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MICHIGAN STATE

I Ill/Illl/II/l/IIll/IIIll/IH/I/l/Ill/iiiliiliiilli/ii/ill ‘

00032

III

 

 

This is to certify that the
dissertation entitled
Retention of Organic Contaminants by Soils and
Clays Exchanged with Organic Cations
presented by I}

Jiunn-qu Lee

has been accepted towards fulfillment {
of the requirements for ‘

 

 

 

 

 

Doctorate degree in Philosophy i
)
g M ‘
Major profggn 5)
Date MU 21 /?ff f:
MSU is an Affirmative Action/Equal Opportuniry Institution 0-12771
f"

 

with v _
Michigan State
University

 

 

 

 

 

' PLACE'IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before date due.

 

DATE DUE DATE DUE DATE DUE
9 1 8 o 4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

   

 

   

MSU is An Affirmative Action/Equal Opportunity Institution

 

RETENTION OF ORGANIC CONTAMINANTS BY SOILS AND

CLAYS EXCHANGED WITH ORGANIC CATIONS

by

Jiunn-qu Lee

A DISSERTATION

Submitted to
Michigan State University
in part fuifiiiment of the requirements
for the degree of

DOCTOR OF PHILOSOPHY

Department of Crop and Soii Sciences

1988

 

ABSTRACT

RETENTION OF ORGANIC CONTAMINANTS BY SOILS AND
CLAYS EXCHANGED WITH ORGANIC CATIONS

by

Jiunn—qu Lee

When organic cations of the form [(CH3)3NR]+ were exchanged for
naturai metai ions such as Ca2+ and Na+ in soii (ciay), the sorptive
properties of the soii (ciay), were greatly modified. When modified with
cations containing Iarge organic moieties (R groups), the surface
properties of soii (ciay) changed from hydrophiiic to hydrophobic. Soiis
(ciays) exchanged with hexadecyitrimethyiammonium (HDTMA) ions (R = CwH33)
had significantiy higher organic matter contents, and greatiy enhanced
sorptive capacities for removing nonionic organic chemicais (NOCs) from
water. In aqueous soiution, the uptake of NOCs by HDTMA treated soii
(ciay) was primariiy by soiute partitioning into the organic medium formed
by congiomeration of iarge aikyi chains of organic cations. The organic
phase derived from exchanged HDTMA was 10 to 30 times more effective than
naturai organic matter for removing NOCs from water.

In organo—ciay compiexes containing smaiier organic cations, such

as tetramethyiammonium (TMA), the organic matter consisted of separate

 

small organic moieties. This phase did not act as an effective partition
medium for removing NOCs from water, and the sorptive behavior of TMA-
smectite were distinctly different from that of HDTMA-smectite. In the
presence of water, TMA-smectite showed a high degree of shape selectivity
resulting in high uptake of benzene and progressively lower uptake of
larger aromatics. The dry TMA-smectite showed comparatively low
selectivity in the uptake of aromatic chemicals. The shape—selectivity
of TMA—smectite could be increased by the presence of water which appeared
to shirnk the interlamellar cavities. Increased shape selectivity was
also manifest in a high—charged smectite (from Arizona) due to the closer
parking of TMA ions. Overall, the main factor affecting the degree of
sorption of aromatic compounds by TMA-smectite was found to be the lateral
free distances between the exchanged cations.

The modification of soils and clays with organic cations might be
used to improved the retardation capabilities of low organic matter soils
for NOCs, and to enhance the containment capabilities of clay landfill

liners and slurry walls.

 

TABLE OF CONTENTS

Page
LIST OF TABLES .................................................... iii
LIST OF FIGURES ................................................... iv
INTRODUCTION ...................................................... 1
References .................................................. 7
CHAPTER I. ATTENUATING ORGANIC CONTAMINANT MOBILITY
BY SOIL MODIFICATION .................................. 11
References ...................................... 29
CHAPTER II. RETENTION OF ORGANIC CONTAMINANTS BY SOILS
EXCHANGED WITH ORGANIC CATIONS ........................ 30
References ...................................... 62
CHAPTER III.SHAPE SELECTIVE ADSORPTION OF AROMATIC MOLECULES
FROM WATER BY TETRAMETHYLAMMONIUM—SMECTITE ............ 64
References ...................................... 91
CHPATER IV. SELECTIVE ADSORPTION OF AROMATIC MOLECULES BY
TMA-SMECTITES OF DIFFERENT CHARGE DENSITY ............. 93
References ...................................... 124

 

LIST OF TABLES

Table
Chapter I Page
1. Properties of the untreated and HDTMA-treated clay and soil... 17

2. Sorption coefficients of benzene, perchloroethene (PCE)
and 1,2-dichlorobenzene (DCB) on untreated and

HDTMA-treated soil ............................................ 24
Chapter II
1. Properties of the untreated Marlette soils and the Marlette

soil treated with three different organic cations ............. 38
2. Properties of three different soils (Bt-horizons) either

untreated or treated with HDTMA ............................... 39

3. Sorption coefficients of organic chemicals on the untreated
Marlette soil and on the Marlette soil treated with three

different organic cations ..................................... 53
4. Sorption coefficients of three different soils (Br—horizons)
either untreated or treated with HDTMA ........................ 54

Chapter III

1. Organic carbon contents and surface areas of the clay

absorbents .................................................... 72
Chapter IV
1. Properties of Wyoming and Arizona TMA-smectites ............... 119

 

LIST OF FIGURES

Figure Page
Chapter I

1. Adsorption of hexadecyltrimethylammonium (HDTMA) iodide

by Ca-smectite and Marlette soil A and B horizons ............ 15
2. Sorption of benzene, perchloroethene (PC ) and

1,2—dichlorobenzene (DCB) by untreated and HDTMA-treated

Marlette soil A horizon ....................................... 19
3. Sorption isotherms of benzene, perchloroethene (PCE) and

1,2—dichlorobenzene (DCB) by untreated and HDTMA-treated

Marlette soil Bt horizon ...................................... 21
4. Schematic representing smectite clay intercalated with

hexadecyltrimethylammonium cations ............................ 26
Chapter II
1. Adsorption isotherms of hexadecyltrimethylammonium (HDTMA)

on untreated soil Bt horizon .................................. 41
2. Sorption of benzene, toluene, and ethylbenzene on untreated

and HDTMA-treated Bt-horizon St. Clair and Oshtemo soils ...... 43
3. Sorption of benzene, toluene, and ethylbenzene on untreated

and HDTMA-treated Marlette soil A and B horizons ............. 46

4. Sorption of benzene, toluene, and ethylbenzene on

HDTMA—treated St. Clair, Marlette, and Oshtemo

soil Bt horizon ............................................... 48
5. Sorption of benzene, toluene, and ethylbenzene on

treated Marlette soil B horizon: NTMA-treated,

DDTMA-treated, and HDTMA—treated .............................. 50

Chapter III

1. Surface area of TMA/Ca-smectites as a function of the

fraction of cation exchange capacity occupied by TMA .......... 74
2. Adsorption of organic compounds from water by TMA-smectite,

mixed TMAzCa (1:3)-smectite, and TMA-illite ................... 76
3. Adsorption of benzene vapor by dry TMA—smectite ............... 79
4. Adsorption of toluene and xylene vapors by dry TMA—smectite... 81
5. Reduced plot showing adsorption of organic compounds from

water by TMA—smectite versus relative concentration (Ce/Cs)... 83
Chapter IV

1. Adsorption of benzene vapor by dry Arizona and Wyoming
TMA-smectites ................................................. 100

 

. Adsorption of toluene vapor by dry Arizona and Wyoming

TMA—smectites ................................................. 102
. Adsorption of o—xylene vapor by dry Arizona and Wyoming

TMA-smectites ................................................. 104
. Adsorption of benzene from aqueous solution by Arizona

and Wyoming TMA-smectites ..................................... 107
. Adsorption of toluene from aqueous solution by Arizona

and Wyoming TMA-smectites ..................................... 109
. Adsorption of o—xylene from aqueous solution by Arizona

and Wyoming TMA-smectites ..................................... 111
. Adsorption of benzene from water in the presence and

absence of toluene by Wyoming TMA-smectite .................... 114
. Adsorption of toluene from water in the presence and

absence of benzene by Wyoming TMA-smectite .................... 116

 

 

INTRODUCTION

The sorption and desorption behaviors of soils, sediments and
suspended particles are important factors affecting the environmental
fates and effects of xenobiotic chemicals. Chemicals that are sorbed
tightly are less available for processes such as leaching, volatization
(Kay and Elrick, 1967; Spencer et al., 1988), transformation (Bartha,
1971; Bordeleau and Bartha, 1972) and degradation (Hsu and Bartha, 1974;
Stevenson, 1982). The factors that may affect the extent to which
chemicals sorb to the sorbents are : (1) physical and chemical properties
of chemicals (Briggs, 1969; Chiou et al., 1979); (2) soil environmental
factors : temperature, pH, water content, etc. (Harris and Warren, 1964;
Bailey, 1968; Yaron and Saltzman, 1972); (3) soils characteristics: type
of clay, content of clay, silt, sand, and soil organic matter (Moreale and
Bladel, 1976; Chiou et al., 1983; Garbarini and Lion, 1986). For nonionic
organic compounds (NOCs), which are by far the most important class of
organic pollutants, the organic matter content of soil appears to be the
most important soil factor in controlling their sorptive behavior. In
studying the sorption of simazine by soil, Burnside et al. (1961)
indicated that both clay and organic matter were involved in adsorption
of simazine. Upchurch et al. (1962) reported that the percentage of

monuron retained against leaching increased from 35% to 95% as the soil

 

2

organic matter content increased from 0.87% to 1.14%. Similar studies
done by Lambert et al.(1965, 1967, 1968) have shown that the organic
fraction of soil was single factor Inost highly related to herbicide
activity. Lambert further suggested that the role of soil organic matter
was similar to that of an organic solvent in solvent extraction and that
the partitioning of a neutral organic compound between soil organic matter
and water correlated well with its partitioning between water and an
immisible organic solvent. The works of Chiou et al (1979, 1983, 1985a)
are of special interest when attempting to resolve the role of soil
organic matter on the sorption of nonionic, nonpolar organic chemicals
from water. These authors interpreted the uptake of nonionic organic
compounds from aqueous system by soils in terms of a partitioning of the
solutes between the solution and a hydrophobic phase consisting of the
soil organic matter. They also suggested that a wide range of sorption
behavior can be accounted for by considering the soil to be a dual
sorbent, in which the mineral fraction of the soil functions as a
conventional solid adsorbent and the organic matter functions as a
partition medium. They further pointed out that in soil water systems,
water is preferentially adsorbed by the mineral surfaces, and the uptake
by soil consists primarily of solute partitioning into the organic matter.
Thus water acts to deactivate the mineral surfaces for the adsorption of
nonionic organic compounds which cannot compete with water for binding
sites in the mineral surfaces. This concept of partitioning allows
accurate estimation of soil-water distribution coefficient from octanol-
water partition coefficients (KOH) (Karickhoff, 1979; CHiou et al., 1982;
Means et al., 1982), water solubilities (Freed et al., 1979; Chiou et al.,

 

3

1983), and bioconcentration factor (BCF) (Chiou, 1985b). Recently, Boyd
et al. (1988a) demonstrated that when large organic cations are exchanged
for metal cations in soil, the sorptive capacity for nonionic compounds
and the organic matter content of soil are greatly enchanced. The
exchanged organic cations were shown to form an organic phase that behaved
as a powerful partition medium for the removal of organic compounds from
water. Therefore, it wound be of interest to study the sorption behavior
of soil after treated with various organic cations.

In clay minerals, much effort has been focused on smectite for the
sorption of xenobiotics because of its high cation exchange capacity (CEC)
and surface area. Many authors (Hilton and Yuen, 1963; Shin et al., 1970;
Pionke and Chesters, 1973; Hassett et al., 1980) have shown that clays
have the ability to reduce drastically the effect of pesticides. For
examples, Yuen et al. (1962) showed that the adsorption of monuron was
highest in those soils in which smectite was the major clay constituent.
Hill et al. (1955) found that more monuron was adsorbed by bentonite than
by kaolinite. Coggins et al. (1959) suggested that the reason for the
differential influence of substituted-ureas was due to the amount of
bentonite present in soil. Although clays have been recognized as
powerful adsorbents for the adsorption of polar, ionic organic compounds
from water (Hance, 1969), few studies have been carried out on clays to
serve as practical adsorbent for the removal nonpolar, nonionic organic
chemicals from aqueous systems. This can be attributed to strong polar
interaction of water with mineral surfaces (Mortland, 1970) which are thus
no longer available for the sorption of relatively nonpolar organic

solutes. However, this limited sorption character of clays toward

 

4

nonionic chemicals can be remedied by replacing natural inorganic exchange
cations by large organic cations through ion exchange reations (Barrer et
al., 1955, 1957; Wolfe et al., 1985; Boyd et al., 1988b). As organic
cations exchange for metal ions on clay surfaces, these organic cations
act as "pillars" which hold the aluminosilicate sheets permanently apart
and greatly increase the available sorbing surface. Also, in the modified
form, the surface properties of clay will change from hydrophilicity to
hydrophobicity because organic cations are not strongly hydrated and
decrease the free mineral surface area. Several workers have indicated
that exchanging quaternary ammonium cations for the metal ions on the clay
surface could considerably modify the hydratic and swelling properties of
clays (Stul et al., 1978, 1979). The properties and structure of alkyl
and aryl ammonium complexes of smectite have been extensively studied with
more attention being given to the alkyl ammonium complexes. The
literature dealing with alkyl ammonium complexes has emphasised organic
vapor uptake. Barrer et al. (1955, 1957) showed that tetramethyl ammonium
(TMA) and methylammonium—montmorillonite are useful sorbents for the
separations of organic vapors by selective intercalation. White and Cowan
(1958) have successfully separated aliphatic from aromatic compounds using
dimethyldioctadecyl ammonium (DMDOA) as the stationary phase in gas
chromatography (GC). These authors also pointed out that DMDOA ions form
pillars when adhered on the interlamellae creating large free intersheet
volumes. The same results were obtained by Barrer et al. (1961).

Until recently, the uptake of organic chemicals by organo-clay
derivates from aqueous systems has received little attention. McBride et

al. (1977) has reported the uptake of aromatics from water by TMA- and

 

 

5

HDTMA-smetite. Slabaugh et al. (1968) reported adsorption of methanol on
DMDOA-montmorillonite from aqueous phase. Stul et al. (1978, 1979) were
able to adsorb a series of alcohols on dodecylammonium montmorillonite in
studies conducted over a wide range of alcohol concentration. Recently,
Mortland et al. (1986) and Boyd et al. (1988c) studied the uptake of
phenol and chlorophenols from aqueous solution by smectite whose cations
were exchanged by quaternary ammonium ions. These authors showed that the
sorptive capacity of modified clays is greatly improved in comparison with
unmodified clays.

To obtained additional perspective of the influence of exchanged
organic cations on the mechanistic function of modified clay, Boyd et al.
(1988a) studied the sorption of nonionic, nonpolar organic compounds on
hexadecyltrimethylammonium (HDTMA)—smectite from water. They found that
the exchanged HDTMA ions functions as a partition medium and the uptake
of organic solutes by HDTMA-smectite is mainly by solute partitioning into
the organic medium that is formed by conglomeration of large C16 alkyl
groups associated with HDTMA. Improvement of the sorptive capacity of
soils with low organic matter contents was similarly achieved by cation
exchange reactions with HDTMA ions.

