Hydrodehalogenation of 1- to 3-carbon halogenated organic compounds in water using a palladium catalyst and hydrogen gas

Article abstract of DOI:10.1021/es980963m

Supported palladium (Pd) metal catalysts-along with H₂ gas show significant potential as a technology which can provide rapid, on-site destruction of halogenated ground-water contaminants. Pd catalyzes the rapid hydrodehalogenation of nine 1- to 3-carbon HOCs, resulting in little or no production of halogenated intermediates. Initial transformation rates were compared for 12 HOCs using 1 w/w% Pd-on-Al₂O₃ (Pd/Al) and metallic Pd catalysts in clean, aqueous batch systems at ambient pressure and temperature. Half-lives of 4-6 min were observed for 5-100 μM (1-10 mg/L) aqueous concentrations of PCE, TCE, cis-, trans-, 1,1-DCE, carbon tetrachloride, and 1,2-dibromo-3-chloropropane at ambient temperature and pressure with 0.22 g/L of catalyst. Using Pd/Al, TCE transformed quantitatively (97%) to ethane without formation of any detectable chlorinated intermediate compounds. This implies a direct conversion of TCE to ethane at the Pd surface. Carbon tetrachloride transformed primarily to methane and ethane and minor amounts of ethylene, propane, and propylene. Chloroform is a reactive intermediate (20%). Formation of C2 and C3 products implies a free radical mechanism. Methylene chloride, 1,1-dichloroethane, and 1,2-dichloroethane were nonreactive. Reaction mechanisms and kinetic models are postulated for TCE, carbon tetrachloride, and chloroform transformation. Supported palladium (Pd) metal catalysts along with H₂ gas show significant potential as a technology which can provide rapid, on-site destruction of halogenated groundwater contaminants. Pd catalyzes the rapid hydrodehalogenation of nine 1- to 3-carbon HOCs, resulting in little or no production of halogenated intermediates. Initial transformation rates were compared for 12 HOCs using 1 w/w% Pd-on-Al₂O₃ (Pd/Al) and metallic Pd catalysts in clean, aqueous batch systems at ambient pressure and temperature. Half-lives of 4-6 min were observed for 5-100 μM (1-10 mg/L) aqueous concentrations of PCE, TCE, cis-, trans-, 1,1-DCE, carbon tetrachloride, and 1,2-dibromo-3-chloropropane at ambient temperature and pressure with 0.22 g/L of catalyst. Using Pd/Al, TCE transformed quantitatively (97%) to ethane without formation of any detectable chlorinated intermediate compounds. This implies a direct conversion of TCE to ethane at the Pd surface. Carbon tetrachloride transformed primarily to methane and ethane and minor amounts of ethylene, propane, and propylene. Chloroform is a reactive intermediate (20%). Formation of C2 and C3 products implies a free radical mechanism. Methylene chloride, 1,1-dichloroethane, and 1,2-dichloroethane were nonreactive. Reaction mechanisms and kinetic models are postulated for TCE, carbon tetrachloride, and chloroform transformation.

Full text of DOI:10.1021/es980963m

Environ. Sci. Technol. 1999, 33, 1905-1910
SCHEME 1. Sequential and Direct Reaction Pathways for
Conversion of TCE to Ethane
Hydrodehalogenation of 1- to
-Carbon Halogenated Organic
3
Compounds in Water Using a
Palladium Catalyst and Hydrogen
Gas
G R E G O R Y V . L O W R Y A N D
M A R T I N R E I N H A R D *
Department of Civil and Environmental Engineering,
Stanford University, Stanford, California 94305-4020
of iron, and accumulation of toxic products in some cases.
