dures were also similar to those of the slurry experiments.
Samples were prepared in triplicate in the anaerobic chamber.
To remove oxygen sorbed on the soil surfaces, the vials
containing the soil were incubated in the chamber for 2 days.
The mass ratio of soil:solution was 1.29. An aliquot of 0.5 N
KOH solution was added to slurries to maintain pH of the
samples at 12.1, which was selected as a target pH in the
solid-phase experiments because PCE degradation reactions
were optimal at pH near 12.1 in the previous study (3). The
headspace volume of the reactor was approximately 0.78
mL, which would render the partitioning of ∼2.7% of PCE
into the gas phase assuming no sorption. In the sorption
kinetic experiment, the initial aqueous phase concentration
of PCE was 60 mg/ L. In the isotherm experiment, samples
with seven different initial PCE concentrations (6.25, 12.5,
25, 52.5, 105, 168, 210 mg/ L) were prepared. The equilibration
time for the isotherm experiments was 7 days, which was
based on results of a kinetic experiment that showed little
change in PCE concentration after 1 day and as long as 12
days. This is consistent with reports that sorption of PCE and
TCE to aquifer sandstones was completed within 1 day (18).
After PCE spiking, the vials were placed on the tumbler
described above. At each sampling time, the vials were
centrifuged at 873 g for 30 min, and an aliquot of the aqueous
phase was extracted for PCE as described previously.
Solid-Phase Degradation Experim ents. Solid-phase
samples were also prepared in the anaerobic chamber by
using the same glass vials used for the slurry experiments.
All reactive samples were prepared in triplicate and controls
in duplicate. The vials containing solids (cement, soil) and
two stainless steel balls of 79 mm diameter were incubated
in the chamber for 2 days and were then filled with
appropriate amounts of the water, Fe(II) stock solution, and
the 5.3 N KOH or 5 N HCl solution, if needed. The mass ratio
of soil:cement:solution was 62.5:7.5:30, which produced a
moderately soft solid that was needed to facilitate crushing,
and extraction of the target compounds. The acid or base
solution was added to maintain pH of the porewater of solid-
phase samples near 12.1. The solid mixtures were then
blended with a stainless steel spatula. Reactions were started
by spiking 10 µL of methanolic stock solutions of PCE into
solid mixtures to yield a PCE concentration of 100 mg/ kg
soil. After PCE spiking, samples were placed on the tumbler
for 2 days. The stainless steel balls in the vials facilitated
initial homogenization of the samples when the vials were
rotated by the tumbler. The samples were then taken from
the tumbler and were cured at the room temperature (19.3
( 0.7 °C).
TABLE 1. Chemical Characteristics of PCE
dimensionless
aqueous
solubilityc
(mg/L)
CAS
registry no.
Henry’s law
b
MW
constanta
log K
ow
127-18-4
165.83
0.525
2.60
149
a
b
c
Reference 9. Reference 10. Reference 11.
TABLE 2. Physical and Chemical Characteristics of the Silawa
Soil
specific surface
areaa (m2/g)
organic
C (%)
Fe(II)c
mg/g)
Fe(III)c
(mg/g)
b
d
pH
13.0
0.69
0.498
5.757
6.1
a
Determ ined by EGME (ethylene glycol m onoethyl ether) m ethod
b
c
(13). Determ ined by dry com bustion m ethod (14). Determ ined by
d
1,10-phenanthroline m ethod (15, 16) after acid extraction. The soil
was equilibrated with water with a soil:water ratio of 1:1 (14).
chemical characteristics, which are presented in Table 2.
The sand, silt, and clay fractions of the soil were 81.1%, 12.2%,
and 6.7%, respectively.
Aqueous stock solutions of Fe(II), (FeCl2), and humic acid
(sodium salt, Aldrich) were prepared using the deoxygenated
deionized water in an anaerobic chamber containing 95%
N2/ 5% H2 (Coy Laboratory Products). The deoxygenated
deionized water (water hereafter) was prepared by sparging
deionized water with the chamber atmosphere for at least
12 h. The stock solution of the humic acid was prepared by
adding 60 g of the humic acid in 1 L of the water, which
yielded a TOC concentration of 23 500 mg / L. Aldrich humic
acid is reported to contain 7.5% of ash (17). Stock solutions
of PCE were prepared in methanol daily.
Slurry Degradation Experim ents with Hum ic Acid. Clear
borosilicate glass vials (24.2 ( 0.10 mL) with the triple-seals
(3) were used as batch slurry reactors. Samples were prepared
in the anaerobic chamber as described below. Triplicate
samples were prepared for reactive experiments and dupli-
cates for controls. Controls contained PCE and water. The
mass ratio of cement:solution was 0.1. Slurry samples were
prepared by filling the vials with appropriate aliquots of the
water and stock solutions of humic acid and/ or Fe(II). The
dose of Fe(II) was 39.2 mM and that of humic acid 500 mg
TOC/ L. Headspace volumes of the vials were minimized (0.3-
0.6 mL), which would allow less than 1.6% of PCE partitioning
into the gas phase assuming no sorption and a dimensionless
Henry’s law constant of 0.525 (9) which is appropriate for
the temperature (19.3 ( 0.7 °C) at which experiments were
conducted. pH values of slurries were maintained at 12.6 (
0.05. To control the pH of slurries without cement, an aliquot
of deoxygenated 5.3 N KOH solution was added to the slurries
when they were prepared. Reactions were initiated by rapidly
introducing 10 µL PCE stock solution into the slurries, and
the vials were capped immediately. The initial concentration
of PCE was 40.6 mg/ L. The vials were then mounted on a
tumbler that provided end-over-end rotation at 7 rpm. To
rule out photochemical effects, the vial containers were
covered with aluminum foil.
At each sampling time, samples were extracted for PCE
and its degradation products in 250-mL high density
polyethylene bottles with fluorinated surface (Nalgene). While
extracted, the sample vials were firmly held onto the bottom
of the bottles by using a cement base. To extract the target
organics, the closures of the vials were removed rapidly, and
the solid-phase samples were crushed with a hammer drill
under layers of water (100 mL) and pentane (40 mL). The
time required to crush the samples did not exceed 1 min.
The bottles were then capped and shaken for 6 h at 250 rpm.
Then pentane layers were analyzed for PCE and its degrada-
tion products. pH of the porewater of the solid-phase samples
was measured when the samples underwent approximately
25, 50, and 75% of PCE degradation reactions. A porewater
expression device (19) was used to extract the porewater
from the samples.
Duplicate or triplicate samples were sacrificed at every
sampling time for the analysis of PCE and its daughter
products. The vials were retrieved from the tumbler and were
centrifuged at 582 g for 2 min. Then ∼5 mL of aqueous
samples were withdrawn from the vials and were then
extracted with 5 mL of a pentane extractant containing
toluene as an internal standard in 20-mL glass vials.
PCE Sorption Experim ents. For sorption experiments,
the same reactor system used for the slurry degradation
experiments was used, and the sample preparation proce-
Analytical Methods. PCE and its chlorinated degradation
products, TCE, 1,1-DCE (dichloroethylene), c-DCE, t-DCE,
vinyl chloride, dichloroacetylene, and chloroacetylene were
analyzed by a Hewlett-Packard G1800A GCD. The analytical
methods were identical to those described in the previous
paper (3). Concentration of the humic acid was determined
9
VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 3 7 9 3