Electrochemical investigation of the rate-limiting mechanisms for trichlomethylene and carbon tetrachloride reduction at iron surfaces

Article abstract of DOI:10.1021/es0019878

The mechanisms involved in reductive dechlorination of carbon tetrachloride (CT) and trichloroethylene (TCE) at iron surfaces were studied to determine if their reaction rates were limited by rates of electron transfer. Chronoamperometry and chronopotentiometry analyses were used to determine the kinetics of CT and TCE reduction by a rotating disk electrode in solutions of constant halocarbon concentration. Rate constants for CT and TCE dechlorination were measured as a function of the electrode potential over a temperature range from 2 to 42 °C. Changes in dechlorination rate constants with electrode potential were used to determine the apparent electron-transfer coefficients at each temperature. The transfer coefficient for CT dechlorination was 0.22 ± 0.02 and was independent of temperature. The temperature independence of the CT transfer coefficient is consistent with a rate-limiting mechanism involving an outer-sphere electron-transfer step. Conversely, the transfer coefficient for TCE was temperature dependent and ranged from 0.06 ± 0.01 at 2 °C to 0.21 ± 0.02 at 42 °C. The temperature-dependent TCE transfer coefficient indicated that its reduction rate was limited by chemical dependent factors and not exclusively by the rate of electron transfer. In accord with a rate-limiting mechanism involving an electron-transfer step, the apparent activation energy (E_a) for CT reduction decreased with decreasing electrode potential and ranged from 33.0 ± 1.6 to 47.8 ± 2.0 kJ/mol. In contrast, the E, for TCE reduction did not decline with decreasing electrode potential and ranged from 29.4 ± 3.4 to 40.3 ± 3.9. The absence of a potential dependence for the TCE E_a supports the conclusion that its reaction rate was not limited by an electron-transfer step. The small potential dependence of TCE reaction rates can be explained by a reaction mechanism in which TCE reacts with atomic hydrogen produced from reduction of water.

Full text of DOI:10.1021/es0019878

Environ. Sci. Technol. 2001, 35, 3560-3565
serves as an electron donor that transforms chlorinated
Electrochemical Investigation of the
Rate-Limiting Mechanisms for
Trichloroethylene and Carbon
Tetrachloride Reduction at Iron
Surfaces
organic compounds to their nonchlorinated analogues and
chloride ions. Redox active metals, water, and other ground-
water constituents, such as nitrate and carbonate, may also
be reduced by reactions with the zerovalent iron (8, 9).
Dechlorination rates and byproducts for most environ-
mentally relevant halocarbons have been determined under
a wide range of experimental conditions (1-7, 10, 11). In
most instances, rates of dechlorination within a homologous
series of compounds increase with the degree of chlorination
(3). This observation has led investigators to develop linear
free energy relationships (LFERs) between reaction rates and
halocarbon physicochemical properties (11-13).
LFERs with high correlation coefficients have been found
between rates of halocarbon dechlorination and physical
parameters such as the energy of the lowest unoccupied
molecular orbital (E-LUMO), two-electron reduction po-
tential (E2), one-electron reduction potential (E1), and
carbon-halogen bond dissociation energies (11-13). These
correlations between reaction rates and potential dependent
parameters, such as E-LUMO, E1, and E2, have led inves-
tigators to conclude that rates of electron transfer control
dechlorination rates. However, the correlation between many
physical properties makes LFERs unsuitable for determining
whether halocarbon reaction rates are limited by rates of
electron transfer. For example, parameters such as E1 and
T I E L I A N D J A M E S F A R R E L L *
Department of Chemical and Environmental Engineering,
College of Engineering and Mines, The University of Arizona,
1209 East 2 Street, Tucson, Arizona 85721-0011
The mechanisms involved in reductive dechlorination of
carbon tetrachloride (CT) and trichloroethylene (TCE) at iron
surfaces were studied to determine if their reaction
rates were limited by rates of electron transfer. Chrono-
amperometry and chronopotentiometry analyses were used
to determine the kinetics of CT and TCE reduction by a
rotating disk electrode in solutions of constant halocarbon
concentration. Rate constants for CT and TCE dechlorination
were measured as a function of the electrode potential
over a temperature range from 2 to 42 °C. Changes in
dechlorination rate constants with electrode potential were
used to determine the apparent electron-transfer
coefficients at each temperature. The transfer coefficient
for CT dechlorination was 0.22 ( 0.02 and was independent
of temperature. The temperature independence of the CT
transfer coefficient is consistent with a rate-limiting
mechanism involving an outer-sphere electron-transfer
step. Conversely, the transfer coefficient for TCE was
temperature dependent and ranged from 0.06 ( 0.01 at 2
°C to 0.21 ( 0.02 at 42 °C. The temperature-dependent TCE
transfer coefficient indicated that its reduction rate was
limited by chemical dependent factors and not exclusively
by the rate of electron transfer. In accord with a rate-
limiting mechanism involving an electron-transfer step, the
E2 correlate with the degree of chlorination within
a
homologous series. Therefore, these parameters also correlate
with halocarbon hydrophobicity and the extent of adsorption
to reactive sites. Thus, faster dechlorination rates for more
highly chlorinated homologues may arise from greater
adsorption to reactive sites and not from a greater thermo-
dynamic driving force for reduction, as indicated by E-LUMO,
E1, and E2.
