Structure relationship for catalytic dechlorination rate of dichlorobenzenes in water

Article abstract of DOI:10.1016/j.chemosphere.2004.11.071

Three isomers of dichlorobenzene (o-, m- and p-DCB) were dechlorinated by Pd/Fe catalyst in aqueous solutions through catalytic reduction. The dechlorination reaction took place on the surface site of the catalyst via a pseudo-first-order kinetics, and resulted in benzene as the final reduction product. The rate constants of the reductive dechlorination for the three dichlorobenzenes (DCBs) in the presence of Pd/Fe as a catalyst were measured experimentally. In all cases, the reaction rate constants were found to increase with the decrease in the Gibbs free energy of the formation of DCBs. The reaction rate constant for o-, m- and p-DCBs in the presence of 0.020% (w/w) Pd/Fe at 25°C was determined to be 0.0213, 0.0223, and 0.0254 min ^-1, respectively. While the activation energy of each dechlorination reaction was measured to be 102.5, 96.6 and 80.0 kJ mol^-1 for o-, m- and p-DCBs, respectively. The results demonstrated that p-DCBs were reduced more easily than o- or m-DCBs, and the order of the tendency of the dechlorination was p-DCB > m-DCB > o-DCB. The presented data show the catalytic reduction using Pd/Fe as a catalyst is a fast and easy approach for the dechlorination of DCBs.

Full text of DOI:10.1016/j.chemosphere.2004.11.071

Chemosphere 58 (2005) 1497–1502
www.elsevier.com/locate/chemosphere
Structure relationship for catalytic dechlorination rate of
dichlorobenzenes in water
a,
a
Xinhua Xu *, Hongyi Zhou a,b, Dahui Wang
a
Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
b
College of Biological & Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China
Received 27August 2004; received in revised form 13 November 2004; accepted 25 November 2004
Abstract
Three isomers of dichlorobenzene (o-, m- and p-DCB) were dechlorinated by Pd/Fe catalyst in aqueous solutions
through catalytic reduction. The dechlorination reaction took place on the surface site of the catalyst via a pseudo-
first-order kinetics, and resulted in benzene as the final reduction product. The rate constants of the reductive dechlo-
rination for the three dichlorobenzenes (DCBs) in the presence of Pd/Fe as a catalyst were measured experimentally. In
all cases, the reaction rate constants were found to increase with the decrease in the Gibbs free energy of the formation
of DCBs. The reaction rate constant for o-, m- and p-DCBs in the presence of 0.020% (w/w) Pd/Fe at 25 ꢀC was deter-
mined to be 0.0213, 0.0223, and 0.0254 minꢀ1, respectively. While the activation energy of each dechlorination reaction
was measured to be 102.5, 96.6 and 80.0 kJ molꢀ1 for o-, m- and p-DCBs, respectively. The results demonstrated that
p-DCBs were reduced more easily than o- or m-DCBs, and the order of the tendency of the dechlorination was
p-DCB > m-DCB > o-DCB. The presented data show the catalytic reduction using Pd/Fe as a catalyst is a fast and easy
approach for the dechlorination of DCBs.
ꢁ 2004 Elsevier Ltd. All rights reserved.
Keywords: Pd/Fe; Dechlorination; Structure; DCBs
1. Introduction
in various waste oils and other organic liquids. By their
nature, they are highly toxic and poorly biodegradable
compounds that are now established as a class of prior-
ity environmental pollutants by the US EPA (1988).
There is a definite need for efficient methods of dechlo-
rination that are suitable for eliminating chlorobenzenes
from both concentrated industrial effluents and diluted
polluted groundwater (Meharg et al., 2000).
Chlorobenzenes are so widely used that they are
nearly ubiquitous in all major environmental compart-
ments. For example, chlorobenzenes have a wide range
of industrial and domestic uses, such as intermediates
in the synthesis of other chemicals, solvents, hygiene
products, and components of dielectric fluids (Meharg
et al., 2000). They are hazardous pollutants contained
Recently, chemical reduction of hazardous com-
pounds such as chlorinated organic compounds (COCs)
and nitroaromatic compounds (NACs) using zero-valent
metals has been intensively studied for either in situ or
aboveground treatment of contaminated water. For in-
stance, the use of zero-valent iron for the treatment of
*
Corresponding author. Tel.: +86 571 8795 1239; fax: +86
571 8795 2771.