For example, carbon tetrachloride is readily converted to
chloroform and more slowly to methylene chloride (MeCl),
but the reaction essentially stops there and MeCl accumulates
Supported palladium (Pd) metal catalysts along with H2
gas show significant potential as a technology which can
provide rapid, on-site destruction of halogenated ground-
water contaminants. Pd catalyzes the rapid hydrodehalo-
genation of nine 1- to 3-carbon HOCs, resulting in little
or no production of halogenated intermediates. Initial
transformation rates were compared for 12 HOCs using 1
w/w% Pd-on-Al2O3 (Pd/Al) and metallic Pd catalysts in
clean, aqueous batch systems at ambient pressure and
temperature. Half-lives of 4-6 min were observed for 5-100
µM (1-10 mg/L) aqueous concentrations of PCE, TCE,
cis-, trans-, 1,1-DCE, carbon tetrachloride, and 1,2-dibromo-
(
2). The destruction of HOCs in water at mild or ambient
conditions using noble metal catalysts and H gas (5-7), or
2
palladized iron (8), has recently afforded some attention in
the literature. Also, noble metal catalysts are effective for the
destruction of halogenated aromatics (1,2-dichlorobenzene),
chlorinated biphenyls (4-chlorobiphenyl), and hydrogenation
of polyaromatic hydrocarbons (PAH’s) at ambient conditions
in hydrogen saturated water (9).
The palladium catalyzed reactions studied are hydrode-
halogenation and hydrogenation. If complete, these reactions
yield saturated hydrocarbons that are of little or no health
concern. Details of these reactions are given elsewhere (6,
3-chloropropane at ambient temperature and pressure
with 0.22 g/L of catalyst. Using Pd/Al, TCE transformed
quantitatively (97%) to ethane without formation of any
detectable chlorinated intermediate compounds. This implies
a direct conversion of TCE to ethane at the Pd surface.
Carbon tetrachloride transformed primarily to methane and
ethane and minor amounts of ethylene, propane, and
propylene. Chloroform is a reactive intermediate (20%).
Formation of C2 and C3 products implies a free radical
mechanism. Methylene chloride, 1,1-dichloroethane, and 1,2-
dichloroethane were nonreactive. Reaction mechanisms
and kinetic models are postulated for TCE, carbon
tetrachloride, and chloroform transformation.
7
, 10-12). As an example, in the absence of byproduct
formation, the hydrodehalogenation and hydrogenation of
trichloroethylene (TCE) can be hypothesized as in Scheme
1. Palladium catalyzes the hydrodehalogenation reactions,
k
1
-k
with hydrogen atoms, along with the formation of HCl. The
double bond is also hydrogenated to yield ethane (k ).
Distinguishing between a sequential pathway through lesser
chlorinated compounds or a direct pathway to ethane (k
7
, in which chlorine atoms are removed and replaced
8
9
)
is important for assessing the potential formation of toxic
intermediates. Although reported studies (4-9) have shown
the potential of palladium catalysts for treating contaminated
aqueous waste streams at ambient temperature and pressure,
few have addressed the potential formation of toxic inter-
mediates and products or the details of the hydrodehalo-
genation pathway such as radical formation, final product
distribution, and catalyst support effects.
Introduction
The widespread contamination of groundwater by chlori-
nated solvents and pesticides is a well-known environmental
problem. Current treatment methods, such as air stripping,
carbon absorption, and bioreactors, are limited in that they
may (i) transfer contaminants from one medium to another
and require further treatment prior to disposal, (ii) lead to
the production of lesser chlorinated products which may be
more toxic than the parent compound, and (iii) are often not
effective on the entire range of contaminants at a site. As a
result of these inadequacies, treatment technologies which
provide on-site destruction of a wide range of HOCs in water
at ambient conditions are needed.
A demonstration of the applicability of palladium metal
catalysts for the destruction of several HOCs was performed
in clean, aqueous batch systems. Initial rates measured in
a zero-headspace reactor are used to compare the relative
reactivity of 11 HOCs, and conclusions are drawn regarding
structure and reactivity. Experiments with TCE, carbon
tetrachloride (CT), chloroform (CF), and methylene chloride
(
MeCl) in headspace reactors are used to (i) elucidate
mechanistic differences in transformation of chlorinated
ethylenes and chlorinated methanes, (ii) assess the potential
for toxic intermediate formation, and (iii) determine product
distributions. Mathematical models and reaction pathways
are postulated. Supported and unsupported palladium
catalysts are used to investigate how the catalyst support
affects the potential formation of halogenated intermediate
compounds.
The use of elemental iron for reductive dechlorination
and nitrate removal has recently emerged in the literature
(
1-4) but is limited by slow rates, the gradual consumption
*
Corresponding author phone: (650)723-0308; fax: (650)723-7058;
e-mail: reinhard@ce.stanford.edu.
