Kinetics of the transformation of halogenated aliphatic compounds by iron sulfide

Article abstract of DOI:10.1021/es980946x

The transformation of nine halogenated aliphatic compounds, i.e., pentachloroethane (PCA), 1,1,2,2- and 1,1,1,2-tetrachloroethanes (1122-TeCA and 1112-TeCA), 1,1,1- and 1,1,2-trichloroethanes (111-TCA and 112-TCA), 1,1- and 1,2-dichloroethanes (11-DCA and 12-DCA), carbon tetrachloride (CT), and tribromomethane (TBM), by 10 g/L FeS at pH 8.3 was investigated in batch experiments. 11-DCA, 12-DCA, and 112-TCA showed no significant transformation by FeS over ~ 120 days, but the other compounds were transformed with half-lives of hours to days. PCA and 1122-TeCA underwent dehydrohalogenation faster than FeS-mediated reductive dehalogenation reactions. The remaining compounds for which considerable transformation was observed underwent FeS-mediated reactions more rapidly than hydrolysis or dehydrohalogenation. For 1112-TeCA, the dihaloelimination product, i.e., 1,1-dichlroethylene, was the only reaction product observed. For 111-TCA, CT, and TBM, hydrogenolysis products were the only products detected, even though their mass recoveries were significantly < 100%. Two simple log-linear correlations between rate constants and either one-electron reduction potentials or homolytic bond dissociation enthalpies were developed, with determination coefficients of 0.48 and 0.82, respectively. These results were consistent with a rate-limiting step involving homolytic bond dissociation. However, neither correlation precisely characterized the reactivity of all the compounds, indicating distinctions among the mechanisms for reductive dehalogenation of the compounds by FeS or the influence of additional molecular or thermodynamic parameters on rate constants.

Full text of DOI:10.1021/es980946x

Environ. Sci. Technol. 2000, 34, 422-429
and tetrachloroethylene (PCE) (11). In related studies, the
Kinetics of the Transformation of
Halogenated Aliphatic Compounds
by Iron Sulfide
rates of transformation of HCA by biotite or vermiculite were
observed to increase significantly when aqueous bisulfide
was added to the experimental system (12), and transforma-
tion of CT by biotite or vermiculite plus aqueous bisulfide
was also observed (13). The reactivity of the biotite/
vermiculite bisulfide systems may have been due to the
formation of a secondary iron sulfide phase upon reaction
of bisulfide with ferrous iron associated with the clay minerals
(12, 13). In contrast to the studies reporting reductive
dehalogenation by FeS, other researchers have reported no
transformation of CT (3), 1,1,1-trichloroethane (111-TCA)
(3), TCE (5), or PCE (3, 5) in the presence of FeS. Differences
in experimental conditions such as pH, dissolved oxygen
concentration, and reaction time among the studies to date
may be responsible for these sometimes inconsistent results.
The objectives of the experiments described here were
2-fold: first, to assess the relative rates and products of
transformation of a variety of halogenated aliphatics by the
soil mineral FeS under a uniform set of experimental
conditions and, second, to establish whether there exists a
relationship between rate constants for these transformations
and free energy or molecular parameters. Such a relationship
was sought to aid in understanding the influence of different
molecular properties on reaction rates, to provide evidence
about rate-limiting processes in these transformations, and
to allow prediction of the rates of FeS-mediated reductive
dehalogenation reactions in natural systems. Previous studies
have correlated rate constants for reductive dehalogenation
by anaerobic sediments (14), transition metal coenzymes
(15), iron metal (16-18), mercaptojuglone (19), and iron
porphyrins (19) with molecular properties such as reduction
potential, bond dissociation energy, and lowest unoccupied
molecular orbital (LUMO) energy.
Rates and products of the transformation of the following
compounds by FeS are reported here: pentachloroethane
(PCA), 1,1,2,2- and 1,1,1,2-tetrachloroethanes (1122-TeCA and
1112-TeCA), 111-TCA, 1,1,2-trichloroethane (112-TCA), 1,1-
and 1,2-dichloroethanes (11-DCA and 12-DCA), CT, and
tribromomethane (TBM). Previously reported results for the
transformation of HCA (9), TCE (11), and PCE (11) by FeS are
also discussed. These compounds were chosen for study
based on their environmental relevance and also to obtain
experimental data for a group of compounds possessing a
range of the physical and chemical properties expected to
influence reaction rates. Many are regulated under the United
States Clean Water and Safe Drinking Water Acts and are
common groundwater pollutants (1, 2).
