Effects of alcohols, anionic and nonionic surfactants on the reduction of PCE and TCE by zero-valent iron

Article abstract of DOI:10.1016/S0043-1354(00)00422-X

The effects of surfactants, sodium dodecyl sulfate (SDS) and Triton X-a00 (TX), and alcohols (methanol, ethanol, and propanol) on the dehalogenation of TCE and PCE by zero-valent iron were examined. Surface concentrations of PCE and TCE on the iron were dependent on aqueous surfactant concentrations. At concentrations above the CMC, sorbed halocarbon concentrations declined and concentrations associated with solution phase micelles increased. The anionic surfactant SDS ([SDS]< CMC) did not affect reduction rates, until the CMC was exceeded after which reactivity decreased, possibly due to sequestering of the TCE and PCE in mobile micelles. The nonionic TX showed a mixed effect on reactivity, increasing the PCE reduction rate, but not affecting TCE removal. Production of TCE from PCE increased in the presence of TX. Similar experiments showed that methanol, ethanol, and propanol inhibited reduction of TCE and PCE by metallic iron. Zero-valent iron may be useful in recycling soil washing effluents contaminated with TCE and PCE. Copyright

Full text of DOI:10.1016/S0043-1354(00)00422-X

Wat. Res. Vol. 35, No. 6, pp. 1453–1460, 2001
# 2001 Elsevier Science Ltd. All rights reserved
Printed in Great Britain
PII: S0043-1354(00)00422-X
0043-1354/01/$ - see front matter
EFFECTS OF ALCOHOLS, ANIONIC AND NONIONIC
SURFACTANTS ON THE REDUCTION OF PCE AND TCE BY
ZERO-VALENT IRON
GREGORY A. LORAINE*
Air Force Research Laboratory, AFRL/MLQR, 139 Barnes Dr. Tyndall AFB, FL 32403, USA
(First received 1 July 1999; accepted in revised form 1 July 2000)
Abstract}The effects of surfactants, sodium dodecyl sulfate (SDS) and Triton X-a00 (TX), and alcohols
(methanol, ethanol, and propanol) on the dehalogenation of TCE and PCE by zero-valent iron were
examined. Surface concentrations of PCE and TCE on the iron were dependent on aqueous surfactant
concentrations. At concentrations above the CMC, sorbed halocarbon concentrations declined and
concentrations associated with solution phase micelles increased. The anionic surfactant SDS ([SDS]5
CMC) did not affect reduction rates, until the CMC was exceeded after which reactivity decreased, possibly
due to sequestering of the TCE and PCE in mobile micelles. The nonionic TX showed a mixed effect on
reactivity, increasing the PCE reduction rate, but not affecting TCE removal. Production of TCE from PCE
increased in the presence of TX. Similar experiments showed that methanol, ethanol, and propanol
inhibited reduction of TCE and PCE by metallic iron. Zero-valent iron may be useful in recycling soil
washing effluents contaminated with TCE and PCE. # 2001 Elsevier Science Ltd. All rights reserved
Key words}zero-valent iron, surfactants, TCE, PCE, cosolvents
INTRODUCTION
two halogens are lost simultaneously. Both of these
processes involve a net exchange of two electrons.
Alcohols and surfactants are increasingly used in the
Hydrogenolysis of PCE produces TCE, while reduc-
remediation of subsurface contamination by spar-
ingly soluble chlorinated organic compounds such
as tetrachloroethene (PCE) and trichloroethene
(TCE) (Jawitz et al., 1998; Smith et al., 1997). If
the chlorinated organics could be removed the
recovered cosolvents or surfactants could be reused.
acetylene. The final products are ethene and ethane.
However, removal of the chlorinated contaminants is
problematic. One method of reducing halogenated
to produce chloroacetylene. Like DCE, chloroacety-
alkenes in aqueous solutions uses zero-valent iron
(Gillham and O’Hannesin, 1994; Campbell et al.,
1997). The feasibility of combining these two
of PCE and the resulting product formations and
technologies, using zero-valent iron to dechlorinate
PCE and TCE in typical enhanced solubilization
(pH=7.2). They determined the relative selectivity of
matrices, was examined.
In anaerobic solutions chlorinated organic com-
iron they used is different that used in this study, so
pounds such as PCE and TCE are dehalogenated by
metallic iron (Campbell et al., 1997). Reductive
dehalogenation occurs via two pathways hydrogeno-
tive b-elimination results in dichloroacetylene. Tri-
chloroethene itself can undergo b-elimination to
produce chloroacetylene, or hydrogenolysis to give
1,1-dichloroethene (DCE), cis 1,2-DCE, or trans 1,2-
DCE. Dichloroethene degrades to vinyl chloride or
Dichloroacetylene rapidly undergoes hydrogenolysis
lene will produce either vinyl chloride or acetylene.
