RESEARCH BRIEF
Transformations of Halogenated Hydrocarbons:
Hydrolysis and Redox Processes
by
N. Lee Wolfe1 and Peter M. Jeffers2
-^Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens GA 30613
2State University of New York, Cortland
Cortland NY 13045
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ABSTRACT
This report describes process models and rate constants that
are necessary to predict the environmental fate of halogenated
aliphatic hydrocarbons. Two important pathways for the
transformation of this class of compounds in sediment and
groundwater systems are abiotic reduction and abiotic hydrolysis.
Models have been developed to explain the reactivity of the
compounds in heterogenous systems. Reduction rate constants are
reported for 16 chloro-, bromo- and iodo- hydrocarbons, along with
a structure reactivity relationship that correlates reactivity to
general molecular descriptors. Redox reactivity of sediment or
ground water samples is related to the organic content of sample
solids. Neutral and alkaline hydrolysis rate constants, along with
activation paramaters have been measured for 18 chlorinated
methanes, ethanes, propanes and ethenes. Hydrolysis half-lives for
these compounds are calculated for typical environmental reaction
conditions.
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INTRODUCTION
In spite of the many biological degradation studies of
halogenated hydrocarbons, a major environmental question of concern
is why some halogenated hydrocarbons persist and why other are
quite reactive and rapidly disappear. Furthermore, it is not clear
what ecosystem properties dictate whether the compounds rapidly
degrade or persist over months or years. It has become increasing
apparent that biological processes are inadequate to account for
the environmental fate of these compounds. It is thus logical that
attention turn to abiotic processes in an attempt to answer these
questions.
The occurrence of halogenated hydrocarbons as ubiquitous
contaminants in the environment is well documented (Vogel et al.,
1987). Specific compounds in this class of organics are widely
used as industrial cleaning and degreasing solvents, high pressure
lubricants, pesticides, and coolants (including proposed freon
substitutes). Assessing the fate of this class of compounds
requires quantitative descriptions of redox and hydrolysis
transformation processes. Such descriptive models have been
developed for homogeneous abiotic processes such as hydrolysis.
Similar algorithms are needed that describe redox transformation
reactions in soil-water ecosystems.
This report presents our recent findings on environmental
redox and hydrolysis fate processes. Abiotic reduction rate
constants are given for 15 halogenated hydrocarbons along with
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structure reactivity relationships that can be used to assess
reactivity of other compounds of the same class. Likewise, neutral
and alkaline hydrolysis rate constants and activation parameters
are given for 18 chlorinated methanes, ethanes, propanes and
ethenes.
DISCUSSION
ABIOTIC REDOX REACTIONS
This discussion is divided into two parts beginning with a
report of pseudo-first-order disappearance rate constants .for
halohydrocarbons and the presentation of a structure reactivity
relationship (SRR) of these rate constants with selected molecular
descriptors. The second part presents a correlation of
hexachloroethane reaction rate constants with system parameters
that vary across a wide range of sediment characteristics to
provide the capability to extrapolate decay rate constants from
one aquatic system to another.
Correlation of Reactivity with Structure
Figure 1 lists the halogenated hydrocarbons along with the
chemical structures that were used in developing the SRR's. Only
compounds for which there are reported first-order disappearance
rate constants obtained under defined reaction conditions are
included. All the rate constants given in Table 1 were obtained
under reducing conditions in sediment, saturated soil or aquifer
samples.
The compounds used in these correlations cover a broad section
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of structural features including halogenated methanes, ethanes,
ethenes and aromatics. These compounds react by a variety of
mechanisms in these reducing systems but they all apparently
involve an initial "slow11 electron transfer step in the reductive
pathways. The different pathways are reflected in the different
products, as has recently been reported (Wolfe et al., 1989).
The disappearance rate constants were correlated with several
molecular descriptors with the best correlation obtained with the
carbon-halogen bond strength (BS), Taft's sigma constant (o*), and
the carbon-halogen bond energy (BE) (Table 1). A plot of Jcobs
versus kcal is given in Figure 2 along with the regression analysis
data. The reactivity covers about 4 orders of magnitude and gives
a regression coefficient, R2, of 0.989. This relationship provides
a convenient way to estimate the relative reactivity of other
halogenated hydrocarbons.
Correlation of Reactivity with Sediment Properties
Hexachloroethane was selected as a model compound to elucidate
system properties that govern the reducing activity of sediment and
ground water samples. Data for the reduction of hexachloroethane
include disappearance rate constants for a large number of
sediment, soil and aquifer samples (Table 2). The data suggest a
correlation between the rate constants and the organic carbon (OC)
content of the samples.