Although most investigators have reported faster dechlo-
rination rates with increasing degree of halogenation, not all
studies are consistent with this trend. For example, Arnold
and Roberts (11) reported that dechlorination rates for a series
of chlorinated ethenes decreased with increasing degree of
chlorination, and Farrell et al. (14) reported faster reaction
rates for trichloroethylene (TCE) than for perchloroethylene.
Other investigators have found that the relative rates of TCE
and dichloroethylene reduction varied along the length of
column reactors and also varied between batch and column
systems (7). These observations suggest that there may be
multiple mechanisms for chlorinated ethene reduction on
iron surfaces.
These multiple mechanisms may include direct and
indirect electron-transfer reactions. Direct reduction may
occur by electron tunneling or by formation of a chemi-
sorption complex of the organic compound with the iron
surface (15). Electron tunneling may occur to halocarbons
that are physically adsorbed at the iron surface or to
halocarbons that are separated from the iron surface by one
or more layers of adsorbed water. Halocarbons may also be
reduced indirectly via reaction with atomic hydrogen pro-
duced from water reduction (16). These reactions may involve
formation of hydrocarbon-hydride complexes on the iron
surface (15).
apparent activation energy (E ) for CT reduction decreased
a
with decreasing electrode potential and ranged from
33.0 ( 1.6 to 47.8 ( 2.0 kJ/mol. In contrast, the E for TCE
a
reduction did not decline with decreasing electrode
potential and ranged from 29.4 ( 3.4 to 40.3 ( 3.9. The
absence of a potential dependence for the TCE E supports
a
the conclusion that its reaction rate was not limited by
an electron-transfer step. The small potential dependence
of TCE reaction rates can be explained by a reaction
mechanism in which TCE reacts with atomic hydrogen
produced from reduction of water.
Introduction
Electrochem ical Kinetics. In the absence of mass transfer
limitations, the potential dependence of electrochemical
reaction rates can be described by the Butler-Volmer
equation (17):
Permeable reactive barriers containing zerovalent iron are
becoming increasingly popular for in situ remediation of
groundwater contaminated with chlorinated organic com-
pounds (1-7). In zerovalent iron remedial systems, the iron
eq)/ RT
i ) i0[e-RbF(E-E )/ RT - eRaF(E-E
]
(1)
eq
* Corresponding author phone: (520)621-2465; fax: (520)621-6048;
e-mail: farrell@engr.arizona.edu.
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3 5 6 0 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 17, 2001
10.1021/es0019878 CCC: $20.00
2001 Am erican Chem ical Society
Published on Web 07/28/2001

where i is net reaction current, i0 is the exchange current, F
is the Faraday constant, R is the gas constant, T is the
temperature, Eis the electrode potential, Eeq is the equilibrium
potential for the redox reaction, and Rb and Ra are the transfer
coefficients for the reduction and oxidation reactions,
respectively. The first term in the brackets represents the
rate of the forward reduction reaction, while the second term
gives the rate of the reverse oxidation reaction. The transfer
coefficients depend on the number of electrons transferred
before (bγ) and after (aγ) the rate-determining step, the number
of times the rate-determining step must occur, and the
symmetry factor (â) for the reaction (18). For overall reactions
involving only a single CT or TCE molecule, the transfer
coefficients may be expressed as
Rb ) bγ + râ
(2)
(3)
FIGURE 1. Diagram of rotating disk electrode reactor.