Arnold and Roberts (1999) examined the reduction
reactions with pure iron granules in buffered water
each pathway by fitting a complex mechanism. The
direct comparisons cannot be made. However, their
results are instructive. They found that of the PCE
reduced 87% proceeded via b-elimination and 13%
via hydrogenolysis and 97% of TCE reduced under-
went b-elimination and 3% hydrogenolysis. b-elim-
ination was the dominant pathway for all of the
above reactions.
lysis (two sequential one eꢀ transfers removing a
halogen and adding a hydrogen from H2O), and
reductive b-elimination in which two eꢀ are added and
It has been shown that surfactants can increase the
surface concentration of sparingly soluble organics
on non-reactive surfaces such as soil and sediment
particles in water (Ko et al., 1998; Sun et al., 1995;
*Author to whom all correspondence should be addressed.
Department of Civil and Environmental Engineering,
San Diego State University, 5500 Campanile Drive, San
Diego, CA 92182, USA. E-mail: galoraine@yahoo.com
1453

1454
Gregory A. Loraine
Burris and Antworth, 1992). Ko et al. (1998) chlorinated organics have made the assumption that
demonstrated that anionic and nonionic surfactants increasing surface concentrations would increase
raised surface concentrations of aromatic compounds reaction rates. Sayles et al. (1997) found that the
on kaolinite particles. The surface concentration was reduction of DDT (1,1,1-trichloro-2, 2-bis (p-chloro-
dependent on the aqueous surfactant concentration. phenyl) ethane) by zero-valent iron increased 40% in
Once the critical micelle concentration (CMC, the the presence of Triton X-114 (an octylphenoxy
surfactant concentration at which monomers aggre- ethoxylate that differs from Triton X-100 in ethoxy-
gate to form micelles) was reached, partitioning to ethanol chain length). The enhanced rate was
the mobile phase micelles began to compete with attributed to increased aqueous solubility of the
sorption into the surfactant layer on the clay hydrophobic DDT and its products leading to
surfaces. Above the CMC the sparingly soluble increased mobility of the DDT. The concentrations
organic concentration on the surface declined on the surface of the iron were not measured. Gu
(Sun et al., 1995). This was especially true for anionic et al. (1997) found that a mixture of 2% Aerosol
surfactants. While enhanced surface sorption has a MA-1 (an anionic surfactant) and 2% ethanol or
negative effect on the mobilization of hydrophobic isopropanol did not affect the kinetics of TCE
organics in groundwater, it may be possible to increase reduction by Fe0. Li (1998) studied the reduction of
the concentration of chlorinated organics on reductive PCE by Fe0 and Fe0 coated with HDTMA (cationic
iron surfaces using surfactants. The reduction rates of surfactant) or SDS. The reduction of PCE with
halogenated compounds by zero-valent iron are slow HDTMA coated iron was three times faster than
and long contact times are required. Surfactants could unmodified iron. The SDS coated iron was only
be used to retard the flow of pollutants through slightly faster than unmodified iron. The surface
permeable barriers thus increasing contact time and concentrations of PCE, TCE or SDS were not
decreasing the required size of the barrier.
determined. The aqueous concentrations of TCE
Micelles of sodium dodecyl sulfate in water form produced from PCE were higher with the surfactant
with the polar OSOꢀ3 head facing the aqueous phase modified irons. It was concluded that increasing the
and the hydrophobic dodecyl tail in the interior. On surface concentration of PCE increased the reaction
surfaces SDS monomers organized into patches of rates. These previous studies suggest that increased
bilayers or hemimicelles with the anionic heads surface concentration increased reaction rate but did
pointed out giving the surface a negative charge. not directly measure the surface concentrations.
On surfaces with a net negative charge, like corroding
In this study, the effects on reduction rates of TCE
iron, electrostatic repulsion could have caused the and PCE with zero-valent iron by modifying the
SDS bilayers to roll up into spherical micelles on surface concentration with surfactants and alcohols
the surface (Rusling, 1997; Chen et al., 1999). The were determined. Surfactant solutions containing an
nonionic octylphenol ethoxylate (TX) was not sub- anionic surfactant, sodium dodecyl sulfate or a
ject to electrostatic repulsions and may have pro- nonionic surfactant, Triton X-100, at concentrations
vided more complete coverage.
above and below the CMCs were used. Reduction
Electron transfer can be affected by non-electro- reactions in solutions of ethanol, methanol, and
active surfactants (Rusling, 1997; Chen et al., 1999; 2-propanol at concentrations typical of field reme-
Possidonio and El Seoud, 1999). For electron diation efforts were also examined.
transfer to occur the substrate must be on or near
the surface. If a surfactant monomer is on an active
site the substrate must either displace it or approach
within one headgroup distance of the surface (Rusl-
ing, 1997). Solutes can enter the bound surfactant
layer either by diffusion from the bulk liquid or by
being transported with micelles merging with the
surface aggregates. Transfer into micelles of SDS by
small nonionic molecules such as PCE and TCE have
been shown to proceed at diffusion controlled rates
(Rusling, 1997). Diffusion out of micelles was
inversely related to the chain length of the hydro-
phobic tail (Possidonio and El Seoud, 1999).
