Effects of AgI, AuIII, and CuII on the reductive dechlorination of carbon tetrachloride by green rust

Article abstract of DOI:10.1021/es030304w

Green rusts (GRs), mixed iron(II)/iron(III) hydroxide minerals found in many suboxic environments, have been shown to reduce a range of organic and inorganic contaminants, including several chlorinated hydrocarbons. Many studies have demonstrated the catalytic activity of transition metal species in the reduction of chlorinated hydrocarbons, suggesting the potential for enhanced reduction by GR in the presence of an appropriate transition metal catalyst. Reductive dechlorination of carbon tetrachloride (CT) was examined in aqueous suspensions of GR amended with Ag^I, Au^III, or Cu^II. The CT reduction rates were greatly increased for systems amended with Cu^II, Au^III, and Ag^I (listed in order of increasing rates) relative to GR alone. Observed intermediates and products included chloroform, dichloromethane, chloromethane, methane, acetylene, ethene, ethane, carbon monoxide, tetrachloroethene, and various nonchlorinated C₃ and C₄ compounds. Product distributions for the reductive dechlorination of CT were highly dependent on the transition metal used. A reaction pathway scheme is proposed in which CT is reduced primarily to methane and other nonchlorinated end products, largely through a series of one-electron reductions forming radicals and carbenes/carbenoids. Recently, X-ray absorption fine structure analysis of aqueous GR suspensions amended with Ag^I, Au^III, or Cu^II showed that the metals were reduced to their zerovalent forms. A possible mechanism for CT reduction is the formation of a galvanic couple involving the zerovalent metal and GR, with reduction of CT occurring on the surface of the metal and GR serving as the bulk electron source. The enhanced reduction of CT by GR suspensions amended with Ag^I, Au^III, or Cu^II may prove useful in the development of improved materials for remediation of chlorinated organic contaminants.

Full text of DOI:10.1021/es030304w

Environ. Sci. Technol. 2003, 37, 2905-2912
hydroxide layers and hydrated anion layers with the general
composition
I
III
II
Effects of Ag , Au , and Cu on the
Reductive Dechlorination of Carbon
Tetrachloride by Green Rust
[FeII(6-x)FeIIIx(OH)12]x+[(A)x/nyH2O]x-
(1)
where x ) 0.9-4.2, Ais an n-valent anion (typically Cl-, SO4
,
,
2-
E D W A R D J . O ’ L O U G H L I N , * , † , ‡
K E N N E T H M . K E M N E R , ‡ A N D
D A V I D R . B U R R I S † , §
or CO32-, although GRs containing Br-, I-, NO3-, ClO4-, SO3
2-
SeO42-, and oxalate have been prepared), and y denotes the
varying amounts of interlayer water (typically y ) 2-4 for
most GRs). Green rusts typically form under neutral to alkaline
conditions in suboxic environments, and they have been
identified as products of both abiotic and microbially induced
corrosion of iron and steel (1-4), the microbially mediated
(5, 6) and abiotic (7, 8) reductive dissolution of ferric
oxyhydroxides by Fe2+, and the biooxidation of Fe2+ (9). Green
rusts are readily synthesized in the laboratory via the oxidation
of ferrous hydroxide by molecular oxygen at circumneutral
pH (10, 11). Oxidation of GRs results in the formation of
ferrihydrite, goethite (R-FeOOH), akaganeite (â-FeOOH),
lepidocrocite (γ-FeOOH), δ-FeOOH, maghemite (γ-Fe2O3),
or magnetite (Fe3O4) depending upon pH, solution compo-
sition, oxidant, rate of oxidation, and degree and rate of
dehydration (12-17).
Air Force Research Laboratory, AFRL/MLQL, 139 Barnes Drive,
Tyndall Air Force Base, Florida, 32403-5323, Environmental
Research Division, Argonne National Laboratory,
9700 South Cass Avenue, Argonne, Illinois 60439-4843, and
Integrated Science and Technology, Inc., 433 Harrison Avenue,
Panama City, Florida 32401
Green rusts (GRs), mixed iron(II)/iron(III) hydroxide
minerals found in many suboxic environments, have been
shown to reduce a range of organic and inorganic
contaminants, including several chlorinated hydrocarbons.
Many studies have demonstrated the catalytic activity of
transition metal species in the reduction of chlorinated
hydrocarbons, suggesting the potential for enhanced reduction
by GR in the presence of an appropriate transition metal
catalyst. Reductive dechlorination of carbon tetrachloride
(CT) was examined in aqueous suspensions of GR
The characteristic bluish-green color of hydromorphic
soils has long been presumed to be due to the presence of
mixed iron(II)/ iron(III) hydroxide species (18); however, the
unambiguous identification of GRs in suboxic soils and
sediments has been complicated by the rapid oxidation of
these compounds upon exposure to air. Direct evidence for
the presence of GRs in hydromorphic soils has recently been
reported (19, 20), and the material has been tentatively
identified as hydroxy-GR (GROH) (21). Recent work by Bourrie´
et al. (22) suggests that the solubility of Fe in soil solutions
amended with AgI, AuIII, or CuII. The CT reduction rates
were greatly increased for systems amended with CuII, AuIII,
and AgI (listed in order of increasing rates) relative to
GR alone. Observed intermediates and products included
chloroform, dichloromethane, chloromethane, methane,
acetylene, ethene, ethane, carbon monoxide, tetrachloro-
ethene, and various nonchlorinated C and C compounds.
Product distributions for the reductive dechlorination of CT
were highly dependent on the transition metal used. A
reaction pathway scheme is proposed in which CT is reduced
primarily to methane and other nonchlorinated end
products, largely through a series of one-electron reductions
forming radicals and carbenes/carbenoids. Recently,
X-ray absorption fine structure analysis of aqueous GR
suspensions amended with AgI, AuIII, or CuII showed that
the metals were reduced to their zerovalent forms. A
possible mechanism for CT reduction is the formation of a
galvanic couple involving the zerovalent metal and GR,
with reduction of CT occurring on the surface of the metal
and GR serving as the bulk electron source. The enhanced
reduction of CT by GR suspensions amended with AgI,
AuIII, or CuII may prove useful in the development of improved
materials for remediation of chlorinated organic contami-
nants.
of hydromorphic soils is controlled by equilibrium with GROH
.
As metastable intermediates in the transformation of FeII to
magnetite and iron(III) oxyhydroxides (e.g., lepidocrocite and
goethite) at near-neutral to slightly alkaline pH, GRs are
believed to play a central role in the redox cycling of Fe in
aquatic and terrestrial environments. In addition, recent
research suggests that GRs may be highly reactive reductants
in suboxic environments. Green rusts are capable of reducing
a number of organic and inorganic contaminants, including
uranyl (23), nitrate and nitrite (24, 25), selenate (26), chromate
(27, 28), and halogenated hydrocarbons (29-31); these
observations suggest that GRs may play an important role
in the fate and transport of contaminants in suboxic soils
and sediments.