1
0.1021/es980963m CCC: $18.00

1999 Am erican Chem ical Society
VOL. 33, NO. 11, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 9 0 5
Published on Web 04/15/1999

TABLE 1. Catalyst Physical Properties
physical properties
Pd-met
Pd/Al
av particle size (µm )
0.25-0.55
38-70
1
155
0.20
Pd content (% w/w)
99.95
5
2
BET surf. area (m /g)
3
pore volum e (cm /g) 0.01
pore size dist (nm )
m etal dispersion (%) N/A
nonporous <8 (78%), 20-80 (8%)
21
TABLE 2. Reactor Operating Conditions
zero-HS reactor
HS reactor
tem p (K)
pressure (atm )
294 ( 1
1
800
1.15
1.15/0.0
295 ( 0.2
1.20-1.34
960-1070
2.0
a
[
H2]aq (µM)
vol. (L)
aq/vap
pH
initial
1.5/0.5
6.5-6.8
4.0-5.0
500
0.22
5-100
6.6-6.8
3.0-4.0
1000
final
agitator speed (rpm )
Ccat (g/L)
0.05-0.34b
25-250
FIGURE 1. Headspace reactor. Total reactor volume is 2 L with 1.5
L of liquid and 0.5 L of headspace.
[HOC]o (µM)
a
4
b
[H
2
]
aq estim ated using Henry’s constant (K
H
)7 ×10 atm ). Catalyst
concentration was adjusted to keep experim ental tim es under 6 h.
Replicate 1-µL samples of the extract were analyzed for the
parent compound and chlorinated intermediates using a HP
5890 series II GC equipped with an electron capture detector
and a 30 m × 0.32 mm i.d. DB-1701 capillary column (J&W).
Nonhalogenated products were not measured in the zero-
headspace experiments. The instrument was calibrated daily
using standards prepared from volumetric dilutions of a stock
solution. The water was unbuffered in all experiments. As a
result of the hydrodehalogenation reactions the reactor pH
decreased from an initial value of 6.5-6.8 to 4-5.
Headspace Reactors. The headspace reactor was a
gastight 2.0-L glass and stainless steel reactor (Bioengineering
AG, Wald, Switzerland) equipped with a magnetically driven
mixer, temperature probe, heat exchanger, and pressure
gauge (Figure 1). Two 5-cm impellers and four 2-cm baffles
at the outer edge provide ample mixing. The impeller speed
and reactor temperature were monitored and controlled by
an attached computer. Reactor operating conditions are given
in Table 2.
Experimental Section
Chem icals. Twelve halogenated organic compounds (RX)
were studied. All reagents were 99+% pure unless noted.
Tetrachloroethylene (PCE), 1,1-dichloroethylene (DCE), cis-
DCE (97%), trans-DCE (98%), 1,2-dichloroethane (DCA), and
1
,1,2-trichlorotrifluoroethane (Freon-113) were supplied by
Aldrich. TCE was supplied by Fisher. CT and MeCl were
supplied by J. T. Baker. The 1,1-DCA was supplied by
Suppelco. The 1,2-dibromo-3-chloropropane (DBCP, 97%)
was supplied by Pfaltz and Baur, and CF was supplied by
Mallinckrodt. All compounds were used as received without
further purification.
2 3
Catalysts. The Pd-on-Al O (Pd/ Al) (Aldrich) and metallic
palladium (Pd-met) (Alfa Aesar) catalysts were supplied and
used in a prereduced powdered form. No special precautions
were taken to avoid catalyst exposure to air prior to kinetic
experiments. Catalyst physical properties are given in Table
1
. BET surface area, pore volume, and pore size distribution
The experimental procedure for the headspace reactor
was similar to that of the zero-headspace reactor except that
(i) headspace was present, (ii) the reactor was kept under a
constant hydrogen pressure (1.2-1.3 atm) throughout the
experiment, and (iii) the catalyst was added last, after the
contaminant had equilibrated between the aqueous and
vapor phases. Headspace samples (200 µL) were withdrawn
from the reactor and analyzed by GC using methods adapted
from Burris et al. (13). Samples were injected splitless at 225
°C onto a HP 5890 GC equipped with a 30-m megabore GSQ-
PLOT column (J&W) and a FID detector. The oven temper-
ature profile was 50 °C for 2 min, ramp 40 °C/ min to 220 °C,
and hold at 220 C for 2 min. This method provided excellent
resolution for the parent compounds, all chlorinated inter-
mediates, and the C1-C3 hydrocarbon products in a single
run. Detection limits using this method were TCE 0.2 µM,
1,1-DCE 0.1 µM, cis-DCE 0.1 µM, trans-DCE 0.1 µM, CT 1.0
µM, and CF 2.5 µM. HOC standards (except vinyl chloride)
were prepared as described above, and calibration factors
were calculated based on the total amount of compound
added to the bottle (25% hs/ 75% aq). Nonhalogenated gas
standards and vinyl chloride were purchased from Scott
Specialty Gases.
