Abiotic reductive dechlorination of chlorinated ethylenes by iron-bearing soil minerals. 2. Green rust

Article abstract of DOI:10.1021/es0258374

Abiotic reductive dechlorination of chlorinated ethylenes by the sulfate form of green rust (GR_SO4) was studied in batch reactors. A modified Langmuir-Hinshelwood model described dechlorination kinetics. The rate constant for reductive dechlorination of chlorinated ethylenes at reactive GR_SO4 surfaces was 0.592-1.59/day The specific reductive capacity of GR_SO4 for target organics was 9.86-18 μM/g and sorption coefficient was at 0.53-1.22/mM. Surface area-normalized pseudo-first-order initial rate constants for chlorinated ethylenes by GR_SO4 were 3.4-8.2 times greater than those by pyrite. Chlorinated ethylenes were chiefly transformed to acetylene. There were no detectable quantities of chlorinated intermediates. The rate constants for the reductive dechlorination of trichloroethylene (TCE) increased with increasing pH but were independent of solid concentration and initial TCE concentration. Magnetite and/or maghemite were produced by the oxidation of GR_SO4 by TCE. The results of reductive dechlorination of chlorinated ethylenes by Fe-bearing soil minerals is important because it can produce basic knowledge that could be utilized in predicting the fate of chlorinated ethylenes in natural environments and to effectively operate remedial technologies.

Full text of DOI:10.1021/es0258374

Environ. Sci. Technol. 2002, 36, 5348-5354
soil minerals. It has been reported that iron sulfide min-
erals (e.g., pyrite (12, 14), troilite (15), and mackinawite
16, 17)) and iron oxide minerals (e.g., magnetite (15, 18, 19))
Abiotic Reductive Dechlorination of
Chlorinated Ethylenes by
Iron-Bearing Soil Minerals. 2. Green
Rust
(
can transform chlorinated ethanes and ethylenes primarily
to non-chlorinated products (acetylene, ethylene, ethane).
The reduction of nitrobenzenes and chlorinated ethyl-
enes at iron oxide surfaces has been reported to be accel-
erated by the addition of Fe(II) (13, 18). Green rust
II
III
x 2
(GR; [Fe 6-xFe (OH)12] O] ) is a mixed Fe(II)-
x+[(A)x/n yH
‚
x-
W O O J I N L E E * , † A N D B I L L B A T C H E L O R ‡
Fe(III) hydroxide that has a pyroaurite-type structure con-
sisting of brucite-like layers i.e., positively charged triocta-
hedral m etal-hydroxide layers and negatively charged
interlayer of anions and water molecules (20). It has attracted
attention because it may be produced in both natural (21)
and engineered systems (22, 23) and it can reduce many
inorganic and organic contaminants. GR has been reported
to occur naturally in reductomorphic soils (21) and during
the corrosion of iron, where it is believed to be an intermediate
in the formation of end products such as goethite, lepi-
docrocite, or magnetite (24, 25). It has been reported that
nitrate (26, 27) and Cr(VI) (28) are transformed by suspensions
Environmental Science Research Center, School of Public and
Environmental Affairs, Indiana University,
Bloomington, Indiana 47405, and Department of Civil
Engineering, Texas A&M University,
College Station, Texas 77843
Abiotic reductive dechlorination of chlorinated ethylenes
by the sulfate form of green rust (GRSO4) was examined in
batch reactors. Dechlorination kinetics were described
by a modified Langmuir-Hinshelwood model. The rate
constant for reductive dechlorination of chlorinated ethylenes
at reactive GRSO4 surfaces was in the range of 0.592
II
III
‚
4
of sulfate GR (GRSO4;Fe
4
Fe
2
(OH)12SO
2
yH O) and carbonate
II
FeIII
(OH)12CO ‚
GR (GRCO3;Fe
4
2
3
yH O) at similar or higher rates
2
than those reported for microorganisms. Carbon tetrachloride
(CT), hexachloroethane (HCA), pentachloroethane (PCA), and
tetrachloroethylene (PCE) have been shown to undergo
reductive transformation in the presence of GRSO4 (18, 29,
-
1
(
(4.4%) to 1.59 ((6.3%) day . The specific reductive
capacity of GRSO4 for target organics was in the range of
.86 ((10.1%) to 18.0 ((4.3%) µM/g and sorption
coefficient was in the range of 0.53 ((2.4%) to 1.22
9
3
0). However, no significant research on the reductive
-
1
dechlorination of chlorinated ethylenes by GR has been
systemically conducted to date.