It has been known that by a simple ion—exchange process, clays can
be activated to give a greatly enhanced sorbing surface toward organic
compounds in aqueous system. However, interesting as these observations
are, the amount of quantitative information is still limited regarding the
sorptive behavior of clay complexes with different organic cations. The
effects of the structure of the exchanged organic cations on the sorptive

properties of the organo—clay (sorbent), as well as the physical/chemical

 

6

properties of nonionic organic compounds (sorbates) that determine their
sorptive uptake by organo-clays, are elucidated in this work. In
addition, the effects of water on the sorptive phenomena are established.
To accomplish this, the sorptive behavior of various organic compounds by
different modified clays from both aqueous and vapor phase were studied.
In order to further clarify the sorptive properties of modified clays for
NOCs, the competitive effects of organic chemicals in multi—solute
systems, and the effect of the charge density (or CEC) of the clay, were
evaluated. In addition, for the sake of environmental interest, the
stability and affinity of exchanged organic cations on modified clays are

shown in this research.

 

REFERENCES

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by montmorillonite: Role of pH and chemical character of adsorbate: Soil
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Barrer, R. M., and Reay, J. S. S. (1957) Sorption and interaction by
methylammonium montmorillonite: Trans. Faraday Soc. 53, 1253-1261.

Barrer, R. M., and Kelsey, K. E. (1961) Thermodynamic of interlamellar
complexes: PART II. Sorption by dimethyldioctadecylammonium bentonite:
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Bartha, R. (1971) Fate of herbicide—derived chloroanilines in soil: J.
Agric. Food Chem. 19, 385—387.

Bordeleau, L. M., and Bartha, M. (1972) Biochemical transformation of
heribicide-derived anilines in culture medium and in soil: Can. J.
Microbiol 18, 1857-1864.

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contaminant mobility by soil modification: Nature 337, 345-347.

Boyd, S. A., Mortland, M. M., and Chiou, C. T. (1988b) Sorption
characteristics of organic compounds on hexadecyltrimethylammonium—
smectite: Soil Sci. Soc. Am. J. 52, 652-657.

Boyd, S. A., Shaobai, S., Lee, J. F., and Mortland, M. M. (1988c)
Pentachlorophenol sorption by organo—clays: Clays and Clay Minerals 36,
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Briggs, G. G. (1969) Molecular structure of herbicides and their sorption
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Burnside, D. C., and Behrens, R. (1961) Phytotoxicity of simazine: Weeds
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832.

Chiou, C. T., and Schmedding, D. W. (1982) Partitioning of organic
compounds in octanol-water systems: Environ. Sci. Technol. 16, 4-10.

Chiou, C. T., Porter, P. E., and Schmedding, D. W. (1983) Partition
equilibria of nonionic organic compounds between soil organic matter and
water: Environ. Sci. Technol. 17, 227-231.

 

8

Chiou, C. T., Shoup, T. D., and Porter, P. E. (1985a) Mechanistic roles
of soil humus and minerals in the sorption of nonionic organic compounds
from aqueous and organic solutions: Org. Geochem. 8, 9-14.

Chiou, C. T. (1985b) Partition coefficients of organic compounds in lipid-
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Coggins, C. W., and Crafts, A. S. (1959) Substituted urea herbicides:
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physical factors in toxicological assessment tests: Environ. Health
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Garbarini, D. R., and Lion, L. W. (1986) Influence of the nature of soil
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Hance, R. J. (1969) An empirical relationship between chemical structure
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Hill, G. D., McGahen, J. W., Baker, H. M., Finnerty, D. W., and Bingeman,
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Hilton, H. W., and Yuen, O. H. (1963) Adsorption of several preemergence
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Hsu, T. S., and Bartha, R. (1974) Biodegradation of chloroaniline-humus
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Karickhoff, S. W., Brown, D. S., and Scott, T. A. (1979) Sorption of
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Kay, B. D., and Elrick, D. E. (1967) Adsorption and movement of lindane
in soil: Soil Sci. 104, 314-322.

Lambert, S. M., Porter, P. E., and Schieferstein, H. (1965) Movement and
sorption of chemicals applied to the soil: Weeds 13, 185—190.

Lambert, S. M. (1967) Functional relationship between sorption in soil and
chemical structure: J. Agric. Food Chem. 15, 572-576.

 

Lambert, S. M. (1968) Omega (9) a useful index of soil sorption
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Means, J. C., Wood, S. G., Hassett, J. J., and Banwart, W. L. (1982)
Sorption of amino- and carboxy—substituted polynuclear aromatic
hydrocarbons by sediments and soils: Environ. Sci. Technol. 16, 93-98.

McBride, M. B., Pinnnavaia, T. J., and Mortland, M. M. (1977) in Fate of
pollutants in the Air and Water Environments, John Wiley and Sons, New
York, part I, pp. 145.

Moreale. A., and Bladel, R. V. (1976) Influence of soil properties on
adsorption of pesticide—derived aniline and p-chloroaniline: J. Soil Sci.
27, 48-57.

Martland, M. M., Shaobai, S., and Boyd, S. A. (1986) Clay—organic
complexes as adsorbents for phenol and chlorophenols: Clays and Clay
Minerals 34, 581—586.

Pionke, H. B., and Chesters, G. (1973) Pesticide sediment water
interactions: J. Environ. Qual. 2, 29—45.

Shin, Y. 0., Chodan, J. J., and Wolcott, A. R. (1970) Adsorption of DDT
by soils, soil fractions and biological methods: J. Agric. Food Chem. 18,
1129-1133.

Slabaugh, W. H., and Carter, L. S. (1968) The hydrophilic-hydrophobic
charater of organomontmorillonites: J. Colloid Interface Sci. 27, 235—238.

Spencer, W. F., Cliath, M. M., Fury, A. W., and Zhang, L. 2.
(1988)Volatilization of organic chemicals from soil as related to their
Henry’ 5 law constants. J. Environ. Qual. 17, 504- 509.

Stevenson, F. J. (1982) in Humus Chemistry: Genesis, cgmpostion,
reactions. Wiley, New York, pp.443.

Stul, M. S. ,Means, A., and Uytterhoeven, J. B. (1978) The sorption of n-
aliphatic alcohols from dilute aqueous solution on RNH —montmorillonite
PART 1. Distribution at infinite dilution: Clays and clay minerals 26,
309-317.

Stul, M. S, Means, A., and Uytterhoeven, J. B. (1979) The sorption of n—
aliphatic alcohols from silute aqueous solution on RNH3 -montmorillonite
PART II. Interlamellar aggregation of the adsorbate: Clays and clay
minerals 33, 350— 356.

White, D., and Cowan, C. T. (1958) The sorption properties of
dimethyldioctadecylammonium bentonite using gas chromatography: Trans.
Faraday Soc. 54, 557—561.

Wolfe, T. A., Demirel, T., and Baumann, E. R. (1985) Interaction of
aliphatic amines with montmorillonite to enhance adsorption of organic
pollutants: Clays and Clay Minerals 33, 301—311.

 

10

Upchurch, R. P., and Manson, D. D. (1962) The influence of soil organic
matter of a podzol: Soil Sci. 93, 225-231.

Yaron, B., and Saltzman, S. (1972) Influence of water and temperature on
adsorption of parathion by soils: Soil Sci. Soc. Amer. Proc. 36, 583—586.

Yuen, O. H., and Hilton, H. W. (1962) THe adsorption of monuron and diuron
by Hawaiian sugar cane soils: J. Agric. Food Chem. 10, 386-392.

 

CHAPTER 1

ATTENUATING ORGANIC CONTAMINANT MOBILITY
BY SOIL MODIFICATION

 

ABSTRACT

Low organic matter clays, soils and aquifer materials have very
little sorptive capability for common groundwater contaminants. Here we
use organic cations of the form [(CH3)3NR]+ to displace naturally occurring
exchange ions resulting in significantly higher organic matter content,
and greatly enhanced sorptive properties for nonionic organic solutes.
The organic phase derived from exchanged hexadecyltrimethylammonium, where
R is a C-16 hydrocarbon, behaved as a powerful partition medium that was
10 to 30 times more effective on a unit weight basis than natural soil
organic matter for removing benzene, dichlorobenzene and perchloroethene
from water. This simple soil modification might be used to improve the
retardation capabilities of low organic matter soils and aquifer
materials, and to enhance the containment capabilities of clay landfill

liners and bentonite slurry walls.

Low organic matter clays, soils and aquifer materials have very
little sorptive capability for common groundwater contaminants. We have
used organic cations of the form [(CH3)3NR]+ to displace naturally
occurring exchange ions (e.g. Na+ or Ca2+) from soil resulting in
significantly higher organic matter contents, and greatly enhanced
sorptive properties for nonionic organic solutes. The organic phase
derived from exchanged hexadecyltrimethylammonium, where R is a C—16
hydrocarbon, behaved as a powerful partitioning medium that was 10 to 30x
more effective on a unit weight basis than natural soil organic matter for
removing benzene, dichlorobenzene and perchloroethene from water. This

simple soil modification may be used to improve the retardation

 

13

capabilities of low organic matter soils and aquifer materials, and to
enhance the containment capabilities of clay landfill liners and bentonite
slurry walls.

The sorption of nonionic organic contaminants from water by soil is
controlled by the soil organic matter content. There is strong evidence
that the mechanism of uptake is primarily partitioning into the soil
organic phase (Chiou et al., 1979,1983) which acts as a solubilizing
medium for nonionic organic solutes. In contrast, the soil mineral
fraction (e.g. oxides and clays) is generally not important in the uptake
of nonionic organic solutes in soil water systems. Dipole interactions
between the mineral surfaces and water prohibit nonionic organic solutes
from interacting with this fraction of soil (Chiou et al., 1985). One
practical result of the strong interaction of water with clays is the lack
of sorptive capability of clay containment barriers (e g. bentonite slurry
walls or clay landfill liners) whose sole function is to retard the
movement of water. Similarly, low organic matter surface soils and
aquifer materials have very little retardation capability for nonionic
organic pollutants. As a result, many anthropogenic organic compounds are
found as groundwater contaminants and represent a serious threat to human
health.

Soil materials contain significant cation exchange capacity (CEC)
derived primarily from clay minerals such as smectite and kaolinite.
Clays possess a net negative electrical charge which is compensated for
by exchange cations on their surfaces. In nature, these exchange cations
are mainly alkali metal and alkaline earth metal cations like Na+ or Ca2+.

Hydration of these metal cations imparts a hydrophilic nature to the

 

14

mineral surfaces and, as described above, these surfaces are not good
sorbents for nonionic organic species which cannot displace the tightly
held water. Organic cations may enter into ion exchange with metal
cations on the exchange sites of clays. When metal ions are exchanged
with certain organic cations which contain sizable organic moieties, the
surface of the clay may be greatly modified to become strongly
organophilic.

Boyd et al. (1988a) have performed such ion exchange reactions using
purified smectite clays and actual soil materials, and organic cations of
the form [(CH3)3NR]+. The R group imparts the desired organophilic
properties needed for sorption of organic solutes, and may be a C—I to
C-20 hydrocarbon (Boyd et al., 1988b). In this report we present data
showing the enhanced uptake of several mobile organic contaminants from
water by soil materials exchanged with hexadecyltrimethylammonium
[(CH3)3N(CH2)15CH3](HDTMA) cations.

Adsorption isotherms of 1[‘C-HDTMA on (a) CaZT-smectite, (b) Marlette
soil A horizon, and (c) Marlette soil Bt horizon are shown in Figure 1.
The adsorption of HDTMA by all three materials is very strong as expected,
with nearly quantitative displacement of Ca2+, in the case of CaZT—smectite
or the naturally occurring inorganic ions present in the Marlette A and
8t horizons. These are type I isotherms characteristic of ion exchange
reactions in which the added ion, in this case HDTMA, is strongly
preferred. Figure 1 shows that HDTMA is very strongly sorbed from
solution until the CEC of the clay or soil material (Table I) is exceeded.
The strong preference for HDTMA is due to its high mass relative to

inorganic exchange ions like Na+ and Cazi. The large alkyl ammonium ions

 

 

Figure

1.

15

Adsorption of hexadecyltrimethylammonium (HDTMA) iodide
(methyl-1H3 American Radiolabeled chemicals, Inc., specific
activity 52 mci/mmole) by Ca—smectite (right scale) and
Marlette soil A and 8t horizons (left scale). Either 0.2 g
Ca—smectite or 1 9 soil and 10 ml water were placed in glass
centrifuge tubes. Portions (50 to 400a] of an ethanol
solution containing 1:1000 1"C-HDTMAzHDTMA were added in an
amount equal to 0.25, 0.5, 0.75, l, 1.5 and 2X the cation
exchange capacity (CEC) of the clay or soil. The samples were
shaken 24h at 25°C. 'The aqueous phas, was separated by

centrifugation and lnfl assayed for 1""C by liquid scintillation

counting.

UPTAKE, Q (“mole/g)

 

1200

 

 

200

1000

160

 

800

 

 

 

 

 

 

 

120
600
80 —
l —400
40}— ~200
0 _L i_L i A i L i I i L
0 3.5 7.0 10.5 14.0 17.5 21

EQUIL. CONC., C (,umole/mL)

.z_w>wpomammg .N.H vcm N
mew: Loaves owcmmco cu Cosimo 0_:wmco <zeo: use Fwom pcm>=ou on now: mcopomi .ocmcoFou
.cmcFow ..0:H .mmwgopmconmn seemezz an umEcoeLma mm: mwm>_m:m Cosimo 0wcwmco .zcowwconmn
mcwumme _rom xpwmaw>wca mpmpm cmmwgowz esp an emsgowcma we: mwm>_mcm onm mFo_pcwa
. «2 + g + m: + no + —< “mcoepmo m_n~aowcpxw we saw me Ammov promamo mmcmcoxw cowpwum

 

a
H.0H N.m mm.e 0.0 m.- m.H .z.o _apoe
a.e o mN.e o m.o~ o .z.o <zeoz
N.m N.m o.o 9.0 m.H m.H .z.o _wom
ARV gmpumz owcmmgo
nu m.o m.N N.m m.o m.NH Hm.o Asv Congas anaemic
e.HN n.0e OOH AMFQ
o.N~ 5.0H o p_wm
m.om o.Ne 0 seem
Axv anm a_owpama
e.mH o.eH Hm Amoofi\caev 0mg

 

vammce umpmmcgcz cwpmmce emammcpca empmmce umpwmcpc:
-<zpo: -<z~o: -<zeo:

mxpgwqoga

co~_co;-< co~_xoc-wm mpwpowam-mu

 

.Fwom vcm zm—o umwmwcp-<2Poz new umpmmgpcs 059 we mmwucmqoca .H 0Fn~h

 

 

18

once bound are also very difficult for smaller inorganic ions to displace
due again to their high mass and because of van der Waals interactions
between the alkyl chains themselves (Theng, 1979). As a result, large
organic cations like HDTMA bind almost irreversibly to the clay surface.

Table I shows properties of the untreated and HDTMA-treated clay
and soil materials. The treated materials were prepared by adding HDTMA
in an amount equivalent to the CEC of the clay or soil. It is obvious
from Table I that the organic matter contents have increased
significantly. The amount of HDTMA organic matter expected in the treated
samples can be calculated from the CEC. For example, the Bt soil should
contain 4% HDTMA derived organic matter after the ion exchange reaction.
Thus, the measured organic matter contents agree well with the expected
(calculated) amount, again showing the stoichiometric displacement of
inorganic ions by HDTMA. Depending on the particular organic cation and
the CEC, organic matter contents in excess of 20 percent by weight can be
achieved (Boyd et al., 1988b).

