United States
Environmental Protection
Agency
Robert S. Kerr Environmental
Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S2-91/033 Aug. 1991
&EPA Project Summary
The Swelling Properties of Soil
Organic Matter and Their
Relation to Sorption of Non-Ionic
Organic Compounds
William G. Lyon and David E. Rhodes
A method has been developed to
measure the swelling properties of con-
centrated natural organic materials In
various organic liquids, and has been
applied to various peat, pollen, chttln
and cellulose samples. The swelling of
these macromolecular materials Is the
volumetric manifestation of bulk sorp-
tlon, I.e., sorptlon by dissolution (or
partitioning) of the sorbsd liquids Into
the macromolecular solid phase. Di-
rect evidence for the existence of this
category of sorbed materials has been
obtained for soil organic materials by
the present research; swelling In liq-
uids has long been known In coals and
polymers.
Bulk sorbed molecules are thought
to be Inaccessible to direct biological
attack, and may represent a continuing
source of low-level, "rebound" contami-
nation of groundwater at a polluted site
following attempted pump-and-treat
remediation. Equilibration of bulk
sorbed molecules with liquid phases
surrounding the particles Is klnetically
slow (diffusion limited) relative to sorp-
tlon and fluid movement, and this slug-
gishness Is probably responsible for
some nonequlllbrlum sorptlon phenom-
ena seen In soil column flow experi-
ments.
Molecules with molar volumes greater
than about 93 cm* mor1 appear to be
strongly excluded from sorptlon inside
the soil organic materials studied In
this work. In contrast, cellulose ex-
cluded molecules with molar volumes
greater than 88 cm1 mor1.
Besides the size exclusion factor, the
degree of swelling of soil organic ma-
terials In different liquids Is controlled
mainly by site-specific, generalized
acid-base Interactions between the
sorbed molecules and the various
acidic sites within soil organic materi-
als. The swelling spectra observed for
soil materials are complex, and com-
pletely unlike the simple Gaussian
swelling spectra obssrved for polymers
like rubber (cross-linked polylsoprene)
and for some coals. In these latter
materials the Intermolecular forces are
dominated by non-specific dispersion
forces (van der Waals Interactions), and
can be adequately treated by simple
equations (Flory-Hugglns-Rehner
theory) Involving the solubility param-
eters of the liquid and the swelling sub-
strate.
Swelling In morphollne appears to
be a characteristic of soil organic ma-
terials containing free cellulose. Un-
fortunately, the cellulose within natural
llgno-celluloslc plant debris apparently
behaves differently from free cellulose,
so that swelling alone does not pro-
vide a simple measurs of humlflcatlon
in soils or peats. We speculate that
the Intimate association of llgnln with
the cellulose at the molecular level
blocks access to the specific sites (al-
cohollc-OH groups) on the cellulose
with which morphollne Interacts most
strongly. Free cellulose does, how-
ever, appear to be present In pollen
Intlne membranes.
This Project Summary was developed
by EPA's Robert 5. Kerr Environmental
Printed on Recycled Paper
-------
Research Laboratory, Ada, OK, to an-
nounce key findings of the research
project that Is fully documented In a
separate report of the same title (see
Project Report ordering Information at
back).
Introduction
The Swelling Phenomenon
Macromolecular substances swell when
placed in contact with various fluids1; the
amount of swelling depends both on the
nature of the fluid, and of the macromo-
lecular material. Strongly cross-linked ma-
terials swell less than weakly cross-
linked materials. Such swelling is a prop-
erty of the insoluble macromolecular net-
work itself, and it occurs even when all
associated soluble materials have been
removed by exhaustive solvent extraction.
Swelling is the dissolution of small mol-
ecules of more mobile substances into
the solid. The macromolecules act as a
solvent for these smaller molecules, and
the swollen phase represents an unusual
kind of solution. An alternate point of
view is that swelling represents the solva-
tion of internal macromolecular "surfaces"
by the smaller fluid molecules.