3
4
The reductive transformation of many organic and
inorganic contaminants is thermodynamically favored in
suboxic environments and often results in the formation of
products considered to be less hazardous than the parent
compound; however, these reactions are often kinetically
constrained. The rate of reductive dehalogenation of chlo-
rinated hydrocarbons by GR is typically slower than the rate
for Fe0 (30, 32), the most commonly used material for the
construction of permeable reactive barriers (PRBs) (33).
However, studies demonstrating the catalytic activity of
transition metal species in the reduction of a range of
contaminants by a number of bulk reductants (34-43) suggest
the potential for enhanced contaminant reduction by GR in
the presence of certain metals. This paper examines the effects
of AgI, AuIII, and CuII on the rate and product distribution for
the reductive dechlorination of carbon tetrachloride in
aqueous suspensions of GR.
Introduction
Green rusts (GRs) are mixed ferrous/ ferric hydroxides that
have structures consisting of alternating positively charged
* Corresponding author e-mail: oloughlin@anl.gov; telephone:
(630)252-9902; fax: (630)252-5498.
† Air Force Research Laboratory.
‡ Argonne National Laboratory.
§
Integrated Science and Technology, Inc.
9
10.1021/es030304w CCC: $25.00
Published on Web 05/17/2003
2003 Am erican Chem ical Society
VOL. 37, NO. 13, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 9 0 5

ionization detector (FID; 240 °C). Samples were injected in
split mode (split ratio 2:1) with the injector at 180 °C. Helium
was used as the carrier gas. The oven temperature was held
at 50 °C for 2 min, ramped at 25 °C min-1, and held at 200
°C for 10 min. The system was calibrated by equilibrating
known masses of analytes and internal standard (n-heptane)
in serum vials containing the same ratio of aqueous phase
to vapor phase as the experimental systems, thus accounting
for water-vapor partitioning and allowing analyte concen-
tration to be expressed as mass per bottle. Vapor-liquid
partitioning in these systems was rapid relative to the reaction
kinetics. For reactions with fast kinetics, CT and CF were
analyzed on an identical GC equipped with a 0.53 mm i.d,,
30 m long DB-1 column under isothermal conditions at 50
°C. Samples were injected in split mode (split ratio 2:1) at
200 °C with helium as the carrier gas. The FID temperature
in this second GC was 200 °C.
Experimental Section
Chem icals and Reagents. Chloromethane (CM, 99.5%);
tetrachloroethene (perchloroethene (PCE), 99.5%); iron(II)
sulfate heptahydrate (99+%); silver(I) acetate (99.999%);
copper(II) chloride (99.999%); propene (99+%); 1-butene
(99+%); n-heptane (99+%); a mixture of cis- and trans-2-
butene (38.1% cis and 61.6% trans); and nanoparticulate
activated powders of Ag0, Au0, and Cu0 (99.9+%) were
obtained from Aldrich. Tetrachloromethane (carbon tetra-
chloride (CT), 100%) and dichloromethane ((DCM), 99.5%)
were purchased from Chem Service. Trichloromethane
(chloroform (CF), HPLC grade) was purchased from Fisher.
Propane (1.002% in N2), n-butane (1.0% in N2), carbon
monoxide (10.0% in He), and a multicomponent gas mixture
containing methane, ethane, ethene, and acetylene (each at
1.0 mol % in N2) were purchased from Scott Specialty Gases.
Gold(III) chloride was purchased from Sigma. Chemicals were
of reagent grade unless otherwise indicated.
Analysis of carbon monoxide in headspace samples was
performed by using a Trace Analytical (Menlo Park, CA) RGA3
reduction gas analyzer with an oven temperature of 266 °C.
Formate was analyzed by using a Dionex LC20 ion chro-
matograph with self-regenerating suppressor control. An
IONPAC AG11 guard column (4 × 50 mm) and an IONPAC
AG11 analytical column (4 ×250 mm) were used with a weakly
basic eluent (NaOH, 350 µmol L-1) at a flow rate of 1.0 mL
Hydroxysulfate GR (GRSO ), a GR in which the interlayer
4
anion is SO42-, was synthesized by air oxidation of a 1.0 M
ferrous sulfate solution. Briefly, 278 g of FeSO4‚7H2O was
dissolved in 1 L of distilled deionized water on a magnetic
stirrer under ambient atmosphere. Upon dissolution, 1.0 M
NaOH was added dropwise until the pH remained stable at
min-1
.
7.0, at which point the GRSO suspension was placed in a
4
glovebox. All subsequent handling occurred in an anoxic
atmosphere (4-6% H2 in N2). The GR was recovered by
centrifugation and subsequently washed four times with
distilled deionized water. The specific surface area of a freeze-
dried subsample of the GRSO4 suspension was measured by
five-point Brunauer-Emmett-Teller (BET) gas adsorption
with N2. Exposure to air during the BET measurement was
minimized by using a Quantachrome cell-seal assem-
bly. Magnetite was prepared as described by Cornell and
Schwertmann (44).
X-ray diffraction (XRD) analysis of GRSO4 and its oxidation
product(s) was performed using a Rigaku MiniFlex X-ray
diffractometer with Ni-filtered Cu KR radiation. Samples for
XRD analysis were collected by centrifugation, and the
resulting wet pastes were mixed with glycerol to minimize
oxidation (45) and prepared as smears on 22 mm wide glass
plates. The samples were scanned between 6° and 80° 2θ at
a speed of 1° 2θ min-1
.
Kinetic Analysis. Apparent pseudo-first-order rate con-
stants (kobs) were obtained by fitting the data for mass of CT
in the system over time to the exponential decay expression:
Experim ental Setup. The reaction system consisted of
160-mL serum vials crimp-sealed with Teflon-lined rubber
septa. Each vial contained 94 mL of distilled deionized water,
5 mL of GRSO suspension (∼0.5 g of GRSO ), 1 mL of either
obst
MCT ) MCT e-k
(2)
t
0
4
4
0.1 M HCl or 10-2 M AuIII or CuII in 0.1 M HCl, and 0.1 mL
of 1.0 M NaOH. Systems containing AgI were prepared as
described above; however, the AgI stock solution was
prepared in distilled deionized water, and NaOH was not
added. All preparative work was performed in an anoxic
glovebox. The vials were sealed and spiked with 5 µmol of
n-heptane (5 µL of a 1.0 M methanolic solution of n-heptane)
that served as an internal standard. Reduction reactions were
initiated by spiking with 2 µmol of a given chlorinated
methane (2 µL of 1.0 M methanolic solutions of either CT,
CF, or DCM or 50 µL of CM gas at standard temperature and
where t refers to time, and MCT and MCT refer to the mass
t
0
of CT in the system at times t and zero, respectively. For a
reaction taking place only in the solution phase, the apparent
pseudo-first-order rate constant that pertains to the reaction
of a volatile constituent capable of rapid partitioning between
the solution phase and the headspace (kobs) is equivalent to
the rate constant that would be obtained in a headspace-
free system (k′obs), as described by Burris et al. (46). Values
of k′obs were calculated using the “dimensionless” Henry’s
law constant for CT from Gossett (47).
