Assessing the Environmental Partitioning of
Organic Acid Compounds
by
Chad T. Jafvert
Environmental research Laboratory
U.S. Environmental Protection Agency
.Athens-, Georgia 30613
.Submitted as an
Environmental Research Bxdef
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Background
The ultimate disposition of organic compounds in the environment is
influenced tremendously by their tendency to partition, or "adsorb" to
sediment particles or other distinct environmental components or phases. To
date, the majority of sorption research for organic chemicals has focused on
describing partitioning processes of neutral compounds, such as the
polycyclicaromatic hydrocarbons, polychlorinated biphenyls, and dioxins. Many
environmentally relevant organic compounds, however, contain acid or base
functional groups. Compounds containing acid functional groups include the
chlorinated phenoxyalkyl acid herbicides, such as 2,4-D and 2,4,5-T; chloro-
and nitro- phenols, such as pentachlorophenol (PCP); and sulfonated compounds,
such as azo and anthraquinone acid dyes, and linear alkylbenzenesulfonate
(LAS) surfactants. Organic bases of significance include the nitrogen
heterocycles, such as quinoline, and aromatic amines, such as benzidine.
This Research Brief focuses on the partitioning of organic acid compounds
in octanol-water and in sediment-water systems. The jnain emphasis is to
characterize the adsorption processes of these compounds "mechanistically," to
permit the development n~F -tpmni rfai-fry^ 4jrt-nac-3trrg-^u^t-jwi.i.y T^J a*~i*?T?*5:h'JiTT; in
lignt of the differences in adsorption reactions among neutral, cationic, and
anxonic-organic compounds, a "briBf theoretical description -is ^iven as t.o -why
these compounds partition as they do in the environment. For example, why do
certain environmental components, such as fish lipid, or the organic material
in soils and sediments preferentially adsorb neutral compounds over charged
species? An explanation is given based on thermodynamic considerations, and
various assumptions and requirements necessary to quantify these partitioning
processes are examined.
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Equilibrium Partitioning
If no kinetic limitations exist, the tendency to partition, expressed as
an equilibrium adsorption coefficient (Kd), is directly related to the
standard free energy of adsorption, AG°ds, as expressed by Equation 1. (See
[1,2] for a more thorough discussion).
AG°dg = - RT In Kd (D
To characterize the adsorption of organic compounds to environmental matrices,
such as soils, sediments, and biota, this "standard free energy" of adsorption
must be factored into adsorption components (i.e. soil organic carbon).
Adsorption to each of these components, in turn, results from additive effects
or" ^arioxts adsorption interactions or "energies" . According to Stern [see
1,2], in the most basic form these energies can be separated into an intrinsic
chemical energy, term (AG°^em) and an electrostatic energy term CAG°^) .
AGads - AGchem + <1 (2)
For neutral organic compounds, the total energy of adsorption contains no
electrostatic component, and hence, is equal to the intrinsic chemical
adsorptiozi energy (AG^famn) - -Sox moderately large .{imoleinilar ragygfofc >• J.OQ}
no-npolar compounds, this adsorption energy is dominated by hydrophobia
forces,. Hence -partitioning occurs "out" of hydrophilic -phases such -as -water
to more hydrophobic phases. Water solubility (corrected for crystal energy
of solids), octanol-water partitioning, connectivity indices, chromatographic
retention times, and estimated molecular surface areas, all have been used to
index chemical activity (or free energy changes) of hydrophobic compounds for
estimating partitioning to media such as soil organic carbon and fish lipid.
Because the intrinsic chemical adsorption energy (AG^^) for organic
compounds, as a rule, results from hydrophobic interactions, ;this term for
both cationic and anionic organic compounds is less than for neutral compounds
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of similar structure. For these charged species, however, the primary change
in the overall adsorption energy may result from electrostatic contributions
to adsorption (AC0^) . This electrostatic contribution can be positive or
negative, and is equal to the product of the compound's valence (z) , Faraday's
constant (F) , and the surface potential (V>) . Most natural soils and sediments
contain mineral and organic surfaces that possess negatively charged
functional groups. Therefore, the overall sign of AG^ is ultimately
determined by the charge of the adsorbing ion.
