WATER RESOURCES RESEARCH, VOL. 38, NO. 12, 1327, doi:10.1029/2002WR001340, 2002

Physical factors affecting the transport and fate ofcolloids in saturated porous media
Scott A. Bradford, Scott R. Yates, Mehdi Bettahar, and Jirka Simunek
George E. Brown, Jr., Salinity Laboratory, U.S. Department of Agriculture, Agricultural Research Service
Riverside, California, USA
Received 28 March 2002; revised 2 July 2002; accepted 2 July 2002; published 31 December 2002.

[1] Saturated soil column experiments were conducted to explore the influence of colloid

size and soil grain size distribution characteristics on the transport and fate of colloid
particles in saturated porous media. Stable monodispersed colloids and porous media thatare negatively charged were employed in these studies. Effluent colloid concentration
curves and the final spatial distribution of retained colloids by the porous media were
found to be highly dependent on the colloid size and soil grain size distribution. Relative
peak effluent concentrations decreased and surface mass removal by the soil increased
when the colloid size increased and the soil median grain size decreased. Theseobservations were attributed to increased straining of the colloids; i.e., blocked pores act asdead ends for the colloids. When the colloid size is small relative to the soil pore sizes,
straining becomes a less significant mechanism of colloid removal and attachment
becomes more important. Mathematical modeling of the colloid transport experiments
using traditional colloid attachment theory was conducted to highlight differences incolloid attachment and straining behavior and to identify parameter ranges that are
applicable for attachment models. Simulated colloid effluent curves using fitted first-orderattachment and detachment parameters were able to describe much of the effluent
concentration data. The model was, however, less adequate at describing systems which
exhibited a gradual approach to the peak effluent concentration and the spatial
distribution of colloids when significant mass was retained in the soil. Current
colloid xfiltration theory did not adequately predict the fitted first-order attachment
INDEX TERMS: 1831 Hydrology:
coefficients, presumably due to straining in these systems.
Groundwater quality; 1832 Hydrology: Groundwater transport
Citation: Bradford, S. A., S. R. Yates, M. Bettahar, and J. Simunek, Physical factors affecting the transport and fate of colloids insaturated porous media, Water Resour. Res., 38(12), 1327, doi:10.1029/2002WR001340, 2002.

1. Introduction[2] Colloids are particles with effective diameters lessthan 10 mm [McCarthy and Zachara, 1989]. A variety of
inorganic, organic, and microbiological colloids exist innatural subsurface systems including silicate clays, iron andaluminum oxides, mineral precipitates, humic materials,
microemulsions of nonaqueous phase liquids, viruses, andbacteria. These colloid particles can be released into soil
solution and groundwater through a variety of hydrologic,
geochemical, and microbiological processes includingtranslocation from the vadose zone [Nyhan et al., 1985],
dissolution of minerals and surface coatings [Ryan and
Gschwend, 1990], precipitation from solution [Gschwend
and Reynolds, 1987], deflocculation of aggregates [McCarthy and Zachara, 1989], and microbial mediated solubilization of humic substances from kerogen and lignitic
materials [Ouyang et al., 1996]. Consequently, colloid
particles will vary widely in concentration, composition,
structure, and size depending on the spatial and temporalvariability of physical, chemical, and microbiological characteristics. The concentration of natural colloids in groundThis paper is not subject to U.S. copyright.
Published in 2002 by the American Geophysical Union.

waters typically ranges from 108 to 1017 particles L1[Kim, 1991].
[3] Knowledge of the processes that control colloid transport and fate is required to efficiently manage and remediate
many environmental contaminants. For example, an accurate description of microbial (colloid) transport is required
to assess contamination potential and to protect drinking
water supplies from pathogenic microorganisms [Bitton andHarvey, 1992], in the development of engineered bioaugmentation and bioremediation strategies [Wilson et al.,1986], and in microbially enhanced oil recovery [MacLeod
et al., 1988]. Furthermore, the high surface area of colloid
particles facilitates the sorption of organic and inorganics
contaminants such as polychlorinated biphenols (PCBs),
polycyclic aromatic hydrocarbons (PAHs), heavy metals,
and radionuclides [Puls and Powell, 1992]. The colloids canthen act as a mobile solid phase that accelerates the transport of sorbed contaminants [Ouyang et al., 1996].
[4] Colloids are affected by many of the same physicaland chemical processes that influence solute transport, i.e.,
advection, diffusion, dispersion, and sorption. In the colloidtransport literature, sorption processes are frequently
alluded to as attachment, deposition, or filtration. In thismanuscript, colloid sorption and desorption will be referred