(
(4.3%) mM . Surface area-normalized pseudo-first-
order initial rate constants for chlorinated ethylenes by
GRSO4 were 3.4 to 8.2 times greater than those by pyrite.
Chlorinated ethylenes were mainly transformed to acetylene,
and no detectable amounts of chlorinated intermediates
were observed. The rate constants for the reductive
dechlorination of trichloroethylene (TCE) increased as pH
increased (6.8 to 10.1) but were independent of solid
concentration and initial TCE concentration. Magnetite and/
or maghemite were produced by the oxidation of GRSO4
by TCE. These findings are relevant to the understanding
of the role of abiotic reductive dechlorination during natural
attenuation in environments that contain GRSO4.
The characterization of reductive dechlorination of chlo-
rinated ethylenes by iron-bearing soil minerals is important
because it can produce basic knowledge that could be used
to predict the fate of chlorinated ethylenes in natural
environments and to effectively operate remedial technolo-
gies. Although GR has not been reported as being present in
natural systems contaminated with chlorinated organics, it
could be formed in such systems under iron-reducing
conditions. The objective of this research was to characterize
the kinetics of reductive dechlorination of chlorinated
ethylenes by the sulfate form of GR and to identify the
products formed by the reaction. A series of chlorinated
ethylenes (PCE, trichloroethylene (TCE), cis-dichloroethylene
(
cis-DCE), and vinyl chloride (VC)) were used as target
chlorinated organics.
Introduction
The well-known prevalence (1, 2) and persistence (3, 4) of
chlorinated ethylenes in natural environments have driven
substantial research to characterize the reductive dechlo-
rination of chlorinated ethylenes under various environ-
mental conditions. Substantial research on biodegradation
of chlorinated organics has been conducted for the last few
decades (5, 6). Recently, the effort to characterize natural
reductants causing abiotic reductive dechlorination has
attracted attention not only due in part to the limitations of
biodegradation but also due to the potential interactions
between biotic and abiotic processes. Sulfides (7, 8), natural
organic matter (8, 9), macrocycles (10, 11), and mineral sur-
faces (12, 13) in natural environments have been reported
to increase the dechlorination rates of chlorinated organics
by acting as electron donors and mediators/ carriers. Research
has been conducted to measure the reactivity of iron-bearing
Experimental Section
Materials. Anaerobic environments required for the prepa-
ration of reagents and samples were previously described
(
31). Chlorinated organics used for this research were ACS
grade: PCE (99.9%, Sigma), TCE (99.6%, Sigma), cis-DCE,
97.0%, Sigma), trans-dichloroethylene (trans-DCE, 98%,
(
Sigma), 1,1-dichloroethylene (1,1-DCE, 99.0%, Sigma), and
VC (20 000 mg/ L, Sigma). Standard gases of ethane (99.0%),
ethylene (1.0%), and acetylene (1000 mg/ L) (Scott Specialty
Gases) were used for the analysis of non-chlorinated trans-
formation products. Methanol (99.8%, HPLC grade, EM) was
used as a diluent for target organic stock solutions.
Deaerated deionized water (ddw) was prepared by
‚
deoxygenating 18 MΩ cm deionized water with 99.99% nitro-
gen for 2 h and then by deoxygenating with the mixed gases
in an anaerobic chamber for 12 h (31). A bicarbonate buffer
(
(
10 mM) was prepared by adding an exact amount of NaHCO
100.3%, Sigma) to ddw. Acid and base solutions were
SO (95.7%, Sigma) and NaOH (97.0%,
EM) with ddw. The following biological buffers were
3
*
Corresponding author phone: (812)855-8486; fax: (812)855-1881;
e-mail: woojlee@indiana.edu.
†
‡
prepared by diluting H
Indiana University.
Texas A&M University.
2
4
5
3 4 8
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 24, 2002
10.1021/es0258374 CCC: $22.00

2002 Am erican Chem ical Society
Published on Web 11/09/2002

used to investigate the effect of pH on the dechlorination
kinetics. They were purchased from Sigma and were ACS or
higher grades: 2-[N-Morpholino]ethanesulfonic acid (MES,
concentration of target organic in aqueous solution was
monitored at each sampling time.
A batch experiment was also conducted in 200-mL amber
bottle (Kimble) with open-top cap and three-layered septum
system to identify oxidation products of GRSO4 after reaction
with TCE. An exact amount of pure TCE was spiked to GRSO4
suspension including a bicarbonate buffer. The initial
concentration of TCE was 0.22 M and mass ratio of GRSO4
was 0.01. Controls were prepared in the same way without
TCE. pH value of the suspension was adjusted and kept
constant at 7 as described for the batch kinetic experiments.