For adsorption of cationic organic compounds (z —Hi) , the overall sign
of AG°^ is negative, and electrostatic interactions can promote adsorption
beyond what is predicted for a similar neutral compound based purely on
hydrophobic interactions. Examples of this in the literature include the
adsorption of benzidine (4,4-diaminobiphenyl, pK& = 4.3) [3] and quinoline
(pKa — 4.94) [4] onto sediments or aquifer material. The adsorption of these
compounds to natural surfaces is highly pH dependent because of their
relatively low pK& values. At pH values where the cationic form of these
compounds predominates, adsorption can be two orders of magnitude greater than
rtbe hydrophohic adsorption «f ±he corresponding TKentral species^ This .type of
.reaction is generally referred to as ligand (or cation) exchange, and modeling
this :process relies on understanding the adsorption behavior of other
• Q _|_0
exchangeable ions, such as Ca and Mg . Also, adsorption can occur to
various subcomponents of soils and sediments, such as the organic matrix and
various mineral components. To effectively describe these processes, model
algorithms must be somewhat complex. Modeling the adsorption of organic
cations may best be accomplished by incorporating adsorption algorithms into
"electrostatic" or ligand exchange adsorption models, similar to those used to
estimate metals adsorption.
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For the adsorption of anionic organic compounds (z = -1), the overall
sign on AG°-^ is positive, and electrostatic interactions act to attenuate
adsorption. Hence, adsorption to natural sediments by the anionic form of an
organic acid is generally less than adsorption of the corresponding neutral
species. Because no specific electrostatic interactions occur to promote
adsorption, the partitioning process must result from the existence of a
significant chemical adsorption energy term. This situation results in
adsorption occurring predominantly through hydrophobic interactions,
attenuated by electrostatic forces. Indeed, in limited cases [5,6], the
adsorption of anionic organic compounds has been normalized to the organic
carbon content of the sediments and soils. In another study [7], adsorption
oi? IAS -surfactants was shown to increase with surfactant alkyl chain length
through the surfactant series investigated, indicating that hydrophobicity
contributes to the total energy of adsorption.
In quantitatively assessing these partitioning reactions, separating the
hydrophobic component of partitioning from the electrostatic component is
essentially impossible. As with neutral organic compounds, however, model
systems can be used to assess :and possibly quantitatively describe sorption
-processes. Model systems include some of the same "systems" or parameters
-used-to assess partitioning of neutral compounds, such as solubility or
octanol-water distribution. Indeed, many of the same factors that influence
the transfer of organic acids between octanol and water (or affect the
apparent water solubility) influence the transfer of these compounds between
the components of the more heterogeneous system of sediment-water [8-10].
These similar factors include the degree of dissociation and the
hydrophobicity and electrostatic influences of both sorbent and sorbate.
As a result of these similarities, the distribution of several organic
acids between octanol and water phases has been examined as a function of
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aqueous phase salt concentration (for KCl, NaCl, LiCl, MgCl2, and CaCl2) and
aqueous phase pH [9]. Compounds that have moderately low (< 5) pKa values
were chosen. From these experiments, equilibrium partition coefficients of
the neutral and charged forms, as well as several pK& values have been
calculated. The sediment-water distribution of selected organic acids also
has been investigated as a function of aqueous phase ionic composition and pH
[10]. Experimental results and model development for each system is briefly
reviewed.
Octanol-Water Partitioning
The distribution of organic acids between water and octanol in the
presence of monovalent inorganic cations can be described by -Reactions 1
through 6 of Figure 1. The reactions in Figure 1 are formulated in terms of
KCl as the monovalent salt, although NaCl and LiCl also have been used. The
mass action equations-for these reactions, along with the material balance and
electroneutrality constraints, comprise the chemical equilibrium model.
