United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S2-90/004 Apr. 1990
&EPA Project Summary
Adsorption of Organic Cations to
Soils and Subsurface Materials
John C. Westall, Bruce J. Brownawell, Hua Chen, John M. Collier,
and Julia Hatfield
A study of the fundamentals of
adsorption of amphiphilic organic
cations on natural and pristine sur-
faces was conducted to elucidate (i)
the factors that influence the extent
of adsorption and (ii) indirect effects
of adsorption of organic cations: the
competitive adsorption of organic
cations and metal cations and
increased adsorption of hydrophobic
organic compounds on mineral
surfaces coated with organic cations.
The sorbents studied were kaolin-
ite, montmorillonite, silica, alumina, a
soil, and two aquifer materials. The
distribution ratio of the alkylpyrid-
iriium depended strongly on the
nature and concentration of inorgan-
ic: cations in solution, but solution pH
had little effect The adsorption iso-
therms were distinctly nonlinear,
even at very low surface concen-
trations of organic cations. The ad-
sorption of dodecylpyridinium in the
presence of various concentrations
of salts was described quantitatively
with a multisite competitive ion-
exchange model; two adsorption
reactions were significant: exchange
of pyridinium with a metal cation, and
adsorption of pyridinium with chlor-
ide counter-ion.
The indirect effects of adsorption
of organic cations were investigated
as well. Dodecylpyridinium was found
to displace Cu(ll) effectively from
Lula aquifer material when the
surface concentration of dodecylpy-
ridinium is within 10 - 100% of the
adsorption maximum. Dodecylpy-
ridinium on Lula aquifer material
increased the sorption of chloro-
benzenes; organic-carbon normalized
distribution ratios of chlorobenzenes
(Koc's) were greater for DP-loaded
sorbents than for natural sorbents,
and the values of K^ were greater for
greater amounts of dodecylpyrid-
inium on the surface.
This Project Summary was
developed by ERA'S Robert S. Kerr
Environmental Research Laboratory,
Ada, OK, 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
Amphiphilic organic cations are used
in a wide range of consumer and
industrial products such as detergents,
fabric softeners, dyes, herbicides, emulsi-
fying agents, and additives to drilling
muds. Adsorption of organic cations
influences their bioavailability, transport,
and fate. Adsorption can have important
indirect effects as well: organic cations
can effectively displace adsorbed metal
ions, and adsorbed organic cations can
increase the affinity of sorbents for
hydrophobic compounds. Despite the
importance of these compounds, there
has not been extensive systematic inves-
tigation of their adsorption to natural
materials.
Direct effects. Organic cations
generally adsorb strongly due to the
electrostatic attraction to negatively
charged surfaces and the favorable
hydrophobic interaction. In the environ-
ment they are likely to accumulate on
surfaces until they are degraded, for
example, in sediments downstream from
a wastewater discharge. The environ-
mental concentrations of organic cations
have not been surveyed as intensively as
/7TV
^gA) Printed on Recycled Paper
-------
the concentrations of many other
compounds. This situation is largely due
to difficulties in separation and analysis of
organic cations in natural water samples.
Organic cations appear to be less
toxic to aquatic organisms in systems
containing suspended particles, presum-
ably due to binding of the organic cations
(Lewis and Wee, 1983). One of the
primary problems in assessing the
environmental safety of organic cations is
the characterization of their physical and
chemical form (speciation) in test sys-
tems. If meaningful interpretation of
laboratory toxicity experiments is to be
made, the speciation of the compounds
in solution must be known.
Indirect effects. In addition to these
direct effects of organic cations in the
environment, the indirect effects must be
considered. While these effects will occur
naturally as organic cations are
introduced into the environment, they
might also be the basis for remedial
action at sites already contaminated with
metals or hydrophobic organic com-
pounds. The observation that adsorbed
organic cations increase the tendency of
mineral surfaces to sorb hydrophobic
organic compounds has led to
consideration of applying organic cations
in the subsurface environment (e.g., clay
liners) to slow the migration of organic
compounds. However, such a treatment
might also lead to desorption and
increased mobility of toxic metal ions. On
the other hand, if a pump and treat
process were being considered, such a
displacement of metals with nontoxic,
biodegradable organic cations would be
beneficial. This study addresses
fundamental questions that need to be
resolved before these applications are
pursued.