The correlation is enhanced when one takes into account the
fact that hexachloroethane can be significantly sorbed to the solid
phase in some of the samples. In most cases, it was not possible
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to measure the distribution coefficient for hexachloroethane
because it was reduced too fast to make the experimental
measurements. It is, however, possible to estimate the fraction
of hexachloroethane sorbed by use of the octanol-water partition
coefficient and known relationships between octanol and organic
carbon.
Linear regression analyses show that correcting the observed
rate constants (k^) (Table 2) for the fraction sorbed (kcorr)
significantly improves the correlation (Weber and Wolfe, 1987).
The best correlation is with fraction organic carbon (OC) (R2 =
0.905) and is shown in Figure 3 as a plot of kcorr versus kcal along
with the linear regression data.
Although other factors contribute to the reduction of
hexachlorethane, organic carbon (or a reducing agent that co-varies
with organic carbon) is the most important descriptor, accounting
for about 90 % of the variance of the data. It is likely that the
correlation could be improved if sand were excluded from the
sediment in the experiments because it has been shown by Jafvert
and Wolfe (1987) to have little reductive activity.
HYDROLYSIS
There are several reasons to measure the homogeneous, abiotic
hydrolysis rate constants even for compounds that react very slowly
under ordinary environmental conditions. First, because the
residence time is long in aquifers, deep reservoirs and oceans,
slow reactions can still be major factors in loss of pollutants
that must travel a significant distance before they become
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potential problems for the environment or human populations.
Second, the study of any biologically mediated process or
heterogeneously catalyzed reaction must allow for the amount of
reaction that occurs by a purely abiotic, homogeneous pathway.
The compounds for which neutral and alkaline hydrolysis rate
constants were measured are given in Figure 4 in order of
increasing reactivity. This suite of compounds includes 18
chlorinated methanes, ethanes, ethenes and propanes and spans 13
orders of magnitude in hydrolytic reactivity.
It is noteworthy that the relative rates of reactivity -are
dissimilar for the neutral and alkaline pathways. In the case of
alkaline hydrolysis, 1,1,2,3,3-pentachloropropane was the most
reactive while no reaction could be detected for carbon
tetrachloride, 2,2-dichloropropane and 1,1,1-trichloroethane. For
neutral hydrolysis, 2,2-dichloropropane was the most reactive and
hexachloroethane was the least reactive.
The order of reactivities suggests that alkaline hydrolysis
rate constants can be correlated with acidity of the proton being
abstracted in the rate determining step. On the other hand, bond
strength and steric factors appear to control the reactivity
towards neutral hydrolysis. Presently, attempts are under way to
correlate the reactivity of the compounds with readily available
molecular descriptors.
Kinetic data along with calculated half-lives at 25°C and pH
7 are given in Table 3. These calculations take into account
contributions from both neutral and alkaline processes at pH 7.
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Half-lives vary from 0.013 years for 1,1,2,3,3-pentachloropropane
to 2.1 x 1010 years for 1,2-dichloroethene (Jeffers et al., 1989).
Thus, under hydrolytic reaction conditions typical of aquatic
environments, reactivity varies over 12 orders of magnitude.
CONCLUSIONS
Redox reactivity is shown to vary 4 orders of magnitude for
16 halogenated hydrocarbons in sediment samples and
hexachloroethane reactivity is shown to vary 4 orders of magnitude
in going from aquifer samples to sediment samples. Hydrolytic
reactivity of halogenated hydrocarbons is shown to vary 12 orders
of magnitude at 25°C and pH 7. It is obvious that to accurately
predict the fate of this class of compounds requires the
appropriate rate constants and knowledge of eco-system parameters.
For example, hexachloroethane does not react by hydrolysis pathways
but is very reactive under reducing conditions. Also, comparison
of the rate constants for the two reaction types provides an
estimate of the contribution of each pathway and consequently the
distribution of products.
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REFERENCES
Jafvert, C. T. and N. L. Wolfe. 1987. Degradation of selected
halogenated ethanes in anoxic sediment-water systems. Environ.
Toxicol. Chem. 4:827-837.
Jeffers, P. M., L. M. Ward, L. M. Woytowitch and N. L. Wolfe.
1989. Homogeneous hydrolysis rate constants for selected
chlorinated methanes, ethanes, ethenes and propanes. Environ. Sci.