Regardless of the transfer mechanism, once inside
the surfactant layer the solute orients itself with
respect to the interface on a microsecond time scale
(Rusling, 1997). This orientation depends on solvent
interactions in the interior of the micelle and the size
and charge of the headgroup.
EXPERIMENTAL SECTION
All alcohol (ethanol, EtOH, Pharmco; methanol, MeOH,
Fisher, HPLC grade; and 2-propanol, IPA, Fisher, HPLC
grade) and surfactant solutions (sodium dodecyl sulfate,
Sigma, sodium salt 99%; Triton X-100, t-Octylphenoxypo-
lyethoxy ethanol, Sigma, SigmaUltra grade) were made in
argon sparged Milli-Q water and degassed under vacuum
for at least 20 min and stored in an anaerobic chamber (5%
H2, 95% N2). The CMCs were found in the literature to be
2.31 g/L (8.0 mM) for SDS and 0.15 g/L for Triton X-100
(Liu and Roy, 1995; Mukerjee and Mysels, 1971).
Aqueous SDS concentrations were determined by TOC
analyzer (Shimadzu, TOC-5000A) after dilution to
TOC51000 mg/L with Milli-Q water (detection limit
0.05 mg/L). Triton X-100 concentrations were measured
spectrophotometrically at 277.5 nm, e ¼ 1220 M/cm (Cary
3E UV-Visible Spectrophotometer, detection limit 0.001 mg/
L). All samples were filtered through a 0.22 mm filter (Lida,
Pro-X 0.22 mm hydrophilic cellulose acetate membrane)
before analysis.
Sorption isotherm experiments for the surfactants were
run in zero headspace batch experiments with duplicate,
Recent work examining surfactants or surfactant/
alcohol mixtures in studies of Fe0 reduction of sacrificial samples. Initial aqueous concentrations for SDS

Effects of PCE and TCE in alcohols and surfactants
were 0.58, 1.73, 2.31, 4.61, and 23.07 g/L and for TX were
1455
0.05, 0.1, 0.15, 0.3, 1.5, and 7.5 g/L. Acid washed iron filings
(5 g, iron metal Fisher Scientific 40 mesh, SA=1.25 m2/g,
3.1% C w/w) were placed in 15 m serum vials (Wheaton).
The vials were filled completely with liquid (ꢁ14.2 mL) in an
anaerobic chamber and crimp sealed with Teflon-faced
rubber septa. Control samples contained no iron. The
samples were mixed by axial rotation on a roller drum at
8 rpm, at 20 Æ 18C in the dark for 24 h. Surface concentra-
tions were taken to be the difference between aqueous
concentrations in the samples with iron and the controls.
Reduction experiments of PCE and TCE were preformed
in a similar manner with the following changes. Immediately
after being sealed, the vials were removed from the chamber
and spiked with methanol solutions of either tetrachlor-
oethene (J. T. Baker, Photrex grade) or trichloroethene
(Fisher Scientific, Certified ACS Grade) (ꢁ500 mg/mL) to
give a final concentration of ꢁ36 mg/L. Duplicate control
and experimental samples were removed after varying
periods of time and analyzed. Each vial was analyzed to
determine aqueous and total PCE or TCE concentrations.
Duplicate 20 mL aliquots of the aqueous phase were sampled
from the vial and spiked into 1.0 mL of acetonitrile (Fisher
Scientific, PHLC Grade) with 10 mg/L p-dichlorobenzene
(Fisher Scientific, Reagent Grade) as an internal standard
and vortexed. Total system concentration was determined
by transferring the aqueous content of the vial (ꢁ14.2 mL)
into a 40 mL vial containing 10 mL of acetonitrile via
cannule. The remaining solids were washed with two 2.5 mL
portions of acetonitrile (spiked with internal standard) and
vortexed for 1 min. The washing solvents were added to the
40 mL vial. A 100 mL aliquot was spiked into 0.90 mL of
acetonitrile and analyzed via GC/ECD. Burris et al. (1995)
reported extraction recoveries for PCE and TCE of
approximately 100% for this method.