E L I Z A B E T H C . B U T L E R † A N D
K I M F . H A YE S *
Department of Civil and Environmental Engineering,
University of Michigan, Ann Arbor, Michigan 48109-2125
The transformation of nine halogenated aliphatic compounds
by 10 g/L (0.5 m2/L) FeS at pH 8.3 was studied in batch
experiments. These compounds were as follows:
pentachloroethane (PCA), 1,1,2,2- and 1,1,1,2-tetrachloro-
ethanes (1122-TeCA and 1112-TeCA), 1,1,1- and 1,1,2-
trichloroethanes (111-TCA and 112-TCA), 1,1- and 1,2-
dichloroethanes (11-DCA and 12-DCA), carbon tetrachloride
(CT), and tribromomethane (TBM). 11-DCA, 12-DCA, and 112-
TCA showed no appreciable transformation by FeS
over approximately 120 days, but the other compounds
were transformed with half-lives of hours to days. PCA and
1122-TeCA underwent dehydrohalogenation faster than FeS-
mediated reductive dehalogenation reactions under the
conditions of these experiments. The remaining compounds
for which significant transformation was observed
underwent FeS-mediated reactions more rapidly than
hydrolysis or dehydrohalogenation. For 1112-TeCA, the
dihaloelimination product (1,1-dichloroethylene) was the
only reaction product detected. For 111-TCA, CT, and TBM,
hydrogenolysis products were the only products detected,
although their mass recoveries were considerably less
than 100%. Two simple log-linear correlations between rate
constants and either one-electron reduction potentials
or homolytic bond dissociation enthalpies were developed,
with coefficients of determination (R 2 values) of 0.48
and 0.82, respectively. These findings are consistent with
a rate-limiting step involving homolytic bond dissociation.
However, neither correlation accurately described the
reactivity of all the compounds that were studied, suggesting
distinctions among the mechanisms for reductive
dehalogenation of these compounds by FeS or the
influence of additional molecular or thermodynamic
parameters on rate constants.
Experimental Section
Materials and Procedures. Procedures for maintenance of
anaerobic conditions and sterilization of aqueous solutions
and glassware are described elsewhere (9). All chemicals were
commercially available reagent or ACS grade and were used
as received. Water was distilled and then purified using a
Milli-Q Plus water system (Millipore Corp., Bedford, MA).
FeS was prepared and characterized as described previously
(9) as poorly crystalline mackinawite with a specific surface
area of 0.05 m2/ g. All samples contained 10 g/ L (0.5 m2/ L)
FeS. The pH was buffered at 8.3 with 0.05 M each of tris-
(hydroxymethyl)aminomethane (Tris) and Tris-HCl. Samples
also contained 0.04 M NaCl and 0.01 N HCl, resulting in an
ionic strength of 0.1 M.
Introduction
Halogenated aliphatic compounds are common groundwater
pollutants (1, 2) that are susceptible to reductive transfor-
mations in anaerobic environments. Many recent studies
have focused on the abiotic transformation of these com-
pounds by naturally occurring soil minerals such as iron
sulfide (FeS) (3-11). Compounds reported to be transformed
by FeS include hexachloroethane (HCA) (9), carbon tetra-
chloride (CT) (4, 8, 10), trichloroethylene (TCE) (6, 7, 11),
* Corresponding author phone: (734)763-9661; fax: (734)763-2275;
e-mail: ford@engin.umich.edu.
Kinetic experiments were conducted in individual 5-mL
flame-sealed glass ampules (9) to maintain anaerobic condi-
tions and to prevent losses of volatile organic compounds
† Present address: School of Civil Engineering and Environmental
Science, University of Oklahoma, Norman, OK 73019-0631.
9
4 2 2 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 3, 2000
10.1021/es980946x CCC: $19.00
2000 Am erican Chem ical Society
Published on Web 01/04/2000

during experiments of up to 4 months duration. After
preparation, ampules were placed on Labindustries (Berkeley,
CA) model T 415-110 rocking platform shaker at ap-
proximately 22 cycles/ min in a temperature-controlled
chamber at 25 °C in the dark. In each ampule, the aqueous
phase volume was 5 mL, and the gas phase volume was
approximately 2.82 mL. Transformation reactions were
monitored over 3 or more half-lives, except for 11-DCA, 12-
DCA, and 112-TCA, for which transformation reactions were
monitored over the course of 120 days.
Samples from 11- and 12-DCA transformation experi-
ments were analyzed using only GC method B. However, at
least some samples from experiments with all other com-
pounds, except PCA, were analyzed using both GC methods
in order to identify as many reaction products as possible.
Samples from PCA experiments were analyzed using only
GC method A.
Treatm ent of Kinetic Data. Values of pseudo-first-order
rate constants (kobs) were determined by best fit of measured
values of aqueous concentration as a function of time to a
pseudo-first-order rate law. Values of kobs′, defined as the
pseudo-first-order rate constant for disappearance of a
compound in a hypothetical headspace-free system, were
calculated using the procedure described in ref11. Conversion
of kobs values to kobs′ values allowed valid comparison of rate
constants for transformation of a series of compounds that
have significantly different Henry’s law constants. Mass
recoveries were corrected as described in ref 11 to account
for equilibrium partitioning of reactants and products
between the aqueous and gas phases.
Dimensionless Henry’s law constants (H values) used in
these calculations were averages of all experimentally
determined values at approximately 25 °C that were reported
in refs 20-26, except that HCT from ref 21 was eliminated as
an outlier based on application of the Q test at the 95%
confidence level. HPCA, H1112-TeCA, Hacetylene, and HDBM were
estimated, not experimentally measured, values reported in
refs 27 (HPCA, H1112-TeCA, and Hacetylene) and 28 (HDBM). These
dimensionless Henry’s law constants are as follows: HCA,
0.285; PCA, 0.0793; 1122-TeCA, 0.015; 1112-TeCA, 0.0989; 111-
TCA, 0.622; 112-TCA, 0.0369; 11-DCA, 0.230; 12-DCA, 0.043;
CT, 1.244; CF, 0.150; PCE, 0.764; TCE, 0.419; cis-DCE, 0.167;
trans-DCE, 0.384; 11-DCE, 1.365; VC, 1.137; acetylene, 0.887;
TBM, 0.023; and DBM, 0.04.