Scope of study. The objectives of this
study were: (i) to characterize the factors
that influence the adsorption of
amphiphilic organic cations to natural
sorbents; (ii) to describe the observed
adsorption isotherms in terms of mechan-
istically based adsorption models, which
can be used for understanding environ-
mental distributions; and (iii) to assess
the effect of organic cation adsorption on
the adsorption of metals and hydrophobic
organic compounds.
The organic cations selected for this
study were alkylpyridinium cations;
selection criteria include environmental
relevance, representativeness of the
class, and ease of analysis. The sorbents
were aquifer materials, a soil, kaolinite
and montmorillonite, and silica and
alumina. Efforts were concentrated on
well characterized, low-organic-carbon
sorbents. Cu(ll) and chlorobenzenes were
chosen as representative metal and
hydrophobic organic compounds for
examination of indirect effects.
Methods
Materials 14C-labeled dodecylpyridin-
ium bromide (DP) was obtained from
Pathfinder Laboratories Inc. (St. Louis,
MO). Natural sorbents were low- organic-
carbon (< 0.03% organic carbon) aquifer
materials Borden sand and Lula N6, and
a relatively high-organic-carbon (2.3%)
soil, EPA-12. Georgia kaolin (KGa-1) and
Wyoming Na-montmorillonite (SWy-1)
were obtained from Source Clays
Repository at the University of Missouri.
Berkeley 5 nm silica and 10 tun MCA-
540 alumina were obtained from
Pennsylvania Glass Sand Corporation
and Alcoa Laboratories, respectively. A
74 - 210 tim fraction of Borden sand,
isolated by dry-sieving, was used in this
work. The organic carbon contents and
cation exchange capacities (CEC) of
these materials are given in Table 1.
Equipment. For equilibration and
phase separation, a thermostatted shaker
(New Brunswick Model G-24) and
thermostatted centrifuge (Beckman Mod-
el J2-21) were used. Concentrations of
unlabeled pyridinium compounds were
determined spectrophotometrically with
an HP 8452A UV-Visible spectrophoto-
meter. Radioactivity of the labeled com-
pounds was determined with a Beckman
Model LS 7800 scintillation counter.
Labeled compounds on sorbents were
converted to 14C02 with a Harvey Model
300 Oxidizer.
Determination of distributions. Dis-
tributions were determined from batch
equilibration experiments. The electro-
lytes were NaCI, CaCI2, and NaN3. NaN3
was used in place of NaCI in experiments
conducted at low concentration of DP to
inhibit microbial degradation. The con-
tents of the tubes, which were sealed with
PTFE-lined caps, were mixed on the
shaker at a speed of 500 RPM at 25°C
for 4 hours, after which the aqueous and
sorbent phases were separated by
centrifugation at 11,000 G at 25°C for 30
- 60 minutes. The time for equilibration
was established in independent experi-
ments on adsorption and desorption
kinetics.
After centrifugation, the amount of
alkylpyridinium in the aqueous phase, on
the sorbent, and adsorbed to the walls of
the vessel was determined. The concen-
tration distribution ratio, Dc, was
calculated from:
(D
C.(s) (nmol/kg)
D = —
c C.(w) (pM)
where Cj(s) and Cj(w) were the
concentrations of the component i
determined directly on the sorbent and in
the solution, respectively. The recovery,
or fraction of alkylpyridinium added to the
tube that could be accounted for by
analysis, was also calculated from these
data.
Adsorption of Cu(ll). Two procedures
were developed for investigating the
effect of dodecylpyridinium on the
adsorption of Cu(ll) on Lula aquifer
material: (i) conventional batch adsorption
experiments with Cu(ll), DP, and Lula N6,
in which concentrations of Cu(ll) and DP,
in solution and on the sorbent, are
determined by our standard procedures;
and (ii) discrete additions of DP to Cu(ll)
and Lula N6 in a pH-stat, in which the
change in Cu(ll) concentration is
monitored with a cupric ion-selective
electrode. The advantage of the first
procedure is that the concentration of DP
in solution and on the surface is
determined simultaneously with the
concentration of Cu(ll); the disadvantage
of this procedure is that there is no
control over pH during the time of batch
equilibration. While the adsorption of DP
is relatively independent of pH, the
adsorption of Cu(ll) depends strongly on
pH. In the second procedure, the pH can
be controlled extremely well (± 0.03 pH
units), but the amount of Cu(ll) on the
sorbent must be determined by
difference rather than by direct analysis,
and the distribution of DP is found from a
separate set of experiments.
