United States
Environmental Protection
Agency
Office of
Research and
Development
Off ice of Sol id Waste
and Emergency
Response
EPA/540/4-90/053
October 1990
&EPA Ground Water Issue
Basic Concepts of Contaminant
Sorption at Hazardous Waste Sites
Marvin D. Piwoni* and Jack W. Keeley**
Introduction
The Regional Superfund Ground Water Forum is a group of
ground-waterscientists, representing EPA's Regional Superfund
Offices, organized to exchange up-to-date information related to
ground-water remediation of Superfund sites. One of the major
issues of concern to the Forum is the transport and fate of
contaminants in soil and ground water as related to subsurface
remediation. Processes which influence the behavior of
contaminants in the subsurface must be considered both in
evaluating the potential for movement as well as in designing
remediation activities at hazardous waste sites. Such factors not
only tend to regulate the mobility of contaminants, but also their
form and stability. Sorption is often the paramount process
controlling the behavior of contaminants in the subsurface. This
paper summarizes the basic concepts of sorption in soil and
ground water with emphasis on nonpolar organic contaminants.
For further information contact: Joe Williams, FTS 743-2246;
Bert Bledsoe, 743-2324; or Dom DiGiulio, 743-2271 at RSKERL-
Ada.
The Concept of Sorption
Sorption can be defined as the interaction of a contaminant with
a solid. More specifically, the term can be further divided into
adsorption and absorption. The former refers to an excess
contaminant concentration at the surface of a solid while the latter
implies a more or less uniform penetration of the solid by a
contaminant. In most environmental settings this distinction
serves little purpose as there is seldom information concerning
the specific nature of the interaction. The term sorption is used
in a generic way to encompass both phenomena.
There are a number of factors which control the interaction of a
contaminant and the surface of soil or aquifer materials. These
include chemical and physical characteristics ofthe contaminant,
composition of the surface of the solid, and the fluid media
encompassing both. By gaining an understanding of these
factors, logical conclusions can often be drawn about the impact
of sorption on the movement and distribution of contaminants in
the subsurface. The failure to take sorption into account can
result in a significant underestimation of the amount of a
contaminant at a site as well as the time required for it to move
from one point to another.
In introducing sorption theory it is necessary to define the terms
sorbate and sorbent. The sorbate is the contaminant that
adheres to the sorbent, orsorbing material. Inthisdiscussionthe
sorbate will usually be an organic molecule and the sorbent will
be the soil or aquifer matrix.
This Issue Paper is condensed from a presentation given at the
EPA/EPRI Workshop on Leachate Testing Methods in Houston,
Texas, in January 1989.
Factors Influencing Sorption
The properties of a contaminant have a profound impact on its
sorption behavior. Some of these include:
Water Solubility
Polar/Ionic Character
Octanol/Water Partition Coefficient
Acid/Base Chemistry
Oxidation/Reduction Chemistry
Laboratory Services Manager, Hazardous Waste Research and
Information Center, Illinois Department of Energy and Natural
Resources.
Environmental Engineer, Dynamac Corporation, Robert S. Kerr
Environmental Research Laboratory.
Superfund Technology Support Center for Ground Water
Robert S. Kerr Environmental
Research Laboratory
Ada, OK
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Contaminant Characteristics
In discussing sorption it is useful to divide chemicals into three
groups. Although there are many ways to divide chemicals into
subgroups, for this purpose three categories are presented
which transcend normal boundaries between inorganic and
organic species. These are: (1) ionic or charged species; (2)
uncharged polar species; and, (3) uncharged nonpolar species.
Most inorganic chemicals in aqueous solution will occur as ionic
or charged species. This applies to metals and metalloids, and
to other molecules such as cyanide and ammonia. However, in
contaminated water, metals and other inorganic constituents can
exist as polar or nonpolar neutral species. In any event, the
chemical form of a contaminant will have a profound effect on its
sorption and, therefore, its environmental mobility.