Samples were spiked with 50 µL of a 0.002 M solution of
each halogenated aliphatic compound that had been pre-
pared in N2-sparged methanol, resulting in an aqueous
solution containing 1% methanol by volume. Initial aqueous
concentrations after partitioning to the sample headspace
were determined by measurement of the aqueous concen-
tration in the ionic medium alone (no FeS). Except for PCA
and 1122-TeCA, aqueous concentrations measured in the
ionic medium were used as the concentrations at time zero
in fitting data to a pseudo-first-order rate law. For PCA and
1122-TeCA, for which dehydrohalogenation took place even
in the absence of FeS, initial concentrations were those
measured at the first sampling time in the presence of FeS,
which was then considered to be time zero for these
compounds.
Because methanol coeluted with vinyl chloride (VC), a
potential reaction product, when using gas chromatography
(GC) method B (described below), certain samples of 11-
DCA, 12-DCA, and 112-TCA were also spiked with a 0.002 M
solution prepared in 2-propanol. These samples were
analyzed alongside samples spiked with methanol solutions
for the purpose of determining whether VC was a reaction
product for these compounds. Data from the 2-propanol-
spiked samples were not used in rate constant determination.
At regular intervals during the course of the reaction,
ampules were centrifuged at approximately 1000 rpm, broken
open, and analyzed. PCA, 1122-TeCA, 1112-TeCA, 111-TCA,
112-TCA, CT, chloroform (CF), TBM, dibromomethane
(DBM), PCE, TCE, and 1,1-dichloroethylene (11-DCE) were
quantified by extraction of the aqueous supernatant into
2,2,4-trimethylpentane followed by analysis using GC method
A(11). For PCA transformation experiments, the volume ratio
of aqueous sample to extraction solvent was 75 µL/ 1.5 mL,
and for all other experiments it was 100 µL/ 400 µL. For 112-
TCA transformation experiments, the extraction solvent
contained 3.2 µM TBM as an internal standard, and for PCA
transformation experiments, it contained 6 µM 1,3,5-trichlo-
robenzene as an internal standard. For 1122-TeCA, 1112-
TeCA, 111-TCA, CT, and TBM transformation experiments,
no internal standard was used. Different internal standards
(or no internal standard) were used in order to accurately
determine concentrations for compounds with different GC
retention times and for a range of GC operating conditions.
Samples were analyzed in duplicate and quantified by
comparison to a five-point standard curve. The results of
duplicate analyses typically agreed within 1% using GC
method A.
Results and Discussion
Reaction Pathways and Products. Table 1 summarizes the
observed first-order rate constants, reaction products, mass
recoveries, and reaction pathways for the transformation of
10 halogenated aliphatic compounds in the presence of FeS.
Certain data for HCA in this table were reported previously
(9). The rates of disappearance of 11-DCA, 12-DCA, and 112-
TCA over 120 days were not sufficient to determine quan-
titative rate constants for disappearance of these compounds.
Evidence for goodness of fit of the remaining data to a pseudo-
first-order rate law includes the relatively small uncertainties
associated with the rate constants reported in Table 1. In
addition, the initial rate method (29, 30) was used previously
(9) to show that transformation of HCA by 10 g/ L FeS was
first-order with respect to HCA (9) at concentrations similar
to those used here.
Table 1 indicates that mass recoveries of reactants and
products were considerably less than 100% in some cases.
Low mass recoveries may be due to the formation of reaction
products that would not have been detected using these
analytical methods, inaccuracies in the Henry’s law constants
used in calculating mass recoveries, losses of volatile reactants
and products during the sampling process, and adsorption
of reactants and products to the FeS surface.
For each compound shown in Table 1, control experiments
were concurrently performed using the ionic medium only
and no FeS. Except for PCA, 1122-TeCA, and 112-TCA, there
was no disappearance of any compound in the absence of
FeS in the time scale of these experiments. PCA, 1122-TeCA,
and 112-TCA underwent dehydrohalogenation (a nonre-
ductive reaction) regardless of whether FeS was present. The
rate constants, products, and mass recoveries for the
dehydrohalogenation of PCA and 1122-TeCA in the ionic
medium alone (no FeS) are reported in Table 2. Dehydro-
halogenation of 1122-TeCA to TCE is also illustrated in Figure
A 1-mL volume of the aqueous supernatant from certain
samples was also analyzed for 11-DCA, 12-DCA, chloro-
ethane, cis-1,2-dichloroethylene (cis-DCE), trans-1,2-dichlo-
roethylene (trans-DCE), ethane, ethene, acetylene, dichlo-
romethane, and methane using GC method B, which
employed a Hewlett-Packard (Palo Alto, CA) 5890 GC with
a HP 19395 headspace autosampler and an analytical method
described elsewhere (9). Some samples were diluted in a
1:10 ratio prior to analysis. 11-DCA, 12-DCA, and 112-TCA
samples spiked with a 2-propanol solution of the halogenated
aliphatic compound were analyzed for VC using GC method
B.