Adsorption of chlorobenzenes. The
adsorption of chlorobenzenes to Lula N6
aquifer material, with varying surface
concentrations of DP, was determined in
conventional batch experiments. The
chlorobenzenes were 1,2-dichloroben-
zene, 1,2,3-trichlorobenzene, 1,2,3,4-
tetrachlorobenzene, and pentachloroben-
zene. Slurries were prepared that
contained 25 g L-1 of Lula N6, total DP
concentrations between 60 nm and 40
mM, and the chlorobenzenes at total
concentrations approximately 1/100 of
their solubility. The experiments were
carried out in centrifuge tubes with PTFE-
lined caps. Chlorobenzene concentrations
in the aqueous phase were determined
by extraction into hexane and gas
chromatography with electron capture
detection, and amount sorbed was
determined by difference.
-------
Table 1. The Cation Exchange Capacities and Organic Carbon Contents of the Sorbents
Used in this Study. Organic Carbon Contents were Determined with a LECO
Carbon Analyzer. Procedures for Cation Exchange Capacity Determination are
Given in the References Cited.
Sorbent material
Kaolinite (KGa-1)
Na-Montmorillonite (SWy-1)
Borden sand
Lula N6 aquifer material
EPA- 12 soil
Organic carbon
%
0.022
0.020
0.026, 0.0200
0.02 r, 0.033b
2.;'2, 2.33d
Cation exchange
capacity
meq/kg
20"
764"
7.0C, 23b
90"
735<*
a van Olphen, H.; Fripiat, J.J. Eds. Data Handbook tor Clay Materials and Other Non-Metallic
Minerals; Pergamon: New York, 1979.
b Bouchard, D.C.; Powell, R.M.; Clark, D.A. J. Environ. Sci. and Health 1988, A23, 585-601.
c Fuller, C.C.; Davis, J.A. Geochim. Cosmochim. Acta 1987, 51, 1491-1502.
d Hassett, J.J.; Means, J.C.; Banwart, W.L; Wood, S.G. Sorption Properties of Sediments
and Energy-Related Pollutants; U.S.Environmental Protection Agency. National Technical
Information Service: Springfield, VA., 1980; EPA-600/3-80-041.
Results and Discussion
Two factors complicate this study.
F:irst, the nonlinearity of the isotherms
precludes a simple interpretation of the
effects of given properties on the energy
of adsorption. We have ascribed much of
the nonlinearity to the heterogeneity of
adsorption sites of the sorbents, and have
used a distribution of adsorption-site
energies in describing experimental
results. Second, it is difficult, if not
impossible, when studying aqueous sus-
pensions of solid sorbents to vary one
experimental parameter and maintain all
others at constant values. For example,
variation in the solution concentration of
one component causes a significant
variation in the solution concentration of
another component, through interaction
with the sorbent.
Adsorption isotherms. Adsorption
isotherms of dodecylpyridinium (DP) at
relatively high concentrations, on several
different sorbents, are shown in Figure 1.
The equilibrium concentrations of
dodecylpyridinium in solution were well
below the critical micelle concentration
(CMC) in all cases. The surface
concentrations approach or reach plateau
levels, except in the case of EPA-12. The
plateau concentrations correspond
closely, but not strictly, to the cation
exchange capacities reported for the five
sorbents (Table 1). From these data, it
appears that adsorption could be
explained as a simple, virtually quan-
titative, ion-exchange reaction. However,
if the data are considered over a wider
range of concentrations, as shown in
Figure 2, it is apparent that the situation
is more complicated.
The logarithmic adsorption isotherms
of DP on five different materials (Borden
sand, Lula N6 aquifer material, EPA-12
soil, kaolin, and montmorillonite) are
shown in Figure 2. The affinities of the
sorbents for dodecylpyridinium, per unit
mass of sorbent, follows the order
montmorillonite > EPA-12 soil *• Lula
aquifer material > kaolinite > Borden
sand. This order corresponds roughly to
the CEC's of the materials. A judgment
about the presence or absence of a
correlation with organic carbon content of
the materials is hardly warranted, since
the organic carbon content of most of
these materials is so very low. However,
the adsorption affinity of Lula aquifer
material is similar to that of EPA-12
despite the 100-fold difference in organic
carbon contents of these sorbents.
At concentrations of DP in solution
below 125 nM for EPA-12 and Borden
sand, and below 20 pM for Lula aquifer
material, the logarithmic isotherms
(Figure 2) are nearly linear and conform
to the logarithmic form of the Freundlich
equation:
log Cs = n log Cw + log K (2)
where K and n are constants. The values
of n determined from the slopes of the
isotherms in the low concentration region
are approximately 0.6. These low values
of n indicate nonlinearity in the isotherms.
