6.3 Chemical Reduction of Chlorinated Organic
Compounds
Chemical reduction is the use of reductants—electron donors—for the remediation
of COCs—electron acceptors. For soil and groundwater remediation, the technology
is referred as In Situ Chemical Reduction (ISCR). ISCR was primarily designed for
plume management and remediation, especially with permeable reactive barriers
applications (Henderson and Demond 2007; Brown 2010; Gillham et al. 2010).
Since then, injections in front of COCs source zone have been developed (Kueper
et al. 2014; Tratnyek et al. 2014).
This section firstly presents different efficient chemical reductants for COCs
remediation, but is thereafter focused on the use of zero-valent iron (ZVI) due to
its large number of effective in situ applications (Kueper et al. 2014). The latter is
particularly focused on iron properties and reactivity enhancement—by using nanoscale or microscale particles, polymetallic particles, sulfidated particles, or a combination of techniques, or a combination of reactants—kinetic laws and mechanism
pathways observed for common COCs.
6.3.1 Reductants Used
The chemical reduction of COCs is generally performed with reduced sulfur species,
reduced metal species or hydrogen. Figure 6.1a illustrates different redox couples
that can be involved in ISCR and the potential range of reductive half-reactions of
COCs.
Electrochemical half-reactions of some chemical reductants and their standard
potential are shown in Table 6.4.
Complete half-reactions for chlorinated compounds (i.e., leading to the formation
of non-chlorinated compounds) can be generally expressed using Eq. (6.16).
C x H y Cl z þ z H
þ
þ 2z e
À
! C x H yþz þ z Cl
À
ð6:27Þ
Table 6.5 lists complete half-reactions for selected COCs. Complete reduction
leads to the formation of non-chlorinated hydrocarbons and chloride ions.
Depending on the type of reagent, it is possible to observe the formation of saturated
compounds from unsaturated compounds (e.g., the transformation of alkenes into
alkanes).
Hydrogen sparging—or gas bubbling—of aquifers contaminated with chlorinated
solvents has been shown as a promising method to enhance in situ microbial
dechlorination. A concern, however, is the ability to distribute hydrogen effectively
throughout the contaminated interval such that complete dechlorination can occur.
Novel technologies are in development in order to incorporate hydrogen into foams
such as to promote and enhance lateral gas distribution and then to enhance the
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
299
Compounds
Chemical reduction is the use of reductants—electron donors—for the remediation
of COCs—electron acceptors. For soil and groundwater remediation, the technology
is referred as In Situ Chemical Reduction (ISCR). ISCR was primarily designed for
plume management and remediation, especially with permeable reactive barriers
applications (Henderson and Demond 2007; Brown 2010; Gillham et al. 2010).
Since then, injections in front of COCs source zone have been developed (Kueper
et al. 2014; Tratnyek et al. 2014).
This section firstly presents different efficient chemical reductants for COCs
remediation, but is thereafter focused on the use of zero-valent iron (ZVI) due to
its large number of effective in situ applications (Kueper et al. 2014). The latter is
particularly focused on iron properties and reactivity enhancement—by using nanoscale or microscale particles, polymetallic particles, sulfidated particles, or a combination of techniques, or a combination of reactants—kinetic laws and mechanism
pathways observed for common COCs.
6.3.1 Reductants Used
The chemical reduction of COCs is generally performed with reduced sulfur species,
reduced metal species or hydrogen. Figure 6.1a illustrates different redox couples
that can be involved in ISCR and the potential range of reductive half-reactions of
COCs.
Electrochemical half-reactions of some chemical reductants and their standard
potential are shown in Table 6.4.
Complete half-reactions for chlorinated compounds (i.e., leading to the formation
of non-chlorinated compounds) can be generally expressed using Eq. (6.16).
C x H y Cl z þ z H
þ
þ 2z e
À
! C x H yþz þ z Cl
À
ð6:27Þ
Table 6.5 lists complete half-reactions for selected COCs. Complete reduction
leads to the formation of non-chlorinated hydrocarbons and chloride ions.
Depending on the type of reagent, it is possible to observe the formation of saturated
compounds from unsaturated compounds (e.g., the transformation of alkenes into
alkanes).
Hydrogen sparging—or gas bubbling—of aquifers contaminated with chlorinated
solvents has been shown as a promising method to enhance in situ microbial
dechlorination. A concern, however, is the ability to distribute hydrogen effectively
throughout the contaminated interval such that complete dechlorination can occur.
Novel technologies are in development in order to incorporate hydrogen into foams
such as to promote and enhance lateral gas distribution and then to enhance the
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
299