E-mail address: xhxu@hzcnc.com (X. Xu).
0045-6535/$ - see front matter ꢁ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.chemosphere.2004.11.071

1498
X. Xu et al. / Chemosphere 58 (2005) 1497–1502
contaminants in wastewaters and ground waters has
been the focus of much recent research. Iron has success-
fully utilized in the treatment of chlorinated organic
solutions. Zero-valent iron was found to serve as a
donor of electrons (or the reducing agent) in the reaction
(Matheson and Tratnyek, 1994). Zero-valent iron also
reacts with water producing hydrogen gas (which can re-
act with COCs) and hydroxide ions (which lead to the
increase of the pH of the water solutions). The treatment
of COCs by zero-valent iron represents one of the latest
innovative technologies for environmental remediation.
A passive remediation technology involving zero-valent
iron as the reactive material in subsurface barriers was
developed (Gillham and OÕHannesin, 1994a,b). In an-
other study, Orth and Gillham (1996) proposed basic
mechanisms of the reductive transformation of COCs
by metallic iron. Recently, it was also found that the
addition of palladium as a catalyst can speed up the
dechlorination. The dechlorination reaction between
Pd/Fe and small molecule hydrocarbons was found to
be fast enough for treating wastewater in situ (Rosy
et al., 1995). The catalytic dechlorination reaction in
the presence of Pd/Fe was also reported for other classes
of compounds, these include halogenated organics
(Muftikian et al., 1995; Arnold and Roberts, 2000; Far-
rell et al., 2000; Kim and Carraway, 2000; Xu and
Zhang, 2000; Dombek et al., 2001; Liu et al., 2001; Xu
et al., 2003), azoaromatics (Weber, 1996; Nam and Trat-
nyek, 2001), nitroaromatics (Mantha et al., 2001;
Scherer et al., 2001) and inorganics (Melitas et al.,
2001). Most of these studies were concerned with chlori-
nated organics, as chlorinated organics are widespread
and mobile, and they represent important environmental
contaminants.
benzene, acetone, sulfuric acid (H2SO4) and other re-
agents are analytical grade.
2.2. Pd/Fe preparation and characterization
Pd/Fe powders were prepared in an anaerobic glove-
box (under nitrogen gas). Prior to palladization, iron
powders were pretreated by washing using 0.1 M
H2SO4 then acetone and rinsed with distilled water in
order to remove the lower Fe-layers and undesired
organic compounds. An aqueous solution of potassium
hexachloropalladate was added to a bottle containing
iron powders. The solution in the bottle was continu-
ously stirred until the dark orange solution turned to
pale yellow. The deposition of palladium on the surface
of iron particles resulted in a bimetallic surface (Pd/Fe).
Then, the palladized iron was rinsed twice with deion-
ized water and used for reaction without drying.
Surface areas (BET area) of iron and palladized iron
were measured by employing the nitrogen adsorption
method with a ST-03 surface analyzer (Beijing, China).
The morphology of the particles was observed by
XL30ESEM (Philips, Netherlands) scanning electron
microscopy (SEM).
2.3. Batch experimental procedures
The stock solutions of DCBs with desired concentra-
tions were prepared using methanol as the solvent, and
stored in a refrigerator at about 4 ꢀC. Individual (o-,
m- and p-) DCB solution of 50 mg lꢀ1 was prepared
by pipetting the corresponding stock solution and using
water as the solvent. Batch experiments for DCBs
dechlorination were carried out in 75-ml bottles. In most
cases, the bottles containing 4 g Pd/Fe were then filled
with DCBs solutions, leaving no headspace, and sealed
immediately with butyl rubber septa. Each bottle was
placed in an incubator shaker (200 rpm, 25 1 ꢀC). Ali-
quots of samples were withdrawn at times from the
supernatant using a syringe, and filtered with a piece
of 0.45 lm filter film for later-on analysis.