9
VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 2 3

TABLE 1. Observed First-Order Rate Constants, Products, Mass Recoveries, and Pathways for Transformation of Halogenated
Aliphatic Compounds in the Presence of FeS
kobs (h-1 a
)
mass recovery
(as %)c
compound
{kobs′ (h-1)}
productsb
pathway
HCAd
(1.065 ( 0.064) × 10-1
{(1.236 ( 0.074) × 10-1
PCE
PCA
HCA rem aining
total
86
<1
<1
86
dihaloelim ination
hydrogenolysis
}
PCA
(5.54 ( 0.31) × 10-1
PCE
TCE
PCA rem aining
total
77
13
0
dehydrohalogenation
dihaloelim ination
{(5.79 ( 0.32) × 10-1
}
90
1122-TeCA
(1.298 ( 0.087) × 10-2
{(1.309 ( 0.088) × 10-2
TCE
cis-DCE
trans-DCE
acetylene
1122-TeCA rem aining
total
58
3
<1
∼14
1
∼76
62
<1
62
dehydrohalogenation
see text
dihaloelim ination
see text
}
1112-TeCA
111-TCA
112-TCA
(3.05 ( 0.51) × 10-2
11-DCE
1112-TeCA rem aining
total
dihaloelim ination
hydrogenolysis
{(3.22 ( 0.54) × 10-2
}
}
(9.8 ( 1.4) × 10-3
11-DCA
111-TCA rem aining
total
∼2
{(1.32 ( 0.19) × 10-2
4
∼6
7
∼ 1
78
∼86
99
not knowne
11-DCE
VC
112-TCA rem aining
total
dehydrohalogenation
dihaloelim ination
f
11-DCA
12-DCA
CT
not knowne
not knowne
11-DCA rem aining
no products detected
no products detected
hydrogenolysis
f
12-DCA rem aining
81
(6.39 ( 0.79) × 10-2
CF
46
0
46
{(1.09 ( 0.13) × 10-1
}
CT rem aining
total
TBM
(1.29 ( 0.15) ×10-1
DBM
TBM rem aining
total
3
0
3
hydrogenolysis
{(1.31 ( 0.15) ×10-1
}
a
b
Uncertainties are 95% confidence intervals. For HCA, subsequent reaction products include TCE, cis-DCE, and acetylene (9). The PCA
c
d
transform ation experim ent did not m onitor for acetylene and DCEs. Refer to text discussion for calculation of m ass recoveries. Data for HCA
e
are from ref 9. There was not sufficient transform ation of this com pound over the course of 120 days to determ ine a quantitative rate constant.
f No reaction products were detected over the course of 120 days.
TABLE 2. Observed First-Order Rate Constants, Products, Mass Recoveries, and Pathways Transformation of Halogenated
Aliphatic Compounds in the Ionic Medium Alone (no FeS)
kobs (h-1 a
)
compound
{kobs′ (h-1)}
productsb
mass recovery (as %)c
pathway
PCA
(4.63 ( 0.27) × 10-1
{(4.84 ( 0.28) × 10-1
PCE
PCA rem aining
total
92
0
92
dehydrohalogenation
}
}
1122-TeCA
(1.11 ( 0.39) × 10-2
{(1.12 ( 0.39) × 10-2
TCE
90
9
99
dehydrohalogenation
1122-TeCA rem aining
total
112-TCA
not knownd
11-DCE
112-TCA rem aining
total
7
83
91
dehydrohalogenation
a
b
c
Uncertainties are 95% confidence intervals. See text for definitions of kobs and kobs ′. Only initial products are reported here. Refer to text
d
discussion for calculation of m ass recoveries. There was not sufficient transform ation of this com pound over the course of 120 days to determ ine
a quantitative rate constant.
1a,b (open circles). Although the reaction of PCA and 1122-
TeCA with FeS produced minor quantities of dihaloelimi-
nation products, rate constants for the disappearance of these
compounds increased only slightly in the presence of FeS
versus the ionic medium alone, as can be seen by comparison
of Tables 1 and 2 and by inspection of Figure 1a, indicating
that dehydrohalogenation by species present in the ionic
medium, such as hydroxide ion, took place considerably faster
than FeS-mediated reduction reactions for these compounds
at pH 8.3 and 10 g/ L FeS.
The mass recovery of the 112-TCA dehydrohalogenation
product 11-DCE in the ionic medium alone is also reported
in Table 2. Calculation of quantitative rate constants for the
disappearance of 112-TCA in the presence or absence of FeS
was not possible due to the slow rates of these reactions. In
the presence of FeS, however, the mass recovery of the 112-
9
4 2 4 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 3, 2000

TABLE 3. Acetylene/cis-DCE Product Distribution Ratio as a
a
Function of Time for 1122-TeCA Transformation by FeS
b
[acetylene]aq facetylene
[
]
[cis - DCE]aq fcis-DCE
time (h)
141.2
189.8
266.4
358.3
2.7
3.5
5.0
4.5
a
b
10 g/L FeS, pH 8.3, 0.1 M Tris Buffer, I ) 0.1 M. This is the acetylene/
cis-DCE product distribution ratio that would result in a hypothetical
headspace-free system . The variable fi is defined as (1 + HiVg/Vaq), where
Hi is the dim ensionless Henry’s law constant of species i and Vg and
Vaq are the gas- and aqueous-phase volum es, respectively, in each
sam ple.
