metal ions (for example, Cr(VI) to Cr(III)) (Powell et al. 1995; Warren et al. 1995;
Deng et al. 1999, 2000; Su and Puls 2001; Litter et al. 2018a and references therein).
ZVI has been recognized as a potential tool for the removal of contaminants owing to
its exceptional properties: it is abundant (iron is the fourth most abundant element in
the earth’s crust), non-toxic, cheap, easy to be produced, and the process requires
little maintenance. Although ZVI is an effective reductant, it offers also good
properties to degrade or oxidize pollutants, when combined with H 2 O 2 in Fentontype processes. ZVI can treat metals, halogenated organics, nitroaromatics, metalloids such as As, nitrate, dyes, phenolic compounds, etc. The use of the technology
has enormously increased in the last years. The first study on the environmental use
of ZVI, describing the kinetics of reduction of Cr(VI) by metallic iron was published
in 1982 (Gould 1982), and the first field application of ZVI in the permeable reactive
barrier (PRB) technology for in situ remediation of contaminated groundwater was
reported in 1991 (O’Hannesin and Gillham 1992). Metallic iron in the form of iron
microparticles (granular or powdered, >0.1 mm in diameter) has been used in packed
bed reactors and PRBs intersecting the contamination plume (Blowes et al. 2000;
Litter et al. 2010). Since then, the use of iron as a reactive material in PRB has been
extensively reported (Khudenko 1985; Senzaki 1991; Gillham and O’Hannesin
1994) (see, e.g., Litter et al. 2014, 2018a; Balko and Tratnyek 1998; Cantrell et al.
1995; Domga et al. 2015; Gillham 1993; Orth and Gillham 1995; Matheson and
Tratnyek 1994; Senzaki and Kumagai 1988). Over the last two decades, the number
of publications has increased exponentially (Fu et al. 2014), including some reviews
(Fu et al. 2014; Noubactep 2008, 2016a, b; Cundy et al. 2008; Guan et al. 2015).
In recent years, the use of nanoparticulate zero-valent iron (nZVI) has been
reported. nZVI possesses inherent characteristics, different from those of the macroscopic or bulk iron forms (Litter et al. 2018a; Kharisov et al. 2016; Crane and Scott
2012; Noubactep and Caré 2010), especially because the reduction of the particle
size of Fe(0) materials from mm to 10–100 nm increases the surface area and thus the
chemical reactivity, leading to improved catalytic properties (Zhang 2003). nZVI
particles exhibit a typical core–shell structure, where the core consists primarily of a
nucleus of zero-valent iron and an external oxide shell, composed of Fe(II) and Fe
(III), formed as a result of the corrosion and oxidation of the metallic iron (Kharisov
et al. 2016). The outer layer acts not only as protection of the nucleus but also as an
adsorption surface for pollutants and participates on the charge transfer processes
between Fe(0) and the compounds in solution (Scherer et al. 1999).
The use of nanoparticles is convenient for in situ applications. In addition, other
metals such as palladium or nickel are usually added to increase the reduction rate,
forming a bimetallic nanometal.
The use of ZVI in a Fenton-type process together with H 2 O 2 for the degradation
of pollutants (known as advanced Fenton process (AFP)) can avoid or reduce the
problems arising in the homogeneous Fenton processes. Oxidation occurs through
iron ions released into solution or via reactions taking place between solutes and
surface-bound species. The working pH can be higher, but the precipitation of iron
hydroxides/oxides can be avoided because few iron ions are present in the aqueous
phase; an easy separation of the catalyst after the application can be done. Although
7 Introduction to Oxidative Technologies for Water Treatment
141
Deng et al. 1999, 2000; Su and Puls 2001; Litter et al. 2018a and references therein).
ZVI has been recognized as a potential tool for the removal of contaminants owing to
its exceptional properties: it is abundant (iron is the fourth most abundant element in
the earth’s crust), non-toxic, cheap, easy to be produced, and the process requires
little maintenance. Although ZVI is an effective reductant, it offers also good
properties to degrade or oxidize pollutants, when combined with H 2 O 2 in Fentontype processes. ZVI can treat metals, halogenated organics, nitroaromatics, metalloids such as As, nitrate, dyes, phenolic compounds, etc. The use of the technology
has enormously increased in the last years. The first study on the environmental use
of ZVI, describing the kinetics of reduction of Cr(VI) by metallic iron was published
in 1982 (Gould 1982), and the first field application of ZVI in the permeable reactive
barrier (PRB) technology for in situ remediation of contaminated groundwater was
reported in 1991 (O’Hannesin and Gillham 1992). Metallic iron in the form of iron
microparticles (granular or powdered, >0.1 mm in diameter) has been used in packed
bed reactors and PRBs intersecting the contamination plume (Blowes et al. 2000;
Litter et al. 2010). Since then, the use of iron as a reactive material in PRB has been
extensively reported (Khudenko 1985; Senzaki 1991; Gillham and O’Hannesin
1994) (see, e.g., Litter et al. 2014, 2018a; Balko and Tratnyek 1998; Cantrell et al.
1995; Domga et al. 2015; Gillham 1993; Orth and Gillham 1995; Matheson and
Tratnyek 1994; Senzaki and Kumagai 1988). Over the last two decades, the number
of publications has increased exponentially (Fu et al. 2014), including some reviews
(Fu et al. 2014; Noubactep 2008, 2016a, b; Cundy et al. 2008; Guan et al. 2015).
In recent years, the use of nanoparticulate zero-valent iron (nZVI) has been
reported. nZVI possesses inherent characteristics, different from those of the macroscopic or bulk iron forms (Litter et al. 2018a; Kharisov et al. 2016; Crane and Scott
2012; Noubactep and Caré 2010), especially because the reduction of the particle
size of Fe(0) materials from mm to 10–100 nm increases the surface area and thus the
chemical reactivity, leading to improved catalytic properties (Zhang 2003). nZVI
particles exhibit a typical core–shell structure, where the core consists primarily of a
nucleus of zero-valent iron and an external oxide shell, composed of Fe(II) and Fe
(III), formed as a result of the corrosion and oxidation of the metallic iron (Kharisov
et al. 2016). The outer layer acts not only as protection of the nucleus but also as an
adsorption surface for pollutants and participates on the charge transfer processes
between Fe(0) and the compounds in solution (Scherer et al. 1999).
The use of nanoparticles is convenient for in situ applications. In addition, other
metals such as palladium or nickel are usually added to increase the reduction rate,
forming a bimetallic nanometal.
The use of ZVI in a Fenton-type process together with H 2 O 2 for the degradation
of pollutants (known as advanced Fenton process (AFP)) can avoid or reduce the
problems arising in the homogeneous Fenton processes. Oxidation occurs through
iron ions released into solution or via reactions taking place between solutes and
surface-bound species. The working pH can be higher, but the precipitation of iron
hydroxides/oxides can be avoided because few iron ions are present in the aqueous
phase; an easy separation of the catalyst after the application can be done. Although
7 Introduction to Oxidative Technologies for Water Treatment
141