(A linear isotherm is one in which the
surface concentration is directly propor-
tional to the solution concentration, i.e., n
= 1 in Equation 2.) The nonlinearity
extends to surface concentrations that
are as low as 0.02 % of the maximum
surface coverage measured in the case
of EPA-12 soil. As will be discussed, this
nonlinearity is attributed to heterogeneity
of adsorption sites.
The effect of pH on adsorption of DP.
The effect of [H*] on the adsorption of
DP by EPA-12, Lula aquifer material, and
kaolin is shown in Figure 3. The logarithm
of Dc is plotted against log [H*]. A very
small dependence of Dc on pH was
observed for the natural sorbents; the
slopes (A log Dc / A log [H*]) determined
from linear regression of the data in
Figure 3 were -0.04 and -0.01 for Lula
and EPA-12, respectively. The depen-
dence of log Dc on pH for kaolinite was
greater than for the other two materials;
at pH values < 6.06 the slope A log Dc /
A log [H + ] was -0.23. The effects of [H + ]
on adsorption of hexadecylpyridinium
(HP) by the two oxides, silica and
alumina (not shown), were also greater
than that shown for Lula and EPA-12.
Two mechanisms through which pH
can influence adsorption are (i) a change
in surface potential from protonation-
deprotonation of pH-dependent surface
functional groups, or (ii) ion-exchange of
H* for pyridinium. The weak effect of pH
on the two heterogeneous sorbents, in
contrast to the clay and oxides, can be
ascribed to the relative abundances of
pH-dependent surface groups and
traditional ion-exchange sites on the
sorbents.
Nonlinearity of the isotherms. All of
the adsorption isotherms determined in
this study were nonlinear, even at very
low surface coverages of dodecylpyrid-
inium. While the Freundlich isotherm can
be used to represent these data
empirically, it provides no mechanistic
basis for evaluating competition effects,
saturation effects, and comparison of
adsorption energies for a homologous
series of organic cations. Thus the
usefulness of the Freundlich isotherm is
limited.
In order to derive a more useful
description of adsorption data, it is
necessary to consider the causes of
nonlinearity: (i) non-uniformity of energies
of adsorption sites, with saturation of
high-energy sites; (ii) sorbate-sorbate
interactions such as repulsive elec-
trostatic interactions or cooperative chain-
-------
200
o
1 700
CO
o
• EPA-12
m Lula N6
A Kaolinite
C(w)
Figure 1. Adsorption isotherms; dodecylpyridinium in 0.01 M Na f.
s
O)
-r
-2
-3
-4
O Montmorillonite
• EPA-12
m Lula N6
A Kaolinite
- A Borden Sand
1 o oo
A***
-9
-8
-7
-6 -5
log C(w) [Ml
Figure 2. Adsorption isotherms; dodecylpyridinium in 001 M Na
-4
-2
chain interactions, and (iii) experimental
artifacts that stem from covariation of
experimental conditions, such as changes
of pH, major ion concentrations, or
aggregation of particles, that accompany
changes in organic cation concentrations.
For reasons to be discussed, we
interpret the nonlinearity of isotherms in
these experiments primarily as the result
of heterogeneity of sites. Particularly in
the low-concentration regions of the
isotherms, the changes in pH, concen-
trations of major ions in solution, and
surface charge that accompanied adsorp-
tion of the organic cation were too small
to account for the isotherm nonlinearity.
Thus, at low concentrations of organic
cation, heterogeneity of sites is the
preferred explanation. At higher concen-
trations it is more difficult to exclude the
influence of other processes on isotherm
nonlinearity.
The effect of electrolyte on adsorp-
tion. The effect of concentration and type
of electrolyte on adsorption of DP was
examined with two complementary types
of data: (i) adsorption isotherms of DP
from solutions with different concen-
trations of salts; and (ii) concentration
distribution ratio of DP as a function of
concentration of NaCI or CaCI2, with a
constant amount of DP in the system. In
view of the nonlinearity of the isotherms,
the first method is preferable for
development of mechanistic models,
while the second provides a quick
estimate of the magnitude of the effect.
A number of experiments were
performed with different sorbents, and
different inorganic salts at different
concentrations. Here we present data for
one representative system. Figure 4
shows the isotherms of DP on Lula
aquifer material in the presence of 0.01
and 0.1 M NaCI. The effect of the salt
concentration was generally much
greater than that of pH.
