Environ. Sci. Technol. 2010, 44, 1662–1669

Hysteresis of Colloid Retention andRelease in Saturated Porous Media
During Transients in Solution
Chemistry
SAEED TORKZABAN,‡HYUNJUNG N. KIM,§ JIRI SIMUNEK,|A N D S C O T T A . B R A D F O R D * ,†Earth Sciences Division, Lawrence, Berkeley National
Laboratory, Berkeley, CA, Department Chemical andEnvironmental Engineering, University of California,
Riverside, CA, Department Environmental Sciences, Universityof California, Riverside, CA, and USDA, ARS, U.S. Salinity
Laboratory, Riverside, CA

Received October 27, 2009. Revised manuscript received
January 19, 2010. Accepted January 21, 2010.

Saturated packed column and micromodel transport studies
were conducted to gain insight on mechanisms of colloid retention
and release under unfavorable attachment conditions. The
initial deposition of colloids in porous media was found to bea strongly coupled process that depended on solution chemistry
and pore space geometry. During steady state chemical
conditions, colloid deposition was not a readily reversible
process, and micromodel photos indicated that colloids were
immobilized in the presence of fluid drag. Upon stepwise reduction
in eluting solution ionic strength (IS), a sharp release ofcolloids occurred in each step which indicates that colloid
retention depends on a balance of applied (hydrodynamic) andresisting (adhesive) torques which varied with pore spacegeometry, surface roughness, and interaction energy. Whenthe eluting fluid IS was reduced to deionized water, the final
retentionlocationsoccurredneargrain-graincontacts,andcolloid
aggregation was sometimes observed in micromodelexperiments. Significant amounts of colloid retention hysteresis
with IS were observed in the column experiments, and it
depended on the porous medium (glass beads compared withsand), the colloid size (1.1 and 0.5 µm), and on the initialdeposition IS. These observations were attributed to weakadhesive interactions that depended on the double layer thickness(e.g., the depth of the secondary minimum and/or nanoscale
heterogeneity), colloid mass transfer on the solid phase to regions
where the torque and force balances were favorable forretention, the number and extent of grain-grain contacts, andsurface roughness.

Introduction
Many environmentally relevant particles such as microorganisms, clays, and collectors (i.e., porous media) are* Corresponding author phone: 951-369-4857; e-mail: Scott.
Bradford@ars.usda.gov.
†
U.S. Salinity Laboratory.
‡
Lawrence, Berkeley National Laboratory.
§
Department Chemical and Environmental Engineering, University of California.
|
Department Environmental Sciences, University of California.
1662 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 5, 2010

