United States
Environmental Protection
Agency
Robert S. Kerr
Environmental Research Laboratory
Ada, OK 74820
Research and Development
EPA/600/S2-90/020 Sept. 1990
&EPA Project Summary
Transport of Macromolecules
and Humate Colloids Through a
Sand and a Clay Amended Sand
Laboratory Column
Candida Cook West
Laboratory experiments were con-
ducted to determine if macromolecules or
humate colloids would transport through
sand columns and if they would exhibit
any variations in their relative velocity
based upon their molecular volumes and
the pore size distribution of the column
packing. Poly(ethylene oxide) standards
ranging In molecular weights from 50,400
to 900,000 were Investigated. Humate
colloids were prepared from a humate
muck as their calcium and sodium salts.
Columns were packed with fine-grained
Oil Creek sand (OCS) of uniform pore size
(« 20yum in diameter) and the same sand
amended with 4% each kaolinite and illtte
clays (amended OCS) resulting in a pore
size distribution in which ~ 10% of the
pores had diameters less than 2/im. The
poly(ethylene oxides) and calcium and
sodium humate colloids were transported
virtually conservatively through the OCS,
with no evidence of size exclusion
phenomena. Calcium humate was
retarded In the amended sand due to
complexation with the clay fraction but
moved through the column with 77%
recovery of the humate mass. The
mobilization of clays was observed as a
temporary increase in column effluent
turbidity and a significant shift in the par-
ticle size distribution of the effluent (150
to 450 nm).
The data presented here neither sup-
ports nor refutes the application of
chromatographic principles to particle
transport in soil columns. The relative
velocities of macromolecules and col-
loids investigated were not described
using a simplistic model discussed in this
study. Subsequent studies may need to
Include dlffusional non-equilibrium con-
ditions and solid surface to particle inter-
actions.
Tfi/s Prq/ecf Summary was developed
by ERA'S Robert S. Kerr Research
Laboratory, Ada, OK, to announce key
findings of the research program that Is
fully documented In a separate report of
the same title (see Project Report order-
Ing Information at back).
Introduction
Evaluating contaminant fate in any en-
vironment necessitates determining the
compartments into which the contaminant
will distribute. In saturated subsurface en-
vironments the partitioning of a contaminant
between the mobile aqueous phase and the
immobile aquifer solid matrix is often the
controlling distribution. The association of a
contaminant with immobile solids results in
a reduction in the dissolved mobile con-
centration of the contaminant, sometimes by
orders of magnitude. However, it has been
suggested that mobile entities such as
suspended organic and inorganic colloids
and macromolecules may increase the "ap-
parent" solubility of some contaminants. If
this is the case, the predictions of con-
taminant transport based upon a two-phase
system may seriously underestimate ob-
served aqueous phase concentrations of
contaminant in laboratory and field studies.
The influence of these entities would need to
be addressed by modeling systems as three
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phases consisting of immobile sorbed, dis-
solved and mobile sorbed phases.
There is a good deal of research activity in
the areas of colloid origination, chemistry,
stability and mobility. Recently, colloidal en-
tities such as macromolecules and viruses
have been observed to be capable of eluting
prior to conservative solutes in column and
field studies. The intent of this study was to
observe the transport of colloid-sized en-
tities and examine the validity of two
proposed mechanisms by which this
phenomenon may occur in the subsurface.
For the most part, discussions of colloid
and macromolecule transport have been
confined to observations influenced by
physical and chemical interactions that
would retard their transport, relative to con-
servative species. As mentioned pre-
viously, there may be mechanisms by which
macromolecules and colloids can elute prior
to conservative species as has been ob-
served in laboratory and field studies. Two
mechanisms which have been proposed to
account for this behavior are discussed.
The first mechanism is that of particle ex-
clusion from pores. Concepts from steric
(size) exclusion chromatography have been
examined with respect to applicability to
classic dispersion/advection theory. A par-
ticle may be excluded from dead end pores
and pores smaller than some critical
diameter based upon the ratio of the particle
diameter to pore diameter and flowrates
(dynamic porosity). The exclusion from
these pores is a function both of molecular
volume and fluid velocity, that is, diffusional
equilibrium into a pore is a function of the
velocity of the fluid as it moves past the pore.
This can result in an "effective" porosity for
that particle (0£) that may be both specific
for the particle (i.e species) involved and
less than the total porosity (0t). A solute
diffusing through less than the total porosity
would elute from a soil column prior to a
smaller chemical specie like water.
