United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada OK 74820
Research and Development
EPA/600/S6-89/002 Sept. 1989
AEPA Project Summary
Assessment of the Potential for
Transport of Dioxins and
Codisposed Materials to
Groundwater
Richard W. Walters, Zohreh Yousefi, Amy L. Tarleton, Stanley A. Ostazeski,
and David C. Barry
Parameters relevant to the sorptive
transport of polychlorodibenzo-p-
dioxins (PCDDs) through soils were
evaluated In laboratory experiments
involving batch snake testing and sat-
urated-flow soil column techniques.
The experiments were conducted
using water/methanol mixtures and
four uncontaminated sorbents (two
surface soils from Times Beach, MO
and aquifer materials from Traverse
City, Ml and Lula, OK). Five "re-
labeled PCOO cogeners, including
2,3,7,8-tetra (T4COO), 1,2,3,4,7-penta,
1,2,3,4,7,8-hexa, 1,2,3,4,6,7,8-hepta,
and octa (O8CDD), and three codls-
posed materials pentachlorophenol
(POP), and chloroberusene (CB) were
used. The partition coefficient (KD)
for sorptien of T«CDD from water was
found to be in good agreement with
the water-phase K0 predicted by log-
linear extrapolation according to the
cosolvent theory by using KD data
generated) for water/methanol mix-
tures. This observation validates the
use of'log-linear extrapolation to esti-
mate water-phase KD values for
PCDDs using cosolvent data. KD
values for sorption of PCDDs by sur-
face soils at volume fraction solvent
(fs) of 0.5-0.9 were reduced by a
factor of up to 2.5 when PCP or CB
were present Reductions in KD for
PCDDs in the presence of PCP and
CB increased with decreasing fs to
enable a better understanding of the
influence of codisposed materials
such as PCP and CB on the mobility
of PCDDs under environmental con-
ditions typified by low f, (i.e., f,
< < 0.5). Sorption KD values for the
aquifer materials were normalized on
organic carbon content (f^) to yield
values of KQC which were in general
agreement with Koc values deter-
mined for the surface soils. This ob-
servation suggests that sorption by
the organic matter content or the
aquifer materials was sufficient to
mask sorption to mineral surfaces.
Desorption of PCOOs from the sur-
face soils appeared to be reversible
but was limited by kinetics, with
roughly 50-90% of reversible-desorp-
tion equilibrium being attained within
a contact period of 30-50 days.
This Project Summary was de-
veloped lay EPA's Robert S. Kerr En-
vironmental Research Laboratory, Ada,
OK, to announce key findings of the
research prefect that is fully docu-
mented in a separate report of the
same title (see Protect Report ord-
ering information at back).
Introduction
Polychlorodibenzo-p-dioxins as en-
vironmental contaminants have perhaps
received most widespread national at-
tention because they were present in
waste oils that were used for dust control
in Missouri. PCDD-contamination of soils
and sediments in various locations
throughout the United States and in other
countries have been documented.
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Concentrations of PCDDs in soil and
sediment samples as high as 1600 ppb
to 100 ppm have been observed.
Until recently it was believed that
PCDDs were relatively immobile in soils
because of their low water solubilities,
and that there was little potential for
PCDDs to be leached from contaminated
soils. Field observations have shown that
PCDDs move faster through soils than
would be expected based on their water
solubilities. Numerous theories have been
proposed to explain the "facilitated
transport" of PCDDs and other highly
hydrophobic organic contaminants, in-
cluding a cosolvent which accounts for
enhanced mobility in terms of the
reduction in soil sorption in the presence
of cosolvents.
The research presented in the full re-
port involved three basic objectives; 1)
determine the relative effect of the
presence of codisposed materials on the
sorptive transport of PCDDs through
surface soils, 2) evaluate the equilibrium
sorption of PCDDs by aquifer materials,
and 3) evaluate the kinetics of desorption
of PCDDs from surface soils.
Procedure
Solutes
All PCDDs used in the experiments
were radiolabeled with C-14. Individual
stock solutions of the radiolabeled
PCDDs were prepared by dissolving
them in methanol.
Other radiolabeled compounds used in
the experiments included pentachloro-
phenol (PCP), methanol and water. All
radiolabeled compounds were used as
received from the supplier.