Tech. In press.
Vogel, T. M., C. S. Griddle and P. L. McCarty. 1937.
Transformations of halogenated aliphatic compounds. Environ. Sci.
Technol. 21:722-736.
Weber, E. J. and N. L. Wolfe. 1987. Kinetic studies of the
reduction of aromatic azo compounds in anaerobic sediment/water
systems. Environ. Toxicol. Chem. 6:911-919.
Wolfe, N. L., W. Peijnenburg, H. den Hollander, H. Verboom
and D. van de Meent. 1989. Kinetics of reductive transformations
of halogenated hydrocarbons under anoxic reaction conditions. In
preparation.
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IP-CI
2 2
Tetraiodoethene
IH C-CH I
2 2
1,2-Diiodoethane
BrCIHC-CHCIBr
1,2-Dibromo-1,2-dichloroethane
Cl C-CCI
3 3
Hexachloroethane
CHBrHC-CHBrCH
3 3
2,3-Dibromobutane
BrH C-CH Br
2 2
1,2-Dibromoethane
C6H6CI6
Gamma-hexachlorocyclohexane
CCI
4
Carbontetrachloride
Cl HC-CHCI
2 2
1,1,2,2-Tetrachloroethane
Cl C=CCI
2 2
Tetrachloroethene
lodobenzene
C6H6CI6
Alpha-hexachlorocyclohexane
CIH C-CH Cl
2 2
1 , 2-Dichloroethane
Cl C=CH
2 2
1 , 1-Dichloroethene
CIHC=CHCI
Trans- 1 , 2-dichloroethene
CIHC=CHCI
Cis-1 , 2-dichloroethene
C6F14
Perfluorohexane
C10F18
Perfluorodecaline
DDT
1. Halogenated aliphatic and aromatic hydrocarbons selected for
abiotic reduction studies in anaerobic sediments.
10
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0 T
LOG Kobs-X1*BS+X2*SIGMA+X3*BE+C
o
*:
-------
03
O
1 -
0 -
-1 -
-2 -
-3 -
-5 -
LOG Kcorr-X1*LOG OC + C
L.5
-3.5
-2.6
Log Kcorr
-1.6
2
R «0.905
X1=1.240
C=-1.734
-0.5
3. Correlation of abiotic reduction rate constants (kcorr) for
hexachlorethane in anaerobic heterogenous systems with fraction
organic carbon (OC).
12
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AfealbM hydrolysis
Neutral hydrolysis
a
NoRaoeUon
«H
H-C-C-C-H
HCt H
"W
PP
a-c-c-H
o-p-a
H
PP
H-p-C-H
HH
a-c-c-c-a
HH|<
PP
^p-p^
H H
aN
CK
M
'«(«
CtCI
!^«c!
Htghest reactivity
I
a-c-c-c-a
i i i
HHH
Cl-p-Ci
Pt1
PP
0-9-9-0
a H
PP
H-p-p-H
H H
PP
CM3-p-«
a H
PP
a-c-c-a
HH
P
PP
a-p-p-H
HH
PP
v*rr^<
Lowest reactivity
X.
No Reaction
D*tocted ax /a
ax ,<*
CK H
4. Neutral and alkaline hydrolysis rate constants and calculated
half-lives at 25°C and pH 7 for chlorinated methanes, ethanes,
ethenes and propanes.
13
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Table i. Kinetic data and molecular descriptors used in the
structure reactivity correlations.
Compound
BSa
0*b
Kca I/mole
TETRAIODOETHENE
1 , 2-DIIODOETHANE
1 , 2-DICLO-DIBRETHANE
HEXACHLOROETHANE
2 , 3-DIBROMOBUTANE
1 , 2-DIBROMOETHANE
g-HEXACHLOROCYCLOHEXANE
CARBONTETRACHLORIDE
SYM-TETRACHLOROETHANE
TETRACHLOROETHENE
IODOBENZENE
a-HEXACHLOROCYCLOHEXANE
1 , 2-DICHLOROETHANE
1 , 1-DICHLOROETHENE
1 , 2-T-DICHLOROETHENE
1 , 2-C-DICHLOROETHENE
55.
55.
68.
81.
68.
68.
81.
81.
81.
81.
64.
81.
81.
81.
81.
81.