negatively charged at the prevailing pH conditions (1). In
this case, Derjaguin-Landau-Verwey-Overbeek (DLVO)
theory (2, 3) indicates the presence of a significant energy
barrier against colloid attachment in the primary minimum
due to electrostatic repulsion. Nevertheless, a finite amountof colloids are still retained in porous media under theseunfavorable attachment conditions. Potential adhesive interactions include the secondary minimum (e.g., refs 4, 5)
and nanoscale heterogeneity (6–8). Secondary minimum
interaction may occur at a separation distance of a few nm
from the collector surface due to the summation of electrostatic repulsion and van der Waals attraction. Chemicalor physical nanoscale heterogeneities that are much smaller
than the diameter of the colloid-surface contact area may
also create favorable patches for interaction. This interaction
occurs at a separation distance from the collector surface
due to electrostatic repulsion from neighboring regions. It
should be mentioned that colloid interaction via the secondary minimum and nanoscale heterogeneity share many
similarities in that they function at a separation distance,
the interaction strength will be a function of the double layer
thickness (ionic strength), and the interaction is relatively
weak in comparison to the primary minimum.
It has recently been demonstrated that colloid retention
does not solely depend on the strength of the adhesiveinteraction as previously postulated (9–13). These new
findings suggest that pore structure and hydrodynamic forcesalong with the physicochemical factors strongly affectcolloidal retention. In particular, it has been reported thatweakly associated colloids with the solid-water interface
via the secondary minimum or nanoscale heterogeneity may
experience significant hydrodynamic forces due to fluid flowthat may result in rolling, sliding, skipping, or detachmentof colloids on/from the collector surface (9–11, 14, 15). Someof these weakly associated colloids can be translated and/or
funneled by fluid drag force to low velocity regions in small
pore spaces and “eddy zones” which occur near somegrain-grain contacts and surface roughness locations where
they can be retained (10, 12). Indeed, recent experimental
evidence by Kuznar and Elimelech (14) demonstrates thatweakly interacting colloids can be translated along thecollector surface via hydrodynamic forces and be retainedin regions near the rear stagnation point.
Although colloid interaction with porous media via thesecondary minimum and nanoscale heterogeneity are gaining
wide acceptance (6–13, 15–18), our understanding of theseweak interactions is still incomplete. For example, somerecent literature indicates that colloids associated with solid
surfaces via the secondary minimum are “freely mobile” inthe presence of fluid drag (19, 20), whereas others haveindicated that immobilization and detachment of thesecolloids will depend on a balance of applied (hydrodynamic)
and resisting (adhesive) torques and forces (9–13). These
subtle differences in the conceptual description of colloid
interaction have important implications for colloid deposition
and release that will be discussed in detail later on.
Furthermore, hysteresis in the amount of colloid retention
with IS has been observed over limited experimental conditions (21–23). The nature and cause for this hysteresis is still
not well understood. To the best of our knowledge, to date,no published studies have systematically investigated colloid
retention hysteresis over a range of IS. One logical way toinvestigate these issues is to study the influence of transientsin solution IS on colloid release. Switching the eluting fluid
to a lower IS has been observed to enhance colloid release
due to increased electrostatic repulsion and a corresponding
10.1021/es903277p

 2010 American Chemical Society

Published on Web 02/05/2010

TABLE 1. Porous Media Experimental Conditions and Mass Balancesa
dc (µm)porous media

IS (mM)approach velocity (cm/min)column length (cm)

PV0

BTC %

mass recovery %glass beads

6
16
36
66
106

0.2
0.2
0.2
0.2
0.2

5
5
5
5
5

5.2
5.2
5.2
5.2
5.2

99
98
85
70
37

100
99
93
86
83

Ottawa sand

6
16
66

0.2
0.2
0.2

5
5
5

6.1
3.5
6.5

98
73
10

100
97
64

glass beads

16
66

0.1
0.1

10
10

5.4
5.3

84
20

99
50

Ottawa sand

16
66

0.1
0.1

10
10

3.5
5.2

61
4

97
41

1.1

0.5

a
dc, colloid diameter; IS, ionic strength; PV0, colloid pulse during phase A in terms of pore volumes; BTC (%), percent
recovery of injected colloids during the phases A and B; Mass recovery (%), percent recovery of injected colloids duringphases A, B, and C.

increase in the double layer thickness (21–24). However, wide
discrepancies in the extent, rate, and shape of release havebeen reported (10, 21–23).
The literature review above indicates that our understanding of the coupled role of solution chemistry, fluidhydrodynamics, pore structure, and surface heterogeneity
on colloid retention and release is still incomplete underunfavorable conditions. The overall research objective of thiswork is to provide experimental evidence and improved
understanding of colloid retention and release under unfavorable attachment conditions. Findings from this work
provided valuable information on plausible mechanisms of
colloid retention, the resisting (adhesive) torque, colloid
retention hysteresis with IS, and colloid mass transfer on thesolid phase. All of this information increases our knowledgeof factors that influence colloid release with transients insolution chemistry in natural environments.