were measured using a Coulter SA 3100 surface area and
pore size analyzer. Particle size distributions were determined
with a Coulter LS 230 laser diffraction particle size analyzer.
Metal dispersion (percent of metal available for catalysis) for
the supported catalyst was determined by a commercial
laboratory (Coulter, Particle Characterization Division). The
1
wt % palladium content of the Pd/ Al catalyst specified by
the manufacturer was assumed to be accurate.
Zero-Headspace Reactors. The zero-headspace reactor
was a 1.15-L glass reactor equipped with two sample ports.
Reactor operating conditions are given in Table 2. The
experimental procedure for the zero-headspace reactors was
as follows. The reactor was filled with Milli-Q water and 0.25
g of the Pd/ Al catalyst was added. The reactor was then
sparged with pure H
oxygen (reactive species) and provide an excess of hydrogen
est. 800 µM). DO was measured with a Hansatech DW1
2
gas for 15 min to remove dissolved
(
oxygen electrode and found to be less than 0.15 mg/ L after
sparging. After sparging, a concentrated aqueous solution of
known contaminant concentration was quickly added to
provide initial contaminant concentrations of 5-100 µM (1-
1
0 mg/ L). One-milliliter samples were withdrawn from the
reactor (displaced by H saturated water to maintain zero
headspace) and extracted into 0.5 mL of hexane or toluene.
2
Mass Transfer Lim itations. Liquid-solid, gas-liquid, and
intraparticle mass transfer limitations were considered. The
1
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 11, 1999

overall liquid-solid mass transfer rate constant, Klsa, was
estimated using the slip velocity approach of Harriott (14).
TABLE 3. HOC Transformation Rate Constants and Half Lives
in Zero-HS Reactor Using Pd/Al
K
lsa was 4-5 orders of magnitude larger than the largest
reaction rate constant, indicating that liquid-solid mass
[
HOC]o
(µM)
krxn
1
t1/2
(min)
RX
(L gcat- min-1
)
R 2
transfer resistance is negligible. This is primarily because
the catalyst particles are small (d < 100 µm) and the mixing
p
Ethylenes
0.53
rates are rapid.
PCE
TCE
7.8
22.8
23.7
76.3
75.3
0.976
0.993
0.997
0.996
0.997
5.9
4.9
4.5
3.8
4.0
The overall gas-liquid mass transfer rate constant, Klg
a
0.64
0.70
0.83
0.78
(
applicable in the headspace reactors), was estimated based
1
,1-DCE
on the results of Roberts and Dandliker who studied mass
transfer of VOCs from aqueous solution to the atmosphere
during surface aeration (15). Klga values in a similar system
were determined for various power inputs. Based on these
results, it is estimated that Klga values in the headspace reactor
are approximately 7-10 times larger than the observed
reaction rate constants using Pd/ Al and three times larger
than the observed reaction rate constants using metallic
palladium. Therefore, gas-liquid mass transfer resistance
may have slowed the absolute reaction rates approximately
cis-DCE
trans-DCE
Methanes
0.58
CT
CF
4.6
35.2
0.998
0.989
5.4
73.3
0.04
Ethane/Propane
Freon-113
DBCP
1,1-DCA
1,2-DCA
21.3
18.2
101
0.15
0.62
0
0.999
0.997
21.0
5.1
101
0
1
4-33% for the fastest reacting compounds. However, kinetic
pathways and final product distributions are relatively
unaffected, because the reactions are first order with respect
to the HOCs and the rate constants should not change.
