United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati, OH 45268
Research and Development
EPA/530/SW-87/006a April 1992
Project Summary
Batch-Type Procedures For
Estimating Soil Adsorption Of
Chemicals
W.R. Roy, I.G. Krapac, S.F.J. Chou, and R.A. Griffin
This Technical Resource Document
(TRD) contains laboratory procedures
and guidelines for conducting experi-
ments using batch-equilibrium tech-
niques to study the adsorption by clays
or soils of chemicals dissolved in solu-
tion (solutes). The procedures were de-
signed for routine use, and can be used
for developing data for constructing
equilibrium adsorption isotherms. Pro-
cedures are given for inorganic and
organic solutes as well as for volatile
organic solutes. The scientific basis for
each procedural step is discussed in
detail. These procedures were based
on scientific literature, and were devel-
oped and tested by the authors and
cooperating laboratories. Examples are
given to show application of major pro-
cedural steps and concepts. Several
types of soil materials and solutes are
featured, as well as the application of
batch-adsorption data in calculations
of solute movement through compacted
landfill liners.
This TRD was submitted by the Illi-
nois State Geological Survey in fulfill-
ment of Cooperative Agreement
CR810245 with the U.S. Environmental
Protection Agency. It has been revised
to address issues that were raised dur-
ing the public comment period on the
draft TRD entitled Batch-Type Adsorp-
tion Procedures for Estimating Soil At-
tenuation of Chemicals (EPA/530-SW-
87-006). The revised TRD also includes
technical information that became avail-
able after the draft TRD was completed
in May 1987.
This Project Summary was developed
by EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to announce
key findings of the research project
that is fully documented in a separate
report of the same title (see Project
Report ordering information at back).
Introduction
The capacity of geological materials to
attenuate potential pollutants had been
studied by many researchers. The batch-
adsorption or static-equilibration technique
has often been used in laboratory studies
to assess the capacity of soils and soil
components to remove chemical constitu-
ents from solution. Batch procedures vary
considerably from one another in terms of
experimental conditions and research ob-
jectives, and sometimes yield different re-
sults even when the same soils, solutes
and concentrations are used.
The simplicity of the batch-adsorption
technique accounts in part for its popular-
ity. With this technique, an aqueous solu-
tion containing solutes of known composi-
tion and concentration(s) is mixed with a
given mass of adsorbent for a given pe-
riod of time. The solution is then sepa-
rated from the adsorbent and chemically
analyzed to determine changes in solute
concentration. The amount of solute
adsorbed by the adsorbent is assumed to
be the difference between the initial con-
centration (before contact with the adsor-
bent) and the solute concentration after
the mixing period. Although the approach
is relatively simple, several experimental
parameters can affect the adsorption of a
given constituent. These parameters in-
clude: contact time between the solute
and adsorbent, temperature, method of
mixing, soihsolution ratio, adsorbent mois-
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ture content, solution pH, hydrolysis, and
the composition and concentration of other
dissolved constituents in the solution. Dis-
solved organic carbon, volatility, photo-
degradation, biodegradation, and com-
pound stability can also affect adsorption
data associated with organic solutes.
In the past adsorption studies, the par-
ticular experimental conditions selected
were probably appropriate for the specific
system under study and for the intended
use of the data. However, the diversity in
experimental conditions can make com-
parisons of adsorption data between stud-
ies difficult. The U.S. EPA perceived a
need to develop a well-documented, com-
prehensive source for conducting batch-
adsorption determinations.
This (TRD) incorporates the experiences
gained during interlaboratory testing pro-
grams and the interactions with the scien-
tists and laboratories affiliated with Ameri-
can Society for Testing and Materials. It
describes procedural steps for inorganic
and organic solutes, and documents their
scientific basis. Most of the procedural
steps recommended here were tested in
the authors' laboratory with a variety of
soils, solutions containing several solutes,
and aqueous extracts of actual wastes.
Characteristics of the soils, clays, and
wastes are described in the appendices
of the full report.
Discussion
A brief review of the physicochemical
forces and mechanisms thought to be re-
sponsible for the adsorption of ions and
molecules from solution is presented in
Chapter 1. The effects of adsorbent prepa-
ration are discussed in Chapter 2. The
process of preparing samples taken in the
field for laboratory investigations can di-
rectly influence adsorption results. Adsor-
bent samples are usually dried so that
they can be homogenized and stored until
needed. Oven-drying of adsorbents is not
an advisable technique to accelerate dry-
ing even though air-drying may take sev-
eral days with large bulk samples. Air-
drying minimizes physicochemical changes
that may occur when drying and is the
most practical approach at this time. Air-
drying anaerobic soils and sediments re-
quires special handling to prevent the rela-
tively reduced materials from oxidizing if
exposed to the atmosphere.