Reaction 1 describes acid dissociation in the aqueous phase. Reaction 2
describes transfer of the neutral species (HA) to the octanol phase. This
:reactian is the Tsasis of linear _±ree energy TElaSonsnips xelating 3CQC (the
sediment or soil equilibrium partition coefficient^ normalized for organic
carbon content) and KOW (the octanol-water partition coefficient) for sorption
of nonpolar organic compounds [11,12]. For any ionizable compound, this
mechanism is important when a significant fraction of the total compound
exists in the neutral form. Reactions 3 and 4 represent transfer of the free
and ion-paired inorganic species, respectively, to the octanol phase. For
both KCl and NaCl, an aqueous phase concentration of 0.2 M (with no organic
compounds present) will result in approximately 40 /zM total salt partitioning
into the octanol phase. Reactions 5 and 6 describe the transfer of the free
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and ion-paired organic species, respectively, and the inorganic counter-ion to
the octanol phase. The transfer of the organic ion, as a free species or as
an ion-pair, is highly dependent upon the counter-ion. Also, as the aqueous
phase inorganic salt concentration is increased, the transfer of the organic
ion as an ion-pair is favored.
To illustrate these effects, the overall distribution ratio (D), of total
compound in the octanol phase to total in the aqueous phase, can be
calculated:
D = U^Jo + [A JQ -t- IJl A JQ; (3)
([HA]W + [A']w)
where, M+ represents the monovalent electrolyte K+, Na+, or Li+. For
experiments-performed at sufficiently high pH (e.g., 12), [HA] is an
insignificant species in either phase, and the overall distribution results
from Reactions 5 and 6 of .Figure 1. Using the calculated partition
coefficients K^x and K^px for the inorganic species KC1, NaCl, or LiCl, and
the measured coefficients K^, and K^ for the organic compounds
pentachlorophenol (PCP), 4-chloro-a-(4-chlorophenyl)-benzeneacetic acid (DDA),
and 2a4,,5-±richloxopiieiiDxyaEet±c acid .^2f^3-T)^ vs can ofarain the overall
distribution of these compounds (Figure 2) as a function of aqueous phase salt
-concentration,. For comparison, the concentration of the organic species IA"]W
was fixed at 10 /iM, and the pH value was set at 12. For the salts NaCl and
KC1, little difference in the calculated distribution is seen, whereas
partitioning into the octanol obviously is favored for the LiCl system. Also,
the importance of the hydrophobicity of these compounds is apparent. The
magnitude of partitioning into the octanol for the organic anions follows the
sequence PCP > DDA > 2,4,5-T for all three salt systems. This is the same
ordering as the octanol-water distribution coefficients of the corresponding
neutral species (KQW). Indeed, for ten organic compounds investigated,
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including various carboxylic acids and phenols, the calculated partition
coefficients for formation of the ion-pair in the octanol phase (Kj_p)
correlates extremely well to KQW values [9].
Results of four experiments in which pH was varied at constant ionic
strength ([K+]w = 0.1 M) are presented in Figure 3. The curves were
calculated using the partition coefficients given in Figure 3 for the organic
compounds arid calculated values for the coefficients Kix and Kipx, for KC1.
At low pH values (approximately pH 1 to 2) , the neutral species is dominant in
both phases, and the value of log D is Independent of pH. At intermediate pH
values (approximately 4 to 6), the neutral species is dominant in the octanol
phase and the am'onic species becomes dominant in the aqueous phase, and the
distribution ratio becomes pH dependent. At high pH (above pH 9), the anion
predominates in both phases, and the distribution ratio is again independent
of pH, but dependent on ionic strength, as previously discussed.
The significance of each component of this equilibrium system can be more
clearly seen in a log[ concentration] versus pH diagram as presented in Figure
4 for 4,6-dinitro-o-cresol (DNOC). All lines in this figure are calculated
;:from ihe .six stpfnThrxom constants 3for ISHDC and 1HH, the :tcrtal jnass of 13HDC
and KC1 in the system, and the volumes of each phase. The difference Between
±he concentration of T5NOC in the octanol at low pH and the concentration in
the water at high pH is due to the difference in the volumes of the two
phases.