The presence of HDTMA as an exchange ion dramatically alters the
sorptive capabilities of the clay and soil materials. Benzene,
dichlorobenzene, and perchloroethene uptake by the Marlette A and Bt
horizons is shown in Figures 2 and 3. Typical soil sorption behavior was
observed for the untreated soil materials. For benzene and
perchloroethene, uptake from water was very low by the untreated Bt
horizon, due to its low organic matter content (0.6%), and could not be
distinguished from the blank (no soil) samples. Uptake from water by the
untreated A horizon was higher owing to the higher A horizon organic

matter content (5.2%). The sorption isotherms for benzene,

 

 

Figure

2.

19

Sorption (H: benzene, perchloroethene (PCE) and 1,2-
dichlorobenzene (DCB) by untreated and HDTMA-treated Marlette
soil A horizon. Treated soil was prepared by mixing 200 9
soil and 1800 ml water, and then adding 50 NH F50 containing
HDTMA in an amount equal to the CEC of the soil. After
thorough mixing, the mixture was filtered, washed with water
and freeze dried. For the isotherms, 3 g treated soil or 15
g untreated soil, and 25 ml water were placed in glass
centrifuge tubes. Three to 45 ul of benzene (neat), or of 0.1
g/ml methanol solutions containing PCE or DCB, were added.
The tubes were closed immediately and shaken 24 h at 25°C.
The aqueous phase was separated by centrifugation and either
analyzed directly for benzene by HPLC with uv—detection 0r
extracted with hexane and this extract analyzed for PCE or DCB

by GC with electron capture detection.

a

. . a.-_I_-_.§_ __

20
UPTAKE, Q (mg/Kg)

 

 

 

     
   

 

 

 

 

 

 

 

 

 

“~00 _ _ _ _ . _ _ _ _ _ «n0 _ _ _ _ _ _ _ _ _ 0000

.l . I. r L
mmZNmzm 00m

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00001 1 0001 1 0000

00001 1 0001 1 0000
1 1 r 1

00001 1. nsor .1isczazm>am0 1 0000

. - ollozoqz>00m>qm0

.\

.0001 1 ‘00 n\\\\ 1 000
r \\I\ 1

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0 ~00 000 000 000 .000 deco 0 00 00 00 so 00 00 0

mOEEme—s OOZOijquOZ. O .35)..

. 11‘"

 

 

21

Figure 3. Sorption isotherms of benzene, perchloroethene (PCE) and 1
,2-dichlor0benzene (DCB) by untreated and HDTMA- treated
Marlette soil Bt horizon. The treated soil and isotherms were

obtained as described in Figure 2.

 

 

22
U PTAKE, Q (mg/Kg)

 

@000

NmoOl

aooo

Awoo

wooo

amoo

 

  
   

meNmZm

L

FLT.— Lpi PLTFIL1I1— LIL
‘00 ~00 000 000 000 000

 

 

 

 

 

awn _ _ _ _ _ _ _ _ _ _ _
- pom -
mac] 1
1 O i
0001 o 1
1 O 1
wwOI I
I . 1.
NMOI l

C

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r flic243m>4m0 1

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mo

 

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Mumo

NNOO

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o

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0 no 00

ILJIHREHfi
00 00 ‘00 ‘00

 

 

 

23

dichlorobenzene and perchloroethene by the untreated A horizon were highly
linear, as observed in previous studies on the sorption of these solutes
by soils and sediments (Chiou et al., 1979, 1983). Uptake of these
solutes is commonly expressed as
Q =KC

where O is the amount sorbed per unit weight of soil and C is the
equilibrium solute concentration in water. The sorption (partition)
coefficient K corresponds to the slope of the line. The K values
normalized for the soil organic matter contents (Kom = lOOK/%0.M.) tend
to converge on a single value for each solute (Chiou et al., 1979, 1983).
The log Km‘values obtained here are very close to those reported for other
surface soils. For example, the log Km1values reported by Chiou et al.
(1983) for benzene and dichlorobenzene were 1.26 and 2.27, respectively,
as compared to our values of 1.04 and 2.14 (Table II). The log Km1values
obtained for dichlorobenzene from the untreated Marlette A and Bt horizon
are also in good agreement (2.14 and 2.12).

After treating the same soil materials with HDTMA in an amount
equivalent to the CEC, the sorption of benzene, dichlorobenzene, and
perchloroethene increased dramatically. This effect can be seen clearly
in Figures 2 and 3 which shows the sorptive capability of the untreated
and HDTMA-treated soils. For the A horizon the K values increased by a
factor of 12 to 16X in the HDTMA treated material (Table II). The
corresponding Kom values increased by about 6.5 to 8.5X. The increase in
K was even greater for the Bt horizon which, in the untreated form,
contains very little natural organic matter. For dichlorobenzene, K was

about 200x greater in the modified Bt soil.

Table II.

 

Sorption coefficients of benzene, perchloroethene (PCE) and
1,2-dichlorobenzene (DCB) on untreated and HDTMA-treated

 

 

Sorption A-horizon Bt-horizon log Kmf
Coefficient HDTMA— HDTMA-
untreated treated untreated treated
K
Benzene 0.57 7.8 0 16.5
PCE 2.95 37.0 0 27.3
DCB 7.25 120.0 0.80 162.6
log Kom
Benzene 1.04 1.89 - 2.53 2.3b
PCE 1.75 2.56 - 2.75 2.6
DCB 2.14 3.07 2 12 3.52 3.38

 

aOctanol-water partition coefficients (Ref. 2).

Hexane-water partition coefficient (Ref.

8).

 

25

An estimate of the relative effectiveness of organic matter derived
from HDTMA versus natural organic matter can be made by comparing the log
Kom values from the untreated A horizon and from the HDTMA-treated Bt
horizon. In the former case, organic matter is entirely natural, and in
the latter organic matter is derived primarily from HDTMA (Table I). This
comparison reveals that the organic matter composed of HDTMA is about 10
to 30 times more effective than natural soil organic matter for removing
the organic solutes from water (Table 11). Thus, not only can the organic
matter content of soil be increased through modification with HDTMA, but
the added organic matter is highly effective for immobilizing common
mobile pollutants like benzene. The results reported here for actual soil
materials are in good agreement with the sorption characteristics on
HDTMA—smectite for benzene and trichloroethene (Boyd et al., 1988a).

The type of organic phase formed by HDTMA is depicted in Figure 4.
Interaction of the non-polar C-16 hydrocarbon tails of HDTMA present on
the exchange sites apparently forms an organic phase that acts as
solubilizing (partitioning) medium for removing organic solutes from
water. This would be functionally and conceptually similar to a bulk
organic solvent like hexane or octanol. This concept is supported by the
linearity of the isotherms (Chiou et al., 1979, 1983) and by the
similarity of the log Km1values (from the HDTMA-treated Bt) for benzene,
dichlorobenzene, and perchloroethene to the corresponding octanol-water
or hexane water-partition coefficients, i.e. log K0H or log Khu (Table
11). That the organic phase derived from exchanged HDTMA is more
effective than soil organic matter for removing nonionic organic solutes

from water is not surprising. The organic phase of HDTMA is derived

 

 

Figure

4.

26

Schematic representing smectite clay intercalated with
hexadecyltrimethylammonium cations. Gallery region resembles
a bulk phase solvent like hexane or octanol, and functions as
a highly effective partitioning medium for removing organic

contaminants from water.

 

WMa/é

33333333333333333333333333333

 

 

 

 

W

 

 

28

entirely from the low polarity C-16 hydrocarbon tails (Fig. 4). Soil
organic matter contains a high polar group content and is, therefore, a
less effective medium solubilizing nonpolar compounds.

Our results demonstrate that HDTMA exchanged soil materials and
clays are effective sorbents for removing common organic groundwater
pollutants like benzene and perchloroethene from water. The soil
modification is accomplished by simple ion exchange of highly preferred
large organic cations like HDTMA for small inorganic cations like Na+ or
Ca2+. Quaternary organic cations like HDTMA are common cationic
surfactants and available commercially. Such modified soils and clays
may be used: (1) to increase the sorptive capacity of low organic matter
soils; (2) to increase the sorptive properties of subsurface materials
under existing waste disposal sites via underground injection of the
organic cation; and (3) as components of bentonite (smectite) slurry walls
and clay landfill liners to enhance the containment capabilities of such

waste disposal reservoirs.

 

REFERENCES

Boyd, S. A., Mortland, M. M., and Chiou, C. T. (1988a) Sorption
characteristics of organic compounds on hexadecyltrimethylammonium-
smectite: Soil Sci. Soc. Am. J. 52, 652—657.

Boyd, S. A., Shaobai, S., Lee, J. F., and Mortland, M. M. (1988b)
Pentachlorophenol sorption by organo—clays: Clays and Clay Minerals 36,
125-130.

Chiou, C. T., Peters, L. J., and Freed, V. H. (1979) A physical concept
of soil—water equilibria for nonionic organic compounds: Science 206,
831-832.

Chiou, C. T., Porter, P. E., and Schmedding, D. W. (1983) Partition
equilibria of nonionic organic compounds between soil organic matter and
water: Environ. Sci. Technol. 17, 227-231.

Chiou, C. T., and Shoup, T. D. (1985) Soil sorption of organic vapors and
effects of humidity on sorptive mechanism and capacity: Environ. Sci.
Technol. 19, 1196—1200.

Hansch, C. and Leo, A. (1979) Substituent Constants for Correlation
Analysis in Chemistry and Biology (John Wiley and Sons, New York).

Soil Conservation Survey. Soil Survey Laboratory Methods and Procedures
for Collecting Soil Samples. Report No. 1, 66 (U.S. Dept. Agr. Soil
Conservation Service, Washington, D.C., 1972).

Theng, B. K. G. (1979) The Chemistry of Clay-Organic Reactions, (John
Wiley and Sons, New York) pp221-238.

29

 

CHAPTER 2

RETENTION OF ORGANIC CONTAMINANTS BY SOILS
EXCHANGED WITH ORGANIC CATIONS

30

 

31

INTRODUCATION

The interactions of organic contaminants and pesticides with soils,
sediments, and aquifer materials are important in determing the
environmental fates and effects of these toxic chemicals. Knowledge of
the sorptive behaviors of such chemicals is necessary to better understand
their fates and the potential biological effects in the soil environment.
Chemical that are sorbed tightly on soils are less available for important
processes such as leaching, volatization, biodegradation, and
bioaccumulation. Two mechanisms, in general, have been involved the
sorption of organic species on soil-water system. First, the sorption of
organic chemicals may be related to highly specific interactions with soil
components such as ion exchange, protonation, hydrogen bindings,
coordination/ion dipole reactions with clays (Mortland, 1970; Theng, 1974)
and nucleophilic addition reactions with soil organic matter (Parris,
1980). These sorptive mechanisms account for the uptake of organic
cations and some polar organic molecules (e.g. amines) on soil. The
second major mechanism of organic chemical-soil interaction involves
nonionic organic compounds (NOCs), which are by far the largest and most
problematic group of organic contaminants. This mechanism has been
described by Chiou et al. (1979, 1983) as a partition process involving
non-polar interactions between soil organic matter and NOCs. Sorption of
this type is characterized by a linear isotherms, low equilibrium heats,
noncompetitive sorption in multisolute systems, and dependence of sorption
on the solute water solubility (Chiou et al., 1979, 1983) and the
properties of soil (Briggs, 1969; Karickhoff et al., 1979; Garbarini et

al., 1986). The sorption of important groundwater pollutants such as

 

32

benzene and trichloroethylene by soil are examples of this type of
interaction.

Many researchers have showed that the uptake of NOCs from water onto
a great variety of soils is related primarily to the soil organic matter
contents (Bowman et al., 1965; Boucher et al., 1972; Means et al., 1982;
Dzombak et al., 1984; Chiou et al., 1983, 1985a) and this relationship
between uptake of NOCs and soil organic matter has greatly simplified the
prediction of NOCs by soil. However, until recently there was no clear
mechanistic understanding of the association of NOCs with soil organic
matter. Lambert et al. (1965, 1967, 1968) did recognize that, in the
sorption of neutral organic pesticides on soils, the role of soil organic
matter was similar to that of an organic solvent extraction. These
authors further suggested that sorption could be attributed to an active
fraction of soil organic matter and considered soil to be analogous to a
chromatographic column with a stationary phase of organic matter.
Recently, Chiou et al. (1979, 1983) in studies of sorption of NOCs from
water on soil showed that: (1) the sorption isotherms of NOCs were linear
over a wide range of aqueous concentrations, (2) soil uptake of NOCs
exhibited a small heat effect, (3) no apparent solute competition occurred
in multisolute systems, and (4) a dependence of sorption on the solute
water solubility. Based on these observations of sorptive characteristics
in soil—water equilibria, Chiou et al. (1979, 1983) suggested that
sorption of NOCs from water on soil is primarily due to the partitioning
of NOCs into the soil organic phase. Adsorption onto the soil mineral
phase is suppressed in the presence of water because the poorly water

soluble organic species cannot compete with strongly held water for

 

33

adsorption sites (Chiou et al., 1985a).

It is now apparent that low organic matter soils, clays, and aquifer
materials have little sorptive capacity for removing of NOCs from soil—
water systems, and are therefore ineffective in attenuating organic
contaminant mobility. However, this limited sorptive capacity of low
organic carbon soils can be greatly improved by replacing natural metal
cations with large organic cations through a ion exchange reaction. This
simple modification for improvement of the retardation capacity of low
organic matter soils has been examined by Boyd and his co-workers (1988a).
These authors used hexadecyltrimethylammonium (HDTMA) ions to replace
naturally occuring metal ions and found that in HDTMA—exchanged soil, not
only had the organic matter content been increased, but the sorptive
capacity for NOCs was greatly enhanced. The organic phase derived from
HDTMA exchanged on both soils (Boyd et al., 1988a) and smectite clays
(Boyd et al., 1988b) was shown to be a powerful partition medium for the
removal of NOCs from water. It was suggested that such modified soils and
clays may be used to improve the retardation capabilities of low organic
matter soils and aquifer materials, and to enhance the containment
capabilities of clay landfill liners and slurry walls. In the present
communication, the studies of soil modification for attenuating organic
contaminant mobility have been expanded to include seven NOCs with water
solubilities ranging over approximately two orders of magnitude. Many of
these have relatively high water solubilities and are common groundwater
contaminants. In addition, the structural effects of different exchanged
organic cations of the general from [(CH3)3NR]+ in relation to the sorption

of NOCs from water is established. Three organic cations, HDTMA (R =

 

 

34

C16H33), dodecyltrimethylammonium (DDTMA) (R = C12H25), and
nonyltrimethylammonium (NTMA) (R = CJC9), have been used for the
preparation of soil-organic complexes from which different sorptive
behaviors derived from different organic cations can be elucidated.
Finally, soils having different textural composition (clay contents) are
used to assess the influence of cation exchange capacity (CEC) on the

sorptive properties of HDTMA-exchanged soils.

EXPERIMENTAL
Soils

The following soils were used in this study: Marlette soil A and Bt
horizon (fine-loamy, mixed, mesic Glossoboric Hapludalfs), Oshtemo soil
Bt horizon (coarse-loamy, mixed, mesic Typic Hapludalfs), and St. Clair
soil Bt horizon (fine, illitic, mesic Typic Hapludalfs). Organo-soil
complexes were prepared by reacting 2009 of soils with HDTMA, DDTMA, or
NTMA chloride salts. The organic cation salts were dissolved in water
(1800ml) and added in an amount equal to CEC of soils. After thorough
mixing, the organo-soil complexes were dialyzed against distilled water
until a negative chloride test was obtained. Then, the exchanged soils
were freeze-dried and stored at room temperature.