The Connection Between
Sorption and Swelling
Swelling represents a volumetric mani-
festation of certain kinds of sorption
involving bulk sorption (partitioning) into
soil organic matter. The various catego-
ries of sorption in soil organic matter are
detailed in the diagram below.
Bulk Sportion and Swelling
Swelling experiments measure the
maximum capacity for bulk sorption of
certain solvents that solid organic materi-
als can hold while in contact with the pure
solvent. This limiting capacity can be
expressed if a variety of units, such as
cm3 sorbed solvent per cm3 sorbent.
Sorbed material in this form represents a
potential source of residual contamination
that is difficult to remove completely by
any known process (e.g., solvent extrac-
tion, vacuum extraction, biodegradation,
etc.) This residual contamination is con-
sidered to be of greatest importance for
the smaller, more polar solvent molecules
such as methanol. Some solid macro-
molecuialr soil organic materials (e.g., cel-
lulose and chitin) can themselves be bio-
Sorption Categoric In Natural Soil Organic Matter
1 Here we restrict the discussion to fluids with a single
molecular constituent. When two constituents are
present, a richer, more complex set of phenomena
can occur, such as gel-collapse, phase transitions,
where a swollen gel can suddenly decrease in volume
when exposed to a fluid of different composition.
Swelling &
Bulk Sorption
(partitioning)
Organic Matter
Surface Sorption
(adsorption
Insoluble Solid Phases *
(Humin, Cellulose, Chitin)
Extractable Phases"
(Waxes, Resins, LJpids, etc)
Polar ° & Hydrogen Bonding
Cation Exchange"
Non-Polar (hydrophobic)
Capillary Condensation1
(micro pores)
Dissolution in these materials is accompanied by size exclusion. Also, within the solid macromolecular structures,
different types of organic molecules will preferentially solvate various sites: polar, non-polar, hydrogen bonding,
cation exchange, etc.
Dissolution of various sorbed molecules occurs without significant size exclusion. Naturally, too many "sorbed*
molecules in the form of excess solvent will extract and mobilize these materials. Here we emphasize the role of
native waxes, resins, etc., rather than that of anthropogenic residual separate-phase liquid contaminants which
can act similarly.
Polar is used here only to denote a dass of molecules. Various authors (for example, Fowkes, 1960) have shown
that dipole-dipole interaction between polar molecules in a liquid (in contrast to the vapor phase) represent only a
very tiny portion of the intermolecular interactions compared to donor-acceptor interactions.
Cation exchange sorption phenomena would include replacement of ionizable, acidic hydrogens by cationic organic
species, but also polarization of some ligands by multi-charged, exchanged cations.
Some size exclusion effects probably operate here also.
degraded, and would presumably release
materials sorbed in bulk as the macromo-
lecular matrix is destroyed.
The extractable organic soil phases are
also capable of bulk sorption (dissolution)
of various hydrophobic chemicals. Unlike
the insoluble solid phases, however, these
materials are potentially capable of mobili-
zation by the right mixtures of nonaqueous
solvents. The sorbed species are prob-
ably also somewhat more accessible to
reoediation measures, especially if they
are sorbed into biodegradable lipid or wax
fractions.
Surface Sorption
True surface sorption can be defined
meaningfully for insoluble soil organic mat-
ter for sufficiently large organic mol-
ecules that are effectively excluded
from the macromolecular framework of
humin and other macromolecules. This
restriction to surface interaction is prob-
ably very important for many agrochemi-
cals such as insecticides, fungicides and
herbicides, which tend to be rather large
molecules2. Note, however, that the lower
molecular weight, microbial metabolites of
these chemicals may distribute themselves
over the available sorption categories in a
completely different way than the parent
chemical.
Sorption in Micropores
The final category of sorbed material is
that which resides in the micropores of
the various solid organic materials without
causing any volumetric increase. No
doubt, some size exclusion effects oper-
Large in this context means somewhat larger than the
aromatic ring of benzene.
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ate in this situation as well, but probably
not as stringently as in the bulk sorption
into macromolecular organic matter. The
difference between certain gravimetric de-
terminations of sorption and volumetric
determinations of sorption (via swelling,
see below) can indicate the magnitude of
the sorption capacity in this form. In many
ways, this is the least understood of the
categories in the diagram. Diffusion in
and out of micropores also could produce
some of the non-equilibrium effects seen
in column studies.