Fig. 1. Sorption Isotherm of SDS (a) and Triton X-100 (b)
on clean cast iron (Fisher, >40 mesh, SA=1.25 m2/g) at
208C after mixing at 8 rpm for 24 h in the dark. Iron/liquid
ratio was 5 g/14.2 mL. The data points are labeled with the
initial surfactant concentration. (a) [SDS]0=0.58, 1.7, 2.3
(CMCAqu), 4.6, and 23.1 g/L. (b) [Triton X-100]0=0.05,
0.10, 0.15 (CMCAqu), 0.30, 1.5, and 7.5 g/L.
Sorption isotherms for TCE and PCE were calculated
assuming the difference between the total and aqueous
concentrations was bound to the iron surface (control samples
with no iron present showed that sorption to the walls of the
glass vial was negligible). Sorption isotherms (208C) were
made by plotting the bound concentration (mg/g) versus the
aqueous concentration (mg/ml) as the reaction progressed.
Since both the aqueous and total concentrations were
determined at the same time point the isotherm calculations
reflect only the equilibrium and were independent of substrate of cast iron at 20 Æ 18C after 24 h. Sorption of SDS
concentration between time samples.
loss. Reaction rates were determined from the change in total
onto the cast iron increased until the CMC in the
aqueous phase was reached. Above the CMC, the
Tetrachloroethene and trichloroethene concentrations
concentration of bound surfactant reach a plateau
were determined by GC on a 15 m 0.53 mm id HP-1 column
and aqueous micelles formed. Surface sorption of TX
continued to increase beyond the published aqueous
CMC concentration. The apparent change in CMC
of non-homogeneous surfactants in water–particu-
late systems has been observed previously (Hayworth
with film thickness of 3 mm. The temperature program was
508C for 2 min, ramp at 258C/min to 2008C. Helium gas flow
rate was 3.7 mL/min. Injector and detector temperatures
were 150 and 2808C, respectively. Samples were injected
splitless (1 mL). An anode purged Ni63 ECD detector was
used, with a 5% CH4 in Ar make up gas flow of 20 mL/min.
Peaks were identified by co-elution with known standards, and Burris, 1997). Triton X-100 is a mixture of
and detector responses were calibrated as the relative
responses of the analytes to the internal standard on a
chain lengths. It is thought that these different
multilevel standard curve. Limits of detection were 0.30 mg/
octylphenol ethoxylates of different ethoxyethanol
monomers exhibit different solubilities and form
L (2.3 mM) for TCE and 05.0 mg/L (3.1 mM) for PCE.
micelles sequentially. The solid phase may contribute
to the differentiation of micellization and the CMC
observed may be greater than that seen in aqueous
solutions. The CMC in this system was not deter-
mined. The actual CMC was assumed to be near
1.5 g/L Triton X-100.
Analyses of products of PCE and TCE reduction reactions
were based on the method of Campbell et al. (1997) with the
following modifications: five grams of acid washed iron were
used instead of 20 g and pyrite was not added as a buffer;
previous studies have shown that the pH increase is negligible
in the Fisher iron/unbuffered DI system (Deng, 1998).
RESULTS AND DISCUSSION
The sorption isotherms for SDS and TX were also
determined after 2 weeks of exposure time (336 h).
The isotherms were essentially identical to those
SDS matrices
Figure 1 shows the sorption isotherms of SDS obtained after 24 h for both surfactants (data not
(Fig. 1(a)) and Triton X-100 (Fig. 1(b)) on the surface shown). After prolonged exposure to anaerobic water

1456
Gregory A. Loraine
the net surface charge of the cast iron would be Although there was loss due to reduction on the
expected to become increasingly negative and inhibit surface of cast iron, the reaction rates were relatively
sorption of the anionic SDS and change the isotherm. slow and quasi-steady-state conditions should have
A simplified conceptual model of the aqueous occurred (Burris et al., 1998). Plotting the mobile
surfactant–iron system consists of five phases: the phase and bound phase concentrations yielded
aqueous phase, mobile surfactant micelles, surfactant sorption isotherms. Figure 3 shows the sorption
hemimicelles or double layers on the iron surface, isotherms for PCE on cast iron in deionized (DI)
reactive binding sites on the iron surface, and non- water and four concentrations of SDS (initial
reactive graphite inclusions on the iron (Fig. 2). concentrations 0.58, 2.31, 4.61, 23.07 g/L). The
Burris et al. have shown that there is significant non- isotherms were nonlinear and were fit using the
reactive, non-linear sorption of hydrophobic organics Langmuir isotherm, see Table 1 (Sposito, 1980).
to graphite inclusions (ꢁ3.1% w/w) in cast iron Burris et al. (1995) fit TCE and PCE sorption
(Burris et al., 1998).
isotherms using the generalized Langmuir equation.