Material and Methods
Colloid, Electrolyte Solution, and Porous Media. Carboxylate-modified polystyrene latex microspheres (Molecular
Probes, Inc., Eugene, Oregon) of two sizes (diameters of 0.5
and 1.1 µm with negative surface charge densities of 0.1419
and 0.0175 meq g-1, respectively) were used in the experiments. Aqueous solutions used in the experiments consistedof deionized (DI) water with its pH adjusted to 10 using 1.7
mM of Na2CO3 and 1.7 mM of NaHCO3. The IS was adjusted
to desired levels using NaCl. The concentration of colloid
suspension (2.5-2.8 × 107 Nc ml-1; where Nc denotes numberof colloids) was selected to minimize any permeability
reductions of the porous medium. Ottawa sand (U.S. Silica,
Ottawa, IL) and spherical soda lime glass beads (USF Surface
Preparation, Rancho Dominguez, CA) were used as porous
media. Ottawa sand and glass beads have median grain sizes(d50) of 240 and 260 µm, respectively. The uniformity indexes(Ui ) d60/d10 where x% of the mass was finer than dx) of thesand and glass beads were measured to be 3.06 and 1.20,
respectively. Ottawa sand typically consists of 99.8% SiO2
(quartz) and trace amounts of metal oxides. Table 1 indicatesthe parameters of each column experiment.
DLVO Calculations. Colloid-colloid and colloid-collector
interaction energy profiles were calculated using DLVO theory(2, 3) as described in the Supporting Information (SI) for thetwo microsphere sizes (0.5 and 1.1 µm) in the various solution
chemistries.
Transport Experiment Protocol. A stainless steel column
with internal diameter of 5 cm was selected for this study.
The packing and pre-equilibration procedures for eachcolumn experiment is described in a previous publication(25). The porosity of the packed columns was determined
gravimetrically and it varied between 35 and 38%. Afterestablishing specified water chemistry, transport experiments
were carried out in three phases. In phase A colloid
suspension was introduced into the column at a constantrate. The colloid suspension was applied for a given numberof pore volumes (PV0) provided in Table 1. During injection,
the colloid suspension reservoir was agitated to minimize
aggregation, as verified by absorbance analyses. During phase
B colloid-free electrolyte solution was applied to the column
at the same flow rate and IS as during phase A until theeffluent colloid concentration returned to a baseline level.
Finally, the influent IS was lowered in several steps (to study
mobilization/release behavior of colloids) during phase C.
After changing the IS in each step, the column was flushedwith the lowered IS solution until the change in effluent
concentration was minimal. During the transport experiment,
effluent samples were collected at selected intervals by afraction collector and analyzed for colloid concentration using
a spectrophotometer (Perkin-Elmer LC95 UV/vis spectrophotometer, Irvine, CA) at a wavelength of 480 nm. The colloid
mass recovery (%) was calculated as the ratio of the amountof eluted colloids during all three stages to the amount of
introduced colloids
Micromodel Experiments. Transport experiments werealso conducted in a specially designed micromodel (26) tomicroscopically examine the retention and release behavior
of colloids in the porous media at different ISs. Details on
experimental micromodel procedures are given in Bradford
et al. (26) and the SI.

Results
DLVO Calculations. Table S1 in the SI presents calculated
colloid-colloid and colloid-collector interaction energies
from DLVO theory when the solution ISs were 15, 60, and100 mM. Results indicate that colloid-colloid and colloidcollector interactions are repulsive for both colloids (0.5 and1.1 µm). However, it can be observed that the depth of thesecondary energy minimum and the height of the repulsive
energy barriers increases and decreases, respectively, withincreasing IS. Diffusion of colloids over these energy barriers
is reported to be highly unlikely (27). Hence, DLVO calculations indicate unfavorable attachment conditions for the
primary minimum, but a potential for a weak association via
the secondary minimum.
Colloid Deposition and Release in Column Studies.
Figure 1 presents representative colloid breakthrough concentrations (BTCs) in the effluent (during phases A and B)
obtained from columns packed with glass beads at the various
VOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1663