The second mechanism by which the elu-
tion velocity of a particle may be a function
of its size can be described by analogy to
hydrodynamic velocity gradients in capillary
flow. Hydrodynamic chromatography is an
analytical technique used to separate par-
ticles based on their size and rate of
transport through a bed packed with solid,
non-porous particles. According to this
theory, the rate of transport of conservative
colloidal particles depends on the size of the
colloid and the interparticle volume of the
column packing. Larger particles move
faster than the smaller ones, which in turn
move faster than the molecules of the elution
fluid. The rate of transport through a column
bed is expressed by the quantity Rf, the
average moving rate of a particle through
the bed relative to the average flow rate of
the mobile phase (VP|W/VW) . Rf is analogous
to the relative velocity of a contaminant in
groundwater (i.e. velocity of the con-
taminant/velocity of the water).
Hydrodynamic chromatography flow was
first verified based on a model derived for
rigid and flexible polymer molecules flowing
in Poiseuille flow through a capillary tube.
There exists across the flow channel a
velocity gradient such that the velocity of the
fluid is maximum at the center and
diminishes toward the wall. The mean
velocity of a particle is a function of the
velocity gradient to which it is subjected,
based upon the particle radius. A small
molecule, such as water, can access the
entire gradient; and hence, its average inter-
stitial velocity is 1/2 vmax. A large particle will
be subject to higher velocities than a relative-
ly smaller particle. The velocity at a point r,
v(r), from the center of the capillary in the
radius of the tube is given by the equation:
velocity of an excluded particle (vp>e) can be
approximated:
v(r) =
where
[1-(r/r0)2]
Ap = pressure drop across the capillary
r0 = radius of the cylinder
rj = dynamic viscosity of the fluid
/ =total length of the cylinder
and the maximum velocity at r = 0 is
Since a particle of radius rp will be subject
only to the portion of the velocity gradient
£ rp from the wall, the average velocity of the
particle (VP|W) can then be calculated as:
vp.w=vmax[1 -1/2(1-a)']
where
a = radius of the particle/radius of the
pore (rp/ro)
This then can be rewritten as:
Based on the theories of size exclusion
and hydrodynamic velocity in a system of
conservative solutes, those particles not
subject to physical straining should
transport according to size and fluid
velocities and the two mechanisms con-
tributing to particle velocity would be opera-
tional. Assuming that the effective porosity
is the total pore volume available to an ex-
cluded macromolecule and that corrections
for the reduced fluid volume in which it
travels is insignificant, then the elution
where
e
Since the average linear velocity of the
excluded particle can be calculated by its
hydrodynamic velocity as defined above
(vw=vpiW) we now have:
Using the guideline that straining will
occur at a particle to pore diameter ratio (a)
of 1 to 9 the limits placed on vpe based on
the hydrodynamic velocity will be:
Ve/vw=1.20(0t/0g) when a = 1/9 and
Vp £/vw=1 as a approaches zero,
6e approaches 0t.
Procedure
Polymers and Colloids
Polyethylene oxide) polymers have the
monomeric formula (-CH2CH2O-)n. Stand-
ards were obtained as a kit containing eight
high purity polymers ranging in average
molecular weight from 14,200 to 900,000.
Polyethylene oxides) were prepared in 0.01
M CaCI2 and 0.1 % by weight NaN3. Sodium
humate was prepared by weighing 100
grams of humate muck (25% solids by
weight as determined by oven drying at
100°C for 24 hours) into 0.01 M NaCIO4 and
bringing the solution up to a total volume of
one liter. The initial pH was 3.92. The solu-
tion pH was then adjusted to match that of
the background NaCIO4 solution, pH 5.76,
using 1.0 M NaOH. After stirring overnight,
the humate solution was allowed to settle for
24 hours. The supernatant was collected by
vacuum suctioning, then centrifuged at
3,000 rpm for one hour and filtered in small
aliquots through 0.45 ^m Millipore filters.
The calcium humate was prepared in the
same manner, except the diluting solution
was 0.01 M CaSO4. The initial pH was 4.04
and was adjusted to 6.41.