Sediment-Water Partitioning
As in octanol-water partitioning, the distribution of organic acids
between sediment and water is highly dependent upon the degree of chemical
dissociation. To assess the effects of pH (and hence, dissociation) on
sediment-water distribution, initial experiments have been performed using a
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pH-stat. The pH-stat can be used to titrate and maintain a sediment slurry to
a given or set pH value. Over the course of an experiment, a slurry can be
sampled and the pH then can be lowered to a new set-point sequentially,
resulting in a "titration curve". Considerable changes in sediment
characteristics result from such a titration. This approach, however, allows
factoring of the overall chemical distribution into discrete coefficients for
the neutral species (Kd) and the anionic species (K^) . These constants may
be defined operationally as,
Kd = sed (4)
[HA]W
(5)
[A'Jw
with,
[HA]W
where, [HAjsed (mo I/kg) and {HA]W (M) are the Tieutral species concentrations
in sediment and aqueous phase, respectively, and [A~]secj and [A"]w are the
anionic species sediment and .aqueous pfaa««»
Because' only total concentration in each phase can le determined analytically,
mass "balance equations fox each phase and for the total system are needed,
[HA]aq = [HA]W + [A']w (7)
[HA]S = [HA]sed + [A']sed (8)
[HA]t = [HA]aq + (m/v)[HA]s (9)
where, (m/v) is the sediment mass (kg) to aqueous volume (L) ratio.
Combination of Equations 4 through 9 results in an expression for the fraction
of total compound in the aqueous phase,
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f = [HA]W+ [A-]w _ (K
[HA]t (Ka + {H+} + (Kd{H+) + KdiKa)(m/v))
Results of initial studies on several organic acid compounds, using a silty
sediment from the Ohio River (EPA sediment 11) suggests a definite pH
dependence on Kd^ [10] . In accord with electrostatic considerations described
previously, some type of pH dependence on the adsorption of the anion may seem
unequivocal, resulting from changes in the surface charge or degree of
protonation of the organic carbon matrix. For the compounds studied on EPA
sediment 11 to date, Kd^ can be expressed as,
log Kdi = ai(pH) + log Ki (11)
where, the average pH dependence or -slope (0^) is equal to 0,30, and JC^ is -the
"intrinsic" chemical adsorption term (given in Table 1 for each compound).
Only one sediment system was examined, however, and clearly more evidence is
needed to confirm this general pH dependence. In total, 5 parameters are used
in Equations IB and 11. One constant., jkKg, is intrinsic ±o the adsorbate; two
constants, T£d and K^, describe "the interactions "between adsorbate and
radsorbent; one constant, a^ : is contained in Kd^ (along with Kj) and is
-""intrinsic '"to the adsorbent; and pH (or {H+}) is the independent variable.
The experimental data for four compounds are shown in Figure 5, along
with model results using the constants given in Table 1. For PCP, DBA and 2-
(2,4,5-trichlorophenoxy)-propionic acid (silvex) , organic carbon-normalized
partition coefficients for the neutral species (Koc) are, within a factor of
two, equal to the KQW. The KQC for DNOC is significantly greater than its
K , likely resulting from polar interactions with the sediment by, the nitro-
groups on DNOC. A similar trend exists for the anionic species.
10
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Partitioning in octanol-water and sediment-water systems by organic
compounds that contain a single, relatively strong (2 < pKa < 5) acidic
functional group has been examined. Clearly, a significant conclusion of this
work is that, at pH values found in most natural environments (i.e., 5 to 9),
Petitioning can occur by both the neutral and ionic forms of these and
similar compounds. Therefore, to determine a priori chemical distribution in
the environment, partitioning must be factored into measurable or predictable
quantities for each adsorbing species, as has been attempted here. The
adsorption of neutral organic compounds is the simplest case, wherein the
-adsorption energy is largely determined by hydrophobic interactions.
Octanol-water distribution (KQW) , in turn, has been used as a measure of
chemical hydrophobicity. Octanol-water distribution for organic anions is
more complex, and is a function of both chemical hydrophobicity and inorganic
ion composition. For the partitioning to sediments, an "ionic" or
electrostatic effect also is observed for organic ions. This effect is most
obvious as a result of varying the pH of sediment systems, and may result from
changes ±n ^surface 4»xopextxB3 (i_,e^.; xhaxge).. '33iese and joiffaer Effects ±faat
determine the adsorption of organic .anions to natural materials must be
examined further.