pressure). Initial solution concentrations were 0.5 g of GRSO
4
in 100 mL and 100 µM for AgI, AuIII, and CuII; the total mass
of chlorinated methane added to each vial was 2 µmol. The
vials were placed on a roller drum (rotating vertically as the
bottle axis remained horizontal) in an incubator maintained
at 20 °C. For reactions with fast kinetics (reaction complete
in <15 min), the vials were vigorously shaken by hand under
ambient conditions. The initial pH of the experimental
systems was 7.6 ( 0.1. During the reductive dehalogenation
experiments the pH was allowed to change freely; however,
the pH was measured at the end of each experiment and in
all cases was within 0.5 pH units of the initial value. Reduction
of CT by magnetite and by zerovalent Ag, Au, or Cu was
examined as described for GR but with 0.5 g of hydrated
magnetite or 0.1 g of Ag0, Au0, or Cu0 in place of the GR.
Results and Discussion
Reduction of Carbon Tetrachloride. CT was reduced by GR,
with approximately 33% of the initial mass remaining after
195 h (Figure 1). Overall, the reduction of CT by GR was not
well described by first-order kinetics; however, CT reduction
within the first 6 h was consistent with first-order kinetics
(Figure 1) with a rate constant of 1.7 × 10-5 s-1. The surface
area of the GRSO was 14.1 m2 g-1, resulting in a surface-
4
area-normalized rate constant of 2.4 × 10-7 L s-1 m-2
.
Although the experimental systems were quite different
(prohibiting direct comparison), the observed first-order rate
constant for the initial reduction of CT by GR in our system
(1.7 × 10-5 s-1) is within the range of rate constants for CT
reduction by GR (0.47 × 10-5-2.18 × 10-5 s-1) reported by
Erbs et al. (29).
Analytical Methods. At selected intervals, 200-µL head-
space samples were removed from the serum vials for analysis
with a Hewlett-Packard 5890 series II gas chromatograph
(GC) with a GSQ column (0.53 mm i.d. by 30 m) and a flame
The initial product in the reductive dechlorination of CT
by GR was CF (Figure 1). After 575 h (24 d) and with 17% of
the initial CT remaining, CF was the major product (Table
9
2 9 0 6 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 13, 2003

dechlorination of CT by GR. However, methane and CO
accounted for 4% and 29% (respectively) of the mass of CT-
derived C in our experiments, but they were not reported as
products of CT reduction by GR by Erbs and co-workers (29).
Additionally, Erbs et al. (29) reported the production of
hexachloroethane (HCA) and PCE, but HCA was not detected
in our experiments. However, O’Loughlin and Burris (31)
reported that HCA is rapidly reduced to PCE by GR; thus, in
our system HCA may have been an intermediate that was
reduced to PCE before reaching detectable levels. The
differences in the minor products observed in our experi-
ments versus those reported by Erbs et al. (29) are likely due
to differences in experimental conditions. Erbs and co-
workers (29) examined CT reduction in GR suspensions
containing aqueous CT concentrations at or near saturation
due to the presence of bulk-phase CT. The relatively high
aqueous solubility of CT and the presence of bulk-phase CT
make competition for reactive sites between CT and HCA
likely. Such competition may have been significant enough
to slow the kinetics of HCA reduction sufficiently for it to
accumulate to detectable levels. Given the substantially lower
levels of CT (in terms of both total CT and the ratio of CT
to GR) in our experiments, competition would likely be
negligible, and thus HCA would be reduced rapidly to PCE.
As with GR alone, CT was reduced to CF with the addition
of AgI, AuIII, or CuII although at dramatically enhanced rates
(Figure 1). CT was reduced by GR to nondetectable levels in
less than 12, 15, and 25 min in the systems amended with
AgI, AuIII, and CuII (hereafter designated AgGR, AuGR, and
CuGR), respectively. The disappearance of CT in AgGR, AuGR,
and CuGR systems was well described by pseudo-first-order
kinetics (with observed first-order rate constants of 8.7 ×
10-2, 8.3 × 10-3, and 2.7 × 10-3 s-1, respectively). Although
there was little subsequent transformation of CF by GR alone,
CF was completely transformed by AgGR and CuGR, and
only 8% remained with AuGR. The terminal products of CT
transformation observed with AgGR, AuGR, or CuGR were
primarily nonchlorinated hydrocarbons (principally methane
with lesser amounts of ethane, ethene, acetylene, and a suite
of C3 and C4 alkanes and alkenes) and carbon monoxide with
minor amounts of DCM, CM, and PCE (Table 1). The final
carbon recovery (the fraction of CT-derived C accounted for
by identified products) in AgGR, AuGR, and CuGR systems
was less than 75%. In experimental controls without GR, AgI,
AuIII, or CuII (i.e., water only), the loss of volatile compounds
from the vials was less than 5% over the time scale of these
experiments with no indication of biotic or abiotic trans-
formation of CT, CF, DCM, or CM. Moreover, sorption of CT
reduction products by GR was not significant; therefore, the
incomplete recovery of C suggests the formation of uniden-
tified products.
FIGURE 1. Degradation of CT by GR, AgGR, AuGR, and CuGR with
appearance of major intermediates/products. Experiments were
initiated by spiking 2 µmol of CT into 160-mL serum vials containing
100 mL of GR suspension (0.5 g of GRSO ) with 100 µmol of AgI,
SO
4
4
AuIII, or CuII added as indicated.