Operational Definition of
Volumetric Swelling
The volumetric swelling3, Q , is defined
as the ratio of the swollen volume to the
unswpllen volume of the sprbent; a value
of unity4 indicates no swelling. The recip-
rocal of the volumgtric swelling is equal to
the volume fraction of macromolecular
material in the swollen, solution phase.
Thus, the volumetric swelling at equilib-
rium is related to the saturated solubility
of the sorbed fluid in the swollen phase.
Knowledge of appropriate densities, al-
lows this volume-fraction solubility to be
converted to the more usual gravimetric
concentration units.
The volumetric swelling5 is the quantity
determined experimentally in the present
work. A swelling spectrum is obtained
when the volumetric swelling for various
solvents is plotted versus some pertinent
solvent property, such as the convention-
ally chosen solubility parameter.
Experimental Procedures
Samples Studied
Michigan Peat
The sample of Michigan peat was com-
mercial horticultural peat from the 'Alpar
' The volumetric swelling, O, should not be confuted
with the volume change, AV^14. aModated with the
solvent + solid sorbent -» swollen sorbent
The values for CL are positive values 21.0 except in
the extremely unlikely situation where the AV u^ll is
negative and exceeds the imbibed volume of solvent.
Many values of A V,_,_ are potentially Qtflatbft, and
they are usually noTknow because of the difficulty of
determination. Usually AV, _ values are some
small percentage of the volume of imbibed liquid.
4 Apparent swelling values less than unity could occur
if some material is extracted into solution by the
applied solvent
• We note that selling can also be determined gutf-
mairieaUy in these cases, the amount of sorbed
material is determined gravimetricaNy, and converted
to a volume basis using densities. Usually the gravi
metrically determined swelling is larger than the volu-
metric quantity because it includes the filling of
micropores without any associated swelling.
Peat Company of Ovid, Michigan (Clinto-
County). The dark, muck peat is a pre-
dominately reed-sedge peat with some
contributions from tamarack trees. The
mined deposit is a layer approximately 8
to 10 feet in thickness in a region once
used for farming.
Canadian Peat
The Canadian peat was a commercial
horticultural peat with a light brown color
and a distinctly fibric texture. Information
on the origin of this material was unavail-
able. Both acid-washed and calcium-ex-
changed versions of this peat were stud-
ied.
Atoka Pine Duff
The Atoka material consisted of
cmposted pine needles (Pinus echinata)
that had collected in small pockets on a
rocky slope located in Atoka County, Okla-
homa (S20, T1S, R13E). The mineral
constituents were in the form of fine, wind-
blown, clay-sized dust.
Pine Pollen (Pinus echinata) &
Oak Pollen (Quercus stellata)
The samples of pine pollen and oak
pollen were btained from a commercial
laboratory that supplies various species of
pollen and mold spores to allergists.
Cellulose
The sample of cellulose was obtained
from Aid rich Chemical Company, Inc. and
was a powder of nominally 20u,m average
particle diameter. Data from the supplier
showed an average assay of about 90%
a-cellulose based on acid hydrolysis and
an average residue on ignition ("ash") of
about 0.05%.
Chitin (Crab Shell Chitin)
The sample of purified crab shell chitin
was obtained from Sigma Chemical Com-
pany.
Solvents Used
The solvents used for swelling mea-
surements are listed in Table I along with
values for their solvent parameter and
molar zolume. All solvents were the best
available commercial grades, and were
used without further purification.
Summary of Swelling
Measurement Method
The basic method has been described
by Green et a/., 1984 for coal samples.
* Mention of trademarks or commercial products does
not constitute endorsement or recommendation for
use by the U.S. Environmental Protection Agency.
The adaption of this method to the present
kind of sample is detailed in the full re-
port. Spectra were taken in pairs and
averaged to obtain the final spectra repro-
duced in this project summary.