[kobs, no FeS (Table 2)] from the rate constant in the presence
of FeS [kobs, FeS (Table 1)]. This assumes that kobs, FeS is equal
to kd + ke, while kobs, no FeS is equal to kd, where kd is the
pseudo-first-order rate constant for dehydrohalogenation and
ke is the pseudo-first-order rate constant for dihaloelimi-
nation. The resulting ke values are (9.1 ( 4.1) × 10-2 h-1
(PCA) and (1.9 ( 4.0) × 10-3 h-1 (1122-TeCA). Plots of [1122
-k
t and [1122 -TeCA]aq,0e-k t are shown along
obs,FeS
d
-TeCA]
e
aq,0
with experimentally measured aqueous concentrations in
Figure 1a. The resulting ke′ values (corrected for partitioning
between the aqueous and gas phases (11)) are (9.5 ( 4.3) ×
10-2 h-1 (PCA) and (1.9 ( 4.0) × 10-3 h-1 (1122-TeCA).
(Uncertainties in these and subsequent values are 95%
confidence intervals.) The relatively large uncertainties in
values of ke and ke′ result from calculating small differences
between two numbers of approximately equal magnitude.
Because the relative uncertainty in the value of ke′ for 1122-
TeCA exceeded 200%, this value was not included in
subsequent analyses.
The products of 1122-TeCA dihaloelimination were trans-
DCE and possibly cis-DCE as illustrated in Figure 1c. Figure
1c also shows the production of acetylene in the transforma-
tion of 1122-TeCA when FeS was present. (Samples from the
PCA experiment included in Table 1 were not monitored for
dichloroethylenes or acetylene.) Significant accumulation
of acetylene corresponds to a decline in the concentration
of TCE (Figure 1b,c), suggesting that acetylene is formed
from reductive elimination of TCE, which in turn is formed
from 1122-TeCA dehydrohalogenation. Transformation of
TCE to acetylene was reported in ref 11. Since cis-DCE was
reported to be a product of TCE transformation by FeS (11),
it is difficult to determine whether the cis-DCE detected as
a minor reaction product in 1122-TeCA transformation by
FeS came directly from 1122-TeCA dihaloelimination or from
hydrogenolysis of the 1122-TeCA dehydrohalogenation prod-
uct TCE. Both cis-DCE and trans-DCE, but not TCE, were
detected in the transformation of 1122-TeCA by vitamin B12
(33), and cis-DCE was the sole product detected in the
transformation of 1122-TeCA by anoxic sediments (34),
indicating that cis-DCE may be produced by 1122-TeCA
dihaloelimination.
FIGURE 1. Aqueous concentrations of 1122-TeCA, TCE, acetylene,
cis-DCE, and trans-DCE versus time; pH 8.3, 0.1 M Tris buffer, I )
0.1 M. The data points are experimentally measured values, the
-k
t
obs,FeS
solid line is equal to [1122 - TeCA]
e
, where kobs, FeS equals
aq,0
kd + ke and the dotted line is equal to [1122 - TeCA]aq,0e-k t (see
text). Aqueous concentrations of acetylene, cis-DCE, and trans-
DCE were not measured at early time points. Acetylene, cis-DCE,
and trans-DCE were detected only in samples containing FeS. Note
that the y-axis scale for panel c differs from panels a and b.
d
TCA dehydrohalogenation product (11-DCE) was approxi-
mately 7% versus approximately 1% for its dihaloelimination
product (VC) (Table 1). This indicates that, like PCA and 1122-
TeCA, the rate of dehydrohalogenation of 112-TCA exceeds
that of dihaloelimination under the conditions of these
experiments. The more rapid dehydrohalogenation of PCA,
1122-TeCA, and 112-TCA as compared to the other halo-
genated organic compounds studied here is consistent with
the results of Walraevens et al. (31) and Jeffers et al. (32), who
found that the rates of dehydrohalogenation of a series of
chlorinated ethanes in water followed the order PCA > 1122-
TeCA > 112-TCA.
To determine the source of cis-DCE detected in the
transformation of 1122-TeCA by FeS, the acetylene/ cis-DCE
product distribution ratios for the four time points in Figure
1c for which both cis-DCE and acetylene were detected were
calculated and are shown in Table 3. While TCE transforma-
tion by FeS produced a constant acetylene/ cis-DCE product
distribution ratio of 11.8 ( 1.1 (11), the product distribution
ratios reported in Table 3 are considerably smaller than 11.8
( 1.1, and they appear to be increasing over time. Both
observations suggest that cis-DCE comes directly from 1122-
TeCA dihaloelimination as well as from TCE hydrogenolysis,
since production of cis-DCE only from TCE would result in
Rate constants for dihaloelimination of PCA and 1122-
TeCA by FeS were estimated by subtracting the rate constant
for PCA or 1122-TeCA disappearance in the absence of FeS
9
VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 2 5

a larger acetylene/ cis-DCE product distribution ratio that
was constant over time. The trans-DCE detected as a minor
reaction product is more certainly the product of 1122-TeCA
dihaloelimination and not TCE hydrogenolysis, since trans-
DCE was not a product of TCE transformation by FeS (11).