Ra ) aγ + r(1 - â)
solution. The solutions were continuously purged with ∼100
mL/ min nitrogen gas in order to maintain anaerobic condi-
tions and to remove reaction byproducts from the solution.
Halocarbon concentrations in the reaction cell were con-
trolled by adding TCE or CT to the nitrogen purge stream.
Experiments to determine the transfer coefficients were
conducted at a constant CT concentration of 2 mM or a
constant TCE concentration of 3 mM. Before each experi-
ment, the iron disk was chemically and mechanically polished
with an EG&G electrode polishing kit. The iron disk was then
conditioned at -785 mV prior to each experiment in order
to reduce any remaining surface oxides. The disk electrode
was rotated at speeds of 100 or 1000 rpm to eliminate the
effect of mass transfer on rates of TCE or CT dechlorination.
Chronoamperometry (CA) analyses (17) were performed
in background electrolyte solutions and in solutions con-
taining TCE or CT at potentials ranging from -600 to -1200
mV. Chronopotentiometry (CP) analyses (17) were also
performed in electrolyte solutions with or without CT by
applying currents ranging from 0.1 to 400 µA to the disk
electrode. Electrode potentials or electrode currents were
controlled and recorded with an EG&G model 273A poten-
tiostat and M270 software. All potentials are reported relative
to the standard hydrogen electrode (SHE), and cathodic
currents are reported as positive.
where r ) 0 if the rate-determining step does not involve
electron transfer, otherwise r ) 1. The â parameter is
dependent on the symmetry of the potential energy surface
between the transition state and the reactant and product
species (18). For a single-step electron-transfer reaction, â
represents the fraction of the applied overpotential that goes
toward overcoming the activation energy of the cathodic
reaction, while (1 - â) represents the fraction of the
overpotential that goes toward increasing the activation
energy for the reverse, anodic reaction (18).
Transfer coefficients for a range of halocarbons on iron
electrodes have been measured by other investigators (13,
19). However, no attempts were made to determine whether
dechlorination rates were actually limited by the rate of
electron transfer. In multistep electrodic reactions, the overall
reaction rate may be limited by rates of reactant chemi-
sorption, rates of bond breaking, or rates of molecular
rearrangement. These are chemical, rather than potential,
dependent factors. If the overall reaction rate is limited by
chemical dependent factors, the measured transfer coefficient
may be only a fitting parameter or an apparent transfer
coefficient.
The goal of this study was to determine if the rate-limiting
mechanisms for iron-mediated reductive dechlorination of
CT and TCE involve electron transfer. Reaction rate constants
and apparent transfer coefficients were measured at different
temperatures and electrode potentials. The effect of tem-
perature on the electron-transfer coefficients and the effect
of potential on the activation energies of the reactions were
used to determine whether dechlorination rates were limited
by chemical or potential dependent factors.
Analyses. TCE and CT concentrations in the reactor were
determined by analysis of 100-µL aqueous samples. The 100-
µL samples were injected into 1 g of pentane and analyzed
by injection into a Hewlett-Packard 5890 series II gas
chromatograph (GC) equipped with an electron capture
detector (ECD) and autosampler. Chloride ion concentrations
were determined by analysis of 5-mL samples using a DX
500 ion chromatograph (Dionex, Sunnyvale, CA). An Orion
pH probe was used to determine the initial and final pH
values of the electrolyte solutions.
Materials and Methods
Rotating Disk Electrode (RDE) Reactor. All kinetic experi-
ments were performed with an EG&G (Oak Ridge, TN) model
616 rotating disk electrode in a custom glass reaction cell,
as depicted in Figure 1. The cylindrical glass reactor was
contained within a water jacket and had an i.d. of 2.2 cm and
a length of 11 cm. The temperature in the reaction cell was
controlled to within (0.2 °C with a circulating water bath.
An iron disk (Metal Samples Co., Mumford, AL) with a
geometric surface area of 1 cm2 was used as the working
electrode. A Hg/ Hg2SO4 electrode (Perkin-Elmer, Oak Ridge,
TN) was used as the reference electrode, and a 0.3 mm
diameter by 4 cm long platinum wire (Aesar, Ward Hill, MA)
was used as the counter electrode. The counter electrode
was encased within a Nafion (Dupont) proton permeable
membrane in order to prevent oxidation of chloride ions or
the target organic compounds.
Because operation of the RDE required that the solution
be exposed to the atmosphere through the electrode shaft
opening, reaction byproducts were determined in separate
experiments in a 25-mL sealed, glass cell. After a short period
of electrolysis at each potential, the solutions were sampled
and analyzed for chloride ions and hydrocarbon concentra-
tions.