So//s and Aquifer Materials
The soils obtained from Times Beach,
MO were the same as those used in pre-
vious investigations of the sorption of
PCDDs to soils. These soils were air
dried, sieved through 0.3 mm standard
sieves, and characterized for pH, cation
exchange capacity, organic matter con-
tent, organic carbon, organic nitrogen and
texture. The soils are referred to here as
soil 91 (low-organic carbon soil) and soil
96 (high-organic carbon soil).
Lula, OK and Traverse City, Ml aquifer
materials were provided by the EPA
Project Officer and were used as
received.
Solvents
Methanol and methylene chloride sol-
vents used in the experiments described
were pesticide grade and were used as
received from the supplier. Water was
treated by reverse osmosis, activated
carbon bed and a pair of mixed bed
deionizers. Further, the water was dosed
with 0.01% NaN3, as a biocide and its
ionic strength was adjusted to 0.01 M
using CaCI2. The pH of the water was
adjusted to 7.0 using NaOH.
Scintillation Cocktail
An insta-gel liquid scintillation cocktail
obtained from United Technologies was
used for all radioisotope analyses.
Analyses
Liquid Scintillation Counting
Analytical determinations were made
by liquid scintillation counting (LSC)
using a Model 1219 Rackbeta counter.
Liquid-phase solute concentrations (C
or Ce) were determined by sampling
three aliquots of 1-3 ml of liquid phase
and adding the sample to 10 ml of Insta-
gel cocktail in 20-ml glass counting vials.
Soil-phase solute concentrations (S of
Se) were determined primarily by the dif-
ference of the total solute minus liquid-
phase solute. However, to confirm mass
balances direct determination of soil-
phase solute concentrations was made to
determine mass balances.
Data Manipulation
Isotherm data were evaluated by linear
regression to determine best-fit param-
eters for sorption constants. Best-fit
estimates of KD, the coefficient of de-
termination (r2), and the standard error of
the estimate (s) were calculated using
standard equations.
Values of Kobs were determined from
the effluent concentration profiles ob-
tained from saturated-flow soil column
experiments. Kobs is the value of KD
calculated from the proper equation
under the assumption of local equilibrium
and a value of Vr equal to the ratio of
sorbing solute to nonsorbing solute re-
tention time as determined by the center-
of-mass of the respective effluent peaks.
Least-squares regression analysis was
used to determine the best-fit estimates
of the slope (m) and intercept (b) for
proposed cosolvent equations. The
values of m and b and the regression sta-
tistics were determined by using
standard techniques.
Desorption data were regressed ac-
cording to empirical exponential and
power-curve models.
Solubility Determinations
Batch techniques already described to
evaluate T4CDD solubility in water/
methanol mixtures were used to evaluate
the solubilities of the other PCDDs. E>
periments were conducted in 1-ml glas
microvials. PCDD crystals were pre
coated onto the inner wail near th
bottom of vial by delivering a workin
solution containing PCDD and the
evaporating the solvent with nitrogei
One ml of liquid phase was then added t
the precoated microvial, the vials wet
capped and covered with aluminum fo
and then placed on a shaking table. Th
vials were shaken daily for 15 minutes ft
contact periods up to 28 days.
Batch Shake Testing
The batch shake testing techniqu
which was used to generate data pn
sented in this report was based on pul
lished procedures to evaluate sorptic
and desorption of PCDDs to/from soils.
Experiments Involving
Water/Methanol Mixtures
Batch experiments involving wate
methanol mixtures were conducted in 1
ml conical glass centrifuge tubes.
Tubes were dosed by direct addition
soil or aquifer material (0.2-1.0 gm), liqu
phase and solute (delivered by dire
dosing of stock solutions). A liquid-pha:
volume of 12-ml was added to the tub<
except for experiments involving PCP <
solute, in which case the tubes were filk
completely with liquid phase.
Experiments conducted to evaluate tl
sorption of PCDDs in the presence
PCP and CB were performed at co
centrations from 1-10% of solubility.
