be explained by a stepwise dechlorination process in which a brief interaction of this
compound with the ZVI surface would promote an electron transfer one at time. On
the other hand, the TCE reduction mechanism starts with a chemisorption step that
controls the overall rate of reaction before the electron transfer.
The degradation of chlorinated ethanes has been observed to be dependent on the
number of chlorine atoms as well as on their position in the molecule. Song and
Carraway (2005) studied the degradation of a series of chlorinated ethanes and
observed that the reactivity increased by increasing chlorination. They also reported
that among tri and tetrasubstituted compounds, the reactivity was higher for compounds with chlorine atoms more localized in only one carbon, e.g., 1,1,1TCA > 1,1,2-TCA. The proposed explanation for this difference in the reactivity is
the shifting of the mechanism, in which the 1,1,1-TCA reacts by a concerted
pathway involving α-elimination and hydrogenolysis, while 1,1,2-TCA reduction
would proceed by β-elimination.
With the aim of increasing the reactivity; mobility; and transport in subsurface,
along with the inhibition of ZVI passivation, many research groups have been
studying the effect of adding metal catalysts (Schrick et al. 2004); coating
nanoparticles with polymers/surfactants (Wang et al. 2010); supporting
nanoparticles on different substrates (Jia et al. 2011); and applying ZVI in waterin-oil emulsions (Berge and Ramsburg 2009).
Schrick et al. (2004), using Fe
0 /Ni bimetallic nanoparticles, observed a TCE
degradation rate constant that was 50 times faster than pure ZVI, indicating that
the bimetallic nanoparticles were more efficient in degrading TCE when compared
to the monometallic iron nanoparticles.
The introduction of a second catalytic metal could also prevent toxic byproduct
formation by dehalogenating TCE via hydrogen reduction rather than via electron
transfer and even enables the reduction of persistent compounds like
monochlorophenols, which cannot be reduced by ZVI itself (Morales et al. 2002).
On the other hand, the presence of different metals increases water promoted
corrosion of iron, which can result in lower reactivity and lifetime. Yan et al.
(2010) observed that Pd-doped ZVI nanoparticles immersed in water for 24 h did
not exhibit any metallic iron component in the XPS spectrum, while the Fe
0 peak,
although with decreased intensity, remained observable for pure ZVI nanoparticles
kept on the same conditions. Besides, in a reactivity TCE reduction study with
Pd-ZVI nanoparticles, the apparent reaction rate constant decreased from 5.7 1/h for
the fresh particles to 0.96 1/h upon 24 h aging.
In bimetallic systems, the atomic hydrogen adsorbed on the reductant surface
(H ads ) is postulated to be responsible for bimetal reactivity and the generation of H ads
species has been proposed by different ways. It could be produced by the dissociative chemisorption of H 2 , itself generated by water reduction, or as an intermediate to
H 2 generation. It is also suggested that absorbed atomic hydrogen (H abs ) within the
metal additive lattice, instead of surface-adsorbed atomic hydrogen (H ads ), would
represent the reactive entity in iron-based bimetallic systems (Cook 2009).
ZVI can also be used for oxidative degradation of organic compounds, either
providing Fe
2+ to Fenton process (Eq. 2.5) or by reacting with oxygen itself through
2 Nanoscale Zero-Valent Iron Particles for Water Treatment: From Basic. . .
27
compound with the ZVI surface would promote an electron transfer one at time. On
the other hand, the TCE reduction mechanism starts with a chemisorption step that
controls the overall rate of reaction before the electron transfer.
The degradation of chlorinated ethanes has been observed to be dependent on the
number of chlorine atoms as well as on their position in the molecule. Song and
Carraway (2005) studied the degradation of a series of chlorinated ethanes and
observed that the reactivity increased by increasing chlorination. They also reported
that among tri and tetrasubstituted compounds, the reactivity was higher for compounds with chlorine atoms more localized in only one carbon, e.g., 1,1,1TCA > 1,1,2-TCA. The proposed explanation for this difference in the reactivity is
the shifting of the mechanism, in which the 1,1,1-TCA reacts by a concerted
pathway involving α-elimination and hydrogenolysis, while 1,1,2-TCA reduction
would proceed by β-elimination.
With the aim of increasing the reactivity; mobility; and transport in subsurface,
along with the inhibition of ZVI passivation, many research groups have been
studying the effect of adding metal catalysts (Schrick et al. 2004); coating
nanoparticles with polymers/surfactants (Wang et al. 2010); supporting
nanoparticles on different substrates (Jia et al. 2011); and applying ZVI in waterin-oil emulsions (Berge and Ramsburg 2009).
Schrick et al. (2004), using Fe
0 /Ni bimetallic nanoparticles, observed a TCE
degradation rate constant that was 50 times faster than pure ZVI, indicating that
the bimetallic nanoparticles were more efficient in degrading TCE when compared
to the monometallic iron nanoparticles.
The introduction of a second catalytic metal could also prevent toxic byproduct
formation by dehalogenating TCE via hydrogen reduction rather than via electron
transfer and even enables the reduction of persistent compounds like
monochlorophenols, which cannot be reduced by ZVI itself (Morales et al. 2002).
On the other hand, the presence of different metals increases water promoted
corrosion of iron, which can result in lower reactivity and lifetime. Yan et al.
(2010) observed that Pd-doped ZVI nanoparticles immersed in water for 24 h did
not exhibit any metallic iron component in the XPS spectrum, while the Fe
0 peak,
although with decreased intensity, remained observable for pure ZVI nanoparticles
kept on the same conditions. Besides, in a reactivity TCE reduction study with
Pd-ZVI nanoparticles, the apparent reaction rate constant decreased from 5.7 1/h for
the fresh particles to 0.96 1/h upon 24 h aging.
In bimetallic systems, the atomic hydrogen adsorbed on the reductant surface
(H ads ) is postulated to be responsible for bimetal reactivity and the generation of H ads
species has been proposed by different ways. It could be produced by the dissociative chemisorption of H 2 , itself generated by water reduction, or as an intermediate to
H 2 generation. It is also suggested that absorbed atomic hydrogen (H abs ) within the
metal additive lattice, instead of surface-adsorbed atomic hydrogen (H ads ), would
represent the reactive entity in iron-based bimetallic systems (Cook 2009).
ZVI can also be used for oxidative degradation of organic compounds, either
providing Fe
2+ to Fenton process (Eq. 2.5) or by reacting with oxygen itself through
2 Nanoscale Zero-Valent Iron Particles for Water Treatment: From Basic. . .
27