Powdered organic matter was placed in
small glass tubes and exposed to various
liquid organic solvents. Centrifuging at a
constant speed for a fixed time period
was used to obtain reproducible compac-
tion of the powders. Length measure-
ments were made on the powder columns
before and after wetting with each sol-
vent, and the final swelling was computed
as a ratio of lengths once equilibrium had
been achieved. Samples were equilibrated
at 30°C in a waterbath prior to swelling
length measurements to avoid known ki-
netic difficulties near room temperature,
especially in the swelling of cellulose.
Preparation and Processing of
Samples
Air-Drying, Diminution of Particles
and Sieving
The swelling experiment requires
samples of finely powdered organic mate-
rials, relatively concentrated in their swell-
ing components. Fibrous materials such
as the Canadian peat cannot be readily
"ground" to a fine powder, but can be
chopped to a sufficiently small size with
the blades in a blender so that a reason-
able harvest of -100 mesh (i.e. less than
100 mesh) material can be obtained by
sieving.
Soxhlet Extractions
Soxhlet extractions were conducted us-
ing a three-stage sequence of solvents
consisting of 1-propanol, 1-propanol-tolu-
ene (28% propanol) mixture, and toluene.
The purpose of this pre-swelling extrac-
tion procedure was two-fold:
1. to remove as much of the extract-
able fraction as possible so that the
various swelling solvents applied to
these organic materials would not
dissolve any further materials frol the
samples, and
2. to remove hydrophobia, low-melting
waxes from the particle surfaces so
that the samples could be dried at
105°C without blocking access to the
particle interiors for some of the more
hydrophilic swelling solvents.
Unfortunately, some highly swelling sol-
vents such as DMSO and the various
amides dissolved considerable amounts
of humic materials from some samples
during the swelling determinations. In sev-
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Table I. Solvents for Obtaining Swelling Spectra
Solvent 6
Molar Volume (cm'/mol)*
n-Pentane
n- Heptane
Methylcyclohexane
Cydohexane
p-Xytene
Toluene
Ethyl Acetate
Benzene
Tetralin
Acetylacetone6
Chlorobenzene
Dichloromethane
Acetone
Carbon DisulfkJe
1 ,4-Dioxane
Nitrobenzene
3- Methyl- 1-butanol
1-Octanol
Pyridine
Morpholine
N , N- Oimethy lacetamicte
1 -Pentanol
Nitroe thane
1-Butand
2-Propanol
Acetonitrile
1-Propanol
Dimethyl sulfoxide
N,N-Dimethylformamkte
Nitromethane
Ethanol (99.9%)
Propytene Carbonate"
Methanol
1 ,2-Ethanediol
1 ,2-Propanediol
N-Methylformamide
Form amide
Water
14.3
15.1
16.0
16.8
18.0
18.2
18.6
18.8
19.4
19.5
19.6
19.8
20.2
20.4
20.5
20.5
20.5
21.1
21.9
22.1
22.1
22.3
22.7
23.3
23.5
24.1
24.3
24.5
24.8
26.0
26.0
27.2
29.6
29.9
30.7
32.9
39.3
47.9
116.3
147.6
127.6
108.1
123.3
106.8
98.5
89.4
137.1
103.4
102.6
64.5
74.0
60.6
85.7
103.4
108.9
156.4
80.9
87.5
93.0
108.8
71.9
91.9
77.0
53.0
75.2
70.9
77.0
53.7
58.7
85.9
40.7
55.9
73.7
58.4
39.7
18.1
• The delta values tabulated here are mostly the simple solubility parameters of the liquids at 25°C tabulated by Barton, 1983; in a few instances where these were missing, the
total solubility parameter, 8 ,, from the same reference was used instead.
" The values for the molar volume (cm'/mol) were in most cases computed from molecular weights (g/mol) and density (g/cm*) values for the liquids at 25°C tabulated by Barton,
1983. A few missing values were computed from similar data tabulated in standard handbooks elsewhere (e.g., Weast, 1984)
c 2,4-Pentanedione
"1,2-Propanediol cyclic carbonate
eral instances these solvents gave super-
natant solutions over the swollen solid
material that were ais dark as coffee.
