1890
Yihui Liu et al.
reveals the difference in molecular structures. They
found that the reduction reaction rate is faster for the
o
f
o
f
smaller DG compounds. While the values of DG of
o-, m-, and p-chlorophenol are ꢀ56.8, ꢀ56.4, and
ꢀ53.1 kJ/mol, respectively, the variation of the
reaction rate constants obtained here are consistent
with the previous analysis by Dolfing et al. (1992).
CONCLUSION
Dechlorination of chlorophenols using Pd/Fe takes
place on the catalyst surface. The conversion rate
depends on the Pd loading. The reduction reaction is
found to be a pseudo-first-order reaction. The
reaction rate constant increases with the decrease in
the Gibbs free energy of the formation of chloro-
phenols, with the rate constant being 0.0215, 0.0155
and 0.0112 min , respectively, for o-, m- and p-
chlorophenols with 0.048% Pd/Fe. Pd/Fe reduction
is a fast and easy dechlorination method for
chlorophenols. The present finding can be valuable
in designing in situ treatment of chlorophenol
contaminated water.
Fig. 6. History of reduction of chlorophenols and respective
formation of phenol.
ꢀ
1
REFERENCES
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Dolfing J. and Harrison B. K. (1992) Gibbs free energy of
formation of halogenated aromatic compounds and their
potential role as electron acceptor in anaerobic envir-
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Fig. 7. Linear decrease of normalized chlorophenol con-
centration (Ln) with reaction time.
Esplugas E., Yue P. L. and Pervez M. I. (1994) Degradation
of 4-chlorophenol by photolytic oxidation. Water Res. 28,
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323–1328.
catalyst were used in each experiment. Apparently, Grittini C., Malcomson M., Farnando Q. and Korte N.
(
1995) Rapid dechlorination of polychlorinated biphenyls
the location of the chlorine atom on the phenol plays
an important role. o-Chlorophenol concentration
drops below detectable limits within 2 h while it
takes nearly 3 h for m-chlorophenol and 5 h for
p-chlorophenol.
on the surface of Pd/Fe bimetallic system. Environ. Sci.
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Matheson L. J. and Tratnyek P. G. (1994) Reductive
dehalogenation of chlorinated methanes by iron metal.
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rapid dechlorination of low molecular weight chlorinated
hydrocarbons in water. Water Res. 29, 2434–2439.
Rearranging the data shown in Fig. 6 with a linear
o
decrease of Ln(C/C ) over time is obtained as shown
in Figure 7. This linear relationship reveals a pseudo- Schreier C. G. and Reinhard M. (1994) Transformation of
first-order reduction reaction regarding the chloro-
phenol concentration. The reaction rate constants
chlorinated organic compounds by iron and manganese
powders in buffered water and in landfill leachate.
Chemosphere 29, 1743–1753.
Smith M. H. and Woods S. L. (1994) Regiospecificity of
chlorophenols reductive dechlorination by vitamin B12s.
Appl. Environ. Microbiol. 60, 4111–4115.
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(
K) can be determined from Fig. 7 as 0.0215, 0.0155
ꢀ
1
and 0.0112 min , respectively, for o-, m-, p-chloro-
phenol with 0.048% Pd/Fe. The standard errors for
the determined rate constants are 0.0010, 0.0005 and
0.0006, respectively.
The dependence of the reaction rate constant on
the structure of the substrates is not surprising. US Environmental Protection Agency. (1980–1988)
Dolfing et al. (1992) have shown that the rate
constant for the reduction of halogenated aromatics
in anaerobic estuarine sediment is proportional to the
National pollutant discharge elimination system, Code
of FederalRe gulations, 40, Part 122. US Government
Printing Office, Washington, DC.
Wilson E. K. (1995) Zero-valent metals provide possible
solution to groundwater problem. Chem. Engng. News
73(27), 19–23.
o
o
Gibbs free energy of formation DG . DG is one of
the physical chemical parameters of compounds that
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