and chemical processes that are well reviewed in (Siegrist et al. 2011). In brief, H 2 O 2
instantly and exothermically decomposes to oxygen and water (H 2 O 2 ! H 2 O + 1/2O 2 )
once in contact with iron and manganese oxides surfaces of aquifer materials.
Therefore, organic acids (e.g., citric acid, EDTA) additions are widely studied for
H 2 O 2 ISCO (Engelmann et al. 2003). This way, via iron and manganese oxide
dissolution and chelation by organic acids, the unfavourable exothermic reaction
may be slowed down. However, with the addition of organic acids, the question of
other (toxic) metals mobilization arises. Even if metals Cu
+
, Cr
2+ , Co
2+ (Tarr 2003)
can substitute iron in
•
OH formation, their presence has to be monitored to exclude
the possibility of mobilisation. Many studies have shown that the mobilisation of
heavy metals is a time and space limited process in case of joint applications of
chelators and H 2 O 2 (Bennedsen et al. 2012).
As pollutants recalcitrant to
•
OH attack, perhalogenated aliphatic hydrocarbons
are reported (Siegrist et al. 2011), especially chlorinated methanes and ethanes.
Small kinetic constants were also found for long chain petroleum hydrocarbons.
9.4 Non-consensual Radical Mechanisms
We provide basic information on two non-consensual radical mechanisms: Involvement of superoxide radical anion (O 2
•– ) in tetrachloromethane (CCl 4 ) degradation
and ferryl (FeO
2+ ) specie reactions.
Carbon tetrachloride is an established toxicity model substance (Weber et al. 2003),
its transformation mechanism is well described in mammalian hepatic cells. Decades
of CCl 4 toxic action research on cellular, tissue, and animal model levels brought
postulation of its free radical nature: Liver cell cytochrome P450 initiates the transformation of CCl 4 to trichloromethyl radical (
•
CCl 3 ) (McCay et al. 1984), which may
form trichloromethylperoxyl radicals (
•
OOCCl 3 ) in the presence of oxygen (Mönig
et al. 1983). Both radicals were identified as the species being responsible for the
specific toxic effects of CCl 4 . Several hundreds of in-vitro and in-vivo studies have
been conducted in order to clearly understand the variability of CCl 4 -induced toxic
effects, including dose-response dependence, preventive or potentiating impact of
other compounds (e.g., antioxidants), oxygen partial pressure, or age of model animals
(Vulimiri et al. 2011). Chloroform, hexachloroethane, phosgene, and alkanes/alkanals/
alkenals belong to CCl 4 metabolism intermediates while carbon monoxide, carbon
dioxide, and hydrochloric acid are its metabolic endproducts (Plaa 2000).
In complex scientific coverage of molecular mechanisms in CCl 4 -induced toxic
effects, no evidence is given for the O 2
•– involvement in the initial step of the CCl 4
transformation, even though O 2
•– is ubiquitously present in living cells (McCord and
Fridovich 1988). Instead, the initial step is attributed to NADPH-cytochrome P-450
reductase. Furthermore, hyperbaric oxygen conditions are claimed to prevent the
initiation of CCl 4 metabolism via trichloromethyl radical mechanism (Manibusan
et al. 2007; Reiner et al. 1972).
9 Radical Reactions and Their Application for Water Treatment
207
instantly and exothermically decomposes to oxygen and water (H 2 O 2 ! H 2 O + 1/2O 2 )
once in contact with iron and manganese oxides surfaces of aquifer materials.
Therefore, organic acids (e.g., citric acid, EDTA) additions are widely studied for
H 2 O 2 ISCO (Engelmann et al. 2003). This way, via iron and manganese oxide
dissolution and chelation by organic acids, the unfavourable exothermic reaction
may be slowed down. However, with the addition of organic acids, the question of
other (toxic) metals mobilization arises. Even if metals Cu
+
, Cr
2+ , Co
2+ (Tarr 2003)
can substitute iron in
•
OH formation, their presence has to be monitored to exclude
the possibility of mobilisation. Many studies have shown that the mobilisation of
heavy metals is a time and space limited process in case of joint applications of
chelators and H 2 O 2 (Bennedsen et al. 2012).
As pollutants recalcitrant to
•
OH attack, perhalogenated aliphatic hydrocarbons
are reported (Siegrist et al. 2011), especially chlorinated methanes and ethanes.
Small kinetic constants were also found for long chain petroleum hydrocarbons.
9.4 Non-consensual Radical Mechanisms
We provide basic information on two non-consensual radical mechanisms: Involvement of superoxide radical anion (O 2
•– ) in tetrachloromethane (CCl 4 ) degradation
and ferryl (FeO
2+ ) specie reactions.
Carbon tetrachloride is an established toxicity model substance (Weber et al. 2003),
its transformation mechanism is well described in mammalian hepatic cells. Decades
of CCl 4 toxic action research on cellular, tissue, and animal model levels brought
postulation of its free radical nature: Liver cell cytochrome P450 initiates the transformation of CCl 4 to trichloromethyl radical (
•
CCl 3 ) (McCay et al. 1984), which may
form trichloromethylperoxyl radicals (
•
OOCCl 3 ) in the presence of oxygen (Mönig
et al. 1983). Both radicals were identified as the species being responsible for the
specific toxic effects of CCl 4 . Several hundreds of in-vitro and in-vivo studies have
been conducted in order to clearly understand the variability of CCl 4 -induced toxic
effects, including dose-response dependence, preventive or potentiating impact of
other compounds (e.g., antioxidants), oxygen partial pressure, or age of model animals
(Vulimiri et al. 2011). Chloroform, hexachloroethane, phosgene, and alkanes/alkanals/
alkenals belong to CCl 4 metabolism intermediates while carbon monoxide, carbon
dioxide, and hydrochloric acid are its metabolic endproducts (Plaa 2000).
In complex scientific coverage of molecular mechanisms in CCl 4 -induced toxic
effects, no evidence is given for the O 2
•– involvement in the initial step of the CCl 4
transformation, even though O 2
•– is ubiquitously present in living cells (McCord and
Fridovich 1988). Instead, the initial step is attributed to NADPH-cytochrome P-450
reductase. Furthermore, hyperbaric oxygen conditions are claimed to prevent the
initiation of CCl 4 metabolism via trichloromethyl radical mechanism (Manibusan
et al. 2007; Reiner et al. 1972).
9 Radical Reactions and Their Application for Water Treatment
207