The concentrations of the chlorinated compounds The sorption of PCE on iron surfaces increased as
were measured as mobile (solubilized and micellar surfactant concentration approached the CMC then
concentrations) and bound (sorbed to iron active declined at higher surfactant concentrations.
A
sites, iron graphite, and iron-surfactant phase). similar profile was observed for TCE (Table 1).
Partitioning of organic compounds from aqueous
phase to surface-bound surfactants has been shown
to be dependent on the hydrophobicity of the
compound. As PCE is less soluble than TCE the
change in sorption of PCE is more pronounced
(solubility at 258C, PCE = 0.15 g/L, TCE =1.10 g/L,
Verschueren, 1983).
The pseudo-first-order rates of reduction of TCE
and PCE in different concentrations of SDS are given
in Table 2. Statistical analysis of the reduction rate
constants showed no significant difference between
DI water and SDS concentrations below 23.07 g/L
(t-test, P ¼ 0:05). Above 23.07 g/L the rate constants
decreased, possibly due to portioning of TCE and
PCE from the iron surface to mobile micelles formed
at this concentration (Fig. 1(a)). As can be seen from
Fig. 2. In a simple model of an aqueous-surfactant-cast iron
Table 2 and Fig. 3, addition of surfactant increased
system a sparingly soluble organic (SSO) can partition into
PCE sorption but did not result in a corresponding
five phases. The mobile phases are the aqueous fraction
increase in reaction rate. There does not appear to be
a direct relationship between the dehalogenation rate
and net surface concentration.
(SSOAqu) and the micelles. The bound phases are the
surfactant hemimicelles on the surface, graphitic inclusions
in the cast iron (3% w/w), and the metallic iron surface.
Triton X-100 matrices
Figure 4 shows the sorption isotherms of PCE in
DI water and Triton X-100 matrices. The TCE
isotherms were similar (Table 1). The surface layer
of TX continued to form until the aqueous concen-
tration reaches 1.5 g/L (Fig. 1(b)), however, the
[PCE]bound decreases with increasing nonionic sur-
factant concentration. This differed from what was
observed with SDS where [PCE]bound increased until
CMC level of SDS was reached.
The reduction rate constants of TCE and PCE in the
presence of Triton X-100 are listed in Table 3. The rate
constants for TCE reduction were statistically identical
(P ¼ 0:05) at all nonionic surfactant concentrations.
On the other hand, the PCE reduction rates increased
with surfactant concentration (rates at the two lowest
TX concentrations were not significantly different than
DI, P ¼ 0:05) and reached a maximum value at
[TX]=1.5 g/L, the concentration where maximum
[TX]bound is reached. At 1.5g/L of Triton X-100 the
Fig. 3. PCE in SDS sorption isotherm on cast iron (>40
mesh cast iron, SA=1.25 m2/g) at 208C in the dark at a
rotation speed of 8 rpm. (^) DI Water; ( ) 0.58 g/L SDS;
(
) 2.31 g/L SDS; ( ) 4.61 g/L SDS; (*) 23.1 g/L SDS.
Curves fit using Langmuir equation (outliers were deter-
mined by residual analysis and omitted from fitting),
samples were taken after 2, 24, 48, 72, 96, 168, and 240 h.

Effects of PCE and TCE in alcohols and surfactants
1457
Table 1. Langmuir coefficients for TCE and PCE sorption isotherms in SDS and TX, b¼ absorption capacity (mg/g) and K¼ equilibrium
constant (mL/mg)
TCE
PCE
(TX) g/L
b (mg/g)
K (mL/mg)
r2
b (mg/g)
K (mL/mg)
r2
0
28.5
36.4
28.4
38.6
26.8
24.3
ND
ND
0.33
0.15
0.13
0.11
0.42
0.29
NDa
ND
0.90
0.91
0.97
0.97
0.81
0.71
ND
ND
46.3
33.1
29.3
27.3
25.7
18.9
23.9
16.0
0.49
0.93
1.03
1.34
1.40
2.36
0.50
0.14
0.94
0.89
0.86
0.84
0.84
0.88
0.6
0.05
0.1
0.15
0.3
1.5
7.5
15.0
0.56
(SDS) g/L
0
0.58
1.73
2.31
b (mg/g)
28.5
30.4
33.6
31.7
K (mL/mg)
0.33
0.32
r2
b (mg/g)
46.3
76.4
91.9
123.1
66.5
K (mL/mg)
0.49
0.11
0.11
0.07
0.07
ꢀ0.04
r2
0.95
0.90
0.92
0.81
0.91
0.86
0.96
0.97
0.93
0.97
0.98
0.20
0.27
0.32
0.28
0.20
4.61
23.1
23.0
12.3
ꢀ0.3
a ND – not determined.