Organic contaminants have representatives in all three of the
sorption categories. Many of the more common organic ground-
water contaminants are of the nonpolar species, including
trichloroethene (TCE), tetrachloroethene (PCE), the chlorinated
benzenes, and the more soluble components of hydrocarbon
fuels such as benzene, toluene and xylene. Other important
organic contaminants including many of the pesticides, phenols
and dyes exist in solution as either charged or polar molecules.
Still other, larger organics, such as surfactants, can have both
polar and nonpolar ends within the same molecule. The
environmental mobility of contaminants with these distinctive
properties has been less thoroughly studied than nonpolar
organics; therefore, site-specific investigations may provide the
most reliable information for their transport characteristics.
Soil Characteristics
If one avoids the difference between positive and negative
charges, a simple rule of sorption might be: for charged species,
"opposites attract" and for uncharged species, "likes interact with
likes." Likes refers to the three categories of contaminants and
to the properties of the soil matrix. Some of the most important
characteristics of soil affecting the sorptivebehaviorof subsurface
materials include:
Mineralogy
Permeability/Porosity
Texture
Homogeneity
Organic Carbon Content
Surface Charge
Surface Area
Soil, in its natural state, is primarily composed of sand, silt, clay,
water, and a highly variable amount of natural organic carbon.
The latter profoundly complicates a soil's sorptive properties.
The combination of these characteristics describes the surfaces
offered as sorptive sites to contaminants in water passing through
the subsurface matrix. For example, silts and clays have much
higher surface areas than sand, usually carry a negative charge,
and almost invariably associate with natural organic matter.
It can be deduced that sandy materials offer little in the way of
sorptive surfaces to passing contaminants while silts and clays,
particularly those having substantial amounts of organic matter,
provide a rich sorptive environment for all three categories of
contaminants. Even the most porous and highly productive
aquifers, composed of sands and gravels, usually have some
fine grained material, and a few percent of silts and clays can
result in a substantial increase in the sorptive behavior of the
aquifer material.
Fluid Media Characteristics
Under most contamination situations the primary transporting
fluid is water. One of the most important properties of this solvent
phase is pH for it dictates the chemical form and, therefore, the
mobility, of all contaminants susceptible to the gain or loss of a
proton. As an example, pentachlorophenol will primarily be an
uncharged polar molecule in an aqueous solution whose pH is
below about 4.7 and an anion when the pH is above that value,
increasing its solubility from 14 to 90 mg/l.
Other characteristics of water that can influence the behavior of
contaminants include the salt content and the dissolved organic
carbon content. Chlorides, for example, which are not usually of
much concern when dealing with organic contaminants, can
have an important effect on the mobility of various metals.
Dissolved organic matter, at relatively high concentrations found
in many leachates, has a significant effect on the mobility of most
nonpolar organics.
Implications of These Characteristics
Although somewhat simplified, it can be assumed for purposes
of this discussion, that charged and polar species tend to interact
with charged and polar surfaces, and nonpolar compounds
interact with nonpolar components of soil, usually the natural
organic carbon. In orderto make a first estimate ofthe significance
of sorption at a site, it is necessary to determine the polar and
nonpolar nature ofthe material with which the contaminant will
come into contact. This is usually done by measuring the cation
exchange capacity and the natural organic carbon content,
respectively.
The cation exchange capacity (CEC) provides an estimate ofthe
total negatively-charged sites on the surface of the soil. It is
determined by measuring the mass of a standard cation, usually
ammonia, that displaces another cation held by the soil. Under
normal field conditions these sites will be occupied by cations
common to the flowing or percolating water, such as Na+, K+,
Ca2+, and Mg2+. Largerorganic cations and highly-charged metal
ions like Hg2+ or Cr3+ will be preferentially retained at these sites
by "exchanging" with their normal occupants. Thus large organic
cations and heavy metals would not normally be expected to
move far through soils with a measurable cation exchange
capacity.
At contaminated sites, however, conditions may not be "normal"
and Hg2+ may be codisposed with high levels of chloride salts. In
the complexation chemistry shown in Figure 1, Hg2+ may be
replaced by the neutral complex HgCI2 orthe negative ion HgCI3,
both of which move through the soil more quickly than the cationic
form.