Intraparticle mass transfer limitations, applicable to the
Pd/ Al catalyst, were estimated using the method of Weisz
and Prater (16) which requires only the observed reaction
in Table 3. The hydrodehalogenation rates for the chlorinated
ethylenes, CT, and DBCP are extremely rapid, with half-lives
between 3.8 and 5.9 min at a Pd/ Al concentration of 0.22
g/ L. Exceptions are chloroform (t1/ 2 ) 73.3 min), Freon-113
(t1/ 2 ) 21.0 min), and the dichloroethanes, which showed
very little reactivity even at elevated catalyst concentrations.
Trace amounts of ethane were observed in DCA experiments
(measured in a headspace experiment, data not shown),
suggesting that dechlorination may occur, but no reduction
in DCA concentration was evident after 24 h. No detectable
levels of chlorinated intermediates or products (>0.1 µM)
resulted from transformation of the chlorinated ethylenes
and DBCP when the Pd/ Al catalyst was used. Chloroform
was produced as a reactive intermediate during the trans-
formation of CT, accounting for about 20% of the initial CT
at the maximum, but no methylene chloride or chloro-
methane was detected.
rate constant and catalyst properties: particle diameter (d
and effective diffusivity (D ). In all cases, the ratio of
intraparticle diffusion and reaction time scales, kobs
p
)
e
2
p e
d / D ,
was 0.1 or less, indicating that intraparticle mass transfer
resistance is minimal.
Evaluation of Reaction Kinetics. Transformation kinetics
in the zero-headspace reactor were modeled with a simple
pseudo-first-order rate law (eq 1)
1
dCa
-
) kobsCa
(1)
Ccat dt
The disparity in reactivity between Freon-113, 1,1-DCA,
DBCP, and 1,2-DCA is of interest because it may correlate
where C
a
and Ccat represent the HOC and catalyst concen-
tration, respectively, and kobs is the observed pseudo-first-
order reaction rate constant. To correct for variances in
catalyst concentration between experiments, the observed
reaction rate constants are normalized by the catalyst
concentration, such that the reported reaction rate constants,
with their structure. Freon-113 (ClF
1,1-DCA (H CCCl H) is not, and DBCP (H
is reactive while 1,2-DCA (H ClCCClH ) is not. This may be
2
CCCl
2
F) is reactive but
3
2
2
BrCH BrCCClH )
2
2
2
2
because the C-X bond dissociation energy decreases with
increasing number of halogen atoms, and the fact that the
ease of reductive dehalogenation generally follows the order
I > Br > Cl . F (18). Interestingly, minor differences in
-1
-1
k
rxn, have units L gcat min . It is presumed that the reaction
rate is directly proportional to the catalyst concentration
over the range of catalyst concentrations used. Hydrogen is
not included in the rate expression, as the rates were found
reactivity of the chlorinated ethylenes (kDCE > kTCE > kPCE
)
contradicts the expected values based on bond dissociation
energies. Reasons for this trend are unclear at this time.
to be independent of the H
conditions used (data not shown). This is because the
activation energy for H chemisorption on noble metals is
close to zero such that the rates of adsorption and desorption
of H onto catalyst sites are sufficiently high (17), and because
the aqueous H concentration remains high (zero-headspace
2
concentration under the
Headspace Reactor Results. Initial contaminant con-
centrations and normalized pseudo-first-order rate constants
determined from mechanistically derived model fits of kinetic
data are given in Table 4. A mechanism was assumed in each
case based on the formation of products and intermediates
(or lack thereof), and the models described by a set of ordinary
differential equations (eqs 2-6). Rate constants were then
determined from the least-squares fitting procedure incor-
porated in the Scientist software package. Rate constant
subscripts coincide with a specific reaction pathway as shown
in Scheme 1 and Figure 5. Reaction intermediates and product
distributions measured at the termination of each experiment
are presented in Table 5.
2
2
2
experiments) or constant (headspace experiments) through-
out each experiment. Observed pseudo-first-order rate
constants for zero-headspace experiments were calculated
a t
from linear regressions of ln{[C ] / [Cao]} vs time and normal-
ized by the catalyst concentration to directly compare the
results from individual experiments. Kinetic data from the
headspace reactor were evaluated using a kinetic modeling
software package, Scientist v. 2.01 (MicroMath) as described
below.