Reaction was initiated by mixing the bottle in an anaerobic
chamber using an orbital shaker at 180 rpm at room
temperature. The oxidation products of GRSO4 were identified
pK
propanesulfonic acid (MOPS, pK
sion), mixture of Tris[hydroxymethyl]aminomethane and
Tris[hydroxymethyl]aminomethane hydrochloride (Tris, pK
8.1, for pH 8.1 and 9.2 suspensions), and 3-[cyclohexy-
lamino]-1-propanesulfonic acid (CAPS, pK ) 10.4, for pH
0.1 suspension). Aqueous solutions (0.05 M) were prepared
a
) 6.1, for pH 6.8 suspension), 3-[N-Morpholino]-
a
) 7.2, for pH 7.3 suspen-
a
)
a
1
by adding exact amounts of buffers to ddw and adjusting to
the exact pH with 1 M NaOH solution.
GRSO4 was prepared by a method modified by Koch et al.
(
4
26) in which a solution of FeSO is partially oxidized by
-
contact with air. The suspension of dark blue-green pre-
cipitates was introduced into an anaerobic chamber im-
mediately after GRSO4 synthesis and washed several times
with ddw until no Fe(II) was detected in supernatant. Because
GRSO4 would be oxidized during freeze-drying, it was spread
on the plate and dried under an anaerobic atmosphere. The
dried GRSO4 was ground with pestle and mortar, sieved (<63
µm), and stored in the anaerobic chamber for no more than
and Fe(II) and Cl concentrations in aqueous solution and
GRSO4 were monitored at each sampling time. All the controls
and samples for this research were run in duplicates or
triplicates.
Analytical Procedures. Fe(II) and total iron in aqueous
solution and in GRSO4 suspension were measured by using
a Hewlett-Packard (HP) 8452A Diode-Array Spectropho-
tometer using the Ferrozine method (33). Aliquots of the
suspension (2.5 mL) were rapidly withdrawn with a 5-mL
gastight syringe (Hamilton) and filtered with a 0.2 µm
membrane filter (Whatman). Each 1 mL filtrate was trans-
ferred to polypropylene tube (VWR) and diluted with 1 to 9
mL ddw. GRSO4 suspension (0.5 mL) without separation of
oxidation products was transferred to 1.2 M acid solution
(9.5 mL) and shaken for 5 min to extract iron. A 10%
hydroxylamine solution was added to the sample to reduce
Fe(III) to Fe(II) for the measurement of total iron.
The product of the reaction of TCE with GRSO4 was
identified by XRD. An aliquot of GRSO4 suspension was
withdrawn with a 5-mL gastight syringe and filtered with a
7
days before being used in order to preclude an aging effect
(
32). The characteristics of GRSO4 used for this research were
identified previously (31). The Fe(II) and Fe(III) contents of
GRSO4 were 464 and 212 mg/ g. The surface area measured by
ethylene glycol monoethyl ether (EGME) method in the
2
anaerobic chamber was 86.3 m / g. The identity and purity
of synthesized GRSO4 was confirmed by X-ray diffraction (XRD)
(
See Figure S-1, Supporting Information).
Experim ental Procedures. Batch kinetic experiments to
investigate the dechlorination kinetics of target organics and
transformation products in GRSO4 suspension were conducted
as previously described (31). The experiments were con-
ducted in 20-mL glass vials at room temperature (22 ( 0.5
0
.2 µm membrane filter. The filtered GRSO4 sample was treated
with 1:1 (v:v) glycerol solution to minimize oxidation by
atmospheric oxygen (26, 27) and was scanned between 0°
and 70° 2θ with a scan speed of 1° 2θ/ min by a Rigaku
automated diffractometer (Cu KR radiation).
°
C). Exact amounts of GRSO4 were transferred to the vials
followed by a bicarbonate buffer in the anaerobic chamber.
The mass ratio of solid to water was 0.007 resulting in the
2
surface area concentration of 604 m / L. pH of GRSO4
suspensions was adjusted to 7 by adding the acid or base
solutions and kept constant during the experiments. A partial
conversion of GRSO4 to mixed GRCO3/ GRSO4 could occur by
Chlorinated ethylenes and chlorinated transformation
products were measured by using a HP G1800A gas chro-
matography detector (GCD) equipped with a DB-VRX
column and a mass spectrometer detector. After extraction
of supernatant and GRSO4 with pentane on an orbital shaker
at 250 rpm, an aliquot of extractant (1 µL) was injected into
an injection port at a split ratio of 30:1. The extraction
efficiency of target organics for aqueous samples was
generally greater than 92% and that for solid samples was
greater than 88%. Non-chlorinated transformation products
2
-
2-
4
the substitution of CO
3
for SO
in GRSO4, but the anion
exchange was not observed under the experimental condition
employed for the experiments. Target organic stock solutions
(
10-50 µL) were spiked into GRSO4 suspensions to obtain
initial concentrations of 0.19 mM PCE, 0.25 mM TCE, 0.41
mM cis-DCE, and 0.40 mM VC.