Results and Discussion
CT Results. Initiation of CT reduction may proceed via a
stepwise mechanism in which the first step involves only
outer-sphere electron transfer, according to (20)
CCl4 + e- 7step 18 CCl4•- 7step 28 CCl3• + Cl-
(4)
Voltam m etric Experim ents. All experiments were per-
formed in 25 mL of a 10 mM CaSO4 background electrolyte
or it may proceed via a concerted electron-transfer and bond-
breaking step, according to (20, 21)
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VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 5 6 1

CCl4 + e- T CCl3• + Cl-
(5)
The electrode potential, electrode material, and solvent
determine which of these initiation steps predominates (20,
21). Over the potential range investigated in this study,
reaction 4 or 5 is followed by a second electron transfer in
which the trichloromethyl radical reacts according to
(21, 22)
CCl3• + e- + H+ f CHCl3
(6)
The reaction byproducts observed in this study are consistent
with these reactions. GC-ECD analyses indicated that CF
accounted for >90% of the CT disappearance, while chloride
analyses indicated that the CF yield was ∼95%. The disparity
between these two measurements can likely be attributed to
an adsorptive loss of CF to Teflon fittings in the sealed, glass
cell.
FIGURE 2. CA profiles for iron disk electrode in a 10 mM CaSO
electrolyte solution with and without 2 mM CT, at a rotating speed
of 100 rpm and an applied potential of -785 mV.
4
The initial pH of the electrolyte solutions was ∼6.5, and
there was a decline of less than 2 pH units over the course
of each experiment. In most cases, the decline in pH was less
than 1 pH unit. The effect of pH on CT reduction rates was
determined at potential of -785 mV at pH values of 4.1, 5.3,
and 7.2. In these experiments, there was less than a 10%
increase in the CT reaction rate for a solution pH decrease
from 7.2 to 4.1. Therefore, the decrease of less than 2 pH
units over the course of each experiment had a very small
impact on the measured rates of CT dechlorination.
The stronger pH dependency of reaction rates observed
with corroding iron media (23, 24) can be attributed to the
effect of pH on the condition of the iron surfaces and on the
corrosion rate, which affects the iron’s free corrosion
potential. Lower pH values lead to cleaner iron surfaces and
faster corrosion rates. Since the iron in this investigation was
cathodically protected and at a fixed potential, there were
minimal oxides on the iron surface, and pH did not affect
the iron potential.
CA experiments were performed at 22 °C to determine
the reaction rate constants for CT as a function of electrode
potential. A profile from a typical CA experiment is illustrated
in Figure 2. At the start of each experiment, the electrode
potential was stepped cathodically from its open circuit
potential in the blank electrolyte solution to the desired
potential. This generated a constant current for water
reduction, as indicated by the blank current in Figure 2. After
30 min of electrolyzing the blank solution, CT was added to
the nitrogen purge gas. Once the solution concentration had
reached its steady state value of 2 mM, the applied potential
was momentarily terminated, and the potential was again
stepped from the open circuit potential to the desired
potential. As shown in Figure 2, this potential step resulted
in a surge in current due to charging of the electrical double
layer at the electrode surface and to reduction of CT adsorbed
to the electrode under open circuit conditions. Throughout
each experiment, there was a slight decline in the cell current
between 3 and 30 elapsed min, as illustrated in Figure 2. This
can be attributed primarily to the buildup of iron oxides and
atomic and molecular hydrogen on the electrode surface.
Under steady-state conditions, the rate of CT reduction
(rCT) equals the rate of CT mass transfer to the electrode
surface. This rate can be expressed as (25)
FIGURE 3. CT reaction rate constants in RDE reactor at different
electrode potentials.
(17). Under the conditions of these experiments, D/ δ was
more than 2 orders of magnitude greater than kCT. Therefore,
at all electrode potentials, CS was greater than 0.99CB, and
thus all experiments were performed at essentially the same
CS despite difference in kCT
.
The kCT values for CT reduction were determined both
amperometrically and analytically at each potential. Ac-
cording to the reaction stoichiometry in eqs 4-6, each CT
molecule dechlorinated to CF requires 2 electrons. Average
amperometric rate constants for each 30-min electrolysis
period were determined from the difference in integrated
currents between blank and 2 mM CT solutions. Figure 3
shows the average amperometrically determined rate con-
stants as a function of electrode potential at 22 °C. Analytic
rate constants for CT reduction were determined from a
chloride balance on each solution and are also shown in
Figure 3. The nearly identical analytic and amperometric
rate constants indicate that CT reduction occurred via direct
electron transfer and that reduction of CT did not measurably
impact water reduction.
