6.2 Chlorinated Solvents
Chlorinated solvents (C x H y Cl z ) are organic compounds containing at least one
carbon–chlorine bond in their structure. The presence of chlorine instead of hydrogen atoms plays a significant role on the physical and chemical properties of COCs,
their toxicity and all transport and fate processes in groundwater.
6.2.1 Physical and Chemical Properties
Table 6.1 lists the names, formulas, abbreviations, and main physical and chemical
properties at atmospheric pressure of the most common COCs encountered in
polluted soil and groundwater. The presence of chlorine atoms affects physical
and chemical properties of organic compounds, such as density, aqueous solubility,
and volatility. Most chlorinated solvents are part of the volatile organic compounds
(VOCs) group, as they have low boiling points. Molecules are susceptible to
evaporate from liquid (or sublimate from solid), resulting in the formation of a
gaseous plume, especially in the vadose zone, which causes pollution to the surface.
Nevertheless, COCs are denser than water and form a non-aqueous phase, so they
are part of dense non-aqueous phase liquid (DNAPL) group. Their low solubilities
s are frequently represented by the octanol/water partition coefficient K ow (also noted
P) characterizing their hydrophobic nature (log K ow > 0), showing an affinity with
organic phases. Knowledge of solubility is important as it permits to predict and
model DNAPL transport and fate in groundwater. In a DNAPL mixture, a cosolvent
effect must be considered to represent the mole fraction of each constituent of the
mixed DNAPL.
In addition to solubility, other physical properties, e.g., diffusion coefficients and
Henry’s law constant, are important to understand their fate.
Ionization potential (IP) represents the energy required to remove the valence
electron of an atom or a molecule. Generally, for the same chemical structure, the
presence of double bonds decreases IP values, while the presence of chlorine atoms
increases them (Brown 2010). COCs are preferentially oxidized when carbon atoms
have a high electron density, and preferentially reduced when carbon atoms have a
low electron density. The IP value is therefore, in a first approach, a good indicator
helping to select the appropriate remediation technology.
In addition, their electrochemical properties are also affected by the nature of the
COCs. Table 6.2 lists reductive half-reactions for COCs redox couple and their
associated standard potential E
(Boethling and Mackay 2000; Dolfing et al. 2006).
First, the standard aqueous-phase free energy of formation is estimated from the
standard gas-phase free energy of formation and the values of Henry’s law constant
H (Vogel et al. 1987; Dean 2004).
286
R. Rodrigues et al.
Chlorinated solvents (C x H y Cl z ) are organic compounds containing at least one
carbon–chlorine bond in their structure. The presence of chlorine instead of hydrogen atoms plays a significant role on the physical and chemical properties of COCs,
their toxicity and all transport and fate processes in groundwater.
6.2.1 Physical and Chemical Properties
Table 6.1 lists the names, formulas, abbreviations, and main physical and chemical
properties at atmospheric pressure of the most common COCs encountered in
polluted soil and groundwater. The presence of chlorine atoms affects physical
and chemical properties of organic compounds, such as density, aqueous solubility,
and volatility. Most chlorinated solvents are part of the volatile organic compounds
(VOCs) group, as they have low boiling points. Molecules are susceptible to
evaporate from liquid (or sublimate from solid), resulting in the formation of a
gaseous plume, especially in the vadose zone, which causes pollution to the surface.
Nevertheless, COCs are denser than water and form a non-aqueous phase, so they
are part of dense non-aqueous phase liquid (DNAPL) group. Their low solubilities
s are frequently represented by the octanol/water partition coefficient K ow (also noted
P) characterizing their hydrophobic nature (log K ow > 0), showing an affinity with
organic phases. Knowledge of solubility is important as it permits to predict and
model DNAPL transport and fate in groundwater. In a DNAPL mixture, a cosolvent
effect must be considered to represent the mole fraction of each constituent of the
mixed DNAPL.
In addition to solubility, other physical properties, e.g., diffusion coefficients and
Henry’s law constant, are important to understand their fate.
Ionization potential (IP) represents the energy required to remove the valence
electron of an atom or a molecule. Generally, for the same chemical structure, the
presence of double bonds decreases IP values, while the presence of chlorine atoms
increases them (Brown 2010). COCs are preferentially oxidized when carbon atoms
have a high electron density, and preferentially reduced when carbon atoms have a
low electron density. The IP value is therefore, in a first approach, a good indicator
helping to select the appropriate remediation technology.
In addition, their electrochemical properties are also affected by the nature of the
COCs. Table 6.2 lists reductive half-reactions for COCs redox couple and their
associated standard potential E
(Boethling and Mackay 2000; Dolfing et al. 2006).
First, the standard aqueous-phase free energy of formation is estimated from the
standard gas-phase free energy of formation and the values of Henry’s law constant
H (Vogel et al. 1987; Dean 2004).
286
R. Rodrigues et al.