11
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.References
1. Stumn, W and J.J. Morgan. 1981. Aquatic Chemistry, An Introduction
Emphasizing Chemical Equilibria in Natural Waters, 2nd Ed. John Wiley and
Sons, New York.
2. Morel, F.M.M. 1983. Principles of Aquatic Chemistry John Wiley and
Sons, New York.
3. Zierath D.L., J.J. Hassett, W.L. Banwart, S.G. Wood, and J.C. Means.
1980. Sorption of benzidine by sediments and soils. Soil Science. 129:277-
281.
4. ZacharaJ.M., C.C. Ainsworth, L.J. Felice, and C.T. Resch. 1986.
Quinoline sorption to subsurface materials: Role of pH and retention of the
organic cation. Environ. Sci. Technol., 20:620-627-
5. Nkedi-Kizza, P., P.S.C. Rao, and J.W. Johnson. 1983. Adsorption of
diuron and 2,4,5-T on soil particle size separates. J. Environ. Qual.
12:195-197.
6. Ogram, A, V., R.E. Jessups, L.T. On, and P.S.C. Rao. 1985. Effects of
sorption vn biological degradation rates of (2,4-dichloropherrency) ace-tic acid
in soils. Appl. Environ. Microbiol. 49:582-587.
7. Hand, V.C., G.K. Williams. 1987. Structure-activity relationships for
sorption of linear alkylbenzenesulfonates, -Environ. £c±. Technol., 21:370-
373.
8. Westall, J.C., C. Leuenberger, amd R.P. Schwarzenbach. 1985. Influence of
pH and ionic strength on the aqueous-nonaqueous distribution of chlorinated
phenols. Environ. Sci. Technol., 19:193-198.
9. Jafvert, C.T., J.C. Westall, E. Grieder, and R.P. Schwarzenbach. 1989.
JJistribution of hydropnobie ianogenic organic compounds between octanol and
-water* "organic adds. To 3>E submitted-ior publication,
10. Jafvert, C.T. 1989, Sorption of organic acid compounds to sediments:
inital model development. Summitted for publication.
11. Leo, A.; Hansch, C.; Elkins, D. 1971. Partition coefficients and their
uses. Chemical Reviews. 71:525-616.
12. Lyman, W,J~; Loreti, C,¥. Prediction of Soil and Sediment Sorption
for Organic Compounds. Final Draft Report to Office of Water, U.S. EPA,
Washington D.C., 1986.
12
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Table 1. Constants Used in Modeling the Adsorption of Organic Acid Compounds
to Sediment 11.
Compound (m/v)a pKab log Kd log Kj_
PCP
DDA
DNOC
Silvex
0.05
0.08
0.10
0.10
4.85
3.66
4.46
3.07
3.0
2.45
1.93
1.87
3.33
2.31
2.83
2.03
.f Sediment mass to water volume ratio (kg/L)
bReference [9].
13
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Reactions of HA Expression
1. [HA]W - [H+]w + [A-]w Ka
2. [HA]W - [HA]Q Kow
Reactions in KC1, NaCl, and LiCl salt systems.
3. [K+]w+ [Cl-]w= [K+]0 + [C1-]Q Kix
4. [K+]w+ [Cl-]w = [K+C1-]0 Kipx
5-. IK+]W + [A-]w = [K+]0 + [A']0 K±
6. IK+]W + [A']w = [K+A']0 Kip
7. ir'-i^-H IOH-]W = IK+]O -H IOH-]O
8. [K+]w + [OH']W = [K+OH-]Q
Figure 1. Equilibrium Reactions of Monovalent Organic Acids
in Octanol-Water. Adapted from reference [8]. HA
represents a neutral organic molecule and A"
represents the corresponding mono-valent organic anioiu
Terms subscripted with "w" or "o" represent aqueous
phase or octanol phase concentrations, repectively.
14
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2.5-
2.0-
1.5-
PCP, LiCI
PCP, NaCI
PCP, KC!
