United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens, GA 30613-7799
Research and Development
EPA/600/S3-91/017 June 1991
EPA Project Summary
Sorption of lonizable Organic
Compounds to Sediments and
Soils
Chad T. Jafvert and Eric J. Weber
An important process affecting the
ultimata fate of pesticides and other
pollutants in the environment Is the
sorption of these compounds to soils,
sediments and aquifer materials. Be-
sides affecting the physical transport
of these compounds in the environ-
ment, sorption may significantly impact
the magnitude of biological and
chemical degradation processes, In-
cluding anaerobic and aerobic microbial
degradation, photodegradation, hy-
drolysis, and chemical reduction.
Therefore, an understanding of sorption
processes is necessary, not only to
understand transport of chemicals
through the environment, but also to
resolve the significance of these other
processes in determining the ultimate
fate of chemicals in the environment.
This report examines the partition re-
actions of organic acid and base com-
pounds to sediments and soils, as well
as the interactions of an anionic sur-
factant with sediment components. In
contrast to the partition reactions of
non-polar organic compounds, the in-
teractions of these classes of com-
pounds with sedimentary materials has
not previously received the same level
of quantitative interpretation necessary
to determine their environmental fate,
a priori. This is despite the significance
of these compounds as pesticides
(such as 2,4-D, silvex, etc.) or as other
pollutants (chlorophenols, nitrophunols,
anilines, etc.).
This Project Summary was developed
by EPA's Environmental Research
Laboratory, Athens, GA, 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).
Organic Acids
Organic acids may sorb to sedimentary
materials as either neutral or ionic species.
The significance of each form (neutral or
ionic) depends upon the pH of the aqueous
phase and the acid dissociation constant
of the particular compound. The sorption
of the neutral form of organic acids is
similar to the sorption of other neutral
organic compounds in that the predominant
interactions result from hydrophobia forces.
As a result, reasonable estimates con-
cerning the extent of sorption can be made
from knowledge of the organic carbon
content of the sedimentary material and
the hydrophobicity of the compound (as
measured by the water solubility or the
octanol-water partition coefficient). Sorption
of the anionic form of organic acids is
thought to occur primarily through modified
hydrophobic interactions. Because most
sediment-associated humic material is
negatively charged, however, the sorption
of the organic anions is less than that of
the corresponding neutral species by 1 to
2 orders-of-magnitude. In addition, the
sorption of organic anions is somewhat
pH-dependent, as changes in pH affect
not only the dissociation of the organic
acid, but also the charge on sedimentary
humic material.
To quantitatively describe these parti-
tion reactions, three mass action equations
and several material balance constraints
are required. The mass action equations
Printed on Recycled Paper
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Include equations describing (1) sorption
of the neutral species, (2) sorption of the
anionic species, and (3) acid dissociation
in the aqueous phase. Model results and
experimental data are given for the sorp-
tion of pentachtorophenol onto two sedi-
ments as a function of sediment concen-
tration and pH.
Organic Bases
Organic bases of environmental con-
cern include the nitrogen-heterocyclic
compounds (NHCs) and the aromatic
amines. Reaction processes of organic
bases with soil and sediment components
Include partition reactions through hydro-
phobic or modified-hydrophobic mecha-
nisms, cation or ligand exchange type re-
actions, and chemical reactions leading to
the formation of covalent bonds through
nucleophilte addition or oxidative processes
(which is important only for the amines).
Differentiating among these processes is
necessary if predictive fate models are to
be developed. However, in soil and sedi-
ment systems, this can be problematic, as
all processes occur simultaneously and
influence the magnitude of one another.
Currently, no models exist that can quan-
titatively estimate a priori, the reactions or
organic base compounds with sedimentary
materials through knowledge of compound
and sorbent properties. Experimental data
are given in the complete report on the
loss of substituted anilines to sediment as
a function of time, pH, and substituent.
The data indicate that sorption processes
(both hydrophobic and ionic), as well as
chemical reaction, occur simultaneously
for these compounds.
Surfactants
Recently, surfactants have received
considerable attention because of their
solubilizing effects on relatively water-in-
Chad T. Jafvert (a!so the EPA Project Officer) and Eric J. Weber are with the
Environmental Research Laboratory, U.S. Environmental Protection Agency,
Athens, GA 30613-7799.
The complete report, entitled "Sorption of lonizable Organic Compounds to Sedi-
ments and Soils," (Order No. PB91- 181 941/AS; Cost: $15.00, 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:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens, GA 30613-7799
soluble compounds. This interest stems
from both a concern over the environmen-
tal fate of surfactants per se and their ef-
fects on other chemicals (surfactants are
contained in most pesticide formulations),
and an appreciation for the potential of
surfactants to enhance the desorption of
pollutants from contaminated soils and
sediments as a stage in decontamination
treatment. For the neutral and anionic sur-
factants, the interactions with sediments
and soils include (1) sorption reactions of
both the anionic and neutral surfactant,
(2) precipitation reactions of anionic sur-
factant with calcium, and (3) aqueous
phase micelle formation at high concen-
trations of both anionic and neutral sur-
factants. Results describing the signifi-
cance of each of these processes for
dodecylsulfate are given in the complete
report.
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
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EPA/600/S3-91/017
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