In the aquifer, CCl 4 undergoes degradation, which is driven by biotic and abiotic
factors (Shao and Butler 2009), whereby oxygen poor conditions are essential.
Microbial transformations of CCl 4 are either accomplished by extracellular coenzyme exudates (Hashsham and Freedman 1999) or by extracellular mineral secretion
(McCormick and Adriaens 2004). In abiotic degradation pathways, reduced species
of iron and sulfur such as pyrite (Kriegman-King and Reinhard 1994), sulfides
(Devlin and Müller 1999), adsorbed Fe
II (Amonette et al. 2000; Elsner et al. 2004;
Kenneke and Weber 2003), or zero-valent iron (ZVI) (Johnson et al. 1998), were
identified as reactants for reductive CCl 4 degradation. The half-life of CCl 4 in the
aquifer is reported to range from a few days to hundreds of days (Howard 1991). For
more information on the degradation of CCl 4 in the aquifer see Pecher et al. (2002)
and Elsner and Hofstetter (2011).
Major products of CCl 4 aquifer degradation are chloroform, formate, carbon
monoxide, and carbon dioxide. Similarly to CCl 4 mammalian metabolism, the initial
step in the aquifer-related CCl 4 degradation—one electron reduction to
trichloromethyl radical—is supposed for all identified intermediates and products
(Elsner et al. 2004). However, in the degradation mediated by Pseudomonas stutzeri
KC only traces of chloroform has been reported, with most of the carbon mass being
transformed to carbon dioxide (Criddle et al. 1990).
Under oxygen-rich conditions, represented by oxidative water treatment such as
ozone and hydroxyl radical based advanced oxidation processes (AOP), CCl 4 is
claimed to exhibit chemical stability (von Sonntag 2008).
On the other hand, contrary to the above-mentioned literature, there is a series of
studies (Che and Lee 2011; Furman et al. 2009; Howsawkeng et al. 2010; Smith
et al. 2004; Stoin et al. 2015; Teel and Watts 2002; Watts et al. 2005) that describe
CCl 4 degradation proceeding under peroxide-based conditions. Similar CCl 4 degradation reactions are believed to occur in peroxydisulfate systems, which was
reported by (Xu et al. 2014a, b). Moreover, O 2
•– was identified as an oxygen species
initiating CCl 4 transformation in both oxidative systems. Furthermore, a technique
employing CCl 4 was established to prove the O 2
•– presence (Corbin III 2008; Watts
2011; US EPA 2014).
We conclude that the CCl 4 reactivity with O 2
•– in oxidative systems remains
controversial. Both peroxide- and peroxydisulfate-based systems are a very complex
interplay of several simultaneous reactions. Since gaseous oxygen is one of the
reaction products in oxidative systems, it is extremely uneasy to conduct CCl 4
degradability experiments in a methodologically correct way. We suggest that it is
the CCl 4 volatilisation loss rather than its degradation that might be the process
leading to the CCl 4 decrease in the reaction systems reported in the previous
paragraph. The CCl 4 degradation in oxidative systems is still questionable, so is its
initiation by O 2
•– .
The other non-consensual species, which is often discussed in studies of Fentonlike reaction systems, is ferryl ion FeO
2+ . Reaction mechanisms in AOP are perceived as radical (
• OH) based by most of the scientific community. However, there is
an alternative to
•
OH—FeO
2+ . High valent (4+, 5+, and 6+) iron-oxo species
chemistry is thoroughly described in Chap. 8. Four-valent iron-oxo complexes are
208
P. Hrabák and S. Wacławek
factors (Shao and Butler 2009), whereby oxygen poor conditions are essential.
Microbial transformations of CCl 4 are either accomplished by extracellular coenzyme exudates (Hashsham and Freedman 1999) or by extracellular mineral secretion
(McCormick and Adriaens 2004). In abiotic degradation pathways, reduced species
of iron and sulfur such as pyrite (Kriegman-King and Reinhard 1994), sulfides
(Devlin and Müller 1999), adsorbed Fe
II (Amonette et al. 2000; Elsner et al. 2004;
Kenneke and Weber 2003), or zero-valent iron (ZVI) (Johnson et al. 1998), were
identified as reactants for reductive CCl 4 degradation. The half-life of CCl 4 in the
aquifer is reported to range from a few days to hundreds of days (Howard 1991). For
more information on the degradation of CCl 4 in the aquifer see Pecher et al. (2002)
and Elsner and Hofstetter (2011).
Major products of CCl 4 aquifer degradation are chloroform, formate, carbon
monoxide, and carbon dioxide. Similarly to CCl 4 mammalian metabolism, the initial
step in the aquifer-related CCl 4 degradation—one electron reduction to
trichloromethyl radical—is supposed for all identified intermediates and products
(Elsner et al. 2004). However, in the degradation mediated by Pseudomonas stutzeri
KC only traces of chloroform has been reported, with most of the carbon mass being
transformed to carbon dioxide (Criddle et al. 1990).
Under oxygen-rich conditions, represented by oxidative water treatment such as
ozone and hydroxyl radical based advanced oxidation processes (AOP), CCl 4 is
claimed to exhibit chemical stability (von Sonntag 2008).
On the other hand, contrary to the above-mentioned literature, there is a series of
studies (Che and Lee 2011; Furman et al. 2009; Howsawkeng et al. 2010; Smith
et al. 2004; Stoin et al. 2015; Teel and Watts 2002; Watts et al. 2005) that describe
CCl 4 degradation proceeding under peroxide-based conditions. Similar CCl 4 degradation reactions are believed to occur in peroxydisulfate systems, which was
reported by (Xu et al. 2014a, b). Moreover, O 2
•– was identified as an oxygen species
initiating CCl 4 transformation in both oxidative systems. Furthermore, a technique
employing CCl 4 was established to prove the O 2
•– presence (Corbin III 2008; Watts
2011; US EPA 2014).
We conclude that the CCl 4 reactivity with O 2
•– in oxidative systems remains
controversial. Both peroxide- and peroxydisulfate-based systems are a very complex
interplay of several simultaneous reactions. Since gaseous oxygen is one of the
reaction products in oxidative systems, it is extremely uneasy to conduct CCl 4
degradability experiments in a methodologically correct way. We suggest that it is
the CCl 4 volatilisation loss rather than its degradation that might be the process
leading to the CCl 4 decrease in the reaction systems reported in the previous
paragraph. The CCl 4 degradation in oxidative systems is still questionable, so is its
initiation by O 2
•– .
The other non-consensual species, which is often discussed in studies of Fentonlike reaction systems, is ferryl ion FeO
2+ . Reaction mechanisms in AOP are perceived as radical (
• OH) based by most of the scientific community. However, there is
an alternative to
•
OH—FeO
2+ . High valent (4+, 5+, and 6+) iron-oxo species
chemistry is thoroughly described in Chap. 8. Four-valent iron-oxo complexes are
208
P. Hrabák and S. Wacławek