At low concentrations of organic
cation, an increase in the salt
concentration decreases the adsorption
by a nearly constant factor. This behavior
is consistent with an ion-exchange
mechanism. At high concentrations of the
organic cation, the isotherms merge and
cross at surface concentrations that
corresponds roughly to the CEC. This
behavior is consistent with the pre-
dominance of adsorption of pyridinium-
chloride at high concentrations of DP.
Similar isotherms (not shown) were
obtained for EPA-12, Borden sand, and
kaolinite.
The effect of Ca2+ on the adsorption
of DP on Lula aquifer material is
-------
• EPA-12
M Lula N6
A Kaolinite
-7
log [H
-3
generally stronger than that of Na* (not
shown), consistent with ion exchange. A
solution of 0.001 M Ca2 * impedes
adsorption of DP more effectively than
0.01 M Na*.
Adsorption model. The mechanistic
models that were considered for these
data were (i) multisite models, (ii)
electrostatic models, and (iii) some
combination of the two. Most of the
phenomena observed in this study can
be explained readily through the multisite
model, with electroneutral reactions and
no explicit electrostatic energy term.
While this approach is obviously an
approximation, it is consistent with a
variety of observations, and it is
conceptually very simple. The role of
electrostatic energy will be examined in a
future study involving adsorption and
electrophoretic mobility. The adsorption
model that we use is based on the
following reactions:
~~ (3)
R + + NaX = RX + Na
RX
R + + Cl + Y = RC1Y
(4)
Figure 3. The effect of pH on distribution of a/kylpyndmium
Dodecylpyndm/um on Lula
2.5 gL
O)
JC
o -2
O
-3
-4
-5
n 0.07 M NaCl
• 0. ^ M NaCf
-9
-8
-7
-6 -5
log C(w) (Ml
-4
-3
-2
Figure 4. Dodecylpyridinium on Lula N6; 2.5 g/L.
2NaX = CaX2
LCaX2
(5)
where the overbar represents a species
associated with the sorbent, X is a
negatively charged cation-exchange site,
Y is an uncharged site (that could be
thought of as a site for hydrophobic
adsorption), and R+ represents the
alkylpyridinium. The mass action equa-
tions for Reactions 3-5 are:
-------
mmol kg'
C
Figure 5. Adsorption of Cu(ll) on Lula A/6; effect of DP.
i 1
1,2-Di
A A 1,2,3-Tn
-A A 1,2,3,4-Tetra
n—Penta
30 40 50 60
COp (s)immol kg'1
Figure 6. The effect of dodecyclpyridinium on adsorption of chlorobenzenes.
(X1( X2, ... Xn), each of which obeys its
own mass action equations and material
balance equations given above. Thus the
model could be described as a multisite
competitive Langmuir model. A similar
ensemble could be invoked for the Y
sites. The adjustable parameters in the
model are the equilibrium constants
given by Equations 6-8 and the total
number of sites, given in Equations 9 and
10. For the X-sites, the discrete
equilibrium-constant-spectrum approach
was used: the values of KRXj were set to
integral powers of ten (10°, 101, ...) and
the total concentration of each type of
sites was determined, Tx,. The objective
of this approach is to represent the
experimental data in terms of a regular
distribution of sites, rather than the
smallest number of adjustable param-
eters. This approach facilitates com-
parison of complex isotherms obtained
for different sorbents.
All parameters were determined simul-
taneously with FITEQL 2.0, a weighted
nonlinear least squares adjustment
procedure for multicomponent chemical
equilibrium problems.
Application of model. This model was
applied first to the adsorption of DP on
Lula aquifer material. In the first step,
data at two different concentrations of
Na+ (Figure 4) were used to determine
the TXj of the equilibrium-constant
spectrum; since only one type of sites Y
was considered, both KY and TY were
determined. The constants are given in
Table 2, and the isotherms calculated
from the constants are given by the lines
in Figure 4. The agreement between the
model and the data is excellent.
This sort of model was applied to data
for adsorption of DP on EPA-12 from
NaCI solutions, and to DP adsorption
onto Lula and EPA-12 from CaCI2/NaCI
solutions. In all cases the agreement
between the models and the data was
excellent.
Certainly this "discrete equilibrium-
constant-spectrum" approach is subject
to the criticism that a model can be fit to
any set of data if enough adjustable
parameters are used. Furthermore, there
is some covariance among the values of
the adjustable parameters; the values are
not unique, but just one of an infinite
number of sets that can be used to
represent the data.