Combining eq 1 with the Nernst equation for reaction 5
shows that the current going toward CT reduction should
depend on electrode potential and the surface activity of
reactants ({CCl4}) and products ({CCl3 }and {Cl-}) according
•
to (17)
D
δ
rCT ) kCTCS ) (CB - CS)
(7)
•
i ) FA[kf °{CCl4}e-RBF(E-E°)/ RT - kb°{CCl3 }
{Cl-}eRaF(E-E°)/ RT] (8)
where kCT is the reduction rate constant, CS is the surface CT
concentration, CB is the bulk solution concentration, δ is the
diffusion layer thickness, and D is the CT aqueous diffusion
coefficient. The diffusion layer thickness can be determined
from the Levich equation and the electrode rotation speed
where A is the electrode area, and kf° and kb° are the forward
and backward reaction rate constants, respectively, at the
standard reduction potential (E°). Since the electrode po-
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FIGURE 4. Overpotentials at different imposed cathodic currents
FIGURE 5. Effect of temperature on the apparent transfer coefficients
for reduction of water, CT, and TCE. Error bars represent 95%
confidence intervals.
from CP analysis of iron disk electrode in 10 mM CaSO electrolyte
4
solution with 2 mM CT in solution. Transfer coefficients were
calculated from eq 1 using absolute overpotential values >100 mV.
tentials were significantly below the E° for reaction 5 (26),
the rate of the reverse anodic reaction was more than 2 orders
of magnitude smaller than the rate of the forward reaction.
Therefore, the second term in eq 8 can be neglected, and the
Rb for CT reduction can be determined from the reaction rate
constant, kCT, according to
d log kCT
-2.3RT
Rb )
(9)
(
)
F
dE
Calculation of the transfer coefficient from the kCT values at
22 °C is illustrated in Figure 3.
Since the CT current efficiencies, defined as the fraction
of the total cell current going toward CT reduction, were
greater than 90%, CP analysis was used to determine the
transfer coefficient for CT at other temperatures. The
advantage of the CP technique is that all experiments may
be performed in the same solution, since blank current values
at each potential are not needed. This eliminates some
variability in the condition of the electrode surface since the
electrode need not be removed from the solution between
experiments. In accord with eq 1, the transfer coefficients
were determined from measurements of the overpotential
(E - Eeq) at each imposed current, as illustrated in Figure 4.
Only absolute overpotential values greater than 100 mV were
used to calculate the slope in Figure 4 because the relationship
between overpotential and log(i) approaches linearity only
at overpotentials where the second term in eq 1 becomes
negligible as compared to the first term. Average electron-
transfer coefficients with 95% confidence intervals measured
with CP analysis for CT are shown in Figure 5. Note that the
transfer coefficient of 0.20 ( 0.02 determined at 22 °C using
CP analysis was close to the bR of 0.24 ( 0.02 measured using
CA analysis at this temperature.
FIGURE 6. CA profiles for iron disk electrode in 10 mM CaSO
electrolyte solutions with and without 3 mM TCE, at a rotation
speed of 100 rpm and applied potentials of -685 and -1085 mV.
4
CA experiments similar to those described for CT were
performed with TCE. In contrast to CT, reaction rates for
TCE could not be determined amperometrically since TCE
interfered with water reduction. Figure 6 shows that at a
potential of -685 mV, the presence of TCE in the solution
resulted in an increase in current as compared to the blank
electrolyte. The current increase indicates that TCE may
undergo reduction via direct electron transfer. However, at
lower potentials, currents in the TCE solutions were less than
those in the blank electrolyte, as illustrated in Figure 6 for
a potential of -1085 mV. This shows that there was
measurable competition between water and TCE for reaction
sites on the iron surface. Therefore, rate constants (kTCE) for
TCE dechlorination were determined from the chloride
generated during CA runs at each potential. Average apparent
transfer coefficients for TCE reduction were determined using
eq 9 and are shown in Figure 5. Note that the transfer
coefficient measured at 22 °C of 0.14 is close to the value of
0.15 measured by other investigators (13).