63 - 1

63 - 2BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

to as attachment and detachment, respectively. Colloid
attachment, the removal of particle mass from solution via
collision with and fixation to the porous media, is typicallyassumed to be a primary process controlling colloid transport. Attachment is dependent on particle-particle, particlesolvent, and particle-porous media interactions including
electrostatic, London – van der Waals, hydrodynamic, hydration (structural forces), hydrophobic, and steric interactions[Elimelech and O’Melia, 1990]. Colloid attachment is also
reported to be influenced by the physically controlled
processes of sedimentation and interception [Logan et al.,
1995]. The kinetics of particle attachment are controlled by
the rate of transport to solid surfaces and subsequent
fixation to these surfaces.
[5] Previous colloid transport studies in the literature
have focused on the quantification of clean bed first-orderattachment coefficients to describe particle removal in filter
beds (porous media) [Fitzpatrick and Spielman, 1973;
Tobiason and O’Melia, 1988; Vaidyanathan and Tien,1991]. According to this approach, colloid removal by thefilter bed decreases exponentially with depth. Colloid
attachment theory also predicts an optimum particle sizefor transport for a given aqueous-porous medium system[Tobiason and O’Melia, 1988]. Smaller particles are predicted to be removed more efficiently by diffusive transport,
and larger particles are predicted to be removed more
efficiently by sedimentation and interception.
[6] Experimental observations of colloid transport are notalways in agreement with colloid attachment theory. For
example, researchers have reported enhanced colloid retention at the soil surface and that the spatial distribution ofretained colloids does not follow a simple exponential
decrease with depth [Bolster et al., 1999; Redman et al.,2001]. Some of these discrepancies have been attributed tosoil surface roughness [Kretzschmar et al., 1997; Redman etal., 2001], charge heterogeneity [Johnson and Elimelech,
1995], and variability in colloid characteristics [Bolster etal., 1999]. A time-dependent attachment rate has also beenreported to occur as a result of differences in the attachment
behavior of colloids on clean porous media and on media
already containing attached colloids [Johnson and Elimelech, 1995]. Predicted attachment coefficients are also
commonly underestimated when repulsive forces exist
between the colloids and porous media [Vaidyanathan andTien, 1991; Ryan and Elimelech, 1996].
[7] Some of the discrepancies between colloid transport
data and attachment theory may also be due to the fact thatcolloid attachment theory does not account for straining.
Straining is the trapping of colloid particles in the downgradient pore throats that are too small to allow particle
passage [McDowell-Boyer et al., 1986]. In contrast to
colloid attachment, the process of colloid straining hasreceived relatively little research attention. The magnitude
of colloid retention by straining depends on both the colloid
and porous medium properties. If the colloid particles are
physically excluded from entering all of the soil pores, thencomplete straining (mechanical filtration) occurs [McDowell-Boyer et al., 1986]. In this case, a filter cake or surface
mat of colloids forms adjacent to the surface of the porous
medium. This filter cake may significantly reduce thepermeability of the porous medium-filter cake system[Willis and Tosun, 1980]. Incomplete straining occurs when

colloids are physically excluded from pores that are smaller
than some critical size. Hence colloid transport may occur inlarger pores, and colloid retention may occur at pore throatsand grain junctions that are below this critical size. Colloids
retained in smaller pores decrease the effective pore size andmay therefore increase subsequent particle retention in thispore. Incomplete straining has a finite capacity for particle
accumulation and permeability reduction since colloids are
retained in only part of the pore space.
[8] Natural porous media typically exhibit a wide range
in pore sizes due to variations in grain size, orientation andconfiguration, and surface roughness. Few studies have,
however, examined the influence of soil pore size distribution characteristics on colloid straining. It is possible thatstraining occurs at both the pore and grain scale. Sakthivadivel [1966, 1969] and Herzig et al. [1970] developed
geometric relations between the effective diameter of colloids and soil grain size distribution characteristics topredict mass removal by straining and the resulting reduction in permeability. Sakthivadivel [1966, 1969] reported
that straining was significant when the colloid diameter was
greater than 5% of the median grain diameter of the porous
medium. If an upper size limit equal to 0.01 mm (10 mm) is
selected for colloids, then this finding indicates that straining will occur when the median grain size is less than 0.2
mm (sand soil texture). Matthess and Pekdeger [1985]
generalized this rule to porous media made up of a distribution of grain sizes. Colloid transport data from Harvey etal. [1993], however, indicate that straining was more
pronounced than predictions based upon the criteria given
by Matthess and Pekdeger [1985]. Harvey et al. [1993]
observed a trend of increasing colloid retention withincreasing colloid size and hypothesized that migration ofclay particles was responsible for this observed straining
behavior.
[9] In principle, colloid straining is controlled only by thesize of the colloid and the pore size distribution characteristics. Many chemical factors (i.e., pH, ionic strength, surface charge, etc.) will, however, affect the aggregating
behavior of colloid particles (colloid size) and the effective
pore size distribution. Colloids are stabilized when theelectrical double layers are expanded and when the net
particle charge does not equal zero [Ouyang et al., 1996].
Increasing electrolyte concentration and ionic strength
decreases the double layer thickness and thereby promotes
colloid aggregation. Aqueous phase pH influences the net
colloid and solid surface charge by changing pH dependent
surface charge sites. McCarthy and Zachara [1989] also
reported that monodispersed colloids may be produced
through disaggregation if the ion balance is shifted fromone dominated by divalent Ca2+ to one dominated bymonovalent Na+. These same chemical factors (pH, ionicstrength, surface charge, and chemical composition) are also
known to influence soil structure (disaggregation) and poresize distribution (shrinking and swelling) when the soil
contains clays and other colloidal materials [Ayers andWestcot, 1989].
[10] The objective of this work is to investigate theinfluence of straining on colloid transport and fate insaturated porous media. For this purpose, soil column
experiments were conducted for a variety of soil grain size
distribution characteristics and colloid sizes. Colloid trans-

63 - 3BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

port and fate was assessed by measuring temporal changes
in colloid effluent concentrations and the final spatial
distribution of colloid particles remaining in the soil columns. These data were interpreted with the aid of conservative tracer (bromide) studies conducted simultaneously on
the same soil columns and numerical modeling of theexperimental systems.

Table 1. Carboxyl Colloid Properties
dp, mm
0.45
1.00
2.00
3.20

rp, g cm3
1.055
1.055
1.055
1.055

rq, mC cm2
14.6
27.0
27.8
11.9

Ci, particles l1
114.24  10
3.86  1010
4.85  109
1.18  109

fw, deg

z, mV

115.2
115.2
115.2
115.2

N/Ma
77.33
71.37
57.25a

Not measured.