1
. Trichloroethylene Transform ation Using the Pd/Al
Catalyst. TCE transformation using the Pd/ Al catalyst is
shown in Figure 2a. Transformation of TCE yielded ethane
as the only reaction product. Minor amounts of ethylene
were formed (<1% data not shown), but no chlorinated
intermediates were detected (<0.1 µM). A carbon mass
balance of 97% was achieved in the system, providing
Results and Discussion
Zero-Headspace Reactor Results. Initial rate data were
determined from the disappearance of contaminant from
solution. The initial concentration, normalized pseudo-first-
order rate constant, and reactor specific half-life are given
VOL. 33, NO. 11, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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1 9 0 7

TABLE 4. Transformation Rate Constants for TCE, CT, and CF
in Headspace Reactor
[
HOC]o
(µM)
Ccat
(g/L)
krxna
1
RX
cat
(L gcat- min-1
)
TCE
TCE
25.3
250
Pd/Al
Pd-m et
0.047
0.017
k9 ) 0.34 ( 0.02
k9 ) 4.4 ( 0.5
kloss ) 0.7 ( 0.2
k10 ) 0.15 ( 0.01
k11 ) 0.02 ( 0.005
k12 ) 0.06 ( 0.008
k13 ) 0.01 ( 0.01
k10 ) 1.5 ( 0.3
CT
57.3
47.3
53.3
Pd/Al
0.067
0.026
0.336
CT
Pd-m et
Pd/Al
k11 ) 0.4 ( 0.15
k12 ) 1.0 ( 0.3
k13 ) 0.1 ( 0.15
k13 ) 0.014 ( 0.002
CF
a
Errors are 95% confidence intervals.
FIGURE 3. (a) Transformation of TCE in headspace reactor using
.017 g/L of metallic palladium. Lines represent first-order model
0
fit (eq 2) for direct conversion of TCE to ethane with an additional
term (kloss) accounting for the lost mass. An 83% carbon mass balance
was achieved. (b) Production and decay of DCE isomers, VC, and
ethylene. Total chlorinated intermediates account for 3-4% of initial
TCE at their maximum. Lines in Figure 3b are interpolated (not fit)
and only meant to guide the eye.
FIGURE 2. (a) Transformation of TCE in headspace reactor using
.047 g/L Pd/Al. Lines represent first-order model fit (eq 2) for direct
0
conversion of TCE to ethane. A 97% carbon mass balance was
achieved. (b) Simulation of TCE transformation and expected
production of DCE isomers if TCE transformation was a sequential
reductive dechlorination through DCE.
FIGURE 4. Carbon tetrachloride transformation using 0.067 g/L Pd/
Al. Lines represent first-order model fit (eqs 3-6) with chloroform
as a reactive intermediate. A 92% carbon mass balance was
achieved.
evidence that chlorinated intermediates did not accumulate
in the system.
TCE transformation data were fitted using a simple
pseudo-first-order model for the complete conversion of TCE
to ethane at the catalyst surface (eq 2)
to a decrease in catalyst activity over the course of the
experiment. In general, this simple model provides a good
fit of the data, and the formation of ethane is coincident with
TCE disappearance.
dCTCE
dCethane
dt
-
)
) k C
(2)
9
TCE
dt
The complete conversion of TCE to ethane at the catalyst
surface contrasts the sequential reductive dechlorination,
TCE to DCE to VC and finally to ethylene, evident in many
anaerobic biological cultures (19), but is similar to TCE
conversion observed in some zero-valent iron systems (20).
and are shown as lines in Figure 2a. Some deviation from
pseudo-first-order behavior is evident, as the model slightly
under predicts TCE transformation at early times and over
predicts TCE transformation at later times. This may be due
1
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 11, 1999

The lack of chlorinated intermediates is an advantage
when considering the use of palladium catalysts for treatment
of contaminated water but may be due to effects of the
alumina support, as activity and selectivity of heterogeneous
catalysts are commonly affected by the support material (21,
2
2). Also, adsorption of chlorinated intermediates (DCEs and
VC) on the support may be suppressing detection of these
compounds. Experiments were performed using metallic
palladium powder to investigate whether the support sup-
presses production of chlorinated intermediates.