Table 2. Effect of SDS anionic surfactant on pseudo-first order rate
constants for reduction of TCE and PCE (total concentrations) with
zero-valent Iron. [TCE]0=36 mg/L, [PCE]0=36 mg/L, [Fe0]0=5.00
Æ 0.005 g, SA=1.25 m2/g, in 14 mL DI water or surfactant solution,
well mixed
Table 3. Effect of triton X-100 nonionic surfactant on pseudo-first
order rate constants for reduction of TCE and PCE with zero-valent
iron. [TCE]0=36 mg/L, [PCE]0=36 mg/L, [Fe0]0=5.00 Æ 0.005 g,
SA=1.25 m2/g in 14 mL DI water or surfactant solution, well mixed
TCE
PCE
TCE
PCE
Triton X-100 g/L
Rate ( Â 107/s m2)
Rate ( Â 107/s m2)
SDS g/L
Rate ( Â 107/s m2)
Rate ( Â 107/s m2)
0
16.7 Æ 0.16
15.6 Æ 0.24
14.1 Æ 1.4
14.2 Æ 0.4
12.8 Æ 0.16
13.0 Æ 0.8
NDa
4.35 Æ 0.42
4.18 Æ 0.11
4.24 Æ 0.27
5.06 Æ 0.46
5.29 Æ 0.36
6.05 Æ 0.38
5.06 Æ 0.14
3.25 Æ 0.13
0
16.7 Æ 0.2
12.4 Æ 0.8
11.4 Æ 0.04
12.1 Æ 0.6
12.6 Æ 0.96
8.8 Æ 0.3
4.35 Æ 0.42
3.62 Æ 0.80
3.32 Æ 0.13
3.81 Æ 0.78
4.00 Æ 0.96
1.55 Æ 0.9
0.05
0.1
0.15
0.3
1.5
7.5
15.0
0.58
1.73
2.31
4.61
23.1
ND
a ND – not determined.
resulting effluent would be a surfactant matrix with
PCE concentrations much higher than those possible
in water. Reduction rates of PCE by Fe0 in 1.5 g/L
Triton X-100 were determined at five initial concen-
trations (Fig. 5). Heterogeneous reactions can be
though of as three step processes, (1) adsorption-
desorption of the substrate on the reactive surface,
(2) reaction of the surface-bound substrate, (3) the
desorption of the products (Carberry, 1976). The
products of PCE and TCE reduction of Fe0 also
absorbed to the surface and could have competed
Fig. 4. PCE in Triton X-100, sorption isotherm on cast iron with the parents for active sites (Arnold and Roberts,
(>40 mesh cast iron, SA=1.25 m2/g) at 208C in the dark at
a rotation speed of 8 rpm. (^) Water; ( ) 0.05 g/L Triton
this competition may be neglected. Due to the long
X-100; (m) 0.15 g/L Triton X-100; ( ) 1.5 g/L Triton X-100;
1999). However, at high initial PCE concentrations
half-life of PCE in this system (53 h), it can be
(
) 15.0 g L Triton X-100. Curves fit using Langmuir
assumed that partitioning equilibrium conditions
were reached and the rate of the surface reaction
was the rate controlling step. Under these conditions
the data can be modeled using a Langmuir equation.
equation (outliers were determined by residual analysis
and omitted from fitting), samples were taken after 2, 24, 48,
72, 96, 168, and 216 h.
pseudo-first order rate for PCE loss is 39% greater
than in water alone. At higher TX concentrations the
rates diminish perhaps due to partitioning of PCE
from the iron surface into mobile micelles.
Were Triton X-100 to be used in soil washing
applications for a PCE contaminated aquifer, the
kÂKa½PCEŠ
d½PCEŠ
dt
Aqu
ꢀ
¼ V ¼
ð1Þ
1 þ Ka½PCEŠ
Aqu
where k is the rate of reaction per surface area and
Ka is the adsorption equilibrium constant of PCE
per gram iron. Values of k ¼ 1:07 nM/s-m2, and

1458
Gregory A. Loraine
Table 4. Pseudo first-order rate constants for TCE ([TCE]0=
22–49 mg/L) and PCE ([PCE]0=35–40 mg/L) reduction in alcohol/
water solutions, [Fe0]0=5.00 Æ 0.005 g, SA=1.25 m2/g, in 14 mL DI
water or alcohol solution, well mixed at 208C
TCE
PCE
Sample
Rate ( Â 107/s m)
Rate ( Â 107/s m)
DI
16.7 Æ 0.16
1.42 Æ 0.16
0.97 Æ 0.16
1.82 Æ 0.11
15.3 Æ 0.01
0.005 Æ 0.016
4.35 Æ 0.42
0.46 Æ 0.02
0.46 Æ 0.16
0.47 Æ 0.03
NDa
MeOH (57%)
IPA (57%)
EtOH (57%)
EtOH (28%)
EtOH (100%)
ND
a NC – not determined.