5
5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
3
1
4
6
1
2
3
4
4
4
1
2
2
2
2
2
BEC
Avg.k^ t./2
Kcal/mole
.52
.76
.14
.30
.84
.04
.15
.20
.20
.20
.12
.10
.10
.10
.10
.10
53
63
63
63
63
63
63
63
63
53
20
63
63
53
53
53
.8
.2
.2
.2
.2
.2
.2
.2
.2
.8
.0
.2
.2
.8
.8
.8
min hr
9
3
4
2
9
5
1
1
7
4
2
2
1
4
3
3
.22E-02
.41E-02
.99E-03
.33E-03
.98E-04
.45E-04
.51E-04
.10E-04
.30E-05
.05E-05
.55E-05
. 37E-05
.40E-05
.37E-06
.47E-06
.40E-06
0.13
0.34
2.31
4.96
11.6
21.2
76.5
105
158
285.
453
487
825
2643
3328
3397
a Carbon-halogen bond energy.
carbon bond energy
Taft's sigma constant. c Carbon-
14
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Table 2. Disappearance rate constants for hexachloroethane in
selected sediments and aquifer samples.
SEDIMENT Fraction
SOURCE
BarH
BarH
HickoryH
Breukelveen
BeaverD
MemorP
BarH
Loosdr. Plassen
Vechten
EPA-B1
Dommel
EPA-13
EPA- 11
EPA-6
Lula, ag
Blythville, aq
Blythville, aq
Lula , aq
O.C.
0.0221
0.022
0.018
0.29
0.056
0.043
0.022
0.33
0.06
0.009
0.0053
0.03
0.015
0.0072
0.000065
0.00012
0.00012
0.000065
Sediment
Cone. ,
g/g
0.08
0.0895
0.11
0.045
0.2
0.055
0.075
0.050
0.087
0.1
0.469
0.1
0.1
0.1
0.161
0.142
0.613
0.689
Rate
Constant
kr rain"1
5.00E-02
3.51E-02
2.60E-02
2.47E-02
2.00E-02
1.90E-02
1.90E-02
7.62E-03
2.33E-03
1.80E-03
1.22E-03
8.15E-04
7.88E-04
4.47E-04
5.41E-05
4.38E-05
4.36E-05
3.01E-05
t,,.
1/2,
hr
=====::
0.23
0.33
0.44
0.47
1.12
0.61
0.61
1.52
4.96
6.42
9.47
14.2
14.7
25.8
213
264
265
384
15
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Table 3. Kinetic data for the neutral and alkaline hydrolysis of
chlorinated methanes,ethanes, ethenes and propanes in distilled
water.
COMPOUND
2,2-DICHLOROPROPANE
PENTACHLOROETHANE
1,1,2,3,3-PENTACHLORO-
PROPANE
1,1,2,3-TETRACHLORO-
PROPANE
1,1,2,2-TETRACHLORO-
ETHANE
1,1,1-TRICHLORO-
ETHANE
1,3-DICHLOROPROPANE
CARBON TETRACHLORIDE
1,1,1,2-TETRACHLORO-
ETHANE
1,1-DICHLOROETHANE
1,2-DICHLOROETHANE
1,1,2-TRICHLOROETHANE
CHLOROFORM
TRICHLOROETHENE
1,1-DICHLOROETHENE
TETRACHLOROETHENE
HEXACHLOROETHANE
1,2-DICHLOROETHENE
k(neut,25) kb(pH7,25)
min-1 min-1
3.18E-04
4.93E-08
4.71E-07
O.OOE+00
9.70E-09
1.24E-06
5.87E-07
3.26E-08
2.60E-08
2.15E-08
1.83E-08
5.19E-11
1.91E-10
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
O.OOE+00
1.31E-04
9.81E-05
7.84E-05
3.02E-06
O.OOE+00
1.67E-12
O.OOE+00
2.15E-09
7.20E-14
1.04E-11
9.42E-09
5.22E-10
1.07E-12
1.09E-14
1.37E-15
7.18E-16
6.32E-17
kobs=k+kb
min-1
3.18E-04
1.31E-04
9.85E-05
7.84E-05
3.03E-06
1.24E-06
5.87E-07
3.26E-08
2.82E-08
2.15E-08
1.83E-08
9.47E-09
7.13E-10
1.07E-12
1.09E-14
1.37E-15
7.18E-16
6.32E-17
t,/2
days
0.61
1.47
1.95
2.46
63.5
155-
328
5905
6826
8953
10519
2.03E4
2.71E5
1.80E8
1.7E10
1.4E11
2.7E11
3.1E12
16
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