DDA, LiCI
O
cr>
O
1.0-
DDA, NaCI
DDA, KCI
0.5-
- 2,4.5-T, LiCI
0.0-
2,4,5-T, NaCI
.2.4,5-T, KCI
0.04 0.08 0.12 0.16 0.20 0.24
Total K, Na, or LI (M) In Aqueous Phase
Figure 2. The octanol-water distribution of 2,4,5-T, DDA, and PCP,
using the calculated partition coefficients from experimental
data, and setting [A"]w at 10 /iM and the pH at 12.
15
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log(K)
6.0-J
4.CH
o' 2.0-1
O.CH
-2.0-
• PCP
n DDA
«• 2,4,5-T
A DNOC
5.07
4.64
3.31
2.14
-4.85
-3.66
-2,83
-4.46
-2.13
-3.60
-3.76
-3.92
Kip
2.67
1.84
0.60
0.016
JDLD
iBLt) ' ' ia!b ' ' 1DJD ' 12.0 14.D
PH
Figure 3.
The octanol-water distribution of four compounds as a function of
aqueous phase pH. Symbols are data points, and lines are
calculated by minimizing residuals on KQW and Ka. In experiments,
the aqueous phase contained 0.1 M KCl, and HC1 or KOH for pH
adjustment. The total mass of compound per tube in each experiment
was constant.
16
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Q)
O
c
O
o
-2.0-
J -4.0H
-8.0H
(o) Total Measured In
Octanol Phase
(•) Total Measured In
Water Phase
in Octanol
IHAH-A- H- A-K+]O
Total m Water
[HA +
.-4JD
Aqueous Phase pH
\
B.O
1DJ)
4 pC - PH diagram of DNOC species partitioning between octanol and
• H^er as a function of pH> symbols are data points and lines are
model fit. Solid lines are for concentrations in the octanol
phase, and dashed lines are for concentrations in the aqueous
phase' The volumes of each aqueous phase - 10 ml, volume of
octanol - 2.5 ml, total mass of DNOC in each tube - 1.25x10'
moles. The aqueous phase contains 0.1 M KC1, and HC1 or KOH for
pH adjustment.
17
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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WASHINGTON, D.C. 20460
OFFICE OF
RESEARCH AND DEVELOPMENT
SUBJECT:
FROM:
TO:
Transmittal of an ORD Research Brief Entitled Assessing
the Environmental Partitioning of Organic Acid
Compounds (Del iverable ,-§149A)
ourtney Riordan
Director, Of f ice MDT Envi ronmental Processes and
Effects Research (RD-682)
Sylvia Lowrance
Director, Office of Solid Waste (OS-300)
The attached research brief is being delivered to your
office in response to the Agency's need for information about
those processes that determine fate and resultant potential
exposures to organic chemicals that may enter natural surface and
ground-water systems from disposal sites.
The objective of the work reported here was to examine
partitioning of organic acids in octanol-water and sediment-water
systems. The main emphasis was to characterize adsorption
processes of these compounds "mechanistically" to permit
development of quantitative structure-activity relationships.
A significant conclusion of the work is that, at pH values
found in most natural environments (i.e., 5 to 9), partitioning
can occur by both neutral and ionic forms of the tested
compounds. Therefore, to determine a priori chemical
distribution in the environment, partitioning must be factored
into measurable or predictable quantities for each adsorbing
species. Effects of pH and other variables that determine
adsorption of organic ions to natural materials will be examined
and quantified in continuing research.
This document is product of the Land Disposal Assessment and
Evaluation of Other Management Systems research program at the
Environmental Research Laboratory, Athens, GA. The results of
this study have been prepared as a research brief in order to
provide interim results in a timely manner and in a form useful
to your office. Thus, this report has been reviewed within ERL-
Athens but has not been subjected to the Agency's peer review
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process; it should not, therefore, be distributed to, or used by,
non-Agency persons. Further along in the research program, when
more variables have been investigated and quantified, and the
structure-activity relationships are better understood, a peer
reviewed, thoroughly scientifically defensible journal article or
report will be written. Dr. Zubair Saleem of your
Characterization and Assessment Division is familiar with the
project.
cc: Chris Zarba (WH-585)
Elizabeth Bryan (TS-798)
Patrick Holden (H-7509)
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