For the LFER analysis, rate constants were fit to a LFER
of the form (49)
E°HAC
log kobs ) b + b
(1)
(
)
0
1
0.059 V
Table 1 indicates that in addition to PCA, 1122-TeCA, and
112-TCA, two of the three remaining vicinal polychlorinated
ethanes (HCA and 1112-TeCA) underwent dihaloelimination
in the presence of FeS. (No products were detected in the
reaction of 12-DCA with FeS after 120 days.) A hydrogenolysis
product was detected for only one vicinal polychlorinated
ethane (HCA) and then only as a minor product (9).
Dihaloelimination has been shown to be favored over
hydrogenolysis in the reductive transformation of vicinal
polychlorinated ethanes by numerous other reductants (19,
33, 35-43), which may reflect the strong driving force for
alkene formation (44) more than considerations associated
with individual reducing species.
where E°HAC is the standard aqueous-phase one-electron
reduction potential of the halogenated aliphatic compound
and b0 and b1 are constants. Application of this LFER was
based on three assumptions: (i) a common rate-limiting
one-electron reduction (50)
R-X + e- f R• + X-
(2)
for the reductive dehalogenation of all compounds studied
here; (ii) similar tendencies for adsorption of each haloge-
nated aliphatic compound to the FeS surface; and (iii) a
consistent proportionality among all compounds between
the activation energy for reductive dehalogenation of the
Hydrogenolysis products were the sole products detected
for one geminal polychlorinated ethane (111-TCA) and the
two halogenated methanes (CT and TBM) that were studied.
(Essentially no transformation by FeS of the other geminal
polychlorinated ethane, 11-DCA, was observed over 120 days.)
However, mass recoveries of these hydrogenolysis products
were low, particularly for 111-TCA and TBM. The best
explanation for the poor mass recoveries is the formation of
products that were not detected by the analytical methods
employed here. For example, it was recently reported that
CT was transformed to CF and CS2 in an approximately 2:1
ratio by freshly precipitated FeS at near-neutral pH values
(10). Considering the possibility that the CF:CS2 product
distribution ratio varies with pH, CS2 might account for a
significant percentage of the incomplete mass recovery of
CT noted in Table 1. CT was also transformed to CS2 by
pyrite (45) and by aqueous bisulfide in the presence of biotite
or vermiculite (13). Although CS2 was not detected in these
experiments, the concentrations at which it would be present
after sample dilution or extraction would likely have been
below analytical detection limits. Other products of CT
transformation in related experimental systems that would
not have been detected using these analytical methods and
that might account for poor mass recovery include CO (13),
CO2 (13, 45), and formate (45).
adsorbed reactant and E°HAC
.
An initial rate-limiting step such as reaction 2 could lead
to the dihaloelimination and hydrogenolysis products re-
ported in Table 1 (51) as well as certain products hypothesized
earlier to account for the poor mass recovery of CT, 111-
TCA, and TBM. For example, the transformation of CT to CF
and products such as CO, CO2, formate, and CS2 has been
explained by an initial one-electron reduction of CT followed
by parallel formation of hydrogenolysis products, CS2, and
dichlorocarbene, or a dichlorocarbene adduct (13, 45, 52,
53). The dichlorocarbene or its adduct can then react with
water to form CO (13, 52, 53) or formate (45), and CS2 may
subsequently undergo hydrolysis to form CO2 (13, 45). In
related experimental systems, the reductive transformation
of 111-TCA has been shown to involve an initial one-electron
reduction yielding a radical intermediate (54, 55). This could
be followed rapidly by parallel formation of 11-DCA and a
carbene intermediate (46, 56), yielding products such as
acetaldehyde and VC (46). For TBM, evidence for an initial
one-electron reduction to a radical intermediate has been
reported (54). Parallel transformation of the resulting radical
to DBM and a carbene intermediate may follow, with the
carbene forming products such as CO (47).
In addition to evidence from related experimental systems,
two observations from experiments with FeS are consistent
with an initial rate-limiting step such as reaction 2. First,
evidence was reported in ref 9 that HCA was transformed by
FeS to PCE via two successive one-electron transfers. This
evidence included the dependence of pseudo-first-order rate
constants for HCA transformation on both HCA and FeS
concentrations, indicating a bimolecular rate-limiting reac-
tion step and detection of PCA as a reaction intermediate,
which could not be produced by E2 (bimolecular) elimination
and was not likely to be produced by E1cb (unimolecular
elimination of the conjugate base) (9). Also, in the trans-
formation of TCE by FeS (troilite), Sivavec et al. (6) detected
hydrocarbon products hypothesized to arise from coupling
of radical intermediates, consistent with a rate-limiting
reaction step such as reaction 2.
Products that could account for incomplete mass recovery
of 111-TCA reported in Table 1 include acetaldehyde and
VC, detected in the transformation of 111-TCA by CrSO4 (46).
Neither acetaldehyde nor VC would have been detected using
these analytical methods due to coelution with methanol
using GC method B. For TBM, possible reaction products
that would not be detected in these experiments include CO,
detected in the transformation of TBM by rat liver microsomes
containing cytochrome P-450 (47). Bromoalkanes such as
TBM are also susceptible to nucleophilic substitutions in
sulfide-rich waters, leading to alcohols, thiols, and dialkyl
sulfides (48) that may not have been detected with these
analytical methods.