Transfer Coefficient and Activation Energy Analysis. The
temperature dependencies of the transfer coefficients can
be used to assess whether the rate-limiting step for TCE or
CT reduction involves outer-sphere electron transfer. Since
cathodic transfer coefficients depend only on the number of
electrons transferred before the rate-limiting step (bγ) and
the symmetry factor (â), the transfer coefficient should show
the same, or weaker, temperature dependence as â. At room
temperatures and above, the shape of the potential energy
surface between products and reactants is normally con-
sidered to be independent of temperature (28). Therefore,
â and the electron-transfer coefficient should also be
independent of temperature (29). If the measured transfer
coefficient is temperature dependent, this indicates that the
Trichloroethylene Results. In contrast to CT that reacts
primarily via sequential dechlorination (19, 22), the dominant
pathway for TCE reactions with iron involves a â-elimination
mechanism (11, 26). In this mechanism, the first detectable
product of TCE reduction is chloroacetylene, which is
produced according to
C2HCl3 + 2e- f C2HCl + 2Cl-
(10)
The chloroacetylene is then rapidly reduced to acetylene
and ethylene (11, 26, 27). The TCE reaction products observed
in this study are consistent with this pathway. GC analyses
and chloride balances indicated complete TCE dechlorina-
tion, and no chlorinated products were detected. The initial
pH of the solutions was ∼6.5, and there was a decline of less
than 1 pH unit over the course of each 30-min experiment.
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VOL. 35, NO. 17, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 5 6 3

Ea for CT reduction decreased with increasingly negative
electrode potentials. This is consistent with a rate-limiting
step that involves electron transfer. In electron-transfer
reactions, a fraction of the polarization energy goes toward
changing the chemical activation energy, and the apparent
Ea depends on the electrode potential according to (18)
eq
Ea ) Ea + bRF(E - Eeq)
(11)
eq
where Ea is the activation energy at the equilibrium
potential. For CT reduction, cathodic polarizations resulted
in negative overpotentials (E - Eeq) and therefore resulted
in decreasing Ea values with increasingly negative electrode
potentials. The trend in Ea values for water reduction is also
consistent with a rate-limiting step that involves electron
transfer. In contrast to the data for CT and water, statistical
analysis at the 99.9% confidence level indicates that the Ea
values for TCE did not decline with decreasing electrode
potential, as required by eq 11 for electron-transfer reactions.
This clearly indicates that the rate-limiting step for TCE
dechlorination did not involve electron transfer.
According to eq 2, the Rb for a multistep reaction in which
the rate-limiting step does not involve electron transfer (i.e.,
r ) 0) should be equal to bγ. However, since bγ must be an
integer, the observed transfer coefficients for TCE in Figure
5 indicate that there were no electrons transferred before
the rate-limiting step. Therefore, according to eq 2, an Rb
value of zero should have been observed for TCE. However,
the Rb value for TCE was not zero, indicating that its rate of
reaction was dependent on the electrode potential. This
apparent paradox can be explained by a TCE reaction
mechanism that involves atomic hydrogen produced from
water reduction.
There are several simultaneous reactions that occur on
the iron electrode in TCE solutions. As illustrated in eqs 12-
15, these reactions include formation of adsorbed atomic
hydrogen by electrolysis of water (Volmer reaction), molecular
hydrogen evolution resulting from electrochemical desorp-
tion (Heyrovsky reaction), hydrogen evolution resulting from
the chemical recombination of atomic hydrogen (Tafel
recombination), and the reaction between adsorbed atomic
hydrogen and the halocarbon molecule (hydrodechlorina-
tion) (31):
FIGURE 7. Effect of electrode potential on the apparent activation
energies for reduction of (a) CT, (b) TCE, and (c) water. Error bars
represent 95% confidence intervals.
Fe + H2O + e- T Fe - H + OH- (Volmer discharge)
(12)
Fe - H + H2O + e- T Fe + H2 + OH-
rate-limiting mechanism is not an outer-sphere electron-
transfer step. In this case, the reaction rate may be limited
by an inner-sphere electron-transfer step that involves bond
breaking or molecular rearrangement (20, 21, 29).