2. Theory[11] Colloid transport and fate models commonly employ
a modified form of the advective-dispersive equation which
incorporates colloid attachment theory [Harvey and Garabedian, 1991; Corapcioglu and Choi, 1996; Bolster et al.,
1999; Schijven and Hassanizadeh, 2000]. In the absence of
colloid inactivation or degradation, the colloid mass transfer
term between the aqueous and solid phases (Esw) [N T1
L3; N, T, and L denote number of colloid particles, time,and length, respectively] is frequently given asEsw ¼ @ ðrb S Þ
¼ qw katt C þ rb kdet S:@t

ð1ÞHere rb [M L3; M denotes mass] is the soil bulk density, S[N M1] is the solid phase colloid concentration, t [T] istime, C [N L3] is the aqueous phase colloid concentration,
qw [-] is the volumetric water content, katt [T1] is the firstorder colloid attachment coefficient, and kdet [T1] is thefirst-order colloid detachment coefficient. Colloid filtration
theory is incorporated into the katt term as [e.g., Logan etal., 1995]
katt ¼

3ð1  qw Þ
hav;
2d50

ð2Þ

where h [ ] is the collector (porous medium) efficiency, a [ ]is the colloid sticking efficiency, d50 [L] is the medianporous medium grain diameter, and v [L T1] is the porewater velocity. The collector efficiency accounts for colloid
removal due to diffusion, interception, and gravitational
sedimentation [Logan et al., 1995]. The value of h isfrequently calculated using the following correlation written
in terms of dimensionless variables [Logan et al., 1995;
Rajagopalan and Tien, 1976]:2=3

h ¼ 4A1=3
s NPe

1=8

15=8

þ As NLo NRþ 0:00338As NG1:2 NR0:4 :ð3ÞHere As is the Happel correction factor, NPe is the Peclet
number, NLo is London-van der Waals attractive forces
number, NR is the interception number, and NG is thegravitional number. The value of a represents the fraction ofparticles colliding with the porous media that remain
attached and therefore reflects the net effect of repulsive
and attractive forces between colloids and solid surfaces.
The value of a is usually derived from experimental
breakthrough curves [Pieper et al., 1997; Ryan et al., 1999]
or from fitted values of katt and calculated values of h [Baleset al., 1991; Redman et al., 1997].

3. Materials and Methods[12] Fluorescent latex colloids (Interfacial Dynamics
Company, Portland, Oregon) were used as model colloid
particles in the experimental studies presented herein. These

spherical colloid particles have been employed in previous
colloid transport studies reported in the literature [Elimelechand O’Melia, 1990; Wan and Wilson, 1994a, 1994b;
Reimus, 1995]. Yellow-green colloids (excitation at 490
nm and emission at 515 nm) were chosen because they
can be more efficiently illuminated than other color types.
Colloid sizes (0.45, 1.0, 2.0, and 3.2 mm) were selected to beconsistent with sizes of typical pathogenic bacteria. These
colloids have carboxyl surface functional groups grafted
onto latex particle surfaces by the manufacturer to create anegatively charged hydrophobic colloid surface.
[13] Specific physical and chemical characteristics for thecolloids are given in Table 1. Here dp denotes the median
diameter of the colloid particle, rp is the particle density, rq
is the particle surface charge density, qw is the air-watercontact angle, and z is the zeta potential. The values of dp,
rp, and rq were provided by the manufacturer. The reported
uniformity of the colloid size distribution was verified forthe 3.2 mm particles using a Coulter Counter (Beckman
Coulter Inc., Fullerton, California). Good agreement wasobserved between the measured and the reported colloid
size distribution. The value of qw was measured (average offive measurements) on a bed of 0.11 mm carboxyl latex
colloids using a Tantec Contact Angle Meter (Tantec Inc.,
Schaumburg, Illinois). This bed was created following a
protocol similar to that presented by Wan and Wilson[1994b]; i.e., several drops of concentrated colloid solution
were placed on a microscope slide, fluid was distributed on
the slide by tilting, and excess water was allowed toevaporated from the surface. The zeta potential was measured using a Zeta-meter 3.0 (Zeta-meter, Inc., Staunton,
Virginia); i.e., measured electrophoretic mobility was converted to zeta potential using the Smoluchowski equation.
The initial influent colloid concentration (Ci) used in the soilcolumn experiments is also given in Table 1. These concentrations were selected to minimize any permeability
reductions in the soils [Sakthivadivel, 1966, 1969; Herzig
et al., 1970] and to provide each column with a nearly equalmass of colloids (approximately 3.28 mg).
[14] The aqueous phase chemistry (pH, ionic strength,
and composition) of the tracer, resident, and eluant solutions
utilized in the soil column experiments was chosen to createa stabilized monodispersed suspension with the selected
colloids. The initial resident and eluant solutions consistedof a 0.001 M NaCl with its pH buffered to 6.98 using
NaHCO3 (5  105 M). The colloid-conservative tracer
solution consisted of 0.001 M NaBr with its pH buffered to
6.73 using NaHCO3 (5  105 M) and the indicated initial
colloid concentration listed in Table 1. The aqueous solventin all the experimental solutions consisted of deaired(boiled) deionized water.
[15] Glass beads (USF Surface Preparation, Rancho
Dominguez, California) and various sieve sizes of Ottawa

63 - 4

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

Table 2. Porous Medium Properties

2030
3550
70110
MIX
GB

F20 – F30

F35

F50

F70

F110

d50, mm

Ui

100%
-

50%
25%
-

50%
25%
-

50%
25%
100%

50%
25%
-

0.71
0.36
0.15
0.24
0.26

1.21
1.88
2.25
3.06
1.20

sand (U.S. Silica, Ottawa, Illinois) were used in the soilcolumn experiments. The Ottawa sands were selected as arepresentative aquifer material and to encompass a range insoil grain size distribution characteristics. The various
Ottawa sands will be designated herein as 2030, 3550,
MIX, and 70110, whereas glass beads will be denoted as
GB. Table 2 summarizes the grain size distribution properties for the various porous media used in the soil column
experiments. The median grain size (d50; mm) and theuniformity index (Ui = d60/d10) are measures of the averagegrain size and the distribution of grain sizes, respectively;
here x% of the mass is finer than dx.
[16] Bradford and Abriola [2001] presented capillarypressure-saturation curves for the 2030, 3550, MIX, and70110 Ottawa sands. An estimate of the pore size distribution of drained pores can be obtained from capillarypressure-saturations curves using Laplace’s equation ofcapillarity. According to this approach, the drained poresfor the various Ottawa sands were all larger than 10 mm indiameter. Smaller pores that may produce colloid straining
could not be characterized by this method. The measured
residual saturations do, however, provide an estimate of thevolume fraction of the pore space that has pores less than 10
mm in diameter. The reported residual saturation values
suggest that 2, 6.5, 16, and 30% of the pore space containpores less than 10 mm in diameter for the 2030, 3550, MIX,
and 70110 sands, respectively. In contrast, the criteriapresented by Matthess and Pekdeger [1985] indicate thatstraining will not occur for the porous media and colloid
sizes considered in this work.