Based on the results of batch kinetic experiments, TCE
was chosen as a target chlorinated organic to investigate the
effects of pH, concentration of GRSO4, and concentration of
target organic on dechlorination kinetics. For a batch
experiment where pH was an experimental parameter, 0.05
M biological buffers were added to vials to keep pH of GRSO4
suspensions constant throughout the experiment. The pH
range investigated was 6.8 to 10.1 and mass ratio of solid to
liquid in the suspension was 0.01. Controls were prepared
by spiking TCE to aqueous solutions of the biological buffers.
To identify the effect of GRSO4 concentration, exact amounts
of GRSO4 were added to vials at mass ratios of 0.001 to 0.04.
Tris buffer solution (0.05 M) was added to keep suspension
pH at 8.1. The experiment started by spiking 10 µL of 0.76
M TCE stock solution to GRSO4 suspension to provide an
initial concentration of 300 µM. To investigate the effect of
target organic concentration, 5 to 20 µL TCE stock solutions
were spiked to GRSO4 suspension with mass ratio of 0.01 at
pH 8.1 resulting in 4 levels of TCE concentrations (150, 300,
2
(C hydrocarbons) were measured by using a HP 6890 gas
chromatograph (GC) equipped with a GS-Alumina column
and a flame ionization detector. After headspace sampling,
an aliquot of gas sample was introduced into the injection
port at a split ratio of 5:1. The concentrations of C
2
hydrocarbons in headspace were converted to those in
aqueous solution by using dimensionless Henry’s law
constants (34, 35).
Chloride was measured by using a Dionex 500 ion
chromatograph (IC) equipped with AS9-HC column and
conductivity detector. An aqueous sample was filtered with
a 0.2 µm membrane filter, and an aliquot amount of filtrate
was introduced to the injection port.
Results and Discussion
Treatm ent of Kinetic Data. The dechlorination kinetics of
chlorinated ethylenes in GRSO4 suspension was similar to
that observed in pyrite and magnetite suspensions (36). Target
organics rapidly disappeared initially, followed by more slow
removals and ultimately approached constant concentra-
tions. This behavior was attributed to consumption of the
4
50, and 600 µM). All experiments were conducted at room
temperature and samples for analysis were prepared in the
same way described in the batch kinetic experiments. The
VOL. 36, NO. 24, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 3 4 9

TABLE 1. Kinetic Parameters, Transformation Products and Their Recoveries, and Target Organic Remaining in GRSO4 Suspensiona
at the Last Sampling Time
target organicsb
last sampling time)
product recovery and target
organic remaining (%)
K (mM-1)
kc (day-1
)
R2d
k1,sa (Lm-2 day
)
-1 e
(
SR (µM/g)
PCE
9.86 ((10.1%)g
1.22 ((4.3%)
1.59 ((6.3%)
0.90 ((8.6%)
0.592 ((4.4%)
0.911
1.62 × 10-4
8.55 × 10-
5.19 × 10-
C2H2: 5.1
C2H4: 0.7
PCE: 64.7
(60.4 day)
f
total : 70.5
5
TCE
60.4 day)
14.4 ((13.2%)
16.9 ((6.0%)
0.76 ((4.4%)
0.633 ((2.0%)
0.841
0.937
C2H2: 8.8
C2H4: 1.2
TCE: 60
(
f
total : 70
5
cis-DCE
64.4 day)
C2H2: 0.8
C2H6: 8.5
C2H4: 5.0
cis-DCE: 71
(
f
total : 85.3
7.77 × 10-
5
C2H4: 20.3
C2H6: 2.5
VC: 68
VC
64.4 day)
18.0 ((4.3%)
0.53 ((2.4%)
0.94 ((2.8%)
0.970
(
f
total : 90.8
a
) ) 7 g/L. b Initial target organic concentration: 0.19 (PCE), 0.25 (TCE), 0.41 (cis-DCE), and 0.40 m M (VC). Partitioning
c
Concentration of GRSO4 (M
factors (pCE) for k: 1.12 (PCE), 1.08 (TCE), 1.06 (cis-DCE), and 1.07 (VC).