TABLE 1. Final Product Distributions for CT Reduction by GR,
a
AgGR, AuGR, and CuGR
b
% of CT-derived C
compound
GR
AgGR
AuGR
CuGR
carbon m onoxide
m ethane
chlorom ethane
dichlorom ethane
chloroform
carbon tetrachloride
total C1 products
ethane
29
4
4
33
3
trace
11
nd
4
22
26
1
nd
nd
45
17
95
nd
nd
nd
2
7
7
Reduction of Less Chlorinated Methanes. Chloroform
was not readily reduced by GR alone. After a slight initial
reduction, CF levels remained relatively constant (Figure 2)
with 83% remaining after 1,152 h (48 d) (Table 2). Chloroform
was completely transformed by AgGR and CuGR (and partially
reduced by AuGR), although at substantially slower rates
than for CT reduction. The distribution of products from the
reduction of CF was similar to that observed for CT with
methane being the primary product along with lesser
amounts of DCM, CM, CO, and C2-C4 alkanes, alkenes, and
alkynes.
nd
nd
47
1
8
10
1
20
1
4
8
nd
nd
56
1
4
nd
1
6
5
6
73
nd
24
1
23
11
nd
35
3
ethene
acetylene
tetrachloroethene
total C2 products
total C3 products
total C4 products
total recovered C
2
nd
nd
97
7
69
72
a
Data are from final m easurem ents m ade at 70 h (AuGR) or 575 h
(GR, AgGR, and CuGR) after the experim ents were initiated by spiking
Within the duration of the experiments (7 d for AuGR and
48 d for GR), DCM and CM were not transformed by GR or
AuGR (Table S1, Supporting Information). Minor amounts
(<10% after 48 d) of DCM were reduced by AgGR to CM and
methane and by CuGR to methane. With AgGR and CuGR,
CM was essentially nonreactive; however, trace levels of
methane (<1%) were observed after 48 d.
b
with 2 µm ol of CT. nd indicates not detected; trace indicates detected
at <1%.
1) with minor amounts of methane and PCE; DCM and CM
were not detected. Our results are in general agreement with
the products reported by Erbs et al. (29) for the reductive
9
VOL. 37, NO. 13, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 9 0 7

magnetic black precipitate with an XRD pattern consistent
with a mixture of GRSO4 and magnetite/ maghemite (the XRD
patterns of magnetite and the cubic form of maghemite are
nearly indistinguishable) (Figure S1, Supporting Information).
This observation is in agreement with the findings of Erbs
et al. (29), who reported that GRSO is oxidized by CT to
4
magnetite.
The FeII species in our suspensions includes aqueous FeII
complexes, FeII sorbed to solid phases, and structural FeII in
GR and magnetite. In our experimental system, GR in
equilibrium with the solution phase supports a dissolved
FeII concentration of 320 µM; however, Ammonette et al.
(48) observed that Fe2+(aq) is unable to reduce CT, suggesting
that CT reduction in GR suspensions entails heterogeneous
reactions. Magnetite also contains structural FeII and has
been reported to reduce CT (49); thus, it may have been a
factor in the reduction of CT observed in our experiments.
We did not observe degradation of CT, CF, DCM, or CM over
the time scale of these experiments when we used magnetite
(or magnetite with added AgI, AuIII, or CuII) as the reductant.
The reactivity of magnetite with respect to the reduction of
chlorinated solvents seems to exhibit considerable variability
(50, 51), much of which appears to be related to differences
in particle size/ surface area.
Reaction Pathways and Mechanism s. The reduction of
CT and CF to methane may be envisioned as resulting from
a series of sequential hydrogenolysis reactions (i.e., the
stepwise replacement of hydrogen for chlorine), such that
CT f CF f DCM f CM f methane. However, our results
suggest that the reduction of CT by GR, AgGR, AuGR, and
CuGR involves processes other than sequential hydro-
genolysis. A potential reaction scheme that accounts for
nearly all of the products observed in our experiments is
illustrated in Figure 3 (boxes indicate observed intermediates
and/ or products). In this reaction sequence, CT is reduced
primarily to methane and other nonchlorinated end products,
largely through a series of one-electron reductions forming
radicals and carbenes/ carbenoids.
The initial step in the reduction of halogenated hydro-
carbons is commonly reported to be a single-electron transfer
with the subsequent release of chloride, resulting in the
formation of a radical (52-57). In the case of CT, this process
results in the formation of a trichloromethyl radical (58-60)
(Figure 3) that is unstable and can react by several mech-
anisms, including (i) accepting a second electron and
abstracting a proton to form CF; (ii) coupling with a second
trichloromethyl radical, forming HCA; and (iii) accepting a
second electron with the subsequent release of another
chloride, forming dichlorocarbene (ref 61 and references
therein). The products we observed from the reduction of
CT by GR (both alone and with the addition of AgI, AuIII, or
CuII) are consistent with an initial one-electron reduction,
resulting in the formation of a trichloromethyl radical. The
levels of CF observed during CT transformation (Figure 1)
suggest that hydrogenolysis is a significant pathway for the
initial reduction reaction. The high levels of carbon monoxide
produced during the reduction of CT by GR and CuGR (Table
1), with lesser amounts observed for AgGR and AuGR, are
consistent with the formation of a dichlorocarbene (or
carbenoid) intermediate; dichlorocarbene reacts readily with
water to form carbon monoxide and formic acid (62). Formate
was not present at detectable levels (i.e., > 2 µM) in any of
our experimental systems; however, Robinson (63) showed
that CO is the primary product of the reaction of dichloro-
carbene with water and that formate is produced by a
subsequent (and slower) reaction of carbon monoxide with
hydroxide. In addition to reduction of trichloromethyl
radicals, dichlorocarbene may also be produced by the
hydrolysis of CF (62), which likely accounts for the carbon
monoxide formed during the reduction of CF by AuGR and
FIGURE 2. Degradation of CF by GR, AgGR, AuGR, and CuGR and
appearance of major intermediates/products. Experiments were
initiated by spiking 2 µmol of CF into 160-mL serum vials containing
100 mL of GR suspension (0.5 g of GRSO ) with 100 µmol of AgI,
SO
4
4
AuIII, or CuII added as indicated.
TABLE 2. Final Product Distributions for CF Reduction by GR,
a
AgGR, AuGR, and CuGR
b
% of CF-derived C
compound
GR
nd
3
nd
1
83
87
trace
1
nd
2
AgGR
AuGR
CuGR
carbon m onoxide
m ethane
chlorom ethane
dichlorom ethane
chloroform
total C1 products
ethane
ethene
trace
60
9
2
nd
nd
19
nd
nd
50
69
2
2
1
4
2
6
36
2
11
nd
56
trace
5
nd
6
3
1
66
71
trace
trace
1
acetylene
total C2 products
total C3 products
total C4 products
total recovered C
2
1
trace
nd
73
trace
90
1
76
a
Data are from final m easurem ents m ade at 144 h (AgGR, AuGR,
and CuGR) or 1152 h (GR) after the experim ents were initiated by spiking
b
with 2 µm ol of CF. nd indicates not detected; trace indicates detected
at <1%.