Chemicals

The organic chemicals used in this work were: benzene, toluene,
ethylbenzene, perchloroethene (PCE), trichloroethene (TCE), o-
dichlorobenzene (o-DCB), and 1,2,4-trichlorobenzene (1,2,4-TCB). All
reagents used in this study were analytical grade or better and purchased

from Aldrich Chemical Co., Milwaukee, Wis. Benzene and toluene were added

 

35
to the soils as the neat liquids, whereas the others were prepared in
methanol stock solutions, using a Hamilton microliter syringe.
Soil properties:

The cation exchange capacity (CEC) of soils was taken to be the sum
of extractable cations: Al + Ca + Mg + K + Na. Exchangeable Ca, Mg, K,
and Na were extracted with 1N NH4OAC at pH 7 and determined by atomic
absorption spectrophotometry. Exchangeable Al from each soil was
determined by displacement with 1N KCl. The quantity of Al in the extract
was analyzed by titrating with standardized base. Organic carbon analysis
was performed by Huffman Laboratories, Inc., Golden, Colorado. Factotrs
used to convert soil, HDTMA, DDTMA, and NTMA organic carbon to organic
matter were 2, 1.24, 1.27, and 1.29, respectively.

Batch equilibrium experiments:

(1) Asdorption isotherms of organic cations on soils: lg of St.
Clair, Marlette, or Oshtemo soils Bt horizon and 10ml of water were placed
in Corex glass tubes. Portions of an ethanol solution containing 1:1000
”C-HDTMA: HDTMA were added in an amount equal to 0.25, 0.5, 0.75, 1, 1.5,
and 2 times the CEC of soils. The mixed suspension were shaken for 24h
at 20°C in a reciprocating shaker. THe aqueous phase were separated by
centrifugation and 1ml assayed for 14C by liquid scintillation counting.

(2) Adsorption isotherms of organic chemicals on soil-organic
complexes: Adsorption isotherms were obtained by adding varying quantities
of test organic chemicals (Lee et al., 1988) to Corex glass tubes which
contained either 25ml of water and 39 of organo-soil complexes or 20ml of
water and 159 of untreated soils. To obtain equilibrium, mixed solutions

were placed on a reciprocating shaker for 24h at 20°C. Preliminary

36
kinetic investigations indicated that the sorption equilibrium was reached
in 20h. Phases were separated by centrifugation at 8000 rpm for 30 min.
using a Sorvall RC5-C centrifuge equipped with an 55-34 rotor. Aliquots
of the equilibrium supernatant (1ml) were then transferred into a glass
vial containing 10ml of hexane (for o-DCB, 1,2,4-TCB, TCE, and PCE) or
carbon disulfide (for benzene, toluene, and ethylbenzene). The vials were
closed with foil—lined screw caps and shaken vigorously for 1h on a
reciprocating shaker.
Analytical procedures:

The hexane and carbon disulfide extracts were analyzed for the
organic compounds by gas chromatography (GC). GC was performed on a
Hewlett Packard model 5890A equipped with both electron capture and flame
ionization detectors. The mass of organic compounds in the extracts was
determined by measurement of peak areas by a Hewlett Packard 3392A
integrator. For 1,2,4-TCB and o-DCB, a 1.5% SP-2250/1.95% SP-2401 0n
100/120 supelcoport packed glass column (2.4m x 6.4mm ID). For TCE and
PCE, the separation was performed on 1% SP-IOOO coated on 60/80 carbopack
B packed steel column (1.8m x 3.2 mm ID). Finally, the column packed with
5% SP-1200/1.75% Bentonite 34 coated on 100-120 mesh supelcoport support
was used for separation of benzene, toluene, and ethylbenzene. Carrier
gas was high-purity N2 at flow of 40ml/min. Operating temperature for
injection port and detector were 270°C and 300°C, respectively. Each run
was isothermal: Oven temperature were 80°C for benzene and toluene, 100°C
for ethylbenzene, 150°C for TCE, o-DCB, and 1,2,4-TCB, and 180°C for PCE.
All samples were run in duplicates and the recovery of these test

chemicals ranged from 90% to 98%. The equilibrium concentration measured

37

were not adjusted for the recovery.

RESULTS

The physical and chemical properties of untreated Marlette soil and
the Marlette soil treated with three different organic cations are shown
in Table l. The three organic cations used were all of the form
[(CH3)3NR]* and R varied in size from a C16 (HDTMA) to a C12 (DDTMA) to a
C9 (NTMA) hydrocarbon. When the Marlette A and Bt horizon soils were
treated with HDTMA, DDTMA, and NTMA ions (in an amount equivalent to the
CEC of the soil), the organic carbon (0C) contents increased
significantly. The organic matter (OM) content of treated soils is the
sum of indigenous soil organic matter (SOM) plus OM derived from the
exchanged organic cations. For example, The tolal OM content of HDTMA—
treated Marlette soil Bt horizon (4.85%) was obtained by adding the
indigenous SOM (0.6%) plus OM derived from HDTMA ions (4.25%). As
expected, larger organic cations (e.g. HDTMA ions) were more effective for
the contribution of OM than small organic cation (e.g. NTMA ion). The
relative increase in organic matter contents was much greated for the Bt
horizon soil, which as a subsurface soil, has low indigenous SOM valves.
Thus, in the treated Bt soils most of the total organic matter is dervied
from the exchanged organic cations. In contrast, in the treated A horizon
soil, natural organic matter exceed the added synthetic organic matter.

The properties of three different soils either untreated or treated
with HDTMA are shown in Table 2. The soils were selected to give a range
of textural classes, CECs and clay contents. Of these, the Oshtemo (sandy

loam) has the lowest clay content and the lowest CEC, and the St. Clair

 

38

 

 

 

 

 

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39

 

 

 

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40
(clay) the highest. Table 2 shows that the OM content of the treated
soils is depentent on the CEC of the soils. Thus, the treated-Oshtemo
soil, with the lowest CEC, had the lowest total OM content. As mentioned
above, the soil-organic complexes were prepared by adding organic cations
in an amount equal to the CEC of soils.

The adsorption of HDTMA ions by the three soils (Bt horizons) is
shown in Figure l. HDTMA was added in varying amount up to twice the CEC
of the soil. Adsorption isotherms of HDTMA ions on soils were typical for
high-affinity ion exchange reactions. The isotherm shape is Type I in
Brunauer’s classification (1944) showing a high of preference of the soil
for the added HDTMA exchange ions. As illustrated in Figure 1, the HDTMA
ions are very strongly sorbed from solution by the soils. The isotherms
show a very little HDTMA in solution at dosages close to the CEC of soils,
and some continued sorption of HDTMA ions in excess of CEC. However, when
HDTMA ions were added at 1.5 and 2 times the CEC of the soil, the solution
equilibrium concentrations increased significantly.

The release of exchanged 1"C-HDTMA from the Marlette Bt horizon soil
after mixing with aqueous solutions of CaCl2 was also measured (data not
shown). Calcium chloride was added in amount equal to 10, 50, and 100
times the amount of exchanged HDTMA. Sorption isotherms of benzene,
toluene, and ethylbenzene on the untreated and HDTMA-treated St. Clair and
Oshtemo soil Bt horizon are shown in Figure 2. As expected, the untreated
soils display very low uptake of these organic chemicals from water,
presumably due to their low OC contents; the isotherms of benzene and
toluene could not be distinguished from the blank (no soil) samples. In

contrast to the lack of sorption by the untreated soils, very high uptake

41

Figure 1. Adsorption isotherms of hexadecyltrimethyl ammonium (HDTMA)

on untreated soils Bt horizon.

42

HDTMA ON SOILS

 

 

 

 

 

 

 

 

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43

Figure 2. Sorption of benzene, toluene, and ethylbenzene on untreated
(triangles) and HDTMA-treated (circles) Bt-horizon St. Clair
(a - c) and Oshtemo (d — f) soils.

 

 

44

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45
of benzene, toluene, and ethylbenzene was found on the HDTMA-treated
soils. This is reflected in the dramatic increase in the slopes of
isotherms shown in Figure 2. All isotherms were linear over a broad range
of aqueous phase concentrations.

Figure 3 shows sorption isotherms for benzene, toluene, and
ethylbenzene on the untreated and HDTMA-treated A and Bt horizons. Again,
uptake by the Bt horizon of all chemicals tested was greatly enhanced in
the HDTMA-treated soil. For the treated A—horizon, uptake was also
enhanced significantly, but the relative increase was lower than for the
Bt horizon because of the highest indigenous SOM content in the A horizon.
Although the HDTMA—treated Marlette A-horizon soil is about 50% natural
SOM and 50% HDTMA organic matter (Table 1) sorption was more than doubled
in the treated soil.

The influence of soil properties on the sorptive behaviors of the
HDTMA—treated St. Clair, Marlette, and Oshtemo soils for benzene, toluene
and ethylbenzene can be seen by comparing the isotherms shown in Figure
4. Increasing the CEC of soils (Oshtemo < Marlette < St. Clair) resulted
in steeper isotherms (high slopes). Further inspection of Figure 4 shows
that in overall sorptive behavior, much larger differences in the slopes
of the isotherms were found between HDTMA-treated Oshtemo and St. Clair
soils as compared to the differences between the treated-Marlette and St.
Clair soils. This trend follows the same trend in total 0M contents of
the HDTMA—treated soils that decrease in the order: Marlette = St. Clair
> Oshtemo.

Figure 5 shows the isotherms of benzene, toluene, and ethylbenzene

uptake by Marlette soil Bt horizon having HDTMA, DDTMA, and NTMA as the

46

Figure 3. Sorption of benzene, toluene, and ethylbenzene on untreated
(triangles) and HDTMA-treated (circles) Marlette soil A (a -

c) and Bt (d - f) horizons.

47

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48

Figure 4. Sorption of benzene (a), toluene (b), and ethylbenzene (c) on
HDTMA-treated St. Clair (circles), Marlette (squares), and

Oshtemo (triangles) soils Bt horizon.

49

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50

Figure 5. Sorption of benzene (a), toluene (b), and ethylbenzene (c) on
treated Marlette soil Bt horizon: NTMA—treated (circles),

DDTMA-treated (triangles), HDTMA-treated (squares).

51

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52

exchanged cations. A positive relation is apparent between the amount of
sorbed chemicals and alkyl chain length of exchanged organic cations. As
the chain length decreases in going from HDTMA (C16) to DDTMA (C12) to NTMA
(Cg), the degree of uptake of organic species fromeater is decreased. 'The
decrease in sorption follows the total 0M contents which decrease in the
same order: HDTMA > DDTMA > NTMA. Again, sorption isotherms were linear
over a wide range of aqueous phase concentration. Except for the uptake
of benzene by NTMA-treated Oshtemo soil, the linear correlation
coefficient (r2) for all isotherms are greater than 0.95.

Table 3 and 4 show the water solubilities, partition coefficients,
K, and the 0M normalized Kan values (K/fraction GM) of the organic
chemicals used in present work. Table 3 shows the effect of three
different exchange cations on the sorptive properties of the Marlette
soil. Table 4 shows the effect of HDTMA treatment on the sorptive
properties of three different soils. The organic compounds tested in this
study span a range of approximately 2 orders of magnitude of water
solubilities, and were selected to represent major organic ground water
contaminants. The sorption coefficients (K values) of the linear
isotherms correspond to the ratio of the concentration of the sorbed
chemical in the soil to its equilibrium solution concentration in water.
These values are obtained by dividing the solute uptake, 0 (mg/kg), by the
equilibrium concentration, C (mg/l) or simply from the slopes of the
linear isotherms.

Examination the K values listed in Table 3 and 4 reveals that the
K values increase with the 00 content of soil and were higher for

compounds with lower water solubilities. This relation between sorption

 

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55

and 0M content can be seen by comparing the K values from the treated and
untreated soils or from soils which have been treated with various organic
cations. For example, the K values obtained from the uptake of PCE on
untreated (0.6% 0M) and HDTMA-treated (4.85% OM) Marlette soil Bt horizon
are 0.17 and 27.3, respectively (Table 3). Similarly, the K values
obtained from treated soils decrease in the order: HDTMA-treated > DDTMA-
treated > NTMA-treated soils. For example, the K values of ethylbenzene
on the treated Marlette Bt horizon soils were 62.51, 25.94 and 4.53 for
HDTMA-, DDTMA—, and NTMA-treated soil, respectively (Table 3). In all
cases, Kom values for the treated soils were higher than the soil Kom
values for the untreated soils. The K values of organic chemicals were
also affected profoundly by their chemical properties. As the water
solubilities decrease in going from benzene (1780 ppm) to 1,2,4-TCB (20
ppm), the K values for sorption by the untreated Marlette A-horizon soil
increases from 0.57 to 40.25 (Table 3). In general, K values for the
treated soils also increased with decreasing water solubilities of the
organic solutes (Table 3 and 4). None of the other parameters examined
(e.g. particle size and pH) were strongly correlated with K values.

The sorption coefficients K normalized for the 0M content (Kmn==
100K/%0M) are also given in Table 3 and 4. Some of Kom values could not
be obtained from untreated soils Bt horizon because the measured partition
coefficients were very small and K values could not be determined with
sufficient accuracy. It can be seen from Table 3 that the log Km‘values
decreased in the order: HDTMA z DDTMA > NTMA > untreated soil. In all
cases, Kom values for treated soils were greater than soil Kom values for

untreated soils.

56
Table 4 gives the calculated log K¢m1values of benzene, toluene, and
ethylbenzene for three untreated and HDTMA-treated soils. For all three
soils, the log Kmlvalues tended to converge on a single value for each
organic chemical. For example, all three HDTMA-treated soils give log Kan
values of 2.5 for benzene and three untreated soils give approximately log
an

K values of 1.9. Overall, for the same solute, the log Km‘values are

higher for treated soils than those for untreated soils.

 

57
DISCUSSION

The results presented here clearly demonstrate that the sorptive
properties of surface and subsurface soils for NOCs can be greatly
enhanced by simple ion exchange reactions of large organic cations for
naturally occurring inorganic exchange ions. The ion exchange reaction
results in the nearly stoichiometric displacement of the small inorganic
exchange ions. This can be seen clearly in the adsorption isotherm of14C-
HDTMA by the three subsurface soils which rise very sharply. Addition of
HDTMA to soil in amounts less than or equal to the CEC of the soil,
resulted in very low equilibrium solution concentrations of14C-HDTMA. It
is apparent from these isotherms that HDTMA is highly preferred as an
exchange ion and will easily displace inorganic ions. The organic
exchange ions once bound are held essentially irreversibly on the clay
surface. This is shown by the inability of high ionic strength CaCl2
solution to displace exchanged 14C—HDTMA.

In this work we confirm and expand our previous results (Boyd et
al., 1988a) on the increased sorptive capabilities of HDTMA—exchanged soil
by examining several additional organic compounds which are representative
of common ground water contaminants. Treatment of surface or subsurface
soil with HDTMA dramatically increased the uptake from water of all
compounds tested. For example, the sorption coefficients of the treated
subsurface soil increased by about 200 fold for toluene, PCE, ethylbenzene
and dichlorobenzene. The more water soluble compounds (e.g. benzene and
TCE) were not measurably removed from water by the untreated subsurface
soils but comparatively high uptake was observed in the treated soils.

