particles, results obtained by Shih et al. (2009) have shown the production of the
three tetrachlorobenzene isomers, 1,2,4-TCB and 1,3,5-TCB, and 1,4-DCB, as end
products. With Cu/Fe particles, Zhu et al. (2010) have observed the formation of
PeCB, the three TeCB isomers, 1,2,3-TCB and 1,2,4-TCD, and 1,2-DBC as end
products, without selectivity via a stepwise process.
Carbon tetrachloride undergoes successive hydrogenolysis via a direct electron
transfer mechanism, leading to the formation of chloroform, dichloromethane, and
methane (Song and Carraway 2006). McCormick and Adriaens (2004) have also
observed two other pathways, leading to the formation of CO and methane with the
formation of chlorocarbene intermediates. Dichloromethane reacts very slowly with
traditional iron particles, and tends to accumulate (Gillham and O’Hannesin 1994;
Matheson and Tratnyek 1994). The simultaneous production of dichloromethane
and methane can be interpreted by concerted reductive elimination steps involving
carbine and charged radical species (Song and Carraway 2006). As for chlorinated
ethanes and ethylenes, a complete dechlorination and a higher yield of methane is
obtained with reduction by Pd/Fe bimetallic particles (Muftikian et al. 1995; Lien
and Zhang 1999). Coupling reactions of two trichloromethyl radicals in HCA are
also referenced (He et al. 2015).
6.3.5 Influence of Operating Conditions and Medium
Composition
Besides iron intrinsic characteristics, the operating conditions as well as the solution
chemistry are major factors influencing ZVI reactivity and mobility (Sun et al. 2016).
All these factors can affect COCs degradation rates, especially pH, temperature, the
use of surfactant, and the medium composition.
6.3.5.1 pH
As indicated in Table 6.5, protons participate in the chemical reduction of COCs.
The reaction rate is generally higher at low pH, as the reaction is thermodynamically
more favorable (Table 6.13).
pH effects can be different depending on the compound. Wang and Farrell (2003)
have shown by an electrochemical analysis that TCE dechlorination is 50 times
higher when pH is lowered from 7 to 3, but only 15 times greater for PCE
dechlorination. As reduction with iron particles is a heterogeneous reaction, this
result can be attributed to both sorption and surface-reaction step. By electrochemical impedance spectroscopy analysis, the authors have investigated the role of
atomic hydrogen on PCE and TCE reduction by using iron. Results suggest different
reductive mechanisms: TCE reduction occurred via atomic hydrogen at low pH and
via direct electron transfer at neutral pH, whereas PCE reduction occurred via direct
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
329
three tetrachlorobenzene isomers, 1,2,4-TCB and 1,3,5-TCB, and 1,4-DCB, as end
products. With Cu/Fe particles, Zhu et al. (2010) have observed the formation of
PeCB, the three TeCB isomers, 1,2,3-TCB and 1,2,4-TCD, and 1,2-DBC as end
products, without selectivity via a stepwise process.
Carbon tetrachloride undergoes successive hydrogenolysis via a direct electron
transfer mechanism, leading to the formation of chloroform, dichloromethane, and
methane (Song and Carraway 2006). McCormick and Adriaens (2004) have also
observed two other pathways, leading to the formation of CO and methane with the
formation of chlorocarbene intermediates. Dichloromethane reacts very slowly with
traditional iron particles, and tends to accumulate (Gillham and O’Hannesin 1994;
Matheson and Tratnyek 1994). The simultaneous production of dichloromethane
and methane can be interpreted by concerted reductive elimination steps involving
carbine and charged radical species (Song and Carraway 2006). As for chlorinated
ethanes and ethylenes, a complete dechlorination and a higher yield of methane is
obtained with reduction by Pd/Fe bimetallic particles (Muftikian et al. 1995; Lien
and Zhang 1999). Coupling reactions of two trichloromethyl radicals in HCA are
also referenced (He et al. 2015).
6.3.5 Influence of Operating Conditions and Medium
Composition
Besides iron intrinsic characteristics, the operating conditions as well as the solution
chemistry are major factors influencing ZVI reactivity and mobility (Sun et al. 2016).
All these factors can affect COCs degradation rates, especially pH, temperature, the
use of surfactant, and the medium composition.
6.3.5.1 pH
As indicated in Table 6.5, protons participate in the chemical reduction of COCs.
The reaction rate is generally higher at low pH, as the reaction is thermodynamically
more favorable (Table 6.13).
pH effects can be different depending on the compound. Wang and Farrell (2003)
have shown by an electrochemical analysis that TCE dechlorination is 50 times
higher when pH is lowered from 7 to 3, but only 15 times greater for PCE
dechlorination. As reduction with iron particles is a heterogeneous reaction, this
result can be attributed to both sorption and surface-reaction step. By electrochemical impedance spectroscopy analysis, the authors have investigated the role of
atomic hydrogen on PCE and TCE reduction by using iron. Results suggest different
reductive mechanisms: TCE reduction occurred via atomic hydrogen at low pH and
via direct electron transfer at neutral pH, whereas PCE reduction occurred via direct
6 In Situ Chemical Reduction of Chlorinated Organic Compounds
329