(Heyrovsky reaction) (13)
2Fe - H T 2Fe + H2 (Tafel recombination) (14)
A multiple partial f-test (30) was performed to determine
if the transfer coefficients for CT, TCE, and water were
functions of temperature. The statistical analysis found that
the transfer coefficient for CT was independent of temper-
ature at the 99.9% confidence level. This indicates that the
rate of CT reduction was limited by the rate of outer-sphere
electron transfer, as illustrated by step 1 in eq 4. In contrast,
the statistical analysis found that Rb values for TCE were
temperature dependent at the 99.9% confidence level. This
indicates that the TCE reaction rates were limited by chemical
dependent factors and not by rates of electron transfer alone.
Analysis of the transfer coefficients for water indicate that
the Rb values in Figure 5 were independent of temperature
at the 99.9% confidence level. This suggests that the rate-
limiting step for water reduction was an outer-sphere
electron-transfer step.
RX + 2Fe - H f RH + 2Fe + X- + H+
(hydrodechlorination) (15)
The rate of the hydrodechlorination reaction (rTCE) for
TCE can be expressed as (31)
n
rTCE ) kTCEΘH ΘTCE
(16)
where ΘH and ΘTCE are the surface coverages of adsorbed
hydrogen and adsorbed TCE, respectively, and n is the
reaction order of hydrogen in the hydrodechlorination
reaction. Equation 16 shows that the rate of TCE reduction
is a function of both ΘH and ΘTCE. The hydrogen coverage
depends on the relative rates of the Volmer, Heyrovsky, and
Tafel reactions. Reaction 12 has been reported as the rate-
limiting reaction for hydrogen evolution at iron surfaces at
circumneutral pH values (32). With this rate-limiting reaction,
ΘH increases with decreasing electrode potential (31-33).
The apparent activation energies (Ea) for CT, TCE, and
water reduction at different electrode potentials are shown
in Figure 7. A regression analysis (30) found that the apparent
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(4) Starr, R. C.; Cherry, J. C. Ground Water 1994, 32, 465-476.
(5) Johnson, T. L.; Scherer, M. M.; Tratnyek, P. G. Environ. Sci.
Technol. 1996, 30, 2634-2640.
(6) U.S. Environmental Protection Agency. Permeable Reactive
Subsurface Barriers for the Interception and Remediation of
Chlorinated Hydrocarbon and Chromium(VI) Plumes in Ground
Water; EPA-600-f97-008; U.S. Goverment Printing Office: Wash-
ington, DC, 1997.
(7) Wust, W. F.; Kober, R.; Schlicker, O.; Dahmke, A. Environ. Sci.
Technol. 1999, 33, 4304-4309.
(8) Cheng, I. F.; Muftikian, R.; Fernando, Q.; Korte, N. Chemosphere
1997, 35, 2689-2695.
Therefore, the increase in TCE reaction rates with decreasing
potential may be attributed to the effect of potential on ΘH.
Results from this study show that at potentials relevant
to dechlorination by iron media in aqueous systems, rates
of CT reduction are limited by the rate of outer-sphere
electron transfer, while rates of TCE reduction are not limited
by rates of electron transfer. At more negative electrode
potentials, CT dechlorination may also become limited by
chemical dependent factors since rates of electron transfer,
but not chemical dependent steps, increase with decreasing
potential. The different reaction mechanisms for CT and TCE
may explain why rates of chloroethene reduction do not
follow LFER trends established for chloroalkanes (11-13).
Reduction via an outer-sphere mechanism, such as
electron tunneling, requires only physical adsorption of CT
on or near the electrode surface. Since physical adsorption
interactions are of short temporal duration and electrons
are transferred one at a time (18), brief interactions with the
electrode surface may explain why chloroalkanes undergo
primarily stepwise dechlorination and produce chlorinated
byproducts. For TCE, rates of chemisorption likely control
its overall rate of dechlorination. TCE chemisorption has
been postulated to involve di-σ bond formation between the
carbon atoms of TCE and the iron surface (11). This
chemisorption step may then be followed by direct electron
transfer through the chemical bonds. The potential depen-
dence of TCE reaction rates suggests that TCE may also be
indirectly reduced via reaction with atomic hydrogen.
Evidence for this mechanism has been reported in previous
investigations (14, 16). This indirect mechanism for reduction
likely involves formation of hydrocarbon-hydride complexes
with atomic hydrogen adsorbed on the iron surface (15).
Both proposed TCE reaction mechanisms require chemi-
sorption and thus involve longer TCE interactions with the
iron surface than mere physical adsorption. This may explain
why the primary pathway for TCE reduction produces
completely dechlorinated byproducts.
(9) Hardy, L. I.; Gillham, R. W. Environ. Sci. Technol. 1996, 30, 57-
65.
