United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens, QA 30613
Research and Development EPA/600/M-89/032 May 1990
ENVIRONMENTAL
RESEARCH BRIEF
Transformations of Halogenated Hydrocarbons:
Hydrolysis and Redox Processes
N. Lee Wolfe" and Peter M. Jeffers"
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.
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 others 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 increasingly apparent
that biological processes are inadequate to account for the
environmental fate of these compounds. It is thus logical
* U.S. Environmental Protection Agency's Environmental Research
Laboratory, Athens. GA, 30613.
~ State University of New York, Cortland, Cortland, NY 13045.
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 freons as
well as 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 homogenous
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 16 halogenated hydrocarbons along
with 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 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
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extrapolate decay rate constants from one aquatic system to
another.
Correlation of Reactivity with Structure
Rgure 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.
I2C-CI2
Telrakxtoelhene
IH2C-CH2I
1,2-Ditodoe thane
BrCIHC-CHCIBr
1,2-Dibramo-l ,2-dichloroethane
CI3C-CCI3
Hexachtoroe thane
CHgBrHC-CHBrCHa
2,3-Dibromobutane
6rH2C-CH2Br
1,2-Dibromoethane
C8H6CI8
Gamma-hexachtorocyclohexane
CC14
Carbon totrachlorida
CI2HC-CHCI2
1,1,2,2-Tetrachloroethane
CI2 = CCI2
Tetrachloroethene
C6H5-I
lodobenzene
C6H6CI6
Alpha-hexachlorocyclohexane
CIH2C-CH2CI
1,2-Dichloroethane
CI2C — CH2
1,1-Dichloroethene
CIHC = CHCI
Trans-1,2-dichloroethene
CIHC = CHCI
Cis-1,2-dichloroethene
C6F14
Perfluorohexane
Perfluorodecaline
DDT
Rgure 1. Halogenated aliphatic and aromatic hydrocarbons
selected for abiotic reduction studies in anaerobic
sediments.
The compounds used in these correlations cover a broad
section 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
"slow" 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 k0bs 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 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 relation-
ships between octanol and organic carbon.
Linear regression analyses show that correcting the
observed rate constants (kobs) (Table 2) for the fraction
sorbed (kcorr) significantly improves th<3 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 kca| along with the linear regression
data.
Although there are other factors that 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 since 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 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
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Table 1. Kinetic Data and Molecular Descriptors Used in the Structure Reactivity
Correlations
Compound
Tetraiodoethene
1,2-Diiodoethane
1 ,2-Diclo-Dibrethane
Hexachloroethane
2,3-Dibromobutane
1 ,2-Dibromoethane
g-Hexachlorocyclohexane
Carbontetrachloride
Sym-Tetrachloroethane
Tetrachloroethene
lodobenzene
a-Hexachlorocyclohexane
1 ,2-Dichloroethane
1,1-Dichloroethene
1 ,2-T-Dichloroethene
1 ,2-C-Dichloroethene
Kcal/mol
e
55.5
55.5
68.0
81.0
68.0
68.0
81.0
81.0
81.0
81.0
64.0
81.0
81.0
81.0
81.0
81.0
0*
3.52
1.76
4.14
6.30
1.84
2.04
3.15
4.20
4.20
4.20
1.12
2.10
2.10
2.10
2.10
2.10
BE"
Kcal/mole
53.8
63.2
63.2
63.2
63.2
63.2
63.2
63.2
63.2
53.8
20.0
63.2
63.2
53.8
53.8
53.8
Avg.k
min -pfas
9.22E-02
3.41 E-02
4.99E-03
2.33E-03
9.98E-04
5.45E-04
1.51E-04
1.10E-04
7.30E-05
4.05E-05
2.55E-05
2.37E-05
1.40E-05
4.37E-06
3.47E-06
3.40E-06
tl/2
hr
0.13
0.34
2.31
4.96
11.6
21.2
76.5
105
158
285
453
487
825
2643
3328
3397
aCarbon-halogen bond energy.
Waft's sigma constant.