Sorption of Nonpolar Organics
As mentioned above the chemicals at many contaminated sites
are nonpolar organics. It was representatives of these types of
compounds (DDTandotherchlorinated hydrocarbon pesticides)
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that first focused attention on the potential hazards of chemicals
in the environment because of their widespread use, potential
human toxicity, and recalcitrance.
Hg2+ + cr
HgCI+ +Cr
0 + r\-
HgCI
HgCI~
HgCI 2~
The complexation reactions are driven right and down by
increasing chloride concentrations, often characteristic of
waste waters. Increased complexation produces increased
environmental mobility of the mercury.
Figure 1. Mercury Ion Complexation in Chloride-Rich Water
Transport and fate characteristics of these compounds have
been well studied, first by the agricultural community and later by
environmental scientists. As a result, an understanding of the
sorptive behavior of these compounds has evolved which can be
used to assess the environmental consequences posed at a
waste disposal site.
Many organics of environmental concern have a limited solubility
in water because of their nonpolarity and molecular size: that is,
thesolubilityofanorganiccontaminantdecreaseswith decreasing
polarity and increasing molecular size. But even with limited
solubilities, many hazardous chemicals at equilibrium are at
measurable, and sometimes toxic concentrations in water. Polar
molecules, such as ethanol, are compatible with water. Their
combination results in a homogeneous solution regardless ofthe
proportions that are mixed.
Nonpolar organic compounds interact with soil organic matter
through a process known as "hydrophobic sorption" which can
be explained as the affinity of organic compounds for phases
other than water. For example, water being a polar molecule is
not compatible with other nonpolar molecules, such as DDT,
which is immiscible with water.
Octanol-Water Partitioning
Organic molecules of increasing size, decreasing polarity and
therefore water solubility, are said to exhibit increasing
"hydrophobicity" which can be quantified by their octanol-water
partition coefficient. It is a measure of the distribution of the
chemical between a water and an organic (octanol) phase with
which it is in contact. The more hydrophobic the contaminant, the
more likely it is to partition into the octanol phase. The partition
coefficient provides a fairly accurate understanding of the sorptive
process occurring between water and the soil, more specifically,
the soil organic matter.
The octanol-water partition coefficient, expressed as K^ in
Figure 2, is determined by measuring the concentration of a
particular compound in the water and the octanol phases after a
period of mixing. It is importantto note that the more hydrophobic
the compound the less accurate the test, and the results should
Octanol-Water Partition Coefficient:
Concentration
K
Concentration
Almost always presented as Logw because the
numbers are so large for hydrophobic compounds.
Sorption Coefficient:
Concentration
K
Concentration
mg/kg
Units are , which is L/kg.
mg/L
Carbon Normalized Sorption Coefficient:
Sorption Coefficient, K
K
Fraction Organic Carbon
Figure 2. Relationships Pertinent to Nonpolar Organic
Contaminant Transport
be viewed accordingly. It is often sufficient to know that an
extremely high coefficient means that the compound is very
hydrophobic. Since measured K^ values can be in the millions
for important environmental contaminants (PCB's, chlorinated
pesticides, dioxins and furans), they are often expressed as the
baselO logarithm, Log K^.
The K^ has two attributes that make it especially useful in
environmental assessments. First, it varies in a predictable way
within classes of organic compounds. For example, as shown in
Figures 3 and 4, if K^ is known for one member of a class of
compounds it can be used reasonably well to estimate a value for
other members of the same family. In the examples shown, the
K^ can be correlated to the number of chlorine atoms or the
number of rings in the molecular structure of classes of
contaminants.
The second attribute results from the work of a number of
agricultural and environmental researchers who correlated
sorption on the organic matter of soils with the K^ of the
compounds involved. By using these attributes ofthe K^, it is
possible to estimate the potential sorption of organic contaminants
based on the structure ofthe compounds and the organic carbon
content ofthe soil or aquifer material.