2
. Trichloroethylene Transform ation Using Metallic
Palladium . TCE transformation using pure palladium powder
is shown in Figure 3a. Equation 2 was used to fit the data
with an additional first-order rate constant kloss added to
account for the lost mass in the system. The normalized
pseudo-first-order rate constants, k and kloss, are given in
9
Table 4. TCE transformation again resulted in the simulta-
neous generation of ethane as the primary product, but Figure
3
b shows that all three DCE isomers, VC, and ethylene were
FIGURE 5. Proposed reaction pathways for transformation of carbon
tetrachloride and chloroform. Solid boxes represent observed
products and intermediates, and dashed boxes represent hypoth-
esized intermediates.
also formed. The total maximum concentration of chlorinated
intermediates (DCEs + VC) was low and never exceeded
3
-4% of the initial TCE. Interestingly, the three DCE isomers
and VC appear in the system simultaneously, reach their
maximum concentrations in the first 8 or 9 min, and continue
to hydrodehalogenate even though TCE is present at
concentrations 100 times greater. The formation of DCE and
VC appears to be a transient phenomena occurring only in
the first few minutes of the reaction.
TABLE 5. Product Distributions for TCE, CT, and CF
Transformation in Headspace Reactor
expt
duration
(min)
products/inter. carbon
at expt
termntn (%)
intermediates
(% max)
M.B.
(%)
RX/cat
The carbon mass balance of 83% is lower than the 97%
achieved with the Pd/ Al catalyst. There were two unidentified
peaks in the chromatogram with a retention time between
VC and 1,1-DCE that appeared to represent stable products.
An attempt to identify these products based on retention
time in GC analysis was unsuccessful, but it was determined
that these products are not butane or hexane, which have
been shown to be final products in the transformation of
TCE by palladized iron (8). The low levels of transient
chlorinated intermediates are further evidence that the
predominant transformation mechanism is via complete
transformation of TCE to ethane at the palladium surface,
but the fact that they are produced in the system indicates
the potential accumulation of these unwanted halogenated
intermediates in a treatment system if the catalyst charac-
teristics change.
TCE/Pd/Al
ethylene (<1)
163
193
ethane (97)
ethylene (<1)
ethane (83)
unknown 1 (?)
unknown 2 (?)
97
TCE/Pd-m et ethylene (<1)
,1-DCE (<1)
1
83
cis-DCE (2)
trans-DCE (<1)
VC (<1)
CF (21)
ethylene (<1)
propylene
CT/Pd/Al
343
113
291
m ethane (60)
CF (18)
ethane (14)
propane (1.0)
m ethane (51)
CF (23)
ethane (12)
propane (2)
m ethane (80)
ethane (<1)
MeCl (trace)
92
88
81
(
trace)
CT/Pd-m et CF (24)
ethylene (trace)
propylene
trace)
(
CF/Pd/Al
3
. Carbon Tetrachloride, Chloroform , and Methylene
Chloride Transform ation Using Pd/Al. Carbon tetrachloride
transformation using the Pd/ Al catalyst is shown in Figure
4
. CT transformation results primarily in the formation of
Complete conversion of TCE to ethane at the catalyst surface
can be further corroborated with a simulation of the expected
transient DCE concentrations (Figure 2b) based on the
independently measured transformation rates for TCE and
methane (60%), but chloroform is a reactive intermediate
accounting for as much as 21% of the initial CT. No methylene
chloride or chloromethane was detected. A total carbon mass
balance of 92% was achieved in the system. Figure 4 also
reveals that a substantial amount of ethane was produced,
accounting for 14% of initial CT. Minor amounts of ethylene,
propylene, and propane were also produced in the system
-
1
-1
-1
-1
DCE (ktce ) 0.34 L g min , kdce ) 0.44 L g min ). TCE
transformation to the three DCE isomers (cis, trans, and 1,1)
is assumed to be stoichiometric and considered together
such that the DCE concentration is a sum of the three
individual isomers. The DCE transformation rate constant
used is that of cis-DCE as this was the most rapid and would
yield the minimum expected transient total DCE concentra-
tion if TCE were to transform sequentially through DCE.
Transient production of VC and ethylene are not included
in the simulation. These compounds were not tested in this
study but have been shown to rapidly transform to ethane
via similar mechanisms in other studies (7, 12). It is apparent
from the simulation that a substantial amount of total DCE
(
Table 5).
Chloroform transformation using the Pd/ Al catalyst
yielded methane as the primary reaction product (80%). Trace
amounts of ethane (<1% of initial CF) and methylene chloride
(
not quantifiable) were detected (Table 5). Although chlo-
roform transformation is 15-20 times slower than that of
carbon tetrachloride (Table 4), 66% transformation of
chloroform was achieved in 291 min with 0.33 g/ L Pd/ Al.
