6.2.4 Degradation Mechanisms
Abiotic degradation reactions of COCs include number of probable reactions, such
as substitution, dehydrochlorination, oxidation, and reduction (Sweeny 1980; Vogel
et al. 1987). The first two are chemical reactions, and the last two are redox reactions,
which require external electron acceptors or donors.
COCs can undergo substitution reactions with water [hydrolysis, Eq. (6.19)] or
with an anionic nucleophile N
À [nucleophilic substitution, Eq. (6.20)]. Generally,
the reaction with water (or hydroxide ions HO
À at high pH), resulting in the
formation of an alcohol, is considered as the main contributor to substitution
reactions because of the environmental abundance of water, despite its low nucleophilic strength (Schwarzenbach et al. 2003).
Hydrolysis:
RCl þ H 2 O ! ROH þ HCl
ð6:19Þ
Nucleophilic reactions:
RCl þ N
À
! RN þ Cl
À
ð6:20Þ
Dehydrochlorination, or non-reductive elimination reaction, is the
elimination of HCl in two vicinal carbon atoms by a moderate (e.g., H 2 O) or
strong (e.g., HO
À ) base, resulting in the formation of a new carbon–carbon
bond (Cwiertny and Scherer 2010). Thus, chlorinated alkanes are transformed to
chlorinated alkenes (Eq. 6.21), and chlorinated alkenes to chlorinated alkynes
(Eq. 6.22).
Dehydrochlorination:
R 2 CCl À HCR
0
2 !
base R 2 C ¼ CR
0
2 þ HCl
ð6:21Þ
RCCl ¼ HCR
0
!
base RC CR
0
þ HCl
ð6:22Þ
COCs can be chemically oxidized or reduced, with an oxidative or a reducing
agent, respectively. Oxidation is a loss of electron, while reduction is a gain of
electron. Oxidation and reduction occur simultaneously as they involve an electron
transfer between species from two redox couples. The redox reaction is spontaneous
if the oxidizing agent has a higher standard electrode potential than the reducing
agent (e.g., the standard cell potential of the reaction is positive). Each COC presents
a different reactivity and predominance for oxidation or reduction according to its
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
297
Abiotic degradation reactions of COCs include number of probable reactions, such
as substitution, dehydrochlorination, oxidation, and reduction (Sweeny 1980; Vogel
et al. 1987). The first two are chemical reactions, and the last two are redox reactions,
which require external electron acceptors or donors.
COCs can undergo substitution reactions with water [hydrolysis, Eq. (6.19)] or
with an anionic nucleophile N
À [nucleophilic substitution, Eq. (6.20)]. Generally,
the reaction with water (or hydroxide ions HO
À at high pH), resulting in the
formation of an alcohol, is considered as the main contributor to substitution
reactions because of the environmental abundance of water, despite its low nucleophilic strength (Schwarzenbach et al. 2003).
Hydrolysis:
RCl þ H 2 O ! ROH þ HCl
ð6:19Þ
Nucleophilic reactions:
RCl þ N
À
! RN þ Cl
À
ð6:20Þ
Dehydrochlorination, or non-reductive elimination reaction, is the
elimination of HCl in two vicinal carbon atoms by a moderate (e.g., H 2 O) or
strong (e.g., HO
À ) base, resulting in the formation of a new carbon–carbon
bond (Cwiertny and Scherer 2010). Thus, chlorinated alkanes are transformed to
chlorinated alkenes (Eq. 6.21), and chlorinated alkenes to chlorinated alkynes
(Eq. 6.22).
Dehydrochlorination:
R 2 CCl À HCR
0
2 !
base R 2 C ¼ CR
0
2 þ HCl
ð6:21Þ
RCCl ¼ HCR
0
!
base RC CR
0
þ HCl
ð6:22Þ
COCs can be chemically oxidized or reduced, with an oxidative or a reducing
agent, respectively. Oxidation is a loss of electron, while reduction is a gain of
electron. Oxidation and reduction occur simultaneously as they involve an electron
transfer between species from two redox couples. The redox reaction is spontaneous
if the oxidizing agent has a higher standard electrode potential than the reducing
agent (e.g., the standard cell potential of the reaction is positive). Each COC presents
a different reactivity and predominance for oxidation or reduction according to its
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
297