[17] Ottawa sands typically consist of 99.8% SiO2
(quartz) and trace amounts of metal oxides, are spheroidal
in shape, and may have rough surfaces. In contrast, glass
beads are relatively chemical homogeneous, spherical, andsmooth. Hence glass beads serve as a control medium tominimize variations in chemical composition and surface
roughness. The glass beads, Ottawa sands, and the carboxyl
colloids (Table 1) possess a net negative surface charge at aneutral pH. Hence attractive electrostatic interactions
between the colloids and the vast majority of the porous
media are minimal. Attractive electrostatic interactions,however, can occur between the carboxyl colloids and theminor fraction of positively charged metal oxide surfaces
distributed on the Ottawa sands.
[18] Aluminum soil columns, 10 cm long and 5 cm inside
diameter, were used in the colloid transport studies that
employed glass beads as the porous medium. These columns were equipped with standard fittings and stainlesssteal wire screen at both of their ends. The soil columns
employed with Ottawa sands were 15 cm long and 4.8 cm
inside diameter Kontes Chromaflex chromatography columns (Kimble/Kontes, Vineland, New Jersey) made ofborosilicate glass. The chromatography columns were
equipped with a standard flangeless end fitting at thecolumn bottom and a flow adapter at the top. The flow
adapter allowed the column bed length to be adjusted,
thereby facilitating the tight sealing of the adapter at thesoil surface. The manufacturer’s bed support at the column
inlet and outlet was replaced with a stainless steal wire
screen (105 mm mesh spacing) supported by a coarser
polyethylene screen to uniformly distribute water flow andcolloids at the soil surfaces. Tubing to and from thecolumns, fittings, column O-rings, and flow adapter werecomposed of chemically resistant materials such as Teflon,
viton, stainless steal, and Kalrez.
[19] The glass beads were thoroughly cleaned and dried
before packing them in the soil columns. The cleaning
procedure consisted of washing with distilled water, followed by ultrasonic cleaning in heated water for 30 min. In
contrast, the Ottawa sands were used as provided by the

Table 3. Soil Column Properties (Darcy Velocity, q; Porosity, e; and Column Length, L), Colloid Release Efficiency (Er), and theRecovered Effluent (ME), Soil (MS), and the Total Colloid Mass Fraction (MBT)
Soil Typedp, mmq, cm min1

e

L, cm

Er

ME

MS

MBT

2030
2030
2030
2030
3550
3550
3550
3550
MIX
MIX
MIX
MIX
70110
70110
70110
70110
GB
GB
GB
GB

0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20

0.100
0.101
0.103
0.103
0.099
0.100
0.102
0.102
0.104
0.105
0.107
0.108
0.110
0.106
0.109
0.110
0.138
0.152
0.169
0.145

0.367
0.378
0.369
0.362
0.341
0.342
0.343
0.341
0.328
0.330
0.344
0.336
0.342
0.342
0.351
0.356
0.377
0.360
0.373
0.369

13.17813.42012.22413.06912.657
12.67212.81112.77412.40712.45912.672
12.53312.676
12.67612.97812.94810.00010.00010.00010.000

0.660
0.780
0.650
0.590
0.680
0.840
0.670
0.630
0.660
0.810
0.600
0.610
0.610
0.800
0.590
0.610
0.800
0.751
1.000
0.863

0.896
0.825
0.468
0.440
0.838
0.872
0.261
0.342
0.958
0.843
0.115
0.118
0.660
0.463
0.072
0.028
0.628
0.232
0.161
0.006

0.109
0.075
0.368
0.637
0.151
0.103
0.522
0.728
0.215
0.181
0.969
1.254
0.321
0.430
0.956
0.884
0.386
0.794
0.579
1.268

1.005
0.900
0.836
1.077
0.989
0.975
0.783
1.070
1.173
1.024
1.084
1.372
0.981
0.893
1.028
0.912
1.014
1.026
0.740
1.274

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

63 - 5Figure 1. Schematic of the experimental setup employed in the colloid transport studies.
supplier. The soil columns were wet packed with the various
porous media; i.e., the water level was always kept a few
centimeters above the soil surface. Care was taken to ensurea uniform packing by completely mixing small quantities of
the appropriate sieve size before addition to the columns.
After each incremental addition of soil, the added soil was
gently mixed with the lower surface layer of soil and thenvibrated to minimize any settling and layering, and toliberate any entrapped air. Table 3 provides porosity (e)values for each experimental soil column. The porosity wasdetermined according to the method of Danielson andSutherland [1986] using the measured soil bulk density
and assuming a specific solid density of 2.65 g cm3.
[20] Before initiating a carboxyl colloid tracer experiment
the soil columns containing Ottawa sand were flushed withseveral pore volumes (PV) of eluant solution to remove
natural colloids particles from the porous media. A more
rigorous procedure was used for the 2.0 and 3.2 mm carboxyl
colloid experiments in 3550, MIX, and 70110 sands. For
these soils and colloid sizes the analytic procedure was moresensitive to natural background colloids levels; i.e., lowfluorescence readings were influenced by the sample opticaldensity. In this case, the soil columns were flushed asfollows: Initial resident solution was 0.026 M NaCl, 2 PV
flush with deionized water, 2 PV flush with 0.026 M NaClsolution, 2 PV flush with deionized water, and finally a 4 PV
flush with eluant solution. Here the 0.026 M NaCl solution
promotes clay aggregation and the deionized water promotes
clay mobilization [Bettahar et al., 1998].
[21] Figure 1 is a schematic of the experimental setup
employed for the colloid transport studies. A Masterflex L/S
multihead drive pump (Barnant Company, Barrington, Illinois) equipped with standard heads was used to pumpcolloid and bromide tracer (colloids + 0.001 M NaBr + 5 
105 M NaHCO3) or eluant (0.001 M NaCl + 5  105 M