R
d
2
values of nonlinear regression for kinetic param eters. e Surface
R
area-norm alized pseudo-first-order initial rate constant for reductive dechlorination of chlorinated ethylenes, surface area concentration of GRSO4
was 604 m /L. Total is total carbon m ass balance in GRSO4 suspension. g Uncertainties represent 95% confidence lim its.
2
f
finite reductive capacity of GRSO4 as it was oxidized. Oxidation
products could have formed as separate phases or as a coating
on the GRSO4 surfaces. A modified Langmuir-Hinshelwood
kinetic model was developed to describe this behavior (see
pp S1-S2, Supporting Information) (36). The material bal-
ance equation of target organic combined with the rate
equation in a batch reactor follows
0
0
dCCE
dt
(k/ pCE){C - pCE(C - CCE)}CCE
RC CE
)
-
(1)
1/ K + CCE
Vg
pCE ) 1 + H
+ Ks
(2)
(3)
(
CE
)
Vaq
0
C
) MRS
R
RC
FIGURE 1. Reductive dechlorination of chlorinated ethylenes in
GRSO4 suspension (0.007 g/g). Error bars are ranges of relative
concentration of target organics. Curves represent kinetic model
fits. Control is the control sample for PCE.
where CCE is the target organic concentration in aqueous
solution; k is the rate constant for reductive dechlorination
0
RC
of target organic at the reactive sites of GRSO4; C is the
ized pseudo-first-order initial rate constant (k1,sa) was also
initial concentration of reductive capacity of GRSO4 for a target
calculated by normalizing k
1
by the surface area of GRSO4
.
organic and represents the total amount of the target organic
0
CE
that GRSO4 can reduce per unit volume of water; C is the
0
RC
(
k/ pCE)C
initial target organic concentration; K is the sorption coef-
ficient of target organic; pCE is the partitioning factor that
considers the partitioning of target organics to gas, aqueous,
and solid phases assuming instantaneous equilibrium in the
phases; HCE is the dimensionless Henry’s law constant for
k1 )
(4)
0
CE
1/ K + C
Reductive Dechlorination of Chlorinated Ethylenes by
Green Rust. The decay of target organics in controls and
GRSO4 suspension is represented in Figure 1. PCE concentra-
tion in controls rapidly decreased due to the partitioning to
the solid phases and reached equilibrium concentration
(91.2% of initial PCE concentration) at the first sampling
point (1.6 day). The concentrations of TCE, cis-DCE, and VC
(not shown) in controls also dropped rapidly and reached
equilibrium (95.9, 97, and 98% of initial target organic
concentrations, respectively) at 1.6 day. When a first-order
rate law is assumed to account for the partitioning of target
organics to the solid phases, the partitioning rates of target
organics to solid phases were at least 11 times faster than the
highest dechlorination rate of target organics in GRSO4
suspension.
chlorinated ethylenes; V
headspace and aqueous solution; K
partitioning coefficient for chlorinated ethylenes (See Table
S-1, Supporting Information); M is the concentration of
GRSO4; and S is the specific initial reductive capacity of GRSO4
g
and Vaq are the volumes of
s
is the solid-phase
R
R
.
0
RC
The parameters (k, K, and C ) were estimated by an
optimization procedure using MATLAB (MathWorks Inc.)
that solves the differential equation numerically by a fourth-
order Runge-Kutta method and calculates the sum of squares
repeatedly to find its minimum value by adjusting values of
the parameters with the Marquardt-Levenberg algorithm.
1
The corrected pseudo-first-order initial rate constants (k )
were calculated using the following eq 4 in order to compare
the dechlorination kinetics of chlorinated ethylenes in GRSO4
suspension to those for other systems that were reported in
terms of first-order rate constants. The surface area-normal-
Reductive dechlorination of chlorinated ethylenes in the
presence of GRSO4 is well fitted by the kinetic model as shown
in Figure 1. The rate constants for the dechlorination of target
5
3 5 0
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 24, 2002

organics in GRSO4 suspension (Table 1) are similar to those
observed in pyrite suspension (36), but differ by factors of
0
.6 to 1.9. The rate constant for the dechlorination of PCE
at the reactive surfaces of GRSO4 was greatest followed by
those of VC, TCE, and cis-DCE, which is a different order
compared to that shown in the reductive dechlorination of
target organics by iron sulfide and iron oxide minerals (36).