(10) Su, C.; Puls, R. W. Environ. Sci. Technol. 1999, 33, 163-168.
(11) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 2000, 34,
1794-1805.
(12) Scherer, M. M.; Balko, B. A.; Gallagher, D. A.; Tratnyek, P. G.
Environ. Sci. Technol. 1998, 32, 3026-3033.
(13) Liu, Z.; Betterton, E. A.; Arnold, R. G. Environ. Sci. Technol.
2000, 34, 804-811.
(14) Farrell, J.; Melitas, N.; Kason, M.; Li, T. Environ. Sci. Technol.
2000, 34, 2549-2556.
(15) Brewster, J. H. J. Am. Chem. Soc. 1954, 76, 6361-6363.
(16) Li, T.; Farrell, J. Environ. Sci. Technol. 2000, 34, 173-179.
(17) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley
and Sons: New York, 1980.
(18) Bockris, J. O’M.; Reddy, A. K. Modern Electrochemistry, Vol. 2;
Plenum Press: New York, 1970.
(19) Liu, Z.; Arnold, R. G.; Betterton, E. A.; Festa, K. D. Environ. Eng.
Sci. 1999, 16, 1-13.
(20) Save´ant, J. M. Adv. Phys. Org. Chem. 1990, 26, 1-130.
(21) Save´ant, J. M. Acc. Chem. Res. 1993, 26, 455-461.
(22) Matheson, L. J.; Tratnyek, P. G. Environ. Sci. Technol. 1994, 28,
2045-2053.
(23) Warren, K. D.; Arnold, R. G.; Bishop, T. L.; Lindholm, L. C.;
Betterton, E. A. J. Hazard. Mater. 1995, 41, 217-227.
(24) Chen, J. L.; Al-Abed, S. R.; Ryan, J. A.; Li, Z. J. Hazard. Mater.
2001, B83, 243-254.
(25) Fogler, H. S. Elements of Chemical Reaction Engineering, 3rd
ed.; Prentice Hall: Upper Saddle River, NJ, 1999.
(26) Roberts, A. L.; Totten, L. A.; Arnold, W. A.; Burris, D. R.; Campbell,
Acknowledgments
T. J. Environ. Sci. Technol. 1996, 30, 2654-2659.
(27) Campbell, T. J.; Burris, D. R.; Roberts, A. L.; Wells, J. R. Environ.
Toxicol. Chem. 1997, 16, 625-630.
(28) Blowers, P.; Masel, R. I. J. Phys. Chem. A 1999, 103, 7725-7729.
(29) Schmickler, W. Interfacial Electrochemistry; Oxford University
Thanks to Nikos Melitas for his assistance with the manu-
script. This research has been supported by the United States
Environmental Protection Agency (U.S. EPA) Office of
Research and Development. Although the research described
in this paper has been funded by the U.S. EPA through Grant
R-825223-01-0 to J.F., it has not been subject to the Agency’s
peer and policy review and therefore does not necessarily
reflect the views of the Agency, and no official endorsement
should be inferred.
Press: New York, 1996.
(30) Kleinbaum, D. G.; Lawrence K. L.; Muller K. E. Applied Regression
Analysis and Other Multivariate Methods, 2nd ed.; Wadsworth
Publishing Company: Belmont CA, 1988.
(31) Lipkowski, J.; Ross, P. N. Electrocatalysis; Wiley-VCH Publish-
ers: New York, 1998.
(32) Bockris, J. O’M.; Carbajal, J. L.; Scharifker, B. R.; Chandrasekaran,
K. J. Electrochem. Soc. 1987, 134, 1957-1963.
(33) Abd Elhamid, M. H.; Ateya, B. G.; Weil, K. G.; Pickering, H. W.
Literature Cited
(1) Gillham, R. W.; O’Hannesin S. F. Modern Trends in Hydrology;
International Association of Hydrologists Conference, Hamilton,
ON, Canada, 1992; pp 10-13.
J. Electrochem. Soc. 2000, 147, 2148-2150.
(2) O’Hannesin, S. F.; Gillham, R. W. Ground Water 1998, 36, 164-
170.
(3) Tratnyek, P. G.; Johnson, T. L.; Scherer, M. M.; Eykholt, G. R.
Ground Water Monit. Remed. 1997, 17, 108-114.
Received for review December 19, 2000. Revised manuscript
received June 5, 2001. Accepted June 11, 2001.
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