After dosing, the tubes were place
horizontally on a shaking table and we
shaken for 15 minutes every hoi
Contact periods ranged from 2 hr to 30
and varied according to the type of e
periment (kinetic/equilibrium) and tl
soil-solute-liquid phase system und
study. Following contact, tubes were r
moved from the shaking table ai
centrifuged at 2,000 x g for 10 minute
to achieve adequate separation of liqi
and solid phases.
Water-Phase Experiments
Two batch procedures were utilized
generate isotherm data for the sorption
T4CDD from water by soil 91. Both s<
of experiments were conducted using
50-ml round bottom glass centrifu
tubes fitted with Teflon-lined screw caj
Soil and water doses to the tubes we
approximately 50 mg and 40 ml.
PCP Screening Experiments
Preliminary experiments were cc
ducted to validate mass balances and I
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centrifugation procedure used to evaluate
sorption of PCP.
Using general batch shake test pro-
cedures mass balance experiments were
conducted with and without soil to assess
recovery of PCP.
Kinetic Evaluations
Time series experiments involving
sorption of PCP from water by soils 91
and 96 and sorption of P5CDD and
OgCDD by the two aquifer materials were
conducted to determine the contact time
necessary to attain sorption equilibrium
and to qualitatively evaluate sorption
kinetics.
Experiments involving PCP were con-
ducted by using 0.3 g of soil and a PCP
dose such that initial concentration in the
liquid phase was 0.035 ng/ml. Liquid
volume in these experiments was 15-17
ml as necessary to completely fill the
centrifuge tube in order to minimize
volatile losses of PCP into the head-
space. Tubes were contacted for various
times and prepared in triplicate for each
contact periods.
Experiments involving aquifer material
utilized contact periods of 2-72 hr. These
experiments were performed at a fs of
0.65 for 08CDD. P5CDD and 08CDD
were chosen on the basis of known
kinetic data for surface soils which
indicated that these PCDDs bound the
kinetic behavior of all PCDDs studied.
Saturated-Flow Soil Column
Testing
Saturated-flow soil column testing was
performed to validate the use of the
column techniques for estimating the KD
of the PCDDs. These column techniques
were then applied to study the sorption of
PCDDs in the presence of PCP and CB.
These soil column experiments used
either a glass column (2.5 cm x 25 cm)
fitted with an adjustable plunger, to ac-
commodate variable soil masses in the
column, or a stainless steel column (0.48
cm x 10 cm).
Column Packing
The glass column was slurry packed
by adding a known mass of soil (15-25 g)
to roughly 20 ml of liquid phase and
slowly pouring portions of the slurry into
the top of the column. Solvent was al-
lowed to flow from the column by gravity,
and small amounts of slurry or fresh
solvent were continuously added to the
top of the column to maintain a liquid
level above the soil at all times.
The steel column was dry packed with
a known mass of soil (2-3g) with the aid
of a vacuum pump.
Pore Volume Determination
Carbon 14 labeled methanol and 3H
labeled water were used as inert tracers
for determination of the hydraulic deten-
tion volume (HDV) of the soil column.
Solute Retention Experiments
Pulsed input of the radiolabeled solute
similar to that used to evaluate column
HDV was used in column retardation ex-
periments. A plot of effluent sample con-
centration versus cumulative effluent
volume was prepared, and the solute ret-
ention volume was determined from the
center-of-mass of the effluent solute
peak.
Results and Discussion
Solubility Determinations
The results of experiments to deter-
mine the solubility of P5CDD, H6CDD,
H7CDD and OaCDD in methanol are sum-
marized m Table 1.
Values of os determined from the
solubility data for each of the PCDDs and
for PCP are also listed in Table 1. These
values represent the slope of the log-
linear relationship between the log of the
mole fraction solubility and the volume
fraction cosolvent.
Sorption to Surface Soils
The time series experiments suggest
that equilibrium for sorption of PCP to
soil 91 was achieved in less than one day
of contact, while a 30-day contact period
was necessary for equilibrium when soil
96 was used.
Table 1. Summary of Results for Solubility Determinations for PCDDs and PCP in WatertMethanol Mixtures
Measured Solubility, mg/L as
f*
T4CDD
Mean
St. Dev.
n
P5CDD
Mean
St. Dev.
n
H6CDD
Mean
St. Dev.
n
H7CDD
Mean
St. Dev.
n
OgCDD
Mean
St. Dev.
n
PCP
Mean
St. Dev.
n
0.50 1.0
10.6
0.5
33
0.10 52
0.2 2
18 6
20
2
15
24
2
12
4.0
0.6
56
3. 1x103 180xl03
0.6
9
6.25
6.09
7.01
7.35
7.35
3.8
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Table 2.