Because the reductive dechlorination by iron-bearing soil
minerals is probably the result of a variety of serial and parallel
reactions, the dechlorination kinetics of each target organic
may appear to be different depending on the type of
reductants and experimental conditions. The rate constant
for the dechlorination of cis-DCE by GRSO4 was the smallest
among the target organics, which is similar to results that
have been reported for the reductive dechlorination of
chlorinated ethylenes by pyrite, magnetite, and Zn(0) (34,
FIGURE 2. The product distribution for the reductive dechlorination
of target organics in GRSO4 suspension (0.007 g/g).
3
6). The corrected pseudo-first-order initial rate constants
for target organics normalized by GRSO4 surface area (Table
) were 3.4 to 8.2 times greater than those in pyrite suspension
36). They were 31 to 850 times smaller than corresponding
1
(
elimination pathway with further reduction to ethylene (PCE
f dichloroacetylene f chloroacetylene f acetylene f
ethylene).
constants for the dechlorination of target organics in the
presence of mackinawite (17), troilite (15), and zerovalent
metals (15, 34). The specific initial reductive capacity of GRSO4
for target organics was between 9.9 and 18 µM/ g. The
reductive capacity measures the ability of the reductant to
react with a specific target compound. Therefore, it may be
different for different compounds, and it does not necessarily
equal the reductive capacity that could be calculated by
assuming all Fe(II) in GRSO4 can be oxidized to Fe(III). The
values calculated from Fe(II) content are approximately 2-3
orders of magnitude greater than those observed in these
experiments, which may be the result of limited reactivity of
different types of Fe(II) in GRSO4 (e.g., Fe(II) species at edges
or external surfaces) or the blocking effect of Fe(III) surface
coatings.
The principal transformation products for the reductive
dechlorination of TCE in the presence of GRSO4 were acetylene
and ethylene. Because transformation products expected by
a hydrogenolysis pathway (e.g., DCEs and VC) were not
observed above the method detection limits, they are not
likely to be an important intermediate for the transformation
of TCE to acetylene. This also suggests that a highly reactive
intermediate (e.g., chloroacetylene) is produced during the
reductive transformation of TCE to acetylene (17, 18). Based
on the product distribution observed, it appears that reductive
elimination with further reduction to ethylene (TCE f
chloroacetylene f acetylene f ethylene) is the principal
reaction pathway for the transformation of TCE by GRSO4
.
The main transformation product for the reductive
dechlorination of cis-DCE was ethylene. Compared to the
product distribution for cis-DCE dechlorination by pyrite
(36) and zerovalent metals, ethylene was further reduced to
ethane and no VC was detected at concentrations above the
method detection limit. A possible reaction pathway has been
suggested to explain abiotic reductive transformation of cis-
DCE that includes reduction to acetylene followed by
sequential reduction to ethylene and ethane. The formation
of acetylene increased until 25.8 days and then decreased as
the formation of ethylene increased. Although the amount
of acetylene produced was smaller than that estimated by
stoichiometric calculation, the pathway of cis-DCE f acety-
lene f ethylene f ethane would be a reasonable pathway
for the reductive transformation of cis-DCE in GRSO4 suspen-
sion.
Approximately, 29 to 40% of initial target organics were
removed (i.e., not measured in aqueous solution) in 70 days
and the total amount of carbon recovered at the last sampling
time was 70 to 90.8% of the initial amount. The low total
carbon recovery may be mainly caused by unrecovered target
organics in a sorbed state and volatilization loss during the
sampling procedure. The formation of nondetectable prod-
ucts and inaccuracy in Henry’s law constants may affect the
incompleteness of total carbon recovery. The main trans-
formation products for the reductive dechlorination of PCE
were acetylene and ethylene, which accounted for 14.5 and
2
.0% of the removed PCE, respectively (See Figure S-2,
Supporting Information). In contrast to what is usually
observed in the microbial hydrogenolysis of PCE (5, 37) and
the reductive transformation of PCE by Vitamin B12 with Ti-
(
III)-citrate (10, 38), no chlorinated daughter products were
The removal of VC by GRSO4 was approximately 30% at 37
days (graphical estimation), which is 4-6 times greater than
the microbial removals of VC by methanogenic and Fe(III)-
reducing bacteria (40). 63.4% of VC removed was transformed
to ethylene as a principal product and 7.8% was transformed
to ethane. No acetylene was detected during the reaction. A
similar product distribution was shown in the reductive
transformation of VC by pyrite (36). These results suggest a
hydrogenolysis pathway with further reduction to ethane
(VC f ethylene f ethane) for the reductive transformation
of VC in GRSO4 suspension. Both cis-DCE and VC were
reductively transformed to ethane in GRSO4 suspension, while
the transformation of PCE and TCE to ethane was not
observed. This could be due to the limitations of stoichi-
ometry and/ or kinetics. In the system with c-DCE or VC,
there was a higher ratio of reductive capacity in GRSO4 than
oxidative capacity in cis-DCE or VC. Therefore, less of the
reductive capacity of GRSO4 was consumed in converting the
target organics to ethylene and there was more available to
convert ethylene to ethane. Lower reductive capacity re-
observed during the reaction. No further reduction of ethylene
to ethane was observed either. Figure 2 represents the
distribution of transformation products for the reductive
transformation of target organics by GRSO4. A possible reaction
pathway for the reductive transformation of PCE by GRSO4
can be suggested based on the observed product distribution.