FIGURE 1. Breakthrough curves for 1.1 µm latex colloids in glass beads when the Darcy velocity was approximately 0.2 cm min-1,
the pH was 10, and the ionic strength was 6, 16, 36, 66, and 106 mM. Here the normalized effluent concentration (C/C0) is plotted as afunction of pore volumes (PV) passed through the column.
solution ISs. Here the normalized effluent concentration (C/C0) is plotted as a function of pore volumes (PV) passed
through the column. An increase in IS resulted in enhanced
colloid deposition, consistent with previous column transport
studies that involved a variety of colloidal particles and porous
media (4, 5, 10, 17). The time dependent trend observed in
the rising limb of the BTC indicates that either the retention
sites were being filled up during colloid injection (blocking)or simultaneous release (detachment) of deposited colloids
was occurring during the transport experiment. However,blocking or filling of available locations for deposition seems
to be a more likely explanation because colloid detachment
produced insignificant tailing in the BTCs.
The BTCs obtained from experiments conducted with
Ottawa sand (SI Figure S1) exhibited a similar trend with IS
as shown in Figure 1, however more colloid retention
occurred in the sand columns compared with the glass beads
at a given IS. Table 1 presents the recovery (%) of injected
0.5 and 1.1 µm colloids during phases A and B (referred toas BTC%) for both glass bead and Ottawa sand at the various
ISs. Most of these colloid transport experiments wereconducted in duplicate at each IS, and exhibited good
reproducibility.
Figure 2 presents typical behavior of colloid release after
lowering the influent IS in several steps during phase C. The
results of other transient IS experiments are summarized inTable 2. A sudden peak in effluent concentration wasobserved following a decrease in solution IS. Further
comparison of the release data with tracer experimentssuggests that the step change in solution IS resulted in a
rapid, pulse-like release of colloids that appears within thefront of the solution with lower IS. However, the colloid
concentration in the effluent sharply dropped after the front
and exhibited a small tail. A significant amount of released
particles occurred when the influent was switched to DI waterat the last step. However, the mass recovery data (Table 2)shows that a significant fraction of the deposited colloids
was not released in the experiments at higher ISs (i.e., 66 and
106 mM) even after flushing the column with DI water. To
further investigate the possible recovery of the retained
colloids, a flow interruption of 12 up to 24 h was applied forselect experiments. To our surprise, the results showed that
only a very small fraction of the remaining colloids wasrecovered after the flow was restarted.
1664 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 5, 2010

Figure 3 summarizes the colloid retention data at thevarious ISs for the 1.1 µm colloids in glass beads and Ottawa
sand. Here the percentages of retained colloids are plotted
as a function of the solution IS history. The arrows indicate
the direction of the IS change, with arrows facing to the rightindicating results obtained during phases A and B, whereas
those facing to the left for phase C. Figure 3 clearly indicatesthat the amount of colloid retention at a given IS was afunction of the solution IS history. In particular, observe thatthe amount of colloid retention at a given IS increased
as the initial IS increased during phases A and B. Furthermore,
the Ottawa sand has much greater colloid retention than theglass beads for a similar IS history. These findings hold forboth 1.1 and 0.5 µm colloids (Table 2), although exact
comparison of these results is not possible due to differencesin the column length and the approach velocity (Table 1). To
the best of our knowledge, this is the first study tosystematically demonstrate colloid retention hysteresis forvarious ISs, porous media types, and colloid sizes.
Colloid Deposition and Release in Micromodel Studies.
Micromodel studies were conducted to better understandthe factors influencing the observed colloid retention hysteresis. Figure 4 presents micromodel photos of retained
colloids in glass beads when the initial deposition (phases
A and B) was conducted at a solution IS of 106 mM, and thenthe eluting solution IS was decreased in a stepwise fashion
to 66, 31, 6, and 0 (DI water) mM (phase C). The photos
demonstrate that the retained colloids were relatively
uniformly distributed on the collector surfaces when the ISs
were 106, 66, 31, and 6 mM. In contrast, under DI waterconditions colloid retention was only possible (almost
exclusively) near grain-grain contact points. A similar
micromodel experiment to that described above was conducted using Ottawa sand (SI Figure S2). The observations
were comparable with those for the glass beads.

