24.2.1 Iron Nanoparticles
Iron is one of the most abundant elements on earth. Elemental iron has been used as an ideal candidate for remediation because it is inexpensive, abundant,
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easy to prepare and apply to a variety of systems, and devoid of any known toxicity induced by its usage. The concept of using metals, such as iron, as remediation agents is based on reduction–oxidation or “redox” reactions, in which a neutral electron donor (a metal) chemically reduces an electron acceptor (a contaminant). Nanoscale iron particles have surface areas signifi cantly greater than larger-sized powders or granular iron, which leads to enhanced reactivity for the redox process. As a result, iron nanoparticles have been extensively investigated for the decomposition of halogenated hydrocarbons to benign hydrocarbons and the remediation of many other contaminants including anions and heavy metals [ 3 ].
The most commonly used method to synthesize nanoscale zero-valent iron nanoparticles is based on reduction of FeCl 3 using borohydride [ 4 , 5 ].
Transmission electron microscopy (TEM) micrographs have shown that iron nanoparticles synthesized using the previously mentioned method can range from 1 to 100 nm in size [ 4 , 5 ], specifi cally a study on perchlorate reduction reported iron particles with an average diameter of 57 ± 16nm [ 6 ]. In most cases excessive borohydride is needed to accelerate the reaction and provide uniform growth of iron crystals [ 4 , 5 ]. Zero-valent iron nanoparticles are highly reactive and react rapidly with surrounding media in the subsurface [ 7 ]. A signifi cant loss of reactivity can occur before the particles are able to reach the target contaminant. In addition, zero-valent iron nanoparticles tend to fl occulate when added to water, resulting in a reduction in eff ective surface area of the metal. Therefore, the eff ectiveness of a remediation depends on the accessibility of the contaminants to the nanoparticles; and the maximum effi ciency of remediation will be achieved only if the metal nanoparticles can eff ectively migrate without oxidation to the contaminant or the water/
contaminant interface. To overcome such diffi culties, a commonly used strategy is to incorporate iron nanoparticles within support materials, such as polymers, porous carbon, and polyelectrolytes [ 8– 10 ].
The presence of a secondary metal on iron nanoparticles leads to the formation of bimetallic nanoparticles with novel catalytic activity [ 11 ]. The deposition of the second metal can enhance the reactivity of iron nanoparticles by changing their surface electronic properties [ 12 ]. Such bimetallic nanoparticles are often prepared by coating iron particles with palladium or gold through a reduction and deposition process, experimentally completed by soaking freshly prepared iron particles with an ethanol solution containing 1 wt% of palladium acetate [ 4 ]. The synthesized nanoparticles are extremely reactive and usually aggregate if their surfaces are left unprotected [ 13 , 14 ].
Common approaches used for stabilization of bimetallic nanoparticles include protection by capping ligands, such as polymers or surfactants, and dispersion in solid supports, such as active carbon, metal oxides, zeolites, or polymer fi lms [ 15 ]. However, immobilization of nanoparticles in the solid matrix may increase diff usion resistance [ 15 ]. To overcome this problem, it is best to synthesize and immobilize bimetallic nanoparticles in an open matrix. Micro- fi ltration membranes are of great interest for this purpose because of their open structure and large pore size (100–500 nm) [ 9 ]. Membranes functionalized
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with bimetallic nanoparticles off er great advantages for the catalytic reaction because diff usion limitation can be minimized under convective fl ow. For example, Fe/Pd bimetallic nanoparticles have been incorporated into a polyacrylic acid (PAA) and polyvinylidene fl uoride (PVDF) membrane matrix by ion exchange and subsequent reduction [ 9 ]. The role of PAA as a chelating (ion exchange) polymer and their dechlorination behavior has also been investigated [ 9 ].
24.2.2 Iron Oxide Nanoparticles
Compared with zero-valent iron nanoparticles, iron oxide nanoparticles (Fe 3 O 4 ) have super-paramagnetic properties and are capable of being separated from purifi ed water by application of a magnetic fi eld [ 16 ]. The representative synthesis method involves a one-pot synthesis from iron precursor, oleic acid, and 1-octadecene at high temperature (e.g., 320°C) [ 17 , 18 ]. The synthesized nanoparticles were then dispersed in water with the assistance of a surfactant, Brij 30, and sonication [ 17 ]. After successive washing with water, the Brij 30 was removed using ultra-high- speed centrifugation [ 17 ]. Using this method, monodispersed 12 nm iron oxide particles have been synthesized from iron (III) oxide monohydrate, FeO(OH). Similarly, surfactants and/or polymers have been used to stabilize the nanoparticles and prevent aggregation [ 19 ]. Enhanced stability can also be achieved by encapsulating iron oxide particles within porous silica support [ 19 ]. In this approach, additional functionality can be further provided by the silica surface, allowing for particles to be hydrophobic or hydrophilic, in order to stay sustained in an aqueous or organic phase [ 19 ].
Aerosol processing has been used to encapsulate Fe 3 O 4 nanoparticles (20–40 nm) in silica microspheres [ 20 ]. The aerosol process initially atomizes a solution of iron and a silica precursor into droplets. The droplets are then dried and solidifi ed forming nanoparticles that are collected into a fi lter [ 20 ].
Iron oxide has also been incorporated into membrane supports for various applications. Membrane technology is considered to be an eff ective alternative to conventional water treatment for the removal of particles, microorganisms, and organic matter [ 21 ]. Ceramic membranes combined with ozonation generate very high and stable permeate fl uxes without causing membrane damage [ 22 ].
In addition they have been shown to achieve complete removal of fecal coliforms and E. coli , eff ectively disinfecting water [ 23 ]. Incorporating iron oxide with catalyzed ozonation and membrane fi ltration will improve inactivation and removal of bacteria from various water sources [ 17 ].
Besides the Fe 3 O 4 nanoparticles, hydrated iron oxide nanoparticles are another important class of material candidates that are nontoxic, inexpensive, readily available, and chemically stable over a wide pH range. Compared to crystalline forms of iron oxyhydroxide (namely, goethite, feroxyhyte, and
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lepidocrocite), amorphous hydrated iron oxides have higher specifi c surface area. Since sorption sites reside primarily on the surface, amorphous hydrated iron oxides (referred to HFO) off er the highest sorption capacity on a mass basis. Ligand sorption capacity can be greatly increased by dispersing hydrate iron oxide nanoparticles within polymeric anion exchangers. Ion exchange membranes, such as fi brous ion-exchanger (FIBAN) and hybrid anion exchanger (HAIX), have been used for this purpose. FIBAN is generally constructed from polypropylene (PP) industrial fi bers radiochemically grafted with styrene- divinylbenzene copolymer [ 8 ]. Subsequent absorbance of Fe 3+ and precipitation led to the supported HFO [ 8 ]. Similarly, commercially available strong-base anion exchange resin, IRA-900, was used to prepare HAIX, then Fe(III) ions from ferric chloride were impregnated onto HAIX [ 24 ]. HFO particles provide high sorption affi nity toward dissolved contaminant species, whereas the fi brous polymer matrix provides durability, mechanical strength, and excellent hydraulic and kinetics characteristics of fi xed beds [ 8 ]. Higher adsorption results have been shown on hydrated iron oxides in comparison to small nanoscaled dispersed iron oxide particles.