NaHCO3) solution upward through the vertically oriented
columns at a steady rate. The average aqueous Darcy
velocity (q) for the various soil column experiments isgiven in Table 3. The solution was pumped upward tominimize any sedimentation that may occur as a result of
the difference in density of the colloid particles (density
equals 1.055 g cm3) and water (density equal to 0.998 g
cm3). The colloid and bromide tracer solution was
pumped through the columns for around 77 min, after
which a three-way valve was used to switch the pumped
solution to the eluant (0.001 M NaCl + 5  105 M
NaHCO3) for a total experiment time of 250 min. A totalof 50 effluent samples were collected in 20 mL glass
scintillation vials over the course of each soil column
experiment. Each sample was gathered during a 5-mintime interval and then capped. The effluent colloid (particles L1) concentration was then determined on thesesamples using a Turner Quantech Fluorometer (Barnstead/
Thermolyne, Dubuque, Iowa) equipped with excitation(490 nm, LE1095  13, NB490) and emission (515 nm,
LE1095  23, SC515) filters. Some of these samples werealso analyzed for bromide (mol L1) using an Orion 720a
pH/ISE meter (Orion research Inc., Beverly, Massachusetts) equipped with an ion specific electrode for bromide.
Colloid (average of triplicate measurements) and bromide
concentrations were determined using linear calibration
curves established between instrument response and standard solutions. The established calibration curves always
had correlation coefficients very close to unity. This
analytic protocol was sensitive to concentrations over 2
orders of magnitude below the initial colloid and bromide
concentrations.
[22] Following completion of the colloid transport experiments, the spatial distribution of colloids in a soil column wasdetermined. The end fitting was removed and the saturated

63 - 6

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

Figure 2. Representative plots of relative bromide concentration in the effluent versus pore volume for theindicated soil types.

porous media was carefully excavated into 20 mL scintillation vials or 50 mL glass centrifuge tubes. Around 16 vials(or eight centrifuge tubes) were used to recover the soil massfrom a particular column. Excess eluant solution was added
to fill the vials or centrifuge tubes. These vials or tubes were
slowly shaken for 4 hours using a Eberbach shaker (Eberbach
corporation, Ann Arbor, Michigan) to liberate strained colloids; this equilibration time interval was selected to beconsistent with the duration of the colloid transport experiments. The concentration of the colloids in the excess
aqueous solution (decanted from the vials) was measured
using the Turner Quantech Fluorometer and the same experimental protocol outlined for the effluent samples. Water andsand filled vials (uncapped) were covered with aluminum foil
(to prevent loss of sand from the vials) and placed in an ovenfor several hours to volatilize all of the remaining water fromthe sand. The volume of water and mass of sand in each vial
or tube was determined from mass balance by measuring theweight of the empty vials (or tubes), water and sand filledvials, and sand filled vials.
[23] Duplicate ‘‘blank’’ soil column experiments wererun for the various Ottawa sands to elucidate the influenceof background clay colloids on the measured spatial
distribution of 2.0 and 3.2 mm carboxyl colloids in 3550,
MIX, and 70110 sands. These blank soil columns were
subjected to the same cleaning, tracer, and excavation
procedures and protocols given above. The only difference
was that carboxyl colloids were not added to the columns.
Background clay concentrations were determined fromthese blank soil columns at the column inlet, outlet, andmiddle. The average background concentration (a few
percent, 2 – 8% in the aqueous phase) for a particular soil
type was then subtracted off the corresponding measured
2.0 and 3.2 mm carboxyl concentrations determine fromexcavated soil.
[24] The release efficiency (Er) of the carboxyl colloids
from the porous media was sometimes less than 100% due
to colloid attachment and/or straining. Colloid release
efficiency was measured by placing a 10 g sample of dry
porous media and 10 mL of colloid tracer solution at a
specified concentration (100, 75, 50, 25, and 0% percent ofCi) into a 20 mL scintillation vial. These vials were shaken
for 4 hours using an Eberbach shaker, and the concentrationof the colloids in the aqueous solution was then measured
using the Turner Quantech Fluorometer. Release efficiencies were determined as the ratio of the final and initial
aqueous colloid concentrations. Table 3 presents the measured average release efficiencies for the various colloids andporous media. The measured colloid concentrations fromexcavated soil were corrected by dividing by the release
efficiency.
[25] At the end of a soil column experiment, a colloid
mass balance was conducted using effluent concentration
data and the final spatial distribution of strained colloids inthe sands. The calculated number of effluent and soil colloid
particles was normalized by the total number of injectedparticles into a column for this purpose. Table 3 presents therecovered colloid mass fraction in the effluent (ME) and soil(MS), as well as the total (effluent + soil) colloid massbalance (MBT). The value of MS reflects uncertainty in therelease efficiency and the background concentration ofnatural clay particles. Hence errors in MBT are believed to
be primarily caused by errors in MS. In general, the valuesof MBT are reasonably close to unity.

4. Results and Discussion
4.1. Bromide Effluent Concentration Curves[26] Figure 2 presents representative plots of the relative
bromide concentration (C/Ci) in the effluent versus porevolume (product of column volume and porosity) of water
passing through the soil column for the indicated porous
media. Observe that bromide serves as a good conservative
tracer; i.e., bromide samples all of the pore water velocity
distribution and moves at the average pore water velocity.
Note also that the bromide breakthrough curves are very
symmetric, indicating that physical nonequilibrium is not
significant for bromide in these soil column experiments.
4.2. Colloid Effluent Concentration Curves[27] Figure 3 presents a plot of the relative colloid concentration in the effluent as a function of pore volume for theindicated colloid sizes in the glass beads. Also shown in thisfigure is a plot of a representative bromide effluent concentration curve in the glass beads. Observe that the break-Figure 3. A plot of the relative colloid concentration in
the effluent as a function of pore volume for the indicatedcolloid sizes in the glass beads.