Summary of Isotherm Parameters Determined for Sorption of PCP by Soils 91 and 96
Volume Fraction Methanol, fa
0.0
0.25«
0.25«
0.5
0.75
Soil 91
KD, mUg
r*
n
pH*>
Km,oc, mollg
log (Km.0c)
Soil 96
KD, mUg
'2
n
pH«>
«m.oc, mol/g
log (Km,oc)
Molar Volume, mUmol
18
0.999
12
7.0
150
2.18
180
0.96
14
6.6
130
2.11
18.0
4.5
0.93
13
6.5
33
1.52
96
0.98
15
6.3
61
1.78
20.5
5.0
0.93
14
6.9
37
1.57
20.5
1.1
0.69
7
6.5
6.9
0.84
12
0.98
15
6.2
6.5
0.87
24.7
2.0
0.60
8
6.0
0.87
-0.06
29.8
aReplicate isotherms for soil 91 at fs of 0.25 were determined for contact periods of 2 days and 4 days,
respectively.
bpH is the value of the pH in solution at the end of the contract period.
Table 3. Summary of Kobs Values Determined for Sorption of P5CDD, H6CDD, H7CDD and O8CDD by Soil
91 from Water/Methanol Mixtures
Volume Fraction Methanol, fs
0.75a
0.9"
Molar Volume, mUmol
P5CDD
Kobs, mUg
Km.oc. mollg
log (Km,oc)
H6CDD
Kobs, mUg
Km oc, mollg
log (Km,oc)
Kobs, mUg
Km,oc, mollg
log (Xmoc)
OgCDD
Kobs, mUg
Km oc, mollg
log (KmtOC)
29.8
1.7
8.6
0.93
3.6
18
1.3
4.8
24
1.38
37.9
0.20
0.79
-0.10
0.23
0.91
-0.04
0.40
1.6
0.20
0.82
3.2
0.51
40.4
0.072,0.057
0.27, 0.27
-0.57, -0.68
0.70, 0.092
0.38, 0.35
-0.42, -0.46
0.72, 0.77
0.45, 0.47
-0.35, -0.39
0.27, 0.79
0.79, 0.77
-0.70,-G 75
Experiments were conducted at a column flow rate of 8 mL/hr (pore velocity of 73.5 m/day), with ps of
1.4 g/mL and e of 0.79.
^Experiments were conducted at a column flow rate of 8 mUhr (pore velocity of 0.77 m/day), with pB of
1.8 g/mL and £ of 0.51.
^Experiments were conducted at column flow rates of 12 mLlhr and 20 mLJhr (pore velocities of 1.78 and 2.96
m/day), respectively, with pB of 1.75 g/mL and e of 0.33.
Equilibrium isotherm data for the sorp-
tion of PCP by soils were generated by
batch techniques at fs of 0.0, 0.25 and 0.5
and for soil 96 at fs of 0.75. The results of
regression analysis of the isotherm data
obtained for all fs studied are summarized
in Table 2.
The Kobs values for sorption of PCP by
soil 91 were also estimated from column
experiments.
The log (Kmoc) values for sorption of
PCP by soils 91 and 96 plotted against fs
agree with the results obtained from
batch and column techniques over the
entire range in fs studied.
Sorption of T4CDD from Water
The value of log (K^.) determined from
the experiment involving unwashed soil
and a contact period of 48 hours w£
6.44, while the value of log (K0
determined from the experiment involvin
prewashed soils and a contact period
ten days was 6.66.
The value of log (Koc) of 6.67 dete
mined by the second procedure co
responds to a value of log (Km oc) of 5.4
The experimental value of the wate
phase partition coefficient is in goc
-------
agreement with the value predicted by
log-linear extrapolation of the water/
methanol data, namely 5.30. Based on
this observation, it appears that for
T4CDD the cosolvent theory applies over
the entire range of fs, 0.0-1.0 for water/
methanol mixtures. This provides strong
support for the validity of this theory to
PCDDs and other highly hydrophobic or-
ganic contaminants, and also provides
support for the use of log-linear extrapo-
lation to estimate water-phase K^. values
for PCDDs by using data generated from
water/methanol mixtures.