Fig. 5. Langmuir–Hinshelwood plot of PCE reduction by
Fe0 in Triton X-100 (1.5 g/L). The surfactant allows
increased loading of PCE in the system and more PCE is
degraded per gram of iron than is possible in water at this
solid/liquid ratio. However, even in the presence of
surfactant, solubility limits the maximum loading before
the active sites are saturated.
of PCE reduced in DI and 1.5 g/L TX (mass balance
for DI=77%, TX=79%). The yield of TCE is higher
in TX than in DI, and the production of ethene,
ethane, and DCE is approximately equal. Fractiona-
tion of TCE and PCE to the surface may have
affected the relative aqueous concentrations. In
addition, TCE was reacting as it was formed and
was in competition for active surface sites with PCE
and other products. Arnold and Roberts (1999) used
a modification of the Langmuir–Hinshelwood–Hou-
gan–Watson (LHHW) equation to model the forma-
tion of reactive products from parallel reactions on
zero-valent iron. This same method was used to
estimate the true yield of TCE.
d½TCEŠ
¼ YÂkP½PCEŠ
Aqu
dt
KaT½TCEŠ
Aqu
P
ꢀ kT½TCEŠ
ð2Þ
Aqu
1 þ KaiCi
where Y is the fraction of PCE going to TCE, kP and
kT are the measured pseudo-first order reaction rates
of PCE and TCE, respectively, KaT is the adsorption
coefficient of TCE from the Langmuir isotherm
(Table 1), and SKaiCi is the sum of the concentra-
tions times adsorption coefficient of the parent and
all the products. Since the major products were TCE,
ethene, and ethane only PCE and TCE were
considered, this might have resulted in an over-
estimation of Y. However, this method was accurate
enough to demonstrate the significant difference in
TCE yield between DI and TX.
Fig. 6. Product yields of PCE reduction by zero-valent iron
in DI (a) and [TX]=1.5 g/L (b). (^) Ethene; (&) Ethane;
(m) TCE, (*) cis-DCE; ( ) 1,1-DCE.
Ka ¼ 0:24 m/M were determined. This equation was
used to describe the system empirically and cannot be
taken as proof of the assumptions made above.
Solubility constraints of PCE in 1.5 g/L TX pre-
vented determining the reduction rates at higher
initial PCE concentrations.
Product analysis of PCE reduction in both Triton
X-100 and in DI water showed that the major
products were TCE, ethene, and ethane. Minor
products were methane, cis-DCE, 1,1-DCE, and
d½TCEŠ
¼ YÂkP½PCEŠAqu ꢀ kT½TCEŠ
Aqu
dt
KT½TCEŠ
Aqu
Â
1 þ KP½PCEŠAqu þ KT½TCEŠ
Aqu
ð3Þ
acetylene. Trace amounts of trans-DCE were de- Table 5 lists the values of Y fitted by Eq. (3) to data
tected. Vinyl chloride and chloroacetylene concen- from five matrices, DI, [TX]=0.15 g/L, [TX]=1.5 g/
trations were below detection limits. The only L, [SDS]=1.7 g/L, [SDS]=4.6 g/L (product concen-
significant difference between DI and the surfactant trations from the alcohol experiments were at or
solutions was the amount of TCE that built up in the below detection limits and were not modeled). The
system. Figure 6 shows the yield of products per mole presence of SDS did not affect the formation of TCE

Effects of PCE and TCE in alcohols and surfactants
1459
Table 5. Fraction of PCE going to TCE estimated using Eq. (3). The
fraction of TCE formed in SDS did not differ significantly from DI
water. Higher concentrations [TX]bound (Fig. 1(b)) corresponded to
higher yields of TCE from PCE reduction
surfaces (Campbell et al., 1997). It may be that in a
Triton X-100 hemimicelle the hydrogenolysis path-
way was favored over b-elimination. The TX may
have acted as a H donor. Arnold and Roberts (1999)
reported that hydrogenolysis is more important in
PCE reduction than in TCE reduction. This is one
possible explanation for why reduction of PCE is
enhanced by TX and TCE is not. If the TX matrix
favors hydrogenolysis then the formation of vinyl
chloride would also be enhanced. Vinyl chloride is of
concern due to its greater toxicity. If the hydro-
genolysis pathway was favored then more DCE and
VC should have been formed. No Vinyl chloride was
detected in these experiments (detection limit
>4 Â 10ꢀ9 M). Perhaps the reduction rates of DCE
and VC in TX are also faster. Verification of this
hypothesis has yet to be obtained.
