Environmental Engineering Reference
In-Depth Information
7.3.1.2 Dehalogenation
Dehalogenation of halogenated organic compounds with bimetallic Pd/Fe
nanoparticles has been tested, including chlorinated methanes, ethylenes, and ethanes,
and trihalomethanes. Results have demonstrated that bimetallic Pd/Fe nanoparticles can
rapidly dehalogenate a wide array of chlorinated organic compounds including: carbon
tetrachloride (CCl
4
), chloroform (CHCl
3
), tetrachloroethylene (C
2
Cl
4
), trichloroethylene
(C
2
HCl
3
), dichloroethylenes (C
2
H
2
Cl
2
), vinyl chloride (C
2
H
3
Cl), hexachloroethane
(C
2
Cl
6
), pentachloroethane (C
2
HCl
5
), tetrachloroethanes (C
2
H
2
Cl
4
), trichloroethane
(C
2
H
3
Cl
3
), dichloroethanes (C
2
H
4
Cl
2
), bromoform (CHBr
3
), dibromomethane (CH
2
Br
2
),
dibromochloromethane (CHBr
2
Cl) and dichlorobromomethane (CHCl
2
Br). In general,
hydrocarbons such as methane and ethane are the major products. It should be noted that
dichloromethane and dichloroethanes are unable to be degraded by bimetallic Pd/Fe
nanoparticles.
A summary of the product distributions and kinetic information in the
dehalogenation is given in Table 7.3. Specifically, for halogenated methanes, methane
was the primary product with significant amounts of halogenated intermediates. For
example, methane, chloroform and dichloromethane were observed in the transformation
of carbon tetrachloride. For chlorinated ethylenes, ethane and ethylene were the primary
products, and few intermediates were detected. For chlorinated ethanes, ethane and
ethylene appeared as the major products. Chlorinated ethylenes were noticed as a minor
intermediate in the transformation of hexachloroethane, pentachloroethane, and
tetrachloroethanes.
7.3.1.3 Formation of Long-Chain Hydrocarbons
Long-chain hydrocarbons (longer than parent compounds) such as C
3
to C
5
alkanes (e.g., propane, hexane) in dehalogenation of chlorinated organic compounds
using micro-sized ZVI and palladium have been reported (Liang, et al., 1997; Campbell,
et al., 1997; Fennelly, and Roberts, 1998). The production of many hydrocarbons in the
presence of iron was also observed even in the absence of chlorinated organic
compounds (Hardy and Gillham, 1996; Deng, et al., 1997). Possible carbon sources for
those hydrocarbons appearing in the absence of chlorinated organic compounds may
include carbide carbon in the iron (Deng, et al., 1997) and aqueous CO
2
(Hardy and
Gillham, 1996). Although the sources of carbon are still an open question, it is believed
that hydrocarbon formation is likely the result of surface-mediated reactions similar to
the well-known Fischer-Tropsch process.
Fischer-Tropsch synthesis is a classic heterogeneous reaction where long-chain
hydrocarbons are produced by contacting CO and hydrogen gas with catalysts such as
iron and nickel (Satterfield, 1991). Hydrocarbons produced from Fischer-Tropsch
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