Sorption to Soils
Thus far it has been suggested that nonpolar organic compounds
are sorbed by soils as a function of their hydrophobicity (K^) and
the organic carbon content of the soil. There has been
considerable research which suggests that the slow kinetics of
the sorption process may be significant in swiftly moving ground
water. Sorption studies using flow-through columns produce
results sensitive to the flow rate, and batch tests indicate that
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OS
J 4
12345
# of Chlorine Atoms
Figure 3. Relationship of Molecular Structure to
Hydrophobic Character
increased sorption occurs with longer exposure times. The
practical implication of these findings may be that sorption is
overestimated in aquifer systems with relatively high flow rates.
Sorption is expressed in terms of a partition coefficient Kp, which
is defined in Figure 2 as the ratio of the concentration of
contaminants associated with the solid phase to that in solution,
and is, therefore, conceptually similar to K^. The usefulness of
K^ in estimating sorption stems fromthe fact that the soil organic
matter serves the same function as octanol in the octanol-water
test. As a result, there have been many empirical relationships
developed for estimating sorption from the K^ and the soil
organic carbon content. One expression, developed in the
laboratory by Piwoni and Banerjee, 1989, for the sorption of
common environmental contaminants with a low aquifer organic
carbon, is:
Log Koc = 0.69 Log K^ + 0.22
When applying such a relationship, it is important to select a
study in which the compounds used are similar to those of
interest at the site under investigation. However, as shown in
Figure 5, even when applying the empirical relationship to a
structurally dissimilar compound such as anthracene, if it is a
nonpolar organic, the error of estimate should be less than a
factor of five.
These estimates of sorption are based, in large measure, on a
good evaluation of the soil organic carbon content at a site which
is obtained from the degradation of naturally occurring organic
matter. In this regard it is importantto realize that soils and aquifer
materials are very heterogeneous and the organic carbon content
can vary considerably both in the vertical and horizontal dimension.
Fortunately, this variability tends to be the greatest in the vertical
soil profile while most site investigations are concerned with
contaminant movement in the ground water away from the
source. While the soil organic carbon content in the horizontal
plane usually differs by a factor of ten or less, it can vary by a
factor of 10 to 100 in the vertical dimension.
1
Benzo(a)pyrene
0
00
Benzanthracene
[Ql Naphthalene
(Q| Benzene
Increasingly Hydrophobic
34
Rings
Figure 4. Relationship of Molecular Structure to Octanol Water
Partition Coefficient
0123456
Log Kow
Figure 5. Partitioning on Soil Organic Carbon as Function of
Octanol-Water Partition Coefficient
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In order to determine the soil organic carbon content at a site,
samples are usually obtained using split spoon sampler or other
standard soil sampling devices. Representatives portions of the
soil are then burned in an O2 atmosphere and the produced CO2
is measured by IR spectrophotometry. Before burning soil
samples must be acidized to remove inorganic carbon. The
accuracy of measuring organic carbon content can also be
questionable, particularly at low levels and in carbonate soils.
Existing analytical methods for measuring soil organic carbon
were developed for the higher concentrations found near the
surface. Therefore, at the low levels found in deeper soils and
ground water, the same quality assurance procedures used in
determining contaminant levels in water should be followed in
determining the subsurface organic carbon content.
The processes driving hydrophobic sorption are nonspecific and
depend upon small amounts of energy gained by moving
contaminants out of the aqueous phase. The extent to which the
process proceeds is dependent upon how receptive the soil
matrix is to the organic molecule, which is a function of the
organic content. But even when the organic carbon content is
very low, some sorption of the most hydrophobic molecules
continues because of the soil's mineral surfaces.
Sorption Estimation
In order to use the information provided above in estimating the
amount of a contaminant associated with the aqueous and solid
phases of an aquifer, it is necessary to develop a contamination
scenario. To that end it is assumed that the contaminant at an
industrial landfill is 1,4-dichlorobenzene, and there is sufficient
data to indicate that: (1) most of the contamination is below the
water table; (2) the contaminant concentration in ground water
averages 1 mg/l; (3) the measured soil organic carbon is 0.2
percent; and (4) the pore water occupies 50 percent of the aquifer
volume. Steps leading to an estimate of the contaminant's
distribution between the aqueous and solid phases are:
Field Measurements:
Average contaminant concentration
in monitoring wells = 1.0 mg/l
Soil organic carbon = 0.2 percent,
therefore foc = 0.002
Pore water occupies 50 percent of
the aquifer's volume.