Oxidation of Green Rust. The reduction of CT and CF by
GR was coupled to the oxidation of FeII to FeIII and resulted
in the transformation of GR, AgGR, AuGR, and CuGR to a
9
2 9 0 8 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 13, 2003

FIGURE 3. Proposed reaction scheme for the reduction of CT by GR, AgGR, AuGR, and CuGR. Boxes indicate observed intermediates/
products. For simplicity, reaction pathways for the formation of C and C hydrocarbon products are not shown.
3
4
9
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CuGR. Although HCA was not observed directly in any of our
experiments, the production of trace levels of PCE suggests
that coupling of trichloromethyl radicals to form HCA might
have occurred, as HCA is rapidly reduced by GR to form PCE
through a â-elimination reaction (31). Additionally, PCE might
be formed by the coupling of dichlorocarbenes.
not observe DCAc and CAc as intermediates in the reduction
of PCE to acetylene, their data are consistent with the reaction
sequence PCE f DCAc f CAc f acetylene.
The coupling of carbenes is typically considered to be
unlikely, largely because of their highly reactive nature and
the correspondingly low probability of two carbene molecules
interacting as well as rapid dimer dissociation resulting from
high excess internal energy (72, 73). Although these limitations
are consistent with conditions in homogeneous systems
(particularly in vapor-phase reactions), in heterogeneous
systems (like the ones examined in this study) the proposed
carbene intermediates likely exist as surface complexes
(carbenoids), which may increase their relative stability. The
presence of solvent molecules also offers a means of
dissipating the excess internal energy of potential coupling
products, lowering the likelihood of dissociation. Potentially
longer lifetimes and localization on surfaces would increase
the likelihood of carbene-coupling reactions. Indeed, several
studies have suggested that carbene coupling does occur in
systems that promote the stability and/ or physical proximity
of carbenes/ carbenoids (74-77). Independent experiments
examining the reduction of acetylene by GR, AgGR, AuGR,
and CuGR indicated that the kinetics of acetylene reduction
to ethene are too slow to account for the accumulation of
ethene in our CT and CF reduction experiments, suggesting
that parallel reactions (e.g., carbene coupling) are involved.
The C3 and C4 hydrocarbons (propane, propene, n-butane,
and butene isomers) produced during the reduction of CT
(and, to a lesser extent, CF) by AgGR, AuGR, and CuGR may
have resulted from coupling reactions of radicals and/ or
carbenes as well as insertion of carbene into C-H bonds of
smaller-chain hydrocarbons (e.g., ethane + carbene f
propane) (73).
Mechanism (s) of the Catalytic Effects of Ag, Au, and Cu.
Although the reductive transformation of many contaminants
(including many halogenated hydrocarbons) is thermody-
namically favorable under suboxic conditions, these reactions
are often kinetically limited. However, in the presence of a
suitable bulk reductant, many transition metal complexes
have been shown to be effective catalysts for redox reactions
(35, 38, 53, 56, 57). Typically, the catalytic effect has been
attributed to facilitated transfer of electrons from a bulk
reductant to the oxidant of interest through a cycling of the
metal between reduced and oxidized forms. Copper(II) has
been shown to catalyze the oxidation by molecular oxygen
of structural FeII in vermiculatized biotite (78); the proposed
mechanism involves facilitated electron transfer via CuII/
CuI cycling. The reduction of nitrate by FeII is catalyzed by
the addition of CuII, AgI, and HgII (39, 79). Although no specific
mechanism has been proposed, Ottley et al. (39), suggested
that the catalytic effect is due to solid-phase Cu, Ag, or Hg,
either sorbed to iron oxide surfaces or as a bulk mineral
phase.
Although DCM and CM are products of the sequential
hydrogenolysis of CT, we do not believe that they are
significant intermediates in the formation of methane during
the reduction of CT and CF by GR. Once formed, DCM and
CM persisted throughout the course of our experiments.
Furthermore, experiments independently examining the
reduction of DCM and CM by GR (alone and with added AgI,
AuIII, or CuII) showed no significant reduction of either DCM
or CM over the time scale of the experiments (Table S1,
Supporting Information), indicating that DCM and CM are
not intermediates in the formation of methane from CT (or
CF). Castro and Kray (55) proposed that the reduction of CT
and CF by CrII in aqueous solution does not proceed via
sequential hydrogenolysis but instead involves a series of
single-electron transfers in which chloromethyl radicals
accept an electron with the subsequent release of chloride,
forming the corresponding carbene/ carbenoid, which may
then be reduced to a methyl radical containing one less
chlorine atom (Figure 3). Such a reaction scheme does not
involve DCM or CM as an intermediate; however, they may
be formed by a side reaction in which dichloromethyl radicals
or chloromethyl radicals accept an additional electron while
abstracting a proton.
In addition to methane, a suite of C2, C3, and C4
hydrocarbons (ethane, ethene, acetylene, propene, propane,
n-butane, and several butene isomers) were products of the
reduction of CT and CF by AgGR, AuGR, and CuGR (Tables
1 and 2); minor amounts (<1%) of ethane, ethene, and
acetylene were observed as products of CT reduction by GR
alone. These compounds were not observed in controls
containing n-heptane and GR (either with or without added
metals) without the addition of chlorinated methanes,
indicating that the hydrocarbon products are derived from
intermediates in the reduction of CT and CF. Compounds
with carbon chains longer than the parent material are often
identified as products of halogenated hydrocarbon reduction
by reduced transition metal species (38, 64-68), and they
are generally attributed to radical coupling reactions (64,
66-68). Although alkyl radicals are generally highly reactive
and thus have very short lifetimes, complexation by transition
metals may stabilize them, thereby increasing the probability
of radical coupling reactions (69). As discussed earlier, the
formation of PCE during CT reduction in many of our
experimental systems could have resulted from the coupling
of trichloromethyl radicals to form HCA, which was sum-
marily reduced to PCE. The trace levels of ethane produced
during the reduction of CT and CF by GR, AgGR, AuGR, and
CuGR were not the result of reduction of ethene (as
determined by independent experiments), but they may have
been formed by coupling of methyl radicals (and/ or carbene
insertion in the C-H bond of methane).
Recently, examination of AgGR, AuGR, and CuGR and by
X-ray absorption fine structure (XAFS) spectroscopy and high-
resolution transmission electron microscopy has revealed
that AgI, AuIII, and CuII are reduced by GR to Ag0, Au0, and
Cu0, which are present as submicron-sized particles (80).