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

63 - 7

Figure 4. A plot of the relative colloid concentration in the effluent as a function of pore volume for theindicated colloid sizes (Figure 4a, 3.2 mm; Figure 4b, 2 mm; Figure 4c, 1 mm; and Figure 4d, 0.45 mm) andfor the various soil types (2030, 3550, MIX, and 70110).

through time for the various colloid sizes is very similar tothat of bromide. Furthermore, the peak colloid concentration
decreases with increasing colloid size. Recall that the glass
beads are chemically homogeneous, spherical, and smooth.
Hence attachment of the carboxyl colloids (negatively
charged) to the glass beads (negatively charged) cannot
explain the observed effluent concentration curves. The highrelease efficiency of the colloids from the glass beads (seeTable 3) also supports this hypothesis. All of these observations suggest that straining (increases with increasing colloidsize) is the primary mechanism for colloid removal in theseglass beads. Colloid transport experiments were also conducted for the various Ottawa sands described in Table 2.
Figures 4a, 4b, 4c, and 4d present plots of the relative colloid
concentration in the effluent as a function of pore volume forthe 3.2, 2.0, 1.0, and 0.45 mm colloids, respectively, in theindicated sands. Bromide (Figure 2) and colloid (Figure 4)
breakthrough times in a particular sand were quite similar.
Differences in transport behavior of bromide and colloid
particles increased with decreasing soil grain size andincreasing colloid size. With decreasing soil grain size thepeak effluent concentration was lower and the removal of aparticular sized colloid enhanced. This is especially true forthe larger 3.2 (Figure 4a) and 2.0 (Figure 4b) mm colloids; i.e.,very little breakthrough occurs for the 3.2 mm colloids in the70110 sand. Comparison of the effluent concentration curves
for the different sized colloids in a particular sand also reveals
a trend of lower peak effluent concentrations with increasing

colloid size. Recall that similar behavior was observed forthese same colloids in the glass beads (see Figure 3). All theseresults provide substantial evidence that straining can be an
important removal mechanism for carboxyl colloids in these
Ottawa sands.
[28] Comparison of Figures 4c and 4d reveals similartransport behavior for 1.0 and 0.45 mm colloids in 2030,
3550, and MIX sands. The peak effluent colloid concentration in these cases was around 80% of the initial influent
concentration, and independent of the colloid and porous
medium size. These observations suggest that little straining
may be occurring over this range of colloid sizes and soil
grain size distribution characteristics. In this case, mass
removal is believed to be controlled by colloid attachment.
Mechanisms of colloid attachment can be due to bothchemical and physical (attachment at solid grain junction
points and rough solid surfaces) processes. The exact
mechanisms for colloid attachment cannot be deduced fromthe experimental data.
[29] The peak effluent concentration of the 1.0 (Figure4c) and 0.45 (Figure 4d) mm colloids is lower in the finer
70110 sand than in the other Ottawa sands (2030, MIX, and70110). Furthermore, a higher peak effluent concentration
occurs for the 0.45 mm colloids (C/Ci = 0.63) than that for
1.0 mm colloids (C/Ci = 0.48) in the 70110 sand. Theseobservations can again be attributed to enhanced straining
with decreasing soil grain size and increasing colloid size.
The effluent concentration curves for the 1.0 and 0.45 mm

63 - 8

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

Figure 5. A plot of the fraction of total colloid mass per gram of dry sand as a function of normalized
distance from the column inlet for the indicated colloid sizes (Figure 5a, 0.45 mm; Figure 5b, 1 mm; Figure
5c, 2 mm; and Figure 5d, 3.2 mm) and for the various soil types (2030, 3550, MIX, and 70110).

colloids in 70110 sand, and for the 2.0 mm colloids in 2030
sand (see Figure 4b), exhibit a much more gradual approachto their peak concentration value than the other curves
shown in Figures 4a– 4d. This behavior may be indicativeof a superposition of straining and attachment processes anda transition from colloid straining to attachment.
[30] Table 2 indicates that the coefficient of uniformity
for the glass beads and the MIX sand is quite different,
whereas the mean grain size is very similar. Comparison of
the effluent concentration curves for a particular sized
colloid in the glass beads (Figure 3) and MIX sand (Figure4) reveals that the peak effluent concentration was generally
less in the glass beads than in the MIX sand. This result is
attributed to differences in the soil grain size distribution
characteristics. More colloid mass removal occurs in theuniform glass beads than in the graded MIX sands. Resultsfrom Schaap and Bouten [1996] indicate that grain and poresize (capillary pressure curves) distribution characteristics
are positively correlated. The enhanced transport potentialof colloids in the MIX sand is therefore believed to occur
due to the presence of larger pores in the more graded MIX
sand.
4.3. Spatial Distribution of Colloids[31] After completion of the colloid tracer experiments,
the remaining colloid mass in the sand was measured with
distance from the soil column inlet. Figure 5 summarizes
this data by presenting plots of the fraction of total colloid

mass per gram of dry soil as a function of normalized
distance from the column inlet for the various Ottawa sands.
Specifically, Figures 5a, 5b, 5c, and 5d are for 0.45, 1.0, 2.0,
and 3.2 mm colloids, respectively.
[32] Observe in Figure 5a that the fraction of 0.45 mm
colloid mass retained in the soil tends to be more or lessconstant with distance for the 2030, 3550, and MIX soils.
Recall from the discussion of Figure 4d that colloid
removal in this case was attributed primarily to attachment.
In contrast, observe in Figures 4d and 5a the behavior of
the 0.45 mm colloids in 70110 sand. In this case, straining
results in significant mass removal over the first few
centimeters of soil from the inlet because blocked pores
act as dead ends for the colloids. After the first few
centimeters, colloids are only mobile in the larger continuous pore networks. Pore exclusion is hypothesized toinhibit further mass removal by straining in these networks, and colloid mass removal with depth therefore
approaches a more uniform value. Since the magnitudeof the retained colloids is similar to that of the other sands,
it is hypothesized that attachment is the primary means ofmass removal for dimensionless depths greater than 0.2.
Comparison of Figures 5a and 5b indicates that very
similar trends occur for the 1.0 and 0.45 mm colloids.
This observation can again be explained by similarities in
the removal mechanism for the colloids (see discussion for
Figures 4c and 4d). Note, however, that for the 70110 soil
a much higher mass fraction of 1.0 mm colloids (Figure 5b)