Matrix
Est. TCE yield
r2 TCE
DI
0.04 Æ 0.04
0.09 Æ 0.01
0.22 Æ 0.02
0.02 Æ 0.01
0.04 Æ 0.02
0.79
0.94
0.99
0.56
0.42
[TX]=0.15 g/L
[TX]=1.5 g/L
[SDS]=1.7 g/L
[SDS]=4.6 g/L
Alcohol matrices
Pseudo-first-order rate constants for the reduction
of TCE and PCE by metallic iron in DI water,
MeOH (57% v/v), IPA (57% v/v), and EtOH (28, 57,
and 100% v/v) are shown in Table 4. The dehalo-
genation rates varied with the type and concentration
of alcohol. None of the alcoholic solutions tested
improved the removal rate of either TCE or PCE.
However, the lowest concentration of ethanol used
(28%) did not significantly inhibit TCE dehalogena-
tion. Application of zero-valent iron permeable
barriers with ethanol soil washing may be practical
if TCE could be efficiently solubilized at relatively
low EtOH concentrations. Surface sorption of TCE
and PCE in alcoholic matrices was inhibited even in
28% EtOH (data not shown).
Fig. 7. Estimation of yield of TCE using LHHW equation
using data from Fig. 6. DI: (&) PCEDI; (&) TCEDI, (yield)
0.042 Æ 0.042, (rT2 CE) 0.79. TX=0.15 g L: (*) PCE0.15, (*)
TCE0.15, (yield) 0.09, (r2TCE) 0.94. TX=1.5 g L: (m) PCE1.5
(4) TCE1.5, (yield) 0.224 Æ 0.017, (r2TCE) 0.99.
;
CONCLUSIONS
relative to DI. The estimated formation of TCE
increased with increasing [TX] ([TX]=0.15 g/L
Increasing the concentrations of PCE/TCE near
Y ¼ 0:09, [TX]=1.5 g/L Y ¼ 0:22). Data from ex- the iron surface by sorption into surfactant layers
periments with DI, [TX]=0.15 g/L, [TX]0=1.5 g/L, was insufficient to increase reduction. The reduction
and the fitted lines are shown in Fig. 7. The observed rates observed in SDS remained low in spite of
enhancement in aqueous TCE concentration cannot increased [PCE]bound and [TCE]bound. Triton X-100
be attributed to the differences in absorption of T enhanced PCE reduction by as much as 40%,
CE/PCE in DI and TX. This implies that TX dependent on the surfactant concentration. The
influences the reaction mechanism of PCE.
correlation between [TX]bound and reduction rate
Hydrogenation reactions catalyzed by heteroge- suggests that Triton X-100 hemimicelles on iron may
neous catalysts are greatly affected by the solvent have acted as a reactive phase for PCE reduction. As
(Rylander, 1980). The solvent can competitively bind the surface coverage of the surfactant increased the
to active sites, change hydrogen availability, di- reduction rate of PCE increased. It was only when
electric constant or reactant solubility. Changes TX concentrations reached a level where unbound
in reaction rates and product yields in catalytic micelles are formed that the rates declined. The TX
hydrogenations are often manipulated by means of affected product yields as well as rates. The observed
changing the solvent. The surfactant hemimicelles on yield of TCE from PCE reduction increased as [TX]
the iron surface provides a solvent phase very increased, which may be a potential drawback in the
different than water. Thus, changes in product yield application of this technique in the field.
and reaction rates are not unexpected.
The difference in the reduction rates of PCE in
Previous work has shown that both TCE and PCE SDS and TX may be due to electrostatic repulsion
undergo concerted two electron b-elimination and hindering electron transfer into anionic SDS surface
hydrogenolysis via stepwise electron addition on iron aggregates (Horvath and Stevenson, 1999). All of the

1460
Gregory A. Loraine
alcohol solutions tested decreased reduction rates,
probably due to inhibited mass transfer to surface
active sites.
ganic contaminants from within cationic surfactant
enhanced sorbent zones. 1. Experiments. Environ. Sci.
Technol. 31, 1277.
Horvath O. and Stevenson K. L. (1999) Micellar effects
on photoinduced redox reactions of Fe(bpy)2(CN)2.
J. Photochem. Photobio. A. 120, 185.
Acknowledgements}This work was supported by a Na-
tional Research Council Resident Research Associateship
provided by the Air Force Office of Scientific Research.
Jawitz J. W., Annable M. D., Rao P. S. C. and Rhue R. D.
(1998) Field implementation of a winsor type 1 surfac-
tant/alcohol mixture for in situ solubilization of
complex LNAPL as single-phase microemulsion.
Environ. Sci. Technol. 32, 523.
a
a
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