United States
Environmental Protection
Agency
Environmental Research
Laboratory
Athens GA 30613
Research and Development
EPA/600/S3-85/050 Sept. t985
Project Summary
Pollutant Sorption to Soils and
Sediments in Organic/Aqueous
Solvent Systems
Jaw-Kwei Fu and Richard G. Luthy
The solubility end sorpiion properties
of aromatic solute* in aqueous system*
solvents were
tddressed the
containing polar organic
investigated. The work
situation in wnich the rellease of hydr o-
phobic organic contamin int* as * result
of « spill or accidental dis charge may be
accompanied by the rektase of water-
soluble erg*nic solvents,
Th« inveatiga-
tion examined the effect* of * polar
solvent in weter on the solubility Qf
relatively hydrophobic solutes and the
•tf«ct of polar solvent-water mixtures
on sorption of organic solutes onto soil.
The results ere interpreted by meant of
chemical thermodynamie models to
predict solubility in water/solvent sys-
tems and by semi-empirical models to
correlate reduction in sorption charac-
teristics with increase in solubility.
This Project Summary WAX developed
by EPA's Environmental Research
Laboratory. Athens, GA, to announce
key findings of the research project that
is fully documented in a separat* tOfunt
Of the same title (see Project Report
ordering information at back).
Introduction
In recent years, several investigators
have reported on measurement and
modeling of the sorption of hydrophobic
organic compounds onto soils. Hence,
much is known about the manner in
which these types of solutes sorb onto
soil/sediment for the case of clean sys-
tems in which the aqueous phase con-
tains a relatively low concentration of the
organic material. This work is useful for
understanding sorption and transport of
organic solutes in clean water systems;
however, it is not broadly applicable to
understanding organic solute sorption
and transport for the case of near-source
release of wastes to the environment. In
these instances, the processes of solubi-
lization, sorption, and transport of hydro-
phobic organic solutes may be influenced
Strongly by the presence of comparatively
high concentrations of miscible organic
solutes originating from the waste.
The phenomenon of organic solute
solubilization in water co-solvent mix-
tures was evaluated for 18 systems. The
systems were comprised of four water/
co-solvent mixtures and seven solutes as
listed in Table 1.
The sorption of solute onto soil in
water/co-solvent systems was evaluated
for two water/co-solvent mixtures
(water/methanol and water/acetone),
With four solutes (naphthalene, naphthol,
quinoline, and 3,5-dichloroaniline), and
three silt/loam soils of varying percentage
fractions of organic carbon.
The solutes represented a range of
physico-chemical characteristics with
respect to polarity and hydrogen-bonding.
The solvents represented common, water
soluble solvents with different hydrogen-
bonding characteristics (i.e., alcohols
being either H-bonding proton donors or
acceptors, and acetone being an H-
bonding proton acceptor).
Results
In general, it was observed that solute
solubility increased in a semi-logarithmic
manner with respect to volume fraction
organic solvent. For example, as. shewn in
Figure 1, naphthalene solubility increases
from approximately 31 mg/l in clean
water to approximately 70,000 mo/1 in
100% methanol. Data presented in Figure
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*
Table 1. Water/Co-Solvent Systems
Methanol
Naphthalene
Naphthol
Quinoline
Aniline
3.5-Dichloroaniline
Xylene
Phenanthrene
Ethanol
Naphthalene
Phenanthrene
Aniline
Xylene
Propanol
Naphthalene
Xylene
Acetone
Naphthalene
Naphthol
Quinoline
3,5-Dichloroaniline
Xylene
100,
so
30
20
10 -
o
to
.7
.05
.03
.02
.01
Solubility Enhancement
Solute = Naphthalene
Solvent = Methanol/Water
0 20 40 60 80 100
Methanol Content [% by Volume]
Figure 1. Naphthalene solubility in meth-
anol/water solution.
2 show regression equations that describe
the semi-logarithmic solubility behavio'r
for four solutes and water/acetone sol-
vent, systems.
