Potassium acid phthalate (KHP)	425	120	79	75
Sepiolite	NA	Decabromodiphenyl ether (BDE-209)	NA	23 040	48.64	76
HDTMA-Mont	NA	Cr(VI)	50	350	82	72
Silica fume	NA	Cr(VI)	40	120	88	83
Graphene oxide	5 : 1	Methyl blue	0.5	15	99	84
Carbon black	NA	Uranium	580	10 080	98	82
Multiwalled carbon nanotube (MWCNT)	2 : 1	Cr(VI)	20	120	98	85
Carboxymethyl cellulose (CMC)	5 : 1	Cr(VI)	8	60	98	66
Polystyrene resins	NA	Nitrate	50	750	97.2	64
Polyvinylpyrrolidone (PVP-K30)	NA	Tetracycline (TC)	35	240	93	63
Polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A)	NA	Trichloroethene (TCE)	7	180	99	37
Polysorbate 20 (Tween® 20)	NA	As(III)	1	5760	100	92
Fly ash	NA	Phosphate ion	2	50	97	93
Starch	NA	Trichloroethene (TCE)	25	60	98	94
		PCB	2.5	60 000	80
Starch	NA	Tetracycline (TC)	500	14 400	99	95
Hydrophilic carbon	NA	Trichloroethene (TCE)	13	180	97	96
PMMA/anisole	NA	Methyl orange	100	60	81.4	7
		Sunset yellow			74
		Acid fuchsine			69
Bamboo	NA	Methylene blue	140	120	79.6	97
Triton X-100 surfactant	NA	Reactive Black 5 (RB5)	500	180	27	98
Cetyl trimethyl ammonium bromide (CTAB) surfactant					35

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It is however noteworthy that the use of carrier may not automatically signify an improved efficiency. Adsorbed polyelectrolyte coatings like poly(styrene sulfonate) (PSS), carboxymethyl cellulose (CMC), and polyaspartate (PAP) have demonstrated their ability in reducing the reactivity of nZVI to degrade TCE in water.⁹⁹ Some surfactant coatings were also found to decrease the ZVI reactivity toward Reactive Black 5.⁹⁸ Additionally PMMA/anisole hybrid coatings demonstrated an inhibition to the discoloration reactivity of nZVI toward organic dyes.⁷ The combinations of carriers do not also necessarily add any synergic effect and can also eventually inhibit the reactivity of the iron particles.¹⁰⁰

Several results from both TEM and SEM showing dispersion and average particle size of nZVI after been synthesized on various carriers have been presented in this study. What is however missing is whether the results are based on the carrier used or on the synthesis method. It would be very interesting to explore this angle using the same carriers but with different synthesis methods. Another part missing in many publications reviewed is the ratio between the carrier used and the nZVI during synthesis. The information is very vital for future replication of similar experiments.

Traceability of nZVI

For a remediation treatment to be effective and fast, the nZVI must be as mobile as possible. The use of carriers to enhance this mobility has proven to work. However in as much as we want to enhance the mobility of nZVI by using carriers, care must be taken not to generate undesirable side-effects specifically on the potential of nZVI to adsorb and transport contaminants away from the primary contamination zone.¹⁰¹ There are strong evidence to buttress this concern as both natural colloids and nZVI have demonstrated the capacity to sorb contaminants.³⁴

It is therefore necessary to track the nZVI used to effectively determine its fate in the environment. This is however a challenge because of the high background of natural iron colloids present in soils and other environmental systems. One suggested solution to this issue would be to label synthesized nZVI to make them easily traceable and to differentiate them from natural sources.³⁴ Very few studies have investigated the fate of synthesized nano particles in the environment though it is estimated that the most released nano particles are titanium dioxide followed by iron and zinc respectively. About 63–91% of the global synthesized nano particles end up into landfills, 8–28% into the soils, 0.4–7% into water bodies and 0.1–1.5% into the atmosphere.¹⁰²

Conclusion

Several carriers have been used with the aim of overcoming challenges like quick agglomeration and oxidation experience in the application of nZVI some of which has achieved very good results. However, the selection and use of a support should be carefully considered because the kind of support material used can decrease the reactivity of the nZVI especially if the optimum balance ratio between the support and the nano iron is not well done. In such instances, the adsorbed surface excess of the thick layers of coating can directly inhibit the mass transfer of target pollutants, block access to reactive surface sites, reduce indirectly the corrosion reaction of the iron particles with water or H⁺ and the electron transfer to target pollutants.^7,99 With the popularity nZVI is gaining in the field of remediation, it is obvious that its use would keep increasing. However some gray areas about this technique still exist especially in the possibility of the carriers used adsorbing pollutants and thus increasing their migration from the plume as well as the toxicity of the injected nZVI. Future researchers need to focus on these unclear areas and seek to provide some answers to these uncertainties.

Acknowledgements

This study was funded by SFR Condorcet-FR CNRS 3417. The authors would like to express their appreciation to the Institut Polytechnique LaSalle Beauvais, France, and Dr Christophe WATERLOT (Equipe Sols et Environnement-LGCgE) (EA 4515) for their support in various ways.

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