Nevertheless, in situ chemical reduction of COCs is generally performed by using
zero-valent metals. The first application has been investigated in the late 1970s to
dechlorinate halogenated organic compounds—dichlorodiphenyltrichloroethane,
chlorobenzene, endrin, heptachlor, chloroform, and hexachlorocyclo-pentadiene—
from a wastewater stream using copper-doped iron and aluminum (Sweeny 1980).
Aluminum and magnesium are widely used for the dechlorination of organic
chemicals in organic synthesis on laboratory and for wastewater treatment (Nidheesh
et al. 2018), but only a few groundwater cleanup technologies has been fully
implemented with these metals (Brown 2010).
Dechlorination of COCs using zero-valent iron (ZVI) has been considered in the
early 1990s, with the demonstration of the efficiency of iron particles for the
degradation of 14 chloromethanes, chloroethanes, and chloroethenes (Gillham and
O’Hannesin 1994). In addition to ZVI, zero-valent zinc (ZVZ) has also been studied
for the degradation of COCs in the 1990s. Reduction of chlorinated methanes has
been observed by Warren et al. (1995), as well as reduction of chlorinated ethanes
(Fennelly and Roberts 1998; Arnold et al. 1999), ethylenes (Arnold and Roberts
1998), and propanes (Sarathy et al. 2010; Cushman 2014). Zinc is a good alternative
to iron for in situ application (Cheng and Wu 2000) as its storage is easier and its
tolerable concentration in drinking water is high (for example, the maximum admissible value in drinking water for iron is 200 μg L
À1 according to the Council
Directive 98/83/EC of 3 November 1998 and 3 mg L
À1 for zinc according to the
Guidelines for Drinking-Water Quality). Even if ZVZ particles have shown better
degradation efficiency on the reduction of chlorinated alkanes—carbon tetrachloride
or 1,2,3-trichloropropane for example—their reactivity is more strongly impacted by
environmental conditions than their size and morphology (Tratnyek et al. 2010).
Also, in presence of a microbial community, dechlorination occurred faster with ZVI
than with ZVZ (Ma and Wu 2008). Moreover, iron is one of the ten most abundant
elements in our planet (Fu et al. 2014a). Iron plays a key role in many biogeochemical processes implying electron transfers (i.e., redox reactions) because of its
abundance, ubiquity, and particular physicochemical properties. Moreover, iron
can be naturally found in a great variety of chemical species, such as elemental
form (ZVI), aqueous ions (Fe
3+ and Fe
2+ ), oxides, oxy-hydroxides, sulfates and
sulfide minerals, silicates, carbonates, etc. (Fig. 6.1a, b). These species form various
redox couples. Iron physicochemical properties (solubility, redox potential) can
dramatically change from one species to another. In all pH conditions, there are
soluble iron species, and many iron couples have redox potential falling within the
domain of water stability, thus explaining the importance of iron in many redox
mechanisms where it can act either as reductant or as oxidant.
The reduction of some alkyl halides with hydrated or complexed ferrous iron is
thermodynamically possible but quite slow (Klečka and Gonsior 1984; Doong and
Wu 1992). However, surface-bounded Fe
2+ and Fe precipitates are stronger reductants (Johnson et al. 1998; Amonette et al. 2000; Jeong et al. 2013; Bae and Hanna
2015) and can degrade carbon tetrachloride and hexachloroethane (Elsner et al.
2004; Shao and Butler 2007). Green rusts (layered mixed Fe(II)/Fe(III) hydroxide
minerals) can also reduce chlorinated ethanes and ethylenes, with a more rapid
302
R. Rodrigues et al.
zero-valent metals. The first application has been investigated in the late 1970s to
dechlorinate halogenated organic compounds—dichlorodiphenyltrichloroethane,
chlorobenzene, endrin, heptachlor, chloroform, and hexachlorocyclo-pentadiene—
from a wastewater stream using copper-doped iron and aluminum (Sweeny 1980).
Aluminum and magnesium are widely used for the dechlorination of organic
chemicals in organic synthesis on laboratory and for wastewater treatment (Nidheesh
et al. 2018), but only a few groundwater cleanup technologies has been fully
implemented with these metals (Brown 2010).
Dechlorination of COCs using zero-valent iron (ZVI) has been considered in the
early 1990s, with the demonstration of the efficiency of iron particles for the
degradation of 14 chloromethanes, chloroethanes, and chloroethenes (Gillham and
O’Hannesin 1994). In addition to ZVI, zero-valent zinc (ZVZ) has also been studied
for the degradation of COCs in the 1990s. Reduction of chlorinated methanes has
been observed by Warren et al. (1995), as well as reduction of chlorinated ethanes
(Fennelly and Roberts 1998; Arnold et al. 1999), ethylenes (Arnold and Roberts
1998), and propanes (Sarathy et al. 2010; Cushman 2014). Zinc is a good alternative
to iron for in situ application (Cheng and Wu 2000) as its storage is easier and its
tolerable concentration in drinking water is high (for example, the maximum admissible value in drinking water for iron is 200 μg L
À1 according to the Council
Directive 98/83/EC of 3 November 1998 and 3 mg L
À1 for zinc according to the
Guidelines for Drinking-Water Quality). Even if ZVZ particles have shown better
degradation efficiency on the reduction of chlorinated alkanes—carbon tetrachloride
or 1,2,3-trichloropropane for example—their reactivity is more strongly impacted by
environmental conditions than their size and morphology (Tratnyek et al. 2010).
Also, in presence of a microbial community, dechlorination occurred faster with ZVI
than with ZVZ (Ma and Wu 2008). Moreover, iron is one of the ten most abundant
elements in our planet (Fu et al. 2014a). Iron plays a key role in many biogeochemical processes implying electron transfers (i.e., redox reactions) because of its
abundance, ubiquity, and particular physicochemical properties. Moreover, iron
can be naturally found in a great variety of chemical species, such as elemental
form (ZVI), aqueous ions (Fe
3+ and Fe
2+ ), oxides, oxy-hydroxides, sulfates and
sulfide minerals, silicates, carbonates, etc. (Fig. 6.1a, b). These species form various
redox couples. Iron physicochemical properties (solubility, redox potential) can
dramatically change from one species to another. In all pH conditions, there are
soluble iron species, and many iron couples have redox potential falling within the
domain of water stability, thus explaining the importance of iron in many redox
mechanisms where it can act either as reductant or as oxidant.
The reduction of some alkyl halides with hydrated or complexed ferrous iron is
thermodynamically possible but quite slow (Klečka and Gonsior 1984; Doong and
Wu 1992). However, surface-bounded Fe
2+ and Fe precipitates are stronger reductants (Johnson et al. 1998; Amonette et al. 2000; Jeong et al. 2013; Bae and Hanna
2015) and can degrade carbon tetrachloride and hexachloroethane (Elsner et al.
2004; Shao and Butler 2007). Green rusts (layered mixed Fe(II)/Fe(III) hydroxide
minerals) can also reduce chlorinated ethanes and ethylenes, with a more rapid
302
R. Rodrigues et al.