Correlation Analysis. While reductive transformation of
the halogenated aliphatic compounds in Table 1 by FeS led
to a variety of reaction products, it is possible that these
transformations share a common rate-limiting elementary
reaction step. Correlation analyses were performed to explore
the possibility of such rate-limiting processes and to assess
the influence of different molecular properties on rate
constants for FeS reductive dehalogenation. Two approaches
to correlation analysis were used: a linear-free energy
relationship (LFER) and a multiple linear correlation analysis.
Rate constants for the transformation of TCE and PCE by
FeS (11) were included in both analyses. For PCA, ke′, i.e., the
rate constant for PCA dihaloelimination by FeS, was used.
Linear regression of experimentally determined log kobs
′
values versus calculated E°HAC values produced the following
correlation (R 2 ) 0.48)
log kobs′ ) -0.9 ((1.0) + 0.14((0.14)E°HAC/ 0.059 V (3)
that is shown in Figure 2. E°HAC values were calculated as
described in refs 28 and 40 and are reported in Table 4.
Sources for the data used in these calculations are given in
the footnote to Table 4. To assess the influence of uncertain-
ties in these values on the regression coefficients in eq 3,
E°HAC values for HCA, PCA, 1112-TeCA, and 111-TCA were
9
4 2 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 3, 2000

TABLE 4. Data Used in Correlation Analysis
•
E°HAC
E°HAC
ELUMO
∆f G°R
DR-X
Saq
(aq)
f
compound
(V vs NHE)a
(V vs NHE)b
(eV)c
(kJ/mol)d
(kJ/mol)d
(mol/L)e
log kobs′
HCA
PCA
1112-TeCA
111-TCA
TCE
PCE
CT
TBM
-0.0513
0.00526
-0.213
-0.318
-0.789
-0.531
-0.160
-0.555
-0.0985
0.00576
-0.0988
-0.192
NA
NA
NA
NA
-0.563
-0.373
-0.229
-0.072
-0.044
-0.320
-0.629
-1.172
75.37
54.24
65.86
84.60
223.3
204.4
86.57
156.2
295.7
287.8
305.4
316.8
357.4
334.6
304.1
283.3
2.11E-4
2.37E-3
6.55E-3
1.12E-2
6.59E-3
1.53E-3
5.16E-3
1.23E-2
-0.908 ( 0.026
-1.02 ( 0.20
-1.492 ( 0.073
-1.878 ( 0.062
-2.827 ( 0.043
-3.241 ( 0.076
-0.964 ( 0.054
-0.884 ( 0.050
a
E°HAC values are the potentials for reaction 2 in the aqueous phase and were calculated as described in refs 28 and 40, assum ing 298.15 K,
1 atm pressure, and unit activities for each reactant and product. E°HAC values in this colum n were used for the correlation in eq 3 and Figure 2.
Gas-phase standard free energies for form ation of R• (where R• is defined in reaction 2) were obtained or calculated from data in refs 71 (HCA,
PCA, 1112-TeCA, PCE, and CT), 28 (111-TCA), 40 (TBM), or 62 (TCE). Data in these references were obtained using both experim ental and theoretical
m ethods. Gas-phase standard free energies of form ation of R-X (where R-X is defined in reaction 2) were obtained from refs 72 (HCA, TCE, PCE,
CT, and TBM) or 57 (PCA, 1112-TeCA, and 111-TCA). All other therm odynam ic data were from ref 72. Henry’s law constants were reported in the
Experim ental Section. Where m ore than one radical isom er was possible upon reduction of the halogenated aliphatic com pound, the one with
•
the sm allest free energy of form ation was assum ed. For the one-electron reduction of PCA and 1112-TeCA, these radical isom ers were CCl2CHCl2
•
b
and CCl2CH2Cl, respectively (57, 71). For TCE, cis-DCE• was assum ed to form based on the calculations in ref 62. E°HAC values in this colum n were
calculated as described in footnote a using the gas-phase standard free energies for form ation of R• from ref 57, which were obtained using the
m ethods of statistical therm odynam ics. These free energies of form ation were not available (NA) for the com pounds in the last four rows of this
c
table. ꢀLUMO values are sem iem pirical values calculated using the program CAChe WorkSystem version 3.9 (Oxford Molecular Group) using PM3
param eters and MOPAC geom etry optim ization. ∆f G°R•(aq) and DR-X values were calculated using the therm odynam ic data and assum ptions
d
given in footnote a. Saq values corresponding to 25 °C are from refs 73 (HCA) and 27 (all other com pounds). kobs′ values (in h-1) are from Table
1, except the values for TCE and PCE (11) and the value for PCA, which is the logarithm of ke′, i.e., the rate constant for PCA dihaloelim ination
reported in the text.
e
f
The assumption that reaction 2 represents a common
rate-limiting elementary reaction step for FeS reductive
transformation would be inaccurate if FeS transformations
take place by mechanism(s) other than reaction 2 or if mass
transport and not chemical processes were rate-limiting. As
discussed earlier, TBM may have undergone nucleophilic
substitution in the presence of FeS, which may explain the
deviation of this compound from the trend exhibited by the
other compounds. Nucleophiles such as sulfides and polysul-
fides have also been shown to promote dihaloelimination
reactions (39, 40, 42, 43, 59) by pathways that may involve
a single two-electron reduction reaction (60), and it is possible
that the sulfide functional groups in FeS promote such a
reaction for one or more of the compounds studied here.