Previous work with sorption of relatively
hydrophobic organic solutes onto soils/
sediments has shown that sorption is
inversely proportional to solubility. Thus
it may be anticipated that sorption of
polycyclic aromatic hydrocarbons and
related compounds may decrease in a
semi-logarithmic manner with respect to
fraction organic solvent in the aqueous
phase. This is substantiated in part by
results shown in Figures 3 and4. Figure 3
shows linear soil sorption isotherms for
naphthalene with 0,10,30,40, and 50%
volume fraction of acetone. The linear
adsorption isotherm coefficients, Kp, for
naphthalene were found to decrease
semi-logarithmically with fraction ace-
tone co-solvent. This was observed also
for sorption of quinoline, naphthol and
3,5-dichloroaniline in methanol/water
systems as shown in Figure 4.
Prediction of Solute Solubility
The experimental data show that the
presence of an appreciable concentration
of organic solvent in the aqueous phase
can have a very large effect on solubility
and sorption of hydrophobic organic
solutes onto soils. To better understand
these phenomena, several thermody-
namic approaches were invoked to eval-
uate the solubility data. Four approaches
were evaluated to predict solute solubility
in water/co-solvent systems. These ap-
proaches are founded on concepts used
to 'describe fluid phase equilibria in
multicomponent systems. A brief descrip-
tion of each of these approaches is
presented below. A thorough discussion
of the results of this study and the
methodologies employed for predicting
solute solubility in mixed solvent systems
is presented in the project report.
1. Log-linear—This approach follows
principally from regular solution
theory wherein the logarithm of
mole fraction solubility is propor-
tional to volume fraction co-solvent.
2. Molecular Surface Area—This ap-
proach is based on consideration of
hydrophobic surface area (HSA) and
polar surface area (PSA) of the
solute, and interfacial free energy
terms for water and solvent.
3. Excess Free Energy—This tech-
nique is based on expressions that
describe the excess free energy
that results from non-ideal mixing.
4. UNIFAC Method— The UNIFAC
method is based on a group contri-
bution approach for prediction of
activity coefficients for solute and
solvents. The method employs a
700
50
20
70
5
5
.05
.02
.01
f\°
Solubility Enhancement
Solute = Naphthalene, Naphthol,
Quinoline, 3,5 Dichloroaniline
Solvent - Acetone/Water
0 10 20 30 40 50
Acetone Content [% by Volume]
Figure 2. Solubility of naphthalene, naph-
thol, quinoline. and3,5dichloro-
aniline in acetone/water solu-
tion.
solution-of-group concept in which
the activity coefficient is computed
from parameters that relate to dif-
ferences in size and shape of a
molecule in a mixture and to the
energy of interaction between vari-
ous groups.
Prediction Results
Solubility predictions for the 18 solute/
water/co-solvent systems were per-
formed to evaluate the four approaches.
In general, the UNIFAC procedure could
predict solute solubility within a factor of
about ±50%. The other predictive pro-
cedures could give improved accuracy,
provided sufficient experimental data
were available. For most purposes, the
UNIFAC procedure was suitable for initial
assessment. The log-linear approach is
attractive for those situations in which
some solute solubility data exist for
water/co-solvent mixtures.
-------
Conclusion
Hydrophobia aromatic solutes display a
semi-logarithmic increase in solubility
with increasing volume fraction of co-
solvent in water/co-solvent mixtures.
This results in a semi-logarithmic de-
crease in tendency for these solutes to
sorb onto soil. The increase in solubility
does not result in a directly proportional
decrease in sorption coefficients for the
solutes studied in this investigation. The
increase in solute solubility in water/co-
solvent mixtures may be predicted by
thermodynamic models. These predic-
tions may be usef utln other applications.
8.0
5.0
&3.0
*2
.s
.1
1.0
o
-------
J-K. Fu and R. G. Luthy are with Carnegie-Mellon University, Pittsburgh. PA
15213.
S. W. Karickhoff is the EPA Project Officer (see below).
The complete report, entitled "Pollutant Sorption to Soils and Sediments in
Organic/Aqueous Solvent Systems," (Order No. PB 85-242 535/AS; Cost:
$22.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Research Laboratory
U.S. Environmental Protection Agency
Athens, GA 306 f3
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S3-85/050
0000329 PS
U S ENVIR PROTECTION AGENCY
RESIGN 5 LIBRAIY
230 S Q£AR§0«N STRiET
CHICAGO It 60604
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