Acid Washing and Cation
Exchange
Acid washing with 0.1 M HCI solu-
tions was performed when i was desired
to have the acid torn of the organic mate-
rials for study. Washing with 0.5 M CaCI2
solution was performed when the calcium-
exchanged version of the organic materi-
als was needed; this was foil wed by water
washing to a chloride-free condition as
determined by tests with aqueous AgNO3
solution.
Vacuum Oven Drying
The drying treatment for all organic ma-
terials consisted of vacuum oven drying at
105°C for 24 hours at a few Torr pres-
sure: once just before loading the pow-
ders in the swelling tubes, and a second
time after loading in the tubes. After the
second drying, the filjed tubes ere quickly
capped with teflon caps to prevent uptake
of moisture from the air.
Results
Swelling Spectra
The choice of solvent solubility param-
eter as the abscissa is traditional in plots
of swelling, but solubility parameter, by
itself, has no special predictive power for
the swelling in these materials. What is
seen instead, are spectra with large jumps
in swelling for very small changes in sol-
vent solubility parameter. These jumps in
swelling are largely attributable to site-
specific chemical interactions of the do-
nor-acceptor type that lower the net free
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DELTA, SOLVENT SOLUBILITY PARAMETER
Figure 2. Swelling vs. 8o, pine and oak pollen. The swelling spectra of pine pollen and oak pollen are compared in this plot. No ash corrections were applied
to these data.
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DELTA, SOLVENT SOLUBILITY PARAMETER
Rgure 3. Swelling vs. 6C, oeNulose and chitin. This figure compares the swelling spectra for cellulose and chitin. The raw spectrum for cellulose was
scaled by a constant factor so that the average background value of the swelling was shifted upwards to 1.0. No corrections for ash were
applied to either spectrum.
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energy of the swollen state for some swell-
ing agents.
Representative swelling spectra for the
major classes of materials studied in the
present work (peat-like materials, pollens,
and polysaccharides) are given in Figures
1, 2, and 3. Ash corrections for the peat-
like materials were made under the as-
sumption that the mineral matter was en-
tirely an inert, non-swelling diluent. The
spectrum for cellulose was scaled by a
constant factor so that the average back-
ground value of the swelling was shifted
upwards to 1.0.
Molecular Size-Exclusion
Effects
The evidence for molecular size-exclu-
sion is simple and direct. Figure 4 dis-
plays a graph of swelling values plotted
against solvent molar volume for all mate-
rials except cellulose. The largest solvent
still capable of significantly swelling these
materials is N,N-dimethylacetamide with a
molar volume of ca. 93 cm3 mor1. Many
solvents with molar volumes less than this
can swell these materials. For cellulose,
the pattern is similar except that
morpholine with a molar volume of ca. 87
cm3 mor' represents the largest swelling
solvent observed.
Consequences for
Environmental Studies
It would be desirable in environmental
studies of the fate and transport of or-
ganic contaminants in the subsurface to
delineate all possible categories of sorp-
tion onto soil and aquifer materials in terms
of capacity, equilibrium, energetics and
kinetics. This is far from being accom-
plished even for the simplest of typical
real systems.
The major application of the present
work to real environmental questions cen-
ters on the direct demonstration of an
additional category of sorbed substances
that can occur in soil and aquifer systems.
Some nonequilibrium partitioning effects
seen in soil column experiments may well
involve a diffusion-limited step between
swollen organic particles and the external
fluid phase. We wonder, also about labo-
ratory bio-degradation studies using small
molecules such as methanol as a carbon
source. Here we would expect methanol
to swell any soil organic matter that was
present, and thus, add a slow diffusion
step to the overall kinetics of the degrada-
tion process for methanol. In dilute sys-
tems, this would be complicated by com-
petitive sorption phenomena involving wa-
ter a|so.