For the surface soils, sorption coefficients of the untreated samples were

58

higher than for the untreated subsurface soil because of the high
indigenous SOM content of the A-horizon soil. However, the HDTMA-
exchanged A-horizon soil also displayed enhanced uptake of all NOCs
tested. For HDTMA-treated surface soil the increase in K was about 15
times. Although the organic matter content of the Marlette A horizon soil
was approximately doubled by exchange with HDTMA, the K values for the
treated soil increased to a much large extent (~15x) suggesting that the
HDTMA-derived organic matter is a much more effective sorbent for NOCs
than natural SOM.

A more accurate evaluation of the sorptive capabilities of natural
SOM and synthetic OM derived from exchanged cations can be obtained by
comparing the corresponding Kunvalues which indicate the effectiveness of
different organic phases on a unit weight basis. For the untreated A
horizon soil, all OM is natural SOM; whereas, for the treated Bt horizon
soils, OM is derived primarily from the exchanged organic cations. The
lg“ values of benzene, TCE, toluene, PCE, ethylbenzene and o-DCB on the
HDTMA-treated Marlette Bt soil are 10 to 30 times higher than the
corresponding Kanvalues on the untreated Marlette A horizon soil. It can
be therefore concluded that OM derived from added HDTMA is approximately
10 to 30 times more effective on a unit weight basis than natural SOM for
removing NOCs from water. Thus, not only can the OM content of soil be
increased significantly by this simple soil modification technique, but
the added OM is highly effective and can be expected to enhance the
retention of NOCs by soil.

The data presented here strongly suggest that the organic matter

derived from the exchanged quaternary ammonium ions behave as a partition

 

59

medium in the uptake of NOCs from water. This is a process of
splubilization of NOCs on the synthetic organic phase formed by the
conglomeration of the alkyl tails of exchanged organic cations. This
process is essentially identical to the partitioning of solutes from water
into an immisible bulk organic solvent phase such as hexane or octanol.
The similarity of the logKom values of the HDTMA—treated Marlette Bt
horizon soil to the corresponding logK0H values (octanol water partition
coefficient) strongly supports this mechanism.

The concept of solute partitioning has previously been used to
describe soil water equilibria of NOCs (Chiou et al., 1979, 1983). The
characteristic of solute partitioning developed for natural soils are also
observed for the treated soils. These include: highly linear isotherms,
dependence of the sorption coefficients on the water solubility of the NOC
and a close correspondence between logKom and logKOH values.

Although natural SOM and the synthetic organic matter phases are
mechanistically similar in uptake of NOCs from water, the effectiveness
of the latter is 10 to 30 times greater than the former, as mentioned
previously. These results obtained from the composition of the two OM
phases with HDTMA—derived 0M being a better nonpolar solvent than natural
SOM which has a high polar functional group content.

An additional objective of the work was to examine the sorptive
behavior of soil exchanged with these different organic cations,
containing alkyl moieties of different size, ranging from C9 to C16.
Obviously, in the size of the organic cation decreases, the increase in
GM after cation exchange is less. However, the effectiveness of the

organic phases derived from the different cations can be compared on an

 

60

equal weight basis by comparing the Km1values. This comparison of the
treated Marlette Bt horizon soils reveals that the organic phases composed
of HDTMA and DDTMA were similar with HDTMA being slightly more effective.
When the C9 quaternary ammonium ion was used (NTMA) a large decrease in the
Km‘value occurred, although these values were still considerably higher
than Kom values for natural SOM. Apparently, as the alkyl chain length
decreases below C12, the hydrocarbon moieties become more isolated and form
a less effective partion medium than with the larger cations, i.e., HDTMA
and DDTMA.

These results appear to be consisted with earlier work on the
interlayer arrangement of exchanged n—alkylammonium ions on smectite
clays. When smectite is exchanged with n-alkylammonium ions, the
increases in basal spacing of the clay occurs in a stepwise fashion as the
number of carbon atoms in the alkyl chain increases. A spacing of about
1.1nm is observed for C2 - C10 alkyl chains, which jumps to 1.8nm for C12 —

C20 alkyl chains (Theng, 1974). For the small alkyl chain, planer surface
coverage is low enough to allow a "keying in" of organic cation on
opposing clay layers. However, with the larger alkyl ion (C12 and greater)
surface coverage is greater than 50% and they cannot fit into the
unoccupied mineral surfaces of the opposing clay surfaces. As a result,
they are forced to interact with one another instead of with the free
mineral surfaces.

The sorptive capabilities for NOCs of three different soils
exchanged with HDTMA were also compared. These subsurface soils differed
primarily in their CECs which were relatively high for the St. Clair and

Marlette soils and low for the Oshtemo soil. As expected, the 0M contents

 

61
of the HDTMA-exchanged Marlette and St. Clair soils were higher than the
Oshtemo soils, and as a result the K values for Benzene, toluene and
ethylbenzene decreased in the order: St. Clair = Marlette > Oshtemo (Table
4). However, the Km‘values for these three HDTMA-treated soils (Table 4)
were nearly identical indicating that the HDTMA—organic phases formed on

were equally effective.

REFERENCES

Boucher, F. R., and Lee, G. F. (1972) Adsorption of lindane and dieldrin
pesticides on unconsolidated aquifer sands: Environ. Sci. Technol. 6, 538-
543.

Bowman, M. C., Schechter, M. S., and Carter, R. L. (1965) Behavior of
chlorinated insectucudes in a broad spectrum of soil types: J. Agric. Fd.
Chem. 13, 360—365.

Boyd, S. A., Lee, J. F., and Mortland, M. M. (1988a) Attenuating organic
contaminant mobility by soil modification: Nature 333, 345-347.

Boyd, S. A., Mortland, M. M., and Chiou, C. T. (1988b) Sorption
characteristics of organic compounds on hexadecyltrimethylammonium-
smectite: Soil Sci. Soc. Am. J. 52, 652-657.

Briggs, G. G. (1969) Molecular structure of herbicides and their sorption
by soils: Nature 223, 1288.

Brunauer, S. (1944) in The adsorption of gases and vapors, Oxford
University Press, pp. 150.

Chiou, C. T., Peters, L. J., and Freees, V. H. (1979) A physical concept
of soil-water equilibrium for nonionic organic compounds: Science 206,
831—832.

Chiou, C. T., Porter, P. E., and Schmedding, D. w. (1983) Partition
equilibria of nonionic organic compounds between soil organic matter and
water: Environ. Sci. Technol. 17,227-231.

Chiou, C. T., Shoup, T. D., and Porter, P. E. (1985a) Mechanistic roles
of soil humus and minerals in the sorption of nonionic organic compounds
from equeous and organic solutions:0rg. Geochem. 8, 9-14.

Chiou, C. T., and Shoup, T. D. (1985b) Soil sorption of organic vapors and
effects of humidity on sorptive mechanism and capacity: Environ. Sci.
Technol. 19, 1196—1200.

Dzombak, D. A., and Luthy, R. G. (1984) Estimating adsorption of
polycyclic aromatic hydrocarbons on soils: Soil Science 137, 292- 308.

Garbarini, D. R., and Lion, L. N. (1986) Influence of the nature of soil
organics on the sorption of toluene and trichloroethylene: Environ. Sci.
Technol. 20, 1263-1269.

Hassett, J. J., Means, J. C., Banwart, w. J., Wood, S. A., and Khan, A.
(1980) Sorptioon of dibenzothiophene by soils and sedimenys: J. Environ.
Qual. 9, 184-186.

Karickhoff, S. M., Brown, D. S., and Scott, T. A. (1979) sorption of
hydrophobic pollutants on natural sediments: Water Res. 13, 241-248.

62

63

Lambert, S. M., Porter, P. E., and Schieferstein, H. (1965) Movement and
sorption of chemicals applied to the soil: Needs 13, 185-190.

Lambert, S. M. (1967) Functional relationship between sorption in soil and
chemical structure: J. Agric. Fd. Chem. 15, 572-576.

Lambert, S. M. (1968) Omega (0) a useful index of soil sorption
equilibria: J. Agric. Fd. Chem. 16, 340—343.

Lee, J. F., Mortland, M. M., Chiou, C. T., and Boys, S. A. (1988) Shape
selective adsorption of aromatic molecules from water by
tetramethylammonium-smectite: J. Physic. Chem. (submitted)

Means, J. C., Wood, S. G., Hassett, J. J., and Banwart, w. L. (1982)
Sorption of amino- and carboxy-substituted polynuclear aromatic
hydrocarbons by sediments and soils: Environ. Sci. Technol. 16, 93—98.

Mortland, M. M. (1970) Clay-organic complexes and interactions: Adv.
Agron. 22, 75-117.

Mortland, M. M., Shaobai, S., and Body, S. A. (1986( Clay-organic
complexes as adsorbents for phenol and chlorophenols: Clays and Clay
Minerals 34, 581,585.

Theng, B. K. G. (1974) The chemistry of clay-organic reactions: Wiley, New
York, pp. 343.

 

CHAPTER 3

SHAPE SELECTIVE ADSORPTION 0F AROMATIC MOLECULES FROM
HATER BY TETRAMETHYLAMMONIUM-SMECTITE

64

65

ABSTRACT

The adsorption of aromatic compounds by smectite exchanged with
tetramethylammonium (TMA) was studied. Aromatic compounds adsorbed by
TMA-smectite appeared to adopt a tilted orientation in a face-to-face
arrangement with the TMA-tetrahedra. The sorptive characteristics of TMA-
smectite were influenced strongly by the presence of water. The dry TMA-
smectite showed little selectivety in the uptake of benzene, toluene and
xylene. In the presence of water, TMA-smectite showed a high degree of
selectivety based on molecular size/shape, resulting in high uptake of
benzene and progressively lower uptake of larger aromatic molecules.
Water appeared ix) shrink the interlamellar cavities resulting iri this

selectivety.

INTRODUCTION

The net negative electrical charge of clays is balanced by exchanged
cations. In nature these are usually alkali metal and alkaline earth ions
such as Na‘ and Ca2+ which are strongly hydrated by the presence of water.
The hydration of inorganic exchange ions present on clays, and the polar
nature of Si-O groups imparts a hydrophilic nature to the mineral
surfaces. As a result, water is preferentially adsorbed by these surfaces
and nonpolar organic compounds cannot compete with strongly held water for
adsorption sites on the clay surfaces.

It is possible to greatly modify the surface properties of clays by
replacing natural inorganic exchange cations by larger alkylammonium ions
(Barrer et al., 1955, 1957). These ions act as "pillars" which hold the

aluminosilicate sheets permanently apart. In the modified form, the clay

66
surface may become organophilic and interact strongly with organic vapors
and with organic compounds dissolved in water. These organo-clays are now
able to sorb paraffins (Barrer et al., 1955) and aliphatic alcohols (Stul
et al., 1978, 1979), to remove organic contaminants from water (Boyd et
al., 1988a, b, c), and to serve as chromatographic stationary phases
(White et al., 1958; McAtee et al., 1977).

Until recently, the literature on the sorptive behavior of organo-
clays has been concerned almost exclusively with the organic vapor uptake
by dry modified-clay samples. Mortland et al. (1986) and Boyd et al.
(1988a) studied the uptake of phenol and chlorophenols from aqueous
solution by smectites whose cations were exchanged by quarterary ammonium
ions of the form [(C}g)3-NR]+, where R is an alkyl group. These authors
showed that in general when R was a large nonpolar alkyl group (e.g., R
= qu53) the modified clay samples exhibited greatly improved sorption
capacities in comparison with unmodified clays or modified clays in which
the exchanged organic ions were small in size. McBride et al. (1977) and
Mortland et al. (1986) found however that the smectite exchanged with
small tetramethylammonium ion (here referred to as TMA-smectite) exhibited
much higher affinity for benzene from water than for less water-soluble
and large sized 1,2,4-trichlorobenzene (1,2,4—TCB). Ihe~extent of benzene
uptake by the TMA-smectite was also much greater than by clays exchanged
with tetraethylammonium ion (TEA), or'with hexadecyltrimethyl ammonium ion
(HDTMA), in the sequence of TMA-smectite > HDTMA-smectite > TEA-smectite.
These studies indicated that the exchanged organic ions affected the
sorptive behavior of clay in some manner that appeared to be related to

the size and molecular arrangement of the exchanged ion in the clay.

 

67

To obtain a more perspective view of the influence of the exchanged
organic ion on the mechanistic function of the modified clay, Boyd et al.
(1988b) studied the sorption characteristics of benzene and
trichloroethylene (TCE) on HDTMA-smectite from both aqueous solution and
vapor phase. They found that the dry HDTMA-smectite behaved as a dual
sorbent, in much the same fashion as dry soil (Chiou et al., 1985), in
which the bare mineral surfaces function as a solid adsorbent and the
exchanged HDTMA organic ions function as a partition medium. In aqueous
solution, adsorption of nonionic organic compounds by free mineral
surfaces is minimized by the strong competitive adsorption of water, and
the uptake of organic solutes by the modified clay is effected mainly by
solute partitioning in the organic medium that is formed by conglomeration
of large C16 alkyl groups associated with HDTMA. The presumed partition
effect with the HDTMA-smectite is supported by the linear sorption
isotherm, lack of a competitive effect between organic solutes, and the
dependence of the sorption capacity on the amount of HDTMA in clay. The
uptake of organic vapors by dry HDTMA-smectite was greater than by water-
saturated HDTMA-smectite because of concurrent adsorption on mineral
surfaces, and consequently, vapor uptake isotherm were not linear. The
improved sorption of benzene and TCE from water by HDTMA-smectite over
that by pure clays was attributed to partition into the highly nonpolar
hydrocarbon medium. Improvement of the sorption capacity of soils with
low organic matter contents was similarly achieved by cation exchange
reactions with HDTMA ions (Boyd et al., 1988c). This study also
demonstrated that HDTMA-derived organic matter added to soil was 10 to 30

times more effective on a unit weight basis than natural soil organic

 

68
matter for removing organic contaminants from water.

Considering the benzene sorption data with TMA—smectite and HDTMA-
smectite in aqueous systems as mentioned above, one may speculate that
the greater uptake by TMA-smectite than by HDTMA-smectite is effected by
some unique mechanism other than partition associated with TMA—modified
clay surfaces, since it is questionable that small TMA ions can function
as an effective partition medium. The research described here was
designed to elucidate that impact of exchanged TMA ions on the mechanistic
function of the modified clay. Sorption isotherm for a series of organic
compounds of varying sizes and water solubilities were performed both from
aqueous solution and from vapor phase in order to evaluate the influence

of water as well as the molecular properties of the compounds on sorption.

EXPERIMENTAL

The sorbents used in these experiments were TMA-smectite and
TMA-illite which were prepared by reacting < Zum fraction of Wyoming
bentonite or Clarence till with TMA chloride salts in the amount of 3-5
fold meq. per meq. of CEC (0.9 meq. x 9'1 and 0.3 meq. x g'1 respectively
for smectite and illite). After reacting for 2 to 4 h, the clay—organic
complexes were dialyzed against distilled water until no chloride appeared
in the dialysate.

The mixed TMA/Ca-smectites were made by reacting aqueous solution
containing both CaCl2 and TMA ions with Na—saturated Wyoming bentonite in
3-5 fold excess of the CEC. The mixed TMA/Ca-smectites were prepared at
three different saturated levels (100, 50 and 25 percent) corresponding

to the different amounts of CEC occupied by TMA ions. For example, the 25

69
percent saturated TMA—smectite was prepared by adding CaCl2 and TMA ions
simultaneously at a 3 : 1 ratio of CazTMA. Additional details for the
preparation of modified clays have been published previously (Boyd et al.,
1988b).