The enhanced reduction of CT by AgGR, AuGR, and CuGR
may be analogous to the enhanced reduction of chlorinated
hydrocarbons by bimetallic reductants (34, 42, 43, 81-83),
which are composed of a noble metal (typically, Pd, Pt, or
Ni) plated onto a more active metal (e.g., Fe, Zn). Although
the exact mechanism(s) by which the catalysis occurs in most
bimetallic systems are not fully understood, coupling of a
less reactive (noble) metal to a more reactive metal to
accelerate the oxidation of the more reactive metal is well-
known (84). The coupled active and noble metals form
galvanic cells where the active metal serves as the anode and
is oxidized, while reduction of the chlorinated hydrocarbon
In our proposed reaction scheme (Figure 3), acetylene
may be formed through further reduction of the products of
the dimerization of trichloromethyl radicals, dichlorocarbene,
and/ or chlorocarbene. The reduction of PCE by â-elimination
produces dichloroacetylene (DCAc), which can be further
reduced to chloroacetylene (CAc) and then acetylene (70,
71). We did not observe DCAc or CAc as intermediates in the
reduction of CT; however, DCAc and CAc are likely to be
highly reactive intermediates in our experimental system,
and thus they would not be expected to accumulate to levels
detectable by our analytical methods. Lee and Batchelor (30)
reported that acetylene is the main transformation product
of the reduction of PCE by GR. Although these authors did
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(or other oxidant) occurs at the noble metal (cathode). As
surface-associated particles, reduced metals coupled to GR
may similarly form galvanic cells in which the GR serves as
the anode and is oxidized, while CT is reduced at the metal
surface.
of synthetic GR suspensions into the subsurface may provide
an alternative to standard Fe0 PRBs. In addition, the enhanced
reduction of CT and CF by AgGR, AuGR, and CuGR may
prove useful in the development of improved materials for
PRBs and other remediation technologies.
The reduction of CT by nanoparticulate Ag0, Au0, and Cu0
was examined to determine whether CT is degraded by these
metals in the absence of GR. There was no measurable change
in the CT concentrations in the vials containing Ag0 or Au0.
This was expected because the coupling of Ag0 or Au0
oxidation to the reduction of CT is not thermodynamically
favorable. However, it is thermodynamically feasible to
reduce CT with Cu0, and 10 µmol CT was reduced below
detectable levels within 26 h by nanoparticulate Cu0.
However, the only identified products of CT reduction by
Cu0 were CO (18%) and formate (27%); CF, DCM, CM, and
nonchlorinated hydrocarbons (which are products of CT
reduction by CuGR) were not observed, suggesting that direct
reduction of CT by Cu0 is not a significant reaction during
the reduction of CT by CuGR.
Differences in the reduction rates of CT among AgGR,
AuGR, and CuGR as well as variations in product distributions
(e.g., the greater abundance of coupling products observed
for CT reduction by AuGR, see Table 1) are likely due to
differences in the surface atomic structure and electronic
properties of Ag0, Au0, and Cu0. Similar observations have
been reported for related systems. Liu et al. have reported
that the kinetics and product distribution of the electrolytic
reduction of CT depend on the type of metal used for the
cathode (85). Differences in reductions rates are also observed
within series of bimetallic reductants (e.g., PdFe, PtFe, NiFe).
The rates of trichloroethene (TCE) reduction by PdFe and
PdZn are greater than those for NiFe or NiZn, respectively
(43); similarly, TCE reduction by PdFe is faster than by PtFe
(34). It has been suggested that the enhanced reduction
observed with PdFe (relative to Fe0 alone) is largely due to
the accumulation of H2 generated from the reduction of water
by Fe0 at the Pd0-water interface (86). Indeed, Pd0 has been
shown to be an effective catalyst for the reduction of
chlorinated solvents by H2 (87). However, in our experiments
the rate of CT reduction in replicate experiments was
independent of H2 concentration in the headspace, and thus
it is unlikely that a similar catalytic process is occurring with
AgGR, AuGR, and CuGR.
Acknowledgments
We thank Eila Burr and Marlene Cantrell of Applied Research
Associates, Inc., for their assistance in the laboratory. We
also thank Karlin Danielsen at the University of Michigan for
her assistance with the carbon monoxide analysis. We thank
Aaron Williams at the University of Iowa for providing the
specific surface area measurements. We also thank Karen
Haugen, Siddhartha Mitra, and the anonymous reviewers
whose insight and thoughtful suggestions greatly improved
the quality and clarity of this paper. Financial support for
this project was provided by the Air Force Office of Scientific
Research (AFOSR); the Environmental Security Technology
Certification Program (ESTCP) of the U.S. Department of
Defense; and the Natural and Accelerated Bioremediation
(NABIR) program of the Office of Biological and Environ-
mental Research, Office of Science, U.S. Department of
Energy, under Contract W-31-109-Eng-38.
Supporting Information Available
One figures and one table. This material is available free of
charge via the Internet at http:/ / pubs.acs.org.
Literature Cited
(1) Bigham, J. M.; Tuovinen, O. H. In Planetary Ecology; Caldwell,
D. E., Brierley, J. A., Brierley, C. L., Eds.; Van Nostrand Reinhold
Co.: New York, 1985; pp 239-250.
(2) Ge´nin, J.-M. R.; Refait, P.; Olowe, A. A.; Abdelmoula, M.; Fall,
I.; Drissi, S. H. Hyperfine Interact. 1998, 112, 47-50.
(3) Kumar, A. V. R.; Singh, R.; Nigam, R. K. J. Radioanal. Nucl. Chem.
1999, 242, 131-137.
(4) Refait, P.; Abdelmoula, M.; Ge´nin, J.-M. R. Corros. Sci. 1998, 40,
1547-1560.
(5) Ona-Nguema, G.; Abdelmoula, M.; Jorand, F.; Benali, O.; Ge´hin,
A.; Block, J.-C.; Ge´nin, J.-M. R. Environ. Sci. Technol. 2002, 36,
16-20.
(6) Fredrickson, J. K.; Zachara, J. M.; Kennedy, D. W.; Dong, H.;
Onstott, T. C.; Hinman, N. W.; Li, S.-M. Geochim. Cosmochim.
Acta 1998, 62, 3239-3257.
(7) Tamaura, Y.; Saturno, M.; Yamada, K.; Katsura, T. Bull. Chem.
Environm ental Significance. Zerovalent iron has been
the most extensively studied reductant for the treatment of
groundwater contaminated with chlorinated hydrocarbons
and is currently the most commonly used material for the
construction of PRBs (33). However, a thorough understand-
ing of the processes involved in the reduction of chlorinated
hydrocarbons by Fe0 is lacking. In particular, little is known
regarding the role of FeII species in the reduction of
chlorinated hydrocarbons in Fe0 systems (88). Green rusts
have been observed as products in Fe0 columns and PRBs
(89-91), and Williams and Scherer (28) suggested that the
formation of FeII-bearing minerals such as GRs contributes
to the long-term effectiveness of Fe0 PRBs despite the
formation of thick oxide films. Moreover, our results showing
the catalytic effects of AgGR, AuGR, and CuGR on the
reduction of CT and CF may enhance our collective under-
standing of the dechlorination pathways for chlorinated
organic contaminants by bimetallic and other catalytic
systems.