The effects of temperature on adsorp-
tion measurements are discussed in Chap-
ter 3. Batch-adsorption determinations
should be conducted under constant-tem-
perature conditions, if available, or in
rooms where the ambient temperature is
fairly constant (e.g., ±3°C). When batch
experiments are performed, the tempera-
ture of the solution should be recorded
and treated as a potential variable that
can influence the data, and therefore may
be useful in the interpretation of the re-
sults. The stability of nonionic solutes is
the subject of Chapter 4.
In conducting batch-adsorption tests, in-
vestigators must consider the physico-
chemical stability of the solute in solution.
Processes such as photodegradation, hy-
drolysis, and/or microbial degradation can
potentially contribute to a decrease in sol-
ute concentration concomitantly with ad-
sorption, and these changes may even
occur before the solution contacts the ad-
sorbent. Procedures are given in this chap-
ter that can be used as simple screening
tests to identify potential solute-stability
problems.
In Chapter 5 on the effects of pH on
adsorption, the adsorptbn behavior of ionic
and ionizable inorganic and organic sol-
utes by soils and soil materials is dis-
cussed. In general, the adsorption of inor-
ganic cations increases with increasing
pH, and the adsorption of anions is gener-
ally enhanced in acidic environments, al-
though some anionic solutes are adsorbed
to a greater extent in alkaline systems.
The adsorption behavior of neutral, non-
polar hydrophobic organic solutes appear
to be largely unaffected by the pH of the
soil-water system. The potential influence
of pH on the results generated by batch-
adsorption procedures depends on the
system under study. The equilibrium pH
of the soil-solute mixtures should be de-
termined before separating the solution
from the soil or soil component suspen-
sion. The measurement should be given
along with the adsorption data. The failure
to measure and report pH data may ren-
der the adsorption data impossible to in-
terpret in a meaningful way.
The ionic strength of the solution may
have several direct and indirect effects on
adsorption data (Chapter 6). The extent of
these effects depends on the magnitude
of the ionic strength and on the concen-
tration, composition, and charge of the
ionic constituents. The position taken in
developing the batch-adsorption proce-
dures was governed by the philosophy
that they should be simple and designed
primarily for routine use. Thus, the use of
a background electrolyte was rejected in
anticipation that the inherent ionic strength
of the solutions will be influenced by the
chemical constituents occurring in the
leachate or extract, and those derived from
soluble constituents in the particular clay
or soil under investigation. The electrical
conductivity (EC) of the equilibrated soil-
solution should be measured so that the
ionic strength of the solution can be cal-
culated. The failure to measure and report
EC data and/or ionic strength can make
adsorption data difficult to interpret.
In the chapter on phase separation
(Chapter 7), a search of the literature indi-
cated that very few researchers have used
a filtration technique to separate the liquid
and solid phases before analyzing the liq-
uid phase in batch-adsorption studies. This
is probably due to the potential of the filter
membranes to retain significant quantities
of the solute, particularly organic com-
pounds. Solid and liquid phases should
be separated by centrifugation unless the
investigator can clearly demonstrate that
the filtration technique does not signifi-
cantly affect the results.
In theory, the mechanical device used
to mix the solid-liquid mixture during the
equilibration interval should have no ef-
fect on the equilibrium distribution of sol-
utes and adsorbates (Chapter 8). How-
ever, studies in the TRD show that the
mixing method can influence the resulting
adsorption data. For all adsorption experi-
ments, National Bureau of Standards
(NBS) rotary extractor or its equivalent
should be used during each step neces-
sary to develop an adsorption isotherm,
i.e., determining a soihsolution ratio (Chap-
ter 9), equilibration time (Chapter 13), and
the adsorption isotherms. Adsorption data
generated with other mixing devices may
be valid; however, these data should not
be routinely accepted unless the investi-
gator can document that the device used
yields data comparable to those from an
NBS rotary extractor or its equivalent. This
recommendation will help standardize re-
sults between laboratories.
The term "soil to solution ratio" refers to
the ratio of the mass of the adsorbent
sample to the volume of liquid. To con-
struct an adsorption isotherm, soihsolution
ratios must be determined that will permit
enough solute to be adsorbed to result in
measurable, statistically significant differ-
ences in solution concentration. If the
soihsolution ratio is low (i.e., too much
adsorbent or too little solution), most of
the solute may be adsorbed, forcing the
investigator to attempt to measure small
differences in concentration at very low
solute concentrations. If the ratio is too
high( i.e., not enough adsorbent for a given
volume), the changes in the initial solute
concentration may be very small, forcing
the investigator to measure small differ-
ences in concentration between large
amounts of solute. Unfortunately, with in-
organic and polar organic compounds, a
suitable soihsolution ratio cannot be de-
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ternined a priori, and procedures for se-
lecting a suitable ratio are given in Chap-
ter 9.
