low chlorinated compounds such as DCE isomers and vinyl chloride (Zhang et al.
1998; Lowry and Reinhard 1999). This effect can be explained by the cleavage of C–
Cl bonds at Pd surface (Park et al. 1997; Sriwatanapongse et al. 2006; He and Zhao
2008; Omar et al. 2011) and the formation of carbene intermediates. Heck et al.
(2008) have observed with surface-enhanced Raman spectroscopy (SERS) the
formation of C–Pd and Cl–Pd bond, indicating that hydrodechlorination pathways
(dechlorination by hydrogen) follow a sequence of dechlorination and hydrogenation. In addition, in presence of dissolved oxygen, the use of Pd or Ni can enhance
the production of ROS, especially H 2 O 2 , to promote oxidation reactions (Lee and
Sedlak 2008; Shih et al. 2016).
As for monometallic particles, the use of a stabilizer is highly recommended to
prevent aggregation and enhance mobility of bimetallic particles (Basnet et al. 2013,
2015). The use of a support such as activated carbon can result in simultaneous
adsorption and dechlorination process (Choi et al. 2008). Trimetallic particles can
also be synthesized, and appears to be more reactive than bimetallic particles due to
synergistic effects between different metals (Ghauch et al. 2011; Abazari et al. 2013;
Sharma et al. 2016).
In 2009, about 30% of reductive remediation by ZVI were performed by bimetallic particles in the USA (Karn et al. 2009), while only regular nZVI particles were
applied in Europe (Mueller et al. 2012). The main reasons are not technical and
concern (1) the cost of the particles and the handling of the suspension and (2) the
toxicity of the catalyst, especially Ni which is considered as a priority substance in
Europe. In addition, the significant improvement in degradation rates in field experiments has not been proven yet (Comba et al. 2011b; Xie and Cwiertny 2013).
6.3.2.4 Sulfidated Particles
Sulfide species, sodium dithionite S 2 O 4
2À , and thiosulfate S 2 O 3
2À can be used for
the sulfidation of ZVI particles (Kim et al. 2011; Fan et al. 2013; Gu et al. 2017),
which is defined as their chemical modification by reducing sulfur compounds on
iron surface. Sulfidation is supposed to mimic naturally occurring microbial sulfate
reduction that occur in the subsurface (Shao et al. 2018). The S/Fe ratio, the
sulfidation duration, and the solution chemistry are three main operational variables
that affect the structure and morphology of the particles (Fan et al. 2017; Li et al.
2017b; Kumar et al. 2018), and thus their reactivity. Sulfidation is mainly used to
improve the selectivity—or electron efficiency—toward the pollutant and the longevity of ZVI particles by decreasing the reactivity with water and/or oxygen
(Rajajayavel and Ghoshal 2015; Fan et al. 2016ab; Li et al. 2018). The slower
corrosion rate is attributed to a poisoning effect of sulfur which inhibit the recombination of atomic hydrogen to slow down H 2 production (Han and Yan 2016).
Sulfidation results in the formation of a FeS x layer which can mediate the
aggregation of the particles, improve the hydrophobicity of the surface, and enhance
the electron transfer from Fe
0 , thus resulting in the improvement of the reactivity for
dechlorination (Park et al. 2006; Rajajayavel and Ghoshal 2015; Li et al. 2016a; Cao
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
311
1998; Lowry and Reinhard 1999). This effect can be explained by the cleavage of C–
Cl bonds at Pd surface (Park et al. 1997; Sriwatanapongse et al. 2006; He and Zhao
2008; Omar et al. 2011) and the formation of carbene intermediates. Heck et al.
(2008) have observed with surface-enhanced Raman spectroscopy (SERS) the
formation of C–Pd and Cl–Pd bond, indicating that hydrodechlorination pathways
(dechlorination by hydrogen) follow a sequence of dechlorination and hydrogenation. In addition, in presence of dissolved oxygen, the use of Pd or Ni can enhance
the production of ROS, especially H 2 O 2 , to promote oxidation reactions (Lee and
Sedlak 2008; Shih et al. 2016).
As for monometallic particles, the use of a stabilizer is highly recommended to
prevent aggregation and enhance mobility of bimetallic particles (Basnet et al. 2013,
2015). The use of a support such as activated carbon can result in simultaneous
adsorption and dechlorination process (Choi et al. 2008). Trimetallic particles can
also be synthesized, and appears to be more reactive than bimetallic particles due to
synergistic effects between different metals (Ghauch et al. 2011; Abazari et al. 2013;
Sharma et al. 2016).
In 2009, about 30% of reductive remediation by ZVI were performed by bimetallic particles in the USA (Karn et al. 2009), while only regular nZVI particles were
applied in Europe (Mueller et al. 2012). The main reasons are not technical and
concern (1) the cost of the particles and the handling of the suspension and (2) the
toxicity of the catalyst, especially Ni which is considered as a priority substance in
Europe. In addition, the significant improvement in degradation rates in field experiments has not been proven yet (Comba et al. 2011b; Xie and Cwiertny 2013).
6.3.2.4 Sulfidated Particles
Sulfide species, sodium dithionite S 2 O 4
2À , and thiosulfate S 2 O 3
2À can be used for
the sulfidation of ZVI particles (Kim et al. 2011; Fan et al. 2013; Gu et al. 2017),
which is defined as their chemical modification by reducing sulfur compounds on
iron surface. Sulfidation is supposed to mimic naturally occurring microbial sulfate
reduction that occur in the subsurface (Shao et al. 2018). The S/Fe ratio, the
sulfidation duration, and the solution chemistry are three main operational variables
that affect the structure and morphology of the particles (Fan et al. 2017; Li et al.
2017b; Kumar et al. 2018), and thus their reactivity. Sulfidation is mainly used to
improve the selectivity—or electron efficiency—toward the pollutant and the longevity of ZVI particles by decreasing the reactivity with water and/or oxygen
(Rajajayavel and Ghoshal 2015; Fan et al. 2016ab; Li et al. 2018). The slower
corrosion rate is attributed to a poisoning effect of sulfur which inhibit the recombination of atomic hydrogen to slow down H 2 production (Han and Yan 2016).
Sulfidation results in the formation of a FeS x layer which can mediate the
aggregation of the particles, improve the hydrophobicity of the surface, and enhance
the electron transfer from Fe
0 , thus resulting in the improvement of the reactivity for
dechlorination (Park et al. 2006; Rajajayavel and Ghoshal 2015; Li et al. 2016a; Cao
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
311