The bulk-sorbed fraction inside macro-
rnolecular organic materials probably can
serve as a source of hard-to-remove re-
sidual contamination in a pump and treat
remediation; however, not all organic mol-
ecules can participate in bulk sorption into
the solid organic matter. Size exclusion
seems to limit the category to fairly small,
polar molecules such as some alcohols
and amides. If these species were present
in a contaminating mixture, they also might
serve as co-sorbents for other small mol-
ecules with less polar character, such as
the smaller chlorinated hydrocarbons. We
do not expect most of the alkanes and
aromatic species present in fuels to
undergo significant bulksorption Into
rnacromolecular organic materials in
soils and aquifers.
The present work with its emphasis on
rnacromolecular materials does not, how-
ever, rule out the possible bulk sorption of
hydrophobia organic molecules into the
wax-resin-lipid fraction of soils, nor does it
rule out significant sorption into micropores
(i.e., capiilary condensation). These sorp-
tion categories, extractable materials (bi-
tumens) and microporosity, represent po-
tentially important separate areas for fur-
ther study, and may be responsible for
much supposed "partitioning" of non-ionic
organic contaminants.
Suggestion for Future Work
Since volume is only a partial constraint
on molecular shape, it is to be expected
that the volume boundary for swelling is a
somewhat blurry barrier; some long rod-
like molecules might retain some swelling
ability though more globular molecules with
similar donor-acceptor capabilities might
not. The magnitude of molecular exclu-
sion has been partially delimited by the
present work, but these limits really need
to be challenged with further swelling stud-
ies using carefully chosen homologous
series of compounds (e.g., substituted pyri-
dines or surfoxide) with similar donor-ac-
ceptor properties.
The present method of obtaining swell-
ing measurements is fairly labor intensive,
and, unfortunately, rather imprecise. Re-
cently, various instrumental methods have
become available for studying the par-
ticle-size distributions of powdered materi-
als dispersed in liquids. It would be very
worthwhile to explore the use of these
instruments for obtaining swelling data on
a given material from particle size distri-
butions taken in different solvents. This
should be relatively simple to do for pow-
ders consisting of a single substance like
cellulose or chrtin. The swelling of a het-
erogeneous mixture of insoluble organic
materials might still be successfully ana-
lyzed by such methods, provided:
1. a series of solvents were used that
affected the components differently,
and
2. the size distribution functions for the
different components were of rela-
tively simple analytical types, such
as tognormal or Gaussian.
References
Barton, A.F.M., 1983, CRC Handbook of
Solubility Parameters and Other Cohe-
sion Parameters, CRC Press, Inc., Boca
Raton, FL.
Fowkes, F.M., 1980, "Donor-Acceptor In-
teractions at Interfaces", Polymer Sci-
ence and Technology, v. 12A, p. 43-52,
Plenum Press, NY.
Green, T.K., Kovac, J., and Larsen, J.W.,
1984, A Rapid and Convenient Method
for Measuring the Swelling of Coals by
Solvents, Fuel v. 63, p. 935-938.
Weast, R.C., 1984, CRC Handbook of
Chemistry and Physics, CRC Press, Inc.,
Boca Raton, FL, p. F8.
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VBAR, SOLVENT MOLAR VOLUME
Flgun 4. Swelling vs. solvent molar volume. S»'elling for Michigan and Canadian peats, Ca-exchanged Canadian Peat, Atoka oak pollen and chitin are
plotted versus solvent molar volume. The data exhibit an abrupt drop-off for solvents with volumes larger than N,N-dimethylacetamide (ca. 93
cm* mot1). Above tins rather fuzzy boundary, swellings tend to be near 1.0 plus some background imprecision.
•tnj.S. GOVERNMENT PRINTING OFFICE: 1992 - 648-080/40221
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William G. Lyon and David E. Rhodes are with ManTech Environmental Technology,
Inc., Ada. OK 74820.
Roger Crosby is the EPA Project Officer (see below).
The complete report, entitled "The Swelling Properties of Soil Organic Matter and thier
Relation to Sorptton of Non-tonic Organic Compounds," (Order No. PB91-217406/
AS; Cost: $23.00; subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Robert S. Kerr Environmental Research Laboratory
U.S. Environmental Protection Agency
Ada, OK 74820
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/600/S2-91/033
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