Discussion
Results presented in Figures 1-4 provide valuable information to better understand the nature of underlying mechanisms for colloid deposition and release under unfavorable
attachment conditions. Discussion provided in the SI indicates that irreversible retention of colloids in the primary
minimum is likely to play only a minor role in colloid retention
under the selected experimental conditions. Below we will

FIGURE 2. Representative plots of the relative effluent concentration (C/C0) as a function of pore volumes (PV) for transient ionicstrength (IS) experiments. Figure 2a and b were for 1.1 µm latex microspheres in glass beads and Ottawa sand, respectively, andFigure 2c for 0.5 µm colloids in glass beads. The sequence of the transient experiments was (i) deposition of colloids in the column
during phase A; (ii) elution with the same electrolyte solution (without colloids) during phase B; and (iii) the eluting solution IS was
lowered in several steps indicated in the figure legend during phase C. Other experimental conditions are summarized in Tables 1
and 2.
examine two potential explanations for observed colloid
retention and release during transient in IS, namely: interac-tion in the secondary minimum, and nanoscale surface
roughness and chemical heterogeneity.
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TABLE 2. Detailed Mass Balance Information for Transient IS Experimentsa
cumulative mass recovery in effluent (%)
dc (µm)porous media

initial IS (mM)glass beads

6
16
36
66
106

1.1

37

66 mM

70
43

36 mM

16 mM

6 mM

DI

85
71
52

98
86
73
66

99
99
90
78
72

100
99
94
86
83

98
11

73
15

28

100
97
64

90
32

100
49

71
28

96
41

Ottawa sand

6
16
66

glass beads

16
66

20

20

84
26

Ottawa sand

16
66

4

8

61
13

0.5

a

106 mM10dc, colloid diameter; IS, ionic strength; DI, deionized water.

FIGURE 3. A summary of the colloid retention data during thetransient IS experiments for the 1.1 µm colloids in glass beads
and Ottawa sand. Here the percentages of retained colloids areplotted as a function of the solution IS history. The arrows
indicate the direction of the IS change, with arrows facing to
the right indicating results obtained during phases A and B,
whereas those facing to the left for phase C.
Secondary Minimum. Table S1 in the SI indicates thepresence of a secondary minimum for all of the conditions
considered herein. Figure 2 indicates that colloid releaseoccurred in a stepwise fashion with IS. This observation isconsistent with the assumption of a resisting (adhesive)
torque and force for colloids associated with the solid surface
via the secondary minimum (9–11, 13). Furthermore, theexperimental observations show a sharp peak of colloid
release upon virtually complete removal of the secondary
minimum (DI water). Hence, the deposited colloids were
not exclusively retained in zones of “flow stagnation” as has
sometimes been assumed in the literature (19, 20) because
the colloid release was not diffusion controlled.
The fraction of the collector surface that is “chemicallyand hydrodynamically” favorable for colloid retention (Sf)may be determined using the torque balance approachassuming rolling is the dominant detachment mechanism(9, 10). In the absence of surface roughness and chemical
heterogeneity, this approach predicts that Sf decreases withdecreasing IS in a nonlinear fashion, and that retention occurs
in progressively lower velocity regions of the pore space.
Torque balance calculations indicate that low velocity regions
near some grain-grain contacts are usually hydrodynamicallyfavorable for retention (10). This prediction is supported by
micromodel observations shown in Figure 4 under DI waterconditions when the secondary minimum was eliminated.
In contrast, Figure 4 indicates that the retained colloids were1666 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 5, 2010

