For bimetallic particles, as the main degradation mechanism is
hydrodechlorination, there is a relation between pH and degradation. For TCE
dechlorination, He and Zhao (2008) have shown that lowering the pH from 9 to
6, the rate-determining step shift from iron corrosion to hydrodechlorination. Indeed,
H 2 production is more rapid in acidic conditions. However, some studies have
shown that strongly acidic conditions (pH < 3) are detrimental due to the more
important iron corrosion, resulting in the accumulation of H 2 bubbles, and the
destabilization of the catalyst (Dong et al. 2011; Huang et al. 2016b). The opposite
observation was reported for TCE degradation with sulfidated particles, where the
constant rate increased from 0.104 h
À1 at pH 7 to 0.137 h
À1 at pH 9 (Rajajayavel and
Ghoshal 2015).
6.3.5.2 Temperature
Temperature is known to have an effect on many physical and chemical properties of
COCs, such as their solubility, Henry’s law constant, density, viscosity, or interfacial
tension (Stephenson 1992; Sleep and Ma 1997; Heron et al. 1998; Knauss et al.
2000; Mackay et al. 2006; Chen et al. 2012; Rodrigues et al. 2017a), thus affecting
their transport and fate processes. Temperature has also an effect on chemical
reactions. A phenomenological law has been proposed in 1889 by Arrhenius to
establish the relation between the constant rate k and the temperature
T (K) (Eq. 6.54).
k ¼ A e
ÀEa=RT
ð6:54Þ
where A is the pre-exponential factor (unit of k), Ea the energy activation (J mol
À1 ),
and R the universal gas constant (J mol
À1 K
À1 ). The Arrhenius equation is used to
calculate activation energy of a reaction, which represents the minimum energy
required to result in a chemical reaction (effective shock between molecules).
Values of reduction constant rates of reduction of four COCs at different temperature are shown in Table 6.14. For TCE reduction, a rise in the temperature from
10 to 55
C increase the rate constant by a factor of about 10.
6.3.5.3 Surfactant
A surfactant is an amphiphilic compound, having both a hydrophilic and a lipophilic
group in its structure. Surfactants have a strong tendency to accumulate at the
interface between two distinct phases, causing alterations of interfacial properties
according to the molecular structure, pH, and temperature (Rosen and Kunjappu
2012). When dissolved in water, surfactants adsorb to the different interfaces (air–
water, COC–water, and solid–water) and reduce the interfacial tension. Once the
interface is saturated, the interfacial tension stabilizes at its minimal value.
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
331
hydrodechlorination, there is a relation between pH and degradation. For TCE
dechlorination, He and Zhao (2008) have shown that lowering the pH from 9 to
6, the rate-determining step shift from iron corrosion to hydrodechlorination. Indeed,
H 2 production is more rapid in acidic conditions. However, some studies have
shown that strongly acidic conditions (pH < 3) are detrimental due to the more
important iron corrosion, resulting in the accumulation of H 2 bubbles, and the
destabilization of the catalyst (Dong et al. 2011; Huang et al. 2016b). The opposite
observation was reported for TCE degradation with sulfidated particles, where the
constant rate increased from 0.104 h
À1 at pH 7 to 0.137 h
À1 at pH 9 (Rajajayavel and
Ghoshal 2015).
6.3.5.2 Temperature
Temperature is known to have an effect on many physical and chemical properties of
COCs, such as their solubility, Henry’s law constant, density, viscosity, or interfacial
tension (Stephenson 1992; Sleep and Ma 1997; Heron et al. 1998; Knauss et al.
2000; Mackay et al. 2006; Chen et al. 2012; Rodrigues et al. 2017a), thus affecting
their transport and fate processes. Temperature has also an effect on chemical
reactions. A phenomenological law has been proposed in 1889 by Arrhenius to
establish the relation between the constant rate k and the temperature
T (K) (Eq. 6.54).
k ¼ A e
ÀEa=RT
ð6:54Þ
where A is the pre-exponential factor (unit of k), Ea the energy activation (J mol
À1 ),
and R the universal gas constant (J mol
À1 K
À1 ). The Arrhenius equation is used to
calculate activation energy of a reaction, which represents the minimum energy
required to result in a chemical reaction (effective shock between molecules).
Values of reduction constant rates of reduction of four COCs at different temperature are shown in Table 6.14. For TCE reduction, a rise in the temperature from
10 to 55
C increase the rate constant by a factor of about 10.
6.3.5.3 Surfactant
A surfactant is an amphiphilic compound, having both a hydrophilic and a lipophilic
group in its structure. Surfactants have a strong tendency to accumulate at the
interface between two distinct phases, causing alterations of interfacial properties
according to the molecular structure, pH, and temperature (Rosen and Kunjappu
2012). When dissolved in water, surfactants adsorb to the different interfaces (air–
water, COC–water, and solid–water) and reduce the interfacial tension. Once the
interface is saturated, the interfacial tension stabilizes at its minimal value.
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
331