In the present work, we report the experimental
determination of first-order rate constants (k) for the
catalytic dechlorination of three isomers of dichloro-
benzene (namely o-, m- and p-DCBs) in the presence
of a zero-valent bimetallic Pd/Fe catalyst, and their
reaction activation energies. The structure relationship
for the catalytic dechlorination of three dichlorobenz-
enes in water is discussed. Thus it is the purpose of this
study to identify further molecular structural character-
istics and their corresponding structural descriptors
relevant to dechlorination of chlorobenzenes by Pd/Fe
bimetal.
2.4. Analytical method
The organic compounds such as benzene, chloroben-
zene, DCBs, etc. were measured by HPLC. Analysis
parameters were as follows: Instrument: Waters High
Performance Liquid Chromatography. Column: Nova-
Pak C18, 4 lm, 3.9 · 150 mm, Mobile phase: MeOH/
H2O (80/20). Flow rate: 1.0 ml minꢀ1. Detector: UV at
254 nm. Column temperature: 35 ꢀC. Sample size: 20 ll.
Chloride analysis was performed by ion chromato-
graphy (792 Basic IC, Metrohm). Column: Metrosep
A Supp4, Column size: 4 · 250 mm. Analysis condition:
2. Experiments and methods
2.1. Chemicals
Potassium hexachloropalladate (99%, Aldrich,
USA), o-DCB, m-DCB and p-DCB (>98.5%), chloro-
benzene (>98.5%), methanol (reagent for HPLC,
>99.9%), iron powder reduced (>200 mesh, >98.0%),
Eluent: 1.7mM NaHCO + 1.8 mM Na2CO3 (with
3
chemical suppression), Sample size: 20 ll, Flow rate:

X. Xu et al. / Chemosphere 58 (2005) 1497–1502
1.0 ml minꢀ1, Detector: suppressed conductivity detec-
1499
0.006
0.005
0.004
0.003
0.002
0.001
0.000
tor. Before injected, sample solutions were always fil-
tered through a 0.45 lm membrane filter.
Each dechlorination efficiency of DCBs was calcu-
lated using the ratio between free chloride measured
experimentally and free chloride calculated theoretically
by the complete dechlorination of DCBs.
3. Results and discussion
3.1. Characterization of Pd/Fe
0
2
4
6
8
10
12
14
16
The surface morphology of Pd/Fe was shown in
Fig. 1. The surface of Pd/Fe became much rough with
palladium deposited on the iron surface. BET specific
surface area of the Pd/Fe particles was 0.62 m2 gꢀ1. In
comparison, the fine iron powder, which is commer-
cially available, has a specific surface area of about
0.49 m2 gꢀ1. The increased surface area indicated that
the Pd was dispersed on the surface rather than clus-
tered, which was consistent with the information re-
vealed in the SEM image of the Pd/Fe catalyst.
Time (min)
Fig. 2. HPLC chromatograms of the reaction mixture during
reaction.
3.2. Dechlorination of DCBs
Fig. 2 illustrates the HPLC chromatograms during
the dechlorination of o-DCB. o-DCB is first transformed
to chlorobenzene then reduced to benzene quickly, be-
cause o-DCB or chlorobenzene was absorbed on the
surface of Pd/Fe bimetal during the dechlorination,
some o-DCB is reduced to chlorobenzene and dechlori-
nated to benzene directly on the surface of Pd/Fe and
did not return to the solution. The reduction products
for m- and p-DCB were also chlorobenzene and
benzene.
A typical concentration profile resulting from the
batch tests was shown in Fig. 3. As seen from the figure,
the aqueous concentration of the o-DCB declined rap-
idly and dropped below detectable limits within 60
min. Final reaction products in the solution were deter-
mined to be benzene and inorganic chloride. In the solu-
tion, little chlorobenzene was identified and remained
50
o-DCB
40
Chloride
Benzene
30
Chlorobenzene
20
10
0
0
10
20
30
40
50
60
Reaction time (min)
Fig. 3. Transformation of o-dichlorobenzene over Pd/Fe cata-
lysts, C0 = 50 mg lꢀ1, Pd loading 0.020%, Pd/Fe powder 4 g 75
mlꢀ1, T = 25 ꢀC, pH = 6.5.