relatively uniformly distributed on the collector surfaces whenthe IS was decreased in a stepwise fashion from 106, 66, 31,
to 6 mM.
Surface roughness has also been demonstrated to play animportant factor in colloid retention (28, 29). Surface
roughness locations may influence the applied hydrodynamic
force and torque. In particular, when the roughness is greaterthan the colloid radius, the applied hydrodynamic torque
will be zero (30), and create locations that are hydrodynamically favorable for retention (similar to regions neargrain-grain contacts) even under very low values of IS. Smartand Leighton (31) reported that the surface roughness ofglass beads is approximately 0.01 to 0.001 times the beadradius depending on the manufacturing process. This yields
an estimate of surface roughness ranging from 1.3 to 0.13
µm. Hence, surface roughness provides one plausible explanation for the relatively uniform distribution of retained
colloids shown in Figure 4 when IS > 0 mM. Differences inthe diffusion distance at grain-grain contacts and surface
roughness may help to explain the relatively large pulse ofcolloid release when the IS was changed from 6 mM to DI
water in Figure 2 as the depth of secondary minimum wascompletely eliminated.
The observed hysteresis in colloid retention can be
explained in part by colloid mass transfer on the solid phaseas will be discussed below. Filtration theory (32) predicts
that the amount of colloid retention during phase A will
depend primarily on the collector efficiency (η) and the
sticking efficiency (R). The value of η is related to the masstransfer rate of colloids toward the solid phase and is typicallyquantified using correlation equations (e.g., ref 33). The valueof R is the probability that colloids colliding with the collector
will remain associated with the solid (24). Under unfavorable
attachment conditions the value of R has been estimatedfrom the depth of the secondary minimum and the distribution of kinetic energies associated with diffusing colloids (34).
In this case, the value of R is predicted to increase in anonlinear fashion with IS. This approach, however, neglects
the influence of hydrodynamics and pore structure on colloid
retention. Colloids that collide with the solid surface andbecome weakly associated with the surface via the secondary
minimum can be translated and/or funneled by fluid dragforces to low velocity regions and “eddy zones” neargrain-grain contact points and surface roughness locations.
A larger number of colloids may entered these low velocity
regions at higher IS due to the greater depth of secondary
minimum.
The amount of colloid mass transfer on the solid surface
during phases A and B is therefore expected to be greater

FIGURE 4. Representative photos of 1.1 µm colloid retention in the glass beads during the various ionic strength (IS) stages of atransient micromodel experiment. The initial deposition stage occurred at an IS of 106 mM, and then the eluting solution IS was
lowered in a stepwise fashion to 56, 31, 6, and 0 mM solutions. The Darcy velocity was 0.2 cm min-1, and the flow was from the left
to the right.
with increasing IS. It is likely that some of the colloids beingtransported on the solid surface and even a fraction of thedetached colloids during phase C may be subsequently
deposited in the low velocity regions. This hypothesis is
supported by results shown in Figure 3 in that the differencein the amount of colloid retention at a given IS increased
with the deviation in the initial (phases A and B) and final(phase C) IS conditions. This trend is more pronounced athigh ISs due to the nonlinear dependence of colloid masstransfer on the solid surface with IS (9, 10, 30). The observed
differences in the colloid retention hysteresis for glass beads
and Ottawa sand (Figure 4) may be due to the fact that theirregular shape and surface roughness of the Ottawa sandhas a greater length and number of low velocity regions
compared with that of spherical glass beads that produces
a larger value of Sf (35).
Table 1 presents the mass recovery of introduced colloids
in the effluent after the completion of the transient IS
experiments. Even after elimination of the secondary mini-

mum following a rinse with DI water and flow interruption,
only a fraction of the input colloids was recovered in theeffluent for the experiments conducted at high ISs such as
66 and 106 mM. One potential explanation is due to theaggregation of those colloids trapped in low velocity regions.
Micromodel observations sometimes indicated that colloid
aggregation may have occurred near grain-grain contacts(see for example the bottom right-hand photo in Figure 4).
Similarly, Bradford et al. (36) and Tong et al. (37) visuallyobserved that colloid aggregation occurred near grain-tograin contact points under unfavorable conditions. It is
plausible that colloid aggregation is facilitated by confinement
in hydrodynamically isolated regions near grain-to-grain
contact points due to an increased probability of collisionand hydrodynamic forces. In other words, the funneling of
colloids into these low velocity regions may allow free colloids
to overcome repulsion between each other, and produce
aggregation. Hence, aggregation is likely to be in partVOL. 44, NO. 5, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1667

