Different models have been reported in the literature to explain the electron
transfer from iron core to the surface: (1) a direct transfer in the absence of oxide
layer; (2) a transfer from metal through defects (pits) in the oxide layer; (3) a transfer
from the oxide layer as a semiconductor; and (4) a transfer from metal-to-ligand
(Scherer et al. 1999). It is important to note that iron particles are not selective, and
very low electron efficiency toward pollutant are reported (Schöftner et al. 2015;
Tang et al. 2017a). The direct reduction at Fe
0 core surface requires the diffusion of
the pollutant through the oxide shell, which is possible because of the porous
structure of the initial shell (Crane and Scott 2012; Mu et al. 2017). However, this
phenomenon is impeded by the diffusion of Fe
2+ resulting from iron corrosion in the
opposite direction of the pollutant. As it was reported that shell-bounded Fe
2+ on
different iron oxides is a strong reductant with specific redox properties (Johnson
et al. 1998; Amonette et al. 2000; Elsner et al. 2004; Silvester et al. 2005; Shao and
Butler 2007; Bae and Hanna 2015; Gorski et al. 2016), reduction may occur before
the pollutant comes into contact with the surface. The spontaneous electron transfer
between Fe
2+ and Fe(III) oxides on the shell, which can be influenced by the
presence of surface defect (Gorski and Scherer 2009; Notini et al. 2018), results in
an acceleration in the interfacial electron transfer between iron species and the
pollutant (Huang and Zhang 2005; Han et al. 2016b). In addition, the presence of
defects in the shell was assumed to act as a catalyst for hydrodechlorination
(dechlorination by hydrogen) (Liu et al. 2005a). However, the accumulation of H 2
bubbles near the surface of the particles can affect the mass transport of H
+ as well as
the pollutant (Matheson and Tratnyek 1994; Jiang et al. 2017). In the case of a
nonconductive shell, which inhibits the electrons transfer from iron core, the chemical reduction occurs only via indirect reduction by Fe
2+ and H 2 adsorbed/bounded
on the shell (Noubactep 2016; Makota et al. 2017).
When introduced in an aqueous environment, studies have shown that a shortterm depassivation is first observed, followed by a progressive repassivation of the
particles (Sarathy et al. 2008). The aging of ZVI particles in anoxic environment
follows the Fe
0
—Fe(OH) 2 —Fe 3 O 4 —γ-Fe 2 O 3 route (Liu et al. 2007; Reinsch et al.
2010; Kumar et al. 2014a; Dong et al. 2016b; Pullin et al. 2017b; Velimirovic et al.
2018). The presence of dissolved oxygen (DO) will result in the rapid oxidation of
Fe
2+ species (Greenlee et al. 2012; Guan et al. 2015), which can lead to the formation
of Fe(III) oxyhydroxide passivation layer on iron surface (Matheson and Tratnyek
1994; Farrell et al. 2000; Noubactep 2008; Greenlee et al. 2012; Kumar et al. 2014a)
and decrease significantly the reduction efficiency (Szecsody et al. 2000). This can
be explained by a decrease of the porosity of the shell, impeding the diffusion of the
pollutant and iron corrosion products (Crane and Scott 2012). In addition, the more
rapid formation of a less conductive iron oxide due to the oxidation of FeO and
Fe 3 O 4 in Fe 2 O 3 or FeOOH, notably lepidocrocite γ-FeOOH (Haneda and Morrish
1977; Greenwood and Earnshaw 1997; Huang and Zhang 2005; Reinsch et al. 2010;
Rebodos and Vikesland 2010; Greenlee et al. 2012; Liu et al. 2014a) inhibit the
electron transfer from iron core to the oxide shell. A good knowledge of the chemical
properties and the structural evolution of this oxide shell is therefore of crucial
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
305
transfer from iron core to the surface: (1) a direct transfer in the absence of oxide
layer; (2) a transfer from metal through defects (pits) in the oxide layer; (3) a transfer
from the oxide layer as a semiconductor; and (4) a transfer from metal-to-ligand
(Scherer et al. 1999). It is important to note that iron particles are not selective, and
very low electron efficiency toward pollutant are reported (Schöftner et al. 2015;
Tang et al. 2017a). The direct reduction at Fe
0 core surface requires the diffusion of
the pollutant through the oxide shell, which is possible because of the porous
structure of the initial shell (Crane and Scott 2012; Mu et al. 2017). However, this
phenomenon is impeded by the diffusion of Fe
2+ resulting from iron corrosion in the
opposite direction of the pollutant. As it was reported that shell-bounded Fe
2+ on
different iron oxides is a strong reductant with specific redox properties (Johnson
et al. 1998; Amonette et al. 2000; Elsner et al. 2004; Silvester et al. 2005; Shao and
Butler 2007; Bae and Hanna 2015; Gorski et al. 2016), reduction may occur before
the pollutant comes into contact with the surface. The spontaneous electron transfer
between Fe
2+ and Fe(III) oxides on the shell, which can be influenced by the
presence of surface defect (Gorski and Scherer 2009; Notini et al. 2018), results in
an acceleration in the interfacial electron transfer between iron species and the
pollutant (Huang and Zhang 2005; Han et al. 2016b). In addition, the presence of
defects in the shell was assumed to act as a catalyst for hydrodechlorination
(dechlorination by hydrogen) (Liu et al. 2005a). However, the accumulation of H 2
bubbles near the surface of the particles can affect the mass transport of H
+ as well as
the pollutant (Matheson and Tratnyek 1994; Jiang et al. 2017). In the case of a
nonconductive shell, which inhibits the electrons transfer from iron core, the chemical reduction occurs only via indirect reduction by Fe
2+ and H 2 adsorbed/bounded
on the shell (Noubactep 2016; Makota et al. 2017).
When introduced in an aqueous environment, studies have shown that a shortterm depassivation is first observed, followed by a progressive repassivation of the
particles (Sarathy et al. 2008). The aging of ZVI particles in anoxic environment
follows the Fe
0
—Fe(OH) 2 —Fe 3 O 4 —γ-Fe 2 O 3 route (Liu et al. 2007; Reinsch et al.
2010; Kumar et al. 2014a; Dong et al. 2016b; Pullin et al. 2017b; Velimirovic et al.
2018). The presence of dissolved oxygen (DO) will result in the rapid oxidation of
Fe
2+ species (Greenlee et al. 2012; Guan et al. 2015), which can lead to the formation
of Fe(III) oxyhydroxide passivation layer on iron surface (Matheson and Tratnyek
1994; Farrell et al. 2000; Noubactep 2008; Greenlee et al. 2012; Kumar et al. 2014a)
and decrease significantly the reduction efficiency (Szecsody et al. 2000). This can
be explained by a decrease of the porosity of the shell, impeding the diffusion of the
pollutant and iron corrosion products (Crane and Scott 2012). In addition, the more
rapid formation of a less conductive iron oxide due to the oxidation of FeO and
Fe 3 O 4 in Fe 2 O 3 or FeOOH, notably lepidocrocite γ-FeOOH (Haneda and Morrish
1977; Greenwood and Earnshaw 1997; Huang and Zhang 2005; Reinsch et al. 2010;
Rebodos and Vikesland 2010; Greenlee et al. 2012; Liu et al. 2014a) inhibit the
electron transfer from iron core to the oxide shell. A good knowledge of the chemical
properties and the structural evolution of this oxide shell is therefore of crucial
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
305