Fig. 1. SEM images of Pd/Fe bimetallic powders: (a) before
dechlorination; (b) after dechlorination.

1500
X. Xu et al. / Chemosphere 58 (2005) 1497–1502
lower concentration throughout the experiment. The
chloride formation detected was only 84.0% of the maxi-
mum attainable when all o-DCB and chlorobenzene
were removed. It was most likely that o-DCB and chlo-
robenzene could be absorbed or covered by surface pas-
sivating layers due to the precipitation of metal
hydroxides and metal carbonates on the surface of iron
and Pd/Fe (see Fig. 1(b)), in fact, chloride ion also can
be covered or absorbed, so the dechlorination efficiency
was commonly less than 100% in all reactions under
study, but o-DCB could be removed 100%. Our results
were in very good agreement with previous findings
(Burris et al., 1995). The dechlorinations of m- and p-
DCB also show the same results as o-DCB.
stant (8.314 J molꢀ1 Kꢀ1), and T is the reaction temper-
ature (K).
The effect of reaction temperature on the effectiveness
of the dechlorination efficiency was studied by varying
the reaction temperature from 287to 313 K. By rear-
ranging the experiment data at 25 ꢀC shown in Fig. 3,
a linear decrease of ln(Co-DCB/Co-DCB0) with time is
obtained as shown in Fig. 4. This linear relationship
reveals a pseudo-first-order reduction reaction regarding
the dechlorination efficiency. As a result, the reaction
rate constants (k) were determined to be 0.0016,
0.0213, 0.0315, 0.0350 and 0.0651 minꢀ1, respectively,
with a reaction temperature of 14, 25, 30, 35 and 40 ꢀC
(Fig. 4).
The reaction rate constants (k) of m-DCB and p-
DCB at difference temperatures also can be determined
from Fig. 5 as 0.0033, 0.0223, 0.0347, 0.0380 and
0.0789 minꢀ1, respectively, with the temperature 17,
25, 30, 35 and 40 ꢀC for m-DCB, and 0.0045, 0.0254,
0.0355, 0.0473 and as 0.0690 minꢀ1, respectively, with
the temperature 15, 25, 30, 35 and 40 ꢀC for p-DCB.
The results are plotted in Figs. 4 and 5. As seen in
figures, an increase in the reaction temperature can
enhance the reaction rates significantly. The k values cal-
culated at various temperatures were correlated by Eqs.
(4)–(6) and shown in Fig. 6, which gives rise to an esti-
mated activation energy of 102.5, 96.6 and 80.0 kJ
molꢀ1, respectively, for o-, m- and p-DCB:
3.3. Catalytic reduction rate constants
In these dechlorination reactions, most DCBs are
first transformed to chlorobenzene then reduced to ben-
zene quickly, while a small amount of DCBs are reduced
to benzene directly. The reaction pathway for the con-
version of DCBs by Pd/Fe bimetal is illustrated as
follows:
ð1Þ
12333
ln k ¼ 36:92 ꢀ
ð4Þ
ð5Þ
ð6Þ
T
The dechlorination rate of DCBs by Pd/Fe in a batch
system can be determined by the following equation:
11616
ln k ¼ 34:685 ꢀ
ꢀ
ꢁ
T
ꢀ
CCl
ln 1 ꢀ
¼ ꢀkobst
ð2Þ
9627:5
2CDCB0
ln k ¼ 28:26 ꢀ
T
where Clꢀ is the concentration of chloride ion generated
in the aqueous phase (mM); CDCB0 is the concentration
of DCBs, and kobs is the observed first-order reaction
rate constant (minꢀ1).
3.0
2.5
2.0
1.5
1.0
0.5
40˚C
35˚C
30˚C
25˚C
14˚C
The dependence of the reaction rate constant on the
structure of the substrates is not surprising. Dolfing and
Harrison (1992) have shown that the rate constant for
the reduction of halogenated aromatics in anaerobic
estuarine sediment was proportional to the Gibbs free
energy or the activation energy of formation Ea. Ea is
one of the physical chemical parameters of compounds
that reveals the difference in molecular structures.