Sorption of PCDDs from Mixtures
of Water and Methanol
Linear sorption isotherms were ob-
served for all PCDDs studied. The KD
values for the four PCDDs are generally
comparable (ranging from 1.8-3.8 ml/g)
and do not show the expected trend of
increasing with increasing hydrophobicity
of the PCDD.
The results of column studies used in
sorption experiments are presented in
Table 3.
The values of KD and Kobs determined
for the PCDDs by batch and column
techniques, respectively were normalized
on foc converted to molar units by
dividing by liquid-phase molar volume.
The results of the batch experiments de-
termined in the present study are in
general agreement with the log-linear re-
lationship based on the data of other
investigators. The results obtained by
column techniques were consistently
lower than expected from the log-linear
relationship of the batch data only.
The relatively low values of Kobs in
comparison to KD were believed to be
attributed to non-attainment of local
equilibrium within the soil column for the
flow rates utilized.
Sorption of PCDDs in the
Presence of PGP and CB
The effect of the presence of PCP on
the sorption of PCDDs was evaluated by
batch and column techniques.
Batch experiments were conducted to
generate sorption isotherm data for
08CDD with both soils at PCP con-
centrations of 25 and 200 ppm. Contact
periods of 2 and 36 days were utilized for
soils 91 and 96, respectively. For both
soils and both PCP concentrations, the
sorption of 08CDD in the presence of
PCP was roughly 30% of the sorption of
08CDD in the absence of PCP.
Column experiments were conducted
for P5CDD, H6CDD and H7CDD using soil
)1, with three liquid phases; 1) fs of 0.75,
2) fs of 0.75 and a PCP concentration of
1000 ppm, and 3) fs of 0.75 and a CB
concentration of 10 ml/L. For each PCDD
studied, the lowest Kobs values were
observed for the solvent/CB system. The
K(,bs for both solvent/CB and solvent/PCP
systems were consistently below Kobs for
the solvent only. These normalized
values show that PCDD sorption
generally decreased by 25% in the
presence of PCP and 50% in the
presence of CB.
Column experiments were also con-
ducted for all PCDDs at fs of 0.9 with and
without PCP present. The results for all
PCDDs indicate that there was generally
no significant change in Kobs in the
presence of PCP relative to values in the
absence of PCP at a fs value of 0.9.
The reduced sorption of PCDDs in the
presence of cosolutes may be explained
by a number of factors, including the
effect of the cosolute on the solubility of
PCDDs and/or the effect of the cosolute
on PCDD-soil organic matter interactions.
However, it is suspected that the reduced
sorption of PCDDs in the presence of the
cosolutes may be explained in terms of
either the effect of soil-phase cosolute on
the soil-phase activity coefficients of the
PCDDs or the results due to the
competition between the sorbed
cosolutes and PCDDs. This speculation
would support the observation that the
effects become more pronounced at
lower fs because greater soil-phase
concentrations of the cosolutes would be
expected via sorption as fs decreases.
Sorption by Aquifer Materials
Linear isotherms were observed for all
PCDDs for fs values of 0.25 and 0.65 with
the exception of T4CDD and O8CDD.
Comparison of the experimental data
points for the aquifer materials with the
data for the surface soils, and their cor-
responding regression equations, in-
dicates that values of log (Km oc) for both
sorbents are well described by a single
curve for T4CDD, H6CDD and H7CDD.
For P5CDD, the experimental values of
lOQ (Km,oc) f°r the aquifer materials at
each fs' are significantly greater than
those determined from surface soil. For
O8CDD, the experimental values of log
(Km.oc) foe the aquifer materials are
slightly below those predicted for the
surface soils.
The general agreement between values
of log (Km oc) for the aquifer materials and
the surface soils for PCDDs suggests that
foc alone is the dominant factor
determining the extent of sorption for
highly hydrophobic organic contaminants
such as PCDDs, presumably because the
organic sorption is sufficiently strong to
dominate the weaker sorption to minerals.