From The Literature:
Log K^ (1,4-dichlorobenzene) = 3.6
Piwoni and Banerjee Regression,
Log Koc = 0.69 K^ + 0.22
Calculated:
Log Koc = 0.69(3.6) + 0.22 = 2.70
therefore: Koc = 506
Kp = Koc(foc) = 506 (0.002) 3 1.0 3 Sorbed C
Solution C
Conclusion;
The contaminant, equally distributed between each phase,
is expressed as mg/kg (soil) and mg/l (water). Since soil is
about 2.5 times more dense than water, 2 liters of aquifer
would contain 1 liter of water and 2.5 kg of soil. Therefore,
1.0 mg/l of the contaminant would be associated with the
water and 2.5 mg (70 percent) would be sorbed to the
aquifer's solid phase.
As can be seen from this example, sorption tends to complicate
remediation techniques that require pumping waterto the surface
for treatment. The desorption process has kinetic constraints
that can render a pump-and-treat system ineffective. Slow
desorption kinetics result in progressively lower contaminant
concentrations atthe surface, and less cost-effective contaminant
removal. It is not uncommon to pump a system until the
contaminant concentration in the pumped water meets a
mandated restoration level, while the aquifer's solid phase still
contains a substantial mass of contaminant. If the pumps are
turned off, concentrations in the ground water will soon return to
their equilibrium level.
Measuring Sorption
It is preferable to obtain the best information possible on which to
base an estimate of sorption. Therefore, tests should be made
with the contaminants of concern, as well as soils and aquifer
material from a specific site. The goal is to obtain a partition
coefficient, Kp, for use in the prediction of contaminant movement.
There are essentially two methods for measuring the partition
coefficient, those being batch and dynamic techniques. Batch
techniques are quicker and easier to perform and, therefore,
more amenable to replication and quality control. Dynamic or
flow through techniques offer the advantage of more closely
representing processes occurring in the field.
The standard approach to determine the partition coefficient is to
generate a sorption isotherm, a graphical representation of the
amount of material sorbed at a variety of solute concentrations.
The Freundlich isotherm, S = KpC1/n, is the representation most
often used for the sorption of nonpolar organics to soils and
aquifer materials. In this equation, Sis the mass sorbed per mass
of sorbent (mg/kg), C is the solute concentration at equilibrium
(mg/l), Kp is the Freundlich partition coefficient, and 1/n is a fitting
factor. The equation can be expressed in a linear form for
convenience:
Log S = Log Kp + 1/n Log C
As shown in Figure 6, Log Kp can be estimated by determining
the intercept of the regression of a Log-Log plot of S and C.
Summary
This has been a discussion of the concepts involved in estimating
contaminantsorption, particularly nonpolarorganics, at hazardous
waste sites. After determining the types of contaminants present
at a site, it is possible to estimate K using Kow values from the
literature, an appropriate sorption coefficient/^
equation, and some organic carbon values.
regression
If sorption determinations are within the scope of the project, site
representative soil samples and contaminants should be selected
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1.00
0.50
0.10
B. 0.05
0.01
• Kp=0.74=lntercept
0.01
0.05 0.10 0.15
C=Solution Concentration mg/L
1.00
Figure 6. Sorption of 1,4-Dichlorobenzene
from the tests. The measured sorption information is best used
to evaluate the validity of preliminary estimates. If the measured
partition coefficients differ from the estimates by more than a
factor of 2 or 3, it may be useful to select other contaminants from
the site and determine K values the same soil samples. A plot
of K values versus Kow values will provide a useful guide for
predicting the sorption characteristics of other contaminants at
the site.
Selected References
American Society of Agronomy Soil Sciences of America. 1982.
Methods of Soil Analyses. Part 2-Chemical and Microbiological
Properties-Second Addition. Agronomy No. 9, Part 2.
Ballard, T.M. 1971. Role of Humic Carrier Substances in DDT
Movement Through Forrest Soil. SoilSci. Soc. Am. Proc. 35:145-
147.