Columns and Packing
The columns were specially fabricated of
316 stainless steel, 1 cm in length and 5.0
cm internal diameter. The columns were
fitted with removable stainless steel porous
endplates (0.078 cm thickness, 40/*m pore
size) and Viton o-rings. Solutions were
pumped into the columns by a proportioning
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pump using tygon pump tubes. The
columns were plumbed to the feed solutions
and pump via teflon tubing. Flowrates were
9.6 ml/hour (0.474 cm/hr linear velocity) for
all column experiments. The column pack-
ing material is a fine-grained sand referred to
as Oil Creek Sand mined commercially in
Oklahoma. It is comprised of 99.6% sand
and 0.4% silt. The total organic carbon con-
tent of the sand is less than 0.01%. Where
noted, the sand was amended with 4% each
kaolinite and illite clay standard (8% total
clay) obtained from the Clay Minerals
Society for the purpose of adjusting the pore
size distribution.
Analytical Methods
Humate concentrations were expressed
as mg/L organic carbon determined using
low temperature ultraviolet/potassium per-
sutfate oxidation after acidification with con-
centrated phosphoric acid. The total
organic carbon in column packings was
determined by digestion in 10% perchloric
acid with subsequent solids and filtrate
analysis using low temperature
ultraviolet/potassium persulfate oxidation.
Fatty acid determinations (C3 - C7) were
performed on column effluents as a qualita-
tive indicator of presence or absence of
microbial growth in poly(ethylene oxide)
column experiments. The colloids prepared
for column studies were examined for par-
ticle size distribution, stability of size distribu-
tion from one batch preparation to another
and overtime, and maintainence of distribu-
tion after transport through columns. Particle
size distributions were determined on a Mal-
vern Autosizer 2C. Tritium concentrations
were analyzed on a liquid scintillation
counter with automatic quench correction.
Soil moisture characteristic curves were per-
formed on both OCS and amended OCS
using standard Tempe cells in which a
drainage curve is established by pairing
volumetric water content with matric pres-
sure head (American Society for Testing and
Materials, F316). From these curves, as-
suming spherical particles, pore size dis-
tributions were calculated using the
equation:
= r2jr(h/>g)
where
r = radius of the pore or capillary
Y = surface tension of water
9 = contact angle (cos0 = 1 for a
wetted sample)
h/>g = hydrostatic pressure
Results and Discussion
The interparticle pore size frequency dis-
tribution for OCS is very uniform, with «74%
of the pore radii between 13 to S^urn and less
than 1% £ 1.0 /*m. When the sand was
amended with 8% clay, -10% of the pores
were less than 1.0 /*m, which could be ex-
pected to provide a significant proportion of
pore openings from which a large colloid
would be excluded if size exclusion is func-
tional.
The particle size distributions for the study
polymers are given in Table 1. The
poly(ethylene oxides) could only be es-
timated due to their non-spherical shapes.
Nevertheless, the range of particle diameters
(« 100 to 200 nm) can be used to estimate
if there would be any significant effect of
molecular volume on breakthrough velocity
in either of the column packings used based
on the theories described above. The cal-
cium humate had a particle size distribution
predominantly between 350 to 525 nm and
a small contribution in the 200 to 250 nm
range. The sodium humate had a significant
particle population in the same size distribu-
tion as the calcium humate, but was bimodal
with a significantly higher population in the
range of 200 to 250 nm. The particle size
distributions of both humates were stable
over a period of five days. The
reproducibility of the sodium humate from
one batch preparation to the next was good,
reaffirming that the bimodal distribution was
not an artifact.
Experimental concentrations, recoveries
and experimental Rf's for column
breakthrough results for the study macro-
molecules are listed in Table 2 for reference.
Except for the 900,000 molecular weight
fraction, all the poly(ethylene oxides) broke
through the Oil Creek Sand after the tritiated
water (R f < 1.0). Recoveries of the 50.7K,
83.8K and 770K polymers were relatively
high (84-98%) although some tailing was
observed. The 900,000 molecular weight
polyethylene oxide) eluted prior to tritium
(Rf = 1.116). It is difficult to determine if this
is real or an artifact, perhaps a reflection of
inaccurate analysis as evidenced by the
high recovery (109%). No polymer was
detected in the effluent when transport
studies of two of the polymers through an
amended Oil Creek sand column was con-
ducted. Batch sorption tests showed that
due to sorption excessive quantities of the
expensive polymers would need to be pur-
chased to conduct further studies. For this
reason the column elution studies were not
pursued.
Both the sodium and calcium humates
transported conservatively (-100%
recovery) through the OCS (Figure 1). The
particle size distribution for the sodium
humate effluent was virtually identical to that
in the influent, however, a percentage of the
particles in the calcium humate effluent
shifted to significantly smaller particle
diameters (Figure 2). This was probably due
to calcium ion loss from the humate complex
by the low cation exchange capacity of the
sand causing disaggregation of the humate
particles. The calcium humate was retarded
in the amended OCS (Figure 3) in an ap-
parently irreversible fashion. Approximately
77% of the humate mass was recovered.