Adsorption isotherms were determined for aqueous solutions of eight
organic pollutants by using a batch equilibration technique. Benzene,
toluene, ethylbenzene, o—xylene, p-xylene, o-dichlorobenzene (o—DCB),
1,2,3-trichlorobenzene (1,2,3-TCB), and hexachlorocyclohexane (r-isomer)
(lindane) were purchased from Aldrich Chemical Co. All reagents used in
this study were analytical grade or better and used without further
purification. Experiments were conducted at ambient temperatures (20 i
1 °C).

To determine the sorptive capacity of TMA—smectite, samples of clay
(0.1-0.3 g) were weighed into 25 ml Corex glass centrifuge tubes with
Teflon-lined screw caps, and 25 ml distilled water added leaving a minimum
headspace to avoid the loss of solutes from evaporation. The initial
concentration ranges for the aqueous solution of eight organic pollutants
were: benzene, 100 to 1500 mg l4; toluene, 20 to 550 mg l4. o—xylene and
p—xylene, 12 to 190 mg l'1; o-DCB and ethylbenzene 12 to 150 mg l'H
1,2,3-TCB, 0.8 to 10 mg l'1; and lindane, 0.4 to 8 mg l'1. Benzene and
toluene were added directly as the neat liquid using a Hamilton microliter
syringe; whereas, the others were prepared in methanol as a stock
solution. The concentration of organic pollutants did not exceed their
water solubility values. After appropriate volumes of eight chemicals
were added, the centrifuge tubes were immediately closed with foil—lined

screw caps then shaken for 24 h on a reciprocating shaker. Preliminary

70

kinetic investigations indicated that sorption equilibrium was reached in
20 h or less and so an equilibration time of 24 h was chosen for all
experiments. After shaking, the tubes were centrifuged for 30 min. at
8000 rpm to separate the liquid and solid phases. The supernatant liquid
(1 ml) was then transferred into a glass vial containing 10 mf of hexane
(for o-DCB, 1,2,3-TCB, and lindane) or carbon disulfide (for benzene,
ethylbenzene, toluene, and xylene). The vials were closed with foil-lined
screw caps and shaken vigorously for 1 h on a reciprocating shaker. An
aliqout of hexane or carbon disulfide layer was then removed for gas
chromatography (GC) analysis.

The G0 was performed on a Hewlett Packard model 5890A GC using
either an electron capture (o-DCB, 1,2,3-TCB and lindane) or~ a flame
ionization detector (benzene, ethylbenzene, toluene and xylene). For 1,
2, 3-TCB, o-DCB and lindane, a 1.5% sp-2250/1.95% sp-2401 on 100/120
Supelcoport packed glass column (2.4m x 6.4mm ID) was used. For benzene,
ethylbenzene, toluene and xylene, a 5% sp-1200/1.75% Bentonite 34 on
100/120 Supelcoport packed steel column (1.8m x 3.2mm ID) was used. The
carrier gas was N2 applied at a linear velocity of 40 ml min'1. The
injector temperature was 250°C and the detector 300°C. Each run was
isothermal: 80°C for benzene and toluene, lOO°C for ethylbenzene and
xylene and 150°C for 1,2,3-TCB, o-DCB and lindane. For a quantitative
measurement of peak areas, a Hewlett Packard 3392A integrator was
employed. All samples were run in duplicates and the recovery of these
tested chemicals ranged from 92 to 98%. The equilibrium concentration

measured were not adjusted for the recovery.

71

A Quantasorb Jr. sorption meter was used for surface area
measurement. The fully to partially saturated TMA-smectites were
dehydrated at 150°C for 12 hours under vacuum. The surface areas were
then determined at liquid N, temperatures using N2 as the adsorbate and
He as the carrier gas. The adsorption data were plotted according to the
BET equation. Approximately 100 mg of sorbents were used for each
determination.

The uptake of benzene, toluene, and o-xylene vapors by dry smectite
fully to partially saturated with TMA was measured by using a static
equilibrium sorption apparatus. The test compounds were purified by
vacuum distillation before their vapors were introduced into the sorption
chamber containing a Cahn electrical microbalance. After vacuum
distillation, the vapor pressures of the test compounds were monitored by
the Baratron pressure gauge of the apparatus. Additional details of the

procedure and equipment have been given elsewhere (Chiou et al., 1988).

RESULTS

The clay samples used in this study were smectite and illite. The
illite was fully exchanged with TMA. Smectite was fully to partially
saturated with TMA; Ca2+ occupied the remaining exchange sites. The
organic carbon contents and surface areas of these clays are given in
Table I. The TMA-smectite with 4.0, 2.5 and 1.7 percent organic carbon
corresponds to approximately 100, 50 and 25 percent saturation of the
exchange complex with TMA ions. A plot of surface area against fraction
of TMA ions on the exchange complex (0, 0.25, 0.5 and 1) reveals that the

mixed TMA/Ca-smectites were above the line connecting homoionic

 

 

 

Table 1. Organic carbon contents and surface areas of the clay
adsorbents.
Sample Organic TMA: Surface
carbon Ca area
(9/1009) (ma/g)
TMA—smectite 4.0 1:0 172.4
TMA/Ca-smectite 2.5 1:1 135.9
TMA/Ca-smectite 1.7 1:3 84.7
Ca-smectite 0.9 0:1 24.8
TMA-illite 1.2 1:0 n.d.*

 

*n.d. is not determined.

73
Ca-smectite and TMA-smectite (Fig. 1). The fact that the surface areas
are not linearly related to the extent of exchange of TMA for Ca”*clearly
demonstrates a degree of randomness in distribution of calcium and TMA
ions on the clay surface. If the calcium and TMA ions were segregated
into different interlamellar regions of smectite, then the points would
lie on the dashed line in Figure 1.

The sorption isotherms (H’ benzene, toluene, o-xylene, p-xylene,
ethylbenzene, o-DCB, 1,2,3-TCB and lindane from aqueous solution by
homoionic 'TMA-smectite and 'TMA-illite are presented in figure 2a-h.
Figure 2a also shows the benzene sorption isotherm for TMA/Ca-smectite
where TMA ions occupy approximately 25 percent of the cation exchange
sites. TMA-smectite showed the highest affinity for benzene; toluene,
ethylbenzene, xylene were intermediate, and lindane and 1,2,3-TCB were
the lowest. The uptake of benzene from water was lower for the mixed
TMA:Ca(l:3)-smectite as compared to smectite fully saturated with TMA
(Figure 2a). For the 1:1 TMA:Ca-smectite, benzene uptake was intermediate
(data not shown). There is also a noticeable difference in the type of
sorption isotherms represented in the graphs. The benzene isotherm is
Type I (Brunauer, 1944). As uptake decreases in going from benzene to
toluene to ethylbenzene and xylene, the isotherm shape changes to Type V.
Lindane and 1,2,3-TCB have linear sorption isotherms. ‘The uptake of
1,2,3-TCB was higher than for lindane, which is the only non-planar
molecule studied. With the exception of lindane, all the organic compound
used in this study show less uptake by TMA-illite as compared to the
TMA-smectite (Fig. 2). For lindane, TMA-illite gave slightly higher
uptake than TMA-smectite.

74

Figure 1. Surface area of TMA/Ca-smectites as a function of the fraction

of cation exchange capacity occupied by TMA.

75

 

180 I I I | I I I I l

150

120

90

Surface Area (m2/9)

60

30

 

 

 

0 1 I 1 l _ 1 I 1 l 1
O 0.2 0.4 0.6 0.8 1

Fraction of CEC Occupied by TMA

 

Figure 2.

76

Adsorption of organic compounds from water by TMA-smectite
(triangles), mixed TMA:Ca(l:3)-smectite (squares), and
TMA—illite (circles). Plot (a) is benzene, (b) is toluene,
(c) is p-xylene, (d) is ethylbenzene, (e) is o-xylene, (f) is
o-dichlorobenzene, (g) is 1,2,3-trichlorobenzene and (h) is

lindane.

 

77

Uptake, Q(mg/g)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

anr

 

1:

1.

_

 

 

 

 

 

 

 

 

 

 

 

AM _ _ . _ . _ _ _ — _ MA
. U 1 .
am] 1 4N.m1
nm1 1 .81
T 4 I
MA 1 1 d0.m1
‘81 1 «1
V I
O
moo O
w.mO .
Q.OOI 1 O.M1Nm1 1
M351 1 O.MM01 4
ewe- 1o;mm1 1
.. + 1 1.
rMO1 1o;ao1 1
0.601 D 0.0mm 1 I
O . . _ _ _ p _ . 0
.m0 0 AC m0 4M0 O m

mnczmczc—s Oosomsfimzos. Gaga;

 

 

 

 

 

78

Figure 3 shows the uptake of benzene vapors by dry TMA-smectite and
mixed TMA/Ca-smectite. Figure 4 shows the uptake of toluene and o-xylene.
vapors by dry TMA- smectite. The amounts of benzene, toluene and o-xylene
uptake by ‘TMA-smectite decreased in the order benzene > toluene >
o-xylene, but overall the differences were relatively small. The uptake
of benzene, toluene, and o-xylene vapors at P/P° = 0.75 were 145, 140, and
132 mg/g, respectively. When there was partial exchange of Caz+ by TMA
ions, the sorptive capacity of TMA-smectite rose in proportion to the
fraction of TMA ions present. The uptake of benzene vapor by TMA-smectite
was approximately twice that of TMA:Ca(l:3)-smectite.

In order to evaluate differences in uptake from water by solutes of
different. water solubilities, and to compare the uptake of’ organic
chemicals from vapor phase to that from aqueous phase, the aqueous-phase
isotherms are expressed in terms of reduced concentration (Kilpling et
al., 1965). Figure 5 is obtained by plotting the amount of uptake by the
clay sample (0) against Ce/Cs, where Ce and Cs are equilibrium
concentration and water solubility respectively, rather than against Ce
as in the standard isotherm plot. The vapor sorption isotherms are
similarly expressed in the form of Q versus P/P°, where P is the
equilibrium partial pressure and P° is the saturation vapor pressure of
the compound at system temperature. The reduced plot provides a
theoretical basis for the order of preference of TMA-smectite for various
solutes and vapors tested under the same chemical activities.

By comparison of the reduced isotherms, uptake of benzene, toluene,
and o-xylene as vapors by dry TMA-smectite is found to be greater than

that of the solutes from aqueous system by the water-saturated TMA-

79

Figure 3. Adsorption of benzene vapor by dry TMA-smectite. The organic
carbon (0C) contents refer to the amount of carbon in TMA

ions. Additional exchange ions were Ca”fi

 

180

150

120

90

Uptake, 0(mg/g)

60

3O

80

 

 

4.0% 0C

   

2.5%OC

   

1.7%OC

  

 

 

P/P°

81

Figure .4, Adsorption of toluene (triangles) and o-xylene (circles)

vapors by dry TMA-smectite.

82

 

 

 

 

180

 

.3950 .833:

0.96

0.64
P/P°

0.32

Figure 5.

83

Reduced plot showing adsorption of organic compounds from
water by TMA-smectite (0) versus relative concentration
(Ce/Cs): benzene (closed circles), toluene (closed triangles),
ethylbenzene (asterisks), p-xylene (closed squares), o-xylene
(open circles), o-dichlorobenzene (open squares), 1,2,3-

trichlorobenzene and lindane (open triangles).

 

 

84

 

 

 

 

_
5
4
6355 .833:

15-

 

0.72

0.48

 

85
smectite, based on the results given in Figures 3-5. For instance, when
P/P° and Ce/Cs = 0.3, the amount of benzene sorbed as a vapor and from

water was 130 and 84 mg/g, respectively.

DISCUSSION

When adhered to the interlamellar surfaces of smectite, TMA ions
because of their small size are isolated from each other. When larger
organic cations are used, such as HDTMA, the hydrophobic tails interact
with each other producing an organic phase which acts as a partition
medium (Boyd et al., 1988b). Nonionic organic molecules partition from
water into this organic phase which acts as a solubilizing medium similar
to a bulk solvent phase such as octanol or hexane. The "mchanism of
uptake of nonionic organic compounds from water by TMA-smectite (i.e. ,
adsorption) is distinctly different from that by HDTMA-smectite (i.e.
partitioning). A curvilinear Type I isotherm is observed for the sorption
of benzene by TMA-smectite; whereas, a linear isotherm is observed for the
sorption of benzene by HDTMA-smectite (Boyd et al., 1988b). Other sorption
characteristics of TMA-smectite contrast with the partition behavior of
HDTMA-smectite. For example, compounds such as benzene and toluene that
have high water solubilities show strong affinities for TMA-smectite. In
a partition mechanism, the higher the water solubility, the lower the
partition coefficient. The uptake of benzene from aqueous solution is
also higher for TMA-smectite than for HDTMA-smectite despite the fact that
TMA-smectite has a much lower organic carbon content (4%) than
HDTMA-smectite (17%). For solute partitioning, it; is expected that a

higher carbon content of sorbent should result in greater solute uptake.

86

However, at Ce/Cs = 0.3, the uptake of benzene from aqueous solution is
84 mg/g for TMA-smectite and 25 mg/g for HDTMA-smectite (Boyd et al.,
1988b).

For the adsorption of organic vapors by dry TMA-smectite, results
from this study were similar to those obtained by Barrer et al. (1957).
Isotherms of benzene, toluene, and o-xylene were all of the same form,
composed of an initial sharp rise followed by a slow increase over a
fairly wide range of P/P°. The very high affinity of TMA-smectite shown
toward aromatic compounds in this paper, compared with aliphatic
hydrocarbon sorption from the vapor phase (Barrer et al., 1955), suggests
that there are some particularly favorable factors promoting the
adsorption of aromatic compounds. The juxtaposition of alternate
tetrahedral TMA ions adhering to the upper and the lower aluminosilica
sheets appears to provide modified "organic—coated" clay surfaces that
are not strongly wetted by water. Benzene may therefore be adsorbed onto
these modified surfaces possibly in a tilted orientation of its planar
aromatic ring onto the triangular faces of the TMA tetrahedra. In
estimating the surface areas of TMA ions and the free interlamellar cation
distances of TMA-clay, Barrer et al. (1961) concluded that the tilted
orientation is necessary for the intercalation of benzene in alkylammonium
montmorillonites. The higher uptake of aromatic hydrocarbons than of
aliphatic hydrocarbons by TMA-smectite could be a result of the more
compact and sheet-like aromatic structure for intercalation and some
interaction of the 111-electron system of the aromatic molecules with

positively charged nitrogen of TMA.

87

Although the greatest uptake of aromatic compounds has been found
by dry TMA-smectite, TMA-smectite does not show any pronounced selectively
in the adsorption of vapors of benzene, toluene or o-xylene (Figures 3 and
4). However, significant differences in selectivities were observed for
the uptake of these aromatic compounds from aqueous solution by
TMA-smectite. This is shown both by the amount of uptake and the shapes
of the isotherms for the aromatic compounds studied. TMA-smectite shows
a clear preference for benzene despite the fact that benzene has the
highest water solubility and lowest molecular weight of the compounds
tested. These factors would be expected to result in less partition
uptake from water as by HDTMA—exchanged clays (Boyd et al., 1988b, c).
Therefore, it appears that this selectivity is related to the shape and
size of the benzene molecule and the free space between the TMA ions on
the surface of water saturated clay. The importance of this shape/size
requirement is further illustrated by the fact that the extent of uptake
decreased as the size of the solute molecules is increased by adding
substituents to the benzene ring. Thus, the degree of uptake from water
by TMA-smectite decreases in the order: benzene > toluene> xylene ~
ethylbenzene > dichlorobenzene > trichlorobenzene > lindane. The isotherm
shapes also change from Type I for benzene to Type V for toluene, xylene,
ethylbenzene and dichlorobenzene, indicating progressively weaker
interactions with the TMA-smectite. The linear isotherms of
trichlorobenzene and lindane manifests very weak adsorptive interaction
of these large molecules with the TMA—smectite in aqueous solution.