responsible for the observed incomplete effluent mass
recovery of colloids.
Nanoscale Surface Roughness and Chemical Heterogeneity. The observed colloid transport and retention behavior
may be explained by the combination of secondary minimum
interaction, mass transfer, pore structure, surface roughness,
and aggregation as described above. An alternative hypothesis
for colloid retention under unfavorable conditions is nanoscalechemical and/or physical (roughness) heterogeneity.
Nanoscale chemical heterogeneity (6–8, 15, 18, 38–40) is
assumed to be much smaller in size than the colloidal
particles. In this case, the interaction force from neighboring
regions will have a significant effect on the adhesive force
and resisting torque that act on colloids near solid surfaces(18). These nanoscale chemical heterogeneities have been
demonstrated to produce attachment on surfaces, even when
the average charge properties are unfavorable for attachment(39, 40). Results presented by Duffadar and Davis (18) indicatethat the magnitude of the attractive electrostatic force
between the colloid and the “favorable nanoscale patches”
increases with IS due to compression of the double layer
thickness and is also a function of the colloid size. Similar
to the secondary minimum discussed above, colloids may
interact with “favorable nanoscale patches” at a separation
distance, and results in attachment, detachment or rollingon the solid surface depending on the local hydrodynamics,
the density and charge of the nanoscale patches, and theBrownian force (38). Hence, it is logical to expect that colloid
attachment to “favorable nanoscale patches” is a functionof the IS. Specifically, a larger number of colloids may interact
with “favorable nanoscale patches” at a thinner (higher IS)
than a thicker (lower IS) double layer as they move near thesolid surface and find a local minima in the interaction energy.
Furthermore, in contrast to the secondary minimum attachment, it is logical to anticipate that it would be moredifficult to detach colloids from local minima found underhigher than lower IS conditions by changing the solution
chemistry or hydrodynamic force. This phenomenon mightalso cause colloid retention hysteresis.
It should also be mentioned that repulsive interaction
between a colloidal particle and a solid surface is lower ona rough surface compared to a smooth surface (41). The
reduction in the energy barrier is strongly correlated withthe magnitude of nanoscale surface roughness (41). Similar
to nanoscale chemical heterogeneity, these findings suggestthat surface roughness can create locally favorable conditions
for colloid deposition on macroscopically unfavorable substrates. It is possible that nanoscale surface roughness andnanoscale chemical heterogeneity may influence colloid
adhesive interactions in a similar way. Nanoscale chemicaland physical heterogeneity also provides an alternative
explanation to the relatively uniform distribution of retained
colloids when the IS g 6 mM (Figure 4), for the incomplete
recovery of the colloids in DI water (i.e., complete eliminationof secondary minimum) following periods of flow interruption, and for the observed colloid retention hysteresis. The
relative importance of secondary minimum, nanoscale
heterogeneity, and surface roughness on colloid retention isdifficult to quantify, and the above discussion suggests all ofthese factors are likely to play a role and to function in asimilar manner.

Acknowledgments
This research was supported by the 206 Manure andByproduct Utilization Project of the USDA-ARS, and by agrant from NRI (NRI 2006-02541). Mention of trade names
and company names in this manuscript does not imply any
endorsement or preferential treatment by the USDA.
1668 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 5, 2010

Supporting Information Available
A brief description and discussion of (i) the DLVO calculations
and results; (ii) the micromodel procedures and results for
Ottawa sand; (iii) representative colloid breakthrough curves
for different IS in the Ottawa sand; and (iv) discussion related
to the potential for colloids to interact via the primary
minimum. This material is available free of charge via theInternet at http://pubs.acs.org.

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