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

Figure 6. A plot of the fraction of total colloid mass pergram of dry porous media as a function of normalized
distance from the column inlet for the variously sized
colloids retained in the glass beads.

are removed in the first few centimeters of soil comparedwith the 0.45 mm colloids (Figure 5a). This is attributable toan increase in pore straining as a result of the increase incolloid size.
[33] For the larger colloid sizes, 2.0 (Figure 5c) and 3.2(Figure 5d) mm, the primary mechanism for colloid removalby the soil is believed to be straining. Recall that straining
increases with decreasing mean grain size. Hence thecoarsest 2030 soil always exhibits the lowest colloid massfractions at the column inlet, and the finest MIX and 70110
sands exhibit the highest colloid mass fractions at thecolumn inlet. The 3550 sand shows intermediate behavior
between these two extremes. After the initial sharp decreasein mass removal with increasing distance from the column
inlet, note that the colloid mass fraction retained in the soil
approaches a more constant value with depth (dimensionless depths greater than 0.2). In this region of more constantretention, pore exclusion may inhibit further mass removal
by straining, and attachment is therefore believed to be theprimary means of mass removal. To further support thishypothesis, note that the mass fraction of 3.2 and 2 mm
colloids retained in the uniform concentration region issimilar in magnitude to that for the 0.45 mm colloids.
[34] Comparison of the spatial distribution of various
sized colloids in a particular sand reveals a trend ofincreasing colloid mass removal at the column inlet withincreasing colloid size (see Figures 5a, 5b, 5c, and 5d). This
result is again attributed to enhanced straining with increasing colloid size and is consistent with the observed effluent
concentration curves shown in Figure 4.
[35] Figure 6 presents a plot of the spatial distribution of
the variously sized colloids retained in the glass beads. The
corresponding colloid effluent concentration curves were
presented earlier in Figure 3. Notice that the mass fractionof retained colloids at the column inlet is greatest for thelargest 3.2 mm colloids and is lowest for the smallest 0.45
mm colloids. This result is again attributed to enhanced
straining with increasing colloid size. The 1.0 and 2.0 mm
colloids exhibit intermediate behavior to these two
extremes. Similar colloid mass distributions occur for the
1.0 and 2.0 mm colloids occur because of their comparable
effluent concentration curves (see Figure 3). In all cases, the63 - 9concentration of retained colloids is relatively low andconstant for dimensionless depths greater than 0.2. As for
the Ottawa sand, this result is ascribed to pore exclusion that
limits straining.
[36] Recall that the mean grain size of the MIX sand andthe glass beads is quite similar, whereas the coefficient of
uniformity is very different (see Table 2). Comparison of thespatial distribution of a particular sized colloid in the MIX
sand (Figure 5) and glass beads (Figure 6) reveals more
colloid retention at the column inlet in the glass beads thanin the MIX sand. This is indicative of enhanced straining in
the glass beads. The uniform glass beads are believed tohave fewer large pores than the graded MIX sands. Differences in the effluent concentration curves also support thishypothesis (see Figures 3 and 4).
4.4. Simulating Colloid Transport[37] Colloid attachment theory has been used to predict
colloid transport behavior for a wide variety of colloid andporous media characteristics. Recall that equations (1) – (3)were developed under the assumption that attachment controls colloid mass removal by porous media. Little attention
has been paid to the colloid and porous media parameter
ranges that may limit attachment model applicability,
because colloid straining has been assumed a priori to be
insignificant. Colloid attachment theory is not designed toquantify colloid mass removal via straining. Comparison of
the attachment model results and experimental data will be
used herein to highlight differences in colloid attachment
and straining behavior and to help identify parameter ranges
that are applicable for attachment models.
[38] A slightly modified version of the Hydrus-1D computer code [Simunek et al., 1998] was utilized to simulate
the colloid transport experiments. Hydrus-1D simulates
water, heat, and multiple solute movement in one-dimensional variably-saturated porous media. The code is coupled
to a nonlinear least squares optimization routine based upon
the Marquardt-Levenberg algorithm [Marquardt, 1963] tofacilitate the estimation of solute transport parameters from

Figure 7. Observed and fitted (lH, katt, and kdet) effluent
concentration curves for several representative colloidtransport experiments indicated in the legend.

63 - 10

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

Figure 8. Observed and simulated fraction of total colloid
mass per gram of dry soil as a function of normalized
distance from the column inlet for several representative
systems indicated in the legend.
experimental data. Attachment and detachment rate coefficients, and the hydrodynamic dispersivity (lH) were simultaneously fitted to each colloid effluent concentration curve.
Since bromide and colloids may not necessarily follow thesame flow paths, a separate bromide tracer hydrodynamic
dispersivity (lHBR) was also fitted to the experimental data.
[39] Figure 7 presents the observed and fitted effluent
concentration curves for several representative colloid transport experiments. Observe that the simulation fits describes
the experimental data fairly well. The fit of the data for 0.45
mm colloids in 70110 sand is described less adequately thanthe other data shown in Figure 7. Here the experimental data
exhibit a more gradual approach to its peak effluent concentration value. Detachment cannot account for this behavior because a higher detachment rate would also lead tohigher concentration tailing. The asymmetric shape of theeffluent concentration curve for 0.45 mm colloids in 70110
sand is therefore believed to reflect a transition from colloid
attachment to straining.
[40] Figure 8 presents the observed and simulated fractionof total colloid mass per gram of dry soil as a function of
normalized distance from the column inlet for several