This disparate affinity of TMA-smectite for the adsorption of

aromatic compounds from aqueous phase can be attributed to the molecular

88

shape as well as the role of water molecules. In an aqueous system water
molecules may associate with TMA ions and also solvate mineral sites of
TMA-smectite resulting in insufficient interlamellar spaces to allow the
accommodation of relatively larger molecules such as o-xylene and
1,2,3-TCB. Therefore, the main factor affecting the degree of adsorption
of aromatic molecules by TMA-smectite was found to be the molecular
shape/size. Organic molecules (e.g. benzene) whose critical dimensions
are less than the lateral distance between interlamellar ions or
intersheet spacing are adsorbed with greater ease and in substantial
amounts. These reasons offered explanation to extremely high affinity of
TMA-smectite for benzene, and the comparatively low affinity for larger
di- and trichlorobenzenes.

The presence of water also decreases the overall uptake of the
aromatic molecules tested. Benzene, toluene and xylene are adsorbed by
TMA-smectite in greater extent from vapor phase than from aqueous
solution. In the dry TMA/Ca-smectites, both free mineral surfaces and
TMA-modified surfaces provide adsorption sites for the aromatic molecules.
The decreased uptake by water-saturated TMA/Ca-smectite can be accounted
for in part by the inability of free mineral surfaces to adsorb nonionic,
nonpolar compounds from water because of the preferential adsorption of
water by the mineral surfaces (Chiou et al., 1985); in this case water
acts to deactivate the mineral surface. In addition, TMA ions may also
be weakly hydrated resulting in less uptake.

As compared with TMA-smectite, TMA-illite shows lower uptake
capacity for all the compounds used in this study except lindane. This

behavior can be attributed to the fact that illite does not have the

 

89

capability of interlamellar expansion to create additional surface for
adsorption. That the adsorption of all aromatic compounds tested is
greater for TMA-smectite than for TMA-illite indicates that these
molecules are being intercalated to some extent by TMA-smectite. However,
as the molecular size increases in going from benzene to trichlorobenzene,
fewer adsorption sites of sufficient size are available, and the larger
molecules have limited access to the interlamellar regions. Thus for
trichlorobenzene, the difference in uptake by TMA-smectite and TMA-illite
are relatively small as compared to benzene. Although both lindane and
benzene possess a six carbon ring, the molecular structure of lindane is
not planar; instead, the carbon ring assumes a "puckered" chair form. The
nonplanar molecular configuration of lindane apparently does not allow it
to be adsorbed on the interlamellar surfaces by TMA-smectite and in this
case the uptake of lindane is actually slightly higher for TMA-illite as
compared to TMA-smectite.

As the amount of TMA in TMA-smectite decreases, the amount of
benzene uptake also decreases. For uptake of vapors by dry TMA-smectite,
this decrease probably results from a concominant decrease in surface area
(Table I) as TMA is replaced by calcium, and a decrease in number of
TMA-ions as adsorption sites for benzene. For benzene uptake from aqueous
solution, the decreased adsorption likely results from having fewer TMA
ions and thus fewer sites for benzene. Here the mineral surface is
preferentially occupied by water so that surface area per se is not a
proper indicator.

The work reported here emphasizes very striking differences in the

adsorptive properties of dry and wet TMA-smectites as evaluated from their

 

 

 

90
uptakes of organic compounds from aqueous and vapor systems. Sieving
effects of TMA-smectite can only be seen from aqueous solution in which
water molecules shrink the interlamellar cavities resulting in a strongly
selective uptake of aromatic compounds. It is suggested that TMA-smectite
could be used for the selective removal of benzene-like compounds from

aqueous system.

 

REFERENCES

Barrer, R. M. and Macleod, D.M. (1955) Activation of montmorillonite by
ion exchange and sorption complexes of tetra-alkyl ammonium
montmorillonite: Trans. Faraday Soc. 51, 1290-1300.

Barrer, R. M., and Perry, G. S. (1961) Sorption of mixtures, and
selectivity in alkylammonium montorillonite part II. Tetramethylammonium
montmorillonite: J. Chem. Soc. 1961, 850—858.

Barrer, R. M., and Reay, J. S. S. (1957) Sorption and intercalation by
methylammonium montmorillonite: Trans. Faraday Soc. 53, 1253-1261.

Brunauer, S. (1944) in The Adsorption of gases and vapors, Oxford
University Press, pp. 150.

Boyd, S. A., Shaobai, S., Lee, J. F., and Mortland, M. M. (1988a)
Pentachlorophenol sorption by organo-clays: Clays and Clay Minerals 36,
125—130.

Boyd, S. A., Mortland, M. M., and Chiou, C. T. (1988b) Sorption
characteristics of organic compounds on hexadecyltrimethylammonium—
smectite: Soil Sci. Am. Proc. 52, 652-657.

Boyd, S. A., Lee, J. F. , and Mortland, M. M. (1988c) Attenuating organic
contaminant mobility by soil modification: Nature 333, 345-347.

Chiou, C. T., Kile, D. E., and Malcolm, R. L. (1988) Sorption of vapors
of some organic liquids on soil humic acid and its relation to
partitioning of organic compounds in soil organic matter: Environ. Sci.
Technol. 22, 298-303.

Chiou C. T., and Shoup, T. D. (1985) Soil sorption of organic vapors and
effects of humidity on sorptive mechanism and capacity: Environ. Sci.
Technol. 19, 1196-1200.

Kilpling, J. J. (1965) in Adsorption from solutions of nonelectrolvses,
Academic Press, London, pp. 328.

McAtee, J. L., Jr, and Harris, B. R. (1977) Gas chromatographic pathway
for certain chloro—alkylammonium montmorillonite: Clays and Clay Minerals
25, 90-93.

McBride, M. B., Pinnavaia, T. J., and Mortland, M. M. (1977) in Fate of
pollutants in the air and water environments, John Wiley and Sons, New
York, Part I, pp. 145.

Mortland, M. M., Shaobai, S., and Boyd, S. A. (1986) Clay-organic

complexes as adsorbents for phenol and chlorophenols: Clays and Clay
Minerals 34, 581-586.

91

92

Stul, M. S., Maes, A., and Uytterhoeven, J. B. (1978) The adsorption of
N-aliphatic alcohols from dilute aqueous solutions on RNH3—
montmorillonites, Part 1, Distribution at infinite dilution: Clays and
Clay Minerals 26, 309-317.

Stul, M. S., Uytterhoeven, J. B., and Bock, J. D. (1979) The adsorption
of N-aliphatic alcohols from dilute aqueous solutions on RNH —

montmorillonites, Part II, Interlamellar association of the adsorbatg:
Clays and Clay Minerals 27,377-386.

White, 0., and Cowan, C. T. (1958) The sorption properties of
dimethyldioctadecyl ammonium bentonite using gas chromatography: Trans.
Faraday Soc. 54, 557-561.

CHAPTER 4

SELECTIVE ADSORPTION OF AROMATIC MOLECULES BY
TMA-SMECTITES OF DIFFERENT CHARGE DENSITY

93

94

INTRODUCTION

The type of exchangeable cations on clays directly influences the
surface properties of clays, and the resulting sorptive characteristics
of clays for the uptake of nonionic organic compounds (NOCs). In nature,
the net negative electrical charges of clays are usually balanced by
inorganic exchange ions such as Na+ and Ca2+ which are strongly hydrated
in the presence of water. The hydration of these exchangeable metal ions
and the polar nature of Si-O groups of clays impart a hydrophilic nature
to the mineral surfaces. As a result, the adsorption of NOCs by clays is
suppressed in the presence of water because the relatively lower polar
organic chemicals cannot compete with strongly held water for adsorption
sites on the clay surface. In the absences of water, the clay acts as a
conventional solid adsorbent, and shows high sorptive capacity related to
total surface area (Chiou and Shoup, 1985; Call, 1957; Jurinak, 1957).

However, this limited sorptive capacity of clays toward NOCs from
aqueous solution can be remedied by replacing natural metal cations with
larger organic cations through ion exchange reactions (Barrer et al.,
1955, 1957; McAtee et al., 1977). Several researchers have indicated that
exchanging quaternary ammonium cations for the metal ions on the clay
surface could considerably modify the sorptive characteristics of clays
(White et al., 1958; Barrer et al., 1961; Wolfe et al., 1985). As organic
cations exchange for metal ions on mineral surface, these organic cations
act as "pillars" and increase the distance between the aluminosilicate
sheets. Importantly, the organic cations are not strongly hydradated and,
due to surface coverage, they also decrease the free mineral surface area.

As a result, the surface properties of the clay may change from

95
hydrophilic, when exchanged with naturally occurring inorganic ions, to
organophilic when organic exchange ions are present.

Mortland et al. (1986) and Boyd et al. (1988a) used quaternary
ammonium cations as exchange ions on smectite to enhance the uptake of
phenol and chlorophenols from water. 'These authors showed that in general
the sorptive capacity of modified clays exchanged with large quaternary
ammonium ions was greatly improved as compared to unmodified clays or
modified clays in which the exchanged organic cations were small in size.

To elucidate the role of exchanged hydrophobic organic cations on
the sorptive behavior of modified clays, Boyd et al. (1988b) studied the
sorption of NOCs on hexadecyltrimethylammonium
[(CH3)3N(CH2)15CH3]+(HDTMA)-smectite from both aqueous and vapor phases.
These authors found that in dry HDTMA-smectite, both the mineral and
organic phase are effective sorbents for the uptake of NOCs, analogous to
the behavior of mineral and organic matter in dry soil (Chiou et el.,
1985). However, in aqueous solution adsorption on mineral surfaces was
suppressed by water and the uptake of NOCs by HDTMA-smectite was primarily
by solute partitioning into the organic medium formed by the
conglomeration of large C56 alkyl groups of HDTMA ions. This partition
behavior of HDTMA-smectite, as shown in the uptake of NOCs from aqueous
solution, can be recognized in many aspects, including: (1) the highly
linear isotherms for the sorption of NOCs by HDTMA-smectite, (2) the
dependence of the sorption coefficient on the water solubility of the NOC,
(3) the general agreement between the octonal-water partition coefficient
(Km) of the NOC and the corresponding organic matter normalized partition

coefficients (Kw) of the organo-clay, and (4) the related dependence of

96
the sorption coefficient of HDTMA-smectite on the organic carbon content
of the modifed clay, i.e. , the proportion of the CEC occupied by HDTMA
ions (Boyd et al., 1988b).

Recently, Lee et al. (1988) in studies of the sorption of aromatic
compounds on water-saturated tetramethyl ammonium (TMA)-smectite found that
the sorptive characteristics of'TMA-smectite contrasted‘with the partition
behavior of HDTMA-smectite for the uptake of NOCs from water. For
instance, the sorption isotherms of benzene, toluene and o-xylene on
TMA-smectite were not linear. In addition, compounds with high water
solubilities (e.g. benzene) showed strong affinities for TMA-smectite,
whereas, lower water solubility compounds such as trichlorobenzene had
very low affinities for TMA-smectite. It was also shown that the uptake
of benzene from aqueous solution was significantly higher for'TMA-smectite
than for HDTMA-smectite, despite the fact that HDTMA-smectite had a much
higher organic carbon content than TMA-smectite.

The work by Lee et al. (1988), also demonstrated that TMA-smectite
showed shape selectivity in the sorption of aromatic compounds from‘water,
with high uptake of benzene and progressively lower uptake of larger
aromatic compounds. This selectivity could only be seen for the sorption
of aromatic compounds from aqueous solution, and not for the sorption of
the corresponding vapors by the dry clay. This unique selective property
of TMA—smectite was attributed to water molecules modifying the sorptive
behavior of TMA-smectite by shrinking the interlamellar cavities where
adsorption of the aromatic molecules occurred.

In this paper, two smectites of differing charge densities were used

for the preparation of TMA-clay complexes. One smectite, obtained from

97
Arizona, has a cation exchange capacity (CEC) of 120 meq per 100 g; the
other, from Wyoming, has a CEC of 90 meq per lOOg (Table 1). When fully
exchanged with TMA ions, both modified clays possess considerable
interlamellar porosity, however, (”1 Arizona TMA-smectite, the surface
density of TMA ions is increased because of its higher CEC. Therefore,
these two materials were used to investigate the effect of layer charge

density on the sorptive behavior of TMA-smectite.

EXPERIMENTAL

Samples for these investigations were prepared in a manner
previously described (Lee et al., 1988). The organo-clay complexes were
prepared by exchanging the TMA for Na ions on Wyoming smectite and Arizona
smectite having CEC values of 90 meq/100g and 120 meq/1009, respectively
(Table 1). The 0.2 um fractions of the original clays were obtained by
standard dispersion and sedimentation methods. After discarding the
impurities of the original clays, the clays (primarily in the Na form)
were treated with an excess of TMA chloride salt in the amount of 3-5 meq
per meq of CEC of the clays for 2-4 h. The TMA-smectites were then
dialyzed against distilled water until a negative chloride test with
silver nitrate for the dialyzate was obtained. Then the TMA-smectites
were freeze dried and stored at room temperature.

Adsorption isotherms were determined using the batch equilibration
technique. Eight initial concentrations were prepared in the range of
100 to 1500 ppm for benzene; 20 to 550 ppm for toluene; 12 to 190 ppm for
o-xylene; 12 to 150 ppm for o-dichlorobenzene (o-DCB) and ethylbenzene,
and 0.8 to 10 ppm for 1,2,3-trichlorobenzene (1,2,3-TCB). Corex glass

98

tubes (25 ml) containing 100 mg of TMA-smectite and 25 ml of the aqueous
solutions containing the organic compounds were closed with foil-lined
screw tops and shaken for 24 h at room temperature in a reciprocating
shaker. Preliminary kinetic investigations indicated that the sorption
equilibrium was reached in less than 20 h. Shaking was followed by
centrifugation at room temperature in a high speed centrifuge at 8000 rpm
for 30 min. A 1 ml portion of the supernatant was then transferred into
a glass vial containing 10 ml of carbon disulfide, closed with foil-lined
screw caps and shaken vigorously for 1 h on a reciprocating shaker. A
portion of carbon disulfide layer'was then removed for gas chromatographic
(GC) analysis.

To measure the competitive effects of benzene and toluene adsorption
on TMA-smectite, the isotherms of binary mixtures were performed as
follows: eight initial concentrations of benzene in the range of 100 to
1500 ppm, and of toluene in the range of 20 to 550 ppm, were added
simultaneously to the corex glass tubes which contain 25 ml of 0.4% w/w
clay-water suspension. The mixed solutions were then shaken and treated
as described above.

The GC was carried out on a model 5890A Hewlett Packard gas
chromatograph equipped with a flame ionization detector. Peak areas were
quantitated using a Hewlett Packard 3392A integrator. The concentration
of the organic compound(s) in the carbon disulfide extract was determined
by using a series external standards. The column used was packed with 5%
sp-1200/1.75% Bentonite 34 coated on 100-120 mesh Supelcoport. Carrier
gas was high-purity nitrogen at a flow of 40 ml/min. The oven temperature

was 65°C for benzene, 80°C for toluene, lOO°C for xylene and ethylbenzene,

 

99
and 125°C for' o-DCB. Operating temperature for injection port and
detector were 270°C and 300°C, respectively. All adsorption measurements
were carried out in duplicate. Blank determinations of organic compounds
in absence of clays were conducted and the recoveries ranged from 92 to
98%. The equilibrium concentrations measured were not adjusted for these
recoveries.