However, the advantages of this
approach are significant, too. The
practical advantage is that the effects of
multiple interactions in complex systems
can be represented in a way that is
completely compatible with chemical
equilibrium models used in environmental
-------
Table 2. Equilibrium Constants for Adsorption of Dodecylpyridinium on Lula Aquifer
Type of site Equilibrium constants"-*3
log KRX log KCaX2 log Kync/
Total site density
mol kg'1
5.00
3.00
2.89 x TO'4
X2 3.00 0.57
Y 4.95
3.21 x JO'2
7.99 X 70'2
a Reactions described in text, Reactions 3, 4, 5. Equilibrium constants valid for 25 ± 2 °C,
with aqueous concentrations in mol L1 and concentrations on sorbent in mol kg-1.
Properties of sorbents listed in Table 1.
b Constants were determined by the following procedure: values of Kpx were set according
to the equilibrium constant spectrum approach, and values of Tx, TY, and KYRCI were
determined from data in Figure 4a with FITEQL. Then values of K"caX2 were determined
from data not shown here.
chemistry. An adsorption isotherm that
conforms closely to the Freundlich
equation over more than six orders of
magnitude range in C(w) can be
described very well by conventional
mass action and material balance
equations; these equations have a
mechanistic basis, which potentially
could be used to understand the factors
that influence adsorption.
Another advantage of such a model is
that it provides a framework for
evaluating quantitatively mechanisms that
could be responsible for the phenomena
that are observed experimentally, such
as, (i) reversal of effect of electrolyte
concentration on isotherms near the CEC;
(ii) lateral shift of isotherms with salt
concentration far below the CEC; (iii)
absence of pH effect; and (iv) relative
effect of Na vs. Ca on the isotherms.
Adsorption of Cu(ll) on Lula A/6. The
adsorption isotherms of Cu(ll) on Lula N6
in 0.01 M NaCI, in the absence and
presence of various concentrations of DP,
are shown in Figure 5. (The isotherms in
the figure are from the batch
experiments; the isotherms from the pH-
stat experiments were quite similar, but
are not shown.)
In the absence of DP, the apparent
distribution ratio (Dc) of Cu(ll) varies
between 2000 and 100 L kg-1 for solution
concentrations between 5 and 100 nM.
Over this range of solution concentrations
the surface concentrations range between
I and 10 mmol kg-1, which is still far
below an apparent cation exchange
capacity of the sorbent. Solution
concentrations were held below 200 pM
and pH below 5.5 in these experiments to
avoid problems with precipitation of
Cu(OH)2(s).
Adsorption isotherms of Cu(ll) with
various total concentrations of DP are
also shown in Figure 5. (The total
concentration refers to the total number
of moles of DP added per liter of solution;
this is not the same as the final
equilibrium concentration in solution. The
approximate relation between total and
adsorbed DP is given in Table 3.) The
effect of DP is to depress the adsorption
of Cu(ll) significantly, particularly above
100 iiM total DP (or 17 mmol kg-1 DP on
the sorbent) At 400 and 1000 nM total
DP, the DP concentrations on the surface
were close to the apparent "CEC" for
adsorption of DP; only under these
conditions did the moles of DP desorbed
per mole of Cu(ll) adsorbed approach the
stoichiometric 2:1 limit characteristic of
an electroneutral ion exchange. It is clear
that the amphiphilic organic cations could
be used to promote desorption of metal
cations. For example, with 1000 nM total
DP in the system, the Dc of Cu(ll) is
about one tenth of its value with no DP,
with the same total amount of Cu(ll) in the
system; at 400 pM total DP, the drop in
Dc of Cu(ll) is to about one fourth. Indica-
tions are that this drop depends on the
surface concentration of DP relative to
the CEC of the sorbent. Since Cu(ll) does
seem to cause some desorption of DP at
concentrations near the CEC, it is
possible that another organic cation that
adsorbs more strongly (such as
tetradecyl- or hexadecylpyridinium) could
displace Cu(ll) more effectively.
Effect of DP on adsorption of
chlorobenzenes by Lula A/6. The
concentration distribution ratio (Dc) of the
chlorobenzenes, as a function of the
concentration of DP on the surface, is
shown in Figure 6. Clearly the presence
of DP on the surface promotes
adsorption. (However, at sufficiently high
concentrations of DP, micelles are
formed in solution and the Dc falls off
again. All of these data are not shown in
the figure.)