representative systems. Observe that the simulated decreasein the colloid concentration with depth is much more
gradual than that shown by the experimental data. This isespecially true for systems in which significant colloid mass
was retained by the soil as a result of straining. For example,
observe the difference between simulated and experimental
2 mm colloids in MIX sand compared to 0.45 mm colloids in
70110 sand. Although the effluent concentration curves
shown in Figure 7 were adequately described using afirst-order attachment and detachment coefficient, Figure 8
suggests that this modeling approach does not describe theobserved spatial colloid distribution well for systems thatexhibit significant straining.
[41] Table 4 presents the estimated parameters values oflHBR, lH, katt, and kdet as well as the correlation coefficient.
Observe that values of lHBR and lH are significantly different for a particular experimental system, with lHBR tending
to be less than lH. The higher lH value may occur tocompensate for the inability of the colloid attachment theory
to perfectly describe the data. Alternatively, the higher lHvalues may also indicate that the colloid particles experience
more spreading in the soil and follow a slightly different
flow path than bromide due to pore exclusion of colloids.
Consistent with colloid straining, the value of katt tends toincrease with increasing colloid size (for a particular porous
medium) and decreasing mean grain size (for a particular
colloid size). Table 4 also indicates that higher values of kdet
occur for the smaller sized colloids. Recall that attachment
was hypothesized to be the dominant mechanism of mass
removal for the smaller sized colloids.
[42] Table 4 also presents calculated values of h according to equation (3). The value of h, for a particular soil type,is predicted to be maximum for the smallest (0.45 mm)
colloids and tends to achieve a minimum value for the 2.0mm colloids. The attachment coefficients given in Table 4,
however, tend to reach a maximum value for the largest
colloids. To compensate for this difference, values of a (asdetermined from equation (2) using the values of h and kattgiven in Table 4) vary with colloid size and soil type. Recall
that a theoretically accounts for the net effect of repulsive

Table 4. Fitted (Bromide Dispersivity, LHBR; Colloid Dispersivity, LH; Attachment Coefficient, katt; Detachment Coefficient, kdet; andColloid Sticking Efficiency, A) and Calculated (Collector Efficiency, H) Model Parameters
Soil Typedp, mm

LHBR, cm

LH, cmkatt  103, min1

kdet  104, min1

H  102A  102

r2

2030
2030
2030
2030
3550
3550
3550
3550
MIX
MIX
MIX
MIX
70110
70110
70110
70110
GB
GB
GB
GB

0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20
0.45
1.00
2.00
3.20

0.080
0.061
0.092
0.076
0.070
0.070
0.148
0.103
0.094
0.074
0.105
0.053
0.104
0.080
0.071
0.179
0.260
0.086
0.209
0.047

0.189
0.237
0.110
0.239
0.151
0.158
0.493
0.288
0.095
0.179
0.385
0.361
0.166
0.071
0.299
0.355
0.135
0.091
0.122
0.100

3.75
4.01
20.1
19.3
5.46
3.57
36.9
29.7
3.06
56.8
63.0
63.0
12.8
22.2
77.0
132.2
14.9
81.9
102.1
232.9

30.2
0.75
10.6
4.10
21.5
6.65
8.61
2.87
45.4
10.0
6.00
3.75
14.4
12.4
7.09
24.0
0.02
20.1
13.5
7.18

2.87
1.80
1.81
2.87
4.80
3.00
2.60
3.48
6.28
3.91
3.14
4.04
8.02
5.22
4.29
5.14
4.44
2.70
2.00
2.68

3.59
6.33
29.8
17.6
1.43
1.48
17.4
10.4
0.37
10.9
15.7
11.7
0.75
2.09
8.91
12.9
2.51
19.1
30.7
59.9

0.998
0.986
0.974
0.990
0.997
0.998
0.885
0.991
0.997
0.992
0.929
0.984
0.985
0.981
0.726
0.001
0.988
0.941
0.927
0.094

BRADFORD ET AL.: FACTORS AFFECTING THE FATE OF COLLOIDS

and attractive forces between colloids and solid surfaces.
Since the electrostatic forces between the various sizedcarboxyl colloids (see Table 1) and Ottawa sands are similar
in magnitude, the scatter in calculated values of a (2 ordersof magnitude) reflects the inability of this approach topredict the experimental data. Ryan and Elimelech [1996]
also noted serious discrepancies between calculated sticking
efficiencies and experimentally measured values. This limitation may occur in part because straining is not adequately
described by this theory.
[43] The numerical simulations demonstrate that traditional colloid attachment theory is not well suited todescribe colloid transport in systems that exhibit straining.
The experimental data strongly suggest that straining is an
important mechanism of colloid retention for the larger 2.0
and 3.2 mm colloids in the various porous media consideredherein. Colloid straining also likely occurred for the 0.45
and 1.0 mm colloids in the finest 70110 sand and the glass
beads. Caution is therefore warranted when applying colloid
attachment theory to colloids and porous media that encompass this range in size; i.e., when the ratio of the colloid
diameter to the mean grain diameter (dp/d50) is greater thanapproximately 0.0017.
[44] The colloid transport studies presented herein wereconducted in water-saturated porous media and with negatively charged colloids. In unsaturated systems, water flow
and colloid transport will be controlled by the spatial
distribution of water saturation. At lower water saturations,water flow occurs primarily in the finer grained materials,
and smaller pores and pore spaces. Hence the significanceof the physical factors (colloid size and grain size distribution) is anticipated to be magnified in unsaturated systems.
Straining may also be influenced by other physical (watervelocity, colloid concentration, colloid and soil grain size
distribution characteristics, heterogeneity, and dimensionality) and chemical (surface charge of colloid and porous
media) factors of the porous media and colloids. Additional
colloid transport studies are currently under way to quantify
these transport processes and to incorporate these findings
into a deterministic colloid transport simulator.

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M. Bettahar, S. A. Bradford, J. Simunek, and S. R. Yates, George E.
Brown, Jr., Salinity Laboratory, U.S. Department of Agriculture, Agricultural Research Service, 450 W. Big Springs Road, Riverside, CA 925074617, USA. (sbradford@ussl.ars.usda.gov)