PCE could be reduced to acetylene via a dechlorination path
and then to ethylene. Because TCE, dichloroethylenes (DCEs),
and VC were not detected above the method detection limits
(
TCE: 0.8 µM, cis-DCE: 2.06 µM, trans-DCE: 8.3 µM, 1,1-
DCE: 2.4 µM, and VC: 16 µM), these compounds are not
likely intermediates for the transformation of PCE. However,
unstable intermediates such as dichloroacetylene and chlo-
roacetylene could be formed (PCE f dichloroacetylene f
chloroacetylene f acetylene) (17, 18, 22). Compared to the
parallel transformation pathways usually found in the
reductive dechlorination of PCE by zerovalent metals (34,
3
9) and iron sulfides (17, 18), the main pathway for the
transformation of PCE by GRSO4 is probably the reductive
VOL. 36, NO. 24, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5 3 5 1

TABLE 2. Kinetic Parameters for the Reductive Dechlorination of TCE as Functions of pH, Mass Ratio, and Initial Target Organic
Concentration
0
TCE
a(mM)
K (mM-1
k (day-1
)
R2c
pH
mass ratio (g/g)
C
SR (µM/g)
)
6
7
8
9
.8
0.01
0.01
0.01
0.01
0.01
0.001
0.005
0.02
0.04
0.01
0.01
0.01
0.3
9.6((2.1%)b
10.2((1.8%)
10.6((3.3%)
12.5((2.5%)
15.1((5.1%)
12.0((3.3%)
13.8((4.1%)
13.0((3.5%)
12.8((4.9%)
10.5((2.7%)
11.2((8.8%)
10.9((7.0%)
0.80((3.6%)
0.80((2.3%)
0.80((3.2%)
0.80((2.6%)
0.801((4.4%)
0.789((3.1%)
0.80((5.3%)
0.80((4.6%)
0.791((4.2%)
0.803((1.9%)
0.798((6.5%)
0.80((8.4%)
0.56((2.7%)
0.92((1.9%)
1.21((1.9%)
1.71((3.3%)
2.22((6.1%)
1.20((3.2%)
1.21((3.6%)
1.18((3.4%)
1.22((4.0%)
1.19((1.3%)
1.24((4.5%)
1.22((5.9%)
0.972
0.98
.3
.1
.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.15
0.45
0.6
0.984
0.968
0.907
0.971
0.962
0.964
0.931
0.987
0.921
0.897
1
0.1
8
8
8
8
8
8
8
.1
.1
.1
.1
.1
.1
.1
a
Initial concentration of TCE (m M). b Uncertainties represent 95% confidence intervals. c R2 values of nonlinear regression for kinetic
param eters.
converted to active sites, the increases in pH would tend to
increase the initial reductive capacity, which was observed
(
Figure 3). Increases in either of these parameters would
increase the rate of TCE dechlorination. Similar effects of pH
have been observed for the reduction of nitroaromatics by
magnetite with Fe(II) (13) and by iron porphyrin with H
2
S
(
9) as well as in the reductive dechlorination of HCA by
mackinawite (16). However, the increase of dechlorination
rate constant due to the pH increase in GRSO4 suspension
was approximately 1.6 to 8.5 times smaller than those
observed in systems (9, 13, 16) containing mackinawite,
magnetite with Fe(II), and iron porphyrin with H
the surface area concentration of GRSO4 was approximately
to 3 orders of magnitude higher than that of the other
2
S although
1
reductants. In contrast to the pH effect described above, it
has been reported that an optimum pH was observed for
PCE dechlorination by suspensions of Portland cement and
Fe(II) (22). An optimum pH for PCE degradation with GRSO4
might be observed if experiments were conducted at higher
pH values.
FIGURE 3. Rate constant (k) and specific initial reductive capacity
of GRSO4 (S
of 0.3 mM TCE in 0.01 g/g GRSO4 suspension. Error bars for k and
represent 95% confidence intervals.