It is significant that the value of Dc
climbs not linearly with concentration of
DP on the surface (and fractional organic-
carbon content of the sorbent, f^) but
almost quadratically, particularly for the
chlorobenzenes with greater numbers of
Cl- substituents. Thus the "quality" of
organic carbon for adsorbing hydropho-
bic compounds increases with surface
coverage. One could associate this beha-
vior with the formation of hydrophobic
domains on the surface; the higher the
surface coverage the greater the likeli-
hood that aggregates of DP are situated
to interact with a chlorobenzene mole-
cule.
The range of experimentally
determined values of Dc can be
compared to those predicted from the
Kow'foc paradigm,
log Dc = a log Kow + log foc + b
(11)
For this comparison we use the values
of a, b, and Kow values from the
correlation of Schwarzenbach and
Westall (1981), which was derived with
chlorobenzenes and alkylbenzenes. The
foe of the sorbent is found from the mass
of carbon in a mole of DP (204 g/mol)
and the concentration of DP on the
surface (mol/kg). The values of Dc
predicted from Equation 11 are all less
than those found experimentally for the
DP-treated Lula N6.
It is clear that organic cations could be
used to increase the organic carbon
content and the ability of an inorganic
substrate to retain a hydrophobic
compound, with as little as 10 mmol kg-1
(0.2% organic carbon for the sorbent). At
this level the distribution of Cu(ll) is
affected, but not severely. Thus by fine
tuning the hydrophobicity of the organic
cation to the specific environmental
problem (properties of the sorbent,
properties of the hydrophobic solutes,
etc.), it should be possible to immobilize
hydrophobic organic compounds without
significantly mobilizing metal cations.
Conclusions and
Recommendations
Large organic cations, such as
dodecylpyridinium, are strongly adsorbed
to sediment, soil, and subsurface
materials. Adsorption depends primarily
on the CEC of the sorbent material, the
nature and concentration of the
electrolyte, and the concentration and
alkyl chain length of the organic cation.
Solution pH has little effect on the
-------
Table 3. Range of Concentration of DP in Solution and on the Surface over the Course
of Cu(ll) Adsorption Isotherms.
Total concentration in solution Solution concentration at
(initial) jjM equilibrium fiM
Surface concentration at
equilibrium mmol kg'1
1
10
50
100
400
1000
0.016
0.355
3.19
12.2
206
778
- 0.020
- 0.424
-4.26
- 15.0
-229
-805
0.180
1.83
8.46
17.0
34.0
39.9
-0.183
- 1.87
-9.03
- 17.5
- 36.6
- 42.4
adsorption of alkylpyridinium on the
natural materials studied.
The adsorption isotherms are distinctly
nonlinear, even at very low surface
concentrations of organic cation. The
data conform to the Freundlich equation
and can be interpreted mechanistically as
a summation of Langmuir isotherms.
Maximum or plateau adsorption values
of adsorbed DP corresponded roughly to
the reported CEC's of the sorbent
materials. Adsorption was somewhat
greater than the CEC for EPA-12 soil; this
effect may be related to an increased role
of organic matter for high foc materials.
The effects of electrolyte
concentration on adsorption could be
explained with two types of electroneutral
reactions: cation exchange and
adsorption of the organic cation with an
inorganic anion. Ca2+ had a larger effect
on adsorption than did Na + , as expected
for ion exchange.
The adsorption of DP to Lula aquifer
material displaces Cu(ll). The
displacement occurs when the surface
concentration of DP is within 10 - 100%
of the adsorption maximum of DP (an
effective CEC) in 0.01 M NaCI. However,
the reaction cannot be described as a
simple ion exchange reaction; pH-
dependent surface complexation of Cu(ll)
and electrostatic competition with DP
seem to be involved.
The adsorption of DP to Lula aquifer
material promotes sorption of
chlorobenzenes. Over the range of DP
equivalent to foc = 0.0001 - 0.005, the
values of Koc were not constant, but were
greater for greater loadings of DP. This
result is attributed to cooperative effects
of DP molecules at higher surface
concentrations.
This work is relevant to two potential
site-remediation methodologies: (i) the
use of amphiphilic organic cations to
increase the tendency of inorganic
substrates to retain hydrophobic organic
compounds, and (ii) the use of
amphiphilic organic cations to promote
desorption ot metal ions in pump-and-
treat type operations. However, further
study and site-specific information is still
necessary for the development of a
treatment program.