Banerjee, P., M.D. Piwoni, and K. Ebeid. 1985. Sorption of
Organic Contaminants to Low Carbon Substrate Core.
Chemosphere 14:1057-1067.
Bouchard, D.C., R.M. Powell, and D.A. Clark. 1988. Organic
Cation Effects on the Sorption of Metals and Neutral Organic
Compounds on Aquifer Material. J. Environ. Sci. Health Part A
23:585-601.
Bouchard, D.C., and A.L. Wood. 1988. Pesticide Sorption on
Geologic Material of Varying Organic Carbon Content. Toxic.
Industr. Health 4:341-349.
Briggs, G.G. 1981. Theoretical and Experimental Relationships
Between Soil Adsorption, Octanol-Water Partition Coefficients,
Water Solubilities, Bioconcentration Factors, and the Parachor.
J. Agric. Food Chem. 29:1050-1059.
Brown, D.S., and E.W. Flagg. 1981. Empirical Prediction of
Organic Pollutant Sorption in Natural Sediments. J. Environ.
Qual. 10:382-386.
Carlson, D.J., L.M. Mayer, M.L. Brann, and T.H. Mague. 1985.
Binding of Monomeric Organic Compounds to Macromolecular
Dissolved Organic Matter In Seawater. Mar. Chem. 16:141-163.
Carlson, R.M., R.E. Carlson, and H.L. Kopperman. 1975.
Determination of Partition Coefficients by Liquid Chromatography.
J. Chromatogr. 107:219-223.
Caron, G., H. Suffet, and T. Belton. 1985. Effect of Dissolved
Organic Carbon on the Environmental Distribution of Nonpolar
Organic Compounds. Chemosphere 14:993-1000.
Carter, C.W., and I.H. Suffet. 1982. Binding of DDT to Dissolved
Humic Materials. Environ. Sci. Technol. 16:735-740.
Chin, Y., W.J. Weber, and T.C. Voice. 1986. Determination of
Partition Coefficients and Aqueous Solubilities by Reverse Phase
Chromatography-ll. Water Res. 20:1443-1450.
Chiou, C.T., R.L. Malcolm, T.I. Brinton, and D.E. Kile. 1986.
Water Solubility Enhancement of Some Organic Pollutants and
Pesticides by Dissolved Humic and Fulvic Acids. Environ. Sci.
Technol. 20:502-508.
Dragun, James. 1988. The Soil Chemistry of Hazardous
Materials. Hazardous Materials Control Research Institute.
Silver Spring, MD.
Enfield, C.G. 1985. Chemical Transport Facilitated by Multiphase
Flow Systems. Water Sci. Technol. 17:1-12.
Enfield, C.G., D.M. Walters, R.F. Carsell, and S.Z. Cohen. 1982.
Approximating Transport of Organic Pollutants to Groundwater.
Ground Water 20:711-722.
Garbarini, D.R., and L.W. Lion. 1986. Influence of the Nature of
Soil Organics on the Sorption of Toluene and Trichloroethylene.
Environ. Sci. Technol. 20:1263-1269.
Gamerdinger, A.P. R.J. Wagonet, and M. th. van Genuchten.
1990. Application of Two-Site/Two-Region Models for Studying
Simultaneous Transport and Degradation of Pesticides. SoilSci.
Soc. Am. J. 54:957-963.
Gauthier, T.D., W.R. Seitz, and C.L. Grant. 1987. Effects of
Structural and Compositional Variations of Dissolved Humic
Materials on Pyrene K Values. Environ. Sci. Technol. 21:243-
248.
Griffin, R.A. andW.R. Roy. 1985. Interaction of Organic Solvents
with Saturated Soil-Water Systems. Open File Report prepared
for the Environmental Institute for Waste Management Studies,
University of Alabama.
Gschwend,P.M.,andS.Wu. 1985. OntheConstancyofSediment-
Water Partition Coefficients of Hydrophobic Organic Pollutants.
Environ. Sci. Technol. 19:90-96.