The integrated form of the proposed pseudo-first-
order kinetic equation was
0.0
0
10
20
30
40
50
60
Ea
RT
Time (min)
ln k ¼ A ꢀ
ð3Þ
Fig. 4. Linear decrease of normalized dechlorination efficiency
(ln) with the reaction time under different temperatures for o-
dichlorobenzene, C0 = 50 mg lꢀ1, Pd loading 0.020%, Pd/Fe
powder 4 g 75 mlꢀ1, pH = 6.5.
where Ea is the activation energy (kJ molꢀ1), A the pre-
exponential factor, k the observed pseudo-first-order
reaction rate constant (minꢀ1), R the universal gas con-

X. Xu et al. / Chemosphere 58 (2005) 1497–1502
1501
tuted congener and the tendency of rate constant follows
p-DCB > m-DCB > o-DCB.
5.0
4.0
3.0
2.0
1.0
0.0
17˚C
25˚C
30˚C
35˚C
40˚C
4. Conclusions
The reductive dechlorination of DCBs in the presence
of Pd/Fe is found to be a pseudo-first-order reaction,
and occurs primarily at the catalyst surface. The reac-
tion rate constant increases with the decrease in the
Gibbs free energy of the formation of DCBs, with the
rate constant being 0.0213, 0.0223 and 0.0254 minꢀ1
,
respectively, for o-, m- and p-DCBs with 0.020% Pd/Fe
at 25 ꢀC. The activation energy of the reaction was
determined to be 102.5 kJ molꢀ1 for o-dichlorobenzene,
96.6 kJ molꢀ1 for m-dichlorobenzene and 80.0 kJ molꢀ1
for p-dichlorobenzene. Molecular structural descriptors
that are significantly relevant to logk and the activation
energy include the descriptors describing the overall
character of the molecules and the descriptors describing
the nature of the chlorine atoms. Our results show p-
DCBs were reduced more easily than o- and m-DCBs,
and the rate constant of the dechlorination for each
DCB was p-DCB > m-DCB > o-DCB. Pd/Fe catalytic
reduction provides a fast and easy means for the dechlo-
rination of all three DCBs. The present finding can be
valuable in designing in situ treatment of dichlorobenz-
enes contaminated water.
0
10
20
30
40
50
60
time (min)
(m-DCB)
15˚C
25˚C
3.0
2.5
2.0
1.5
1.0
0.5
0.0
30˚C
35˚C
40˚C
0
10
20
30
40
50
60
time (min)
(p-DCB)
Acknowledgment
Fig. 5. Linear decrease of normalized dechlorination efficiency
(ln) with the reaction time under different temperatures for m-
dichlorobenzene and p-dichlorobenzene, C0 = 50 mg lꢀ1, Pd
loading 0.020%, Pd/Fe powder 4 g 75 mlꢀ1, pH = 6.5.
The authors are grateful for the financial support
provided by the National Natural Science Foundation
of China (No. 20407015) and the Science and Technol-
ogy Project of Zhejiang Province, PR China (No.
2004C34006).
p-DCB
m-DCB
References
6.0
o-DCB
Arnold, W.A., Roberts, A.L., 2000. Pathways and kinetics of
chlorinated ethylene and chlorinated acetylene reaction with
Fe (0) particles. Environ. Sci. Technol. 34 (9), 1794–1805.
Burris, D.R., Campbell, T.J., Manoranjan, V.S., 1995. Sorption
of trichloroethylene and tetrachloroethylene in a batch
reactive metallic iron–water system. Environ. Sci. Technol.
29 (11), 2850–2855.
4.0
2.0
0.0
Dolfing, J., Harrison, B.K., 1992. Gibbs free energy of
formation of halogenated aromatic compounds and their
potential role as electron acceptor in anaerobic environ-
ments. Environ. Sci. Technol. 26 (11), 2213–2216.
0.00315
0.00325
0.00335
1/T
0.00345
Dombek, T., Dolan, E., Schultz, J., 2001. Rapid reductive
dechlorination of atrazine by zero-valent iron under acidic
conditions. Environ. Pollut. 111, 21–27.
