progress in nanoscale technologies. In parallel and for similar reason, the development and use of polymetallic particles was also studied.
6.3.2.2 Nanoscale and Microscale Particles
Several studies focused on the development of nanoscale particles (nZVI particles,
10 nm < d < 100 nm) mainly for source zone targeted injection. Intrinsic characteristics, reactivity, and aging of nZVI particles are strongly impacted by their
synthesis conditions (Liu et al. 2005b; Hwang et al. 2011; Kim et al. 2012; Han
et al. 2015). Various physical and chemical methods of synthesis are reported
(Stefaniuk et al. 2016). nZVI particles are generally prepared from a bottom-up
approach (formation of nanomaterials from atoms/molecules) by the reduction of
dissolved iron species—Fe(II) or Fe(III) salts—using sodium borohydride NaBH 4
(Wang and Zhang 1997). The top-down approach (breaking down of bulk materials)
can also be used to create nanomaterials by physical or chemical methods, such as
milling or etching (Li et al. 2009; Mu et al. 2017). The third route for nZVI
production is the thermal reduction, which consists in the reduction of iron oxide
precursor in hydrogen at high temperature. Direct electrochemical synthesis of nZVI
with ultrasonication appears as a promising process for an economic and large-scale
production (Chen et al. 2004; Iranzo et al. 2015). Finally, green synthesis by using
food industry wastes of leaf extracts have been recently reported (Kharissova et al.
2013; Machado et al. 2015; Saif et al. 2016). The use of vacuum annealing at 500
C
after the synthesis led to the formation of a thin, uniform, and conductive oxide shell,
which may be responsible for the improvement of the reactive lifetime of nZVI
particles.
The increase in the reactivity of nZVI particles was mainly attributed to the
increase in the specific surface area, which provide a greater number of active sites
on which reactions occur (Xie and Cwiertny 2010; Amir and Lee 2011). Specific
surface areas of nZVI particles are generally reported in the range 20–55 m
2 g
À1
(Wang and Zhang 1997; Zhang 2003; Zhang and Elliott 2006; Shih et al. 2011b).
However, the specific surface area can be dramatically decreased by the vacuum
annealing, if performed (Scott et al. 2010). As illustrated in Fig. 6.2, the lower the
particle size, the higher the specific surface area. Also, particles mobility increases as
their size decreases (Tratnyek and Johnson 2006). Depending on particles size,
sticking coefficient (defined as the ratio of the rate of adsorption to the rate at
which the adsorptive strikes the total surface), and environmental conditions, transport distances range from millimeters to centimeters (Tratnyek and Johnson 2006).
More recently, microscale zero-valent iron (mZVI) particles (diameter < 100 μm)
have been developed because of the high cost of nZVI particles and their very fast
corrosion rate with water (Liu et al. 2005b; Liu and Lowry 2006; Li et al. 2006b;
Noubactep et al. 2012), resulting in a rapid depletion of Fe
0 and a limited degradation efficiency in field application (Noubactep and Caré 2010; Comba et al. 2011b).
Specific surface areas of microparticles are generally lower than 2 m
2 g
À1 (Wang and
Zhang 1997; Lien and Zhang 2001). However, the reactivity is not only linked to the
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
307
6.3.2.2 Nanoscale and Microscale Particles
Several studies focused on the development of nanoscale particles (nZVI particles,
10 nm < d < 100 nm) mainly for source zone targeted injection. Intrinsic characteristics, reactivity, and aging of nZVI particles are strongly impacted by their
synthesis conditions (Liu et al. 2005b; Hwang et al. 2011; Kim et al. 2012; Han
et al. 2015). Various physical and chemical methods of synthesis are reported
(Stefaniuk et al. 2016). nZVI particles are generally prepared from a bottom-up
approach (formation of nanomaterials from atoms/molecules) by the reduction of
dissolved iron species—Fe(II) or Fe(III) salts—using sodium borohydride NaBH 4
(Wang and Zhang 1997). The top-down approach (breaking down of bulk materials)
can also be used to create nanomaterials by physical or chemical methods, such as
milling or etching (Li et al. 2009; Mu et al. 2017). The third route for nZVI
production is the thermal reduction, which consists in the reduction of iron oxide
precursor in hydrogen at high temperature. Direct electrochemical synthesis of nZVI
with ultrasonication appears as a promising process for an economic and large-scale
production (Chen et al. 2004; Iranzo et al. 2015). Finally, green synthesis by using
food industry wastes of leaf extracts have been recently reported (Kharissova et al.
2013; Machado et al. 2015; Saif et al. 2016). The use of vacuum annealing at 500
C
after the synthesis led to the formation of a thin, uniform, and conductive oxide shell,
which may be responsible for the improvement of the reactive lifetime of nZVI
particles.
The increase in the reactivity of nZVI particles was mainly attributed to the
increase in the specific surface area, which provide a greater number of active sites
on which reactions occur (Xie and Cwiertny 2010; Amir and Lee 2011). Specific
surface areas of nZVI particles are generally reported in the range 20–55 m
2 g
À1
(Wang and Zhang 1997; Zhang 2003; Zhang and Elliott 2006; Shih et al. 2011b).
However, the specific surface area can be dramatically decreased by the vacuum
annealing, if performed (Scott et al. 2010). As illustrated in Fig. 6.2, the lower the
particle size, the higher the specific surface area. Also, particles mobility increases as
their size decreases (Tratnyek and Johnson 2006). Depending on particles size,
sticking coefficient (defined as the ratio of the rate of adsorption to the rate at
which the adsorptive strikes the total surface), and environmental conditions, transport distances range from millimeters to centimeters (Tratnyek and Johnson 2006).
More recently, microscale zero-valent iron (mZVI) particles (diameter < 100 μm)
have been developed because of the high cost of nZVI particles and their very fast
corrosion rate with water (Liu et al. 2005b; Liu and Lowry 2006; Li et al. 2006b;
Noubactep et al. 2012), resulting in a rapid depletion of Fe
0 and a limited degradation efficiency in field application (Noubactep and Caré 2010; Comba et al. 2011b).
Specific surface areas of microparticles are generally lower than 2 m
2 g
À1 (Wang and
Zhang 1997; Lien and Zhang 2001). However, the reactivity is not only linked to the
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
307