Hassett, J.P., and M.A. Anderson. 1982. Effect of Dissolved
Organic Matter on Adsorption of Hydrophobic Organic
Compounds by River-and Sewage-Borne Particulate Matter.
Water Res. 16:681-686.
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Karickhoff, S.W. 1981. Semi-Empirical Estimation of Sorption of
Hydrophobic Pollutants on Natural Sediments and Soils.
Chemosphere 10:833-846.
Karickhoff, S.W., D.S. Brown, and T.A. Scott. 1979. Sorption of
Hydrophobic Pollutants on Natural Sediments. Water Res. 13:241 -
248.
Landrum, P.P., S.R. Nihart, B.J. Eadie, and W.S. Gardner. 1984.
Reverse-Phase Separation Method for Determining Pollutant
Binding to Aldrich Humic Acid and Dissolved Organic Carbon of
Natural Waters. Environ. Sci. Technol. 18:187-192.
McCarty, P.L., M. Reinhard, and B.E. Rittman. 1981. Trace
Organics in Groundwater. Environ. Sci. Technol. 15:40-51.
Morrow, N.R., and I. Chatzis. 1982. Measurement and Correlation
of Conditions for Entrapment and Mobilization of Residual Oil.
DOE/BC/10310-20.
Mortland, M.M., S. Shaobai, and S.A. Boyd. 1986. Clay-Organic
Adsorbents for Phenol and Chlorophenol. Clays and Clay Minerals.
34:581-585.
Piwoni, M.D., and P. Banerjee. 1989. Sorption of Volatile
Organic Solvents from Aquifer Solution onto Subsurface Solids.
Journal of Contaminant Hydrology. 4:163-179.
Poirrier, M.A., B.R. Bordelon, andJ.L. Laseter. 1972. Adsorption
and Concentration of Dissolved Carbon-14 DDT by Coloring
Colloids in Surface Waters. Environ. Sci. Technol. 6:1033-1035.
Rao, P.S.C., and R.E. Jessup. 1983. Sorption and Movement of
Pesticides and other Toxic Substances in Soils, p. 183-201. In
D.W. Nelson et al. (ed.) Chemical Mobility and Reactivity in Soil
Systems. SSASpec. Publ. II, Madison, Wl.
Schwarzenbach, R.P.andJ. Westall. 1981.TransportofNonpolar
Organic Compounds from Surface Water to Groundwater.
Laboratory Sorption Studies. Environ. Sci. Technol. 15:1360-
1367.
Thurman, E.M. 1985. Humic Substances in Groundwater. p. 87-
103. In G.R.Aikenetal.(ed.)HumicSubstances in Soil, Sediment,
and Water. Wiley-lnterscience, New York.
Voice, T.C., C.P. Rice, and W.J. Weber, Jr. 1983. Effect of Solids
Concentration on the Sorptive Partitioning of Hydrophobic
Pollutants in Aquatic Systems. Environ. Sci. Technol. 17:513-
518.
Weber, W.J., Y. Chin, and C.P. Rice. 1986. Determination of
Partition Coefficients and Aqueous Solubilities by Reverse Phase
Chromatography-l. Water Res. 20:1433-1442.
Wershaw, R.L, P.J. Burcar, and M.C. Goldberg. 1969. Interaction
of Pesticides with Natural Organic Material. Environ. Sci. Technol.
3:271-273.
Whitehouse, B. 1985. The Effects of Dissolved Organic Matteron
the Aqueous Partitioning of Polynuclear Aromatic Hydrocarbons.
Estuarine Coastal Shelf Sci. 20:393-402.
Wilson, J.L., and S.H. Conrad. 1984. Is Physical Displacement of
Residual Hydrocarbons a Realistic Possibility in Aquifer
Restoration? p. 274-298. In Proc. Petroleum Hydrocarbons
Organic Chemicals Groundwater, Houston, TX.
Wolfe, T.A., T. Demiral, and E.R. Baumann. 1985. Interaction of
Aliphatic Amines with Montmorilloniteto Enhance Adsorption of
Organic Pollutants. Clays and Clay Minerals. 33:301-311.
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