Soc. Jpn. 1984, 57, 2417-2421.
(8) Taylor, R. M. Clay Miner. 1980, 15, 369-382.
(9) Chaudhuri, S. K.; Lack, J. G.; Coates, J. D. Appl. Environ. Microbiol.
2001, 67, 2844-2848.
(10) Refait, P.; Drissi, S. H.; Pytkiewicz, J.; Ge´nin, J.-M. R. Corros. Sci.
1997, 39, 1699-1710.
(11) Schwertmann, U.; Fechter, H. Clay Miner. 1994, 29, 87-92.
(12) Olowe, A. A.; Marie, Y.; Refait, P.; Ge´nin, J.-M. R. Hyperfine
Interact. 1994, 93, 1783-1788.
(13) Perez, O. P.; Umetsu, Y.; Sasaki, H. Hydrometallurgy 1998, 50,
223-242.
(14) Refait, P.; Ge´nin, J.-M. R. Corros. Sci. 1997, 39, 539-553.
(15) Schwertmann, U.; Taylor, R. M. In Minerals in Soil Environments;
Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America:
Madison, WI, 1977; pp 145-180.
(16) Srinivasan, R.; Lin, R.; Spicer, R. L.; Davis, B. H. Colloids Surf.
A 1996, 113, 97-105.
(17) Lin, R.; Spicer, R. L.; Tungate, F. L.; Davis, B. H. Colloids Surf.
A 1996, 113, 79-96.
(18) Ponnamperuma, F. N.; Tianco, E. M.; Loy, T. Soil Sci. 1967, 103,
374-382.
(19) Abdelmoula, M.; Trolard, F.; Bourrie´, G.; Ge´nin, J.-M. R. Hyperfine
Interact. 1998, 112, 235-238.
(20) Trolard, F.; Ge´nin, J.-M. R.; Abdelmoula, M.; Bourrie´, G.;
Humbert, B.; Herbillon, A. Geochim. Cosmochim. Acta 1997, 61,
1107-1111.
(21) Ge´nin, J.-M. R.; Bourrie´, G.; Trolard, F.; Abdelmoula, M.; Jaffrezic,
A.; Refait, P.; Maitre, V.; Humbert, B.; Herbillon, A. Environ. Sci.
Technol. 1998, 32, 1058-1068.
Our results along with other studies of the reduction of
CT, halogenated ethanes, and chlorinated ethenes by GR
(29-31) suggest that GR may be useful for the remediation
of chlorinated hydrocarbon contamination in the subsurface.
Green rusts can be easily prepared from relatively inexpensive
commodity chemicals (e.g., ferrous sulfate (copperas) and
sodium hydroxide (caustic soda)). Injection and dispersion
9
VOL. 37, NO. 13, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 9 1 1

(22) Bourrie´, G.; Trolard, F.; Ge´nin, J.-M. R.; Jaffrezic, A.; Maˆıtre, V.;
Abdelmoula, M. Geochim. Cosmochim. Acta 1999, 63, 3417-
3427.
(23) O’Loughlin, E. J.; Kelly, S. D.; Csencsits, R.; Cook, R. E.; Kemner,
K. M. Environ. Sci. Technol. 2003, 37, 721-727.
(24) Hansen, H. C. B.; Borggaard, O. K.; Sorensen, J. Geochim.
Cosmochim. Acta 1994, 58, 2599-2608.
(25) Koch, C. B.; Hansen, H. C. B. Adv. GeoEcol. 1997, 30, 373-393.
(26) Myneni, S. C. B.; Tokunaga, T. K.; Brown, G. E., Jr. Science 1997,
278, 1106-1109.
(27) Loyaux-Lawniczak, S.; Refait, P.; Lecomte, P.; Ehrhardt, J.-J.;
Ge´nin, J.-M. R. Hydrol. Earth Syst. Sci. 1999, 3, 593-599.
(28) Williams, A. G. B.; Scherer, M. M. Environ. Sci. Technol. 2001,
35, 3488-3494.
(58) Assaf-Anid, N.; Hayes, K. F.; Vogel, T. M. Environ. Sci. Technol.
1994, 28, 246-252.
(59) Lewis, T. A.; Paszczynski, A.; Gordon-Wylie, S. W.; Jeedigunta,
S.; Lee, C.-H.; Crawford, R. L. Environ. Sci. Technol. 2001, 35,
552-559.
(60) Poyer, J. L.; Floyd, R. A.; McCay, P. B.; Janzen, E. G.; Davis, E.
R. Biochim. Biophys. Acta 1978, 539, 402-409.
(61) Criddle, C. S.; McCarty, P. L. Environ. Sci. Technol. 1991, 25,
973-978.
(62) Hine, J. J. Am. Chem. Soc. 1950, 72, 2438-2445.
(63) Robinson, E. A. J. Chem. Soc. 1961, 1663-1671.
(64) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1986, 108, 719-723.
(65) Campbell, T. J.; Burris, D. R.; Roberts, A. L.; Wells, J. R. Environ.
Toxicol. Chem. 1997, 16, 625-630.
(29) Erbs, M.; Hansen, H. C. B.; Olsen, C. E. Environ. Sci. Technol.
1999, 33, 307-311.
(66) Castro, C. E.; Kray, W. C., Jr. J. Am. Chem. Soc. 1963, 85, 2768-
2773.
(30) Lee, W.; Batchelor, B. Environ. Sci. Technol. 2002, 36, 5348-
(67) Fennelly, J. P.; Roberts, A. L. Environ. Sci. Technol. 1998, 32,
1980-1988.
(68) Helvenston, M. C.; Castro, C. E. J. Am. Chem. Soc. 1992, 114,
8490-8496.
(69) Kochi, J. K.; Rust, F. F. J. Am. Chem. Soc. 1961, 83, 2017-2018.
(70) Arnold, W. A.; Roberts, A. L. Environ. Sci. Technol. 1998, 32,
3017-3025.
(71) Roberts, A. L.; Totten, L. A.; Arnold, W. A.; Burris, D. R.; Campbell,
T. J. Environ. Sci. Technol. 1996, 30, 2654-2659.
(72) Harris, J. M.; Wamser, C. C. Fundamentals of Organic Reaction
Mechanisms; John Wiley & Sons: New York, 1976.
(73) March, J. Advanced Organic Chemistry: Reactions, Mechanisms,
and Structure, 4th ed.; John Wiley & Sons: New York, 1992.
(74) Baratta, W.; Del Zotto, A.; Rigo, P. Organometallics 1999, 18,
5091-5096.