The possibility that adsorption or desorption of reactants or
products to or from the FeS surface were rate-limiting
processes seems unlikely, since the observed rate constants
are relatively small and differ from each other by 1 or more
orders of magnitude (Table 1, Figure 2), while rate constants
for mass transfer to or from the FeS surface are likely to be
larger and more uniform in magnitude.
FIGURE 2. Log kobs′ versus E°HAC/0.059 V. Dotted lines represent the
95% confidence interval. The linear equation is log kobs′ ) -0.9
((1.0) + 0.14((0.14) × E°HAC/0.059 V (R 2 ) 0.48). Units of kobs′ are
h-1
.
Significantly different tendencies for adsorption of each
compound to the FeS surface could also contribute to the
relatively poor correlation in Figure 2, since rate constants
for transformation of adsorbed reactants are proportional to
equilibrium adsorption constants under conditions where
pseudo-first-order behavior with respect to the adsorbed
reactant is observed (61). Different mechanisms of adsorption
may also have influenced rate constants and thereby
contributed to the poor correlation in Figure 2, since bonding
interactions between each compound and the FeS surface
could raise or lower activation energies and negate the
assumption of a consistent proportionality between activa-
tion energies and E°HAC values. These effects may be most
important for compounds with structure and properties
significantly different from the others, such as TCE and PCE.
Significant adsorption to nonreactive surface sites may also
have influenced rate constants and contributed to the poor
correlation in Figure 2.
recalculated using a different source of thermodynamic data
(57) and are reported in the third column of Table 4. (Ref 57
does not contain the thermodynamic data necessary for
recalculation of E°HAC values for the remaining compounds.)
Replotting Figure 2 using these revised E°HAC values for HCA,
PCA, 1112-TeCA, and 111-TCA and the original E°HAC values
for the remaining compounds yielded a linear equation [log
k
obs′ ) -1.0 ((1.0) + 0.12 ((0.14)E°HAC/ 0.059 (R 2 ) 0.43)]
that lies within the 95% confidence interval of the line
illustrated in Figure 2. Although not an exhaustive uncertainty
analysis, this result indicates that differences among pub-
lished thermodynamic data for haloaliphatic radicals do not
significantly affect the trend illustrated in Figure 2.
The slope of eq 3 is not statistically different from zero
at the 95% confidence level. It is, however, significantly
different from zero at the 90% confidence level, indicating
a significant correlation between log kobs′ and E°HAC values
at this relatively conservative confidence level. However, for
one-electron reduction reactions for which the assumptions
stated earlier apply, a line of the form of eq 3 will have a slope
of 1 (58). The slope of 0.14 in eq 3 indicates the limitations
of one or more of the previously stated assumptions.
Because the results illustrated in Figure 2 suggested that
properties other than E°HAC influence rate constants, an
empirical correlation that related rate constants to one or
more molecular property was sought. The regression tech-
9
VOL. 34, NO. 3, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 4 2 7

predicting the transformation of halogenated aliphatic pol-
lutants by FeS in natural and engineered systems. However,
no thermodynamic or empirical relationship accurately
described the reactivity of all the halogenated aliphatic
compounds that were investigated, suggesting differences
among the mechanisms for reductive dehalogenation of these
compounds by FeS or suggesting that parameters not
addressed in the correlations also influence rate constants.
This is particularly evident with PCE and TBM. It is likely
that different tendencies or mechanisms of adsorption to
the FeS surface play a role in determining relative reaction
rates. An important goal of future studies will be to further
investigate these questions.
Acknowledgments
We thank Tom Yavaraski for invaluable technical assistance
in the laboratory. Funding for this research was provided by
the Great Lakes and Mid-Atlantic Center for Hazardous
Substance Research under Grant R-819605 from the Office
of Research and Development, U.S. Environmental Protection
Agency (U.S. EPA). Partial funding of the research activities
of the center was provided by the State of Michigan
Department of Natural Resources. Funding for this research
was also provided by the U.S. EPA, the U.S. Department of
Energy, the National Science Foundation, and the Office of
Naval Research Joint Program on Bioremediation (EPA-G-
R-825958). The content of this publication does not neces-
sarily reflect the views of these agencies.
FIGURE 3. Log kobs′ versus DR-X. Dotted lines represent the 95%
confidence interval. The linear equation is log kobs′ ) 8.8 ((4.9) -
3.4 ((1.6) × 10-2 × DR-X (R 2 ) 0.82). Units of kobs′ are h-1
.
niques described in ref 62 were used to determine the simple
or multiple linear equation that best described the relation-
ship between log kobs′ values and one or more of the following
properties of the halogenated aliphatic compound: E°HAC
,
the LUMO energy (ꢀLUMO), the free energy of formation of the
aqueous-phase radical (∆f G°R•(aq), where R• has the meaning
shown in reaction 2), the gas-phase homolytic bond dis-
sociation enthalpy (DR-X), and aqueous solubility (Saq). Values
of these properties are reported in Table 4. The first four
properties are closely related and represent one or more
components of the thermodynamic driving force for one-
electron reduction, while the fifth, Saq, is typically related to
the extent of adsorption of hydrophobic organic compounds
to mineral surfaces that do not have significant organic
carbon content (63).
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Received for review September 14, 1998. Revised manuscript
received November 12, 1999. Accepted November 17, 1999.
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