Desorption From Surface Soils
From limited data, it appears that the
sorption of PCDDs from the surface soils
into water/methanol mixtures may be
reversible, but that desorption is char-
acterized by very low rates.
Preliminary experiments conducted to
assess batch and column techniques for
the study of sorptive transport of PCDDs
through soils in the presence of diesel
fuel, were found to be infeasible, owing to
the formation of multiple phases. These
experiments suggested that relatively
stable water/methanol/diesel fuel emul-
sions were readily transported through
the soil column. Further, although the soil
column had been apparently flushed of
diesel fuel at a high relative velocity, it
was observed that the column had a
residual capacity to retain significant
amounts of diesel fuel which was
released from the column as a stable
emulsion at low liquid-phase flow rates.
Conclusions
The sorption and desorption of PCDDs
by surface soils and the sorption of
PCDDs by aquifer materials has been in-
vestigated by using batch shake testing
and saturated-flow soil column tech-
niques. Experiments were conducted
using water and water/methanol mixtures
and two model codisposed materials
(pentachlorophenol (PCP) and chloroben-
zene (CB)).
Sorption isotherm data were generated
by batch techniques to evaluate the sorp-
tion of T4CDD from water by soil 91. A
contact period of 10 days, and a water
prewash procedure which was used to re-
move nonseparable suspended particles
(NSP) from the water phase, appeared to
be adequate for evaluating an equilibrium
KD value which was not biased by the
inadvertent sampling of NSP during liquid
phase analysis. A linear sorption isotherm
was obtained which indicated that KD was
30,600 (log (KD) = 4.49). A value of Km oc
of 2.58x105 (log (Kmoc) = 5.41) was de-
termined by dividing this KD value by f^
and by the molar volume of water. This
corresponds to a value of log (Km oc) of
5.30 predicted by log-linear extrapolation
of KD data generated by other in-
vestigators for T4CDD and soils 91 and
96 and water/ methanol mixtures. This
observation provides support for the ap-
plicability of the cosolvent theory to the
sorption of PCDDs to soils from
water/methanol mixtures over the entire
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range in fs from 0.0-1.0, and provides
justification for log-linear extrapolation
using data generated for water/methanol
mixtures to predict equilibrium water-
phase KD values for PCDDs and other
highly hydrophobic organic contaminants.
The effect of the presence of PCP on
the sorptive transport of PCDDs through
soils was studied by using batch and
column techniques, in which the liquid
phase consisted of water/methanol mix-
tures ranging in fs from 0.5-0.9 and con-
taining PCP at concentrations ranging
from 1-10% of solubility. These experi-
ments indicated that the sorptive reten-
tion of PCDDs by soils was reduced by
about a factor of 3 when PCP was
present in the liquid phase to that ob-
served in the absence of PCP.
The effect of the presence of CB on
the sorptive transport of PCDDs through
soils was studied by using column tech-
niques, in which the liquid phase con-
sisted of water/methanol mixtures ranging
in fs from of 0.75-0.9 and a CB concen-
tration of 10 mL/L. These experiments in-
dicated that the sorptive retention of
PCDDs by soils was reduced by a factor
of 2 in the presence of CB at an fs of 0.75
relative to that observed in the absence
of CB. Experiments to evaluate the
sorption of PCDDs by aquifer materials
included an assessment of sorption
kinetics and equilibrium KDs. The results
of these experiments suggested that a
two-day contact period was sufficient to
achieve sorption equilibrium for all
PCDDs studied for these low carbon soil
materials. Sorption isotherm data were
generally linear, and equilibrium KDs,
when converted to Km (partition coef-
ficient), were log-linearly related to fs in
accordance with the cosolvent theory.
When Km values were normalized on f^
of the aquifer material, the resulting KmiOC
values were in general agreement with
Km.oc values determined for surface soils
from the Times Beach, MO area. It is
suspected that the agreement between
aquifer material and surface soil sorption
for PCDDs indicates that sorption to or-
ganic carbon is sufficient to mask
sorption to mineral surfaces.
Experiments to evaluate the desorption
of PCDDs from surface soil were con-
ducted by batch techniques, in which
PCDDs were previously sorbed by soil
for thirty days prior to the initiation of the
desorption experiments. The results of
these experiments suggested that de-
sorption appeared to be generally re-
versible but was limited by kinetics, with
roughly 50-90% of reversible-desorption
equilibrium being attained within a 30-day
contact period.