The uptake of benzene, toluene, and o-xylene vapor by dry
TMA-smectite was measured by using a static equilibrium sorption apparatus
described previously (Chiou et al., 1988).

A Philips diffractometer model XRG3lOO equipped with a copper target
x-ray tube was used for x-ray diffraction examination. Samples were
prepared by placing clay aqueous suspension onto glass microscope slides
and drying them at room temperature (Table 1). Organic carbon analyses
were performed in duplicate by Huffman Laboratories, Golden, Colorado

(Table 1).

RESULTS

Adsorption isotherms for the uptake of benzene, toluene and o-xylene
organic vapors by dry TMA-smectites are shown in Figures 1-3. The
adsorption isotherms on both Wyoming and Arizona TMA-smectites are sigmoid
curves approaching Type IV in Brunauer’s classification (Brunauer, 1944).
For both clays, the amount of organic vapor uptake by dry TMA-smectites
decreased in the order benzene > toluene > o-xylene. The magnitude of
this decrease was much greater for Arizona TMA-smectite as compared to
Wyoming TMA-smectite, where differences in the uptake of benzene, toluene

and xylene vapors were relatively small. Further inspection of Figures

100

Figure 1. Adsorption of benzene vapor by dry Arizona and Wyoming TMA-

smectites.

UPTAKE O (MG/G)

101

BENZENE VAPOR ON TMA—SMECTITE

 

188

158

128

98

 

68- -

38—- -

8 l 1 l 1 l 1 l 1 l 1 l
8 8.15 8.3 8.45 8.6 8.75 8.9

 

 

 

P /P°

102

Figure 2. Adsorption of toluene vapor by dry Arizona and Wyoming TMA—

smectites.

UPTAKE O (MG/G)

188

158

128

98

68

38

8
8 8.1E3E3fl313.45 8.6 8.7EJELEB

1m
TOLUENE VAPOR ON TMA—SMECTITE

 

0‘0

E
-<
u

a

 

 

I 1 l 1 1 L 1 1 1 1 I

 

P /P°

104

Figure 3. Adsorption of o—xylene vapor by dry Arizona and Wyoming TMA-

smectites.

UPTAKE O (MG/G)

105

O—XYLENE VAPOR ON TMA—SMECTITE

1881—111'111'11

 

158 —- —
128
98

68

 

38

 

 

l 1

g I 1 1 1 I 1 I l
8 8.15 8.3 8.45 8.6 8.75 8.9

1

 

P /P°

 

 

106

1—3 shows that the amount of benzene uptake was nearly identical for the
Wyoming and Arizona TMA-smectites, whereas the amounts of toluene vapor
sorbed by Wyoming TMA-smectite exceed the amount taken up by Arizona
TMA-smectite, especially at lower P/P’. For o—xylene, the largest of the
three compounds tested, the uptake by Wyoming TMA-smectite was
considerably higher than by Arizona TMA-smectite, and this difference
extended to relatively high P/P°. The amount of organic vapor uptake of
all three compounds tested indicated that adsorption occurred at both the
internal and external surfaces of the TMA-smectites. In addition, the
sorptive behavior of both TMA-smectites did not show threshold pressure.

Figures 4-6 show adsorption isotherms for the uptake of benzene,
toluene and o-xylene from water by Wyoming and Arizona TMA-smectites.
The uptake of each compound from water was lower than the uptake of the
corresponding vapors by the dry samples. A noticeable difference in the
degree of uptake of these organic chemicals by both Wyoming and Arizona
TMA-smectites can be seen in the isotherms. In each case, the amount of
uptake decreases in the order: benzene > toluene > xylene. However, the
uptake of benzene from water was much lower for Arizona TMA—smectite than
for Wyoming TMA-smectite. This obvious difference is in contrast with the
uptake of benzene vapor by the dry samples which was nearly identical for
both clays. The uptake of toluene and o-xylene by Arizona TMA-smectite
was very low, and differences between the sorptive uptake by Wyoming and
Arizona TMA-smectites were magnified in going from benzene to toluene to
o-xylene. As expected from the results with o-xylene (Figure 2), o-DCB
and ethylbenzene were entirely excluded from the Arizona TMA-smectite

(data not shown) ; they were, however, sorbed slightly by Wyoming

107

Figure 4. Adsorption of benzene from aqueous solution by Arizona and

Wyoming TMA-smectites.

UPTAKE O (MG/G)

108

BENZENE ON TMA-SMECTITE

 

188 I I I I I l I I I l I

98

72

54

36

 

 

 

 

8 8.2 £3A4 8.6 8.8 1 1

EQUILIBRIUM CONC. (MG/ML)

.2

109

Figure 5. Adsorption of toluene from aqueous solution by Arizona and

Wyoming TMA-smectites.

UPTAKE 0 (MG / G)

 

42

35

28

21

14

110

TOLUENE ON TMA—SNECTITE

 

 

1 I 1 I r h 1 f 1
\NY
0 AZ .—
- A x
o . .-——"1‘*"'TTF 1 I 1
E3 8.89 8.18 8.27 8.36 8.45

EQUILIBRIUM CONCrCe

 

 

 

 

 

 

(MG / ML)

 

 

111

Figure 6. Adsorption of o-xylene from aqueous solution by Arizona and

Wyoming TMA-smectites.

UPTAKE 0 (MG / G)

112

O—XYLENE ON TMA—SMECTITE
‘8 11‘1'1'1'11

 

 

 

 

1 A I l 1 l

8
8 8.8258.858.8758.18.1258.15

 

EQUILIBRIUM CONC. (MG ~/ ML)

113
TMA-smectite (Lee et al., 1988).

There is also a pronounced difference in the shape of adsorption
isotherms shown in Figure 4-6 indicating progressively weaker interactions
with TMA-smectite in the order: benzene > toluene > o-xylene. It is clear
from both the shape of the isotherms and the degree of uptake that, in the
presence of water, the aromatic molecules have weaker interactions with
Arizona TMA-smectite than with Wyoming TMA-smectite. The adsorption
isotherms of toluene and o-xylene suggest that the uptake of these two
chemicals from aqueous solution by Arizona TMA-smectite is restricted to
external surfaces. 0n the other hand, the uptake of benzene from water
appears to involve the interlamellar surfaces of the TMA-clays.

Competitive effects in the sorption of benzene-toluene mixtures from
aqueous solution by Wyoming TMA-smectite are presented in Figures 7 and
8. When the adsorption isotherms for benzene alone and in a binary solute
mixture with toluene are plotted on the same diagram, the isotherms are
nearly superimposable (Figure 7). This demonstrates that toluene, at any
concentration, is unable to compete with benzene for adsorption sites on
Wyoming TMA-smectite. However, benzene shows very effective competition
with toluene for the adsorption sites of Wyoming TMA-smectite, and in the
presence of benzene, the adsorption of toluene is greatly depressed
(Figure 8). As the initial concentration of benzene increased, the
differences in uptake of toluene from water by Wyoming TMA-smectite are

increased.

114

Figure 7. Adsorption of benzene from water in the presence (triangles)

and absence (circles) of toluene by Wyoming TMA-smectite.

UPTAKE Q (MG/G)

115

BENZENE ON TMA—SMECTITE

 

198111111r111'
—-

98 T- W’t toluene —
,.. /5°WI toluene .1

g/ 53

54- —

36 // ._
L

 

H
18 _

 

[3.11111111411
8 E312 8.4-1313 8.8 1 1

EQUILIBRIUM CONC. (MG/ML)

.2

|‘—

116

Figure 8. Adsorption of toluene from water in the presence (triangles)

and absence (circles) of benzene by Wyoming TMA-smectite.

UPTAKE O (MG/G)

42

35

28

14

8
8

117

TOLUENE ON TMA—SMECTITE

 

 

I I I I I I I

th benzene

l

‘ 1

WI benzene
A

 

8.85 8.1 8.15 8.2 8.25 8.3

EQUILIBRIUM CONC.

(MG/ML)

 

118
DISCUSSION

The relation between the sorptive behavior of TMA—smectites and the
density of exchanged TMA ions was studied using two smectites of different
charge densities. The closer packing of TMA ions on the higher charge
density Arizona-smectite was demonstrated by an organic carbon content of
5 percent as compared to 4 percent in the lower charge density
Wyoming-smectite (Table 1). The difference in organic carbon contents of
20 percent was in agreement with the approximate differences in cation
exchange capacities of 25 percent for the two smectites.

Consideration of the surface areas of TMA ions, and the free space
between exchanged TMA ions on Wyoming smectite, led Barrer et al (1961)
to conclude that it was necessary for benzene to adopt a tilted
orientation when adsorbed in the interlamellar regions of TMA-smectite.
Thus, benzene and presumably other aromatic molecules appear to be
oriented in a face—to-face arrangement with the planar faces of the
TMA-tetrahedra. Size restrictions in the interlamellar regions were
previously shown to result in high uptake of benzene and progressively
lower uptake of larger (more substituted) aromatic molecules from water
(Lee et al., 1988). This type of shape—selective adsorptive behavior of
TMA-smectite is much different than the sorptive behavior of smectite
exchanged with quaternary ammonium ions containing large hydrocarbon
moieties, such as hexadecyltrimethylammonium [(CH3)3N(CH2)15CH3]+ (HDTMA).
When absorbed on the surface of smectite, the hydrocarbon tails of such
large organic cations (e.g. HDTMA) appear to form an organic phase that
acts as a partition medium for the uptake of nonionic organic compounds

(NOCs) (Boyd et al., 1988). This is a process of solubilization of NOCs

119

 

Table 1. Properties of Wyoming and Arizona TMA-smectites.
Property Wyominga Arizonab
CEC (meq/lOOg) 90 120
Organic carbon (g/lOOg)c 4.0 5.0
d(001) (nm) 1.38 1.38

(nm)=d(001)—0.95 0.43 0.43
Formulae

Wyoming (A1153F90A6M9033)($1335A10p5101010H12
Arizona (A1L4F90J7Mgox3)($1188A10J2101010H12

 

3Supplied by American Colloid Co.
bRegerence Clay sample SAz-l from C.M.S. repository.

COrganic carbon derived from TMA ions.

 

120
in the HDTMA-derived organic phase, and no shape selectivity is observed.
Rather the degree of uptake is related exclusively to the water solubility
of the NOC. The partitioning of NOCs also manifests highly linear
isotherms as compared to the generally nonlinear isotherms observed for
TMA-smectite.

The closer packing of TMA ions in the Arizona-smectite as compared
to Wyoming-smectite resulted in a higher degree of shape selectivity in
the adsorption of aromatic compounds. For the sorption of organic vapors,
both dry TMA—smectites showed similar affinities for the sorption of
benzene. However, as the size of sorbates increased in going from benzene
to toluene to o-xylene, the uptake of organic vapors by Arizona
TMA—smectite decreased; whereas, the amount of vapor uptake by Wyoming
TMA—smectite remained relatively constant. Thus, Arizona TMA-smectite
showed an obvious shape selectivity in the adsorption of aromatic vapors
in the absence of water; whereas, dry Wyoming TMA smectite showed little
selectivity among benzene, toluene, and xylene. As the values of
TMA-smectites are 4.3A (Table 1), both TMA-smectites have essentially
identical interlayer distances to accommodate the benzene-like compounds.
Therefore, the selectivity of Arizona TMA-smectite can be attributed to
the effects of charge density on the clay surfaces.

The closer packing of TMA ions in the Arizona smectite apparently
resulted in a significant change in the size distribution of cavities in
the interlamellae. Arizona TMA-smectite appears to contain fewer sites
(between TMA ions) of sufficient size to accommodate the larger aromatic
molecules. Thus overall, the selectivity of Arizona TMA—smectite was

greater than the more loosley packed Wyoming TMA—smectite. In the former

 

121

case, shape selectivity was observed for the adsorption of organic vapors
by 'the dry sample, and to a greater extent for the sorption of the
aromatic compounds from water. However, for Wyoming TMA—smectite shape
selectivity was only prominent in the aqueous system. Thus, in addition
to size distribution of cavities resulting from the packing of TMA ions,
water played a direct role in determining the shape selective adsorption
of aromatic compounds.

The different affinity of both TMA—smectites for the sorption of
aromatic compounds from water can be seen in Figures 4-6. In the aqueous
system, aromatic compounds showed a clear preference for Wyoming
TMA—smectite. This is demonstrated by both the amount of uptake and the
shapes of isotherms. All aromatic compounds tested were sorbed by Wyoming
TMA-smectite, however, toluene and o—xylene were nearly completely
excluded from Arizona TMA-smectite. Since the vertical free distance
between lamellae is 4.3A in both TMA-smectites, it must be the lateral
free distance between TMA ions which causes the difference. The higher
charge density of Arizona smectite means a greater population of TMA ions
fixed per unit area, with resulting lower amounts of free surface
available for adsorption and smaller cavities on the interlamellae.
Sieving effects of the TMA-smectite were either smaller or not
demonstrated for the uptake of organic vapors because the internal
surfaces and cavities of dry TMA-smectite were more available. However,
as the adsorption is taking place in an aqueous system, the water
molecules will associate with the free mineral surfaces and to some extent
with the exchanged TMA ions. Not only does the average lateral distances

between adjacent interlamellar cations decrease as the amount of TMA

 

 

122

exchanged increases, but the lateral distance will further shrink due to
hydration. Therefore, the free distances between nearest neighbor pairs
of organic cations are not large enough in the Arizona TMA-smectite for
significant uptake substituted aromatic molecules to occur. It is thus
suggested that these aromatic compounds occupy only the largest few
interlamellar gaps of Arizona TMA-smectite. In this event the sorptive
capacity of the Arizona TMA-smectite in aqueous system will be much
reduced as compared with that of Wyoming TMA-smectite. As expected from
the results of o-xylene, o-DCB and ethylbenzene were also excluded from
Arizona TMA-smectite, however, they were intercalated by Wyoming
TMA-smectite.

The proposed adsorptive mechanism for the uptake of organic
compounds from water should manifest competitive effects in multisolute
systems. This was examined by studying adsorption of sorbate mixtures of
benzene and toluene over a range of concentrations of each solute. The
adsorption isotherms of the pure single sorbates show that more benzene
than toluene is taken up by both TMA-smectites from water (Figure 3).
Thus, in the binary solute system benzene should compete effectively with
toluene for adsorption sites. This effect was observed as the uptake of
toluene was greatly depressed by the presence of benzene. However, the
uptake of benzene was not affected by toluene because the affinity of
benzene for the water-saturated TMA-smectite is greater than that of
toluene. These results suggest that the adsorption of toluene in the
presence of benzene is restricted to the external surfaces with benzene
preferentially occupying the intralamellar regions of Wyoming

TMA.smectite. As a result, the adsorption isotherm of toluene in the

123
presence of benzene by Wyoming TMA-smectite (Fig. 8) is similar to the
adsorption isotherm of toluene by Arizona TMA-smectite (Fig. 5) where
adsorption is also limited to external surfaces.

The work presented here, in conjunction with earlier studies, has
demonstrated the potential versatility of modified clay minerals for the
separation of organic compounds in aqueous systems. By ion—exchange
reactions of TMA for inorganic exchange ions on smectite, molecular sieves
of widely differing behavior may be prepared which demonstrate
shape-selective adsorption of aromatic compounds. The degree of
shape—selectivity can be increased by using smectites of higher charge
density. In these experiments, the main factor affecting the degree of
sorption of aromatic compounds from water by TMA-smectite was found to be
the lateral distances between the interlamellar cations on water-saturated

TMA-smectite.

 

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