Finding a suitable soihsolution ratio for
ionic and polar solutes requires laboratory
work, but a simple calculation can be used
to estimate a suitable ratio for nonionic
solutes, particularly hydrophobic organic
species. This estimation technique requires
a value for the organic carbon content of
the adsorbent and for the organic carbon
partition coefficient (KJ of the solute.
In Chapter 11, the effects of the
soihsolution ratio on adsorption data are
discussed. The soihsolution ratio may be
one of the most important experimental
variables to consider when developing an
adsorption isotherm and evaluating the
data, particularly when comparing results
from different investigators who used dif-
ferent ratios. Specific soihsolution ratios
provided in Chapter 17 should be adopted
as standard ratios 'for the construction of
adsorption isotherms. Adherence to this
recommendation will enable direct com-
parisons of adsorption data generated by
different investigators.
Two experimental techniques are used
to generate batch-adsorption data:
I.The constant soihsolution ratio
method: mixing a batch of aqueous
solutions—each solution containing
progressively decreasing solute con-
centrations—with adsorbent, keeping
the amount of adsorbent (by weight)
constant in all solutions.
2. The variable soiksolution ratio method:
mixing a batch of solutions, all con-
taining the same solute concentra-
tion, with progressively increasing
amounts of adsorbent.
The first technique presumably is based
on a standard ratio selected from the pro-
cedures given in Chapters 9, 10, and 17.
The second is similar to the technique
used to select a soihsolution ratio for ionic
solutes (Chapter 9). An isotherm produced
by variable soihsolution ratios was consid-
ered to be more environmentally conser-
vative.
Adsorption at the solid-liquid interface
is a thermodynamic process, and adsorp-
tion measurements are taken when the
system has equilibrated (Chapter 13). Ad-
sorption is generally regarded as a fast
reaction, and subsequent removal of sol-
ute from solution may be attributed to
other processes. Adsorption processes at
solid-liquid interfaces are often initially
rapid; further reduction in solute concen-
tration continues at a decreasing rate,
asymptomatically approaching a constant
concentration. In some cases, equilibrium
is never clearly attained. The ambiguity in
the definition and measurement of equili-
bration times has been acknowledged as
a major problem in adsorption studies.
The EPA suggested that the equilibration
time should be the minimum amount of
time needed to establish a rate of change
of the solute concentration in solution equal
to or less than 5% per 24-hr interval. This
definition is an operational definition of
equilibrium and is equivalent to a steady
state.
The selection and construction of ad-
sorption isotherms is discussed in Chap-
ters 14 and 15. An adsorption isotherm is
a graphic representation showing the
amount of solute adsorbed by an adsor-
bent as a function of the equilibrium con-
centration of the solute. This relationship
is quantitatively defined by some type of
partition function or adsorption isotherm
equation that is statistically applied to gen-
eralize the data.
Chapter 16 covers the use of batch-
adsorption data. Adsorption data are used
in describing the partitioning of chemicals
between soils and water, and have been
used successfully as input parameters in
models describing the movement of chemi-
cals through soil to groundwater. This
chapter is a brief introduction to possible
applications of batch-adsorption data. An
example to calculate solute movement
through a compacted soil liner of a landfill
is provided.
The procedures for the determination of
the soihsolution ratio, equilibration time,
and other parameters necessary for the
construction of adsorption isotherms are
contained in Chapter 17 and are written in
the ASTM format. The rationale for these
procedures, presented in previous chap-
ters, should be studied before attempting
to use them. Throughout this chapter, ref-
erences are made to other parts of the
TRD that elucidate topics relevant to a
specific procedural step. The references
cited include 169 publications that were
consulted in the evolution of this TRD.
The full report was submitted in fulfill-
ment of Cooperative Agreement No.
CR810245 by the Illinois State Geological
Survey under the sponsorship of the U.S.
Environmental Protection Agency.
•&V.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40232
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W. R. Roy, I.G. Krapac, andS. F.J. Chou are with the Illinois State Geological Survey,
Champaign, IL 61820; and R.A. Griffin is with the University of Alabama,
Tuscabosa, AL 35487-0203.
Michael H. Roulier is the EPA Project Officer (see below).
The complete report, entitled "Batch-Type Procedures for Estimating Soil Adsorp-
tion of Chemicals," (Order No. PB92-146190/AS; Cost: $26.00; subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, V'A 221'61
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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