Fig. 6. Correlation for reaction rate constant k with reaction
temperature for three isomers of dichlorobenzene.
Farrell, J., Kason, M., Melitas, N., 2000. Electrochemical and
column investigation of iron-mediated reductive dechlori-
nation of trichloroethylene and perchloroethylene. Environ.
Sci. Technol. 34 (12), 2549–2556.
The results revealed higher chlorine-substituted
homologues were reduced more easily than lower-substi-

1502
X. Xu et al. / Chemosphere 58 (2005) 1497–1502
Gillham, R.W., OÕHannesin, S.F., 1994a. Enhanced degrada-
tion of halogenated aliphatics by zero-valent iron. Ground
Water 32, 958–967.
Muftikian, R., Fernando, Q., Korte, N., 1995. A method for
the rapid dechlorination of low molecular weight chlori-
nated hydrocarbons in water. Water Res. 29 (10), 2434–
2439.
Gillham, R.W., OÕHannesin, S.F., 1994b. Development of zero
valent iron as an in situ reactant for remediation of VOC
contaminated groundwater. Groundwater Remediation:
Existing Technology and Future Directions, Association
of Groundwater Scientists and Engineers, Las Vegas, NV,
9–12 October.
Nam, S., Tratnyek, P.G., 2001. Reduction of azo dyes with
zero-valent iron. Water Res. 34, 1837–1845.
Orth, W.S., Gillham, R.W., 1996. Dechlorination of trichloro-
ethene in aqueous solution using Fe0. Environ. Sci. Technol.
30 (1), 66–71.
Kim, Y., Carraway, E.R., 2000. Dechlorination of pentachlo-
rophenol by zero valent iron and modified zero valent irons.
Environ. Sci. Technol. 34 (10), 2014–2017.
Rosy, M., Quintus, F., Nic, K., 1995. A method for the rapid
dechlorination of low molecular weight chlorinated hydro-
carbons in water. Water Res. 29, 2434–2439.
Liu, Y.H., Yang, F.L., Yue, P.L., 2001. Catalytic dechlorina-
tion of chlorophenols in water by palladium/iron. Water
Res. 35 (8), 1887–1890.
Scherer, M.M., Johnson, K.M., Westall, J.C., 2001. Mass
transport effects on the kinetics of nitrobenzene reduction
by iron metal. Environ. Sci. Technol. 35 (13), 2804–2811.
US Environmental Protection Agency, 1988. National pollu-
tant discharge elimination system, Code of Federal Regu-
lations, 40, Part 122. US Government Printing Office,
Washington, DC.
Mantha, R., Taylor, K., Biswas, N., 2001. A continuous system
for Fe0 reduction of nitrobenzene in synthetic wastewater.
Environ. Sci. Technol. 35 (15), 3231–3236.
Matheson, L.J., Tratnyek, P.G., 1994. Reductive dehalogen-
ation of chlorinated methanes by iron metal. Environ. Sci.
Technol. 28 (12), 2045–2053.
Weber, E.J., 1996. Iron-mediated reductive transformations:
investigation of reaction mechanism. Environ. Sci. Technol.
30 (2), 716–719.
Meharg, A.A., Wright, J., Osborn, D., 2000. Chlorobenzenes in
rivers draining industrial catchments. Sci. Total Environ.,
251–252, 243–253.
Xu, Y., Zhang, W.X., 2000. Subcolloidal Fe/Ag particles for
reductive dehalogenation of chlorinated benzenes. Ind. Eng.
Chem. Res. 39 (7), 2238–2244.
Melitas, N., Chuffe-Moscoso, O., Farrell, J., 2001. Kinetics of
soluble chromium removal from contaminated water by
zerovalent iron media: corrosion inhibition and passive
oxide effects. Environ. Sci. Technol. 35 (19), 3948–3953.
Xu, X.H., Zhou, H.Y., Wang, D.H., 2003. Catalytic dechlori-
nation of chlorobenzene in water by Pd/Fe system. Chin.
Chem. Lett. 14 (7), 700–703.