(75) Beckhaus, R. J. Chem. Soc., Dalton Trans. 1997, 1991-2001.
(76) Borer, M.; Loosli, T.; Minger, A.; Neuenschwander, M.; Engel,
P. Helv. Chim. Acta 1995, 78, 1311-1324.
(77) Do¨tz, K. H.; Siemoneit, S.; Hohmann, F.; Nieger, M. J. Organomet.
Chem. 1997, 541, 285-290.
(78) Sayin, M. Clays Clay Miner. 1982, 30, 287-290.
(79) Buresh, R. J.; Moraghan, J. T. J. Environ. Qual. 1976, 5, 320-325.
(80) O’Loughlin, E. J.; Kelly, S. D.; Kemner, K. M.; Csencsits, R.; Cook,
R. E. Chemosphere (in press).
(81) Lien, H.-L.; Zhang, W.-x. J. Environ. Eng. 1999, 125, 1042-1047.
(82) Muftikian, R.; Fernando, Q.; Korte, N. Water Res. 1995, 29, 2434-
2439.
(83) Wan, C.; Chen, Y. H.; Wei, R. Environ. Toxicol. Chem. 1999, 18,
1091-1096.
(84) Jones, D. A. Principles and Prevention of Corrosion; Prentice-
Hall: Englewood Cliffs, NJ, 1992.
5354.
(31) O’Loughlin, E. J.; Burris, D. R. Environ. Toxicol. Chem. (in press).
(32) Scherer, M. M.; Balko, B. A.; Gallagher, D. A.; Tratnyek, P. G.
Environ. Sci. Technol. 1998, 32, 3026-3033.
(33) Scherer, M. M.; Richter, S.; Valentine, R. L.; Alvarez, P. J. J. Crit.
Rev. Environ. Sci. Technol. 2000, 30, 363-411.
(34) Cheng, S.-F.; Wu, S.-C. Chemosphere 2000, 41, 1263-1270.
(35) Gantzer, C. J.; Wackett, L. P. Environ. Sci. Technol. 1991, 25,
715-722.
(36) Ma, H.; O’Loughlin, E. J.; Burris, D. R. Environ. Sci. Technol.
2001, 35, 717-724.
(37) Nelson, D. W.; Bremner, J. M. Soil Biol. Biochem. 1970, 2, 1-8.
(38) O’Loughlin, E. J.; Burris, D. R.; Delcomyn, C. A. Environ. Sci.
Technol. 1999, 33, 1145-1147.
(39) Ottley, C. J.; Davison, W.; Edmunds, W. M. Geochim. Cosmochim.
Acta 1997, 61, 1819-1828.
(40) Pettine, M.; Barra, I.; Campanella, L.; Millero, F. Water Res. 1998,
32, 2807-2813.
(41) Schwarzenbach, R. P.; Stierli, R.; Lanz, K.; Zeyer, J. Environ. Sci.
Technol. 1990, 24, 1566-1574.
(42) Xu, Y.; Zhang, W.-x. Ind. Eng. Chem. Res. 2000, 39, 2238-2244.
(43) Zhang, W.-x.; Wang, C.-B.; Lien, H.-L. Catal. Today 1998, 40,
387-395.
(44) Cornell, R. M.; Schwertmann, U. The Iron Oxides: Structure,
Properties, Reactions, Occurrence and Uses; VCH Publishers:
New York, 1996.
(45) Hansen, H. C. B.; Koch, C. B. Clay Miner. 1998, 33, 87-101.
(46) Burris, D. R.; Delcolymn, C. A.; Smith, M. H.; Roberts, A. L.
Environ. Sci. Technol. 1996, 30, 3047-3052.
(47) Gossett, J. M. Environ. Sci. Technol. 1987, 21, 202-208.
(48) Amonette, J. E.; Workman, D. J.; Kennedy, D. W.; Fruchter, J.
S.; Gorby, Y. A. Environ. Sci. Technol. 2000, 34, 4606-4613.
(49) McCormick, M. L.; Bouwer, E. J.; Adriaens, P. Environ. Sci.
Technol. 2002, 36, 403-410.
(85) Liu, Z.; Arnold, R. G.; Betterton, E. A.; Festa, K. D. Environ. Eng.
Sci. 1999, 16, 1-13.
(86) Grittini, C.; Malcomson, M.; Fernando, Q.; Korte, N. Environ.
Sci. Technol. 1995, 29, 2898-2900.
(50) Curtis, G. P. Ph.D. Dissertation, Stanford University, Palo Alto,
CA, 1991.
(87) Lowry, G. V.; Reinhard, M. Environ. Sci. Technol. 1999, 33, 1905-
1910.
(51) McCormick, M. L.; Kim, H. S.; Bouwer, E. J.; Adriaens, P. Abiotic
transformation of chlorinated solvents as a consequence of
microbial iron reduction: An investigation of the role of biogenic
magnetite in mediating reductive dechlorination. Thirtieth Mid-
Atlantic Industrial and Hazardous Waste Conference-Hazardous
and Industrial Wastes; 1998; pp 339-348.
(88) Scherer, M. M.; Balko, B. A.; Tratnyek, P. G. In Kinetics and
Mechanisms of Reactions at the Mineral-Water Interface; Sparks,
D. L., Grundl, T. J., Eds.; American Chemical Society: Wash-
ington, DC, 1998; pp 301-332.
(89) Furukawa, Y.; Kim, J.-W.; Watkins, J.; Wilkin, R. T. Environ. Sci.
Technol. 2002, 36, 5469-5475.
(52) Andrieux, C. P.; Gallardo, I.; Save´ant, J.-M.; Su, K.-B. J. Am. Chem.
Soc. 1986, 108, 638-647.
(90) Gu, B.; Phelps, T. J.; Liang, L.; Dickey, M. J.; Roh, Y.; Kinsall, B.
L.; Palumbo, A. V.; Jacobs, G. K. Environ. Sci. Technol. 1999, 33,
2170-2177.
(53) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1986, 108, 713-719.
(54) Buschmann, J.; Angst, W.; Schwarzenbach, R. P. Environ. Sci.
Technol. 1999, 33, 1015-1020.
(91) Roh, Y.; Lee, S. Y.; Elless, M. P. Environ. Geol. 2000, 40, 184-194.
(55) Castro, C. E.; Kray, W. C., Jr. J. Am. Chem. Soc. 1966, 88, 4447-
4455.
Received for review January 7, 2003. Revised manuscript
received April 16, 2003. Accepted April 22, 2003.
(56) Glod, G.; Angst, W.; Holliger, C.; Schwarzenbach, R. Environ.
Sci. Technol. 1997, 31, 253-260.
(57) Wade, R. S.; Castro, C. E. J. Am. Chem. Soc. 1973, 95, 226-230.
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