The effluent temporarily became turbid and
the particle size distribution (Figure 3)
showed a marked change from a primary
particle size of 150 nm to « 450 nm indicat-
ing that the elution species was probably a
clay/humate complex. There was consider-
able scatter in the tritium data for this
breakthrough curve which was probably
due to interference by the clay particles.
When calcium humate was pulsed a second
time it was observed that the humate broke
through with less retardation, possibly due
to saturation of some fraction of the im-
mobile clay sites (Figure 3).
Conclusions
The observation of column transport of the
colloids and macromolecules present addi-
tional evidence for colloidal mobility. The
ability to predict the rate at which these par-
ticles transport in media other than non-in-
teractive is highly complex and may
ultimately be of minor significance except
under very specific circumstances.
The calcium humate colloid was retarded
due to irreversible clay-humate interactions.
Subsequent flow of calcium humate through
the column showed significantly less retar-
dation of the humate. Based on the theories
of relative velocity presented here, it would
be expected to see early breakthrough of the
calcium humate colloid. However, this
could not have been investigated without
driving the system to saturation complicat-
ing the interpretation of results.
The behavior of the tritiated water was
erratic as evidenced by data-scatter,
breakthrough curve asymmetry and exces-
sive tailing. This may have been due to the
equilibrium of water molecules from the bulk
phase to the hydrated colloid. If this is the
case, then tritiated water may not be an
appropriate conservative tracer for the
transport studies.
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Table 1. Particle Size Frequency Distribution for Study Macromolecules
Primary Particle Size
Macromolecule Cone, (ppm) Frequency Distribution (nm)
Poly(ethylene oxides)
50,700 MW1
83,800 MW
770,000 MW
900,000 MW
Humates
NaCIO4 / humate
CaSO4 1 humate
1261
1119
1044
961
327
285
ND2
190
140
201
200-250, 350-525
350-525
1 molecular weight
2 Not determined
Table 2. Concentrations, Recoveries and Relative Velocities of Study Macromolecules
Observed Relative
Macromolecule Cone (ppm) Column Packing % Recovery Velocity (Rf)1
Poly(ethylene oxides)
50,700 MW2 1261 OCS 95 0.86
83,800 MW 119 OCS 98 0.98
770,000 MW 1044 OCS 84 0.96
900,000 MW 961 OCS 109 1.12
Calcium humate
285 OCS 107 1.01
320 AOCS 100
Sodium humate 325 OCS 96 0.98
molecular weight
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Relative
Concentration
T.O-,
0.8 --
0.6 --
Relative
Concentration
Pore Volumes
Figure 1. Sodium humate (upper figure, closed circles) and calcium humate (lower figure, closed
circles) transport through Oil Creek sand. Tritiated water data (open circles) fitted using a
non-linear least squares procedure (double line). Porosity 0.32; Darcy velocity 0.47 cm/hr;
dispersMty 0.09 cm for calcium humate and 1.2 cm for sodium humate.
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% by Number
0 -
200
400
600
800
% by Number
200
400
Particle Size (nm)
600
800
Figure 2. Particle size distributions of humates transported through Oil Creek sand. Sodium humate
(upper figure); Influent, solid line and effluent, dashed line. Calcium humates (lower figure);
Influent, single line and effluents, double line (1.0 pore volume) and dotted line (1.9 pore
volume).
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Relative
Concentration
Pore Volumes
% by Number
20-~
10--
o-t-
200
400
600
800
Particle Size (nm)
Figure 3. Upper figure: Calcium humate transport (closed circles) through amended Oil Creek
sand. Tritiated water data (open circles) fitted using a non-linear sqares procedure
(double line). Porosity 0.32; dispersivity 0.26 cm; Darcy velocity 0.47 cm/hr.
Lower figure: Corresponding particle size distribution. Influent, solid line. Effluents, 1.13,
1.73 and 2.27 pore volumes, double, dotted and dashed lines, respectively.
'U.S. Government Printing Office: 1993— 760-071/60862
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Candida Cook West is the EPA author and Project Officer (see below).
The complete report, entitled "Transport of Macromolecules and Humate Colloids
Through a Sand and a Clay Amended Sand Laboratory Column," (Order
No. PB 90-219 205/AS; Cost: $17.00, 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 Aqency
P.O. Box 1198
Ada, OK 74820
United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S2-90/020
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