Recommendations
Additional research in six areas of the
transport of PCDDs andrelated con-
taminants through soils is recommended:
1. Sorption of PCDFs. No data have
been reported on the sorptive partitioning
of polychlorodibenzofurans (PCDFs).
High-purity standards of these contami-
nants are now available in radiolabeled
form, and it is suggested that issues
relevant to sorptive partitioning be inves-
tigated using several PCDF congeners.
2. Sorption from Water/Methanol Mix-
tures. The sorption of PCDDs as a func-
tion of volume fraction cosolvent (fs) for
water-miscible cosolvents is generally
well understood. However, further re-
search is necessary to better understand
solubility and sorption behavior at low fs
(e.g., fs below 0.05-0.1). This work should
be conducted using PCDDs and PCDFs
as well as slightly less hydrophobic
contaminants, such as polycyclic
aromatic hydrocarbons (PAHs) and poly-
chlorinated biphenyls (PCBs). Research
should also be initiated to evaluate the
cosolvent effects attributed to ethylene
glycol, a water-miscible solvent which is
relevant to the manufacturing process of
PCP and thus relevant to waste sites
contaminated by PCDDs.
3. Sorption in the Presence of Codis-
posed Materials. Additional research is
needed to better understand the sorption
of PCDDs and related contaminants in
the presence of codisposed materials.
Three groups of codisposed materials are
important. The first group consists of
slightly soluble solvents. Much is known
regarding the sorption of hydrophobic
contaminants in the presence of miscible
cosolvents such as methanol and ace-
tone, but little is known regarding the
effect of immiscible solvent such as
methylene chloride, benzene, and aniline.
The second group consists of slightly
soluble solids. For example, the effect of
the presence of 2,4,5-trichlorophenol and
terpenes, as well as PCP, at low fs should
be evaluated. The third group consists of
oil/carrier liquids. There is a significant
requirement to further understand the
movement of oil through soils, and the
manner in which oil movement affects the
sorptive transport of PCDDs and
codisposed materials. Thus, research
should be conducted to evaluate the
sorptive transport of oils, oils and PCP,
and oils, PCP and terpenes, as well as
the sorptive transport of PCDDs in the
presence of these materials. This re-
search should focus on evaluating the
relative contributions to transport attribu-
ted to dissolved oil constituents versus oil
droplets and oil/water emulsions, ir
eluding water-phase sorption-desorptioi
dislodging of entrapped droplets, an
potential for emulsion formation.
4. Sorption of PCDDs in the present
of colloids. Much is known regarding tti
sorptive transport of moderately hydrc
phobic organic contaminants in the pre:
ence of colloidal organic matter, and th
work suggests that colloids can substar
tially increase the mobility of organi
contaminants through soils. Addition
work is necessary to extend this researc
to include highly hydrophobic organic
such as PCDDs.
5. Sorption of PCDDs by aquifer mi
terials. Additional research is needed I
evaluate the relative mineral contributior
to sorptive transport of PCDDs an
similar organics through low f^ aquif<
materials. This research should focus c
organic contaminants that are very hydrc
phobic, such as PAHs and PCBs, as we
as PCDDs. The research should include
variety of sorbents to enable a bett<
fundamental understanding of the relativ
effects of KOW, mineral surface area, ar
foe on sorption to low-foe sorbents.
6. Desorption of PCDDs. Desorption i
PCDDs from soils at low fs in the pre:
ence of codisposed materials, such <
oil, should be investigated. This researc
should include development of a mod
which incorporates desorption kinetics
enable better prediction of the moveme
of PCDDs through soils under real-wor
conditions.
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R. W. Walters, Zohreh Yousefi, Amy L. Tarleton, Stanley A. Ostazeski, and David
C. Barry are with University of Maryland, College Park, MD 20742
Carl G. En field is the EPA Project Officer (see below).
The complete report, entitled "Assessment of the Potential for Transport of
Dioxins and Codisposed Materials to Groundwater," (Order No. PB 89-
166 6071 AS; Cost: $21.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S6-89/002
0000*5833
60604
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