R
) as a function of pH for the reductive dechlorination
S
R
maining in a system with PCE or TCE could limit the observed
production of ethane by slowing dechlorination kinetics or
if very low, by stoichiometry.
Influence of Solid Concentration. A statistical inference
test showed that the slopes of the rate constant (k), specific
initial reductive capacity (S ) and sorption coefficient (K)
R
Factors Affecting Dechlorination Kinetics. The effect of
pH, solid concentration, and target organic concentration
on the reductive dechlorination of TCE was investigated in
GRSO4 suspensions. The estimated parameters to describe
the dechlorination kinetics are reported in Table 2.
Influence of pH. The pH dependence of dechlorination
rate constant is demonstrated in Figure 3. The uncertainties
of estimated rate constants representing 95% confidence
versus concentration of GRSO4 solids were not significantly
different from zero at the 95% confidence level (See Figure
S-3, Supporting Information). The independence of these
parameters from affect of solid concentration supports the
validity of the kinetic model. An average specific initial
reductive capacity was calculated as 12.4 ( 1.5 µM/ g, which
is generally consistent with that measured in previous
experiments (31), differing by about a factor of 1.1. The kinetic
model predicts that the pseudo-first-order initial rate constant
2
interval were less than 6.1% and the R values are greater
1
(k ) should be a linear function of mass ratio. Similar behavior
than 0.90, which indicate that the kinetic model adequately
described the reductive dechlorination of TCE by GRSO4 in
these experiments. An increase of suspension pH increased
dechlorination rate constants. This result may be described
by the effect of pH on thermodynamics of the redox reaction.
Since the dechlorination reaction produces hydrogen ions,
an increase of pH would increase the driving force for the
reaction and possibly increase its rate. Alternatively, the pH
dependence of TCE dechlorination by GRSO4 could be
has been reported for other reductants/ target organics, such
as Fe(0)/ chlorinated ethylenes (42, 43) and magnetite/
nitroaromatics (13).
The results of this kinetic study can be extrapolated to
predict first-order decay coefficients and half-lives of TCE in
aquifers that contain green rust. The initial reductive capacity
0
RC
concentration (C ) in the aquifer can be calculated by
assuming that green rust represents 1% of the iron content
of the soil. The mass of iron per volume water in a typical
saturated soil can be calculated as 91 g/ L by assuming an
iron content of 2.6%, bulk density of 1.4 kg/ L, and porosity
of 0.40. This provides a green rust concentration of 1.73 g/ L
explained by assuming that the hydrated GRSO4 surfaces are
+
composed of hydroxide functional groups (e.g., tFe(II)OH
2
,
+
tFe(III)OH
2
, tFe(II)OH, and tFe(III)OH). These groups
originate from the hydrolysis of partially uncoordinated
surface iron and undergo protonation and deprotonation as
pH changes, resulting in changes in surface charge (41).
Deprotonated surface groups are more reactive than pro-
tonated ones (16); therefore, the increase of deprotonated
species on GRSO4 surfaces due to the increases of pH would
tend to increase the rate constant. If nonreactive sites were
assuming that GR is 52.6% iron. Combining these results
0
RC
with those in Table 2 produces a value for C of 0.0225 mM.
A partitioning factor (pCE) for TCE of 4.66 can be calculated
by assuming a soil organic fraction of 0.005 and an organic
carbon partition coefficient of 206 L/ kg. Equation 4 can be
used to calculate a pseudo-first-order rate constant of 0.0037
5
3 5 2
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 24, 2002

Acknowledgments
This research has been funded entirely with funds from the
State of Texas as part of the program of the Texas Hazardous
Waste Research Center. The contents do not necessarily
reflect the views and policies of the sponsor nor does the
mention of trade names or commercial products constitute
endorsement or recommendation for use.
Supporting Information Available
Table of measured solid phase partitioning coefficients and
partitioning factors of chlorinated ethylenes in GRSO4 sus-
pension and figures of diffractogram of GRSO4, reductive
transformation of PCE by GRSO4 and rate constant, specific
initial reductive capacity, and sorption coefficient as a
function of mass ratio and as a function of initial target organic
concentration for the reductive dechlorination of TCE. This
material is available free of charge via the Internet at http:/ /
pubs.acs.org.
FIGURE 4. The change of XRD patterns for the oxidation product
during the reaction between GRSO4 and TCE. The values near peaks
represent their d-spacing values (Å).
day- which gives an apparent half-life of 190 days. This is
on the order of reported half-lives of chlorinated organics
during natural attenuation and demonstrates the potential
for soil minerals such as green rust to play an important role
in determining the fate of chlorinated organics in natural
systems.
1
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