0 Carbon-carbon bond energy.
g>-4-
-5-
-6
-6
Log Kobs = XI'BS + X2*SIGMA + X3*BE + C
R2 = 0.985
X1=-0.142
X2 = 0.483
X3= 0.039
C = 3.031
-4 -2
Log Kobs, Min-1
Figure 2. Structure reactivity relationship for abiotic reductive
rate constants (Kobs) of halogenated hydrocarbons
with carbon-halogen bond energies (BE), Taft sigma
constants (o*) and carbon-carbon bond energies
(BE).
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. Half-lives vary from 0.013 years for 1,1,2,3,3-penta-
chloropropane to 2.1 x 101° 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.
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Table 2. Disappearance Rate Constants for Hexachloroethane in
Selected Sediments and Aquifer Samples
Sediment Rate
Sediment Fraction Cone., Constant t1/2,
Source O.C. g/g k, min-1 hr
BarH
BarH
HickoryH
Broukolvoen
BcavorD
MemorP
BarH
Loosdr.
Plassen
Vochten
EPA-B1
Domrnel
EPA-13
EPA-11
EPA-6
Lula, aq
Blythville, aq
Blythville.aq
Lula, aq
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
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
5.00E-02
3.51 E-02
2.60E-02
2.47E-02
2.00E-0 2
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
5.41 E-05
4.38E-05
4.36E-05
3.01 E-05
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
Log Kcorr = X1"Log OC + C
O
s
1 -
0-
-1 -
-2 -
-3-
-4 m*-"-"
•* •
-5 •
-b T i
-4.5
^•^^
-3.5
^"
• ^^'^
" ^'^'
^^
R2 = 0.905
X1 =1.240
C = -1.734
-2.5 -1.5 -0.5
Log Kcorr
Figure 3. Correlation of abiotic reduction rate constants (kcorr) for
hexachlorethane in anaerobic hetergeneous systems with
fraction organic carbon (OC).
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Alkaline Hydrolysis
c*\ r*\ c*\
919191
Neutral Hydrolysis
99
CI-C-C-CI
cm
CICI
CI-C-C-CI
99
CI-C-C-H
CI-C-C-H
CI H
9'
CI-C-CI
a CI
H-C-C-H
H A
CI-C-C-C-CI
HHH
qi H
CI-C-C-H
H H'
Ck
ci-6-ci
CI
No Reaction c, H
Detected ^^
CI H
HCII^
H-C-CC-H
i!i /^t ili
l_,lx /*-"
crc=Cxci
99
CI-C-C-CI
Clx sC\
H H
Highest Reactivity
Lowest Reactivity
H-C-C-C-H
AdlH
91 H
CI-C-C-H
cm
CI-C-C-H
H H
^|-S-CI
CI-C-CI
CI
CIH
CI-C-C-H
H H
99
CI-C-C-H
CIH
99
H-C-C-H
HH
99
CI-C-C-H
CIH
99
CI-C-C-CI
HH
91
CI-C-CI
9191
CI-C-C-H
H H
9191
CI-C-C-CI
No Reaction
Detected CIN
Figure 4. Neutral and alkaline hydrolysis rate constants and calculated half-lives at 25 °C and pH 7 for chlorinated methanes,
ethanes, ethenes and propanes.
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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-
Pentachloropropane
1.1,2,3-
Tetrachloropropane '
1,1,2,2-
Tetrachloroethane
1,1,1 -Trichloroethane
1 ,3-Dichloropropane
Carbon Tetrachloride
1,1,1,2-
Tetrachloroethane
1 ,1 -Oichloroethane
1 ,2-Dichloroethane
1 ,1 ,2-Trichloroethane
Chloroform
Trichloroethene
1,1-Dichloroethene
Tetrachloroethene
Hexachloroethane
1 ,2-Dichloroethene
k (neut,25)
min-1
3.18E-04
4.93E-08
4.71 E-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
kb (pH7,25)
min-1
O.OOE + OQ
1.31E-04
9.81 E-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
tl/2
days
0.61
1.47
1.95
2.46
63.5
155
328
5905
6826
8953
10519
2.03E4
2.71 E5
1.80E8
1.7E10
1.4E11
2.7E1 1
3.1E12
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.
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. 1987.
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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