2.2 Chemical Pathways of Pollutants Removal by
Zerovalent Iron
Under anaerobic conditions, Fe
0 can be oxidized by H 2 O or H
+ yielding Fe
2+ and H 2 ,
both of which are also potential reducing agents for contaminants. There are two
main dehalogenation reactions by which the organic compounds can be reduced by
ZVI: hydrogenolysis (replacement of a halogen atom by a hydrogen) and reductive
elimination, in which two halide ions are released. In both reactions, there is a net
transfer of two electrons and they can be mediated either by atomic hydrogen
transfer or by direct electron transfer.
The mechanism and reactivity of chlorinated compounds reduction by ZVI is
somewhat controversial and still not very well understood. While some papers show
an increased reactivity with an increasing number of halogen atoms in the organic
compounds, others show the opposite trend. It seems that these conflicting data result
from differences in the materials—ZVI produced by borohydride reduction versus
ZVI produced by hydrogen reduction from iron minerals—as well as from differences in the experimental conditions (Elsner and Hofstetter 2011).
Wang and Farrell (2003), for example, observed that TCE reduction occurred
almost exclusively by atomic hydrogen transfer at low pH values and by atomic
hydrogen transfer and direct electron transfer at neutral pH values, while PCE
reacted mainly via direct electron transfer at both low and neutral pH values.
However, in acid conditions and micromolar concentrations, TCE reaction rates
were faster than those of PCE due to faster reduction of TCE by atomic hydrogen
transfer, while in neutral environment and millimolar concentrations, PCE reaction
rates were faster than those of TCE. This variation of relative reaction rates was
explained by a lower contribution of the atomic hydrogen reaction mechanism with
increasing pH values and pollutant concentrations.
In the case of chlorinated methanes, the degradation pathways by reaction with
zerovalent iron may differ from the chlorinated ethenes. Song and Carraway (2006),
for example, observed that CCl 4 (CT), CHCl 3 , and CH 2 Cl 2 degradation rates were
not affected by changing the hydrogen concentration in water or reaction atmosphere. So, in contrast to TCE degradation by nano ZVI synthesized by borohydride
(Liu et al. 2005), no catalytic hydrogenation was found to be the degradation route
for any of these compounds. Actually, CH 2 Cl 2 is the main degradation product
during CCl 4 reduction by ZVI and it is considered a as the final product since no
degradation was observed by ZVI reaction. However, CH 4 is also produced but in
very low amounts and is claimed to be generated directly through CCl 4 via a
concerted elimination steps mechanism mediated by carbon radicals and carbanions.
Li and Farrell (2001) published an electrochemical investigation of the rate
limiting mechanisms for TCE and CCl 4 reduction and concluded that rates of CT
reduction were limited by the rate of outer-sphere electron transfer, while rates of
TCE reduction were not limited by rates of electron transfer. Reduction via an outersphere mechanism requires only physical adsorption of CT on or near the ZVI
surface. Then the production of chlorinated byproducts from chloroalkanes would
26
T. Phenrat et al.
Zerovalent Iron
Under anaerobic conditions, Fe
0 can be oxidized by H 2 O or H
+ yielding Fe
2+ and H 2 ,
both of which are also potential reducing agents for contaminants. There are two
main dehalogenation reactions by which the organic compounds can be reduced by
ZVI: hydrogenolysis (replacement of a halogen atom by a hydrogen) and reductive
elimination, in which two halide ions are released. In both reactions, there is a net
transfer of two electrons and they can be mediated either by atomic hydrogen
transfer or by direct electron transfer.
The mechanism and reactivity of chlorinated compounds reduction by ZVI is
somewhat controversial and still not very well understood. While some papers show
an increased reactivity with an increasing number of halogen atoms in the organic
compounds, others show the opposite trend. It seems that these conflicting data result
from differences in the materials—ZVI produced by borohydride reduction versus
ZVI produced by hydrogen reduction from iron minerals—as well as from differences in the experimental conditions (Elsner and Hofstetter 2011).
Wang and Farrell (2003), for example, observed that TCE reduction occurred
almost exclusively by atomic hydrogen transfer at low pH values and by atomic
hydrogen transfer and direct electron transfer at neutral pH values, while PCE
reacted mainly via direct electron transfer at both low and neutral pH values.
However, in acid conditions and micromolar concentrations, TCE reaction rates
were faster than those of PCE due to faster reduction of TCE by atomic hydrogen
transfer, while in neutral environment and millimolar concentrations, PCE reaction
rates were faster than those of TCE. This variation of relative reaction rates was
explained by a lower contribution of the atomic hydrogen reaction mechanism with
increasing pH values and pollutant concentrations.
In the case of chlorinated methanes, the degradation pathways by reaction with
zerovalent iron may differ from the chlorinated ethenes. Song and Carraway (2006),
for example, observed that CCl 4 (CT), CHCl 3 , and CH 2 Cl 2 degradation rates were
not affected by changing the hydrogen concentration in water or reaction atmosphere. So, in contrast to TCE degradation by nano ZVI synthesized by borohydride
(Liu et al. 2005), no catalytic hydrogenation was found to be the degradation route
for any of these compounds. Actually, CH 2 Cl 2 is the main degradation product
during CCl 4 reduction by ZVI and it is considered a as the final product since no
degradation was observed by ZVI reaction. However, CH 4 is also produced but in
very low amounts and is claimed to be generated directly through CCl 4 via a
concerted elimination steps mechanism mediated by carbon radicals and carbanions.
Li and Farrell (2001) published an electrochemical investigation of the rate
limiting mechanisms for TCE and CCl 4 reduction and concluded that rates of CT
reduction were limited by the rate of outer-sphere electron transfer, while rates of
TCE reduction were not limited by rates of electron transfer. Reduction via an outersphere mechanism requires only physical adsorption of CT on or near the ZVI
surface. Then the production of chlorinated byproducts from chloroalkanes would
26
T. Phenrat et al.