Methylene chloride showed virtually no reactivity when
using the Pd/ Al catalyst. No decrease in methylene chloride
concentration was evident, and only trace levels of methane
were detected in the reactor after 78 min, even at elevated
catalyst concentrations. This contrasts results of Muftikian
et al. (8), where a 75% reduction of methylene chloride was
observed in a system with 3.6 g of palladized iron in 10 mL
(
32% at max) should appear if the hydrodehalogenation
reaction were sequential. Coincident ethane formation, the
lack of any detectable halogenated intermediates, and good
carbon mass balances are evidence that a complete conver-
sion of TCE to ethane occurs at the catalyst surface.
VOL. 33, NO. 11, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1 9 0 9

of a 200 mg/ L methylene chloride solution, but is similar to
results with iron metal alone which showed very little
methylene chloride transformation after several months (2).
k10/ (k10 + k11 + k12) and approximately 35% via formation of
the trichloromethyl radical (k11 + k12)/ (k10 + k11 + k12).
Rapid dehalogenation rates and the absence of toxic
intermediates or products, in most cases, indicate the
potential of palladium catalysts for environmental or in-
dustrial applications. Crucial application considerations still
remain, however, such as (i) groundwater matrix effects on
catalyst activity and lifetime, (ii) hydrogen dosing require-
ments, (iii) competing substrates, and (iv) catalyst fouling
and deactivation. Current research efforts are focused on
these issues.
4
. Carbon Tetrachloride Transform ation Using Metallic
Palladium . Carbon tetrachloride transformation using me-
tallic palladium resulted in a nearly identical product
distribution as with Pd/ Al (Table 5). The carbon mass balance
of 88% was similar to that achieved with Pd/ Al. This result
reveals that palladium metal is indeed responsible for the
transformation, and the alumina support appears to have
little effect on the transformation of carbon tetrachloride.
5
. Proposed Reaction Pathway and Kinetic Model for
Carbon Tetrachloride and Chloroform . Based on observed
reaction products and intermediates, hypothesized reaction
pathways for the transformation of CT and chloroform are
shown in Figure 5. Carbon tetrachloride transforms in part
via a direct pathway to methane (k10), but production of C2
and C3 hydrocarbons implies that CT transformation also
occurs in part via production of a radical species (k11 + k12).
Trichloromethyl radical formation is most likely and may
account for the formation of chloroform (k12), as this pathway
has been observed in biotic and abiotic transformations of
CT in aqueous systems (23-25).
Acknowledgments
The authors also thank Professor Paul Roberts, LLNL
researchers, and members of the Reinhard Research Group
for their helpful insight and comments. Although the research
described in this article has been funded in part by the United
states Environmental Protection Agency through R825421-
01-0 to Martin Reinhard, it has not been subject to the
Agency’s required peer and policy review and therefore does
not necessarily reflect the views of the Agency, and no official
endorsement should be inferred.
Chloroform transforms predominantly via a direct con-
version to methane (k13) because only trace amounts of radical
coupling products (ethane) were observed, and there was no
accumulation of the nonreactive species methylene chloride.
A single reduction to methylene chloride is possible, however,
as trace amounts of methylene chloride were produced from
chloroform. Trace amounts of ethane observed in chloroform
transformation also indicate that the dichloromethyl radical
may form. The lower reactivity of chloroform relative to CT
may reflect the increased carbon-halogen bond dissociation
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•
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(
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(
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(
(
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-
) (k10 + k11 + k )C
(3)
(4)
(5)
(6)
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CT
dt
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(
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(k C - k CCF)
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2
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(
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dt
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9
73.
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(k C + k CCF)
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2
Different model alternatives were tested and rejected based
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(
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4
09.
-
1
-1
because kloss < 0.005 L g min . The model fit is shown as
lines in Figure 4, and best fit values for individual rate
constants are given in Table 4. Based on the fitted parameters,
carbon tetrachloride transforms approximately 65% via direct
conversion to methane as calculated by the selectivity ratio
Received for review September 18, 1998. Revised manuscript
received February 24, 1999. Accepted March 1, 1999.
ES980963M
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 11, 1999