For further study we recommend that
efforts be concentrated on: (i) the nature
of competition in the system organic
cation/NaCI, CaC^transition metal cation,
i.e., clarification of the role of electrostatic
and surface-complexation interactions in
the sorption of organic cations and metal
cations on heterogeneous sorbents; (ii)
the role of the "quality" of organic carbon
in the sorption of hydrophobic solutes
such as chlorobenzenes, and the
dependence of apparent Dc on coverage
of the surface by organic cations; (iii)
adsorption-desorption kinetics of organic
cations in soil-column experiments, and
the concomitant effect on metal ion and
hydrophobic organic compound adsorp-
tion-desorption; and (iv) the effect of
organic cations on the hydraulic
conductivity of soil columns.
References
Lewis, M. A. and V. T. Wee. 1983.
Aquatic Safety Assessment for Cationic
Surfactants Environ. Toxicol. Chem.
2:105-118.
Schwarzenbach, R. P. and J. Westall.
1981. Transport of Nonpolar Organic
Compounds from Surface Water to
Ground Water. Environ. Sci. Technol.
15:1360-1367
Figures.
1. Adsorption isotherms of dodecylpyri-
dinium on different materials; data are
from the high concentration range of
Figure 2, plotted here on a linear
scale. Note the correspondence be-
tween the plateaus of the adsorption
isotherms and the cation exchange
capacities listed in Table 1. Ranges of
pH were: kaolinite, pH = 5.39 - 4.72;
soil EPA-12, pH = 7.43 - 7.00; and
Lula aquifer material, pH = 6.81 -
4.49. Solution contained 0.01 M Na*;
ratio of solids to liquids are given in
Figure 2.
2. Logarithmic adsorption isotherms of
dodecylpyridinium on different
materials: montmorillonite, ratio of
solids to liquids (Cs(w)) was 0.0050
kg/L; kaolinite, Cs(w) = 0.025; soil
EPA-12, Cs(w) = 0.025; Lula aquifer
material, Cs(w) = 0.0025; and Borden
sand, Cs(w) = 0.025 or 0.050 kg/L.
Solutions contained 0.01 M Na+.
3. Effect of log [H + ] on the concentration
distribution ratio Dc of dodecyl-
pyridinium between aqueous solutions
and selected sorbents: EPA-12, Cs(w)
= 0.015 kg/L; Lula, Cs(w) = 0.0025;
and kaolinite, Cs(w) = 0 0125. Solution
contained 0.01 M Na*. The amounts of
dodecylpyridinium adsorbed to the
surface were nearly constant for EPA-
12 and Lula sorbents. CR(s) = 0.20
and 29 pmol/g, respectively. CR(s)
ranged from 7.6 - 15 nmol/g for
kaolinite.
4. Logarithmic adsorption isotherms of
dodecylpyridinium on Lula aquifer
material from 0.1 M Na* and 0.01 M
Ma*. The ratio of solids to liquid was
0.0025 kg/L. The solid lines in Figure 4
were computed from the model
defined in Table 2.
5. Adsorption of Cu(ll) on Lula N6 (0.005
kg L-1) in 0.01 NaCI at pH in the range
about 4.9 - 5.6, with various total
concentrations of DP (given on the
figure), shown on linear and log-
arithmic scales. The approximate rela-
tion between total and adsorbed DP is
given in Table 3. Total Cu(ll) was in
the range 10 - 200 ^M for the
isotherms. Isotherms were obtained
from the batch method, in which both
solution and adsorbed Cu(ll) and DP
were determined directly.
6. Concentration distribution ratios (Dc) of
chlorobenzenes between Lula N6
(0.025 kg L-i) and 0.01 M NaCI, as a
function of amount of DP on the
sorbent.
-------
John C. Westall, Bruce J. Brownawell Hua Chen, John M. Collier, and Julia
Hatfield are with Oregon State University Corvallis, OR 97331-4003
Robert Puls is the EPA Project Officer (see below).
The complete report, entitled "Adsorption of Organic Cations to Soils and
Subsurface Materials," (Order No. PB 90-171 927/AS; Cost: $17.00,
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 Officers can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
*U.S. Government Printing Office: 1993— 750-071/60263
-------
-------
-------
m
T)
£
O)
o
o
C/)
N>
ci>
o
3 —•
0) Q.
s: 05.
O?
-oi
=- CD
< M
CD
C
CD
O
O
CO 3 D
CD < =V
3 =;• CD
O O Q.
O 0)
= CD
0) (/)
TJ
O
CD
O
^' 2* CD
o 2 =
5' 3 ®
3 w
O
o
03
m
3
3J
CD
------- |